What is the protein structure of the SARS-CoV-2 spike?

Binding Ability Prediction between Spike Protein and Human ACE2 Reveals the Adaptive Strategy of SARS-CoV-2 in Humans

SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2) is a novel coronavirus causing an outbreak of COVID-19 globally in the past six months. A relatively higher divergence on the spike protein of SASR-CoV-2 enables it to transmit across species efficiently. We particularly believe that the adaptive mutations of the receptor-binding domain (RBD) of spike protein in SARS-CoV-2 might be essential to its high transmissibility among humans. Thus here we collected 2,142 high-quality genome sequences of SARS-CoV-2 from 160 regions in over 50 countries and reconstructed their phylogeny, and also analyzed the interaction between the polymorphisms of spike protein and human ACE2 (hACE2). Phylogenetic analysis of SARS-CoV-2 and coronavirus in other hosts show SARS-CoV-2 is highly possible originated from Bat-CoV (RaTG13) found in horseshoe bat and a recombination event may occur on the spike protein of Pangolin-CoV to imbue it the ability to infect humans. Moreover, compared to the S gene of SARS-CoV-2, it is more conserved in the direct-binding sites of RBD and we noticed that spike protein of SARS-CoV-2 may under a consensus evolution to adapt to human hosts better. 3,860 amino acid mutations in spike protein RBD (T333-C525) of SARS-CoV-2 were simulated and their stability and affinity binding to hACE2 (S19-D615) were calculated. Our analysis indicates SARS-CoV-2 could infect humans from different populations with no preference, and a higher divergence in the spike protein of SARS-CoV-2 at the early stage of this pandemic may be a good indicator that could show the pathway of SARS-CoV-2 transmitting from the natural reservoir to human beings.

Molecular architecture of the SARS-CoV-2 virus

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is an enveloped virus responsible for the COVID-19 pandemic. Despite recent advances in the structural elucidation of SARS-CoV-2 proteins and the complexes of the spike (S) proteins with the cellular receptor ACE2 or neutralizing antibodies, detailed architecture of the intact virus remains to be unveiled. Here we report the molecular assembly of the authentic SARS-CoV-2 virus using cryo-electron tomography (cryo-ET) and subtomogram averaging (STA). Native structures of the S proteins in both pre- and postfusion conformations were determined to average resolutions of 9-11 Å. Compositions of the N-linked glycans from the native spikes were analyzed by mass spectrometry, which revealed highly similar overall processing states of the native glycans to that of the recombinant glycoprotein glycans. The in situ architecture of the ribonucleoproteins (RNP) and its higher-order assemblies were revealed. These characterizations have revealed the architecture of the SARS-CoV-2 virus to an unprecedented resolution, and shed lights on how the virus packs its ~30 Kb long single-segmented RNA in the ~80 nm diameter lumen. Overall, the results unveiled the molecular architecture and assembly of the SARS-CoV-2 in native context.

Structural Basis of SARS-CoV-2 Spike Protein Priming by TMPRSS2

Entry of SARS-CoV-2, etiological agent of COVID-19, in the host cell is driven by the interaction of its spike protein with human ACE2 receptor and a serine protease, TMPRSS2. Although complex between SARS-CoV-2 spike protein and ACE2 has been structurally resolved, the molecular details of the SARS-CoV-2 and TMPRSS2 complex are still elusive. TMPRSS2 is responsible for priming of the viral spike protein that entails cleavage of the spike protein at two potential sites, Arg685/Ser686 and Arg815/Ser816. The present study aims to investigate the conformational details of complex between TMPRSS2 and SARS-CoV-2 spike protein, in order to discern the finer details of the priming of viral spike and to point candidate drug targets. Briefly, full length structural model of TMPRSS2 was developed and docked against the resolved structure of SARS-CoV-2 spike protein with directional restraints of both cleavage sites. The docking simulations showed that TMPRSS2 interacts with the two different loops of SARS-CoV-2 spike protein, each containing different cleavage sites. Key functional residues of TMPRSS2 (His296, Ser441 and Ser460) were found to interact with immediate flanking residues of cleavage sites of SARS-CoV-2 spike protein. Compared to the N-terminal cleavage site (Arg685/Ser686), TMPRSS2 region that interact with C-terminal cleavage site (Arg815/Ser816) of the SARS-CoV-2 spike protein was predicted as relatively more druggable. In summary, the present study provide structural characteristics of molecular complex between human TMPRSS2 and SARS-CoV-2 spike protein and points to the candidate drug targets that could further be exploited to direct structure base drug designing.

Structure-based Design of Prefusion-stabilized SARS-CoV-2 Spikes

The COVID-19 pandemic caused by the novel coronavirus SARS-CoV-2 has led to accelerated efforts to develop therapeutics, diagnostics, and vaccines to mitigate this public health emergency. A key target of these efforts is the spike (S) protein, a large trimeric class I fusion protein that is metastable and difficult to produce recombinantly in large quantities. Here, we designed and expressed over 100 structure-guided spike variants based upon a previously determined cryo-EM structure of the prefusion SARS-CoV-2 spike. Biochemical, biophysical and structural characterization of these variants identified numerous individual substitutions that increased protein yields and stability. The best variant, HexaPro, has six beneficial proline substitutions leading to ~10-fold higher expression than its parental construct and is able to withstand heat stress, storage at room temperature, and multiple freeze-thaws. A 3.2 Å-resolution cryo-EM structure of HexaPro confirmed that it retains the prefusion spike conformation. High-yield production of a stabilized prefusion spike protein will accelerate the development of vaccines and serological diagnostics for SARS-CoV-2.

Structural variations in human ACE2 may influence its binding with SARS-CoV-2 spike protein

The recent pandemic of COVID-19, caused by SARS-CoV-2, is unarguably the most fearsome compared with the earlier outbreaks caused by other coronaviruses, SARS-CoV and MERS-CoV. Human ACE2 is now established as a receptor for the SARS-CoV-2 spike protein. Where variations in the viral spike protein, in turn, lead to the cross-species transmission of the virus, genetic variations in the host receptor ACE2 may also contribute to the susceptibility and/or resistance against the viral infection. This study aims to explore the binding of the proteins encoded by different human ACE2 allelic variants with SARS-CoV-2 spike protein. Briefly, coding variants of ACE2 corresponding to the reported binding sites for its attachment with coronavirus spike protein were selected and molecular models of these variants were constructed by homology modeling. The models were then superimposed over the native ACE2 and ACE2-spike protein complex, to observe structural changes in the ACE2 variants and their intermolecular interactions with SARS-CoV-2 spike protein, respectively. Despite strong overall structural similarities, the spatial orientation of the key interacting residues varies in the ACE2 variants compared with the wild-type molecule. Most ACE2 variants showed a similar binding affinity for SARS-CoV-2 spike protein as observed in the complex structure of wild-type ACE2 and SARS-CoV-2 spike protein. However, ACE2 alleles, rs73635825 (S19P) and rs143936283 (E329G) showed noticeable variations in their intermolecular interactions with the viral spike protein. In summary, our data provide a structural basis of potential resistance against SARS-CoV-2 infection driven by ACE2 allelic variants.

Critical Differences between the Binding Features of the Spike Proteins of SARS-CoV-2 and SARS-CoV

The COVID-19 caused by SARS-CoV-2 has spread globally and caused tremendous loss of lives and properties, and it is of utmost urgency to understand its propagation process and to find ways to slow down the epidemic. In this work, we used a coarse-grained model to calculate the binding free energy of SARS-CoV-2 or SARS-CoV to their human receptor ACE2. The investigation of the free energy contribution of the interacting residues indicates that the residues located outside the receptor binding domain are the source of the stronger binding of the novel virus. Thus, the current results suggest that the essential evolution of SARS-CoV-2 happens remotely from the binding domain at the spike protein trimeric body. Such evolution may facilitate the conformational change and the infection process that occurs after the virus is bound to ACE2. By studying the binding pattern between SARS-CoV antibody m396 and SARS-CoV-2, it is found that the remote energetic contribution is missing, which might explain the absence of cross-reactivity of such antibodies.

[Image: see text] The COVID-19 caused by SARS-CoV-2 has spread globally and caused tremendous loss of lives and properties, and it is of utmost urgency to understand its propagation process and to find ways to slow down the epidemic. In this work, we used a coarse-grained model to calculate the binding free energy of SARS-CoV-2 or SARS-CoV to their human receptor ACE2. The investigation of the free energy contribution of the interacting residues indicates that the residues located outside the receptor binding domain are the source of the stronger binding of the novel virus. Thus, the current results suggest that the essential evolution of SARS-CoV-2 happens remotely from the binding domain at the spike protein trimeric body. Such evolution may facilitate the conformational change and the infection process that occurs after the virus is bound to ACE2. By studying the binding pattern between SARS-CoV antibody m396 and SARS-CoV-2, it is found that the remote energetic contribution is missing, which might explain the absence of cross-reactivity of such antibodies.

Novel ACE2-Independent Carbohydrate-Binding of SARS-CoV-2 Spike Protein to Host Lectins and Lung Microbiota

The immediate call for translational research in the field of coronavirus disease (COVID-19) pandemic, needs new and unexplored angles to support and contribute to this important worldwide health problem. The aim of this study is to better understand the pathogenic mechanisms underlying COVID-19, deciphering the carbohydrate-mediated interactions of the SARS-CoV-2 spike protein. We studied the carbohydrate-binding receptors that could be important for viral entry and for immune-modulatory responses, and we studied the interactions of the spike protein with the host lung microbiota. Exploring solid-phase immunoassays, we evaluated the interactions between the SARS-CoV-2 spike protein and a library of 12 different human carbohydrate-binding proteins (C-type lectins and Siglecs) involved in binding, triggering and modulation of innate and adaptive immune-responses. We revealed a specific binding of the SARS-CoV-2 spike protein to the receptors DC-SIGN, MGL, Siglec-9 and Siglec-10 that are all expressed on myeloid immune cells. In addition, because the lung microbiota can promote or modulate viral infection, we studied the interactions between the SARS-CoV-2 spike protein and a library of Streptococcus pneumoniae capsular polysaccharides, as well as other bacterial glyco-conjugates. We show specific binding of the spike protein to different S. pneumoniae capsular polysaccharides (serotypes 19F and 23F but not to serotype 14). Moreover we demonstrated a specific binding of SARS-CoV-2 spike protein to the lipopolysaccharide from the opportunistic human pathogen Pseudomonas aeruginosa, one of the leading cause of acute nosocomial infections and pneumonia. Interestingly, we identified rhamnosylated epitopes as one of the discriminating structures in lung microbiota to bind SARS-CoV-2 spike protein. In conclusion, we revealed novel ACE2-independent carbohydrate-mediated interactions with immune modulating lectins expressed on myeloid cells, as well as host lung microbiota glyco-conjugates. Our results identified new molecular pathways using host lectins and signalling, that may contribute to viral infection and subsequent immune exacerbation. Moreover we identified specific rhamnosylated epitopes in lung microbiota to bind SARS-CoV-2, providing a hypothetical link between the presence of specific lung microbiota and SARS-CoV-2 infection and severity.

Considerations around the SARS-CoV-2 Spike Protein with particular attention to COVID-19 brain infection and neurological symptoms

Spike protein (S protein) is the virus 'key' to infect cells being able to strongly bind to the human angiotensin-converting enzyme2 (ACE2), as it has been reported. In fact, Spike structure and function is known to be highly important for cell infection as well as entering the brain. Growing evidence indicates that different types of coronaviruses not only affect the respiratory system, but they might also invade the central nervous system (CNS). However, very few evidence have been so far reported on the presence of COVID-19 in the brain and the potential exploitation, by this virus, of lung to brain axis to reach neurons has not completely understood. In this article we assessed the SARS-CoV and SARS-CoV-2 Spike protein sequence, structure and electrostatic potential using computational approaches. Our results showed that the S proteins of SARS-CoV-2 and SARS-CoV are highly similar, sharing a sequence identity of 77%. In addition, we found that the SARS-CoV-2 S protein is slightly more positively charged than that of SARS-CoV since it contains four more positively charged residues and five less negatively charged residues which may lead to an increased affinity to bind to negatively charged regions of other molecules through non-specific and specific interactions. Analyzing of the S protein binds to the host ACE2 receptor showed a 30% higher binding energy for SARS-CoV-2 than the SARS-CoV S protein. These results might be useful for understanding the mechanism of cell entry, blood brain barrier crossing and clinical features related to the CNS infection by SARS-CoV-2.

An engineered stable mini-protein to plug SARS-Cov-2 Spikes

The novel betacoronavirus SARS-CoV-2 is the etiological agent of the current pandemic COVID-19. Like other coronaviruses, this novel virus relies on the surface Spike glycoprotein to access the host cells, mainly through the interaction of its Receptor Binding Domain (RBD) with the human angiotensin-converting enzyme 2 (ACE2). Therefore, molecular entities able to interfere with binding of the SARS-CoV-2 Spike protein to ACE2 have a great potential to inhibit viral entry. Starting from the available structural data on the interaction between SARS-CoV-2 Spike protein and the host ACE2 receptor, we here engineered a mini-protein with the aim of creating a soluble and stable Spike interactor. This mini-protein, which was recombinantly produced in high yields, possesses a stable α helical conformation and is able to interact with the RBD of glycosylated Spike protein from SARS-CoV-2 with nanomolar affinity, as measured by microscale thermophoresis. By plugging the Spike protein, our mini-protein constitutes a valid tool for the development of treatments against different types of coronavirus.

Interaction of the spike protein RBD from SARS-CoV-2 with ACE2: similarity with SARS-CoV, hot-spot analysis and effect of the receptor polymorphism

The spread of COVID-19 caused by the SARS-CoV-2 outbreak has been growing since its first identification in December 2019. The publishing of the first SARS-CoV-2 genome made a valuable source of data to study the details about its phylogeny, evolution, and interaction with the host. Protein-protein binding assays have confirmed that Angiotensin-converting enzyme 2 (ACE2) is more likely to be the cell receptor through which the virus invades the host cell. In the present work, we provide an insight into the interaction of the viral spike Receptor Binding Domain (RBD) from different coronavirus isolates with host ACE2 protein. By calculating the binding energy score between RBD and ACE2, we highlighted the putative jump in the affinity from a progenitor form of SARS-CoV-2 to the current virus responsible for COVID-19 outbreak. Our result was consistent with previously reported phylogenetic analysis and corroborates the opinion that the interface segment of the spike protein RBD might be acquired by SARS-CoV-2 via a complex evolutionary process rather than a progressive accumulation of mutations. We also highlighted the relevance of Q493 and P499 amino acid residues of SARS-CoV-2 RBD for binding to human ACE2 and maintaining the stability of the interface. Moreover, we show from the structural analysis that it is unlikely for the interface residues to be the result of genetic engineering. Finally, we studied the impact of eight different variants located at the interaction surface of ACE2, on the complex formation with SARS-CoV-2 RBD. We found that none of them is likely to disrupt the interaction with the viral RBD of SARS-CoV-2.

Interaction of the spike protein RBD from SARS-CoV-2 with ACE2: Similarity with SARS-CoV, hot-spot analysis and effect of the receptor polymorphism

Considerations around the SARS-CoV-2 Spike Protein with particular attention to COVID-19 brain infection and neurological symptoms.

Spike protein recognition of mammalian ACE2 predicts the host range and an optimized ACE2 for SARS-CoV-2 infection

Abstract SARS-CoV-2 causes the recent global COVID-19 public health emergency. ACE2 is the receptor for both SARS-CoV-2 and SARS-CoV. To predict the potential host range of SARS-CoV-2, we analyzed the key residues of ACE2 for recognizing S protein. We found that most of the selected mammals including pets (dog and cat), pangolin and Circetidae mammals remained the most of key residues for association with S protein from SARS-CoV and SARS-CoV-2. The interaction interface between cat/dog/pangolin/Chinese hamster ACE2 and SARS-CoV/SARS-CoV-2 S protein was simulated through homology modeling. We identified that N82 in ACE2 showed a closer contact with SARS-CoV-2 S protein than M82 in human ACE2. Our finding will provide important insights into the host range of SARS-CoV-2 and a new strategy to design an optimized ACE2 for SARS-CoV-2 infection.

Distinct Structural Flexibility within SARS-CoV-2 Spike Protein Reveals Potential Therapeutic Targets

The emergence and rapid worldwide spread of the novel coronavirus disease, COVID-19, has prompted concerted efforts to find successful treatments. The causative virus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), uses its spike (S) protein to gain entry into host cells. Therefore, the S protein presents a viable target to develop a directed therapy. Here, we deployed an integrated artificial intelligence with molecular dynamics simulation approach to provide new details of the S protein structure. Based on a comprehensive structural analysis of S proteins from SARS-CoV-2 and previous human coronaviruses, we found that the protomer state of S proteins is structurally flexible. Without the presence of a stabilizing beta sheet from another protomer chain, two regions in the S2 domain and the hinge connecting the S1 and S2 subunits lose their secondary structures. Interestingly, the region in the S2 domain was previously identified as an immunodominant site in the SARS-CoV-1 S protein. We anticipate that the molecular details elucidated here will assist in effective therapeutic development for COVID-19.

Cross-reactive antibody response between SARS-CoV-2 and SARS-CoV infections

Summary The World Health Organization has declared the ongoing outbreak of COVID-19, which is caused by a novel coronavirus SARS-CoV-2, as pandemic. There is currently a lack of knowledge about the antibody response elicited from SARS-CoV-2 infection. One major immunological question concerns antigenic differences between SARS-CoV-2 and SARS-CoV. We address this question by analyzing plasma from patients infected by SARS-CoV-2 or SARS-CoV, and from infected or immunized mice. Our results show that, while cross-reactivity in antibody binding to the spike protein is common, cross-neutralization of the live viruses may be rare, indicating the presence of non-neutralizing antibody response to conserved epitopes in the spike. Whether such low or non-neutralizing antibody response leads to antibody-dependent disease enhancement needs to be addressed in the future. Overall, this study not only addresses a fundamental question regarding antigenicity differences between SARS-CoV-2 and SARS-CoV, but also has implications for immunogen design and vaccine development.

Key residues of the receptor binding motif in the spike protein of SARS-CoV-2 that interact with ACE2 and neutralizing antibodies

Coronavirus disease 2019 (COVID-19), caused by the novel human coronavirus SARS-CoV-2, is currently a major threat to public health worldwide. The viral spike protein binds the host receptor angiotensin-converting enzyme 2 (ACE2) via the receptor-binding domain (RBD), and thus is believed to be a major target to block viral entry. Both SARS-CoV-2 and SARS-CoV share this mechanism. Here we functionally analyzed the key amino acid residues located within receptor binding motif of RBD that may interact with human ACE2 and available neutralizing antibodies. The in vivo experiments showed that immunization with either the SARS-CoV RBD or SARS-CoV-2 RBD was able to induce strong clade-specific neutralizing antibodies in mice; however, the cross-neutralizing activity was much weaker, indicating that there are distinct antigenic features in the RBDs of the two viruses. This finding was confirmed with the available neutralizing monoclonal antibodies against SARS-CoV or SARS-CoV-2. It is worth noting that a newly developed SARS-CoV-2 human antibody, HA001, was able to neutralize SARS-CoV-2, but failed to recognize SARS-CoV. Moreover, the potential epitope residues of HA001 were identified as A475 and F486 in the SARS-CoV-2 RBD, representing new binding sites for neutralizing antibodies. Overall, our study has revealed the presence of different key epitopes between SARS-CoV and SARS-CoV-2, which indicates the necessity to develop new prophylactic vaccine and antibody drugs for specific control of the COVID-19 pandemic although the available agents obtained from the SARS-CoV study are unneglectable.

Biophysical characterization of the SARS-CoV-2 spike protein binding with the ACE2 receptor and implications for infectivity

SARS-CoV-2 is a novel highly virulent pathogen which gains entry to human cells by binding with the cell surface receptor – angiotensin converting enzyme (ACE2). We computationally contrasted the binding interactions between human ACE2 and coronavirus spike protein receptor binding domain (RBD) of the 2002 epidemic-causing SARS-CoV-1, SARS-CoV-2, and bat coronavirus RaTG13 using the Rosetta energy function. We find that the RBD of the spike protein of SARS-CoV-2 is highly optimized to achieve very strong binding with human ACE2 (hACE2) which is consistent with its enhanced infectivity. SARS-CoV-2 forms the most stable complex with hACE2 compared to SARS-CoV-1 (23% less stable) or RaTG13 (11% less stable) while occupying the greatest number of residues in the ATR1 binding site. Notably, the SARS-CoV-2 RBD out-competes the angiotensin 2 receptor type I (ATR1) which is the native binding partner of ACE2 by 35% in terms of the calculated binding affinity. Strong binding is mediated through strong electrostatic attachments with every fourth residue on the N-terminus alpha-helix (starting from Ser19 to Asn53) as the turn of the helix makes these residues solvent accessible. By contrasting the spike protein SARS-CoV-2 Rosetta binding energy with ACE2 of different livestock and pet species we find strongest binding with bat ACE2 followed by human, feline, equine, canine and finally chicken. This is consistent with the hypothesis that bats are the viral origin and reservoir species. These results offer a computational explanation for the increased infectivity of SARS-CoV-2 and allude to therapeutic modalities by identifying and rank-ordering the ACE2 residues involved in binding with the virus.

SARS-CoV-2 Spike Glycoprotein Receptor Binding Domain is Subject to Negative Selection with Predicted Positive Selection Mutations

COVID-19 is a highly contagious disease caused by a novel coronavirus SARS-CoV-2. The interaction between SARS-CoV-2 spike protein and the host cell surface receptor ACE2 is responsible for mediating SARS-CoV-2 infection. By analyzing the spike-hACE2 interacting surface, we predicted many hot spot residues that make major contributions to the binding affinity. Mutations on most of these residues are likely to be deleterious, leading to less infectious virus strains that may suffer from negative selection. Meanwhile, several residues with mostly advantageous mutations have been predicted. It is more probable that mutations on these residues increase the transmission ability of the virus by enhancing spike-hACE2 interaction. So far, only a limited number of mutations has been reported in this region. However, the list of hot spot residues with predicted downstream effects from this study can still serve as a tracking list for SARS-CoV-2 evolution studies. Coincidentally, one advantageous mutation, p.476G>S, started to surge in the last couple of weeks based on the data submitted to the public domain, indicating that virus strains with increased transmission ability may have already spread.

Structure of the SARS-CoV-2 spike receptor-binding domain bound to the ACE2 receptor

A new and highly pathogenic coronavirus (severe acute respiratory syndrome coronavirus-2, SARS-CoV-2) caused an outbreak in Wuhan city, Hubei province, China, starting from December 2019 that quickly spread nationwide and to other countries around the world1-3. Here, to better understand the initial step of infection at an atomic level, we determined the crystal structure of the receptor-binding domain (RBD) of the spike protein of SARS-CoV-2 bound to the cell receptor ACE2. The overall ACE2-binding mode of the SARS-CoV-2 RBD is nearly identical to that of the SARS-CoV RBD, which also uses ACE2 as the cell receptor4. Structural analysis identified residues in the SARS-CoV-2 RBD that are essential for ACE2 binding, the majority of which either are highly conserved or share similar side chain properties with those in the SARS-CoV RBD. Such similarity in structure and sequence strongly indicate convergent evolution between the SARS-CoV-2 and SARS-CoV RBDs for improved binding to ACE2, although SARS-CoV-2 does not cluster within SARS and SARS-related coronaviruses1-3,5. The epitopes of two SARS-CoV antibodies that target the RBD are also analysed for binding to the SARS-CoV-2 RBD, providing insights into the future identification of cross-reactive antibodies.

Spike protein fusion loop controls SARS-CoV-2 fusogenicity and infectivity

The Spike is a hallmark coronavirus protein that determines virus fusion, entry and spread in the host, and thus holds clues for the rapid spread of the SARS-CoV-2 pandemic. We have investigated the Spike from six β-coronaviruses, including the SARS-CoV-2, and find that their surface-exposed fusion peptides constituting the fusion loop are spatially organized contiguous to each other to work synergistically for triggering the virus-host membrane fusion process. The SARS-CoV-2 fusion peptides have unique physicochemical properties, accrued in part from the presence of consecutive prolines that impart backbone rigidity which aids the virus fusogenicity. The specific contribution of these prolines has been inferred from comparative studies of their deletion mutant in a fellow murine β-coronavirus MHV-A59 that show significantly diminished fusogenicity in vitro and associated pathogenesis in vivo. The Spike cleavage-linked priming and fusogenic conformational transition steered by the fusion loop may be critical for the SARS-CoV-2 spread. Summary The Spike protein on the SARS-CoV-2 surface is the prime mediator of COVID-191 because of its central role in virus-host attachment, virus-entry, and virus-spread2. The contagious nature of SARS-CoV-2 infection has been attributed to dense glycosylation of the Spike glycoprotein3, its high affinity of binding to human ACE2 receptor4, and cleavage5. While these may be imperative, it does not explain the uncontrolled infectivity. Here we show that properties of the fusion peptides constituting the fusion loop of SARS-CoV-2 Spike that triggers the virus fusion, distinguishes it from the other five β-coronaviruses. The SARS-CoV-2 Spike trimer surface is relatively more hydrophobic, including the surface contributed by the fusion peptides. The fusion peptides are structurally rigid owing to its consecutive prolines, aromatic/hydrophobic clusters, a stretch of consecutive β-branched amino acids, and the hydrogen bonds. The role of rigidity accrued from the presence of consecutive prolines contributing to virus fusogenicity can be deciphered from our previous murine β-coronavirus, MHV-A59 studies6. The synergy brought about by the global location of the surface exposed fusion peptides, their physicochemical features, and the fusogenic conformational transition appears to drive the fusion process, which may explain the severity of the infection and widespread nature of the COVID-19 pandemic.

A highly conserved cryptic epitope in the receptor-binding domains of SARS-CoV-2 and SARS-CoV

The outbreak of COVID-19, which is caused by SARS-CoV-2 virus, continues to spread globally, but there is currently very little understanding of the epitopes on the virus. In this study, we have determined the crystal structure of the receptor-binding domain (RBD) of the SARS-CoV-2 spike (S) protein in complex with CR3022, a neutralizing antibody previously isolated from a convalescent SARS patient. CR3022 targets a highly conserved epitope that enables cross-reactive binding between SARS-CoV-2 and SARS-CoV. Structural modeling further demonstrates that the binding site can only be accessed when at least two RBDs on the trimeric S protein are in the “up” conformation. Overall, this study provides structural and molecular insight into the antigenicity of SARS-CoV-2. ONE SENTENCE SUMMARY Structural study of a cross-reactive SARS antibody reveals a conserved epitope on the SARS-CoV-2 receptor-binding domain.

Bioinformatics studies on a function of the SARS-CoV-2 spike glycoprotein as the binding of host sialic acid glycans

SARS-CoV and SARS-CoV-2 do not appear to have functions of a hemagglutinin and neuraminidase. This is a mystery, because sugar binding activities appear essential to many other viruses including influenza and even most other coronaviruses in order to bind to and escape from the glycans (sugars, oligosaccharides or polysaccharides) characteristic of cell surfaces and saliva and mucin. The S1 N terminal Domains (S1-NTD) of the spike protein, largely responsible for the bulk of the characteristic knobs at the end of the spikes of SARS-CoV and SARS-CoV-2, are here predicted to be “hiding” sites for recognizing and binding glycans containing sialic acid. This may be important for infection and the ability of the virus to locate ACE2 as its known main host cell surface receptor, and if so it becomes a pharmaceutical target. It might even open up the possibility of an alternative receptor to ACE2. The prediction method developed, which uses amino acid residue sequence alone to predict domains or proteins that bind to sialic acids, is naïve, and will be advanced in future work. Nonetheless, it was surprising that such a very simple approach was so useful, and it can easily be reproduced in a very few lines of computer program to help make quick comparisons between SARS-CoV-2 sequences and to consider the effects of viral mutations.

Comparing the Binding Interactions in the Receptor Binding Domains of SARS-CoV-2 and SARS-CoV

[Image: see text] SARS-CoV-2, since emerging in Wuhan, China, has been a major concern because of its high infection rate and has left more than six million infected people around the world. Many studies endeavored to reveal the structure of the SARS-CoV-2 compared to the SARS-CoV, in order to find solutions to suppress this high infection rate. Some of these studies showed that the mutations in the SARS-CoV spike (S) protein might be responsible for its higher affinity to the ACE2 human cell receptor. In this work, we used molecular dynamics simulations and Monte Carlo sampling to compare the binding affinities of the S proteins of SARS-CoV and SARS-CoV-2 to the ACE2. Our results show that the protein surface of the ACE2 at the receptor binding domain (RBD) exhibits negative electrostatic potential, while a positive potential is observed for the S proteins of SARS-CoV/SARS-CoV-2. In addition, the binding energies at the interface are slightly higher for SARS-CoV-2 because of enhanced electrostatic interactions. The major contributions to the electrostatic binding energies result from the salt bridges forming between R426 and ACE-2-E329 in the case of SARS-CoV and K417 and ACE2-D30 in the SARS-CoV-2. In addition, our results indicate that the enhancement in the binding energy is not due to a single mutant but rather because of the sophisticated structural changes induced by all these mutations together. This finding suggests that it is implausible for the SARS-CoV-2 to be a lab-engineered virus.

Molecular Mechanism of Evolution and Human Infection with SARS-CoV-2

The outbreak of a novel coronavirus, which was later formally named the severe acute respiratory coronavirus 2 (SARS-CoV-2), has caused a worldwide public health crisis. Previous studies showed that SARS-CoV-2 is highly homologous to SARS-CoV and infects humans through the binding of the spike protein to ACE2. Here, we have systematically studied the molecular mechanisms of human infection with SARS-CoV-2 and SARS-CoV by protein-protein docking and MD simulations. It was found that SARS-CoV-2 binds ACE2 with a higher affinity than SARS-CoV, which may partly explain that SARS-CoV-2 is much more infectious than SARS-CoV. In addition, the spike protein of SARS-CoV-2 has a significantly lower free energy than that of SARS-CoV, suggesting that SARS-CoV-2 is more stable and may survive a higher temperature than SARS-CoV. This provides insights into the evolution of SARS-CoV-2 because SARS-like coronaviruses have originated in bats. Our computation also suggested that the RBD-ACE2 binding for SARS-CoV-2 is much more temperature-sensitive than that for SARS-CoV. Thus, it is expected that SARS-CoV-2 would decrease its infection ability much faster than SARS-CoV when the temperature rises. These findings would be beneficial for the disease prevention and drug/vaccine development of SARS-CoV-2.

Computational simulations reveal the binding dynamics between human ACE2 and the receptor binding domain of SARS-CoV-2 spike protein

A novel coronavirus (the SARS-CoV-2) has been identified in January 2020 as the causal pathogen for COVID-19 pneumonia, an outbreak started near the end of 2019 in Wuhan, China. The SARS-CoV-2 was found to be closely related to the SARS-CoV, based on the genomic analysis. The Angiotensin converting enzyme 2 protein (ACE2) utilized by the SARS-CoV as a receptor was found to facilitate the infection of SARS-CoV-2 as well, initiated by the binding of the spike protein to the human ACE2. Using homology modeling and molecular dynamics (MD) simulation methods, we report here the detailed structure of the ACE2 in complex with the receptor binding domain (RBD) of the SARS-CoV-2 spike protein. The predicted model is highly consistent with the experimentally determined complex structures. Plausible binding modes between human ACE2 and the RBD were revealed from all-atom MD simulations. The simulation data further revealed critical residues at the complex interface and provided more details about the interactions between the SARS-CoV-2 RBD and human ACE2. Two mutants mimicking rat ACE2 were modeled to study the mutation effects on RBD binding to ACE2. The simulations showed that the N-terminal helix and the K353 of the human ACE2 alter the binding modes of the CoV2-RBD to the ACE2.

Comparing the binding interactions in the receptor binding domains of SARS-CoV-2 and SARS-CoV

COVID-19, since emerged in Wuhan, China, has been a major concern due to its high infection rate, leaving more than one million infected people around the world. Huge number of studies tried to reveal the structure of the SARS-CoV-2 compared to the SARS-CoV-1, in order to suppress this high infection rate. Some of these studies showed that the mutations in the SARS-CoV-1 Spike protein might be responsible for its higher affinity to the ACE2 human cell receptor. In this work, we used molecular dynamics simulations and Monte Carlo sampling to compare the binding affinities of the spike proteins of SARS-CoV and SARS-CoV-2 to the ACE2. We found that the SARS-CoV-2 binds to ACE2 stronger than SARS-CoV by 7 kcal/mol, due to enhanced electrostatic interactions. The major contributions to the electrostatic binding energies are resulting from the salt-bridges formed between R426 and ACE2-E329 in case of SARS-CoV and K417 and ACE2-D30 for SARS-CoV2. In addition, there is no significant contribution from a single mutant to the binding energies. However, these mutations induce sophisticated structural changes that enhance the binding energies. Our results also indicate that the SARS-CoV-2 is unlikely a lab engineered virus.

Minimal system for assembly of SARS-CoV-2 virus like particles

SARS-CoV-2 virus is the causative agent of COVID-19. Here we demonstrate that non-infectious SARS-CoV-2 virus like particles (VLPs) can be assembled by co-expressing the viral proteins S, M and E in mammalian cells. The assembled SARS-CoV-2 VLPs display numerous S protein spikes ideal for vaccine development. The particles have a spike to spike size of 103 ± 6 nm and a membrane diameter of 63 ± 5 nm. We further show that SARS-CoV-2 VLPs dried in ambient conditions can retain their structural integrity upon repeated scans with Atomic Force Microscopy up to a peak force of 1 nN.

Computational analysis on the ACE2-derived peptides for neutralizing the ACE2 binding to the spike protein of SARS-CoV-2

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of the COVID-19, is spreading globally and has infected more than 3 million people. It has been discovered that SARS-CoV-2 initiates the entry into cells by binding to human angiotensin-converting enzyme 2 (hACE2) through the receptor binding domain (RBD) of its spike glycoprotein. Hence, drugs that can interfere the SARS-CoV-2-RBD binding to hACE2 potentially can inhibit SARS-CoV-2 from entering human cells. Here, based on the N-terminal helix α1 of human ACE2, we designed nine short peptides that have potential to inhibit SARS-CoV-2 binding. Molecular dynamics simulations of peptides in the their free and SARS-CoV-2 RBD-bound forms allow us to identify fragments that are stable in water and have strong binding affinity to the SARS-CoV-2 spike proteins. The important interactions between peptides and RBD are highlighted to provide guidance for the design of peptidomimetics against the SARS-CoV-2.

Structure of the SARS-CoV-2 spike receptor-binding domain bound to the ACE2 receptor.

A novel and highly pathogenic coronavirus (SARS-CoV-2) has caused an outbreak in Wuhan city, Hubei province of China since December 2019, and soon spread nationwide and spilled over to other countries around the world1-3. To better understand the initial step of infection at an atomic level, we determined the crystal structure of the SARS-CoV-2 spike receptor-binding domain (RBD) bound to the cell receptor ACE2 at 2.45 Å resolution. The overall ACE2-binding mode of the SARS-CoV-2 RBD is nearly identical to that of the SARS-CoV RBD, which also utilizes ACE2 as the cell receptor4. Structural analysis identified residues in the SARS-CoV-2 RBD that are critical for ACE2 binding, the majority of which either are highly conserved or share similar side chain properties with those in the SARS-CoV RBD. Such similarity in structure and sequence strongly argue for convergent evolution between the SARS-CoV-2 and SARS-CoV RBDs for improved binding to ACE2, although SARS-CoV-2 does not cluster within SARS and SARS-related coronaviruses1-3,5. The epitopes of two SARS-CoV antibodies targeting the RBD are also analysed with the SARS-CoV-2 RBD, providing insights into the future identification of cross-reactive antibodies.

Investigation of the effect of temperature on the structure of SARS-Cov-2 Spike Protein by Molecular Dynamics Simulations

Statistical and epidemiological data imply temperature sensitivity of the SARS-CoV-2 coronavirus. However, the molecular level understanding of the virus structure at different temperature is still not clear. Spike protein is the outermost structural protein of the SARS-CoV-2 virus which interacts with the Angiotensin Converting Enzyme 2 (ACE2), a human receptor, and enters the respiratory system. In this study, we performed an all atom molecular dynamics simulation to study the effect of temperature on the structure of the Spike protein. After 200ns of simulation at different temperatures, we came across some interesting phenomena exhibited by the protein. We found that the solvent exposed domain of Spike protein, namely S1, is more mobile than the transmembrane domain, S2. Structural studies implied the presence of several charged residues on the surface of N-terminal Domain of S1 which are optimally oriented at 10-30 °C. Bioinformatics analyses indicated that it is capable of binding to other human receptors and should not be disregarded. Additionally, we found that receptor binding motif (RBM), present on the receptor binding domain (RBD) of S1, begins to close around temperature of 40 °C and attains a completely closed conformation at 50 °C. The closed conformation disables its ability to bind to ACE2, due to the burying of its receptor binding residues. Our results clearly show that there are active and inactive states of the protein at different temperatures. This would not only prove beneficial for understanding the fundamental nature of the virus, but would be also useful in the development of vaccines and therapeutics. Graphical Abstract Highlights Statistical and epidemiological evidence show that external climatic conditions influence the SARS-CoV infectivity, but we still lack a molecular level understanding of the same. Here, we study the influence of temperature on the structure of the Spike glycoprotein, the outermost structural protein, of the virus which binds to the human receptor ACE2. Results show that the Spike’s S1 domain is very sensitive to external atmospheric conditions compared to the S2 transmembrane domain. The N-terminal domain comprises of several solvent exposed charged residues that are capable of binding to human proteins. The region is specifically stable at temperatures ranging around 10-30° C. The Receptor Binding Motif adopts a closed conformation at 40°C and completely closes at higher temperatures making it unsuitable of binding to human receptors

Analysis of the mutation dynamics of SARS-CoV-2 reveals the spread history and emergence of RBD mutant with lower ACE2 binding affinity

Monitoring the mutation dynamics of SARS-CoV-2 is critical for the development of effective approaches to contain the pathogen. By analyzing 106 SARS-CoV-2 and 39 SARS genome sequences, we provided direct genetic evidence that SARS-CoV-2 has a much lower mutation rate than SARS. Minimum Evolution phylogeny analysis revealed the putative original status of SARS-CoV-2 and the early-stage spread history. The discrepant phylogenies for the spike protein and its receptor binding domain proved a previously reported structural rearrangement prior to the emergence of SARS-CoV-2. Despite that we found the spike glycoprotein of SARS-CoV-2 is particularly more conserved, we identified a mutation that leads to weaker receptor binding capability, which concerns a SARS-CoV-2 sample collected on 27th January 2020 from India. This represents the first report of a significant SARS-CoV-2 mutant, and raises the alarm that the ongoing vaccine development may become futile in future epidemic if more mutations were identified. Highlights Based on the currently available genome sequence data, we proved that SARS-COV-2 genome has a much lower mutation rate and genetic diversity than SARS during the 2002-2003 outbreak. The spike (S) protein encoding gene of SARS-COV-2 is found relatively more conserved than other protein-encoding genes, which is a good indication for the ongoing antiviral drug and vaccine development. Minimum Evolution phylogeny analysis revealed the putative original status of SARS-CoV-2 and the early-stage spread history. We confirmed a previously reported rearrangement in the S protein arrangement of SARS-COV-2, and propose that this rearrangement should have occurred between human SARS-CoV and a bat SARS-CoV, at a time point much earlier before SARS-COV-2 transmission to human. We provided first evidence that a mutated SARS-COV-2 with reduced human ACE2 receptor binding affinity have emerged in India based on a sample collected on 27th January 2020.

Protein structure analysis of the interactions between SARS-CoV-2 spike protein and the human ACE2 receptor: from conformational changes to novel neutralizing antibodies

The recent severe acute respiratory syndrome, known as Coronavirus Disease 2019 (COVID-19) has spread so much rapidly and severely to induce World Health Organization (WHO) to declare a state of emergency over the new coronavirus SARS-CoV-2 pandemic. While several countries have chosen the almost complete lock-down for slowing down SARS-CoV-2 spread, the scientific community is called to respond to the devastating outbreak by identifying new tools for diagnosis and treatment of the dangerous COVID-19. With this aim, we performed an in silico comparative modeling analysis, which allows gaining new insights into the main conformational changes occurring in the SARS-CoV-2 spike protein, at the level of the receptor-binding domain (RBD), along interactions with human cells angiotensin-converting enzyme 2 (ACE2) receptor, that favor human cell invasion. Furthermore, our analysis provides (1) an ideal pipeline to identify already characterized antibodies that might target SARS-CoV-2 spike RBD, aiming to prevent interactions with the human ACE2, and (2) instructions for building new possible neutralizing antibodies, according to chemical/physical space restraints and complementary determining regions (CDR) mutagenesis of the identified existing antibodies. The proposed antibodies show in silico high affinity for SARS-CoV-2 spike RBD and can be used as reference antibodies also for building new high-affinity antibodies against present and future coronaviruses able to invade human cells through interactions of their spike proteins with the human ACE2. More in general, our analysis provides indications for the set-up of the right biological molecular context for investigating spike RBD-ACE2 interactions for the development of new vaccines, diagnostic kits, and other treatments based on the targeting of SARS-CoV-2 spike protein.

Dynamics of the ACE2 - SARS-CoV/SARS-CoV-2 spike protein interface reveal unique mechanisms

The coronavirus disease 2019 (COVID-19) pandemic, caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is a major public health concern. A handful of static structures now provide molecular insights into how SARS-CoV-2 and SARS-CoV interact with its host target, which is the angiotensin converting enzyme 2 (ACE2). Molecular recognition, binding and function are dynamic processes. To evaluate this, multiple all atom molecular dynamics simulations of at least 500 ns each were performed to better understand the structural stability and interfacial interactions between the receptor binding domain of the spike protein of SARS-CoV-2 and SARS-CoV bound to ACE2. Several contacts were observed to form, break and reform in the interface during the simulations. Our results indicate that SARS-CoV and SARS-CoV-2 utilizes unique strategies to achieve stable binding to ACE2. Several differences were observed between the residues of SARS-CoV-2 and SARS-CoV that consistently interacted with ACE2. Notably, a stable salt bridge between Lys417 of SARS-CoV-2 spike protein and Asp30 of ACE2 as well as three stable hydrogen bonds between Tyr449, Gln493, and Gln498 of SARS-CoV-2 and Asp38, Glu35, and Lys353 of ACE2 were observed, which were absent in the SARS-CoV-ACE2 interface. Some previously reported residues, which were suggested to enhance the binding affinity of SARS-CoV-2, were not observed to form stable interactions in these simulations. Stable binding to the host receptor is crucial for virus entry. Therefore, special consideration should be given to these stable interactions while designing potential drugs and treatment modalities to target or disrupt this interface.

Cross-reactive Antibody Response between SARS-CoV-2 and SARS-CoV Infections

The World Health Organization has declared the ongoing outbreak of COVID-19, which is caused by a novel coronavirus SARS-CoV-2, a pandemic. There is currently a lack of knowledge about the antibody response elicited from SARS-CoV-2 infection. One major immunological question concerns antigenic differences between SARS-CoV-2 and SARS-CoV. We address this question by analyzing plasma from patients infected by SARS-CoV-2 or SARS-CoV and from infected or immunized mice. Our results show that, although cross-reactivity in antibody binding to the spike protein is common, cross-neutralization of the live viruses may be rare, indicating the presence of a non-neutralizing antibody response to conserved epitopes in the spike. Whether such low or non-neutralizing antibody response leads to antibody-dependent disease enhancement needs to be addressed in the future. Overall, this study not only addresses a fundamental question regarding antigenicity differences between SARS-CoV-2 and SARS-CoV but also has implications for immunogen design and vaccine development.

Role of changes in SARS-CoV-2 spike protein in the interaction with the human ACE2 receptor: An in silico analysis

Many human viral diseases are a consequence of a zoonotic event. Some of the diseases caused by these zoonotic events have affected millions of people around the world, some of which have resulted in high rates of morbidity/mortality in humans. Changes in the viral proteins that function as ligands of the host receptor may promote the spillover between species. The most recent of these zoonotic events that have caused an ongoing epidemic of high magnitude is the Covid-19 epidemics caused by SARS-CoV-2. The aim of this study was to determine the mutation(s) in the sequence of the spike protein of the SARS-CoV-2 that might be favoring human to human transmission. An in silico approach was performed, and changes were detected in the S1 subunit of the receptor-binding domain of spike. The observed changes have significant effect on SARS-CoV-2 spike/ACE2 interaction and produce a reduction in the binding energy, compared to the one of the Bat-CoV to this receptor. The data presented in this study suggest a higher affinity of the SARS-Cov-2 spike protein to the human ACE2 receptor, compared to the one of Bat-CoV spike and ACE2. This could be the cause of the rapid viral spread of SARS-CoV-2 in humans.

High affinity binding of SARS-CoV-2 spike protein enhances ACE2 carboxypeptidase activity

A novel coronavirus (SARS-CoV-2) has emerged to a global pandemic and caused significant damages to public health. Human angiotensin-converting enzyme 2(ACE2) was identified as the entry receptor for SARS-CoV-2. As a carboxypeptidase, ACE2 cleaves many biological substrates besides Ang II to control vasodilatation and permeability. Given the nanomolar high affinity between ACE2 and SARS-CoV-2 spike protein, we wonder how this interaction would affect the enzymatic activity of ACE2. Surprisingly, SARS-CoV-2 trimeric spike protein increased ACE2 proteolytic activity ~3-10 fold when fluorogenic caspase-1 substrate and Bradykinin-analog peptides were used to characterize ACE2 activity. In addition, the enhancement was mediated by ACE2 binding of RBD domain of SARS-CoV-2 spike. These results highlighted the altered activity of ACE2 during SARS-CoV-2 infection and would shed new lights on the pathogenesis of COVID-19 and its complications for better treatments.

The recent severe acute respiratory syndrome, known as Coronavirus Disease 2019 (COVID-19) has spread so much rapidly and severely to induce World Health Organization (WHO) to declare a state of emergency over the new coronavirus SARS-CoV-2 pandemic. While several countries have chosen the almost complete lock-down for slowing down SARS-CoV-2 spread, the scientific community is called to respond to the devastating outbreak by identifying new tools for diagnosis and treatment of the dangerous COVID-19. With this aim, we performed an in silico comparative modeling analysis, which allows gaining new insights into the main conformational changes occurring in the SARS-CoV-2 spike protein, at the level of the receptor-binding domain (RBD), along interactions with human cells angiotensin-converting enzyme 2 (ACE2) receptor, that favor human cell invasion. Furthermore, our analysis provides (1) an ideal pipeline to identify already characterized antibodies that might target SARS-CoV-2 spike RBD, aiming to prevent interactions with the human ACE2, and (2) instructions for building new possible neutralizing antibodies, according to chemical/physical space restraints and complementary determining regions (CDR) mutagenesis of the identified existing antibodies. The proposed antibodies show in silico high affinity for SARS-CoV-2 spike RBD and can be used as reference antibodies also for building new high-affinity antibodies against present and future coronaviruses able to invade human cells through interactions of their spike proteins with the human ACE2. More in general, our analysis provides indications for the set-up of the right biological molecular context for investigating spike RBD–ACE2 interactions for the development of new vaccines, diagnostic kits, and other treatments based on the targeting of SARS-CoV-2 spike protein. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1007/s00018-020-03580-1) contains supplementary material, which is available to authorized users.

The insert sequence in SARS-CoV-2 enhances spike protein cleavage by TMPRSS

At the end of 2019, the SARS-CoV-2 induces an ongoing outbreak of pneumonia in China1, even more spread than SARS-CoV infection2. The entry of SARS-CoV into host cells mainly depends on the cell receptor (ACE2) recognition and spike protein cleavage-induced cell membrane fusion3,4. The spike protein of SARS-CoV-2 also binds to ACE2 with a similar affinity, whereas its spike protein cleavage remains unclear5,6. Here we show that an insertion sequence in the spike protein of SARS-CoV-2 enhances the cleavage efficiency, and besides pulmonary alveoli, intestinal and esophagus epithelium were also the target tissues of SARS-CoV-2. Compared with SARS-CoV, we found a SPRR insertion in the S1/S2 protease cleavage sites of SARS-CoV-2 spike protein increasing the cleavage efficiency by the protein sequence aligment and furin score calculation. Additionally, the insertion sequence facilitates the formation of an extended loop which was more suitable for protease recognition by the homology modeling and molicular docking. Furthermore, the single-cell transcriptomes identified that ACE2 and TMPRSSs are highly coexpressed in AT2 cells of lung, along with esophageal upper epithelial cells and absorptive enterocytes. Our results provide the bioinformatics evidence for the increased spike protein cleavage of SARS-CoV-2 and indicate its potential target cells.

Characterization of spike glycoprotein of SARS-CoV-2 on virus entry and its immune cross-reactivity with SARS-CoV

Since 2002, beta coronaviruses (CoV) have caused three zoonotic outbreaks, SARS-CoV in 2002-2003, MERS-CoV in 2012, and the newly emerged SARS-CoV-2 in late 2019. However, little is currently known about the biology of SARS-CoV-2. Here, using SARS-CoV-2 S protein pseudovirus system, we confirm that human angiotensin converting enzyme 2 (hACE2) is the receptor for SARS-CoV-2, find that SARS-CoV-2 enters 293/hACE2 cells mainly through endocytosis, that PIKfyve, TPC2, and cathepsin L are critical for entry, and that SARS-CoV-2 S protein is less stable than SARS-CoV S. Polyclonal anti-SARS S1 antibodies T62 inhibit entry of SARS-CoV S but not SARS-CoV-2 S pseudovirions. Further studies using recovered SARS and COVID-19 patients' sera show limited cross-neutralization, suggesting that recovery from one infection might not protect against the other. Our results present potential targets for development of drugs and vaccines for SARS-CoV-2.

Immunological characterization of the spike protein of the severe acute respiratory syndrome coronavirus.

Severe acute respiratory syndrome (SARS) is a novel infectious disease caused by the SARS-associated coronavirus (SARS-CoV). There are four major structural proteins in the SARS-CoV, including the nucleocapsid, spike, membrane, and small envelope proteins. In this study, two sets of truncated fragments of spike protein were generated, the first were approximately 210-bp nonoverlapping fragments and the second were overlapping segments of 750 to 900 bp. From these 23 fragments, we identified a fragment of 259 amino acids (amino acids 441 to 700) that is a major immunodominant epitope. This fragment was highly expressed, and the purified fragment C could detect all 33 SARS patient serum samples tested, collected from 7 to 60 days after the onset of fever, but had no reactivity with all 66 healthy human serum samples tested. Thus, fragment C of spike protein was identified as an immunodominant antigen and could be used for serological detection of SARS-CoV infection.

Increasing host cellular receptor-angiotensin-converting enzyme 2 expression by coronavirus may facilitate 2019-nCoV (or SARS-CoV-2) infection

The ongoing outbreak of a new coronavirus (2019-nCoV, or severe acute respiratory syndrome coronavirus 2 [SARS-CoV-2]) has caused an epidemic of the acute respiratory syndrome known as coronavirus disease (COVID-19) in humans. SARS-CoV-2 rapidly spread to multiple regions of China and multiple other countries, posing a serious threat to public health. The spike (S) proteins of SARS-CoV-1 and SARS-CoV-2 may use the same host cellular receptor, angiotensin-converting enzyme 2 (ACE2), for entering host cells. The affinity between ACE2 and the SARS-CoV-2 S protein is much higher than that of ACE2 binding to the SARS-CoV S protein, explaining why SARS-CoV-2 seems to be more readily transmitted from human to human. Here, we report that ACE2 can be significantly upregulated after infection of various viruses, including SARS-CoV-1 and SARS-CoV-2, or by the stimulation with inflammatory cytokines such as interferons. We propose that SARS-CoV-2 may positively induce its cellular entry receptor, ACE2, to accelerate its replication and spread; high inflammatory cytokine levels increase ACE2 expression and act as high-risk factors for developing COVID-19, and the infection of other viruses may increase the risk of SARS-CoV-2 infection. Therefore, drugs targeting ACE2 may be developed for the future emerging infectious diseases caused by this cluster of coronaviruses.

SARS-CoV-2 and bat RaTG13 spike glycoprotein structures inform on virus evolution and furin-cleavage effects.

SARS-CoV-2 is thought to have emerged from bats, possibly via a secondary host. Here, we investigate the relationship of spike (S) glycoprotein from SARS-CoV-2 with the S protein of a closely related bat virus, RaTG13. We determined cryo-EM structures for RaTG13 S and for both furin-cleaved and uncleaved SARS-CoV-2 S; we compared these with recently reported structures for uncleaved SARS-CoV-2 S. We also biochemically characterized their relative stabilities and affinities for the SARS-CoV-2 receptor ACE2. Although the overall structures of human and bat virus S proteins are similar, there are key differences in their properties, including a more stable precleavage form of human S and about 1,000-fold tighter binding of SARS-CoV-2 to human receptor. These observations suggest that cleavage at the furin-cleavage site decreases the overall stability of SARS-CoV-2 S and facilitates the adoption of the open conformation that is required for S to bind to the ACE2 receptor.

SARS-CoV-2 and bat RaTG13 spike glycoprotein structures inform on virus evolution and furin-cleavage effects

Investigation of ACE2 N-terminal fragments binding to SARS-CoV-2 Spike RBD

Coronavirus disease 19 (COVID-19) is an emerging global health crisis. With over 7 million confirmed cases to date, this pandemic continues to expand, spurring research to discover vaccines and therapies. SARS-CoV-2 is the novel coronavirus responsible for this disease. It initiates entry into human cells by binding to angiotensin-converting enzyme 2 (ACE2) via the receptor binding domain (RBD) of its spike protein (S). Disrupting the SARS-CoV-2-RBD binding to ACE2 with designer drugs has the potential to inhibit the virus from entering human cells, presenting a new modality for therapeutic intervention. Peptide-based binders are an attractive solution to inhibit the RBD-ACE2 interaction by adequately covering the extended protein contact interface. Using molecular dynamics simulations based on the recently solved cryo-EM structure of ACE2 in complex with SARS-CoV-2-RBD, we observed that the ACE2 peptidase domain (PD) α1 helix is important for binding SARS-CoV-2-RBD. Using automated fast-flow peptide synthesis, we chemically synthesized a 23-mer peptide fragment of the ACE2 PD α1 helix (SBP1) composed entirely of proteinogenic amino acids. Chemical synthesis of SBP1 was complete in 1.5 hours, and after work up and isolation >20 milligrams of pure material was obtained. Bio-layer interferometry (BLI) revealed that SBP1 associates with micromolar affinity to insect-derived SARS-CoV-2-RBD protein obtained from Sino Biological. Association of SBP1 was not observed to an appreciable extent to HEK cell-expressed SARS-CoV-2-RBD proteins and insect-derived variants acquired from other vendors. Moreover, competitive BLI assays showed SBP1 does not outcompete ACE2 binding to Sino Biological insect-derived SARS-CoV-2-RBD. Further investigations are ongoing to gain insight into the molecular and structural determinants of the variable binding behavior to different SARS-CoV-2-RBD protein variants.

Cross-reactive antibody response between SARS-CoV-2 and SARS-CoV infections

The World Health Organization has recently declared the ongoing outbreak of COVID-19, which is caused by a novel coronavirus SARS-CoV-2, as pandemic. There is currently a lack of knowledge in the antibody response elicited from SARS-CoV-2 infection. One major immunological question is concerning the antigenic differences between SARS-CoV-2 and SARS-CoV. We address this question by using plasma from patients infected by SARS-CoV-2 or SARS-CoV, and plasma obtained from infected or immunized mice. Our results show that while cross-reactivity in antibody binding to the spike protein is common, cross-neutralization of the live viruses is rare, indicating the presence of non-neutralizing antibody response to conserved epitopes in the spike. Whether these non-neutralizing antibody responses will lead to antibody-dependent disease enhancement needs to be addressed in the future. Overall, this study not only addresses a fundamental question regarding the antigenicity differences between SARS-CoV-2 and SARS-CoV, but also has important implications in vaccine

Since 2002, beta coronaviruses (CoV) have caused three zoonotic outbreaks, SARS-CoV in 2002–2003, MERS-CoV in 2012, and the newly emerged SARS-CoV-2 in late 2019. However, little is currently known about the biology of SARS-CoV-2. Here, using SARS-CoV-2 S protein pseudovirus system, we confirm that human angiotensin converting enzyme 2 (hACE2) is the receptor for SARS-CoV-2, find that SARS-CoV-2 enters 293/hACE2 cells mainly through endocytosis, that PIKfyve, TPC2, and cathepsin L are critical for entry, and that SARS-CoV-2 S protein is less stable than SARS-CoV S. Polyclonal anti-SARS S1 antibodies T62 inhibit entry of SARS-CoV S but not SARS-CoV-2 S pseudovirions. Further studies using recovered SARS and COVID-19 patients’ sera show limited cross-neutralization, suggesting that recovery from one infection might not protect against the other. Our results present potential targets for development of drugs and vaccines for SARS-CoV-2.

Cryo-EM structure of the SARS coronavirus spike glycoprotein in complex with its host cell receptor ACE2

The trimeric SARS coronavirus (SARS-CoV) surface spike (S) glycoprotein consisting of three S1-S2 heterodimers binds the cellular receptor angiotensin-converting enzyme 2 (ACE2) and mediates fusion of the viral and cellular membranes through a pre- to postfusion conformation transition. Here, we report the structure of the SARS-CoV S glycoprotein in complex with its host cell receptor ACE2 revealed by cryo-electron microscopy (cryo-EM). The complex structure shows that only one receptor-binding domain of the trimeric S glycoprotein binds ACE2 and adopts a protruding “up” conformation. In addition, we studied the structures of the SARS-CoV S glycoprotein and its complexes with ACE2 in different in vitro conditions, which may mimic different conformational states of the S glycoprotein during virus entry. Disassociation of the S1-ACE2 complex from some of the prefusion spikes was observed and characterized. We also characterized the rosette-like structures of the clustered SARS-CoV S2 trimers in the postfusion state observed on electron micrographs. Structural comparisons suggested that the SARS-CoV S glycoprotein retains a prefusion architecture after trypsin cleavage into the S1 and S2 subunits and acidic pH treatment. However, binding to the receptor opens up the receptor-binding domain of S1, which could promote the release of the S1-ACE2 complex and S1 monomers from the prefusion spike and trigger the pre- to postfusion conformational transition.

Structural and Functional Basis of SARS-CoV-2 Entry by Using Human ACE2

The recent emergence of a novel coronavirus (SARS-CoV-2) in China has caused significant public health concerns. Recently, ACE2 was reported as an entry receptor for SARS-CoV-2. In this study, we present the crystal structure of the C-terminal domain of SARS-CoV-2 (SARS-CoV-2-CTD) spike (S) protein in complex with human ACE2 (hACE2), which reveals a hACE2-binding mode similar overall to that observed for SARS-CoV. However, atomic details at the binding interface demonstrate that key residue substitutions in SARS-CoV-2-CTD slightly strengthen the interaction and lead to higher affinity for receptor binding than SARS-RBD. Additionally, a panel of murine monoclonal antibodies (mAbs) and polyclonal antibodies (pAbs) against SARS-CoV-S1/receptor-binding domain (RBD) were unable to interact with the SARS-CoV-2 S protein, indicating notable differences in antigenicity between SARS-CoV and SARS-CoV-2. These findings shed light on the viral pathogenesis and provide important structural information regarding development of therapeutic countermeasures against the emerging virus.

Structural insight into the putative role of novel SARS CoV-2 E protein in viral infection: a potential target for LAV development and therapeutic strategies

The outbreak of COVID-19 across the world has posed unprecedented and global challenges on multiple fronts. Most of the vaccine and drug development has focused on the spike proteins and viral RNA-polymerases. Using the bioinformatics and structural modeling approach, we modeled the structure of the envelope (E)-protein of novel SARS-CoV-2. The E-protein of this virus shares sequence similarity with that of SARS-CoV-1, and is highly conserved in the N-terminal regions. Incidentally, compared to spike proteins, E proteins demonstrate lower disparity and mutability among the isolated sequences. Using homology modeling, we found that the most favorable structure could function as a gated proton channel. Combining pocket estimation and docking with water, we determined that GLU 8 and ASN 15 in the N-terminal region were in close proximity to form H-bonds. Additionally, two distinct “core” structures were visible, the hydrophobic core and the central core, which may regulate the opening/closing of the channel. We propose this as a mechanism of viral proton channeling activity which may play a critical role in viral infection. In addition, it provides a structural basis and additional avenues for LAV development and generating therapeutic interventions against the virus. Significance Statement Structural modeling of the novel SARS-CoV-2 envelope protein (E-protein) demonstrating its possible proton channeling activity

The recent severe acute respiratory syndrome, known as Corona Virus Disease 2019 (COVID-19) has spread so much rapidly and severely to induce World Health Organization (WHO) to declare state of emergency over the new coronavirus SARS-CoV-2 pandemic. While several countries have chosen the almost complete lock-down for slowing down SARS-CoV-2 spread, scientific community is called to respond to the devastating outbreak by identifying new tools for diagnosis and treatment of the dangerous COVID-19. With this aim we performed an in silico comparative modeling analysis, which allows to gain new insights about the main conformational changes occurring in the SARS-CoV-2 spike protein, at the level of the receptor binding domain (RBD), along interactions with human cells angiotensin converting enzyme 2 (ACE2) receptor, that favour human cell invasion. Furthermore, our analysis provides i) an ideal pipeline to identify already characterized antibodies that might target SARS-CoV-2 spike RBD, for preventing interactions with the human ACE2, and ii) instructions for building new possible neutralizing antibodies, according to chemical/physical space restraints and complementary determining regions (CDR) mutagenesis of the identified existing antibodies. The proposed antibodies show in silico a high affinity for SARS-CoV-2 spike RBD and can be used as reference antibodies also for building new high affinity antibodies against present and future coronavirus able to invade human cells through interactions of their spike proteins with the human ACE2. More in general, our analysis provides indications for the set-up of the right biological molecular context for investigating spike RBD-ACE2 interactions for the development of new vaccines, diagnosis kits and other treatments based on the usage or the targeting of SARS-CoV-2 spike protein.

Rapid in silico design of antibodies targeting SARS-CoV-2 using machine learning and supercomputing

Rapidly responding to novel pathogens, such as SARS-CoV-2, represents an extremely challenging and complex endeavor. Numerous promising therapeutic and vaccine research efforts to mitigate the catastrophic effects of COVID-19 pandemic are underway, yet an efficacious countermeasure is still not available. To support these global research efforts, we have used a novel computational pipeline combining machine learning, bioinformatics, and supercomputing to predict antibody structures capable of targeting the SARS-CoV-2 receptor binding domain (RBD). In 22 days, using just the SARS-CoV-2 sequence and previously published neutralizing antibody structures for SARS-CoV-1, we generated 20 initial antibody sequences predicted to target the SARS-CoV-2 RBD. As a first step in this process, we predicted (and publicly released) structures of the SARS-CoV-2 spike protein using homology-based structural modeling. The predicted structures proved to be accurate within the targeted RBD region when compared to experimentally derived structures published weeks later. Next we used our in silico design platform to iteratively propose mutations to SARS-CoV-1 neutralizing antibodies (known not to bind SARS-Cov-2) to enable and optimize binding within the RBD of SARS-CoV-2. Starting from a calculated baseline free energy of −48.1 kcal/mol (± 8.3), our 20 selected first round antibody structures are predicted to have improved interaction with the SARS-CoV-2 RBD with free energies as low as −82.0 kcal/mole. The baseline SARS-CoV-1 antibody in complex with the SARS-CoV-1 RBD has a calculated interaction energy of −52.2 kcal/mole and neutralizes the virus by preventing it from binding and entering the human ACE2 receptor. These results suggest that our predicted antibody mutants may bind the SARS-CoV-2 RBD and potentially neutralize the virus. Additionally, our selected antibody mutants score well according to multiple antibody developability metrics. These antibody designs are being expressed and experimentally tested for binding to COVID-19 viral proteins, which will provide invaluable feedback to further improve the machine learning–driven designs. This technical report is a high-level description of that effort; the Supplementary Materials includes the homology-based structural models we developed and 178,856 in silico free energy calculations for 89,263 mutant antibodies derived from known SARS-CoV-1 neutralizing antibodies.

Structural variations in human ACE2 may influence its binding with SARS‐CoV‐2 spike protein

The recent pandemic of COVID‐19, caused by SARS‐CoV‐2, is unarguably the most fearsome compared with the earlier outbreaks caused by other coronaviruses, SARS‐CoV and MERS‐CoV. Human ACE2 is now established as a receptor for the SARS‐CoV‐2 spike protein. Where variations in the viral spike protein, in turn, lead to the cross‐species transmission of the virus, genetic variations in the host receptor ACE2 may also contribute to the susceptibility and/or resistance against the viral infection. This study aims to explore the binding of the proteins encoded by different human ACE2 allelic variants with SARS‐CoV‐2 spike protein. Briefly, coding variants of ACE2 corresponding to the reported binding sites for its attachment with coronavirus spike protein were selected and molecular models of these variants were constructed by homology modeling. The models were then superimposed over the native ACE2 and ACE2‐spike protein complex, to observe structural changes in the ACE2 variants and their intermolecular interactions with SARS‐CoV‐2 spike protein, respectively. Despite strong overall structural similarities, the spatial orientation of the key interacting residues varies in the ACE2 variants compared with the wild‐type molecule. Most ACE2 variants showed a similar binding affinity for SARS‐CoV‐2 spike protein as observed in the complex structure of wild‐type ACE2 and SARS‐CoV‐2 spike protein. However, ACE2 alleles, rs73635825 (S19P) and rs143936283 (E329G) showed noticeable variations in their intermolecular interactions with the viral spike protein. In summary, our data provide a structural basis of potential resistance against SARS‐CoV‐2 infection driven by ACE2 allelic variants.

Evolutionary relationships and sequence-structure determinants in human SARS coronavirus-2 spike proteins for host receptor recognition

Coronavirus disease 2019 (COVID-19) is a pandemic infectious disease caused by novel severe acute respiratory syndrome coronavirus-2 (SARS CoV-2). The SARS CoV-2 is transmitted more rapidly and readily than SARS CoV. Both, SARS CoV and SARS CoV-2 via their glycosylated spike proteins recognize the human angiotensin converting enzyme-2 (ACE-2) receptor. We generated multiple sequence alignments and phylogenetic trees for representative spike proteins of SARS CoV and SARS CoV-2 from various host sources in order to analyze the specificity in SARS CoV-2 spike proteins required for causing infection in humans. Our results show that among the genomes analyzed, two sequence regions in the N-terminal domain "MESEFR" and "SYLTPG" are specific to human SARS CoV-2. In the receptor-binding domain, two sequence regions "VGGNY" and "EIYQAGSTPCNGV" and a disulfide bridge connecting 480C and 488C in the extended loop are structural determinants for the recognition of human ACE-2 receptor. The complete genome analysis of representative SARS CoVs from bat, civet, human host sources, and human SARS CoV-2 identified the bat genome (GenBank code: MN996532.1) as closest to the recent novel human SARS CoV-2 genomes. The bat SARS CoV genomes (GenBank codes: MG772933 and MG772934) are evolutionary intermediates in the mutagenesis progression toward becoming human SARS CoV-2.

Structural basis of SARS-CoV-2 spike protein induced by ACE2

Motivation The recent emergence of the novel SARS-coronavirus 2 (SARS-CoV-2) and its international spread pose a global health emergency. The viral spike (S) glycoprotein binds the receptor (angiotensin-converting enzyme 2) ACE2 and promotes SARS-CoV-2 entry into host cells. The trimeric S protein binds the receptor using the distal receptor-binding domain (RBD) causing conformational changes in S protein that allow priming by host cell proteases. Unravelling the dynamic structural features used by SARS-CoV-2 for entry might provide insights into viral transmission and reveal novel therapeutic targets. Using structures determined by X-ray crystallography and cryo-EM, we performed structural analysis and atomic comparisons of the different conformational states adopted by the SARS-CoV-2-RBD. Results Here, we determined the key structural components induced by the receptor and characterized their intramolecular interactions. We show that κ-helix (also known as polyproline II) is a predominant structure in the binding interface and in facilitating the conversion to the active form of the S protein. We demonstrate a series of conversions between switch-like κ-helix and β-strand, and conformational variations in a set of short α-helices which affect the proximal hinge region. This conformational changes lead to an alternating pattern in conserved disulfide bond configurations positioned at the hinge, indicating a possible disulfide exchange, an important allosteric switch implicated in viral entry of various viruses, including HIV and murine coronavirus. The structural information presented herein enables us to inspect and understand the important dynamic features of SARS-CoV-2-RBD and propose a novel potential therapeutic strategy to block viral entry. Overall, this study provides guidance for the design and optimization of structure-based intervention strategies that target SARS-CoV-2.

Amino acids 1 to 422 of the spike protein of SARS associated coronavirus are required for induction of cyclooxygenase-2

The causative agent of severe acute respiratory syndrome (SARS) has been identified as SARS-associated coronavirus (SARS-CoV). To evaluate the molecular mechanisms involved in the viral infection, in this study, we investigated the role of SARS-CoV Spike (S) protein in the regulation of cyclooxygenase-2 (COX-2). Expression of COX-2 stimulated by the S protein was verified by RT-PCR and western blot assay. To explore the relationship between S and COX-2, we constructed a series of plasmids containing truncated N-terminal fragments of the SARS-CoV S gene (designated from Sa to Si), which encoded truncated S proteins, and investigated whether these truncated proteins could induce effective expression of COX-2 in 293T cells. Our results showed that S(d) that encoded a truncated S protein with 422 amino acid residues (from 1 to 422 aa), a part of 672 amino-acid S1 subunit is crucial for the induction of COX-2 expression. Immunofluorescence examinations also give the evidence that these N terminal 422 amino acids of the S protein were also required for the correct localization of the protein. We also compared S protein sequences of SARS-CoV isolated during the SARS break with that from palm civets, a possible source of SARS-CoV found in humans. S protein residues (344, 360), which mutated in the epitome from palm civet to human being were characterized in 3D modeling of 252–375 amino acid fragment. Collectively, these results indicate that S protein of SARS-CoV induces the expression of COX-2 and an N-terminal fragment of the Spike protein is crucial for the induction. Our finding may provide clue for the induction of inflammation by SARS-CoV and cast insight into the severity of the SARS epidemic.

The Distal Polybasic Cleavage Sites of SARS-CoV-2 Spike Protein Enhance Spike Protein-ACE2 Binding

The receptor-binding domain (RBD) of the SARS-CoV-2 spike protein plays a crucial role in binding the human cell receptor ACE2 that is required for viral entry. Many studies have been conducted to target the structures of RBD-ACE2 binding and to design RBD-targeting vaccines and drugs. Nevertheless, mutations distal from the SARS-CoV-2 RBD also impact its transmissibility and antibody can target non-RBD regions, suggesting the incomplete role of the RBD region in the spike protein-ACE2 binding. Here, in order to elucidate distant binding mechanisms, we analyze complexes of ACE2 with the wild type spike protein and with key mutants via large-scale all-atom explicit solvent molecular dynamics simulations. We find that though distributed approximately 10 nm away from the RBD, the SARS-CoV-2 polybasic cleavage sites enhance, via electrostatic interactions and hydration, the RBD-ACE2 binding affinity. A negatively charged tetrapeptide (GluGluLeuGlu) is then designed to neutralize the positively charged arginine on the polybasic cleavage sites. We find that the tetrapeptide GluGluLeuGlu binds to one of the three polybasic cleavage sites of the SARS-CoV-2 spike protein lessening by 34% the RBD-ACE2 binding strength. This significant binding energy reduction demonstrates the feasibility to neutralize RBD-ACE2 binding by targeting this specific polybasic cleavage site. Our work enhances understanding of the binding mechanism of SARS-CoV-2 to ACE2, which may aid the design of therapeutics for COVID-19 infection. TOC: The SARS-CoV-2 spike protein-ACE2 complex showing the polybasic cleavage sites

Spike protein binding prediction with neutralizing antibodies of SARS-CoV-2

Coronavirus disease 2019 (COVID-19) is a new emerging human infectious disease caused by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2, also previously known as 2019-nCoV), originated in Wuhan seafood and animal market, China. Since December 2019, more than 69,000 cases of COVID-19 have been confirmed in China and quickly spreads to other counties. Currently, researchers put their best efforts to identify effective drugs for COVID-19. The neutralizing antibody, which binds to viral capsid in a manner that inhibits cellular entry of virus and uncoating of the genome, is the specific defense against viral invaders. In this study, we investigate to identify neutralizing antibodies that can bind to SARS-CoV-2 Sipke (S) protein and interfere with the interaction between viral S protein and a host receptor by bioinformatic methods. The sequence analysis of S protein showed two major differences in the RBD region of the SARS-CoV-2 S protein compared to SARS-CoV and SARS-CoV related bat viruses (btSARS-CoV). The insertion regions were close to interacting residues with the human ACE2 receptor. Epitope analysis of neutralizing antibodies revealed that SARS-CoV neutralizing antibodies used conformational epitopes, whereas MERS-CoV neutralizing antibodies used a common linear epitope region, which contributes to form the β-sheet structure in MERS-CoV S protein and deleted in SARS-CoV-2 S protein. To identify effective neutralizing antibodies for SARS-CoV-2, the binding affinities of neutralizing antibodies with SARS-CoV-2 S protein were predicted and compared by antibody-antigen docking simulation. The result showed that CR3022 neutralizing antibody from human may have higher binding affinity with SARS-CoV-2 S protein than SARS-CoV S protein. We also found that F26G19 and D12 mouse antibodies could bind to SARS-CoV S protein with high affinity. Our findings provide crucial clues towards the development of antigen diagnosis, therapeutic antibody, and the vaccine against SARS-CoV-2.

SARS-CoV-2 mutations and where to find them: An in silico perspective of structural changes and antigenicity of the Spike protein

The recent emergence of a novel coronavirus (SARS-CoV-2) is causing a severe global health threat characterized by severe acute respiratory syndrome (Covid-19). At the moment, there is no specific treatment for this disease, and vaccines are still under development. The structural protein Spike is essential for virus infection and has been used as the main target for vaccine and serological diagnosis test development. We analysed 2363 sequences of the Spike protein from SARS-CoV-2 isolates and identified variability in 44 amino acid residues and their worldwide distribution in all continents. We used the three-dimensional structure of the homo-trimer model to predict conformational epitopes of B-cell, and sequence of Spike protein Wuhan-Hu-1 to predict linear epitopes of T-Cytotoxic and T-Helper cells. We identified 45 epitopes with amino acid variations. Finally, we showed the distribution of mutations within the epitopes. Our findings can help researches to identify more efficient strategies for the development of vaccines, therapies, and serological diagnostic tests based on the Spike protein of Sars-Cov-2.

In situ structural analysis of SARS-CoV-2 spike reveals flexibility mediated by three hinges

The spike (S) protein of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is required for cell entry and is the major focus for vaccine development. We combine cryo electron tomography, subtomogram averaging and molecular dynamics simulations to structurally analyze S in situ. Compared to recombinant S, the viral S is more heavily glycosylated and occurs predominantly in a closed pre-fusion conformation. We show that the stalk domain of S contains three hinges that give the globular domain unexpected orientational freedom. We propose that the hinges allow S to scan the host cell surface, shielded from antibodies by an extensive glycan coat. The structure of native S contributes to our understanding of SARS-CoV-2 infection and the development of safe vaccines. The large scale tomography data set of SARS-CoV-2 used for this study is therefore sufficient to resolve structural features to below 5 Ångstrom, and is publicly available at EMPIAR-10453.

A Cryptic Site of Vulnerability on the Receptor Binding Domain of the SARS-CoV-2 Spike Glycoprotein

SARS-CoV-2 is a zoonotic virus that has caused a pandemic of severe respiratory disease—COVID-19— within several months of its initial identification. Comparable to the first SARS-CoV, this novel coronavirus’s surface Spike (S) glycoprotein mediates cell entry via the human ACE-2 receptor, and, thus, is the principal target for the development of vaccines and immunotherapeutics. Molecular information on the SARS-CoV-2 S glycoprotein remains limited. Here we report the crystal structure of the SARS-CoV-2 S receptor-binding-domain (RBD) at a the highest resolution to date, of 1.95 Å. We identified a set of SARS-reactive monoclonal antibodies with cross-reactivity to SARS-CoV-2 RBD and other betacoronavirus S glycoproteins. One of these antibodies, CR3022, was previously shown to synergize with antibodies that target the ACE-2 binding site on the SARS-CoV RBD and reduce viral escape capacity. We determined the structure of CR3022, in complex with the SARS-CoV-2 RBD, and defined a broadly reactive epitope that is highly conserved across betacoronaviruses. This epitope is inaccessible in the “closed” prefusion S structure, but is accessible in “open” conformations. This first-ever resolution of a human antibody in complex with SARS-CoV-2 and the broad reactivity of this set of antibodies to a conserved betacoronavirus epitope will allow antigenic assessment of vaccine candidates, and provide a framework for accelerated vaccine, immunotherapeutic and diagnostic strategies against SARS-CoV-2 and related betacoronaviruses. HIGHLIGHTS High resolution structure of the SARS-CoV-2 Receptor-Binding-Domain (RBD). Recognition of the SARS-CoV-2 RBD by SARS-CoV antibodies. Structure of the SARS-COV-2 RBD in complex with antibody CR3022. Identification of a cryptic site of vulnerability on the SARS-CoV-2 Spike.

High seroreactivity against SARS-CoV-2 Spike epitopes in a pre SARS-CoV-2 cohort: implications for antibody testing and vaccine design

Little is known about the quality of polyclonal antibody responses in COVID-19 patients, and how it correlates with disease severity or patients' prior exposure to other pathogens. The whole polyclonal antibody repertoire in a retrospective cohort of 538 individuals was mapped against SARS-CoV-2 spike (S) glycoprotein, the main target of antibody immune responses in SARS-CoV-2 infection. Bioinformatic predictions identified 15 major B cell epitopes for S of SARS-CoV-2. Several epitopes localised in RBD of S including those spanning the ACE2-binding site, the highly conserved cryptic epitope of the neutralizing antibody of SARS-CoV, and fusion/entry domains of HR1 and HR2 of S protein of SARS-CoV-2. Intriguingly, some of these epitopes have cross-reactivity to antigens of common pathogens, potentially affecting SARS-CoV-2 infection outcome. High level of anti-Spike SARS-CoV-2 seroreactivity in populations with no history of exposure to SARS-CoV-2 is of clinical relevance and could underpin better understanding of COVID-19 pathophysiology in different populations and provide a blueprint for design of effective vaccines and developing better strategies for antibody testing.

Cell entry mechanisms of SARS-CoV-2

A novel severe acute respiratory syndrome (SARS)-like coronavirus (SARS-CoV-2) is causing the global coronavirus disease 2019 (COVID-19) pandemic. Understanding how SARS-CoV-2 enters human cells is a high priority for deciphering its mystery and curbing its spread. A virus surface spike protein mediates SARS-CoV-2 entry into cells. To fulfill its function, SARS-CoV-2 spike binds to its receptor human ACE2 (hACE2) through its receptor-binding domain (RBD) and is proteolytically activated by human proteases. Here we investigated receptor binding and protease activation of SARS-CoV-2 spike using biochemical and pseudovirus entry assays. Our findings have identified key cell entry mechanisms of SARS-CoV-2. First, SARS-CoV-2 RBD has higher hACE2 binding affinity than SARS-CoV RBD, supporting efficient cell entry. Second, paradoxically, the hACE2 binding affinity of the entire SARS-CoV-2 spike is comparable to or lower than that of SARS-CoV spike, suggesting that SARS-CoV-2 RBD, albeit more potent, is less exposed than SARS-CoV RBD. Third, unlike SARS-CoV, cell entry of SARS-CoV-2 is preactivated by proprotein convertase furin, reducing its dependence on target cell proteases for entry. The high hACE2 binding affinity of the RBD, furin preactivation of the spike, and hidden RBD in the spike potentially allow SARS-CoV-2 to maintain efficient cell entry while evading immune surveillance. These features may contribute to the wide spread of the virus. Successful intervention strategies must target both the potency of SARS-CoV-2 and its evasiveness.

Novel antibody epitopes dominate the antigenicity of spike glycoprotein in SARS-CoV-2 compared to SARS-CoV

Comparison of SARS-CoV-2 spike protein binding to ACE2 receptors from human, pets, farm animals, and putative intermediate hosts.

The emergence of a novel coronavirus, SARS-CoV-2, resulted in a pandemic. Here, we used X-ray structures of human ACE2 bound to the receptor-binding domain (RBD) of the spike protein (S) from SARS-CoV-2 to predict its binding to ACE2 proteins from different animals, including pets, farm animals, and putative intermediate hosts of SARS-CoV-2. Comparing the interaction sites of ACE2 proteins known to serve or not serve as receptor allows to define residues important for binding. From the 20 amino acids in ACE2 that contact S up to seven can be replaced and ACE2 can still function as the SARS-CoV-2 receptor. These variable amino acids are clustered at certain positions, mostly at the periphery of the binding site, while changes of the invariable residues prevent S-binding or infection of the respective animal. Some ACE2 proteins even tolerate the loss or the acquisition of N-glycosylation sites located near the S-interface. Of note, Pigs and dogs, which are not or not effectively infected and have only a few changes in the binding site, exhibit relatively low levels of ACE2 in the respiratory tract. Comparison of the RBD of S of SARS-CoV-2 with viruses from Bat-CoV-RaTG13 and Pangolin-CoV revealed that the latter contains only one substitution, whereas the Bat-CoV-RaTG13 exhibits five. However, ACE2 of pangolin exhibit seven changes relative to human ACE2, a similar number of substitutions is present in ACE2 of bats, raccoon, and civet suggesting that SARS-CoV-2 may not especially adapted to ACE2 of any of its putative intermediate hosts. These analyses provide new insight into the receptor usage and animal source/origin of SARS-CoV-2.IMPORTANCE SARS-CoV-2 is threatening people worldwide and there are no drugs or vaccines available to mitigate its spread. The origin of the virus is still unclear and whether pets and livestock can be infected and transmit SARS-CoV-2 are important and unknown scientific questions. Effective binding to the host receptor ACE2 is the first prerequisite for infection of cells and determines the host range. Our analysis provides a framework for the prediction of potential hosts of SARS-CoV-2. We found that ACE2 from species known to support SARS-CoV-2 infection tolerate many amino acid changes indicating that the species barrier might be low. An exception are dogs and especially pigs, which, however, revealed relatively low ACE2 expression levels in the respiratory tract. Monitoring of animals is necessary to prevent the generation of a new coronavirus reservoir. Finally, our analysis also showed that SARS-CoV-2 may not be specifically adapted to any of its putative intermediate hosts.

Impact of Thiol–Disulfide Balance on the Binding of Covid-19 Spike Protein with Angiotensin-Converting Enzyme 2 Receptor

[Image: see text] The novel coronavirus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has led to an ongoing pandemic of coronavirus disease (COVID-19), which started in 2019. This is a member of Coronaviridae family in the genus Betacoronavirus, which also includes SARS-CoV and Middle East respiratory syndrome coronavirus (MERS-CoV). The angiotensin-converting enzyme 2 (ACE2) is the functional receptor for SARS-CoV and SARS-CoV-2 to enter the host cells. In particular, the interaction of viral spike proteins with ACE2 is a critical step in the viral replication cycle. The receptor-binding domain of the viral spike proteins and ACE2 have several cysteine residues. In this study, the role of thiol–disulfide balance on the interactions between SARS-CoV/CoV-2 spike proteins and ACE2 was investigated using molecular dynamics simulations. The study revealed that the binding affinity was significantly impaired when all of the disulfide bonds of both ACE2 and SARS-CoV/CoV-2 spike proteins were reduced to thiol groups. The impact on the binding affinity was less severe when the disulfide bridges of only one of the binding partners were reduced to thiols. This computational finding possibly provides a molecular basis for the differential COVID-19 cellular recognition due to the oxidative stress.

Delving deep into the structural aspects of a furin cleavage site inserted into the spike protein of SARS-CoV-2: A structural biophysical perspective

One notable feature of the SARS-CoV-2 genome, the spike (S) protein of SARS-CoV-2 has a polybasic furin cleavage site (FCS) at its S1-S2 boundary through the insertion of 12 nucleotides encoding four amino acid residues PRRA. Quite intriguingly, this polybasic FCS is absent in coronaviruses of the same clade as SARS-CoV-2. Thus, with currently available experimental structural data for S protein, this short article presents a set of comprehensive structural characterization of the insertion of FCS into S protein, and argues against a hypothesis of the origin of SARS-CoV-2 from purposeful manipulation: (1), the inserted FCS is spatially located at a random coil loop region, mostly distantly solvent-exposed (instead of deeply buried), with no structural proximity to the other part of the S protein; (2), the insertion of FCS itself does not alter, neither stabilize nor de-stabilize, the three-dimensional structure of S; (3), the net result here is the insertion of a furin cleavage site into S protein, whose S1 and S2 subunits will still be strongly electrostatically bonded together from a structural and biophysical point of view, even if the polybasic FCS is actually cleaved by furin protease before or after viral cell entry.

SARS-CoV-2 and SARS-CoV Spike-RBD Structure and Receptor Binding Comparison and Potential Implications on Neutralizing Antibody and Vaccine Development

SARS-CoV-2 and SARS-CoV share a common human receptor ACE2. Protein-protein interaction structure modeling indicates that spike-RBD of the two viruses also has similar overall binding conformation and binding free energy to ACE2. In vitro assays using recombinant ACE2 proteins and ACE2 expressing cells confirmed the two coronaviruses’ similar binding affinities to ACE2. The above studies provide experimental supporting evidences and possible explanation for the high transmissibility observed in the SARS-CoV-2 outbreak. Potent ACE2-blocking SARS-CoV neutralizing antibodies showed limited cross-binding and neutralizing activities to SARS-CoV-2. ACE2-non-blocking SARS-CoV RBD antibodies, though with weaker neutralizing activities against SARS-CoV, showed positive cross-neutralizing activities to SARS-CoV-2 with an unknown mechanism. These findings suggest a trade-off between the efficacy and spectrum for therapeutic antibodies to different coronaviruses, and hence highlight the possibilities and challenges in developing broadly protecting antibodies and vaccines against SARS-CoV-2 and its future mutants.

Binding of the SARS-CoV-2 Spike Protein to Glycans

The pandemic of SARS-CoV-2 has caused a high number of deaths in the world. To combat it, it is necessary to develop a better understanding of how the virus infects host cells. Infection normally starts with the attachment of the virus to cell-surface glycans like heparan sulfate (HS) and sialic acid-containing oligosaccharides. In this study, we examined and compared the binding of the subunits and spike (S) proteins of SARS-CoV-2 and SARS-CoV, MERS-CoV to these glycans. Our results revealed that the S proteins and subunits can bind to HS in a sulfation-dependent manner, the length of HS is not a critical factor for the binding, and no binding with sialic acid residues was detected. Overall, this work suggests that HS binding may be a general mechanism for the attachment of these coronaviruses to host cells, and supports the potential importance of HS in infection and in the development of antiviral agents against these viruses.

Structural basis of receptor recognition by SARS-CoV-2

A novel severe acute respiratory syndrome (SARS)-like coronavirus (SARS-CoV-2) recently emerged and is rapidly spreading in humans, causing COVID-191,2. A key to tackling this pandemic is to understand the receptor recognition mechanism of the virus, which regulates its infectivity, pathogenesis and host range. SARS-CoV-2 and SARS-CoV recognize the same receptor-angiotensin-converting enzyme 2 (ACE2)-in humans3,4. Here we determined the crystal structure of the receptor-binding domain (RBD) of the spike protein of SARS-CoV-2 (engineered to facilitate crystallization) in complex with ACE2. In comparison with the SARS-CoV RBD, an ACE2-binding ridge in SARS-CoV-2 RBD has a more compact conformation; moreover, several residue changes in the SARS-CoV-2 RBD stabilize two virus-binding hotspots at the RBD-ACE2 interface. These structural features of SARS-CoV-2 RBD increase its ACE2-binding affinity. Additionally, we show that RaTG13, a bat coronavirus that is closely related to SARS-CoV-2, also uses human ACE2 as its receptor. The differences among SARS-CoV-2, SARS-CoV and RaTG13 in ACE2 recognition shed light on the potential animal-to-human transmission of SARS-CoV-2. This study provides guidance for intervention strategies that target receptor recognition by SARS-CoV-2.

Adenoviral expression of a truncated S1 subunit of SARS-CoV spike protein results in specific humoral immune responses against SARS-CoV in rats

Abstract The causative agent of severe acute respiratory syndrome (SARS) has been identified as SARS-associated coronavirus (SARS-CoV), but the prophylactic treatment of SARS-CoV is still under investigation. We constructed a recombinant adenovirus containing a truncated N-terminal fragment of the SARS-CoV Spike (S) gene (from −45 to 1469, designated Ad-SN), which encoded a truncated S protein (490 amino-acid residues, a part of 672 amino-acid S1 subunit), and investigated whether this construct could induce effective immunity against SARS-CoV in Wistar rats. Rats were immunized either subcutaneously or intranasally with Ad-SN once a week for three consecutive weeks. Our results showed that all of the immunized animals generated humoral immunity against the SARS-CoV spike protein, and the sera of immunized rats showed strong capable of protecting from SARS-CoV infection in vitro. Histopathological examination did not find evident side effects in the immunized animals. These results indicate that an adenoviral-based vaccine carrying an N-terminal fragment of the Spike gene is able to elicit strong SARS-CoV-specific humoral immune responses in rats, and may be useful for the development of a protective vaccine against SARS-CoV infection.

Comparison of SARS-CoV-2 spike protein binding to ACE2 receptors from human, pets, farm animals, and putative intermediate hosts

SARS-CoV-2, SARS-CoV, and MERS-COV: A comparative overview

The recent outbreak of SARS-CoV-2 that started in Wuhan, China, has now spread to several other countries and is in its exponential phase of spread. Although less pathogenic than SARS-CoV, it has taken several lives and taken down the economies of many countries. Before this outbreak, the most recent coronavirus outbreaks were the SARS-CoV and the MERS-CoV outbreaks that happened in China and Saudi Arabia, respectively. Since the SARS-CoV-2 belongs to the same family as of SARS-CoV and MERS-CoV, they share several similarities. So, this review aims at understanding the new scenario of SARS-CoV-2 outbreak and compares the epidemiology, clinical presentations, and the genetics of these coronaviruses. Studies reveal that SARS-CoV-2 is very similar in structure and pathogenicity with SARS-CoV, but the most important structural protein, i.e., the spike protein (S), is slightly different in these viruses. The presence of a furin-like cleavage site in SARS-CoV-2 facilitates the S protein priming and might increase the efficiency of the spread of SARS-CoV-2 as compared to other beta coronaviruses. So, furin inhibitors can be targeted as potential drug therapies for SARS-CoV.

Structural Proteins in Severe Acute Respiratory Syndrome Coronavirus-2

Abstract What began with a sign of pneumonia-related respiratory disorders in China has now become a pandemic named by WHO as Covid-19 known to be caused by Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2). The SARS-CoV-2 are newly emerged β coronaviruses belonging to the Coronaviridae family. SARS-CoV-2 has a positive viral RNA genome expressing open reading frames that code for structural and non-structural proteins. The spike, nucleocapsid, membrane, and envelope proteins are structural proteins. The S1 subunit of spike protein facilitates ACE2 mediated virus attachment and S2 subunit for membrane fusion. The presence of glutamine, asparagine, leucine, phenylalanine and serine amino in SARS-CoV-2 enhanced ACE2 binding. The N protein is composed of a serine-rich linker region sandwiched between N terminal (NTD) and C terminal (CTD). These terminals play a role in viral entry and its processing post entry. The NTD of SARS-CoV-2 N protein forms orthorhombic crystals and binds to the viral genome. The linker region contains phosphorylation sites that regulate its functioning. The CTD promotes nucleocapsid formation. Envelope proteins contain an NTD, hydrophobic domain and C terminal which form viroporins needed for viral assembly. Membrane proteins hydrophilic C terminal and amphipathic N terminal. Its long-form promotes spike incorporations and interaction with E facilitate virion production. As each protein is essential in viral functioning, this review describes the insights of SARS-CoV-2 structural proteins that would help in developing therapeutic strategies by targeting each protein to curb the rapidly growing pandemic.

Conformational Reorganization of the SARS Coronavirus Spike Following Receptor Binding: Implications for Membrane Fusion

The SARS coronavirus (SARS-CoV) spike is the largest known viral spike molecule, and shares a similar function with all class 1 viral fusion proteins. Previous structural studies of membrane fusion proteins have largely used crystallography of static molecular fragments, in isolation of their transmembrane domains. In this study we have produced purified, irradiated SARS-CoV virions that retain their morphology, and are fusogenic in cell culture. We used cryo-electron microscopy and image processing to investigate conformational changes that occur in the entire spike of intact virions when they bind to the viral receptor, angiotensin-converting enzyme 2 (ACE2). We have shown that ACE2 binding results in structural changes that appear to be the initial step in viral membrane fusion, and precisely localized the receptor-binding and fusion core domains within the entire spike. Furthermore, our results show that receptor binding and subsequent membrane fusion are distinct steps, and that each spike can bind up to three ACE2 molecules. The SARS-CoV spike provides an ideal model system to study receptor binding and membrane fusion in the native state, employing cryo-electron microscopy and single-particle image analysis.

Immunological, structural, and preliminary X-ray diffraction characterizations of the fusion core of the SARS-coronavirus spike protein

Abstract The SARS-CoV spike protein, a glycoprotein essential for viral entry, is a primary target for vaccine and drug development. Two peptides denoted HR-N(SN50) and HR-C(SC40), corresponding to the Leu/Ile/Val-rich heptad-repeat regions from the N-terminal and C-terminal segments of the SARS-CoV spike S2 sequence, respectively, were synthesized and predicted to form trimeric assembly of hairpin-like structures. The polyclonal antibodies produced by recombinant S2 protein were tested for antigenicity of the two heptad repeats. We report here the first crystallographic study of the SARS spike HR-N/HR-C complex. The crystal belongs to the triclinic space group P1 and the data-set collected to 2.98Å resolution showed noncrystallographic pseudo-222 and 3-fold symmetries. Based on these data, comparative modeling of the SARS-CoV fusion core was performed. The immunological and structural information presented herein may provide a more detailed understanding of the viral fusion mechanism as well as the development of effective therapy against SARS-CoV infection.

COVID-19 spike-host cell receptor GRP78 binding site prediction

OBJECTIVES: Understanding the novel coronavirus (COVID-19) mode of host cell recognition may help to fight the disease and save lives. The spike protein of coronaviruses is the main driving force for host cell recognition. METHODS: In this study, the COVID-19 spike binding site to the cell-surface receptor (Glucose Regulated Protein 78 (GRP78)) is predicted using combined molecular modeling docking and structural bioinformatics. The COVID-19 spike protein is modeled using its counterpart, the SARS spike. RESULTS: Sequence and structural alignments show that four regions, in addition to its cyclic nature have sequence and physicochemical similarities to the cyclic Pep42. Protein-protein docking was performed to test the four regions of the spike that fit tightly in the GRP78 Substrate Binding Domain ß (SBDß). The docking pose revealed the involvement of the SBDß of GRP78 and the receptor-binding domain of the coronavirus spike protein in recognition of the host cell receptor. CONCLUSIONS: We reveal that the binding is more favorable between regions III (C391-C525) and IV (C480-C488) of the spike protein model and GRP78. Region IV is the main driving force for GRP78 binding with the predicted binding affinity of -9.8â¯kcal/mol. These nine residues can be used to develop therapeutics specific against COVID-19.

Phylogenetic Analysis and Structural Modeling of SARS-CoV-2 Spike Protein Reveals an Evolutionary Distinct and Proteolytically Sensitive Activation Loop

The 2019 novel coronavirus (2019-nCoV/SARS-CoV-2) originally arose as part of a major outbreak of respiratory disease centered on Hubei province, China. It is now a global pandemic and is a major public health concern. Taxonomically, SARS-CoV-2 was shown to be a Betacoronavirus (lineage B) closely related to SARS-CoV and SARS-related bat coronaviruses, and it has been reported to share a common receptor with SARS-CoV (ACE-2). Subsequently, betacoronaviruses from pangolins were identified as close relatives to SARS-CoV-2. Here, we perform structural modeling of the SARS-CoV-2 spike glycoprotein. Our data provide support for the similar receptor utilization between SARS-CoV-2 and SARS-CoV, despite a relatively low amino acid similarity in the receptor binding module. Compared to SARS-CoV and all other coronaviruses in Betacoronavirus lineage B, we identify an extended structural loop containing basic amino acids at the interface of the receptor binding (S1) and fusion (S2) domains. We suggest this loop confers fusion activation and entry properties more in line with betacoronaviruses in lineages A and C, and be a key component in the evolution of SARS-CoV-2 with this structural loop affecting virus stability and transmission.

Difference in receptor usage between severe acute respiratory syndrome (SARS) coronavirus and SARS-like coronavirus of bat origin.

Severe acute respiratory syndrome (SARS) is caused by the SARS-associated coronavirus (SARS-CoV), which uses angiotensin-converting enzyme 2 (ACE2) as its receptor for cell entry. A group of SARS-like CoVs (SL-CoVs) has been identified in horseshoe bats. SL-CoVs and SARS-CoVs share identical genome organizations and high sequence identities, with the main exception of the N terminus of the spike protein (S), known to be responsible for receptor binding in CoVs. In this study, we investigated the receptor usage of the SL-CoV S by combining a human immunodeficiency virus-based pseudovirus system with cell lines expressing the ACE2 molecules of human, civet, or horseshoe bat. In addition to full-length S of SL-CoV and SARS-CoV, a series of S chimeras was constructed by inserting different sequences of the SARS-CoV S into the SL-CoV S backbone. Several important observations were made from this study. First, the SL-CoV S was unable to use any of the three ACE2 molecules as its receptor. Second, the SARS-CoV S failed to enter cells expressing the bat ACE2. Third, the chimeric S covering the previously defined receptor-binding domain gained its ability to enter cells via human ACE2, albeit with different efficiencies for different constructs. Fourth, a minimal insert region (amino acids 310 to 518) was found to be sufficient to convert the SL-CoV S from non-ACE2 binding to human ACE2 binding, indicating that the SL-CoV S is largely compatible with SARS-CoV S protein both in structure and in function. The significance of these findings in relation to virus origin, virus recombination, and host switching is discussed.

The MERS-CoV receptor DPP4 as a candidate binding target of the SARS-CoV-2 spike

SUMMARY The ongoing outbreak of the novel coronavirus pneumonia COVID-19 has caused great number of cases and deaths, but our understanding about the pathogen SARS-CoV-2 remains largely unclear. The attachment of the virus with the cell-surface receptor and a co-factor is the first step for the infection. Here, bioinformatics approaches combining human-virus protein interaction prediction and protein docking based on crystal structures have revealed the high affinity between human dipeptidyl peptidase 4 (DPP4) and the spike (S) receptor-binding domain of SARS-CoV-2. Intriguingly, the crucial binding residues of DPP4 are identical to those as bound to the MERS-CoV-S. Moreover, E484 insertion and adjacent substitutions should be most essential for this DPP4-binding ability acquirement of SARS-CoV-2-S compared with SARS-CoV-S. This potential utilization of DPP4 as a binding target for SARS-CoV-2 may offer novel insight into the viral pathogenesis, and help the surveillance and therapeutics strategy for meeting the challenge of COVID-19.

Morphology, Genome Organization, Replication, and Pathogenesis of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2)

SARS-CoV-2 is a single-stranded RNA virus of ~30 kb genome size which belongs to genus Coronavirus and family Coronaviridae. SARS-CoV-2 has recently emerged and has been declared as a pandemic by the World Health Organization. Genomic characterization of SARS-CoV-2 has shown that it is of zoonotic origin. The structure of SARS-CoV-2 is found to be similar to SARS-CoV with virion size ranging from 70 to 90 nm. Spike, membrane, and envelope surface viral proteins of coronavirus are embedded in host membrane-derived lipid bilayer encapsulating the helical nucleocapsid comprising viral RNA. The genome comprises of 6–11 open reading frames (ORFs) with 5′ and 3′ flanking untranslated regions (UTRs). Sequence variation among SARS-CoV-2 and SARS-CoV revealed no significant difference in ORFs and nsps. The nsps includes two viral cysteine proteases including papain-like protease (nsp3), chymotrypsin-like, 3C-like, or main protease (nsp5), RNA-dependent RNA polymerase (nsp12), helicase (nsp13), and others likely to be involved in the transcription and replication of SARS-CoV-2. The structure of spike glycoprotein structure of SARS-CoV-2 resembles that of the spike protein of SARS-CoV with an root-mean-square deviation (RMSD) of 3.8 Å. Like SARS-CoV, SARS-CoV-2 uses the ACE2 receptor for internalization and TMPRSS2 serine proteases for S protein priming. Histopathological investigation of tissues from SARS-CoV-2 infected patients showed virus-induced cytopathic effect with signs of acute respiratory distress syndrome in lung cells. This chapter discusses about the morphology, genome organization, replication, and pathogenesis of SARS-CoV-2 that may help us understand the disease that may leads to identification of effective antiviral drugs and vaccines.

SARS-CoV-2 spike protein binds heparan sulfate in a length- and sequence-dependent manner

Severe acute respiratory syndrome-related coronavirus 2 (SARS-CoV-2) is causing an unprecedented global pandemic demanding the urgent development of therapeutic strategies. Microarray binding experiments using an extensive heparan sulfate (HS) oligosaccharide library showed the spike of SARS-CoV-2 can bind HS in a length- and sequence-dependent manner. Hexa- and octasaccharides composed of IdoA2S-GlcNS6S repeating units were identified as optimal ligands. Surface plasma resonance (SPR) showed the SARS-CoV-2 spike protein binds with higher affinity to heparin (KD 55 nM) compared to the receptor binding domain (RBD, KD 1 µM) alone. An octasaccharide composed of IdoA2S-GlcNS6S could inhibit spike-heparin interaction with an IC50 of 38 nM. Our data supports a model in which the RBD of the spike of SARS-CoV-2 confers sequence specificity for HS expressed by target cells whereas an additional HS binding site in the S1/S2 proteolytic cleavage site enhances the avidity of binding. Collectively, our results highlight the potential of using HS oligosaccharides as a therapeutic agent by inhibiting SARS-CoV-2 binding to target cells.

Whole-genome sequence analysis and homology modelling of the main protease and non-structural protein 3 of SARS-CoV-2 reveal an aza-peptide and a lead inhibitor with possible antiviral properties

Viruses belonging to the family Coronaviridae consist of virulent pathogens that have a zoonotic property. Severe acute respiratory syndrome coronaviruses (SARS-CoVs) and Middle East respiratory syndrome coronaviruses (MERS-CoVs) of this family have emerged before and SARS-CoV-2 has emerged now globally. The characterization of spike glycoproteins, polyproteins and other viral proteins from viruses is important for antiviral drug development. Homology modelling of these proteins with known templates offers the opportunity to discover ligand-binding sites and explore the possible antiviral properties of these protein-ligand complexes. In this study, we performed a complete bioinformatic analysis, sequence alignment, comparison of multiple sequences and modelling of the SARS-CoV-2 whole-genome sequences, the spike protein and the polyproteins for homology with known proteins. We also analysed binding sites in these models for possible binding with ligands that exhibit antiviral properties. Our results indicated that the sequence of the polyprotein isolate SARS-CoV-2_HKU-SZ-001_2020 showed 98.94 percent identity to SARS-coronavirus NSP12 bound to NSP7 and NSP8 co-factors. The results also indicated that a part of the viral genome (residues 3268-3573 in Frame 2 with 306 amino acids) of the SARS-CoV-2 isolate Wuhan-Hu-1 (GenBank Accession Number MN908947.3) when modelled with templateof the PDB database showed 96 percent identity to a 3C-like peptidase of SARS-CoVs, which has the ability to bind with an aza-peptide epoxide (APE) known for the irreversible inhibition of SARS-CoV main peptidase. A docking profile with 9 different conformations of the ligand with the protein model using Autodock Vina showed an affinity of −7.1 kcal mol−1. This region was conserved in 831 genomes of SARS-CoV-2. The part of the genome (residues 1568-1882 in Frame 2 with 315 amino acids) when modelled with template of the PDB database showed 82 percent identity to a papain-like protease/deubiquitinase, which when complexed with ligand GRL0617 acts as an inhibitor and can block SARS-CoV replication. A docking profile with 9 different conformations of the ligand with the protein model using Autodock Vina showed an affinity of −7.9 kcal mol−1. This region was conserved in 831 genomes of SARS-CoV-2. It is possible that these ligands can be used as antivirals of SARS-CoV-2.

A highly conserved cryptic epitope in the receptor-binding domains of SARS-CoV-2 and SARS-CoV

The outbreak of COVID-19 caused by SARS-CoV-2 virus has now become a pandemic, but there is currently very little understanding of the antigenicity of the virus. We therefore determined the crystal structure of CR3022, a neutralizing antibody previously isolated from a convalescent SARS patient, in complex with the receptor-binding domain (RBD) of the SARS-CoV-2 spike (S) protein to 3.1 Å. CR3022 targets a highly conserved epitope, distal from the receptor-binding site, that enables cross-reactive binding between SARS-CoV-2 and SARS-CoV. Structural modeling further demonstrates that the binding epitope can only be accessed by CR3022 when at least two RBD on the trimeric S protein are in the “up” conformation and slightly rotated. Overall, this study provides molecular insights into antibody recognition of SARS-CoV-2.

Characterization of the SARS-CoV-2 Spike in an Early Prefusion Conformation

Pandemic coronavirus disease 2019 (COVID-19) is caused by the emerging severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), for which there are no efficacious vaccines or therapeutics that are urgently needed. We expressed three versions of spike (S) proteins—receptor binding domain (RBD), S1 subunit and S ectodomain—in insect cells. RBD appears monomer in solutions, whereas S1 and S associate into homotrimer with substantial glycosylation. The three proteins confer excellent antigenicity with six convalescent COVID-19 patient sera. Cryo-electron microscopy (cryo-EM) analyses indicate that the SARS-CoV-2 S trimer dominate in a unique conformation distinguished from the classic prefusion conformation of coronaviruses by the upper S1 region at lower position ~15 Å proximal to viral membrane. Such conformation is proposed as an early prefusion state for the SARS-CoV-2 spike that may broaden the knowledge of coronavirus and facilitate vaccine development.

Naturally mutated spike proteins of SARS-CoV-2 variants show differential levels of cell entry

The causative agent of the coronavirus disease 2019 (COVID-19) pandemic, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is steadily mutating during continuous transmission among humans. Such mutations can occur in the spike (S) protein that binds to the angiotensin-converting enzyme-2 (ACE2) receptor and is cleaved by transmembrane protease serine 2 (TMPRSS2). However, whether S mutations affect SARS-CoV-2 infectivity remains unknown. Here, we show that naturally occurring S mutations can reduce or enhance cell entry via ACE2 and TMPRSS2. A SARS-CoV-2 S-pseudotyped lentivirus exhibits substantially lower entry than SARS-CoV S. Among S variants, the D614G mutant shows the highest cell entry, as supported by structural observations. Nevertheless, the D614G mutant remains susceptible to neutralization by antisera against prototypic viruses. Taken together, these data indicate that the D614G mutation enhances viral infectivity while maintaining neutralization susceptibility.

Summary Objectives Understanding the novel coronavirus (COVID-19) mode of host cell recognition may help to fight the disease and save lives. The spike protein of coronaviruses is the main driving force for host cell recognition. Methods In this study, the COVID-19 spike binding site to the cell-surface receptor (Glucose Regulated Protein 78 (GRP78)) is predicted using combined molecular modeling docking and structural bioinformatics. The COVID-19 spike protein is modeled using its counterpart, the SARS spike. Results Sequence and structural alignments show that four regions, in addition to its cyclic nature have sequence and physicochemical similarities to the cyclic Pep42. Protein-protein docking was performed to test the four regions of the spike that fit tightly in the GRP78 Substrate Binding Domain β (SBDβ). The docking pose revealed the involvement of the SBDβ of GRP78 and the receptor-binding domain of the coronavirus spike protein in recognition of the host cell receptor. Conclusions We reveal that the binding is more favorable between regions III (C391-C525) and IV (C480-C488) of the spike protein model and GRP78. Region IV is the main driving force for GRP78 binding with the predicted binding affinity of -9.8 kcal/mol. These nine residues can be used to develop therapeutics specific against COVID-19.

Expression of ACE2 and TMPRSS2 proteins in the upper and lower aerodigestive tracts of rats

Objective Patients with coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), exhibit not only respiratory symptoms but also symptoms of chemo-sensitive disorders and kidney failure. Cellular entry of SARS-CoV-2 depends on the binding of its spike protein to a cellular receptor named angiotensin-converting enzyme 2 (ACE2), and the subsequent spike protein-priming by host cell proteases, including transmembrane protease serine 2 (TMPRSS2). Thus, high expression of ACE2 and TMPRSS2 are considered to enhance the invading capacity of SARS-CoV-2. Methods To elucidate the underlying histological mechanisms of the aerodigestive disorders caused by SARS-CoV-2, we investigated the expression of ACE2 and TMPRSS2 proteins in the aerodigestive tracts of the tongue, hard palate with partial nasal tissue, larynx with hypopharynx, trachea, esophagus, lung, and kidney of rats through immunohistochemistry. Results Strong co-expression of ACE2 and TMPRSS2 proteins was observed in the nasal respiratory epithelium, trachea, bronchioles, alveoli, kidney, and taste buds of the tongue. Remarkably, TMPRSS2 expression was much stronger in the peripheral alveoli than in the central alveoli. These results coincide with the reported clinical symptoms of COVID-19, such as the loss of taste, loss of olfaction, respiratory dysfunction, and acute nephropathy. Conclusions A wide range of organs have been speculated to be affected by SARS-CoV-2 depending on the expression levels of ACE2 and TMPRSS2. Differential distribution of TMPRSS2 in the lung indicated the COVID-19 symptoms to possibly be exacerbated by TMPRSS2 expression. This study might provide potential clues for further investigation of the pathogenesis of COVID-19. Level of Evidence NA

Synthetic Peptide Studies on the Severe Acute Respiratory Syndrome (SARS) Coronavirus Spike Glycoprotein: Perspective for SARS Vaccine Development

Background: The S (spike) protein of the etiologic coronavirus (CoV) agent of severe acute respiratory syndrome (SARS) plays a central role in mediating viral infection via receptor binding and membrane fusion between the virion and the host cell. We focused on using synthetic peptides for developing antibodies against SARS-CoV, which aimed to block viral invasion by eliciting an immune response specific to the native SARS-CoV S protein. Methods: Six peptide sequences corresponding to the surface regions of SARS-CoV S protein were designed and investigated by use of combined bioinformatics and structural analysis. These synthetic peptides were used to immunize both rabbits and monkeys. Antisera collected 1 week after the second immunization were analyzed by ELISA and tested for antibody specificity against SARS-CoV by immunofluorescent confocal microscopy. Results: Four of our six synthetic peptides (S2, S3, S5, and S6) elicited SARS-CoV-specific antibodies, of which S5 (residues 788–820) and S6 (residues 1002–1030) exhibited immunogenic responses similar to those found in a parallel investigation using truncated recombinant protein analogs of the SARS-CoV S protein. This suggested that our S5 and S6 peptides may represent two minimum biologically active sequences of the immunogenic regions of the SARS-CoV S protein. Conclusions: Synthetic peptides can elicit specific antibodies to SARS-CoV. The study provides insights for the future development of SARS vaccine via the synthetic-peptide-based approach.

A virus that has gone viral: amino acid mutation in S protein of Indian isolate of Coronavirus COVID-19 might impact receptor binding, and thus, infectivity

Since 2002, ß coronaviruses (CoVs) have caused three zoonotic outbreaks, SARS-CoV in 2002, MERS-CoV in 2012, and the recent outbreak of SARS-CoV-2 late in 2019 (also named as COVID-19 or novel coronavirus 2019 or nCoV2019). Spike (S) protein, one of the structural proteins of this virus plays key role in receptor (ACE2) binding and thus virus entry. Thus, this protein has attracted scientists for detailed study and therapeutic targeting. As the nCoV2019 takes its course throughout the world, more and more sequence analyses are being done and genome sequences are being deposited in various databases. From India, two clinical isolates have been sequenced and the full genome has been deposited in GenBank. We have performed sequence analyses of the Spike protein of the Indian isolates and compared with that of the Wuhan, China (where the outbreak was first reported). While all the sequences of Wuhan isolates are identical, we found point mutations in the Indian isolates. Out of the two isolates, one was found to harbor a mutation in its receptor-binding domain (RBD) at position 407. At this site, arginine (a positively charged amino acid) was replaced by isoleucine (a hydrophobic amino acid that is also a C-ß branched amino acid). This mutation has been seen to change the secondary structure of the protein at that region and this can potentially alter receptor binding of the virus. Although this finding needs further validation and more sequencing, the information might be useful in rational drug designing and vaccine engineering.

SARS-CoV-2 spike protein favors ACE2 from Bovidae and Cricetidae

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) causes the recent COVID-19 public health crisis. Bat is the widely believed original host of SARS-CoV-2. However, its intermediate host before transmitting to humans is not clear. Some studies proposed pangolin, snake, or turtle as the intermediate hosts. Angiotensin-converting enzyme 2 (ACE2) is the receptor for SARS-CoV-2, which determines the potential host range for SARS-CoV-2. On the basis of structural information of the complex of human ACE2 and SARS-CoV-2 receptor-binding domain (RBD), we analyzed the affinity to S protein of the 20 key residues in ACE2 from mammal, bird, turtle, and snake. Several ACE2 proteins from Primates, Bovidae, Cricetidae, and Cetacea maintained the majority of key residues in ACE2 for associating with SARS-CoV-2 RBD. The simulated structures indicated that ACE2 proteins from Bovidae and Cricetidae were able to associate with SARS-CoV-2 RBD. We found that nearly half of the key residues in turtle, snake, and bird were changed. The simulated structures showed several key contacts with SARS-CoV-2 RBD in turtle and snake ACE2 were abolished. This study demonstrated that neither snake nor turtle was the intermediate hosts for SARS-CoV-2, which further reinforced the concept that the reptiles are resistant against infection of coronavirus. This study suggested that Bovidae and Cricetidae should be included in the screening of intermediate hosts for SARS-CoV-2.

Structure, function and antigenicity of the SARS-CoV-2 spike glycoprotein

The recent emergence of a novel coronavirus associated with an ongoing outbreak of pneumonia (Covid-2019) resulted in infections of more than 72,000 people and claimed over 1,800 lives. Coronavirus spike (S) glycoprotein trimers promote entry into cells and are the main target of the humoral immune response. We show here that SARS-CoV-2 S mediates entry in VeroE6 cells and in BHK cells transiently transfected with human ACE2, establishing ACE2 as a functional receptor for this novel coronavirus. We further demonstrate that the receptor-binding domains of SARS-CoV-2 S and SARS-CoV S bind with similar affinities to human ACE2, which correlates with the efficient spread of SARS-CoV-2 among humans. We found that the SARS-CoV-2 S glycoprotein harbors a furin cleavage site at the boundary between the S1/S2 subunits, which is processed during biogenesis and sets this virus apart from SARS-CoV and other SARS-related CoVs. We determined a cryo-electron microscopy structure of the SARS-CoV-2 S ectodomain trimer, demonstrating spontaneous opening of the receptor-binding domain, and providing a blueprint for the design of vaccines and inhibitors of viral entry. Finally, we demonstrate that SARS-CoV S murine polyclonal sera potently inhibited SARS-CoV-2 S-mediated entry into target cells, thereby indicating that cross-neutralizing antibodies targeting conserved S epitopes can be elicited upon vaccination.

The SARS-CoV-2 Spike protein has a broad tropism for mammalian ACE2 proteins

SARS-CoV-2 emerged in late 2019, leading to the COVID-19 pandemic that continues to cause significant global mortality in human populations. Given its sequence similarity to SARS-CoV, as well as related coronaviruses circulating in bats, SARS-CoV-2 is thought to have originated in Chiroptera species in China. However, whether the virus spread directly to humans or through an intermediate host is currently unclear, as is the potential for this virus to infect companion animals, livestock and wildlife that could act as viral reservoirs. Using a combination of surrogate entry assays and live virus we demonstrate that, in addition to human ACE2, the Spike glycoprotein of SARS-CoV-2 has a broad host tropism for mammalian ACE2 receptors, despite divergence in the amino acids at the Spike receptor binding site on these proteins. Of the twenty-two different hosts we investigated, ACE2 proteins from dog, cat and rabbit were the most permissive to SARS-CoV-2, while bat and bird ACE2 proteins were the least efficiently used receptors. The absence of a significant tropism for any of the three genetically distinct bat ACE2 proteins we examined indicates that SARS-CoV-2 receptor usage likely shifted during zoonotic transmission from bats into people, possibly in an intermediate reservoir. Interestingly, while SARS-CoV-2 pseudoparticle entry was inefficient in cells bearing the ACE2 receptor from bats or birds the live virus was still able to enter these cells, albeit with markedly lower efficiency. The apparently broad tropism of SARS-CoV-2 at the point of viral entry confirms the potential risk of infection to a wide range of companion animals, livestock and wildlife.

Antibody repertoire induced by SARS-CoV-2 spike protein immunogens

Multiple vaccine candidates against SARS-CoV-2 based on viral spike protein are under development. However, there is limited information on the quality of antibody response generated following vaccination by these vaccine modalities. To better understand antibody response induced by spike protein-based vaccines, we immunized rabbits with various SARS-CoV-2 spike protein antigens: S-ectodomain (S1+S2) (aa 16-1213), which lacks the cytoplasmic and transmembrane domains (CT-TM), the S1 domain (aa 16-685), the receptor-binding domain (RBD) (aa 319-541), and the S2 domain (aa 686-1213 as control). Antibody response was analyzed by ELISA, Surface Plasmon Resonance (SPR) against different Spike proteins in native conformation, and a pseudovirion neutralization assay to measure the quality and function of the antibodies elicited by the different Spike antigens. All three antigens (S1+S2 ectodomain, S1 domain, and RBD) generated strong neutralizing antibodies against SARS-CoV-2. Vaccination induced antibody repertoire was analyzed by SARS-CoV-2 spike Genome Fragment Phage Display Libraries (SARS-CoV-2 GFPDL), which identified immunodominant epitopes in the S1, S1-RBD and S2 domains. Furthermore, these analyses demonstrated that surprisingly the RBD immunogen elicited a higher antibody titer with 5-fold higher affinity antibodies to native spike antigens compared with other spike antigens. These findings may help guide rational vaccine design and facilitate development and evaluation of effective therapeutics and vaccines against COVID-19 disease. One Sentence Summary SARS-CoV-2 Spike induced immune response

Structure-based preliminary analysis of immunity and virulence of SARS coronavirus.

The research on SARS-associated coronavirus (SARS-CoV) has not stopped since its discovery, but the pathogenesis of SARS is still unclear. To explore the possible molecular mechanisms of the invasion and virulence of SARS-CoV, we investigated the structural basis of the viral proteins using computational biology. Forty-five motifs relating to superantigens, toxins and other bioactive molecules were detected in the proteins of SARS-CoV. The results showed that the distribution of the motifs varied in different proteins. Enzyme-like motifs were located in the R protein, while ICAM- 1-like and toxin-like molecules were located in the spike, envelop, nucleocapsid, PUP1, PUP 2 and PUP 4 proteins. Comparison of SARS-CoV with other viruses (OC43, PEDV, HRSV, HHerpV and HAdenoV) showed that each group of motifs was different for each type of virus. Data suggest that the proteins of SARS-CoV with toxic motifs might play crucial roles in targeting host cells and interfering with the immune system. This study provides new information for drug and vaccine design, as well as therapeutic strategies against SARS.

Multi-epitope-based peptide vaccine design against SARS-CoV-2 using its spike protein

SARS CoV-2 has particularly been efficient in ensuring that many countries are brought to a standstill. With repercussions ranging from rampant mortality, fear, paranoia and economic recession, the virus has brought together countries in order to look at possible therapeutic countermeasures. With prophylactic interventions possibly months away from being particularly effective, a slew of measures and possibilities concerning the design of vaccines are being worked upon. We attempted a structure-based approach utilizing a combination of epitope prediction servers to develop a multi-epitope-based subunit vaccine that involves the two major domains of the spike glycoprotein of SARS CoV-2 (S1 and S2) coupled with a substantially effective chimeric adjuvant to create stable vaccine constructs through MD simulations. The designed constructs were evaluated based on their docking with Toll Like Receptor (TLR) 4. Our findings provide an epitope-based peptide fragment; which can be a potential candidate for the development of a vaccine against SARS-CoV-2. Recent experimental studies based on determining immunodominant regions across the spike glycoprotein of SARS-CoV-2 indicate the presence of the predicted epitopes included in this study.

The MERS-CoV Receptor DPP4 as a Candidate Binding Target of the SARS-CoV-2 Spike

The ongoing outbreak of the novel coronavirus pneumonia COVID-19 has caused great number of cases and deaths, but our understanding about the pathogen SARS-CoV-2 remains largely unclear. The attachment of the virus with the cell-surface receptor and a cofactor is the first step for the infection. Here, bioinformatics approaches combining human-virus protein interaction prediction and protein docking based on crystal structures have revealed the high affinity between human dipeptidylpeptidase 4 (DPP4) and the spike (S) receptor-binding domain of SARS-CoV-2. Intriguingly, the crucial binding residues of DPP4 are identical to those that are bound to the MERS-CoV-S. Moreover, E484 insertion and adjacent substitutions should be most essential for this DPP4-binding ability acquirement of SARS-CoV-2-S compared with SARS-CoV-S. This potential utilization of DPP4 as a binding target for SARS-CoV-2 may offer novel insight into the viral pathogenesis and help the surveillance and therapeutics strategy for meeting the challenge of COVID-19.

Structure of SARS coronavirus spike receptor-binding domain complexed with receptor.

The spike protein (S) of SARS coronavirus (SARS-CoV) attaches the virus to its cellular receptor, angiotensin-converting enzyme 2 (ACE2). A defined receptor-binding domain (RBD) on S mediates this interaction. The crystal structure at 2.9 angstrom resolution of the RBD bound with the peptidase domain of human ACE2 shows that the RBD presents a gently concave surface, which cradles the N-terminal lobe of the peptidase. The atomic details at the interface between the two proteins clarify the importance of residue changes that facilitate efficient cross-species infection and human-to-human transmission. The structure of the RBD suggests ways to make truncated disulfide-stabilized RBD variants for use in the design of coronavirus vaccines.

Structural basis of receptor recognition by SARS-CoV-2.

A novel SARS-like coronavirus (SARS-CoV-2) recently emerged and is rapidly spreading in humans1,2. A key to tackling this epidemic is to understand the virus's receptor recognition mechanism, which regulates its infectivity, pathogenesis and host range. SARS-CoV-2 and SARS-CoV recognize the same receptor - human ACE2 (hACE2)3,4. Here we determined the crystal structure of the SARS-CoV-2 receptor-binding domain (RBD) (engineered to facilitate crystallization) in complex with hACE2. Compared with the SARS-CoV RBD, a hACE2-binding ridge in SARS-CoV-2 RBD takes a more compact conformation; moreover, several residue changes in SARS-CoV-2 RBD stabilize two virus-binding hotspots at the RBD/hACE2 interface. These structural features of SARS-CoV-2 RBD enhance its hACE2-binding affinity. Additionally, we show that RaTG13, a bat coronavirus closely related to SARS-CoV-2, also uses hACE2 as its receptor. The differences among SARS-CoV-2, SARS-CoV and RaTG13 in hACE2 recognition shed light on potential animal-to-human transmission of SARS-CoV-2. This study provides guidance for intervention strategies targeting receptor recognition by SARS-CoV-2.