rhomboid proteases are the largest family of enzymes that hydrolyze peptide bonds within the cell membrane .
although discovered to be serine proteases only a decade ago , rhomboid proteases are already considered to be the best understood intramembrane proteases .
the presence of rhomboid proteins in all domains of life emphasizes their importance but makes their evolutionary history difficult to chart with confidence .
the near ubiquity of rhomboid proteases across evolution suggests broad , organizational roles that are not directly essential for cell survival .
functions have been deciphered in only about a dozen organisms and fall into four general categories : initiating cell signaling in animals , facilitating bacterial quorum sensing , regulating mitochondrial homeostasis , and dismantling adhesion complexes of parasitic protozoa .
although in no organism has the full complement of rhomboid function yet been elucidated , links to devastating human disease are emerging rapidly , including to parkinson 's disease , type ii diabetes , cancer , and bacterial and malaria infection .
rhomboid proteases are unlike most proteolytic enzymes , because they are membrane - immersed ; understanding how the membrane immersion affects their function remains a key challenge .
proteolysis within the membrane was discovered in seemingly rare contexts nearly 15 years ago [ 1 - 3 ] .
it is now widely appreciated that this fascinating regulatory paradigm permeates most areas of modern cell biology [ 4 - 7 ] .
of the three protease families that catalyze intramembrane proteolysis , rhomboid enzymes are the only family that were not discovered from the direct study of human disease .
the name ' rhomboid ' has its origin deep in the rich folklore of drosophila genetics .
rhomboid emerged from the historic quest to identify all genes required to organize construction of a free - living organism from a single cell . because genes were named after the altered appearance of the mutant larval cuticle , the mis - shaped , rhombus - like head skeleton of the mutant embryo earned rhomboid its name . mutating
the growth factor that rhomboid activates yielded indistinguishable head - skeleton defects , and was named spitz ( ' pointed ' in german ) .
the rhomboid gene was cloned and sequenced by bier and colleagues in 1990 , revealing a seven transmembrane ( 7tm ) protein with no homology to any sequence known at the time .
the spitz sequence was more informative , encoding a clear epidermal growth factor ( egf)-like protein .
the fact that rhomboid mirrored spitz phenotypically , and encoded a seven tm protein , led to the proposal that it might be a serpentine receptor for spitz signaling .
but as sequencing of genomes from diverse organisms began to reveal rhomboid homologs in every form of cellular life , it became clear that rhomboid proteins may be at the core of very diverse biological regulation .
sequence analysis , however , yielded no clues about the underlying biochemical function of rhomboid proteins , and no other homologs were as well studied as drosophila rhomboid .
a decade of drosophila genetics had , however , set the stage for a biochemical approach : rhomboid was definitively implicated as an upstream activator of spitz in the signal - sending cell , providing a framework for analyzing its molecular function [ 12 - 15 ] .
a focused analysis of spitz activation eventually yielded four key pieces of the puzzle : ( i ) substochiometric levels of rhomboid triggered spitz proteolysis , implying that rhomboid acts enzymatically ; ( ii ) proteolysis depends absolutely only on four rhomboid residues , and their identity is consistent with serine protease catalysis ; ( iii ) spitz proteolysis is blocked only by serine protease inhibitors ; and ( iv ) spitz is cleaved within its tm segment at a depth similar to that of the putative rhomboid catalytic serine .
these pieces fit together into a model in which rhomboid acts as an intramembrane serine protease for spitz , which was confirmed 4 years later by reconstituting cleavage with pure proteins [ 17 - 19 ] .
the ability to study spitz proteolysis as a direct test of rhomboid activity was used to determine that even distant bacterial homologs are functional intramembrane serine proteases .
most bacterial species are now known to encode one rhomboid protease , while some encode two , and very few encode three .
rhomboid proteases are also present in many if not most archaea , but the greatest expansion occurred in multicellular organisms and some parasitic protozoa .
although the human , mouse and drosophila genomes encode at least seven rhomboid genes , the largest number of rhomboid genes are encoded by plants ( 13 in arabidopsis ) , which do not have egf signaling [ 21 - 23 ] . in many of these diverse organisms , at least one rhomboid has directly been demonstrated to have proteolytic activity ( table 1 ) [ 16,20,23 - 26 ] .
known rhomboid protease substrates and functions across evolution only those rhomboid proteases whose proteolytic activity has been detected are included in the table .
rhomboid proteases are found in all branches of life , yet the sequence identity across all family members is strikingly low , around only 6% .
we suggest that this is not despite rhomboid proteases being so widespread but because of it .
this divergence is exacerbated by their sequences being predominantly transmembrane and thus experiencing a different evolutionary pressure .
this has made phylogenetic analyses noisy , yielding few incontrovertible conclusions and inevitably fueling debate .
of particular intrigue is their evolutionary origin : rhomboid proteins have been argued to be perhaps the most widely distributed membrane proteins in nature   ( figure 1 ) .
this near ubiquity is instinctively viewed as evidence of an ancient enzyme family that evolved early .
although this is likely if the last universal common ancestor already encoded several different rhomboid proteins , phylogenetic analysis has also raised the possibility of a different history in which rhomboid proteins are a later invention of bacteria that rapidly spread to most other organisms .
this scenario requires a controversial amount of horizontal gene transfer to populate all kingdoms of life .
currently the true nature of rhomboid phylogenetic history remains a point of inconsolable debate , but three observations serve as valuable guiding principles .
a subset of 109 rhomboid and derlin family protein reference sequences , retrieved from the ncbi refseq database , was chosen to illustrate their diversity .
mega 5.05 was used to align sequences by muscle and construct an unrooted neighbor - joining phylogenetic tree .
branches are labeled according to their common characteristics and are shaded or outlined to denote active or inactive protease sequences , respectively .
individual sequence names are colored black , blue , or red to indicate a 6tm , 6 + 1tm , or 1 + 6tm arrangement , respectively , with each refseq accession number included within parentheses . despite the tremendous number and diversity of rhomboid proteins , structures of only two 6tm
the first organizing principle emerging from sequence analysis is the observation that rhomboid proteases come in three distinct topological flavors ( figure 2 ) .
the simplest consists of the 6tm core , which itself is the smallest catalytically active unit .
this form predominates in bacteria , but is also represented , albeit more rarely , in eukaryotic organisms , including animals . to this basic unit
eukaryotes add a seventh tm segment following the 6tm core ( 6 + 1tm form ) .
five of the seven drosophila , human , and mouse rhomboid proteins are of this form
lastly , a distinct form of 7tm rhomboid proteases exists in endosymbiotic organelles , adding the seventh tm preceding the 6tm core ( 1 + 6tm form ) .
the best studied are those imported into mitochondria [ 28 - 30 ] , although interest in plastid - resident rhomboid proteins has recently been sparked .
although the sequence analysis is clear on these three topological distinctions , their functional relevance is unclear .
the expectation is that they confer different biochemical properties , although current evidence , albeit limited , indicates that many bacterial 6tm forms and eukaryotic 6 + 1tm forms show similar activity against surrogate substrates , including spitz .
the smallest , catalytically active form of a rhomboid protease consists of a 6tm core , with variable amino termini ( dashed lines ) .
most eukaryotic and mitochondrial rhomboid proteases have an additional tm segment , added either carboxy - terminally ( eukaryotes , blue ) or amino - terminally ( mitochondria , red ) to the 6tm core , as depicted .
catalytic residues are in yellow for nucleophilic chemistry ( hydrolysis ) and white for electrophilic residues ( oxyanion transition state stabilization ) .
although protease activity has been reconstituted with both the 6tm and 6 + 1tm rhomboid forms in vitro , it is only a 6tm form called glpg from escherichia coli and haemophilus influenzae that has proven amenable to structural analysis [ 34 - 37 ] .
this major breakthrough - the first atomic - resolution structure of any intramembrane protease - not only confirmed that proteolysis is intramembrane and catalyzed by a serine protease apparatus , but revealed an unanticipated and complex architecture .
although a thorough description is beyond the scope of the current discussion ( see   for a comprehensive review ) , two features are characteristic ( figure 3 ) : although most tm helices are long and run roughly perpendicular to the membrane , the fourth tm segment runs slanted relative to the others and enters the center of the protein as an extended loop , converting to an  helix at the catalytic serine .
more unexpected was the orientation of the long l1 loop connecting tms 1 and 2 , which forms a lateral hairpin that lies half submerged in the membrane .
this feature , which has not been encountered before or since , has major structural implications and results in a highly asymmetric protein .
it is assumed that the structure of the other rhomboid forms will be analogous , and recent modeling of the mitochondrial 1 + 6tm form on e. coli glpg hints at an unanticipated level of similarity .
the crystal structure of the 6tm core of the e. coli rhomboid protease glpg ( pdb 2nrf molecule a ) is shown from three vantage points ( ' top view ' is looking at the cell from the outside with the membrane in the plane of the page ) .
the protein forms a compact helical bundle , with two characteristic features . a short and slanted tm4 ( black ) forms a helix below the catalytic serine ( circled in the ' back ' view ) , but an extended loop ( l3 ) above it .
the l1 loop ( purple ) forms a hairpin structure that nestles between tms 1 and 3 and protrudes laterally into the outer leaflet of the membrane ( red dashed lines representing the membrane interface are provided only for reference ) .
catalytic dyad residues serine and histidine are in cyan ; putative oxyanion - stabilizing electrophilic asparagine and histidine residues are in red .
in addition to the number of tms , two further variations provide potential for additional rhomboid diversity .
first , in all three forms , the cytosolic amino termini are highly variable , ranging from large domains to being non - existent .
the implications , however , remain unclear , at least partly because achieving well - diffracting crystals required absence of this domain , making its relationship to the catalytic core speculative . on the simplest level
two predominant clusters are a distinct 6 + 1tm form in animals , called irhom proteins , and a 6tm form that is represented widely in eukaryotes by the derlin proteins [ 42 - 45 ] .
both of these have been implicated in endoplasmic reticulum - associated degradation ( erad ) .
derlins have clear sequence homology near the membrane - submerged l1 loop , but also less conspicuously along their entire length , and are thus likely to adopt a glpg - like 6tm structure .
although clearly not proteolytic , their potential similarity to other aspects of the rhomboid protease mechanism should not be discounted at this early stage ( but lie beyond the scope of this review ) .
the second guiding principle stems from the tremendous diversity of organisms that encode rhomboid enzymes .
since these include organisms that do not encode any known forms of cell - to - cell communication , sequence information implies that rhomboid proteins perform an ancient and fundamental role in cell biology .
this function is not essential for cell survival , however , because several lineages are missing rhomboid genes entirely , presumably by gene loss .
although defining the cellular functions of rhomboid proteases has proven a persistent challenge , focused investigations have succeeded in documenting the function of at least one rhomboid in nearly a dozen organisms ( table 1 ) .
these functions are usually regulated by substrate trafficking , and fall into four broad categories ( figure 4 ) .
rhomboid-1 is localized in the golgi apparatus , and cleaves spitz ( green ) after it is transported from the endoplasmic reticulum by star ( purple ) .
top right : the mitochondrial rhomboid parl cleaves pink1 to reduce parkin recruitment to mitochondria and downregulate mitophagy .
bottom right : malaria parasite - encoded rhomboid proteases cleave adhesins to disassemble the junction formed between parasite and host erythrocyte at the end of invasion .
note that adhesins ( in black ) , initially held in internal organelles , encounter rhomboid only when they are secreted onto the surface and motored to the posterior of the parasite .
bottom left : the providencia rhomboid protease aara activates tata by removing a small amino - terminal extension .
this allows tata to assemble into the machinery required for protein ( and presumably quorum - sensing signal ) export . in the left two roles rhomboid cleavage activates a latent factor
first , rhomboid proteases initiate animal cell signaling by releasing growth factors from the membrane .
this function emerged from detailed genetic study of drosophila development ; rhomboid proteases are localized in the golgi apparatus and act as the signal - generating component by cleaving spitz to initiate the pathway in neighboring cells .
although a role in regulating egf signaling is also seen in caenorhabditis elegans vulval development , cerom-1 has a surprisingly minor role as a target of egf signaling that sets up a paracrine loop to amplify and spread the signal .
even less is clear in mammals : recent investigations have localized rhomboid proteins to the secretory pathway and cell surface and begun to uncover increased rhomboid expression in cancer cells with potential links to growth factor signaling .
however , this is not limited to active rhomboid proteases ; expression of the irhom rhbdf1 , which is localized in the endoplasmic reticulum in human epithelial cancer cells , increased secretion of the egf ligand transforming growth factor- .
accordingly , rhbdf1 silencing decreased pathway activation through egf receptor ( egfr ) , erk and akt phosphorylation , and limited tumor growth in mice .
the drosophila homolog , however , was recently found to have the opposite effect of decreasing egfr signaling by promoting the erad - mediated degradation of egf ligands .
the basis of this remarkable discrepancy is currently unclear ; knockout mouse studies are expected to provide clarity on the physiological roles of rhomboid proteins .
recent studies have also placed the mitochondrial rhomboid protease at the nexus of key pathways that govern mitochondrial fusion , mitophagy and apoptosis .
all mitochondrial rhomboid proteins are encoded in the nuclear genome , and imported into mitochondria .
the main function of the mitochondrial rhomboid pcp1 is to release the dynamin - like gtpase mgm1 from the membrane .
because mgm1 is essential for mitochondrial fusion and mgm1 cleavage occurs only in healthy mitochondria , this limits fusion to occurring between healthy organelles .
a similar function was described in drosophila , but genetic interactions soon revealed further complexity in metazoans ; the mitochondrial rhomboid dmrho-7 also participates in the parkin / pink1 pathway that malfunctions in parkinson 's disease .
it has recently become clear that the human mitochondrial rhomboid parl cleaves pink1 to suppress its ability to recruit the parkin ubiquitin ligase onto mitochondria [ 56 - 58 ] . without parl cleavage
a parl knockout mouse suffers tremendous atrophy several months after birth resulting from malformed mitochondria and elevated apoptosis , although without mitochondrial fusion defects .
parl has also been implicated in suppressing apoptosis in lymphocytes , potentially through a different substrate , high - temperature regulated a ( htra , also called omi ) .
intriguingly , mutations in parl have recently been found in parkinson 's disease patients   and diabetes patients , although the significance of these mutations for disease remains speculative .
the third category of rhomboid function was revealed in providencia stuartii , a gram - negative bacterial pathogen .
genetic screens identified its rhomboid homolog , aara , to be required for production of an unidentified signal for quorum sensing . once the similarity to rhomboid was noted , proteolytic activity of aara was demonstrated against spitz , and aara was found to partially rescue tissue development of drosophila mutant in rhomboid .
historically , the intriguing similarity of activating drosophila egf signaling and producing an auto - inducer for bacterial quorum sensing , both by a rhomboid , received much attention .
but the similarity proved to be superficial when the substrate was identified to be tata , a component of the twin - arginine translocation machinery .
as such , aara removes a short amino - terminal extension , presumably to activate the machinery for signal secretion , rather than activating the signal itself .
tata from other bacteria , including e. coli , lacks this short extension and is immediately active , and the aara function is therefore an exception .
nevertheless , this is the only known function for a rhomboid protease in any prokaryote , and it dramatically highlights the apparent diversity of rhomboid function even within similar bacteria .
this is the only role that was discovered by searching for rhomboid targets using substrate specificity determinants .
the adhesins of plasmodium and toxoplasma are necessary for host - cell invasion , making them essential proteins for the survival of these obligate intracellular parasites .
these parasites encode six or more rhomboid proteases , two of which in each organism are known to process these adhesins at the end of the invasion program [ 25,26,68 - 70 ] . the precise need for this dismantling
is not entirely clear , but has been thought to free the parasite from being tethered to the host plasma membrane .
recent knockdown experiments indicate that this processing is important for efficient invasion , although the full extent is incompletely understood and may involve later functions during parasite replication within the host cell .
even the non - cell - invasive entamoeba histolytica encodes a highly active rhomboid protease , which is localized to the parasite surface but which relocalizes to phagosomes during feeding and the bud neck during immune evasion , perhaps to shed surface proteins , including lectins .
the functions of other plasmodium or toxoplasma rhomboid proteases not involved in invasion are not yet understood , and many other parasites encode rhomboid enzymes whose functions have never been explored .
perhaps the most powerful , yet subtle , guiding principle that can be deduced from the near ubiquity of rhomboid proteases is that they possess a biochemical property that is both very rare and highly useful : but what ?
solving this riddle requires understanding the enzymatic features of rhomboid proteases , and remarkable progress has been made towards these goals ( reviewed in ) .
this includes reconstitution of proteolysis with pure proteins , protease inhibitor profiling , extensive analysis of residues essential for activity , and structural visualization of catalytic residues and with a covalently bound inhibitor [ 34 - 37,78 ] .
moreover , the initial paradox of how water is delivered to the membrane - immersed active site for hydrolysis was largely addressed by structural analyses [ 34 - 37 ] : the active site lies submerged about 10  below the presumed membrane surface , but with an open cavity above the active site for water access ( figure 3 ) .
structure - function analyses of rhomboid proteases have also revealed several unusual proteolytic properties that make them unlike most serine proteases .
these differences are clear evidence of convergent evolution to a serine protease mechanism down an independent path .
first , structural analysis indicates that nucleophilic catalysis is achieved by a histidine - serine catalytic pair , rather than the more common aspartate - histidine - serine catalytic triad [ 34 - 37 ] .
the identity of the residues that stabilize the oxyanion transition state is uncertain , but this stabilization is most likely mediated by asparagine and/or histidine side - chains   ( figure 3 ) .
use of an asparagine for oxyanion stabilization is uncommon but strikingly analogous to the mechanism of the conventional serine protease subtilisin .
the third unusual catalytic property of rhomboid proteases relates to the direction in which substrates lie across the active site cleft relative to the catalytic residues . although initially thought to be similar to nearly all other serine proteases , identification of the substrate gate on the opposite side of glpg relative to expectation mandated that substrates approach the catalytic residues from the so - called ' si ' face .
consistent with this stereochemistry are rhomboid 's resistance to most canonical serine protease inhibitors and a weak but specific sensitivity to monocyclic -lactams .
it should be stressed that the definitive evidence for substrate orientation , identity of the oxyanion hole , and the nature of substrate stabilization await a co - structure with a peptide substrate .
rhomboid proteases have been studied largely within the framework of an established serine protease precedent as a way to interpret rhomboid mechanism , which is instructive but does not help to understand how they are different .
although deciphering the specifics of the catalytic chemistry is essential for designing effective inhibitors , the key functional properties of rhomboid enzymes that are relevant to the cell are unlikely to be determined by its catalytic mechanism .
these defining features most likely result from membrane - immersion of the enzyme , and more recent investigations have started to study rhomboid proteases as integral membrane proteins directly .
the greatest impact of membrane immersion is on how substrates and rhomboid proteases behave ( as reviewed in ) .
the closed ring of tm segments observed in the first crystal structure suggested that something must move to clear a path for lateral substrate entry [ 34 - 37 ] .
only mutations that weaken tm5 packing with tm2 were found to enhance protease activity by up to ten - fold , thereby identifying the gate functionally .
this dramatic enhancement also revealed that gate opening is the rate - limiting step for intramembrane proteolysis .
molecular dynamics simulations and structural analysis in a bicelle also suggest membrane thinning surrounding glpg , but its mechanistic implications remain unclear . investigating the role of the membrane in greater detail promises to reveal the defining features of the rhomboid proteolysis system .
perhaps the most powerful , yet subtle , guiding principle that can be deduced from the near ubiquity of rhomboid proteases is that they possess a biochemical property that is both very rare and highly useful : but what ?
solving this riddle requires understanding the enzymatic features of rhomboid proteases , and remarkable progress has been made towards these goals ( reviewed in ) .
this includes reconstitution of proteolysis with pure proteins , protease inhibitor profiling , extensive analysis of residues essential for activity , and structural visualization of catalytic residues and with a covalently bound inhibitor [ 34 - 37,78 ] .
moreover , the initial paradox of how water is delivered to the membrane - immersed active site for hydrolysis was largely addressed by structural analyses [ 34 - 37 ] : the active site lies submerged about 10  below the presumed membrane surface , but with an open cavity above the active site for water access ( figure 3 ) .
structure - function analyses of rhomboid proteases have also revealed several unusual proteolytic properties that make them unlike most serine proteases .
these differences are clear evidence of convergent evolution to a serine protease mechanism down an independent path .
first , structural analysis indicates that nucleophilic catalysis is achieved by a histidine - serine catalytic pair , rather than the more common aspartate - histidine - serine catalytic triad [ 34 - 37 ] .
the identity of the residues that stabilize the oxyanion transition state is uncertain , but this stabilization is most likely mediated by asparagine and/or histidine side - chains   ( figure 3 ) .
use of an asparagine for oxyanion stabilization is uncommon but strikingly analogous to the mechanism of the conventional serine protease subtilisin .
the third unusual catalytic property of rhomboid proteases relates to the direction in which substrates lie across the active site cleft relative to the catalytic residues .
although initially thought to be similar to nearly all other serine proteases , identification of the substrate gate on the opposite side of glpg relative to expectation mandated that substrates approach the catalytic residues from the so - called ' si ' face .
consistent with this stereochemistry are rhomboid 's resistance to most canonical serine protease inhibitors and a weak but specific sensitivity to monocyclic -lactams .
it should be stressed that the definitive evidence for substrate orientation , identity of the oxyanion hole , and the nature of substrate stabilization await a co - structure with a peptide substrate .
rhomboid proteases have been studied largely within the framework of an established serine protease precedent as a way to interpret rhomboid mechanism , which is instructive but does not help to understand how they are different .
although deciphering the specifics of the catalytic chemistry is essential for designing effective inhibitors , the key functional properties of rhomboid enzymes that are relevant to the cell are unlikely to be determined by its catalytic mechanism .
these defining features most likely result from membrane - immersion of the enzyme , and more recent investigations have started to study rhomboid proteases as integral membrane proteins directly .
the greatest impact of membrane immersion is on how substrates and rhomboid proteases behave ( as reviewed in ) .
the closed ring of tm segments observed in the first crystal structure suggested that something must move to clear a path for lateral substrate entry [ 34 - 37 ] .
only mutations that weaken tm5 packing with tm2 were found to enhance protease activity by up to ten - fold , thereby identifying the gate functionally .
this dramatic enhancement also revealed that gate opening is the rate - limiting step for intramembrane proteolysis .
molecular dynamics simulations and structural analysis in a bicelle also suggest membrane thinning surrounding glpg , but its mechanistic implications remain unclear . investigating the role of the membrane in greater detail promises to reveal the defining features of the rhomboid proteolysis system .
the rhomboid gene was identified in the drosophila screens of the late 1970s and early 1980s , and it was cloned and sequenced about a decade later .
it took another decade , until 2001 , for its biochemical function as an intramembrane serine protease to be revealed .
it has now been a decade since that turning point , and advances in the intervening period have culminated in rhomboid proteases becoming widely regarded as the best understood of all intramembrane proteases . biochemical insights and defined roles in parasitic protozoa ( reviewed in ) place rhomboid study on the cusp of becoming applicable in a therapeutic setting .
a major lingering obstacle is a rudimentary understanding of its unusual enzymatic mechanism , but these questions are being pursued intensively , and momentum towards a sophisticated understanding is building . by contrast , defining the cellular roles of rhomboid proteases has been a slow process .
although even early biochemical insights have led to the identification of substrates that can be cleaved , whether these candidates are indeed physiological targets , and if so , whether they truly represent a major rhomboid function , remain unknown .
for example , although the study of human rhbdl2 over the past 7 years has uncovered at least three well - cleaved substrates ( thrombomodulin , b - type ephrins , and egf ) , it is still unclear which , if any , are actual physiological targets , and whether cleavage represents a bona fide contribution to cellular function .
perhaps the most humbling example is e. coli glpg , whose atomic details have been revealed in over a dozen structures and countless mutants , yet its cellular function remains a complete mystery . in reality , it is not the ability to find substrate candidates but rather their validation that has proven to be the bottleneck in these studies .
the urgent need is for approaches with which to study enzymes under physiological settings on a higher throughput scale .
this , in turn , will focus biochemical investigations by providing physiological targets and new functional contexts .
work in our laboratory is supported by the howard hughes medical institute , and the david and lucile packard foundation .
 summaryrhomboid proteases are the largest family of enzymes that hydrolyze peptide bonds within the cell membrane . 
 although discovered to be serine proteases only a decade ago , rhomboid proteases are already considered to be the best understood intramembrane proteases . 
 the presence of rhomboid proteins in all domains of life emphasizes their importance but makes their evolutionary history difficult to chart with confidence . 
 phylogenetics nevertheless offers three guiding principles for interpreting rhomboid function . 
 the near ubiquity of rhomboid proteases across evolution suggests broad , organizational roles that are not directly essential for cell survival . 
 functions have been deciphered in only about a dozen organisms and fall into four general categories : initiating cell signaling in animals , facilitating bacterial quorum sensing , regulating mitochondrial homeostasis , and dismantling adhesion complexes of parasitic protozoa . 
 although in no organism has the full complement of rhomboid function yet been elucidated , links to devastating human disease are emerging rapidly , including to parkinson 's disease , type ii diabetes , cancer , and bacterial and malaria infection . 
 rhomboid proteases are unlike most proteolytic enzymes , because they are membrane - immersed ; understanding how the membrane immersion affects their function remains a key challenge . 
Summary
Gene organization and evolutionary history
Characteristic structural features
Localization and function
Mechanism
Frontiers
Competing interests
Acknowledgements