RIG-I discriminates between host and viral RNA by sensing differences at the 5ʹ end of single-stranded RNA transcripts. Eukaryotic RNA is transcribed in the nucleus and contains a 5ʹ-triphosphate group on its initial nucleotide that undergoes subsequent enzymatic modification called capping by the addition of a 7-methylguanosine to the 5ʹ-triphosphate. Most RNA viruses, however, do not replicate in the nucleus, where capping normally occurs, and their RNA genomes do not undergo this modification. Biochemical studies have determined that RIG-I senses the unmodified 5ʹ-triphosphate end of the ssRNA viral genome. Flavivirus RNA transcripts have the unmodified 5ʹ-triphosphate, as do the transcripts of many other ssRNA viruses, and they are detected by RIG-I. In contrast, the picornaviruses, which include poliovirus and hepatitis A, replicate by a mechanism that involves the covalent attachment of a viral protein to the 5ʹ end of the viral RNA, so that the 5ʹ-triphosphate is absent, which explains why RIG-I is not involved in sensing them.
MDA-5 (melanoma differentiation-associated 5), also called helicard, is similar in structure to RIG-I, but it senses dsRNA. In contrast to RIG-I-deficient mice, mice deficient in MDA-5 are susceptible to picornaviruses, indicating that these two sensors of viral RNAs have crucial but distinct roles in host defense. Inactivating mutations in alleles of human RIG-I or MDA-5 have been reported, but these mutations were not associated with immunodeficiency. The RLR family member LGP2 (encoded by DHX58) retains a helicase domain but lacks CARD domains. LGP2 appears to cooperate with RIG-I and MDA-5 in the recognition of viral RNA, since mice lacking LGP2 have impaired antiviral responses normally mediated by RIG-I or MDA-5. This cooperative viral recognition by LGP2 appears to depend on its helicase domain, since in mice, mutations that disrupt its ATPase activity result in impaired IFN-β production in response to various RNA viruses.