Regulation of innate antiviral defenses through a shared repressor domain in RIG-I and LGP2

• Published in: Proceedings of the National Academy of Sciences - PNAS 2007-01, Vol.104 (2), p.582-587
• Main Authors: Saito, Takeshi, Hirai, Reiko, Loo, Yueh-Ming, Owen, David, Johnson, Cynthia L, Sinha, Sangita C, Akira, Shizuo, Fujita, Takashi, Gale, Michael Jr
• Format: Article
• Language: English
• Subjects: Amino acids; Antiviral Agents - metabolism; Binding sites; Biological Sciences; Cell Line; Cell lines; Cellular immunity; DEAD Box Protein 58; DEAD-box RNA Helicases - chemistry; DEAD-box RNA Helicases - genetics; DEAD-box RNA Helicases - immunology; Double stranded RNA; Hepacivirus - immunology; Hepatitis; Hepatitis C virus; Humans; Immune system; Immunity, Innate; Infections; Interferon-Induced Helicase, IFIH1; Models, Immunological; Permissiveness; Plasmids; Protein Structure, Tertiary; Proteins; Receptors, Immunologic; Ribonucleic acid; Ribozymes; RNA; RNA Helicases - chemistry; RNA Helicases - genetics; RNA Helicases - immunology; RNA, Viral - metabolism; Signal Transduction; Viruses
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.0606699104

RIG-I is an RNA helicase containing caspase activation and recruitment domains (CARDs). RNA binding and signaling by RIG-I are implicated in pathogen recognition and triggering of IFN-α/β immune defenses that impact cell permissiveness for hepatitis C virus (HCV). Here we evaluated the processes that control RIG-I signaling. RNA binding studies and analysis of cells lacking RIG-I, or the related MDA5 protein, demonstrated that RIG-I, but not MDA5, efficiently binds to secondary structured HCV RNA to confer induction of IFN-β expression. We also found that LGP2, a helicase related to RIG-I and MDA5 but lacking CARDs and functioning as a negative regulator of host defense, binds HCV RNA. In resting cells, RIG-I is maintained as a monomer in an autoinhibited state, but during virus infection and RNA binding it undergoes a conformation shift that promotes self-association and CARD interactions with the IPS-1 adaptor protein to signal IFN regulatory factor 3- and NF-κB-responsive genes. This reaction is governed by an internal repressor domain (RD) that controls RIG-I multimerization and IPS-1 interaction. Deletion of the RIG-I RD resulted in constitutive signaling to the IFN-β promoter, whereas RD expression alone prevented signaling and increased cellular permissiveness to HCV. We identified an analogous RD within LGP2 that interacts in trans with RIG-I to ablate self-association and signaling. Thus, RIG-I is a cytoplasmic sensor of HCV and is governed by RD interactions that are shared with LGP2 as an on/off switch controlling innate defenses. Modulation of RIG-I/LGP2 interaction dynamics may have therapeutic implications for immune regulation.