| Summary: | RIG-I is a cytosolic helicase that senses 5’-ppp-RNA contained in negative strand RNA viruses and triggers innate antiviral immune responses. Calorimetric binding studies establish that the RIG-I C-terminal regulatory domain (CTD) binds to blunt-end double-stranded 5’-ppp-RNA a factor of 17 more tightly than to its single-stranded counterpart. Here we report on the crystal structure of RIG-I CTD domain bound to both blunt-ends of a self-complementary 5’-ppp-dsRNA 12-mer, with interactions involving 5’-pp clearly visible in the complex. The structure, supported by mutation studies, defines how a lysine-rich basic cleft within the RIG-I CTD domain sequesters the observable 5’-pp of the bound RNA, with a stacked Phe capping the terminal base pair. Key intermolecular interactions observed in the crystalline state are retained in the complex of 5’-ppp-dsRNA 24-mer and full-length RIG-I under in vivo conditions, as evaluated from the impact of binding pocket RIG-I mutations and 2’-OCH3 RNA modifications on the interferon response.
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