Defining the functional determinants for RNA surveillance by RIG‐I

Retinoic acid‐inducible gene‐I (RIG‐I) is an intracellular RNA sensor that activates the innate immune machinery in response to infection by RNA viruses. Here, we report the crystal structure of distinct conformations of a RIG‐I:dsRNA complex, which shows that HEL2i‐mediated scanning allows RIG‐I to sense the length of RNA targets. To understand the implications of HEL2i scanning for catalytic activity and signalling by RIG‐I, we examined its ATPase activity when stimulated by duplex RNAs of varying lengths and 5′ composition. We identified a minimal RNA duplex that binds one RIG‐I molecule, stimulates robust ATPase activity, and elicits a RIG‐I‐mediated interferon response in cells. Our results reveal that the minimal functional unit of the RIG‐I:RNA complex is a monomer that binds at the terminus of a duplex RNA substrate. This behaviour is markedly different from the RIG‐I paralog melanoma differentiation‐associated gene 5 (MDA5), which forms cooperative filaments.