Structural Insights Into Rna Recognition And Activation By Innate Immune Pattern-Recognition Receptor Rig-I

RIG‐I (Retinoic acid Inducible Gene‐I) is a cytoplasmic pathogen recognition receptor that differentiates between viral and cellular RNAs to trigger the innate immune response. RIG‐I comprises N‐terminal CAspase Recruitment Domains (CARDs), a DExH/D box helicase/ATPase domain and a C‐terminal Repressor Domain (RD). The helicase and repressor domains (RD) of RIG‐I recognize double‐stranded (ds) RNA and 5′‐triphosphate (ppp) RNA as foreign and activate the RIG‐I CARDs domain for downstream signaling. To understand the synergy between helicase and RD for RNA binding and how ATP hydrolysis contributes to RIG‐I activation, we determined the crystal structure of human RIG‐I helicase‐RD in complex with dsRNA and an ATP‐analog. Helicase‐RD organizes into a ring around dsRNA that caps the blunt‐end with the helicase utilizing previously uncharacterized motifs to recognize dsRNA. Small angle X‐ray scattering (SAXS), limited proteolysis, and differential scanning fluorimetry suggest that RIG‐I is in an extended and flexible conformation that compacts upon binding RNA. These results provide a detailed view of the role of helicase in dsRNA recognition, the synergy between the RD and the helicase for RNA binding and the organization of full‐length RIG‐I bound to dsRNA, and provide evidence of a conformational change upon RNA binding. The RIG‐I helicase‐RD structure is consistent with dsRNA translocation without unwinding and cooperative binding to RNA. The structure yields unprecedented insight into innate immunity and has a broader impact on other areas of biology, including RNA interference and DNA repair, which utilize homologous helicase domains within DICER and FANCM.