Structural Basis for the Activation of RIG-I/MAVS Antiviral Immune Signaling
Retinoic acid inducible gene-I (RIG-I) is a key cytosolic pathogen RNA sensor that activates mitochondrial antiviral signaling protein (MAVS) to trigger rapid innate immune responses. Using RNAs of different lengths as model ligands, we showed that RIG-I oligomerized on dsRNA in an ATP hydrolysis-dependent and dsRNA length-dependent manner, which correlated with the strength of type-I interferon (IFN-I) activation. The obtained negative stain EM structure of full-length RIG-I in complex with a 5'ppp stem-loop RNA and the crystal structure of RIG-I/Ub complex elucidated a two-step oligomerization and conformational change of RIG-I for activation. RIG-I oligomers nucleate MAVS through homotypic interaction of the N-terminal caspase activation and recruitment domains (CARDs) and induce the formation of prion-like aggregates. The obtained cryoEM structure of left-handed helical filaments of MAVS CARD revealed specific interfaces between individual CARD subunits that are dictated by a combination of electrostatic and hydrophobic interactions and hydrogen bonding. Point mutations at multiple locations of these interfaces impaired filament formation and antiviral signaling. Super-resolution imaging of virus-infected cells revealed rod-shaped MAVS clusters on mitochondria. These results elucidated the structural mechanism of RIG-I activation by RNA and K63-linked ubiquitin chains as well as the activation of MAVS through polymerization, revealing a highly efficient signaling cascade for viral RNA sensing.