196 Mechanisms of viral dsRNA detection and antiviral signal activation by MDA5 filament

Abstract

MDA5, a viral double-stranded RNA (dsRNA) receptor, shares sequence similarity and interferon signaling pathways with RIG-I, yet plays essential functions in antiviral immunity by recognizing largely distinct groups of viruses and viral RNAs. We have previously shown that MDA5 forms novel filamentous oligomers by stacking monomers along the length of dsRNA head-to-tail, and that such filament formation is distinct from monomeric binding observed with RIG-I. We have recently determined the first crystal structure of MDA5 in complex with dsRNA, which revealed the molecular basis for functional divergence between RIG-I and MDA5, and in particular, the divergent mechanism for dsRNA recognition and filament formation. We have further demonstrated that MDA5 filament formation is (1) required for high affinity interaction with dsRNA, (2) provides a molecular framework to measure dsRNA length that is orders of magnitude larger than the individual protein itself, and finally, (3) brings the signaling domain, tandem CARD (2CARD), into proximity to promote oligomerization of 2CARD, which in turn activates interferon signaling pathways by inducing MAVS filament formation. Interestingly, during each of these processes, ATP plays critical roles by allowing conformational change and oligomerization of 2CARD within the filament, while triggering dissociation of MDA5 from filament ends which then regulates the global stability of the filament (and 2CARD oligomers) in a dsRNA length-dependent manner. Our discoveries of the novel roles of ATP hydrolysis, filament dynamics, and the structure of the MDA5:dsRNA complex together provide comprehensive understanding of the complex molecular mechanisms of MDA5 function during self and non-self RNA discrimination, and offer novel insights into the divergent evolution of MDA5 and RIG-I.