An alternative mechanism for activation of innate immune signaling by MDA5
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Long double-stranded (ds) RNA in the cytosol acts as a potent inflammatory molecule recognized by the receptor MDA5, triggering the innate immune response. Mutations in MDA5 affecting dsRNA recognition can lead to increased infection sensitivity or autoimmune disease. The current model proposes that MDA5 nucleoprotein filament assembly-disassembly dynamics regulates long dsRNA recognition and signaling. We show that MDA5 preferentially loads onto dsRNA via a 3’ recessed end and uses ATP hydrolysis to translocate towards the 5’-end until obstructed, such as by another MDA5 on the opposite strand. Multiple MDA5 monomers accumulate at the blockade, forming a partial filament that extrudes the associated RNA in single-stranded loops and thereby compacting the MDA5-RNA complex. The compacted state is further stabilized by oligomerization of the MDA5’s caspase recruitment domain (CARD) and can withstand significant forces, offering an alternative intermediate in the activation of MDA5-dependent innate immunity.
