MDA5, an RIG-I-like helicase, is a conserved cytoplasmic viral RNA sensor, which recognizes dsRNA from a wide-range of viruses in a length-dependent manner. It has been proposed that MDA5 forms higher-order structures upon viral dsRNA recognition or during antiviral signaling, however the organization and nature of this proposed oligomeric state is unknown. We report here that MDA5 cooperatively assembles into a filamentous oligomer composed of a repeating segmental arrangement of MDA5 dimers along the length of dsRNA. Binding of MDA5 to dsRNA stimulates its ATP hydrolysis activity with little coordination between neighboring molecules within a filament. Individual ATP hydrolysis in turn renders an intrinsic kinetic instability to the MDA5 filament, triggering dissociation of MDA5 from dsRNA at a rate inversely proportional to the filament length. These results suggest a previously unrecognized role of ATP hydrolysis in control of filament assembly and disassembly processes, thereby autoregulating the interaction of MDA5 with dsRNA, and provides a potential basis for dsRNA length-dependent antiviral signaling. (1). Upon viral RNA recognition, RIG-I and MDA5 interact with a common signaling adaptor, MAVS and activate NF-kB and IRF3/7 signaling pathways, resulting in the expression of type I interferon and proinflammatory cytokines (2).Previous studies suggested that RIG-I and MDA5 recognize largely distinct groups of viruses through their divergent RNA specificity (3, 4). RIG-I detects a variety of positive and negative strand viruses through recognition of a 5′ triphosphate group and blunt ends of genomic RNAs (3, 5, 6). By contrast, MDA5 detects several positive strand and dsRNA viruses such as picornaviruses and reoviruses through recognition of dsRNA replication intermediates and genomic dsRNAs (3,4,7). This viral RNA recognition by MDA5 is independent of 5′ triphosphate or blunt end, but instead depends on dsRNA length in a range of approximately 1-7 kb (8).The precise mechanism for length-dependent dsRNA recognition by MDA5 is poorly understood. Previous studies suggest that ATP hydrolysis could play an important role in both interferon signaling and RNA specificity (1, 2). The current model of MDA5 (or RIG-I) mediated signal activation is that binding of viral RNA to MDA5 triggers ATP hydrolysis in the conserved helicase domain, which then alters the receptor conformations or accessibility of the signaling domain (a tandem caspase activation recruitment domain) to allow its interaction with MAVS (1, 9). The proposed role of ATP hydrolysis as a conformational switch for signaling is supported by the requirement of ATP in the reconstituted signaling system of RIG-I (9, 10) and the observation that mutations in the active site for ATP hydrolysis either constitutively activate or inactivate interferon signaling by 12). In addition, induction of ATP hydrolysis by dsRNA has been shown to correlate with stimulation of interferon signaling (6, 13).It has been proposed that MDA5 (and RIG-I) forms a higher-order st...