Fig. 6.

Model of ATPase-mediated discrimination of non-self RNA by RIG-I. a Activation model of wt RIG-I. In resting state, RIG-I is in an auto-repressed form with CARD2 binding to hel2i, which prevents RNA binding to the hel domain. Upon a viral infection, RIG-I CTD selectively catches non-self ^5′pppdsRNA (1). This favors the cis-binding of ^5′pppdsRNA to the hel domain that allows/enhances ATP binding by allosteric effect (2). ATP binding promotes RIG-I folding into closed state with release of CARD2 away from hel2i and exposure of the CARDs (3a). ATP hydrolysis causes ADP+Pi release, hel opening, allosteric weakening of the RNA to hel interaction and CARD resequestration (3b). Because remaining bound to CTD by its ^5′pppdsRNA end, RNA rebinding to hel in cis is favored (3a), which resumes another cycle of ATP fixation, CARD release and ATP hydrolysis (3a and 3b). Rapid repetition of RNA/hel association/dissociation powered by the ATPase motor with on/off cycling of CARD exposure (4a) ensures a sustained level of activated RIG-I molecules per time unit above the critical threshold required for effective signalling. b A fortuitous binding of self RNA to hel (2b) induces ATP fixation, closed conformation and ATP hydrolysis that irreversibly evicts the RNA from hel, because it lacks CTD anchoring. Consequently, the CARDs are exposed too transiently, if ever, to reach the threshold number of transduction units per time unit required for signalling (4b). c Model of the constitutive activity of RIG-I-E373Q. Surrounding (self) RNA serendipitously binds to hel (2b), allows ATP binding with eviction of CARD2 from hel2i (3a). The slow ATP hydrolysis refrains the eviction of the RNA from the hel and CARD2 resequestering (step 3b is very slow). Consequently, a sustained level of activated RIG-I-E373Q molecules with their CARDs exposed accumulate above the threshold level required for IFNβ promoter activation (4c). CARD: caspase activation and recruitment domain; CTD: C-terminal domain; hel: helicase