Current view of the RIG-I activation pathway. (A) A color-coded schematic representation of the domain structure of RIG-I, as used throughout. Bare lines represent flexible linkers. (B) Cartoon of auto-inhibited RIG-I and its substrates, 5′ppp-bpRNA and ATP. (C) RIG-I binds via its high-affinity 5′ppp blunt-end composite binding site even on relatively short dsRNAs, or to lower-affinity sites on longer dsRNAs independent of their 5′ ends, releasing the CARDs. In the latter case, cycles of ATP hydrolysis (round arrows) can lead to translocation (straight arrows). (D) The free CARDs are covalently or noncovalently bound by K63-linked polyubiquitin chains generated by E3-ligase TRIM25. This promotes tetramerization of RIG-I via its ubiquitin-bound CARDs. Tetramers are shown either with RIG-Is bound to different 5′ppp-bpRNAs (left) or the same long dsRNA (right). The mitochondrial membrane chaperone 14-3-3ɛ directs the RNA-bound multimers to mitochondrial membranes, enabling MAVS and RIG-I CARDs to interact. This promotes MAVS aggregation and propagation of the activation signal downstream.
