Figure 2. Model for RIG-I signaling activation and its regulation.

In the resting state, RIG-I is held in a “closed” conformation in the resting state by casein kinase II phosphorylation and autoregulatory intramolecular interactions with the C-terminal RD within the CTD holds the CARDs unavailable for signaling. During virus infection, RD engagement of a 5′ppp RNA PAMP containing and other nonself signature(s), concommitant with K63-linked ubiquitination of RIG-I by TRIM25 and RING finger protein leading to RIG-I activation (Ripletotherwise known as REUL or RNF135), leads to conformational changes that release the CARDs from autoregulation. The model shows RIG-I binding to 5′ppp RNA containing poly-U/UCmotif such as the HCV RNA genome (Saito, Owen et al. 2008; Uzri and Gehrke 2009). RIG-I then assumes an “open” conformation that allows for oligomerization and CARD-dependent interaction with IPS-1 which activates signaling molecules at the IPS-1 singalosome. These interactions trigger a signaling cascade that culminates in IFN production and expression of proteins with direct antiviral or immune-modulating activities to control infection. To prevent excessive activation of innate immune responses, RIG-I signaling activity is inhibited by (1) phosphorylation events that inhibit the K63-linked ubiquitination required for signaling activation, (2) negative regulators that sequester PAMP from RIG-I, (3) association with negative regulators or molecules that disrupt/destabilize its interaction with IPS-1, and (4) K43-linked ubiquitination by RNF125 which targets RIG-I for proteasomal degradation.