I11 RIG-I signaling in RNA virus infection and immunity

RIG-I-like receptors (RLRs) are a class of cytosolic pathogen recognition receptors including RIG-I, MDA5, and LGP2. RIG-I and MDA5 are essential for trigging immunity towards distinct RNA viruses, while LGP2 may function as a cofactor or regulator of these processes. Our studies have shown that RIG-I in particular initiates immune defenses against hepatitis C virus (HCV), a major human pathogen. RIG-I engages pathogen-associated molecular pattern motif(s) within HCV RNA in infected cells to trigger innate immune defenses through its adaptor protein MAVS/IPS-1. However, HCV blocks RIG-I signaling through the viral NS3/4A protease cleavage of MAVS to support persistent infection. While MAVS resides on mitochondria and peroxisomes, how its signaling is coordinated among these organelles has been unknown. We have found that the mitochondrial-associated ER membrane (MAM), a distinct membrane subdomain which links ER to mitochondria, is a major site of MAVS localization and intracellular signaling of innate immune defenses against RNA viruses, including HCV. Upon RIG-I pathway activation, RIG-I is recruited to the MAM to bind MAVS. The MAM then dynamically organizes mitochondria and peroxisomes into MAVS-anchored synapses to regulate antiviral signaling. The importance of the MAM in anchoring these “innate immune synapses” is highlighted by the fact that the HCV NS3/4A protease cleaves MAVS on the MAM, but not the mitochondria, to evade immunity. Thus, an intracellular immune synapse anchored by the MAM coordinates innate immunity during RNA virus infection and is targeted by the HCV NS3/4A protease. While the interaction between RIG-I and MAVS at the MAM is essential for regulated innate immune signaling, it is unclear how RIG-I, a cytosolic protein, can identify and interact with MAM-localized MAVS. We have also focused on defining the mechanisms of RIG-I translocation to the MAM for intracellular signaling. We now show that RIG-I interacts with the E3 ubiquitin ligase TRIM25 and the molecular chaperone 14-3-3ε to form a translocon that moves RIG-I from cytosol to MAM during the acute phase of RNA virus infection. Our results demonstrate that RIG-I signaling is mediated through this ternary complex of RIG-I/TRIM25/14-3-3ε to mediate engagement of RIG-I with MAM to bind MAVS and initiate signaling of immunity to HCV infection. Overall, our studies indicate that the RIG-I translocon-MAM signaling network is essential for immunity against HCV and other pathogeni