Pattern Recognition and Signaling Mechanisms of RIG-I and MDA5

Abstract

Most organisms rely on innate immune receptors to recognize conserved molecular structures from invading microbes. Two essential innate immune receptors, RIG-I and MDA5, detect viral double-stranded RNA in the cytoplasm. The inflammatory response triggered by these RIG-I-like receptors (RLRs) is one of the first and most important lines of defense against infection. RIG-I recognizes short RNA ligands with 5'-triphosphate caps. MDA5 recognizes long kilobase-scale genomic RNA and replication intermediates. Ligand binding induces conformational changes and oligomerization of RLRs that activate the signaling partner MAVS on the mitochondrial and peroxisomal membranes. This signaling process is under tight regulation, dependent on post-translational modifications of RIG-I and MDA5, and on regulatory proteins including unanchored ubiquitin chains and a third RLR, LGP2. Here, we review recent advances that have shifted the paradigm of RLR signaling away from the conventional linear signaling cascade. In the emerging RLR signaling model, large multimeric signaling platforms generate a highly cooperative, self-propagating, and context-dependent signal, which varies with the subcellular localization of the signaling platform.

Keywords: RecA-like DEAD-box (DExD/H-box) RNA helicase; amyloid-like aggregation; caspase recruitment domain; nucleic-acid sensor; pathogen-associated molecular pattern; prion-like switch; signal transduction; signalosome.

Figure 1

The RLR signaling pathway is shown. RIG-I and MDA5 recognize a complementary set of cytosolic viral dsRNA ligands. Their activation is tightly regulated by phosphorylation, ubiquitination, and host proteins such as LGP2. RIG-I and MDA5 signal to MAVS, which initiates the production of interferon signaling. Circled “P” indicates phosphorylation and slashed circled “P” indicates dephosphorylation.

Figure 2

Assembly of the RLR signalosome is shown. (A) The domain architecture of RIG-I [colored as in Ref. (34)]. (B) Two orthogonal views of the RIG-I (left) and MDA5 (right) helicase domains and CTD bound to a dsRNA ligand (17). The CTD of RIG-I caps the 5′ end of the dsRNA ligand, however, in MDA5 the CTD is rotated by 20° relative to Hel2, allowing for MDA5 to polymerize along the dsRNA. (C) Two orthogonal views of the RIG-I tandem CARDs, which assemble into a “lock-washer” with three K63-di-ubiquitin molecules are shown (35). (D) RIG-I recognizes viral dsRNA in the cytosol and undergoes a conformational change, releasing the CARDs from an auto-repressed state. Four RIG-I molecules come together and their CARDs assemble into an oligomer stabilized by unanchored K63-linked polyubiquitin chains. The RIG-I CARDs serve as a scaffold for MAVS, which forms a filament that is tethered on the mitochondrial or peroxisomal membrane (36).