RIG-I-like Receptors (RLRs)
RIG-I-like Receptors (RLRs) are a subset of cytosolic pattern recognition receptors that are essential for antiviral immunity. The RLR family consists of three members: RIG-I, MDA5, and LGP2. Unlike the NOD-like Receptor (NLR) family proteins, the RLR proteins are RNA helicases that recognize viral RNA. RIG-I and MDA5 contain two N-terminal caspase recruitment domains (CARDs), an internal RNA helicase domain, and a C-terminal repressor/RNA-binding domain. Signaling through both receptors activates NF-kappa B- and IRF3-/IRF7-dependent gene expression, leading to the secretion of pro-inflammatory cytokines, and type I and type III interferons, respectively. Unlike RIG-I and MDA5, LGP2 does not contain a CARD domain at its N-terminal end and has not been shown to bind to RNA. As a result, LGP2 is currently thought to serve as a regulator of RIG-I and MDA5 signaling.