Regulation of interferon production by RIG-I and LGP2: a lesson in self-control.

The cytoplasmic CARD-containing DExD/H box RNA helicases RIG-I and MDA5 act as sensors of viral infections through recognition of viral double-stranded (ds) RNAs. They both associate with the mitochondrial adaptor IPS-1 (also referred to as MAVS, VISA, and CARDIF) through homotypic CARD-CARD interactions. IPS-1, in turn, triggers signaling pathways, including activation of the protein kinases TBK1 and IKKepsilon, responsible for the phosphorylation of IRF3, a key transcription factor involved in interferon (IFN) synthesis, one essential element of the innate immune response. RIG-I remains in an autoinhibited state in the absence of dsRNA, through an internal repressor domain (RD) that binds within both its CARD and its RNA helicase domains and therefore acts in cis to control its multimerization and interaction with IPS-1. Ectopic expression of the RD prevents signaling and increases cell permissiveness to viruses, including hepatitis C virus. LGP2, which is another DExD/H RNA helicase of the RIG-I and MDA5 family and which is devoid of CARD domain, negatively controls IFN induction at different levels: by sequestering dsRNA, by blocking RIG-I's multimerization in trans through a domain analogous to the RIG-I RD, and by competing with the protein kinase IKKepsilon for a common interaction site on IPS-1. The ability of RIG-I and LGP2 to exert such a feedback control at the earliest steps of IFN synthesis allows the cells to exert a tight regulation of the induction of the innate immune response.