InterPro domain: IPR038557

Intracellular RIG-I-like receptors (RLRs: retinoic acid-inducible gene I, RIG-I; melanoma differenciation-associated gene 5, MDA-5; and laboratory of genetics and physiology 2, LGP2) recognise viral RNAs as pathogen-associated molecular patterns (PAMPs) and initiate an antiviral immune response. RLRs belong to the superfamily 2 (SF2) helicases or ATPases. Among SF2 ATPases, RIG-I, MDA5, and LGP2 possess a unique domain structure. RIG-I and MDA5 consist of two N-terminal caspase activation and recruitment domains (CARDs), a central SF2 type DECH box ATPase domain (consisting oftwo RecA-like helicase domains, Hel1 and Hel2, and an insert domain, Hel2i), and a C-terminal regulatory (CTR) domain. LGP2 lacks the two N-terminal CARDs but contains the DECH box domain, as well as the CTR domain. The CTR domain helps recognise non-self RNAs within the cellular environment. To facilitate the detection of a broad range of non-self RNAtargets, each RLR contains a similar but divergent CTR domain that mediates RLR-specific interactions between bound nucleic acids or neighbouring patterns [ 1 , 2 , 3 , 4 , 5 , 6 ].

The RLR CTR domain is a globular, slightly flattened domain with a concave and a convex side. It is structurally organised in three leaves consisting of two four-stranded (beta1, beta2, beta9, beta10 and beta5, beta6, beta7, beta8) and one two-stranded (beta3, beta4) antiparallel beta sheets. Small helical turns connect the three beta sheets. The two four stranded beta sheets are laterally connected by two protruding loops, each containing two highly conserved cysteine residues. Together, the four thiol groups of these cysteines coordinate a zinc ion [ 7 , 7 ].

This superfamily represents the C-terminal regulatory domain of the RIG-I-like receptor.

1. An evolving arsenal: viral RNA detection by RIG-I-like receptors. Curr. Opin. Microbiol. 20, 76-81
2. Pattern Recognition and Signaling Mechanisms of RIG-I and MDA5. Front Immunol 5, 342
3. Structural insights into RNA recognition by RIG-I. Cell 147, 409-22
4. The C-terminal regulatory domain is the RNA 5'-triphosphate sensor of RIG-I. Mol. Cell 29, 169-79
5. Crystal structure of RIG-I C-terminal domain bound to blunt-ended double-strand RNA without 5' triphosphate. Nucleic Acids Res 39, 1565-75
6. The regulatory domain of the RIG-I family ATPase LGP2 senses double-stranded RNA. Nucleic Acids Res. 37, 2014-25