RIG-I uses its intrinsically disordered CARDs-Helicase linker in RNA proofreading functions

Abstract

The innate immune receptor RIG-I provides the first line of defense against viral infections. Viral RNAs are recognized by the C-terminal domain (CTD) of RIG-I, but the RNA must engage the helicase domain to release the signaling domain CARDs from their autoinhibitory CARD2:Hel2i interactions. Because the helicase lacks RNA specificity, there must be mechanisms to proofread RNAs entering the helicase domain. Although such mechanisms are crucial in preventing aberrant immune responses by non-specific RNAs, they remain largely unknown. This study reveals a previously unknown proofreading mechanism that RIG-I uses to ensure the helicase engages RNAs chosen explicitly by the CTD. A crucial part of this mechanism involves the intrinsically disordered CARDs-Helicase Linker (CHL), which uses its negatively charged regions to electrostatically antagonize incoming RNAs. In addition to RNA gating, CHL is essential in stabilizing the CARD2:Hel2i interface. The CHL and CARD2:Hel2i interface work together, establishing a tunable gating mechanism that allows CTD-chosen RNAs to bind into the helicase while blocking non-specific RNAs. With its critical regulatory functions, CHL represents a novel target for RIG-I-based therapeutics.