A Conserved Histidine in the RNA Sensor RIG-I Controls Immune Tolerance to N1-2′O-Methylated Self RNA

Abstract

The cytosolic helicase retinoic acid-inducible gene-I (RIG-I) initiates immune responses to most RNA viruses by detecting viral 5′-triphosphorylated RNA (pppRNA). Although endogenous mRNA is also 5′-triphosphorylated, backbone modifications and the 5′-ppp-linked methylguanosine (m7G) cap prevent immunorecognition. Here we show that the methylation status of endogenous capped mRNA at the 5′-terminal nucleotide (N1) was crucial to prevent RIG-I activation. Moreover, we identified a single conserved amino acid (H830) in the RIG-I RNA binding pocket as the mediator of steric exclusion of N1-2′O-methylated RNA. H830A alteration (RIG-I(H830A)) restored binding of N1-2′O-methylated pppRNA. Consequently, endogenous mRNA activated the RIG-I(H830A) mutant but not wild-type RIG-I. Similarly, knockdown of the endogenous N1-2′O-methyltransferase led to considerable RIG-I stimulation in the absence of exogenous stimuli. Studies involving yellow-fever-virus-encoded 2′O-methyltransferase and RIG-I(H830A) revealed that viruses exploit this mechanism to escape RIG-I. Our data reveal a new role for cap N1-2′O-methylation in RIG-I tolerance of self-RNA.