2′-O methylation of the viral mRNA cap evades host restriction by IFIT family members

Abstract

Cellular mRNA of higher eukaryotes and many viral RNA are methylated at the N-7 and 2′-O positions of the 5′ guanosine cap by specific nuclear and cytoplasmic methyltransferases (MTases), respectively. Whereas N-7 methylation is essential for RNA translation and stability 1, the function of 2′-O methylation has remained uncertain since its discovery 35 years ago 2-4. Here, we show that a West Nile virus (WNV) mutant (E218A) that lacks 2′-O MTase activity was attenuated in wild type primary cells and mice but was pathogenic in the absence of type I interferon (IFN) signaling. 2′-O methylation of viral RNA did not affect IFN induction in WNV-infected fibroblasts but instead modulated the antiviral effects of IFN-induced proteins with tetratricopeptide repeats (IFIT), which are interferon-stimulated genes (ISG) implicated in regulation of protein translation. Poxvirus and coronavirus mutants that lacked 2′-O MTase activity similarly showed enhanced sensitivity to the antiviral actions of IFN and specifically, IFIT proteins. Our results demonstrate that the 2′-O methylation of the 5′ cap of viral RNA functions to subvert innate host antiviral responses through escape of IFIT-mediated suppression, and suggest an evolutionary explanation for 2′-O methylation of cellular mRNA: to distinguish self from non-self RNA. Differential methylation of cytoplasmic RNA likely serves as a paradigm for pattern recognition and restriction of propagation of foreign viral RNA in host cells.

Innate Barriers to Viral Infection

“…Both HCoV-229E and MHV mutants lacking 2´-O-methyltransferase activity triggered a much higher expression of type I IFN than wild-type viruses, and these mutant viruses were highly sensitive to IFN. The induction of type I IFN was dependent on the RIG-I-like receptor family member, MDA-5 [26,27]. Thiel concluded by highlighting how coronaviruses evade the MDA-5-triggered innate immune response.…”

Inhibitor of κB Kinase ϵ (IKKϵ), STAT1, and IFIT2 Proteins Define Novel Innate Immune Effector Pathway against West Nile Virus Infection

“…ISGF3 translocates to the nucleus and triggers the transcription of hundreds of ISGs (13,15). ISG products serve as immunomodulators and restriction factors against virus infection (22)(23)(24).…”

“…Among the ISGs, IFN-induced protein with tetratricopeptide repeats (IFIT)2, also known as ISG54, has been identified as an ISG restriction factor of WNV (23). IFIT2 belongs to the IFIT gene family whose members function to restrict virus infection through alteration of cellular protein synthesis (reviewed in Ref.…”

Crystal structure of IFIT2 (ISG54) predicts functional properties of IFITs

“…These findings are in contrast with those of Pichlmair et al [3] which reported that IFIT2 can bind to RNA only indirectly by complexing with IFIT1 and the complex binds to 5′-triphosphorylated RNA only. The above studies were done in vitro, whereas in vivo studies by Daffis et al [4] demonstrated that mouse Ifit1 can functionally distinguish mRNAs, which have 2′-O methylation, from those that do not. Many protein partners of IFITs have been reported.…”

“…However, the underlying mechanism remains unclear. Daffis et al [4] and Szretter et al [14] demonstrated that 2′-O-methylation of viral RNAs, from both RNA and DNA viruses, promotes evasion of the antiviral effects of Ifit1 in a cell type-specific way. Although the specific requirements for RNA recognition by Ifit1 seem to be different in the two reports, both point to the 5′ end of viral RNAs as the potential target of recognition.…”