Sunday, May 11, 2025
5/11/2025
Cap specific N6 methylation of viral mRNA by the cellular methyltransferase PCIF1 - The long-term goal of this project is to define the function(s) of the cap-specific N6, 2'-O-dimethyladenosine (m6 Am) present at the transcription start site of eukaryotic mRNAs. We and others recently identified the cellular mRNA methyltransferase responsible for methylation of the cap-proximal adenosine at the N6 position as phosphorylated carboxy-terminal domain interacting factor 1 (PCIF1). PCIF1 binds the phosphorylated C- terminal domain of host RNA polymerase II to selectively modify the cap proximal A, following the sequential methylation of the cap-structure by the guanine-N7-methylase and ribose-2'-O methylase. The functional significance of the cap-proximal m6Am modification is uncertain with published literature reaching different conclusions regarding mRNA stability and translation. Viruses that replicate in the cytoplasm such as the negative-strand RNA virus vesicular stomatitis virus (VSV) also contain this cap-proximal m6Am modification on mRNA synthesized in infected cells despite the absence of a viral encoded N6, 2'-O-dimethyltransferase. In preliminary data we have found that PCIF1 is relocalized to the cytoplasm in VSV infected cells and methylates VSV mRNA. The 5 VSV mRNAs are well characterized and we have developed tools necessary to define how m6Am influences the function of each of those mRNAs. Our preliminary data shows that neither mRNA stability nor mRNA translation is impacted by the loss of m6Am, and that in 293T and Hela cells in culture, virus replication is unaffected under basal conditions. Pretreatment of cells with interferon, however, demonstrates that loss of PCIF1 results in the further translational suppression of viral mRNA and a more pronounced reduction in viral growth. This PCIF1 dependent phenotype suggests that one function of this cap-proximal m6Am is to discriminate self from non-self mRNA. The mRNA cap has been hypothesized to have emerged with eukaryotic evolution, when PCIF1 is first detected, to replace the Shine-Dalgarno sequence for directing ribosomes to mRNAs and to protect mRNAs from digestion by 5' exoribonucleases thus providing an early method for distinguishing self- versus foreign mRNAs. It is likely that extant viruses have evolved in the face of this RNA methylation to evade the eukaryotic self-defense system. To further probe the role of PCIF1 modification of viral RNA we generated a PCIF1 -/- mouse providing an additional unique reagent to mechanistically dissect the role of m6Am of viral mRNA in vivo. Capitalizing on this preliminary data we will use genetic, biochemical, cell biological and virological approaches both in cell-culture and in vivo to dissect the role of m6Am and PCIF1 mediated mRNA methylation. Our underlying hypothesis is that PCIF1 modification of mRNA contributes to distinguishing self from non-self mRNA, and that viruses have coopted PCIF1 to ensure efficient replication.
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