Interferons induce cell intrinsic responses associated with resistance to viral infection. To overcome the suppressive action of interferons and their downstream effectors viruses have evolved diverse mechanisms. Working with vesicular stomatitis virus (VSV) we report a role for the host cell N6-adenosine mRNA cap-methylase, phosphorylated C-terminal domain interacting factor 1 (PCIF1), in attenuating the antiviral activity of interferon-{beta}. Using cell based and in vitro biochemical assays we demonstrate that PCIF1 efficiently modifies VSV mRNA cap structures to m7Gpppm6Am, and we identify the cis-acting elements required for this modification. Under basal conditions, N6-methylation of VSV mRNA cap structures is functionally inert with regard to mRNA stability, translation and viral infectivity. Induction of an antiviral state by treatment of cells with interferon-{beta} prior to infection uncovered a functional role for PCIF1 in attenuation of the antiviral response. Cells lacking PCIF1 or expressing a catalytically inactive PCIF1, exhibit an augmented effect of interferon-{beta} in the inhibition of viral replication and gene expression. This work identifies a function of PCIF1 and cap-proximal m6Am in attenuation of the host response to VSV infection that likely extends to other viruses. SignificanceThe cap structure present at the 5 end of eukaryotic mRNAs regulates RNA stability, translation, and marks mRNA as self, thereby impeding recognition by the innate immune system. Cellular transcripts beginning with adenosine are additionally modified at the N6 position of the 2-O methylated cap-proximal residue by the methyltransferase PCIF1 to m7Gpppm6Am. We define a function for this N6-adenosine methylation in attenuating the interferon-{beta} mediated suppression of viral infection. Cells lacking PCIF1, or defective in its enzymatic activity, augment the cell intrinsic suppressive effect of interferon-{beta} treatment on vesicular stomatitis virus gene expression. VSV mRNAs are efficiently methylated by PCIF1, suggesting this contributes to viral evasion of innate immune suppression.

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