Vaccinia virus D10 protein has mRNA decapping activity, providing a mechanism for control of host and viral gene expression

Previous studies indicated that the vaccinia virus D10 protein, which is conserved in all sequenced poxviruses, participates in the rapid turnover of host and viral mRNAs. D10 contains a motif present in the family of Nudix/MutT enzymes, a subset of which has been shown to enhance mRNA turnover in eukaryotic cells through cleavage of the 5' cap (m⁷GpppNm-). Here, we demonstrate that a purified recombinant D10 fusion protein possesses an intrinsic activity that liberates m⁷GDP from capped RNA substrates. Furthermore, point mutations in the Nudix/MutT motif abolished decapping activity. D10 has a strong affinity for capped RNA substrates (Km [almost equal to] 3 nm). RNAs of 24-309 nt were decapped to comparable extents, whereas the cap of a 12-nt RNA was uncleaved. At large molar ratios relative to capped RNA substrate, competitor m⁷GpppG, m⁷GTP, or m⁷GDP inhibited decapping, whereas even higher concentrations of unmethylated analogs did not. High concentrations of uncapped RNA were also inhibitory, suggesting that D10 recognizes its substrate through interaction with both cap and RNA moieties. Thus far, poxviruses represent the only virus family shown to encode a Nudix hydrolase-decapping enzyme. Although it may seem self-destructive for a virus to encode a decapping and a capping enzyme, accelerated mRNA turnover helps eliminate competing host mRNAs and allows stage-specific synthesis of viral proteins.