The vaccinia virus K3L gene product potentiates translation by inhibiting double-stranded-RNA-activated protein kinase and phosphorylation of the alpha subunit of eukaryotic initiation factor 2

Interferon resistance of vaccinia virus is mediated by specific inhibition of phosphorylation of the alpha subunit of eukaryotic initiation factor 2 (eIF-2α) by the double-stranded-RNA-activated (DAI) protein kinase. Vaccinia virus encodes a homolog of eIF-2α, K3L, the deletion of which renders the virus sensitive to interferon treatment. We have studied the mechanism by which this protein product elicits interferon resistance in a transient DNA transfection system designed to evaluate regulators of eIF-2α phosphorylation. In this system, translation of a reporter gene mRNA is inefficient because of eIF-2 phosphorylation mediated by the DAI protein kinase. Cotransfection of the K3L gene enhances translation of the reporter mRNA in this system. The K3L protein inhibits eIF-2α phosphorylation and DAI kinase activation, apparently without being phosphorylated itself. Inhibition of protein synthesis, elicited by expression of a mutant Ser-51→Asp eIF-2α designed to mimic a phosphorylated serine, is not relieved by the presence of K3L, suggesting that K3L cannot bypass a block imposed by eIF-2α phosphorylation. The results suggest that K3L acts as a decoy of eIF-2α to inhibit DAI kinase autophosphorylation and activation. Another vaccinia virus gene product, K1L, which is required for growth of vaccinia virus on human cells, does not enhance translation in this assay.