The Herpes Simplex Virus U(S)11 Protein Effectively Compensates for the γ(1)34.5 Gene if Present before Activation of Protein Kinase R by Precluding Its Phosphorylation and That of the α Subunit of Eukaryotic Translation Initiation Factor 2

In herpes simplex virus-infected cells, viral γ(1)34.5 protein blocks the shutoff of protein synthesis by activated protein kinase R (PKR) by directing the protein phosphatase 1α to dephosphorylate the α subunit of eukaryotic translation initiation factor 2 (eIF-2α). The amino acid sequence of the γ(1)34.5 protein which interacts with the phosphatase has high homology to a domain of the eukaryotic protein GADD34. A class of compensatory mutants characterized by a deletion which results in the juxtaposition of the α47 promoter next to U(S)11, a γ(2) (late) gene in wild-type virus-infected cells, has been described. In cells infected with these mutants, protein synthesis continues even in the absence of the γ(1)34.5 gene. In these cells, PKR is activated but eIF-2α is not phosphorylated, and the phosphatase is not redirected to dephosphorylate eIF-2α. We report the following: (i) in cells infected with these mutants, U(S)11 protein was made early in infection; (ii) U(S)11 protein bound PKR and was phosphorylated; (iii) in in vitro assays, U(S)11 blocked the phosphorylation of eIF-2α by PKR activated by poly(I-C); and (iv) U(S)11 was more effective if present in the reaction mixture during the activation of PKR than if added after PKR had been activated by poly(I-C). We conclude the following: (i) in cells infected with the compensatory mutants, U(S)11 made early in infection binds to PKR and precludes the phosphorylation of eIF-2α, whereas U(S)11 driven by its natural promoter and expressed late in infection is ineffective; and (ii) activation of PKR by double-stranded RNA is a common impediment countered by most viruses by different mechanisms. The γ(1)34.5 gene is not highly conserved among herpesviruses. A likely scenario is that acquisition by a progenitor of herpes simplex virus of a portion of the cellular GADD34 gene resulted in a more potent and reliable means of curbing the effects of activated PKR. U(S)11 was retained as a γ(2) gene because, like many viral proteins, it has multiple functions.