The herpes simplex virus US11 protein effectively compensates for the gamma1(34.5) gene if present before activation of protein kinase R by precluding its phosphorylation and that of the alpha subunit of eukaryotic translation initiation factor 2.
In herpes simplex virus-infected cells, viral gamma134.5 protein blocks the shutoff of protein synthesis by activated protein kinase R (PKR) by directing the protein phosphatase 1alpha to dephosphorylate the alpha subunit of eukaryotic translation initiation factor 2 (eIF-2alpha). The amino acid sequence of the gamma134.5 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 alpha47 promoter next to US11, a gamma2 (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 gamma134.5 gene. In these cells, PKR is activated but eIF-2alpha is not phosphorylated, and the phosphatase is not redirected to dephosphorylate eIF-2alpha. We report the following: (i) in cells infected with these mutants, US11 protein was made early in infection; (ii) US11 protein bound PKR and was phosphorylated; (iii) in in vitro assays, US11 blocked the phosphorylation of eIF-2alpha by PKR activated by poly(I-C); and (iv) US11 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, US11 made early in infection binds to PKR and precludes the phosphorylation of eIF-2alpha, whereas US11 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 gamma134.5 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. US11 was retained as a gamma2 gene because, like many viral proteins, it has multiple functions.
The Marjorie B. Kovler Viral Oncology Laboratories, The University of Chicago, Chicago, Illinois 60637, USA.
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D000818:Animals; D002460:Cell Line; D002522:Chlorocebus aethiops; D004789:Enzyme Activation / Q000187:drug effects; D015852:Eukaryotic Initiation Factor-2 / Q000378:metabolism*; D005814:Genes, Viral; D006367:HeLa Cells; D018259:Herpesvirus 1, Human / Q000235:genetics* / Q000378:metabolism*; D006801:Humans; D009154:Mutation; D010766:Phosphorylation; D011070:Poly I-C / Q000494:pharmacology; D016601:RNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D014709:Vero Cells; D014764:Viral Proteins / Q000235:genetics* / Q000378:metabolism*; D019892:eIF-2 Kinase / Q000378:metabolism*
D015852:Eukaryotic Initiation Factor-2; D016601:RNA-Binding Proteins; D011993:Recombinant Fusion Proteins; C074546:US11 protein, herpesvirus; D014764:Viral Proteins; C066469:gamma 34.5 protein, Human herpesvirus 1; D019892:eIF-2 Kinase; D011070:Poly I-C