Inhibition of PKR activation by the proline-rich RNA binding domain of theherpes simplex virus type 1 Us11 protein

Upon activation by double-stranded RNA in virus-infected cells, the cellula r PKR kinase phosphorylates the translation initiation factor eukaryotic in itiation factor 2 (eIF2) and thereby inhibits protein synthesis. The gamma 34.5 and Us11 gene products encoded by herpes simplex virus type I (HSV-1) are dedicated to preventing the accumulation of phosphorylated eIF2. While the gamma 34.5 gene specifies a regulatory subunit for protein phosphatase 1 alpha, the Us11 gene encodes an RNA binding protein that also prevents PK R activation. gamma 34.5 mutants fail to grow on a variety of human cells a s phosphorylated eIF2 accumulates and protein synthesis ceases prior to the completion of the viral life cycle. We demonstrate that expression of a 68 -amino-acid fragment of Us11 containing a novel proline-rich basic RNA bind ing domain allows for sustained protein synthesis and enhanced growth of ga mma 34.5 mutants. Furthermore, this fragment is sufficient to inhibit activ ation of the cellular PKR kinase in a cell-free system, suggesting that the intrinsic activities of this small fragment, notably RNA binding and ribos ome association, may be required to prevent PKR activation.