BINDING OF THE INFLUENZA-VIRUS NS1 PROTEIN TO DOUBLE-STRANDED-RNA INHIBITS THE ACTIVATION OF THE PROTEIN-KINASE THAT PHOSPHORYLATES THE ELF-2 TRANSLATION INITIATION-FACTOR

The NS1 protein of influenza A virus binds not only to poly(A) and a s tem-bulge region in U6 small nuclear RNA (snRNA), but also to double-s tranded (ds) RNA. Binding assays with NS1 protein mutants established that the previously identified RNA-binding domain of the NS1 protein i s required for binding to ds RNA as well as for binding to poly(A) and U6 snRNA. In addition, dsRNA competed with U6 snRNA for binding to th e NS1 protein, consistent with both RNAs sharing the same binding site on the protein. As a consequence of its binding to dsRNA, the NS1 pro tein blocks the activation of the dsRNA-activated protein kinase (PKR) in vitro. This kinase phosphorylates the cu subunit of eukaryotic tra nslation initiation factor 2 (elF-2 alpha), leading to a decrease in t he rate of initiation of translation. Assays using purified PKR and pu rified elF2 demonstrated that the NS1 protein blocks the dsRNA activat ion of PKR, and experiments using reticulocyte extracts showed that th e NS1 protein blocks the inhibition of translation caused by dsRNA act ivation of PKR. The implications of these results for control mechanis ms occurring in influenza virus-infected cells are discussed. (C) 1995 Academic Press, Inc.