ROLE OF THE AMINO-TERMINAL RESIDUES OF THE INTERFERON-INDUCED PROTEIN-KINASE IN ITS ACTIVATION BY DOUBLE-STRANDED-RNA AND HEPARIN

We have previously reported that the amino-terminal residues 1-34 of t he interferon-induced protein kinase (RNA-activated) (PKR) are necessa ry for its binding to and activation by double-stranded RNA (dsRNA) (P atel, R. C., and Sen, G. C. (1992) J. Biol. Chen. 267, 7671-7676). Her e, we report that the amino-terminal 24 residues are indispensable for these properties of the enzyme. The replacement of these residues wit h 14 unrelated residues fully restored the protein's dsRNA binding act ivity, but only partially restored the enzyme activity. Mutation of re sidues 18 and 19 revealed their importance in determining the affinity of PKR for dsRNA and its ability to phosphorylate eukaryotic initiati on factor 2 alpha. These mutations, however, did not affect PKR's auto phosphorylation activity. Deletion mutants that failed to bind to and be activated by dsRNA could be fully activated by the alternative acti vator, heparin. Thus, activation of PKR by dsRNA and heparin is mediat ed through different mechanisms that require different domains of the protein.