REGULATION OF THE INTERFERON-INDUCIBLE PROTEIN-KINASE PKR AND (2'-5')OLIGO(ADENYLATE) SYNTHETASE BY A CATALYTICALLY INACTIVE PKR MUTANT THROUGH COMPETITION FOR DOUBLE-STRANDED-RNA BINDING

The interferon-inducible double-stranded RNA-dependent protein kinase PKR has been suggested to function as a tumour suppressor gene product . Catalytically inactive mutants of PKR give rise to a tumorigenic phe notype when overexpressed in NIH-3T3 fibroblasts and this has been att ributed to a dominant negative effect on the activity of the wild-type enzyme. Here we show that the mutant with Lys296 replaced by Arg, [K2 96R]PKR, not only inhibits the protein kinase activity of wild-type PK R but is also inhibitory towards another double-stranded RNA-dependent enzyme, the 40-kDa form of (2'-5')oligo(adenylate) synthetase. Inhibi tion of both wild-type PKR and (2'-5')oligo(adenylate) synthetase is r eversed by adding higher concentrations of double-stranded RNA. These results suggest competition between [K296R]PKR and wild-type PKR or (2 '-5')oligo(adenylate) synthetase for limiting amounts of double-strand ed RNA. Moreover, the data imply that the tumorigenic effect of this P KR mutant could be due to inhibition of additional pathways requiring low levels of double-stranded RNA for activation and cannot be unambig uously attributed to inhibition of endogenous PKR itself.