REVERSAL OF THE DOUBLE-STRANDED-RNA-INDUCED INHIBITION OF PROTEIN-SYNTHESIS BY A CATALYTICALLY INACTIVE MUTANT OF THE PROTEIN-KINASE PKR

The interferon-inducible double-stranded-RNA(dsRNA)-dependent protein kinase PKR has been implicated in both the antiviral and cell growth-regulatory effects of the interferons. Over-expression of the wild-type form of this protein inhibits cell proliferation, whereas over-expression of inactive mutant forms transforms cells to a tumourigenic phenotype. It has been suggested that mutant PKR exerts a dominant negative effect on the activity of the wild-type protein kinase. We have investigated this possibility using the rabbit reticulocyte cell-free translation system in which protein synthesis is inhibited by dsRNA due to activation of PKR and phosphorylation of initiation factor eIF-2. Addition of a highly purified inactive PKR mutant, synthesised in a baculovirus-infected insect cell system, rescues protein synthesis from inhibition by low concentrations of dsRNA in a dose-dependent manner. The PKR mutant has no effect on protein synthesis in the absence of dsRNA or in the presence of another inhibitory protein kinase, the haem-controlled repressor. Inhibition of translation can be re-established in the presence of the mutant PKR by adding a higher concentration of dsRNA. These results suggest that inactive mutant PKR does exert a dominant negative effect on wild-type PKR and that this may be due to competition for dsRNA binding.