trans-Autophosphorylation by the isolated kinase domain is not sufficient for dimerization or activation of the dsRNA-activated protein kinase PKR.

The double-stranded (ds) RNA-activated protein kinase PKR phosphorylates the alpha-subunit of the eukaryotic initiation factor 2 (eIF2alpha) and inhibits translation initiation. PKR contains two dsRNA binding domains in its amino terminus and a kinase domain in its carboxy terminus. dsRNA binding activates PKR from a latent state by inducing dimerization and trans-autophosphorylation. Recent studies show that PKR is also activated by caspase cleavage to remove the inhibitory dsRNA binding domains. In this report, we show that the isolated kinase domain of PKR is a constitutively active monomeric kinase that has an activity similar to that of wild-type PKR. We used a solid-phase kinase assay system to show that PKR does not transfer its own phosphate to either PKR or eIF2alpha but rather uses the gamma-phosphate from ATP. In addition, the isolated autophosphorylated kinase domain of PKR phosphorylated intact monomeric PKR in trans in a reaction that did not require dsRNA binding. However, this trans-phosphorylation did not occur at the critical Thr446/451 sites and was not sufficient to induce dimerization and/or activation of PKR. The results show that dsRNA binding domains of PKR are not only required for dimerization of PKR but also required for phosphorylation of Thr446/451 sites of PKR. The results imply that even though the isolated kinase domain of PKR phosphorylates intact PKR and eIF2alpha, it is unable to activate PKR.