Mechanism of interferon action: characterization of the intermolecular autophosphorylation of PKR, the interferon-inducible, RNA-dependent protein kinase.

The interferon-inducible, RNA-dependent protein kinase (PKR) is activated by autophosphorylation, a process mediated by double-stranded RNA. A catalytically deficient, histidine-tagged mutant PKR protein [His-PKR(K296R)] was used as the substrate for characterization of the intermolecular phosphorylation catalyzed by purified wild-type PKR [PKR(Wt)]. The intermolecular autophosphorylation of His-PKR(K296R) by PKR(Wt) was RNA dependent. Excess His-PKR(K296R) substrate inhibited both the auto- and the trans-phosphorylation activities of PKR(Wt). Inhibition of PKR(Wt) by His-PKR(K296R) was relieved by higher concentrations of activator double-stranded RNA. Phosphopeptide analysis revealed that the sites of intermolecular autophosphorylation in His-PKR(K296R) were very similar, if not identical, to the sites that were autophosphorylated in PKR(Wt) and suggest a multiple of four major phosphorylation sites per PKR molecule.