Identification by two-dimensional gel electrophoresis of vaccinia virus and cellular phosphoproteins modified after inducible expression of the dsRNA-activated protein kinase.

The interferon (IFN)-induced double-stranded (ds) RNA-activated protein kinase (PKR) is a serine/threonine kinase that plays an important role in the biology of IFN, exerting antiviral and anticellular actions. These effects have been correlated with phosphorylation of the eukaryotic initiation factor eIF-2alpha and the NF-kappaB inhibitor IkappaB, although it has not been demonstrated that IkappaB is a direct target of PKR in vivo. In view of the various biological effects of PKR, it is likely that other cellular substrates are involved in PKR action. To identify novel substrates of PKR, we have carried out a systematic study of the phosphorylated proteins from cultured cells following PKR activation using high-resolution two-dimensional gel electrophoresis (2D-PAGE). We have used metabolic labeling with [32P]orthophosphate of HeLa cells infected with vaccinia virus (VV) recombinants expressing wild type (wt) or the catalytically inactive mutant form (K296R) of PKR under regulation of the Escherichia coli lacI operator/repressor system. Upon induction of PKR in the presence of isopropyl-beta-D-thiogalactoside (IPTG), the 68-kDA wt enzyme and eIF-2alpha are phosphorylated. These events lead to changes in the phosphorylation state of viral and cellular proteins. A distinct set of VV-induced phosphoproteins remained phophorylated, while the labeling of other viral proteins decreased markedly, probably as a result of a PKR-dependent translational block. Five proteins of unknown origin (68, 26, 20, 19, 15-16 kDA) appeared to be newly phosphorylated after PKR activation. Expression of the catalytically inactive K296R mutant form of PKR did not induce changes in the phosphorylation of VV proteins. Thus, by 2D-PAGE, we identified cellular and VV-induced phosphoproteins modified after PKR activation. Some or all of the phosphoproteins appearing or increasing in amount after PKR activation might not be direct targets of PKR, but rather indirect consequences of PKR activation.