Specific phosphorylation of eukaryotic initiation factor 2 alpha, eIF-2 alpha, by bovine adrenocortical cyclic nucleotide-independent protein kinase, PK 380

Recently, we characterized a novel cyclic nucleotide-independent protein kinase, PK 380, from bovine adrenal cortex (Y. Kuroda and R. K. Sharma (1980) Biochem. Biophys. Res. Commun.96, 601–610). PK 380 is independent of cyclic AMP, cyclic GMP, calcium, and calcium-calmodulin for its activity. It does not phosphorylate any of the commonly used exogenous substrates but phosphorylates an endogenous 120,000-dalton peptide. In the present study we demonstrate that PK 380 specifically phosphorylates the serine residue of eukaryotic initiation factor 2α, eIF-2α. PK 380 can be differentiated from two other protein kinases, hemin-controled repressor (HCR) or double-stranded RNA-activated inhibitor (dsRI), that are also known to phosphorylate eIF-2α. Unlike the activity of HCR, PK 380 activity is independent of hemin (5–10 μm) and dependent on sulfhydryl groups. Poly(I) · poly(C), which is known to activate dsRI, does not affect the activity of PK 380. The possibility exists that the physiological substrate of PK 380 is eIF-2α. Thus, this novel protein kinase could play an important role in the translational control processes of adrenocortical cell.