Phospholipid-sensitive Ca2+-dependent protein kinase phosphorylates the beta subunit of eukaryotic initiation factor...

The ability of homogeneous phospholipid-sensitive Ca2+-dependent protein kinase (PL-Ca-PK) from pig spleen to phosphorylate eukaryotic initiation factor 2 (eIF-2) was examined. PL-Ca-PK phosphorylated the beta-subunit of eIF-2, whereas myosin light chain kinase (MLCK) and cyclic AMP- and cyclic GMP-dependent protein kinases (cA-PK and cG-PK) did not. PL-Ca-PK could incorporate a maximum of 1.6 mol phosphate/mol eIF-2. The app. Km and Vmax for PL-Ca-PK phosphorylation of eIF-2 were 0.13 microM and 0.02 mumol.min-1.mg enzyme-1, respectively. Phosphoamino acid analysis revealed that incorporation of phosphate into eIF-2 occurred almost exclusively at serine residues. These findings indicate that eIF-2 was an effective substrate for PL-Ca-PK, suggesting that this enzyme may play a role in the regulation of protein synthesis.