Structure and phosphorylation of eukaryotic initiation factor 2. Casein kinase 2 and protein kinase C phosphorylate distinct but adjacent sites in the beta-subunit

Eukaryotic initiation factor 2 (eIF-2) from rabbit reticulocytes can be phosphorylated on its beta-subunit by two different protein kinases, protein kinase C and casein kinase 2. Phosphorylation by these kinases is additive, suggesting that they phosphorylate different sites (serine residues) in eIF-2 beta. Two-dimensional peptide mapping of the phosphopeptides generated from labelled eIF-2 beta by digestion with trypsin, cyanogen bromide or Staphylococcus aureus V8 proteinase showed that protein kinase C and casein kinase 2 phosphorylated distinct and different sites in this protein. This conclusion was supported by the results of analysis of the phosphopeptides on reverse-phase chromatography. Analysis of the phosphopeptides derived from eIF-2 beta labelled by both kinases together strongly suggested that the sites labelled by protein kinase C and casein kinase 2 are adjacent in the primary sequence. These data are discussed in the light of the present understanding of the sequence specificity of the kinases. Rat liver eIF-2 beta was also found to be a substrate for protein kinase C and casein kinase 2, which were again shown to label different serine residues.