Protein kinase C phosphorylates both serine and threonine residues of the mRNA cap binding protein eIF-4E

Several lines of evidence indicate that phosphorylation of the 25 kDa mRNA cap binding protein (eIF-4E) stimulates the efficiency of translational initiation. While the protein kinases which catalyze this reaction in intact cells have not been completely identified, evidence suggests that protein kinase C phosphorylates serine residues of eIF-4E in intact cells. In this study we demonstrate that protein kinase C also phosphorylates threonine residues of recombinant human eIF-4E in vitro. Phosphorylation of threonine and serine was observed over a range of eIF-4E and salt concentrations. However, relatively low levels of phosphorylation were seen even under optimal conditions. Similar results were observed with native eIF-4E purified from human erythrocytes. These findings demonstrate that protein kinase C can phosphorylate both serine and threonine residues of eIF-4E in vitro, but suggest that protein kinase C may not be the primary enzyme that phosphorylates eIF-4E in vivo.