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.
Haas
D W
DW
Department of Medicine, Vanderbilt University School of Medicine, Nashville, TN 37232.
Hagedorn
C H
CH
D000595:Amino Acid Sequence; D000596:Amino Acids / Q000032:analysis; D002384:Catalysis; D039561:Eukaryotic Initiation Factor-4E; D006801:Humans; D008969:Molecular Sequence Data; D010448:Peptide Initiation Factors / Q000737:chemistry / Q000378:metabolism*; D010449:Peptide Mapping / Q000379:methods; D010748:Phosphopeptides / Q000737:chemistry*; D010766:Phosphorylation; D011485:Protein Binding; D011493:Protein Kinase C / Q000378:metabolism*; D012333:RNA, Messenger / Q000378:metabolism*; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D012694:Serine / Q000378:metabolism*; D013912:Threonine / Q000378:metabolism*
D000596:Amino Acids; D039561:Eukaryotic Initiation Factor-4E; D010448:Peptide Initiation Factors; D010748:Phosphopeptides; D012333:RNA, Messenger; D011994:Recombinant Proteins; D013912:Threonine; D012694:Serine; D011493:Protein Kinase C