Ras transformation of cloned rat embryo fibroblasts results in increased rates of protein synthesis and phosphorylation of eukaryotic initiation factor 4E.
Eukaryotic initiation factor 4E (eIF-4E) is a 25-kDa phosphoprotein that binds to the 7-methylguanosine cap of mRNA and acts, along with other eIF-4 polypeptides, to unwind mRNA secondary structure at the 5' terminus. Recent studies have indicated that eIF-4E acts as a protooncogene, but only in its phosphorylated state. In order to determine the role of eIF-4E in oncogenesis, we examined its regulation and expression in cloned rat embryo fibroblasts transformed with the Harvey ras (Ha-ras) oncogene. The expression of Ha-ras increased the rate of protein synthesis but did not increase the levels of eIF-4E mRNA or protein. However, a dramatic increase (7-fold) in phosphate incorporation into eIF-4E was observed. The percentage of eIF-4E in the phosphorylated state was the same in transfected and control cells, indicating that both phosphorylation and dephosphorylation of eIF-4E were increased. Phosphopeptide mapping of eIF-4E from transformed cells indicated a single site of phosphorylation at Ser-53, which is the same as that identified previously in eIF-4E from reticulocytes and HeLa cells. These results indicate that p21ras is part of the signal transduction pathway leading to phosphorylation of eIF-4E. These findings also provide a potential mechanism for cell transformation by p21ras which involves the preferential stimulation of translation of certain mRNAs.
Rinker-Schaeffer
C W
CW
Department of Biochemistry, College of Medicine, University of Kentucky, Lexington 40536.
Austin
V
V
Zimmer
S
S
Rhoads
R E
RE
D000818:Animals; D015153:Blotting, Western; D002461:Cell Line, Transformed; D002999:Clone Cells; D004591:Electrophoresis, Polyacrylamide Gel; D039561:Eukaryotic Initiation Factor-4E; D015870:Gene Expression; D011905:Genes, ras; D015689:Oncogene Protein p21(ras) / Q000235:genetics / Q000378:metabolism*; D010448:Peptide Initiation Factors / Q000378:metabolism*; D010449:Peptide Mapping; D010766:Phosphorylation; D016133:Polymerase Chain Reaction; D014176:Protein Biosynthesis; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D015398:Signal Transduction
D039561:Eukaryotic Initiation Factor-4E; D010448:Peptide Initiation Factors; D012333:RNA, Messenger; D015689:Oncogene Protein p21(ras)