Novel phosphorylation sites of eukaryotic initiation factor-4F and evidence that phosphorylation stabilizes interactions of the p25 and p220 subunits.
Only serine phosphorylation of eukaryotic initiation factor-4E (eIF-4E) has been previously reported in intact cells. We found that treatment of HepG2 cells with okadaic acid resulted in as much as 20% of eukaryotic initiation factor (eIF)-4E phosphorylation occurring on threonine residues and that tryptic phosphopeptide maps showed several previously unrecognized phosphopeptides. Analysis of p220 from control and okadaic acid-treated cells demonstrated serine and threonine phosphorylation under both conditions. However, a unique pattern of phosphopeptides in okadaic acid-treated cells was observed. The most notable finding was that hyperphosphorylation of eIF-4E and p220 increased binding of p220 but not eIF-4E to the m7GTP cap structure. We suggest that phosphorylation of eIF-4E is more complicated than previously recognized and that hyperphosphorylation of eIF-4E and p220 recruits more p220 into the protein complex that associates with mRNA caps. A better understanding of these protein-protein and protein-mRNA interactions may aid the design of anti-sense directed chemistries that disrupt such interactions for a specific target mRNA (Baker, B.F., Miraglia, L., and Hagedorn, C. H. (1992) J. Biol. Chem. 267, 11495-11499).
Bu
X
X
Department of Medicine, Vanderbilt University School of Medicine, Nashville, Tennessee.
Haas
D W
DW
Hagedorn
C H
CH
D002478:Cells, Cultured; D004988:Ethers, Cyclic / Q000494:pharmacology; D039562:Eukaryotic Initiation Factor-4F; D019319:Okadaic Acid; D010448:Peptide Initiation Factors / Q000737:chemistry / Q000378:metabolism*; D010766:Phosphorylation; D012333:RNA, Messenger / Q000378:metabolism
D004988:Ethers, Cyclic; D039562:Eukaryotic Initiation Factor-4F; D010448:Peptide Initiation Factors; D012333:RNA, Messenger; D019319:Okadaic Acid