Erythropoietin stimulates phosphorylation of eIF-4E and identification of a 37-kD phosphoprotein that binds mRNA caps in erythroblasts.
To explore the mechanism of erythropoietin action on differentiation of erythroblasts, we have examined its effect on regulating phosphorylation of the 25-kD mRNA cap binding protein (eIF-4E). Erythroblasts from the spleens of mice infected with the anemia strain of Friend virus (FVA cells) were studied. Erythropoietin stimulated phosphorylation of eIF-4E in FVA cells within 30 minutes, and this effect was maximal at 60 minutes. Phosphoamino acid analysis and tryptic phosphopeptide map analysis of eIF-4E isolated from both control and erythropoietin-treated cells identified a predominant phosphopeptide containing phosphoserine. However, when cells were incubated with 1 muM okadaic acid, eIF-4E was phosphorylated on both serine and threonine residues and three additional tryptic phosphopeptides were detected. We also identified a 37-kD phosphoprotein (pp37) that bound specifically to the m7GTP cap structure and coimmunoprecipitated with eIF-kD protein was phosphorylated on both serine and threonine residues. These results indicate that phosphorylation of eIF-4E is a target in erythropoietin-initiated signal transduction events and that this phosphorylation precedes observable effects of erythropoietin on macromolecular biosynthesis. Although of pp37 remains to be studied, it may represent a developmentally regulated mRNA cap binding protein.
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Department of Medicine and Cell Biology, Vanderbilt University School of Medicine , Nashville, TN, USA.
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Hagedorn
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D000740:Anemia / Q000378:metabolism / Q000821:virology; D000818:Animals; D004900:Erythroblasts / Q000378:metabolism*; D004921:Erythropoietin / Q000494:pharmacology*; D004988:Ethers, Cyclic / Q000494:pharmacology; D039561:Eukaryotic Initiation Factor-4E; D005260:Female; D005622:Friend murine leukemia virus; D006153:Guanosine Diphosphate / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000378:metabolism; D051379:Mice; D008970:Molecular Weight; D019319:Okadaic Acid; D010448:Peptide Initiation Factors / Q000378:metabolism*; D010750:Phosphoproteins / Q000378:metabolism*; D010766:Phosphorylation; D012315:RNA Caps / Q000378:metabolism*; D012333:RNA, Messenger / Q000378:metabolism*; D013154:Spleen / Q000166:cytology; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology
D004988:Ethers, Cyclic; D039561:Eukaryotic Initiation Factor-4E; D010448:Peptide Initiation Factors; D010750:Phosphoproteins; D012315:RNA Caps; D012333:RNA, Messenger; D004921:Erythropoietin; D006153:Guanosine Diphosphate; D019319:Okadaic Acid; D006160:Guanosine Triphosphate; D013755:Tetradecanoylphorbol Acetate