Maturation hormone induced an increase in the translational activity of starfish oocytes coincident with the phosphorylation of the mRNA cap binding protein, eIF-4E, and the activation of several kinases.
The stimulation of translation in starfish oocytes by the maturation hormone, 1-methyladenine (1-MA), requires the activation or mobilization of both initiation factors and mRNAs [Xu and Hille, Cell Regul. 1:1057, 1990]. We identify here the translational initiation complex, eIF-4F, and the guanine nucleotide exchange factor for eIF-2, eIF-2B, as the rate controlling components of protein synthesis in immature oocytes of the starfish, Pisaster orchraceus. Increased phosphorylation of eIF-4E, the cap binding subunit of the eIF-4F complex, is coincident with the initial increase in translational activity during maturation of these oocytes. Significantly, protein kinase C activity increased during oocyte maturation in parallel with the increase in eIF-4E phosphorylation and protein synthesis. An increase in the activities of cdc2 kinase and mitogen-activated myelin basic protein kinase (MBP kinase) similarly coincide with the increase in eIF-4E phosphorylation. However, neither cdc2 kinase nor MBP kinase phosphorylates eIF-4E in vitro. Casein kinase II activity does not change during oocyte maturation, and therefore, cannot be responsible for the activation of translation. Treatment of oocytes with phorbol 12-myristate 13-acetate, an activator of protein kinase C, for 30 min prior to the addition of 1-MA resulted in the inhibition of 1-MA-induced phosphorylation of eIF-4E, translational activation, and germinal vesicle breakdown. Therefore, protein kinase C may phosphorylate eIF-4E, after very early events of maturation. Another possibility is that eIF-4E is phosphorylated by an unknown kinase that is activated by the cascade of reactions stimulated by 1-MA. In conclusion, our results suggest a role for the phosphorylation of eIF-4E in the activation of translation during maturation, similar to translational regulation during the stimulation of growth in mammalian cells.
Xu
Z
Z
Department of Zoology, University of Washington, Seattle 98195.
Dholakia
J N
JN
Hille
M B
MB
D000225:Adenine / Q000031:analogs & derivatives* / Q000494:pharmacology; D000818:Animals; D004789:Enzyme Activation / Q000187:drug effects; D039561:Eukaryotic Initiation Factor-4E; D005260:Female; D020662:Guanine Nucleotide Exchange Factors; D009865:Oocytes / Q000187:drug effects* / Q000254:growth & development / Q000378:metabolism*; D010448:Peptide Initiation Factors / Q000378:metabolism*; D010766:Phosphorylation; D010770:Phosphotransferases / Q000378:metabolism; D014176:Protein Biosynthesis / Q000187:drug effects*; D011506:Proteins / Q000378:metabolism; D039381:RNA Cap-Binding Proteins; D012315:RNA Caps / Q000378:metabolism; D016601:RNA-Binding Proteins / Q000378:metabolism; D013215:Starfish / Q000235:genetics / Q000378:metabolism
D039561:Eukaryotic Initiation Factor-4E; D020662:Guanine Nucleotide Exchange Factors; D010448:Peptide Initiation Factors; D011506:Proteins; D039381:RNA Cap-Binding Proteins; D012315:RNA Caps; D016601:RNA-Binding Proteins; C008407:1-methyladenine; D010770:Phosphotransferases; D000225:Adenine