Tyrosine Phosphorylation of MPF and Membrane Proteins During Meiotic Maturation of Starfish Oocytes
It has been reported that the starfish homolog of the fission yeast cdc2 protein is a component of maturation promoting factor (MPF) that controls entry of eukaryotic cells into M-phase. The p34 cdc2 protein is phosphorylated during interphase and dephosphorylated during M-phase. We show that starfish p34 cdc2 is phosphorylated in vivo on tyrosine, threonine and serine in G2 (prophase I) and Gl (after completion of meiotic divisions). Rephosphorylation of p34 cdc2 is not prevented or modified by inhibition of protein synthesis during M-phase.
Tyrosine phosphorylation was found to occur in proteins from oocytes cortices, by immunoprecipitations with an antibody specific for phosphotyrosine. In 32P-preloaded oocytes, labeled phosphotyrosine containing proteins were noticeable only after hormonal induction of meiotic divisions. Labeling increased in five major phosphoproteins of 195, 155, 100, 45 and 35 kDa until first polar body emission, then decreased upon completion of meiosis. Endogenous in vitro phosphorylation of cortices showed a high tyrosine kinase activity towards a 68-kDa protein but no difference between cortices from oocytes treated or not with the hormone.
These findings suggest that tyrosine phosphorylation contributes both to the transduction of the hormonal signal for meiosis resumption and to several steps of the ensuing divisions.
KeywordsTyrosine Phosphorylation Fission Yeast Meiotic Division Cortical Granule Meiotic Maturation
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