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It has been known for some time that maturation-promoting factor (MPF) appears in a wide variety of eukaryotic cells at M phase and exerts equal M-phase-promoting activity in both meiotic cells and mitotic cells in a non-specific manner. MPF was extracted from cdc20 mutant cells of the yeast Saccharomyces cerevisiae synchronized at M phase by incubation at the restrictive temperature. When injected into immature oocytes of Xenopus laevis, yeast MPF caused meiosis reinitiation in a dose-dependent manner and even in the presence of cycloheximide. Yeast MPF exerted its activity in starfish oocytes as well. MPF activity was obtained only from cells in M phase and not from G1, S or G2 phase cells, indicating cyclical changes during the yeast mitotic cell cycle. Preliminary characterization of yeast MPF revealed that its activity was associated with a heat-labile protein having a sedimentation coefficient value of 6 S. In contrast to the current assumption that MPF is a Ca-sensitive phosphoprotein stabilized by phosphorylated small molecules, such as ATP and Na-beta-glycerophosphate, the present study revealed that yeast MPF was still active even after treatment with either Ca2+ or alkaline phosphatase. Furthermore, it was found that yeast MPF and these phosphorylated small molecules were complementary in inducing reinitiation of meiosis, since the meiosis-reinitiating activity was detected only when both were present simultaneously and almost undetectable when either of them was present alone.(ABSTRACT TRUNCATED AT 250 WORDS)
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