Meiosis reinitiation in the mollusc Patella vulgata. Regulation of MPF, CSF and chromosome condensation activity by intracellular pH, protein synthesis and phosphorylation

The dependency of some key events of the cell cycle upon pH, phosphorylation and protein synthesis was investigated during meiosis reinitiation of Patella vulgata oocytes stimulated by ammonia. In this report, we show, through heterologous microinjection experiments, that a stable maturation promoting factor (MPF) is actually produced under these conditions. This factor, which may be amplified in recipient germinal vesicle (GV)-blocked oocytes of the starfish Asterias rubens, but not of Patella, remains present in the metaphase-1-blocked oocytes for at least 2á5h. Using effective concentrations of the protein synthesis inhibitor emetine, we further demonstrate that the appearance of this factor does not depend on newly made proteins, whereas the maintenance of metaphase-1 conditions and chromosome condensation activity (CCA), which has been related to the simultaneous presence of a cytostatic factor (CSF), requires a continuous supply of new short-lived proteins. Without this, the chromosomes decondense and the nuclear envelope reforms. Finally, we show that the entry into M-phase is accompanied by extensive protein phosphorylation, while the drug 6-dimethylaminopurine (6-DMAP), which induces protein dephosphorylation without affecting protein synthesis or phosphoprotein phosphatase activity, produces the same cytological effects as emetine. The fact that the effect of 6-DMAP, both on protein phosphorylation and chromosome structure, is perfectly reversible indicates that Patella CSF activity must also be controlled at the posttranslational level, possibly through the activation of relevant protein kinases.