Control of cell division in eucaryotes

In eucaryotes, M-phase promoting factor (MPF) triggers meiosis in germ cells and mitosis in somatic cells. MPF is composed of two proteins of which one is homologous with the protein kinase encoded by gene cdc2 of Schizosaccharomyces pombe (p34cdc2) and the other is a cyclin whose concentration oscillates during the cell cycle. Inactivation of p34cdc2 (MPF) requires cyclin degradation, which occurs during the metaphase-anaphase transition of the M-phase. Cyclin degradation is not only associated with cell cycle progression, but is also required for this event. At the G2/M transition, p34cdc2 protein kinase is activated and catalyzes phosphorylation of numerous key proteins, thus enabling cell changes to occur. p34cdc2 undergoes multiple-site phosphorylation in a cell cycle-dependent manner. At onset of mitosis, the protein phosphatase cdc25 catalyzes dephosphorylation of the p34cdc2 kinase at the threonine 14 and tyrosine 15 sites. This event may be the rate-limiting step controlling onset of mitosis in cells of vertebrates. A second protein kinase, encoded by the proto-oncogene c-mos, acts as a cytostatic factor preventing cyclin degradation and keeping unfertilized eggs from progressing beyond the second meiotic metaphase.