Abstract
Maturation-promoting factor (MPF) is a cytoplasmic factor that is capable of inducing a maturation response involving nuclear envelope breakdown, chromosome condensation and spindle formation when microinjected into immature oocytes. MPF activity is found in a wide variety of eukaryotic cells at M-phase, such as mitotically dividing cells as well as maturing oocytes. MPF acts non-species-specifically among the animal kingdom. MPF has been partially purified and characterized as a heatlabile protein with a molecular size of approx. 5s. MPF activity oscillates with the same period as the cell cycle, with peaks during metaphase. MPF has the ability to amplify itself by activating its precursor, which is stored in fully grown oocytes. But replenishing MPF after its fall during cell cycle requires protein synthesis. MPF can cause nuclear envelope breakdown and chromosome condensation in vitro with isolated nuclei. Such cell-free system requires the cytoplasmic component extracted from eggs in addition to MPF. Further, MPF dissociates the junction between oocyte surface and surrounding follicle cells. Thus, MPF seems to act directly not on nucleus but on cytoplasm. Taken together, MPF might more generally be described as "metaphase-promoting factor" rather than a maturation-promoting factor.
