Reports by Masui (1967) and others in the late 1960s on the ability to obtain complete oocyte maturation in vitro by addition of progesterone were a great stimulus to work in the field. Additional stimulation came in 1971 from the work of Masui & Markert (1971) and Smith & Ecker (1971) that described a cytoplasmic activity capable of inducing complete maturation when microinjected into resting oocytes in the absence of progesterone. This activity, known as maturation-promoting factor (MPF), could be detected in all types of M phase cells. It was ultimately purified in this laboratory from Xenopus eggs (Lohka et al 1988) and shown to consist of a cyclin B/Cdc2 complex (Gautier et al 1988, Dunphy et al 1988). Combined with genetic work in yeast showing that cdc2 is a universal mitotic control gene (Lee & Nurse 1987), the field of cell cycle control entered a new, more energetic phase that continues today. The system in which MPF was characterized and purified, the frog oocyte system, has continued as a leading model for cell cycle control from at least three perspectives: mechanism of MPF activation; mechanism of MPF inactivation, and the linkage of consecutive M phases in meiosis (Fig. 1).
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