Release from this arrest, which is equivalent to the G2/M phase transition in somatic cells, produces mature, fertilizable haploid eggs.
Immature oocytes generally arrest their cell cycle at prophase of the first meiosis. MPF, designated “maturation-promoting factor”, was first demonstrated over four decades ago by Masui and Markert ( 1971) during investigations on oocytes and eggs of the frog Rana pipiens. Originally, however, MPF was defined as a transferable activity that is not only present in the donor M phase cell but that also can induce the G2/M phase transition in the recipient G2 phase cell in the absence of new protein synthesis. Today, the term maturation or M phase-promoting factor (MPF) is assumed simply to describe a molecule or molecular complex that triggers M phase within the eukaryotic cell. The current view that MPF is a synonym for cyclin B-Cdk1 in donor cells is thus imprecise instead, MPF is best regarded as the entire pathway involved in the autoregulatory activation of cyclin B-Cdk1, with specifics depending on the experimental system. MPF, as originally defined, is thus not synonymous with cyclin B-Cdk1, but is instead a system consisting of both cyclin B-Cdk1 that directs mitotic entry and Gwl that suppresses the anti-cyclin B-Cdk1 phosphatase. To accomplish these tasks, Gwl helps cyclin B-Cdk1 by suppressing protein phosphatase 2A (PP2A)-B55 that counteracts cyclin B-Cdk1. Involvement of Gwl in MPF can be explained by its contribution to the autoregulatory activation of cyclin B-Cdk1 and by its stabilization of phosphorylations on cyclin B-Cdk1 substrates, both of which are essential when MPF induces the G2/M phase transition in recipient cells. The enigma is now resolved through the elucidation that MPF, defined as an activity that exhibits its effect in recipient cells, consists of at least two separate kinases, cyclin B-Cdk1 and Greatwall (Gwl). For over three decades, this inconsistency has remained a long-standing puzzle. In some conditions, however, MPF, as originally defined, is undetectable even though cyclin B-Cdk1 is fully active. Today, however, MPF is assumed to describe an activity that exhibits its effect in donor cells, and furthermore, MPF is consistently equated with the kinase cyclin B-Cdk1. MPF was originally defined as a transferable activity that can induce the G2/M phase transition in recipient cells. Maturation or M phase-promoting factor (MPF) is the universal inducer of M phase common to eukaryotic cells.
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