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Maturation promoting factor
Maturation-promoting factor (abbreviated MPF, also called mitosis-promoting factor or M-Phase-promoting factor) is the cyclin-Cdk complex that was discovered first in frog eggs. It stimulates the mitotic and meiotic phases of the cell cycle. MPF promotes the entrance into mitosis (the M phase) from the G2 phase by phosphorylating multiple proteins needed during mitosis. MPF is activated at the end of G2 by a phosphatase, which removes an inhibitory phosphate group added earlier.
The MPF is also called the M phase kinase because of its ability to phosphorylate target proteins at a specific point in the cell cycle and thus control their ability to function.
==Discovery==
In 1971, two independent teams of researchers (Yoshio Masui and Clement Markert, as well as Dennis Smith and Robert Ecker) found that frog oocytes arrested in G2 could be induced to enter M phase by microinjection of cytoplasm from oocytes that had been hormonally stimulated with progesterone.〔Masui Y, Markert CL (1971), Cytoplasmic control of nuclear behavior during meiotic maturation of frog oocytes. J Exp Zool 177:2, 129-145〕〔Smith LD, Ecker RE (1971), The interaction of steroids with Rana pipiens oocytes in the induction of maturation. Dev Biol 25:2, 232-247〕 Because the entry of oocytes into meiosis is frequently referred to as oocyte maturation, this cytoplasmic factor was called maturation promoting factor (MPF). Further studies showed, however, that the activity of MPF is not restricted to the entry of oocytes into meiosis. To the contrary, MPF is also present in somatic cells, where it induces entry into M phase of the mitotic cycle.
Evidence that a diffusible factor regulates the entry into mitosis had been previously obtained in 1966 using the slime mold ''Physarum polycephalum'' in which the nuclei of the multi-nucleate plasmodial form undergo synchronous mitoses. Fusing plasmodia whose cell cycles were out of phase with each other led to a synchronous mitosis in the next mitotic cycle. This result demonstrated that mitotic entry was controlled by a diffusible cytoplasmic factor and not by a "nuclear clock." 〔Rusch HP, Sachsenmaier W, Behrens K, Gruter V. (1966). Synchronization of mitosis by the fusion of the plasmodia of Physarum polycephalum. J Cell Biol.31:204-9.〕
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