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Cyclin B in Xenopus oocytes: implications for the mechanism of pre‐MPF activation.

Using a polyclonal antibody raised against B2 cyclin from Xenopus laevis, we show that prophase‐arrested Xenopus oocytes contain a stockpile of cyclin B2 protein. During progesterone‐induced maturation, an increase in the synthesis of cyclin B2 is observed, although Western blotting experiments show that this new synthesis does not significantly increase the mass of cyclin over the maternal stockpile. In the oocyte cyclin B2 is already present in two forms which differ in the extent of phosphorylation, but the phosphorylated form becomes predominant as oocytes progress towards germinal vesicle breakdown (GVBD), coincident with cdc2 protein kinase activation. These two events do not depend upon formation of a new complex between cyclin and cdc2 protein kinase, since these two proteins are already found associated in resting oocytes, prior to activation of the kinase.