Non-dependence of cyclin E/Cdk2 kinase activity on the initiation of oocyte maturation in goldfish

Cdk2 kinase activity increases during oocyte maturation but neither cyclin A nor B is associated with Cdk2 in mature oocytes in goldfish. As a potenti al Cdk2 partner in meiosis, a cyclin E homolog was isolated from a goldfish oocyte cDNA library. A monoclonal antibody was raised against bacterially produced full-length goldfish cyclin E. Both cyclin E and Cdk2 were already present in immature oocytes and their protein levels did not change remark ably during oocyte maturation. Cyclin E formed a complex mainly with Cdk2 j ust at the time of germinal vesicle breakdown (GVBD) in association with th e increase in Cdk2 kinase activity, although a fraction of cyclin E bound t o Cdk(s) other than Cdk2 and Cdc2. Ectopic activation of cyclin E/Cdk2 by t he injection of cyclin E messenger RNA (mRNA) into immature oocytes did not induce maturation-promoting factor (MPF) activation and GVBD. Furthermore, inhibition of cyclin E/Cdk2 kinase activity by the injection of p21(SDI1) into the oocytes treated with 17 alpha,20 beta-dihydroxy-4-pregnen-3-one ha d no effect on MPF activation and GVBD. These results indicate that cyclin E/Cdk2 kinase activity is insufficient and unnecessary for initiating goldf ish oocyte maturation.