MEIOTIC CELL-CYCLE IN XENOPUS OOCYTES IS INDEPENDENT OF CDK2 KINASE

In vertebrates, M phase-promoting factor (MPF), a universal G(2)/M reg ulator in eukaryotic cells, drives meiotic maturation of oocytes, whil e cytostatic factor (CSF) arrests mature oocytes at metaphase II until fertilization, Cdk2 kinase, a G(1)/S regulator in higher eukaryotic c ells, is activated during meiotic maturation of Xenopus oocytes and, l ike Mos (an essential component of CSF), is proposed to be involved in metaphase II arrest in mature oocytes, In addition, cdk2 kinase has b een shown recently to be essential for MPF activation in Xenopus embry onic mitosis, Here we report injection of Xenopus oocytes with the cdk 2 kinase inhibitor p21(Cip) in order to (re)evaluate the role of cdk2 kinase in oocyte meiosis, Immature oocytes injected with (p)21(Cip) ca n enter both meiosis I and meiosis II normally, as evidenced by the ty pical fluctuations in MPF activity. Moreover, mature oocytes injected with p21(Cip) are retained normally in metaphase II for a prolonged pe riod, whereas those injected with neutralizing anti-Mos antibody are r eleased readily from metaphase II arrest, These results argue strongly against a role for cdk2 kinase in MPF activation and its proposed rol e in metaphase II arrest, in Xenopus oocyte meiosis, We discuss the po ssibility that cdk2 kinase stored in oocytes may function, as a matern al protein, solely for early embryonic cell cycles.