MATERNAL XENOPUS CDK2-CYCLIN-E COMPLEXES FUNCTION DURING MEIOTIC AND EARLY EMBRYONIC-CELL CYCLES THAT LACK A G(1) PHASE

Earlier work demonstrated that cyclins A1, B1, and B2 are not associat ed with Cdk2 from unfertilized Xenopus eggs. As a potential Cdk2 partn er during meiosis, a cyclin E homolog was cloned from a Xenopus oocyte cDNA library and found to be 60% identical at the amino acid level to human cyclin E. Cyclin E1 protein was detected in resting oocytes, an d the level increased severalfold in meiosis II, concomitant with the appearance of forms with decreased electrophoretic mobility. During oo cyte maturation, the patterns of cyclin E1-associated kinase activity and Cdk2 activity were identical, with activity low until after germin al vesicle breakdown, peaking during meiosis II. Cyclin E1 complexes i mmunoprecipitated from unfertilized Xenopus eggs contained Cdk2 but no t Cdc2. In cycling egg extracts Cdk2-cyclin E1-associated kinase activ ity oscillated, but the level of cyclin E1 protein and its association with Cdk2 did not vary appreciably; complex activity appeared to be r egulated neither by the synthesis and destruction of the cyclin subuni t nor by association/disassociation of the two subunits. During the ea rly cleavage divisions in embryos, cyclin E1 and Cdk2 remained associa ted. The data indicate that the Cdk2-cyclin E complex functions during meiotic and embryonic cell cycles in addition to performing its estab lished role during G(1) in somatic cells.