CHARACTERIZATION AND PHYSIOLOGICAL IMPORTANCE OF A NOVEL CELL-CYCLE-REGULATED PROTEIN-KINASE IN XENOPUS-LAEVIS OOCYTES THAT PHOSPHORYLATES CYCLIN B2

We have partially purified a specific cyclin B2 kinase (cyk) from prop hase oocytes of Xenopus laevis after an ATP-gamma-S activation step. P hosphopeptide analysis identified Ser53 as the major in vitro phosphor ylation site for cyk in cyclin B2. Using a synthetic peptide derived f rom cyclin B2 encompassing Ser53 (cyktide) as a substrate, cyk was sho wn to be activated during progesterone-induced maturation, with a peak of activity between 40 and 50% maturation, A sustained high cyk activ ity was observed in oscillating egg extracts. Microinjection of cyk-ph osphorylated cyclin B2 into pro-phase oocytes accelerated progesterone -induced maturation by about 2 h, indicating that cyclin B2 is a relev ant substrate for cyk and that the function of cyk is situated upstrea m of cdc2-cyclin B activation. Microinjection of cyk-phosphorylated cy ktide or a combination of cyk and cyclin B1 into G(2) fibroblasts indu ced significant changes in cell morphology, reminiscent of a premature prophase-like phenotype. Similarly, addition of cyk-phosphorylated cy ktide in cyclin B1-dependent interphase extracts resulted in histone H 1 kinase activation. (C) 1996 Academic Press.