DEPHOSPHORYLATION AND ACTIVATION OF A P34CDC2 CYCLIN-B COMPLEX INVITRO BY HUMAN CDC25 PROTEIN

OOCYTES arrested in the G2 phase of the cell cycle contain a p34cdc2/cyclin B complex which is kept in an inactive form by phosphorylation of its p34cdc2 subunit on tyrosine, threonine and perhaps serine residues (see refs 1 and 2 for review). The phosphatase(s) involved in p34cdc2 dephosphorylation is unknown, but the product of the fission yeast cdc25+ gene 3,4, and its homologues in budding yeast 5 and Drosophila 6 are probably positive regulators of the transition from G2 to M phase. We have purified the inactive p34cdc2/cyclin B complex from G2-arrested starfish oocytes. Addition of the purified bacterially expressed product of the human homologue of the fission yeast cdc25+ gene 7 (p54CDC25H) triggers p34cdc2 dephosphorylation and activates H1 histone kinase activity in this preparation. We propose that the cdc25+ gene product directly activates the p34cdc2-cyclin B complex.