THE CHARACTERIZATION OF HUMAN CDC2 KINASE AND CDK2

p34cdc2 kinase plays a key role in the initiation of mitosis. The acti vity of this kinase requires the binding of a protein, named cyclin, t o it. The kinase forms a heterodimer with cyclin. Cyclin A or cyclin B is the counterpart of this complex. The differences in the activity b etween cyclin A/cdc2 kinase and cyclin B/cdc2 kinase have not been cle ared. In recent years, the other cdc2-like kinases were identified. On e of them was CDK2 (cyclin dependent kinase 2). CDK2 could rescue the defect of the budding yeast CDC28 mutation, which arrested the cells a t a point named START, in G1 phase. Then, CDK2 was thought to be worke d at G1 through S phase in a cell cycle, but the details on the role o f this kinase has not been cleared so far. In this study, we separated the human cyclin A/cdc2 kinase, cyclin B/cdc2 kinase and CDK2, each o ther by use of column chromatography, and characterized the each kinas e. These kinases had the same substrate specificities when the synthes ized peptides were tested. They phosphorylated the threonine residue i n the sequence -Thr-Pro-Lys-Lys-Ala- but hardly phosphorylated threoni ne residue the sequence -Thr-Pro-Lys-Ala-Lys-. They had some differenc es in the substrate-proference when the native proteins were tested. I n a cell cycle of human cells, the activity of cdc2 kinase increased a t G2/M phase and the activity of CDK2 was high from S through M phase. These data suggested that cdc2 kinase works at the transition from G2 to M phase and that CDK2 works from G1 through G2/M phase. They could phosphorylate different protein-substrates having the common phosphor ylated sequence -Thr-Pro-X-Lys-.