cdc2 protein kinase: structure-function relationships.

Activation of the cdc2 kinase in the cell cycle occurs upon binding to a regulatory subunit called cyclin. Cyclin A associates with both Cdc2 and its homologue Cdk2. The two complexes appear in S phase but cyclin A/Cdk2 is activated earlier than cyclin A/Cdc2. Several regions in Cdc2 are involved in binding cyclins A and B. Phosphorylation of cyclin/Cdk complexes ensures that the kinase activity peaks at a specific time in the cell cycle. Phosphorylation of Thr161 in Cdc2 is required for strong cyclin binding and kinase activity in vitro; its dephosphorylation is necessary for cells to exit mitosis. We have identified a novel 'Activating factor' that stimulates binding between cyclin and Cdc2 by inducing phosphorylation of Cdc2 on Thr161. We propose that Thr161 is targeted by an additional cell cycle regulatory pathway.