Cdc25 is a protein phosphatase: it removes inhibitory phosphate groups from CDK1, relieving inhibition of an appropriately prepared cyclin B–CDK1 assembly. Its reaction is removal, whereas Wee1 is a protein kinase adding inhibitory modification. Removing the inhibitory sites is different from removing the activating phosphorylation supplied by CAK.[ref]
Components of the Cell-Cycle Control System. Components of the Cell-Cycle Control System: inhibitory phosphorylation and Figure 17-18, Cdc25-mediated removal of inhibitory phosphates
Cdc25 action on CDK1 must also be distinguished from phosphorylation of Cdc25 itself. In cell-cycle activation feedback, CDK1-dependent regulation promotes Cdc25 activity, which can then activate additional inhibited CDK1. The complete regulatory path can involve intermediates. A histone H1 kinase assay of a mixture reports downstream protein kinase action; assigning its change to Cdc25 requires a perturbation or an independent protein phosphatase assay, rather than interpreting histone H1 labeling as phosphate removal.
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