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Joined 2026-09-21 Message
Cdc25 Created 2026-10-14 Updated 2026-10-14
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.
Cyclin B Created 2026-10-14 Updated 2026-10-14
Cyclin B is a regulatory protein partner of CDK1. The protein kinase catalytic component is CDK1; bound cyclin B helps make that component competent to act. Changes in cyclin B synthesis and destruction regulate how much assembly can form, while CDK1 phosphorylation states determine whether bound assemblies are inhibited. This separates the supply of regulatory partners from release of an already assembled brake.[ref]
Components of the Cell-Cycle Control System. Components of the Cell-Cycle Control System: cyclically activated protein kinases, Figures 17-15–17-17, and “Cdk Activity Can Be Suppressed Both by Inhibitory Phosphorylation and by Inhibitory Proteins”
More cyclin B therefore need not give a proportional increase in CDK1 activity. Compare the matched-input cell-cycle hysteresis experiment, where different histories produced different activities, and the distinct measurements under CDK1 activity and protein abundance. Neither comparison makes cyclin B itself the catalytic protein kinase.
Cyclin-dependent kinase 1 Created 2026-10-14 Updated 2026-10-14
Cyclin-dependent kinase 1, abbreviated CDK1 and historically called Cdc2, is the catalytic protein kinase in the CDK1–cyclin B assembly. CDK1 transfers a phosphate group from ATP to a protein substrate; cyclin B supplies a regulatory binding partner. Binding and phosphorylation regulate catalytic competence, so detecting CDK1 protein alone does not establish activity. The distinction makes a durable catalytic component reusable as the cell cycle changes its regulatory inputs.[ref]
Components of the Cell-Cycle Control System. Molecular Biology of the Cell, 4th edition, Components of the Cell-Cycle Control System: “The Cell-Cycle Control System Is Based on Cyclically Activated Protein Kinases”, Figures 17-15–17-16
The cell-cycle activation feedback uses CDK1 activity as both an output and a regulatory input: activity promotes Cdc25 activation and suppresses Wee1, reinforcing release from inhibitory phosphorylation. The already described cell-cycle hysteresis experiment tests how that output depends on history at matched cyclin B input; the histone H1 kinase assay below explains what its output measures. A binding diagram alone cannot establish hysteresis.