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.
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