Cyclin-dependent kinase 1

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]

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.

A CDK1 phosphorylation state identifies modified amino acid residues. Different sites have opposite effects: in studied Xenopus laevis Cdc2, Thr14/Tyr15 are inhibitory, while Thr161 phosphorylation is activating. T denotes a threonine residue, Y a tyrosine residue, and numbers are protein primary structure positions. Do not assume this numbering for every species.^[ref]

The 1992 study supplies a useful separation of mechanisms: replacing Thr14, Tyr15 and Thr161 still permitted cyclin B binding, but replacing Thr161 prevented activation of histone H1 phosphorylation. Binding therefore did not suffice for catalytic activity in that preparation. Activating phosphorylation also occurred with catalytically inactive Cdc2, supporting a separate CDK-activating kinase rather than a requirement for intramolecular autophosphorylation.^[ref]

In this deliberately coarse classification, bound cyclin B, activating phosphorylation, and cleared inhibitory sites must coincide for the activity-permissive state. Removing inhibitory phosphate groups does not supply missing cyclin B or activating phosphorylation. Conversely, adding activating phosphorylation does not erase inhibitory modification. Actual preparations contain mixtures of states and additional regulators; this classification gives no numerical reaction rate.

A CDK-activating kinase is a protein kinase supplying activating phosphorylation to a cyclin-dependent kinase 1 preparation. The Xenopus laevis experiment above distinguished this activity from the catalytic activity of Cdc2 itself. CAK names that function here, without asserting that every species uses the same protein assembly.

Myt1 is a protein kinase distinct from Wee1. The cloned Xenopus laevis Myt1 phosphorylated Cdc2 at both inhibitory sites Thr14 and Tyr15; depletion experiments implicated it as the predominant Thr14-directed activity in Xenopus laevis egg extracts.^[ref]

A histone H1 kinase assay measures protein kinase action on supplied histone H1, a protein substrate. When the transferred phosphate carries a radioactive isotope label, label incorporated into histone H1 reports catalytic action over the assay interval. It does not count CDK1 molecules, and a crude preparation can contain other protein kinases acting on histone H1. Attribution therefore needs controls.

A 2010 study prepared human HeLa S3 extracts after a double thymidine block, using swelling, a Dounce homogenizer, centrifugation, dialysis and concentration. Added recombinant cyclin B1 incubated for $20$ minutes.^[ref]

Aliquots received histone H1 and terminal-${}^{32}\mathrm{P}$-labeled ATP for a $15$-minute room-temperature reaction, stopped with Laemmli buffer. Dried SDS-PAGE gels exposed a phosphorimager screen; a BioRad Molecular Imager FX quantified labeling. No-added-cyclin B activity set the normalization baseline; RO-3306 reduced H1 activity approximately $85\%$. Companion Western blots monitored Tyr15 phosphorylation, with actin Western blot loading control.^[ref]

Mixed extracts lose compartment boundaries; inhibitor sensitivity supports a major CDK1 contribution, with residual signal.^[ref] Substrate choice and inhibitor specificity limit attribution. The cell-cycle hysteresis experiment supplies a separate history comparison with this kind of histone H1 readout; its dose thresholds should not be transferred to human biological cells.

CDK1 activity and protein abundance are different observables. A total-CDK1 Western blot estimates protein amount under its calibration and loading assumptions. A phosphosite-specific Western blot asks about a particular CDK1 phosphorylation state. A histone H1 kinase assay asks how much substrate modification occurs under specified assay conditions. Any conversion between them needs a model and suitable controls.

An original counting example stipulates $100$ CDK1 molecules, with $60$ bound to cyclin B in each of two hypothetical samples. In sample A, $10$ bound molecules are activity-permissive and $50$ are inhibited; in sample B, $40$ are activity-permissive and $20$ are inhibited. The other $40$ are unbound in both samples. Total CDK1 and bound CDK1 agree, but activity-permissive counts differ fourfold. These are invented counts for bookkeeping, not measured biological populations.

If each activity-permissive molecule contributes the same reaction rate, substrate and ATP remain available, and the assay stays linear in time, a simple illustrative readout isHere $R$ is the recorded signal, $R_0$ is a matched blank signal, $N_{\mathrm{active}}$ is the activity-permissive count, $t$ the reaction interval, and $\alpha$ a common signal contribution per active molecule per unit time. The corrected signals in the stipulated samples have ratio $40/10=4$, although their total protein amounts agree. This is an assay accounting assumption, not an ordinary differential equation for the cell cycle. The no-added-cyclin B baseline in the inspected experiment can itself contain catalytic activity; it need not equal an enzyme-free blank $R_0$.

Substrate depletion, differing inhibitory conditions or a saturated detector can break the simple proportionality. Comparing samples therefore requires matched reaction intervals, substrate supply, loading and detector response, rather than assuming that brighter histone H1 labeling automatically means more total CDK1.