Key takeaways

• Cyclins are non-enzymatic activating subunits; a CDK has essentially no activity until a cyclin binds, so oscillating cyclin levels set the timing of the cycle.
• The pairings are specific — cyclin D with CDK4 and CDK6, cyclin E with CDK2, cyclin A with CDK2 then CDK1, and cyclin B with CDK1.
• CDK1-cyclin B, originally identified as maturation promoting factor in Xenopus oocytes, drives mitotic entry.
• Mitotic entry is switch-like because of positive feedback: CDK1 activates its activator Cdc25 and inhibits its inhibitors Wee1 and Myt1. Cdc25 activates CDK1 — it is not a CDK-opposing phosphatase.
• CDK1 phosphorylates a S/T-P-X-K/R motif, and because the Plk1 polo-box domain reads phosphorylated S/T-P, CDK1 priming creates Plk1 docking sites — the basis of a sequential kinase cascade.
• Anaphase begins when APC/C-Cdc20 destroys cyclin B, removing CDK1 activity and making mitotic exit irreversible.
• CDK activity is also restrained by inhibitor proteins — the INK4 and CIP/KIP families — which are the basis of CDK4/6 inhibitor drugs now used in breast cancer.

Contents

• Cyclins and CDKs
• Which cyclin pairs with which CDK
• CDK inhibitor proteins
• The CDK1-cyclin B complex
• Positive feedback and the mitotic switch
• CDK1 substrates and the phosphorylation motif
• Priming: the CDK1-Plk1 cascade
• Mitotic timing and the kinase hierarchy
• Anaphase and cyclin destruction
• Phosphatases and mitotic exit
• Therapeutic targeting
• Choosing antibodies
• Frequently asked questions

Antibodies for cell cycle work

The panel follows the article’s own hierarchy — the cyclins that set timing, CDK1 as the mitotic engine, then the Plk1 and Aurora B kinases it primes, plus a definitive mitotic marker.

Cyclin B1

Cyclin B1 Rabbit Polyclonal Antibody

Rabbit pAbWB / IF

Accumulates through G2, peaks at mitosis and is destroyed at anaphase — a direct readout of cell cycle position.

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Cyclin D1

Cyclin D1 Rabbit Polyclonal Antibody

Rabbit pAbWB / IF

The G1 cyclin partnering CDK4 and CDK6; frequently amplified in cancer and the target of CDK4/6 inhibitor therapy.

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CDK1

CDK1 Rabbit Polyclonal Antibody

Rabbit pAbWB / IF

The archetypal CDK and the essential driver of mitotic entry when bound to cyclin B.

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PLK1

PLK1 Rabbit Polyclonal Antibody

Rabbit pAbWB / IF

Acts downstream of CDK1 priming via its polo-box domain; central to spindle assembly and mitotic progression.

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Aurora B

Human Aurora Kinase B ELISA Kit

Sandwich ELISAHuman

The chromosomal passenger kinase governing kinetochore attachment correction and error-free segregation.

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p-H3 Ser10

Anti-Phospho-Histone H3 (Ser10) Monoclonal Antibody

MonoclonalWB / IF / Flow

The definitive mitotic marker — distinguishes true M phase from G2, which DNA content cannot.

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Cyclins and CDKs

Cyclin-dependent kinases are serine/threonine kinases that are, on their own, essentially inactive. They require a cyclin — a non-enzymatic regulatory subunit — to adopt an active conformation and to acquire substrate specificity. Because cyclin concentrations rise and fall on a schedule through the cycle, this arrangement converts a set of constitutively present kinases into a timing mechanism.

Cyclins were discovered by Timothy Hunt in 1982 in sea urchin eggs, identified as proteins whose abundance oscillated with each division — the observation that gave them their name and later a share of the 2001 Nobel Prize.

Within the kinome, CDKs sit in the CMGC group alongside GSK3, the DYRK family and the CDK-like kinases. They differ from related kinases such as the MAPKs in how specificity is achieved: MAPKs use docking sites separate from the catalytic cleft, whereas CDKs depend on a bound regulatory subunit to supply the sequences required for activity. Systematic renaming of the family as CDK1 through CDK20 has made the literature considerably easier to navigate.

Which cyclin pairs with which CDK

The original article notes that each cyclin partners a corresponding CDK without saying which. The pairings are specific and worth setting out, because they are what assign each complex to a phase.

The cyclin D-CDK4/6 step deserves particular attention because it is where mitogenic signals enter the cycle. Growth factor signalling raises cyclin D, the resulting kinase activity phosphorylates the retinoblastoma protein, and phosphorylated Rb releases E2F transcription factors that drive expression of the S-phase machinery — including cyclin E. That is the molecular substance of the restriction point, and it is the pathway CDK4/6 inhibitors target.

CDK inhibitor proteins

Cyclin binding is not the only control. Two families of endogenous inhibitor proteins restrain CDK activity directly, and their omission from most summaries leaves the regulation looking simpler than it is.

The CIP/KIP proteins are not straightforwardly inhibitory: at low concentrations they assist assembly of cyclin D-CDK4/6 complexes while inhibiting CDK2, which is one reason CDK inhibition produces context-dependent effects.

The CDK1-cyclin B complex

CDK1 is the archetypal member of the family and the only CDK strictly essential for the mammalian cell cycle — it can substitute for the others, whereas they cannot substitute for it.

Bound to cyclin B, CDK1 forms the complex originally characterised as maturation promoting factor in Xenopus oocytes, an identification that established the molecular basis of mitotic entry. The complex phosphorylates the substrates that produce the physical events of early mitosis: nuclear lamins, whose phosphorylation disassembles the nuclear envelope; condensin, driving chromosome condensation; and numerous spindle and centrosomal proteins.

Cyclin B accumulates through G2 while CDK1 is held inactive by inhibitory phosphorylation, so the cell assembles a large pool of complex that is ready but restrained. Mitotic entry is then triggered by removing that restraint rather than by synthesising more complex — which is what makes the transition abrupt.

Positive feedback and the mitotic switch

CDK1-cyclin B activity is governed by two opposing sets of enzymes, and CDK1 acts on both in the direction that favours its own activation.

• Wee1 and Myt1 phosphorylate CDK1 at inhibitory sites, holding the complex inactive during G2. CDK1 phosphorylates and inactivates them.
• Cdc25 phosphatases remove those inhibitory phosphates and thereby activate CDK1. CDK1 phosphorylates and activates Cdc25.

Both loops are positive, so once a threshold of CDK1 activity is crossed the system commits rapidly and completely — a bistable switch rather than a gradual rise. This is why mitotic entry appears all-or-nothing at the level of a single cell.

One point of terminology is worth being explicit about, because it is easy to state backwards: Cdc25 is a phosphatase that activates CDK1. It is not an antagonist of CDK activity. The phosphatases that oppose CDK-driven phosphorylation are PP1 and PP2A, which reverse CDK1 substrate phosphorylation and are covered below.

CDK1 substrates and the phosphorylation motif

CDK1 phosphorylates serine or threonine within the consensus motif S/T-P-X-K/R, where the proline immediately following the target residue is the critical determinant. In yeast and some other organisms a minimal S/T-P motif is often sufficient, indicating some divergence in stringency between species.

Knowing the motif made systematic substrate discovery possible. A phosphoproteomic study of the mitotic spindle identified 736 distinct phosphorylation sites falling within the CDK1 consensus, indicating how broadly the kinase acts during mitosis. Parallel work in Schizosaccharomyces pombe — which is fission yeast, not budding yeast — identified roughly 200 CDK1 substrate proteins, many of them homologues of spindle assembly checkpoint components and other mitotic regulators.

The scale of this substrate set is the point: CDK1 does not regulate mitosis by controlling a few key proteins but by simultaneously altering the phosphorylation state of hundreds, remodelling the cell wholesale within minutes.

Priming: the CDK1-Plk1 cascade

One of the more elegant findings in mitotic signalling concerns how CDK1 and Plk1 are coupled. The Plk1 polo-box domain binds a phosphorylated serine or threonine followed immediately by proline — which is precisely the product of CDK1 phosphorylation. CDK1 therefore creates its own downstream docking sites: it primes a substrate, and Plk1 is recruited to that substrate as a direct consequence.

This priming mechanism enforces order. Plk1 cannot act on a target until CDK1 has marked it, so the two kinases operate in sequence rather than in parallel, and the sequence is encoded in the substrate itself rather than requiring separate timing machinery.

Worked examples

• Nedd1. A component of the gamma-tubulin ring complex, sequentially phosphorylated by CDK1 then Plk1. This recruits gamma-TuRC to the centrosome and enables microtubule nucleation at the spindle pole.
• BubR1. Sequential CDK1 and Plk1 phosphorylation at the kinetochore regulates spindle assembly checkpoint signalling, contributing to the wait-anaphase signal that prevents premature segregation.
• INCENP. CDK1 priming at threonine 59 and threonine 388 creates Plk1 docking sites, bringing Plk1 into proximity with Aurora B within the chromosomal passenger complex.

Mitotic timing and the kinase hierarchy

Three kinases dominate mitosis, and they are activated in a defined order: CDK1, then Plk1, then Aurora B. The INCENP example above shows the mechanism connecting the second and third — CDK1 priming recruits Plk1, and Plk1 in turn regulates Aurora B activity within the chromosomal passenger complex.

Aurora B governs kinetochore-microtubule attachment: it destabilises incorrect attachments so they can be re-formed correctly, generating the tension that satisfies the spindle assembly checkpoint. Errors here produce chromosome missegregation and aneuploidy.

Timing is enforced at several levels simultaneously — transcriptional control of kinase expression, cyclin availability, activating and inhibitory phosphorylation, subcellular localisation, and opposing phosphatase activity. No single mechanism accounts for the precision; the redundancy is the point.

Anaphase and cyclin destruction

Mitosis ends by destroying the protein that started it. Once the spindle assembly checkpoint is satisfied, the anaphase-promoting complex/cyclosome with its co-activator Cdc20 ubiquitinates cyclin B, targeting it for proteasomal degradation.

Losing cyclin B abolishes CDK1 activity, and the consequences follow immediately: sister chromatids separate, the spindle disassembles, the nuclear envelope reforms and cytokinesis proceeds. APC/C-Cdc20 also destroys securin, releasing separase to cleave the cohesin holding sister chromatids together — which is the direct trigger for their separation.

Proteolysis is used here rather than dephosphorylation because it is irreversible. A cell cannot re-enter mitosis by reversing this step; it must synthesise cyclin B afresh. That irreversibility is what makes the cycle unidirectional, and it is why proteasome inhibitors such as MG-132 arrest cells in mitosis — they prevent cyclin B destruction and so prevent exit.

Phosphatases and mitotic exit

Removing CDK1 activity stops new phosphorylation but does not reverse what has already been done. Hundreds of CDK1 substrates must be dephosphorylated for the cell to exit mitosis, and that requires phosphatases — principally PP1 and PP2A-B55.

These are not constitutively active. PP2A-B55 is held inhibited during early mitosis by the Greatwall kinase pathway, and is released as CDK1 activity falls, so dephosphorylation is timed rather than continuous. Mitotic exit is therefore an actively scheduled process in its own right, not merely the absence of kinase activity.

This has therapeutic relevance. Because many phosphatases are most active during mitotic exit, targeting them has been proposed as an anti-mitotic strategy that would not be undermined by mitotic slippage — the route by which cells escape conventional anti-mitotic drugs by degrading cyclin B slowly and exiting mitosis without dividing.

Therapeutic targeting

Because uncontrolled proliferation defines cancer, the cyclin-CDK machinery has been an obvious drug target. The record is instructive about which parts of it are tractable.

The pattern is consistent: targets that a tumour is selectively dependent on succeed, while targets essential to every dividing cell do not. CDK4/6 inhibitors work because Rb-proficient, cyclin D-driven tumours need that specific complex; CDK1 inhibitors fail because every proliferating cell needs CDK1.

Frequently asked questions