G2–M Control (Cyclin B–Cdk1, Wee1/Cdc25)

Core Concept

Entry into mitosis is controlled by the cyclin B–Cdk1 complex (historically, "MPF" — maturation-promoting factor). Cyclin B accumulates through G2 and binds Cdk1. The complex is held inactive by Wee1/Myt1 kinases, which phosphorylate Cdk1 on inhibitory sites (Thr14/Tyr15). At the G2/M boundary, the phosphatase Cdc25 removes these phosphates, rapidly activating Cdk1. Active Cdk1 then phosphorylates and activates more Cdc25 while inhibiting Wee1, creating a positive-feedback loop that flips the system abruptly into mitosis. DNA damage and unreplicated DNA activate checkpoints (ATM/ATR → Chk1/Chk2) that phosphorylate Cdc25 (for degradation/sequestration) and block mitotic entry.

Key Components

• Cyclin B–Cdk1 (MPF): The master mitotic kinase.
• Wee1 and Myt1: Kinases that phosphorylate Cdk1 at Thr14/Tyr15 (inhibitory).
• Cdc25: Phosphatase that removes Thr14/Tyr15 phosphates (activating).
• CAK: Adds the activating T-loop phosphate (Thr161 in Cdk1).
• Plk1 and Aurora kinases: Cooperate to amplify Cdk1 activation.
• Chk1/Chk2: Effector kinases of the DNA-damage/replication checkpoints.
• 14-3-3 proteins: Sequester phosphorylated Cdc25 in the cytoplasm, inactivating it.

Mechanism / How It Works

1. Priming: Through G2, cyclin B is synthesized and binds Cdk1. CAK phosphorylates the T-loop (activating), but Wee1/Myt1 keep the complex off by phosphorylating Thr14/Tyr15 near the ATP-binding site.
2. Trigger: At the G2/M boundary, Cdc25 is activated (dephosphorylated/recruited to the nucleus) and removes the inhibitory phosphates, flipping Cdk1 on.
3. Positive feedback: Active cyclin B–Cdk1 phosphorylates Cdc25 (activating it further) and phosphorylates Wee1/Myt1 (inactivating them). More Cdk1 activity → more Cdc25 and less Wee1 → a self-reinforcing, switch-like, irreversible commitment to mitosis.
4. Mitotic events: Cyclin B–Cdk1 phosphorylates lamins (nuclear envelope breakdown), condensin and histone H3 (chromosome condensation), proteins driving centrosome separation and spindle assembly, and the Golgi disassembly machinery.
5. Checkpoint brake: If DNA is damaged or replication incomplete, ATM/ATR activate Chk1/Chk2, which phosphorylate Cdc25. Phosphorylated Cdc25 binds 14-3-3 and is sequestered (or degraded), so it cannot activate Cdk1 — the cell arrests in G2 until repair is complete.

Energy and Directionality

The G2/M transition consumes ATP at every phosphorylation/dephosphorylation step (Wee1, CAK, Cdk1 substrates) and, at mitotic exit, ATP for ubiquitin-dependent destruction of cyclin B by the APC/C. The switch-like character comes from the positive-feedback loop: Cdk1 and Cdc25 activate each other, so once the loop tips past a threshold the transition is rapid, all-or-none, and effectively irreversible — a bistable system. Reversal requires destroying cyclin B, not merely dephosphorylating it.

Experimental Evidence / Technique

• MPF discovery: Masui and Markert (1971) found that cytoplasm from a maturing frog oocyte injected into an immature oocyte induced maturation — a transferable "maturation-promoting factor."
• Xenopus extracts and cyclin B: Non-degradable cyclin B holds extracts in mitosis; adding cyclin B alone drives interphase nuclei into mitosis, showing Cdk1 is sufficient.
• wee1 and cdc25 yeast mutants: wee1 mutants enter mitosis prematurely (small cells — "wee" in Scottish), while cdc25 mutants fail to enter mitosis, defining the antagonistic kinase/phosphatase pair.
• Phospho-specific antibodies (anti-pCdk1-Tyr15) monitor the abrupt dephosphorylation at mitotic entry.
• Caffeine overrides the G2/M checkpoint by inhibiting ATM/ATR, showing the checkpoint is kinase-dependent.

How it works

1. Priming: Through G2, cyclin B is synthesized and binds Cdk1. CAK phosphorylates the T-loop (activating), but Wee1/Myt1 keep the complex off by phosphorylating Thr14/Tyr15 near the ATP-binding site.
2. Trigger: At the G2/M boundary, Cdc25 is activated (dephosphorylated/recruited to the nucleus) and removes the inhibitory phosphates, flipping Cdk1 on.
3. Positive feedback: Active cyclin B–Cdk1 phosphorylates Cdc25 (activating it further) and phosphorylates Wee1/Myt1 (inactivating them). More Cdk1 activity → more Cdc25 and less Wee1 → a self-reinforcing, switch-like, irreversible commitment to mitosis.
4. Mitotic events: Cyclin B–Cdk1 phosphorylates lamins (nuclear envelope breakdown), condensin and histone H3 (chromosome condensation), proteins driving centrosome separation and spindle assembly, and the Golgi disassembly machinery.
5. Checkpoint brake: If DNA is damaged or replication incomplete, ATM/ATR activate Chk1/Chk2, which phosphorylate Cdc25. Phosphorylated Cdc25 binds 14-3-3 and is sequestered (or degraded), so it cannot activate Cdk1 — the cell arrests in G2 until repair is complete.

Common confusions

• Wee1 vs. Cdc25: Wee1 is a kinase that inhibits Cdk1; Cdc25 is a phosphatase that activates Cdk1. They are antagonists, both acting on the same residues.
• Activating vs. inhibitory phosphates on Cdk1: CAK's T-loop phosphate activates; Wee1's Thr14/Tyr15 phosphates inhibit. Do not conflate them.
• G2/M checkpoint vs. spindle checkpoint: The G2/M checkpoint senses DNA damage/replication and acts via Chk1/Cdc25; the spindle assembly checkpoint senses chromosome attachment and acts via Mad/Bub → APC/C inhibition.
• Cyclin B degradation = mitotic exit, not entry: Cdk1 activation triggers mitosis; cyclin B destruction ends it.

Quick review

• Cyclin B accumulates in G2 → binds Cdk1 → CAK activates T-loop, Wee1 inhibits → Cdc25 removes inhibition → active Cdk1 → mitosis.
• Feedback: Cdk1 ↔ Cdc25 (mutual activation) makes entry switch-like.
• Damage → Chk1/Chk2 → Cdc25 inhibited → G2 arrest.