Cell Cycle Regulation: M Phase and APCC Activation

MCDK (M-phase Cyclin-Dependent Kinase) phosphorylates specific targets, working in conjunction with other molecules (non-MCDK proteins).
This phosphorylation contributes to:
- Chromosome condensation.
- Breakdown of the nuclear envelope.
- Mitotic spindle assembly.
MCDK's phosphorylation of specific substrates is crucial for these processes to advance.
Other regulatory kinases are involved; for example, Ran-GTP aids in mitotic spindle organization.

MCDK phosphorylates APCC (Anaphase-Promoting Complex/Cyclosome), initiating its activation.
The first coactivator to bind APCC is CDC20.
APCC bound to CDC20 has two primary targets:
- Cyclin in the MCDK complex: This leads to MCDK breakdown and reduced availability, decreasing its activity.
- Securin: When securin is degraded, separase is activated, leading to sister chromatid separation.
- Sister chromatid separation is abbreviated as SC separation.

As APCC diminishes MCDK activity and availability:
- The activity of specific phosphatases increases.
- There is a balance between MCDK activity and phosphatase activity.
- High MCDK activity phosphorylates components that decrease phosphatase activation.
- As MCDK activity declines, phosphatase availability increases.
Decreasing MCDK leads to decreased phosphorylation of APCC.
- APCC is then more likely to release CDC20.
- CDH1, a secondary binding partner, is dephosphorylated.
- $APCC + CDH1 \rightarrow APCC:CDH1$
APCC bound to CDH1 targets:
- Residual MCDK activity.
- Other unidentified substrates.
This allows the cell to progress through telophase and back to G1.

MCDK is inactive in G1 and held in an inactive state.
Dephosphorylation activates MCDK, initiating prophase.
Phosphorylation of cellular proteins by MCDK drives prophase events:
- Chromosome condensation.
- Nuclear envelope breakdown.
- Mitotic spindle assembly.
MCDK starts the activation of APCC bound to CDC20.
Provided the cell meets checkpoint requirements, APCC bound to CDC20 reduces MCDK activity, targeting additional substrates.
If sister chromatids are NOT correctly attached to the mitotic spindle, regulatory proteins (like MAD proteins) block APCC activation.
The transition between MCDK and APCC activation involves a crucial cell cycle checkpoint, dependent on proper chromosome attachment to the mitotic spindle.
If chromosome attachment is incorrect, the cell cycle halts, preventing premature sister chromatid separation and potential aneuploidy.
This transition is between metaphase and anaphase.

Motor proteins (kinesins and dyneins) are essential for:
- Moving mitotic spindle elements.
- Regulating chromatid positioning.
- Moving chromosomes to the metaphase plate during metaphase.
The kinetochore connects microtubules (hollow tubes of 13 protofilaments) to DNA.
The mechanism is tension-driven; the cell detects equivalent high tension on each sister chromatid before proceeding through the checkpoint.
One model suggests that protein proximity and phosphorylation patterns change based on tension levels.
- Low Tension: Proteins are in closer proximity, resulting in one phosphorylation pattern.

The second half of M phase is regulated by APCC, which is a ubiquitin ligase that tags specific substrates for degradation.
Major APCC targets:
- Cyclins: Decreasing MCDK activation.
- Securin: Degradation releases separase, enabling sister chromatid separation.
APCC is activated by phosphorylation and binding to a co-activator with CDK playing a role in this activation.

After separase activation:
- Microtubules rearrange.
- Motor proteins are activated.
- Kinetochore microtubules shorten, pulling sister chromatids apart.
- The mitotic spindle elongates.
This creates two distinct poles, each containing a set of chromosomes.
Inappropriate attachment should halt this process via proteins like MAD, preventing cells with incorrect chromosome numbers.

Telophase reverses prophase events:
- Mitotic spindle disassembles.
- Nuclear envelope reforms.
- Condensed chromosomes decondense.
Cytokinesis (cell division) is driven by actin rearrangement in animal cells.
- A ring of linear actin filaments is formed via Rho-driven formin-nucleated actin polymerization, along with associated motor proteins.
- APCC bound to CDH1 targets many proteins for degradation, resetting the cell to G1.
  - Examples: Aurora kinases, CDC20, Polo-like kinases

Cell division in multicellular organisms must be controlled.
Signals trigger transcriptional changes, leading to production of cyclin-CDK complexes.
These complexes phosphorylate targets, causing further transcriptional events and activation of S-CDK (S-phase CDK).
S-CDK regulates DNA replication.
- Helicase loading is tightly controlled.
- Mechanisms prevent re-replication.
M-CDK (Mitotic CDK) activates APCC bound to CDC20, leading to securin breakdown, separase release, and sister chromatid separation.
As M-CDK activity drops, phosphatase activity increases, APCC switches from CDC20 to CDH1, and targets other proteins.

APCC and SCF are ubiquitin ligases active at different cell cycle phases.
APCC targets cyclin and securin when bound to CDC20 and additional substrates like Aurora kinases and Polo-like kinases when bound to CDH1.
SCF, with various coactivators, targets different substrates during the other half of the cell cycle.

As MCDK activity declines, phosphatases dephosphorylate targets.
CDK:MCDK phosphorylates APCC, making it receptive to CDC20 binding. As CDK declines, phosphatases dephosphorylate APCC and CDH1.
MCDK phosphorylates and inactivates phosphatases; as MCDK declines, phosphatase inhibition is relieved.
High MCDK activity = low phosphatase activity.
Declining MCDK activity = increased phosphatase activity.