chromosomes

Phosphorylation and Cohesin Complexes

  • Phosphorylation Role

    • Phosphorylation is sufficient for removing cohesin complexes from chromosome ends.

    • Cohesin complexes may be recycled after cell division since they are in functional shape but are phosphorylated, preventing binding to DNA.

  • Cohesin Pools

    • A small pool of cohesin remains at the central region (centromere) where sister chromatids are held together.

    • Cohesin is already released from the chromosomal arms, contributing to the classic X-shaped structure of mitotic chromosomes.

  • Protection Mechanism

    • A specialized mechanism protects centromere cohesin from degradation.

    • Proteins involved include:

      • Kinase AurA

      • Protein Phosphatase 2A

    • Kinases add phosphate groups; thus, the removal of these groups by phosphatases ensures protection of cohesin at centromeres.

Separation of Cohesin

  • Separase Pathway

    • After most cohesin is removed, an entirely different pathway, called the separase pathway, is responsible for degrading remaining cohesin.

    • Once degraded, cohesin cannot be reused.

  • Regulation of Cell Cycle

    • Controlled proteolysis is crucial in regulating the cell cycle during mitosis, involving the Anaphase Promoting Complex (APC).

    • APC either in its active form (APC/C) or inactive form is critical for the transition to anaphase.

  • Requirements for Cells Exiting Mitosis

    • Cells must inactivate Cyclin-dependent kinase 1 (CDK1) and activate Separase.

    • Both processes rely on proteolytic pathways managed by APC/C.

  • Checkpoints

    • The APC/C is regulated by spindle assembly checkpoint (SAC) ensuring all kinetochores are attached to microtubules before proceeding to anaphase.

    • When kinetochores are not properly attached, they produce MCC (Mitosis Checkpoint Complex) which inhibits APC/C, halting degradation signals.

Activation of APC/C and Its Consequences

  • Activation Process

    • Properly attached kinetochores signal the APC/C relieving it from MCC inhibition.

    • APC/C activation leads to degradation of:

    • Cyclin B (activator of CDK1)

    • Securin (inhibitor of separase)

  • Outcomes

    • Activation of APC/C triggers two essential outcomes:

    • CDK1 inactivation allows exit from mitosis by ceasing phosphorylation events.

    • Separase becomes active, cleaving cohesin at the centromere, allowing chromatids to separate.

The Spindle Assembly Checkpoint (SAC)

  • Functionality

    • The SAC senses the attachment status of kinetochores to microtubules, ensuring that all kinetochores are properly attached before allowing progression to anaphase.

  • SAC Components

    • Critical components are actively recruited to unattached kinetochores.

    • Some components include:

      • MPS1 kinase, which detects whether kinetochores are bound to microtubules.

  • Visualizing Kinetochores

    • Techniques like antibody staining (e.g., using anti-MAP2) allow visualization of unattached kinetochores in live cells, aiding in checkpoint function and delaying cell division as necessary.

Cohesin’s Multifunctional Role

  • Cohesin in Cell Biology

    • Cohesin is crucial in various processes beyond chromosome segregation, including:

    • DNA repair: Acts to maintain genomic integrity.

    • Chromatin organization: Influences the spatial arrangement of chromatin in the nucleus.

    • Centrosome Biology: Cohesin also functions at centrosomes, critical for microtubule organization and development.

  • Connection to Cancer

    • Mutations in cohesin components have been linked to various cancers, with implications in cell division and other cellular processes.

    • Notably, these mutations may not necessarily alter chromosome numbers but can still affect tumor development due to their roles in DNA and chromatin regulation.

  • Unexpected Cohesin Functions

    • Recent research indicates that cohesin is also required for the proper functioning of centrosomes, especially in cell cycle regulation.

    • Cohesin maintains centrioles together; similar to sister chromatids, they need to separate to ensure proper cell division.

Antimitotic Therapies

  • Clinical Relevance

    • Antimitotic drugs aim to inhibit cell division, particularly in cancer therapies, by targeting various aspects of the cellular mitotic machinery:

    • Components involved in microtubule organization.

    • Regulatory proteins like kinases involved in checkpoints and separation processes.

    • Examples may include drugs that target Cyclin-dependent kinases, APC components, and other mitotic factors.

  • Conclusion

    • These antimitotic drugs provide therapeutic strategies to slow down or halt the over-proliferation characteristic of cancerous tissues, demonstrating the interconnected nature of cohesin function and cancer cell proliferation.