L6 cohesion complex

Lecture 6: Cohesin Complex and Its Role in Chromosome Biology

Overview of Lecture Topics

  • Aspects of Chromosome Biology Focus on Cancer

    • Genome organisation and epigenetics in cancer

    • Chromosome segregation in normal and cancer cells

    • Cohesin complex and its role in chromosome biology

    • Aneuploidy in cancer cells

    • Case study

Biology of the Cohesin Complex

  • Components of the Cohesin Complex

    • SMC Proteins:

    • SMC1

    • SMC3

    • Non-SMC Subunits:

    • Rad21/Scc1 (binds to additional regulatory proteins)

    • Scc3 (binds further regulatory proteins)

  • Function of Cohesin:

    • Loaded onto chromosomes during the G1 phase of the cell cycle.

    • After DNA replication, cohesin keeps sister chromatids together.

    • Works with CTCF (CCCTC-binding factor) to establish chromatin unit borders during interphase.

    • Release from chromosome arms occurs in prophase, aligning with chromosomal axial compression during mitosis.

Stages of the Cohesin Cycle

  • Loading:

    • Cohesin is loaded onto DNA in a non-cohesive form just after mitosis, aided by the cohesin loading factor Scc2.

  • Acetylation:

    • In S phase, enzymes Eco1 (Esco1/Esco2) acetylate cohesin, leading to its cohesive form which holds sister chromatids together.

  • Removal:

    • Total cohesin removal occurs during cell division. Controlled through two phases:

    1. Removal of cohesin from chromosomal arms

    2. Removal of cohesin from centromeres, regulated by Separase.

Regulation of Cohesin Removal

  • Wave-like Removal:

    • Cohesin removal enabling chromosome segregation occurs in two distinct waves.

    • Shugoshin and Protein Phosphatase 2A protect centromeric cohesin throughout the process.

Separase Activity Control

  • Metaphase to Anaphase Transition Prerequisites:

    • Inactivation of Cdk1

    • Activation of Separase

  • Triggered by degradation of regulatory proteins through the proteasome pathway, necessitating ubiquitylation of targeted proteins.

  • Anaphase-Promoting Complex/Cyclosome (APC/C):

    • E3-type ubiquitin ligase, attaches ubiquitin to substrates for degradation.

    • Controlled by the Spindle Assembly Checkpoint (SAC).

Spindle Assembly Checkpoint (SAC)

  • Mechanism of Action:

    • Unattached kinetochores generate a STOP signal that inhibits APC/C activity.

    • This involves the formation of the Mitotic Checkpoint Complex (MCC), comprising:

    • BubR1

    • Bub3

    • Mad2

    • Cdc20

  • MCC inhibits APC/C until all kinetochores are successfully attached to spindle microtubules.

Cohesin Removal and Chromosome Segregation

  • Upon APC/C activation, a cascade of events leads to Cdk1 inactivation and Separase activation, essential for separating centromeric cohesin.

  • A single unattached kinetochore is sufficient to maintain the STOP signal, delaying anaphase initiation.

Key Phases of Mitosis

  • Prometaphase, Metaphase, Anaphase:

    • SAC status dictates the transitions:

    • SAC On: Active during unattached kinetochores.

    • SAC Off: Transition state allowing for anaphase onset.

Components of the Spindle Assembly Checkpoint

  • Key Proteins Involved:

    • Bub1, Bub3, BubR1, Cdc20, Mad1, Mad2.

    • Other functional proteins:

    • Plk1 (Polo-like kinase 1)

    • Aurora B (kinase critical for SAC function)

  • Recruitment of SAC components occurs exclusively at unattached kinetochores, commencing MCC generation.

Experimentation and SAC Functionality

  • Delayed chromosome congression leads to increased binding of Mad2 at unattached kinetochores.

  • The SAC provides additional time to ensure proper microtubule-kinetochore attachment.

Error Correction in SAC

  • The SAC responds both to unattached kinetochores and to erroneous attachments, detecting both lack of kinetochore occupancy and tension.

  • Aurora B is crucial for SAC component function, as its inhibition affects Mps1 recruitment to kinetochores.

Importance of Aurora B in Cancer

  • Overexpression of Aurora B correlates with tumorigenesis.

  • Research is focused on developing Aurora B inhibitors as potential cancer therapeutics.

Mutations in Cohesin Components and Cancer

  • Cohesin component mutations are common in cancer cells, particularly in the STAG2 subunit.

  • These mutations do not necessarily correlate with changes in chromosome number in daughter cells.

  • Alterations in cohesin gene expression and mutations may influence cancer pathways in complex manners.

Multifunctionality of Cohesin

  • Cohesion-dependent Functions:

    • DNA replication, chromosome biorientation, transcription regulation, etc.

  • Cohesion-independent Functions:

    • Genome compartmentalization, DNA damage repair, centromere maintenance.

Centrosome Functionality and Cohesin

  • Recent findings identify cohesin as a centrosomal protein.

  • Cohesin's role extends to spindle assembly and the regulation of centrosomes during mitosis.

Conclusion

  • Cohesin plays critical roles in both architectural structures during interphase and regulatory functions during mitosis.

  • Cohesins are built from a combination of SMC and non-SMC components that modulate their functionality.

  • Genes encoding cohesin proteins are often mutated in cancer, impacting various functional pathways.

  • Understanding the nuanced functions of cohesin can aid the development of targeted cancer treatments, particularly through the exploration of spindle assembly checkpoint mechanisms and mitotic drug targeting strategies.