#cohesin

Cell Division

Which enzyme is responsible for breaking down cohesin to allow sister chromatid separation?

Sister chromatid cohesion mediated by cohesin ring complex with SMC1 and SMC3 coiled-coil proteins forming V-shaped dimer bridged by kleisin Rad21 that closes tripartite ring embracing both sister DNAs from replication until anaphase. Opening requires proteolytic cleavage rather than dissociation. Separase, large 230 kDa cysteine endopeptidase belonging to CD clan with catalytic histidine-cysteine dyad analogous to caspases, serves as cleaving enzyme. Before anaphase, separase kept inactive through two layers: binding of securin pseudosubstrate occupying active site, and cyclin B-CDK1 mediated phosphorylation at serine 1126 causing cytoplasmic retention and auto-inhibition. At anaphase onset, APC/C-Cdc20 ubiquitinates securin with K11 chains for proteasomal destruction, cyclin B degradation reduces CDK1 activity allowing PP2A-dependent dephosphorylation, releasing separase to chromosomes where it cleaves Rad21 at conserved EXXR sequences separating N-terminal and C-terminal fragments unable to hold ring closed. Non-cleavable Rad21 mutant blocks sister separation causing metaphase arrest, proving cleavage essential. This circuitry is highly conserved across eukaryotes, integrating growth factor signals, DNA damage surveillance, and developmental cues, and its disruption frequently underlies oncogenesis, providing targets for checkpoint inhibitors and cancer therapeutics.

Ref: Uhlmann et al., Nature 2000, Mechanism of Separase. Nasmyth, Science 2002, Cohesin Cleavage.

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Cell Cycle Regulation

Which enzyme cleaves cohesin at anaphase onset?

Persistent cohesion between sister chromatids depends on cohesin, tetrameric ring of Smc1, Smc3 ATPases, kleisin Scc1/Rad21 and stromalin SA1/2 subunits embracing replicated DNA strands. During prophase most arm cohesin dissociates via Wapl-dependent opening after Plk1 and Aurora B phosphorylation of SA2, while centromeric pool protected by Shugoshin recruiting PP2A-B56 counteracting phosphorylation remains for biorientation. Final dissolution at anaphase onset requires proteolytic cleavage. Separase, 230 kDa cysteine protease of CD clan, executes this. Kept inactive by dual mechanisms including pseudosubstrate binding of securin chaperone covering conserved active site and phosphorylation by CDK1-Cyclin B1 promoting cyclin B interaction, separase activates only after APC/C-Cdc20 polyubiquitinates securin and cyclin B for proteasomal destruction. Freed separase recognizes conserved separase cleavage motif Glu-X-X-Arg in Scc1 and cleaves after Arg arginine 172 and 450 in human Rad21, opening ring structure and allowing spontaneous dissociation. Cleaved cohesin fragments removed, permitting microtubule pulling forces to drive disjunction. Separase also cleaves itself, cohesin-independent roles in FEAR activation and centriole disengagement. Additional feedback loops involving polo-like kinases, phosphatases and SCF-mediated degradation reinforce irreversibility and protect against premature progression that would compromise genome integrity and viability.

Ref: Nasmyth & Haering, Cohesin and Separase Regulation, Annu Rev Genet 2009; Uhlmann, Separase in Anaphase and Beyond.

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Nucleosome Packing

SMC proteins are involved in:

Structural Maintenance of Chromosomes proteins represent large ATPase machines forming elongated coiled-coil dimers with hinge domain closing ring-like structures embracing DNA duplexes. Condensin complexes SMC2/SMC4 compact mitotic chromosomes via loop extrusion, cohesin SMC1/SMC3 holds sister chromatids together from S phase until anaphase cleavage by separase, SMC5/SMC6 aids homologous recombination repair at collapsed forks. Mutations produce chromosome missegregation, aneuploidy and repair defects. Proteins localize to scaffold attachment regions and utilize ATP hydrolysis to extrude loops organizing chromatin far beyond nucleosome scale into chromosome level maintenance.

Ref: Hirano T 2016 Nat Rev Mol Cell Biol SMC; Lodish et al., Chapter 19: SMC Proteins Chromosome Maintenance

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Nucleosome Packing

Major scaffold protein involved in loop formation is:

Chromosome condensation during prophase requires scaffold proteins shaping 30 nanometer fiber loops into cylindrical chromatids 700 nanometers wide. Topoisomerase II alpha, type II enzyme cleaving both DNA strands, passing duplex through break and resealing, resolves sister chromatid catenations, adjusts supercoiling and forms axial scaffold along chromatid center overlapping condensin complex. Immunofluorescence shows topoisomerase II colocalizes with condensin along axis. DNA polymerase synthesizes DNA, RNA polymerase transcribes, ligase joins Okazaki fragments, but scaffold architecture and loop compaction depend critically on topoisomerase II plus condensin activity during mitotic entry.

Ref: Earnshaw and Laemmli 1983 Scaffold; Alberts et al., Molecular Biology of the Cell, Chapter 19: Scaffold Protein Topoisomerase II

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