Practice question

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.