Practice question

Separase activation integrates checkpoint satisfaction with physical separation of chromatids. Newly synthesized separase folds with assistance of securin chaperone, which inserts its own reactive site loop into catalytic pocket, blocking proteolysis and ensuring proper localization to nucleus. Parallel inhibition involves CDK1-Cyclin B1 binding phosphorylating separase at Ser1126 creating docking site for cyclin B. At metaphase, with all kinetochores amphitelically attached, spindle assembly checkpoint silencing allows APC/C-Cdc20 E3 ligase to become active. APC/C-Cdc20 recognizes KEN-box and D-box degron sequences on securin, catalyzing assembly of K11 and K48 linked polyubiquitin chains. Polyubiquitinated securin targeted to 26S proteasome with half-life decreasing from hours to under ten minutes. Simultaneously cyclin B degradation removes CDK1-mediated inhibition. Removal of securin exposes catalytic dyad histidine and cysteine residues allowing cleavage of cohesin kleisin Scc1/Rad21 at EXXR motifs. Some separase autocleavage yields C-terminal fragment with retained activity. Securin therefore acts both as inhibitor and as folding assistant, while APC/C functions as trigger linking ubiquitination to proteolysis-driven anaphase initiation. 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.