Practice question

Sister chromatid cohesion, mediated by the ring-shaped cohesin complex composed of Smc1, Smc3, kleisin Scc1 or Rad21 and SA proteins, must be abruptly destroyed only after bioriented attachment. Separase provides the decisive enzymatic activity. This large cysteine endopeptidase recognizes a conserved E-x-x-R motif within the kleisin subunit and cleaves after arginine, effectively opening the ring. During early mitosis separase remains constrained by two inhibitory locks: stoichiometric binding of securin which occupies the catalytic groove as pseudosubstrate and chaperone, and CDK1-Cyclin B1 dependent phosphorylation that promotes cyclin B association. When the spindle assembly checkpoint is satisfied, free APC/C-Cdc20 polyubiquitinates securin and cyclin B through D-box and KEN-box degrons for rapid proteasomal degradation. Liberated separase becomes active, cleaves centromeric cohesin that escaped Wapl-mediated removal from chromosome arms earlier in prophase. Cleavage allows kinetochore microtubules to generate pulling forces unopposed, driving chromatids toward opposite poles. Failure to regulate separase causes premature separation, aneuploidy and tumorigenesis, underscoring tight cell cycle control by APC/C and securin balance. 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.