Practice question

Experiments in amphibian oocytes and in yeast genetics converged upon identification of universal M-phase regulator historically called Maturation Promoting Factor for its ability to induce meiotic maturation when cytoplasm from M-phase cells injected into G2 oocytes. Molecular composition resolved as complex of catalytic subunit CDK1 originally named cdc2 or p34cdc2 and regulatory subunit Cyclin B. CDK1 provides serine-threonine kinase activity requiring activating phosphorylation on activation loop Thr161 by CAK complex CDK7-Cyclin H-MAT1 and removal of inhibitory phosphates Thr14 Tyr15 via Cdc25 phosphatase. Cyclin B synthesized during S and G2 accumulates at G2/M, contains destruction box recognized by APC/C-Cdc20 for degradation providing temporal restriction, supplies cyclin box for CDK activation and contributes substrate docking through hydrophobic patch recognizing RXL motifs plus nuclear localization signal translocating complex into nucleus at prophase. MPF substrates include lamins causing envelope breakdown, condensin subunits driving chromosome compaction, microtubule regulators forming bipolar spindle and APC/C components regulating exit. Degradation of Cyclin B at anaphase inactivates MPF, allowing phosphatases to restore interphase state.