Practice question

Cell cycle progression driven by oscillating cyclin-dependent kinase activities where temporal regulation derived from cyclin synthesis and degradation while catalytic function resides within CDK subunit. Cyclins contain cyclin box fold binding hydrophobic patch on CDK but lack enzymatic residues for phosphate transfer. CDKs share bilobal kinase architecture with ATP binding pocket between N and C lobes, PSTAIRE helix aligning catalytic residues and activation T-loop blocking substrate binding when unphosphorylated. Cyclin binding rotates PSTAIRE helix inward positioning Glu51 to coordinate Lys33 for ATP orientation and moves T-loop aside exposing substrate binding groove. Full activation additionally requires phosphorylation at conserved threonine 160 in CDK2 or 161 in CDK1 by CAK complex CDK7-Cyclin H-MAT1 stabilizing T-loop interaction with substrate peptide through arginine pocket, and removal of inhibitory phosphorylations at Thr14 Tyr15 added by Wee1 Myt1 kinases via Cdc25 phosphatases. Active CDK then transfers gamma phosphate of ATP to serine-threonine-proline motifs on hundreds of targets, powering cell cycle transitions. Hence catalytic subunit capable of phosphotransfer is CDK, cyclins provide allosteric activation, localization and substrate specificity.