Practice question

CDK activation involves multiple ordered modifications ensuring integration of multiple regulatory inputs. Monomeric CDK presents T-loop obstructing peptide substrate binding and PSTAIRE helix oriented outward preventing coordination of ATP phosphates. Cyclin association through extensive hydrophobic interface rotates PSTAIRE helix inward aligning Glu51 with Lys33 for ATP positioning and partially displaces T-loop. However maximal substrate affinity requires phosphorylation of conserved threonine within T-loop, Thr160 in CDK2, Thr161 in CDK1, Thr172 in CDK4, catalyzed by CDK-activating kinase complex composed of CDK7, Cyclin H and assembly factor MAT1 part of transcription factor TFIIH. Phospho-threonine coordinates network of three arginine residues Arg50 Arg126 Arg150 creating stable beta-sheet platform for peptide binding increasing affinity hundredfold and aligning catalytic aspartate Asp145 for phosphotransfer. Dephosphorylation of inhibitory Thr14 Tyr15 by Cdc25 family further required for full activity. CAK activity largely constitutive linking cell cycle progression to transcriptional capacity, yet cyclin availability limits timing. Structural studies by Pavletich reveal stabilization mechanism explaining how Thr160 phosphorylation boosts catalytic efficiency and substrate recruitment during cell cycle transitions.