Regulation of activity of cyclin- CDK complex by SCF and APC/C

Role of SCF and APC/C in Regulation of Cyclin–CDK Complex Activity The activity of cyclin–CDK complexes is regulated by two important protein complexes: SCF (Skp1–Cullin–F-box complex) and APC/C (Anaphase-Promoting Complex/Cyclosome). Both act as E3 ubiquitin ligases, attaching ubiquitin to specific proteins and targeting them for degradation by the proteasome. 1. SCF Complex Functions mainly during the G1/S transition. Targets CDK inhibitors (CKIs) such as p27 and Sic1 for degradation. Removal of CKIs activates cyclin–CDK complexes. Promotes progression from G1 phase to S phase, allowing DNA replication to begin. 2. APC/C Complex Functions mainly during mitosis (M phase). Targets securin and mitotic cyclins (Cyclin A and Cyclin B) for degradation. Degradation of securin activates separase, enabling sister chromatid separation during anaphase. Degradation of mitotic cyclins inactivates CDKs, leading to exit from mitosis and entry into G1 phase. Summary SCF promotes cell cycle progression by degrading CDK inhibitors and activating cyclin–CDK complexes. APC/C ensures proper chromosome segregation and mitotic exit by degrading securin and mitotic cyclins. Together, they maintain accurate and orderly progression of the cell cycle.