Double-negative feedback between S-phase cyclin-CDK and CKI generates abruptness in the G1/S switch

摘要: The G1/S transition is a crucial decision point in the cell cycle. At G1/S, there an abrupt switch from state of high cyclin-dependent kinases (CDK) inhibitor (CKI) levels and low S-phase CDK activity to degraded CKI. In budding yeast, this triggered by phosphorylation Cdk1 Sic1 at multiple sites G1-phase (Cln1,2-Cdk1) (Clb5,6-Cdk1) complexes. Using mathematical modeling we demonstrate that mechanistic basis for abruptness highly specific complex. This generated double-negative feedback loop which S-CDK1 phosphorylates Sic1, thus targeting it destruction, thereby liberating further inhibitory Sic1-S-CDK1 Our model predicts depends upon strong binding affinity within Sic1-S-CDK vitro analysis using purified yeast proteins revealed free Clb5-Cdk1 can create positive phosphorylating bound complex, inhibits with sub-nanomolar inhibition constant. also if complex too efficient then becomes smooth readily reversible transition. We propose optimal role would not be act as kinase directly responsible degradation CKI, but rather priming signal initiates driven emerging CDK.