Design principles of the yeast G1/S switch

Area of Science:

• Cell Biology
• Molecular Biology
• Biochemistry

Background:

• The G1/S transition in budding yeast is regulated by the degradation of Sic1, a cyclin-dependent kinase inhibitor.
• Dysregulation of Sic1 degradation leads to genomic instability.
• The roles of specific kinases (Cln1/2-Cdk1 and Clb5/6-Cdk1) in Sic1 phosphorylation and degradation are debated.

Purpose of the Study:

• To elucidate the in vivo mechanism of Sic1 degradation and its role in the G1/S cell cycle switch.
• To clarify the distinct contributions of Cln1/2-Cdk1 and Clb5/6-Cdk1 to Sic1 destruction dynamics.
• To investigate the impact of substrate sharing between kinases on circuit robustness.

Main Methods:

• Real-time monitoring of Sic1 destruction in individual yeast cells under various perturbations.
• In vivo analysis of Sic1 degradation dynamics.
• Mathematical modeling to confirm experimental findings and explore circuit design principles.

Main Results:

• Cln1/2-Cdk1 acts as a trigger for Sic1 destruction, setting the timing but not the speed.
• Clb5/6-Cdk1 controls the speed of Sic1 destruction via a double-negative feedback loop, ensuring an all-or-none switch.
• Degradation of a single-phosphosite Sic1 mutant is rapid and switch-like, similar to wild-type.
• Substrate sharing between kinases is crucial for balancing degradation timing and sharpness.

Conclusions:

• The yeast G1/S switch operates through a precisely regulated degradation mechanism involving distinct kinase roles.
• A double-negative feedback loop ensures robust cell cycle transitions.
• The study provides mechanistic insights into the design principles of biological switches.