Phosphate-binding pocket on cyclin B governs CDK substrate phosphorylation and mitotic timing

Area of Science:

• Molecular Biology
• Cell Biology
• Biochemistry

Background:

• Cell cycle progression relies on cyclin-dependent kinases (CDKs) and their regulatory partners, cyclin and Cks1.
• CDKs regulate numerous substrates through multisite phosphorylation, a process dependent on Cks1 for efficient secondary site modification.

Purpose of the Study:

• To investigate the role of a newly identified phosphate-binding pocket (PP) on B-type cyclins in regulating CDK substrate phosphorylation.
• To determine if this cyclin-associated PP influences cell cycle timing and substrate modification patterns.

Main Methods:

• Site-directed mutagenesis of the phosphate-binding pocket in Clb2, a key mitotic cyclin in budding yeast.
• Analysis of cell morphology and cell cycle progression (anaphase onset) in mutant strains.
• In vitro phosphorylation assays of CDK substrates using wild-type and mutant Clb2.

Main Results:

• Mutation of the Clb2 phosphate-binding pocket led to altered bud morphology and delayed anaphase onset.
• The mutated PP significantly reduced multi-site phosphorylation of CDK substrates, including components of the anaphase-promoting complex/cyclosome and polarisome.
• These results indicate the cyclin PP is essential for efficient multisite phosphorylation of specific CDK targets.

Conclusions:

• The phosphate-binding pocket on B-type cyclins plays a critical role in controlling the pattern of multisite phosphorylation on CDK substrates.
• This cyclin-mediated regulation, analogous to Cks1's function, contributes to the precise and robust timing of essential cell cycle events.