PP2A^Cdc55 Phosphatase Imposes Ordered Cell-Cycle Phosphorylation by Opposing Threonine Phosphorylation

Area of Science:

• Molecular Biology
• Cell Biology
• Biochemistry

Background:

• Cell-cycle progression is regulated by increasing cyclin-dependent kinase (Cdk) activity, establishing ordered transitions through distinct phases.
• The precise mechanisms by which Cdk activity thresholds are translated into substrate phosphorylation timing remain incompletely understood.

Purpose of the Study:

• To investigate how cell-cycle progression is ordered by Cdk activity thresholds.
• To elucidate the role of phosphatases in regulating Cdk substrate phosphorylation timing.
• To understand the contribution of specific amino acid residues to cell-cycle control.

Main Methods:

• Utilized the budding yeast model system.
• Investigated the interplay between PP2A(Cdc55) phosphatase and Cdk activity.
• Analyzed the phosphorylation timing of late Cdk substrates, focusing on threonine residues.
• Examined the phosphorylation of the model substrate Ndd1.

Main Results:

• The PP2A(Cdc55) phosphatase counteracts Cdk phosphorylation during interphase, delaying the phosphorylation of late Cdk substrates.
• PP2A(Cdc55) specifically inhibits phosphorylation on threonine residues, leading to late-occurring threonine-directed phosphorylation.
• The late phosphorylation of the Ndd1 substrate is dependent on the threonine identity of its Cdk target sites.

Conclusions:

• Cdk-counteracting phosphatases, like PP2A(Cdc55), contribute to cell-cycle ordering by enforcing Cdk activity thresholds.
• Phosphoacceptor amino acid identity, specifically threonine, represents a novel regulatory principle in cell-cycle control.
• This regulatory principle may extend to other signaling pathways beyond cell-cycle regulation.