Multisite phosphorylation of a CDK inhibitor sets a threshold for the onset of DNA replication

Area of Science:

• Cell Biology
• Molecular Biology
• Biochemistry

Background:

• SCF ubiquitin ligases mediate protein degradation through F-box protein adaptors.
• The F-box protein Cdc4 targets Sic1, a cyclin-dependent kinase inhibitor, for degradation during the G1 phase.
• This degradation is crucial for initiating DNA replication and cell cycle progression.

Purpose of the Study:

• To investigate the mechanism by which Cdc4 binds to its substrate Sic1.
• To understand how phosphorylation sites regulate the timing of Sic1 degradation.
• To explore the role of suboptimal binding motifs in establishing cell cycle thresholds.

Main Methods:

• Analysis of protein-protein interactions between Cdc4 and Sic1.
• Characterization of the Cdc4 phospho-degron (CPD) motif.
• In vivo studies to assess the impact of phosphorylation site mutations on Sic1 degradation and cell cycle progression.

Main Results:

• Cdc4 binds to phosphorylated Sic1 via its WD40 repeat domain, recognizing the CPD motif.
• Sic1 possesses multiple suboptimal CPD motifs that collectively mediate Cdc4 binding.
• These weak binding sites create a phosphorylation-dependent threshold, delaying Sic1 degradation and ensuring a minimal G1 phase duration.

Conclusions:

• Multisite phosphorylation and suboptimal binding motifs serve as a general mechanism for establishing regulatory thresholds in protein-protein interactions.
• This mechanism ensures precise control over cell cycle events, such as the timing of DNA replication.
• The findings provide insights into the regulation of protein turnover and cell cycle control.