Mutations at multiple CDK phosphorylation consensus sites on Cdt2 increase the affinity of CRL4^Cdt2 for PCNA and its...

Area of Science:

• Molecular Biology
• Cell Cycle Regulation
• Ubiquitin Ligase Activity

Background:

• The CRL4Cdt2 ubiquitin ligase is crucial for maintaining genome integrity during the cell cycle.
• Previous studies suggested a negative regulatory role for Cdk1 via Cdt2 phosphorylation, but this mechanism remained unclear.

Purpose of the Study:

• To investigate the role of CDK-mediated phosphorylation of Cdt2 in regulating CRL4Cdt2 activity.
• To determine how Cdt2 phosphorylation affects its interaction with PCNA and substrate degradation.

Main Methods:

• Site-directed mutagenesis of Cdt2 to create a non-phosphorylatable mutant (Cdt2-18A).
• In vitro and in vivo phosphorylation assays using cyclinA/Cdk2 and cyclinB/Cdk1.
• Analysis of Cdt2-PCNA interaction, substrate degradation (Cdt1, Set8, thymine DNA glycosylase), and ubiquitination activity.

Main Results:

• CDK phosphorylation of Cdt2 was confirmed in vitro and in vivo, and significantly reduced in the Cdt2-18A mutant.
• The Cdt2-18A mutation enhanced Cdt2's affinity for PCNA, leading to increased colocalization with PCNA foci during S phase.
• CRL4Cdt2-mediated poly-ubiquitination of Cdt1 was enhanced, and degradation of Cdt1, Set8, and thymine DNA glycosylase was prevented in Cdt2-18A cells.

Conclusions:

• CDK-mediated phosphorylation of Cdt2 inactivates CRL4Cdt2 ubiquitin ligase activity by decreasing its affinity to PCNA.
• This phosphorylation-dependent regulation is essential for controlling the levels of key cell cycle proteins.
• The findings reveal a critical mechanism for maintaining genome stability and proper cell cycle progression.