Requirement for ATP by the DNA damage checkpoint clamp loader

Area of Science:

• Molecular Biology
• Biochemistry
• DNA Repair

Background:

• The DNA damage clamp loader, replication factor C (RFC-Rad24), facilitates DNA repair by loading the Rad17/3/1 clamp onto DNA.
• Its function is proposed to mirror the replication clamp loader RFC and proliferating cell nuclear antigen (PCNA) system.
• In the RFC-PCNA system, ATP binding to Rfc2-4 is crucial for PCNA loading, while Rfc1 is not required.

Purpose of the Study:

• To investigate the role of ATP binding in the RFC-Rad24 complex's function.
• To compare the ATP utilization mechanisms between the DNA damage checkpoint and replication clamp loader systems.

Main Methods:

• Site-directed mutagenesis was used to create mutations in the ATP-binding domains of Rad24 and Rfc4.
• ATPase activity assays were performed to assess the enzymatic function of the mutant complexes.
• Interaction studies were conducted to evaluate the binding of RFC-Rad24 to the Rad17/3/1 clamp.
• In vitro clamp loading assays were used to measure the efficiency of checkpoint clamp loading.

Main Results:

• A rad24-K115E mutation disrupted RFC-Rad24 ATPase activity, impaired interaction with Rad17/3/1, and abolished checkpoint clamp loading.
• A rfc4K55R mutation similarly affected ATPase activity and clamp loading, while rfc4K55E showed partial activity.
• These findings indicate that ATP binding to Rad24 is essential for RFC-Rad24 function.

Conclusions:

• ATP utilization by the DNA damage checkpoint clamp/clamp loader system differs significantly from the replication clamp loader system.
• Rad24's ATP-binding site plays a critical role in the loading of the DNA damage clamp, unlike Rfc1 in the replication system.