Loading dynamics of a sliding DNA clamp.

Area of Science:

• Molecular Biology
• Biophysics
• Structural Biology

Background:

• Sliding DNA clamps are essential protein rings that encircle DNA, acting as processivity factors for DNA polymerases.
• Clamp loading requires ATP hydrolysis by a clamp loader complex to open the clamp, allowing it to encircle DNA.
• Understanding the real-time dynamics of this loading process is crucial for comprehending DNA replication fidelity and regulation.

Purpose of the Study:

• To visualize and characterize the real-time conformational changes of the E. coli β clamp during its loading onto DNA.
• To elucidate the temporal sequence of events and the role of ATP hydrolysis in clamp closure and release.

Main Methods:

• Single-molecule Förster Resonance Energy Transfer (smFRET) was employed to monitor clamp conformational states in real-time.
• Single-molecule polarization studies were used to investigate rotational dynamics within the clamp during closure.
• The study focused on the loading of the E. coli β clamp at a single-stranded/double-stranded DNA junction.

Main Results:

• Three distinct FRET states corresponding to clamp loading intermediates were observed, with durations of 0.3 s, 0.7 s, and >9 min.
• ATP hydrolysis by the clamp loader induced clamp closure within 0.3 s, followed by release and diffusion on dsDNA.
• A ~8° in-plane rotation of the clamp's interfacial domain was detected during closure, indicating a significant 3D conformational change.

Conclusions:

• The study provides the first real-time visualization of the ATP-hydrolysis-dependent 3D conformational dynamics of a sliding DNA clamp during loading.
• The findings reveal a rapid and sequential mechanism for clamp loading, involving distinct conformational states and a precise rotational movement.
• These insights advance our understanding of the molecular mechanisms governing DNA replication and repair.