Structure of the PCNA unloader Elg1-RFC

Area of Science:

• Molecular Biology
• Structural Biology
• Biochemistry

Background:

• DNA replication requires proliferating cell nuclear antigen (PCNA) clamp loading by replication factor C (RFC).
• PCNA forms a stable ring around DNA and requires a dedicated complex, Elg1-RFC, for its removal.
• Understanding PCNA unloading is crucial for DNA replication fidelity.

Purpose of the Study:

• To elucidate the structural basis of Elg1-RFC's dedicated PCNA unloading activity.
• To reveal the molecular mechanisms distinguishing PCNA unloading from loading.

Main Methods:

• Cryo-electron microscopy (cryo-EM) to determine the structure of Elg1-RFC with PCNA.
• Biochemical assays to assess PCNA unloading activity.

Main Results:

• The cryo-EM structures revealed unique Elg1-RFC features, including external loops and an "Elg1 plug" domain.
• These structural elements were shown to block DNA binding and fill the central chamber, ensuring exclusive PCNA unloading.
• Elg1-RFC demonstrated PCNA unloading capability even with non-hydrolyzable AMP-PNP, and both RFC and Elg1-RFC removed PCNA from closed circular DNA.

Conclusions:

• Elg1-RFC possesses distinct structural adaptations for dedicated PCNA unloading.
• PCNA unloading by Elg1-RFC operates via a mechanism separate from PCNA loading by RFC.
• These findings provide critical insights into the regulation of DNA replication and genome stability.