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PCNA is a nucleotide exchange factor for the clamp loader ATPase complex.

Joshua Pajak1, Jacob T Landeck1, Xingchen Liu1

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Summary

The Saccharomyces cerevisiae clamp loader, Replication Factor C (RFC), tightly binds ADP. The sliding clamp, Proliferating Cell Nuclear Antigen (PCNA), acts as a nucleotide exchange factor, facilitating ADP release for RFC recycling.

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Area of Science:

  • Molecular Biology
  • Biochemistry
  • Structural Biology

Background:

  • All life depends on loading ring-shaped sliding clamp protein complexes onto DNA.
  • Sliding clamp loaders, like Replication Factor C (RFC), are conserved AAA+ ATPases essential for this process.
  • The mechanism of clamp loader recycling after multiple loading events remains poorly understood.

Purpose of the Study:

  • To investigate the recycling mechanism of the Saccharomyces cerevisiae clamp loader, RFC.
  • To elucidate the role of the sliding clamp, Proliferating Cell Nuclear Antigen (PCNA), in RFC function.
  • To understand how RFC releases ADP to prepare for subsequent clamp loading cycles.

Main Methods:

  • X-ray crystallography to determine the structures of RFC in the absence of PCNA or nucleotide.
  • Molecular dynamics simulations to analyze RFC-PCNA interactions and nucleotide exchange.
  • Biochemical assays to validate the functional implications of structural findings.

Main Results:

  • RFC structures reveal tight ADP binding in multiple ATPase active sites, indicating regulated nucleotide exchange.
  • PCNA binding induces rapid ADP exchange, suggesting PCNA acts as a nucleotide exchange factor (NEF).
  • PCNA likely pries apart RFC subunits, facilitating ADP release and enabling RFC recycling.

Conclusions:

  • PCNA functions as a NEF for RFC, promoting efficient ADP release and clamp loader recycling.
  • This substrate-mediated NEF mechanism prevents premature DNA binding by RFC before PCNA loading.
  • The findings provide critical insights into the regulation and efficiency of DNA replication machinery.