PCNA stimulates catalysis by structure-specific nucleases using two distinct mechanisms: substrate targeting and...

Area of Science:

• Molecular Biology
• Biochemistry
• Enzymology

Background:

• Proliferating Cell Nuclear Antigen (PCNA) is a sliding clamp protein involved in DNA replication and repair.
• PCNA interacts with various DNA-modifying enzymes, including nucleases, polymerases, and helicases.
• Fen-1 (5'-flap endonuclease) and XPF (3'-flap endonuclease) are nucleases implicated in DNA processing and are known to interact with PCNA.

Purpose of the Study:

• To elucidate the distinct mechanisms by which PCNA activates the nucleases Fen-1 and XPF.
• To investigate the functional role of PCNA beyond its established targeting and organizing functions.
• To explore the implications of PCNA's cofactor-like activity in regulating nuclease function at DNA replication and repair sites.

Main Methods:

• Utilized a novel continuous fluorimetric assay to monitor nuclease activity.
• Quantified the effects of PCNA on enzyme kinetics, including binding affinity (K(M)) and catalytic rate constant (k(cat)).
• Compared the activation mechanisms of PCNA on Fen-1 and XPF.

Main Results:

• PCNA enhances Fen-1 activity by increasing its substrate binding affinity.
• PCNA dramatically increases the catalytic rate constant of XPF by four orders of magnitude, without altering its binding affinity.
• PCNA may act as a platform to facilitate DNA distortion by XPF, thereby enhancing catalysis.

Conclusions:

• PCNA employs fundamentally different mechanisms to activate Fen-1 and XPF.
• PCNA can function as an essential catalytic cofactor, a novel role beyond its organizing function.
• This cofactor activity provides a mechanism for precise control of nuclease activity in specific cellular contexts like replication forks.