Structures of monomeric, dimeric and trimeric PCNA: PCNA-ring assembly and opening

Area of Science:

• Biochemistry
• Structural Biology
• Molecular Biology

Background:

• DNA sliding clamps are crucial protein rings that encircle DNA, acting as platforms for DNA-processing enzymes.
• The dynamic process of clamp ring opening and closing is fundamental for their function but remains incompletely understood at a molecular level.

Purpose of the Study:

• To investigate the molecular mechanisms underlying the assembly and opening of sliding clamp rings.
• To elucidate the structural basis for the dynamic conformational changes of PCNA homologues.

Main Methods:

• X-ray crystallography was employed to determine the structures of distinct oligomeric states (monomeric, dimeric, trimeric) of three PCNA homologues from Sulfolobus solfataricus.
• Ultracentrifugation analysis was used to validate the oligomeric states and conformational changes in solution.

Main Results:

• The study revealed distinct structures of monomeric, dimeric (V-shaped, 130° opening), and trimeric (ring formation, 120° rotation) PCNA assemblies.
• The association of a rigid monomer with a dimer induced ring closure and introduced tension, potentially facilitating subsequent opening upon dissociation.

Conclusions:

• A molecular model for sliding clamp ring assembly and opening is proposed, based on the observed structural transitions.
• The findings provide insights into the dynamic conformational changes that enable clamp loading and unloading during DNA replication.