Unlocking the PIP-box: A peptide library reveals interactions that drive high-affinity binding to human PCNA

Area of Science:

• Molecular Biology
• Biochemistry
• Structural Biology

Background:

• Proliferating Cell Nuclear Antigen (hPCNA) is a crucial protein involved in DNA replication and repair.
• hPCNA interacts with over 200 proteins via a conserved motif known as the PIP-box.
• The precise mechanisms by which PIP-box variations modulate hPCNA binding affinity and downstream cellular processes remain unclear.

Purpose of the Study:

• To systematically investigate how modifications at each position within the PIP-box affect hPCNA binding affinity.
• To understand the structural basis for varying affinities between hPCNA and its interacting peptides.
• To identify sequence-specific features of the PIP-box that contribute to high-affinity hPCNA binding.

Main Methods:

• Synthesis of 27 peptides derived from p21 with PIP-box modifications and 19 peptides with PIP-box motifs from other proteins.
• Characterization of hPCNA-binding affinities using surface plasmon resonance (SPR) to determine K_D values.
• Determination of hPCNA-bound peptide structures via X-ray crystallography and computational modeling.

Main Results:

• hPCNA-binding affinities of the synthesized peptides spanned a 4000-fold range (1.83 nM to 7.59 μM).
• Structural analyses revealed specific intermolecular and intramolecular interactions correlating with high hPCNA affinity.
• Rational design based on these findings yielded a novel peptide with the highest affinity hPCNA-binding partner to date (K_D = 1.12 nM).

Conclusions:

• Sequence-specific variations within the PIP-box are critical for tuning hPCNA binding affinity.
• These findings enhance our understanding of how hPCNA affinity is regulated for DNA replication and repair.
• The study provides valuable insights for the rational design of future hPCNA inhibitors.