Unlocking the PIP-box: A peptide library reveals interactions that drive high-affinity binding to human PCNA
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Updated: Nov 5, 2025
Semi-automated Biopanning of Bacterial Display Libraries for Peptide Affinity Reagent Discovery and Analysis of Resulting Isolates
Published on: December 6, 2017
Aimee J Horsfall1, Beth A Vandborg2, Wioleta Kowalczyk3
1ARC Centre of Excellence for Nanoscale BioPhotonics, Institute of Photonics and Advanced Sensing, School of Physical Sciences, The University of Adelaide, Adelaide, South Australia, Australia.
Abstract:
The human sliding clamp, Proliferating Cell Nuclear Antigen (hPCNA), interacts with over 200 proteins through a conserved binding motif, the PIP-box, to orchestrate DNA replication and repair. It is not clear how changes to the features of a PIP-box modulate protein binding and thus how they fine-tune downstream processes. Here, we present a systematic study of each position within the PIP-box to reveal how hPCNA-interacting peptides bind with drastically varied affinities. We synthesized a series of 27 peptides derived from the native protein p21 with small PIP-box modifications and another series of 19 peptides containing PIP-box binding motifs from other proteins. The hPCNA-binding affinity of all peptides, characterized as KD values determined by surface plasmon resonance, spanned a 4000-fold range, from 1.83 nM to 7.59 μM. The hPCNA-bound peptide structures determined by X-ray crystallography and modeled computationally revealed intermolecular and intramolecular interaction networks that correlate with high hPCNA affinity. These data informed rational design of three new PIP-box sequences, testing of which revealed the highest affinity hPCNA-binding partner to date, with a KD value of 1.12 nM, from a peptide with PIP-box QTRITEYF. This work showcases the sequence-specific nuances within the PIP-box that are responsible for high-affinity hPCNA binding, which underpins our understanding of how nature tunes hPCNA affinity to regulate DNA replication and repair processes. In addition, these insights will be useful to future design of hPCNA inhibitors.
Insights
Understanding how the Proliferating Cell Nuclear Antigen (hPCNA) binding motif, the PIP-box, affects protein interactions is key for DNA replication and repair. This study reveals sequence-specific nuances in the PIP-box that fine-tune hPCNA binding affinity.
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 KD 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 (KD = 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.
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