John B Bruning

The University of Adelaide, Australia

Title: The human sliding clamp as a therapeutic target


John B Bruning received BSc from Texas A&M University in 1997. He began crystallography in the Laboratory of Yousif Shamoo at Rice University. During his
graduate studies he worked on the structural mechanism of the human sliding clamp and its interactions with DNA replication proteins. He received his PhD in 2005
and completed two successful Post-docs; the fi rst was at the Scripps Research Institute from 2005-2007 working on structural studies of nuclear receptors including
PPAR, RXR, ER and TR and his second Post-doc was with Jim Sacchettini in the Houston Medical Centre and as a part of the TB structural genomics consortium.
He received his fi rst faculty position at the University of Adelaide as a Lecturer in 2012. He was tenured in 2015 and promoted as Senior Lecturer in 2016. Due to
his continued collaboration with Scripps Research Institute, he was also appointed as Adjunct Professor of the Scripps Research Institute in 2016.


The human sliding clamp (also known as PCNA) controls access to DNA of many of the proteins involved in essential
processes such as DNA replication, DNA repair and cell cycle control. Proteins compete for interaction with the PCNA
surface by means of a short, conserved peptide sequence known as the PCNA-interacting protein motif (or PIP-box). Binding
to PCNA via the PIP box allows access to DNA. For example, the major replicative polymerase, pol delta, requires PCNA for
processive DNA synthesis, without interaction with PCNA the polymerase dissociates from DNA and is incapable of processive
DNA synthesis. As such, many groups have proposed the usefulness of PIP box mimetics for use as cancer therapeutics given
they would block upregulated PCNA form allowing interaction with pol delta and hence would inhibit DNA replication.
However, no peptide mimetics of PCNA have been forthcoming to date. Here we describe the design and synthesis of the fi rst
PCNA peptidomimetic. Our mimetic, ACR2, was designed through synthetic lactam chemistry to constrain the secondary
structure of the peptide for optimized binding to PCNA. NMR solution studies show that the wild type p21 peptide from which
ACR2 was designed adopts no defi ned secondary structure in solution, while our mimetic adopts a 310 helix in solution, which
has been shown in previous studies to be essential for PIP box binding to PCNA. Binding experiments determined a KD of 200
nM of ACR2 for PCNA, which is higher than the wild type peptide. A co-crystal structure of ACR2 bound to hPCNA revealed
the mechanism of interaction of this mimetic with PCNA.