Structural Biology

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.