Discovery of SHANK1-PDZ Peptide-Fragment Inhibitors Using a Dynamic Ligation Screening Strategy

The development of ligands that modulate protein–protein interactions (PPIs) remains an ongoing challenge in chemical biology and drug discovery. While several approaches have been elaborated to target α-helix-mediated PPIs, methods for β-strand-mediated PPIs are less well developed. In addition to the shallow and extended interfaces characteristic of PPIs, β-strand-mediated PPIs exhibit topographical complexity, with side chains oriented above and below the plane of the strand, alongside hydrogen-bond donor and acceptor groups oriented perpendicular to the side chains. One class of β-strand-mediated PPIs involves the structurally conserved PDZ domains, which recognize protein partners through a β-strand containing a short consensus motif; canonical PDZ binding motifs (PBMs) recognize their substrates through a C-terminal carboxylate, offering a particularly challenging motif to mimic. Peptides and peptidomimetics represent a promising template for the design of ligands that target β-strand-mediated PPIs. In this work, we replaced segments of a peptide-based template using target/structure-agnostic fragments to achieve β-strand mimicry. Using reversible hydrazone exchange reactions allowed us to identify fragments at both the C- and N-terminus of an internal PDZ recognition motif with affinity for the SHANK1-PDZ domain. When combined into ligands bearing two different fragments, negative co-operativity was observed. In addition to broadening the acylhydrazone-fragment approach to screen for PDZ-binding ligands, this workflow for successive screening and combination of fragments should have broader applicability to other targets in future.