Sortase A-mediated synthesis of ligand-grafted cyclized peptides for modulating a model protein-protein interaction

Specific ligand-grafted cyclic peptides are promising drug candidates that can modulate protein-protein interactions (PPIs) with increased proteolytic stability. In this study, we aimed to demonstrate that Sortase A (SrtA)-mediated peptide transpeptidation can be applied to produce bioactive sequence-grafted, stable, cyclic peptides. A naturally occurring cyclic peptide, sunflower trypsin inhibitor 1 (SFTI-1), was selected as the scaffold, and a tetrapeptide motif, Glu-Ser-Asp-Val (ESDV), was grafted into the scaffold as a model ligand. The linear precursor of the grafted peptide with SrtA-recognition motifs at the N- and C-termini was cyclized in good yield simply by co-incubation with SrtA. The ESDV-grafted cyclic SFTI-1 obtained was confirmed to have high stability against proteolysis by human serum and bound to the target PDZ2 domain of postsynaptic density-95 protein. An optimized sequence-grafted cyclic SFTI-1 could competitively suppress the interaction of PDZ2 with its natural ligand, the C-terminal peptide of the NR2B subunit of the N-methyl-D-aspartate receptor. These results show that a strategy combining peptide grafting into the SFTI-1 scaffold with SrtA-catalyzed cyclization can be a simple and effective method for producing stable peptide drugs.