Leveraging peptide substrate libraries to design inhibitors of bacterial Lon protease
Lon is a widely-conserved housekeeping protease found in all domains of life. Bacterial Lon is involved in the recovery from various types of stress, including tolerance to fluoroquinolone antibiotics, and is linked to pathogenesis in a number of organisms. However, detailed functional studies of Lon have been limited by the lack of selective, cell-permeable inhibitors. Here we describe the use of positional scanning libraries of hybrid peptide substrates to profile the primary sequence specificity of bacterial Lon. In addition to identifying optimal natural amino acid binding preferences, we identified several non-natural residues that were leveraged to develop optimal peptide substrates as well as a potent peptidic boronic acid inhibitor of Lon. Treatment of E. coli with this inhibitor promotes UV-induced filamentation and reduces tolerance to ciprofloxacin, phenocopying established lon-deletion phenotypes. It is also non-toxic to mammalian cells due to its increased selectivity for Lon over the proteasome. Our results provide new insight into the primary substrate specificity of Lon and identify substrates and an inhibitor that will serve as useful tools for dissecting the diverse cellular functions of Lon.