The Host Antimicrobial Peptide Bac71-35 Binds to Bacterial Ribosomal Proteins and Inhibits Protein Synthesis

Antimicrobial peptides (AMPs) are molecules from innate immunity with high potential as novel anti-infective agents. Most of them inactivate bacteria through pore formation or membrane barrier disruption, but others cross the membrane without damages and act inside the cells, affecting vital processes. However, little is known about their intracellular bacterial targets. Here we report that Bac7(1-35), a proline-rich AMP belonging to the cathelicidin family, can reach high concentrations (up to 340 mu M) inside the E. coli cytoplasm. The peptide specifically and completely inhibits in vitro translation in the micromolar concentration range. Experiments of incorporation of radioactive precursors in macromolecules with E. coli cells confirmed that Bac7(1-35) affects specifically protein synthesis. Ribosome coprecipitation and crosslinking assays showed that the peptide interacts with ribosomes, binding to a limited subset of ribosomal proteins. Overall, these results indicate that the killing mechanism of Bac7(1-35) is based on a specific block of protein synthesis.