Mechanisms of Action of Ribosome-Binding Antimicrobial Peptides

Today’s urgent need for better antibiotics calls for multifaceted approaches not only to search for new compounds but also to understand their mode of action to develop them as effective inhibitors of microbial growth. As contributors to this task, our lab focuses on deciphering the molecular mechanisms of antimicrobials that target the protein synthesis process in bacteria. Antimicrobial peptides (AMPs) have recently gained attention as promising candidates for new antibiotics. AMPs are produced by organisms of all kingdoms to protect themselves from bacterial infections. AMPs are structurally dissimilar and have a diverse array of bacterial targets. In these studies, we are demonstrating that two different types of AMPs, odilorhabdins (ODLs) and proline-rich antimicrobial peptides (PrAMPs), inhibit protein synthesis by targeting the ribosome. However, their binding sites within the ribosome and their strategies to stop protein synthesis are completely different and novel. The bacterium Xenorhabdus nematophila produces ODLs to kill competing bacteria co-residing in the nematode host. By binding to a unique location of the ribosome, ODLs can affect protein synthesis in two ways: at low concentrations, they cause ribosomes to make mistakes, while high concentrations block the assembly of new proteins. Importantly, the inhibitory action of ODLs effectively eradicates infections caused by drug-resistant bacteria in mice, a property that has advanced ODLs into the antibiotic development pipeline. As part of their immune systems, insects, crustaceans, and mammals produce PrAMPs. A common feature of PrAMPs is that they enter the exit tunnel through which newly made proteins leave the ribosome. Once inside the tunnel, while type I PrAMPs, i.e. Onc112 and Bac7, stop ribosomes from starting to make proteins, type II PrAMP Apidaecin allows assembly of the entire protein but impedes its release from the ribosome. Besides contributing to design of treatments for bacterial infections, the knowledge of the detailed mechanisms of action of AMPs extends beyond their application as pharmaceutical drugs. For example, Onc112 was used as a tool to discover new bacterial genes and we are taking advantage of the inhibitory properties of Apidaecin to develop molecular biology screens and to study translation regulation in a variety of organisms.