Proline-rich antimicrobial peptides (PrAMPs) are produced as part of the innate immune response of animals, insects, and plants. The well-characterized mammalian PrAMP bactenecin-5 (Bac5) has been shown to help fight bacterial infection by binding to the bacterial ribosome and inhibiting protein synthesis. In the absence of Bac5-ribosome structures, the binding mode of Bac5 and exact mechanism of action has remained unclear. Here, we present a cryo-electron microscopy structure of Bac5 in complex with the 70S ribosome from the Gram-negative marine bacterium Vibrio natriegens. The structure shows that, despite sequence similarity to Bac7 and other type I PrAMPs, Bac5 displays a completely distinct mode of interaction with the ribosomal exit tunnel. Bac5 overlaps with the binding site of both A- and P-site transfer RNAs bound at the peptidyltransferase center, suggesting that this type I PrAMP can interfere with late stages of translation initiation as well as early stages of elongation. Collectively, our study presents a ribosome structure from V. natriegens, a fast-growing bacterium that has interesting biotechnological and synthetic biology applications, as well as providing additional insights into the diverse binding modes that type I PrAMPs can utilize to inhibit protein synthesis.
