Structure and Function of the PiuA and PirA Siderophore-Drug Receptors from Pseudomonas aeruginosa and Acinetobacter baumannii.

The outer membrane of Gram-negative bacteria presents an efficient barrier to the permeation of antimicrobial molecules. One strategy pursued to circumvent this obstacle is to hijack transport systems for essential nutrients, such as iron. BAL30072 and MC-1 are two monobactams conjugated to a dihydroxypyridone siderophore that are active against and Here, we investigated the mechanism of action of these molecules in We identified two novel TonB-dependent receptors, termed -PiuA and -PirA, that are required for the antimicrobial activity of both agents. Deletion of either or in resulted in 4- to 8-fold-decreased susceptibility, while their overexpression in the heterologous host increased susceptibility to the two siderophore-drug conjugates by 4- to 32-fold. The crystal structures of PiuA and PirA from and their orthologues from were determined. The structures revealed similar architectures; however, structural differences between PirA and PiuA point to potential differences between their cognate siderophore ligands. Spontaneous mutants, selected upon exposure to BAL30072, harbored frameshift mutations in either the ExbD3 or the TonB3 protein of , forming the cytoplasmic-membrane complex providing the energy for the siderophore translocation process. The results of this study provide insight for the rational design of novel siderophore-drug conjugates against problematic Gram-negative pathogens.