Structural Insights into Cir-mediated Killing by the Antimicrobial Protein Microcin V.

Drug-resistant bacterial infections have become a high-priority concern in the global healthcare landscape. Novel treatments for such infections are needed, but can be especially difficult to develop for Gram-negative species owing to the need to traverse the outer membrane to reach targets beneath. A promising solution is found in natural antibiotics such as albomycin, which can bind outer membrane siderophore receptors and co-opt them for import into the periplasm. Exploring this and similar mechanisms may open avenues for antibiotic development. An underappreciated class of natural antibiotics are the microcins, which are small antimicrobial proteins secreted by certain bacterial species as a means of inter-species competition. Once secreted, the microcins bind specific outer-membrane receptors of prey species and cross into the periplasm. Microcins have potent activity, bind specific targets, and have been shown to control pathobiont expansion and pathogen colonization in animal models. One such microcin, MccV, has previously been shown to utilize the colicin Ia receptor, Cir, for periplasmic import. Here, we report the first high-resolution structure of the Cir/MccV complex by Cryo-EM, revealing an interaction centered on an electropositive cavity within a usually occluded binding pocket in Cir. We also report the calculated affinity of MccV for Cir. We mutagenized putative interacting residues at this interface and identified key contacts that are critical to MccV binding and Cir-mediated import and bacteriolysis. Future efforts in this project will help us better understand the mechanisms of microcin killing, and will assess relationships between other microcins and their respective importers with the aim of understanding the potential for microcins to be used as treatment for drug-resistant pathogens.