Acinetobacter baumannii is a high-risk pathogen due to the rapid global spread of multi-drug resistant lineages. It is phylogenetically divergent from other Gram-negative bacteria meaning that determinants of its antimicrobial resistance can be difficult to extrapolate from other widely studied bacteria. A recent study showed that A. baumannii upregulates the production of an outer-membrane lipoprotein, which we designate BonA, in response to challenge with polymyxins. In this work, we show that BonA has limited sequence similarity and distinct structural features compared to lipoproteins from other bacterial species. We use X-ray crystallography, small-angle X-ray scattering, electron microscopy, and multiangle light scattering to demonstrate that BonA has a dual BON-domain architecture and forms a decamer via an unusual oligomerization mechanism. This analysis also indicates this decamer is transient, suggesting dynamic oligomerization plays a role in BonA function. Antisera recognizing BonA shows it is an outer membrane protein localized to the divisome. Loss of BonA modulates the density of the outer membrane, consistent with a change in its structure or link to the peptidoglycan, and prevents motility in a clinical strain (ATCC 17978). Consistent with these findings, the dimensions of the BonA decamer are sufficient to permeate the peptidoglycan layer. We hypothesize that BonA forms a membrane-spanning complex during cell division, and may stabilize the outer envelope through interaction with the peptidoglycan and other membrane proteins.
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