BonA fromAcinetobacter baumanniiforms a divisome-localized decamer that supports outer envelope function

Abstract

Acinetobacter baumannii is a high-risk pathogen due to the rapid global spread of multi-drug resistant lineages. Its phylogenetic divergence from other ESKAPE pathogens means that determinants of its antimicrobial resistance can be difficult to extrapolate from other widely studied bacteria. A recent study showed that A. baumannii upregulates production of an outer-membrane lipoprotein, which we designate BonA, in response to challenge with polymyxins. Here we show that BonA has limited sequence similarity and distinct structural features compared to lipoproteins from other bacterial species. Analyses through X-ray crystallography, small-angle X-ray scattering, electron microscopy, and multiangle light scattering 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, with the potential to form a membrane-spanning complex during cell division.

Modeling of protein complexes in CASP14 with emphasis on the interaction interface prediction

“…Interestingly, when during the post‐CASP analysis we repeated the docking experiment using the same monomer model, but without the N‐terminal helix, the docking was highly successful [Figure 7(C)]. Of note, in solution this protein exists as a mixture of a monomer and a decamer, and the dimer observed in the crystal might represent an intermediate state in decamerization 43 …”

Target highlights in CASP14: Analysis of models by structure providers

“…BonA is tethered to the membrane by an N‐terminal cysteine‐linked acyl chain, which is connected to the first BON domain by a 27 amino acid linker. BonA forms a decamer, composed of a pentamer of dimers, with the 27 amino acid N‐terminal linker playing an important but undefined role in decamer formation 73 …”

“…However, protein structural information can provide insight into cryptic actives sites, not easily discernible from analysis of amino acid sequence alone. Additionally, the initial purification and analysis of BonA showed that it forms a decamer 73 . Other BON domain‐containing proteins had not previously been shown to oligomerize, so it was unclear how the decamer of BonA formed.…”

“…BonA is an outer‐membrane lipoprotein from the opportunistic pathogen A. baumannii that is important for maintaining the structure and function of the outer membrane 73 . In A. baumannii , the loss of BonA causes the loss of cell motility and a change in the structure of the outer membrane 73 .…”

Modeling of protein complexes in CASP14 with emphasis on the interaction interface prediction

“…Of note, the structure and the oligomeric state of this protein are dynamic. It exists as a monomer, a dimer or a decamer in solution, and the dimer observed in the crystal might represent an intermediate state in decamerization 42 .…”