How do bacteria change their coats? Structural analysis of acyltransferases involved in O-antigen modification

Acyltransferase-3 (AT3) domain-containing proteins are involved in acylation of a diverse range of carbohydrates across all domains of life. In bacteria they are essential in processes including symbiosis, antimicrobial resistance, and antibiotic biosynthesis. Despite this, the mechanism of action is largely unknown. AT3 proteins fromSalmonellaspp. are responsible for acetylation of lipopolysaccharides which can generate a specific immune response upon infection. Here we analysed two AT3 proteins fromSalmonellaspp., some differences exist but both contain an integral membrane AT3 domain fused to a periplasmic SGNH domain. Identification of essential residues from each domain suggests both domains are required for acylation.

The crystal structure of the SGNH domain and periplasmic linking region was determined. Novel structural features are seen in comparison to other SGNH domains. In particular, the periplasmic linking region is structured and forms an extension of the SGNH domain (SGNH-extension). Removal of the SGNH-extension suggests that this region is important for stability of the SGNH domain. The structure of the SGNH-extension suggests the SGNH domain is in close proximity to the acyltransferase domain and the domains may interact. In silicoco-evolution analysis, used to make predictions about the structure of both domains, suggests likely inter-domain interactions. This analysis also predicted which transmembrane helices in the acyltransferase domain interact giving an insight into the overall structure.

Combining these data we propose a refined model of AT3-SGNH proteins which enhances our understanding of the mechanism and function of AT3 proteins required for modification of cell-surface carbohydrates.