Diverse functions for acyltransferase-3 proteins in the modification of bacterial cell surfaces

Abstract

The acylation of sugars, most commonly via acetylation, is a widely used mechanism in bacteria that uses a simple chemical modification to confer useful traits. For structures like lipopolysaccharide, capsule and peptidoglycan, that function outside of the cytoplasm, their acylation during export or post-synthesis requires transport of an activated acyl group across the membrane. In bacteria this function is most commonly linked to a family of integral membrane proteins – acyltransferase-3 (AT3). Numerous studies examining production of diverse extracytoplasmic sugar-containing structures have identified roles for these proteins in O-acylation. Many of the phenotypes conferred by the action of AT3 proteins influence host colonisation and environmental survival, as well as controlling the properties of biotechnologically important polysaccharides and the modification of antibiotics and antitumour drugs by Actinobacteria. Herein we present the first systematic review, to our knowledge, of the functions of bacterial AT3 proteins, revealing an important protein family involved in a plethora of systems of importance to bacterial function that is still relatively poorly understood at the mechanistic level. By defining and comparing this set of functions we draw out common themes in the structure and mechanism of this fascinating family of membrane-bound enzymes, which, due to their role in host colonisation in many pathogens, could offer novel targets for the development of antimicrobials.

A novel fold for acyltransferase-3 (AT3) proteins provides a framework for transmembrane acyl-group transfer

“…In addition, the core of the SGNH domains are also similar (79) with variation occurring in the length of the linking regions that connect the AT3 to the SGNH. While the PglI-NG protein closely resembles OafB with a short, structured linker (1), OatA proteins show a more extended, and potentially more mobile and flexible, structure, locating the SGNH domain further from the AT3 domain ( Fig. 2A ).…”

“…The realisation that diverse bacteria use AT3 proteins in a plethora of different biological processes for the modification of extracytoplasmically localised glycans, directed our efforts into describing the fundamental mechanism of these proteins (1). While the role of AT3 proteins in these diverse processes is often implicated from genetics only, the OafB protein has been studied in much more detail, along with other selected AT3 proteins including the OatA protein from S. aureus (10,57,58).…”

“…Furthermore, standalone AT3 proteins contribute an acyl group in the biosynthesis of macrolide antibiotics in Streptomyces species which increases antibiotic efficacy (5,(53)(54)(55). These selected examples (see (1) for a recent review) illustrate that AT3 domain-containing proteins are involved in a range of highly relevant processes for bacterial pathogens and may be relevant for biotechnological applications.…”

“…Acyltransferase family 3 (Acyl_transf_3, AT3) domain (Interpro: IPR002656, PFAM: PF01757) containing proteins are found in all domains of life. These membrane-bound AT3 proteins are involved in acylation of a wide range of extracytoplasmic and surface bacterial polysaccharides (1), but are also important in Eukarya, for example in the regulation of lifespan in Caenorhabditis elegans (2) and in Drosophila development (3). In bacteria, where these proteins have been primarily studied, the resulting acylations have been shown to be involved in root nodulation (4), increase the efficacy of macrolide antibiotics (5), conferring resistance to lysozyme (6), influencing bacteriophage sensitivity (7), and altering antibody recognition (8)(9)(10).…”

Microbial Musings – Spring 2022

“…Modification of LPS through O-acetylation is another chemical alteration used by bacteria to change their cell surface and is one of many polysaccharides modified by a family of membrane-bound acyltransferases of the acyltransferase-3 (AT3) family [13]. The functions of these proteins in modifying LPS, peptidoglycan and other polysaccharides, as well as secreted chemicals and also pilin proteins, is reviewed by Sarah Tindall (@sarahsci28), Caroline Pearson (@CarolineRosePea), Marjan van der Woude and myself (@GavinHThomas), all based at the Unversity of York, UK, in an article where we tried to bring together for the first time the diverse literature on cell surface glycans that are modified by AT3 proteins [14]. While many catalyze O-acetylation of sugars, we have also identified an AT3 protein, IcaC, that we think catalyzes the O-succinylation of poly- N -acetylglucosamine (PNAG), an important biofilm-forming component in Staphylococcus aureus and Staphylococcus epidermidis [15].…”