O-Acetylation of Cell Wall Glycans in Gram-Positive Bacteria
The cell wall of Gram-positive bacteria is vital for maintaining cell shape, integrity, and virulence. It is composed of a matrix of peptidoglycan (PG) and secondary cell wall polysaccharides (SCWP). These polymers are often modified through the process of O-acetylation, which mediates protein-carbohydrate interactions at the cell surface, and is responsible for pathogenicity. Little was known about the biochemistry of cell wall O-acetylation, and how these modified polymers affect protein-carbohydrate interactions. Hence, the primary purpose of this study was to examine the structure-function relationships of putative cell wall O-acetyltransferases and cell wall binding proteins. The crystal structures of polysaccharide O-acetyltransferase B and PG O-acetyltransferase A from Bacillus cereus and Streptococcus pneumoniae will be presented. A model for polysaccharide O-acetylation is proposed based on enzymatic analyses and structure guided mutagenesis of each enzyme. Additionally, a structure of a cell wall binding domain (Surface-layer homology domain) in complex with a modified SCWP unit from Bacillus anthracis will also be presented. This structure provides insight into why modified SCWP are essential for the assembly of proteinaceous surface layers. The significance of this work is underpinned by the ongoing antimicrobial resistance crisis, as the findings described could guide the development of new strategies to overcome this major healthcare problem.