PatB1 is an O-acetyltransferase that decorates secondary cell wall polysaccharides

O-Acetylation of the secondary cell wall polysaccharides (SCWP) of the Bacillus cereus group of pathogens, which includes Bacillus anthracis, is essential for the proper attachment of surface-layer (S-layer) proteins to their cell walls. Using a variety of pseudosubstrates and a chemically synthesized analog of SCWP, we report here the identification of PatB1 as a SCWP O-acetyltransferase in Bacillus cereus. Additionally, we report the crystal structure of PatB1, which provides detailed insights into the mechanism of this enzyme and defines a novel subfamily of the SGNH family of esterases and lipases. We propose a model for the O-acetylation of SCWP requiring the translocation of acetyl groups from a cytoplasmic source across the plasma membrane by PatA1 and PatA2 for their transfer to SCWP by PatB1.