Identification of critical residues of O-antigen-modifying O-acetyltransferase B (OacB) of Shigella flexneri

Shigellosis is an acute gastrointestinal disease caused primarily by the bacterium Shigella flexneri. Upon ingestion, S. flexneri initiates a serotype-specific immune response that targets the O-antigen of the pathogen's lipopolysaccharide. O-antigen subunits are modified by the addition of chemical moieties, which give rise to new serotypes of S. flexneri. Nineteen different serotypes of S. flexneri have been recognized. A recently identified O-antigen-modifying enzyme, O-acetyltransferase B (OacB), which adds an acetyl residue at either position 3 or 4 of RhamnoseIII (3/4-O-acetylation) in serotypes 1a, 1b, 2a, 5a, 7a, Y, and 6 and position 6 of N- acetylglucosamine (6-O-acetylation) in serotypes 2a, 3a, Y and Yv of the O-antigen subunits. Critical residues in other proteins involved in O-antigen modifications such as glucosyltransferases (Gtrs) and acetyltransferase (Oac) of S. flexneri have been identified, whereas identification of important amino acids in OacB function is yet to be determined.

In the absence of a three-dimensional structure of the serotype converting enzyme, O-acetyltransferase B (OacB), a clear role of important residues in the mechanism of action is precluded. Therefore, in this study, using site-directed mutagenesis, seven residues critical to the function of OacB were identified. The lack of agglutination of cell expressing mutant OacB in the presence of the antiserum indicated the functional role of the corresponding residues. Hence, this study provides significant information about key residues in OacB which might be involved in forming the catalytic sites of this O-antigen modifying enzyme of S. flexneri.

Hydrophobicity analysis showed that OacB is a transmembrane protein with 11 transmembrane segments, 12 loops, and periplasmic N- and cytoplasmic C- termini. Bioinformatics analyses revealed that OacB contains acetyltransferase-3 domain and several conserved residues. Using site-directed mutagenesis, selected amino acids were mutated to alanine to elucidate their role in the mechanism of action of OacB. Seven amino acids R47, H58, F98, W71, R116, R119, and S146 were found critical for the OacB function.