Identification of active site residues in the Shigella flexneri glucosyltransferase GtrV

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

Background

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 Rhamnose^III (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.

Results

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.

Conclusion

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.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12860-022-00415-8.

Topological investigation of glucosyltransferase V in Shigella flexneri using the substituted cysteine accessibility method

Modification of the lipopolysaccharide O-antigen of Shigella converts the serotype, which is significant as acquired immune responses are serotype specific. Glucosyltransferases (Gtrs) modify the O-antigen by the addition of glucosyl-groups; however the precise mechanism of O-antigen modification is not fully understood. This study aims to substantiate inferences made on the GtrV topological structure using the substituted cysteine accessibility method (SCAM). Twenty-one amino acid residues were tested to clarify three features of GtrV: the extramembrane regions, a proposed reentrant loop, and a membrane border region. Overall, the results agreed with a previous topology proposed for GtrV. The topology of GtrV consists of 11 extramembrane regions with a cytoplasmic N-terminus, periplasmic C-terminus and 9 transmembrane (TM) helices. The existence of a reentrant loop between TM helices IV and V was verified, and the cytoplasmic membrane border region of TM helix II was examined in depth.