The O-antigens of S. flexneri undergo certain modifications through the addition of glucosyl, O-acetyl and the recently discovered phosphoethanolamine (PEtN) residues. These modifications cause serotype conversion in S. flexneri,giving rise to 19 different serotypes. This serotype conversion present in S. flexneri has made the immune response generated against shigellosis to become extremely serotype specific and the sheer number of serotypes has caused the production of a vaccine against any Shigella species to be a challenge.

The PEtN residues are transferred to the O-antigen through the use of a protein in the inner-membrane of S. flexneri called the phosphoethanolamine transferase (Opt). Opt has been identified to contain 506 amino acids and using topology prediction software such as DAS, HMMTOP, SOSUI, Predict Protein, PSORTb, TMHMM, TMPred and TOpPred II, we have produced a model with both its N-terminus and C-terminus situated in the cytoplasmic region and four transmembrane regions.

In order to confirm the accuracy of this computer generated topology model of Opt, we have produced functional Opt mutants where any available cysteine amino acids are replaced by either alanine or serine in order to perform a substituted cysteine accessibility method (SCAM) analysis.

Also, through the use of nucleotide and protein databases, we were able to identify homologous and conserved regions with the Opt and we wish to carry out site directed mutagenesis is currently being carried out to identify residues within these regions important for the structure and function of Opt.