The Gram-negative bacterium Shigella flexneri (S. flexneri) causes bacillary dysentery mainly in developing countries. Its pathogenesis is related to production of the Shiga toxin. Live attenuated strains as oral vaccines showed limited success rates and the best protection against shigellosis remains wild type infection. Here, strong reactions against O‑antigen were observed, stressing its important role for immunization. The S. flexneri O‑antigen has a backbone with high (75 %) rhamnose content and a growing body of evidence suggests that this specific rhamnose composition creates a polymer with highly dynamic glycosidic torsion angles. Several serotypes of this O‑antigen exist that mainly vary in the substitutions of the backbone with acetylations or glucosylations and virulence of S. flexneri is intimately linked to its specific and dynamic O‑antigen composition.
We analyzed complex formation of the tailspike protein (TSP) of S. flexneri phage Sf6 with octasaccharides of the basic unbranched S. flexneri serotype Y O‑antigen. Crystal structure analysis had previously shown an intersubunit carbohydrate binding groove in the homotrimeric Sf6TSP (1). Surface immobilized Sf6TSP bound octasaccharides with very low affinities as analyzed by surface plasmon resonance; MD simulations of octasaccharides in the binding site showed a stable complex. The low affinity of the Sf6TSP system is reasonable given the small number of protein-carbohydrate contacts to serotype Y oligosaccharides. By contrast, Sf6TSP rapidly cleaves long polysaccharides into small oligosaccharide fragments. From these results we conclude that Sf6TSP must recognize specifically the large conformational epitopes formed by the long polysaccharide chains on S. flexneri.
1. Muller, J. J., Barbirz, S., Heinle, K., Freiberg, A., Seckler, R., and Heinemann, U. (2008) An intersubunit active site between supercoiled parallel beta helices in the trimeric tailspike endorhamnosidase of Shigella flexneri Phage Sf6. Structure 16, 766-775
