Abstract:
Bacteriophages use fibrous adhesion organelles for the initial contact to the bacterial host. Many of these tailspike proteins (TSP) recognize glycan structures on the bacterial cell surface with high specificity. In O-antigen specific phages TSP bind and enzymatically cleave the O- polysaccharide part of the lipopolysaccharide (LPS) as an essential step prior to DNA release. The short-tailed podovirus P22 and the long-tailed siphovirus 9NA use the same Salmonella Typhimurium host and have similar TSP. Both phages have adapted their TSP specificity to their life style: P22 is a temperate phage and binds to O-antigen only in the absence of prophage glucosylation. Phage 9NA is lytic and selects host cells according to a different phase variable glucosylation phenotype. These O-antigen modifications are not detected by antibody serotyping and 9NATSP and P22TSP are therefore well suited to monitor O-antigen glucosylation in Salmonella Typhimurium. For a broad application of TSP as glycan binding and diagnosis tools it would be desirable to design their affinities and specificities. However, fundamental mechanisms of protein-glycan binding are still not well understood on a molecular level given that complex formation is the result of a multidimensional process influenced by thermodynamic and kinetic contributions of all individual molecular events. We have explored TSP-oligosaccharide interfaces both of high and low affinity with isothermal titration calorimetry, X-ray crystallography, STD-NMR and molecular dynamics simulations. Our results emphasize that overall dynamics of the solvent shell in the binding site as well as dynamics of the carbohydrate ligand largely govern the recognition process.