Anomer-Specific Recognition and Dynamics in a Fucose-Binding Lectin

Sugar binding by a cell surface similar to 29 kDa lectin (RSL) from the bacterium Ralstonia solanacearum was characterized by NMR spectroscopy. The complexes formed with four monosaccharides and four fucosides were studied. Complete resonance assignments and backbone dynamics were determined for RSL in the sugar-free form and when bound to L-fucose or D-mannose. RSL was found to interact with both the alpha- and the beta-anomer of L-fucose and the "fucose like" sugars D-arabinose and L-galactose. Peak splitting was observed for some resonances of the binding site residues. The assignment of the split signals to the alpha- or beta-anomer was confirmed by comparison with the spectra of RSL bound to methyl-alpha-L-fucoside or methyl-beta-L-fucoside. The backbone dynamics of RSL were sensitive to the presence of ligand, with the protein adopting a more compact structure upon binding to L-fucose. Taking advantage of tryptophan residues in the binding sites, we show that the indole resonance is an excellent reporter on ligand binding. Each sugar resulted in a distinct signature of chemical shift perturbations, suggesting that tryptophan signals are a sufficient probe of sugar binding.