Structural basis of preferential binding of fucose-containing saccharide by the Caenorhabditis elegans galectin LEC-6.

Galectins are a group of lectins that can bind carbohydrate chains containing β-galactoside units. LEC-6, a member of galectins of Caenorhabditis elegans, binds fucose-containing saccharides. We solved the crystal structure of LEC-6 in complex with galactose-β1,4-fucose (Galβ1-4Fuc) at 1.5 Å resolution. The overall structure of the protein and the identities of the amino-acid residues binding to the disaccharide are similar to those of other galectins. However, further structural analysis and multiple sequence alignment between LEC-6 and other galectins indicate that a glutamic acid residue (Glu67) is important for the preferential binding between LEC-6 and the fucose moiety of the Galβ1-4Fuc unit. Frontal affinity chromatography analysis indicated that the affinities of E67D and E67A mutants for Galβ1-4Fuc are lower than that of wild-type LEC-6. Furthermore, the affinities of Glu67 mutants for an endogenous oligosaccharide, which contains a Galβ1-4Fuc unit, are drastically reduced relative to that of the wild-type protein. We conclude that the Glu67 in the oligosaccharide-binding site assists the recognition of the fucose moiety by LEC-6.