Identification of a second, non-conserved amino acid that contributes to the unique sugar binding properties of the nematode galectin LEC-1.

The basic disaccharide structure recognized by galectin family members is the lactosamine-like structure Galbeta1-4(3)Glc(NAc). The 32-kDa galectin LEC-1 of the nematode Caenorhabditis elegans is composed of two domains, each of which is homologous to vertebrate 14-kDa-type galectins. The N-terminal lectin domain of LEC-1 recognizes blood group A saccharide (GalNAcalpha1-3(Fucalpha1-2)Galbeta1-3GlcNAc), whereas this saccharide is poorly recognized by the C-terminal domain. Using a combination of site-directed mutagenesis and analysis of the sugar-binding profile by frontal affinity chromatography, we previously found that Thr41 in the N-terminal lectin domain of LEC-1 is important for its affinity for A-hexasaccharide. Thr41 is located on beta-strand S3, next to the three beta-strands S4-S6, where the conserved amino acids form the binding site for the basic Galbeta1-4(3)Glc(NAc) structure. Here, we report that a second amino acid, Asp133, in the N-terminal lectin domain of LEC-1, located on the beta-strand S2 adjacent to that containing Thr41, is important for LEC-1-specific recognition of A-hexasaccharide. These results suggest that amino acid residues other than those located on the three beta-strands S4-S6, contribute to the unique sugar binding specificity of individual galectins.