Identification of a Second, Non-conserved Amino Acid That Contributes to the Unique Sugar Binding Properties of the Nematode Galectin LEC-1

Abstract

The basic disaccharide structure recognized by galectin family members is the lactosamine-like structure Galβ1-4(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 (GalNAcα1-3(Fucα1-2)Galβ1-3GlcNAc), 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 β-strand S3, next to the three β-strands S4—S6, where the conserved amino acids form the binding site for the basic Galβ1-4(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 β-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 β-strands S4—S6, contribute to the unique sugar binding specificity of individual galectins.