Identification of the amino acid residue in the nematode galectin LEC-1 responsible for its unique sugar binding property: Analysis by combination of site-directed mutagenesis and frontal affinity chromatography

The basic disaccharide structure recognized by galectin family members is the lactosamine-like structure Galβ1-4(3)Glc(NAc). In galectins, eight highly conserved amino acid residues participate in the recognition of this basic structure. Each galectin seems to mediate diverse biological functions due to recognition of different modifications of the basic disaccharide Galβ1-4(3)Glc(NAc), but there is very little information about which amino acid residue in galectin is responsible for recognizing these modifications. 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. Although both lectin domains have an affinity for N-acetyllactosamine (Galβ1-4GlcNAc)-containing, N-linked, complex-type sugar chains, the N-terminal lectin domain of LEC-1 recognizes blood group A saccharide (GalNAcα1-3(Fucα1-2)Galβ1-3GlcNAc), whereas this saccharide is only poorly recognized by the C-terminal domain. Here, we used a combination of site-directed mutagenesis of the N-terminal lectin domain of galectin LEC-1 and an analysis of the sugar-binding profile by frontal affinity chromatography to identify the amino acid residues important for this recognition. Our results indicate that Thr⁴¹ in the N-terminal lectin domain of LEC-1 is important for its affinity for A-hexasaccharide.