Carbohydrate specificity of a galectin from chicken liver (CG-16)

Owing to the expression of more than one type of galectin in animal tissues , the delineation of the functions of individual members of this lectin fam ily requires the precise definition of their carbohydrate specificities. Th us, the binding properties of chicken liver galectin (CG-16) to glycoprotei ns (gps) and Streptococcus pneumoniae type 14 polysaccharide were studied b y the biotin/avidin-mediated micro titre-plate lectin-binding assay and by the inhibition of lectin-glycan interactions with sugar ligands. Among 33 g lycans tested for lectin binding, CG-16 reacted best with human blood group ABO (H) precursor gps and their equivalent gps, which contain a high densi ty of D-galactopyranose(beta1-4)2-acetamido-2-deoxy-D-glucopyranose [Gal(be ta1-4)GlcNAc] and Gal(beta1-3)GlcNAc residues at the nonreducing end, but t his lectin reacted weakly or not at all with A-, H-type and sialylated gps. Among the oligosaccharides tested by the inhibition assay, the tri-antenna ry Gal(beta1-4)GlcNAc (Tri-II) was the best. It was 2.1 x 10(3) nM and 3.0 times more potent than Gal and Gal(beta1-4)GlcNAc (II)/Gal(beta1-3) GlcNAc( beta1-3)Gal(beta1-4)Glc (lacto-N-tetraose) respectively. CG-16 has a prefer ence for the beta -anomer of Gal at the nonreducing end of oligosaccharides with a Gal(beta1-4) linkage > Gal(beta1-3) greater than or equal to Gal(be ta1-6). From the results, it can be concluded that the combining site of th is agglutinin should be a cavity type, and that a hydrophobic interaction i n the vicinity of the binding site for sugar accommodation increases the af finity. The binding site of CG-16 is as large as a tetrasaccharide of the b eta -anomer of Gal, and is most complementary to lacto-N-tetraose and Gal(b eta1-4)GlcNAc related sequences.