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 family requires the precise definition of their carbohydrate specificities. Thus, the binding properties of chicken liver galectin (CG-16) to glycoproteins (gps) and Streptococcus pneumoniae type 14 polysaccharide were studied by the biotin/avidin-mediated microtitre-plate lectin-binding assay and by the inhibition of lectin-glycan interactions with sugar ligands. Among 33 glycans tested for lectin binding, CG-16 reacted best with human blood group ABO (H) precursor gps and their equivalent gps, which contain a high density of D- galactopyranose(β1-4)2-acetamido-2-deoxy-D-glucopyranose [Gal(β1-4)GlcNAc] and Gal(β1-3)GlcNAc residues at the non-reducing end, but this lectin reacted weakly or not at all with A-, H-type and sialylated gps. Among the oligosaccharides tested by the inhibition assay, the tri-antennary Gal(β1-4)GlcNAc (Tri-II) was the best. It was 2.1 × 10³ nM and 3.0 times more potent than Gal and Gal(β1-4)GlcNAc (II)/Gal(β1-3) GlcNAc(β1-3)Gal(β1-4)Glc (lacto-N-tetraose) respectively. CG-16 has a preference for the β-anomer of Gal at the non-reducing end of oligosaccharides with a Gal(β1-4) linkage > Gal(β1-3) ≥ Gal(β1-6). From the results, it can be concluded that the combining site of this agglutinin should be a cavity type, and that a hydrophobic interaction in the vicinity of the binding site for sugar accommodation increases the affinity. The binding site of CG-16 is as large as a tetrasaccharide of the β-anomer of Gal, and is most complementary to lacto-N-tetraose and Gal(β1-4)GlcNAc related sequences.