Comparative study of the glycan specificities of cell-bound human tandem-repeat-type galectin-4, -8 and -9.

Adhesion/growth-regulatory galectins (gals) exert their functionality by the cis/trans-cross-linking of distinct glycans after initial one-point binding. In order to define the specificity of ensuing association events leading to cross-linking, we recently established a cell-based assay using fluorescent glycoconjugates as flow cytometry probes and tested it on two human gals (gal-1 and -3). Here we present a systematic study of tandem-repeat-type gal-4, -8 and -9 loaded on Raji cells resulting in the following key insights: (i) all three gals bound to oligolactosamines; (ii) binding to ligands with Galβ1-3GlcNAc or Galβ1-3GalNAc as basic motifs was commonly better than that to canonical Galβ1-4GlcNAc; (iii) all three gals bound to 3'-O-sulfated and 3'-sialylated disaccharides mentioned above better than that to parental neutral forms and (iv) histo-blood group ABH antigens were the highest affinity ligands in both the cell and the solid-phase assay. Fine specificity differences were revealed as follows: (i) gal-8 and -9, but not gal-4, bound to disaccharide Galβ1-3GlcNAc; (ii) increase in binding due to negatively charged substituents was marked only in the case of gal-4 and (iii) gal-4 and -8 bound preferably to histo-blood group A glycans, whereas gal-9 targeted B-type glycans. Experiments with single carbohydrate recognition domains (CRDs) of gal-4 showed that the C-CRD preferably bound to ABH glycans, whereas the N-CRD associated with oligolactosamines. In summary, the comparative analysis disclosed the characteristic profiles of glycan reactivity for the accessible CRD of cell-bound gals. These results indicate the distinct sets of functionality for these three members of the same subgroup of human gals.