Crystal structure of the putative carbohydrate recognition domain of human galectin‐related protein

Galectins are b-galactoside binding proteins sharing a conserved carbohydrate recognition domain (CRD).1 Galectin-encoding genes are present in the genomes of a wide variety of eukaryotic organisms ranging from marine sponges up to humans.2 They are however restricted to the animal kingdom and to the subclass of agaricomycetidae in the fungal kingdom. Members of the galectin family can be assigned to one of three subgroups: The prototype subgroup having only one CRD per monomer, the tandem-repeat type having two CRDs and the chimera type which possesses a long peptide extension in addition to its CRD.3 The highly conserved residues responsible for b-galactoside association have been defined based on the X-ray structures of galectin family members or respective CRDs in complex with their carbohydrate ligands.4–9 Residues His49, Asn51, Arg53, Asn62, and Glu72 recognize the b-galactoside ligand via specific hydrogen bonds, while Trp69 confers binding affinity through hydrophobic stacking with the galactose moiety [numbering of human galectin-7 (hGal-7)]. However, within the galectin family, there are a growing number of proteins containing a galectin CRD based on sequence similarity, but which lack some of the strictly conserved amino acids responsible for b-galactoside binding and accordingly do not display significant affinity for b-galactosides. Members of this so-called galectin-related protein (GRP) family are galectin-related interfiber protein (GRIFIN), the mannose-binding Charcot-Leyden crystal protein, Coprinopsis cinerea GRP (CGL3) and human GRP, to name a few. Human GRP contains a short N-terminal segment of unknown function and a putative CRD, which shares consensus amino acids at 51 out of the 64 most highly conserved residues in other galectins2 (see Fig. 1). No ligand has so far been identified. To get insight into the function of this GRP highly conserved among animals, we have determined the three-dimensional structure of the GRP CRD fragment at 2.0 Å resolution.