The 2.15 angstrom crystal structure of CG-16, the developmentally regulated homodimeric chicken galectin

Differential developmental regulation of expression, fine-specificity diffe rences in ligand recognition and disparate capacity for homodimerization ar e characteristics of the two currently known proto-type chicken galectins. The X-ray crystal structure of the first avian galectin, the homodimeric ag glutinin from chicken liver (CG-16), has been solved in the absence of liga nd in two crystal forms. Although the arrangement of lectin dimers in the t wo crystals is different, the structure of the monomers and their associati on into the extended beta-sandwich that characterises the dimer are virtual ly identical. The fold establishes a beta-sandwich motif composed of a five -stranded and a six-stranded beta-sheet evocative of proto-type mammalian g alectins. The carbohydrate-binding site is occupied by six water molecules that take the place of the sugar in the complex. They help to stabilise in the absence of the ligand the spatial arrangement of the amino acid side-ch ains involved in sugar recognition. Docking of N-acetyllactosamine into the binding site reveals that three of these water molecules, which are in dir ect contact with the protein, occupy positions equivalent to the key sugar hydroxyl groups, namely the hydroxyls at positions 4 and 6 of the galactose unit and at position 3 of the N-acetylglucosamine unit. Crystallographic d ata are fully consistent with the binding features in solution previously d erived from chemical mapping with deoxy, fluoro and O-methyl derivatives an d laser photo-CIDNP (chemically induced dynamic nuclear polarisation) studi es. The possible molecular basis for the monomeric character of the chicken intestinal galectin as well as potential mechanisms of oxidative inactivat ion by disulphide bridging are evaluated on the basis of the given structur al information concerning the CG-16 dimer interface and the cysteine residu es, respectively. (C) 1999 Academic Press.