STRUCTURE OF S-LECTIN, A DEVELOPMENTALLY-REGULATED VERTEBRATE BETA-GALACTOSIDE-BINDING PROTEIN

The crystal structure of a 14 kDa bovine spleen S-lectin complexed with the disaccharide N acetyllactosamine at 1.9-Angstrom resolution reveals a surprising structural relationship to legume lectins, despite the lack of sequence homology. Two monomers associate to form an extended beta-sandwich, each with the same jelly roll topology typical of legume lectins but with dramatically trimmed loops and with different dimer association. Each monomer binds one N-acetyl lactosamine molecule in a topologically and spatially different site than that of legume lectins. The carbohydrate-binding site provides an unprecedented paradigm for carbohydrate binding, with a unique network of salt bridges. The specificity for beta-galactose arises from intricate interactions that constrain the position of the O4 atom.