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

The crystal structure of a 14 kDa bovine spleen S-lectin complexed wit h the disaccharide N acetyllactosamine at 1.9-Angstrom resolution reve als a surprising structural relationship to legume lectins, despite th e lack of sequence homology. Two monomers associate to form an extende d beta-sandwich, each with the same jelly roll topology typical of leg ume lectins but with dramatically trimmed loops and with different dim er association. Each monomer binds one N-acetyl lactosamine molecule i n a topologically and spatially different site than that of legume lec tins. 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.