Departments of Molecular and Medical Genetics and Biochemistry, University of Toronto, Toronto, Ontario, M5S 1A8, Canada.
Galectins are a family of lectins which share similar carbohydrate recognition domains (CRDs) and affinity for small beta-galactosides, but which show significant differences in binding specificity for more complex glycoconjugates. We report here the x-ray crystal structure of the human galectin-3 CRD, in complex with lactose and N-acetyllactosamine, at 2.1-A resolution. This structure represents the first example of a CRD determined from a galectin which does not show the canonical 2-fold symmetric dimer organization. Comparison with the published structures of galectins-1 and -2 provides an explanation for the differences in carbohydrate-binding specificity shown by galectin-3, and for the fact that it fails to form dimers by analogous CRD-CRD interactions.