Carbohydrate specificity and quaternary association in basic winged bean lectin: X-ray analysis of the lectin at 2.5 A resolution.

The structure of basic Winged Bean Agglutinin (WBAI) with two dimeric molecules complexed with methyl-alpha-D-galactopyranoside in the asymmetric unit, has been determined by the molecular replacement method and refined with 2.5 A X-ray intensity data. The polypeptide chain of each monomer has the characteristic legume lectin tertiary fold. The structure clearly defines the lectin-carbohydrate interactions. It reveals how the unusually long variable loop in the binding region endows the lectin with its characteristic sugar specificity. The lectin forms non-canonical dimers of the type found in Erythrina corallodendron lectin (EcorL) even though glycosylation, unlike in EcorL, does not prevent the formation of canonical dimers. The structure thus further demonstrates that the mode of dimerisation of legume lectins is not necessarily determined by the covalently bound carbohydrate but is governed by features intrinsic to the protein. The present analysis and our earlier work on peanut lectin (PNA), show that legume lectins are a family of proteins in which small alterations in essentially the same tertiary structure lead to wide variations in quaternary association. A relationship among the non-canonical modes of dimeric association in legume lectins is presented.