Carbohydrate binding, quaternary structure and a novel hydrophobic bindingsite in two legume lectin oligomers from Dolichos biflorus

The seed lectin (DBL) from the leguminous plant Dolichos biflorus has a uni que specificity among the members of the legume lectin family because of it s high preference for GalNAc over Gal. In addition, precipitation of blood group A + H substance by DBL is slightly better inhibited by a blood group A trisaccharide (GalNAc(alpha 1-3)[Fuc(alpha 1-2)]Gal) containing pentasacc haride, and about 40 times better by the Forssman disaccharide (GalNAc(alph a 1-3)GalNAc) than by GalNAc. We report the crystal structures of the DBL-b lood group A trisaccharide complex and the DBL-Forssman disaccharide comple x. A comparison with the binding sites of Gal-binding legume lectins indicates that the low affinity of DBL for Gal is due to the substitution of a conse rved aromatic residue by an aliphatic residue (Leu127). Binding studies wit h a Leu127Phe mutant corroborate these conclusions. DBL has a higher affini ty for GalNAc because the N-acetyl group compensates for the loss of aromat ic stacking in DBL by making a hydrogen bond with the backbone amide group of Gly103 and a hydrophobic contact with the side-chains of Trp132 and Tyr1 04. Some legume lectins possess a hydrophobic binding site that binds adenine a nd adenine-derived plant hormones, i.e; cytokinins. The exact function of t his binding site is unknown, but adenine/cytokinin-binding legume lectins m ight be involved in storage of plant hormones or plant growth regulation. T he structures of DBL in complex with adenine and of the dimeric stem and le af lectin (DB58) from the same plant provide the first structural data on t hese binding sites. Both oligomers possess an unusual architecture, featuri ng an ct-helix sandwiched between two monomers. In both oligomers, this a-h elix is directly involved in the formation of the hydrophobic binding site. DB58 adopts a novel quaternary structure, related to the quaternary struct ure of the DBL heterotetramer, and brings the number of know legume lectin dimer types to four. (C) 1999 Academic Press.