The epitope of the H-type 2 trisaccharide recognized by Erythrina corallodendron lectin. Evidence for both attractive polar and strong hydrophobic interactions for complex formation involving a lectin

Erythrina corallodendron lectin (ECorL), originally regarded as specific fo r N-acetyllactosamine (LacNAc), shows preferential binding of the II-type 2 human blood group determinant (alpha -L-Fuc-(1-2)-beta -D-Gal-(1-4)-beta - D-GlcNAc-OH). In the present study, various deoxy- and mono-O-methyl deriva tives of beta -H-type 2 methyl glycoside and beta -LacNAc methyl glycoside were employed in an enzyme-linked lectin assay to further examine the speci ficity requirements of ECorL and the contribution of the fucose moiety to t he binding of the trisaccharide to the lectin. Based on the relative affini ties of the ligands, the X-ray structure of the ECorL/ LacNAc complex, and the site-directed mutagenesis studies of the lectin, the epitope of the II- type 2 trisaccharide recognized by ECorL has been delineated. The fucose mo iety is shown to provide a strong hydrophobic interaction at its 2-OH, most likely with Trp 135 of the lectin combining site, in addition to the polar interactions of the key hydroxyl groups at positions 3 and 4 of the galact ose and the stacking of the latter residue on Phe131. Comparison of the epi topes of H-type-2-OMe recognized by three other legume lectins, from Galact ia tenuiflora, Psophocarpus tetragonolobus, and Ulex europaeus, shows that all four are different, in spite of the high similarity between their terti ary structures. Our findings provide a striking illustration of how a singl e oligosaccharide can carry different messages for communication with diffe rent recognition molecules.