Isolectins I-A and I-B of Griffonia (Bandeiraea) simplicifolia -: Crystal structure of metal-free GS I-B4 and molecular basis for metal binding and monosaccharide specificity

Seeds from the African legume shrub Griffonia simplicifolia contain several lectins. Among them the tetrameric lectin GS I-B-4 has strict specificity for terminal alphaGal residues, whereas the closely related lectin GS I-A(4) can also bind to alphaGaINAc. These two lectins are commonly used as markers in histology or for research in xenotransplantation. To elucidate the basis for the fine difference in specificity, the amino acid sequences of both lectins have been determined and show 89% identity. The crystal structure of GS I-B4, determined at 2.5-Angstrom resolution, reveals a new quaternary structure that has never been observed in other legume lectins. An unexpected loss of both Ca2+ and Mn2+ ions, which are necessary for carbohydrate binding in legume lectins, may be related to a particular amino acid sequence Pro-Glu-Pro in the metal binding loop. Comparison with demetallized concanavalin A reveals a different process for the loss of metal ions and for the subsequent loss of carbohydrate binding activity. The GS I-A.alphaGaINAc and GS I-B.alphaGal complexes were constructed using homology modeling and docking approaches. The unusual presence of an aromatic amino acid at position 47 (Tyr in I-A and Trp in I-B) explains the strong preference for alpha-anomeric sugars in both isolectins. Alteration at one amino acid position, Ala(106) in I-A versus Glu(106) in I-B, is the basis for the observed specificities toward alphaGalNAc and alphaGal.