title: Crystal structure of a wheat germ agglutinin/glycophorin-sialoglycopeptide receptor complex. Structural basis for cooperative lectin-cell binding.
abstract: The crystal structure of wheat germ agglutinin isolectin 1 (WGA1) complexed with a tryptic sialoglycopeptide fragment (T-5) from its erythrocyte receptor glycophorin A, which contains the O-linked tetrasaccharide NeuNAc-alpha 2,3-Gal-beta 1,3-(alpha 2,6-NeuNAc) Gal-NAc-alpha 1-O-Thr, has been determined by molecular replacement techniques and refined at 2.0-A resolution (R = 18.1%). The structure reveals that association between WGA1 dimers, composed of two identical four-domain (A-D) monomers, and T-5 is asymmetric and involves sialic acid binding at three nonequivalent aromatic residue-rich sites. Two independent binding modes are observed. In the dominant ("major") binding mode, the two highest affinity sites are utilized to cross-link neighboring crystallographically related WGA1 dimers. The branched tetrasaccharide has an extended rigid conformation, and its terminal alpha 2,6-NeuNAc and alpha 2,3-NeuNAc residues occupy specificity sites in domains B1 (monomer 1) and C2 (monomer 2) on opposing dimers, respectively. This asymmetric selection of binding sites leads to infinite open-ended arrays of interlinked lectin molecules. In the subsidiary "minor" binding mode, only the terminal alpha 2,6-NeuNAc, anchored to the aromatic residue-rich binding site in domain A2, is clearly visible. The remaining portion of T-5 is disordered. This structure presents the first evidence for NeuNAc binding in the aromatic residue-rich sites of domains A and C and suggests a preference of WGA for alpha 2,6-linked NeuNAc. Moreover, the unusual asymmetric WGA1-tetrasaccharide association, involving domain binding sites that differ in their binding affinities for NeuNAc, offers explanations for the widely observed cooperative cell binding behavior of WGA.
authors.affiliation: Department of Biochemistry and Molecular Biophysics, Virginia Commonwealth University, Richmond 23298.
authors.forename: C S
authors.initials: CS
authors.lastname: Wright
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D002236:Carbohydrate Conformation; D002240:Carbohydrate Sequence; D006021:Glycophorins / Q000737:chemistry* / Q000378:metabolism; D006801:Humans; D046911:Macromolecular Substances; D008958:Models, Molecular; D008969:Molecular Sequence Data; D019158:N-Acetylneuraminic Acid; D009844:Oligosaccharides / Q000032:analysis; D011487:Protein Conformation; D011956:Receptors, Cell Surface / Q000737:chemistry* / Q000378:metabolism; D012794:Sialic Acids / Q000032:analysis; D012795:Sialoglycoproteins / Q000737:chemistry / Q000378:metabolism; D014909:Wheat Germ Agglutinins / Q000737:chemistry* / Q000378:metabolism; D014961:X-Ray Diffraction
chemical_list: D006021:Glycophorins; D046911:Macromolecular Substances; D009844:Oligosaccharides; D011956:Receptors, Cell Surface; D012794:Sialic Acids; D012795:Sialoglycoproteins; D014909:Wheat Germ Agglutinins; D019158:N-Acetylneuraminic Acid