title: Tachylectin-2: crystal structure of a specific GlcNAc/GalNAc-binding lectin involved in the innate immunity host defense of the Japanese horseshoe crab Tachypleus tridentatus.
abstract: Tachylectin-2, isolated from large granules of the hemocytes of the Japanese horseshoe crab (Tachypleus tridentatus), is a 236 amino acid protein belonging to the lectins. It binds specifically to N-acetylglucosamine and N-acetylgalactosamine and is a part of the innate immunity host defense system of the horseshoe crab. The X-ray structure of tachylectin-2 was solved at 2.0 A resolution by the multiple isomorphous replacement method and this molecular model was employed to solve the X-ray structure of the complex with N-acetylglucosamine. Tachylectin-2 is the first protein displaying a five-bladed beta-propeller structure. Five four-stranded antiparallel beta-sheets of W-like topology are arranged around a central water-filled tunnel, with the water molecules arranged as a pentagonal dodecahedron. Tachylectin-2 exhibits five virtually identical binding sites, one in each beta-sheet. The binding sites are located between adjacent beta-sheets and are made by a large loop between the outermost strands of the beta-sheets and the connecting segment from the previous beta-sheet. The high number of five binding sites within the single polypeptide chain strongly suggests the recognition of carbohydrate surface structures of pathogens with a fairly high ligand density. Thus, tachylectin-2 employs strict specificity for certain N-acetyl sugars as well as the surface ligand density for self/non-self recognition.
authors.affiliation: Max-Planck-Institut für Biochemie, Am Klopferspitz 18a, 82152 Martinsried, Germany. beisel@biochem.mpg.de
authors.forename: H G
authors.initials: HG
authors.lastname: Beisel
authors.forename: S
authors.initials: S
authors.lastname: Kawabata
authors.forename: S
authors.initials: S
authors.lastname: Iwanaga
authors.forename: R
authors.initials: R
authors.lastname: Huber
authors.forename: W
authors.initials: W
authors.lastname: Bode
mesh_terms: D000116:Acetylgalactosamine / Q000737:chemistry* / Q000378:metabolism; D000117:Acetylglucosamine / Q000737:chemistry* / Q000378:metabolism; D000818:Animals; D001665:Binding Sites; D001798:Blood Proteins / Q000737:chemistry* / Q000378:metabolism; D018360:Crystallography, X-Ray; D006434:Hemocytes / Q000737:chemistry; D006737:Horseshoe Crabs / Q000276:immunology*; D037102:Lectins / Q000737:chemistry* / Q000378:metabolism; D008958:Models, Molecular; D008969:Molecular Sequence Data
chemical_list: D001798:Blood Proteins; C097500:L10 protein, Tachypleus tridentatus; D037102:Lectins; D000116:Acetylgalactosamine; D000117:Acetylglucosamine