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| The EMBO Journal
(1999) 18, 2313–2322, doi:10.1093/emboj/18.9.2313 |
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| 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 |
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| Hans-Georg Beisel1, Shun-ichiro Kawabata2, 3, Sadaaki Iwanaga2, Robert Huber1 and Wolfram Bode1 |
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1 Max-Planck-Institut für Biochemie, Am Klopferspitz 18a, 82152 Martinsried, Germany 2 Department of Biology, Faculty of Science, Kyushu University, Fukuoka 812-82, Japan 3 CREST, Japan Science and Technology Corporation, Japan To whom correspondence should be addressed Hans-Georg Beisel, beisel@biochem.mpg.de Received 12 December 1998; Revised 2 March 1999; Accepted 2 March 1999. |
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| Abstract |
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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 Å 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  -propeller structure. Five four-stranded antiparallel -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 -sheet. The binding sites are located between adjacent -sheets and are made by a large loop between the outermost strands of the -sheets and the connecting segment from the previous -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. |
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| Keywords: animal lectin, crystal structure, horseshoe crab, N-acetylglucosamine, -propeller |
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