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Five types of lectins, named tachylectins, have been identified from circulating hemocytes and hemolymph plasma of the Japanese horseshoe crab, Tachypleus tridentatus (1). Tachylectin-1 interacts with Gram-negative bacteria through 2-keto-3-deoxyoctonate, one of the constituents of lipopolysaccharides (LPS). Tachylectin-1 also binds to polysaccharides such as agarose and dextran with broad specificity. Tachylectin-2 binds to D-GlcNAc or D-GalNAc and recognizes staphylococcal lipoteichoic acids and LPS from several Gram-negative bacteria. In contrast, tachylectins-3 and 4 specifically bind to S-type LPS from several Gram-negative bacteria through a certain sugar moiety on the O-specific polysaccharides (O-antigens). Tachylectin-5 identified in hemolymph plasma has the strongest agglutinating activity, and it recognizes acetyl group-containing substances including non-carbohydrates; the acetyl group is required and is sufficient for recognition (2). Tachylectin-5 consists of a short N-terminal Cys-containing segment and a C-terminal fibrinogen-like domain with the highest sequence identity (51%) to that of mammalian ficolins. Electron microscopy revealed that tachylectin-5 forms two- to four-bladed propeller structures. Tachylectin-5, however, lacks the collagenous domain found in a kind of "bouquet arrangement" of ficolins and collectins. The innate immune system of horseshoe crab may recognize invading pathogens through a combinatorial method using these lectins with different specificities against carbohydrates exposed on pathogens.
The X-ray structure of tachylectin-2 has been established (3). Tachylectin-2 is the first protein displaying a five-bladed beta-propeller structure and exhibits five virtually identical binding sites, one in each 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 quite high ligand density (see figure). Thus, tachylectin-2 employs strict specificity to certain N-acetyl sugars as well as the surface ligand density for self/nonself recognition. |
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| References |
(1) |
Kawabata S, Iwanaga S: Role of lectins in the innate immunity of horseshoe crab. Dev. Comp. Immunol. 23, 391-400, 1999 |
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(2) |
Gokudan S, Muta T, Tsuda R, Koori K, Kawahara T, Seki N, Mizunoe Y, Wai SN, Iwanaga S, Kawabata S: Horseshoe crab acetyl group -recognizing lectins involved in innate immunity are structurally related to fibrinogen. Proc. Natl. Acad. Sci. USA 96, 10086-10091, 1999 |
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(3) |
Beisel H-G, Kawabata S, Iwanaga S, Huber R, Bode W: 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. EMBO J. 18, 2313-2322, 1999 |
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