Tachylectins: Horseshoe Crab Lectins Involved in Innate Immunity

 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.
Shun-ichiro Kawabata
(Department of Biology, Kyushu University)
References (1) Kawabata S, Iwanaga S: Role of lectins in the innate immunity of horseshoe crab. Dev. Comp. Immunol. 23, 391-400, 1999
(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
(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
Dec. 15, 2000

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