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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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 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 β‐propeller structure. Five four‐stranded antiparallel β‐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 β‐sheet. The binding sites are located between adjacent β‐sheets and are made by a large loop between the outermost strands of the β‐sheets and the connecting segment from the previous β‐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.