Horseshoe crab acetyl group-recognizing lectins involved in innate immunity are structurally related to fibrinogen

Abstract

We have characterized and cloned newly isolated lectins from hemolymph plasma of the horseshoe crab Tachypleus tridentatus, which we named tachylectins 5A and 5B (TLs-5). TLs-5 agglutinated all types of human erythrocytes and Gram-positive and Gram-negative bacteria. TLs-5 specifically recognize acetyl group-containing substances including noncarbohydrates; the acetyl group is required and is sufficient for recognition. TLs-5 enhanced the antimicrobial activity of a horseshoe crab-derived big defensin. cDNA sequences of TLs-5 indicated that they consist 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. TLs-5, however, lack the collagenous domain found in a kind of ''bouquet arrangement'' of ficolins and collectins. Electron microscopy revealed that TLs-5 form two-to four-bladed propeller structures. The horseshoe crab is equipped with a unique functional homologue of vertebrate fibrinogen, coagulogen, as the target protein of the clotting cascade. Our observations clearly show that the horseshoe crab has fibrinogen-related molecules in hemolymph plasma and that they function as nonselfrecognizing lectins. An ancestor of fibrinogen may have functioned as a nonself-recognizing protein.

The 2.0-Å crystal structure of tachylectin 5A provides evidence for the common origin of the innate immunity and the blood coagulation systems

“…Interestingly, we have previously observed the formation of hexameric and octameric bouquet-like arrangements of purified TL5A by electron microscopy (7). Although TL5A crystallized with a monomer in the asymmetric unit, it is noteworthy that the crystallographic symmetry imposes a tetrameric arrangement (Fig.…”

“…Furthermore, TL5A exhibits strong bacterial agglutinating activity against both Gram-positive and Gram-negative bacteria. Therefore, it is likely that TL5A functions as a first-line host defense protein (7).…”

Fibrinogen-bearing protein genes in the snail Biomphalaria glabrata: Characterization of two novel genes and expression studies during ontogenesis and trematode infection

“…For example, ficolins possess a short N-terminal collagen-like domain whereas snail FREPs have an N-terminal IgSF domain. The N-terminal regions of some FBG-bearing proteins have no homology to known proteins, for instance, horseshoe crab tachylectin [7], and most FBG-like sequences in the fruitfly Drosophila melanogaster and the mosquito Anopheles gambiae [42]. Alternatively, the molecules contain only FBG-like domain(s), as in some mosquito FBG-like sequences [42] and the slug FBG-like lectins [43].…”

“…It has been shown that ascidian ficolins, the FBG-bearing proteins, bind to N-acetyl-D-glucosamine (GlcNAc) and N-acetyl-D-galactosamine (GalNAc) and activate immune responses [6]. In horseshoe crab, tachylections have antibacterial activities and also function in non-self recognition [7]. In mosquitoes of the genera Anopheles and Armigeres, FBG-like proteins have been implicated in immune responses to microbes or malaria parasites because their transcript levels are upregulated after infection [8][9][10].…”

Head-to-Tail Polymerization of Coagulin, a Clottable Protein of the Horseshoe Crab

“…The two proteins, however, are quite different in molecular size and amino acid sequence. Interestingly, a structural homologue of fibrinogen has recently been identified in horseshoe crab plasma, although it functions as a non-self-recognizing protein (20). The crystal structure of coagulogen suggested a possible polymerization mechanism, by which, it was theorized, the removal of the peptide C would expose a hydrophobic cove on the head of one coagulin, thereby providing a new interaction site for the hydrophobic surface of the second molecule (10).…”