A newly identified horseshoe crab lectin with binding specificity to O- antigen of bacterial lipopolysaccharides

We identified a novel horseshoe crab hemocyte-derived lectin, which we named tachylectin-4. It has more potent hemagglutinating activity against human A-type erythrocytes than a previously identified hemocyte lectin with an affinity to N-acetylglucosamine, tachylectin-2. The purified tachylectin- 4 is an oligomeric glycoprotein of 470 kDa, composed of subunits of 30 and 31.5 kDa. Ca²⁺ at 10 mM enhanced the hemagglutinating activity 4-fold, and the activity was inhibited by EDTA and o-phenanthroline. L-Fucose and N- acetylneuraminic acid at 100 mM completely inhibited the activity of tachylectin-4. The activity was also inhibited more strongly by bacterial S- type lipopolysaccharides (LPS) but not by R-type LPS lacking O-antigen. The most effective S-type LPS was from Escherichia coli 0111:B4, and the minimum concentration required for inhibiting agglutination against human A-type erythrocytes (0.1 μg/ml) was 160-fold lower than those of S-type LPS from Salmonella minnesota. Therefore, colitose (3-deoxy-L-fucose), a unique sugar present in the O-antigen of E. coli 0111:B4 with structural similarity to L- fucose, is the most probable candidate for a specific ligand of tachylectin- 4. A cDNA coding for tachylectin-4 was isolated from a hemocyte cDNA library. The open reading frame of the 1344-base pair cDNA coded for the mature protein with 232 amino acids. There is no significant sequence similarity to any other known LPS-binding lectins, whereas tachylectin-4 is homologous to the NH₂-terminal domain with unknown functions of Xenopus laevis pentraxin 1.