The solution structure of antimicrobial peptide tachystatin A from the Japanese horseshoe crab (Tachypleus tridentatus) was determined by two-dimensional nuclear magnetic resonance measurements and distance-restrained simulated annealing calculations. The correct pairs of disulfide bonds were also confirmed in this study. The obtained structure has a cysteine-stabilized triple-stranded β-sheet as a dominant secondary structure and shows an amphiphilic folding observed in many membrane-interactive peptides. Interestingly, tachystatin A shares structural similarities with the calcium channel antagonist ω-agatoxin IVA isolated from spider toxin and mammalian defensins, and we predicted that ω-agatoxin IVA also have the antifungal activity. These structural comparisons and functional correspondences suggest that tachystatin A and ω-agatoxin IVA may exert the antimicrobial activity in a manner similar to defensins, and we have confirmed such activity using fungal culture assays. Furthermore, tachystatin A is a chitin-binding peptide, and ω-agatoxin IVA also showed chitin-binding activities in this study. Tachystatin A and ω-agatoxin IVA showed no structural homology with well known chitin-binding motifs, suggesting that their structures belong to a novel family of chitin-binding peptides. Comparison of their structures with those of cellulose-binding proteins indicated that Phe9 of tachystatin A might be an essential residue for binding to chitin.
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