PROTEIN STRUCTURE AND FOLDING
Structure of the Antimicrobial Peptide Tachystatin A*

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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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Published, JBC Papers in Press, April 16, 2002, DOI 10.1074/jbc.M111120200

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This work was supported in part by the Center of Excellence Research Program of the Science and Technology Agency and grants-in-aid for scientific research from the Ministry of Education, Science, and Culture of Japan (10680625); by a grant from the Integrated Research Program for the Development of Insect Technology of the Ministry of Agriculture, Forestries, and Fisheries of Japan; and by the Program for Promotion of Basic Research Activities for Innovative Biosciences (Japan).

The assignment data have been deposited in the BioMagResBank (BMRB), a National Institutes of Health-funded bioinformatics resource (Department of Biochemistry, University of Wisconsin, Madison, WI; www.bmrb.wisc.edu) under accession code 5268.