Horseshoe crab hemocyte-derived antimicrobial polypeptides, tachystatins, with sequence similarity to spider neurotoxins

Antimicrobial peptides, named tachystatins A, B, and C, were identified fro m hemocytes of the horseshoe crab Tachypleus tridentatus. Tachystatins exhi bited a broad spectrum of antimicrobial activity against Gram-negative and Gram-positive bacteria and fungi. Of these tachystatins, tachystatin C was most effective. Tachystatin A is homologous to tachystatin B, but tachystat in C has no significant sequence similarity to tachystatins A and B. Tachys tatins A and B showed sequence similarity to omega-agatoxin-IVA of funnel w eb spider venom, a potent blocker of voltage-dependent calcium channels. Ho wever, they exhibited no blocking activity of the P-type calcium channel in rat Purkinje cells. Tachystatin C also showed sequence similarity to sever al insecticidal neurotoxins of spider venoms. Tachystatins A, B, and C boun d significantly to chitin, A causal relationship was observed between chiti n binding activity and antifungal activity. Tachystatins caused morphologic al changes against a budding yeast, and tachystatin C had a strong cell lys is activity. The septum between mother cell and bud, a chitin-rich region, was stained by fluorescence-labeled tachystatin C, suggesting that the prim ary recognizing substance on the cell wall is chitin. As horseshoe crab is a close relative of the spider, tachystatins and spider neurotoxins may hav e evolved from a common ancestral peptide, with adaptive functions.