Antimicrobial tachyplesin peptide precursor. cDNA cloning and cellular localization in the horseshoe crab (Tachypleus tridentatus).

The hemocytes of the horseshoe crab have been found to contain a new family of Arthropodous antibiotics, termed the "tachyplesin family." These peptides are composed of 17-18 amino acid residues with a carboxyl-terminal arginine alpha-amide. We report here the entire cDNA sequence coding for the tachyplesin precursors and their distribution in various tissues of the horseshoe crab. Sequence analysis of the cloned cDNAs revealed that the tachyplesin precursors consist of 77 amino acids with 23 residues in a presegment, and that there are two types of mRNAs corresponding to the isopeptides tachyplesins I and II. Both precursors contain a putative signal peptide, a processing peptide sequence and a carboxyl-terminal amidation signal "Gly-Lys-Arg" connected to the mature tachyplesin peptide. Moreover, an unusual acidic amino acid cluster, Asp-Glu-Asp-Glu-Asp-Asp-Asp-Glu-Glu-COOH, is present in the carboxyl-terminal portions of both precursors. These results suggest that the two types of tachyplesin precursors are first synthesized as preproproteins and are then incorporated into the intracellular organelle, accompanied by various processing events. Northern blot analysis on a total RNA from various tissues of the horseshoe crab revealed that the tachyplesin precursors are expressed mainly in hemocytes and cardiac and brain tissues. Tachyplesin was immunohistochemically localized in the smaller dense granules rather than the typical large granules present in abundance in the hemocytes.