Conformation of tachyplesin I from Tachypleus tridentatus when interacting with lipid matrices

The mode of action of tachyplesin I, an antimicrobial cationic heptadecapeptide amide isolated from the hemocyte debris of a horseshoe crab, Tachypleus tridentatus, toward lipid matrices was studied with synthetic tachyplesin I, its analogs with Phe in place of Trp or Tyr, a linear analog with no disulfide bonds, and two linear short fragments. Circular dichroism spectra showed that tachyplesin I took an antiparallel beta-structure in buffer solution and a certain less ordered structure in acidic liposomes composed of egg phosphatidylcholine and egg phosphatidylglycerol (3:1). Spectrophotometric titration of the peptides with laurylphosphorylcholine revealed that both Trp and Tyr residues orient toward the inside of lipid matrices, suggesting that they are on the same side of the peptide backbone. The carboxyfluorescein leakage experiment and fluorescence data indicated that tachyplesin I interacted strongly with neutral and acidic lipid bilayers and an aromaticity-rich hydrophobic part of the peptide was embedded in lipid membranes. All the peptides except for the short fragments were almost equally active in lipopolysaccharide binding. The energy-transfer experiment showed that a conformational change occurred such that the Tyr and Trp residues are positioned more closely to each other in acidic liposomes than in buffer solution. The present study strongly suggested that amphipathic lipid bilayers induced a conformational change of tachyplesin I from an energetically stable beta-structure to a less ordered, probably more amphipathic structure.