Structural Characterization and Cytolytic Activity of a Potent Antimicrobial Motif in Longicin, a Defensin-Like Peptide in the Tick Haemaphysalis longicornis

Longicin, a defensin-like peptide, was recently identified in the hard tick Haemaphysalis longicornis. Longicin and one of its synthetic partial analogs (P4) displayed antimicrobial/fungicidal/parasiticidal activity. In the present study, we compared longicin-derived synthetic analogs in order to characterize the antimicrobial motif (P4) by analyzing some structural features using various bioinformatic tools and/or CD spectroscopy. According to the chemicophysical characteristics, P4 is suggested to be a cationic peptide with hydrophobic and amphipathic character. The predicted secondary structure indicated the existence of a β-sheet, which was also observed in the modeled tertiary structure. CD spectroscopic results also showed the existence of a β-sheet and transition to a helical conformation in the presence of membrane-mimicking conditions. These structural observations on P4 suggested that the antimicrobial activity could be due to the β-sheet as well as the α-helix. In addition, a sequence homology search showed that molecules identified in other ticks and organisms also have the P4 analogous domain at their C-terminal, which indicates P4 as a conserved domain. The peptide P4 also showed low cytolytic activity. Based on the present result and previously reported studies, the peptide P4 could be suggested as a novel antimicrobial domain indicating future therapeutic agent against bacteria.