Investigations into the ability of the peptide, HAL18, to interact with bacterial membranes

Abstract

Halocidin was isolated from hemocytes, Halocynthia aurantium as a heterodimeric peptide consisting of two α-helical subunits, Hal15 and Hal18. Hal18 was shown to have antibacterial properties against Bacillus subtilis (MLC = 15 μM) and Escherichia coli (MLC = 100 μM). The peptide was shown to produce stable monolayers, which were characteristic of α-helical peptides predicted to orientate parallel to the surface of the interface. Constant area assays showed that Hal18 was surface active (4 μM) inducing surface pressure changes >30 mN m−1 characteristic of membrane interactive peptides. The peptide induced stable surface pressure changes in monolayers that were mimetic of B. subtilis membranes (circa 7 mN m−1) and E. coli membrane-mimics (circa 4 mN m−1). Hal18 inserted readily into zwitterionic DOPE and anionic DOPG monolayers inducing surface pressure changes circa 8 mN m−1 in both cases, providing evidence that interaction is not headgroup specific. Thermodynamic analysis of compression isotherms showed that the presence of Hal18 destabilised B. subtilis membranes (ΔG Mix > 0), which is in contrast to its stabilising effect on E. coli lipid extract implying the differential antimicrobial efficacy may be driven by lipid packing.