The innate immunity to pathogenic invasion of organisms in the plant and animal kingdoms relies upon cationic
antimicrobial peptides (AMPs) as the first line of defense. In addition to these natural peptide antibiotics, similar
cationic peptides, such as the bee venom toxin melittin, act as nonspecific toxins. Molecular details of AMP and
peptide toxin action are not known, but the universal function of these peptides to disrupt cell membranes of
pathogenic bacteria (AMPs) or a diverse set of eukaryotes and prokaryotes (melittin) is widely accepted. Here, we
have utilized spectroscopic techniques to elucidate peptide-membrane interactions of alpha-helical human and
mouse AMPs of the cathelicidin family as well as the peptide toxin melittin. The activity of these natural peptides
and their engineered analogs was studied on eukaryotic and prokaryotic membrane mimics consisting of <200-nm
bilayer vesicles composed of anionic and neutral lipids as well as cholesterol. Vesicle disruption, or peptide
potency, was monitored with a sensitive fluorescence leakage assay. Detailed molecular information on peptidemembrane
interactions and peptide structure was further gained through vibrational spectroscopy combined with
circular dichroism. Finally, steady-state fluorescence experiments yielded insight into the local environment of
native or engineered tryptophan residues in melittin and human cathelicidin embedded in bilayer vesicles.
Collectively, our results provide clues to the functional structures of the engineered and toxic peptides and may
impact the design of synthetic antibiotic peptides that can be used against the growing number of antibiotic-resistant
pathogens.
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