Abstract:

Antimicrobial peptides (AMPs) are components of the innate immune system of nearly every living thing. These molecules exhibit broad spectrum cidal activity against pathogenic agents and a notable lack of activity against eukaryotic cells. This activity is believed to result from deleterious interactions with the lipid bilayer of the pathogenic membrane and can be manipulated by tweaking the amino acid sequence of the peptides. As a result, AMPs have gained popularity as possible candidates for novel antibiotics.

This thesis investigates the membrane activity of a ?-sheet AMP from mice known as cryptdin-4 (Crp4). Specifically, the membrane perturbative capacity of this peptide is shown to be directly related to its microbicidal activity in many cases. Additionally, the mechanism of perturbation is revealed as a process whereby dimers of Crp4 translocate across the membrane bilayer. A model is presented that maps the kinetics of this translocation process onto the leakage that occurs from fluorophoreloaded lipid vesicles upon exposure to the peptide. The selectivity of Crp4 for pathogenic membranes over eukaryotic membranes is shown to be due to the electrostatic attraction between the peptide, which is highly cationic, and the bilayers surrounding microbes, which often harbor large amounts of negatively charged lipids. However, the antimicrobial potency of the peptide is due to the strategic placement of individual amino acid residues within the peptide structure and their position with respect to the membrane; potency is not necessarily due to overall peptide characteristics such as net charge. Finally, Crp4 demonstrates the rare ability to induce the formation of hemi-fused v lipid vesicle aggregates, structures that could potentially be important for practical applications such as drug delivery.