Antimicrobial peptides are evolutionarily highly preserved elements of the innate immune system in animals. They kill a wide range of microbial organisms such as bacteria, viruses, and fungi with high potency and speed. Their remarkable ability to prevent pathogenic resistance makes these peptides a viable alternative to conventional antibiotics. The broad objective of this research is to elucidate the structural basis for antimicrobial action, so that new and improved antibiotics with strong microbiocidal activity but low cytotoxicity to mammalian cells may be designed. ? ? The central hypothesis of this project is that antimicrobial peptides share common mechanisms of action that derive from the special characteristics of microbial membranes such as radius of curvature, surface charge, and lack of cholesterol, characteristics that are distinct from mammalian membranes. To test this hypothesis, we will specifically determine the interactions of two representative peptides with lipid bilayers that mimic bacterial, retroviral, and human erythrocyte membranes; determine the orientation topology of these peptides in these various lipid membranes; and measure the secondary structure and aggregation state of these peptides. The peptides of choice are protegrin-1 (PG-1) and rhesus theta-defensin 1 (RTD-1), which both possess a disulfide-bond stabilized beta-sheet conformation that is common to a large number of antimicrobial peptides, including the defensins found in humans. ? ? We will use an integrated solid-state nuclear magnetic resonance (NMR) approach to study the mechanism of action of these two beta-sheet peptides. The important lipid factors in antimicrobial selectivity will be identified by studying the lipid-peptide interactions in lipids with defined membrane curvature, cholesterol content, and anionic surface charges. 31P and 2H NMR will be used as the main probes for the lipid-peptide interaction. Information on the peptide orientation in the lipid bilayer is important for understanding whether the peptides disrupt the cell membrane by pore formation or by micellization. This information will be obtained by 13C and 15N NMR experiments using both oriented and unoriented static samples. The ability to extract molecular orientation using unoriented samples will allow us to measure the concentration-dependence and membrane-curvature-dependence of the peptide orientation. The secondary structure and aggregation of PG-1 and RTD-1 in lipid bilayers will be determined from NMR isotropic chemical shifts and multiple-quantum experiments, respectively. Together, the new structural information, correlated with the characteristics of lipid membranes, will significantly advance our understanding of the mechanism of action of beta-sheet antimicrobial peptides. Moreover, the proposed research will fill our knowledge gap of how beta-sheet peptides interact with lipid bilayers in general.
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