Membrane-dependent oligomeric structure and pore formation of a β-hairpin antimicrobial peptide in lipid bilayers from solid-state NMR

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Abstract
We used solid-state NMR spectroscopy to investigate the oligomeric structure and insertion of protegrin-1 (PG-1), a β-hairpin antimicrobial peptide, in lipid bilayers that mimic either the bacterial inner membrane [palmitoyloleoylphosphatidyl ethanolamine and palmitoyloleoylphosphatidylglycerol (POPE/POPG) bilayers] or the red blood cell membrane [neutral palmitoyloleoylphosphatidylcholine (POPC)/cholesterol bilayers]. 1 H spin diffusion from lipids to the peptide indicates that PG-1 contacts both the lipid acyl chains and the headgroups in the anionic membrane but resides far from the lipid chains in the POPC/cholesterol bilayer. 19 F spin diffusion data indicates that 75% of the β-hairpins have homodimerized N strands and C strands in the anionic membrane. The resulting (NCCN) n multimer suggests a membrane-inserted β-barrel enclosing a water pore. The lipids surrounding the β-barrel have high orientational disorder and chain upturns, thus they may act as fillers for the pore. These results revise several features of the toroidal pore model, first proposed for magainin and subsequently applied to PG-1. In the POPC/cholesterol membrane, the N and C strands of PG-1 cluster into tetramers, suggesting the formation of β-sheets on the membrane surface. Thus, the membrane composition plays a decisive role in defining the assembly and insertion of PG-1. The different oligomeric structures of PG-1 help to explain its greater toxicity for bacteria than for eukaryotic cells.