Conformations and Dynamic Transitions of a Melittin Derivative That Forms Macromolecule-Sized Pores in Lipid Bilayers

Systematically evolved from the primary active component of bee venom, MelP5 is a lipophilic peptide with important physical properties that differ from wild-type melittin, including the ability to create large equilibrium pores in lipid bilayers at low peptide to lipid ratios. Self-assembly into stable membrane spanning pores makes MelP5 a promising candidate for future applications in the pharmaceutical arena. Despite significant interest, little is known about the mechanism by which MelP5 remodels the lipid bilayer upon binding. We demonstrate by direct atomic force microscope imaging of supported lipid bilayers in solution that MelP5 remodels 1-palmitoyl-2-oleoyl- sn-glycero-3-phosphocholine (POPC) in one of two ways. It creates either highly localized voids in the bilayer or diffuse nonlocalized thinning. Thinning of the bilayer was measured to be 3.0 ± 1.4 Å (mean ± standard deviation) below the surface of the upper leaflet of the bilayer. Pores, defined as highly localized voids in the bilayer, exhibited several sizes. Approximately 20% of pores exhibited large footprint areas (47 ± 20 nm²) which appear capable of passing bulky macromolecules. The peptide-effected bilayer was observed to reversibly exchange between membrane-thinned and pore states in an apparent dynamic equilibrium. Analysis of time-lapsed images suggested upper and lower bounds (0.2 < τ < 180 s) on the characteristic time scale of transitions between the membrane-thinned and pore states. Moreover, pores were found to colocalize with membrane-thinned regions, a novel observation that is consistent with the notion of cooperativity among membrane-bound peptides when forming pores.