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Study of vesicle leakage induced by melittin
1Département de Chimie, Université de Montréal, Québec, Canada.
Abstract:
The leakage induced by melittin, a membrane-perturbing amphipathic peptide, from large unilamellar 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) vesicles was studied using calcein as fluorescent marker. The extent of leakage has been found to be regulated by the melittin/lipid molar ratio. Melittin leads to the complete release of trapped calcein from some vesicles. This all-or-none mechanism leads to the co-existence of two different vesicle populations: the 'empty' and the intact one. Intervesicular migration of melittin was not observed. The results reveal a specific targeting of the lysed vesicles by melittin. The presence of negatively charged lipids (unprotonated palmitic acid or 1-palmitoyl-2-oleoylphosphatidylglycerol) in the neutral POPC matrix inhibits the lytic power of melittin; this inhibition increases with increasing surface charge density. It is proposed that the anchorage of the peptide on the charged surface prevents the formation of defects allowing leakage. A statistical model based on a random distribution of the peptide molecules on the vesicles is proposed to describe the release induced by melittin. It is proposed that about 250 melittin molecules per vesicle are required to affect the bilayer permeability and to empty a vesicle of its content. This large number suggests that leakage is more likely due to collective membrane perturbation by the peptide rather than to the formation of a well-defined pore.
Insights
Melittin causes leakage from lipid vesicles in an all-or-none manner, with about 250 molecules needed per vesicle. Negatively charged lipids inhibit this melittin-induced membrane perturbation.
Area of Science:
- Biophysics
- Membrane biophysics
- Lipid-peptide interactions
Background:
- Melittin is a peptide known to perturb biological membranes.
- Understanding peptide-lipid interactions is crucial for drug delivery and membrane studies.
Purpose of the Study:
- To investigate the mechanism of melittin-induced membrane leakage from large unilamellar vesicles.
- To determine the factors regulating melittin's lytic activity.
Main Methods:
- Studied leakage from 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) vesicles using calcein as a fluorescent marker.
- Varied the melittin/lipid molar ratio and lipid composition.
Main Results:
- Melittin induced an all-or-none leakage, creating populations of empty and intact vesicles.
- Leakage was inhibited by negatively charged lipids, with inhibition increasing with surface charge density.
- Approximately 250 melittin molecules per vesicle were estimated to be required for significant leakage.
Conclusions:
- Melittin's lytic activity is regulated by lipid composition and peptide concentration.
- Membrane perturbation by melittin likely involves collective effects rather than a defined pore.
- Anchorage to charged surfaces may prevent melittin-induced membrane defects.