Mechanism of leakage of phospholipid vesicle contents induced by the peptide GALA.
The synthetic, amphipathic peptide GALA undergoes a pH-dependent conformational change and induces leakage of contents from large unilamellar phosphatidylcholine vesicles when in a helical conformation. The kinetics of this process have been investigated over a wide range of pH and lipid and peptide concentrations. Leakage from lipid vesicles is rapidly initiated (within 2 s) when the pH is lowered below 6 and is rapidly terminated when the pH is raised to 7.5. The leakage shows a selectivity to the size of the entrapped molecules and occurs by an all or none mechanism; vesicles either leak or retain all of their contents. Using this experimental data, we have developed a mathematical description of the kinetics of leakage induced by GALA. This model assumes that GALA becomes incorporated into the vesicle bilayer and aggregates to form a pore. Leakage occurs when a critical number of peptides assemble into a supramolecular aggregate in the bilayer. Leakage curves generated at lipid/peptide ratios ranging from 500/1 to 30000/1 can be well described by this formalism. On the basis of the results and the model, we suggest that GALA forms a transbilayer channel composed of 8-12 monomers. The channel diameter ranges from 5 to 10 A. To the best of our knowledge, this is the first model that can predict the leakage kinetics of solutes entrapped in lipid vesicles induced by a pore-forming peptide. The analysis may be of general use in defining the kinetics and state of aggregation of similarly acting peptides and proteins which form multimeric assemblies in membranes.
Parente
R A
RA
Department of Pharmacy, School of Pharmacy, University of California, San Francisco 94143.
Nir
S
S
Szoka
F C
FC
D000595:Amino Acid Sequence; D003198:Computer Simulation; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008051:Lipid Bilayers; D008081:Liposomes; D008968:Molecular Conformation; D008969:Molecular Sequence Data; D010455:Peptides / Q000494:pharmacology*; D010539:Permeability / Q000187:drug effects; D013329:Structure-Activity Relationship
D008051:Lipid Bilayers; D008081:Liposomes; D010455:Peptides; C066951:GALA peptide