A lipid vesicle system for probing voltage-dependent peptide-lipid interactions: application to alamethicin channel formation.

A membrane potential is shown to be established in phosphatidylcholine/cholesterol unilamellar vesicles using valinomycin in conjunction with a potassium ion gradient; this potential is monitored using the externally added fluorescent dye Safranine O. In the same system, transmembrane calcium fluxes are then detected using the (internally trapped) fluorescent dye Quin-2. The calcium-transport behavior of the channel-forming peptide alamethicin is shown to be potential dependent in this system, in contrast to calcium transport by the ionophore Br-A23187, which is unaffected by the potential. The observation of this potential-dependent behavior for alamethicin suggests that this vesicle system may be suitable for direct spectroscopic observation of the voltage-gating process.