Direction of proton translocation in proteoliposomes formed from purple membrane and acidic lipids depends on the pH during reconstitution.

The reconstitution of proton pumping activity in proteoliposomes formed by brief sonication of purple membrane and lipid dispersions was studied as a function of pH. Proteoliposomes reconstituted using cardiolipin showed light-dependent proton extrusion when formed at a pH below 2.75 and proton uptake when formed above pH 2.75. Several other acidic lipids including halobacterial lipids behaved similarly. The experiments suggest that the degree of dissociation of the lipid phosphate groups determines the preferential orientation of bacteriorhodopsin in reconstituted proteoliposomes.