Purple membrane vesicles: Morphology and proton translocation

Abstract

Purple membrane vesicles prepared by different techniques differ widely in their morphology and ability to establish a proton gradient in the light. The procedures used to prepare active vesicles do not completely dissociate the purple membrane and thus preserve a preferential orientation of the protein, while most of the lipid is exchanged for added lipid. Responses to illumination are largely determined by the size of the vesicles and the degree to which bacteriorhodopsin is preferentially oriented. Any attempt to compare the interaction of different lipids with bacteriorhodopsin by measuring the pH response must take these factors into account. With an improved technique we have obtained vesicles of rather uniform size and bacteriorhodopsin orientation, which accumulate protons with an initial rate of 160 ng H⁺ sec⁻¹ mg⁻¹ protein at light intensities of 10⁶ erg cm⁻² sec⁻¹. The kinetics of the process are complex and at present insufficiently understood.