Compartmental analysis of light-induced proton movement in reconstituted bacteriorhodopsin vesicles

Purified bacteriorhodopsin from purple membrane sheets isolated from Halobacter halobium was solubilized with a bile salt detergent, 3-[(3-cholamidopropyl) dimethyl-ammonium]-1-propanesulfonate (CHAPS). The detergent-solubilized protein was then incorporated into lecithin vesicles at either high (450:1) or low (65:1) lipid to protein ratios. Circular dichroism studies showed that the bacteriorhodopsin incorporated was in a monomeric form in the 450:1 vesicles. The 65:1 vesicles exhibited an exciton splitting characteristic of the aggregated state of bacteriorhodopsin. We then examined the light-induced movement of protons for these two preparations. Compartmental analysis was used to derive a kinetic model for the observed proton movement. The pumping was qualitatively the same for monomeric and aggregated protein. A three-compartment model provided an excellent description of proton movement in both sets of vesicles and at four different light intensities. This model demands two independent processes to account for the proton movement. The rate coefficients for both are linearly related to light intensity. However, the total flux of protons via one of these processes diminishes as a function of the hydrogen ion accumulation within the vesicles.