Purple membrane lipid control of bacteriorhodopsin conformational flexibility and photocycle activity

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Abstract
Specific lipids of the purple membrane of Halobacteria are required for normal bacteriorhodopsin structure, function, and photocycle kinetics [Hendler, R.W. & Dracheva, S. (2001) Biochemistry (Moscow)66, 1623–1627]. The decay of the M‐fast intermediate through a path including the O intermediate requires the presence of a hydrophobic environment near four charged aspartic acid residues within the cytoplasmic loop region of the protein (R. W. Hendler & S. Bose, unpublished results). On the basis of the unique ability of squalene, the most hydrophobic purple membrane lipid, to induce recovery of M‐fast activity in Triton‐treated purple membrane, we proposed that this uncharged lipid modulates an electrostatic repulsion between the membrane surface of the inner trimer space and the nearby charged aspartic acids of the cytoplasmic loop region to promote transmembrane α‐helical mobility with a concomitant increase in the speed of the photocycle. We examined Triton‐treated purple membranes in various stages of reconstitution with native lipid suspensions using infrared spectroscopic techniques. We demonstrate a correlation between the vibrational half‐width parameter of the protein α‐helical amide I mode at 1660 cm−1, reflecting the motional characteristics of the transmembrane helices, and the lipid‐induced recovery of native bacteriorhodopsin properties in terms of the visible absorbance maxima of ground state bacteriorhodopsin and the mean decay times of the photocycle M‐state intermediates.