Protein-Protein Interactions in the Purple Membrane

Abstract

The purple membrane from Halobacterium halobium is a specialized area of the bacterial membrane. The purified purple membranes are flat oval sheets with an average diameter of 0.5 μm. They contain only a single type of protein, bacteriorhodopsin, of mol wt 26,000. The proteins make up 75% of the total mass of the membrane, the lipids, 25%. The intense purple color is due to the chromophore all-trans or 13-cis retinal, which is covalently bound to a lysine residue of the opsin. In the light-adapted state of the membrane, the longest wavelength absorption band occurs at 570 nm. When light is absorbed in this band, a photochemical cycle is initiated. Via a number of intermediates, the system returns to the 570 nm complex. The intermediate with the longest lifetime absorbs at 412 nm and has a lifetime of several msec. In contrast to the 570 nm complex, the 412 nm complex reacts with hydroxylamine to form retinaloxime. By illuminating the purple membrane with 570 nm light in the presence of hydroxylamine, it is thus possible to produce a colorless chromophore-free membrane, the apomembrane (Oesterhelt et al., 1974).