Absorption spectral properties of acetylated bacteriorhodopsin in purple membrane depending on pH

The dark-adapted form of bacteriorhodopsin in the purple membrane of Halobacterium halobium changes its absorption maximum from 560 to 600 nm if the pH is lowered to about 2 [Oesterhelt, D., & Stoeckenius, W. (1971) Nature (London), New Biol. 233, 149; Moore, T. A., Edgerton, M. E., Parr, G., Greenwood, C., & Perham, R. N. (1978) Biochem. J. 171, 469; Mowery, P. C., Lozier, R. H., Chae, Q., Tseng, T.-W., Taylor, M., & Stoeckenius, W. (1979) Biochemistry 18, 4100; Fischer, U., & Oesterhelt, D. (1979) Biophys. J. 28, 211; Muccio, D. D., & Cassim, J. Y. (1979) J. Mol. Biol. 135, 595]. We compared the pH dependence of the absorption spectra of acetylated membrane with that of unacetylated native membrane. The completely acetylated membrane showed a midpoint of pH 4.8 for the conversion to the acidic form; that of the native membrane was 3.4. On acetylation, the absorption maximum at neutral pH moved from 560 to 555 nm with about 20% decreases in extinction coefficients as compared with that of the native membrane, whereas the spectrum in acid was not affected. The chloride-dependent blue shift from the acidic form of the acetylated membrane was largely suppressed. The CD spectrum of the acetylated membrane was composed of two bands of an opposite sign with slightly decreased amplitudes. The chromophore of the acetylated membrane was sensitive to hydroxylamine, and the spectrum before bleaching was restored on addition of all-trans-retinal to the bleached membrane followed by dark incubation. Blue light irradiation accelerated the conversion to the acidic form in the native membrane but not in the acetylated membrane. Reductive ethylation did not affect the pH dependence of the absorption spectra.