Oriented purple-membrane films as a probe for studies of the mechanism of bacteriorhodopsin functioning. I. The vectorial character of the external electric-field effect on the dark state and the photocycle of bacteriorhodopsin

Oriented purple-membrane preparations from Halobacterium halobium were obtained by electrophoretic sedimentation of a purple-membrane suspension on a transparent current-conducting surface. Light exposure of orderly oriented purple-membrane films causes the generation of a photopotential amounting to several volts. The effects of external electric field on the dark state and photocycle of bacteriorhodopsin is studied in dry orderly oriented purple-membrane films. In contrast to nonuniformly oriented preparations (Borisevich, G.P., Lakashev, E.P., Kononenko, A.A. and Rubin, A.B. (1979) Biochim. Biophys. Acta 546, 171–174 and Lukashev, E.P., Vozary, E., Kononenko, A.A. and Rubin, A.B. (1980) Biochim. Biophys. Acta 590, 258–266), a specific feature of the field-induced phenomena observed in orderly oriented films is their vectorial character. The field-induced bathochromic shift of the maximum absorbance of bacteriorhodopsin is observed in an electric field, directed from the periplasmatic to cytoplasmatic side of the purple membrane and the field-induced rise of the photo-stationary M₄₁₂ concentration in a field of opposite sign. This field-induced rise is a result of slowering of M₄₁₂ decay. The observed effects seem likely to reflect the existence of the potential-dependent regulation of the bacteriorhodopsin photocycle in intact purple membranes.