A light-induced conformational change in rod photoreceptor disc membrane

Abstract

Turbidity, viscosity and light-scattering intensity in aqueous suspensions of cattle rod photoreceptor disc membrane were measured before and after the bleaching at various suspension conditions. Turbidity of the disc membrane decreased after the bleaching about 2% at the wavelength 800 nm and at the pH range from 6 to 8. Similarly, an increase in viscosity of the suspensions and a decrease in angular light scattering intensities of the suspensions by a HeNe laser light were observed after the bleaching of the suspensions by a white light. These observations suggests that the rod photoreceptor disc membrane is expanded radially about 1% during bleaching. The extent of the turbidity decrease was suppressed considerably by glutaraldehyde fixation of unbleached disc membrane. The extent of the turbidity decrease was very slightly affected by the prior addition of 0·4 m monovalent salts. An appreciable increase in specific viscosity of digitonin-treated rhodopsin was also observed after the bleaching, which means that the dissolved rhodopsin molecule is somewhat expanded by the bleaching of its chromophore. Taking account of the above observations and others, a schematic model of photoreceptor disc membrane was proposed and discussed.