Photoreceptor Disk Membranes Research Articles

Cooperative conformational change in rod photoreceptor disk membrane induced by bleaching

Cooperative conformational change in rod photoreceptor disk membrane induced by bleaching

Intact photoreceptor membrane from bovine rod outer segment: size and shape in bleached state.

Photoreceptor disk membranes isolated from bovine rod outer segments are suspended in dilute aqueous sucrose (4.36 X 10(-3%)) and bleached, and their size and shape are determined with quasielastic and elastic light scattering. They are found to be extremely homogeneous spherical vesicles with a radius of 0.49 +/- 0.07 mum which appear to have derived from the native disk shape by osmotic swelling.

A light-induced conformational change in rod photoreceptor disc membrane

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.

Interactions of retinol-binding protein with various chromophores and with thyroxine-binding protein. A model for visual pigments

The properties of retinol-binding protein from human serum were compared with those of rhodopsin in an attempt to learn more about the role of protein in visual pigments. In the serum, retinol-binding protein (mol wt. 21 000) tightly binds one molecule of all- trans-retinol and one molecule of the tetrameric protein prealbumin (mol. wt. of tetramer, 54 000). Apo-retinol binding protein was found to be completely dissociated from prealbumin under conditions in which native retinol-retinol-binding protein and reconstituted all- trans-retinol-retinol-binding protein were tightly bound. This difference in binding to prealbumin between apo- and holo-retinol-binding protein is due most probably to different conformations of the apo- and holo-protein. By analogy, it is suggested that the incorporation of the retinal chromophore into apo-visual pigment might act as recognition marker for the incorporation of visual pigment into the photoreceptor disc membrane. Apo-retinol binding protein was shown to form stable 1 : 1 molar complexes with retinols, retinals, retinoic acid, retinyl acetate and retinyl oxime, but not with retinyl palmitate. Only the retinol isomer and retinoic acid-retinol-binding protein complexes were bound to prealbumin at physiological ionic strength, while the other chromophores-retinol·binding protein complexes were not. The various chromophores were all bound to the same site on the retinol-binding chromophore, the binding was noncovalent and irreversible under normal physiological conditions. All the chromophores-retinol-binding protein complexes showed a large induced optical activity of the chromophore absorption band upon binding to the protein. The rotatory strength of the circular dichroism bands of the various chromophore retinol-binding protein complexes was of the same order of magnitude as that of rhodopsin. The binding of retinol and retinal to bovine serum albumin did not produce optical activity in the chromophore absorption band.