Abstract
Rod outer segment membranes from fresh bovine retinas have been separated into different fractions by centrifugation on discontinuous sucrose gradients buffered with either 5 m m-Tris acetate or 100 m m-sodium phosphate. In the Tris-buffered gradient one fraction contained predominantly free discs which had retained their original morphology. Intact appearing outer segments were found in all fractions of the phosphate-buffered gradient. Gel electrophoresis showed that in the disc fraction more than 95% of the total protein stain can be assigned to rhodopsin/opsin; in the rod outer segment fractions, this number is about 85%. Electron micrographs of negatively stained disc membranes fixed with glutaraldehyde suggest that polar channels (about 1·5−2 nm wide) exist in the membrane. The repeat distance of the channels is approximately 6 nm. Freeze-fractured rod outer segments reveal a rough fracture face on the plasmic leaflet of both the disc and of the plasma membrane. In the fracture face of the inner leaflet of the disc membrane a fine granularity could be resolved (20,000 grains/μm −2). Freeze-dried and then shadowed dises carry some big particles (20 nm in diameter) protruding from the otherwise smooth external surface. The results are consistent with the assumption that rhodopsin is a transmembrane protein.
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