Photoreceptor membrane carbohydrate on the intradiscal surface of retinal rod disks

Abstract

RHODOPSIN is an integral constituent of the photoreceptor membrane where it is believed to function as a light-activated gate for the release of ions or other substances. The shape, localisation and orientation of the rhodopsin molecule in the membrane is therefore of great importance in understanding the primary events of light perception. Uncertainties in the present state of knowledge are reflected in the numerous models which localise rhodopsin variously on the internal, external or both sides of the disk membrane. In the last few years it has become increasingly accepted that rhodopsin is an elongated molecule, partially embedded in or entirely spanning the disk membrane. Even this model leaves many uncertainties as to the orientation of the molecule in the membrane. Besides the chromophore retinal, the oligosaccharide chain^1,2 is a good orientation point on the rhodopsin molecule. Since rhodopsin is the dominant protein of the photoreceptor membrane^3–8 where it forms about 85–95% of the total protein content, localisation of the carbohydrate component of the membrane would indicate the location of the rhodopsin carbohydrate moiety. Here I present an electron microscopic histochemical demonstration of the carbohydrate component of the photoreceptor membrane; a part of this investigation was reported briefly earlier⁹.