Bovine Photoreceptor Outer Segments Research Articles

Light-induced, reversible binding of proteins to bovine photoreceptor membranes. Influence of nucleotides

At least four polypeptides are extractable, using centrifugation in aqueous buffers, from dark adapted bovine photoreceptor outer segments but become membrane-bound upon illumination of rhodopsin. Their approximate molecular weights are 68,000, 48,000, 37,000, and 35,000 daltons, as estimated by gel electrophoresis. The 68 K polypeptide was previously identified to carry rhodopsin kinase activity. Both the 37 K and 35 K polypeptides together are shown to carry GTPase activity. The GTPase in its soluble form is inactive but is reactivated if washed rod outer segment membranes are added to it in the light. In the dark after bleaching, the bound polypeptides (enzymes) slowly become soluble again. Nucleotides strongly interact with the light-induced binding of all four of the polypeptides. GTP, for instance, prevents and reverses the light-induced binding of the GTPase. The binding of the GTPase to bleached membranes and its subsequent specific elution with GTP was used to purify the GTPase to homogeneity. It is postulated that these light-induced changes in interaction of proteins with the photoreceptor membrane reflect part of the mechanism by which the enzymes are light-activated via bleaching of rhodopsin.

Rhodopsin as a glycoprotein: a possible role for the oligosaccharide in phagocytosis

Isolated bovine photoreceptor outer segment preparations are capable of transferring both galactose and fucose from the appropriate sugar nucleotides to rhodopsin. Both manganese chloride and Triton, a non-ionic detergent, are required for full activity. A further stimulation of transfer is produced by ATP. No effect is produced by cyclic-AMP, cyclic-GMP, taurine or calcium chloride on either galactose or fucose transfer. The incorporated radio-active sugars co-migrate with rhodopsin on agarose chromatography. Intact retinas incubated in vitro also incorporate galactose into opsin and rhodopsin. Visual pigment labeled with galactose by either method can be purified on agarose columns and hydrolyzed, releasing 94% of the label which co-chromatographs with authentic galactose in two solvent systems. Neither fucose nor galactose is found in rhodopsin but may be added to rhodopsin as a prelude to disc shedding, thereby providing a new recognition site which allows the pigment epithelium to initiate phagocytosis.