STRUCTURE-FUNCTION-RELATIONSHIPS AND LOCALIZATION OF THE NA CA-K EXCHANGER IN ROD PHOTORECEPTORS/

The structural and functional properties of the bovine rod photorecept or Na/Ca-K exchanger and its distribution in vertebrate photoreceptor cells were studied using a panel of monoclonal antibodies. Antibodies that bind to distinct epitopes along the large hydrophilic N-terminal segment of the exchanger labeled the extracellular surface of the rod outer segment plasma membrane, whereas antibodies against a large hydr ophilic loop between the two membrane domains labeled the intracellula r side, Enzymatic deglycosylation studies indicated that the exchanger primarily contains O-linked sialo-oligosaccharides located within the N-terminal domain. Removal of the extracellular domain with trypsin o r the large intracellular domain with kallikrein did not alter the Na- or K+-dependent Ca2+ efflux activity of the exchanger when reconstit uted into lipid vesicles. Anti-exchanger antibodies were also used to visualize the distribution of the exchanger in the retina by light and electron microscopy, The exchanger was localized to the plasma membra ne of rod outer segments. No labeling was observed in the disk membran es, cone photoreceptor cells, or other retinal neurons, and only faint staining was seen in the rod inner segment. These results indicate th at the O-linked glycosylated rod Na/Ca-K exchanger is specifically tar geted to the plasma membrane of rod photoreceptors and has a topologic al organization similar to that reported for the cardiac Na/Ca exchang er. The large intracellular and extracellular domains do not directly function in the transport of ions across the rod outer segment plasma membrane, but instead may play a role in protein-protein interactions that maintain the spatial organization of the exchanger in the plasma membrane or possibly regulate transport activity of the exchanger.