Partial characterisation of high molecular weight membrane proteins of bovine retinal rod outer segment (ROS), with emphasis on their possible involvement in the phagocytosis of ROS by retinal pigment epithelial cells
The ageing tips of vertebrate rod photoreceptors, (known as rod outer segments, or ROS), are phagocytosed by the retinal pigment epithelial (RPE) cells, which lie in close apposition to them. It has been shown by several workers, using in vitro RPE cell cultures, that ROS bind to the RPE cells with a higher affinity than other particles. On the basis of this and other evidence in the literature, it has been proposed that there is a 'ROS receptor' on the RPE plasma membrane, and a corresponding ligand on the ROS plasma membrane. In this project, an attempt has been made to identify and characterise the ROS plasma membrane ligand. At an early stage in this project, another research group published an abstract in which they proposed that a high molecular weight ROS integral plasma membrane protein, now known to be a Na+/Ca2+ exchanger, was the ligand referred to above. In this project, the proteins of bovine ROS membranes were analysed by SDS-PAGE. It appeared that one particular Coomassie blue staining band, by virtue of its molecular weight, might have been the Na+/Ca2+exchanger and putative ligand. Moreover, the results of a vectorial ROS membrane protein radio-iodination study were apparently consistent with the band representing a plasma membrane spanning protein, rather than a protein located in the much more abundant ROS disc membranes, which are enclosed by the plasma membrane. However, neither an extension of the vectorial radio-iodination study, which involved the chemical cleavage of the radiolabelled Coomassie staining band into peptides, nor a Ficoll Flotation study, in which an attempt was made to fractionate ROS membranes into disc and plasma membrane components, provided evidence that the Coomassie staining band referred to above represented a plasma membrane spanning protein.