Evidence against the role of rhodopsin in rod outer segment binding to RPE cells

The possible role of rhodopsin in the binding and phagocytosis of rod outer segments (ROS) by cultured bovine retinal pigment epithelial (RPE) cells was studied using both quantitative phagocytosis assays and electron microscopy. In inhibition studies an immunoaffinity purified 2-39 N-terminal rhodopsin glycopeptide, a synthetic 1-16 peptide analogue of rhodopsin and purified, unsealed ROS disc membranes were found to be ineffective in inhibiting the binding of 125I-labeled ROS to RPE cells. A two-fold excess of unlabeled intact ROS, however, inhibited 125I-labeled ROS binding to RPE cells by over 40%. In another series of experiments, rhodopsin on the surface of fixed ROS was densely labeled with gold-dextran particles conjugated to an N-terminal-specific (rho 4D2) rhodopsin monoclonal antibody or its F(ab')2 fragment in an effort to block binding and phagocytosis by RPE cells. As visualized by both transmission and scanning electron microscopy using secondary and backscatter electron imaging, these antibody-gold-dextran-labeled ROS were effectively phagocytized by RPE cells. These results provide compelling evidence that rhodopsin in the ROS plasma membrane does not function as the ligand for recognition by RPE cells.