Components of the cytoskeleton in the retinal pigmented epithelium of the chick

The retinal pigmented epithelium (RPE) is a simple cuboidal epithelium with apical processes which, unlike many epithelia, do not extend freely into a lumen but rather interdigitate closely with the outer segments of the neural retina. To determine whether this close association was reflected in the cytoskeletal organization of the RPE, we studied the components of the cytoskeleton of the RPE and their localization in the body of the cell and in the apical processes. By relative mobility on SDS gels and by immunoblotting, we identified actin, vimentin, myosin, spectrin (240/235), and alpha-actinin as major components, and vinculin as a minor component. In addition, the RPE cytoskeleton contains polypeptides of Mr 280,000 and 250,000; the latter co-electrophoreses with actin-binding protein. By immunofluorescence, the terminal web region appeared similar to the comparable region of the intestinal epithelium that consists of broad belts of microfilaments containing myosin, actin, spectrin, and alpha- actinin. However, the components of the apical processes were very different from those of intestinal microvilli. We observed staining along the process for myosin, actin, spectrin, alpha-actinin, and vinculin. The presence in the apical processes of contractile proteins and also of proteins typically found at sites of cell attachments suggests that the RPE may actively adhere to, and exert tension on, the neural retina.