A monoclonal antibody that binds to photoreceptors in the turtle retina

We have raised monoclonal antibodies to photoreceptor cells in the retina of the turtle (Pseudemys scripta elegans). One of these antibodies, 15-18 (an IgG1), was studied by immunoelectron microscopy using colloidal gold, and found to bind to the outer segments of all rods and some single cones, but did not stain turtle double cones. Immunoblotting and immunoprecipitation show that antibody 15-18 binds to an antigen of apparent Mr approximately 34,5000 which is probably turtle opsin. Antibody 15-18 binds visual pigments from several species, including bovine opsin. In order to determine the antigenic site bound by 15-18 in bovine opsin, synthetic peptides were used as competitors in an enzyme-linked immunoassay (ELISA). The antigenic site is located in the surface loop connecting rhodopsin helices IV-V, in the sequence 190-197. Antibody 15-18 binds to the external surface of rod cell outer segments, thus providing direct evidence for the predicted orientation of rhodopsin in the plasma membrane.