Monoclonal antibody labels both rod and cone outer segments of turtle photoreceptors.

We have studied the immunoreactivity of turtle photoreceptors to a monoclonal antibody (MAb 15-18) which binds to the external loop connecting bovine rhodopsin helices IV-V. Three chromatic types of cone photoreceptors were identified by the presence and color of oil droplets. MAb 15-18 intensely labeled the outer segments of both rods and green cones. In addition, a weak cross-reactivity was also found in the outer segments of red cones having a pale-green oil droplet, and of blue cones. Other morphological subtypes of red cones, cones with a red oil droplet and both members of double cones, showed no labeling. Our results indicate that rhodopsin and green cone opsin have a similar antigenic determinant, and that two different structural forms of red cone opsin may be present in the turtle retina.