Organization of rhodopsin in photoreceptor membranes. 1. Proteolysis of bovine rhodopsin in native membranes and the distribution of sulfhydryl groups in the fragments.

Papain and thermolysin are shown to cleave bovine rhodopsin in native membranes in two temporally distinct steps at room temperature. The final product of the proteolysis consists of two membrane-bound fragments of molecular weights 27 000 (Rh27) and 12 500 (Rh12). The molecular weights are not changed by reduction with dithiothreitol. The two fragments remain closely associated in both the membrane and nondenaturing detergents before and after bleaching and can be selectively cross-linked with carbodiimides. The sulfhydryl chemistry of the cleaved protein in nearly indistinguishable from native rhodopsin, and of the total of six sulfhydryl groups, two are located on Rh12 and four on Rh27. In the membrane-bound protein, two sulfhydryl groups are accessible for modification, one on Rh12 and the other on Rh27. The sulfhydryl on Rh12 is particularly reactive and may be selectively labeled with maleimides. Continuous irradiation with white light induces additional sulfhydryl reactivity on Rh27.