Different states of aggregation for unbleached and bleached rhodopsin after isolation in two different detergents

Phospholipid-free rhodopsin has been purified in the detergents sodium cholate and octaethylene glycol n-dodecyl ether (C12E8). In both detergents, the native absorption spectrum of the unbleached protein is maintained; however, upon photolysis, the preparation in C12E8 loses its ability to recombine with 11-cis-retinal, whereas the preparation in cholate does not. The circular dichroic spectra of the protein in the two detergents are nearly identical, indicating that the secondary structure of the protein is the same in the two detergents. The state of association of the protein in the two detergents is different. In sodium cholate, the smallest species present was found to be a trimer of the rhodopsin polypeptide chain, and this association was unaffected by exposure to light. On the other hand, in C12E8, the protein is monomeric and undergoes a nonspecific aggregation process on exposure to light. These results suggest that protein--protein interactions may play an important role in the stabilization of the native structure of rhodopsin.