RHODOPSIN: ITS MOLECULAR SUBSTRUCTURE AND PHOSPHOLIPID INTERACTIONS*

Abstract

Abstract—Rhodopsin in retinal rod outer segment disc membranes, was proteolyzed by treatment with papain. This treatment left three fragments of apparent mol wt of 26,000, 19,000 and 10,000 in the membrane. The circular dichroism (CD) of solubilized, proteolyzed rhodopsin, in both the UV and visible spectral regions, was essentially identical to that of native rhodopsin. This indicates that the retinal binding site configuration is essentially unchanged by proteolysis and that the proteolyzed form of rhodopsin retained the helical content of native rhodopsin. Far UV CD measurements on the fragments indicate that the secondary structural features of the proteolyzed complex were largely maintained when the complex was dissociated. This finding suggests that the proteolytic fragments represent independently stabilized domains within rhodopsin.Measurements of the dependence of the activation free energy of the unfolding of opsin (as determined by the rate of loss of regenerability of opsin) and the meta I to meta II transition on the level of phospholipid associated with opsin and rhodopsin. respectively, have allowed for a determination of the mode of stabilization of these proteins by phospholipid. This dependence has been shown to have a linear form for opsin and rhodopsin. Hence, it appears that the stabilization of the tertiary structure of both solubilized opsin and rhodopsin is attributable to the sum of their interactions with individual phospholipid molecules, interacting with the protein in a non‐cooperative manner.