Rhodopsin-cholesterol interactions in bovine rod outer segment disk membranes

Cholesterol modulates the function of rhodopsin in the retinal rod outer segment (ROS) disk membranes. One mechanism for such modulation is cholesterol modulation of the properties of the membrane bilayer. This has been explored previously. Another possible mechanism is an interaction between the sterol and the protein, which has not been previously explored. In this study, the fluorescent sterol, cholestatrienol, was used to probe interactions between cholesterol and rhodopsin in bovine ROS disk membranes. Cholestatrienol was incorporated into the disk membranes by exchange from donor phospholipid vesicles. Fluorescence energy transfer from protein tryptophans to cholestatrienol was observed indicating close approach of this fluorescent sterol to the tryptophan. The effectiveness of the energy transfer was measured by the quenching of tryptophan fluorescence by cholestatrienol. The quenching of tryptophan fluorescence was directly related to the cholestatrienol content of the membranes. Cholesterol was incorporated into the disk membranes by exchange from donor phospholipid vesicles. The effect of increasing membrane cholesterol on the ability of cholestatrienol to quench rhodopsin tryptophan fluorescence was determined. This quenching was inversely proportional to the membrane cholesterol content. Furthermore the observed quenching was greater than could be explained by a simple dilution of the cholestatrienol by the addition of cholesterol to the membrane. These data suggest an interaction between the sterol and the protein. The specificity of this interaction was explored by the addition of ergosterol, instead of cholesterol, to the disk membranes. Ergosterol was not able to inhibit the quenching of protein trytophans beyond that due to dilution of the cholestatrienol by addition of ergosterol to the membrane. The ability of cholesterol to compete with cholestatrienol for that interaction suggests a 'site' at which cholesterol contacts rhodopsin. The inability of ergosterol to compete with cholestatrienol for this 'site' suggested that the site was specific for the structure of cholesterol.