Location of two sulfhydryl groups in the rhodopsin molecule by use of the spin label technique

Sulfhydryl groups of membrane-bound rhodopsin are studied with the spin label technique by using five maleimide derivative probes of different lengths. Two sulfhydryl groups are titrated per molecule of rhodopsin, These groups are located in protected but probably different environments, less than 12 A away from the aqueous phase. A distance of about 37 A is measured between the two groups. These results are consistent with a model in which the two groups would be located close by the external surface of the protein but embedded within the membrane layer, the strong immobilization of the label molecules resulting from phosphlipid-protein interactions.