Rhodopsin in reconstituted phospholipid vesicles. 2. Rhodopsin-rhodopsin interactions detected by resonance energy transfer.

The interactions between rhodopsin molecules in a micellar detergent solution (octyl glucoside) and in reconstituted phospholipid vesicles were studied in the dark and after bleaching. Resonance energy transfer measurements were used to monitor the proximity between rhodopsin monomers conjugated with a fluorescent donor or a fluorescent acceptor. Reactive sulfhydryl groups of rhodopsin were labeled with pyrenylmaleimide (donor) or monobromobimane (acceptor), whereas amino groups were labeled with dansyl chloride (donor) or fluorescein isothiocyanate (acceptor). The results suggest that in the micellar solution rhodopsin was monomeric in the dark and aggregated after bleaching. If the aggregate were to be a dimer, the labeled sulfhydryl groups of the monomers would be approximately 40 A apart, while the labeled amino groups would be at least 68 A distant from each other. Rhodopsin reconstituted in phospholipid vesicles appeared aggregated both in the dark and after bleaching. The proximity between the sulfhydryl groups of the monomers was not influenced by illumination. In contrast, the labeled amino groups seemed to be largely separated in the dark and closer to each other once the vesicles were bleached. If the aggregate were to be a dimer, the labeled sulfhydryl groups would be approximately 40 A apart both in the dark and after bleaching, whereas the labeled amino groups would be greater than 60 A apart in the dark and approximately 44 A from each other after bleaching. These findings are discussed in the context of rhodopsin structure, its ability to regenerate after bleaching, and the light-induced events initiated by rhodopsin photoexcitation.