Studies on the Light-transducing Pigment Bacteriorhodopsin

Until recently, the visual pigments were the only known light-transducing proteins that use retinal (vitamin A aldehyde) as the chromophore. George Wald (1968), in his pioneering studies, identified the nature of the chromophore and showed that opsin is a protein. In addition, Wald and co-workers also demonstrated the photochemical cycle in visual transduction by recognizing short-lived, spectroscopically distinct intermediates at low temperatures. Recently, three additional proteins that use retinal as the chromophore have been discovered. All these are found in the extremely halophilic bacteria, e.g., Halobacterium halobium (Stoeckenius and Bogmolni 1982). Of these, the most studied is bacteriorhodopsin. This is the only protein that is present in the specialized purple membrane, which carries out light-dependent proton translocation. The resulting electrochemical gradient is used for ATP synthesis by the cell. Because of its many attractive features as a model for light-transduction studies, we have devoted considerable effort to the study of...