Membrane potential modulates photocycling rates of bacterial rhodopsins

Effects of membrane potential on photochemical reactions of three retinal-containing chromoproteins in Halobacterium halobium, sensory rhodopsin I (sR-I), bacteriorhodopsin, and halorhodopsin, are described. Each of the three exhibits a decreased rate of thermal decay of its principal intermediate when photoactivated in an artificially energized compared to a deenergized membrane. The similar response of the three pigments suggests a voltage-dependent conformational change common to their respective photocycles. Spectral and kinetic properties of the sR-I photochemical reaction cycle were measured in phototactic H. halobium cells, and differences from in vitro photocycle kinetics were attributable to the electrical membrane potential present in vivo. In vivo sR-I photocycling rates were reproduced in envelope vesicle preparations in the presence of a valinomycin-induced potassium diffusion potential.