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Abstract

Halorhodopsin-containing membrane fragments were isolated from Halobacterium halobium strain L-33, a bacteriorhodopsin-deficient mutant. When these so called Tween-washed membrane fragments are added to one side of a positively charged planar lipid bilayer the system becomes photoelectrically active. Under stationary light conditions photocurrent transients are obtained while the steady-state photocurrent is extremely low. The steady-state photocurrent, however, is considerably increased when the lipid bilayer is doped with the Cl⁻/OH⁻ exchanging carrier triphenyltin together with a proton carrier. These results suggest that halorhodopsin is associated with the underlying black lipid membrane in a sandwich-like structure. The photoresponse occurs only in the presence of Cl⁻, Br⁻ and I⁻ with half-saturation concentrations in the range of 1–10 mM and is virtually independent on the type of the cation (Na⁺, K⁺, Mg²⁺) present. With other anions, such as SO₄⁻², F⁻and NO₃⁻ no photoresponse was obtained. The results provide direct evidence that halorhodopsin is a light-driven electrogenic pump with a high specificity for halides.