THE CHROMOPROTEIN OF HALORHODOPSIN IS THE LIGHT-DRIVEN ELECTROGENIC CHLORIDE PUMP IN HALOBACTERIUM-HALOBIUM

The chromoprotein of halorhodopsin was isolated from Halobacterium halobium strain L-33, a bacteriorhodopsin-deficient mutant, and incorporated into asolectin lipid vesicles. When these vesicles are added to one side of a planar lipid membrane, the membrane system becomes photoelectrically active. The observed photoresponse occurs only in the presence of Cl- (and other halides). The action spectrum of the photoresponse is identical with the visible absorption band of the chromoprotein in lipid vesicles. The photoresponse consists of a transient photocurrent, which indicates that the lipid vesicles are absorbed to the surface of the planar lipid membrane but not integrated into it. The stationary photocurrent is extremely low because the underlying lipid membrane is virtually impermeable to the transported ion. The stationary photocurrent increases drastically upon the addition of the lipophilic anion tetraphenyl borate or of the protonophore tetrachloro-2-(trifluoromethyl)benzimidazole (TTFB, HA) to the system. The TTFB-enhanced stationary photocurrent is caused by the transport of an HA2- species. The chromoprotein of halorhodopsin is the light-driven Cl- pump in H. halobium.