The chromoprotein of halorhodopsin is the light-driven electrogenic chloride pump in Halobacterium halobium

Abstract

The chromoprotein of halorhodopsin was isolated from Halobacteriqm halobium strain L-33, a bacteriorhodopsin-deficient mutant, and incorporated into asolectin lipid vesicles. When these vesicles are added to one side ofa planar lipid membrane, the membrane system becomes photoelectrically active. The observed photoresponse occurs only in the presence of chloride (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 adsorbed to the surface of the planar lipid membrane but not integrated into it. The stationary photocurrent is extremely low because the underlyinglipid membrane is virtually impermeable to the transported ion. The stationary photocurrent, however, increases drastically upon the addition of the lipophilic anion tetraphenylborate or of the protonophore tetrachloro-2-(trifluoromethy1)benzimidazole (TTFB, HA) to the system. The TTFB-enhanced stationary photocurrent is caused by the transport of an HA2- species. The results obtained demonstrate that the chromoprotein of halorhodopsin is the lightdriven C1- pump in H. halobiurn.