Halorhodopsin is a light-driven chloride pump.

Light-dependent membrane potentials, ionic fluxes, and volume changes were measured in two kinds of Halobacterium halobium cell envelope vesicles: one containing bacteriorhodopsin and another halorhodopsin. Bacteriorhodopsin-containing vesicles extruded protons by a primary electrogenic mechanism and an energized volume decrease was observed. This was shown to be the consequences of sodium extrusion via proton/sodium antiport (which recirculated protons) and the accompanying passive chloride extrusion. Halorhodopsin-containing vesicles, in contrast, exhibited a volume increase during illumination, apparently caused by primary inward transport of chloride, and accompanied by passive cation (sodium or potassium, and proton) uptake. It was demonstrated that the chloride transport will occur against both electrical and concentration gradients across the vesicle membrane. Moreover, chloride was required on the vesicle exterior for the light-dependent generation of membrane potential, pH change, and swelling. These observations are inconsistent with an earlier proposal that halorhodopsin is an outward directed sodium pump, but suggest very strongly that it is an inward directed chloride pump. Quantitative arguments from the present work rule out a significant role of sodium in the functioning of halorhodopsin.