Characterization of the light-driven sodium pump of Halobacterium halobium. Consequences of sodium efflux as the primary light-driven event.

A class of cell envelope vesicles (M vesicles) prepared from Halobacterium halobium Rl contain a light-driven sodium pump that serves to eject sodium ions and to create an electrical potential difference in direct proportion to the intensity of illumination. The M vesicles take up protons as an immediate consequence of this electrical potential difference. Predictably, the sodium efflux rate and extent are not inhibited by proton ionophores, whereas the proton uptake is enhanced. In the presence of valinomycin and K+, the electrical potential difference and the proton uptake are abolished, but the light-driven sodium extrusion is only slightly decreased. M vesicles are essentially free (>95%) from contamination by a light-driven proton pump. The participation of a reversed bacteriorhodopsin pump is unlikely since absolutely no internally positive electrical potential difference is detectable. The ejection of sodium by M vesicles is independent of protonmotive force and the rate of the Na+ efflux does not show a gating effect in response to membrane potential. In sharp contrast, another class of cell envelope vesicles (L vesicles) accomplish sodium extrusion via an electrogenic H’/Na’ antiporter that is driven by protonmotive force and activated by membrane potential (Lanyi, J. K., and MacDonald, R. E. (1976) Biochemistry, 15, 4608-4614). We conclude that at least two mechanisms exist in H. halobium to create electrical and chemical potential energy by the transduction of light energy: the bacteriorhodopsin proton pump and a light-driven sodium pump. The consequences of this second pump to the circulation of ions and to Na’/ amino acid symport are considerable and cannot be neglected.