Light-induced glutamate transport in Halobacterium halobium envelope vesicles. II. Evidence that the driving force is a light-dependent sodium gradient.

Abstract

Illumination of cell envelope vesicles from H. halobium causes the development of protonmotive force and energizes the uphill transport of glutamate. Although the uncoupler, p-trifluoromethoxycarbonyl cyanide phenylhydrazone (FCCP), and the membrane-permeant cation, triphenylmethylphosphonium (TPMP+), are inhibitory to the effect of light, the time course and kinetics of the production of the energized state for transport, and its rate of decay after illumination, are inconsistent with the idea that glutamate accumulation is driven directly by the protonmotive force. Similarities between the light-induced transport and the Na+-gradient-induced transport of glutamate in these vesicles suggest that the energized state for the amino acid uptake in both cases consists of a transmembrane Na+ gradient (Na+out/Na+in greater than 1). Rapid efflux of 22Na from the envelope vesicles is induced by illumination. FCCP and TPMP+ inhibit the light-induced efflux of Na+ but accelerate the post-illumination relaxation of the Na+ gradient created, suggesting electrogenic antiport of Na+ with another cation, or electrogenic symport with an anion. The light-induced protonmotive force in the H. halobium cell envelope vesicles is thus coupled to Na+ efflux and thereby indirectly to glutamate uptake as well.