Light-dependent cation gradients and electrical potential in Halobacterium halobium cell envelope vesicles

Vesicles can be prepared from Halobacterium halobium cell envelopes, which contain properly oriented bacteriorhodopsin and which extrude H+ during illumination. The pH difference that is generated across the membranes is accompanied by an electrical potential of 90-100 mV (interior negative) and the movements of other cations. Among these is the efflux of Na+, which proceeds against its electrochemical potential. The relationship between the size and direction of the light-induced pH gradient and the rate of depletion of Na+ from the vesicles, as well as other evidence, suggest that the active Na+-extrusion is facilitated by a membrane component that exchanges H+ for Na+ with a stoichiometry greater than 1. The gradients of H+ and Na+ are thus coupled to one another. The Na+-gradient (Na+out greater than Na+in), which arises during illumination, plays a major role in energizing the active transport of amino acids.