Light-Dependent ΔpH and Membrane Potential Changes in Halobacterial Vesicles Coupled to Sodium Transport

Abstract

Bacteriorhodopsin and Halorhodopsin present in Halobacterium halobium strains have been investigated in relation to Na⁺/H⁺ exchange in isolated cell envelope vesicles. Upon illumination, these retinal proteins result in extrusion of sodium ions by either an electrogenic Na⁺/H⁺ antiporter and/or a direct sodium pump. Since a molecular characterization of these mechanism(s) of sodium extrusion has not yet been realized, it was of interest to measure directly the light- and sodium-dependent changes in ΔpH and membrane potential under nearly identical conditions in S₉ and R¹mR cell membrane vesicles to gain information on the relation of these retinal proteins to sodium extrusion. These activities were evaluated in terms of their dependence on light intensity, and on the inhibitory effect of chemical modifiers of carboxyl groups (carbodiimides); electroneutral exchanges (monensin and triphenyltin); digitoxin and some analogues; and phloretin. Under most of the conditions and treatments employed, light- and sodium-dependent ΔpH led to similar effects in both membrane vesicle types. Hence, it is concluded that the ΔpH and Δξ which arise from sodium transport occur by either a single mechanism or by one which shares common features.