Patterns of electrochemical proton gradient formation by membrane vesicles from an obligately acidophilic bacterium

Abstract

Isolated membrane vesicles from the obligately acidophilic bacterium Bacillus acidocaldarius generated an electrochemical gradient of protons (Δ(μ-)(H+)) upon energization with ascorbate-phenazine methosulfate at pH 6.0 or 3.0. At pH 6.0, there was little or no transmembrane pH gradient (ΔpH), but a transmembrane electrical potential (ΔΨ) of ca. -77 mV, positive out, was observed. At pH 3.0 a ΔpH equivalent to -100 mV, acid out, and a ΔΨ of -73 mV, positive out, were observed upon energization. The total magnitude of the Δ(μ-)(H+) was higher than that of whole cells at acid pH, but the very large ΔpHs and the reversed ΔΨs, i.e., inside positive, that are typical of acidophile cells were not observed in the vesicles. The vesicles exhibited energy-dependent accumulation of α-aminoisobutyric acid that was inhibited by both nigericin and valinomycin (plus K+) at pH 3.0 but was inhibited little by nigericin at pH 6.0.