Indications for a K⁺/H⁺ cotransport system in plasma membranes from two acidophilic microorganisms

The unicellular green alga Dunaliella acidophila grows optimally at pH 1.0 while maintaining a neutral cytoplasmic pH and a positive membrane potential. To study the mechanism of K⁺ uptake in this organism and in the acidophilic yeast Metschnikowia reukaufii, ⁸⁶Rb was trapped in plasma membrane vesicles of these organisms, by freezing and thawing with phospholipid vesicles. Both preparations possess K⁺-stimulated, vanadate-sensitive H⁺-ATPases. ATP stimulates ⁸⁶Rb efflux from the reconstituted vesicles, and the stimulation is abolished by vanadate. The protonophore SF-6847 also inhibits the ATP-dependent ⁸⁶Rb efflux. In contrast, the lipophilic anion SCN^-, which abolishes the ATP-induced membrane potential, does not inhibit ⁸⁶Rb efflux. An artificial pH difference, acid inside, stimulates ⁸⁶Rb efflux in reconstituted D. acidophila plasma membranes and the stimulation is not affected by vanadate but is inhibited by SF-6847. These results suggest the existence of a K⁺/H⁺ cotransport system acting in parallel to the H⁺-ATPase in the plasma membranes of these organisms. It can explain how acidophiles accumulate K⁺ against a large electrochemical potential gradient.