Proton movements and electric potential generation in reconstituted ATPase proteoliposomes from the thermophilic...

ATP hydrolysis-induced proton translocation and electric potential generation have been studied in ATPase proteoliposomes by means of various optical probes. The proteoliposomes consisted of reconstituted ATPase complex and native lipid mixture isolated from the thermophilic cyanobacterium Synechococcus 6716 [Van Walraven et al. (1983) Eur. J. Biochem. 137, 101-106]. The native cartenoids and added oxonol VI served as probes for the electric membrane potential generated by the net charge separation (negative outside, positive inside). Their responses, with similar half-times as 9-tetradecylamino-6-chloro-2-methoxyacridine, are sensitive to valinomycin and stimulated by nigericin, as expected. The proton concentrations of extraliposomal and intraliposomal aqueous spaces were monitored by neutral red and cresol red; for internal measurements these pH indicators were trapped inside the vesicles during detergent dialysis. Internal acidification and external alkalinization induced by ATP hydrolysis are inhibited by nigericin and enhanced by valinomycin; at the commonly used higher valinomycin concentrations the neutral red response becomes transient, while the much slower cresol red response is diminished right from its onset. At smaller preset pH gradients both ATP hydrolysis activity and neutral red response are diminished. At increasing MgCl2 concentrations the neutral red responses are slowed down and the cresol red responses are slightly enhanced; this is observed for both internal and external dye responses. Neutral red permeation through the membrane is insignificant under our experimental conditions but is enhanced at temperatures below the lipid-phase transition. In the case of externally added neutral red the non-permeant buffer Hepes is only effective at high MgCl2 concentration, whereas some external cresol red response is visible only at high MgCl2 concentration in the presence of Hepes. The kinetics of the pH indicator and electric potential probe responses clearly distinguish fast interfacial and intra-membrane proton displacements from slow bulk proton equilibration. The data are summarized in a model that supports the importance of localized proton displacements for the primary energy-transducing events.