The mechanism and energetics of the secondary Pi transport system of A. johnsonii were studied in membrane vesicles and proteoliposomes in which the transport protein was functionally reconstituted. Pi uptake is strictly dependent on the presence of divalent cations, like Mg2+, Ca2+, Mn2+, or Co2+. These cations form a MeHPO4 complex with up to 87% of the Pi present in the incubation mixture, suggesting that divalent cations and Pi are co-transported via a metal-phosphate chelate. Metal-phosphate uptake is driven by the proton motive force (interior negative and alkaline). The metal-phosphate/proton stoichiometry was close to unity. The transport system mediates efflux and homologous exchange of metal-phosphate, but not heterologous exchange of metal-phosphate and glycerol-3-P or glucose-6-P. Exchange and counterflow were essentially pH-independent while efflux and uptake increased with increasing pH. Efflux was inhibited by the proton motive force, whereas exchange was inhibited by the membrane potential only. These observations are consistent with an ordered mechanism for binding and dissociation of metal-phosphate and proton to and from the carrier protein and point to the recycling of a positively charged, protonated carrier protein during exchange.
