Calcium Transport Driven by a Proton Gradient in Inverted Membrane Vesicles of Escherichia coZi*

Calcium transport into inverted vesicles of Escherichia coli was observed to occur without an exogenous energy source when an artificial proton gradient was used. The orientation of the proton gradient was acid inside and alkaline outside. Either phosphate or oxalate was necessary for transport, as was found for respiratory-driven or ATP-driven uptake (Tsuchiya, T., and Rosen, B. P. (1975) J. Biol. Chem. 250, 7687-7692). Phosphate accumulation was found to occur in conjunction with calcium accumulation. Calcium transport driven by an artificial proton gradient was stimulated by dicyclohexylcarbodiimide, an inhibitor of the Mg’+ATPase (EC 3.6.1.3). Valinomycin, which catalyzes electrogenic potassium movement, stimulated calcium accumulation, while nigericin, which catalyzes electroneutral exchange of potassium and protons, inhibited both artificial proton gradient-driven transport and respiratory-driven transport. Other properties of the proton gradient-driven system and the previously reported energy-linked calcium transport system are similar, indicating that calcium is transported by the same carrier whether energy is supplied through an artificial proton gradient or an energized membrane state. These results suggest the existence of a calcium/proton antiport.