Calcium transport driven by a proton gradient and inverted membrane vesicles of Escherichia coli.

T Tsuchiya,B P Rosen

doi:10.1016/s0021-9258(17)33786-9

Abstract

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 Mg2+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.

Highlights

Calcium transport into inverted vesicles of Escherichia coli was observed to occur without an exogenous energy source when an artificial proton gradient was used
Active transport of small molecules in E. coli has been shown to occur by two different mechanisms, one utilizing directly phosphate bond energy and the other utilizing the “high energy state of the membrane” [34, 35]
The calcium transport system falls into the second category for several reasons: (a) it is present in membrane vesicles [14], while systems of the first type are not; (b) it can utilize the energy of oxidation by the respiratory chain even in the absence of the Mg*+ATPase [33], while phosphate bond-linked systems can use respiration only when the MgS+ATPase is functional; and (c) it can utilize ATP only when the Mg*+ATPase is functional [15], while systems of the other type use the energy of ATP by some other, undefined mechanism

Summary

Introduction

Calcium transport into inverted vesicles of Escherichia coli was observed to occur without an exogenous energy source when an artificial proton gradient was used. 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. Of the several models which have been proposed to explain energy coupling [1, 2], the chemiosmotic hypothesis of Mitchell [2, 3] appears to us the most likely According to this model, the driving force for active transport is the electrochemical gradient of protons established either by the electron transport chain during the oxidation of substrates or by the energy-transducing. Under anaerobic conditions specialized electron transport chains, using nitrate as the terminal electron acceptor, may fill this role [4]

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Conclusion