Cation/proton antiport systems in Escherichia coli. Properties of the calcium/proton antiporter.

The potassium/proton antiport system of Escherichia coli has been characterized by the effect of monovalent cations on the pH gradient formed by oxidation of lactate in everted membrane vesicles. Substrates of the system include K+, Na+, Li+, Rb+, and Tl+. The antiporter could also be assayed by uptake of 204Tl+ into everted vesicles. The antiporter exhibits a basic pH optimum and catalyzes electroneutral proton/cation exchange. Antiporter activity is trypsin-sensitive, but trypsin inactivation is prevented by prior formation of an electrochemical proton gradient. Two other proton/cation exchangers, the Na+/H+ and Ca2+/H+ antiporters, were unaffected by the trypsin treatment. Regulation of cytosolic pH by K+/H+ exchange is postulated, where proton return to the cytosol by the K+/H+ antiporter prevents alkalinization of the cytosol during proton extrusion associated with the formation of a protonmotive force or during growth at alkaline pH.