Calcium efflux from Escherichia coli. Evidence for two systems

Calcium transport into everted membrane vesicles of Escherichia coli was found to have two components, one phosphate-dependent and the other phosphate-independent. In vesicles prepared in a glycerol buffer, calcium/proton exchange was phosphate independent but net uptake of 45Ca2+ required phosphate. In vesicles prepared in a sucrose buffer both phosphate-independent and phosphate-dependent accumulation of 45Ca2+ occurred. Both calcium/proton exchange and phosphate-independent uptake of 45Ca2+ were inactivated by treatment with trypsin, chymotrypsin, or N,N'-dicyclohexylcarbodiimide but not by N-ethylmaleimide. Phosphate-dependent uptake of 45Ca2+ was inhibited by N-ethylmaleimide but not by trypsin, chymotrypsin, or N,N'-dicyclohexylcarbodiimide. Under conditions permitting only phosphate-dependent uptake of 45Ca2+, concomitant uptake of 32Pi was also observed. Both uptake and efflux of the two ions occurred with a 1:1 ratio. These results imply the existence of two calcium transport systems in everted membrane vesicles, one of which catalyzes exchange of calcium ions and protons, the other of which catalyzes cotransport of calcium and phosphate.