Calcium transport in membrane vesicles of Streptococcus cremoris

Abstract

Rightside‐out membrane vesicles of Streptococcus cremoris were fused with proteoliposomes containing the light‐driven proton pump bacteriorhodopsin by a low‐pH fusion procedure reported earlier [Driessen, A. J. M., Hellingwerf, K. J. & Konings, W. N. (1985) Biochim. Biophys. Acta 808, 1–12]. In these fused membranes a proton motive force, interior positive and acid, can be generated in the light and this proton motive force can drive the uptake of Ca²⁺. Collapsing ΔΨ with a concomitant increase in ΔpH stimulates Ca²⁺ uptake while dissipation of the ΔpH results in a reduced rate of Ca²⁺ uptake. Also an artificially generated ΔpH, interior acid, can drive Ca²⁺ uptake in S. cremoris membrane vesicles. Ca²⁺ uptake depends strongly on the presence of external phosphate while Ca²⁺‐efflux‐induced proton flux is independent of the presence of external phosphate. Ca²⁺ accumulation is abolished by the divalent cation ionophore A23187. Calcium extrusion from intact cells is accelerated by lactose. Collapse of the proton motive force by the uncoupler carbonylcyanide p‐trifluoromethoxyphenylhydrazone or inhibition of the membrane‐bound ATPase by N,N′‐dicyclohexylcarbodiimide strongly inhibits Ca²⁺ release. Further studies on Ca²⁺ efflux at different external pH values in the presence of either valinomycin or nigericin suggested that Ca²⁺ exit from intact cells is an electrogenic process. It is concluded that Ca²⁺ efflux in S. cremoris is mediated by a secondary transport system catalyzing exchange of calcium ions and protons.