Calcium transport in membrane vesicles ofStreptococcus cremoris

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Abstract
Rightside‐out membrane vesicles ofStreptococcus cremoriswere 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 inS. cremorismembrane 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 carbonylcyanidep‐trifluoromethoxyphenylhydrazone or inhibition of the membrane‐bound ATPase byN,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 inS. cremorisis mediated by a secondary transport system catalyzing exchange of calcium ions and protons.

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