Calcium transport mediated by NhaA, a Na+/H+ antiporter from Escherichia coli

[摘要]

In everted membrane vesicles of E. coli strain EP432/pGM42, which has only one Na⁺/H⁺ antiporter (NhaA), external CaC4 inhibits dissipation of the respiration-dependent ΔpH in response to the addition of NaCl at pH 7.5, and decreases equilibrium concentration of the intravesicular Na⁺. In the NhaA proteoliposomes, imposition of an artificial ΔpH (acid inside) leads to the several-fold accumulation of calcium. The apparent K _m for this ΔpH-driven Ca²⁺ uptake at pH 8.5 is 2 mM, and the V _max is 1.79 of protein. Dissipation of ΔpH causes release of calcium from the vesicles. CaCl₂ was found to inhibit the ΔpH-driven Na⁺ uptake mediated by reconstituted NhaA, and vice versa. Further, heterological Ca²⁺/Na⁺ exchange has been demonstrated in proteoliposomes containing NhaA. Transmembrane electric potential difference proved to drive this process. All these data are consistent with the assumption that NhaA can also catalyze Ca²⁺/H⁺ exchange.

[学科分类] 生物化学/生物物理

[关键词] Na+H+ antiporter;Calcium transport;NhaA;Escherichia coli;Δψ transmembrane electric potential difference;ΔpH;ΔNa;and ΔpCa;transmembrane H+;Na+;and Ca2+ concentration gradients;DTT;dithiothreitol