CALCIUM-TRANSPORT MEDIATED BY NHAA, A NA+/H+ ANTIPORTER FROM ESCHERICHIA-COLI

In everted membrane vesicles of E. coli strain EP432/pGM42, which has only one Na+/H+ antiporter (NhaA), external CaCl2 inhibits dissipation of the respiration-dependent Delta pH 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 Delta pH (acid inside) leads to the several-fold accumulation of calcium. The apparent K-m for this Delta pH-driven Ca2+ uptake at pH 8.5 is 2 mM, and the V-max is 1.79 mu mol/min/mg of protein. Dissipation of Delta pH causes release of calcium from the vesicles. CaCl2 was found to inhibit the Delta pH-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.