Ca2+/H+ exchange in the plasma membrane of Arabidopsis thaliana leaves

Abstract

A large number of plant Ca2+/H+ exchangers have been identified in endomembranes, but far fewer have been studied for Ca2+/H+ exchange in plasma membrane so far. To investigate the Ca2+/H+ exchange in plasma membrane here, inside-out plasma membrane vesicles were isolated from Arabidopsis thaliana leaves using aqueous two-phase partitioning method. Ca2+/H+ exchange in plasma membrane vesicles was measured by Ca2+-dependent dissipation of a pre-established pH gradient. The results showed that transport mediated by the Ca2+/H+ exchange was optimal at pH 7.0, and displayed transport specificity for Ca2+ with saturation kinetics at Km = 47 μM. Sulfate and vanadate inhibited pH gradient across vesicles and decreased the Ca2+-dependent transport of H+ out of vesicles significantly. When the electrical potential across plasma membrane was dissipated with valinomycin and potassium, the rate of Ca2+/H+ exchange increased comparing to control without valinomycin effect, suggesting that the Ca2+/H+ exchange generated a membrane potential (interior negative), i.e. that the stoichiometric ratio for the exchange is greater than 2H+:Ca2+. Eosin Y, a Ca2+-ATPase inhibitor, drastically inhibited Ca2+/H+ exchange in plasma membrane as it does for the purified Ca2+-ATPase in proteoliposomes, indicating that measured Ca2+/H+ exchange activity is mainly due to a plasma membrane Ca2+ pump. These suggest that calcium (Ca2+) is transported out of Arabidopsis cells mainly through a Ca2+-ATPase-mediated Ca2+/H+ exchange system that is driven by the proton-motive force from the plasma membrane H+-ATPase.