The Ca2+-Transport ATPase of Plant Plasma Membrane Catalyzes a nH+/Ca2+ Exchange

Microsomal vesicles from 24-hour-old radish (Raphanus sativus L.) seedlings accumulate Ca(2+) upon addition of MgATP. MgATP-dependent Ca(2+) uptake co-migrates with the plasma membrane H(+)-ATPase on a sucrose gradient. Ca(2+) uptake is insensitive to oligomycin, inhibited by vanadate (IC(50) 40 micromolar) and erythrosin B (IC(50) 0.2 micromolar) and displays a pH optimum between pH 6.6 and 6.9. MgATP-dependent Ca(2+) uptake is insensitive to protonophores. These results indicate that Ca(2+) transport in these microsomal vesicles is catalyzed by a Mg(2+)-dependent ATPase localized on the plasma membrane. Ca(2+) strongly reduces DeltapH generation by the plasma membrane H(+)-ATPase and increases MgATP-dependent membrane potential difference (Deltapsi) generation. These effects of Ca(2+) on DeltapH and Deltapsi generation are drastically reduced by micromolar erythrosin B, indicating that they are primarily a consequence of Ca(2+) uptake into plasma membrane vesicles. The Ca(2+)-induced increase of Deltapsi is collapsed by permeant anions, which do not affect Ca(2+)-induced decrease of DeltapH generation by the plasma membrane H(+)-ATPase. The rate of decay of MgATP-dependent DeltapH, upon inhibition of the plasma membrane H(+)-ATPase, is accelerated by MgATP-dependent Ca(2+) uptake, indicating that the decrease of DeltapH generation induced by Ca(2+) reflects the efflux of H(+) coupled to Ca(2+) uptake into plasma membrane vesicles. It is therefore proposed that Ca(2+) transport at the plasma membrane is mediated by a Mg(2+)-dependent ATPase which catalyzes a nH(+)/Ca(2+) exchange.