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

Abstract

Microsomal vesicles from 24-hour-old radish (Raphanus sativus L.) seedlings accumulate Ca2+ upon addition of MgATP. MgATP-dependent Ca2+ uptake co-migrates with the plasma membrane H+-ATPase on a sucrose gradient. Ca2+ uptake is insensitive to oligomycin, inhibited by vanadate (IC50 40 micromolar) and erythrosin B (IC50 0.2 micromolar) and displays a pH optimum between pH 6.6 and 6.9. MgATP-dependent Ca2+ uptake is insensitive to protonophores. These results indicate that Ca2+ transport in these microsomal vesicles is catalyzed by a Mg2+-dependent ATPase localized on the plasma membrane. Ca2+ strongly reduces .DELTA.pH generation by the plasma membrane H+-ATPase and increases MgATP-dependent membrane potential difference (.DELTA..psi.) generation. These effects of Ca2+ on .DELTA.pH and .DELTA..psi. generation are drastically reduced by micromolar erythrosin B, indicating that they are primarily a consequence of Ca2+ uptake into plasma membrane vesicles. The Ca2+-induced increase of .DELTA..psi. is collapsed by permeant anions, which do not affect Ca2+-induced decrease of .DELTA.pH generation by the plasma membrane H+-ATPase. The rate of decay of MgATP-dependent .DELTA.pH, upon inhibition of the plasma membrane H+-ATPase, is accelerated by MgATP-dependent Ca2+ uptake, indicating that the decrease of .DELTA.pH generation induced by Ca2+ reflects the efflux of H+ coupled to Ca2+ uptake into plasma membrane vesicles. It is therefore proposed that Ca2+ transport at the plasma membrane is mediated by a Mg2+-dependent ATPase which catalyzes a nH+/Ca2+ exchange.