Activities of Ca2+ pump and low affinity Ca2+/H+ antiport in plasma membrane vesicles of corn roots

Abstract

ATP-dependent Ca 2+ -uptake was investigated in sealed plasma membrane vesicles isolated from corn roots (Zea mays L. cv. Hybrid-3352/Palma-Pioneer). In a chloride-containing medium, at high calcium concentrations, about 30% of the total Ca 2+ accumulation ∼ 4 nmol Ca 2+ mg -1 protein) was shown to be protonophore-sensitive and corresponded to the fraction of Ca 2+ not accumulated in a sulphate-containing medium. Furthermore, vesicles in the presence of nitrate, which stimulates H + transport, or vesicles preloaded with H + , take up Ca 2+ more rapidly, suggesting that, at high calcium concentrations, there is a mechanism for Ca 2+ transport which depends on the magnitude of the proton gradient across the membrane. The traction of Ca 2+ uptake shown to be sensitive to the protonophore CCCP increased by about 150-200% as the Ca 2+ concentration in the medium increased from 50 μM to 250 μM. Under the same conditions, the CCCP-insensitive fraction of Ca 2+ accumulated was reduced by about 25-30%, suggesting that different Ca 2+ affinities exist in the two Ca 2+ uptake processes. Although calmodulin stimulation was not observed, the sensitivity to Ca 2+ and external pH indicates that H + gradient-independent Ca 2+ accumulation reflects activity of the Ca 2+ -pump. These results indicate that the plasma membrane of corn roots contain two distinct mechanisms of Ca 2+ transport: a high Ca 2+ affinity, proton gradient-independent Ca 2+ pump and a low Ca 2+ affinity, proton gradient-dependent Ca 2+ /H + antiport, which have greatest activity at concentrations of Ca 2+ below and above 50 μM, respectively.