Activities of Ca2+ pump and low affinity Ca2+/H+ antiport in plasma membrane vesicles of corn roots
1995
Vicente, J.A.F. | Vale, M.G.P.
ATP-dependent Ca2+-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 Ca2+ accumulation (approximately 4 nmol Ca2+ mg-1 protein) was shown to be protonophore-sensitive and corresponded to the fraction of Ca2+ 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 Ca2+ more rapidly, suggesting that, at high calcium concentrations, there is a mechanism for Ca2+ transport which depends on the magnitude of the proton gradient across the membrane. The fraction of Ca2+ uptake shown to be sensitive to the protonophore CCCP increased by about 150-200% as the Ca2+ concentration in the medium increased from 50 micromolar to 250 micromolar. Under the same conditions, the CCCP-insensitive fraction of Ca2+ accumulated was reduced by about 25-30%, suggesting that different Ca2+ affinities exist in the two Ca2+ uptake processes. Although calmodulin stimulation was not observed, the sensitivity to Ca2+ and external pH indicates that H+ gradient independent Ca2+ accumulation reflects activity of the Ca2+-pump. These results indicate that the plasma membrane of corn roots contain two distinct mechanisms of Ca2+ transport: a high Ca2+ affinity, proton gradient-independent Ca2+ pump and a low Ca2+ affinity, proton gradient-dependent Ca2+/H+ antiport, which have greatest activity at concentrations of Ca2+ below and above 50 micromolar, respectively.
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