Active calcium transporters in isolated membranes of wheat root cells

Plant endomembranes carry both primary and secondary ATP-dependent Ca2+ transporters, but conflicting results have been obtained as to their subcellular localization and means of regulation. This study describes how these transporters can be given a respective characterization, by proper choice of pH and inhibitors/activators, even under conditions when they cannot physically be separated from each other. The characterization was done with a light endomembrane fraction from the outer 1—1.5 cm of wheat root tips (Triticum aestivum L.), free from plasma membranes. The endomembrane secondary active ATP-driven Ca2+ transport was totally inhibited by bafilomycin, was increased several-fold by oxalate and had a pH optimum at 7.4. The primary active ATP-dependent Ca2+ transport was inhibited by vanadate and erythrosin B, was not increased by oxalate, and had a relatively lower pH optimum (≤pH 6.8). The two endomembrane Ca2+ transporters were located in different cellular compartments, the secondary H+/Ca2+ anti-port located in the vacuolar membranes, and the primary Ca2+ ATPase located in the endoplasmic reticulum. A Ca2+ ATPase in wheat root plasma membranes was characterized in an earlier study (Olbe and Sommarin, 1991). Thus, two primary Ca2+ ATPases with different locations can be distinguished in wheat root cells. The endomembrane primary Ca2+ transport showed many similarities to that of the plasma membranes, e.g. sensitivity to vanadate and to erythrosin B (although higher erythrosin B was needed to inhibit the endomembrane activity), and stimulation by calmodulin (provided the plasma membranes had been extensively washed). Both enzymes showed affinity for Ca2+ in the low micromolar range, but differed in pH preference and substrate specificity. Taken together, they are clearly distinct enzymes located in different subcellular compartments.