Ca2+-ATPase in the symbiosome membrane from broad bean root nodules: further evidence for its functioning as ATP-driven Ca2+/H+ exchanger

To date, it has been established that the symbiosome membrane (SM), i.e., plant-derived membrane of symbiosomes, nitrogen-fixing compartments of legume root nodules, is equipped with Ca²⁺-ATPase transporting Ca²⁺ ions through the SM from the cytosol of infected cells into the symbiosome space (SS). Earlier in the experiments on the SM vesicles isolated from broad bean root nodules some data indicating the action of the Ca²⁺-ATPase as ATP-driven Ca²⁺/H⁺ antiporter were obtained. In the present work performed on isolated symbiosomes from the same plant object, further evidence in favor of calcium-proton countertransport mechanism of the pump operation was obtained. These were expressed in vanadate-sensitive alkalinization of the SS coupled with Ca²⁺ uptake by symbiosomes catalyzed by the SM Ca²⁺-ATPase, stimulation of the kinetics of the latter process in the response to artificial acidification of the SS and expectable modulation of ITP-hydrolyzing activity of this enzyme caused by the variation of pH within this compartment. The above findings are discussed in the framework of the model describing the mechanism of Ca²⁺-ATPase operation as an ATP-driven Ca²⁺/H⁺ exchanger and on this base allow us to put forward the hypothesis about the involvement of this enzyme in symbiosome signaling in a Ca²⁺- and pH-dependent manner.