Coordination of V-ATPase and V-PPase at the Vacuolar Membrane of Plant Cells

The vacuole is one of the most conspicuous multifunctional compartments within a plant cell. In most mature plant cells the central vacuole occupies as much as 90% of the cell volume, or even more in plants with crassulacean acid metabolism (CAM). The vacuole is involved in the control of cell volume and cell turgor, the regulation of cytoplasmic ion concentrations, and pH, sequestration of toxic ions and detoxification of xenobiotics, and transient storage of metabolites (Sze et al. 1992; Blumwald and Gelli 1997; Martinoia and Ratajczak 1997; Wink 1997). Molecules stored in the vacuole include inorganic ions such as sodium, potassium, calcium, magnesium, heavy metals, chloride, and nitrate; organic molecules such as carbohydrates, organic acids, amino acids, polyamines, peptides; and secondary plant products. Regulation of content and volume of plant vacuoles depends on the coordinated action of transporters and channels located in the vacuolar membrane, i.e. the tonoplast (Maeshima 2001). Since transport in most cases has to be driven against a concentration gradient, energisation of the tonoplast is required. This task is achieved by two key transport proteins of the tonoplast: the vacuolar H+-translocating adenosine triphosphatase (V-ATPase, EC 3.6.1.34) and the vacuolar H+-translocating pyrophosphatase (V-PPase, EC 3.6.1.1).