Contributes to the transtonoplast (from cytosol to vacuole lumen) H(+)-electrochemical potential difference. It establishes a proton gradient of similar and often greater magnitude than the H(+)-ATPase on the same membrane. In addition, facilitates auxin transport by modulating apoplastic pH and regulates auxin-mediated developmental processes. Confers tolerance to NaCl and to drought by increasing ion retention. diphosphate + H(+)(in) + H2O = 2 H(+)(out) + 2 phosphate Activated by K(+) and Mg(2+). Inhibited by Ca(2+), N,N'-dicyclohexylcarbodiimide (DCCD), N-ethylmaleimide (NEM) and aminomethylenediphosphonate (AMDP), and, to a lower extent, by fluoride (KF). Monomer. Mostly vacuolar, tonoplast. Also present in endosomes and plasma membrane. A number of isoforms are produced. According to EST sequences. Ubiquitous (at protein level). Mostly expressed in vascular tissues, meristems and root pericycle. Increase of expression in pollen during flower development. Expressed in developing leaves, sepals, petals, stamens and carpels. Repressed by light. Induced by CAMTA1 and/or CAMTA5 in pollen. Has 16 transmembrane helices and a cytoplasmic domain that contains the active site. Has few direct interactions with pyrophosphate. Interacts with the substrate via divalent metal cations, such as magnesium ions, that are bound to the pyrophosphate (By similarity). Belongs to the H(+)-translocating pyrophosphatase (TC 3.A.10) family. K(+)-stimulated subfamily.