Pyrophosphate Stimulates the Phosphate-Sodium Symporter of Trypanosoma brucei Acidocalcisomes and Saccharomyces cerevisiae Vacuoles

ABSTRACT Inorganic pyrophosphate (PP i ) is a by-product of biosynthetic reactions and has bioenergetic and regulatory roles in a variety of cells. Here we show that PP i and other pyrophosphate-containing compounds, including polyphosphate (polyP), can stimulate sodium-dependent depolarization of the membrane potential and P i conductance in Xenopus oocytes expressing a Saccharomyces cerevisiae or Trypanosoma brucei Na + /P i symporter. PP i is not taken up by Xenopus oocytes, and deletion of the TbPho91 SPX domain abolished its depolarizing effect. PP i generated outward currents in Na + /P i -loaded giant vacuoles prepared from wild-type or pho91 Δ yeast strains expressing TbPHO91 but not from the pho91Δ strains. Our results suggest that PP i , at physiological concentrations, can function as a signaling molecule releasing P i from S. cerevisiae vacuoles and T. brucei acidocalcisomes. IMPORTANCE Acidocalcisomes, first described in trypanosomes and known to be present in a variety of cells, have similarities with S. cerevisiae vacuoles in their structure and composition. Both organelles share a Na + /P i symporter involved in P i release to the cytosol, where it is needed for biosynthetic reactions. Here we show that PP i , at physiological cytosolic concentrations, stimulates the symporter expressed in either Xenopus oocytes or yeast vacuoles via its SPX domain, revealing a signaling role of this molecule.