Pyrophosphate Stimulates the Phosphate-Sodium Symporter of <i>Trypanosoma brucei</i> Acidocalcisomes and <i>Saccharomyces cerevisiae</i> 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 <i>Xenopus</i> oocytes expressing a <i>Saccharomyces cerevisiae</i> or <i>Trypanosoma brucei</i> Na⁺/P_i symporter. PP_i is not taken up by <i>Xenopus</i> 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 <i>pho91</i>Δ yeast strains expressing <i>TbPHO91</i> but not from the <i>pho91Δ</i> strains. Our results suggest that PP_i, at physiological concentrations, can function as a signaling molecule releasing P_i from <i>S. cerevisiae</i> vacuoles and <i>T. brucei</i> acidocalcisomes.<b>IMPORTANCE</b> Acidocalcisomes, first described in trypanosomes and known to be present in a variety of cells, have similarities with <i>S. cerevisiae</i> 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 <i>Xenopus</i> oocytes or yeast vacuoles via its SPX domain, revealing a signaling role of this molecule.