5-Diphosphoinositol pentakisphosphate (5-IP7) regulates phosphate release from acidocalcisomes and yeast vacuoles

Acidocalcisomes of Trypanosoma brucei and the acidocalcisome-like vacuoles of Saccharomyces cerevisiae are acidic calcium compartments that store polyphosphate (polyP). Both organelles possess a phosphate-sodium symporter (TbPho91 and Pho91p in T. brucei and yeast, respectively), but the roles of these transporters in growth and orthophosphate (P-i) transport are unclear. We found here that Tbpho91(-/-) trypanosomes have a lower growth rate under phosphate starvation and contain larger acidocalcisomes that have increased P-i content. Heterologous expression of TbPHO91 in Xenopus oocytes followed by two-electrode voltage clamp recordings disclosed that myo-inositol polyphosphates stimulate both sodium-dependent depolarization of the oocyte membrane potential and P-i conductance. Deletion of the SPX domain in TbPho91 abolished this stimulation. Inositol pyrophosphates such as 5-diphosphoinositol pentakisphosphate generated outward currents in Na+/P-i-loaded giant vacuoles prepared from WT or from TbPHO91-expressing pho91 Delta strains but not from the pho91 Delta yeast strains or from the pho91 Delta strains expressing PHO91 or TbPHO91 with mutated SPX domains. Our results indicate that TbPho91 and Pho91p are responsible for vacuolar P-i and Na+ efflux and that myo-inositol polyphosphates stimulate the Na+/P-i symporter activities through their SPX domains.