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 Δ strains but not from the pho91 Δ yeast strains or from the pho91 Δ 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.