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Acidocalcisomes of <i>Trypanosoma brucei</i> and the acidocalcisome-like vacuoles of <i>Saccharomyces cerevisiae</i> are acidic calcium compartments that store polyphosphate (polyP). Both organelles possess a phosphate-sodium symporter (TbPho91 and Pho91p in <i>T. brucei</i> and yeast, respectively), but the roles of these transporters in growth and orthophosphate (P<sub>i</sub>) transport are unclear. We found here that <i>Tbpho91</i><sup>-/-</sup> trypanosomes have a lower growth rate under phosphate starvation and contain larger acidocalcisomes that have increased P<sub>i</sub> content. Heterologous expression of <i>TbPHO91</i> in <i>Xenopus</i> oocytes followed by two-electrode voltage clamp recordings disclosed that <i>myo</i>-inositol polyphosphates stimulate both sodium-dependent depolarization of the oocyte membrane potential and P<sub>i</sub> conductance. Deletion of the SPX domain in TbPho91 abolished this stimulation. Inositol pyrophosphates such as 5-diphosphoinositol pentakisphosphate generated outward currents in Na<sup>+</sup>/P<sub>i</sub>-loaded giant vacuoles prepared from WT or from <i>TbPHO91</i>-expressing <i>pho91</i>Δ strains but not from the <i>pho91</i>Δ yeast strains or from the <i>pho91</i>Δ strains expressing <i>PHO91</i> or <i>TbPHO91</i> with mutated SPX domains. Our results indicate that TbPho91 and Pho91p are responsible for vacuolar P<sub>i</sub> and Na<sup>+</sup> efflux and that <i>myo</i>-inositol polyphosphates stimulate the Na<sup>+</sup>/P<sub>i</sub> symporter activities through their SPX domains.
Published in: Prentice-Hall, Inc eBooks
Volume 293, Issue 49, pp. 19101-19112