The Role of Polyphosphates in Toxoplasma gondii

Polyphosphate (PolyP) is a linear polymer of tens to hundreds of phosphate residues that can reach millimolar levels in protozoan parasites while the concentration in host cells is at the micromolar level. In bacteria, PolyP functions as phosphate reservoir, energy source, chelator of metal ions, regulator of metabolism, stress response, differentiation, and gene expression. In eukaryotes, PolyP plays an equally large number of diverse roles. This catalog of apparently unrelated functions may be the result of PolyP"s ability to function as a protein-stabilizing scaffold. T. gondii stores PolyP at molar levels in acidocalcisomes which are hydrolyzed upon alkalinization followed by release of calcium into the cytosol. Nothing is known about the function of acidocalcisome PolyP in the T. gondii infection cycle, virulence, or the establishment of the chronic infection. In Saccharomyces cerevisiae, a vacuolar transporter chaperone complex (VTC complex) synthesizes PolyP, in expense of Pi and ATP, and translocates PolyP to the yeast vacuole and acidocalcisomes. The VTC complex is composed of 5 subunits (VTC1-5), in which VTC4 acts as the catalytic subunit. ToxoDB supports the presence of two homologous genes: TGGT1_298630 (TgVTC2) and TGGT1_299080 (TgVTC4). We created mutants for both genes and found that TgVTC2 and TgVCT4 are essential for the T. gondii lytic cycle, and that TgVTC2 and TgVTC4 co-localize with the plant-like vacuolar compartment (PLVAC) markers VP1 and CPL, and with the Zinc transporter previously localized to the PLVAC and acidocalcisomes. Both mutants showed a significant decrease in PolyP content. We are characterizing the role of polyP as a protein-like chaperone and its role in acidic calcium storage.