Transport of inorganic phosphate in Leishmania infantum and compensatory regulation at low inorganic phosphate concentration

BACKGROUND

Proliferation of Leishmania infantum depends on exogenous inorganic phosphate (P(i)) but little is known about energy metabolism and transport of P(i) across the plasma membrane in Leishmania sp.

METHODS

We investigated the kinetics of 32P(i) transport, the influence of H+ and K+ ionophores and inhibitors, and expression of the genes for the Na+:P(i) and H+:P(i) cotransporters.

RESULTS

The proton ionophore FCCP, bafilomycin A1 (vacuolar ATPase inhibitor), nigericin (K+ ionophore) and SCH28080 (an inhibitor of H+, K(+)-ATPase) all inhibited the transport of P(i). This transport showed Michaelis-Menten kinetics with K0.5 and V(max) values of 0.016 +/- 0.002 mM and 564.9 +/- 18.06 pmol x h(-1) x 10(-7) cells, respectively. These values classify the P(i) transporter of L. infantum among the high-affinity transporters, a group that includes Pho84 of Saccharomyces cerevisiae. Two sequences were identified in the L. infantum genome that code for phosphate transporters. However, transcription of the PHO84 transporter was 10-fold higher than the PHO89 transporter in this parasite. Accordingly, P(i) transport and LiPho84 gene expression were modulated by environmental P(i) variations.

CONCLUSIONS

These findings confirm the presence of a P(i) transporter in L. infantum, similar to PHO84 in S. cerevisiae, that contributes to the acquisition of inorganic phosphate and could be involved in growth and survival of the promastigote forms of L. infantum.

GENERAL SIGNIFICANCE

This work provides the first description of a PHO84-like P(i) transporter in a Trypanosomatide parasite of the genus Leishmania, responsible for many infections worldwide.

Effects of methylphosphonate, a phosphate analogue, on the expression and degradation of the high-affinity phosphate transporter Pho84, in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, the Pho84 high-affinity transport system is the major phosphate transporter activated when the cells experience a limitation in external phosphate. In this study, we have compared the phosphate-responsive mechanism of cells expressing PHO84 with a Deltapho84 strain by use of a phosphate analogue, methylphosphonate, which was judged to be suitable for assessment of phosphate homeostasis in the cells. Intracellular levels of the analogue, which in several respects mimicks phosphate, were monitored by (31)P NMR spectroscopy. Results show that methylphosphonate is a nonhydrolyzable and nonutilizable analogue that cannot be used to replenish phosphate or polyphosphate in yeast cells grown under conditions of phosphate limitation. However, the presence of methylphosphonate under such conditions represses the Pho5 acidic phosphatase activity of PHO84 cells, a finding that implies a direct role of the analogue in the regulation of phosphate-responsive genes and/or proteins. Likewise, accumulation of the Pho84 protein at the plasma membrane of the same cells is inhibited by methylphosphonate, although the derepressive expression of the PHO84 gene is unperturbed. Thus, a post-transcriptional regulation is suggested. Supportive of this suggestion is the fact that addition of methylphosphonate to cells with abundant and active Pho84 at the plasma membrane causes enhanced internalization of the Pho84 protein. Altogether, these observations suggest that the Pho84 transporter is regulated not only at the transcriptional level but also by a direct molecule-sensing mechanism at the protein level.