Characterization of purified and unidirectionally reconstituted Pho84 phosphate permease of Saccharomyces cerevisiae

Functional expression, purification and reconstitution of the recombinant phosphate transporter Pho89 of Saccharomyces cerevisiae

The Saccharomyces cerevisiae high-affinity phosphate transporter Pho89 is a member of the inorganic phosphate (Pi) transporter (PiT) family, and shares significant homology with the type III Na(+)/Pi symporters, hPit1 and hPit2. Currently, detailed biochemical and biophysical analyses of Pho89 to better understand its transport mechanisms are limited, owing to the lack of purified Pho89 in an active form. In the present study, we expressed functional Pho89 in the cell membrane of Pichia pastoris, solubilized it in Triton X-100 and foscholine-12, and purified it by immobilized nickel affinity chromatography combined with size exclusion chromatography. The protein eluted as an oligomer on the gel filtration column, and SDS/PAGE followed by western blotting analysis revealed that the protein appeared as bands of approximately 63, 140 and 520 kDa, corresponding to the monomeric, dimeric and oligomeric masses of the protein, respectively. Proteoliposomes containing purified and reconstituted Pho89 showed Na(+)-dependent Pi transport activity driven by an artificially imposed electrochemical Na(+) gradient. This implies that Pho89 operates as a symporter. Moreover, its activity is sensitive to the Na(+) ionophore monensin. To our knowledge, this study represents the first report on the functional reconstitution of a Pi-coupled PiT family member.

Mutational analysis of putative phosphate- and proton-binding sites in the Saccharomyces cerevisiae Pho84 phosphate:H(+) transceptor and its effect on signalling to the PKA and PHO pathways

In Saccharomyces cerevisiae, the Pho84 phosphate transporter acts as the main provider of phosphate to the cell using a proton symport mechanism, but also mediates rapid activation of the PKA (protein kinase A) pathway. These two features led to recognition of Pho84 as a transceptor. Although the physiological role of Pho84 has been studied in depth, the mechanisms underlying the transport and sensor functions are unclear. To obtain more insight into the structure-function relationships of Pho84, we have rationally designed and analysed site-directed mutants. Using a three-dimensional model of Pho84 created on the basis of the GlpT permease, complemented with multiple sequence alignments, we selected Arg(168) and Lys(492), and Asp(178), Asp(358) and Glu(473) as residues potentially involved in phosphate or proton binding respectively, during transport. We found that Asp(358) (helix 7) and Lys(492) (helix 11) are critical for the transport function, and might be part of the putative substrate-binding pocket of Pho84. Moreover, we show that alleles mutated in the putative proton-binding site Asp(358) are still capable of strongly activating PKA pathway targets, despite their severely reduced transport activity. This indicates that signalling does not require transport and suggests that mutagenesis of amino acid residues involved in binding of the co-transported ion may constitute a promising general approach to separate the transport and signalling functions in transceptors.

The effect of phosphate accumulation on metal ion homeostasis in Saccharomyces cerevisiae

Much of what is currently understood about the cell biology of metals involves their interactions with proteins. By comparison, little is known about interactions of metals with intracellular inorganic compounds such as phosphate. Here we examined the role of phosphate in metal metabolism in vivo by genetically perturbing the phosphate content of Saccharomyces cerevisiae cells. Yeast pho80 mutants cannot sense phosphate and have lost control of phosphate uptake, storage, and metabolism. We report here that pho80 mutants specifically elevate cytosolic and nonvacuolar levels of phosphate and this in turn causes a wide range of metal homeostasis defects. Intracellular levels of the hard-metal cations sodium and calcium increase dramatically, and cells become susceptible to toxicity from the transition metals manganese, cobalt, zinc, and copper. Disruptions in phosphate control also elicit an iron starvation response, as pho80 mutants were seen to upregulate iron transport genes. The iron-responsive transcription factor Aft1p appears activated in cells with high phosphate content in spite of normal intracellular iron levels. The high phosphate content of pho80 mutants can be lowered by mutating Pho4p, the transcription factor for phosphate uptake and storage genes. Such lowering of phosphate content by pho4 mutations reversed the high calcium and sodium content of pho80 mutants and prevented the iron starvation response. However, pho4 mutations only partially reversed toxicity from heavy metals, representing a novel outcome of phosphate dysregulation. Overall, these studies underscore the importance of maintaining a charge balance in the cell; a disruption in phosphate metabolism can dramatically impact on metal homeostasis.

Kinetics of charge translocation in the passive downhill uptake mode of the Na+/H+ antiporter NhaA of Escherichia coli

The Na+/H+ antiporter NhaA is the main Na+ extrusion system in E. coli. Using direct current measurements combined with a solid supported membrane (SSM), we obtained electrical data of the function of NhaA purified and reconstituted in liposomes. These measurements demonstrate NhaA's electrogenicity, its specificity for Li+ and Na+ and its pronounced pH dependence in the range pH 6.5-8.5. The mutant G338S, in contrast, presents a pH independent profile, as reported previously. A complete right-side-out orientation of the NhaA antiporter within the proteoliposomal membrane was determined using a NhaA-specific antibody based ELISA assay. This allowed for the first time the investigation of NhaA in the passive downhill uptake mode corresponding to the transport of Na+ from the periplasmic to the cytoplasmic side of the membrane. In this mode, the transporter has kinetic properties differing significantly from those of the previously investigated efflux mode. The apparent Km values were 11 mM for Na+ and 7.3 mM for Li+ at basic pH and 180 mM for Na+ and 50 mM for Li+ at neutral pH. The data demonstrate that in the passive downhill uptake mode pH regulation of the carrier affects both apparent Km as well as turnover (Vmax).

Structure and function of the GTP binding protein Gtr1 and its role in phosphate transport in Saccharomyces cerevisiae

The Pho84 high-affinity phosphate permease is the primary phosphate transporter in the yeast Saccharomyces cerevisiae under phosphate-limiting conditions. The soluble G protein, Gtr1, has previously been suggested to be involved in the derepressible Pho84 phosphate uptake function. This idea was based on a displayed deletion phenotype of Deltagtr1 similar to the Deltapho84 phenotype. As of yet, the mode of interaction has not been described. The consequences of a deletion of gtr1 on in vivo Pho84 expression, trafficking and activity, and extracellular phosphatase activity were analyzed in strains synthesizing either Pho84-green fluorescent protein or Pho84-myc chimeras. The studies revealed a delayed response in Pho84-mediated phosphate uptake and extracellular phosphatase activity under phosphate-limiting conditions. EPR spectroscopic studies verified that the N-terminal G binding domain (residues 1-185) harbors the nucleotide responsive elements. In contrast, the spectra obtained for the C-terminal part (residues 186-310) displayed no evidence of conformational changes upon GTP addition.

Structural modeling of dual-affinity purified Pho84 phosphate transporter

The phosphate transporter Pho84 of Saccharomyces cerevisiae is predicted to contain 12 transmembrane (TM) regions, divided into two partially duplicated parts of 6 TM segments. The three-dimensional (3D) organization of the Pho84 protein has not yet been determined. However, the 3D crystal structure of the Escherichia coli MFS glycerol-3-phosphate/phosphate antiporter, GlpT, and lactose transporter, LacY, has recently been determined. On the basis of extensive prediction and fold recognition analyses (at the MetaServer), GlpT was proposed as the best structural template on which the arrangement of TM segments of the Pho84 transporter was fit, using the comparative structural modeling program MODELLER. To initiate an evaluation of the appropriateness of the Pho84 model, we have performed two direct tests by targeting spin labels to putative TM segments 8 and 12. Electron paramagnetic resonance spectroscopy was then applied on purified and spin labeled Pho84. The line shape from labels located at both positions is consistent with the structural environment predicted by the template-generated model, thus supporting the model.

A new alkalitolerant Yarrowia lipolytica yeast strain is a promising model for dissecting properties and regulation of Na+-dependent phosphate transport systems

A newly isolated osmo-, salt-, and alkalitolerant Yarrowia lipolytica yeast strain is distinguished from other yeast species by its capacity to grow vigorously at alkaline pH values (9.7), which makes it a promising model organism for studying Na+-dependent phosphate transport systems in yeasts. Phosphate uptake by Y. lipolytica cells grown at pH 9.7 was mediated by several kinetically discrete Na+-dependent systems specifically activated by Na+. One of these, a low-affinity transporter, operated at high concentrations of extracellular phosphate. The other two, high-affinity systems, maximally active in phosphate-starved cells, were repressed or derepressed depending on the prevailing extracellular phosphate concentration and pH value. The contribution of Na+/P(i)-cotransport systems to the total cellular phosphate uptake progressively increased with increasing pH, reaching its maximum at pH >/= 9.

Regulation of phosphate acquisition in Saccharomyces cerevisiae

Membrane transport systems active in cellular inorganic phosphate (P(i)) acquisition play a key role in maintaining cellular P(i) homeostasis, independent of whether the cell is a unicellular microorganism or is contained in the tissue of a higher eukaryotic organism. Since unicellular eukaryotes such as yeast interact directly with the nutritious environment, regulation of P(i) transport is maintained solely by transduction of nutrient signals across the plasma membrane. The individual yeast cell thus recognizes nutrients that can act as both signals and sustenance. The present review provides an overview of P(i) acquisition via the plasma membrane P(i) transporters of Saccharomyces cerevisiae and the regulation of internal P(i) stores under the prevailing P(i) status.

Phosphate transport and sensing in Saccharomyces cerevisiae

Cellular metabolism depends on the appropriate concentration of intracellular inorganic phosphate; however, little is known about how phosphate concentrations are sensed. The similarity of Pho84p, a high-affinity phosphate transporter in Saccharomyces cerevisiae, to the glucose sensors Snf3p and Rgt2p has led to the hypothesis that Pho84p is an inorganic phosphate sensor. Furthermore, pho84Delta strains have defects in phosphate signaling; they constitutively express PHO5, a phosphate starvation-inducible gene. We began these studies to determine the role of phosphate transporters in signaling phosphate starvation. Previous experiments demonstrated a defect in phosphate uptake in phosphate-starved pho84Delta cells; however, the pho84Delta strain expresses PHO5 constitutively when grown in phosphate-replete media. We determined that pho84Delta cells have a significant defect in phosphate uptake even when grown in high phosphate media. Overexpression of unrelated phosphate transporters or a glycerophosphoinositol transporter in the pho84Delta strain suppresses the PHO5 constitutive phenotype. These data suggest that PHO84 is not required for sensing phosphate. We further characterized putative phosphate transporters, identifying two new phosphate transporters, PHO90 and PHO91. A synthetic lethal phenotype was observed when five phosphate transporters were inactivated, and the contribution of each transporter to uptake in high phosphate conditions was determined. Finally, a PHO84-dependent compensation response was identified; the abundance of Pho84p at the plasma membrane increases in cells that are defective in other phosphate transporters.

Properties of the cysteine-less Pho84 phosphate transporter of Saccharomyces cerevisiae

The derepressible Pho84 high-affinity phosphate permease of Saccharomyces cerevisiae, encoded by the PHO84 gene belongs to a family of phosphate:proton symporters (PHS). The protein contains 12 native cysteine residues of which five are predicted to be located in putative transmembrane regions III, VI, VIII, IX, and X, and the remaining seven in the hydrophilic domains of the protein. Here we report on the construction of a Pho84 transporter devoid of cysteine residues (C-less) in which all 12 native residues were replaced with serines using PCR mutagenesis and the functional consequences of this. Our results clearly demonstrate that the C-less Pho84 variant is able to support growth of yeast cells to the same extent as the wild-type Pho84 and is stably expressed under derepressible conditions and is fully active in proton-coupled phosphate transport across the yeast plasma membrane.