Conservation of PHO pathway in ascomycetes and the role of Pho84.

In budding yeast, Saccharomyces cerevisiae, the phosphate signalling and response pathway, known as PHO pathway, monitors phosphate cytoplasmic levels by controlling genes involved in scavenging, uptake and utilization of phosphate. Recent attempts to understand the phosphate starvation response in other ascomycetes have suggested the existence of both common and novel components of the budding yeast PHO pathway in these ascomycetes. In this review, we discuss the components of PHO pathway, their roles in maintaining phosphate homeostasis in yeast and their conservation across ascomycetes. The role of high-affinity transporter, Pho84, in sensing and signalling of phosphate has also been discussed.

Yap1 mediates tolerance to cobalt toxicity in the yeast Saccharomyces cerevisiae.

BACKGROUND: Cobalt has a rare occurrence in nature, but may accumulate in cells to toxic levels. In the present study, we have investigated how the transcription factor Yap1 mediates tolerance to cobalt toxicity. METHODS: Fluorescence microscopy was used to address how cobalt activates Yap1. Using microarray analysis, we compared the transcriptional profile of a strain lacking Yap1 to that of its parental strain. To evaluate the extent of the oxidative damage caused by cobalt, GSH was quantified by HPLC and protein carbonylation levels were assessed. RESULTS: Cobalt activates Yap1 under aerobiosis and anaerobiosis growth conditions. This metal generates a severe oxidative damage in the absence of Yap1. However, when challenged with high concentrations of cobalt, yap1 mutant cells accumulate lower levels of this metal. Accordingly, microarray analysis revealed that the expression of the high affinity phosphate transporter, PHO84, a well-known cobalt transporter, is compromised in the yap1 mutant. Moreover, we show that Yap1 is a repressor of the low affinity iron transporter, FET4, which is also known to transport cobalt. CONCLUSIONS: Cobalt activates Yap1 that alleviates the oxidative damage caused by this metal. Yap1 partially controls cobalt cellular uptake via the regulation of PHO84. Although FET4 repression by Yap1 has no effect on cobalt uptake, it may be its first line of defense against other toxic metals. GENERAL SIGNIFICANCE: Our results emphasize the important role of Yap1 in mediating cobalt-induced oxidative damages and reveal new routes for cell protection provided by this regulator.

Roles of major facilitator superfamily transporters in phosphate response in Drosophila.

The major facilitator superfamily (MFS) transporter Pho84 and the type III transporter Pho89 are responsible for metabolic effects of inorganic phosphate in yeast. While the Pho89 ortholog Pit1 was also shown to be involved in phosphate-activated MAPK in mammalian cells, it is currently unknown, whether orthologs of Pho84 have a role in phosphate-sensing in metazoan species. We show here that the activation of MAPK by phosphate observed in mammals is conserved in Drosophila cells, and used this assay to characterize the roles of putative phosphate transporters. Surprisingly, while we found that RNAi-mediated knockdown of the fly Pho89 ortholog dPit had little effect on the activation of MAPK in Drosophila S2R+ cells by phosphate, two Pho84/SLC17A1-9 MFS orthologs (MFS10 and MFS13) specifically inhibited this response. Further, using a Xenopus oocyte assay, we show that MSF13 mediates uptake of [(33)P]-orthophosphate in a sodium-dependent fashion. Consistent with a role in phosphate physiology, MSF13 is expressed highest in the Drosophila crop, midgut, Malpighian tubule, and hindgut. Altogether, our findings provide the first evidence that Pho84 orthologs mediate cellular effects of phosphate in metazoan cells. Finally, while phosphate is essential for Drosophila larval development, loss of MFS13 activity is compatible with viability indicating redundancy at the levels of the transporters.

Role of mitochondrial phosphate carrier in metabolism-secretion coupling in rat insulinoma cell line INS-1.

In pancreatic ß-cells, glucose-induced mitochondrial ATP production plays an important role in insulin secretion. The mitochondrial phosphate carrier PiC is a member of the SLC25 (solute carrier family 25) family and transports Pi from the cytosol into the mitochondrial matrix. Since intramitochondrial Pi is an essential substrate for mitochondrial ATP production by complex V (ATP synthase) and affects the activity of the respiratory chain, Pi transport via PiC may be a rate-limiting step for ATP production. We evaluated the role of PiC in metabolism-secretion coupling in pancreatic ß-cells using INS-1 cells manipulated to reduce PiC expression by siRNA (small interfering RNA). Consequent reduction of the PiC protein level decreased glucose (10 mM)-stimulated insulin secretion, the ATP:ADP ratio in the presence of 10 mM glucose and elevation of intracellular calcium concentration in response to 10 mM glucose without affecting the mitochondrial membrane potential (Δψm) in INS-1 cells. In experiments using the mitochondrial fraction of INS-1 cells in the presence of 1 mM succinate, PiC down-regulation decreased ATP production at various Pi concentrations ranging from 0.001 to 10 mM, but did not affect Δψm at 3 mM Pi. In conclusion, the Pi supply to mitochondria via PiC plays a critical role in ATP production and metabolism-secretion coupling in INS-1 cells.

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.

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.