A putative new membrane protein, Pho86p, in the inorganic phosphate uptake system of Saccharomyces cerevisiae

The PHO84 gene in Saccharomyces cerevisiae encodes the P-i transporter Pho84p. The other three genes, GTR1, PHO86 and PHO87, are also suggested to be involved in the P-i uptake system. We cloned and sequenced PHO86 and found that it encodes a 34-kDa protein consisting of 311 amino acid residues with two strongly hydrophobic segments in its N-terminal half. Western blotting analysis of cell extracts revealed that Pho86p, tagged with c-Myc, was fractionated into a water-insoluble fraction. Disruption of PHO86 did not affect cell viability even in combination with the pho84 and/or pho87 disruptions. The triple disruptants showed high levels of constitutive rAPase synthesis and arsenate resistance similar to the pho84 mutant, but showed slower cell growth than the pho84 mutant. PHO86 has two putative binding sites for the transcriptional activator, Pho4p, at nucleotide positions -191 and -497 relative to the ATG start codon, and showed substantial levels of transcription under high-P-i conditions and more enhanced levels in low-P-i medium.