Disease relevance of PHO4

• Upon derepression of PHO8, the chromatin structure changes significantly: The two upstream hypersensitive sites containing the PHO4 binding sites merge, resulting in a long region of hypersensitivity [1].
• Chromatin reorganization of the PHO5 and murine mammary tumor virus (MMTV) promoters is triggered by binding of either Pho4 or the glucocorticoid receptor (GR), respectively [2].

High impact information on PHO4

• Phosphorylation of a fourth site blocks the interaction of Pho4 with the transcription factor Pho2 [3].
• Here it is shown that multiple phosphorylation sites on the budding yeast transcription factor Pho4 play distinct and separable roles in regulating the factor's activity [3].
• Phosphorylation of Pho4 at two sites promotes the factor's nuclear export and phosphorylation at a third site inhibits its nuclear import [3].
• A complex consisting of the cyclin-dependent kinase (CDK) PHO85 and the cyclin PHO80 phosphorylates and is thought to inactivate the transcription factor PHO4 when yeast cells are grown in medium containing high concentrations of phosphate [4].
• Pse1 binds directly to Pho4 and is required for its import in vivo [5].

Biological context of PHO4

• Determination of the nucleotide sequences of four PHO4c mutant alleles and insertion and deletion analyses of PHO4 DNA indicated that a region from aa 163 to 202 is involved in interaction with a negative regulatory factor PHO80 [6].
• Four functional domains were found in the PHO4 protein, which consists of 312 amino acid (aa) residues as deduced from the open reading frame of PHO4 [6].
• The suppressing plasmids contained either CEP1 or DNA derived from the PHO4 locus [7].
• Using GAL4-PHO4 fusions, we demonstrate that a functional interaction between these two proteins is necessary for transcriptional activation to occur [8].
• Thus, both the Pho80p-Pho85p kinase complex (by Pho4p phosphorylation) and a novel component of the N glycosylation pathway contribute to basal levels of aminoglycoside resistance in Saccharomyces cerevisiae [9].

Anatomical context of PHO4

• We conclude that the vac5-1 allele directs the Pho85 kinase to regulate, via transcription factors Pho4 and Pho2, genes that affect vacuole inheritance but which are not known to be under normal PHO pathway control [10].

Associations of PHO4 with chemical compounds

• In addition, we also demonstrate that an additional factor, Pho81, interacts in high phosphate with both the Pho80 cyclin and with Pho4 [11].
• PHO4 binding specificity is altered by mutation at any of three different positions in the basic region, including a single Glu to Asp substitution [12].
• PHO4 binds to DNA as a homodimer with direct reading of both the core E-box sequence CACGTG and its 3'-flanking bases [13].
• We have identified a genetic locus, pho4, in Schizosaccharomyces pombe which encodes a minor expressed cell surface acid phosphatase that is repressed by low concentrations (0.5 microM) of thiamin [14].
• A cysteine residue in helixII of the bHLH domain is essential for homodimerization of the yeast transcription factor Pho4p [15].

Physical interactions of PHO4

• The two positively acting regulatory proteins PHO2 and PHO4 physically interact with PHO5 upstream activation regions [16].
• Gel retardation assays with the PHO2 protein revealed a binding region that lay between the two PHO4-binding sites [16].
• Specific cis-acting sequence for PHO8 expression interacts with PHO4 protein, a positive regulatory factor, in Saccharomyces cerevisiae [17].
• A 300-base pair region of the GIT1 promoter containing potential Pho4p binding sites was shown to be required for full GIT1 expression [18].
• Chromatin remodeling is equally delayed in a galactose-inducible PHO5 promoter variant in which the Pho4 binding sites have been replaced by Gal4 binding sites [19].

Enzymatic interactions of PHO4

• Furthermore, immunoprecipitated Pcl9 complexes from yeast lysates were capable of phosphorylating the exogenous substrate Pho4 [20].

Regulatory relationships of PHO4

• More detailed investigation of APase synthesis is a conditional PHO80(Ts) mutant indicated that neither PHO4 nor any other protein factor necessary for APase mRNA synthesis is transcriptionally regulated by PHO80 [21].
• Phosphorylation of Pho4 inhibits its interaction with Pse1, providing a mechanism by which phosphorylation may regulate import of Pho4 in vivo [5].
• In every case, the ability of the Pho4 mutants to activate transcription correlates with their ability to disrupt nucleosome structure in the PHO5 promoter [22].
• Expression of VTC4 is regulated by Pho4, a transcription factor that is inhibited by the Pho80-Pho85 kinase [23].
• The ability of Pho85 to regulate the transcription factor Pho4 is mediated by its association the Pho80 cyclin [24].

Other interactions of PHO4

• Analysis of the sequence data uncovered striking homology regions with PHO4, another protein necessary for the induction of PHO5 [25].
• Deletion of PHO4 abolished GIT1 transcription in a wild-type strain [26].
• 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 [27].
• In low phosphate, Pho80 and Pho81 dissociate from Pho4, but retain the ability to interact with each other [11].
• NIN1 is mapped on chromosome VI, 16 cM centromere-distal to PHO4 [28].

Analytical, diagnostic and therapeutic context of PHO4

• Gel retardation assays using promoter deletions revealed two regions involved in PHO4 binding [16].
• Immunoprecipitation experiments showed that Pho4p and Pho2p form a complex on a 36-bp sequence bearing an upstream activation site (UAS) and protein binding assays indicated that these proteins interact directly [29].
• The DNA-binding activity of NUC-1 has not been directly demonstrated; however, results of deletion analysis, sequence analysis of the nuc-1 mutant alleles, and strong sequence similarity with the Saccharomyces cerevisiae PHO4 protein strongly suggest that the basic helix-loop-helix motif of NUC-1 forms a DNA-binding domain [30].
• Genetic tests and electron microscopy experiments suggest that VTC4 is a key target of Pho4 and that Pho80-Pho85-mediated regulation of VTC4 expression is required for proper vacuole function and for yeast cell survival under a variety of suboptimal conditions [23].

References