Disease relevance of STE6

• STE6, the yeast a-factor transporter, is a member of the ATP binding cassette protein superfamily, which also includes the mammalian multidrug resistance protein and the cystic fibrosis gene product [1].
• To determine stoichiometry of Tup1p associated with the gene, a yeast plasmid containing varying lengths of the STE6 gene including flanking control regions and an Escherichia coli lac operator sequence was constructed [2].
• Confocal microscopy showed the presence of RNF2 in the cytoplasm of the Pgp-negative, drug-sensitive MCF-7 breast cancer cells [3].

High impact information on STE6

• Identification of revertants for the cystic fibrosis delta F508 mutation using STE6-CFTR chimeras in yeast [4].
• Homologues of ste6 and CDC25 could regulate ras activity in other eukaryotic cells [5].
• Epistatic interactions indicate that the ste6 gene functions upstream of ras1 [5].
• Subsequent analysis revealed that the yeast P-glycoprotein is the product of the STE6 gene, a locus previously shown to be required in MATa cells for production of a-factor pheromone [6].
• Mammalian tumours displaying multidrug resistance overexpress a plasma membrane protein (P-glycoprotein), which is encoded by the MDR1 gene and apparently functions as an energy-dependent drug efflux pump [6].

Chemical compound and disease context of STE6

• Glibenclamide also inhibits the cystic fibrosis transmembrane conductance regulator, p-glycoprotein in animals and guard cell ion channels in plants [7].

Biological context of STE6

• We also examined the phenotype of a mutant carrying an insertion mutation of the STE6 gene, the ste6::lacZ allele [8].
• Negative regulation of STE6 gene expression by the alpha 2 product of Saccharomyces cerevisiae [8].
• This observation suggests that the methyl group is likely to be a critical recognition determinant for the a-factor transporter, STE6, thus providing insight into the substrate specificity of STE6 and also supporting the hypothesis that carboxyl methylation can have a dramatic impact on protein-protein interactions [9].
• Metabolic instability and constitutive endocytosis of STE6, the a-factor transporter of Saccharomyces cerevisiae [10].
• A high degree of structural conservation between the STE6 and the HST6 loci with respect to DNA sequence, physical linkage and transcriptional arrangement indicates that HST6 is the C. albicans orthologue of the S. cerevisiae STE6 gene [11].

Anatomical context of STE6

• The punctate pattern is consistent with the view that most of the STE6 molecules present in a cell at any given moment could be en route either to or from the plasma membrane [10].
• We report here that STE6 is metabolically unstable in a wild-type strain, and that this instability is blocked in a pep4 mutant, suggesting that degradation of STE6 occurs in the vacuole and is dependent upon vacuolar proteases [10].
• Saccharomyces cerevisiae STE6 gene product: a novel pathway for protein export in eukaryotic cells [12].
• One model consistent with our results is that the degradation of Ste6p, the bulk of which is exposed to the cytosol, requires the activity of both the cytosolic proteasomal degradative machinery and the vacuolar lumenal proteases, acting in a synergistic fashion [13].
• In contrast to wild-type Ste6, which was associated mainly with internal membranes, the ubiquitination-deficient mutants accumulated at the plasma membrane, as demonstrated by immunofluorescence and cell fractionation experiments [14].

Associations of STE6 with chemical compounds

• ET-18-OCH3 was also found capable of blocking a-peptide pheromone transport and STE6 complementation by these ABC proteins [15].
• Other a-specific sterile strains with ste6 and ste16 mutations also have wild-type levels of the farnesyl cysteine carboxyl methyltransferase activity [16].
• The amino acid sequence of Ste6 predicts two ATP-binding folds; correspondingly, Ste6 was photoaffinity-labeled specifically with 8-azido-[alpha-32P]ATP [17].
• The apparent molecular weight of Ste6 was unaffected by tunicamycin treatment, and the radiolabeled protein did not bind to concanavalin A, indicating that Ste6 is not glycosylated and that glycosylation is not required either for its membrane delivery or its function [17].
• Here we establish the requirements for the ERAD of Ste6p*, a multispanning membrane protein with a cytosolic mutation, and compare them with those for mutant form of carboxypeptidase Y (CPY*), a soluble luminal protein [18].

Physical interactions of STE6

• Overexpression of the soluble Ubp1 variant stabilizes the ATP-binding cassette-transporter Ste6, which is transported to the lysosome-like vacuole for degradation, and whose transport is regulated by ubiquitination [19].
• The cis-acting GAL4 protein-binding site contained in the hairpin-TFO is targeted in vivo to the 5' upstream sequence of STE6 and CBT1 genes that are transcribed in opposite directions and share a poly(pu/py) sequence that can form triple helical structure [20].
• Interestingly, yeast Pdr5p interacted with flavonoids recently found to bind to cancer cell P-glycoprotein and to the protozoan parasite multidrug transporter [21].
• Ste6 is a very unstable protein (half-life 13 min) which is stabilized approximately 3-fold in a ubc4 ubc5 mutant, implicating the ubiquitin system in the degradation of Ste6 [22].
• Thus, it appears that these mutations interfere with the ability of Ste6p to transport a-factor out of the MATa cell [23].

Regulatory relationships of STE6

• We show that TRT2 acts as a barrier to repression, protecting the upstream CBT1 gene from the influence of the STE6 alpha2 operator in MATalpha cells [24].
• The yeast a-factor mating peptide and its transporter Ste6 are normally expressed only in MATa haploid cells [25].
• Similarly, glycerol enhanced protein levels of P-gp expressed under control of the GAL1 promoter [26].
• Uptake of the ATP-binding cassette (ABC) transporter Ste6 into the yeast vacuole is blocked in the doa4 Mutant [27].

Other interactions of STE6

• RPD3 is required for both full repression and full activation of transcription of target genes including PHO5, STE6, and TY2 [28].
• Examination of STE6 localization by indirect immunofluorescence indicates that STE6 is found in a punctate, possibly vesicular, intracellular pattern, distinct from the rim-staining pattern characteristic of PMA1 [10].
• In order to understand how SIN3 functions in STE6 regulation, we have performed a genetic analysis [29].
• Mutations in AXL1, STE6 and FUS3 were identified in the screen [30].
• Expression of MFA1 and STE6 is sufficient for mating type-independent secretion of yeast a-factor, but not mating competence [25].

Analytical, diagnostic and therapeutic context of STE6

• Chromatin immunoprecipitation assays using Tup1p antibodies showed Tup1p to be associated with the entire genomic STE6 coding region [2].
• We show by co-immunoprecipitation that STE6 half-molecules interact physically, supporting the view that they co-assemble in vivo to form a functional transporter [31].
• Sequence analysis of the isolated Arabidopsis complementing cDNA however showed no homology to the STE6 gene [32].
• Fractionation and immunofluorescence data are compatible with a Golgi localization of Ste6 [22].
• Subcellular fractionation, various extraction procedures, and immunoblotting showed that Ste6 is an intrinsic plasma membrane-associated protein [17].

References