Sterol metabolism and ERG2 gene regulation in the yeast Saccharomyces cerevisiae

249

307

Sterol levels affect the expression of many genes in yeast and humans. We found that the paralogous transcription factors Upc2p and Ecm22p of yeast were sterol regulatory element (SRE) binding proteins (SREBPs) responsible for regulating transcription of the sterol biosynthetic genes ERG2 and ERG3. We defined a 7-bp SRE common to these and other genes, including many genes involved in sterol biosynthesis. Upc2p and Ecm22p activated ERG2 expression by binding directly to this element in the ERG2 promoter. Upc2p and Ecm22p may thereby coordinately regulate genes involved in sterol homeostasis in yeast. Ecm22p and Upc2p are members of the fungus-specific Zn[2]-Cys[6] binuclear cluster family of transcription factors and share no homology to the analogous proteins, SREBPs, that are responsible for transcriptional regulation by sterols in humans. These results suggest that Saccharomyces cerevisiae and human cells regulate sterol synthesis by different mechanisms.Sterols, essential for life in most, if not all, eukaryotes, are synthesized by a pathway known variously as the mevalonate pathway, the isoprenoid pathway, or the sterol biosynthetic pathway. Although sterols (which include molecules ranging from cholesterol to steroid hormones) are the major products of the sterol biosynthetic pathway, this pathway also produces many important nonsterol compounds that are necessary for cellular processes such as respiration, glycosylation, protein prenylation and photosynthesis. In the heavily studied mammalian sterol biosynthetic pathway, regulation occurs at both the transcriptional and the posttranscriptional levels. At present, the best-understood aspect of this regulation is the transcriptional response to changes in sterol levels. Sterol depletion in mammalian cells causes activation of the transcription factors known as sterol regulatory element (SRE) binding proteins (SREBPs) (8), encoded by the homologous genes SREBP-1 and SREBP-2. When sterols are abundant, the SREBPs are inactive, tethered to the endoplasmic reticulum (ER) membrane by two intrinsic transmembrane helices. When sterol levels drop, regulated proteolysis releases the transcriptional activation domains of the SREBPs from the membrane tether, allowing the activation domains to translocate to the nucleus. Once in the nucleus, these domains activate transcription of genes involved in sterol and fatty acid homeostasis.The SREBPs bind the same DNA sequences in vitro (21). In vivo, the SREBPs activate an overlapping set of target genes but may have slightly different DNA binding specificities and/or activities (8,22). Hence, differential activation and synthesis of the SREBPs may make it possible for mammalian cells to respond to a demand for different levels of sterol and nonsterol products.Many genes in the sterol biosynthetic pathway in Saccharomyces cerevisiae are transcriptionally regulated in response to changes in sterol levels (6, 13). However, less is known about this regulatory mechanism in yeast. Also, yeast lack convincing homologues of the mamm...

Section: Resultsmentioning

confidence: 99%

“…When this signal failed to upregulate sterol biosynthesis, because the transcription factors responsible for the effect had been deleted, the cells died. This conclusion is supported by the induction of ERG2 expression in an erg24⌬ mutant (30). Moreover, ERG3-lacZ expression is induced in an erg2⌬ mutant (3).…”

Section: Fig 7 (A)mentioning

confidence: 86%

Upc2p and Ecm22p, Dual Regulators of Sterol Biosynthesis in Saccharomyces cerevisiae

Vik

Rine

2001

249

307

“…When cells are treated with lovastatin, a competitive inhibitor of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase, sterols are depleted and the expression of several ERG genes, including ERG2 and ERG3, is induced (5,10,36,37,40). The induction of ERG2 and ERG3 in response to lovastatin depends on Upc2p and Ecm22p (40).…”

Section: Upc2p and Ecm22p Contained Discrete Transcriptional Activatimentioning

confidence: 99%

“…When sterol levels are reduced, the expression of ergosterol biosynthesis (ERG) genes encoding the enzymes that catalyze late steps in ergosterol biosynthesis, such as ERG2 and ERG3, is substantially increased (5,36,37,40). This induction can be mediated by either Upc2p or Ecm22p; mutants defective in either protein can induce these genes in response to sterol depletion, but the deletion of both genes eliminates induction (40).…”

mentioning

confidence: 99%

Dual Activators of the Sterol Biosynthetic Pathway of Saccharomyces cerevisiae: Similar Activation/Regulatory Domains but Different Response Mechanisms

Davies

Wang

Rine

2005

118

144

Genes encoding biosynthetic enzymes that make ergosterol, the major fungal membrane sterol, are regulated, in part, at the transcriptional level. Two transcription factors, Upc2p and Ecm22p, bind to the promoters of most ergosterol biosynthetic (ERG) genes, including ERG2 and ERG3, and activate these genes upon sterol depletion. We have identified the transcriptional activation domains of Upc2p and Ecm22p and found that UPC2-1, a mutation that allows cells to take up sterols aerobically, increased the potency of the activation domain. The equivalent mutation in ECM22 also greatly enhanced transcriptional activation. The C-terminal regions of Upc2p and Ecm22p, which contained activation domains, also conferred regulation in response to sterol levels. Hence, the activation and regulatory domains of these proteins overlapped. However, the two proteins differed markedly in how they respond to an increased need for sterols. Upon inducing conditions, Upc2p levels increased, and chromatin immunoprecipitation experiments revealed more Upc2p at promoters even when the activation/regulatory domains were tethered to a different DNA-binding domain. However, induction resulted in decreased Ecm22p levels and a corresponding decrease in the amount of Ecm22p bound to promoters. Thus, these two activators differ in their contributions to the regulation of their targets.

“…Apparently exogenous ergosterol is imported and cycles between membranes and lipid droplets but does not affect control points in the endoplasmic reticulum for its synthesis (5,75), given that ergosterol synthesis is not possible without oxygen. In terms of their regulation, Upc2 and Rox1 control many of them (51,84,87), and binding sites for these factors were significantly enriched in clusters containing these genes (Tables 1 and 3).…”

Section: Vol 25 2005 Genomic Response Of Yeast To Anaerobiosis 4085mentioning

confidence: 99%

Dynamical Remodeling of the Transcriptome during Short-Term Anaerobiosis in Saccharomyces cerevisiae: Differential Response and Role of Msn2 and/or Msn4 and Other Factors in Galactose and Glucose Media

Lai¹,

Kosorukoff

Burke³

et al. 2005

117

In contrast to previous steady-state analyses of the O 2 -responsive transcriptome, here we examined the dynamics of the response to short-term anaerobiosis (2 generations) in both catabolite-repressed (glucose) and derepressed (galactose) cells, assessed the specific role that Msn2 and Msn4 play in mediating the response, and identified gene networks using a novel clustering approach. Upon shifting cells to anaerobic conditions in galactose medium, there was an acute (ϳ10 min) yet transient (<45 min) induction of Msn2-and/or Msn4-regulated genes associated with the remodeling of reserve energy and catabolic pathways during the switch from mixed respiro-fermentative to strictly fermentative growth. Concomitantly, MCB-and SCB-regulated networks associated with the G 1 /S transition of the cell cycle were transiently down-regulated along with rRNA processing genes containing PAC and RRPE motifs. Remarkably, none of these gene networks were differentially expressed when cells were shifted in glucose, suggesting that a metabolically derived signal arising from the abrupt cessation of respiration, rather than O 2 deprivation per se, elicits this "stress response." By ϳ0.2 generation of anaerobiosis in both media, more chronic, heme-dependent effects were observed, including the down-regulation of Hap1-regulated networks, derepression of Rox1-regulated networks, and activation of Upc2-regulated ones. Changes in these networks result in the functional remodeling of the cell wall, sterol and sphingolipid metabolism, and dissimilatory pathways required for long-term anaerobiosis. Overall, this study reveals that the acute withdrawal of oxygen can invoke a metabolic state-dependent "stress response" but that acclimatization to oxygen deprivation is a relatively slow process involving complex changes primarily in heme-regulated gene networks.