Expression Of Phospholipid Biosynthetic Genes Research Articles

The stability of the Opi1p repressor for phospholipid biosynthetic gene expression in Saccharomyces cerevisiae is dependent on its interactions with Scs2p and Ino2p

The yeast Saccharomyces cerevisiae Opi1p negatively regulates phospholipid biosynthetic genes. Under derepressing conditions, Opi1p binds to the endoplasmic reticulum/nuclear membrane with the aid of the membrane protein Scs2p and phosphatidic acids under derepressing conditions. Under repressing conditions, it enters the nucleus to inhibit the positive transcription factors Ino2p and Ino4p. While the spatial regulation of Opi1p is understood, the regulation of its abundance remains unclear. We investigated the role of Scs2p and Ino2p in Opi1p stability by overexpressing these proteins in yeast cells. Opi1p was stable in the presence of Scs2p, but mutations in residues required for interaction with Scs2p caused Opi1p unstable. Even in the absence of Scs2p, Opi1p remained stable in the strain having a mutation to increase phosphatidic acid levels. Conversely, overproduction of Ino2p reduced Opi1p stability, whereas a mutant Ino2p that cannot interact with Opi1p did not. Additionally, Opi1p was stable in strains lacking Ino2p or with a mutated Ino2p-binding domain. These findings suggest that regulation, adding another layer to the regulation of phospholipid biosynthetic gene expression by Opi1p.

Transcriptional activation domains interact with ATPase subunits of yeast chromatin remodelling complexes SWI/SNF, RSC and INO80

Chromatin remodelling complexes (CRC) are ATP-dependent molecular machines important for the dynamic organization of nucleosomes along eukaryotic DNA. CRCs SWI/SNF, RSC and INO80 can move positioned nucleosomes in promoter DNA, leading to nucleosome-depleted regions which facilitate access of general transcription factors. This function is strongly supported by transcriptional activators being able to interact with subunits of various CRCs. In this work we show that SWI/SNF subunits Swi1, Swi2, Snf5 and Snf6 can bind to activation domains of Ino2 required for expression of phospholipid biosynthetic genes in yeast. We identify an activator binding domain (ABD) of ATPase Swi2 and show that this ABD is functionally dispensable, presumably because ABDs of other SWI/SNF subunits can compensate for the loss. In contrast, mutational characterization of the ABD of the Swi2-related ATPase Sth1 revealed that some conserved basic and hydrophobic amino acids within this domain are essential for the function of Sth1. While ABDs of Swi2 and Sth1 define separate functional protein domains, mapping of an ABD within ATPase Ino80 showed co-localization with its HSA domain also required for binding actin-related proteins. Comparative interaction studies finally demonstrated that several unrelated activators each exhibit a specific binding pattern with ABDs of Swi2, Sth1 and Ino80.

Multiple Taf subunits of TFIID interact with Ino2 activation domains and contribute to expression of genes required for yeast phospholipid biosynthesis.

Expression of phospholipid biosynthetic genes in yeast requires activator protein Ino2 which can bind to the UAS element inositol/choline-responsive element (ICRE) and trigger activation of target genes, using two separate transcriptional activation domains, TAD1 and TAD2. However, it is still unknown which cofactors mediate activation by TADs of Ino2. Here, we show that multiple subunits of basal transcription factor TFIID (TBP-associated factors Taf1, Taf4, Taf6, Taf10 and Taf12) are able to interact in vitro with activation domains of Ino2. Interaction was no longer observed with activation-defective variants of TAD1. We were able to identify two nonoverlapping regions in the N-terminus of Taf1 (aa 1-100 and aa 182-250) each of which could interact with TAD1 of Ino2 as well as with TAD4 of activator Adr1. Specific missense mutations within Taf1 domain aa 182-250 affecting basic and hydrophobic residues prevented interaction with wild-type TAD1 and caused reduced expression of INO1. Using chromatin immunoprecipitation we demonstrated Ino2-dependent recruitment of Taf1 and Taf6 to ICRE-containing promoters INO1 and CHO2. Transcriptional derepression of INO1 was no longer possible with temperature-sensitive taf1 and taf6 mutants cultivated under nonpermissive conditions. This result supports the hypothesis of Taf-dependent expression of structural genes activated by Ino2.

Induction of intranuclear membranes by overproduction of Opi1p and Scs2p, regulators for yeast phospholipid biosynthesis, suggests a mechanism for Opi1p nuclear translocation.

In the yeast Saccharomyces cerevisiae, the expression of phospholipid biosynthetic genes is suppressed by the Opi1p negative regulator. Opi1p enters into the nucleoplasm from the nuclear membrane to suppress the gene expression under repressing conditions. The binding of Opi1p to the nuclear membrane requires an integral membrane protein, Scs2p and phosphatidic acid (PA). Although it is demonstrated that the association of Opi1p with membranes is affected by PA levels, how Opi1p dissociates from Scs2p is unknown. Here, we found that fluorescently labelled Opi1p accumulated on a perinuclear region in an Scs2p-dependent manner. Electron microscopic analyses indicated that the perinuclear region consists of intranuclear membranes, which may be formed by the invagination of the nuclear membrane due to the accumulation of Opi1p and Scs2p in a restricted area. As expected, localization of Opi1p and Scs2p in the intranuclear membranes was detected by immunoelectron microscopy. Biochemical analysis showed that Opi1p recovered in the membrane fraction was detergent insoluble while Scs2p was soluble, implying that Opi1p behaves differently from Scs2p in the fraction. We hypothesize that Opi1p dissociates from Scs2p after targeting to the nuclear membrane, making it possible to be released from the membrane quickly when PA levels decrease.

Protective role of phosphatidic acid in the cellular response to lysophosphatidylcholine analogues

The synthetic lysophosphatidylcholine (LPC) analogue edelfosine and the naturally occurring platelet activating factor (PAF) are structurally related lipids. Edelfosine is a potent antitumor drug, while PAF is a neurotoxic lipid that has been implicated in Alzheimer's disease. These LPC analogues are also toxic to budding yeast, which can be used as an excellent model system to investigate their mode of action.LPC analogues induce alterations in membrane organization, lipid metabolism and signaling in yeast. Phosphatidic acid (PA) produced by the activity of PLD alleviates sensitivity of yeast cells to PAF and edelfosine. PA is a key lipid signalling and precursor of all glycerolipids. Recent lipid analysis indicates that diacylglycerol (DAG) accumulates in cells treated with LPC analogues and this could contribute to their toxicity. In fact, sensitivity assays in yeast genetically manipulated to alter PA and DAG levels clearly support a protective role of PA in the response to these lipid drugs. Altogether, the evidence suggests that cells are able to sense altered PA: DAG ratios induced by these metabolically stable lipids. While DAG binding proteins have not been clearly identified in yeast, several yeast proteins recognize PA with high specificity. One such protein is the transcriptional repressor Opi1, which regulates expression of phospholipid biosynthetic genes through its interaction with PA. We provide evidence supporting a role of Opi1 in sensing the membrane changes induced by edelfosine, including i) altered sensitivity of cells lacking Opi1, ii) prevalent nuclear localization and iii) Opi1‐dependent UASINO gene repression induced by edelfosine.

Differential roles of Opi1 and Pah1 in the metabolism and regulation of phospholipid biosynthetic genes

In yeast, phosphatidic acid (PA), a common metabolic precursor to both phospholipids and triacylglycerol, influences the localization of Opi1, a negative regulator of transcription of phospholipid biosynthetic genes. PA levels are also regulated in part by the activity of the yeast lipin Pah1. We are investigating the extent to which Opi1 and Pah1 compete for the same PA pool in the endoplasmic reticulum. Since changes in PA levels regulate expression of phospholipid biosynthetic genes, such as INO1, we analyzed the derepression of this gene in wild type and pah1Δ strains during the transition from inositol supplementation to medium lacking inositol. Wild type cells exhibit a peak of INO1 derepression at about 3 hours after removal of inositol while the pah1Δ strain reaches its maximum at 5 hours. In both strains INO1 expression decreased precipitously after reaching its peak. We are currently exploring the relationships between changes in PA content and relocalization of Opi1 to the perinuclear endoplasmic reticulum in wild type, pah1Δ and a series of Pah1 nonphosphorylatable alanine mutants from Carman laboratory.Supported by NIH grant GM‐19629 to SAH

Expression of the INO2 regulatory gene of Saccharomyces cerevisiae is controlled by positive and negative promoter elements and an upstream open reading frame

The INO2 gene encodes a transcriptional activator of the phospholipid biosynthetic genes of Saccharomyces cerevisiae. Complete derepression of phospholipid biosynthetic gene expression in response to inositol/choline deprivation requires both INO2 and INO4. Ino2p dimerizes with Ino4p to bind the upstream activating sequence (UAS)INO element found in the promoters of the target genes. We have demonstrated previously that transcription from the INO2 promoter is autoregulated 12-fold in a manner identical to that of the target genes. Here, we show that this regulation occurs at the levels of transcription and translation. Transcription accounts for fourfold regulation, whereas translation accounts for an additional threefold regulation. Regulation of transcription requires a UAS(INO) element. Additional promoter elements include an upstream essential sequence (UES) located upstream of the UAS(INO) element and a negative regulatory element in the vicinity of the UAS(INO) element. Regulation of translation is dependent on an upstream open reading frame (uORF) in the INO2 leader. These data support the model that regulatory gene promoters may display unusual organizations and may be subject to multiple levels of regulation. We have shown previously that the UME6 gene positively regulates INO2 expression. Here, we limit the UME6-responsive region of the INO2 promoter to nucleotides -217 to -56.

A network of yeast basic helix-loop-helix interactions.

The Ino4 protein belongs to the basic helix-loop-helix (bHLH) family of proteins. It is known to form a dimer with Ino2p, which regulates phospholipid biosynthetic genes. Mammalian bHLH proteins have been shown to form multiple dimer combinations. However, this flexibility in dimerization had not been documented for yeast bHLH proteins. Using the yeast two-hybrid assay and a biochemical assay we show that Ino4p dimerizes with the Pho4p, Rtg1p, Rtg3p and Sgc1p bHLH proteins. Screening a yeast cDNA library identified three additional proteins that interact with Ino4p: Bck2p, YLR422W and YNR064C. The interaction with Bck2p prompted us to examine if any of the Bck2p-associated functions affect expression of phospholipid biosynthetic genes. We found that hyperosmotic growth conditions altered the growth phase regulation of a phospholipid biosynthetic gene, CHO1. There are two recent reports of initial whole genome yeast two-hybrid interactions. Interestingly, one of these reports identified five proteins that interact with Ino4p: Ino2p, Hcs1p, Apl2p, YMR317W and YNL279W. Ino2p is the only protein in common with the data presented here. Our finding that Ino4p interacts with five bHLH proteins suggests that Ino4p is likely to be a central player in the coordination of multiple biological processes.

Combinatorial regulation of phospholipid biosynthetic gene expression by the UME6, SIN3 and RPD3 genes.

The Ume6p-Sin3p-Rpd3p complex negatively regulates expression of genes containing a Ume6p binding site. However, these regulatory proteins also function independently to regulate gene expression both negatively and positively. The model system for this combinatorial regulation is the yeast phospholipid biosynthetic pathway. Sin3p negatively regulates the INO1, CHO1, CHO2 and OPI3 genes while Ume6p negatively regulates the INO1 gene and positively regulates the other genes. We have suggested that the positive regulation results from indirect effects on expression of the INO2 transcriptional activator gene. Here, we demonstrate that the effect of Ume6p on INO2 gene expression is also indirect. We also show that Rpd3p is a negative regulator of phospholipid biosynthetic gene expression. The ability of Ume6p, Sin3p and Rpd3p to differentially regulate expression of the phospholipid biosynthetic genes affects phospholipid composition. A sin3 mutant strain lacks detectable levels of phosphatidylethanolamine and elevated levels of phosphatidylcholine (PC) and a rpd3 mutant strain has reduced levels of PC. These alterations in membrane composition suggest that there may exist additional differences in regulation of phospholipid biosynthetic gene expression and that membrane compositions may be coordinated with other biological processes regulated by Ume6p, Sin3p and Rpd3p.

Overproduction of the Opi1 repressor inhibits transcriptional activation of structural genes required for phospholipid biosynthesis in the yeast Saccharomyces cerevisiae.

Transcription of structural genes required for phospholipid biosynthesis in the yeast Saccharomyces cerevisiae is repressed by high concentrations of inositol and choline. The ICRE (inositol/choline-responsive element), which is necessary and sufficient for regulation by phospholipid precursors, functions as a binding site for the heterodimeric Ino2/Ino4 activator. ICRE-dependent transcription becomes constitutive in the absence of the Opi1 repressor. Opi1 contains a leucine zipper motif and two glutamine-rich stretches. In this work we describe a molecular analysis of OPI1 function and expression. Opi1 mutant variants altered at the leucine zipper and a glutamine-rich region, respectively, were no longer functional repressors. In contrast, an Opi1 deletion variant lacking the N-terminal 106 amino acids still mediated negative regulation. Although the leucine zipper suggests that Opi1 may act as a DNA-binding protein, our data do not support a direct interaction with the ICRE. Despite its function as an antagonist of INO2 and INO4, expression of OPI1 is stimulated by an upstream ICRE. Overexpression of OPI1 under control of the GAL1 promoter severely inhibited activation of ICRE-dependent genes, leading to inositol-requiring cells. Growth inhibition of GAL1-OPI1 was observed with INO2 and INO4 alleles activated by either the natural promoter or a heterologous control region. Although induction of GAL1-OPI1 strongly repressed ICRE-dependent gene expression, the concentration of the Ino2/Ino4 activator remained unchanged. This finding suggests that differential expression of phospholipid biosynthetic genes may occur even in the presence of a constant amount of the specific activator.

The Yeast BSD2-1 Mutation Influences Both the Requirement for Phosphatidylinositol Transfer Protein Function and Derepression of Phospholipid Biosynthetic Gene Expression in Yeast

The BSD2-1 allele renders Saccharomyces cerevisiae independent of its normally essential requirement for phosphatidylinositol transfer protein (Sec14p) in the stimulation of Golgi secretory function and cell viability. We now report that BSD2-1 yeast mutants also exhibit yet another phenotype, an inositol auxotrophy. We demonstrate that the basis for this Ino- phenotype is the inability of BSD2-1 strains to derepress transcription of INO1, the structural gene for the enzyme that catalyzes the committed step in de novo inositol biosynthesis in yeast. This constitutive repression of INO1 expression is mediated through specific inactivation of Ino2p, a factor required for trans-activation of INO1 transcription, and we show that these transcriptional regulatory defects can be uncoupled from the "bypass Sec14p" phenotype of BSD2-1 strains. Finally, we present evidence that newly synthesized phosphatidylinositol is subject to accelerated turnover in BSD2-1 mutants and that prevention of this accelerated phosphatidyl-inositol turnover in turn negates suppression of Sec14p defects by BSD2-1. We propose that, in BSD2-1 strains, a product(s) generated by phosphatidylinositol turnover coordinately modulates the activities of both the Sec14p/Golgi pathway and the pathway through which transcription of phospholipid biosynthetic genes is derepressed.

The Yeast UME6 Gene Is Required for Both Negative and Positive Transcriptional Regulation of Phospholipid Biosynthetic Gene Expression

In Saccharomyces cerevisiae, regulation of the phospholipid biosynthetic genes, INO1, CHO1, CHO2 and OPI3, is known to occur at the level of transcript abundance. Derepression in response to inositol deprivation requires the INO2 and INO4 regulatory genes. Repression in response to inositol supplementation requires the OPI1 regulatory gene. Here, we examined the role of the UME6 global negative regulatory gene in expression of the phospholipid biosynthetic genes. These studies were stimulated by the finding that the INO1 promoter included a UME6 cognate cis-acting regulatory sequence (URS1). We found that the UME6 negative regulatory gene was involved in regulation of phospholipid biosynthetic gene expression through two distinct regulatory pathways. One pathway was the direct repression of INO1 expression through the URS1 element. Surprisingly, the UME6 gene was also required for derepression of CHO1, CHO2 and OPI3 gene expression. Consistent with this observation, the UME6 gene was required for wild-type levels of expression of the INO2 positive regulatory gene. Therefore, the UME6 gene has both a negative and a positive role in regulating phospholipid biosynthesis.

Regulation of yeast phospholipid biosynthetic gene expression in response to inositol involves two superimposed mechanisms.

Transcription of phospholipid biosynthetic genes in the yeast Saccharomyces cerevisiae is maximally derepressed when cells are grown in the absence of inositol and repressed when the cells are grown in its presence. We have previously suggested that this response to inositol may be dictated by regulating transcription of the cognate activator gene, INO2. However, it was also known that cells which harbor a mutant opi1 allele express constitutively derepressed levels of target genes (INO1 and CHO1), implicating the OPI1 negative regulatory gene in the response to inositol. These observations suggested that the response to inositol may involve both regulation of INO2 transcription as well as OPI1-mediated repression. We investigated these possibilities by examining the effect of inositol on target gene expression in a strain containing the INO2 gene under control of the GAL1 promoter. In this strain, transcription of the INO2 gene was regulated in response to galactose but was insensitive to inositol. The expression of the INO1 and CHO1 target genes was still responsive to inositol even though expression of the INO2 gene was unresponsive. However, the level of expression of the INO1 and CHO1 target genes correlated with the level of INO2 transcription. Furthermore, the effect of inositol on target gene expression was eliminated by deleting the OPI1 gene in the GAL1-INO2-containing strain. These data suggest that the OPI1 gene product is the primary target (sensor) of the inositol response and that derepression of INO2 transcription determines the degree of expression of the target genes.

DNA binding site of the yeast heteromeric Ino2p/Ino4p basic helix-loop-helix transcription factor: structural requirements as defined by saturation mutagenesis

The inositol/choline-responsive element (ICRE) is an 11 bp cis-activating sequence motif with central importance for the regulated expression of phospholipid biosynthetic genes in the yeast Saccharomyces cerevisiae. The ICRE containing the CANNTG core binding sequence (E-box) of basic helix-loop-helix (bHLH) regulatory proteins is recognized by the heteromeric bHLH transcription factor Ino2p/Ino4p. In this study, we define the Ino2p/Ino4p consensus binding sequence (5′-WYTTCAYR-TGS-3′) based on the characterization of all possible single nucleotide substitutions. Interestingly, this analysis also identified a single functional deviation (CACATTC) from the CANNTG core recognition element of bHLH proteins. The DNA binding specificities of different yeast bHLH proteins may now be explained by distinct nucleotide preferences especially at two positions immediately preceding the CANNTG core motif.

Autoregulated expression of the yeast INO2 and INO4 helix-loop-helix activator genes effects cooperative regulation on their target genes.

In the yeast Saccharomyces cerevisiae, the phospholipid biosynthetic genes are highly regulated at the transcriptional level in response to the phospholipid precursors inositol and choline. In the absence of inositol and choline (derepressing), the products of the INO2 and INO4 genes form a heteromeric complex which binds to a 10-bp element, upstream activation sequence INO (UASINO), in the promoters of the phospholipid biosynthetic genes to activate their transcription. In the presence of inositol and choline (repressing), the product of the OPI1 gene represses transcription dictated by the UASINO element. Curiously, we identified a UASINO-like element in the promoters of both the INO2 and INO4 genes. The presence of the UASINO element in these two promoters suggested that the mechanism for the inositol-choline response would involved regulating expression of the two activator genes. Using a cat reporter gene, we find that INO2-cat expression was regulated 12-fold in response to inositol and choline but that INO4-cat was constitutively expressed. We further observed that INO2-cat was not expressed in either an ino2 or an ino4 mutant strain and was constitutively overexpressed in an opi1 mutant strain. Expression of the INO4-cat gene was affected only by mutation in the INO4 gene itself. Therefore, INO2-cat transcription is regulated by the products of both the INO2 and INO4 genes whereas INO4 must interact with another protein to activate its own transcription. Our data show that derepression of phospholipid biosynthetic gene expression involves two mechanisms: increasing the levels of the INO2 and INO4 gene products and inactivating the OPI1-mediated repression mechanism. We propose a model suggesting that this dual mechanism of regulation accounts for the observed cooperative stimulation of IN01 and CH01 gene expression (phospholipids biosynthetic genes).

Yeast transcriptional activator INO2 interacts as an Ino2p/Ino4p basic helix-loop-helix heteromeric complex with the inositol/choline-responsive element necessary for expression of phospholipid biosynthetic genes in Saccharomyces cerevisiae.

Coordinate transcriptional control of yeast genes involved in phospholipid biosynthesis is mediated by the inositol/choline-responsive element (ICRE) contained in the respective promoter regions. Regulatory genes INO2 and INO4, both encoding basic helix-loop-helix (bHLH) proteins, are necessary for ICRE-dependent gene activation. By the use of size variants and by heterologous expression in E. coli we demonstrate that Ino2p and Ino4p are both necessary and sufficient for the formation of the previously described FAS binding factor 1, Fbf1, interacting with the ICRE. Formation of a heteromeric complex between Ino2p and Ino4p by means of the respective bHLH domains was demonstrated in vivo by the interaction of appropriate two-hybrid constructs and in vitro by Far-Western analyses. Neither Ino2p nor Ino4p binds to the ICRE as a homodimer. When fused to the DNA-binding domain of Gal4p, Ino2p but not Ino4p was able to activate a UASGAL-containing reporter gene even in the absence of the heterologous Fbf1 subunit. By deletion studies, two separate transcriptional activation domains were identified in the N-terminal part of Ino2p. Thus, the bHLH domains of Ino2p and Ino4p constitute the dimerization/DNA-binding module of Fbf1 mediating its interaction with the ICRE, while transcriptional activation is effected exclusively by Ino2p.