Effect of growth phase on phospholipid biosynthesis in Saccharomyces cerevisiae

The Saccharomyces cerevisiae Spo7 basic tail is required for Nem1-Spo7/Pah1 phosphatase cascade function in lipid synthesis

The Saccharomyces cerevisiae Nem1-Spo7 protein phosphatase complex dephosphorylates and thereby activates Pah1 at the nuclear/endoplasmic reticulum membrane. Pah1, a phosphatidate phosphatase catalyzing the dephosphorylation of phosphatidate to produce diacylglycerol, is one of the most highly regulated enzymes in lipid metabolism. The diacylglycerol produced in the lipid phosphatase reaction is utilized for the synthesis of triacylglycerol that is stored in lipid droplets. Disruptions of the Nem1-Spo7/Pah1 phosphatase cascade cause a plethora of physiological defects. Spo7, the regulatory subunit of the Nem1-Spo7 complex, is required for the Nem1 catalytic function and interacts with the acidic tail of Pah1. Spo7 contains three conserved homology regions (CR1-3) that are important for the interaction with Nem1, but its region for the interaction with Pah1 is unknown. Here, by deletion and site-specific mutational analyses of Spo7, we revealed that the C-terminal basic tail (residues 240-259) containing five arginine and two lysine residues is important for the Nem1-Spo7 complex-mediated dephosphorylation of Pah1 and its cellular function (triacylglycerol synthesis, lipid droplet formation, maintenance of nuclear/endoplasmic reticulum membrane morphology, and cell growth at elevated temperatures). The glutaraldehyde cross-linking analysis of synthetic peptides indicated that the Spo7 basic tail interacts with the Pah1 acidic tail. This work advances our understanding of the Spo7 function and the Nem1-Spo7/Pah1 phosphatase cascade in yeast lipid synthesis.

Phosphatidate-mediated regulation of lipid synthesis at the nuclear/endoplasmic reticulum membrane

In yeast and higher eukaryotes, phospholipids and triacylglycerol are derived from phosphatidate at the nuclear/endoplasmic reticulum membrane. In de novo biosynthetic pathways, phosphatidate is channeled into membrane phospholipids via its conversion to CDP-diacylglycerol. Its dephosphorylation to diacylglycerol is required for the synthesis of triacylglycerol as well as for the synthesis of phosphatidylcholine and phosphatidylethanolamine via the Kennedy pathway. In addition to the role of phosphatidate as a precursor, it is a regulatory molecule in the transcriptional control of phospholipid synthesis genes via the Henry regulatory circuit. Pah1 phosphatidate phosphatase and Dgk1 diacylglycerol kinase are key players that function counteractively in the control of the phosphatidate level at the nuclear/endoplasmic reticulum membrane. Loss of Pah1 phosphatidate phosphatase activity not only affects triacylglycerol synthesis but also disturbs the balance of the phosphatidate level, resulting in the alteration of lipid synthesis and related cellular defects. The pah1Δ phenotypes requiring Dgk1 diacylglycerol kinase exemplify the importance of the phosphatidate level in the misregulation of cellular processes. The catalytic function of Pah1 requires its translocation from the cytoplasm to the nuclear/endoplasmic reticulum membrane, which is regulated through its phosphorylation in the cytoplasm by multiple protein kinases as well as through its dephosphorylation by the membrane-associated Nem1-Spo7 protein phosphatase complex. This article is part of a Special Issue entitled Endoplasmic reticulum platforms for lipid dynamics edited by Shamshad Cockcroft and Christopher Stefan.

Phosphatidate phosphatase regulates membrane phospholipid synthesis via phosphatidylserine synthase

The yeast Saccharomyces cerevisiae serves as a model eukaryote to elucidate the regulation of lipid metabolism. In exponentially growing yeast, a diverse set of membrane lipids are synthesized from the precursor phosphatidate via the liponucleotide intermediate CDP-diacylglycerol. As cells exhaust nutrients and progress into the stationary phase, phosphatidate is channeled via diacylglycerol to the synthesis of triacylglycerol. The CHO1-encoded phosphatidylserine synthase, which catalyzes the committed step in membrane phospholipid synthesis via CDP-diacylglycerol, and the PAH1-encoded phosphatidate phosphatase, which catalyzes the committed step in triacylglycerol synthesis are regulated throughout cell growth by genetic and biochemical mechanisms to control the balanced synthesis of membrane phospholipids and triacylglycerol. The loss of phosphatidate phosphatase activity (e.g., pah1Δ mutation) increases the level of phosphatidate and its conversion to membrane phospholipids by inducing Cho1 expression and phosphatidylserine synthase activity. The regulation of the CHO1 expression is mediated through the inositol-sensitive upstream activation sequence (UAS_INO), a cis-acting element for the phosphatidate-controlled Henry (Ino2-Ino4/Opi1) regulatory circuit. Consequently, phosphatidate phosphatase activity regulates phospholipid synthesis through the transcriptional regulation of the phosphatidylserine synthase enzyme.

Yeast PAH1-encoded phosphatidate phosphatase controls the expression of CHO1-encoded phosphatidylserine synthase for membrane phospholipid synthesis

The PAH1-encoded phosphatidate phosphatase (PAP), which catalyzes the committed step for the synthesis of triacylglycerol in Saccharomyces cerevisiae, exerts a negative regulatory effect on the level of phosphatidate used for the de novo synthesis of membrane phospholipids. This raises the question whether PAP thereby affects the expression and activity of enzymes involved in phospholipid synthesis. Here, we examined the PAP-mediated regulation of CHO1-encoded phosphatidylserine synthase (PSS), which catalyzes the committed step for the synthesis of major phospholipids via the CDP-diacylglycerol pathway. The lack of PAP in the pah1Δ mutant highly elevated PSS activity, exhibiting a growth-dependent up-regulation from the exponential to the stationary phase of growth. Immunoblot analysis showed that the elevation of PSS activity results from an increase in the level of the enzyme encoded by CHO1 Truncation analysis and site-directed mutagenesis of the CHO1 promoter indicated that Cho1 expression in the pah1Δ mutant is induced through the inositol-sensitive upstream activation sequence (UAS_INO), a cis-acting element for the phosphatidate-controlled Henry (Ino2-Ino4/Opi1) regulatory circuit. The abrogation of Cho1 induction and PSS activity by a CHO1 UAS_INO mutation suppressed pah1Δ effects on lipid synthesis, nuclear/endoplasmic reticulum membrane morphology, and lipid droplet formation, but not on growth at elevated temperature. Loss of the DGK1-encoded diacylglycerol kinase, which converts diacylglycerol to phosphatidate, partially suppressed the pah1Δ-mediated induction of Cho1 and PSS activity. Collectively, these data showed that PAP activity controls the expression of PSS for membrane phospholipid synthesis.

Examining the role of membrane lipid composition in determining the ethanol tolerance of Saccharomyces cerevisiae

Yeast (Saccharomyces cerevisiae) has an innate ability to withstand high levels of ethanol that would prove lethal to or severely impair the physiology of other organisms. Significant efforts have been undertaken to elucidate the biochemical and biophysical mechanisms of how ethanol interacts with lipid bilayers and cellular membranes. This research has implicated the yeast cellular membrane as the primary target of the toxic effects of ethanol. Analysis of model membrane systems exposed to ethanol has demonstrated ethanol's perturbing effect on lipid bilayers, and altering the lipid composition of these model bilayers can mitigate the effect of ethanol. In addition, cell membrane composition has been correlated with the ethanol tolerance of yeast cells. However, the physical phenomena behind this correlation are likely to be complex. Previous work based on often divergent experimental conditions and time-consuming low-resolution methodologies that limit large-scale analysis of yeast fermentations has fallen short of revealing shared mechanisms of alcohol tolerance in Saccharomyces cerevisiae. Lipidomics, a modern mass spectrometry-based approach to analyze the complex physiological regulation of lipid composition in yeast and other organisms, has helped to uncover potential mechanisms for alcohol tolerance in yeast. Recent experimental work utilizing lipidomics methodologies has provided a more detailed molecular picture of the relationship between lipid composition and ethanol tolerance. While it has become clear that the yeast cell membrane composition affects its ability to tolerate ethanol, the molecular mechanisms of yeast alcohol tolerance remain to be elucidated.

Ethanol production and maximum cell growth are highly correlated with membrane lipid composition during fermentation as determined by lipidomic analysis of 22 Saccharomyces cerevisiae strains

Optimizing ethanol yield during fermentation is important for efficient production of fuel alcohol, as well as wine and other alcoholic beverages. However, increasing ethanol concentrations during fermentation can create problems that result in arrested or sluggish sugar-to-ethanol conversion. The fundamental cellular basis for these problem fermentations, however, is not well understood. Small-scale fermentations were performed in a synthetic grape must using 22 industrial Saccharomyces cerevisiae strains (primarily wine strains) with various degrees of ethanol tolerance to assess the correlation between lipid composition and fermentation kinetic parameters. Lipids were extracted at several fermentation time points representing different growth phases of the yeast to quantitatively analyze phospholipids and ergosterol utilizing atmospheric pressure ionization-mass spectrometry methods. Lipid profiling of individual fermentations indicated that yeast lipid class profiles do not shift dramatically in composition over the course of fermentation. Multivariate statistical analysis of the data was performed using partial least-squares linear regression modeling to correlate lipid composition data with fermentation kinetic data. The results indicate a strong correlation (R(2) = 0.91) between the overall lipid composition and the final ethanol concentration (wt/wt), an indicator of strain ethanol tolerance. One potential component of ethanol tolerance, the maximum yeast cell concentration, was also found to be a strong function of lipid composition (R(2) = 0.97). Specifically, strains unable to complete fermentation were associated with high phosphatidylinositol levels early in fermentation. Yeast strains that achieved the highest cell densities and ethanol concentrations were positively correlated with phosphatidylcholine species similar to those known to decrease the perturbing effects of ethanol in model membrane systems.

Phosphorylation of yeast phosphatidylserine synthase by protein kinase A: identification of Ser46 and Ser47 as major sites of phosphorylation

The CHO1-encoded phosphatidylserine synthase from Saccharomyces cerevisiae is phosphorylated and inhibited by protein kinase A in vitro. CHO1 alleles bearing Ser(46) --> Ala and/or Ser(47) --> Ala mutations were constructed and expressed in a cho1Delta mutant lacking phosphatidylserine synthase. In vitro, the S46A/S47A mutation reduced the total amount of phosphorylation by 90% and abolished the inhibitory effect protein kinase A had on phosphatidylserine synthase activity. The enzyme phosphorylation by protein kinase A, which was time- and dose-dependent and dependent on the concentration of ATP, caused a electrophoretic mobility shift from a 27-kDa form to a 30-kDa form. The two electrophoretic forms of phosphatidylserine synthase were present in exponential phase cells, whereas only the 27-kDa form was present in stationary phase cells. In vivo labeling with (32)P(i) and immune complex analysis showed that the 30-kDa form was predominantly phosphorylated when compared with the 27-kDa form. However, the S46A/S47A mutations abolished the protein kinase A-mediated electrophoretic mobility shift. The S46A/S47A mutations also caused a 55% reduction in the total amount of phosphatidylserine synthase in exponential phase cells and a 66% reduction in the amount of enzyme in stationary phase cells. In phospholipid composition analysis, cells expressing the S46A/S47A mutant enzyme exhibited a 57% decrease in phosphatidylserine and a 40% increase in phosphatidylinositol. These results indicate that phosphatidylserine synthase is phosphorylated on Ser(46) and Ser(47) by protein kinase A, which results in a higher amount of enzyme for the net effect of stimulating the synthesis of phosphatidylserine.

Characterization of the yeast DGK1-encoded CTP-dependent diacylglycerol kinase

The Saccharomyces cerevisiae DGK1 gene encodes a diacylglycerol kinase enzyme that catalyzes the formation of phosphatidate from diacylglycerol. Unlike the diacylglycerol kinases from bacteria, plants, and animals, the yeast enzyme utilizes CTP, instead of ATP, as the phosphate donor in the reaction. Dgk1p contains a CTP transferase domain that is present in the SEC59-encoded dolichol kinase and CDS1-encoded CDP-diacylglycerol synthase enzymes. Deletion analysis showed that the CTP transferase domain was sufficient for diacylglycerol kinase activity. Point mutations (R76A, K77A, D177A, and G184A) of conserved residues within the CTP transferase domain caused a loss of diacylglycerol kinase activity. Analysis of DGK1 alleles showed that the in vivo functions of Dgk1p were specifically due to its diacylglycerol kinase activity. The DGK1-encoded enzyme had a pH optimum at 7.0-7.5, required Ca(2+) or Mg(2+) ions for activity, was potently inhibited by N-ethylmaleimide, and was labile at temperatures above 40 degrees C. The enzyme exhibited positive cooperative (Hill number = 2.5) kinetics with respect to diacylglycerol (apparent K(m) = 6.5 mol %) and saturation kinetics with respect to CTP (apparent K(m) = 0.3 mm). dCTP was both a substrate (apparent K(m) = 0.4 mm) and competitive inhibitor (apparent K(i) = 0.4 mm) of the enzyme. Diacylglycerol kinase activity was stimulated by major membrane phospholipids and was inhibited by CDP-diacylglycerol and sphingoid bases.

Coordination of the dynamics of yeast sphingolipid metabolism during the diauxic shift

BACKGROUND

The diauxic shift in yeast requires cells to coordinate a complicated response that involves numerous genes and metabolic processes. It is unknown whether responses of this type are mediated in vivo through changes in a few "key" genes and enzymes, which are mathematically characterized by high sensitivities, or whether they are based on many small changes in genes and enzymes that are not particularly sensitive. In contrast to global assessments of changes in gene or protein interaction networks, we study here control aspects of the diauxic shift by performing a detailed analysis of one specific pathway-sphingolipid metabolism-which is known to have signaling functions and is associated with a wide variety of stress responses.

RESULTS

The approach uses two components: publicly available sets of expression data of sphingolipid genes and a recently developed Generalized Mass Action (GMA) mathematical model of the sphingolipid pathway. In one line of exploration, we analyze the sensitivity of the model with respect to enzyme activities, and thus gene expression. Complementary to this approach, we convert the gene expression data into changes in enzyme activities and then predict metabolic consequences by means of the mathematical model. It was found that most of the sensitivities in the model are low in magnitude, but that some stand out as relatively high. This information was then deployed to test whether the cell uses a few of the very sensitive pathway steps to mount a response or whether the control is distributed throughout the pathway. Pilot experiments confirm qualitatively and in part quantitatively the predictions of a group of metabolite simulations.

CONCLUSION

The results indicate that yeast coordinates sphingolipid mediated changes during the diauxic shift through an array of small changes in many genes and enzymes, rather than relying on a strategy involving a few select genes with high sensitivity. This study also highlights a novel approach in coupling data mining with mathematical modeling in order to evaluate specific metabolic pathways.

Respiratory deficiency mediates the regulation of CHO1-encoded phosphatidylserine synthase by mRNA stability in Saccharomyces cerevisiae

The CHO1-encoded phosphatidylserine synthase (CDP-diacylglycerol:l-serine O-phosphatidyltransferase, EC 2.7.8.8) is one of the most highly regulated phospholipid biosynthetic enzymes in the yeast Saccharomyces cerevisiae. CHO1 expression is regulated by nutrient availability through a regulatory circuit involving a UAS(INO) cis-acting element in the CHO1 promoter, the positive transcription factors Ino2p and Ino4p, and the transcriptional repressor Opi1p. In this work, we examined the post-transcriptional regulation of CHO1 by mRNA stability. CHO1 mRNA was stabilized in mutants defective in deadenylation (ccr4Delta), mRNA decapping (dcp1), and the 5'-3'-exonuclease (xrn1), indicating that the CHO1 transcript is primarily degraded through the general 5'-3' mRNA decay pathway. In respiratory-sufficient cells, the CHO1 transcript was moderately stable with a half-life of 12 min. However, the CHO1 transcript was stabilized to a half-life of >45 min in respiratory-deficient (rho(-) and rho(o)) cells, the cox4Delta mutant defective in the cytochrome c oxidase, and wild type cells treated with KCN (a cytochrome c oxidase inhibitor). The increased CHO1 mRNA stability in response to respiratory deficiency caused increases in CHO1 mRNA abundance, phosphatidylserine synthase protein and activity, and the synthesis of phosphatidylserine in vivo. Respiratory deficiency also caused increases in the activities of CDP-diacylglycerol synthase, phosphatidylserine decarboxylase, and the phospholipid methyltransferases. Phosphatidylinositol synthase and choline kinase activities were not affected by respiratory deficiency. This work advances our understanding of phosphatidylserine synthase regulation and underscores the importance of mitochondrial respiration to the regulation of phospholipid synthesis in S. cerevisiae.

Expression of the Saccharomyces cerevisiae PIS1 gene is modulated by multiple ATGs in the promoter

The PIS1 gene encodes a key branchpoint phospholipid biosynthetic enzyme, phosphatidylinositol synthase. The PIS1 promoter contains the unusual feature of three ATG codons (ATGs1, 2, and 3) in-frame with three stop codons, located just before the authentic start codon (ATG4). Using a PIS1(promoter)-lacZ reporter expression system and site-directed mutagenesis, we investigated the role the "upstream" ATG codons play in modulation of PIS1 expression. Of the single codon changes, mutation of the first ATG (ATG1) resulted in the largest increase of the reporter gene PIS1(promoter)-lacZ expression. All combinations of altered upstream ATG codons also resulted in greater reporter expression. Reverse transcription-PCR revealed that at least some PIS1 transcripts include all AUG codons, and their synthesis is probably directed by a second TATA box upstream of the putative TATA box. These results indicate that the multiple upstream AUG codons are present in at least some PIS1 transcripts and negatively impact PIS1 expression.

Regulation of phospholipid synthesis in Saccharomyces cerevisiae by zinc depletion

The synthesis of phospholipids in the yeast Saccharomyces cerevisiae is regulated by zinc, an essential mineral required for growth and metabolism. Cells depleted of zinc contain increased levels of phosphatidylinositol and decreased levels of phosphatidylethanolamine. In addition to the major phospholipids, the levels of the minor phospholipids phosphatidate and diacylglycerol pyrophosphate decrease in the vacuole membrane of zinc-depleted cells. Alterations in phosphatidylinositol and phosphatidylethanolamine can be ascribed to an increase in PIS1-encoded phosphatidylinositol synthase activity and to decreases in the activities of CDP-diacylglycerol pathway enzymes including the CHO1-encoded phosphatidylserine synthase, respectively. Alterations in the minor vacuole membrane phospholipids are due to the induction of the DPP1-encoded diacylglycerol pyrophosphate phosphatase. These changes in the activities of phospholipid biosynthetic enzymes result from differential regulation of gene expression at the level of transcription. Under zinc-deplete conditions, the positive transcription factor Zap1p stimulates the expression of the DPP1 and PIS1 genes through the cis-acting element UAS(ZRE). In contrast, the negative regulatory protein Opi1p, which is involved in inositol-mediated regulation of phospholipid synthesis, represses the expression of the CHO1 gene through the cis-acting element UAS(INO). Regulation of phospholipid synthesis may provide an important mechanism by which cells cope with the stress of zinc depletion, given the roles that phospholipids play in the structure and function of cellular membranes.

Regulation of the Saccharomyces cerevisiae EKI1-encoded ethanolamine kinase by zinc depletion

Ethanolamine kinase catalyzes the committed step in the synthesis of phosphatidylethanolamine via the CDP-ethanolamine branch of the Kennedy pathway. Regulation of the EKI1-encoded ethanolamine kinase by the essential nutrient zinc was examined in Saccharomyces cerevisiae. The level of ethanolamine kinase activity increased when zinc was depleted from the growth medium. This regulation correlated with increases in the CDP-ethanolamine pathway intermediates phosphoethanolamine and CDP-ethanolamine, and an increase in the methylated derivative of phosphatidylethanolamine, phosphatidylcholine. The beta-galactosidase activity driven by the P(EKI1)-lacZ reporter gene was elevated in zinc-depleted cells, indicating that the increase in ethanolamine kinase activity was attributed to a transcriptional mechanism. The expression level of P(EKI1)-lacZ reporter gene activity in the zrt1deltazrt2delta mutant (defective in plasma membrane zinc transport) cells grown with zinc was similar to the activity expressed in wild-type cells grown without zinc. This indicated that EKI1 expression was sensitive to intracellular zinc. The zinc-mediated regulation of EKI1 expression was attenuated in the zap1delta mutant defective in the zinc-regulated transcription factor Zap1p. Direct interactions between Zap1p and putative zinc-responsive elements in the EKI1 promoter were demonstrated by electrophoretic mobility shift assays. Mutations of these elements to a nonconsensus sequence abolished Zap1p-DNA interactions. Taken together, this work demonstrated that the zinc-mediated regulation of ethanolamine kinase and the synthesis of phospholipids via the CDP-ethanolamine branch of the Kennedy pathway were controlled in part by Zap1p.

The Saccharomyces cerevisiae Lipin homolog is a Mg2+-dependent phosphatidate phosphatase enzyme

Mg(2+)-dependent phosphatidate (PA) phosphatase (3-sn-phosphatidate phosphohydrolase, EC 3.1.3.4) catalyzes the dephosphorylation of PA to yield diacylglycerol and P(i). In this work, we identified the Saccharomyces cerevisiae PAH1 (previously known as SMP2) gene that encodes Mg(2+)-dependent PA phosphatase using amino acid sequence information derived from a purified preparation of the enzyme (Lin, Y.-P., and Carman, G. M. (1989) J. Biol. Chem. 264, 8641-8645). Overexpression of PAH1 in S. cerevisiae directed elevated levels of Mg(2+)-dependent PA phosphatase activity, whereas the pah1Delta mutation caused reduced levels of enzyme activity. Heterologous expression of PAH1 in Escherichia coli confirmed that Pah1p is a Mg(2+)-dependent PA phosphatase enzyme and showed that its enzymological properties were very similar to those of the enzyme purified from S. cerevisiae. The PAH1-encoded enzyme activity was associated with both the membrane and cytosolic fractions of the cell, and the membrane-bound form of the enzyme was salt-extractable. Lipid analysis showed that mutants lacking PAH1 accumulated PA and had reduced amounts of diacylglycerol and its derivative triacylglycerol.ThePAH1-encoded Mg(2+)-dependent PA phosphatase shows homology to mammalian lipin, a fat-regulating protein whose molecular function is unknown. Heterologous expression of human LPIN1 in E. coli showed that lipin 1 is also a Mg(2+)-dependent PA phosphatase enzyme.

Phosphatidylinositol biosynthesis: biochemistry and regulation

Phosphatidylinositol (PI) is a ubiquitous membrane lipid in eukaryotes. It is becoming increasingly obvious that PI and its metabolites play a myriad of very diverse roles in eukaryotic cells. The Saccharomyces cerevisiae PIS1 gene is essential and encodes PI synthase, which is required for the synthesis of PI. Recently, PIS1 expression was found to be regulated in response to carbon source and oxygen availability. It is particularly significant that the promoter elements required for these responses are conserved evolutionarily throughout the Saccharomyces genus. In addition, several genome-wide strategies coupled with more traditional screens suggest that several other factors regulate PIS1 expression. The impact of regulating PIS1 expression on PI synthesis will be discussed along with the possible role(s) that this may have on diseases such as cancer.

Regulation of the PIS1-encoded phosphatidylinositol synthase in Saccharomyces cerevisiae by zinc

In the yeast Saccharomyces cerevisiae, the mineral zinc is essential for growth and metabolism. Depletion of zinc from the growth medium of wild type cells results in changes in phospholipid metabolism, including an increase in phosphatidylinositol content (Iwanyshyn, W. M., Han, G.-S., and Carman, G. M. (2004) J. Biol. Chem. 279, 21976-21983). We examined the effects of zinc depletion on the regulation of the PIS1-encoded phosphatidylinositol synthase, the enzyme that catalyzes the formation of phosphatidylinositol from CDP-diacylglycerol and inositol. Phosphatidylinositol synthase activity increased when zinc was depleted from the growth medium. Analysis of a zrt1Delta zrt2Delta mutant defective in plasma membrane zinc transport indicated that the cytoplasmic levels of zinc were responsible for the regulation of phosphatidylinositol synthase. PIS1 mRNA, its encoded protein Pis1p, and the beta-galactosidase activity driven by the P(PIS1)-lacZ reporter gene were elevated in zinc-depleted cells. This indicated that the increase in phosphatidylinositol synthase activity was the result of a transcriptional mechanism. The zinc-mediated induction of the P(PIS1)-lacZ reporter gene, Pis1p, and phosphatidylinositol synthase activity was lost in zap1Delta mutant cells. These data indicated that the regulation of PIS1 gene expression by zinc depletion was mediated by the zinc-regulated transcription factor Zap1p. Direct interaction between glutathione S-transferase (GST)-Zap1p(687-880) and a putative upstream activating sequence (UAS) zinc-responsive element in the PIS1 promoter was demonstrated by electrophoretic mobility shift assays. Mutations in the UAS zinc-responsive element in the PIS1 promoter abolished the GST-Zap1p(687-880)-DNA interaction in vitro and abolished the zinc-mediated regulation of the PIS1 gene in vivo. This work advances understanding of phospholipid synthesis regulation by zinc and the transcription control of the PIS1 gene.

Neuropathy Target Esterase and Its Yeast Homologue Degrade Phosphatidylcholine to Glycerophosphocholine in Living Cells

Eukaryotic cells control the levels of their major membrane lipid, phosphatidylcholine (PtdCho), by balancing synthesis with degradation via deacylation to glycerophosphocholine (GroPCho). Here we present evidence that in both yeast and mammalian cells this deacylation is catalyzed by neuropathy target esterase (NTE), a protein originally identified by its reaction with organophosphates, which cause nerve axon degeneration. YML059c, a Saccharomyces cerevisiae protein with sequence homology to NTE, had similar catalytic properties to the mammalian enzyme in assays of microsome preparations and, like NTE, was localized to the endoplasmic reticulum. Yeast lacking YML059c were viable under all conditions examined but, unlike the wild-type strain, did not convert PtdCho to GroPCho. Despite the absence of the deacylation pathway, the net rate of [(14)C]choline incorporation into PtdCho in YML059c-null yeast was not greater than that in the wild type; this was because, in the null strain diminished net uptake of extracellular choline and decreased formation of the rate-limiting intermediate, CDP-choline, resulted in a reduced rate of PtdCho synthesis. In [(14)C]choline labeling experiments with cultured mammalian cell lines, production of [(14)C]GroPCho was enhanced by overexpression of catalytically active NTE and was diminished by reduction of endogenous NTE activity mediated either by RNA interference or organophosphate treatment. We conclude that NTE and its homologues play a central role in membrane lipid homeostasis.

Regulation of phospholipid synthesis in Saccharomyces cerevisiae by zinc

Zinc is an essential nutrient required for the growth and metabolism of eukaryotic cells. In this work, we examined the effects of zinc depletion on the regulation of phospholipid synthesis in the yeast Saccharomyces cerevisiae. Zinc depletion resulted in a decrease in the activity levels of the CDP-diacylglycerol pathway enzymes phosphatidylserine synthase, phosphatidylserine decarboxylase, phosphatidylethanolamine methyltransferase, and phospholipid methyltransferase. In contrast, the activity of phosphatidylinositol synthase was elevated in response to zinc depletion. The level of Aut7p, a marker for the induction of autophagy, was also elevated in zinc-depleted cells. For the CHO1-encoded phosphatidylserine synthase, the reduction in activity in response to zinc depletion was controlled at the level of transcription. This regulation was mediated through the UAS(INO) element and by the transcription factors Ino2p, Ino4p, and Opi1p that are responsible for the inositol-mediated regulation of UAS(INO)-containing genes involved in phospholipid synthesis. Analysis of the cellular composition of the major membrane phospholipids showed that zinc depletion resulted in a 66% decrease in phosphatidylethanolamine and a 29% increase in phosphatidylinositol. A zrt1Delta zrt2Delta mutant (defective in the plasma membrane zinc transporters Zrt1p and Zrt2p) grown in the presence of zinc exhibited a phospholipid composition similar to that of wild type cells depleted for zinc. These results indicated that a decrease in the cytoplasmic levels of zinc was responsible for the alterations in phospholipid composition.

Regulation of phospholipid synthesis in the yeast cki1Delta eki1Delta mutant defective in the Kennedy pathway. The Cho1-encoded phosphatidylserine synthase is regulated by mRNA stability

In the yeast Saccharomyces cerevisiae, the most abundant phospholipid phosphatidylcholine is synthesized by the complementary CDP-diacylglycerol and Kennedy pathways. Using a cki1Delta eki1Delta mutant defective in choline kinase and ethanolamine kinase, we examined the consequences of a block in the Kennedy pathway on the regulation of phosphatidylcholine synthesis by the CDP-diacylglycerol pathway. The cki1Delta eki1Delta mutant exhibited increases in the synthesis of phosphatidylserine, phosphatidylethanolamine, and phosphatidylcholine via the CDP-diacylglycerol pathway. The increase in phospholipid synthesis correlated with increased activity levels of the CDP-diacylglycerol pathway enzymes phosphatidylserine synthase, phosphatidylserine decarboxylase, phosphatidylethanolamine methyltransferase, and phospholipid methyltransferase. However, other enzyme activities, including phosphatidylinositol synthase and phosphatidate phosphatase, were not affected in the cki1Delta eki1Delta mutant. For phosphatidylserine synthase, the enzyme catalyzing the committed step in the pathway, activity was regulated by increases in the levels of mRNA and protein. Decay analysis of CHO1 mRNA indicated that a dramatic increase in transcript stability was a major component responsible for the elevated level of phosphatidylserine synthase. These results revealed a novel mechanism that controls phospholipid synthesis in yeast.

Regulation of phosphatidylglycerophosphate synthase by inositol in Saccharomyces cerevisiae is not at the level of PGS1 mRNA abundance

Phosphatidylglycerophosphate synthase catalyzes the committed step in the synthesis of the mitochondrial phospholipid cardiolipin. We showed previously that phosphatidylglycerophosphate synthase activity in Saccharomyces cerevisiae is increased in conditions favoring mitochondrial development and during growth in the absence of inositol. Interestingly, the regulatory effects of inositol were not altered in ino2, ino4, or opi1 mutants suggesting that regulation in response to inositol is not at the level of gene transcription. We report here that steady state mRNA levels of the PGS1 gene, which encodes phosphatidylglycerophosphate synthase, were not altered by inositol or choline. Growth in the presence of the inositol-depleting drug valproate led to an increase in phosphatidylglycerophosphate synthase activity unaccompanied by increased PGS1 mRNA. PGS1 mRNA abundance was not decreased in ino2 or ino4 mutants and was unaffected in an opi1 mutant. Therefore, regulation of phosphatidylglycerophosphate synthase by inositol is not mediated at the level of mRNA abundance and does not require the INO2-INO4-OPI1 regulatory circuit. PGS1 was increased in glycerol/ethanol compared with glucose media and was maximally expressed as cells entered the stationary phase. Deletion of the mitochondrial genome did not affect PGS1 expression. Thus, whereas inositol controls phosphatidylglycerophosphate synthase activity, regulation of PGS1 expression occurs primarily in response to mitochondrial development cues.

Phosphorylation of Saccharomyces cerevisiae CTP synthetase at Ser424 by protein kinases A and C regulates phosphatidylcholine synthesis by the CDP-choline pathway

The Saccharomyces cerevisiae URA7-encoded CTP synthetase is phosphorylated and stimulated by protein kinases A and C. Previous studies have revealed that Ser424 is the target site for protein kinase A. Using a purified S424A mutant CTP synthetase enzyme, we examined the effect of Ser424 phosphorylation on protein kinase C phosphorylation. The S424A mutation in CTP synthetase caused a 50% decrease in the phosphorylation of the enzyme by protein kinase C and an 80% decrease in the stimulatory effect on CTP synthetase activity by protein kinase C. The S424A mutation caused increases in the apparent Km values of CTP synthetase and ATP of 20-and 2-fold, respectively, in the protein kinase C reaction. The effect of the S424A mutation on the phosphorylation reaction was dependent on time and protein kinase C concentration. A CTP synthetase synthetic peptide (SLGRKDSHSA) containing Ser424 was a substrate for protein kinase C. Comparison of phosphopeptide maps of the wild type and S424A mutant CTP synthetase enzymes phosphorylated by protein kinases A and C indicated that Ser424 was also a target site for protein kinase C. Phosphorylation of Ser424 accounted for 10% of the total phosphorylation of CTP synthetase by protein kinase C. The incorporation of [methyl-3H]choline into phosphocholine, CDP-choline, and phosphatidylcholine in cells carrying the S424A mutant CTP synthetase enzyme was reduced by 48, 32, and 46%, respectively, when compared with control cells. These data indicated that phosphorylation of Ser424 by protein kinase A or by protein kinase C was required for maximum phosphorylation and stimulation of CTP synthetase and that the phosphorylation of this site played a role in the regulation of phosphatidylcholine synthesis by the CDP-choline pathway.

Phosphorylation of CTP synthetase on Ser36, Ser330, Ser354, and Ser454 regulates the levels of CTP and phosphatidylcholine synthesis in Saccharomyces cerevisiae

The Saccharomyces cerevisiae URA7-encoded CTP synthetase is phosphorylated and stimulated by protein kinase C. We examined the hypothesis that Ser36, Ser330, Ser354, and Ser454, contained in a protein kinase C sequence motif in CTP synthetase, were target sites for the kinase. Synthetic peptides containing a phosphorylation motif at these serine residues served as substrates for protein kinase C in vitro. Ser --> Ala (S36A, S330A, S354A, and S454A) mutations in CTP synthetase were constructed by site-directed mutagenesis and expressed normally in a ura7 ura8 double mutant that lacks CTP synthetase activity. The CTP synthetase activity in extracts from cells bearing the S36A, S354A, and S454A mutant enzymes was reduced when compared with cells bearing the wild type enzyme. Kinetic analysis of purified mutant enzymes showed that the S36A and S354A mutations caused a decrease in the Vmax of the reaction. This regulation could be attributed in part by the effects phosphorylation has on the nucleotide-dependent oligomerization of CTP synthetase. In contrast, CTP synthetase activity in cells bearing the S330A mutant enzyme was elevated, and kinetic analysis of purified enzyme showed that the S330A mutation caused an elevation in the Vmax of the reaction. In vitro data indicated that phosphorylation of CTP synthetase at Ser330 affected the phosphorylation of the enzyme at another site. The phosphorylation of CTP synthetase at Ser36, Ser330, Ser354, and Ser454 residues was physiologically relevant. Cells bearing the S36A, S354A, and S454A mutations had reduced CTP levels, whereas cells with the S330A mutation had elevated CTP levels. The alterations in CTP levels correlated with the regulatory effects CTP has on the pathways responsible for the synthesis of the membrane phospholipid phosphatidylcholine.

Molecular and cell biology of phosphatidylserine and phosphatidylethanolamine metabolism

In this review, the pathways for phosphatidylserine (PS) and phosphatidylethanolamine (PE) biosynthesis, as well as the genes and proteins involved in these pathways, are described in mammalian cells, yeast, and prokaryotes. In mammalian cells, PS is synthesized by a base-exchange reaction in which phosphatidylcholine or PE is substrate for PS synthase-1 or PS synthase-2, respectively. Isolation of Chinese hamster ovary cell mutants led to the cloning of cDNAs and genes encoding these two PS synthases. In yeast and prokaryotes PS is produced by a biosynthetic pathway completely different from that in mammals: from a reaction between CDP-diacylglycerol and serine. The major route for PE synthesis in cultured cells is from the mitochondrial decarboxylation of PS. Alternatively, PE can be synthesized in the endoplasmic reticulum (ER) from the CDP-ethanolamine pathway. Genes and/or cDNAs encoding all the enzymes in these two pathways for PE synthesis have been isolated and characterized. In mammalian cells, PS is synthesized on the ER and/or mitochondria-associated membranes (MAM). PS synthase-1 and -2 are highly enriched in MAM compared to the bulk of ER. Since MAM are a region of the ER that appears to be in close juxtaposition to the mitochondrial outer membrane, it has been proposed that MAM act as a conduit for the transfer of newly synthesized PS into mitochondria. A similar pathway appears to operate in yeast. The use of yeast mutants has led to identification of genes involved in the interorganelle transport of PS and PE in yeast, but so far none of the corresponding genes in mammalian cells has been identified. PS and PE do not act solely as structural components of membranes. Several specific functions have been ascribed to these two aminophospholipids. For example, cell-surface exposure of PS during apoptosis is thought to be the signal by which apoptotic cells are recognized and phagocytosed. Translocation of PS from the inner to outer leaflet of the plasma membrane of platelets initiates the blood-clotting cascade, and PS is an important activator of several enzymes, including protein kinase C. Recently, exposure of PE on the cell surface was identified as a regulator of cytokinesis. In addition, in Escherichia coli, PE appears to be involved in the correct folding of membrane proteins; and in Drosophila, PE regulates lipid homeostasis via the sterol response element-binding protein.

Nonradioactive analysis of phosphatidylinositides and other anionic phospholipids by anion-exchange high-performance liquid chromatography with suppressed conductivity detection

Phosphatidylinositol 4,5-biphosphate (PIP(2)) modulates the function of numerous ion transporters and channels, as well as cell signaling and cytoskeletal proteins. To study PIP(2) levels of cells without radiolabeling, we have developed a new method to quantify anionic phospholipid species. Phospholipids are extracted and deacylated to glycero-head groups, which are then separated by anion-exchange HPLC and detected by suppressed conductivity measurements. The major anionic head groups can be quantified in single runs with practical detection limits of about 100 pmol, and the D3 isoforms of phosphatidylinositol phosphate (PIP) and PIP(2) are detected as shoulder peaks. In HeLa, Hek 293 and COS cells, as well as intact heart, PIP(2) amounts to 0.5 to 1.5% of total anionic phospholipid (10 to 30 micromol/liter cell water or 0.15 to 0.45 nmol/mg protein). In cell cultures, overexpression of Type I PIP5-kinase specifically increases PIP(2), whereas overexpression of Type II PI4-kinase can increase both PIP and PIP(2). Phosphatidylinositol 3,4,5-trisphosphate (PIP(3)) and the D3 isomers of PIP(2) are detected after treatment of cells with pervanadate; in yeast, overexpression of a phosphatidylinositol 3-kinase (VPS34) specifically increases phosphatidylinositol 3-phosphate (PI3P). Using isolated cardiac membranes, lipid kinase and lipid phosphatase activities can be monitored with the same methods. Upon addition of ATP, PIP increases while PIP(2) remains low; exogenous PIP(2) is rapidly degraded to PIP and phosphatidylinositol (PI). In summary, the HPLC methods described here can be used to probe multiple aspects of phosphatidylinositide (Ptide) metabolism without radiolabeling.

Synthesis of phosphatidylserine in carrot cells cultured under carbon-source starvation

When carrot suspension cells were cultured on medium containing no carbon source (starvation), the levels of phosphatidylserine (PS) increased transiently 3-4 d after the initiation of starvation while levels of most other phospholipid (PL) species decreased. We previously reported that fatty acids of these PLs served as an alternative carbon source during starvation. The present study showed that cells possess two different biosynthetic pathways involving phosphatidylcholine (PC)/phosphatidylethanolamine (PE) exchange enzymes and PS synthase to synthesize PS. These activities peaked similarly 4 d after the initiation of starvation and coincided with the peak of PS level. The synthesis of serine was also significantly activated during starvation. The activity of phosphoserine aminotransferase (PSAT) which is involved in serine synthesis increased with a time course similar to that of the increase in the PS level. These observations suggest that the increase in PS level plays an important role in membranes which are degraded during starvation.

Expression of yeast INM1 encoding inositol monophosphatase is regulated by inositol, carbon source and growth stage and is decreased by lithium and valproate

Inositol monophosphatase plays a vital role in the de novo biosynthesis of inositol and in the phosphoinositide second messenger signalling pathway. We cloned the Saccharomyces cerevisiae open reading frame (ORF) YHR046c (termed INM1), which encodes inositol monophosphatase, characterized the protein Inm1p and analysed expression of the INM1 gene. INM1 was expressed in bacteria under the control of the lacZ promoter. The purified protein has inositol monophosphatase activity that is inhibited by the antibipolar drug lithium, but not valproate. In the inm1Delta:URA3 null mutant, inositol monophosphatase activity was reduced but not eliminated. The disruption had little effect on growth in the presence of lithium or valproate and no effect on growth in the absence of inositol. To characterize the regulation of INM1, we examined the effects of inositol, carbon source, growth phase, and the antibipolar drugs lithium and valproate on INM1 expression using an INM1-lacZ reporter gene. Unlike all other phospholipid biosynthetic enzyme-encoding genes studied, which contain the UASINO regulatory element, INM1 expression is increased in the presence of inositol. In addition, INM1 expression was repressed during growth in glycerol and derepressed as glucose-grown cells entered stationary. Both lithium and valproate, which cause a decrease in intracellular inositol, effect a decrease in INM1 expression. A model is presented to account for regulation of INM1 expression.

The phosphatidylcholine to phosphatidylethanolamine ratio of Saccharomyces cerevisiae varies with the growth phase

This study compares the effect of the growth phase on the phospholipid composition and the activity of several phospholipid biosynthetic enzymes in a wild-type yeast grown in fermentable (glucose) and non-fermentable (lactate) semi-synthetic and complete synthetic media. Several distinct differences as well as similarities were found. The cellular phosphatidylcholine: phosphatidylethanolamine (PC:PE) ratio was found to vary with the growth phase, with increases in PC levels at the expense of PE during the transition to stationary phase. The variation was most pronounced in semi-synthetic lactate medium, which is routinely used for the isolation of mitochondria, where the PC:PE ratio changed from 0.9 to 2.2 during this transition. Similar growth phase-dependent changes in PC and PE content were observed in isolated organelles such as mitochondria, mitochondria-associated membranes and microsomes. Phosphatidylinositol (PI) levels were much higher in cells grown on lactate compared to cells grown on glucose (20% vs. 5-10%). Irrespective of the medium, PI levels increased upon entering stationary phase. The activities of the phospholipid biosynthetic enzymes phosphatidylserine synthase and the phospholipid-N-methyltransferases were found to be maximal at the end of logarithmic growth and to decrease upon entering stationary phase in all media. Cells grown on lactate displayed a significantly higher phospholipid:protein ratio than cells grown on glucose. The results are discussed in terms of regulation of phospholipid biosynthesis and membrane biogenesis in response to growth phase and carbon source.

Phospholipid biosynthesis in the yeast Saccharomyces cerevisiae and interrelationship with other metabolic processes

In this review, we have discussed recent progress in the study of the regulation that controls phospholipid metabolism in S. cerevisiae. This regulation occurs on multiple levels and is tightly integrated with a large number of other cellular processes and related metabolic and signal transduction pathways. Progress in deciphering this complex regulation has been very rapid in the last few years, aided by the availability of the sequence of the entire Saccharomyces genome. The assignment of functions to the remaining unassigned open reading frames, as well as ascertainment of remaining gene-enzyme relationships in phospholipid biosynthesis in yeast, promises to provide detailed understanding of the genetic regulation of a crucial area of metabolism in a key eukaryotic model system. Since the processes of lipid metabolism, secretion, and signal transduction show fundamental similarities in all eukaryotes, the dissection of this regulation in yeast promises to have wide application to our understanding of metabolic control in all eukaryotes.

Isolation and characterization of the Saccharomyces cerevisiae EKI1 gene encoding ethanolamine kinase

Ethanolamine kinase (ATP:ethanolamine O-phosphotransferase, EC 2.7.1. 82) catalyzes the committed step of phosphatidylethanolamine synthesis via the CDP-ethanolamine pathway. The gene encoding ethanolamine kinase (EKI1) was identified from the Saccharomyces Genome Data Base (locus YDR147W) based on its homology to the Saccharomyces cerevisiae CKI1-encoded choline kinase, which also exhibits ethanolamine kinase activity. The EKI1 gene was isolated and used to construct eki1Delta and eki1Delta cki1Delta mutants. A multicopy plasmid containing the EKI1 gene directed the overexpression of ethanolamine kinase activity in wild-type, eki1Delta mutant, cki1Delta mutant, and eki1Delta cki1Delta double mutant cells. The heterologous expression of the S. cerevisiae EKI1 gene in Sf-9 insect cells resulted in a 165,500-fold overexpression of ethanolamine kinase activity relative to control insect cells. The EKI1 gene product also exhibited choline kinase activity. Biochemical analyses of the enzyme expressed in insect cells, in eki1Delta mutants, and in cki1Delta mutants indicated that ethanolamine was the preferred substrate. The eki1Delta mutant did not exhibit a growth phenotype. Biochemical analyses of eki1Delta, cki1Delta, and eki1Delta cki1Delta mutants showed that the EKI1 and CKI1 gene products encoded all of the ethanolamine kinase and choline kinase activities in S. cerevisiae. In vivo labeling experiments showed that the EKI1 and CKI1 gene products had overlapping functions with respect to phospholipid synthesis. Whereas the EKI1 gene product was primarily responsible for phosphatidylethanolamine synthesis via the CDP-ethanolamine pathway, the CKI1 gene product was primarily responsible for phosphatidylcholine synthesis via the CDP-choline pathway. Unlike cki1Delta mutants, eki1Delta mutants did not suppress the essential function of Sec14p.

The yeast inositol-sensitive upstream activating sequence, UASINO, responds to nitrogen availability

The INO1 gene of yeast is expressed in logarithmically growing, wild-type cells when inositol is absent from the medium. However, the INO1 gene is repressed when inositol is present during logarithmic growth and it is also repressed as cells enter stationary phase whether inositol is present or not. In this report, we demonstrate that transient nitrogen limitation also causes INO1 repression. The repression of INO1 in response to nitrogen limitation shares many features in common with repression in response to the presence of inositol. Specifically, the response to nitrogen limitation is dependent upon the presence of a functional OPI1 gene product, it requires ongoing phosphatidylcholine biosynthesis and it is mediated by the repeated element, UASINO, found in the promoter of INO1 and other co-regulated genes of phospholipid biosynthesis. Thus, we propose that repression of INO1 in response to inositol and in response to nitrogen limitation occurs via a common mechanism that is sensitive to the status of ongoing phospholipid metabolism.

Cloning and expression of a wheat (Triticum aestivum L.) phosphatidylserine synthase cDNA. Overexpression in plants alters the composition of phospholipids

We describe the cloning of a wheat cDNA (TaPSS1) that encodes a phosphatidylserine synthase (PSS) and provides the first strong evidence for the existence of this enzyme in a higher eukaryotic cell. The cDNA was isolated on its ability to confer increased resistance to aluminum toxicity when expressed in yeast. The sequence of the predicted protein encoded by TaPSS1 shows homology to PSS from both yeast and bacteria but is distinct from the animal PSS enzymes that catalyze base-exchange reactions. In wheat, Southern blot analysis identified the presence of a small family of genes that cross-hybridized to TaPSS1, and Northern blots showed that aluminum induced TaPSS1 expression in root apices. Expression of TaPSS1 complemented the yeast cho1 mutant that lacks PSS activity and altered the phospholipid composition of wild type yeast, with the most marked effect being increased abundance of phosphatidylserine (PS). Arabidopsis thaliana leaves overexpressing TaPSS1 showed a marked enhancement in PSS activity, which was associated with increased biosynthesis of PS at the expense of both phosphatidylinositol and phosphatidylglycerol. Unlike mammalian cells where PS accumulation is tightly regulated even when the capacity for PS biosynthesis is increased, plant cells accumulated large amounts of PS when TaPSS1 was overexpressed. High levels of TaPSS1 expression in Arabidopsis and tobacco (Nicotiana tabacum) led to the appearance of necrotic lesions on leaves, which may have resulted from the excessive accumulation of PS. The cloning of TaPSS1 now provides evidence that the yeast pathway for PS synthesis exists in some plant tissues and provides a tool for understanding the pathways of phospholipid biosynthesis and their regulation in plants.

Cardiolipin synthase expression is essential for growth at elevated temperature and is regulated by factors affecting mitochondrial development

Cardiolipin (CL) is a unique dimeric phospholipid localized primarily in the mitochondrial membrane. In eukaryotes, the enzyme CL synthase catalyses the synthesis of CL from two lipid substrates, CDP-diacylglycerol and phosphatidylglycerol. In earlier studies, we reported the purification of CL synthase from Saccharomyces cerevisiae and the cloning of the gene CRD1 (previously called CLS1) that encodes the enzyme. Because CL is an important component of the mitochondrial membrane, knowledge of its regulation will provide insight into the biogenesis of this organelle. To understand how CL synthesis is regulated, we analysed CRD1 expression by Northern blot analysis of RNA extracted from cells under a variety of growth conditions. CRD1 expression is regulated by mitochondrial development factors. CRD1 levels were 7- to 10-fold greater in stationary than in logarithmic growth phase, and threefold greater in wild-type than in rho 0 mutants. Expression was somewhat elevated during growth in glycerol/ethanol versus glucose media. In contrast, CRD1 expression was not regulated by the phospholipid precursors inositol and choline, and was not altered in the regulatory mutants ino2, ino4 and opi1. Mutations in cytochrome oxidase assembly, which led to reduced Crd1p enzyme activity, did not affect CRD1 expression. The crd1 null mutant makes a truncated CRD1 message. Although the null mutant can grow on both fermentable and non-fermentable carbon sources at lower temperatures, it cannot form colonies at 37 degrees C. In conclusion, CRD1 expression is controlled by factors affecting mitochondrial development, but not by the phospholipid precursors inositol and choline. Expression of CRD1 is essential for growth at elevated temperatures, suggesting that either CL or Crd1p is required for an essential cellular function.

Pleiotropic effects of the opi1 regulatory mutation of yeast: its effects on growth and on phospholipid and inositol metabolism

Key factors which impact on the biosynthesis and subsequent fate of the phospholipid precursor inositol were studied as a function of growth phase in the yeast Saccharomyces cerevisiae. Both wild-type and strains disrupted for the OPI1 gene, the principal negative regulator of the phospholipid biosynthetic genes, were examined. Overexpression of the INO1 gene and overproduction of both inositol and the major inositol-containing phospholipid, phosphatidylinositol, varied as a function of growth phase. In opi1 cells, INO1 expression was constitutive at a high level throughout growth, although the level of transcript was reduced at stationary phase when the cells were grown in defined medium. In the wild-type strain, INO1 expression was limited to a peak in the exponential phase of growth in cells grown in the absence of inositol. Interestingly, the pattern of OPI1 expression in the wild-type strain resembled that of its putative target, INO1. Intracellular inositol contents of the opi1 strain were higher than those of the wild-type strain, with peak levels occurring in the stationary phase. Membrane phosphatidylinositol content paralleled intracellular inositol content, with opi1 strains having a higher phosphatidylinositol content in stationary phase. The proportion of the predominant phospholipid, phosphatidylcholine, exhibited a profile that was the inverse of the phosphatidylinositol content: phosphatidylcholine content was lowest in opi1 cells in stationary phase. The opi1 mutation was also found to have effects beyond phospholipid biosynthesis. opi1 cells were smaller, and opi1 cultures achieved a cell density twice as high as comparable wild-type cultures. opi1 cells were also more salt tolerant than wild-type cells: they were partly resistant to shrinking, more rapidly resumed growth, and attained a higher culture density after upshift to medium supplemented with 8% NaCl.

Genetic regulation of phospholipid metabolism: yeast as a model eukaryote

Baker's yeast, Saccharomyces cerevisiae, is an excellent and an increasingly important model for the study of fundamental questions in eukaryotic cell biology and genetic regulation. The fission yeast, Schizosaccharomyces pombe, although not as intensively studied as S. cerevisiae, also has many advantages as a model system. In this review, we discuss progress over the past several decades in biochemical and molecular genetic studies of the regulation of phospholipid metabolism in these two organisms and higher eukaryotes. In S. cerevisiae, following the recent completion of the yeast genome project, a very high percentage of the gene-enzyme relationships in phospholipid metabolism have been assigned and the remaining assignments are expected to be completed rapidly. Complex transcriptional regulation, sensitive to the availability of phospholipid precusors, as well as growth phase, coordinates the expression of the structural genes encoding these enzymes in S. cerevisiae. In this article, this regulation is described, the mechanism by which the cell senses the ongoing metabolic activity in the pathways for phospholipid biosynthesis is discussed, and a model is presented. Recent information relating to the role of phosphatidylcholine turnover in S. cerevisiae and its relationship to the secretory pathway, as well as to the regulation of phospholipid metabolism, is also presented. Similarities in the role of phospholipase D-mediated phosphatidylcholine turnover in the secretory process in yeast and mammals lend further credence to yeast as a model system.

Effect of CTP synthetase regulation by CTP on phospholipid synthesis in Saccharomyces cerevisiae

CTP synthetase (EC 6.3.4.2, UTP:ammonia ligase (ADP-forming)) activity in Saccharomyces cerevisiae is allosterically regulated by CTP product inhibition. Amino acid residue Glu161 in the URA7-encoded and URA8-encoded CTP synthetases was identified as being involved in the regulation of these enzymes by CTP product inhibition. The specific activities of the URA7-encoded and URA8-encoded enzymes with a Glu161 --> Lys (E161K) mutation were 2-fold greater when compared with the wild-type enzymes. The E161K mutant URA7-encoded and URA8-encoded CTP synthetases were less sensitive to CTP product inhibition with inhibitor constants for CTP of 8.4- and 5-fold greater, respectively, than those of their wild-type counterparts. Cells expressing the E161K mutant enzymes on a multicopy plasmid exhibited an increase in resistance to the pyrimidine poison and cancer therapeutic drug cyclopentenylcytosine and accumulated elevated (6-15-fold) levels of CTP when compared with cells expressing the wild-type enzymes. Cells expressing the E161K mutation in the URA7-encoded CTP synthetase exhibited an increase (1.5-fold) in the utilization of the Kennedy pathway for phosphatidylcholine synthesis when compared with control cells. Cells bearing the mutation also exhibited an increase in the synthesis of phosphatidylcholine (1.5-fold), phosphatidylethanolamine (1.3-fold), and phosphatidate (2-fold) and a decrease in the synthesis of phosphatidylserine (1.7-fold). These alterations were accompanied by an inositol excretion phenotype due to the misregulation of the INO1 gene. Moreover, cells bearing the E161K mutation exhibited an increase (1.6-fold) in the ratio of total neutral lipids to phospholipids, an increase in triacylglycerol (1.4-fold), free fatty acids (1.7-fold), and ergosterol ester (1.8-fold), and a decrease in diacylglycerol (1. 3-fold) when compared with control cells. These data indicated that the regulation of CTP synthetase activity by CTP plays an important role in the regulation of phospholipid synthesis.

Isolation and characterization of the Saccharomyces cerevisiae LPP1 gene encoding a Mg2+-independent phosphatidate phosphatase

The DPP1-encoded diacylglycerol pyrophosphate (DGPP) phosphatase enzyme accounts for half of the Mg2+-independent phosphatidate (PA) phosphatase activity in Saccharomyces cerevisiae. The LPP1 (lipid phosphate phosphatase) gene encodes a protein that contains a novel phosphatase sequence motif found in DGPP phosphatase and in the mouse Mg2+-independent PA phosphatase. A genomic copy of the S. cerevisiae LPP1 gene was isolated and was used to construct lpp1Delta and lpp1Delta dpp1Delta mutants. A multicopy plasmid containing the LPP1 gene directed a 12.9-fold overexpression of Mg2+-independent PA phosphatase activity in the S. cerevisiae lpp1Delta dpp1Delta double mutant. The heterologous expression of the S. cerevisiae LPP1 gene in Sf-9 insect cells resulted in a 715-fold overexpression of Mg2+-independent PA phosphatase activity relative to control insect cells. The Mg2+-independent PA phosphatase activity encoded by the LPP1 gene was associated with the membrane fraction of the cell. The LPP1 gene product also exhibited lyso-PA phosphatase and DGPP phosphatase activities. The order of substrate preference was PA > lyso-PA > DGPP. Like the dpp1Delta mutant, the lpp1Delta mutant and the lpp1Delta dpp1Delta double mutant were viable and did not exhibit obvious growth defects. Biochemical analyses of lpp1Delta, dpp1Delta, and lpp1Delta dpp1Delta mutants showed that the LPP1 and DPP1 gene products encoded nearly all of the Mg2+-independent PA phosphatase and lyso-PA phosphatase activities and all of the DGPP phosphatase activity in S. cerevisiae. Moreover, the analyses of the mutants showed that the LPP1 and DPP1 gene products played a role in the regulation of phospholipid metabolism and the cellular levels of phosphatidylinositol and PA.

The PEL1 gene (renamed PGS1) encodes the phosphatidylglycero-phosphate synthase of Saccharomyces cerevisiae

Phosphatidylglycerophosphate (PG-P) synthase catalyzes the synthesis of PG-P from CDP-diacylglycerol and sn-glycerol 3-phosphate and functions as the committed and rate-limiting step in the biosynthesis of cardiolipin (CL). In eukaryotic cells, CL is found predominantly in the inner mitochondrial membrane and is generally thought to be an essential component of many mitochondrial functions. We have determined that the PEL1 gene (now renamed PGS1), previously proposed to encode a second phosphatidylserine synthase of yeast (Janitor, M., Jarosch, E., Schweyen, R. J., and Subik, J. (1995) Yeast 13, 1223-1231), in fact encodes a PG-P synthase of Saccharomyces cerevisiae. Overexpression of the PGS1 gene product under the inducible GAL1 promoter resulted in a 14-fold increase in in vitro PG-P synthase activity. Disruption of the PGS1 gene in a haploid strain of yeast did not lead to a loss of viability but did result in a dependence on a fermentable carbon source for growth, a temperature sensitivity for growth, and a petite lethal phenotype. The pgs1 null mutant exhibited no detectable in vitro PG-P synthase activity and no detectable CL or phosphatidylglycerol (PG); significant CL synthase activity was still present. The growth arrest phenotype and lack of PG-P synthase activity of a pgsA null allele of Escherichia coli was corrected by an N-terminal truncated derivative of the yeast PG-P synthase. These results unequivocally demonstrate that the PGS1 gene encodes the major PG-P synthase of yeast and that neither PG nor CL are absolutely essential for cell viability but may be important for normal mitochondrial function.

Phosphatidylserine synthase from yeast

Whereas mammalian cells produce PS by a base exchange reaction from preexisting phospholipids, yeast cells synthesize PS from CDP-diacylglycerol and serine by the PS synthase reaction. Yeast PS synthase was purified to homogeneity and shown to have a molecular mass of 23 kDa. The activity is dependent on either Mg2+ or Mn2+ and Triton X-100. The enzyme specifically transfers the phosphatidyl group from CDP-diacylglycerol or dCDP-diacylglycerol to L-serine, but not to threonine, cysteine and ethanolamine. The PSS/CHO1 gene encoding the enzyme was cloned by the complementation of the choline auxotrophic pss/cho1 mutant. The deduced protein comprises 279 amino acids with a calculated molecular mass of 30,804. The primary translate undergoes proteolytic processing to the enzymatically more active 23-kDa enzyme. The deduced amino acid sequence contains several putative membrane-spanning regions and resembles that of the Bacillus subtilis enzyme, but not those of the E. coli and Haemophilus influenzae enzymes. The sequence also contains the local, conserved region found in enzymes catalyzing the transfer of the phosphoalcohol moiety from CDP-alcohol, such as PI synthase, cholinephosphotransferase and phosphatidylglycerolphosphate synthase. The activity of PS synthase is maximal in the exponential phase, but decreases when cells enter the stationary phase. The enzyme is phosphorylated at a single serine residue by cyclic AMP-dependent protein kinase with a 60-70% decrease in enzymatic activity, but the primary translation product is not phosphorylated. PS synthase is inhibited by CTP, probably due to the chelation of the divalent cations, Mg2+ and Mn2+, and also by sphingoid bases, such as sphinganine and phytosphingosine. Phosphatidate, phosphatidylcholine and phosphatidylinositol are stimulatory, whereas cardiolipin and diacylglycerol are inhibitory. The expression of yeast PS synthase is transcriptionally repressed by myo-inositol and choline in a coordinate manner with other phospholipid-synthesizing enzymes. The upstream regulatory region of the PSS/CHO1 gene responsible for the myo-inositol-choline regulation was identified. An octameric sequence, CATRTGAA (R = A or G), plays an important role in the conferral of the myo-inositol-choline transcriptional regulation.

Phosphatidylinositol synthase from yeast

PI is an important precursor for polyphosphoinositides and some sphingolipids and is also involved in the glycolipid anchoring of plasma membrane proteins. This lipid is synthesized from CDP-diacylglycerol and myo-inositol by PI synthase, an enzyme localized in the outer mitochondrial membranes and microsomes in yeast. PI synthase was highly purified from yeast microsomes after solubilization with Triton X-100. The activity is dependent on Mn2+ or Mg2+ and Triton X-100. The reaction follows a sequential Bi-Bi mechanism with binding to CDP-diacylglycerol before myo-inositol and releasing PI prior to CMP. Unlike most of the yeast phospholipid-synthesizing enzymes, PI synthase is a constitutive enzyme. Its expression is insensitive to the addition of myo-inositol and choline to culture medium or the transition of growth phase. The primary translate deduced from the encoding gene, PIS, comprises 220 amino acid residues with a calculated molecular mass of 23,613. The sequence contains several hydrophobic regions and resembles that of the human enzyme. The sequence also contains the local, conserved region found in enzymes catalyzing the transfer of the phosphoalcohol moiety from CDP-alcohol, such as phosphatidylserine synthase, cholinephosphotransferase and phosphatidylglycerolphosphate synthase. Substitution of amino acid at position 114 from His (CAC) to Gln (CAA) results in a 200-fold increase in Km of the enzyme for myo-inositol, making cells auxotrophic for myo-inositol. Disruption of the PIS locus in the genome is lethal, indicating that PI is essential for the survival and growth of yeast cells.

IP6: a novel anti-cancer agent

Inositol hexaphosphate (InsP6 or IP6) is ubiquitous. At 10 microM to 1 mM concentrations, IP6 and its lower phosphorylated forms (IP(1-5)) as well as inositol (Ins) are contained in most mammalian cells, wherein they are important in regulating vital cellular functions such as signal transduction, cell proliferation and differentiation. A striking anti-cancer action of IP6 has been demonstrated both in vivo and in vitro, which is based on the hypotheses that exogenously administered IP6 may be internalized, dephosphorylated to IP(1-5), and inhibit cell growth. There is additional evidence that Ins alone may further enhance the anti-cancer effect of IP6. Besides decreasing cellular proliferation, IP6 also causes differentiation of malignant cells often resulting in a reversion to normal phenotype. These data strongly point towards the involvement of signal transduction pathways, cell cycle regulatory genes, differentiation genes, oncogenes and perhaps, tumor suppressor genes in bringing about the observed anti-neoplastic action of IP6.

Reduction of CDP-diacylglycerol synthase activity results in the excretion of inositol by Saccharomyces cerevisiae

A yeast mutant, cdg1, was isolated on the basis of an inositol excretion phenotype. This mutant exhibited pleiotropic deficiencies in phospholipid biosynthesis, including reduced levels of CDP-diacylglycerol (DAG) synthase activity (Klig, L. S., Homann, M. J., Kohlwein, S. D., Kelley, M. J., Henry, S. A., and Carman, G. M. (1988) J. Bacteriol. 170, 1878-1886). In this study we present evidence that the molecular basis for the inositol excretion phenotype is a G305/A305 point mutation (Cys102 --> Tyr substitution) within the CDS1 gene (encodes CDP-DAG synthase) of this mutant. Expression of CDP-DAG synthase activity from a plasmid-borne copy of the CDS1 gene in the cdg1 mutant was not down-regulated, and this expression also corrected the inositol excretion phenotype. Introduction of the above mutated gene (CDS1*) controlled by its endogenous promoter on a single copy plasmid into a cds1-null background reconstituted a transformant with the cdg1 phenotype, including reduced CDP-DAG synthase activity, elevated phosphatidylserine synthase activity, and inositol excretion into the growth medium. Expression of CDS1* in a single copy in the cdg1 mutant raised CDP-DAG synthase activity from 15 to 30% of derepressed wild-type yeast levels but still did not correct the inositol excretion phenotype. CDP-DAG synthase activity was not regulated in response to precursors of phospholipid biosynthesis in the cdg1 mutant either with or without a trans copy of the CDS1* gene. An open reading frame was identified 5' to the CDS1 locus, YBR0314, which also resulted in inositol excretion when present in trans in multiple copies.

Inositol transport in Saccharomyces cerevisiae is regulated by transcriptional and degradative endocytic mechanisms during the growth cycle that are distinct from inositol-induced regulation

Regulation of inositol uptake activity in Saccharomyces cerevisiae during the growth cycle was examined. Activity increased as the cell population transited from lag phase to exponential growth, and continued to increase until late exponential phase. The increase in activity was due to increased transcription of the ITR1 gene and synthesis of the Itr1 permease. When the culture reached stationary phase, uptake activity decreased and dropped to a minimum within 4 h. The decrease was due to repression of ITR1 transcription, independent of the negative regulator Opi1p, and degradation of the existing permease. Degradation depended on delivery of the permease to the vacuole through the END3/END4 endocytic pathway. During exponential growth in inositol-containing medium the permease is also rapidly degraded, whereas in inositol-free medium the permease is highly stable. Rapid degradation of the permease at stationary phase occurred in inositol-free medium, indicating that there are two distinct mechanisms that trigger endocytosis and degradation in response to different physiological stimuli. In addition, the level of the enzyme required for inositol biosynthesis, inositol-1-phosphate synthase, encoded by INO1, is not reduced in stationary-phase cells, and this contrast in the regulation of inositol supply is discussed.

Regulation of phospholipid biosynthesis in Saccharomyces cerevisiae by CTP

In the yeast Saccharomyces cerevisiae, the major membrane phospholipid phosphatidylcholine is synthesized by the CDP-diacylglycerol and CDP-choline pathways. We examined the regulation of phosphatidylcholine synthesis by CTP. The cellular concentration of CTP was elevated (2.4-fold) by overexpressing CTP synthetase, the enzyme responsible for the synthesis of CTP. The overexpression of CTP synthetase resulted in a 2-fold increase in the utilization of the CDP-choline pathway for phosphatidylcholine synthesis. The increase in CDP-choline pathway usage was not due to an increase in the expression of any of the enzymes in this pathway. CDP-choline, the product of the phosphocholine cytidylyltransferase reaction, was the limiting intermediate in the CDP-choline pathway. The apparent Km of CTP (1.4 mM) for phosphocholine cytidylyltransferase was 2-fold higher than the cellular concentration of CTP (0.7 mM) in control cells. This provided an explanation of why the overexpression of CTP synthetase caused an increase in the cellular concentration of CDP-choline. Phosphatidylserine synthase activity was reduced in cells overexpressing CTP synthetase. This was not due to a transcriptional repression mechanism. Instead, the decrease in phosphatidylserine synthase activity was due, at least in part, to a direct inhibition of activity by CTP. These results show that CTP plays a role in the regulation of the pathways by which phosphatidylcholine is synthesized. This regulation includes the supple of CTP for the phosphocholine cytidylyltransferase reaction in the CDP-choline pathway and the inhibition of the phosphatidylserine synthase reaction in the CDP-diacylglycerol pathway.

Regulation of lipid biosynthesis in Saccharomyces cerevisiae by fumonisin B1

The regulation of lipid biosynthesis in the yeast Saccharomyces cerevisiae by fumonisin B1 was examined. Fumonisin B1 inhibited the growth of yeast cells. Cells supplemented with fumonisin B1 accumulated free sphinganine and phytosphingosine in a dose-dependent manner. The cellular concentration of ceramide was reduced in fumonisin B1-supplemented cells. Ceramide synthase activity was found in yeast cell membranes and was inhibited by fumonisin B1. Fumonisin B1 inhibited the synthesis of the inositol-containing sphingolipids inositol phosphorylceramide, mannosylinositol phosphorylceramide, and mannosyldiinositol phosphorylceramide. Fumonisin B1 also caused a decrease in the synthesis of the major phospholipids synthesized via the CDP-diacylglycerol-dependent pathway and the synthesis of neutral lipids. The effects of fumonisin B1 and sphingoid bases on the activities of enzymes in the pathways leading to the synthesis of sphingolipids, phospholipids, and neutral lipids were also examined. Other than ceramide synthase, fumonisin B1 did not affect the activities of any of the enzymes examined. However, sphinganine and phytosphingosine inhibited the activities of inositol phosphorylceramide synthase, phosphatidylserine synthase, and phosphatidate phosphatase. These are key enzymes responsible for the synthesis of lipids in yeast. The data reported here indicated that the biosynthesis of sphingolipids, phospholipids and neutral lipids was coordinately regulated by fumonisin B1 through the regulation of lipid biosynthetic enzymes by sphingoid bases.

Generation, modulation and maintenance of the plasma membrane asymmetric phospholipid composition in yeast cells during growth: their relation to surface potential and membrane protein activity

During growth a cyclic exposure of anionic phospholipids to the external surface of the plasma membrane was found. The surface charge density (sigma) increased gradually reaching a maximum in the first 5 h of growth and returned gradually to their initial value at the end of the logarithmic phase of growth (10-12 h). Phosphatidylinositol, that determines to a large extent the magnitude of the sigma, increased 83% in the yeast cells during the first 4 h of growth and returned gradually to their initial level at 10-12 h. During the stationary phase (12-24 h), both sigma and the anionic/zwitterionic phospholipid ratio, remained without any significant variation. The high-affinity H-linked glutamate transport system that behaves as a sensor of the changes in the membrane surface potential (phi) increased its activity in the first 5 h and then decreased it, following with great accuracy the sigma variations and remained without changes during the stationary phase of growth. The phosphatidylserine (PS) relative concentration in the cells (9.0%) did not significantly change during the whole growth curve, but their asymmetric distribution varied, contributing to the changes in sigma. PS facing the outer membrane surface increased 2.45-times during the first 5 h of growth and then returned to their original value at the end of the log phase (12 h). Phosphatidylcholine (PC) remained constant during the whole growth curve (50%), while phosphatidylethanolamine (PE) decreased 3-fold in the first 4 h and then increased to its original value at 10 h. Interestingly, PE at the outer membrane surface remained constant (3% of the total phospholipids) during the whole growth curve. During growth yeast cells change their phospholipid composition originating altered patterns of the plasma membrane phospholipid composition and IN-OUT distribution. This dynamic asymmetry is involved in the regulation of the surface potential and membrane protein activity.