The membrane-associated enzyme phosphatidylserine synthase is regulated at the level of mRNA abundance

Mapping the Substrate-Binding Sites in the Phosphatidylserine Synthase in Candida albicans

The fungal phosphatidylserine (PS) synthase, a membrane protein encoded by the CHO1 gene, is a potential drug target for pathogenic fungi, such as Candida albicans. However, both substrate-binding sites of C. albicans Cho1 have not been characterized. Cho1 has two substrates: cytidyldiphosphate-diacylglycerol (CDP-DAG) and serine. Previous studies identified a conserved CDP-alcohol phosphotransferase (CAPT) binding motif, which is present within Cho1. We tested the CAPT motif for its role in PS synthesis by mutating conserved residues using alanine substitution mutagenesis. PS synthase assays revealed that mutations in all but one conserved amino acid within the CAPT motif resulted in decreased Cho1 function. In contrast, there were no clear motifs in Cho1 for binding serine. Therefore, to identify the serine binding site, PS synthase sequences from three fungi were aligned with sequences of a similar enzyme, phosphatidylinositol (PI) synthase, from the same fungi. This revealed a motif that was unique to PS synthases. Using alanine substitution mutagenesis, we found that some of the residues in this motif are required for Cho1 function. Two alanine substitution mutants, L184A and R189A, exhibited contrasting impacts on PS synthase activity, and were characterized for their Michaelis-Menten kinetics. The L184A mutant displayed enhanced PS synthase activity and showed an increased V_max. In contrast, R189A showed decreased PS synthase activity and increased K_m for serine, suggesting that residue R189 is involved in serine binding. These results help to characterize PS synthase substrate binding, and should direct rational approaches for finding Cho1 inhibitors that may lead to better antifungals.

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.

Genome-wide analysis of Saccharomyces cerevisiae identifies cellular processes affecting intracellular aggregation of Alzheimer's amyloid-β42: importance of lipid homeostasis

Amyloid-β (Aβ)-containing plaques are a major neuropathological feature of Alzheimer's disease (AD). The two major isoforms of Aβ peptide associated with AD are Aβ40 and Aβ42, of which the latter is highly prone to aggregation. Increased presence and aggregation of intracellular Aβ42 peptides is an early event in AD progression. Improved understanding of cellular processes affecting Aβ42 aggregation may have implications for development of therapeutic strategies. Aβ42 fused to green fluorescent protein (Aβ42-GFP) was expressed in ∼4600 mutants of a Saccharomyces cerevisiae genome-wide deletion library to identify proteins and cellular processes affecting intracellular Aβ42 aggregation by assessing the fluorescence of Aβ42-GFP. This screening identified 110 mutants exhibiting intense Aβ42-GFP-associated fluorescence. Four major cellular processes were overrepresented in the data set, including phospholipid homeostasis. Disruption of phosphatidylcholine, phosphatidylserine, and/or phosphatidylethanolamine metabolism had a major effect on intracellular Aβ42 aggregation and localization. Confocal microscopy indicated that Aβ42-GFP localization in the phospholipid mutants was juxtaposed to the nucleus, most likely associated with the endoplasmic reticulum (ER)/ER membrane. These data provide a genome-wide indication of cellular processes that affect intracellular Aβ42-GFP aggregation and may have important implications for understanding cellular mechanisms affecting intracellular Aβ42 aggregation and AD disease progression.

The response to inositol: regulation of glycerolipid metabolism and stress response signaling in yeast

This article focuses on discoveries of the mechanisms governing the regulation of glycerolipid metabolism and stress response signaling in response to the phospholipid precursor, inositol. The regulation of glycerolipid lipid metabolism in yeast in response to inositol is highly complex, but increasingly well understood, and the roles of individual lipids in stress response are also increasingly well characterized. Discoveries that have emerged over several decades of genetic, molecular and biochemical analyses of metabolic, regulatory and signaling responses of yeast cells, both mutant and wild type, to the availability of the phospholipid precursor, inositol are discussed.

Phosphatidate phosphatase plays role in zinc-mediated regulation of phospholipid synthesis in yeast

In the yeast Saccharomyces cerevisiae, the synthesis of phospholipids is coordinately regulated by mechanisms that control the homeostasis of the essential mineral zinc (Carman, G.M., and Han, G. S. (2007) Regulation of phospholipid synthesis in Saccharomyces cerevisiae by zinc depletion. Biochim. Biophys. Acta 1771, 322-330; Eide, D. J. (2009) Homeostatic and adaptive responses to zinc deficiency in Saccharomyces cerevisiae. J. Biol. Chem. 284, 18565-18569). The synthesis of phosphatidylcholine is balanced by the repression of CDP-diacylglycerol pathway enzymes and the induction of Kennedy pathway enzymes. PAH1-encoded phosphatidate phosphatase catalyzes the penultimate step in triacylglycerol synthesis, and the diacylglycerol generated in the reaction may also be used for phosphatidylcholine synthesis via the Kennedy pathway. In this work, we showed that the expression of PAH1-encoded phosphatidate phosphatase was induced by zinc deficiency through a mechanism that involved interaction of the Zap1p zinc-responsive transcription factor with putative upstream activating sequence zinc-responsive elements in the PAH1 promoter. The pah1Δ mutation resulted in the derepression of the CHO1-encoded phosphatidylserine synthase (CDP-diacylglycerol pathway enzyme) and loss of the zinc-mediated regulation of the enzyme. Loss of phosphatidate phosphatase also resulted in the derepression of the CKI1-encoded choline kinase (Kennedy pathway enzyme) but decreased the synthesis of phosphatidylcholine when cells were deficient of zinc. This result confirmed the role phosphatidate phosphatase plays in phosphatidylcholine synthesis via the Kennedy pathway.

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.

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.

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 cardiolipin synthase levels in Saccharomyces cerevisiae

The Saccharomyces cerevisiae cardiolipin (CL) synthase encoded by the CRD1 gene catalyses the synthesis of CL, which is localized to the inner mitochondrial membrane and plays an important role in mitochondrial function. To investigate how CRD1 expression is regulated, a lacZ reporter gene was placed under control of the CRD1 promoter and the 5'-untranslated region of its mRNA (P(CRD1)-lacZ). P(CRD1)-lacZ expression was 2.5 times higher in early stationary phase than in logarithmic phase for glucose grown cells. Non-fermentable growth resulted in a two-fold elevation in expression relative to glucose grown cells. A shift from glycerol to glucose rapidly repressed expression, whereas a shift from glucose to glycerol had the opposite effect. The derepression of P(CRD1)-lacZ expression by non-fermentable carbon sources was dependent on mitochondrial respiration. These results support a tight coordination between translation and transcription of the CRD1 gene, since similar effects by the above factors on CRD1 mRNA levels have been reported. In glucose-grown cells, P(CRD1)-lacZ expression was repressed 70% in a pgs1delta strain (lacks phosphatidylglycerol and CL) compared with wild-type and rho- cells and elevated 2.5-fold in crd1delta cells, which have increased phosphatidylglycerol levels, suggesting a role for phosphatidylglycerol in regulating CRD1 expression. Addition of inositol to the growth medium had no effect on expression. However, expression was elevated in an ino4delta mutant but not in ino2delta cells, suggesting multiple and separate functions for the inositol-responsive INO2/INO4 gene products, which normally function as a dimer in regulating gene function.

Genome-wide analysis reveals inositol, not choline, as the major effector of Ino2p-Ino4p and unfolded protein response target gene expression in yeast

In the yeast Saccharomyces cerevisiae, the transcription of many genes encoding enzymes of phospholipid biosynthesis are repressed in cells grown in the presence of the phospholipid precursors inositol and choline. A genome-wide approach using cDNA microarray technology was used to profile the changes in the expression of all genes in yeast that respond to the exogenous presence of inositol and choline. We report that the global response to inositol is completely distinct from the effect of choline. Whereas the effect of inositol on gene expression was primarily repressing, the effect of choline on gene expression was activating. Moreover, the combination of inositol and choline increased the number of repressed genes compared with inositol alone and enhanced the repression levels of a subset of genes that responded to inositol. In all, 110 genes were repressed in the presence of inositol and choline. Two distinct sets of genes exhibited differential expression in response to inositol or the combination of inositol and choline in wild-type cells. One set of genes contained the UASINO sequence and were bound by Ino2p and Ino4p. Many of these genes were also negatively regulated by OPI1, suggesting a common regulatory mechanism for Ino2p, Ino4p, and Opi1p. Another nonoverlapping set of genes was coregulated by the unfolded protein response pathway, an ER-localized stress response pathway, but was not dependent on OPI1 and did not show further repression when choline was present together with inositol. These results suggest that inositol is the major effector of target gene expression, whereas choline plays a minor role.

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.

Maintenance and integrity of the mitochondrial genome: a plethora of nuclear genes in the budding yeast

Instability of the mitochondrial genome (mtDNA) is a general problem from yeasts to humans. However, its genetic control is not well documented except in the yeast Saccharomyces cerevisiae. From the discovery, 50 years ago, of the petite mutants by Ephrussi and his coworkers, it has been shown that more than 100 nuclear genes directly or indirectly influence the fate of the rho(+) mtDNA. It is not surprising that mutations in genes involved in mtDNA metabolism (replication, repair, and recombination) can cause a complete loss of mtDNA (rho(0) petites) and/or lead to truncated forms (rho(-)) of this genome. However, most loss-of-function mutations which increase yeast mtDNA instability act indirectly: they lie in genes controlling functions as diverse as mitochondrial translation, ATP synthase, iron homeostasis, fatty acid metabolism, mitochondrial morphology, and so on. In a few cases it has been shown that gene overexpression increases the levels of petite mutants. Mutations in other genes are lethal in the absence of a functional mtDNA and thus convert this petite-positive yeast into a petite-negative form: petite cells cannot be recovered in these genetic contexts. Most of the data are explained if one assumes that the maintenance of the rho(+) genome depends on a centromere-like structure dispensable for the maintenance of rho(-) mtDNA and/or the function of mitochondrially encoded ATP synthase subunits, especially ATP6. In fact, the real challenge for the next 50 years will be to assemble the pieces of this puzzle by using yeast and to use complementary models, especially in strict aerobes.

Regulation of yeast phospholipid biosynthetic genes in phosphatidylserine decarboxylase mutants

In the yeast Saccharomyces cerevisiae, the products of two genes (PSD1 and PSD2) are able to catalyze the decarboxylation of phosphatidylserine (PS) to produce phosphatidylethanolamine (PE) (C. J. Clancey, S. Chang, and W. Dowhan, J. Biol. Chem. 268:24580-24590, 1993; P. J. Trotter, J. Pedretti, and D. R. Voelker, J. Biol. Chem. 268:21416-21424, 1993; P.J. Trotter, and D. R. Voelker, J. Biol. Chem. 270:6062-6070, 1995). I report that the major mitochondrial PS decarboxylase gene (PSD1) is transcriptionally regulated by inositol in a manner similar to that reported for other coregulated phospholipid biosynthetic genes. The second PS decarboxylase gene (PSD2) is not regulated on a transcriptional level by inositol and/or ethanolamine. In yeast, phosphatidylcholine (PC) biosynthesis is required for the repression of the phospholipid biosynthetic genes, including the INO1 gene, in response to inositol. I show that the presence of a functional major mitochondrial PS decarboxylase encoded by the PSD1 gene is necessary for proper regulation of INO1 in response to inositol in the absence of ethanolamine. Disruption of the second PS decarboxylase gene (PSD2) does not affect the INO1 regulation. Analysis of phospholipid content of PS decarboxylase mutants suggests that the proportion of PC on total cellular phospholipids is not correlated to the cell's ability to repress INO1 in response to inositol. Rather, yeast cells are apparently able to monitor the flux through the phospholipid biosynthetic pathway and modify the transcription of phospholipid biosynthetic genes accordingly.

The unfolded protein response coordinates the production of endoplasmic reticulum protein and endoplasmic reticulum membrane

The endoplasmic reticulum (ER) is a multifunctional organelle responsible for production of both lumenal and membrane components of secretory pathway compartments. Secretory proteins are folded, processed, and sorted in the ER lumen and lipid synthesis occurs on the ER membrane itself. In the yeast Saccharomyces cerevisiae, synthesis of ER components is highly regulated: the ER-resident proteins by the unfolded protein response and membrane lipid synthesis by the inositol response. We demonstrate that these two responses are intimately linked, forming different branches of the same pathway. Furthermore, we present evidence indicating that this coordinate regulation plays a role in ER biogenesis.

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.

INO1-100: an allele of the Saccharomyces cerevisiae INO1 gene that is transcribed without the action of the positive factors encoded by the INO2, INO4, SWI1, SWI2 and SWI3 genes

A dominant allele of the INO1 locus, INO1-100, does not require the positive regulators encoded by INO2 and INO4 for expression. Sequence analysis showed that INO1-100 had a 239 bp deletion in the INO1 promoter. INO1-100 suppressed the inositol auxotrophy of ino2, ino4, swi1, swi2 and swi3 mutants. Transcription of INO1-100 was constitutive and independent of these regulators. A 20 bp deletion from -247 to -228 caused a similar phenotype. A 38 bp deletion from -245 to -208 suppressed the inositol auxotrophy of an ino2 mutant, but not an ino4 mutant, indicating that Ino2p and Ino4p may function alone as well as in a complex. A 40 bp deletion from -287 to -248 that removed a URS1 site caused constitutive transcription that required INO2 and INO4. A deletion from -167 to -128 suppressed the inositol auxotrophy of swi, ino2 and ino4 mutants, indicating the presence of a previously unidentified URS1. Mutation of the specific negative regulator of phospholipid synthesis encoded by OPI1 suppressed the inositol auxotrophy of swi2 mutants. This study indicates that negative regulation of INO1 is chromatin mediated and provides in vivo information on the interaction of both general and specific regulatory factors that function to accomplish negative and positive regulation of the INO1 promoter in response to inositol.

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).

The INO1 promoter of Saccharomyces cerevisiae includes an upstream repressor sequence (URS1) common to a diverse set of yeast genes

The INO1 promoter of Saccharomyces cerevisiae includes a copy of an upstream repression sequence (URS1; 5'AGCCGCCGA 3') observed in the promoters of several unrelated yeast genes. Expression of INO1-lacZ and CYC1-lacI'Z, activated by the INO1 UASINO, is significantly decreased by the INO1 URS1.

Molecular cloning of the yeast OPI3 gene as a high copy number suppressor of the cho2 mutation

By functional complementation of the auxotrophic requirements for choline of a cdg1, cho2 double-mutant, by transformation with a genomic DNA library in a high copy number plasmid, two different types of complementing DNA inserts were identified. One type of insert was earlier shown to represent the CHO2 structural gene. In this report we describe the molecular and biochemical chemical characterization of the second type of complementing activity. The transcript encoded by the cloned gene was about 1000-nt in length and was regulated in response to the soluble phospholipid precursors, inositol and choline. A gene disruption resulted in no obvious growth phenotype at 23 degrees C or 30 degrees C, but in a lack of growth at 37 degrees C in the presence of monomethylethanolamine. Null-mutants exhibited an inositol-secretion phenotype, indicative of mutations in the lipid biosynthetic pathway. Complementation analysis, biochemical analysis of the phospholipid methylation pathway in vivo, and comparison of the restriction pattern of the cloned gene to published sequences, unequivocally identified the cloned gene as the OPI3 gene, encoding phospholipid-N-methyltransferase in yeast. When present in multiple copies the OPI3 gene efficiently suppresses the phospholipid methylation defect of a cho2 mutation. As a result of impaired synthesis of phosphatidylcholine, the INO1-deregulation phenotype is abolished in cho2 mutants transformed with the OPI3 gene on a high copy number plasmid.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulatory gene INO4 of yeast phospholipid biosynthesis is positively autoregulated and functions as a transactivator of fatty acid synthase genes FAS1 and FAS2 from Saccharomyces cerevisiae

The sequence motif 5' TYTTCACATGY 3' functions as an upstream activation site common to both yeast fatty acid synthase genes, FAS1 and FAS2. In addition, this UASFAS element is shared by all so far characterized genes of yeast phospholipid biosynthesis. We have investigated the influence of a functional INO4 gene previously described as a regulator of inositol biosynthesis on the expression of FAS1 and FAS2. In a delta ino4 null allele strain, both genes are expressed at only 50% of wild type level. Using individual UASFAS sequence motifs inserted into a heterologous test system, a drastic decrease of reporter gene expression to 2-10% of the wild type reference was observed in the delta ino4 mutant. In gel retardation assays, the protein-DNA complex involving the previously described FAS binding factor 1, Fbf1, was absent when using a protein extract from the delta ino4 mutant. On the other hand, this signal was enhanced with an extract from cells grown under conditions of inositol/choline derepression. Subsequent experiments demonstrated that INO4 expression is itself affected by phospholipid precursors, mediated by an UASFAS element in the INO4 upstream region. Thus, in addition of being an activator of phospholipid biosynthetic genes, INO4 is also subject to a positive autoregulatory loop in its own biosynthesis.

Regulation of CDP-diacylglycerol synthesis and utilization by inositol and choline in Schizosaccharomyces pombe

CDP-diacylglycerol (CDP-DG) is an important branchpoint intermediate in eucaryotic phospholipid biosynthesis and could be a key regulatory site in phospholipid metabolism. Therefore, we examined the effects of growth phase, phospholipid precursors, and the disruption of phosphatidylcholine (PC) synthesis on the membrane-associated phospholipid biosynthetic enzymes CDP-DG synthase, phosphatidylglycerolphosphate (PGP) synthase, phosphatidylinositol (PI) synthase, and phosphatidylserine (PS) synthase in cell extracts of the fission yeast Schizosaccharomyces pombe. In complete synthetic medium containing inositol, maximal expression of CDP-DG synthase, PGP synthase, PI synthase, and PS synthase in wild-type cells occurred in the exponential phase of growth and decreased two- to fourfold in the stationary phase of growth. In cells starved for inositol, this decrease in PGP synthase, PI synthase, and PS synthase expression was not observed. Starvation for inositol resulted in a twofold derepression of PGP synthase and PS synthase expression, while PI synthase expression decreased initially and then remained constant. Upon the addition of inositol to inositol-starved cells, there was a rapid and continued increase in PI synthase expression. We examined expression of these enzymes in cho2 and cho1 mutants, which are blocked in the methylation pathway for synthesis of PC. Choline starvation resulted in a decrease in PS synthase and CDP-DG synthase expression in cho1 but not cho2 cells. Expression of PGP synthase and PI synthase was not affected by choline starvation. Inositol starvation resulted in a 1.7-fold derepression of PGP synthase expression in cho2 but not cho1 cells when PC was synthesized. PS synthase expression was not depressed, while CDP-DG synthase and PI synthase expression decreased in cho2 and cho1 cells in the absence of inositol. These results demonstrate that (i) CDP-DG synthase, PGP synthase, PI synthase, and PS synthase are similarly regulated by growth phase; (ii) inositol affects the expression of PGP synthase, PI synthase, and PS synthase; (iii) disruption of the methylation pathway results in aberrant patterns of regulation of growth phase and phospholipid precursors. Important differences between S. pombe and Saccharomyces cerevisiae with regard to regulation of these enzymes are discussed.

Cis and trans regulatory elements required for regulation of the CHO1 gene of Saccharomyces cerevisiae

A 34 base-pair (bp) fragment spanning sequences -154 to -120 of the promoter of the CHO1 gene (structural gene for phosphatidylserine synthase) from the yeast Saccharomyces cerevisiae has been shown to place transcription of a promoter-less Escherichia coli lacZ gene under control of the phospholipid precursors inositol and choline. Furthermore, in deletion experiments the CHO1 UASINO was localized to sequences between -151 and -123 of the CHO1 promoter. A nine bp sequence was identified in the promoter region of the CHO1 gene that shares an eight out of nine bp match with a sequence (consensus 5' ATGTGAAAT 3') that is repeated a total of 23 times upstream from several coregulated phospholipid biosynthetic genes. This sequence is contained within the -151 to -123 region to which the CHO1 UAS has been localized. The nine bp repeated element is believed to be involved in the control of phospholipid biosynthetic gene transcription in response to changing levels of inositol and choline in the growth medium. This control has been shown to require activities encoded by the products of the three regulatory genes: INO2, INO4, and OPI1. A mutation in any of these regulatory genes results in aberrant CHO1-lacZ gene regulation, and affects regulation of the construct containing the 34 bp (-154 to -120) CHO1 fragment demonstrating that the regulatory signal generated by these genes interacts with the 5' end of the CHO1 gene.

Functional analysis of the regulatory region of the yeast phosphatidylserine synthase gene, PSS

The upstream region of the PSS gene contains three positive cis-acting elements, upstream activation sequences 1 and 2 (UAS1 and UAS2) and a TATA box. The 5' end of UAS1 occurs between positions -239 and -209, and that of UAS2 is between positions -172 and -164. UAS2 contains 5'-TTCACATG-3' as a core sequence at positions -161 to -154. Mutational analysis revealed that this octamer is responsible for the control of PSS expression by inositol and choline. The TATA box is located at positions -112 to -108. In addition, PSS contains a negative cis-acting sequence between UAS2 and the TATA box.

Coordinate regulation of phosphatidylserine decarboxylase in Saccharomyces cerevisiae

Regulation of the activity of the mitochondrial enzyme phosphatidylserine decarboxylase (PSD) was measured in vitro by using membrane preparations from wild-type and mutant strains of Saccharomyces cerevisiae. PSD specific activity was not affected by carbon source, and on all carbon sources, the highest specific activity was observed in cells entering the stationary phase of growth. However, PSD activity was found to be regulated in response to soluble precursors of phospholipid biosynthesis. PSD specific activity was reduced to about 63% of the level observed in unsupplemented wild-type cells when the cells were grown in the presence of 75 microM inositol. The presence of 1 mM choline alone had no repressing effect, but the presence of 1 mM choline and 75 microM inositol together led to further repression to a level of about 28% of the derepressed activity. Regulatory mutations known to affect regulation or expression of genes encoding phospholipid-synthesizing enzymes also affected PSD specific activity. opi1 mutants, which are constitutive for a number of phospholipid-biosynthetic enzymes, were found to have high, constitutive levels of PSD. Likewise, in ino2 or ino4 regulatory mutants, PSD activity was found to be at the fully repressed level regardless of growth condition. Regulation of PSD activity was also affected in several structural-gene mutants under conditions of impaired phosphatidylcholine biosynthesis. Together, these data strongly suggest that PSD expression is controlled by the mechanism of general control of phospholipid biosynthesis that regulates many enzymes of phospholipid biosynthesis.

Interaction of trans and cis regulatory elements in the INO1 promoter of Saccharomyces cerevisiae

Electrophoretic mobility shift assays (EMSA) were used to define the regions of the INO1 promoter that interact with factors present in extracts prepared from the yeast, Saccharomyces cerevisae. These experiments identified three different types of protein:DNA complexes that assemble with the INO1 promoter. Formation of one type of complex depended on functional alleles of previously described regulatory genes, INO2 and INO4, that encode positive regulatory factors. Formation of the INO2/INO4-dependent complexes was increased when extracts prepared from cells grown under derepressing conditions (i.e. absence of inositol and choline). A second type of complex was dependent on the OPI1 gene, that encodes a negative regulator. Computer-aided sequence analysis of the promoters of several genes encoding phospholipid biosynthetic enzymes revealed a highly conserved nine basepair element (5'-ATGTGAAAT-3'), designated 'nonamer' motif, that is similar to the octamer motif identified in the promoters of mammalian immunoglobulin genes. The nonamer motif was shown to form a specific complex with a factor, designated nonamer binding factor (NBF). Extracts prepared from Schizosaccharomyces pombe formed a nonamer-specific complex.

Analysis of sequences in the INO1 promoter that are involved in its regulation by phospholipid precursors

The promoter region of the highly regulated INO1 structural gene of yeast has been investigated. The major transcription initiation start site (+1) was mapped to a position located five nucleotides upstream of the previously identified initiation codon. The INO1 TATA is located at -116 to -111. The INO1 promoter region was used to construct fusions to the Escherichia coli lacZ gene. All INO1 fusion constructs that retained regulation in response to the phospholipid precursors inositol and choline, contained at least one copy of a nine bp repeated element (consensus, 5'-ATGTG-AAAT-3'). The smallest fragment of the INO1 promoter found to activate and regulate transcription of the fusion gene from a heterologous TATA element was 40 nucleotides in length. This fragment contained one copy of the nine bp repeat and spanned the INO1 promoter region from -259 to -219. However, when an oligonucleotide containing the nine bp repeated sequence was inserted 5' to the CYC1 TATA element, it failed to activate transcription.

Repression of choline kinase by inositol and choline in Saccharomyces cerevisiae

The regulation of choline kinase (EC 2.7.1.32), the initial enzyme in the CDP-choline pathway, was examined in Saccharomyces cerevisiae. The addition of myo-inositol to a culture of wild-type cells resulted in a significant decrease in choline kinase activity. Additional supplementation of choline caused a further reduction in the activity. The coding frame of the choline kinase gene, CK1, was joined to the carboxyl terminus of lacZ and expressed in Escherichia coli as a fusion protein, which was then used to prepare an anti-choline kinase antibody. Upon Western (immuno-) and Northern (RNA) blot analyses using the antibody and a CK1 probe, respectively, the decrease in the enzyme activity was found to be correlated with decreases in the enzyme amount and mRNA abundance. The molecular mass of the enzyme was estimated to be 66 kilodaltons, in agreement with the value predicted previously from the nucleotide sequence of the gene. The coding region of CK1 was replaced with that of lacZ, and CK1 expression was measured by assaying beta-galactosidase. The expression of beta-galactosidase from this fusion was repressed by myo-inositol and choline and derepressed in a time-dependent manner upon their removal. The present findings indicate that yeast choline kinase is regulated by myo-inositol and choline at the level of mRNA abundance.

Regulation of phospholipid biosynthesis in Saccharomyces cerevisiae by cyclic AMP-dependent protein kinase

The addition of cyclic AMP (cAMP) to Saccharomyces cerevisiae cyr1 mutant cells resulted in an increase in the rate of phosphatidylinositol synthesis at the expense of phosphatidylserine synthesis. The decrease in phosphatidylserine synthesis correlated with the down regulation of phosphatidylserine synthase activity by cAMP-dependent protein kinase phosphorylation. The increase in phosphatidylinositol synthesis was not due to the regulation of phosphatidylinositol synthase by cAMP-dependent protein kinase.

Inositol regulates phosphatidylglycerolphosphate synthase expression in Saccharomyces cerevisiae

The enzyme phosphatidylglycerolphosphate synthase (PGPS; CDPdiacylglycerol-glycerol-3-phosphate 3-phosphatidyltransferase; EC 2.7.8.5) catalyzes the committed step in the synthesis of cardiolipin, a phospholipid found predominantly in the mitochondrial inner membrane. To determine whether PGPS is regulated by cross-pathway control, we analyzed PGPS expression under conditions in which the regulation of general phospholipid synthesis could be examined. The addition of inositol resulted in a three- to fivefold reduction in PGPS expression in wild-type cells in the presence or absence of exogenous choline. The reduction in enzyme activity in response to inositol was seen in minutes, suggesting that inactivation or degradation of the enzyme plays an important role in inositol-mediated repression of PGPS. In cho2 and opi3 mutants, which are blocked in phosphatidylcholine synthesis, inositol-mediated repression of PGPS did not occur unless choline was added to the media. Three previously identified genes that regulate general phospholipid synthesis, INO2, INO4, and OP11, did not affect PGPS expression. Thus, ino2 and ino4 mutants, which are unable to derepress biosynthetic enzymes involved in general phospholipid synthesis, expressed wild-type levels of PGPS activity under derepressing conditions. PGPS expression in the opi1 mutant, which exhibits constitutive synthesis of general phospholipid biosynthetic enzymes, was fully repressed in the presence of inositol and partially repressed even in the absence of inositol. These results demonstrate for the first time that an enzymatic step in cardiolipin synthesis is coordinately controlled with general phospholipid synthesis but that this control is not mediated by the same genetic regulatory circuit.

Phosphorylation of yeast phosphatidylserine synthase in vivo and in vitro by cyclic AMP-dependent protein kinase

Evidence is presented that demonstrates that phosphatidylserine synthase (CDPdiacylglycerol:L-serine O-phosphatidyltransferase, EC 2.7.8.8) from Saccharomyces cerevisiae is phosphorylated in vivo and in vitro by cAMP-dependent protein kinase. Phosphatidylserine synthase activity in cell extracts was reduced in the bcy1 mutant (which has high cAMP-dependent protein kinase activity) and elevated in the cyr1 mutant (which has low cAMP-dependent protein kinase activity) when compared with wild-type cells. The reduced phosphatidylserine synthase activity in the bcy1 mutant correlated with elevated levels of a phosphorylated form of the phosphatidylserine synthase Mr 23,000 subunit. The elevated phosphatidylserine synthase activity in the cyr1 mutant correlated with reduced levels of the phosphorylated form of the enzyme. There was negligible phosphorylation of the phosphatidylserine synthase Mr 23,000 subunit from stationary-phase cells. Pure phosphatidylserine synthase was phosphorylated by the cAMP-dependent protein kinase catalytic subunit, which resulted in a 60-70% reduction in phosphatidylserine synthase activity. The cAMP-dependent protein kinase catalytic subunit catalyzed the incorporation of 0.7 mol of phosphate per mol of phosphatidylserine synthase Mr 23,000 subunit. The specific cAMP-dependent protein kinase inhibitor prevented the phosphorylation of phosphatidylserine synthase and the inhibition of its activity by the catalytic subunit. Analysis of peptides derived from protease-treated labeled phosphatidylserine synthase showed only one labeled peptide. Phospho amino acid analysis of labeled phosphatidylserine synthase showed that the enzyme was phosphorylated at a serine residue.

Inositol regulates phosphatidylglycerolphosphate synthase expression in Saccharomyces cerevisiae

The enzyme phosphatidylglycerolphosphate synthase (PGPS; CDPdiacylglycerol-glycerol-3-phosphate 3-phosphatidyltransferase; EC 2.7.8.5) catalyzes the committed step in the synthesis of cardiolipin, a phospholipid found predominantly in the mitochondrial inner membrane. To determine whether PGPS is regulated by cross-pathway control, we analyzed PGPS expression under conditions in which the regulation of general phospholipid synthesis could be examined. The addition of inositol resulted in a three- to fivefold reduction in PGPS expression in wild-type cells in the presence or absence of exogenous choline. The reduction in enzyme activity in response to inositol was seen in minutes, suggesting that inactivation or degradation of the enzyme plays an important role in inositol-mediated repression of PGPS. In cho2 and opi3 mutants, which are blocked in phosphatidylcholine synthesis, inositol-mediated repression of PGPS did not occur unless choline was added to the media. Three previously identified genes that regulate general phospholipid synthesis, INO2, INO4, and OP11, did not affect PGPS expression. Thus, ino2 and ino4 mutants, which are unable to derepress biosynthetic enzymes involved in general phospholipid synthesis, expressed wild-type levels of PGPS activity under derepressing conditions. PGPS expression in the opi1 mutant, which exhibits constitutive synthesis of general phospholipid biosynthetic enzymes, was fully repressed in the presence of inositol and partially repressed even in the absence of inositol. These results demonstrate for the first time that an enzymatic step in cardiolipin synthesis is coordinately controlled with general phospholipid synthesis but that this control is not mediated by the same genetic regulatory circuit.

Regulation of phosphatidate phosphatase activity by inositol in Saccharomyces cerevisiae

Regulation of phosphatidate phosphatase (EC 3.1.34) activity was examined in Saccharomyces cerevisiae cells supplemented with phospholipid precursors. Addition of inositol to the growth medium of wild-type cells resulted in a twofold increase in phosphatidate phosphatase activity. The increase in phosphatidate phosphatase activity was not due to soluble effector molecules, and inositol did not have a direct effect on enzyme activity. The phosphatidate phosphatase activity associated with the mitochondrial, microsomal, and cytosolic fractions of the cell was regulated by inositol in the same manner. Cells supplemented with inositol had elevated phospholipid levels and reduced triacylglycerol levels compared with unsupplemented cells. Serine, ethanolamine, and choline did not significantly affect the phosphatidate phosphatase activity of cells grown in the absence or presence of inositol. Enzyme activity was not regulated in inositol biosynthesis regulatory mutants, suggesting that regulation by inositol is coupled to regulation of inositol biosynthesis. Phosphatidate phosphatase activity was pleiotropically expressed in structural gene mutants defective in phospholipid biosynthesis. These results suggested that phosphatidate phosphatase was regulated by inositol at a genetic level.

Identification of mitochondrial and microsomal phosphatidylserine synthase in Saccharomyces cerevisiae as the gene product of the CHO1 structural gene

In Saccharomyces cerevisiae, the membrane-associated enzyme phosphatidylserine synthase (EC 2.7.8.8) is present in the mitochondria and the endoplasmic reticulum. The enzyme from both membrane fractions reacted with antiserum raised against a hybrid protein expressed from a TRPE-CHO1 fusion gene in Escherichia coli and was absent in a cho1 null mutant, strongly suggesting that both the mitochondrial and microsomal forms of phosphatidylserine synthase are the products of the CHO1 gene. The highest degree of purification of enzymatically active protein was 380- and 420-fold from the mitochondrial and the microsomal compartments, respectively. In both cases, the enzymatically active and immunoreactive material comigrated with a protein band of 30,000 apparent molecular weight. In the absence of protease inhibitors during the preparation of membranes, the enzyme underwent degradation to an enzymatically active protein of 23,000 apparent molecular weight.

Saccharomyces cerevisiae mutant with a partial defect in the synthesis of CDP-diacylglycerol and altered regulation of phospholipid biosynthesis

A Saccharomyces cerevisiae mutant (cdg1 mutation) was isolated on the basis of an inositol excretion phenotype and exhibited pleiotropic deficiencies in phospholipid biosynthesis. Genetic analysis of the mutant confirmed that the cdg1 mutation represents a new genetic locus and that a defect in a single gene was responsible for the Cdg1 phenotype. CDP-diacylglycerol synthase activity in mutant haploid cells was 25% of the wild-type derepressed level. Biochemical and immunoblot analyses revealed that the defect in CDP-diacylglycerol synthase activity in the cdg1 mutant was due to a reduced level of the CDP-diacylglycerol synthase Mr-56,000 subunit rather than to an alteration in the enzymological properties of the enzyme. This defect resulted in a reduced rate of CDP-diacylglycerol synthesis, an elevated phosphatidate content, and alterations in overall phospholipid synthesis. Unlike wild-type cells, CDP-diacylglycerol synthase was not regulated in response to water-soluble phospholipid precursors. The cdg1 lesion also caused constitutive expression of inositol-1-phosphate synthase and elevated phosphatidylserine synthase. Phosphatidylinositol synthase was not affected in the cdg1 mutant.

Coordinate regulation of phospholipid biosynthesis by serine in Saccharomyces cerevisiae

The addition of L-serine to inositol-containing growth medium repressed membrane-associated CDPdiacylglycerol synthase (CTP:phosphatidate cytidylyltransferase, EC 2.7.7.41) and phosphatidylserine synthase (CDPdiacylglycerol:L-serine O-phosphatidyltransferase, EC 2.7.8.8) activities and subunit levels in wild-type Saccharomyces cerevisiae. Enzyme activities and subunit levels were not repressed when inositol was absent from the growth medium. The addition of L-serine to the growth medium did not affect the phospholipid composition of wild-type cells. CDPdiacylglycerol synthase and phosphatidylserine synthase were not regulated in the S. cerevisiae inositol biosynthesis ino2, ino4, and opi1 regulatory mutants, suggesting that regulation by inositol plus L-serine is coupled to inositol synthesis. Inositol and L-serine did not affect the activities of purified CDPdiacylglycerol synthase and phosphatidylserine synthase. The addition of compounds structurally related to L-serine to the growth medium of wild-type cells also resulted in a repression of CDPdiacylglycerol synthase and phosphatidylserine synthase but only in the presence of inositol. Phosphatidylinositol synthase (CDPdiacylglycerol:myo-inositol 3-phosphatidyltransferase, EC 2.7.8.11) was not regulated by inositol plus L-serine.