Inositol metabolism in yeasts

Vitamin requirements and biosynthesis in Saccharomyces cerevisiae

Chemically defined media for yeast cultivation (CDMY) were developed to support fast growth, experimental reproducibility, and quantitative analysis of growth rates and biomass yields. In addition to mineral salts and a carbon substrate, popular CDMYs contain seven to nine B-group vitamins, which are either enzyme cofactors or precursors for their synthesis. Despite the widespread use of CDMY in fundamental and applied yeast research, the relation of their design and composition to the actual vitamin requirements of yeasts has not been subjected to critical review since their first development in the 1940s. Vitamins are formally defined as essential organic molecules that cannot be synthesized by an organism. In yeast physiology, use of the term "vitamin" is primarily based on essentiality for humans, but the genome of the Saccharomyces cerevisiae reference strain S288C harbours most of the structural genes required for synthesis of the vitamins included in popular CDMY. Here, we review the biochemistry and genetics of the biosynthesis of these compounds by S. cerevisiae and, based on a comparative genomics analysis, assess the diversity within the Saccharomyces genus with respect to vitamin prototrophy.

Inositol Hexakisphosphate Kinase 1 (IP6K1) Regulates Inositol Synthesis in Mammalian Cells

myo-Inositol, the precursor of all inositol compounds, has pivotal roles in cell metabolism and signaling pathways. Although physiological studies indicate a strong correlation between abnormal intracellular inositol levels and neurological disorders, very little is known about the regulation of inositol synthesis in mammalian cells. In this study, we report that IP6K1, an inositol hexakisphosphate kinase that catalyzes the synthesis of inositol pyrophosphate, regulates inositol synthesis in mammalian cells. Ip6k1 ablation led to profound changes in DNA methylation and expression of Isyna1 (designated mIno1), which encodes the rate-limiting enzyme inositol-3-phosphate synthase. Interestingly, IP6K1 preferentially bound to the phospholipid phosphatidic acid, and this binding was required for IP6K1 nuclear localization and the regulation of mIno1 transcription. This is the first demonstration of IP6K1 as a novel negative regulator of inositol synthesis in mammalian cells.

Determining the effects of inositol supplementation and the opi1 mutation on ethanol tolerance of Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae is an important microorganism for the ethanol fuel industry. As with many microorganisms, the production and accumulation of certain metabolites, such as ethanol, can have a detrimental effect on cell growth and productivity. Yeast cells containing a higher concentration of phosphatidylinositol (PI) in the cellular membrane, due to inositol supplementation in the growth media, have been shown to tolerate and produce higher concentrations of ethanol. The specific goal of our research was to assess the effects of inositol supplementation in the growth media as well as to compare the ethanol tolerance of the wild-type S. cerevisiae to a mutant, the opi1 strain (opi=overproduction of inositol). The OPI1 gene product is a negative regulatory factor that controls the transcription of the INO1 structural gene, which encodes the enzyme catalyzing the limiting step in the biosynthesis of inositol, that is, the conversion of glucose-6-phosphate to inositol-3-phosphate. Upon the deletion of the OPI1 gene, the cell will constitutively produce inositol, regardless of the extracellular inositol concentration. Inositol supplementation in cultures of wild-type cells increased ethanol tolerance in terms of cell viability. Cells grown in -I media had a 20% higher specific death rate than cells grown in +I media when exposed to 15% ethanol. The opi1 strain, with the ability to constitutively produce inositol regardless of media composition, showed less inhibition of cell growth in the presence of ethanol than did the wild-type strain, particularly in inositol-free media. We conclude that the introduction of an opi1 mutation in yeast results in an inherent increase in PI levels and constitutive biosynthesis of inositol that, in turn, will reduce the cost of supplementing inositol into the media to achieve a higher ethanol tolerance.

Inositol Induces a Profound Alteration in the Pattern and Rate of Synthesis and Turnover of Membrane Lipids in Saccharomyces cerevisiae

The addition of inositol to actively growing yeast cultures causes a rapid increase in the rate of synthesis of phosphatidylinositol and, simultaneously, triggers changes in the expression of hundreds of genes. We now demonstrate that the addition of inositol to yeast cells growing in the presence of choline leads to a dramatic reprogramming of cellular lipid synthesis and turnover. The response to inositol includes a 5-6-fold increase in cellular phosphatidylinositol content within a period of 30 min. The increase in phosphatidylinositol content appears to be dependent upon fatty acid synthesis. Phosphatidylcholine turnover increased rapidly following inositol addition, a response that requires the participation of Nte1p, an endoplasmic reticulum-localized phospholipase B. Mass spectrometry revealed that the acyl species composition of phosphatidylinositol is relatively constant regardless of supplementation with inositol or choline, whereas phosphatidylcholine acyl species composition is influenced by both inositol and choline. In medium containing inositol, but lacking choline, high levels of dimyristoylphosphatidylcholine were detected. Within 60 min following the addition of inositol, dimyristoylphosphatidylcholine levels had decreased from approximately 40% of total phosphatidylcholine to a basal level of less than 5%. nte1Delta cells grown in the absence of inositol and in the presence of choline exhibited lower levels of dimyristoylphosphatidylcholine than wild type cells grown under these same conditions, but these levels remained largely constant after the addition of inositol. These results are discussed in relationship to transcriptional regulation known to be linked to lipid metabolism in yeast.

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.

Casein kinase II phosphorylation of the yeast phospholipid synthesis transcription factor Opi1p

The transcription factor Opi1p regulates phospholipid synthesis in the yeast Saccharomyces cerevisiae by repressing the expression of several UAS(INO)-containing genes (e.g. INO1). Opi1p repressor activity is most active in inositol-supplemented cells. Regulation of Opi1p repressor activity is mediated by multiple phosphorylations catalyzed by protein kinases A and C. In this work, we showed that Opi1p was also phosphorylated by casein kinase II. Using purified maltose-binding protein-Opi1p as a substrate, casein kinase II activity was dose-and time-dependent and dependent on the concentrations of maltose-binding protein-Opi1p (Km = 25 microg/ml) and ATP (Km = 7 microM). Of three mutations (S10A, S38A, and S239A) in putative phosphorylation sites, 10 only the S10A mutation affected Opi1p phosphorylation. That Ser10 was a specific target of casein kinase II was confirmed by the loss of a phosphopeptide in the S10A mutant protein. The S10A mutation did not affect phosphorylation of Opi1p by either protein kinase A or protein kinase C. Likewise, phosphorylation of Opi1p by casein kinase II was not affected by mutations in protein kinase A (S31A and S251A) and protein (S10A) kinase C (S26A) phosphorylation sites. Expression of the OPI1 allele in an opi1Delta mutant attenuated (2-fold) the repressive effect of Opi1p on INO1 expression, and this effect was only observed when cells were grown in the absence of inositol. These data supported the conclusion that casein kinase II phosphorylation at Ser10 played a role in stimulating the repression of INO1 when Opi1p was not in its most active state (i.e. in inositol-deprived cells).

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.

IRE1-independent gain control of the unfolded protein response

Nonconventional splicing of the gene encoding the Hac1p transcription activator regulates the unfolded protein response (UPR) in Saccharomyces cerevisiae. This simple on/off switch contrasts with a more complex circuitry in higher eukaryotes. Here we show that a heretofore unrecognized pathway operates in yeast to regulate the transcription of HAC1. The resulting increase in Hac1p production, combined with the production or activation of a putative UPR modulatory factor, is necessary to qualitatively modify the cellular response in order to survive the inducing conditions. This parallel endoplasmic reticulum–to–nucleus signaling pathway thereby serves to modify the UPR-driven transcriptional program. The results suggest a surprising conservation among all eukaryotes of the ways by which the elements of the UPR signaling circuit are connected. We show that by adding an additional signaling element to the basic UPR circuit, a simple switch is transformed into a complex response.

The unfolded protein response in yeast was thought to be a simple on/off switch. Here, a second signaling element is revealed, transforming this simple switch into a complex response

Formation of myo-inositol phosphates by Aspergillus niger 3-phytase

Kinetics of phytate hydrolysis by Aspergillus niger phytase and correlation between the amount of released phosphate and creation of lower myo-inositol phosphates were investigated. Phytase was able to hydrolyze myo-inositol hexakis-, pentakis-, tetrakis-, and trisphosphates. Finally, about 56% of total phosphate were released and myo-inositol bisphosphate was detected as the end-product.

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.

Signal transduction from the endoplasmic reticulum to the cell nucleus

The endoplasmic reticulum (ER) serves several important functions. Cholesterol, an essential component of cellular membranes, is synthesized on the ER surface. Inside the organelle, proteins destined for secretion or transport to the cell surface are folded and become glycosylated. Because these processes are essential for cell viability, a disturbance in ER function presents significant stress to the cell. In response to ER stress, three distinct signal transduction pathways can be activated. Two of these, the unfolded protein response and the ER-overload response, respond to disturbances in protein processing. The third, the sterol regulatory cascade, is activated by depletion of cholesterol. This review summarizes the recent advances in our understanding of these ER-nuclear signal transduction pathways. In addition, it points to novel regulatory mechanisms discovered in these pathways, which may be widely used in other systems.

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.

Khafrefungin, a novel inhibitor of sphingolipid synthesis

In the course of screening for antifungal agents we have discovered a novel compound isolated from an endophytic fungus that inhibits fungal sphingolipid synthesis. Khafrefungin, which is composed of aldonic acid linked via an ester to a C22 modified alkyl chain, has fungicidal activity against Candida albicans, Cryptococcus neoformans, and Saccharomyces cerevisiae. Sphingolipid synthesis is inhibited in these organisms at the step in which phosphoinositol is transferred to ceramide, resulting in accumulation of ceramide and loss of all of the complex sphingolipids. In vitro, khafrefungin inhibits the inositol phosphoceramide synthase of C. albicans with an IC50 of 0.6 nM. Khafrefungin does not inhibit the synthesis of mammalian sphingolipids thus making this the first reported compound that is specific for the fungal pathway.

1L-myo-inositol-1-phosphate synthase

1L-myo-Inositol-1-phosphate synthase catalyzes the conversion of D-glucose 6-phosphate to 1L-myo-inositol-1-phosphate, the first committed step in the production of all inositol-containing compounds, including phospholipids, either directly or by salvage. The enzyme exists in a cytoplasmic form in a wide range of plants, animals, and fungi. It has also been detected in several bacteria and a chloroplast form is observed in alga and higher plants. The enzyme has been purified from a wide range of organisms and its active form is a multimer of identical subunits ranging in molecular weight from 58,000 to 67,000. The activity of the synthase is stimulated by NH4Cl and inhibited by glucitol 6-phosphate and 2-deoxyglucose 6-phosphate. Structural genes (INO1) encoding the 1L-myo-inositol-1-phosphate synthase subunit have been isolated from several eukaryotic microorganisms and higher plants. In baker's yeast, Saccharomyces cerevisiae, the transcriptional regulation of the INO1 gene has been studied in detail and its expression is sensitive to the availability of phospholipid precursors as well as growth phase. The regulation of the structural gene encoding 1L-myo-inositol-1-phosphate synthase has also been analyzed at the transcriptional level in the aquatic angiosperm, Spirodela polyrrhiza and the halophyte, Mesembryanthemum crystallinum.

A conditional sterol esterification defect in yeast having either a sec1 or sec5 mutation in the secretory pathway

Two temperature-conditional secretory mutations, sec1 and sec5, cause the accumulation of post-Golgi vesicles when strains containing these mutations are grown at 37 degrees C. In addition to accumulating vesicles, the mutants do not esterify free sterol on rich media at the restrictive temperature. It is the high level of inositol in the media that causes this condition in the yeast Saccharomyces cerevisiae, not a defective steryl ester synthase or lack of substrates. When strains containing the sec1 or sec5 mutation were transformed separately with a plasmid carrying SEC1 and SEC5, the esterification and secretory defects were alleviated. Double mutants containing sec6, sec14 or sec18 with either a sec1 or sec5 mutation have normal esterification levels. Strains with suppressor mutations were isolated that grew at 37 degrees C, esterified sterols and had diminished accumulation of vesicles, when grown at the restrictive temperature on defined media with additional inositol. Electron microscopy was used to examine vesicle accumulation, the number of lipid droplets, and to further characterize the esterification defect. When grown at 37 degrees C on defined medium, the strains with sec5 or sec1 accumulated the usual secretory vesicles, but when grown under similar conditions with elevated levels of inositol, accumulated an additional vesicular-like body.

Endoplasmicreticulum-induced signal transduction and gene expression

Cells can respond to perturbations in endoplasmic reticulum (ER) function by activating two distinct signal-transduction pathways: one responds to unfolded proteins, the other to an overload of the organelle with membrane proteins. A third pathway is activated upon sterol depletion of cells and involves the cleavage and subsequent nuclear translocation of an ER membrane-bound transcription factor. Thus, three distinct pathways each activated by a different signal are currently known to project from the ER into the nucleus. This review summarizes the current understanding of these three pathways.

Candida albicans phosphatidylinositol synthase has common features with both Saccharomyces cerevisiae and mammalian phosphatidylinositol synthases

Phosphatidylinositol (PI) synthase (cytidine 5'-diphospho (CDP)-1,2-diacyl-sn-glycerol:myo-inositol 3-phosphatidyltransferase, EC 2.7.8.11) was isolated from the microsomal cell fraction of Candida albicans. The Triton X-100 extracted enzyme was enriched 140-fold by affinity chromatography on CDP-diacylglycerol-Sepharose. The enzyme had a pH optimum at 9.5 in glycine/NaOH buffer. It had an absolute requirement for Mg2+ or Mn2+ and was inhibited by Ca2+ and Zn2+. Maximal activity was at 0.2-0.6 mM-CDP-diacylglycerol, higher concentrations inhibited the enzyme. With 2'-deoxy-CDP-diacylglycerol as the lipid substrate, optimal activity was at 0.7 mM. The K(m) for myo-inositol was determined to be 0.55 mM. The optimal temperature for the PI synthase reaction was 55 degrees C. The C. albicans PI synthase shows differences to the Saccharomyces cerevisiae enzyme, such as activation by bivalent cations, inhibition by nucleotides, temperature optimum and activation energy, but also to the human PI synthase in preference for the lipid substrates, inhibition by nucleoside monophosphates and stabilization by Mn2+ and phospholipids.

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.

Production and reutilization of an extracellular phosphatidylinositol catabolite, glycerophosphoinositol, by Saccharomyces cerevisiae

Phosphatidylinositol catabolism in Saccharomyces cerevisiae is known to result in the formation of extracellular glycerophosphoinositol (GroPIns). We now report that S. cerevisiae not only produces but also reutilizes extracellular GroPIns and that these processes are regulated in response to inositol availability. A wild-type strain uniformly prelabeled with [3H] inositol displayed dramatically higher extracellular GroPIns levels when cultured in medium containing inositol than when cultured in medium lacking inositol. This difference in extracellular accumulation of GroPIns in response to inositol availability was shown to be a result of both regulated production and regulated reutilization. In a strain in which a negative regulator of phospholipid and inositol biosynthesis had been deleted (an opi1 mutant), this pattern of extracellular GroPIns accumulation in response to inositol availability was altered. An inositol permease mutant (itr1 itr2), which is unable to transport free inositol, was able to incorporate label from exogenous glycerophospho [3H]inositol, indicating that the inositol label did not enter the cell solely via the transporters encoded by itr1 and itr2. Kinetic studies of a wild-type strain and an itr1 itr2 mutant strain revealed that at least two mechanisms exist for the utilization of exogenous GroPIns: an inositol transporter-dependent mechanism and an inositol transporter-independent mechanism. The inositol transporter-independent pathway of exogenous GroPIns utilization displayed saturation kinetics and was energy dependent. Labeling studies employing [14C]glycerophospho[3H] inositol indicated that, while GroPIns enters the cell intact, the inositol moiety but not the glycerol moiety is incorporated into lipids.

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.

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).

Regulation of phosphatidylinositol:ceramide phosphoinositol transferase in Saccharomyces cerevisiae

Maximal phosphatidylinositol:ceramide phosphoinositol transferase activity was measured in yeast cells harvested during the exponential phase of growth. The addition of inositol to the growth medium resulted in a twofold increase in IPC synthase activity in cells grown in the presence or absence of exogenous choline. Enzyme activity was not regulated in yeast inositol biosynthesis regulatory mutants by the addition of inositol to the growth medium.

Phosphatidylinositol 4-kinase: gene structure and requirement for yeast cell viability

Phosphatidylinositol (PtdIns) 4-kinase catalyzes the first step in the biosynthesis of PtdIns-4,5-bisphosphate (PtdIns[4,5]P2). Hydrolysis of PtdIns[4,5]P2 in response to extracellular stimuli is thought to initiate intracellular signaling cascades that modulate cell proliferation and differentiation. The PIK1 gene encoding a PtdIns 4-kinase from the yeast Saccharomyces cerevisiae was isolated by polymerase chain reaction (PCR) with oligonucleotides based on the sequence of peptides derived from the purified enzyme. The sequence of the PIK1 gene product bears similarities to that of PtdIns 3-kinases from mammals (p110) and yeast (Vps34p). Expression of PIK1 from a multicopy plasmid elevated PtdIns 4-kinase activity and enhanced the response to mating pheromone. A pik1 null mutant was inviable, indicating that PtdIns4P and presumably PtdIns[4,5]P2 are indispensable phospholipids.

Transcriptional induction of genes encoding endoplasmic reticulum resident proteins requires a transmembrane protein kinase

The transcription of genes encoding soluble proteins that reside in the endoplasmic reticulum (ER) is induced when unfolded proteins accumulate in the ER. Thus, an intracellular signal transduction pathway must exist that mediates communication between the ER lumen and the nucleus. We have identified a gene in S. cerevisiae, IRE1, that is required for this pathway: ire1- mutants cannot activate transcription of KAR2 and PDI1, which encode the ER resident proteins BiP and protein disulfide isomerase. Moreover, IRE1 is essential for cell viability under stress conditions that cause unfolded proteins to accumulate in the ER. IRE1 encodes a transmembrane serine/threonine kinase that we propose transmits the unfolded protein signal across the ER or inner nuclear membrane. IRE1 is also required for inositol prototrophy, suggesting that the induction of ER resident proteins is coupled to the biogenesis of new ER membrane.

Multiple isomers of phosphatidyl inositol monophosphate and inositol bis- and trisphosphates from filamentous fungi

The range of inositol phosphates and inositol phospholipids present in three filamentous fungi, Neurospora crassa, Fusarium graminearum and Phanerochaete chrysosporium has been investigated by HPLC analysis. The profiles obtained demonstrate that two isomers of phosphatidyl inositol monophosphate are present, and that an apparent complexity in the number of isomers of inositol bis- and trisphosphates is found in filamentous fungi that has not been observed in animal or plant cells.

Yeast sugar transporters

Transport of sugars is a fundamental property of all eukaryotic cells. Of particular importance is the uptake of glucose, a preferred carbon and energy source. The rate of glucose utilization in yeast is often dictated by the activity and concentration of glucose transporters in the plasma membrane. Given the importance of transport as a site of control of glycolytic flux, the regulation of glucose transporters is necessarily complex. The molecular analysis of these transporters in Saccharomyces has revealed the existence of a multigene family of sugar carriers. Recent data have raised the question of the actual role of all of these proteins in sugar catabolism, as some appear to be lowly expressed, and point mutations of these genes may confer pleiotropic phenotypes, inconsistent with a simple role as catabolic transporters. The transporters themselves appear to be intimately involved in the process of sensing glucose, a model for which there is growing support.

Emerging role of lipids of Candida albicans, a pathogenic dimorphic yeast

It is clear that C. albicans lipids have gained tremendous importance in recent years. In addition to being a barrier for entrance of various metabolites, it also provides the site of action for the synthesis of enzyme(s) involved in cell wall morphogenesis and antifungal action. While alterations in lipid composition during a yeast to mycelia transition have been observed, in most of the studies, lipid fluctuations reported could have been due to various environmental factors involved in the induction of morphogenesis [4,5]. A clear understanding of lipid biosynthesis and metabolic blocks due to antifungal action is likely to shed further light on selective interactions of antifungals. Despite the multifacet role of lipids in various functions of this pathogenic yeast, their exact involvement is poorly understood. The situation is little better with regard to ergosterol and its metabolism. Ergosterol is, indeed, important for anti-candidal activity and appears to be involved in the morphogenesis of C. albicans. The fluctuation in phospholipid composition have led to altered properties of plasma membrane namely, membrane fluidity, transport activities and drug sensitivity, which suggest that-a critical level of individual phospholipid is important for proper functioning of the plasma membrane. What the exact role is of individual phospholipid is far from clear. Many unanswered questions relating to the role of PI and sphingomyelin in signal transduction, involvement of phospholipases in the maintenance of phospholipid composition, and role of lipid transfer proteins in assembly and asymmetry of lipids are some aspects which merit further work.

Sphingolipid long-chain-base auxotrophs of Saccharomyces cerevisiae: genetics, physiology, and a method for their selection

A selection method for sphingolipid long-chain-base auxotrophs of Saccharomyces cerevisiae was devised after observing that strains that require a long-chain base for growth become denser when starved for this substance. Genetic analysis of over 60 such strains indicated only two complementation classes, lcb1 and lcb2. Mutant strains from each class grew equally well with 3-ketodihydrosphingosine, erythrodihydrosphingosine or threodihydrosphingosine, or phytosphingosine. Since these metabolites represent the first, second, and last components, respectively, of the long-chain-base biosynthetic pathway, it is likely that the LCB1 and LCB2 genes are involved in the first step of long-chain-base synthesis. The results of long-chain-base starvation in the Lcb- strains suggest that one or more sphingolipids have a vital role in S. cerevisiae. Immediate sequelae of long-chain-base starvation were loss of viability, exacerbated in the presence of alpha-cyclodextrin, and loss of phosphoinositol sphingolipid synthesis but not phosphatidylinositol synthesis. Loss of viability with long-chain-base starvation could be prevented by also blocking either protein or nucleic acid synthesis. Without a long-chain-base, cell division, dry mass accumulation, and protein synthesis continued at a diminished rate and were further inhibited by the detergent Tergitol. The cell density increase induced by long-chain-base starvation is thus explained as a differential loss of cell division and mass accumulation. Long-chain-base starvation in Lcb- S. cerevisiae and inositol starvation of Inos- S. cerevisiae share common features: an increase in cell density and a loss of cell viability overcome by blocking macromolecular synthesis.

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.

Regulation of phosphatidylserine decarboxylase in Saccharomyces cerevisiae by inositol and choline: kinetics of repression and derepression

The biosynthesis of phosphatidylserine (PS) and its conversion to phosphatidylcholine (PC) are regulated coordinately by inositol and choline in Saccharomyces cerevisiae (G. M. Carman and S. A. Henry, 1989, Annu. Rev. Biochem. 58, 635-669). In this study, PS decarboxylase activity is shown to be partially repressed when inositol is added to the medium of cells in the log phase of growth, and the extent of repression is augmented by the inclusion of choline, but not ethanolamine. The kinetics of repression and derepression of PS decarboxylase, PS synthase, and phospholipid N-methyltransferase (PNMT) activities, as regulatory responses to the availability of exogenous inositol and choline, have been characterized. When inositol was added to the medium of cell cultures growing exponentially, the three biosynthetic enzyme activities reached an intermediate level of repression (50-85% of control) within 60 min. After the addition of the combination of inositol and choline, PS decarboxylase, PS synthase, and PNMT activities decreased to the intermediate levels of repression in 60 min and were subsequently reduced to 15-40% of control values during a later stage of regulation (2-3 h). In a derepression study, the three enzyme activities remained relatively stable for approximately 60 min following the removal of choline and/or inositol from the growth medium, but the specific activities of PS decarboxylase, PS synthase, and PNMT increased to maximally derepressed levels within 2-3 h. The induction of the three biosynthetic activities was blocked by cycloheximide, but not by chloramphenicol. In summary, the level of PS decarboxylase activity in S. cerevisiae is partially and reversibly suppressed by inositol and further diminished by the combination of inositol and choline. The biphasic kinetics of repression by inositol and choline suggest that the effect of choline is dependent on earlier events mediated by inositol and possibly involves a separate regulatory factor(s).

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.

Incorporation of unsaturated fatty acids by Saccharomyces cerevisiae: conservation of fatty-acyl saturation in phosphatidylinositol

Saccharomyces cerevisiae was grown anaerobically in media supplemented with myristoleic 14:1(9c), palmitoleic 16:1(9c), oleic 18:1(9c), linoleic 18:2(9,12c), gamma-linolenic 18:3(9,12,15c) or eicosenoic 20:1(11c) acid. Cells from exponential-phase cultures contained approximately the same proportions of the major phospholipid classes, namely phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol and phosphatidylserine, the greatest differences being detected in cells grown in the presence of 14:1(9c) or 20:1(11c) acids. The extent to which phospholipids from cells were enriched with residues of the exogenously supplied acid varied from 52% in cells grown in the presence of 14:1(9c) acid to 13% in cells grown in media supplemented with 20:1(11c) acid. Analysis of the fatty-acyl composition of the four major phospholipid classes revealed that the degree of unsaturation varied considerably in three of the classes, while phosphatidylinositol conserved a high degree of saturation. The possible significance of the latter finding in relation to the physiological role of phosphatidylinositol in the plasma membrane is discussed.