Metabolic interconversion of free sterols and steryl esters in Saccharomyces cerevisiae

Glutamine metabolism modulates azole susceptibility in Trypanosoma cruzi amastigotes

The mechanisms underlying resistance of the Chagas disease parasite, Trypanosoma cruzi, to current therapies are not well understood, including the role of metabolic heterogeneity. We found that limiting exogenous glutamine protects actively dividing amastigotes from ergosterol biosynthesis inhibitors (azoles), independent of parasite growth rate. The antiparasitic properties of azoles are derived from inhibition of lanosterol 14α-demethylase (CYP51) in the endogenous sterol synthesis pathway. We find that carbons from ¹³C-glutamine feed into amastigote sterols and into metabolic intermediates that accumulate upon CYP51 inhibition. Incorporation of ¹³C-glutamine into endogenously synthesized sterols is increased with BPTES treatment, an inhibitor of host glutamine metabolism that sensitizes amastigotes to azoles. Similarly, amastigotes are re-sensitized to azoles following addition of metabolites upstream of CYP51, raising the possibility that flux through the sterol synthesis pathway is a determinant of sensitivity to azoles and highlighting the potential role for metabolic heterogeneity in recalcitrant T. cruzi infection.

The wide-ranging phenotypes of ergosterol biosynthesis mutants, and implications for microbial cell factories

Yeast strains have been used extensively as robust microbial cell factories for the production of bulk and fine chemicals, including biofuels (bioethanol), complex pharmaceuticals (antimalarial drug artemisinin and opioid pain killers), flavours, and fragrances (vanillin, nootkatone, and resveratrol). In many cases, it is of benefit to suppress or modify ergosterol biosynthesis during strain engineering, for example, to increase thermotolerance or to increase metabolic flux through an alternate pathway. However, the impact of modifying ergosterol biosynthesis on engineered strains is discussed sparsely in literature, and little attention has been paid to the implications of these modifications on the general health and well-being of yeast. Importantly, yeast with modified sterol content exhibit a wide range of phenotypes, including altered organization and dynamics of plasma membrane, altered susceptibility to chemical treatment, increased tolerance to high temperatures, and reduced tolerance to other stresses such as high ethanol, salt, and solute concentrations. Here, we review the wide-ranging phenotypes of viable Saccharomyces cerevisiae strains with altered sterol content and discuss the implications of these for yeast as microbial cell factories.

Understanding colonization and proliferation potential of endophytes and pathogen in planta via plating, polymerase chain reaction and ergosterol assay

This study aimed to establish the colonization behavior and proliferation potential of three endophytes and one pathogen Ganoderma boninense (Gb) introduced into oil palm ramets (host model). The endophytes selected were Diaporthe phaseolorum (WAA02), Trichoderma asperellum (T2), and Penicillium citrinum (BTF08). Ramets were first inoculated with 100 mL of fungal cells (106 cfu mL-1) via soil drenching. For the next 7 days, ramets were sampled and subjected to three different assays to detect and identify fungal colonization, and establish their proliferation potential in planta. Plate assay revealed the presence of endophytes in root, stem and leaf tissues within 7 days after inoculation. Polymerase Chain Reaction (PCR) detected and identified the isolates from the plant tissues. The ergosterol assay (via high performance liquid chromatography, HPLC) confirmed the presence of endophytes and Gb in planta. The increase in ergosterol levels throughout 49 days was however insignificant, suggesting that proliferation may be absent or may occur very slowly in planta. This study strongly suggests that the selected endophytes could colonize the host upon inoculation, but proliferation occurs at a slower rate, which may subsequently influence the biocontrol expression of endophytes against the pathogen.

Age-related obesity is a heritage of the evolutionary past

In the process of human aging, an increase in the total amount of fat is observed mainly due to accumulation of lipids in non-adipose tissues. Insulin resistance, provoked by the intracellular accumulation of triglycerides, is often associated with development of such age-related diseases as atherosclerosis, type 2 diabetes, cancer, osteoporosis, and also with systemic inflammation and lipo- and glucose toxicity. Accumulation of lipids and lipophilic compounds is a biological phenomenon common for both prokaryotes and eukaryotes. Initially, it arose as an adaptation to starvation and shortage of nitrogen-containing nutrients, but later it converted into a depot of membrane material, needed on recommencement of cell division. In rodents and humans, the accumulation of non-metabolized fat in non-adipose tissues can be regarded as an adaptation to changes in the internal medium on a certain stage of ontogenesis as a result of age-related dysfunction of adipose tissue.

Genome-wide screens for gene products regulating lipid droplet dynamics

Lipid droplets (LDs) are emerging as dynamic cellular organelles that play a key role in lipid and membrane homeostasis. Abnormal lipid droplet dynamics are associated with the pathophysiology of many metabolic diseases, such as obesity, diabetes, atherosclerosis, fatty liver, and even cancer. Understanding the molecular mechanisms governing the dynamics of LDs, namely, their biogenesis, growth, maintenance, and degradation, will not only shed light on the cellular functions of LDs, but also provide additional clues to treatment of metabolic diseases. Genome-wide screen is a powerful approach to identify genetic factors that regulate lipid droplet dynamics. Here, we summarize recent genome-wide studies using yeast and Drosophila cells to understand the cellular dynamics of LDs. The results suggest that the genome-wide screens should be carried out in multiple organisms or cells, and using different nutritional conditions.

Mathematical modeling and validation of the ergosterol pathway in Saccharomyces cerevisiae

The de novo biosynthetic machinery for both sphingolipid and ergosterol production in yeast is localized in the endoplasmic reticulum (ER) and Golgi. The interconnections between the two pathways are still poorly understood, but they may be connected in specialized membrane domains, and specific knockouts strongly suggest that both routes have different layers of mutual control and are co-affected by drugs. With the goal of shedding light on the functional integration of the yeast sphingolipid-ergosterol (SL-E) pathway, we constructed a dynamic model of the ergosterol pathway using the guidelines of Biochemical Systems Theory (BST) (Savageau., J. theor. Biol., 25, 365–9, 1969). The resulting model was merged with a previous mathematical model of sphingolipid metabolism in yeast (Alvarez-Vasquez et al., J. theor. Biol., 226, 265–91, 2004; Alvarez-Vasquez et al., Nature433, 425–30, 2005). The S-system format within BST was used for analyses of consistency, stability, and sensitivity of the SL-E model, while the GMA format was used for dynamic simulations and predictions. Model validation was accomplished by comparing predictions from the model with published results on sterol and sterol-ester dynamics in yeast. The validated model was used to predict the metabolomic dynamics of the SL-E pathway after drug treatment. Specifically, we simulated the action of drugs affecting sphingolipids in the endoplasmic reticulum and studied changes in ergosterol associated with microdomains of the plasma membrane (PM).

Bioconversion of xylan to triglycerides by oil-rich yeasts

A series of lipid-accumulating yeasts was examined for their potential to saccharify xylan and accumulate triglyceride. Of the genera tested, including Candida, Cryptococcus, Lipomyces, Rhodosporidium, Rhodotorula, and Trichosporon, only Cryptococcus and Trichosporon isolates saccharified xylan. All of the strains could assimilate xylose and accumuate triglyceride under nitrogen-limiting conditions. Strains of Cryptococcus albidus were found to be especially useful for a one-step saccharification of xylan coupled to triglyceride synthesis. Cryptococcus terricolus, a strain constitutive for lipid accumulation, lacked extracellular xylanase, but did assimilate xylose and xylobiose and was able to continuously convert xylan to triglyceride if the culture medium was supplemented with xylanase.

Distribution of free and esterified ergosterols in the medicinal fungus Ganoderma lucidum

The fruiting bodies, spores, and lipid from the spores of Ganoderma lucidum have been widely used for medicinal purpose in China. Ergosterol content may be a suitable marker for evaluating the quality of ganoderma spore and ganoderma spore lipid (GSL) products. A gradient reversed-phase high-performance liquid chromatography method was developed for the simultaneous determination of free and esterified ergosterols in G. lucidum. The contents of free and esterified ergosterols in the different parts (the stipe, pileus, tubes, and spores) of G. lucidum and GSL were determined. The results showed that total ergosterol levels in the stipe, pileus, tubes, and spores of G. lucidum were between 0.8 and 1.6 mg/g. The relative abundances of free to esterified ergosterol were different in the different parts of G. lucidum. The spores and the tubes, the hymenophore tissue that contains the spore-producing cells, have a considerably higher percentage of ergosteryl esters (41.9 and 39.7% of total ergosterol) in comparison with the pileus and stipe tissues (3.6 and 6.2%).

Yeh1 constitutes the major steryl ester hydrolase under heme-deficient conditions in Saccharomyces cerevisiae

Steryl esters are stored in intracellular lipid droplets from which they are mobilized upon demand and hydrolyzed to yield free sterols and fatty acids. The mechanisms that control steryl ester mobilization are not well understood. We have previously identified a family of three lipases of Saccharomyces cerevisiae that are required for efficient steryl ester hydrolysis, Yeh1, Yeh2, and Tgl1 (R. Köffel, R. Tiwari, L. Falquet, and R. Schneiter, Mol. Cell. Biol. 25:1655-1668, 2005). Both Yeh1 and Tgl1 localize to lipid droplets, whereas Yeh2 is localized to the plasma membrane. To characterize the precise function of these three partially redundant lipases, we examined steryl ester mobilization under heme-deficient conditions. S. cerevisiae is a facultative anaerobic organism that becomes auxotrophic for sterols and unsaturated fatty acids in the absence of molecular oxygen. Anaerobic conditions can be mimicked in cells that are deficient for heme synthesis. We here report that Yeh1 is the sole active steryl ester hydrolase under such heme-deficient conditions, indicating that Yeh1 is activated whereas Yeh2 and Tgl1 are inactivated by the lack of heme. The heme-dependent activation of Yeh1 is mediated at least in part by an increase in steady-state levels of Yeh1 at the expense of Yeh2 and Tgl1 in exponentially growing cells. This increase in steady-state levels of Yeh1 requires Rox3, a component of the mediator complex that regulates transcription by RNA polymerase II. These data thus provide the first link between fat degradation and the transcriptional control of lipase activity in yeast.

Metabolic flux analysis of the sterol pathway in the yeast Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae is a useful model system for examining the biosynthesis of sterols in eukaryotic cells. To investigate underlying regulation mechanisms, a flux analysis of the ergosterol pathway was performed. A stoichiometric model was derived based on well known biochemistry of the pathway. The model was integrated in the Software COMPFlux which uses a global optimization algorithm for the estimation of intracellular fluxes. Sterol concentration patterns were determined by gas chromatography in aerobic and anaerobic batch cultivations, when the sterol metabolism was suppressed due to the absence of oxygen. In addition, the sterol concentrations were observed in a cultivation which was shifted from anaerobic to aerobic growth conditions causing the sterol pools in the cell to be filled. From time-dependent flux patterns, possible limitations in the pathway could be localized and the esterification of sterols was identified as an integral part of regulation in ergosterol biosynthesis.

The Saccharomyces cerevisiae YLL012/YEH1, YLR020/YEH2, and TGL1 genes encode a novel family of membrane-anchored lipases that are required for steryl ester hydrolysis

Sterol homeostasis in eukaryotic cells relies on the reciprocal interconversion of free sterols and steryl esters. The formation of steryl esters is well characterized, but the mechanisms that control steryl ester mobilization upon cellular demand are less well understood. We have identified a family of three lipases of Saccharomyces cerevisiae that are required for efficient steryl ester mobilization. These lipases, encoded by YLL012/YEH1, YLR020/YEH2, and TGL1, are paralogues of the mammalian acid lipase family, which is composed of the lysosomal acid lipase, the gastric lipase, and four novel as yet uncharacterized human open reading frames. Lipase triple-mutant yeast cells are completely blocked in steryl ester hydrolysis but do not affect the mobilization of triacylglycerols, indicating that the three lipases are required for steryl ester mobilization in vivo. Lipase single mutants mobilize steryl esters to various degrees, indicating partial functional redundancy of the three gene products. Lipase double-mutant cells in which the third lipase is expressed from the inducible GAL1 promoter have greatly reduced steady-state levels of steryl esters, indicating that overexpression of any of the three lipases is sufficient for steryl ester mobilization in vivo. The three yeast enzymes constitute a novel class of membrane-anchored lipases that differ in topology and subcellular localization.

Affinity of singleS. cerevisiaecells to 2-NBDglucose under changing substrate concentrations

BACKGROUND

Saccharomyces cerevisiae is a widely employed microorganism in biotechnological processes. Since proliferation and product formation depend on the capacity of the cell to access and metabolize a carbon source, a technique was developed to enable for analyzing the S. cerevisiae H155 cells' affinity to extracellular glucose concentrations.

METHODS

The fluorescent glucose analogue 2-NBDglucose was employed as a functional parameter to analyze the cells' affinity to glucose. Structural parameters (proliferation, neutral lipid content, granularity, and cell size) were also investigated. Cells were grown both in batches and in chemostat regimes.

RESULTS

The 2-NBDglucose uptake in individual cells proceeds in a time- and concentration-dependent manner and is affected by respiratory and respirofermentative modes of growth. The process is inhibited by D-glucose, D-fructose, D-mannose, and sucrose, but not L-glucose, D-galactose or lactose; maltose is a weak inhibitor. The affinity of the individual cells to 2-NBDglucose was found to be high at low extracellular glucose concentrations, and weak at high concentrations. An additional, underlying pattern in the cells' affinity to glucose was detected, illustrated by the recurrent appearance of two subpopulations showing distinctly differing quantities of this substrate.

CONCLUSIONS

A multiparameter flow cytometry approach is presented that enables, for the first time, for analysis of the affinity of individual S. cerevisiae cells to glucose. Besides the adjustment of the yeast cell metabolism to extracellular glucose concentrations by altering their affinity to glucose, at least one further mechanism is clearly involved. Two subpopulations of cells were resolved, with different affinities not correlated with other cellular parameters measured.

Anaerobiosis induces complex changes in sterol esterification pattern in the yeast Saccharomyces cerevisiae

Yeast Saccharomyces cerevisiae is auxotrophic for ergosterol in the absence of oxygen. We showed that complex changes in esterification of exogenously supplied sterols were also induced by anaerobiosis. Utilization of oleic acid for sterol esterification was significantly impaired in anaerobic cells. Furthermore, anaerobic cells fed different sterols exhibited striking variation in esterification efficiency (high levels of sterol esters for cholesterol and sitosterol, low levels for ergosterol, lanosterol or stigmasterol). Relative activities of two yeast acylCoA:sterol acyltransferases (Are1p and Are2p) changed in response to anaerobiosis: while Are2p was dominant under aerobic conditions, Are1p provided the major activity in the absence of oxygen. Our results indicate that sterol esters may fulfil different roles in aerobic and anaerobic cells.

Intracellular lipid particles of eukaryotic cells

In this review article we describe characterization of intracellular lipid particles of three different eukaryotic species, namely mammalian cells, plants and yeast. Lipid particles of all types of cells share a general structure. A hydrophobic core of neutral lipids is surrounded by a membrane monolayer of phospholipids which contains a minor amount of proteins. Whereas lipid particles from mammalian cells and plants harbor specific classes of polypeptides, mainly perilipins and oleosins, respectively, yeast lipid particles contain a more complex set of enzymes which are involved in lipid biosynthesis. Function of lipid particles as storage compartment and metabolic organelle, and their interaction with other subcellular fractions are discussed. Furthermore, models for the biogenesis of lipid particles are presented and compared among the different species.

Sterol metabolism and ERG2 gene regulation in the yeast Saccharomyces cerevisiae

Certain exogenously-supplied sterols, like ergost-8-enol, are efficiently converted into ergosterol in yeast. We have taken advantage of this property to study the regulation of the Delta8-Delta7-sterol isomerase-encoding ERG2 gene in an ergosterol auxotrophic mutant devoid of squalene-synthase activity. Ergosterol starvation leads to an 8-16-fold increase in ERG2 gene expression. Such an increase was also observed in wild-type cells either grown anaerobically or treated with SR31747A a sterol isomerase inhibitor. Exogenously-supplied zymosterol is entirely transformed into ergosterol, which represses ERG2 transcription. By contrast, exogenously-supplied ergosterol has little or no effect on ERG2 transcription.

Biochemical characterization and subcellular localization of the sterol C-24(28) reductase, erg4p, from the yeast saccharomyces cerevisiae

The yeast ERG4 gene encodes sterol C-24(28) reductase which catalyzes the final step in the biosynthesis of ergosterol. Deletion of ERG4 resulted in a complete lack of ergosterol and accumulation of the precursor ergosta-5,7,22,24(28)-tetraen-3beta-ol. An erg4 mutant strain exhibited pleiotropic defects such as hypersensitivity to divalent cations and a number of drugs such as cycloheximide, miconazole, 4-nitroquinoline, fluconazole, and sodium dodecyl sulfate. Similar to erg6 mutants, erg4 mutants are sensitive to the Golgi-destabilizing drug brefeldin A. Enzyme activity measurements with isolated subcellular fractions revealed that Erg4p is localized to the endoplasmic reticulum. This view was confirmed in vivo by fluorescence microscopy of a strain expressing a functional fusion of Erg4p to enhanced green fluorescent protein. We conclude that ergosterol biosynthesis is completed in the endoplasmic reticulum, and the final product is supplied from there to its membranous destinations.

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

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

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.

Isolation and characterization of the Saccharomyces cerevisiae SUT1 gene involved in sterol uptake

A new gene (SUT1) of Saccharomyces cerevisiae, implicated in sterol uptake, was isolated from a yeast genomic library constructed in a high-copy-number vector by virtue of conferring resistance to fenpropimorph in medium supplemented with ergosterol. The high expression of SUT1 in sterol auxotrophic mutant strains alleviates the requirement for accessory mutations affecting heme biosynthesis and allows sterol uptake in aerobiosis. Measurements of [14C]cholesterol uptake confirmed that SUT1 is involved in sterol absorption. Within the 4.1-kb insert isolated, the functional gene was localised on a 1.7-kb DNA fragment. The nucleotide sequence encodes a predicted protein of 299 amino acids. Northern blot analysis revealed that SUT1 is a new member of the hypoxic gene family. Gene disruption showed that SUT1 is not essential for aerobic or anaerobic yeast growth.

Degradation of HMG-CoA reductase-induced membranes in the fission yeast, Schizosaccharomyces pombe

Elevated levels of certain membrane proteins, including the sterol biosynthetic enzyme HMG-CoA reductase, induce proliferation of the endoplasmic reticulum. When the amounts of these proteins return to basal levels, the proliferated membranes are degraded, but the molecular details of this degradation remain unknown. We have examined the degradation of HMG-CoA reductase-induced membranes in the fission yeast, Schizosaccharomyces pombe. In this yeast, increased levels of the Saccharomyces cerevisiae HMG-CoA reductase isozyme encoded by HMG1 induced several types of membranes, including karmellae, which formed a cap of stacked membranes that partially surrounded the nucleus. When expression of HMG1 was repressed, the karmellae detached from the nucleus and formed concentric, multilayered membrane whorls that were then degraded. During the degradation process, CDCFDA-stained compartments distinct from preexisting vacuoles formed within the interior of the whorls. In addition to these compartments, particles that contained neutral lipids also formed within the whorl. As the thickness of the whorl decreased, the lipid particle became larger. When degradation was complete, only the lipid particle remained. Cycloheximide treatment did not prevent the formation of whorls. Thus, new protein synthesis was not needed for the initial stages of karmellae degradation. On the contrary, cycloheximide promoted the detachment of karmellae to form whorls, suggesting that a short lived protein may be involved in maintaining karmellae integrity. Taken together, these results demonstrate that karmellae membranes differentiated into self-degradative organelles. This process may be a common pathway by which ER membranes are turned over in cells.

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.

Export of steryl esters from lipid particles and release of free sterols in the yeast, Saccharomyces cerevisiae

Fatty acyl esters of the yeast specific sterol, ergosterol, are exclusively stored in lipid particles. Under conditions of sterol deficiency, e.g., in the presence of terbinafine, an inhibitor of fungal squalene epoxidase, steryl esters are hydrolyzed, and sterols are set free for membrane formation. Lipid particles do not contain steryl-ester hydrolase activity themselves; the highest specific activity of this enzyme is found in the plasma membrane. Therefore, steryl esters have to be exported from lipid particles to their site of hydrolytic cleavage. This process of translocation and metabolic conversion was studied in vivo. Addition of nocodazole to terbinafine-treated cells did not disturb the mobilization of steryl esters, indicating that this process is not mediated by microtubuli-dependent vesicle flux. Under the influence of inhibitors of cellular energy production (azide and fluoride) and protein biosynthesis (cycloheximide) mobilization of steryl esters came to an halt. These results support the view that ongoing membrane proliferation may be a driving force for the release of sterols from steryl esters of lipid particles.

Characterization of lipid particles of the yeast, Saccharomyces cerevisiae

Lipid particles of the yeast, Saccharomyces cerevisiae, were isolated to high purity and their components were analysed. The hydrophobic core of this organelle consists of triacylglycerols and steryl esters, which are almost exclusively located to that compartment. Lipid particles are stabilized by a surface membrane consisting of phospholipids and proteins. Electron microscopy confirmed the purity of the preparations and the proposed structure deduced from biochemical experiments. Major proteins of lipid particles have molecular weights of 72, 52, 43 and 34 kDa, respectively. The 43 kDa protein reacts with an antiserum against human apolipoprotein AII. In lipid particles of the yeast mutant strain S. cerevisiae erg6, which is deficient in sterol delta 24-methyltransferase, this protein is missing thereby identifying the protein and confirming our previous finding (Zinser et al., 1993) that sterol delta 24-methylation is associated with lipid particles. A possible involvement of surface proteins of lipid particles in the interaction with other organelles is discussed with respect to sterol translocation in yeast.

Fibril assembly and carotenoid overaccumulation in chromoplasts: a model for supramolecular lipoprotein structures

Chromoplast development in ripening bell pepper fruits is characterized by a massive synthesis of carotenoid pigments, resulting in their distinctive red color. We have shown that 95% of these pigments accumulate in chromoplasts in specific lipoprotein fibrils. In addition to carotenoids, purified fibrils contain galactolipids, phospholipids, and a single, 32-kD protein, designated fibrillin, which has antigenically related counterparts in other species. Fibrils were reconstituted in vitro when purified fibrillin was combined with carotenoids and polar lipids in the same stoichiometric ratio found in fibrils in vivo. Antibodies directed against fibrillin were used to isolate a fibrillin cDNA clone and, in immunological studies, to follow its accumulation during the chloroplast-to-chromoplast transition under different conditions. A model for fibril architecture is proposed wherein carotenoids accumulate in the center of the fibrils and are surrounded by a layer of polar lipids, which in turn are surrounded by an outer layer of fibrillin. Topological analysis of purified fibrils verified this structure. Collectively, these results suggest that the process of fibril self-assembly in chromoplasts is an example of a general phenomenon shared among cells that target excess membrane lipids into deposit structures to avoid their destabilizing or toxic effects. In addition, we have shown that abscisic acid stimulates this phenomenon in chromoplasts, whereas gibberellic acid and auxin delay it.

Sterol composition of yeast organelle membranes and subcellular distribution of enzymes involved in sterol metabolism

Organelles of the yeast Saccharomyces cerevisiae were isolated and analyzed for sterol composition and the activity of three enzymes involved in sterol metabolism. The plasma membrane and secretory vesicles, the fractions with the highest sterol contents, contain ergosterol as the major sterol. In other subcellular membranes, which exhibit lower sterol contents, intermediates of the sterol biosynthetic pathway were found at higher percentages. Lipid particles contain, in addition to ergosterol, large amounts of zymosterol, fecosterol, and episterol. These sterols are present esterified with long-chain fatty acids in this subcellular compartment, which also harbors practically all of the triacylglycerols present in the cell but very little phospholipids and proteins. Sterol delta 24-methyltransferase, an enzyme that catalyzes one of the late steps in sterol biosynthesis, was localized almost exclusively in lipid particles. Steryl ester formation is a microsomal process, whereas steryl ester hydrolysis occurs in the plasma membrane and in secretory vesicles. The fact that synthesis, storage, and hydrolysis of steryl esters occur in different subcellular compartments gives rise to the view that ergosteryl esters of lipid particles might serve as intermediates for the supply of ergosterol from internal membranes to the plasma membrane.

Quantitative separation of Trichoderma lipid classes on a bonded phase column

Bond Elut aminopropyl columns were used to purify the different lipid classes of Trichoderma harzianum and Trichoderma viride. This methodology permitted good separation of the fungal lipid classes in less time than traditional techniques. The incorporation of [1 (14)C]linoleic acid into neutral lipids, free fatty acids and phospholipids was quantified for both strains. The fatty acid profile of the different lipid classes of these fungal strains was determined as a function of culture time.

Stimulation by heme of steryl ester synthase and aerobic sterol exclusion in the yeast Saccharomyces cerevisiae

Saccharomyces cerevisiae sterol and heme auxotrophs were used to elucidate a role for hemes in sterol esterification. Steryl ester synthase (SES) activity was stimulated on average fourfold in cells supplemented with 50 micrograms/ml delta-aminolevulinic acid (ALA). This stimulation was not dependent on ALA per se, but on the ability of this precursor to effect heme competency. The addition of ALA stimulated SES activity of yeast on either fermentative or respiratory carbon sources. The elevation of SES activity was independent of intracellular free sterol, unsaturated fatty acid, or methionine levels. SES activity increases as the cells enter stationary phase, and this increase is enhanced by heme competency. SES was directly inhibited by the hypocholesterolemic drug lovastatin (mevinolin). The inhibition of SES activity by lovastatin was enhanced in heme-competent cells.

A defect in the sterol:steryl ester interconversion in a mutant of the yeast, Saccharomyces cerevisiae

A culture cycle dependent interconversion of sterols and steryl esters is disturbed in a mutant of Saccharomyces cerevisiae. Independent extragenic suppressors to this mutant return the mutant's pleiotropic phenotype to that of the parental wild type. Concomitant with the alterations in interconversion, modifications were found in the yeast proteins that antigenically react with antibodies elicited against mammalian apolipoproteins. Suppressor mutations returned the aberrant immunoblot banding pattern of the mutant to that of the wild type in apolipoprotein B.

Genetic study and further biochemical characterization of a tobacco mutant that overproduces sterols

A genetic and biochemical characterization is presented of a tobacco mutant that was previously shown to have an increased sterol content with an accumulation of biosynthetic intermediates. We first show that a precise regulation of the membrane sterol composition occurs in this mutant, via a selective esterification process. Indeed, sterols representing the usual end-products of the biosynthetic pathway are preferably integrated into the membranes as free sterols, whereas most of the intermediates pool is esterified and stored in cytoplasmic lipid droplets. It is further demonstrated that overproduction of sterols by the LAB1-4 mutant is due to a single nuclear and semi-dominant mutation. Finally, increase of biosynthesis and esterification of unusual sterols are shown to be responsible for the resistance of LAB1-4 calli to LAB170 250F, the triazole pesticide used to select this mutant. However, differentiated LAB1-4 tissues do not express the resistance trait, suggesting that sterol biosynthesis might not be the only site of action for the triazole at the plant level.

Involvement of heme components in sterol metabolism of Saccharomyces cerevisiae

There is an intimate association between sterol biosynthesis in yeast and aerobicity. Besides the requirement for molecular oxygen for the epoxidation of squalene, cytochrome hemoproteins are involved in demethylation and desaturation steps. Regulatory effects of hemes on sterol formation have been demonstrated using specifically defective mutants of yeast. Heme competency participates in a mechanism whereby wild-type cells are prevented from taking exogenous sterols from the growth media. The multiple interactions of hemes and sterols appear to be associated with the variously defined functions for sterols in the yeast cells.

Lipid Content and Cryotolerance of Bakers' Yeast in Frozen Doughs

The relationship between lipid content and tolerance to freezing at −50°C was studied in Saccharomyces cerevisiae grown under batch or fed-batch mode and various aeration and temperature conditions. A higher free-sterol-to-phospholipid ratio as well as higher free sterol and phospholipid contents correlated with the superior cryoresistance in dough or in water of the fed-batch-grown compared with the batch-grown cells. For both growth modes, the presence of excess dissolved oxygen in the culture medium greatly improved yeast cryoresistance and trehalose content (P. Gélinas, G. Fiset, A. LeDuy, and J. Goulet, Appl. Environ. Microbiol. 26:2453-2459, 1989) without significantly changing the lipid profile. Under the batch or fed-batch modes, no correlation was found between the cryotolerance of bakers' yeast and the total cellular lipid content, the total sterol content, the phospholipid unsaturation index, the phosphate or crude protein content, or the yeast cell morphology (volume and roundness).

Yeast proteins with reactivity to antibodies elicited against mammalian apolipoproteins

Hyperimmunized rabbit anti-rat apolipoprotein antibodies showed reactivity to proteins in Saccharomyces cerevisiae. Antiapolipoproteins A1, B and E reacted with proteins in both a crude extract and a lipid enriched fraction. Protein reactivity was dependent on antisera dilution. Furthermore, the different antiapolipoproteins tested reacted with individually distinct yeast proteins, possibly suggesting the presence of a variety of yeast apolipoproteins with distinct cellular functions as is the case with mammalian apolipoproteins. The specificity of the antibody was directed toward the yeast protein and not a lipid moiety.

Effects of lovastatin (mevinolin) on sterol levels and on activity of azoles in Saccharomyces cerevisiae

The hypocholesterolemic drug lovastatin (mevinolin) was found to be very effective in lowering the sterol levels of the wild-type yeast Saccharomyces cerevisiae. Lovastatin dramatically decreased the steryl ester content from 2.62 to 0.8 micrograms/mg (dry weight), whereas the free sterol content decreased only from 2.79 to 2.24 micrograms/mg (dry weight) when lovastatin was present in the medium at 10 micrograms/ml. At higher concentrations (100 micrograms/ml), lovastatin nearly abolished the accumulation of steryl esters and decreased the free sterol concentration to less than 1.3 micrograms/mg (dry weight). As a result of the lowered sterol levels, proportional amounts of exogenous sterol were taken up from the medium during aerobic, respiratory conditions. Nearly all of the exogenous sterol taken up was partitioned into the free sterol fraction. The inhibition of sterol esterification in the presence of lovastatin was dependent on heme synthesis. The result of these combined effects caused the MICs of three azole antifungal drugs (ketoconazole, clotrimazole, and miconazole) to be lowered from 6- to 32-fold when lovastatin was present in the medium at 10 micrograms/ml.

Gas chromatographic analysis of intact steryl esters in wild type Saccharomyces cerevisiae and in an ester accumulating mutant

The steryl ester faction from wild type and mutant strains of the yeast Saccharomyces cerevisiae was analyzed without saponification by a non-polar capillary gas chromatographic column. When expressed as microgram ester/mg dry wt, the total ester fraction remained constant or declined slightly from log to stationary phase in the wild type. In the mutant the decrease was more dramatic. No individual ergosteryl ester species was dominant throughout the culture cycle in the wild type. A compound tentatively identified as zymosteryl palmitate was the most prevalent ester in wild type log phase cells, ergosta-5,7-dienyl palmitate and ergosta-5,7-dienyl palmitoleate were the major esters in stationary cells. In the mutant strain, ergosteryl esters of palmitate, palmitoleate, oleate, and stearate were the major ester components throughout the culture cycle. Like the wild type, however, the mutant showed an increase in the proportion of ergosta-5,7-dienyl esters in the stationary phase of the culture cycle. The data did not indicate a sterol/fatty acid specificity during the culture cycle.

Pleiotropic mutations in Saccharomyces cerevisiae affecting sterol uptake and metabolism

Sterol uptake control mutants (upc-) have been isolated via ethylmethanesulfonate mutagenesis from wild-type Saccharomyces cerevisiae. These mutants are heme and sterol competent but possess the ability to accumulate exogenous sterol(s) under aerobic conditions. Previous studies demonstrate sterol uptake only in a hem-, erg- background; however, the Upc- strains described here are Hem+ and do not require exogenous sterol for growth. We were unable to obtain viable hem+, erg-, upc+ recombinants; such combinations appear to be lethal. Isolates of Upc mutants demonstrated different levels of sterol uptake, and sterol analysis revealed a broad phenotypic range with regard to amounts and accumulation of ergosterol and non-ergosterol sterols. Assays of acyl CoA: ergosterol acyltransferase and sterol ester hydrolase showed no apparent difference in activity between Upc mutants and the wild type.

Relationship between intracellular sterol content and sterol esterification and hydrolysis in Saccharomyces cerevisiae

The relationship between the supply of free sterol and the synthesis of steryl esters by an auxotroph of Saccharomyces cerevisiae has been examined in order to understand the role of cellular free sterol content in the regulatory interactions of sterol esterification. Our results show that the yeast cells must maintain an essential, low level of free sterol that is critical for growth. An additional, expandable pool of free sterol is maintained by the cells, provided there is adequate available sterol. As the quantity of sterol in the expandable pool increases, there is a progressively increasing rate of sterol esterification, which is consistent with the results from in vitro assays of acyl-Co A:ergosterol acyltransferase. Some increases in acyltransferase activity were insensitive to the protein synthetic inhibitor, cycloheximide. The data support the conclusion that sterol interconversion between the free and esterified forms is directed toward maintaining the essential amount of free sterol and that the activity of the relevant sterol enzymes in this organism are modulated in response to intracellular sterol content.

Regulation of ergosterol biosynthesis and sterol uptake in a sterol-auxotrophic yeast

Inhibition of sterol uptake in Saccharomyces cerevisiae sterol auxotroph FY3 (alpha hem1 erg7 ura) by delta-aminolevulinic acid (ALA) is dependent on the ability of the organism to synthesize heme from ALA. Sterol-depleted cells not exposed to ALA or strain PFY3 cells, with a double heme mutation, exposed to ALA did not exhibit inhibition of sterol uptake. Addition of ALA to sterol-depleted FY3 stimulated production of a high endogenous concentration of 2,3-oxidosqualene (25.55 micrograms mg-1 [dry weight]) at 24 h, whereas FY3 not exposed to ALA or PFY3 exposed to ALA did not accumulate 2,3-oxidosqualene. The high concentration of 2,3-oxidosqualene in FY3 with ALA decreased, and 2,3;22,23-dioxidosqualene increased to a very high level. The elevation of 2,3-oxidosqualene by ALA was correlated with a fivefold increase in the activity of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (EC 1.1.1.34). The enhanced activity of 3-hydroxy-3-methylglutaryl-coenzyme A reductase was prevented by cycloheximide but not chloramphenicol and was dependent on a fermentative energy source. Inhibition of sterol uptake could not be attributed to 2,3-oxidosqualene or 2,3;22,23-dioxidosqualene but was due to a nonsaturating level of ergosterol produced as a consequence of heme competency through a leaky erg7 mutation.

Characteristics of sterol uptake in Saccharomyces cerevisiae

A Saccharomyces cerevisiae sterol auxotroph, FY3 (alpha hem1 erg7 ura), was used to probe the characteristics of sterol uptake in S. cerevisiae. The steady-state cellular concentration of free sterol at the late exponential phase of growth could be adjusted within a 10-fold range by varying the concentration of exogenously supplied sterol. When cultured on 1 microgram of sterol ml-1, the cells contained a minimal cellular free-cholesterol concentration of 0.85 nmol/mg (dry weight) and were termed sterol depleted. When cultured on 11 micrograms of sterol ml-1 or more, the cells contained a maximal cellular free-cholesterol concentration of 6.8 nmol/mg (dry weight) and were termed free sterol saturated. Cells with free-sterol concentrations below the maximal level were capable of accumulating free sterol from the medium. The capacity of the cells for cholesterol uptake was inversely proportional to the initial intracellular concentration. The uptake of sterol was shown to be a nonactive process that is independent of cellular energy sources or viability. The intracellular transport of sterol for esterification is not sensitive to anti-microtubule agents.

Ethanol-sensitive mutants of Saccharomyces cerevisiae

Saccharomyces cerevisiae mutants unable to grow at ethanol concentrations at which the wild type strain S288C does grow, have been isolated. Some of them show additional phenotypic alterations in colony size, temperature sensitivity and viability in ethanol, which cosegregate with the growth sensitivity in ethanol. 21 selected monogenic ethanol-sensitive mutants define 20 complementation groups, denominated ETA1 to ETA20, which indicates that there is a high number of genes involved in the ethanol tolerance/sensitivity mechanism. Out of 21 selected monogenic mutants, 20 are not altered in the glycolytic pathway since, when maintained in glucose-supplemented medium, they can produce as much ethanol as the wild type and at about the same velocity. Nor do any of the mutants seem to be altered in the lipid biosynthetic pathway since, whether grown in the absence or in the presence of ethanol, their concentration of fatty acids and ergosterol is similar to that of the wild type under the same conditions. Therefore growth sensitivity to ethanol does not seem necessarily to be related to carbohydrate or lipid metabolism.