The yeast mic2 mutant is defective in the formation of mannosyl-diinositolphosphorylceramide

Deciphering the distinct biocontrol activities of lipopeptides fengycin and surfactin through their differential impact on lipid membranes

Lipopeptides produced by beneficial bacilli present promising alternatives to chemical pesticides for plant biocontrol purposes. Our research explores the distinct plant biocontrol activities of lipopeptides surfactin (SRF) and fengycin (FGC) by examining their interactions with lipid membranes. Our study shows that FGC exhibits a direct antagonistic activity against Botrytis cinerea and no marked immune-eliciting activity in Arabidopsis thaliana while SRF only demonstrates an ability to stimulate plant immunity. It also reveals that SRF and FGC exhibit diverse effects on membrane integrity and lipid packing. SRF primarily influences membrane physical state without significant membrane permeabilization, while FGC permeabilizes membranes without significantly affecting lipid packing. From our results, we can suggest that the direct antagonistic activity of lipopeptides is linked to their capacity to permeabilize lipid membrane while the stimulation of plant immunity is more likely the result of their ability to alter the mechanical properties of the membrane. Our work also explores how membrane lipid composition modulates the activities of SRF and FGC. Sterols negatively impact both lipopeptides' activities while sphingolipids mitigate the effects on membrane lipid packing but enhance membrane leakage. In conclusion, our findings emphasize the importance of considering both membrane lipid packing and leakage mechanisms in predicting the biological effects of lipopeptides. It also sheds light on the intricate interplay between the membrane composition and the effectiveness of the lipopeptides, providing insights for targeted biocontrol agent design.

Cellular Responses and Targets in Food Spoilage Yeasts Exposed to Antifungal Prenylated Isoflavonoids

Prenylated isoflavonoids are phytochemicals with promising antifungal properties. Recently, it was shown that glabridin and wighteone disrupted the plasma membrane (PM) of the food spoilage yeast Zygosaccharomyces parabailii in distinct ways, which led us to investigate further their modes of action (MoA). Transcriptomic profiling with Z. parabailii showed that genes encoding transmembrane ATPase transporters, including Yor1, and genes homologous to the pleiotropic drug resistance (PDR) subfamily in Saccharomyces cerevisiae were upregulated in response to both compounds. Gene functions involved in fatty acid and lipid metabolism, proteostasis, and DNA replication processes were overrepresented among genes upregulated by glabridin and/or wighteone. Chemogenomic analysis using the genome-wide deletant collection for S. cerevisiae further suggested an important role for PM lipids and PM proteins. Deletants of gene functions involved in biosynthesis of very-long-chain fatty acids (constituents of PM sphingolipids) and ergosterol were hypersensitive to both compounds. Using lipid biosynthesis inhibitors, we corroborated roles for sphingolipids and ergosterol in prenylated isoflavonoid action. The PM ABC transporter Yor1 and Lem3-dependent flippases conferred sensitivity and resistance, respectively, to the compounds, suggesting an important role for PM phospholipid asymmetry in their MoAs. Impaired tryptophan availability, likely linked to perturbation of the PM tryptophan permease Tat2, was evident in response to glabridin. Finally, substantial evidence highlighted a role of the endoplasmic reticulum (ER) in cellular responses to wighteone, including gene functions associated with ER membrane stress or with phospholipid biosynthesis, the primary lipid of the ER membrane. IMPORTANCE Preservatives, such as sorbic acid and benzoic acid, inhibit the growth of undesirable yeast and molds in foods. Unfortunately, preservative tolerance and resistance in food spoilage yeast, such as Zygosaccharomyces parabailii, is a growing challenge in the food industry, which can compromise food safety and increase food waste. Prenylated isoflavonoids are the main defense phytochemicals in the Fabaceae family. Glabridin and wighteone belong to this group of compounds and have shown potent antifungal activity against food spoilage yeasts. The present study demonstrated the mode of action of these compounds against food spoilage yeasts by using advanced molecular tools. Overall, the cellular actions of these two prenylated isoflavonoids share similarities (at the level of the plasma membrane) but also differences. Tryptophan import was specifically affected by glabridin, whereas endoplasmic reticulum membrane stress was specifically induced by wighteone. Understanding the mode of action of these novel antifungal agents is essential for their application in food preservation.

Glycosphingolipids in Filamentous Fungi: Biological Roles and Potential Applications in Cosmetics and Health Foods

Filamentous fungi are a group of economically important fungi used in the production of fermented foods, industrial enzymes, and secondary metabolites. Glycosphingolipids (GSLs) as constituents of lipid rafts are involved in growth, differentiation, and response to environment stress in filamentous fungi. In addition to these key roles, GSLs are also important in the barrier function of skin to retain moisture as a moisturizing ingredient in cosmetics or health products for their strong biological activity as a functional component. GSLs found in filamentous fungi are divided in two major classes: neutral GSLs (glycosylceramides), glucosylceramides (GlcCers), and/or galactosylceramides (GalCers) and acidic GSLs, mannosylinositol phosphorylceramide (MIPC) and mannosyldiinositol phosphorylceramide [M(IP)₂C]. Glycosylceramides are one of the abundant GSLs in Aspergillus and known to improve skin-barrier function and prevent intestinal impairment as a prebiotic. Some filamentous fungi of Aspergillus spp., synthesizing both GlcCer and GalCer, would be an amenable source to exploit glycosylceramides that wildly adding in cosmetics as moisturizing ingredients or health food as dietary supplements. In this minireview, the types, structures, and biosynthetic pathways of GSLs in filamentous fungi, and the relevance of GSLs in fungal growth, spore formation, and environmental stress response are explained. Furthermore, the advantage, potential development, and application of GlcCer and GalCer from filamentous fungi Aspergillus spp. are also investigate based on the use of plant GlcCer in health foods and cosmetics.

Yeast Sphingolipid-Enriched Domains and Membrane Compartments in the Absence of Mannosyldiinositolphosphorylceramide

The relevance of mannosyldiinositolphosphorylceramide [M(IP)₂C] synthesis, the terminal complex sphingolipid class in the yeast Saccharomyces cerevisiae, for the lateral organization of the plasma membrane, and in particular for sphingolipid-enriched gel domains, was investigated by fluorescence spectroscopy and microscopy. We also addressed how changing the complex sphingolipid profile in the plasma membrane could influence the membrane compartments (MC) containing either the arginine/ H⁺ symporter Can1p (MCC) or the proton ATPase Pma1p (MCP). To achieve these goals, wild-type (wt) and ipt1Δ cells, which are unable to synthesize M(IP)₂C accumulating mannosylinositolphosphorylceramide (MIPC), were compared. Living cells, isolated plasma membrane and giant unilamellar vesicles reconstituted from plasma membrane lipids were labelled with various fluorescent membrane probes that report the presence and organization of distinct lipid domains, global order, and dielectric properties. Can1p and Pma1p were tagged with GFP and mRFP, respectively, in both yeast strains, to evaluate their lateral organization using confocal fluorescence intensity and fluorescence lifetime imaging. The results show that IPT1 deletion strongly affects the rigidity of gel domains but not their relative abundance, whereas no significant alterations could be perceived in ergosterol-enriched domains. Moreover, in these cells lacking M(IP)₂C, a clear alteration in Pma1p membrane distribution, but no significant changes in Can1p distribution, were observed. Thus, this work reinforces the notion that sphingolipid-enriched domains distinct from ergosterol-enriched regions are present in the S. cerevisiae plasma membrane and suggests that M(IP)₂C is important for a proper hydrophobic chain packing of sphingolipids in the gel domains of wt cells. Furthermore, our results strongly support the involvement of sphingolipid domains in the formation and stability of the MCP, possibly being enriched in this compartment.

The molecular mechanism of Nystatin action is dependent on the membrane biophysical properties and lipid composition

Nystatin-induced membrane permeabilization is related to its effects on membrane properties and organization.

Sphingolipids as targets for treatment of fungal infections

Invasive fungal infections have significantly increased in the last few decades. Three classes of drugs are commonly used to treat these infections: polyenes, azoles and echinocandins. Unfortunately each of these drugs has drawbacks; polyenes are toxic, resistance against azoles is emerging and echinocandins have narrow spectrum of activity. Thus, the development of new antifungals is urgently needed. In this context, fungal sphingolipids have emerged as a potential target for new antifungals, because their biosynthesis in fungi is structurally different than in mammals. Besides, some fungal sphingolipids play an important role in the regulation of virulence in a variety of fungi. This review aims to highlight the diverse strategies that could be used to block the synthesis or/and function of fungal sphingolipids.

Initiation of phospholipomannan β-1,2 mannosylation involves Bmts with redundant activity, influences its cell wall location and regulates β-glucans homeostasis but is dispensable for Candida albicans systemic infection

Pathogenic and non-pathogenic fungi synthesize glycosphingolipids, which have a crucial role in growth and viability. Glycosphingolipids also contribute to fungal-associated pathogenesis. The opportunistic yeast pathogen Candida albicans synthesizes phospholipomannan (PLM), which is a glycosphingolipid of the mannosylinositol phosphorylceramide family. Through its lipid and glycan moieties, PLM contributes to the initial recognition of the yeast, causing immune system disorder and persistent fungal disease through activation of host signaling pathways. The lipid moiety of PLM activates the deregulation signaling pathway involved in yeast phagocytosis whereas its glycan moiety, composed of β-1,2 mannosides (β-Mans), participates to inflammatory processes through a mechanism involving Galectin-3. Biosynthesis of PLM β-Mans involves two β-1,2 mannosyltransferases (Bmts) that initiate (Bmt5) and elongate (Bmt6) the glycan chains. After generation of double bmtsΔ mutants, we show that Bmt5 has redundant activity with Bmt2, which can replace Bmt5 in bmt5Δ mutant. We also report that PLM is located in the inner layer of the yeast cell wall. PLM seems to be not essential for systemic infection of the yeast. However, defect of PLM β-mannosylation increases resistance of C. albicans to inhibitors of β-glucans and chitin synthesis, highlighting a role of PLM in cell wall homeostasis.

Gel domains in the plasma membrane of Saccharomyces cerevisiae: highly ordered, ergosterol-free, and sphingolipid-enriched lipid rafts

The plasma membrane of Saccharomyces cerevisiae was studied using the probes trans-parinaric acid and diphenylhexatriene. Diphenylhexatriene anisotropy is a good reporter of global membrane order. The fluorescence lifetimes of trans-parinaric acid are particularly sensitive to the presence and nature of ordered domains, but thus far they have not been measured in yeast cells. A long lifetime typical of the gel phase (>30 ns) was found in wild-type (WT) cells from two different genetic backgrounds, at 24 and 30 °C, providing the first direct evidence for the presence of gel domains in living cells. To understand their nature and location, the study of WT cells was extended to spheroplasts, the isolated plasma membrane, and liposomes from total lipid and plasma membrane lipid extracts (with or without ergosterol extraction by cyclodextrin). It is concluded that the plasma membrane is mostly constituted by ordered domains and that the gel domains found in living cells are predominantly at the plasma membrane and are formed by lipids. To understand their composition, strains with mutations in sphingolipid and ergosterol metabolism and in the glycosylphosphatidylinositol anchor remodeling pathway were also studied. The results strongly indicate that the gel domains are not ergosterol-enriched lipid rafts; they are mainly composed of sphingolipids, possibly inositol phosphorylceramide, and contain glycosylphosphatidylinositol-anchored proteins, suggesting an important role in membrane traffic and signaling, and interactions with the cell wall. The abundance of the sphingolipid-enriched gel domains was inversely related to the cellular membrane system global order, suggesting their involvement in the regulation of membrane properties.

Membrane raft lipid constituents affect drug susceptibilities of Candida albicans

By exploiting the biosynthetic pathways of raft lipid constituents, in this study we demonstrate that fluctuations in either sphingolipid or ergosterol levels result in increased drug sensitivity and morphological defects in Candida albicans cells. We show that any change in either ergosterol composition by conditionally disrupting ERG1 or in sphingolipid composition by homozygously disrupting its biosynthetic gene IPT1 leads to improper surface localization of a major ABC (ATP-binding cassette) drug efflux protein, Cdr1p. Results suggest that sterol/sphingolipid-rich membrane microdomains play an important role in positioning and functional maintenance of the integral efflux protein. The impaired ability of erg1/ipt1 mutant cells to efflux drugs mediated through Cdr1p appears to be the main cause of increased drug sensitivity of Candida cells.

Functional analysis of CaIPT1, a sphingolipid biosynthetic gene involved in multidrug resistance and morphogenesis of Candida albicans

In the present study we describe the isolation and functional analysis of a sphingolipid biosynthetic gene, IPT1, of Candida albicans. The functional consequence of the disruption of both alleles of IPT1 was confirmed by mass analysis of its sphingolipid composition. The disruption of both alleles or a single allele of IPT1 did not lead to any change in growth phenotype or total sphingolipid, ergosterol, or phospholipid content of the mutant cells. The loss of mannosyl diinositol diphosphoceramide [M(IP)(2)C] in the ipt1 disruptant, however, resulted in increased sensitivity to drugs like 4-nitroquinoline oxide, terbinafine, o-phenanthroline, fluconazole, itraconazole, and ketoconazole. The increase in drug susceptibilities of ipt1 cells was linked to an altered sphingolipid composition, which appeared to be due to the impaired functionality of Cdr1p, a major drug efflux pump of C. albicans that belongs to the ATP binding cassette superfamily. Our confocal and Western blotting results demonstrated that surface localization of green fluorescent protein-tagged Cdr1p was affected in ipt1 disruptant cells. Poor surface localization of Cdr1p resulted in an impaired ability to efflux fluconazole and rhodamine 6G. The effect of mannosyl inositol phosphoceramide accumulation in the ipt1 mutant and the absence of M(IP)(2)C from the ipt1 mutant on the efflux of drug substrates was very selective. The efflux of methotrexate, a specific substrate of CaMdr1p, another major efflux pump of major facilitator superfamily, remained unaffected in ipt1 mutant cells. Interestingly, changes in sphingolipid composition affected the ability of mutant cells to form proper hyphae in various media. Taken together, our results demonstrate that an altered composition of sphingolipid, which is among the major constituents of membrane rafts, affects the drug susceptibilities and morphogenesis of C. albicans.

HOR7, a multicopy suppressor of the Ca2+-induced growth defect in sphingolipid mannosyltransferase-deficient yeast

Yeast mutants defective in sphingolipid mannosylation accumulate inositol phosphorylceramide C (IPC-C), which renders cells Ca(2+)-sensitive. A screen for loss of function suppressors of the Ca(2+)-sensitive phenotype previously led to the identification of numerous genes involved in IPC-C synthesis. To better understand the molecular basis of the Ca(2+)-induced growth defect in IPC-C-overaccumulating cells, we searched for genes whose overexpression restored Ca(2+) tolerance in a mutant lacking the IPC mannosyltransferases Csg1p and Csh1p. Here we report the isolation of HOR7 as a multicopy suppressor of the Ca(2+)-sensitive phenotype of Deltacsg1Deltacsh1 cells. HOR7 belongs to a group of hyperosmolarity-responsive genes and encodes a small (59-residue) type I membrane protein that localizes at the plasma membrane. Hor7p is not required for high Ca(2+) or Na(+) tolerance. Instead, we find that Hor7p-overproducing cells display an increased resistance to high salt, sensitivity to low pH, and a reduced uptake of methylammonium, an indicator of the plasma membrane potential. These phenotypes are induced through a mechanism independent of the plasma membrane H(+)-ATPase, Pma1p. Our findings suggest that induction of Hor7p causes a depolarization of the plasma membrane that may counteract a Ca(2+)-induced influx of toxic cations in IPC-C-overaccumulating cells.

Sphingolipid functions in Saccharomyces cerevisiae

Recent advances in understanding sphingolipid metabolism and function in Saccharomyces cerevisiae have moved the field from an embryonic, descriptive phase to one more focused on molecular mechanisms. One advance that has aided many experiments has been the uncovering of genes for the biosynthesis and breakdown of sphingolipids. S. cerevisiae seems on the verge of becoming the first organism in which all sphingolipid metabolic genes are identified. Other advances include the demonstration that S. cerevisiae cells have lipid rafts composed of sphingolipids and ergosterol and that specific proteins associate with rafts. Roles for phytosphingosine (PHS) and dihydrosphingosine (DHS) in heat stress continue to be uncovered including regulation of the transient cell cycle arrest, control of putative signaling pathways that govern cell integrity, endocytosis, movement of the cortical actin cytoskeleton and regulation of protein breakdown in the plasma membrane. Other studies suggest roles for sphingolipids in exocytosis, growth regulation and longevity. Finally, some progress has been made in understanding how sphingolipid synthesis is regulated and how sphingolipid levels are maintained.

A gene encoding a sphingolipid biosynthesis enzyme determines the sensitivity of Saccharomyces cerevisiae to an antifungal plant defensin from dahlia (Dahlia merckii)

We have previously identified a Saccharomyces cerevisiae mutant that is markedly more resistant than wild-type to Dahlia merckii antimicrobial peptide 1 (DmAMP1), an antifungal plant defensin isolated from seeds of dahlia (Dahlia merckii). A complementation approach was followed that consisted of the introduction of a genomic library of DmAMP1-sensitive wild-type yeast into the DmAMP1-resistant yeast mutant and screening for restored sensitivity to DmAMP1. The gene determining sensitivity of S. cerevisiae to DmAMP1 was identified as IPT1, a gene encoding an enzyme involved in the last step of the synthesis of the sphingolipid mannose-(inositol-phosphate)(2)-ceramide. Strains with a nonfunctional IPT1 allele lacked mannose-(inositol-phosphate)(2)-ceramide in their plasma membranes, bound significantly less DmAMP1 compared with wild-type strains, and were highly resistant to DmAMP1-mediated membrane permeabilization. All of these phenotypic deviations could be restored by reintroduction of a functional IPT1 gene. Our data support a model in which membrane patches containing sphingolipids act as binding sites for DmAMP1 or, alternatively, are required to anchor membrane or cell wall-associated proteins, which themselves interact with DmAMP1.

Antifungals targeted to sphingolipid synthesis: focus on inositol phosphorylceramide synthase

Currently available antifungal drugs for serious infections have essentially two molecular targets, 14alpha demethylase (azoles) and ergosterol (polyenes). The former is a fungistatic target, vulnerable to resistance development; the latter, while a fungicidal target, is not sufficiently different from the host to ensure high selectivity. Antifungals in clinical development have a third molecular target, beta-1,3-glucan synthase. Drugs aimed at totally new targets are required to increase our chemotherapeutic options and to forestall, alone or in combination chemotherapy, the emergence of drug resistance. Sphingolipids, essential membrane components in eukaryotic cells, but distinct in mammalian and fungal cells, present an attractive new target. Several natural product inhibitors of sphingolipid biosynthesis have been discovered in recent years, some of which act at a step unique to fungi and have potent and selective antifungal activity.

Syringomycin E inhibition of Saccharomyces cerevisiae: requirement for biosynthesis of sphingolipids with very-long-chain fatty acids and mannose- and phosphoinositol-containing head groups

Syringomycin E is an antifungal cyclic lipodepsinonapeptide that inhibits the growth of Saccharomyces cerevisiae by interaction with the plasma membrane. A screen conducted to find the yeast genes necessary for its fungicidal action identified two novel syringomycin E response genes, SYR3 and SYR4. A syr3 mutant allele was complemented by ELO2 and ELO3. These genes encode enzymes that catalyze the elongation of sphingolipid very long chain fatty acids. Tetrad analysis showed that SYR3 was ELO2. Strains with deletions of SYR3/ELO2 and ELO3 were resistant to syringomycin E, and lipid analyses of both mutants revealed shortened fatty acid chains and lower levels of sphingolipids. SYR4 was identified by Tn5 inactivation of genomic library plasmids that complemented a syr4 mutant allele. SYR4 was found to be identical to IPT1, which encodes the terminal sphingolipid biosynthetic enzyme, mannosyl-diinositolphosphoryl-ceramide synthase. Deletion Deltasyr4/ipt1 strains were viable, were resistant to syringomycin E, did not produce mannosyl-diinositolphosphoryl-ceramide, and accumulated mannosyl-inositolphosphoryl-ceramide. Accumulation of mannosyl-inositolphosphoryl-ceramide was not responsible for resistance since a temperature-sensitive secretory pathway mutant (sec14-3(ts)) accumulated this sphingolipid and was sensitive to syringomycin E. Finally, Deltacsg1/sur1 and Deltacsg2 strains defective in the transfer of mannose to inositolphosphoryl-ceramide were resistant to syringomycin E. These findings show that syringomycin E growth inhibition of yeast is promoted by the production of sphingolipids with fully elongated fatty acid chains and the mannosyl and terminal phosphorylinositol moieties of the polar head group.

Brave little yeast, please guide us to thebes: sphingolipid function in S. cerevisiae

Sphingolipids typically cover the exoplasmic leaflet of the plasma membrane of eukaryotic cells. They differ from the more abundant glycerophospholipids in that they contain ceramide instead of diacylglycerol as a hydrophobic anchor. Why did nature choose to invent this complex class of lipids, and why do eukaryotic cells follow elaborate remodelling pathways in order to generate dozens to hundreds of different molecular species of sphingolipid, depending on cell type? Yeast may, once again, serve as a model to dissect sphingolipid function at various levels. Almost the complete pathway for sphingolipid synthesis in yeast has been uncovered during the past two decades. More recently, key enzymes in sphingolipid degradation and signalling have been identified. Together with a wealth of genetic data obtained from the characterization of various suppressor mutants, this information now allows for an unprecedented analysis of sphingolipid function in this organism. This overview summarizes recent data on sphingolipid function in cell signalling, their role in the heat-stress response and Ca(2+) homeostasis, and addresses their function in transport of glycosylphosphatidylinositol-anchored proteins.

Specific sterols required for the internalization step of endocytosis in yeast

Sterols are major components of the plasma membrane, but their functions in this membrane are not well understood. We isolated a mutant defective in the internalization step of endocytosis in a gene (ERG2) encoding a C-8 sterol isomerase that acts in the late part of the ergosterol biosynthetic pathway. In the absence of Erg2p, yeast cells accumulate sterols structurally different from ergosterol, which is the major sterol in wild-type yeast. To investigate the structural requirements of ergosterol for endocytosis in more detail, several erg mutants (erg2Delta, erg6Delta, and erg2Deltaerg6Delta) were made. Analysis of fluid phase and receptor-mediated endocytosis indicates that changes in the sterol composition lead to a defect in the internalization step. Vesicle formation and fusion along the secretory pathway were not strongly affected in the ergDelta mutants. The severity of the endocytic defect correlates with changes in sterol structure and with the abundance of specific sterols in the ergDelta mutants. Desaturation of the B ring of the sterol molecules is important for the internalization step. A single desaturation at C-8,9 was not sufficient to support internalization at 37 degrees C whereas two double bonds, either at C-5,6 and C-7,8 or at C-5,6 and C-8,9, allowed internalization.

Metabolism and selected functions of sphingolipids in the yeast Saccharomyces cerevisiae

Our knowledge of sphingolipid metabolism and function in Saccharomyces cerevisiae is growing rapidly. Here we discuss the current status of sphingolipid metabolism including recent evidence suggesting that exogenous sphingoid long-chain bases must first be phosphorylated and then dephosphorylated before incorporation into ceramide. Phenotypes of strains defective in sphingolipid metabolism are discussed because they provide hints about the undiscovered functions of sphingolipids and are one of the major reasons for studying this model eukaryote. The long-chain base phosphates, dihydrosphingosine-1-phosphate and phytosphingosine-1-phosphate, have been hypothesized to play roles in heat stress resistance, perhaps acting as signaling molecules. We evaluate the data supporting this hypothesis and suggest future experiments needed to verify it. Finally, we discuss recent clues that may help to reveal how sphingolipid synthesis and total cellular sphingolipid content are regulated.

Systematic analysis of yeast strains with possible defects in lipid metabolism

Lipids are essential components of all living cells because they are obligate components of biological membranes, and serve as energy reserves and second messengers. Many but not all genes encoding enzymes involved in fatty acid, phospholipid, sterol or sphingolipid biosynthesis of the yeast Saccharomyces cerevisiae have been cloned and gene products have been functionally characterized. Less information is available about genes and gene products governing the transport of lipids between organelles and within membranes or the turnover and degradation of complex lipids. To obtain more insight into lipid metabolism, regulation of lipid biosynthesis and the role of lipids in organellar membranes, a group of five European laboratories established methods suitable to screen for novel genes of the yeast Saccharomyces cerevisiae involved in these processes. These investigations were performed within EUROFAN (European Function Analysis Network), a European initiative to identify the functions of unassigned open reading frames that had been detected during the Yeast Genome Sequencing Project. First, the methods required for the complete lipid analysis of yeast cells based on chromatographic techniques were established and standardized. The reliability of these methods was demonstrated using tester strains with established defects in lipid metabolism. During these investigations it was demonstrated that different wild-type strains, among them FY1679, CEN.PK2-1C and W303, exhibit marked differences in lipid content and lipid composition. Second, several candidate genes which were assumed to encode proteins involved in lipid metabolism were selected, based on their homology to genes of known function. Finally, lipid composition of mutant strains deleted of the respective open reading frames was determined. For some genes we found evidence suggesting a possible role in lipid metabolism.

Yeast sphingolipids

Many advances in our understanding of fungal sphingolipids have been made in recent years. This review focuses on the types of sphingolipids that have been found in fungi and upon the genes in Saccharomyces cerevisiae, the common baker's yeast, that are necessary for sphingolipid metabolism. While only a small number of fungi have been examined, most contain sphingolipids composed of ceramide derivatized at carbon-1 with inositol phosphate. Further additions include mannose and then other carbohydrates. The second major class of fungal sphingolipids is the glycosylceramides, having either glucose or galactose attached to ceramide rather than inositol phosphate. The glycosylceramides sometimes contain additional carbohydrates. Knowledge of the genome sequence has expedited identification of S. cerevisiae genes necessary for sphingolipid metabolism. At least one gene is known for most steps in S. cerevisiae sphingolipid metabolism, but more are likely to be identified so that the 13 known genes are likely to grow in number. The AUR1 gene is necessary for addition of inositol phosphate to ceramide and has been identified as a target of several potent antifungal compounds. This essential step in yeast sphingolipid synthesis, which is not found in humans, appears to be an excellent target for the development of more effective antifungal compounds, both for human and for agricultural use.

Biochemistry, cell biology and molecular biology of lipids of Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae is a powerful experimental system to study biochemical, cell biological and molecular biological aspects of lipid synthesis. Most but not all genes encoding enzymes involved in fatty acid, phospholipid, sterol or sphingolipid biosynthesis of this unicellular eukaryote have been cloned, and many gene products have been functionally characterized. Less information is available about genes and gene products governing the transport of lipids between organelles and within membranes, turnover and degradation of complex lipids, regulation of lipid biosynthesis, and linkage of lipid metabolism to other cellular processes. Here we summarize current knowledge about lipid biosynthetic pathways in S. cerevisiae and describe the characteristic features of the gene products involved. We focus on recent discoveries in these fields and address questions on the regulation of lipid synthesis, subcellular localization of lipid biosynthetic steps, cross-talk between organelles during lipid synthesis and subcellular distribution of lipids. Finally, we discuss distinct functions of certain key lipids and their possible roles in cellular processes.

Sphingolipid functions in Saccharomyces cerevisiae: comparison to mammals

Many roles for sphingolipids have been identified in mammals. Available data suggest that sphingolipids and their intermediates also have diverse roles in Saccharomyces cerevisiae. These roles include signal transduction during the heat stress response, regulation of calcium homeostasis or components in calcium-mediated signaling pathways, regulation of the cell cycle, and functions as components in trafficking of secretory vesicles from the endoplasmic reticulum to the Golgi apparatus and as the lipid moiety in many glycosylphosphatidylinositol-anchored proteins. S. cerevisiae is likely to be the first organism in which all genes involved in sphingolipid metabolism are identified. This information will provide an unprecedented opportunity to determine, for the first time in any organism, how sphingolipid synthesis is regulated. Through the use of both genetic and biochemical techniques, the identification of the complete array of processes regulated by sphingolipid signals is likely to be possible, as is the quantification of the physiological contribution of each.

Synthesis of mannose-(inositol-P)2-ceramide, the major sphingolipid in Saccharomyces cerevisiae, requires the IPT1 (YDR072c) gene

Knowledge of the Saccharomyces cerevisiae genes and proteins necessary for sphingolipid biosynthesis is far from complete. Such information should expedite studies of pathway regulation and sphingolipid functions. Using the Aur1 protein sequence, recently identified as necessary for synthesis of the sphingolipid inositol-P-ceramide (IPC), we show that a homolog (open reading frame YDR072c), termed Ipt1 (inositolphosphotransferase 1) is necessary for synthesis of mannose-(inositol-P)2-ceramide (M(IP)2C), the most abundant and complex sphingolipid in S. cerevisiae. This conclusion is based upon analysis of an ipt1-deletion strain, which fails to accumulate M(IP)2C and instead accumulates increased amounts of the precursor mannose-inositol-P-ceramide. The mutant also fails to incorporate radioactive precursors into M(IP)2C, and membranes prepared from it do not incorporate [3H-inositol]phosphatidylinositol into M(IP)2C, indicating a lack of M(IP)2C synthase activity (putatively phosphatidylinositol:mannose-inositol-P-ceramide phosphoinositol transferase). M(IP)2C synthase activity is inhibited in the micromolar range by aureobasidin A, but drug sensitivity is over 1000-fold lower than reported for IPC synthase activity. An ipt1-deletion mutant has no severe phenotypic effects but is slightly more resistant to growth inhibition by calcium ions. Identification of the IPT1 gene should be helpful in determining the function of the M(IP)2C sphingolipid and in determining the catalytic mechanism of IPC and M(IP)2C synthases.