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

Foot traffic on turf primarily shaped the endophytic bacteriome of the soil-rhizosphere-root continuum

Foot traffic on turf can cause grass wear-stress and soil compaction, adversely impacting turf health. The root microbiome, consisting of diverse microbes, plays a crucial role in enhancing plant resilience to abiotic stressors. However, the effects of foot traffic on these microbes and the mechanisms they employ to help plant survival remain largely unknown. Here, we investigated how foot traffic affected microbial communities of the root endosphere, rhizosphere, and bulk soil in Bermudagrass (Cynodon spp.) and Zoysiagrass (Zoysia spp.) turfs. Foot traffic was simulated to mimic six professional football games per week using a modified Baldree traffic simulator. High-throughput amplicon sequencing targeting 16S rRNA for bacteria and ITS for fungi was employed to analyze microbial communities. Foot traffic slightly and significantly reduced soil moisture and inorganic nitrogen, likely due to soil compaction and associated impairment on microbial activity. Microbial alpha diversity varied across microhabitats, with no discernible effect of foot traffic. However, microbial community composition was impacted by foot traffic, being more pronounced on bacteria of the root endosphere and on fungi of the bulk soil. In light of the genetic potential predicted by PICRUSt2, foot traffic enriched a few pathways of the endophytic bacteriome, including nitrifier denitrification (PWY7084) and mannosylglycerate biosynthesis (PWY5656). This indicated that root endophytes could help turfgrass to tolerate foot traffic via controls on the concentration of nitric oxide, the signaling molecule for root growth, and mannosylglycerate, the compatible solute for protecting enzymes against osmotic stress. Foot traffic also enhanced degradation pathways of carbohydrates and 4-coumarate, the constituent of turfgrass cell walls (PWY-3801, PWY-2221, PWY-7046), indicating the faster turnover of root tissues. Along the root-rhizosphere-bulk soil continuum, the bacteriome varied substantially in composition and also exhibited contrasting genetic potentials from stress alleviation to nutrient supply in coping with grass growth. But foot traffic had little effect on the genetic potential of bacteriome in rhizosphere and bulk soil. Our findings indicated that the endophytic bacteriome was more sensitive to foot traffic than the bacteriome in the rhizosphere and bulk soil and could potentially help turf survival via influences on plant signal molecules and compatible solutes.

Biological Importance of Complex Sphingolipids and Their Structural Diversity in Budding Yeast Saccharomyces cerevisiae

Complex sphingolipids are components of eukaryotic biomembranes and are involved in various physiological functions. In addition, their synthetic intermediates and metabolites, such as ceramide, sphingoid long-chain base, and sphingoid long-chain base 1-phosphate, play important roles as signaling molecules that regulate intracellular signal transduction systems. Complex sphingolipids have a large number of structural variations, and this structural diversity is considered an important molecular basis for their various physiological functions. The budding yeast Saccharomyces cerevisiae has simpler structural variations in complex sphingolipids compared to mammals and is, therefore, a useful model organism for elucidating the physiological significance of this structural diversity. In this review, we focus on the structure and function of complex sphingolipids in S. cerevisiae and summarize the response mechanisms of S. cerevisiae to metabolic abnormalities in complex sphingolipids.

Combatting Fungal Infections: Advances in Antifungal Polymeric Nanomaterials

Fungal pathogens cause over 6.5 million life-threatening systemic infections annually, with mortality rates ranging from 20 to 95%, even with medical intervention. The World Health Organization has recently emphasized the urgent need for new antifungal drugs. However, the range of effective antifungal agents remains limited and resistance is increasing. This Review explores the current landscape of fungal infections and antifungal drugs, focusing on synthetic polymeric nanomaterials like nanoparticles that enhance the physicochemical properties of existing drugs. Additionally, we examine intrinsically antifungal polymers that mimic naturally occurring peptides. Advances in polymer characterization and synthesis now allow precise design and screening for antifungal activity, biocompatibility, and drug interactions. These antifungal polymers represent a promising new class of drugs for combating fungal infections.

Antifungal Activity of Disalt of Epipyrone A from Epicoccum nigrum Likely via Disrupted Fatty Acid Elongation and Sphingolipid Biosynthesis

Multidrug-resistant fungal pathogens and antifungal drug toxicity have challenged our current ability to fight fungal infections. Therefore, there is a strong global demand for novel antifungal molecules with the distinct mode of action and specificity to service the medical and agricultural sectors. Polyenes are a class of antifungal drugs with the broadest spectrum of activity among the current antifungal drugs. Epipyrone A, a water-soluble antifungal molecule with a unique, linear polyene structure, was isolated from the fungus Epiccocum nigrum. Since small changes in a compound structure can significantly alter its cell target and mode of action, we present here a study on the antifungal mode of action of the disalt of epipyrone A (DEA) using chemical-genetic profiling, fluorescence microscopy, and metabolomics. Our results suggest the disruption of sphingolipid/fatty acid biosynthesis to be the primary mode of action of DEA, followed by the intracellular accumulation of toxic phenolic compounds, in particular p-toluic acid (4-methylbenzoic acid). Although membrane ergosterol is known to be the main cell target for polyene antifungal drugs, we found little evidence to support that is the case for DEA. Sphingolipids, on the other hand, are known for their important roles in fungal cell physiology, and their biosynthesis has been recognized as a potential fungal-specific cell target for the development of new antifungal drugs.

Lipid-Centric Approaches in Combating Infectious Diseases: Antibacterials, Antifungals and Antivirals with Lipid-Associated Mechanisms of Action

One of the global challenges of the 21st century is the increase in mortality from infectious diseases against the backdrop of the spread of antibiotic-resistant pathogenic microorganisms. In this regard, it is worth targeting antibacterials towards the membranes of pathogens that are quite conservative and not amenable to elimination. This review is an attempt to critically analyze the possibilities of targeting antimicrobial agents towards enzymes involved in pathogen lipid biosynthesis or towards bacterial, fungal, and viral lipid membranes, to increase the permeability via pore formation and to modulate the membranes' properties in a manner that makes them incompatible with the pathogen's life cycle. This review discusses the advantages and disadvantages of each approach in the search for highly effective but nontoxic antimicrobial agents. Examples of compounds with a proven molecular mechanism of action are presented, and the types of the most promising pharmacophores for further research and the improvement of the characteristics of antibiotics are discussed. The strategies that pathogens use for survival in terms of modulating the lipid composition and physical properties of the membrane, achieving a balance between resistance to antibiotics and the ability to facilitate all necessary transport and signaling processes, are also considered.

Beyond membrane components: uncovering the intriguing world of fungal sphingolipid synthesis and regulation

Sphingolipids (SLs) are essential to fungal survival and represent a major class of structural and signaling lipids. Unique SL structures and their biosynthetic enzymes in filamentous fungi make them an ideal drug target. Several studies have contributed towards the functional characterization of specific SL metabolism genes, which have been complemented by advanced lipidomics methods which allow accurate identification and quantification of lipid structures and pathway mapping. These studies have provided a better understanding of SL biosynthesis, degradation and regulation networks in filamentous fungi, which are discussed and elaborated here.

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.

The ER calcium channel Csg2 integrates sphingolipid metabolism with autophagy

Sphingolipids are ubiquitous components of membranes and function as bioactive lipid signaling molecules. Here, through genetic screening and lipidomics analyses, we find that the endoplasmic reticulum (ER) calcium channel Csg2 integrates sphingolipid metabolism with autophagy by regulating ER calcium homeostasis in the yeast Saccharomyces cerevisiae. Csg2 functions as a calcium release channel and maintains calcium homeostasis in the ER, which enables normal functioning of the essential sphingolipid synthase Aur1. Under starvation conditions, deletion of Csg2 causes increases in calcium levels in the ER and then disturbs Aur1 stability, leading to accumulation of the bioactive sphingolipid phytosphingosine, which specifically and completely blocks autophagy and induces loss of starvation resistance in cells. Our findings indicate that calcium homeostasis in the ER mediated by the channel Csg2 translates sphingolipid metabolism into autophagy regulation, further supporting the role of the ER as a signaling hub for calcium homeostasis, sphingolipid metabolism and autophagy.

Ecological Niche-Inspired Genome Mining Leads to the Discovery of Crop-Protecting Nonribosomal Lipopeptides Featuring a Transient Amino Acid Building Block

Investigating the ecological context of microbial predator-prey interactions enables the identification of microorganisms, which produce multiple secondary metabolites to evade predation or to kill the predator. In addition, genome mining combined with molecular biology methods can be used to identify further biosynthetic gene clusters that yield new antimicrobials to fight the antimicrobial crisis. In contrast, classical screening-based approaches have limitations since they do not aim to unlock the entire biosynthetic potential of a given organism. Here, we describe the genomics-based identification of keanumycins A-C. These nonribosomal peptides enable bacteria of the genus Pseudomonas to evade amoebal predation. While being amoebicidal at a nanomolar level, these compounds also exhibit a strong antimycotic activity in particular against the devastating plant pathogen Botrytis cinerea and they drastically inhibit the infection of Hydrangea macrophylla leaves using only supernatants of Pseudomonas cultures. The structures of the keanumycins were fully elucidated through a combination of nuclear magnetic resonance, tandem mass spectrometry, and degradation experiments revealing an unprecedented terminal imine motif in keanumycin C extending the family of nonribosomal amino acids by a highly reactive building block. In addition, chemical synthesis unveiled the absolute configuration of the unusual dihydroxylated fatty acid of keanumycin A, which has not yet been reported for this lipodepsipeptide class. Finally, a detailed genome-wide microarray analysis of Candida albicans exposed to keanumycin A shed light on the mode-of-action of this potential natural product lead, which will aid the development of new pharmaceutical and agrochemical antifungals.

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.

Anti-fungal properties and mechanisms of melittin

Many fungal diseases remain poorly addressed by public health authorities, despite posing a substantial threat to humans, animals, and plants. More worryingly, few classes of anti-fungals have been developed to combat fungal infections thus far. These medications also have certain drawbacks in terms of toxicity, spectrum of activity, and pharmacokinetic properties. Hence, there is a dire need for discovery of novel anti-fungal agents. Melittin, the main constituent in the venom of European honeybee Apis mellifera, has attracted considerable attention among researchers owing to its potential therapeutic applications. To our knowledge, there has been no review pertinent to anti-fungal properties of melittin, prompting us to synopsize the results of experimental investigations with a special emphasis upon underlying mechanisms. In this respect, melittin inhibits a broad spectrum of fungal genera including Aspergillus, Botrytis, Candida, Colletotrichum, Fusarium, Malassezia, Neurospora, Penicillium, Saccharomyces, Trichoderma, Trichophyton, and Trichosporon. Melittin hinders fungal growth by several mechanisms such as membrane permeabilization, apoptosis induction by reactive oxygen species-mediated mitochondria/caspase-dependent pathway, inhibition of (1,3)-β-D-glucan synthase, and alterations in fungal gene expression. Overall, melittin will definitely open up new avenues for various biomedical applications, from medicine to agriculture. KEYPOINTS: • Venom-derived peptides have potential for development of anti-microbial agents. • Many fungal pathogens are susceptible to melittin at micromolar concentrations. • Melittin possesses multi-target mechanism of action against fungal cells.

Chemical diversity and mode of action of natural products targeting lipids in the eukaryotic cell membrane

Covering: up to 2019Nature furnishes bioactive compounds (natural products) with complex chemical structures, yet with simple, sophisticated molecular mechanisms. When natural products exhibit their activities in cells or bodies, they first have to bind or react with a target molecule in/on the cell. The cell membrane is a major target for bioactive compounds. Recently, our understanding of the molecular mechanism of interactions between natural products and membrane lipids progressed with the aid of newly-developed analytical methods. New technology reconnects old compounds with membrane lipids, while new membrane-targeting molecules are being discovered through the screening for antimicrobial potential of natural products. This review article focuses on natural products that bind to eukaryotic membrane lipids, and includes clinically important molecules and key research tools. The chemical diversity of membrane-targeting natural products and the molecular basis of lipid recognition are described. The history of how their mechanism was unveiled, and how these natural products are used in research are also mentioned.

Cyclic Lipodepsipeptides From Pseudomonas spp. - Biological Swiss-Army Knives

Cyclic lipodepsipeptides produced by Pseudomonas spp. (Ps-CLPs) are biosurfactants that constitute a diverse class of versatile bioactive natural compounds with promising application potential. While chemically diverse, they obey a common structural blue-print, allowing the definition of 14 distinct groups with multiple structurally homologous members. In addition to antibacterial and antifungal properties the reported activity profile of Ps-CLPs includes their effect on bacterial motility, biofilm formation, induced defense responses in plants, their insecticidal activity and anti-proliferation effects on human cancer cell-lines. To further validate their status of potential bioactive substances, we assessed the results of 775 biological tests on 51 Ps-CLPs available from literature. From this, a fragmented view emerges. Taken as a group, Ps-CLPs present a broad activity profile. However, reports on individual Ps-CLPs are often much more limited in the scope of organisms that are challenged or activities that are explored. As a result, our analysis shows that the available data is currently too sparse to allow biological function to be correlated to a particular group of Ps-CLPs. Consequently, certain generalizations that appear in literature with respect to the biological activities of Ps-CLPs should be nuanced. This notwithstanding, the data for the two most extensively studied Ps-CLPs does indicate they can display activities against various biological targets. As the discovery of novel Ps-CLPs accelerates, current challenges to complete and maintain a useful overview of biological activity are discussed.

Antifungal amphiphilic kanamycins: new life for an old drug

Classical aminoglycoside antibiotics are obsolete or hampered by the emergence of drug resistant bacteria. Recent discoveries of antifungal amphiphilic kanamycins offer new strategies for reviving and repurposing these old drugs. A simple structural modification turns the clinically obsolete antibacterial kanamycin into an antifungal agent. Structure-activity relationship studies have led to the production of K20, an antifungal kanamycin that can be mass-produced for uses in agriculture as well as in animals. This review delineates the path to the discovery of K20 and other related antifungal amphiphilic kanamycins, determination of its mode of action, and findings in greenhouse and field trials with K20 that could lead to crop disease protection strategies.

Recovering Genomics Clusters of Secondary Metabolites from Lakes Using Genome-Resolved Metagenomics

Metagenomic approaches became increasingly popular in the past decades due to decreasing costs of DNA sequencing and bioinformatics development. So far, however, the recovery of long genes coding for secondary metabolites still represents a big challenge. Often, the quality of metagenome assemblies is poor, especially in environments with a high microbial diversity where sequence coverage is low and complexity of natural communities high. Recently, new and improved algorithms for binning environmental reads and contigs have been developed to overcome such limitations. Some of these algorithms use a similarity detection approach to classify the obtained reads into taxonomical units and to assemble draft genomes. This approach, however, is quite limited since it can classify exclusively sequences similar to those available (and well classified) in the databases. In this work, we used draft genomes from Lake Stechlin, north-eastern Germany, recovered by MetaBat, an efficient binning tool that integrates empirical probabilistic distances of genome abundance, and tetranucleotide frequency for accurate metagenome binning. These genomes were screened for secondary metabolism genes, such as polyketide synthases (PKS) and non-ribosomal peptide synthases (NRPS), using the Anti-SMASH and NAPDOS workflows. With this approach we were able to identify 243 secondary metabolite clusters from 121 genomes recovered from our lake samples. A total of 18 NRPS, 19 PKS, and 3 hybrid PKS/NRPS clusters were found. In addition, it was possible to predict the partial structure of several secondary metabolite clusters allowing for taxonomical classifications and phylogenetic inferences. Our approach revealed a high potential to recover and study secondary metabolites genes from any aquatic ecosystem.

PP2A Controls Genome Integrity by Integrating Nutrient-Sensing and Metabolic Pathways with the DNA Damage Response

Mec1^ATR mediates the DNA damage response (DDR), integrating chromosomal signals and mechanical stimuli. We show that the PP2A phosphatases, ceramide-activated enzymes, couple cell metabolism with the DDR. Using genomic screens, metabolic analysis, and genetic and pharmacological studies, we found that PP2A attenuates the DDR and that three metabolic circuits influence the DDR by modulating PP2A activity. Irc21, a putative cytochrome b5 reductase that promotes the condensation reaction generating dihydroceramides (DHCs), and Ppm1, a PP2A methyltransferase, counteract the DDR by activating PP2A; conversely, the nutrient-sensing TORC1-Tap42 axis sustains DDR activation by inhibiting PP2A. Loss-of-function mutations in IRC21, PPM1, and PP2A and hyperactive tap42 alleles rescue mec1 mutants. Ceramides synergize with rapamycin, a TORC1 inhibitor, in counteracting the DDR. Hence, PP2A integrates nutrient-sensing and metabolic pathways to attenuate the Mec1^ATR response. Our observations imply that metabolic changes affect genome integrity and may help with exploiting therapeutic options and repositioning known drugs.

Yeast lacking the amphiphysin family protein Rvs167 is sensitive to disruptions in sphingolipid levels

Rvs167 and Rvs161 in Saccharomyces cerevisiae are amphiphysin family proteins, which are involved in several important cellular events, such as invagination and scission of endocytic vesicles, and actin cytoskeleton organization. It has been reported that cellular dysfunctions caused by deletion of RVS167 or RVS161 are rescued by deletion of specific nonessential sphingolipid-metabolizing enzyme genes. Here, we found that yeast cells lacking RVS167 or RVS161 exhibit a decrease in sphingolipid levels. In rvs167∆ cells, the expression level of Orm2, a negative regulator of serine palmitoyltransferase (SPT) catalyzing the initial step of sphingolipid biosynthesis, was increased in a calcineurin-dependent manner, and the decrease in sphingolipid levels in rvs167∆ cells was reversed on deletion of ORM2. Moreover, repression of both ORM1 and ORM2 expression or overexpression of SPT caused a strong growth defect of rvs167∆ cells, indicating that enhancement of de novo sphingolipid biosynthesis is detrimental to rvs167∆ cells. In contrast, partial repression of LCB1-encoding SPT suppressed abnormal phenotypes caused by the deletion of RVS167, including supersensitivity to high temperature and salt stress, and impairment of endocytosis and actin cytoskeleton organization. In addition, the partial repression of SPT activity suppressed the temperature supersensitivity and abnormal vacuolar morphology caused by deletion of VPS1 encoding a dynamin-like GTPase, which is required for vesicle scission and is functionally closely related to Rvs167/Rvs161, whereas repression of both ORM1 and ORM2 expression in vps1∆ cells caused a growth defect. Thus, it was suggested that proper regulation of SPT activity is indispensable for amphiphysin-deficient cells.

Antifungal peptides: To be or not to be membrane active

Most antifungal peptides (AFPs), if not all, have membrane activity, while some also have alternative targets. Fungal membranes share many characteristics with mammalian membranes with only a few differences, such as differences in sphingolipids, phosphatidylinositol (PI) content and the main sterol is ergosterol. Fungal membranes are also more negative and a better target for cationic AFPs. Targeting just the fungal membrane lipids such as phosphatidylinositol and/or ergosterol by AFPs often translates into mammalian cell toxicity. Conversely, a specific AFP target in the fungal pathogen, such as glucosylceramide, mannosyldiinositol phosphorylceramide or a fungal protein target translates into high pathogen selectivity. However, a lower target concentration, absence or change in the specific fungal target can naturally lead to resistance, although such resistance in turn could result in reduced pathogen virulence. The question is then to be or not to be membrane active - what is the best choice for a successful AFP? In this review we deliberate on this question by focusing on the recent advances in our knowledge on how natural AFPs target fungi.

The global effect of exposing bakers' yeast to 5-fluoruracil and nystatin; a view to Toxichip

A genome-wide screen of a haploid deletion library of bakers' yeast (Saccharomyces cerevisiae) was conducted to document the phenotypic and transcriptional impact of exposure to each of the two pharmaceutical products 5-fluorouracil (an anti-tumor agent) and nystatin (an anti-fungal agent). The combined data set was handled by applying a systems biology perspective. A Gene Ontology analysis identified functional categories previously characterized as likely targets for both compounds. Induced transcription profiles were well correlated in yeast and human HepG2 cells. The identified molecular targets for both compounds were used to suggest a small set of human orthologues as appropriate for testing on human material. The yeast system developed here (denoted "Toxichip") has likely utility for identifying biomarkers relevant for health and environmental risk assessment applications required as part of the development process for novel pharmaceuticals.

Deletion of PdMit1, a homolog of yeast Csg1, affects growth and Ca(2+) sensitivity of the fungus Penicillium digitatum, but does not alter virulence

GDP-mannose:inositol-phosphorylceramide (MIPC) and its derivatives are important for Ca(2+) sensitization of Saccharomyces cerevisiae and for the virulence of Candida albicans, but its role in the virulence of plant fungal pathogens remains unclear. In this study, we report the identification and functional characterization of PdMit1, the gene encoding MIPC synthase in Penicillium digitatum, one of the most important pathogens of postharvest citrus fruits. To understand the function of PdMit1, a PdMit1 deletion mutant was generated. Compared to its wild-type control, the PdMit1 deletion mutant exhibited slow radial growth, decreased conidia production and delayed conidial germination, suggesting that PdMit1 is important for the growth of mycelium, sporulation and conidial germination. The PdMit1 deletion mutant also showed hypersensitivity to Ca(2+). Treatment with 250 mmol/l Ca(2+) induced vacuole fusion in the wild-type strain, but not in the PdMit1 deletion mutant. Treatment with 250mmol/lCaCl2 upregulated three Ca(2+)-ATPase genes in the wild-type strain, and this was significantly inhibited in the PdMit1 deletion mutant. These results suggest that PdMit1 may have a role in regulating vacuole fusion and expression of Ca(2+)-ATPase genes by controlling biosynthesis of MIPC, and thereby imparts P. digitatum Ca(2+) tolerance. However, we found that PdMit1 is dispensable for virulence of P. digitatum.

Antifungal amphiphilic aminoglycoside K20: bioactivities and mechanism of action

K20 is a novel amphiphilic antifungal aminoglycoside that is synthetically derived from the antibiotic kanamycin A. Reported here are investigations of K20's antimicrobial activities, cytotoxicity, and fungicidal mechanism of action. In vitro growth inhibitory activities against a variety of human and plant pathogenic yeasts, filamentous fungi, and bacteria were determined using microbroth dilution assays and time-kill curve analyses, and hemolytic and animal cell cytotoxic activities were determined. Effects on Cryptococcus neoformans H-99 infectivity were determined with a preventive murine lung infection model. The antifungal mechanism of action was studied using intact fungal cells, yeast lipid mutants, and small unilamellar lipid vesicles. K20 exhibited broad-spectrum in vitro antifungal activities but not antibacterial activities. Pulmonary, single dose-administration of K20 reduced C. neoformans lung infection rates 4-fold compared to controls. Hemolysis and half-maximal cytotoxicities of mammalian cells occurred at concentrations that were 10 to 32-fold higher than fungicidal MICs. With fluorescein isothiocyanate (FITC), 20-25 mg/L K20 caused staining of >95% of C. neoformans and Fusarium graminearum cells and at 31.3 mg/L caused rapid leakage (30-80% in 15 min) of calcein from preloaded small unilamellar lipid vesicles. K20 appears to be a broad-spectrum fungicide, capable of reducing the infectivity of C. neoformans, and exhibits low hemolytic activity and mammalian cell toxicity. It perturbs the plasma membrane by mechanisms that are lipid modulated. K20 is a novel amphiphilic aminoglycoside amenable to scalable production and a potential lead antifungal for therapeutic and crop protection applications.

The yeast sphingolipid signaling landscape

Sphingolipids are recognized as signaling mediators in a growing number of pathways, and represent potential targets to address many diseases. The study of sphingolipid signaling in yeast has created a number of breakthroughs in the field, and has the potential to lead future advances. The aim of this article is to provide an inclusive view of two major frontiers in yeast sphingolipid signaling. In the first section, several key studies in the field of sphingolipidomics are consolidated to create a yeast sphingolipidome that ranks nearly all known sphingolipid species by their level in a resting yeast cell. The second section presents an overview of most known phenotypes identified for sphingolipid gene mutants, presented with the intention of illuminating not yet discovered connections outside and inside of the field.

The protein kinase Sch9 is a key regulator of sphingolipid metabolism in Saccharomyces cerevisiae

Sphingolipids play crucial roles in the determination of growth and survival of eukaryotic cells. The budding yeast protein kinase Sch9 is not only an effector, but also a regulator of sphingolipid metabolism. This new function provides a crucial link between nutrient and sphingolipid signaling.

The Saccharomyces cerevisiae protein kinase Sch9 is an in vitro and in vivo effector of sphingolipid signaling. This study examines the link between Sch9 and sphingolipid metabolism in S. cerevisiae in vivo based on the observation that the sch9Δ mutant displays altered sensitivity to different inhibitors of sphingolipid metabolism, namely myriocin and aureobasidin A. Sphingolipid profiling indicates that sch9Δ cells have increased levels of long-chain bases and long-chain base-1 phosphates, decreased levels of several species of (phyto)ceramides, and altered ratios of complex sphingolipids. We show that the target of rapamycin complex 1–Sch9 signaling pathway functions to repress the expression of the ceramidase genes YDC1 and YPC1, thereby revealing, for the first time in yeast, a nutrient-dependent transcriptional mechanism involved in the regulation of sphingolipid metabolism. In addition, we establish that Sch9 affects the activity of the inositol phosphosphingolipid phospholipase C, Isc1, which is required for ceramide production by hydrolysis of complex sphingolipids. Given that sphingolipid metabolites play a crucial role in the regulation of stress tolerance and longevity of yeast cells, our data provide a model in which Sch9 regulates the latter phenotypes by acting not only as an effector but also as a regulator of sphingolipid metabolism.

Membrane lipid-modulated mechanism of action and non-cytotoxicity of novel fungicide aminoglycoside FG08

A novel aminoglycoside, FG08, that differs from kanamycin B only by a C8 alkyl chain at the 4″-O position, was previously reported. Unlike kanamycin B, FG08 shows broad-spectrum fungicidal but not anti-bacterial activities. To understand its specificity for fungi, the mechanism of action of FG08 was studied using intact cells of the yeast Saccharomyces cerevisiae and small unilamellar membrane vesicles. With exposure to FG08 (30 µg mL⁻¹), 8-fold more cells were stained with fluorescein isothiocyanate, cells had 4 to 6-fold higher K⁺ efflux rates, and 18-fold more cells were stained with SYTOX Green in comparison to exposure to kanamycin B (30 µg mL⁻¹). Yeast mutants with aberrant membrane sphingolipids (no sphingoid base C4 hydroxyl group, truncated very long fatty acid chain, or lacking the terminal phosphorylinositol group of mannosyl-diinositolphosphorylphytoceramide were 4 to 8-fold less susceptible to growth inhibition with FG08 and showed 2 to 10-fold lower SYTOX Green dye uptake rates than did the isogenic wild-type strain. FG08 caused leakage of pre-loaded calcein from 50% of small unilamellar vesicles with glycerophospholipid and sterol compositions that mimic the compositions of fungal plasma membranes. Less than 5 and 10% of vesicles with glycerophospholipid and sterol compositions that mimic bacterial and mammalian cell plasma membranes, respectively, showed calcein leakage. In tetrazolium dye cytotoxicity tests, mammalian cell lines NIH3T3 and C8161.9 showed FG08 toxicity at concentrations that were 10 to 20-fold higher than fungicidal minimal inhibitory concentrations. It is concluded that FG08’s growth inhibitory specificity for fungi lie in plasma membrane permeability changes involving mechanisms that are modulated by membrane lipid composition.

The synthetic amphipathic peptidomimetic LTX109 is a potent fungicide that disturbs plasma membrane integrity in a sphingolipid dependent manner

The peptidomimetic LTX109 (arginine-tertbutyl tryptophan-arginine-phenylethan) was previously shown to have antibacterial properties. Here, we investigated the activity of this novel antimicrobial peptidomimetic on the yeast Saccharomyces cerevisiae. We found that LTX109 was an efficient fungicide that killed all viable cells in an exponentially growing population as well as a large proportion of cells in biofilm formed on an abiotic surface. LTX109 had similar killing kinetics to the membrane-permeabilizing fungicide amphotericin B, which led us to investigate the ability of LTX109 to disrupt plasma membrane integrity. S. cerevisiae cells exposed to a high concentration of LTX109 showed rapid release of potassium and amino acids, suggesting that LTX109 acted by destabilizing the plasma membrane. This was supported by the finding that cells were permeable to the fluorescent nucleic acid stain SYTOX Green after a few minutes of LTX109 treatment. We screened a haploid S. cerevisiae gene deletion library for mutants resistant to LTX109 to uncover potential molecular targets. Eight genes conferred LTX109 resistance when deleted and six were involved in the sphingolipid biosynthetic pathway (SUR1, SUR2, SKN1, IPT1, FEN1 and ORM2). The involvement of all of these genes in the biosynthetic pathway for the fungal-specific lipids mannosylinositol phosphorylceramide (MIPC) and mannosyl di-(inositol phosphoryl) ceramide (M(IP)2C) suggested that these lipids were essential for LTX109 sensitivity. Our observations are consistent with a model in which LTX109 kills S. cerevisiae by nonspecific destabilization of the plasma membrane through direct or indirect interaction with the sphingolipids.

Isolation, structure, and biological activity of Phaeofungin, a cyclic lipodepsipeptide from a Phaeosphaeria sp. Using the Genome-Wide Candida albicans Fitness Test

Phaeofungin (1), a new cyclic depsipeptide isolated from Phaeosphaeria sp., was discovered by application of reverse genetics technology, using the Candida albicans fitness test (CaFT). Phaeofungin is comprised of seven amino acids and a β,γ-dihydroxy-γ-methylhexadecanoic acid arranged in a 25-membered cyclic depsipeptide. Five of the amino acids were assigned with d-configurations. The structure was elucidated by 2D-NMR and HRMS-MS analysis of the natural product and its hydrolyzed linear peptide. The absolute configuration of the amino acids was determined by Marfey's method by complete and partial hydrolysis of 1. The CaFT profile of the phaeofungin-containing extract overlapped with that of phomafungin (3), another structurally different cyclic lipodepsipeptide isolated from a Phoma sp. using the same approach. Comparative biological characterization further demonstrated that these two fungal lipodepsipeptides are functionally distinct. While phomafungin was potentiated by cyclosporin A (an inhibitor of the calcineurin pathway), phaeofungin was synergized with aureobasidin A (2) (an inhibitor of the sphingolipid biosynthesis) and to some extent caspofungin (an inhibitor of glucan synthase). Furthermore, phaeofungin caused ATP release in wild-type C. albicans strains but phomafungin did not. It showed modest antifungal activity against C. albicans (MIC 16-32 μg/mL) and better activity against Aspergillus fumigatus (MIC 8-16 μg/mL) and Trichophyton mentagrophytes (MIC 4 μg/mL). The linear peptide was inactive, suggesting that the macrocyclic depsipeptide ring is essential for target engagement and antifungal activity.

Members 5 and 6 of the Candida albicans BMT family encode enzymes acting specifically on β-mannosylation of the phospholipomannan cell-wall glycosphingolipid

A family of nine genes encoding proteins involved in the synthesis of β-1,2 mannose adhesins of Candida albicans has been identified. Four of these genes, BMT1-4, encode enzymes acting stepwise to add β-mannoses on to cell-wall phosphopeptidomannan (PPM). None of these acts on phospholipomannan (PLM), a glycosphingolipid member of the mannose-inositol-phosphoceramide family, which contributes with PPM to β-mannose surface expression. We show that deletion of BMT5 and BMT6 led to a dramatic reduction of PLM glycosylation and accumulation of PLM with a truncated β-oligomannoside chain, respectively. Disruptions had no effect on sphingolipid biosynthesis and on PPM β-mannosylation. β-Mannose surface expression was not affected, confirming that β-mannosylation is a process based on specificity of acceptor molecules, but liable to global regulation.

Improving upon nature: active site remodeling produces highly efficient aldolase activity toward hydrophobic electrophilic substrates

The substrate specificity of enzymes is frequently narrow and constrained by multiple interactions, limiting the use of natural enzymes in biocatalytic applications. Aldolases have important synthetic applications, but the usefulness of these enzymes is hampered by their narrow reactivity profile with unnatural substrates. To explore the determinants of substrate selectivity and alter the specificity of Escherichia coli 2-keto-3-deoxy-6-phosphogluconate (KDPG) aldolase, we employed structure-based mutagenesis coupled with library screening of mutant enzymes localized to the bacterial periplasm. We identified two active site mutations (T161S and S184L) that work additively to enhance the substrate specificity of this aldolase to include catalysis of retro-aldol cleavage of (4S)-2-keto-4-hydroxy-4-(2'-pyridyl)butyrate (S-KHPB). These mutations improve the value of k(cat)/K(M)(S-KHPB) by >450-fold, resulting in a catalytic efficiency that is comparable to that of the wild-type enzyme with the natural substrate while retaining high stereoselectivity. Moreover, the value of k(cat)(S-KHPB) for this mutant enzyme, a parameter critical for biocatalytic applications, is 3-fold higher than the maximal value achieved by the natural aldolase with any substrate. This mutant also possesses high catalytic efficiency for the retro-aldol cleavage of the natural substrate, KDPG, and a >50-fold improved activity for cleavage of 2-keto-4-hydroxy-octonoate, a nonfunctionalized hydrophobic analogue. These data suggest a substrate binding mode that illuminates the origin of facial selectivity in aldol addition reactions catalyzed by KDPG and 2-keto-3-deoxy-6-phosphogalactonate aldolases. Furthermore, targeting mutations to the active site provides a marked improvement in substrate selectivity, demonstrating that structure-guided active site mutagenesis combined with selection techniques can efficiently identify proteins with characteristics that compare favorably to those of naturally occurring enzymes.

Screening for yeast mutants defective in recipient ability for transkingdom conjugation with Escherichia coli revealed importance of vacuolar ATPase activity in the horizontal DNA transfer phenomenon

Proteobacterium Escherichia coli strains harboring wide-transfer-range conjugative plasmids are able to transfer these plasmids to several yeast species. Whole plasmid DNA is mobilizable in the transkingdom conjugation phenomenon. Owing to the availability of various conjugative plasmids in bacteria, the horizontal DNA transfer has potential to occur between various bacteria and eukaryotes. In order to know host factor genes involved in such conjugation, we systematically tested the conjugability of strains among a yeast library comprising single-gene-knockout mutants in this study. This genome-wide screen identified 26 host chromosomal genes whose absence reduced the efficiency of the transkingdom conjugation. Among the 26 genes, 20 are responsible for vacuolar ATPase activity, while 5 genes (SHP1, CSG2, CCR4, NOT5, and HOF1) seem to play a role in maintaining the cell surface. Lack of either ZUO1 gene or SSZ1 gene, each of which encodes a component of the ribosome-associated cytoplasmic molecular chaperone, also strongly affected transkingdom conjugation.

Fungicidal Activities and Mechanisms of Action of Pseudomonas syringae pv. syringae Lipodepsipeptide Syringopeptins 22A and 25A

The plant-associated bacterium Pseudomonas syringae pv. syringae simultaneously produces two classes of metabolites: the small cyclic lipodepsinonapeptides such as the syringomycins and the larger cyclic lipodepsipeptide syringopeptins SP22 or SP25. The syringomycins inhibit a broad spectrum of fungi (but particularly yeasts) by lipid-dependent membrane interaction. The syringopeptins are phytotoxic and inhibitory to Gram-positive bacteria. In this study, the fungicidal activities of two major syringopeptins, SP22A and SP25A, and their mechanisms of action were investigated and compared to those of syringomycin E. SP22A and SP25A were observed to inhibit the fungal yeasts Saccharomyces cerevisiae and Candida albicans although less effectively than syringomycin E. S. cerevisiae mutants defective in ergosterol and sphingolipid biosyntheses were less susceptible to SP22A and SP25A but the relative inhibitory capabilities of SRE vs. SP22A and SP25A were maintained. Similar differences were observed for capabilities to cause cellular K(+) and Ca(2+) fluxes in S. cerevisiae. Interestingly, in phospholipid bilayers the syringopeptins are found to induce larger macroscopic ionic conductances than syringomycin E but form single channels with similar properties. These findings suggest that the syringopeptins target the yeast plasma membrane, and, like syringomycin E, employ a lipid-dependent channel-forming mechanism of action. The differing degrees of growth inhibition by these lipodepsipeptides may be explained by differences in their hydrophobicities. The more hydrophobic SP22A and SP25A might interact more strongly with the yeast cell wall that would create a selective barrier for their incorporation into the plasma membrane.

Directed evolution of a pyruvate aldolase to recognize a long chain acyl substrate

The use of biological catalysts for industrial scale synthetic chemistry is highly attractive, given their cost effectiveness, high specificity that obviates the need for protecting group chemistry, and the environmentally benign nature of enzymatic procedures. Here we evolve the naturally occurring 2-keto-3-deoxy-6-phosphogluconate (KDPG) aldolases from Thermatoga maritima and Escherichia coli, into enzymes that recognize a nonfunctionalized electrophilic substrate, 2-keto-4-hydroxyoctonoate (KHO). Using an in vivo selection based on pyruvate auxotrophy, mutations were identified that lower the K(M) value up to 100-fold in E. coli KDPG aldolase, and that enhance the efficiency of retro-aldol cleavage of KHO by increasing the value of k(cat)/K(M) up to 25-fold in T. maritima KDPG aldolase. These data indicate that numerous mutations distal from the active site contribute to enhanced 'uniform binding' of the substrates, which is the first step in the evolution of novel catalytic activity.

Structure of the lipodepsipeptide syringomycin E in phospholipids and sodium dodecylsulphate micelle studied by circular dichroism, NMR spectroscopy and molecular dynamics

Syringomycin E (SRE) is a member of a family of lipodepsipeptides that characterize the secondary metabolism of the plant-associated bacteria Pseudomonas syringae pv. syringae. It displays phytotoxic, antifungal and haemolytic activities, due to the membrane interaction and ion channel formation. To gain an insight into the conformation of SRE in the membrane environment, we studied the conformation of SRE bound to SDS micelle, a suitable model for the membrane-bound SRE. In fact, highly similar circular dichroism (CD) spectra were obtained for SRE bound to sodium dodecylsulphate (SDS) and to a phospholipid bilayer, indicating the conformational equivalence of SRE in these two media, at difference with the CD spectrum of SRE in water solution. The structure of SDS-bound SRE was determined by NMR spectroscopy combined with molecular dynamics calculations in octane environment. The results of this study highlight the influence of the interaction with lipids in determining the three-dimensional structure of SRE and provide the basis for further investigations on structural determinants of syringomycin E-membrane interaction.

Interruption of inositol sphingolipid synthesis triggers Stt4p-dependent protein kinase C signaling

The protein kinase C (PKC)-MAPK signaling cascade is activated and is essential for viability when cells are starved for the phospholipid precursor inositol. In this study, we report that inhibiting inositol-containing sphingolipid metabolism, either by inositol starvation or treatment with agents that block sphingolipid synthesis, triggers PKC signaling independent of sphingoid base accumulation. Under these same growth conditions, a fluorescent biosensor that detects the necessary PKC signaling intermediate, phosphatidylinositol (PI)-4-phosphate (PI4P), is enriched on the plasma membrane. The appearance of the PI4P biosensor on the plasma membrane correlates with PKC activation and requires the PI 4-kinase Stt4p. Like other mutations in the PKC-MAPK pathway, mutants defective in Stt4p and the PI4P 5-kinase Mss4p, which generates phosphatidylinositol 4,5-bisphosphate, exhibit inositol auxotrophy, yet fully derepress INO1, encoding inositol-3-phosphate synthase. These observations suggest that inositol-containing sphingolipid metabolism controls PKC signaling by regulating access of the signaling lipids PI4P and phosphatidylinositol 4,5-bisphosphate to effector proteins on the plasma membrane.

A genomic approach highlights common and diverse effects and determinants of susceptibility on the yeast Saccharomyces cerevisiae exposed to distinct antimicrobial peptides

Background

The mechanism of action of antimicrobial peptides (AMP) was initially correlated with peptide membrane permeation properties. However, recent evidences indicate that action of a number of AMP is more complex and involves specific interactions at cell envelopes or with intracellular targets. In this study, a genomic approach was undertaken on the model yeast Saccharomyces cerevisiae to characterize the antifungal effect of two unrelated AMP.

Results

Two differentiated peptides were used: the synthetic cell-penetrating PAF26 and the natural cytolytic melittin. Transcriptomic analyses demonstrated distinctive gene expression changes for each peptide. Quantitative RT-PCR confirmed differential expression of selected genes. Gene Ontology (GO) annotation of differential gene lists showed that the unique significant terms shared by treatment with both peptides were related to the cell wall (CW). Assays with mutants lacking CW-related genes including those of MAPK signaling pathways revealed genes having influence on sensitivity to peptides. Fluorescence microscopy and flow cytometry demonstrated PAF26 interaction with cells and internalization that correlated with cell killing in sensitive CW-defective mutants such as Δecm33 or Δssd1. GO annotation also showed differential responses between peptides, which included ribosomal biogenesis, ARG genes from the metabolism of amino groups (specifically induced by PAF26), or the reaction to unfolded protein stress. Susceptibility of deletion mutants confirmed the involvement of these processes. Specifically, mutants lacking ARG genes from the metabolism of arginine pathway were markedly more resistant to PAF26 and had a functional CW. In the deletant in the arginosuccinate synthetase (ARG1) gene, PAF26 interaction occurred normally, thus uncoupling peptide interaction from cell killing. The previously described involvement of the glycosphingolipid gene IPT1 was extended to the peptides studied here.

Conclusions

Reinforcement of CW is a general response common after exposure to distinct AMP, and likely contributes to shield cells from peptide interaction. However, a weakened CW is not necessarily indicative of a higher sensitivity to AMP. Additional processes modulate susceptibility to specific peptides, exemplified in the involvement of the metabolism of amino groups in the case of PAF26. The relevance of the response to unfolded protein stress or the sphingolipid biosynthesis, previously reported for other unrelated AMP, was also independently confirmed.

Coordination of storage lipid synthesis and membrane biogenesis: evidence for cross-talk between triacylglycerol metabolism and phosphatidylinositol synthesis

Despite the importance of triacylglycerols (TAG) and steryl esters (SE) in phospholipid synthesis in cells transitioning from stationary-phase into active growth, there is no direct evidence for their requirement in synthesis of phosphatidylinositol (PI) or other membrane phospholipids in logarithmically growing yeast cells. We report that the dga1Δlro1Δare1Δare2Δ strain, which lacks the ability to synthesize both TAG and SE, is not able to sustain normal growth in the absence of inositol (Ino(-) phenotype) at 37 °C especially when choline is present. Unlike many other strains exhibiting an Ino(-) phenotype, the dga1Δlro1Δare1Δare2Δ strain does not display a defect in INO1 expression. However, the mutant exhibits slow recovery of PI content compared with wild type cells upon reintroduction of inositol into logarithmically growing cultures. The tgl3Δtgl4Δtgl5Δ strain, which is able to synthesize TAG but unable to mobilize it, also exhibits attenuated PI formation under these conditions. However, unlike dga1Δlro1Δare1Δare2Δ, the tgl3Δtgl4Δtgl5Δ strain does not display an Ino(-) phenotype, indicating that failure to mobilize TAG is not fully responsible for the growth defect of the dga1Δlro1Δare1Δare2Δ strain in the absence of inositol. Moreover, synthesis of phospholipids, especially PI, is dramatically reduced in the dga1Δlro1Δare1Δare2Δ strain even when it is grown continuously in the presence of inositol. The mutant also utilizes a greater proportion of newly synthesized PI than wild type for the synthesis of inositol-containing sphingolipids, especially in the absence of inositol. Thus, we conclude that storage lipid synthesis actively influences membrane phospholipid metabolism in logarithmically growing cells.

Mannosylinositol phosphorylceramide is a major sphingolipid component and is required for proper localization of plasma-membrane proteins in Schizosaccharomyces pombe

In Saccharomyces cerevisiae, three classes of sphingolipids contain myo-inositol--inositol phosphorylceramide (IPC), mannosylinositol phosphorylceramide (MIPC) and mannosyldiinositol phosphorylceramide [M(IP)(2)C]. No fission yeast equivalent of Ipt1p, the inositolphosphotransferase that synthesizes M(IP)(2)C from MIPC, has been found in the Schizosaccharomyces pombe genome. Analysis of the sphingolipid composition of wild-type cells confirmed that MIPC is the terminal and most abundant complex sphingolipid in S. pombe. Three proteins (Sur1p, Csg2p and Csh1p) have been shown to be involved in the synthesis of MIPC from IPC in S. cerevisiae. The S. pombe genome has three genes (SPAC2F3.01, SPCC4F11.04c and SPAC17G8.11c) that are homologues of SUR1, termed imt1(+), imt2(+) and imt3(+), respectively. To determine whether these genes function in MIPC synthesis in S. pombe, single and multiple gene disruptants were constructed. Single imt disruptants were found to be viable. MIPC was not detected and IPC levels were increased in the triple disruptant, indicating that the three SUR1 homologues are involved in the synthesis of MIPC. GFP-tagged Imt1p, Imt2p and Imt3p localized to Golgi apparatus membranes. The MIPC-deficient mutant exhibited pleiotropic phenotypes, including defects in cellular and vacuolar morphology, and in localization of ergosterols. MIPC seemed to be required for endocytosis of a plasma-membrane-localized amino acid transporter, because sorting of the transporter from the plasma membrane to the vacuole was severely impaired in the MIPC-deficient mutant grown under nitrogen-limiting conditions. These results suggest that MIPC has multiple functions not only in the maintenance of cell and vacuole morphology but also in vesicular trafficking in fission yeast.

Roles for sphingolipids in Saccharomyces cerevisiae

Studies using Saccharomyces cerevisiae, the common baker's or brewer's yeast, have progressed over the past twenty years from knowing which sphingolipids are present in cells and a basic outline of how they are made to a complete or nearly complete directory of the genes that catalyze their anabolism and catabolism. In addition, cellular processes that depend upon sphingolipids have been identified including protein trafficking/exocytosis, endocytosis and actin cytoskeleton dynamics, membrane microdomains, calcium signaling, regulation of transcription and translation, cell cycle control, stress resistance, nutrient uptake and aging. These will be summarized here along with new data not previously reviewed. Advances in our knowledge of sphingolipids and their roles in yeast are impressive but molecular mechanisms remain elusive and are a primary challenge for further progress in understanding the specific functions of sphingolipids.

Isolation, structure and biological activity of phomafungin, a cyclic lipodepsipeptide from a widespread tropical Phoma sp

We isolated a cyclic lipodepsipeptide, phomafungin, from a Phoma sp. The distinct antifungal activity of phomafungin in the crude extract was initially discovered by mechanistic profiling in the Candida albicans fitness test. The purified compound contains a 28 member ring consisting of eight amino acids and a beta-hydroxy-gamma-methyl-hexadecanoic acid, and displays a broad spectrum of antifungal activity against Candida spp., Aspergillus fumigatus and Trichophyton mentagrophytes with MIC of 2-8 microg/ml, and toxicity to mice at 25 mg/kg. The linear peptide derived from opening of the lactone ring was devoid of antifungal activity as well as toxicity. Phomafungin has been identified in a number of Phoma spp. collected from Africa and the Indian and Pacific Ocean islands.