[39] Cerulenin

Cryptic Cerulenin Rearrangement in Ketosynthase Covalent Inhibition

The antibiotic cerulenin is a fungal natural product identified as a covalent inhibitor of ketosynthases within fatty acid and polyketide biosynthesis. Due to its selective and potent inhibitory activity, cerulenin has found significant utility in multidisciplinary biochemical, biomedical, and clinical studies. Although its covalent inhibition profile has been confirmed, cerulenin's mechanism has not been fully determined at a molecular level, frustrating the drug development of related analogues. Herein, we describe the use of stable isotopic tracking with NMR and MS methods to unravel the covalent mechanism of cerulenin against type II fatty acid ketosynthases. We detail the discovery of a unique C2-C3 retro-aldol bond cleavage and a structural rearrangement upon covalent inhibition of cerulenin at the active cysteine residue in E. coli type II fatty acid ketosynthases FabB and FabF.

Masked cerulenin enables a dual-site selective protein crosslink

Protein-reactive natural products such as the fungal metabolite cerulenin are recognized for their value as therapeutic candidates, due to their ability to selectively react with catalytic residues within a protein active site or a complex of protein domains. Here, we explore the development of fatty-acid and polyketide-synthase probes by synthetically modulating cerulenin's functional moieties. Using a mechanism-based approach, we reveal unique reactivity within cerulenin and adapt it for fluorescent labeling and crosslinking of fatty-acid and iterative type-I polyketide synthases. We also describe two new classes of silylcyanohydrin and silylhemiaminal masked crosslinking probes that serve as new tools for activity and structure studies of these biosynthetic pathways.

Arf1 coordinates fatty acid metabolism and mitochondrial homeostasis

Lipid mobilization through fatty acid β-oxidation is a central process essential for energy production during nutrient shortage. In yeast, this catabolic process starts in the peroxisome from where β-oxidation products enter mitochondria and fuel the tricarboxylic acid cycle. Little is known about the physical and metabolic cooperation between these organelles. Here we found that expression of fatty acid transporters and of the rate-limiting enzyme involved in β-oxidation is decreased in cells expressing a hyperactive mutant of the small GTPase Arf1, leading to an accumulation of fatty acids in lipid droplets. Consequently, mitochondria became fragmented and ATP synthesis decreased. Genetic and pharmacological depletion of fatty acids phenocopied the arf1 mutant mitochondrial phenotype. Although β-oxidation occurs in both mitochondria and peroxisomes in mammals, Arf1's role in fatty acid metabolism is conserved. Together, our results indicate that Arf1 integrates metabolism into energy production by regulating fatty acid storage and utilization, and presumably organelle contact sites.

Perilipin 3 promotes the formation of membrane domains enriched in diacylglycerol and lipid droplet biogenesis proteins

Lipid droplets (LDs) serve as intracellular stores of energy-rich neutral lipids. LDs form at discrete sites in the endoplasmic reticulum (ER) and they remain closely associated with the ER during lipogenic growth and lipolytic consumption. Their hydrophobic neutral lipid core is covered by a monolayer of phospholipids, which harbors a specific set of proteins. This LD surface is coated and stabilized by perilipins, a family of soluble proteins that specifically target LDs from the cytosol. We have previously used chimeric fusion proteins between perilipins and integral ER membrane proteins to test whether proteins that are anchored to the ER bilayer could be dragged onto the LD monolayer. Expression of these chimeric proteins induces repositioning of the ER membrane around LDs. Here, we test the properties of membrane-anchored perilipins in cells that lack LDs. Unexpectedly, membrane-anchored perilipins induce expansion and vesiculation of the perinuclear membrane resulting in the formation of crescent-shaped membrane domains that harbor LD-like properties. These domains are stained by LD-specific lipophilic dyes, harbor LD marker proteins, and they transform into nascent LDs upon induction of neutral lipid synthesis. These ER domains are enriched in diacylglycerol (DAG) and in ER proteins that are important for early steps of LD biogenesis, including seipin and Pex30. Formation of the domains in vivo depends on DAG levels, and we show that perilipin 3 (PLIN3) binds to liposomes containing DAG in vitro. Taken together, these observations indicate that perilipin not only serve to stabilize the surface of mature LDs but that they are likely to exert a more active role in early steps of LD biogenesis at ER subdomains enriched in DAG, seipin, and neutral lipid biosynthetic enzymes.

Chemical Highlights Supporting the Role of Lipid A in Efficient Biological Adaptation of Gram-Negative Bacteria to External Stresses

The outer membrane (OM) of Gram-negative bacteria provides an efficient barrier against external noxious compounds such as antimicrobial agents. Associated with drug target modification, it contributes to the overall failure of chemotherapy. In the complex OM architecture, Lipid A plays an essential role by anchoring the lipopolysaccharide in the membrane and ensuring the spatial organization between lipids, proteins, and sugars. Currently, the targets of almost all antibiotics are intracellularly located and require translocation across membranes. We report herein an integrated view of Lipid A synthesis, membrane assembly, a structure comparison at the molecular structure level of numerous Gram-negative bacterial species, as well as its recent use as a target for original antibacterial molecules. This review paves the way for a new vision of a key membrane component that acts during bacterial adaptation to environmental stresses and for the development of new weapons against microbial resistance to usual antibiotics.

Engineering Escherichia coli for Glutarate Production as the C5 Platform Backbone

Glutarate is a linear-chain dicarboxylic acid with wide applications in the production of polyesters and polyamides such as nylon-4,5 and nylon-5,5. Previous studies focused on the biological production of glutarate from lysine with low yields and titers. Here, we report on glutarate production by Escherichia coli using a five-step reverse adipate degradation pathway (RADP) identified in Thermobifida fusca By expressing the enzymes of RADP, the glutarate was detected by strain Bgl146 in shaken flasks. After fermentation optimization, the titer of glutarate by Bgl146 was increased to 4.7 ± 0.2 mM in shaken flasks. We further eliminated pathways for the major metabolites competing for carbon flux by CRISPR/Cas9 (ΔarcA, ΔldhA, ΔatoB, and ΔpflB). Moreover, the final strain Bgl4146 produced 36.5 ± 0.3 mM glutarate by fed-batch fermentation. These results constitute the highest glutarate titer reported in E. coliIMPORTANCE Glutarate is an important C₅ linear-chain dicarboxylic acid, which is widely used in polyesters and polyamides such as nylon-4,5 and nylon-5,5 in the chemical industry. Glutarate is currently produced from the feedstocks derived from petroleum, specifically by oxidation of a mixture of cyclohexanone and cyclohexanol catalyzed by nitric acid. However, the chemical synthesis results in high pollution and dramatic greenhouse gas emission. Thus, the biological production of glutarate directly from the substrate is of great importance. Although there have been reports using Corynebacterium glutamicum to produce glutarate, it has serious limitations due to the limited lysine supply and long fermentation time. To solve this problem, a novel synthetic pathway was constructed in this study, and the highest glutarate titer was reported in Escherichia coli using a short fermentation time without lysine addition, making bio-based glutarate production much more feasible.

The pathogen-related yeast protein Pry1, a member of the CAP protein superfamily, is a fatty acid-binding protein

Members of the CAP superfamily (cysteine-rich secretory proteins, antigen 5, and pathogenesis-related 1 proteins), also known as SCP superfamily (sperm-coating proteins), have been implicated in many physiological processes, including immune defenses, venom toxicity, and sperm maturation. Their mode of action, however, remains poorly understood. Three proteins of the CAP superfamily, Pry1, -2, and -3 (pathogen related in yeast), are encoded in the Saccharomyces cerevisiae genome. We have shown previously that Pry1 binds cholesterol in vitro and that Pry function is required for sterol secretion in yeast cells, indicating that members of this superfamily may generally bind sterols or related small hydrophobic compounds. On the other hand, tablysin-15, a CAP protein from the horsefly Tabanus yao, has been shown to bind leukotrienes and free fatty acids in vitro Therefore, here we assessed whether the yeast Pry1 protein binds fatty acids. Computational modeling and site-directed mutagenesis indicated that the mode of fatty acid binding is conserved between tablysin-15 and Pry1. Pry1 bound fatty acids with micromolar affinity in vitro, and its function was essential for fatty acid export in cells lacking the acyl-CoA synthetases Faa1 and Faa4. Fatty acid binding of Pry1 is independent of its capacity to bind sterols, and the two sterol- and fatty acid-binding sites are nonoverlapping. These results indicate that some CAP family members, such as Pry1, can bind different lipids, particularly sterols and fatty acids, at distinct binding sites, suggesting that the CAP domain may serve as a stable, secreted protein domain that can accommodate multiple ligand-binding sites.

Stalk cell differentiation without polyketides in the cellular slime mold

Polyketides induce prestalk cell differentiation in Dictyostelium. In the double-knockout mutant of the SteelyA and B polyketide synthases, most of the pstA cells-the major part of the prestalk cells-are lost, and we show by whole mount in situ hybridization that expression of prestalk genes is also reduced. Treatment of the double-knockout mutant with the PKS inhibitor cerulenin gave a further reduction, but some pstA cells still remained in the tip region, suggesting the existence of a polyketide-independent subtype of pstA cells. The double-knockout mutant and cerulenin-treated parental Ax2 cells form fruiting bodies with fragile, single-cell layered stalks after cerulenin treatment. Our results indicate that most pstA cells are induced by polyketides, but the pstA cells at the very tip of the slug are induced in some other way. In addition, a fruiting body with a single-cell layered, vacuolated stalk can form without polyketides.

High-Copy Overexpression Screening Reveals PDR5 as the Main Doxorubicin Resistance Gene in Yeast

Doxorubicin is one of the most potent anticancer drugs used in the treatment of various cancer types. The efficacy of doxorubicin is influenced by the drug resistance mechanisms and its cytotoxicity. In this study, we performed a high-copy screening analysis to find genes that play a role in doxorubicin resistance and found several genes (CUE5, AKL1, CAN1, YHR177W and PDR5) that provide resistance. Among these genes, overexpression of PDR5 provided a remarkable resistance, and deletion of it significantly rendered the tolerance level for the drug. Q-PCR analyses suggested that transcriptional regulation of these genes was not dependent on doxorubicin treatment. Additionally, we profiled the global expression pattern of cells in response to doxorubicin treatment and highlighted the genes and pathways that are important in doxorubicin tolerance/toxicity. Our results suggest that many efflux pumps and DNA metabolism genes are upregulated by the drug and required for doxorubicin tolerance.

Transcription factor Reb1p regulates DGK1-encoded diacylglycerol kinase and lipid metabolism in Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, the DGK1-encoded diacylglycerol kinase catalyzes the CTP-dependent phosphorylation of diacylglycerol to form phosphatidate. This enzyme, in conjunction with PAH1-encoded phosphatidate phosphatase, controls the levels of phosphatidate and diacylglycerol for phospholipid synthesis, membrane growth, and lipid droplet formation. In this work, we showed that a functional level of diacylglycerol kinase is regulated by the Reb1p transcription factor. In the electrophoretic mobility shift assay, purified recombinant Reb1p was shown to specifically bind its consensus recognition sequence (CGGGTAA, -166 to -160) in the DGK1 promoter. Analysis of cells expressing the PDGK1-lacZ reporter gene showed that mutations (GT→TG) in the Reb1p-binding sequence caused an 8.6-fold reduction in β-galactosidase activity. The expression of DGK1(reb1), a DGK1 allele containing the Reb1p-binding site mutation, was greatly lower than that of the wild type allele, as indicated by analyses of DGK1 mRNA, Dgk1p, and diacylglycerol kinase activity. In the presence of cerulenin, an inhibitor of de novo fatty acid synthesis, the dgk1Δ mutant expressing DGK1(reb1) exhibited a significant defect in growth as well as in the synthesis of phospholipids from triacylglycerol mobilization. Unlike DGK1, the DGK1(reb1) expressed in the dgk1Δ pah1Δ mutant did not result in the nuclear/endoplasmic reticulum membrane expansion, which occurs in cells lacking phosphatidate phosphatase activity. Taken together, these results indicate that the Reb1p-mediated regulation of diacylglycerol kinase plays a major role in its in vivo functions in lipid metabolism.

Thiolactomycin-based β-ketoacyl-AcpM synthase A (KasA) inhibitors: fragment-based inhibitor discovery using transient one-dimensional nuclear overhauser effect NMR spectroscopy

Thiolactomycin (TLM) is a natural product inhibitor of KasA, the β-ketoacyl synthase A from Mycobacterium tuberculosis. To improve the affinity of TLM for KasA, a series of TLM analogs have been synthesized based on interligand NOEs between TLM and a pantetheine analog when both are bound simultaneously to the enzyme. Kinetic binding data reveal that position 3 of the thiolactone ring is a suitable position for elaboration of the TLM scaffold, and the structure-activity relationship studies provide information on the molecular features that govern time-dependent inhibition in this enzyme system. These experiments also exemplify the utility of transient one-dimensional NOE spectroscopy for obtaining interligand NOEs compared with traditional steady state two-dimensional NOESY spectroscopy.

Dehydratase-specific probes for fatty acid and polyketide synthases

We targeted the development of a dehydratase (DH)-specific reactive probe that can facilitate detection, enrichment, and identification of DH enzymes in fatty acid synthases (FASs) and polyketide synthases (PKSs). The first reported mechanism-based inactivator, 3-decynoyl-N-acetylcysteamine (3-decynoyl-NAC), while active against the Escherichia coli β-hydroxydecanoyl thiol ester DH FabA, translates poorly to an activity-based probe because of nonspecific reactivity of the thioester moiety. Here we describe the design, synthesis, and utility of a DH-specific probe that contains a sulfonyl 3-alkyne reactive warhead engineered to avoid hydrolysis or nonenzymatic inactivation. When coupled with a fluorescent tag, this probe targets DH enzymes from recombinant type I and type II FAS and PKS enzyme systems and in whole proteomes. Activity studies, including FabA inactivation and antibiotic susceptibility, suggest that this sulfonyl 3-alkyne scaffold selectively targets a common DH mechanism. These studies indicate that the DH-specific mechanism-based probe can greatly accelerate both the functional characterization and molecular identification of virtually any type of FAS and PKS in complex proteomes.

DGK1-encoded diacylglycerol kinase activity is required for phospholipid synthesis during growth resumption from stationary phase in Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, triacylglycerol mobilization for phospholipid synthesis occurs during growth resumption from stationary phase, and this metabolism is essential in the absence of de novo fatty acid synthesis. In this work, we provide evidence that DGK1-encoded diacylglycerol kinase activity is required to convert triacylglycerol-derived diacylglycerol to phosphatidate for phospholipid synthesis. Cells lacking diacylglycerol kinase activity (e.g. dgk1Δ mutation) failed to resume growth in the presence of the fatty acid synthesis inhibitor cerulenin. Lipid analysis data showed that dgk1Δ mutant cells did not mobilize triacylglycerol for membrane phospholipid synthesis and accumulated diacylglycerol. The dgk1Δ phenotypes were partially complemented by preventing the formation of diacylglycerol by the PAH1-encoded phosphatidate phosphatase and by channeling diacylglycerol to phosphatidylcholine via the Kennedy pathway. These observations, coupled to an inhibitory effect of dioctanoyl-diacylglycerol on the growth of wild type cells, indicated that diacylglycerol kinase also functions to alleviate diacylglycerol toxicity.

Integral membrane proteins Brr6 and Apq12 link assembly of the nuclear pore complex to lipid homeostasis in the endoplasmic reticulum

Cells of Saccharomyces cerevisiae lacking Apq12, a nuclear envelope (NE)-endoplasmic reticulum (ER) integral membrane protein, are defective in assembly of nuclear pore complexes (NPCs), possibly because of defects in regulating membrane fluidity. We identified BRR6, which encodes an essential integral membrane protein of the NE-ER, as a dosage suppressor of apq12 Delta. Cells carrying the temperature-sensitive brr6-1 allele have been shown to have defects in nucleoporin localization, mRNA metabolism and nuclear transport. Electron microscopy revealed that brr6-1 cells have gross NE abnormalities and proliferation of the ER. brr6-1 cells were hypersensitive to compounds that affect membrane biophysical properties and to inhibitors of lipid biosynthetic pathways, and displayed strong genetic interactions with genes encoding non-essential lipid biosynthetic enzymes. Strikingly, brr6-1 cells accumulated, in or near the NE, elevated levels of the two classes of neutral lipids, steryl esters and triacylglycerols, and over-accumulated sterols when they were provided exogenously. Although neutral lipid synthesis is dispensable in wild-type cells, viability of brr6-1 cells was fully dependent on neutral lipid production. These data indicate that Brr6 has an essential function in regulating lipid homeostasis in the NE-ER, thereby impacting NPC formation and nucleocytoplasmic transport.

Ypk1, the yeast orthologue of the human serum- and glucocorticoid-induced kinase, is required for efficient uptake of fatty acids

Fatty acids constitute an important energy source for various tissues. The mechanisms that mediate and control uptake of free fatty acids from the circulation, however, are poorly understood. Here we show that efficient fatty-acid uptake by yeast cells requires the protein kinase Ypk1, the orthologue of the human serum- and glucocorticoid-induced kinase Sgk1. ypk1Delta mutant cells fail to grow under conditions that render cells auxotrophic for fatty acids, show a reduced uptake of radiolabelled or fluorescently labelled fatty acids, lack the facilitated component of the uptake activity, and have elevated levels of fatty acids in a bovine serum albumin (BSA) back-extractable compartment. Efficient fatty-acid uptake and/or incorporation requires the protein-kinase activity of Ypk1, because a kinase-dead point-mutant allele of YPK1 is defective in this process. This function of Ypk1 in fatty-acid uptake and/or incorporation is functionally conserved, because expression of the human Sgk1 kinase rescues ypk1Delta mutant yeast. These observations suggest that Ypk1 and possibly the human Sgk1 kinase affect fatty-acid uptake and thus energy homeostasis through regulating endocytosis. Consistent with such a proposition, mutations that block early steps of endocytosis display reduced levels of fatty-acid uptake.

Essentiality of FASII pathway for Staphylococcus aureus

Recently, Brinster et al. suggested that type II fatty-acid biosynthesis (FASII) is not a suitable antibacterial target for Gram-positive pathogens because they use fatty acids directly from host serum rather than de novo synthesis. Their findings, if confirmed, are relevant for further scientific and financial investments in the development of new drugs targeting FASII. We present here in vitro and in vivo data demonstrating that their observations do not hold for Staphylococcus aureus, a major Gram-positive pathogen causing several human infections. The observed differences among Gram-positive pathogens in FASII reflects heterogeneity either in fatty-acid synthesis or in the capacity for fatty-acid uptake from the environment.

Production of triacylglycerol and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) by the toluene-degrading bacterium Rhodococcus aetherivorans IAR1

Poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) produced by various bacteria has been intensively investigated as a promising biodegradable plastic, but required a supply of an expensive precursor as a secondary carbon source for its production. In a previous study, we identified a new bacterial strain, Rhodococcus aetherivorans IAR1, which synthesizes PHBV from toluene without the supply of a precursor. Toluene is the volatile organic compound most abundantly emitted to the environment. In the present paper, we show that R. aetherivorans IAR1 produces triacylglycerols (TAGs) simultaneously with PHBV. Both PHBV and TAGs were synthesized before the nitrogen source is completely exhausted. The cellular content of PHBV reached 10% of cell dry weight (CDW) and its synthesis ceased even during intermittent supply of toluene. However, accumulation of TAGs continued during cultivation and their cellular content reached 24% of CDW at the end of cultivation. Cerulenin inhibited TAG production and increased PHBV cellular content up to 30% of CDW. The mole fraction of 3-hydroxyvalerate (3HV) in PHBV produced from toluene increased from 60% to 80% during its accumulation. Fatty acid compositions of TAGs produced from acetate and toluene were different. At the end of cultivation, the mole fraction of C17:0, one of odd-carbon number fatty acids, was 5% on toluene or 10% on acetate while the mole fraction of 3HV in PHBV from toluene was as high as that in PHBV from acetate, suggesting that a C5 intermediate of toluene degradation might directly become a precursor of 3HV whereas propionyl-CoA is required for the incorporation of C17:0 into TAGs.

Stringent response in Vibrio cholerae: genetic analysis of spoT gene function and identification of a novel (p)ppGpp synthetase gene

RelA and SpoT of Gram-negative organisms critically regulate cellular levels of (p)ppGpp. Here, we have dissected the spoT gene function of the cholera pathogen Vibrio cholerae by extensive genetic analysis. Unlike Escherichia coli, V. choleraeDeltarelADeltaspoT cells accumulated (p)ppGpp upon fatty acid or glucose starvation. The result strongly suggests RelA-SpoT-independent (p)ppGpp synthesis in V. cholerae. By repeated subculturing of a V. choleraeDeltarelADeltaspoT mutant, a suppressor strain with (p)ppGpp(0) phenotype was isolated. Bioinformatics analysis of V. cholerae whole genome sequence allowed identification of a hypothetical gene (VC1224), which codes for a small protein (approximately 29 kDa) with a (p)ppGpp synthetase domain and the gene is highly conserved in vibrios; hence it has been named relV. Using E. coliDeltarelA or DeltarelADeltaspoT mutant we showed that relV indeed codes for a novel (p)ppGpp synthetase. Further analysis indicated that relV gene of the suppressor strain carries a point mutation at nucleotide position 676 of its coding region (DeltarelADeltaspoT relV676), which seems to be responsible for the (p)ppGpp(0) phenotype. Analysis of a V. choleraeDeltarelADeltaspoTDeltarelV triple mutant confirmed that apart from canonical relA and spoT genes, relV is a novel gene in V. cholerae responsible for (p)ppGpp synthesis.

Role and regulation of fatty acid biosynthesis in the response of Shewanella piezotolerans WP3 to different temperatures and pressures

Members of the genus Shewanella inhabit various environments; they are capable of synthesizing various types of low-melting-point fatty acids, including monounsaturated fatty acids (MUFA) and branched-chain fatty acids (BCFA) with and without eicosapentanoic acid (EPA). The genes involved in fatty acid synthesis in 15 whole-genome-sequenced Shewanella strains were identified and compared. A typical type II fatty acid synthesis pathway in Shewanella was constructed. A complete EPA synthesis gene cluster was found in all of the Shewanella genomes, although only a few of them were found to produce EPA. The roles and regulation of fatty acids synthesis in Shewanella were further elucidated in the Shewanella piezotolerans WP3 response to different temperatures and pressures. The EPA and BCFA contents of WP3 significantly increased when it was grown at low temperature and/or under high pressure. EPA, but not MUFA, was determined to be crucial for its growth at low temperature and high pressure. A gene cluster for a branched-chain amino acid ABC transporter (LIV-I) was found to be upregulated at low temperature. Combined approaches, including mutagenesis and an isotopic-tracer method, revealed that the LIV-I transporter played an important role in the regulation of BCFA synthesis in WP3. The LIV-I transporter was identified only in the cold-adapted Shewanella species and was assumed to supply an important strategy for Shewanella cold adaptation. This is the first time the molecular mechanism of BCFA regulation in bacteria has been elucidated.

In Vitro Biosynthesis of the Antitumor Agent Azinomycin B

[reaction: see text] Azinomycins have potential therapeutic value as antitumor agents; however, their biosynthesis is poorly understood. Here, we provide the first demonstration of a protein cell-free system capable of supporting complete in vitro biosynthesis of the antitumor agent azinomycin B. The cell-free system is utilized to probe the cofactor dependence and substrate requirements of the pathway en route to azinomycin.

New prestalk and prespore inducing signals in Dictyostelium

The differentiation-inducing signals (DIFs) currently known in Dictyostelium appear unable to account for the full diversity of cell types produced in development. To search for new signals, we analyzed the differentiation in monolayers of cells expressing prestalk (ecmAO, ecmA, ecmO, ecmB and cAR2) and prespore (psA) markers. Expression of each marker drops off as the cell density is reduced, suggesting that cell interaction is required. Expression of each marker is inhibited by cerulenin, an inhibitor of polyketide synthesis, and can be restored by conditioned medium. However, the known stalk-inducing polyketide, DIF-1, could not replace conditioned medium and induce the ecmA or cAR2 prestalk markers, suggesting that they require different polyketide inducers. Polyketide production by fungi is stimulated by cadmium ions, which also dramatically stimulates differentiation in Dictyostelium cell cultures and the accumulation of medium factors. Factors produced with cadmium present were extracted from conditioned medium and fractionated by HPLC. A new factor inducing prespore cell differentiation, called PSI-2, and two inducing stalk cell differentiation (DIFs 6 and 7) were resolved. All are distinct from currently identified factors. DIF-6, but not DIF-7 or PSI-2, appears to have an essential carbonyl group. Thus Dictyostelium may use extensive polyketide signaling in its development.

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

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

Futile cycling of intermediates of fatty acid biosynthesis toward peroxisomal beta-oxidation in Saccharomyces cerevisiae

The flux of fatty acids toward beta-oxidation was analyzed in Saccharomyces cerevisiae by monitoring polyhydroxyalkanoate synthesis in the peroxisome from the polymerization, by a bacterial polyhydroxyalkanoate synthase, of the beta-oxidation intermediates 3-hydroxyacyl-CoAs. Synthesis of polyhydroxyalkanoate was dependent on the beta-oxidation enzymes acyl-CoA oxidase and enoyl-CoA hydratase/3-hydroxyacyl-CoA dehydrogenase multifunctional protein, which are involved in generating 3-hydroxyacyl-CoAs, and on the peroxin PEX5, which is involved in the import of proteins into the peroxisome. In wild type cells grown in media containing fatty acids, the polyhydroxyalkanoate monomer composition was largely influenced by the nature of the external fatty acid, such that even-chain monomers are generated from oleic acid and odd-chain monomers are generated from heptadecenoic acid. In contrast, polyhydroxyalkanoate containing predominantly 3-hydroxyoctanoate, 3-hydroxydecanoate, and 3-hydroxydodecanoate was synthesized in a mutant deficient in the peroxisomal 3-ketothiolase (fox3 Delta 0) growing either on oleic acid or heptadecenoic acid as well as in wild type and fox3 Delta 0 mutants grown on glucose or raffinose, indicating that 3-hydroxyacyl-CoAs used for polyhydroxyalkanoate synthesis were generated from the degradation of intracellular short- and medium-chain fatty acids by the beta-oxidation cycle. Inhibition of fatty acid biosynthesis with cerulenin blocked the synthesis of polyhydroxyalkanoate from intracellular fatty acids but still enabled the use of extracellular fatty acids for polymer production. Mutants affected in the synthesis of lipoic acid showed normal polyhydroxyalkanoate synthesis capacity. Together, these results uncovered the existence of a substantial futile cycle whereby short- and medium-chain intermediates of the cytoplasmic fatty acid biosynthetic pathway are directed toward the peroxisomal beta-oxidation pathway.

Fatty acid synthesizing enzymes intrinsic to myelin

A recent study showing incorporation of acetyl groups from neuronal N-acetylaspartate into myelin lipids suggested the presence of fatty acid synthesizing enzymes in myelin that utilize the acetyl groups liberated by myelin-associated aspartoacylase [J. Neurochem. 78 (2001) 736]. We report here detection of the fatty acid synthase (FAS) complex and acetyl-CoA carboxylase (ACC) in purified myelin. The activity of myelin FAS was approximately half that of cytosolic FAS and, unlike the latter, required detergent for activation. Intrinsic association of FAS with myelin was indicated by failure to remove the activity with NaCl or Na-taurocholate. Myelin-associated ACC was approximately 10% of cytosolic ACC in myelin isolated by gradient centrifugation, and this was reduced by half following osmotic shock; this suggested bimodal distribution of myelin ACC, some being loosely associated within inter-lamellar cytoplasmic spaces and the remainder more firmly associated in a manner that resists NaCl/Na-taurocholate treatments. These results, in combination with earlier findings, provide a possible mechanism for the observed incorporation of neuronal NAA acetyl groups into myelin lipids.

The role of KasA and KasB in the biosynthesis of meromycolic acids and isoniazid resistance in Mycobacterium tuberculosis

Mycobacterium tuberculosis has two discrete beta-ketoacyl synthases encoded by kasA and kasB that are located in tandem within a five-gene operon that has been implicated in isoniazid-sensitivity and mycolic acid synthesis. We have developed an in vitro meromycolic acid synthase assay to elucidate the anabolic role of these enzymes. Overproduction of KasA and KasB individually and together in M. smegmatis enabled cell-free incorporation of [(14)C]malonyl-CoA into lipids whose chain length was dependent upon the M. tuberculosis elongating enzyme used. KasA specifically elongated palmitoyl-CoA to monounsaturated fatty acids that averaged 40 carbons in length. KasB hyperproduction in the presence of KasA produced longer chain multiunsaturated hydrocarbons averaging 54 carbons in length. These products comigrated with a synthetic standard of meromycolic acid and their production was sensitive to isoniazid, thiolactomycin, and triclosan. KasA mutations associated with isoniazid resistance produced an enzyme that had a diminished overall catalytic activity but conferred enhanced resistance to isoniazid. In vivo analysis confirmed that overexpression of each of the four mutant KasAs enhanced isoniazid resistance when compared to overexpression of wild-type KasA. These results suggest discrete anabolic roles for both KasA and KasB in mycolic acid synthesis and substantiate the involvement of KasA mutations in isoniazid resistance.

Structure and regulation of the omega-3 polyunsaturated fatty acid synthase genes from the deep-sea bacterium Photobacterium profundum strain SS9

Omega-3 polyunsaturated fatty acids (PUFAs) such as eicosapentaenoic acid (20:5n-3; EPA) and docosahexaenoic acid (22:6n-3; DHA) have been shown to be of major importance in the promotion of cardiovascular health, proper human development and the prevention of some cancers. A high proportion of bacterial isolates from low-temperature and high-pressure marine environments produce EPA or DHA. This paper presents the sequence of a 33 kbp locus from the deep-sea bacterium Photobacterium profundum strain SS9 which includes four of the five genes required for EPA biosynthesis. As with other bacterial pfa (polyunsaturated fatty acid) genes, the deduced amino acid sequences encoded by the SS9 genes reveal large multidomain proteins that are likely to catalyse EPA biosynthesis by a novel polyketide synthesis mechanism. RNase protection experiments separated the SS9 pfa genes into two transcriptional units, pfaA-C and pfaD. The pfaA transcriptional start site was identified. Cultivation at elevated hydrostatic pressure or reduced temperature did not increase pfa gene expression despite the resulting increase in percentage composition of EPA under these conditions. However, a regulatory mutant was characterized which showed both increased expression of pfaA-D and elevated EPA percentage composition. This result suggests that a regulatory factor exists which coordinates pfaA-D transcription. Additional consideration regarding the activities required for PUFA synthesis is provided together with comparative analyses of bacterial pfa genes and gene products.

A missense mutation in the fabB (beta-ketoacyl-acyl carrier protein synthase I) gene confers tiolactomycin resistance to Escherichia coli

Thiolactomycin (TLM) is an antibiotic that inhibits bacterial type II fatty acid synthesis at the condensing enzyme step, and beta-ketoacyl-acyl carrier protein synthase I (FabB) is the relevant target in Escherichia coli. TLM resistance is associated with the upregulation of efflux pumps. Therefore, a tolC knockout mutant (strain ANS1) was constructed to eliminate the contribution of type I secretion systems to TLM resistance. Six independent TLM-resistant clones of strain ANS1 were isolated, and all possessed the same missense mutation in the fabB gene (T1168G) that directed the expression of a mutant protein, FabB(F390V). FabB(F390V) was resistant to TLM in vitro. Leucine is the only other amino acid found at position 390 in nature, and the Staphylococcus aureus FabF protein, which contains this substitution, was sensitive to TLM. Structural modeling predicted that the CG2 methyl group of the valine side chain interfered with the positioning of the C11 methyl on the isoprenoid side chain of TLM in the binary complex, whereas the absence of a bulky methyl group on the leucine side chain permitted TLM binding. These data illustrate that missense mutations that introduce valine at position 390 confer TLM resistance while maintaining the vital catalytic properties of FabB.

Effects of pyrazinamide on fatty acid synthesis by whole mycobacterial cells and purified fatty acid synthase I

The effects of low extracellular pH and intracellular accumulation of weak organic acids were compared with respect to fatty acid synthesis by whole cells of Mycobacterium tuberculosis and Mycobacterium smegmatis. The profile of fatty acids synthesized during exposure to benzoic, nicotinic, or pyrazinoic acids, as well as that observed during intracellular hydrolysis of the corresponding amides, was not a direct consequence of modulation of fatty acid synthesis by these compounds but reflected the response to inorganic acid stress. Analysis of fatty acid synthesis in crude mycobacterial cell extracts demonstrated that pyrazinoic acid failed to directly modulate the fatty acid synthase activity catalyzed by fatty acid synthase I (FAS-I). However, fatty acid synthesis was irreversibly inhibited by 5-chloro-pyrazinamide in a time-dependent fashion. Moreover, we demonstrate that pyrazinoic acid does not inhibit purified mycobacterial FAS-I, suggesting that this enzyme is not the immediate target of pyrazinamide.

Role of spoT-dependent ppGpp accumulation in the survival of light-exposed starved bacteria

In bacteria, cytoplasmic levels of the effector nucleotide ppGpp are regulated in response to changes in growth conditions. This study describes the involvement of SpoT-mediated ppGpp accumulation in the survival of light-exposed bacteria during fatty acid starvation. In contrast to isogenic wild-type strains and relA mutants, the 'Vibrio angustum' S14 spoT and Escherichia coli relA spoT mutants displayed significant losses in viability in response to cerulenin-induced fatty acid starvation under cool-white fluorescent light. However, when starvation experiments were performed in complete darkness, or under light filtered through a UV-resistant perspex sheet, only a minor decline in viability was observed for the wild-type and mutant strains. This finding indicated that the lethal effect was mediated by weak UV emission. In contrast to the E. coli relA spoT mutant, which lacks ppGpp, the 'V. angustum' S14 spoT mutant exhibited higher ppGpp levels and lower RNA synthesis rates during fatty acid starvation, features that might be correlated with its lethality. In agreement with this finding, fatty acid starvation lethality also occurred upon induction of ppGpp overaccumulation in E. coli. These data suggest that the precise regulation of ppGpp levels in the stressed cell is crucial, and that both the absence and the overaccumulation of ppGpp impair fatty acid starvation survival of light-exposed cells. Moreover, the UV-induced lethal effect during fatty acid starvation was also observed for E. coli strains mutated in rpoS and dps, which, in the wild-type, are regulated directly or indirectly by ppGpp, respectively. The restoration of viability of fatty-acid-starved spoT mutant cells through the addition of exogenous catalase suggested that the observed light-dependent lethal effect was, at least in part, caused by UV-imposed oxidative stress. Based on these results, it is proposed that fatty acid starvation adaptation of light-exposed bacterial cells depends on the development of resistance to UV-induced oxidative stress. This stress resistance was found to require appropriate ppGpp levels, ppGpp-induced RpoS expression and, hence, upregulation of RpoS-regulated stress-defending genes, such as dps.

Regulation of catalytic activity of a multifunctional polyketide biosynthetic enzyme, 6-hydroxymellein synthase, by interaction between NADPH and phenylglyoxal-sensitive amino acid residue at the reaction center

Treatment of 6-hydroxymellein synthase, a multifunctional polyketide biosynthetic enzyme in carrot cells, with phenylglyoxal yielded a chemically modified protein in which approximately two moles of the reagent were covalently attached to each subunit of the enzyme. Only NADH- but not NADPH-associated form of native 6-hydroxymellein synthase was inhibited by cerulenin; however, the NADPH-synthase complex lost the insensitivity by the chemical modification of the enzyme protein with phenylglyoxal. Appreciable differences in K(m) values observed between the NADPH- and NADH-associated enzymes were greatly reduced by the treatment with phenylglyoxal. Although the catalytic activity of the NADPH-associated synthase was enhanced by the addition of free CoA, the compound exhibited a significant inhibitory activity to the phenylglyoxal-modified enzyme. A marked deuterium isotope effect in the catalytic reaction of the native synthase-NADPH complex was appreciably decreased in the chemically modified enzyme. These results strongly suggest that an electrostatic interaction between the phosphate group attached to the 2'-position of adenosyl moiety of NADPH and the phenylglyoxal-sensitive amino acid residue, probably arginine, at the reaction center of 6-hydroxymellein synthase regulates several biochemical properties of this multifunctional enzyme.

Bacterial fatty-acid biosynthesis: a genomics-driven target for antibacterial drug discovery

In this review we demonstrate how the interplay of genomics, bioinformatics and genomic technologies has enabled an in-depth analysis of the component enzymes of the bacterial fatty-acid biosynthesis pathway as a source of novel antibacterial targets. This evaluation has revealed that many of the enzymes are potentially selective, broad-spectrum antibacterial targets. We also illustrate the suitability of some of these targets for HTS. Furthermore, we discuss how the availability of a robust selectivity assay, mode-of-action assays and numerous crystal structures provide an excellent set of tools with which to initiate integrated programs of research to identify novel antibiotics targeted at these enzymes.

Response of Bacillus subtilis to cerulenin and acquisition of resistance

Cerulenin is a fungal mycotoxin that potently inhibits fatty acid synthesis by covalent modification of the active site thiol of the chain-elongation subtypes of beta-ketoacyl-acyl carrier protein (ACP) synthases. The Bacillus subtilis fabF (yjaY) gene (fabF(b)) encodes an enzyme that catalyzes the condensation of malonyl-ACP with acyl-ACP to extend the growing acyl chain by two carbons. There were two mechanisms by which B. subtilis adapted to exposure to this antibiotic. First, reporter gene analysis demonstrated that transcription of the operon containing the fabF gene increased eightfold in response to a cerulenin challenge. This response was selective for the inhibition of fatty acid synthesis, since triclosan, an inhibitor of enoyl-ACP reductase, triggered an increase in fabF reporter gene expression while nalidixic acid did not. Second, spontaneous mutants arose that exhibited a 10-fold increase in the MIC of cerulenin. The mutation mapped at the B. subtilis fabF locus, and sequence analysis of the mutant fabF allele showed that a single base change resulted in the synthesis of FabF(b)[I108F]. The purified FabF(b) and FabF(b)[I108F] proteins had similar specific activities with myristoyl-ACP as the substrate. FabF(b) exhibited a 50% inhibitory concentration (IC(50)) of cerulenin of 0.1 microM, whereas the IC(50) for FabF(b)[I108] was 50-fold higher (5 microM). These biochemical data explain the absence of an overt growth defect coupled with the cerulenin resistance phenotype of the mutant strain.

In vitro activity of a novel antimycobacterial compound, N-octanesulfonylacetamide, and its effects on lipid and mycolic acid synthesis

beta-Sulfonyl carboxamides have been proposed to serve as transition-state analogues of the beta-ketoacyl synthase reaction involved in fatty acid elongation. We tested the efficacy of N-octanesulfonylacetamide (OSA) as an inhibitor of fatty acid and mycolic acid biosynthesis in mycobacteria. Using the BACTEC radiometric growth system, we observed that OSA inhibits the growth of several species of slow-growing mycobacteria, including Mycobacterium tuberculosis (H37Rv and clinical isolates), the Mycobacterium avium complex (MAC), Mycobacterium bovis BCG, Mycobacterium kansasii, and others. Nearly all species and strains tested, including isoniazid and multidrug resistant isolates of M. tuberculosis, were susceptible to OSA, with MICs ranging from 6.25 to 12.5 microg/ml. Only three clinical isolates of M. tuberculosis (CSU93, OT2724, and 401296), MAC, and Mycobacterium paratuberculosis required an OSA MIC higher than 25.0 microg/ml. Rapid-growing mycobacterial species, such as Mycobacterium smegmatis, Mycobacterium fortuitum, and others, were not susceptible at concentrations of up to 100 microg/ml. A 2-dimensional thin-layer chromatography system showed that OSA treatment resulted in a significant decrease in all species of mycolic acids present in BCG. In contrast, mycolic acids in M. smegmatis were relatively unaffected following exposure to OSA. Other lipids, including polar and nonpolar extractable classes, were unchanged following exposure to OSA in both BCG and M. smegmatis. Transmission electron microscopy of OSA-treated BCG cells revealed a disruption in cell wall synthesis and incomplete septum formation. Our results indicate that OSA inhibits the growth of several species of mycobacteria, including both isoniazid-resistant and multidrug resistant strains of M. tuberculosis. This inhibition may be the result of OSA-mediated effects on mycolic acid synthesis in slow-growing mycobacteria or inhibition via an undescribed mechanism. Our results indicate that OSA may serve as a promising lead compound for future antituberculous drug development.

Inhibition of beta-ketoacyl-acyl carrier protein synthases by thiolactomycin and cerulenin. Structure and mechanism

The beta-ketoacyl-acyl carrier protein (ACP) synthases are key regulators of type II fatty acid synthesis and are the targets for two natural products, thiolactomycin (TLM) and cerulenin. The high resolution structures of the FabB-TLM and FabB-cerulenin binary complexes were determined. TLM mimics malonyl-ACP in the FabB active site. It forms strong hydrogen bond interactions with the two catalytic histidines, and the unsaturated alkyl side chain interaction with a small hydrophobic pocket is stabilized by pi stacking interactions. Cerulenin binding mimics the condensation transition state. The subtle differences between the FabB-cerulenin and FabF-cerulenin (Moche, M., Schneider, G., Edwards, P., Dehesh, K., and Lindqvist, Y. (1999) J. Biol. Chem. 244, 6031-6034) structures explain the differences in the sensitivity of the two enzymes to the antibiotic and may reflect the distinct substrate specificities that differentiate the two enzymes. The FabB[H333N] protein was prepared to convert the FabB His-His-Cys active site triad into the FabH His-Asn-Cys configuration to test the importance of the two His residues in TLM and cerulenin binding. FabB[H333N] was significantly more resistant to both antibiotics than FabB and had an affinity for TLM an order of magnitude less than the wild-type enzyme, illustrating that the two-histidine active site architecture is critical to protein-antibiotic interaction. These data provide a structural framework for understanding antibiotic sensitivity within this group of enzymes.

Involvement of fatty acid synthase in axonal development in mouse embryos

BACKGROUND

Dynamic protein palmitoylation of signalling proteins is thought to be an important step in the regulation of signal transduction in eukaryotic cells. The enzyme responsible for protein palmitoylation in vertebrates, however, has not been identified. I have previously reported that p260/270, which is expressed in embryos of the silkworm Bombyx mori, has protein palmitoylase activity.

RESULTS

A homologue of Bombyx p260/270, mouse fatty-acid synthase (FAS), was shown to be expressed specifically throughout most of the central and peripheral nervous system in mouse embryos. Mouse FAS was expressed specifically in cultured primary neurones in which growth-associated protein (GAP)-43 was expressed. GAP-43, by protein palmitoylation, regulates Go signal transduction and neural axonal growth. In a cell-free system, purified FAS from mouse embryos transferred palmitate to GAP-43 through cysteine residues. Furthermore, cerulenin, an inhibitor of FAS, reduced axonal growth and in vivo palmitoylation of GAP-43 in cultured neurones.

CONCLUSIONS

Mouse FAS was hypothesized to be responsible for the palmitoylation of GAP-43 and subsequent regulation of axonal growth in mouse embryos.

Pyrazinamide inhibits the eukaryotic-like fatty acid synthetase I (FASI) of Mycobacterium tuberculosis

Tuberculosis treatment is shortened to six months by the indispensable addition of pyrazinamide (PZA) to the drug regimen that includes isoniazid and rifampin. PZA is a pro-drug of pyrazinoic acid (POA) (ref. 3), whose target of action has never been identified. Although PZA is active only against Mycobacterium tuberculosis, the PZA analog 5-chloro-pyrazinamide (5-Cl-PZA) displays a broader range of anti-mycobacterial activity. We have found that the eukaryotic-like fas1 gene (encoding fatty acid synthetase I, FASI) from M. avium, M. bovis BCG or M. tuberculosis confers resistance to 5-Cl-PZA when present on multi-copy vectors in M. smegmatis. 5-Cl-PZA and PZA markedly inhibited the activity of M. tuberculosis FASI, the biosynthesis of C16 to C24/C26 fatty acids from acetyl-CoA (ref. 6). Importantly, PZA inhibited FASI in M. tuberculosis in correlation with PZA susceptibility. These results indicate that FASI is a primary target of action for PZA in M. tuberculosis. Further characterization of FASI as a drug target for PZA may allow the development of new drugs to shorten the therapy against M. tuberculosis and may provide more options for treatment against M. bovis, M. avium and drug resistant M. tuberculosis.

Monounsaturated but not polyunsaturated fatty acids are required for growth of the deep-sea bacterium Photobacterium profundum SS9 at high pressure and low temperature

There is considerable evidence correlating the production of increased proportions of membrane unsaturated fatty acids (UFAs) with bacterial growth at low temperatures or high pressures. In order to assess the importance of UFAs to microbial growth under these conditions, the effects of conditions altering UFA levels in the psychrotolerant piezophilic deep-sea bacterium Photobacterium profundum SS9 were investigated. The fatty acids produced by P. profundum SS9 grown at various temperatures and pressures were characterized, and differences in fatty acid composition as a function of phase growth, and between inner and outer membranes, were noted. P. profundum SS9 was found to exhibit enhanced proportions of both monounsaturated (MUFAs) and polyunsaturated (PUFAs) fatty acids when grown at a decreased temperature or elevated pressure. Treatment of cells with cerulenin inhibited MUFA but not PUFA synthesis and led to a decreased growth rate and yield at low temperature and high pressure. In addition, oleic acid-auxotrophic mutants were isolated. One of these mutants, strain EA3, was deficient in the production of MUFAs and was both low-temperature sensitive and high-pressure sensitive in the absence of exogenous 18:1 fatty acid. Another mutant, strain EA2, produced little MUFA but elevated levels of the PUFA species eicosapentaenoic acid (EPA; 20:5n-3). This mutant grew slowly but was not low-temperature sensitive or high-pressure sensitive. Finally, reverse genetics was employed to construct a mutant unable to produce EPA. This mutant, strain EA10, was also not low-temperature sensitive or high-pressure sensitive. The significance of these results to the understanding of the role of UFAs in growth under low-temperature or high-pressure conditions is discussed.

The biosynthesis of differentiation-inducing factor, a chlorinated signal molecule regulating Dictyostelium development

Differentiation-inducing factor (DIF)-1 is a chlorinated alkyl phenone released by developing Dictyostelium amoebae, which induces them to differentiate into stalk cells. A biosynthetic pathway for DIF-1 is proposed from labeling, inhibitor, and enzymological experiments. Cells incorporate 36Cl- into DIF-1 during development, showing that the chlorine atoms originate from chloride ions; peak incorporation is at the first finger stage. DIF-1 synthesis can be blocked by cerulenin, a polyketide synthase inhibitor, suggesting that it is made from a polyketide. This is most likely the C12 polyketide (2,4,6-trihydroxyphenyl)-1-hexan-1-one (THPH). Feeding experiments confirm that living cells can convert THPH to DIF-1. Conversion requires both chlorination and methylation of THPH, and enzymatic activities able to do this exist in cell lysates. The chlorinating activity, assayed using 36Cl-, is stimulated by H2O2 and requires both soluble and particulate components. It is specific for THPH and does not use this compound after O-methylation. The methyltransferase is soluble, uses S-adenosyl-L-methionine as a co-substrate, has a Km for dichloro-THPH of about 1.1 microM, and strongly prefers this substrate to close analogues. Both chlorinating and methyltransferase activities increase in development in parallel with DIF-1 production, and both are greatly reduced in a mutant strain that makes little DIF-1. It is proposed that DIF-1 is made by the initial assembly of a C12 polyketide skeleton, which is then chlorinated and methylated.

Role of precursor translocation in coordination of murein and phospholipid synthesis in Escherichia coli

Inhibition of phospholipid synthesis in Escherichia coli by either cerulenin treatment or glycerol starvation of a glycerol-auxotrophic mutant resulted in a concomitant block of murein synthesis. The intracellular pool of cytoplasmic and lipid-linked murein precursors was not affected by an inhibition of phospholipid synthesis, nor was the activity of the penicillin-binding proteins. In addition, a decrease in the activity of the two lipoprotein murein hydrolases, the lytic transglycosylases A and B, could not be demonstrated. The indirect inhibition of murein synthesis by cerulenin resulted in a 68% decrease of trimeric muropeptide structures, proposed to represent the attachment points of newly added murein. Importantly, inhibition of phospholipid synthesis also inhibited O-antigen synthesis with a sensitivity and kinetics similar to those of murein synthesis. It is concluded that the step common for murein and O-antigen synthesis, the translocation of the respective bactoprenolphosphate-linked precursor molecules, is affected by an inhibition of phospholipid synthesis. Consistent with this assumption, it was shown that murein synthesis no longer depends on ongoing phospholipid synthesis in ether-permeabilized cells. We propose that the assembly of a murein-synthesizing machinery, a multienzyme complex consisting of murein hydrolases and synthases, at specific sites of the membrane, where integral membrane proteins such as RodA and FtsW facilitate the translocation of the lipid-linked murein precursors to the periplasm, depends on ongoing phospholipid synthesis. This would explain the well-known phenomenon that both murein synthesis and antibiotic-induced autolysis depend on phospholipid synthesis and thereby indirectly on the stringent control.

Inhibitory effects of cerulenin on protein palmitoylation and insulin internalization in rat adipocytes

Protein acylation by long-chain fatty acids has been suggested as a necessary step in membrane trafficking. Because several insulin effects are dependent upon membrane trafficking, the cellular effects of the protein acylation inhibitor cerulenin were examined. Cerulenin blocked palmitoylation of selected rat adipocyte proteins including CD36, the dominant marker for palmitoylation in adipocytes. To measure cerulenin's effects on insulin internalization, rat adipocytes were incubated with 125I-insulin at 37 degrees C in the presence or absence of cerulenin. Surface-bound and intracellular insulin were discriminated by the sensitivity of the former to rapid dissociation by a pH 3 buffer at 4 degrees C. Insulin internalization was inhibited 85% by 0.3 mM cerulenin. Inhibition required preincubation with the agent, was irreversible, was not dependent upon protein synthesis, and was not the result of ATP depletion. Cerulenin was also found to inhibit insulin-stimulated glucose uptake and acetyl-CoA carboxylase activity. Cerulenin had no effect on basal glucose uptake and utilization or on the uptake and retention of fatty acids. In summary, protein acylation may be an important step in insulin-regulated cellular functions dependent upon membrane trafficking, such as insulin internalization.

Analysis and expression of the Candida albicans FAS2 gene

Candida albicans (Ca) FAS2, the gene encoding the alpha-subunit of fatty acid synthase (FAS), has been isolated and characterized. Saccharomyces cerevisiae (Sc) FAS2 was used as a probe to screen genomic libraries of Ca, strain 4918. Clones were obtained that contained all but the first 1 kb of the gene. The 5' end, as well as upstream sequences, were subsequently isolated by PCR. The Ca FAS2 gene is comprised of an open reading frame (ORF) of 5655 bp that is free of introns and encodes a 207, 587-Da protein of 1885 amino acids. The Ca FAS2 gene and the encoded polypeptide exhibit 70 and 67% homology with the Sc FAS2 gene and protein, respectively. The gene specifies a single transcript of approx. 6 kb, and transcript levels are regulated independently of morphogenesis. Chromosome analysis localized the gene to chromosome 3. In addition, no differences were noted when comparing the FAS2 sequence, that encompasses the cerulenin-binding domain of FAS, between strain 4918 and two derived cerulenin-resistant (CerR) mutants. Thus, alteration within this region is not responsible for the increased CerR of the mutant strains.