Inositol phosphorylceramide synthase is located in the Golgi apparatus of Saccharomyces cerevisiae

Local control of phosphatidylinositol 4-phosphate signaling in the Golgi apparatus by Vps74 and Sac1 phosphoinositide phosphatase

Signaling by phosphatidylinositol 4-kinases (PI4Ks) in the Golgi apparatus controls lipid homeostasis and protein-sorting pathways. Signaling is shown to be terminated on the medial cisterna by a complex of a PI4K effector, Vps74, and Sac1, the major PtdIns4P phosphatase in the cell.

In the Golgi apparatus, lipid homeostasis pathways are coordinated with the biogenesis of cargo transport vesicles by phosphatidylinositol 4-kinases (PI4Ks) that produce phosphatidylinositol 4-phosphate (PtdIns4P), a signaling molecule that is recognized by downstream effector proteins. Quantitative analysis of the intra-Golgi distribution of a PtdIns4P reporter protein confirms that PtdIns4P is enriched on the trans-Golgi cisterna, but surprisingly, Vps74 (the orthologue of human GOLPH3), a PI4K effector required to maintain residence of a subset of Golgi proteins, is distributed with the opposite polarity, being most abundant on cis and medial cisternae. Vps74 binds directly to the catalytic domain of Sac1 (K_D = 3.8 μM), the major PtdIns4P phosphatase in the cell, and PtdIns4P is elevated on medial Golgi cisternae in cells lacking Vps74 or Sac1, suggesting that Vps74 is a sensor of PtdIns4P level on medial Golgi cisternae that directs Sac1-mediated dephosphosphorylation of this pool of PtdIns4P. Consistent with the established role of Sac1 in the regulation of sphingolipid biosynthesis, complex sphingolipid homeostasis is perturbed in vps74Δ cells. Mutant cells lacking complex sphingolipid biosynthetic enzymes fail to properly maintain residence of a medial Golgi enzyme, and cells lacking Vps74 depend critically on complex sphingolipid biosynthesis for growth. The results establish additive roles of Vps74-mediated and sphingolipid-dependent sorting of Golgi residents.

Amino acid determinants of substrate selectivity in the Trypanosoma brucei sphingolipid synthase family

The substrate selectivity of four Trypanosoma brucei sphingolipid synthases was examined. TbSLS1, an inositol phosphorylceramide (IPC) synthase, and TbSLS4, a bifunctional sphingomyelin (SM)/ethanolamine phosphorylceramide (EPC) synthase, were inactivated by Ala substitutions of a conserved triad of residues His210, His253, and Asp257 thought to form part of the active site. TbSLS4 also catalyzed the reverse reaction, production of ceramide from sphingomyelin, but none of the Ala substitutions of the catalytic triad in TbSLS4 were able to do so. Site-directed mutagenesis identified residues proximal to the conserved triad that were responsible for the discrimination between charge and size of the different head groups. For discrimination between anionic (phosphoinositol) and zwitterionic (phosphocholine, phosphoethanolamine) head groups, doubly mutated V172D/S252F TbSLS1 and D172V/F252S TbSLS3 showed reciprocal conversion between IPC and bifunctional SM/EPC synthases. For differentiation of zwitterionic headgroup size, N170A TbSLS1 and A170N/N187D TbSLS4 showed reciprocal conversion between EPC and bifunctional SM/EPC synthases. These studies provide a mapping of the SLS active site and demonstrate that differences in catalytic specificity of the T. brucei enzyme family are controlled by natural variations in as few as three residue positions.

Role for Golgi reassembly and stacking protein (GRASP) in polysaccharide secretion and fungal virulence

Secretion of virulence factors is a critical mechanism for the establishment of cryptococcosis, a disease caused by the yeast pathogen Cryptococcus neoformans. One key virulence strategy of C. neoformans is the release of glucuronoxylomannan (GXM), a capsule-associated immune-modulatory polysaccharide that reaches the extracellular space through secretory vesicles. Golgi reassembly and stacking protein (GRASP) is required for unconventional protein secretion mechanisms in different eukaryotic cells, but its role in polysaccharide secretion is unknown. This study demonstrates that a C. neoformans functional mutant of a GRASP orthologue had attenuated virulence in an animal model of cryptococcosis, in comparison with wild-type (WT) and reconstituted cells. Mutant cells manifested altered Golgi morphology, failed to produce typical polysaccharide capsules and showed a reduced ability to secrete GXM both in vitro and during animal infection. Isolation of GXM from cultures of WT, reconstituted or mutant strains revealed that the GRASP orthologue mutant produced polysaccharides with reduced dimensions. The mutant was also more efficiently associated to and killed by macrophages than WT and reconstituted cells. These results demonstrate that GRASP, a protein involved in unconventional protein secretion, is also required for polysaccharide secretion and virulence in C. neoformans.

The Sphingolipid Biosynthetic Pathway Is a Potential Target for Chemotherapy against Chagas Disease

The protozoan parasite Trypanosoma cruzi is the causative agent of human Chagas disease, for which there currently is no cure. The life cycle of T. cruzi is complex, including an extracellular phase in the triatomine insect vector and an obligatory intracellular stage inside the vertebrate host. These phases depend on a variety of surface glycosylphosphatidylinositol-(GPI-) anchored glycoconjugates that are synthesized by the parasite. Therefore, the surface expression of GPI-anchored components and the biosynthetic pathways of GPI anchors are attractive targets for new therapies for Chagas disease. We identified new drug targets for chemotherapy by taking the available genome sequence information and searching for differences in the sphingolipid biosynthetic pathways (SBPs) of mammals and T. cruzi. In this paper, we discuss the major steps of the SBP in mammals, yeast and T. cruzi, focusing on the IPC synthase and ceramide remodeling of T. cruzi as potential therapeutic targets for Chagas disease.

A comprehensive comparison of transmembrane domains reveals organelle-specific properties

The various membranes of eukaryotic cells differ in composition, but it is at present unclear if this results in differences in physical properties. The sequences of transmembrane domains (TMDs) of integral membrane proteins should reflect the physical properties of the bilayers in which they reside. We used large datasets from both fungi and vertebrates to perform a comprehensive comparison of the TMDs of proteins from different organelles. We find that TMDs are not generic but have organelle-specific properties with a dichotomy in TMD length between the early and late parts of the secretory pathway. In addition, TMDs from post-ER organelles show striking asymmetries in amino acid compositions across the bilayer that is linked to residue size and varies between organelles. The pervasive presence of organelle-specific features among the TMDs of a particular organelle has implications for TMD prediction, regulation of protein activity by location, and sorting of proteins and lipids in the secretory pathway.

The essential neutral sphingomyelinase is involved in the trafficking of the variant surface glycoprotein in the bloodstream form of Trypanosoma brucei

Sphingomyelin is the main sphingolipid in Trypanosoma brucei, the causative agent of African sleeping sickness. In vitro and in vivo characterization of the T. brucei neutral sphingomyelinase demonstrates that it is directly involved in sphingomyelin catabolism. Gene knockout studies in the bloodstream form of the parasite indicate that the neutral sphingomyelinase is essential for growth and survival, thus highlighting that the de novo biosynthesis of ceramide is unable to compensate for the loss of sphingomyelin catabolism. The phenotype of the conditional knockout has given new insights into the highly active endocytic and exocytic pathways in the bloodstream form of T. brucei. Hence, the formation of ceramide in the endoplasmic reticulum affects post-Golgi sorting and rate of deposition of newly synthesized GPI-anchored variant surface glycoprotein on the cell surface. This directly influences the corresponding rate of endocytosis, via the recycling endosomes, of pre-existing cell surface variant surface glycoprotein. The trypanosomes use this coupled endocytic and exocytic mechanism to maintain the cell density of its crucial variant surface glycoprotein protective coat. TbnSMase is therefore genetically validated as a drug target against African trypanosomes, and suggests that interfering with the endocytic transport of variant surface glycoprotein is a highly desirable strategy for drug development against African trypanosomasis.

Lipid metabolism in Trypanosoma brucei

Trypanosoma brucei membranes consist of all major eukaryotic glycerophospholipid and sphingolipid classes. These are de novo synthesized from precursors obtained either from the host or from catabolised endocytosed lipids. In recent years, substantial progress has been made in the molecular and biochemical characterisation of several of these lipid biosynthetic pathways, using gene knockout or RNA interference strategies or by enzymatic characterization of individual reactions. Together with the completed genome, these studies have highlighted several possible differences between mammalian and trypanosome lipid biosynthesis that could be exploited for the development of drugs against the diseases caused by these parasites.

Characterization of membrane properties of inositol phosphorylceramide

Inositol phosphorylceramides (IPCs) are a class of anionic sphingolipids with a single inositol-phosphate head group coupled to ceramide. IPCs and more complex glycosylated IPCs have been identified in fungi, plants and protozoa but not in mammals. IPCs have also been identified in detergent resistant membranes in several organisms. Here we report on the membrane properties of the saturated N-palmitoyl-IPC (P-IPC) in one component bilayers as well as in complex bilayers together with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine and cholesterol. The membrane properties of P-IPC were shown to be affected by calcium. According to anisotropy changes reported by DPH, the gel-to-liquid transition temperature (T(m)) of P-IPC was 48 degrees C. Addition of 5 mM CaCl(2) during vesicle preparation markedly increased the T(m) (65 degrees C). According to fluorescence quenching experiments in complex lipid mixtures, P-IPC formed sterol containing domains in an otherwise fluid environment. The P-IPC containing domains melted at a lower temperature and appeared to contain less sterol as compared to domains containing N-palmitoyl-sphingomyelin. Calcium further reduced the sterol content of the ordered domains and also increased the thermal stability of the domains. Calcium also induced vesicle aggregation of unilamellar vesicles containing P-IPC, as was observed by 4D confocal microscopy and dynamic light scattering. We believe that IPCs and the calcium induced effects could be important in numerous membrane associated cellular processes such as membrane fusion and in membrane raft linked processes.

Kei1: a novel subunit of inositolphosphorylceramide synthase, essential for its enzyme activity and Golgi localization

Fungal sphingolipids have inositol-phosphate head groups, which are essential for the viability of cells. These head groups are added by inositol phosphorylceramide (IPC) synthase, and AUR1 has been thought to encode this enzyme. Here, we show that an essential protein encoded by KEI1 is a novel subunit of IPC synthase of Saccharomyces cerevisiae. We find that Kei1 is localized in the medial-Golgi and that Kei1 is cleaved by Kex2, a late Golgi processing endopeptidase; therefore, it recycles between the medial- and late Golgi compartments. The growth defect of kei1-1, a temperature-sensitive mutant, is effectively suppressed by the overexpression of AUR1, and Aur1 and Kei1 proteins form a complex in vivo. The kei1-1 mutant is hypersensitive to aureobasidin A, a specific inhibitor of IPC synthesis, and the IPC synthase activity in the mutant membranes is thermolabile. A part of Aur1 is missorted to the vacuole in kei1-1 cells. We show that the amino acid substitution in kei1-1 causes release of Kei1 during immunoprecipitation of Aur1 and that Aur1 without Kei1 has hardly detectable IPC synthase activity. From these results, we conclude that Kei1 is essential for both the activity and the Golgi localization of IPC synthase.

Mechanisms of transport through the Golgi complex

The Golgi complex is the central sorting and processing station of the secretory pathway, ensuring that cargo proteins, which are synthesized in the endoplasmic reticulum, are properly glycosylated and packaged into carriers for transport to their final destinations. Two recent studies highlight the fact that properties of membrane lipids play key roles in Golgi structural organization and trafficking. The Antonny laboratory has demonstrated the mechanism by which a Golgi tether containing a membrane-curvature-sensing domain at one end can link highly curved and flat membranes together in a reversible manner. In this way, a strong interaction that binds membranes together in an oriented fashion can easily be disrupted as the properties of the membranes change. The Lippincott-Schwartz laboratory has developed a new model for intra-Golgi trafficking, called the rapid-partitioning model, which incorporates lipid trafficking as an integral part. Simulations reveal that the sorting of lipids into processing and export domains that are connected to each Golgi cisterna, and bidirectional trafficking throughout the Golgi to allow proteins to associate with their preferred lipid environment, is sufficient to drive protein transport through the secretory pathway. Although only a proof in principle, this model for the first time invokes lipid sorting as the driving force in intra-Golgi trafficking, and provides a framework for future experimental work.

Modulation of sphingolipid metabolism by the phosphatidylinositol-4-phosphate phosphatase Sac1p through regulation of phosphatidylinositol in Saccharomyces cerevisiae

Sphingolipids and phosphoinositides both play signaling roles in Saccharomyces cerevisiae. Although previous data indicate independent functions for these two classes of lipids, recent genetic studies have suggested interactions between phosphatidylinositol (PtdIns) phosphate effectors and sphingolipid biosynthetic enzymes. The present study was undertaken to further define the effects of phosphatidylinositol 4-phosphate (PtdIns(4)P) metabolism on cell sphingolipid metabolism. The data presented indicate that deletion of SAC1, a gene encoding a PtdIns(4)P phosphatase, increased levels of most sphingolipid species, including sphingoid bases, sphingoid base phosphates, and phytoceramide. In contrast, sac1Delta dramatically reduced inositol phosphosphingolipids, which result from the addition of a PtdIns-derived phosphoinositol head group to ceramides through Aur1p. Deletion of SAC1 decreased PtdIns dramatically in both steady-state and pulse labeling studies, suggesting that the observed effects on sphingolipids may result from modulation of the availability of PtdIns as a substrate for Aur1p. Supporting this hypothesis, acute attenuation of PtdIns(4)P production through Stt4p immediately increased PtdIns and subsequently reduced sphingoid bases. This reduction was overcome by the inhibition of Aur1p. Moreover, modulation of sphingoid bases through perturbation of PtdIns(4)P metabolism initiated sphingolipid-dependent biological effects, supporting the biological relevance for this route of regulating sphingolipids. These findings suggest that, in addition to potential signaling effects of PtdInsP effectors on sphingolipid metabolism, PtdIns kinases may exert substantial effects on cell sphingolipid profiles at a metabolic level through modulation of PtdIns available as a substrate for complex sphingolipid synthesis.

Inhibition of inositol phosphorylceramide synthase by the cyclic peptide aureobasidin A

By using a detergent-washed membrane preparation, the interaction of the fungal natural product inhibitor aureobasidin A (AbA) with inositol phosphorylceramide synthase (IPC synthase) was studied by kinetic analysis of wild-type and mutant enzyme-catalyzed reactions. AbA inhibited the wild-type enzyme from both Candida albicans and Saccharomyces cerevisiae in an irreversible, time-dependent manner, with apparent K(i) values of 183 and 234 pM, respectively. Three synthetic chemistry-derived AbA derivatives, PHA-533179, PHA-556655, and PHA-556656, had affinities 4 to 5 orders of magnitude lower and were reversible inhibitors that competed with the donor substrate phosphatidylinositol (PI). AbA was a reversible, apparently noncompetitive inhibitor, with a K(i) of 1.4 microM, of the IPC synthase from an AbA-resistant S. cerevisiae mutant. The K(m) values for both substrates (ceramide and PI) were similar when they interacted with the mutant and the wild-type enzymes. By contrast, the V(max) for the mutant enzyme was less than 10% of that for the wild-type enzyme. A comparison of the results obtained with AbA with those obtained with two other natural products inhibitors, rustmicin and khafrefungin, revealed that while rustmicin appeared to be a reversible, noncompetitive inhibitor of the wild-type enzyme, with a K(i) of 16.0 nM, khafrefungin had the kinetic properties of a time-dependent inhibitor and an apparent K(i) of 0.43 nM. An evaluation of the efficiencies of these compounds as inhibitors of the mutant enzyme revealed for both a drop in the apparent affinity for the enzyme of more than 2 orders of magnitude.

An inositolphosphorylceramide synthase is involved in regulation of plant programmed cell death associated with defense in Arabidopsis

The Arabidopsis thaliana resistance gene RPW8 triggers the hypersensitive response (HR) to restrict powdery mildew infection via the salicylic acid-dependent signaling pathway. To further understand how RPW8 signaling is regulated, we have conducted a genetic screen to identify mutations enhancing RPW8-mediated HR-like cell death (designated erh). Here, we report the isolation and characterization of the Arabidopsis erh1 mutant, in which the At2g37940 locus is knocked out by a T-DNA insertion. Loss of function of ERH1 results in salicylic acid accumulation, enhanced transcription of RPW8 and RPW8-dependent spontaneous HR-like cell death in leaf tissues, and reduction in plant stature. Sequence analysis suggests that ERH1 may encode the long-sought Arabidopsis functional homolog of yeast and protozoan inositolphosphorylceramide synthase (IPCS), which converts ceramide to inositolphosphorylceramide. Indeed, ERH1 is able to rescue the yeast aur1 mutant, which lacks the IPCS, and the erh1 mutant plants display reduced ( approximately 53% of wild type) levels of leaf IPCS activity, indicating that ERH1 encodes a plant IPCS. Consistent with its biochemical function, the erh1 mutation causes ceramide accumulation in plants expressing RPW8. These data reinforce the concept that sphingolipid metabolism (specifically, ceramide accumulation) plays an important role in modulating plant programmed cell death associated with defense.

Regulation of telomere length by fatty acid elongase 3 in yeast. Involvement of inositol phosphate metabolism and Ku70/80 function

In this study, we investigated the roles of very long-chain fatty acid (VLCFA) synthesis by fatty acid elongase 3 (ELO3) in the regulation of telomere length and life span in the yeast Saccharomyces cerevisiae. Loss of VLCFA synthesis via deletion of ELO3 reduced telomere length, and reconstitution of the expression of wild type ELO3, and not by its mutant with decreased catalytic activity, rescued telomere attrition. Further experiments revealed that alterations of phytoceramide seem to be dispensable for telomere shortening in response to loss of ELO3. Interestingly, telomere shortening in elo3Delta cells was almost completely prevented by deletion of IPK2 or KCS1, which are involved in the generation of inositol phosphates (IP4, IP5, and inositol pyrophosphates). Deletion of IPK1, which generates IP6, however, did not affect regulation of telomere length. Further data also suggested that elo3Delta cells exhibit accelerated chronologic aging, and reduced replicative life span compared with wild type cells, and deletion of KCS1 helped recover these biological defects. Importantly, to determine downstream mechanisms, epistasis experiments were performed, and data indicated that ELO3 and YKU70/80 share a common pathway for the regulation of telomere length. More specifically, chromatin immunoprecipitation assays revealed that the telomere binding and protective function of YKu80p in vivo was reduced in elo3Delta cells, whereas its non-homologues end-joining function was not altered. Deletion of KCS1 in elo3Delta cells recovered the telomere binding and protective function of Ku, consistent with the role of KCS1 mutation in the rescue of telomere length attrition. Thus, these findings provide initial evidence of a possible link between Elo3-dependent VLCFA synthesis, and IP metabolism by KCS1 and IPK2 in the regulation of telomeres, which play important physiological roles in the control of senescence and aging, via a mechanism involving alterations of the telomere-binding/protection function of Ku.

Developmentally regulated sphingolipid synthesis in African trypanosomes

Sphingolipids are essential components of eukaryotic membranes, and many unicellular eukaryotes, including kinetoplastid protozoa, are thought to synthesize exclusively inositol phosphorylceramide (IPC). Here we characterize sphingolipids from Trypanosoma brucei, and a trypanosome sphingolipid synthase gene family (TbSLS1-4) that is orthologous to Leishmania IPC synthase. Procyclic trypanosomes contain IPC, but also sphingomyelin, while surprisingly bloodstream-stage parasites contain sphingomyelin and ethanolamine phosphorylceramide (EPC), but no detectable IPC. In vivo fluorescent ceramide labelling confirmed stage-specific biosynthesis of both sphingomyelin and IPC. Expression of TbSLS4 in Leishmania resulted in production of sphingomyelin and EPC suggesting that the TbSLS gene family has bi-functional synthase activity. RNAi silencing of TbSLS1-4 in bloodstream trypanosomes led to rapid growth arrest and eventual cell death. Ceramide levels were increased more than threefold by silencing suggesting a toxic downstream effect mediated by this potent intracellular messenger. Topology predictions support a revised six-transmembrane domain model for the kinetoplastid sphingolipid synthases consistent with the proposed mammalian sphingomyelin synthase structure. This work reveals novel diversity and regulation in sphingolipid metabolism in this important group of human parasites.

Mathematical modeling of pathogenicity of Cryptococcus neoformans

Cryptococcus neoformans (Cn) is the most common cause of fungal meningitis worldwide. In infected patients, growth of the fungus can occur within the phagolysosome of phagocytic cells, especially in non-activated macrophages of immunocompromised subjects. Since this environment is characteristically acidic, Cn must adapt to low pH to survive and efficiently cause disease. In the present work, we designed, tested, and experimentally validated a theoretical model of the sphingolipid biochemical pathway in Cn under acidic conditions. Simulations of metabolic fluxes and enzyme deletions or downregulation led to predictions that show good agreement with experimental results generated post hoc and reconcile intuitively puzzling results. This study demonstrates how biochemical modeling can yield testable predictions and aid our understanding of fungal pathogenesis through the design and computational simulation of hypothetical experiments.

Thematic review series: sphingolipids. New insights into sphingolipid metabolism and function in budding yeast

Our understanding of sphingolipid metabolism and functions in the baker's yeast Saccharomyces cerevisiae has progressed substantially in the past 2 years. Yeast sphingolipids contain a C26-acyl moiety, all of the genes necessary to make these long-chain fatty acids have been identified, and a mechanism for how chain length is determined has been proposed. Advances in understanding how the de novo synthesis of ceramide and complex sphingolipids is regulated have been made, and they demonstrate that the Target Of Rapamycin Complex 2 (TORC2) controls ceramide synthase activity. Other work shows that TORC2 regulates the level of complex sphingolipids in a pathway using the Slm1 and Slm2 proteins to control the protein phosphatase calcineurin, which regulates the breakdown of complex sphingolipids. The activity of Slm1 and Slm2 has also been shown to be regulated during heat stress by phosphoinositides and TORC2, along with sphingoid long-chain bases and the Pkh1 and Pkh2 protein kinases, to control the actin cytoskeleton, the trafficking of nutrient transporters, and cell viability. Together, these results provide the first molecular insights into understanding previous genetic interaction data that indicated a connection between sphingolipids and the TORC2 and phosphoinositide signaling networks. This new knowledge provides a foundation for greatly advancing our understanding of sphingolipid biology in yeast.

Yeast ARV1 is required for efficient delivery of an early GPI intermediate to the first mannosyltransferase during GPI assembly and controls lipid flow from the endoplasmic reticulum

Glycosylphosphatidylinositol (GPI), covalently attached to many eukaryotic proteins, not only acts as a membrane anchor but is also thought to be a sorting signal for GPI-anchored proteins that are associated with sphingolipid and sterol-enriched domains. GPI anchors contain a core structure conserved among all species. The core structure is synthesized in two topologically distinct stages on the leaflets of the endoplasmic reticulum (ER). Early GPI intermediates are assembled on the cytoplasmic side of the ER and then are flipped into the ER lumen where a complete GPI precursor is synthesized and transferred to protein. The flipping process is predicted to be mediated by a protein referred as flippase; however, its existence has not been proven. Here we show that yeast Arv1p is an important protein required for the delivery of an early GPI intermediate, GlcN-acylPI, to the first mannosyltransferase of GPI synthesis in the ER lumen. We also provide evidence that ARV1 deletion and mutations in other proteins involved in GPI anchor synthesis affect inositol phosphorylceramide synthesis as well as the intracellular distribution and amounts of sterols, suggesting a role of GPI anchor synthesis in lipid flow from the ER.

Membrane topology and essential amino acid residues of Phs1, a 3-hydroxyacyl-CoA dehydratase involved in very long-chain fatty acid elongation

Yeast Phs1 is the 3-hydroxyacyl-CoA dehydratase that catalyzes the third reaction of the four-step cycle in the elongation of very long-chain fatty acids (VLCFAs). In yeast, the hydrophobic backbone of sphingolipids, ceramide, consists of a long-chain base and an amide-linked C26 VLCFA. Therefore, defects in VLCFA synthesis would be expected to greatly affect sphingolipid synthesis. In fact, in this study we found that reduced Phs1 levels result in significant impairment of the conversion of ceramide to inositol phosphorylceramide. Phs1 proteins are conserved among eukaryotes, constituting a novel protein family. Phs1 family members exhibit no sequence similarity to other dehydratase families, so their active site sequence and catalytic mechanism have been completely unknown. Here, by mutating 22 residues conserved among Phs1 family members, we identified six amino acid residues important in Phs1 function, two of which (Tyr-149 and Glu-156) are indispensable. We also examined the membrane topology of Phs1 using an N-glycosylation reporter assay. Our results suggest that Phs1 is a membrane-spanning protein that traverses the membrane six times and has an N terminus and C terminus facing the cytosol. The important amino acids are concentrated in or near two of the six proposed transmembrane regions. Thus, we also propose a catalytic mechanism for Phs1 that is not unlike mechanisms used by other hydratases active in lipid synthesis.

Isc1 regulates sphingolipid metabolism in yeast mitochondria

The Saccharomyces cerevisiae inositol sphingolipid phospholipase C (Isc1p), a homolog of mammalian neutral sphingomyelinases, hydrolyzes complex sphingolipids to produce ceramide in vitro. Epitope-tagged Isc1p associates with the mitochondria in the post-diauxic phase of yeast growth. In this report, the mitochondrial localization of Isc1p and its role in regulating sphingolipid metabolism were investigated. First, endogenous Isc1p activity was enriched in highly purified mitochondria, and western blots using highly purified mitochondrial membrane fractions demonstrated that epitope-tagged Isc1p localized to the outer mitochondrial membrane as an integral membrane protein. Next, LC/MS was employed to determine the sphingolipid composition of highly purified mitochondria which were found to be significantly enriched in alpha-hydroxylated phytoceramides (21.7 fold) relative to the whole cell. Mitochondria, on the other hand, were significantly depleted in sphingoid bases. Compared to the parental strain, mitochondria from isc1Delta in the post-diauxic phase showed drastic reduction in the levels of alpha-hydroxylated phytoceramide (93.1% loss compared to WT mitochondria with only 2.58 fold enrichment in mitochondria compared to whole cell). Functionally, isc1Delta showed a higher rate of respiratory-deficient cells after incubation at high temperature and was more sensitive to hydrogen peroxide and ethidium bromide, indicating that isc1Delta exhibits defects related to mitochondrial function. These results suggest that Isc1p generates ceramide in mitochondria, and the generated ceramide contributes to the normal function of mitochondria. This study provides a first insight into the specific composition of ceramides in mitochondria.

Parallel Synthesis and Yeast Growth Inhibition Screening of Succinamic Acid Libraries

Libraries of succinamic acid derivatives resulting from the condensation of a series of succinic acid derivatives with amines are reported as putative khafrefungin analogues. A total of 480 compounds derived from the initial condensation of 8 scaffolds with 60 different amines have been synthesized using automated technology with the help of scavenger resins. A simple acetate hydrolysis of five of the above sublibraries afforded additional 300 compounds for a total of 780 compounds. Around 55% of the library members showed purities higher than 70% (HPLC-ELS-MS) thus proving the generality of this approach. Results on growth inhibition of the yeast Saccharomyces cerevisiae in the presence of selected library members are also reported as a preliminary evaluation of the antifungal activity.

Both sphingomyelin synthases SMS1 and SMS2 are required for sphingomyelin homeostasis and growth in human HeLa cells

Sphingomyelin (SM) is a vital component of cellular membranes in organisms ranging from mammals to protozoa. Its production involves the transfer of phosphocholine from phosphatidylcholine to ceramide, yielding diacylglycerol in the process. The mammalian genome encodes two known SM synthase (SMS) isoforms, SMS1 and SMS2. However, the relative contributions of these enzymes to SM production in mammalian cells remained to be established. Here we show that SMS1 and SMS2 are co-expressed in a variety of cell types and function as the key Golgi- and plasma membrane-associated SM synthases in human cervical carcinoma HeLa cells, respectively. RNA interference-mediated depletion of either SMS1 or SMS2 caused a substantial decrease in SM production levels, an accumulation of ceramides, and a block in cell growth. Although SMS-depleted cells displayed a reduced SM content, external addition of SM did not restore growth. These results indicate that the biological role of SM synthases goes beyond formation of SM.

Lipid requirements for endocytosis in yeast

Endocytosis is, besides secretion, the most prominent membrane transport pathway in eukaryotic cells. In membrane transport, defined areas of the donor membranes engulf solutes of the compartment they are bordering and bud off with the aid of coat proteins to form vesicles. These transport vehicles are guided along cytoskeletal paths, often matured and, finally, fuse to the acceptor membrane they are targeted to. Lipids and proteins are equally important components in membrane transport pathways. Not only are they the structural units of membranes and vesicles, but both classes of molecules also participate actively in membrane transport processes. Whereas proteins form the cytoskeleton and vesicle coats, confer signals and constitute attachment points for membrane-membrane interaction, lipids modulate the flexibility of bilayers, carry protein recognition sites and confer signals themselves. Over the last decade it has been realized that all classes of bilayer lipids, glycerophospholipids, sphingolipids and sterols, actively contribute to functional membrane transport, in particular to endocytosis. Thus, abnormal bilayer lipid metabolism leads to endocytic defects of different severity. Interestingly, there seems to be a great deal of interdependence and interaction among lipid classes. It will be a challenge to characterize this plenitude of interactions and find out about their impact on cellular processes.

Yeast sphingolipids do not need to contain very long chain fatty acids

Synthesis of VLCFAs (very long chain fatty acids) and biosynthesis of DHS (dihydrosphingosine) both are of vital importance for Saccharomyces cerevisiae. The bulk of VLCFAs and DHS are used for ceramide synthesis by the Lag1p (longevity-assurance gene 1)/Lac1p (longevity-assurance gene cognate 1)/Lip1p (Lag1p/Lac1p interacting protein) ceramide synthase. LAG1 and LAC1 are redundant but LIP1 is essential. Here we show that 4Delta (lag1Deltalac1Deltaypc1Deltaydc1Delta) cells devoid of all known endogenous ceramide synthesis pathways are unviable but can be rescued by the expression of Lass5, a mouse LAG1 homologue. Ceramide synthase activity of 4Delta.Lass5 cells only utilizes C16 and C18 fatty acids and does not require the help of Lip1p, an essential cofactor of Lag1p/Lac1p. HPLC-electrospray ionization-MS/MS analysis demonstrated that in IPCs (inositolphosphorylceramides) of 4Delta.Lass5, the very long chain fatty acids (C26 and C24) account for <1% instead of the normal >97%. Notwithstanding, IPCs incorporated into glycosylphosphatidylinositol anchors of 4Delta.Lass5 show normal mobility on TLC and the ceramide- and raft-dependent traffic of Gas1p (glycophospholipid-anchored surface protein) from endoplasmic reticulum to Golgi remains almost normal. Moreover, the biosynthesis of C24:0 fatty acids remains essential. Thus, C(24:0) and dihydrosphingosine are both necessary for survival of yeast cells even if they utilize C16 and C18 fatty acids for sphingolipid biosynthesis.

Regulation of the transport and protein levels of the inositol phosphorylceramide mannosyltransferases Csg1 and Csh1 by the Ca2+-binding protein Csg2

Complex sphingolipids in yeast are known to function in cellular adaptation to environmental changes. One of the yeast complex sphingolipids, mannosylinositol phosphorylceramide (MIPC), is produced by the redundant inositol phosphorylceramide (IPC) mannosyltransferases Csg1 and Csh1. The Ca2+-binding protein Csg2 can form a complex with either Csg1 or Csh1 and is considered to act as a regulatory subunit. However, the role of Csg2 in MIPC synthesis has remained unclear. In this study, we found that Csg1 and Csh1 are N-glycosylated with core-type and mannan-type structures, respectively. Further identification of the glycosylated residues suggests that both Csg1 and Csh1 exhibit membrane topology with their C termini in the cytosol and their mannosyltransferase domains in the lumen. After complexing with Csg2, both Csg1 and Csh1 function in the Golgi, and then are delivered to the vacuole for degradation. However, uncomplexed Csh1 cannot exit from the endoplasmic reticulum. We also demonstrated that Ca2+ stimulates IPC-to-MIPC conversion, because of a Csg2-dependent increase in Csg1 levels. Thus, Csg2 has several regulatory functions for Csg1 and Csh1, including stability, transport, and gene expression.

The role of lipids in the biogenesis of integral membrane proteins

Most integral membrane proteins are cotranslationally inserted into the lipid bilayer. In prokaryotes, membrane insertion of the nascent chain takes place at the plasma membrane, whereas in eukaryotes insertion takes place into the endoplasmatic reticulum. In both kingdoms of life, however, the same membrane that acquaints the newly born membrane protein also synthesizes the bilayer lipids and thus ensures the balanced growth of the membrane as a whole. Recent evidence indicates that the lipid composition of the host membrane can determine the fate of the newborn membrane protein, as it can affect (1) the efficiency of translocation, (2) the topology of the resulting membrane protein, (3) its stability, (4) its assembly into oligomeric complexes, (5) its transport and sorting along the secretory pathway, and (6) its enzymatic activity. The lipid composition of the membrane thus can affect the biogenesis and function of integral membrane proteins at multiple steps along its biogenetic pathway. While understanding this interdependence between bilayer lipids and protein biogenesis is interesting in its own right, careful consideration of a potential host's membrane lipid composition may also allow optimization of the yield and activity of membrane proteins that are expressed in a heterologous organism. Here, we review and discuss some examples that illustrate the interdependence between bilayer lipids and the biogenesis of integral membrane proteins.

Maturation of Golgi cisternae directly observed

Newly synthesized secretory proteins pass through the Golgi apparatus, which consists of multiple cisternae containing distinct populations of enzymes. Are the cargo proteins shuttled between cisternae in vesicles or do they remain in a cisterna while it is the Golgi enzymes that are removed and replaced? As predicted by the latter model--the cisternal maturation hypothesis--two groups have directly observed the replacement of one Golgi protein with another in individual cisternae, thus answering the question. However, its solution raises many more unknowns.

Transmembrane topology of ceramide synthase in yeast

Ceramide plays a crucial role as a basic building block of sphingolipids, but also as a signalling molecule mediating cell-fate decisions. Three genes, LAG1, LAC1 and LIP1, have been shown to be required for ceramide synthase activity in Saccharomyces cerevisiae [Guillas, Kirchman, Chuard, Pfefferli, Jiang, Jazwinski and Conzelman (2001) EMBO J. 20, 2655-2665; Schorling, Vallee, Barz, Reizman and Oesterhelt (2001) Mol. Biol. Cell 12, 3417-3427; Vallee and Riezman (2005) EMBO J. 24, 730-741]. In the present study, the topology of the Lag1p and Lac1p subunits was investigated. The N- and C-termini of the proteins are in the cytoplasm and eight putative membrane-spanning domains were identified in Lag1p and Lac1p by insertion of glycosylation and factor Xa cleavage sites at various positions. The conserved Lag motif, potentially containing the active site, is most likely embedded in the membrane. We also present evidence that histidine and aspartic acid residues in the Lag motif are essential for the function of Lag1p in vivo.

Very long-chain fatty acid-containing lipids rather than sphingolipids per se are required for raft association and stable surface transport of newly synthesized plasma membrane ATPase in yeast

The proton-pumping H+-ATPase, Pma1p, is an abundant and very long lived polytopic protein of the yeast plasma membrane. Pma1p constitutes a major cargo of the secretory pathway and thus serves as a model to study plasma membrane biogenesis. Pma1p associates with detergent-resistant membrane domains (lipid "rafts") already in the ER, and a lack of raft association correlates with mistargeting of the protein to the vacuole, where it is degraded. We are analyzing the role of specific lipids in membrane domain formation and have previously shown that surface transport of Pma1p is independent of newly synthesized sterols but that sphingolipids with C26 very long chain fatty acid are crucial for raft association and surface transport of Pma1p (Gaigg, B., Timischl, B., Corbino, L., and Schneiter, R. (2005) J. Biol. Chem. 280, 22515-22522). We now describe a more detailed analysis of the function that sphingolipids play in this process. Using a yeast strain in which the essential function of sphingolipids is substituted by glycerophospholipids containing C26 very long chain fatty acids, we find that sphingolipids per se are dispensable for raft association and surface delivery of Pma1p but that the C26 fatty acid is crucial. We thus conclude that the essential function of sphingolipids for membrane domain formation and stable surface delivery of Pma1p is provided by the C26 fatty acid that forms part of the yeast ceramide.

Palmitoylation by the DHHC protein Pfa4 regulates the ER exit of Chs3

The yeast chitin synthase Chs3 provides a well-studied paradigm for polytopic membrane protein trafficking. In this study, high-throughput analysis of the yeast deletion collection identifies a requirement for Pfa4, which is an uncharacterized protein with protein acyl transferase (PAT) homology, in Chs3 transport. PATs, which are the enzymatic mediators of protein palmitoylation, have only recently been discovered, and few substrates have been identified. We find that Chs3 is palmitoylated and that this modification is Pfa4-dependent, indicating that Pfa4 is indeed a PAT. Chs3 palmitoylation is required for ER export, but not for interaction with its dedicated ER chaperone, Chs7. Nonetheless, both palmitoylation and chaperone association are required to prevent the accumulation of Chs3 in high–molecular mass aggregates at the ER. Our data indicate that palmitoylation is necessary for Chs3 to attain an export-competent conformation, and suggest the possibility of a more general role for palmitoylation in the ER quality control of polytopic membrane proteins.

Organization and functions of sphingolipid biosynthesis in yeast

Sphingolipids are important signalling molecules and thus it is very important to understand how they are generated. Sphingolipid biosynthesis shows a conserved compartmentalization in eukaryotic cells. Their synthesis begins in the endoplasmic reticulum and is completed in the Golgi apparatus. We now know quite a bit about the topology of the reactions in the pathway, but certain critical steps, such as ceramide synthesis, are still poorly understood. In the present paper, we discuss the latest views on this subject.

Lipid-dependent surface transport of the proton pumping ATPase: a model to study plasma membrane biogenesis in yeast

The proton pumping H+-ATPase, Pma1, is one of the most abundant integral membrane proteins of the yeast plasma membrane. Pma1 activity controls the intracellular pH and maintains the electrochemical gradient across the plasma membrane, two essential cellular functions. The maintenance of the proton gradient, on the other hand, also requires a specialized lipid composition of this membrane. The plasma membrane of eukaryotic cells is typically rich in sphingolipids and sterols. These two lipids condense to form less fluid membrane microdomains or lipid rafts. The yeast sphingolipid is peculiar in that it invariably contains a saturated very long-chain fatty acid with 26 carbon atoms. During cell growth and plasma membrane expansion, both C26-containing sphingolipids and Pma1 are first synthesized in the endoplasmatic reticulum from where they are transported by the secretory pathway to the cell surface. Remarkably, shortening the C26 fatty acid to a C22 fatty acid by mutations in the fatty acid elongation complex impairs raft association of newly synthesized Pma1 and induces rapid degradation of the ATPase by rerouting the enzyme from the plasma membrane to the vacuole, the fungal equivalent of the lysosome. Here, we review the role of lipids in mediating raft association and stable surface transport of the newly synthesized ATPase, and discuss a model, in which the newly synthesized ATPase assembles into a membrane environment that is enriched in C26-containing lipids already in the endoplasmatic reticulum. The resulting protein-lipid complex is then transported and sorted as an entity to the plasma membrane. Failure to successfully assemble this lipid-protein complex results in mistargeting of the protein to the vacuole.

Human TPST1 Transmembrane Domain Triggers Enzyme Dimerisation and Localisation to the Golgi Compartment

TPST1 is a human tyrosylprotein sulfotransferase that uses 3'phosphoadenosine-5'phosphosulfate (PAPS) to transfer the sulfate moiety to proteins predominantly designated for secretion. To achieve a general understanding of the cellular role of human tyrosine-directed sulfotransferases, we investigated targeting, structure and posttranslational modification of TPST1. Golgi localisation of the enzyme in COS-7 and HeLa cells was visualised by fluorescence imaging techniques. PNGase treatment and mutational studies determined that TPST1 bears N-linked glycosyl residues exclusively at position Asn60 and Asn262. By alanine mutation of these asparagine residues, we could determine that the N-linked oligosaccharides do not have an influence on Golgi retention of TPST1. In concert with N and C-terminal flanking residues, the transmembrane domain of TPST1 was determined to act in targeting and retention of the enzyme to the trans-Golgi compartment. This domain exhibits a pronounced secondary structure in a lipid environment. Further in vivo FRET studies using the transmembrane domain suggest that the human tyrosylprotein sulfotransferase may be functional as homodimer/oligomer in the trans-Golgi compartment.

The protozoan inositol phosphorylceramide synthase: a novel drug target that defines a new class of sphingolipid synthase

Sphingolipids are ubiquitous and essential components of eukaryotic membranes, particularly the plasma membrane. The biosynthetic pathway for the formation of these lipid species is conserved up to the formation of sphinganine. However, a divergence is apparent in the synthesis of complex sphingolipids. In animal cells, ceramide is a substrate for sphingomyelin (SM) production via the enzyme SM synthase. In contrast, fungi utilize phytoceramide in the synthesis of inositol phosphorylceramide (IPC) catalyzed by IPC synthase. Because of the absence of a mammalian equivalent, this essential enzyme represents an attractive target for anti-fungal compounds. In common with the fungi, the kinetoplastid protozoa (and higher plants) synthesize IPC rather than SM. However, orthologues of the gene believed to encode the fungal IPC synthase (AUR1) are not readily identified in the complete genome data bases of these species. By utilizing bioinformatic and functional genetic approaches, we have isolated a functional orthologue of AUR1 in the kinetoplastids, causative agents of a range of important human diseases. Expression of this gene in a mammalian cell line led to the synthesis of an IPC-like species, strongly indicating that IPC synthase activity is reconstituted. Furthermore, the gene product can be specifically inhibited by an anti-fungal-targeting IPC synthase. We propose that the kinetoplastid AUR1 functional orthologue encodes an enzyme that defines a new class of protozoan sphingolipid synthase. The identification and characterization of the protozoan IPC synthase, an enzyme with no mammalian equivalent, will raise the possibility of developing anti-protozoal drugs with minimal toxic side affects.

Loss of P4 ATPases Drs2p and Dnf3p disrupts aminophospholipid transport and asymmetry in yeast post-Golgi secretory vesicles

Eukaryotic plasma membranes generally display asymmetric lipid distributions with the aminophospholipids concentrated in the cytosolic leaflet. This arrangement is maintained by aminophospholipid translocases (APLTs) that use ATP hydrolysis to flip phosphatidylserine (PS) and phosphatidylethanolamine (PE) from the external to the cytosolic leaflet. The identity of APLTs has not been established, but prime candidates are members of the P4 subfamily of P-type ATPases. Removal of P4 ATPases Dnf1p and Dnf2p from budding yeast abolishes inward translocation of 6-[(7-nitrobenz-2-oxa-1,3-diazol-4-yl)aminocaproyl] (NBD)-labeled PS, PE, and phosphatidylcholine (PC) across the plasma membrane and causes cell surface exposure of endogenous PE. Here, we show that yeast post-Golgi secretory vesicles (SVs) contain a translocase activity that flips NBD-PS, NBD-PE, and NBD-PC to the cytosolic leaflet. This activity is independent of Dnf1p and Dnf2p but requires two other P4 ATPases, Drs2p and Dnf3p, that reside primarily in the trans-Golgi network. Moreover, SVs have an asymmetric PE arrangement that is lost upon removal of Drs2p and Dnf3p. Our results indicate that aminophospholipid asymmetry is created when membrane flows through the Golgi and that P4-ATPases are essential for this process.

Retrograde regulation of multidrug resistance in Saccharomyces cerevisiae

Communication between the mitochondria and the nucleus is essential to ensure correct metabolic coordination of the cell. Signaling pathways leading from the mitochondria to the nucleus are referred to as retrograde signaling and were first discovered in the yeast Saccharomyces cerevisiae. Cells that lack their mitochondrial genome (rho0 cells) trigger expression of the nuclear CIT2 gene in order to ensure adequate amino acid biosynthesis. More recently, it has been found that a different set of genes involved in multidrug resistance in S. cerevisiae is strongly induced in rho0 cells. During a search for negative regulators of the ATP-binding cassette (ABC) transporter-encoding gene PDR5, it was observed that rho0 mutants exhibited dramatic up-regulation of the transcript of this gene. This induction was due to the post-translational activation of a direct upstream regulator of PDR5 that was designated Pdr3p. Loss of the LGE1 gene led to a block in rho0-mediated induction of PDR5 expression. Lge1p has been observed by others to be involved in histone H2B ubiquitination along with the ubiquitin-conjugating enzyme Rad6p and the ubiquitin ligase Bre1p. Our studies provide evidence that Lge1p has another function unique from H2B ubiquitination that is required for retrograde regulation of PDR5 transcription. We have also found that the Pdr pathway regulates expression of several genes involved in sphingolipid biosynthesis. These findings suggest that the physiological role of the PDR genes might be to regulate membrane homeostasis and rho0-triggered changes in this parameter may be the signal controlling PDR gene expression.

Molecular mechanisms and regulation of ceramide transport

De novo biosynthesis of sphingolipids begins in the endoplasmic reticulum (ER) and continues in the Golgi apparatus and plasma membrane. A crucial step in sphingolipid biosynthesis is the transport of ceramide by vesicular and non-vesicular mechanisms from its site of synthesis in the ER to the Golgi apparatus. The recent discovery of the ceramide transport protein CERT has revealed a novel pathway for the delivery of ceramide to the Golgi apparatus for sphingomyelin (SM) synthesis. In addition to a ceramide-binding START domain, CERT has FFAT (referring to two phenylalanines [FF] in an acidic tract) and pleckstrin homology (PH) domains that recognize the ER integral membrane protein VAMP-associated protein (VAP) and Golgi-associated PtdIns 4-phosphate, respectively. Mechanisms for vectorial transport involving dual-organellar targeting and sites of deposition of ceramide in the Golgi apparatus are proposed. Similar Golgi-ER targeting motifs are also present in the oxysterol-binding protein (OSBP), which regulates ceramide transport and SM synthesis in an oxysterol-dependent manner. Consequently, this emerges as a potential mechanism for integration of sphingolipid and cholesterol metabolism. The identification of organellar targeting motifs in other related lipid-binding/transport proteins indicate that concepts learned from the study of ceramide transport can be applied to other lipid transport processes.

The role and mechanism of diacylglycerol-protein kinase C1 signaling in melanogenesis by Cryptococcus neoformans

The fungus Cryptococcus neoformans is an opportunistic human pathogen that causes a life-threatening meningoencephalitis by expression of virulence factors such as melanin, a black pigment produced by the cell wall-associated enzyme laccase. In previous studies (Heung, L. J., Luberto, C., Plowden, A., Hannun, Y. A., and Del Poeta, M. (2004) J. Biol. Chem. 279, 21144-21153) we proposed that the sphingolipid enzyme inositol-phosphoryl ceramide synthase 1 (Ipc1) regulates melanin production through the generation of diacylglycerol (DAG), which was found to activate in vitro protein kinase C1 (Pkc1). Here, we investigated the molecular mechanisms by which DAG regulates Pkc1 in vivo and the effect of this regulation on laccase activity and melanin synthesis. To this end we deleted the putative DAG binding C1 domain of C. neoformans Pkc1 and found that the C1 deletion abolished the activation of Pkc1 by DAG. Deletion of the C1 domain repressed laccase activity and, consequently, melanin production. Finally, we show that these biological effects observed in the C1 deletion mutant are mediated by alteration of cell wall integrity and displacement of laccase from the cell wall. These studies define novel molecular mechanisms addressing Pkc1-laccase regulation by the sphingolipid pathway of C. neoformans, with important implications for understanding and targeting the Ipc1-Pkc1-laccase cascade as a regulator of virulence of this important human pathogen.

Multiple lipid transport pathways to the plasma membrane in yeast

The plasma membrane of the yeast Saccharomyces cerevisiae is devoid of lipid-synthesizing enzymes, but contains all classes of bilayer-forming lipids. As the lipid composition of the plasma membrane does not match any of the intracellular membranes, specific trafficking of lipids from internal membranes, especially the endoplasmic reticulum and the Golgi, to the cell periphery is required. Although the secretory pathway is an obvious route to translocate glycerophospholipids, sphingolipids and sterols to the plasma membrane, experimental evidence for the role of this pathway in lipid transport is rare. Addressing this issue in a systematic way, we labeled temperature-sensitive secretory yeast mutants (sec mutants) with appropriate lipid precursors, isolated the plasma membranes at high purity and quantified labeled lipids of this compartment. Shifting sec mutants to the restrictive temperature reduced transport of both proteins and lipids to the plasma membrane, indicating that the latter compounds are also trafficked to the cell periphery through the protein secretory pathway. However, efficient sec blocks did not abrogate protein and lipid transport, suggesting that parallel pathway(s) for the translocation of membrane components to the plasma membrane of yeast must exist.

Characterization of the inositol phosphorylceramide synthase activity from Trypanosoma cruzi

IPC (inositol phosphorylceramide) synthase is an enzyme essential for fungal viability, and it is the target of potent antifungal compounds such as rustmicin and aureobasidin A. Similar to fungi and some other lower eukaryotes, the protozoan parasite Trypanosoma cruzi is capable of synthesizing free or protein-linked glycoinositolphospholipids containing IPC. As a first step towards understanding the importance and mechanism of IPC synthesis in T. cruzi, we investigated the effects of rustmicin and aureobasidin A on the proliferation of different life-cycle stages of the parasite. The compounds did not interfere with the axenic growth of epimastigotes, but aureobasidin A decreased the release of trypomastigotes from infected murine peritoneal macrophages and the number of intracellular amastigotes in a dose-dependent manner. We have demonstrated for the first time that all forms of T. cruzi express an IPC synthase activity that is capable of transferring inositol phosphate from phosphatidylinositol to the C-1 hydroxy group of C6-NBD-cer {6-[N-(7-nitro-2,1,3-benzoxadiazol-4-yl)-amino]hexanoylceramide} to form inositol phosphoryl-C6-NBD-cer, which was purified and characterized by its chromatographic behaviour on TLC and HPLC, sensitivity to phosphatidylinositol-specific phospholipase C and resistance to mild alkaline hydrolysis. Unlike the Saccharomyces cerevisiae IPC synthase, the T. cruzi enzyme is stimulated by Triton X-100 but not by bivalent cations, CHAPS or fatty-acid-free BSA, and it is not inhibited by rustmicin or aureobasidin A, or the two in combination. Further studies showed that aureobasidin A has effects on macrophages independent of the infecting T. cruzi cells. These results suggest that T. cruzi synthesizes its own IPC, but by a mechanism that is not affected by rustmicin and aureobasidin A.

Synthesis of sphingolipids with very long chain fatty acids but not ergosterol is required for routing of newly synthesized plasma membrane ATPase to the cell surface of yeast

The proton pumping H(+)-ATPase, Pma1p, is an abundant and very long-lived polytopic protein of the Saccharomyces cerevisiae plasma membrane. Pma1p constitutes a major cargo of the secretory pathway and thus serves as an excellent model to study plasma membrane biogenesis. We have previously shown that newly synthesized Pma1p is mistargeted to the vacuole in an elo3Delta mutant that affects the synthesis of the ceramide-bound C26 very long chain fatty acid (Eisenkolb, M., Zenzmaier, C., Leitner, E., and Schneiter, R. (2002) Mol. Biol. Cell 13, 4414-4428) and now describe a more detailed analysis of the role of lipids in Pma1p biogenesis. Remarkably, a block at various steps of sterol biosynthesis, a complete block in sterol synthesis, or the substitution of internally synthesized ergosterol by externally supplied ergosterol or even by cholesterol does not affect Pma1p biogenesis or its association with detergent-resistant membrane domains (lipid "rafts"). However, a block in sphingolipid synthesis or any perturbation in the synthesis of the ceramide-bound C26 very long chain fatty acid results in mistargeting of newly synthesized Pma1p to the vacuole. Mistargeting correlates with a lack of newly synthesized Pma1p to acquire detergent resistance, suggesting that sphingolipids with very long acyl chains affect sorting of Pma1p to the cell surface.

Polytopic Proteins: Preventing Aggregation in the Membrane

It has been proposed that the aggregation of nascent transmembrane segments of polytopic proteins is prevented by chaperones present in the endoplasmic reticulum membrane; now the first experimental support for this proposal has been reported.

Specialized membrane-localized chaperones prevent aggregation of polytopic proteins in the ER

The integral endoplasmic reticulum (ER) membrane protein Shr3p is required for proper plasma membrane localization of amino acid permeases (AAPs) in yeast. In the absence of Shr3p AAPs are uniquely retained in the ER with each of their twelve membrane-spanning segments correctly inserted in the membrane. Here, we show that the membrane domain of Shr3p specifically prevents AAPs from aggregating, and thus, plays a critical role in assisting AAPs to fold and correctly attain tertiary structures required for ER exit. Also, we show that the integral ER proteins, Gsf2p, Pho86p, and Chs7p, function similarly to Shr3p. In cells individually lacking one of these components only their cognate substrates, hexose transporters, phosphate transporters, and chitin synthase-III, respectively, aggregate and consequently fail to exit the ER membrane. These findings indicate that polytopic membrane proteins depend on specialized membrane-localized chaperones to prevent inappropriate interactions between membrane-spanning segments as they insert and fold in the lipid bilayer of the ER membrane.

Yeast sphingolipid metabolism: clues and connections

This review of sphingolipid metabolism in the budding yeast Saccharomyces cerevisiae contains information on the enzymes and the genes that encode them, as well as connections to other metabolic pathways. Particular attention is given to yeast homologs, domains, and motifs in the sequence, cellular localization of enzymes, and possible protein-protein interactions. Also included are genetic interactions of special interest that provide clues to the cellular biological roles of particular sphingolipid metabolic pathways and specific sphingolipids.

Rafts and sphingolipid biosynthesis in the kinetoplastid parasitic protozoa

Summary relatively large rafts are a feature of activated mammalian cells. These studies allow us to consider the functional role of lipid rafts in kinetoplastid parasites, which are particularly rich in lipid-anchored surface molecules. Morphological, biochemical and genetic studies indicate that lipid rafts (and sphingolipid biosythesis) are important in the differentiation of extracellular Leishmania to mammalian-infective metacyclic promastigotes, perhaps orchestrating the clearly observable reorganization of the plasma membrane during this process that leads to an activated metacyclic primed for invasion. However, the first step in the sphingolipid biosynthetic pathway (mediated by serine palmitoyltransferase), and at least regulated, de novo sphingoid base and ceramide synthesis, are not essential for the pathogenesis of intramacrophage Leishmania amastigotes.