Transcriptional control of yeast plasma membrane H(+)-ATPase by glucose. Cloning and characterization of a new gene involved in this regulation

Multi-Omic Profiling, Structural Characterization, and Potent Inhibitor Screening of Evasion-Related Proteins of a Parasitic Nematode, Haemonchus contortus, Surviving Vaccine Treatment

The emergence of drug-resistant parasitic nematodes in both humans and livestock calls for development of alternative and cost-effective control strategies. Barbervax^® is the only registered vaccine for the economically important ruminant strongylid Haemonchus contortus. In this study, we compared the microbiome, genome-wide diversity, and transcriptome of H. contortus adult male populations that survived vaccination with an experimental vaccine after inoculation in sheep. Our genome-wide SNP analysis revealed 16 putative candidate vaccine evasion genes. However, we did not identify any evidence for changes in microbial community profiling based on the 16S rRNA gene sequencing results of the vaccine-surviving parasite populations. A total of fifty-eight genes were identified as significantly differentially expressed, with six genes being long non-coding (lnc) RNAs and none being putative candidate SNP-associated genes. The genes that highly upregulated in surviving parasites from vaccinated animals were associated with GO terms belonging to predominantly molecular functions and a few biological processes that may have facilitated evasion or potentially lessened the effect of the vaccine. These included five targets: astacin (ASTL), carbonate dehydratase (CA2), phospholipase A2 (PLA2), glutamine synthetase (GLUL), and fatty acid-binding protein (FABP3). Our tertiary structure predictions and modelling analyses were used to perform in silico searches of all published and commercially available inhibitor molecules or substrate analogs with potential broad-spectrum efficacy against nematodes of human and veterinary importance.

Regulation of yeast Snf1 (AMPK) by a polyhistidine containing pH sensing module

Cellular regulation of pH is crucial for internal biological processes and for the import and export of ions and nutrients. In the yeast Saccharomyces cerevisiae, the major proton pump (Pma1) is regulated by glucose. Glucose is also an inhibitor of the energy sensor Snf1/AMPK, which is conserved in all eukaryotes. Here, we demonstrate that a poly-histidine (polyHIS) tract in the pre-kinase region (PKR) of Snf1 functions as a pH-sensing module (PSM) and regulates Snf1 activity. This regulation is independent from, and unaffected by, phosphorylation at T210, the major regulatory control of Snf1, but is controlled by the Pma1 plasma-membrane proton pump. By examining the PKR from additional yeast species, and by varying the number of histidines in the PKR, we determined that the polyHIS functions progressively. This regulation mechanism links the activity of a key enzyme with the metabolic status of the cell at any given moment.

Cellobiose Consumption Uncouples Extracellular Glucose Sensing and Glucose Metabolism in Saccharomyces cerevisiae

Glycolysis is central to energy metabolism in most organisms and is highly regulated to enable optimal growth. In the yeast Saccharomyces cerevisiae, feedback mechanisms that control flux through glycolysis span transcriptional control to metabolite levels in the cell. Using a cellobiose consumption pathway, we decoupled glucose sensing from carbon utilization, revealing new modular layers of control that induce ATP consumption to drive rapid carbon fermentation. Alterations of the beta subunit of phosphofructokinase-1 (PFK2), H⁺-plasma membrane ATPase (PMA1), and glucose sensors (SNF3 and RGT2) revealed the importance of coupling extracellular glucose sensing to manage ATP levels in the cell. Controlling the upper bound of cellular ATP levels may be a general mechanism used to regulate energy levels in cells, via a regulatory network that can be uncoupled from ATP concentrations under perceived starvation conditions.

Living cells are fine-tuned through evolution to thrive in their native environments. Genome alterations to create organisms for specific biotechnological applications may result in unexpected and undesired phenotypes. We used a minimal synthetic biological system in the yeast Saccharomyces cerevisiae as a platform to reveal novel connections between carbon sensing, starvation conditions, and energy homeostasis.

Quaternary ammonium salt N-(dodecyloxycarboxymethyl)-N,N,N-trimethyl ammonium chloride induced alterations in Saccharomyces cerevisiae physiology

We investigated the influence of the quaternary ammonium salt (QAS) called IM (N-(dodecyloxycarboxymethyl)- N,N,N-trimethyl ammonium chloride) on yeast cells of the parental strain and the IM-resistant mutant (EO25 IMR) growth. The phenotype of this mutant was pleiotropic. The IMR mutant exhibited resistance to ethanol, osmotic shock and oxidative stress, as well as increased sensitivity to UV. Moreover, it was noted that mutant EO25 appears to have an increased resistance to clotrimazole, ketoconazole, fluconazole, nystatin and cycloheximide. It also tolerated growth in the presence of crystal violet, DTT and metals (selenium, tin, arsenic). It was shown that the presence of IM decreased ergosterol level in mutant plasma membrane and increased its unsaturation. These results indicate changes in the cell lipid composition. Western blot analysis showed the induction of Pma1 level by IM. RT-PCR revealed an increased PMA1 expression after IM treatment.

Overexpression of PMA1 enhances tolerance to various types of stress and constitutively activates the SAPK pathways in Saccharomyces cerevisiae

PMA1 encodes a transmembrane polypeptide that functions to pump protons out of the cell. Ectopic PMA1 overexpression in Saccharomyces cerevisiae enhances tolerance to weak acids, reactive oxygen species (ROS) and ethanol, and changes the following physiological properties: better proton efflux, lower membrane permeability, and lessened internal hydrogen peroxide production. The enhanced stress tolerance was dependent on the mitogen-activated protein kinase (MAPK) Hog1 of the high osmolarity glycerol (HOG) pathway, but not the MAPK Slt2 of the cell wall integrity (CWI) pathway; however, a PMA1 overexpression constitutively activated both Hog1 and Slt2. The constitutive Hog1 activation required the MAPK kinase kinase (MAP3K) Ssk2 of the HOG pathway, but not Ste11 and Ssk22, two other MAP3Ks of the same pathway. The constitutive Slt2 activation did not require Rom2 and the membrane sensors of the CWI pathway, whereas Bck1 was indispensable. The PMA1 overexpression activated the stress response element but not the cyclic AMP response element and the Rlm1 transcription factor. PMA1 overexpression may facilitate the construction of industrial strains with simultaneous tolerance to weak acids, ROS, and ethanol.

CytoASP: a Cytoscape app for qualitative consistency reasoning, prediction and repair in biological networks

Background

Qualitative reasoning frameworks, such as the Sign Consistency Model (SCM), enable modelling regulatory networks to check whether observed behaviour can be explained or if unobserved behaviour can be predicted. The BioASP software collection offers ideal tools for such analyses. Additionally, the Cytoscape platform can offer extensive functionality and visualisation capabilities. However, specialist programming knowledge is required to use BioASP and no methods exist to integrate both of these software platforms effectively.

Results

We report the implementation of CytoASP, an app that allows the use of BioASP for influence graph consistency checking, prediction and repair operations through Cytoscape. While offering inherent benefits over traditional approaches using BioASP, it provides additional advantages such as customised visualisation of predictions and repairs, as well as the ability to analyse multiple networks in parallel, exploiting multi-core architecture. We demonstrate its usage in a case study of a yeast genetic network, and highlight its capabilities in reasoning over regulatory networks.

Conclusion

We have presented a user-friendly Cytoscape app for the analysis of regulatory networks using BioASP. It allows easy integration of qualitative modelling, combining the functionality of BioASP with the visualisation and processing capability in Cytoscape, and thereby greatly simplifying qualitative network modelling, promoting its use in relevant projects.

Specific phosphoantibodies reveal two phosphorylation sites in yeast Pma1 in response to glucose

Glucose triggers post-translational modifications of the Saccharomyces cerevisiae plasma membrane H(+)-ATPase (Pma1) that lead to an increase in enzyme activity. The activation results from changes in two kinetic parameters: an increase in the affinity of the enzyme for ATP, depending on Ser899, and an increase in the Vmax involving Ser911/Thr912. Using phosphospecific antibodies, we show that Ser899 and Ser911/Thr912 are phosphorylated in vivo during glucose activation and that protein phosphatase Glc7 is involved in the dephosphorylation of Ser899 upon glucose starvation.

A role for FACT in repopulation of nucleosomes at inducible genes

Xenobiotic drugs induce Pleiotropic Drug Resistance (PDR) genes via the orthologous Pdr1/Pdr3 transcription activators. We previously identified the Mediator transcription co-activator complex as a key target of Pdr1 orthologs and demonstrated that Pdr1 interacts directly with the Gal11/Med15 subunit of the Mediator complex. Based on an interaction between Pdr1 and the FACT complex, we show that strains with spt16 or pob3 mutations are sensitive to xenobiotic drugs and display diminished PDR gene induction. Although FACT acts during the activation of some genes by assisting in the nucleosomes eviction at promoters, PDR promoters already contain nucleosome-depleted regions (NDRs) before induction. To determine the function of FACT at PDR genes, we examined the kinetics of RNA accumulation and changes in nucleosome occupancy following exposure to a xenobiotic drug in wild type and FACT mutant yeast strains. In the presence of normal FACT, PDR genes are transcribed within 5 minutes of xenobiotic stimulation and transcription returns to basal levels by 30–40 min. Nucleosomes are constitutively depleted in the promoter regions, are lost from the open reading frames during transcription, and the ORFs are wholly repopulated with nucleosomes as transcription ceases. While FACT mutations cause minor delays in activation of PDR genes, much more pronounced and significant defects in nucleosome repopulation in the ORFs are observed in FACT mutants upon transcription termination. FACT therefore has a major role in nucleosome redeposition following cessation of transcription at the PDR genes, the opposite of its better-known function in nucleosome disassembly.

The yeast sphingolipid signaling landscape

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

Comparative proteomic analysis of transition of saccharomyces cerevisiae from glucose-deficient medium to glucose-rich medium

Background

When glucose is added to Saccharomyces cerevisiae grown in non-fermentable carbon sources, genes encoding ribosomal, cell-cycle, and glycolytic proteins are induced. By contrast, genes involved in mitochondrial functions, gluconeogenesis, and the utilization of other carbon sources are repressed. Glucose also causes the activation of the plasma membrane ATPase and the inactivation of gluconeogenic enzymes and mitochondrial enzymes. The goals of this study were to use the iTRAQ-labeling mass spectrometry technique to identify proteins whose relative levels change in response to glucose re-feeding and to correlate changes in protein abundance with changes in transcription and enzymatic activities. We used an experimental condition that causes the degradation of gluconeogenic enzymes when glucose starved cells are replenished with glucose. Identification of these enzymes as being down-regulated by glucose served as an internal control. Furthermore, we sought to identify new proteins that were either up-regulated or down-regulated by glucose.

Results

We have identified new and known proteins that change their relative levels in cells that were transferred from medium containing low glucose to medium containing high glucose. Up-regulated proteins included ribosomal subunits, proteins involved in protein translation, and the plasma membrane ATPase. Down-regulated proteins included small heat shock proteins, mitochondrial proteins, glycolytic enzymes, and gluconeogenic enzymes. Ach1p is involved in acetate metabolism and is also down-regulated by glucose.

Conclusions

We have identified known proteins that have previously been reported to be regulated by glucose as well as new glucose-regulated proteins. Up-regulation of ribosomal proteins and proteins involved in translation may lead to an increase in protein synthesis and in nutrient uptake. Down-regulation of glycolytic enzymes, gluconeogenic enzymes, and mitochondrial proteins may result in changes in glycolysis, gluconeogenesis, and mitochondrial functions. These changes may be beneficial for glucose-starved cells to adapt to the addition of glucose.

Regulation of Saccharomyces cerevisiae Plasma membrane H(+)-ATPase (Pma1) by Dextrose and Hsp30 during Exposure to Thermal Stress

Pma1p is an essential plasma membrane H(+)-pump in Saccharomyces cerevisiae that pumps out H(+) at the expense of cellular ATP. Its activity is induced by glucose at 30°C and is inhibited by Hsp30 during exposure to heat shock conditions. To further investigate the regulation of Pma1 function by glucose and Hsp30 during exposure to thermal stress, we estimated Pma1 activity, its protein levels and ser-phosphorylation status in membrane fractions isolated from BY4741 and hsp30Δ cells grown in dextrose and sorbitol at 30°C, and following exposure at 40°C for 30 min. Our results demonstrate that Pma1 activity and protein levels were reduced in Hsp30(+) cells following exposure to thermal stress in dextrose media. The above was not observed in hsp30Δ cells wherein Pma1 activity did not decrease following exposure to similar conditions. Although Pma1p levels decreased in heat-shocked hsp30Δ cells, it was lower compared to that observed in Hsp30(+) cells. Total ser-phosphorylation of Pma1 also showed a decrease following exposure to heat shock condition in dextrose media in both BY4741 and hsp30Δ cells. Its levels were also reduced in BY4741 cells upon heat shock treatment in sorbitol unlike that observed in hsp30Δ cells wherein it was increased. Taken together the above indicate that heat shock induced reduction in Pma1 activity and protein levels in dextrose media required Hsp30. To examine functional interactions between dextrose utilization, Hsp30 and the regulation of various aspects of Pma1, we determined if dextrose regulated other functions attributed to Hsp30. Results demonstrate that the deletion of HSP30 rendered cells dependent on dextrose utilization for survival during exposure to lethal heat stress. Our study has hence been able to establish a functional relationship between glucose utilization, Hsp30 function and the regulation of Pma1 activity. Finally, since the deletion of HSP30 renders Pma1p levels and its activity unresponsive to thermal stress in dextrose media, we concluded that Hsp30 is necessary to maintain Pma1 in a regulation competent conformation. Hsp30 may thus act as a chaperone in the S. cerevisiae plasma membrane.

Identification of yeast proteins necessary for cell-surface function of a potassium channel

Inwardly rectifying potassium (Kir) channels form gates in the cell membrane that regulate the flow of K(+) ions into and out of the cell, thereby influencing the membrane potential and electrical signaling of many cell types, including neurons and cardiomyocytes. Kir-channel function depends on other cellular proteins that aid in the folding of channel subunits, assembly into tetrameric complexes, trafficking of quality-controlled channels to the plasma membrane, and regulation of channel activity at the cell surface. We used the yeast Saccharomyces cerevisiae as a model system to identify proteins necessary for the functional expression of a mammalian Kir channel at the cell surface. A screen of 376 yeast strains, each lacking one nonessential protein localized to the early secretory pathway, identified seven deletion strains in which functional expression of the Kir channel at the plasma membrane was impaired. Six deletions were of genes with known functions in trafficking and lipid biosynthesis (sur4Delta, csg2Delta, erv14Delta, emp24Delta, erv25Delta, and bst1Delta), and one deletion was of an uncharacterized gene (yil039wDelta). We provide genetic and functional evidence that Yil039wp, a conserved, phosphoesterase domain-containing protein, which we named "trafficking of Emp24p/Erv25p-dependent cargo disrupted 1" (Ted1p), acts together with Emp24p/Erv25p in cargo exit from the endoplasmic reticulum (ER). The seven yeast proteins identified in our screen likely impact Kir-channel functional expression at the level of vesicle budding from the ER and/or the local lipid environment at the plasma membrane.

Direct in vivo access to potential gene targets of the RPD3 histone deactylase using fitness-based interferential genetics

Using the fitness-based interferential genetics (FIG) approach in yeast, potential in vivo gene targets of the Rpd3 histone deacetylase were selected. In agreement with previous studies using different methods, three genes were found to be involved in the translational machinery (MRPL27, FHL1 and RDN1). Moreover, other selected genes are linked to cell-cycle control (CSE4, AMN1, VAC17 and GRR1). In addition to playing a crucial role in cell cycle progression to the S phase and participating in the G(2)-M transition, GRR1 has important functions related to nutrient import to the cell via the the derepression of hexose transporters and the induction of amino acid permeases. Consistent with this, FIG selection also retrieved: the PMA1 gene, encoding the plasma H(+)-membrane ATPase; FOL2 and FOL3, involved in folic acid biosynthesis; and UBR2, which indirectly downregulates the proteasome genes. Finally, the other selected genes, ISU1, involved in the biosynthesis of the iron-sulphur cluster in mitochondria, and the less well functionally defined BSC5 and YBR270c, may participate in the cell's antioxidant and stress defence. The genes emerging from this FIG selection thus appear to be part of the downstream molecular mechanisms of the TOR signalling pathway, accounting for its effects on cell proliferation and longevity. From our results on gene expression under conditions of RPD3 overexpression, and by comparison with the available pharmacogenomics studies, it is proposed that FIG could be an invaluable approach for contributing to our understanding of complex cell regulatory systems.

Elovl3: a model gene to dissect homeostatic links between the circadian clock and nutritional status

The ELOVL3 protein is a very long-chain fatty acid elongase found in liver, skin, and brown adipose tissues. Circadian expression of the Elovl3 gene in the liver is perturbed in mutant CLOCK mice but persists in mice with severe hepatic dysfunction. A reliance on an intact clock, combined with the refractoriness to liver decompensation and the finding of a robust sexually dimorphic pattern of expression, evince a particularly complex mode of transcriptional control. The Elovl3 gene upstream region was repressed by RevErbalpha and activated by sterol-regulatory element binding protein-1 (SREBP1) transcription factors. We propose that the temporal coordination of RevErbalpha and SREBP1 activities integrates clock and nutrition signals to drive a subset of oscillatory transcripts in the liver. Proteolytic activation of SREBP1 is circadian in the liver, and because the cycle of SREBP1 activation was reversed after restricting meals to the inactive phase of the day, this factor could serve as an acute sensor of nutritional state. SREBP1 regulates many known lipogenic and cholesterogenic circadian genes; hence, our results could explain how feeding can override brain-derived entraining signals in the liver. This mechanism would permit a rapid adjustment in the sequence of key aspects of the absorptive and postabsorptive phases in the liver.

Yeast protein kinase Ptk2 localizes at the plasma membrane and phosphorylates in vitro the C-terminal peptide of the H+-ATPase

Glucose triggers posttranslational modifications that increase the activity of the Saccharomyces cerevisiae plasma membrane H+-ATPase (Pma1). Glucose activation of yeast H+-ATPase results from the change in two kinetic parameters: an increase in the affinity of the enzyme for ATP, depending on Ser899, and an increase in the Vmax involving Thr912. Our previous studies suggested that Ptk2 mediates the Ser899-dependent part of the activation. In this study we find that Ptk2 localized to the plasma membrane in a Triton X-100 insoluble fraction. In vitro phosphorylation assays using a recombinant GST-fusion protein comprising 30 C-terminal amino acids of Pma1 suggest that Ser899 is phosphorylated by Ptk2. Furthermore, we show that the Ptk2 carboxyl terminus is essential for glucose-dependent Pma1 activation and for the phosphorylation of Ser899.

Activation of the plasma membrane H+-ATPase of Saccharomyces cerevisiae by glucose is mediated by dissociation of the H+-ATPase-acetylated tubulin complex

In the yeast Saccharomyces cerevisiae, plasma membrane H(+)-ATPase is activated by d-glucose. We found that in the absence of glucose, this enzyme forms a complex with acetylated tubulin. Acetylated tubulin usually displays hydrophilic properties, but behaves as a hydrophobic compound when complexed with H(+)-ATPase, and therefore partitions into a detergent phase. When cells were treated with glucose, the H(+)-ATPase-tubulin complex was disrupted, with two consequences, namely (a) the level of acetylated tubulin in the plasma membrane decreased as a function of glucose concentration and (b) the H(+)-ATPase activity increased as a function of glucose concentration, as measured by both ATP-hydrolyzing capacity and H(+)-pumping activity. The addition of 2-deoxy-d-glucose inhibited the above glucose-induced phenomena, suggesting the involvement of glucose transporters. Whereas total tubulin is distributed uniformly throughout the cell, acetylated tubulin is concentrated near the plasma membrane. Results from immunoprecipitation experiments using anti-(acetylated tubulin) and anti-(H(+)-ATPase) immunoglobulins indicated a physical interaction between H(+)-ATPase and acetylated tubulin in the membranes of glucose-starved cells. When cells were pretreated with 1 mm glucose, this interaction was disrupted. Double immunofluorescence, observed by confocal microscopy, indicated that H(+)-ATPase and acetylated tubulin partially colocalize at the periphery of glucose-starved cells, with predominance at the outer and inner sides of the membrane, respectively. Colocalization was not observed when cells were pretreated with 1 mm glucose, reinforcing the idea that glucose treatment produces dissociation of the H(+)-ATPase-tubulin complex. Biochemical experiments using isolated membranes from yeast and purified tubulin from rat brain demonstrated inhibition of H(+)-ATPase activity by acetylated tubulin and concomitant increase of the H(+)-ATP ase-tubulin complex.

Ethanol and thermotolerance in the bioconversion of xylose by yeasts

The mechanisms underlying ethanol and heat tolerance are complex. Many different genes are involved, and the exact basis is not fully understood. The integrity of cytoplasmic and mitochondrial membranes is critical to maintain proton gradients for metabolic energy and nutrient uptake. Heat and ethanol stress adversely affect membrane integrity. These factors are particularly detrimental to xylose-fermenting yeasts because they require oxygen for biosynthesis of essential cell membrane and nucleic acid constituents, and they depend on respiration for the generation of ATP. Physiological responses to ethanol and heat shock have been studied most extensively in S. cerevisiae. However, comparative biochemical studies with other organisms suggest that similar mechanisms will be important in xylose-fermenting yeasts. The composition of a cell's membrane lipids shifts with temperature, ethanol concentration, and stage of cultivation. Levels of unsaturated fatty acids and ergosterol increase in response to temperature and ethanol stress. Inositol is involved in phospholipid biosynthesis, and it can increase ethanol tolerance when provided as a supplement. Membrane integrity determines the cell's ability to maintain proton gradients for nutrient uptake. Plasma membrane ATPase generates the proton gradient, and the biochemical characteristics of this enzyme contribute to ethanol tolerance. Organisms with higher ethanol tolerance have ATPase activities with low pH optima and high affinity for ATP. Likewise, organisms with ATPase activities that resist ethanol inhibition also function better at high ethanol concentrations. ATPase consumes a significant fraction of the total cellular ATP, and under stress conditions when membrane gradients are compromised the activity of ATPase is regulated. In xylose-fermenting yeasts, the carbon source used for growth affects both ATPase activity and ethanol tolerance. Cells can adapt to heat and ethanol stress by synthesizing trehalose and heat-shock proteins, which stabilize and repair denatured proteins. The capacity of cells to produce trehalose and induce HSPs correlate with their thermotolerance. Both heat and ethanol increase the frequency of petite mutations and kill cells. This might be attributable to membrane effects, but it could also arise from oxidative damage. Cytoplasmic and mitochondrial superoxide dismutases can destroy oxidative radicals and thereby maintain cell viability. Improved knowledge of the mechanisms underlying ethanol and thermotolerance in S. cerevisiae should enable the genetic engineering of these traits in xylose-fermenting yeasts.

A specific structural requirement for ergosterol in long-chain fatty acid synthesis mutants important for maintaining raft domains in yeast

Fungal sphingolipids contain ceramide with a very-long-chain fatty acid (C26). To investigate the physiological significance of the C26-substitution on this lipid, we performed a screen for mutants that are synthetically lethal with ELO3. Elo3p is a component of the ER-associated fatty acid elongase and is required for the final elongation cycle to produce C26 from C22/C24 fatty acids. elo3delta mutant cells thus contain C22/C24- instead of the natural C26-substituted ceramide. We now report that under these conditions, an otherwise nonessential, but also fungal-specific, structural modification of the major sterol of yeast, ergosterol, becomes essential, because mutations in ELO3 are synthetically lethal with mutations in ERG6. Erg6p catalyzes the methylation of carbon atom 24 in the aliphatic side chain of sterol. The lethality of an elo3delta erg6delta double mutant is rescued by supplementation with ergosterol but not with cholesterol, indicating a vital structural requirement for the ergosterol-specific methyl group. To characterize this structural requirement in more detail, we generated a strain that is temperature sensitive for the function of Erg6p in an elo3delta mutant background. Examination of raft association of the GPI-anchored Gas1p and plasma membrane ATPase, Pma1p, in the conditional elo3delta erg6(ts) double mutant, revealed a specific defect of the mutant to maintain raft association of preexisting Pma1p. Interestingly, in an elo3delta mutant at 37 degrees C, newly synthesized Pma1p failed to enter raft domains early in the biosynthetic pathway, and upon arrival at the plasma membrane was rerouted to the vacuole for degradation. These observations indicate that the C26 fatty acid substitution on lipids is important for establishing raft association of Pma1p and stabilizing the protein at the cell surface. Analysis of raft lipids in the conditional mutant strain revealed a selective enrichment of ergosterol in detergent-resistant membrane domains, indicating that specific structural determinants on both sterols and sphingolipids are required for their association into raft domains.

ARL1 and membrane traffic in Saccharomyces cerevisiae

To examine the functions of the Arf-like protein, Arl1p, in Saccharomyces cerevisiae, a null allele, arl1delta::HIS3, was constructed in two strains. In one background only, loss of ARL1 resulted in temperature-sensitive (ts) growth (suppressed on high-osmolarity media). Allelic variation at the SSD1 locus accounted for differences between strains. Strains lacking ARL1 exhibited several defects in membrane traffic. First, arl1delta strains secreted less protein as measured by TCA-precipitable radioactivity found in the media of [(35)S]-labelled cells. A portion of newly synthesized carboxypeptidase Y (CPY) was secreted rather than correctly targeted to the vacuole. Uptake of the fluid-phase marker, lucifer yellow, was reduced. All these phenotypes were exacerbated in an ssd1 background. The ts phenotype of the arl1deltassd1 strain was suppressed by YPT1, the yeast Rab1a homologue, suggesting that ARL1 and YPT1 have partially overlapping functions. These findings demonstrate that ARL1 encodes a regulator of membrane traffic.

Protein phosphatase type 1 regulates ion homeostasis in Saccharomyces cerevisiae

Protein phosphatase type 1 (PP1) is encoded by the essential gene GLC7 in Saccharomyces cerevisiae. glc7-109 (K259A, R260A) has a dominant, hyperglycogen defect and a recessive, ion and drug sensitivity. Surprisingly, the hyperglycogen phenotype is partially retained in null mutants of GAC1, GIP2, and PIG1, which encode potential glycogen-targeting subunits of Glc7. The R260A substitution in GLC7 is responsible for the dominant and recessive traits of glc7-109. Another mutation at this residue, glc7-R260P, confers only salt sensitivity, indicating that the glycogen and salt traits of glc7-109 are due to defects in distinct physiological pathways. The glc7-109 mutant is sensitive to cations, aminoglycosides, and alkaline pH and exhibits increased rates of l-leucine and 3,3'-dihexyloxacarbocyanine iodide uptake, but it is resistant to molar concentrations of sorbitol or KCl, indicating that it has normal osmoregulation. KCl suppresses the ion and drug sensitivities of the glc7-109 mutant. The CsCl sensitivity of this mutant is suppressed by recessive mutations in PMA1, which encodes the essential plasma membrane H(+)ATPase. Together, these results indicate that Glc7 regulates ion homeostasis by controlling ion transport and/or plasma membrane potential, a new role for Glc7 in budding yeast.

New aspects of the glucose activation of the H(+)-ATPase in the yeast Saccharomyces cerevisiae

The glucose-induced activation of plasma membrane ATPase from Saccharomyces cerevisiae was first described by Serrano in 1983. Many aspects of this signal transduction pathway are still obscure. In this paper, evidence is presented for the involvement of Snf3p as the glucose sensor related to this activation process. It is shown that, in addition to glucose detection by Snf3p, sugar transport is also necessary for activation of the ATPase. The participation of the G protein, Gpa2p, in transducing the internal signal (phosphorylated sugars) is also demonstrated. Moreover, the involvement of protein kinase C in the regulation of ATPase activity is confirmed. Finally, a model pathway is presented for sensing and transmission of the glucose activation signal of the yeast H(+)-ATPase.

The organizing potential of sphingolipids in intracellular membrane transport

Eukaryotes are characterized by endomembranes that are connected by vesicular transport along secretory and endocytic pathways. The compositional differences between the various cellular membranes are maintained by sorting events, and it has long been believed that sorting is based solely on protein-protein interactions. However, the central sorting station along the secretory pathway is the Golgi apparatus, and this is the site of synthesis of the sphingolipids. Sphingolipids are essential for eukaryotic life, and this review ascribes the sorting power of the Golgi to its capability to act as a distillation apparatus for sphingolipids and cholesterol. As Golgi cisternae mature, ongoing sphingolipid synthesis attracts endoplasmic reticulum-derived cholesterol and drives a fluid-fluid lipid phase separation that segregates sphingolipids and sterols from unsaturated glycerolipids into lateral domains. While sphingolipid domains move forward, unsaturated glycerolipids are retrieved by recycling vesicles budding from the sphingolipid-poor environment. We hypothesize that by this mechanism, the composition of the sphingolipid domains, and the surrounding membrane changes along the cis-trans axis. At the same time the membrane thickens. These features are recognized by a number of membrane proteins that as a consequence of partitioning between domain and environment follow the domains but can enter recycling vesicles at any stage of the pathway. The interplay between protein- and lipid-mediated sorting is discussed.

Tsc13p is required for fatty acid elongation and localizes to a novel structure at the nuclear-vacuolar interface in Saccharomyces cerevisiae

The TSC13/YDL015c gene was identified in a screen for suppressors of the calcium sensitivity of csg2Delta mutants that are defective in sphingolipid synthesis. The fatty acid moiety of sphingolipids in Saccharomyces cerevisiae is a very long chain fatty acid (VLCFA) that is synthesized by a microsomal enzyme system that lengthens the palmitate produced by cytosolic fatty acid synthase by two carbon units in each cycle of elongation. The TSC13 gene encodes a protein required for elongation, possibly the enoyl reductase that catalyzes the last step in each cycle of elongation. The tsc13 mutant accumulates high levels of long-chain bases as well as ceramides that harbor fatty acids with chain lengths shorter than 26 carbons. These phenotypes are exacerbated by the deletion of either the ELO2 or ELO3 gene, both of which have previously been shown to be required for VLCFA synthesis. Compromising the synthesis of malonyl coenzyme A (malonyl-CoA) by inactivating acetyl-CoA carboxylase in a tsc13 mutant is lethal, further supporting a role of Tsc13p in VLCFA synthesis. Tsc13p coimmunoprecipitates with Elo2p and Elo3p, suggesting that the elongating proteins are organized in a complex. Tsc13p localizes to the endoplasmic reticulum and is highly enriched in a novel structure marking nuclear-vacuolar junctions.

The cell wall integrity/remodeling MAPK cascade is involved in glucose activation of the yeast plasma membrane H(+)-ATPase

Glucose triggers transcriptional and post-transcriptional mechanisms that increase the amount and the activity of Saccharomyces cerevisiae plasma membrane H(+)-ATPase. In a previous study, we found that a mutation in the Rsp5 ubiquitin-protein ligase enzyme affected the post-transcriptional activation of the enzyme by glucose. Mutations at the RSP5 locus alter the glucose-triggered K(m) decrease. In a genetic screening for multicopy suppressors of the rsp5 mutation, we identified the WSC2/YNL283c gene. Deletion of the WSC2 gene disturbs ATPase activation by glucose, abolishing the K(m) decrease that occurs during this process. Wsc2 is a component of the PKC1-MPK1 mitogen-activated protein kinase (MAPK) signaling pathway that controls the cell wall integrity. Deletion of the MPK1/SLT2 gene disturbs the glucose-triggered K(m) decrease in ATPase.

Regulation of yeast H(+)-ATPase by protein kinases belonging to a family dedicated to activation of plasma membrane transporters

The regulation of electrical membrane potential is a fundamental property of living cells. This biophysical parameter determines nutrient uptake, intracellular potassium and turgor, uptake of toxic cations, and stress responses. In fungi and plants, an important determinant of membrane potential is the electrogenic proton-pumping ATPase, but the systems that modulate its activity remain largely unknown. We have characterized two genes from Saccharomyces cerevisiae, PTK2 and HRK1 (YOR267c), that encode protein kinases implicated in activation of the yeast plasma membrane H(+)-ATPase (Pma1) in response to glucose metabolism. These kinases mediate, directly or indirectly, an increase in affinity of Pma1 for ATP, which probably involves Ser-899 phosphorylation. Ptk2 has the strongest effect on Pma1, and ptk2 mutants exhibit a pleiotropic phenotype of tolerance to toxic cations, including sodium, lithium, manganese, tetramethylammonium, hygromycin B, and norspermidine. A plausible interpretation is that ptk2 mutants have a decreased membrane potential and that diverse cation transporters are voltage dependent. Accordingly, ptk2 mutants exhibited reduced uptake of lithium and methylammonium. Ptk2 and Hrk1 belong to a subgroup of yeast protein kinases dedicated to the regulation of plasma membrane transporters, which include Npr1 (regulator of Gap1 and Tat2 amino acid transporters) and Hal4 and Hal5 (regulators of Trk1 and Trk2 potassium transporters).

Role of a new mammalian gene family in the biosynthesis of very long chain fatty acids and sphingolipids

Whereas the physiological significance of microsomal fatty acid elongation is generally appreciated, its molecular nature is poorly understood. Here, we describe tissue-specific regulation of a novel mouse gene family encoding components implicated in the synthesis of very long chain fatty acids. The Ssc1 gene appears to be ubiquitously expressed, whereas Ssc2 and Cig30 show a restricted expression pattern. Their translation products are all integral membrane proteins with five putative transmembrane domains. By complementing the homologous yeast mutants, we found that Ssc1 could rescue normal sphingolipid synthesis in the sur4/elo3 mutant lacking the ability to synthesize cerotic acid (C(26:0)). Similarly, Cig30 reverted the phenotype of the fen1/elo2 mutant that has reduced levels of fatty acids in the C(20)-C(24) range. Further, we show that Ssc1 mRNA levels were markedly decreased in the brains of myelin-deficient mouse mutants known to have very low fatty acid chain elongation activity. Conversely, the dramatic induction of Cig30 expression during brown fat recruitment coincided with elevated elongation activity. Our results strongly implicate this new mammalian gene family in tissue-specific synthesis of very long chain fatty acids and sphingolipids.

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

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

The genome of fowlpox virus

Here we present the genomic sequence, with analysis, of a pathogenic fowlpox virus (FPV). The 288-kbp FPV genome consists of a central coding region bounded by identical 9.5-kbp inverted terminal repeats and contains 260 open reading frames, of which 101 exhibit similarity to genes of known function. Comparison of the FPV genome with those of other chordopoxviruses (ChPVs) revealed 65 conserved gene homologues, encoding proteins involved in transcription and mRNA biogenesis, nucleotide metabolism, DNA replication and repair, protein processing, and virion structure. Comparison of the FPV genome with those of other ChPVs revealed extensive genome colinearity which is interrupted in FPV by a translocation and a major inversion, the presence of multiple and in some cases large gene families, and novel cellular homologues. Large numbers of cellular homologues together with 10 multigene families largely account for the marked size difference between the FPV genome (260 to 309 kbp) and other known ChPV genomes (178 to 191 kbp). Predicted proteins with putative functions involving immune evasion included eight natural killer cell receptors, four CC chemokines, three G-protein-coupled receptors, two beta nerve growth factors, transforming growth factor beta, interleukin-18-binding protein, semaphorin, and five serine proteinase inhibitors (serpins). Other potential FPV host range proteins included homologues of those involved in apoptosis (e.g., Bcl-2 protein), cell growth (e.g., epidermal growth factor domain protein), tissue tropism (e.g., ankyrin repeat-containing gene family, N1R/p28 gene family, and a T10 homologue), and avian host range (e.g., a protein present in both fowl adenovirus and Marek's disease virus). The presence of homologues of genes encoding proteins involved in steroid biogenesis (e.g., hydroxysteroid dehydrogenase), antioxidant functions (e.g., glutathione peroxidase), vesicle trafficking (e.g., two alpha-type soluble NSF attachment proteins), and other, unknown conserved cellular processes (e.g., Hal3 domain protein and GSN1/SUR4) suggests that significant modification of host cell function occurs upon viral infection. The presence of a cyclobutane pyrimidine dimer photolyase homologue in FPV suggests the presence of a photoreactivation DNA repair pathway. This diverse complement of genes with likely host range functions in FPV suggests significant viral adaptation to the avian host.

Regulation of plasma membrane H(+)-ATPase in fungi and plants

The plasma membrane H+-ATPase from fungi and plants is a proton pump which plays a key role in the physiology of these organisms controlling essential functions such as nutrient uptake and intracellular pH regulation. In fungal and plant cells the activity of the proton pump is regulated by a large number of environmental factors at both transcriptional and post-translational levels. During the last years the powerful tools of molecular biology have been successfully used in fungi and plants allowing the cloning of a wide diversity of H+-ATPase genes and rapid progress on the molecular basis of reaction mechanism and regulation of the proton pump. This review focuses on recent results on regulation of plasma membrane H+-ATPase obtained by molecular approaches.

The growth of mdp1/rsp5 mutants of Saccharomyces cerevisiae is affected by mutations in the ATP-binding domain of the plasma membrane H+ -ATPase

Mutations in the PMA1 gene, encoding plasma membrane H+ -ATPase, were isolated that are able to suppress the temperature sensitivity (ts) phenotype of mdp1 mutations located in RSP5, the ubiquitin-protein ligase gene. The mdp1 mutants were previously found to change the mitochondrial/cytosolic distribution of Mod5p-I, the tRNA modifying enzyme, and to affect fluid phase endocytosis. The data presented reveal that mdp1 mutants are also pH sensitive, and hypersensitive to hygromycin B and paromomycin. The ts phenotype, hygromycin B and paromomycin sensitivity are suppressed by pmal-t, but the pH sensitivity, the effect of mdp1 on Mod5p-I cytoplasmic/mitochondrial localization and endocytosis are not. Characterization of pmal-t revealed the substitution of amino acid G(653)V in the ATP-binding domain of the H+ -ATPase. Our results indicate that Rsp5 ubiquitin-protein ligase may also influence, in addition to protein distribution, the functioning of plasma membrane H+ -ATPase and the response of cells to stress.

An alternative model for the transmembrane segments of the yeast H+-ATPase

An alternative topological model for the yeast plasma membrane H(+)-ATPase from K. lactis was deduced by joint prediction, using 11 algorithms for the prediction of transmembrane segments complemented with hydrophobic moment analysis. Similarly to the model currently used in the literature, this alternative model contains 10 transmembrane segments, four in the N-half and six in the C-half of the protein. However, the distribution of the membrane-associated segments on the C-half of the enzyme differs in both models. Nine of the 10 transmembrane segments are highly hydrophobic with low hydrophobic moments, and are probably involved in structural roles. The fifth transmembrane segment is, on the other hand, less hydrophobic, with the highest hydrophobic moment, suggesting that this segment might have a dynamic role in the coupling of the hydrolysis of ATP with the translocation of protons across the membrane. The alignment of the Ca(2+)-ATPase, the Na(+)/K(+)-ATPase and the H(+)-ATPase sequences showed that these proteins have the same topology in the N-half, but important differences were found at the C-half of the enzymes. In contrast with the mammalian ATPases, the fifth transmembrane segment in the H(+)-ATPase appears early in the sequence, giving rise to a shorter cytoplasmic central loop. This alternative model will be useful in the designing of site-directed mutagenesis experiments and contains information for the fitting of the amino acid sequence into the transmembrane region of the three-dimensional model of the ATPase.

Electrospray ionization tandem mass spectrometry (ESI-MS/MS) analysis of the lipid molecular species composition of yeast subcellular membranes reveals acyl chain-based sorting/remodeling of distinct molecular species en route to the plasma membrane

Nano-electrospray ionization tandem mass spectrometry (nano-ESI-MS/MS) was employed to determine qualitative differences in the lipid molecular species composition of a comprehensive set of organellar membranes, isolated from a single culture of Saccharomyces cerevisiae cells. Remarkable differences in the acyl chain composition of biosynthetically related phospholipid classes were observed. Acyl chain saturation was lowest in phosphatidylcholine (15.4%) and phosphatidylethanolamine (PE; 16.2%), followed by phosphatidylserine (PS; 29.4%), and highest in phosphatidylinositol (53.1%). The lipid molecular species profiles of the various membranes were generally similar, with a deviation from a calculated average profile of approximately +/- 20%. Nevertheless, clear distinctions between the molecular species profiles of different membranes were observed, suggesting that lipid sorting mechanisms are operating at the level of individual molecular species to maintain the specific lipid composition of a given membrane. Most notably, the plasma membrane is enriched in saturated species of PS and PE. The nature of the sorting mechanism that determines the lipid composition of the plasma membrane was investigated further. The accumulation of monounsaturated species of PS at the expense of diunsaturated species in the plasma membrane of wild-type cells was reversed in elo3Delta mutant cells, which synthesize C24 fatty acid-substituted sphingolipids instead of the normal C26 fatty acid-substituted species. This observation suggests that acyl chain-based sorting and/or remodeling mechanisms are operating to maintain the specific lipid molecular species composition of the yeast plasma membrane.

Metabolism and selected functions of sphingolipids in the yeast Saccharomyces cerevisiae

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

A novel mechanism of ion homeostasis and salt tolerance in yeast: the Hal4 and Hal5 protein kinases modulate the Trk1-Trk2 potassium transporter

The regulation of intracellular ion concentrations is a fundamental property of living cells. Although many ion transporters have been identified, the systems that modulate their activity remain largely unknown. We have characterized two partially redundant genes from Saccharomyces cerevisiae, HAL4/SAT4 and HAL5, that encode homologous protein kinases implicated in the regulation of cation uptake. Overexpression of these genes increases the tolerance of yeast cells to sodium and lithium, whereas gene disruptions result in greater cation sensitivity. These phenotypic effects of the mutations correlate with changes in cation uptake and are dependent on a functional Trk1-Trk2 potassium transport system. In addition, hal4 hal5 and trk1 trk2 mutants exhibit similar phenotypes: (i) they are deficient in potassium uptake; (ii) their growth is sensitive to a variety of toxic cations, including lithium, sodium, calcium, tetramethylammonium, hygromycin B, and low pH; and (iii) they exhibit increased uptake of methylammonium, an indicator of membrane potential. These results suggest that the Hal4 and Hal5 protein kinases activate the Trk1-Trk2 potassium transporter, increasing the influx of potassium and decreasing the membrane potential. The resulting loss in electrical driving force reduces the uptake of toxic cations and improves salt tolerance. Our data support a role for regulation of membrane potential in adaptation to salt stress that is mediated by the Hal4 and Hal5 kinases.

Involvement of long chain fatty acid elongation in the trafficking of secretory vesicles in yeast

Members of the synaptobrevin/VAMP family of v-SNAREs are thought to be essential for vesicle docking and exocytosis in both lower and higher eukaryotes. Here, we describe yeast mutants that appear to bypass the known v-SNARE requirement in secretion. Recessive mutations in either VBM1 or VBM2, which encode related ER-localized membrane proteins, allow yeast to grow normally and secrete in the absence of Snc v-SNAREs. These mutants show selective alterations in protein transport, resulting in the differential trafficking and secretion of certain protein cargo. Yet, processing of the vacuolar marker, carboxypeptidase Y, and the secreted protein, invertase, appear normal in these mutants indicating that general protein trafficking early in the pathway is unaffected. Interestingly, VBM1 and VBM2 are allelic to ELO3 and ELO2, two genes that have been shown recently to mediate the elongation of very long chain fatty acids and subsequent ceramide and inositol sphingolipid synthesis. Thus, the v-SNARE requirement in constitutive exocytosis is abrogated by mutations in early components of the secretory pathway that act at the level of lipid synthesis to affect the ability of secretory vesicles to sort and deliver protein cargo.

Regulation of a plant plasma membrane sucrose transporter by phosphorylation

The protein phosphatase inhibitor okadaic acid (OA) either provided directly to sugar beet (Beta vulgaris L.) leaf discs or infiltrated in the leaf blade rapidly inhibited sucrose uptake. Methyl okadaic acid, a biologically inactive analogue of OA, had only a marginal effect on uptake. OA inhibited proton-motive force-driven uptake of sucrose into plasma membrane vesicles, without affecting their proton permeability. OA did not significantly affect the amount of sucrose transporters present in the vesicles, as estimated by ELISA with specific antibodies. It is concluded that OA directly inhibits the activity of a H+-sucrose cotransporter of the plant plasma membrane, likely by maintaining it in a phosphorylated form.

Yeast gene YOR137c is involved in the activation of the yeast plasma membrane H+-ATPase by glucose

Glucose triggers transcriptional and post-transcriptional mechanisms that increase the level and activity of Saccharomyces cerevisiae plasma membrane H+-ATPase. We have studied the post-transcriptional activation of the enzyme by glucose and have found that the YOR137c gene product is implicated in this activation. Deletion of YOR137c does not affect the level of Pma1 at the plasma membrane, but disturbs the glucose-triggered Vmax increase of the enzyme. We propose that at least two independent mechanisms are involved in glucose activation of the H+-ATPase.

Cig30, a mouse member of a novel membrane protein gene family, is involved in the recruitment of brown adipose tissue

We have identified a previously uncharacterized gene that is implicated in the thermogenic function of brown adipose tissue of mice. This gene, termed Cig30, is the first mammalian member of a novel gene family comprising several nematode and yeast genes, such as SUR4 and FEN1, mutation of which is associated with highly pleiotropic phenotypes. It codes for a 30-kDa plasma membrane glycoprotein with five putative transmembrane domains. The Cig30 mRNA was readily detected only in brown fat and liver. When animals were exposed to a 3-day cold stress, the Cig30 expression was selectively elevated in brown fat more than 200-fold. Similar increases were brought about in two other conditions of brown fat recruitment, namely during perinatal development and after cafeteria diet. The magnitude of Cig30 mRNA induction in the cold could be mimicked by chronic norepinephrine treatment in vivo. However, in primary cultures of brown adipocytes, a synergistic action of norepinephrine and dexamethasone was required for full expression of the gene, indicating that both catecholamines and glucocorticoids are required for the induction of Cig30. We propose that the CIG30 protein is involved in a pathway connected with brown fat hyperplasia.

Glucose activation of the yeast plasma membrane H+-ATPase requires the ubiquitin-proteasome proteolytic pathway

Glucose triggers transcriptional and post-transcriptional mechanisms that increase the level and activity of Saccharomyces cerevisiae plasma membrane H+-ATPase. We have studied the post-transcriptional activation of the enzyme by glucose and have found that Rsp5, a ubiquitin-protein ligase enzyme, Ubc4, a ubiquitin-conjugating enzyme, and the 26S proteasome complex are implicated in this activation. These results suggest that ATPase activation by glucose requires the ubiquitin-proteasome proteolytic pathway. This is supported by the fact that over-expression of the ubiquitin-specific protease Ubp2, which cleaves ubiquitin from its branched conjugates, inhibits this activation. We propose that glucose triggers degradation of an inhibitory protein resulting in enzyme activation.

ELO2 and ELO3, homologues of the Saccharomyces cerevisiae ELO1 gene, function in fatty acid elongation and are required for sphingolipid formation

ELO2 and ELO3 were identified from the Saccharomyces cerevisiae genome data base as homologues of ELO1, a gene involved in the elongation of the fatty acid 14:0 to 16:0. Mutations in these genes have previously been shown to produce pleiotropic effects involving a number of membrane functions. The simultaneous disruption of ELO2 and ELO3 has also been shown to produce synthetic lethality, indicating that they have related and/or overlapping functions. Gas chromatography and gas chromatography/mass spectroscopy analyses reveal that null mutations of ELO2 and ELO3 produce defects in the formation of very long chain fatty acids. Analysis of the null mutants indicates that these genes encode components of the membrane-bound fatty acid elongation systems that produce the 26-carbon very long chain fatty acids that are precursors for ceramide and sphingolipids. Elo2p appears to be involved in the elongation of fatty acids up to 24 carbons. It appears to have the highest affinity for substrates with chain lengths less than 22 carbons. Elo3p apparently has a broader substrate specificity and is essential for the conversion of 24-carbon acids to 26-carbon species. Disruption of either gene reduces cellular sphingolipid levels and results in the accumulation of the long chain base, phytosphingosine. Null mutations in ELO3 result in accumulation of labeled precursors into inositol phosphoceramide, with little labeling in the more complex mannosylated sphingolipids, whereas disruption of ELO2 results in reduced levels of all sphingolipids.

Modulation of maize roots H(+)-ATPase by sulfated polysaccharides

Vesicles derived from maize roots retain a membrane bound H(+)-ATPase that is able to pump H+ at the expense of ATP hydrolysis. In this work it is shown that heparin, fucose-branched chondroitin sulfate and dextran sulfate 8000 promote a shift of the H(+)-ATPase optimum pH from 6.0 to 7.0. This shift is a result of a dual effect of the sulfated polysaccharides, inhibition at pH 6.0 and activation at pH 7.0. At pH 6.0 dextran 8000 promotes an increase of the apparent K(m) for ATP from 0.28 to 0.95 mM and a decrease of the Vmax from 14.5 to 7.1 mumol Pi/mg x 30 min-1. At pH 7.0 dextran 8000 promotes an increase in Vmax from 6.7 to 11.7 mumol Pi/mg x 30 min-1. In the presence of lysophosphatidylcholine the inhibitory effect of the sulfated polysaccharides observed at pH 6.0 was not altered but the activation of pH 7.0 decreased. It was found that in the presence of sulfated polysaccharides the ATPase became highly sensitive to K+ and Na+. Both the inhibition at pH 6.0 and the activation promoted by the polysaccharide were antagonized by monovalent cations (K+ > Na+ > > Li+).

Characterization of a new gene family developing pleiotropic phenotypes upon mutation in Saccharomyces cerevisiae

The aim of this paper is to gather and complete data about four members of a new gene family. Mutation in SUR4 gene was originally selected as a suppressor of defects caused by mutations in RVS161 or RVS167 genes. Cloning and sequencing of the SUR4 gene were performed. The deduced protein contains six putative transmembrane domains. Sequence comparison revealed that two yeast genes, FEN1 and JO343, shared significant similarities with SUR4. Mutants for SUR4 and FEN1 have the same pleiotropic phenotype, including bud localization defects, resistance to an immunosuppressor and resistance to ergosterol biosynthesis inhibitors. The double inactivation of SUR4 and FEN1 genes is lethal. These data and other aspects implicating SUR4 in glucose metabolism, suggest an involvement of these genes in the dynamics of cortical actin cytoskeleton in response to nutrient availability. Moreover, the existence of a fourth homologous gene in C. elegans extends the family to pluricellular organisms.