Applications of a microplate reader in yeast physiology research

'Small volume-big problem': culturing Yarrowia lipolytica in high-throughput micro-formats

With the current progress in the 'design' and 'build' stages of the 'design-build-test-learn' cycle, many synthetic biology projects become 'test-limited'. Advances in the parallelization of microbes cultivations are of great aid, however, for many species down-scaling leaves a metabolic footprint. Yarrowia lipolytica is one such demanding yeast species, for which scaling-down inevitably leads to perturbations in phenotype development. Strictly aerobic metabolism, propensity for filamentation and adhesion to hydrophobic surfaces, spontaneous flocculation, and high acidification of media are just several characteristics that make the transfer of the micro-scale protocols developed for the other microbial species very challenging in this case. It is well recognized that without additional 'personalized' optimization, either MTP-based or single-cell-based protocols are useless for accurate studies of Y. lipolytica phenotypes. This review summarizes the progress in the scaling-down and parallelization of Y. lipolytica cultures, highlighting the challenges that occur most frequently and strategies for their overcoming. The problem of Y. lipolytica cultures down-scaling is illustrated by calculating the costs of micro-cultivations, and determining the unintentionally introduced, thus uncontrolled, variables. The key research into culturing Y. lipolytica in various MTP formats and micro- and pico-bioreactors is discussed. Own recently developed and carefully pre-optimized high-throughput cultivation protocol is presented, alongside the details from the optimization stage. We hope that this work will serve as a practical guide for those working with Y. lipolytica high-throughput screens.

Heterologous expression reveals unique properties of Trk K+ importers from nonconventional biotechnologically relevant yeast species together with their potential to support Saccharomyces cerevisiae growth

In the model yeast Saccharomyces cerevisiae, Trk1 is the main K⁺ importer. It is involved in many important physiological processes, such as the maintenance of ion homeostasis, cell volume, intracellular pH, and plasma-membrane potential. The ScTrk1 protein can be of great interest to industry, as it was shown that changes in its activity influence ethanol production and tolerance in S. cerevisiae and also cell performance in the presence of organic acids or high ammonium under low K⁺ conditions. Nonconventional yeast species are attracting attention due to their unique properties and as a potential source of genes that encode proteins with unusual characteristics. In this work, we aimed to study and compare Trk proteins from Debaryomyces hansenii, Hortaea werneckii, Kluyveromyces marxianus, and Yarrowia lipolytica, four biotechnologically relevant yeasts that tolerate various extreme environments. Heterologous expression in S. cerevisiae cells lacking the endogenous Trk importers revealed differences in the studied Trk proteins' abilities to support the growth of cells under various cultivation conditions such as low K⁺ or the presence of toxic cations, to reduce plasma-membrane potential or to take up Rb⁺ . Examination of the potential of Trks to support the stress resistance of S. cerevisiae wild-type strains showed that Y. lipolytica Trk1 is a promising tool for improving cell tolerance to both low K⁺ and high salt and that the overproduction of S. cerevisiae's own Trk1 was the most efficient at improving the growth of cells in the presence of highly toxic Li⁺ ions.

A high-throughput yeast approach to characterize aquaporin permeabilities: Profiling the Arabidopsis PIP aquaporin sub-family

INTRODUCTION

Engineering membrane transporters to achieve desired functionality is reliant on availability of experimental data informing structure-function relationships and intelligent design. Plant aquaporin (AQP) isoforms are capable of transporting diverse substrates such as signaling molecules, nutrients, metalloids, and gases, as well as water. AQPs can act as multifunctional channels and their transport function is reliant on many factors, with few studies having assessed transport function of specific isoforms for multiple substrates.

METHODS

High-throughput yeast assays were developed to screen for transport function of plant AQPs, providing a platform for fast data generation and cataloguing of substrate transport profiles. We applied our high-throughput growth-based yeast assays to screen all 13 Arabidopsis PIPs (AtPIPs) for transport of water and several neutral solutes: hydrogen peroxide (H2O2), boric acid (BA), and urea. Sodium (Na+) transport was assessed using elemental analysis techniques.

RESULTS

All AtPIPs facilitated water and H2O2 transport, although their growth phenotypes varied, and none were candidates for urea transport. For BA and Na+ transport, AtPIP2;2 and AtPIP2;7 were the top candidates, with yeast expressing these isoforms having the most pronounced toxicity response to BA exposure and accumulating the highest amounts of Na+. Linking putative AtPIP isoform substrate transport profiles with phylogenetics and gene expression data, enabled us to align possible substrate preferences with known and hypothesized biological roles of AtPIPs.

DISCUSSION

This testing framework enables efficient cataloguing of putative transport functionality of diverse AQPs at a scale that can help accelerate our understanding of AQP biology through big data approaches (e.g. association studies). The principles of the individual assays could be further adapted to test additional substrates. Data generated from this framework could inform future testing of AQP physiological roles, and address knowledge gaps in structure-function relationships to improve engineering efforts.

A 4 × 4 Array of Complementary Split-Ring Resonators for Label-Free Dielectric Spectroscopy

In this study, complementary split-ring resonator (CSRR) metamaterial structures are proposed for label-free dielectric spectroscopy of liquids in microplates. This novel combination of an array of sensors and microplates is readily scalable and thus offers a great potential for non-invasive, rapid, and label-free dielectric spectroscopy of liquids in large microplate arrays. The proposed array of sensors on a printed circuit board consists of a microstrip line coupled to four CSRRs in cascade with resonant frequencies ranging from 7 to 10 GHz, spaced around 1 GHz. The microwells were manufactured and bonded to the CSRR using polydimethylsiloxane, whose resonant frequency is dependent on a complex relative permittivity of the liquid loaded in the microwell. The individual microstrip lines with CSRRs were interconnected to the measurement equipment using two electronically controllable microwave switches, which enables microwave measurements of the 4 × 4 CSRR array using only a two-port measurement system. The 4 × 4 microwell sensor arrays were calibrated and evaluated using water-ethanol mixtures with different ethanol concentrations. The proposed measurement setup offers comparable results to ones obtained using a dielectric probe, confirming the potential of the planar sensor array for large-scale microplate experiments.

Minority potassium-uptake system Trk2 has a crucial role in yeast survival of glucose-induced cell death

The existence of programmed cell death in Saccharomyces cerevisiae has been reported for many years. Glucose induces the death of S. cerevisiae in the absence of additional nutrients within a few hours, and the absence of active potassium uptake makes cells highly sensitive to this process. S. cerevisiae cells possess two transporters, Trk1 and Trk2, which ensure a high intracellular concentration of potassium, necessary for many physiological processes. Trk1 is the major system responsible for potassium acquisition in growing and dividing cells. The contribution of Trk2 to potassium uptake in growing cells is almost negligible, but Trk2 becomes crucial for stationary cells for their survival of some stresses, e.g. anhydrobiosis. As a new finding, we show that both Trk systems contribute to the relative thermotolerance of S. cerevisiae BY4741. Our results also demonstrate that Trk2 is much more important for the cell survival of glucose-induced cell death than Trk1, and that stationary cells deficient in active potassium uptake lose their ATP stocks more rapidly than cells with functional Trk systems. This is probably due to the upregulated activity of plasma-membrane Pma1 H⁺-ATPase, and consequently, it is the reason why these cells die earlier than cells with functional active potassium uptake.

K+-specific importers Trk1 and Trk2 play different roles in Ca2+ homeostasis and signalling in Saccharomyces cerevisiae cells

The maintenance of K+ and Ca2+ homeostasis is crucial for many cellular functions. Potassium is accumulated in cells at high concentrations, while the cytosolic level of calcium, to ensure its signalling function, is kept at low levels and transiently increases in response to stresses. We examined Ca2+ homeostasis and Ca2+ signalling in Saccharomyces cerevisiae strains lacking plasma-membrane K+ influx (Trk1 and Trk2) or efflux (Tok1, Nha1 and Ena1-5) systems. The lack of K+ exporters slightly increased the cytosolic Ca2+, but did not alter the Ca2+ tolerance or Ca2+-stress response. In contrast, the K+-importers Trk1 and Trk2 play important and distinct roles in the maintenance of Ca2+ homeostasis. The presence of Trk1 was vital mainly for the growth of cells in the presence of high extracellular Ca2+, whilst the lack of Trk2 doubled steady-state intracellular Ca2+ levels. The absence of both K+ importers highly increased the Ca2+ response to osmotic or CaCl2 stresses and altered the balance between Ca2+ flux from external media and intracellular compartments. In addition, we found Trk2 to be important for the tolerance to high KCl and hygromycin B in cells growing on minimal media. All the data describe new interconnections between potassium and calcium homeostasis in S. cerevisiae.

Astaxanthin protects oxidative stress mediated DNA damage and enhances longevity in Saccharomyces cerevisiae

Reactive oxygen species (ROS) have long been found to play an important role in oxidative mediated DNA damage. Fortunately, cells possess an antioxidant system that can neutralize ROS. However, oxidative stress occurs when antioxidants are overwhelmed by ROS or impaired antioxidant pathways. This study was carried out to find the protective effect of astaxanthin on the yeast DNA repair-deficient mutant cells under hydrogen peroxide stress. The results showed that astaxanthin enhances the percent cell growth of rad1∆, rad51∆, apn1∆, apn2∆ and ogg1∆ cells. Further, the spot test and colony-forming unit count results confirmed that astaxanthin protects DNA repair mutant cells from oxidative stress. The DNA binding property of astaxanthin studied by in silico and in vitro methods indicated that astaxanthin binds to the DNA in the major and minor groove, and that might protect DNA against oxidative stress induced by Fenton's reagent. The intracellular ROS, 8-OHdG level and the DNA fragmentation as measured by comet tail was reduced by astaxanthin under oxidative stress. Similarly, reduced nuclear fragmentation and chromatin condensation results suggest that astaxanthin might reduce apoptosis. Finally, we show that astaxanthin decreases the accumulation of mutation rate and enhances the longevity of DNA repair-deficient mutants' cells during a chronological lifespan.

Astaxanthin supplementation reduces dichlorvos-induced cytotoxicity in Saccharomyces cerevisiae

This study evaluates the protective effect of astaxanthin against dichlorvos cytotoxicity in yeast Saccharomyces cerevisiae. Dichlorvos induce a dose-dependent cytotoxicity in yeast cells, which is mediated by oxidative stress. Our experimental results showed pre-treatment with astaxanthin enhances cell viability by 20-30% in yeast cells exposed to dichlorvos. A decrease in DCF fluorescence intensity and lipid peroxidation, increased SOD activity, and glutathione levels in astaxanthin-treated cells indicate that astaxanthin protected the cells against dichlorvos-induced oxidative stress. Reduced chromatin condensation and nuclear fragmentation in astaxanthin pre-treated cells also indicate that astaxanthin rescued the cells from dichlorvos-induced apoptosis. Our overall results suggest that dichlorvos induces oxidative stress-mediated cytotoxicity in yeast cells, and that was rescued by astaxanthin pre-treatment.

A simple and inexpensive quantitative technique for determining chemical sensitivity in Saccharomyces cerevisiae

Chemical sensitivity, growth inhibition in response to a chemical, is a powerful phenotype that can reveal insight into diverse cellular processes. Chemical sensitivity assays are used in nearly every model system, however the yeast Saccharomyces cerevisiae provides a particularly powerful platform for discovery and mechanistic insight from chemical sensitivity assays. Here we describe a simple and inexpensive approach to determine chemical sensitivity quantitatively in yeast in the form of half maximal inhibitory concentration (IC50) using common laboratory equipment. We demonstrate the utility of this method using chemicals commonly used to monitor changes in membrane traffic. When compared to traditional agar-based plating methods, this method is more sensitive and can detect defects not apparent using other protocols. Additionally, this method reduces the experimental protocol from five days to 18 hours for the toxic amino acid canavanine. Furthermore, this method provides reliable results using lower amounts of chemicals. Finally, this method is easily adapted to additional chemicals as demonstrated with an engineered system that activates the spindle assembly checkpoint in response to rapamycin with differing efficiencies. This approach provides researchers with a cost-effective method to perform chemical genetic profiling without specialized equipment.

Osmotolerance of Dekkera bruxellensis and the role of two Stl glycerol-proton symporters

Dekkera bruxellensis is important for lambic beer fermentation but is considered a spoilage yeast in wine fermentation. We compared two D. bruxellensis strains isolated from wine and found that they differ in some basic properties, including osmotolerance. The genomes of both strains contain two highly similar copies of genes encoding putative glycerol-proton symporters from the STL family that are important for yeast osmotolerance. Cloning of the two DbSTL genes and their expression in suitable osmosensitive Saccharomyces cerevisiae mutants revealed that both identified genes encode functional glycerol uptake systems, but only DbStl2 has the capacity to improve the osmotolerance of S. cerevisiae cells.

Characterization of the Candida albicans Amino Acid Permease Family: Gap2 Is the Only General Amino Acid Permease and Gap4 Is an S-Adenosylmethionine (SAM) Transporter Required for SAM-Induced Morphogenesis

Candida albicans is a commensal organism that can thrive in many niches in its human host. The environmental conditions at these different niches differ quite a bit, and this fungus must be able to sense these changes and adapt its metabolism to them. Apart from glucose and other sugars, the uptake of amino acids is very important. This is underscored by the fact that the C. albicans genome encodes 6 orthologues of the Saccharomyces. cerevisiae general amino acid permease Gap1 and many other amino acid transporters. In this work, we characterize these six permeases and we show that C. albicans Gap2 is the functional orthologue of ScGap1 and that C. albicans Gap4 is an orthologue of ScSam3, an S-adenosylmethionine (SAM) transporter. Furthermore, we show that Gap4 is required for SAM-induced morphogenesis, an important virulence factor of C. albicans.

Amino acids are key sources of nitrogen for growth of Candida albicans. In order to detect and take up these amino acids from a broad range of different and changing nitrogen sources inside the host, this fungus must be able to adapt via its expression of genes for amino acid uptake and further metabolism. We analyzed six C. albicans putative general amino acid permeases based on their homology to the Saccharomyces cerevisiae Gap1 general amino acid permease. We generated single- and multiple-deletion strains and found that, based on growth assays and transcriptional or posttranscriptional regulation, Gap2 is the functional orthologue to ScGap1, with broad substrate specificity. Expression analysis showed that expression of all GAP genes is under control of the Csy1 amino acid sensor, which is different from the situation in S. cerevisiae, where the expression of ScGAP1 is not regulated by Ssy1. We show that Gap4 is the functional orthologue of ScSam3, the only S-adenosylmethionine (SAM) transporter in S. cerevisiae, and we report that Gap4 is required for SAM-induced morphogenesis.

IMPORTANCECandida albicans is a commensal organism that can thrive in many niches in its human host. The environmental conditions at these different niches differ quite a bit, and this fungus must be able to sense these changes and adapt its metabolism to them. Apart from glucose and other sugars, the uptake of amino acids is very important. This is underscored by the fact that the C. albicans genome encodes 6 orthologues of the Saccharomyces. cerevisiae general amino acid permease Gap1 and many other amino acid transporters. In this work, we characterize these six permeases and we show that C. albicans Gap2 is the functional orthologue of ScGap1 and that C. albicans Gap4 is an orthologue of ScSam3, an S-adenosylmethionine (SAM) transporter. Furthermore, we show that Gap4 is required for SAM-induced morphogenesis, an important virulence factor of C. albicans.

Vcx1 and ESCRT components regulate intracellular pH homeostasis in the response of yeast cells to calcium stress

Endosomal sorting complexes required for transport (ESCRTs) are involved in the formation of multivesicular bodies and sorting of targeted proteins to the yeast vacuole. The deletion of seven genes encoding components of the ESCRT machinery render Saccharomyces cerevisiae cells sensitive to high extracellular CaCl2 concentrations as well as to low pH in media. In this work, we focused on intracellular pH (pHin) homeostasis of these mutants. None of the studied ESCRT mutants exhibited an altered pHin level compared to the wild type under standard growth conditions. Nevertheless, 60 min of CaCl2 treatment resulted in a more significant drop in pHin levels in these mutants than in the wild type, suggesting that pHin homeostasis is affected in ESCRT mutants upon the addition of calcium. Similarly, CaCl2 treatment caused a bigger pHin decrease in cells lacking the vacuolar Ca(2+)/H(+) antiporter Vcx1 which indicates a role for this protein in the maintenance of proper pHin homeostasis when cells need to cope with a high CaCl2 concentration in media. Importantly, ESCRT gene deletions in the vcx1Δ strain did not result in an increase in the CaCl2-invoked drop in the pHin levels of cells, which demonstrates a genetic interaction between VCX1 and studied ESCRT genes.

The high-capacity specific fructose facilitator ZrFfz1 is essential for the fructophilic behavior of Zygosaccharomyces rouxii CBS 732T

Zygosaccharomyces rouxii is a fructophilic yeast that consumes fructose preferably to glucose. This behavior seems to be related to sugar uptake. In this study, we constructed Z. rouxii single-, double-, and triple-deletion mutants in the UL4 strain background (a ura3 strain derived from CBS 732(T)) by deleting the genes encoding the specific fructose facilitator Z. rouxii Ffz1 (ZrFfz1), the fructose/glucose facilitator ZrFfz2, and/or the fructose symporter ZrFsy1. We analyzed the effects on the growth phenotype, on kinetic parameters of fructose and glucose uptake, and on sugar consumption profiles. No growth phenotype was observed on fructose or glucose upon deletion of FFZ genes. Deletion of ZrFFZ1 drastically reduced fructose transport capacity, increased glucose transport capacity, and eliminated the fructophilic character, while deletion of ZrFFZ2 had almost no effect. The strain in which both FFZ genes were deleted presented even higher consumption of glucose than strain Zrffz1Δ, probably due to a reduced repressing effect of fructose. This study confirms the molecular basis of the Z. rouxii fructophilic character, demonstrating that ZrFfz1 is essential for Z. rouxii fructophilic behavior. The gene is a good candidate to improve the fructose fermentation performance of industrial Saccharomyces cerevisiae strains.

Fluconazole affects the alkali-metal-cation homeostasis and susceptibility to cationic toxic compounds of Candida glabrata

Candida glabrata is a salt-tolerant and fluconazole (FLC)-resistant yeast species. Here, we analyse the contribution of plasma-membrane alkali-metal-cation exporters, a cation/proton antiporter and a cation ATPase to cation homeostasis and the maintenance of membrane potential (ΔΨ). Using a series of single and double mutants lacking CNH1 and/or ENA1 genes we show that the inability to export potassium and toxic alkali-metal cations leads to a slight hyperpolarization of the plasma membrane of C. glabrata cells; this hyperpolarization drives more cations into the cells and affects cation homeostasis. Surprisingly, a much higher hyperpolarization of C. glabrata plasma membrane was produced by incubating cells with subinhibitory concentrations of FLC. FLC treatment resulted in a substantially increased sensitivity of cells to various cationic drugs and toxic cations that are driven into the cell by negative-inside plasma-membrane potential. The effect of the combination of FLC plus cationic drug treatment was enhanced by the malfunction of alkali-metal-cation transporters that contribute to the regulation of membrane potential and cation homeostasis. In summary, we show that the combination of subinhibitory concentrations of FLC and cationic drugs strongly affects the growth of C. glabrata cells.

Osmotolerant yeast species differ in basic physiological parameters and in tolerance of non-osmotic stresses

Osmotolerance is the ability to grow in an environment with a high osmotic pressure. In this study we compared the physiological parameters and tolerance to osmotic and non-osmotic stresses of three osmotolerant yeast species, Debaryomyces hansenii, Pichia farinosa (sorbitophila) and Zygosaccharomyces rouxii, with those of wild-type Saccharomyces cerevisiae. Although the osmotolerant species did not differ significantly in their basic parameters, such as cell size or growth capacity, they had different abilities to survive anhydrobiosis, potassium limitation or the presence of toxic cationic drugs. When their osmotolerance was compared, the results revealed that some of the species isolated as sugar/polyol-tolerant (e.g.  P. farinosa) are also highly tolerant to salts and, vice versa, some strains isolated from an environment with high concentration of salt (e.g. Z. rouxii ATCC 42981) tolerate high concentrations of sugars. None of the tested strains and species was osmophilic. Taken together, our results showed that P. farinosa (sorbitophila) is the most robust species when coping with various stresses, while Z. rouxii CBS 732, although osmotolerant in general, is not specifically salt-tolerant and is quite sensitive to most of the tested stress conditions.

ZrFsy1, a high-affinity fructose/H+ symporter from fructophilic yeast Zygosaccharomyces rouxii

Zygosaccharomyces rouxii is a fructophilic yeast than can grow at very high sugar concentrations. We have identified an ORF encoding a putative fructose/H⁺ symporter in the Z. rouxii CBS 732 genome database. Heterologous expression of this ORF in a S. cerevisiae strain lacking its own hexose transporters (hxt-null) and subsequent kinetic characterization of its sugar transport activity showed it is a high-affinity low-capacity fructose/H⁺ symporter, with K_m 0.45±0.07 mM and V_max 0.57±0.02 mmol h⁻¹ (gdw) ⁻¹. We named it ZrFsy1. This protein also weakly transports xylitol and sorbose, but not glucose or other hexoses. The expression of ZrFSY1 in Z. rouxii is higher when the cells are cultivated at extremely low fructose concentrations (<0.2%) and on non-fermentable carbon sources such as mannitol and xylitol, where the cells have a prolonged lag phase, longer duplication times and change their microscopic morphology. A clear phenotype was determined for the first time for the deletion of a fructose/H⁺ symporter in the genome where it occurs naturally. The effect of the deletion of ZrFSY1 in Z. rouxii cells is only evident when the cells are cultivated at very low fructose concentrations, when the ZrFsy1 fructose symporter is the main active fructose transporter system.

Real-time monitoring of relative peptide-protein interaction strengths in the yeast two-hybrid system

The yeast two-hybrid (Y2H) system is one of the most technically straightforward, effective, and widely used tools for the discovery of the binary peptide or protein interactions. However, its exceptional detection sensitivity poses a serious challenge for affinity ranking and hence prioritizing the resultant large number of putative interactors for follow-up analyses. To overcome this apparent bottleneck, we describe here a novel yeast growth curve-based interaction-monitoring approach that permits semiautomatic quantification, comparison, and statistically ascertained scoring of a large collection of Y2H interactions under real-time conditions. Initially, we conducted a proof-of-concept test of five literature-validated peptide-protein interactions with known affinities in the low μM range, and subsequently used the method to classify 88 novel vitamin D receptor-binding peptides derived from high-throughput screening of a highly diverse artificial peptide aptamer library. Based on our in-depth data evaluation, we conclude that real-time monitoring of clone growth as a measure of relative binding strength offers a facile, cost-effective, accurate, reproducible, and further adaptable complement to standard Y2H-derived clone management.

Comparison of the influence of small GTPases Arl1 and Ypt6 on yeast cells' tolerance to various stress factors

The GTPases Arl1 and Ypt6 are involved in the intracellular transport of vesicles and their fusion with the trans-Golgi network. This work is focused on comparing the roles of these GTPases in the tolerance of Saccharomyces cerevisiae cells to an increased concentration of alkali metal cations and other stress factors. We studied the phenotypes of arl1 or ypt6 deletions in combination with the deletions of genes encoding alkali-metal-cation transporters (ena1-4, nha1, nhx1, and kha1). Salt sensitivity of the arl1 and ypt6 mutants was shown to be independent of the tested cation transporters and electrochemical membrane potential. Phenotype manifestations of ypt6 deletion were usually more prominent than those of arl1 (cells were more sensitive to KCl, NaCl, LiCl, hygromycin B, increased temperature, and increased pH). At suboptimal temperature, the growth inhibition of arl1 and ypt6 mutants was approximately the same, and low pH was the only condition where arl1 mutants grew even worse than ypt6 mutants. Overexpression of the ARL1 gene suppressed the phenotypes of ypt6 deletion; however, this did not work vice versa (additional copies of YPT6 could not replace ARL1). Our results suggest partially overlapping functions of the GTPases in resistance to various stress factors, with Ypt6 being more efficient under physiological conditions and Arl1 more versatile when overexpressed.

Absorbance and fluorometric sensing with capillary wells microplates

Detection and readout from small volume assays in microplates are a challenge. The capillary wells microplate approach [Ng et al., Appl. Phys. Lett. 93, 174105 (2008)] offers strong advantages in small liquid volume management. An adapted design is described and shown here to be able to detect, in a nonimaging manner, fluorescence and absorbance assays minus the error often associated with meniscus forming at the air-liquid interface. The presence of bubbles in liquid samples residing in microplate wells can cause inaccuracies. Pipetting errors, if not adequately managed, can result in misleading data and wrong interpretations of assay results; particularly in the context of high throughput screening. We show that the adapted design is also able to detect for bubbles and pipetting errors during actual assay runs to ensure accuracy in screening.

The osmotolerant fructophilic yeast Zygosaccharomyces rouxii employs two plasma-membrane fructose uptake systems belonging to a new family of yeast sugar transporters

Owing to its high resistance to weak-acid preservatives and extreme osmotolerance, Zygosaccharomyces rouxii is one of the main spoilage yeasts of sweet foods and beverages. In contrast with Saccharomyces cerevisiae, Z. rouxii is a fructophilic yeast; it consumes fructose faster than glucose. So far, to our knowledge, no specific Z. rouxii proteins responsible for this fructophilic behaviour have been characterized. We have identified two genes encoding putative fructose transporters in the Z. rouxii CBS 732 genome. Heterologous expression of these two Z. rouxii ORFs in a S. cerevisiae strain lacking its own hexose transporters (hxt-null) and subsequent kinetic analysis of sugar transport showed that both proteins are functionally expressed at the plasma membrane: ZrFfz1 is a high-capacity fructose-specific facilitator (K(m)∼400&emsp14;mM and V(max)∼13&emsp14;mmol h(-1) g(-1)) and ZrFfz2 is a facilitator transporting glucose and fructose with similar capacity and affinity (K(m)∼200&emsp14;mM and V(max)∼4&emsp14;mmol h(-1) g(-1)). These two proteins together with the Zygosaccharomyces bailii Ffz1 fructose-specific transporter belong to a new family of sugar transport systems mediating the uptake of hexoses via the facilitated diffusion mechanism, and are more homologous to drug/H(+) antiporters (regarding their primary protein structure) than to other yeast sugar transporters of the Sugar Porter family.

New applications of pHluorin--measuring intracellular pH of prototrophic yeasts and determining changes in the buffering capacity of strains with affected potassium homeostasis

pHluorin is a pH-sensitive variant of green fluorescent protein for measuring intracellular pH (pH(in)) in living cells. We constructed a new pHluorin plasmid with the dominant selection marker KanMX. This plasmid allows pH measurements in cells without auxotrophic mutations and/or grown in chemically indefinite media. We observed differing values of pH(in) for three prototrophic wild-types. The new construct was also used to determine the pH(in) in strains differing in the activity of the plasma membrane Pma1 H(+)-ATPase and the influence of glucose on pH(in). We describe in detail pHluorin measurements performed in a microplate reader, which require much less hands-on time and much lower cell culture volumes compared to standard cuvettes measurements. We also utilized pHluorin in a new method of measuring the buffering capacity of yeast cell cytosol in vivo, shown to be ca. 52 mM/pH for wild-type yeast and moderately decreased in mutants with affected potassium transport.

Genetic interactions among the Arl1 GTPase and intracellular Na(+) /H(+) antiporters in pH homeostasis and cation detoxification

The roles of intracellular GTPase Arl1 and organellar cation/H(+) antiporters (Kha1 and Nhx1) in Saccharomyces cerevisiae tolerance to various stress factors were investigated and interesting new phenotypes of strains devoid of these proteins were found. The role of Arl1 GTPase in their tolerance to various cations is not caused by an altered plasma-membrane potential. Besides the known sensitivity of arl1 mutants to high temperature, we discovered their sensitivity to low temperature. We found for the first time that in the absence of Arl1p, Kha1p increases potassium, sodium and lithium tolerance, and can thus be categorized as an antiporter with broad substrate specificity. Kha1p also participates in the detoxification of undesired chemical compounds, pH regulation and growth at nonoptimal temperatures. Cells with the combined deletions of all three genes have considerable difficulty growing under nonoptimal conditions. We conclude that Arl1p, Kha1p and Nhx1p collaborate in survival strategies at nonoptimal pH, temperatures and cation concentrations, but work independent of each other.

Laser nephelometry applied in an automated microplate system to study filamentous fungus growth

By contrast with photometry (i.e., the measurement of light transmitted through a particle suspension), nephelometry is a direct method of measuring light scattered by particles in suspension. Since the scattered light intensity is directly proportional to the suspended particle concentration, nephelometry is a promising method for recording microbial growth and especially for studying filamentous fungi, which cannot be efficiently investigated through spectrophotometric assays. We describe herein for the first time a filamentous fungi–tailored procedure based on microscale liquid cultivation and automated nephelometric recording of growth, followed by extraction of relevant variables (lag time and growth rate) from the obtained growth curves. This microplate reader technique is applicable for the evaluation of antifungal activity and for large-scale phenotypic profiling.

Subinhibitory concentrations of fluconazole increase the intracellular sodium content in both fluconazole-resistant and -sensitiveCandida albicansstrains

Fluconazole-sensitive (SC 5314) and -resistant (clinical isolate 1173) Candida albicans strains were compared in terms of their osmotolerance and tolerance to toxic sodium cations. The two strains did not differ in their tolerance to high osmotic pressure in general but exhibited distinct sensitivities to sodium cations. Although the fluconazole-resistant 1173 strain contained, under all conditions tested, significantly lower intracellular amounts of Na+, it was much more sodium sensitive than the SC 5314 strain. The addition of subinhibitory concentrations of fluconazole to media supplemented with NaCl significantly influenced the growth of both strains and resulted in substantially elevated intracellular sodium concentrations compared with growth in medium containing NaCl but no fluconazole.

Membrane hyperpolarization drives cation influx and fungicidal activity of amiodarone

Cationic amphipathic drugs, such as amiodarone, interact preferentially with lipid membranes to exert their biological effect. In the yeast Saccharomyces cerevisiae, toxic levels of amiodarone trigger a rapid influx of Ca(2+) that can overwhelm cellular homeostasis and lead to cell death. To better understand the mechanistic basis of antifungal activity, we assessed the effect of the drug on membrane potential. We show that low concentrations of amiodarone (0.1-2 microm) elicit an immediate, dose-dependent hyperpolarization of the membrane. At higher doses (>3 microm), hyperpolarization is transient and is followed by depolarization, coincident with influx of Ca(2+) and H(+) and loss in cell viability. Proton and alkali metal cation transporters play reciprocal roles in membrane polarization, depending on the availability of glucose. Diminishment of membrane potential by glucose removal or addition of salts or in pma1, tok1Delta, ena1-4Delta, or nha1Delta mutants protected against drug toxicity, suggesting that initial hyperpolarization was important in the mechanism of antifungal activity. Furthermore, we show that the link between membrane hyperpolarization and drug toxicity is pH-dependent. We propose the existence of pH- and hyperpolarization-activated Ca(2+) channels in yeast, similar to those described in plant root hair and pollen tubes that are critical for cell elongation and growth. Our findings illustrate how membrane-active compounds can be effective microbicidals and may pave the way to developing membrane-selective agents.