Yeast ATP-binding cassette transporters: cellular cleaning pumps

Transcriptional dynamics of Fusarium pseudograminearum under high fungicide stress and the important role of FpZRA1 in fungal pathogenicity and DON toxin production

Fusarium pseudograminearum, the causal agent of Fusarium crown rot, poses a significant threat to cereal crops. Building upon our previous investigation of the transcriptional response of this pathogen to four key fungicides (carbendazim, phenamacril, pyraclostrobin, and tebuconazole), this study delves into the impact of elevated fungicide concentrations using RNA-seq. Global transcriptomic analysis and gene clustering revealed significant enrichment of genes involved in the ABC transporter pathway. Among these transporters, FPSE_06011 (FpZRA1), a conserved gene in eukaryotes, exhibited consistent upregulation at both low and high fungicide concentrations. Targeted deletion of FpZRA1 resulted in reduced sporulation, spore germination, and tolerance to cell wall stress, osmotic stress, and oxidative stress. Furthermore, the FpZRA1 knockout mutants exhibited decreased pathogenicity on wheat coleoptiles and reduced production of the mycotoxin deoxynivalenol (DON), as evidenced by the markedly down-regulated expression of TRI5, TRI6, and TRI10 in the RT-qPCR analysis. In summary, our findings highlight the impact of fungicide concentration on transcriptional reprogramming in F. pseudograminearum and identify FpZRA1 as a critical regulator of fungal development, stress tolerance, and pathogenicity.

Bioelectrochemical biosensors for water quality assessment and wastewater monitoring

Bioelectrochemical biosensors offer a promising approach for real-time monitoring of industrial bioprocesses. Many bioelectrochemical biosensors do not require additional labelling reagents for target molecules. This simplifies the monitoring process, reduces costs, and minimizes potential contamination risks. Advancements in materials science and microfabrication technologies are paving the way for smaller, more portable bioelectrochemical biosensors. This opens doors for integration into existing bioprocessing equipment and facilitates on-site, real-time monitoring capabilities. Biosensors can be designed to detect specific heavy metals such as lead, mercury, or chromium in wastewater. Early detection allows for the implementation of appropriate removal techniques before they reach the environment. Despite these challenges, bioelectrochemical biosensors offer a significant leap forward in wastewater monitoring. As research continues to improve their robustness, selectivity, and cost-effectiveness, they have the potential to become a cornerstone of efficient and sustainable wastewater treatment practices.

Integrated omics of Saccharomyces cerevisiae CENPK2-1C reveals pleiotropic drug resistance and lipidomic adaptations to cannabidiol

Yeast metabolism can be engineered to produce xenobiotic compounds, such as cannabinoids, the principal isoprenoids of the plant Cannabis sativa, through heterologous metabolic pathways. However, yeast cell factories continue to have low cannabinoid production. This study employed an integrated omics approach to investigate the physiological effects of cannabidiol on S. cerevisiae CENPK2-1C yeast cultures. We treated the experimental group with 0.5 mM CBD and monitored CENPK2-1C cultures. We observed a latent-stationary phase post-diauxic shift in the experimental group and harvested samples in the inflection point of this growth phase for transcriptomic and metabolomic analysis. We compared the transcriptomes of the CBD-treated yeast and the positive control, identifying eight significantly overexpressed genes with a log fold change of at least 1.5 and a significant adjusted p-value. Three notable genes were PDR5 (an ABC-steroid and cation transporter), CIS1, and YGR035C. These genes are all regulated by pleiotropic drug resistance linked promoters. Knockout and rescue of PDR5 showed that it is a causal factor in the post-diauxic shift phenotype. Metabolomic analysis revealed 48 significant spectra associated with CBD-fed cell pellets, 20 of which were identifiable as non-CBD compounds, including fatty acids, glycerophospholipids, and phosphate-salvage indicators. Our results suggest that mitochondrial regulation and lipidomic remodeling play a role in yeast's response to CBD, which are employed in tandem with pleiotropic drug resistance (PDR). We conclude that bioengineers should account for off-target product C-flux, energy use from ABC-transport, and post-stationary phase cell growth when developing cannabinoid-biosynthetic yeast strains.

Ubiquiton-An inducible, linkage-specific polyubiquitylation tool

The posttranslational modifier ubiquitin regulates most cellular processes. Its ability to form polymeric chains of distinct linkages is key to its diverse functionality. Yet, we still lack the experimental tools to induce linkage-specific polyubiquitylation of a protein of interest in cells. Here, we introduce a set of engineered ubiquitin protein ligases and matching ubiquitin acceptor tags for the rapid, inducible linear (M1-), K48-, or K63-linked polyubiquitylation of proteins in yeast and mammalian cells. By applying the so-called "Ubiquiton" system to proteasomal targeting and the endocytic pathway, we validate this tool for soluble cytoplasmic and nuclear as well as chromatin-associated and integral membrane proteins and demonstrate how it can be used to control the localization and stability of its targets. We expect that the Ubiquiton system will serve as a versatile, broadly applicable research tool to explore the signaling functions of polyubiquitin chains in many biological contexts.

Green Tea Catechin Potentiating Doxorubicine Effects against BE(2)C Neuroblastoma Cells In Vitro

Background: Neuroblastoma (NB), a malignant sympathetic nervous system cancer, is the second most common type of pediatric tumor. Increasing the number of NB death emerges to design a new strategy for NB treatment. Nowadays, the development of natural compounds has gradually increased due to their ability to apoptosis induction. Tea catechin, a flavonoid compound, is one of the natural combinations which inhibit tumor growth and enhance tumor cell apoptosis. In the current study, the effects of pure catechin, doxorubicin (DOX), and their combination on a cellular model of NB [BE(2)C cells] are perused. (NB) a malignant sympathetic nervous system cancer is the second most common type of pediatric tumor. Increasing the number of NB death emerges to design a new strategy for NB treatment. Nowadays, the development of natural compounds has gradually increased due to their ability to apoptosis induction. Tea catechin, a flavonoid compound, is one of the natural combinations which inhibit tumor growth and enhance tumor cell apoptosis. Objectives: In the current study, the effects of pure catechin, doxorubicin (DOX), and their combination on a cellular model of NB [BE(2)C cells] are perused. Methods: The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay was done to assess a response dose for each drug. Fluorescent Microscopic and cell cycle analyses were performed for apoptosis detection. Finally, Colony formation was performed to examine cell migration and invasion. Results: The MTT assay showed that catechin and DOX treatment inhibited the viability of the cells while the combination of their ineffective doses had more cytotoxic effects. However, these treatments could not inhibit the cell growth of the normal human fibroblast. Moreover, this combination reduced cell attachment, chromatin fragmentation, and G/S arrest in the cell cycle. The clonogenic assay demonstrated that colony size and numbers obviously reduced after ten days; therefore, Catchin and its combination with DOX suppressed cell capacities of clone formation and migration. Conclusions: These results suggest that catechin, DOX, and their combination may inhibit the proliferation, invasion, and migration of BE(2)C neuroblastoma cells in vitro while inducing cell apoptosis by arresting the cell cycle.

iTRAQ-based proteomics revealed baicalein enhanced oxidative stress of Candida albicans by up-regulating CPD2 expression

Baicalein could inhibit the growth and biofilm formation of Candida albicans, the most common clinical fungal pathogen. However, the antifungal mechanism of baicalein has not been elucidated. In this study, isobaric tags for relative and absolute quantification (iTRAQ) was used to verify the mechanism of antifungal fluconazole and baicalein. A total of 58 common proteins were detected in cells treated with fluconazole. These proteins encompassed fluconazole-targeted sterol synthesis pathway, including Erg11p, Erg6p, Erg3p, Erg25p, Erg5p, Erg10p and Ncp1p. Next, iTRAQ was applied to the comparison of baicalein-treated C. albicans proteins, which detected 16 common proteins. The putative NADH dehydrogenase Cpd2p and the ATP-binding cassette transporter Snq2p were the most up-regulated proteins with the treatment of baicalein. Our results showed that CPD2 disruption elevated C. albicans resistance to baicalein significantly both in vitro and in vivo. Further in-depth studies revealed that CPD2 disruption reduced the activation of C. albicans metacaspase and partially restored the mitochondrial membrane potential reduction caused by the treatment of baicalein, which indicated that CPD2 was involved in the apoptosis induced by baicalein. Consistently, under the treatment of baicalein, CPD2Δ/Δ mutant produced lower reactive oxygen species (ROS) that was critical in causing oxidative damage and apoptosis in C. albicans. These results indicated that baicalein could increase intracellular oxidative damage by up-regulating the expression of Cpd2p so as to inhibit the growth of C. albicans, which provides new insights for investigating the antifungal target of baicalein.

Flipping and other astonishing transporter dance moves in fungal drug resistance

In all domains of life, transmembrane proteins from the ATP-binding cassette (ABC) transporter family drive the translocation of diverse substances across lipid bilayers. In pathogenic fungi, the ABC transporters of the pleiotropic drug resistance (PDR) subfamily confer antibiotic resistance and so are of interest as therapeutic targets. They also drive the quest for understanding how ABC transporters can generally accommodate such a wide range of substrates. The Pdr5 transporter from baker's yeast is representative of the PDR group and, ever since its discovery more than 30 years ago, has been the subject of extensive functional analyses. A new perspective of these studies has been recently provided in the framework of the first electron cryo-microscopy structures of Pdr5, as well as emergent applications of machine learning in the field. Taken together, the old and the new developments have been used to propose a mechanism for the transport process in PDR proteins. This mechanism involves a "flippase" step that moves the substrates from one leaflet of the bilayer to the other, as a central element of cellular efflux.

Novel ABC Transporter Associated with Fluconazole Resistance in Aging of Cryptococcus neoformans

Cryptococcus neoformans causes meningoencephalitis in immunocompromised individuals, which is treated with fluconazole (FLC) monotherapy when resources are limited. This can lead to azole resistance, which can be mediated by overexpression of ABC transporters, a class of efflux pumps. ABC pump-mediated efflux of FLC is also augmented in 10-generation old C. neoformans cells. Here, we describe a new ABC transporter Afr3 (CNAG_06909), which is overexpressed in C. neoformans cells of advanced generational age that accumulate during chronic infection. The Δafr3 mutant strain showed higher FLC susceptibility by FLC E-Test strip testing and also by a killing test that measured survival after 3 h FLC exposure. Furthermore, Δafr3 cells exhibited lower Rhodamine 6G efflux compared to the H99 wild-type cells. Afr3 was expressed in the Saccharomyces cerevisiae ADΔ strain, which lacks several drug transporters, thus reducing background transport. The ADΔ + Afr3 strain demonstrated a higher efflux with both Rhodamine 6G and Nile red, and a higher FLC resistance. Afr3-GFP localized in the plasma membrane of the ADΔ + Afr3 strain, further highlighting its importance as an efflux pump. Characterization of the Δafr3 mutant revealed unattenuated growth but a prolongation (29%) of the replicative life span. In addition, Δafr3 exhibited decreased resistance to macrophage killing and attenuated virulence in the Galleria mellonella infection model. In summary, our data indicate that a novel ABC pump Afr3, which is upregulated in C. neoformans cells of advanced age, may contribute to their enhanced FLC tolerance, by promoting drug efflux. Lastly, its role in macrophage resistance may also contribute to the selection of older C. neoformans cells during chronic infection.

Vacuole-Specific Lipid Release for Tracking Intracellular Lipid Metabolism and Transport in Saccharomyces cerevisiae

Lipid metabolism is spatiotemporally regulated within cells, yet intervention into lipid functions at subcellular resolution remains difficult. Here, we report a method that enables site-specific release of sphingolipids and cholesterol inside the vacuole in Saccharomyces cerevisiae. Using this approach, we monitored real-time sphingolipid metabolic flux out of the vacuole by mass spectrometry and found that the endoplasmic reticulum-vacuole-tethering protein Mdm1 facilitated the metabolism of sphingoid bases into ceramides. In addition, we showed that cholesterol, once delivered into yeast using our method, could restore cell proliferation induced by ergosterol deprivation, overcoming the previously described sterol-uptake barrier under aerobic conditions. Together, these data define a new way to study intracellular lipid metabolism and transport from the vacuole in yeast.

Structure and efflux mechanism of the yeast pleiotropic drug resistance transporter Pdr5

Pdr5, a member of the extensive ABC transporter superfamily, is representative of a clinically relevant subgroup involved in pleiotropic drug resistance. Pdr5 and its homologues drive drug efflux through uncoupled hydrolysis of nucleotides, enabling organisms such as baker’s yeast and pathogenic fungi to survive in the presence of chemically diverse antifungal agents. Here, we present the molecular structure of Pdr5 solved with single particle cryo-EM, revealing details of an ATP-driven conformational cycle, which mechanically drives drug translocation through an amphipathic channel, and a clamping switch within a conserved linker loop that acts as a nucleotide sensor. One half of the transporter remains nearly invariant throughout the cycle, while its partner undergoes changes that are transmitted across inter-domain interfaces to support a peristaltic motion of the pumped molecule. The efflux model proposed here rationalises the pleiotropic impact of Pdr5 and opens new avenues for the development of effective antifungal compounds.

Pdr5 is an ABC transporter conferring multidrug resistance to pathogenic fungi. Here, structural analysis of Pdr5 provides insights into the transport mechanism featuring asymmetric movements of Pdr5 domain and enabling efflux of a broad spectrum of compounds.

The ABC transporter G subfamily in Arabidopsis thaliana

ABC transporters are ubiquitously present in all kingdoms and mediate the transport of a large spectrum of structurally different compounds. Plants possess high numbers of ABC transporters in relation to other eukaryotes; the ABCG subfamily in particular is extensive. Earlier studies demonstrated that ABCG transporters are involved in important processes influencing plant fitness. This review summarizes the functions of ABCG transporters present in the model plant Arabidopsis thaliana. These transporters take part in diverse processes such as pathogen response, diffusion barrier formation, or phytohormone transport. Studies involving knockout mutations reported pleiotropic phenotypes of the mutants. In some cases, different physiological roles were assigned to the same protein. The actual transported substrate(s), however, still remain to be determined for the majority of ABCG transporters. Additionally, the proposed substrate spectrum of different ABCG proteins is not always reflected by sequence identities between ABCG members. Applying only reverse genetics is thereby insufficient to clearly identify the substrate(s). We therefore stress the importance of in vitro studies in addition to in vivo studies in order to (i) clarify the substrate identity; (ii) determine the transport characteristics including directionality; and (iii) identify dimerization partners of the half-size proteins, which might in turn affect substrate specificity.

Quantitative Characterization of α-Synuclein Aggregation in Living Cells through Automated Microfluidics Feedback Control

Aggregation of α-synuclein and formation of inclusions are hallmarks of Parkinson’s disease (PD). Aggregate formation is affected by cellular environment, but it has been studied almost exclusively in cell-free systems. We quantitatively analyzed α-synuclein inclusion formation and clearance in a yeast cell model of PD expressing either wild-type (WT) α-synuclein or the disease-associated A53T mutant from the galactose (Gal)-inducible promoter. A computer-controlled microfluidics device regulated α-synuclein in cells by means of closed-loop feedback control. We demonstrated that inclusion formation is strictly concentration dependent and that the aggregation threshold of the A53T mutant is about half of the WT α-synuclein (56%). We chemically modulated the proteasomal and autophagic pathways and demonstrated that autophagy is the main determinant of A53T α-synuclein inclusions’ clearance. In addition to proposing a technology to overcome current limitations in dynamically regulating protein expression levels, our results contribute to the biology of PD and have relevance for therapeutic applications.

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In silico feedback control enables regulation of α-synuclein expression in yeast

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α-Synuclein inclusion formation is strictly concentration, but not time, dependent

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The aggregation threshold of the α-synuclein A53T mutant is 56% of the wild-type

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Autophagy induction speeds up inclusion clearance in the A53T α-synuclein strain

Yeast-Based Biosensors: Current Applications and New Developments

Biosensors are regarded as a powerful tool to detect and monitor environmental contaminants, toxins, and, more generally, organic or chemical markers of potential threats to human health. They are basically composed of a sensor part made up of either live cells or biological active molecules coupled to a transducer/reporter technological element. Whole-cells biosensors may be based on animal tissues, bacteria, or eukaryotic microorganisms such as yeasts and microalgae. Although very resistant to adverse environmental conditions, yeasts can sense and respond to a wide variety of stimuli. As eukaryotes, they also constitute excellent cellular models to detect chemicals and organic contaminants that are harmful to animals. For these reasons, combined with their ease of culture and genetic modification, yeasts have been commonly used as biological elements of biosensors since the 1970s. This review aims first at giving a survey on the different types of yeast-based biosensors developed for the environmental and medical domains. We then present the technological developments currently undertaken by academic and corporate scientists to further drive yeasts biosensors into a new era where the biological element is optimized in a tailor-made fashion by in silico design and where the output signals can be recorded or followed on a smartphone.

In vitro and in vivo Efficacy of a Synergistic Combination of Itraconazole and Verapamil Against Aspergillus fumigatus

The incidence of aspergillosis continues to rise sharply, while the progress made in expanding the antifungal drug arsenal remains extremely slow, indicating an urgent need for new strategies. Previous studies have shown that the calcium signaling pathway, which is evolutionarily conserved in mammals and fungi, is involved in regulating the tolerance of azoles in fungi. In this study, we performed a preliminary screening among various combinations of different clinical calcium channel blockers and different antifungal drugs. We found that the combination of itraconazole and verapamil showed the best synergistic effect against Aspergillus fumigatus. Thereafter, using the checkboard assays we observed synergistic effects of the combination treatment against most of the A. fumigatus strains tested, including itraconazole-sensitive and itraconazole-resistant strains, with a fractional inhibitory concentration index (FICI) < 0.5. Furthermore, we showed that verapamil strongly decreased the cytosolic calcium transients following itraconazole stimulation by an aequorin-mediated method. Moreover, verapamil influenced the efflux of rhodamine 6G, an azole mimic substance. An ergosterol assay revealed that verapamil alone had no effect on ergosterol biosynthesis, but the combination of itraconazole and verapamil treatment decreased the ergosterol level. Further murine assays were performed using a luciferase-probed bioluminescence imaging method. Drug combination therapy reduced lung burden and improved survival rate. In conclusion, verapamil is a promising candidate to enhance the antifungal activity of itraconazole against A. fumigatus. In addition, our study suggests the effectiveness of an emerging approach based on bioluminescence imaging in monitoring the efficacy of drug combination therapy for invasive aspergillosis.

In vitro NTPase activity of highly purified Pdr5, a major yeast ABC multidrug transporter

The ABC transporter Pdr5 of S. cerevisiae is a key player of the PDR network that works as a first line of defense against a wide range of xenobiotic compounds. As the first discovered member of the family of asymmetric PDR ABC transporters, extensive studies have been carried out to elucidate the molecular mechanism of drug efflux and the details of the catalytic cycle. Pdr5 turned out to be an excellent model system to study functional and structural characteristics of asymmetric, uncoupled ABC transporters. However, to date studies have been limited to in vivo or plasma membrane systems, as it was not possible to isolate Pdr5 in a functional state. Here, we describe the solubilization and purification of Pdr5 to homogeneity in a functional state as confirmed by in vitro assays. The ATPase deficient Pdr5 E1036Q mutant was used as a control and proves that detergent-purified wild-type Pdr5 is functional resembling in its activity the one in its physiological environment. Finally, we show that the isolated active Pdr5 is monomeric in solution. Taken together, our results described in this study will enable a variety of functional investigations on Pdr5 required to determine molecular mechanism of this asymmetric ABC transporter.

Novel 3,4-Dihydroisocoumarins Inhibit Human P-gp and BCRP in Multidrug Resistant Tumors and Demonstrate Substrate Inhibition of Yeast Pdr5

Multidrug resistance (MDR) in tumors and pathogens remains a major problem in the efficacious treatment of patients by reduction of therapy options and subsequent treatment failure. Various mechanisms are described to be involved in the development of MDR with overexpression of ATP-binding cassette (ABC) transporters reflecting the most extensively studied. These membrane transporters translocate a wide variety of substrates utilizing energy from ATP hydrolysis leading to decreased intracellular drug accumulation and impaired drug efficacy. One treatment strategy might be inhibition of transporter-mediated efflux by small molecules. Isocoumarins and 3,4-dihydroisocoumarins are a large group of natural products derived from various sources with great structural and functional variety, but have so far not been in the focus as potential MDR reversing agents. Thus, three natural products and nine novel 3,4-dihydroisocoumarins were designed and analyzed regarding cytotoxicity induction and inhibition of human ABC transporters P-glycoprotein (P-gp), multidrug resistance-associated protein 1 (MRP1) and breast cancer resistance protein (BCRP) in a variety of human cancer cell lines as well as the yeast ABC transporter Pdr5 in Saccharomyces cerevisiae. Dual inhibitors of P-gp and BCRP and inhibitors of Pdr5 were identified, and distinct structure-activity relationships for transporter inhibition were revealed. The strongest inhibitor of P-gp and BCRP, which inhibited the transporters up to 80 to 90% compared to the respective positive controls, demonstrated the ability to reverse chemotherapy resistance in resistant cancer cell lines up to 5.6-fold. In the case of Pdr5, inhibitors were identified that prevented substrate transport and/or ATPase activity with IC₅₀ values in the low micromolar range. However, cell toxicity was not observed. Molecular docking of the test compounds to P-gp revealed that differences in inhibition capacity were based on different binding affinities to the transporter. Thus, these small molecules provide novel lead structures for further optimization.

Saccharomyces cerevisiae as a platform for assessing sphingolipid lipid kinase inhibitors

Successful medicinal chemistry campaigns to discover and optimize sphingosine kinase inhibitors require a robust assay for screening chemical libraries and for determining rank order potencies. Existing assays for these enzymes are laborious, expensive and/or low throughput. The toxicity of excessive levels of phosphorylated sphingoid bases for the budding yeast, Saccharomyces cerevisiae, affords an assay wherein inhibitors added to the culture media rescue growth in a dose-dependent fashion. Herein, we describe our adaptation of a simple, inexpensive, and high throughput assay for assessing inhibitors of sphingosine kinase types 1 and 2 as well as ceramide kinase and for testing enzymatic activity of sphingosine kinase type 2 mutants. The assay was validated using recombinant enzymes and generally agrees with the rank order of potencies of existing inhibitors.

Emerging roles for sphingolipids in cellular aging

Aging is a gradual loss of physiological functions as organisms' progress in age. Although aging in multicellular organisms is complex, some fundamental mechanisms and pathways may be shared from the single cellular yeast to human. Budding yeast Saccharomyces cerevisiae has been established model system for aging studies. A yeast cell divides asymmetrically to produce two cells that differ in size and age. The one that is smaller coming from bud is a newborn cell that with a full replicative potential head irrespective of the replicative age of its mother-the larger cell from which the bud grows out before division. The age asymmetry between daughter and mother is thought to be dependent on asymmetric segregation of certain factors such as protein aggregates, extrachromosomal DNA (ERCs) and dysfunctional organelles during successive cell divisions of the yeast replicative lifespan (RLS). It is also thought that certain plasma membrane proteins, in particular multidrug-resistant (MDR) proteins, asymmetrically partition between the mother and the bud based on the age of the polypeptides. Functional decline associated with the molecular aging of those proteins contributes to the fitness decline at advance age. In our recent study, we showed that sphingolipids facilitate the age-dependent segregation of MDRs between daughter and mother cell. In this review, we highlight and discuss the potential mechanisms by which sphingolipids regulate the aging process in yeast and cells of vertebrate animals including human.

Sphingolipids facilitate age asymmetry of membrane proteins in dividing yeast cells

One proposed mechanism of cellular aging is the gradual loss of certain cellular components that are insufficiently renewed. In an earlier study, multidrug resistance transporters (MDRs) were postulated to be such aging determinants during the yeast replicative life span (RLS). Aged MDR proteins were asymmetrically retained by the aging mother cell and did not diffuse freely into the bud, whereas newly synthesized MDR proteins were thought to be deposited mostly in the bud before cytokinesis. In this study, we further demonstrate the proposed age asymmetry of MDR proteins in dividing yeast cells and investigate the mechanism that controls diffusive properties of MDR proteins to maintain this asymmetry. We found that long-chain sphingolipids, but not the septin/endoplasmic reticulum-based membrane diffusion barrier, are important for restricting MDR diffusion. Depletion of sphingolipids or shortening of their long acyl chains resulted in an increase in the lateral mobility of MDR proteins, causing aged MDR protein in the mother cell to enter the bud. We used a mathematical model to understand the effect of diminished MDR age asymmetry on yeast cell aging, the result of which was qualitatively consistent with the observed RLS shortening in sphingolipid mutants.

Transmitting the energy: interdomain cross-talk in Pdr5

ABC (ATP-binding cassette) transporters are ubiquitous integral membrane proteins catalyzing the active export or import of structurally and functionally unrelated compounds. In humans, these proteins are clinically and economically important, as their dysfunction is responsible for a number of diseases. In the case of multidrug resistance (MDR) ABC exporters, they particularly confer resistance to a broad spectrum of toxic compounds, placing them in the focus of clinical research. However, ABC-mediated drug resistance is not only restricted to humans. In yeast for example, MDR is called pleiotropic drug resistance (PDR). Important and well-studied members of the PDR subfamily of ABC transporters are Pdr5 from Saccharomyces cerevisiae and its homolog Cdr1 from Candida albicans. Mutational studies of these two transporters provided many insights into the complexity and conceivable mechanism of the interdomain cross-talk that transmits the energy gained from ATP hydrolysis to the substrate translocation process across the membrane. In this review, we summarize and discuss our current knowledge of the interdomain cross-talk as well as new results obtained for asymmetric ABC transporters and derive possible structural and functional implications for Pdr5.

A general strategy to construct small molecule biosensors in eukaryotes

Biosensors for small molecules can be used in applications that range from metabolic engineering to orthogonal control of transcription. Here, we produce biosensors based on a ligand-binding domain (LBD) by using a method that, in principle, can be applied to any target molecule. The LBD is fused to either a fluorescent protein or a transcriptional activator and is destabilized by mutation such that the fusion accumulates only in cells containing the target ligand. We illustrate the power of this method by developing biosensors for digoxin and progesterone. Addition of ligand to yeast, mammalian, or plant cells expressing a biosensor activates transcription with a dynamic range of up to ~100-fold. We use the biosensors to improve the biotransformation of pregnenolone to progesterone in yeast and to regulate CRISPR activity in mammalian cells. This work provides a general methodology to develop biosensors for a broad range of molecules in eukaryotes.

DOI:http://dx.doi.org/10.7554/eLife.10606.001

Small molecules play essential roles in organisms, and so methods to sense these molecules within living cells could have wide-ranging uses in both biology and biotechnology. However, current methods for making new “biosensors” are limited and only a narrow range of small molecules can be detected.

One approach to biosensor design in yeast and other eukaryotic organisms uses proteins called ligand-binding domains, which bind to small molecules. Here, Feng, Jester, Tinberg, Mandell et al. have developed a new method to make biosensors from ligand-binding domains that could, in principle, be applied to any target small molecule. The new method involves taking a ligand-binding domain that is either engineered or occurs in nature and linking it to something that can be readily detected, such as a protein that fluoresces or that controls gene expression. This combined biosensor protein is then engineered, via mutations, such that it is unstable unless it binds to the small molecule. This means that, in the absence of the small molecule, these proteins are destroyed inside living cells. However, the binding of a target molecule to one of these proteins protects it from degradation, which allows the signal to be detected.

Feng, Jester, Tinberg, Mandell et al. use this method to create biosensors for a human hormone called progesterone and a drug called digoxin, which is used to treat heart disease. Further experiments used the biosensors to optimize the production of progesterone in yeast and to regulate the activity of a gene editing protein called Cas9 in human cells. The biosensors can be also used to produce long-term environmental sensors in plant cells.

This approach makes it possible to produce a wide variety of biosensors for different organisms. The next step is to continue to explore the ability of various proteins to be converted into biosensors, and to find out how easy it is to transfer a biosensor produced in one species to another.

DOI:http://dx.doi.org/10.7554/eLife.10606.002

Evaluation of mRNA Expression Levels of cyp51A and mdr1, Candidate Genes for Voriconazole Resistance in Aspergillus flavus

Background:

Voriconazole Resistance (VRC-R) in Aspergillus flavus isolates impacts the management of aspergillosis, since azoles are the first choice for prophylaxis and therapy. However, to the best of our knowledge, the mechanisms underlying voriconazole resistance are poorly understood.

Objectives:

The present study was designed to evaluate mRNA expression levels of cyp51A and mdr1 genes in voriconazole resistant A. flavus by a Real-Time Reverse Transcriptase Polymerase Chain Reaction (RT-PCR) technique.

Materials and Methods:

Five A. flavus isolates with resistance to VRC were examined by a RT-PCR approach.

Results:

Four out of five isolates revealed cyp51A and mdr1 mRNA overexpression. Interestingly, the isolate, which was negative for cyp51A and mdr1 mRNA expression showed a high voriconazole Minimum Inhibitory Concentration (MIC). Furthermore, a computational-based analysis predicted that voriconazole resistance could be mediated through cooperation with a network protein interaction.

Conclusions:

Our experimental and in silico findings may provide new insight in the complex molecular pathways of drug resistance and also could assist design an efficient therapeutic strategy for aspergillosis treatment.

Puromycin- and methotrexate-resistance cassettes and optimized Cre-recombinase expression plasmids for use in yeast

Here we expand the set of tools for genetically manipulating Saccharomyces cerevisiae. We show that puromycin-resistance can be achieved in yeast through expression of a bacterial puromycin-resistance gene optimized to the yeast codon bias, which in turn serves as an easy-to-use dominant genetic marker suitable for gene disruption. We have constructed a similar DNA cassette expressing yeast codon-optimized mutant human dihydrofolate reductase (DHFR), which confers resistance to methotrexate and can also be used as a dominant selectable marker. Both of these drug-resistant marker cassettes are flanked by loxP sites, allowing for their excision from the genome following expression of Cre-recombinase. Finally, we have created a series of plasmids for low-level constitutive expression of Cre-recombinase in yeast that allows for efficient excision of loxP-flanked markers.

Drug transporters in spermatogenesis: A re-evaluation of recent data on P-glycoprotein

Drug transporters are integral membrane proteins expressed by a variety of organs, including the liver, kidney, small intestine and testis, and they are generally known to mediate drug or xenobiotic transport into and out of cells. Previous studies have also reported the presence of several drug transporters at blood-tissue barriers where they are thought to protect organs from harmful agents. In this editorial, we briefly discuss and re-evaluate recent findings that show P-glycoprotein, an efflux pump, to function at the blood-testis barrier. We also put forth a mechanistic model, hoping this information will form a strong basis for future studies.

Generating symmetry in the asymmetric ATP-binding cassette (ABC) transporter Pdr5 from Saccharomyces cerevisiae

Pdr5 is a plasma membrane-bound ABC transporter from Saccharomyces cerevisiae and is involved in the phenomenon of resistance against xenobiotics, which are clinically relevant in bacteria, fungi, and humans. Many fungal ABC transporters such as Pdr5 display an inherent asymmetry in their nucleotide-binding sites (NBS) unlike most of their human counterparts. This degeneracy of the NBSs is very intriguing and needs explanation in terms of structural and functional relevance. In this study, we mutated nonconsensus amino acid residues in the NBSs to its consensus counterpart and studied its effect on the function of the protein and effect on yeast cells. The completely "regenerated" Pdr5 protein was severely impaired in its function of ATP hydrolysis and of rhodamine 6G transport. Moreover, we observe alternative compensatory mechanisms to counteract drug toxicity in some of the mutants. In essence, we describe here the first attempts to restore complete symmetry in an asymmetric ABC transporter and to study its effects, which might be relevant to the entire class of asymmetric ABC transporters.

Alcohols are inhibitors of Saccharomyces cerevisiae multidrug-resistance pumps Pdr5p and Snq2p

The effect of alcohols on cell membrane proteins has originally been assumed to be mediated by their primary action on membrane lipid matrix. Many studies carried out later on both animal and yeast cells have revealed that ethanol and other alcohols inhibit the functions of various membrane channels, receptors and solute transport proteins, and a direct interaction of alcohols with these membrane proteins has been proposed. Using our fluorescence diS-C3 (3) diagnostic assay for multidrug-resistance pump inhibitors in a set of isogenic yeast Pdr5p and Snq2p mutants, we found that n-alcohols (from ethanol to hexanol) variously affect the activity of both pumps. Beginning with propanol, these alcohols have an inhibitory effect that increases with increasing length of the alcohol acyl chain. While ethanol does not exert any inhibitory effect at any of the concentration used (up to 3%), hexanol exerts a strong inhibition at 0.1%. The alcohol-induced inhibition of MDR pumps was detected even in cells whose membrane functional and structural integrity were not compromised. This supports a notion that the inhibitory action does not necessarily involve only changes in the lipid matrix of the membrane but may entail a direct interaction of the alcohols with the pump proteins.

The transcriptomic signature of RacA activation and inactivation provides new insights into the morphogenetic network of Aspergillus niger

RacA is the main Rho GTPase in Aspergillus niger regulating polarity maintenance via controlling actin dynamics. Both deletion and dominant activation of RacA (Rac(G18V)) provoke an actin localization defect and thereby loss of polarized tip extension, resulting in frequent dichotomous branching in the ΔracA strain and an apolar growing phenotype for Rac(G18V). In the current study the transcriptomics and physiological consequences of these morphological changes were investigated and compared with the data of the morphogenetic network model for the dichotomous branching mutant ramosa-1. This integrated approach revealed that polar tip growth is most likely orchestrated by the concerted activities of phospholipid signaling, sphingolipid signaling, TORC2 signaling, calcium signaling and CWI signaling pathways. The transcriptomic signatures and the reconstructed network model for all three morphology mutants (ΔracA, Rac(G18V), ramosa-1) imply that these pathways become integrated to bring about different physiological adaptations including changes in sterol, zinc and amino acid metabolism and changes in ion transport and protein trafficking. Finally, the fate of exocytotic (SncA) and endocytotic (AbpA, SlaB) markers in the dichotomous branching mutant ΔracA was followed, demonstrating that hyperbranching does not per se result in increased protein secretion.

Transcriptome response to alkane biofuels in Saccharomyces cerevisiae: identification of efflux pumps involved in alkane tolerance

BACKGROUND

Hydrocarbon alkanes have been recently considered as important next-generation biofuels because microbial production of alkane biofuels was demonstrated. However, the toxicity of alkanes to microbial hosts can possibly be a bottleneck for high productivity of alkane biofuels. To tackle this toxicity issue, it is essential to understand molecular mechanisms of interactions between alkanes and microbial hosts, and to harness these mechanisms to develop microbial host strains with improved tolerance against alkanes. In this study, we aimed to improve the tolerance of Saccharomyces cerevisiae, a model eukaryotic host of industrial significance, to alkane biofuels by exploiting cellular mechanisms underlying alkane response.

RESULTS

To this end, we first confirmed that nonane (C9), decane (C10), and undecane (C11) were significantly toxic and accumulated in S. cerevisiae. Transcriptome analyses suggested that C9 and C10 induced a range of cellular mechanisms such as efflux pumps, membrane modification, radical detoxification, and energy supply. Since efflux pumps could possibly aid in alkane secretion, thereby reducing the cytotoxicity, we formed the hypothesis that those induced efflux pumps could contribute to alkane export and tolerance. In support of this hypothesis, we demonstrated the roles of the efflux pumps Snq2p and Pdr5p in reducing intracellular levels of C10 and C11, as well as enhancing tolerance levels against C10 and C11. This result provided the evidence that Snq2p and Pdr5p were associated with alkane export and tolerance in S. cerevisiae.

CONCLUSIONS

Here, we investigated the cellular mechanisms of S. cerevisiae response to alkane biofuels at a systems level through transcriptome analyses. Based on these mechanisms, we identified efflux pumps involved in alkane export and tolerance in S. cerevisiae. We believe that the results here provide valuable insights into designing microbial engineering strategies to improve cellular tolerance for highly efficient alkane biofuel production.

Functional impact of a single mutation within the transmembrane domain of the multidrug ABC transporter Pdr5

The pleiotropic drug resistance network in budding yeast presents a first line of defense against xenobiotics, which is formed by primary and secondary active membrane transporters. Among these transporters, the ABC transporter Pdr5 is a key component, because it confers resistance against a broad spectrum of such cytotoxic agents. Furthermore, it represents a model system for homologous transporters from pathogenic fungi and has been intensively studied in the past. In addition to other mutational studies, the S1360F mutation of Pdr5 was found to modulate substrate specificity and resistance. Notably, in the S1360F background, the resistance against the immunosuppressant FK506 is drastically increased. We present a detailed analysis of this mutation that is located in the predicted cytosolic part of transmembrane helix 11. Our data demonstrate that kinetic and thermodynamic parameters of the S1360F mutant are similar to those of the wild-type protein, except for FK506-inhibited ATPase activity and the degree of competitive inhibition. In summary, our results indicate that the S1360F mutation within the transmembrane domain interferes drastically with the ability of the nucleotide-binding domains to hydrolyze ATP by interfering with interdomain crosstalk.

Steroid toxicity and detoxification in ascomycetous fungi

In the last couple of decades fungal infections have become a significant clinical problem. A major interest into fungal steroid action has been provoked since research has proven that steroid hormones are toxic to fungi and affect the host/fungus relationship. Steroid hormones were found to differ in their antifungal activity in ascomycetous fungi Hortaea werneckii, Saccharomyces cerevisiae and Aspergillus oryzae. Dehydroepiandrosterone was shown to be the strongest inhibitor of growth in all three varieties of fungi followed by androstenedione and testosterone. For their protection, fungi use several mechanisms to lower the toxic effects of steroids. The efficiency of biotransformation in detoxification depended on the microorganism and steroid substrate used. Biotransformation was a relatively slow process as it also depended on the growth phase of the fungus. In addition to biotransformation, steroid extrusion out of the cells contributed to the lowering of the active intracellular steroid concentration. Plasma membrane Pdr5 transporter was found to be the most effective, followed by Snq2 transporter and vacuolar transporters Ybt1 and Ycf1. Proteins Aus1 and Dan1 were not found to be involved in steroid import. The research of possible targets of steroid hormone action in fungi suggests that steroid hormones inhibit ergosterol biosynthesis in S. cerevisiae and H. werneckii. Results of this inhibition caused changes in the sterol content of the cellular membrane. The presence of steroid hormones most probably causes the degradation of the Tat2 permease and impairment of tryptophan import.

Specific interactions between the Candida albicans ABC transporter Cdr1p ectodomain and a D-octapeptide derivative inhibitor

Overexpression of the Candida albicans ATP-binding cassette transporter CaCdr1p causes clinically significant resistance to azole drugs including fluconazole (FLC). Screening of a ~1.89 × 10(6) member D-octapeptide combinatorial library that concentrates library members at the yeast cell surface identified RC21v3, a 4-methoxy-2,3,6-trimethylbenzenesulphonyl derivative of the D-octapeptide D-NH(2) -FFKWQRRR-CONH(2) , as a potent and stereospecific inhibitor of CaCdr1p. RC21v3 chemosensitized Saccharomyces cerevisiae strains overexpressing CaCdr1p but not other fungal ABC transporters, the C. albicans MFS transporter CaMdr1p or the azole target enzyme CaErg11p, to FLC. RC21v3 also chemosensitized clinical C. albicans isolates overexpressing CaCDR1 to FLC, even when CaCDR2 was overexpressed. Specific targeting of CaCdr1p by RC21v3 was confirmed by spontaneous RC21v3 chemosensitization-resistant suppressor mutants of S. cerevisiae expressing CaCdr1p. The suppressor mutations introduced a positive charge beside, or within, extracellular loops 1, 3, 4 and 6 of CaCdr1p or an aromatic residue near the extracytoplasmic end of transmembrane segment 5. The mutations did not affect CaCdr1p localization or CaCdr1p ATPase activity but some increased susceptibility to the CaCdr1p substrates FLC, rhodamine 6G, rhodamine 123 and cycloheximide. The suppressor mutations showed that the drug-like CaCdr1p inhibitors FK506, enniatin, milbemycin α11 and milbemycin β9 have modes of action similar to RC21v3.

Response of pathogenic and non-pathogenic yeasts to steroids

Steroids are known to induce pleiotropic drug resistance states in hemiascomycetes, with tremendous potential consequences on human fungal infections. The proteins capable of binding to steroids such as progesterone binding protein (PBP), estradiol binding proteins (ESP) are found in yeasts, however, the well known receptor mediated signaling present in higher eukaryotic cells is absent in yeasts and fungi. Steroids are perceived as stress by yeast cells which triggers general stress response leading to activation of heat shock proteins, cell cycle regulators, MDR transporters, etc. In this article, we review the response of yeast to human steroid hormones which affects its cell growth, morphology and virulence. We discuss that a fairly conserved response to steroids at the level of transcription and translation exists between pathogenic and non-pathogenic yeasts. Article from a special issue on steroids and microorganisms.

A mutation in intracellular loop 4 affects the drug-efflux activity of the yeast multidrug resistance ABC transporter Pdr5p

Multidrug resistance protein Pdr5p is a yeast ATP-binding cassette (ABC) transporter in the plasma membrane. It confers multidrug resistance by active efflux of intracellular drugs. However, the highly polymorphic Pdr5p from clinical strain YJM789 loses its ability to expel azole and cyclohexmide. To investigate the role of amino acid changes in this functional change, PDR5 chimeras were constructed by segmental replacement of homologous BY4741 PDR5 fragments. Functions of PDR5 chimeras were evaluated by fluconazole and cycloheximide resistance assays. Their expression, ATPase activity, and efflux efficiency for other substrates were also analyzed. Using multiple lines of evidence, we show that an alanine-to-methionine mutation at position 1352 located in the predicted short intracellular loop 4 significantly contributes to the observed transport deficiency. The degree of impairment is likely correlated to the size of the mutant residue.

Bugs, drugs and chemical genomics

The serendipitous discovery of penicillin inspired intensive research into how small molecules affect basic cellular processes and their potential to treat disease. Biochemical and genetic approaches have been fundamental for clarifying small-molecule modes of action. Genomic technologies have permitted the use of chemical-genetic strategies that comprehensively study compound-target relationships in the context of a living cell, providing a systems biology view of both the cellular targets and the interdependent networks that respond to chemical stress. These studies highlight the fact that in vitro determinations of mechanism rarely translate into a complete understanding of drug behavior in the cell. Here, we review key discoveries that gave rise to the field of chemical genetics, with particular attention to chemical-genetic strategies developed for bakers' yeast, their extension to clinically relevant microbial pathogens, and the potential of these approaches to affect antimicrobial drug discovery.

Chimeras of Candida albicans Cdr1p and Cdr2p reveal features of pleiotropic drug resistance transporter structure and function

Members of the pleiotropic drug resistance (PDR) family of ATP binding cassette (ABC) transporters consist of two homologous halves, each containing a nucleotide binding domain (NBD) and a transmembrane domain (TMD). The PDR transporters efflux a variety of hydrophobic xenobiotics and despite the frequent association of their overexpression with the multidrug resistance of fungal pathogens, the transport mechanism of these transporters is poorly understood. Twenty-eight chimeric constructs between Candida albicans Cdr1p (CaCdr1p) and Cdr2p (CaCdr2p), two closely related but functionally distinguishable PDR transporters, were expressed in Saccharomyces cerevisiae. All chimeras expressed equally well, localized properly at the plasma membrane, retained their transport ability, but their substrate and inhibitor specificities differed significantly between individual constructs. A detailed characterization of these proteins revealed structural features that contribute to their substrate specificities and their transport mechanism. It appears that most transmembrane spans of CaCdr1p and CaCdr2p provide or affect multiple, probably overlapping, substrate and inhibitor binding site(s) similar to mammalian ABC transporters. The NBDs, in particular NBD1 and/or the ∼150 amino acids N-terminal to NBD1, can also modulate the substrate specificities of CaCdr1p and CaCdr2p.

Genome-wide association analysis of clinical vs. nonclinical origin provides insights into Saccharomyces cerevisiae pathogenesis

Because domesticated Saccharomyces cerevisiae strains have been used to produce fermented food and beverages for centuries without apparent health implications, S. cerevisiae has always been considered a Generally Recognized As Safe (GRAS) microorganism. However, the number of reported mucosal and systemic S. cerevisiae infections in the human population has increased and fatal infections have occurred even in relatively healthy individuals. In order to gain insight into the pathogenesis of S. cerevisiae and improve our understanding of the emergence of fungal pathogens, we performed a population-based genome-wide environmental association analysis of clinical vs. nonclinical origin in S. cerevisiae. Using tiling array-based, high-density genotypes of 44 clinical and 44 nonclinical S. cerevisiae strains from diverse geographical origins and source substrates, we identified several genetic loci associated with clinical background in S. cerevisiae. Associated polymorphisms within the coding sequences of VRP1, KIC1, SBE22 and PDR5, and the 5' upstream region of YGR146C indicate the importance of pseudohyphal formation, robust cell wall maintenance and cellular detoxification for S. cerevisiae pathogenesis, and constitute good candidates for follow-up verification of virulence and virulence-related factors underlying the pathogenicity of S. cerevisiae.

Knocking out multigene redundancies via cycles of sexual assortment and fluorescence selection

Phenotypes that might otherwise reveal a gene’s function can be obscured by genes with overlapping function. This phenomenon is best-known within gene families, where an important shared function may only be revealed by mutating all family members. Here we describe the ‘Green Monster’ technology enabling the precise deletion of many genes. In this method, a population of deletion strains with each deletion marked by an inducible green fluorescent protein (GFP) reporter gene, is subjected to repeated rounds of mating, meiosis, and flow-cytometric enrichment. This results in the aggregation of multiple deletion loci within single cells. The Green Monster strategy is potentially applicable to assembling other engineered alterations in any species with sex or alternative means of allelic assortment. To demonstrate the technology, we generated a single broadly drug-sensitive strain of Saccharomyces cerevisiae bearing precise deletions of all 16 adenosine triphosphate-binding cassette transporters within clades associated with multi-drug resistance.

Asymmetrically inherited multidrug resistance transporters are recessive determinants in cellular replicative ageing

Cellular ageing is known to correlate with the accumulation of many harmful agents, but it is unclear whether ageing can also result from the deterioration of components that are beneficial to the cell, but have a low rate of renewal. Here, we report a group of plasma membrane-associated transporters in yeast, belonging to the multidrug resistance (MDR) protein families, that may represent the latter type of ageing determinants. During the division of a yeast cell, newly synthesized transporter proteins are deposited mostly into the growing bud, whereas previously synthesized MDR proteins remain tightly associated with the mother cortex. Thus, the new and old pools of membrane-bound MDR proteins are spatially segregated during yeast asymmetric cell division, with the older pool stably inherited by the ageing mother. A model based on the observed dynamics of MDR protein inheritance and turnover predicted a decline in MDR activity as the mother cell advances in replicative age. As MDR proteins have crucial roles in cellular metabolism, detoxification and stress response, their collective decline may lead to fitness loss at an advanced age. Supporting this hypothesis, mutants lacking certain MDR genes exhibited a reduced replicative lifespan (RLS), whereas introduction of only one extra copy of these MDR genes extended RLS.

Antagonistic changes in sensitivity to antifungal drugs by mutations of an important ABC transporter gene in a fungal pathogen

Fungal pathogens can be lethal, especially among immunocompromised populations, such as patients with AIDS and recipients of tissue transplantation or chemotherapy. Prolonged usage of antifungal reagents can lead to drug resistance and treatment failure. Understanding mechanisms that underlie drug resistance by pathogenic microorganisms is thus vital for dealing with this emerging issue. In this study, we show that dramatic sequence changes in PDR5, an ABC (ATP-binding cassette) efflux transporter protein gene in an opportunistic fungal pathogen, caused the organism to become hypersensitive to azole, a widely used antifungal drug. Surprisingly, the same mutations conferred growth advantages to the organism on polyenes, which are also commonly used antimycotics. Our results indicate that Pdr5p might be important for ergosterol homeostasis. The observed remarkable sequence divergence in the PDR5 gene in yeast strain YJM789 may represent an interesting case of adaptive loss of gene function with significant clinical implications.

ABC transporters in Saccharomyces cerevisiae and their interactors: new technology advances the biology of the ABCC (MRP) subfamily

Members of the ATP-binding cassette (ABC) transporter superfamily exist in bacteria, fungi, plants, and animals and play key roles in the efflux of xenobiotic compounds, physiological substrates, and toxic intracellular metabolites. Based on sequence relatedness, mammalian ABC proteins have been divided into seven subfamilies, ABC subfamily A (ABCA) to ABCG. This review focuses on recent advances in our understanding of ABC transporters in the model organism Saccharomyces cerevisiae. We propose a revised unified nomenclature for the six yeast ABC subfamilies to reflect the current mammalian designations ABCA to ABCG. In addition, we specifically review the well-studied yeast ABCC subfamily (formerly designated the MRP/CFTR subfamily), which includes six members (Ycf1p, Bpt1p, Ybt1p/Bat1p, Nft1p, Vmr1p, and Yor1p). We focus on Ycf1p, the best-characterized yeast ABCC transporter. Ycf1p is located in the vacuolar membrane in yeast and functions in a manner analogous to that of the human multidrug resistance-related protein (MRP1, also called ABCC1), mediating the transport of glutathione-conjugated toxic compounds. We review what is known about Ycf1p substrates, trafficking, processing, posttranslational modifications, regulation, and interactors. Finally, we discuss a powerful new yeast two-hybrid technology called integrated membrane yeast two-hybrid (iMYTH) technology, which was designed to identify interactors of membrane proteins. iMYTH technology has successfully identified novel interactors of Ycf1p and promises to be an invaluable tool in future efforts to comprehensively define the yeast ABC interactome.

Multidrug efflux pumps: substrate selection in ATP-binding cassette multidrug efflux pumps--first come, first served?

Multidrug resistance is a major challenge in the therapy of cancer and pathogenic fungal infections. More than three decades ago, P-glycoprotein was the first identified multidrug transporter. It has been studied extensively at the genetic and biochemical levels ever since. Pdr5, the most abundant ATP-binding cassette transporter in Saccharomyces cerevisiae, is highly homologous to azole-resistance-mediating multidrug transporters in fungal pathogens, and a focus of clinical drug resistance research. Despite functional equivalences, P-glycoprotein and Pdr5 exhibit striking differences in their architecture and mechanisms. In this minireview, we discuss the mechanisms of substrate selection and multidrug transport by comparing the fraternal twins P-glycoprotein and Pdr5. We propose that substrate selection in eukaryotic multidrug ATP-binding cassette transporters is not solely determined by structural features of the transmembrane domains but also by their dynamic behavior.

Efflux-mediated antifungal drug resistance

Fungi cause serious infections in the immunocompromised and debilitated, and the incidence of invasive mycoses has increased significantly over the last 3 decades. Slow diagnosis and the relatively few classes of antifungal drugs result in high attributable mortality for systemic fungal infections. Azole antifungals are commonly used for fungal infections, but azole resistance can be a problem for some patient groups. High-level, clinically significant azole resistance usually involves overexpression of plasma membrane efflux pumps belonging to the ATP-binding cassette (ABC) or the major facilitator superfamily class of transporters. The heterologous expression of efflux pumps in model systems, such Saccharomyces cerevisiae, has enabled the functional analysis of efflux pumps from a variety of fungi. Phylogenetic analysis of the ABC pleiotropic drug resistance family has provided a new view of the evolution of this important class of efflux pumps. There are several ways in which the clinical significance of efflux-mediated antifungal drug resistance can be mitigated. Alternative antifungal drugs, such as the echinocandins, that are not efflux pump substrates provide one option. Potential therapeutic approaches that could overcome azole resistance include targeting efflux pump transcriptional regulators and fungal stress response pathways, blockade of energy supply, and direct inhibition of efflux pumps.