Vmr 1p is a novel vacuolar multidrug resistance ABC transporter in Saccharomyces cerevisiae

Multilevel Regulation of Membrane Proteins in Response to Metal and Metalloid Stress: A Lesson from Yeast

In the face of flourishing industrialization and global trade, heavy metal and metalloid contamination of the environment is a growing concern throughout the world. The widespread presence of highly toxic compounds of arsenic, antimony, and cadmium in nature poses a particular threat to human health. Prolonged exposure to these toxins has been associated with severe human diseases, including cancer, diabetes, and neurodegenerative disorders. These toxins are known to induce analogous cellular stresses, such as DNA damage, disturbance of redox homeostasis, and proteotoxicity. To overcome these threats and improve or devise treatment methods, it is crucial to understand the mechanisms of cellular detoxification in metal and metalloid stress. Membrane proteins are key cellular components involved in the uptake, vacuolar/lysosomal sequestration, and efflux of these compounds; thus, deciphering the multilevel regulation of these proteins is of the utmost importance. In this review, we summarize data on the mechanisms of arsenic, antimony, and cadmium detoxification in the context of membrane proteome. We used yeast Saccharomyces cerevisiae as a eukaryotic model to elucidate the complex mechanisms of the production, regulation, and degradation of selected membrane transporters under metal(loid)-induced stress conditions. Additionally, we present data on orthologues membrane proteins involved in metal(loid)-associated diseases in humans.

Molecular study on the role of vacuolar transporters in glycyrrhetinic acid production in engineered Saccharomyces cerevisiae

Glycyrrhetinic acid (GA) is one of the major bioactive components of the leguminous plant, Glycyrrhiza spp. (Chinese licorice). Owing to GA's complicated chemical structure, its production by chemical synthesis is challenging and requires other efficient strategies such as microbial synthesis. Earlier investigations employed numerous approaches to improve GA yield by refining the synthetic pathway and improving the metabolic flux. Nevertheless, the metabolic role of transporters in GA biosynthesis in microbial cell factories has not been studied so far. In this study, we investigated the role of yeast ATP binding cassette (ABC) vacuolar transporters in GA production. Molecular docking of GA and its precursors, β-Amyrin and 11-oxo-β-amyrin, was performed with five vacuolar ABC transporters (Bpt1p, Vmr1p, Ybt1p, Ycf1p and Nft1p). Based on docking scores, two top scoring transporters were selected (Bpt1p and Vmr1p) to investigate transporters' functions on GA production via overexpression and knockout experiments in one GA-producing yeast strain (GA166). Results revealed that GA and its precursors exhibited the highest predicted binding affinity towards BPT1 (ΔG = -10.9, -10.6, -10.9 kcal/mol for GA, β-amyrin and 11-oxo-β-amyrin, respectively). Experimental results showed that the overexpression of BPT1 and VMR1 restored the intracellular as well as extracellular GA production level under limited nutritional conditions, whereas knockout of BPT1 resulted in a total loss of GA production. These results suggest that the activity of BPT1 is required for GA production in engineered Saccharomyces cerevisiae.

ABC-finder: A containerized web server for the identification and topology prediction of ABC proteins

In view of the multiple clinical and physiological implications of ABC transporter proteins, there is a considerable interest among researchers to characterize them functionally. However, such characterizations are based on the premise that ABC proteins are accurately identified in the proteome of an organism, and their topology is correctly predicted. With this objective, we have developed ABC-finder, i.e., a Docker-based package for the identification of ABC proteins in all organisms, and visualization of the topology of ABC proteins using a web browser. ABC-finder is built and deployed in a Linux container, making it scalable for many concurrent users on our servers and enabling users to download and run it locally. Overall, ABC-finder is a convenient, portable, and platform-independent tool for the identification and topology prediction of ABC proteins. ABC-finder is accessible at http://abc-finder.osdd.jnu.ac.in.

Multidrug Resistance in Mammals and Fungi-From MDR to PDR: A Rocky Road from Atomic Structures to Transport Mechanisms

Multidrug resistance (MDR) can be a serious complication for the treatment of cancer as well as for microbial and parasitic infections. Dysregulated overexpression of several members of the ATP-binding cassette transporter families have been intimately linked to MDR phenomena. Three paradigm ABC transporter members, ABCB1 (P-gp), ABCC1 (MRP1) and ABCG2 (BCRP) appear to act as brothers in arms in promoting or causing MDR in a variety of therapeutic cancer settings. However, their molecular mechanisms of action, the basis for their broad and overlapping substrate selectivity, remains ill-posed. The rapidly increasing numbers of high-resolution atomic structures from X-ray crystallography or cryo-EM of mammalian ABC multidrug transporters initiated a new era towards a better understanding of structure-function relationships, and for the dynamics and mechanisms driving their transport cycles. In addition, the atomic structures offered new evolutionary perspectives in cases where transport systems have been structurally conserved from bacteria to humans, including the pleiotropic drug resistance (PDR) family in fungal pathogens for which high resolution structures are as yet unavailable. In this review, we will focus the discussion on comparative mechanisms of mammalian ABCG and fungal PDR transporters, owing to their close evolutionary relationships. In fact, the atomic structures of ABCG2 offer excellent models for a better understanding of fungal PDR transporters. Based on comparative structural models of ABCG transporters and fungal PDRs, we propose closely related or even conserved catalytic cycles, thus offering new therapeutic perspectives for preventing MDR in infectious disease settings.

Multiple roles of ABC transporters in yeast

The ATP binding cassette (ABC) transporters, first discovered as high-affinity nutrient importers in bacteria, rose to prominence when their ability to confer multidrug resistance (MDR) to cancer cells was realized. The most characterized human permeability glycoprotein (P-gp) is a dominant exporter of anti-cancer drugs and its overexpression is directly linked to MDR. The overexpression of drug efflux pumps belonging to the ABC superfamily is also a frequent cause of resistance to antifungals. Fungi has a battery of ABC proteins, but in variable numbers and at different subcellular locations. These proteins perform many critical functions, from serving as gatekeepers for xenobiotic cleansing to translocating various structurally unrelated cargoes, including lipids, fatty acids, ions, peptides, sterols, metabolites and toxins. Their emerging additional roles in cellular physiology and virulence call for attention to analyze and re-examine their divergent functions in yeast. In brief, this review traces the history of ABC transporters in yeast and discusses their typical physiological functions that go beyond their well-known role as antifungal drug efflux pumps.

An update on ABC transporters of filamentous fungi - from physiological substrates to xenobiotics

The superfamily of ATP-binding cassette (ABC) transporters is a large family of proteins with a wide substrate repertoire and range of functions. The main role of these proteins is in the transportation of different molecules across biological membranes. Due to the broad range of substrates, ABC transporters can transport not only natural metabolites but also various xenobiotics, including antifungal compounds, which makes some ABC transporters key players in antifungal resistance. Alternatively, ABC proteins without transport function seem to be essential for fungal cell viability. In this work, we review the individual subfamilies of ABC transporters in filamentous fungi regarding physiological substrates, clinical and agricultural significance. Subfamilies are defined using well-studied transporters in yeast, which may help to clarify their role in filamentous fungi.

Transcription factors and ABC transporters: from pleiotropic drug resistance to cellular signaling in yeast

Budding yeast Saccharomyces cerevisiae survives in microenvironments utilizing networks of regulators and ATP-binding cassette (ABC) transporters to circumvent toxins and a variety of drugs. Our understanding of transcriptional regulation of ABC transporters in yeast is mainly derived from the study of multidrug resistance protein networks. Over the past two decades, this research has not only expanded the role of transcriptional regulators in pleiotropic drug resistance (PDR) but evolved to include the role that regulators play in cellular signaling and environmental adaptation. Inspection of the gene networks of the transcriptional regulators and characterization of the ABC transporters has clarified that they also contribute to environmental adaptation by controlling plasma membrane composition, toxic-metal sequestration, and oxidative stress adaptation. Additionally, ABC transporters and their regulators appear to be involved in cellular signaling for adaptation of S. cerevisiae populations to nutrient availability. In this review, we summarize the current understanding of the S. cerevisiae transcriptional regulatory networks and highlight recent work in other notable fungal organisms, underlining the expansion of the study of these gene networks across the kingdom fungi.

The yeast (Saccharomyces cerevisiae) YCF1 vacuole transporter: Evidence on its implication into the yeast resistance to flusilazole as revealed by GC/EI/MS metabolomics

The development of plant protection product (PPPs)-resistant populations of plant pathogens, pests, and weeds, represents a major challenge that the crop protection sector is facing. Focusing on plant pathogenic fungi, the increased efflux of the active ingredients (a.i.) from the cytoplasm is highly correlated to elevated resistance levels to the applied fungicides. Such mechanism is regulated by ATP-binding cassette transporters (ABC transporters), and although it has been investigated for the past two decades, the latest developments in "omics" technologies could provide new insights with potential applications in crop protection. Within this context, and based on results from preliminary experiments, we have undertaken the task of mining the involvement of the ABC transporter YCF1, which is located in the vacuole membrane, in the fungicide resistance development, applying a functional genomics approach and using yeast (Saccharomyces cerevisiae) as the model organism. Among the fungicides being assessed, flusilazole, which belongs to the azole group of dimethylation inhibitors (DMIs), was discovered as a possible substrate of the YCF1. GC/EI/MS metabolomics analysis revealed the effect of the fungicide's toxicity and that of genotype on yeast's metabolism, confirming the role of this transporter. Fluctuations in the activity of various yeast biosynthetic pathways associated with stress responses were recorded, and corresponding metabolites-biomarkers of flusilazole toxicity were discovered. The metabolites α,α-trehalose, glycerol, myo-inositol-1-phosphate, GABA, l-glutamine, l-tryptophan, l-phenylalanine, l-tyrosine, and phosphate, were the major identified biomarkers of toxicity. Among these, are metabolites that play important roles in fungal metabolism (e.g., cell responses to osmotic stress) or serve as signaling molecules. To the best of our knowledge, this is the first report on the implication of YCF1 in fungal resistance to PPPs. Additionally, the results of GC/EI/MS yeast metabolomics confirmed the robustness of the method and its applicability in the high-throughput study of fungal resistance to fungicides.

The Reduced Level of Inorganic Polyphosphate Mobilizes Antioxidant and Manganese-Resistance Systems in Saccharomyces cerevisiae

Inorganic polyphosphate (polyP) is crucial for adaptive reactions and stress response in microorganisms. A convenient model to study the role of polyP in yeast is the Saccharomyces cerevisiae strain CRN/PPN1 that overexpresses polyphosphatase Ppn1 with stably decreased polyphosphate level. In this study, we combined the whole-transcriptome sequencing, fluorescence microscopy, and polyP quantification to characterize the CRN/PPN1 response to manganese and oxidative stresses. CRN/PPN1 exhibits enhanced resistance to manganese and peroxide due to its pre-adaptive state observed in normal conditions. The pre-adaptive state is characterized by up-regulated genes involved in response to an external stimulus, plasma membrane organization, and oxidation/reduction. The transcriptome-wide data allowed the identification of particular genes crucial for overcoming the manganese excess. The key gene responsible for manganese resistance is PHO84 encoding a low-affinity manganese transporter: Strong PHO84 down-regulation in CRN/PPN1 increases manganese resistance by reduced manganese uptake. On the contrary, PHM7, the top up-regulated gene in CRN/PPN1, is also strongly up-regulated in the manganese-adapted parent strain. Phm7 is an unannotated protein, but manganese adaptation is significantly impaired in Δphm7, thus suggesting its essential function in manganese or phosphate transport.

Rsp5-dependent endocytosis and degradation of the arsenite transporter Acr3 requires its N-terminal acidic tail as an endocytic sorting signal and arrestin-related ubiquitin-ligase adaptors

The yeast plasma membrane transporter Acr3 mediates efflux of toxic arsenite and antimonite. Here, we investigated the mechanisms of Acr3 turnover. We found that after arrival and residence at the plasma membrane, Acr3 is subjected to internalization followed by proteolysis in the vacuole. Endocytic degradation of Acr3 is promoted by the ubiquitin ligase Rsp5 and requires polyubiquitination of Acr3 at multiple lysine residues via lysine 63-linked ubiquitin chains. The turnover of Acr3 also depends on two arrestin-related proteins, Art3/Aly2 and Art4/Rod1, that enable recruitment of Rsp5 to its targets. Finally, we found that a short acidic patch located in the N-terminal tail of Acr3 is needed for its ubiquitination and internalization. We propose that this motif serves as an endocytic signal that facilitates binding of the arrestin-Rsp5 complexes to the Acr3 cargo.

Glutathione: subcellular distribution and membrane transport 1

Glutathione (γ-l-glutamyl-l-cysteinylglycine) is a small tripeptide found at millimolar concentrations in nearly all eukaryotes as well as many prokaryotic cells. Glutathione synthesis is restricted to the cytosol in animals and fungi and to the cytosol and plastids in plants. Nonetheless, glutathione is found in virtually all subcellular compartments. This implies that transporters must exist that facilitate glutathione transport into and out of the various subcellular compartments. Glutathione may also be exported and imported across the plasma membrane in many cells. However, in most cases, the molecular identity of these transporters remains unclear. Whilst glutathione transport is essential for the supply and replenishment of subcellular glutathione pools, recent evidence supports a more active role for glutathione transport in the regulation of subcellular glutathione redox homeostasis. However, our knowledge of glutathione redox homeostasis at the level of specific subcellular compartments remains remarkably limited and the role of glutathione transport remains largely unclear. In this review, we discuss how new tools and techniques have begun to yield insights into subcellular glutathione distribution and glutathione redox homeostasis. In particular, we discuss the known and putative glutathione transporters and examine their contribution to the regulation of subcellular glutathione redox homeostasis.

Exploring the biological roles of Dothideomycetes ABC proteins: Leads from their phylogenetic relationships with functionally-characterized Ascomycetes homologs

BACKGROUND

The ATP-binding cassette (ABC) superfamily is one of the largest, ubiquitous and diverse protein families in nature. Categorized into nine subfamilies, its members are important to most organisms including fungi, where they play varied roles in fundamental cellular processes, plant pathogenesis or fungicide tolerance. However, these proteins are not yet well-understood in the class Dothideomycetes, which includes several phytopathogens that infect a wide range of food crops including wheat, barley and maize and cause major economic losses.

RESULTS

We analyzed the genomes of 14 Dothideomycetes fungi (Test set) and seven well-known Ascomycetes fungi (Model set- that possessed gene expression/ functional analysis data about the ABC genes) and predicted 578 and 338 ABC proteins from each set respectively. These proteins were classified into subfamilies A to I, which revealed the distribution of the subfamily members across the Dothideomycetes and Ascomycetes genomes. Phylogenetic analysis of Dothideomycetes ABC proteins indicated evolutionary relationships among the subfamilies within this class. Further, phylogenetic relationships among the ABC proteins from the Model and the Test fungi within each subfamily were analyzed, which aided in classifying these proteins into subgroups. We compiled and curated functional and gene expression information from the previous literature for 118 ABC genes and mapped them on the phylogenetic trees, which suggested possible roles in pathogenesis and/or fungicide tolerance for the newly identified Dothideomycetes ABC proteins.

CONCLUSIONS

The present analysis is one of the firsts to extensively analyze ABC proteins from Dothideomycetes fungi. Their phylogenetic analysis and annotating the clades with functional information indicated a subset of Dothideomycetes ABC genes that could be considered for experimental validation for their roles in plant pathogenesis and/or fungicide tolerance.

Sensitivity of Yeast Mutants Deficient in Mitochondrial or Vacuolar ABC Transporters to Pathogenesis-Related Protein TcPR-10 of Theobroma cacao

Pathogenesis-related proteins (PRs) are induced in plants after infection by pathogens and/or abiotic stress. Among these proteins, the family 10 (PR-10) influences the biosynthesis of secondary metabolites and shows antimicrobial ribonuclease activity. TcPR-10p (Pathogenesis-related Protein 10 of Theobroma cacao) was isolated from resistant and susceptible Moniliophthora perniciosa cacao cultivars. Cell survival with Saccharomyces cerevisiae mutant lines deficient in ATP-binding cassette (ABC) transporter proteins indicated the influence on resistance to TcPR-10p. Proteins of the ABC transport type are considered important in the process of resistance to antimicrobials and toxins. Thus, the objective of this work was to observe the sensitivity of ABC transporter yeast mutants in the presence of the TcPR-10p. Chronic exposure of S. cerevisiae mitochondrial (BYatm1Δ and BYmdl1Δ) and vacuole (BYnft1Δ, BYvmr1Δ, BYybt1Δ, BYycf1Δ and BYbpt1Δ) ABC transporter mutants to TcPR-10p (3 μg/mL, 0, 6, 12 and 24 h) was performed. Two TcPR-10p sensitive strains (BYmdl1Δ and BYnft1Δ) were submitted to a fluorescence test with the fluorogenic dihydroethidium (DHE), to visualize the presence of oxidative stress in the cells. Oxidative stress-increased sensitivity was confirmed by flow cytometry indicating induced cell death either via apoptosis or necrosis. This yeast data combined with previous data of literature (of M. perniciosa sensitivity to TcPR-10p) show that increased sensitivity to TcPR-10p in these mutants could be due to the TcPR10p-generated higher levels of intracellular reactive oxygen species (ROS), leading to increased cell death either via necrosis or apoptosis.

Vba4p, a vacuolar membrane protein, is involved in the drug resistance and vacuolar morphology of Saccharomyces cerevisiae

In the vacuolar basic amino acid (VBA) transporter family of Saccharomyces cerevisiae, VBA4 encodes a vacuolar membrane protein with 14 putative transmembrane helices. Transport experiments with isolated vacuolar membrane vesicles and estimation of the amino acid contents in vacuoles showed that Vba4p is not likely involved in the transport of amino acids. We found that the vba4Δ cells, as well as vba1Δ and vba2Δ cells, showed increased susceptibility to several drugs, particularly to azoles. Although disruption of the VBA4 gene did not affect the salt tolerance of the cells, vacuolar fragmentation observed under high salt conditions was less prominent in vba4Δ cells than in wild type, vba1Δ, and vba2Δ cells. Vba4p differs from Vba1p and Vba2p as a vacuolar transporter but is important for the drug resistance and vacuolar morphology of S. cerevisiae.

Candida Efflux ATPases and Antiporters in Clinical Drug Resistance

An enhanced expression of genes encoding ATP binding cassette (ABC) and major facilitator superfamily (MFS) transport proteins are known to contribute to the development of tolerance to antifungals in pathogenic yeasts. For example, the azole resistant (AR) clinical isolates of the opportunistic human fungal pathogen Candida albicans show an overexpression of CDR1 and/or CaMDR1 belonging to ABC and MFS, superfamilies, respectively. The reduced accumulation (due to rapid efflux) of drugs in AR isolates confirms the role of efflux pump proteins in the development of drug tolerance. Considering the importance of major multidrug transporters, the focus of recent research has been to understand the structure and function of these proteins which could help to design inhibitors/modulators of these pump proteins. This chapter focuses on some aspects of the structure and function of yeast transporter proteins particularly in relation to MDR in Candida.

Unravelling glutathione conjugate catabolism in Saccharomyces cerevisiae: the role of glutathione/dipeptide transporters and vacuolar function in the release of volatile sulfur compounds 3-mercaptohexan-1-ol and 4-mercapto-4-methylpentan-2-one

Sulfur-containing aroma compounds are key contributors to the flavour of a diverse range of foods and beverages, such as wine. The tropical fruit characters of Sauvignon Blanc wines are attributed to the presence of the aromatic thiols 3-mercaptohexan-1-ol (3-MH), its acetate ester 3-mercaptohexyl acetate (3-MHA), and 4-mercapto-4-methylpentan-2-one (4-MMP). These aromatic thiols are not detectable in grape juice to any significant extent but are released by yeast during alcoholic fermentation. While the processes involved in the release of 3-MH and 4-MMP from their cysteinylated precursors have been studied extensively, degradation pathways for glutathione S-conjugates (GSH-3-MH and GSH-4-MMP) have not. In this study, a candidate gene approach was taken, focusing on genes known to play a role in glutathione and glutathione-S-conjugate turnover in Saccharomyces cerevisiae. Our results confirm the role of Opt1p as the major transporter responsible for uptake of GSH-3-MH and GSH-4-MMP, and identify vacuolar Ecm38p as a key determinant of 3-MH release from GSH-3-MH. ECM38 was unimportant, on the other hand, for release of 4-MMP, and abolition of vacuolar biogenesis caused an increase in the amount of 4-MMP released. The alternative cytosolic glutathione degradation pathway was not involved in release of either thiol from their glutathionylated precursors. Finally, cycling of GSH-3-MH and/or its breakdown intermediates between the cytosol and the vacuole or extracellular space was implicated in modulation of 3-MH formation. Together, these results provide new targets for development of yeast strains that optimize release of these potent volatile sulfur compounds, and further our understanding of the processes involved in glutathione-S-conjugate turnover.

Repression of the Low Affinity Iron Transporter Gene FET4: A NOVEL MECHANISM AGAINST CADMIUM TOXICITY ORCHESTRATED BY YAP1 VIA ROX1

Cadmium is a well known mutagenic metal that can enter cells via nonspecific metal transporters, causing several cellular damages and eventually leading to death. In the yeast Saccharomyces cerevisiae, the transcription factor Yap1 plays a key role in the regulation of several genes involved in metal stress response. We have previously shown that Yap1 represses the expression of FET4, a gene encoding a low affinity iron transporter able to transport metals other than iron. Here, we have studied the relevance of this repression in cell tolerance to cadmium. Our results indicate that genomic deletion of Yap1 increases FET4 transcript and protein levels. In addition, the cadmium toxicity exhibited by this strain is completely reversed by co-deletion of FET4 gene. These data correlate well with the increased intracellular levels of cadmium observed in the mutant yap1. Rox1, a well known aerobic repressor of hypoxic genes, conveys the Yap1-mediated repression of FET4. We further show that, in a scenario where the activity of Yap1 or Rox1 is compromised, cells activate post-transcriptional mechanisms, involving the exoribonuclease Xrn1, to compensate the derepression of FET4. Our data thus reveal a novel protection mechanism against cadmium toxicity mediated by Yap1 that relies on the aerobic repression of FET4 and results in the impairment of cadmium uptake.

The diversity of membrane transporters encoded in bacterial arsenic-resistance operons

Transporter-facilitated arsenite extrusion is the major pathway of arsenic resistance within bacteria. So far only two types of membrane-bound transporter proteins, ArsB and ArsY (ACR3), have been well studied, although the arsenic transporters in bacteria display considerable diversity. Utilizing accumulated genome sequence data, we searched arsenic resistance (ars) operons in about 2,500 bacterial strains and located over 700 membrane-bound transporters which are encoded in these operons. Sequence analysis revealed at least five distinct transporter families, with ArsY being the most dominant, followed by ArsB, ArsP (a recently reported permease family), Major Facilitator protein Superfamily (MFS) and Major Intrinsic Protein (MIP). In addition, other types of transporters encoded in the ars operons were found, but in much lower frequencies. The diversity and evolutionary relationships of these transporters with regard to arsenic resistance will be discussed.

ABCC subfamily vacuolar transporters are involved in Pb (lead) detoxification in Saccharomyces cerevisiae

The present work has as objective to contribute for the elucidation of the mechanism associated with Pb detoxification, using the yeast Saccharomyces cerevisiae as a model organism. The deletion of GTT1 or GTT2 genes, coding for functional glutathione transferases (GST) enzymes in S. cerevisiae, caused an increased susceptibility to high Pb concentrations (500-1000 μmol L(-1)). These results suggest that the formation of glutathione-Pb conjugate (GS-Pb), dependent of GSTs, is important in Pb detoxification. The involvement of ATP-binding cassette (ABC) vacuolar transporters, belonging to class C subfamily (ABCC) in vacuolar compartmentalization of Pb, was evaluated. For this purpose, mutant strains disrupted in YCF1, VMR1, YBT1 or BPT 1 genes were used. All mutants tested, without vacuolar ABCC transporters, presented an increased sensitivity to 500-1000 μmol L(-1) Pb comparative to wild-type strain. Taken together, the obtained results suggest that Pb detoxification, by vacuolar compartmentalization, can occur as a result of the concerted action of GSTs and vacuolar ABCC transporters. Pb is conjugated with glutathione, catalysed by glutathione transferases and followed to the transport of GS-Pb conjugate to the vacuole by ABCC transporters.

Cadmium

Cadmium is not an essential element for life. It is geologically marginal but anthropogenic activities have contributed significantly to its dispersion in the environment and to cadmium exposure of living species. The natural speciation of the divalent cation Cd2+ is dominated by its high propensity to bind to sulfur ligands, but Cd2+ may also occupy sites providing imidazole and carboxylate ligands. It binds to cell walls by passive adsorption (bio-sorption) and it may interact with surface receptors. Cellular uptake can occur by ion mimicry through a variety of transporters of essential divalent cations, but not always. Once inside cells, Cd2+ preferentially binds to thiol-rich molecules. It can accumulate in intracellular vesicles. It may also be transported over long distances within multicellular organisms and be trapped in locations devoid of efficient excretion systems. These locations include the renal cortex of animals and the leaves of hyper-accumulating plants. No specific regulatory mechanism monitors Cd2+ cellular concentrations. Thiol recruitment by cadmium is a major interference mechanism with many signalling pathways that rely on thiolate-disulfide equilibria and other redox-related processes. Cadmium thus compromises the antioxidant intracellular response that relies heavily on molecules with reactive thiolates. These biochemical features dominate cadmium toxicity, which is complex because of the diversity of the biological targets and the consequent pleiotropic effects. This chapter compares the cadmium-handling systems known throughout phylogeny and highlights the basic principles underlying the impact of cadmium in biology.

Class C ABC transporters and Saccharomyces cerevisiae vacuole fusion

Membrane fusion is carried out by core machinery that is conserved throughout eukaryotes. This is comprised of Rab GTPases and their effectors, and SNARE proteins, which together are sufficient to drive the fusion of reconstituted proteoliposomes. However, an outer layer of factors that are specific to individual trafficking pathways in vivo regulates the spatial and temporal occurrence of fusion. The homotypic fusion of Saccharomyces cerevisiae vacuolar lysosomes utilizes a growing set of factors to regulate the fusion machinery that include members of the ATP binding cassette (ABC) transporter family. Yeast vacuoles have five class C ABC transporters that are known to transport a variety of toxins into the vacuole lumen as part of detoxifying the cell. We have found that ABCC transporters can also regulate vacuole fusion through novel mechanisms. For instance Ybt1 serves as negative regulator of fusion through its effects on vacuolar Ca²⁺ homeostasis. Additional studies showed that Ycf1 acts as a positive regulator by affecting the efficient recruitment of the SNARE Vam7. Finally, we discuss the potential interface between the translocation of lipids across the membrane bilayer, also known as lipid flipping, and the efficiency of fusion.

Yeast ABC proteins involved in multidrug resistance

Pleiotropic drug resistance is a complex phenomenon that involves many proteins that together create a network. One of the common mechanisms of multidrug resistance in eukaryotic cells is the active efflux of a broad range of xenobiotics through ATP-binding cassette (ABC) transporters. Saccharomyces cerevisiae is often used as a model to study such activity because of the functional and structural similarities of its ABC transporters to mammalian ones. Numerous ABC transporters are found in humans and some are associated with the resistance of tumors to chemotherapeutics. Efflux pump modulators that change the activity of ABC proteins are the most promising candidate drugs to overcome such resistance. These modulators can be chemically synthesized or isolated from natural sources (e.g., plant alkaloids) and might also be used in the treatment of fungal infections. There are several generations of synthetic modulators that differ in specificity, toxicity and effectiveness, and are often used for other clinical effects.

Functional expression of Schizosaccharomyces pombe Vba2p in the vacuolar membrane of Saccharomyces cerevisiae

A vacuolar membrane protein, Vba2p of Schizosaccharomyces pombe, is involved in basic amino acid uptake by intact cells. Here we found evidence that Vba2p mediated ATP-dependent lysine uptake by vacuolar membrane vesicles of Saccharomyces cerevisiae. Vba2p was also responsible for quinidine sensitivity, and the addition of lysine improved cell growth on quinidine-containing media. These findings should be useful for further characterization of Vba2p.

Glutathione transporters

BACKGROUND

Glutathione (GSH) is synthesized in the cytoplasm but there is a requirement for glutathione not only in the cytoplasm, but in the other organelles and the extracellular milieu. GSH is also imported into the cytoplasm. The transports of glutathione across these different membranes in different systems have been biochemically demonstrated. However the molecular identity of the transporters has been established only in a few cases.

SCOPE OF REVIEW

An attempt has been made to present the current state of knowledge of glutathione transporters from different organisms as well as different organelles. These include the most well characterized transporters, the yeast high-affinity, high-specificity glutathione transporters involved in import into the cytoplasm, and the mammalian MRP proteins involved in low affinity glutathione efflux from the cytoplasm. Other glutathione transporters that have been described either with direct or indirect evidences are also discussed.

MAJOR CONCLUSIONS

The molecular identity of a few glutathione transporters has been unambiguously established but there is a need to identify the transporters of other systems and organelles. There is a lack of direct evidence establishing transport by suggested transporters in many cases. Studies with the high affinity transporters have led to important structure-function insights.

GENERAL SIGNIFICANCE

An understanding of glutathione transporters is critical to our understanding of redox homeostasis in living cells. By presenting our current state of understanding and the gaps in our knowledge the review hopes to stimulate research in these fields. This article is part of a Special Issue entitled Cellular functions of glutathione.

Arsenic and antimony transporters in eukaryotes

Arsenic and antimony are toxic metalloids, naturally present in the environment and all organisms have developed pathways for their detoxification. The most effective metalloid tolerance systems in eukaryotes include downregulation of metalloid uptake, efflux out of the cell, and complexation with phytochelatin or glutathione followed by sequestration into the vacuole. Understanding of arsenic and antimony transport system is of high importance due to the increasing usage of arsenic-based drugs in the treatment of certain types of cancer and diseases caused by protozoan parasites as well as for the development of bio- and phytoremediation strategies for metalloid polluted areas. However, in contrast to prokaryotes, the knowledge about specific transporters of arsenic and antimony and the mechanisms of metalloid transport in eukaryotes has been very limited for a long time. Here, we review the recent advances in understanding of arsenic and antimony transport pathways in eukaryotes, including a dual role of aquaglyceroporins in uptake and efflux of metalloids, elucidation of arsenic transport mechanism by the yeast Acr3 transporter and its role in arsenic hyperaccumulation in ferns, identification of vacuolar transporters of arsenic-phytochelatin complexes in plants and forms of arsenic substrates recognized by mammalian ABC transporters.

New insights into the Ca2+-ATPases that contribute to cadmium tolerance in yeast

Cadmium (Cd(2+)) is a toxic heavy metal which triggers several toxic effects in eukaryotes, including neurotoxicity and impaired calcium metabolism. In the model organism Saccharomyces cerevisiae, the best characterized pathway for Cd(2+) detoxification involves conjugation with glutathione (GSH) and subsequent transport to vacuoles by Ycf1p, an ATPase homologous to human MRP1 (Multidrug resistance associated protein 1). However, Cd(2+) tolerance also can be mediated by Pmr1p, a Ca(2+) pump located in the Golgi membrane, possibly through to the secretory pathway. Herein, we showed that inactivation of the PMR1 gene, alone or simultaneously with YCF1, delayed initial Cd(2+) capture compared to wild-type (WT) cells. In addition, Cd(2+) treatment altered the expression profile of yeast internal Ca(2+) transporters; specifically, PMC1 gene expression is induced substantially by the metal in WT cells, and this induction is stronger in mutants lacking YCF1. Taken together, these results indicate that, in addition to Pmr1p, the vacuolar Ca(2+)-ATPase Pmc1p also helps yeast cells cope with Cd(2+) toxicity. We propose a model where Pmc1p and Pmr1p Ca(2+)-ATPase function in cooperation with Ycf1p to promote Cd(2+) detoxification.

Vacuolar import of phosphatidylcholine requires the ATP-binding cassette transporter Ybt1

ATP-binding cassette (ABC) transporters are well known for their roles as multidrug resistance determinants but also play important roles in regulation of lipid levels. In the yeast Saccharomyces cerevisiae, the plasma membrane ABC transporter proteins Pdr5 and Yor1 are required for normal rates of transport of phosphatidyethanolamine to the surface of the cell. Loss of these ABC transporters causes a defect in phospholipid asymmetry across the plasma membrane and has been linked with slowed rates of trafficking of other membrane proteins. Four ABC transporter proteins are found on the limiting membrane of the yeast vacuole and loss of one of these vacuolar ABC transporters, Ybt1, caused a major defect in the normal delivery of the phosphatidylcholine (PC) analog NBD-PC (7-nitro-2,1,3-benzoxadiazol-PC) to the lumen of the vacuole. NBD-PC accumulates on cytosolic membranes in an ybt1Δ strain. We demonstrated that Ybt1 is required to import NBD-PC into vacuoles in the presence of ATP in vitro. Loss of Ybt1 prevented vacuolar remodeling of PC analogs. Turnover of Ybt1 was reduced under conditions in which function of this vacuolar remodeling pathway was required. Our data describe a novel vacuolar route for lipid remodeling and reutilization in addition to previously described enzymatic avenues in the cytoplasm.