The Zrc1 is involved in zinc transport system between vacuole and cytosol in Saccharomyces cerevisiae

Structures, Mechanisms, and Physiological Functions of Zinc Transporters in Different Biological Kingdoms

Zinc transporters take up/release zinc ions (Zn2+) across biological membranes and maintain intracellular and intra-organellar Zn2+ homeostasis. Since this process requires a series of conformational changes in the transporters, detailed information about the structures of different reaction intermediates is required for a comprehensive understanding of their Zn2+ transport mechanisms. Recently, various Zn2+ transport systems have been identified in bacteria, yeasts, plants, and humans. Based on structural analyses of human ZnT7, human ZnT8, and bacterial YiiP, we propose updated models explaining their mechanisms of action to ensure efficient Zn2+ transport. We place particular focus on the mechanistic roles of the histidine-rich loop shared by several zinc transporters, which facilitates Zn2+ recruitment to the transmembrane Zn2+-binding site. This review provides an extensive overview of the structures, mechanisms, and physiological functions of zinc transporters in different biological kingdoms.

Zinc homeostasis in Schizosaccharomyces pombe

Most metal ions such as iron, calcium, zinc, or copper are essential for all eukaryotes. Organisms must maintain homeostasis of these metal ions because excess or deficiency of metal ions could cause damage to organisms. The steady state of many metal ions such as iron and copper has been well studied in detail. However, how to regulate zinc homeostasis in Schizosaccharomyces pombe is still confusing. In this review, we provide an overview of the molecular mechanisms that how S. pombe is able to maintain the balance of zinc levels in the changes of environment. In response to high levels of zinc, the transcription factor Loz1 represses the expression of several genes involved in the acquisition of zinc. Meanwhile, the CDF family proteins transport excess zinc to the secretory pathway. When zinc levels are limited, Loz1 was inactivated and could not inhibit the expression of zinc acquisition genes, and zinc stored in the secretory pathway is released for use by the cells. Besides, other factors that regulate zinc homeostasis are also discussed.

Fungal Zinc Homeostasis and Its Potential as an Antifungal Target: A Focus on the Human Pathogen Aspergillus fumigatus

Aspergillus fumigatus is an opportunistic airborne fungus that causes severe invasive aspergillosis in immunocompromised patients. Zinc is an essential micronutrient for the growth of A. fumigatus and even for all microorganisms. An increasing number of studies have reported that fungal zinc acquisition ability plays a key role in fungal survival in hosts with an extremely zinc-limited microenvironment. The ability to fight scarcity and excess of zinc are tightly related to fungal virulence and may be used as new potential targets. Because the regulation of zinc homeostasis is important, a thorough understanding of the functional genes involved in the regulatory network for zinc homeostasis is required for fungal pathogens. The current mini-review summarized potential zinc homeostasis regulators in A. fumigatus and classified these regulators according to localization and function, which were identified or predicted based on A. fumigatus or deduced from homologs in model yeasts. Future perspectives for zinc homeostasis regulators as potential antifungal targets to treat invasive aspergillosis are also discussed.

Characterization and functional analysis of zinc trafficking in the human fungal pathogen Candida parapsilosis

The zinc restriction and zinc toxicity are part of host defence, called nutritional immunity. The crucial role of zinc homeostasis in microbial survival within a host is established, but little is known about these processes in the opportunistic human fungal pathogen Candida parapsilosis. Our in silico predictions suggested the presence of at least six potential zinc transporters (ZnTs) in C. parapsilosis-orthologues of ZRC1, ZRT3 and ZRT101-but an orthologue of PRA1 zincophore was not found. In addition, we detected a species-specific gene expansion of the novel ZnT ZRT2, as we identified three orthologue genes in the genome of C. parapsilosis. Based on predictions, we created homozygous mutant strains of the potential ZnTs and characterized them. Despite the apparent gene expansion of ZRT2 in C. parapsilosis, only CpZRT21 was essential for growth in a zinc-depleted acidic environment, in addition we found that CpZrc1 is essential for zinc detoxification and also protects the fungi against the elimination of murine macrophages. Significantly, we demonstrated that C. parapsilosis forms zincosomes in a Zrc1-independent manner and zinc detoxification is mediated by the vacuolar importer CpZrc1. Our study defines the functions of C. parapsilosis ZnTs, including a species-specific survival and zinc detoxification system.

Zinc Transporter ZmLAZ1-4 Modulates Zinc Homeostasis on Plasma and Vacuolar Membrane in Maize

Zinc is an essential micronutrient for plant growth and development, and functions as a cofactor for hundreds of transcription factors and enzymes in numerous biological processes. Zinc deficiency is common abiotic stress resulting in yield loss and quality deterioration of crops, but zinc excess causes toxicity for biological systems. In plants, zinc homeostasis is tightly modulated by zinc transporters and binding compounds that uptake/release, transport, localize, and store zinc, as well as their upstream regulators. Lazarus 1 (LAZ1), a member of DUF300 protein family, functions as transmembrane organic solute transporter in vertebrates. However, the function of LAZ1 in plants is still obscure. In the present study, the ZmLAZ1-4 protein was confirmed to bind to zinc ions by bioinformatic prediction and thermal shift assay. Heterologous expression of ZmLAZ1-4 in the zinc-sensitive yeast mutant, Arabidopsis, and maize significantly facilitated the accumulation of Zn2+ in transgenic lines, respectively. The result of subcellular localization exhibited that ZmLAZ1-4 was localized on the plasma and vacuolar membrane, as well as chloroplast. Moreover, the ZmLAZ1-4 gene was negatively co-expressed with ZmBES1/BZR1-11 gene through co-expression and real-time quantitative PCR analysis. The results of yeast one-hybrid and dual-luciferase assay suggested that ZmBES1/BZR1-11 could bind to ZmLAZ1-4 promoter to inhibit its transcription. All results indicated that ZmLAZ1-4 was a novel zinc transporter on plasma and vacuolar membrane, and transported zinc under negative regulation of the ZmBES1/BZR1-11 transcription factor. The study provides insights into further underlying the mechanism of ZmLAZ1-4 regulating zinc homeostasis.

Transcriptome characterization of candidate genes related to chromium uptake, transport and accumulation in Miscanthus sinensis

Miscanthus sinensis is a C4 perennial grass species that is widely used as forage, ornamental grass and bioenergy crop due to its broad adaption and great biological traits. Recent studies indicated that M. sinensis could also grow in marginal lands which were contaminated with heavy metals, and exhibited important ecological restoration potential. In this study, transcriptome characterization of candidate genes related to chromium (Cr) uptake, transport and accumulation in M. sinensis were employed to investigate the molecular mechanism of plant tolerance to heavy metal stress. The result showed that following treatment of 200 mg/L of Cr, plant roots could accumulate most Cr and localize mainly in cell walls and soluble fractions, whereas Cr in stems and leaves was primarily in soluble fractions. A total of 83,645 differentially expressed genes (DEGs) were obtained after the treatment. Many genes involved in heavy metal transport, metal ion chelation and photosynthesis were found to be Cr-induced DEGs. Co-expression and weighted correlation network analysis revealed that Glutathion metabolism and ABC transporters pathways play an important role in Cr tolerance of M. sinensis. A hypothesis schematic diagram for the Cr uptake, transport and accumulation of M. sinensis cells were suggested, which could provide a molecular and genetic basis for future candidate genes validation and breeding of such crops.

Enhancement of Zn tolerance and accumulation in plants mediated by the expression of Saccharomyces cerevisiae vacuolar transporter ZRC1

Transgenic Arabidopsis thaliana and Populus alba plants overexpressing the zinc transporter ScZRC1 in shoots exhibit Zn tolerance. Increased Zn concentrations were observed in shoots of P. alba, a species suitable for phytoremediation. Genetic engineering of plants for phytoremediation is worth to consider if genes leading to heavy metal accumulation and tolerance are expressed in high biomass producing plants. The Saccharomyces cerevisiae ZRC1 gene encodes a zinc transporter which is primarily involved in the uptake of Zn into the vacuole. The ZRC1 gene was expressed in the model species A. thaliana and P. alba (cv. Villafranca). Both species were transformed with constructs carrying ScZRC1 under the control of either the CaMV35S promoter for constitutive expression or the active promoter region of the tobacco Rubisco small subunit (pRbcS) to limit the expression to the above-ground tissues. In hydroponic cultures, A. thaliana and poplar ScZRC1-expressing plants accumulated more Zn in vegetative tissues and were more tolerant than untransformed plants. No differences were found between plants carrying the CaMV35::ScZRC1 or pRbcS::ScZRC1 constructs. The higher Zn accumulation in transgenic plants was accompanied by an increased superoxide dismutase (SOD) activity, indicating the activation of defense mechanisms to prevent cellular damage. In the presence of cadmium in addition to Zn, plants did not show symptoms of metal toxicity, neither in hydroponic cultures nor in soil. Zn accumulation increased in shoots, while no differences were observed for Cd accumulation, in comparison to control plants. These data suggest that ectopic expression of ScZRC1 can increase the potential of poplar for the remediation of Zn-polluted soils, although further tests are required to assay its application in remediating multimetal polluted soils.

Zinc at the Host-Fungus Interface: How to Uptake the Metal?

Zinc is an essential nutrient for all living organisms. However, firm regulation must be maintained since micronutrients also can be toxic in high concentrations. This notion is reinforced when we look at mechanisms deployed by our immune system, such as the use of chelators or membrane transporters that capture zinc, when threatened with pathogens, like fungi. Pathogenic fungi, on the other hand, also make use of a variety of transporters and specialized zinc captors to survive these changes. In this review, we sought to explain the mechanisms, grounded in experimental analysis and described to date, utilized by pathogenic fungi to maintain optimal zinc levels.

Isolation of multi-metal tolerant ubiquitin fusion protein from metal polluted soil by metatranscriptomic approach

Release of heavy metals into the soil pose a significant threat to the environment and public health because of their toxicity accumulation in the food chain and persistence in nature. The potential of soil microbial diversity of cadmium (Cd) contaminated site was exploited through functional metatranscriptomics by construction of cDNA libraries A (0.1-0.5 kb), B (0.5-1.0 kb), and C (1-4 kb) of variable size, from the eukaryotic mRNA. The cDNA library B was further screened for cadmium tolerant transcripts through yeast complementation system. We are reporting one of the transformants ycf1^ΔPLBe1 capable of tolerating high concentrations of Cd (40 μM - 80 μM). Sequence analysis revealed that PLBe1 cDNA showed homology with ubiquitin domain containing protein fused with AN1 type zinc finger protein of Acanthameoba castellani. Further, this cDNA was tested for its tolerance towards other heavy metals such as copper (Cu), zinc (Zn) and cobalt (Co). Functional complementation assay of cDNA PLBe1 showed a range of tolerance towards copper (150 μM - 300 μM), zinc (10 mM - 12 mM) and cobalt (2 mM - 4 mM). This study promulgates PLBe1 as credible member of multi-metal tolerant gene in the eukaryotic soil microbial community and can be used as potential member to revitalise the heavy metal contaminated sites or can be used as a biomarker to detect heavy metal contamination in the soil environment.

Vacuolar zinc transporter Zrc1 is required for detoxification of excess intracellular zinc in the human fungal pathogen Cryptococcus neoformans

Zinc is an important transition metal in all living organisms and is required for numerous biological processes. However, excess zinc can also be toxic to cells and cause cellular stress. In the model fungus Saccharomyces cerevisiae, a vacuolar zinc transporter, Zrc1, plays important roles in the storage and detoxification of excess intracellular zinc to protect the cell. In this study, we identified an ortholog of the S. cerevisiae ZRC1 gene in the human fungal pathogen Cryptococcus neoformans. Zrc1 was localized in the vacuolar membrane in C. neoformans, and a mutant lacking ZRC1 showed significant growth defects under high-zinc conditions. These results suggested a role for Zrc1 in zinc detoxification. However, contrary to our expectation, the expression of Zrc1 was induced in cells grown in zinc-limited conditions and decreased upon the addition of zinc. These expression patterns were similar to those of Zip1, the high-affinity zinc transporter in the plasma membrane of C. neoformans. Furthermore, we used the zrc1 mutant in a murine model of cryptococcosis to examine whether a mammalian host could inhibit the survival of C. neoformans using zinc toxicity. We found that the mutant showed no difference in virulence compared with the wildtype strain. This result suggests that Zrc1-mediated zinc detoxification is not required for the virulence of C. neoformans, and imply that zinc toxicity may not be an important aspect of the host immune response to the fungus.

A mutagenic study identifying critical residues for the structure and function of rice manganese transporter OsMTP8.1

Rice (Oryza sativa) MTP8.1 (OsMTP8.1) is a tonoplast-localized manganese transporter of the cation diffusion facilitator family. Here we present a structure-function analysis of OsMTP8.1 based on the site-directed and random mutagenesis and complementation assays in manganese hypersensitive yeast, in combination with three-dimensional (3D) structure modeling based on the crystal structure of the Escherichia coli CDF family member, EcYiiP. Two metal-binding sites are conserved in OsMTP8.1 with EcYiiP, one is between transmembrane helices TM2 and TM5, the other is the cytoplasmic C-terminus. In addition to these two metal-binding sites, there may exist other Mn-binding sites such as that at the very end of the CTD. Two residues (R167 and L296) may play an important role for the hinge-like movement of CTDs. Several mutations such as E357A and V374D may affect dimer formation, and S132A may induce a conformational change, resulting in a loss of transport function or modification in metal selectivity. The N-terminus of OsMTP8.1 was not functional for Mn transport activity, and the real function of NTD remains to be investigated in the future. The findings of the present study illustrate the structure-function relationship of OsMTP8.1 in Mn transport activity, which may also be applied to other plant Mn-CDF proteins.

The ZIP family zinc transporters support the virulence of Cryptococcus neoformans

Zinc is an essential element in living organisms and a cofactor for various metalloproteins. To disseminate and survive, a pathogenic microbe must obtain zinc from the host, which is an environment with extremely limited zinc availability. In this study, we investigated the roles of the ZIP family zinc transporters Zip1 and Zip2 in the human pathogenic fungus Cryptococcus neoformans Zip1 and Zip2 are homologous to Zrt1 and Zrt2 of the model fungus, Saccharomyces cerevisiae, respectively. We found that the expression of ZIP1 was regulated by the zinc concentration in the environment. Furthermore, the mutant lacking ZIP1 displayed a severe growth defect under zinc-limited conditions, while the mutant lacking ZIP2 displayed normal growth. Inductively coupled plasma-atomic emission spectroscopy analysis showed that the absence of Zip1 expression significantly reduced total cellular zinc levels relative to that in the wild type, while overexpression of Zip1 was associated with increased cellular zinc levels. These findings suggested that Zip1 plays roles in zinc uptake in C. neoformans We also constructed a Zip1-FLAG fusion protein and found, by immunofluorescence, not only that the protein was localized to the periphery implying it is a membrane transporter, but also that the protein was N-glycosylated. Furthermore, the mutant lacking ZIP1 showed attenuated virulence in a murine inhalation model of cryptococcosis and reduced survival within murine macrophages. Overall, our data suggest that Zip1 plays essential roles in zinc transport and the virulence of C. neoformans.

The Zinc Transport Systems and Their Regulation in Pathogenic Fungi

Zinc is an essential micronutrient required for many enzymes that play essential roles in a cell. It was estimated that approximately 3% of the total cellular proteins are required for zinc for their functions. Zinc has long been considered as one of the key players in host-pathogen interactions. The host sequesters intracellular zinc by utilizing multiple cellular zinc importers and exporters as a means of nutritional immunity. To overcome extreme zinc limitation within the host environment, pathogenic microbes have successfully evolved a number of mechanisms to secure sufficient concentrations of zinc for their survival and pathogenesis. In this review, we briefly discuss the zinc uptake systems and their regulation in the model fungus Saccharomyces cerevisiae and in major human pathogenic fungi such as Aspergillus fumigatus, Candida albicans, and Cryptococcus gattii.

Involvement of the adaptor protein 3 complex in lignocellulase secretion in Neurospora crassa revealed by comparative genomic screening

BACKGROUND

Lignocellulase hypersecretion has been achieved in industrial fungal workhorses such as Trichoderma reesei, but the underlying mechanism associated with this process is not well understood. Although previous comparative genomic studies have revealed that the mutagenic T. reesei strain RUT-C30 harbors hundreds of mutations compared with its parental strain QM6a, how these mutations actually contribute to the hypersecretion phenotype remains to be elucidated.

RESULTS

In this study, we systematically screened gene knockout (KO) mutants in the cellulolytic fungus Neurospora crassa, which contains orthologs of potentially defective T. reesei RUT-C30 mutated genes. Of the 86 deletion mutants screened in N. crassa, 12 exhibited lignocellulase production more than 25% higher than in the wild-type (WT) strain and 4 showed nearly 25% lower secretion. We observed that the deletion of Ncap3m (NCU03998), which encodes the μ subunit of the adaptor protein 3 (AP-3) complex in N. crassa, led to the most significant increase in lignocellulase secretion under both Avicel and xylan culture conditions. Moreover, strains lacking the β subunit of the AP-3 complex, encoded by Ncap3b (NCU06569), had a similar phenotype to ΔNcap3m, suggesting that the AP-3 complex is involved in lignocellulase secretion in N. crassa. We also found that the transcriptional abundance of major lignocellulase genes in ΔNcap3m was maintained at a relatively higher level during the late stage of fermentation compared with the WT, which might add to the hypersecretion phenotype. Finally, we found that importation of the T. reesei ap3m ortholog Trap3m into ΔNcap3m can genetically restore secretion of lignocellulases to normal levels, which suggests that the effect of the AP-3 complex on lignocellulase secretion is conserved in cellulolytic ascomycetes.

CONCLUSIONS

Using the model cellulolytic fungus N. crassa, we explored potential hypersecretion-related mutations in T. reesei strain RUT-C30. Through systematic genetic screening of 86 corresponding orthologous KO mutants in N. crassa, we identified several genes, particularly those encoding the AP-3 complex that contribute to lignocellulase secretion. These findings will be useful for strain improvement in future lignocellulase and biomass-based chemical production.

Cation Diffusion Facilitator family: Structure and function

The Cation Diffusion Facilitators (CDFs) form a family of membrane-bound proteins capable of transporting zinc and other heavy metal ions. Involved in metal tolerance/resistance by efflux of ions, CDF proteins share a two-modular architecture consisting of a transmembrane domain (TMD) and C-terminal domain (CTD) that protrudes into the cytoplasm. Discovery of a Zn²⁺ and Cd²⁺ CDF transporter from a marine bacterium Maricaulis maris that does not possess the CTD questions current perceptions regarding this family of proteins. This article describes a new, CTD-lacking subfamily of CDFs and our current knowledge about this family of proteins in the view of these findings.

Subcellular metal imaging identifies dynamic sites of Cu accumulation in Chlamydomonas

We identified a Cu-accumulating structure with a dynamic role in intracellular Cu homeostasis. During Zn limitation, Chlamydomonas reinhardtii hyperaccumulates Cu, a process dependent on the nutritional Cu sensor CRR1, but it is functionally Cu deficient. Visualization of intracellular Cu revealed major Cu accumulation sites coincident with electron-dense structures that stained positive for low pH and polyphosphate, suggesting that they are lysosome-related organelles. Nano-secondary ion MS showed colocalization of Ca and Cu, and X-ray absorption spectroscopy was consistent with Cu(+) accumulation in an ordered structure. Zn resupply restored Cu homeostasis concomitant with reduced abundance of these structures. Cu isotope labeling demonstrated that sequestered Cu(+) became bioavailable for the synthesis of plastocyanin, and transcriptome profiling indicated that mobilized Cu became visible to CRR1. Cu trafficking to intracellular accumulation sites may be a strategy for preventing protein mismetallation during Zn deficiency and enabling efficient cuproprotein metallation or remetallation upon Zn resupply.

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.

Phosphatidate phosphatase, a key regulator of lipid homeostasis

Yeast Pah1p phosphatidate phosphatase (PAP) catalyzes the penultimate step in the synthesis of triacylglycerol. PAP plays a crucial role in lipid homeostasis by controlling the relative proportions of its substrate phosphatidate and its product diacylglycerol. The cellular amounts of these lipid intermediates influence the synthesis of triacylglycerol and the pathways by which membrane phospholipids are synthesized. Physiological functions affected by PAP activity include phospholipid synthesis gene expression, nuclear/endoplasmic reticulum membrane growth, lipid droplet formation, and vacuole homeostasis and fusion. Yeast lacking Pah1p PAP activity are acutely sensitive to fatty acid-induced toxicity and exhibit respiratory deficiency. PAP is distinguished in its cellular location, catalytic mechanism, and physiological functions from Dpp1p and Lpp1p lipid phosphate phosphatases that utilize a variety of substrates that include phosphatidate. Phosphorylation/dephosphorylation is a major mechanism by which Pah1p PAP activity is regulated. Pah1p is phosphorylated by cytosolic-associated Pho85p-Pho80p, Cdc28p-cyclin B, and protein kinase A and is dephosphorylated by the endoplasmic reticulum-associated Nem1p-Spo7p phosphatase. The dephosphorylation of Pah1p stimulates PAP activity and facilitates the association with the membrane/phosphatidate allowing for its reaction and triacylglycerol synthesis. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

Possible involvement of GDI1 protein, a GDP dissociation inhibitor related to vesicle transport, in an amelioration of zinc toxicity in Saccharomyces cerevisiae

The GDI1 protein related vesicle transport system was studied to investigate the possibility that an exclusion of toxic zinc (Zn) from the cytoplasm ameliorates Zn toxicity in Saccharomyces cerevisiae (yeast). A temperature-sensitive gdi1 mutant (originally called sec19), in which the GDP dissociation inhibitor becomes inactive at the non-permissive temperature (37 °C), was more sensitive to Zn than its parental GDI1 strain at 32 °C (a moderately non-permissive temperature). The relative efflux of cytoplasmic Zn in the gdi1 mutant was lower than that in the control strain. Treatment with a vesicle transport-specific inhibitor, Brefeldin A, caused an increase of Zn sensitivity and a decrease of Zn efflux in these strains. It is therefore suggested that the GDI1-related vesicle transport system contributes to Zn tolerance in yeast. Furthermore, changes in the number of Zn-specific fluorescent granules (zincosomes) were observed by zinquin staining in the mutant cells under Zn treatment at 32 °C and 37 °C. We concluded that the GDI1 protein is implicated in control of vesicle numbers. Collectively, the results suggest that the GDI1protein is involved in Zn efflux via small vesicle trafficking and contributes to the control of cytoplasmic Zn content, allowing yeast to survive in the presence of toxic Zn.

Phosphatidate phosphatase plays role in zinc-mediated regulation of phospholipid synthesis in yeast

In the yeast Saccharomyces cerevisiae, the synthesis of phospholipids is coordinately regulated by mechanisms that control the homeostasis of the essential mineral zinc (Carman, G.M., and Han, G. S. (2007) Regulation of phospholipid synthesis in Saccharomyces cerevisiae by zinc depletion. Biochim. Biophys. Acta 1771, 322-330; Eide, D. J. (2009) Homeostatic and adaptive responses to zinc deficiency in Saccharomyces cerevisiae. J. Biol. Chem. 284, 18565-18569). The synthesis of phosphatidylcholine is balanced by the repression of CDP-diacylglycerol pathway enzymes and the induction of Kennedy pathway enzymes. PAH1-encoded phosphatidate phosphatase catalyzes the penultimate step in triacylglycerol synthesis, and the diacylglycerol generated in the reaction may also be used for phosphatidylcholine synthesis via the Kennedy pathway. In this work, we showed that the expression of PAH1-encoded phosphatidate phosphatase was induced by zinc deficiency through a mechanism that involved interaction of the Zap1p zinc-responsive transcription factor with putative upstream activating sequence zinc-responsive elements in the PAH1 promoter. The pah1Δ mutation resulted in the derepression of the CHO1-encoded phosphatidylserine synthase (CDP-diacylglycerol pathway enzyme) and loss of the zinc-mediated regulation of the enzyme. Loss of phosphatidate phosphatase also resulted in the derepression of the CKI1-encoded choline kinase (Kennedy pathway enzyme) but decreased the synthesis of phosphatidylcholine when cells were deficient of zinc. This result confirmed the role phosphatidate phosphatase plays in phosphatidylcholine synthesis via the Kennedy pathway.

The cation-efflux transporter BjCET2 mediates zinc and cadmium accumulation in Brassica juncea L. leaves

Brassica juncea L. is a Zn/Cd accumulator. To determine the physiological basis of its metal accumulation phenotype, the functional properties and role of the metal efflux transporter BjCET2 were investigated using transgenic technology. Heterologous expression of BjCET2 in the double mutant yeast strain Deltazrc1Deltacot1 enhanced the metal tolerance of the yeast strain and led to decrease in Zn or Cd accumulation. Detection of green fluorescence from green fluorescent protein (GFP) in the root tip of transgenic tobacco further revealed that BjCET2::GFP is localized at the plasma membrane. Semi-quantitative RT-PCR analysis showed that BjCET2 was most abundant in the root and was weakly expressed in the stem and leaves. The expression of BjCET2 was up-regulated by heavy metals. However, exposure to low temperature, salt and drought did not affect the expression of BjCET2. Overexpression of BjCET2 in transgenic B. juncea plants conferred heavy metal tolerance and increased Cd/Zn accumulation in the leaves. BjCET2-deficient B. juncea mediated by antisense RNA resulted in hypersensitivity to heavy metals and decreased Zn/Cd accumulation in the plants. These results suggest that the heavy metal efflux of BjCET2 plays important roles in the metal tolerance of B. juncea and in Zn/Cd accumulation in B. juncea.

Zinc status and vacuolar zinc transporters control alkaline phosphatase accumulation and activity in Saccharomyces cerevisiae

Little is known about how metalloproteins in the secretory pathway obtain their metal ion cofactors. We used the Pho8 alkaline phosphatase of the yeast Saccharomyces cerevisiae to probe this process in vivo. We found that both Pho8 activity and protein accumulation are zinc-dependent and decrease in zinc-limited cells. Low Pho8 accumulation was the result of degradation by vacuolar proteases. Surprisingly, the protective effect of zinc on Pho8 stability was not solely due to Zn(2+) binding to the active-site ligands suggesting that the Pho8 protein is targeted for degradation in zinc-limited cells by another mechanism. Pho8 appears to be a rare example of a metalloprotein whose stability is regulated by its metal cofactor independently of active-site binding. We also assessed which zinc transporters are responsible for supplying zinc to Pho8. We found that the Zrc1 and Cot1 vacuolar zinc transporters play the major role while the Msc2/Zrg17 zinc transporter complex active in the endoplasmic reticulum is not involved. These results demonstrate that the vacuolar zinc transporters, previously implicated in metal detoxification, also deliver zinc to certain metalloproteins within intracellular compartments. These data suggest that Pho8 receives its metal cofactor in the vacuole rather than in earlier compartments of the secretory pathway.

Regulation of the Saccharomyces cerevisiae CKI1-encoded choline kinase by zinc depletion

In the yeast Saccharomyces cerevisiae, the CKI1-encoded choline kinase catalyzes the committed step in the synthesis of phosphatidylcholine via the CDP-choline branch of the Kennedy pathway. Analysis of a P(CKI1)-lacZ reporter gene revealed that CKI1 expression was regulated by intracellular levels of the essential mineral zinc. Zinc depletion resulted in a concentration-dependent induction of CKI1 expression. This regulation was mediated by the zinc-sensing and zinc-inducible transcriptional activator Zap1p. A purified Zap1p probe interacted with two putative UAS(ZRE) sequences (ZRE1 and ZRE2) in the CKI1 promoter. Mutations of ZRE1 and ZRE2 to a nonconsensus UAS(ZRE) attenuated the induction of CKI1 expression in response to zinc depletion. A UAS(INO) element in the CKI1 promoter was responsible for stimulating CKI1 expression, but this element was not involved with the regulation by zinc depletion. The induction of CKI1 expression in zinc-depleted cells translated into increased choline kinase activity in vitro and in vivo, and an increase in phosphatidylcholine synthesis via the Kennedy pathway.

Physiological and transcriptional responses of Saccharomyces cerevisiae to zinc limitation in chemostat cultures

Transcriptional responses of the yeast Saccharomyces cerevisiae to Zn availability were investigated at a fixed specific growth rate under limiting and abundant Zn concentrations in chemostat culture. To investigate the context dependency of this transcriptional response and eliminate growth rate-dependent variations in transcription, yeast was grown under several chemostat regimens, resulting in various carbon (glucose), nitrogen (ammonium), zinc, and oxygen supplies. A robust set of genes that responded consistently to Zn limitation was identified, and the set enabled the definition of the Zn-specific Zap1p regulon, comprised of 26 genes and characterized by a broader zinc-responsive element consensus (MHHAACCBYNMRGGT) than so far described. Most surprising was the Zn-dependent regulation of genes involved in storage carbohydrate metabolism. Their concerted down-regulation was physiologically relevant as revealed by a substantial decrease in glycogen and trehalose cellular content under Zn limitation. An unexpectedly large number of genes were synergistically or antagonistically regulated by oxygen and Zn availability. This combinatorial regulation suggested a more prominent involvement of Zn in mitochondrial biogenesis and function than hitherto identified.

Saccharomyces cerevisiae vacuole in zinc storage and intracellular zinc distribution

Previous studies of the yeast Saccharomyces cerevisiae indicated that the vacuole is a major site of zinc storage in the cell. However, these studies did not address the absolute level of zinc that was stored in the vacuole nor did they examine the abundances of stored zinc in other compartments of the cell. In this report, we describe an analysis of the cellular distribution of zinc by use of both an organellar fractionation method and an electron probe X-ray microanalysis. With these methods, we determined that zinc levels in the vacuole vary with zinc status and can rise to almost 100 mM zinc (i.e., 7 x 10(8) atoms of vacuolar zinc per cell). Moreover, this zinc can be mobilized effectively to supply the needs of as many as eight generations of progeny cells under zinc starvation conditions. While the Zrc1 and Cot1 zinc transporters are essential for zinc uptake into the vacuole under steady-state growth conditions, additional transporters help mediate zinc uptake into the vacuole during "zinc shock," when zinc-limited cells are resupplied with zinc. In addition, we found that other compartments of the cell do not provide significant stores of zinc. In particular, zinc accumulation in mitochondria is low and is homeostatically regulated independently of vacuolar zinc storage. Finally, we observed a strong correlation between zinc status and the levels of magnesium and phosphorus accumulated in cells. Our results implicate zinc as a major determinant of the ability of the cell to store these other important nutrients.

Genome-wide screening of yeast metal homeostasis genes involved in mitochondrial functions

Metal ions are essential for mitochondria to execute their roles. Yeast mutants that are sensitive to metals (either excess or deficiency) on non-fermentable media but not on fermentable media may carry mutations in genes that participate in metal homeostasis involving mitochondrial functions. A collection of approximately 4,800 haploid yeast deletion mutants was screened for metal ion homeostasis genes linked to mitochondrial respiration. In addition to several well-characterized metal homeostasis genes, 45 new mutants, impaired in various molecular functions, were identified on non-fermentable media that were sensitive to adscititious metals or metal deficiency. While 35 of these mutants displayed metal-sensitivity only on non-fermentable media, the remaining 10 also exhibited metal sensitivity on fermentable media, suggesting metal-sensitivity of the latter is not due to mitochondrial dysfunction. Inductively coupled plasma optical emission spectrometry (ICP-OES) was conducted for 12 mutants that were sensitive to metal excess to analyze their metal contents. Among these 12 mutants 7 were sensitive to metal excess on non-fermentable but not on fermentable media. All the seven respiration-dependent mutants displayed abnormal levels of metal ions inside mitochondria, indicative of disrupted mitochondrial metal homeostasis. This study therefore effectively identified multiple new genes involved in metal homeostasis pathways possibly pertinent to mitochondrial functions, and should be helpful for future studies to further understand their molecular roles.

Regulation of phospholipid synthesis in Saccharomyces cerevisiae by zinc depletion

The synthesis of phospholipids in the yeast Saccharomyces cerevisiae is regulated by zinc, an essential mineral required for growth and metabolism. Cells depleted of zinc contain increased levels of phosphatidylinositol and decreased levels of phosphatidylethanolamine. In addition to the major phospholipids, the levels of the minor phospholipids phosphatidate and diacylglycerol pyrophosphate decrease in the vacuole membrane of zinc-depleted cells. Alterations in phosphatidylinositol and phosphatidylethanolamine can be ascribed to an increase in PIS1-encoded phosphatidylinositol synthase activity and to decreases in the activities of CDP-diacylglycerol pathway enzymes including the CHO1-encoded phosphatidylserine synthase, respectively. Alterations in the minor vacuole membrane phospholipids are due to the induction of the DPP1-encoded diacylglycerol pyrophosphate phosphatase. These changes in the activities of phospholipid biosynthetic enzymes result from differential regulation of gene expression at the level of transcription. Under zinc-deplete conditions, the positive transcription factor Zap1p stimulates the expression of the DPP1 and PIS1 genes through the cis-acting element UAS(ZRE). In contrast, the negative regulatory protein Opi1p, which is involved in inositol-mediated regulation of phospholipid synthesis, represses the expression of the CHO1 gene through the cis-acting element UAS(INO). Regulation of phospholipid synthesis may provide an important mechanism by which cells cope with the stress of zinc depletion, given the roles that phospholipids play in the structure and function of cellular membranes.

Zinc transporters and the cellular trafficking of zinc

Zinc is an essential nutrient for all organisms because this metal serves as a catalytic or structural cofactor for many different proteins. Zinc-dependent proteins are found in the cytoplasm and within many organelles of the eukaryotic cell including the nucleus, the endoplasmic reticulum, Golgi, secretory vesicles, and mitochondria. Thus, cells require zinc transport mechanisms to allow cells to efficiently accumulate the metal ion and distribute it within the cell. Our current knowledge of these transport systems in eukaryotes is the focus of this review.

Solution Structure of a Zap1 Zinc-responsive Domain Provides Insights into Metalloregulatory Transcriptional Repression in Saccharomyces cerevisiae

The Zap1 transcription factor controls expression of genes that regulate zinc homeostasis in Saccharomyces cerevisiae. The solution structure of two zinc fingers (zf1-2(CA3)) derived from a zinc-responsive domain of Zap1 (zf1-2) has been determined. Under zinc-limiting conditions, zinc finger 2 (zf2) from this domain has been shown to be a constitutive transcriptional activator. Moreover, repression of zf2 function in zinc-replete cells required zinc coordination to both canonical finger 1 (zf1) and zf2 metal sites, suggesting zf1-zf2 cooperativity underlies Zap1 metalloregulation. A structural basis for this cooperativity is identified here. Favorable inter-helical contacts in zf1-2(CA3) extend the individual finger hydrophobic cores through the zf1-zf2 interface. Tryptophan residues at position 5 in each finger provide numerous non-helical inter-finger contacts reminiscent of those observed in GLI1 zinc fingers 1 and 2. The molecular mechanism for zf1-dependent repression of zf2 transcriptional activation is explored further using NMR and CD titration studies. While zf1 independently forms a betabetaalpha solution structure, the majority of zf2 ensemble solution states do not adopt the canonical betabetaalpha zinc finger fold without zf1-zf2 interactions. Cooperative effects on Zn(II) affinities stemming from these finger-finger interactions are observed also in calorimetric studies, in which the 160(+/-20)nM (zf1) and 250(+/-40)nM (zf2) K(d) values for each individual finger increased substantially in the context of the zf1-2 protein (apparent K(dzf1-2WT)=4.6(+/-1.2)nM). On the basis of the above observations, we propose a mechanism for Zap1 transcriptional regulation in which zf1-zf2 interactions stabilize the betabetaalpha folded "repressed state" of the zf2 activation domain in the presence of cellular Zn(II) excess. Moreover, in contrast to earlier reports of <<1 labile zinc ion/Escherichia coli cell, the zf1-zf2 zinc affinities determined calorimetrically are consistent with Zn(II) levels >>1 labile zinc ion/eukaryotic cell.

Regulation of the Saccharomyces cerevisiae EKI1-encoded ethanolamine kinase by zinc depletion

Ethanolamine kinase catalyzes the committed step in the synthesis of phosphatidylethanolamine via the CDP-ethanolamine branch of the Kennedy pathway. Regulation of the EKI1-encoded ethanolamine kinase by the essential nutrient zinc was examined in Saccharomyces cerevisiae. The level of ethanolamine kinase activity increased when zinc was depleted from the growth medium. This regulation correlated with increases in the CDP-ethanolamine pathway intermediates phosphoethanolamine and CDP-ethanolamine, and an increase in the methylated derivative of phosphatidylethanolamine, phosphatidylcholine. The beta-galactosidase activity driven by the P(EKI1)-lacZ reporter gene was elevated in zinc-depleted cells, indicating that the increase in ethanolamine kinase activity was attributed to a transcriptional mechanism. The expression level of P(EKI1)-lacZ reporter gene activity in the zrt1deltazrt2delta mutant (defective in plasma membrane zinc transport) cells grown with zinc was similar to the activity expressed in wild-type cells grown without zinc. This indicated that EKI1 expression was sensitive to intracellular zinc. The zinc-mediated regulation of EKI1 expression was attenuated in the zap1delta mutant defective in the zinc-regulated transcription factor Zap1p. Direct interactions between Zap1p and putative zinc-responsive elements in the EKI1 promoter were demonstrated by electrophoretic mobility shift assays. Mutations of these elements to a nonconsensus sequence abolished Zap1p-DNA interactions. Taken together, this work demonstrated that the zinc-mediated regulation of ethanolamine kinase and the synthesis of phospholipids via the CDP-ethanolamine branch of the Kennedy pathway were controlled in part by Zap1p.

Characterization of the yeast ionome: a genome-wide analysis of nutrient mineral and trace element homeostasis in Saccharomyces cerevisiae

BACKGROUND

Nutrient minerals are essential yet potentially toxic, and homeostatic mechanisms are required to regulate their intracellular levels. We describe here a genome-wide screen for genes involved in the homeostasis of minerals in Saccharomyces cerevisiae. Using inductively coupled plasma-atomic emission spectroscopy (ICP-AES), we assayed 4,385 mutant strains for the accumulation of 13 elements (calcium, cobalt, copper, iron, potassium, magnesium, manganese, nickel, phosphorus, selenium, sodium, sulfur, and zinc). We refer to the resulting accumulation profile as the yeast 'ionome'.

RESULTS

We identified 212 strains that showed altered ionome profiles when grown on a rich growth medium. Surprisingly few of these mutants (four strains) were affected for only one element. Rather, levels of multiple elements were altered in most mutants. It was also remarkable that only six genes previously shown to be involved in the uptake and utilization of minerals were identified here, indicating that homeostasis is robust under these replete conditions. Many mutants identified affected either mitochondrial or vacuolar function and these groups showed similar effects on the accumulation of many different elements. In addition, intriguing positive and negative correlations among different elements were observed. Finally, ionome profile data allowed us to correctly predict a function for a previously uncharacterized gene, YDR065W. We show that this gene is required for vacuolar acidification.

CONCLUSION

Our results indicate the power of ionomics to identify new aspects of mineral homeostasis and how these data can be used to develop hypotheses regarding the functions of previously uncharacterized genes.

Characterization of a Glomus intraradices gene encoding a putative Zn transporter of the cation diffusion facilitator family

A full-length cDNA (GintZnT1) encoding a putative Zn transporter was isolated from the extraradical mycelium of Glomus intraradices. Based on its sequence analysis, GintZnT1 was classified as a member of the cation diffusion facilitator (CDF) family of heavy metal transporters. Functional analysis of GintZnT1 was performed by heterologous expression in yeast mutants defective in different CDFs. Although Zn sensitivity of the mutants was not reverted, an effect of GintZnT1 on the labile regulatory Zn pool was detected by using a Zn-regulated beta-galactosidase reporter gene. GintZnT1 expression was studied in the extraradical mycelium obtained from a symbiotic root organ culture. Gin +/- ZnT1 was up-regulated in the extraradical mycelium of G. intraradices upon short-time exposure to Zn and when the mycelia were developed in 75 microM Zn supplemented plates. These data suggest a role of GintZnT1 in Zn compartmentalization and in the protection of G. intraradices against Zn stress.

Exchangeable zinc ions transiently accumulate in a vesicular compartment in the yeast Saccharomyces cerevisiae

The baker's yeast Saccharomyces cerevisiae was used as a model to visualize intracellular labile zinc under conditions of nutritional zinc imbalance. Zinc-specific staining was performed in yeast cells using both Zinquin fluorescence and zinc-selenium autometallography. Both techniques resulted in specific labeling of an intracellular vesicular compartment that was present in wild type cells as well as in the vacuolar Zn transporter mutants Deltazrc1 and Deltacot1. This compartment, that closely resembles mammalian zincosomes, appeared rapidly under conditions of zinc availability and was independent of endocytosis. However, persistence of the zinc loaded vesicles in nutritional zinc deficiency was dependent on the presence of functional Zrc1 and Cot1 vacuolar transporters. Overall our findings indicate that labile zinc in yeast cells enters a dynamic vesicular compartment which could represent an extremely important defence to buffer both zinc excess and deficiency.

Zinc and the Msc2 zinc transporter protein are required for endoplasmic reticulum function

In this report, we show that zinc is required for endoplasmic reticulum function in Saccharomyces cerevisiae. Zinc deficiency in this yeast induces the unfolded protein response (UPR), a system normally activated by unfolded ER proteins. Msc2, a member of the cation diffusion facilitator (CDF) family of metal ion transporters, was previously implicated in zinc homeostasis. Our results indicate that Msc2 is one route of zinc entry into the ER. Msc2 localizes to the ER when expressed at normal levels. UPR induction in low zinc is exacerbated in an msc2 mutant. Genetic and biochemical evidence indicates that this UPR induction is due to genuine ER dysfunction. Notably, we found that ER-associated protein degradation is defective in zinc-limited msc2 mutants. We also show that the vacuolar CDF proteins Zrc1 and Cot1 are other pathways of ER zinc acquisition. Finally, zinc deficiency up-regulates the mammalian ER stress response indicating a conserved requirement for zinc in ER function among eukaryotes.

The yeast genes, ARL1 and CCZ1, interact to control membrane traffic and ion homeostasis

The yeast ARL1 gene, encoding a guanine-nucleotide binding protein of the Arf-like family, exhibits a synthetic genetic interaction with CCZ1. An arl1 Delta ccz1 Delta double mutant was viable but grew slowly, was more sensitive to caffeine, Ca(2+), Zn(2+), and hygromycin B than either single mutant, and had a more severe vacuolar protein sorting phenotype. Overexpression of ARL1 did not suppress ccz1 Delta mutant phenotypes, nor did overexpression of CCZ1 suppress arl1 Delta mutant phenotypes. We conclude that ARL1 and CCZ1 independently contribute to both ion homeostasis and protein sorting.

Zinc fingers can act as Zn2+ sensors to regulate transcriptional activation domain function

The yeast Zap1 transcription factor controls the expression of genes involved in zinc accumulation and storage. Zap1 is active in zinc-limited cells and repressed in replete cells. Zap1 has two activation domains, AD1 and AD2, which are both regulated by zinc. AD2 function was mapped to a region containing two Cys2His2 zinc fingers, ZF1 and ZF2, that are not involved in DNA binding. More detailed mapping placed AD2 almost precisely within the endpoints of ZF2, suggesting a role for these fingers in regulating activation domain function. Consistent with this hypothesis, ZF1 and ZF2 bound zinc in vitro but less stably than did zinc fingers involved in DNA binding. Furthermore, mutations predicted to disrupt zinc binding to ZF1 and/or ZF2 rendered AD2 constitutively active. Our results also indicate that the repressed form of AD2 requires an intramolecular interaction between ZF1 and ZF2. These studies suggest that these zinc fingers play an unprecedented role as zinc sensors to control activation domain function.

Efflux-mediated heavy metal resistance in prokaryotes

What makes a heavy metal resistant bacterium heavy metal resistant? The mechanisms of action, physiological functions, and distribution of metal-exporting proteins are outlined, namely: CBA efflux pumps driven by proteins of the resistance-nodulation-cell division superfamily, P-type ATPases, cation diffusion facilitator and chromate proteins, NreB- and CnrT-like resistance factors. The complement of efflux systems of 63 sequenced prokaryotes was compared with that of the heavy metal resistant bacterium Ralstonia metallidurans. This comparison shows that heavy metal resistance is the result of multiple layers of resistance systems with overlapping substrate specificities, but unique functions. Some of these systems are widespread and serve in the basic defense of the cell against superfluous heavy metals, but some are highly specialized and occur only in a few bacteria. Possession of the latter systems makes a bacterium heavy metal resistant.

Induction of the ZRC1 metal tolerance gene in zinc-limited yeast confers resistance to zinc shock

Zinc is an essential nutrient but toxic to cells with overaccumulation. For this reason, intracellular zinc levels are tightly controlled. In the yeast Saccharomyces cerevisiae, the Zrc1 and Cot1 proteins have been implicated in the storage and detoxification of excess zinc in the vacuole. Surprisingly, transcription of ZRC1 is induced in zinc-limited cells by the zinc-responsive transcription factor Zap1. We show here that this increase in ZRC1 expression is a novel mechanism of zinc homeostasis and stress tolerance. Zinc-limited cells also express high levels of the plasma membrane zinc uptake transporters. As a consequence, when zinc-limited cells are resupplied with small amounts of zinc, large quantities quickly accumulate in the cell, a condition we refer to as "zinc shock." We show here that ZRC1 and its induction in zinc-limited cells are required for resistance to this zinc shock. Experiments using the zinc-responsive fluorophore FuraZin-1 as an indicator of vacuolar zinc levels indicated that Zrc1 is required for the rapid transport of zinc into the vacuole during zinc shock. We also present evidence that cytosolic zinc rises to higher levels in cells unable to sequester this excess zinc. Thus, the increase in ZRC1 expression occurs prior to the zinc shock stress for which this induction is important. We propose that this "proactive" strategy of homeostatic regulation, such as we document here for ZRC1, may represent a common but largely unrecognized phenomenon.

Biochemical properties of vacuolar zinc transport systems of Saccharomyces cerevisiae

The yeast vacuole plays an important role in zinc homeostasis by storing zinc for later use under deficient conditions, sequestering excess zinc for its detoxification, and buffering rapid changes in intracellular zinc levels. The mechanisms involved in vacuolar zinc sequestration are only poorly characterized. Here we describe the properties of zinc transport systems in yeast vacuolar membrane vesicles. The major zinc transport activities in these vesicles were ATP-dependent, requiring a H+ gradient generated by the V-ATPase for function. One system we identified was dependent on the ZRC1 gene, which encodes a member of the cation diffusion facilitator family of metal transporters. These data are consistent with the proposed role of Zrc1 as a vacuolar zinc transporter. Zrc1-independent activity was also observed that was not dependent on the closely related vacuolar Cot1 protein. Both Zrc1-dependent and independent activities showed a high specificity for Zn(2+) over other physiologically relevant substrates such as Ca2+, Fe2+, and Mn2+. Moreover, these systems had high affinities for zinc with apparent K(m) values in the 100-200 nm range. These results provide biochemical insight into the important role of Zrc1 and related proteins in eukaryotic zinc homeostasis.