YKE4 (YIL023C) Encodes a Bidirectional Zinc Transporter in the Endoplasmic Reticulum of Saccharomyces cerevisiae

AtIAR1 is a Zn transporter that regulates auxin metabolism in Arabidopsis thaliana

Root growth in Arabidopsis is inhibited by exogenous auxin-amino acid conjugates, and mutants resistant to one such conjugate [indole-3-acetic acid (IAA)-Ala] map to a gene (AtIAR1) that is a member of a metal transporter family. Here, we test the hypothesis that AtIAR1 controls the hydrolysis of stored conjugated auxin to free auxin through zinc transport. AtIAR1 complements a yeast mutant sensitive to zinc, but not manganese- or iron-sensitive mutants, and the transporter is predicted to be localized to the endoplasmic reticulum/Golgi in plants. A previously identified Atiar1 mutant and a non-expressed T-DNA mutant both exhibit altered auxin metabolism, including decreased IAA-glucose conjugate levels in zinc-deficient conditions and insensitivity to the growth effect of exogenous IAA-Ala conjugates. At a high concentration of zinc, wild-type plants show a novel enhanced response to root growth inhibition by exogenous IAA-Ala which is disrupted in both Atiar1 mutants. Furthermore, both Atiar1 mutants show changes in auxin-related phenotypes, including lateral root density and hypocotyl length. The findings therefore suggest a role for AtIAR1 in controlling zinc release from the secretory system, where zinc homeostasis plays a key role in regulation of auxin metabolism and plant growth regulation.

Considerations in production of the prokaryotic ZIP family transporters for structural and functional studies

Zinc ions play essential roles as components of enzymes and many other important biomolecules, and are associated with numerous diseases. The uptake of Zn2+ and other metal ions require a widely distributed transporter protein family called Zrt/Irt-like Proteins (ZIP family), the majority members of which tend to have eight transmembrane helices with both N- and C- termini located on the extracellular or periplasmic side. Their small sizes and dynamic conformations bring many difficulties in their production for structural studies either by crystallography or Cryo-EM. Here, we summarize the problems that may encounter at the various steps of processing the ZIP proteins from gene to structural and functional studies, and provide some solutions and examples from our and other labs for the cloning, expression, purification, stability screening, metal ion transport assays and structural studies of prokaryotic ZIP family transporters using Escherichia coli as a heterologous host.

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.

Impact of Zinc Transport Mechanisms on Embryonic and Brain Development

The trace element zinc (Zn) binds to over ten percent of proteins in eukaryotic cells. Zn flexible chemistry allows it to regulate the activity of hundreds of enzymes and influence scores of metabolic processes in cells throughout the body. Deficiency of Zn in humans has a profound effect on development and in adults later in life, particularly in the brain, where Zn deficiency is linked to several neurological disorders. In this review, we will summarize the importance of Zn during development through a description of the outcomes of both genetic and early dietary Zn deficiency, focusing on the pathological consequences on the whole body and brain. The epidemiology and the symptomology of Zn deficiency in humans will be described, including the most studied inherited Zn deficiency disease, Acrodermatitis enteropathica. In addition, we will give an overview of the different forms and animal models of Zn deficiency, as well as the 24 Zn transporters, distributed into two families: the ZIPs and the ZnTs, which control the balance of Zn throughout the body. Lastly, we will describe the TRPM7 ion channel, which was recently shown to contribute to intestinal Zn absorption and has its own significant impact on early embryonic development.

Nutritional immunity: targeting fungal zinc homeostasis

Transition metals, such as Zn²⁺, are essential dietary constituents of all biological life, including mammalian hosts and the pathogens that infect them. Therefore, to thrive and cause infection, pathogens must successfully assimilate these elements from the host milieu. Consequently, mammalian immunity has evolved to actively restrict and/or pool metals to toxic concentrations in an effort to attenuate microbial pathogenicity - a process termed nutritional immunity. Despite host-induced Zn²⁺ nutritional immunity, pathogens such as Candida albicans, are still capable of causing disease and thus must be equipped with robust Zn²⁺ sensory, uptake and detoxification machinery. This review will discuss the strategies employed by mammalian hosts to limit Zn²⁺ during infection, and the subsequent fungal interventions that counteract Zn²⁺ nutritional immunity.

Toward unzipping the ZIP metal transporters: structure, evolution, and implications on drug discovery against cancer

The Zrt-/Irt-like protein (ZIP) family consists of divalent metal transporters, ubiquitous in all kingdoms of life. Since the discovery of the first ZIPs in the 1990s, the ZIP family has been expanding to contain tens of thousands of members playing key roles in uptake and homeostasis of life-essential trace elements, primarily zinc, iron and manganese. Some family members are also responsible for toxic metal (particularly cadmium) absorption and distribution. Their central roles in trace element biology, and implications in many human diseases, including cancers, have elicited interest across multiple disciplines for potential applications in biomedicine, agriculture and environmental protection. In this review and perspective, selected areas under rapid progress in the last several years, including structural biology, evolution, and drug discovery against cancers, are summarised and commented. Future research to address the most prominent issues associated with transport and regulation mechanisms are also discussed.

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.

MCHM Acts as a Hydrotrope, Altering the Balance of Metals in Yeast

While drugs and other industrial chemicals are routinely studied to assess risks, many widely used chemicals have not been thoroughly evaluated. One such chemical, 4-methylcyclohexane methanol (MCHM), is an industrial coal-cleaning chemical that contaminated the drinking water supply in Charleston, WV, USA in 2014. While a wide range of ailments was reported following the spill, little is known about the molecular effects of MCHM exposure. We used the yeast model to explore the impacts of MCHM on cellular function. Exposure to MCHM dramatically altered the yeast transcriptome and the balance of metals in yeast. Underlying genetic variation in the response to MCHM, transcriptomics and, mutant analysis uncovered the role of the metal transporters, Arn2 and Yke4, to MCHM response. Expression of Arn2, which is involved in iron uptake, was lower in MCHM-tolerant yeast and loss of Arn2 further increased MCHM tolerance. Genetic variation within Yke4, an ER zinc transporter, also mediated response to MCHM, and loss of Yke4 decreased MCHM tolerance. The addition of zinc to MCHM-sensitive yeast rescued growth inhibition. In vitro assays demonstrated that MCHM acted as a hydrotrope and prevented protein interactions, while zinc induced the aggregation of proteins. We hypothesized that MCHM altered the structures of extracellular domains of proteins, and the addition of zinc stabilized the structure to maintain metal homeostasis in yeast exposed to MCHM.

Transcription factors and transporters in zinc homeostasis: lessons learned from fungi

Zinc is an essential nutrient for all organisms because this metal serves as a critical structural or catalytic cofactor for many proteins. These zinc-dependent proteins are abundant in the cytosol as well as within organelles of eukaryotic cells such as the nucleus, mitochondria, endoplasmic reticulum, Golgi, and storage compartments such as the fungal vacuole. Therefore, cells need zinc transporters so that they can efficiently take up the metal and move it around within cells. In addition, because zinc levels in the environment can vary drastically, the activity of many of these transporters and other components of zinc homeostasis is regulated at the level of transcription by zinc-responsive transcription factors. Mechanisms of post-transcriptional control are also important for zinc homeostasis. In this review, the focus will be on our current knowledge of zinc transporters and their regulation by zinc-responsive transcription factors and other mechanisms in fungi because these organisms have served as useful paradigms of zinc homeostasis in all organisms. With this foundation, extension to other organisms will be made where warranted.

Identification and characterization of the ZRT, IRT-like protein (ZIP) family genes reveal their involvement in growth and kojic acid production in Aspergillus oryzae

The ZRT, IRT-like protein (ZIP) family exists in many species and plays an important role in many biological processes, but little is known about ZIP genes in Aspergillus oryzae. Here, 10 ZIP genes in A. oryzae were identified and these were classified into four groups based on phylogenetic analysis. The structures of these AoZip genes were determined, which indicated a great divergence of AoZip members from different groups. Synteny analysis revealed that AoZip7, AoZip8, and AoZip10 are conserved among Aspergillus species. We also found that the promoter regions of AoZip2, AoZip7, AoZip8, and AoZip10 contain multiple conserved response elements. Expression analysis revealed that AoZips exhibited different expression patterns in response to different metal treatments. Moreover, overexpression and RNA-interference (RNAi) of AoZip2 led to a decrease in mycelium growth diameter and inhibited conidia formation. AoZip2 overexpression and RNAi strains showed distinct sensitivity to severely Zn/Mn-depleted stress. In addition, kojic acid production was markedly lower in AoZip2 overexpression and RNAi strains than in the control strains, and the expression of kojA, kojR, and kojT was down-regulated in AoZip2 overexpression and RNAi strains. This study provides new insights into our understanding of ZIP genes and lays a foundation for further investigation of their roles in Aspergillus oryzae.

A Brief Overview from the Physiological and Detrimental Roles of Zinc Homeostasis via Zinc Transporters in the Heart

Zinc (mostly as free/labile Zn²⁺) is an essential structural constituent of many proteins, including enzymes in cellular signaling pathways via functioning as an important signaling molecule in mammalian cells. In cardiomyocytes at resting condition, intracellular labile Zn²⁺ concentration ([Zn²⁺]_i) is in the nanomolar range, whereas it can increase dramatically under pathological conditions, including hyperglycemia, but the mechanisms that affect its subcellular redistribution is not clear. Therefore, overall, very little is known about the precise mechanisms controlling the intracellular distribution of labile Zn²⁺, particularly via Zn²⁺ transporters during cardiac function under both physiological and pathophysiological conditions. Literature data demonstrated that [Zn²⁺]_i homeostasis in mammalian cells is primarily coordinated by Zn²⁺ transporters classified as ZnTs (SLC30A) and ZIPs (SLC39A). To identify the molecular mechanisms of diverse functions of labile Zn²⁺ in the heart, the recent studies focused on the discovery of subcellular localization of these Zn²⁺ transporters in parallel to the discovery of novel physiological functions of [Zn²⁺]_i in cardiomyocytes. The present review summarizes the current understanding of the role of [Zn²⁺]_i changes in cardiomyocytes under pathological conditions, and under high [Zn²⁺]_i and how Zn²⁺ transporters are important for its subcellular redistribution. The emerging importance and the promise of some Zn²⁺ transporters for targeted cardiac therapy against pathological stimuli are also provided. Taken together, the review clearly outlines cellular control of cytosolic Zn²⁺ signaling by Zn²⁺ transporters, the role of Zn²⁺ transporters in heart function under hyperglycemia, the role of Zn²⁺ under increased oxidative stress and ER stress, and their roles in cancer are discussed.

The trafficking of metal ion transporters of the Zrt- and Irt-like protein family

Metal ion transporters of the Zrt- and Irt-like protein (ZIP, or SLC39A) family transport zinc, iron, manganese and/or cadmium across cellular membranes and into the cytosol. The 14 human ZIP family proteins are expressed in a wide variety of tissues and function in many different cellular processes. Many of these proteins (including ZIP1, 2, 3, 4, 5, 6/10, 8, 9, 11, 12, 14) are situated, at least some of the time, on the plasma membrane, where they mediate metal ion uptake into cells. Their level on the cell surface can be controlled rapidly via protein trafficking in response to the ions they transport. For example, the cell surface level of many ZIPs (including ZIP1, 3, 4, 8 and 12) is mediated by the available concentration of zinc. Zinc depletion causes a decrease in endocytosis and degradation, resulting in more ZIP on the surface to take up the essential ion. ZIP levels on the cell surface are a balance between endocytosis, recycling and degradation. We review the trafficking mechanisms of human ZIP proteins, highlighting possible targeting motifs and suggesting a model of zinc-mediated endocytic trafficking. We also provide two possible models for ZIP14 trafficking and degradation.

ZIP13: A Study of Drosophila Offers an Alternative Explanation for the Corresponding Human Disease

The fruit fly Drosophila melanogaster has become an important model organism to investigate metal homeostasis and human diseases. Previously we identified dZIP13 (CG7816), a member of the ZIP transporter family (SLC39A) and presumably a zinc importer, is in fact physiologically primarily responsible to move iron from the cytosol into the secretory compartments in the fly. This review will discuss the implication of this finding for the etiology of Spondylocheirodysplasia-Ehlers-Danlos Syndrome (SCD–EDS), a human disease defective in ZIP13. We propose an entirely different model in that lack of iron in the secretory compartment may underlie SCD-EDS. Altogether three different working models are discussed, supported by relevant findings made in different studies, with uncertainties, and questions remained to be solved. We speculate that the distinct ZIP13 sequence features, different from those of all other ZIP family members, may confer it special transport properties.

The SlZRT1 Gene Encodes a Plasma Membrane-Located ZIP (Zrt-, Irt-Like Protein) Transporter in the Ectomycorrhizal Fungus Suillus luteus

Zinc (Zn) is an essential micronutrient but may become toxic when present in excess. In Zn-contaminated environments, trees can be protected from Zn toxicity by their root-associated micro-organisms, in particular ectomycorrhizal fungi. The mechanisms of cellular Zn homeostasis in ectomycorrhizal fungi and their contribution to the host tree's Zn status are however not yet fully understood. The aim of this study was to identify and characterize transporters involved in Zn uptake in the ectomycorrhizal fungus Suillus luteus, a cosmopolitan pine mycobiont. Zn uptake in fungi is known to be predominantly governed by members of the ZIP (Zrt/IrtT-like protein) family of Zn transporters. Four ZIP transporter encoding genes were identified in the S. luteus genome. By in silico and phylogenetic analysis, one of these proteins, SlZRT1, was predicted to be a plasma membrane located Zn importer. Heterologous expression in yeast confirmed the predicted function and localization of the protein. A gene expression analysis via RT-qPCR was performed in S. luteus to establish whether SlZRT1 expression is affected by external Zn concentrations. SlZRT1 transcripts accumulated almost immediately, though transiently upon growth in the absence of Zn. Exposure to elevated concentrations of Zn resulted in a significant reduction of SlZRT1 transcripts within the first hour after initiation of the exposure. Altogether, the data support a role as cellular Zn importer for SlZRT1 and indicate a key role in cellular Zn uptake of S. luteus. Further research is needed to understand the eventual contribution of SlZRT1 to the Zn status of the host plant.

Unravelling the role of protein kinase CK2 in metal toxicity using gene deletion mutants

Metal ions, biologically essential or toxic, are present in the surrounding environment of living organisms. Understanding their uptake, homeostasis or detoxification is critical in cell biology and human health. In this study, we investigated the role of protein kinase CK2 in metal toxicity using gene deletion strains of Saccharomyces cerevisiae against a panel of six metal ions. The deletion of CKA2, the yeast orthologue of mammalian CK2α', leads to a pronounced resistant phenotype against Zn²⁺ and Al³⁺, whilst the deletion of CKB1 or CKB2 results in tolerance to Cr⁶⁺ and As³⁺. The individual deletion mutants of CK2 subunits (CKA1, CKA2, CKB1 and CKB2) did not have any benefit against Co²⁺ and Cd²⁺. The metal ion content in the treated cells was then measured by inductively coupled plasma mass spectrometry. Two contrasting findings were obtained for the CKA2 deletion mutant (cka2Δ) against Al³⁺ or Zn²⁺. Upon exposure to Al³⁺, cka2Δ had markedly lower Al³⁺ content than the wild type and other CK2 mutants, congruous to the resistant phenotype of cka2Δ against Al³⁺, indicating that CKA2 is responsible for Al³⁺ uptake. Upon zinc exposure the same mutant showed similar Zn²⁺ content to the wild type and cka1Δ. Strikingly, the selective inhibitor of CK2 TBB (4,5,6,7-tetrabromo-1H-benzotriazole) abolished the resistant phenotype of cka2Δ against Zn²⁺. Hence, the CK2 subunit CKA1 plays a key role in Zn²⁺ sequestration of the cell. Given that both zinc and CK2 are implicated in cancer development, the findings herein are of significance to cancer research and anticancer drug development.

Understanding the Contribution of Zinc Transporters in the Function of the Early Secretory Pathway

More than one-third of newly synthesized proteins are targeted to the early secretory pathway, which is comprised of the endoplasmic reticulum (ER), Golgi apparatus, and other intermediate compartments. The early secretory pathway plays a key role in controlling the folding, assembly, maturation, modification, trafficking, and degradation of such proteins. A considerable proportion of the secretome requires zinc as an essential factor for its structural and catalytic functions, and recent findings reveal that zinc plays a pivotal role in the function of the early secretory pathway. Hence, a disruption of zinc homeostasis and metabolism involving the early secretory pathway will lead to pathway dysregulation, resulting in various defects, including an exacerbation of homeostatic ER stress. The accumulated evidence indicates that specific members of the family of Zn transporters (ZNTs) and Zrt- and Irt-like proteins (ZIPs), which operate in the early secretory pathway, play indispensable roles in maintaining zinc homeostasis by regulating the influx and efflux of zinc. In this review, the biological functions of these transporters are discussed, focusing on recent aspects of their roles. In particular, we discuss in depth how specific ZNT transporters are employed in the activation of zinc-requiring ectoenzymes. The means by which early secretory pathway functions are controlled by zinc, mediated by specific ZNT and ZIP transporters, are also subjects of this review.

Effects of zinc transporters on Cryptococcus gattii virulence

Zinc is an essential nutrient for all living organisms because it is a co-factor of several important proteins. Furthermore, zinc may play an essential role in the infectiousness of microorganisms. Previously, we determined that functional zinc metabolism is associated with Cryptococcus gattii virulence. Here, we characterized the ZIP zinc transporters in this human pathogen. Transcriptional profiling revealed that zinc levels regulated the expression of the ZIP1, ZIP2 and ZIP3 genes, although only the C. gattii zinc transporter Zip1 was required for yeast growth under zinc-limiting conditions. To associate zinc uptake defects with virulence, the most studied cryptococcal virulence factors (i.e., capsule, melanin and growth at 37 °C) were assessed in ZIP mutant strains; however, no differences were detected in these classical virulence-associated traits among the mutant and WT strains. Interestingly, higher levels of reactive oxygen species were detected in the zip1Δ and in the zip1Δ zip2Δ double mutants. In line with these phenotypic alterations, the zip1Δ zip2Δ double mutant displayed attenuated virulence in a murine model of cryptococcosis. Together, these results indicate that adequate zinc uptake is necessary for cryptococcal fitness and virulence.

Pumping iron

The primary role of the ZIP13 metal transporter in flies is to move iron ions out of cells, rather than moving zinc ions into cells, as is the case in human cells.

Genome-wide analysis of copper, iron and zinc transporters in the arbuscular mycorrhizal fungus Rhizophagus irregularis

Arbuscular mycorrhizal fungi (AMF), belonging to the Glomeromycota, are soil microorganisms that establish mutualistic symbioses with the majority of higher plants. The efficient uptake of low mobility mineral nutrients by the fungal symbiont and their further transfer to the plant is a major feature of this symbiosis. Besides improving plant mineral nutrition, AMF can alleviate heavy metal toxicity to their host plants and are able to tolerate high metal concentrations in the soil. Nevertheless, we are far from understanding the key molecular determinants of metal homeostasis in these organisms. To get some insights into these mechanisms, a genome-wide analysis of Cu, Fe and Zn transporters was undertaken, making use of the recently published whole genome of the AMF Rhizophagus irregularis. This in silico analysis allowed identification of 30 open reading frames in the R. irregularis genome, which potentially encode metal transporters. Phylogenetic comparisons with the genomes of a set of reference fungi showed an expansion of some metal transporter families. Analysis of the published transcriptomic profiles of R. irregularis revealed that a set of genes were up-regulated in mycorrhizal roots compared to germinated spores and extraradical mycelium, which suggests that metals are important for plant colonization.

Compensatory mutations in predicted metal transporters modulate auxin conjugate responsiveness in Arabidopsis

Levels of the phytohormone indole-3-acetic acid (IAA) can be altered by the formation and hydrolysis of IAA conjugates. The isolation and characterization of Arabidopsis thaliana mutants with reduced IAA-conjugate sensitivity and wild-type IAA responses is advancing the understanding of auxin homeostasis by uncovering the factors needed for conjugate metabolism. For example, the discovery that the IAA-Ala-resistant mutant iar1 is defective in a protein in the ZIP family of metal transporters uncovered a link between metal homeostasis and IAA-conjugate sensitivity. To uncover additional factors impacting auxin conjugate metabolism, we conducted a genetic modifier screen and isolated extragenic mutations that restored IAA-amino acid conjugate sensitivity to the iar1 mutant. One of these suppressor mutants is defective in a putative cation diffusion facilitator, MTP5 (At3g12100; formerly known as MTPc2). Loss of MTP5 function restored IAA conjugate sensitivity to iar1 but not to mutants defective in IAA-amino acid conjugate amidohydrolases. Our results are consistent with a model in which MTP5 and IAR1 transport metals in an antagonistic fashion to regulate metal homeostasis within the subcellular compartment in which the IAA-conjugate amidohydrolases reside, and support previous suggestions that the ion composition in this compartment influences hydrolase activity.

The ins and outs of algal metal transport

Metal transporters are a central component in the interaction of algae with their environment. They represent the first line of defense to cellular perturbations in metal concentration, and by analyzing algal metal transporter repertoires, we gain insight into a fundamental aspect of algal biology. The ability of individual algae to thrive in environments with unique geochemistry, compared to non-algal species commonly used as reference organisms for metal homeostasis, provides an opportunity to broaden our understanding of biological metal requirements, preferences and trafficking. Chlamydomonas reinhardtii is the best developed reference organism for the study of algal biology, especially with respect to metal metabolism; however, the diversity of algal niches necessitates a comparative genomic analysis of all sequenced algal genomes. A comparison between known and putative proteins in animals, plants, fungi and algae using protein similarity networks has revealed the presence of novel metal metabolism components in Chlamydomonas including new iron and copper transporters. This analysis also supports the concept that, in terms of metal metabolism, algae from similar niches are more related to one another than to algae from the same phylogenetic clade. This article is part of a Special Issue entitled: Cell Biology of Metals.

Yap5 protein-regulated transcription of the TYW1 gene protects yeast from high iron toxicity

The budding yeast Saccharomyces cerevisiae responds to high cytosolic iron by inducing Yap5-mediated transcription. We identified genes regulated by Yap5 in response to iron and show that one of the genes induced is TYW1, which encodes an iron-sulfur cluster enzyme that participates in the synthesis of wybutosine-modified tRNA. Strains deleted for TYW1 do not show a phenotype in standard yeast medium. In contrast, overexpression of TYW1 results in decreased cell growth and induction of the iron regulon, leading to increased expression of the high affinity iron transporters. We identified a minimal domain of S. cerevisiae Tyw1 that is sufficient to induce the iron regulon. CCC1, a vacuolar iron importer, is a Yap5-regulated gene, and deletion of either CCC1 or YAP5 resulted in high iron sensitivity. Deletion of TYW1 in a Δccc1 strain led to increased iron sensitivity. The increased iron sensitivity of Δccc1Δtyw1 could be suppressed by overexpression of iron-sulfur cluster enzymes. We conclude that the Yap5-mediated induction of TYW1 provides protection from high iron toxicity by the consumption of free cytosolic iron through the formation of protein-bound iron-sulfur clusters.

Differential subcellular localization of the splice variants of the zinc transporter ZnT5 is dictated by the different C-terminal regions

BACKGROUND

Zinc is emerging as an important intracellular signaling molecule, as well as fulfilling essential structural and catalytic functions through incorporation in a myriad of zinc metalloproteins so it is important to elucidate the molecular mechanisms of zinc homeostasis, including the subcellular localizations of zinc transporters.

PRINCIPAL FINDINGS

Two splice variants of the human SLC30A5 Zn transporter gene (ZnT5) have been reported in the literature. These variants differ at their N- and C-terminal regions, corresponding with the use of different 5' and 3' exons. We demonstrate that full length human ZnT5 variant B is a genuine transcript in human intestinal cells and confirm expression of both variant A and variant B in a range of untreated human tissues by splice variant-specific RT-PCR. Using N- or C-terminal GFP or FLAG fusions of both isoforms of ZnT5 we identify that the differential subcellular localization to the Golgi apparatus and ER respectively is a function of their alternative C-terminal sequences. These different C-terminal regions result from the incorporation into the mature transcript of either the whole of exon 14 (variant B) or only the 5' region of exon 14 plus exons 15-17 (variant A).

CONCLUSIONS

We thus propose that exons 15 to 17 include a signal that results in trafficking of ZnT5 to the Golgi apparatus and that the 3' end of exon 14 includes a signal that leads to retention in the ER.

Genome-wide analysis of plant metal transporters, with an emphasis on poplar

The specific transport of metal ions, mediated by membrane-localized metal transporters, is of fundamental importance in all eukaryotes. Genome-wide analysis of metal transporters was undertaken, making use of whole genome sequences of the green alga Chlamydomonas reinhardtii, the moss Physcomitrella patens, the lycophyte Selaginella moellendorffii, the monocots rice and sorghum, and the dicots Arabidopsis thaliana, poplar, grapevine, as well as of the yeast Saccharomyces cerevisiae. A repertoire of 430 metal transporters was found in total across eight photosynthetic plants, as well as in S. cerevisiae. Seventy-two full-length metal transporter genes were identified in the Populus genome alone, which is the largest number of metal transporters genes identified in any single species to date. Diversification of some transporter family gene clusters appears to have occurred in a lineage-specific manner. Expression analysis of Populus metal transporters indicates that some family members show tissue-specific transcript abundance. Taken together, the data provide a picture into the diversification of these important gene families.

The putative Arabidopsis zinc transporter ZTP29 is involved in the response to salt stress

Salt stress leads to a stress response, called the unfolded protein response (UPR), in the endoplasmic reticulum (ER). UPR is also induced in a wide range of organisms by zinc deficiency. However, it is not clear whether regulation of zinc levels is involved in the initiation of the UPR in plant response to salt stress. In this study, a putative zinc transporter, ZTP29, was identified in Arabidopsis thaliana. ZTP29 localizes to the ER membrane and is expressed primarily in hypocotyl and cotyledon tissues, but its expression can be induced in root tissue by salt stress. T-DNA insertion into the ZTP29 gene led to NaCl hypersensitivity in seed germination and seedling growth, leaf etiolation, and widening of cells in the root elongation zone. In addition, in ztp29 mutant plants, salt stress-induced upregulation of the UPR pathway genes BiP2 and bZIP60 was inhibited. Furthermore, under conditions of salt stress, upregulation of BiP2 and bZIP60 was inhibited by treatment with high concentrations of zinc in both control and ztp29 plants. However, zinc chelation restored salt stress-induced BiP2 and bZIP60 upregulation in ztp29 mutant plants. These experimental results suggest that ZTP29 is involved in the response to salt stress, perhaps through regulation of zinc levels required to induce the UPR pathway.

Demonstration and characterization of the heterodimerization of ZnT5 and ZnT6 in the early secretory pathway

The majority of CDF/ZnT zinc transporters form homo-oligomers. However, ZnT5, ZnT6, and their orthologues form hetero-oligomers in the early secretory pathway where they load zinc onto zinc-requiring enzymes and maintain secretory pathway functions. The details of this hetero-oligomerization remain to be elucidated, and much more is known about homo-oligomerization that occurs in other CDF/ZnT family proteins. Here, we addressed this issue using co-immunoprecipitation experiments, mutagenesis, and chimera studies of hZnT5 and hZnT6 in chicken DT40 cells deficient in ZnT5, ZnT6, and ZnT7 proteins. We found that hZnT5 and hZnT6 combine to form heterodimers but do not form complexes larger than heterodimers. Mutagenesis of hZnT6 indicated that the sites present in transmembrane domains II and V in which many CDF/ZnT proteins have conserved hydrophilic amino acid residues are not involved in zinc binding of hZnT6, although they are required for zinc transport in other CDF/ZnT family homo-oligomers. We also found that the long N-terminal half of hZnT5 is not necessary for its functional interaction with hZnT6, whereas the cytosolic C-terminal tail of hZnT5 is important in determining hZnT6 as a partner molecule for heterodimer formation. In DT40 cells, cZnT5 variant lacking the N-terminal half was endogenously induced during periods of endoplasmic reticulum stress and so seemed to function to supply zinc to zinc-requiring enzymes under these conditions. The results outlined here provide new information about the mechanism of action through heterodimerization of CDF/ZnT proteins that function in the early secretory pathway.

SLC39A9 (ZIP9) regulates zinc homeostasis in the secretory pathway: characterization of the ZIP subfamily I protein in vertebrate cells

The SLC39A family of zinc transporters can be divided into four subfamilies (I, II, LIV-1, and gufA) in vertebrates, but studies of their functions have been restricted exclusively to members of subfamilies II and LIV-1. In this study, we characterized SLC39A9 (ZIP9), the only member of subfamily I in vertebrates. Confocal microscopy demonstrated that transiently expressed, HA-tagged human ZIP9 (hZIP9-HA) was localized to the trans-Golgi network regardless of zinc status. Disruption of the ZIP9 gene in DT40 cells did not change the growth rate, sensitivity to high zinc and manganese concentrations during long-term culture, or cellular zinc status after short-term incubation with zinc. The alkaline phosphatase activity of ZIP9(-/-) cells did not change in cells cultured in medium containing normal zinc levels. In contrast, the activity of this enzyme decreased in wild-type cells cultured in zinc deficient medium but less so in ZIP9(-/-) cells under these conditions. Stable over-expression of hZIP9-HA moderately decreased alkaline phophatase activity. These results suggest that ZIP9 functions to regulate zinc homeostasis in the secretory pathway without significantly altering cytosolic zinc homeostasis.

Arabidopsis IRT2 cooperates with the high-affinity iron uptake system to maintain iron homeostasis in root epidermal cells

Iron is an essential nutrient for all organisms but toxic when present in excess. Consequently, plants carefully regulate their iron uptake, dependent on the FRO2 ferric reductase and the IRT1 transporter, to control its homeostasis. Arabidopsis IRT2 gene, whose expression is induced in root epidermis upon iron deprivation, was shown to encode a functional iron/zinc transporter in yeast, and proposed to function in iron acquisition from the soil. In this study, we demonstrate that, unlike its close homolog IRT1, IRT2 is not involved in iron absorption from the soil since overexpression of IRT2 does not rescue the iron uptake defect of irt1-1 mutant and since a null irt2 mutant shows no chlorosis in low iron. Consistently, an IRT2-green fluorescent fusion protein, transiently expressed in culture cells, localizes to intracellular vesicles. However, IRT2 appears strictly co-regulated with FRO2 and IRT1, supporting the view that IRT2 is an integral component of the root response to iron deficiency in root epidermal cells. We propose a model where IRT2 likely prevents toxicity from IRT1-dependent iron fluxes in epidermal cells, through compartmentalization.

Dietary zinc absorption is mediated by ZnT1 in Drosophila melanogaster

Zinc is an essential nutritional factor involved in many key biological processes. However, the physiological function of zinc transporters at the organismal level is not well characterized. Early embryonic lethality of Znt1 knockout mice precludes functional analysis of the role of ZnT1 in dietary zinc absorption. Here, we report the identification and characterization of the Drosophila ZnT1 orthologue, dZnT1, for its role in Drosophila dietary zinc absorption. In cell culture, dZnT1 promoted zinc transport to reduce cytoplasmic zinc levels. Ubiquitous RNA interference of dZnT1 in Drosophila resulted in developmental arrest under restriction of dietary zinc, while dZnT1-overexpressing flies exhibited hypersensitivity to zinc. dZnT1 was prominently expressed in restricted regions of the midgut and exhibited a distribution on the basolateral membrane of the enterocytes. Gut-specific silencing of dZnT1 was sufficient to evoke lethality under zinc scarcity. Human ZnT1, but not ZnT7 or ZnT4, could rescue the zinc-acquiring defects caused by dZnT1 silencing. Taken together, our results proved that dZnT1 is a key zinc transporter in dietary zinc absorption, functioning by pumping zinc out of the enterocytes across the basolateral membrane. This study will be helpful in understanding the fundamental process of acquiring dietary zinc in higher eukaryotes.

Differential control of Zap1-regulated genes in response to zinc deficiency in Saccharomyces cerevisiae

BACKGROUND

The Zap1 transcription factor is a central player in the response of yeast to changes in zinc status. We previously used transcriptome profiling with DNA microarrays to identify 46 potential Zap1 target genes in the yeast genome. In this new study, we used complementary methods to identify additional Zap1 target genes.

RESULTS

With alternative growth conditions for the microarray experiments and a more sensitive motif identification algorithm, we identified 31 new potential targets of Zap1 activation. Moreover, an analysis of the response of Zap1 target genes to a range of zinc concentrations and to zinc withdrawal over time demonstrated that these genes respond differently to zinc deficiency. Some genes are induced under mild zinc deficiency and act as a first line of defense against this stress. First-line defense genes serve to maintain zinc homeostasis by increasing zinc uptake, and by mobilizing and conserving intracellular zinc pools. Other genes respond only to severe zinc limitation and act as a second line of defense. These second-line defense genes allow cells to adapt to conditions of zinc deficiency and include genes involved in maintaining secretory pathway and cell wall function, and stress responses.

CONCLUSION

We have identified several new targets of Zap1-mediated regulation. Furthermore, our results indicate that through the differential regulation of its target genes, Zap1 prioritizes mechanisms of zinc homeostasis and adaptive responses to zinc deficiency.

Zinc regulates the stability of repetitive minisatellite DNA tracts during stationary phase

Repetitive minisatellite DNA tracts are stable in mitotic cells but unstable in meiosis, altering in repeat number and repeat composition. As relatively little is known about the factors that influence minisatellite stability, we isolated mutations that destabilize a minisatellite repeat tract in the ADE2 gene of Saccharomyces cerevisiae. One mutant class exhibited a novel color segregation phenotype, "blebbing," characterized by minisatellite instability during stationary phase. Minisatellite tract alterations in blebbing strains consist exclusively of the loss of one 20-bp repeat. Timing experiments suggest that these tract alterations occur only after cells have entered stationary phase. Two complementation groups identified in this screen have mutations in either the high-affinity zinc transporter ZRT1 or its zinc-dependent transcriptional regulator ZAP1. The Deltazrt1 mutant specifically affects the stability of minisatellite tracts; microsatellites or simple insertions in the ADE2 reading frame are not destabilized by loss of ZRT1. The Deltazrt1 blebbing phenotype is partially dependent on a functional RAD50. Zinc is known for its role as an essential cofactor in many DNA-binding proteins. We describe possible models by which zinc can influence minisatellite stability. Our findings directly implicate zinc homeostasis in the maintenance of genomic stability during stationary phase.

Cation diffusion facilitator Cis4 is implicated in Golgi membrane trafficking via regulating zinc homeostasis in fission yeast

We screened for mutations that confer sensitivities to the calcineurin inhibitor FK506 and to a high concentration of MgCl(2) and isolated the cis4-1 mutant, an allele of the gene encoding a cation diffusion facilitator (CDF) protein that is structurally related to zinc transporters. Consistently, the addition of extracellular Zn(2+) suppressed the phenotypes of the cis4 mutant cells. The cis4 mutants and the mutant cells of another CDF-encoding gene SPBC16E9.14c (we named zrg17(+)) shared common and nonadditive zinc-suppressible phenotypes, and Cis4 and Zrg17 physically interacted. Cis4 localized at the cis-Golgi, suggesting that Cis4 is responsible for Zn(2+) uptake to the cis-Golgi. The cis4 mutant cells showed phenotypes such as weak cell wall and decreased acid phosphatase secretion that are thought to be resulting from impaired membrane trafficking. In addition, the cis4 deletion cells showed synthetic growth defects with all the four membrane-trafficking mutants tested, namely ypt3-i5, ryh1-i6, gdi1-i11, and apm1-1. Interestingly, the addition of extracellular Zn(2+) significantly suppressed the phenotypes of the ypt3-i5 and apm1-1 mutant cells. These results suggest that Cis4 forms a heteromeric functional complex with Zrg17 and that Cis4 is implicated in Golgi membrane trafficking through the regulation of zinc homeostasis in fission yeast.

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.

The emerging role of the LIV-1 subfamily of zinc transporters in breast cancer

Zinc transporter LIV-1 (SLC39A6) is estrogen regulated and present in increased amounts in estrogen receptor-positive breast cancer as well as in tumors that spread to the lymph nodes. The LIV-1 subfamily of ZIP zinc transporters consists of nine human sequences that share considerable homology across transmembrane domains. Many of these sequences have been shown to transport zinc and/or other ions across cell membranes. Increasingly, studies have implicated members of the LIV-1 transporter subfamily in a variety of diseases. We review these studies and report our own investigations of the role in breast cancer of the nine LIV-1 zinc transporters. We have documented the response of these transporters to estrogen and antiestrogens, and also their presence in our models of resistance to antiestrogens. Resistance to antiestrogen drugs such as tamoxifen and fulvestrant often occurs in advanced breast cancer. In these models we observed differential expression of individual LIV-1 family members, which may be related to their observed variable tissue expression. We were unable detect ZIP4, which is known to be expressed in the intestine. HKE4/SLC39A7 had elevated expression in both antiestrogen-resistant cell lines, and ZIP8 had elevated expression in fulvestrant-resistant cells. In addition, we investigated the expression of the nine LIV-1 family members in a clinical breast cancer series. Although a number of different LIV-1 family members showed some association with growth factor receptors, LIV-1 was solely associated with estrogen receptor and a variety of growth factors commonly associated with clinical breast cancer. HKE4, however, did show an association with the marker of cell proliferation Ki67 the spread of breast cancer to lymph nodes.

Survival in the presence of antifungals: genome-wide expression profiling of Aspergillus niger in response to sublethal concentrations of caspofungin and fenpropimorph

How yeast cells respond to cell wall stress is relatively well understood; however, how filamentous fungi cope with cell wall damage is largely unexplored. Here we report the first transcriptome analysis of Aspergillus niger exposed to the antifungal compounds caspofungin, an inhibitor of beta-1,3-glucan synthesis, and fenpropimorph, which inhibits ergosterol synthesis. The presence of sublethal drug concentrations allowed A. niger to adapt to the stress conditions and to continue growth by the establishment of new polarity axes and formation of new germ tubes. By comparing the expression profile between caspofungin-exposed and nonexposed A. niger germlings, we identified a total of 172 responsive genes out of 14,509 open reading frames present on the Affymetrix microarray chips. Among 165 up-regulated genes, mainly genes predicted to function in (i) cell wall assembly and remodeling, (ii) cytoskeletal organization, (iii) signaling, and (iv) oxidative stress response were affected. Fenpropimorph modulated expression of 43 genes, of which 41 showed enhanced expression. Here, genes predicted to function in (i) membrane reconstruction, (ii) lipid signaling, (iii) cell wall remodeling, and (iv) oxidative stress response were identified. Northern analyses of selected genes were used to confirm the microarray analyses. The results further show that expression of the agsA gene encoding an alpha-1,3-glucan synthase is up-regulated by both compounds. Using two PagsA-GFP reporter strains of A. niger and subjecting them to 16 different antifungal compounds, including caspofungin and fenpropimorph, we could show that agsA is specifically activated by compounds interfering directly or indirectly with cell wall biosynthesis.

ZnT5 Variant B Is a Bidirectional Zinc Transporter and Mediates Zinc Uptake in Human Intestinal Caco-2 Cells

Zinc is an essential micronutrient, so it is important to elucidate the molecular mechanisms of zinc homeostasis, including the functional properties of zinc transporters. Mammalian zinc transporters are classified in two major families: the SLC30 (ZnT) family and the SLC39 family. The prevailing view is that SLC30 family transporters function to reduce cytosolic zinc concentration, either through efflux across the plasma membrane or through sequestration in intracellular compartments, and that SLC39 family transporters function in the opposite direction to increase cytosolic zinc concentration. We demonstrated that human ZnT5 variant B (ZnT5B (hZTL1)), an isoform expressed at the plasma membrane, operates in both the uptake and the efflux directions when expressed in Xenopus laevis oocytes. We measured increased activity of the zinc-responsive metallothionein 2a (MT2a) promoter when ZnT5b was co-expressed with an MT2a promoter-reporter plasmid construct in human intestinal Caco-2 cells, indicating increased total intracellular zinc concentration. Increased cytoplasmic zinc concentration mediated by ZnT5B, in the absence of effects on intracellular zinc sequestration by the Golgi apparatus or endoplasmic reticulum, was also confirmed by a dramatically enhanced signal from the zinc fluorophore Rhodzin-3 throughout the cytoplasm of Caco-2 cells overexpressing ZnT5B at the plasma membrane when compared with control cells. Our findings demonstrate clearly that, in addition to mediating zinc efflux, ZnT5B at the plasma membrane can function to increase cytoplasmic zinc concentration, thus indicating a need to reevaluate the current paradigm that SLC30 family zinc transporters operate exclusively to decrease cytosolic zinc concentration.