Functional dissection of Ctr4 and Ctr5 amino-terminal regions reveals motifs with redundant roles in copper transport

Arabidopsis COPT1 copper transporter uses a single histidine to regulate transport activity and protein stability

Copper acquisition and subsequent delivery to target proteins are essential for many biological processes. However, the cellular levels of this trace element must be controlled because of its potential toxicity. The COPT1 protein rich in potential metal-binding amino acids functions in high affinity copper uptake at the plasma membrane of Arabidopsis cells. The functional role of these putative metal-binding residues is largely unknown. Through truncations and site-directed mutagenesis, we identified His43, a single residue within the extracellular N-terminal domain as absolutely critical for copper uptake of COPT1. Substitution of this residue with leucine, methionine or cysteine almost inactivated transport function of COPT1, implying that His43 fails to serves as a copper ligand in the regulation of COPT1 activity. Deletion of all extracellular N-terminal metal-binding residues completely blocked copper-stimulated degradation but did not alter the subcellular distribution and multimerization of COPT1. Although mutation of His43 to alanine and serine retained the transporter activity in yeast cells, the mutant protein was unstable and degraded in the proteasome in Arabidopsis cells. Our results demonstrate a pivotal role for the extracellular residue His43 in high affinity copper transport activity, and suggest common molecular mechanisms for regulating both metal transport and protein stability of COPT1.

Septin filament compaction into rings requires the anillin Mid2 and contractile ring constriction

Septin filaments assemble into high-order molecular structures that associate with membranes, acting as diffusion barriers and scaffold proteins crucial for many cellular processes. How septin filaments organize in such structures is still not understood. Here, we used fission yeast to explore septin filament organization during cell division and its cell cycle regulation. Live-imaging and polarization microscopy analysis uncovered that septin filaments are initially recruited as a diffuse meshwork surrounding the acto-myosin contractile ring (CR) in anaphase, which undergoes compaction into two rings when CR constriction is initiated. We found that the anillin-like protein Mid2 is necessary to promote this compaction step, possibly acting as a bundler for septin filaments. Moreover, Mid2-driven septin compaction requires inputs from the septation initiation network as well as CR constriction and the β(1,3)-glucan synthase Bgs1. This work highlights that anillin-mediated septin ring assembly is under strict cell cycle control.

Fission yeast RNA-binding proteins Puf2 and Puf4 are involved in repression of ferrireductase Frp1 expression in response to iron

This study identifies a post-transcriptional mechanism of iron uptake regulation by Puf2 and Puf4 of the Pumilio and FBF (Puf) family of RNA-binding proteins in Schizosaccharomyces pombe. Cells expressing Puf2 and Puf4 stimulate decay of the frp1⁺ mRNA encoding a key enzyme of the reductive iron uptake pathway. Results consistently showed that frp1⁺ mRNA is stabilized in puf2Δ puf4Δ mutant cells under iron-replete conditions. As a result, puf2Δ puf4Δ cells exhibit an increased sensitivity to iron accompanied by enhanced ferrireductase activity. A pool of GFP-frp1⁺ 3'UTR RNAs was generated using a reporter gene containing the 3' untranslated region (UTR) of frp1⁺ that was under the control of a regulatable promoter. Results showed that Puf2 and Puf4 accelerate the destabilization of mRNAs containing the frp1⁺ 3'UTR which harbors two Pumilio response elements (PREs). Binding studies revealed that the PUM-homology RNA-binding domain of Puf2 and Puf4 expressed in Escherichia coli specifically interacts with PREs in the frp1⁺ 3'UTR. Using RNA immunoprecipitation in combination with reverse transcription qPCR assays, results showed that Puf2 and Puf4 interact preferentially with frp1⁺ mRNA under basal and iron-replete conditions, thereby contributing to inhibit Frp1 production and protecting cells against toxic levels of iron.

Molecular characterization of the COPT/Ctr-type copper transporter family under heavy metal stress in alfalfa

In nature, heavy metals significantly affect crop growth and quality. Among various heavy metals, copper (Cu) is both essential and toxic to plants depending on the concentration and complex homeostatic networks. The Cu transporter family (COPT) plays important roles in Cu homeostasis, including absorption, transportation, and growth in plants; however, this gene family is still poorly understood in alfalfa (Medicago sativa L.). In this study, a total of 12 MsCOPTs were identified and characterized. Based on the conserved motif and phylogenetic analysis, MsCOPTs could be divided into four subgroups (A1, A2, A3, and B). Gene structure, chromosomal location, and synteny analyses of MsCOPTs showed that segmental and tandem duplications likely contributed to their evolution. Tissue-specific expression analysis of MsCOPT genes indicated diverse spatiotemporal expression patterns. Most MsCOPT genes had high transcription levels in roots and nodules, indicating that these genes may play vital roles in the absorption and transport of Cu through root. The complementary heterologous expression function of yeast once again indicates that root-specific COPT can supplement the growth of defective yeast strains on YPEG medium, suggesting that these genes are Cu transporters. In summary, for the first time, our research identified COPT family genes at the whole-genome level to provide guidance for effectively improving the problem of Cu deficiency in the grass-livestock chain and provide theoretical support for the subsequent development of grass and animal husbandry.

Iron deficiency leads to repression of a non-canonical methionine salvage pathway in Schizosaccharomyces pombe

The methionine salvage pathway (MSP) regenerates methionine from 5'-methylthioadenosine (MTA). Aerobic MSP consists of six enzymatic steps. The mug14⁺ and adi1⁺ genes that are involved in the third and fifth steps of the pathway are repressed when Schizosaccharomyces pombe undergoes a transition from high- to low-iron conditions. Results consistently show that methionine auxotrophic cells (met6Δ) require iron for growth in the presence of MTA as the sole source of methionine. Inactivation of the iron-using protein Adi1 leads to defects in the utilization of MTA. In the case of the third step of the pathway, co-expression of two distinct proteins, Mta3 and Mde1, is required. These proteins are interdependent to rescue MTA-dependent growth deficit of met6Δ cells. Coimmunoprecipitation experiments showed that Mta3 is a binding partner of Mde1. Meiotic met6Δ cells co-expressing mta3⁺ and mde1⁺ or mta3⁺ and mug14⁺ produce comparable levels of spores in the presence of MTA, revealing that Mde1 and Mug14 share a common function when co-expressed with Mta3 in sporulating cells. In sum, our findings unveil several novel features of MSP, especially with respect to its regulation by iron and the discovery of a non-canonical third enzymatic step in the fission yeast.

Heme acquisition by Shu1 requires Nbr1 and proteins of the ESCRT complex in Schizosaccharomyces pombe

Assimilation of heme is mediated by the cell surface protein Shu1 in Schizosaccharomyces pombe. Shu1 undergoes internalization from the cell surface to the vacuole in response to high concentrations of hemin. Here, we have identified cellular components that are involved in mediating vacuolar targeting of Shu1. Cells deficient in heme biosynthesis and lacking the polyubiquitin gene ubi4⁺ exhibit poor growth in the presence of exogenous hemin as a sole source of heme. Microscopic analyses of hem1Δ shu1Δ ubi4Δ cells expressing a functional HA₄ -tagged Shu1 show that Shu1 localizes to the cell surface. Ubiquitinated Nbr1 functions as a receptor for the endosomal sorting complexes required for transport (ESCRT) that delivers cargos to the vacuole. Inactivation of nbr1⁺ , ESCRT-0 hse1⁺ or ESCRT-I sst6⁺ results in hem1Δ cells being unable to use exogenous hemin for the growth. Using lysate preparations from hemin-treated cells, Shu1-Nbr1 and Shu1-Hse1 complexes are detected by coimmunoprecipitation experiments. Further analysis by immunofluorescence microscopy shows that Shu1 is unable to reach vacuoles of hemin-treated cells harboring a deletion for one of the following genes: ubi4⁺ , nbr1⁺ , hse1⁺ and sst6⁺ . Together, these results reveal that hemin-mediated vacuolar targeting of Shu1 requires Ubi4-dependent ubiquitination, the receptor Nbr1 and the ESCRT proteins Hse1 and Sst6.

A role for the transcription factor Mca1 in activating the meiosis-specific copper transporter Mfc1

When reproduction in fungi takes place by sexual means, meiosis enables the formation of haploid spores from diploid precursor cells. Copper is required for completion of meiosis in Schizosaccharomyces pombe. During the meiotic program, genes encoding copper transporters exhibit distinct temporal expression profiles. In the case of the major facilitator copper transporter 1 (Mfc1), its maximal expression is induced during middle-phase meiosis and requires the presence of the Zn₆Cys₂ binuclear cluster-type transcription factor Mca1. In this study, we further characterize the mechanism by which Mca1 affects the copper-starvation-induced expression of mfc1⁺. Using a chromatin immunoprecipitation (ChIP) approach, results showed that a functional Mca1-TAP occupies the mfc1⁺ promoter irrespective of whether this gene is transcriptionally active. Under conditions of copper starvation, results showed that the presence of Mca1 promotes RNA polymerase II (Pol II) occupancy along the mfc1⁺ transcribed region. In contrast, Pol II did not significantly occupy the mfc1⁺ locus in meiotic cells that were incubated in the presence of copper. Further analysis by ChIP assays revealed that binding of Pol II to chromatin at the chromosomal locus of mfc1⁺ is exclusively detected during meiosis and absent in cells proliferating in mitosis. Protein function analysis of a series of internal mutants compared to the full-length Mca1 identified a minimal form of Mca1 consisting of its DNA-binding domain (residues 1 to 150) fused to the amino acids 299 to 600. This shorter form is sufficient to enhance Pol II occupancy at the mfc1⁺ locus under low copper conditions. Taken together, these results revealed novel characteristics of Mca1 and identified an internal region of Mca1 that is required to promote Pol II-dependent mfc1⁺ transcription during meiosis.

The major facilitator transporter Str3 is required for low-affinity heme acquisition in Schizosaccharomyces pombe

In the fission yeast Schizosaccharomyces pombe, acquisition of exogenous heme is largely mediated by the cell membrane-associated Shu1. Here, we report that Str3, a member of the major facilitator superfamily of transporters, promotes cellular heme import. Using a strain that cannot synthesize heme de novo (hem1Δ) and lacks Shu1, we found that the heme-dependent growth deficit of this strain is rescued by hemin supplementation in the presence of Str3. Microscopic analyses of a hem1Δ shu1Δ str3Δ mutant strain in the presence of the heme analog zinc mesoporphyrin IX (ZnMP) revealed that ZnMP fails to accumulate within the mutant cells. In contrast, Str3-expressing hem1Δ shu1Δ cells could take up ZnMP at a 10-μm concentration. The yeast Saccharomyces cerevisiae cannot efficiently transport exogenously supplied hemin. However, heterologous expression of Str3 from S. pombe in S. cerevisiae resulted in ZnMP accumulation within S. cerevisiae cells. Moreover, hemin-agarose pulldown assays revealed that Str3 binds hemin. In contrast, an Str3 mutant in which Tyr and Ser residues of two putative heme-binding motifs (⁵³⁰YX₃Y⁵³⁴ and ⁵⁵²SX₄Y⁵⁵⁷) had been replaced with alanines exhibited a loss of affinity for hemin. Furthermore, this Str3 mutant failed to rescue the heme-dependent growth deficit of a hem1Δ shu1Δ str3Δ strain. Further analysis by absorbance spectroscopy disclosed that a predicted extracellular loop region in Str3 containing the two putative heme-binding motifs interacts with hemin, with a K_D of 6.6 μm Taken together, these results indicate that Str3 is a second cell-surface membrane protein for acquisition of exogenous heme in S. pombe.

Analysis of ambient pH stress response mediated by iron and copper intake in Schizosaccharomyces pombe

The molecular mechanism of tolerance to alkaline pH is well studied in model fungi Aspergillus nidulans and Saccharomyces cerevisiae. However, how fission yeast Schizosaccharomyces pombe survives under alkaline stress remains largely unknown, as the genes involved in the alkaline stress response pathways of A. nidulans and S. cerevisiae were not found in the genome of this organism. Since uptake of iron and copper into cells is important for alkaline tolerance in S. cerevisiae, here we examined whether iron and copper uptake processes were involved in conferring tolerance to alkaline stress in S. pombe. We first revealed that S. pombe wild-type strain could not grow at a pH higher than 6.7. We further found that the growths of mutants harboring disruption in the iron uptake-related gene frp1⁺, fio1⁺ or fip1⁺ were severely inhibited under ambient pH stress condition. In contrast, derepression of these genes, by deletion of their repressor gene fep1⁺, caused cells to acquire resistance to pH stress. Together, these results suggested that uptake of iron is essential for ambient pH tolerance in S. pombe. We also found that copper is required for the pH stress response because disruptants of ctr4⁺, ctr5⁺, ccc2⁺ and cuf1⁺ genes, all of which are needed for regulating intracellular Cu⁺, displayed ambient pH sensitivity. Furthermore, supplementing Fe²⁺ and Cu²⁺ ions to the culture media improved growth under ambient pH stress. Taken together, our results suggested that uptake of iron and copper is the crucial factor needed for the adaptation of S. pombe to ambient pH stress.

Cell-surface copper transporters and superoxide dismutase 1 are essential for outgrowth during fungal spore germination

During fungal spore germination, a resting spore returns to a conventional mode of cell division and resumes vegetative growth, but the requirements for spore germination are incompletely understood. Here, we show that copper is essential for spore germination in Schizosaccharomyces pombe Germinating spores develop a single germ tube that emerges from the outer spore wall in a process called outgrowth. Under low-copper conditions, the copper transporters Ctr4 and Ctr5 are maximally expressed at the onset of outgrowth. In the case of Ctr6, its expression is broader, taking place before and during outgrowth. Spores lacking Ctr4, Ctr5, and the copper sensor Cuf1 exhibit complete germination arrest at outgrowth. In contrast, ctr6 deletion only partially interferes with formation of outgrowing spores. At outgrowth, Ctr4-GFP and Ctr5-Cherry first co-localize at the spore contour, followed by re-location to a middle peripheral spore region. Subsequently, they move away from the spore body to occupy the periphery of the nascent cell. After breaking of spore dormancy, Ctr6 localizes to the vacuole membranes that are enriched in the spore body relative to the germ tube. Using a copper-binding tracker, results showed that labile copper is preferentially localized to the spore body. Further analysis showed that Ctr4 and Ctr6 are required for copper-dependent activation of the superoxide dismutase 1 (SOD1) during spore germination. This activation is critical because the loss of SOD1 activity blocked spore germination at outgrowth. Taken together, these results indicate that cell-surface copper transporters and SOD1 are required for completion of the spore germination program.

Heme Assimilation in Schizosaccharomyces pombe Requires Cell-surface-anchored Protein Shu1 and Vacuolar Transporter Abc3

The Schizosaccharomyces pombe shu1⁺ gene encodes a cell-surface protein required for assimilation of exogenous heme. In this study, shaving experiments showed that Shu1 is released from membrane preparations when spheroplast lysates are incubated with phosphoinositide-specific phospholipase C (PI-PLC). Shu1 cleavability by PI-PLC and its predicted hydropathy profile strongly suggested that Shu1 is a glycosylphosphatidylinositol-anchored protein. When heme biosynthesis is selectively blocked in hem1Δ mutant cells, the heme analog zinc mesoporphyrin IX (ZnMP) first accumulates into vacuoles and then subsequently, within the cytoplasm in a rapid and Shu1-dependent manner. An HA₄-tagged shu1⁺ allele that retained wild-type function localizes to the cell surface in response to low hemin concentrations, but under high hemin concentrations, Shu1-HA₄ re-localizes to the vacuolar membrane. Inactivation of abc3⁺, encoding a vacuolar membrane transporter, results in hem1Δ abc3Δ mutant cells being unable to grow in the presence of hemin as the sole iron source. In hem1Δ abc3Δ cells, ZnMP accumulates primarily in vacuoles and does not sequentially accumulate in the cytosol. Consistent with a role for Abc3 as vacuolar hemin exporter, results with hemin-agarose pulldown assays showed that Abc3 binds to hemin. In contrast, an Abc3 mutant in which an inverted Cys-Pro motif had been replaced with Ala residues fails to bind hemin with high affinity. Taken together, these results show that Shu1 undergoes rapid hemin-induced internalization from the cell surface to the vacuolar membrane and that the transporter Abc3 participates in the mobilization of stored heme from the vacuole to the cytosol.

Comparison of extracellular Cys/Trp motif between Schizosaccharomyces pombe Ctr4 and Ctr5

The reduction and binding of copper ions to the Cys/Trp motif, which is characterized by two cysteines and two tryptophans, in the extracellular N-terminal domain of the copper transporter (Ctr) protein of fungi are investigated using the model peptides of Ctr4 and Ctr5 from Schizosaccharomyces pombe. The Cys/Trp motif of Ctr5 can reduce Cu(II) and ligate Cu(I), which is the same as that of Ctr4 previously reported. Titration of Cu(II) and Cu(I) ions indicates that both the Cys/Trp motifs of Ctr4 and Ctr5 reduce two Cu(II) and bind two Cu(I) per one peptide. However, the coordination structure of the Cu(I)-peptide complex differs between Ctr4 and Ctr5. Cu(I) is bound to the Cys/Trp motif of Ctr5 via cysteine thiolate-Cu(I) bonds and cation-π interaction with tryptophan, as reported for Ctr4, and a histidine residue in the Cys/Trp motif of Ctr5 is suggested to interact with Cu(I) via its Nτ atom. Ctr4 and Ctr5 exhibit a heterotrimeric form within cell membranes and the copper transport mechanism of the Ctr4/Ctr5 heterotrimer is discussed along with quantitative evaluation of the Cu(I)-binding constant of the Cys/Trp motif.

The copper transport-associated protein Ctr4 can form prion-like epigenetic determinants in Schizosaccharomyces pombe

Prions are protein-based infectious entities associated with fatal brain diseases in animals, but also modify a range of host-cell phenotypes in the budding yeast, Saccharomyces cerevisiae. Many questions remain about the evolution and biology of prions. Although several functionally distinct prion-forming proteins exist in S. cerevisiae, [HET-s] of Podospora anserina is the only other known fungal prion. Here we investigated prion-like, protein-based epigenetic transmission in the fission yeast Schizosaccharomyces pombe. We show that S. pombe cells can support the formation and maintenance of the prion form of the S. cerevisiae Sup35 translation factor [PSI⁺], and that the formation and propagation of these Sup35 aggregates is inhibited by guanidine hydrochloride, indicating commonalities in prion propagation machineries in these evolutionary diverged yeasts. A proteome-wide screen identified the Ctr4 copper transporter subunit as a putative prion with a predicted prion-like domain. Overexpression of the ctr4 gene resulted in large Ctr4 protein aggregates that were both detergent and proteinase-K resistant. Cells carrying such [CTR⁺] aggregates showed increased sensitivity to oxidative stress, and this phenotype could be transmitted to aggregate-free [ctr^-] cells by transformation with [CTR⁺] cell extracts. Moreover, this [CTR⁺] phenotype was inherited in a non-Mendelian manner following mating with naïve [ctr^-] cells, but intriguingly the [CTR⁺] phenotype was not eliminated by guanidine-hydrochloride treatment. Thus, Ctr4 exhibits multiple features diagnostic of other fungal prions and is the first example of a prion in fission yeast. These findings suggest that transmissible protein-based determinants of traits may be more widespread among fungi.

Php4 Is a Key Player for Iron Economy in Meiotic and Sporulating Cells

Meiosis is essential for sexually reproducing organisms, including the fission yeast Schizosaccharomyces pombe In meiosis, chromosomes replicate once in a diploid precursor cell (zygote), and then segregate twice to generate four haploid meiotic products, named spores in yeast. In S. pombe, Php4 is responsible for the transcriptional repression capability of the heteromeric CCAAT-binding factor to negatively regulate genes encoding iron-using proteins under low-iron conditions. Here, we show that the CCAAT-regulatory subunit Php4 is required for normal progression of meiosis under iron-limiting conditions. Cells lacking Php4 exhibit a meiotic arrest at metaphase I. Microscopic analyses of cells expressing functional GFP-Php4 show that it colocalizes with chromosomal material at every stage of meiosis under low concentrations of iron. In contrast, GFP-Php4 fluorescence signal is lost when cells undergo meiosis under iron-replete conditions. Global gene expression analysis of meiotic cells using DNA microarrays identified 137 genes that are regulated in an iron- and Php4-dependent manner. Among them, 18 genes are expressed exclusively during meiosis and constitute new putative Php4 target genes, which include hry1⁺ and mug14⁺ Further analysis validates that Php4 is required for maximal and timely repression of hry1⁺ and mug14⁺ genes. Using a chromatin immunoprecipitation approach, we show that Php4 specifically associates with hry1⁺ and mug14⁺ promoters in vivo Taken together, the results reveal that in iron-starved meiotic cells, Php4 is essential for completion of the meiotic program since it participates in global gene expression reprogramming to optimize the use of limited available iron.

Essential and Beneficial Trace Elements in Plants, and Their Transport in Roots: a Review

The essentiality of 14 mineral elements so far have been reported in plant nutrition. Eight of these elements were known as micronutrients due to their lower concentrations in plants (usually ≤100 mg/kg/dw). However, it is still challenging to mention an exact number of plant micronutrients since some elements have not been strictly proposed yet either as essential or beneficial. Micronutrients participate in very diverse metabolic processes, including from the primary and secondary metabolism to the cell defense, and from the signal transduction to the gene regulation, energy metabolism, and hormone perception. Thus, the attempt to understand the molecular mechanism(s) behind their transport has great importance in terms of basic and applied plant sciences. Moreover, their deficiency or toxicity also caused serious disease symptoms in plants, even plant destruction if not treated, and many people around the world suffer from the plant-based dietary deficiencies or metal toxicities. In this sense, shedding some light on this issue, the 13 mineral elements (Fe, B, Cu, Mn, Mo, Si, Zn, Ni, Cl, Se, Na, Al, and Co), required by plants at trace amounts, has been reviewed with the primary focus on the transport proteins (transporters/channels) in plant roots. So, providing the compiled but extensive information about the structural and functional roles of micronutrient transport genes/proteins in plant roots.

Shu1 is a cell-surface protein involved in iron acquisition from heme in Schizosaccharomyces pombe

Iron is an essential metal cofactor that is required for many biological processes. Eukaryotic cells have consequently developed different strategies for its acquisition. Until now, Schizosaccharomyces pombe was known to use reductive iron uptake and siderophore-bound iron transport to scavenge iron from the environment. Here, we report the identification of a gene designated shu1(+) that encodes a protein that enables S. pombe to take up extracellular heme for cell growth. When iron levels are low, the transcription of shu1(+) is induced, although its expression is repressed when iron levels rise. The iron-dependent down-regulation of shu1(+) requires the GATA-type transcriptional repressor Fep1, which strongly associates with a proximal promoter region of shu1(+) in vivo in response to iron repletion. HA4-tagged Shu1 localizes to the plasma membrane in cells expressing a functional shu1(+)-HA4 allele. When heme biosynthesis is selectively blocked in mutated S. pombe cells, their ability to acquire exogenous hemin or the fluorescent heme analog zinc mesoporphyrin IX is dependent on the expression of Shu1. Further analysis by absorbance spectroscopy and hemin-agarose pulldown assays showed that Shu1 interacts with hemin, with a KD of ∼2.2 μm. Taken together, results reported here revealed that S. pombe possesses an unexpected pathway for heme assimilation, which may also serve as a source of iron for cell growth.

Copper transport and regulation in Schizosaccharomyces pombe

The fission yeast Schizosaccharomyces pombe has been successfully used as a model to gain fundamental knowledge in understanding how eukaryotic cells acquire copper during vegetative growth. These studies have revealed the existence of a heteromeric Ctr4-Ctr5 plasma membrane complex that mediates uptake of copper within the cells. Furthermore, additional studies have led to the identification of one of the first vacuolar copper transporters, Ctr6, as well as the copper-responsive Cuf1 transcription factor. Recent investigations have extended the use of S. pombe to elucidate new roles for copper metabolism in meiotic differentiation. For example, these studies have led to the discovery of Mfc1, which turned out to be the first example of a meiosis-specific copper transporter. Whereas copper-dependent transcriptional regulation of the Ctr family members is under the control of Cuf1 during mitosis or meiosis, meiosis-specific copper transporter Mfc1 is regulated by the recently discovered transactivator Mca1. It is foreseeable that identification of novel meiotic copper-related proteins will serve as stepping stones to unravel fundamental aspects of copper homoeostasis.

Fra2 is a co-regulator of Fep1 inhibition in response to iron starvation

Iron is required for several metabolic functions involved in cellular growth. Although several players involved in iron transport have been identified, the mechanisms by which iron-responsive transcription factors are controlled are still poorly understood. In Schizosaccharomyces pombe, the Fep1 transcription factor represses genes involved in iron acquisition in response to high levels of iron. In contrast, when iron levels are low, Fep1 becomes inactive and loses its ability to associate with chromatin. Although the molecular basis by which Fep1 is inactivated under iron starvation remains unknown, this process requires the monothiol glutaredoxin Grx4. Here, we demonstrate that Fra2 plays a role in the negative regulation of Fep1 activity. Disruption of fra2⁺ (fra2Δ) led to a constitutive repression of the fio1⁺ gene transcription. Fep1 was consistently active and constitutively bound to its target gene promoters in cells lacking fra2⁺. A constitutive activation of Fep1 was also observed in a php4Δ fra2Δ double mutant strain in which the behavior of Fep1 is freed of its transcriptional regulation by Php4. Microscopic analyses of cells expressing a functional Fra2-Myc₁₃ protein revealed that Fra2 localized throughout the cells with a significant proportion of Fra2 being observed within the nuclei. Further analysis by coimmunoprecipitation showed that Fra2, Fep1 and Grx4 are associated in a heteroprotein complex. Bimolecular fluorescence complementation experiments brought further evidence that an interaction between Fep1 and Fra2 occurs in the nucleus. Taken together, results reported here revealed that Fra2 plays a role in the Grx4-mediated pathway that inactivates Fep1 in response to iron deficiency.

Characterization of Schizosaccharomyces pombe copper transporter proteins in meiotic and sporulating cells

Meiosis requires copper to undertake its program in which haploid gametes are produced from diploid precursor cells. In Schizosaccharomyces pombe, copper is transported by three members of the copper transporter (Ctr) family, namely Ctr4, Ctr5, and Ctr6. Although central for sexual differentiation, very little is known about the expression profile, cellular localization, and physiological contribution of the Ctr proteins during meiosis. Analysis of gene expression of ctr4(+) and ctr5(+) revealed that they are primarily expressed in early meiosis under low copper conditions. In the case of ctr6(+), its expression is broader, being detected throughout the entire meiotic process with an increase during middle- and late-phase meiosis. Whereas the expression of ctr4(+) and ctr5(+) is exclusively dependent on the presence of Cuf1, ctr6(+) gene expression relies on two distinct regulators, Cuf1 and Mei4. Ctr4 and Ctr5 proteins co-localize at the plasma membrane shortly after meiotic induction, whereas Ctr6 is located on the membrane of vacuoles. After meiotic divisions, Ctr4 and Ctr5 disappear from the cell surface, whereas Ctr6 undergoes an intracellular re-location to co-localize with the forespore membrane. Under copper-limiting conditions, disruption of ctr4(+) and ctr6(+) results in altered SOD1 activity, whereas these mutant cells exhibit substantially decreased levels of CAO activity mostly in early- and middle-phase meiosis. Collectively, these results emphasize the notion that Ctr proteins exhibit differential expression, localization, and contribution in delivering copper to SOD1 and Cao1 proteins during meiosis.

Cryptococcus neoformans copper detoxification machinery is critical for fungal virulence

Copper (Cu) is an essential metal that is toxic at high concentrations. Thus, pathogens often rely on host Cu for growth, but host cells can hyperaccumulate Cu to exert antimicrobial effects. The human fungal pathogen Cryptococcus neoformans encodes many Cu-responsive genes, but their role in infection is unclear. We determined that pulmonary C. neoformans infection results in Cu-specific induction of genes encoding the Cu-detoxifying metallothionein (Cmt) proteins. Mutant strains lacking CMTs or expressing Cmt variants defective in Cu-coordination exhibit severely attenuated virulence and reduced pulmonary colonization. Consistent with the upregulation of Cmt proteins, C. neoformans pulmonary infection results in increased serum Cu concentrations and increases and decreases alveolar macrophage expression of the Cu importer (Ctr1) and ATP7A, a transporter implicated in phagosomal Cu compartmentalization, respectively. These studies indicate that the host mobilizes Cu as an innate antifungal defense but C. neoformans senses and neutralizes toxic Cu to promote infection.

Transcriptional regulation of the copper transporter mfc1 in meiotic cells

Mfc1 is a meiosis-specific protein that mediates copper transport during the meiotic program in Schizosaccharomyces pombe. Although the mfc1(+) gene is induced at the transcriptional level in response to copper deprivation, the molecular determinants that are required for its copper starvation-dependent induction are unknown. Promoter deletion and site-directed mutagenesis have allowed identification of a new cis-regulatory element in the promoter region of the mfc1(+) gene. This cis-acting regulatory sequence containing the sequence TCGGCG is responsible for transcriptional activation of mfc1(+) under low-copper conditions. The TCGGCG sequence contains a CGG triplet known to serve as a binding site for members of the Zn(2)Cys(6) binuclear cluster transcriptional regulator family. In agreement with this fact, one member of this group of regulators, denoted Mca1, was found to be required for maximum induction of mfc1(+) gene expression. Analysis of Mca1 cellular distribution during meiosis revealed that it colocalizes with both chromosomes and sister chromatids during early, middle, and late phases of the meiotic program. Cells lacking Mca1 exhibited a meiotic arrest at metaphase I under low-copper conditions. Binding studies revealed that the N-terminal 150-residue segment of Mca1 expressed as a fusion protein in Escherichia coli specifically interacts with the TCGGCG sequence of the mfc1(+) promoter. Taken together, these results identify the cis-regulatory TCGGCG sequence and the transcription factor Mca1 as critical components for activation of the meiotic copper transport mfc1(+) gene in response to copper starvation.

Cuf2 is a novel meiosis-specific regulatory factor of meiosis maturation

Background

Meiosis is the specialized form of the cell cycle by which diploid cells produce the haploid gametes required for sexual reproduction. Initiation and progression through meiosis requires that the expression of the meiotic genes is precisely controlled so as to provide the correct gene products at the correct times. During meiosis, four temporal gene clusters are either induced or repressed by a cascade of transcription factors.

Principal Findings

In this report a novel copper-fist-type regulator, Cuf2, is shown to be expressed exclusively during meiosis. The expression profile of the cuf2⁺ mRNA revealed that it was induced during middle-phase meiosis. Both cuf2⁺ mRNA and protein levels are unregulated by copper addition or starvation. The transcription of cuf2⁺ required the presence of a functional mei4⁺ gene encoding a key transcription factor that activates the expression of numerous middle meiotic genes. Microscopic analyses of cells expressing a functional Cuf2-GFP protein revealed that Cuf2 co-localized with both homologous chromosomes and sister chromatids during the meiotic divisions. Cells lacking Cuf2 showed an elevated and sustained expression of several of the middle meiotic genes that persisted even during late meiosis. Moreover, cells carrying disrupted cuf2Δ/cuf2Δ alleles displayed an abnormal morphology of the forespore membranes and a dramatic reduction of spore viability.

Significance

Collectively, the results revealed that Cuf2 functions in the timely repression of the middle-phase genes during meiotic differentiation.

Mfc1 is a novel forespore membrane copper transporter in meiotic and sporulating cells

To gain insight in the molecular basis of copper homeostasis during meiosis, we have used DNA microarrays to analyze meiotic gene expression in the model yeast Schizosaccharomyces pombe. Profiling data identified a novel meiosis-specific gene, termed mfc1(+), that encodes a putative major facilitator superfamily-type transporter. Although Mfc1 does not exhibit any significant sequence homology with the copper permease Ctr4, it contains four putative copper-binding motifs that are typically found in members of the copper transporter family of copper transporters. Similarly to the ctr4(+) gene, the transcription of mfc1(+) was induced by low concentrations of copper. However, its temporal expression profile during meiosis was distinct to ctr4(+). Whereas Ctr4 was observed at the plasma membrane shortly after induction of meiosis, Mfc1 appeared later in precursor vesicles and, subsequently, at the forespore membrane of ascospores. Using the fluorescent copper-binding tracker Coppersensor-1 (CS1), labile cellular copper was primarily detected in the forespores in an mfc1(+)/mfc1(+) strain, whereas an mfc1Δ/mfc1Δ mutant exhibited an intracellular dispersed punctate distribution of labile copper ions. In addition, the copper amine oxidase Cao1, which localized primarily in the forespores of asci, was fully active in mfc1(+)/mfc1(+) cells, but its activity was drastically reduced in an mfc1Δ/mfc1Δ strain. Furthermore, our data showed that meiotic cells that express the mfc1(+) gene have a distinct developmental advantage over mfc1Δ/mfc1Δ mutant cells when copper is limiting. Taken together, the data reveal that Mfc1 serves to transport copper for accurate and timely meiotic differentiation under copper-limiting conditions.

The copper regulon of the human fungal pathogen Cryptococcus neoformans H99

Cryptococcus neoformans is a human fungal pathogen that is the causative agent of cryptococcosis and fatal meningitis in immuno-compromised hosts. Recent studies suggest that copper (Cu) acquisition plays an important role in C. neoformans virulence, as mutants that lack Cuf1, which activates the Ctr4 high affinity Cu importer, are hypo-virulent in mouse models. To understand the constellation of Cu-responsive genes in C. neoformans and how their expression might contribute to virulence, we determined the transcript profile of C. neoformans in response to elevated Cu or Cu deficiency. We identified two metallothionein genes (CMT1 and CMT2), encoding cysteine-rich Cu binding and detoxifying proteins, whose expression is dramatically elevated in response to excess Cu. We identified a new C. neoformans Cu transporter, CnCtr1, that is induced by Cu deficiency and is distinct from CnCtr4 and which shows significant phylogenetic relationship to Ctr1 from other fungi. Surprisingly, in contrast to other fungi, we found that induction of both CnCTR1 and CnCTR4 expression under Cu limitation, and CMT1 and CMT2 in response to Cu excess, are dependent on the CnCuf1 Cu metalloregulatory transcription factor. These studies set the stage for the evaluation of the specific Cuf1 target genes required for virulence in C. neoformans.

Molecular and functional analyses of COPT/Ctr-type copper transporter-like gene family in rice

BACKGROUND

The copper (Cu) transporter (COPT/Ctr) gene family has an important role in the maintenance of Cu homeostasis in different species. The rice COPT-type gene family consists of seven members (COPT1 to COPT7). However, only two, COPT1 and COPT5, have been characterized for their functions in Cu transport.

RESULTS

Here we report the molecular and functional characterization of the other five members of the rice COPT gene family (COPT2, COPT3, COPT4, COPT6, and COPT7). All members of the rice COPT family have the conserved features of known COPT/Ctr-type Cu transporter genes. Among the proteins encoded by rice COPTs, COPT2, COPT3, and COPT4 physically interacted with COPT6, respectively, except for the known interaction between COPT1 and COPT5. COPT2, COPT3, or COPT4 cooperating with COPT6 mediated a high-affinity Cu uptake in the yeast Saccharomyces cerevisiae mutant that lacked the functions of ScCtr1 and ScCtr3 for Cu uptake. COPT7 alone could mediate a high-affinity Cu uptake in the yeast mutant. None of the seven COPTs alone or in cooperation could complement the phenotypes of S. cerevisiae mutants that lacked the transporter genes either for iron uptake or for zinc uptake. However, these COPT genes, which showed different tissue-specific expression patterns and Cu level-regulated expression patterns, were also transcriptionally influenced by deficiency of iron, manganese, or zinc.

CONCLUSION

These results suggest that COPT2, COPT3, and COPT4 may cooperate with COPT6, respectively, and COPT7 acts alone for Cu transport in different rice tissues. The endogenous concentrations of iron, manganese, or zinc may influence Cu homeostasis by influencing the expression of COPTs in rice.

Grx4 monothiol glutaredoxin is required for iron limitation-dependent inhibition of Fep1

The expression of iron transport genes in Schizosaccharomyces pombe is controlled by the Fep1 transcription factor. When iron levels exceed those needed by the cells, Fep1 represses iron transport genes. In contrast, Fep1 is unable to bind chromatin under low-iron conditions, and that results in activation of genes involved in iron acquisition. Studies of fungi have revealed that monothiol glutaredoxins are required to inhibit iron-dependent transcription factors in response to high levels of iron. Here, we show that the monothiol glutaredoxin Grx4 plays an important role in the negative regulation of Fep1 activity in response to iron deficiency. Deletion of the grx4(+) gene led to constitutive promoter occupancy by Fep1 and caused an invariable repression of iron transport genes. We found that Grx4 and Fep1 physically interact with each other. Grx4 contains an N-terminal thioredoxin (TRX)-like domain and a C-terminal glutaredoxin (GRX)-like domain. Deletion mapping analysis revealed that the TRX domain interacts strongly and constitutively with the C-terminal region of Fep1. As opposed to the TRX domain, the GRX domain associates weakly and in an iron-dependent manner with the N-terminal region of Fep1. Further analysis showed that Cys35 of Grx4 is required for the interaction between the Fep1 C terminus and the TRX domain, whereas Grx4 Cys172 is necessary for the association between the Fep1 N terminus and the GRX domain. Our results describe the first example of a monothiol glutaredoxin that acts as an inhibitory partner for an iron-regulated transcription factor under conditions of low iron levels.

Advances in the understanding of mammalian copper transporters

Copper (Cu) is an essential micronutrient. Its ability to exist in 2 oxidation states (Cu(1+) and Cu(2+)) allows it to function as an enzymatic cofactor in hydrolytic, electron transfer, and oxygen utilization reactions. Cu transporters CTR1, ATP7A, and ATP7B play key roles in ensuring that adequate Cu is available for Cu-requiring processes and the prevention of excess Cu accumulation within cells. Two diseases of Cu metabolism, Menkes disease and Wilson disease, which are caused by mutations in ATP7A and ATP7B, respectively, exemplify the critical importance of regulating Cu balance in humans. Herein, we review recent studies of the biochemical and cell biological characteristics of CTR1, ATP7A, and ATP7B, as well as emerging roles for Cu in new areas of physiology.

Dissection of the relative contribution of the Schizosaccharomyces pombe Ctr4 and Ctr5 proteins to the copper transport and cell surface delivery functions

The Ctr1 family of proteins mediates high-affinity copper (Cu) acquisition in eukaryotic organisms. In the fission yeast Schizosaccharomyces pombe, Cu uptake is carried out by a heteromeric complex formed by the Ctr4 and Ctr5 proteins. Unlike human and Saccharomyces cerevisiae Ctr1 proteins, Ctr4 and Ctr5 are unable to function independently in Cu acquisition. Instead, both proteins physically interact with each other to form a Ctr4-Ctr5 heteromeric complex, and are interdependent for secretion to the plasma membrane and Cu transport activity. In this study, we used S. cerevisiae mutants that are defective in high-affinity Cu uptake to dissect the relative contribution of Ctr4 and Ctr5 to the Cu transport function. Functional complementation and localization assays show that the conserved Met-X(3)-Met motif in transmembrane domain 2 of the Ctr5 protein is dispensable for the functionality of the Ctr4-Ctr5 complex, whereas the Met-X(3)-Met motif in the Ctr4 protein is essential for function and for localization of the hetero-complex to the plasma membrane. Moreover, Ctr4/Ctr5 chimeric proteins reveal unique properties found either in Ctr4 or in Ctr5, and are sufficient for Cu uptake on the cell surface of Sch. pombe cells. Functional chimeras contain the Ctr4 central and Ctr5 carboxyl-terminal domains (CTDs). We propose that the Ctr4 central domain mediates Cu transport in this hetero-complex, whereas the Ctr5 CTD functions in the regulation of trafficking of the Cu transport complex to the cell surface.

The bacterial pathogen Xanthomonas oryzae overcomes rice defenses by regulating host copper redistribution

Pathogen effectors are virulence factors causing plant diseases. How the host targets of these effectors facilitate pathogen infection is largely unknown. An effector of Xanthomonas oryzae pv oryzae (Xoo) transcriptionally activates rice (Oryza sativa) susceptibility gene Xa13 to cause bacterial blight disease. Xa13 encodes an indispensable plasma membrane protein of the MtN3/saliva family, which is prevalent in eukaryotes with unknown biochemical function. We show that the XA13 protein cooperates with two other proteins, COPT1 and COPT5, to promote removal of copper from xylem vessels, where Xoo multiplies and spreads to cause disease. Copper, an essential micronutrient of plants and an important element for a number of pesticides in agriculture, suppresses Xoo growth. Xoo strain PXO99 is more sensitive to copper than other strains; its infection of rice is associated with activation of XA13, COPT1, and COPT5, which modulate copper redistribution in rice. The involvement of XA13 in copper redistribution has led us to propose a mechanism of bacterial virulence.

Copper-dependent trafficking of the Ctr4-Ctr5 copper transporting complex

BACKGROUND

In Schizosaccharomyces pombe, copper uptake is carried out by a heteromeric complex formed by the Ctr4 and Ctr5 proteins. Copper-induced differential subcellular localization may play a critical role with respect to fine tuning the number of Ctr4 and Ctr5 molecules at the cell surface.

METHODOLOGY/PRINCIPAL FINDINGS

We have developed a bimolecular fluorescence complementation (BiFC) assay to analyze protein-protein interactions in vivo in S. pombe. The assay is based on the observation that N- and C-terminal subfragments of the Venus fluorescent protein can reconstitute a functional fluorophore only when they are brought into tight contact. Wild-type copies of the ctr4(+) and ctr5(+) genes were inserted downstream of and in-frame with the nonfluorescent C-terminal (VC) and N-terminal (VN) coding fragments of Venus, respectively. Co-expression of Ctr4-VC and Ctr5-VN fusion proteins allowed their detection at the plasma membrane of copper-limited cells. Similarly, cells co-expressing Ctr4-VN and Ctr4-VC in the presence of Ctr5-Myc(12) displayed a fluorescence signal at the plasma membrane. In contrast, Ctr5-VN and Ctr5-VC co-expressed in the presence of Ctr4-Flag(2) failed to be visualized at the plasma membrane, suggesting a requirement for a combination of two Ctr4 molecules with one Ctr5 molecule. We found that plasma membrane-located Ctr4-VC-Ctr5-VN fluorescent complexes were internalized when the cells were exposed to high levels of copper. The copper-induced internalization of Ctr4-VC-Ctr5-VN complexes was not dependent on de novo protein synthesis. When cells were transferred back from high to low copper levels, there was reappearance of the BiFC fluorescent signal at the plasma membrane.

SIGNIFICANCE

These findings reveal a copper-dependent internalization and recycling of the heteromeric Ctr4-Ctr5 complex as a function of copper availability.

Simon Labbé's work on iron and copper homeostasis

Iron and copper have a wealth of functions in biological systems, which makes them essential micronutrients for all living organisms. Defects in iron and copper homeostasis are directly responsible for diseases, and have been linked to impaired development, metabolic syndromes and fungal virulence. Consequently, it is crucial to gain a comprehensive understanding of the molecular bases of iron- and copper-dependent proteins in living systems. Simon Labbé maintains parallel programs on iron and copper homeostasis using the fission yeast Schizosaccharomyces pombe (Schiz. pombe) as a model system. The study of fission yeast transition-metal metabolism has been successful, not only in discerning the genes and pathways functioning in Schiz. pombe, but also the genes and pathways that are active in mammalian systems and for other fungi.

Tryptophan scanning analysis of the membrane domain of CTR-copper transporters

Membrane proteins of the CTR family mediate cellular copper uptake in all eukaryotic cells and have been shown to participate in uptake of platinum-based anticancer drugs. Despite their importance for life and the clinical treatment of malignancies, directed biochemical studies of CTR proteins have been difficult because high-resolution structural information is missing. Building on our recent 7A structure of the human copper transporter hCTR1, we present the results of an extensive tryptophan-scanning analysis of hCTR1 and its distant relative, yeast CTR3. The comparative analysis supports our previous assignment of the transmembrane helices and shows that most functionally and structurally important residues are clustered around the threefold axis of CTR trimers or engage in helix packing interactions. The scan also identified residues that may play roles in interactions between CTR trimers and suggested that the first transmembrane helix serves as an adaptor that allows evolutionarily diverse CTRs to adopt the same overall structure. Together with previous biochemical and biophysical data, the results of the tryptophan scan are consistent with a mechanistic model in which copper transport occurs along the center of the trimer.

Regulation of copper transport in Arabidopsis thaliana: a biochemical oscillator?

Plants are among the most versatile higher eukaryotes in accommodating environmental copper availability to largely variable demands. In particular, copper deficiency in soils is a threat for plant survival since it mostly affects reproductive structures. One of the strategies that plant cells use to overcome this situation is to increase copper levels by expressing high-affinity copper transporters delivering the metal to the cytosol. In this minireview, we discuss recent advances in the structure, function, and regulation of the CTR/COPT family of copper transporters, and pay special attention to the Arabidopsis thaliana counterparts. These are constituted by transmembrane polypeptides, containing several copper-binding sequences of functional and/or regulatory value, and assembling as trimers. Copper deficiency activates the expression of some members of the COPT family via the interaction of the SPL7 transcription factor with reiterative GTAC motifs present in their promoters. Interestingly, the regulation of the synthesis of these transporters by copper itself constitutes a negative-feedback loop that could cause a sustained oscillation in the cytosolic copper levels. We analyze the theoretical conditions required for this hypothetical copper oscillation and the potential advantages of synchronization with other cycles. Diverse data in other organisms point to the relationship between copper homeostasis and circadian cycles.

Copper distributed by Atx1 is available to copper amine oxidase 1 in Schizosaccharomyces pombe

Copper amine oxidases (CAOs) have been proposed to be involved in the metabolism of xenobiotic and biogenic amines. The requirement for copper is absolute for their activity. In the fission yeast Schizosaccharomyces pombe, cao1(+) and cao2(+) genes are predicted to encode members of the CAO family. While both genes are expressed in wild-type cells, we determined that the expression of only cao1(+) but not cao2(+) results in the production of an active enzyme. Site-directed mutagenesis identified three histidine residues within the C-terminal region of Cao1 that are necessary for amine oxidase activity. By use of a cao1(+)-GFP allele that retained wild-type function, Cao1-GFP was localized in the cytosol (GFP is green fluorescent protein). Under copper-limiting conditions, disruption of ctr4(+), ctr5(+), and cuf1(+) produced a defect in amine oxidase activity, indicating that a functionally active Cao1 requires Ctr4/5-mediated copper transport and the transcription factor Cuf1. Likewise, atx1 null cells exhibited substantially decreased levels of amine oxidase activity. In contrast, deletion of ccc2, cox17, and pccs had no significant effect on Cao1 activity. Residual amine oxidase activity in cells lacking atx1(+) can be restored to normal levels by returning an atx1(+) allele, underscoring the critical importance of the presence of Atx1 in cells. Using two-hybrid analysis, we demonstrated that Cao1 physically interacts with Atx1 and that this association is comparable to that of Atx1 with the N-terminal region of Ccc2. Collectively, these results describe the first example of the ability of Atx1 to act as a copper carrier for a molecule other than Ccc2 and its critical role in delivering copper to Cao1.

[The molecular bases for copper uptake and distribution: lessons from yeast]

Copper exists in two oxidation states, cuprous (Cu1+) and cupric (Cu2+), which, respectively, can donate or accept electrons. The fact that copper has two readily interconvertible redox states makes it a catalytic co-factor for many important enzymes. Over the past years, work in a number of laboratories has clearly demonstrated that studies in yeast have served as a springboard for identifying cellular components and processes involved in copper uptake and distribution. In several cases, it has been shown that mammalian proteins are capable of functionally replacing yeast proteins, thereby revealing their remarkable functional conservation. For high-affinity copper transport into cells, it has been shown that copper transporters of the Ctr family are required. Upon entering the cell, copper is partitioned to different proteins and into different compartments within the cell. Given the potential toxicity of copper, specialized proteins bind copper after it enters the cell and subsequently donate the bound copper to their corresponding recipient proteins. Three copper-binding proteins, Ccs1, Cox17, and Atx1, have been identified that serve as "copper chaperones" to deliver copper. double dagger.

Crm1-mediated nuclear export of the Schizosaccharomyces pombe transcription factor Cuf1 during a shift from low to high copper concentrations

In this study, we examine the fate of the nuclear pool of the Schizosaccharomyces pombe transcription factor Cuf1 in response to variations in copper levels. A nuclear pool of Cuf1-green fluorescent protein (GFP) was generated by expressing a functional cuf1(+)-GFP allele in the presence of a copper chelator. We then extinguished cuf1(+)-GFP expression and tracked the changes in the localization of the nuclear pool of Cuf1-GFP in the presence of low or high copper concentrations. Treating cells with copper as well as silver ions resulted in the nuclear export of Cuf1. We identified a leucine-rich nuclear export signal (NES), (349)LAALNHISAL(358), within the C-terminal region of Cuf1. Mutations in this sequence abrogated Cuf1 export from the nucleus. Furthermore, amino acid substitutions that impair Cuf1 NES function resulted in increased target gene expression and a concomitant cellular hypersensitivity to copper. Export of the wild-type Cuf1 protein was inhibited by leptomycin B (LMB), a specific inhibitor of the nuclear export protein Crm1. We further show that cells expressing a temperature-sensitive mutation in crm1(+) exhibit increased nuclear accumulation of Cuf1 at the nonpermissive temperature. Although wild-type Cuf1 is localized in the nucleus in both conditions, we observed that the protein can still be inactivated by copper, resulting in the repression of ctr4(+) gene expression in the presence of exogenous copper. These results demonstrate that nuclear accumulation of Cuf1 per se is not sufficient to cause the unregulated expression of the copper transport genes like ctr4(+). In addition to nuclear localization, a functional Cys-rich domain or NES element in Cuf1 is required to appropriately regulate copper transport gene expression in response to changes in intracellular copper concentration.

A structural perspective on copper uptake in eukaryotes

Over a decade ago, genetic studies identified a family of small integral membrane proteins, commonly referred to as copper transporters (CTRs) that are both required and sufficient for cellular copper uptake in a yeast genetic complementation assay. We recently used electron crystallography to determine a projection density map of the human high affinity transporter hCTR1 embedded into a lipid bilayer. At 6 A resolution, this first glimpse of the structure revealed that hCTR1 is trimeric and possesses the type of radial symmetry that traditionally has been associated with the structure of certain ion channels such as potassium or gap junction channels. Representative for this particular type of architecture, a region of low protein density at the center of the trimer is consistent with the existence of a copper permeable pore along the center three-fold axis of the trimer. In this contribution, we will briefly discuss how recent structure-function studies correlate with the projection density map, and provide a perspective with respect to the cellular uptake of other transition metals.

Mechanisms of copper loading on the Schizosaccharomyces pombe copper amine oxidase 1 expressed in Saccharomyces cerevisiae

Copper amine oxidases (CAOs) are found in almost every living kingdom. Although Saccharomyces cerevisiae is one of the few yeast species that lacks an endogenous CAO, heterologous gene expression of CAOs from other organisms produces a functional enzyme. To begin to characterize their function and mechanisms of copper acquisition, two putative cao(+) genes from Schizosaccharomyces pombe were expressed in S. cerevisiae. Expression of spao1(+) resulted in the production of an active enzyme capable of catalysing the oxidative deamination of primary amines. On the other hand, expression of spao2(+) failed to produce an active CAO. Using a functional spao1(+)-GFP fusion allele, the SPAO1 protein was localized in the cytosol. Under copper-limiting conditions, yeast cells harbouring deletions of the MAC1, CTR1 and CTR3 genes were defective in amine oxidase activity. Likewise, atx1Delta null cells exhibited no CAO activity, while ccc2Delta mutant cells exhibited decreased levels of amine oxidase activity, and mutations in cox17Delta and ccs1Delta did not cause any defects in this activity. Copper-deprived S. cerevisiae cells expressing spao1(+) required a functional atx1(+) gene for growth on minimal medium containing ethylamine as the sole nitrogen source. Under these conditions, the inability of the atx1Delta cells to utilize ethylamine correlated with the lack of SPAO1 activity, in spite of the efficient expression of the protein. Cells carrying a disrupted ccc2Delta allele exhibited only weak growth on ethylamine medium containing a copper chelator. The results of these studies reveal that expression of the heterologous spao1(+) gene in S. cerevisiae is required for its growth in medium containing ethylamine as the sole nitrogen source, and that expression of an active Schiz. pombe SPAO1 protein in S. cerevisiae depends on the acquisition of copper through the high-affinity copper transporters Ctr1 and Ctr3, and the copper chaperone Atx1.