Molecular mechanisms of copper uptake and distribution

Solution structure of Sco1: a thioredoxin-like protein Involved in cytochrome c oxidase assembly

Sco1, a protein required for the proper assembly of cytochrome c oxidase, has a soluble domain anchored to the cytoplasmic membrane through a single transmembrane segment. The solution structure of the soluble part of apoSco1 from Bacillus subtilis has been solved by NMR and the internal mobility characterized. Its fold places Sco1 in a distinct subgroup of the functionally unrelated thioredoxin proteins. In vitro Sco1 binds copper(I) through a CXXXCP motif and possibly His 135 and copper(II) in two different species, thus suggesting that copper(II) is adventitious more than physiological. The Sco1 structure represents the first structure of this class of proteins, present in a variety of eukaryotic and bacterial organisms, and elucidates a link between copper trafficking proteins and thioredoxins. The availability of the structure has allowed us to model the homologs Sco1 and Sco2 from S. cerevisiae and to discuss the physiological role of the Sco family.

The Schizosaccharomyces pombe Pccs protein functions in both copper trafficking and metal detoxification pathways

Because copper is both an essential cofactor and a toxic metal, different strategies have evolved to appropriately regulate its homeostasis as a function of changing environmental copper levels. In this report, we describe a metallochaperone-like protein from Schizosaccharomyces pombe that maintains the delicate balance between essentiality and toxicity. This protein, designated Pccs, has four distinct domains. SOD activity assays reveal that the first three domains of Pccs are necessary and sufficient to deliver copper to its target, copper-zinc superoxide dismutase (SOD1). Pccs domain IV, which is absent in Saccharomyces cerevisiae CCS1, contains seventeen cysteine residues, eight pairs of which are in a potential metal coordination arrangement, Cys-Cys. We show that S. cerevisiae ace1Delta mutant cells expressing the full-length Pccs molecule are resistant to copper toxicity. Furthermore, we demonstrate that the Pccs domain IV enhances copper resistance of the ace1Delta cells by an order of magnitude compared with that observed in the same strain expressing a pccs+ I-II-III allele encoding Pccs domains I-III. We consistently found that S. pombe cells disrupted in the pccs+ gene exhibit an increased sensitivity to copper and cadmium. Furthermore, we demonstrate that overexpression of pccs+ is associated with increased copper resistance in fission yeast cells. Taken together, our findings suggest that Pccs activates apo-SOD1 under copper-limiting conditions through the use of its first three domains and protects cells against metal ion toxicity via its fourth domain.

Cell Surface Expression of the Prion Protein in Yeast Does Not Alter Copper Utilization Phenotypes

Prion diseases are fatal neurodegenerative disorders that result from conversion of a normal, cell surface glycoprotein (PrP(C)) into a conformationally altered isoform (PrP(Sc)) that is thought to be infectious. Although a great deal is known about the role of PrP(Sc) in the disease process, the physiological function of PrP(C) has remained enigmatic. In this report, we have used the yeast Saccharomyces cerevisiae to test one hypothesized function of PrP(C), as a receptor for the uptake or efflux of copper ions. We first modified the PrP signal peptide by replacing its hydrophobic core with the signal sequence from the yeast protein dipeptidyl aminopeptidase B, so that the resulting protein was targeted cotranslationally to the secretory pathway when synthesized in yeast. PrP molecules with the modified signal peptide were efficiently glycosylated, glycolipid-anchored, and localized to the plasma membrane. We then tested whether PrP expression altered the growth deficiency phenotypes of yeast strains harboring deletions in genes that encode key components of copper utilization pathways, including transporters, chaperones, pumps, reductases, and cuproenzymes. We found that PrP did not rescue any of these mutant phenotypes, arguing against a direct role for the protein in copper utilization. Our results provide further clarification of the physiological function of PrP(C), and lay the groundwork for using PrP-expressing yeast to study other aspects of prion biology.

From aging to virulence: forging connections through the study of copper homeostasis in eukaryotic microorganisms

Recent years have witnessed an explosion in the breadth of investigations on transition metal homeostasis and the subsequent depth of our understanding of metals in biology. Many genes and proteins that serve in the uptake, distribution, sensing and detoxification of one such transition metal, copper, have been identified. Through genetic and biochemical studies, the molecular details of copper uptake are being elucidated, and evidence suggests a largely conserved mechanism for copper acquisition and distribution from yeast to humans. Investigations of the mitochondrial copper pathway reveal the complexity surrounding copper delivery to cytochrome oxidase and highlight additional roles for some of the participants in copper homeostasis, such as a copper chaperone that influences the subcellular distribution of its target for copper incorporation. Furthermore, our understanding of the structure and function of copper transporters, chaperones and cupro-proteins, coupled with the emergence of additional model systems, is providing surprising examples of the integration of copper homeostasis with other physiological and pathophysiological processes and states, such as cancer, aging and virulence.

Solution Structures of a Cyanobacterial Metallochaperone

The Atx1 copper metallochaperone from Synechocystis PCC 6803, ScAtx1, interacts with two P(1)-type copper ATPases to supply copper proteins within intracellular compartments, avoiding ATPases for other metals en route. Here we report NMR-derived solution structures for ScAtx1. The monomeric apo form has a betaalphabetabetaalpha fold with backbone motions largely restricted to loop 1 containing Cys-12 and Cys-15. The tumbling rate of Cu(I)ScAtx1 (0.1-0.8 mm) implies dimers. Experimental restraints are satisfied by symmetrical dimers with Cys-12 or His-61, but not Cys-15, invading the copper site of the opposing subunit. A full sequence of copper ligands from the cell surface to thylakoid compartments is proposed, considering in vitro homodimer liganding to mimic in vivo liganding in ScAtx1-ATPase heterodimers. A monomeric high resolution structure for Cu(I)ScAtx1, with Cys-12, Cys-15, and His-61 as ligands, is calculated without violations despite the rotational correlation time. (2)J(NH) couplings in the imidazole ring of His-61 establish coordination of N(epsilon2) to copper. His-61 is analogous to Lys-65 in eukaryotic metallochaperones, stabilizing Cu(I)S(2) complexes but by binding Cu(I) rather than compensating charge. Cys-Cys-His ligand sets are an emergent theme in some copper metallochaperones, although not in related Atx1, CopZ, or Hah1. Surface charge (Glu-13) close to the metal-binding site of ScAtx1 is likely to support interaction with complementary surfaces of copper-transporting ATPases (PacS-Arg-11 and CtaA-Lys-14) but to discourage interaction with zinc ATPase ZiaA and so inhibit aberrant formation of copper-ZiaA complexes.

Copper transport across pea thylakoid membranes

The initial rate of Cu2+ movement across the thylakoid membrane of pea (Pisum sativum) chloroplasts was directly measured by stopped-flow spectrofluorometry using membranes loaded with the Cu(2+)-sensitive fluorophore Phen Green SK. Cu2+ transport was rapid, reaching completion within 0.5 s. The initial rate of uptake was dependent upon Cu2+ concentration and saturated at about 0.6 microm total Cu2+. Cu2+ uptake was maximal at a thylakoid lumen pH of 7.0. Cu2+ transport was inhibited by Zn2+ but was largely unaffected by Mn2+ and Cu+. Zn2+ inhibited Cu2+ transport to a maximum of 60%, indicating that there may be more than one transporter for copper in pea thylakoid membranes.

Yeast contain a non-proteinaceous pool of copper in the mitochondrial matrix

The yeast mitochondrion is shown to contain a pool of copper that is distinct from that associated with the two known mitochondrial cuproenzymes, superoxide dismutase (Sod1) and cytochrome c oxidase (CcO) and the copper-binding CcO assembly proteins Cox11, Cox17, and Sco1. Only a small fraction of mitochondrial copper is associated with these cuproproteins. The bulk of the remainder is localized within the matrix as a soluble, anionic, low molecular weight complex. The identity of the matrix copper ligand is unknown, but the bulk of the matrix copper fraction is not protein-bound. The mitochondrial copper pool is dynamic, responding to changes in the cytosolic copper level. The addition of copper salts to the growth medium leads to an increase in mitochondrial copper, yet the expansion of this matrix pool does not induce any respiration defects. The matrix copper pool is accessible to a heterologous cuproenzyme. Co-localization of human Sod1 and the metallochaperone CCS within the mitochondrial matrix results in suppression of growth defects of sod2Delta cells. However, in the absence of CCS within the matrix, the activation of human Sod1 can be achieved by the addition of copper salts to the growth medium.

Identification of Methionine-rich Clusters That Regulate Copper-stimulated Endocytosis of the Human Ctr1 Copper Transporter

Copper uptake and subsequent delivery to copper-dependent enzymes are essential for many cellular processes. However, the intracellular levels of this nutrient must be controlled because of its potential toxicity. The hCtr1 protein functions in high affinity copper uptake at the plasma membrane of human cells. Recent studies have shown that elevated copper stimulates the endocytosis and degradation of the hCtr1 protein, and this response is likely an important homeostatic mechanism that prevents the overaccumulation of copper. The domains of hCtr1 involved in copper-stimulated endocytosis and degradation are unknown. In this study we examined the importance of potential copper-binding sequences in the extracellular domain and a conserved transmembrane (150)MXXXM(154) motif for copper-stimulated endocytosis and degradation of hCtr1. The endocytic response of hCtr1 to low copper concentrations required an amino-terminal methionine cluster ((40)MMMMPM(45)) closest to the transmembrane region. However, this cluster was not required for the endocytic response to higher copper levels, suggesting this motif may function as a high affinity copper-sensing domain. Moreover, the transmembrane (150)MXXXM(154) motif was absolutely required for copper-stimulated endocytosis and degradation of hCtr1 even under high copper concentrations. Together with previous studies demonstrating a role for these motifs in high affinity copper transport activity, our findings suggest common biochemical mechanisms regulate both transport and trafficking functions of hCtr1.

C-Terminal Domain of the Membrane Copper Transporter Ctr1 from Saccharomyces cerevisiae Binds Four Cu(I) Ions as a Cuprous-Thiolate Polynuclear Cluster: Sub-femtomolar Cu(I) Affinity of Three Proteins Involved in Copper Trafficking

The cytosolic C-terminal domain of the membrane copper transporter Ctr1 from the yeast Saccharomyces cerevisiae, Ctr1c, was expressed in E. coli as an oxygen-sensitive soluble protein with no significant secondary structure. Visible-UV spectroscopy demonstrated that Ctr1c bound four Cu(I) ions, structurally identified as a Cu(I)(4)(micro-S-Cys)(6) cluster by Xray absorption spectroscopy. This was the only metalated form detected by electrospray ionization mass spectrometry. An average dissociation constant K(D) = (K(1)K(2)K(3)K(4))(1/4) = 10(-)(19) for binding of Cu(I) to Ctr1c was estimated via competition with the ligand bathocuproine disulfonate bcs (beta(2) = 10(19.8)). Equivalent experiments for the yeast chaperone Atx1 and an N-terminal domain of the yeast Golgi pump Ccc2, which both bind a single Cu(I) ion, provided similar K(D) values. The estimates of K(D) were supported by independent estimates of the equilibrium constants K(ex) for exchange of Cu(I) between pairs of these three proteins. It is apparent that, in vitro, the three proteins buffer "free" Cu(I) concentrations in a narrow range around 10(-)(19) M. The results provide quantitative support for the proposals that, in yeast, (a) "free" copper concentrations are very low in the cytosol and (b) the Cu(I) trafficking gradient is shallow along the putative Ctrlc --> Atx1 --> Ccc2n metabolic pathway. In addition, both Ctr1c and its copper-responsive transcription factor Mac1 contain similar clusters which may be important in signaling copper status in yeast.

The Arabidopsis copper transporter COPT1 functions in root elongation and pollen development

Copper plays a dual role in aerobic organisms, as both an essential and a potentially toxic element. To ensure copper availability while avoiding its toxic effects, organisms have developed complex homeostatic networks to control copper uptake, distribution, and utilization. In eukaryotes, including yeasts and mammals, high affinity copper uptake is mediated by the Ctr family of copper transporters. This work is the first report on the physiological function of copper transport in Arabidopsis thaliana. We have studied the expression pattern of COPT1 in transgenic plants expressing a reporter gene under the control of the COPT1 promoter. The reporter gene is highly expressed in embryos, trichomes, stomata, pollen, and root tips. The involvement of COPT1 in copper acquisition was investigated in CaMV35S::COPT1 antisense transgenic plants. Consistent with a decrease in COPT1 expression and the associated copper deprivation, these plants exhibit increased mRNA levels of genes that are down-regulated by copper, decreased rates of (64)Cu uptake by seedlings and reduced steady state levels of copper as measured by atomic absorption spectroscopy in mature leaves. Interestingly, COPT1 antisense plants also display dramatically increased root length, which is completely and specifically reversed by copper addition, and an increased sensitivity to growth inhibition by the copper-specific chelator bathocuproine disulfonic acid. Furthermore, COPT1 antisense plants exhibit pollen development defects that are specifically reversed by copper. Taken together, these studies reveal striking plant growth and development roles for copper acquisition by high affinity copper transporters.

A novel role for XIAP in copper homeostasis through regulation of MURR1

XIAP is a potent suppressor of apoptosis that directly inhibits specific members of the caspase family of cysteine proteases. Here we demonstrate a novel role for XIAP in the control of intracellular copper levels. XIAP was found to interact with MURR1, a factor recently implicated in copper homeostasis. XIAP binds to MURR1 in a manner that is distinct from that utilized by XIAP to bind caspases, and consistent with this, MURR1 did not affect the antiapoptotic properties of XIAP. However, cells and tissues derived from Xiap-deficient mice were found to contain reduced copper levels, while suppression of MURR1 resulted in increased intracellular copper in cultured cells. Consistent with these opposing effects, XIAP was observed to negatively regulate MURR1 protein levels by the formation of K48 polyubiquitin chains on MURR1 that promote its degradation. These findings represent the first described phenotypic alteration in Xiap-deficient mice and demonstrate that XIAP can function through MURR1 to regulate copper homeostasis.

Structural Basis for the Function of the N-terminal Domain of the ATPase CopA from Bacillus subtilis

The solution structure of the N-terminal region (151 amino acids) of a copper ATPase, CopA, from Bacillus subtilis, is reported here. It consists of two domains, CopAa and CopAb, linked by two amino acids. It is found that the two domains, which had already been separately characterized, interact one to the other through a hydrogen bond network and a few hydrophobic interactions, forming a single rigid body. The two metal binding sites are far from one another, and the short link between the domains prevents them from interacting. This and the surface electrostatic potential suggest that each domain receives copper from the copper chaperone, CopZ, independently and transfers it to the membrane binding site of CopA. The affinity constants of silver(I) and copper(I) are similar for the two sites as monitored by NMR. Because the present construct "domain-short link-domain" is shared also by the last two domains of the eukaryotic copper ATPases and several residues at the interface between the two domains are conserved, the conclusions of the present study have general validity for the understanding of the function of copper ATPases.

A copper-regulated transporter required for copper acquisition, pigmentation, and specific stages of development in Drosophila melanogaster

The trace element copper is required for normal growth and development, serving as an essential catalytic co-factor for enzymes involved in energy generation, oxidative stress protection, neuropeptide maturation, and other fundamental processes. In yeast and mammals copper acquisition occurs through the action of the Ctr1 family of high affinity copper transporters. Here we describe studies using Drosophila melanogaster to investigate the role of copper acquisition through Ctr1 in normal growth and development. Three distinct Drosophila Ctr1 genes (Ctr1A, Ctr1B, and Ctr1C) have been identified, which have unique expression patterns over the course of development. Interestingly, Ctr1B, which is expressed exclusively during the late embryonic and larval stages of development, is transcriptionally activated in response to nutritionally induced copper deprivation and down-regulated in response to copper adequacy. The generation of Ctr1B mutant flies results in decreased larval copper accumulation, marked body pigmentation defects that parallel defects in tyrosinase activity, and specific developmental arrest under conditions of both nutritional copper limitation and excess. These studies establish that copper acquisition through the Drosophila Ctr1B transporter is crucial for normal growth and in early and specific stages of metazoan development.

Cuprous oxidase activity of yeast Fet3p and human ceruloplasmin: implication for function

The Fet3 protein in Saccharomyces cerevisiae and mammalian ceruloplasmin are multicopper oxidases (MCO) that are required for iron homeostasis via their catalysis of the ferroxidase reaction, 4Fe(2+)+O(2)+4H(+)-->4Fe(3+)+2H(2)O. The enzymes may play an essential role in copper homeostasis since they exhibit a strikingly similar kinetic activity towards Cu(1+) as substrate. In contrast, laccase, an MCO that exhibits weak activity towards Fe(2+), exhibits a similarly weak activity towards Cu(1+). Kinetic analyses of the Fet3p reaction demonstrate that the ferroxidase and cuprous oxidase activities are due to the same electron transfer site on the enzyme. These two ferroxidases are fully competent kinetically to play a major role in maintaining the cuprous-cupric redox balance in aerobic organisms.

Transport of trace elements in lenses of normal and hereditary cataract UPL rats

The multitracer technique was applied to the determination of the uptake of trace elements in the lenses of normal and hereditary cataract UPL rats to investigate the transport mechanisms of trace elements during cataract development. Be, Na, Sc, V, Cr, Mn, Fe, Co, Zn, As, Se, Rb, Sr, Y, Zr, Tc, Ru and Rh accumulate in normal and UPL cataract rat lenses. The rates of uptake of trace elements differ among species and also differ between normal and UPL rat lenses. The uptakes of V and Sr are greater in normal rat lenses, while the uptakes of Mn and Co are greater in UPL rat lenses. High concentrations of Zn are transported into normal rat lenses in comparison with other elements. However, the uptake of Se was highest in the lenses of UPL cataract rats. In addition, the difference in Se uptake between the normal and UPL rat lenses was greatest among the tested trace elements. The present study suggests that the transport characteristics of trace elements are different in the lenses of normal and UPL cataract rats. The different transport characteristics of trace elements in the lenses of normal and UPL cataract rats, especially the higher accumulation of Se in UPL rat lenses, may be implicated in cataract development.

Methods for studying synaptosomal copper release

Cu is thought to play an important role in the pathogenesis of several neurodegenerative diseases, such as Wilson's, Alzheimer's, and probably in prion protein diseases like Creutzfeld-Jakob's disease. Until now, no method existed to determine the concentration of this cation in vivo. Here, we present two possible approaches combined with a critical comparison of the results. The successful use of fluorescent ligands for the determination of Ca2+-concentrations in recent years encouraged us to seek a fluorophore which specifically reacts to Cu2+ and to characterize it for our purposes. We found that the emission of TSPP (tetrakis-(4-sulfophenyl)porphine) at an emission wavelength of 645 nm is in vitro highly specific to Cu2+ (apparent dissociation constant Kd=0.43 +/- 0.07 microM at pH 7.4). It does not react with the most common divalent cations in the brain, Ca2+ and Mg2+, unlike most of the other dyes examined. In addition, Zn2+ quenches TSPP fluorescence at a different emission wavelength (605 nm) with a Kd of 50 +/- 2.5 microM (pH 7.0). With these findings, we applied the measurement of Cu with TSPP to a biological system, showing for the first time in vivo that there is release of copper by synaptosomes upon depolarisation. Our findings were validated with a completely independent analytical approach based on ICP-MS (inductively-coupled-plasma mass-spectrometry).

Spectropotentiometric analysis of metal binding to structural zinc-binding sites: accounting quantitatively for pH and metal ion buffering effects

Studies of the metal-binding affinity of protein sites are ubiquitous in bioinorganic chemistry and are valuable for the information that they can provide about metal speciation and exchange in biological systems. The potential for error in these studies is high, however, since many competing equilibria are present in solution and must be taken into consideration. Here, we report a new spectropotentiometric titration apparatus that allows pH and UV-vis absorption to be monitored simultaneously on small samples under inert atmosphere. In addition, we explain how data obtained from the complex equilibria can be combined with tabulated information about the protonation and metal-binding constants for common buffers to provide detailed, quantitative information about metal-protein interactions. Application of this approach to the investigation of metal binding to structural zinc-binding domains and common pitfalls encountered when performing these experiments are also discussed. We have used this approach to reevaluate the metal-binding constants of the N-terminal zinc-binding peptide from the HIV-1 nucleocapsid protein (10(-8)M</=K(d)(Co)</=10(-7)M; 10(-11)M</=K(d)(Zn)</=10(-10)M).

Neurodegeneration in amyotrophic lateral sclerosis: the role of oxidative stress and altered homeostasis of metals

Amyotrophic lateral sclerosis is one of the most common neurodegenerative disorders, with an incidence of about 1/100,000. One of the typical features of this progressive, lethal disease, occurring both sporadically and as a familial disorder, is degeneration of cortical and spinal motor neurones. Present evidence indicates that loss of neurones in patients results from a complex interplay among oxidative injury, excitotoxic stimulation, dysfunction of critical proteins and genetic factors. This review focuses on existing evidence that oxidative stress is a major culprit in the pathogenesis of amyotrophic lateral sclerosis. An increase in reactive oxygen species and in products of oxidation has been observed both in post-mortem samples and in experimental models for ALS. This increase may be consequent to altered metabolism of copper and iron ions, that share the property to undergo redox cycling and generate reactive oxygen species. Metal-mediated oxidative stress would lead to several intracellular alterations and contribute to the induction of cell death pathways.

A labile regulatory copper ion lies near the T1 copper site in the multicopper oxidase CueO

CueO, a multicopper oxidase, is part of the copper-regulatory cue operon in Escherichia coli, is expressed under conditions of copper stress and shows enhanced oxidase activity when additional copper is present. The 1.7-A resolution structure of a crystal soaked in CuCl2 reveals a Cu(II) ion bound to the protein 7.5 A from the T1 copper site in a region rich in methionine residues. The trigonal bipyramidal coordination sphere is unusual, containing two methionine sulfur atoms, two aspartate carboxylate oxygen atoms, and a water molecule. Asp-439 both ligates the labile copper and hydrogen-bonds to His-443, which ligates the T1 copper. This arrangement may mediate electron transfer from substrates to the T1 copper. Mutation of residues bound to the labile copper results in loss of oxidase activity and of copper tolerance, confirming a regulatory role for this site. The methionine-rich portion of the protein, which is similar to that of other proteins involved in copper homeostasis, does not display additional copper binding. The type 3 copper atoms of the trinuclear cluster in the structure are bridged by a chloride ion that completes a square planar coordination sphere for the T2 copper atom but does not affect oxidase activity.

A core mutation affecting the folding properties of a soluble domain of the ATPase protein CopA from Bacillus subtilis

The two N-terminal domains of the P-type copper ATPase, CopAa and CopAb, from Bacillus subtilis differ in their folding capabilities in vitro. Whereas CopAb has the typical betaalphabetabetaalphabeta structure and is a rigid protein, CopAa is found to be largely unfolded. A sequence analysis of the two and of orthologue homologous proteins indicates that Ser46 in CopAa may destabilise the hydrophobic core, as also confirmed through a bioinformatic energy study. CopAb has a Val in the corresponding position. The S46V and S46A mutants are found to be folded, although the latter displays multiple conformations. S46VCopAa, in both apo and copper(I) loaded forms, has very similar structural and dynamic properties with respect to CopAb, besides a different length of strand beta2 and beta4. It is intriguing that the oxygen of Thr16 is found close, though at longer than bonding distance, to copper in both domains, as it also occurs in a human orthologue domain. This study contributes to understanding the behaviour of proteins that do not properly fold in vitro. A possible biological significance of the peculiar folding behaviour of this domain is discussed.

Schizosaccharomyces pombe as a model for metal homeostasis in plant cells: the phytochelatin-dependent pathway is the main cadmium detoxification mechanism

Sequestration of metal ions by phytochelatins is an important metal tolerance mechanism in a wide range of organisms including plants and certain fungi. Substantial progress in understanding phytochelatin formation at the molecular level has been made in Schizosaccharomyces pombe. The genome of S. pombe has been completely sequenced and all the necessary tools of functional genomics are available. Since most other proteins implicated in plant metal tolerance and homeostasis are also present in this yeast, it represents a very powerful system to elucidate basic mechanisms of metal buffering, sequestration, and toxicity in cells that form phytochelatins. Here, we summarize the work on phytochelatin formation and metal homeostasis in S. pombe. We describe examples of molecular insights obtained from experiments with S. pombe that will be useful in guiding studies with plants. We also provide evidence for the dominance of the phytochelatin pathway in Cd detoxification in S. pombe.

Escherichia coli mechanisms of copper homeostasis in a changing environment

Escherichia coli is equipped with multiple systems to ensure safe copper handling under varying environmental conditions. The Cu(I)-translocating P-type ATPase CopA, the central component in copper homeostasis, is responsible for removing excess Cu(I) from the cytoplasm. The multi-copper oxidase CueO and the multi-component copper transport system CusCFBA appear to safeguard the periplasmic space from copper-induced toxicity. Some strains of E. coli can survive in copper-rich environments that would normally overwhelm the chromosomally encoded copper homeostatic systems. Such strains possess additional plasmid-encoded genes that confer copper resistance. The pco determinant encodes genes that detoxify copper in the periplasm, although the mechanism is still unknown. Genes involved in copper homeostasis are regulated by MerR-like activators responsive to cytoplasmic Cu(I) or two-component systems sensing periplasmic Cu(I). Pathways of copper uptake and intracellular copper handling are still not identified in E. coli.

Transition metal speciation in the cell: insights from the chemistry of metal ion receptors

The essential transition metal ions are avidly accumulated by cells, yet they have two faces: They are put to use as required cofactors, but they also can catalyze cytotoxic reactions. Several families of proteins are emerging that control the activity of intracellular metal ions and help confine them to vital roles. These include integral transmembrane transporters, metalloregulatory sensors, and diffusible cytoplasmic metallochaperone proteins that protect and guide metal ions to targets. It is becoming clear that many of these proteins use atypical coordination chemistry to accomplish their unique goals. The different coordination numbers, types of coordinating residues, and solvent accessibilities of these sites are providing insight into the inorganic chemistry of the cytoplasm.

Structure of the Alzheimer's disease amyloid precursor protein copper binding domain. A regulator of neuronal copper homeostasis

A major source of free radical production in the brain derives from copper. To prevent metal-mediated oxidative stress, cells have evolved complex metal transport systems. The Alzheimer's disease amyloid precursor protein (APP) is a major regulator of neuronal copper homeostasis. APP knockout mice have elevated copper levels in the cerebral cortex, whereas APP-overexpressing transgenic mice have reduced brain copper levels. Importantly, copper binding to APP can greatly reduce amyloid beta production in vitro. To understand this interaction at the molecular level we solved the structure of the APP copper binding domain (CuBD) and found that it contains a novel copper binding site that favors Cu(I) coordination. The surface location of this site, structural homology of CuBD to copper chaperones, and the role of APP in neuronal copper homeostasis are consistent with the CuBD acting as a neuronal metallotransporter.

Integrating trace element metabolism from the cell to the whole organism

The redox chemistry of copper (Cu) makes this both a powerful enzyme catalyst and a dangerous reactant that generates hydroxyl radical. Although virtually all cells from microbes to mammals must acquire Cu to drive important biochemical reactions, the potential toxicity of Cu demands an exquisite level of vectorial transport and homeostatic control. Our laboratory is interested in how organisms acquire Cu through the action of high-affinity plasma membrane Cu transporters of the copper transport protein (Ctr) class of proteins. We have isolated Ctr Cu transporters from baker's yeast and fission yeast and from flies, mice and mammals. This review will focus on understanding how the Ctr high-affinity Cu transport proteins function, from their biochemical mechanism of action in yeast and cultured metazoan cells to their roles in Cu delivery and mammalian embryonic development.

The Schizosaccharomyces pombe Cuf1 is composed of functional modules from two distinct classes of copper metalloregulatory transcription factors

In fission yeast, the genes encoding proteins that are components of the copper transporter family are controlled at the transcriptional level by the Cuf1 transcription factor. Under low copper availability, Cuf1 induces expression of the copper transporter genes. In contrast, sufficient levels of copper inactivate Cuf1 and expression of its target genes. Our study reveals that Cuf1 harbors a putative copper-binding motif, Cys-X-Cys-X(3)-Cys-X-Cys-X(2)-Cys-X(2)-His, within its carboxyl-terminal region to sense changing environmental copper levels. Binding studies reveal that the amino-terminal 174-residue segment of Cuf1 expressed as a fusion protein in Escherichia coli specifically interacts with the cis-acting copper transporter promoter element CuSE (copper-signaling element). Within this region, the first 61 amino acids of Cuf1 exhibit more overall homology to the Saccharomyces cerevisiae Ace1 copper-detoxifying factor (from residues 1 to 63) than to Mac1, its functional ortholog. Consistently, we demonstrate that a chimeric Cuf1 protein bearing the amino-terminal 63-residue segment of Ace1 complements cuf1 Delta null phenotypes. Furthermore, we show that Schizosaccharomyces pombe cuf1Delta mutant cells expressing the full-length S. cerevisiae Ace1 protein are hypersensitive to copper ions, with a concomitant up-regulation of CuSE-mediated gene expression in fission yeast. Taken together, these studies reveal that S. cerevisiae Ace1 1-63 is functionally exchangeable with S. pombe Cuf1 1-61, and the nature of the amino acids located downstream of this amino-terminal conserved region may be crucial in dictating the type of regulatory response required to establish and maintain copper homeostasis.

A redox switch in CopC: an intriguing copper trafficking protein that binds copper(I) and copper(II) at different sites

The protein CopC from Pseudomonas syringae has been found capable of binding copper(I) and copper(II) at two different sites, occupied either one at a time or simultaneously. The protein, consisting of 102 amino acids, is known to bind copper(II) in a position that is now found consistent with a coordination arrangement including His-1, Glu-27, Asp-89, and His-91. A full solution structure analysis is reported here for Cu(I)-CopC. The copper(I) site is constituted by His-48 and three of the four Met residues (40, 43, 46, 51), which are clustered in a Met-rich region. Both copper binding sites have been characterized through extended x-ray absorption fine structure studies. They represent novel coordination environments for copper in proteins. The two sites are approximately 30 A far apart and have little affinity for the ion in the other oxidation state. Oxidation of Cu(I)-CopC or reduction of Cu(II)-CopC causes migration of copper from one site to the other. This behavior is observed both in NMR and EXAFS studies and indicates that CopC can exchange copper between two sites activated by a redox switch. CopC resides in the periplasm of Gram-negative bacteria where there is a multicopper oxidase, CopA, which may modulate the redox state of copper. CopC and CopA are coded in the same operon, responsible for copper resistance. These peculiar and novel properties of CopC are discussed with respect to their relevance for copper homeostasis.

A redox switch in CopC: an intriguing copper trafficking protein that binds copper(I) and copper(II) at different sites

The protein CopC from Pseudomonas syringae has been found capable of binding copper(I) and copper(II) at two different sites, occupied either one at a time or simultaneously. The protein, consisting of 102 amino acids, is known to bind copper(II) in a position that is now found consistent with a coordination arrangement including His-1, Glu-27, Asp-89, and His-91. A full solution structure analysis is reported here for Cu(I)-CopC. The copper(I) site is constituted by His-48 and three of the four Met residues (40, 43, 46, 51), which are clustered in a Met-rich region. Both copper binding sites have been characterized through extended x-ray absorption fine structure studies. They represent novel coordination environments for copper in proteins. The two sites are approximately 30 A far apart and have little affinity for the ion in the other oxidation state. Oxidation of Cu(I)-CopC or reduction of Cu(II)-CopC causes migration of copper from one site to the other. This behavior is observed both in NMR and EXAFS studies and indicates that CopC can exchange copper between two sites activated by a redox switch. CopC resides in the periplasm of Gram-negative bacteria where there is a multicopper oxidase, CopA, which may modulate the redox state of copper. CopC and CopA are coded in the same operon, responsible for copper resistance. These peculiar and novel properties of CopC are discussed with respect to their relevance for copper homeostasis.

The N-terminus of the human copper transporter 1 (hCTR1) is localized extracellularly, and interacts with itself

We have used indirect immunofluorescense studies and glycosylation-site insertion and deletion mapping to characterize the topology of human copper transporter 1 (hCTR1), the putative human high-affinity copper-import protein. Both approaches indicated that hCTR1 contains three transmembrane domains and that the N-terminus of hCTR1, which contains several putative copper-binding sites, is localized extracellularly, whereas the C-terminus is exposed to the cytosol. Based on previous observations that CTR1 proteins form high-molecular-mass complexes, we investigated directly whether CTR1 proteins interact with themselves. Yeast two-hybrid studies showed that interaction of yeast, mouse, rat and human CTR1 occurs at the sites of their N-terminal domains, and is not dependent on the copper concentration in the growth media. Analysis of deletion constructs indicated that multiple regions in the N-terminus are essential for this self-interaction. In contrast, the N-terminal tail of the presumed low-affinity copper transporter, hCTR2, does not interact with itself. Taken together, these results suggest that CTR1 spans the membrane at least six times, permitting formation of a channel, which is consistent with its proposed role as a copper transporter.

CopZ from Bacillus subtilis interacts in vivo with a copper exporting CPx-type ATPase CopA

The structure of the hypothetical copper-metallochaperone CopZ from Bacillus subtilis and its predicted partner CopA have been studied but their respective contributions to copper export, -import, -sequestration and -supply are unknown. DeltacopA was hypersensitive to copper and contained more copper atoms cell(-1) than wild-type. Expression from the copA operator-promoter increased in elevated copper (not other metals), consistent with a role in copper export. A bacterial two-hybrid assay revealed in vivo interaction between CopZ and the N-terminal domain of CopA but not that of a related transporter, YvgW, involved in cadmium-resistance. Activity of copper-requiring cytochrome caa(3) oxidase was retained in deltacopZ and deltacopA. DeltacopZ was only slightly copper-hypersensitive but deltacopZ/deltacopA was more sensitive than deltacopA, implying some action of CopZ that is independent of CopA. Significantly, deltacopZ contained fewer copper atoms cell(-1) than wild-type under these conditions. CopZ makes a net contribution to copper sequestration and/or recycling exceeding any donation to CopA for export.

Ctr1 and its role in body copper homeostasis

Members of the Cu transporter (Ctr) family have been reported to be part of the copper uptake machinery in several organisms. Recently it has been suggested that human Ctr1 (hCtr1) may act as a copper transporter in several tissues including the intestine. hCtr1 is a 190 amino acid protein and is predicted to have three transmembrane-spanning domains and exist in the plasma membrane as a homo-trimer. Ctr1-transfected cell lines exhibit saturable, pH-dependent Cu(I) uptake indicating a role in copper transport. Recent studies with Ctr1 knockout mice have highlighted an essential function in mammalian embryonic development since homozygous mutants die in utero. Heterozygotes are indistinguishable from wild-type littermates but have a severely reduced brain copper content, suggesting that Ctr1 is a key component of the copper uptake pathway in the brain. However, its role in other tissues remains elusive.

Altered Cu metabolism and differential transcription of Cu/ZnSod genes in a Cu/ZnSOD-deficient mutant of maize: evidence for a Cu-responsive transcription factor

Maize inbred line A351 exhibits extremely low levels of Cu/Zn superoxide dismutase (SOD) isozymes, three cytosolic and one chloroplastic, which are increased by supplying copper to near-toxic concentrations. Activities of the copper enzymes cytochrome c oxidase and ascorbate oxidase are also reduced. The level of expression of the maize copper chaperone for SOD is normal to elevated. The gene transcript encoding chloroplastic SOD-1 is present at normal levels, whereas RNA levels of the cytosolic SODs are low and increase with added copper, suggesting a promoter element and copper-dependent transcription factor common to the three genes. Although a reduced level of high-affinity copper transport in A351 cannot be ruled out, high transcript levels of a constitutively expressed metallothionein, suggesting increased copper chelation capacity and creating a general copper-deprivation effect, seem to be a likely cause of the reduced levels of copper enzyme activity and Cu/ZnSod gene transcripts. While exogenous copper does not affect the wild-type SOD activity or protein, it increases wild-type Cu/ZnSod transcript levels in a response similar to that of several yeast genes involved in copper sequestration and antioxidant defense. A sequence that is highly homologous to those of the copper-responsive transcription factors ACE1 (Saccharomyces cerevisiae) and AMT1 (Candida glabrata) is present in the promoters of three maize Cu/ZnSod genes.

Reaction of elemental sulfur with a copper(I) complex forming a trans-mu-1,2 end-on disulfide complex: new directions in copper-sulfur chemistry

Elemental sulfur (S8) was found to react with [(TMPA)CuI(CH3CN)]+ to form the trans-mu-1,2 end-on disulfide complex [(TMPA)Cu-S-S-Cu(TMPA)]2+. The X-ray structure of this centrosymmetric disulfide complex shows a Cu(1)-S(1) bond length of 2.280(2) A and a S(1)-S(1A) bond length of 2.044(4) A.

Knockout of 'metal-responsive transcription factor' MTF-1 in Drosophila by homologous recombination reveals its central role in heavy metal homeostasis

'Metal-responsive transcription factor-1' (MTF-1), a zinc finger protein, is conserved from mammals to insects. In the mouse, it activates metallothionein genes and other target genes in response to several cell stress conditions, notably heavy metal load. The knockout of MTF-1 in the mouse has an embryonic lethal phenotype accompanied by liver degeneration. Here we describe the targeted disruption of the MTF-1 gene in Drosophila by homologous recombination. Unlike the situation in the mouse, knockout of MTF-1 in Drosophila is not lethal. Flies survive well under laboratory conditions but are sensitive to elevated concentrations of copper, cadmium and zinc. Basal and metal-induced expression of Drosophila metallothionein genes MtnA (Mtn) and MtnB (Mto), and of two new metallothionein genes described here, MtnC and MtnD, is abolished in MTF-1 mutants. Unexpectedly, MTF-1 mutant larvae are sensitive not only to copper load but also to copper depletion. In MTF-1 mutants, copper depletion prevents metamorphosis and dramatically extends larval development/lifespan from normally 4-5 days to as many as 32 days, possibly reflecting the effects of impaired oxygen metabolism. These findings expand the roles of MTF-1 in the control of heavy metal homeostasis.

Nutritive metal uptake in teleost fish

Transition metals are essential for health, forming integral components of proteins involved in all aspects of biological function. However, in excess these metals are potentially toxic, and to maintain metal homeostasis organisms must tightly coordinate metal acquisition and excretion. The diet is the main source for essential metals, but in aquatic organisms an alternative uptake route is available from the water. This review will assess physiological, pharmacological and recent molecular evidence to outline possible uptake pathways in the gills and intestine of teleost fish involved in the acquisition of three of the most abundant transition metals necessary for life; iron, copper, and zinc.

Ctr6, a vacuolar membrane copper transporter in Schizosaccharomyces pombe

Aerobic organisms possess efficient systems for the transport of copper. This involves transporters that mediate the passage of copper across biological membranes to reach essential intracellular copper-requiring enzymes. In this report, we identify a new copper transporter in Schizosaccharomyces pombe, encoded by the ctr6(+) gene. The transcription of ctr6(+) is induced under copper-limiting conditions. This regulation is mediated by the cis-acting promoter element CuSE (copper-signaling element) through the copper-sensing transcription factor Cuf1. An S. pombe strain bearing a disrupted ctr6Delta allele displays a strong reduction of copper,zinc superoxide dismutase activity. When the ctr6+ gene is overexpressed from the thiamine-inducible nmt1(+) promoter, the cells are unable to grow on medium containing exogenous copper. Surprisingly, this copper-sensitive growth phenotype is not due to an increase of copper uptake at the cell surface. Instead, copper delivery across the plasma membrane is reduced. Consistently, this results in repressing ctr4(+) gene expression. By using a functional ctr6(+) epitope-tagged allele expressed under the control of its own promoter, we localize the Ctr6 protein on the membrane of vacuoles. Furthermore, we demonstrate that Ctr6 is an integral membrane protein that can trimerize. Moreover, we show that Ctr6 harbors a putative copper-binding Met-X-His-Cys-X-Met-X-Met motif in the amino terminus, which is essential for its function. Our findings suggest that under conditions in which copper is scarce, Ctr6 is required as a means to mobilize stored copper from the vacuole to the cytosol.

Characterization of mouse embryonic cells deficient in the ctr1 high affinity copper transporter. Identification of a Ctr1-independent copper transport system

The trace metal copper is an essential cofactor for a number of enzymes that have critical roles in biological processes, but it is highly toxic when allowed to accumulate in excess of cellular needs. Consequently, homeostatic copper metabolism is maintained by molecules involved in copper uptake, distribution, excretion, and incorporation into copper-requiring enzymes. Previously, we reported that overexpression of the human or mouse Ctr1 copper transporter stimulates copper uptake in mammalian cells, and deletion of one Ctr1 allele in mice gives rise to tissue-specific defects in copper accumulation and in the activities of copper-dependent enzymes. To investigate the physiological roles for mammalian Ctr1 protein in cellular copper metabolism, we characterized wild type, Ctr1 heterozygous, and Ctr1 homozygous knock-out cells isolated from embryos obtained by the inter-cross of Ctr1 heterozygous mice. Ctr1-deficient mouse embryonic cells are viable but exhibit significant defects in copper uptake and accumulation and in copper-dependent enzyme activities. Interestingly, Ctr1-deficient cells exhibit approximately 30% residual copper transport activity that is saturable, with a K(m) of approximately 10 microm, with biochemical features distinct from that of Ctr1. These observations demonstrate that, although Ctr1 is critical for both cellular copper uptake and embryonic development, mammals possess additional biochemically distinct functional copper transport activities.

The ABCDs of periplasmic copper trafficking

The structure of the CopC protein of Pseudomonas syringae pathovar tomato provides fascinating clues, not only to its role in the periplasmic space in copper resistance, but also to features important for copper trafficking and homeostasis that may be conserved in a variety of biological systems.

Biochemical and genetic analyses of yeast and human high affinity copper transporters suggest a conserved mechanism for copper uptake

The redox active metal copper is an essential cofactor in critical biological processes such as respiration, iron transport, oxidative stress protection, hormone production, and pigmentation. A widely conserved family of high affinity copper transport proteins (Ctr proteins) mediates copper uptake at the plasma membrane. However, little is known about Ctr protein topology, structure, and the mechanisms by which this class of transporters mediates high affinity copper uptake. In this report, we elucidate the topological orientation of the yeast Ctr1 copper transport protein. We show that a series of clustered methionine residues in the hydrophilic extracellular domain and an MXXXM motif in the second transmembrane domain are important for copper uptake but not for protein sorting and delivery to the cell surface. The conversion of these methionine residues to cysteine, by site-directed mutagenesis, strongly suggests that they coordinate to copper during the process of metal transport. Genetic evidence supports an essential role for cooperativity between monomers for the formation of an active Ctr transport complex. Together, these results support a fundamentally conserved mechanism for high affinity copper uptake through the Ctr proteins in yeast and humans.