Dissecting the role of the N-terminal metal-binding domains in activating the yeast copper ATPasein vivo

Physiology and proteomics reveal Fulvic acid mitigates Cadmium adverse effects on growth and photosynthetic properties of lettuce

Understanding the molecular mechanisms of plants in response to Cd stress is crucial for improving plants adaptation to Cd stress. Fulvic acid (FA) is an active humic substance that is often used as a soil conditioner. However, there are few reports on the role of FA against Cd stress. The aim of this study was to determine the effects of Fulvic acid on alleviation of Cd toxicity in lettuce (Lactuca sativa L) under hydroponic conditions. Our results showed that 20 μmol/L Cd stress significantly reduced photosynthetic pigment metabolism and the expression of photosynthetic apparatus-related proteins, thereby inhibiting photosynthetic electron transport, net photosynthetic rate and negatively affecting photosynthetic carbon assimilation and growth of lettuce. However, proteomic findings suggest that the application of FA can reduce the adverse effects of Cd contamination. Compared to Cd stress alone, FA significantly increased the expression of Light-harvesting proteins, reaction center and electron transport-related proteins. Further results showed that FA at 0.5 g/L reduced the uptake of Cd by the roots, resulting in a 23.5% reduction in total Cd content in lettuce. Moreover, FA enhanced S metabolism and rebuilt redox homeostasis in cells. Overall, these findings provide new insights into the mechanism of cadmium toxicity mitigation in lettuce by FA. Which is recommended as an eco-friendly tool for improving the photosynthesis performance and biomass of lettuce under Cd stress.

Structure and ion-release mechanism of PIB-4-type ATPases

Transition metals, such as zinc, are essential micronutrients in all organisms, but also highly toxic in excessive amounts. Heavy-metal transporting P-type (P_IB) ATPases are crucial for homeostasis, conferring cellular detoxification and redistribution through transport of these ions across cellular membranes. No structural information is available for the P_IB-4-ATPases, the subclass with the broadest cargo scope, and hence even their topology remains elusive. Here we present structures and complementary functional analyses of an archetypal P_IB‑4‑ATPase, sCoaT from Sulfitobacter sp. NAS14-1. The data disclose the architecture, devoid of classical so-called heavy metal binding domains, and provides fundamentally new insights into the mechanism and diversity of heavy-metal transporters. We reveal several novel P-type ATPase features, including a dual role in heavy-metal release and as an internal counter ion of an invariant histidine. We also establish that the turn-over of P_IB‑ATPases is potassium independent, contrasting to many other P-type ATPases. Combined with new inhibitory compounds, our results open up for efforts in e.g. drug discovery, since P_IB-4-ATPases function as virulence factors in many pathogens.

Long-Term Correction of Copper Metabolism in Wilson's Disease Mice with AAV8 Vector Delivering Truncated ATP7B

Wilson's disease (WD) is an autosomal recessive disorder of copper metabolism caused by mutations in the ATP7B gene encoding a liver active copper transport enzyme. Gene therapy with adeno-associated virus (AAV) carrying full-length ATP7B, which is about 4.4 kb, was shown to rescue copper metabolism disorder in WD mouse model. However, due to its relatively large size, the AAV vector containing full-length ATP7B could be oversized for its packaging capacity, which could lead to inefficient packaging. To this purpose, we engineered a truncated ATP7B mutant (tATP7B) that is about 3.3 kb in length and used for AAV gene therapy for WD mice. In vitro test showed that the excretion of copper outside the cells could be achieved with tATP7B as efficient as the full-length ATP7B. In vivo delivery of tATP7B to WD mice by AAV8 vectors corrected their copper metabolisms and significantly rescued copper accumulation-related syndromes, including reduced urinary copper excretion, increased serum ceruloplasmin, and improved liver damages. Thus, our study demonstrated that AAV gene therapy based on truncated ATP7B is a promising strategy in the treatment of WD.

Wilson disease missense mutations in ATP7B affect metal-binding domain structural dynamics

Wilson disease (WD) is caused by mutations in the gene for ATP7B, a copper transport protein that regulates copper levels in cells. A large number of missense mutations have been reported to cause WD but genotype-phenotype correlations are not yet established. Since genetic screening for WD may become reality in the future, it is important to know how individual mutations affect ATP7B function, with the ultimate goal to predict pathophysiology of the disease. To begin to assess mechanisms of dysfunction, we investigated four proposed WD-causing missense mutations in metal-binding domains 5 and 6 of ATP7B. Three of the four variants showed reduced ATP7B copper transport ability in a traditional yeast assay. To probe mutation-induced structural dynamic effects at the atomic level, molecular dynamics simulations (1.5 μs simulation time for each variant) were employed. Upon comparing individual metal-binding domains with and without mutations, we identified distinct differences in structural dynamics via root-mean square fluctuation and secondary structure content analyses. Most mutations introduced distant effects resulting in increased dynamics in the copper-binding loop. Taken together, mutation-induced long-range alterations in structural dynamics provide a rationale for reduced copper transport ability.

Folding of copper proteins: role of the metal?

Copper is a redox-active transition metal ion required for the function of many essential human proteins. For biosynthesis of proteins coordinating copper, the metal may bind before, during or after folding of the polypeptide. If the metal binds to unfolded or partially folded structures of the protein, such coordination may modulate the folding reaction. The molecular understanding of how copper is incorporated into proteins requires descriptions of chemical, thermodynamic, kinetic and structural parameters involved in the formation of protein-metal complexes. Because free copper ions are toxic, living systems have elaborate copper-transport systems that include particular proteins that facilitate efficient and specific delivery of copper ions to target proteins. Therefore, these pathways become an integral part of copper protein folding in vivo. This review summarizes biophysical-molecular in vitro work assessing the role of copper in folding and stability of copper-binding proteins as well as protein-protein copper exchange reactions between human copper transport proteins. We also describe some recent findings about the participation of copper ions and copper proteins in protein misfolding and aggregation reactions in vitro.

Copper relay path through the N-terminus of Wilson disease protein, ATP7B

In human cells, copper (Cu) ions are transported by the cytoplasmic Cu chaperone Atox1 to the Wilson disease protein (ATP7B) in the Golgi for loading of Cu-dependent enzymes. ATP7B is a membrane-spanning protein which, in contrast to non-mammalian homologs, has six cytoplasmic metal-binding domains (MBDs). To address the reason for multiple MBDs, we introduced strategic mutations in which one, two or three MBDs had been blocked for Cu binding via cysteine-to-serine mutations (but all six MBDs are present in all) in a yeast system that probes Cu flow through Atox1 and ATP7B. The results, combined with earlier work, support a mechanistic model in which MBD1-3 forms a regulatory unit of ATP7B Cu transport. Cu delivery via Atox1 to this unit, followed by loading of Cu in MBD3, promotes release of inhibitory interactions. Whereas the Cu site in MBD4 can be mutated without a large effect, an intact Cu site in either MBD5 or MBD6 is required for Cu transport. All MBDs, expressed as single-domain proteins, can replace Atox1 and deliver Cu to full-length ATP7B. However, only MBD6 can deliver Cu to truncated ATP7B where all six MBDs are removed, suggesting a docking role for this structural unit.

The Cu chaperone CopZ is required for Cu homeostasis in Rhodobacter capsulatus and influences cytochrome cbb3 oxidase assembly

Cu homeostasis depends on a tightly regulated network of proteins that transport or sequester Cu, preventing the accumulation of this toxic metal while sustaining Cu supply for cuproproteins. In Rhodobacter capsulatus, Cu-detoxification and Cu delivery for cytochrome c oxidase (cbb₃ -Cox) assembly depend on two distinct Cu-exporting P_1B -type ATPases. The low-affinity CopA is suggested to export excess Cu and the high-affinity CcoI feeds Cu into a periplasmic Cu relay system required for cbb₃ -Cox biogenesis. In most organisms, CopA-like ATPases receive Cu for export from small Cu chaperones like CopZ. However, whether these chaperones are also involved in Cu export via CcoI-like ATPases is unknown. Here we identified a CopZ-like chaperone in R. capsulatus, determined its cellular concentration and its Cu binding activity. Our data demonstrate that CopZ has a strong propensity to form redox-sensitive dimers via two conserved cysteine residues. A ΔcopZ strain, like a ΔcopA strain, is Cu-sensitive and accumulates intracellular Cu. In the absence of CopZ, cbb₃ -Cox activity is reduced, suggesting that CopZ not only supplies Cu to P_1B -type ATPases for detoxification but also for cuproprotein assembly via CcoI. This finding was further supported by the identification of a ~150 kDa CcoI-CopZ protein complex in native R. capsulatus membranes.

Mass spectrometric studies of Cu(I)-binding to the N-terminal domains of B. subtilis CopA and influence of bacillithiol

CopA is a Cu(I)-exporting transmembrane P_1B-type ATPase from Bacillus subtilis. It contains two N-terminal cytoplasmic domains, CopAab, which bind Cu(I) with high affinity and to form higher-order complexes with multiple Cu(I) ions. To determine the precise nature of these species, electrospray ionisation mass spectrometry (ESI-MS) under non-denaturing conditions was employed. Up to 1 Cu per CopAab resulted in Cu coordination to one or both CopAab domains. At >1 Cu/CopAab, two distinct dimeric charge state envelopes were observed, corresponding to distinct conformations, each with Cu₆(CopAab)₂ as its major form. The influence of the physiologically relevant low molecular weight thiol bacillithiol (BSH) on Cu(I)-binding to CopAab was assessed. Dimeric CopAab persisted in the presence of BSH, with previously undetected Cu₇(CopAab)₂ and Cu₆(CopAab)₂(BSH) forms apparent.

A Luminal Loop of Wilson Disease Protein Binds Copper and Is Required for Protein Activity

The copper-transporting ATPase ATP7B is essential for loading of copper ions to copper-dependent enzymes in the secretory pathway; its inactivation results in Wilson disease. In contrast to copper-ion uptake by the cytoplasmic domains, ATP7B-mediated copper-ion release in the Golgi has not been explored yet. We demonstrate here that a luminal loop in ATP7B, rich in histidine/methionine residues, binds reduced copper (Cu(I)) ions, and identified copper-binding residues play an essential role in ATP7B-mediated metal ion release. NMR experiments on short-peptide models demonstrate that three methionine and two histidine residues are specifically involved in Cu(I) ion binding; with these residues replaced by alanines, no Cu(I) ion interaction is detected. Although more than one Cu(I) ion can interact with the wild-type peptide, removing either all histidine or all methionine residues reduces the stoichiometry to one Cu(I) ion binding per peptide. Using a yeast complementation assay, we show that for efficient copper transport by full-length ATP7B, the complete set of histidine and methionine residues in the lumen loop are required. The replacement of histidine/methionine residues by alanines does not perturb overall ATP7B structure, as the localization of ATP7B variants in yeast cells matches that of the wild-type protein. Thus, in similarity to ATP7A, ATP7B also appears to have a luminal “exit” copper ion site.

Probing functional roles of Wilson disease protein (ATP7B) copper-binding domains in yeast

After Ctr1-mediated uptake into human cells, copper (Cu) ions are transported by the cytoplasmic Cu chaperone Atox1 to the Wilson disease protein (ATP7B) in the Golgi network. Cu transfer occurs via direct protein-protein interactions and leads to incorporation of Cu into Cu-dependent enzymes. ATP7B is a large multi-domain membrane-spanning protein which, in contrast to homologs, has six cytoplasmic metal-binding domains (MBDs). The reason for multiple MBDs is proposed to be indirect modulation of activity but mechanistic studies of full-length ATP7B are limited. We here developed a system that probes Cu flow through human Atox1 and ATP7B proteins when expressed in yeast. Using this assay, we assessed the roles of the different MBDs in ATP7B and found that the presence of the most N-terminal MBD increased, whereas the third MBD decreased, overall ATP7B-mediated Cu transport activity. Upon removal of all MBDs in ATP7B, the ability to transport Cu disappeared. The designed system can be expanded to include other yeast viability parameters and will be a useful tool for further mechanistic insights on human Cu transport as well as diseases involving Cu imbalance.

Copper Acquisition and Utilization in Fungi

Fungal cells colonize and proliferate in distinct niches, from soil and plants to diverse tissues in human hosts. Consequently, fungi are challenged with the goal of obtaining nutrients while simultaneously elaborating robust regulatory mechanisms to cope with a range of availability of nutrients, from scarcity to excess. Copper is essential for life but also potentially toxic. In this review we describe the sophisticated homeostatic mechanisms by which fungi acquire, utilize, and control this biochemically versatile trace element. Fungal pathogens, which can occupy distinct host tissues that have their own intrinsic requirements for copper homeostasis, have evolved mechanisms to acquire copper to successfully colonize the host, disseminate to other tissues, and combat host copper bombardment mechanisms that would otherwise mitigate virulence.

Disease-causing point-mutations in metal-binding domains of Wilson disease protein decrease stability and increase structural dynamics

After cellular uptake, Copper (Cu) ions are transferred from the chaperone Atox1 to the Wilson disease protein (ATP7B) for incorporation into Cu-dependent enzymes in the secretory pathway. Human ATP7B is a large multi-domain membrane-spanning protein which, in contrast to homologues in other organisms, has six similar cytoplasmic metal-binding domains (MBDs). The reason for multiple MBDs is proposed to be indirect modulation of enzymatic activity and it is thus intriguing that point mutations in MBDs can promote Wilson disease. We here investigated, in vitro and in silico, the biophysical consequences of clinically-observed Wilson disease mutations, G85V in MBD1 and G591D in MBD6, incorporated in domain 4. Because G85 and G591 correspond to a conserved Gly found in all MBDs, we introduced the mutations in the well-characterized MBD4. We found the mutations to dramatically reduce the MBD4 thermal stability, shifting the midpoint temperature of unfolding by more than 20 °C. In contrast to wild type MBD4 and MBD4D, MBD4V adopted a misfolded structure with a large β-sheet content at high temperatures. Molecular dynamic simulations demonstrated that the mutations increased backbone fluctuations that extended throughout the domain. Our findings imply that reduced stability and enhanced dynamics of MBD1 or MBD6 is the origin of ATP7B dysfunction in Wilson disease patients with the G85V or G591D mutation.

Unresolved questions in human copper pump mechanisms

Copper (Cu) is an essential transition metal providing activity to key enzymes in the human body. To regulate the levels and avoid toxicity, cells have developed elaborate systems for loading these enzymes with Cu. Most Cu-dependent enzymes obtain the metal from the membrane-bound Cu pumps ATP7A/B in the Golgi network. ATP7A/B receives Cu from the cytoplasmic Cu chaperone Atox1 that acts as the cytoplasmic shuttle between the cell membrane Cu importer, Ctr1 and ATP7A/B. Biological, genetic and structural efforts have provided a tremendous amount of information for how the proteins in this pathway work. Nonetheless, basic mechanistic-biophysical questions (such as how and where ATP7A/B receives Cu, how ATP7A/B conformational changes and domain-domain interactions facilitate Cu movement through the membrane, and, finally, how target polypeptides are loaded with Cu in the Golgi) remain elusive. In this perspective, unresolved inquiries regarding ATP7A/B mechanism will be highlighted. The answers are important from a fundamental view, since mechanistic aspects may be common to other metal transport systems, and for medical purposes, since many diseases appear related to Cu transport dysregulation.

Mass spectrometry of B. subtilis CopZ: Cu(i)-binding and interactions with bacillithiol

Mass spectrometry reveals a high resolution overview of species formed by CopZ and Cu(i), and the effects of the physiological low molecular weight thiol bacillithiol.

Enthalpy-entropy compensation at play in human copper ion transfer

Copper (Cu) is an essential trace element but toxic in free form. After cell uptake, Cu is transferred, via direct protein-protein interactions, from the chaperone Atox1 to the Wilson disease protein (WD) for incorporation into Cu-dependent enzymes. Cu binds to a conserved C(1)XXC(2) motif in the chaperone as well as in each of the cytoplasmic metal-binding domains of WD. Here, we dissect mechanism and thermodynamics of Cu transfer from Atox1 to the fourth metal binding domain of WD. Using chromatography and calorimetry together with single Cys-to-Ala variants, we demonstrate that Cu-dependent protein heterocomplexes require the presence of C(1) but not C(2). Comparison of thermodynamic parameters for mutant versus wild type reactions reveals that the wild type reaction involves strong entropy-enthalpy compensation. This property is explained by a dynamic inter-conversion of Cu-Cys coordinations in the wild type ensemble and may provide functional advantage by protecting against Cu mis-ligation and bypassing enthalpic traps.

An expanding range of functions for the copper chaperone/antioxidant protein Atox1

SIGNIFICANCE

Antioxidant protein 1 (Atox1 in human cells) is a copper chaperone for the copper export pathway with an essential role in cellular copper distribution. In vitro, Atox1 binds and transfers copper to the copper-transporting ATPases, stimulating their catalytic activity. Inactivation of Atox1 in cells inhibits maturation of secreted cuproenzymes as well as copper export from cells.

RECENT ADVANCES

Accumulating data suggest that cellular functions of Atox1 are not limited to its copper-trafficking role and may include storage of labile copper, modulation of transcription, and antioxidant defense. The conserved metal binding site of Atox1, CxGC, differs from the metal-binding sites of copper-transporting ATPases and has a physiologically relevant redox potential that equilibrates with the GSH:GSSG pair.

CRITICAL ISSUES

Tight relationship appears to exist between intracellular copper levels and glutathione (GSH) homeostasis. The biochemical properties of Atox1 place it at the intersection of cellular networks that regulate copper distribution and cellular redox balance. Mechanisms through which Atox1 facilitates copper export and contributes to oxidative defense are not fully understood.

FUTURE DIRECTIONS

The current picture of cellular redox homeostasis and copper physiology will be enhanced by further mechanistic studies of functional interactions between the GSH:GSSG pair and copper-trafficking machinery.

Metal transport across biomembranes: emerging models for a distinct chemistry

Transition metals are essential components of important biomolecules, and their homeostasis is central to many life processes. Transmembrane transporters are key elements controlling the distribution of metals in various compartments. However, due to their chemical properties, transition elements require transporters with different structural-functional characteristics from those of alkali and alkali earth ions. Emerging structural information and functional studies have revealed distinctive features of metal transport. Among these are the relevance of multifaceted events involving metal transfer among participating proteins, the importance of coordination geometry at transmembrane transport sites, and the presence of the largely irreversible steps associated with vectorial transport. Here, we discuss how these characteristics shape novel transition metal ion transport models.

Biochemical characterization of AtHMA6/PAA1, a chloroplast envelope Cu(I)-ATPase

Copper is an essential plant micronutrient playing key roles in cellular processes, among them photosynthesis. In Arabidopsis thaliana, copper delivery to chloroplasts, mainly studied by genetic approaches, is thought to involve two P(IB)-type ATPases: AtHMA1 and AtHMA6/PAA1. The lack of biochemical characterization of AtHMA1 and PAA1, and more generally of plant P(IB)-type ATPases, is due to the difficulty of getting high amounts of these membrane proteins in an active form, either from their native environment or after expression in heterologous systems. In this study, we report the first biochemical characterization of PAA1, a plant copper-transporting ATPase. PAA1 produced in Lactococcus lactis is active, forming an aspartyl phosphate intermediate in the presence of ATP and the adequate metal ion. PAA1 can also be phosphorylated using inorganic phosphate in the absence of transition metal. Both phosphorylation types allowed us to demonstrate that PAA1 is activated by monovalent copper ions (and to a lower extent by silver ions) with an apparent affinity in the micromolar range. In agreement with these biochemical data, we also demonstrate that when expressed in yeast, PAA1 induces increased sensitivities to copper and silver. These data provide the first enzymatic characterization of a P(IB-1)-type plant ATPase and clearly identify PAA1 as a high affinity Cu(I) transporter of the chloroplast envelope.

Thermodynamics of copper and zinc distribution in the cyanobacterium Synechocystis PCC 6803

Copper is supplied to plastocyanin for photosynthesis and cytochrome c oxidase for respiration in the thylakoids of Synechocystis PCC 6803 by the membrane-bound P-type ATPases CtaA and PacS, and the metallochaperone Atx1. We have determined the Cu(I) affinities of all of the soluble proteins and domains in this pathway. The Cu(I) affinities of the trafficking proteins range from 5 × 10(16) to 5 × 10(17) M(-1) at pH 7.0, consistent with values for homologues. Unusually, Atx1 binds Cu(I) significantly tighter than the metal-binding domains (MBDs) of CtaA and PacS (CtaA(N) and PacS(N)), and equilibrium copper exchange constants of approximately 0.2 are obtained for transfer to the MBDs. Dimerization of Atx1 increases the affinity for Cu(I), but the loop 5 His61 residue has little influence. The MBD of the zinc exporter ZiaA (ZiaA(N)) exhibits an almost identical Cu(I) affinity, and Cu(I) exchange with Atx1, as CtaA(N) and PacS(N), and the relative stabilities of the complexes must enable the metallochaperone to distinguish between the MBDs. The binding of potentially competing zinc to the trafficking proteins has been studied. ZiaA(N) has the highest Zn(II) affinity and thermodynamics could be important for zinc removal from the cell. Plastocyanin has a Cu(I) affinity of 2.6 × 10(17) M(-1), 15-fold tighter than that of the Cu(A) site of cytochrome c oxidase, highlighting the need for specific mechanisms to ensure copper delivery to both of these targets. The narrow range of Cu(I) affinities for the cytoplasmic copper proteins in Synechocystis will facilitate relocation when copper is limiting.

Crystal structure of a copper-transporting PIB-type ATPase

Heavy-metal homeostasis and detoxification is crucial for cell viability. P-type ATPases of the class IB (PIB) are essential in these processes, actively extruding heavy metals from the cytoplasm of cells. Here we present the structure of a PIB-ATPase, a Legionella pneumophila CopA Cu(+)-ATPase, in a copper-free form, as determined by X-ray crystallography at 3.2 Å resolution. The structure indicates a three-stage copper transport pathway involving several conserved residues. A PIB-specific transmembrane helix kinks at a double-glycine motif displaying an amphipathic helix that lines a putative copper entry point at the intracellular interface. Comparisons to Ca(2+)-ATPase suggest an ATPase-coupled copper release mechanism from the binding sites in the membrane via an extracellular exit site. The structure also provides a framework to analyse missense mutations in the human ATP7A and ATP7B proteins associated with Menkes' and Wilson's diseases.

Proteomic tools for the analysis of transient interactions between metalloproteins

Metalloproteins play major roles in cell metabolism and signalling pathways. In many cases, they show moonlighting behaviour, acting in different processes, depending on the physiological state of the cell. To understand these multitasking proteins, we need to discover the partners with which they carry out such novel functions. Although many technological and methodological tools have recently been reported for the detection of protein interactions, specific approaches to studying the interactions involving metalloproteins are not yet well developed. The task is even more challenging for metalloproteins, because they often form short-lived complexes that are difficult to detect. In this review, we gather the different proteomic techniques and biointeractomic tools reported in the literature. All of them have shown their applicability to the study of transient and weak protein-protein interactions, and are therefore suitable for metalloprotein interactions.

Two serine residues control sequential steps during catalysis of the yeast copper ATPase through different mechanisms that involve kinase-mediated phosphorylations

Ccc2, the yeast copper-transporting ATPase, pumps copper from the cytosol to the Golgi lumen. During its catalytic cycle, Ccc2 undergoes auto-phosphorylation on Asp(627) and uses the energy gained to transport copper across the cell membrane. We previously demonstrated that cAMP-dependent protein kinase (PKA) controls the energy interconversion (Cu)E∼P → E-P + Cu when Ser(258) is phosphorylated. We now demonstrate that Ser(258) is essential in vivo for copper homeostasis in extremely low copper and iron concentrations. The S258A mutation abrogates all PKA-mediated phosphorylations of Ccc2, whereas the S971A mutation leads to a 100% increase in its global regulatory phosphorylation. With S258A, the first-order rate constant of catalytic phosphorylation by ATP decreases from 0.057 to 0.030 s(-1), with an 8-fold decrease in the burst of initial phosphorylation. With the S971A mutant, the rate constant decreases to 0.007 s(-1). PKAi(5-24) decreases the amount of the aspartylphosphate intermediate (EP) in Ccc2 wt by 50% within 1 min, but not in S258A, S971A, or S258A/S971A. The increase of the initial burst and the extremely slow phosphorylation when the "phosphomimetic" mutant S258D was assayed (k = 0.0036 s(-1)), indicate that electrostatic and conformational (non-electrostatic) mechanisms are involved in the regulatory role of Ser(258). Accumulation of an ADP-insensitive form in S971A demonstrates that Ser(971) is required to accelerate the hydrolysis of the E-P form during turnover. We propose that Ser(258) and Ser(971) are under long-range intramolecular, reciprocal and concerted control, in a sequential process that is crucial for catalysis and copper transport in the yeast copper ATPase.

Human copper homeostasis: a network of interconnected pathways

Copper plays an essential role in normal human physiology. Copper misbalance affects heart development, CNS and liver function, influences lipid metabolism, inflammation, and resistance to chemotherapeutic drugs. Recent studies yielded new information on the structure, function, and regulation of human copper transporters, uncovered unanticipated functions for copper chaperones, and established connections between copper homeostasis and other metabolic pathways. It has become apparent that the copper trafficking machinery is regulated at several levels and that the cross-talk between cell compartments contributes to the intracellular copper balance. The human copper regulon is emerging.