Domains I and III of the human copper chaperone for superoxide dismutase interact via a cysteine-bridged Dicopper(I) cluster

Insights on the endogenous labile iron pool binding properties

Under normal physiological conditions, the endogenous Labile Iron Pool (LIP) constitutes a ubiquitous, dynamic, tightly regulated reservoir of cellular ferrous iron. Furthermore, LIP is loaded into new apo-iron proteins, a process akin to the activity of metallochaperones. Despite such importance on iron metabolism, the LIP identity and binding properties have remained elusive. We hypothesized that LIP binds to cell constituents (generically denoted C) and forms an iron complex termed CLIP. Combining this binding model with the established Calcein (CA) methodology for assessing cytosolic LIP, we have formulated an equation featuring two experimentally quantifiable parameters (the concentrations of the cytosolic free CA and CA and LIP complex termed CALIP) and three unknown parameters (the total concentrations of LIP and C and their thermodynamic affinity constant Kd). The fittings of cytosolic CALIP × CA concentrations data encompassing a few cellular models to this equation with floating unknown parameters were successful. The computed adjusted total LIP (LIPT) and C (CT) concentrations fall within the sub-to-low micromolar range while the computed Kd was in the 10-2 µM range for all cell types. Thus, LIP binds and has high affinity to cellular constituents found in low concentrations and has remarkably similar properties across different cell types, shedding fresh light on the properties of endogenous LIP within cells.

The molecular and cellular basis of copper dysregulation and its relationship with human pathologies

Copper (Cu) is an essential micronutrient required for the activity of redox-active enzymes involved in critical metabolic reactions, signaling pathways, and biological functions. Transporters and chaperones control Cu ion levels and bioavailability to ensure proper subcellular and systemic Cu distribution. Intensive research has focused on understanding how mammalian cells maintain Cu homeostasis, and how molecular signals coordinate Cu acquisition and storage within organs. In humans, mutations of genes that regulate Cu homeostasis or facilitate interactions with Cu ions lead to numerous pathologic conditions. Malfunctions of the Cu⁺ -transporting ATPases ATP7A and ATP7B cause Menkes disease and Wilson disease, respectively. Additionally, defects in the mitochondrial and cellular distributions and homeostasis of Cu lead to severe neurodegenerative conditions, mitochondrial myopathies, and metabolic diseases. Cu has a dual nature in carcinogenesis as a promotor of tumor growth and an inducer of redox stress in cancer cells. Cu also plays role in cancer treatment as a component of drugs and a regulator of drug sensitivity and uptake. In this review, we provide an overview of the current knowledge of Cu metabolism and transport and its relation to various human pathologies.

Copper-zinc superoxide dismutase is activated through a sulfenic acid intermediate at a copper ion entry site

Metallochaperones are a diverse family of trafficking molecules that provide metal ions to protein targets for use as cofactors. The copper chaperone for superoxide dismutase (Ccs1) activates immature copper-zinc superoxide dismutase (Sod1) by delivering copper and facilitating the oxidation of the Sod1 intramolecular disulfide bond. Here, we present structural, spectroscopic, and cell-based data supporting a novel copper-induced mechanism for Sod1 activation. Ccs1 binding exposes an electropositive cavity and proposed "entry site" for copper ion delivery on immature Sod1. Copper-mediated sulfenylation leads to a sulfenic acid intermediate that eventually resolves to form the Sod1 disulfide bond with concomitant release of copper into the Sod1 active site. Sod1 is the predominant disulfide bond-requiring enzyme in the cytoplasm, and this copper-induced mechanism of disulfide bond formation obviates the need for a thiol/disulfide oxidoreductase in that compartment.

How Mammalian Cells Acquire Copper: An Essential but Potentially Toxic Metal

Cu is an essential micronutrient, and its role in an array of critical physiological processes is receiving increasing attention. Among these are wound healing, angiogenesis, protection against reactive oxygen species, neurotransmitter synthesis, modulation of normal cell and tumor growth, and many others. Free Cu is absent inside cells, and a network of proteins has evolved to deliver this essential, but potentially toxic, metal ion to its intracellular target sites following uptake. Although the total body content is low (∼100 mg), dysfunction of proteins involved in Cu homeostasis results in several well-characterized human disease states. The initial step in cellular Cu handling is its transport across the plasma membrane, a subject of study for only about the last 25 years. This review focuses on the initial step in Cu homeostasis, the properties of the major protein, hCTR1, that mediates Cu uptake, and the status of our understanding of this highly specialized transport system. Although a high-resolution structure of the protein is still lacking, an array of biochemical and biophysical studies have provided a picture of how hCTR1 mediates Cu(I) transport and how Cu is delivered to the proteins in the intracellular milieu. Recent studies provide evidence that the transporter also plays a key protective role in the regulation of cellular Cu via regulatory endocytosis, lowering its surface expression, in response to elevated Cu loads.

Molecular mechanisms underlying copper homeostasis in Mammalian cells

Copper (Cu) is an essential metal for living organisms that utilize oxygen for respiration and is required as a cofactor of redox-regulating enzymes, such as superoxide dismutase, ceruloplasmin, lysyl oxidase, tyrosinase, and dopamine β-hydroxylase. However, the redox-active property of this metal may have toxic effects on cells due to the generation of harmful reactive oxygen species. Given these circumstances, it is said that cells have a dependable system for Cu homeostasis that efficiently distributes this essential metal to cuproenzymes, thereby preventing damage to proteins, nucleic acids, sugars, and lipids. In particular, influx, efflux, and intracellular distribution with maintenance of the oxidation state of Cu are strictly regulated. Several groups of Cu-regulating factors have been identified in mammalian cells, i.e., Cu transporters, Cu chaperones, Cu-binding proteins/peptides, and others. In this review, the features of the Cu-regulating factors are concisely examined in terms of molecular mechanisms underlying Cu homeostasis in cells.

Copper in Eukaryotes

Copper is essential for normal growth and development of eukaryotic organisms. Numerous physiological processes rely on sufficient availability of copper: from indispensable reactions such as mitochondrial respiration to more highly specialized processes such as pigment development in a skin. Copper misbalance has been linked to a variety of metabolic and neurodegenerative disorders in humans. Complex cellular machinery has evolved to mediate copper uptake, compartmentalization and incorporation into target proteins. Extensive studies revealed a predominant utilization of methionines and histidines by copper handling molecules for copper capture at the extracellular surface and delivery to cuproenzymes in the lumen of cellular compartments, respectively. Cu(I) is a predominant form within the cell, and copper binding and distribution inside the cell at the cytosolic sites relies heavily on cysteines. The selectivity and directionality of copper transfer reactions is determined by thermodynamic and kinetic factors as well as spatial distribution of copper donors and acceptors. In this chapter, we review current structural and mechanistic data on copper transport and distribution in yeast and mammalian cells and highlight important issues and questions for future studies.

Upregulated copper transporters in hypoxia-induced pulmonary hypertension

Pulmonary vascular remodeling and increased arterial wall stiffness are two major causes for the elevated pulmonary vascular resistance and pulmonary arterial pressure in patients and animals with pulmonary hypertension. Cellular copper (Cu) plays an important role in angiogenesis and extracellular matrix remodeling; increased Cu in vascular smooth muscle cells has been demonstrated to be associated with atherosclerosis and hypertension in animal experiments. In this study, we show that the Cu-uptake transporter 1, CTR1, and the Cu-efflux pump, ATP7A, were both upregulated in the lung tissues and pulmonary arteries of mice with hypoxia-induced pulmonary hypertension. Hypoxia also significantly increased expression and activity of lysyl oxidase (LOX), a Cu-dependent enzyme that causes crosslinks of collagen and elastin in the extracellular matrix. In vitro experiments show that exposure to hypoxia or treatment with cobalt (CoCl2) also increased protein expression of CTR1, ATP7A, and LOX in pulmonary arterial smooth muscle cells (PASMC). In PASMC exposed to hypoxia or treated with CoCl2, we also confirmed that the Cu transport is increased using 64Cu uptake assays. Furthermore, hypoxia increased both cell migration and proliferation in a Cu-dependent manner. Downregulation of hypoxia-inducible factor 1α (HIF-1α) with siRNA significantly attenuated hypoxia-mediated upregulation of CTR1 mRNA. In summary, the data from this study indicate that increased Cu transportation due to upregulated CTR1 and ATP7A in pulmonary arteries and PASMC contributes to the development of hypoxia-induced pulmonary hypertension. The increased Cu uptake and elevated ATP7A also facilitate the increase in LOX activity and thus the increase in crosslink of extracellular matrix, and eventually leading to the increase in pulmonary arterial stiffness.

Species-specific activation of Cu/Zn SOD by its CCS copper chaperone in the pathogenic yeast Candida albicans

Candida albicans is a pathogenic yeast of important public health relevance. Virulence of C. albicans requires a copper and zinc containing superoxide dismutase (SOD1), but the biology of C. albicans SOD1 is poorly understood. To this end, C. albicans SOD1 activation was examined in baker's yeast (Saccharomyces cerevisiae), a eukaryotic expression system that has proven fruitful for the study of SOD1 enzymes from invertebrates, plants, and mammals. In spite of the 80% similarity between S. cerevisiae and C. albicans SOD1 molecules, C. albicans SOD1 is not active in S. cerevisiae. The SOD1 appears incapable of productive interactions with the copper chaperone for SOD1 (CCS1) of S. cerevisiae. C. albicans SOD1 contains a proline at position 144 predicted to dictate dependence on CCS1. By mutation of this proline, C. albicans SOD1 gained activity in S. cerevisiae, and this activity was independent of CCS1. We identified a putative CCS1 gene in C. albicans and created heterozygous and homozygous gene deletions at this locus. Loss of CCS1 resulted in loss of SOD1 activity, consistent with its role as a copper chaperone. C. albicans CCS1 also restored activity to C. albicans SOD1 expressed in S. cerevisiae. C. albicans CCS1 is well adapted for activating its partner SOD1 from C. albicans, but not SOD1 from S. cerevisiae. In spite of the high degree of homology between the SOD1 and CCS1 molecules in these two fungal species, there exists a species-specific barrier in CCS-SOD interactions which may reflect the vastly different lifestyles of the pathogenic versus the noninfectious yeast.

Copper chaperones. The concept of conformational control in the metabolism of copper

Copper chaperones compose a specific class of proteins assuring safe handling and specific delivery of potentially harmful copper ions to a variety of essential copper proteins. Copper chaperones are structurally heterogeneous and can exist in multiple metal-loaded as well as oligomeric forms. Moreover, many copper chaperones can exist in various oxidative states and participate in redox catalysis, connected with their functioning. This review is focused on the analysis of the structural and functional properties of copper chaperones and their partners, which allowed us to define specific regulatory principles in copper metabolism connected with copper-induced conformational control of copper proteins.

The structural plasticity of the human copper chaperone for SOD1: insights from combined size-exclusion chromatographic and solution X-ray scattering studies

The incorporation of copper into biological macromolecules such as SOD1 (Cu,Zn superoxide dismutase) is essential for the viability of most organisms. However, copper is toxic and therefore the intracellular free copper concentration is kept to an absolute minimum. Several proteins, termed metallochaperones, are charged with the responsibility of delivering copper from membrane transporters to its intracellular destination. The CCS (copper chaperone for SOD1) is the major pathway for SOD1 copper loading. We have determined the first solution structure of hCCS (human CCS) by SAXS (small-angle X-ray scattering) in conjunction with SEC (size-exclusion chromatography). The findings of the present study highlight the importance of this combined on-line chromatographic technology with SAXS, which has allowed us to unambiguously separate the hCCS dimer from other oligomeric and non-physiological aggregated states that would otherwise adversely effect measurements performed on bulk solutions. The present study exposes the dynamic molecular conformation of this multi-domain chaperone in solution. The metal-binding domains known to be responsible for the conveyance of copper to SOD1 can be found in positions that would expedite this movement. Domains I and III of a single hCCS monomer are able to interact and can also move into positions that would facilitate initial copper binding and ultimately transfer to SOD1. Conversely, the interpretation of our solution studies is not compatible with an interaction between these domains and their counterparts in an hCCS dimer. Overall, the results of the present study reveal the plasticity of this multi-domain chaperone in solution and are consistent with an indispensable flexibility necessary for executing its dual functions of metal binding and transfer.

The essential role of the Cu(II) state of Sco in the maturation of the Cu(A) center of cytochrome oxidase: evidence from H135Met and H135SeM variants of the Bacillus subtilis Sco

Sco is a red copper protein that plays an essential yet poorly understood role in the metalation of the Cu(A) center of cytochrome oxidase, and is stable in both the Cu(I) and Cu(II) forms. To determine which oxidation state is important for function, we constructed His135 to Met or selenomethionine (SeM) variants that were designed to stabilize the Cu(I) over the Cu(II) state. H135M was unable to complement a scoΔ strain of Bacillus subtilis, indicating that the His to Met substitution abrogated cytochrome oxidase maturation. The Cu(I) binding affinities of H135M and H135SeM were comparable to that of the WT and 100-fold tighter than that of the H135A variant. The coordination chemistry of the H135M and H135SeM variants was studied by UV/vis, EPR, and XAS spectroscopy in both the Cu(I) and the Cu(II) forms. Both oxidation states bound copper via the S atoms of C45, C49 and M135. In particular, EXAFS data collected at both the Cu and the Se edges of the H135SeM derivative provided unambiguous evidence for selenomethionine coordination. Whereas the coordination chemistry and copper binding affinity of the Cu(I) state closely resembled that of the WT protein, the Cu(II) state was unstable, undergoing autoreduction to Cu(I). H135M also reacted faster with H(2)O(2) than WT Sco. These data, when coupled with the complete elimination of function in the H135M variant, imply that the Cu(I) state cannot be the sole determinant of function; the Cu(II) state must be involved in function at some stage of the reaction cycle.

Copper in diseases and treatments, and copper-based anticancer strategies

Copper is found in all living organisms and is a crucial trace element in redox chemistry, growth and development. It is important for the function of several enzymes and proteins involved in energy metabolism, respiration, and DNA synthesis, notably cytochrome oxidase, superoxide dismutase, ascorbate oxidase, and tyrosinase. The major functions of copper-biological molecules involve oxidation-reduction reactions in which they react directly with molecular oxygen to produce free radicals. Therefore, copper requires tightly regulated homeostatic mechanisms to ensure adequate supplies without any toxic effects. Overload or deficiency of copper is associated, respectively, with Wilson disease (WD) and Menkes disease (MD), which are of genetic origin. Researches on Menkes and Wilson disorders have provided useful insights in the field of copper homeostasis and in particular into the understanding of intracellular trafficking and distribution of copper at molecular levels. Therapies based on metal supplementation with copper histidine or removal of copper excess by means of specific copper chelators are currently effective in treating MD and WD, respectively. Copper chelation therapy is now attracting much attention for the investigation and treatment of various neurodegenerative disorders such as Alzheimer, Parkinson and CreutzfeldtJakob. An excess of copper appears to be an essential co-factor for angiogenesis. Moreover, elevated levels of copper have been found in many types of human cancers, including prostate, breast, colon, lung, and brain. On these basis, the employment of copper chelators has been reported to be of therapeutic value in the treatment of several types of cancers as anti-angiogenic molecules. More recently, mixtures of copper chelators with copper salts have been found to act as efficient proteasome inhibitors and apoptosis inducers, specifically in cancer cells. Moreover, following the worldwide success of platinum(II) compounds in cancer chemotherapy, several families of individual copper complexes have been studied as potential antitumor agents. These investigations, revealing the occurrence of mechanisms of action quite different from platinum drugs, head toward the development of new anticancer metallodrugs with improved specificity and decreased toxic side effects.

The right to choose: multiple pathways for activating copper,zinc superoxide dismutase

Since the discovery of SOD1 in 1969, there have been numerous achievements made in our understanding of the enzyme's biochemical reactivity and its role in oxidative stress protection and as a genetic determinant in amyotrophic lateral sclerosis. Many recent advances have also been made in understanding the "activation" of SOD1, i.e. the process by which an inert polypeptide is converted to a mature active enzyme through post-translational modifications. To date, two such activation pathways have been identified: one requiring the CCS copper chaperone and one that works independently of CCS to insert copper and activate SOD1 through oxidation of an intramolecular disulfide. Depending on an organism's lifestyle and complexity, different eukaryotes have evolved to favor one pathway over the other. Some organisms rely solely on CCS for activating SOD1, and others can only activate SOD1 independently of CCS, whereas the majority of eukaryotes appear to have evolved to use both pathways. In this minireview, we shall highlight recent advances made in understanding the mechanisms by which the CCS-dependent and CCS-independent pathways control the activity, structure, and intracellular localization of copper,zinc superoxide dismutase, with relevance to amyotrophic lateral sclerosis and an emphasis on evolutionary biology.

Selenocysteine positional variants reveal contributions to copper binding from cysteine residues in domains 2 and 3 of human copper chaperone for superoxide dismutase

The human copper chaperone for superoxide dismutase binds copper both in an Atx1-like MTCQSC motif in domain 1 and via a multinuclear cluster formed by two CXC motifs at the D3 dimer interface. The composition of the Cu(I) cluster has been investigated previously by mutagenesis of the CXC motif, and by construction of a CXU selenocysteine derivative, which has permitted XAS studies at both Cu and Se absorption edges. Here, we report the semisynthesis and spectroscopic characterization of a series of derivatives with the sequences 243-CACA, 243-CAUA, 243-UACA, and 243-UAUA in the D1 double mutant (C22AC25A) background, prepared by expressed protein ligation of Sec-containing tetrapeptides to an hCCS-243 truncation. By varying the position of the Se atom in the CXC motif, we have been able to show that Se is always bridging (2 Se-Cu) rather than terminal (1 Se-Cu). Substitution of both D3 Cys residues by Sec in the UAUA variant does not eliminate the Cu-S contribution, confirming our previous description of the cluster as most likely a Cu(4)S(6) species, and suggesting that D2 Cys residues contribute to the cluster. As predicted by this model, when Cys residues C141, C144, and C227 are mutated to alanine either individually or together as a triple mutant, the cluster nuclearity is dramatically attenuated. These data suggest that Cys residues in D2 of hCCS are involved in the formation, stability, and redox potential of the D3 cluster. The significance of these finding to the SOD1 thiol/disulfide oxidase activity are discussed in terms of a model in which a similar multinuclear cluster may form in the CCS-SOD heterodimer.

Instability of superoxide dismutase 1 of Drosophila in mutants deficient for its cognate copper chaperone

Copper,zinc superoxide dismutase (SOD1) in mammals is activated principally via a copper chaperone (CCS) and to a lesser degree by a CCS-independent pathway of unknown nature. In this study, we have characterized the requirement for CCS in activating SOD1 from Drosophila. A CCS-null mutant (Ccs(n)(29)(E)) of Drosophila was created and found to phenotypically resemble Drosophila SOD1-null mutants in terms of reduced adult life span, hypersensitivity to oxidative stress, and loss of cytosolic aconitase activity. However, the phenotypes of CCS-null flies were less severe, consistent with some CCS-independent activation of Drosophila SOD1 (dSOD1). Yet SOD1 activity was not detectable in Ccs(n)(29)(E) flies, due largely to a striking loss of SOD1 protein. In contrast, human SOD1 expressed in CCS-null flies is robustly active and rescues the deficits in adult life span and sensitivity to oxidative stress. The dependence of dSOD1 on CCS was also observed in a yeast expression system where the dSOD1 polypeptide exhibited unusual instability in CCS-null (ccs1Delta) yeast. The residual dSOD1 polypeptide in ccs1Delta yeast was nevertheless active, consistent with CCS-independent activation. Stability of dSOD1 in ccs1Delta cells was readily restored by expression of either yeast or Drosophila CCS, and this required copper insertion into the enzyme. The yeast expression system also revealed some species specificity for CCS. Yeast SOD1 exhibits preference for yeast CCS over Drosophila CCS, whereas dSOD1 is fully activated with either CCS molecule. Such variation in mechanisms of copper activation of SOD1 could reflect evolutionary responses to unique oxygen and/or copper environments faced by divergent species.

A selenocysteine variant of the human copper chaperone for superoxide dismutase. A Se-XAS probe of cluster composition at the domain 3-domain 3 dimer interface

We report the semisynthesis of a selenocysteine (Sec) derivative of the human copper chaperone for superoxide dismutase, substituted with Sec at the C-terminal C246 residue. Measurements of hCCS-induced SOD1 activation were used to show that the C-terminal CXC sequence is both necessary and sufficient for EZn-SOD maturation. Therefore, an active CAU variant carrying Sec as the terminal amino acid was prepared by expressed protein ligation of a single selenocysteine amino acid to a 243-CA truncation. This reaction proceeded in high yield and generated the desired 243-CAX (X = C or U) protein with the expected mass. Se-edge XAS of the apoprotein indicated that both Se-S and Se-Se interactions were present in a 0.3:0.7 ratio, indicating an equilibrium between species with either a selenosulfide or a diselenide cross-link. After reduction on immobilized TCEP, the ligated Cys and Sec apoproteins bound up to 2.5 Cu(I) ions per hCCS monomer with both Cu and Se as constituent atoms of the cluster which forms at the domain 3 interface of a hCCS dimer. Merging of XAS data at the Cu and Se K-absorption edges provided additional details of the cluster composition, specifically the fact that both Se atoms occupied bridging positions between two Cu(I) atoms. Further, the requirement for identical Cu-Se bond lengths and Debye-Waller factors at each absorption edge allowed us to rule out simple models for the cluster composition such as a bis-Cys(Sec)-bridged dinuclear cluster and was indicative of a more complex cluster with a nuclearity of >or=3.

Oxidative switches in functioning of mammalian copper chaperone Cox17

Cox17, a copper chaperone for cytochrome-c oxidase, is an essential and highly conserved protein in eukaryotic organisms. Yeast and mammalian Cox17 share six conserved cysteine residues, which are involved in complex redox reactions as well as in metal binding and transfer. Mammalian Cox17 exists in three oxidative states, each characterized by distinct metal-binding properties: fully reduced mammalian Cox17(0S-S) binds co-operatively to four Cu+; Cox17(2S-S), with two disulfide bridges, binds to one of either Cu+ or Zn2+; and Cox17(3S-S), with three disulfide bridges, does not bind to any metal ions. The E(m) (midpoint redox potential) values for two redox couples of Cox17, Cox17(3S-S)<-->Cox17(2S-S) (E(m1)) and Cox17(2S-S)<-->Cox17(0S-S) (E(m2)), were determined to be -197 mV and -340 mV respectively. The data indicate that an equilibrium exists in the cytosol between Cox17(0S-S) and Cox17(2S-S), which is slightly shifted towards Cox17(0S-S). In the IMS (mitochondrial intermembrane space), the equilibrium is shifted towards Cox17(2S-S), enabling retention of Cox17(2S-S) in the IMS and leading to the formation of a biologically competent form of the Cox17 protein, Cox17(2S-S), capable of copper transfer to the copper chaperone Sco1. XAS (X-ray absorption spectroscopy) determined that Cu4Cox17 contains a Cu4S6-type copper-thiolate cluster, which may provide safe storage of an excess of copper ions.

ArsD Residues Cys12, Cys13, and Cys18 Form an As(III)-binding Site Required for Arsenic Metallochaperone Activity

The ArsA ATPase is the catalytic subunit of the ArsAB pump encoded by the arsRDABC operon of Escherichia coli plasmid R773. ArsD is a metallochaperone that delivers As(III) to ArsA, increasing its affinity for As(III), thus conferring resistance to environmental concentrations of arsenic. R773 ArsD is a homodimer with three vicinal cysteine pairs, Cys(12)-Cys(13), Cys(112)-Cys(113), and Cys(119)-Cys(120), in each subunit. Each vicinal pair binds As(III) or Sb(III). Alignment of the primary sequence of homologues of ArsD indicates that only the first vicinal cysteine pair, Cys(12)-Cys(13), and an additional cysteine, Cys(18), are conserved. The effect of cysteine-to-alanine substitutions and truncations were examined. By yeast two-hybrid analysis, nearly all of the ArsD mutants were able to interact with wild type ArsD, indicating that the mutations do not interfere with dimerization. ArsD mutants with alanines substituting for Cys(112), Cys(113), Cys(119), or Cys(120) individually or in pairs or truncations lacking the vicinal pairs retained ability to interact with ArsA and to activate its ATPase activity. Cells expressing these mutants retained ArsD-enhanced As(III) efflux and resistance. In contrast, mutants with substitutions of conserved Cys(12), Cys(13), or Cys(18), individually or in pairs, were unable to activate ArsA or to enhance the activity of the ArsAB pump. We propose that ArsD residues Cys(12), Cys(13), and Cys(18), but not Cys(112), Cys(113), Cys(119), or Cys(120), are required for delivery of As(III) to and activation of the ArsAB pump.

Copper entry into human cells: progress and unanswered questions

In this brief review we summarize what is known about the role of hCTR1 in mediating the entry of copper into human cells. There is a body of information that clearly identifies this protein as being a major source (though not the only source) of copper entry into human cells, and thus a crucial element of copper homeostasis. However, much remains that is poorly understood and key aspects of the physiological roles of hCTR1 and its regulation are only superficially appreciated. The particular characteristics of a transport process that in vivo involves the binding, transmembrane transport and release of a substrate that is not present in a free form in the intracellular or extracellular compartments poses particular challenges that are not encountered in the transport of more familiar physiologically important metal cations. Thus much of what we have learned about the more commonly encountered transported ions provides an inadequate model for studies of copper homeostasis. In this article we review progress made and identify the major questions that need to be resolved before an adequate description is attained of how copper entry into human cells is mediated and regulated by hCTR1.

Copper homeostasis in eukaryotes: Teetering on a tightrope

The transition metal copper is an essential trace element for both prokaryotes and eukaryotes. However, intracellular free copper has to be strictly limited due to its toxic side effects, not least the generation of reactive oxygen species (ROS) via redox cycling. Thus, all organisms have sophisticated copper homeostasis mechanisms that regulate uptake, distribution, sequestration and export of copper. From insects to mammals, metal-responsive transcription factor (MTF-1), a zinc finger transcription factor, controls expression of metallothioneins and other components involved in heavy metal homeostasis. In the fruit fly Drosophila, MTF-1 paradoxically acts as an activator under both high and low copper concentrations. Namely, under high copper conditions, MTF-1 activates metallothioneins in order to protect the cell, while under low copper conditions MTF-1 activates the copper importer Ctr1B in order to acquire scarce copper from the surroundings. This review highlights the current knowledge of copper homeostasis in eukaryotes with a focus on Drosophila and the role of MTF-1.

Posttranslational modifications in Cu,Zn-superoxide dismutase and mutations associated with amyotrophic lateral sclerosis

Activation of the enzyme Cu,Zn-superoxide dismutase (SOD1) involves several posttranslational modifications including copper and zinc binding, as well as formation of the intramolecular disulfide bond. The copper chaperone for SOD1, CCS, is responsible for intracellular copper loading in SOD1 under most physiological conditions. Recent in vitro and in vivo assays reveal that CCS not only delivers copper to SOD1 under stringent copper limitation, but it also facilitates the stepwise conversion of the disulfide-reduced immature SOD1 to the active disulfide-containing enzyme. The two new functions attributed to CCS, (i.e., O(2)-dependent sulfhydryl oxidase- and disulfide isomerase-like activities) indicate that this protein has attributes of the larger class of molecular chaperones. The CCS-dependent activation of SOD1 is dependent upon oxygen availability, suggesting that the cell only loads copper and activates this enzyme when O(2)-based oxidative stress is present. Thiol/disulfide status as well as metallation state of SOD1 significantly affects its structure and protein aggregation, which are relevant in pathologies of a neurodegenerative disease, amyotrophic lateral sclerosis (ALS). The authors review here a mechanism for posttranslational activation of SOD1 and discuss models for ALS in which the most immature forms of the SOD1 polypeptide exhibits propensity to form toxic aggregates.

Structure and coordination of CuB in the Acidianus ambivalens aa3 quinol oxidase heme-copper center

The coordination environment of the Cu(B) center of the quinol oxidase from Acidianus ambivalens, a type B heme-copper oxygen reductase, was investigated by Fourier transform (FT) IR and extended X-ray absorption fine structure (EXAFS) spectroscopy. The comparative structural chemistry of dinuclear Fe-Cu sites of the different types of oxygen reductases is of great interest. Fully reduced A. ambivalens quinol oxidase binds CO at the heme a (3) center, with nu(CO)=1,973 cm(-1). On photolysis, the CO migrated to the Cu(B) center, forming a Cu (B) (I) -CO complex with nu(CO)=2,047 cm(-1). Raising the temperature of the samples to 25 degrees C did not result in a total loss of signal in the FTIR difference spectrum although the intensity of these signals was reduced sevenfold. This observation is consistent with a large energy barrier against the geminate rebinding of CO to the heme iron from Cu(B), a restricted limited access at the active-site pocket for a second binding, and a kinetically stable Cu(B)-CO complex in A. ambivalens aa (3). The Cu(B) center was probed in a number of different states using EXAFS spectroscopy. The oxidized state was best simulated by three histidines and a solvent O scatterer. On reduction, the site became three-coordinate, but in contrast to the bo (3) enzyme, there was no evidence for heterogeneity of binding of the coordinated histidines. The Cu(B) centers in both the oxidized and the reduced enzymes also appeared to contain substoichiometric amounts (0.2 mol equiv) of nonlabile chloride ion. EXAFS data of the reduced carbonylated enzyme showed no difference between dark and photolyzed forms. The spectra could be well fit by 2.5 imidazoles, 0.5 Cl(-) and 0.5 CO ligands. This arrangement of scatterers would be consistent with about half the sites remaining as unligated Cu(his)(3) and half being converted to Cu(his)(2)Cl(-)CO, a 50/50 ratio of Cu(his)(2)Cl(-) and Cu(his)(3)CO, or some combination of these formulations.

Copper transfer to the N-terminal domain of the Wilson disease protein (ATP7B): X-ray absorption spectroscopy of reconstituted and chaperone-loaded metal binding domains and their interaction with exogenous ligands

The copper-transporting ATPases are 165-175 kDa membrane proteins, composed of 8 transmembrane segments and two large cytosolic domains, the N-terminal copper-binding domain and the catalytic ATP-hydrolyzing domain. In ATP7B, the Wilson disease protein, the N-terminal domain is made up of six metal-binding sub-domains containing the MXCXXC motif which is known to coordinate copper via the two cysteine residues. We have expressed the N-terminal domain of ATP7B as a soluble C-terminal fusion with the maltose binding protein. This expression system produces a protein which can be reconstituted with copper without recourse to the harsh denaturing conditions or low pH reported by other laboratories. Here we describe the reconstitution of the metal binding domains (MBD) with Cu(I) using a number of different protocols, including copper loading via the chaperone, Atox1. X-ray absorption spectra have been obtained on all these derivatives, and their ability to bind exogenous ligands has been assessed. The results establish that the metal-binding domains bind Cu(I) predominantly in a bis cysteinate environment, and are able to bind exogenous ligands such as DTT in a similar fashion to Atox1. We have further observed that exogenous ligand binding induces the formation of a Cu-Cu interaction which may signal a conformational change of the N-terminal domain.

The Unusually Stable Quaternary Structure of Human Cu,Zn-Superoxide Dismutase 1 Is Controlled by Both Metal Occupancy and Disulfide Status

The eukaryotic copper,zinc superoxide dismutases are remarkably stable dimeric proteins that maintain an intrasubunit disulfide bond in the reducing environment of the cytosol and are active under a variety of stringent denaturing conditions. The structural interplay of conserved disulfide bond and metal-site occupancy in human copper,zinc superoxide dismutase (hSOD1) is of increasing interest as these post-translational modifications are known to dramatically alter the catalytic chemistry, the subcellular localization, and the susceptibility of the protein to aggregation. Using biophysical methods, we find no significant change in the gross secondary or tertiary structure of the demetallated form upon reduction of the disulfide. Interestingly, reduction does lead to a dramatic change in the quaternary structure, decreasing the monomer-to-dimer equilibrium constant by at least four orders of magnitude. This reduced form of hSOD1 is monomeric, even at concentrations well above the physiological range. Either the addition of Zn(II) or the formation of the disulfide leads to a shift in equilibrium that favors the dimeric species, even at low protein concentrations (i.e. micromolar range). We conclude that only the most immature form of hSOD1, i.e. one without any post-translational modifications, favors the monomeric state under physiological conditions. This finding provides a basis for understanding the selectivity of mitochondrial SOD1 import and may be relevant to the toxic properties of mutant forms of hSOD1 that can cause the familial form of amyotrophic lateral sclerosis.

Solution structure of Cox11, a novel type of beta-immunoglobulin-like fold involved in CuB site formation of cytochrome c oxidase

Cytochrome c oxidase assembly process involves many accessory proteins including Cox11, which is a copper-binding protein required for Cu incorporation into the Cu(B) site of cytochrome c oxidase. In a genome wide search, a number of Cox11 homologs are found in all of the eukaryotes with complete genomes and in several Gram-negative bacteria. All of them possess a highly homologous soluble domain and contain an N-terminal fragment that anchors the protein to the membrane. An anchor-free construct of 164 amino acids was obtained from Sinorhizobium meliloti, and the first structure of this class of proteins is reported here. The apoform has an immunoglobulin-like fold with a novel type of beta-strand organization. The copper binding motif composed of two highly conserved cysteines is located on one side of the beta-barrel structure. The apoprotein is monomeric in the presence of dithiothreitol, whereas it dimerizes in the absence of the reductant. When copper(I) binds, NMR and extended x-ray absorption fine structure (EXAFS) data indicate a dimeric protein state with two thiolates bridging two copper(I) ions. The present results advance the knowledge on the poorly understood molecular aspects of cytochrome c oxidase assembly.

Oxygen and the copper chaperone CCS regulate posttranslational activation of Cu,Zn superoxide dismutase

Oxidative stress leads to the up-regulation of many antioxidant enzymes including Cu,Zn superoxide dismutase (SOD1) via transcriptional mechanisms; however, few examples of posttranslational regulation are known. The copper chaperone for SOD1 (CCS) is involved in physiological SOD1 activation, and its primary function is thought to be delivery of copper to the enzyme. Data presented here are consistent with a previously uncharacterized function for CCS in the SOD1 pathway, namely mediating enzyme activation in response to increases in oxygen tension. Activity assays with pure proteins and cell extracts reveal that O(2) (or superoxide) is required for activation of SOD1 by CCS. Dose-response studies with a translational blocking agent demonstrate that the cellular oxidative response to O(2) is multitiered: existing apo-pools of SOD1 are activated by CCS in the early response, followed by increasing expression of SOD1 protein with persistent oxidative stress. This CCS function provides oxidant-responsive posttranslational regulation of SOD1 activity and may be relevant to a wide array of physiological stresses that involve a sudden elevation of oxygen availability.

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.

Copper chaperones: personal escorts for metal ions

Copper serves as the essential cofactor for a number of enzymes involved in redox chemistry and virtually all organisms must accumulate trace levels of copper in order to survive. However, this metal can also be toxic and a number of effective methods for sequestering and detoxifying copper prevent the metal from freely circulating inside a cell. Copper metalloenzymes are therefore faced with the challenge of acquiring their precious metal cofactor in the absence of available copper. To overcome this dilemma, all eukaryotic organisms have evolved with a family of intracellular copper binding proteins that help reserve a bioavailable pool of copper for the metalloenzymes, escort the metal to appropriate targets, and directly transfer the copper ion. These proteins have been collectively called "copper chaperones." The identification of such molecules has been made possible through molecular genetic studies in the bakers' yeast Saccharomyces cerevisiae. In this review, we highlight the findings that led to a new paradigm of intracellular trafficking of copper involving the action of copper chaperones. In particular, emphasis will be placed on the ATX1 and CCS copper chaperones that act to deliver copper to the secretory pathway and to Cu/Zn superoxide dismutase in the cytosol, respectively.

Metallochaperones: bind and deliver

Metallochaperones deliver metal ions directly to target proteins via specific protein-protein interactions. Recent research has led to a molecular picture of how some metallochaperones bind metal ions, recognize their partner proteins, and accomplish metal ion transfer.

Human CCS gene: genomic organization and exclusion as a candidate for amyotrophic lateral sclerosis (ALS)

BACKGROUND

Amyotrophic lateral sclerosis (ALS) is a progressive lethal disorder of large motor neurons of the spinal cord and brain. In approximately 20% of the familial and 2% of sporadic cases the disease is due to a defect in the gene encoding the cytosolic antioxidant enzyme Cu, Zn-superoxide dismutase (SOD1). The underlying molecular defect is known only in a very small portion of the remaining cases and therefore involvement of other genes is likely. As SOD1 receives copper, essential for its normal function, by the copper chaperone, CCS (Copper Chaperone for SOD), we considered CCS as a potential candidate gene for ALS.

RESULTS

We have characterized the genomic organization of CCS and determined exon-intron boundaries. The 823 bp coding region of the CCS is organized in 8 exons. We have evaluated involvement of the CCS in ALS by sequencing the entire coding region for mutations in 20 sporadic ALS patients.

CONCLUSIONS

No causative mutations for the ALS have been detected in the CCS gene in 20 sporadic ALS patients analyzed, but an intragenic single nucleotide polymorphism has been identified.

Function, structure, and mechanism of intracellular copper trafficking proteins

Genetic, biochemical, and spectroscopic studies have established a new function for an intracellular protein, i.e., guiding and inserting a copper cofactor into the active site of a target enzyme. Studies of these new proteins have revealed a fundamental aspect of copper physiology, namely the vast overcapacity of the cytoplasm for copper sequestration. This finding framed the mechanistic, energetic, and structural aspects of intracellular copper trafficking proteins. One hallmark of the copper chaperones is the similarity of the protein fold between the chaperone and its target enzyme. The surface residues presented by each partner, however, are quite different, and some initial findings concerning the complementarity of these interfaces have led to mechanistic insights. The copper chaperones appear to lower the activation barrier for metal transfer into specific protein-binding sites. The manner in which they facilitate metal insertion appears to involve a docking of the metal donor and acceptor sites in close proximity to one another. Although the intimate mechanism is still open, it appears that a low activation barrier for metal transfer is achieved by a network of coordinate-covalent, electrostatic, and hydrogen bonding interactions in the vicinity of the metal-binding site itself.

Copper stabilizes a heterodimer of the yCCS metallochaperone and its target superoxide dismutase

The copper chaperone for superoxide dismutase (CCS) activates the antioxidant enzyme Cu,Zn-SOD (SOD1) by directly inserting the copper cofactor into the apo form of SOD1. Neither the mechanism of protein-protein recognition nor of metal transfer is clear. The metal transfer step has been proposed to occur within a transient copper donor/acceptor complex that is either a heterodimer or heterotetramer (i.e. a dimer of dimers). To determine the nature of this intermediate, we generated a mutant form of SOD1 by replacing a copper binding residue His-48 with phenylalanine. This protein cannot accept copper from CCS but does form a stable complex with apo- and Cu-CCS, as observed by immunoprecipitation and native gel electrophoresis. Fluorescence anisotropy measurements corroborate the formation of this species and further indicate that copper enhances the stability of the dimer by an order of magnitude. The copper form of the heterodimer was isolated by gel filtration chromatography and contains one copper and one zinc atom per heterodimer. These results support a mechanism for copper transfer in which CCS and SOD1 dock via their highly conserved dimer interfaces in a manner that precisely orients the Cys-rich copper donor sites of CCS and the His-rich acceptor sites of SOD1 to form a copper-bridged intermediate.

Mechanism of Cu,Zn-superoxide dismutase activation by the human metallochaperone hCCS

The mechanism for copper loading of the antioxidant enzyme copper, zinc superoxide dismutase (SOD1) by its partner metallochaperone protein is not well understood. Here we show the human copper chaperone for Cu,Zn-SOD1 (hCCS) activates either human or yeast enzymes in vitro by direct protein to protein transfer of the copper cofactor. Interestingly, when denatured with organic solvents, the apo-form of human SOD1 cannot be reactivated by added copper ion alone, suggesting an additional function of hCCS such as facilitation of an active folded state of the enzyme. While hCCS can bind several copper ions, metal binding studies in the presence of excess copper scavengers that mimic the intracellular chelation capacity indicate a limiting stoichiometry of one copper and one zinc per hCCS monomer. This protein is active and unlike the yeast protein, is a homodimer regardless of copper occupancy. Matrix-assisted laser desorption ionization-mass spectrometry and metal binding studies suggest that Cu(I) is bound by residues from the first and third domains and no bound copper is detected for the second domain of hCCS in either the full-length or truncated forms of the protein. Copper-induced conformational changes in the essential C-terminal peptide of hCCS are consistent with a "pivot, insert, and release" mechanism that is similar to one proposed for the well characterized metal handling enzyme, mercuric ion reductase.

Copper delivery by metallochaperone proteins

Copper is an essential element in all living organisms, serving as a cofactor for many important proteins and enzymes. Metallochaperone proteins deliver copper ions to specific physiological partners by direct protein-protein interactions. The Atx1-like chaperones transfer copper to intracellular copper transporters, and the CCS chaperones shuttle copper to copper,zinc superoxide dismutase. Crystallographic studies of these two copper chaperone families have provided insights into metal binding and target recognition by metallochaperones and have led to detailed molecular models for the copper transfer mechanism.

A pivotal role of Zn-binding residues in the function of the copper chaperone for SOD1

A Cu chaperone for SOD1 (CCS) is required for the incorporation of copper ion into the protein. To investigate the roles of the conserved metal-binding residues in CCS, we introduced amino acid substitutions into human CCS and examined the function of the mutant CCS by transforming a mutant yeast strain, SY2950, which lacks the lys7 gene, a yeast orthologue of the mammalian CCS. Mutant CCS in which amino acid residues His147 and Asp167 were substituted by Ala exhibited a decreased ability to complement the growth of SY2950 under Lys-deficient conditions. This is because the mutations made the human CCS function in a less efficient manner, especially under metal-restricted conditions, leaving Cu,Zn-SOD in an apo-form. Since the His and Asp residues are both responsible for binding Zn which would serve to maintain the folded structure, the structural integrity supported by the coordinated Zn ion would be essential for CCS function.