Solution structure of the N-domain of Wilson disease protein: distinct nucleotide-binding environment and effects of disease mutations

Involvement of protein kinase D in expression and trafficking of ATP7B (copper ATPase)

ATP7B is a P-type ATPase involved in copper transport and homeostasis. In experiments with microsomes isolated from COS-1 cells or HepG2 hepatocytes sustaining ATP7B heterologous expression, we found that ATP7B utilization of ATP includes autophosphorylation of an aspartyl residue serving as ATPase catalytic intermediate as well as phosphorylation of serine residues by protein kinase D (PKD). The latter was abolished by specific PKD inhibition with CID755673. The presence of PKD protein in the microsomal fraction was demonstrated by Western blotting. PKD is a serine/threonine kinase that associates with the trans-Golgi network, regulating fission of transport carriers destined to the cell surface. Parallel studies on cultured cells showed that nascent WT ATP7B transits to the Golgi complex where it undergoes serine phosphorylation by PKD. Misfolded ATP7B protein (especially if subjected to deletions) underwent proteasome-mediated degradation, which provides effective quality control. Inhibition of proteasome-mediated degradation with MG132 yielded additional, but nonfunctional protein. On the other hand, serine phosphorylation protected WT ATP7B from degradation. Protection was enhanced by PKD activation with phorbol esters and limited by PKD inhibition with CID75673. As a final step, phosphorylated ATP7B was transferred from the Golgi complex to cytosolic trafficking vesicles. Phosphorylation and trafficking were completely prevented by mutations of critical copper binding sites, demonstrating copper dependence of both PKD-assisted phosphorylation and trafficking. ATP7B trafficking was markedly reduced by the Ser-478/481/1121/1453 to Ala mutation. We conclude that PKD plays a key role in copper-dependent serine phosphorylation, permitting high levels of ATP7B protein expression and trafficking.

Genetics of Wilsons disease

Wilson's disease is a rare autosomal recessive disorder of copper transport due to mutations in the ATP7B gene, responsible for transport of copper into bile from hepatocytes and its incorporation into apoceruloplasmin to form ceruloplasmin resulting in excessive accumulation of copper in the liver and extrahepatic tissues. Clinical features of WD result from toxic accumulation of copper in liver, brain and kidney. Early diagnosis is mandatory to initiate early treatment to prevent morbidity and mortality. More than 400 mutations have been reported, some of which are rather characteristic of geographical regions and ethnic population. Genetic testing is not useful as a routine procedure, but has its role in at risk individuals such as siblings and children of probands and in individuals with suggestive symptoms but where other tests are contradictory.

Cellular copper distribution: a mechanistic systems biology approach

Copper is an essential but potentially harmful trace element required in many enzymatic processes involving redox chemistry. Cellular copper homeostasis in mammals is predominantly maintained by regulating copper transport through the copper import CTR proteins and the copper exporters ATP7A and ATP7B. Once copper is imported into the cell, several pathways involving a number of copper proteins are responsible for trafficking it specifically where it is required for cellular life, thus avoiding the release of harmful free copper ions. In this study we review recent progress made in understanding the molecular mechanisms of copper transport in cells by analyzing structural features of copper proteins, their mode of interaction, and their thermodynamic and kinetic parameters, thus contributing to systems biology of copper within the cell.

The binding mode of ATP revealed by the solution structure of the N-domain of human ATP7A

We report the solution NMR structures of the N-domain of the Menkes protein (ATP7A) in the ATP-free and ATP-bound forms. The structures consist of a twisted antiparallel six-stranded beta-sheet flanked by two pairs of alpha-helices. A protein loop of 50 amino acids located between beta 3 and beta 4 is disordered and mobile on the subnanosecond time scale. ATP binds with an affinity constant of (1.2 +/- 0.1) x 10(4) m(-1) and exchanges with a rate of the order of 1 x 10(3) s(-1). The ATP-binding cavity is considerably affected by the presence of the ligand, resulting in a more compact conformation in the ATP-bound than in the ATP-free form. This structural variation is due to the movement of the alpha1-alpha2 and beta2-beta 3 loops, both of which are highly conserved in copper(I)-transporting P(IB)-type ATPases. The present structure reveals a characteristic binding mode of ATP within the protein scaffold of the copper(I)-transporting P(IB)-type ATPases with respect to the other P-type ATPases. In particular, the binding cavity contains mainly hydrophobic aliphatic residues, which are involved in van der Waal's interactions with the adenine ring of ATP, and a Glu side chain, which forms a crucial hydrogen bond to the amino group of ATP.

Structural organization of human Cu-transporting ATPases: learning from building blocks

Copper-transporting ATPases (Cu-ATPases) ATP7A and ATP7B play an essential role in human physiological function. Their primary function is to deliver copper to the secretory pathway and export excess copper from the cell for removal or further utilization. Cells employ Cu-ATPases in numerous physiological processes that include the biosynthesis of copper-dependent enzymes, lactation, and response to hypoxia. Biochemical studies of human Cu-ATPases and their orthologs have demonstrated that Cu-ATPases share many common structural and mechanistic characteristics with other members of the P-type ATPase family. Nevertheless, the Cu-ATPases have a unique coordinate environment for their ligands, copper and ATP, and additional domains that are required for sophisticated regulation of their intracellular localization and activity. Here, we review recent progress that has been made in understanding the structure of Cu-ATPases from the analysis of their individual domains and orthologs from microorganisms, and speculate about the implications of these findings for the function and regulation of human copper pumps.

Reaction cycle of Thermotoga maritima copper ATPase and conformational characterization of catalytically deficient mutants

Copper transport ATPases sustain important roles in homeostasis of heavy metals and delivery of copper to metalloenzymes. The copper transport ATPase from Thermotoga maritima (CopA) provides a useful system for mechanistic studies, due to its heterologous expression and stability. Its sequence comprises 726 amino acids, including the N-terminal metal binding domain (NMBD), three catalytic domains (A, N, and P), and a copper transport domain formed by eight helices, including the transmembrane metal binding site (TMBS). We performed functional characterization and conformational analysis by proteolytic digestion of WT and mutated (NMBD deletion or mutation) T. maritima CopA, comparing it with Archaeoglobus fulgidus CopA and Ca²⁺ ATPase. A specific feature of T. maritima CopA is ATP utilization in the absence of copper, to form a low-turnover phosphoenzyme intermediate, with a conformation similar to that obtained by phosphorylation with P_i or phosphate analogues. On the other hand, formation of an activated state requires copper binding to both NMBD and TMBS, with consequent conformational changes involving the NMBD and A domain. Proteolytic digestion analysis demonstrates A domain movements similar to those of other P-type ATPases to place the conserved TGES motif in the optimal position for catalytic assistance. We also studied an H479Q mutation (analogous to one of human copper ATPase ATP7B in Wilson disease) that inhibits ATPase activity. We found that, in spite of the H479Q mutation within the nucleotide binding domain, the mutant still binds ATP, yielding a phosphorylation transition state conformation. However, covalent phosphoryl transfer is not completed, and no catalytic turnover is observed.

Copper transport in mammalian cells: special care for a metal with special needs

Copper plays an essential role in human physiology. It is required for respiration, radical defense, neuronal myelination, angiogenesis, and many other processes. Copper has distinct physicochemical properties that pose uncommon challenges for its transport across biological membranes. Only small amounts of copper are present in biological fluids, and essentially none of it exists in a free ion form. These properties and the low redox potential of copper dictate special structural and mechanistic features in copper transporters. This minireview discusses molecular mechanisms through which copper enters and exits human cells.

Chaperone-mediated Cu+ delivery to Cu+ transport ATPases: requirement of nucleotide binding

Cu(+)-ATPases drive the efflux of Cu(+) from the cell cytoplasm. During their catalytic/transport cycle, cytoplasmic Cu(+)-chaperones deliver the metal to the two transmembrane metal-binding sites (TM-MBSs) responsible for Cu(+) translocation. Here, using Archaeoglobus fulgidus Cu(+)-ATPase CopA and the C-terminal Cu(+)-chaperone domain of CopZ (Ct-CopZ), we describe the mechanism of Cu(+) transfer to both TM-MBSs. In absence of other ligands, Ct-CopZ transfers Cu(+) to wild-type CopA and to various CopA constructs lacking or having mutated cytoplasmic metal-binding domains, in a fashion consistent with occupancy of a single TM-MBS. Similar experiments performed in the presence of 2.5 mm ADP-Mg(2+), stabilizing an E1.ADP, lead to full occupancy of both TM-MBSs. In both cases, the transfer is largely stoichiometric, i.e. equimolar amounts of Ct-CopZ.Cu(+) saturated the TM-MBSs. Experiments performed with CopA mutants lacking either TM-MBS showed that both sites are loaded independently, and nucleotide binding does not affect their availability. The nucleotide-induced E2-->E1 transition is structurally characterized by a large displacement of the A and N domains opening the cytoplasmic region of P-type ATPases. Then, it is apparent that, whereas the first Cu(+)-chaperone can bind an ATPase form available in the absence of ligands, the second requires the E1.nucleotide intermediary to interact and deliver the metal. Interestingly, independent of TM-MBS Cu(+) loading, nucleotide binding also prevents the regulatory interaction of the N-terminal cytoplasmic metal-binding domain with the nucleotide binding domain.

Nucleotide recognition by CopA, a Cu+-transporting P-type ATPase

Heavy metal pumps constitute a large subgroup in P-type ion-transporting ATPases. One of the outstanding features is that the nucleotide binding N-domain lacks residues critical for ATP binding in other well-studied P-type ATPases. Instead, they possess an HP-motif and a Gly-rich sequence in the N-domain, and their mutations impair ATP binding. Here, we describe 1.85 A resolution crystal structures of the P- and N-domains of CopA, an archaeal Cu(+)-transporting ATPase, with bound nucleotides. These crystal structures show that CopA recognises the adenine ring completely differently from other P-type ATPases. The crystal structure of the His462Gln mutant, in the HP-motif, a disease-causing mutation in human Cu(+)-ATPases, shows that the Gln side chain mimics the imidazole ring, but only partially, explaining the reduction in ATPase activity. These crystal structures lead us to propose a role of the His and a mechanism for removing Mg(2+) from ATP before phosphoryl transfer.

Crystallization and preliminary X-ray studies of the N-domain of the Wilson disease associated protein

Wilson disease associated protein (ATP7B) is essential for copper transport in human cells. Mutations that affect ATP7B function result in Wilson's disease, a chronic copper toxicosis. Disease-causing mutations within the N-domain of ATP7B (WND) are known to disrupt ATP binding, but a high-resolution X-ray structure of the ATP-binding site has not been reported. The N-domain was modified to delete the disordered loop comprising residues His1115-Asp1138 (WNDDelta(1115-1138)). Unlike the wild-type N-domain, WNDDelta(1115-1138) formed good-quality crystals. Synchrotron diffraction data have been collected from WNDDelta(1115-1138) at the Canadian Light Source. A native WNDDelta(1115-1138) crystal diffracted to 1.7 A resolution and belonged to space group P4(2)2(1)2, with unit-cell parameters a = 39.2, b = 39.2, c = 168.9 A. MAD data were collected to 2.7 A resolution from a SeMet-derivative crystal with unit-cell parameters a = 38.4, b = 38.4, c = 166.7 A. The WNDDelta(1115-1138) structure is likely to be solved by phasing from multiwavelength anomalous diffraction (MAD) experiments.

Atp7b-/- mice as a model for studies of Wilson's disease

Wilson's disease is a severe human disorder of copper homoeostasis. The disease is associated with various mutations in the ATP7B gene that encodes a copper-transporting ATPase, and a massive accumulation of copper in the liver and several other tissues. The most frequent disease manifestations include a wide spectrum of liver pathologies as well as neurological and psychiatric abnormalities. A combination of copper chelators and zinc therapy has been used to prevent disease progression; however, accurate and timely diagnosis of the disease remains challenging. Similarly, side effects of treatments are common. To understand better the biochemical and cellular basis of Wilson's disease, several animal models have been developed. This review focuses on genetically engineered Atp7b(-/-) mice and describes the properties of these knockout animals, insights into the disease progression generated using Atp7b(-/-) mice, as well as advantages and limitations of Atp7b(-/-) mice as an experimental model for Wilson's disease.

Zebrafish mutants calamity and catastrophe define critical pathways of gene-nutrient interactions in developmental copper metabolism

Nutrient availability is an important environmental variable during development that has significant effects on the metabolism, health, and viability of an organism. To understand these interactions for the nutrient copper, we used a chemical genetic screen for zebrafish mutants sensitive to developmental copper deficiency. In this screen, we isolated two mutants that define subtleties of copper metabolism. The first contains a viable hypomorphic allele of atp7a and results in a loss of pigmentation when exposed to mild nutritional copper deficiency. This mutant displays incompletely penetrant skeletal defects affected by developmental copper availability. The second carries an inactivating mutation in the vacuolar ATPase that causes punctate melanocytes and embryonic lethality. This mutant, catastrophe, is sensitive to copper deprivation revealing overlap between ion metabolic pathways. Together, the two mutants illustrate the utility of chemical genetic screens in zebrafish to elucidate the interaction of nutrient availability and genetic polymorphisms in cellular metabolism.

Copper is an essential nutrient required for multiple biologic functions. Proper uptake, transport, and excretion of copper are critical for use of this metal while reducing its inherent toxicity. While several key proteins involved in this process have been identified, there are still gaps in our understanding of copper metabolism—particularly during early development. We have used zebrafish, a genetically useful animal model system, to study genetic interactions with copper deficiency during development. We treated mutant embryonic zebrafish with a chelator that reduces the level of available copper and screened for mutants that displayed a copper deficient phenotype only in the presence of the chelator. We identified and characterized two mutants that advance our understanding of copper metabolism during the early periods of development, as well as show an interaction between copper metabolism and another fundamental pathway—that of proton transport. Our results expand our knowledge of copper metabolism and illustrate the power of this type of genetic screen in zebrafish to elucidate mechanisms of nutrient metabolism.

Structure of the two transmembrane Cu+ transport sites of the Cu+ -ATPases

Cu(+)-ATPases drive metal efflux from the cell cytoplasm. Paramount to this function is the binding of Cu(+) within the transmembrane region and its coupled translocation across the permeability barrier. Here, we describe the two transmembrane Cu(+) transport sites present in Archaeoglobus fulgidus CopA. Both sites can be independently loaded with Cu(+). However, their simultaneous occupation is associated with enzyme turnover. Site I is constituted by two Cys in transmembrane segment (TM) 6 and a Tyr in TM7. An Asn in TM7 and Met and Ser in TM8 form Site II. Single site x-ray spectroscopic analysis indicates a trigonal coordination in both sites. This architecture is distinct from that observed in Cu(+)-trafficking chaperones and classical cuproproteins. The high affinity of these sites for Cu(+) (Site I K(a)=1.3 fM(-1), Site II K(a)=1.1 fM(-1)), in conjunction with reversible direct Cu(+) transfer from chaperones, points to a transport mechanism where backward release of free Cu(+) to the cytoplasm is largely prevented.

Structure of a copper pump suggests a regulatory role for its metal-binding domain

P-type ATPases play an important role in Cu homeostasis, which provides sufficient Cu for metalloenzyme biosynthesis but prevents oxidative damage of free Cu to the cell. The P(IB) group of P-type ATPases includes ATP-dependent pumps of Cu and other transition metal ions, and it is distinguished from other family members by the presence of N-terminal metal-binding domains (MBD). We have determined structures of two constructs of a Cu pump from Archaeoglobus fulgidus (CopA) by cryoelectron microscopy of tubular crystals, which reveal the overall architecture and domain organization of the molecule. By comparing these structures, we localized its N-terminal MBD within the cytoplasmic domains that use ATP hydrolysis to drive the transport cycle. We have built a pseudoatomic model by fitting existing crystallographic structures into the cryoelectron microscopy maps for CopA, which suggest a Cu-dependent regulatory role for the MBD.

Intermediate phosphorylation reactions in the mechanism of ATP utilization by the copper ATPase (CopA) of Thermotoga maritima

Recombinant and purified Thermotoga maritima CopA sustains ATPase velocity of 1.78-2.73 micromol/mg/min in the presence of Cu+ (pH 6, 60 degrees C) and 0.03-0.08 micromol/mg/min in the absence of Cu+. High levels of enzyme phosphorylation are obtained by utilization of [gamma-32P]ATP in the absence of Cu+. This phosphoenzyme decays at a much slower rate than observed with Cu.E1 approximately P. In fact, the phosphoenzyme is reduced to much lower steady state levels upon addition of Cu+, due to rapid hydrolytic cleavage. Negligible ATPase turnover is sustained by CopA following deletion of its N-metal binding domain (DeltaNMBD) or mutation of NMBD cysteines (CXXC). Nevertheless, high levels of phosphoenzyme are obtained by utilization of [gamma-3)P]ATP by the DeltaNMBD and CXXC mutants, with no effect of Cu+ either on its formation or hydrolytic cleavage. Phosphoenzyme formation (E2P) can also be obtained by utilization of Pi, and this reaction is inhibited by Cu+ (E2 to E1 transition) even in the DeltaNMBD mutant, evidently due to Cu+ binding at a (transport) site other than the NMBD. E2P undergoes hydrolytic cleavage faster in DeltaNMBD and slower in CXXC mutant. We propose that Cu+ binding to the NMBD is required to produce an "active" conformation of CopA, whereby additional Cu+ bound to an alternate (transmembrane transport) site initiates faster cycles including formation of Cu.E1 approximately P, followed by the E1 approximately P to E2-P conformational transition and hydrolytic cleavage of phosphate. An H479Q mutation (analogous to one found in Wilson disease) renders CopA unable to utilize ATP, whereas phosphorylation by Pi is retained.

Stability and ATP binding of the nucleotide-binding domain of the Wilson disease protein: effect of the common H1069Q mutation

Perturbation of the human copper-transporter Wilson disease protein (ATP7B) causes intracellular copper accumulation and severe pathology, known as Wilson disease (WD). Several WD mutations are clustered within the nucleotide-binding subdomain (N-domain), including the most common mutation, H1069Q. To gain insight into the biophysical behavior of the N-domain under normal and disease conditions, we have characterized wild-type and H1069Q recombinant N-domains in vitro and in silico. The mutant has only twofold lower ATP affinity compared to that of the wild-type N-domain. Both proteins unfold in an apparent two-state reaction at 20 degrees C and ATP stabilizes the folded state. The thermal unfolding reactions are irreversible and, for the same scan rate, the wild-type protein is more resistant to perturbation than the mutant. For both proteins, ATP increases the activation barrier towards thermal denaturation. Molecular dynamics simulations identify specific differences in both ATP orientation and protein structure that can explain the absence of catalytic activity for the mutant N-domain. Taken together, our results provide biophysical characteristics that may be general to N-domains in other P(1B)-ATPases as well as identify changes that may be responsible for the H1069Q WD phenotype in vivo.

Cellular multitasking: the dual role of human Cu-ATPases in cofactor delivery and intracellular copper balance

The human copper-transporting ATPases (Cu-ATPases) are essential for dietary copper uptake, normal development and function of the CNS, and regulation of copper homeostasis in the body. In a cell, Cu-ATPases maintain the intracellular concentration of copper by transporting copper into intracellular exocytic vesicles. In addition, these P-type ATPases mediate delivery of copper to copper-dependent enzymes in the secretory pathway and in specialized cell compartments such as secretory granules or melanosomes. The multiple functions of human Cu-ATPase necessitate complex regulation of these transporters that is mediated through the presence of regulatory domains in their structure, posttranslational modification and intracellular trafficking, as well as interactions with the copper chaperone Atox1 and other regulatory molecules. In this review, we summarize the current information on the function and regulatory mechanisms acting on human Cu-ATPases ATP7A and ATP7B. Brief comparison with the Cu-ATPase orthologs from other species is included.

Sequence variation in the ATP-binding domain of the Wilson disease transporter, ATP7B, affects copper transport in a yeast model system

ATP7B is a copper transporting P-type ATPase defective in the autosomal recessive copper storage disorder, Wilson disease (WND). Functional assessment of variants helps to distinguish normal from disease-causing variants and provides information on important amino acid residues. A total of 11 missense variants of ATP7B, originally identified in WND patients, were examined for their capacity to functionally complement a yeast mutant strain in which the yeast gene ortholog, CCC2, was disrupted. Solution structures of ATP7B domains were used to predict the effects of each variant on ATP7B structure. Three variants lie within the copper-binding domain and eight within the ATP-binding domain of ATP7B. All three ATP7B variants within the copper-binding domain and four within the ATP-binding domain showed full complementation of the yeast ccc2 phenotype. For the remaining four located in the ATP-binding domain, p.Glu1064Lys and p.Val1106Asp were unable to complement the yeast ccc2 high-affinity iron uptake deficiency phenotype, apparently due to mislocalization and/or change in conformation of the variant protein. p.Leu1083Phe exhibited a temperature-sensitive phenotype with partial complementation at 30 degrees C and a severe deficit at 37 degrees C. p.Met1169Val only partially complemented the ccc2 phenotype at 30 degrees C and 37 degrees C. Therefore, four variant positions were identified as important for copper transport and as disease-causing changes. Since the yeast assay specifically evaluates copper transport function, variants with normal transport could be defective in some other aspect of ATP7B function, particularly trafficking in mammalian cells. Functional assessment is critical for reliable use of mutation analysis as an aid to diagnosis of this clinically variable condition.

Delivery of the Cu-transporting ATPase ATP7B to the plasma membrane in Xenopus oocytes

Cu-transporting ATPase ATP7B (Wilson disease protein) is essential for the maintenance of intracellular copper concentration. In hepatocytes, ATP7B is required for copper excretion, which is thought to occur via a transient delivery of the ATP7B- and copper-containing vesicles to the apical membrane. The currently available experimental systems do not allow analysis of ATP7B at the cell surface. Using epitope insertion, we identified an extracellular loop into which the HA-epitope can be introduced without inhibiting ATP7B activity. The HA-tagged ATP7B was expressed in Xenopus oocytes and the presence of ATP7B at the plasma membrane was demonstrated by electron microscopy, freeze-fracture experiments, and surface luminescence measurements in intact cells. Neither the deletion of the entire N-terminal copper-binding domain nor the inactivating mutation of catalytic Asp1027 affected delivery to the plasma membrane of oocytes. In contrast, surface targeting was decreased for the ATP7B variants with mutations in the ATP-binding site or the intra-membrane copper-binding site, suggesting that ligand-stabilized conformation(s) are important for ATP7B trafficking. The developed system provides significant advantages for studies that require access to both sides of ATP7B in the membrane.

Common patterns and unique features of P-type ATPases: a comparative view on the KdpFABC complex from Escherichia coli (Review)

P-type ATPases are ubiquitously abundant primary ion pumps, which are capable of transporting cations across the cell membrane at the expense of ATP. Since these ions comprise a large variety of vital biochemical functions, nature has developed rather sophisticated transport machineries in all kingdoms of life. Due to the importance of these enzymes, representatives of both eu- and prokaryotic as well as archaeal P-type ATPases have been studied intensively, resulting in detailed structural and functional information on their mode of action. During catalysis, P-type ATPases cycle between the so-called E1 and E2 states, each of which comprising different structural properties together with different binding affinities for both ATP and the transport substrate. Crucial for catalysis is the reversible phosphorylation of a conserved aspartate, which is the main trigger for the conformational changes within the protein. In contrast to the well-studied and closely related eukaryotic P-type ATPases, much less is known about their homologues in bacteria. Whereas in Eukarya there is predominantly only one subunit, which builds up the transport system, in bacteria there are multiple polypeptides involved in the formation of the active enzyme. Such a rather unusual prokaryotic P-type ATPase is the KdpFABC complex of the enterobacterium Escherichia coli, which serves as a highly specific K(+) transporter. A unique feature of this member of P-type ATPases is that catalytic activity and substrate transport are located on two different polypeptides. This review compares generic features of P-type ATPases with the rather unique KdpFABC complex and gives a comprehensive overview of common principles of catalysis as well as of special aspects connected to distinct enzyme functions.

Multiple metal binding domains enhance the Zn(II) selectivity of the divalent metal ion transporter AztA

Transition metal-transporting P1B-type CPx ATPases play crucial roles in mediating metal homeostasis and resistance in all cells. The degree to which N-terminal metal binding domains (MBDs) confer metal specificity to the transporter is unclear. We show that the two MBDs of the Zn/Cd/Pb effluxing pump Anabaena AztA are functionally nonequivalent, but only with respect to zinc resistance. Inactivation of the a-MBD largely abrogates resistance to high intracellular Zn(II) levels, whereas inactivation of the b-MBD is not as deleterious. In contrast, inactivation of either the a- or b-MBD has little measurable impact on Cd(II) and Pb(II) resistance. The membrane proximal b-MBD binds Zn(II) with a higher affinity than the distal N-terminal a-MBD. Facile Zn(II)-specific intermolecular transfer from the a-MBD to the higher-affinity b-MBD is readily observed by 1H-15N HSQC spectroscopy. Unlike Zn(II), Cd(II) and Pb(II) form saturated 1:1 S4 or S3(O/N) complexes with AztAaHbH, where a single metal ion bridges the two MBDs. We propose that the tandem MBDs enhance Zn(II)-specific transport, while stabilizing a non-native inter-MBD Cd/Pb cross-linked structure that is a poor substrate and/or regulator for the transporter.

Function and regulation of human copper-transporting ATPases

Copper-transporting ATPases (Cu-ATPases) ATP7A and ATP7B are evolutionarily conserved polytopic membrane proteins with essential roles in human physiology. The Cu-ATPases are expressed in most tissues, and their transport activity is crucial for central nervous system development, liver function, connective tissue formation, and many other physiological processes. The loss of ATP7A or ATP7B function is associated with severe metabolic disorders, Menkes disease, and Wilson disease. In cells, the Cu-ATPases maintain intracellular copper concentration by transporting copper from the cytosol across cellular membranes. They also contribute to protein biosynthesis by delivering copper into the lumen of the secretory pathway where metal ion is incorporated into copper-dependent enzymes. The biosynthetic and homeostatic functions of Cu-ATPases are performed in different cell compartments; targeting to these compartments and the functional activity of Cu-ATPase are both regulated by copper. In recent years, significant progress has been made in understanding the structure, function, and regulation of these essential transporters. These studies raised many new questions related to specific physiological roles of Cu-ATPases in various tissues and complex mechanisms that control the Cu-ATPase function. This review summarizes current data on the structural organization and functional properties of ATP7A and ATP7B as well as their localization and functions in various tissues, and discusses the current models of regulated trafficking of human Cu-ATPases.

Molecular pathogenesis of Wilson and Menkes disease: correlation of mutations with molecular defects and disease phenotypes

The trace metal copper is essential for a variety of biological processes, but extremely toxic when present in excessive amounts. Therefore, concentrations of this metal in the body are kept under tight control. Central regulators of cellular copper metabolism are the copper-transporting P-type ATPases ATP7A and ATP7B. Mutations in ATP7A or ATP7B disrupt the homeostatic copper balance, resulting in copper deficiency (Menkes disease) or copper overload (Wilson disease), respectively. ATP7A and ATP7B exert their functions in copper transport through a variety of interdependent mechanisms and regulatory events, including their catalytic ATPase activity, copper-induced trafficking, post-translational modifications and protein-protein interactions. This paper reviews the extensive efforts that have been undertaken over the past few years to dissect and characterise these mechanisms, and how these are affected in Menkes and Wilson disease. As both disorders are characterised by an extensive clinical heterogeneity, we will discus how the underlying genetic defects correlate with the molecular functions of ATP7A and ATP7B and with the clinical expression of these disorders.

Biochemical basis of regulation of human copper-transporting ATPases

Copper is essential for cell metabolism as a cofactor of key metabolic enzymes. The biosynthetic incorporation of copper into secreted and plasma membrane-bound proteins requires activity of the copper-transporting ATPases (Cu-ATPases) ATP7A and ATP7B. The Cu-ATPases also export excess copper from the cell and thus critically contribute to the homeostatic control of copper. The trafficking of Cu-ATPases from the trans-Golgi network to endocytic vesicles in response to various signals allows for the balance between the biosynthetic and copper exporting functions of these transporters. Although significant progress has been made towards understanding the biochemical characteristics of human Cu-ATPase, the mechanisms that control their function and intracellular localization remain poorly understood. In this review, we summarize current information on structural features and functional properties of ATP7A and ATP7B. We also describe sequence motifs unique for each Cu-ATPase and speculate about their role in regulating ATP7A and ATP7B activity and trafficking.

Domain organization and movements in heavy metal ion pumps: papain digestion of CopA, a Cu+-transporting ATPase

To study domain organization and movements in the reaction cycle of heavy metal ion pumps, CopA, a bacterial Cu+-ATPase from Thermotoga maritima was cloned, overexpressed, and purified, and then subjected to limited proteolysis using papain. Stable analogs of intermediate states were generated using AMPPCP as a nonhydrolyzable ATP analog and AlFx as a phosphate analog, following conditions established for Ca2+-ATPase (SERCA1). Characteristic digestion patterns obtained for different analog intermediates show that CopA undergoes domain rearrangements very similar to those of SERCA1. Digestion sites were identified on the loops connecting the A-domain and the transmembrane helices M2 and M3 as well as on that connecting the N-terminal metal binding domain (NMBD) and the first transmembrane helix, Ma. These digestion sites were protected in the E1P.ADP and E2P analogs, whereas the M2-A-domain loop was cleaved specifically in the absence of ions to be transported, just as in SERCA1. ATPase activity was lost when the link between the NMBD and the transmembrane domain was cleaved, indicating that the NMBD plays a critical role in ATP hydrolysis in T. maritima CopA. The change in susceptibility of the loop between the NMBD and Ma helix provides evidence that the NMBD is associated to the A-domain and recruited into domain rearrangements and that the Ma helix is the counterpart of the M1 helix in SERCA1 and Mb and Mc are uniquely inserted before M2.

Identification and subcellular localization of the soybean copper P1B-ATPase GmHMA8 transporter

We have identified a copper P(1B)-ATPase transporter in soybean (Glycine max), named as GmHMA8, homologue to cyanobacterial PacS and Arabidopsis thaliana AtHMA8 (PAA2) transporters. A novel specific polyclonal anti-GmHMA8 antibody raised against a synthetic peptide reacted with a protein of an apparent mass of around 180-200 kDa in chloroplast and thylakoid membrane preparations isolated from soybean cell suspensions. Immunoblot analysis with this antibody also showed a band with similar apparent molecular mass in chloroplasts from Lotus corniculatus. Immunofluorescence labelling with the anti-GmHMA8 antibody and double immunofluorescence labelling with anti-GmHMA8 and anti-RuBisCo antibodies revealed the localization of the GmHMA8 transporter within the chloroplast organelle. Furthermore, the precise ultrastructural distribution of GmHMA8 within the chloroplast subcompartments was demonstrated by using electron microscopy immunogold labelling. The GmHMA8 copper transporter from soybean was localized in the thylakoid membranes showing a heterogeneous distribution in small clusters.

Sulfate acts as phosphate analog on the monomeric catalytic fragment of the CPx-ATPase CopB from Sulfolobus solfataricus

The crystal structure of the catalytic fragment of a Sulfolobus solfataricus P-type ATPase, CopB-B, was determined with a 2.6 A resolution. CopB-B is the major soluble fragment of the archaeal CPx-ATPase CopB and is comprized of a nucleotide and a phosphorylation domain. In the crystalline state two molecules of CopB-B are in close contact to each other, although the presence of dimers in free solution could be ruled out by analytical ultracentrifugation. The overall architecture of CopB-B is similar to that of other P-type ATPases such as Ca-ATPase. Short peptide segments are linking the nucleotide binding to the phosphorylation domain. CopB-B exhibits 33% sequence identity (of 216 aligned residues) with the respective fragment of the Archaeoglobus fulgidus ATPase CopA. The CopB-B nucleotide-binding domain has the most primitive fold yet identified for this enzyme class. It is 24% identical to the nucleotide-binding domain of the disease-related Wilson ATPase ATP7B (80 structurally aligned residues). Structural superposition with Ca-ATPase suggests a putative nucleotide-binding site in CopB-B. The phosphorylation domain of CopB-B is structurally related to the corresponding part of Ca-ATPase in the anion-bound E2 state. In CopB-B crystals, a bound sulfate anion was identified at the phosphate-binding location. In solution state, the potential binding of CopB-B to phosphate was probed with (32)P(i). Bound phosphate could be readily displaced by orthovanadate at submillimolar concentration as well as by sulfate at millimolar concentration. It is possible therefore to assign the structure of the sulfate-bound phosphorylation domain of CopB-B to a state related to the E2.P(i) intermediate state of the catalytic cycle.

Hepatic copper-transporting ATPase ATP7B: function and inactivation at the molecular and cellular level

Copper-transporting ATPase ATP7B (Wilson disease protein) is a member of the P-type ATPase family with characteristic domain structure and distinct ATP-binding site. ATP7B plays a central role in the regulation of copper homeostasis in the liver by delivering copper to the secretory pathway and mediating export of excess copper into the bile. The dual function of ATP7B in hepatocytes is coupled with copper-dependent intracellular relocalization of the transporter. The final destination of ATP7B in hepatocytes during the copper-induced trafficking process is still under debate. We show the results of immunocytochemistry experiments in polarized HepG2 cells that support the model in which elevated copper induces trafficking of ATP7B to sub-apical vesicles, and transiently to the canalicular membrane. In Atp7b-/- mice, an animal model of Wilson disease, both copper delivery to the trans-Golgi network and copper export into the bile are disrupted despite large accumulation of copper in the cytosol. We review the biochemical and physiological changes associated with Atp7b inactivation in mouse liver and discuss the pleiotropic consequences of the common Wilson disease mutation, His1069Gln.

Atx1-like chaperones and their cognate P-type ATPases: copper-binding and transfer

Copper is an essential yet toxic metal ion. To satisfy cellular requirements, while, at the same time, minimizing toxicity, complex systems of copper trafficking have evolved in all cell types. The best conserved and most widely distributed of these involve Atx1-like chaperones and P(1B)-type ATPase transporters. Here, we discuss current understanding of how these chaperones bind Cu(I) and transfer it to the Atx1-like N-terminal domains of their cognate transporter.

The structure and function of heavy metal transport P1B-ATPases

P(1B)-type ATPases transport heavy metals (Cu+, Cu2+, Zn2+, Co2+, Cd2+, Pb2+) across membranes. Present in most organisms, they are key elements for metal homeostasis. P(1B)-type ATPases contain 6-8 transmembrane fragments carrying signature sequences in segments flanking the large ATP binding cytoplasmic loop. These sequences made possible the differentiation of at least four P(1B)-ATPase subgroups with distinct metal selectivity: P(1B-1): Cu+, P(1B-2): Zn2+, P(1B-3): Cu2+, P(1B-4): Co2+. Mutagenesis of the invariant transmembrane Cys in H6, Asn and Tyr in H7 and Met and Ser in H8 of the Archaeoglobus fulgidus Cu+-ATPase has revealed that their side chains likely coordinate the metals during transport and constitute a central unique component of these enzymes. The structure of various cytoplasmic domains has been solved. The overall structure of those involved in enzyme phosphorylation (P-domain), nucleotide binding (N-domain) and energy transduction (A-domain), appears similar to those described for the SERCA Ca2+-ATPase. However, they show different features likely associated with singular functions of these proteins. Many P(1B)-type ATPases, but not all of them, also contain a diverse arrangement of cytoplasmic metal binding domains (MBDs). In spite of their structural differences, all N- and C-terminal MBDs appear to control the enzyme turnover rate without affecting metal binding to transmembrane transport sites. In addition, eukaryotic Cu+-ATPases have multiple N-MBD regions that participate in the metal dependent targeting and localization of these proteins. The current knowledge of structure-function relationships among the different P(1B)-ATPases allows for a description of selectivity, regulation and transport mechanisms. Moreover, it provides a framework to understand mutations in human Cu+-ATPases (ATP7A and ATP7B) that lead to Menkes and Wilson diseases.

A structural perspective on copper uptake in eukaryotes

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

A cadmium-transporting P1B-type ATPase in yeast Saccharomyces cerevisiae

Detoxification and homeostatic acquisition of metal ions are vital for all living organisms. We have identified PCA1 in yeast Saccharomyces cerevisiae as an overexpression suppressor of copper toxicity. PCA1 possesses signatures of a P1B-type heavy metal-transporting ATPase that is widely distributed from bacteria to humans. Copper resistance conferred by PCA1 is not dependent on catalytic activity, but it appears that a cysteine-rich region located in the N terminus sequesters copper. Unexpectedly, when compared with two independent natural isolates and an industrial S. cerevisiae strain, the PCA1 allele of the common laboratory strains we have examined possesses a missense mutation in a predicted ATP-binding residue conserved in P1B-type ATPases. Consistent with a previous report that identifies an equivalent mutation in a copper-transporting P1B-type ATPase of a Wilson disease patient, the PCA1 allele found in laboratory yeast strains is nonfunctional. Overexpression or deletion of the functional allele in yeast demonstrates that PCA1 is a cadmium efflux pump. Cadmium as well as copper and silver, but not other metals examined, dramatically increase PCA1 protein expression through post-transcriptional regulation and promote subcellular localization to the plasma membrane. Our study has revealed a novel metal detoxification mechanism in yeast mediated by a P1B-type ATPase that is unique in structure, substrate specificity, and mode of regulation.