A NMR Study of the Interaction of a Three-domain Construct of ATP7A with Copper(I) and Copper(I)-HAH1

Unraveling Copper Exchange in the Atox1-Cu(I)-Mnk1 Heterodimer: A Simulation Approach

Copper, an essential metal for various cellular processes, requires tight regulation to prevent cytotoxicity. Intracellular pathways crucial for maintaining optimal copper levels involve soluble and membrane transporters, namely, metallochaperones and P-type ATPases, respectively. In this study, we used a simulation workflow based on free-energy perturbation (FEP) theory and parallel bias metadynamics (PBMetaD) to predict the Cu(I) exchange mechanism between the human Cu(I) chaperone, Atox1, and one of its two physiological partners, ATP7A. ATP7A, also known as the Menkes disease protein, is a transmembrane protein and one of the main copper-transporting ATPases. It pumps copper into the trans-Golgi network for the maturation of cuproenzymes and is also essential for the efflux of excess copper across the plasma membrane. In this analysis, we utilized the nuclear magnetic resonance (NMR) structure of the Cu(I)-mediated complex between Atox1 and the first soluble domain of the Menkes protein (Mnk1) as a starting point. Independent free-energy simulations were conducted to investigate the dissociation of both Atox1 and Mnk1. The calculations revealed that the two dissociations require free energy values of 6.3 and 6.2 kcal/mol, respectively, following a stepwise dissociation mechanism.

Structure and mechanism of the human copper transporting ATPases: Fitting the pieces into a moving puzzle

Human copper transporters ATP7B and ATP7A deliver copper to biosynthetic pathways and maintain copper homeostasis in the cell. These enzymes combine several challenges for structural biology because they are large low abundance membrane proteins with many highly mobile domains and long disordered loops. No method has yet succeeded in solving the structure of the complete fully functional protein. Still, X-ray crystallography, Cryo-EM and NMR helped to piece together a structure based model of the enzyme activity and regulation by copper. We review the structures of ATP7B and ATP7A with an emphasis on the mechanistic insights into the unique aspects of the transport function and regulation of the human copper ATPases that have emerged from more than twenty years of research.

Copper chaperone antioxidant 1: multiple roles and a potential therapeutic target

Copper (Cu) was recently demonstrated to play a critical role in cellular physiological and biochemical processes, including energy production and maintenance, antioxidation and enzymatic activity, and signal transduction. Antioxidant 1 (ATOX1), a chaperone of Cu previously named human ATX1 homologue (HAH1), has been found to play an indispensable role in maintaining cellular Cu homeostasis, antioxidative stress, and transcriptional regulation. In the past decade, it has also been found to be involved in a variety of diseases, including numerous neurodegenerative diseases, cancers, and metabolic diseases. Recently, increasing evidence has revealed that ATOX1 is involved in the regulation of cell migration, proliferation, autophagy, DNA damage repair (DDR), and death, as well as in organism development and reproduction. This review summarizes recent advances in the research on the diverse physiological and cytological functions of ATOX1 and the underlying mechanisms of its action in human health and diseases. The potential of ATOX1 as a therapeutic target is also discussed. This review aims to pose unanswered questions related to ATOX1 biology and explore the potential use of ATOX1 as a therapeutic target.

Metal trafficking in the cell: Combining atomic resolution with cellular dimension

Metals are widely present in biological systems as simple ions or complex cofactors, and are involved in a variety of processes essential for life. Their transport inside cells and insertion into the binding sites of the proteins that need metals to function occur through complex and selective pathways involving dedicated multiprotein machineries specifically and transiently interacting with each other, often sharing the coordination of metal ions and/or cofactors. The understanding of these machineries requires integrated approaches, ranging from bioinformatics to experimental investigations, possibly in the cellular context. In this review, we report two case studies where the use of integrated in vitro and in cellulo approaches is necessary to clarify at atomic resolution essential aspects of metal trafficking in cells.

Disrupting Cu trafficking as a potential therapy for cancer

Copper ions play a crucial role in various cellular biological processes. However, these copper ions can also lead to toxicity when their concentration is not controlled by a sophisticated copper-trafficking system. Copper dys-homeostasis has been linked to a variety of diseases, including neurodegeneration and cancer. Therefore, manipulating Cu-trafficking to trigger selective cancer cell death may be a viable strategy with therapeutic benefit. By exploiting combined in silico and experimental strategies, we identified small peptides able to bind Atox1 and metal-binding domains 3-4 of ATP7B proteins. We found that these peptides reduced the proliferation of cancer cells owing to increased cellular copper ions concentration. These outcomes support the idea of harming copper trafficking as an opportunity for devising novel anti-cancer therapies.

Chlamydomonas ATX1 is essential for Cu distribution to multiple cupro-enzymes and maintenance of biomass in conditions demanding cupro-enzyme-dependent metabolic pathways

Copper (Cu) chaperones, of which yeast ATX1 is a prototype, are small proteins with a Cu(I) binding MxCxxC motif and are responsible for directing intracellular Cu toward specific client protein targets that use Cu as a cofactor. The Chlamydomonas reinhardtii ATX1 (CrATX1) was identified by its high sequence similarity with yeast ATX1. Like the yeast homologue, CrATX1 accumulates in iron-deficient cells (but is not impacted by other metal-deficiencies). N- and C-terminally YFP-ATX1 fusion proteins are distributed in the cytoplasm. Reverse genetic analysis using artificial microRNA (amiRNA) to generate lines with reduced CrATX1 abundance and CRISPR/Cpf1 to generate atx1 knockout lines validated a function for ATX1 in iron-poor cells, again reminiscent of yeast ATX1, most likely because of an impact on metalation of the multicopper oxidase FOX1, which is an important component in high-affinity iron uptake. We further identify other candidate ATX1 targets owing to reduced growth of atx1 mutant lines on guanine as a sole nitrogen source, which we attribute to loss of function of UOX1, encoding a urate oxidase, a cupro-enzyme involved in guanine assimilation. An impact of ATX1 on Cu distribution in atx1 mutants is strikingly evident by a reduced amount of intracellular Cu in all conditions probed in this work.

Functional characterization of copper transporters zCtr1, zAtox1, zAtp7a and zAtp7b in zebrafish liver cell line ZFL

Copper (Cu) is an essential element for all organisms, serving as an enzyme cofactor to maintain cellular activity and vitality. However, Cu homeostasis must be maintained at the physiological and cellular levels as Cu ions can be highly toxic. In mammals, ATP7A is expressed in most tissues, but relatively lower expression is found in the liver, and is responsible for the intestinal uptake of Cu, while ATP7B is highly expressed in the liver, kidneys and placenta, and is responsible for removal of Cu in the liver. CTR1 and ATOX1 are responsible for cellular Cu uptake and intracellular Cu transport, respectively. Here, using a zebrafish liver cell line (ZFL), we studied the cellular functions of four zebrafish Cu transporters. In zebrafish, zAtp7a is expressed mainly in the liver and zAtp7b is expressed mainly in the intestines, different from that of humans which have a high ATP7b level in the liver and high ATP7a level in the intestines. We here found that zctr1 or zatox1 overexpression increased Cu accumulation in ZFL cells. Moreover, zctr1 overexpression made ZFL cells more sensitive to Cu and Zn exposure, and overexpression of zatox1 or zatp7b increased Cu uptake and Cu tolerance in ZFL cells. Overexpression of zatp7a made ZFL cells more sensitive to Zn. Taken together, our findings suggest that zatp7b is responsible for Cu export despite its expression level being much lower than zatp7a in ZFL cells.

Unraveling the Impact of Cysteine-to-Serine Mutations on the Structural and Functional Properties of Cu(I)-Binding Proteins

Appropriate maintenance of Cu(I) homeostasis is an essential requirement for proper cell function because its misregulation induces the onset of major human diseases and mortality. For this reason, several research efforts have been devoted to dissecting the inner working mechanism of Cu(I)-binding proteins and transporters. A commonly adopted strategy relies on mutations of cysteine residues, for which Cu(I) has an exquisite complementarity, to serines. Nevertheless, in spite of the similarity between these two amino acids, the structural and functional impact of serine mutations on Cu(I)-binding biomolecules remains unclear. Here, we applied various biochemical and biophysical methods, together with all-atom simulations, to investigate the effect of these mutations on the stability, structure, and aggregation propensity of Cu(I)-binding proteins, as well as their interaction with specific partner proteins. Among Cu(I)-binding biomolecules, we focused on the eukaryotic Atox1-ATP7B system, and the prokaryotic CueR metalloregulator. Our results reveal that proteins containing cysteine-to-serine mutations can still bind Cu(I) ions; however, this alters their stability and aggregation propensity. These results contribute to deciphering the critical biological principles underlying the regulatory mechanism of the in-cell Cu(I) concentration, and provide a basis for interpreting future studies that will take advantage of cysteine-to-serine mutations in Cu(I)-binding systems.

ATP7A and ATP7B copper transporters have distinct functions in the regulation of neuronal dopamine-β-hydroxylase

The copper (Cu) transporters ATPase copper-transporting alpha (ATP7A) and ATPase copper-transporting beta (ATP7B) are essential for the normal function of the mammalian central nervous system. Inactivation of ATP7A or ATP7B causes the severe neurological disorders, Menkes disease and Wilson disease, respectively. In both diseases, Cu imbalance is associated with abnormal levels of the catecholamine-type neurotransmitters dopamine and norepinephrine. Dopamine is converted to norepinephrine by dopamine-β-hydroxylase (DBH), which acquires its essential Cu cofactor from ATP7A. However, the role of ATP7B in catecholamine homeostasis is unclear. Here, using immunostaining of mouse brain sections and cultured cells, we show that DBH-containing neurons express both ATP7A and ATP7B. The two transporters are located in distinct cellular compartments and oppositely regulate the export of soluble DBH from cultured neuronal cells under resting conditions. Down-regulation of ATP7A, overexpression of ATP7B, and pharmacological Cu depletion increased DBH retention in cells. In contrast, ATP7B inactivation elevated extracellular DBH. Proteolytic processing and the specific activity of exported DBH were not affected by changes in ATP7B levels. These results establish distinct regulatory roles for ATP7A and ATP7B in neuronal cells and explain, in part, the lack of functional compensation between these two transporters in human disorders of Cu imbalance.

Shared gene-network signatures between the human heavy metal proteome and neurological disorders and cancer types

In this work, for the first time, the human heavy metal proteome was predicted.

The structural flexibility of the human copper chaperone Atox1: Insights from combined pulsed EPR studies and computations

Metallochaperones are responsible for shuttling metal ions to target proteins. Thus, a metallochaperone's structure must be sufficiently flexible both to hold onto its ion while traversing the cytoplasm and to transfer the ion to or from a partner protein. Here, we sought to shed light on the structure of Atox1, a metallochaperone involved in the human copper regulation system. Atox1 shuttles copper ions from the main copper transporter, Ctr1, to the ATP7b transporter in the Golgi apparatus. Conventional biophysical tools such as X-ray or NMR cannot always target the various conformational states of metallochaperones, owing to a requirement for crystallography or low sensitivity and resolution. Electron paramagnetic resonance (EPR) spectroscopy has recently emerged as a powerful tool for resolving biological reactions and mechanisms in solution. When coupled with computational methods, EPR with site-directed spin labeling and nanoscale distance measurements can provide structural information on a protein or protein complex in solution. We use these methods to show that Atox1 can accommodate at least four different conformations in the apo state (unbound to copper), and two different conformations in the holo state (bound to copper). We also demonstrate that the structure of Atox1 in the holo form is more compact than in the apo form. Our data provide insight regarding the structural mechanisms through which Atox1 can fulfill its dual role of copper binding and transfer.

Understanding copper sensitivity in zebrafish (Danio rerio) through the intracellular localization of copper transporters in a hepatocyte cell-line ZFL and the tissue expression profiles of copper transporters

Zebrafish (Danio rerio) is a freshwater fish species of Cyprinidae known for its copper (Cu) intolerance, yet the underlying mechanisms of the sensitivity remain unclear. In this study, we examined the highly conserved molecular machineries in the copper handling system, namely ATOX1, ATP7A, ATP7B, and CTR1, by profiling their gene expression patterns among tissues before and after acute waterborne Cu exposure, and investigating their intracellular localization patterns using a zebrafish hepatocyte cell line, ZFL. We found that ATP7B was weak in its response toward Cu exposure to elicit its copper efflux function. Tissue distribution of these Cu transporters, however, revealed a distinct expression profile compared with mammals and other fish, particularly ATP7A, which unlike ATP7B was highly expressed in the liver, while ATP7B, not ATP7A, was specifically expressed in the intestine. ATOX1 transcript expression was also found to be significantly up-regulated with acute waterborne Cu, in contrast to the decreased expression found in other fish. A possible explanation for the Cu sensitivity in zebrafish is discussed.

Structural and mechanistic insights into an extracytoplasmic copper trafficking pathway in Streptomyces lividans

In Streptomyces lividans an extracytoplasmic copper-binding Sco protein plays a role in two unlinked processes: (i) initiating a morphological development switch and (ii) facilitating the co-factoring of the CuA domain of CcO (cytochrome c oxidase). How Sco obtains copper once secreted to the extracytoplasmic environment is unknown. In the present paper we report on a protein possessing an HX₆MX₂₁HXM motif that binds a single cuprous ion with subfemtomolar affinity. High-resolution X-ray structures of this extracytoplasmic copper chaperone-like protein (ECuC) in the apo- and Cu(I)-bound states reveal that the latter possesses a surface-accessible cuprous-ion-binding site located in a dish-shaped region of β-sheet structure. A cuprous ion is transferred under a favourable thermodynamic gradient from ECuC to Sco with no back transfer occurring. The ionization properties of the cysteine residues in the Cys⁸⁶xxxCys⁹⁰ copper-binding motif of Sco, together with their positional locations identified from an X-ray structure of Sco, suggests a role for Cys⁸⁶ in initiating an inter-complex ligand-exchange reaction with Cu(I)-ECuC. Generation of the genetic knockouts, Δsco, Δecuc and Δsco/ecuc, and subsequent in vivo assays lend support to the existence of a branched extracytoplasmic copper-trafficking pathway in S. lividans. One branch requires both Sco and to a certain extent ECuC to cofactor the CuA domain, whereas the other uses only Sco to deliver copper to a cuproenzyme to initiate morphological development.

Single-molecule dynamics and mechanisms of metalloregulators and metallochaperones

Understanding how cells regulate and transport metal ions is an important goal in the field of bioinorganic chemistry, a frontier research area that resides at the interface of chemistry and biology. This Current Topic reviews recent advances from the authors' group in using single-molecule fluorescence imaging techniques to identify the mechanisms of metal homeostatic proteins, including metalloregulators and metallochaperones. It emphasizes the novel mechanistic insights into how dynamic protein-DNA and protein-protein interactions offer efficient pathways via which MerR-family metalloregulators and copper chaperones can fulfill their functions. This work also summarizes other related single-molecule studies of bioinorganic systems and provides an outlook toward single-molecule imaging of metalloprotein functions in living cells.

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.

A new structural paradigm in copper resistance in Streptococcus pneumoniae

Copper resistance has emerged as an important virulence determinant of microbial pathogens. In Streptococcus pneumoniae, copper resistance is mediated by the copper-responsive repressor CopY, CupA and the copper-effluxing P(1B)-type ATPase CopA. We show here that CupA is a previously uncharacterized cell membrane-anchored Cu(I) chaperone and that a Cu(I) binding-competent, membrane-localized CupA is obligatory for copper resistance. The crystal structures of the soluble domain of CupA and the N-terminal metal-binding domain (MBD) of CopA (CopA(MBD)) reveal isostructural cupredoxin-like folds that each harbor a binuclear Cu(I) cluster unprecedented in bacterial copper trafficking. NMR studies reveal unidirectional Cu(I) transfer from the low-affinity site on the soluble domain of CupA to the high-affinity site of CopA(MBD). However, copper binding by CopA(MBD) is not essential for cellular copper resistance, consistent with a primary role of CupA in cytoplasmic Cu(I) sequestration and/or direct delivery to the transmembrane site of CopA for cellular efflux.

Human superoxide dismutase 1 (hSOD1) maturation through interaction with human copper chaperone for SOD1 (hCCS)

Copper chaperone for superoxide dismutase 1 (SOD1), CCS, is the physiological partner for the complex mechanism of SOD1 maturation. We report an in vitro model for human CCS-dependent SOD1 maturation based on the study of the interactions of human SOD1 (hSOD1) with full-length WT human CCS (hCCS), as well as with hCCS mutants and various truncated constructs comprising one or two of the protein's three domains. The synergy between electrospray ionization mass spectrometry (ESI-MS) and NMR is fully exploited. This is an in vitro study of this process at the molecular level. Domain 1 of hCCS is necessary to load hSOD1 with Cu(I), requiring the heterodimeric complex formation with hSOD1 fostered by the interaction with domain 2. Domain 3 is responsible for the catalytic formation of the hSOD1 Cys-57-Cys-146 disulfide bond, which involves both hCCS Cys-244 and Cys-246 via disulfide transfer.

Dynamic multibody protein interactions suggest versatile pathways for copper trafficking

As part of intracellular copper trafficking pathways, the human copper chaperone Hah1 delivers Cu(+) to the Wilson's Disease Protein (WDP) via weak and dynamic protein-protein interactions. WDP contains six homologous metal binding domains (MBDs) connected by flexible linkers, and these MBDs all can receive Cu(+) from Hah1. The functional roles of the MBD multiplicity in Cu(+) trafficking are not well understood. Building on our previous study of the dynamic interactions between Hah1 and the isolated fourth MBD of WDP, here we study how Hah1 interacts with MBD34, a double-domain WDP construct, using single-molecule fluorescence resonance energy transfer (smFRET) combined with vesicle trapping. By alternating the positions of the smFRET donor and acceptor, we systematically probed Hah1-MBD3, Hah1-MBD4, and MBD3-MBD4 interaction dynamics within the multidomain system. We found that the two interconverting interaction geometries were conserved in both intermolecular Hah1-MBD and intramolecular MBD-MBD interactions. The Hah1-MBD interactions within MBD34 are stabilized by an order of magnitude relative to the isolated single-MBDs, and thermodynamic and kinetic evidence suggest that Hah1 can interact with both MBDs simultaneously. The enhanced interaction stability of Hah1 with the multi-MBD system, the dynamic intramolecular MBD-MBD interactions, and the ability of Hah1 to interact with multiple MBDs simultaneously suggest an efficient and versatile mechanism for the Hah1-to-WDP pathway to transport Cu(+).

The architecture of CopA from Archeaoglobus fulgidus studied by cryo-electron microscopy and computational docking

CopA uses ATP to pump Cu(+) across cell membranes. X-ray crystallography has defined atomic structures of several related P-type ATPases. We have determined a structure of CopA at 10 Å resolution by cryo-electron microscopy of a new crystal form and used computational molecular docking to study the interactions between the N-terminal metal-binding domain (NMBD) and other elements of the molecule. We found that the shorter-chain lipids used to produce these crystals are associated with movements of the cytoplasmic domains, with a novel dimer interface and with disordering of the NMBD, thus offering evidence for the transience of its interaction with the other cytoplasmic domains. Docking identified a binding site that matched the location of the NMBD in our previous structure by cryo-electron microscopy, allowing a more detailed view of its binding configuration and further support for its role in autoinhibition.

Relating dynamic protein interactions of metallochaperones with metal transfer at the single-molecule level

Metallochaperones undertake specific interactions with their target proteins to deliver metal ions inside cells. Understanding how these protein interactions are coupled with the underlying metal transfer process is important, but challenging because they are weak and dynamic. Here we use a nanovesicle trapping scheme to enable single-molecule FRET measurements of the weak, dynamic interactions between the copper chaperone Hahl and the fourth metal binding domain (MBD4) of WDP. By monitoring the behaviors of single interacting pairs, we visualize their interactions in real time in both the absence and the presence of various equivalents of Cu(1+). Regardless of the proteins' metallation state, we observe multiple, interconverting interaction complexes between Hah1 and MBD4. Within our experimental limit, the overall interaction geometries of these complexes appear invariable, but their stabilities are dependent on the proteins' metallation state. In apo-holo Hah1-MBD4 interactions, the complexes are stabilized relative to that observed in the apo-apo interactions. This stabilization is indiscernible when Hah1's Cu(1+)-binding is eliminated or when both proteins have Cu(1+) loaded. The nature of this Cu(1+)-induced complex stabilization and of the interaction complexes are discussed. These Cu(1+)-induced effects on the Hah1-MBD4 interactions provide a step toward understanding how the dynamic protein interactions of copper chaperones are coupled with their metal transfer function.

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.

Tackling metal regulation and transport at the single-molecule level

To maintain normal metal metabolism, organisms utilize dynamic cooperation of many biomacromolecules for regulating metal ion concentrations and bioavailability. How these biomacromolecules work together to achieve their functions is largely unclear. For example, how do metalloregulators and DNA interact dynamically to control gene expression to maintain healthy cellular metal level? And how do metal transporters collaborate dynamically to deliver metal ions? Here we review recent advances in studying the dynamic interactions of macromolecular machineries for metal regulation and transport at the single-molecule level: (1) The development of engineered DNA Holliday junctions as single-molecule reporters for metalloregulator-DNA interactions, focusing onMerR-family regulators. And (2) The development of nanovesicle trapping coupled with single molecule fluorescence resonance energy transfer (smFRET) for studying weak, transient interactions between the copper chaperone Hah1 and the Wilson disease protein. We describe the methodologies,the information content of the single-molecule results, and the insights into the biological functions of the involved biomacromolecules for metal regulation and transport. We also discuss remaining challenges from our perspective.

Copper(I)-mediated protein–protein interactions result from suboptimal interaction surfaces

The homoeostasis of metal ions in cells is the result of the contribution of several cellular pathways that involve transient, often weak, protein–protein interactions. Metal transfer typically implies the formation of adducts where the metal itself acts as a bridge between proteins, by co-ordinating residues of both interacting partners. In the present study we address the interaction between the human copper(I)-chaperone HAH1 (human ATX1 homologue) and a metal-binding domain in one of its partners, namely the P-type copper-transporting ATPase, ATP7A (ATPase, Cu+ transporting, α polypeptide). The adduct was structurally characterized in solution, in the presence of copper(I), and through X-ray crystallography, upon replacing copper(I) with cadmium(II). Further insight was obtained through molecular modelling techniques and site-directed mutagenesis. It was found that the interaction involves a relatively small interface (less than 1000 Å2, 1 Å=0.1 nm) with a low fraction of non-polar atoms. These observations provide a possible explanation for the low affinity of the two apoproteins. It appears that electrostatics is important in selecting which domain of the ATPase is able to form detectable amounts of the metal-mediated adduct with HAH1.

An NMR study of the interaction of the N-terminal cytoplasmic tail of the Wilson disease protein with copper(I)-HAH1

ATP7B is a human P(1B)-type ATPase that has a crucial role in maintaining copper(I) homeostasis. Mutations in the corresponding gene are the cause of Wilson disease. Among its various distinguishing features is a long ( approximately 630 amino acids) N-terminal cytosolic tail containing six domains that are individually folded and capable of binding one copper(I) ion each. We expressed the entire tail as a single construct in Escherichia coli and investigated its interaction with its copper chaperone (i.e. HAH1) by solution NMR spectroscopy. We observed that all six of the metal-binding domains were metallated by Cu(I)-HAH1, with the first, the second, and the fourth domains forming an adduct with it. This behavior is different from that of the highly similar human ATPase ATP7A, in which only two domains form such an adduct. The distinct behaviors of the different domains were analyzed in terms of the energetics of Cu(I) transfer, hinting at a specific role of the interaction with copper(I)-HAH1 in the overall functional process.

Copper trafficking in biology: an NMR approach

Copper ions are essential for living organisms because they are involved in several fundamental biological processes. Biomolecules interacting with copper ions have to be characterized as such, when bound to the metal ion, and when they interact with other biomolecules or substrates. The characterization is both structural and dynamic. In this context, NMR is a preferred tool of investigation because it allows shedding light on what happens in solution. Here, the NMR contribution to the copper trafficking is described, providing precious information on biochemical pathways, which are essential to understand the mechanisms of life at the molecular level.

Metal binding domains 3 and 4 of the Wilson disease protein: solution structure and interaction with the copper(I) chaperone HAH1

The Wilson disease protein or ATP7B is a P 1B-type ATPase involved in human copper homeostasis. The extended N-terminus of ATP7B protrudes into the cytosol and contains six Cu(I) binding domains. This report presents the NMR structure of the polypeptide consisting of soluble Cu(I) binding domains 3 and 4. The two domains exhibit ferredoxin-like folds, are linked by a flexible loop, and act independently of one another. Domains 3 and 4 tend to aggregate in a concentration-dependent manner involving nonspecific intermolecular interactions. Both domains can be loaded with Cu(I) when provided as an acetonitrile complex or by the chaperone HAH1. HAH1 forms a 70% complex with domain 4 that is in fast exchange with the free protein in solution. The ability of HAH1 to form a complex only with some domains of ATP7B is an interesting property of this class of proteins and may have a signaling role in the function of the ATPases.

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.

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.

Trafficking of the copper-ATPases, ATP7A and ATP7B: Role in copper homeostasis

Copper is essential for human health and copper imbalance is a key factor in the aetiology and pathology of several neurodegenerative diseases. The copper-transporting P-type ATPases, ATP7A and ATP7B are key molecules required for the regulation and maintenance of mammalian copper homeostasis. Their absence or malfunction leads to the genetically inherited disorders, Menkes and Wilson diseases, respectively. These proteins have a dual role in cells, namely to provide copper to essential cuproenzymes and to mediate the excretion of excess intracellular copper. A unique feature of ATP7A and ATP7B that is integral to these functions is their ability to sense and respond to intracellular copper levels, the latter manifested through their copper-regulated trafficking from the transGolgi network to the appropriate cellular membrane domain (basolateral or apical, respectively) to eliminate excess copper from the cell. Research over the last decade has yielded significant insight into the enzymatic properties and cell biology of the copper-ATPases. With recent advances in elucidating their localization and trafficking in human and animal tissues in response to physiological stimuli, we are progressing rapidly towards an integrated understanding of their physiological significance at the level of the whole animal. This knowledge in turn is helping to clarify the biochemical and cellular basis not only for the phenotypes conferred by individual Menkes and Wilson disease patient mutations, but also for the clinical variability of phenotypes associated with each of these diseases. Importantly, this information is also providing a rational basis for the applicability and appropriateness of certain diagnostic markers and therapeutic regimes. This overview will provide an update on the current state of our understanding of the localization and trafficking properties of the copper-ATPases in cells and tissues, the molecular signals and posttranslational interactions that govern their trafficking activities, and the cellular basis for the clinical phenotypes associated with disease-causing mutations.

The roles of copper transporters in cisplatin resistance

Platinum-based antitumor agents have been effective in the treatments of many human malignancies but the ultimate success of these agents is often compromised by development of drug resistance. One mechanism associated with resistance to platinum drugs is reduced intracellular accumulation owing to impaired drug intake, enhanced outward transport, or both. Mechanisms for transporting platinum drugs were not known until recent demonstrations that import and export transporters involved in maintenance copper homeostasis are also involved in the transport of these drugs. Ctr1, the major copper influx transporter, has been convincingly demonstrated to transport cisplatin and its analogues, carboplatin, and oxaliplatin. Evidence also suggests that the two copper efflux transporters ATP7A and ATP7B regulate the efflux of cisplatin. These observations are intriguing, because conventional thinking of the inorganic physiologic chemistry of cisplatin and copper is quite different. Hence, understanding the underlying mechanistic aspects of these transporters is critically important. While the mechanisms by which hCtr1, ATP7A and ATP7B transport copper ions have been studied extensively, very little is known about the mechanisms by which these transporters shuffle platinum-based antitumor agents. This review discusses the identification of copper transporters as platinum drug transporters, the structural-functional and mechanistic aspects of these transporters, the mechanisms that regulate their expression, and future research directions that may eventually lead to improved efficacy of platinum-based-based drugs in cancer chemotherapy through modulation of their transporters' activities.