Identification of the transmembrane metal binding site in Cu+-transporting PIB-type ATPases

Copper resistance in the cold: Genome analysis and characterisation of a PIB-1 ATPase in Bizionia argentinensis

Copper homeostasis is a fundamental process in organisms, characterised by unique pathways that have evolved to meet specific needs while preserving core resistance mechanisms. While these systems are well-documented in model bacteria, information on copper resistance in species adapted to cold environments is scarce. This study investigates the potential genes related to copper homeostasis in the genome of Bizionia argentinensis (JUB59-T), a psychrotolerant bacterium isolated from Antarctic seawater. We identified several genes encoding proteins analogous to those crucial for copper homeostasis, including three sequences of copper-transport P1B-type ATPases. One of these, referred to as BaCopA1, was chosen for cloning and expression in Saccharomyces cerevisiae. BaCopA1 was successfully integrated into yeast membranes and subsequently extracted with detergent. The purified BaCopA1 demonstrated the ability to catalyse ATP hydrolysis at low temperatures. Structural models of various BaCopA1 conformations were generated and compared with mesophilic and thermophilic homologous structures. The significant conservation of critical residues and structural similarity among these proteins suggest a shared reaction mechanism for copper transport. This study is the first to report a psychrotolerant P1B-ATPase that has been expressed and purified in a functional form.

Structural basis of ion uptake in copper-transporting P1B-type ATPases

Copper is essential for living cells, yet toxic at elevated concentrations. Class 1B P-type (P1B-) ATPases are present in all kingdoms of life, facilitating cellular export of transition metals including copper. P-type ATPases follow an alternating access mechanism, with inward-facing E1 and outward-facing E2 conformations. Nevertheless, no structural information on E1 states is available for P1B-ATPases, hampering mechanistic understanding. Here, we present structures that reach 2.7 Å resolution of a copper-specific P1B-ATPase in an E1 conformation, with complementing data and analyses. Our efforts reveal a domain arrangement that generates space for interaction with ion donating chaperones, and suggest a direct Cu+ transfer to the transmembrane core. A methionine serves a key role by assisting the release of the chaperone-bound ion and forming a cargo entry site together with the cysteines of the CPC signature motif. Collectively, the findings provide insights into P1B-mediated transport, likely applicable also to human P1B-members.

The CopA2-Type P1B-Type ATPase CcoI Serves as Central Hub for cbb 3-Type Cytochrome Oxidase Biogenesis

Copper (Cu)-transporting P_1B-type ATPases are ubiquitous metal transporters and crucial for maintaining Cu homeostasis in all domains of life. In bacteria, the P_1B-type ATPase CopA is required for Cu-detoxification and exports excess Cu(I) in an ATP-dependent reaction from the cytosol into the periplasm. CopA is a member of the CopA1-type ATPase family and has been biochemically and structurally characterized in detail. In contrast, less is known about members of the CopA2-type ATPase family, which are predicted to transport Cu(I) into the periplasm for cuproprotein maturation. One example is CcoI, which is required for the maturation of cbb ₃-type cytochrome oxidase (cbb ₃-Cox) in different species. Here, we reconstituted purified CcoI of Rhodobacter capsulatus into liposomes and determined Cu transport using solid-supported membrane electrophysiology. The data demonstrate ATP-dependent Cu(I) translocation by CcoI, while no transport is observed in the presence of a non-hydrolysable ATP analog. CcoI contains two cytosolically exposed N-terminal metal binding sites (N-MBSs), which are both important, but not essential for Cu delivery to cbb ₃-Cox. CcoI and cbb ₃-Cox activity assays in the presence of different Cu concentrations suggest that the glutaredoxin-like N-MBS1 is primarily involved in regulating the ATPase activity of CcoI, while the CopZ-like N-MBS2 is involved in Cu(I) acquisition. The interaction of CcoI with periplasmic Cu chaperones was analyzed by genetically fusing CcoI to the chaperone SenC. The CcoI-SenC fusion protein was fully functional in vivo and sufficient to provide Cu for cbb ₃-Cox maturation. In summary, our data demonstrate that CcoI provides the link between the cytosolic and periplasmic Cu chaperone networks during cbb ₃-Cox assembly.

Transmembrane Cu(I) P-type ATPase pumps are electrogenic uniporters

Cu(i) P-type ATPases are transmembrane primary active ion pumps that catalyze the extrusion of copper ions across cellular membranes. Their activity is critical in controlling copper levels in all kingdoms of life. Biochemical and structural characterization established the structural framework by which Cu-pumps perform their function. However, the details of the overall mechanism of transport (uniporter vs. cotransporter) and electrogenicity still remain elusive. In this work, we developed a platform to reconstitute the model Cu(i)-pump from E. coli (EcCopA) in artificial lipid bilayer small unilamellar vesicles (SUVs) to quantitatively characterize the metal substrate, putative counter-ions and charge translocation. By encapsulating in the liposome lumen fluorescence detector probes (CTAP-3, pyranine and oxonol VI) responsive to diverse stimuli (Cu(i), pH and membrane potential), we correlated substrate, secondary-ion translocation and charge movement events in EcCopA proteoliposomes. This platform centered on multiple fluorescence reporters allowed study of the mechanism and translocation kinetic parameters in real-time for wild-type EcCopA and inactive mutants. The maximal initial Cu(i) transport rate of 165 nmol Cu(i) mg-1 min-1 and KM, Cu(I) = 0.15 ± 0.07 μM was determined with this analysis. We reveal that Cu(i) pumps are primary-active uniporters and electrogenic. The Cu(i) translocation cycle does not require proton counter-transport resulting in electrogenic generation of transmembrane potential upon translocation of one Cu(i) per ATP hydrolysis cycle. Thus, mechanistic differences between Cu(i) pumps and other better characterized P-type ATPases are discussed. The platform opens the venue to study translocation events and mechanisms of transport in other transition metal P-type ATPase pumps.

Identification of genes required for gold and silver tolerance in Burkholderia cenocepacia H111 by transposon sequencing

Members of the genus Burkholderia show remarkable abilities to adapt to a wide range of environmental conditions and is frequently isolated from soils contaminated with heavy metals. In this study, we used a transposon sequencing approach to identify 138 and 164 genes that provide a benefit for growth of the opportunistic pathogen Burkholderia cenocepacia H111 in the presence of silver and gold ions respectively. The data suggest that arginine metabolism and citrate biosynthesis are important for silver tolerance, while components of an ABC transporter (BCAL0307-BCAL0308) and de novo cysteine biosynthesis are required for tolerance to gold ions. We show that determinants that affect tolerance to both metal ions include the two-component systems BCAL0497/99 and BCAL2830/31 and genes that are involved in maintaining the integrity of the cell envelope, suggesting that membrane proteins represent important targets of silver and gold ions. Furthermore, we show that that the P-type ATPase CadA (BCAL0055), which confers tolerance to cadmium contributes to silver but not gold tolerance. Our results may be useful for improving the antibacterial effect of silver and gold ions to combat drug-resistant pathogens.

A conserved, buried cysteine near the P-site is accessible to cysteine modifications and increases ROS stability in the P-type plasma membrane H+-ATPase

Sulfur-containing amino acid residues function in antioxidative responses, which can be induced by the reactive oxygen species generated by excessive copper and hydrogen peroxide. In all Na+/K+, Ca2+, and H+ pumping P-type ATPases, a cysteine residue is present two residues upstream of the essential aspartate residue, which is obligatorily phosphorylated in each catalytic cycle. Despite its conservation, the function of this cysteine residue was hitherto unknown. In this study, we analyzed the function of the corresponding cysteine residue (Cys-327) in the autoinhibited plasma membrane H+-ATPase isoform 2 (AHA2) from Arabidopsis thaliana by mutagenesis and heterologous expression in a yeast host. Enzyme kinetics of alanine, serine, and leucine substitutions were identical with those of the wild-type pump but the sensitivity of the mutant pumps was increased towards copper and hydrogen peroxide. Peptide identification and sequencing by mass spectrometry demonstrated that Cys-327 was prone to oxidation. These data suggest that Cys-327 functions as a protective residue in the plasma membrane H+-ATPase, and possibly in other P-type ATPases as well.

Characterization of ATP7A missense mutants suggests a correlation between intracellular trafficking and severity of Menkes disease

Menkes disease (MD) is caused by mutations in ATP7A, encoding a copper-transporting P-type ATPase which exhibits copper-dependent trafficking. ATP7A is found in the Trans-Golgi Network (TGN) at low copper concentrations, and in the post-Golgi compartments and the plasma membrane at higher concentrations. Here we have analyzed the effect of 36 ATP7A missense mutations identified in phenotypically different MD patients. Nine mutations identified in patients with severe MD, virtually eliminated ATP7A synthesis, in most cases due to aberrant RNA splicing. A group of 21 predominantly severe mutations led to trapping of the protein in TGN and displayed essentially no activity in a yeast-based functional assay. These were predicted to inhibit the catalytic phosphorylation of the protein. Four mutants showed diffuse post-TGN localization, while two displayed copper dependent trafficking. These six variants were identified in patients with mild MD and typically displayed activity in the yeast assay. The four post-TGN located mutants were presumably affected in the catalytic dephosphorylation of the protein. Together these results indicate that the severity of MD correlate with cellular localization of ATP7A and support previous studies indicating that phosphorylation is crucial for the exit of ATP7A from TGN, while dephosphorylation is crucial for recycling back to TGN.

Bacterial Cu(+)-ATPases: models for molecular structure-function studies

The early discovery of the human Cu(+)-ATPases and their link to Menkes and Wilson's diseases brought attention to the unique role of these transporters in copper homeostasis. The characterization of bacterial Cu(+)-ATPases has significantly furthered our understanding of the structure, selectivity and transport mechanism of these enzymes, as well as their interplay with other elements of Cu(+) distribution networks. This review focuses on the structural-functional insights that have emerged from studies of bacterial Cu(+)-ATPases at the molecular level and how these observations have contributed to drawing up a comprehensive picture of cellular copper homeostasis.

The promiscuous phosphomonoestearase activity of Archaeoglobus fulgidus CopA, a thermophilic Cu+ transport ATPase

Membrane transport P-type ATPases display two characteristic enzymatic activities: a principal ATPase activity provides the driving force for ion transport across biological membranes, whereas a promiscuous secondary activity catalyzes the hydrolysis of phosphate monoesters. This last activity is usually denoted as the phosphatase activity of P-ATPases. In the present study, we characterize the phosphatase activity of the Cu(+)-transport ATPase from Archaeglobus fulgidus (Af-CopA) and compare it with the principal ATPase activity. Our results show that the phosphatase turnover number was 20 times higher than that corresponding to the ATPase activity, but it is compensated by a high value of Km, producing a less efficient catalysis for pNPP. This secondary activity is enhanced by Mg(2+) (essential activator) and phospholipids (non-essential activator), and inhibited by salts and Cu(+). Transition state analysis of the catalyzed and noncatalyzed hydrolysis of pNPP indicates that Af-CopA enhances the reaction rates by a factor of 10(5) (ΔΔG(‡)=38 kJ/mol) mainly by reducing the enthalpy of activation (ΔΔH(‡)=30 kJ/mol), whereas the entropy of activation is less negative on the enzyme than in solution. For the ATPase activity, the decrease in the enthalpic component of the barrier is higher (ΔΔH(‡)=39 kJ/mol) and the entropic component is small on both the enzyme and in solution. These results suggest that different mechanisms are involved in the transference of the phosphoryl group of p-nitrophenyl phosphate and ATP.

Copper resistance and its regulation in the sulfate-reducing bacterium Desulfosporosinus sp. OT

Desulfosporosinus sp. OT is a Gram-positive, acidophilic sulfate-reducing firmicute isolated from copper tailings sediment in the Norilsk mining-smelting area in Siberia and represents the first Desulfosporosinus species whose genome has been sequenced. Desulfosporosinus sp. OT is exceptionally copper resistant, which made it of interest to study the resistance mechanism. It possesses a copUAZ operon which is shown here to be involved in copper resistance. The copU gene encodes a CsoR-type homotetrameric repressor. By electrophoretic mobility shift assay, it was shown that CopU binds to the operator/promoter region of the copUAZ operon in the absence of copper and is released from the DNA by Cu⁺ or Ag⁺, implying that CopU regulates the operon in a copper/silver-dependent manner. DOT_CopA is a P1B-type ATPase related to other characterized, bacterial copper ATPases. When expressed in a copper-sensitive Escherichia coli ΔcopA mutant, it restores copper resistance to WT levels. His-tagged DOT_CopA was expressed from a plasmid in E. coli and purified by Ni-NTA affinity chromatography. The purified enzyme was most active in the presence of Cu(I) and bacterial phospholipids. These findings indicate that the copUAZ operon confers copper resistance to Desulfosporosinus sp. OT, but do not per se explain the basis of the high copper resistance of this strain.

Development of [⁶⁴Cu]-DOTA-PR81 radioimmunoconjugate for MUC-1 positive PET imaging

OBJECTIVE

Breast cancer radioimmunoscintigraphy targeting MUC1 expression is a growing field of work in nuclear medicine research. PR81 is a monoclonal antibody that binds with high affinity to MUC1, which is over expressed on breast tumors. In this study, we report production, quality control and preclinical qualifications of a copper-64 labeled PR81 for PET imaging of breast cancer.

METHODS

PR81 was conjugated with DOTA-NHS-ester and purified by molecular filtration followed by chelate:mAb ratio determination by spectrophotometric method. DOTA-PR81 was labeled with (64)Cu followed by radiochemical purity, in vitro stability, in vitro internalization and immunoreactivity determination. The tissue biodistribution of the (64)Cu-DOTA-PR81 and (64)Cu-DOTA-hIgG was evaluated in BALB/c mice with breast carcinoma tumors using tissue counting and imaging.

RESULTS

The radiochemical purity of radioimmunoconjugate was >95±1.9% (ITLC) (specific activity; 4.6 μCi/μg). The average number of chelators per antibody was 3.4±0.3:1. The (64)Cu-DOTA-PR81 showed immunoreactivity towards MUC1 antigen and MCF7 cell line with significant in vitro stability (>89% in PBS and 78±0.5% in human serum) over 48 h. Maximum internalized activity of radiolabeled PR81 in 4-8 h was 81.5%. The biodistribution and scintigraphy studies showed the accumulation of the complex at the site of tumors with high sensitivity and specificity compared to control probes.

CONCLUSION

The results showed that (64)Cu-DOTA-PR81 may be considered as a potential PET tracer for diagnosis and follow-up of MUC1 expression in oncology.

Functional and Biochemical Characterization of Cucumber Genes Encoding Two Copper ATPases CsHMA5.1 and CsHMA5.2

Plant copper P1B-type ATPases appear to be crucial for maintaining copper homeostasis within plant cells, but until now they have been studied mostly in model plant systems. Here, we present the molecular and biochemical characterization of two cucumber copper ATPases, CsHMA5.1 and CsHMA5.2, indicating a different function for HMA5-like proteins in different plants. When expressed in yeast, CsHMA5.1 and CsHMA5.2 localize to the vacuolar membrane and are activated by monovalent copper or silver ions and cysteine, showing different affinities to Cu(+) (Km ∼1 or 0.5 μM, respectively) and similar affinity to Ag(+) (Km ∼2.5 μM). Both proteins restore the growth of yeast mutants sensitive to copper excess and silver through intracellular copper sequestration, indicating that they contribute to copper and silver detoxification. Immunoblotting with specific antibodies revealed the presence of CsHMA5.1 and CsHMA5.2 in the tonoplast of cucumber cells. Interestingly, the root-specific CsHMA5.1 was not affected by copper stress, whereas the widely expressed CsHMA5.2 was up-regulated or down-regulated in roots upon copper excess or deficiency, respectively. The copper-induced increase in tonoplast CsHMA5.2 is consistent with the increased activity of ATP-dependent copper transport into tonoplast vesicles isolated from roots of plants grown under copper excess. These data identify CsHMA5.1 and CsHMA5.2 as high affinity Cu(+) transporters and suggest that CsHMA5.2 is responsible for the increased sequestration of copper in vacuoles of cucumber root cells under copper excess.

A sulfur-based transport pathway in Cu+-ATPases

Cells regulate copper levels tightly to balance the biogenesis and integrity of copper centers in vital enzymes against toxic levels of copper. PIB -type Cu(+)-ATPases play a central role in copper homeostasis by catalyzing the selective translocation of Cu(+) across cellular membranes. Crystal structures of a copper-free Cu(+)-ATPase are available, but the mechanism of Cu(+) recognition, binding, and translocation remains elusive. Through X-ray absorption spectroscopy, ATPase activity assays, and charge transfer measurements on solid-supported membranes using wild-type and mutant forms of the Legionella pneumophila Cu(+)-ATPase (LpCopA), we identify a sulfur-lined metal transport pathway. Structural analysis indicates that Cu(+) is bound at a high-affinity transmembrane-binding site in a trigonal-planar coordination with the Cys residues of the conserved CPC motif of transmembrane segment 4 (C382 and C384) and the conserved Met residue of transmembrane segment 6 (M717 of the MXXXS motif). These residues are also essential for transport. Additionally, the studies indicate essential roles of other conserved intramembranous polar residues in facilitating copper binding to the high-affinity site and subsequent release through the exit pathway.

HMA6 and HMA8 are two chloroplast Cu+-ATPases with different enzymatic properties

Copper (Cu) plays a key role in the photosynthetic process as cofactor of the plastocyanin (PC), an essential component of the chloroplast photosynthetic electron transfer chain. Encoded by the nuclear genome, PC is translocated in its apo-form into the chloroplast and the lumen of thylakoids where it is processed to its mature form and acquires Cu. In Arabidopsis, Cu delivery into the thylakoids involves two transporters of the PIB-1 ATPases family, heavy metal associated protein 6 (HMA6) located at the chloroplast envelope and HMA8 at the thylakoid membrane. To gain further insight into the way Cu is delivered to PC, we analysed the enzymatic properties of HMA8 and compared them with HMA6 ones using in vitro phosphorylation assays and phenotypic tests in yeast. These experiments reveal that HMA6 and HMA8 display different enzymatic properties: HMA8 has a higher apparent affinity for Cu(+) but a slower dephosphorylation kinetics than HMA6. Modelling experiments suggest that these differences could be explained by the electrostatic properties of the Cu(+) releasing cavities of the two transporters and/or by the different nature of their cognate Cu(+) acceptors (metallochaperone/PC).

Copper Delivery to Chloroplast Proteins and its Regulation

Copper is required for photosynthesis in chloroplasts of plants because it is a cofactor of plastocyanin, an essential electron carrier in the thylakoid lumen. Other chloroplast copper proteins are copper/zinc superoxide dismutase and polyphenol oxidase, but these proteins seem to be dispensable under conditions of low copper supply when transcripts for these proteins undergo microRNA-mediated down regulation. Two ATP-driven copper transporters function in tandem to deliver copper to chloroplast compartments. This review seeks to summarize the mechanisms of copper delivery to chloroplast proteins and its regulation. We also delineate some of the unanswered questions that still remain in this field.

Metal ion homeostasis in Listeria monocytogenes and importance in host-pathogen interactions

Listeria monocytogenes is responsible for one of the most life-threatening food-borne infections and the leading cause of food-poisoning associated deaths in the UK. Infection may be of the unborn/newly born infant where disease may manifest as listeric abortion, stillbirth or late-onset neonatal listeriosis, while in adults, infection usually affects the central nervous system causing meningitis. Crucial to the survival of L. monocytogenes, both inside and outside the host, is its ability to acquire metals which act as cofactors for a broad range of its cellular proteins. However, L. monocytogenes must also protect itself against the innate toxicity of metals. The importance of metals in host-pathogen interactions is illustrated by the restriction of metals (including zinc and iron) in vertebrates in response to infection and the use of high levels of metals (copper and zinc) as part of the antimicrobial defences within host phagocytes. As such, L. monocytogenes is equipped with various mechanisms to tightly control its cellular metal pools and avoid metal poisoning. These include multiple DNA-binding metal-responsive transcription factors, metal-acquisition, metal-detoxification and metal-storage systems, some of which represent key L. monocytogenes virulence determinants. This review discusses current knowledge of the role of metals in L. monocytogenes infections, with a focus on the mechanisms that contribute to zinc and copper homeostasis in this organism. The requirement to precisely control cellular metal levels may impose a vulnerability to L. monocytogenes which can be exploited in antimicrobials and therapeutics.

Copper homeostasis in grapevine: functional characterization of the Vitis vinifera copper transporter 1

MAIN CONCLUSION

The Vitis vinifera copper transporter 1 is capable of self-interaction and mediates intracellular copper transport. An understanding of copper homeostasis in grapevine (Vitis vinifera L.) is particularly relevant to viticulture in which copper-based fungicides are intensively used. In the present study, the Vitis vinifera copper transporter 1 (VvCTr1), belonging to the Ctr family of copper transporters, was cloned and functionally characterized. Amino acid sequence analysis showed that VvCTr1 monomers are small peptides composed of 148 amino acids with 3 transmembrane domains and several amino acid residues typical of Ctr transporters. Bimolecular fluorescence complementation (BiFC) demonstrated that Ctr monomers are self-interacting and subcellular localization studies revealed that VvCTr1 is mobilized via the trans-Golgi network, through the pre-vacuolar compartment and located to the vacuolar membrane. The heterologous expression of VvCTr1 in a yeast strain lacking all Ctr transporters fully rescued the phenotype, while a deficient complementation was observed in a strain lacking only plasma membrane-bound Ctrs. Given the common subcellular localization of VvCTr1 and AtCOPT5 and the highest amino acid sequence similarity in comparison to the remaining AtCOPT proteins, Arabidopsis copt5 plants were stably transformed with VvCTr1. The impairment in root growth observed in copt5 seedlings in copper-deficient conditions was fully rescued by VvCTr1, further supporting its involvement in intracellular copper transport. Expression studies in V. vinifera showed that VvCTr1 is mostly expressed in the root system, but transcripts were also present in leaves and stems. The functional characterization of VvCTr-mediated copper transport provides the first step towards understanding the physiological and molecular responses of grapevines to copper-based fungicides.

Functional diversity of five homologous Cu+-ATPases present in Sinorhizobium meliloti

Copper is an important element in host-microbe interactions, acting both as a catalyst in enzymes and as a potential toxin. Cu(+)-ATPases drive cytoplasmic Cu(+) efflux and protect bacteria against metal overload. Many pathogenic and symbiotic bacteria contain multiple Cu(+)-ATPase genes within particular genetic environments, suggesting alternative roles for each resulting protein. This hypothesis was tested by characterizing five homologous Cu(+)-ATPases present in the symbiotic organism Sinorhizobium meliloti. Mutation of each gene led to different phenotypes and abnormal nodule development in the alfalfa host. Distinct responses were detected in free-living S. meliloti mutant strains exposed to metal and redox stresses. Differential gene expression was detected under Cu(+), oxygen or nitrosative stress. These observations suggest that CopA1a maintains the cytoplasmic Cu(+) quota and its expression is controlled by Cu(+) levels. CopA1b is also regulated by Cu(+) concentrations and is required during symbiosis for bacteroid maturation. CopA2-like proteins, FixI1 and FixI2, are necessary for the assembly of two different cytochrome c oxidases at different stages of bacterial life. CopA3 is a phylogenetically distinct Cu(+)-ATPase that does not contribute to Cu(+) tolerance. It is regulated by redox stress and required during symbiosis. We postulated a model where non-redundant homologous Cu(+)-ATPases, operating under distinct regulation, transport Cu(+) to different target proteins.

Identification of functionally important conserved trans-membrane residues of bacterial PIB -type ATPases

Powered by ATP hydrolysis, P_IB-ATPases drive the energetically uphill transport of transition metals. These high affinity pumps are essential for heavy metal detoxification and delivery of metal cofactors to specific cellular compartments. Amino acid sequence alignment of the trans-membrane (TM) helices of P_IB-ATPases reveals a high degree of conservation, with ∼60–70 fully conserved positions. Of these conserved positions, 6–7 were previously identified to be important for transport. However, the functional importance of the majority of the conserved TM residues remains unclear. To investigate the role of conserved TM residues of P_IB-ATPases we conducted an extensive mutagenesis study of a Zn²⁺ Cd²⁺ P_IB-ATPase from Rhizobium radiobacter (rrZntA) and seven other P_IB-ATPases. Of the 38 conserved positions tested, 24 had small effects on metal tolerance. Fourteen mutations compromised in vivo metal tolerance and in vitro metal-stimulated ATPase activity. Based on structural modelling, the functionally important residues line a constricted ‘channel’, tightly surrounded by the residues that were found to be inconsequential for function. We tentatively propose that the distribution of the mutable and immutable residues marks a possible trans-membrane metal translocation pathway. In addition, by substituting six trans-membrane amino acids of rrZntA we changed the in vivo metal specificity of this pump from Zn²⁺ Cd²⁺ to Ag⁺.

Origin and evolution of metal P-type ATPases in Plantae (Archaeplastida)

Metal ATPases are a subfamily of P-type ATPases involved in the transport of metal cations across biological membranes. They all share an architecture featuring eight transmembrane domains in pairs of two and are found in prokaryotes as well as in a variety of Eukaryotes. In Arabidopsis thaliana, eight metal P-type ATPases have been described, four being specific to copper transport and four displaying a broader metal specificity, including zinc, cadmium, and possibly copper and calcium. So far, few efforts have been devoted to elucidating the origin and evolution of these proteins in Eukaryotes. In this work, we use large-scale phylogenetics to show that metal P-type ATPases form a homogenous group among P-type ATPases and that their specialization into either monovalent (Cu) or divalent (Zn, Cd…) metal transport stems from a gene duplication that took place early in the evolution of Life. Then, we demonstrate that the four subgroups of plant metal ATPases all have a different evolutionary origin and a specific taxonomic distribution, only one tracing back to the cyanobacterial progenitor of the chloroplast. Finally, we examine the subsequent evolution of these proteins in green plants and conclude that the genes thoroughly characterized in model organisms are often the result of lineage-specific gene duplications, which calls for caution when attempting to infer function from sequence similarity alone in non-model organisms.

On allosteric modulation of P-type Cu(+)-ATPases

P-type ATPases perform active transport of various compounds across biological membranes and are crucial for ion homeostasis and the asymmetric composition of lipid bilayers. Although their functional cycle share principles of phosphoenzyme intermediates, P-type ATPases also show subclass-specific sequence motifs and structural elements that are linked to transport specificity and mechanistic modulation. Here we provide an overview of the Cu(+)-transporting ATPases (of subclass PIB) and compare them to the well-studied sarco(endo)plasmic reticulum Ca(2+)-ATPase (of subclass PIIA). Cu(+) ions in the cell are delivered by soluble chaperones to Cu(+)-ATPases, which expose a putative "docking platform" at the intracellular interface. Cu(+)-ATPases also contain heavy-metal binding domains providing a basis for allosteric control of pump activity. Database analysis of Cu(+) ligating residues questions a two-site model of intramembranous Cu(+) binding, and we suggest an alternative role for the proposed second site in copper translocation and proton exchange. The class-specific features demonstrate that topological diversity in P-type ATPases may tune a general energy coupling scheme to the translocation of compounds with remarkably different properties.

The mechanism of Cu+ transport ATPases: interaction with CU+ chaperones and the role of transient metal-binding sites

Cu(+)-ATPases are membrane proteins that couple the hydrolysis of ATP to the efflux of cytoplasmic Cu(+). In cells, soluble chaperone proteins bind and distribute cytoplasmic Cu(+), delivering the ion to the transmembrane metal-binding sites in the ATPase. The structure of Legionella pneumophila Cu(+)-ATPase (Gourdon, P., Liu, X. Y., Skjørringe, T., Morth, J. P., Møller, L. B., Pedersen, B. P., and Nissen, P. (2011) Nature 475, 59-64) shows that a kinked transmembrane segment forms a "platform" exposed to the cytoplasm. In addition, neighboring invariant Met, Asp, and Glu are located at the "entrance" of the ion path. Mutations of amino acids in these regions of the Archaeoglobus fulgidus Cu(+)-ATPase CopA do not affect ATPase activity in the presence of Cu(+) free in solution. However, Cu(+) bound to the corresponding chaperone (CopZ) could not activate the mutated ATPases, and in parallel experiments, CopZ was unable to transfer Cu(+) to CopA. Furthermore, mutation of a specific electronegative patch on the CopZ surface abolishes the ATPase activation and Cu(+) transference, indicating that the region is required for the CopZ-CopA interaction. Moreover, the data suggest that the interaction is driven by the complementation of the electropositive platform in the ATPase and the electronegative Cu(+) chaperone. This docking likely places the Cu(+) proximal to the conserved carboxyl and thiol groups in the entrance site that induce metal release from the chaperone via ligand exchange. The initial interaction of Cu(+) with the pump is transient because Cu(+) is transferred from the entrance site to transmembrane metal-binding sites involved in transmembrane translocation.

A structural model of the copper ATPase ATP7B to facilitate analysis of Wilson disease-causing mutations and studies of the transport mechanism

The copper-transporting ATPase ATP7B has an essential role in human physiology, particularly for the liver and brain function. Inactivation of ATP7B is associated with a severe hepato-neurologic disorder, Wilson disease (WD). Hundreds of WD related mutations have been identified in ATP7B to date. The low frequency and the compound-heterozygous nature of causative mutations complicate the analysis of individual mutants and the establishment of genotype-phenotype correlations. To facilitate studies of disease-causing mutations and mechanistic understanding of WD, we have homology-modelled the ATP7B core (residues 643-1377) using the recent structure of the bacterial copper-ATPase LCopA as a template. The model, supported by evolutionary conservation and hydrophobicity analysis, as well as existing and new mutagenesis data, allows molecular interpretations of experimentally characterized clinical mutations. We also illustrate that structure and conservation can be used to grade potential deleterious effects for many WD mutations, which were clinically detected but have not yet been experimentally characterized. Finally, we compare the structural features of ATP7B and LCopA and discuss specific features of the eukaryotic copper pump.

Metal transport across biomembranes: emerging models for a distinct chemistry

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

A tetrahedral coordination of Zinc during transmembrane transport by P-type Zn(2+)-ATPases

Zn(2+) is an essential transition metal required in trace amounts by all living organisms. However, metal excess is cytotoxic and leads to cell damage. Cells rely on transmembrane transporters, with the assistance of other proteins, to establish and maintain Zn(2+) homeostasis. Metal coordination during transport is key to specific transport and unidirectional translocation without the backward release of free metal. The coordination details of Zn(2+) at the transmembrane metal binding site responsible for transport have now been established. Escherichia coli ZntA is a well-characterized Zn(2+)-ATPase responsible for intracellular Zn(2+) efflux. A truncated form of the protein lacking regulatory metal sites and retaining the transport site was constructed. Metrical parameters of the metal-ligand coordination geometry for the zinc bound isolated form were characterized using x-ray absorption spectroscopy (XAS). Our data support a nearest neighbor ligand environment of (O/N)(2)S(2) that is compatible with the proposed invariant metal coordinating residues present in the transmembrane region. This ligand identification and the calculated bond lengths support a tetrahedral coordination geometry for Zn(2+) bound to the TM-MBS of P-type ATPase transporters.

Toward a molecular understanding of metal transport by P(1B)-type ATPases

The P(1B) family of P-type ATPases couples the transport of cytoplasmic transition metals across biological membranes to the hydrolysis of ATP. These ubiquitous transporters function in maintaining cytoplasmic metal quotas and in the assembly of metalloproteins, and have been classified into subfamilies (P(1B-1)-P(1B-5)) on the basis of their transported substrates (Cu(+), Zn(2+), Cu(2+), and Co(2+)) and signature sequences in their transmembrane segments. In addition, each subgroup presents a characteristic membrane topology and specific regulatory cytoplasmic metal-binding domains. In recent years, significant major aspects of their transport mechanism have been described, including the stoichiometry of transport and the delivery of substrates to transport sites by metallochaperones. Toward understanding their structure, the metal coordination by transport sites has been characterized for Cu(+) and Zn(2+)-ATPases. In addition, atomic resolution structures have been determined, providing key insight into the elements that enable transition metal transport. Because the Cu(+)-transporting ATPases are found in humans and are linked to disease, this subfamily has been the focus of intense study. As a result, significant progress has been made toward understanding Cu(+)-ATPase function on the molecular level, using both the human proteins and the bacterial homologs, most notably the CopA proteins from Archaeoglobus fulgidus, Bacillus subtilis, and Thermotoga maritima. This chapter thus focuses on the mechanistic and structural information obtained by studying these latter Cu(+)-ATPases, with some consideration of how these aspects might differ for the other subfamilies of P(1B)-ATPases.

Altered intracellular localization and valosin-containing protein (p97 VCP) interaction underlie ATP7A-related distal motor neuropathy

ATP7A is a P-type ATPase that regulates cellular copper homeostasis by activity at the trans-Golgi network (TGN) and plasma membrane (PM), with the location normally governed by intracellular copper concentration. Defects in ATP7A lead to Menkes disease or its milder variant, occipital horn syndrome or to a newly discovered condition, ATP7A-related distal motor neuropathy (DMN), for which the precise pathophysiology has been obscure. We investigated two ATP7A motor neuropathy mutations (T994I, P1386S) previously associated with abnormal intracellular trafficking. In the patients' fibroblasts, total internal reflection fluorescence microscopy indicated a shift in steady-state equilibrium of ATP7A(T994I) and ATP7A(P1386S), with exaggerated PM localization. Transfection of Hek293T cells and NSC-34 motor neurons with the mutant alleles tagged with the Venus fluorescent protein also revealed excess PM localization. Endocytic retrieval of the mutant alleles from the PM to the TGN was impaired. Immunoprecipitation assays revealed an abnormal interaction between ATP7A(T994I) and p97/VCP, an ubiquitin-selective chaperone which is mutated in two autosomal dominant forms of motor neuron disease: amyotrophic lateral sclerosis and inclusion body myopathy with early-onset Paget disease and fronto-temporal dementia. Small-interfering RNA (SiRNA) knockdown of p97/VCP corrected ATP7A(T994I) mislocalization. Flow cytometry documented that non-permeabilized ATP7A(P1386S) fibroblasts bound a carboxyl-terminal ATP7A antibody, consistent with relocation of the ATP7A di-leucine endocytic retrieval signal to the extracellular surface and partially destabilized insertion of the eighth transmembrane helix. Our findings illuminate the mechanisms underlying ATP7A-related DMN and establish a link between p97/VCP and genetically distinct forms of motor neuron degeneration.

Bacterial transition metal P(1B)-ATPases: transport mechanism and roles in virulence

P(1B)-type ATPases are polytopic membrane proteins that couple the hydrolysis of ATP to the efflux of cytoplasmic transition metals. This paper reviews recent progress in our understanding of the structure and function of these proteins in bacteria. These are members of the P-type superfamily of transport ATPases. Cu(+)-ATPases are the most frequently observed and best-characterized members of this group of transporters. However, bacterial genomes show diverse arrays of P(1B)-type ATPases with a range of substrates (Cu(+), Zn(2+), Co(2+)). Furthermore, because of the structural similarities among transitions metals, these proteins can also transport nonphysiological substrates (Cd(2+), Pb(2+), Au(+), Ag(+)). P(1B)-type ATPases have six or eight transmembrane segments (TM) with metal coordinating amino acids in three core TMs flanking the cytoplasmic domain responsible for ATP binding and hydrolysis. In addition, regulatory cytoplasmic metal binding domains are present in most P(1B)-type ATPases. Central to the transport mechanism is the binding of the uncomplexed metal to these proteins when cytoplasmic substrates are bound to chaperone and chelating molecules. Metal binding to regulatory sites is through a reversible metal exchange among chaperones and cytoplasmic metal binding domains. In contrast, the chaperone-mediated metal delivery to transport sites appears as a largely irreversible event. P(1B)-ATPases have two overarching physiological functions: to maintain cytoplasmic metal levels and to provide metals for the periplasmic assembly of metalloproteins. Recent studies have shown that both roles are critical for bacterial virulence, since P(1B)-ATPases appear key to overcome high phagosomal metal levels and are required for the assembly of periplasmic and secreted metalloproteins that are essential for survival in extreme oxidant environments.

Bacterial ATP-driven transporters of transition metals: physiological roles, mechanisms of action, and roles in bacterial virulence

Maintaining adequate intracellular levels of transition metals is fundamental to the survival of all organisms. While all transition metals are toxic at elevated intracellular concentrations, metals such as iron, zinc, copper, and manganese are essential to many cellular functions. In prokaryotes, the concerted action of a battery of membrane-embedded transport proteins controls a delicate balance between sufficient acquisition and overload. Representatives from all major families of transporters participate in this task, including ion-gradient driven systems and ATP-utilizing pumps. P-type ATPases and ABC transporters both utilize the free energy of ATP hydrolysis to drive transport. Each of these very different families of transport proteins has a distinct role in maintaining transition metal homeostasis: P-type ATPases prevent intracellular overloading of both essential and toxic metals through efflux while ABC transporters import solely the essential ones. In the present review we discuss how each system is adapted to perform its specific task from mechanistic and structural perspectives. Despite the mechanistic and structural differences between P-type ATPases and ABC transporters, there is one important commonality: in many clinically relevant bacterial pathogens, transporters of transition metals are essential for virulence. Here we present several such examples and discuss how these may be exploited for future antibacterial drug development.

The transport mechanism of bacterial Cu+-ATPases: distinct efflux rates adapted to different function

Cu(+)-ATPases play a key role in bacterial Cu(+) homeostasis by participating in Cu(+) detoxification and cuproprotein assembly. Characterization of Archaeoglobus fulgidus CopA, a model protein within the subfamily of P(1B-1) type ATPases, has provided structural and mechanistic details on this group of transporters. Atomic resolution structures of cytoplasmic regulatory metal binding domains (MBDs) and catalytic actuator, phosphorylation, and nucleotide binding domains are available. These, in combination with whole protein structures resulting from cryo-electron microscopy analyses, have enabled the initial modeling of these transporters. Invariant residues in helixes 6, 7 and 8 form two transmembrane metal binding sites (TM-MBSs). These bind Cu(+) with high affinity in a trigonal planar geometry. The cytoplasmic Cu(+) chaperone CopZ transfers the metal directly to the TM-MBSs; however, loading both of the TM-MBSs requires binding of nucleotides to the enzyme. In agreement with the classical transport mechanism of P-type ATPases, occupancy of both transmembrane sites by cytoplasmic Cu(+) is a requirement for enzyme phosphorylation and subsequent transport into the periplasmic or extracellular milieus. Recent transport studies have shown that all Cu(+)-ATPases drive cytoplasmic Cu(+) efflux, albeit with quite different transport rates in tune with their various physiological roles. Archetypical Cu(+)-efflux pumps responsible for Cu(+) tolerance, like the Escherichia coli CopA, have turnover rates ten times higher than those involved in cuproprotein assembly (or alternative functions). This explains the incapability of the latter group to significantly contribute to the metal efflux required for survival in high copper environments.

Structure and interactions of the C-terminal metal binding domain of Archaeoglobus fulgidus CopA

The Cu(+)-ATPase CopA from Archaeoglobus fulgidus belongs to the P(1B) family of the P-type ATPases. These integral membrane proteins couple the energy of ATP hydrolysis to heavy metal ion translocation across membranes. A defining feature of P(1B-1)-type ATPases is the presence of soluble metal binding domains at the N-terminus (N-MBDs). The N-MBDs exhibit a conserved ferredoxin-like fold, similar to that of soluble copper chaperones, and bind metal ions via a conserved CXXC motif. The N-MBDs enable Cu(+) regulation of turnover rates apparently through Cu-sensitive interactions with catalytic domains. A. fulgidus CopA is unusual in that it contains both an N-terminal MBD and a C-terminal MBD (C-MBD). The functional role of the unique C-MBD has not been established. Here, we report the crystal structure of the apo, oxidized C-MBD to 2.0 A resolution. In the structure, two C-MBD monomers form a domain-swapped dimer, which has not been observed previously for similar domains. In addition, the interaction of the C-MBD with the other cytoplasmic domains of CopA, the ATP binding domain (ATPBD) and actuator domain (A-domain), has been investigated. Interestingly, the C-MBD interacts specifically with both of these domains, independent of the presence of Cu(+) or nucleotides. These data reinforce the uniqueness of the C-MBD and suggest a distinct structural role for the C-MBD in CopA transport.

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.

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.

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.

Copper homeostasis

Copper (Cu) is a cofactor in proteins that are involved in electron transfer reactions and is an essential micronutrient for plants. Copper delivery is accomplished by the concerted action of a set of evolutionarily conserved transporters and metallochaperones. As a result of regulation of transporters in the root and the rarity of natural soils with high Cu levels, very few plants in nature will experience Cu in toxic excess in their tissues. However, low Cu bioavailability can limit plant productivity and plants have an interesting response to impending Cu deficiency, which is regulated by an evolutionarily conserved master switch. When Cu supply is insufficient, systems to increase uptake are activated and the available Cu is utilized with economy. A number of Cu-regulated small RNA molecules, the Cu-microRNAs, are used to downregulate Cu proteins that are seemingly not essential. On low Cu, the Cu-microRNAs are upregulated by the master Cu-responsive transcription factor SPL7, which also activates expression of genes involved in Cu assimilation. This regulation allows the most important proteins, which are required for photo-autotrophic growth, to remain active over a wide range of Cu concentrations and this should broaden the range where plants can thrive.

Response to excess copper in the hyperthermophile Sulfolobus solfataricus strain 98/2

Copper is an essential micronutrient, but toxic in excess. Sulfolobus solfataricus cells have the ability to adapt to fluctuations of copper levels in their external environment. To better understand the molecular mechanism behind the organismal response to copper, the expression of the cluster of genes copRTA, which encodes the copper-responsive transcriptional regulator CopR, the copper-binding protein CopT, and CopA, has been investigated and the whole operon has been shown to be cotranscribed at low levels from the copR promoter under all conditions, whereas increased transcription from the copTA promoter occurs in the presence of excess copper. Furthermore, the expression of the copper-transporting ATPase CopA over a 27-h interval has been monitored by quantitative real-time RT-PCR and compared to the pattern of cellular copper accumulation, as determined in a parallel analysis by Inductively Coupled Plasma Optical Emission spectrometry (ICP-OES). The results provide the basis for a model of the molecular mechanisms of copper homeostasis in Sulfolobus, which relies on copper efflux and sequestration.

A P-type ATPase importer that discriminates between essential and toxic transition metals

Transition metals, although being essential cofactors in many physiological processes, are toxic at elevated concentrations. Among the membrane-embedded transport proteins that maintain appropriate intracellular levels of transition metals are ATP-driven pumps belonging to the P-type ATPase superfamily. These metal transporters may be differentiated according to their substrate specificities, where the majority of pumps can extrude either silver and copper or zinc, cadmium, and lead. In the present report, we have established the substrate specificities of nine previously uncharacterized prokaryotic transition-metal P-type ATPases. We find that all of the newly identified exporters indeed fall into one of the two above-mentioned categories. In addition to these exporters, one importer, Pseudomonas aeruginosa Q9I147, was also identified. This protein, designated HmtA (heavy metal transporter A), exhibited a different substrate recognition profile from the exporters. In vivo metal susceptibility assays, intracellular metal measurements, and transport experiments all suggest that HmtA mediates the uptake of copper and zinc but not of silver, mercury, or cadmium. The substrate selectivity of this importer ensures the high-affinity uptake of essential metals, while avoiding intracellular contamination by their toxic counterparts.

Structural model of the CopA copper ATPase of Enterococcus hirae based on chemical cross-linking

The CopA copper ATPase of Enterococcus hirae belongs to the family of heavy metal pumping CPx-type ATPases and shares 43% sequence similarity with the human Menkes and Wilson copper ATPases. Due to a lack of suitable protein crystals, only partial three-dimensional structures have so far been obtained for this family of ion pumps. We present a structural model of CopA derived by combining topological information obtained by intramolecular cross-linking with molecular modeling. Purified CopA was cross-linked with different bivalent reagents, followed by tryptic digestion and identification of cross-linked peptides by mass spectrometry. The structural proximity of tryptic fragments provided information about the structural arrangement of the hydrophilic protein domains, which was integrated into a three-dimensional model of CopA. Comparative modeling of CopA was guided by the sequence similarity to the calcium ATPase of the sarcoplasmic reticulum, Serca1, for which detailed structures are available. In addition, known partial structures of CPx-ATPase homologous to CopA were used as modeling templates. A docking approach was used to predict the orientation of the heavy metal binding domain of CopA relative to the core structure, which was verified by distance constraints derived from cross-links. The overall structural model of CopA resembles the Serca1 structure, but reveals distinctive features of CPx-type ATPases. A prominent feature is the positioning of the heavy metal binding domain. It features an orientation of the Cu binding ligands which is appropriate for the interaction with Cu-loaded metallochaperones in solution. Moreover, a novel model of the architecture of the intramembranous Cu binding sites could be derived.

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.

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.

Mechanism of Cu+-transporting ATPases: soluble Cu+ chaperones directly transfer Cu+ to transmembrane transport sites

As in other P-type ATPases, metal binding to transmembrane metal-binding sites (TM-MBS) in Cu(+)-ATPases is required for enzyme phosphorylation and subsequent transport. However, Cu(+) does not access Cu(+)-ATPases in a free (hydrated) form but is bound to a chaperone protein. Cu(+) transfer from Cu(+) chaperones to regulatory cytoplasmic metal-binding domains (MBDs) present in these ATPases has been described, but there is no evidence of a proposed subsequent Cu(+) movement from the MBDs to the TM-MBS. Alternatively, we postulate the parsimonious Cu(+) transfer by the chaperone directly to TM-MBS. Testing both models, the delivery of Cu(+) by Archaeoglobus fulgidus Cu(+) chaperone CopZ to the corresponding Cu(+)-ATPase, CopA, was studied. As expected, CopZ interacted with and delivered the metal to CopA MBDs. Cu(+)-loaded MBDs, acting as metal donors, were unable to activate CopA or a truncated CopA lacking MBDs. Conversely, Cu(+)-loaded CopZ activated the CopA ATPase and CopA constructs in which MBDs were rendered unable to bind Cu(+). Furthermore, under nonturnover conditions, CopZ transferred Cu(+) to the TM-MBS of a CopA lacking MBDs. These data are consistent with a model where MBDs serve a regulatory function without participating in metal transport and the chaperone delivers Cu(+) directly to transmembrane transport sites of Cu(+)-ATPases.

Membrane structure of CtrA3, a copper-transporting P-type-ATPase from Aquifex aeolicus

We have produced and characterized two new copper-transporting ATPases, CtrA2 and CtrA3 from Aquifex aeolicus, that belong to the family of heavy metal ion-transporting P(IB)-type ATPases. CtrA2 has a CPC metal-binding sequence in TM6 and a CxxC metal-binding N-terminal domain, while CtrA3 has a CPH metal-binding motif in TM6 and a histidine-rich N-terminal metal-binding domain. We have cloned both copper pumps, expressed them in Escherichia coli and characterized them functionally. CtrA2 is activated by Ag(+) and Cu(+) and presumably transports reduced Cu(+), while CtrA3 is activated by, and presumably transports, the oxidized copper ion. Both CtrA2 and CtrA3 are thermophilic proteins with an activity maximum at 75 degrees C. Electron cryomicroscopy of two-dimensional crystals of CtrA3 yielded a projection map at approximately 7 A resolution with density peaks, indicating eight membrane-spanning alpha-helices per monomer. A fit of the Ca-ATPase structure to the projection map indicates that the arrangement of the six central helices surrounding the ion-binding site in the membrane is conserved, and suggests the position of the two additional N-terminal transmembrane helices that are characteristic of the heavy metal, eight-helix P(1B)-type ATPases.

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.

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.