Purification and membrane reconstitution of catalytically active Menkes copper-transporting P-type ATPase (MNK; ATP7A)

CtpB is a plasma membrane copper (I) transporting P-type ATPase of Mycobacterium tuberculosis

BACKGROUND

The intracellular concentration of heavy-metal cations, such as copper, nickel, and zinc is pivotal for the mycobacterial response to the hostile environment inside macrophages. To date, copper transport mediated by P-type ATPases across the mycobacterial plasma membrane has not been sufficiently explored.

RESULTS

In this work, the ATPase activity of the putative Mycobacterium tuberculosis P1B-type ATPase CtpB was associated with copper (I) transport from mycobacterial cells. Although CtpB heterologously expressed in M. smegmatis induced tolerance to toxic concentrations of Cu2+ and a metal preference for Cu+, the disruption of ctpB in M. tuberculosis cells did not promote impaired cell growth or heavy-metal accumulation in whole mutant cells in cultures under high doses of copper. In addition, the Cu+ ATPase activity of CtpB embedded in the plasma membrane showed features of high affinity/slow turnover ATPases, with enzymatic parameters KM 0.19 ± 0.04 µM and Vmax 2.29 ± 0.10 nmol/mg min. In contrast, the ctpB gene transcription was activated in cells under culture conditions that mimicked the hostile intraphagosomal environment, such as hypoxia, nitrosative and oxidative stress, but not under high doses of copper.

CONCLUSIONS

The overall results suggest that M. tuberculosis CtpB is associated with Cu+ transport from mycobacterial cells possibly playing a role different from copper detoxification.

Trace metal metabolism in plants

Many trace metals are essential micronutrients, but also potent toxins. Due to natural and anthropogenic causes, vastly different trace metal concentrations occur in various habitats, ranging from deficient to toxic levels. Therefore, one focus of plant research is on the response to trace metals in terms of uptake, transport, sequestration, speciation, physiological use, deficiency, toxicity, and detoxification. In this review, we cover most of these aspects for the essential micronutrients copper, iron, manganese, molybdenum, nickel, and zinc to provide a broader overview than found in other recent reviews, to cross-link aspects of knowledge in this very active research field that are often seen in a separated way. For example, individual processes of metal usage, deficiency, or toxicity often were not mechanistically interconnected. Therefore, this review also aims to stimulate the communication of researchers following different approaches, such as gene expression analysis, biochemistry, or biophysics of metalloproteins. Furthermore, we highlight recent insights, emphasizing data obtained under physiologically and environmentally relevant conditions.

Protein Biochemistry and Expression Regulation of Cadmium/Zinc Pumping ATPases in the Hyperaccumulator Plants Arabidopsis halleri and Noccaea caerulescens

P_1B-ATPases are decisive for metal accumulation phenotypes, but mechanisms of their regulation are only partially understood. Here, we studied the Cd/Zn transporting ATPases NcHMA3 and NcHMA4 from Noccaea caerulescens as well as AhHMA3 and AhHMA4 from Arabidopsis halleri. Protein biochemistry was analyzed on HMA4 purified from roots of N. caerulescens in active state. Metal titration of NcHMA4 protein with an electrochromic dye as charge indicator suggested that HMA4 reaches maximal ATPase activity when all internal high-affinity Cd²⁺ binding sites are occupied. Although HMA4 was reported to be mainly responsible for xylem loading of heavy metals for root to shoot transport, the current study revealed high expression of NcHMA4 in shoots as well. Further, there were additional 20 and 40 kD fragments at replete Zn²⁺ and toxic Cd²⁺, but not at deficient Zn²⁺ concentrations. Altogether, the protein level expression analysis suggested a more multifunctional role of NcHMA4 than previously assumed. Organ-level transcription analysis through quantitative PCR of mRNA in N. caerulescens and A. halleri confirmed the strong shoot expression of both NcHMA4 and AhHMA4. Further, in shoots NcHMA4 was more abundant in 10 μM Zn²⁺ and AhHMA4 in Zn²⁺ deficiency. In roots, NcHMA4 was up-regulated in response to deficient Zn²⁺ when compared to replete Zn²⁺ and toxic Cd²⁺ treatment. In both species, HMA3 was much more expressed in shoots than in roots, and HMA3 transcript levels remained rather constant regardless of Zn²⁺ supply, but were up-regulated by 10 μM Cd²⁺. Analysis of cellular expression by quantitative mRNA in situ hybridisation showed that in A. halleri, both HMA3 and HMA4 mRNA levels were highest in the mesophyll, while in N. caerulescens they were highest in the bundle sheath of the vein. This is likely related to the different final storage sites for hyperaccumulated metals in both species: epidermis in N. caerulescens, mesophyll in A. halleri.

Mechanisms of charge transfer in human copper ATPases ATP7A and ATP7B

ATP7A and ATP7B are Cu⁺ -transporting ATPases of subclass IB and play a fundamental role in intracellular copper homeostasis. ATP7A/B transfer Cu⁺ ions across the membrane from delivery to acceptor proteins without establishing a free Cu⁺ gradient. Transfer of copper across the membrane is coupled to ATP hydrolysis. Current measurements on solid supported membranes (SSM) were performed to investigate the mechanism of copper-related charge transfer across ATP7A and ATP7B. SSM measurements demonstrated that electrogenic copper displacement occurs within ATP7A/B following addition of ATP and formation of the phosphorylated intermediate. Comparison of the time constants for cation displacement in ATP7A/B and sarcoplasmic reticulum Ca²⁺ -ATPase is consistent with the slower phosphoenzyme formation in copper ATPases. Moreover, ATP-dependent copper transfer in ATP7A/B is not affected by varying the pH, suggesting that net proton counter-transport may not occur in copper ATPases. Platinum anticancer drugs activate ATP7A/B and are subjected to ATP-dependent vectorial displacement with a mechanism analogous to that of copper. © 2016 IUBMB Life, 69(4):218-225, 2017.

Protein Biochemistry and Expression Regulation of Cadmium/Zinc Pumping ATPases in the Hyperaccumulator Plants Arabidopsis halleri and Noccaea caerulescens

P_1B-ATPases are decisive for metal accumulation phenotypes, but mechanisms of their regulation are only partially understood. Here, we studied the Cd/Zn transporting ATPases NcHMA3 and NcHMA4 from Noccaea caerulescens as well as AhHMA3 and AhHMA4 from Arabidopsis halleri. Protein biochemistry was analyzed on HMA4 purified from roots of N. caerulescens in active state. Metal titration of NcHMA4 protein with an electrochromic dye as charge indicator suggested that HMA4 reaches maximal ATPase activity when all internal high-affinity Cd²⁺ binding sites are occupied. Although HMA4 was reported to be mainly responsible for xylem loading of heavy metals for root to shoot transport, the current study revealed high expression of NcHMA4 in shoots as well. Further, there were additional 20 and 40 kD fragments at replete Zn²⁺ and toxic Cd²⁺, but not at deficient Zn²⁺ concentrations. Altogether, the protein level expression analysis suggested a more multifunctional role of NcHMA4 than previously assumed. Organ-level transcription analysis through quantitative PCR of mRNA in N. caerulescens and A. halleri confirmed the strong shoot expression of both NcHMA4 and AhHMA4. Further, in shoots NcHMA4 was more abundant in 10 μM Zn²⁺ and AhHMA4 in Zn²⁺ deficiency. In roots, NcHMA4 was up-regulated in response to deficient Zn²⁺ when compared to replete Zn²⁺ and toxic Cd²⁺ treatment. In both species, HMA3 was much more expressed in shoots than in roots, and HMA3 transcript levels remained rather constant regardless of Zn²⁺ supply, but were up-regulated by 10 μM Cd²⁺. Analysis of cellular expression by quantitative mRNA in situ hybridisation showed that in A. halleri, both HMA3 and HMA4 mRNA levels were highest in the mesophyll, while in N. caerulescens they were highest in the bundle sheath of the vein. This is likely related to the different final storage sites for hyperaccumulated metals in both species: epidermis in N. caerulescens, mesophyll in A. halleri.

Identification of Two Conserved Residues Involved in Copper Release from Chloroplast PIB-1-ATPases

Copper is an essential transition metal for living organisms. In the plant model Arabidopsis thaliana, half of the copper content is localized in the chloroplast, and as a cofactor of plastocyanin, copper is essential for photosynthesis. Within the chloroplast, copper delivery to plastocyanin involves two transporters of the PIB-1-ATPases subfamily: HMA6 at the chloroplast envelope and HMA8 in the thylakoid membranes. Both proteins are high affinity copper transporters but share distinct enzymatic properties. In the present work, the comparison of 140 sequences of PIB-1-ATPases revealed a conserved region unusually rich in histidine and cysteine residues in the TMA-L1 region of eukaryotic chloroplast copper ATPases. To evaluate the role of these residues, we mutated them in HMA6 and HMA8. Mutants of interest were selected from phenotypic tests in yeast and produced in Lactococcus lactis for further biochemical characterizations using phosphorylation assays from ATP and Pi Combining functional and structural data, we highlight the importance of the cysteine and the first histidine of the CX3HX2H motif in the process of copper release from HMA6 and HMA8 and propose a copper pathway through the membrane domain of these transporters. Finally, our work suggests a more general role of the histidine residue in the transport of copper by PIB-1-ATPases.

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.

CtpA, a putative Mycobacterium tuberculosis P-type ATPase, is stimulated by copper (I) in the mycobacterial plasma membrane

The transport of heavy-metal ions across the plasma membrane is essential for mycobacterial intracellular survival; in this context, P-type ATPases are pivotal for maintenance of ionic gradients and the plasma membrane homeostasis of mycobacteria. To date, the copper ion transport that is mediated by P-type ATPases in mycobacteria is poorly understood. In this work, the ion-specific activation of CtpA, a putative plasma membrane Mycobacterium tuberculosis P-type ATPase, with different heavy-metal cations was assessed. Mycobacterium smegmatis mc(2)155 cells heterologously expressing the M. tuberculosis ctpA gene displayed an increased tolerance to toxic levels of the Cu(2+) ion (4 mM) compared to control cells, suggesting that CtpA is possibly involved in the copper detoxification of mycobacterial cells. In contrast, the tolerance of M. smegmatis recombinant cells against other heavy-metal divalent cations, such as Co(2+), Mn(2+), Ni(2+) and Zn(2+), was not detected. In addition, the ATPase activity of plasma membrane vesicles that were obtained from M. smegmatis cells expressing CtpA was stimulated by Cu(+) (4.9 nmol of Pi released/mg of protein.min) but not by Cu(2+) ions; therefore, Cu(2+) reduction to Cu(+) inside mycobacterial cells is suggested. Finally, the plasma membrane vesicles of M. smegmatis that were enriched with CtpA exhibited an optimal activity at 37 °C and pH 7.9; the apparent kinetic parameters of the enzyme were a K(1/2) of 4.68 × 10(-2) µM for Cu(+), a Vmax of 10.3 U/mg of protein, and an h value of 1.91.

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).

Compartmentation and complexation of metals in hyperaccumulator plants

Hyperaccumulators are being intensely investigated. They are not only interesting in scientific context due to their "strange" behavior in terms of dealing with high concentrations of metals, but also because of their use in phytoremediation and phytomining, for which understanding the mechanisms of hyperaccumulation is crucial. Hyperaccumulators naturally use metal accumulation as a defense against herbivores and pathogens, and therefore deal with accumulated metals in very specific ways of complexation and compartmentation, different from non-hyperaccumulator plants and also non-hyperaccumulated metals. For example, in contrast to non-hyperaccumulators, in hyperaccumulators even the classical phytochelatin-inducing metal, cadmium, is predominantly not bound by such sulfur ligands, but only by weak oxygen ligands. This applies to all hyperaccumulated metals investigated so far, as well as hyperaccumulation of the metalloid arsenic. Stronger ligands, as they have been shown to complex metals in non-hyperaccumulators, are in hyperaccumulators used for transient binding during transport to the storage sites (e.g., nicotianamine) and possibly for export of Cu in Cd/Zn hyperaccumulators [metallothioneins (MTs)]. This confirmed that enhanced active metal transport, and not metal complexation, is the key mechanism of hyperaccumulation. Hyperaccumulators tolerate the high amount of accumulated heavy metals by sequestering them into vacuoles, usually in large storage cells of the epidermis. This is mediated by strongly elevated expression of specific transport proteins in various tissues from metal uptake in the shoots up to the storage sites in the leaf epidermis. However, this mechanism seems to be very metal specific. Non-hyperaccumulated metals in hyperaccumulators seem to be dealt with like in non-hyperaccumulator plants, i.e., detoxified by binding to strong ligands such as MTs.

Role of the P-Type ATPases, ATP7A and ATP7B in brain copper homeostasis

Over the past two decades there have been significant advances in our understanding of copper homeostasis and the pathological consequences of copper dysregulation. Cumulative evidence is revealing a complex regulatory network of proteins and pathways that maintain copper homeostasis. The recognition of copper dysregulation as a key pathological feature in prominent neurodegenerative disorders such as Alzheimer's, Parkinson's, and prion diseases has led to increased research focus on the mechanisms controlling copper homeostasis in the brain. The copper-transporting P-type ATPases (copper-ATPases), ATP7A and ATP7B, are critical components of the copper regulatory network. Our understanding of the biochemistry and cell biology of these complex proteins has grown significantly since their discovery in 1993. They are large polytopic transmembrane proteins with six copper-binding motifs within the cytoplasmic N-terminal domain, eight transmembrane domains, and highly conserved catalytic domains. These proteins catalyze ATP-dependent copper transport across cell membranes for the metallation of many essential cuproenzymes, as well as for the removal of excess cellular copper to prevent copper toxicity. A key functional aspect of these copper transporters is their copper-responsive trafficking between the trans-Golgi network and the cell periphery. ATP7A- and ATP7B-deficiency, due to genetic mutation, underlie the inherited copper transport disorders, Menkes and Wilson diseases, respectively. Their importance in maintaining brain copper homeostasis is underscored by the severe neuropathological deficits in these disorders. Herein we will review and update our current knowledge of these copper transporters in the brain and the central nervous system, their distribution and regulation, their role in normal brain copper homeostasis, and how their absence or dysfunction contributes to disturbances in copper homeostasis and neurodegeneration.

Crosstalk between Cu(I) and Zn(II) homeostasis via Atx1 and cognate domains

The copper metallochaperone Atx1 and the N-terminal metal-binding domain of a copper-transporting ATP-ase can form tight Zn(II)-mediated hetero-complexes in both cyanobacteria and humans. Copper and zinc homeostasis could be linked by metal binding to these CXXC-containing proteins.

Links between copper and cholesterol in Alzheimer's disease

Altered copper homeostasis and hypercholesterolemia have been identified independently as risk factors for Alzheimer's disease (AD). Abnormal copper and cholesterol metabolism are implicated in the genesis of amyloid plaques and neurofibrillary tangles (NFT), which are two key pathological signatures of AD. Amyloidogenic processing of a sub-population of amyloid precursor protein (APP) that produces Aβ occurs in cholesterol-rich lipid rafts in copper deficient AD brains. Co-localization of Aβ and a paradoxical high concentration of copper in lipid rafts fosters the formation of neurotoxic Aβ:copper complexes. These complexes can catalytically oxidize cholesterol to generate H₂O₂, oxysterols and other lipid peroxidation products that accumulate in brains of AD cases and transgenic mouse models. Tau, the core protein component of NFTs, is sensitive to interactions with copper and cholesterol, which trigger a cascade of hyperphosphorylation and aggregation preceding the generation of NFTs. Here we present an overview of copper and cholesterol metabolism in the brain, and how their integrated failure contributes to development of AD.

Copper export from cultured astrocytes

Copper is an essential trace metal that is required as a catalytic co-factor or a structural component of several important enzymes. However, since excess of copper can also harm cells due to its potential to catalyse the generation of toxic reactive oxygen species, transport of copper and the cellular copper content are tightly regulated. Astrocytes are known to efficiently take up copper ions, but it was not known whether these cells are also able to export copper. Treatment of astrocyte-rich primary cultures for 24 h with copper chloride caused a concentration-dependent increase in the specific cellular copper content. During further 24 h incubation in the absence of copper chloride, the copper-loaded astrocytes remained viable and released up to 45% of the accumulated copper. The rate of copper export was proportional to the amount of cellular copper, was almost completely prevented by lowering the incubation temperature to 4 °C and was partly prevented by the endocytosis inhibitor amiloride. Copper export is most likely mediated by the copper ATPase ATP7A, since this transporter is expressed in astrocyte cultures and its cellular location is strongly affected by the absence or the presence of extracellular copper. The potential of cultured astrocytes to export copper suggests that astrocytes provide neighbouring cells in brain with this essential trace element.

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

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

Calcium and copper transport ATPases: analogies and diversities in transduction and signaling mechanisms

The calcium transport ATPase and the copper transport ATPase are members of the P-ATPase family and retain an analogous catalytic mechanism for ATP utilization, including intermediate phosphoryl transfer to a conserved aspartyl residue, vectorial displacement of bound cation, and final hydrolytic cleavage of Pi. Both ATPases undergo protein conformational changes concomitant with catalytic events. Yet, the two ATPases are prototypes of different features with regard to transduction and signaling mechanisms. The calcium ATPase resides stably on membranes delimiting cellular compartments, acquires free Ca²⁺ with high affinity on one side of the membrane, and releases the bound Ca²⁺ on the other side of the membrane to yield a high free Ca²⁺ gradient. These features are a basic requirement for cellular Ca²⁺ signaling mechanisms. On the other hand, the copper ATPase acquires copper through exchange with donor proteins, and undergoes intracellular trafficking to deliver copper to acceptor proteins. In addition to the cation transport site and the conserved aspartate undergoing catalytic phosphorylation, the copper ATPase has copper binding regulatory sites on a unique N-terminal protein extension, and has also serine residues undergoing kinase assisted phosphorylation. These additional features are involved in the mechanism of copper ATPase intracellular trafficking which is required to deliver copper to plasma membranes for extrusion, and to the trans-Golgi network for incorporation into metalloproteins. Isoform specific glyocosylation contributes to stabilization of ATP7A copper ATPase in plasma membranes.

Comparative features of copper ATPases ATP7A and ATP7B heterologously expressed in COS-1 cells

ATP7A and ATP7B are P-type ATPases required for copper homeostasis and involved in the etiology of Menkes and Wilson diseases. We used heterologous expression of ATP7A or ATP7B in COS-1 cells infected with adenovirus vectors to characterize differential features pertinent to each protein expressed in the same mammalian cell type, rather than to extrinsic factors related to different cells sustaining expression. Electrophoretic analysis of the expressed protein, before and after purification, prior or subsequent to treatment with endoglycosidase, and evidenced by protein or glycoprotein staining as well as Western blotting, indicates that the ATP7A protein is glycosylated while ATP7B is not. This is consistent with the prevalence of glycosylation motifs in the ATP7A sequence, and not in ATP7B. ATP7A and ATP7B undergo copper-dependent phosphorylation by utilization of ATP, forming equal levels of an "alkali labile" phosphoenzyme intermediate that undergoes similar catalytic (P-type ATPase) turnover in both enzymes. In addition, incubation with ATP yields an "alkali stable" phosphoprotein fraction, attributed to phosphorylation of serines. Alkali stable phosphorylation occurs at lower levels in ATP7A, consistent with a different distribution of serines in the amino acid sequence. Immunostaining of COS-1 cells sustaining heterologous expression shows initial association of both ATP7A and ATP7B with Golgi and the trans-Golgi network. However, in the presence of added copper, ATP7A undergoes prevalent association with the plasma membrane while ATP7B exhibits intense trafficking with cytosolic vesicles. Glycosylation of ATP7A and phosphorylation of ATP7B apparently contribute to their different trafficking and membrane association when expressed in the same cell type.

Golgi membranes from liver express an ATPase with femtomolar copper affinity, inhibited by cAMP-dependent protein kinase

Copper-stimulated P-type ATPases are essential in the fine-tuning of intracellular copper. In the present work we characterized a copper-dependent ATPase hydrolysis in a native Golgi-enriched preparation from liver and investigated its modulation by cyclic AMP-dependent protein kinase (PKA). The very high-affinity Atp7b copper pump presented here shows a K(0.5) for free copper of 2.5×10(-17) M in bathocuproine disulfonate/copper buffer and ATP hydrolysis was inhibited 50% upon stimulation of PKA pathway, using forskolin, cAMP or cholera toxin. Incubation with PKA inhibitor (PKAi(5-24) peptide) raises Cu(I)-ATPase activity by 50%. Addition of purified PKA α-catalytic subunit increases K(0.5) for free copper (6.2×10(-17) M) without modification in the affinity for ATP in the low-affinity range of the substrate curve (∼1 mM). The Hill coefficient for free copper activation also remains unchanged if exogenous PKA is added (2.7 and 2.3 in the absence and presence of PKA, respectively). The results demonstrate that this high-affinity copper pump in its natural environment is a target of the liver PKA pathway, being regulatory phosphorylation able to influence both turnover rate and ion affinity.

ATP dependent charge movement in ATP7B Cu+-ATPase is demonstrated by pre-steady state electrical measurements

ATP7B is a copper dependent P-type ATPase, required for copper homeostasis. Taking advantage of high yield heterologous expression of recombinant protein, we investigated charge transfer in ATP7B. We detected charge displacement within a single catalytic cycle upon ATP addition and formation of phosphoenzyme intermediate. We attribute this charge displacement to movement of bound copper within ATP7B. Based on specific mutations, we demonstrate that enzyme activation by copper requires occupancy of a site in the N-terminus extension which is not present in other transport ATPases, as well as of a transmembrane site corresponding to the cation binding site of other ATPases.

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.

Abundant expression and purification of biologically active mitochondrial citrate carrier in baculovirus-infected insect cells

Heterologous expression of recombinant proteins is an essential technology for protein characterization. A major obstacle to investigating the biochemical properties of membrane proteins is the difficulty in obtaining sufficient amounts of functional protein. Here we report the successful expression of the tricarboxylate (or citrate) carrier (CIC) of eel (Anguilla anguilla) from Spodoptera frugiperda (Sf9) cells using the baculovirus expression system. The recombinant CIC was purified by affinity chromatography on Ni(2+)-NTA agarose; the yield of the purified active protein was 0.4-0.5 mg/l of culture. The transport characteristics of the recombinant CIC and the effects of inhibitors on transport are similar to those determined for eel liver mitochondrial CIC. Because the CIC is one member of an extensive family of mitochondrial transport proteins, it is likely that the procedure used in this study to express and purify this carrier can be successfully applied to other mitochondrial transport proteins, thus providing sufficient protein for functional characterization.

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

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

High yield heterologous expression of wild-type and mutant Cu+-ATPase (ATP7B, Wilson disease protein) for functional characterization of catalytic activity and serine residues undergoing copper-dependent phosphorylation

ATP7B is a P-type ATPase required for copper homeostasis and related to Wilson disease of humans. In addition to various domains corresponding to other P-type ATPases, ATP7B includes an N terminus extension (NMBD) with six copper binding sites. We obtained high yield expression of WT and mutant ATP7B in COS1 cells infected with adenovirus vector. ATP7B, isolated with the microsomal fraction of cell homogenates, accounts for 10–20% of the total protein. Copper-dependent, steady-state ATPase yields 30 nmol of P_i/mg of protein/min at 37 °C, pH 6.0. ATP7B phosphorylation with ATP occurs with diphasic kinetics and is totally copper-dependent. Alkali labile phosphoenzyme (catalytic intermediate of P-ATPases) accounts for a small fraction of the total phosphoprotein and is prevented by D1027N (P domain) or C983A/C985A (CXC copper binding motif in TM6) mutations. Decay of [³²P]phosphoenzyme following chase with non-radioactive ATP occurs with an initial burst involving alkali labile phosphoenzyme (absent in D1027N and C983A/C985A mutants) and continues at a slow rate involving alkali-resistant phosphoenzyme. If a copper chelator is added with the ATP chase, the initial burst is smaller, and further cleavage is totally inhibited. Analysis by proteolysis and mass spectrometry demonstrates that the alkali stable phosphoenzyme involves Ser⁴⁷⁸ and Ser⁴⁸¹ (NMBD), Ser¹¹²¹ (“N” domain) and Ser¹⁴⁵³ (C terminus), and occurs with the same pattern ex vivo (COS-1) and in vitro (microsomes). The overall copper dependence of phosphorylation and hydrolytic cleavage suggests long range conformational effects, including interactions of NMBD and headpiece domains, with strong influence on catalytic turnover.

The multi-layered regulation of copper translocating P-type ATPases

The copper-translocating Menkes (ATP7A, MNK protein) and Wilson (ATP7B, WND protein) P-type ATPases are pivotal for copper (Cu) homeostasis, functioning in the biosynthetic incorporation of Cu into copper-dependent enzymes of the secretory pathway, Cu detoxification via Cu efflux, and specialized roles such as systemic Cu absorption (MNK) and Cu excretion (WND). Essential to these functions is their Cu and hormone-responsive distribution between the trans-Golgi network (TGN) and exocytic vesicles located at or proximal to the apical (WND) or basolateral (MNK) cell surface. Intriguingly, MNK and WND Cu-ATPases expressed in the same tissues perform distinct yet complementary roles. While intramolecular differences may specify their distinct roles, cellular signaling components are predicted to be critical for both differences and synergy between these enzymes. This review focuses on these mechanisms, including the cell signaling pathways that influence trafficking and bi-functionality of Cu-ATPases. Phosphorylation events are hypothesized to play a central role in Cu homeostasis, promoting multi-layered regulation and cross-talk between cuproenzymes and Cu-independent mechanisms.

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.

Transport of cisplatin by the copper efflux transporter ATP7B

ATP7B is a P-type ATPase that mediates the efflux of copper. Recent studies have demonstrated that ATP7B regulates the cellular efflux of cisplatin (DDP) and controls sensitivity to the cytotoxic effects of this drug. To determine whether DDP is a substrate for ATP7B, DDP transport was assayed in vesicles isolated from Sf9 cells infected with a baculovirus that expressed either the wild-type ATP7B or a mutant ATP7B that was unable to transport copper as a result of conversion of the transmembrane metal binding CPC motif to CPA. Only the wild-type ATP7B-expressing vesicles exhibited copper-dependent ATPase activity, copper-induced acyl-phosphate formation, and ATP-dependent transport of copper. The amount of DDP that became bound was higher for vesicles expressing either type of ATP7B than for those not expressing either form of ATP7B, but only the vesicles expressing wild-type ATP7B mediated ATP-dependent accumulation of the drug. At pH 4.6, the vesicles expressing the wild-type ATP7B exhibited ATP-dependent accumulation of DDP with an apparent K(m) of 1.2 +/- 0.5 (S.E.M.) muM and V(max) of 0.03 +/- 0.002 (S.E.M.) nmol/mg of protein/min. DDP also induced the acyl-phosphorylation of ATP7B but at a much slower rate than copper. Copper and DDP each inhibited the ATP-dependent transport of the other. These results establish that DDP is a substrate for ATP7B but is transported at a much slower rate than copper.

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

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

Biochemical basis of regulation of human copper-transporting ATPases

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