The Lys1010-Lys1325 fragment of the Wilson's disease protein binds nucleotides and interacts with the N-terminal domain of this protein in a copper-dependent manner

Structural Basis for the Function of the N-terminal Domain of the ATPase CopA from Bacillus subtilis

The solution structure of the N-terminal region (151 amino acids) of a copper ATPase, CopA, from Bacillus subtilis, is reported here. It consists of two domains, CopAa and CopAb, linked by two amino acids. It is found that the two domains, which had already been separately characterized, interact one to the other through a hydrogen bond network and a few hydrophobic interactions, forming a single rigid body. The two metal binding sites are far from one another, and the short link between the domains prevents them from interacting. This and the surface electrostatic potential suggest that each domain receives copper from the copper chaperone, CopZ, independently and transfers it to the membrane binding site of CopA. The affinity constants of silver(I) and copper(I) are similar for the two sites as monitored by NMR. Because the present construct "domain-short link-domain" is shared also by the last two domains of the eukaryotic copper ATPases and several residues at the interface between the two domains are conserved, the conclusions of the present study have general validity for the understanding of the function of copper ATPases.

Archaeoglobus fulgidus CopB is a thermophilic Cu2+-ATPase: functional role of its histidine-rich-N-terminal metal binding domain

P1B-type ATPases transport heavy metal ions across cellular membranes. Archaeoglobus fulgidus CopB is a member of this subfamily. We have cloned, expressed in Escherichia coli, and functionally characterized this enzyme. CopB and its homologs are distinguished by a metal binding sequence Cys-Pro-His in their sixth transmembrane segment (H6) and a His-rich N-terminal metal binding domain (His-N-MBD). CopB is a thermophilic protein active at 75 degrees C and high ionic strength. It is activated by Cu2+ with high apparent affinity (K1/2 = 0.28 microm) and partially by Cu+ and Ag+ (22 and 55%, respectively). The higher turnover was associated with a faster phosphorylation rate in the presence of Cu2+. A truncated CopB lacking the first 54 amino acids was constructed to characterize the His-N-MBD. This enzyme showed reduced ATPase activity (50% of wild type) but no changes in metal selectivity, ATP dependence, or phosphorylation levels. However, a slower rate of dephosphorylation of the E2P(Cu2+) form was observed for truncated CopB. The data suggest that the presence of the His residue in the putative transmembrane metal binding site of CopB determines a selectivity for this enzyme that is different for that observed in Cu+/Ag+-ATPases carrying a Cys-Pro-Cys sequence. The His-NMBD appears to have a regulatory role affecting the metal transport rate by controlling the metal release/dephosphorylation rates.

Functional roles of metal binding domains of the Archaeoglobus fulgidus Cu(+)-ATPase CopA

CopA, a thermophilic membrane ATPase from Archaeoglobus fulgidus, drives the outward movement of Cu(+) or Ag(+) [Mandal et al. (2002) J. Biol. Chem. 277, 7201-7208]. This, as other P(IB)-ATPases, is characterized by a putative metal binding sequence (C(380)PC(382)) in its sixth transmembrane fragment and cytoplasmic metal binding sequences in its NH(2)- and COOH-terminal ends (C(27)AMC(30) and C(751)HHC(754)). Using isolated CopA, we have studied the functional role of these three putative metal binding domains. Replacement of transmembrane Cys residues by Ala results in nonfunctional enzymes that are unable to hydrolyze ATP. However, the CPC --> APA substituted enzyme binds ATP, indicating its correct folding and suggesting that enzyme turnover is prevented by the lack of metal binding to the transmembrane site. Replacement of C-terminal Cys by Ala (C(751,754)A) has no significant effect on ATPase activity, enzyme phosphorylation, apparent binding affinities of ligands, or E1-E2 equilibrium. In contrast, replacement of Cys in the N-terminal metal binding domain (N-MBD) (C(27,30)A) leads to 40% reduction in enzyme turnover. The C(27,30)A enzyme binds Cu(+), Ag(+), and ATP with the same high apparent affinities as the wild-type CopA. Evidence that N-MBD disruption has no effect on the E1-E2 equilibrium is provided by the normal interaction of ATP acting with low affinity and the unaffected IC(50) for vanadate inhibition observed in the C(27,30)A-substituted enzyme. However, replacement C(27,30)A slowed the dephosphorylation of the E2P(metal) form of the enzyme, suggesting a reduction in the rate of metal release. Other investigators have shown the Cu-dependent interaction of isolated N-MBDs from the Wilson disease Cu-ATPase with the ATP binding cytoplasmic domain [Tsivkovskii et al. (2001) J. Biol. Chem. 276, 2234-2242]. Therefore, the data suggest a regulatory mechanism in which the Cu-dependent N-MBD/ATP binding domain interaction would accelerate cation release, the enzyme rate-limiting step, and consequently Cu(+) transport.

Fluorescent and colored trinitrophenylated analogs of ATP and GTP

Fluorescent and colored trinitrophenylated (TNP) analogs of ATP and GTP can interact with nucleotide-requiring enzymes and proteins as a substitute for the parent nucleotide. These analogs have strong binding affinities for most nucleotide-requiring systems. Their bindings are easily detected by absorption and fluorescence changes in the visible region. Recent years have seen dramatic developments in the application of the TNP nucleotide analogs as spectroscopic probes for the study on the nucleotide-interacting properties of various enzymes and proteins including their mutants. This review is intended as a broad overview of currently extensively used applications of the nucleotide analogs in various biological systems.

The distinct roles of the N-terminal copper-binding sites in regulation of catalytic activity of the Wilson's disease protein

Wilson's disease protein (WNDP) is a copper-transporting ATPase essential for normal distribution of copper in human cells. Recent studies demonstrate that copper regulates WNDP through several mechanisms. Six metal-binding sites (MBS) at the N terminus of WNDP are predicted to be involved in copper-dependent regulation of WNDP; however, specific roles of MBS remain poorly understood. To address this issue, we generated WNDP variants with mutations or truncation in the N-terminal region and characterized their functional properties. We show that copper cooperatively stimulates catalytic activity of WNDP and that this effect requires the presence of both MBS5 and MBS6. Mutations of MBS6 or MBS1-5 result in non-cooperative activation of the enzyme by copper, whereas the deletion of MBS1-4 does not abolish cooperativity. Our data further suggest that MBS5 and MBS6 together regulate the affinity of the intramembrane-binding site(s) for copper. Analysis of the copper-dependent stimulation of catalytic phosphorylation demonstrate that the MBS6 and MBS1-5 mutants have a 7-8-fold lower EC50 for copper activation, suggesting that their affinity for copper is increased. This conclusion is confirmed by a markedly decreased inhibition of these mutants by a copper chelator bathocuproine disulphonate. In contrast, deletion of MBS1-4 does not affect the affinity of sites important for catalytic phosphorylation. Rather, the MBS1-4 region appears to control access of copper to the functionally important metal-binding sites. The implications of these findings for intracellular regulation of WNDP are discussed.

A core mutation affecting the folding properties of a soluble domain of the ATPase protein CopA from Bacillus subtilis

The two N-terminal domains of the P-type copper ATPase, CopAa and CopAb, from Bacillus subtilis differ in their folding capabilities in vitro. Whereas CopAb has the typical betaalphabetabetaalphabeta structure and is a rigid protein, CopAa is found to be largely unfolded. A sequence analysis of the two and of orthologue homologous proteins indicates that Ser46 in CopAa may destabilise the hydrophobic core, as also confirmed through a bioinformatic energy study. CopAb has a Val in the corresponding position. The S46V and S46A mutants are found to be folded, although the latter displays multiple conformations. S46VCopAa, in both apo and copper(I) loaded forms, has very similar structural and dynamic properties with respect to CopAb, besides a different length of strand beta2 and beta4. It is intriguing that the oxygen of Thr16 is found close, though at longer than bonding distance, to copper in both domains, as it also occurs in a human orthologue domain. This study contributes to understanding the behaviour of proteins that do not properly fold in vitro. A possible biological significance of the peculiar folding behaviour of this domain is discussed.

Zn, Cu and Co in cyanobacteria: selective control of metal availability

Homeostatic systems for essential and non-essential metals create the cellular environments in which the correct metals are acquired by metalloproteins while the incorrect ones are somehow avoided. Cyanobacteria have metal requirements often absent from other bacteria; copper in thylakoidal plastocyanin, zinc in carboxysomal carbonic anhydrase, cobalt in cobalamin but magnesium in chlorophyll, molybdenum in heterocystous nitrogenase, manganese in thylakoidal water-splitting oxygen-evolving complex. This article reviews: an intracellular trafficking pathway for inward copper supply, the sequestration of surplus zinc by metallothionein (also present in other bacteria) and the detection and export of excess cobalt. We consider the influence of homeostatic proteins on selective metal availability.

The Methanocaldococcus jannaschii protein Mj0968 is not a P-type ATPase

The Methanocaldococcus jannaschii (formerly Methanococcus jannaschii) protein Mj0968 has been reported to represent a soluble P-type ATPase [Ogawa et al., FEBS Lett. 471 (2000) 99-102]. In this study, we report that the heterologously expressed Mj0968-His(10) protein exhibits high rates of phosphatase activity, whereas only very low ATPase activity was measured. Replacement of the aspartate residue in the DSAGT motif (D7A), which becomes phosphorylated during the reaction cycle of P-type ATPases, does not affect the V(max), but only the K(M) of the reaction. Labeling studies with [gamma-(32)P]ATP and [alpha-(32)P]ATP revealed that the previously reported labeling experiments [Ogawa et al., 2000] do not necessarily show phosphorylation of Mj0968, but rather point to ATP binding. Binding studies with trinitrophenyl adenosine nucleotides showed low apparent K(d) values for those molecules. These results provide evidence that the native function of Mj0968 seems to be that of a phosphatase, rather than that of an ATP-hydrolyzing enzyme.

The role of the invariant His-1069 in folding and function of the Wilson's disease protein, the human copper-transporting ATPase ATP7B

The copper-transporting ATPase ATP7B is essential for normal distribution of copper in human cells. Mutations in ATP7B lead to Wilson's disease, a severe disorder with neurological and hepatic manifestations. One of the most common disease mutations, a H1069Q substitution, causes intracellular mislocalization of ATP7B (the Wilson's disease protein, WNDP). His-1069 is located in the nucleotide-binding domain of WNDP and is conserved in all copper-transporting ATPases from bacteria to mammals; however, the specific role of this His in the structure and function of WNDP remains unclear. We demonstrate that substitution of His-1069 for Gln, Ala, or Cys does not significantly alter the folding of the WNDP nucleotide-binding domain or the proteolytic resistance of the full-length WNDP. In contrast, the function of WNDP is markedly affected by the mutations. The ability to form an acylphosphate intermediate in the presence of ATP is entirely lost in all three mutants, suggesting that His-1069 is important for ATP-dependent phosphorylation. Other steps of the WNDP enzymatic cycle are less dependent on His-1069. The H1069C mutant shows normal phosphorylation in the presence of inorganic phosphate; it binds an ATP analogue, beta,gamma-imidoadenosine 5'-triphosphate (AMP-PNP), and copper and undergoes nucleotide-dependent conformational transitions similar to those of the wild-type WNDP. Although binding of AMP-PNP is not disrupted by the mutation, the apparent affinity for the nucleotide is decreased by 4-fold. We conclude that His-1069 is responsible for proper orientation of ATP in the catalytic site of WNDP prior to ATP hydrolysis.

Functional properties of the human copper-transporting ATPase ATP7B (the Wilson's disease protein) and regulation by metallochaperone Atox1

Wilson's disease protein (WNDP) is a copper-transporting P(1)-type ATPase which plays a key role in normal distribution of copper in a number of tissues, particularly in the liver and the brain. Copper has numerous effects on WNDP, altering its structure, activity, and intracellular localization. To better understand the function of this copper-transporting ATPase and its regulation by copper, we have recently developed the functional expression systems for WNDP and for Atox1, a cytosolic protein that serves as an intracellular donor of copper for WNDP. Here we summarize the results of our experiments on characterization of the enzymatic properties of WNDP and the effects of Atox1 on the WNDP activity.

Heavy metal transport CPx-ATPases from the thermophile Archaeoglobus fulgidus

PIB-type ATPases transport diverse heavy metals (Cu(+), Ag(+), Cu(2+). Zn(2+), Cd(2+), Pb(2+), Co(2+)) across membranes. Toward understanding their mechanisms of metal selectivity, we are studying thermophilic archaeal PIB-type ATPases. Like other PIB ATPases, these are characterized by the presence of a cation binding CPX sequence in their 6th transmembrane segment and by cytoplasmic N-terminus metal binding domains (N-MBDs). CopA and CopB from the thermophile Archaeoglobus fulgidus were cloned and expressed in E. coli. The resulting proteins were purified in a soluble active form. Typical yields were in the order of 3-5 mg of pure protein per liter of bacterial culture. Both enzymes showed maximum activity at 75-85 degrees C. CopA was activated by Ag(+)>Cu(+) while CopB was activated by Cu(2+)>Ag(+)>Cu(+). The differences in enzyme selectivity can be explained by different consensus sequences in the transmembrane cation binding domain (CopA: CPC, CopB: CPH). Mutagenesis studies show that the cysteines in the transmembrane CPC site of CopA are necessary for enzyme function, while those in the N-MBD (CXXC), although not essential, are required for maximum enzyme activity. Different from CopA, CopB has a His-rich N-MBD. Removal of this domain reduced enzyme activity without affecting enzyme selectivity. These studies show that these enzymes are an excellent system for structural functional studies directed to explain the mechanisms of metal selectivity by PIB ATPases.

Cd2+ and the N-terminal metal-binding domain protect the putative membranous CPC motif of the Cd2+-ATPase of Listeria monocytogenes

CadA, the Cd(2+)-ATPase of Listeria monocytogenes, contains four cysteine residues: two in the CTNC (Cys-Thr-Asn-Cys) sequence in the cytoplasmic metal-binding domain (MBD), and two in the CPC (Cys-Pro-Cys) sequence in the membrane domain. Taking advantage of DeltaMBD, a truncated version of CadA that lacks the MBD but which still acts as a functional Cd(2+)-ATPase [Bal, Mintz, Guillain and Catty (2001) FEBS Lett. 506, 249-252], we analysed the role of the membrane cysteine residues (studied using DeltaMBD) separately from that of the cysteine residues of the MBD, which were studied using full-length CadA. The role of the cysteines was assessed by reacting DeltaMBD and CadA with N -ethylmaleimide (NEM), an SH-specific reagent, in the presence or absence of Cd(2+). We show here that (i) in both DeltaMBD and CadA, the cysteine residues in the CPC motif are essential for phosphorylation; (ii) in both proteins, Cd(2+) protects against alkylation by NEM; and (iii) in the absence of Cd(2+), the MBD of CadA also protects against alkylation by NEM. Our results suggest that the CPC motif is present in the membrane Cd(2+) transport site(s) and that the MBD protects these site(s).

A conditional mutation affecting localization of the Menkes disease copper ATPase. Suppression by copper supplementation

Copper is an essential co-factor for several key metabolic processes. This requirement in humans is underscored by Menkes disease, an X-linked copper deficiency disorder caused by mutations in the copper transporting P-type ATPase, MNK. MNK is located in the trans-Golgi network where it transports copper to secreted cuproenzymes. Increases in copper concentration stimulate the trafficking of MNK to the plasma membrane where it effluxes copper. In this study, a Menkes disease mutation, G1019D, located in the large cytoplasmic loop of MNK, was characterized in transfected cultured cells. In copper-limiting conditions the G1019D mutant protein was retained in the endoplasmic reticulum. However, this mislocalization was corrected by the addition of copper to cells via a process that was dependent upon the copper binding sites at the N-terminal region of MNK. Reduced growth temperature and the chemical chaperone, glycerol, were found to correct the mislocalization of the G1019D mutant, suggesting this mutation interferes with protein folding in the secretory pathway. These findings identify G1019D as the first conditional mutation associated with Menkes disease and demonstrate correction of the mislocalized protein by copper supplementation. Our findings provide a molecular framework for understanding how mutations that affect the proper folding of the MNK transporter in Menkes patients may be responsive to parenteral copper therapy.

Human copper-transporting ATPase ATP7B (the Wilson's disease protein): biochemical properties and regulation

Wilson's disease protein (WNDP) is a product of a gene ATP7B that is mutated in patients with Wilson's disease, a severe genetic disorder with hepatic and neurological manifestations caused by accumulation of copper in the liver and brain. In a cell, WNDP transports copper across various cell membranes using energy of ATP-hydrolysis. Copper regulates WNDP at several levels, modulating its catalytic activity, posttranslational modification, and intracellular localization. This review summarizes recent studies on enzymatic function and copper-dependent regulation of WNDP. Specifically, we describe the molecular architecture and major biochemical properties of WNDP, discuss advantages of the recently developed functional expression of WNDP in insect cells, and summarize the results of the ligand-binding studies and molecular modeling experiments for the ATP-binding domain of WNDP. In addition, we speculate on how copper binding may regulate the activity and intracellular distribution of WNDP, and what role the human copper chaperone Atox1 may play in these processes.

Menkes copper-translocating P-type ATPase (ATP7A): biochemical and cell biology properties, and role in Menkes disease

The Menkes copper-translocating P-type ATPase (ATP7A; MNK) is a ubiquitous protein that regulates the absorption of copper in the gastrointestinal tract. Inside cells the protein has a dual function: it delivers copper to cuproenzymes in the Golgi compartment and effluxes excess copper. The latter property is achieved through copper-dependent vesicular trafficking of the Menkes protein to the plasma membrane of the cell. The trafficking mechanism and catalytic activity combine to facilitate absorption and intercellular transport of copper. The mechanism of catalysis and copper-dependent trafficking of the Menkes protein are the subjects of this review. Menkes disease, a systemic copper deficiency disorder, is caused by mutations in the gene encoding the Menkes protein. The effect of these mutations on the catalytic cycle and the cell biology of the Menkes protein, as well as predictions of the effect of particular mutant MNKs on observed Menkes disease symptoms will also be discussed.

Structural and functional insights of Wilson disease copper-transporting ATPase

Wilson disease is an autosomal recessive disorder of copper metabolism. The gene for this disorder has been cloned and identified to encode a copper-transporting ATPase (ATP7B), a member of a large family of cation transporters, the P-type ATPases. In addition to the core elements common to all P-type ATPases, the Wilson copper-transporting ATPase has a large cytoplasmic N-terminus comprised six heavy metal associated (HMA) domains, each of which contains the copper-binding sequence motif GMT/HCXXC. Extensive studies addressing the functional, regulatory, and structural aspects of heavy metal transport by heavy metal transporters in general, have offered great insights into copper transport by Wilson copper-transporting ATPase. The findings from these studies have been used together with homology modeling of the Wilson disease copper-transporting ATPases based on the X-ray structure of the sarcoplasmic reticulum (SR) calcium-ATPase, to present a hypothetical model of the mechanism of copper transport by copper-transporting ATPases.

Characterization of a thermophilic P-type Ag+/Cu+-ATPase from the extremophile Archaeoglobus fulgidus

The thermophilic, sulfur metabolizing Archaeoglobus fulgidus contains two genes, AF0473 and AF0152, encoding for PIB-type heavy metal transport ATPases. In this study, we describe the cloning, heterologous expression, purification, and functional characterization of one of these ATPases, CopA (NCB accession number AAB90763), encoded by AF0473. CopA is active at high temperatures (75 degrees C; E(a) = 103 kJ/mol) and inactive at 37 degrees C. It is activated by Ag+ (ATPase V(max) = 14.82 micromol/mg/h) and to a lesser extent by Cu+ (ATPase V(max) = 3.66 micromol/mg/h). However, Cu+ interacts with the enzyme with higher apparent affinity (ATPase stimulation, Ag+ K(12) = 29.4 microm; Cu+ K(12) = 2.1 microm). This activation by Ag+ or Cu+ is dependent on the presence of millimolar amounts of cysteine. In the presence of ATP, these metals drive the formation of an acid-stable phosphoenzyme with apparent affinities similar to those observed in the ATPase activity determinations (Ag+, K(12) = 23.0 microm; Cu+, K(12) = 3.9 microm). However, comparable levels of phosphoenzyme are reached in the presence of both cations (Ag+, 1.40 nmol/mg; Cu+, 1.08 nmol/mg). The stimulation of phosphorylation by the cations suggests that CopA drives the outward movement of the metal. CopA presents additional functional characteristics similar to other P-type ATPases. ATP interacts with the enzyme with two apparent affinities (ATPase K(m) = 0.25 mm; phosphorylation K(m) = 4.81 microm), and the presence of vanadate leads to enzyme inactivation (IC(50) = 24 microm). This is the first Ag+/Cu+ -ATPase expressed and purified in a functional form. Thus, it provides a model for structure-functional studies of these transporters. Moreover, its characterization will also contribute to an understanding of thermophilic ion transporters.

Functional properties of the copper-transporting ATPase ATP7B (the Wilson's disease protein) expressed in insect cells

Copper-transporting ATPase ATP7B is essential for normal distribution of copper in human cells. Mutations in the ATP7B gene lead to copper accumulation in a number of tissues and to a severe multisystem disorder, known as Wilson's disease. Primary sequence analysis suggests that the copper-transporting ATPase ATP7B or the Wilson's disease protein (WNDP) belongs to the large family of cation-transporting P-type ATPases, however, the detailed characterization of its enzymatic properties has been lacking. Here, we developed a baculovirus-mediated expression system for WNDP, which permits direct and quantitative analysis of catalytic properties of this protein. Using this system, we provide experimental evidence that WNDP has functional properties characteristic of a P-type ATPase. It forms a phosphorylated intermediate, which is sensitive to hydroxylamine, basic pH, and treatments with ATP or ADP. ATP stimulates phosphorylation with an apparent K(m) of 0.95 +/- 0.25 microm; ADP promotes dephosphorylation with an apparent K(m) of 3.2 +/- 0.7 microm. Replacement of Asp(1027) with Ala in a conserved sequence motif DKTG abolishes phosphorylation in agreement with the proposed role of this residue as an acceptor of phosphate during the catalytic cycle. Catalytic phosphorylation of WNDP is inhibited by the copper chelator bathocuproine; copper reactivates the bathocuproine-treated WNDP in a specific and cooperative fashion confirming that copper is required for formation of the acylphosphate intermediate. These studies establish the key catalytic properties of the ATP7B copper-transporting ATPase and provide a foundation for quantitative analysis of its function in normal and diseased cells.

Copper transportion of WD protein in hepatocytes from Wilson disease patients in vitro

AIM

To study the effect of copper transporting P-type ATPase in copper metabolism of hepatocyte and pathogenesis of Wilson disease (WD).

METHODS

WD copper transporting properties in some organelles of the cultured hepatocytes were studied from WD patients and normal controls.These cultured hepatocytes were incubated in the media of copper 15 mg x L(-1) only, copper 15 mg x L(-1) with vincristine (agonist of P-type ATPase) 0.5mg x L(-1), or copper 15 mg x L(-1) with vanadate (antagonist of P-type ATPase) 18.39 mg x L(-1) separately. Microsome (endoplasmic reticulum and Golgi apparatus), lysosome, mitochondria, and cytosol were isolated by differential centrifugation. Copper contents in these organelles were measured with atomic absorption spectrophotometer, and the influence in copper transportion of these organelles by vanadate and vincristine were comparatively analyzed between WD patients and controls. WD copper transporting P-type ATPase was detected by SDS-PAGE in conjunction with Western blot in liver samples of WD patients and controls.

RESULTS

The specific WD proteins (M(r)155,000 lanes) were expressed in human hepatocytes, including the control and WD patients. After incubation with medium containing copper for 2 h or 24 h, the microsome copper concentration in WD patients was obviously lower than that of controls, and the addition of vanadate or vincristine would change the copper transporting of microsomes obviously. When incubated with vincristine, levels of copper in microsome were significantly increased, while incubated with vanadate, the copper concentrations in microsome were obviously decreased. The results indicated that there were WD proteins, the copper transportion P-type ATPase in the microsome of hepatocytes. WD patients possessed abnormal copper transporting function of WD protein in the microsome, and the agonist might correct the defect of copper transportion by promoting the activity of copper transportion P-type ATPase.

CONCLUSION

Copper transportion P-type ATPase plays an important role in hepatocytic copper metabolism. Dysfunction of hepatocytic WD protein copper transportion might be one of the most important factors for WD.

Functional analysis of chimeric proteins of the Wilson Cu(I)-ATPase (ATP7B) and ZntA, a Pb(II)/Zn(II)/Cd(II)-ATPase from Escherichia coli

ATP7B, the Wilson disease-associated Cu(I)-transporter, and ZntA from Escherichia coli are soft metal P1-type ATPases with mutually exclusive metal ion substrates. P1-type ATPases have a distinctive amino-terminal domain containing the conserved metal-binding motif GXXCXXC. ZntA has one copy of this motif while ATP7B has six copies. The effect of interchanging the amino-terminal domains of ATP7B and ZntA was investigated. Chimeric proteins were constructed in which either the entire amino-terminal domain of ATP7B or only its sixth metal-binding motif replaced the amino-terminal domain of ZntA. Both chimeras conferred resistance to lead, zinc, and cadmium salts but not to copper salts. The purified chimeras displayed activity with lead, cadmium, zinc, and mercury, which are substrates of ZntA. There was no activity with copper or silver, which are substrates of ATP7B. The chimeras were 2-3-fold less active than ZntA. Thus, the amino-terminal domain of P1-type ATPases cannot alter the metal specificity determined by the transmembrane segment. Also, these results suggest that this domain interacts with the rest of the transporter in a metal ion-specific manner; the amino-terminal domain of ATP7B cannot replace that of ZntA in restoring full catalytic activity.

Copper specifically regulates intracellular phosphorylation of the Wilson's disease protein, a human copper-transporting ATPase

Copper is a trace element essential for normal cell homeostasis. The major physiological role of copper is to serve as a cofactor to a number of key metabolic enzymes. In humans, genetic defects of copper distribution, such as Wilson's disease, lead to severe pathologies, including neurodegeneration, liver lesions, and behavior abnormalities. Here, we demonstrate that, in addition to its role as a cofactor, copper can regulate important post-translational events such as protein phosphorylation. Specifically, in human cells copper modulates phosphorylation of a key copper transporter, the Wilson's disease protein (WNDP). Copper-induced phosphorylation of WNDP is rapid, specific, and reversible and correlates with the intracellular location of this copper transporter. WNDP is found to have at least two phosphorylation sites, a basal phosphorylation site and a site modified in response to increased copper concentration. Comparative analysis of WNDP, the WNDP pineal isoform, and WNDP C-terminal truncation mutants revealed that the basal phosphorylation site is located in the C-terminal Ser(796)-Tyr(1384) region of WNDP. The copper-induced phosphorylation appears to require the presence of the functional N-terminal domain of this protein. The novel physiological role of copper as a modulator of protein phosphorylation could be central to understanding how copper transport is regulated in mammalian cells.

Escherichia coli CopA N-terminal Cys(X)(2)Cys motifs are not required for copper resistance or transport

Escherichia coli CopA is a Cu(I)-translocating P-type ATPase that is involved in copper export and resistance. It is an orthologue of the human Menkes and Wilson disease-related proteins. Each of those two human copper pumps has six N-terminal Cys(X)(2)Cys sequences, but their function in transport is unclear. CopA has two N-terminal Cys(X)(2)Cys sequences, GLSC(14)GHC(17) and GMSC(110)ASC(113). The requirement of these cysteine motifs was investigated by mutagenesis of the codons for all four cysteine residues, singly and in combination. Cells of a copA deletion strain expressing genes for the mutant genes were nearly as resistant to copper as the wild type. In addition, everted membrane vesicles from cells expressing the mutant copA genes exhibited ATP-coupled accumulation of copper similar to that of the wild type. The results indicate that neither of two N-terminal Cys(X)(2)Cys motifs is required for either resistance or transport.