Purification and functional analysis of the copper ATPase CopA of Enterococcus hirae

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.

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).

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.

Response of gram-positive bacteria to copper stress

The Gram-positive bacteria Enterococcus hirae, Lactococcus lactis, and Bacillus subtilis have received wide attention in the study of copper homeostasis. Consequently, copper extrusion by ATPases, gene regulation by copper, and intracellular copper chaperoning are understood in some detail. This has provided profound insight into basic principles of how organisms handle copper. It also emerged that many bacterial species may not require copper for life, making copper homeostatic systems pure defense mechanisms. Structural work on copper homeostatic proteins has given insight into copper coordination and bonding and has started to give molecular insight into copper handling in biological systems. Finally, recent biochemical work has shed new light on the mechanism of copper toxicity, which may not primarily be mediated by reactive oxygen radicals.

High yield expression of biologically active recombinant full length human tuftelin protein in baculovirus-infected insect cells

Tuftelin is an acidic protein expressed at very early stages of mouse odontogenesis. It was suggested to play a role during epithelial-mesenchymal interactions, and later, when enamel formation commences, to be involved in enamel mineralization. Tuftelin was also detected in several normal soft tissues of different origins and some of their corresponding cancerous tissues. Tuftelin is expressed in low quantities, and undergoes degradation in the enamel extracellular matrix. To investigate the structure and function of tuftelin, the full length recombinant human tuftelin protein was produced. The full length human tuftelin cDNA was cloned using Gateway recombination into the Bac-to-Bac system compatible transfer vector pDest10. This vector adds a hexahistidine tag to the N-terminus of the expressed protein, enabling one-step affinity purification on nickel column. The recombinant human tuftelin protein was transposed into the bacmid and expressed in Spodoptera frugiperda (Sf9) insect cells. The yield of the purified, his-tagged recombinant full length human Tuftelin (rHTuft+) was 5-8 mg/L culture. rHTuft+ was characterized by SDS-PAGE, Western blot, ESI-TOF spectrometry, restriction mapping and MS/MS sequencing. The availability of the purified, full length recombinant human tuftelin protein opened up the possibility to investigate novel functions of tuftelin. Application of rHTuft+ agarose beads onto embryonic mouse mandibular explants caused changes in the surrounding epithelial cells, including morphology, orientation and spatial organization. Further studies using DiI labeling, revealed that rHTuft+, placed on the tooth germ region, brought about recruitment of adjacent embryonic mesenchymal cells. These findings support the hypothesis that tuftelin plays an important role during embryogenesis.

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.

Differences of Cu accumulation and Cu-induced ATPase activity in roots of two populations of Elsholtzia haichowensis Sun

Two populations of Elsholtzia haichowensis, which are from an uncontaminated site (Hong'an) and a Cu mine site (Tonglushan) in Hubei province, China, were studied for ATPase activities and Cu accumulation of roots under hydroponic conditions. The root tolerance indices were markedly higher in Tonglushan population than in Hong'an population under Cu stress conditions. The root Cu concentration in Tonglushan population was significantly lower than that in Hong'an population in all Cu treatments. The Cu-stimulated ATPase activity increased significantly under 1 microM Cu, reached a peak at 5 microM Cu and then, began to decrease. However, no significant change of ATPase activity was observed in Hong'an population except for an obvious decrease in 80 microM Cu treatment. These results showed that Cu-ATPase might exist in the root plasma membrane of Tonglushan population and play a possible role in resistance to copper.

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.

A native Zn/Cd pumping P1B ATPase from natural overexpression in a hyperaccumulator plant

We report here the first purification of a P(1B) type ATPase, a group of transporters that occurs in bacteria, plants and animals incl. humans, from a eukaryotic organism in native state. TcHMA4 is a P(1B) type ATPase that is highly expressed in the Cd/Zn hyperaccumulator plant Thlaspi caerulescens and contains a C-terminal 9-histidine repeat. After isolation from roots, we purified TcHMA4 protein via metal affinity chromatography. The purified protein exhibited Cd- and Zn-activated ATPase activity after reconstitution into lipid vesicles, showing that it was in its native state. Gels of crude root extract and of the purified protein revealed TcHMA4-specific bands of about 50 and 60kDa, respectively, while the TcHMA4 mRNA predicts a single protein with a size of 128kDa. This indicates the occurrence of post-translational processing; the properties of the two bands were characterised by their activity and binding properties.

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

The MNK (Menkes disease protein; ATP7A) is a major copper- transporting P-type ATPase involved in the delivery of copper to cuproenzymes in the secretory pathway and the efflux of excess copper from extrahepatic tissues. Mutations in the MNK (ATP7A) gene result in Menkes disease, a fatal neurodegenerative copper deficiency disorder. Currently, detailed biochemical and biophysical analyses of MNK to better understand its mechanisms of copper transport are not possible due to the lack of purified MNK in an active form. To address this issue, we expressed human MNK with an N-terminal Glu-Glu tag in Sf9 [Spodoptera frugiperda (fall armyworm) 9] insect cells and purified it by antibody affinity chromatography followed by size-exclusion chromatography in the presence of the non-ionic detergent DDM (n-dodecyl beta-D-maltopyranoside). Formation of the classical vanadate-sensitive phosphoenzyme by purified MNK was activated by Cu(I) [EC50=0.7 microM; h (Hill coefficient) was 4.6]. Furthermore, we report the first measurement of Cu(I)-dependent ATPase activity of MNK (K0.5=0.6 microM; h=5.0). The purified MNK demonstrated active ATP-dependent vectorial 64Cu transport when reconstituted into soya-bean asolectin liposomes. Together, these data demonstrated that Cu(I) interacts with MNK in a co-operative manner and with high affinity in the sub-micromolar range. The present study provides the first biochemical characterization of a purified full-length mammalian copper-transporting P-type ATPase associated with a human disease.

Expression and purification of HtpX-like small heat shock integral membrane protease of an unknown organism related to Methylobacillus flagellatus

The M48 conserved family of peptidases contains a single catalytic zinc ion tetrahedrally co-ordinated by two histidines within an HEXXH motif. The proteases of this class are generally toxic to the cell and thus difficult to express and purify. Here, we report the expression and purification of the small HtpX-like heat shock metalloprotease from an unknown organism related to the obligate methylotrophic anaerobic bacterium, Methylobacillus flagellatus. The protease was expressed in the Escherichia coli vector - pT7. Optimization of expression was done to increase the yield and solubility of the expressed protein. Improved refolding procedures from inclusion bodies of pT7 E. coli system were devised to get the protease in an active and stable form. The protease was purified to near homogeneity in its active form from the refolded proteins of the inclusion bodies by a two-step (cation exchange followed by gel filtration) high performance liquid chromatography (HPLC). The purified protease was active on zymography and casein hydrolysis assays. The activity of the protease was found to be optimum at pH 7.4 and at a temperature of 37 degrees C but significant activity was also retained at higher temperatures of 45-50 degrees C. Centrifugal fractionation showed that it is a membrane localized endopeptidase. The methods described here can serve as guidelines to express and purify other homologues of M48 family of proteases for functional and structural studies.

Induction of heavy-metal-transporting CPX-type ATPases during acid adaptation in Lactobacillus bulgaricus

Lactobacillus bulgaricus is a lactic acid bacteria (LAB) that, through the production of lactic acid, gradually acidifies its environment during growth. In the course of this process, L. bulgaricus acquires an improved tolerance to acidity. A survey of the recently established genome sequence shows that this bacterium possesses few of the pH control functions that have been described in other LAB and raises the question of what other mechanisms could be involved in its adaptation to the decreasing environmental pH. In some bacteria other than LAB, ion transport systems have been implicated in acid adaptation. We therefore studied the expression of this type of transport system during acid adaptation in L. bulgaricus by reverse transcription and real-time quantitative PCR and mapped transcription start sites. Intriguingly, the most significantly induced were three ATPases carrying the CPX signature of heavy-metal transporters. Protein homology and the presence of a conserved sequence motif in the promoter regions of the genes encoding these proteins strongly suggest that they are involved in copper homeostasis. Induction of this system is thought to assist in avoiding indirect damage that could result from medium acidification.

Role of metal-binding domains of the copper pump from Archaeoglobus fulgidus

CopA from the extreme thermophile Archaeoglobus fulgidus is a P-type ATPase that transports Cu(+) and Ag(+) and has individual metal-binding domains (MBDs) at both N- and C-termini. We expressed and purified full-length CopA as well as constructs with MBDs deleted either individually or collectively. Cu(+) and Ag(+)-dependent ATPase assays showed that full-length CopA had submicromolar affinity for both ions, but was inhibited by concentrations above 1muM. Deletion of both MBDs had no effect on affinity but resulted in loss of this inhibition. Individual deletions implicated the N-terminal MBD in causing the inhibition at concentrations >1muM. Rates of phosphoenzyme decay indicated that neither the dephosphorylation step, nor the E1P-E2P equilibrium accounted for this inhibition, suggesting the involvement of a different catalytic step. Alternative hypotheses are discussed by which the N-terminal MBD could influence the catalytic activity of CopA.

High yield of biologically active recombinant human amelogenin using the baculovirus expression system

The amelogenins are secreted by the ameloblast cells of developing teeth; they constitute about 90% of the enamel matrix proteins and play an important role in enamel biomineralization. Recent evidence suggests that amelogenin may also be involved in the regeneration of the periodontal tissues and that different isoforms may have cell-signalling effects. During enamel development and mineralization, the amelogenins are lost from the tissue due to sequential degradation by specific proteases, making isolation of substantial purified quantities of full-length amelogenin challenging. The aim of the present study was to express and characterize a recombinant human amelogenin protein in the eukaryotic baculovirus system in quantities sufficient for structural and functional studies. Human cDNA coding for a 175 amino acid amelogenin protein was subcloned into the pFastBac HTb vector (Invitrogen), this system adds a hexa-histidine tag and an rTEV protease cleavage site to the amino terminus of the expressed protein, enabling effective one-step purification by Ni2+-NTA affinity chromatography. The recombinant protein was expressed in Spodoptera frugiperda (Sf9) insect cells and the yield of purified his-tagged human amelogenin (rHAM+) was up to 10 mg/L culture. Recombinant human amelogenin (rHAM+) was characterized by SDS-PAGE, Western blot, ESI-TOF spectrometry, peptide mapping, and MS/MS sequencing. Production of significant amounts of pure, full-length amelogenin opened up the possibility to investigate novel functions of amelogenin. Our recent in vivo regeneration studies reveal that the rHAM+ alone could bring about regeneration of the periodontal tissues; cementum, periodontal ligament, and bone.

The tcrB gene is part of the tcrYAZB operon conferring copper resistance in Enterococcus faecium and Enterococcus faecalis

The plasmid-localized tcrB (transferable copper-resistance gene B) gene from Enterococcus faecium was identified to be part of an operon called the tcrYAZB operon, which has a genetic organization similar to the copYZAB copper-homeostasis gene cluster from Enterococcus hirae. Putative promoter (P(tcr))- and repressor-binding sites highly similar to the E. hirae cop-promoter region were identified upstream of the tcrYAZB genes. The P(tcr) promoter was cloned in both the absence and the presence of the proximal repressor-encoding tcrY gene into a promoter-probe vector. Induction of the promoter was shown in liquid growth medium containing increasing concentrations of copper sulphate. To determine the growth advantage conferred by the tcrYAZB genes in a copper environment, a tcr-deletion mutant was isolated, and its growth was compared with that of its copper-resistant ancestor (strain A17sv1) in sublethal concentrations of copper sulphate. A competition assay using these two isogenic strains showed that copper sulphate concentrations of 3 mmol l(-1) and above are sufficient to select for copper resistance.

Role of a Streptococcus gordonii copper-transport operon, copYAZ, in biofilm detachment

Streptococcus gordonii is a pioneer oral bacterium that is associated with the initiation of dental plaque development. Located downstream of the S. gordonii adc operon, which is involved in competence and biofilm formation, were three open reading frames, designated copY, copA and copZ. These open reading frames were homologous to the copYAZ genes in Streptococcus mutans that are involved in copper homeostasis and biofilm detachment. This study examined whether copYAZ genes play any role in the biofilm formation and detachment of S. gordonii. The copY gene encodes a 143-amino acid protein homologous to the negative transcriptional regulator of a copper-transport operon, copA encodes a 748-amino acid copper-transporting P-type ATPase, and copZ encodes a 69-amino acid putative metallochaperone protein in S. mutans. Each open reading frame in the copYAZ operon in S. gordonii was inactivated by insertional mutation and the growth, biofilm formation and detachment of each mutant were examined. S. gordonii copY::specR, copA::specR, and copZ::specR mutants were able to form biofilms on both polystyrene and glass surfaces. However, inactivation of copZ and to a lesser extent copY resulted in phenotypes that were defective in biofilm detachment, which is consistent with previous observations in S. mutans and suggests that the trace element copper might influence biofilm detachment of bacterial biofilms.

A mutational study in the transmembrane domain of Ccc2p, the yeast Cu(I)-ATPase, shows different roles for each Cys-Pro-Cys cysteine

Ccc2p is homologous to the human Menkes and Wilson copper ATPases and is herein studied as a model for human copper transport. Most studies to date have sought to understand how mutations in the human Menkes or Wilson genes impair copper homeostasis and induce disease. Here we analyze whether eight conserved amino acids of the transmembrane domain are important for copper transport. Wild-type Ccc2p and variants were expressed in a ccc2-Delta yeast strain to check whether they were able to restore copper transport by complementation. Wild-type Ccc2p and variants were also expressed in Sf9 cells using baculovirus to study their enzymatic properties on membrane preparations. The latter system allowed us to measure a copper-activated ATPase activity of about 20 nmol/mg/min for the wild-type Ccc2p at 37 degrees C. None of the variants was as efficient as the wild type in restoring copper homeostasis. The mutation of each cysteine of the (583)CPC(585) motif into a serine resulted in nonfunctional proteins that could not restore copper homeostasis in yeast and had no ATPase activity. Phosphorylation by ATP was still possible with the C583S variant, although it was not possible with the C585S variant, suggesting that the cysteines of the CPC motif have a different role in copper transport. Cys(583) would be necessary for copper dissociation and/or enzyme dephosphorylation and Cys(585) would be necessary for ATP phosphorylation, suggesting a role in copper binding.

The metal specificity and selectivity of ZntA from Escherichia coli using the acylphosphate intermediate

ZntA from Escherichia coli is a P-type ATPase that confers resistance to Pb(II), Zn(II), and Cd(II) in vivo. We had previously shown that purified ZntA shows ATP hydrolysis activity with the metal ions Pb(II), Zn(II), and Cd(II). In this study, we utilized the acylphosphate formation activity of ZntA to further investigate the substrate specificity of ZntA. The site of phosphorylation was Asp-436, as expected from sequence alignments. We show that in addition to Pb(II), Zn(II), and Cd(II), ZntA is active with Ni(II), Co(II), and Cu(II), but not with Cu(I) and Ag(I). Thus, ZntA is specific for a broad range of divalent soft metal ions. The activities with Ni(II), Co(II), and Cu(II) are extremely low; the activities with these non-physiological substrates are 10-20-fold lower compared with the values obtained with Pb(II), Zn(II), and Cd(II). Similar results were obtained with DeltaN-ZntA, a ZntA derivative lacking the amino-terminal metal binding domain. By characterizing the acylphosphate formation reaction in ZntA in detail, we show that a step prior to enzyme phosphorylation, most likely the metal ion binding step, is the slow step in the reaction mechanism in ZntA. The low activities with Ni(II), Co(II), and Cu(II) are because of a further decrease in the rate of binding of these metal ions. Thus, metal ion selectivity in ZntA and possibly other P1-type ATPases is based on the charge and the ligand preference of particular metal ions but not on their size.

Copper homeostasis in Enterococcus hirae

Copper is an essential component of life because of its convenient redox potential of 200-800 mV when bound to protein. Extensive insight into copper homeostasis has only emerged in the last decade and Enterococcus hirae has served as a paradigm for many aspects of the process. The cop operon of E. hirae regulates copper uptake, availability, and export. It consists of four genes that encode a repressor, CopY, a copper chaperone, CopZ, and two CPx-type copper ATPases, CopA and CopB. Most of these components have been conserved across the three evolutionary kingdoms. The four Cop proteins have been studied in vivo as well as in vitro and their function is understood in some detail.

Efflux-mediated heavy metal resistance in prokaryotes

What makes a heavy metal resistant bacterium heavy metal resistant? The mechanisms of action, physiological functions, and distribution of metal-exporting proteins are outlined, namely: CBA efflux pumps driven by proteins of the resistance-nodulation-cell division superfamily, P-type ATPases, cation diffusion facilitator and chromate proteins, NreB- and CnrT-like resistance factors. The complement of efflux systems of 63 sequenced prokaryotes was compared with that of the heavy metal resistant bacterium Ralstonia metallidurans. This comparison shows that heavy metal resistance is the result of multiple layers of resistance systems with overlapping substrate specificities, but unique functions. Some of these systems are widespread and serve in the basic defense of the cell against superfluous heavy metals, but some are highly specialized and occur only in a few bacteria. Possession of the latter systems makes a bacterium heavy metal resistant.

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).

Biochemical characterization of CopA, the Escherichia coli Cu(I)-translocating P-type ATPase

Escherichia coli CopA is a copper ion-translocating P-type ATPase that confers copper resistance. CopA formed a phosphorylated intermediate with [gamma-(32)P]ATP. Phosphorylation was inhibited by vanadate and sensitive to KOH and hydroxylamine, consistent with acylphosphate formation on conserved Asp-523. Phosphorylation required a monovalent cation, either Cu(I) or Ag(I). Divalent cations Cu(II), Zn(II), or Co(II) could not substitute, signifying that the substrate of this copper-translocating P-type ATPase is Cu(I) and not Cu(II). CopA purified from dodecylmaltoside-solubilized membranes similarly exhibited Cu(I)/Ag(I)-stimulated ATPase activity, with a K(m) for ATP of 0.5 mm. CopA has two N-terminal Cys(X)(2)Cys sequences, Gly-Leu-Ser-Cys(14)-Gly-His-Cys(17), and Gly-Met-Ser-Cys(110)-Ala-Ser-Cys(113), and a Cys(479)-Pro-Cys(481) motif in membrane-spanning segment six. The requirement of these cysteine residues was investigated by the effect of mutations and deletions. Mutants with substitutions of the N-terminal cysteines or deletion of the first Cys-(X)(2)-Cys motif formed acylphosphate intermediates. From the copper dependence of phosphoenzyme formation, the mutants appear to have 2-3 fold higher affinity for Cu(I) than wild type CopA. In contrast, substitutions in Cys(479) or Cys(481) resulted in loss of copper resistance, transport and phosphoenzyme formation. These results imply that the cysteine residues of the Cys-Pro-Cys motif (but not the N-terminal cysteine residues) are required for CopA function.

Role for zinc(II) in the copper(I) regulated protein CopY

Despite the importance of copper-thiolate clusters in the regulation of copper metabolism the formation chemistry of these clusters in proteins is not well understood. The number of Cu(I) ions that can be incorporated within a given molecule and their coordination number varies. CopY is a repressor protein from Enterococcus hirae which utilises a copper-thiolate cluster in the regulation of the copper homeostasis genes. Physical, biological assays of purified native reconstituted apoCopY suggest that the formation of a Zn(II)-protein prior to Cu(I) incorporation is necessary to achieve the native Cu(I)-S cluster. In this protein the Zn(II) is readily displaced by the Cu(I). CopY proteins with homologous metal binding motifs are being used to investigate cluster formation stabilisation.

Interaction of the CopZ copper chaperone with the CopA copper ATPase of Enterococcus hirae assessed by surface plasmon resonance

Intracellular copper routing in Enterococcus hirae can be accomplished by the CopZ metallochaperone. Using surface plasmon resonance analysis, we show here that CopZ interacts with the CopA copper ATPase. The binding affinity of CopZ for CopA was increased in the presence of copper, due to a 15-fold lower dissociation rate constant. Mutating the N-terminal copper binding motif of CopA from CxxC to SxxS abolished this copper-induced effect. Moreover, CopZ failed to show an interaction with an unrelated copper binding protein used as a control. These results show that (i) the CopA copper ATPase specifically interacts with the CopZ chaperone, (ii) this interaction is based on protein-protein interaction, and (iii) surface plasmon resonance is a novel tool for quantitative analysis of metallochaperone-target interactions.

The regulation of catalytic activity of the menkes copper-translocating P-type ATPase. Role of high affinity copper-binding sites

The Menkes protein is a transmembrane copper translocating P-type ATPase. Mutations in the Menkes gene that affect the function of the Menkes protein may cause Menkes disease in humans, which is associated with severe systemic copper deficiency. The catalytic mechanism of the Menkes protein, including the formation of transient acylphosphate, is poorly understood. We transfected and overexpressed wild-type and targeted mutant Menkes protein in yeast and investigated its transient acyl phosphorylation. We demonstrated that the Menkes protein is transiently phosphorylated by ATP in a copper-specific and copper-dependent manner and appears to undergo conformational changes in accordance with the classical P-type ATPase model. Our data suggest that the catalytic cycle of the Menkes protein begins with the binding of copper to high affinity binding sites in the transmembrane channel, followed by ATP binding and transient phosphorylation. We propose that putative copper-binding sites at the N-terminal domain of the Menkes protein are important as sensors of low concentrations of copper but are not essential for the overall catalytic activity.