Activation of Archaeoglobus fulgidus Cu+-ATPase CopA by cysteine

Functional characterization of Legionella pneumophila Cu+ transport ATPase. The activation by Cu+ and ATP

Cu⁺-ATPases are integral membrane proteins belonging to the IB subfamily of the P-type ATPases that couple Cu⁺ transport to the hydrolysis of ATP. As some structural and functional particularities arise for Cu⁺-ATPases, several authors suggest that some of the reaction steps of the Albers-Post model postulated for other P-ATPases may be different. In this work we describe a functional characterization of Legionella pneumophila Cu⁺-ATPase (LpCopA), the first P_IB-ATPase whose structure was determined by X-ray crystallography. Cu⁺-ATPase activity of the enzyme presents a maximum at ∼37 °C and pH 6.6-6.8. Phospholipids enhance LpCopA Cu⁺-ATPase activity in a non-essential mode where optimal activity is achieved at an asolectin molar fraction of 0.15 and an amphiphile-protein ratio of ~30,000. As described for other P-ATPases, Mg²⁺ acts as an essential activator. Furthermore, Cu⁺-ATPase activity dependence on [Cu⁺] and [ATP] can both be described by a sum of two hyperbolic functions. Based on that, and the [Cu⁺] and [ATP] dependencies of the best fitting parameters of the hyperbolae pointed above, we propose a minimal reaction scheme for the catalytic mechanism that shares the basic reaction steps of the Albers-Post model for P-type ATPases. The reaction scheme postulated contemplates two different binding affinities for a single ATP (apparent affinities of 0.66 and 550 μM at [Cu⁺] → ∞) and binding of at least 2 Cu⁺ with different affinities as well (apparent affinities of 1.4 and 102.5 μM at [ATP] → ∞).

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

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

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.

Membrane Anchoring and Ion-Entry Dynamics in P-type ATPase Copper Transport

Cu+-specific P-type ATPase membrane protein transporters regulate cellular copper levels. The lack of crystal structures in Cu+-binding states has limited our understanding of how ion entry and binding are achieved. Here, we characterize the molecular basis of Cu+ entry using molecular-dynamics simulations, structural modeling, and in vitro and in vivo functional assays. Protein structural rearrangements resulting in the exposure of positive charges to bulk solvent rather than to lipid phosphates indicate a direct molecular role of the putative docking platform in Cu+ delivery. Mutational analyses and simulations in the presence and absence of Cu+ predict that the ion-entry path involves two ion-binding sites: one transient Met148-Cys382 site and one intramembranous site formed by trigonal coordination to Cys384, Asn689, and Met717. The results reconcile earlier biochemical and x-ray absorption data and provide a molecular understanding of ion entry in Cu+-transporting P-type ATPases.

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

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

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

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

Molecular and biochemical properties of two P1B2-ATPases, CsHMA3 and CsHMA4, from cucumber

P1B-ATPases (heavy metal ATPases, HMAs) constitute a multigenic subfamily of P-ATPases involved in the transport of monovalent and divalent heavy metals in plant cells. Here, we present the organization of genes encoding the HMA family in the cucumber genome and report the function and biochemical properties of two cucumber proteins homologous to the HMA2-4-like plant HMAs. Eight genes encoding putative P1B -ATPases were identified in the cucumber genome. Among them, CsHMA3 was predominantly expressed in roots and up-regulated by Pb, Zn and Cd excess, whereas the CsHMA4 transcript was most abundant in roots and flowers of cucumber plants, and elevated under Pb and Zn excess. Expression of CsHMA3 in Saccharomyces cerevisiae enhanced yeast tolerance to Cd and Pb, whereas CsHMA4 conferred increased resistance of yeast cells to Cd and Zn. Immunostaining with specific antibodies raised against cucumber proteins revealed tonoplast localization of CsHMA3 and plasma membrane localization of CsHMA4 in cucumber root cells. Kinetic studies of CsHMA3 and CsHMA4 in yeast membranes indicated differing heavy metal cation affinities of these two proteins. Altogether, the results suggest an important role of CsHMA3 and CsHMA4 in Cd and Pb detoxification and Zn homeostasis in cucumber cells.

Distinct functions of serial metal-binding domains in the Escherichia coli P1 B -ATPase CopA

P1 B -ATPases are among the most common resistance factors to metal-induced stress. Belonging to the superfamily of P-type ATPases, they are capable of exporting transition metal ions at the expense of adenosine triphosphate (ATP) hydrolysis. P1 B -ATPases share a conserved structure of three cytoplasmic domains linked by a transmembrane domain. In addition, they possess a unique class of domains located at the N-terminus. In bacteria, these domains are primarily associated with metal binding and either occur individually or as serial copies of each other. Within this study, the roles of the two adjacent metal-binding domains (MBDs) of CopA, the copper export ATPase of Escherichia coli were investigated. From biochemical and physiological data, we deciphered the protein-internal pathway of copper and demonstrate the distal N-terminal MBD to possess a function analogous to the metallochaperones of related prokaryotic copper resistance systems, that is its involvement in the copper transfer to the membrane-integral ion-binding sites of CopA. In contrast, the proximal domain MBD2 has a regulatory role by suppressing the catalytic activity of CopA in absence of copper. Furthermore, we propose a general functional divergence of tandem MBDs in P1 B -ATPases, which is governed by the length of the inter-domain linker.

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.

An expanding range of functions for the copper chaperone/antioxidant protein Atox1

SIGNIFICANCE

Antioxidant protein 1 (Atox1 in human cells) is a copper chaperone for the copper export pathway with an essential role in cellular copper distribution. In vitro, Atox1 binds and transfers copper to the copper-transporting ATPases, stimulating their catalytic activity. Inactivation of Atox1 in cells inhibits maturation of secreted cuproenzymes as well as copper export from cells.

RECENT ADVANCES

Accumulating data suggest that cellular functions of Atox1 are not limited to its copper-trafficking role and may include storage of labile copper, modulation of transcription, and antioxidant defense. The conserved metal binding site of Atox1, CxGC, differs from the metal-binding sites of copper-transporting ATPases and has a physiologically relevant redox potential that equilibrates with the GSH:GSSG pair.

CRITICAL ISSUES

Tight relationship appears to exist between intracellular copper levels and glutathione (GSH) homeostasis. The biochemical properties of Atox1 place it at the intersection of cellular networks that regulate copper distribution and cellular redox balance. Mechanisms through which Atox1 facilitates copper export and contributes to oxidative defense are not fully understood.

FUTURE DIRECTIONS

The current picture of cellular redox homeostasis and copper physiology will be enhanced by further mechanistic studies of functional interactions between the GSH:GSSG pair and copper-trafficking machinery.

Sinorhizobium meliloti Nia is a P(1B-5)-ATPase expressed in the nodule during plant symbiosis and is involved in Ni and Fe transport

The P1B-ATPases are a ubiquitous family of metal transporters. These transporters are classified into subfamilies on the basis of substrate specificity, which is conferred by conserved amino acids in the last three transmembrane domains. Five subfamilies have been identified to date, and representative members of four (P1B-1 to P1B-4) have been studied. The fifth family (P1B-5), of which some members contain a C-terminal hemerythrin (Hr) domain, is less well characterized. The S. meliloti Sma1163 gene encodes for a P1B-5-ATPase, denoted Nia (Nickel-iron ATPase), that is induced by exogenous Fe(2+) and Ni(2+). The nia mutant accumulates nickel and iron, suggesting a possible role in detoxification of these two elements under free-living conditions, as well as in symbiosis, when the highest expression levels are measured. This function is supported by an inhibitory effect of Fe(2+) and Ni(2+) on the pNPPase activity, and by the ability of Nia to bind Fe(2+) in the transmembrane domain. Optical and X-ray absorption spectroscopic studies of the isolated Hr domain confirm the presence of a dinuclear iron center and suggest that this domain might function as an iron sensor.

Differential gene expression and bioinformatics analysis of copper resistance gene afe_1073 in Acidithiobacillus ferrooxidans

Copper resistance of acidophilic bacteria is very significant in bioleaching of copper ore since high concentration of copper are harmful to the growth of organisms. Copper resistance gene afe_1073 was putatively considered to be involved in copper homeostasis in Acidithiobacillus ferrooxidans ATCC23270. In the present study, differential expression of afe_1073 in A. ferrooxidans strain DY26 and DC was assessed with quantitative reverse transcription polymerase chain reaction. The results showed the expression of afe_1073 in two strains increased with the increment of copper concentrations. The expression of DY26 was lower than that of DC at the same copper concentration although A. ferrooxidans strain DY26 possessed higher copper resistance than strain DC. In addition, bioinformatics analysis showed AFE_1073 was a typical transmembrane protein P1b1-ATPase, which could reduce the harm of Cu(+) by pumping it out from the cell. There were two mutation sites in AFE_1073 between DY26 and DC and one may change the hydrophobicity of AFE_1073, which could enhance the ability of DY26 to pump out Cu(+). Therefore, DY26 needed less gene expression of afe_1073 for resisting copper toxicity than that of DC at the same copper stress. Our study will be beneficial to understanding the copper resistance mechanism of A. ferrooxidans.

Characterization of the PIB-Type ATPases present in Thermus thermophilus

P(IB)-type ATPases transport heavy metals (Cu(2+), Cu(+), Ag(+), Zn(2+), Cd(2+), Co(2+)) across biomembranes, playing a key role in homeostasis and in the mechanisms of biotolerance of these metals. Three genes coding for putative P(IB)-type ATPases are present in the genome of Thermus thermophilus (HB8 and HB27): the TTC1358, TTC1371, and TTC0354 genes; these genes are annotated, respectively, as two copper transporter (CopA and CopB) genes and a zinc-cadmium transporter (Zn(2+)/Cd(2+)-ATPase) gene. We cloned and expressed the three proteins with 8His tags using a T. thermophilus expression system. After purification, each of the proteins was shown to have phosphodiesterase activity at 65°C with ATP and p-nitrophenyl phosphate (pNPP) as substrates. CopA was found to have greater activity in the presence of Cu(+), while CopB was found to have greater activity in the presence of Cu(2+). The putative Zn(2+)/Cd(2+)-ATPase was truncated at the N terminus and was, surprisingly, activated in vitro by copper but not by zinc or cadmium. When expressed in Escherichia coli, however, the putative Zn(2+)/Cd(2+)-ATPase could be isolated as a full-length protein and the ATPase activity was increased by the addition of Zn(2+) and Cd(2+) as well as by Cu(+). Mutant strains in which each of the three P-type ATPases was deleted singly were constructed. In each case, the deletion increased the sensitivity of the strain to growth in the presence of copper in the medium, indicating that each of the three can pump copper out of the cells and play a role in copper detoxification.

Comparative study of the active cadmium efflux systems operating at the plasma membrane and tonoplast of cucumber root cells

The strategies developed by plants to avoid the toxicity of cadmium (Cd) and other heavy metals involve active sequestration of metals into the apoplast and vacuoles. The protein systems excluding heavy metals from the cell cytosol localize to the plasma membrane and tonoplast and are energized either by ATP or by the electrochemical gradient generated by H(+)-ATPase or by V-ATPase and pyrophosphatase (PPase), respectively. In this work, a comparative study on the contribution of both the plasma membrane and tonoplast in the active detoxification of plant cells after treatment with Cd was performed. The studies using plants treated and untreated with Cd reveal that both, H(+)-coupled and MgATP-driven efflux of Cd across plasma membranes and tonoplast is markedly stimulated in the presence of Cd in the environment. Previous studies on plasma-membrane localized H(+)-coupled Cd efflux together with the present data demonstrating tonoplast H(+)/Cd(2+) antiport activity suggest that H(+)-coupled secondary transport of Cd displays a lower affinity for Cd when compared with Cd primary pumps driven by MgATP. In addition, it is shown that MgATP-energized Cd efflux across both membranes is significantly enhanced by cysteine, dithiothreitol, and glutathione. These results suggest that Cd is excluded from the cytosol through an energy-dependent system as a free ion as well as a complexed form. Although both membranes contribute in the active exclusion of ionized and complexed Cd from the cytosol, the overall calculation of Cd accumulation in the everted plasma membranes and vacuolar vesicles suggests that the tonoplast and vacuole have a major function in Cd efflux from the cytosol in the roots of cucumber subjected to Cd stress.

Distinct functional roles of homologous Cu+ efflux ATPases in Pseudomonas aeruginosa

In bacteria, most Cu(+) -ATPases confer tolerance to Cu by driving cytoplasmic metal efflux. However, many bacterial genomes contain several genes coding for these enzymes suggesting alternative roles. Pseudomonas aeruginosa has two structurally similar Cu(+) -ATPases, CopA1 and CopA2. Both proteins are essential for virulence. Expressed in response to high Cu, CopA1 maintains the cellular Cu quota and provides tolerance to this metal. CopA2 belongs to a subgroup of ATPases that are expressed in association with cytochrome oxidase subunits. Mutation of copA2 has no effect on Cu toxicity nor intracellular Cu levels; but it leads to higher H(2) O(2) sensitivity and reduced cytochrome oxidase activity. Mutation of both genes does not exacerbate the phenotypes produced by single-gene mutations. CopA1 does not complement the copA2 mutant strain and vice versa, even when promoter regions are exchanged. CopA1 but not CopA2 complements an Escherichia coli strain lacking the endogenous CopA. Nevertheless, transport assays show that both enzymes catalyse cytoplasmic Cu(+) efflux into the periplasm, albeit CopA2 at a significantly lower rate. We hypothesize that their distinct cellular functions could be based on the intrinsic differences in transport kinetic or the likely requirement of periplasmic partner Cu-chaperone proteins specific for each Cu(+) -ATPase.

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.

Reaction cycle of Thermotoga maritima copper ATPase and conformational characterization of catalytically deficient mutants

Copper transport ATPases sustain important roles in homeostasis of heavy metals and delivery of copper to metalloenzymes. The copper transport ATPase from Thermotoga maritima (CopA) provides a useful system for mechanistic studies, due to its heterologous expression and stability. Its sequence comprises 726 amino acids, including the N-terminal metal binding domain (NMBD), three catalytic domains (A, N, and P), and a copper transport domain formed by eight helices, including the transmembrane metal binding site (TMBS). We performed functional characterization and conformational analysis by proteolytic digestion of WT and mutated (NMBD deletion or mutation) T. maritima CopA, comparing it with Archaeoglobus fulgidus CopA and Ca²⁺ ATPase. A specific feature of T. maritima CopA is ATP utilization in the absence of copper, to form a low-turnover phosphoenzyme intermediate, with a conformation similar to that obtained by phosphorylation with P_i or phosphate analogues. On the other hand, formation of an activated state requires copper binding to both NMBD and TMBS, with consequent conformational changes involving the NMBD and A domain. Proteolytic digestion analysis demonstrates A domain movements similar to those of other P-type ATPases to place the conserved TGES motif in the optimal position for catalytic assistance. We also studied an H479Q mutation (analogous to one of human copper ATPase ATP7B in Wilson disease) that inhibits ATPase activity. We found that, in spite of the H479Q mutation within the nucleotide binding domain, the mutant still binds ATP, yielding a phosphorylation transition state conformation. However, covalent phosphoryl transfer is not completed, and no catalytic turnover is observed.

Structure of the two transmembrane Cu+ transport sites of the Cu+ -ATPases

Cu(+)-ATPases drive metal efflux from the cell cytoplasm. Paramount to this function is the binding of Cu(+) within the transmembrane region and its coupled translocation across the permeability barrier. Here, we describe the two transmembrane Cu(+) transport sites present in Archaeoglobus fulgidus CopA. Both sites can be independently loaded with Cu(+). However, their simultaneous occupation is associated with enzyme turnover. Site I is constituted by two Cys in transmembrane segment (TM) 6 and a Tyr in TM7. An Asn in TM7 and Met and Ser in TM8 form Site II. Single site x-ray spectroscopic analysis indicates a trigonal coordination in both sites. This architecture is distinct from that observed in Cu(+)-trafficking chaperones and classical cuproproteins. The high affinity of these sites for Cu(+) (Site I K(a)=1.3 fM(-1), Site II K(a)=1.1 fM(-1)), in conjunction with reversible direct Cu(+) transfer from chaperones, points to a transport mechanism where backward release of free Cu(+) to the cytoplasm is largely prevented.

Expressional control of a cadmium-transporting P1B-type ATPase by a metal sensing degradation signal

Cadmium is a highly toxic environmental contaminant implicated in various diseases. Our previous data demonstrated that Pca1, a P1B-type ATPase, plays a critical role in cadmium resistance in yeast S. cerevisiae by extruding intracellular cadmium. This illustrates the first cadmium-specific efflux pump in eukaryotes. In response to cadmium, yeast cells rapidly enhance expression of Pca1 by a post-transcriptional mechanism. To gain mechanistic insights into the cadmium-dependent control of Pca1 expression, we have characterized the pathway for Pca1 turnover and the mechanism of cadmium sensing that leads to up-regulation of Pca1. Pca1 is a short-lived protein (t1/2 < 5 min) and is subject to ubiquitination when cells are growing in media lacking cadmium. Distinct from many plasma membrane transporters targeted to the vacuole for degradation via endocytosis, cells defective in this pathway did not stabilize Pca1. Rather, Pca1 turnover was dependent on the proteasome. These data suggest that, in the absence of cadmium, Pca1 is targeted for degradation before reaching the plasma membrane. Mapping of the N terminus of Pca1 identified a metal-responding degradation signal encompassing amino acids 250-350. Fusion of this domain to a stable protein demonstrated that it functions autonomously in a metal-responsive manner. Cadmium sensing by cysteine residues within this domain circumvents ubiquitination and degradation of Pca1. These data reveal a new mechanism for substrate-mediated control of P1B-type ATPase expression. Cells have likely evolved this mode of regulation for a rapid and specific cellular response to cadmium.

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

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

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

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

Domain organization and movements in heavy metal ion pumps: papain digestion of CopA, a Cu+-transporting ATPase

To study domain organization and movements in the reaction cycle of heavy metal ion pumps, CopA, a bacterial Cu+-ATPase from Thermotoga maritima was cloned, overexpressed, and purified, and then subjected to limited proteolysis using papain. Stable analogs of intermediate states were generated using AMPPCP as a nonhydrolyzable ATP analog and AlFx as a phosphate analog, following conditions established for Ca2+-ATPase (SERCA1). Characteristic digestion patterns obtained for different analog intermediates show that CopA undergoes domain rearrangements very similar to those of SERCA1. Digestion sites were identified on the loops connecting the A-domain and the transmembrane helices M2 and M3 as well as on that connecting the N-terminal metal binding domain (NMBD) and the first transmembrane helix, Ma. These digestion sites were protected in the E1P.ADP and E2P analogs, whereas the M2-A-domain loop was cleaved specifically in the absence of ions to be transported, just as in SERCA1. ATPase activity was lost when the link between the NMBD and the transmembrane domain was cleaved, indicating that the NMBD plays a critical role in ATP hydrolysis in T. maritima CopA. The change in susceptibility of the loop between the NMBD and Ma helix provides evidence that the NMBD is associated to the A-domain and recruited into domain rearrangements and that the Ma helix is the counterpart of the M1 helix in SERCA1 and Mb and Mc are uniquely inserted before M2.