The M4M5 cytoplasmic loop of the Na,K-ATPase, overexpressed in Escherichia coli, binds nucleoside triphosphates with the same selectivity as the intact native protein

Identification of cisplatin-binding sites on the large cytoplasmic loop of the Na+/K+-ATPase

Cisplatin is the most widely used chemotherapeutic drug for the treatment of various types of cancer; however, its administration brings also numerous side effects. It was demonstrated that cisplatin can inhibit the Na⁺/K⁺-ATPase (NKA), which can explain a large part of the adverse effects. In this study, we have identified five cysteinyl residues (C452, C456, C457, C577, and C656) as the cisplatin binding sites on the cytoplasmic loop connecting transmembrane helices 4 and 5 (C45), using site-directed mutagenesis and mass spectrometry experiments. The identified residues are known to be susceptible to glutathionylation indicating their involvement in a common regulatory mechanism.

RH421 binds into the ATP-binding site on the Na+/K+-ATPase

The Na⁺/K⁺-ATPase plays a key role in ion transport across the plasma membrane of all animal cells. The voltage-sensitive styrylpyrimidium dye RH421 has been used in several laboratories for monitoring of Na⁺/K⁺-ATPase kinetics. It is known, that RH421 can interact with the enzyme and it can influence its activity at micromolar concentrations, but structural details of this interaction are only poorly understood. Experiments with isolated large cytoplasmic loop (C45) of Na⁺/K⁺-ATPase revealed that RH421 can interact with this part of the protein with dissociation constant 1μM. The Trp-to-RH421 FRET performed on six single-tryptophan mutants revealed that RH421 binds directly into the ATP-binding site. This conclusion was further supported by results from molecular docking, site-directed mutagenesis and by competitive experiments using ATP. Experiments with C45/DPPC mixture revealed that RH421 can bind to both C45 and lipids, but only the former interaction was influenced by the presence of ATP.

Selective Assembly of Na,K-ATPase α2β2 Heterodimers in the Heart: DISTINCT FUNCTIONAL PROPERTIES AND ISOFORM-SELECTIVE INHIBITORS

The Na,K-ATPase α₂ subunit plays a key role in cardiac muscle contraction by regulating intracellular Ca²⁺, whereas α₁ has a more conventional role of maintaining ion homeostasis. The β subunit differentially regulates maturation, trafficking, and activity of α-β heterodimers. It is not known whether the distinct role of α₂ in the heart is related to selective assembly with a particular one of the three β isoforms. We show here by immunofluorescence and co-immunoprecipitation that α₂ is preferentially expressed with β₂ in T-tubules of cardiac myocytes, forming α₂β₂ heterodimers. We have expressed human α₁β₁, α₂β₁, α₂β₂, and α₂β₃ in Pichia pastoris, purified the complexes, and compared their functional properties. α₂β₂ and α₂β₃ differ significantly from both α₂β₁ and α₁β₁ in having a higher K_0.5K⁺ and lower K_0.5Na⁺ for activating Na,K-ATPase. These features are the result of a large reduction in binding affinity for extracellular K⁺ and shift of the E₁P-E₂P conformational equilibrium toward E₁P. A screen of perhydro-1,4-oxazepine derivatives of digoxin identified several derivatives (e.g. cyclobutyl) with strongly increased selectivity for inhibition of α₂β₂ and α₂β₃ over α₁β₁ (range 22-33-fold). Molecular modeling suggests a possible basis for isoform selectivity. The preferential assembly, specific T-tubular localization, and low K⁺ affinity of α₂β₂ could allow an acute response to raised ambient K⁺ concentrations in physiological conditions and explain the importance of α₂β₂ for cardiac muscle contractility. The high sensitivity of α₂β₂ to digoxin derivatives explains beneficial effects of cardiac glycosides for treatment of heart failure and potential of α₂β₂-selective digoxin derivatives for reducing cardiotoxicity.

ATP protects against FITC labeling of Solanum lycopersicon and Arabidopsis thaliana Ca2+-ATPase ATP binding domains

Ca(2+)-ATPases are integral membrane proteins that actively transport Ca(2+) against substantial concentration gradients in eukaryotic cells. This active transport is energized by coupling ion translocation with ATP hydrolysis. In order to better understand this coupling mechanism, we studied the nucleotide specificities of isolated ATP binding domains (ABDs) of Solanum lycopersicon Ca(2+)-ATPase (LCA), a type IIA non-calmodulin regulated P-type pump found in tomato plants that is very similar to mammalian sarco/endoplasmic reticulum Ca(2+)-ATPase (SERCA), and Arabidopsis Ca(2+)-ATPase, isoform 2 (ACA2), a type IIB calmodulin regulated P-type ATPase found in the endoplasmic reticulum of Arabidopsis cells. We used nucleotide protection against FITC labeling as a measure of binding since both LCA and ACA contained the KGAP(S,V,F)E motif, which has been shown to be modified by fluorescein isothiocyanate (FITC) in P-type pumps from animal cells. We demonstrated that the heterologously expressed GST-tagged ABDs from both LCA and ACA2 were modified by FITC and that ATP protects against this modification. Moreover, GTP was able to reduce, but not eliminate, the level of FITC labeling in both ABD constructs, suggesting that these plant pumps may also bind GTP with low affinity, which is in contrast to mammalian SERCA and PMCA type pumps which do not bind GTP.

Fluorone dyes have binding sites on both cytoplasmic and extracellular domains of Na,K-ATPase

Combination of fluorescence techniques and molecular docking was used to monitor interaction of Na,K-ATPase and its large cytoplasmic loop connecting fourth and fifth transmembrane helices (C45) with fluorone dyes (i.e. eosin Y, 5(6)-carboxyeosin, rose bengal, fluorescein, and erythrosine B). Our data suggested that there are at least two binding sites for all used fluorone dyes, except of 5(6)-carboxyeosin. The first binding site is located on C45 loop, and it is sensitive to the presence of nucleotide. The other site is located on the extracellular part of the enzyme, and it is sensitive to the presence of Na(+) or K(+) ions. The molecular docking revealed that in the open conformation of C45 loop (which is obtained in the presence of ATP) all used fluorone dyes occupy position directly inside the ATP-binding pocket, while in the closed conformation (i.e. in the absence of any ligand) they are located only near the ATP-binding site depending on their different sizes. On the extracellular part of the protein, the molecular docking predicts two possible binding sites with similar binding energy near Asp897(α) or Gln69(β). The former was identified as a part of interaction site between α- and β-subunits, the latter is in contact with conserved FXYD sequence of the γ-subunit. Our findings provide structural explanation for numerous older studies, which were performed with fluorone dyes before the high-resolution structures were known. Further, fluorone dyes seem to be good probes for monitoring of intersubunit interactions influenced by Na(+) and K(+) binding.

Subunit isoform selectivity in assembly of Na,K-ATPase α-β heterodimers

To catalyze ion transport, the Na,K-ATPase must contain one α and one β subunit. When expressed by transfection in various expression systems, each of the four α subunit isoforms can assemble with each of the three β subunit isoforms and form an active enzyme, suggesting the absence of selective α-β isoform assembly. However, it is unknown whether in vivo conditions the α-β assembly is random or isoform-specific. The α(2)-β(2) complex was selectively immunoprecipitated by both anti-α(2) and anti-β(2) antibodies from extracts of mouse brain, which contains cells co-expressing multiple Na,K-ATPase isoforms. Neither α(1)-β(2) nor α(2)-β(1) complexes were detected in the immunoprecipitates. Furthermore, in MDCK cells co-expressing α(1), β(1), and β(2) isoforms, a greater fraction of the β(2) subunits was unassembled with α(1) as compared with that of the β(1) subunits, indicating preferential association of the α(1) isoform with the β(1) isoform. In addition, the α(1)-β(2) complex was less resistant to various detergents than the α(1)-β(1) complex isolated from MDCK cells or the α(2)-β(2) complex isolated from mouse brain. Therefore, the diversity of the α-β Na,K-ATPase heterodimers in vivo is determined not only by cell-specific co-expression of particular isoforms, but also by selective association of the α and β subunit isoforms.

Covalent binding of cisplatin impairs the function of Na(+)/K(+)-ATPase by binding to its cytoplasmic part

This study was aimed at verifying the hypothesis that acute kidney failure accompanying cisplatin administration in the cancer therapy could be due to cisplatin interaction with the cytoplasmic part of Na(+)/K(+)-ATPase. Our results demonstrated that cisplatin-binding caused inhibition of Na(+)/K(+)-ATPase, in contrast to other platinated chemotherapeutics such as carboplatin and oxaliplatin, which are known to be much less nephrotoxic. To acquire more detailed structural information, we performed a series of experiments with the isolated large cytoplasmic segment connecting transmembrane helices 4 and 5 (C45 loop) of Na(+)/K(+)-ATPase. Electrochemistry showed that cisplatin is bound to the cysteine residues of the C45 loop, mass spectrometry revealed a modification of the C45 peptide fragment GSHMASLEAVETLGSTSTICSDK, which contains the conserved phosphorylated residue Asp369. Hence, we hypothesize that binding of cisplatin to Cys367 can cause sterical obstruction during the phosphorylation or dephosphorylation step of the Na(+)/K(+)-ATPase catalytic cycle.

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

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

Changes in electrostatic surface potential of Na+/K+-ATPase cytoplasmic headpiece induced by cytoplasmic ligand(s) binding

A set of single-tryptophan mutants of the Na(+)/K(+)-ATPase isolated, large cytoplasmic loop connecting transmembrane helices M4 and M5 (C45) was prepared to monitor effects of the natural cytoplasmic ligands (i.e., Mg(2+) and/or ATP) binding. We introduced a novel method for the monitoring of the changes in the electrostatic surface potential (ESP) induced by ligand binding, using the quenching of the intrinsic tryptophan fluorescence by acrylamide or iodide. This approach opens a new way to understanding the interactions within the proteins. Our experiments revealed that the C45 conformation in the presence of the ATP (without magnesium) substantially differed from the conformation in the presence of Mg(2+) or MgATP or in the absence of any ligand not only in the sense of geometry but also in the sense of the ESP. Notably, the set of ESP-sensitive residues was different from the set of geometry-sensitive residues. Moreover, our data indicate that the effect of the ligand binding is not restricted only to the close environment of the binding site and that the information is in fact transmitted also to the distal parts of the molecule. This property could be important for the communication between the cytoplasmic headpiece and the cation binding sites located within the transmembrane domain.

Regulation of Na,K-ATPase subunit abundance by translational repression

The Na,K-ATPase is an alphabeta heterodimer responsible for maintaining fluid and electrolyte homeostasis in mammalian cells. We engineered Madin-Darby canine kidney cell lines expressing alpha(1)FLAG, beta(1)FLAG, or beta(2)MYC subunits via a tetracycline-regulated promoter and a line expressing both stable beta(1)MYC and tetracycline-regulated beta(1)FLAG to examine regulatory mechanisms of sodium pump subunit expression. When overexpression of exogenous beta(1)FLAG increased total beta subunit levels by >200% without changes in alpha subunit abundance, endogenous beta(1) subunit (beta(1)E) abundance decreased. beta(1)E down-regulation did not occur during beta(2)MYC overexpression, indicating isoform specificity of the repression mechanism. Measurements of RNA stability and content indicated that decreased beta subunit expression was not accompanied by any change in mRNA levels. In addition, the degradation rate of beta subunits was not altered by beta(1)FLAG overexpression. Cells stably expressing beta(1)MYC, when induced to express beta(1)FLAG subunits, showed reduced beta(1)MYC and beta(1)E subunit abundance, indicating that these effects occur via the coding sequences of the down-regulated polypeptides. In a similar way, Madin-Darby canine kidney cells overexpressing exogenous alpha(1)FLAG subunits exhibited a reduction of endogenous alpha(1) subunits (alpha(1)E) with no change in alpha mRNA levels or beta subunits. The reduction in alpha(1)E compensated for alpha(1)FLAG subunit expression, resulting in unchanged total alpha subunit abundance. Thus, regulation of alpha subunit expression maintained its native level, whereas beta subunit was not as tightly regulated and its abundance could increase substantially over native levels. These effects also occurred in human embryonic kidney cells. These data are the first indication that cellular sodium pump subunit abundance is modulated by translational repression. This mechanism represents a novel, potentially important mechanism for regulation of Na,K-ATPase expression.

ATP and magnesium drive conformational changes of the Na+/K+-ATPase cytoplasmic headpiece

Conformational changes of the Na(+)/K(+)-ATPase isolated large cytoplasmic segment connecting transmembrane helices M4 and M5 (C45) induced by the interaction with enzyme ligands (i.e. Mg(2+) and/or ATP) were investigated by means of the intrinsic tryptophan fluorescence measurement and molecular dynamic simulations. Our data revealed that this model system consisting of only two domains retained the ability to adopt open or closed conformation, i.e. behavior, which is expected from the crystal structures of relative Ca(2+)-ATPase from sarco(endo)plasmic reticulum for the corresponding part of the entire enzyme. Our data revealed that the C45 is found in the closed conformation in the absence of any ligand, in the presence of Mg(2+) only, or in the simultaneous presence of Mg(2+) and ATP. Binding of the ATP alone (i.e. in the absence of Mg(2+)) induced open conformation of the C45. The fact that the transmembrane part of the enzyme was absent in our experiments suggested that the observed conformational changes are consequences only of the interaction with ATP or Mg(2+) and may not be related to the transported cations binding/release, as generally believed. Our data are consistent with the model, where ATP binding to the low-affinity site induces conformational change of the cytoplasmic part of the enzyme, traditionally attributed to E2-->E1 transition, and subsequent Mg(2+) binding to the enzyme-ATP complex induces in turn conformational change traditionally attributed to E1-->E2 transition.

beta-Subunit overexpression alters the stoicheometry of assembled Na-K-ATPase subunits in MDCK cells

In eukaryotic cells, the apparent maintenance of 1:1 stoicheometry between the Na-K-ATPase alpha- and beta-subunits led us to question whether this was alterable and thus if some form of regulation was involved. We have examined the consequences of overexpressing Na-K-ATPase beta1-subunits using Madin-Darby canine kidney (MDCK) cells expressing flag-tagged beta1-subunits (beta1flag) or Myc-tagged beta1-subunits (beta1myc) under the control of a tetracycline-dependent promoter. The induction of beta1flag subunit synthesis in MDCK cells, which increases beta1-subunit expression at the plasma membrane by more than twofold, while maintaining stable alpha1 expression levels, revealed that all mature beta1-subunits associate with alpha1-subunits, and no evidence of "free" beta1-subunits was obtained. Consequently, the ratio of assembled beta1- to alpha1-subunits is significantly increased when "extra" beta-subunits are expressed. An increased beta1/alpha1 stoicheometry is also observed in cells treated with tunicamycin, suggesting that the protein-protein interactions involved in these complexes are not dependent on glycosylation. Confocal images of cocultured beta1myc-expressing and beta1flag-expressing MDCK cells show colocalization of beta1myc and beta1flag subunits at the lateral membranes of neighboring cells, suggesting the occurrence of intercellular interactions between the beta-subunits. Immunoprecipitation using MDCK cells constitutively expressing beta1myc and tetracycline-regulated beta1flag subunits confirmed beta-beta-subunit interactions. These results demonstrate that the equimolar ratio of assembled beta1/alpha1-subunits of the Na-K-ATPase in kidney cells is not fixed by the inherent properties of the interacting subunits. It is likely that cellular mechanisms are present that regulate the individual Na-K-ATPase subunit abundance.

Cellular multitasking: the dual role of human Cu-ATPases in cofactor delivery and intracellular copper balance

The human copper-transporting ATPases (Cu-ATPases) are essential for dietary copper uptake, normal development and function of the CNS, and regulation of copper homeostasis in the body. In a cell, Cu-ATPases maintain the intracellular concentration of copper by transporting copper into intracellular exocytic vesicles. In addition, these P-type ATPases mediate delivery of copper to copper-dependent enzymes in the secretory pathway and in specialized cell compartments such as secretory granules or melanosomes. The multiple functions of human Cu-ATPase necessitate complex regulation of these transporters that is mediated through the presence of regulatory domains in their structure, posttranslational modification and intracellular trafficking, as well as interactions with the copper chaperone Atox1 and other regulatory molecules. In this review, we summarize the current information on the function and regulatory mechanisms acting on human Cu-ATPases ATP7A and ATP7B. Brief comparison with the Cu-ATPase orthologs from other species is included.

Common patterns and unique features of P-type ATPases: a comparative view on the KdpFABC complex from Escherichia coli (Review)

P-type ATPases are ubiquitously abundant primary ion pumps, which are capable of transporting cations across the cell membrane at the expense of ATP. Since these ions comprise a large variety of vital biochemical functions, nature has developed rather sophisticated transport machineries in all kingdoms of life. Due to the importance of these enzymes, representatives of both eu- and prokaryotic as well as archaeal P-type ATPases have been studied intensively, resulting in detailed structural and functional information on their mode of action. During catalysis, P-type ATPases cycle between the so-called E1 and E2 states, each of which comprising different structural properties together with different binding affinities for both ATP and the transport substrate. Crucial for catalysis is the reversible phosphorylation of a conserved aspartate, which is the main trigger for the conformational changes within the protein. In contrast to the well-studied and closely related eukaryotic P-type ATPases, much less is known about their homologues in bacteria. Whereas in Eukarya there is predominantly only one subunit, which builds up the transport system, in bacteria there are multiple polypeptides involved in the formation of the active enzyme. Such a rather unusual prokaryotic P-type ATPase is the KdpFABC complex of the enterobacterium Escherichia coli, which serves as a highly specific K(+) transporter. A unique feature of this member of P-type ATPases is that catalytic activity and substrate transport are located on two different polypeptides. This review compares generic features of P-type ATPases with the rather unique KdpFABC complex and gives a comprehensive overview of common principles of catalysis as well as of special aspects connected to distinct enzyme functions.

Nucleotide binding to Na,K-ATPase: pK values of the groups affecting the high affinity site

Investigation of the ionic strength effect on the interactions between nucleotides (ATP and ADP) and Na,K-ATPase in a broad pH range was aimed at revealing pK values of the charged groups of the interacting species. Ionic strength experiments suggested that an amino acid residue with a pK > 8.0 is part of the protein binding site. A combination of equilibrium and transient experiments at various pH values allowed for the characterization of the groups electrostatically involved in either the association process (kon) or the stability of the preformed complexes (koff). Two groups (pK1 = 6.7 and pK2 = 8.4) appear to be important for the proper organization of the binding site and, therefore, the association reaction. Moreover, deprotonation of the basic group completely precludes association. pH dependencies of the dissociation rate constants for ATP and ADP are very different. An increase in pH from 5 to 9.5 induces a 9-fold increase in koff for ATP, whereas koff for ADP decreases 4-fold between pH 5 and 8, and decreases further in the alkaline region. A comparison of the pH dependencies for koff for ATP and ADP suggests two effects: (1) at acidic pH, the value of the total negative charge of the nucleotide determines the tightness of binding; and (2) short-range interactions involving the terminal phosphate group are important for nucleotide dissociation from the site. The difference in the pH dependencies of koff for the nucleotides suggests the existence of positive charges in close proximity to Asp369, relieving the repulsion between the gamma-phosphate of ATP and Asp369.

Metals in membranes

In this critical review we discuss recent advances in understanding the modes of interaction of metal ions with membrane proteins, including channels, pumps, transporters, ATP-binding cassette proteins, G-protein coupled receptors, kinases and respiratory enzymes. Such knowledge provides a basis for elucidating the mechanism of action of some classes of metallodrugs, and a stimulus for the further exploration of the coordination chemistry of metal ions in membranes. Such research offers promise for the discovery of new drugs with unusual modes of action. The article will be of interest to bioinorganic chemists, chemical biologists, biochemists, pharmacologists and medicinal chemists. (247 references).

Self-association of isolated large cytoplasmic domain of plasma membrane H+ -ATPase from Saccharomyces cerevisiae: role of the phosphorylation domain in a general dimeric model for P-ATPases

Large cytoplasmic domain (LCD) plasma membrane H+ -ATPase from S. cerevisiae was expressed as two fusion polypeptides in E. coli: a DNA sequence coding for Leu353-Ileu674 (LCDh), comprising both nucleotide (N) and phosphorylation (P) domains, and a DNA sequence coding for Leu353-Thr543 (LCDDeltah, lacking the C-terminus of P domain), were inserted in expression vectors pDEST-17, yielding the respective recombinant plasmids. Overexpressed fusion polypeptides were solubilized with 6 M urea and purified on affinity columns, and urea was removed by dialysis. Their predicted secondary structure contents were confirmed by CD spectra. In addition, both recombinant polypeptides exhibited high-affinity 2',3'-O-(2,4,6-trinitrophenyl)adenosine-5'-triphosphate (TNP-ATP) binding (Kd = 1.9 microM and 2.9 microM for LCDh and LCDDeltah, respectively), suggesting that they have native-like folding. The gel filtration profile (HPLC) of purified LCDh showed two main peaks, with molecular weights of 95 kDa and 39 kDa, compatible with dimeric and monomeric forms, respectively. However, a single elution peak was observed for purified LCDDeltah, with an estimated molecular weight of 29 kDa, as expected for a monomer. Together, these data suggest that LCDh exist in monomer-dimer equilibrium, and that the C-terminus of P domain is necessary for self-association. We propose that such association is due to interaction between vicinal P domains, which may be of functional relevance for H+ -ATPase in native membranes. We discuss a general dimeric model for P-ATPases with interacting P domains, based on published crystallography and cryo-electron microscopy evidence.

Effects of C-terminal truncations on trafficking of the yeast plasma membrane H+-ATPase

Within the large family of P-type cation-transporting ATPases, members differ in the number of C-terminal transmembrane helices, ranging from two in Cu2+-ATPases to six in H+-, Na+,K+-, Mg2+-, and Ca2+-ATPases. In this study, yeast Pma1 H+-ATPase has served as a model to examine the role of the C-terminal membrane domain in ATPase stability and targeting to the plasma membrane. Successive truncations were constructed from the middle of the major cytoplasmic loop to the middle of the extended cytoplasmic tail, adding back the C-terminal membrane-spanning helices one at a time. When the resulting constructs were expressed transiently in yeast, there was a steady increase in half-life from 70 min in Pma1 delta452 to 348 min in Pma1 delta901, but even the longest construct was considerably less stable than wild-type ATPase (t(1/2) = 11 h). Confocal immunofluorescence microscopy showed that 11 of 12 constructs were arrested in the endoplasmic reticulum and degraded in the proteasome. The only truncated ATPase that escaped the ER, Pma1 delta901, traveled slowly to the plasma membrane, where it hydrolyzed ATP and supported growth. Limited trypsinolysis showed Pma1 delta901 to be misfolded, however, resulting in premature delivery to the vacuole for degradation. As model substrates, this series of truncations affirms the importance of the entire C-terminal domain to yeast H+-ATPase biogenesis and defines a sequence element of 20 amino acids in the carboxyl tail that is critical to ER escape and trafficking to the plasma membrane.

Structure of the ATP binding domain from the Archaeoglobus fulgidus Cu+-ATPase

The P-type ATPases translocate cations across membranes using the energy provided by ATP hydrolysis. CopA from Archaeoglobus fulgidus is a hyperthermophilic ATPase responsible for the cellular export of Cu+ and is a member of the heavy metal P1B-type ATPase subfamily, which includes the related Wilson and Menkes diseases proteins. The Cu+-ATPases are distinct from their P-type counter-parts in ion binding sequences, membrane topology, and the presence of cytoplasmic metal binding domains, suggesting that they employ alternate forms of regulation and novel mechanisms of ion transport. To gain insight into Cu+-ATPase function, the structure of the CopA ATP binding domain (ATPBD) was determined to 2.3 A resolution. Similar to other P-type ATPases, the ATPBD includes nucleotide binding (N-domain) and phosphorylation (P-domain) domains. The ATPBD adopts a closed conformation similar to the nucleotide-bound forms of the Ca2+-ATPase. The CopA ATPBD is much smaller and more compact, however, revealing the minimal elements required for ATP binding, hydrolysis, and enzyme phosphorylation. Structural comparisons to the AMP-PMP-bound form of the Escherichia coli K+-transporting Kdp-ATPase and to the Wilson disease protein N-domain indicate that the five conserved N-domain residues found in P1B-type ATPases, but not in the other families, most likely participate in ATP binding. By contrast, the P-domain includes several residues conserved among all P-type ATPases. Finally, the CopA ATPBD structure provides a basis for understanding the likely structural and functional effects of various mutations that lead to Wilson and Menkes diseases.

Thr-774 (transmembrane segment M5), Val-920 (M8), and Glu-954 (M9) are involved in Na+ transport, and Gln-923 (M8) is essential for Na,K-ATPase activity

The highly conserved amino acids of rat Na,K-ATPase, Thr-774 in the transmembrane helices M5, Val-920 and Gln-923 in M8, and Glu-953 and Glu-954 in M9, the side chains of which appear to be in close proximity, were mutated, and the resulting proteins, T774A, E953A/K, and E954A/K, V920E and Q923N/E/D/L, were expressed in HeLa cells. Ouabain-resistant cell lines were obtained from T774A, V920E, E953A, and E954A, whereas Q923N/E/D/L, E953K, and E954K could only be transiently expressed as fusion proteins with an enhanced green fluorescent protein. The apparent K0.5 values for Na+, as estimated by the Na+-dependent phosphoenzyme formation (K0.5(Na,EP)) or Na,K-ATPase activity (K(0.5)(Na,ATPase)), were increased by around 2 approximately 8-fold in the case of T774A, V920E, and E954A. The apparent K0.5 values for K+, as estimated by the Na,K-ATPase (K0.5(K,ATPase)) or p-nitrophenylphosphatase activity (K0.5(K,pNPPase)), were affected only slightly by the 3 mutations, except that V920E showed a 1.7-fold increase in the K0.5(K,ATPase). The apparent K0.5 values for ATP (K0.5(EP)), as estimated by phosphorylation (a high affinity ATP effect), were increased by 1.6 approximately 2.6-fold in the case of T774A, V920E, and E954A. Those estimated by Na,K-ATPase activity (K0.5(ATPase)) and ATP-induced inhibition (K(i,0.5)(pNPPase)) of K-pNPPase activity (low affinity ATP effects) were, respectively, increased by 1.8-fold and unchanged in the case of T774A but decreased by 2- and 4.8-fold in the case of V920E and were slightly changed and increased by 1.7-fold in the case of E954A. The E953A showed little significant change in the apparent affinities. These results suggest that Gln-923 in M8 is crucial for the active transport of Na+ and/or K+ across membranes and that the side chain oxygen atom of Thr-774 in M5, the methyl group(s) of Val-920 in M8, and the carboxyl oxygen(s) of Glu-954 in M9 mainly play some role in the transport of Na+ and also in the high and low affinity ATP effects rather than the transport of K+.

The phosphatase activity of the isolated H4-H5 loop of Na+/K+ ATPase resides outside its ATP binding site

The structural stability of the large cytoplasmic domain (H(4)-H(5) loop) of mouse alpha(1) subunit of Na(+)/K(+) ATPase (L354-I777), the number and the location of its binding sites for 2'-3'-O-(trinitrophenyl) adenosine 5'-triphosphate (TNP-ATP) and p-nitrophenylphosphate (pNPP) were investigated. C- and N-terminal shortening revealed that neither part of the phosphorylation (P)-domain are necessary for TNP-ATP binding. There is no indication of a second ATP site on the P-domain of the isolated loop, even though others reported previously of its existence by TNP-N(3)ADP affinity labeling of the full enzyme. Fluorescein isothiocyanate (FITC)-anisotropy measurements reveal a considerable stability of the nucleotide (N)-domain suggesting that it may not undergo a substantial conformational change upon ATP binding. The FITC modified loop showed only slightly diminished phosphatase activity, most likely due to a pNPP site on the N-domain around N398 whose mutation to D reduced the phosphatase activity by 50%. The amino acids forming this pNPP site (M384, L414, W411, S400, S408) are conserved in the alpha(1-4) isoforms of Na(+)/K(+) ATPase, whereas N398 is only conserved in the vertebrates' alpha(1) subunit. The phosphatase activity of the isolated H(4)-H(5) loop was neither inhibited by ATP, nor affected by mutation of D369, which is phosphorylated in native Na(+)/K(+) ATPase.

Functional modules of KdpB, the catalytic subunit of the Kdp-ATPase from Escherichia coli

The large cytoplasmic domain (H4H5) of KdpB of the KdpFABC complex (P-type ATPase) from Escherichia coli consists of two separate modules, the phosphorylation domain (KdpBP) and the nucleotide binding domain (KdpBN). The H4H5 and the KdpBN domains were purified as soluble 10His-tagged fusion proteins. Both proteins exhibit a mainly alpha-helical secondary structure as judged by CD spectroscopy. Fluorescein 5-isothiocyanate (FITC) labeling studies revealed that both proteins form a proper nucleotide binding site. Adenosine nucleotides protect the H4H5 loop but not KdpBN against FITC modification. Trinitrophenyl (TNP)-nucleotide binding studies revealed that both H4H5 and KdpBN bind nucleotides with high affinity. Furthermore, the H4H5 loop was still able to hydrolyze ATP, as well as p-nitrophenyl phosphate (pNPP). These results lend support to the notion that the separately synthesized H4H5 and KdpBN domains retain their native structure and that they reveal properties of both P2-type ATPases (e.g., Na(+),K(+)-ATPase and Ca(2+)-ATPase) and P1b-type ATPases (e.g., heavy metal transporting ATPases). Furthermore, this report also emphasizes the unique position of the Kdp-ATPase within the P-type ATPase family.

The hydrogen bonds between Arg423 and Glu472 and other key residues, Asp443, Ser477, and Pro489, are responsible for the formation and a different positioning of TNP-ATP and ATP within the nucleotide-binding site of Na(+)/K(+)-ATPase

Mutation of Arg(423) at the N-domain of Na(+)/K(+)-ATPase resulted in a large decrease of both TNP-ATP and ATP binding. Thus, this residue, localized outside the binding pocket, seems to play a key role in supporting the proper structure and shape of the binding site. In addition, mutation of Glu(472) also caused a large decrease of both TNP-ATP and ATP binding. On the basis of our computer model, we hypothesized that a hydrogen bond between Arg(423) and Glu(472) supports the connection of two opposite halves of the ATP-binding pocket. To verify this hypothesis, we have also prepared the construct containing both these mutations. Binding of neither TNP-ATP nor ATP to this double mutant differed from binding to any of the single mutants. This strongly supported the existence of the hydrogen bond between Arg(423) and Glu(472). Similarly, the conserved residue Pro(489) seems to be substantial for the proper interaction of the third and fourth beta-strands of the N-domain, which both contain residues that take part in ATP binding. Mutation of Asp(443) affected only ATP, but not TNP-ATP, binding, suggesting that these ligands adopt different positions in the nucleotide-binding pocket. On the basis of a recently published crystal structure [Håkansson, K. O. (2003) J. Mol. Biol. 332, 1175-1182], we improved our model and computed the interaction of these two ligands with the N-domain. This model is in good agreement with all previously reported spectroscopic data and revealed that Asp(443) forms a hydrogen bond with the NH(2) group of the adenosine moiety of ATP, but not TNP-ATP.

FITC binding site and p-nitrophenyl phosphatase activity of the Kdp-ATPase of Escherichia coli

The KdpFABC complex of Escherichia coli, which belongs to the P-type ATPase family, has a unique structure, since catalytic activity (KdpB) and the capacity to transport potassium ions (KdpA) are located on different subunits. We found that fluorescein 5-isothiocyanate (FITC) inhibits ATPase activity, probably by covalently modifying lysine 395 in KdpB. In addition, we observed that the KdpFABC complex is able to hydrolyze p-nitrophenyl phosphate (pNPP) in a Mg(2+)-dependent reaction. The pNPPase activity is inhibited by FITC and o-vanadate. Low concentrations of ATP (1-30 microM) stimulate the pNPPase activity, while concentrations of >500 microM are inhibitory. This behavior can be explained either by a regulatory ATP binding site, where ATP hydrolysis is required, or by proposing an interactive dimer. The notion that FITC inhibits pNPPase and ATPase activity supports the idea that the catalytic domain of KdpB is much more compact than other P-type ATPases, like Na(+),K(+)-ATPase, H(+),K(+)-ATPase, and Ca(2+)-ATPase.

Oligomerization of the Na,K-ATPase in cell membranes

The higher order oligomeric state of the Na,K-ATPase alphabeta heterodimer in cell membranes is the subject of controversy. We have utilized the baculovirus-infected insect cell system to express Na,K-ATPase with alpha-subunits bearing either His(6) or FLAG epitopes at the carboxyl terminus. Each of these constructs produced functional Na,K-ATPase alphabeta heterodimers that were delivered to the plasma membrane (PM). Cells were simultaneously co-infected with viruses encoding alpha-His/beta and alpha-FLAG/beta Na,K-ATPases. Co-immunoprecipitation of the His-tagged alpha-subunit in the endoplasmic reticulum (ER) and PM fractions of co-infected cells by the anti-FLAG antibody demonstrates that protein-protein associations exist between these heterodimers. This suggests the Na,K-ATPase is present in cell membranes in an oligomeric state of at least (alphabeta)(2) composition. Deletion of 256 amino acid residues from the central cytoplasmic loop of the alpha-subunit results in the deletion alpha-4,5-loop-less (alpha-4,5LL), which associates with beta but is confined to the ER. Co-immunoprecipitation demonstrates that when this inactive alpha-4,5LL/beta heterodimer is co-expressed with wild-type alphabeta, oligomers of wild-type alphabeta and alpha-4,5LL/beta form in the ER, but the alpha-4,5LL mutant remains retained in the ER, and the wild-type protein is still delivered to the PM. We conclude that the Na,K-ATPase is present as oligomers of the monomeric alphabeta heterodimer in native cell membranes.

The distinct functional properties of the nucleotide-binding domain of ATP7B, the human copper-transporting ATPase: analysis of the Wilson disease mutations E1064A, H1069Q, R1151H, and C1104F

Copper transport by the P(1)-ATPase ATP7B, or Wilson disease protein (WNDP),1 is essential for human metabolism. Perturbation of WNDP function causes intracellular copper accumulation and severe pathology, known as Wilson disease (WD). Several WD mutations are clustered within the WNDP nucleotide-binding domain (N-domain), where they are predicted to disrupt ATP binding. The mechanism by which the N-domain coordinates ATP is presently unknown, because residues important for nucleotide binding in the better characterized P(2)-ATPases are not conserved within the P(1)-ATPase subfamily. To gain insight into nucleotide binding under normal and disease conditions, we generated the recombinant WNDP N-domain and several WD mutants. Using isothermal titration calorimetry, we demonstrate that the N-domain binds ATP in a Mg(2+)-independent manner with a relatively high affinity of 75 microm, compared with millimolar affinities observed for the P(2)-ATPase N-domains. The WNDP N-domain shows minimal discrimination between ATP, ADP, and AMP, yet discriminates well between ATP and GTP. Similar results were obtained for the N-domain of ATP7A, another P(1)-ATPase. Mutations of the invariant WNDP residues E1064A and H1069Q drastically reduce nucleotide affinities, pointing to the likely role of these residues in nucleotide coordination. In contrast, the R1151H mutant exhibits only a 1.3-fold reduction in affinity for ATP. The C1104F mutation significantly alters protein folding, whereas C1104A does not affect the structure or function of the N-domain. Together, the results directly demonstrate the phenotypic diversity of WD mutations within the N-domain and indicate that the nucleotide-binding properties of the P(1)-ATPases are distinct from those of the P(2)-ATPases.

Limitations in linearized analyses of binding equilibria: binding of TNP-ATP to the H4-H5 loop of Na/K-ATPase

Binding of TNP-ATP [2',3'- O-(2,4,6-trinitrophenyl)adenosine 5'-triphosphate, a fluorescent analogue of ATP] to the K605 protein was studied. This is an isolated N-domain in the cytoplasmic loop of the Na/K-ATPase alpha-subunit, lying between membrane-spanning segments 4 and 5 (sequence L(354)-I(604)). A titration equation is derived that explicitly makes it possible to relate the fluorescence of TNP-ATP and K605 solutions to total probe concentration in the sample. Using this, it is possible to obtain the value of the dissociation constant from the titration experiment without resorting to the Scatchard plot, which is far from optimal from the statistical point of view. Using the new formula with non-linear regression analysis, a value of the dissociation constant K(D) for TNP-ATP binding to the K605 protein of 3.03 +/- 0.28 microM at 22 degrees C was obtained. A series of fits to simulated data with added noise demonstrated clearly the advantage of non-linear regression using the new formula over the commonly employed linear regression using the Scatchard plot. The procedure presented is generally applicable to binding assays using changes in the fluorescence of ligands on binding.

Structure and mechanism of Na,K-ATPase: functional sites and their interactions

The cell membrane Na,K-ATPase is a member of the P-type family of active cation transport proteins. Recently the molecular structure of the related sarcoplasmic reticulum Ca-ATPase in an E1 conformation has been determined at 2.6 A resolution. Furthermore, theoretical models of the Ca-ATPase in E2 conformations are available. As a result of these developments, these structural data have allowed construction of homology models that address the central questions of mechanism of active cation transport by all P-type cation pumps. This review relates recent evidence on functional sites of Na,K-ATPase for the substrate (ATP), the essential cofactor (Mg(2+) ions), and the transported cations (Na(+) and K(+)) to the molecular structure. The essential elements of the Ca-ATPase structure, including 10 transmembrane helices and well-defined N, P, and A cytoplasmic domains, are common to all PII-type pumps such as Na,K-ATPase and H,K-ATPases. However, for Na,K-ATPase and H,K-ATPase, which consist of both alpha- and beta-subunits, there may be some detailed differences in regions of subunit interactions. Mutagenesis, proteolytic cleavage, and transition metal-catalyzed oxidative cleavages are providing much evidence about residues involved in binding of Na(+), K(+), ATP, and Mg(2+) ions and changes accompanying E1-E2 or E1-P-E2-P conformational transitions. We discuss this evidence in relation to N, P, and A cytoplasmic domain interactions, and long-range interactions between the active site and the Na(+) and K(+) sites in the transmembrane segments, for the different steps of the catalytic cycle.

Eight amino acids form the ATP recognition site of Na(+)/K(+)-ATPase

Point mutations of a part of the H(4)-H(5) loop (Leu(354)-Ile(604)) of Na(+)/K(+)-ATPase have been used to study the ATP and TNP-ATP binding affinities. Besides the previously reported amino acid residues Lys(480), Lys(501), Gly(502), and Cys(549), we have found four more amino acid residues, viz., Glu(446), Phe(475), Gln(482), and Phe(548), completing the ATP-binding pocket of Na(+)/K(+)-ATPase. Moreover, mutation of Arg(423) has also resulted in a large decrease in the extent of ATP binding. This residue, localized outside the binding pocket, seems to play a key role in supporting the proper structure and shape of the binding site, probably due to formation of a hydrogen bond with Glu(472). On the other hand, only some minor effects were caused by mutations of Ile(417), Asn(422), Ser(445), and Glu(505).

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.

Ion pump-interacting proteins: promising new partners

The sorting and regulation of the Na,K and H,K-ATPases requires that the pump proteins must associate, at least transiently, with kinases, phosphatases, scaffolding molecules, and components of the cellular trafficking machinery. The identities of these interacting proteins and the nature of their associations with the pump polypeptides have yet to be elucidated. We have begun a series of yeast two-hybrid screens employing structurally defined segments of pump polypeptides as baits in order to gain insight into the nature and function of these interacting proteins.

Interactions between Na,K-ATPase alpha-subunit ATP-binding domains

The reaction mechanism of the Na,K-ATPase is thought to involve a number of ligand-induced conformational changes. The specific amino acid residues responsible for binding many of the important ligands have been identified; however, details of the specific conformational changes produced by ligand binding are largely undescribed. The experiments described in this paper begin to identify interactions between domains of the Na,K-ATPase alpha-subunit that depend on the presence of particular ligands. The major cytoplasmic loop (between TM4 and TM5), which we have previously shown contains the ATP-binding domain, was overexpressed in bacteria either with a His(6) tag or as a fusion protein with glutathione S-transferase. We have observed that these polypeptides associate in the presence of MgATP. Incubation with [gamma-(32)P]ATP under conditions that result in phosphorylation of the full-length Na,K-ATPase did not result in (32)P incorporation into either the His(6) tag or glutathione S-transferase fusion proteins. The MgATP-induced association was strongly inhibited by prior modification of the fusion proteins with fluorescein isothiocyanate or by simultaneous incubation with 10 microm eosin, indicating that the effect of MgATP is due to interactions within the nucleotide-binding domain. These data are consistent with Na,K-ATPase associating within cells via interactions in the nucleotide-binding domains. Although any functional significance of these associations for ion transport remains unresolved, they may play a role in cell function and in modulating interactions between the Na,K-ATPase and other proteins.

Biochemistry of Na,K-ATPase

The Na,K-ATPase or sodium pump carries out the coupled extrusion and uptake of Na and K ions across the plasma membranes of cells of most higher eukaryotes. It is a member of the P-type ATPase superfamily. This heterodimeric integral membrane protein is composed of a 100-kDa alpha-subunit with ten transmembrane segments and a heavily glycosylated beta subunit of about 55 kDa, which is a type II membrane protein. Current ideas on how the protein achieves active transport are based on a fusion of results of transport physiology, protein chemistry, and heterologous expression of mutant proteins. Recently acquired high resolution structural information provides an important new avenue for a more complete understanding of this protein. In this review, the current status of knowledge of Na,K-ATPase is discussed, and areas where there is still considerable uncertainty are highlighted.

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.

Phe(475) and Glu(446) but not Ser(445) participate in ATP-binding to the alpha-subunit of Na(+)/K(+)-ATPase

The ATP-binding site of Na(+)/K(+)-ATPase is localized on the large cytoplasmic loop of the alpha-subunit between transmembrane helices H(4) and H(5). Site-directed mutagenesis was performed to identify residues involved in ATP binding. On the basis of our recently developed model of this loop, Ser(445), Glu(446), and Phe(475) were proposed to be close to the binding pocket. Replacement of Phe(475) with Trp and Glu(446) with Gln profoundly reduced the binding of ATP, whereas the substitution of Ser(445) with Ala did not affect ATP binding. Fluorescence measurements of the fluorescent analog TNP-ATP, however, indicated that Ser(445) is close to the binding site, although it does not participate in binding.

Characterization of the ATP-binding domain of the sarco(endo)plasmic reticulum Ca(2+)-ATPase: probing nucleotide binding by multidimensional NMR

The skeletal muscle sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA1a) mediates muscle relaxation by pumping Ca(2+) from the cytosol to the ER/SR lumen. In efforts aimed at understanding the structural basis for the conformational changes accompanying the reaction cycle catalyzed by SERCA1a, we have studied the ATP-binding domain of SERCA1a in both nucleotide-bound and -free forms by NMR. Limited proteolysis analyses guided us to express a 28 kDa stably folded fragment containing the nucleotide-binding domain of SERCA1a spanning residues Thr357-Leu600. ATP binding activity was demonstrated for this fragment by a FITC competition assay. A nearly complete backbone resonance assignment of this 28 kDa ATP-binding fragment, in both the AMP-PNP-bound and -free forms, was obtained by means of heteronuclear multidimensional NMR techniques. NMR titration experiments with AMP-PNP revealed a confined nucleotide-binding site which coincides with a cytoplasmic pocket region identified in the crystal structure of apo-SERCA1a. These results are consistent with previous site-directed mutagenesis studies of SERCA1a.

The functional role of beta subunits in oligomeric P-type ATPases

Na,K-ATPase and gastric and nongastric H,K-ATPases are the only P-type ATPases of higher organisms that are oligomeric and are associated with a beta subunit, which is obligatory for expression and function of enzymes. Topogenesis studies suggest that beta subunits have a fundamental and unique role in K+-transporting P-type ATPases in that they facilitate the correct membrane integration and packing of the catalytic a subunit of these P-type ATPases, which is necessary for their resistance to cellular degradation, their acquisition of functional properties, and their routing to the cell surface. In addition to this chaperone function, beta subunits also participate in the determination of intrinsic transport properties of Na,K- and H,K-ATPases. Increasing experimental evidence suggests that beta assembly is a highly ordered, beta isoform-specific process, which is mediated by multiple interaction sites that contribute in a coordinate, multistep process to the structural and functional maturation of Na,K- and H,K-ATPases.

Conformational coupling: the moving parts of an ion pump

The Na,K-ATPase carries out the coupled functions of ATP hydrolysis and cation transport. These functions are performed by two distinct regions of the protein. ATP binding and hydrolysis is mediated by the large central cytoplasmic loop of about 430 amino-acids. Transmembrane cation transport is accomplished via coordination of the Na and K ions by side-chains of the amino-acids of several of the transmembrane segments. The way in which these two protein domains interact lies at the heart of the molecular mechanism of active transport, or ion pumping. We summarize evidence obtained from protein chemistry studies of the purified renal Na,K-ATPase and from bacterially expressed polypeptides which characterize these separate functions and point to various movements which may occur as the protein transits through its reaction cycle. We then describe recent work using heterologous expression of renal Na,K-ATPase in baculovirus-infected insect cells which provides a suitable system to characterize such protein motions and which can be employed to test specific models arising from recently acquired high resolution structural information on related ion pumps.

Heterologous expression of Na(+)-K(+)-ATPase in insect cells: intracellular distribution of pump subunits

The Na(+)-K(+)-ATPase is a heterodimeric plasma membrane protein responsible for cellular ionic homeostasis in nearly all animal cells. It has been shown that some insect cells (e.g., High Five cells) have no (or extremely low) Na(+)-K(+)-ATPase activity. We expressed sheep kidney Na(+)-K(+)-ATPase alpha- and beta-subunits individually and together in High Five cells via the baculovirus expression system. We used quantitative slot-blot analyses to determine that the expressed Na(+)-K(+)-ATPase comprises between 0.5% and 2% of the total membrane protein in these cells. Using a five-step sucrose gradient (0.8-2.0 M) to separate the endoplasmic reticulum, Golgi apparatus, and plasma membrane fractions, we observed functional Na(+) pump molecules in each membrane pool and characterized their properties. Nearly all of the expressed protein functions normally, similar to that found in purified dog kidney enzyme preparations. Consequently, the measurements described here were not complicated by an abundance of nonfunctional heterologously expressed enzyme. Specifically, ouabain-sensitive ATPase activity, [(3)H]ouabain binding, and cation dependencies were measured for each fraction. The functional properties of the Na(+)-K(+)-ATPase were essentially unaltered after assembly in the endoplasmic reticulum. In addition, we measured ouabain-sensitive (86)Rb(+) uptake in whole cells as a means to specifically evaluate Na(+)-K(+)-ATPase molecules that were properly folded and delivered to the plasma membrane. We could not measure any ouabain-sensitive activities when either the alpha-subunit or beta-subunit were expressed individually. Immunostaining of the separate membrane fractions indicates that the alpha-subunit, when expressed alone, is degraded early in the protein maturation pathway (i.e., the endoplasmic reticulum) but that the beta-subunit is processed normally and delivered to the plasma membrane. Thus it appears that only the alpha-subunit has an oligomeric requirement for maturation and trafficking to the plasma membrane. Furthermore, assembly of the alpha-beta heterodimer within the endoplasmic reticulum apparently does not require a Na(+) pump-specific chaperone.

Cytoplasmic segment interactions in the alpha1-subunit of the rat Na+, K+-atpase

The currently accepted topographical model for the organization of the alpha-subunit of the Na+, K+-ATPase in the membrane considers that the protein has ten transmembrane segments and six cytoplasmic loops. Evidence of interaction between the cytoplasmic regions may contribute to a better understanding of the structure/function relationship of this protein. In this study, the first four cytoplasmic segments (C1, C2, C3 and C4) of the rat alpha1 subunit were expressed in Escherichia Coli. The large cytoplasmic loop between transmembrane segments four and five (C3) retained its native structure as demonstrated by the ability of ATP to protect against chemical modification by Fluorescein 5-isothiocyanate (FITC). Interaction studies were conducted by an overlay assay (Far Western blots) and surface plasmon resonance technology. We observed that C3 interacts with the N-terminal segment of the Na+, K+-ATPase, C1; and that both C1 and C3 interact with the cytoplasmic segments C2 and C4.

Structural similarities of Na,K-ATPase and SERCA, the Ca(2+)-ATPase of the sarcoplasmic reticulum

The crystal structure of SERCA1a (skeletal-muscle sarcoplasmic-reticulum/endoplasmic-reticulum Ca(2+)-ATPase) has recently been determined at 2.6 A (note 1 A = 0.1 nm) resolution [Toyoshima, Nakasako, Nomura and Ogawa (2000) Nature (London) 405, 647-655]. Other P-type ATPases are thought to share key features of the ATP hydrolysis site and a central core of transmembrane helices. Outside of these most-conserved segments, structural similarities are less certain, and predicted transmembrane topology differs between subclasses. In the present review the homologous regions of several representative P-type ATPases are aligned with the SERCA sequence and mapped on to the SERCA structure for comparison. Homology between SERCA and the Na,K-ATPase is more extensive than with any other ATPase, even PMCA, the Ca(2+)-ATPase of plasma membrane. Structural features of the Na,K-ATPase are projected on to the Ca(2+)-ATPase crystal structure to assess the likelihood that they share the same fold. Homology extends through all ten transmembrane spans, and most insertions and deletions are predicted to be at the surface. The locations of specific residues are examined, such as proteolytic cleavage sites, intramolecular cross-linking sites, and the binding sites of certain other proteins. On the whole, the similarity supports a shared fold, with some particular exceptions.

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

Wilson's disease, an autosomal disorder associated with vast accumulation of copper in tissues, is caused by mutations in a gene encoding a copper-transporting ATPase (Wilson's disease protein, WNDP). Numerous mutations have been identified throughout the WNDP sequence, particularly in the Lys(1010)-Lys(1325) segment; however, the biochemical properties and molecular mechanism of WNDP remain poorly characterized. Here, the Lys(1010)-Lys(1325) fragment of WNDP was overexpressed, purified, and shown to form an independently folded ATP-binding domain (ATP-BD). ATP-BD binds the fluorescent ATP analogue trinitrophenyl-ATP with high affinity, and ATP competes with trinitrophenyl-ATP for the binding site; ADP and AMP appear to bind to ATP-BD at the site separate from ATP. Purified ATP-BD hydrolyzes ATP and interacts specifically with the N-terminal copper-binding domain of WNDP (N-WNDP). Strikingly, copper binding to N-WNDP diminishes these interactions, suggesting that the copper-dependent change in domain-domain contact may represent the mechanism of WNDP regulation. In agreement with this hypothesis, N-WNDP induces conformational changes in ATP-BD as evidenced by the altered nucleotide binding properties of ATP-BD in the presence of N-WNDP. Significantly, the effects of copper-free and copper-bound N-WNDP on ATP-BD are not identical. The implications of these results for the WNDP function are discussed.

Abolishment of proton pumping and accumulation in the E1P conformational state of a plant plasma membrane H+-ATPase by substitution of a conserved aspartyl residue in transmembrane segment 6

The plasma membrane H(+)-ATPase AHA2 of Arabidopsis thaliana, which belongs to the P-type ATPase superfamily of cation-transporting ATPases, pumps protons out of the cell. To investigate the mechanism of ion transport by P-type ATPases we have mutagenized Asp(684), a residue in transmembrane segment M6 of AHA2 that is conserved in Ca(2+)-, Na(+)/K(+)-, H(+)/K(+)-, and H(+)-ATPases and which coordinates Ca(2+) ions in the SERCA1 Ca(2+)-ATPase. We describe the expression, purification, and biochemical analysis of the Asp(684) --> Asn mutant, and provide evidence that Asp(684) in the plasma membrane H(+)-ATPase is required for any coupling between ATP hydrolysis, enzyme conformational changes, and H(+)-transport. Proton pumping by the reconstituted mutant enzyme was completely abolished, whereas ATP was still hydrolyzed. The mutant was insensitive to the inhibitor vanadate, which preferentially binds to P-type ATPases in the E(2) conformation. During catalysis the Asp(684) --> Asn enzyme accumulated a phosphorylated intermediate whose stability was sensitive to addition of ADP. We conclude that the mutant enzyme is locked in the E(1) conformation and is unable to proceed through the E(1)P-E(2)P transition.

Site-directed chemical labeling of extracellular loops in a membrane protein. The topology of the Na,K-ATPase alpha-subunit

We have mapped the membrane topology of the renal Na,K-ATPase alpha-subunit by using a combination of introduced cysteine mutants and surface labeling with a membrane impermeable Cys-directed reagent, N-biotinylaminoethyl methanethiosulfonate. To begin our investigation, two cysteine residues (Cys(911) and Cys(964)) in the wild-type alpha-subunit were substituted to create a background mutant devoid of exposed cysteines (Lutsenko, S., Daoud, S., and Kaplan, J. H. (1997) J. Biol. Chem. 272, 5249-5255). Into this background construct were then introduced single cysteines in each of the five putative extracellular loops (P118C, T309C, L793C, L876C, and M973C) and the resulting alpha-subunit mutants were co-expressed with the beta-subunit in baculovirus-infected insect cells. All of our expressed Na,K-ATPase mutants were functionally active. Their ATPase, phosphorylation, and ouabain binding activities were measured, and the turnover of the phosphoenzyme intermediate was close to the wild-type enzyme, suggesting that they are folded properly in the infected cells. Incubation of the insect cells with the cysteine-selective reagent revealed essentially no labeling of the alpha-subunit of the background construct and labeling of all five mutants with single cysteine residues in putative extracellular loops. Two additional mutants, V969C and L976C, were created to further define the M9M10 loop. The lack of labeling for these two mutants showed that although Met(973) is apparently exposed, Val(969) and Leu(976) are not, demonstrating that this method may also be utilized to define membrane aqueous boundaries of membrane proteins. Our labeling studies are consistent with a specific 10-transmembrane segment model of the Na,K-ATPase alpha-subunit. This strategy utilized only functional Na,K-ATPase mutants to establish the membrane topology of the entire alpha-subunit, in contrast to most previously applied methods.

Soluble P-type ATPase from an archaeon, Methanococcus jannaschii

MJ0968 has been proposed to be an ancestor of P-type ATPase, because its primary structure is highly homologous to that of the core catalytic domain of P-type ATPase. However it completely lacks amino acid sequences that possibly constitute transmembrane domains. To examine if MJ0968 is indeed a P-type ATPase, it was overexpressed in Escherichia coli and purified. It did show ATPase activity, autophosphorylation and inhibition by vanadate. All these properties support the idea that MJ0968 is indeed a soluble P-type ATPase.

The carbonyl group of glutamic acid-795 is essential for gastric H+,K+-ATPase activity

To study the role of Glu795offresent in the fifth transmembrane domain of the alpha-subunit of gastric H+,K+-ATPase, several mutants were generated and expressed in Sf9 insect cells. The E795Q mutant had rather similar properties as the wild-type enzyme. The apparent affinity for K+ in both the ATPase reaction and the dephosphorylation of the phosphorylated intermediate was even slightly enhanced. This indicates that the carbonyl group of Glu795 is sufficient for enzymatic activity. This carbonyl group, however, has to be at a particular position with respect to the other liganding groups, since the E795D and E795N mutants showed a strongly reduced ATPase activity, a lowered apparent K+ affinity, and a decreased steady-state phosphorylation level. In the absence of a carbonyl residue at position 795, the K+ sensitivity was either strongly decreased (E795A) or completely absent (E795L). The mutant E795L, however, showed a SCH 28080 sensitive ATPase activity in the absence of K+, as well as an enhanced spontaneous dephosphorylation rate, that could not be further enhanced by K+, suggesting that this mutant mimicks the filled K+ binding pocket. The results indicate that the Glu795 residue is involved in K+-stimulated ATPase activity and K+-induced dephosphorylation of the phosphorylated intermediate. Glu795 might also be involved in H+ binding during the phosphorylation step, since the mutants E795N, E795D, and E795A showed a decrease in the phosphorylation rate as well as in the apparent ATP affinity in the phosphorylation reaction. This indicates that Glu795 is not only involved in K+ but might also play a role in H+ binding.

Identification of fluorescein-5'-isothiocyanate-modification sites in proteins by electrospray-ionization mass spectrometry

Model peptides and proteins, such as hen eggwhite lysozyme, have been modified with fluorescein-5'-isothiocyanate (FITC) to yield the corresponding fluorescein-thiocarbamoyl (FTC) conjugates (N, N'-disubstituted thiourea and dithiourethane adducts). The extent of FITC incorporation, i.e., number of modified residues, has been identified by direct molecular weight determination using matrix-assisted laser desorption-ionization and electrospray-ionization mass spectrometry (MALDI-MS; ESI-MS). A specific fragmentation by cleavage of the FTC moiety from modified residues occurs by nozzle-skimmer dissociation in ESI mass spectra at increased declustering potential. This fragmentation pathway is easily obtained and renders ESI-MS an efficient tool for the characterization of FITC-modified proteins, and identification of modification sites in FTC-peptide mixtures.

Cys(577) is a conformationally mobile residue in the ATP-binding domain of the Na,K-ATPase alpha-subunit

2-[4'-Maleimidylanilino]naphthalene 6-sulfonic acid (MIANS) irreversibly inactivates Na,K-ATPase in a time- and concentration-dependent manner. Inactivation is prevented by 3 mM ATP or low K(+) (<1 mM); the protective effect K(+) is reversed at higher concentrations. This biphasic effect was also observed with K(+) congeners. In contrast, Na(+) ions did not protect. MIANS inactivation disrupted high affinity ATP binding. Tryptic fragments of MIANS-labeled protein were analyzed by reversed phase high performance liquid chromatography. ATP clearly protected one major labeled peptide peak. This observation was confirmed by separation of tryptic peptides in SDS-polyacrylamide gel electrophoresis revealing a single fluorescently-labeled peptide of approximately 5 kDa. N-terminal amino acid sequencing identified the peptide (V(545)LGFCH...). This hydrophobic peptide contains only two Cys residues in all sodium pump alpha-subunit sequences and is found in the major cytoplasmic loop between M4 and M5, a region previously associated with ATP binding. Subsequent digestion of the tryptic peptide with V8 protease and N-terminal amino acid sequencing identified the modified residue as Cys(577). The cation-dependent change in reactivity of Cys(577) implies structural alterations in the ATP-binding domain following cation binding and occlusion in the intramembrane domain of Na,K-ATPase and expands our knowledge of the extent to which cation binding and occlusion are sensed in the ATP hydrolysis domain.

Catalytic activity of an isolated domain of Na,K-ATPase expressed in Escherichia coli

Fusion proteins of glutathione-S-transferase and fragments from the large cytoplasmic domain of the sheep Na,K-ATPase alpha1-subunit were expressed in Escherichia coli. The Na,K-ATPase sequences begin at Ala345 and terminate at either Arg600 (DP600f), Thr610 (DP610f), Gly731 (DP731f), or Glu779 (DP779f). After affinity purification on glutathione-Sepharose, the fusion proteins were labeled with [alpha-32P]-2-N3-ATP, and incorporation of the radiolabel into the fusion proteins was measured by scintillation counting after sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Kd values of 220-290 microM for 2-N3-ATP binding to the fusion proteins were obtained from the photolabeling experiments. Approximately 1 mol of 2-N3-ATP was calculated to be incorporated per mole of fusion protein after correction for photochemical incorporation efficiency. Labeling of all of the fusion proteins by 25 microM 2-N3-ATP was reduced in the presence of MgATP, Na2ATP, MgCl2, 2',3'-O-(2,4, 6-trinitrophenyl)-ATP, and p-nitrophenylphosphate, and Ki values of 2-11 mM for Na2ATP, 0.2-5 mM for MgCl2, 0.1-5 mM for MgATP, and 20-300 microM for p-nitrophenylphosphate were calculated for these ligands. All of the fusion proteins catalyze the hydrolysis of p-nitrophenylphosphate. The reaction requires MgCl2 and is inhibited by inorganic phosphate, which is similar to the hydrolysis of p-nitrophenylphosphate by native Na,K-ATPase. Based on these observations, it appears that the soluble fragments from the large cytoplasmic domain of Na,K-ATPase expressed in bacterial cells are folded in an E2-like conformation and are likely to retain much of the native structure.