Purification and characterization of (Na+, K+)-ATPase. V. Conformational changes in the enzyme Transitions between the Na-form and the K-form studied with tryptic digestion as a tool

Effect of Cations on ATP Binding to the N-domain of Na+, K+-ATPase

The nucleotide-binding domain (N-domain) of the Na+, K+-ATPase (NKA) is physicochemically characterized by a high content of Glu and Asp residues, resulting in a low isoelectric point (pI = 5.0). Acidic proteins are known to interact with cations. The analysis in silico revealed potential cation interaction sites in the NKA N-domain structure. The interaction with cations was tested in vitro by using a recombinant NKA N-domain. The N-domain contains two Trp residues at the protein surface, as determined by acrylamide-mediated fluorescence quenching, that are useful for structural studies through fluorescence changes. Intrinsic fluorescence of the N-domain was decreased by the presence of cations (Na+, K+, Ca2+) indicating an effect on the protein structure. ATP binding also decreased the N-domain intrinsic fluorescence, which allowed nucleotide affinity determination. In the presence of cations, the N-domain affinity for ATP was increased. Molecular docking of fluorescein isothiocyanate (FITC) with the N-domain showed two binding modes with the isothiocyanate group located 5-6 Å close to Lys485 and Lys506 in the nucleotide-binding site. The presence of ATP prevented the FITC covalent labeling of the N-domain demonstrating the competitive behavior for the binding site. It is proposed that cations interact with the N-domain structure and thereby modulate nucleotide (ATP) affinity and possibly affecting NKA catalysis.

Electrostatic switch mechanisms of membrane protein trafficking and regulation

Lipid-protein interactions are normally classified as either specific or general. Specific interactions refer to lipid binding to specific binding sites within a membrane protein, thereby modulating the protein's thermal stability or kinetics. General interactions refer to indirect effects whereby lipids affect membrane proteins by modulating the membrane's physical properties, e.g., its fluidity, thickness, or dipole potential. It is not widely recognized that there is a third distinct type of lipid-protein interaction. Intrinsically disordered N- or C-termini of membrane proteins can interact directly but nonspecifically with the surrounding membrane. Many peripheral membrane proteins are held to the cytoplasmic surface of the plasma membrane via a cooperative combination of two forces: hydrophobic anchoring and electrostatic attraction. An acyl chain, e.g., myristoyl, added post-translationally to one of the protein's termini inserts itself into the lipid matrix and helps hold peripheral membrane proteins onto the membrane. Electrostatic attraction occurs between positively charged basic amino acid residues (lysine and arginine) on one of the protein's terminal tails and negatively charged phospholipid head groups, such as phosphatidylserine. Phosphorylation of either serine or tyrosine residues on the terminal tails via regulatory protein kinases allows for an electrostatic switch mechanism to control trafficking of the protein. Kinase action reduces the positive charge on the protein's tail, weakening the electrostatic attraction and releasing the protein from the membrane. A similar mechanism regulates many integral membrane proteins, but here only electrostatic interactions are involved, and the electrostatic switch modulates protein activity by altering the stabilities of different protein conformational states.

P-type ATPases: Many more enigmas left to solve

P-type ATPases constitute a large ancient super-family of primary active pumps that have diverse substrate specificities ranging from H+ to phospholipids. The significance of these enzymes in biology cannot be overstated. They are structurally related, and their catalytic cycles alternate between high- and low-affinity conformations that are induced by phosphorylation and dephosphorylation of a conserved aspartate residue. In the year 1988, all P-type sequences available by then were analyzed and five major families, P1 to P5, were identified. Since then, a large body of knowledge has accumulated concerning the structure, function, and physiological roles of members of these families, but only one additional family, P6 ATPases, has been identified. However, much is still left to be learned. For each family a few remaining enigmas are presented, with the intention that they will stimulate interest in continued research in the field. The review is by no way comprehensive and merely presents personal views with a focus on evolution.

Involvement of the alpha-subunit N-terminus in the mechanism of the Na+,K+-ATPase

Previous studies have shown that cytoplasmic K+ release and the associated E2 → E1 conformational change of the Na+,K+-ATPase is a major rate-determining step of the enzyme's ion pumping cycle and hence a prime site of acute regulatory intervention. From the ionic strength dependence of the enzyme's distribution between the E2 and E1 states, it has also been found that E2 is stabilized by an electrostatic attraction. Any disruption of this electrostatic attraction would, thus, have profound effects on the rate of ion pumping. The aim of this paper is to identify the location of this interaction. Using enhanced-sampling molecular dynamics simulations with a predicted N-terminal structure added to the X-ray crystal structure of the Na+,K+-ATPase, a previously postulated salt bridge between Lys32 and Glu233 (rat sequence numbering) of the enzyme's α-subunit can be excluded. The residues never approach closely enough to form a salt bridge. In contrast, strong interactions with anionic lipid head groups were seen. To investigate the possibility of a protein-lipid interaction experimentally, the surface charge density of Na+,K+-ATPase-containing membrane fragments was estimated from zeta potential measurements to be 0.019 (± 0.001) C m-2. This is in good agreement with the charge density previously determined to be responsible for stabilization of the E2 state of 0.023 (± 0.009) C m-2 and the membrane charge density estimated here from published electron-microscopic images of 0.018C m-2. The results are, therefore, consistent with an interaction of the Na+,K+-ATPase α-subunit N-terminus with negatively-charged lipid head groups of the neighbouring cytoplasmic membrane surface as the origin of the electrostatic interaction stabilising the E2 state.

Bioinformatics approaches for classification and investigation of the evolution of the Na/K-ATPase alpha-subunit

BACKGROUND

Na,K-ATPase is a key protein in maintaining membrane potential that has numerous additional cellular functions. Its catalytic subunit (α), found in a wide range of organisms from prokaryotes to complex eukaryote. Several studies have been done to identify the functions as well as determining the evolutionary relationships of the α-subunit. However, a survey of a larger collection of protein sequences according to sequences similarity and their attributes is very important in revealing deeper evolutionary relationships and identifying specific amino acid differences among evolutionary groups that may have a functional role.

RESULTS

In this study, 753 protein sequences using phylogenetic tree classification resulted in four groups: prokaryotes (I), fungi and various kinds of Protista and some invertebrates (II), the main group of invertebrates (III), and vertebrates (IV) that was consisted with species tree. The percent of sequences that acquired a specific motif for the α/β subunit assembly increased from group I to group IV. The vertebrate sequences were divided into four groups according to isoforms with each group conforming to the evolutionary path of vertebrates from fish to tetrapods. Data mining was used to identify the most effective attributes in classification of sequences. Using 1252 attributes extracted from the sequences, the decision tree classified them in five groups: Protista, prokaryotes, fungi, invertebrates and vertebrates. Also, vertebrates were divided into four subgroups (isoforms). Generally, the count of different dipeptides and amino acid ratios were the most significant attributes for grouping. Using alignment of sequences identified the effective position of the respective dipeptides in the separation of the groups. So that 208GC is apparently involved in the separation of vertebrates from the four other organism groups, and 41DH, 431FK, and 451KC were involved in separation vertebrate isoform types.

CONCLUSION

The application of phylogenetic and decision tree analysis for Na,K-ATPase, provides a better understanding of the evolutionary changes according to the amino acid sequence and its related properties that could lead to the identification of effective attributes in the separation of sequences in different groups of phylogenetic tree. In this study, key evolution-related dipeptides are identified which can guide future experimental studies.

Structural basis for gating mechanism of the human sodium-potassium pump

P2-type ATPase sodium-potassium pumps (Na⁺/K⁺-ATPases) are ion-transporting enzymes that use ATP to transport Na⁺ and K⁺ on opposite sides of the lipid bilayer against their electrochemical gradients to maintain ion concentration gradients across the membranes in all animal cells. Despite the available molecular architecture of the Na⁺/K⁺-ATPases, a complete molecular mechanism by which the Na⁺ and K⁺ ions access into and are released from the pump remains unknown. Here we report five cryo-electron microscopy (cryo-EM) structures of the human alpha3 Na⁺/K⁺-ATPase in its cytoplasmic side-open (E1), ATP-bound cytoplasmic side-open (E1•ATP), ADP-AlF₄⁻ trapped Na⁺-occluded (E1•P-ADP), BeF₃⁻ trapped exoplasmic side-open (E2P) and MgF₄²⁻ trapped K⁺-occluded (E2•P_i) states. Our work reveals the atomically resolved structural detail of the cytoplasmic gating mechanism of the Na⁺/K⁺-ATPase.

Through cryo-EM analysis, here authors reveal conformational rearrangements that are critical for the gating mechanism of the human alpha3 Na+/K+−ATPase

Cryo-EM structures of recombinant human sodium-potassium pump determined in three different states

Sodium-Potassium Pump (Na⁺/K⁺-ATPase, NKA) is an ion pump that generates an electrochemical gradient of sodium and potassium ions across the plasma membrane by hydrolyzing ATP. During each Post-Albers cycle, NKA exchanges three cytoplasmic sodium ions for two extracellular potassium ions through alternating changes between the E1 and E2 states. Hitherto, several steps remained unknown during the complete working cycle of NKA. Here, we report cryo-electron microscopy (cryo-EM) structures of recombinant human NKA (hNKA) in three distinct states at 2.7–3.2 Å resolution, representing the E1·3Na and E1·3Na·ATP states with cytosolic gates open and the basic E2·[2K] state, respectively. This work provides the insights into the cytoplasmic Na⁺ entrance pathway and the mechanism of cytoplasmic gate closure coupled with ATP hydrolysis, filling crucial gaps in the structural elucidation of the Post-Albers cycle of NKA.

Sodium-Potassium Pump (Na⁺/K⁺-ATPase, NKA) generates an electrochemical gradient of sodium and potassium ions across the plasma membrane by hydrolyzing ATP. Here, the authors report structures of human NKA providing insight into the cytoplasmic Na⁺ entrance and the cytoplasmic gate closure coupled to ATP hydrolysis.

Order-disorder transitions of cytoplasmic N-termini in the mechanisms of P-type ATPases

Membrane protein structure and function are modulated via interactions with their lipid environment. This is particularly true for integral membrane pumps, the P-type ATPases. These ATPases play vital roles in cell physiology, where they are associated with the transport of cations and lipids, thereby generating and maintaining crucial (electro-)chemical potential gradients across the membrane. Several pumps (Na⁺, K⁺-ATPase, H⁺, K⁺-ATPase and the plasma membrane Ca²⁺-ATPase) which are located in the asymmetric animal plasma membrane have been found to possess polybasic (lysine-rich) domains on their cytoplasmic surfaces, which are thought to act as phosphatidylserine (PS) binding domains. In contrast, the sarcoplasmic reticulum Ca²⁺-ATPase, located within an intracellular organelle membrane, does not possess such a domain. Here we focus on the lysine-rich N-termini of the plasma-membrane-bound Na⁺, K⁺- and H⁺, K⁺-ATPases. Synthetic peptides corresponding to the N-termini of these proteins were found, via quartz crystal microbalance and circular dichroism measurements, to interact via an electrostatic interaction with PS-containing membranes, thereby undergoing an increase in helical or other secondary structure content. As well as influencing ion pumping activity, it is proposed that this interaction could provide a mechanism for sensing the lipid asymmetry of the plasma membrane, which changes drastically when a cell undergoes apoptosis, i.e. programmed cell death. Thus, polybasic regions of plasma membrane-bound ion pumps could potentially perform the function of a "death sensor", signalling to a cell to reduce pumping activity and save energy.

Structure and Mechanism of P-Type ATPase Ion Pumps

P-type ATPases are found in all kingdoms of life and constitute a wide range of cation transporters, primarily for H⁺, Na⁺, K⁺, Ca²⁺, and transition metal ions such as Cu(I), Zn(II), and Cd(II). They have been studied through a wide range of techniques, and research has gained very significant insight on their transport mechanism and regulation. Here, we review the structure, function, and dynamics of P2-ATPases including Ca²⁺-ATPases and Na,K-ATPase. We highlight mechanisms of functional transitions that are associated with ion exchange on either side of the membrane and how the functional cycle is regulated by interaction partners, autoregulatory domains, and off-cycle states. Finally, we discuss future perspectives based on emerging techniques and insights.

Polarity of the ATP binding site of the Na+,K+-ATPase, gastric H+,K+-ATPase and sarcoplasmic reticulum Ca2+-ATPase

A fluorescence ratiometric method utilizing the probe eosin Y is presented for estimating the ATP binding site polarity of P-type ATPases in different conformational states. The method has been calibrated by measurements in a series of alcohols and tested using complexation of eosin Y with methyl-β-cyclodextrin. The results obtained with the Na⁺,K⁺-, H⁺,K⁺- and sarcoplasmic reticulum Ca²⁺-ATPases indicate that the ATP binding site, to which eosin is known to bind, is significantly more polar in the case of the Na⁺,K⁺- and H⁺,K⁺-ATPases compared to the Ca²⁺-ATPase. This result was found to be consistent with docking calculations of eosin with the E2 conformational state of the Na⁺,K⁺-ATPase and the Ca²⁺-ATPase. Fluorescence experiments showed that eosin binds significantly more strongly to the E1 conformation of the Na⁺,K⁺-ATPase than the E2 conformation, but in the case of the Ca²⁺-ATPase both fluorescence experiments and docking calculations showed no significant difference in binding affinity between the two conformations. This result could be due to the fact that, in contrast to the Na⁺,K⁺- and H⁺,K⁺-ATPases, the E2-E1 transition of the Ca²⁺-ATPase does not involve the movement of a lysine-rich N-terminal tail which may affect the overall enzyme conformation. Consistent with this hypothesis, the eosin affinity of the E1 conformation of the Na⁺,K⁺-ATPase was significantly reduced after N-terminal truncation. It is suggested that changes in conformational entropy of the N-terminal tail of the Na⁺, K⁺- and the H⁺,K⁺-ATPases during the E2-E1 transition could affect the thermodynamic stability of the E1 conformation and hence its ATP binding affinity.

Penetration of phospholipid membranes by poly-l-lysine depends on cholesterol and phospholipid composition

Clusters of positively-charged basic amino acid residues, particularly lysine, are known to promote the interaction of many peripheral membrane proteins with the cytoplasmic surface of the plasma membrane via electrostatic interactions. In this work, cholesterol's effects on the interaction between lysine residues and membranes have been studied. Using poly-l-lysine (PLL) and vesicles as models to mimic the interaction between lysine-rich protein domains and the plasma membrane, light scattering measurements indicated cholesterol enhanced the electrostatic interaction through indirectly affecting the negatively charged phospholipid dioleoylphosphatidylserine, DOPS. Addition of PLL to lipid vesicles containing DOPS showed an initial increase in static light scattering (SLS), attributed to binding of PLL to the vesicle surface, followed by a slower continuously declining SLS signal, which, from comparison with fluorescent dye leakage studies could be attributed to vesicle lysis. Although electrostatic interactions between PLL and the membrane were not necessary for penetration to occur, cholesterol promoted membrane disruption of negatively charged vesicles, possibly by increasing the electrostatic interactions between PLL and the membrane. In contrast, cholesterol lowered the susceptibility of uncharged vesicles (formed using dioleoylphosphatidylcholine, DOPC) to PLL penetration. This can be explained by the absence of electrostatic interactions and cholesterol's known ability to increase membrane thickness and mechanical strength. Thus, the ability of cationic peptides to penetrate membranes including cholesterol is likely to depend on the membrane's PS:PC ratio.

External Ion Access in the Na/K Pump: Kinetics of Na+, K+, and Quaternary Amine Interaction

Na/K pumps build essential ion gradients across the plasmalemma of animal cells by coupling the extrusion of three Na⁺, with the import of two K⁺ and the hydrolysis of one ATP molecule. The mechanisms of selectivity and competition between Na⁺, K⁺, and inhibitory amines remain unclear. We measured the effects of external tetrapropylammonium (TPA⁺) and ethylenediamine (EDA²⁺) on three different Na/K pump transport modes in voltage-clamped Xenopus oocytes: 1) outward pump current (I_P), 2) passive inward H⁺ current at negative voltages without Na⁺ or K⁺ (I_H), and 3) transient charge movement reporting the voltage-dependent extracellular binding/release of Na⁺ (Q_Na). Both amines competed with K⁺ to inhibit I_P. TPA⁺ inhibited I_H without competing with H⁺, whereas EDA²⁺ did not alter I_H at pH 7.6. TPA⁺ competed with Na⁺ in Q_Na measurements, reducing Na⁺-apparent affinity, evidenced by a ∼-75 mV shift in the charge-voltage curve (at 20 mM TPA⁺) without reduction of the total charge moved (Q_tot). In contrast, EDA²⁺ and K⁺ did not compete with Na⁺ to inhibit Q_Na; both reduced Q_tot without decreasing Na⁺-apparent affinity. EDA²⁺ (15 mM) right-shifted the charge-voltage curve by ∼+50 mV. Simultaneous occlusion of EDA²⁺ and Na⁺ by an E2P conformation unable to reach E1P was demonstrated by voltage-clamp fluorometry. Trypsinolysis experiments showed that EDA²⁺-bound pumps are much more proteolysis-resistant than Na⁺-, K⁺-, or TPA⁺-bound pumps, therefore uncovering unique EDA²⁺-bound conformations. K⁺ effects on Q_Na and I_H were also evaluated in pumps inhibited with beryllium fluoride, a phosphate mimic. K⁺ reduced Q_tot without shifting the charge-voltage curve, indicating noncompetitive effects, and partially inhibited I_H to the same extent as TPA⁺ in non-beryllium-fluorinated pumps. These results demonstrate that K⁺ interacts with beryllium-fluorinated pumps inducing conformational changes that alter Q_Na and I_H, suggesting that there are two external access pathways for proton transport by I_H.

Cholesterol depletion inhibits Na+,K+-ATPase activity in a near-native membrane environment

Cholesterol's effects on Na+,K+-ATPase reconstituted in phospholipid vesicles have been extensively studied. However, previous studies have reported both cholesterol-mediated stimulation and inhibition of Na+,K+-ATPase activity. Here, using partial reaction kinetics determined via stopped-flow experiments, we studied cholesterol's effect on Na+,K+-ATPase in a near-native environment in which purified membrane fragments were depleted of cholesterol with methyl-β-cyclodextrin (mβCD). The mβCD-treated Na+,K+-ATPase had significantly reduced overall activity and exhibited decreased observed rate constants for ATP phosphorylation (ENa3+ → E2P, i.e. phosphorylation by ATP and Na+ occlusion from the cytoplasm) and K+ deocclusion with subsequent intracellular Na+ binding (E2K2+ → E1Na3+). However, cholesterol depletion did not affect the observed rate constant for K+ occlusion by phosphorylated Na+,K+-ATPase on the extracellular face and subsequent dephosphorylation (E2P → E2K2+). Thus, partial reactions involving cation binding and release at the protein's intracellular side were most dependent on cholesterol. Fluorescence measurements with the probe eosin indicated that cholesterol depletion stabilizes the unphosphorylated E2 state relative to E1, and the cholesterol depletion-induced slowing of ATP phosphorylation kinetics was consistent with partial conversion of Na+,K+-ATPase into the E2 state, requiring a slow E2 → E1 transition before the phosphorylation. Molecular dynamics simulations of Na+,K+-ATPase in membranes with 40 mol % cholesterol revealed cholesterol interaction sites that differ markedly among protein conformations. They further indicated state-dependent effects on membrane shape, with the E2 state being likely disfavored in cholesterol-rich bilayers relative to the E1P state because of a greater hydrophobic mismatch. In summary, cholesterol extraction from membranes significantly decreases Na+,K+-ATPase steady-state activity.

Glutathionylation of Na,K-ATPase Alpha-Subunit Alters Enzyme Conformation and Sensitivity to Trypsinolysis

We found earlier that Na,K-ATPase is purified from duck salt glands in partially glutathionylated state (up to 13 of the 23 cysteine residues of the Na,K-ATPase catalytic α-subunit can be S-glutathionylated). To determine the effect of glutathionylation on the enzyme conformation, we have analyzed the products of trypsinolysis of Na,K-ATPase α-subunit in different conformations with different extent of glutathionylation. Incubation of the protein in the E1 conformation with trypsin produced a large fragment with a molecular mass (MM) of 80 kDa with the following formation of smaller fragments with MM 40, 35.5, and 23 kDa. Tryptic digestion of Na,K-ATPase in the E2 conformation also resulted in the generation of the fragments with MM 40, 35.5, and 23 kDa. Deglutathionylation of Na,K-ATPase α-subunit increases the rate of proteolysis of the enzyme in both E1 and E2 conformations. The pattern of tryptic digestion of the α-subunit in E2 conformation additionally glutathionylated with oxidized glutathione is similar to that of partially deglutathionylated Na,K-ATPase. The pattern of tryptic digestion of the additionally glutathionylated α-subunit in E1 conformation is similar to that of the native enzyme. The highest rate of trypsinolysis was observed for the α-subunit in complex with ouabain (E2-OBN conformation). Additional glutathionylation increased the content of high-molecular-weight fragments among the digestion products, as compared to the native and deglutathionylated enzymes. The data obtained were confirmed using molecular modeling that revealed that number of sites accessible for trypsinolysis is higher in the E2P-OBN conformation than in the E1- and E2-conformations and that glutathionylation decreases the number of sites accessible for trypsin. Therefore, glutathionylation affects enzyme conformation and its sensitivity to trypsinolysis. The mechanisms responsible for the changes in the Na,K-ATPase sensitivity to trypsinolysis depending on the level of enzyme glutathionylation and increase in the enzyme sensitivity to proteolysis upon its binding to ouabain, as well as physiological role of these phenomena are discussed.

Evolutionary Analysis of the Lysine-Rich N-terminal Cytoplasmic Domains of the Gastric H+,K+-ATPase and the Na+,K+-ATPase

The catalytic α-subunits of both the Na⁺,K⁺-ATPase and the gastric H⁺,K⁺-ATPase possess lysine-rich N-termini which project into the cytoplasm. Due to conflicting experimental results, it is currently unclear whether the N-termini play a role in ion pump function or regulation, and, if they do, by what mechanism. Comparison of the lysine frequencies of the N-termini of both proteins with those of all of their extramembrane domains showed that the N-terminal lysine frequencies are far higher than one would expect simply from exposure to the aqueous solvent. The lysine frequency was found to vary significantly between different vertebrate classes, but this is due predominantly to a change in N-terminal length. As evidenced by a comparison between fish and mammals, an evolutionary trend towards an increase of the length of the N-terminus of the H⁺,K⁺-ATPase on going from an ancestral fish to mammals could be identified. This evolutionary trend supports the hypothesis that the N-terminus is important in ion pump function or regulation. In placental mammals, one of the lysines is replaced by serine (Ser-27), which is a target for protein kinase C. In most other animal species, a lysine occupies this position and hence no protein kinase C target is present. Interaction with protein kinase C is thus not the primary role of the lysine-rich N-terminus. The disordered structure of the N-terminus may, via increased flexibility, facilitate interaction with another binding partner, e.g. the surrounding membrane, or help to stabilise particular enzyme conformations via the increased entropy it produces.

Steady-state analysis of enzymes with non-Michaelis-Menten kinetics: The transport mechanism of Na+/K+-ATPase

Procedures to define kinetic mechanisms from catalytic activity measurements that obey the Michaelis-Menten equation are well established. In contrast, analytical tools for enzymes displaying non-Michaelis-Menten kinetics are underdeveloped, and transient-state measurements, when feasible, are therefore preferred in kinetic studies. Of note, transient-state determinations evaluate only partial reactions, and these might not participate in the reaction cycle. Here, we provide a general procedure to characterize kinetic mechanisms from steady-state determinations. We described non-Michaelis-Menten kinetics with equations containing parameters equivalent to k_cat and K_m and modeled the underlying mechanism by an approach similar to that used under Michaelis-Menten kinetics. The procedure enabled us to evaluate whether Na⁺/K⁺-ATPase uses the same sites to alternatively transport Na⁺ and K⁺ This ping-pong mechanism is supported by transient-state studies but contradicted to date by steady-state analyses claiming that the release of one cationic species as product requires the binding of the other (ternary-complex mechanism). To derive robust conclusions about the Na⁺/K⁺-ATPase transport mechanism, we did not rely on ATPase activity measurements alone. During the catalytic cycle, the transported cations become transitorily occluded (i.e. trapped within the enzyme). We employed radioactive isotopes to quantify occluded cations under steady-state conditions. We replaced K⁺ with Rb⁺ because ⁴²K⁺ has a short half-life, and previous studies showed that K⁺- and Rb⁺-occluded reaction intermediates are similar. We derived conclusions regarding the rate of Rb⁺ deocclusion that were verified by direct measurements. Our results validated the ping-pong mechanism and proved that Rb⁺ deocclusion is accelerated when Na⁺ binds to an allosteric, nonspecific site, leading to a 2-fold increase in ATPase activity.

Electrostatic Stabilization Plays a Central Role in Autoinhibitory Regulation of the Na+,K+-ATPase

The Na⁺,K⁺-ATPase is present in the plasma membrane of all animal cells. It plays a crucial role in maintaining the Na⁺ and K⁺ electrochemical potential gradients across the membrane, which are essential in numerous physiological processes, e.g., nerve, muscle, and kidney function. Its cellular activity must, therefore, be under tight metabolic control. Consideration of eosin fluorescence and stopped-flow kinetic data indicates that the enzyme's E2 conformation is stabilized by electrostatic interactions, most likely between the N-terminus of the protein's catalytic α-subunit and the adjacent membrane. The electrostatic interactions can be screened by increasing ionic strength, leading to a more evenly balanced equilibrium between the E1 and E2 conformations. This represents an ideal situation for effective regulation of the Na⁺,K⁺-ATPase's enzymatic activity, because protein modifications, which perturb this equilibrium in either direction, can then easily lead to activation or inhibition. The effect of ionic strength on the E1:E2 distribution and the enzyme's kinetics can be mathematically described by the Gouy-Chapman theory of the electrical double layer. Weakening of the electrostatic interactions and a shift toward E1 causes a significant increase in the rate of phosphorylation of the enzyme by ATP. Electrostatic stabilization of the Na⁺,K⁺-ATPase's E2 conformation, thus, could play an important role in regulating the enzyme's physiological catalytic turnover.

Protein Interaction and Na/K-ATPase-Mediated Signal Transduction

The Na/K-ATPase (NKA), or Na pump, is a member of the P-type ATPase superfamily. In addition to pumping ions across cell membrane, it is engaged in assembly of multiple protein complexes in the plasma membrane. This assembly allows NKA to perform many non-pumping functions including signal transduction that are important for animal physiology and disease progression. This article will focus on the role of protein interaction in NKA-mediated signal transduction, and its potential utility as target for developing new therapeutics.

In silico identification and characterization of the ion transport specificity for P-type ATPases in the Mycobacterium tuberculosis complex

Background

P-type ATPases hydrolyze ATP and release energy that is used in the transport of ions against electrochemical gradients across plasma membranes, making these proteins essential for cell viability. Currently, the distribution and function of these ion transporters in mycobacteria are poorly understood.

Results

In this study, probabilistic profiles were constructed based on hidden Markov models to identify and classify P-type ATPases in the Mycobacterium tuberculosis complex (MTBC) according to the type of ion transported across the plasma membrane. Topology, hydrophobicity profiles and conserved motifs were analyzed to correlate amino acid sequences of P-type ATPases and ion transport specificity. Twelve candidate P-type ATPases annotated in the M. tuberculosis H37Rv proteome were identified in all members of the MTBC, and probabilistic profiles classified them into one of the following three groups: heavy metal cation transporters, alkaline and alkaline earth metal cation transporters, and the beta subunit of a prokaryotic potassium pump. Interestingly, counterparts of the non-catalytic beta subunits of Hydrogen/Potassium and Sodium/Potassium P-type ATPases were not found.

Conclusions

The high content of heavy metal transporters found in the MTBC suggests that they could play an important role in the ability of M. tuberculosis to survive inside macrophages, where tubercle bacilli face high levels of toxic metals. Finally, the results obtained in this work provide a starting point for experimental studies that may elucidate the ion specificity of the MTBC P-type ATPases and their role in mycobacterial infections.

Distribution of Na/K-ATPase alpha 3 isoform, a sodium-potassium P-type pump associated with rapid-onset of dystonia parkinsonism (RDP) in the adult mouse brain

The Na(+)/K(+)-ATPase1 alpha subunit 3 (ATP1α(3)) is one of many essential components that maintain the sodium and potassium gradients across the plasma membrane in animal cells. Mutations in the ATP1A3 gene cause rapid-onset of dystonia parkinsonism (RDP), a rare movement disorder characterized by sudden onset of dystonic spasms and slowness of movement. To achieve a better understanding of the pathophysiology of the disease, we used immunohistochemical approaches to describe the regional and cellular distribution of ATP1α(3) in the adult mouse brain. Our results show that localization of ATP1α(3) is restricted to neurons, and it is expressed mostly in projections (fibers and punctuates), but cell body expression is also observed. We found high expression of ATP1α(3) in GABAergic neurons in all nuclei of the basal ganglia (striatum, globus pallidus, subthalamic nucleus, and substantia nigra), which is a key circuitry in the fine movement control. Several thalamic nuclei structures harboring connections to and from the cortex expressed high levels of the ATP1α(3) isoform. Other structures with high expression of ATP1α(3) included cerebellum, red nucleus, and several areas of the pons (reticulotegmental nucleus of pons). We also found high expression of ATP1α(3) in projections and cell bodies in hippocampus; most of these ATP1α(3)-positive cell bodies showed colocalization to GABAergic neurons. ATP1α(3) expression was not significant in the dopaminergic cells of substantia nigra. In conclusion, and based on our data, ATP1α(3) is widely expressed in neuronal populations but mainly in GABAergic neurons in areas and nuclei related to movement control, in agreement with RDP symptoms.

A structural overview of the plasma membrane Na+,K+-ATPase and H+-ATPase ion pumps

Plasma membrane ATPases are primary active transporters of cations that maintain steep concentration gradients. The ion gradients and membrane potentials derived from them form the basis for a range of essential cellular processes, in particular Na(+)-dependent and proton-dependent secondary transport systems that are responsible for uptake and extrusion of metabolites and other ions. The ion gradients are also both directly and indirectly used to control pH homeostasis and to regulate cell volume. The plasma membrane H(+)-ATPase maintains a proton gradient in plants and fungi and the Na(+),K(+)-ATPase maintains a Na(+) and K(+) gradient in animal cells. Structural information provides insight into the function of these two distinct but related P-type pumps.

Neutralization of the charge on Asp 369 of Na+,K+-ATPase triggers E1 <--> E2 conformational changes

This work investigates the role of charge of the phosphorylated aspartate, Asp(369), of Na(+),K(+)-ATPase on E(1) <--> E(2) conformational changes. Wild type (porcine alpha(1)/His(10)-beta(1)), D369N/D369A/D369E, and T212A mutants were expressed in Pichia pastoris, labeled with fluorescein 5'-isothiocyanate (FITC), and purified. Conformational changes of wild type and mutant proteins were analyzed using fluorescein fluorescence (Karlish, S. J. (1980) J. Bioenerg. Biomembr. 12, 111-136). One central finding is that the D369N/D369A mutants are strongly stabilized in E(2) compared with wild type and D369E or T212A mutants. Stabilization of E(2)(Rb) is detected by a reduced K(0.5)Rb for the Rb(+)-induced E(1) <--> E(2)(2Rb) transition. The mechanism involves a greatly reduced rate of E(2)(2Rb) --> E(1)Na with no effect on E(1) --> E(2)(2Rb). Lowering the pH from 7.5 to 5.5 strongly stabilizes wild type in E(2) but affects the D369N mutant only weakly. Thus, this "Bohr" effect of pH on E(1) <--> E(2) is due largely to protonation of Asp(369). Two novel effects of phosphate and vanadate were observed with the D369N/D369A mutants as follows. (a) E(1) --> E(2).P is induced by phosphate without Mg(2+) ions by contrast with wild type, which requires Mg(2+). (b) Both phosphate and vanadate induce rapid E(1) --> E(2) transitions compared with slow rates for the wild type. With reference to crystal structures of Ca(2+)-ATPase and Na(+),K(+)-ATPase, negatively charged Asp(369) favors disengagement of the A domain from N and P domains (E(1)), whereas the neutral D369N/D369A mutants favor association of the A domain (TGES sequence) with P and N domains (E(2)). Changes in charge interactions of Asp(369) may play an important role in triggering E(1)P(3Na) <--> E(2)P and E(2)(2K) --> E(1)Na transitions in native Na(+),K(+)-ATPase.

Specific Sites in the Cytoplasmic N Terminus Modulate Conformational Transitions of the Na,K-ATPase

The cytoplasmic N terminus of the Na,K-ATPase is a highly charged and flexible structure that comprises three predicted helical regions including H1 spanning residues 27 to 33 and H2 spanning residues 42 to 50. Previous deletion mutagenesis experiments showed that deletion of residues up to and including most of H2 shifts the E(1)/E(2) conformational equilibrium toward E(1). The present study describes a clustered charge-to-alanine mutagenesis approach designed to delineate specific sites within the N terminus that modulate the steady-state E(1) <--> E(2) and E(1)P <--> E(2)P poise. Criteria to assess shifts in poise include (i) sensitivity to inhibition by inorganic orthovanadate to assess overall poise; (ii) K(+)-sensitivity of Na-ATPase measured at micromolar ATP to assess changes in the E(2)(K) + ATP --> E(1) x ATP + K(+) rate; (iii) K'(ATP) for low-affinity ATP binding at the latter step; (iv) overall catalytic turnover, and (v) the E(1)P --> E(2)P transition. The results of alanine replacements in H1 (31KKE) suggest that this site stabilizes E(2)P and to a lesser extent E(2). In H2, residues within 47HRK have a role in stabilizing E(2) but not E(2)P as revealed with double mutants 31KKE --> AAA/47H --> A and 31KKE --> AAA/47HRK --> AAA. Taken together, these observations suggest that sites 31KKE in H1 and 47HRK in H2 have distinct roles in modulating the enzyme's conformational transitions during the catalytic cycle of the enzyme.

Stabilization of trypsin by association to plasma membranes: Implications for tryptic cleavage of membrane-bound Na,K-ATPase

Tryptic cleavage has been a potential method for studying the structure and mechanism of many membrane transport proteins. Here, we report tight association of trypsin to pig kidney plasma membranes enriched in Na,K-ATPase. Trypsin also associated with protein-free vesicles prepared from plasma membrane lipids. Membrane-associated trypsin was found to be highly resistant to autolysis and insensitive to inhibition by PMSF. Na,K-ATPase substrate ions differentially influenced the level of trypsin membrane association. Thus, NaCl significantly increased trypsin membrane association compared to KCl. The ions seem to exert direct effects on the membrane independent of their effects on protein conformation. Bicarbonate anions, which detach peripheral membrane proteins, efficiently released trypsin from the membrane. Trypsin membrane association was found to enhance the cleavage of the Na,K-ATPase gamma-subunit. Comparison between membranes from shark rectal gland and pig kidney showed that trypsin association was significantly higher in the former. This was found to be partly due to the presence of higher cholesterol levels in the membrane. In conclusion, the differential membrane association of trypsin may affect the outcome of proteolytic cleavage of membrane-bound proteins.

The identification of the sodium pump

The identification of the sodium potassium pump as a Na+, K+-ATPase is described.

The crystallographic structure of Na,K-ATPase N-domain at 2.6A resolution

The structure of the N-domain of porcine alpha(2) Na,K-ATPase was determined crystallographically to 3.2A resolution by isomorphous heavy-atom replacement using a single mercury derivative. The structure was finally refined against 2.6A resolution synchrotron data. The domain forms a seven-stranded antiparallel beta-sheet with two additional beta-strands forming a hairpin and five alpha-helices. Approximately 75% of the residues were superimposable with residues from the structure of Ca-ATPase N-domain, and a structure-based sequence alignment is presented. The positions of key residues are discussed in relation to the pattern of hydrophobicity, charge and sequence conservation of the molecular surface. The structure of a hexahistidine tag binding to nickel ions is presented.

Inactivation of Na,K-ATPase following Co(NH3)4ATP binding at a low affinity site in the protomeric enzyme unit

The Na(+)-dependent or E1 stages of the Na,K-ATPase reaction require a few micromolar ATP, but submillimolar concentrations are needed to accelerate the K(+)-dependent or E2 half of the cycle. Here we use Co(NH(3))(4)ATP as a tool to study ATP sites in Na,K-ATPase. The analogue inactivates the K(+) phosphatase activity (an E2 partial reaction) and the Na,K-ATPase activity in parallel, whereas ATP-[(3)H]ADP exchange (an E1 reaction) is affected less or not at all. Although the inactivation occurs as a consequence of low affinity Co(NH(3))(4)ATP binding (K(D) approximately 0.4-0.6 mm), we can also measure high affinity equilibrium binding of Co(NH(3))(4)[(3)H]ATP (K(D) = 0.1 micro m) to the native enzyme. Crucially, we find that covalent enzyme modification with fluorescein isothiocyanate (which blocks E1 reactions) causes little or no effect on the affinity of the binding step preceding Co(NH(3))(4)ATP inactivation and only a 20% decrease in maximal inactivation rate. This suggests that fluorescein isothiocyanate and Co(NH(3))(4)ATP bind within different enzyme pockets. The Co(NH(3))(4)ATP enzyme was solubilized with C(12)E(8) to a homogeneous population of alphabeta protomers, as verified by analytical ultracentrifugation; the solubilization did not increase the Na,K-ATPase activity of the Co(NH(3))(4)ATP enzyme with respect to parallel controls. This was contrary to the expectation for a hypothetical (alphabeta)(2) membrane dimer with a single ATP site per protomer, with or without fast dimer/protomer equilibrium in detergent solution. Besides, the solubilized alphabeta protomer could be directly inactivated by Co(NH(3))(4)ATP, to less than 10% of the control Na,K-ATPase activity. This suggests that the inactivation must follow Co(NH(3))(4)ATP binding at a low affinity site in every protomeric unit, thus still allowing ATP and ADP access to phosphorylation and high affinity ATP sites.

Structural basis for alpha1 versus alpha2 isoform-distinct behavior of the Na,K-ATPase

We showed earlier that the kinetic behavior of the alpha2 isoform of the Na,K-ATPase differs from the ubiquitous alpha1 isoform primarily by a shift in the steady-state E(1)/E(2) equilibrium of alpha2 in favor of E(1) form(s). The aim of the present study was to identify regions of the alpha chain that confer the alpha1/alpha2 distinct behavior using a mutagenesis and chimera approach. Criteria to assess shifts in conformational equilibrium included (i) K(+) sensitivity of Na-ATPase measured at micromolar ATP, under which condition E(2)(K(+)) --> E(1) + K(+) becomes rate-limiting, (ii) changes in K'(ATP) for low affinity ATP binding, (iii) vanadate sensitivity of Na,K-ATPase activity, and (iv) the rate of the partial reaction E(1)P --> E(2)P. We first confirmed that interactions between the cytoplasmic domains of alpha2 that modulate conformational shifts are fundamentally similar to those of alpha1, suggesting that the predilection of alpha2 for E(1) state(s) is due to differences in primary structure of the two isoforms. Kinetic behavior of the alpha1/alpha2 chimeras indicates that the difference in E(1)/E(2) poise of the two isoforms cannot be accounted for by their notably distinct N termini, but rather by the front segment extending from the cytoplasmic N terminus to the C-terminal end of the extracellular loop between transmembranes 3 and 4, with a lesser contribution of the alpha1/alpha2 divergent portion within the M4-M5 loop near the ATP binding domain. In addition, we show that the E(1) shift of alpha2 results primarily from differences in the conformational transition of the dephosphoenzyme, (E(2)(K(+)) --> E(1) + K(+)), rather than phosphoenzyme (E(1)P --> E(2)P).

Conformational dynamics of the Na+/K+-ATPase probed by voltage clamp fluorometry

The method of voltage clamp fluorometry combined with site-directed fluorescence labeling was used to detect local protein motions of the fully active Na(+)K(+)-ATPase in real time under physiological conditions. Because helix M5 extends from the cytoplasmic site of ATP hydrolysis into the cation binding region, we chose the extracellular M5-M6 loop of the sheep alpha(1)-subunit for the insertion of cysteine residues to identify reporter positions for conformational rearrangements during the catalytic cycle. After expression of the single cysteine mutants in Xenopus oocytes and covalent attachment of tetramethylrhodamine-6-maleimide, only mutant N790C reported molecular rearrangements of the M5-M6 loop by showing large, ouabain-sensitive fluorescence changes ( approximately 5%) on addition of extracellular K(+). When the enzyme was subjected to voltage jumps under Na(+)Na(+)-exchange conditions, we observed fluorescence changes that directly correlated to transient charge movements originating from the E(1)P-E(2)P transition of the transport cycle. The voltage jump-induced fluorescence changes and transient currents were abolished after replacement of Na(+) by tetraethylammonium or on addition of ouabain, showing that conformational flexibility is impaired under these conditions. Voltage-dependent fluorescence changes could also be observed in the presence of subsaturating K(+) concentrations. This allowed to monitor the time course of voltage-dependent relaxations into a new stationary distribution of states under turnover conditions, showing the acceleration of relaxation kinetics with increasing K(+) concentrations. As a result, the stationary distribution between E(1) and E(2) states and voltage-dependent relaxation times can be determined at any time and membrane potential under Na(+)Na(+) exchange as well as Na(+)K(+) turnover conditions.

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.

New insights into the role of the N terminus in conformational transitions of the Na,K-ATPase

The deletion of 32 residues from the N terminus of the alpha1 catalytic subunit of the rat Na,K-ATPase (mutant alpha1M32) shifts the E(1)/E(2) conformational equilibrium toward E(1), and the combination of this deletion with mutation E233K in the M2-M3 loop acts synergistically to shift the conformation further toward E(1) (Boxenbaum, N., Daly, S. E., Javaid, Z. Z., Lane, L. K., and Blostein, R. (1998) J. Biol. Chem. 273, 23086-23092). To delimit the region of the cytoplasmic N terminus involved in these interactions, the consequences of a series of N-terminal deletions of alpha1 beyond Delta32 were evaluated. Criteria to assess shifts in conformational equilibrium were based on effects of perturbation of the entire catalytic cycle ((i) sensitivity to vanadate inhibition, (ii) K(+) sensitivity of Na-ATPase measured at micromolar ATP, (iii) changes in K'(ATP), and (iv) catalytic turnover), as well as estimates of the rates of the conformational transitions of phospho- and dephosphoenzyme (E(1)P --> E(2)P and E(2)(K(+)) --> E(1) + K(+)). The results show that, compared with alpha1M32, the deletion of up to 40 residues (alpha1M40) further shifts the poise toward E(1). Remarkably, further deletions (mutants alpha1M46, alpha1M49, and alpha1M56) reverse the effect, such that these mutants increasingly resemble the wild type alpha1. These results suggest novel intramolecular interactions involving domains within the N terminus that impact the manner in which the N terminus/M2-M3 loop regulatory domain interacts with the M4-M5 catalytic loop to effect E(1) <--> E(2) transitions.

A hundred years of sodium pumping

This article gives a history of the evidence (a) that animal cell membranes contain pumps that expel sodium ions in exchange for potassium ions; (b) that the pump derives energy from the hydrolysis of ATP; (c) that it is thermodynamically reversible-artificially steep transmembrane ion gradients make it run backward synthesizing ATP from ADP and orthophosphate; (d) that its mechanism is a ping-pong one, in which phosphorylation of the pump by ATP is associated with an efflux of three sodium ions, and hydrolysis of the phosphoenzyme is associated with an influx of two potassium ions; (e) that each half of the working cycle involves both the transfer of a phosphate group and a conformational change-the phosphate transfer being associated with the occlusion of ions bound at one surface and the conformational change releasing the occluded ions at the opposite surface.

Dimethyl sulfoxide-induced conformational state of Na(+)/K(+)-ATPase studied by proteolytic cleavage

Effects of dimethyl sulfoxide (Me(2)SO) on substrate affinity for phosphorylation by inorganic phosphate, on phosphorylation by ATP in the absence of Na(+), and on ouabain binding to the free form of the Na(+)/K(+)-ATPase have been attributed to changes in solvation of the active site or Me(2)SO-induced changes in the structure of the enzyme. Here we used selective trypsin cleavage as a procedure to determine the conformations that the Na(+)/K(+)-ATPase acquires in Me(2)SO medium. In water or in Me(2)SO medium, Na(+)/K(+)-ATPase exhibited after partial proteolysis two distinct groups of fragments: (1) in the presence of 0.1 M Na(+) or 0.1 M Na(+) + 3 mM ADP (enzyme in the E1 state) cleavage produced a main fragment of about 76 kDa; and (2) in the presence of 20 mM K(+) (E2 state) a 58-kDa fragment plus two or three fragments of 39-41 kDa were obtained. Cleavage in Me(2)SO medium in the absence of Na(+) and K(+) exhibited the same breakdown pattern as that obtained in the presence of K(+), but a 43-kDa fragment was also observed. An increase in the K(+) concentration to 0.5 mM eliminated the 43-kDa fragment, while a 39- to 41-kDa doublet was accumulated. Both in water and in Me(2)SO medium, a strong enhancement of the 43-kDa band was observed in the presence of either P(i) + ouabain or vanadate, suggesting that the 43-kDa fragment is closely related to the conformation of the phosphorylated enzyme. These results indicate that Me(2)SO acts not only by promoting the release of water from the ATP site, but also by inducing a conformation closely related to the phosphorylated state, even when the enzyme is not phosphorylated.

The Occlusion of Rb(+) in the Na(+)/K(+)-ATPase. I. The identity of occluded states formed by the physiological or the direct routes: occlusion/deocclusion kinetics through the direct route

Occlusion of K(+) or its congeners in the Na(+)/K(+)-ATPase occurs after K(+)-dependent dephosphorylation (physiological route) or in media lacking ATP and Na(+) (direct route). The effects of P(i) or ATP on the kinetics of deocclusion of the K(+)-congener Rb(+) formed by each of the above mentioned routes was independent of the route of occlusion, which suggests that both routes lead to the same enzyme intermediate. The time course of occlusion via the direct route can be described by the sum of two exponential functions plus a small component of very high velocity. At equilibrium, occluded Rb(+) is a hyperbolic function of free [Rb(+)] suggesting that the direct route results in enzyme states holding either one or two occluded Rb(+). Release of occluded Rb(+) follows the sum of two decreasing exponential functions of time, corresponding to two phases with similar sizes. These phases are not caused by independent physical compartments. The rate constant of one of the phases is reduced up to 30 times by free Rb(+). When Rb(+) is the only pump ligand, the kinetics of occlusion and deocclusion through the direct route are consistent with an ordered-sequential process with additional independent step(s) interposed between the uptake or the release of each occluded Rb(+).

Three-dimensional structure of renal Na,K-ATPase from cryo-electron microscopy of two-dimensional crystals

The structure of Na, K-ATPase was determined by electron crystallography at 9.5 A from multiple small 2-D crystals induced in purified membranes isolated from the outer medulla of pig kidney. The density map shows a protomer stabilized in the E(2) conformation which extends approximately 65 A x 75 A x 150 A in the asymmetric unit of the P2 type unit cell. The alpha, beta, and gamma subunits were demonstrated in the membrane crystals with Western blotting and related to distinct domains in the density map. The alpha subunit corresponds to most of the density in the transmembrane region as well as the large hydrophilic headpiece on the cytoplasmic side of the membrane. The headpiece is divided into three separated domains, which are similar in overall shape to the domains of the calcium pump of the sarcoplasmic reticulum. One of these domains gives rise to a characteristic elongated projection onto the membrane plane while the putative nucleotide binding and phosphorylation domains form comparatively compact densities in the rest of the cytoplasmic part of the structure. Density on the extracellular face corresponds to the protein part of the beta subunit and is located as an extension of the transmembrane region perpendicular to the membrane plane. The structure of the lipid bilayer spanning part suggests the positions for the transmembrane helix from the beta subunit as well as the small gamma subunit present in this Na,K-ATPase. Two groups of ten helices from the catalytic alpha subunit corresponds to the remaining density in the transmembrane region. The present results demonstrate distinct similarities between the structure of the alpha subunit of Na,K-ATPase as determined here by cryo-electron microscopy and the reported X-ray structure of Ca-ATPase. However, conformational changes between the E(1) and E(2) forms are suggested by different relative positions of cytoplasmatic domains.

Role of conserved TGDGVND-loop in Mg2+ binding, phosphorylation, and energy transfer in Na,K-ATPase

In the P-domain, the 369-DKTGTLT and the 709-GDGVNDSPALKK segment are highly conserved during evolution of P-type E1-E2-ATPase pumps irrespective of their cation specificities. The focus of this article is on evaluation of the role of the amino acid residues in the P domain of the alpha subunit of Na,K-ATPase for the E1P[3Na]--> E2P[2Na] conversion, the K+-activated dephosphorylation, and the transmission of these changes to and from the cation binding sites. Mutations of residues in the TGDGVND loop show that Asp710 is essential, and Asn713 is important, for Mg2+ binding and formation of the high-energy MgE1P[3Na] intermediate. In contrast Asp710 and Asp713 do not contribute to Mg2+ binding in the E2P-ouabain complex. Transition to E2P thus involves a shift of Mg2+ coordination away from Asp710 and Asn713 and the two residues become more important for K+-activated hydrolysis of the acyl phosphate bond at Asp369. Transmission of structural changes between the P-domain and cation sites in the membrane domain is evaluated in light of the protein structure, and the information from proteolytic or metal-catalyzed cleavage and mutagenesis studies.

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.

Rate limitation of the Na(+),K(+)-ATPase pump cycle

The kinetics of Na(+)-dependent phosphorylation of the Na(+),K(+)-ATPase by ATP were investigated via the stopped-flow technique using the fluorescent label RH421 (saturating [ATP], [Na(+)], and [Mg(2+)], pH 7.4, and 24 degrees C). The well-established effect of buffer composition on the E(2)-E(1) equilibrium was used as a tool to investigate the effect of the initial enzyme conformation on the rate of phosphorylation of the enzyme. Preincubation of pig kidney enzyme in 25 mM histidine and 0.1 mM EDTA solution (conditions favoring E(2)) yielded a 1/tau value of 59 s(-1). Addition of MgCl(2) (5 mM), NaCl (2 mM), or ATP (2 mM) to the preincubation solution resulted in increases in 1/tau to values of 129, 167, and 143 s(-1), respectively. The increases can be attributed to a shift in the enzyme conformational equilibrium before phosphorylation from the E(2) state to an E(1) or E(1)-like state. The results thus demonstrate conclusively that the E(2) --> E(1) transition does in fact limit the rate of subsequent reactions of the pump cycle. Based on the experimental results, the rate constant of the E(2) --> E(1) transition under physiological conditions could be estimated to be approximately 65 s(-1) for pig kidney enzyme and 90 s(-1) for enzyme from rabbit kidney. Taking into account the rates of other partial reactions, computer simulations show these values to be consistent with the turnover number of the enzyme cycle (approximately 48 s(-1) and approximately 43 s(-1) for pig and rabbit, respectively) calculated from steady-state measurements. For enzyme of the alpha(1) isoform the E(2) --> E(1) conformational change is thus shown to be the major rate-determining step of the entire enzyme cycle.

Structure-function relationships of Na(+), K(+), ATP, or Mg(2+) binding and energy transduction in Na,K-ATPase

The focus of this article is on progress in establishing structure-function relationships through site-directed mutagenesis and direct binding assay of Tl(+), Rb(+), K(+), Na(+), Mg(2+) or free ATP at equilibrium in Na,K-ATPase. Direct binding may identify residues coordinating cations in the E(2)[2K] or E(1)P[3Na] forms of the ping-pong reaction sequence and allow estimates of their contributions to the change of Gibbs free energy of binding. This is required to understand the molecular basis for the pronounced Na/K selectivity at the cytoplasmic and extracellular surfaces. Intramembrane Glu(327) in transmembrane segment M4, Glu(779) in M5, Asp(804) and Asp(808) in M6 are essential for tight binding of K(+) and Na(+). Asn(324) and Glu(327) in M4, Thr(774), Asn(776), and Glu(779) in 771-YTLTSNIPEITP of M5 contribute to Na(+)/K(+) selectivity. Free ATP binding identifies Arg(544) as essential for high affinity binding of ATP or ADP. In the 708-TGDGVND segment, mutations of Asp(710) or Asn(713) do not interfere with free ATP binding. Asp(710) is essential and Asn(713) is important for coordination of Mg(2+) in the E(1)P[3Na] complex, but they do not contribute to Mg(2+) binding in the E(2)P-ouabain complex. Transition to the E(2)P form involves a shift of Mg(2+) coordination away from Asp(710) and Asn(713) and the two residues become more important for hydrolysis of the acyl phosphate bond at Asp(369).

Structure of Na+,K+-ATPase at 11-A resolution: comparison with Ca2+-ATPase in E1 and E2 states

Na+,K+-ATPase is a heterodimer of alpha and beta subunits and a member of the P-type ATPase family of ion pumps. Here we present an 11-A structure of the heterodimer determined from electron micrographs of unstained frozen-hydrated tubular crystals. For this reconstruction, the enzyme was isolated from supraorbital glands of salt-adapted ducks and was crystallized within the native membranes. Crystallization conditions fixed Na+,K+-ATPase in the vanadate-inhibited E2 conformation, and the crystals had p1 symmetry. A large number of helical symmetries were observed, so a three-dimensional structure was calculated by averaging both Fourier-Bessel coefficients and real-space structures of data from the different symmetries. The resulting structure clearly reveals cytoplasmic, transmembrane, and extracellular regions of the molecule with densities separately attributable to alpha and beta subunits. The overall shape bears a remarkable resemblance to the E2 structure of rabbit sarcoplasmic reticulum Ca2+-ATPase. After aligning these two structures, atomic coordinates for Ca2+-ATPase were fit to Na+,K+-ATPase, and several flexible surface loops, which fit the map poorly, were associated with sequences that differ in the two pumps. Nevertheless, cytoplasmic domains were very similarly arranged, suggesting that the E2-to-E1 conformational change postulated for Ca2+-ATPase probably applies to Na+,K+-ATPase as well as other P-type ATPases.

Preparation of Na,K-ATPase specifically modified on the anti-fluorescein antibody-inaccessible site by fluorescein 5'-isothiocyanate

Specific labeling is required for energy transfer measurements and to avoid artifacts in the use of fluorophores as reporter groups. Therefore, a method for specific modification by one of the most popular reagents for P-type ATPases (fluorescein 5'-isothiocyanate) has been developed. Sulfhydryl reagents protected against modification of cysteine residues, and treatment with dithiothreitol eliminated a slow doubling of the fluorescence of conventionally modified Na,K-ATPase upon dilution that is attributed to disappearance of self-energy transfer. Removal of nonspecifically bound fluorescein was also confirmed by titration of the modified Na, K-ATPase with anti-fluorescein antibody and by time resolution of the fluorescence change when the modified enzyme was mixed with Na(+) in a stopped-flow instrument. The only fluorescence change when specifically modified Na,K-ATPase was mixed with Na(+) was the signal from fluorescein at the antibody-inaccessible, substrate-protectable site that reports the conformational change in unphosphorylated enzyme. The magnitude of the fluorescence change reporting the conformational change increased from between 8 and 12% to between 25 and 30% without affecting the kinetic constants estimated from titrations with Na(+) and K(+). The method should be generally applicable to the preparation of specifically labeled P-type pumps for use in kinetic and equilibrium titrations or energy transfer measurements.

Ligands presumed to label high affinity and low affinity ATP binding sites do not interact in an (alpha beta)2 diprotomer in duck nasal gland Na+,K+-ATPase, nor Do the sites coexist in native enzyme

The interaction of ligands deemed to be ATP analogues with renal Na(+),K(+)-ATPase suggests that two ATP binding sites coexist on each functional unit. Previous studies in which fluorescein 5-isothiocyanate (FITC) was used to label the high affinity ATP site and 2'(3')-O-(2,4,6-trinitrophenyl)adenosine 5'-diphosphate (TNP-ADP) was used to probe the low affinity site suggested that the two sites coexist on the same alphabeta protomer. Other studies in which FITC labeled the high affinity site and erythrosin-5-isothiocyanate (ErITC) labeled the low affinity site led to the conclusion that the high and low affinity sites exist on separate interacting protomers in a functional diprotomer. We report here that at 100% inhibition of ATPase activity by FITC, each alphabeta protomer of duck nasal gland enzyme has a single bound FITC. Both TNP-ADP and ErITC interact with FITC-bound protomers, which unambiguously demonstrates that putative high and low affinity ATP sites coexist on the same protomer. In unlabeled nasal gland enzyme, TNP-ADP and ErITC inhibit both ATPase activity and p-nitrophenyl phosphatase activity, functions attributed to the putative high and low affinity ATP site, respectively, by interacting with a single site with characteristics of the high affinity ATP binding site. In FITC-labeled enzyme, TNP-ADP and ErITC inhibit p- nitrophenyl phosphatase activity but at much higher concentrations than with the unmodified enzyme. Low affinity sites do not exist on the unmodified enzyme but can be detected only after the high affinity site is modified by FITC.

Affinity labelling with MgATP analogues reveals coexisting Na+ and K+ forms of the alpha-subunits of Na+/K+-ATPase

To test the hypothesis that Na+/K+-ATPase works as an (alpha beta)2-diprotomer with interacting catalytic alpha-subunits, tryptic digestion of pig kidney enzyme, that had been inactivated with substitution-inert MgATP complex analogues, was performed. This led to the demonstration of coexisting C-terminal Na+-like 80-kDa as well as K+-like 60-kDa peptides and N-terminal 40-kDa peptides of the alpha-subunit. To localize the ATP binding sites on tryptic peptides, studies with radioactive MgATP complex analogues were performed: Co(NH3)4-8-N3-ATP specifically modified the E2ATP (low affinity) binding site of Na+/K+-ATPase with an inactivation rate constant (k2) of 12 x 10-3.min-1 at 37 degrees C and a dissociation constant (Kd) of 207 +/- 28 microm. Tryptic digestion of the [gamma32P]Co(NH3)4-8-N3-ATP-inactivated and photolabelled alpha-subunit (Mr = 100 kDa) led, in the absence of univalent cations, to a K+-like C-terminal 60-kDa fragment which was labelled in addition to an unlabelled Na+-like C-terminal 80-kDa fragment. Tryptic digestion of [alpha32P]-or [gamma32P]Cr(H2O)4ATP - bound to the E1ATP (high affinity) site - led to the labelling of a Na+-like 80-kDa fragment besides the immediate formation of an unlabelled K+-like N-terminal 40-kDa fragment and a C-terminal 60-kDa fragment. Because a labelled Na+-like 80-kDa fragment cannot result from an unlabelled K+-like 60-kDa fragment, and because unlabelled alpha-subunits did not show any catalytic activity, the findings are consistent with a situation in which Na+- and K+-like conformations are stabilized by tight binding of substitution-inert MgATP complex analogues to the E1ATP and E2ATP sites. Hence, all data are consistent with the hypothesis that ATP binding induces coexisting Na+ and K+ conformations within an (alphabeta)2-diprotomeric Na+/K+-ATPase.

Reconstitution of detergent-solubilized Na,K-ATPase and formation of two-dimensional crystals

Very pure, detergent-solubilized Na,K-ATPase from dog or lamb kidneys has been successfully reconstituted at high protein-to-lipid weight ratios. Studies have been conducted to establish the orientation of the Na,K-ATPase molecules in the reconstituted membranes and to assess the functional activity and the conformational state of the reconstituted enzyme. Results indicate that reincorporation of the Na,K-ATPase molecules in the lipid bilayer is unidirectional and that the reconstituted enzyme retains its functional and structural integrity. Two-dimensional crystals have been induced in these preparations by vanadate ions. The arrays, with a dimeric structure in the unit cell, have a morphology similar to that of the crystals that had previously formed in the native membranes. Filtered images show that in projection, the molecule had an asymmetrical mass distribution, which at the resolution of 2.5 nm is identical to that of the earlier crystals. These sheets, although small, represent the first crystals of Na, K-ATPase to be formed by reconstitution. We expect that optimization of the reconstitution and crystallization parameters will lead to larger and better-ordered sheets, suitable for electron crystallography.