A negative slope in the current-voltage relationship of the Na+/K+ pump in Xenopus oocytes produced by reduction of external [K+]

Structure and Function of Na,K-ATPase-The Sodium-Potassium Pump

Na,K-ATPase is an ubiquitous enzyme actively transporting Na-ions out of the cell in exchange for K-ions, thereby maintaining their concentration gradients across the cell membrane. Since its discovery more than six decades ago the Na-pump has been studied extensively and its vital physiological role in essentially every cell has been established. This article aims at providing an overview of well-established biochemical properties with a focus on Na,K-ATPase isoforms, its transport mechanism and principle conformations, inhibitors, and insights gained from crystal structures. © 2021 American Physiological Society. Compr Physiol 11:1-21, 2021.

Comparative analysis of alternating hemiplegia of childhood and rapid-onset dystonia-parkinsonism ATP1A3 mutations reveals functional deficits, which do not correlate with disease severity

Heterozygous mutations in the ATP1A3 gene, coding for an alpha subunit isoform (α3) of Na⁺/K⁺-ATPase, are the primary genetic cause for rapid-onset dystonia-parkinsonism (RDP) and alternating hemiplegia of childhood (AHC). Recently, cerebellar ataxia, areflexia, pes cavus, optic atrophy and sensorineural hearing loss (CAPOS), early infantile epileptic encephalopathy (EIEE), childhood rapid onset ataxia (CROA) and relapsing encephalopathy with rapid onset ataxia (RECA) extend the clinical spectrum of ATP1A3 related disorders. AHC and RDP demonstrate distinct clinical features, with AHC symptoms being generally more severe compared to RDP. Currently, it is largely unknown what determines the disease severity, and whether severity is linked to the degree of functional impairment of the α3 subunit. Here we compared the effect of twelve different RDP and AHC specific mutations on the expression and function of the α3 Na⁺/K⁺-ATPase in transfected HEK cells and oocytes. All studied mutations led to functional impairment of the pump, as reflected by lower survival rate and reduced pump current. No difference in the extent of impairment, nor in the expression level, was found between the two phenotypes, suggesting that these measures of pump dysfunction do not exclusively determine the disease severity.

Transient Electrical Currents Mediated by the Na+/K+-ATPase: A Tour from Basic Biophysics to Human Diseases

The Na⁺/K⁺-ATPase is a chemical molecular machine responsible for the movement of Na⁺ and K⁺ ions across the cell membrane. These ions are moved against their electrochemical gradients, so the protein uses the free energy of ATP hydrolysis to transport them. In fact, the Na⁺/K⁺-ATPase is the single largest consumer of energy in most cells. In each pump cycle, the protein sequentially exports 3Na⁺ out of the cell, then imports 2K⁺ into the cell at an approximate rate of 200 cycles/s. In each half cycle of the transport process, there is a state in which ions are stably trapped within the permeation pathway of the protein by internal and external gates in their closed states. These gates are required to open alternately; otherwise, passive ion diffusion would be a wasteful end of the cell's energy. Once one of these gates open, ions diffuse from their binding sites to the accessible milieu, which involves moving through part of the electrical field across the membrane. Consequently, ions generate transient electrical currents first discovered more than 30 years ago. They have been studied in a variety of preparations, including native and heterologous expression systems. Here, we review three decades' worth of work using these transient electrical signals to understand the kinetic transitions of the movement of Na⁺ and K⁺ ions through the Na⁺/K⁺-ATPase and propose the significance that this work might have to the understanding of the dysfunction of human pump orthologs responsible for some newly discovered neurological pathologies.

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.

Intracellular Requirements for Passive Proton Transport through the Na+,K+-ATPase

The Na⁺,K⁺-ATPase (NKA or Na/K pump) hydrolyzes one ATP to exchange three intracellular Na+ (Na⁺_i) for two extracellular K+ (K⁺_o) across the plasma membrane by cycling through a set of reversible transitions between phosphorylated and dephosphorylated conformations, alternately opening ion-binding sites externally (E2) or internally (E1). With subsaturating [Na⁺]_o and [K⁺]_o, the phosphorylated E2P conformation passively imports protons generating an inward current (I_H), which may be exacerbated in NKA-subunit mutations associated with human disease. To elucidate the mechanisms of I_H, we studied the effects of intracellular ligands (transported ions, nucleotides, and beryllium fluoride) on I_H and, for comparison, on transient currents measured at normal Na⁺_o (Q_Na). Utilizing inside-out patches from Xenopus oocytes heterologously expressing NKA, we observed that 1) in the presence of Na⁺_i, I_H and Q_Na were both activated by ATP, but not ADP; 2) the [Na⁺]_i dependence of I_H in saturating ATP showed K_0.5,Na = 1.8 ± 0.2 mM and the [ATP] dependence at saturating [Na⁺]_i yielded K_0.5,ATP = 48 ± 11 μM (in comparison, Na⁺_i-dependent Q_Na yields K_0.5,Na = 0.8 ± 0.2 mM and K_0.5,ATP = 0.43 ± 0.03 μM; 3) ATP activated I_H in the presence of K⁺_i (∼15% of the I_H observed in Na⁺_i) only when Mg²⁺_i was also present; and 4) beryllium fluoride induced maximal I_H even in the absence of nucleotide. These data indicate that I_H occurs when NKA is in an externally open E2P state with nucleotide bound, a conformation that can be reached through forward Na/K pump phosphorylation of E1, with Na⁺_i and ATP, or by backward binding of K⁺_i to E1, which drives the pump to the occluded E2(2K), where free P_i (at the micromolar levels found in millimolar ATP solutions) promotes external release of occluded K⁺ by backdoor NKA phosphorylation. Maximal I_H through beryllium-fluorinated NKA indicates that this complex mimics ATP-bound E2P states.

Arginine substitution of a cysteine in transmembrane helix M8 converts Na+,K+-ATPase to an electroneutral pump similar to H+,K+-ATPase

Na⁺,K⁺-ATPase and H⁺,K⁺-ATPase are electrogenic and nonelectrogenic ion pumps, respectively. The underlying structural basis for this difference has not been established, and it has not been revealed how the H⁺,K⁺-ATPase avoids binding of Na⁺ at the site corresponding to the Na⁺-specific site of the Na⁺,K⁺-ATPase (site III). In this study, we addressed these questions by using site-directed mutagenesis in combination with enzymatic, transport, and electrophysiological functional measurements. Replacement of the cysteine C932 in transmembrane helix M8 of Na⁺,K⁺-ATPase with arginine, present in the H⁺,K⁺-ATPase at the corresponding position, converted the normal 3Na⁺:2K⁺:1ATP stoichiometry of the Na⁺,K⁺-ATPase to electroneutral 2Na⁺:2K⁺:1ATP stoichiometry similar to the electroneutral transport mode of the H⁺,K⁺-ATPase. The electroneutral C932R mutant of the Na⁺,K⁺-ATPase retained a wild-type-like enzyme turnover rate for ATP hydrolysis and rate of cellular K⁺ uptake. Only a relatively minor reduction of apparent Na⁺ affinity for activation of phosphorylation from ATP was observed for C932R, whereas replacement of C932 with leucine or phenylalanine, the latter of a size comparable to arginine, led to spectacular reductions of apparent Na⁺ affinity without changing the electrogenicity. From these results, in combination with structural considerations, it appears that the guanidine⁺ group of the M8 arginine replaces Na⁺ at the third site, thus preventing Na⁺ binding there, although allowing Na⁺ to bind at the two other sites and become transported. Hence, in the H⁺,K⁺-ATPase, the ability of the M8 arginine to donate an internal cation binding at the third site is decisive for the electroneutral transport mode of this pump.

ATP1A2 Mutations in Migraine: Seeing through the Facets of an Ion Pump onto the Neurobiology of Disease

Mutations in four genes have been identified in familial hemiplegic migraine (FHM), from which CACNA1A (FHM type 1) and SCN1A (FHM type 3) code for neuronal voltage-gated calcium or sodium channels, respectively, while ATP1A2 (FHM type 2) encodes the α2 isoform of the Na(+),K(+)-ATPase's catalytic subunit, thus classifying FHM primarily as an ion channel/ion transporter pathology. FHM type 4 is attributed to mutations in the PRRT2 gene, which encodes a proline-rich transmembrane protein of as yet unknown function. The Na(+),K(+)-ATPase maintains the physiological gradients for Na(+) and K(+) ions and is, therefore, critical for the activity of ion channels and transporters involved neuronal excitability, neurotransmitter uptake or Ca(2+) signaling. Strikingly diverse functional abnormalities have been identified for disease-linked ATP1A2 mutations which frequently lead to changes in the enzyme's voltage-dependent properties, kinetics, or apparent cation affinities, but some mutations are truly deleterious for enzyme function and thus cause full haploinsufficiency. Here, we summarize structural and functional data about the Na(+),K(+)-ATPase available to date and an overview is provided about the particular properties of the α2 isoform that explain its physiological relevance in electrically excitable tissues. In addition, current concepts about the neurobiology of migraine, the correlations between primary brain dysfunction and mechanisms of headache pain generation are described, together with insights gained recently from modeling approaches in computational neuroscience. Then, a survey is given about ATP1A2 mutations implicated in migraine cases as documented in the literature with focus on mutations that were described to completely destroy enzyme function, or lead to misfolded or mistargeted protein in particular model cell lines. We also discuss whether or not there are correlations between these most severe mutational effects and clinical phenotypes. Finally, perspectives for future research on the implications of Na(+),K(+)-ATPase mutations in human pathologies are presented.

Inhibitory efficacy of bufadienolides on Na+,K+-pump activity versus cell proliferation

Bufadienolides are cytotoxic drugs that may form the basis for anticancer agents. Due to structural and functional similarity to cardiotonic glycosides, application is restricted. We, therefore, investigated correlation of their putative anticancer effects with inhibition of Na⁺,K⁺pumps. The natural bufalin and three derivatives were tested. The anticancer effects of the drugs were checked by observing their inhibitory effects on proliferation of rat liver cancer cells using MTT assay. Inhibition of Na⁺,K⁺-pump was determined by measuring pump-mediated current of rat α1/β1 and α2/β1 Na⁺,K⁺pumps expressed in Xenopus oocytes.

All tested bufadienolides inhibited cell proliferation and Na⁺,K⁺pump activity. An activity coefficient A=100xIC₅₀^{Na,K pump}/IC₅₀^{proliferation} was used to describe drug effectivity as anticancer drug. Natural bufalin exhibited lowest effectivity on cell proliferation, and also the A value for rat α1 isoform was the lowest (0.08), the α2 isoform was much less sensitive (A=1.00). The highest A values were obtained for the BF238 derivative with A=0.88 and 2.64 for the α1 and α2 isoforms, respectively. Therefore, we suggest that search for bufalin derivatives with high anticancer effect and low affinity for both Na⁺,K⁺pump isoforms may be a promising strategy for development of anticancer drugs.

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Effects of bufadienolides on Na pump are not correlated with their cytotoxicity.

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Bufadienolides with high anticancer effect but low side effect may exist.

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BF238 may form a basis for further anticancer drug research and development.

Tuning of the Na,K-ATPase by the beta subunit

The vital gradients of Na⁺ and K⁺ across the plasma membrane of animal cells are maintained by the Na,K-ATPase, an αβ enzyme complex, whose α subunit carries out the ion transport and ATP hydrolysis. The specific roles of the β subunit isoforms are less clear, though β2 is essential for motor physiology in mammals. Here, we show that compared to β1 and β3, β2 stabilizes the Na⁺-occluded E1P state relative to the outward-open E2P state, and that the effect is mediated by its transmembrane domain. Molecular dynamics simulations further demonstrate that the tilt angle of the β transmembrane helix correlates with its functional effect, suggesting that the relative orientation of β modulates ion binding at the α subunit. β2 is primarily expressed in granule neurons and glomeruli in the cerebellum, and we propose that its unique functional characteristics are important to respond appropriately to the cerebellar Na⁺ and K⁺ gradients.

The α4 isoform of the Na⁺,K⁺-ATPase is tuned for changing extracellular environments

In their journey from the male to the female reproductive tract, spermatozoa are confronted with a constantly changing environment. To cope with the associated challenges, spermatozoa express a distinct set of transporters, channels and pumps. One of the membrane proteins unique to spermatozoa is the α4 isoform of the Na(+) ,K(+) -ATPase. In addition to α4, spermatozoa express the ubiquous α1 variant. To get a detailed understanding of how α1 and α4 differ, and why spermatozoa need an additional Na(+) ,K(+) -ATPase, we have conducted an electrophysiological comparison of the rodent isoforms (rat α4 versus mouse α1-3) using the Xenopus oocyte expression system. We demonstrate isoform-specific differences in the voltage sensitivity of steady-state turnover, with α2 being the more sensitive, and α1 and α2 having faster Na(+) release kinetics than α3 and α4. Our data further show that the α1 and α2 turnover rates are fast compared with those of α3 and α4. Finally, α4 is less influenced by changes in extracellular Na(+) and temperature than α1. Based on these findings, we discuss the possibility that evolution has selected robust activity rather than rapid turnover for α4.

Mechanism of potassium ion uptake by the Na(+)/K(+)-ATPase

The Na⁺/K⁺-ATPase restores sodium (Na⁺) and potassium (K⁺) electrochemical gradients dissipated by action potentials and ion-coupled transport processes. As ions are transported, they become transiently trapped between intracellular and extracellular gates. Once the external gate opens, three Na⁺ ions are released, followed by the binding and occlusion of two K⁺ ions. While the mechanisms of Na⁺ release have been well characterized by the study of transient Na⁺ currents, smaller and faster transient currents mediated by external K⁺ have been more difficult to study. Here we show that external K⁺ ions travelling to their binding sites sense only a small fraction of the electric field as they rapidly and simultaneously become occluded. Consistent with these results, molecular dynamics simulations of a pump model show a wide water-filled access channel connecting the binding site to the external solution. These results suggest a mechanism of K⁺ gating different from that of Na⁺ occlusion.

During transport by the Na⁺/K⁺-ATPase, Na⁺ and K⁺ ions become occluded between intra- and extracellular gates. Here Castillo et al. measure transient electrical signals arising from K⁺ occlusion and use molecular simulations to describe a K⁺ gating mechanism fundamentally different to that of Na⁺.

Probing the extracellular access channel of the Na,K-ATPase

When the Na,K-ATPase pumps at each turnover two K(+) ions into the cytoplasm, this translocation consists of several reaction steps. First, the ions diffuse consecutively from the extracellular phase through an access pathway to the binding sites where they are coordinated. In the next step, the enzyme is dephosphorylated and the ions are occluded inside the membrane domain. The subsequent transition to the E1 conformation produces a deocclusion of the binding sites to the cytoplasmic side of the membrane and allows in the last steps ion dissociation and diffusion to the aqueous phase. The interaction and competition of K(+) with various quaternary organic ammonium ions have been used to gain insight into the molecular mechanism of the ion binding process from the extracellular side in the P-E2 conformation of the enzyme. Using the electrochromic styryl dye RH421, evidence has been obtained that the access pathway consists of a wide and water-filled funnel-like part that is accessible also for bulky cations such as the benzyltriethylammonium ion, and a narrow part that permits passage only of small cations such as K(+) and NH4(+) in a distinct electrogenic way. Benzyltriethylammonium ions inhibit K(+) binding in a competitive manner that can be explained by a stopper-like function at the interface between the wide and narrow parts of the access pathway. In contrast to other quaternary organic ammonium ions, benzyltriethylammonium ions show a specific binding to the ion pump in a position inside the access pathway where it blocks effectively the access to the binding sites.

Identification of electric-field-dependent steps in the Na(+),K(+)-pump cycle

The charge-transporting activity of the Na(+),K(+)-ATPase depends on its surrounding electric field. To isolate which steps of the enzyme's reaction cycle involve charge movement, we have investigated the response of the voltage-sensitive fluorescent probe RH421 to interaction of the protein with BTEA (benzyltriethylammonium), which binds from the extracellular medium to the Na(+),K(+)-ATPase's transport sites in competition with Na(+) and K(+), but is not occluded within the protein. We find that only the occludable ions Na(+), K(+), Rb(+), and Cs(+) cause a drop in RH421 fluorescence. We conclude that RH421 detects intramembrane electric field strength changes arising from charge transport associated with conformational changes occluding the transported ions within the protein, not the electric fields of the bound ions themselves. This appears at first to conflict with electrophysiological studies suggesting extracellular Na(+) or K(+) binding in a high field access channel is a major electrogenic reaction of the Na(+),K(+)-ATPase. All results can be explained consistently if ion occlusion involves local deformations in the lipid membrane surrounding the protein occurring simultaneously with conformational changes necessary for ion occlusion. The most likely origin of the RH421 fluorescence response is a change in membrane dipole potential caused by membrane deformation.

Sodium and proton effects on inward proton transport through Na/K pumps

The Na/K pump hydrolyzes ATP to export three intracellular Na (Nai) as it imports two extracellular K (Ko) across animal plasma membranes. Within the protein, two ion-binding sites (sites I and II) can reciprocally bind Na or K, but a third site (site III) exclusively binds Na in a voltage-dependent fashion. In the absence of Nao and Ko, the pump passively imports protons, generating an inward current (IH). To elucidate the mechanisms of IH, we used voltage-clamp techniques to investigate the [H]o, [Na]o, and voltage dependence of IH in Na/K pumps from ventricular myocytes and in ouabain-resistant pumps expressed in Xenopus oocytes. Lowering pHo revealed that Ho both activates IH (in a voltage-dependent manner) and inhibits it (in a voltage-independent manner) by binding to different sites. Nao effects depend on pHo; at pHo where no Ho inhibition is observed, Nao inhibits IH at all concentrations, but when applied at pHo that inhibits pump-mediated current, low [Na]o activates IH and high [Na]o inhibits it. Our results demonstrate that IH is a property inherent to Na/K pumps, not linked to the oocyte expression environment, explains differences in the characteristics of IH previously reported in the literature, and supports a model in which 1), protons leak through site III; 2), binding of two Na or two protons to sites I and II inhibits proton transport; and 3), pumps with mixed Na/proton occupancy of sites I and II remain permeable to protons.

Route, mechanism, and implications of proton import during Na+/K+ exchange by native Na+/K+-ATPase pumps

The Na⁺/K⁺ pump is a hybrid transporter that can also import protons at physiological K⁺ and Na⁺ concentrations.

A single Na⁺/K⁺-ATPase pumps three Na⁺ outwards and two K⁺ inwards by alternately exposing ion-binding sites to opposite sides of the membrane in a conformational sequence coupled to pump autophosphorylation from ATP and auto-dephosphorylation. The larger flow of Na⁺ than K⁺ generates outward current across the cell membrane. Less well understood is the ability of Na⁺/K⁺ pumps to generate an inward current of protons. Originally noted in pumps deprived of external K⁺ and Na⁺ ions, as inward current at negative membrane potentials that becomes amplified when external pH is lowered, this proton current is generally viewed as an artifact of those unnatural conditions. We demonstrate here that this inward current also flows at physiological K⁺ and Na⁺ concentrations. We show that protons exploit ready reversibility of conformational changes associated with extracellular Na⁺ release from phosphorylated Na⁺/K⁺ pumps. Reversal of a subset of these transitions allows an extracellular proton to bind an acidic side chain and to be subsequently released to the cytoplasm. This back-step of phosphorylated Na⁺/K⁺ pumps that enables proton import is not required for completion of the 3 Na⁺/2 K⁺ transport cycle. However, the back-step occurs readily during Na⁺/K⁺ transport when external K⁺ ion binding and occlusion are delayed, and it occurs more frequently when lowered extracellular pH raises the probability of protonation of the externally accessible carboxylate side chain. The proton route passes through the Na⁺-selective binding site III and is distinct from the principal pathway traversed by the majority of transported Na⁺ and K⁺ ions that passes through binding site II. The inferred occurrence of Na⁺/K⁺ exchange and H⁺ import during the same conformational cycle of a single molecule identifies the Na⁺/K⁺ pump as a hybrid transporter. Whether Na⁺/K⁺ pump–mediated proton inflow may have any physiological or pathophysiological significance remains to be clarified.

Kinetic comparisons of heart and kidney Na+,K(+)-ATPases

Most kinetic measurements of the partial reactions of Na(+),K(+)-ATPase have been conducted on enzyme from mammalian kidney. Here we present a kinetic model that is based on the available equilibrium and kinetic parameters of purified kidney enzyme, and allows predictions of its steady-state turnover and pump current in intact cells as a function of ion and ATP concentrations and the membrane voltage. Using this model, we calculated the expected dependence of the pump current on voltage and extracellular Na(+) concentration. The simulations indicate a lower voltage dependence at negative potentials of the kidney enzyme in comparison with heart muscle Na(+),K(+)-ATPase, in agreement with experimental results. The voltage dependence is enhanced at high extracellular Na(+) concentrations. This effect can be explained by a voltage-dependent depopulation of extracellular K(+) ion binding sites on the E2P state and an increase in the proportion of enzyme in the E1P(Na(+))(3) state in the steady state. This causes a decrease in the effective rate constant for occlusion of K(+) by the E2P state and hence a drop in turnover. Around a membrane potential of zero, negligible voltage dependence is observed because the voltage-independent E2(K(+))(2) → E1 + 2K(+) transition is the major rate-determining step.

Control of gastric H,K-ATPase activity by cations, voltage and intracellular pH analyzed by voltage clamp fluorometry in Xenopus oocytes

Whereas electrogenic partial reactions of the Na,K-ATPase have been studied in depth, much less is known about the influence of the membrane potential on the electroneutrally operating gastric H,K-ATPase. In this work, we investigated site-specifically fluorescence-labeled H,K-ATPase expressed in Xenopus oocytes by voltage clamp fluorometry to monitor the voltage-dependent distribution between E₁P and E₂P states and measured Rb⁺ uptake under various ionic and pH conditions. The steady-state E₁P/E₂P distribution, as indicated by the voltage-dependent fluorescence amplitudes and the Rb⁺ uptake activity were highly sensitive to small changes in intracellular pH, whereas even large extracellular pH changes affected neither the E₁P/E₂P distribution nor transport activity. Notably, intracellular acidification by approximately 0.5 pH units shifted V_0.5, the voltage, at which the E₁P/E₂P ratio is 50∶50, by −100 mV. This was paralleled by an approximately two-fold acceleration of the forward rate constant of the E₁P→E₂P transition and a similar increase in the rate of steady-state cation transport. The temperature dependence of Rb⁺ uptake yielded an activation energy of ∼90 kJ/mol, suggesting that ion transport is rate-limited by a major conformational transition. The pronounced sensitivity towards intracellular pH suggests that proton uptake from the cytoplasmic side controls the level of phosphoenzyme entering the E₁P→E₂P conformational transition, thus limiting ion transport of the gastric H,K-ATPase. These findings highlight the significance of cellular mechanisms contributing to increased proton availability in the cytoplasm of gastric parietal cells. Furthermore, we show that extracellular Na⁺ profoundly alters the voltage-dependent E₁P/E₂P distribution indicating that Na⁺ ions can act as surrogates for protons regarding the E₂P→E₁P transition. The complexity of the intra- and extracellular cation effects can be rationalized by a kinetic model suggesting that cations reach the binding sites through a rather high-field intra- and a rather low-field extracellular access channel, with fractional electrical distances of ∼0.5 and ∼0.2, respectively.

Energy landscape of the reactions governing the Na+ deeply occluded state of the Na+/K+-ATPase in the giant axon of the Humboldt squid

The Na(+)/K(+) pump is a nearly ubiquitous membrane protein in animal cells that uses the free energy of ATP hydrolysis to alternatively export 3Na(+) from the cell and import 2K(+) per cycle. This exchange of ions produces a steady-state outwardly directed current, which is proportional in magnitude to the turnover rate. Under certain ionic conditions, a sudden voltage jump generates temporally distinct transient currents mediated by the Na(+)/K(+) pump that represent the kinetics of extracellular Na(+) binding/release and Na(+) occlusion/deocclusion transitions. For many years, these events have escaped a proper thermodynamic treatment due to the relatively small electrical signal. Here, taking the advantages offered by the large diameter of the axons from the squid Dosidicus gigas, we have been able to separate the kinetic components of the transient currents in an extended temperature range and thus characterize the energetic landscape of the pump cycle and those transitions associated with the extracellular release of the first Na(+) from the deeply occluded state. Occlusion/deocclusion transition involves large changes in enthalpy and entropy as the ion is exposed to the external milieu for release. Binding/unbinding is substantially less costly, yet larger than predicted for the energetic cost of an ion diffusing through a permeation pathway, which suggests that ion binding/unbinding must involve amino acid side-chain rearrangements at the site.

THE ROLE OF MEMBRANE ELECTROSTATICS IN THE REGULATION OF CELL VOLUME AND ION CONCENTRATIONS

We present a model to study how membrane surface negative charges can affect the electro-osmotic regulation properties of a cell. This model is based on the cellular analog proposed by Jakobsson, which includes passive and active ion transports; we further introduce the effect of membrane surface charges, using a generalized formulation of the Gouy–Chapman theory. We derive a system of nonlinear differential-algebraic equations (DAEs) which describes the dynamics of the cellular analog. The system admits a unique asymptotically stable stationary state, in which the Na-pump rate, which is crucial for electro-osmotic regulation, is inversely related to the Ca2+level in the extracellular milieu; numerical integration shows that this apparent inhibition of the Na-pump by external Ca2+results from a decrease in the electrostatic field produced by surface charges at the external side of the membrane. Furthermore, the degree of stability of the stationary state dramatically depends on the amount of negative charges on the membrane; a maximal stability is obtained for densities around - e /500 Å2, where the Na-pump is maximally activated by an increase in the Na content of the cytoplasm.

Physiological adaptation of an Antarctic Na+/K+-ATPase to the cold

Because enzymatic activity is strongly suppressed by the cold, polar poikilotherms face significant adaptive challenges. For example, at 0°C the catalytic activity of a typical enzyme from a temperate organism is reduced by more than 90%. Enzymes embedded in the plasma membrane, such as the Na(+)/K(+)-ATPase, may be even more susceptible to the cold because of thermal effects on the lipid bilayer. Accordingly, adaptive changes in response to the cold may include adjustments to the enzyme or the surrounding lipid environment, or synergistic changes to both. To assess the contribution of the enzyme itself, we cloned orthologous Na(+)/K(+)-ATPase α-subunits from an Antarctic (Pareledone sp.; -1.8°C) and a temperate octopus (Octopus bimaculatus; ∼18°C), and compared their turnover rates and temperature sensitivities in a heterologous expression system. The primary sequences of the two pumps were found to be highly similar (97% identity), with most differences being conservative changes involving hydrophobic residues. The physiology of the pumps was studied using an electrophysiological approach in intact Xenopus oocytes. The voltage dependence of the pumps was equivalent. However, at room temperature the maximum turnover rate of the Antarctic pump was found to be 25% higher than that of the temperate pump. In addition, the Antarctic pump exhibited a lower temperature sensitivity, leading to significantly higher relative activity at lower temperatures. Orthologous Na(+)/K(+) pumps were then isolated from two tropical and two Arctic octopus. The temperature sensitivities of these pumps closely matched those of the temperate and Antarctic pumps, respectively. Thus, reduced thermal sensitivity appears to be a common mechanism driving cold adaptation in the Na(+)/K(+)-ATPase.

Kinetics of K(+) occlusion by the phosphoenzyme of the Na(+),K(+)-ATPase

Investigations of K(+)-occlusion by the phosphoenzyme of Na(+),K(+)-ATPase from shark rectal gland and pig kidney by stopped-flow fluorimetry reveal major differences in the kinetics of the two enzymes. In the case of the pig enzyme, a single K(+)-occlusion step could be resolved with a rate constant of 342 (± 26) s⁻¹. However, in the case of the shark enzyme, two consecutive K(+)-occlusions were detected with rate constants of 391 (± 19) s⁻¹ and 48 (± 2) s⁻¹ at 24°C and pH 7.4. A conformational change of the phosphoenzyme associated with K(+)-occlusion is, thus, the major rate-determining step of the shark enzyme under saturating concentrations of all substrates, whereas for the pig enzyme the major rate-determining step under the same conditions is the E2 → E1 transition and its associated K(+) deocclusion and release to the cytoplasm. The differences in rate constants of the K(+) occlusion reactions of the two enzymes are paralleled by compensating changes to the rate constant for the E2 → E1 transition, which explains why the differences in the enzymes' kinetic behaviors have not previously been identified.

The effect of holding potential on charge translocation by the Na+/K +-ATPase in the absence of potassium

The Na(+)/K(+)-ATPase exports 3Na(+) and imports 2K(+) at the expense of the hydrolysis of 1 ATP. In the absence of K(+), it carries on electroneutral, Na(+)-dependent transient charge movement (also known as "electroneutral Na(+)/Na(+) exchange mode") and produces a transient current containing faster and slower components in response to a sudden voltage step. Components with different speeds represent sequential release of Na(+) ions from three binding sites. The effect of holding potential on slow charge movement was studied in the presence of different concentrations of ADP(i), Na (i) (+) and Na (o) (+) with the intention of improving our understanding of Na (i) (+) binding. However, the manipulation of [ADP](i) and [Na(+)](i) did not cause as pronounced changes as predicted in the magnitude of charge movement (Q (tot)), which indicated that our experimental conditions were not able to backwardly drive reaction across the energy barrier to Na (i) (+) release/rebinding steps. On the contrary, lowering [Na(+)](o) caused evident dependence of Q (tot) on holding potential, with characteristics suggesting that pumps were escaping from E2P through the uncoupled Na(+) efflux activity.

The two C-terminal tyrosines stabilize occluded Na/K pump conformations containing Na or K ions

Interactions of the three transported Na ions with the Na/K pump remain incompletely understood. Na/K pump crystal structures show that the extended C terminus of the Na,K-adenosine triphosphatase (ATPase) alpha subunit directly contacts transmembrane helices. Deletion of the last five residues (KETYY in almost all Na/K pumps) markedly lowered the apparent affinity for Na activation of pump phosphorylation from ATP, a reflection of cytoplasmic Na affinity for forming the occluded E1P(Na3) conformation. ATPase assays further suggested that C-terminal truncations also interfere with low affinity Na interactions, which are attributable to extracellular effects. Because extracellular Na ions traverse part of the membrane's electric field to reach their binding sites in the Na/K pump, their movements generate currents that can be monitored with high resolution. We report here electrical measurements to examine how Na/K pump interactions with extracellular Na ions are influenced by C-terminal truncations. We deleted the last two (YY) or five (KESYY) residues in Xenopus laevis alpha1 Na/K pumps made ouabain resistant by either of two kinds of point mutations and measured their currents as 10-mM ouabain-sensitive currents in Xenopus oocytes after silencing endogenous Xenopus Na/K pumps with 1 microM ouabain. We found the low affinity inhibitory influence of extracellular Na on outward Na/K pump current at negative voltages to be impaired in all of the C-terminally truncated pumps. Correspondingly, voltage jump-induced transient charge movements that reflect pump interactions with extracellular Na ions were strongly shifted to more negative potentials; this signals a several-fold reduction of the apparent affinity for extracellular Na in the truncated pumps. Parallel lowering of Na affinity on both sides of the membrane argues that the C-terminal contacts provide important stabilization of the occluded E1P(Na3) conformation, regardless of the route of Na ion entry into the binding pocket. Gating measurements of palytoxin-opened Na/K pump channels additionally imply that the C-terminal contacts also help stabilize pump conformations with occluded K ions.

Interaction of ATP with the phosphoenzyme of the Na+,K+-ATPase

The interaction of ATP with the phosphoenzyme of Na(+),K(+)-ATPase from pig kidney, rabbit kidney, and shark rectal gland was investigated using the voltage-sensitive fluorescent probe RH421. In each case, ATP concentrations >or=100 microM caused a drop in fluorescence intensity, which, because RH421 is sensitive to the formation of enzyme in the E2P state, can be attributed to ATP binding to the E2P phosphoenzyme. Simulations of the experimental behavior using kinetic models based on either a monomeric or a dimeric enzyme mechanism yielded a K(d) for ATP binding in the range 140-500 muM. Steady-state activity measurements and independent measurements of the phosphoenzyme level via a radioactive assay indicated that ATP binding to E2P causes a deceleration in its dephosphorylation when acting in the Na(+)-ATPase mode, i.e., in the absence of K(+) ions. Both the ATP-induced drop in RH421 fluorescence and the effect on the dephosphorylation reaction could be attributed to an inhibition of dissociation from the E2P(Na(+))(3) state of the one Na(+) ion necessary to allow dephosphorylation. Stopped-flow studies on the shark enzyme indicated that the ATP-induced inhibition of dephosphorylation is abolished in the presence of 1 mM KCl. A possible physiological role of allosteric binding of ATP to the phosphoenzyme could be to stabilize the E2P state and stop the enzyme running backward, which would cause dissipation of the Na(+) electrochemical potential gradient and the resynthesis of ATP from ADP. ATP binding to E2P could also fix ATP within the enzyme ready to phosphorylate it in the subsequent turnover.

Hyperpolarization-activated inward leakage currents caused by deletion or mutation of carboxy-terminal tyrosines of the Na+/K+-ATPase {alpha} subunit

The Na(+)/K(+)-ATPase mediates electrogenic transport by exporting three Na(+) ions in exchange for two K(+) ions across the cell membrane per adenosine triphosphate molecule. The location of two Rb(+) ions in the crystal structures of the Na(+)/K(+)-ATPase has defined two "common" cation binding sites, I and II, which accommodate Na(+) or K(+) ions during transport. The configuration of site III is still unknown, but the crystal structure has suggested a critical role of the carboxy-terminal KETYY motif for the formation of this "unique" Na(+) binding site. Our two-electrode voltage clamp experiments on Xenopus oocytes show that deletion of two tyrosines at the carboxy terminus of the human Na(+)/K(+)-ATPase alpha(2) subunit decreases the affinity for extracellular and intracellular Na(+), in agreement with previous biochemical studies. Apparently, the DeltaYY deletion changes Na(+) affinity at site III but leaves the common sites unaffected, whereas the more extensive DeltaKETYY deletion affects the unique site and the common sites as well. In the absence of extracellular K(+), the DeltaYY construct mediated ouabain-sensitive, hyperpolarization-activated inward currents, which were Na(+) dependent and increased with acidification. Furthermore, the voltage dependence of rate constants from transient currents under Na(+)/Na(+) exchange conditions was reversed, and the amounts of charge transported upon voltage pulses from a certain holding potential to hyperpolarizing potentials and back were unequal. These findings are incompatible with a reversible and exclusively extracellular Na(+) release/binding mechanism. In analogy to the mechanism proposed for the H(+) leak currents of the wild-type Na(+)/K(+)-ATPase, we suggest that the DeltaYY deletion lowers the energy barrier for the intracellular Na(+) occlusion reaction, thus destabilizing the Na(+)-occluded state and enabling inward leak currents. The leakage currents are prevented by aromatic amino acids at the carboxy terminus. Thus, the carboxy terminus of the Na(+)/K(+)-ATPase alpha subunit represents a structural and functional relay between Na(+) binding site III and the intracellular cation occlusion gate.

Altered Na+ transport after an intracellular alpha-subunit deletion reveals strict external sequential release of Na+ from the Na/K pump

The Na/K pump actively exports 3 Na(+) in exchange for 2 K(+) across the plasmalemma of animal cells. As in other P-type ATPases, pump function is more effective when the relative affinity for transported ions is altered as the ion binding sites alternate between opposite sides of the membrane. Deletion of the five C-terminal residues from the alpha-subunit diminishes internal Na(+) (Na(i)(+)) affinity approximately 25-fold [Morth et al. (2007) Nature 450:1043-1049]. Because external Na(+) (Na(o)(+)) binding is voltage-dependent, we studied the reactions involving this process by using two-electrode and inside-out patch voltage clamp in normal and truncated (DeltaKESYY) Xenopus-alpha1 pumps expressed in oocytes. We observed that DeltaKESYY (i) decreased both Na(o)(+) and Na(i)(+) apparent affinities in the absence of K(o)(+), and (ii) did not affect apparent Na(o)(+) affinity at high K(o)(+). These results support a model of strict sequential external release of Na(+) ions, where the Na(+)-exclusive site releases Na(+) before the sites shared with K(+) and the DeltaKESYY deletion only reduces Na(o)(+) affinity at the shared sites. Moreover, at nonsaturating K(o)(+), DeltaKESYY induced an inward flow of Na(+) through Na/K pumps at negative potentials. Guanidinium(+) can also permeate truncated pumps, whereas N-methyl-D-glucamine cannot. Because guanidinium(o)(+) can also traverse normal Na/K pumps in the absence of both Na(o)(+) and K(o)(+) and can also inhibit Na/K pump currents in a Na(+)-like voltage-dependent manner, we conclude that the normal pathway transited by the first externally released Na(+) is large enough to accommodate guanidinium(+).

Distribution of the NA/K pumps' turnover rates as a function of membrane potential, temperature, and ion concentration gradients and effect of fluctuations

Because of structural independence of the Na/K pump molecules, the pumping rates of individual pumps may not be the same, instead showing some sort of distribution. Detailed information about the distribution has not previously been reported. The pumping rate of Na/K pumps depends on many parameters, such as membrane potential, temperature, and ion concentration gradients across the cell membrane. Fluctuation of any of the variables will change the pumping rate, resulting in a distribution. On the basis of a simplified six-state model, a steady-state pumping flux and therefore the pumping rate were obtained. Parameters were determined based on previous experimental results on amphibian skeletal muscle and theoretical work. Gaussian fluctuations of all the variables were considered to determine the changes in the pumping rate. These variable fluctuations may be totally independent or correlated to each other. The results showed that the pumping rates of the Na/K pumps are distributed in an asymmetric profile, which has a higher probability at the lower pumping rate. We present a model distribution of pumping rates as a function of temperature, membrane potential, and ion concentration.

Interactions of Na+,K+-ATPase and co-expressed delta-opioid receptor

To investigate interference of delta-opioid receptor with the Na(+),K(+)-ATPase in a simple model system, we used the Xenopus oocytes as an expression system. Our results indicate that expression of the delta-opioid receptor (DOR) results in reduction of endogenous sodium-pump activity. Stimulation of DOR by the DOR agonist [(D)-Pen(2,5)]-enkephalin (DPDPE) had no pronounced additional effect on pump activity. Qualitatively similar results were obtained in experiments with a variety of co-expressed exogenous sodium pumps. We suggest that reduced pump activity with DOR expression is brought about by an interaction of the pump with DOR. Direct interaction is also supported by co-immunoprecipitation, not only in the Xenopus oocytes but also in rat hippocampal neurons. The interaction may be responsible for altered agonist sensitivity of DOR; activation of the sodium pump led to an increase of the K(m) value for DOR activation by DPDPE from about 0.17 to 0.27muM. In conclusion, pump activity not only affects neural activity directly but our results also suggest that pump activity is affected through functional interaction with DOR that will modulate pain sensation.

Diverse functional consequences of mutations in the Na+/K+-ATPase alpha2-subunit causing familial hemiplegic migraine type 2

Mutations in ATP1A2, the gene coding for the Na(+)/K(+)-ATPase alpha(2)-subunit, are associated with both familial hemiplegic migraine and sporadic cases of hemiplegic migraine. In this study, we examined the functional properties of 11 ATP1A2 mutations associated with familial or sporadic hemiplegic migraine, including missense mutations (T263M, T376M, R383H, A606T, R763H, M829R, R834Q, R937P, and X1021R), a deletion mutant (del(K935-S940)ins(I)), and a frameshift mutation (S966fs). According to the Na(+)/K(+)-ATPase crystal structure, a subset of the mutated residues (Ala(606), Arg(763), Met(829), and Arg(834)) is involved in important interdomain H-bond networks, and the C terminus of the enzyme, which is elongated by the X1021R mutation, has been implicated in voltage dependence and formation of a third Na(+)-binding site. Upon heterologous expression in Xenopus oocytes, the analysis of electrogenic transport properties, Rb(+) uptake, and protein expression revealed pronounced and markedly diverse functional alterations in all ATP1A2 mutants. Abnormalities included a complete loss of function (T376M), impaired plasma membrane expression (del(K935-S940)ins(I) and S966fs), and altered apparent affinities for extracellular cations or reduced enzyme turnover (R383H, A606T, R763H, R834Q, and X1021R). In addition, changes in the voltage dependence of pump currents and the increased rate constants of the voltage jump-induced redistribution between E(1)P and E(2)P states were observed. Thus, mutations that disrupt distinct interdomain H-bond patterns can cause abnormal conformational flexibility and exert long range consequences on apparent cation affinities or voltage dependence. Of interest, the X1021R mutation severely impaired voltage dependence and kinetics of Na(+)-translocating partial reactions, corroborating the critical role of the C terminus of Na(+)/K(+)-ATPase in these processes.

Charge transfer in P-type ATPases investigated on planar membranes

Planar lipid bilayers, e.g., black lipid membranes (BLM) and solid supported membranes (SSM), have been employed to investigate charge movements during the reaction cycle of P-type ATPases. The BLM/SSM method allows a direct measurement of the electrical currents generated by the cation transporter following chemical activation by a substrate concentration jump. The electrical current transients provides information about the reaction mechanism of the enzyme. In particular, the BLM/SSM technique allows identification of electrogenic steps which in turn may be used to localize ion translocation during the reaction cycle of the pump. In addition, using the high time resolution of the technique, especially when rapid activation via caged ATP is employed, rate constants of electrogenic and electroneutral steps can be determined. In the present review, we will discuss the main results obtained by the BLM and SSM methods and how they have contributed to unravel the transport mechanism of P-type ATPases.

Entrainment of Na/K pumps by a synchronization modulation electric field

We studied entrainment of the catalytic cycle of the Na/K pumps by an imposed external AC electric field. Our results show that a well designed dichotomous oscillating electric field with a frequency close to the pumps' natural turnover rate can synchronize the pump molecules. Characteristics of the synchronized pumps include: (1) outward pump currents responding to Na-extrusion and inward pump currents responding to K-pumping in are separated; (2) magnitude of the outward pump currents can be up to three times higher than that of the randomly paced pump currents; (3) magnitude ratio of the outward over inward pump currents reveals the 3:2 stoichiometry of the pumps. We, further, gradually increased the field oscillating frequency in a stepwise pattern and kept pump synchronization in each step. We found that the pumps' turnover rate could be modulated up as the field frequency increased. Consequently, the pump currents significantly increased by many fold. In summary, these results show that the catalytic cycle of Na/K pumps can be synchronized and modulated by a well designed oscillating electric field resulting in activation of the pump functions.

Structural basis of Na(+)/K(+)-ATPase adaptation to marine environments

Throughout evolution, enzymes have adapted to perform in different environments. The Na(+)/K(+) pump, an enzyme crucial for maintaining ionic gradients across cell membranes, is strongly influenced by the ionic environment. In vertebrates, the pump sees much less external Na(+) (100-160 mM) than it does in osmoconformers such as squid (450 mM), which live in seawater. If the extracellular architecture of the squid pump were identical to that of vertebrates, then at the resting potential, the pump's function would be severely compromised because the negative voltage would drive Na(+) ions back to their binding sites, practically abolishing forward transport. Here we show that four amino acids that ring the external mouth of the ion translocation pathway are more positive in squid, thereby reducing the pump's sensitivity to external Na(+) and explaining how it can perform optimally in the marine environment.

The third sodium binding site of Na,K-ATPase is functionally linked to acidic pH-activated inward current

Sodium- and potassium-activated adenosine triphosphatases (Na,K-ATPase) is the ubiquitous active transport system that maintains the Na(+) and K(+) gradients across the plasma membrane by exchanging three intracellular Na(+) ions against two extracellular K(+) ions. In addition to the two cation binding sites homologous to the calcium site of sarcoplasmic and endoplasmic reticulum calcium ATPase and which are alternatively occupied by Na(+) and K(+) ions, a third Na(+)-specific site is located close to transmembrane domains 5, 6 and 9, and mutations close to this site induce marked alterations of the voltage-dependent release of Na(+) to the extracellular side. In the absence of extracellular Na(+) and K(+), Na,K-ATPase carries an acidic pH-activated, ouabain-sensitive "leak" current. We investigated the relationship between the third Na(+) binding site and the pH-activated current. The decrease (in E961A, T814A and Y778F mutants) or the increase (in G813A mutant) of the voltage-dependent extracellular Na(+) affinity was paralleled by a decrease or an increase in the pH-activated current, respectively. Moreover, replacing E961 with oxygen-containing side chain residues such as glutamine or aspartate had little effect on the voltage-dependent affinity for extracellular Na(+) and produced only small effects on the pH-activated current. Our results suggest that extracellular protons and Na(+) ions share a high field access channel between the extracellular solution and the third Na(+) binding site.

Synchronization of Na/K pump molecules by a train of squared pulses

We experimentally studied the Na/K pump currents evoked by a train of squared pulses whose pulse-duration is about the time course of Na-extrusion at physiological conditions. The magnitude of the measured pump current can be as much as three-fold of that induced by the traditional single pulse measurement. The increase in the pump current is directly dependent on the number of pre-pulses. The larger the number of the pre-pulses is, the higher the current magnitude can be obtained. At a particular number of pre-pulses, the pump current becomes saturated. These results suggest that a large number of pre-pulses may synchronize the pump molecules to work at the same pace. As a result, the pump molecules may extrude Na ions at the same time corresponding to the stimulation pulses, and pump in K ions at the same time during the pulse intervals. Therefore, the measured pump current is three-fold of that measured by a single pulse where the outward and inward pump currents are canceled each other.

Access of extracellular cations to their binding sites in Na,K-ATPase: role of the second extracellular loop of the alpha subunit

Na,K-ATPase, the main active transport system for monovalent cations in animal cells, is responsible for maintaining Na⁺ and K⁺ gradients across the plasma membrane. During its transport cycle it binds three cytoplasmic Na⁺ ions and releases them on the extracellular side of the membrane, and then binds two extracellular K⁺ ions and releases them into the cytoplasm. The fourth, fifth, and sixth transmembrane helices of the α subunit of Na,K-ATPase are known to be involved in Na⁺ and K⁺ binding sites, but the gating mechanisms that control the access of these ions to their binding sites are not yet fully understood. We have focused on the second extracellular loop linking transmembrane segments 3 and 4 and attempted to determine its role in gating. We replaced 13 residues of this loop in the rat α1 subunit, from E314 to G326, by cysteine, and then studied the function of these mutants using electrophysiological techniques. We analyzed the results using a structural model obtained by homology with SERCA, and ab initio calculations for the second extracellular loop. Four mutants were markedly modified by the sulfhydryl reagent MTSET, and we investigated them in detail. The substituted cysteines were more readily accessible to MTSET in the E1 conformation for the Y315C, W317C, and I322C mutants. Mutations or derivatization of the substituted cysteines in the second extracellular loop resulted in major increases in the apparent affinity for extracellular K⁺, and this was associated with a reduction in the maximum activity. The changes produced by the E314C mutation were reversed by MTSET treatment. In the W317C and I322C mutants, MTSET also induced a moderate shift of the E1/E2 equilibrium towards the E1(Na) conformation under Na/Na exchange conditions. These findings indicate that the second extracellular loop must be functionally linked to the gating mechanism that controls the access of K⁺ to its binding site.

Charge translocation by the Na+/K+ pump under Na+/Na+ exchange conditions: intracellular Na+ dependence

The effect of intracellular (i) and extracellular (o) Na+ on pre-steady-state transient current associated with Na+/Na+ exchange by the Na+/K+ pump was investigated in the vegetal pole of Xenopus oocytes. Current records in response to 40-ms voltage pulses from -180 to +100 mV in the absence of external Na+ were subtracted from current records obtained under Na+/Na+ exchange conditions. Na+-sensitive transient current and dihydroouabain-sensitive current were equivalent. The quantity of charge moved (Q) and the relaxation rate coefficient (ktot) of the slow component of the Nao+-sensitive transient current were measured for steps to various voltages (V). The data were analyzed using a four-state kinetic model describing the Na+ binding, occlusion, conformational change, and release steps of the transport cycle. The apparent valence of the Q vs. V relationship was near 1.0 for all experimental conditions. When extracellular Na+ was halved, the midpoint voltage of the charge distribution (Vq) shifted -25.3+/-0.4 mV, which can be accounted for by the presence of an extracellular ion-well having a dielectric distance delta=0.69+/-0.01. The effect of changes of Nai+ on Nao+-sensitive transient current was investigated. The midpoint voltage (Vq) of the charge distribution curve was not affected over the Nao+ concentration range 3.13-50 mM. As Nai+ was decreased, the amount of charge measured and its relaxation rate coefficient decreased with an apparent Km of 3.2+/-0.2 mM. The effects of lowering Nai+ on pre-steady-state transient current can be accounted for by decreasing the charge available to participate in the fast extracellular Na+ release steps, by a slowly equilibrating (phosphorylation/occlusion) step intervening between intracellular Na+ binding and extracellular Na+ release.

Voltage dependence of the carrier-mediated ion transport

With regards to the common features of carrier-mediated transport, voltage dependence was studied, using an asymmetric, six-state model. Our study shows that for an ion exchanger, transporting one kind of ion via exchange with another kind, the ion flux as a function of the membrane potential shows a sigmoidal curve with a shallow slope, saturation behavior, and possibly a negative slope. These features are mainly due to the transport of ions with charges of the same sign in the opposite direction. Membrane potential depolarization can facilitate only one transport and hinder another. As a result, the ion flux cannot increase dramatically and has an upper limitation because the exchanging rate depends on competition of the two inversely voltage-dependent transport processes. In contrast, for unidirectional ion transporters, the ion flux will monotonically increase as a function of the membrane potential. Both the maximum ion flux and the voltage sensitivity are much higher than those of the ion exchanger.

The beta subunit of the Na+/K+-ATPase follows the conformational state of the holoenzyme

The Na+/K+-ATPase is a ubiquitous plasma membrane ion pump that utilizes ATP hydrolysis to regulate the intracellular concentration of Na+ and K+. It is comprised of at least two subunits, a large catalytic alpha subunit that mediates ATP hydrolysis and ion transport, and an ancillary beta subunit that is required for proper trafficking of the holoenzyme. Although processes mediated by the alpha subunit have been extensively studied, little is known about the participation of the beta subunit in conformational changes of the enzyme. To elucidate the role of the beta subunit during ion transport, extracellular amino acids proximal to the transmembrane region of the sheep beta1 subunit were individually replaced for cysteines. This enabled sulfhydryl-specific labeling with the environmentally sensitive fluorescent dye tetramethylrhodamine-6-maleimide (TMRM) upon expression in Xenopus oocytes. Investigation by voltage-clamp fluorometry identified three reporter positions on the beta1 subunit that responded with fluorescence changes to alterations in ionic conditions and/or membrane potential. These experiments for the first time show real-time detection of conformational rearrangements of the Na+/K+-ATPase through a fluorophore-labeled beta subunit. Simultaneous recording of presteady-state or stationary currents together with fluorescence signals enabled correlation of the observed environmental changes of the beta subunit to certain reaction steps of the Na+/K+-ATPase, which involve changes in the occupancy of the two principle conformational states, E1P and E2P. From these experiments, evidence is provided that the beta1-S62C mutant can be directly used to monitor the conformational state of the enzyme, while the F64C mutant reveals a relaxation process that is triggered by sodium transport but evolves on a much slower time scale. Finally, shifts in voltage dependence and kinetics observed for mutant K65C show that this charged lysine residue, which is conserved in beta1 isoforms, directly influences the effective potential that determines voltage dependence of extracellular cation binding and release.

Na,K-ATPase mutations in familial hemiplegic migraine lead to functional inactivation

The Na,K-ATPase is an ion-translocating transmembrane protein that actively maintains the electrochemical gradients for Na+ and K+ across the plasma membrane. The functional protein is a heterodimer comprising a catalytic alpha-subunit (four isoforms) and an ancillary beta-subunit (three isoforms). Mutations in the alpha2-subunit have recently been implicated in familial hemiplegic migraine type 2, but almost no thorough studies of the functional consequences of these mutations have been provided. We investigated the functional properties of the mutations L764P and W887R in the human Na,K-ATPase alpha2-subunit upon heterologous expression in Xenopus oocytes. No Na,K-ATPase-specific pump currents could be detected in cells expressing these mutants. The binding of radiolabelled [3H]ouabain to intact cells suggested that this could be due to a lack of plasma membrane expression. However, plasma membrane isolation showed that the mutated pumps are well expressed at the plasma membrane. 86Rb+-flux and ATPase activity measurements demonstrated that the mutants are inactive. Therefore, the primary disease-causing mechanism is loss-of-function of the Na,K-ATPase alpha2-isoform.

Effect of extracellular pH on presteady-state and steady-state current mediated by the Na+/K+ pump

A ouabain sensitive inward current occurs in Xenopus oocytes in Na+ and K(+)-free solutions. Several laboratories have investigated the properties of this current and suggested that acidic extracellular pH (pHo) produces a conducting pathway through the Na+/K+ pump that is permeable to H+ and blocked by [Na+]o. An alternative suggestion is that the current is mediated by an electrogenic H(+)-ATPase. Here we investigate the effect of pHo and [Na+]o on both transient and steady-state ouabain-sensitive current. At alkaline or neutral pHo the relaxation rate of pre-steady-state current is an exponential function of voltage. Its U-shaped voltage dependence becomes apparent at acidic pHo, as predicted by a model in which protonation of the Na+/K+ pump reduces the energy barrier between the internal solution and the Na+ occluded state. The model also predicts that acidic pHo increases steady-state current leak through the pump. The apparent pK of the titratable group(s) is approximately 6, suggesting that histidine is involved in induction of the conductance pathway. 22Na efflux experiments in squid giant axon and current measurements in oocytes at acidic pHo suggest that both Na+ and H+ are permeant. The acid-induced inward current is reduced by high [Na+]o, consistent with block by Na+. A least squares analysis predicts that H+ is four orders of magnitude more permeant than Na+, and that block occurs when 3 Na+ ions occupy a low affinity binding site (K(0.5) = 130 +/- 30 m M) with a dielectric coefficient of 0.23 +/- 0.03. These data support the conclusion that the ouabain-sensitive conducting pathway is a result of passive leak of both Na+ and H+ through the Na+/K+ pump.

Quaternary organic amines inhibit Na,K pump current in a voltage-dependent manner: direct evidence of an extracellular access channel in the Na,K-ATPase

The effects of organic quaternary amines, tetraethylammonium (TEA) chloride and benzyltriethylammonium (BTEA) chloride, on Na,K pump current were examined in rat cardiac myocytes superfused in extracellular Na(+)-free solutions and whole-cell voltage-clamped with patch electrodes containing a high Na(+)-salt solution. Extracellular application of these quaternary amines competitively inhibited extracellular K(+) (K(+)(o)) activation of Na,K pump current; however, the concentration for half maximal inhibition of Na,K pump current at 0 mV (K(0)(Q)) by BTEA, 4.0 +/- 0.3 mM, was much lower than the K(0)(Q) for TEA, 26.6 +/- 0.7 mM. Even so, the fraction of the membrane electric field dissipated during K(+)(o) activation of Na,K pump current (lambda(K)), 39 +/- 1%, was similar to lambda(K) determined in the presence of TEA (37 +/- 2%) and BTEA (35 +/- 2%), an indication that the membrane potential (V(M)) dependence for K(+)(o) activation of the Na,K pump current was unaffected by TEA and BTEA. TEA was found to inhibit the Na,K pump current in a V(M)-independent manner, i.e., inhibition of current dissipated 4 +/- 2% of the membrane electric field. In contrast, BTEA dissipated 40 +/- 5% of the membrane electric field during inhibition of Na,K pump current. Thus, BTEA inhibition of the Na,K-ATPase is V(M)-dependent. The competitive nature of inhibition as well as the similar fractions of the membrane electric field dissipated during K(+)(o)-dependent activation and BTEA-dependent inhibition of Na,K pump current suggest that BTEA inhibits the Na,K-ATPase at or very near the enzyme's K(+)(o) binding site(s) located in the membrane electric field. Given previous findings that organic quaternary amines are not occluded by the Na,K-ATPase, these data clearly demonstrate that an ion channel-like structure provides access to K(+)(o) binding sites in the enzyme.

Modulation of Na(+),K(+) pumping and neurotransmitter uptake by beta-amyloid

Micromolar concentrations of beta-amyloid (Abeta), a 40/42-amino-acid-long proteolytic fragment (Abeta(1-40/42)) of the amyloid precursor protein, was shown previously to play a crucial role in pathogenesis of Alzheimer's disease. We used the Xenopus oocyte expression system to investigate specific effects of micromolar concentrations of Abeta(1-42) on the neurotransmitter transporters for gamma-aminobutyric acid (GABA), GAT1, and for the excitatory amino acid glutamate, EAAC1, which are driven by the transmembrane Na(+) gradient that is regulated by the Na(+),K(+)-ATPase. Brief treatment with Abeta(1-42), up to 80 min, leads to a significant inhibition of ion translocation by the Na(+),K(+)-ATPase (30-40%); also glutamate uptake is inhibited (20%) while GABA uptake is not affected. Since reduced glutamate uptake will result in elevated, neurotoxic concentrations of extracellular glutamate, we investigated the effects of Abeta(1-42) and the smaller fragments, Abeta(12-28) and Abeta(25-35), on EAAC1 in more detail. Prolonged incubation in 1 microM Abeta(1-42) leads to further, strong inhibition of glutamate uptake and EAAC1-mediated current (after 4 h inhibition amounts to more than 80%). Abeta(12-28) is less effective with 50% inhibition after 4 h of incubation at 20 microM. Abeta(1-42) and Abeta(12-28) affect EAAC1-mediated current to a similar extent as the rate of glutamate uptake. The effects on EAAC1-mediated current are irreversible if Abeta were applied for longer time periods. Peptides directly microinjected into the oocyte are ineffective suggesting that the observed effect were mediated by extracellular proteins. Abeta(25-35) hardly affects EAAC1-mediated current or glutamate uptake. The results demonstrate that Abeta specifically inhibits the Na(+),K(+) pump and EAAC1. The domain between amino acids 12 and 28 of Abeta seems to play a crucial role for inhibition of EAAC1. The inhibition of EAAC1 by neurotoxic, elevated extracellular glutamate levels may contribute to Alzheimer's pathogenesis.

Electrophysiological analysis of the mutated Na,K-ATPase cation binding pocket

Na,K-ATPase mediates net electrogenic transport by extruding three Na+ ions and importing two K+ ions across the plasma membrane during each reaction cycle. We mutated putative cation coordinating amino acids in transmembrane hairpin M5-M6 of rat Na,K-ATPase: Asp776 (Gln, Asp, Ala), Glu779 (Asp, Gln, Ala), Asp804 (Glu, Asn, Ala), and Asp808 (Glu, Asn, Ala). Electrogenic cation transport properties of these 12 mutants were analyzed in two-electrode voltage-clamp experiments on Xenopus laevis oocytes by measuring the voltage dependence of K+-stimulated stationary currents and pre-steady-state currents under electrogenic Na+/Na+ exchange conditions. Whereas mutants D804N, D804A, and D808A hardly showed any Na+/K+ pump currents, the other constructs could be classified according to the [K+] and voltage dependence of their stationary currents; mutants N776A and E779Q behaved similarly to the wild-type enzyme. Mutants E779D, E779A, D808E, and D808N had in common a decreased apparent affinity for extracellular K+. Mutants N776Q, N776D, and D804E showed large deviations from the wild-type behavior; the currents generated by mutant N776D showed weaker voltage dependence, and the current-voltage curves of mutants N776Q and D804E exhibited a negative slope. The apparent rate constants determined from transient Na+/Na+ exchange currents are rather voltage-independent and at potentials above -60 mV faster than the wild type. Thus, the characteristic voltage-dependent increase of the rate constants at hyperpolarizing potentials is almost absent in these mutants. Accordingly, dislocating the carboxamide or carboxyl group of Asn776 and Asp804, respectively, decreases the extracellular Na+ affinity.

Found: Na(+) and K(+) binding sites of the sodium pump

Homology modeling and valence mapping have been used to predict the location and structure of Na(+) and K(+) binding sites in the Na(+)-K(+)-ATPase on the basis of the known atomic resolution structure of SERCA. Additional sites are predicted that may be associated with intracellular access and extracellular egress pathways for Na(+). The model predictions are in excellent agreement with previous structure-function and electrical studies.

Investigation of Na(+),K(+)-ATPase on a solid supported membrane: the role of acylphosphatase on the ion transport mechanism

Charge translocation by Na(+),K(+)-ATPase was investigated by adsorbing membrane fragments containing Na(+),K(+)-ATPase from pig kidney on a solid supported membrane (SSM). Upon adsorption, the ion pumps were activated by performing ATP concentration jumps at the surface of the SSM, and the capacitive current transients generated by Na(+),K(+)-ATPase were measured under potentiostatic conditions. To study the behavior of the ion pump under multiple turnover conditions, ATP concentration jump experiments were carried out in the presence of Na(+) and K(+) ions. Current transients induced by ATP concentration jumps were also recorded in the presence of the enzyme alpha-chymotrypsin. The effect of acylphosphatase (AcP), a cytosolic enzyme that may affect the functioning of Na(+),K(+)-ATPase by hydrolyzing its acylphosphorylated intermediate, was investigated by performing ATP concentration jumps both in the presence and in the absence of AcP. In the presence of Na(+) but not of K(+), the addition of AcP causes the charge translocated as a consequence of ATP concentration jumps to decrease by about 50% over the pH range from 6 to 7, and to increase by about 20% at pH 8. Conversely, no appreciable effect of pH upon the translocated charge is observed in the absence of AcP. The above behavior suggests that protons are involved in the AcP-catalyzed dephosphorylation of the acylphosphorylated intermediate of Na(+),K(+)-ATPase.