Pre-steady-state transient currents mediated by the Na/K pump in internally perfused Xenopus oocytes

Comparative analysis of the functional properties of human and mouse ferroportin

Ferroportin (Fpn)-expressed at the plasma membrane of macrophages, enterocytes, and hepatocytes-mediates the transfer of cellular iron into the blood plasma. Under the control of the iron-regulatory hormone hepcidin, Fpn serves a critical role in systemic iron homeostasis. Although we have previously characterized human Fpn, a great deal of research in iron homeostasis and disorders uses mouse models. By way of example, the flatiron mouse, a model of classical ferroportin disease, bears the mutation H32R in Fpn and is characterized by systemic iron deficiency and macrophage iron retention. The flatiron mouse also appears to exhibit a manganese phenotype, raising the possibility that mouse Fpn serves a role in manganese metabolism. At odds with this observation, we have found that human Fpn does not transport manganese, so we considered the possibility that a species difference could explain this discrepancy. We tested the hypothesis that mouse but not human Fpn can transport manganese and performed a comparative analysis of mouse and human Fpn. We examined the functional properties of human Fpn, mouse Fpn, and mutant mouse Fpn by using radiotracer assays in RNA-injected Xenopus oocytes. We found that neither mouse nor human Fpn transports manganese. Mouse and human Fpn share identical properties with respect to substrate profile, calcium dependence, optimal pH, and hepcidin sensitivity. We have also demonstrated that Fpn is not an ATPase pump. Our findings validate the use of mouse models of ferroportin function in iron homeostasis and disease.

A comparative review on the well-studied GAT1 and the understudied BGT-1 in the brain

γ-aminobutyric acid (GABA) is the primary inhibitory neurotransmitter in the central nervous system (CNS). Its homeostasis is maintained by neuronal and glial GABA transporters (GATs). The four GATs identified in humans are GAT1 (SLC6A1), GAT2 (SLC6A13), GAT3 (SLC6A11), and betaine/GABA transporter-1 BGT-1 (SLC6A12) which are all members of the solute carrier 6 (SLC6) family of sodium-dependent transporters. While GAT1 has been investigated extensively, the other GABA transporters are less studied and their role in CNS is not clearly defined. Altered GABAergic neurotransmission is involved in different diseases, but the importance of the different transporters remained understudied and limits drug targeting. In this review, the well-studied GABA transporter GAT1 is compared with the less-studied BGT-1 with the aim to leverage the knowledge on GAT1 to shed new light on the open questions concerning BGT-1. The most recent knowledge on transporter structure, functions, expression, and localization is discussed along with their specific role as drug targets for neurological and neurodegenerative disorders. We review and discuss data on the binding sites for Na⁺, Cl^-, substrates, and inhibitors by building on the recent cryo-EM structure of GAT1 to highlight specific molecular determinants of transporter functions. The role of the two proteins in GABA homeostasis is investigated by looking at the transport coupling mechanism, as well as structural and kinetic transport models. Furthermore, we review information on selective inhibitors together with the pharmacophore hypothesis of transporter substrates.

Role of a conserved ion-binding site tyrosine in ion selectivity of the Na+/K+ pump

The essential transmembrane Na+ and K+ gradients in animal cells are established by the Na+/K+ pump, a P-type ATPase that exports three Na+ and imports two K+ per ATP hydrolyzed. The mechanism by which the Na+/K+ pump distinguishes between Na+ and K+ at the two membrane sides is poorly understood. Crystal structures identify two sites (sites I and II) that bind Na+ or K+ and a third (site III) specific for Na+. The side chain of a conserved tyrosine at site III of the catalytic α-subunit (Xenopus-α1 Y780) has been proposed to contribute to Na+ binding by cation-π interaction. We substituted Y780 with natural and unnatural amino acids, expressed the mutants in Xenopus oocytes and COS-1 cells, and used electrophysiology and biochemistry to evaluate their function. Substitutions disrupting H-bonds impaired Na+ interaction, while Y780Q strengthened it, likely by H-bond formation. Utilizing the non-sense suppression method previously used to incorporate unnatural derivatives in ion channels, we were able to analyze Na+/K+ pumps with fluorinated tyrosine or phenylalanine derivatives inserted at position 780 to diminish cation-π interaction strength. In line with the results of the analysis of mutants with natural amino acid substitutions, the results with the fluorinated derivatives indicate that Na+-π interaction with the phenol ring at position 780 contributes minimally, if at all, to the binding of Na+. All Y780 substitutions decreased K+ apparent affinity, highlighting that a state-dependent H-bond network is essential for the selectivity switch at sites I and II when the pump changes conformational state.

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.

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.

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.

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.

Effects of oligomycin on transient currents carried by Na+ translocation of Bufo Na+/K(+)-ATPase expressed in Xenopus oocytes

Na(+)/K(+)-ATPase (NKA) exports 3Na(+) and imports 2K(+) at the expense of the hydrolysis of 1ATP under physiological conditions. In the absence of K(+), it can mediate electroneutral Na(+)/Na(+) exchange. In the electroneutral Na(+)/Na(+) exchange mode, NKA produces a transient current containing fast, medium and slow components in response to a sudden voltage step. These three components of the transient current demonstrate the sequential release of Na(+) ions from three binding sites. Our data from oocytes provide further experimental support for the existence of these components. Oligomycin is an NKA inhibitor that favors the 2Na(+)-occluded state without affecting the conformational state of the NKA. We studied the effects of oligomycin on both K(+)-activated currents and transient currents in wild-type Bufo NKA and a mutant form of Bufo NKA, NKA: G813A. Oligomycin blocked almost all of the K(+)-activated current, although the three components of the transient current showed different sensitivities to oligomycin. The oligomycin-inhibited charge movement measured using a P/4 protocol had a rate coefficient similar to the medium transient component. The fast component of the transient current elicited by a short voltage step also showed sensitivity to oligomycin. However, the slow component was not totally inhibited by oligomycin. Our results indicate that the second and third sodium ions might be released to the extracellular medium by a mechanism that is not shared by the first sodium ion.

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.

Quaternary benzyltriethylammonium ion binding to the Na,K-ATPase: a tool to investigate extracellular K+ binding reactions

This study examined how the quaternary organic ammonium ion, benzyltriethylamine (BTEA), binds to the Na,K-ATPase to produce membrane potential (V(M))-dependent inhibition and tested the prediction that such a V(M)-dependent inhibitor would display electrogenic binding kinetics. BTEA competitively inhibited K(+) activation of Na,K-ATPase activity and steady-state (86)Rb(+) occlusion. The initial rate of (86)Rb(+) occlusion was decreased by BTEA to a similar degree whether it was added to the enzyme prior to or simultaneously with Rb(+), a demonstration that BTEA inhibits the Na,K-ATPase without being occluded. Several BTEA structural analogues reversibly inhibited Na,K-pump current, but none blocked current in a V(M)-dependent manner except BTEA and its para-nitro derivative, pNBTEA. Under conditions that promoted electroneutral K(+)-K(+) exchange by the Na,K-ATPase, step changes in V(M) elicited pNBTEA-activated ouabain-sensitive transient currents that had similarities to those produced with the K(+) congener, Tl(+). pNBTEA- and Tl(+)-dependent transient currents both displayed saturation of charge moved at extreme negative and positive V(M), equivalence of charge moved during and after step changes in V(M), and similar apparent valence. The rate constant (k(tot)) for Tl(+)-dependent transient current asymptotically approached a minimum value at positive V(M). In contrast, k(tot) for pNBTEA-dependent transient current was a "U"-shaped function of V(M) with a minimum value near 0 mV. Homology models of the Na,K-ATPase alpha subunit suggested that quaternary amines can bind to two extracellularly accessible sites, one of them located at K(+) binding sites positioned between transmembrane helices 4, 5, and 6. Altogether, these data revealed important information about electrogenic ion binding reactions of the Na,K-ATPase that are not directly measurable during ion transport by this enzyme.

Measuring ion transport activities in Xenopus oocytes using the ion-trap technique

The ion-trap technique is an experimental approach allowing measurement of changes in ionic concentrations within a restricted space (the trap) comprised of a large-diameter ion-selective electrode apposed to a voltage-clamped Xenopus laevis oocyte. The technique is demonstrated with oocytes expressing the Na(+)/glucose cotransporter (SGLT1) using Na(+)- and H(+)-selective electrodes and with the electroneutral H(+)/monocarboxylate transporter (MCT1). In SGLT1-expressing oocytes, bath substrate diffused into the trap within 20 s, stimulating Na(+)/glucose influx, which generated a measurable decrease in the trap Na(+) concentration ([Na(+)](T)) by 0.080 +/- 0.009 mM. Membrane hyperpolarization produced a further decrease in [Na(+)](T), which was proportional to the increased cotransport current. In a Na(+)-free, weakly buffered solution (pH 5.5), H(+) drives glucose transport through SGLT1, and this was monitored with a H(+)-selective electrode. Proton movements can also be clearly detected on adding lactate to an oocyte expressing MCT1 (pH 6.5). For SGLT1, time-dependent changes in [Na(+)](T) or [H(+)](T) were also detected during a membrane potential pulse (150 ms) in the presence of substrate. In the absence of substrate, hyperpolarization triggered rapid reorientation of SGLT1 cation binding sites, accompanied by cation capture from the trap. The resulting change in [Na(+)](T) or [H(+)](T) is proportional to the pre-steady-state charge movement. The ion-trap technique can thus be used to measure steady-state and pre-steady-state transport activities and provides new opportunities for studying electrogenic and electroneutral ion transport mechanisms.

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.

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.

A General Channel Model Accounts for Channel, Carrier, Countertransport and Cotransport Kinetics

In this work we propose a unifying model of mediated membrane transport, based upon the idea that the integral membrane proteins involved in these processes operate via complex channel mechanisms. In the first part, we briefly review literature about the structural aspects of membrane transporters. We conclude that there is a substantial amount of evidence suggesting that most membrane proteins performing transport are embodied with channel-like structures that may constitute the translocation paths. This includes cases where the phenomenological transport kinetics do not correspond to the classical channel behavior. In the second part of this article we introduce the general channel model of mediated transport and employ it to derive specific examples, like simple one- or two-ligand channels, water-ligand channels, simple carriers, co- and counter-transport systems and more complex water-ligand carriers. We show that, for the most part, these particular cases can be obtained by the application of the techniques of diagram reduction to the full model. The necessary conditions for diagram reduction reflect physical properties of the protein and its surroundings.

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.

Effect of ADP on Na(+)-Na(+) exchange reaction kinetics of Na,K-ATPase

The whole-cell voltage-clamp technique was used in rat cardiac myocytes to investigate the kinetics of ADP binding to phosphorylated states of Na,K-ATPase and its effects on presteady-state Na(+)-dependent charge movements by this enzyme. Ouabain-sensitive transient currents generated by Na,K-ATPase functioning in electroneutral Na(+)-Na(+) exchange mode were measured at 23 degrees C with pipette ADP concentrations ([ADP]) of up to 4.3 mM and extracellular Na(+) concentrations ([Na](o)) between 36 and 145 mM at membrane potentials (V(M)) from -160 to +80 mV. Analysis of charge-V(M) curves showed that the midpoint potential of charge distribution was shifted toward more positive V(M) both by increasing [ADP] at constant Na(+)(o) and by increasing [Na](o) at constant ADP. The total quantity of mobile charge, on the other hand, was found to be independent of changes in [ADP] or [Na](o). The presence of ADP increased the apparent rate constant for current relaxation at hyperpolarizing V(M) but decreased it at depolarizing V(M) as compared to control (no added ADP), an indication that ADP binding facilitates backward reaction steps during Na(+)-Na(+) exchange while slowing forward reactions. Data analysis using a pseudo three-state model yielded an apparent K(d) of approximately 6 mM for ADP binding to and release from the Na,K-ATPase phosphoenzyme; a value of 130 s(-1) for k(2), a rate constant that groups Na(+) deocclusion/release and the enzyme conformational transition E(1) approximately P --> E(2)-P; a value of 162 s(-1)M(-1) for k(-2), a lumped second-order V(M)-independent rate constant describing the reverse reactions; and a Hill coefficient of approximately 1 for Na(+)(o) binding to E(2)-P. The results are consistent with electroneutral release of ADP before Na(+) is deoccluded and released through an ion well. The same approach can be used to study additional charge-moving reactions and associated electrically silent steps of the Na,K-pump and other transporters.

Structure-function relationship in P-type ATPases--a biophysical approach

P-type ATPases are a large family of membrane proteins that perform active ion transport across biological membranes. In these proteins the energy-providing ATP hydrolysis is coupled to ion-transport that builds up or maintains the electrochemical potential gradients of one or two ion species across the membrane. P-type ATPases are found in virtually all eukaryotic cells and also in bacteria, and they are transporters of a broad variety of ions. So far, a crystal structure with atomic resolution is available only for one species, the SR Ca-ATPase. However, biochemical and biophysical studies provide an abundance of details on the function of this class of ion pumps. The aim of this review is to summarize the results of preferentially biophysical investigations of the three best-studied ion pumps, the Na,K-ATPase, the gastric H,K-ATPase, and the SR Ca-ATPase, and to compare functional properties to recent structural insights with the aim of contributing to the understanding of their structure-function relationship.

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.

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.

Role of the intracellular domain of the beta subunit in Na,K pump function

The catalytic alpha subunit of the (Na,K)- and (H,K)-ATPases needs to be coexpressed with a beta subunit in order to produce cation transport activity. Although the isoform of the beta subunit is known to influence the functional characteristics of the Na,K pump, the role of the different domains of the beta subunit is not fully understood. We have studied the function of a Na,K pump resulting from the expression of a wild-type alpha subunit with a N-terminally truncated mutant of the beta subunit using the two-electrode voltage clamp and the cut-open oocyte techniques. While the maximal activity, measured as the K+-activated outward current, was not significantly altered, the beta N-terminal truncation induced an ouabain-sensitive conductance in the absence of extracellular K+. The voltage dependence of the ouabain-sensitive charge distribution indicated that in the Na/Na exchange conditions, the E1-E2 conformation equilibrium was shifted towards the E2 conformation, a change resulting from alteration of both the forward and the backward reaction rate. Removal of the intracellular domain of the beta subunit modifies several aspects of the whole enzyme function by a mechanism that must imply the state of the extracellular and/or transmembrane parts of the alpha/beta subunit complex.

The voltage dependence of a cloned mammalian renal type II Na+/Pi cotransporter (NaPi-2)

The voltage dependence of the rat renal type II Na+/Pi cotransporter (NaPi-2) was investigated by expressing NaPi-2 in Xenopus laevis oocytes and applying the two-electrode voltage clamp. In the steady state, superfusion with inorganic phosphate (Pi) induced inward currents (Ip) in the presence of 96 mM Na+ over the potential range -140 </= V </= +40 mV. With Pi as the variable substrate, the apparent affinity constant (KmPi) was strongly dependent on Na+, increasing sixfold for a twofold reduction in external Na+. KmPi increased with depolarizing voltage and was more sensitive to voltage at reduced Na+. The Hill coefficient was close to unity and the predicted maximum Ip (Ipmax) was 40% smaller at 50 mM Na+. With Na+ as the variable substrate, KmNa was weakly dependent on both Pi and voltage, the Hill coefficient was close to 3 and Ipmax was independent of Pi at -50 mV. The competitive inhibitor phosphonoformic acid suppressed the steady state holding current in a Na+-dependent manner, indicating the existence of uncoupled Na+ slippage. Voltage steps induced pre-steady state relaxations typical for Na+-coupled cotransporters. NaPi-2-dependent relaxations were quantitated by a single, voltage-dependent exponential. At 96 mM Na+, a Boltzmann function was fit to the steady state charge distribution (Q-V) to give a midpoint voltage (V0.5) in the range -20 to -50 mV and an apparent valency of approximately 0.5 e-. V0.5 became more negative as Na+ was reduced. Pi suppressed relaxations in a dose-dependent manner, but had little effect on their voltage dependence. Reducing external pH shifted V0.5 to depolarizing potentials and suppressed relaxations in the absence of Na+, suggesting that protons interact with the unloaded carrier. These findings were incorporated into an ordered kinetic model whereby Na+ is the first and last substrate to bind, and the observed voltage dependence arises from the unloaded carrier and first Na+ binding step.

The Na+,K+-ATPase carrying the carboxy-terminal Ca2+/calmodulin binding domain of the Ca2+ pump has 2Na+,2K+ stoichiometry and lost charge movement in Na+/Na+ exchange

An altered ion-transport stoichiometry from 3Na+,2K+ to 2Na+,2K+ is observed in a chimeric Na+,K+ATPase, which carries the Ca2+/calmodulin binding domain (CBD) of the plasma membrane Ca2+-ATPase at its carboxy-terminus [Zhao et al., FEBS Lett. 408 (1997) 271-2751. The ouabain-resistant mutant of this chimera (ORalpha1-CBD) was constructed to further investigate the effect of the CBD on ion-transport properties. The ORalpha1-CBD still shows the 2Na+,2K+ stoichiometry. The loss of electrogenicity is accompanied by the disappearance of transient charge movements in the Na+/Na+ exchange mode. We conclude that the binding of the third Na+ ion, but not of the two others, in 3Na+,2K+ transport mode apparently senses the electric field, and that the voltage-dependent Na+ binding is likely to be lost in the chimera with CBD.

Comparison of Na+/K(+)-ATPase pump currents activated by ATP concentration or voltage jumps

Using the giant patch technique, we combined two fast relaxation methods on excised patches from guinea pig cardiomyocytes to compare the rate constants of the involved reaction steps. Experiments were done in the absence of intra- or extracellular K+. Fast ATP concentration jumps were generated by photolysis of caged ATP at pH 6.3 with laser flash irradiation at a wavelength of 308 nm and 10 ns duration, as described previously. Transient outward currents with a fast rising phase, followed by a slower decay and a small stationary current, were obtained. Voltage pulses were applied to the same patch in the presence or absence of intracellular ATP. Subtraction of the voltage jump-induced currents in the absence of ATP from those taken in the presence of ATP yielded monoexponential transient current signals, which were dependent on external Na+ but did not differ between intracellular pH (pHi) values 6.3 or 7.4. Rate constants showed a characteristic voltage dependence, i.e., saturating at positive potentials (approximately 200 s-1, 24 degrees C) and exponentially rising with increasing negative potentials. Rate constants of the fast component from transient currents obtained after an ATP concentration jump agree well with rate constants from currents obtained after a voltage jump to zero or positive potentials (pHi 6.3), and the two exhibit the same activation energy of approximately 80 kJ.mol-1. For a given membrane patch, the amount of charge that is moved across the plasma membrane is roughly the same for each of the two relaxation techniques.

Ion binding and permeation at the GABA transporter GAT1

This study addresses the binding of ions and the permeation of substrates during function of the GABA transporter GAT1. GAT1 was expressed in Xenopus oocytes and studied electrophysiologically as well as with [3H]GABA flux; GAT1 was also expressed in mammalian cells and studied with [3H]GABA and [3H]tiagabine binding. Voltage jumps, Na+ and Cl- concentration jumps, and exposure to high-affinity blockers (NO-05-711 and SKF-100330A) all produce capacitive charge movements. Occlusive interactions among these three types of perturbations show that they all measure the same population of charges. The concentration dependences of the charge movements reveal (1) that two Na+ ions interact with the transporter even in the absence of GABA, and (2) that Cl- facilitates the binding of Na+. Comparison between the charge movements and the transport-associated current shows that this initial Na(+)-transporter interaction limits the overall transport rate when [GABA] is saturating. However, two classes of manipulation--treatment with high-affinity uptake blockers and the W68L mutation-"lock" Na+ onto the transporter by slowing or preventing the subsequent events that release the substrates to the intracellular medium. The Na+ substitutes Li+ and Cs+ do not support charge movements, but they can permeate the transporter in an uncoupled manner. Our results (1) support the hypothesis that efficient removal of synaptic transmitter by the GABA transporter GAT1 depends on the previous binding of Na+ and Cl-, and (2) indicate the important role of the conserved putative transmembrane domain 1 in interactions with the permeant substrates.

Na+,K(+)-ATPase pump currents in giant excised patches activated by an ATP concentration jump

The giant-patch technique was used to study the Na+,K(+)-ATPase in excised patches from rat or guinea pig ventricular myocytes. Na+,K(+)-pump currents showed a saturable ATP dependence with aK(m) of approximately 150 microM at 24 degrees C. The pump current can be completely abolished by ortho-vanadate. Dissociation of vanadate from the enzyme in the absence of extracellular Na+ was slow, with a Koff of 3.10(-4) S-1 (K1 approximately 0.5 microM, at 24 degrees C). Stationary currents were markedly dependent on intracellular pH, with a maximum at pH 7.9. Temperature-dependence measurements of the stationary pump current yielded an activation energy of approximately 100 kJ mol-1. Partial reactions in the transport cycle were investigated by generating ATP concentration jumps through photolytic release of ATP from caged ATP at pH 7.4 and 6.3. Transient outward currents were obtained at pH 6.3 with a fast rising phase followed by a slower decay to a stationary current. It was concluded that the fast rate constant of approximately 200 s-1 at 24 degrees C (pH 6.3) reflects a step rate-limiting the electrogenic Na+ release. Simulating the data with a simple three-state model enabled us to estimate the turnover rate under saturating substrate concentrations, yielding rates (at pH 7.4) of approximately 60 s-1 and 200 s-1 at 24 degrees C and 36 degrees C, respectively.

Ion binding and permeation at the GABA transporter GAT1

This study addresses the binding of ions and the permeation of substrates during function of the GABA transporter GAT1. GAT1 was expressed in Xenopus oocytes and studied electrophysiologically as well as with [3H]GABA flux; GAT1 was also expressed in mammalian cells and studied with [3H]GABA and [3H]tiagabine binding. Voltage jumps, Na+ and Cl- concentration jumps, and exposure to high-affinity blockers (NO-05-711 and SKF-100330A) all produce capacitive charge movements. Occlusive interactions among these three types of perturbations show that they all measure the same population of charges. The concentration dependences of the charge movements reveal (1) that two Na+ ions interact with the transporter even in the absence of GABA, and (2) that Cl- facilitates the binding of Na+. Comparison between the charge movements and the transport-associated current shows that this initial Na(+)-transporter interaction limits the overall transport rate when [GABA] is saturating. However, two classes of manipulation--treatment with high-affinity uptake blockers and the W68L mutation-"lock" Na+ onto the transporter by slowing or preventing the subsequent events that release the substrates to the intracellular medium. The Na+ substitutes Li+ and Cs+ do not support charge movements, but they can permeate the transporter in an uncoupled manner. Our results (1) support the hypothesis that efficient removal of synaptic transmitter by the GABA transporter GAT1 depends on the previous binding of Na+ and Cl-, and (2) indicate the important role of the conserved putative transmembrane domain 1 in interactions with the permeant substrates.

Role in cation translocation of the N-terminus of the alpha-subunit of the Na(+)-K+ pump of Bufo

1. We have studied the effects on the physiological properties of the Na(+)-K+ pump of both 31- and 40-amino acid N-terminal truncated forms of the alpha-subunit of the Na(+)-K(+)-ATPase. 2. Na(+)-K+ pumps that were moderately ouabain resistant (K1 = 50 microM) were expressed in the Xenopus oocyte by injection of wild-type or truncated variants of the Bufo marinus Na(+)-K(+)-ATPase alpha-subunit cRNA with Bufo beta-subunit cRNA. The function of the Na(+)-K+ pump was studied by electrophysiological methods after Na+ loading and inhibition of the endogenous Xenopus Na(+)-K(+)-ATPase by exposure to a low concentration (0.2 microM) of ouabain. 3. The voltage-dependent potassium activation kinetics of the Na(+)-K+ pump current and the ouabain-sensitive proton-dependent inward current were studied using the two-electrode voltage-clamp technique. A novel technique involving permeabilization of part of the oocyte membrane with digitonin was developed to enable study of the pre-steady-state current following fast voltage perturbation. 4. By comparison with the wild type, the 40-amino acid N-terminal truncation induced a lower level of Na(+)-K+ pump current, a 2- to 3-fold reduction in the apparent external K+ affinity when measured in the presence of extracellular Na+, a relative increase in the proton-dependent inward current, and a reduction in the rate constant of the pre-steady-state current following a voltage step towards a positive membrane potential. The 31-amino acid truncation induced changes that were qualitatively similar but of smaller magnitude. 5. We have analysed these results using a kinetic model of the Na(+)-K+ pump cycle and have shown that all these effects can be explained by the change in a single rate constant in the cycle kinetics, namely a reduction in the rate of the main charge translocating part of the Na(+)-K+ pump cycle, i.e. the forward E1 to E2 conformational change, the deocclusion and release of Na+ to the external side. 6. The highly charged N-terminal segment seems to be directly involved in the mechanism that translocates Na+ ions across the membrane's electrical field.

Mouse sperm membrane potential: changes induced by Ca2+

Mouse sperm resting membrane potential (Er) (-42 +/- 8.8 mV), determined with a potential sensitive dye, depended on extracellular K+ and, in the absence of extracellular Ca2+ ([Ca2+]e), on external Na+ ([Na+]e). Ca2+ addition (> 5 microM) to sperm in Ca-free media induced a transient hyperpolarization (Ca-ith) which strongly depended on [Na+]e and less on external Cl- ([Cl-]e). Cd2+ and Mn2+ (microM) mimicked the Ca2+ effect, but not Ba2+. The Ca-ith was partially inhibited by ouabain (74%, IC50 = 5.8 microM) and niflumic acid (38%, IC50 = 240 microM), indicating the participation of the Na-K ATPase and Cl- channels. In Ca-free low-Na+ media, Ca2+ addition caused a depolarization sensitive to: nimodipine (25 microM), trifluoperazine (12.5 microM) and Mg2+ (1.2 mM), suggesting the participation of Ca2+ channels. Since some inhibitors of the sperm Ca-ith block the acrosome reaction (AR), both processes may share transport systems.

Specific inhibition of Na-Ca exchange function by antisense oligodeoxynucleotides

The Na-Ca exchanger is essential for the Ca2+ homeostasis in many cell types. This transporter has been difficult to investigate because no specific inhibitor is available. We have synthesized an antisense oligodeoxynucleotide directed against the rat cardiac Na-Ca exchanger mRNA. To estimate the activity of the Na-Ca exchange in single cultured myocytes, the exchange current (INaCa) was measured with the voltage-clamp technique while the intracellular Ca2+ concentration ([Ca2+]i) was simultaneously recorded. Most cells exposed to antisense oligodeoxynucleotide showed neither an INaCa nor an increase of [Ca2+]i upon extracellular Na+ removal. Liberation of Ca2+ by flashphotolysis of caged Ca2+ was not followed by a decay of [Ca2+]i in cells exposed to the antisense oligonucleotide, whereas in control cells resting [Ca2+]i was reached 6 s after the flash. Control experiments with non-sense and mismatched oligonucleotides were performed to exclude unspecific inhibitory effects. These results demonstrate that the Na-Ca exchange was specifically and completely suppressed and that antisense oligodeoxynucleotides represent a useful tool to investigate the cellular and molecular properties of the Na-Ca exchanger.

A conformation of Na(+)-K+ pump is permeable to proton

The Na(+)-K+ pump is thought to operate through a two-conformation (E1-E2) transport cycle in which the cation binding sites are accessible only from one side at a time. Using Na(+)-loaded Xenopus oocytes in which Na(+)-K+ pumps were overexpressed by injection of cRNA of the Xenopus Na(+)-K+ pump alpha-and beta-sub units, we observed a Na(+)-K+ pump-mediated (ouabain-sensitive) inward current in the absence of other transportable cations, except H+, in the external solution. This inward current was strongly inwardly rectifying, pH dependent, and larger at acid pH. Under conditions favoring a large ouabain-sensitive inward current, we observed a ouabain-sensitive intracellular acidification, and the amplitude of the acidification was significantly related to the ouabain-sensitive current, indicating that this current was carried by protons. The reversal potential of the ouabain-sensitive current was dependent on external pH as expected for a proton-conductive pathway. We conclude that in the absence of external K+ the Na(+)-K+ pump can mediate a large inward electrogenic transport of proton. This is most easily explained by the hypothesis that the E2 conformation of the Na(+)-K+ pump with cation binding sites exposed to the outside is accessible to protons from both sides and thus provides a channellike pathway for protons.