The role of Na,K-ATPase alpha subunit serine 775 and glutamate 779 in determining the extracellular K+ and membrane potential-dependent properties of the Na,K-pump

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.

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.

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.

An ion gating mechanism of gastric H,K-ATPase based on molecular dynamics simulations

Gastric H,K-ATPase is an electroneutral transmembrane pump that moves protons from the cytoplasm of the parietal cell into the gastric lumen in exchange for potassium ions. The mechanism of transport against the established electrochemical gradients includes intermediate conformations in which the transferred ions are trapped (occluded) within the membrane domain of the pump. The pump cycle involves switching between the E1 and E2P states. Molecular dynamics simulations on homology models of the E2P and E1 states were performed to investigate the mechanism of K(+) movement in this enzyme. We performed separate E2P simulations with one K(+) in the luminal channel, one K(+) ion in the occlusion site, two K(+) ions in the occlusion site, and targeted molecular dynamics from E2P to E1 with two K(+) ions in the occlusion site. The models were inserted into a lipid bilayer system and were stable over the time course of the simulations, and K(+) ions in the channel moved to a consistent location near the center of the membrane domain, thus defining the occlusion site. The backbone carbonyl oxygen from residues 337 through 342 on the nonhelical turn of M4, as well as side-chain oxygen from E343, E795, and E820, participated in the ion occlusion. A single water molecule was stably bound between the two K(+) ions in the occlusion site, providing an additional ligand and partial shielding the positive charges from one another. Targeted molecular dynamics was used to transform the protein from the E2P to the E1 state (two K(+) ions to the cytoplasm). This simulation identified the separation of the water column in the entry channel as the likely gating mechanism on the luminal side. A hydrated exit channel also formed on the cytoplasmic side of the occlusion site during this simulation. Hence, water molecules became available to hydrate the ions. The movement of the M1M2 transmembrane segments, and the displacement of residues Q159, E160, Q110, and T152 during the conformational change, as well as the motions of E343 and L346, acted as the cytoplasmic-side gate.

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.

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.

Large diameter of palytoxin-induced Na/K pump channels and modulation of palytoxin interaction by Na/K pump ligands

Palytoxin binds to Na/K pumps to generate nonselective cation channels whose pore likely comprises at least part of the pump's ion translocation pathway. We systematically analyzed palytoxin's interactions with native human Na/K pumps in outside-out patches from HEK293 cells over a broad range of ionic and nucleotide conditions, and with or without cardiotonic steroids. With 5 mM internal (pipette) [MgATP], palytoxin activated the conductance with an apparent affinity that was highest for Na⁺-containing (K⁺-free) external and internal solutions, lowest for K⁺-containing (Na⁺-free) external and internal solutions, and intermediate for the mixed external Na⁺/internal K⁺, and external K⁺/internal Na⁺ conditions; with Na⁺ solutions and MgATP, the mean dwell time of palytoxin on the Na/K pump was about one day. With Na⁺ solutions, the apparent affinity for palytoxin action was low after equilibration of patches with nucleotide-free pipette solution. That apparent affinity was increased in two phases as the equilibrating [MgATP] was raised over the submicromolar, and submillimolar, ranges, but was increased by pipette MgAMPPNP in a single phase, over the submillimolar range; the apparent affinity at saturating [MgAMPPNP] remained ∼30-fold lower than at saturating [MgATP]. After palytoxin washout, the conductance decay that reflects palytoxin unbinding was accelerated by cardiotonic steroid. When Na/K pumps were preincubated with cardiotonic steroid, subsequent activation of palytoxin-induced conductance was greatly slowed, even after washout of the cardiotonic steroid, but activation could still be accelerated by increasing palytoxin concentration. These results indicate that palytoxin and a cardiotonic steroid can simultaneously occupy the same Na/K pump, each destabilizing the other. The palytoxin-induced channels were permeable to several large organic cations, including N-methyl-d-glucamine⁺, suggesting that the narrowest section of the pore must be ∼7.5 Å wide. Enhanced understanding of palytoxin action now allows its use for examining the structures and mechanisms of the gates that occlude/deocclude transported ions during the normal Na/K pump cycle.

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.

Electrogenicity of Na,K- and H,K-ATPase activity and presence of a positively charged amino acid in the fifth transmembrane segment

The transport activity of the Na,K-ATPase (a 3 Na+ for 2 K+ ion exchange) is electrogenic, whereas the closely related gastric and non-gastric H,K-ATPases perform electroneutral cation exchange. We have studied the role of a highly conserved serine residue in the fifth transmembrane segment of the Na,K-ATPase, which is replaced with a lysine in all known H,K-ATPases. Ouabain-sensitive 86Rb uptake and K+-activated currents were measured in Xenopus oocytes expressing the Bufo bladder H,K-ATPase or the Bufo Na,K-ATPase in which these residues, Lys800 and Ser782, respectively, were mutated. Mutants K800A and K800E of the H,K-ATPase showed K+-stimulated and ouabain-sensitive electrogenic transport. In contrast, when the positive charge was conserved (K800R), no K+-induced outward current could be measured, even though rubidium transport activity was present. Conversely, the S782R mutant of the Na,K-ATPase had non-electrogenic transport activity, whereas the S782A mutant was electrogenic. The K800S mutant of the H,K-ATPase had a more complex behavior, with electrogenic transport only in the absence of extracellular Na+. Thus, a single positively charged residue in the fifth transmembrane segment of the alpha-subunit can determine the electrogenicity and therefore the stoichiometry of cation transport by these ATPases.

Cation stoichiometry and cation pathway in the Na,K-ATPase and nongastric H,K-ATPase

The mechanism of cation translocation by the Na,K-ATPase was investigated by cysteine scanning mutagenesis and measurements of accessibility through exposure to cysteine reagents. In the native protein, accessible residues were found only at the most extracellular residues of the 5th and 6th transmembrane segments (TMS) and the short loop between them. However, after modification by palytoxin a number of residues became accessible along the whole length of the 5th TMS and in the outer half of the 6th TMS, showing the contribution of each of these segments to the "channel" formed by the palytoxin-transformed Na,K-pump. Assuming that this structure is similar in the native and the palytoxin-transformed pump, our data allow us to determine the residues lining the cation pathway from the extracellular solution to their binding sites. A critical position in the 5th TMS contains a lysine conserved in all known nonelectrogenic H,K-ATPases, and a serine in all known electrogenic Na,K-ATPase sequences. Wild-type or mutant Na,K-or H,K-ATPase a subunits were coinjected with the Bufo beta2 subunit in Xenopus oocytes and Rb(86) uptake and electrophysiological measurements were performed. An electrogenic activity was recorded for the H,K-ATPase mutants in which the positively charged lysine had been replaced by neutral or negatively charged residues, while nonelectrogenic transport was observed with the S(782)R mutant of the Na,K-ATPase. The presence or the absence of a positively charged residue at the S(782) position appears to be critical for the stoichiometry of cation exchange.

Functional differences between alpha subunit isoforms of the rat Na,K-ATPase expressed in Xenopus oocytes

The functional properties of the three most widely distributed alpha subunit isoforms of the Na,K-ATPase are not well known, particularly concerning the voltage dependence of their activity and cation binding kinetics. We measured the electrogenic activity generated by Na,K-ATPases resulting from co-expression of the rat alpha1, alpha2* or alpha3* subunits with the rat beta1 subunit in Xenopus oocytes; alpha2* and alpha3* are ouabain-resistant mutants of the alpha2 and alpha3 isoform, which allowed selective inhibition of the endogenous Na(+),K(+)-pump of the oocyte. In oocytes expressing the three isoforms of the alpha subunit, K(+) induced robust outward currents that were largely ouabain-sensitive. In addition, ouabain-sensitive inward currents were recorded for all three isoforms in sodium-free and potassium-free acid solutions. The very similar voltage dependence of the Na(+),K(+)-pump activity observed in the absence of extracellular Na(+) indicated a similar stoichiometry of the transported cations by the three isoforms. The affinity for extracellular K(+) was slightly lower for the alpha2* and alpha3* than for the alpha1 isoform. The alpha2* isoform was, however, more sensitive to voltage-dependent inhibition by extracellular Na(+), indicating a higher affinity of the extracellular Na(+) site in this isoform. We measured and controlled [Na(+)](i) using a co-expressed amiloride-sensitive Na(+) channel. The intracellular affinity for Na(+) was slightly higher in the alpha2* than in the alpha1 or alpha3* isoforms. These results suggest that the alpha2 isoform could have an activity that is strongly dependent upon [Na(+)](o) and [K(+)](o). These concentrations could selectively modulate its activity when large variations are present, for instance in the narrow intercellular spaces of brain or muscle tissues.

Site-Directed Mutagenesis of Cation Coordinating Residues in the Gastric H,K-ATPase

Site-mutations were introduced into putative cation binding site 1 of the H,K-ATPase at glu-797, thr-825, and glu-938. The side chain oxygen of each was not essential but the mutations produced different activation and inhibition kinetics. Site mutations thr-825 (ala, leu) and glu-938 (ala, gln) modestly decreased the apparent affinity to K+, while glu-797 (gln) was equivalent to wild type. As expected of competitive inhibition, mutations of thr-825 and glu-938 that decreased the apparent affinity for K+ also increased the apparent affinity for SCH28080. This is consistent with the participation of thr-825 and glu-938 in a cation binding domain. The sidechain geometry, but not the sidechain charge of glu-797, is essential to ATPase function as the site mutant glu-797 (gly) inactivated the H,K-ATPase, while glu-797 (gln) was active but the apparent affinity to SCH 28080 was decreased by four-fold. Lys-793, a unique residue of the H,K-ATPase, was essential for ATPase function. Since this residue is adjacent to site 1, the result suggests that charge pairing between lys-793 and residues at or near this site may be essential to ATPase function.

Electrogenic sodium-sodium exchange carried out by Na,K-ATPase containing the amino acid substitution Glu779Ala

Na,K-ATPase containing the amino acid substitution glutamate to alanine at position 779 of the alpha subunit (Glu779Ala) supports a high level of Na-ATPase and electrogenic Na+-Na+ exchange activity in the absence of K+. In microsomal preparations of Glu779Ala enzyme, the Na+ concentration for half maximal activation of Na-ATPase activity was 161 +/- 14 mM (n = 3). Furthermore, enzyme activity with 800 mM Na+ was found to be similar in the presence and absence of 20 mM K+. These results showed that Na+, with low affinity, could stimulate enzyme turnover as effectively as K+. To gain further insight into the mechanism of this enzyme activity, HeLa cells expressing Glu779Ala enzyme were voltage clamped with patch electrodes containing 115 mM Na+ during superfusion in K+-free solutions. Electrogenic Na+-Na+ exchange was observed as an ouabain-inhibitable outward current whose amplitude was proportional to extracellular Na+ (Na+(o)) concentration. At all Na+(o) concentrations tested (3-148 mM), exchange current was maximal at negative membrane potentials (V(M)), but decreased as V(M) became more positive. Analyzing this current at each V(M) with a Hill equation showed that Na+-Na+ exchange had a high-affinity, low-capacity component with an apparent Na+(o) affinity at 0 mV (K0(0.5)) of 13.4 +/- 0.6 mM and a low-affinity, high-capacity component with a K0(0.5) of 120 +/- 13 mM (n = 17). Both high- and low-affinity exchange components were V(M) dependent, dissipating 30 +/- 3% and 82 +/- 6% (n = 17) of the membrane dielectric, respectively. The low-affinity, but not the high-affinity exchange component was inhibited with 2 mM free ADP in the patch electrode solution. These results suggest that the high-affinity component of electrogenic Na+-Na+ exchange could be explained by Na+(o) acting as a low-affinity K+ congener; however, the low-affinity component of electrogenic exchange appeared to be due to forward enzyme cycling activated by Na+(o) binding at a Na+-specific site deep in the membrane dielectric. A pseudo six-state model for the Na,K-ATPase was developed to simulate these data and the results of the accompanying paper (Peluffo, R.D., J.M. Argüello, and J.R. Berlin. 2000. J. Gen. Physiol. 116:47-59). This model showed that alterations in the kinetics of extracellular ion-dependent reactions alone could explain the effects of Glu779Ala substitution on the Na,K-ATPase.