Crystal structures of Na+ ,K+ -ATPase reveal the mechanism that converts the K+ -bound form to Na+ -bound form and opens and closes the cytoplasmic gate.

Na+ ,K+ -ATPase (NKA) plays a pivotal role in establishing electrochemical gradients for Na+ and K+ across the cell membrane by alternating between the E1 (showing high affinity for Na+ and low affinity for K+ ) and E2 (low affinity to Na+ and high affinity to K+ ) forms. Presented here are two crystal structures of NKA in E1·Mg2+ and E1·3Na+ states at 2.9 and 2.8 Å resolution, respectively. These two E1 structures fill a gap in our description of the NKA reaction cycle based on the atomic structures. We describe how NKA converts the K+ -bound E2·2K+ form to an E1 (E1·Mg2+ ) form, which allows high-affinity Na+ binding, eventually closing the cytoplasmic gate (in E1 ~ P·ADP·3Na+ ) after binding three Na+ , while keeping the extracellular ion pathway sealed. We now understand previously unknown functional roles for several parts of NKA and that NKA uses even the lipid bilayer for gating the ion pathway.

Cryo-electron microscopy of Na+ ,K+ -ATPase reveals how the extracellular gate locks in the E2·2K+ state.

Na+ ,K+ -ATPase (NKA) is one of the most important members of the P-type ion-translocating ATPases and plays a pivotal role in establishing electrochemical gradients for Na+ and K+ across the cell membrane. Presented here is a 3.3 Å resolution structure of NKA in the E2·2K+ state solved by cryo-electron microscopy. It is a stable state with two occluded K+ after transferring three Na+ into the extracellular medium and releasing inorganic phosphate bound to the cytoplasmic P domain. We describe how the extracellular ion pathway wide open in the E2P state becomes closed and locked in E2·2K+ , linked to events at the phosphorylation site more than 50 Å away. We also show, although at low resolution, how ATP binding to NKA in E2·2K+ relaxes the gating machinery and thereby accelerates the transition into the next step, that is, the release of K+ into the cytoplasm, more than 100 times.

Cryoelectron microscopy of Na+,K+-ATPase in the two E2P states with and without cardiotonic steroids.

Cryoelectron microscopy (cryo-EM) was applied to Na+,K+-ATPase (NKA) to determine the structures of two E2P states, one (E2PATP) formed by ATP and Mg2+ in the forward reaction, and the other (E2PPi) formed by inorganic phosphate (Pi) and Mg2+ in the backward reaction, with and without ouabain or istaroxime, representatives of classical and new-generation cardiotonic steroids (CTSs). These two E2P states exhibit different biochemical properties. In particular, K+-sensitive acceleration of the dephosphorylation reaction is not observed with E2PPi, attributed to the presence of a Mg2+ ion in the transmembrane cation binding sites. The cryo-EM structures of NKA demonstrate that the two E2P structures are nearly identical but Mg2+ in the transmembrane binding cavity is identified only in E2PPi, corroborating the idea that it should be denoted as E2PPi·Mg2+. We can now explain why the absence of transmembrane Mg2+ in E2PATP confers the K+ sensitivity in dephosphorylation. In addition, we show that ATP bridges the actuator (A) and nucleotide binding (N) domains, stabilizing the E2PATP state; CTS binding causes hardly any changes in the structure of NKA, both in E2PATP and E2PPi·Mg2+, indicating that the binding mechanism is conformational selection; and istaroxime binds to NKA, extending its aminoalkyloxime group deep into the cation binding site. This orientation is upside down compared to that of classical CTSs with respect to the steroid ring. Notably, mobile parts of NKA are resolved substantially better in the electron microscopy (EM) maps than in previous X-ray structures, including sugars sticking out from the ß-subunit and many phospholipid molecules.

Displacement of Native FXYD Protein From Na+/K+-ATPase With Novel FXYD Peptide Derivatives: Effects on Doxorubicin Cytotoxicity.

The seven mammalian FXYD proteins associate closely with α/ß heterodimers of Na+/K+-ATPase. Most of them protect the ß1 subunit against glutathionylation, an oxidative modification that destabilizes the heterodimer and inhibits Na+/K+-ATPase activity. A specific cysteine (Cys) residue of FXYD proteins is critical for such protection. One of the FXYD proteins, FXYD3, confers treatment resistance when overexpressed in cancer cells. We developed two FXYD3 peptide derivatives. FXYD3-pep CKCK retained the Cys residue that can undergo glutathionylation and that is critical for protecting the ß1 subunit against glutathionylation. FXYD3-pep SKSK had all Cys residues mutated to Serine (Ser). The chemotherapeutic doxorubicin induces oxidative stress, and suppression of FXYD3 with siRNA in pancreatic- and breast cancer cells that strongly express FXYD3 increased doxorubicin-induced cytotoxicity. Exposing cells to FXYD3-pep SKSK decreased co-immunoprecipitation of FXYD3 with the α1 Na+/K+-ATPase subunit. FXYD3-pep SKSK reproduced the increase in doxorubicin-induced cytotoxicity seen after FXYD3 siRNA transfection in pancreatic- and breast cancer cells that overexpressed FXYD3, while FXYD3-pep CKCK boosted the native protein's protection against doxorubicin. Neither peptide affected doxorubicin's cytotoxicity on cells with no or low FXYD3 expression. Fluorescently labeled FXYD3-pep SKSK was detected in a perinuclear distribution in the cells overexpressing FXYD3, and plasmalemmal Na+/K+-ATPase turnover could not be implicated in the increased sensitivity to doxorubicin that FXYD3-pep SKSK caused. FXYD peptide derivatives allow rapid elimination or amplification of native FXYD protein function. Here, their effects implicate the Cys residue that is critical for countering ß1 subunit glutathionylation in the augmentation of cytotoxicity with siRNA-induced downregulation of FXYD3.

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

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

Binding of cardiotonic steroids to Na+,K+-ATPase in the E2P state.

The sodium pump (Na+, K+-ATPase, NKA) is vital for animal cells, as it actively maintains Na+ and K+ electrochemical gradients across the cell membrane. It is a target of cardiotonic steroids (CTSs) such as ouabain and digoxin. As CTSs are almost unique strong inhibitors specific to NKA, a wide range of derivatives has been developed for potential therapeutic use. Several crystal structures have been published for NKA-CTS complexes, but they fail to explain the largely different inhibitory properties of the various CTSs. For instance, although CTSs are thought to inhibit ATPase activity by binding to NKA in the E2P state, we do not know if large conformational changes accompany binding, as no crystal structure is available for the E2P state free of CTS. Here, we describe crystal structures of the BeF3- complex of NKA representing the E2P ground state and then eight crystal structures of seven CTSs, including rostafuroxin and istaroxime, two new members under clinical trials, in complex with NKA in the E2P state. The conformations of NKA are virtually identical in all complexes with and without CTSs, showing that CTSs bind to a preformed cavity in NKA. By comparing the inhibitory potency of the CTSs measured under four different conditions, we elucidate how different structural features of the CTSs result in different inhibitory properties. The crystal structures also explain K+-antagonism and suggest a route to isoform specific CTSs.

Peptide Ligation at High Dilution via Reductive Diselenide-Selenoester Ligation.

Peptide ligation chemistry has revolutionized protein science by providing access to homogeneously modified peptides and proteins. However, lipidated polypeptides and integral membrane proteins-an important class of biomolecules-remain enormously challenging to access synthetically owing to poor aqueous solubility of one or more of the fragments under typical ligation conditions. Herein we describe the advent of a reductive diselenide-selenoester ligation (rDSL) method that enables efficient ligation of peptide fragments down to low nanomolar concentrations, without resorting to solubility tags or hybridizing templates. The power of rDSL is highlighted in the efficient synthesis of the FDA-approved therapeutic lipopeptide tesamorelin and palmitylated variants of the transmembrane lipoprotein phospholemman (FXYD1). Lipidation of FXYD1 was shown to critically modulate inhibitory activity against the Na+/K+ pump.

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

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

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

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

Interaction of N-terminal peptide analogues of the Na+,K+-ATPase with membranes.

The Na+,K+-ATPase, which is present in the plasma membrane of all animal cells, plays a crucial role in maintaining the Na+ and K+ electrochemical potential gradients across the membrane. Recent studies have suggested that the N-terminus of the protein's catalytic α-subunit is involved in an electrostatic interaction with the surrounding membrane, which controls the protein's conformational equilibrium. However, because the N-terminus could not yet be resolved in any X-ray crystal structures, little information about this interaction is so far available. In measurements utilising poly-l-lysine as a model of the protein's lysine-rich N-terminus and using lipid vesicles of defined composition, here we have identified the most likely origin of the interaction as one between positively charged lysine residues of the N-terminus and negatively charged headgroups of phospholipids (notably phosphatidylserine) in the surrounding membrane. Furthermore, to isolate which segments of the N-terminus could be involved in membrane binding, we chemically synthesized N-terminal fragments of various lengths. Based on a combination of results from RH421 UV/visible absorbance measurements and solid-state 31P and 2H NMR using these N-terminal fragments as well as MD simulations it appears that the membrane interaction arises from lysine residues prior to the conserved LKKE motif of the N-terminus. The MD simulations indicate that the strength of the interaction varies significantly between different enzyme conformations.

Distinct pH dependencies of Na+/K+ selectivity at the two faces of Na,K-ATPase.

The sodium pump (Na,K-ATPase) in animal cells is vital for actively maintaining ATP hydrolysis-powered Na+ and K+ electrochemical gradients across the cell membrane. These ion gradients drive co- and countertransport and are critical for establishing the membrane potential. It has been an enigma how Na,K-ATPase discriminates between Na+ and K+, despite the pumped ion on each side being at a lower concentration than the other ion. Recent crystal structures of analogs of the intermediate conformations E2·Pi·2K+ and Na+-bound E1∼P·ADP suggest that the dimensions of the respective binding sites in Na,K-ATPase are crucial in determining its selectivity. Here, we found that the selectivity at each membrane face is pH-dependent and that this dependence is unique for each face. Most notable was a strong increase in the specific affinity for K+ at the extracellular face (i.e. E2 conformation) as the pH is lowered from 7.5 to 5. We also observed a smaller increase in affinity for K+ on the cytoplasmic side (E1 conformation), which reduced the selectivity for Na+ Theoretical analysis of the pKa values of ion-coordinating acidic amino acid residues suggested that the face-specific pH dependences and Na+/K+ selectivities may arise from the protonation or ionization of key residues. The increase in K+ selectivity at low pH on the cytoplasmic face, for instance, appeared to be associated with Asp808 protonation. We conclude that changes in the ionization state of coordinating residues in Na,K-ATPase could contribute to altering face-specific ion selectivity.

Capturing suboptical dynamic structures in lipid bilayer patches formed from free-standing giant unilamellar vesicles.

There is accumulating evidence that the small-scale lateral organization of biological membranes has a crucial role in signaling and trafficking in cells. However, it has been difficult to characterize these features with existing methods for preparing and analyzing freestanding membranes, because the dynamics occurs below the optical resolution possible with these protocols. We have developed a protocol that permits the imaging of lipid nanodomains and lateral protein organization in membranes of giant unilamellar vesicles (GUVs). Freestanding GUVs are transferred onto a mica support, and after treatment with magnesium chloride, they collapse to form planar lipid bilayer (PLB) patches. Rapid GUV collapse onto the mica preserves the lateral organization of freestanding membranes and thus makes it possible to image 'snapshots' of GUVs up to nanometer resolution by high-resolution microscopy. The method has been applied to classical lipid raft mixtures in which suboptical domain fluctuations have been imaged in both the liquid-ordered and liquid-disordered membrane phases. High-resolution scanning by atomic force microscopy (AFM) of membranes composed of binary and ternary lipid mixtures reconstituted with Na+/K+-ATPase (NKA) has revealed the spatial distribution and orientations of individual proteins, as well as details of membrane lateral structure. Immunolabeling followed by confocal microscopy can also provide information about the spatial distribution of proteins. The protocol opens up a new avenue for quantitative biophysical studies of suboptical dynamic structures in biomembranes, which are local and short-lived. Preparation of GUVs, PLB patches and their imaging takes <24 h.

The voltage-sensitive dye RH421 detects a Na+,K+-ATPase conformational change at the membrane surface.

RH421 is a voltage-sensitive fluorescent styrylpyridinium dye which has often been used to probe the kinetics of Na+,K+-ATPase partial reactions. The origin of the dye's response has up to now been unclear. Here we show that RH421 responds to phosphorylation of the Na+,K+-ATPase by inorganic phosphate with a fluorescence increase. Analysis of the kinetics of the fluorescence response indicates that the probe is not detecting phosphorylation itself but rather a shift in the protein's E1/E2 conformational equilibrium induced by preferential phosphate binding to and phosphorylation of enzyme in the E2 conformation. Molecular dynamics simulations of crystal structures in lipid bilayers indicate some change in the protein's hydrophobic thickness during the E1-E2 transition, which may influence the dye response. However, the transition is known to involve significant rearrangement of the protein's highly charged lysine-rich cytoplasmic N-terminal sequence. Using poly-l-lysine as a model of the N-terminus, we show that an analogous response of RH421 to the E1→E2P conformational change is produced by poly-l-lysine binding to the surface of the Na+,K+-ATPase-containing membrane fragments. Thus, it seems that the prime origin of the RH421 fluorescence response is a change in the interaction of the protein's N-terminus with the surrounding membrane. Quantum mechanical calculations of the dye's visible absorption spectrum give further support to this conclusion. The results obtained indicate that membrane binding and release of the N-terminus of the Na+,K+-ATPase α-subunit are intimately involved in the protein's catalytic cycle and could represent an effective site of regulation.

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

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

Glutathionylation-Dependence of Na(+)-K(+)-Pump Currents Can Mimic Reduced Subsarcolemmal Na(+) Diffusion.

The existence of a subsarcolemmal space with restricted diffusion for Na(+) in cardiac myocytes has been inferred from a transient peak electrogenic Na(+)-K(+) pump current beyond steady state on reexposure of myocytes to K(+) after a period of exposure to K(+)-free extracellular solution. The transient peak current is attributed to enhanced electrogenic pumping of Na(+) that accumulated in the diffusion-restricted space during pump inhibition in K(+)-free extracellular solution. However, there are no known physical barriers that account for such restricted Na(+) diffusion, and we examined if changes of activity of the Na(+)-K(+) pump itself cause the transient peak current. Reexposure to K(+) reproduced a transient current beyond steady state in voltage-clamped ventricular myocytes as reported by others. Persistence of it when the Na(+) concentration in patch pipette solutions perfusing the intracellular compartment was high and elimination of it with K(+)-free pipette solution could not be reconciled with restricted subsarcolemmal Na(+) diffusion. The pattern of the transient current early after pump activation was dependent on transmembrane Na(+)- and K(+) concentration gradients suggesting the currents were related to the conformational poise imposed on the pump. We examined if the currents might be accounted for by changes in glutathionylation of the ß1 Na(+)-K(+) pump subunit, a reversible oxidative modification that inhibits the pump. Susceptibility of the ß1 subunit to glutathionylation depends on the conformational poise of the Na(+)-K(+) pump, and glutathionylation with the pump stabilized in conformations equivalent to those expected to be imposed on voltage-clamped myocytes supported this hypothesis. So did elimination of the transient K(+)-induced peak Na(+)-K(+) pump current when we included glutaredoxin 1 in patch pipette solutions to reverse glutathionylation. We conclude that transient K(+)-induced peak Na(+)-K(+) pump current reflects the effect of conformation-dependent ß1 pump subunit glutathionylation, not restricted subsarcolemmal diffusion of Na(+).

Spatial distribution and activity of Na(+)/K(+)-ATPase in lipid bilayer membranes with phase boundaries.

We have reconstituted functional Na(+)/K(+)-ATPase (NKA) into giant unilamellar vesicles (GUVs) of well-defined binary and ternary lipid composition including cholesterol. The activity of the membrane system can be turned on and off by ATP. The hydrolytic activity of NKA is found to depend on membrane phase, and the water relaxation in the membrane on the presence of NKA. By collapsing and fixating the GUVs onto a solid support and using high-resolution atomic-force microscopy (AFM) imaging we determine the protein orientation and spatial distribution at the single-molecule level and find that NKA is preferentially located at lo/ld interfaces in two-phase GUVs and homogeneously distributed in single-phase GUVs. When turned active, the membrane is found to unbind from the support suggesting that the protein function leads to softening of the membrane.

Membrane accessibility of glutathione.

Regulation of the ion pumping activity of the Na+,K+-ATPase is crucial to the survival of animal cells. Recent evidence has suggested that the activity of the enzyme could be controlled by glutathionylation of cysteine residue 45 of the ß-subunit. Crystal structures so far available indicate that this cysteine is in a transmembrane domain of the protein. Here we have analysed via fluorescence and NMR spectroscopy as well as molecular dynamics simulations whether glutathione is able to penetrate into the interior of a lipid membrane. No evidence for any penetration of glutathione into the membrane was found. Therefore, the most likely mechanism whereby the cysteine residue could become glutathionylated is via a loosening of the α-ß subunit association, creating a hydrophilic passageway between them to allow access of glutathione to the cysteine residue. By such a mechanism, glutathionylation of the protein would be expected to anchor the modified cysteine residue in a hydrophilic environment, inhibiting further motion of the ß-subunit during the enzyme's catalytic cycle and suppressing enzymatic activity, as has been experimentally observed. The results obtained, therefore, suggest a possible structural mechanism of how the Na+,K+-ATPase could be regulated by glutathione.

Sequential substitution of K(+) bound to Na(+),K(+)-ATPase visualized by X-ray crystallography.

Na(+),K(+)-ATPase transfers three Na(+) from the cytoplasm into the extracellular medium and two K(+) in the opposite direction per ATP hydrolysed. The binding and release of Na(+) and K(+) are all thought to occur sequentially. Here we demonstrate by X-ray crystallography of the ATPase in E2·MgF4(2-)·2K(+), a state analogous to E2·Pi·2K(+), combined with isotopic measurements, that the substitution of the two K(+) with congeners in the extracellular medium indeed occurs at different rates, substantially faster at site II. An analysis of thermal movements of protein atoms in the crystal shows that the M3-M4E helix pair opens and closes the ion pathway leading to the extracellular medium, allowing K(+) at site II to be substituted first. Taken together, these results indicate that site I K(+) is the first cation to bind to the empty cation-binding sites after releasing three Na(+).

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.

Inhibition of K+ transport through Na+, K+-ATPase by capsazepine: role of membrane span 10 of the α-subunit in the modulation of ion gating.

Capsazepine (CPZ) inhibits Na+,K+-ATPase-mediated K+-dependent ATP hydrolysis with no effect on Na+-ATPase activity. In this study we have investigated the functional effects of CPZ on Na+,K+-ATPase in intact cells. We have also used well established biochemical and biophysical techniques to understand how CPZ modifies the catalytic subunit of Na+,K+-ATPase. In isolated rat cardiomyocytes, CPZ abolished Na+,K+-ATPase current in the presence of extracellular K+. In contrast, CPZ stimulated pump current in the absence of extracellular K+. Similar conclusions were attained using HEK293 cells loaded with the Na+ sensitive dye Asante NaTRIUM green. Proteolytic cleavage of pig kidney Na+,K+-ATPase indicated that CPZ stabilizes ion interaction with the K+ sites. The distal part of membrane span 10 (M10) of the α-subunit was exposed to trypsin cleavage in the presence of guanidinum ions, which function as Na+ congener at the Na+ specific site. This effect of guanidinium was amplified by treatment with CPZ. Fluorescence of the membrane potential sensitive dye, oxonol VI, was measured following addition of substrates to reconstituted inside-out Na+,K+-ATPase. CPZ increased oxonol VI fluorescence in the absence of K+, reflecting increased Na+ efflux through the pump. Surprisingly, CPZ induced an ATP-independent increase in fluorescence in the presence of high extravesicular K+, likely indicating opening of an intracellular pathway selective for K+. As revealed by the recent crystal structure of the E1.AlF4-.ADP.3Na+ form of the pig kidney Na+,K+-ATPase, movements of M5 of the α-subunit, which regulate ion selectivity, are controlled by the C-terminal tail that extends from M10. We propose that movements of M10 and its cytoplasmic extension is affected by CPZ, thereby regulating ion selectivity and transport through the K+ sites in Na+,K+-ATPase.