Cysteine-scanning mutagenesis study of the sixth transmembrane segment of the Na,K-ATPase alpha subunit

Visualizing the mapped ion pathway through the Na,K-ATPase pump

The Na(+),K(+)-ATPase pump achieves thermodynamically uphill exchange of cytoplasmic Na(+) ions for extracellular K(+) ions by using ATP-mediated phosphorylation, followed by autodephosphorylation, to power conformational changes that allow ion access to the pump's binding sites from only one side of the membrane at a time. Formally, the pump behaves like an ion channel with two tightly coupled gates that are constrained to open and close alternately. The marine agent palytoxin disrupts this coupling, allowing both gates to sometimes be open, so temporarily transforming a pump into an ion channel. We made a cysteine scan of Na(+),K(+)-ATPase transmembrane (TM) segments TM1 to TM6, and used recordings of Na(+) current flow through palytoxin-bound pump-channels to monitor accessibility of introduced cysteine residues via their reaction with hydrophilic methanethiosulfonate (MTS) reagents. To visualize the open-channel pathway, the reactive positions were mapped onto a homology model of Na(+),K(+)-ATPase based on the structure of the related sarcoplasmicand endoplasmic-reticulum (SERCA) Ca(2+)-ATPase in a BeF(3)(-)-trapped state,(1,2) in which the extra-cytoplasmic gate is wide open (although the cytoplasmic access pathway is firmly shut). The results revealed a single unbroken chain of reactive positions that traverses the pump from the extracellular surface to the cytoplasm, comprises residues from TM1, TM2, TM4 and TM6, and passes through the equivalent of cation binding site II in SERCA, but not through site I. Cavity search analysis of the homology model validated its use for mapping the data by yielding a calculated extra-cytoplasmic pathway surrounded by MTS-reactive residues. As predicted by previous experimental results, that calculated extra-cytoplasmic pathway abruptly broadens above residue T806, at the outermost end of TM6 that forms the floor of the extracellular-facing vestibule. These findings provide a structural basis for further understanding cation translocation by the Na(+),K(+)-ATPase and by other P-type pumps like the Ca(2+)- and H(+),K(+)-ATPases.

Palytoxin: membrane mechanisms of action

Palytoxin is a marine toxin originally isolated from the zoantharians of the genus Palythoa, but now is found in marine organisms ranging from dinoflagellates to fishes. With a MW of 2680, it is one of the largest nonpolymeric natural products ever found. Its complex structure has been elucidated and total synthesis has been achieved. With an LD(50) of 25 ng/kg for rabbits (the most sensitive species), it is one of the most lethal marine toxins. It binds to the Na,K-ATPase specifically with a K(D) of 20 pM. It has a unique action on the Na,K-ATPase, converting the pump into an ion channel and resulting in K(+) efflux, Na(+) influx and membrane depolarization. As a result palytoxin causes a wide spectrum of secondary pharmacological actions. By acting like a key to unlock the internal structure of the Na,K-ATPase, palytoxin holds promise as a useful tool for investigation of the pump molecule.

Review. Peering into an ATPase ion pump with single-channel recordings

In principle, an ion channel needs no more than a single gate, but a pump requires at least two gates that open and close alternately to allow ion access from only one side of the membrane at a time. In the Na+,K+-ATPase pump, this alternating gating effects outward transport of three Na+ ions and inward transport of two K+ ions, for each ATP hydrolysed, up to a hundred times per second, generating a measurable current if assayed in millions of pumps. Under these assay conditions, voltage jumps elicit brief charge movements, consistent with displacement of ions along the ion pathway while one gate is open but the other closed. Binding of the marine toxin, palytoxin, to the Na+,K+-ATPase uncouples the two gates, so that although each gate still responds to its physiological ligand they are no longer constrained to open and close alternately, and the Na+,K+-ATPase is transformed into a gated cation channel. Millions of Na+ or K+ ions per second flow through such an open pump-channel, permitting assay of single molecules and allowing unprecedented access to the ion transport pathway through the Na+,K+-ATPase. Use of variously charged small hydrophilic thiol-specific reagents to probe cysteine targets introduced throughout the pump's transmembrane segments allows mapping and characterization of the route traversed by transported ions.

The ion pathway through the opened Na(+),K(+)-ATPase pump

P-type ATPases pump ions across membranes, generating steep electrochemical gradients that are essential for the function of all cells. Access to the ion-binding sites within the pumps alternates between the two sides of the membrane to avoid the dissipation of the gradients that would occur during simultaneous access. In Na(+),K(+)-ATPase pumps treated with the marine agent palytoxin, this strict alternation is disrupted and binding sites are sometimes simultaneously accessible from both sides of the membrane, transforming the pumps into ion channels (see, for example, refs 2, 3). Current recordings in these channels can monitor accessibility of introduced cysteine residues to water-soluble sulphydryl-specific reagents. We found previously that Na(+),K(+) pump-channels open to the extracellular surface through a deep and wide vestibule that emanates from a narrower pathway between transmembrane helices 4 and 6 (TM4 and TM6). Here we report that cysteine scans from TM1 to TM6 reveal a single unbroken cation pathway that traverses palytoxin-bound Na(+),K(+) pump-channels from one side of the membrane to the other. This pathway comprises residues from TM1, TM2, TM4 and TM6, passes through ion-binding site II, and is probably conserved in structurally and evolutionarily related P-type pumps, such as sarcoplasmic- and endoplasmic-reticulum Ca(2+)-ATPases and H(+),K(+)-ATPases.

Characterization of the palytoxin effect on Ca2+-ATPase from sarcoplasmic reticulum (SERCA)

The effect of palytoxin was studied in a microsomal fraction enriched in longitudinal tubules of the sarcoplasmic reticulum membrane. Half-maximal effect of palytoxin on Ca(2+)-ATPase activity yielded an apparent inhibition constant of approx. 0.4 microM. The inhibition process exhibited the following characteristics: (i) the degree of inhibition was dependent on membrane protein concentration; (ii) no protection was observed when the ATP concentration was raised; (iii) dependence on Ca(2+) concentration with a decreased maximum catalytic rate; (iv) it occurred in the absence of Ca(2+) ionophoric activity. Likewise, the inhibition mechanism was linked to: (i) rapid enzyme phosphorylation from ATP in the presence of Ca(2+) but lower steady-state levels of phosphoenzyme; (ii) more drastic effect on phosphoenzyme levels when the toxin was added to the enzyme in the absence of Ca(2+); (iii) decreased phosphoenzyme levels at saturating Ca(2+) concentrations; (iv) no effect on kinetics of phosphoenzyme decomposition. The palytoxin effect is related with lock of the enzyme in the Ca(2+)-free conformation so that progression of the catalytic cycle is impeded.

Temporal and steric analysis of ionic permeation and binding in SERCA via molecular dynamic simulations

The availability of the crystal structure of the sarco(endo)plasmic reticulum calcium ATPase (SERCA) has allowed atomic-level molecular dynamic (MD) simulations of this membrane transport protein to be done. The biomedical and nanotechnological implications of this work are discussed as well as the methods of performing the simulations and analysis. We have performed nanosecond timescale simulations of SERCA for several of its known conformations in a lipid/water environment. One simulation contained Ca(2+) ions, while others without ions were analyzed by techniques such as steric pathway determination. We discuss details of the resulting putative cytoplasmic and lumenal pathways, along with experimental evidence from the literature to support our conclusions. Finally, we give a brief overview of future research directions, as they pertain to MD simulations and their analysis. The methodology used in this work shows that significant insight into the structure-function relationship of ion-motive transmembrane pumps can be derived by a combination of simulation tools and analysis techniques including MD trajectories, steric analysis and electrostatic potentials.

Palytoxin acts on Na(+),K (+)-ATPase but not nongastric H(+),K (+)-ATPase

Palytoxin (PTX) opens a pathway for ions to pass through Na,K-ATPase. We investigate here whether PTX also acts on nongastric H,K-ATPases. The following combinations of cRNA were expressed in Xenopus laevis oocytes: Bufo marinus bladder H,K-ATPase alpha(2)- and Na,K-ATPase beta(2)-subunits; Bufo Na,K-ATPase alpha(1)- and Na,K-ATPase beta(2)-subunits; and Bufo Na,K-ATPase beta(2)-subunit alone. The response to PTX was measured after blocking endogenous Xenopus Na,K-ATPase with 10 microM ouabain. Functional expression was confirmed by measuring (86)Rb uptake. PTX (5 nM: ) produced a large increase of membrane conductance in oocytes expressing Bufo Na,K-ATPase, but no significant increase occurred in oocytes expressing Bufo H,K-ATPase or in those injected with Bufo beta(2)-subunit alone. Expression of the following combinations of cDNA was investigated in HeLa cells: rat colonic H,K-ATPase alpha(1)-subunit and Na,K-ATPase beta(1)-subunit; rat Na,K-ATPase alpha(2)-subunit and Na,K-ATPase beta(2)-subunit; and rat Na,K-ATPase beta(1)- or Na,K-ATPase beta(2)-subunit alone. Measurement of increases in (86)Rb uptake confirmed that both rat Na,K and H,K pumps were functional in HeLa cells expressing rat colonic HKalpha(1)/NKbeta(1) and NKalpha(2)/NKbeta(2). Whole-cell patch-clamp measurements in HeLa cells expressing rat colonic HKalpha(1)/NKbeta(1) exposed to 100 nM PTX showed no significant increase of membrane current, and there was no membrane conductance increase in HeLa cells transfected with rat NKbeta(1)- or rat NKbeta(2)-subunit alone. However, in HeLa cells expressing rat NKalpha(2)/NKbeta(2), outward current was observed after pump activation by 20 mM K(+) and a large membrane conductance increase occurred after 100 nM PTX. We conclude that nongastric H,K-ATPases are not sensitive to PTX when expressed in these cells, whereas PTX does act on Na,K-ATPase.

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

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

Palytoxin-induced effects on partial reactions of the Na,K-ATPase

The interaction of palytoxin with the Na,K-ATPase was studied by the electrochromic styryl dye RH421, which monitors the amount of ions in the membrane domain of the pump. The toxin affected the pump function in the state P-E2, independently of the type of phosphorylation (ATP or inorganic phosphate). The palytoxin-induced modification of the protein consisted of two steps: toxin binding and a subsequent conformational change into a transmembrane ion channel. At 20 degrees C, the rate-limiting reaction had a forward rate constant of 10(5) M(-1)s(-1) and a backward rate constant of about 10(-3) s(-1). In the palytoxin-modified state, the binding affinity for Na+ and H+ was increased and reached values between those obtained in the E1 and P-E2 conformation under physiological conditions. Even under saturating palytoxin concentrations, the ATPase activity was not completely inhibited. In the Na/K mode, approximately 50% of the enzyme remained active in the average, and in the Na-only mode 25%. The experimental findings indicate that an additional exit from the inhibited state exists. An obvious reaction pathway is a slow dephosphorylation of the palytoxin-inhibited state with a time constant of approximately 100 s. Analysis of the effect of blockers of the extracellular and cytoplasmic access channels, TPA+ and Br2-Titu3+, respectively, showed that both access channels are part of the ion pathway in the palytoxin-modified protein. All experiments can be explained by an extension of the Post-Albers cycle, in which three additional states were added that branch off in the P-E2 state and lead to states in which the open-channel conformation is introduced and returns into the pump cycle in the occluded E2 state. The previously suggested molecular model for the channel state of the Na,K-ATPase as a conformation in which both gates between binding sites and aqueous phases are simultaneously in their open state is supported by this study.

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

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

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.

The role of the third extracellular loop of the Na+,K+-ATPase alpha subunit in a luminal gating mechanism

Na+,K+-ATPase is responsible for maintaining the cross-membrane Na+ and K+ gradients of animal cells. This P-type ATPase works via a complex transport cycle, during which it binds and occludes three intracellular Na+ ions and then two extracellular K+ ions, which it then releases on the other side of the membrane. The cation pathway through the protein, and the structures responsible for occluding cations inside the protein, have not yet been definitely identified. We used cysteine mutagenesis to explore the accessibility and the role of five conserved residues in the short third extracellular loop, between the fifth and the sixth transmembrane helices. The P801C and L802C mutants were not affected by extracellular sulfhydryl reagents. The presence of cysteine residues at three positions (G803C, T804C and V805C) conferred sensitivity to omeprazole, a known inhibitor of the gastric proton pump, and to [2-(trimethylammonium)-ethyl]methanethiosulphonate bromide (MTSET). The effects of omeprazole and MTSET were modulated by the presence of extracellular K+, indicating that the accessibility of these positions depended on the conformational state of the protein. MTSET binding to cysteine at position 803 partially inhibited the Na+,K+-pump function by decreasing its affinity towards extracellular K+, suggesting a restriction of the access of extracellular K+ ions to their binding sites. In contrast, MTSET binding to cysteine at position 805 partially inhibited the Na+,K+-pump function by reducing its maximum turnover rate, probably by slowing a rate-limiting conformational change. These residues occupy positions that are critical for either the cation pathway or the conformational modifications.

Recent Insights into the Structure and Mechanism of the Sodium Pump

The sodium pump (or Na-K-ATPase) is essential to the function of animal cells. Publication of the related calcium pump (SERCA) structure together with several recent results from a variety of approaches allow us to propose a mechanistic model to answer the question: “How does the sodium pump pump?”

The small heart mutation reveals novel roles of Na+/K+-ATPase in maintaining ventricular cardiomyocyte morphology and viability in zebrafish

Forward genetic screens in zebrafish have been used to identify mutations in genes with important roles in organogenesis. One of these mutants, small heart, develops a diminutive and severely malformed heart and multiple developmental defects of the brain, ears, eyes, and kidneys. Using a positional cloning approach, we identify that the mutant gene encodes the zebrafish Na+/K+-ATPase alpha1B1 protein. Disruption of Na+/K+-ATPase alpha1B1 function via morpholino "knockdown" or pharmacological inhibition with ouabain phenocopies the mutant phenotype, in a dose-dependent manner. Heterozygosity for the mutation sensitizes embryos to ouabain treatment. Our findings present novel genetic and morphological details on the function of the Na+/K+-ATPase alpha1B1 in early cardiac morphogenesis and the pathogenesis of the small heart malformation. We demonstrate that the reduced size of the mutant heart is caused by dysmorphic ventricular cardiomyocytes and an increase in ventricular cardiomyocyte apoptosis. This study provides a new insight that Na+/K+-ATPase alpha1B1 is required for maintaining ventricular cardiomyocyte morphology and viability.

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.

The fourth transmembrane segment of the Na,K-ATPase alpha subunit: a systematic mutagenesis study

The Na,K-ATPase is a major ion-motive ATPase of the P-type family responsible for many aspects of cellular homeostasis. To determine the structure of the pathway for cations across the transmembrane portion of the Na,K-ATPase, we mutated 24 residues of the fourth transmembrane segment into cysteine and studied their function and accessibility by exposure to the sulfhydryl reagent 2-aminoethyl-methanethiosulfonate. Accessibility was also examined after treatment with palytoxin, which transforms the Na,K-pump into a cation channel. Of the 24 tested cysteine mutants, seven had no or a much reduced transport function. In particular cysteine mutants of the highly conserved "PEG" motif had a strongly reduced activity. However, most of the non-functional mutants could still be transformed by palytoxin as well as all of the functional mutants. Accessibility, determined as a 2-aminoethyl-methanethiosulfonate-induced reduction of the transport activity or as inhibition of the membrane conductance after palytoxin treatment, was observed for the following positions: Phe(323), Ile(322), Gly(326), Ala(330), Pro(333), Glu(334), and Gly(335). In accordance with a structural model of the Na,K-ATPase obtained by homology modeling with the two published structures of sarcoplasmic and endoplasmic reticulum calcium ATPase (Protein Data Bank codes 1EUL and 1IWO), the results suggest the presence of a cation pathway along the side of the fourth transmembrane segment that faces the space between transmembrane segments 5 and 6. The phenylalanine residue in position 323 has a critical position at the outer mouth of the cation pathway. The residues thought to form the cation binding site II ((333)PEGL) are also part of the accessible wall of the cation pathway opened by palytoxin through the Na,K-pump.

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.

Effects of palytoxin on cation occlusion and phosphorylation of the (Na+,K+)-ATPase

Palytoxin (PTX) inhibits the (Na(+) + K+)-driven pump and simultaneously opens channels that are equally permeable to Na+ and K+ in red cells and other cell membranes. In an effort to understand the mechanism by which PTX induces these fluxes, we have studied the effects of PTX on: 1) K+ and Na+ occlusion by the pump protein; 2) phosphorylation and dephosphorylation of the enzyme when a phosphoenzyme is formed from ATP and from P(i); and 3) p-nitro phenyl phosphatase (p-NPPase) activity associated with the (Na+, K+)-ATPase. We have found that palytoxin 1) increases the rate of deocclusion of K+(Rb+) in a time- and concentration-dependent manner, whereas Na+ occluded in the presence of oligomycin is unaffected by the toxin; 2) makes phosphorylation from P(i) insensitive to K+, and 3) stimulates the p-NPPase activity. The results are consistent with the notion that PTX produces a conformation of the Na+, K(+)-pump that resembles the one observed when ATP is bound to its low-affinity binding site. Further, they suggest that the channels that are formed by PTX might arise as a consequence of a perturbation in the ATPase structure, leading to the loss of control of the outside "gate" of the enzyme and hence to an uncoupling of the ion transport from the catalytic function of the ATPase.

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.

Ion occlusion/deocclusion partial reactions in individual palytoxin-modified Na/K pumps

In P-type ion-motive ATPases, transported ions approach their binding sites from one membrane surface, become buried deep within "occluded" conformations in which the sites are inaccessible from either membrane side, and are then deoccluded and released to the opposite membrane surface. This describes an alternating-gate transport mechanism, in which the pump acts like an ion channel with two gates that open and close alternately. The occluded states ensure that one gate closes before the other can open, thus preventing the large electrodiffusive ion fluxes that would otherwise quickly undo the pump's electrochemical work. High-resolution crystal structures of two conformations of the SERCA (sarcoplasmic and endoplasmic reticulum Ca(2+)) P-type ATPase, together with mutagenesis results and analyses of structural models based on homology, have begun to provide a picture of the ion coordination sites in related P-type ATPases, including the Na/K pump. However, in no P-type ATPase are the structures and mechanisms of the gates known. The marine toxin, palytoxin (PTX), is known to bind to the Na/K pump and elicit a nonselective cation leak pathway, possibly by disrupting the strict coupling between the pump's inner and outer gates, allowing them to both be open. We recently found that ion flow through PTX-modified Na/K pump-channels appears to be modulated by two gates that can be regulated by the pump's physiological ligands in a manner suggesting that gating reflects underlying ion occlusion/deocclusion partial reactions. We review that work here and provide evidence that the pore of the PTX-induced pump-channel has a diameter > 6 A.

Neural dysfunction and neurodegeneration in Drosophila Na+/K+ ATPase alpha subunit mutants

The Na+/K+ ATPase asymmetrically distributes sodium and potassium ions across the plasma membrane to generate and maintain the membrane potential in many cell types. Although these pumps have been hypothesized to be involved in various human neurological disorders, including seizures and neurodegeneration, direct genetic evidence has been lacking. Here, we describe novel mutations in the Drosophila gene encoding the alpha (catalytic) subunit of the Na+/K+ ATPase that lead to behavioral abnormalities, reduced life span, and severe neuronal hyperexcitability. These phenotypes parallel the occurrence of extensive, age-dependent neurodegeneration. We have also discovered that the ATPalpha transcripts undergo alternative splicing that substantially increases the diversity of potential proteins. Our data show that maintenance of neuronal viability is dependent on normal sodium pump activity and establish Drosophila as a useful model for investigating the role of the pump in human neurodegenerative and seizure disorders.

Na+/K+-pump ligands modulate gating of palytoxin-induced ion channels

The Na+/K+ pump is a ubiquitous P-type ATPase that binds three cytoplasmic Na+ ions deep within its core where they are temporarily occluded before being released to the extracellular surface. The 3Na+/2K+ -exchange transport cycle is completed when two extracellular K+ ions bind and become temporarily occluded within the protein and subsequently released to the cytoplasm. Coupling of Na+ -ion occlusion to phosphorylation of the pump by ATP and of K+ -ion occlusion to its dephosphorylation ensure the vectorial nature of net transport. The occluded-ion conformations, with binding sites inaccessible from either side, represent intermediate states in these alternating-access descriptions of transport. They afford protection against potentially catastrophic effects of inadvertently allowing simultaneous access from both membrane sides. The marine toxin, palytoxin, converts Na+/K+ pumps into nonselective cation channels, possibly by disrupting the normal strict coupling between opening of one access pathway in the Na+/K+ ATPase and closing of the other. We show here that gating of the channels in palytoxin-bound Na+/K+ pumps in excised membrane patches is modulated by the pump's physiological ligands: cytoplasmic application of ATP promotes opening of the channels, and extracellular replacement of Na+ ions by K+ ions promotes closing of the channels. This suggests that, despite the presence of bound palytoxin, certain partial reactions of the normal Na+/K+ -transport cycle persist and remain capable of effecting the conformational changes that control access to the pump's cation-binding sites. These findings affirm the alternating-access model of ion pumps and offer the possibility of examining ion occlusion/deocclusion reactions in single pump molecules.

Ion channel-like properties of the Na+/K+ Pump

Ion pumps and exchangers are considered to be different from ion channels for two principal reasons. Ion pumps move ions against, whereas ion channels allow ions to move with, the electrochemical potential gradient, and pumps transport ions relatively slowly, approximately 10(2) s(-1), whereas channels conduct ions rapidly, approximately 10(7) s(-1). However, the latter high rate refers only to the open pore, and yet all ion channels contain at least one gate. Not surprisingly, the conformational changes associated with channel gating occur with kinetics similar to those of ion pumping. Indeed, ion pumps may be viewed as ion channels with two gates, one external to, and the other internal to, the ion binding cavity. The simple operational rule for such a pump is that the two gates should never be open simultaneously; otherwise, the pump would become a channel and conduct dissipative fluxes several orders of magnitude larger than, and in the opposite direction to, the active transport fluxes. Analyses of Na(+) ion movements mediated by the Na(+)/K(+) pump under various conditions have suggested that in at least one, short-lived, conformation of the pump, an ion-channel-like structure, closed at its intracellular end, connects the extracellular solution with the ion binding sites deep in the protein core. Here we use the marine toxin, palytoxin, to act on Na(+)/K(+) pumps in outside-out patches excised from cardiac myocytes and so transform the pumps into nonselective cation channels which we study using macroscopic, and single-channel, recording. We find that gating of the palytoxin-induced channels is regulated by the pump's natural ligands. Thus, external K(+) congeners tend to close, and external Na(+) tends to open, an extracellular gate, whereas ATP acts from the cytoplasmic solution to open an intracellular gate. These gating influences echo the normal ion occlusion and deocclusion reactions that first entrap two extracellular K(+) ions within the interior of the pump (between the two gates) and then release them to the cytoplasmic side in a step accelerated by ATP. These results offer the promise of being able to examine ion occlusion and deocclusion steps at the microscopic level in single Na(+)/K(+) pump molecules.