A 19-kDa C-terminal tryptic fragment of the alpha chain of Na/K-ATPase is essential for occlusion and transport of cations

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

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

External Ion Access in the Na/K Pump: Kinetics of Na+, K+, and Quaternary Amine Interaction

Na/K pumps build essential ion gradients across the plasmalemma of animal cells by coupling the extrusion of three Na⁺, with the import of two K⁺ and the hydrolysis of one ATP molecule. The mechanisms of selectivity and competition between Na⁺, K⁺, and inhibitory amines remain unclear. We measured the effects of external tetrapropylammonium (TPA⁺) and ethylenediamine (EDA²⁺) on three different Na/K pump transport modes in voltage-clamped Xenopus oocytes: 1) outward pump current (I_P), 2) passive inward H⁺ current at negative voltages without Na⁺ or K⁺ (I_H), and 3) transient charge movement reporting the voltage-dependent extracellular binding/release of Na⁺ (Q_Na). Both amines competed with K⁺ to inhibit I_P. TPA⁺ inhibited I_H without competing with H⁺, whereas EDA²⁺ did not alter I_H at pH 7.6. TPA⁺ competed with Na⁺ in Q_Na measurements, reducing Na⁺-apparent affinity, evidenced by a ∼-75 mV shift in the charge-voltage curve (at 20 mM TPA⁺) without reduction of the total charge moved (Q_tot). In contrast, EDA²⁺ and K⁺ did not compete with Na⁺ to inhibit Q_Na; both reduced Q_tot without decreasing Na⁺-apparent affinity. EDA²⁺ (15 mM) right-shifted the charge-voltage curve by ∼+50 mV. Simultaneous occlusion of EDA²⁺ and Na⁺ by an E2P conformation unable to reach E1P was demonstrated by voltage-clamp fluorometry. Trypsinolysis experiments showed that EDA²⁺-bound pumps are much more proteolysis-resistant than Na⁺-, K⁺-, or TPA⁺-bound pumps, therefore uncovering unique EDA²⁺-bound conformations. K⁺ effects on Q_Na and I_H were also evaluated in pumps inhibited with beryllium fluoride, a phosphate mimic. K⁺ reduced Q_tot without shifting the charge-voltage curve, indicating noncompetitive effects, and partially inhibited I_H to the same extent as TPA⁺ in non-beryllium-fluorinated pumps. These results demonstrate that K⁺ interacts with beryllium-fluorinated pumps inducing conformational changes that alter Q_Na and I_H, suggesting that there are two external access pathways for proton transport by I_H.

General and specific lipid-protein interactions in Na,K-ATPase

The molecular activity of Na,K-ATPase and other P2 ATPases like Ca(2+)-ATPase is influenced by the lipid environment via both general (physical) and specific (chemical) interactions. Whereas the general effects of bilayer structure on membrane protein function are fairly well described and understood, the importance of the specific interactions has only been realized within the last decade due particularly to the growing field of membrane protein crystallization, which has shed new light on the molecular details of specific lipid-protein interactions. It is a remarkable observation that specific lipid-protein interactions seem to be evolutionarily conserved, and conformations of specifically bound lipids at the lipid-protein surface within the membrane are similar in crystal structures determined with different techniques and sources of the protein, despite the rather weak lipid-protein interaction energy. Studies of purified detergent-soluble recombinant αβ or αβFXYD Na,K-ATPase complexes reveal three separate functional effects of phospholipids and cholesterol with characteristic structural selectivity. The observations suggest that these three effects are exerted at separate binding sites for phophatidylserine/cholesterol (stabilizing), polyunsaturated phosphatidylethanolamine (stimulatory), and saturated PC or sphingomyelin/cholesterol (inhibitory), which may be located within three lipid-binding pockets identified in recent crystal structures of Na,K-ATPase. The findings point to a central role of direct and specific interactions of different phospholipids and cholesterol in determining both stability and molecular activity of Na,K-ATPase and possible implications for physiological regulation by membrane lipid composition. This article is part of a special issue titled "Lipid-Protein Interactions."

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.

K+ congeners that do not compromise Na+ activation of the Na+,K+-ATPase: hydration of the ion binding cavity likely controls ion selectivity

The Na(+),K(+)-ATPase is essential for ionic homeostasis in animal cells. The dephosphoenzyme contains Na(+) selective inward facing sites, whereas the phosphoenzyme contains K(+) selective outward facing sites. Under normal physiological conditions, K(+) inhibits cytoplasmic Na(+) activation of the enzyme. Acetamidinium (Acet(+)) and formamidinium (Form(+)) have been shown to permeate the pump through the outward facing sites. Here, we show that these cations, unlike K(+), are unable to enter the inward facing sites in the dephosphorylated enzyme. Consistently, the organic cations exhibited little to no antagonism to cytoplasmic Na(+) activation. Na(+),K(+)-ATPase structures revealed a previously undescribed rotamer transition of the hydroxymethyl side chain of the absolutely conserved Thr(772) of the α-subunit. The side chain contributes its hydroxyl to Na(+) in site I in the E1 form and rotates to contribute its methyl group toward K(+) in the E2 form. Molecular dynamics simulations to the E1·AlF4 (-)·ADP·3Na(+) structure indicated that 1) bound organic cations differentially distorted the ion binding sites, 2) the hydroxymethyl of Thr(772) rotates to stabilize bound Form(+) through water molecules, and 3) the rotamer transition is mediated by water traffic into the ion binding cavity. Accordingly, dehydration induced by osmotic stress enhanced the interaction of the congeners with the outward facing sites and profoundly modified the organization of membrane domains of the α-subunit. These results assign a catalytic role for water in pump function, and shed light on a backbone-independent but a conformation-dependent switch between H-bond and dispersion contact as part of the catalytic mechanism of the Na(+),K(+)-ATPase.

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.

Structure-function relationships in the NA+,K+-pump

The Na,K-pump was discovered about 50 years ago. Since then there has been a methodic investigation of its structure and functional characteristics. The development of the Albers-Post model for the transport cycle was a milestone that provided the framework for detailed understanding of the transport process. The pump is composed of 2 subunits that exist in the membrane as an alphabeta heterodimer. All known enzymatic functions of the pump occur through the alpha subunit. Although necessary for activity, the complete role of the beta subunit is not understood fully. Numerous studies have established that the alphabeta protomer is the minimal functional unit needed to perform the Albers-Post reaction cycle. However, higher orders of aggregation [(alphabeta)n] are commonly detected. There is little evidence that oligomerization has functional consequence for ion transport. The Na+,K+-adenosine triphosphatase (ATPase) is a member of the P-type ATPase family of transporters. Proteins within this family have common amino acid sequence motifs that share functional characteristics and structure. Low-resolution 3-dimensional reconstruction of 2-dimensional crystal diffractions provide evidence for the similarity in tertiary structure of the alpha subunit and the Ca2+ATPase (a closely related P-type ATPase). The spatial location of the beta subunit also is obvious in these reconstructions. Recent high-resolution reconstructions from 3-dimensional crystals of the Ca2+ATPase provide structural details at the atomic level. It now is possible to interpret structurally some of the key steps in the Albers-Post reaction. Some of these high-resolution interpretations are translatable to the Na+,K+-ATPase, but a high-resolution structure of the Na,K-pump is needed for the necessary details of those aspects that are unique to this transporter.

Stabilization of trypsin by association to plasma membranes: Implications for tryptic cleavage of membrane-bound Na,K-ATPase

Tryptic cleavage has been a potential method for studying the structure and mechanism of many membrane transport proteins. Here, we report tight association of trypsin to pig kidney plasma membranes enriched in Na,K-ATPase. Trypsin also associated with protein-free vesicles prepared from plasma membrane lipids. Membrane-associated trypsin was found to be highly resistant to autolysis and insensitive to inhibition by PMSF. Na,K-ATPase substrate ions differentially influenced the level of trypsin membrane association. Thus, NaCl significantly increased trypsin membrane association compared to KCl. The ions seem to exert direct effects on the membrane independent of their effects on protein conformation. Bicarbonate anions, which detach peripheral membrane proteins, efficiently released trypsin from the membrane. Trypsin membrane association was found to enhance the cleavage of the Na,K-ATPase gamma-subunit. Comparison between membranes from shark rectal gland and pig kidney showed that trypsin association was significantly higher in the former. This was found to be partly due to the presence of higher cholesterol levels in the membrane. In conclusion, the differential membrane association of trypsin may affect the outcome of proteolytic cleavage of membrane-bound proteins.

Curcumin modulation of Na,K-ATPase: phosphoenzyme accumulation, decreased K+ occlusion, and inhibition of hydrolytic activity

1 Curcumin, the major constitute of tumeric, is an important nutraceutical that has been shown to be useful in the treatment of many diseases. As an inhibitor of the sarcoplasmic reticulum Ca(2+)-ATPase, curcumin was shown to correct cystic fibrosis (CF) defects in some model systems, whereas others have reported no or little effects on CF after curcumin treatment, suggesting that curcumin effect is not due to simple inhibition of the Ca(2+)-ATPase. 2 We tested the hypothesis that curcumin may modulate other members of the P(2)-type ATPase superfamily by studying the effects of curcumin on the activity and kinetic properties of the Na,K-ATPase. 3 Curcumin treatment inhibited Na,K-ATPase activity in a dose-dependent manner (K(0.5) approximately 14.6 microM). Curcumin decreased the apparent affinity of Na,K-ATPase for K(+) and increased it for Na(+) and ATP. Kinetic analyses indicated that curcumin induces a three-fold reduction in the rate of E1P --> E2P transition, thereby increasing the steady-state phosphoenzyme level. Curcumin treatment significantly abrogated K(+) occlusion to the enzyme as evidenced from kinetic and proteolytic cleavage experiments. Curcumin also significantly decreased the vanadate sensitivity of the enzyme. 4 Thus, curcumin partially blocks the K(+) occlusion site, and induces a constitutive shift in the conformational equilibrium of the enzyme, towards the E1 conformation. 5 The physiological consequences of curcumin treatment previously reported in different epithelial model systems may, at least in part, be related to the direct effects of curcumin on Na,K-ATPase activity.

Covalent Cross-links between the γ Subunit (FXYD2) and α and β Subunits of Na,K-ATPase

This study describes specific intramolecular covalent cross-linking of the gamma to alpha and gamma to beta subunits of pig kidney Na,K-ATPase and rat gamma to alpha co-expressed in HeLa cells. For this purpose pig gammaa and gammab sequences were determined by cloning and mass spectrometry. Three bifunctional reagents were used: N-hydroxysuccinimidyl-4-azidosalicylic acid (NHS-ASA), disuccinimidyl tartrate (DST), and 1-ethyl-3-[3dimethylaminopropyl]carbodiimide (EDC). NHS-ASA induced alpha-gamma, DST induced alpha-gamma and beta-gamma, and EDC induced primarily beta-gamma cross-links. Specific proteolytic and Fe(2+)-catalyzed cleavages located NHS-ASA- and DST-induced alpha-gamma cross-links on the cytoplasmic surface of the alpha subunit, downstream of His(283) and upstream of Val(440). Additional considerations indicated that the DST-induced and NHS-ASA-induced cross-links involve either Lys(347) or Lys(352) in the S4 stalk segment. Mutational analysis of the rat gamma subunit expressed in HeLa cells showed that the DST-induced cross-link involves Lys(55) and Lys(56) in the cytoplasmic segment. DST and EDC induced two beta-gamma cross-links, a major one at the extracellular surface within the segment Gly(143)-Ser(302) of the beta subunit and another within Ala(1)-Arg(142). Based on the cross-linking and other data on alpha-gamma proximities, we modeled interactions of the transmembrane alpha-helix and an unstructured cytoplasmic segment SKRLRCGGKKHR of gamma with a homology model of the pig alpha1 subunit. According to the model, the transmembrane segment fits in a groove between M2, M6, and M9, and the cytoplasmic segment interacts with loops L6/7 and L8/9 and stalk S5.

The role of loop 6/7 in folding and functional performance of Na,K-ATPase

Alanine substitutions were made for 15 amino acids in the cytoplasmic loop between transmembrane helices 6 and 7 (L6/7) of the human alpha(1)-subunit of Na,K-ATPase. Most mutations reduced Na,K-ATPase activity by less than 50%; however, the mutations R834A, R837A, and R848A reduced Na,K-ATPase activity by 75, 89, and 66%, respectively. Steady-state phosphoenzyme formation from ATP was reduced in mutants R834A, R837A, and R848A, and R837A also had a faster E(2)P --> E(2) dephosphorylation rate compared with the wild-type enzyme. Effects of L6/7 mutations on the phosphorylation domain of the protein were also demonstrated by (18)O exchange, which showed that intrinsic rate constants for P(i) binding and/or reaction with the protein were altered. Although most L6/7 mutations had no effect on the interaction of Na(+) or K(+) with Na,K-ATPase, the E825A, E828A, R834A, and R837A mutations reduced the apparent affinity of the enzyme for both Na(+) and K(+) by 1.5-3-fold. 1-Bromo-2,4,6-tris(methylisothiouronium)benzene (Br-TITU(3+)), a competitive antagonist of Rb(+) and Na(+) occlusion, was used to test whether charged residues in L6/7 are involved in binding monovalent cations and cation antagonists. Br-TITU(3+) inhibited ouabain binding to wild type Na,K-ATPase with an IC(50) of 30 microM. Ouabain binding to the E825A, E828A, R834A, or R837A mutants was still inhibited by Br-TITU(3+), indicating that Br-TITU(3+) does not bind to charged residues in L6/7. This observation makes it unlikely that L6/7 functions as a cytoplasmic cation binding site in Na,K-ATPase, and together with the effects of L6/7 mutations on phosphate interactions with the enzyme suggests that L6/7 is important in stabilizing the phosphorylation domain and its relationship to the ion binding sites of the protein.

Structure and mechanism of Na,K-ATPase: functional sites and their interactions

The cell membrane Na,K-ATPase is a member of the P-type family of active cation transport proteins. Recently the molecular structure of the related sarcoplasmic reticulum Ca-ATPase in an E1 conformation has been determined at 2.6 A resolution. Furthermore, theoretical models of the Ca-ATPase in E2 conformations are available. As a result of these developments, these structural data have allowed construction of homology models that address the central questions of mechanism of active cation transport by all P-type cation pumps. This review relates recent evidence on functional sites of Na,K-ATPase for the substrate (ATP), the essential cofactor (Mg(2+) ions), and the transported cations (Na(+) and K(+)) to the molecular structure. The essential elements of the Ca-ATPase structure, including 10 transmembrane helices and well-defined N, P, and A cytoplasmic domains, are common to all PII-type pumps such as Na,K-ATPase and H,K-ATPases. However, for Na,K-ATPase and H,K-ATPase, which consist of both alpha- and beta-subunits, there may be some detailed differences in regions of subunit interactions. Mutagenesis, proteolytic cleavage, and transition metal-catalyzed oxidative cleavages are providing much evidence about residues involved in binding of Na(+), K(+), ATP, and Mg(2+) ions and changes accompanying E1-E2 or E1-P-E2-P conformational transitions. We discuss this evidence in relation to N, P, and A cytoplasmic domain interactions, and long-range interactions between the active site and the Na(+) and K(+) sites in the transmembrane segments, for the different steps of the catalytic cycle.

Direct activation of gastric H,K-ATPase by N-terminal protein kinase C phosphorylation. Comparison of the acute regulation mechanisms of H,K-ATPase and Na,K-ATPase

In this study we compared the protein kinase dependent regulation of gastric H,K-ATPase and Na,K-ATPase. The protein kinase A/protein kinase C (PKA/PKC) phosphorylation profile of H,K-ATPase was very similar to the one found in the Na,K-ATPase. PKC phosphorylation was taking place in the N-terminal part of the alpha-subunit with a stoichiometry of approximately 0.6 mol Pi/mole alpha-subunit. PKA phosphorylation was in the C-terminal part and required detergent, as is also found for the Na,K-ATPase. The stoichiometry of PKA-induced phosphorylation was approximately 0.7 mol Pi/mole alpha-subunit. Controlled proteolysis of the N-terminus abolished PKC phosphorylation of native H,K-ATPase. However, after detergent treatment additional C-terminal PKC sites became exposed located at the beginning of the M5M6 hairpin and at the cytoplasmic L89 loop close to the inner face of the plasma membrane. N-terminal PKC phosphorylation of native H,K-ATPase alpha-subunit was found to stimulate the maximal enzyme activity by 40-80% at saturating ATP, depending on pH. Thus, a direct modulation of enzyme activity by PKC phosphorylation could be demonstrated that may be additional to the well-known regulation of acid secretion by recruitment of H,K-ATPase to the apical membranes of the parietal cells. Moreover, a distinct difference in the regulation of H,K-ATPase and Na,K-ATPase is the apparent absence of any small regulatory proteins associated with the H,K-ATPase.

Biochemistry of Na,K-ATPase

The Na,K-ATPase or sodium pump carries out the coupled extrusion and uptake of Na and K ions across the plasma membranes of cells of most higher eukaryotes. It is a member of the P-type ATPase superfamily. This heterodimeric integral membrane protein is composed of a 100-kDa alpha-subunit with ten transmembrane segments and a heavily glycosylated beta subunit of about 55 kDa, which is a type II membrane protein. Current ideas on how the protein achieves active transport are based on a fusion of results of transport physiology, protein chemistry, and heterologous expression of mutant proteins. Recently acquired high resolution structural information provides an important new avenue for a more complete understanding of this protein. In this review, the current status of knowledge of Na,K-ATPase is discussed, and areas where there is still considerable uncertainty are highlighted.

Role of the self-association of beta subunits in the oligomeric structure of Na+/K+-ATPase

The two subunits of Na(+)/K(+)-ATPase that are essential for function are alpha and beta. Previous cross-linking studies on the oligomeric structure of the membrane-bound enzyme identified alpha,beta and alpha,alpha associations, but only the former and not the latter could be detected after solubilization. To study the possibility of direct beta,beta association, the purified membrane enzyme and a trypsin-digested enzyme that occludes cations and contains an essentially intact beta and fragments of alpha were subjected to oxidative cross-linking in the presence of Cu(2+)-phenanthroline. Resolution of products on polyacrylamide gels, N-terminal analysis and reactivity with anti-beta antibody showed that, in addition to previously identified products (e.g. alpha,alpha and alpha,beta dimers), a beta,beta dimer, most likely linked through intramembrane Cys(44) residues of two chains, is also formed. This dimer was also noted when digitonin-solubilized intact enzyme, and the trypsin-digested enzyme solubilized with digitonin or polyoxyethylene 10-laurylether were subjected to cross-linking, indicating that the detected beta,beta association was not due to random collisions. In the digested enzyme, K(+) but not Na(+) enhanced beta,beta dimer formation. The alternative cross-linking of beta-Cys(44) to a Cys residue of a transmembrane alpha-helix was antagonized specifically by K(+) or Na(+). The findings (i) indicate the role of beta,beta association in maintaining the minimum oligomeric structure of (alpha,beta)(2), (ii) provide further support for conformation-dependent flexibilities of the spatial relations of the transmembrane helices of alpha and beta and (iii) suggest the possibility of significant differences between the quaternary structures of the P-type ATPases that do and do not contain a beta subunit.

Protein kinase C phosphorylation of purified Na,K-ATPase: C-terminal phosphorylation sites at the alpha- and gamma-subunits close to the inner face of the plasma membrane

The alpha-subunit of the Na,K-ATPase is phosphorylated at specific sites by protein kinases A and C. Phosphorylation by protein kinase C (PKC) is restricted to the N terminus and takes place to a low stoichiometry, except in rat. Here we show that the alpha-subunit of shark Na,K-ATPase can be phosphorylated by PKC at C-terminal sites to stoichiometric levels in the presence of detergents. Two novel phosphorylation sites are possible candidates for this PKC phosphorylation: Thr-938 in the M8/M9 loop located very close to the PKA site, and Ser-774, in the proximal part of the M5/M6 hairpin. Both sites are highly conserved in all known alpha-subunits, indicating a physiological role. A similar pattern of detergent-mediated phosphorylation by PKC was found in pig kidney Na,K-ATPase alpha-subunit. Interestingly, the kidney-specific gamma-subunit was phosphorylated by PKC in the presence of detergent. The close proximity of the novel PKC sites to the membrane suggests that targeting proteins to tether PKC into the membrane phase is important in controlling the in vivo phosphorylation of this novel class of membrane-adjacent PKC sites. It is suggested that in purified preparations where functional targeting may be impaired detergents are needed to expose the sites.

Structural organization and energy transduction mechanism of Na+,K+-ATPase studied with transition metal-catalyzed oxidative cleavage

This chapter describes contributions of transition metal-catalyzed oxidative cleavage of Na+,K+-ATPase to our understanding of structure-function relations. In the presence of ascorbate/H2O2, specific cleavages are catalyzed by the bound metal and because more than one peptide bond close to the metal can be cleaved, this technique reveals proximity of the different cleavage positions within the native structure. Specific cleavages are catalyzed by Fe2+ bound at the cytoplasmic surface or by complexes of ATP-Fe2+, which directs the Fe2+ to the normal ATP-Mg2+ site. Fe2+- and ATP-Fe2+-catalyzed cleavages reveal large conformation-dependent changes in interactions between cytoplasmic domains, involving conserved cytoplasmic sequences, and a change of ligation of Mg2+ ions between E1P and E2P, which may be crucial in facilitating hydrolysis of E2P. The pattern of domain interactions in E1 and E2 conformations, and role of Mg2+ ions, may be common to all P-type pumps. Specific cleavages can also be catalyzed by Cu2+ ions, bound at the extracellular surfaces, or a hydrophobic Cu2+-diphenyl phenanthroline (DPP) complex, which directs the Cu2+ to the membrane-water interface. Cu2+ or Cu2+-DPP-catalyzed cleavages are providing information on alpha/beta subunit interactions and spatial organization of transmembrane segments. Transition metal-catalyzed cleavage could be widely used to investigate other P-type pumps and membrane proteins and, especially, ATP binding proteins.

The functional role of beta subunits in oligomeric P-type ATPases

Na,K-ATPase and gastric and nongastric H,K-ATPases are the only P-type ATPases of higher organisms that are oligomeric and are associated with a beta subunit, which is obligatory for expression and function of enzymes. Topogenesis studies suggest that beta subunits have a fundamental and unique role in K+-transporting P-type ATPases in that they facilitate the correct membrane integration and packing of the catalytic a subunit of these P-type ATPases, which is necessary for their resistance to cellular degradation, their acquisition of functional properties, and their routing to the cell surface. In addition to this chaperone function, beta subunits also participate in the determination of intrinsic transport properties of Na,K- and H,K-ATPases. Increasing experimental evidence suggests that beta assembly is a highly ordered, beta isoform-specific process, which is mediated by multiple interaction sites that contribute in a coordinate, multistep process to the structural and functional maturation of Na,K- and H,K-ATPases.

Conformational coupling: the moving parts of an ion pump

The Na,K-ATPase carries out the coupled functions of ATP hydrolysis and cation transport. These functions are performed by two distinct regions of the protein. ATP binding and hydrolysis is mediated by the large central cytoplasmic loop of about 430 amino-acids. Transmembrane cation transport is accomplished via coordination of the Na and K ions by side-chains of the amino-acids of several of the transmembrane segments. The way in which these two protein domains interact lies at the heart of the molecular mechanism of active transport, or ion pumping. We summarize evidence obtained from protein chemistry studies of the purified renal Na,K-ATPase and from bacterially expressed polypeptides which characterize these separate functions and point to various movements which may occur as the protein transits through its reaction cycle. We then describe recent work using heterologous expression of renal Na,K-ATPase in baculovirus-infected insect cells which provides a suitable system to characterize such protein motions and which can be employed to test specific models arising from recently acquired high resolution structural information on related ion pumps.

Slippage and uncoupling in P-type cation pumps; implications for energy transduction mechanisms and regulation of metabolism

P-type ATPases couple scalar and vectorial events under optimized states. A number of procedures and conditions lead to uncoupling or slippage. A key branching point in the catalytic cycle is at the cation-bound form of E(1)-P, where isomerization to E(2)-P leads to coupled transport, and hydrolysis leads to uncoupled release of cations to the cis membrane surface. The phenomenon of slippage supports a channel model for active transport. Ability to occlude cations within the channel is essential for coupling. Uncoupling and slippage appear to be inherent properties of P-type cation pumps, and are significant contributors to standard metabolic rate. Heat production is favored in the uncoupled state. A number of disease conditions, include ageing, ischemia and cardiac failure, result in uncoupling of either the Ca(2+)-ATPase or Na(+)/K(+)-ATPase.

Structural similarities of Na,K-ATPase and SERCA, the Ca(2+)-ATPase of the sarcoplasmic reticulum

The crystal structure of SERCA1a (skeletal-muscle sarcoplasmic-reticulum/endoplasmic-reticulum Ca(2+)-ATPase) has recently been determined at 2.6 A (note 1 A = 0.1 nm) resolution [Toyoshima, Nakasako, Nomura and Ogawa (2000) Nature (London) 405, 647-655]. Other P-type ATPases are thought to share key features of the ATP hydrolysis site and a central core of transmembrane helices. Outside of these most-conserved segments, structural similarities are less certain, and predicted transmembrane topology differs between subclasses. In the present review the homologous regions of several representative P-type ATPases are aligned with the SERCA sequence and mapped on to the SERCA structure for comparison. Homology between SERCA and the Na,K-ATPase is more extensive than with any other ATPase, even PMCA, the Ca(2+)-ATPase of plasma membrane. Structural features of the Na,K-ATPase are projected on to the Ca(2+)-ATPase crystal structure to assess the likelihood that they share the same fold. Homology extends through all ten transmembrane spans, and most insertions and deletions are predicted to be at the surface. The locations of specific residues are examined, such as proteolytic cleavage sites, intramolecular cross-linking sites, and the binding sites of certain other proteins. On the whole, the similarity supports a shared fold, with some particular exceptions.

Thermal denaturation of the Na,K-ATPase provides evidence for alpha-alpha oligomeric interaction and gamma subunit association with the C-terminal domain

Thermal denaturation can help elucidate protein domain substructure. We previously showed that the Na,K-ATPase partially unfolded when heated to 55 degrees C (Arystarkhova, E., Gibbons, D. L., and Sweadner, K. J. (1995) J. Biol. Chem. 270, 8785-8796). The beta subunit unfolded without leaving the membrane, but three transmembrane spans (M8-M10) and the C terminus of the alpha subunit were extruded, while the rest of alpha retained its normal topology with respect to the lipid bilayer. Here we investigated thermal denaturation further, with several salient results. First, trypsin sensitivity at both surfaces of alpha was increased, but not sensitivity to V8 protease, suggesting that the cytoplasmic domains and extruded domain were less tightly packed but still retained secondary structure. Second, thermal denaturation was accompanied by SDS-resistant aggregation of alpha subunits as dimers, trimers, and tetramers without beta or gamma subunits. This implies specific alpha-alpha contact. Third, the gamma subunit, like the C-terminal spans of alpha, was selectively lost from the membrane. This suggests its association with M8-M10 rather than the more firmly anchored transmembrane spans. The picture that emerges is of a Na,K-ATPase complex of alpha, beta, and gamma subunits in which alpha can associate in assemblies as large as tetramers via its cytoplasmic domain, while beta and gamma subunits associate with alpha primarily in its C-terminal portion, which has a unique structure and thermal instability.

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.

Mechanisms of sodium pump regulation

The Na(+)-K(+)-ATPase, or sodium pump, is the membrane-bound enzyme that maintains the Na(+) and K(+) gradients across the plasma membrane of animal cells. Because of its importance in many basic and specialized cellular functions, this enzyme must be able to adapt to changing cellular and physiological stimuli. This review presents an overview of the many mechanisms in place to regulate sodium pump activity in a tissue-specific manner. These mechanisms include regulation by substrates, membrane-associated components such as cytoskeletal elements and the gamma-subunit, and circulating endogenous inhibitors as well as a variety of hormones, including corticosteroids, peptide hormones, and catecholamines. In addition, the review considers the effects of a range of specific intracellular signaling pathways involved in the regulation of pump activity and subcellular distribution, with particular consideration given to the effects of protein kinases and phosphatases.

Replacement of glycine 232 by aspartic acid in the KdpA subunit broadens the ion specificity of the K(+)-translocating KdpFABC complex

Replacement of glycine residue 232 with aspartate in the KdpA subunit of the K(+)-translocating KdpFABC complex of Escherichia coli leads to a transport complex that has reduced affinity for K(+) and has lost the ability to discriminate Rb(+) ions (, J. Biol. Chem. 270:6678-6685). This glycine residue is the first in a highly conserved GGG motif that was aligned with the GYG sequence of the selectivity filter (P- or H5-loop) of K(+) channels (, Nature. 371:119-122). Investigations with the purified and reconstituted KdpFABC complex using the potential sensitive fluorescent dye DiSC(3)(5) and the "caged-ATP/planar bilayer method" confirm the altered ion specificity observed in uptake measurements with whole cells. In the absence of cations a transient current was observed in the planar bilayer measurements, a phenomenon that was previously observed with the wild-type enzyme and with another kdpA mutant (A:Q116R) and most likely represents the movement of a protein-fixed charge during a conformational transition. After addition of K(+) or Rb(+), a stationary current could be observed, representing the continuous pumping activity of the KdpFABC complex. In addition, DiSC(3)(5) and planar bilayer measurements indicate that the A:G232D Kdp-ATPase also transports Na(+), Li(+), and H(+) with a reduced rate. Similarities to mutations in the GYG motif of K(+) channels are discussed.

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

The roles of Ser775 and Glu779, two amino acids in the putative fifth transmembrane segment of the Na,K-ATPase alpha subunit, in determining the voltage and extracellular K+ (K+(o)) dependence of enzyme-mediated ion transport, were examined in this study. HeLa cells expressing the alpha1 subunit of sheep Na,K-ATPase were voltage clamped via patch electrodes containing solutions with 115 mM Na+ (37 degrees C). Na,K-pump current produced by the ouabain-resistant control enzyme (RD), containing amino acid substitutions Gln111Arg and Asn122Asp, displayed a membrane potential and K+(o) dependence similar to wild-type Na,K-ATPase during superfusion with 0 and 148 mM Na+-containing salt solutions. Additional substitution of alanine at Ser775 or Glu779 produced 155- and 15-fold increases, respectively, in the K+(o) concentration that half-maximally activated Na,K-pump current at 0 mV in extracellular Na+-free solutions. However, the voltage dependence of Na,K-pump current was unchanged in RD and alanine-substituted enzymes. Thus, large changes in apparent K+(o) affinity could be produced by mutations in the fifth transmembrane segment of the Na,K-ATPase with little effect on voltage-dependent properties of K+ transport. One interpretation of these results is that protein structures responsible for the kinetics of K+(o) binding and/or occlusion may be distinct, at least in part, from those that are responsible for the voltage dependence of K+(o) binding to the Na,K-ATPase.

Site-directed chemical labeling of extracellular loops in a membrane protein. The topology of the Na,K-ATPase alpha-subunit

We have mapped the membrane topology of the renal Na,K-ATPase alpha-subunit by using a combination of introduced cysteine mutants and surface labeling with a membrane impermeable Cys-directed reagent, N-biotinylaminoethyl methanethiosulfonate. To begin our investigation, two cysteine residues (Cys(911) and Cys(964)) in the wild-type alpha-subunit were substituted to create a background mutant devoid of exposed cysteines (Lutsenko, S., Daoud, S., and Kaplan, J. H. (1997) J. Biol. Chem. 272, 5249-5255). Into this background construct were then introduced single cysteines in each of the five putative extracellular loops (P118C, T309C, L793C, L876C, and M973C) and the resulting alpha-subunit mutants were co-expressed with the beta-subunit in baculovirus-infected insect cells. All of our expressed Na,K-ATPase mutants were functionally active. Their ATPase, phosphorylation, and ouabain binding activities were measured, and the turnover of the phosphoenzyme intermediate was close to the wild-type enzyme, suggesting that they are folded properly in the infected cells. Incubation of the insect cells with the cysteine-selective reagent revealed essentially no labeling of the alpha-subunit of the background construct and labeling of all five mutants with single cysteine residues in putative extracellular loops. Two additional mutants, V969C and L976C, were created to further define the M9M10 loop. The lack of labeling for these two mutants showed that although Met(973) is apparently exposed, Val(969) and Leu(976) are not, demonstrating that this method may also be utilized to define membrane aqueous boundaries of membrane proteins. Our labeling studies are consistent with a specific 10-transmembrane segment model of the Na,K-ATPase alpha-subunit. This strategy utilized only functional Na,K-ATPase mutants to establish the membrane topology of the entire alpha-subunit, in contrast to most previously applied methods.

Functional role of cysteine residues in the (Na,K)-ATPase alpha subunit

The structural-functional roles of 23 cysteines present in the sheep (Na,K)-ATPase alpha1 subunit were studied using site directed mutagenesis, expression, and kinetics analysis. Twenty of these cysteines were individually substituted by alanine or serine. Cys452, Cys455 and Cys456 were simultaneously replaced by serine. These substitutions were introduced into an ouabain resistant alpha1 sheep isoform and expressed in HeLa cells under ouabain selective pressure. HeLa cells transfected with a cDNA encoding for replacements of Cys242 did not survive ouabain selective pressure. Single substitutions of the remaining cysteines yielded functional enzymes, although some had reduced turnover rates. Only minor variations were observed in the enzyme Na(+) and K(+) dependence as a result of these replacements. Some substitutions apparently affect the E1<-->E2 equilibrium as suggested by changes in the K(m) of ATP acting at its low affinity binding site. These results indicate that individual cysteines, with the exception of Cys242, are not essential for enzyme function. Furthermore, this suggests that the presence of putative disulfide bridges is not required for alpha1 subunit folding and subsequent activity. A (Na,K)-ATPase lacking cysteine residues in the transmembrane region was constructed (Cys104, 138, 336, 802, 911, 930, 964, 983Xxx). No alteration in the K(1/2) of Na(+) or K(+) for (Na,K)-ATPase activation was observed in the resulting enzyme, although it showed a 50% reduction in turnover rate. ATP binding at the high affinity site was not affected. However, a displacement in the E1<-->E2 equilibrium toward the E1 form was indicated by a small decrease in the K(m) of ATP at the low affinity site accompanied by an increase in IC(50) for vanadate inhibition. Thus, the transmembrane cysteine-deficient (Na,K)-ATPase appears functional with no critical alteration in its interactions with physiological ligands.

Entrance port for Na(+) and K(+) ions on Na(+),K(+)-ATPase in the cytoplasmic loop between trans-membrane segments M6 and M7 of the alpha subunit. Proximity Of the cytoplasmic segment of the beta subunit

Based on the following observations we propose that the cytoplasmic loop between trans-membrane segments M6 and M7 (L6/7) of the alpha subunit of Na(+),K(+)-ATPase acts as an entrance port for Na(+) and K(+) ions. 1) In defined conditions chymotrypsin specifically cleaves L6/7 in the M5/M6 fragment of 19-kDa membranes, produced by extensive proteolysis of Na(+),K(+)-ATPase, and in parallel inactivates Rb(+) occlusion. 2) Dissociation of the M5/M6 fragment from 19-kDa membranes is prevented either by occluded cations or by competitive antagonists such as Ca(2+), Mg(2+), La(3+), p-xylylene bisguanidinium and m-xylylene bisguanidinium, or 1-bromo-2,4, 6-tris(methylisothiouronium)benzene and 1,3-dibromo-2,4,6-tris (methylisothiouronium)benzene (Br(2)-TITU(3+)). 3) Ca(2+) ions raise electrophoretic mobility of the M5/M6 fragment but not that of the other fragments of the alpha subunit. It appears that negatively charged residues in L6/7 recognize either Na(+) or K(+) ions or the competitive cation antagonists. Na(+) and K(+) ions are then occluded within trans-membrane segments and can be transported, whereas the cation antagonists are not occluded and block transport at the entrance port. The cytoplasmic segment of the beta subunit appears to be close to or contributes to the entrance port, as inferred from the following observations. 1) Specific chymotryptic cleavage of the 16-kDa fragment of the beta subunit to 15-kDa at 20 degrees C (Shainskaya, A., and Karlish, S. J. D. (1996) J. Biol. Chem. 271, 10309-10316) markedly reduces affinity for Br(2)-TITU(3+) and for Na(+) ions, detected by Na(+) occlusion assays or electrogenic Na(+) binding, whereas Rb(+) occlusion is unchanged. 2) Na(+) ions specifically protect the 16-kDa fragment against this chymotryptic cleavage.

The energy transduction mechanism of Na,K-ATPase studied with iron-catalyzed oxidative cleavage

This paper extends our recent report on specific iron-catalyzed oxidative cleavages of renal Na,K-ATPase and effects of E1 left arrow over right arrow E2 conformational transitions (Goldshleger, R. , and Karlish, S. J. D. (1997) Proc. Natl. Acad. Sci. U. S. A. 94, 9596-9601). The experiments indicate that only peptide bonds close to a bound Fe2+ ion are cleaved, and provide evidence on proximity of the different cleavage positions in the native enzyme. A sequence HFIH near trans-membrane segment M3 appears to be involved in Fe2+ binding. Previously we hypothesized that E2 and E1 conformations are characterized by formation or relaxation of interactions within the alpha subunit at or near highly conserved sequences, TGES in the minor cytoplasmic loop and CSDK, MVTGD, and VNDSPALKK in the major cytoplasmic loop. This concept has been tested by examining iron-catalyzed cleavage in both non-phosphorylated and phosphorylated conformations and effects of phosphate, vanadate, and ouabain. The results imply that both E1 left arrow over right arrow E2 and E1P left arrow over right arrow E2P transitions are indeed associated with formation and relaxation of interactions between cytoplasmic domains, comprising the minor loop plus N-terminal tail leading into M1 and major loop, respectively. Furthermore, it appears that either non-covalently or covalently bound phosphate bind near CSDK and MVTGD, and Mg2+ ions may bind to residues within TGES and VNDSPALKK and to bound phosphate. Thus cytoplasmic domain interactions seem to occur within or near the active site. We discuss the relationship between structural changes in the cytoplasmic domain and movements of trans-membrane segments that lead to cation transport. Presumably conformation-dependent formation and relaxation of domain interactions underlie energy transduction in all P-type pumps.

Glu-857 moderates K+-dependent stimulation and SCH 28080-dependent inhibition of the gastric H,K-ATPase

The rabbit H,K-ATPase alpha- and beta-subunits were transiently expressed in HEK293 T cells. The co-expression of the H,K-ATPase alpha- and beta-subunits was essential for the functional H,K-ATPase. The K+-stimulated H,K-ATPase activity of 0.82 +/- 0.2 micromol/mg/h saturated with a K0.5 (KCl) of 0.6 +/- 0.1 mM, whereas the 2-methyl-8-(phenylmethoxy)imidazo[1,2a]pyridine-3-acetonitrile (SCH 28080)-inhibited ATPase of 0.62 +/- 0.07 micromol/mg/h saturated with a Ki (SCH 28080) of 1.0 +/- 0.3 microM. Site mutations were introduced at the N,N-dicyclohexylcarbodiimide-reactive residue, Glu-857, to evaluate the role of this residue in ATPase function. Variations in the side chain size and charge of this residue did not inhibit the specific activity of the H,K-ATPase, but reversal of the side chain charge by substitution of Lys or Arg for Glu produced a reciprocal change in the sensitivity of the H,K-ATPase to K+ and SCH 28080. The K0.5 for K+stimulated ATPase was decreased to 0.2 +/-.05 and 0.2 +/-.03 mM, respectively, in Lys-857 and Arg-857 site mutants, whereas the Ki for SCH 28080-dependent inhibition was increased to 6.5 +/- 1.4 and 5.9 +/- 1.5 microM, respectively. The H,K-ATPase kinetics were unaffected by the introduction of Ala at this site, but Leu produced a modest reciprocal effect. These data indicate that Glu-857 is not an essential residue for cation-dependent activity but that the residue influences the kinetics of both K+ and SCH 28080-mediated functions. This finding suggests a possible role of this residue in the conformational equilibrium of the H,K-ATPase.

Stabilization of the H,K-ATPase M5M6 membrane hairpin by K+ ions. Mechanistic significance for p2-type atpases

The integral membrane protein, the gastric H,K-ATPase, is an alpha-beta heterodimer, with 10 putative transmembrane segments in the alpha-subunit and one such segment in the beta-subunit. All transmembrane segments remain within the membrane domain following trypsinization of the intact gastric H,K-ATPase in the presence of K+ ions, identified as M1M2, M3M4, M5M6, and M7, M8, M9, and M10. Removal of K+ ions from this digested preparation results in the selective loss of the M5M6 hairpin from the membrane. The release of the M5M6 fragment is directed to the extracellular phase as evidenced by the accumulation of the released M5M6 hairpin inside the sealed inside out vesicles. The stabilization of the M5M6 hairpin in the membrane phase by the transported cation as well as loss to the aqueous phase in the absence of the transported cation has been previously observed for another P2-type ATPase, the Na, K-ATPase (Lutsenko, S., Anderko, R., and Kaplan, J. H. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 7936-7940). Thus, the effects of the counter-transported cation on retention of the M5M6 segment in the membrane as compared with the other membrane pairs may be a general feature of P2-ATPase ion pumps, reflecting a flexibility of this region that relates to the mechanism of transport.

Characterization of disulfide cross-links between fragments of proteolyzed Na,K-ATPase. Implications for spatial organization of trans-membrane helices

This study characterizes disulfide cross-links between fragments of a well defined tryptic preparation of Na,K-ATPase, 19-kDa membranes solubilized with C12E10 in conditions preserving an intact complex of fragments and Rb occlusion (Or, E., Goldshleger, R., Tal, D. M., and Karlish, S. J. D. (1996) Biochemistry 35, 6853-6864). Upon solubilization, cross-links form spontaneously between the beta subunit, 19- and 11.7-kDa fragments of the alpha subunit, containing trans-membrane segments M7-M10 and M1/M2, respectively. Treatment with Cu2+-phenanthroline (CuP) improves efficiency of cross-linking. Sequencing and immunoblot analysis have shown that the cross-linked products consist of a mixture of beta-19 kDa dimers ( approximately 65%) and beta-19 kDa-11.7 kDa trimers ( approximately 35%). The alpha-beta cross-link has been located within the 19-kDa fragment to a 6.5-kDa chymotryptic fragment containing M8, indicating that betaCys44 is cross-linked to either Cys911 or Cys930. In addition, an internal cross-link between M9 and M10, Cys964-Cys983, has been found by sequencing tryptic fragments of the cross-linked product. The M1/M2-M7/M10 cross-link has not been identified directly. However, we propose that Cys983 in M10 is cross-linked either to Cys104 in M1 or internally to Cys964 in M9. Based on this study, cross-linking induced by o-phthalaldehyde (Or, E., Goldshleger, R., and Karlish, S. J. D. (1998) Biochemistry 37, 8197-8207), and information from the literature, we propose an approximate spatial organization of trans-membrane segments of the alpha and beta subunits.

Specific Cu2+-catalyzed oxidative cleavage of Na,K-ATPase at the extracellular surface

This paper describes specific Cu2+-catalyzed oxidative cleavage of alpha and beta subunits of Na,K-ATPase at the extracellular surface. Incubation of right side-out renal microsomal vesicles with Cu2+ ions, ascorbate, and H2O2 produces two major cleavages of the alpha subunit within the extracellular loop between trans-membrane segments M7 and M8 and L7/8. Minor cleavages are also detected in loops L9/10 and L5/6. In the beta subunit two cleavages are detected, one before the first S-S bridge and the other between the second and third S-S bridges. Na,K-ATPase and Rb+ occlusion are inactivated after incubation with Cu2+/ascorbate/H2O2. These observations are suggestive of a site-specific mechanism involving cleavage of peptide bonds close to a bound Cu2+ ion. This mechanism allows several inferences on subunit interactions and spatial organization. The two cleavage sites in L7/8 of the alpha subunit and two cleavage sites of the beta subunit identify interacting segments of the subunits. L7/8 is also close to L9/10 and to cation occlusion sites. Comparison of the locations of Cu2+-catalyzed cleavages with Fe2+-catalyzed cleavages (Goldshleger, R., and Karlish, S. J. D. (1997) Proc. Natl. Acad. Sci. U. S. A. 94, 9596-9601) suggests division of the membrane sector into two domains comprising M1-M6 and M7-M10/Mbeta, respectively.

Alpha 2 isoform of the Na,K-adenosine triphosphatase is reduced in temporal cortex of bipolar individuals

BACKGROUND

The pathophysiology of bipolar illness has been associated with changes in transmembrane ion flux and redistribution of biologically active ions. The recent identification of multiple isoforms of Na,K-adenosine triphosphatase (ATPase) alpha and beta subunits raises the possibility of altered pump isoform expression.

METHODS

We determined Na,K-ATPase alpha subunit expression in postmortem temporal cortex gray matter from individuals suffering from bipolar disorder, schizoaffective disorder, schizophrenia, and matched normal controls. Quantification of isoform expression was accomplished via densitometric scanning of Western blots utilizing isoform-specific antibodies.

RESULTS

Bipolar individuals exhibited a significant reduction in the abundance of the alpha 2 isoform of Na,K-ATPase compared to normal controls. Schizophrenic and schizo-affective brains were not significantly different from normal controls.

CONCLUSION

These data suggest that previously observed abnormalities in regulation and distribution of ions in bipolar illness may be related to specific alpha 2 dysregulation.

Photoinactivation of fluorescein isothiocyanate-modified Na,K-ATPase by 2'(3')-O-(2,4,6-trinitrophenyl)8-azidoadenosine 5'-diphosphate. Abolition of E1 and E2 partial reactions by sequential block of high and low affinity nucleotide sites

The Na,K-ATPase activity of the sodium pump exhibits apparent multisite kinetics toward ATP, a feature that is inherent to the minimal enzyme unit, the alpha beta protomer. We have argued that this should arise from separate catalytic and noncatalytic sites on the alpha beta protomer as fluorescein isothiocyanate (FITC) blocks a high affinity ATP site on all alpha subunits and yet the modified Na, K-ATPase retains a low affinity response to nucleotides (Ward, D. G., and Cavieres, J. D. (1996) J. Biol. Chem. 271, 12317-12321). We now find that 2'(3')-O-(2,4,6-trinitrophenyl)8-azido-adenosine 5'-diphosphate (TNP-8N3-ADP), a high affinity photoactivatable analogue of ATP, can inhibit the K+-phosphatase activity of the FITC-modified enzyme during assays in dimmed light. The inhibition occurs with a Ki of 140 microM at 20 mM K+; it requires the adenine ring as 2'(3')-O-(2,4 6-trinitrophenyl) (TNP)-UDP or TNP-uridine are less potent and 2,4,6-trinitrobenzene-sulfonate is ineffective. Under irradiation with UV light, TNP-8N3-ADP inactivates the K+-phosphatase activity of the fluorescein-enzyme and also its phosphorylation by [32P]Pi. The photoinactivation process is stimulated by Na+ or Mg2+, and is inhibited by K+ or excess TNP-ADP. In the presence of 50 mM Na+ and 1 mM Mg2+, TNP-8N3-ADP photoinactivates with a K0.5 of 15 microM. Furthermore, TNP-8N3-ADP photoinactivates the FITC-modified, solubilized alpha beta protomers, even more effectively than the membrane-bound fluorescein-enzyme. These results strongly suggest that catalytic and allosteric ATP sites coexist on the alpha beta protomer of Na,K-ATPase.

Selectivity of lipid-protein interactions with trypsinized Na, K-ATPase studied by spin-label EPR

The selectivity of the lipid-protein interactions in trypsinised Na, K-ATPase membranes from Squalus acanthias has been determined by using EPR spectroscopy with different lipid probes spin-labelled on the 14-C atom of the fatty acid chain. From measurements at low ionic strength and different pH values, the pattern of selectivity is: (stearic acid)->(phosphatidylserine)->(stearic acid)0>(phosphatidylcholine)+/-, where superscripts indicate the formal electrostatic charge on the lipid headgroup. This is in the same order as that determined with native Na,K-ATPase membranes [M. Esmann, D. Marsh, Biochemistry 24 (1985) 3572-3578]. The selectivity for phosphatidylserine is independent of pH, over the range pH 6.0-9. 0, as found also for native membranes. For membranes trypsinised in the presence of Rb+ ions, and in the presence of Na+ (which allows more extensive proteolysis), the relative association constants, Kr, of all lipids are the same as for control membranes, with the exception of ionised (stearic acid)- that shows the highest specificity. Therefore, both the stoichiometry and the principal determinants of the specificity of lipid-protein interaction are preserved on extensive trypsinisation of Na,K-ATPase membranes. This has implications for the location and arrangement of those amino acid side chains that determine the lipid selectivity of the native Na,K-ATPase.

Interactions between fragments of trypsinized Na,K-ATPase detected by thermal inactivation of Rb+ occlusion and dissociation of the M5/M6 fragment

This work provides evidence for interactions between fragments of "19-kDa membranes," a trypsinized preparation of Na,K-ATPase that retains cation occlusion and ouabain binding. Previously, we reported rapid thermal inactivation of Rb+ occlusion at 37 degreesC (Or, E., David, P., Shainskaya, A., Tal, D. M., and Karlish, S. J. D. (1993) J. Biol. Chem. 268, 16929-16937). We describe here the detailed kinetics of thermal inactivation. In the range 25-35 degreesC, a two-step model (N left and right arrow U --> I, where N is the native species, U is the reversibly unfolded intermediate, and I is the irreversibly denatured form) fits the data. Reversibility of inactivation has been observed at 25 degreesC, consistent with the model. At 37 degreesC and higher temperatures, the data can be fitted to the simple mechanism N --> I, i.e. U is not significant as an intermediate. Occluded cations (Na+, Rb+, K+, Tl+, NH4+, and Cs+) and ouabain protect strongly against thermal inactivation. Ca2+, Ba2+, and La3+ ions do not protect. Proteolysis experiments provide independent evidence that disorganization can occur in stages, first in transmembrane segments and then in extra-membrane segments of the fragments. Analysis of selective dissociation of the M5/M6 fragment at 37 degreesC (Lutsenko, S., and Kaplan, J. H. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 7936-7940), using a specific antibody, showed that inactivation of Rb+ occlusion precedes dissociation of the fragment, and only approximately 50% of the fragment is released when occlusion is fully inactivated. In the presence of Ca2+ ions, occlusion is inactivated, but the M5/M6 fragment is not released. The experiments demonstrate that occlusion is inactivated by disruption of interactions between fragments of 19-kDa membranes, and only then does the M5/M6 fragment dissociate. Interactions between the M5/M6 and M7/M10 fragments seem to be essential for maintenance of Rb+ occlusion.

Tissue-specific distribution and modulatory role of the gamma subunit of the Na,K-ATPase

The Na,K-ATPase comprises a catalytic alpha subunit and a glycosylated beta subunit. Another membrane polypeptide, gamma, first described by Forbush et al. (Forbush, B., III, Kaplan, J. H., and Hoffman, J. F. (1978) Biochemistry 17, 3667-3676) associates with alpha and beta in purified kidney enzyme preparations. In this study, we have used a polyclonal anti-gamma antiserum to define the tissue specificity and topology of gamma and to address the question of whether gamma has a functional role. The trypsin sensitivity of the amino terminus of the gamma subunit in intact right-side-out pig kidney microsomes has confirmed that it is a type I membrane protein with an extracellular amino terminus. Western blot analysis shows that gamma subunit protein is present only in membranes from kidney tubules (rat, dog, pig) and not those from axolemma, heart, red blood cells, kidney glomeruli, cultured glomerular cells, alpha1-transfected HeLa cells, all derived from the same (rat) species, nor from three cultured cell lines derived from tubules of the kidney, namely NRK-52E (rat), LLC-PK (pig), or MDCK (dog). To gain insight into gamma function, the effects of the anti-gamma serum on the kinetic behavior of rat kidney sodium pumps was examined. The following evidence suggests that gamma stabilizes E1 conformation(s) of the enzyme and that anti-gamma counteracts this effect: (i) anti-gamma inhibits Na,K-ATPase, and the inhibition increases at acidic pH under which condition the E2(K) --> E1 phase of the reaction sequence becomes more rate-limiting, (ii) the oligomycin-stimulated increase in the level of phosphoenzyme was greater in the presence of anti-gamma indicating that the antibody shifts the E1 left and right arrow left and right arrow E2P equilibria toward E2P, and (iii) when the Na+-ATPase reaction is assayed with the Na+ concentration reduced to levels (</=2 mM) which limit the rate of the E1 --> --> E2P transition, anti-gamma is stimulatory. These observations taken together with evidence that the pig gamma subunit, which migrates as a doublet on polyacrylamide gels, is sensitive to digestion by trypsin, and that Rb+ ions partially protect it against this effect, indicate that the gamma subunit is a tissue-specific regulator which shifts the steady-state equilibria toward E1. Accordingly, binding of anti-gamma disrupts alphabeta-gamma interactions and counteracts these modulatory effects of the gamma subunit.

Probing of the membrane topology of sarcoplasmic reticulum Ca2+-ATPase with sequence-specific antibodies. Evidence for plasticity of the c-terminal domain

The topology of Ca2+-ATPase in sarcoplasmic reticulum (SR) vesicles was investigated with the aid of sequence-specific antibodies, produced against oligopeptides corresponding to sequences close to the membranous portions of the protein. The antisera in competitive enzyme-linked immunosorbent assays only reacted with intact SR vesicles to a limited extent, but most epitopic regions were exposed by low concentrations of nondenaturing detergent, octaethylene glycol dodecyl ether (C12E8) or after removal of cytosolic regions by proteinase K. In particular, these treatments exposed the loop regions in the C-terminal domain, including L7-8, the loop region located between transmembrane segments M7 and M8, with a putative intravesicular position, which had immunochemical properties very similar to those of the C terminus with a documented cytosolic exposure. In contrast to this, the reactivity of the N-terminal intravesicular loop regions L1-2 and L3-4 was only increased by C12E8 treatment but not by proteinase K proteolysis. Complexation of Ca2+-ATPase with beta,gamma-CrATP stabilized the C-terminal domain of Ca2+-ATPase against proteinase K proteolysis and reaction with most of the antisera, but immunoreactivity was maintained by the L6-7 and L7-8 loops. Immunoelectron microscopic analyses of vesicles following negative staining, thin sectioning, and the SDS-digested freeze-fracture labeling method suggested that the L7-8 epitope, in contrast to L6-7 and the C terminus, can be exposed on either the intravesicular or cytosolic side of the membrane. A preponderant intravesicular location of L7-8 in intact vesicles is suggested by the susceptibility of this region to proteolytic cleavage after disruption of the vesicular barrier with C12E8 and in symmetrically reconstituted Ca2+-ATPase proteoliposomes. In conclusion, our data suggest an adaptable membrane insertion of the C-terminal Ca2+-ATPase domain, which under some conditions permits sliding of M8 through the membrane with cytosolic exposure of L7-8, of possible functional significance in connection with Ca2+ translocation. On the technical side, our data emphasize that extreme caution is needed when using nondenaturing detergents or other treatments like EGTA at alkaline pH to open up vesicles for probing of intravesicular location with antibodies.

Characterization of the secondary structure and assembly of the transmembrane domains of trypsinized Na,K-ATPase by Fourier transform infrared spectroscopy

Fourier transform infrared spectroscopy has been used to compare native Na,K-ATPase-containing membranes with those trypsinized in the presence of either Rb+ or Na+ ions to remove the extramembranous parts of the protein. The protein secondary structure content deduced from the amide I band is approximately 30-35% alpha-helix, 37-40% beta-structure, and 13-15% random coil for native membranes from shark rectal gland and from pig kidney, in both the Na- and K-forms. Trypsinization in either Rb+ (a K+ congener) or Na+ removes approximately 35% of the amide I band intensity of native membranes from shark rectal gland. The protein secondary structural content of the trypsinized membranes lies in the range of approximately 23-32% alpha-helix, 37-46% beta-structure, and 12-18% random coil for the shark and kidney enzymes. The distribution of intensity between the bands corresponding to protonated and deuterium-exchanged alpha-helices, and between the component bands attributed to beta-structure, changes considerably on trypsinization, in the direction of a greater proportion of protonated alpha-helix and a broader range of frequencies for beta-structure. The kinetics of deuteration of the slowly exchanging population of protein amide groups is also changed on trypsinization. The mean rate constant for deuteration of trypsinized membranes is approximately half that for native membranes, whereas the proportion of amides contributing to this population increases on trypsinization. The temperature dependence of the amide I band in the Fourier transform infrared spectra indicates that the onset of thermal denaturation occurs at 58 degrees C for native membranes (in either Na+ or K+) and for membranes trypsinized in Rb+, but the major denaturation event for membranes trypsinized in Na+ occurs at approximately 84 degrees C. These results correlate with the functional properties of the intramembranous section of the enzyme.

Fe-catalyzed cleavage of the alpha subunit of Na/K-ATPase: evidence for conformation-sensitive interactions between cytoplasmic domains

Incubation of Na/K-ATPase with ascorbate plus H2O2 produces specific cleavage of the alpha subunit. Five fragments with intact C termini and complementary fragments with intact N termini were observed. The beta subunit is not cleaved. Cleavages depend on the presence of contaminant or added Fe2+ ions, as inferred by suppression of cleavages with nonspecific metal complexants (histidine, EDTA, phenanthroline) or the Fe3+-specific complexant desferrioxamine, or acceleration of cleavages by addition of low concentrations of Fe2+ but not of other heavy metal ions. Na/K-ATPase is inactivated in addition to cleavage, and both effects are insensitive to OH. radical scavengers. Cleavages are sensitive to conformation. In low ionic strength media (E2) or media containing Rb ions [E2(Rb)], cleavage is much faster than in high ionic strength media (E1) or media containing Na ions (E1Na). N-terminal fragments and two C-terminal fragments (N-terminals E214 and V712) have been identified by amino acid sequencing. Approximate positions of other cleavages were determined with specific antibodies. The results suggest that Fe2+ (or Fe3+) ions bind with high affinity at the cytoplasmic surface and catalyze cleavages of peptide bonds close to the Fe2+ (or Fe3+) ion. Thus, cleavage patterns can provide information on spatial organization of the polypeptide chain. We propose that highly conserved regions of the alpha subunit, within the minor and major cytoplasmic loops, interact in the E2 or E2(Rb) conformations but move apart in the E1 or E1Na conformations. We discuss implications of domain interactions for the energy transduction mechanism. Fe-catalyzed cleavages may be applicable to other P-type pumps or membrane proteins.

Evidence for the existence of two ATP-sensitive Rb+ occlusion pockets within the transmembrane domains of Na+/K+-ATPase

A trypsin-digested Na+/K+-ATPase that has lost ATPase activity and about half of its protein content retains an essentially intact beta-subunit, the 10 transmembrane domains of the alpha-subunit, and the full capacity to occlude Na+ and Rb+ (a congener of K+). When this preparation was incubated at 37 degrees C in the absence of Rb+, it lost half of its Rb+ occluding capacity and two-thirds of its Na+ occluding capacity. Comparison of the Rb+ occlusion-deocclusion kinetics of the digested enzyme before and after partial inactivation indicated that (a) the affinities of the labile and the stable halves of occluded Rb+ were the same; (b) occlusion and deocclusion rates of the stable pool were lower than those of the labile pool; (c) ATP at a low affinity site (K0.5 = 25-300 microM) increased deocclusion rate in the stable pool and occlusion rate in the labile pool; (d) Na+ increased Rb+ deocclusion rate of the sum of the two pools but not that of the stable pool; and (e) occlusion and deocclusion rates of both pools were decreased by ouabain. These findings suggest that (a) the peptide complex of the digested enzyme contains two distinct but interacting cation occlusion pockets, one occluding two Na+ or one Rb+, and the other occluding one Na+ or one Rb+; (b) this peptide complex that is devoid of the catalytic ATP site retains an allosteric ATP site; and (c) the access channels of the two pockets are regulated differently by ATP but similarly by ouabain. Analyses of the gel electrophoretic patterns of the digested enzyme and the N termini of the appropriate bands showed that inactivation of the labile occlusion pocket was accompanied by 60-70% loss of two alpha-fragments containing H3-H4 and H5-H6 transmembrane domains. This and the previously established interactions among the transmembrane helices of alpha- and beta-subunits suggest that one occlusion pocket is associated with H3-H6 domains and that the other is located within a complex of beta-subunit and two alpha-fragments containing H1-H2 and H7-H10 transmembrane domains.

Equilibrium of phosphointermediates of sodium and potassium ion transport adenosine triphosphatase: action of sodium ion and Hofmeister effect

Sodium and potassium ion transport adenosine triphosphatase accepts and donates a phosphate group in the course of its reaction sequence. The phosphorylated enzyme has two principal reactive states, E1P and E2P. E1P is formed reversibly from ATP in the presence of Na+ and is precursor to E2P, which equilibrates with P(i) in the presence of K+. We studied equilibrium between these states at 4 degrees C and the effect of Na+ on it. To optimize the reaction system we used a Hofmeister effect, replacing the usual anion, chloride, with a chaotropic anion, usually nitrate. We phosphorylated enzyme from canine kidney with [32P]ATP. We estimated interconversion rate constants for the reaction E1P <--> E2P and their ratio. To estimate rate constants we terminated phosphorylation and observed decay kinetics. We observed E1P or E2P selectively by adding K+ or ADP respectively. K+ dephosphorylates E2P leaving E1P as observable species; ADP dephosphorylates E1P leaving E2P as observable species. We fitted a 2-pool model comprising two reactive species or a twin 2-pool model, comprising a pair of independent 2-pool models, to the data and obtained interconversion and hydrolysis rate constants for each state. Replacing Na+ with Tris+ or lysine+ did not change the ratio of interconversion rate constants between E1P and E2P. Thus Na+ binds about equally strongly to E1P and E2P. This conclusion is consistent with a model of Pedemonte (1988. J. Theor. Biol. 134:165-182.). We found that Na+ affected another equilibrium, that of transphosphorylation between ATP x dephosphoenzyme and ADP x E1P. We used the reactions and model of Pickart and Jencks (1982. J. Biol. Chem. 257:5319-5322.) to generate and fit data. Decreasing the concentration of Na+ 10-fold shifted the equilibrium constant 10-fold favoring ADP x E1P over ATP x dephosphoenzyme. Thus Na+ can dissociate from E1P x Na3. Furthermore, we found two characteristics of Hofmeister effects on this enzyme.