A cytoplasmic region of the Na,K-ATPase alpha-subunit is necessary for specific alpha/alpha association

Multimeric species in equilibrium in detergent-solubilized Na,K-ATPase

In this work, we find an equilibrium between different Na,K-ATPase (NKA) oligomeric species solubilized in a non-ionic detergent C12E8 by means of Dynamic Light Scattering (DLS), Analytical Ultracentrifugation (AUC), Small Angle X-ray Scattering (SAXS), Spectrophotometry (absorption at 280/350nm) and enzymatic activity assay. The NKA sample after chromatography purification presented seven different populations as identified by AUC, with monomers and tetramers amounting to ∼55% of the total protein mass in solution. These two species constituted less than 40% of the total protein mass after increasing the NKA concentration. Removal of higher-order oligomer/aggregate species from the NKA solution using 220nm-pore filter resulted in an increase of the specific enzymatic activity. Nevertheless, the enzyme forms new large aggregates over an elapsed time of 20h. The results thus point out that C12E8-solubilized NKA is in a dynamic equilibrium of monomers, tetramers and high-order oligomers/subunit aggregates. These latter have low or null activity. High amount of detergent leads to the dissociation of NKA into smaller aggregates with no enzymatic activity.

Endothelial and smooth muscle cell ion channels in pulmonary vasoconstriction and vascular remodeling

The pulmonary circulation is a low resistance and low pressure system. Sustained pulmonary vasoconstriction and excessive vascular remodeling often occur under pathophysiological conditions such as in patients with pulmonary hypertension. Pulmonary vasoconstriction is a consequence of smooth muscle contraction. Many factors released from the endothelium contribute to regulating pulmonary vascular tone, while the extracellular matrix in the adventitia is the major determinant of vascular wall compliance. Pulmonary vascular remodeling is characterized by adventitial and medial hypertrophy due to fibroblast and smooth muscle cell proliferation, neointimal proliferation, intimal, and plexiform lesions that obliterate the lumen, muscularization of precapillary arterioles, and in situ thrombosis. A rise in cytosolic free Ca(2+) concentration ([Ca(2+)]cyt) in pulmonary artery smooth muscle cells (PASMC) is a major trigger for pulmonary vasoconstriction, while increased release of mitogenic factors, upregulation (or downregulation) of ion channels and transporters, and abnormalities in intracellular signaling cascades are key to the remodeling of the pulmonary vasculature. Changes in the expression, function, and regulation of ion channels in PASMC and pulmonary arterial endothelial cells play an important role in the regulation of vascular tone and development of vascular remodeling. This article will focus on describing the ion channels and transporters that are involved in the regulation of pulmonary vascular function and structure and illustrating the potential pathogenic role of ion channels and transporters in the development of pulmonary vascular disease.

Ouabain activates the Na-K-ATPase signalosome to induce autosomal dominant polycystic kidney disease cell proliferation

The Na-K-ATPase is part of a cell signaling complex, the Na-K-ATPase signalosome, which upon activation by the hormone ouabain regulates the function of different cell types. We previously showed that ouabain induces proliferation of epithelial cells derived from renal cysts of patients with autosomal dominant polycystic kidney disease (ADPKD cells). Here, we investigated the signaling pathways responsible for mediating the effects of ouabain in these cells. Incubation of ADPKD cells with ouabain, in concentrations similar to those found in blood, stimulated phosphorylation of the epidermal growth factor receptor (EGFR) and promoted its association to the Na-K-ATPase. In addition, ouabain activated the kinase Src, but not the related kinase Fyn. Tyrphostin AG1478 and PP2, inhibitors of EGFR and Src, respectively, blocked ouabain-dependent ADPKD cell proliferation. Treatment of ADPKD cells with ouabain also caused phosphorylation of the caveolar protein caveolin-1, and disruption of cell caveolae with methyl-β-cyclodextrin prevented Na-K-ATPase-EGFR interaction and ouabain-induced proliferation of the cells. Downstream effects of ouabain in ADPKD cells included activation of B-Raf and MEK and phosphorylation of the extracellular regulated kinase ERK, which translocated into the ADPKD cell nuclei. Finally, ouabain reduced expression of the cyclin-dependent kinase inhibitors p21 and p27, which are suppressors of cell proliferation. Different from ADPKD cells, ouabain showed no significant effect on B-Raf, p21, and p27 in normal human kidney epithelial cells. Altogether, these results identify intracellular pathways of ouabain-dependent Na-K-ATPase-mediated signaling in ADPKD cells, including EGFR-Src-B-Raf-MEK/ERK, and establish novel mechanisms involved in ADPKD cell proliferation.

Increased expression of the Na,K-ATPase alpha4 isoform enhances sperm motility in transgenic mice

The Na,K-ATPase alpha4 (ATP1A4) isoform is specifically expressed in male germ cells and is highly prevalent in spermatozoa. Although selective inhibition of alpha4 activity with ouabain has been shown to affect sperm motility, a more direct analysis of the role of this isoform in sperm movement has not yet been demonstrated. To establish this, we engineered transgenic mice that express the rat alpha4 isoform fused to green fluorescent protein in male germ cells, under the control of the mouse protamine 1 promoter. We showed that the rat Atp1a4 transgene is expressed in mouse spermatozoa and that it is localized to the sperm flagellum. In agreement with increased expression of the alpha4 isoform, sperm from transgenic mice displayed higher alpha4-specific Na,K-ATPase activity and binding of fluorescently labeled ouabain than wild-type mice. In contrast, expression and activity of ATP1A1 (alpha1), the other Na,K-ATPase alpha isoform present in sperm, remained unchanged. Similar to wild-type mice, mice expressing the alpha4 transgene exhibited normal testis and sperm morphology and no differences in fertility. However, compared to wild-type mice, sperm from transgenic mice displayed plasma membrane hyperpolarization and higher total and progressive motility. Other parameters of motility also increased, including straight-line, curvilinear, and average path velocities and amplitude of lateral head displacement. In addition, sperm from the transgenic mice showed enhanced sperm hyperactive motility, but no changes in progesterone-induced acrosome reaction. Altogether, these results provide new genetic evidence for the role of the ATP1A4 isoform in sperm motility, under both noncapacitating and capacitating conditions.

Self-association of isolated large cytoplasmic domain of plasma membrane H+ -ATPase from Saccharomyces cerevisiae: role of the phosphorylation domain in a general dimeric model for P-ATPases

Large cytoplasmic domain (LCD) plasma membrane H+ -ATPase from S. cerevisiae was expressed as two fusion polypeptides in E. coli: a DNA sequence coding for Leu353-Ileu674 (LCDh), comprising both nucleotide (N) and phosphorylation (P) domains, and a DNA sequence coding for Leu353-Thr543 (LCDDeltah, lacking the C-terminus of P domain), were inserted in expression vectors pDEST-17, yielding the respective recombinant plasmids. Overexpressed fusion polypeptides were solubilized with 6 M urea and purified on affinity columns, and urea was removed by dialysis. Their predicted secondary structure contents were confirmed by CD spectra. In addition, both recombinant polypeptides exhibited high-affinity 2',3'-O-(2,4,6-trinitrophenyl)adenosine-5'-triphosphate (TNP-ATP) binding (Kd = 1.9 microM and 2.9 microM for LCDh and LCDDeltah, respectively), suggesting that they have native-like folding. The gel filtration profile (HPLC) of purified LCDh showed two main peaks, with molecular weights of 95 kDa and 39 kDa, compatible with dimeric and monomeric forms, respectively. However, a single elution peak was observed for purified LCDDeltah, with an estimated molecular weight of 29 kDa, as expected for a monomer. Together, these data suggest that LCDh exist in monomer-dimer equilibrium, and that the C-terminus of P domain is necessary for self-association. We propose that such association is due to interaction between vicinal P domains, which may be of functional relevance for H+ -ATPase in native membranes. We discuss a general dimeric model for P-ATPases with interacting P domains, based on published crystallography and cryo-electron microscopy evidence.

Functional consequences of the oligomeric form of the membrane-bound gastric H,K-ATPase

Cross-linking and two-dimensional crystallization studies have suggested that the membrane-bound gastric H,K-ATPase might be a dimeric alpha,beta-heterodimer. Effects of an oligomeric structure on the characteristics of E(1), E(2), and phosphoenzyme conformations were examined by measuring binding stoichiometries of acid-stable phosphorylation (EP) from [gamma-(32)P]ATP or (32)P(i) or of binding of [gamma-(32)P]ATP and of a K(+)-competitive imidazonaphthyridine (INT) inhibitor to an enzyme preparation containing approximately 5 nmol of ATPase/mg of protein. At <10 microM MgATP, E(1)[ATP].Mg.(H(+)):E(2) is formed at a high-affinity site, and is then converted to E(1)P.Mg.(H(+)):E(2) and then to E(2)P.Mg:E(1) with luminal proton extrusion. Maximal acid-stable phosphorylation yielded 2.65 nmol/mg of protein. Luminal K(+)-dependent dephosphorylation returns this conformation to the E(1) form. At high MgATP concentrations (>0.1 mM), the oligomer forms E(2)P.Mg:E(1)[ATP].Mg.(H(+)). The sum of the levels of maximal EP formation and ATP binding was 5.3 nmol/mg. The maximal amount of [(3)H]INT bound was 2.6 nmol/mg in the presence of MgATP, Mg(2+), Mg-P(i), or Mg-vanadate with complete inhibition of activity. K(+) displaced INT only in nigericin-treated vesicles, and thus, INT binds to the luminal surface of the E(2) form. INT-bound enzyme also formed 2.6 nmol of EP/mg at high ATP concentrations by formation of E(2).Mg.(INT)(exo):E(1)[ATP].Mg.(H(+)) which is converted to E(2).Mg.(INT)(exo):E(1)P.Mg.(H(+))(cyto), but this E(1)P form was K(+)-insensitive. Binding of the inhibitor fixes half the oligomer in the E(2) form with full inhibition of activity, while the other half of the oligomer is able to form E(1)P only when the inhibitor is bound. It appears that the catalytic subunits of the oligomer during turnover in intact gastric vesicles are restricted to a reciprocal E(1):E(2) configuration.

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.

Structural interactions between FXYD proteins and Na+,K+-ATPase: alpha/beta/FXYD subunit stoichiometry and cross-linking

Interactions of rat FXYD4 (corticosteroid hormone-induced factor (CHIF)), FXYD2 (gamma), or FXYD1 (phospholemman (PLM)) proteins with rat alpha1 subunits of Na(+),K(+)-ATPase have been analyzed by co-immunoprecipitation and covalent cross-linking. In detergent-solubilized membranes from HeLa cells expressing both gamma and CHIF or CHIF and hemagglutinin A-tagged CHIF, mixed complexes of CHIF and gamma or CHIF and hemagglutinin A-tagged CHIF with alpha/beta subunits are undetectable. This implies that the alpha/beta/FXYD protomer is the major species in detergent solution. A lipid-soluble cysteine-cysteine bifunctional reagent, dibromobimane, cross-links CHIF to alpha in colonic membranes but not gamma or PLM to alpha in kidney or heart membranes, respectively. Sequence comparisons of the FXYD proteins suggested that Cys-49 in the trans-membrane segment of CHIF could be involved. In detergent-solubilized HeLa cell membranes, dibromobimane cross-links wild-type CHIF to alpha but not the C49F mutant, and also the corresponding F36C mutant but not wild-type gammab, and F48C but not wild-type PLM. C140S, C338A, C804A, and C966S mutants of the alpha subunit have been expressed. Only the C140S mutant prevents cross-linking with CHIF. The data demonstrated the proximity of trans-membrane segments of CHIF, gamma, and PLM to M2 of alpha. Molecular modeling is consistent with location of the trans-membrane segment of all FXYD proteins between M2, M6, and M9 and the proximity of Cys-49 of CHIF or Phe-36 of gamma with Cys-140 of M2. Cross-linking also demonstrated CHIF-alpha and CHIF-beta proximities in extra-membrane regions, similar to the evidence for gamma-alpha and gamma-beta cross-links.

Interaction of SDS with Na+/K+-ATPase

Because nearly all structure/function studies on Na(+)/K(+)-ATPase have been done on enzymes prepared in the presence of SDS, we have studied previously unrecognized consequences of SDS interaction with the enzyme. When the purified membrane-bound kidney enzyme was solubilized with SDS or TDS concentrations just sufficient to cause complete solubilization, but not at concentrations severalfold higher, the enzyme retained quaternary structure, exhibiting alpha,alpha-, alpha,beta-, beta,beta-, and alpha,gamma-associations as detected by chemical cross-linking. The presence of solubilized oligomers was confirmed by sucrose density gradient centrifugation. This solubilized enzyme had no ATPase activity and was not phosphorylated by ATP, but it retained the ability to occlude Rb(+) and Na(+). This, and comparison of cross-linking patterns obtained with different reagents, suggested that the transmembrane domains of the enzyme are more resistant to SDS-induced unfolding than its other domains. These findings (a). indicate that the partially unfolded oligomer(s) retaining partial function is the intermediate in the SDS-induced denaturation of the native membrane enzyme having the minimum oligomeric structure of (alpha,beta,gamma)(2) and (b). suggest potential functions for Na(+)/K(+)-ATPase with intrinsically unfolded domains. Mixtures of solubilized/partially unfolded enzyme and membrane-bound enzyme exhibited cross-linking patterns and Na(+) occlusion capacities different from those of either enzyme species, suggesting that the two interact. Formation of the partially unfolded enzyme during standard purification procedure for the preparation of the membrane-bound enzyme was shown, indicating that it is necessary to ensure the separation of the partially unfolded enzyme from the membrane-bound enzyme to avoid the distortion of the properties of the latter.

Oligomerization of the Na,K-ATPase in cell membranes

The higher order oligomeric state of the Na,K-ATPase alphabeta heterodimer in cell membranes is the subject of controversy. We have utilized the baculovirus-infected insect cell system to express Na,K-ATPase with alpha-subunits bearing either His(6) or FLAG epitopes at the carboxyl terminus. Each of these constructs produced functional Na,K-ATPase alphabeta heterodimers that were delivered to the plasma membrane (PM). Cells were simultaneously co-infected with viruses encoding alpha-His/beta and alpha-FLAG/beta Na,K-ATPases. Co-immunoprecipitation of the His-tagged alpha-subunit in the endoplasmic reticulum (ER) and PM fractions of co-infected cells by the anti-FLAG antibody demonstrates that protein-protein associations exist between these heterodimers. This suggests the Na,K-ATPase is present in cell membranes in an oligomeric state of at least (alphabeta)(2) composition. Deletion of 256 amino acid residues from the central cytoplasmic loop of the alpha-subunit results in the deletion alpha-4,5-loop-less (alpha-4,5LL), which associates with beta but is confined to the ER. Co-immunoprecipitation demonstrates that when this inactive alpha-4,5LL/beta heterodimer is co-expressed with wild-type alphabeta, oligomers of wild-type alphabeta and alpha-4,5LL/beta form in the ER, but the alpha-4,5LL mutant remains retained in the ER, and the wild-type protein is still delivered to the PM. We conclude that the Na,K-ATPase is present as oligomers of the monomeric alphabeta heterodimer in native cell membranes.

The carboxy terminus of the colonic H+,K+-ATPase α-subunit is required for stable β subunit assembly and function

BACKGROUND

The present experiments were designed to study the importance of the carboxy-terminus of colonic H(+), K(+)-ATPase alpha-subunit (HKalpha(2)), for both function as well as integrity of assembly with beta1-Na(+), K(+)-ATPase.

METHODS

For this purpose, a mutation of 84 amino acids in the carboxy-terminus was created (DeltaHKalpha(2)) and HEK-293 cells were used as expression systems for functional studies using (86)Rb(+)-uptake, coimmunoprecipitation using specific antibodies and fluorescence microscopy using green fluorescent protein.

RESULTS

The results demonstrate that comparable levels of expression of HKalpha(2) and DeltaHKalpha(2) mRNA were observed when cells were cotransfected with beta1 subunit. However, the abundance of expression of full length HKalpha(2) protein exceeded that of the truncated protein DeltaHKalpha(2). Ouabain-sensitive (86)Rb(+)-uptake was present only in cells cotransfected with HKalpha(2)/beta(1), indicating that the mutation was incapable of sustaining functionality. Coimmunoprecipitation experiments demonstrated that HKalpha(2) protein was immunoprecipitated more abundantly than DeltaHKalpha(2) when coexpressed with beta1. The use of sucrose gradients and green fluorescence protein immunofluorescence demonstrated that while the DeltaHKalpha(2)/beta(1) complex was confined to the endoplasmic reticulum, the HKalpha(2)/beta(1) complex translocated to the plasma membrane.

CONCLUSION

Taken together, our results are consistent with the view that the carboxy-terminus of HKalpha(2) facilitates the proper folding of the HKalpha(2)/beta(1) complex allowing translocation of the heterodimer to the plasma membrane where potassium uptake occurs. Otherwise, the alpha/beta complex is destined for degradation.

Interactions between Na,K-ATPase alpha-subunit ATP-binding domains

The reaction mechanism of the Na,K-ATPase is thought to involve a number of ligand-induced conformational changes. The specific amino acid residues responsible for binding many of the important ligands have been identified; however, details of the specific conformational changes produced by ligand binding are largely undescribed. The experiments described in this paper begin to identify interactions between domains of the Na,K-ATPase alpha-subunit that depend on the presence of particular ligands. The major cytoplasmic loop (between TM4 and TM5), which we have previously shown contains the ATP-binding domain, was overexpressed in bacteria either with a His(6) tag or as a fusion protein with glutathione S-transferase. We have observed that these polypeptides associate in the presence of MgATP. Incubation with [gamma-(32)P]ATP under conditions that result in phosphorylation of the full-length Na,K-ATPase did not result in (32)P incorporation into either the His(6) tag or glutathione S-transferase fusion proteins. The MgATP-induced association was strongly inhibited by prior modification of the fusion proteins with fluorescein isothiocyanate or by simultaneous incubation with 10 microm eosin, indicating that the effect of MgATP is due to interactions within the nucleotide-binding domain. These data are consistent with Na,K-ATPase associating within cells via interactions in the nucleotide-binding domains. Although any functional significance of these associations for ion transport remains unresolved, they may play a role in cell function and in modulating interactions between the Na,K-ATPase and other proteins.

Identification of the cofilin-binding sites in the large cytoplasmic domain of Na,K-ATPase

Na,K-ATPase, an alpha, beta heterodimer, is found in the plasma membrane of all animal cells. The alpha chain is believed to have 10 transmembrane regions and a large cytoplasmic domain between the 4th and 5th transmembrane regions (H4-H5). In our previous report, the large (3rd) cytoplasmic domains of the alpha1 and alpha2 isoform were found to interact with cofilin, an actin-modulating protein, by the yeast two-hybrid system. Here we show that cofilin interacts only with the 3rd cytoplasmic domain of the alpha2 subunit but not with the 2nd, 4th, and 5th cytoplasmic domains or the cytoplasmic region of the beta subunit of Na,K-ATPase. We also demonstrate that cofilin interacts with the large cytoplasmic domains of the alpha1, alpha2 and alpha3 isoforms of Na,K-ATPase, but not with those of glucose transporter 1, glucose transporter 4, cystic fibrosis transmembrane conductance regulator and plasma membrane Ca-ATPase. We introduced 10 mutations into the 3rd cytoplasmic domain of Na,K-ATPase to identify the binding sites with cofilin. Eight of these mutants were single amino acid substitutions (R417Q, K470Q, K654G, D672A, K691A, R700G, R700A and D710G) and two were double mutant (K654GR700G and K719AK720A). Analysis of the activity of the reporter gene of these mutants shows that residues D672 and R700 of the 3rd cytoplasmic domain of Na,K-ATPase are involved in the interaction with cofilin.

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.

Structure of Na+,K+-ATPase at 11-A resolution: comparison with Ca2+-ATPase in E1 and E2 states

Na+,K+-ATPase is a heterodimer of alpha and beta subunits and a member of the P-type ATPase family of ion pumps. Here we present an 11-A structure of the heterodimer determined from electron micrographs of unstained frozen-hydrated tubular crystals. For this reconstruction, the enzyme was isolated from supraorbital glands of salt-adapted ducks and was crystallized within the native membranes. Crystallization conditions fixed Na+,K+-ATPase in the vanadate-inhibited E2 conformation, and the crystals had p1 symmetry. A large number of helical symmetries were observed, so a three-dimensional structure was calculated by averaging both Fourier-Bessel coefficients and real-space structures of data from the different symmetries. The resulting structure clearly reveals cytoplasmic, transmembrane, and extracellular regions of the molecule with densities separately attributable to alpha and beta subunits. The overall shape bears a remarkable resemblance to the E2 structure of rabbit sarcoplasmic reticulum Ca2+-ATPase. After aligning these two structures, atomic coordinates for Ca2+-ATPase were fit to Na+,K+-ATPase, and several flexible surface loops, which fit the map poorly, were associated with sequences that differ in the two pumps. Nevertheless, cytoplasmic domains were very similarly arranged, suggesting that the E2-to-E1 conformational change postulated for Ca2+-ATPase probably applies to Na+,K+-ATPase as well as other P-type ATPases.

Hyperphosphorylation of Na-pump contributes to defective renal dopamine response in old rats

Dopamine D1-like receptor activation causes phosphorylation and inhibition of Na,K-ATPase (Na-pump) activity in the proximal tubules, which is associated with an increase in sodium excretion. It has been shown that dopamine and SKF 38393, a D1-like receptor agonist, caused inhibition of Na,K-ATPase activity in the proximal tubules of adult (6 mo) but not of old (24 mo) Fischer 344 rats. The present study demonstrated that SKF 38393 and PDBu, a phorbol ester and protein kinase C (PKC) activator, increased phosphorylation of the alpha(1)-subunit of Na,K-ATPase in adult but not in old rats. In adult rats, SKF 38393-mediated phosphorylation was antagonized by SCH 23390, a D1-like receptor antagonist. Similarly, Na,K-ATPase activity was inhibited by SKF 38393 and PDBu in adult but not in old rats. The basal activity of Na,K-ATPase was decreased and the basal phosphorylation state of the enzyme was increased in old compared with adult rats. Basal activity of PKC was higher in old compared with adult rats, and SKF 38393 and PDBu stimulated PKC activity in adult but not in old rats. The conclusion is that the failure of D1-like receptor agonist and phorbol ester to stimulate PKC and inhibit Na,K-ATPase activity in old rats is due, at least in part, to the higher basal PKC activity and Na,K-ATPase phosphorylation in old compared with adult rats.

Activation of dopamine D(1)-like receptor causes phosphorylation of alpha(1)-subunit of Na(+),K(+)-ATPase in rat renal proximal tubules

Dopamine causes inhibition of Na(+),K(+)-ATPase activity via activation of dopamine D(1)-like receptors. It is the phosphorylation of Serine(18) of the alpha(1)-subunit of Na(+),K(+)-ATPase which results in the inhibition of the enzyme activity; however, such a phosphorylation by dopamine D(1)-like receptor agonist has not been demonstrated in the proximal tubules. We show here by immunoprecipitation and detection with phosphoserine antibody that SKF 38393, a dopamine D(1)-like receptor agonist, causes phosphorylation of the alpha(1)-subunit of Na(+),K(+)-ATPase. The effect of (+/-)-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol hydrochloride, SKF 38393, is blocked by R(+)-7-choro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-benzazepine hydrochloride, SCH 23390, a dopamine D(1)-like receptor antagonist, and staurosporin, a protein kinase C inhibitor. The phosphorylation is also increased by phorbol 12-13 dibutyrate ester. However, Rp-cAMP triethylamine, an inhibitor of protein kinase A, does not affect the SKF 38393-mediated phosphorylation of Na(+),K(+)-ATPase. Therefore, these results provide the evidence that dopamine D(1)-like receptor activation causes phosphorylation of the alpha(1)-subunit of Na(+),K(+)-ATPase in renal proximal tubules via protein kinase C pathway.

Regulation of the Na+/K+-ATPase by insulin: why and how?

The sodium-potassium ATPase (Na+/K+-ATPase or Na+/K+-pump) is an enzyme present at the surface of all eukaryotic cells, which actively extrudes Na+ from cells in exchange for K+ at a ratio of 3:2, respectively. Its activity also provides the driving force for secondary active transport of solutes such as amino acids, phosphate, vitamins and, in epithelial cells, glucose. The enzyme consists of two subunits (alpha and beta) each expressed in several isoforms. Many hormones regulate Na+/K+-ATPase activity and in this review we will focus on the effects of insulin. The possible mechanisms whereby insulin controls Na+/K+-ATPase activity are discussed. These are tissue- and isoform-specific, and include reversible covalent modification of catalytic subunits, activation by a rise in intracellular Na+ concentration, altered Na+ sensitivity and changes in subunit gene or protein expression. Given the recent escalation in knowledge of insulin-stimulated signal transduction systems, it is pertinent to ask which intracellular signalling pathways are utilized by insulin in controlling Na+/K+-ATPase activity. Evidence for and against a role for the phosphatidylinositol-3-kinase and mitogen activated protein kinase arms of the insulin-stimulated intracellular signalling networks is suggested. Finally, the clinical relevance of Na+/K+-ATPase control by insulin in diabetes and related disorders is addressed.

Polarization of the Na+, K(+)-ATPase in epithelia derived from the neuroepithelium

The neuroepithelium generates a fascinating group of epithelia. One of their intriguing properties is how they polarize the distribution of the Na+, K(+)-ATPase. Typically, this ion pump is concentrated in the basolateral membrane, but it is concentrated in the apical membranes of the retinal pigment epithelium and the epithelium of the choroid plexus. A comparison of their development with that of systemic epithelia yields insights into how cells polarize the distribution of this and other membrane proteins. The polarization of the Na+, K(+)-ATPase depends upon the interplay between different sorting signals and different types of polarity mechanisms. These include intracellular targeting signals that direct the delivery of newly synthesized proteins, and maintenance signals that stabilize proteins in the proper membrane domain. Conflicting signals appear to be arranged in a hierarchy that can be rearranged as cells respond to certain environmental stimuli. Part of this response is mediated by changes in the distribution and composition of the cortical cytoskeleton.

Na/K-ATPase-mediated 86Rb+ uptake and asymmetrical trophectoderm localization of alpha1 and alpha3 Na/K-ATPase isoforms during bovine preattachment development

This study evaluated Na/K-ATPase alpha 1- and alpha 3-subunit isoform polypeptide expression and localization during bovine preattachment development. Na/K-ATPase cation transport activity from the one-cell to blastocyst stage was also determined by measuring ouabain-sensitive 86Rb+ uptake. Both alpha1- and alpha 3-subunit polypeptides were detected by immunofluorescence to encircle the entire cell margins of each blastomere of inseminated zygotes, cleavage stage embryos, and morulae. Immunofluorescent localization of alpha1-subunit polypeptide in bovine blastocysts revealed an alpha1 immunofluorescence signal confined to the basolateral membrane margins of the trophectoderm and encircling the cell periphery of each inner cell mass (ICM) cell. In contrast, alpha 3-subunit polypeptide immunofluorescence was localized primarily to the apical cell surfaces of the trophectoderm with a reduced signal present in basolateral trophectoderm regions. There was no apparent alpha 3-subunit signal in the ICM. Analysis of 86Rb+ transport in vitro demonstrated ouabain-sensitive activity throughout development from the one-cell to the six- to eight-cell stage of bovine development. 86Rb+ uptake by morulae (day 6 postinsemination) did not vary significantly from uptake detected in cleavage stage embryos; however, a significant increase was measured at the blastocyst stage (P < 0.05). Treatment of embryos with cytochalasin D (5 micrograms/ml) did not influence 86Rb+ uptake in cleavage stage embryos. Cytochalasin D treatment however was associated with a significant rise in ion transport in morulae and blastocysts (13.49 and 61.57 fmol/embryo/min, respectively) compared to untreated controls (2.65 and 22.83 fmol/embryo/min, respectively). Our results, for the first time, demonstrate that multiple Na/K-ATPase alpha-subunit isoforms are distributed throughout the first week of mammalian development and raise the possibility that multiple isozymes of the Na/K-ATPase contribute to blastocyst formation.

Does binding of ouabain to human alpha1-subunit of Na+, K+-ATPase affect the ATPase activity of adjacent rat alpha1-subunit?

To ascertain whether ouabain binding to human alpha1-subunit influences coexpression of rat alpha1-subunit, the ouabain-sensitive profiles of Na+,K+-ATPase activity and 86Rb+ uptake activity and ouabain binding capacity were measured in HeLa cells stably expressing rat alpha1-subunit. The ouabain-sensitive profile of ATPase and 86Rb+ uptake activity seemed to be the sum of two components, one with high and one with low apparent affinity to ouabain, which were similar to that observed in HeLa and NRK-52E cells derived from human and rat, respectively. The ATPase activity with low sensitivity to ouabain increased in simple proportion to the amount of the rat alpha1 mRNA derived from transfected cDNA, which was determined by the reverse transcription-polymerase chain reaction method. The turnover number of the human Na+,K+-ATPase activity obtained from the ratio of the Na+,K+-ATPase activity to the ouabain binding capacity is about 150/sec. The expression of the rat alpha1-subunit had no effect on the turnover numbers of the Na+,K+-ATPase activity with high affinity to ouabain estimated from the ouabain binding capacity as the active site concentration. These results suggested that the ouabain bound to human alpha1-subunit did not inhibit the ATPase activity of the coexpressing rat alpha1 in these cells.

Biochemical characterization of the subunits of the Na+/K+ ATPase expressed in insect cells

The Na+/K+ ATPase is composed of two subunits called alpha and beta chains. In insect cells, independently expressed alpha and beta chains are localized to intracellular membranes. Sucrose density gradient sedimentation, crosslinking analysis, and immunoprecipitation of radio-labeled proteins show that the alpha chains expressed alone are in large aggregates of different molecular weights with less than 4% being monomeric. Analysis by non-reducing SDS-PAGE and immunoblotting show that the beta chains expressed alone are in Triton X-100 insoluble, disulfide-linked aggregates. Co-expression of both subunits in insect cells results in only a small fraction (less than 15%) of the alpha chains being assembled as the active recombinant enzyme, with at least 22% of the active recombinant enzyme localized to the plasma membrane as determined by a biochemical assay. The small amount of beta chain at the plasma membrane in cells that express both subunits is beyond the limit of detection by the biochemical assay. Immunoprecipitation of Triton X-100 soluble alpha chains from radio-labeled cells expressing both subunits shows that the alpha chains are mostly in large aggregates containing beta chains. These results suggest that, in insect cells, the availability of correctly folded beta chains is the rate limiting step in the assembly of active Na+/K+ ATPase.

Parallel detection of Na,K-ATPase alpha subunit isoforms by pan-specific monoclonal mAb 9A7

While emphasis has been placed upon those proteins which either mediate or respond to the rapid influx of calcium following depolarization, there has been little emphasis upon those proteins which aid in the reequilibration of the membrane potential. In an effort to identify presynaptic membrane proteins implicated in neurosecretion, monoclonal antibodies were screened against proteins which cosegregated with neuronal voltage-dependent calcium channels (VDCC) following immunoprecipitation. One monoclonal antibody (mAb 9A7) identified a 110-kDa protein. Micropeptide sequencing of (i) the mAb 9A7 immunoaffinity purified antigen and (ii) the 110-kDa protein present in the neuronal (N-type) VDCC preparation (McEnery et al., 1991, Proc. Natl. Acad. Sci. 88, 11095-11099) indicated identity with the alpha subunit(s) of the Na,K-ATPase. Further characterization by Western blotting, immunochemical localization, and immunoaffinity purification indicated that mAb 9A7 not only recognized the alpha3 isoform which is predominant in neuronal tissues but also identified the alpha1 and alpha2 isoforms. mAb 9A7 exhibited a wide cross-species reactivity and recognized human, rat, and mouse alpha subunit isoforms at an internal epitope. The pan-specificity of mAb 9A7 and the differential mobility of the alpha1 isoform relative to the alpha2 and alpha3 permitted parallel detection of multiple alpha isoforms. Western blot analysis of undifferentiated rat pheochromocytoma cell line (PC12) and human neuroblastoma (IMR32) cells indicated coexpression of the alpha1 and alpha3 isozymes. Upon differentiation of IMR32 cells by dibutrylyl-cAMP, a substantial increase in the alpha3 relative to the alpha1 isoform was observed. While the enrichment of total Na,K-ATPase may reflect the increased demand for ATP-dependent ion transport as IMR32 cells become more excitable, the specific increase in the alpha3 isoform suggests a unique role of this isoform during IMR32 cell differentiation.

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.

Subunit interactions in the Na,K-ATPase explored with the yeast two-hybrid system

Subunit interactions of the alpha1- and beta1-subunits of the chicken Na,K-ATPase were explored with the yeast two-hybrid system. Gal4-fusion proteins containing domains of the alpha1- and beta1-subunits were designed for examining both intersubunit and intrasubunit protein-protein interactions. Regions of the alpha- and beta-subunits known to be involved in alpha-beta-subunit assembly were positive in two-hybrid assay, supporting the validity of the assays. A library of beta-subunit ectodomains with C-terminal truncations was screened to find the maximal truncation retaining an interaction with the alpha-subunit extracellular H7H8 loop (where H7 refers to the seventh membrane span, and so on). The maximal truncation removed all the cysteines involved in disulfide bridges, leaving only 63 amino acids of the beta-subunit ectodomain. Scanning alanine mutagenesis led to identification of an evolutionarily conserved sequence of four amino acids (SYGQ) in the extracellular H7H8 loop of the alpha-subunit that is crucial to alpha-beta-intersubunit interactions. Oligomerization studies with single domains failed to detect self-association of either of the two large cytosolic loops (H2H3 and H4H5) within the alpha-subunit. However, evidence was found for an interaction between these two cytoplasmic loops.

Interpretation of extraordinary kinetics of Na(+)-K(+)-ATPase by a phase change

We interpret at a molecular level an extraordinary response in the transient kinetics of the phosphointermediate of Na(+)-K(+)-ATPase (I. Klodos, R. L. Post, and B. Forbush III. J. Biol. Chem. 269: 1734-1743, 1994). The phosphointermediate comprises two principal states. The partition between these states varies with salt concentration. A jump in salt concentration changes the partition of some of the molecules more rapidly than they interconvert in a steady state at constant salt concentration. We propose that interconversion is limited by free volume in the lipid of the surrounding membrane. This lipid is partitioned into phases that vary with salt concentration. Free volume is larger at the interface between these phases than within the phases themselves. Na(+)-K(+)-ATPase molecules are distributed at random in the membrane. When the phase boundary moves in response to a jump in salt concentration, it crosses some Na+ -K+ -ATPase molecules, which transiently experience an increase in free volume of the surrounding lipid. Thus their phosphointermediate states equilibrate more rapidly than at a constant salt concentration. Functional and structural heterogeneity of Na(+)-K(+)-ATPase molecules is discussed.

Oligomeric regulation of gastric H+,K+-ATPase

The H+,K+-ATPase of intact gastric vesicles has two Km values for ATP hydrolysis, 7 and 80 microM. Irradiation of vesicles with ultraviolet light in the presence of 1 mM ATP resulted in K+-ATPase activity that shows only the low affinity ATP binding. The irradiation stimulated or inhibited proton uptake rate compared with control vesicles at high or low ATP concentrations, respectively. The relation between proton uptake rate and K+-ATPase activity at different ATP concentrations was linear with irradiated vesicles and nonlinear with control vesicles. These results indicate that hydrolysis at the high affinity ATP binding site regulates the energy-transport coupling in negative and positive manners at high and low ATP concentrations, respectively. The complete inhibition of K+-ATPase by a specific proton pump inhibitor E3810 (rabeprazole) (2-([4-(3-methoxypropoxy)-3-methylpyridin-2-yl]methylsulf i nyl)-1H-benzimidazole sodium salt) occurred when E3810 bound to half of the alpha-subunit of H+,K+-ATPase in unirradiated vesicles at both 200 and 10 microM ATP, whereas the complete inhibition of proton uptake occurred when E3810 bound to half or a quarter of the alpha-subunit at 200 or 10 microM ATP, respectively. These results suggest that dimeric interaction between the alpha-subunits is necessary for the enzyme activity at all ATP concentrations and that dimeric or tetrameric interaction is necessary for proton transport at high or low ATP concentrations, respectively.

Dimerization of the gastric H+, K(+)-ATPase

When the gastric H+, K(+)-ATPase was solubilized by n-dodecyl beta-D-maltoside and electrophoresed in blue native-polyacrylamide gels (BN-PAGE), one major band at about 360 kDa was observed. Since this band was recognized by both monoclonal antibodies 1218 (anti-alpha) and wheat germ agglutinin (anti-beta), the H+, K(+)-ATPase in its native state exists in a dimeric (alpha beta)2 form. The site of interaction between the heterodimers was determined using Cu(2+)-phenanthroline cross-linking. The Cu(2+)-phenanthroline reagent reacted with the H+, K(2+)-ATPase activity. This cross-linking and enzyme inhibition were prevented by ATP. Cross-linking followed by N-ethylmaleimide blockade of maleimide-reactive SH groups, then reduction and fluorescein 5-maleimide labeling, then reduction and fluorescent tryptic peptide of about 6.5 kDa that had been cross-linked. Since its N-terminal amino acid is Val561, the peptide probably ends at Arg616 or Arg621 and Cys565 and/or Cys615 are probably within the region of closest contact between the two alpha-subunits.

Modeling of the three-dimensional structure of the digitalis intercalating matrix in Na+/K(+)-ATPase protodimer

Based on the knowledge that the digitalis receptor site in Na+/K(+)-ATPase is the interface between two interacting alpha-subunits of the protodimer (alpha beta)2, the present review makes an approach towards modeling the three-dimensional structure of the digitalis intercalating matrix by exploiting the information on: the primary structure and predicted membrane topology of the catalytic alpha-subunit; the determinants of the secondary, tertiary and quaternary structure of the membrane-spanning protein domains; the impact of mutational amino acid substitutions on the affinity of digitalis compounds, and the structural characteristics in potent representatives. The designed model proves its validity by allowing quantitative interpretations of the contributions of distinct amino acid side chains to the special bondings of the three structural elements of digitalis compounds.