Chromosome-mediated transfer of the murine Na,K-ATPase alpha subunit confers ouabain resistance.

We transferred murine NIH 3T3 metaphase chromosomes into monkey CV-1 cells to investigate the different ouabain sensitivities of rodent and primate cells. In 16 ouabain-resistant transferents, the mouse Na,K-ATPase alpha 1 subunit gene was detected, suggesting that structural differences between the rodent and primate alpha 1 subunits determine the different ouabain sensitivities.     

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.     

Mutation of a cysteine in the first transmembrane segment of Na,K-ATPase alpha subunit confers ouabain resistance.

The cardiac glycoside ouabain inhibits Na,K-ATPase by binding to the alpha subunit. In a highly ouabain resistant clone from the MDCK cell line, we have found two alleles of the alpha subunit in which the cysteine, present in the wild-type first transmembrane segment, is replaced by a tyrosine (Y) or a phenylalanine (F). We have studied the kinetics of ouabain inhibition by measuring the current generated by the Na,K-pump in Xenopus oocytes injected with wild-type and mutated alpha 1 and wild-type beta 1 subunit cRNAs. When these mutations, alpha 1C113Y and alpha 1C113F [according to the published sequence [Verrey et al. (1989) Am. J. Physiol., 256, F1034] were introduced in the alpha 1 subunit of the Na,K-ATPase from Xenopus laevis, the inhibition constant (Ki) of ouabain increased greater than 1000-fold compared with wild-type. A more conservative mutation, serine alpha 1C113S did not change the Ki. We observed that the decreased affinity for ouabain was mainly due to a faster dissociation, but probably also to a slower association. Thus we propose that an amino acid residue of the first transmembrane segment located deep in the plasma membrane participates in the structure and the function of the ouabain binding site.     

CV-1 cell recipients of the mouse ouabain resistance gene express a ouabain-insensitive Na,K-ATPase after growth in cardioactive steroids

The cation-transporting activity and Na,K-ATPase activity of CV-1 cell recipients of the mouse ouabain resistance gene (ouaR6, or OR6 cells; see Levenson, R., Racaniello, V., Albritton, L., and Housman, D. (1984) Proc. Natl. Acad. Sci. U. S. A. 81, 1489-1493) have been further characterized. OR6 cells grown in strophanthidin (a cardiac aglycon which may be removed rapidly from the Na,K-ATPase) possess both ouabain-sensitive and -insensitive 86Rb+ uptake activities. The ouabain-sensitive 86Rb+ uptake activity of these cells (OR6-S cells) exhibits the same Ki for ouabain as that of the CV-1 parent cells (Ki(app) = 3 x 10(-7) M ouabain), but accounts for only approximately 30% of total 86Rb+ uptake into Na+-loaded OR6-S cells, compared to 80% for CV-1 cells. Most of the ouabain-resistant 86Rb+ uptake in OR6-S cells is dependent on internal Na+ and is insensitive to furosemide, suggesting that it is due to an ouabain-resistant Na,K pump. In OR6-S cell lysates, 50% of Na+-dependent ATPase activity is insensitive to 1 mM ouabain, compared to less than 5% in CV-1 cell lysates. In addition, purified plasma membranes from OR6-S cells contain a 100-kDa protein which is transiently phosphorylated by ATP in an Na+-dependent, K+-sensitive manner, like the alpha subunit of the CV-1 Na,K-ATPase and the canine renal Na,K-ATPase, but which is unaffected by preincubation in 1 mM ouabain. All of these data suggest that OR6-S cells possess a ouabain-insensitive Na,K pump with characteristics similar to the ouabain-sensitive pump of CV-1 parent cells. Since the mouse ouabain resistance gene does not encode either subunit of the Na,K-ATPase, these results suggest that the ouabain resistance gene product may modify the ouabain sensitivity of the endogenous CV-1 Na,K pump.     

Maturation of the catalytic alpha-subunit of Na,K-ATPase during intracellular transport

The protease sensitivity of the catalytic alpha-subunit of Na,K-ATPase during intracellular transport along the exocytic pathway has been investigated in two amphibian epithelial cell lines. Controlled trypsinolysis followed by immunoprecipitation of cell homogenates or microsomal fractions from [35S]methionine pulse-chased A6 kidney cells revealed distinct cleavage patterns by SDS-PAGE. Shortly after synthesis (7-min pulse), the 98-kD alpha-subunit is fully sensitive to trypsin digestion and is cleaved into a 35-kD membrane-bound and a 27.5- kD soluble peptide. With a 15-min pulse, 10% of the newly synthesized polypeptide becomes resistant to trypsin digestion. With longer chase time, the proportion of protease-resistant alpha-subunit further increases. Concomitantly, the alpha-subunit acquires the ability to undergo cation-dependent conformational transitions, as reflected by distinct tryptic digest patterns in the presence of Na+ or K+. Similar results were obtained in TBM cells, a toad bladder cell line. Our data indicate that the catalytic subunit of Na,K-ATPase is structurally rearranged during intracellular transport from its site of synthesis to its site of action at the cell surface, a modification which might mark the functional maturation of the enzyme.     

Ouabain-resistant mutants of the rat Na,K-ATPase alpha 2 isoform identified by using an episomal expression vector.

Site-directed mutagenesis was used to identify residues responsible for the greater than 1,000-fold difference in ouabain sensitivity between the rat Na,K-ATPase alpha 1 and alpha 2 isoforms. A series of mutagenized cDNAs was constructed that replaced residues of the rat alpha 2 subunit with the corresponding residues from the rat alpha 1 subunit. These cDNAs were cloned into a mammalian episomal expression vector (EBOpLPP) and expressed in ouabain-sensitive primate cells. Either of two single substitutions introduced into the rat alpha 2 subunit cDNA (Leu-111----Arg or Asn-122----Asp) conferred partial resistance (approximately 10 microM ouabain) upon transformed cells. This resistance was intermediate between the levels conferred by the rat alpha 1 cDNA (approximately 500 microM ouabain) and the rat alpha 2 cDNA (approximately 0.2 microM ouabain). A double substitution of the rat alpha 2 cDNA (Leu-111----Arg and Asn-122----Asp) conferred a resistance level equivalent to that obtained with rat alpha 1. These results demonstrate that the residues responsible for isoform-specific differences in ouabain sensitivity are located at the end of the H1-H2 extracellular domain. The combination of site-directed mutagenesis and episomal expression provides a useful system for the selection and analysis of mutants.     

Amplification of DNA sequences coding for the Na,K-ATPase alpha-subunit in ouabain-resistant C+ cells.

We have studied the mechanism of cellular resistance to cardiac glycosides in C+ cells. C+ cells were resistant to ouabain and overproduced plasma membrane-bound Na,K-ATPase relative to parental HeLa cells. Overexpression of Na,K-ATPase in C+ cells correlated with increased ATPase mRNA levels and amplification (approximately 100 times) of the ATPase gene. Growth of C+ cells in ouabain-free medium resulted in a marked decline in ATPase mRNA and DNA levels. However, when cells were reexposed to ouabain, they proliferated and ATPase mRNA and DNA sequences were reamplified. Restriction analysis of C+ and other human DNA samples revealed the occurrence of rearrangements in the region of the Na,K-ATPase gene in C+ cells. Furthermore, C+ cells expressed an ATPase mRNA species not found in HeLa cells. These results suggest that amplification of the gene coding for Na,K-ATPase results in overproduction of Na,K-ATPase polypeptides. Amplification of the ATPase gene or the expression of new ATPase mRNA sequences or both may also be responsible for acquisition of the ouabain-resistant phenotype.     

Newly synthesized Na,K-ATPase alpha-subunit has no cytosolic intermediate in MDCK cells

Recently published data indicate that the alpha-subunit of Na,K-ATPase, a transmembrane protein of animal cell plasma membranes, is synthesized as a soluble precursor. In the present experiments we demonstrate that an apparent "soluble" form can indeed be detected in crude cytosolic fractions prepared by centrifugation from MDCK cells disrupted by sonication. We find, however, that this form has no precursor-product relationship with membrane-associated alpha-subunit. The quantity of unsedimentable alpha-subunit can be greatly diminished by increasing the centrifugal field employed to remove membranous vesicles from the cytosol fraction. Sonication of membrane pellets generates alpha-subunit which, like the "soluble" form, resists pelleting. Finally, cytosol fractions prepared from cells disrupted by sonication contain membrane-bound vesicles which can be immunoadsorbed on Staphylococcus aureus cells coated with a monoclonal antibody directed against alpha-subunit. We find, therefore, that the previously observed soluble precursor of alpha-subunit is actually a component of small unpelleted membrane vesicles generated by harsh disruption conditions. When cells are disrupted by less violent techniques no newly synthesized alpha-subunit can be detected in the cytosol fraction. We calculate that to escape detection under our experimental conditions a bona fide soluble precursor of alpha-subunit must have a cytosolic t1/2 less than 20 s. We conclude that the alpha-subunit is most probably inserted into the bilayer cotranslationally.     

Primary structure of the alpha-subunit of human Na,K-ATPase deduced from cDNA sequence

Clones carrying cDNA sequences for the alpha-subunit of the Na,K-ATPase from HeLa cells have been isolated. Nucleotide sequence analysis of the cloned cDNA has revealed the primary structure of this polypeptide, which consists of 1,023 amino acids. The alpha-subunit of the human Na,K-ATPase exhibited 87% homology with its Torpedo counterpart and 98% homology with its sheep counterpart. The six putative transmembrane segments M1-M6 showed higher conservation than the total segments. Total genomic Southern hybridization indicated the existence of at most two copies, possibly only one, of the gene encoding the Na,K-ATPase alpha-subunit in the human genome.     

Photoaffinity labeling of the ouabain binding site in Na, K-ATPase in developing brine shrimp

Analysis of purified Na,K-ATPase from brine shrimp nauplii by sodium dodecyl sulfate-polyacrylamide gel electrophoresis reveals two large (alpha) subunits [G.L. Peterson, R.D. Ewing, S.R. Hootman, and F.P. Conte (1978) J. Biol. Chem. 253:4762]. The band with lower mobility in a neutral or alkaline gel is designated alpha 1 and the band with higher mobility alpha 2. Ouabain prevents dephosphorylation of both alpha 1 and alpha 2 as documented by gel analysis, but a higher concentration of ouabain is required to prevent dephosphorylation of alpha 2. The photoaffinity label, [3H]4'(2-ethyldiazomalonyl) digitoxigenin monodigitoxiside, specifically labels alpha in a ouabain-protectable manner without labeling other contaminating proteins in the preparation. Greater than 93% of the total ouabain-protectable labeling of the alpha subunits is associated with alpha 1. The photoaffinity label, [3H]4"' (2-ethyldiazomalonyl) digitoxin, specifically labels alpha 1 and beta in a ouabain-protectable manner without labeling other contaminating proteins. These data show that in the brine shrimp the third digitoxose residue of digitoxin binds in a region in which the alpha 1 and beta chains are in close proximity. Less than 5% of the specific ouabain-protectable labeling of total alpha is associated with alpha 2. These studies indicate that cardioactive steroids have higher affinity for the alpha 1 subunit.     

A protein whose binding to Na,K-ATPase is regulated by ouabain

Immunoprecipitation of Na,K-ATPase from kidney homogenate by antibodies against alpha1-subunit results in the precipitation of several proteins together with the Na,K-ATPase. A protein with molecular mass of about 67 kD interacting with antibodies against melittin (melittin-like protein, MLP) was found in the precipitate when immunoprecipitation was done in the presence of ouabain. If immunoprecipitation was done using antibodies against melittin, MLP and Na,K-ATPase alpha1-subunit were detected in the precipitate, and the amount of alpha1-subunit in the precipitate was increased after the addition of ouabain to the immunoprecipitation medium. MLP was purified from mouse kidney homogenate using immunoaffinity chromatography with antibodies against melittin. The addition of MLP to purified FITC-labeled Na,K-ATPase decreases fluorescence in medium with K+ and increases it in medium with Na+. The enhancement of fluorescence depends upon the MLP concentration. The N-terminal sequence of MLP determined by the Edman method is the following: HPPKRVRSRLNG. No proteins with such N-terminal sequence were found in the protein sequence databases. However, we revealed five amino acid sequences that contain this peptide in the middle part of the chain at distance 553 amino acids from the C-terminus (that corresponds to protein with molecular mass of about 67 kD). Analysis of amino acid sequence located between C-terminus and HPPKRVRSRLNG in all found sequences has shown that they were highly conservative and include WD40 repeats. It is suggested that the 67-kD MLP either belongs to the found protein family or was a product of proteolysis of one of them.     

The C-terminal tail of the polycystin-1 protein interacts with the Na,K-ATPase alpha-subunit

Polycystin-1 (PC-1) is the product of the PKD1 gene, which is mutated in autosomal dominant polycystic kidney disease. We show that the Na,K-ATPase alpha-subunit interacts in vitro and in vivo with the final 200 amino acids of the polycystin-1 protein, which constitute its cytoplasmic C-terminal tail. Functional studies suggest that this association may play a role in the regulation of the Na,K-ATPase activity. Chinese hamster ovary cells stably expressing the entire PC-1 protein exhibit a dramatic increase in Na,K-ATPase activity, although the kinetic properties of the enzyme remain unchanged. These data indicate that polycystin-1 may contribute to the regulation of Na,K-ATPase activity in kidneys in situ, thus modulating renal tubular fluid and electrolyte transport.     

Effects of ouabain on the rotational dynamics of renal Na,K-ATPase studied by saturation-transfer EPR.

The interaction of a nitroxide spin-labeled derivative of ouabain with sheep kidney Na,K-ATPase and the motional behavior of the ouabain spin label-Na,K-ATPase complex have been studied by means of electron paramagnetic resonance (EPR) and saturation-transfer EPR (ST-EPR). Spin-labeled ouabain binds with high affinity to the Na,K-ATPase with concurrent inhibition of ATPase activity. Enzyme preparations retain 0.61 +/- 0.1 mol of bound ouabain spin label per mole of ATP-dependent phosphorylation sites, even after repeated centrifugation and resuspension of the purified ATPase-containing membrane fragments. The conventional EPR spectrum of the ouabain spin label bound to the ATPase consists almost entirely (greater than 99%) of a broad resonance at 0 degrees C, characteristic of a tightly bound spin label which is strongly immobilized by the protein backbone. Saturation-transfer EPR measurements of the spin-labeled ATPase preparations yield effective correlation times for the bound labels significantly longer than 100 microseconds at 0 degrees C. Since the conventional EPR measurements of the ouabain spin-labeled Na,K-ATPase indicated the label was strongly immobilized, these rotational correlation times most likely represent the motion of the protein itself rather than the independent motion of mobile spin probes relative to a slower moving protein. Additional ST-EPR measurements of ouabain spin-labeled Na,K-ATPase (a) cross-linked with glutaraldehyde and (b) crystallized in two-dimensional arrays indicated that the observed rotational correlation times predominantly represented the motion of large Na,K-ATPase-containing membrane fragments, as opposed to the motion of individual monomeric or dimeric polypeptides within the membrane fragment. The results suggest that the binding of spin-labeled ouabain to the ATPase induces the protein to form large aggregates, implying that cardiac glycoside induced enzyme aggregation may play a role in the mechanism of action of the cardiac glycosides in inhibiting the Na,K-ATPase.     

Na,K-ATPase extracellular surface probed with a monoclonal antibody that enhances ouabain binding.

The Na,K-stimulated ATPase is inhibited by extracellular cardiac glycosides, which bind to the enzyme's alpha subunit. We used a monoclonal antibody, VG4, as a probe of the extracellular surface. The antibody was specific for Na,K-ATPase and bound to intact cells. The epitope was mapped to the first extracellular loop (H1-H2) of alpha, using a combination of techniques including trypsinolysis, N-terminal sequence of a fragment containing the determinant, and analysis of the effects of species-specific sequence differences. The antibody inhibited Na,K-ATPase activity under certain circumstances, indicating that the H1-H2 loop participates in conformational changes that are transmitted to the active site. Mutations in the H1-H2 loop have been shown by others to affect ouabain affinity. Ouabain and the antibody acted synergistically to inhibit the enzyme, which seemingly supported the hypothesis that the H1-H2 loop is an essential part of the cardiac glycoside binding site. Direct measurements of the binding of [3H]ouabain, however, indicated that VG4 enhanced rather than inhibited binding, presumably by promoting favorable conformation changes. The data suggest the possibility that the cardiac glycoside binding site may be intramembrane rather than extracellular.     

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.     

The carboxyl-terminal 161 amino acids of the Na,K-ATPase alpha-subunit are sufficient for assembly with the beta-subunit.

Chimeric cDNAs encoding regions of the Na,K-ATPase alpha-subunit and a sarcoplasmic reticulum Ca(2+)-ATPase were constructed and expressed together with the avian Na,K-ATPase beta-subunit cDNA in COS-1 cells to determine which regions of the alpha-subunit are required for assembly with the beta-subunit. Assembly was assayed by immune precipitation of the chimeric subunit with a monoclonal antibody to the avian beta-subunit. A chimera composed of the amino-terminal two-thirds of the Na,K-ATPase and carboxyl-terminal one-third of the Ca(2+)-ATPase did not assemble with the avian beta-subunit. In contrast, the reciprocal chimera, containing the carboxyl-terminal one-third of the Na,K-ATPase, assembled with the beta-subunit. A third chimera, in which 161 amino acids of the Na,K-ATPase carboxyl terminus replaced the corresponding amino acids of the Ca(2+)-ATPase carboxyl terminus, also assembled with the beta-subunit. These results suggest that the aminoacyl residues of the Na,K-ATPase alpha-subunit critical for subunit assembly lie within the carboxyl-terminal 16% of the sequence.     

Postnatal changes in Na,K-ATPase isoform expression in rat cardiac ventricle. Conservation of biphasic ouabain affinity.

The cardiac glycoside sensitivity of the rat heart changes during postnatal maturation and in response to certain pathological conditions. The Na,K-ATPase is thought to be the receptor for cardiac glycosides, and there are three isozymes of its catalytic (alpha) subunit with different cardiac glycoside affinities: alpha 1 (low affinity) and alpha 2 and alpha 3 (high affinity). We examined the developmental expression of the alpha subunit isozymes in rat ventricular membrane preparations by immunoblotting with isozyme-specific antibodies. The alpha 1 isozyme was present throughout all stages of maturation. A developmental switch from alpha 3 to alpha 2 occurred between 14 and 21 days after birth. Measurements of [3H]ouabain binding and inhibition of Na,K-ATPase activity indicated that alpha 2 and alpha 3 should make equivalent contributions to ion pump capacity; in both neonatal natal and adult preparations, ouabain interacted with a single class of high-affinity binding sites (KD = 15 or 40 nM, respectively; Bmax = 4-5 pmol/mg protein), and at low concentrations produced a similar degree of Na,K-ATPase inhibition (25%). The results indicate that the developmental difference in cardiac glycoside sensitivity cannot be explained by quantitative differences in the proportion of high-affinity isozymes of the Na,K-ATPase. The switch from alpha 3 to alpha 2 coincides with other major changes in cardiac electrophysiology and calcium metabolism.     

Cys(577) is a conformationally mobile residue in the ATP-binding domain of the Na,K-ATPase alpha-subunit.

2-[4'-Maleimidylanilino]naphthalene 6-sulfonic acid (MIANS) irreversibly inactivates Na,K-ATPase in a time- and concentration-dependent manner. Inactivation is prevented by 3 mM ATP or low K(+) (<1 mM); the protective effect K(+) is reversed at higher concentrations. This biphasic effect was also observed with K(+) congeners. In contrast, Na(+) ions did not protect. MIANS inactivation disrupted high affinity ATP binding. Tryptic fragments of MIANS-labeled protein were analyzed by reversed phase high performance liquid chromatography. ATP clearly protected one major labeled peptide peak. This observation was confirmed by separation of tryptic peptides in SDS-polyacrylamide gel electrophoresis revealing a single fluorescently-labeled peptide of approximately 5 kDa. N-terminal amino acid sequencing identified the peptide (V(545)LGFCH...). This hydrophobic peptide contains only two Cys residues in all sodium pump alpha-subunit sequences and is found in the major cytoplasmic loop between M4 and M5, a region previously associated with ATP binding. Subsequent digestion of the tryptic peptide with V8 protease and N-terminal amino acid sequencing identified the modified residue as Cys(577). The cation-dependent change in reactivity of Cys(577) implies structural alterations in the ATP-binding domain following cation binding and occlusion in the intramembrane domain of Na,K-ATPase and expands our knowledge of the extent to which cation binding and occlusion are sensed in the ATP hydrolysis domain.     

Na,K-ATPase from Xenopus laevis kidney and epidermis: ouabain interaction studies

A satisfactory purification from Xenopus laevis epidermis is presented. Ki and Kd values for the ouabain-enzyme interaction have been evaluated. Both parameters appear very low, as compared with those demonstrated in other anurans. Very low digitalis-like compound level was demonstrated in Xenopus; on the contrary in Bufo, in which these substances play a defensive role, it is very high. Our results strengthen the hypothesis of a physiological action of such compounds in regulating enzyme-driven Na+/K+ exchanges.