Na+,K(+)-ATPase

Significance of lysine/glycine cluster structure in gastric H+,K+-ATPase

Gastric H+,K+-ATPase consists of alpha- and beta-subunits. The catalytic alpha-subunit contains a very unique structure consisting of lysine and glycine clusters, KKK(or KKKK)AG(G/R)GGGK-(K/R)K, in the amino-terminal cytoplasmic region. This structure is well conserved in all gastric H+,K+-ATPases from different animal species, and was postulated to be the site controlling the access of cations (or proton) to its binding site. In this report, we studied the role of this unique structure by expressing several H+,K+-ATPase mutants of the alpha-subunit together with the wild-type beta-subunit in HEK-293 cells. Even after replacing all the positively-charged amino acid residues (six lysines and one arginine) in the cluster with alanine or removing all the glycine residues in the cluster, the mutants preserved the H+,K+-ATPase activity, and showed similar affinity for ATP and K+ as well as similar pH profiles as those of wild-type H+,K+-ATPase, indicating that the cluster is not indispensable for H+,K+-ATPase activity and not directly involved in determination of the affinity for cation (proton).

Binding domain of oligomycin on Na(+),K(+)-ATPase

Oligomycin inhibits Na(+),K(+)-ATPase activity by stabilizing the Na(+) occlusion but not the K(+) occlusion. To locate the binding domain of oligomycin on Na(+),K(+)-ATPase, the tryptic-digestion profile of Na(+),K(+)-ATPase was compared with the profile of Na(+) occlusion within the digested Na(+),K(+)-ATPase in the presence of oligomycin. The Na(+) occlusion profile is responsible for the digestion profile of the alpha-subunit, which is the catalytic subunit of the ATPase. The effect of oligomycin on chimeric Ca(2+)-ATPase activity was examined. The chimera used, in which the 163 N-terminal amino acids of chicken sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase 1 were replaced with the 200 N-terminal amino acids of the chicken Na(+),K(+)-ATPase alpha1-subunit, partially retains the Na(+)-dependent characteristics of Na(+), K(+)-ATPase, because the chimeric Ca(2+)-ATPase activity is activated by Na(+) but inhibited by ouabain, a specific inhibitor of Na(+),K(+)-ATPase (Ishii, T., Lemas, M.V., Takeyasu, K., 1994, Proc. Natl. Acad. Sci. U. S. A., 91, 6103-6107). Oligomycin depressed the activation by Na(+) of the chimeric Ca(2+)-ATPase activity. These findings suggest that the 200 N-terminal amino acids of the Na(+), K(+)-ATPase alpha-subunit include a binding domain for oligomycin.

Sperm motility is dependent on a unique isoform of the Na,K-ATPase

The Na,K-ATPase, a member of the P-type ATPases, is composed of two subunits, alpha and beta, and is responsible for translocating Na(+) out of the cell and K(+) into the cell using the energy of hydrolysis of one molecule of ATP. The electrochemical gradient it generates is necessary for many cellular functions, including establishment of the plasma membrane potential and transport of sugars and ions in and out of the cell. Families of isoforms for both the alpha and beta subunits have been identified, and specific functional roles for individual isoforms are just beginning to emerge. The alpha4 isoform is the most recently identified Na, K-ATPase alpha isoform, and its expression has been found only in testis. Here we show that expression of the alpha4 isoform in testis is localized to spermatozoa and that inhibition of this isoform alone eliminates sperm motility. These data describe for the first time a biological function for the alpha4 isoform of the Na,K-ATPase, revealing a critical role for this isoform in sperm motility.

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.

Identification and purification of aminophospholipid flippases

Transbilayer phospholipid asymmetry is a common structural feature of most biological membranes. This organization of lipids is generated and maintained by a number of phospholipid transporters that vary in lipid specificity, energy requirements and direction of transport. These transporters can be divided into three classes: (1) bidirectional, non-energy dependent 'scramblases', and energy-dependent transporters that move lipids (2) toward ('flippases') or (3) away from ('floppases') the cytofacial surface of the membrane. One of the more elusive members of this family is the plasma membrane aminophospholipid flippase, which selectively transports phosphatidylserine from the external to the cytofacial monolayer of the plasma membrane. This review summarizes the characteristics of aminophospholipid flippase activity in intact cells and describes current strategies to identify and isolate this protein. The biochemical characteristics of candidate flippases are critically compared and their potential role in flippase activity is evaluated.

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.

Regulation of Na+,K+-ATPase by persistent sodium accumulation in adult rat thalamic neurones

The present study investigated the regulatory mechanism of the Na+, K+-ATPase and the level of internal Na+ and Ca2+ in response to persistent Na+ influx in acutely dissociated rat thalamic neurones. Whole-cell patch-clamp recordings and Na+ imaging revealed a stable [Na+]i and low background pump activity. Exposure to veratridine (50 microM) for 1 h resulted in a progressive rise in [Na+]i (DeltaFNa = 64 +/-22%) and [Ca2+]i (DeltaFCa = 44 +/- 14%) over 3 h. Increases in [Na+]i and [Ca2+]i were also observed during neuronal exposure to the Na+ ionophore monensin (50 microM). Subcellular confocal immunofluorescence quantification of alpha3 catalytic Na+-K+ pump subunits showed that a veratridine-induced rise in [Na+]i was accompanied by a significant increase in pump density in both membrane and cytoplasmic compartments, by 39 and 54%, respectively. Similar results were also obtained in experiments when neurones were treated with monensin. A fluorescent 9-anthroylouabain binding assay detected a 60 and 110% increase in phosphorylated (active) pumps after veratridine and monensin exposure, respectively. During the entire experiment, application of ouabain or veratridine alone induced little cell swelling and death, but pump inhibition in cells pre-loaded with Na+ led to rapid cell swelling and necrosis. The above results indicate that a persistent influx of Na+ may trigger rapid enhancement of pump synthesis, membrane redistribution and functional activity. However, these compensatory mechanisms failed to prevent persistent Na+ accumulation.

Paradoxical effects of mineralocorticoids on the ion gated sodium channel in embryologically diverse cells

PCR analysis and Western blotting revealed the expression of the mineralocorticoid receptor (MCR) and the epithelial sodium channel (ENaC) genes at the level of RNA, DNA, and protein in several leukemic cell lines, fibroblasts from human cornea, and epithelial cells from ocular tissues. Following immunofluorescence, the MCR appeared to be primarily nuclear whereas the ENaC was almost exclusively membrane-bound. Paradoxically, the MCR-specific antagonist ZK 91587 actually stimulated the multiplication of human erythroblastic leukemia cells, contrary to the inhibitory effect of the antagonist RU 26752 on the multiplication of corneal fibroblasts; both effects were opposed by aldosterone. In quantitative PCR, both basal and aldosterone-induced levels of ENaC were diminished by ZK 91587 in the corneal fibroblast, in contrast to the stimulation observed in the retinal pigmentary epithelium. Thus, contrary to the existing notions, (a) antimineralocorticoids can act both as agonists and antagonists, and (b) the receptor-mediated action of mineralocorticoids on the sodium channel is not restricted to the epithelial cell.

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.

Specific up-regulation of mitochondrial F0F1-ATPase activity after short episodes of atrial fibrillation in sheep

INTRODUCTION

Ventricular fibrillation induced by either digitalis intoxication or electrical stimulation is reported to alter myocardial energy by impairing the sarcolemmal Na,K-ATPase or the receptor for digitalis and the mitochondrial ATPase synthase or F0F1-ATPase. However, little is known about these membrane functions during atrial fibrillation (AF).

METHODS AND RESULTS

We analyzed the effects of electrically induced AF on biochemical activities of atrial F0F1-ATPase and Na,K-ATPase in sheep. A group of six sheep was subjected to direct short electrical stimulation of the right atrium to induce AF. Sham-operated sheep served as a control group. Microsomal and mitochondrial membranes of atrial muscle were isolated and tested for enzymatic activity. All paced sheep developed multiple episodes of sustained AF, with a mean total duration of 110 minutes over a 2-hour period. Data showed that short-term pacing-induced AF significantly activated membrane F0F1-ATPase activity (P < 0.05) without changes in cytochrome-c oxidase activity, Na,K-ATPase activity, ouabain sensitivity, and alpha1-subunit expression.

CONCLUSION

Specific activation of F0F1-ATPase activity is an early molecular consequence of sustained AF in sheep.

Inhibition of Na,K-ATPase by external electrical cardioversion in a sheep model of atrial fibrillation

INTRODUCTION

Electrical external cardioversion commonly used to treat atrial fibrillation (AF) is associated with myocardial membrane damage and disturbances in ionic homeostasis (hemodynamically unstable). The present study was designed to investigate whether alterations in ionic homeostasis observed were due in part to changes in the myocardial activity of Na,K-ATPase.

METHODS AND RESULTS

AF was induced by pacing in ten anesthetized sheep divided into two groups. Group I (n = 4) received a single external countershock of 360 J after three episodes of AF lasting 10 minutes. Group II (n = 6) served as controls. Activity, responsiveness to ouabain, and membrane expression of catalytic alpha and beta subunits of Na,K-ATPase in sarcolemmal myocardial membrane fractions were investigated. Membrane fluidity and fatty acid composition, and plasma levels of atrial natriuretic factor (ANF) also were measured. One shock after episodes of AF significantly decreased ventricular Na,K-ATPase activity up to 50% (P < 0.001) without modification of atrial activity at the membrane level. Sites with low affinity to ouabain showed a fivefold lower affinity for ouabain in the cardioversion group than in the control group (IC50 = 7.9 micromol/L vs 40 micromol/L ouabain, P < 0.05). Plasma levels of ANF were significantly increased in the cardioversion group compared with the control group. These changes were independent of membrane modulation in terms of expression of Na,K-ATPase, membrane fluidity, and fatty acid composition.

CONCLUSION

This study suggests that left ventricular perturbation of ionic homeostasis subsequent to transthoracic cardioversion could result from inactivation of Na,K-ATPase activity.

Effect of streptozotocin-induced diabetes on rat liver Na+/K+-ATPase

Na+/K+-ATPase during diabetes may be regulated by synthesis of its alpha and beta subunits and by changes in membrane fluidity and lipid composition. As these mechanisms were unknown in liver, we studied in rats the effect of streptozotocin-induced diabetes on liver Na+/K+-ATPase. We then evaluated whether fish oil treatment prevented the diabetes-induced changes. Diabetes mellitus induced an increased Na+/K+-ATPase activity and an enhanced expression of the beta1 subunit; there was no change in the amount of the alpha1 and beta3 isoenzymes. Biphasic ouabain inhibition curves were obtained for diabetic groups indicating the presence of low and high affinity sites. No alpha2 and alpha3 isoenzymes could be detected. Diabetes mellitus led to a decrease in membrane fluidity and a change in membrane lipid composition. The diabetes-induced changes are not prevented by fish oil treatment. The results suggest that the increase of Na+/K+-ATPase activity can be associated with the enhanced expression of the beta1 subunit in the diabetic state, but cannot be attributed to changes in membrane fluidity as typically this enzyme will increase in response to an enhancement of membrane fluidity. The presence of a high-affinity site for ouabain (IC50 = 10-7 M) could be explained by the presence of (alphabeta)2 diprotomeric structure of Na+/K+-ATPase or an as yet unknown alpha subunit isoform that may exist in diabetes mellitus. These stimulations might be related, in part, to the modification of fatty acid content during diabetes.

Effects of fluorescent pseudo-ATP and ATP-metal analogs on secondary structure of Na(+)/K(+)-ATPase

The secondary structure of Na(+)/K(+)-ATPase after modification of the ATP-binding sites was analyzed. Consistently with recent reports, we found in trypsin-treated Na(+)/K(+)-ATPase additionally to alpha-helix also beta-sheet structures in the transmembrane segments. However, binding of fluorescein 5'-isothiocyanate (FITC), the pseudo-ATP analog, to the ATP-binding site did not affect the secondary structure of undigested Na(+)/K(+)-ATPase. Consequently, fluorescence intensity changes of FITC-labeled Na(+)/K(+)-ATPase commonly used to observe conformational transitions of the enzyme reflect physiological changes of the native structure. The metal complex analogues of ATP, Cr(H(2)O)(4)ATP and Co(NH(3))(4)ATP, on the other hand, affected the secondary structure of Na(+)/K(+)-ATPase. We propose that these changes in the secondary structure are responsible for inhibition of backdoor phosphorylation.

Regulation of the plasma membrane potential in Pneumocystis carinii

Many protists use a H(+) gradient across the plasma membrane, the proton motive force, to drive nutrient uptake. This force is generated in part by the plasma membrane potential (DeltaPsi). We investigated the regulation of the DeltaPsi in Pneumocystis carinii using the potentiometric fluorescent dye bisoxonol. The steady state DeltaPsi in a buffer containing Na(+) and K(+) (standard buffer) was found to be -78+/-8 mV. In the absence of Na(+) and K(+) (NMG buffer) or Cl(-) (gluconate buffer), DeltaPsi was not significantly changed suggesting that cation and anion conductances do not play a significant role in the regulation of DeltaPsi in P. carinii. The DeltaPsi was also not affected by inhibitors of the Na(+)/K(+)-ATPase, ouabain (1 mM), and the K(+)/H(+)-ATPase, omeprazole (1 mM). In contrast, inhibitors of the plasma membrane H(+)-ATPase, dicyclohexylcarbodiimide (100 microM), N-ethylmaleimide (100 microM) and diethylstilbestrol (25 microM), significantly depolarized the DeltaPsi to -43+/-7, -56+/-5 and -40+/-12 mV, respectively. The data support that the plasma membrane H(+)-ATPase plays a significant role in the regulation of DeltaPsi in P. carinii.

High DNA sequence variability at the alpha 1 Na/K-ATPase locus of Artemia franciscana (brine shrimp): polymorphism in a gene for salt-resistance in a salt-resistant organism

We previously reported that the Na/K-ATPase alpha 1 subunit coding gene showed signs of being a very polymorphic locus in Artemia franciscana. This species is adapted to highly saline waters, and the Na/K-ATPase alpha 1 isoform presumably plays a key role in this adaptation. Therefore, we were interested in further study of the alpha 1 Na/K-ATPase polymorphisms to examine whether they might be due to an adaptation to salt resistance driven by natural selection. Using coding sequences from 10 genomic clones and 3 cDNAs, we observed that most substitutions are in synonymous positions (88.8%). The 12 nonsynonymous substitutions code for conservative amino acid replacements with an apparent scattered distribution across functional domains of the protein. Interspecific comparison between these sequences and two genomic clones from Artemia parthenogenetica containing 1,122 bp of the alpha 1 Na/K-ATPase locus coding sequence showed independence of the synonymous/nonsynonymous ratio in the comparison within A. franciscana and between A. franciscana and A. parthenogenetica, which fits the neutral model of evolution. Since there were no previous studies on DNA polymorphism for other A. franciscana genes, we also studied variability at the Actin 302 locus for comparison. Both loci were amplified by reverse transcription-polymerase chain reaction, and 20 sequences were obtained for each. This study shows that the amplified region of the alpha 1 Na/K-ATPase gene is 3.5 times as polymorphic as the Actin 302 gene and 2.9 times as heterozygotic. Interestingly, under a model of neutral evolution, the data observed would be expected with a probability of approximately 0.05, suggesting an excess of intraspecific variation of alpha 1 Na/K-ATPase with respect to Actin 302. Restriction fragment length polymorphism studies show similar patterns of polymorphism along the approximately 41-kb span of the alpha 1 Na/K-ATPase locus. Most of the nucleotide differences are linked in a few haplotypes, although recombination events are also inferred from the data. We propose a possible explanation for the high polymorphic levels at the alpha 1 Na/K-ATPase locus which invokes positive selection acting tightly to the locus in transiently isolated or semi-isolated subpopulations.

Synthesis of gill Na(+)-K(+)-ATPase in Atlantic salmon smolts: differences in alpha-mRNA and alpha-protein levels

Several parameters were analyzed to determine the mechanisms responsible for the enhancement of the gill Na(+)-K(+)-ATPase activity of Atlantic salmon smolts. A major alpha-subunit transcript of 3.7 kb was revealed by Northern blot in both parr and smolt gills when hybridized with two distinct cDNA probes. The alpha-mRNA abundance demonstrated an increase to maximal levels in smolts at an early stage of the parr-smolt transformation. This was followed by a gradual rise in alpha-protein levels, revealed by Western blots with specific antibodies and by an increase in gill Na(+)-K(+)-ATPase hydrolytic activity, both only reaching maximum levels a month later, at the peak of the transformation process. Parr fish experienced a decrease in alpha-mRNA abundance and had basal levels of alpha-protein and enzyme activity. Measurement of the binding of [(3)H]ouabain to Na(+)-K(+)-ATPase was characterized in smolts and parr gill membranes showing more than a twofold elevation in smolts and was of high affinity in both groups (dissociation constant = 20-23 nM). Modulation of the enzyme due to increased salinity was also observed in seawater-transferred smolts, as demonstrated by an increase in alpha-mRNA levels after 24 h with a rise in Na(+)-K(+)-ATPase activity occurring only after 11 days. No qualitative change in alpha-expression was revealed at either the mRNA or protein level. Immunological identification of the alpha-protein was performed with polyclonal antibodies directed against the rat alpha-specific isoforms, revealing that parr, freshwater, and seawater smolts have an alpha(3)-like isoform. This study shows that the increase in Na(+)-K(+)-ATPase activity in smolt gills depends first on an increase in the alpha-mRNA expression and is followed by a slower rise in alpha-protein abundance that eventually leads to a higher synthesis of Na(+)-K(+) pumps.

General overview of mineralocorticoid hormone action

The adrenal cortex elaborates two major groups of steroids that have been arbitrarily classified as glucocorticoids and mineralocorticoids, despite the fact that carbohydrate metabolism is intimately linked to mineral balance in mammals. In fact, glucocorticoids assured both of these functions in all living cells, animal and photosynthetic, prior to the appearance of aldosterone in teleosts at the dawn of terrestrial colonization. The evolutionary drive for a hormone specifically designed for hydromineral regulation led to zonation for the conversion of 18-hydroxycorticosterone into aldosterone through the catalytic action of a synthase in the secluded compartment of the adrenal zona glomerulosa. Corticoid hormones exert their physiological action by binding to receptors that belong to a transcription factor superfamily, which also includes some of the proteins regulating steroid synthesis. Steroids stimulate sodium absorption by the activation and/or de novo synthesis of the ion-gated, amiloride-sensitive sodium channel in the apical membrane and that of the Na+/K+-ATPase in the basolateral membrane. Receptors, channels, and pumps apparently are linked to the cytoskeleton and are further regulated variously by methylation, phosphorylation, ubiquination, and glycosylation, suggesting a complex system of control at multiple checkpoints. Mutations in genes for many of these different proteins have been described and are known to cause clinical disease.

Mechanistic studies of phosphoserine phosphatase, an enzyme related to P-type ATPases

Phosphoserine phosphatase belongs to a new class of phosphotransferases forming an acylphosphate during catalysis and sharing three motifs with P-type ATPases and haloacid dehalogenases. The phosphorylated residue was identified as the first aspartate in the first motif (DXDXT) by mass spectrometry analysis of peptides derived from the phosphorylated enzyme treated with NaBH(4) or alkaline [(18)O]H(2)O. Incubation of native phosphoserine phosphatase with phosphoserine in [(18)O]H(2)O did not result in (18)O incorporation in residue Asp-20, indicating that the phosphoaspartate is hydrolyzed, as in P-type ATPases, by attack of the phosphorus atom. Mutagenesis studies bearing on conserved residues indicated that four conservative changes either did not affect (S109T) or caused a moderate decrease in activity (G178A, D179E, and D183E). Other mutations inactivated the enzyme by >80% (S109A and G180A) or even by >/=99% (D179N, D183N, K158A, and K158R). Mutations G178A and D179N decreased the affinity for phosphoserine, suggesting that these residues participate in the binding of the substrate. Mutations of Asp-179 decreased the affinity for Mg(2+), indicating that this residue interacts with the cation. Thus, investigated residues appear to play an important role in the reaction mechanism of phosphoserine phosphatase, as is known for equivalent residues in P-type ATPases and haloacid dehalogenases.

The Na,K-ATPase alpha4 gene (Atp1a4) encodes a ouabain-resistant alpha subunit and is tightly linked to the alpha2 gene (Atp1a2) on mouse chromosome 1

We have isolated and characterized cDNA clones encoding the murine homologue of a putative fourth Na,K-ATPase alpha subunit isoform (alpha4). The predicted polypeptide is 1032 amino acids in length and exhibits 75% amino acid sequence identity to the rat alpha1, alpha2, and alpha3 subunits. Within the first extracellular loop, the alpha4 subunit is highly divergent from other Na,K-ATPase alpha subunits. Because this region of Na,K-ATPase is a major determinant of ouabain sensitivity, we tested the ability of the rodent alpha4 subunit to transfer ouabain resistance in a transfection protocol. We find that a cDNA containing the complete rodent alpha4 ORF is capable of conferring low levels of ouabain resistance upon HEK 293 cells, an indication that the alpha4 subunit can substitute for the endogenous ouabain-sensitive alpha subunit of human cells. Nucleotide sequences specific for the murine alpha4 subunit were used to identify the chromosomal position of the alpha4 subunit gene. By hybridizing an alpha4 probe with a series of BACs, we localized the alpha4 subunit gene (Atp1a4) to the distal portion of mouse chromosome 1, in very close proximity to the murine Na,K-ATPase alpha2 subunit gene. In adult mouse tissues, we detected expression of the alpha4 subunit gene almost exclusively in testis, with low levels of expression in epididymis. The close similarities in the organization and expression pattern of the murine and human alpha4 subunit genes suggest that these two genes are orthologous. Together, our studies indicate that the alpha4 subunit represents a functional Na,K-ATPase alpha subunit isoform.

K(+)/Na(+) antagonism at cytoplasmic sites of Na(+)-K(+)-ATPase: a tissue-specific mechanism of sodium pump regulation

Tissue-distinct interactions of the Na(+)-K(+)-ATPase with Na(+) and K(+), independent of isoform-specific properties, were reported previously (A. G. Therien, N. B. Nestor, W. J. Ball, and R. Blostein. J. Biol. Chem. 271: 7104-7112, 1996). In this paper, we describe a detailed analysis of tissue-specific kinetics particularly relevant to regulation of pump activity by intracellular K(+), namely K(+) inhibition at cytoplasmic Na(+) sites. Our results show that the order of susceptibilities of alpha(1) pumps of various rat tissues to K(+)/Na(+) antagonism, represented by the ratio of the apparent affinity for Na(+) binding at cytoplasmic activation sites in the absence of K(+) to the affinity constant for K(+) as a competitive inhibitor of Na(+) binding at cytoplasmic sites, is red blood cell < axolemma approximately rat alpha(1)-transfected HeLa cells < small intestine < kidney < heart. In addition, we have carried out an extensive analysis of the kinetics of K(+) binding and occlusion to the cytoplasmic cation binding site and find that, for most tissues, there is a relationship between the rate of K(+) binding/occlusion and the apparent affinity for K(+) as a competitive inhibitor of Na(+) activation, the order for both parameters being heart >/= kidney > small intestine approximately rat alpha(1)-transfected HeLa cells. The notion that modulations in cytoplasmic K(+)/Na(+) antagonism are a potential mode of pump regulation is underscored by evidence of its reversibility. Thus the relatively high K(+)/Na(+) antagonism characteristic of kidney pumps was reduced when rat kidney microsomal membranes were fused into the dog red blood cell.

Functional characterization of a testes-specific alpha-subunit isoform of the sodium/potassium adenosinetriphosphatase

Different isoforms of the sodium/potassium adenosinetriphosphatase (Na,K-ATPase) alpha and beta subunits have been identified in mammals. The association of the various alpha and beta polypeptides results in distinct Na,K-ATPase isozymes with unique enzymatic properties. We studied the function of the Na,K-ATPase alpha4 isoform in Sf-9 cells using recombinant baculoviruses. When alpha4 and the Na pump beta1 subunit are coexpressed in the cells, Na, K-ATPase activity is induced. This activity is reflected by a ouabain-sensitive hydrolysis of ATP, by a Na(+)-dependent, K(+)-sensitive, and ouabain-inhibitable phosphorylation from ATP, and by the ouabain-inhibitable transport of K(+). Furthermore, the activity of alpha4 is inhibited by the P-type ATPase blocker vanadate but not by compounds that inhibit the sarcoplasmic reticulum Ca-ATPase or the gastric H,K-ATPase. The Na,K-ATPase alpha4 isoform is specifically expressed in the testis of the rat. The gonad also expresses the beta1 and beta3 subunits. In insect cells, the alpha4 polypeptide is able to form active complexes with either of these subunits. Characterization of the enzymatic properties of the alpha4beta1 and alpha4beta3 isozymes indicates that both Na,K-ATPases have similar kinetics to Na(+), K(+), ATP, and ouabain. The enzymatic properties of alpha4beta1 and alpha4beta3 are, however, distinct from the other Na pump isozymes. A Na, K-ATPase activity with similar properties as the alpha4-containing enzymes was found in rat testis. This Na,K-ATPase activity represents approximately 55% of the total enzyme of the gonad. These results show that the alpha4 polypeptide is a functional isoform of the Na,K-ATPase both in vitro and in the native tissue.

Lipid restructuring does not contribute to elevated activities of Na(+)/K(+)-ATPase in basolateral membranes from the gill of seawater-acclimated eel (Anguilla rostrata)

In teleost fishes, increases in gill Na(+)/K(+)-ATPase activity accompanying the transition from fresh water to sea water may be attributed to changes in either the numbers of enzyme molecules present or to turnover number (k(cat)). The sensitivity of Na(+)/K(+)-ATPase to its chemical/physical environment in the membrane makes it plausible that modulation of enzyme activity may be driven, in part, by changes in membrane properties. In the current study, I test the hypothesis that lipid compositional changes (restructuring) contribute to the modulation of gill Na(+)/K(+)-ATPase activity. An enriched preparation of basolateral membranes was prepared from the gills of freshwater- and seawater-acclimated American eel (Anguilla rostrata). Phospholipid class distribution, fatty acyl chain compositions and cholesterol contents were determined. Phosphatidylcholine, the most abundant phospholipid present in gill basolateral membranes, makes up more than 60 % of the total phospholipid content in both freshwater- and seawater-acclimated animals. The contents of other phospholipids and major fatty acyl chains are also similar for the two acclimation groups. Cholesterol/phospholipid molar ratios are 0.28 for freshwater and 0.29 for seawater animals. The similarity between lipid compositions in membranes from freshwater- and seawater-acclimated eels indicates that lipid restructuring is not a mechanism for modulation of gill Na(+)/K(+)-ATPase activity in Anguilla rostrata, at least during the acclimation time course used in the present study.

Expression of the beta2-subunit and apical localization of Na+-K+-ATPase in metanephric kidney

During kidney organogenesis, the Na+-K+-ATPase pump is not restricted to the basolateral plasma membrane of the renal epithelial cell but is instead either localized to the apical and lateral membrane sites of the early nephron or expressed in a nonpolarized distribution in the newly formed collecting ducts. The importance of Na+-K+-ATPase beta-subunit expression in the translocation of the Na+-K+-ATPase to the plasma membrane raises the question as to which beta-subunit isoform is expressed during kidney organogenesis. Immunocytochemical, Western analysis and RNase protection studies showed that both beta2-subunit protein and beta2 mRNA are expressed in the early gestation to midgestation human metanephric kidney. In contrast, although beta1 mRNA abundance is equivalent to that of the beta2-subunit in the metanephric kidney, the beta1-subunit protein was not detected in early to midgestation metanephric kidney samples. Immunocytochemical analysis revealed that both alpha1- and beta2-subunits were present in the apical epithelial plasma membranes of distal nephron segments of early stage nephrons, maturing loops of Henle, and collecting ducts during kidney development. We also detected a significant increase in alpha1 and beta1 mRNA after birth with a marked reduction in beta2 mRNA abundance associated with an increase in alpha1- and beta1-subunit proteins and loss of beta2 protein expression. These studies support the conclusion that the expression of the beta2-subunit in the fetal kidney may be an important mechanism controlling polarization of the Na+-K+-ATPase pump in the epithelia of the developing nephron during kidney organogenesis.

Regulation of expression of Na+,K+-ATPase in androgen-dependent and androgen-independent prostate cancer

The beta 1-subunit of Na+,K+-ATPase was isolated and identified as an androgen down-regulated gene. Expression was observed at high levels in androgen-independent as compared to androgen-dependent (responsive) human prostate cancer cell lines and xenografts when grown in the presence of androgens. Down-regulation of the beta 1-subunit was initiated at concentrations between 0.01 nM and 0.03 nM of the synthetic androgen R1881 after relatively long incubation times (> 24 h). Using polyclonal antibodies, the concentration of beta 1-subunit protein, but not of the alpha 1-subunit protein, was markedly reduced in androgen-dependent human prostate cancer cells (LNCaP-FGC) cultured in the presence of androgens. In line with these observations it was found that the protein expression of total Na+,K+-ATPase in the membrane (measured by 3H-ouabain binding) was also markedly decreased. The main function of Na+,K+-ATPase is to maintain sodium and potassium homeostasis in animal cells. The resulting electrochemical gradient is facilitative for transport of several compounds over the cell membrane (for example cisplatin, a chemotherapeutic agent experimentally used in the treatment of hormone-refractory prostate cancer). Here we observed that a ouabain-induced decrease of Na+,K+-ATPase activity in LNCaP-FGC cells results in reduced sensitivity of these cells to cisplatin-treatment. Surprisingly, androgen-induced decrease of Na+,K+-ATPase expression, did not result in significant protection against the chemotherapeutic agent.

Mutant Phe788 --> Leu of the Na+,K+-ATPase is inhibited by micromolar concentrations of potassium and exhibits high Na+-ATPase activity at low sodium concentrations

Mutant Phe788 --> Leu of the rat kidney Na+,K(+)-ATPase was expressed in COS cells to active-site concentrations between 40 and 60 pmol/mg of membrane protein. Analysis of the functional properties showed that the discrimination between Na+ and K+ on the two sides of the system is severely impaired in the mutant. Micromolar concentrations of K+ inhibited ATP hydrolysis (K(0.5) for inhibition 107 microM for the mutant versus 76 mM for the wild-type at 20 mM Na+), and at 20 mM K+, the molecular turnover number for Na+,K(+)-ATPase activity was reduced to 11% that of the wild-type. This inhibition was counteracted by Na+ in high concentrations, and in the total absence of K+, the mutant catalyzed Na(+)-activated ATP hydrolysis ("Na(+)-ATPase activity") at an extraordinary high rate corresponding to 86% of the maximal Na+,K(+)-ATPase activity. The high Na(+)-ATPase activity was accounted for by an increased rate of K(+)-independent dephosphorylation. Already at 2 mM Na+, the dephosphorylation rate of the mutant was 8-fold higher than that of the wild-type, and the maximal rate of Na(+)-induced dephosphorylation amounted to 61% of the rate of K(+)-induced dephosphorylation. The cause of the inhibitory effect of K+ on ATP hydrolysis in the mutant was an unusual stability of the K(+)-occluded E2(K2) form. Hence, when E2(K2) was formed by K+ binding to unphosphorylated enzyme, the K(0.5) for K+ occlusion was close to 1 microM in the mutant versus 100 microM in the wild-type. In the presence of 100 mM Na+ to compete with K+ binding, the K(0.5) for K+ occlusion was still 100-fold lower in the mutant than in the wild-type. Moreover, relative to the wild-type, the mutant exhibited a 6-7-fold reduced rate of release of occluded K+, a 3-4-fold increased apparent K+ affinity in activation of the pNPPase reaction, a 10-11-fold lower apparent ATP affinity in the Na+,K(+)-ATPase assay with 250 microM K+ present (increased K(+)-ATP antagonism), and an 8-fold reduced apparent ouabain affinity (increased K(+)-ouabain antagonism).

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.

Localization of Na,K-ATPase alpha/beta isoforms in rat sciatic nerves: effect of diabetes and fish oil treatment

The localization of the Na,K-ATPase isoenzymes in sciatic nerve remains controversial, as well as diabetes-induced changes in Na,K-ATPase isoforms. Some of these changes could be prevented by fish oil therapy. The aim of this study was to determine by confocal microscopy the distribution of Na,K-ATPase isoforms (alpha1, alpha2, alpha3, beta1, and beta2) in the sciatic nerve, the changes induced by diabetes, and the preventive effect of fish oil in diabetic neuropathy. This study was performed in three groups of rats. In the first two groups, diabetes was induced by streptozotocin and rats were supplemented daily with fish oil or olive oil at a dosage of 0.5 g/kg of body weight. The third one was a control group that was supplemented with olive oil. Five antibodies against specific epitopes of Na,K-ATPase isoenzymes were applied to stained dissociated nerve fibers with fluorescent secondary antibodies. The five isoenzymes were documented in nonspecific regions, Schwann cells (myelin), and the node of Ranvier. The localization of the alpha1, alpha2, and beta1 isoenzymes was not affected by diabetes. In contrast, diabetes induced a decrease of the alpha2 subunit (p < 0.05) and an up-regulation of the beta2 subunit (p < 0.05). These modifications were noted in both regions for alpha2 and were localized at the myelin domain only for the beta2. Fish oil supplementation prevented the diabetes-induced changes in the alpha2 subunit with an additional up-regulation. The beta2 subunit was not modified. A phenotypic change similar to nerve injury was induced by diabetes. Fish oil supplementation partially prevented some of these changes.

Alanine scanning mutagenesis of oxygen-containing amino acids in the transmembrane region of the Na,K-ATPase

Oxygen-containing amino acids in the transmembrane region of the Na, K-ATPase alpha subunit were studied to identify residues involved in Na+ and/or K+ coordination by the enzyme. Conserved residues located in the polar face of transmembrane helices were selected using helical wheel and topological models of the enzyme. Alanine substitution of these residues were introduced into an ouabain-resistant sheep alpha1 isoform and expressed in HeLa cells. The capacity to generate essential Na+ and K+ gradients and thus support cell growth was used as an initial indication of the functionality of heterologous enzymes. Enzymes carrying alanine substitution of Ser94, Thr136, Ser140, Gln143, Glu144, Glu282, Thr334, Thr338, Thr340, Ser814, Tyr817, Glu818, Glu821, Ser822, Gln854, and Tyr994 supported cell growth, while those carrying substitutions Gln923Ala, Thr955Ala, and Asp995Ala did not. To study the effects of these latter replacements on cation binding, they were introduced into the wild-type alpha1 sheep isoform and expressed in mouse NIH3T3 cells where [3H]ouabain binding was utilized to probe the heterologous proteins. These substitutions did not affect ouabain, K+, or Na+ binding. Expression levels of these enzymes were similar to that of control. However, the level of Gln923Ala-, Thr955Ala-, or Asp995Ala-substituted enzyme at the plasma membrane was significantly lower than that of the wild-type isoform. Thus, these substitutions appear to impair the maturation process or targeting of the enzyme to the plasma membrane, but not cation-enzyme interactions. These results complete previous studies which have identified Ser755, Asp804, and Asp808 as absolutely essential for Na+ and K+ transport by the enzyme. Thus, it is significant that most transmembrane conserved-oxygen-containing residues in the Na,K-ATPase can be replaced without substantially affecting cation-enzyme interactions to the extent of preventing enzyme function. Consequently, other chemical groups, aromatic rings or backbone carbonyls, should be considered in models of cation-binding sites.

Intracellular reactive oxygen species mediate the linkage of Na+/K+-ATPase to hypertrophy and its marker genes in cardiac myocytes

We showed before that in cardiac myocytes partial inhibition of Na+/K+-ATPase by nontoxic concentrations of ouabain causes hypertrophy and transcriptional regulations of growth-related marker genes through multiple Ca2+-dependent signal pathways many of which involve Ras and p42/44 mitogen-activated protein kinases. The aim of this work was to explore the roles of intracellular reactive oxygen species (ROS) in these ouabain-initiated pathways. Ouabain caused a rapid generation of ROS within the myocytes that was prevented by preexposure of cells to N-acetylcysteine (NAC) or vitamin E. These antioxidants also blocked or attenuated the following actions of ouabain: inductions of the genes of skeletal alpha-actin and atrial natriuretic factor, repression of the gene of the alpha3-subunit of Na+/K+-ATPase, activation of mitogen-activated protein kinases, activation of Ras-dependent protein synthesis, and activation of transcription factor NF-kappaB. Induction of c-fos and activation of AP-1 by ouabain were not sensitive to NAC. Ouabain-induced inhibition of active Rb+ uptake through Na+/K+-ATPase and the resulting rise in intracellular Ca2+ were also not prevented by NAC. A phorbol ester that also causes myocyte hypertrophy did not increase ROS generation, and its effects on marker genes and protein synthesis were not affected by NAC. We conclude the following: (a) ROS are essential second messengers within some but not all signal pathways that are activated by the effect of ouabain on Na+/K+-ATPase; (b) the ROS-dependent pathways are involved in ouabain-induced hypertrophy; (c) increased ROS generation is not a common response of the myocyte to all hypertrophic stimuli; and (d) it may be possible to dissociate the positive inotropic effect of ouabain from its growth-related effects by alteration of the redox state of the cardiac myocyte.

Catalytic activity of an isolated domain of Na,K-ATPase expressed in Escherichia coli

Fusion proteins of glutathione-S-transferase and fragments from the large cytoplasmic domain of the sheep Na,K-ATPase alpha1-subunit were expressed in Escherichia coli. The Na,K-ATPase sequences begin at Ala345 and terminate at either Arg600 (DP600f), Thr610 (DP610f), Gly731 (DP731f), or Glu779 (DP779f). After affinity purification on glutathione-Sepharose, the fusion proteins were labeled with [alpha-32P]-2-N3-ATP, and incorporation of the radiolabel into the fusion proteins was measured by scintillation counting after sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Kd values of 220-290 microM for 2-N3-ATP binding to the fusion proteins were obtained from the photolabeling experiments. Approximately 1 mol of 2-N3-ATP was calculated to be incorporated per mole of fusion protein after correction for photochemical incorporation efficiency. Labeling of all of the fusion proteins by 25 microM 2-N3-ATP was reduced in the presence of MgATP, Na2ATP, MgCl2, 2',3'-O-(2,4, 6-trinitrophenyl)-ATP, and p-nitrophenylphosphate, and Ki values of 2-11 mM for Na2ATP, 0.2-5 mM for MgCl2, 0.1-5 mM for MgATP, and 20-300 microM for p-nitrophenylphosphate were calculated for these ligands. All of the fusion proteins catalyze the hydrolysis of p-nitrophenylphosphate. The reaction requires MgCl2 and is inhibited by inorganic phosphate, which is similar to the hydrolysis of p-nitrophenylphosphate by native Na,K-ATPase. Based on these observations, it appears that the soluble fragments from the large cytoplasmic domain of Na,K-ATPase expressed in bacterial cells are folded in an E2-like conformation and are likely to retain much of the native structure.

Cation selectivity of gastric H,K-ATPase and Na,K-ATPase chimeras

Chimeras of the catalytic subunits of the gastric H,K-ATPase and Na, K-ATPase were constructed and expressed in LLC-PK1 cells. The chimeras included the following: (i) a control, H85N (the first 85 residues comprising the cytoplasmic N terminus of Na,K-ATPase replaced by the analogous region of H,K-ATPase); (ii) H85N/H356-519N (the N-terminal half of the cytoplasmic M4-M5 loop also replaced); and (iii) H519N (the entire front half replaced). The latter two replacements confer a decrease in apparent affinity for extracellular K+. The 356-519 domain and, to a greater extent, the H519N replacement confer increased apparent selectivity for protons relative to Na+ at cytoplasmic sites as shown by the persistence of K+ influx when the proton concentration is increased and the Na+ concentration decreased. The pH and K+ dependence of ouabain-inhibitable ATPase of membranes derived from the transfected cells indicate that the H519N and, to a lesser extent, the H356-519N substitution decrease the effectiveness of K+ to compete for protons at putative cytoplasmic H+ activation sites. Notable pH-independent behavior of H85N/H356-519N at low Na+ suggests that as pH is decreased, Na+/K+ exchange is replaced largely by (Na+ + H+)/K+ exchange. With H519N, the pH and Na+ dependence of pump and ATPase activities suggest relatively active H+/K+ exchange even at neutral pH. Overall, this study provides evidence for important roles in cation selectivity for both the N-terminal half of the M4-M5 loop and the adjacent transmembrane helice(s).

The nongastric H+-K+-ATPases: molecular and functional properties

The Na-K/H-K-ATPase gene family is divided in three subgroups including the Na-K-ATPases, mainly involved in whole body and cellular ion homeostasis, the gastric H-K-ATPase involved in gastric fluid acidification, and the newly described nongastric H-K-ATPases for which the identification of physiological roles is still in its infancy. The first member of this last subfamily was first identified in 1992, rapidly followed by the molecular cloning of several other members. The relationship between each member remains unclear. The functional properties of these H-K-ATPases have been studied after their ex vivo expression in various functional expression systems, including the Xenopus laevis oocyte, the insect Sf9 cell line, and the human HEK 293 cells. All these H-K-ATPase alpha-subunits appear to encode H-K-ATPases when exogenously expressed in such expression systems. Recent data suggest that these H-K-ATPases could also transport Na+ in exchange for K+, revealing a complex cation transport selectivity. Moreover, they display a unique pharmacological profile compared with the canonical Na-K-ATPases or the gastric H-K-ATPase. In addition to their molecular and functional characterizations, a major goal is to correlate the molecular expression of these cloned H-K-ATPases with the native K-ATPases activities described in vivo. This appears to be more complex than anticipated. The discrepancies between the functional data obtained by exogenous expression of the nongastric H-K-ATPases and the physiological data obtained in native organs could have several explanations as discussed in the present review. Extensive studies will be required in the future to better understand the physiological role of these H-K-ATPases, especially in disease processes including ionic or acid-base disorders.

Temporal responses of oxidative vs. glycolytic skeletal muscles to K+ deprivation: Na+ pumps and cell cations

When K+ output exceeds input, skeletal muscle releases intracellular fluid K+ to buffer the fall in extracellular fluid (ECF) K+. To investigate the mechanisms and muscle specificity of the K+ shift, rats were fed K+-deficient chow for 2-10 days, and two muscles at phenotypic extremes were studied: slow-twitch oxidative soleus and fast-twitch glycolytic white gastrocnemius (WG). After 2 days of low-K+ chow, plasma K+ concentration ([K+]) fell from 4.6 to 3.7 mM, and Na+-K+-ATPase alpha2 (not alpha1) protein levels in both muscles, measured by immunoblotting, decreased 36%. Cell [K+] decreased from 116 to 106 mM in soleus and insignificantly in WG, indicating that alpha2 can decrease before cell [K+]. After 5 days, there were further decreases in alpha2 (70%) and beta2 (22%) in WG, not in soleus, whereas cell [K+] decreased and cell [Na+] increased by 10 mM in both muscles. By 10 days, plasma [K+] fell to 2.9 mM, with further decreases in WG alpha2 (94%) and beta2 (70%); cell [K+] fell 19 mM in soleus and 24 mM in WG compared with the control, and cell [Na+] increased 9 mM in soleus and 15 mM in WG; total homogenate Na+-K+-ATPase activity decreased 19% in WG and insignificantly in soleus. Levels of alpha2, beta1, and beta2 mRNA were unchanged over 10 days. The ratios of alpha2 to alpha1 protein levels in both control muscles were found to be nearly 1 by using the relative changes in alpha-isoforms vs. beta1- (soleus) or beta2-isoforms (WG). We conclude that the patterns of regulation of Na+ pump isoforms in oxidative and glycolytic muscles during K+ deprivation mediated by posttranscriptional regulation of alpha2beta1 and alpha2beta2 are distinct and that decreases in alpha2-isoform pools can occur early enough in both muscles to account for the shift of K+ to the ECF.

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.

Identification of a specific role for the Na,K-ATPase alpha 2 isoform as a regulator of calcium in the heart

It is well accepted that inhibition of the Na,K-ATPase in the heart, through effects on the Na/Ca exchanger, raises the intracellular Ca2+ concentration and strengthens cardiac contraction. However, the contribution that individual isoforms make to this calcium regulatory role is unknown. Assessing the phenotypes of mouse hearts with genetically reduced levels of Na,K-ATPase alpha 1 or alpha 2 isoforms clearly demonstrates different functional roles for these isoforms in vivo. Heterozygous alpha 2 hearts are hypercontractile as a result of increased calcium transients during the contractile cycle. In contrast, heterozygous alpha 1 hearts are hypocontractile. The different functional roles of these two isoforms are further demonstrated since inhibition of the alpha 2 isoform with ouabain increases the contractility of heterozygous alpha 1 hearts. These results definitively illustrate a specific role for the alpha 2 Na,K-ATPase isoform in Ca2+ signaling during cardiac contraction.

Functional role of oxygen-containing residues in the fifth transmembrane segment of the Na,K-ATPase alpha subunit

The functional roles of Tyr771, Thr772, and Asn776 in the fifth transmembrane segment of the Na, K-ATPase alpha subunit were studied using site-directed mutagenesis, expression, and kinetics analysis. Nonconservative replacements Thr772Tyr and Asn776Ala led to reduced Na,K-ATPase turnover. Replacements at these positions (Asn776Ala, Thr772Leu, and Thr772Tyr) also led to high Na-ATPase activity (in the absence of K+). However, Thr772- and Asn776-substituted enzymes showed only small alterations in the apparent Na+ and K+ affinities (K1/2 for Na,K-ATPase activation). Thus, the high Na-ATPase activity does not appear related to cation-binding alterations. It is probably associated with conformational alterations which lead to an acceleration of enzyme dephosphorylation by Na+ acting at the extracellular space (Argüello et al. J. Biol. Chem. 271, 24610-24616, 1996). Nonconservative substitutions at position 771 (Tyr771Ala and Tyr771Ser) produced a significant decrease of enzyme turnover. Enzyme-Na+ interaction was greatly changed in these enzymes, while their activation by K+ did not appear affected. Although the Na+ K1/2 for Na,K-ATPase stimulation was unchanged (Tyr771Ala, Tyr771Ser), the activation by this cation showed no cooperativity (Tyr771Ala, nHill = 0.75; Tyr771Ser, nHill = 0.92; Control, nHill = 2.28). Substitution Tyr771Phe did not lead to a significant reduction in the cooperativity of the ATPase Na+ dependence (nHill = 1.91). All Tyr771-substituted enzymes showed low steady-state levels of phosphoenzyme during Na-activated phosphorylation by ATP. Phosphorylation levels were not increased by oligomycin, although the drug bound and inactivated Tyr771-substituted enzymes. No E1 left and right arrow E2 equilibrium alterations were detected using inhibition by vanadate as a probe. The data suggest that Tyr771 might play a central role in Na+ binding and occlusion without participating in K+-enzyme interactions.

Decreased plasma membrane thiol concentration is associated with increased osmotic fragility of erythrocytes in zinc-deficient rats

Zinc deficiency leads to pathological signs that are related to impaired function of plasma membrane proteins. The purpose of this study was to assess the effect of dietary zinc status on the sulfhydryl (SH) content of erythrocyte plasma membranes and erythrocyte function. Three experiments were performed. In the first, immature male rats were fed for 21 d either a low-zinc (<1.0 mg/kg) diet free choice (-ZnAL), an adequate-zinc (100 mg/kg) diet free choice (+ZnAL), or the adequate-zinc diet limited to the intake of -ZnAL pair-mates (+ZnPF). Tail blood was sampled to measure osmotic fragility and SH concentration of erythrocyte membrane proteins. The zinc-deficient rats were then repleted for 2 d and erythrocytes assayed for fragility and SH content. In the second experiment blood was sampled at 3-d intervals to determine the time course of change in fragility and SH concentration. In the third experiment the SH concentration of erythrocyte band 3 protein and the binding of zinc to isolated plasma membranes were measured. SH concentration decreased from approximately 75 nmol/mg protein to 68 nmol/mg protein during 21 d of depletion and returned to control level within 2 d of repletion. There was an inverse relationship between osmotic fragility and SH concentration of erythrocyte membrane proteins. Maximal decrease in SH occurred within 6 d of consuming the low-zinc diet. The SH content of band 3 protein isolated from deficient rats was also significantly lower than that of pair-fed controls (45 vs. 51 nmol/mg protein). The zinc-binding affinity of plasma membrane proteins tended to be decreased by zinc deficiency. In summary, low-zinc status lowers the plasma membrane SH concentration, and the decreased reducing potential is inversely related to osmotic fragility, and presumably, with impaired volume recovery of erythrocytes.

Affinity labelling with MgATP analogues reveals coexisting Na+ and K+ forms of the alpha-subunits of Na+/K+-ATPase

To test the hypothesis that Na+/K+-ATPase works as an (alpha beta)2-diprotomer with interacting catalytic alpha-subunits, tryptic digestion of pig kidney enzyme, that had been inactivated with substitution-inert MgATP complex analogues, was performed. This led to the demonstration of coexisting C-terminal Na+-like 80-kDa as well as K+-like 60-kDa peptides and N-terminal 40-kDa peptides of the alpha-subunit. To localize the ATP binding sites on tryptic peptides, studies with radioactive MgATP complex analogues were performed: Co(NH3)4-8-N3-ATP specifically modified the E2ATP (low affinity) binding site of Na+/K+-ATPase with an inactivation rate constant (k2) of 12 x 10-3.min-1 at 37 degrees C and a dissociation constant (Kd) of 207 +/- 28 microm. Tryptic digestion of the [gamma32P]Co(NH3)4-8-N3-ATP-inactivated and photolabelled alpha-subunit (Mr = 100 kDa) led, in the absence of univalent cations, to a K+-like C-terminal 60-kDa fragment which was labelled in addition to an unlabelled Na+-like C-terminal 80-kDa fragment. Tryptic digestion of [alpha32P]-or [gamma32P]Cr(H2O)4ATP - bound to the E1ATP (high affinity) site - led to the labelling of a Na+-like 80-kDa fragment besides the immediate formation of an unlabelled K+-like N-terminal 40-kDa fragment and a C-terminal 60-kDa fragment. Because a labelled Na+-like 80-kDa fragment cannot result from an unlabelled K+-like 60-kDa fragment, and because unlabelled alpha-subunits did not show any catalytic activity, the findings are consistent with a situation in which Na+- and K+-like conformations are stabilized by tight binding of substitution-inert MgATP complex analogues to the E1ATP and E2ATP sites. Hence, all data are consistent with the hypothesis that ATP binding induces coexisting Na+ and K+ conformations within an (alphabeta)2-diprotomeric Na+/K+-ATPase.

Isoform-specific effects of charged residues at borders of the M1-M2 loop of the Na,K-ATPase alpha subunit

The Na,K-ATPase is specifically inhibited by the cardiac glycoside, ouabain. Via a largely undefined mechanism, the ouabain affinity of the Na,K-ATPase can be manipulated by mutating the residues at the borders of the first extracellular (M1-M2) loop of the alpha subunit [Price, E. M., Rice, D. A., and Lingrel, J. B. (1990) J. Biol. Chem. 265, 6638-6641]. To address this issue, we compared the effects of two combinations of charged residues at the M1-M2 loop border, R113, D124 and D113,R124 (numbered according to the rat alpha1 subunit), on the ouabain sensitivity of the alpha1 and alpha2 isoforms. We report that ouabain sensitivity is dependent not only upon the identity of the residues at the M1-M2 loop border but also upon the context into which they are introduced. Furthermore, at low concentrations of ATP, the identity of the residues at the M1-M2 loop border affects the regulation of ATP hydrolysis by potassium in an isoform-specific manner. Analysis of chimeric alpha subunits reveals that the effects of potassium are determined primarily by the interaction of the N-terminus and M1-M2 loop with the C-terminal third of the alpha subunit. M1-M2 loop border residues may, therefore, influence ouabain sensitivity indirectly by altering the stability or structure of the intermediate of the Na,K-ATPase catalytic cycle which is competent to bind ouabain.

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.

Ginkgo biloba extract (EGb 761) protects Na,K-ATPase activity during cerebral ischemia in mice

Neuroprotective drugs such as Ginkgo biloba extract (EGb 761) could prevent the ischemia-induced impairment of the Na,K-ATPase activity. In this study, Na,K-ATPase activity and expression, contents in fatty acids and malondialdehyde, an index of lipoperoxidation, were compared in the ipsilateral (ischemic) and the contralateral (unlesioned) cortices after 1 h of unilateral focal cortices cerebral ischemia in the mouse. EGb 761 (110 mg/kg) was administered daily to half of the animals for 10 days before ischemia. Ischemia significantly reduced Na,K-ATPase activity by about 40% and increased malondialdehyde content; EGb 761 pretreatment abolished these effects. The free radical scavenger properties of EGb 761 are a potential mechanism by which Na,K-ATPase injury and lipoperoxidation are prevented.

Microenvironment of the high affinity ATP-binding site of Na+/K+-ATPase is slightly acidic

Fluorescein-5'-isothiocyanate (FITC) was used to study the high-affinity ATP-binding site of Na+/K+-ATPase. The molar ratio of specifically bound FITC per alpha-subunit of Na+/K+-ATPase was found to be 0.5 as followed from pretreatment experiments with another specific E1ATP-inhibitor Cr(H2O)4AdoPP[CH2]P. This indicated an existence of one high affinity ATP-binding site (E1ATP-binding site) in the native (alphabeta)2-diprotomer of Na+/K+-ATPase. Fluorescence dual-excitation ratio of specifically bound FITC revealed that at external pH 7.5, the pH value inside the E1ATP-binding site is 6.95 +/- 0.18. In addition, FITC fluorescence quenching by anti-fluorescein and by iodide choline indicated the limited access of water into the small pocket of the E1ATP-binding site.

Contribution to Tl+, K+, and Na+ binding of Asn776, Ser775, Thr774, Thr772, and Tyr771 in cytoplasmic part of fifth transmembrane segment in alpha-subunit of renal Na,K-ATPase

The sequence Y771TLTSNIPEIT781P in the fifth transmembrane segment of the alpha-subunit of Na,K-ATPase is unique among cation pump proteins. Here, in search of the molecular basis for Na,K specificity, alanine and conservative substitutions were directed to six oxygen-carrying residues in this segment. The contribution of the residues to cation binding was estimated from direct binding of Tl+ [Nielsen, et al. (1998) Biochemistry 37, 1961-1968], K+ displacement of ATP binding at equilibrium, and Na+-dependent phosphorylation from ATP in the presence of oligomycin. As an intrinsic control, substitution of Thr781 had no effect on Tl+(K+) or Na+ binding. There are several novel observations from this work. First, the carboxamide group of Asn776 is equally important for binding Tl+(K+) or Na+, whereas a shift of the position of the carboxamide of Asn776 (Asn776Gln) causes a large depression of Na+ binding without affecting the binding of Tl+(K+). Second, Thr774 is important for Na+ selectivity because removal of the hydroxyl group reduces the binding of Na+ with no effect on binding of Tl+(K+). Removal of the methyl groups of Thr774 or Thr772 reduces binding of both Tl+(K+) and Na+, whereas the hydroxyl group of Thr772 does not contribute to cation binding. Furthermore, the hydroxyl groups of Ser775 and Tyr771 are important for binding both Tl+(K+) and Na+. The data suggest that rotating or tilting of the cytoplasmic part of the fifth transmembrane segment may adapt distances between coordinating groups and contribute to the distinctive Na+/K+ selectivity of the pump.

Affinity labeling of two nucleotide sites on Na,K-ATPase using 2'(3')-O-(2,4,6-trinitrophenyl)8-azidoadenosine 5'-[alpha-32P]diphosphate (TNP-8N3-[alpha-32P]ADP) as a photoactivatable probe. Label incorporation before and after blocking the high affinity ATP site with fluorescein isothiocyanate

ATP and its analogues act on the minimal functional unit of Na, K-ATPase, the alpha beta protomer, with high and low affinity effects. Fluorescein isothiocyanate (FITC) irreversibly blocks the high affinity, or catalytic, ATP site, and yet the surviving K+-phosphatase activity of soluble FITC-modified alphabeta protomers can be photoinactivated by 2'(3')-O-trinitrophenyl (TNP)-8N3-ADP (Ward, D. G., and Cavieres, J. D. (1998) J. Biol. Chem. 273, 14277-14284). We have now used TNP-8N3-[alpha-32P]ADP as a photoaffinity label for Na,K-ATPase. The native enzyme can be photolabeled at 5 microM TNP-8N3-[alpha-32P]ADP, and ATP or FITC treatment prevents labeling of the alpha chain. At 25 microM, however, TNP-8N3-[alpha-32P]ADP can be incorporated in the FITC-modified alpha chain, concurrently with the inactivation of the K+-phosphatase activity, to an extrapolated level of 0.5-1.2 mol of 32P-probe per mol of alpha chain. Photoinactivation and labeling are prevented by TNP-ADP, vanadate, or strophanthidin and are promoted by Na+ or Mg2+, but not K+. The cation effects suggest that the fluorescein-modified enzyme incorporates the TNP-8N3-[alpha-32P]ADP. Mg complex preferentially, and the free probe when in the E1 enzyme form and after occupation of a low-affinity Na+ site. Partial trypsinolysis reveals that the point of TNP-8N3-[alpha-32P]ADP attachment is on the C-terminal 58-kDa fragment of the FITC-modified alpha chain. The affinity labeling of the fluorescein enzyme by TNP-8N3-[alpha-32P]ADP endorses the view that two nucleotide sites can be occupied simultaneously in each alpha subunit of Na,K-ATPase.

Erythrosin 5'-isothiocyanate labels Cys549 as part of the low-affinity ATP binding site of Na+/K+-ATPase

The high-affinity E1ATP site of Na+/K+-ATPase labeled with fluorescein 5'-isothiocyanate and its E2ATP site labeled with erythrosin 5'-isothiocyanate (ErITC), as was shown recently [Linnertz et al. (1998) J. Biol. Chem. 273, 28813-28821], reside on separate and adjacent catalytic alpha subunits. This paper provides evidence that specific labeling of the E2ATP binding site with ErITC resulted in a modification of the Cys549 residue in the tryptic fragment with the sequence Val545-Leu-Gly-Phe-Cys549-His550. Hence, Cys549 is part of or close to the low-affinity E2ATP binding site of Na+/K+-ATPase.

Possible implication for an indirect interaction between basic fibroblast growth factor and (Na,K)ATPase

The (Na,K)ATPase is responsible for generating the ionic gradients and membrane potentials by the exchange of intracellular Na+ for K+. It has been recently shown that (Na,K)ATPase is involved in the exocytic pathway of basic fibroblast growth factor (bFGF), although it is not known that bFGF is secreted to the outside of cell through direct interaction with (Na,K)ATPase. To understand the role for (Na,K)ATPase in the secretory pathway of bFGF, we have sought to identify the cytoplasmic domains of the alpha 1 isoform of (Na,K)ATPase interacting with bFGF by yeast two-hybrid system. We have also investigated the interaction between the alpha 2 isoform of (Na,K)ATPase and bFGF to find out whether the interaction is isoform-specific. We found that none of the cytoplasmic domains of (Na,K)ATPase isoforms interacted with bFGF. The result suggests that the interaction between bFGF and (Na,K)ATPase might be indirect, thus requiring other proteins which are involved in the formation of protein complexes for the interaction, although we cannot exclude the possibility that the interaction requires the element of the whole alpha subunit structure that was not present in the isolated alpha subunit cytoplasmic domains.

Adrenal-dependent modulation of the catalytic subunit isoforms of the Na+-K+-ATPase in aorta

Na+-K+-ATPase gene expression and activity were studied in aortas from adrenalectomized (ADX) rats and ADX rats with deoxycorticosterone supplement (ADX-DOCA). Northern analysis of RNA from ADX rats revealed a significant decrease in alpha2-mRNA levels (38.5 +/- 8.3% of control, P < 0.01) that was prevented by DOCA (P < 0.05). A decrease to 55.8 +/- 7.7% in alpha2-isoform protein was observed 8 days after adrenal removal (P < 0.05); DOCA reversed this effect (90.8 +/- 10.5%). Adrenalectomy induced a decrease of 68.5 +/- 4.5% in beta1-mRNA (P < 0.01) and 52.7 +/- 8.3% in ADX-DOCA rats (P < 0.01). Also, a reduction in beta1-isoform protein that was not prevented by DOCA was detected after adrenalectomy (47.1 +/- 11%, P < 0.01). In contrast, no differences in alpha1-mRNA or -protein levels were observed. Vascular sodium pump activity was reduced to 59.8 +/- 4.6% of control values after adrenalectomy (P < 0.01); this reduction was reversed by DOCA. Our data indicate that corticosteroids regulate Na+-K+-ATPase isoform expression and activity in vascular tissue in vivo, suggesting a mineralocorticoid-dependent modulation of alpha2-Na+-K+-ATPase gene expression in aorta, with beta1-isoform expression dependent on the presence of glucocorticoids.

Expression of alpha- and beta-subunits and activity of Na+K+ ATPase in pig thyroid cells in primary culture: modulation by thyrotropin and thyroid hormones

Na+ K+ ATPase located at the basolateral pole of thyroid epithelial cells, contributes to thyroid hormone synthesis by generating the driving force for the uptake of the substrate, iodide. We have investigated whether the expression of the alpha- and beta-subunits and activity of Na+ K+ ATPase were subjected to variations in response, (a) to TSH, that controls the expression of differentiation in thyroid cells and (b) to thyroid hormones as potential autocrine factors. Studies were carried out on pig thyroid cells cultured (a) without TSH to obtain thyroid cell monolayers (TCM) in basal state or (b) with TSH in the form of cell monolayers (TCM-T) or as reconstituted thyroid follicles (RTF). Iodide uptake activity, thyroperoxidase protein and thyroglobulin mRNA taken as parameters of thyroid cell differentiation were 6 to 25-fold higher in RTF and TCM-T than in TCM. Western blot analyses of Na+ K+ ATPase subunits revealed that the alpha-subunit (105 kDa) content of TCM-T and RTF was similar but 8-fold higher than that of TCM. In contrast, the beta-subunit (50 kDa) content of TCM-T and RTF was only about twice that of TCM. Similar relative variations were observed at the mRNA level for both alpha- and beta-subunits. Na+ K+ ATPase activity was only 40% higher in RTF and TCM-T than in TCM. A 48 h treatment of RTF by either T4 or T3 (1-100 nM) induced a 3-fold increase of the alpha-subunit but did neither alter the beta-subunit nor the Na+ K+ ATPase activity. In conclusion, Na+ K+ ATPase activity and the level of expression of its beta-subunit, known to control the assembly and targetting of alpha-beta oligomers and thus the amount of functional sodium pump at the plasma membrane, are only moderately altered when thyroid cells undergo major changes in their differentiation status. Our data show that the expression of the alpha-subunit of Na+ K+ ATPase by thyroid cells is up-regulated by TSH and thyroid hormones.