Rat hepatic (Na+, K+)-ATPase: alpha-subunit isolation by immunoaffinity chromatography and structural analysis by peptide mapping

Activation of the plasma membrane H+-ATPase of Saccharomyces cerevisiae by glucose is mediated by dissociation of the H+-ATPase-acetylated tubulin complex

In the yeast Saccharomyces cerevisiae, plasma membrane H(+)-ATPase is activated by d-glucose. We found that in the absence of glucose, this enzyme forms a complex with acetylated tubulin. Acetylated tubulin usually displays hydrophilic properties, but behaves as a hydrophobic compound when complexed with H(+)-ATPase, and therefore partitions into a detergent phase. When cells were treated with glucose, the H(+)-ATPase-tubulin complex was disrupted, with two consequences, namely (a) the level of acetylated tubulin in the plasma membrane decreased as a function of glucose concentration and (b) the H(+)-ATPase activity increased as a function of glucose concentration, as measured by both ATP-hydrolyzing capacity and H(+)-pumping activity. The addition of 2-deoxy-d-glucose inhibited the above glucose-induced phenomena, suggesting the involvement of glucose transporters. Whereas total tubulin is distributed uniformly throughout the cell, acetylated tubulin is concentrated near the plasma membrane. Results from immunoprecipitation experiments using anti-(acetylated tubulin) and anti-(H(+)-ATPase) immunoglobulins indicated a physical interaction between H(+)-ATPase and acetylated tubulin in the membranes of glucose-starved cells. When cells were pretreated with 1 mm glucose, this interaction was disrupted. Double immunofluorescence, observed by confocal microscopy, indicated that H(+)-ATPase and acetylated tubulin partially colocalize at the periphery of glucose-starved cells, with predominance at the outer and inner sides of the membrane, respectively. Colocalization was not observed when cells were pretreated with 1 mm glucose, reinforcing the idea that glucose treatment produces dissociation of the H(+)-ATPase-tubulin complex. Biochemical experiments using isolated membranes from yeast and purified tubulin from rat brain demonstrated inhibition of H(+)-ATPase activity by acetylated tubulin and concomitant increase of the H(+)-ATP ase-tubulin complex.

Involvement of 3Na+/2K+ ATP-ase and Pi-3 kinase in the response of skeletal muscle ATP-sensitive K+ channels to insulin

The modulation of ATP-sensitive K+ channel (K(ATP)) by insulin plays a role in neuromuscular disorders associated to altered K+ homeostasis. However, the mechanisms by which insulin modulates K(ATP) channels are not known. Here, the insulin-dependent 3Na+/2K+ ATP-ase and Pi-3 kinase pathways were explored by using patch-clamp techniques. High and low affinity inhibition of K(ATP) channels by ouabain was observed in the insulin-stimulated and resting fibers, respectively. The 9A5 antibody directed against the alpha1-subunit of the pump inhibited the K(ATP) channel in the resting fibers but fails to inhibit it in the insulin-stimulated fibers. In contrast, the RT2NKATPabr, an alpha2-subunit specific antibody, inhibited the K(ATP) channels in the insulin-stimulated fibers failing to inhibit it in the resting fibers. The insulin-dependent stimulation of K(ATP) channel was prevented by Pi-3 kinase inhibitors Wortmannin and LY294002. In conclusion, insulin stimulating the 3Na+/2K+ ATP-ase activates K(ATP) channels through a membrane-delimited interaction thus controlling the K+ homeostasis. The Pi-3 kinase is the intracellular insulin signal linking the glucose homeostasis to the K(ATP) channel.

Microwave cell death: Immunohistochemical and enzyme histochemical evaluation

In Japan, microwave coagulation therapy (MCT) has been used for the management of primary and metastatic liver cancer. Needle biopsy examination from the lesion has frequently shown the presence of nucleated cancer cells in histopathological examinations, prompting the conclusion that cancer cells are not completely eliminated by microwave therapy, whereas computed tomography and ultrasonography examinations show tumor regression. To determine whether microwave-treated tissue contains functionally viable cells, an examination of the Na+-K+-ATPase protein and its activity using immunohistochemical and enzyme histochemical methods were carried out in microwave-treated rat liver. Four concentric, morphologically identifiable zones around the microwave probe needle appeared 2 days after treatment. Zone A, which was between the innermost spongy zone and the outer necrotic zone, contained only slight morphological alterations in the hepatocytes, which had slightly hyperchromatic nuclei and mildly eosinophilic cytoplasm. The hepatocytes in zone A were found to be positive for the Na+-K+-ATPase antigenicity but negative for enzyme activity, indicating that zone A was undergoing cell death, although morphologically this was not discernible. This type of cell death caused by microwave treatment is morphologically different from previously known types of cell death, either oncosis or apoptosis.

Extracellular matrix and Na+,K+-ATPase in human corneas following cataract surgery: comparison with bullous keratopathy and Fuchs' dystrophy corneas

PURPOSE

To examine the distribution of extracellular matrix (ECM) and basement membrane (BM) components and of Na+,K+-ATPase in postcataract surgery (PCS) corneas. These corneas were from patients who never developed pseudophakic or aphakic bullous keratopathy (PBK/ABK) after cataract surgery. PCS corneas were compared with PBK/ABK and Fuchs' dystrophy corneas.

METHODS

The distribution of PBKIABK ECM and BM markers and of all three Na+,K+-ATPase alpha subunits was studied by immunofluorescence in 10 healthy, 11 PCS, 16 PBK/ABK, and 12 Fuchs' dystrophy corneas.

RESULTS

Fibrotic ECM proteins, tenascin-C and fibrillin-1, were found in only 1 of 10 healthy and in 2 of 11 PCS corneas. In contrast, these proteins were expressed in all PBK/ABK and more than half of the Fuchs' dystrophy corneas. BM components in PCS corneas were altered to a greater extent (40-60%), especially fibronectin and laminin-10. A decreased epithelial immunostaining for Na+,K+-ATPase alpha subunits was seen in approximately 40% of PCS corneas and in approximately two thirds of PBK/ABK and Fuchs' dystrophy corneas. However, the endothelial staining was normal in all groups.

CONCLUSIONS

Because tenascin-C and fibrillin-1 were mostly found in diseased but not in PCS corneas, their expression may be related to later, clinical stages of corneal edema development. However, BM components abnormal in PBK/ABK and Fuchs' dystrophy corneas were also altered in PCS corneas without clinical evidence of ocular disease. This may result from subclinical corneal changes resulting from cataract surgery, lens removal, exposure to the intraocular lens, or endothelial cell damage. Alterations of epithelial Na+,K+-ATPase point to the importance of epithelial changes in the development of corneal edematous diseases.

Brain plasma membrane Na+,K+-ATPase is inhibited by acetylated tubulin

Membranes from brain tissue contain tubulin that can be isolated as a hydrophobic compound by partitioning into Triton X-114. The hydrophobic behavior of this tubulin is due to the formation of a complex with the alpha-subunit of Na+,K+-ATPase. In the present work we show that the interaction of tubulin with Na+K+-ATPase inhibits the enzyme activity. We found that the magnitude of the inhibition is correlated with: (1) concentration of the acetylated tubulin isoform present in the tubulin preparation used, and (2) amount of acetylated tubulin isoform isolated as a hydrophobic compound. In addition, some compounds involved in the catalytic action of Na+K+-ATPase were assayed to determine their effects on the inhibitory capability of tubulin on this enzyme. The inhibitory effect of tubulin was only slightly decreased by ATP at relatively low nucleotide concentration (0.06 mM). NaCl (1-160 mM) and KCl (0.2-10 mM) showed no effect whereas inorganic phosphate abolished the inhibitory effect of tubulin in a concentration-dependent manner.

Tissue-specific expression of the Na,K-ATPase beta3 subunit. The presence of beta3 in lung and liver addresses the problem of the missing subunit

The Na,K-ATPase belongs to a family of P-type ion-translocating ATPases sharing homologous catalytic subunits (alpha) that traverse the membrane several times and contain the binding sites for ATP and cations. In this family, only Na,K- and H,K-ATPases have been shown to have a second subunit, a single-span glycoprotein called beta. Recently a new isoform (beta3) has been identified in mammals. Here we describe structural features and tissue distribution of the beta3 protein, utilizing an antiserum specific for its N terminus. beta3 was the only beta detected in Na,K-ATPase purified from C6 glioma. Treatment with N-glycosidase F confirmed that beta3 is a glycoprotein containing N-linked carbohydrate chains. Molecular masses of the glycosylated protein and core protein were estimated to be 42 and 35 kDa, respectively, which are different from those of the beta1 and beta2 subunits. Detection of beta subunits has historically been difficult in certain tissues. Sensitivity was improved by deglycosylating, and expression was evaluated by obtaining estimates of beta3/alpha ratio. The proportion of beta3 protein in the rat was highest in lung and testis. It was also present in liver and skeletal muscle, whereas kidney, heart, and brain contained it only as a minor component of the Na,K-ATPase. In P7 rat, we found skeletal muscle and lung Na,K-ATPase to be the most enriched in beta3 subunit, whereas expression in liver was very low, illustrating developmentally regulated changes in expression. The substantial expression in lung and adult liver very likely explains long-standing puzzles about an apparent paucity of beta subunit in membranes or in discrete cellular or subcellular structures.

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

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

Increased hepatic Na,K-ATPase activity during hepatic regeneration is associated with induction of the beta1-subunit and expression on the bile canalicular domain

Cellular and molecular mechanisms regulating the activity of the sodium pump or Na,K-ATPase during proliferation of hepatocytes following 70% liver resection have not been defined. Na,K-ATPase may be regulated by synthesis of its alpha- and beta-subunits, by sorting to either the sinusoidal or apical plasma membrane domains, or by increasing membrane lipid fluidity. This study investigated the time course of changes during hepatic regeneration for Na, K-ATPase activity, lipid composition and fluidity, and protein content of liver plasma membrane subfractions. As early as 4 h after hepatic resection, Na,K-ATPase activity was increased selectively in the bile canalicular fraction. It reached a new steady state at 12 h and remained elevated for 2 days. Although hepatic regeneration was associated with a reduced cholesterol/phospholipid molar ratio and increased fluidity, measured with two different probes, these changes in lipid metabolism were in the sinusoidal membrane domain. The Na,K-ATPase beta1-subunit, but not the alpha1-subunit, was increased selectively at the bile canalicular surface as shown by immunoblotting of liver plasma membrane subfractions and the morphological demonstration at both the light and electron microscopic levels. Furthermore, cycloheximide blocked the rise in beta1-subunit mRNA levels. Since the time course for beta1-subunit accumulation was similar to that for activation of Na,K-ATPase activity, this change implicated the beta1-subunit in activating sodium pump activity.

Hepatic Na(+)-K(+)-ATPase enzyme activity correlates with polarized beta-subunit expression

We have examined underlying causes for observations made in hepatocytes in which catalytic subunits of Na(+)-K(+)-ATPase are found both in bile canalicular (apical) and sinusoidal (basolateral) membrane domains, whereas functional activity is associated preferentially with sinusoidal membrane sites. In a series of parallel studies, we determined by both light and electron microscopy that Na(+)-K(+)-ATPase alpha-subunits were localized to both membrane domains of hepatocytes. With the use of purified liver plasma membrane subfractions, ouabain inhibition curves demonstrated similar inhibition constants (inhibition constant 10(-5) M), and immunoblots using alpha 1-, alpha 2-, and alpha 3-polyclonal and monoclonal antibodies demonstrated antigenic sites predominantly for alpha 1 in both membrane fractions. Also, Northern blot hybridization analysis revealed only the alpha 1-isoform in hepatocytes. In contrast to the bipolar distribution of the alpha 1-subunit, the beta-subunit was identified only at the sinusoidal surface using fluorescence labeling with a monoclonal antibody. The beta 1-isoform was demonstrated by Northern blot analysis and was present predominantly at the sinusoidal domain by immunoblotting with polyclonal antibodies. In addition to the bipolar distribution of alpha 1, immunoblotting of liver plasma membrane subfractions demonstrated a symmetrical distribution of fodrin, ankyrin, actin, and E-cadherin at both domains. These results suggest that functionally competent alpha/beta-complexes form at the sinusoidal domain, whereas only alpha 1-subunits are present at the apical pole.

Purification and immunochemical properties of human Na,K-ATPase alpha subunits and formic acid-derived polypeptide fragments

In this study, alpha (alpha) isoform proteins were purified from the partially purified Na,K-ATPase by SDS-PAGE and electroelution. Peptide mapping showed subtle biochemical differences between alpha subunit proteins of rat and human origin. The purified alpha proteins were treated with formic acid, the cleaved polypeptide fragments were separated by SDS-PAGE, the bands corresponding to 40, 50, and 60 kDa were excised, and the proteins were electroeluted. The purified 40, 50, and 60 kDa polypeptides were essentially homogeneous, and were used for preparation of polyclonal antibodies in rabbits. The antisera to alpha proteins (R alpha) and 60 & 40 kDa polypeptides (R60 & R40) were obtained and characterized by Western blotting. All three antisera were highly specific, since they cross-reacted with only the 100 kDa bands of the crude brainstem homogenates, of the axolemma, and of the cerebral cortex synaptosomes and microsomes. R alpha and R40 were successfully used for immunohistochemical staining of fibers in the white matter of the human brain frontal cortex. These antisera were not isoform-specific, they cross-reacted with 40, 50, and 60 kDa polypeptides as well as the three alpha bands.

Determination of Na(+)-K(+)-ATPase alpha- and beta-isoforms and kinetic properties in mammalian liver

While Western blot analysis clearly revealed the presence of the alpha- and beta-subunits of Na(+)-K(+)-ATPase in a variety of rat tissues, beta was not readily detectable in liver. This observation was consistent with a previous report indicating that Na(+)-K(+)-ATPase immunoprecipitated from rat liver gives no clear evidence for the presence of a beta-subunit (Hubert et al. Biochemistry 25: 4156-4163, 1986). However, Western blot analysis of density gradient-purified lamb and rat liver microsomes showed the presence of a protein with an approximate molecular mass of 42 kDa that was immunoreactive with beta-specific polyclonal antibodies as well as beta-directed monoclonal antibodies. Deglycosylation of this protein by N-glycosidase F generated a core protein (beta c, M(r) approximately 32,000) that had the identical electrophoretic mobility as the beta c protein of the purified kidney enzyme. Isoform-specific monoclonal and synthetic peptide-directed polyclonal antibodies were used to demonstrate the presence of only the alpha 1- and beta 1-proteins in the liver and the presence of beta 2 in rat brain. Functional studies then showed that although both rat and lamb liver enzymes had sensitivities to cardiac glycoside inhibition similar to that of their corresponding kidney enzyme, the lamb liver enzyme had higher affinities for Na+, K+, and ATP than the kidney enzyme.

Evidence for coordinate regulation of the A system for amino acid transport and the mRNA for the alpha 1 subunit of the Na+,K(+)-ATPase gene in Chinese hamster ovary cells

Previous work suggested that the structural gene for the A system transporter and the mRNA for the alpha subunit of the Na+,K(+)-ATPase in Chinese hamster ovary cells CHO-K1 [wild type (WT)] are coordinately controlled by regulatory gene R1. This conclusion was based on analysis of a mutant for the A system, alar4. This mutant had a constitutive level of A system transport activity equal to the level found in derepressed WT cells and a 4 times increase in abundance of the alpha 1 subunit of Na+,K(+)-ATPase mRNA over that found in repressed WT. The level of Na+ per cell in alar4 was not significantly greater than that found in the WT. To further characterize the likely coregulation of both genes, we have studied the A system activity and Na+,K(+)-ATPase mRNA alpha 1-subunit levels in cells grown under various conditions that result in repression or derepression of the A system in the WT. System A activity increased up to 2-3 times the basal transport rate (repressed state) and Na+,K(+)-ATPase mRNA alpha 1-subunit levels showed a 3-fold increase after amino acid starvation (derepressed state). These changes occurred along with a decrease in intracellular Na+ levels. N-Methyl-alpha-aminoisobutyric acid and beta-alanine, previously shown to be corepressors for the A system, prevented to a similar extent A system derepression and Na+,K(+)-ATPase mRNA alpha 1-subunit accumulation. On the other hand, phenylalanine and lysine, amino acids that are not corepressors of the A system, failed to significantly prevent derepression of both genes. Hybrids between the WT and alar4 have the phenotype of the WT when grown under repressed conditions. These results give further support to the proposition that both the A system transporter and mRNA for the alpha 1 subunit of the Na+,K(+)-ATPase are coordinately controlled by regulatory gene R1 and elevated Na+ concentrations are not involved. No Na+,K(+)-ATPase activity was detected in derepressed cells. Activity was restored by the addition of monensin. However, this activity was no greater than that obtained in repressed cells. Indications are that the reduced Na+ content in derepressed cells inhibits Na+,K(+)-ATPase activity and that conditions that favored derepression do not allow for de novo synthesis of the Na+,K(+)-ATPase.

Thyroid hormone regulation of Na,K-ATPase subunit-mRNA expression in neonatal rat myocardium

Regulation of Na,K-ATPase mRNA alpha isoform and mRNA beta expression by thyroid hormone (T3) in neonatal rat myocardium was examined. In euthyroid neonates between ages of 2 and 5 days, mRNA alpha 1, mRNA alpha 3, and mRNA beta 1 abundances were nearly constant while mRNA alpha 2 was undetectable. During the interval between postnatal days 5 and 15, mRNA alpha 3 decreased to negligible levels and mRNA alpha 2 became expressed and increased in abundance to account for approximately 20% of the mRNA alpha pool by the 15th postnatal day. To examine the effect of T3 on this developmental program, neonates were injected with 75 micrograms T3/100 g body weight or diluent alone on the second and third postnatal days and myocardial Na,K-ATPase subunit-mRNA abundances were determined on the third and fourth postnatal days. Because T3 treatment increased the RNA/DNA ratios of myocardial tissue, the subunit-mRNA abundances were normalized per unit DNA. Following 24 and 48 hr of T3 treatment, the abundances of mRNA alpha 1, mRNA alpha 3, and mRNA beta 1 increased, while mRNA alpha 2 continued to remain undetectable during the 2-day interval between the second to fourth postnatal days. It is concluded that T3 augments the abundance of Na,K-ATPase subunit mRNAs that are already being expressed in the neonatal rat myocardium. The results further suggest that T3 does not act as a "molecular switch" in the developmental expression of the mRNA alpha isoforms in rat myocardium during the first four postnatal days.

Na,K-ATPase isoform expression in sheep red blood cell precursors

Isoform expression of mammalian red cell Na,K-ATPase was analyzed using messenger RNA isolated from red cell precursor-enriched bone marrow of anemic sheep. Expression of the catalytic alpha subunit was analyzed using rat isoform-specific cDNA probes and expression of the beta 1 subunit, using a sheep beta 1-specific cDNA probe. RNA isolated from sheep kidney and brain were analyzed concurrently. In the red cell, as in the kidney, messenger RNA encoding only one isoform (alpha 1) of the catalytic subunit is detected; neither of the other isoforms (alpha 2 or alpha 3) could be detected. This holds true for bone marrow of sheep of either the low potassium or high potassium phenotype. Relative to the expression of alpha 1, beta subunit-specific message (beta 1) was extremely low in the red cell compared to either kidney (less than 5%) or brain (less than 3%). Using a rat cDNA probe specific for a beta 1-like subunit, beta 2, message was detected in brain but not in either kidney or bone marrow.

Thyroidal enhancement of rat myocardial Na,K-ATPase: preferential expression of alpha 2 activity and mRNA abundance

In hypothyroid rat myocardium, the low-ouabain-sensitivity Na,K-ATPase activity had a KI = 10(-4) M and accounted for approximately 95% of the enzyme activity, while the high-ouabain-sensitivity activity contributed approximately 5% to the total activity, with a KI = 3 x 10(-7) M. mRNA alpha 1 was 7.2- and 5.5-fold more abundant than mRNA alpha 2 and mRNA beta, respectively, in hypothyroid ventricles while mRNA alpha 3 was undetectable. Administration of T3 increased total Na,K-ATPase activity 1.6-fold; the low-ouabain-sensitivity activity increased 1.5-fold while high-ouabain-sensitivity activity was stimulated 3.2-fold. T3 increased the number of high-affinity ouabain-binding sites 2.9-fold with no change in Kd (approximately 2 x 10(-7) M). The abundances of mRNA alpha 1, mRNA alpha 2, and mRNA beta (per unit RNA) following T3 treatment increased 3.6-, 10.6-, and 12.7-fold, respectively. The larger increments in subunit mRNA abundances than in Na,K-ATPase activity suggests the involvement of translational and/or post-translational regulatory steps in Na,K-ATPase biogenesis in response to T3. It is concluded that T3 enhances myocardial Na,K-ATPase subunit mRNA abundances and Na,K-ATPase activity, and that the expression of the high- and low-ouabain-sensitivity activities are probably a reflection of the abundances of the alpha 2 and alpha 1 isoforms, respectively. The physiological role played by the beta subunit remains uncertain.

Thyroid hormone induction of Na(+)-K(+)-ATPase and its mRNAs in a rat liver cell line

The expression of mRNAs encoding the alpha- and beta-subunits of Na(+)-K(+)-ATPase (Na(+)-K+ pump) was examined in a rat liver cell line, Clone 9, in various thyroidal states. Northern blot analysis of total RNA isolated from cells incubated in hypothyroid serum-containing medium revealed the expression of mRNAs encoding Na(+)-K(+)-ATPase alpha 1-(mRNA alpha 1) and beta- (mRNA beta) subunits; mRNAs encoding the alpha 2- and alpha 3-subunits were undetectable. There was a discrepancy in the abundance of mRNA alpha 1 relative to mRNA beta such that mRNA alpha 1 exceeded the sum of the multiple mRNA beta bands by approximately 35-fold. 3,3',5-Triiodothyronine (T3) produced a coordinate augmentation of mRNA alpha 1 and mRNA beta contents that was demonstrable within 2 h and preceded the stimulation of Na(+)-K(+)-ATPase activity. After incubation of cells with T3 for 48 h, Na(+)-K(+)-ATPase activity was stimulated by 1.32-fold, whereas mRNA alpha 1 and mRNA beta abundances were increased 1.46- and 2.87-fold, respectively. Treatment of cells for 6 h with 10 micrograms/ml cycloheximide, a concentration sufficient to inhibit protein synthesis by 95%, elicited a 3.5- and 5.1-fold increase in mRNA alpha 1 and mRNA beta content, respectively. Cycloheximide abrogated the stimulatory effect of T3 on mRNA beta abundance, whereas the T3-induced increase in mRNA alpha 1 content was not prevented.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification of membrane proteins

As stated at the beginning of this chapter, it is not possible to present a single step-by-step protocol for the purification of all membrane proteins. We have discussed many of the individual techniques employed to purify membrane proteins and the problems associated with their application. A successful purification protocol will require the use of a combination of these techniques. It should be clear from the examples given, however, that affinity chromatography is by far the most useful technique available. The greatest single obstacle to performing a successful purification is the ability to maintain solubilized proteins in fully dispersed monomeric micelles without inactivating the protein. The best combination of techniques and the optimal conditions for their use can be determined only by trial and error. This process should become easier as a greater number of examples become available and as greater use is made of HPLC and FPLC techniques.

Human Na+,K+-ATPase genes. Beta-subunit gene family contains at least one gene and one pseudogene

The existence of a chromosome gene family containing at least one gene and one pseudogene was shown for the Na+,K+-ATPase beta-subunit. A partial structure of the beta 1-gene was determined, the coding part of which was completely homologous to cDNA of the Na+,K+-ATPase beta I-subunit from HeLa cells. The region encoding the putative protein transmembrane domain was shown to be bordered by two introns. The structure of a pseudogene (beta psi) was determined. This pseudogene is processed and contains multiple stop codons. Its homology to the beta I-subunit cDNA from HeLa cells is about 88%.

alar4, a constitutive mutant of the A system for amino acid transport, has increased abundance of the Na+,K+-ATPase and mRNA for alpha 1 subunit of this enzyme

A constitutive mutant, alar4, for the A system of amino acid transport, has increased activity and amount of the A system. This is accompanied by increased sensitivity to ouabain, as measured by efficiency of plating, and increased activity and abundance of the Na+,K+-ATPase that is present in the parental cell line, CHO-K1 (wild type). The latter was shown by increases in (i) ouabain-inhibitable 86Rb uptake in intact cells, (ii) ouabain-inhibitable ATPase activity in mixed membrane vesicles, and (iii) number of ouabain-binding sites and by similar Kd values for ouabain binding and K1/2 for ouabain inhibition of Na+,K+-ATPase as compared to the wild type. The increase in abundance of the Na+ pump is associated with a 4-fold increase in abundance of the mRNA for the alpha 1 subunit of the Na+,K+-ATPase. We could not detect mRNA for alpha 2 or alpha 3 or for the beta subunits. The increase in abundance of the A system and Na+,K+-ATPase is associated with a negligible increase in intracellular Na+ concentration. We propose that the increase in the abundance of the A system and the Na+,K+-ATPase is the result of a mutation in regulatory gene R1 that controls the A system and the Na+,K+-ATPase and is not due to a primary effect of a possible initial increase in Na+ concentration.

A monoclonal antibody against horse kidney (Na+ + K+)-ATPase inhibits sodium pump and E2K to E1 conversion of (Na+ + K+)-ATPase from outside of the cell membrane

Monoclonal antibodies against horse kidney outer medulla (Na+ + K+)-ATPase were prepared. One of these antibodies (M45-80), was identified as an IgM, recognized the alpha subunit of the enzyme. M45-80 had the following effects on horse kidney (Na+ + K+)-ATPase: (1) it inhibited the enzyme activity by 50% in 140 mM Na+ and by 80% in 8.3 mM Na+; (2) it increased the Na+ concentration necessary for half-maximal activation (K0.5 for Na+) from 12.0 to 57.6 mM, but did not affect K0.5 for K+; (3) it slightly increased the K+-dependent p-nitrophenylphosphatase (K-pNPPase) activity; (4) it inhibited phosphorylation of the enzyme with ATP by 30%, but did not affect the step of dephosphorylation; and (5) it enhanced the ouabain binding rate. These data are compatible with a stabilizing effect on the E2 form of (Na+ + K+)-ATPase. M45-80 was concluded to bind to the extracellular surface of the plasmamembrane, based on the following evidence: (1) M45-80 inhibited by 50% the ouabain-sensitive 86Rb+ uptake in human intact erythrocytes from outside of the cells; (2) the inhibition of (Na+ + K+)-ATPase activity in right-side-out vesicles of human erythrocytes was greater than that in inside-out vesicles; and (3) the fluorescence intensity due to FITC-labeled rabbit anti-mouse IgM that reacted with M45-80 bound to the right-side-out vesicles was much greater than that in the case of the inside-out vesicles.

Biochemical localization of hepatic surface-membrane Na+,K+-ATPase activity depends on membrane lipid fluidity

Membrane proteins of transporting epithelia are often distributed between apical and basolateral surfaces to produce a functionally polarized cell. The distribution of Na+,K+-ATPase [ATP phosphohydrolase (Na+/K+-transporting), EC 3.6.1.37] between apical and basolateral membranes of hepatocytes has been controversial. Because Na+,K+-ATPase activity is fluidity dependent and the physiochemical properties of the apical membrane reduces its fluidity, we investigated whether altering membrane fluidity might uncover cryptic Na+,K+-ATPase in bile canalicular (apical) surface fractions free of detectable Na+,K+-ATPase and glucagon-stimulated adenylate cyclase activities. Apical fractions exhibited higher diphenylhexatriene-fluorescence polarization values when compared with sinusoidal (basolateral) membrane fractions. When 2-(2-methoxyethoxy)ethyl 8-(cis-2-n-octylcyclopropyl)octanoate (A2C) was added to each fraction, Na+,K+-ATPase, but not glucagon-stimulated adenylate cyclase activity, was activated in the apical fraction. In contrast, further activation of both enzymes was not seen in sinusoidal fractions. The A2C-induced increase in apical Na+,K+-ATPase approached 75% of the sinusoidal level. Parallel increases in apical Na+,K+-ATPase were produced by benzyl alcohol and Triton WR-1339. All three fluidizing agents decreased the order component of membrane fluidity. Na+,K+-ATPase activity in each subfraction was identically inhibited by the monoclonal antibody 9-A5, a specific inhibitor of this enzyme. These findings suggest that hepatic Na+,K+-ATPase is distributed in both surface membranes but functions more efficiently and, perhaps, specifically in the sinusoidal membranes because of their higher bulk lipid fluidity.

Increased abundance of Na+-K+-ATPase mRNAs in response to low external K+

Exposure of ARL 15 cells, an established line from adult rat liver, to external K+ concentrations less than 1 mM for 24 h increases Na+-K+ pump abundance (Na+-K+-ATPase) (J. Gen. Physiol. 87:591-606, 1986). We found that treatment of confluent monolayers of ARL 15 cells with low-K+ medium (0.65 mM) caused a 100% increase in total RNA content per plate after 24 h, as well as a 25% increase in DNA and protein content per plate. Concomitant with this growth effect, low-K+ exposure for 6 h elicited 60% increases in mRNA alpha and mRNA beta, the mRNAs that encode the constituent subunits of the Na+-K+-ATPase, in a polyadenylated RNA fraction. At 24 h, however, the abundance of mRNA alpha increased by 290%, whereas mRNA beta increased by only 70%. Moreover, in both control and low-K+-treated cells, mRNA alpha was 30-fold or more greater in abundance than mRNA beta. This discrepancy in abundance was also present in rat liver, but not in cultured MDCK cells. The differences in abundance of mRNA alpha and mRNA beta suggest that the liver may have an unusual subunit composition or biosynthetic mechanism. Nevertheless, the increases in the abundance of mRNA alpha and mRNA beta are sufficient to account for the observed 70-100% increase in Na+-K+-ATPase activity in response to low external K+.

Calcium-mobilizing hormones and phorbol myristate acetate mediate heterologous desensitization of the hormone-sensitive hepatic Na+/K+ pump

The Na+/K+ pump in rat hepatocytes is stimulated in response to Ca2+-mobilizing hormones such as [arginine]vasopressin (AVP), angiotensin II and adrenaline, as well as tumour promoters such as 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (PMA). The ability of these agents to increase cellular contents of diacylglycerol and activate protein kinase C may be necessary to observe this response. In the present work, ouabain-sensitive 86Rb+ uptake was studied in isolated rat hepatocytes to help to explain why stimulation of the Na+/K+ pump by Ca2+-mobilizing hormones and tumour promoters is not temporally sustained relative to other hormone responses. A transient stimulation (3-4 min) of the Na+/K+ pump was observed in hepatocytes exposed to high (10 nM), but not low (0.1 nM), concentrations of AVP. Experiments with the Ca2+ chelator EGTA and the Na+ ionophore monensin indicate that the rapid secondary decrease in Na+/K+-pump activity which occurs after AVP stimulation is not due to changes in cytosolic Ca2+ and Na+ concentrations. When added after the stimulation and rapid decrease in Na+/K+-pump activity induced in hepatocytes by a high concentration of AVP, a second challenge with AVP or PMA failed to stimulate the pump. Similarly, previous exposure of hepatocytes to angiotensin, adrenaline or PMA attenuated the subsequent Na+/K+-pump responses to AVP and PMA. In contrast, previous exposure to AVP had no significant effect on subsequent stimulation of the Na+/K+-pump by monensin, glucagon, forskolin or 8-p-chlorophenylthio cyclic AMP. In addition, exposure to monensin had no effect on subsequent responses to AVP and PMA. These data indicate that high concentrations of Ca2+-mobilizing hormones and PMA result in heterologous desensitization of the hepatic Na+/K+ pump to subsequent stimulation by Ca2+-mobilizing hormones and PMA, but not by cyclic-AMP-dependent agonists or monensin.

Differential expression of Na+,K+-ATPase alpha- and beta-subunit mRNAs in rat tissues and cell lines

We have analyzed Na+,K+-ATPase (EC 3.6.1.3) alpha- and beta-subunit mRNA expression in rat tissues and cell lines derived from the rat central nervous system. Substantial differences in the tissue and developmental specificity of expression were found for the genes encoding three isoforms of the alpha subunit. Transcripts of the alpha 1-subunit gene were detected in all tissues tested, whereas alpha 2- and alpha 3-subunit mRNA species were expressed predominantly in brain. The pattern of expression of beta-subunit mRNA also was complex and tissue specific but was distinct from that of any of the alpha-subunit mRNAs. Cell lines derived from the rat central nervous system and the pheochromocytoma PC12 expressed the mRNAs for all three alpha-subunit isoforms, whereas beta-subunit mRNA was detected only in PC12 cells. The distinct expression patterns of rat Na+,K+-ATPase mRNAs suggest that different members of the ATPase family may have specialized functions.

Distribution of (Na+ + K+)ATPase and sodium channels in skeletal muscle and electroplax

The distributions of (Na+ + K+)ATPase and sodium channels in skeletal muscle fibres and electrocytes were determined by immunofluorescent and immunoelectron microscopic techniques using antibodies against rat and eel (Na+ + K+)ATPase and the eel electric organ sodium channel. The extrajunctional sarcolemma of skeletal muscle was uniformly stained by polyclonal antibodies against (Na+ + K+)ATPase and the sodium channel. The T-tubule system of skeletal muscle was also labelled heavily for both (Na+ + K+)ATPase and the sodium channel. The terminal cisternae of the sarcoplasmic reticulum was stained for (Na+ + K+)ATPase but not sodium channels. At the motor endplate, (Na+ + K+)ATPase-like immunoreactivity was present along the plasmalemma of motor nerve terminals but not along the postsynaptic junctional sarcolemma. Paradoxically, a monoclonal antibody that binds to the alpha form of the catalytic subunit of (Na+ + K+)ATPase from rat hepatocytes and renal tubule cells did not label the enzyme in rat skeletal muscle. In electrocytes, (Na+ + K+)ATPase-like immunoreactivity was concentrated primarily along the plasmalemma and calveolae of the non-innervated face. In contrast, sodium channel-like immunoreactivity was concentrated along the plasmalemma of the innervated face except in the clefts of the postsynaptic membrane. Thus, we conclude that at endplates both the (Na+ + K+)ATPase of rat skeletal muscle and sodium channels of eel electrocytes are not concentrated in the juxtaneuronal postsynaptic membrane. We also interpret the failure of the monoclonal anti-alpha (Na+ + K+)ATPase antibodies to bind to the enzyme in muscle to indicate that the catalytic subunit of skeletal muscle (Na+ + K+)ATPase displays different epitopes than does the alpha subunit of kidney and liver.

Molecular cloning and sequence analysis of the (Na+ + K+)-ATPase beta subunit from dog kidney

cDNA complementary to mRNA coding for the beta subunit of dog renal (Na+ + K+)-ATPase has been cloned into lambda gt11 and the nucleotide sequence of the DNA has been determined. The amino acid sequence of the beta subunit polypeptide has also been deduced from the DNA. The mature form of the dog kidney beta subunit contains 302 amino acids with three potential asparagine-linked attachment sites for carbohydrate. The initiation methionine is removed during processing of the polypeptide to its mature form. Although the beta subunit is an integral membrane protein there is no signal sequence for the polypeptide, and hydropathy analysis predicts that the beta subunit polypeptide spans the cell membrane only once. Secondary structure predictions and a model for the structure of the beta subunit are proposed. DNA sequencing of the 5' non-coding region of the mRNA revealed a 200 bp inverted repeat from the coding region. Blot hybridization of a fragment of the beta subunit cDNA identified a single mRNA species of 2.7 kb in dog kidney and several rat tissues. RNA from rat liver was deficient in mRNA that hybridized to the dog kidney beta subunit cDNA, although mRNA that hybridized to an alpha subunit cDNA was detected. RNA from a human hepatoma cell line, HepG2, however, contained comparable levels of mRNA for both the alpha and the beta subunits.

The distribution of (Na+ + K+)ATPase is continuous along the axolemma of unensheathed axons from spinal roots of 'dystrophic' mice

(Na+ + K+)ATPase-like immunoreactivity along the axolemma of sensory and motor neurons and the plasmalemma of Schwann cells from spinal roots of dystrophic mice (129 ReJ Dy/Dy) was determined using polyclonal antibodies specific for guinea pig renal (Na+ + K+)ATPase (GP-17), along with polyclonal (439-2) and monoclonal (9A5) antibodies specific for rat renal (Na+ + K+)ATPase. In normal and dystrophic mice, (Na+ + K+)ATPase-like immunoreactivity was observed along the axolemma at nodes of Ranvier using GP-17 and 439-2, each of which binds to isozymes of (Na+ + K+)ATPase composed of the alpha and alpha + forms of the catalytic subunit. Staining was not seen along the nodal axolemma with 9A5, a preparation that binds to the alpha form of the catalytic subunit. The terminal processes and microvilli of Schwann cells were stained using all three antibody probes. The axolemma of unensheathed axons in dystrophic mice was continuously and uniformly labelled with GP-17 and 439-2, but not 9A5. Concentrations of (Na+ + K+)ATPase-like immunoreactivity along Schwann cell processes were observed most often in areas adjacent to unensheathed axolemma. At heminodes, staining abruptly decreased along Schwann cell processes in areas that were separated from the unensheathed axolemma by other intervening Schwann cell processes. It was concluded from these data that in dystrophic mice (Na+ + K+)ATPase is uniformly distributed along unensheathed portions of axons without evidence of detectable focal concentrations of the enzyme, and that the catalytic subunit of (Na+ + K+)ATPase along unensheathed axons is distinct from the alpha form found in Schwann cells and other organs. In addition, (Na+ + K+)ATPase is concentrated along the plasmalemma of Schwann cells in regions of close apposition to axolemmal areas associated with large ionic fluxes.

Rat-brain Na,K-ATPase beta-chain gene: primary structure, tissue-specific expression, and amplification in ouabain-resistant HeLa C+ cells

We deduced the complete amino acid sequence of the rat brain Na,K-ATPase beta-subunit from cDNA. The rat brain beta-subunit exhibits a high degree of primary sequence and secondary structural homology with the human and Torpedo beta-subunit polypeptides. Analysis of rat tissue RNA reveals that the beta-subunit gene encodes four separate mRNA species which are expressed in a tissue-specific fashion. In ouabain-resistant HeLa C+ cells, beta-subunit DNA sequences are amplified (approximately 20-fold) and beta-subunit mRNAs are overproduced relative to levels in parental HeLa cells. These results suggest that the beta-subunit plays an important role in Na,K-ATPase structure-function and in the mechanism underlying cellular resistance to the cardiac glycosides.

Rat-brain Na,K-ATPase beta-chain gene: primary structure, tissue-specific expression, and amplification in ouabain-resistant HeLa C+ cells

We deduced the complete amino acid sequence of the rat brain Na,K-ATPase beta-subunit from cDNA. The rat brain beta-subunit exhibits a high degree of primary sequence and secondary structural homology with the human and Torpedo beta-subunit polypeptides. Analysis of rat tissue RNA reveals that the beta-subunit gene encodes four separate mRNA species which are expressed in a tissue-specific fashion. In ouabain-resistant HeLa C+ cells, beta-subunit DNA sequences are amplified (approximately 20-fold) and beta-subunit mRNAs are overproduced relative to levels in parental HeLa cells. These results suggest that the beta-subunit plays an important role in Na,K-ATPase structure-function and in the mechanism underlying cellular resistance to the cardiac glycosides.