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

Insulin Stimulates the Activity of Na+/K+-Atpase in Human Peritoneal Mesothelial Cells

Objective

To assess the effect of insulin on the Na+/ K+-ATPase expression and activity in human peritoneal mesothelial cells (HPMC).

Methods

HPMC were isolated from the omental tissue of non-uremic patients, grown to confluence and rendered quiescent by serum deprivation for 24 hours. The activity of Na+/K+-ATPase was determined by measuring the ouabain-sensitive86Rb uptake. To assess whether the effect of insulin was related to changes in [Na+]i the sodium influx was measured with 22Na and the activity of Na+/K+ -A TPase was assessed in the presence of amiloride. Expression of Na+/K+ -A TPaseα1’ α2 and β1-subunit mRNAs was determined by RT/PCR.

Results

Exposure of HPMC to insulin resulted in a time and dose-dependent increase in the Na+/K+-ATPase activity. After 60 minutes the ouabain-sensitive 86Rb up take (cpm/104 cells) was increased from 6650±796 in control cells to 9763±1212 in HPMC exposed to 100 mU/ mL insulin (1.5-fold increase; n=4, P<0.05). In addition, incubation of HPMC with 100 mU/mL insulin resulted in a time-dependent increase in the 22Na influx. Pre-exposure of HPMC to 1 mM amiloride reduced the activity of Na+/K+-A TPase but did not block the stimulatory effect of insulin. RT/PCR analysis revealed that HPMC constitutively expressed α1 and β1-subunit mRNAs while the α2-subunit mRNA was barely detectable. Exposure of HPMC to insulin for up to 24 hours was not associated with any changes in the expression of either α1’ α2 or B1-subunit.

Conclusion

Insulin stimulates the Na+/K+-ATPase activity in HPMC in a time and dose-dependent manner. This effect appears to mediated by an increase in [Na+]i and is not related to alterations in Na+/K+-ATPase subunit mRNAs expression.

The Na+-K+-ATPase as self-adhesion molecule and hormone receptor

Thanks to the homeostasis of the internal milieu, metazoan cells can enormously simplify their housekeeping efforts and engage instead in differentiation and multiple forms of organization (tissues, organs, systems) that enable them to produce an astonishing diversity of mammals. The stability of the internal milieu despite drastic variations of the external environment (air, fresh or seawater, gastrointestinal fluids, glomerular filtrate, bile) is due to transporting epithelia that can adjust their specific permeability to H(2)O, H(+), Na(+), K(+), Ca(2+), and Cl(-) over several orders of magnitude and exchange substances with the outer milieu with exquisite precision. This exchange is due to the polarized expression of membrane proteins, among them Na(+)-K(+)-ATPase, an oligomeric enzyme that uses chemical energy from ATP molecules to translocate ions across the plasma membrane of epithelial cells. Na(+)-K(+)-ATPase presents two types of asymmetries: the arrangement of its subunits, and its expression in one pole of the epithelial cell ("polarity"). In most epithelia, polarity consists of the expression of Na(+)-K(+)-ATPase towards the intercellular space and arises in part from the interaction of the extracellular segment of the β-subunit with another β-subunit present in a Na(+)-K(+)-ATPase molecule expressed by a neighboring cell. In addition to enabling the Na(+)-K(+)-ATPase to transport ions and water vectorially, this position exposes its receptors to ouabain and analogous cardiotonic steroids, which are present in the internal milieu because these were secreted by endocrine cells.

Regulation of the Na,K-ATPase in MDCK cells by prostaglandin E1: a role for calcium as well as cAMP

Prostaglandins (PGs) play a significant role in the regulation of sodium reabsorption by the kidney, in addition to accumulating during inflammation as well as in several solid tumors. Previously, we presented evidence indicating that prostaglandin E(1) (PGE(1)), a supplement in the serum-free medium for MDCK cells, increases the activity of the Na,K-ATPase in MDCK cells, in addition to its growth stimulatory effect [J. Cell. Physiol. 151 (1992) 337]. This report defines the molecular mechanisms, and signaling pathways responsible for the increased Na,K-ATPase activity. Our results indicate that the increased activity of the Na,K-ATPase in MDCK monolayers treated with either PGE(1) or 8Bromocyclic AMP (8Br-cAMP) can be attributed to an increase in the rate of biosynthesis of the Na,K-ATPase, and an increase in the levels of Na,K-ATPase alpha and beta subunit mRNAs. As beta subunit mRNA increased to a larger extent than alpha subunit mRNA, transient transfection studies were conducted using a human beta1 promoter/luciferase construct [Nucleic Acids Res. 21 (1993) 2619]. While an 8Br-cAMP stimulation was observed (suggesting the involvement of cAMP), our results also suggest that the observed PGE(1) stimulation could be explained by the involvement of Ca(2+) as well protein kinase C (PKC). Consistent with the involvement of Ca(2+), TMB-8 (which inhibits Ca(2+) efflux from intracellular stores) inhibited the PGE(1) stimulation. Moreover, PGE(1) was observed to stimulate the translocation of PKC beta1 from the soluble to the particulate fraction. The translocation of PKC, the PGE(1) stimulation of transcription, and the PGE(1)-mediated increase in the beta subunit mRNA level were all inhibited by the PKC inhibitor Gö6989. These results can be explained by the involvement of two classes of cell surface receptors in mediating the PGE(1) stimulation, including the EP1subtype (which activates phospholipase C), as well as the EP2 subtype (which activates adenylate cyclase).

Both mitogen activated protein kinase and the mammalian target of rapamycin modulate the development of functional renal proximal tubules in matrigel

Tubules may arise during branching morphogenesis through several mechanisms including wrapping, budding, cavitation and cord hollowing. In this report we present evidence that is consistent with renal proximal tubule formation through a process of cord hollowing (a process that requires the concomitant establishment of apicobasal polarity and lumen formation). Pockets of lumen filled with Lucifer Yellow were observed within developing cords of rabbit renal proximal tubule cells in matrigel. The observation of Lucifer Yellow accumulation suggests functional polarization. In the renal proximal tubule Lucifer Yellow is initially transported intracellularly by means of a basolaterally oriented p-aminohippurate transport system, followed by apical secretion into the lumen of the nephron. Consistent with such polarization in developing tubules, Triticum vulgare was observed to bind to the lumenal membranes within pockets of Lucifer Yellow-filled lumens. As this lectin binds apically in the rabbit renal proximal tubule, T. vulgare binding is indicative of the emergence of an apical domain before the formation of a contiguous lumen. Both epidermal growth factor and hepatocyte growth factor stimulated the formation of transporting tubules. The stimulatory effect of both epidermal growth factor and hepatocyte growth factor on tubulogenesis was inhibited by PD98059, a mitogen activated protein kinase kinase inhibitor, rather than by wortmannin, an inhibitor of phosphoinositide 3-kinase. Nevertheless, Lucifer Yellow-filled lumens were observed in tubules that formed in the presence of PD98059 as well as with wortmannin, indicating that these drugs did not prevent the process of cavitation. By contrast, rapamycin, an inhibitor of the mammalian target of rapamycin, prevented the process of cavitation without affecting the frequency of formation of developing cords. Multicellular cysts were observed to form in 8-bromocyclic AMP-treated cultures. As these cysts did not similarly accumulate Lucifer Yellow lumenally, it is very likely that processes other than organic anion accumulation are involved in the process of cystogenesis, including the Na,K-ATPase.

Packing of the transmembrane helices of Na,K-ATPase: direct contact between beta-subunit and H8 segment of alpha-subunit revealed by oxidative cross-linking

Spatial relationships among the transmembrane (TM) segments of alpha- and beta-subunits of the Na,K-ATPase molecule have been investigated using oxidative induction of disulfide bonds. The catalytic alpha-subunit contains 10 TM alpha-helices (H1-H10) with 9 Cys residues located within or close to the membrane moiety. There is one Cys residue in the single TM segment of beta-subunit (Hbeta). Previously, the cross-linking products containing the beta-subunit and two fragments of alpha-subunit (the N-terminal containing H1-H2 helices and the C-terminal containing H7-H10 helices) have been identified in experiments with membrane-bound or detergent-solubilized preparations of the membrane moiety of trypsin-digested Na,K-ATPase [Sarvazyan, N. A., Modyanov, N. N., and Askari, A. (1995) J. Biol. Chem. 270, 26528-26532 and Sarvazyan, N. A., Ivanov, A., Modyanov, N. N., and Askari, A. (1997) J. Biol. Chem. 272, 7855-7858]. Here, we have shown that Cu(2+)-phenanthroline treatment of digitonin-solubilized preparation provides the most efficient formation of intersubunit cross-linked product that is predominantly a dimer of beta-subunit and a 22-kDa C-terminal alpha-fragment containing H7-H10 helices. This cross-linked product was isolated and subjected to CNBr cleavage. The resulting fragments were electrophoretically separated and sequenced. A 17-kDa peptide composed of Ile853-Met942 alpha-fragment and Ala5-Met56 beta-fragment was identified as a product of intersubunit disulfide cross-link between Cys44 of Hbeta and either Cys911 or Cys930, located in H8. This provides the first direct experimental evidence of the juxtaposition of Hbeta and H8 within the Na,K-ATPase molecule. The second detected cross-linked product was composed of alpha-fragments Lys947-Met963 and Tyr974-Tyr1016 linked by induced disulfide bridge between Cys964 (H9) and Cys983 (H10). The spatial proximity of these Cys residues defines the mutual orientation of H9 and H10 helices of alpha-subunit.

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.

cDNA cloning and expression of HIP, a novel cell surface heparan sulfate/heparin-binding protein of human uterine epithelial cells and cell lines

Heparan sulfate proteoglycans and their corresponding binding sites have been suggested to play an important role during the initial attachment of murine blastocysts to uterine epithelium and human trophoblastic cell lines to uterine epithelial cell lines. Previous studies on RL95 cells, a human uterine epithelial cell line, had characterized a single class of cell surface heparin/heparan sulfate (HP/HS)-binding sites. Three major HP/HS-binding peptide fragments were isolated from cell surfaces by tryptic digestion, and partial amino-terminal amino acid sequence for each peptide fragment was obtained (Raboudi, N., Julian, J., Rohde, L. H., and Carson, D. D. (1992) J. Biol. Chem. 267, 11930-11939). In the current study, using approaches of reverse transcription-polymerase chain reaction and cDNA library screening, we have cloned and expressed a novel, cell surface HP/HS-binding protein, named HP/HS interacting protein (HIP), from RL95 cells. The full-length cDNA of HIP encodes a protein of 159 amino acids with a calculated molecular mass of 17,754 Da and pI of 11.75. Transfection of HIP full-length cDNA into NIH-3T3 cells demonstrated cell surface expression and a size similar to that of HIP expressed by human cells. Predicted amino acid sequence indicates that HIP lacks a membrane spanning region and has no consensus sites for glycosylation. Northern blot analysis detected a single transcript of 1.3 kilobases in both total RNA and poly(A+) RNA. Examination of human cell lines and normal tissues using both Northern blot and Western blot analyses revealed that HIP is expressed at different levels in a variety of human cell lines and normal tissues but absent in some cell lines and some cell types of normal tissues examined. HIP has relatively high homology (approximately 80% both at the levels of nucleotide and protein sequence) to a rodent ribosomal protein L29. Thus, members of the L29 family may be displayed on cell surfaces where they may participate in HP/HS binding events.

4-hydroxynonenal inhibits Na(+)-K(+)-ATPase

4-Hydroxynonenal binds rapidly to Na(+)-K(+)-ATPase, and this was accompanied by a decrease in measurable sulfhydryl groups and a loss of enzyme activity. The I50 value for Na(+)-K(+)-ATPase inhibition by 4-hydroxynonenal was found to be 120 microM. Although the sulfhydryl groups could be completely restored with beta-mercaptoethanol during the reaction of the Na(+)-K(+)-ATPase-HNE-adduct, the Na(+)-K(+)-ATPase activity was only partially restored by this reducing agent. A combination of hydroxylamine and beta-mercaptoethanol yielded the greatest recovery of enzyme activity, 85% of original. Thus, 4-hydroxynonenal binding to Na(+)-K(+)-ATPase led to an irreversible decrease of enzyme activity under the conditions employed. It is hypothesized that 4-hydroxynonenal reacts with sulfhydryls at sites on the enzyme that are inaccessible by beta-mercaptoethanol. Furthermore, evidence was obtained that 4-hydroxynonenal reacts with other amino acids such as lysine to form adducts that also interfere with protein function.

The carbohydrate moieties of the beta-subunit of Na+, K(+)-ATPase: their lateral motions and proximity to the cardiac glycoside site

The beta-subunit associated with the catalytic (alpha) subunit of the mammalian Na+, K(+) -ATPase is a transmembrane glycoprotein with three extracellularly located N-glycosylation sites. Although beta appears to be essential for a functional enzyme, the role of beta and its sugars remains unknown. In these studies, steady-state and dynamic fluorescence measurements of the fluorophore lucifer yellow (LY) covalently linked to the carbohydrate chains of beta have demonstrated that the bound probes are highly solvent exposed but restricted in their diffusional motions. Furthermore, the probes' environments on beta were not altered by Na+ or K+ or ouabain-induced enzyme conformational changes, but both divalent cation and oligomycin addition evoked modest changes in LY fluorescence. Frequency domain measurements reflecting the Förster fluorescence energy transfer (FET) occurring between anthroylouabain (AO) bound to the cardiac glycoside receptor site on alpha and the carbohydrate-linked LY demonstrated their close proximity (18 A). Additional FET determinations made between LY as donor and erythrosin-5-isothiocyanate, covalently bound at the enzyme's putative ATP binding site domain, indicated that a distance of about 85 A separates these two regions and that this distance is reduced upon divalent cation binding and increased upon the Na+E1-->K+E2 conformational transition. These data suggest a model for the localization of the terminal moieties of the oligosaccharides that places them, on average, about 18 A from the AO binding site and this distance or less from the extracellular membrane surface.

Intersubunit and intrasubunit contact regions of Na+/K(+)-ATPase revealed by controlled proteolysis and chemical cross-linking

To identify interfaces of alpha- and beta-subunits of Na+/K(+)-ATPase, and contact points between different regions of the same alpha-subunit, purified kidney enzyme preparations whose alpha-subunits were subjected to controlled proteolysis in different ways were solubilized with digitonin to disrupt intersubunit alpha,alpha-interactions, and oxidatively cross-linked. The following disulfide cross-linked products were identified by gel electrophoresis, staining with specific antibodies, and N-terminal analysis. 1) In the enzyme that was partially cleaved at Arg438-Ala439, the cross-linked products were an alpha,beta-dimer, a dimer of N-terminal and C-terminal alpha fragments, and a trimer of beta and the two alpha fragments. 2) From an extensively digested enzyme that contained the 22-kDa C-terminal and several smaller fragments of alpha, two cross-linked products were obtained. One was a dimer of the 22-kDa C-terminal peptide and an 11-kDa N-terminal peptide containing the first two intramembrane helices of alpha (H1-H2). The other was a trimer of beta, the 11-kDa, and the 22-kDa peptides. 3) The cross-linked products of a preparation partially cleaved at Leu266-Ala267 were an alpha,beta-dimer and a dimer of beta and the 83-kDa C-terminal fragment. Assuming the most likely 10-span model of alpha, these findings indicate that (a) the single intramembrane helix of beta is in contact with portions of H8-H10 intramembrane helices of alpha; and (b) there is close contact between N-terminal H1-H2 and C-terminal H8-H10 segments of alpha; with the most probable interacting helices being the H1,H10-pair and the H2,H8-pair.

Two Drosophila nervous system antigens, Nervana 1 and 2, are homologous to the beta subunit of Na+,K(+)-ATPase

A nervous system-specific glycoprotein antigen from adult Drosophila heads, designated Nervana (Nrv), has been purified on the basis of reactivity of its carbohydrate epitope(s) with anti-horseradish peroxidase (HRP) antibodies that are specific markers for Drosophila neurons. Anti-Nrv monoclonal antibodies (mAbs), specific for the protein moiety of Nrv, were used to screen a Drosophila embryo cDNA expression library. Three cDNA clones (designated Nrv1, Nrv2.1, and Nrv2.2) were isolated that code for proteins recognized by anti-Nrv mAbs on Western blots. DNA sequencing and Southern blot analyses established that the cDNA clones are derived from two different genes. In situ hybridization to Drosophila polytene chromosomes showed that the cDNA clones map to the third chromosome near 92C-D. Nrv1 and Nrv2.1/2.2 have open reading frames of 309 and 322/323 amino acids, respectively, and they are 43.4% identical at the amino acid level. The proteins deduced from these clones exhibit significant homology in both primary sequence and predicted topology to the beta subunit of Na+,K(+)-ATPase. Immunoaffinity-purified Nrv is associated with a protein (M(r) 100,000) recognized on Western blots by anti-ATPase alpha-subunit mAb. Our results suggest that the Drosophila nervous system-specific antigens Nrv1 and -2 are neuronal forms of the beta subunit of Na+,K(+)-ATPase.

Major organ-specific glycoproteins in isolated brain and kidney membranes identified as Na,K-ATPase subunits by combined glycan-, lectin-, and immunoblotting

In the present work combined glycan-, lectin-, and immunoblotting of isolated brain and kidney membranes shows that the alpha and beta subunits of Na,K-ATPase are the most abundant glycoproteins. Further, Datura stramonium and Galanthus nivalis agglutinins recognize the Na,K-ATPase subunits in a mutually exclusive manner in membranes from human, rabbit and rat brain or human, rabbit, rat, pig and dog kidney indicating the presence of species-independent organ-typical glycoforms. The glycosylation status is not related to the ouabain-sensitivity. Taken together, the data reveals organ-specific glycoforms of Na,K-ATPase which might have roles for organ identification and recognition.

The functional roles of disulfide bonds in the beta-subunit of (Na,K)ATPase as studied by site-directed mutagenesis

The beta-subunit of Torpedo californica (Na,K)ATPase contains seven cysteine residues; one (Cys46) is in the single transmembrane segment and the other six (Cys127, Cys150, Cys160, Cys176, Cys215 and Cys278) are in the extracellular domain and form three highly conserved disulfide bonds. A beta-subunit mutant with replacement of Cys46 by Ser could assemble with the alpha-subunit, and the resulting alpha beta-complex was catalytically active. Mutants in which either the N-terminal side or both Cys residues of the Cys127-Cys150 bond were replaced by Ser could also tightly assemble with the alpha-subunit, but the resulting alpha beta-complex was catalytically inactive. On the other hand, disruption of either the Cys160-Cys176 or Cys215-Cys278 bond by substituting the N-terminal side only or both Cys residues with Ser led to a beta-subunit that could not assemble with the alpha-subunit. We conclude that the structure of the beta-subunit around the Cys160-Cys176 and Cys215-Cys278 loops is indispensable for assembly with the alpha-subunit, whereas the Cys127-Cys150 loop is not essential for assembly but is required for enzyme activity.

Cloning and sequence analysis of the alpha subunit of the cat flea sodium pump

We have cloned a cDNA that encodes the sodium pump alpha-subunit of the cat flea Ctenocephalides felis. The deduced amino acid sequence of 1037 residues is 90% homologous with the Drosophila alpha-subunit sequence and shows 75% similarity to the sheep kidney alpha-subunit. One characteristic unique to the insect polypeptides is an extended sequence at the N-terminus. Northern blot analysis of C. felis RNA detects two alpha-subunit transcripts with lengths of 3.8 and 4.4 kb. Sodium pump alpha-subunit sequences were readily detected in homogenates of flea midguts and in cDNA synthesized from midgut RNA.

Surplus Na+ pumps: how low-K(+)-incubated LLC-PK1 cells respond to K+ restoration

We have previously shown that a pig kidney cell line (LLC-PK1/Cl4) responds to chronic exposure to 0.25 mM extracellular K+ by increasing the beta-, not alpha-, subunit mRNA levels and both alpha- and beta-abundance twofold over control. Our objective in the present study was to determine how the LLC-PK1/Cl4 cells respond when returned to control (5.5 mM) medium. A 1.8-fold increase in ouabain binding established that the induced pumps were expressed at the cell surface following 24-h incubation in low K+. On restoration to 5.5 mM K+, intracellular Na+ and K+ concentrations ([Na+]i and [K+]i, respectively) rapidly returned to control levels within 15 min. The doubled pool size of pumps in the chronic low K+ cells had no significant influence on the rate of ion restoration when compared with the rate in cells acutely exposed to low K+. Despite the rapid return of ions to control values, beta-mRNA levels remained elevated for 2 h, then sharply declined to control levels by 6 h of K+ restoration. From these data, we estimate that the half-life of beta-mRNA is 2-3 h during restoration. alpha-Subunit mRNA remained essentially unchanged from control after return of K+ to the medium and restoration of intracellular ions.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolation and characterization of the high-affinity K(+)-translocating ATPase from Rhodobacter sphaeroides

Cells of the purple nonsulfur bacterium Rhodobacter sphaeroides express a high-affinity K+ uptake system when grown in media with low K+ concentrations. A vanadate-sensitive, K(+)-stimulated and Mg(2+)-stimulated ATPase was purified from membranes of these cells by solubilization with decyl-beta-D-maltoside in the presence of Escherichia coli phospholipids followed by triazine-dye affinity chromatography. This primary transport system has a substrate specificity and an inhibitor sensitivity closely similar to those of the Kdp ATPase from E. coli and is composed of three subunits with molecular masses of 70.0, 43.5, and 23.5 kDa.

Synthesis and translocation of Na(+)-K(+)-ATPase alpha- and beta-subunits to plasma membrane in MDCK cells

Synthesis and translocation of Na(+)-K(+)-ATPase alpha-catalytic and beta-glycoprotein subunits from intracellular membranes to the plasma membrane were studied in Madin-Darby canine kidney cells (MDCK-T) by combining the methods of pulse-chase labeling, subcellular fractionation on sorbitol gradients, and immunoprecipitation. Immunoprecipitation from homogenates revealed that radioactive methionine incorporated into beta-subunit was equal to that incorporated into alpha-subunit after 15 min of labeling. Because the ratio of total methionines in alpha- vs. beta-subunit is approximately 5:1, these results suggest that beta-subunit is synthesized in molar excess over alpha-subunit. Half of the newly synthesized beta-subunit, likely unassembled units, were degraded by 60 min after labeling, while alpha-subunits were stable through 120 min after synthesis, suggesting alpha may be limiting for alpha beta-assembly. By 120 min the ratio of counts incorporated into alpha vs. beta approached 5, which is predicted by a 1:1 ratio of alpha to beta. The sorbitol gradient resolved two major membrane samples: a mixture of endoplasmic reticulum and Golgi populations and a plasma membrane-enriched sample. Immature beta (beta i) could not be detected in the plasma membrane-enriched samples at levels greater than could be attributed to cross-contamination by intracellular membranes. Mature beta (beta m) became detectable after 30 min, and conversion of beta i to beta m was 90% complete at 120 min. A peak of labeled alpha-subunit appeared in the plasma membrane-enriched sample at 60 min, coincident with the appearance of labeled beta m-subunit in this sample, suggesting movement as alpha beta-heterodimers.

The functional role of the beta-subunit in the maturation and intracellular transport of Na,K-ATPase

The minimal functional enzyme unit of Na,K-ATPase consists of an alpha-beta complex. The alpha-subunit bears all functional domains of the enzyme and so far a regulatory role for the beta-subunit in the catalytic cycle has not been established. On the other hand, increasing experimental evidence suggests that the beta-subunit is an indispensable element for the structural and functional maturation of the enzyme as well as its intracellular transport to the plasma membrane. This brief review summarizes the experimental data supporting the hypothesis that assembly of the beta-subunit is needed for the alpha-subunit to acquire the correct, stable configuration necessary for the acquisition of functional properties and its exit from the ER.

Molecular cloning of the beta-subunit of the Na,K-ATPase in the brine shrimp, Artemia. The cDNA-derived amino acid sequence shows low homology with the beta-subunits of vertebrates except in the single transmembrane and the carboxy-terminal domains

A cDNA encoding the beta-subunit of the Na,K-ATPase of brine shrimp (Artemia) has been cloned. Its nucleotide sequence and predicted amino acid sequence have been determined. The amino acid sequence shows considerable divergence from that of chicken, dog, human, pig, rat, sheep, Torpedo, and Xenopus. This is not entirely unexpected since brine shrimp is a 'fast clock' organism which diverged from the precursor of the vertebrates 0.5-1.0 billion years ago. However, a highly hydrophobic putative transmembrane domain and the carboxy-terminal domain show considerable conservation. The relatively small degree of conservation in the beta-subunit of Artemia should provide information about the functional significance of this protein.

Isolation and characterization of gastric microsomal glycoproteins. Evidence for a glycosylated beta-subunit of the H+/K(+)-ATPase

Detergent-solubilization of hog gastric microsomal membrane proteins followed by affinity chromatography using wheat germ agglutinin or Ricinus communis I agglutinin resulted in the isolation of five glycoproteins with the apparent molecular masses on sodium dodecyl sulfate polyacrylamide gels of (in kDa): 60-80 (two glycoproteins sharing this molecular mass); 125-150; and 190-210. In the nonionic detergent Nonidet P-40 (NP-40), the 94 kDa H+/K(+)-ATPase was recovered exclusively in the lectin-binding fraction; however, in the cationic detergent dodecyltrimethylammonium bromide, most of the ATPase was recovered in the nonbinding fraction. Detection of glycoproteins either by periodic acid-dansyl hydrazine staining of carbohydrate in polyacrylamide gels or by Western blots probed with lectins indicated that the majority of the ATPase molecules are not glycosylated. In addition, in the absence of microsomal glycoproteins, the NP-40-solubilized ATPase does not bind to a lectin column. Taken together, these results suggest that the recovery of NP-40-solubilized ATPase in the lectin-binding fraction is due to its noncovalent interaction with a gastric microsomal glycoprotein. Immunoprecipitation of the ATPase from NP-40-solubilized microsomal membrane proteins resulted in the co-precipitation of a single 60-80 kDa glycoprotein. Characterization of the 60-80 kDa glycoprotein associated with the ATPase revealed that: it is a transmembrane protein; it has an apparent core molecular mass of 32 kDa; and, it has five asparagine-linked oligosaccharide chains. Given its similarity to the glycosylated beta-subunit of the Na+/K(+)-ATPase, this 60-80 kDa gastric microsomal glycoprotein is suggested to be a beta-subunit of the H+/K(+)-ATPase.

Beta subunit of (Na+ + K+)-ATPase contains three disulfide bonds

Previous studies of titratable (Na+ + K+)-ATPase sulfhydryl groups have indicated the presence of one disulfide bond per mole of holoenzyme. This single disulfide cross-link was assigned to the beta subunit on the basis of the difference between the number of titrated "free" sulfhydryl groups and the total number of titrated sulfhydryl groups for each subunit [Esmann, M. (1982) Biochim. Biophys. Acta 688, 251; Kawamura, M., & Nagano, K. (1984) Biochim. Biophys. Acta 694, 27]. In the present study, beta-subunit tryptic peptides containing disulfide cross-links were identified and purified by HPLC. Two new peptides were generated from each disulfide-bonded peptide by reduction with dithiothreitol, and the amino acid compositions of these reduced peptides were determined. The data demonstrate that there are three disulfide bonds in the native beta subunit: 125Cys-148Cys, 158Cys-174Cys, and 212Cys-275Cys. The number of disulfide bonds in the beta subunit was also estimated by titration of sulfhydryl groups with [14C]iodoacetamide. Six sulfhydryl groups were identified: two sulfhydryl groups were titrated without prior reduction, and four were identified only after reduction of the protein with dithiothreitol. These data, suggesting that the beta subunit contains two disulfide bonds, are inconsistent with the peptide isolation experiments, which directly identified three disulfide bonds in the beta subunit. This inconsistency was resolved by demonstrating that approximately 20% of each disulfide bond in the beta subunit was reduced prior to the start of the experiment, resulting in an underestimation of the number of disulfide-bonded sulfhydryl groups in the beta subunit from the titration experiments.(ABSTRACT TRUNCATED AT 250 WORDS)

The adhesion molecule on glia (AMOG) is a homologue of the beta subunit of the Na,K-ATPase

AMOG (adhesion molecule on glia) is a Ca2(+)-independent adhesion molecule which mediates selective neuron-astrocyte interaction in vitro (Antonicek, H., E. Persohn, and M. Schachner. 1987. J. Cell Biol. 104:1587-1595). Here we report the structure of AMOG and its association with the Na,K-ATPase. The complete cDNA sequence of mouse AMOG revealed 40% amino acid identity with the previously cloned beta subunit of rat brain Na,K-ATPase. Immunoaffinity-purified AMOG and the beta subunit of detergent-purified brain Na,K-ATPase had identical apparent molecular weights, and were immunologically cross-reactive. Immunoaffinity-purified AMOG was associated with a protein of 100,000 Mr. Monoclonal antibodies revealed that this associated protein comprised the alpha 2 (and possibly alpha 3) isoforms of the Na,K-ATPase catalytic subunit, but not alpha 1. The monoclonal AMOG antibody that blocks adhesion was shown to interact with Na,K-ATPase in intact cultured astrocytes by its ability to increase ouabain-inhibitable 86Rb+ uptake. AMOG-mediated adhesion occurred, however, both at 4 degrees C and in the presence of ouabain, an inhibitor of the Na,K-ATPase. Both AMOG and the beta subunit are predicted to be extracellularly exposed glycoproteins with single transmembrane segments, quite different in structure from the Na,K-ATPase alpha subunit or any other ion pump. We hypothesize that AMOG or variants of the beta subunit of the Na,K-ATPase, tightly associated with an alpha subunit, are recognition elements for adhesion that subsequently link cell adhesion with ion transport.

Molecular genetics of Na,K-ATPase

Researchers in the past few years have successfully used molecular-genetic approaches to determine the primary structures of several P-type ATPases. The amino-acid sequences of distinct members of this class of ion-transport ATPases (Na,K-, H,K-, and Ca-ATPases) have been deduced by cDNA cloning and sequencing. The Na,K-ATPase belongs to a multiple gene family, the principal diversity apparently resulting from distinct catalytic alpha isoforms. Computer analyses of the hydrophobicity and potential secondary structure of the alpha subunits and primary sequence comparisons with homologs from various species as well as other P-type ATPases have identified common structural features. This has provided the molecular foundation for the design of models and hypotheses aimed at understanding the relationship between structure and function. Development of a hypothetical transmembrane organization for the alpha subunit and application of site-specific mutagenesis techniques have allowed significant progress to be made toward identifying amino acids involved in cardiac glycoside resistance and possibly binding. However, the complex structural and functional features of this protein indicate that extensive research is necessary before a clear understanding of the molecular basis of active cation transport is achieved. This is complicated further by the paucity of information regarding the structural and functional contributions of the beta subunit. Until such information is obtained, the proposed model and functional hypotheses should be considered judiciously. Considerable progress also has been made in characterizing the regulatory complexity involved in expression of multiple alpha-isoform and beta-subunit genes in various tissues and cells during development and in response to hormones and cations. The regulatory mechanisms appear to function at several molecular levels, involving transcriptional, posttranscriptional, translational, and posttranslational processes in a tissue- or cell-specific manner. However, much research is needed to precisely define the contributions of each of these mechanisms. Recent isolation of the genes for these subunits provides the framework for future advances in this area. Continued application of biochemical, biophysical, and molecular genetic techniques is required to provide a detailed understanding of the mechanisms involved in cation transport of this biologically and pharmacologically important enzyme.

Advances in Na+,K+-ATPase studies: from protein to gene and back to protein

Complete primary structures of both subunits of Na+,K+-ATPase from various sources have been established by a combination of the methods for molecular cloning and protein chemistry. The gene family homologous to the alpha-subunit cDNA of animal Na+,K+-ATPases has been found in the human genome. Some genes of this family encode the known isoforms (alpha I and alpha II) of the Na+,K+-ATPase catalytic subunit. The proteins coded by other genes can be either new isoforms of the Na+,K+-ATPase catalytic subunit or other ion-transporting ATPases. Expression of the genes of this family proceeds in a tissue-specific manner and changes during the postnatal development and neoplastic transformation. The complete exon-intron structure of one of the genes of this family has been established. This gene codes for the form of the catalytic subunit, the existence of which has been unknown. Apparently, all the genes of the discovered family have a similar intron-exon structure. There is certain correlation between the gene structure and the proposed domain arrangement of the alpha-subunit. The results obtained have become the basis for the experiments which prove the existence of the earlier unknown alpha III isoform of the Na+,K+-ATPase catalytic subunit and have made possible the study of its function.

Functional domains of the gastric HK ATPase

The gastric H+ + K+ ATPase is a member of the phosphorylating class of transport ATPase. Based on sequence homologies and CHO content, there may be a b subunit associated with the catalytic subunit of the H+ + K+ ATPase. Its function, if present, is unknown. The pump catalyzes a stoichiometric exchange of H+ for K+, but is also able to transport Na+ in the forward direction. This suggests that the transport step involves hydronium rather than protons. The initial binding site is likely to contain a histidine residue to account for the high affinity of the cellular site. The extracellular site probably lacks this histidine, so that a low affinity for hydronium allows release into a solution of pH 0.8. Labelling with positively charge, luminally reactive reagents that block ATPase and pump activity has shown that a region containing H5 and H6 and the intervening luminal loop is involved in necessary conformational changes for normal pump activity. The calculated structure of this loop shows the presence of an a helical, b turn, and b strand sector, with negative charges close to the membrane domain. This sector provides a possible site of interaction of drugs with the H+ + K+ ATPase, and may be part of the K+ pathway in the enzyme.

(Na+ + K+)-ATPase and plasma membrane polarity of intestinal epithelial cells: presence of a brush border antigen in the distal large intestine that is immunologically related to beta subunit

The previously produced monoclonal antibody IEC 1/48 against cultured rat intestinal crypt cells (Quaroni, A., and K. J. Isselbacher. 1981. J. Natl. Cancer Inst. 67:1353-1362) was extensively characterized and found to be directed against the beta subunit of (Na+ + K+)-ATPase as assessed by immunological and enzymatic criteria. Under nondenaturing conditions the antibody precipitated the alpha-beta enzyme complex (98,000 and 48,000 Mr). This probe, together with the monoclonal antibody C 62.4 against the alpha subunit (Kashgarian, M., D. Biemesderfer, M. Caplan, and B. Forbush. 1985. Kidney Int. 28:899-913), was used to localize (Na+ + K+)-ATPase in epithelial cells along the rat intestinal tract by immunofluorescence and immunoelectron microscopy. Both antibodies exclusively labeled the basolateral membrane of small intestine and proximal colon epithelial cells. However, in the distal colon, IEC 1/48, but not C 62.4, also labeled the brush border membrane. The cross-reacting beta-subunit-like antigen on the apical cell pole was tightly associated with isolated brush borders but was apparently devoid of (Na+ + K+)-ATPase activity. Subcellular fractionation of colonocytes in conjunction with limited proteolysis and surface radioiodination of intestinal segments suggested that the cross-reacting antigen in the brush border may be very similar to the beta subunit. The results support the notion that in the small intestine and proximal colon the enzyme subunits are exclusively targeted to the basolateral membrane while in the distal colon nonassembled beta subunit or a beta-subunit-like protein is also transported to the apical cell pole.

Identification of a disulfide between cysteine 214 and cysteine 277 in the beta subunit of native (Na+ + K+)ATPase

Two peptides, produced during tryptic digestion and thermolytic digestion, respectively, and containing the same intact disulfide from the beta polypeptide of (Na+ + K+)ATPase from Torpedo californica, were isolated and unambiguously identified. The disulfide is between Cysteine 214 and Cysteine 277.

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.

AAQUANT: a computer program for quantitative amino acid analysis of proteins and peptides

Quantitative amino acid analysis is an important tool used in the characterization and structural determination of peptides and proteins. A new computer program, AAQUANT, has been developed specifically to aid researchers in analyzing amino acid composition data. AAQUANT calculates amino acid recoveries, including 95% confidence intervals, following acid hydrolysis of peptides and proteins, and also includes useful routines to locate regions of a specified amino acid composition in known protein sequences, compute amino acid composition reports of known protein sequences, generate proteolytic digestion maps of proteins, and create and edit protein sequence data files. This report describes the AAQUANT routines, and demonstrates the use of the program.

Functional activity of oligosaccharide-deficient (Na,K)ATPase expressed in Xenopus oocytes

(Na,K)ATPase from Torpedo californica was expressed in Xenopus laevis oocytes in the presence of tunicamycin by injecting mRNAs for the alpha- and beta-subunits derived from the cloned cDNAs into the oocytes. The oligosaccharide-deficient ATPase thus synthesized was transported to the oocyte plasma membrane, where it exhibited virtually the same ATPase activity, ouabain-binding capacity and 86Rb+ transport activity as the fully glycosylated enzyme. We conclude that the oligosaccharide chains on the beta-subunit has no effect on the catalytic activities of (Na,K)ATPase.

In vitro biosynthesis of the beta-subunit of the Na+/K+-ATPase in developing brine shrimp: glycosylation and membrane insertion

We demonstrate here translation, glycosylation, and membrane insertion of the beta-subunit of the Na+/K+-ATPase of the developing brine shrimp, Artemia, in a reticulocyte lysate translation system. The apparent molecular weight of the primary translation product as determined by SDS-PAGE is 33,000 +/- 1000 (n = 7). When microsomal membranes are present during the entire translation period, a new band with an apparent molecular weight of 37,000 +/- 1000 (n = 7) appears. This change in apparent molecular weight is due to the addition of about two N-linked oligosaccharides. The temporal relationship between protein synthesis and glycosylation have also been examined. Glycosylation and membrane insertion could be achieved if membranes were added after completion of about 70% of the peptide chain. However, glycosylation did not occur if membranes were added after the completion of translation of the beta-subunit. The beta-subunit was synthesized on membrane-bound polysomes, where about two N-linked oligosaccharides were added to the growing polypeptide chain. These studies demonstrate that in vitro translation systems will be useful for studying the biosynthesis of the beta-subunit of the brine shrimp, which is a good model system to examine the developmental regulation of the Na+/K+-ATPase.

All three potential N-glycosylation sites of the dog kidney (Na+ + K+)-ATPase beta-subunit contain oligosaccharide

The beta-subunit of dog kidney (Na+ + K+)-ATPase is a sialoglycoprotein and contains three potential N-glycosylation sites. In this study, the oligosaccharide chains of purified dog kidney beta-subunit were labeled with tritium by oxidation with sodium periodate or galactose oxidase followed by NaB3H4 reduction. The beta-subunit was extensively digested by trypsin and the radioactive peptides were purified by HPLC. The enzyme, glycopeptidase A, which catalyzes the removal of N-linked oligosaccharide chains and the conversion of the glycosylated Asn residue to Asp, was used to demonstrate that a number of purified beta-subunit tryptic peptides were glycosylated. Amino-acid analysis of these beta-subunit peptides following glycopeptidase-A treatment revealed the expected Asn to Asp conversion for Asn-157, Asn-192 and Asn-264, demonstrating that all three potential N-glycosylation sites of the dog kidney beta-subunit are glycosylated. In addition, amino-acid sequence data suggest that a disulfide bond exists between Cys-158 and Cys-174.

Thyroid hormone coordinately regulates Na+-K+-ATPase alpha- and beta-subunit mRNA levels in kidney

Synthesis of the sodium pump, Na+-K+-ATPase, is regulated by thyroid hormone in responsive tissues. The purpose of this study was to determine if triiodothyronine (T3) regulates the concentration of the mRNAs coding for the two enzyme subunits, alpha and beta, and the time course of the response. A single dose of T3 (250 micrograms/100 g body wt) was administered to hypothyroid rats that were killed at various times after injection. In the kidney cortexes of the T3-injected animals, as well as hypothyroid and euthyroid rats, alpha- and beta-mRNA concentrations were measured by dot blot using cDNAs corresponding to the two mRNAs; alpha-subunit abundance was measured by Western blot using antibodies to the enzyme, and Na+-K+-ATPase activity was measured enzymatically. alpha- and beta-mRNAs increased coordinately, after a 6-h time lag to 1.6-fold over hypothyroid levels by 12 h after T3. alpha-Subunit abundance increased significantly by 48 h and to 1.4-fold over hypothyroid by 72 h after T3. Na+-K+-ATPase activity increased with the same time course as the increase in alpha-subunit abundance to 1.3-fold over hypothyroid by 72 h after T3. We conclude that T3 regulates Na+-K+-ATPase synthesis and activity by coordinately increasing the mRNAs of both the alpha- and beta-subunits of the enzyme.

Chromosomal localization of human Na+, K+-ATPase alpha- and beta-subunit genes

Na+, K+-ATPase is a heterodimeric enzyme responsible for the active maintenance of sodium and potassium gradients across the plasma membrane. Recently, cDNAs for several tissue-specific isoforms of the larger catalytic alpha-subunit and the smaller beta-subunit have been cloned. We have hybridized rat brain and human kidney cDNA probes, as well as human genomic isoform-specific DNA fragments, to Southern filters containing panels of rodent X human somatic cell hybrid lines. The results obtained have allowed us to assign the loci for the ubiquitously expressed alpha-chain (ATP1A1) to human chromosome 1, region 1p21----cen, and for the alpha 2 isoform that predominates in neural and muscle tissues (ATP1A2) to chromosome 1, region cen----q32. A common PstI RFLP was detected with the ATP1A2 probe. The alpha 3 gene, which is expressed primarily in neural tissues (ATP1A3), was assigned to human chromosome 19. A fourth alpha gene of unknown function (alpha D) that was isolated by molecular cloning (ATP1AL1) was mapped to chromosome 13. Although evidence to date had suggested a single gene for the beta-subunit, we found hybridizing restriction fragments derived from two different human chromosomes. On the basis of knowledge of conserved linkage groups on human and murine chromosomes, we propose that the coding gene ATP 1B is located on the long arm of human chromosome 1 and that the sequence on human chromosome 4 (ATP 1BL1) is either a related gene or a pseudogene.