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

Identified and potential internalization signals involved in trafficking and regulation of Na+/K+ ATPase activity

The sodium-potassium pump (NKA) or Na+/K+ ATPase consumes around 30-40% of the total energy expenditure of the animal cell on the generation of the sodium and potassium electrochemical gradients that regulate various electrolyte and nutrient transport processes. The vital role of this protein entails proper spatial and temporal regulation of its activity through modulatory mechanisms involving its expression, localization, enzymatic activity, and protein-protein interactions. The residence of the NKA at the plasma membrane is compulsory for its action as an antiporter. Despite the huge body of literature reporting on its trafficking between the cell membrane and intracellular compartments, the mechanisms controlling the trafficking process are by far the least understood. Among the molecular determinants of the plasma membrane proteins trafficking are intrinsic sequence-based endocytic motifs. In this review, we (i) summarize previous reports linking the regulation of Na+/K+ ATPase trafficking and/or plasma membrane residence to its activity, with particular emphasis on the endocytic signals in the Na+/K+ ATPase alpha-subunit, (ii) map additional potential internalization signals within Na+/K+ ATPase catalytic alpha-subunit, based on canonical and noncanonical endocytic motifs reported in the literature, (iii) pinpoint known and potential phosphorylation sites associated with NKA trafficking, (iv) highlight our recent studies on Na+/K+ ATPase trafficking and PGE2-mediated Na+/K+ ATPase modulation in intestine, liver, and kidney cells.

The β2-Subunit (AMOG) of Human Na+, K+-ATPase Is a Homophilic Adhesion Molecule

The β₂ subunit of Na⁺, K⁺-ATPase was originally identified as the adhesion molecule on glia (AMOG) that mediates the adhesion of astrocytes to neurons in the central nervous system and that is implicated in the regulation of neurite outgrowth and neuronal migration. While β₁ isoform have been shown to trans-interact in a species-specific mode with the β₁ subunit on the epithelial neighboring cell, the β₂ subunit has been shown to act as a recognition molecule on the glia. Nevertheless, none of the works have identified the binding partner of β₂ or described its adhesion mechanism. Until now, the interactions pronounced for β₂/AMOG are heterophilic cis-interactions. In the present report we designed experiments that would clarify whether β₂ is a cell–cell homophilic adhesion molecule. For this purpose, we performed protein docking analysis, cell–cell aggregation, and protein–protein interaction assays. We observed that the glycosylated extracellular domain of β₂/AMOG can make an energetically stable trans-interacting dimer. We show that CHO (Chinese Hamster Ovary) fibroblasts transfected with the human β₂ subunit become more adhesive and make large aggregates. The treatment with Tunicamycin in vivo reduced cell aggregation, suggesting the participation of N-glycans in that process. Protein–protein interaction assay in vivo with MDCK (Madin-Darby canine kidney) or CHO cells expressing a recombinant β₂ subunit show that the β₂ subunits on the cell surface of the transfected cell lines interact with each other. Overall, our results suggest that the human β₂ subunit can form trans-dimers between neighboring cells when expressed in non-astrocytic cells, such as fibroblasts (CHO) and epithelial cells (MDCK).

Role of Na+/K+-ATPase in ischemic stroke: in-depth perspectives from physiology to pharmacology

Na⁺/K⁺-ATPase (NKA) is a large transmembrane protein expressed in all cells. It is well studied for its ion exchanging function, which is indispensable for the maintenance of electrochemical gradients across the plasma membrane and herein neuronal excitability. The widely recognized pump function of NKA closely depends on its unique structure features and conformational changes upon binding of specific ions. Various Na⁺-dependent secondary transport systems are rigorously controlled by the ionic gradients generated by NKA and are essential for multiple physiological processes. In addition, roles of NKA as a signal transducer have also been unveiled nowadays. Plethora of signaling cascades are defined including Src-Ras-MAPK signaling, IP3R-mediated calcium oscillation, inflammation, and autophagy though most underlying mechanisms remain elusive. Ischemic stroke occurs when the blood flow carrying nutrients and oxygen into the brain is disrupted by blood clots, which is manifested by excitotoxicity, oxidative stress, inflammation, etc. The protective effect of NKA against ischemic stress is emerging gradually with the application of specific NKA inhibitor. However, NKA-related research is limited due to the opposite effects caused by NKA inhibitor at lower doses. The present review focuses on the recent progression involving different aspects about NKA in cellular homeostasis to present an in-depth understanding of this unique protein. Moreover, essential roles of NKA in ischemic pathology are discussed to provide a platform and bright future for the improvement in clinical research on ischemic stroke.

Na+/K+-ATPase Revisited: On Its Mechanism of Action, Role in Cancer, and Activity Modulation

Maintenance of Na⁺ and K⁺ gradients across the cell plasma membrane is an essential process for mammalian cell survival. An enzyme responsible for this process, sodium-potassium ATPase (NKA), has been currently extensively studied as a potential anticancer target, especially in lung cancer and glioblastoma. To date, many NKA inhibitors, mainly of natural origin from the family of cardiac steroids (CSs), have been reported and extensively studied. Interestingly, upon CS binding to NKA at nontoxic doses, the role of NKA as a receptor is activated and intracellular signaling is triggered, upon which cancer cell death occurs, which lies in the expression of different NKA isoforms than in healthy cells. Two major CSs, digoxin and digitoxin, originally used for the treatment of cardiac arrhythmias, are also being tested for another indication—cancer. Such drug repositioning has a big advantage in smoother approval processes. Besides this, novel CS derivatives with improved performance are being developed and evaluated in combination therapy. This article deals with the NKA structure, mechanism of action, activity modulation, and its most important inhibitors, some of which could serve not only as a powerful tool to combat cancer, but also help to decipher the so-far poorly understood NKA regulation.

Alteration of renal Na,K-ATPase in rats following the mediastinal γ-irradiation

Na,K-ATPase represents the key enzyme that maintains the homeostasis of sodium and potassium ions in the cells. It was documented that in directly irradiated organs the activity of this enzyme is decreased. The aim of present study was to clarify the remote effect of irradiation in mediastinal area on the activity of the Na,K-ATPase in kidneys in rats. Ionizing radiation in single dose 25 Gy resulted in consequent decrease of the body weight gain as well as the size of kidneys in Wistar rats. In addition, radiation induced alterations in the oxidative status of blood plasma. Irradiation also decreased the activity of renal Na,K-ATPase. Measurements of enzyme kinetics that were dependent on the concentration of energy substrate ATP or cofactor Na+ indicated that the lowered enzyme activity is probably a consequence of decreased number of active molecules of the enzyme, as suggested by lowered Vmax values. Immunoblot analysis confirmed the lowered expression of the catalytic alpha subunit together with decreased content of the glycosylated form of beta subunit in the renal tissue of irradiated rats. The ability of the enzyme to bind the substrate ATP, as well as Na+ was not affected, as shown by unaltered values of Km and KNa . Irradiation of the body in the mediastinal area despite protection of kidneys by lead plates during application of X-ray was followed by significant decline of activity of the renal Na,K-ATPase, what may result in deteriorated homeostasis in the organism.

Isolation, crystallization and crystal structure determination of bovine kidney Na(+),K(+)-ATPase

Na(+),K(+)-ATPase is responsible for the transport of Na(+) and K(+) across the plasma membrane in animal cells, thereby sustaining vital electrochemical gradients that energize channels and secondary transporters. The crystal structure of Na(+),K(+)-ATPase has previously been elucidated using the enzyme from native sources such as porcine kidney and shark rectal gland. Here, the isolation, crystallization and first structure determination of bovine kidney Na(+),K(+)-ATPase in a high-affinity E2-BeF3(-)-ouabain complex with bound magnesium are described. Crystals belonging to the orthorhombic space group C2221 with one molecule in the asymmetric unit exhibited anisotropic diffraction to a resolution of 3.7 Å with full completeness to a resolution of 4.2 Å. The structure was determined by molecular replacement, revealing unbiased electron-density features for bound BeF3(-), ouabain and Mg(2+) ions.

The Na-K-ATPase α₁β₁ heterodimer as a cell adhesion molecule in epithelia

The ion gradients generated by the Na-K-ATPase play a critical role in epithelia by driving transepithelial transport of various solutes. The efficiency of this Na-K-ATPase-driven vectorial transport depends on the integrity of epithelial junctions that maintain polar distribution of membrane transporters, including the basolateral sodium pump, and restrict paracellular diffusion of solutes. The review summarizes the data showing that, in addition to pumping ions, the Na-K-ATPase located at the sites of cell-cell junction acts as a cell adhesion molecule by interacting with the Na-K-ATPase of the adjacent cell in the intercellular space accompanied by anchoring to the cytoskeleton in the cytoplasm. The review also discusses the experimental evidence on the importance of a specific amino acid region in the extracellular domain of the Na-K-ATPase β(1) subunit for the Na-K-ATPase trans-dimerization and intercellular adhesion. Furthermore, a possible role of N-glycans linked to the Na-K-ATPase β(1) subunit in regulation of epithelial junctions by modulating β(1)-β(1) interactions is discussed.

Diverse pathways for maturation of the Na,K-ATPase β1 and β2 subunits in the endoplasmic reticulum of Madin-Darby canine kidney cells

Proper folding of the Na,K-ATPase β subunits followed by assembly with the α subunits is necessary for their export from the endoplasmic reticulum (ER). Here we examine roles of the ER lectin chaperone, calnexin, and non-lectin chaperone, BiP, in folding and quality control of the β(1) and β(2) subunits in Madin-Darby canine kidney cells. Short term prevention of glycan-calnexin interactions by castanospermine slightly increases ER retention of β(1), suggesting minor involvement of calnexin in subunit folding. However, both prolonged incubation with castanospermine and removal of N-glycosylation sites do not affect the α(1)-assembly or trafficking of β(1) but increase the amount of the β(1)-bound BiP, showing that BiP can compensate for calnexin in assisting β(1) folding. In contrast to β(1), prevention of either N-glycosylation or glycan-calnexin interactions abolishes the α(1)-assembly and export of β(2) from the ER despite increased β(2)-BiP binding. Mutations in the α(1)-interacting regions of β(1) and β(2) subunits impair α(1) assembly but do not affect folding of the β subunits tested by their sensitivity to trypsin. At the same time, these mutations increase the amount of β-bound BiP but not of β-bound calnexin and increase ER retention of both β-isoforms. BiP, therefore, prevents the ER export of folded but α(1)-unassembled β subunits. These α(1)-unassembled β subunits are degraded faster than α(1)-bound β subunits, preventing ER overload. In conclusion, folding of the β(1) and β(2) subunits is assisted predominantly by BiP and calnexin, respectively. Folded β(1) and β(2) either assemble with α(1) or bind BiP. The α(1)-bound β subunits traffic to the Golgi, whereas BiP-bound β subunits are retained and degraded in the ER.

Na(+)/K (+)-ATPase β1 subunit interacts with M2 proteins of influenza A and B viruses and affects the virus replication

Interplay between the host and influenza virus has a pivotal role for the outcome of infection. The matrix proteins M2/BM2 from influenza (A and B) viruses are small type III integral membrane proteins with a single transmembrane domain, a short amino-terminal ectodomain and a long carboxy-terminal cytoplasmic domain. They function as proton channels, mainly forming a membrane-spanning pore through the transmembrane domain tetramer, and are essential for virus assembly and release of the viral genetic materials in the endosomal fusion process. However, little is known about the host factors which interact with M2/BM2 proteins and the functions of the long cytoplasmic domain are currently unknown. Starting with yeast two-hybrid screening and applying a series of experiments we identified that the β1 subunit of the host Na(+)/K(+)-ATPase β1 subunit (ATP1B1) interacts with the cytoplasmic domain of both the M2 and BM2 proteins. A stable ATP1B1 knockdown MDCK cell line was established and we showed that the ATP1B1 knockdown suppressed influenza virus A/WSN/33 replication, implying that the interaction is crucial for influenza virus replication in the host cell. We propose that influenza virus M2/BM2 cytoplasmic domain has an important role in the virus-host interplay and facilitates virus replication.

Regulation of Na,K-ATPase subunit abundance by translational repression

The Na,K-ATPase is an alphabeta heterodimer responsible for maintaining fluid and electrolyte homeostasis in mammalian cells. We engineered Madin-Darby canine kidney cell lines expressing alpha(1)FLAG, beta(1)FLAG, or beta(2)MYC subunits via a tetracycline-regulated promoter and a line expressing both stable beta(1)MYC and tetracycline-regulated beta(1)FLAG to examine regulatory mechanisms of sodium pump subunit expression. When overexpression of exogenous beta(1)FLAG increased total beta subunit levels by >200% without changes in alpha subunit abundance, endogenous beta(1) subunit (beta(1)E) abundance decreased. beta(1)E down-regulation did not occur during beta(2)MYC overexpression, indicating isoform specificity of the repression mechanism. Measurements of RNA stability and content indicated that decreased beta subunit expression was not accompanied by any change in mRNA levels. In addition, the degradation rate of beta subunits was not altered by beta(1)FLAG overexpression. Cells stably expressing beta(1)MYC, when induced to express beta(1)FLAG subunits, showed reduced beta(1)MYC and beta(1)E subunit abundance, indicating that these effects occur via the coding sequences of the down-regulated polypeptides. In a similar way, Madin-Darby canine kidney cells overexpressing exogenous alpha(1)FLAG subunits exhibited a reduction of endogenous alpha(1) subunits (alpha(1)E) with no change in alpha mRNA levels or beta subunits. The reduction in alpha(1)E compensated for alpha(1)FLAG subunit expression, resulting in unchanged total alpha subunit abundance. Thus, regulation of alpha subunit expression maintained its native level, whereas beta subunit was not as tightly regulated and its abundance could increase substantially over native levels. These effects also occurred in human embryonic kidney cells. These data are the first indication that cellular sodium pump subunit abundance is modulated by translational repression. This mechanism represents a novel, potentially important mechanism for regulation of Na,K-ATPase expression.

[Energetic applications: Na+/K+-ATPase and neuromuscular transmission]

Na/K-ATPase electrogenic activity and its indispensable role in maintaining gradients suggest that the modifications in isoform distribution and the functioning of the sodium pump have a major influence on both the neuronal functions, including excitability, and motor efficiency. This article proposes to clarify the involvement of Na/K-ATPase in the transmission of nerve influx within the peripheral nerve and then in the genesis, the maintenance, and the physiology of muscle contraction by comparing the data found in the literature with our work on neuron and muscle characterization of Na/K-ATPase activity and isoforms.

Expression, regulation and function of Na,K-ATPase in the lens

Na,K-ATPase is responsible for maintaining the correct concentrations of sodium and potassium in lens cells. Na,K-ATPase activity is different in the two cell types that make up the lens, epithelial cells and fibers; specific activity in the epithelium is higher than in fibers. In some parts of the fiber mass Na,K-ATPase activity is barely detectable. There is a large body of evidence that suggests Na,K-ATPase-mediated ion transport by the epithelium contributes significantly to the regulation of ionic composition in the entire lens. In some species different Na,K-ATPase isoforms are present in epithelium and fibers but in general, fibers and epithelium express a similar amount of Na,K-ATPase protein. Turnover of Na,K-ATPase by protein synthesis may contribute to preservation of high Na,K-ATPase activity in the epithelium. In ageing lens fibers, oxidation, and glycation may decrease Na,K-ATPase activity. Na,K-ATPase activity in lens fibers and epithelium also may be subject to regulation as the result of protein tyrosine phosphorylation. Moreover, activation of G protein-coupled receptors by agonists such as endothelin-1 elicits changes of Na,K-ATPase activity. The asymmetrical distribution of Na,K-ATPase activity in the epithelium and fibers may contribute to ionic currents that flow in and around the lens. Studies on human cataract and experimental cataract in animals reveal changes of Na,K-ATPase activity but no clear pattern is evident. However, there is a convincing link between abnormal elevation of lens sodium and the opacification of the lens cortex that occurs in age-related human cataract.

Mutational analysis of alpha-beta subunit interactions in the delivery of Na,K-ATPase heterodimers to the plasma membrane

The beta-subunit of the Na,K-ATPase is required to deliver functional alpha beta-heterodimers to the plasma membrane (PM) of baculovirus-infected insect cells. We have investigated the molecular determinants in the beta-subunit for the assembly and delivery processes. Trafficking of both subunits was analyzed by Western blots of fractionated membranes enriched in endoplasmic reticulum (ER), Golgi, and PM. Heterodimer assembly was evaluated by co-immunoprecipitation, and enzymatic activity was measured by ATPase assay. Elimination of enzymatic activity by D369A point mutation of the alpha-subunit had no effect on the compartmental distribution of the Na,K-ATPase, demonstrating that enzymatic functioning is not a prerequisite for PM delivery. Replacement of all three N-glycosylation site asparagines with glutamines produced no effect on the delivery to the PM or the activity of the enzyme, but increased susceptibility to degradation was observed. Analysis of beta-subunits in which the disulfide bonds were removed through substitution reveals that the bridges are important for PM targeting but not for assembly of the heterodimer. Assembly is supported by beta-subunits with greatly truncated extracellular domains. The presence of the amino-terminal domain and transmembrane segment is sufficient for assembly and PM delivery. Intermediate length truncated beta-subunits and some disulfide bridge substitution mutants assemble with the alpha-subunit but are not able to exit the ER. We conclude that there are different and separable requirements for the assembly of Na,K-ATPase heterodimer complexes, exit of the dimer from the ER, delivery to the PM, and catalytic activity of the dimer.

The thermal unfolding and domain structure of Na+/K+-exchanging ATPase. A scanning calorimetry study

The thermal unfolding and domain structure of Na+/K+-ATPase from pig kidney were studied by high-sensitivity differential scanning calorimetry (HS-DSC). The excess heat capacity function of Na+/K+-ATPase displays the unfolding of three cooperative domains with midpoint transition temperatures (Td) of 320.6, 327.5, 331.5 K, respectively. The domain with Td = 327.5 K was identified as corresponding to the beta subunit, while two other domains belong to the alpha subunit. The thermal unfolding of the low-temperature domain leads to large changes in the amplitude of the short-circuit current, but has no effect on the ATP hydrolysing activity. Furthermore, dithiothreitol or 2-mercaptoethanol treatment causes destruction of this domain, accompanied by significant disruption of the ion transporting function and a 25% loss of ATPase activity. The observed total unfolding enthalpy of the protein is rather low (approximately 12 J.g-1), suggesting that thermal denaturation of Na+/K+-ATPase does not lead to complete unfolding of the entire molecule. Presumably, transmembrane segments retain most of their secondary structure upon thermal denaturation. The binding of physiological ligands results in a pronounced increase in the conformational stability of both enzyme subunits.

Characterisation of tissue-specific oligosaccharides from rat brain and kidney membrane preparations enriched in Na+,K+-ATPase

The organ-specific nature of the glycosylation of Na+,K+-ATPase-enriched preparations from kidney and brain tissues has earlier been indicated by the use of lectin-staining techniques. Na+,K+-ATPase is ubiquitous and abundant, and subject to upregulation during cell-division and in certain pathological conditions. Lectins specific for the different carbohydrates displayed by the Na+,K+-ATPases may, therefore, be useful carriers/mediators in tissue-specific targeting. N-linked oligosaccharides purified from Na+,K+-ATPase-enriched preparations from rat brain and kidney were consequently characterised in detail in this study using weak anion exchange and normal phase HPLC (combined with serial glycosidase digestions) and matrix-assisted laser desorption/ionisation mass spectrometry. The oligomannose series of glycans were most abundant in the brain tissue preparation and this contrasted with the renal-associated oligosaccharides that were dominated by families of tetra-antennary glycans (with/without a core fucose) with up to four lactosaminylglycan residues in either branched or linear formation.

Insect Na(+)/K(+)-ATPase

Na(+)/K(+)-ATPase (sodium/potassium pump) is a P-type ion-motive ATPase found in the plasma membranes of animal cels. In vertebrates, the functions of this enzyme in nerves, heart and kidney are well characterized and characteristics a defined by different isoforms. In contrast, despite different tissue distributions, insects possess a single isoform of the alpha-subunit. A comparison of insect and vertebrate Na(+)/K(+)-ATPases reveals that although the mode of action and structure are very highly conserved, the specific roles of the enzyme in most tissues varies. However, the enzyme is essential for the function of nerve cells, and in this respect Na(+)/K(+)-ATPase appears to be fundamental in metazoan evolution.

Role of glycosylation and disulfide bond formation in the beta subunit in the folding and functional expression of Na,K-ATPase

Initial folding is a prerequisite for subunit assembly in oligomeric proteins. In this study, we have compared the role of co-translational modifications in the acquisition of an assembly-competent conformation of the beta subunit, the assembly of which is required for the structural and functional maturation of the catalytic Na,K-ATPase alpha subunit. Cysteine or asparagine residues implicated in disulfide bond formation or N-glycosylation, respectively, in the Xenopus beta1 subunit were eliminated by site-directed mutagenesis, and the assembly efficiency of the mutants and the functional expression of Na+,K+ pumps were studied after expression in Xenopus oocytes. Our results show that lack of each one of the two most C-terminal disulfide bonds indeed permits short term but completely abolishes long term assembly of the beta subunit. On the other hand, lack of the most N-terminal disulfide bonds allows the expression of a small number of functional Na+,K+ pumps at the cell surface. Elimination of all three but not of one or two glycosylation sites produces beta subunits that remain stably expressed in the endoplasmic reticulum, in association with binding protein but not as irreversible aggregates. The assembly efficiency of nonglycosylated beta subunits is decreased but a reduced number of functional Na+,K+ pumps is expressed at the cell surface. The lack of sugars does not influence the apparent K+ or ouabain affinity of the Na+,K+ pumps. Thus, these data show that disulfide bond formation and N-glycosylation may play important but qualitatively distinct roles in the initial folding of oligomeric protein subunits. Moreover, the results suggest that an endoplasmic reticulum degradation pathway exists, which is glycosylation-dependent.

Transmembrane topology of alpha- and beta-subunits of Na+,K+-ATPase derived from beta-galactosidase fusion proteins expressed in yeast

Various models of the transmembrane topology of the Na+,K+-ATPase predict either 8 or 10 membrane spans for the alpha-subunit and one to three membrane spans for the beta-subunit. Structure/function analysis, however, requires precise knowledge about the folding of enzymes. Therefore, the intention of this work was to establish a transmembrane topology model for the subunits of Na+,K+-ATPase. The bacterial enzyme beta-galactosidase was fused to the C termini of truncated alpha- and beta-subunits of Na+,K+-ATPase. Fusions were generated at Arg60 (LTTAR60), Glu116 (AATEE116), Ala247 (VEGTA247), Leu311 (YTWEL311), Ala444 (VAGDA444), Ala789 (IFIIA789), Met809 (LGTDM809), Asp884 (RVTWD884), Ile946 (MKNKI946), and Arg972 (GVALR972) of the sheep alpha1-subunit and at Pro236 (LGGYP236) of the dog beta-subunit. The fusion constructs were expressed in yeast cells for studies on the localization of the fused reporter enzyme. Activity measurements of the reporter enzyme revealed that only intracellular fusion sites lead to active beta-galactosidase. Indirect immunofluorescence microscopy with cells expressing alpha1/beta-galactosidase and beta/beta-galactosidase hybrid proteins demonstrated that inactive beta-galactosidase is associated with the yeast plasma membrane and can be detected from the extracellular side. The data obtained suggest that Pro236 of the beta-subunit is located on the extracellular surface, corresponding to a model with one transmembrane segment, and that the alpha-subunit of the Na+,K+-ATPase consists of 10 membrane-associated spans. They also suggest that a stretch of the alpha1-subunit between membrane spans M7 and M8 might be hidden within the membrane, surrounded by the other hydrophobic spans, in analogy to the P-loop of Na+ or K+ channels and to the "hourglass" structure of water channels.

Topology of the Na,K-ATPase. Evidence for externalization of a labile transmembrane structure during heating

The topological organization of the Na,K-ATPase alpha subunit is controversial. Detection of extracellular proteolytic cleavage sites would help define the topology, and so attempts were made to find conditions and proteases that would permit digestion of Na,K-ATPase in sealed right-side-out vesicles from renal medulla. The beta subunit is predominantly extracellular and could mask the surface of the alpha subunit. Most of the tested proteases cleaved beta, and some digested it extensively. However, without further disruption of structure, there was still no digestion of the alpha subunit. Reduction (at 50 degrees C) of disulfide bonds that might stabilize the beta subunit fragments, or heating alone at 55 degrees C, permitted tryptic digestion of alpha at a site close to the C terminus, while simultaneously increasing digestion of beta. A 90-kDa N-terminal fragment of alpha was recovered, but the C-terminal fragment was further digested. Heating and reduction resulted in the extracellular exposure of a protein kinase A phosphorylation site, Ser-938, and the C terminus, both of which have been proposed to be located on the intracellular surface. At the same time, access to a distant protein kinase C phosphorylation site was not increased. The data suggest that the harsh treatment simultaneously resulted in alteration of the beta subunit and the extrusion of a segment of alpha that normally spans the membrane, without causing complete denaturation or opening the sealed vesicles. Preincubation with Rb+ was protective, consistent with prior evidence that it stabilizes the protein segments in the C-terminal third of alpha. We conclude that this portion of the alpha subunit contains a transmembrane structure with unique lability to heating.

Purification of the alpha and beta subunits of (Na,K)-ATPase by continuous elution electrophoresis

Covalent structural information on membrane proteins is not easily acquired since it is difficult to obtain pure membrane proteins in sufficient quantities. We have therefore examined the Bio-Rad 491 prep cell continuous elution electrophoresis apparatus as a method for providing the quantities of purified alpha and beta subunits from (Na,K)-ATPase required for these studies. Twenty-four milligrams of crude (Na,K)-ATPase preparation was applied to the prep cell which consisted of a 7% Laemmli separating gel 4.5 cm in length. The prep cell was run under constant power and continuous cooling conditions. Those fractions containing the beta subunit were combined and further purified by wheat germ agglutinin affinity chromatography. Fractions containing the alpha subunit were combined and did not require further purification. The identity and the degree of purity of the proteins obtained using this approach was assessed utilizing SDS-PAGE, amino acid analysis and N-terminal sequencing. This simple and fast method provides approximately 1.8 milligrams of each purified subunit from 24 milligrams of relatively crude microsomes. Recovery of the alpha and beta subunits from the crude (Na,K)-ATPase preparation was estimated to be 28% and 81%, respectively.

Sequence analysis of DNA encoding an avian Na+,K(+)-ATPase beta 2-subunit

The DNA encoding a chicken Na+,K(+)-ATPase beta 2-subunit was cloned and sequenced. The deduced amino acid sequence has structural features common to all known Na+,K(+)-ATPase beta-subunits. It is proposed to belong to the beta 2-isoform family, though the amino acid sequence has significantly diverged from mammalian beta 2-subunit sequences. Similar to other Na+,K(+)-ATPase beta 2-isoforms, the chicken beta 2-isoform mRNA is predominantly expressed in brain tissue.

Isolation and purification of the extracellular and intracellular portions of the beta subunit of (Na+,K+)-ATPase

The beta subunit of lamb kidney (Na+,K+)-ATPase was isolated by size exclusion high performance liquid chromatography. Treatment of the beta subunit with formic acid yielded two peptide fragments which were purified via reversed phase high performance liquid chromatography. These peptides were identified by sodium dodecylsulfate polyacrylamide gel electrophoresis, amino acid analysis and N-terminal sequencing as (Pro 94-Ser 302), a largely hydrophilic peptide which comprises the major portion of the extracellular domain including six Cys residues which participate in disulfide bond formation and three glycosylation sites and a smaller peptide (Ala 1-Asp 93) which contains the single membrane spanning region and the intracellular domain.

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)

Chemical modification as an approach to elucidation of sodium pump structure-function relations

Chemical modification of specific residues in enzymes, with the characterization of the type of inhibition and properties of the modified activity, is an established approach in structure-function studies of proteins. This strategy has become more productive in recent years with the advances made in obtaining primary sequence information from gene-cloning technologies. This article discusses the application of chemical modification procedures to the study of the Na(+)-K(+)-ATPase protein. A wide array of information has become available about the kinetics, enzyme structure, and various conformational states as a result of the combined use of inhibitors, ligands, modifiers, and proteolytic enzymes. We will review a variety of reagents and approaches that have been employed to arrive at structure-function correlates and discuss critically the limits and ambiguities in the type of information obtained from these methodologies. Chemical modification of the Na(+)-pump protein has already provided a body of data and will, we anticipate, guide the efforts of mutagenesis studies in the future when suitable expression systems become available.

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.

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.