Photoaffinity labeling of the ouabain-binding site on (Na+ plus K+) adenosinetriphosphatase

The Na+,K+-ATPase and its stoichiometric ratio: some thermodynamic speculations

 Almost seventy years after its discovery, the sodium-potassium adenosine triphosphatase (the sodium pump) located in the cell plasma membrane remains a source of novel mechanistic and physiologic findings. A noteworthy feature of this enzyme/transporter is its robust stoichiometric ratio under physiological conditions: it sequentially counter-transports three sodium ions and two potassium ions against their electrochemical potential gradients per each hydrolyzed ATP molecule. Here we summarize some present knowledge about the sodium pump and its physiological roles, and speculate whether energetic constraints may have played a role in the evolutionary selection of its characteristic stoichiometric ratio.

Ouabain Suppresses IL-6/STAT3 Signaling and Promotes Cytokine Secretion in Cultured Skeletal Muscle Cells

The cardiotonic steroids (CTS), such as ouabain and marinobufagenin, are thought to be adrenocortical hormones secreted during exercise and the stress response. The catalytic α-subunit of Na,K-ATPase (NKA) is a CTS receptor, whose largest pool is located in skeletal muscles, indicating that muscles are a major target for CTS. Skeletal muscles contribute to adaptations to exercise by secreting interleukin-6 (IL-6) and plethora of other cytokines, which exert paracrine and endocrine effects in muscles and non-muscle tissues. Here, we determined whether ouabain, a prototypical CTS, modulates IL-6 signaling and secretion in the cultured human skeletal muscle cells. Ouabain (2.5–50 nM) suppressed the abundance of STAT3, a key transcription factor downstream of the IL-6 receptor, as well as its basal and IL-6-stimulated phosphorylation. Conversely, ouabain (50 nM) increased the phosphorylation of ERK1/2, Akt, p70S6K, and S6 ribosomal protein, indicating activation of the ERK1/2 and the Akt-mTOR pathways. Proteasome inhibitor MG-132 blocked the ouabain-induced suppression of the total STAT3, but did not prevent the dephosphorylation of STAT3. Ouabain (50 nM) suppressed hypoxia-inducible factor-1α (HIF-1α), a modulator of STAT3 signaling, but gene silencing of HIF-1α and/or its partner protein HIF-1β did not mimic effects of ouabain on the phosphorylation of STAT3. Ouabain (50 nM) failed to suppress the phosphorylation of STAT3 and HIF-1α in rat L6 skeletal muscle cells, which express the ouabain-resistant α1-subunit of NKA. We also found that ouabain (100 nM) promoted the secretion of IL-6, IL-8, GM-CSF, and TNF-α from the skeletal muscle cells of healthy subjects, and the secretion of GM-CSF from cells of subjects with the type 2 diabetes. Marinobufagenin (10 nM), another important CTS, did not alter the secretion of these cytokines. In conclusion, our study shows that ouabain suppresses the IL-6 signaling via STAT3, but promotes the secretion of IL-6 and other cytokines, which might represent a negative feedback in the IL-6/STAT3 pathway. Collectively, our results implicate a role for CTS and NKA in regulation of the IL-6 signaling and secretion in skeletal muscle.

Ouabain resistance of the epithelial cell line (Ma104) is not due to lack of affinity of its pumps for the drug

Na+, K(+)-pumps of most eukaryotic animal cells bind ouabain with high affinity, stop pumping, and consequently loose K+, detach from each other and from the substrate, and die. Lack of affinity for the drug results in ouabain resistance. In this work, we report that Ma104 cells (epithelial from Rhesus monkey kidney) have a novel form of ouabain-resistance: they bind the drug with high affinity (Km about 4 x 10(-8) M), they loose their K+ and stop proliferating but, in spite of these, up to 100% of the cells remain attached in 1.0 microM ouabain, and 53% in 1.0 mM. When 4 days later ouabain is removed from the culture medium, cells regain K+ and resume proliferation. Strophanthidin, a drug that attaches less firmly than ouabain, produces a similar phenomenon, but allows a considerably faster recovery. This reversal may be associated to the fact that, while in ouabain-sensitive MDCK cells Na+, K(+)-ATPases blocked by the drug are retrieved from the plasma membrane, those in Ma104 cells remain at the cell-cell border, as if they were cell-cell attaching molecules. Cycloheximide (10 micrograms/ml) and chloroquine (10 microM) impair this recovery, suggesting that it also depends on the synthesis and insertion of a crucial protein component, that may be different from the pump itself. Therefore ouabain resistance of Ma104 cells is not due to a lack of affinity for the drug, but to a failure of its Na+, K(+)-ATPases to detach from the plasma membrane in spite of being blocked by ouabain.

Involvement of Na+,K(+)-ATPase inhibition in K562 cell differentiation induced by bufalin

Human leukemia K562 cell differentiation induction by naturally occurring bufadienolides purified from the Chinese drug Senso and synthetic bufalin derivatives was examined by a nitro blue tetrazolium reduction assay. Bufalin showed the strongest activity among all the bufadienolides tested in this study. The degree of the induction of nitro blue diformazan positive cells by the bufadienolides correlated well with their inhibitory activities against Na+,K(+)-ATPase prepared from K562 cells in vitro. N+,K(+)-ATPases from a variant K562 clone (ouabain resistant, OuaR) and murine leukemia cell line M1-T22, which were insensitive to the bufadienolides in terms of growth inhibition and cell differentiation, appeared to be refractory to bufalin in vitro. A binding study of 3H-bufalin and 3H-ouabain revealed that saturated levels of both ligands associated with K562 cells were virtually similar; however, affinity of 3H-bufalin was considerably higher than 3H-ouabain. The saturated level of 3H-bufalin observed in the OuaR cells was approximately half of that observed in K562 cells without a change in its affinity. Association of 3H-bufalin with K562 cells was completely blocked by pretreatment of the cells with cold ouabain at concentrations saturating the binding sites. These results suggest that bufalin acts on the cells by binding to sites on the cell membrane which also bind ouabain. It is thus proposed that N+,K(+)-ATPase inhibition is closely related to the initiation process in the induction of K562 cell differentiation induced by bufalin.

Analysis of amino acid residues in the H5-H6 transmembrane and extracellular domains of Na,K-ATPase alpha subunit identifies threonine 797 as a determinant of ouabain sensitivity

Several amino acid residues of the alpha subunit of the Na,K-ATPase have been identified which alter ouabain sensitivity. These residues are located in the N-terminal half of the alpha 1 subunit suggesting that this portion of the molecule may represent the binding site for cardiac glycosides. However, not all extracellular and transmembrane regions have been investigated, including the H5-H6 membrane-spanning region. To determine if this region of the alpha subunit contributes to ouabain sensitivity, amino acids which have the potential to form hydrogen bonds were substituted with alanine, a non-hydrogen-bonding amino acid. cDNAs encoding enzyme containing these individual amino acid replacements were expressed in ouabain-sensitive HeLa cells, and the ability of the altered enzymes to confer ouabain resistance was examined. Nineteen amino acid substitutions were investigated. T797A (Thr 797 to Ala) was the only substitution which conferred ouabain resistance to sensitive HeLa cells. Three additional substitutions at this position (T797V, T797S, and T797D) were generated in order to examine the effects of the replacements of Thr 797 on ouabain inhibition of Na,K-ATPase activity. The T797V substitution conferred ouabain resistance, but T797S and T797D substitutions did not. The ouabain-resistant cell lines expressing the T797A and T797V substitutions exhibited Na,K-ATPase activity that was 60 and 70 times more resistant to ouabain than the endogenous HeLa or sheep enzymes. The absence of a hydroxyl group at amino acid 797 may be responsible for the reduced sensitivity of the enzyme with substitutions at this position.(ABSTRACT TRUNCATED AT 250 WORDS)

The carboxy terminus of sodium and potassium ion transporting ATPase is located on the cytoplasmic surface of the membrane

The positions, with respect to the plasma membrane, of lysine 905, contained in the peptide QRKIVE, and of lysine 1012, contained in the carboxy-terminal peptide, RPGGWVEKETYY, of ovine Na+/K(+)-transporting ATPase have been reported to be cytoplasmic and extracytoplasmic, respectively [Bayer, R. (1990) Biochemistry 29, 2551-2256]. These results from our laboratory have been reexamined using an extension of the same procedure. Sealed right-side-out vesicles were modified with pyridoxal phosphate and sodium [3H]borohydride in the presence and absence of saponin or cholate. The modified alpha polypeptide was isolated and digested with the proteinase from Staphylococcus aureus strain V8 or trypsin to produce one or the other of these two peptides. These digests were passed over immunoadsorbents, identical to those used by Bayer, directed against pyroglutamylRXIVE or -ETYY. Unlike in the earlier studies, however, in the present studies the modified, radioactive peptides bound and eluted from the immunoadsorbents were submitted to HPLC, and their respective mobilities were compared to those of the synthetic peptides that had also been modified with pyridoxal phosphate. In this manner, the correct, modified peptide could be positively identified, and its specific radioactivity could be estimated. When cholate was added to sealed vesicles, prior to modification, there was at least a 3-fold increase in the incorporation of radioactivity into lysine 1012, consistent with a cytoplasmic location for this residue.(ABSTRACT TRUNCATED AT 250 WORDS)

Substitution of transmembrane residues with hydrogen-bonding potential in the alpha subunit of Na,K-ATPase reveals alterations in ouabain sensitivity

The role of H-bonding amino acids as determinants of ouabain affinity in the Na,K-ATPase was examined. Site-directed mutagenesis was used to substitute 21 conserved amino acid residues in the sheep alpha-subunit transmembrane regions. The amino acids were changed from residues which possess side chains capable of forming H-bonds with specific cardiac glycoside moieties such as the lactone ring or sugar(s) to residues unable to participate in H-bonding. The effect of each of these amino acid replacements on the affinity of the Na,K-ATPase for ouabain was initially assessed by screening the altered enzymes for the ability to confer ouabain resistance when expressed in otherwise sensitive HeLa cells. Three of the substitutions (Tyr-108 to Ala, Cys-104 to Ala, and Cys-104 to Phe) were able to confer resistance to the normally sensitive HeLa cells. Stable cell lines, each expressing one of the altered enzymes, were further characterized in terms of ouabain-inhibitable cell growth and Na,K-ATPase activity. Cell lines expressing the alpha 1-isoform substitution Y108A, C104A, or C104F contained a Na,K-ATPase activity which gave an I50 value for enzyme inhibition 9-, 6-, and 150-fold greater, respectively, than the endogeneous HeLa or sheep enzyme. These data show that Tyr-108 and Cys-104 of the alpha subunit are determinants of ouabain affinity. Cys-104 has also been shown to be a determinant of ouabain sensitivity in Xenopus laevis [Canessa, C. M., Horisberger, J.-D., Louvard, D., & Rossier, B. C. (1992) EMBO J. 11, 1681-1687].(ABSTRACT TRUNCATED AT 250 WORDS)

Up-regulation of sodium pump activity in Xenopus laevis oocytes by expression of heterologous beta 1 subunits of the sodium pump

Recent evidence suggests that the beta subunit of the Na+ pump is essential for the alpha subunit to express catalytic activity and for assembly of the holoenzyme in the plasma membrane. We report here that injection into Xenopus laevis oocytes of cRNAs specific for beta 1 subunit isoforms of the Na+ pump of four species (Torpedo californica, chicken, mouse and rat) causes a time-dependent increase in the number of ouabain-binding sites, both in the plasma membrane and in internal membranes. Expression of the beta 1 subunit of the Na+ pump of mouse and rat in the oocytes could be substantiated by immunoprecipitation using a polyclonal antiserum against the mouse beta 1 subunit. Scatchard analysis in permeabilized cells disclosed that the affinity for ouabain is unchanged after expression of each of the beta 1 subunits. A proportional increase in ouabain-sensitive 86Rb+ uptake indicates that the additionally expressed ouabain-binding sites on the cell surface represent functional Na+ pumps. The findings support the concept of Geering. Theulaz, Verrey, Häuptle & Rossier [(1989) Am. J. Physiol. 257, C851-C858] that beta 1 subunits expressed in oocytes associate with an excess of endogenous alpha subunits of the Na+ pump to form a hybrid enzyme. In addition, all of the beta 1 isoforms investigated in the present study were also capable of combining with the co-expressed alpha 1 subunit of the Torpedo Na+ pump to produce a functional enzyme. Injection of cRNA encoding for the Torpedo alpha 1 subunit alone had no effect on the ouabain-binding capacity of the surface and intracellular membranes of the oocyte.

Functional expression of the genomic DNA sequences encoding mouse Na,K-ATPase alpha 1 gene by cotransfection of overlapping genomic DNA segments

The entire 33-kb coding region of the mouse Na,K-ATPase alpha 1 subunit gene was cloned in two overlapping cosmids which contain inserts of 40 kb. To assess the functional expression of the mouse alpha 1 gene, the two cosmids were cotransfected into ouabain-sensitive CV-1 monkey cells yielding an average of 64 resistant colonies per 10(6) cells per microgram of DNA. Analysis of the DNA transferred to the ouabain-resistant transformants by the two cosmids suggests that the generation of a functional gene can occur by homologous recombination between the two introduced segments, as demonstrated by generation of a novel diagnostic restriction fragment. The ability to reconstruct the intact mouse alpha 1 gene in a heterologous host cell and to monitor its functional expression with a selection protocol permits direct identification and isolation of regulatory sequences for the gene.

Functional expression of the genomic DNA sequences encoding mouse Na,K-ATPase alpha 1 gene by cotransfection of overlapping genomic DNA segments

The entire 33-kb coding region of the mouse Na,K-ATPase alpha 1 subunit gene was cloned in two overlapping cosmids which contain inserts of 40 kb. To assess the functional expression of the mouse alpha 1 gene, the two cosmids were cotransfected into ouabain-sensitive CV-1 monkey cells yielding an average of 64 resistant colonies per 10(6) cells per microgram of DNA. Analysis of the DNA transferred to the ouabain-resistant transformants by the two cosmids suggests that the generation of a functional gene can occur by homologous recombination between the two introduced segments, as demonstrated by generation of a novel diagnostic restriction fragment. The ability to reconstruct the intact mouse alpha 1 gene in a heterologous host cell and to monitor its functional expression with a selection protocol permits direct identification and isolation of regulatory sequences for the gene.

Topological disposition of the sequences -QRKIVE- and -KETYY in native (Na+ + K+)-ATPase

The dispositions with respect to the plane of the membrane of lysine-905 in the internal sequence -EQRKIVE- and of lysine-1012 in the carboxy-terminal sequence -RRPGGWVEKETYY of the alpha-polypeptide of sodium and potassium ion activated adenosinetriphosphatase have been determined. These lysines are found in peptides released from the intact alpha-polypeptide by the extracellular protease from Staphylococcus aureus strain V8 and by trypsin, respectively. Synthetic peptides containing terminal sequences of these were used to prepare polyclonal antibodies, which were then used to prepare immunoadsorbents directed against the respective peptides. Sealed, right-side-out membrane vesicles containing native (Na+ + K+)-ATPase were labeled with pyridoxal phosphate and sodium [3H]borohydride in the absence or presence of saponin. The labeled alpha-polypeptide was isolated from these vesicles and digested with appropriate proteases. The incorporation of radioactivity into the peptides binding to the immunoadsorbent directed against the sequence pyrERXIVE increased 3-fold in the presence of saponin as a result of the increased accessibility of this portion of the protein to the reagent when the vesicles were breached by saponin; hence, this sequence is located on the cytoplasmic face of the membrane. It was inferred that the carboxy-terminal sequence -KETYY is on the extracytoplasmic face since the incorporation of radioactivity into peptides binding to the immunoadsorbent directed against the sequence -ETYY did not change when the vesicles were breached with saponin.

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.

Identification of a region within the Na,K-ATPase alpha subunit that contributes to differential ouabain sensitivity

To analyze determinants within the Na,K-ATPase alpha subunit that contribute to differential ouabain sensitivity, we constructed and expressed a panel of chimeric cDNA molecules between ouabain-resistant and ouabain-sensitive alpha subunit cDNAs. When introduced into ouabain-sensitive monkey CV-1 cells, ouabain-resistant rat alpha 1 subunit cDNA and chimeras in which the 5' end of ouabain-sensitive human alpha 1 or rat alpha 2 subunit cDNA was replaced by the 5' end of rat alpha 1 subunit cDNA conferred resistance to 100 microM ouabain. Monkey cells transfected with the reciprocal chimeras were unable to survive selection in 1 microM ouabain. Rat alpha 2 subunit cDNA and a chimera in which the 5' end of rat alpha 1 subunit cDNA was replaced by the 5' end of rat alpha 2 subunit cDNA conferred resistance to 0.5 microM ouabain. These results suggest that determinants of ouabain resistance reside within the amino-terminal portions of the rat alpha 1 and alpha 2 subunits. Expression of chimeric alpha subunit cDNAs should prove useful for elucidating the structural basis of Na,K-ATPase function.

Identification of a region within the Na,K-ATPase alpha subunit that contributes to differential ouabain sensitivity

To analyze determinants within the Na,K-ATPase alpha subunit that contribute to differential ouabain sensitivity, we constructed and expressed a panel of chimeric cDNA molecules between ouabain-resistant and ouabain-sensitive alpha subunit cDNAs. When introduced into ouabain-sensitive monkey CV-1 cells, ouabain-resistant rat alpha 1 subunit cDNA and chimeras in which the 5' end of ouabain-sensitive human alpha 1 or rat alpha 2 subunit cDNA was replaced by the 5' end of rat alpha 1 subunit cDNA conferred resistance to 100 microM ouabain. Monkey cells transfected with the reciprocal chimeras were unable to survive selection in 1 microM ouabain. Rat alpha 2 subunit cDNA and a chimera in which the 5' end of rat alpha 1 subunit cDNA was replaced by the 5' end of rat alpha 2 subunit cDNA conferred resistance to 0.5 microM ouabain. These results suggest that determinants of ouabain resistance reside within the amino-terminal portions of the rat alpha 1 and alpha 2 subunits. Expression of chimeric alpha subunit cDNAs should prove useful for elucidating the structural basis of Na,K-ATPase function.

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.

Synthesis of virus-specific RNA in permeabilized murine coronavirus-infected cells

We have developed a permeabilized cell system for assaying mouse hepatitis virus-specific RNA polymerase activity. This activity was characterized as to its requirements for mono- and divalent cations, requirements for an exogenous energy source, and pH optimum. This system faithfully reflects MHV-specific RNA synthesis in the intact cell, with regard to both its time of appearance during the course of infection and the products synthesized. The system is efficient and the RNA products were identical to those observed in intact MHV-infected cells as judged by agarose gel electrophoresis and hybridization. Permeabilized cells appear to be an ideal system for studying coronavirus RNA synthesis since they closely mimic in vivo conditions while allowing much of the experimental flexibility of truly cell-free systems.

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

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

Ouabain resistance conferred by expression of the cDNA for a murine Na+, K+-ATPase alpha subunit

The molecular basis for the marked difference between primate and rodent cells in sensitivity to the cardiac glycoside ouabain has been established by genetic techniques. A complementary DNA encoding the entire alpha 1 subunit of the mouse Na+- and K+-dependent adenosine triphosphatase (ATPase) was inserted into the expression vector pSV2. This engineered DNA molecule confers resistance against 10(-4) M ouabain to monkey CV-1 cells. Deletion of sequences encoding the carboxyl terminus of the alpha 1 subunit abolish the activity of the complementary DNA. The ability to assay the biological activity of this ATPase in a transfection protocol permits the application of molecular genetic techniques to the analysis of structure-function relationships for the enzyme that establishes the internal Na+/K+ environment of most animal cells. The full-length alpha 1 subunit complementary DNA will also be useful as a dominant selectable marker for somatic cell genetic studies utilizing ouabain-sensitive cells.

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

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

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

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

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

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

Particles and pits matched in native membranes

We have obtained high resolution electron microscopic images of complementary membrane surface replicas of purified microsomal vesicles from pig kidney outer medulla containing Na+,K+-ATPase. Ultra-rapid freezing of a membrane suspension was followed by fracturing and replicating of the liquid helium cooled specimen under ultra-high vacuum conditions free of hydrocarbon contaminants. The protoplasmic fracture faces are populated with intramembrane particles while the external fracture faces reveal complementary pits. This is the first demonstration of extended precise matching of individual intramembrane particles and their corresponding pits in biological membranes containing transmembrane proteins. The data are also consistent with the theory that the majority of Na+,K+-ATPase mass is located at the protoplasmic half of the membrane.

Demonstration of an Mg2+-induced conformational change by photoaffinity labelling of the high-affinity ATP-binding site of (Na+ + K+)-ATPase with 8-azido-ATP

8-Azido-ATP (8-N3ATP) is a substrate of (Na+ + K+)-ATPase from pork kidney and photoinactivates it by binding to the Mr = 100 000 alpha-subunit. The photoinactivation requires the presence of Mg2+ even though 8-azido-ATP is recognized by the high-affinity ATP binding site (Kd = 3.1 microM). K+ ions protect the enzyme against photoinactivation as does excess ATP. To see whether the Mg2+-requirement of the photoinactivation is due to the action of free Mg2+ or to the existence of an Mg X 8-azido-ATP complex, the action of the stable Mg X ATP complex analogue, chromium X 8-N3ATP (Cr X 8-N3ATP), was studied. Cr X 8-N3ATP photoinactivates (Na+ + K+)-ATPase in the absence of Mg2+, but the photoinactivation is enhanced by Mg2+, indicating that the formation of a Mg X ATP complex is an absolute requirement for photoinactivation. However, the interaction of Mg2+ with a low-affinity site also enhances the photoinactivation. It is therefore concluded that interactions with MgATP and free Mg induce conformational changes in the purine subsite of the high-affinity ATP binding site. Controlled trypsinolysis of the [alpha-32P]8-N3ATP-photolabelled enzyme in the presence of K+ results in the formation of an Mr = 56 000 radioactive peptide, whereas trypsinolysis of a [gamma-32P]Cr X ATP-labelled enzyme under identical conditions forms an Mr = 41 000 radioactive peptide. Extensive trypsinolysis of the [alpha-32P] 8-N3ATP-photolabelled alpha-subunit leads to the formation of a radioactive peptide of Mr = 1800.

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

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

Structural organization of (Na+ + K+)-ATPase in purified membranes

The structural organization of crystalline, membrane-bound (Na+ + K+)-ATPase was studied by negative staining and thin sectioning. The enzyme molecules were induced to form crystalline arrays within fragments of membrane by incubation in defined ionic conditions. The enzyme remained fully active after crystallization. Negative staining and computer processing of images of the crystalline specimens identified two discrete crystalline arrays. The dimensions of the unit cell of one of the arrays were large enough to accommodate an alpha beta protomer; those of the other array, an (alpha beta)2 diprotomer . Thin sections of the crystalline fraction contained a unique membrane complex that was formed from two apposed plasma membranes. The paired membranes in this complex were separated by a center-to-center space of 15 nm containing evenly spaced septa that connected the membrane surfaces; the overall thickness of the entire structure was 22-25 nm. The agglutinin from Ricinus communis, a lectin that binds to the carbohydrate moiety of the beta-subunit of (Na+ + K+)-ATPase, decorated the free surfaces of the complex. Therefore, this complex of paired membranes is the result of interactions between the cytoplasmic domains of the enzyme. From measurements of the dimensions of these structures, we estimate the overall length of the enzyme to be approximately 11.5 nm along the axis perpendicular to the plane of the membrane, and the molecular protrudes more (approximately 5 nm) on the cytoplasmic surface than on the extracytoplasmic surface (approximately 2 nm).

Recognition of sodium- and potassium-dependent adenosine triphosphatase in organs of the mouse by means of a monoclonal antibody

The antigen detected by the rat anti-mouse monoclonal antibody (m Ab), anti-BSP-3, has been initially described as a brain cell-surface protein. Evidence is presented that this m Ab recognizes mouse (Na+ + K+)-ATPase (ATP phosphohydrolase, E.C.3.6.1.3). The antigen, purified from mouse brain by means of affinity chromatography, migrated in SDS-polyacrylamide gels in the form of two polypeptide chains of 100 000 and 48 000 molecular weight, which could be shown to react with subunit-specific polyclonal antisera against ATPase in immunoblotting experiments. Purified BSP-3 antigen was bound to the specific (Na+ + K+)-ATPase inhibitor ouabain. Finally, the anti-BSP-3 m Ab was capable of immunoprecipitating the ATPase activity of a microsomal fraction from mouse kidney. The m Ab was used to study the localization of (Na+ + K+)-ATPase in different organs of the mouse. It stained the basolateral plasma membranes of polarized cells in immunofluorescence experiments, while the entire cell surface of unpolarized cells was labeled. Interestingly, several cell types did not react with the m Ab, indicating a possible heterogeneity of ATPases. Such a m Ab could prove to be a useful tool for studying localization, structure and function of (Na+ + K+)-ATPase.

Effects of base exchange reaction on the Na+, K+ ATPase in rat brain microsomes

Incorporation of ethanolamine and monomethylethanolamine into their corresponding phospholipid by the base exchange enzymes activated an Na+, K+-ATPase associated with a rat brain microsomes enriched preparation. The serine and dimethylethanolamine base exchange catalyzed incorporation reactions inhibited this particular Na+, K+-ATPase. These effects require Ca2+ and several other structural analogues which are not incorporated into phospholipid were without affect on this ATPase.

Structural and biosynthetic studies on the two molecular forms of the (Na+ + K+)-activated adenosine triphosphatase large subunit in Artemia salina Nauplii

The large subunit of (Na+ + K+)-activated ATPase from brine shrimp, Artemia salina, migrates as two bands in sodium dodecyl sulfate-polyacrylamide gels. The slower migrating band, as observed in neutral or alkaline gel systems, is designated alpha 1 and the faster, alpha 2. Structural and biosynthetic studies have been performed to determine if these two bands represent independent molecular forms or precursor products. Peptide mapping of partial proteolytic digests of alpha 1 and alpha 2 showed no distinguishable difference between them whereas this technique produced very distinct differences in the large subunit derived from three different species. The two large subunit bands also behaved identically when cross linked with cupric phenanthroline either in the presence or absence of digitonin, whereas other proteins in these preparations were unaffected. The peptide mapping and cross-linking experiments demonstrate that alpha 1 and alpha 2 have identical or nearly identical primary and probably higher order structure. Their different mobilities may be due to post-translational modification leading, for example, to different oligosaccharide composition. During development of the brine shrimp nauplius, alpha 1 increases in relative abundance while alpha 2 decreases. NaH14CO3 incorporation and pulse-chase experiments indicate that alpha 1 and alpha 2, as well as the small subunit of the brine shrimp (Na+ + K+)-activated ATPase, are synthesized at the same time during development and that all changes in the rates of synthesis of these subunits occur at the same time. The apparent rates of degradation of the subunits are also similar. These results are inconsistent with a precursor-product relationship between alpha 1 and alpha 2.