A proteolipid associated with Na,K-ATPase is not essential for ATPase activity

Some Biological Consequences of the Inhibition of Na,K-ATPase by Translationally Controlled Tumor Protein (TCTP)

Na,K-ATPase is an ionic pump that regulates the osmotic equilibrium and membrane potential of cells and also functions as a signal transducer. The interaction of Na,K-ATPase with translationally controlled tumor protein (TCTP) results, among others, in the inhibition of the former's pump activity and in the initiation of manifold biological and pathological phenomena. These phenomena include hypertension and cataract development in TCTP-overexpressing transgenic mice, as well as the induction of tumorigenesis signaling pathways and the activation of Src that ultimately leads to cell proliferation and migration. This review attempts to collate the biological effects of Na,K-ATPase and TCTP interaction and suggests that this interaction has the potential to serve as a possible therapeutic target for selected diseases.

On the Many Actions of Ouabain: Pro-Cystogenic Effects in Autosomal Dominant Polycystic Kidney Disease

Ouabain and other cardenolides are steroidal compounds originally discovered in plants. Cardenolides were first used as poisons, but after finding their beneficial cardiotonic effects, they were rapidly included in the medical pharmacopeia. The use of cardenolides to treat congestive heart failure remained empirical for centuries and only relatively recently, their mechanisms of action became better understood. A breakthrough came with the discovery that ouabain and other cardenolides exist as endogenous compounds that circulate in the bloodstream of mammals. This elevated these compounds to the category of hormones and opened new lines of investigation directed to further study their biological role. Another important discovery was the finding that the effect of ouabain was mediated not only by inhibition of the activity of the Na,K-ATPase (NKA), but by the unexpected role of NKA as a receptor and a signal transducer, which activates a complex cascade of intracellular second messengers in the cell. This broadened the interest for ouabain and showed that it exerts actions that go beyond its cardiotonic effect. It is now clear that ouabain regulates multiple cell functions, including cell proliferation and hypertrophy, apoptosis, cell adhesion, cell migration, and cell metabolism in a cell and tissue type specific manner. This review article focuses on the cardenolide ouabain and discusses its various in vitro and in vivo effects, its role as an endogenous compound, its mechanisms of action, and its potential use as a therapeutic agent; placing especial emphasis on our findings of ouabain as a pro-cystogenic agent in autosomal dominant polycystic kidney disease (ADPKD).

The gamma subunit of Na+, K+-ATPase: role on ATPase activity and regulatory phosphorylation by PKA

In kidney, Na+, K+-ATPase is an oligomer (alphabeta gamma) with equimolar amounts of essential alpha and beta subunits and one small hydrophobic FXYD protein (gamma subunit). This report describes gamma subunit as an activator of pig kidney outer medulla Na+, K+-ATPase in aqueous medium. The effects of gamma subunit on Na+, K+-ATPase were dose-dependent and preincubation-dependent. Changes in alphabeta/gamma stoichiometry did not alter Km1 for ATP, and slightly increased Km2, but Vmax was increased at both catalytic and regulatory sites. Hydroxylamine treatment of enzyme phosphorylated by ATP (E-P), in the presence of additional gamma subunit, revealed that 52% of the E-P accumulation was not via acyl-phosphate formation. The gamma subunit was phosphorylated by endogenous kinases and by commercial catalytic subunit of protein kinase A (PKA). Additionally, we demonstrated that PKA phosphorylation of gamma subunit increased its capacity to stimulate ATP hydrolysis. These results suggest that gamma subunit can act as an intrinsic Na+, K+-ATPase regulator in kidney.

Cell Adhesion, Polarity, and Epithelia in the Dawn of Metazoans

Transporting epithelia posed formidable conundrums right from the moment that Du Bois Raymond discovered their asymmetric behavior, a century and a half ago. It took a century and a half to start unraveling the mechanisms of occluding junctions and polarity, but we now face another puzzle: lest its cells died in minutes, the first high metazoa (i.e., higher than a sponge) needed a transporting epithelium, but a transporting epithelium is an incredibly improbable combination of occluding junctions and cell polarity. How could these coincide in the same individual organism and within minutes? We review occluding junctions (tight and septate) as well as the polarized distribution of Na+-K+-ATPase both at the molecular and the cell level. Junctions and polarity depend on hosts of molecular species and cellular processes, which are briefly reviewed whenever they are suspected to have played a role in the dawn of epithelia and metazoan. We come to the conclusion that most of the molecules needed were already present in early protozoan and discuss a few plausible alternatives to solve the riddle described above.

Distribution and oligomeric association of splice forms of Na(+)-K(+)-ATPase regulatory gamma-subunit in rat kidney

Renal Na(+)-K(+)-ATPase is associated with the gamma-subunit (FXYD2), a single-span membrane protein that modifies ATPase properties. There are two splice variants with different amino termini, gamma(a) and gamma(b). Both were found in the inner stripe of the outer medulla in the thick ascending limb. Coimmunoprecipitation with each other and the alpha-subunit indicated that they were associated in macromolecular complexes. Association was controlled by ligands that affect Na(+)-K(+)-ATPase conformation. In the cortex, the proportion of the gamma(b)-subunit was markedly lower, and the gamma(a)-subunit predominated in isolated proximal tubule cells. By immunofluorescence, the gamma(b)-subunit was detected in the superficial cortex only in the distal convoluted tubule and connecting tubule, which are rich in Na(+)-K(+)-ATPase but comprise a minor fraction of cortex mass. In the outer stripe of the outer medulla and for a short distance in the deep cortex, the thick ascending limb predominantly expressed the gamma(b)-subunit. Because different mechanisms maintain and regulate Na(+) homeostasis in different nephron segments, the splice forms of the gamma-subunit may have evolved to control the renal Na(+) pump through pump properties, gene expression, or both.

Molecular and functional studies of the gamma subunit of the sodium pump

This article reviews our studies of the gamma subunit of the sodium pump. Gamma is a member of the FXYD family of small, single transmembrane proteins and is expressed predominantly in the kidney tubule. There are two major variants of gamma which function similarly to bring about two distinct effects, one on K'(ATP) and the other, on K(K), the affinity of the pump for K+ acting as a competitor of cytoplasmic Na+. In this way, gamma is believed to provide a self-regulatory mechanism for maintaining the steady-state activity of the pump in the kidney. Our studies also suggest that K+ antagonism of cytoplasmic Na+ activation of the pump is relevant not only to the presence of gamma in the kidney, but probably some hitherto undefined factor(s) in other tissues, most notably heart. The interesting possibility that not only gamma but other members of the FXYD family regulate ion transport in a tissue-specific manner is discussed.

Functional role and immunocytochemical localization of the gamma a and gamma b forms of the Na,K-ATPase gamma subunit

The gamma subunit of the Na,K-ATPase is a member of the FXYD family of type 2 transmembrane proteins that probably function as regulators of ion transport. Rat gamma is present primarily in the kidney as two main splice variants, gamma(a) and gamma(b), which differ only at their extracellular N termini (TELSANH and MDRWYL, respectively; Kuster, B., Shainskaya, A., Pu, H. X., Goldshleger, R., Blostein, R., Mann, M., and Karlish, S. J. D. (2000) J. Biol. Chem. 275, 18441-18446). Expression in cultured cells indicates that both variants affect catalytic properties, without a detectable difference between gamma(a) and gamma(b). At least two singular effects are seen, irrespective of whether the variants are expressed in HeLa or rat alpha1-transfected HeLa cells, i.e. (i) an increase in apparent affinity for ATP, probably secondary to a left shift in E(1) <--> E(2) conformational equilibrium and (ii) an increase in K(+) antagonism of cytoplasmic Na(+) activation. Antibodies against the C terminus common to both variants (anti-gamma) abrogate the first effect but not the second. In contrast, gamma(a) and gamma(b) show differences in their localization along the kidney tubule. Using anti-gamma (C-terminal) and antibodies to the rat alpha subunit as well as antibodies to identify cell types, double immunofluorescence showed gamma in the basolateral membrane of several tubular segments. Highest expression is in the medullary portion of the thick ascending limb (TAL), which contains both gamma(a) and gamma(b). In fact, TAL is the only positive tubular segment in the medulla. In the cortex, most tubules express gamma but at lower levels. Antibodies specific for gamma(a) and gamma(b) showed differences in their cortical location; gamma(a) is specific for cells in the macula densa and principal cells of the cortical collecting duct but not cortical TAL. In contrast, gamma(b) but not gamma(a) is present in the cortical TAL only. Thus, the importance of gamma(a) and gamma(b) may be related to their partially overlapping but distinct expression patterns and tissue-specific functions of the pump that these serve.

Mechanisms of sodium pump regulation

The Na(+)-K(+)-ATPase, or sodium pump, is the membrane-bound enzyme that maintains the Na(+) and K(+) gradients across the plasma membrane of animal cells. Because of its importance in many basic and specialized cellular functions, this enzyme must be able to adapt to changing cellular and physiological stimuli. This review presents an overview of the many mechanisms in place to regulate sodium pump activity in a tissue-specific manner. These mechanisms include regulation by substrates, membrane-associated components such as cytoskeletal elements and the gamma-subunit, and circulating endogenous inhibitors as well as a variety of hormones, including corticosteroids, peptide hormones, and catecholamines. In addition, the review considers the effects of a range of specific intracellular signaling pathways involved in the regulation of pump activity and subcellular distribution, with particular consideration given to the effects of protein kinases and phosphatases.

A new variant of the gamma subunit of renal Na,K-ATPase. Identification by mass spectrometry, antibody binding, and expression in cultured cells

The gamma subunit is a specific regulator of Na,K-ATPase expressed mainly in kidney. On SDS-polyacryylamide gel electrophoresis, gamma runs as a doublet, but the origin and significance of the doublet is obscure. Mass spectrometry of the gamma chains of rat kidney Na, K-ATPase shows that gamma(a) (upper) has a mass of 7184.0 +/- 1 Da (carbamidomethyl cysteine), corresponding closely to that for the published sequence without the initiator methionine, while gamma(b) (lower) has a mass of 7337.9 +/- 1Da. Tryptic peptide mapping and sequencing by mass spectrometry reveals that the seven N-terminal residues of gamma(a), TELSANH, are replaced by Ac-MDRWYL in gamma(b), but otherwise the chains are identical. Antibodies raised against peptides TELSANHC and MDRWYLC recognize either gamma(a) or gamma(b) of the Na,K-ATPase, respectively. gamma(a) or gamma(b) cDNAs have been expressed in human embryonic kidney and HeLa cells. The major bands expressed correspond to gamma(a) or gamma(b) of renal Na, K-ATPase. Additional minor bands seen after transfection, namely gamma(a)' in human embryonic kidney and gamma(b)' in HeLa, are presumably cell-specific modifications. The present work clarifies earlier uncertainty regarding doublets seen in kidney and in transfected cells. In particular, the results show that renal Na, K-ATPase contains two variants of the gamma subunit with different sequences but otherwise are unmodified. We discuss the possible functional significance of the two variants.

The KdpF subunit is part of the K(+)-translocating Kdp complex of Escherichia coli and is responsible for stabilization of the complex in vitro

The kdpABC operon codes for the high affinity K(+)-translocating Kdp complex (P-type ATPase) of Escherichia coli. Upon expression of this operon in minicells, a so far unrecognized small hydrophobic polypeptide, KdpF, could be identified on high resolution SDS-polyacrylamide gels in addition to the subunits KdpA, KdpB, and KdpC. Furthermore, it could be demonstrated that KdpF remains associated with the purified complex. As determined by mass spectrometry, this peptide is present in its formylated form and has a molecular mass of 3100 Da. KdpF is not essential for growth on low K(+) (0.1 mM) medium, as shown by deletion analysis of kdpF, but proved to be indispensable for a functional enzyme complex in vitro. In the absence of KdpF, the ATPase activity of the membrane-bound Kdp complex was almost indistinguishable from that of the wild type. In contrast, the purified detergent-solubilized enzyme complex showed a dramatic decrease in enzymatic activity. However, addition of purified KdpF to the KdpABC complex restored the activity up to wild type level. It is interesting to note that the addition of high amounts of E. coli lipids had a similar effect. Although KdpF is not essential for the function of the Kdp complex in vivo, it is part of the complex and functions as a stabilizing element in vitro. The corresponding operon should now be referred to as kdpFABC.

The gamma subunit modulates Na(+) and K(+) affinity of the renal Na,K-ATPase

The Na(+),K(+)-ATPase catalyzes the active transport of ions. It has two necessary subunits, alpha and beta, but in kidney it is also associated with a 7.4-kDa protein, the gamma subunit. Stable transfection was used to determine the effect of gamma on Na, K-ATPase properties. When isolated from either kidney or transfected cells, alphabetagamma had lower affinities for both Na(+) and K(+) than alphabeta. A post-translational modification of gamma selectively eliminated the effect on Na(+) affinity, suggesting three configurations (alphabeta, alphabetagamma, and alphabetagamma*) conferring different stable properties to Na, K-ATPase. In the nephron, segment-specific differences in Na(+) affinity have been reported that cannot be explained by the known alpha and beta subunit isoforms of Na,K-ATPase. Immunofluorescence was used to detect gamma in rat renal cortex. Cortical ascending limb and some cortical collecting tubules lacked gamma, correlating with higher Na(+) affinities in those segments reported in the literature. Selective expression in different segments of the nephron is consistent with a modulatory role for the gamma subunit in renal physiology.

Stimulation of ouabain binding to Na,K-ATPase in 40% dimethyl sulfoxide by a factor from Na,K-ATPase preparations

In 40% dimethyl sulfoxide (Me2SO) high-affinity ouabain (O) binding to Na,K-ATPase (E) is promoted by Mg2+ in the absence of inorganic phosphate (Pi) (Fontes et al., Biochim. Biophys. Acta 1104, 215-225, 1995). Furthermore, in Me2SO the EO complex reacts very slowly with Pi and this ouabain binding can therefore be measured by the degree of inhibition of rapid phosphoenzyme formation. Here we found that, unexpectedly, the ouabain binding decreased with the enzyme concentration in the Me2SO assay medium. We extracted the enzyme preparation with Me2SO or chloroform/methanol and demonstrated that the extracted (depleted) enzyme bound ouabain poorly. Addition of such extracts to assays with low enzyme concentration or depleted enzyme fully restored the high-affinity ouabain binding. Dialysis experiments indicated that the active principle had a molecular mass between 3.5 and 12 kDa. It was highly resistant to proteolysis. It was suggested that the active principle could either be a low-molecular-weight, proteolysis-resistant-peptide (e.g., a proteolipid) or a factor with a nonproteinaceous nature. A polyclonal antibody raised against the C-terminal 10 amino acids of the rat kidney gamma-subunit was able to recognize this low-molecular-weight peptide present in the extracts. The previously depleted enzyme displayed lower amounts of the gamma-proteolipid in comparison to the native untreated enzyme, as demonstrated by immunoreaction with the antibody.

Expression and functional role of the gamma subunit of the Na, K-ATPase in mammalian cells

The functional role of the gamma subunit of the Na,K-ATPase was studied using rat gamma cDNA-transfected HEK-293 cells and an antiserum (gammaC33) specific for gamma. Although the sequence for gamma was verified and shown to be larger (7237 Da) than first reported, it still comprises a single initiator methionine despite the expression of a gammaC33-reactive doublet on immunoblots. Kinetic analysis of the enzyme of transfected compared with control cells and of gammaC33-treated kidney pumps shows that gamma regulates the apparent affinity for ATP. Thus, gamma-transfected cells have a decreased K'ATP as shown in measurements of (i) K'ATP of Na,K-ATPase activity and (ii) K+ inhibition of Na-ATPase at 1 microM ATP. Consistent with the behavior of gamma-transfected cells, gammaC33 pretreatment increases K'ATP of the kidney enzyme and K+ inhibition (1 microM ATP) of both kidney and gamma-transfected cells. These results are consistent with previous findings that an antiserum raised against the pig gamma subunit stabilizes the E2(K) form of the enzyme (Therien, A. G., Goldshleger, R., Karlish, S. J., and Blostein, R. (1997) J. Biol. Chem. 272, 32628-32634). Overall, our data demonstrate that gamma is a tissue (kidney)-specific regulator of the Na,K-ATPase that can increase the apparent affinity of the enzyme for ATP in a manner that is reversible by anti-gamma antiserum.

A novel small protein associated with a conjugated trienoic chromophore from membranes of scallop adductor muscle: phosphorylation by protein kinase A

Membranes enriched in sarcolemma from the cross-striated adductor muscle of the deep sea scallop have been found to contain a previously undescribed small protein of 6-8 kDa that can be released by treatment with organic solvent mixtures. This proteolipid co-purified with a non-amino acid chromophore containing a conjugated trienoic moiety. Although common in plants and algae, such a stable conjugated trienoic group is unusual for an animal cell. The N-terminal amino acid sequence of the protein was XEFQHGLFGXF/ADNIGLQ, which most strongly resembles sequences in the triacyl glycerol lipase precursor and the product of the human breast cancer susceptibility gene BRCA 1, but does not show similarity to previously described proteolipids. The protein was found to be one of the major substrates in its parent membrane for the catalytic subunit of protein kinase A, which may imply a regulatory function for this molecule.

Isozymes of the Na-K-ATPase: heterogeneity in structure, diversity in function

The Na-K-ATPase is characterized by a complex molecular heterogeneity that results from the expression and differential association of multiple isoforms of both its alpha- and beta-subunits. At present, as many as four different alpha-polypeptides (alpha1, alpha2, alpha3, and alpha4) and three distinct beta-isoforms (beta1, beta2, and beta3) have been identified in mammalian cells. The stringent constraints on the structure of the Na pump isozymes during evolution and their tissue-specific and developmental pattern of expression suggests that the different Na-K-ATPases have evolved distinct properties to respond to cellular requirements. This review focuses on the functional properties, regulation, and possible physiological relevance of the Na pump isozymes. The coexistence of multiple alpha- and beta-isoforms in most cells has hindered the understanding of the roles of the individual polypeptides. The use of heterologous expression systems has helped circumvent this problem. The kinetic characteristics of different Na-K-ATPase isozymes to the activating cations (Na+ and K+), the substrate ATP, and the inhibitors Ca2+ and ouabain demonstrate that each isoform has distinct properties. In addition, intracellular messengers differentially regulate the activity of the individual Na-K-ATPase isozymes. Thus the regulation of specific Na pump isozymes gives cells the ability to precisely coordinate Na-K-ATPase activity to their physiological requirements.

The gamma subunit of the Na,K-ATPase induces cation channel activity

The gamma subunit of the Na,K-ATPase is a hydrophobic protein of approximately 10 kDa. The gamma subunit was expressed in Sf-9 insect cells and Xenopus oocytes to ascertain its role in Na,K-ATPase function. Immunoblotting has shown that the gamma subunit is expressed in Sf-9 cells infected with recombinant baculovirus containing the cDNA for the human gamma subunit. Confocal microscopy demonstrates that the gamma subunit can be delivered to the plasma membrane of Sf-9 cells independently of the other Na,K-ATPase subunits and that gamma colocalizes with alpha1 when these proteins are coexpressed. When Sf-9 cells were coinfected with alpha1 and gamma, antibodies to the gamma subunit were able to coimmunoprecipitate the alpha1 subunit, suggesting that gamma is able to associate with alpha1. The gamma subunit is a member of a family of single-pass transmembrane proteins that induces ion fluxes in Xenopus oocytes. Evidence that the gamma subunit is a functional component was supported by experiments showing gamma-induced cation channel activity when expressed in oocytes and increases in Na+ and K+ uptake when expressed in Sf-9 cells.

Tissue-specific distribution and modulatory role of the gamma subunit of the Na,K-ATPase

The Na,K-ATPase comprises a catalytic alpha subunit and a glycosylated beta subunit. Another membrane polypeptide, gamma, first described by Forbush et al. (Forbush, B., III, Kaplan, J. H., and Hoffman, J. F. (1978) Biochemistry 17, 3667-3676) associates with alpha and beta in purified kidney enzyme preparations. In this study, we have used a polyclonal anti-gamma antiserum to define the tissue specificity and topology of gamma and to address the question of whether gamma has a functional role. The trypsin sensitivity of the amino terminus of the gamma subunit in intact right-side-out pig kidney microsomes has confirmed that it is a type I membrane protein with an extracellular amino terminus. Western blot analysis shows that gamma subunit protein is present only in membranes from kidney tubules (rat, dog, pig) and not those from axolemma, heart, red blood cells, kidney glomeruli, cultured glomerular cells, alpha1-transfected HeLa cells, all derived from the same (rat) species, nor from three cultured cell lines derived from tubules of the kidney, namely NRK-52E (rat), LLC-PK (pig), or MDCK (dog). To gain insight into gamma function, the effects of the anti-gamma serum on the kinetic behavior of rat kidney sodium pumps was examined. The following evidence suggests that gamma stabilizes E1 conformation(s) of the enzyme and that anti-gamma counteracts this effect: (i) anti-gamma inhibits Na,K-ATPase, and the inhibition increases at acidic pH under which condition the E2(K) --> E1 phase of the reaction sequence becomes more rate-limiting, (ii) the oligomycin-stimulated increase in the level of phosphoenzyme was greater in the presence of anti-gamma indicating that the antibody shifts the E1 left and right arrow left and right arrow E2P equilibria toward E2P, and (iii) when the Na+-ATPase reaction is assayed with the Na+ concentration reduced to levels (</=2 mM) which limit the rate of the E1 --> --> E2P transition, anti-gamma is stimulatory. These observations taken together with evidence that the pig gamma subunit, which migrates as a doublet on polyacrylamide gels, is sensitive to digestion by trypsin, and that Rb+ ions partially protect it against this effect, indicate that the gamma subunit is a tissue-specific regulator which shifts the steady-state equilibria toward E1. Accordingly, binding of anti-gamma disrupts alphabeta-gamma interactions and counteracts these modulatory effects of the gamma subunit.

The gamma subunit is a specific component of the Na,K-ATPase and modulates its transport function

The role of small, hydrophobic peptides that are associated with ion pumps or channels is still poorly understood. By using the Xenopus oocyte as an expression system, we have characterized the structural and functional properties of the gamma peptide which co-purifies with Na,K-ATPase. Immuno-radiolabeling of epitope-tagged gamma subunits in intact oocytes and protease protection assays show that the gamma peptide is a type I membrane protein lacking a signal sequence and exposing the N-terminus to the extracytoplasmic side. Co-expression of the rat or Xenopus gamma subunit with various proteins in the oocyte reveals that it specifically associates only with isozymes of Na,K-ATPase. The gamma peptide does not influence the formation and cell surface expression of functional Na,K-ATPase alpha-beta complexes. On the other hand, the gamma peptide itself needs association with Na,K-ATPase in order to be stably expressed in the oocyte and to be transported efficiently to the plasma membrane. Gamma subunits do not associate with individual alpha or beta subunits but only interact with assembled, transport-competent alpha-beta complexes. Finally, electrophysiological measurements indicate that the gamma peptide modulates the K+ activation of Na,K pumps. These data document for the first time the membrane topology, the specificity of association and a potential functional role for the gamma subunit of Na,K-ATPase.

Genetic evidence for two sequentially occupied K+ binding sites in the Kdp transport ATPase

Substrate binding sites in Kdp, a P-type ATPase of Escherichia coli, were identified by the isolation and characterization of mutants with reduced affinity for K+, its cation substrate. Most of the mutants have an altered KdpA subunit, a hydrophobic subunit not found in other P-type ATPases. Topological analysis of KdpA and the locations of the residues changed in the mutants suggest that KdpA has 10 membrane-spanning segments and forms two separate and distinct sites where K+ is bound. One site is formed by three periplasmic loops of the protein and is inferred to be the site of initial binding. The other site is cytoplasmic. We believe K+ moves from the periplasmic site through the membrane to the cytoplasmic site where it becomes "occluded," i.e. inexchangeable with K+ outside the membrane. Membrane-spanning parts of KdpA probably form the path for transmembrane movement of K+. The kinetics of cation transport in the mutants indicate that each of the two binding sites contributes to the observed Km for cations as well as to the marked discrimination between K+ and Rb+ characteristic of wild-type Kdp. Energy coupling in Kdp, mediated by the KdpB subunit, is performed by a different subunit from the one that mediates transport.

Subunit requirements for expression of functional sodium pumps in yeast cells

Na+/K(+)-ATPase from animal cell membranes is known to consist of an alpha-subunit and a beta-subunit. Amino acids within the alpha-subunit have been shown to participate in the catalytic functions of the enzyme and in the binding of cardioactive steroids. Although the function of the beta-subunit is not known, expression of both alpha- and beta-subunits is required for the functional enzyme. A putative third subunit, the gamma-subunit, has been suggested to be a part of the functional Na+/K(+)-ATPase complex, based on experiments showing that both the catalytic alpha-subunit and a small peptide of M(r) = 11,000 can be labeled by a photoreactive ouabain analog. Although the primary structure for the putative gamma-subunit from rat and sheep was recently deduced from cDNA clones, participation of this small protein in the catalytic activity of the Na+/K(+)-ATPase has not been demonstrated. In experiments described here, the heterologous expression of Na+/K(+)-ATPase in yeast cells was used to investigate whether the gamma-subunit is an essential component of the Na+/K(+)-ATPase. Yeast cells do not contain an endogenous Na+/K(+)-ATPase. The alpha- and beta-subunits or the alpha-, beta- and the putative gamma-subunits of Na+/K(+)-ATPase were expressed in the yeast Saccharomyces cerevisiae and ouabain-sensitive ATPase, p-nitrophenylphosphatase, and 86Rb uptake activities were measured either in membranes prepared from transformed yeast cells, or in intact yeast cells. Nontransformed yeast cells or yeast cells transformed with the gamma-subunit alone served as controls. Northern analysis and Western blots demonstrated that yeast cells do not contain an endogenous peptide with significant sequence homology to the putative gamma-subunit. Yeast samples containing only Na+/K(+)-ATPase alpha and beta subunits were capable of ouabain-inhibitable enzymatic activity and 86Rb transport. No gamma-subunit-dependent differences in the measured enzymatic activities or transport properties were detected in the different samples. These observations establish that the alpha beta-subunit complex is the minimum structural unit required for all the ouabain-sensitive reactions of Na+/K(+)-ATPase.

Molecular cloning and immunological characterization of the gamma polypeptide, a small protein associated with the Na,K-ATPase

The gamma subunit of the Na,K-ATPase is a small membrane protein that copurifies with the alpha and beta subunits of the enzyme. Strong evidence that the gamma subunit is a component of the Na,K-ATPase comes from studies indicating that the subunit is involved in forming the site for cardiac glycoside binding. We have isolated and characterized the cDNAs coding the gamma subunit from several species. The gamma subunit is a highly conserved protein consisting of 58 amino acids with a molecular weight of 6500. Hydropathy analysis reveals the presence of a single hydrophobic domain that is sufficient to cross the membrane. There are no sites for N-linked glycosylation. Northern blot analysis revealed that the gamma subunit mRNA is expressed in a tissue-specific fashion and is present in all tissues characterized. gamma-specific antibodies have been used to verify that the sequenced protein is the same protein labeled by [3H]nitroazidobenzoyl-ouabain (NAB-ouabain), and that this protein, the gamma subunit of the Na,K-ATPase, has a distribution pattern along nephron segments that is identical with the alpha subunit. In addition, coimmunoprecipitation of the alpha, beta and gamma subunits demonstrate specific association of the subunits. These results are consistent with the notion that the gamma subunit is specifically associated with and may be an important component of the Na,K-ATPase.

Origin of the gamma polypeptide of the Na+/K+-ATPase

The Na+/K+-ATPase purified from lamb kidney contains a gamma polypeptide fraction which is a collection of fragments derived from the alpha and beta polypeptides of the enzyme. This fraction has the solubility characteristics of a proteolipid and was isolated either by high performance liquid chromatography (size exclusion chromatography) in 1% sodium dodecyl sulfate or by sequential organic extraction of purified lamb kidney Na+/K+-ATPase. Formation of gamma polypeptide(s) from detergent solubilized holoenzyme was accelerated by sulfhydryl containing reagents and was unaffected by addition of inhibitors of proteolytic enzymes. Treatment of the holoenzyme with the photoaffinity reagent N-(2-nitro-4-azidophenyl)[3H]ouabain ([3H]NAP-ouabain) labeled the alpha polypeptide and the gamma polypeptide fraction but not the beta polypeptide. Amino acid sequence analysis of one gamma polypeptide preparation revealed homology of one component of this fraction with the N-terminus of the beta subunit of the Na+/K+-ATPase. Amino acid analysis of two preparations of proteolipid showed similar amino acid compositions with a peptide derived from the alpha subunit. The insolubility and complexity of the gamma polypeptide(s)/proteolipid fraction appears to preclude a conclusive sequence analysis of all components of this fraction.

Partial immunochemical identity between (Na+ + K+)-ATPase and a membrane component extractable with chloroform: methanol

An IgG fraction prepared from an antiserum against a holoenzyme preparation of (Na+ + K+)-ATPase precipitated a single antigen when samples of holoenzyme were subjected to crossed immunoelectrophoresis but precipitated an additional, immunochemically-related antigen when a plasma membrane-enriched fraction was subjected to crossed immunoelectrophoresis under the same conditions. The immunochemically-related antigen could be extracted from the plasma membrane fraction with CHCl3:CH3OH.

Molecular weights of alpha beta-protomeric and oligomeric units of soluble (Na+, K+)-ATPase determined by low-angle laser light scattering after high-performance gel chromatography

The (Na+, K+)-ATPase of canine renal outer medulla was solubilized with a nonionic surfactant, octaethylene glycol n-dodecyl ether (C12E8), in the presence of 0.2 M sodium ion. The solubilized ATPase retained 74% of the enzymatic activity expressed before solubilization. Molecular species of the solubilized ATPase were analyzed by high-performance chromatography through a TSK-GEL G3000SW column in the presence of 1 mg/ml C12E8 at 23 degrees C. The eluate was monitored by one or two monitors chosen from the following: an ultraviolet absorption monitor, a precision differential refractometer and a low-angle laser light scattering photometer. The three kinds of elution pattern thus obtained can best be interpreted by assuming the presence of at least four kinds of protein component with molecular weights 1 740 000 +/- 230 000, 836 000 +/- 82 000, 286 000 +/- 30 000 and 123 000 +/- 8 000, respectively. Among them, those with the last two molecular weight were the major components. The amounts of the first three components were found to increase with time during the incubation before application to the column at the expense of that of the last one. The amounts of the last two were 18 and 73%, respectively, when measured immediately after the solubilization. A stoichiometric composition of 1:1 molar ratio for the alpha and beta polypeptide chains was obtained for the two major components as well as for the intact ATPase by high-performance gel chromatography in the presence of sodium dodecyl sulfate using the same column as above. The (Na+, K+)-ATPase was, thus, indicated to be solubilized with C12E8 to give the alpha beta-protomer and its dimer as the main components.

Crystalline structure of sarcoplasmic reticulum from scallop

Negatively stained sarcoplasmic reticulum from the scallop Placopecten magellanicus presented a variety of crystalline forms, the most common being tubular structures. These were characterized by paired rows of morphological units, spaced at approximately 120 A, running diagonally across the tubules. The orthogonal unit cell (120 X 55 A) contained two units, related by a twofold axis, which probably represented the part of the Ca2+-ATPase molecule projecting from the outer surface of the membrane.

Isolation of two proteolipids from rabbit skeletal muscle sarcoplasmic reticulum

We have isolated two proteolipids from rabbit skeletal muscle sarcoplasmic reticulum by chromatography on columns of Sepharose CL-6B and Sephadex LH-60. One, PL-II, is identical to the proteolipid previously obtained by others using organic solvent extraction. The other, PL-I, has an amino acid composition very similar to those of proteolipids we previously isolated from canine cardiac SR and lamb kidney (Na,K)-ATPase.