The alpha subunit of the Na,K-ATPase specifically and stably associates into oligomers.

The Na,K-ATPase is a heterodimer consisting of an alpha and a beta subunit. Both Na,K-ATPase subunits are encoded by multigene families. Several isoforms for the alpha (alpha 1, alpha 2, and alpha 3) and beta (beta 1, beta 2, and beta 3) subunits have been identified. All these isoforms are capable of forming functionally active enzyme. Although there is general agreement that the Na,K-ATPase consists of alpha and beta subunits in equimolar amounts, the quaternary structure of the Na,K-ATPase and its functional significance is unknown. Several studies have demonstrated that the enzyme exists within the plasma membrane as an oligomer of alpha beta dimers. However, because the alpha beta protomer seems to be catalytically competent, the possibility exists that higher oligomers are irrelevant to function. The ability to express different alpha isoforms in insect cells and the availability of isoform-specific antibodies has provided the opportunity to test for the existence of stable and specific associations among alpha subunits. By coexpressing different alpha-subunit isoforms in cultured cells, we demonstrate that the Na,K-ATPase alpha subunits specifically and stably associate into oligomeric complexes. This same association among alpha-subunit isoforms was demonstrated in the native enzyme from rat brain. The interaction between Na,K-ATPase alpha subunits is highly specific. When the Na,K-ATPase alpha subunit is coexpressed with the alpha subunit from the H,K-ATPase, the H,K subunit does not associate with the Na,K subunit. Moreover, expression of the truncated alpha 1T isoform with the full-length alpha subunit demonstrates that the C-terminal portion of the polypeptide is important in the alpha-subunit association. Although these results do not clarify the functional role of alpha alpha associations, they do establish their highly specific nature and suggest that oligomerization of alpha beta protomers may be important to the stability and physiological regulation of the enzyme.     

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

The γ subunit of the Na,K-ATPase is a hydrophobic protein of approximately 10 kDa. The γ 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 γ subunit is expressed in Sf-9 cells infected with recombinant baculovirus containing the cDNA for the human γ subunit. Confocal microscopy demonstrates that the γ subunit can be delivered to the plasma membrane of Sf-9 cells independently of the other Na,K-ATPase subunits and that γ colocalizes with α1 when these proteins are coexpressed. When Sf-9 cells were coinfected with α1 and γ, antibodies to the γ subunit were able to coimmunoprecipitate the α1 subunit, suggesting that γ is able to associate with α1. The γ subunit is a member of a family of single-pass transmembrane proteins that induces ion fluxes in Xenopus oocytes. Evidence that the γ subunit is a functional component was supported by experiments showing γ-induced cation channel activity when expressed in oocytes and increases in Na⁺ and K⁺ uptake when expressed in Sf-9 cells.     

Na,K-ATPase alpha4 isoform is essential for sperm fertility

Regulation of ion balance in spermatozoa has been shown to be essential for sperm motility and fertility. Control of intracellular ion levels requires the function of distinct ion-transport mechanisms at the cell plasma membrane. Active Na(+) and K(+) exchange in sperm is under the control of the Na,K-ATPase. Two molecular variants of the catalytic subunit of the Na,K-ATPase, α1 and α4, coexist in sperm. These isoforms exhibit different biochemical properties; however, their function in sperm fertility is unknown. In this work, we show that Na,K-ATPase α4 is essential for sperm fertility. Knockout male mice lacking α4 are completely sterile and spermatozoa from these mice are unable of fertilizing eggs in vitro. Furthermore, α4 deletion results in severe reduction in sperm motility and hyperactivation typical of sperm capacitation. In addition, absence of α4 causes a characteristic bend in the sperm flagellum, indicative of abnormal sperm ion regulation. Accordingly, α4-null sperm present increased intracellular Na(+) and cell plasma membrane depolarization. These results are unique in demonstrating the absolute requirement of α4 for sperm fertility. Moreover, the inability of α1 to compensate for α4 suggests that α4 is the Na,K-ATPase-α isoform directly involved in sperm fertility. Our findings show α4 as an attractive target for male contraception and open the possibility for the potential use of this Na,K-ATPase isoform as a biomarker for male fertility.     

The Na,K-ATPase alpha 2 subunit gene displays restriction fragment length polymorphisms between the genomes of normotensive and hypertensive rats

The Na,K-adenosine triphosphatase (ATPase) alpha 2 subunit gene was found to display restriction fragment length polymorphisms (RFLPs) between the genomes of normotensive and hypertensive rats when digested with the restriction enzymes Bgl II and Hind III. In normotensive rats, we tested the spontaneously hypertensive rat (SHR) and its substrain, the stroke-prone spontaneously hypertensive rat (SHR-SP). Rat (SD) complementary (c) DNA encoding the alpha 2 subunit of Na,K-ATPase was used as a probe. When the probe was dissected these RFLPs were found to occur in the vicinity of the genomic locus encoding the middle part of the messenger (m) RNA for the alpha 2 subunit of Na,K-ATPase. A Northern blot analysis indicated that these RFLPs did not influence the alpha 2 subunit with regard to either size or amount of mRNA.     

Effect of palmitylcarnitine on ouabain binding to Na, K-ATPase.

Palmitylcarnitine, an endogenous long-chain fatty acyl ester, inhibited cardiac Na, K-ATPase activity and binding of [³H]ouabain to the enzyme. The inhibitory effects on enzyme hydrolytic activity and drug binding were time and concentration dependent, but also dependent upon the ratio of palmitylcarnitine to protein. Palmitylcarnitine inhibitory effects were irreversible, but could be prevented by bovine serum albumin. In the presence of Mg²⁺ + ATP or Mg²⁺ + Pi, [³H]ouabain binding was fully inhibited by 100 μm palmitylcarnitine. The addition of sodium, or sodium plus potassium to the drug-binding medium reduced the inhibitory effect. The protective action of Na⁺ was concentration dependent and was optimal at 75 μm palmitylcarnitine. Equimolar amounts of choline chloride did not have the same protective effect as sodium chloride. Binding of ouabain to the enzyme in the absence of palmitylcarnitine prevented the protective effect of Na⁺. Inhibition of Na, K-ATPase functional properties occurred at a concentration range of palmitylcarnitine reported to occur in the cytosol of ischemic cells during episodes of experimental myocardial ischemia. It is suggested that elevated levels of palmitylcarnitine in ischemic myocardium may play a role in altering cellular function as well as the inotropic response of ischemic cardiac muscle to digitalis glycosides.     

KATP channel inhibition by ATP requires distinct functional domains of the cytoplasmic C terminus of the pore-forming subunit

ATP-sensitive potassium (“K_ATP”) channels are rapidly inhibited by intracellular ATP. This inhibition plays a crucial role in the coupling of electrical activity to energy metabolism in a variety of cells. The K_ATP channel is formed from four each of a sulfonylurea receptor (SUR) regulatory subunit and an inwardly rectifying potassium (K_ir6.2) pore-forming subunit. We used systematic chimeric and point mutagenesis, combined with patch-clamp recording, to investigate the molecular basis of ATP-dependent inhibition gating of mouse pancreatic β cell K_ATP channels expressed in Xenopus oocytes. We identified distinct functional domains of the presumed cytoplasmic C-terminal segment of the K_ir6.2 subunit that play an important role in this inhibition. Our results suggest that one domain is associated with inhibitory ATP binding and another with gate closure.     

In vivo phosphorylation of the Na,K-ATPase alpha subunit in sciatic nerves of control and diabetic rats: effects of protein kinase modulators.

The phosphorylation state of the Na,K-ATPase alpha subunit has been examined in 32P-labeled sciatic nerves of control and streptozotocin-treated diabetic rats. Intact nerves were challenged with protein kinase (PK) modulators and alpha-subunit 32P labeling was analyzed after immunoprecipitation. In control nerves, the PKC activator phorbol 12-myristate 13-acetate (PMA) had little effect on alpha-subunit 32P labeling. In contrast, staurosporine, a PKC inhibitor, and extracellular calcium omission decreased it. In Ca(2+)-free conditions, PMA restored the labeling to basal levels. The cAMP-raising agent forskolin reduced the 32P labeling of the alpha subunit. The results suggest that nerve Na,K-ATPase is tonically phosphorylated by PKC in a Ca(2+)-dependent manner and that PKA modulates the phosphorylation process. In nerves of diabetic rats, PMA increased 32P labeling of the alpha subunit. In contrast to staurosporine or extracellular calcium omission, the decreased state of phosphorylation seen with forskolin was no longer significant in diabetic nerves. No change in the level of alpha-subunit isoforms (alpha 1 or alpha 2) was detected by Western blot analysis in such nerves. In conclusion, the altered effect of PK activators on Na,K-ATPase phosphorylation state is consistent with the view that a defect in PKC activation exists in diabetic nerves.     

Functional expression of the alpha 2 and alpha 3 isoforms of the Na,K-ATPase in baculovirus-infected insect cells.

Multiple isoforms of both the alpha and beta subunits of Na,K-ATPase have been identified. Elucidating their roles has been complicated by the fact that most tissues express multiple isoforms and purification techniques specific for each isoform have not been achieved. The baculovirus expression system, which uses the baculovirus Autographica californica to infect insect cells, is an ideal system for studying the Na,K-ATPase isoforms since high amounts of foreign proteins can be produced and some insect cell lines have low levels of endogenous Na,K-ATPase. Recombinant baculoviruses containing the cDNAs for the alpha 2, alpha 3, and beta 1 isoforms of the rat Na,K-ATPase were prepared and used to infect Sf-9 cells, an insect cell line derived from the ovary of the fall armyworm Spodoptera frugiperda. By using this system, Na,K-ATPase alpha 2 and alpha 3 subunits that were antigenically and electrophoretically indistinguishable from the native subunits were produced. When each subunit is expressed independently in the Sf-9 cells, it is primarily delivered to the plasma membrane. Although the isolated expression of each Na,K-ATPase subunit did not render active Na,K-ATPase molecules, the coexpression of alpha 2 or alpha 3 with beta 1 resulted in catalytically active molecules. This activity could be measured as a ouabain-sensitive ATPase activity or directly demonstrated using either [gamma-32P]ATP or 32Pi to identify the phosphorylated intermediates of the alpha 2 and alpha 3 isoforms. [3H]Ouabain binding studies showed that both isoforms are capable of binding the cardiotonic steroid with high affinity, alpha 3 being more sensitive to ouabain. These results demonstrate that the baculovirus system is suitable for the expression of the Na,K-ATPase isoforms and should provide a useful method for the characterization of the enzymatic properties of each isoform.     

Cell type-specific integrin variants with alternative alpha chain cytoplasmic domains.

The integrin heterodimers composed of the alpha 6 subunit with the beta 1 or beta 4 subunit (alpha 6 beta 1 and alpha 6 beta 4) are receptors for laminin and basement membrane components, respectively. The alpha 3 beta 1 integrin recognizes laminin, collagen, fibronectin, or epiligrin. We report the identification of structural variants (A and B) of the alpha 6 and alpha 3 subunits, containing distinct cytoplasmic domains. The expression of one cytoplasmic domain or the other, based probably on alternative exon usage, is cell-type dependent. Most transformed cell lines express both alpha 6A and alpha 6B isoforms, as determined by mRNA amplification or antibody immunoprecipitation. In contrast, embryonic fibroblasts express exclusively alpha 6A, and embryonic stem cells express exclusively alpha 6B. In most normal tissues, both alpha 6 isoforms are detectable, but several tissues express either alpha 6A or alpha 6B. The alpha 3B mRNA was amplified from heart and brain, while all other tissues and cell lines tested contained only alpha 3A mRNA. Alternative cytoplasmic domains may provide a means for varying the cellular responses to the ligands of alpha 6 and alpha 3 integrins according to the cell type.     

Dynein binds to and crossbridges cytoplasmic microtubules.

Dynein isolated from Chlamydomonas flagellar axonemes binds to microtubules assembled in vitro from 6S brain tubulin dimers. The dynein arms bind periodically along the length of the microtubules with a center-to-center spacing of 24 nm, equal to the periodicity of dynein arms on intact axonemes. The arms project from the in vitro assembled microtubules at an angle of approximately 55 degrees, thereby defining microtubule polarity. Dynein cosediments with microtubules through a sucrose gradient, as demonstrated by electron microscopy, gel electrophoresis, and ATPase analysis. In addition, dynein induces crossbridging between adjacent microtubules. Darkfield microscopy reveals that microtubules containing dynein are aggregated into large bundles; electron microscopy indicates that microtubules of the same polarity are crossbridged by a regular array of arms. Viewed by darkfield microscopy, addition of ATP to crossbridged microtubules causes their disaggregation; electron microscopy shows that the majority of these microtubules are no longer crossbridged. These observations are applicable to the determination of microtubule polarity and directionality of microtubule assembly in situ and suggest a role for dynein in cytoplasmic microtubule-based cellular movements.     

Alveolar type 1 cells express the alpha2 Na,K-ATPase,which contributes to lung liquid clearance

The alveolar epithelium is composed of alveolar type 1 (AT1) and alveolar type 2 (AT2) cells, which represent approximately 95% and approximately 5% of the alveolar surface area, respectively. Lung liquid clearance is driven by the osmotic gradient generated by the Na,K-ATPase. AT2 cells have been shown to express the alpha1 Na,K-ATPase. We postulated that AT1 cells, because of their larger surface area, should be important in the regulation of active Na+ transport. By immunofluorescence and electron microscopy, we determined that AT1 cells express both the alpha1 and alpha2 Na,K-ATPase isoforms. In isolated, ouabain-perfused rat lungs, the alpha2 Na,K-ATPase in AT1 cells mediated 60% of the basal lung liquid clearance. The beta-adrenergic agonist isoproterenol increased lung liquid clearance by preferentially upregulating the alpha2 Na,K-ATPase protein abundance in the plasma membrane and activity in alveolar epithelial cells (AECs). Rat AECs and human A549 cells were infected with an adenovirus containing the rat Na,K-ATPase alpha2 gene (Adalpha2), which resulted in the overexpression of the alpha2 Na,K-ATPase protein and caused a 2-fold increase in Na,K-ATPase activity. Spontaneously breathing rats were also infected with Adalpha2, which increased alpha2 protein abundance and resulted in a approximately 250% increase in lung liquid clearance. These studies provide the first evidence that alpha2 Na,K-ATPase in AT1 cells contributes to most of the active Na+ transport and lung liquid clearance, which can be further increased by stimulation of the beta-adrenergic receptor or by adenovirus-mediated overexpression of the alpha2 Na,K-ATPase.     

Phosphoinositide-3 kinase binds to a proline-rich motif in the Na+, K+-ATPase alpha subunit and regulates its trafficking

Endocytosis of Na(+),K(+)-ATPase molecules in response to G protein-coupled receptor stimulation requires activation of class I(A) phosphoinositide-3 kinase (PI3K-I(A)) in a protein kinase C-dependent manner. In this paper, we report that PI3K-I(A), through its p85alpha subunit-SH3 domain, binds to a proline-rich region in the Na(+),K(+)-ATPase catalytic alpha subunit. This interaction is enhanced by protein kinase C-dependent phosphorylation of a serine residue that flanks the proline-rich motif in the Na(+),K(+)-ATPase alpha subunit and results in increased PI3K-I(A) activity, an effect necessary for adaptor protein 2 binding and clathrin recruitment. Thus, Ser-phosphorylation of the Na(+),K(+)-ATPase catalytic subunit serves as an anchor signal for regulating the location of PI3K-I(A) and its activation during Na(+),K(+)-ATPase endocytosis in response to G protein-coupled receptor signals.     

An acetylcholine receptor precursor alpha subunit that binds alpha-bungarotoxin but not d-tubocurare.

We have identified an intracellular form of the alpha subunit of the acetylcholine receptor that binds alpha-bungarotoxin with high affinity. Unlike the mature receptor complex, an alpha 2 beta gamma delta pentamer that migrates as a 9S species in velocity sedimentation analysis, the intracellular species moves as a 5S component. The kinetics of appearance of alpha subunit in the 5S component and the mature receptor complex indicate that the intracellular 5S component is a precursor of the mature receptor. The precursor species differs from 9S receptor in two critical features: (i) the precursor alpha subunit is not associated with beta subunit and (ii) alpha-bungarotoxin binding to the precursor alpha subunit is not inhibited by the cholinergic ligands decamethonium or d-tubocurarine. The properties of the precursor suggest that the acquisition of the ligand binding site by alpha subunit occurs at a distinct stage in the posttranslational development of functional acetylcholine receptor.     

Solubilized alpha beta Na,K-ATPase remains protomeric during turnover yet shows apparent negative cooperativity toward ATP.

A prominent feature of the Na,K-ATPase reaction is an ATP dependence that suggests high- and low-affinity ATP requirements during the enzymic cycle. As only one ATP-binding domain has been identified in the alpha subunit and none has been identified in the beta subunit, it has seemed likely that the apparent negative cooperativity results from subunit interactions in an (alpha beta)2 diprotomer. To test this possibility, we have examined the behavior of solubilized alpha beta protomers of Na,K-ATPase down to 50 nM [gamma-32P]ATP. Active-enzyme analytical ultracentrifugation shows that the protomer is the active species and that no oligomerization occurs during turnover. However, we find that dual ATP effects can be clearly demonstrated and that nonhydrolyzable ATP analogs can stimulate the Na,K-ATPase activity of the soluble protomer. We conclude that the apparent negative cooperativity is inherent to the alpha beta protomer and that this should explain some of the complexities found with membrane-bound Na,K-ATPase and, perhaps, other P-type cation pumps.     

Antagonism of ligand-gated ion channel receptors: two domains of the glycine receptor alpha subunit form the strychnine-binding site.

The inhibitory glycine receptor (GlyR) is a member of the ligand-gated ion channel receptor superfamily. Glycine activation of the receptor is antagonized by the convulsant alkaloid strychnine. Using in vitro mutagenesis and functional analysis of the cDNA encoding the alpha 1 subunit of the human GlyR, we have identified several amino acid residues that form the strychnine-binding site. These residues were identified by transient expression of mutated cDNAs in mammalian (293) cells and examination of resultant [3H]strychnine binding, glycine displacement of [3H]strychnine, and electrophysiological responses to the application of glycine and strychnine. This mutational analysis revealed that residues from two separate domains within the alpha 1 subunit form the binding site for the antagonist strychnine. The first domain includes the amino acid residues Gly-160 and Tyr-161, and the second domain includes the residues Lys-200 and Tyr-202. These results, combined with analyses of other ligand-gated ion channel receptors, suggest a conserved tertiary structure and a common mechanism for antagonism in this receptor superfamily.     

Nonclathrin coat protein gamma, a subunit of coatomer, binds to the cytoplasmic dilysine motif of membrane proteins of the early secretory pathway.

Coatomer, a cytosolic heterooligomeric protein complex that consists of seven subunits [alpha-, beta-, beta'-, gamma-, delta-, epsilon-, and zeta-COP (nonclathrin coat protein)], has been shown to interact with dilysine motifs typically found in the cytoplasmic domains of various endoplasmic-reticulum-resident membrane proteins [Cosson, P. & Letourneur, F. (1994) Science 263, 1629-1631]. We have used a photo-cross-linking approach to identify the site of coatomer that is involved in binding to the dilysine motifs. An octapeptide corresponding to the C-terminal tail of Wbp1p, a component of the yeast N-oligosaccharyltransferase complex, has been synthesized with a photoreactive phenylalanine at position -5 and was radioactively labeled with [125I]iodine at a tyrosine residue introduced at the N terminus of the peptide. Photolysis of isolated coatomer in the presence of this peptide and immunoprecipitation of coatomer from photo-cross-linked cell lysates reveal that gamma-COP is the predominantly labeled protein. From these results, we conclude that coatomer is able to bind to the cytoplasmic dilysine motifs of membrane proteins of the early secretory pathway via its gamma-COP subunit, whose complete cDNA-derived amino acid sequence is also presented.     

Attenuation of cardiac contractility in Na,K-ATPase alpha1 isoform-deficient hearts under reduced calcium conditions

We have previously reported that genetic reduction of the Na,K-ATPase alpha1 isoform (alpha1(+/-)) results in a hypocontractile cardiac phenotype. This observation was surprising and unexpected. In order to determine if calcium overload contributes to the depressed phenotype, cardiac performance was examined by perfusing the hearts with buffer containing 2 or 1.5 mM calcium. At 2 mM calcium, +dP/dt for the alpha1(+/-) hearts (1374 +/- 180) was significantly less than that of wild-type (2656 +/- 75, P < 0.05). At 1.5 mM calcium, a larger decrease in +dP/dt occurred (vs. 2 mM calcium) for the alpha1(+/-) hearts (517 +/- 92) compared to wild-type (2238 +/- 157). At 2 mM calcium, -dP/dt was 50% lower in alpha1(+/-) hearts (-1903 +/- 141) than wild-type (-982 +/- 143). At 1.5 mM calcium relaxation was further reduced in alpha1(+/-) compared to wild-type (-443 +/- 56 vs. - 1691 +/- 109). We also tested whether the compensatory upregulation of the Na,K-ATPase alpha2 isoform in the alpha1(+/-) hearts contributes to the hypocontractile phenotype. At 8 x 10(-6) M ouabain, that would completely inhibit the alpha2 isoform, a 30% increase in contractility was obtained in alpha1(+/-) hearts compared to no ouabain treatment, while a 63% faster time-to-peak (TTP) and 67% faster half-time-to-relaxation (RT(1/2)) were observed in alpha1(+/-) hearts treated with ouabain. These results suggest that upregulation of the alpha2 isoform may play a role in slower TTP and RT(1/2) in the alpha1(+/-) hearts. Furthermore, lowering extracellular calcium in the perfusate did not alleviate the depressed contractile phenotype in the alpha1(+/-) hearts and resulted in further depressed cardiac contractility suggesting that these hearts are not calcium overloaded.