Heterologous expression of Na(+)-K(+)-ATPase in insect cells: intracellular distribution of pump subunits

Na,K-ATPase Expression Can Be Limited Post-Transcriptionally: A Test of the Role of the Beta Subunit, and a Review of Evidence

The Na,K-ATPase is an α-β heterodimer. It is well known that the Na,K-ATPase β subunit is required for the biosynthesis and trafficking of the α subunit to the plasma membrane. During investigation of properties of human ATP1A3 mutations in 293 cells, we observed a reciprocal loss of endogenous ATP1A1 when expressing ATP1A3. Scattered reports going back as far as 1991 have shown that experimental expression of one subunit can result in reduction in another, suggesting that the total amount is strictly limited. It seems logical that either α or β subunit should be rate-limiting for assembly and functional expression. Here, we present evidence that neither α nor β may be limiting and that there is another level of control that limits the amount of Na,K-ATPase to physiological levels. We propose that α subunits compete for something specific, like a private chaperone, required to finalize their biosynthesis or to prevent their degradation in the endoplasmic reticulum.

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

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

ATP1A1-linked diseases require a malfunctioning protein product from one allele

Heterozygous germline variants in ATP1A1, the gene encoding the α1 subunit of the Na+/K+-ATPase (NKA), have been linked to diseases including primary hyperaldosteronism and the peripheral neuropathy Charcot-Marie-Tooth disease (CMT). ATP1A1 variants that cause CMT induce loss-of-function of NKA. This heterodimeric (αβ) enzyme hydrolyzes ATP to establish transmembrane electrochemical gradients of Na+ and K+ that are essential for electrical signaling and cell survival. Of the 4 catalytic subunit isoforms, α1 is ubiquitously expressed and is the predominant paralog in peripheral axons. Human population sequencing datasets indicate strong negative selection against both missense and protein-null ATP1A1 variants. To test whether haploinsufficiency generated by heterozygous protein-null alleles are sufficient to cause disease, we tested the neuromuscular characteristics of heterozygous Atp1a1+/- knockout mice and their wildtype littermates, while also evaluating if exercise increased CMT penetrance. We found that Atp1a1+/- mice were phenotypically normal up to 18 months of age. Consistent with the observations in mice, we report clinical phenotyping of a healthy adult human who lacks any clinical features of known ATP1A1-related diseases despite carrying a plasma-membrane protein-null early truncation variant, p.Y148*. Taken together, these results suggest that a malfunctioning gene product is required for disease induction by ATP1A1 variants and that if any pathology is associated with protein-null variants, they may display low penetrance or high age of onset.

A Na pump with reduced stoichiometry is up-regulated by brine shrimp in extreme salinities

Brine shrimp (Artemia) are the only animals to thrive at sodium concentrations above 4 M. Salt excretion is powered by the Na+,K+-ATPase (NKA), a heterodimeric (αβ) pump that usually exports 3Na+ in exchange for 2 K+ per hydrolyzed ATP. Artemia express several NKA catalytic α-subunit subtypes. High-salinity adaptation increases abundance of α2KK, an isoform that contains two lysines (Lys308 and Lys758 in transmembrane segments TM4 and TM5, respectively) at positions where canonical NKAs have asparagines (Xenopus α1's Asn333 and Asn785). Using de novo transcriptome assembly and qPCR, we found that Artemia express two salinity-independent canonical α subunits (α1NN and α3NN), as well as two β variants, in addition to the salinity-controlled α2KK. These β subunits permitted heterologous expression of the α2KK pump and determination of its CryoEM structure in a closed, ion-free conformation, showing Lys758 residing within the ion-binding cavity. We used electrophysiology to characterize the function of α2KK pumps and compared it to that of Xenopus α1 (and its α2KK-mimicking single- and double-lysine substitutions). The double substitution N333K/N785K confers α2KK-like characteristics to Xenopus α1, and mutant cycle analysis reveals energetic coupling between these two residues, illustrating how α2KK's Lys308 helps to maintain high affinity for external K+ when Lys758 occupies an ion-binding site. By measuring uptake under voltage clamp of the K+-congener 86Rb+, we prove that double-lysine-substituted pumps transport 2Na+ and 1 K+ per catalytic cycle. Our results show how the two lysines contribute to generate a pump with reduced stoichiometry allowing Artemia to maintain steeper Na+ gradients in hypersaline environments.

ATP1A1 -linked diseases require a malfunctioning protein product from one allele

Heterozygous germline variants in ATP1A1 , the gene encoding the α1 subunit of the Na + /K + -ATPase (NKA), have been linked to diseases including primary hyperaldosteronism and the peripheral neuropathy Charcot-Marie-Tooth disease (CMT). ATP1A1 variants that cause CMT induce loss-of-function of NKA. This heterodimeric (αβ) enzyme hydrolyzes ATP to establish transmembrane electrochemical gradients of Na + and K + that are essential for electrical signaling and cell survival. Of the 4 catalytic subunit isoforms, α1 is ubiquitously expressed and is the predominant paralog in peripheral axons. Human population sequencing datasets indicate strong negative selection against both missense and protein-null ATP1A1 variants. To test whether haploinsufficiency generated by heterozygous protein-null alleles are sufficient to cause disease, we tested the neuromuscular characteristics of heterozygous Atp1a1 +/- knockout mice and their wildtype littermates, while also evaluating if exercise increased CMT penetrance. We found that Atp1a1 +/- mice were phenotypically normal up to 18 months of age. Consistent with the observations in mice, we report clinical phenotyping of a healthy adult human who lacks any clinical features of known ATP1A1 -related diseases despite carrying a protein-null early truncation variant, p.Y148*. Taken together, these results suggest that a malfunctioning gene product is required for disease induction by ATP1A1 variants and that if any pathology is associated with protein-null variants, they may display low penetrance or high age of onset.

Displacement of the Na+/K+ pump's transmembrane domains demonstrates conserved conformational changes in P-type 2 ATPases

Cellular survival requires the ion gradients built by the Na⁺/K⁺ pump, an ATPase that alternates between two major conformations (E1 and E2). Here we use state-specific engineered-disulfide cross-linking to demonstrate that transmembrane segment 2 (M2) of the pump's α-subunit moves in directions that are inconsistent with distances observed in existing crystal structures of the Na⁺/K⁺ pump in E1 and E2. We characterize this movement with voltage-clamp fluorometry in single-cysteine mutants. Most mutants in the M1-M2 loop produced state-dependent fluorescence changes upon labeling with tetramethylrhodamine-6-maleimide (TMRM), which were due to quenching by multiple endogenous tryptophans. To avoid complications arising from multiple potential quenchers, we analyzed quenching of TMRM conjugated to R977C (in the static M9-M10 loop) by tryptophans introduced, one at a time, in M1-M2. This approach showed that tryptophans introduced in M2 quench TMRM only in E2, with D126W and L130W on the same helix producing the largest fluorescence changes. These observations indicate that M2 moves outward as Na⁺ is deoccluded from the E1 conformation, a mechanism consistent with cross-linking results and with proposals for other P-type 2 ATPases.

Diseases caused by mutations in the Na+/K+ pump α1 gene ATP1A1

Human cell survival requires function of the Na⁺/K⁺ pump; the heteromeric protein that hydrolyzes ATP to extrude Na⁺ and import K⁺ across the plasmalemma, thereby building and maintaining these ions' electrochemical gradients. Numerous dominant diseases caused by mutations in genes encoding for Na⁺/K⁺ pump catalytic (α) subunit isoforms highlight the importance of this protein. Here, we review literature describing disorders caused by missense mutations in ATP1A1, the gene encoding the ubiquitously expressed α1 isoform of the Na⁺/K⁺ pump. These various maladies include primary aldosteronism with secondary hypertension, an endocrine syndrome, Charcot-Marie-Tooth disease, a peripheral neuropathy, complex spastic paraplegia, another neuromuscular disorder, as well as hypomagnesemia accompanied by seizures and cognitive delay, a condition affecting the renal and central nervous systems. This article focuses on observed commonalities among these mutations' functional effects, as well as on the special characteristics that enable each particular mutation to exclusively affect a certain system, without affecting others. In this respect, it is clear how somatic mutations localized to adrenal adenomas increase aldosterone production without compromising other systems. However, it remains largely unknown how and why some but not all de novo germline or familial mutations (where the mutant must be expressed in numerous tissues) produce a specific disease and not the other diseases. We propose hypotheses to explain this observation and the approaches that we think will drive future research on these debilitating disorders to develop novel patient-specific treatments by combining the use of heterologous protein-expression systems, patient-derived pluripotent cells, and gene-edited cell and mouse models.

The inositol pyrophosphate 5-InsP7 drives sodium-potassium pump degradation by relieving an autoinhibitory domain of PI3K p85α

Sodium/potassium-transporting adenosine triphosphatase (Na⁺/K⁺-ATPase) is one of the most abundant cell membrane proteins and is essential for eukaryotes. Endogenous negative regulators have long been postulated to play an important role in regulating the activity and stability of Na⁺/K⁺-ATPase, but characterization of these regulators has been elusive. Mechanisms of regulating Na⁺/K⁺-ATPase homeostatic turnover are unknown. Here, we report that 5-diphosphoinositol 1,2,3,4,6-pentakisphosphate (5-InsP₇), generated by inositol hexakisphosphate kinase 1 (IP6K1), promotes physiological endocytosis and downstream degradation of Na⁺/K⁺-ATPase-α1. Deletion of IP6K1 elicits a twofold enrichment of Na⁺/K⁺-ATPase-α1 in plasma membranes of multiple tissues and cell types. Using a suite of synthetic chemical biology tools, we found that 5-InsP₇ binds the RhoGAP domain of phosphatidylinositol 3-kinase (PI3K) p85α to disinhibit its interaction with Na⁺/K⁺-ATPase-α1. This recruits adaptor protein 2 (AP2) and triggers the clathrin-mediated endocytosis of Na⁺/K⁺-ATPase-α1. Our study identifies 5-InsP₇ as an endogenous negative regulator of Na⁺/K⁺-ATPase-α1.

Factors in the disease severity of ATP1A3 mutations: Impairment, misfolding, and allele competition

Dominant mutations of ATP1A3, a neuronal Na,K-ATPase α subunit isoform, cause neurological disorders with an exceptionally wide range of severity. Several new mutations and their phenotypes are reported here (p.Asp366His, p.Asp742Tyr, p.Asp743His, p.Leu924Pro, and a VUS, p.Arg463Cys). Mutations associated with mild or severe phenotypes [rapid-onset dystonia-parkinsonism (RDP), alternating hemiplegia of childhood (AHC), or early infantile epileptic encephalopathy (EIEE)] were expressed in HEK-293 cells. Paradoxically, the severity of human symptoms did not correlate with whether there was enough residual activity to support cell survival. We hypothesized that distinct cellular consequences may result not only from pump inactivation but also from protein misfolding. Biosynthesis was investigated in four tetracycline-inducible isogenic cell lines representing different human phenotypes. Two cell biological complications were found. First, there was impaired trafficking of αβ complex to Golgi apparatus and plasma membrane, as well as changes in cell morphology, for two mutations that produced microcephaly or regions of brain atrophy in patients. Second, there was competition between exogenous mutant ATP1A3 (α3) and endogenous ATP1A1 (α1) so that their sum was constant. This predicts that in patients, the ratio of normal to mutant ATP1A3 proteins will vary when misfolding occurs. At the two extremes, the results suggest that a heterozygous mutation that only impairs Na,K-ATPase activity will produce relatively mild disease, while one that activates the unfolded protein response could produce severe disease and may result in death of neurons independently of ion pump inactivation.

On the effect of hyperaldosteronism-inducing mutations in Na/K pumps

Mutated Na/K pumps in adrenal adenomas are thought to cause hyperaldosteronism via a gain-of-function effect involving a depolarizing inward current. The findings of Meyer et al. suggest instead that the common mechanism by which Na/K pump mutants lead to hyperaldosteronism is a loss-of-function.

Primary aldosteronism, a condition in which too much aldosterone is produced and that leads to hypertension, is often initiated by an aldosterone-producing adenoma within the zona glomerulosa of the adrenal cortex. Somatic mutations of ATP1A1, encoding the Na/K pump α1 subunit, have been found in these adenomas. It has been proposed that a passive inward current transported by several of these mutant pumps is a "gain-of-function" activity that produces membrane depolarization and concomitant increases in aldosterone production. Here, we investigate whether the inward current through mutant Na/K pumps is large enough to induce depolarization of the cells that harbor them. We first investigate inward currents induced by these mutations in Xenopus Na/K pumps expressed in Xenopus oocytes and find that these inward currents are similar in amplitude to wild-type outward Na/K pump currents. Subsequently, we perform a detailed functional evaluation of the human Na/K pump mutants L104R, delF100-L104, V332G, and EETA963S expressed in Xenopus oocytes. By combining two-electrode voltage clamp with [³H]ouabain binding, we measure the turnover rate of these inward currents and compare it to the turnover rate for outward current through wild-type pumps. We find that the turnover rate of the inward current through two of these mutants (EETA963S and L104R) is too small to induce significant cell depolarization. Electrophysiological characterization of another hyperaldosteronism-inducing mutation, G99R, reveals the absence of inward currents under many different conditions, including in the presence of the regulator FXYD1 as well as with mammalian ionic concentrations and body temperatures. Instead, we observe robust outward currents, but with significantly reduced affinities for intracellular Na⁺ and extracellular K⁺. Collectively, our results point to loss-of-function as the common mechanism for the hyperaldosteronism induced by these Na/K pump mutants.

Na+/K+-ATPase interaction with methylglyoxal as reactive metabolic side product

Proteins are subject to oxidative modification and the formation of adducts with a broad spectrum of reactive species via enzymatic and non-enzymatic mechanisms. Here we report that in vitro non-enzymatic methylglyoxal (MGO) binding causes the inhibition and formation of MGO advanced glycation end-products (MAGEs) in Na⁺/K⁺-ATPase (NKA). Concretely, MGO adducts with NKA amino acid residues (mainly Arg) and N^ε-(carboxymethyl)lysine (CML) formation were found. MGO is not only an inhibitor for solubilized NKA (IC₅₀=91±16μM), but also for reconstituted NKA in the lipid bilayer environment, which was clearly demonstrated using a DPPC/DPPE liposome model in the presence or absence of the NKA-selective inhibitor ouabain. High-resolution mass spectrometric analysis of a tryptic digest of NKA isolated from pig (Sus scrofa) kidney indicates that the intracellular α-subunit is naturally (post-translationally) modified by MGO in vivo. In contrast to this, the β-subunit could only be modified by MGO artificially, and the transmembrane part of the protein did not undergo MGO binding under the experimental setup used. As with bovine serum albumin, serving as the water-soluble model, we also demonstrated a high binding capacity of MGO to water-poorly soluble NKA using a multi-spectral methodology based on electroanalytical, immunochemical and fluorimetric tools. In addition, a partial suppression of the MGO-mediated inhibitory effect could be observed in the presence of aminoguanidine (pimagedine), a glycation suppressor and MGO-scavenger. All the results here were obtained with the X-ray structure of NKA in the E1 conformation (3WGV) and could be used in the further interpretation of the functionality of this key enzyme in the presence of highly-reactive metabolic side-products, glycation agents and generally under oxidative stress conditions.

Gene-expression profile and localization of Na+/K(+)-ATPase in rat enamel organ cells

The sodium pump Na(+)/K(+)-ATPase, expressed in virtually all cells of higher organisms, is involved in establishing a resting membrane potential and in creating a sodium gradient to facilitate a number of membrane-associated transport activities. Na(+)/K(+)-ATPase is an oligomer of α, β, and γ subunits. Four unique genes encode each of the α and β subunits. In dental enamel cells, the spatiotemporal expression of Na(+)/K(+)-ATPase is poorly characterized. Using the rat incisor as a model, this study provides a comprehensive expression profile of all four α and all four β Na(+)/K(+)-ATPase subunits throughout all stages of amelogenesis. Real-time PCR, western blot analysis, and immunolocalization revealed that α1, β1, and β3 are expressed in the enamel organ and that all three are most highly expressed during late-maturation-stage amelogenesis. Expression of β3 was significantly higher than expression of β1, suggesting that the dominant Na(+)/K(+)-ATPase consists of an α1β3 dimer. Localization of α1, β1, and β3 subunits in ameloblasts was primarily to the cytoplasm and occasionally along the basolateral membranes. Weaker expression was also noted in papillary layer cells during early maturation. Our data support that Na(+)/K(+)-ATPase is functional in maturation-stage ameloblasts.

ATP protects against FITC labeling of Solanum lycopersicon and Arabidopsis thaliana Ca2+-ATPase ATP binding domains

Ca(2+)-ATPases are integral membrane proteins that actively transport Ca(2+) against substantial concentration gradients in eukaryotic cells. This active transport is energized by coupling ion translocation with ATP hydrolysis. In order to better understand this coupling mechanism, we studied the nucleotide specificities of isolated ATP binding domains (ABDs) of Solanum lycopersicon Ca(2+)-ATPase (LCA), a type IIA non-calmodulin regulated P-type pump found in tomato plants that is very similar to mammalian sarco/endoplasmic reticulum Ca(2+)-ATPase (SERCA), and Arabidopsis Ca(2+)-ATPase, isoform 2 (ACA2), a type IIB calmodulin regulated P-type ATPase found in the endoplasmic reticulum of Arabidopsis cells. We used nucleotide protection against FITC labeling as a measure of binding since both LCA and ACA contained the KGAP(S,V,F)E motif, which has been shown to be modified by fluorescein isothiocyanate (FITC) in P-type pumps from animal cells. We demonstrated that the heterologously expressed GST-tagged ABDs from both LCA and ACA2 were modified by FITC and that ATP protects against this modification. Moreover, GTP was able to reduce, but not eliminate, the level of FITC labeling in both ABD constructs, suggesting that these plant pumps may also bind GTP with low affinity, which is in contrast to mammalian SERCA and PMCA type pumps which do not bind GTP.

A novel cholesterol-producing Pichia pastoris strain is an ideal host for functional expression of human Na,K-ATPase α3β1 isoform

The heterologous expression of mammalian membrane proteins in lower eukaryotes is often hampered by aberrant protein localization, structure, and function, leading to enhanced degradation and, thus, low expression levels. Substantial quantities of functional membrane proteins are necessary to elucidate their structure-function relationships. Na,K-ATPases are integral, human membrane proteins that specifically interact with cholesterol and phospholipids, ensuring protein stability and enhancing ion transport activity. In this study, we present a Pichia pastoris strain which was engineered in its sterol pathway towards the synthesis of cholesterol instead of ergosterol to foster the functional expression of human membrane proteins. Western blot analyses revealed that cholesterol-producing yeast formed enhanced and stable levels of human Na,K-ATPase α3β1 isoform. ATPase activity assays suggested that this Na,K-ATPase isoform was functionally expressed in the plasma membrane. Moreover, [(3)H]-ouabain cell surface-binding studies underscored that the Na,K-ATPase was present in high numbers at the cell surface, surpassing reported expression strains severalfold. This provides evidence that the humanized sterol composition positively influenced Na,K-ATPase α3β1 stability, activity, and localization to the yeast plasma membrane. Prospectively, cholesterol-producing yeast will have high potential for functional expression of many mammalian membrane proteins.

Fluorone dyes have binding sites on both cytoplasmic and extracellular domains of Na,K-ATPase

Combination of fluorescence techniques and molecular docking was used to monitor interaction of Na,K-ATPase and its large cytoplasmic loop connecting fourth and fifth transmembrane helices (C45) with fluorone dyes (i.e. eosin Y, 5(6)-carboxyeosin, rose bengal, fluorescein, and erythrosine B). Our data suggested that there are at least two binding sites for all used fluorone dyes, except of 5(6)-carboxyeosin. The first binding site is located on C45 loop, and it is sensitive to the presence of nucleotide. The other site is located on the extracellular part of the enzyme, and it is sensitive to the presence of Na(+) or K(+) ions. The molecular docking revealed that in the open conformation of C45 loop (which is obtained in the presence of ATP) all used fluorone dyes occupy position directly inside the ATP-binding pocket, while in the closed conformation (i.e. in the absence of any ligand) they are located only near the ATP-binding site depending on their different sizes. On the extracellular part of the protein, the molecular docking predicts two possible binding sites with similar binding energy near Asp897(α) or Gln69(β). The former was identified as a part of interaction site between α- and β-subunits, the latter is in contact with conserved FXYD sequence of the γ-subunit. Our findings provide structural explanation for numerous older studies, which were performed with fluorone dyes before the high-resolution structures were known. Further, fluorone dyes seem to be good probes for monitoring of intersubunit interactions influenced by Na(+) and K(+) binding.

Nuclear Na+/K+-ATPase plays an active role in nucleoplasmic Ca2+ homeostasis

Na(+)/K(+)-ATPase, an integral membrane protein, has been studied for over a half century with respect to its transporter function in the plasma membrane, where it expels three Na(+) ions from the cell in exchange for two K(+) ions. In this study, we demonstrate a functioning Na(+)/K(+)-ATPase within HEK293 cell nuclei. This subcellular localization was confirmed by western blotting, ouabain-sensitive ATPase activity of the nuclear membrane fraction, immunocytochemistry and delivery of fluorescently tagged Na(+)/K(+)-ATPase α- and β-subunits. In addition, we observed an overlap between nuclear Na(+)/K(+)-ATPase and Na/Ca-exchanger (NCX) when nuclei were immunostained with commercially available Na(+)/K(+)-ATPase and NCX antibodies, suggesting a concerted physiological coupling between these transporters. In keeping with this, we observed an ATP-dependent, strophanthidin-sensitive Na(+) flux into the nuclear envelope (NE) lumen loaded with the Na-sensitive dye, CoroNa-Green. Analogous experiments using Fluo-5N, a low affinity Ca(2+) indicator, demonstrated a similar ATP-dependent and strophanthidin-sensitive Ca(2+) flux into the NE lumen. Our results reveal an intracellular physiological role for the coordinated efforts of the Na(+)/K(+)-ATPase and NCX to actively remove Ca(2+) from the nucleoplasm into the NE lumen (i.e. the nucleoplasmic reticulum).

The beta1 subunit of Na+/K+-ATPase interacts with BKCa channels and affects their steady-state expression on the cell surface

Large conductance Ca2+-activated K+ channels (BKCa) encoded by the Slo1 gene play a role in the physiological regulation of many cell types. Here, we show that the beta1 subunit of Na+/K+-ATPase (NKbeta1) interacts with the cytoplasmic COOH-terminal region of Slo1 proteins. Reduced expression of endogenous NKbeta1 markedly inhibits evoked BKCa currents with no apparent effect on their gating. In addition, NKbeta1 down-regulated cells show decreased density of Slo1 subunits on the cell surface.

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

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

Identification and characterization of retinoid-active short-chain dehydrogenases/reductases in Drosophila melanogaster

BACKGROUND

In chordates, retinoid metabolism is an important target of short-chain dehydrogenases/reductases (SDRs). It is not known whether SDRs play a role in retinoid metabolism of protostomes, such as Drosophila melanogaster.

METHODS

Drosophila genome was searched for genes encoding proteins with approximately 50% identity to human retinol dehydrogenase 12 (RDH12). The corresponding proteins were expressed in Sf9 cells and biochemically characterized. Their phylogenetic relationships were analyzed using PHYLIP software.

RESULTS

A total of six Drosophila SDR genes were identified. Five of these genes are clustered on chromosome 2 and one is located on chromosome X. The deduced proteins are 300 to 406 amino acids long and are associated with microsomal membranes. They recognize all-trans-retinaldehyde and all-trans-3-hydroxyretinaldehyde as substrates and prefer NADPH as a cofactor. Phylogenetically, Drosophila SDRs belong to the same branch of the SDR superfamily as human RDH12, indicating a common ancestry early in bilaterian evolution, before a protostome-deuterostome split.

CONCLUSIONS

Similarities in the substrate and cofactor specificities of Drosophila versus human SDRs suggest conservation of their function in retinoid metabolism throughout protostome and deuterostome phyla.

GENERAL SIGNIFICANCE

The discovery of Drosophila retinaldehyde reductases sheds new light on the conversion of beta-carotene and zeaxantine to visual pigment and provides a better understanding of the evolutionary roots of retinoid-active SDRs.

A tomato ER-type Ca2+-ATPase, LCA1, has a low thapsigargin-sensitivity and can transport manganese

Recombinant Ca(2+)-ATPase from tomato (i.e. LCA1 for Lycopersicon esculentum [Since the identification and naming of LCA1, the scientific name for the tomato has been changed to Solanum lycopersicum.] Ca-ATPase) was heterologously expressed in yeast for structure-function characterization. We investigate the differences between plant and animal Ca pumps utilizing comparisons between chicken and rabbit SERCA-type pumps with Arabidopsis (ECA1) and tomato plant (LCA1) Ca(2+)-ATPases. Enzyme function was confirmed by the ability of each Ca(2+)-ATPase to rescue K616 growth on EGTA-containing agar and directly via in vitro ATP hydrolysis. We found LCA1 to be approximately 300-fold less sensitive to thapsigargin than animal SERCAs, whereas ECA1 was thapsigargin-resistant. LCA1 showed typical pharmacological sensitivities to cyclopiazonic acid, vanadate, and eosin, consistent with it being a P(IIA)-type Ca(2+)-ATPase. Possible amino acid changes responsible for the reduced plant thapsigargin-sensitivity are discussed. We found that LCA1 also complemented K616 yeast growth in the presence of Mn(2+), consistent with moving Mn(2+) into the secretory pathway and functionally compensating for the lack of secretory pathway Ca-ATPases (SPCAs) in plants.

Expression and trafficking of the gamma subunit of Na,K-ATPase in hypertonically challenged IMCD3 cells

The gamma subunit (FXYD2) of Na,K-ATPase is an important regulator of the sodium pump. In this investigation we have analysed the trafficking of gamma to the plasma membrane in cultures of inner medullary collecting duct cells (IMCD3) following acute hypertonic challenge and brefeldin A (BFA) treatment. Following hypertonic challenging for 24 hr immunofluorescence labeling revealed initial co-localization of the gamma subunit and 58K Golgi protein in the cytoplasm, but no co-localization of alpha1 and Golgi protein. Exposure of the challenged cells to BFA prevented the subsequent incorporation of gamma into the basolateral plasma membrane. The gamma subunit instead remained in cytoplasmic vesicles while cell proliferation and cell viability decreased simultaneously. Following removal of BFA from the hypertonic medium the IMCD3 cells recovered with distinct expression of gamma in the basolateral membrane. The alpha1 subunit was only marginally influenced by BFA. The results demonstrate that the gamma subunit trafficks to the plasma membrane via the Golgi apparatus, despite the absence of a signal sequence. The results also suggest that the gamma and alpha subunits do not traffic together to the plasma membrane, and that the gamma and alpha subunit have different turnover rates during these experimental conditions.

beta-Subunit overexpression alters the stoicheometry of assembled Na-K-ATPase subunits in MDCK cells

In eukaryotic cells, the apparent maintenance of 1:1 stoicheometry between the Na-K-ATPase alpha- and beta-subunits led us to question whether this was alterable and thus if some form of regulation was involved. We have examined the consequences of overexpressing Na-K-ATPase beta1-subunits using Madin-Darby canine kidney (MDCK) cells expressing flag-tagged beta1-subunits (beta1flag) or Myc-tagged beta1-subunits (beta1myc) under the control of a tetracycline-dependent promoter. The induction of beta1flag subunit synthesis in MDCK cells, which increases beta1-subunit expression at the plasma membrane by more than twofold, while maintaining stable alpha1 expression levels, revealed that all mature beta1-subunits associate with alpha1-subunits, and no evidence of "free" beta1-subunits was obtained. Consequently, the ratio of assembled beta1- to alpha1-subunits is significantly increased when "extra" beta-subunits are expressed. An increased beta1/alpha1 stoicheometry is also observed in cells treated with tunicamycin, suggesting that the protein-protein interactions involved in these complexes are not dependent on glycosylation. Confocal images of cocultured beta1myc-expressing and beta1flag-expressing MDCK cells show colocalization of beta1myc and beta1flag subunits at the lateral membranes of neighboring cells, suggesting the occurrence of intercellular interactions between the beta-subunits. Immunoprecipitation using MDCK cells constitutively expressing beta1myc and tetracycline-regulated beta1flag subunits confirmed beta-beta-subunit interactions. These results demonstrate that the equimolar ratio of assembled beta1/alpha1-subunits of the Na-K-ATPase in kidney cells is not fixed by the inherent properties of the interacting subunits. It is likely that cellular mechanisms are present that regulate the individual Na-K-ATPase subunit abundance.

Delivery of the Cu-transporting ATPase ATP7B to the plasma membrane in Xenopus oocytes

Cu-transporting ATPase ATP7B (Wilson disease protein) is essential for the maintenance of intracellular copper concentration. In hepatocytes, ATP7B is required for copper excretion, which is thought to occur via a transient delivery of the ATP7B- and copper-containing vesicles to the apical membrane. The currently available experimental systems do not allow analysis of ATP7B at the cell surface. Using epitope insertion, we identified an extracellular loop into which the HA-epitope can be introduced without inhibiting ATP7B activity. The HA-tagged ATP7B was expressed in Xenopus oocytes and the presence of ATP7B at the plasma membrane was demonstrated by electron microscopy, freeze-fracture experiments, and surface luminescence measurements in intact cells. Neither the deletion of the entire N-terminal copper-binding domain nor the inactivating mutation of catalytic Asp1027 affected delivery to the plasma membrane of oocytes. In contrast, surface targeting was decreased for the ATP7B variants with mutations in the ATP-binding site or the intra-membrane copper-binding site, suggesting that ligand-stabilized conformation(s) are important for ATP7B trafficking. The developed system provides significant advantages for studies that require access to both sides of ATP7B in the membrane.

Differential expression of Na+/K+-ATPase alpha-subunits in mouse hippocampal interneurones and pyramidal cells

The sodium pump (Na+/K+-ATPase), maintains intracellular and extracellular concentrations of sodium and potassium by catalysing ATP. Three sodium pump alpha subunits, ATP1A1, ATP1A2 and ATP1A3, are expressed in brain. We compared their role in pyramidal cells and a subset of interneurones in the subiculum. Interneurones were identified by their expression of GFP under the GAD-65 promoter. We used the sensitivity to the cardiac glycoside, ouabain, to discriminate between different alpha subunit isoforms. GFP-positive interneurones were depolarized by nanomolar doses of ouabain, but higher concentrations were needed to depolarize pyramidal cells. Comparison of pump currents in these cells revealed a current sensitive to low doses of ouabain in interneurones, while micromolar doses of ouabain were needed to suppress the pump current in subicular pyramidal cells. As predicted, nanomolar doses of ouabain increased the frequency but not the amplitudes of IPSPs in pyramidal cells. Immunostaining confirmed a differential distribution of alpha-subunits of the Na+/K+-ATPase in subicular interneurones and pyramidal cells. In conclusion, these data suggest that while ATP1A3-isoforms regulate sodium and potassium homeostasis in subicular interneurones, ATP1A1-isoforms assume this function in pyramidal cells. This differential expression of sodium pump isoforms may contribute to differences in resting membrane potential of subicular interneurones and pyramidal cells.

Selective basolateral localization of overexpressed Na-K-ATPase β1- and β2- subunits is disrupted by butryate treatment of MDCK cells

The exclusive basolateral localization of the Na-K-ATPase in kidney epithelium is a critical aspect of nephron function. It has been suggested that mislocalized delivery of the Na-K-ATPase to the apical surface in autosomal dominant polycystic kidney disease (ADPKD) is due to the inappropriate expression of an alternative isoform of the β-subunit, the β2-isoform. It has been reported that heterologous expression of this β2-isoform in Madin-Darby canine kidney (MDCK) cells results in apical delivery of the Na-K-ATPase. We created a MDCK cell line containing a tetracycline-inducible promoter and expressed either myc-tagged β2- or flag-tagged β1-subunits to study the surface localization of these β-subunit isoforms in polarized monolayers. We find that the β2-isoform is targeted to the basolateral surface of the plasma membrane in a polarization pattern indistinguishable from the β1-isoform. However, inclusion of butyrate in the growth medium leads to upregulation of overexpressed β1- or β2-subunits and to their appearance at the apical surface. The β2-isoform expressed in MDCK cells does not assemble into α1β2heterodimers with the endogenous α1. Our findings demonstrate that expression of the β2-isoform does not lead to apical localization of the Na-K-ATPase in MDCK cells and provides evidence for an unexpected effect of butyrate on the trafficking of Na pump subunits.

O-linked glycosylation at threonine 27 protects the copper transporter hCTR1 from proteolytic cleavage in mammalian cells

The major human copper uptake protein, hCTR1, has 190 amino acids and a predicted mass of 21 kDa. hCTR1 antibodies recognize multiple bands in SDS-PAGE centered at 35 kDa. Part of this increased mass is due to N-linked glycosylation at Asn-15. We show that in mammalian cells the N15Q mutant protein trafficked to the plasma membrane and mediated copper uptake at 75% of the rate of wild-type hCTR1. We demonstrate that the extracellular amino terminus of hCTR1 also contains O-linked polysaccharides. Glycosidase treatment that removed O-linked sugars reduced the apparent mass of hCTR1 or N15Q mutant protein by 1-2 kDa. Expression of amino-terminal truncations and alanine substitution mutants of hCTR1 in HEK293 and MDCK cells localized the site of O-linked glycosylation to Thr-27. Expression of alanine substitutions at Thr-27 resulted in proteolytic cleavage of hCTR1 on the carboxyl side of the T27A mutations. This cleavage produced a 17-kDa polypeptide missing approximately the first 30 amino acids of hCTR1. Expression of wild-type hCTR1 in mutant Chinese hamster ovary cells that were unable to initiate O-glycosylation also resulted in hCTR1 cleavage to produce the 17-kDa polypeptide. The 17-kDa hCTR1 polypeptide was located in the plasma membrane and mediated copper uptake at about 50% that of the rate of wild-type hCTR1. Thus, O-linked glycosylation at Thr-27 is necessary to prevent proteolytic cleavage that removes half of the extracellular amino terminus of hCTR1 and significantly impairs transport activity of the remaining polypeptide.

Identification of a pool of non-pumping Na/K-ATPase

Recent studies have ascribed many non-pumping functions to the Na/K-ATPase. Here, we present experimental evidence demonstrating that over half of the plasma membrane Na/K-ATPase in LLC-PK1 cells is performing cellular functions other than ion pumping. This "non-pumping" pool of Na/K-ATPase, like the pumping pump, binds ouabain. Depletion of either cholesterol or caveolin-1 moves some of the "non-pumping" Na/K-ATPase into the pumping pool. Graded knock-down of the alpha1 subunit of the Na/K-ATPase eventually results in loss of this "non-pumping" pool while preserving the pumping pool. Our prior studies indicate that a loss of the non-pumping pool is associated with a loss of receptor function as evidenced by the failure of ouabain administration to induce the activation of Src and/or ERK. Therefore, our new findings suggest that a substantial amount of surface-expressed Na/K-ATPase, at least in some types of cells, may function as non-canonical ouabain-binding receptors.

Structural Genomics

Drug discovery based on structural knowledge has proven useful as several structure-based medicines are already on the market. Structural genomics aims at studying a large number of gene products including whole genomes, topologically similar proteins, protein families and protein subtypes in parallel. Particularly, therapeutically relevant targets have been selected for structural genomics initiatives. In this context, integral membrane proteins, which represent 60–70% of the current drug targets, have been of major interest. Paradoxically, membrane proteins present the last frontier to conquer in structural biology as some 100 high resolution structures among the 30,000 entries in public structural databases are available. The modest success rate on membrane proteins relates to the difficulties in their expression, purification and crystallography. To facilitate technology development large networks providing expertise in molecular biology, protein biochemistry and structural biology have been established. The privately funded MePNet program has studied 100 G protein-coupled receptors, which resulted in high level expression of a large number of receptors at structural biology compatible levels. Currently, selected GPCRs have been purified and subjected to crystallization attempts

Loss of N-linked glycosylation reduces urea transporter UT-A1 response to vasopressin

The vasopressin-regulated urea transporter (UT)-A1 is a transmembrane protein with two glycosylated forms of 97 and 117 kDa; both are derived from a single 88-kDa core protein. However, the precise molecular sites and the function for UT-A1 N-glycosylation are not known. In this study, we compared Madin-Darby canine kidney cells stably expressing wild-type (WT) UT-A1 to Madin-Darby canine kidney cell lines stably expressing mutant UT-A1 lacking one (A1m1, A1m2) or both glycosylation sites (m1m2). Site-directed mutagenesis revealed that UT-A1 has two glycosylation sites at Asn-279 and -742. Urea flux is stimulated by 10 nM vasopressin (AVP) or 10 microM forskolin (FSK) in WT cells. In contrast, m1m2 cells have a delayed and significantly reduced maximal urea flux. A 15-min treatment with AVP and FSK significantly increased UT-A1 cell surface expression in WT but not in m1m2 cells, as measured by biotinylation. We confirmed this finding using immunostaining. Membrane fractionation of the plasma membrane, Golgi, and endoplasmic reticulum revealed that AVP or FSK treatment increases UT-A1 abundance in both Golgi and plasma membrane compartments in WT but not in m1m2 cells. Pulse-chase experiments showed that UT-A1 half-life is reduced in m1m2 cells compared with WT cells. Our results suggest that mutation of the N-linked glycosylation sites reduces urea flux by reducing UT-A1 half-life and decreasing its accumulation in the apical plasma membrane. In vivo, inner medullary collecting duct cells may regulate urea uptake by altering UT-A1 glycosylation in response to AVP stimulation.

Biogenesis of functional antigenic peptide transporter TAP requires assembly of pre-existing TAP1 with newly synthesized TAP2

The transporter associated with antigen processing (TAP) is essential for the delivery of antigenic peptides from the cytosol into the endoplasmic reticulum (ER), where they are loaded onto major histocompatibility complex class I molecules. TAP is a heterodimeric transmembrane protein that comprises the homologous subunits TAP1 and TAP2. As for many other oligomeric protein complexes, which are synthesized in the ER, the process of subunit assembly is essential for TAP to attain a native functional state. Here, we have analyzed the individual requirements of TAP1 and TAP2 for the formation of a functional TAP complex. Unlike TAP1, TAP2 is very unstable when expressed in isolation. We show that heterodimerization of TAP subunits is required for maintaining a stable level of TAP2. By using an in vitro expression system we demonstrate that the biogenesis of functional TAP depends on the assembly of preexisting TAP1 with newly synthesized TAP2, but not vice versa. The pore forming core transmembrane domain (core TMD) of in vitro expressed TAP2 is necessary and sufficient to allow functional complex formation with pre-existing TAP1. We propose that the observed assembly mechanism of TAP protects newly synthesized TAP2 from rapid degradation and controls the number of transport active transporter molecules. Our findings open up new possibilities to investigate functional and structural properties of TAP and provide a powerful model system to address the biosynthetic assembly of oligomeric transmembrane proteins in the ER.

Beta-subunit of cardiac Na+-K+-ATPase dictates the concentration of the functional enzyme in caveolae

Previous studies showed the presence of a significant fraction of Na(+)-K(+)-ATPase alpha-subunits in cardiac myocyte caveolae, suggesting the caveolar interactions of Na(+)-K(+)-ATPase with its signaling partners. Because both alpha- and beta-subunits are required for ATPase activity, to clarify the status of the pumping function of caveolar Na(+)-K(+)-ATPase, we have examined the relative distribution of two major subunit isoforms (alpha(1) and beta(1)) in caveolar and noncaveolar membranes of adult rat cardiac myocytes. When cell lysates treated with high salt (Na(2)CO(3) or KCl) concentrations were fractionated by a standard density gradient procedure, the resulting light caveolar membranes contained 30-40% of alpha(1)-subunits and 80-90% of beta(1)-subunits. Use of Na(2)CO(3) was shown to inactivate Na(+)-K(+)-ATPase; however, caveolar membranes obtained by the KCl procedure were not denatured and contained approximately 75% of total myocyte Na(+)-K(+)-ATPase activity. Sealed isolated caveolae exhibited active Na(+) transport. Confocal microscopy supported the presence of alpha,beta-subunits in caveolae, and immunoprecipitation showed the association of the subunits with caveolin oligomers. The findings indicate that cardiac caveolar inpocketings are the primary portals for active Na(+)-K(+) fluxes, and the sites where the pumping and signaling functions of Na(+)-K(+)-ATPase are integrated. Preferential concentration of beta(1)-subunit in caveolae was cell specific; it was also noted in neonatal cardiac myocytes but not in fibroblasts and A7r5 cells. Uneven distributions of alpha(1) and beta(1) in early and late endosomes of myocytes suggested different internalization routes of two subunits as a source of selective localization of active Na(+)-K(+)-ATPase in cardiac caveolae.

Structure-function relationships in the NA+,K+-pump

The Na,K-pump was discovered about 50 years ago. Since then there has been a methodic investigation of its structure and functional characteristics. The development of the Albers-Post model for the transport cycle was a milestone that provided the framework for detailed understanding of the transport process. The pump is composed of 2 subunits that exist in the membrane as an alphabeta heterodimer. All known enzymatic functions of the pump occur through the alpha subunit. Although necessary for activity, the complete role of the beta subunit is not understood fully. Numerous studies have established that the alphabeta protomer is the minimal functional unit needed to perform the Albers-Post reaction cycle. However, higher orders of aggregation [(alphabeta)n] are commonly detected. There is little evidence that oligomerization has functional consequence for ion transport. The Na+,K+-adenosine triphosphatase (ATPase) is a member of the P-type ATPase family of transporters. Proteins within this family have common amino acid sequence motifs that share functional characteristics and structure. Low-resolution 3-dimensional reconstruction of 2-dimensional crystal diffractions provide evidence for the similarity in tertiary structure of the alpha subunit and the Ca2+ATPase (a closely related P-type ATPase). The spatial location of the beta subunit also is obvious in these reconstructions. Recent high-resolution reconstructions from 3-dimensional crystals of the Ca2+ATPase provide structural details at the atomic level. It now is possible to interpret structurally some of the key steps in the Albers-Post reaction. Some of these high-resolution interpretations are translatable to the Na+,K+-ATPase, but a high-resolution structure of the Na,K-pump is needed for the necessary details of those aspects that are unique to this transporter.

Effects of warm acclimation on Na+,K+-ATPase α-subunit expression in chloride cells of Antarctic fish

The teleosts Trematomus bernacchii thrive in southern oceanic waters with temperatures below 0 degrees C. These fish have serum osmolalities almost double those found in fish of temperate waters, thereby lowering their serum's freezing point and the energy needed for ionic homeostasis. Upon warm acclimation to 4 degrees C, T. bernacchii decrease their serum osmolality and increase the Na+,K+-ATPase activity in their gills. Na+,K+-ATPase alpha1-, alpha2-, and alpha3-subunit isoforms are expressed in the gills of T. bernacchii and it is thought that Na+,K+-ATPase subunit composition in chloride cells changes with warm acclimation. Using immunohistochemistry, we compared the number of chloride cells expressing various alpha-isoforms of the Na+,K+-ATPase in the gills of cold- and warm-acclimated T. bernacchii. We found no change in the number of alpha2- or alpha3-immunopositive cells in warm-acclimated fish gills or in the number of cells immunopositive for the Na+,K+,2Cl- cotransporter. However, the number of pan-alpha-immunopositive (recognizing all three alpha-isoforms) and alpha1-immunopositive cells both increased in warm-acclimated fish. This suggests that changes in the number of alpha1-isoform-expressing chloride cells could contribute to the increased Na+,K+-ATPase activity that occurs with warm-acclimation.

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.

Development of an HTS Assay for Na+, K+-ATPase Using Nonradioactive Rubidium Ion Uptake

A high-throughput screening (HTS) assay was developed for the Na(+),K(+)-ATPase channel in order to study rubidium uptake as a measure of the functional activity and modulation of this exchanger. The assay uses elemental rubidium as a tracer for K(+) ions. Three cell lines were used to study the exchanger, and the assay was performed in a 96-well microtiter plate format. Rb(+) uptake was carried by the CHO-K1 cells at 37 degrees C; the maximum ion influx was at 80 min of incubation of the cell line in the medium containing 5.4 mM RbCl. The cells were incubated in Rb(+) uptake buffer (5.4 mM) and with the pump blocker ouabain for 1, 2, and 3 h, respectively. A complete block of the Rb(+) uptake was observed with a 5 mM concentration of ouabain for all the three time intervals. The ouabain 50% inhibitory concentration (IC(50)) value for CHO-K1 cell line ATPase was observed to be 298 microM after 3 h of incubation. In addition, IC(50) values of 94 and 89 microM were observed at 30 min of incubation, indicating that the protocol shows reproducible results. A Z' factor higher than 0.7 was observed in the assays. These studies extend the profile of Na(+),K(+)-ATPases and demonstrate the feasibility of this HTS assay system to screen for compounds that pharmacologically modulate the function of Na(+),K(+)-ATPase.

Oligomerization of the Na,K-ATPase in cell membranes

The higher order oligomeric state of the Na,K-ATPase alphabeta heterodimer in cell membranes is the subject of controversy. We have utilized the baculovirus-infected insect cell system to express Na,K-ATPase with alpha-subunits bearing either His(6) or FLAG epitopes at the carboxyl terminus. Each of these constructs produced functional Na,K-ATPase alphabeta heterodimers that were delivered to the plasma membrane (PM). Cells were simultaneously co-infected with viruses encoding alpha-His/beta and alpha-FLAG/beta Na,K-ATPases. Co-immunoprecipitation of the His-tagged alpha-subunit in the endoplasmic reticulum (ER) and PM fractions of co-infected cells by the anti-FLAG antibody demonstrates that protein-protein associations exist between these heterodimers. This suggests the Na,K-ATPase is present in cell membranes in an oligomeric state of at least (alphabeta)(2) composition. Deletion of 256 amino acid residues from the central cytoplasmic loop of the alpha-subunit results in the deletion alpha-4,5-loop-less (alpha-4,5LL), which associates with beta but is confined to the ER. Co-immunoprecipitation demonstrates that when this inactive alpha-4,5LL/beta heterodimer is co-expressed with wild-type alphabeta, oligomers of wild-type alphabeta and alpha-4,5LL/beta form in the ER, but the alpha-4,5LL mutant remains retained in the ER, and the wild-type protein is still delivered to the PM. We conclude that the Na,K-ATPase is present as oligomers of the monomeric alphabeta heterodimer in native cell membranes.

Bretylium, an organic quaternary amine, inhibits the Na,K-ATPase by binding to the extracellular K-site

The quaternary amine, bretylium, is a class III antiarrhythmic drug used to treat ventricular tachycardia and fibrillation. The primary mode of action for bretylium is thought to be inhibition of voltage-gated K(+) channels. While the Na,K-ATPase has been the pharmacological target of cardiac glycosides for over a century, recent evidence has shown that bretylium may also inhibit the Na pump. Our experimental findings support and extend these previous reports and provide definitive evidence supporting the previous suggestion that bretylium and K compete for the Na pump. We find that bretylium inhibits the Na pump in a dose-dependent manner in both Na,K-ATPase (IC(50) 4.5 mM) and Rb flux experiments (IC(50) 3.5 mM). Furthermore, we show that bretylium and Rb(+) competes for an extracellular site by measuring ouabain-sensitive (86)Rb flux in intact human red blood cells; that is, there is an apparent increase in K(m) for Rb(+) in the presence of 5 mM bretylium, while V(max) remains unchanged. We also determined that unlike K(+), bretylium does not facilitate the hydrolysis of E2-P. However, it stabilizes this conformation by reducing the ability of K(+) to facilitate dephosphorylation. Finally, we show that bretylium, like K(+), reduces [(3)H]ouabain binding to the Na pump. Taken together, these data are consistent with bretylium binding to the extracellular facing cation site within the E2-P state of the enzyme. Moreover, these findings suggest that bretylium may serve as an effective tool for freezing the pump in an extracellularly cation-bound phosphorylated intermediate, which will aid in future structural analyses.

Structure-function relationship in P-type ATPases--a biophysical approach

P-type ATPases are a large family of membrane proteins that perform active ion transport across biological membranes. In these proteins the energy-providing ATP hydrolysis is coupled to ion-transport that builds up or maintains the electrochemical potential gradients of one or two ion species across the membrane. P-type ATPases are found in virtually all eukaryotic cells and also in bacteria, and they are transporters of a broad variety of ions. So far, a crystal structure with atomic resolution is available only for one species, the SR Ca-ATPase. However, biochemical and biophysical studies provide an abundance of details on the function of this class of ion pumps. The aim of this review is to summarize the results of preferentially biophysical investigations of the three best-studied ion pumps, the Na,K-ATPase, the gastric H,K-ATPase, and the SR Ca-ATPase, and to compare functional properties to recent structural insights with the aim of contributing to the understanding of their structure-function relationship.

The role of bacterial and non-bacterial toxins in the induction of changes in membrane transport: implications for diarrhea

Bacterial toxins induce changes in membrane transport which underlie the loss of electrolyte homeostasis associated with diarrhea. Bacterial- and their secreted toxin-types which have been linked with diarrhea include: (a) Vibrio cholerae (cholera toxin, E1 Tor hemolysin and accessory cholera enterotoxin); (b) Escherichia coli (heat stable enterotoxin, heat-labile enterotoxin and colicins); (c) Shigella dysenteriae (shiga-toxin); (d) Clostridium perfringens (C. perfringens enterotoxin, alpha-toxin, beta-toxin and theta-toxin); (e) Clostridium difficile (toxins A and B); (f) Staphylococcus aureus (alpha-haemolysin); (g) Bacillus cereus (cytotoxin K and haemolysin BL); and (h) Aeromonas hydrophila (aerolysin, heat labile cytotoxins and heat stable cytotoxins). The mechanisms of toxin-induced diarrhea include: (a) direct effects on ion transport in intestinal epithelial cells, i.e. direct toxin interaction with intrinsic ion channels in the membrane and (b) indirect interaction with ion transport in intestinal epithelial cells mediated by toxin binding to a membrane receptor. These effects consequently cause the release of second messengers, e.g. the release of adenosine 3',5'-cyclic monophosphate/guanosine 3',5'-monophosphate, IP(3), Ca2+ and/or changes in second messengers that are the result of toxin-formed Ca2+ and K+ permeable channels, which increase Ca2+ flux and augment changes in Ca2+ homeostasis and cause depolarisation of the membrane potential. Consequently, many voltage-dependent ion transport systems, e.g. voltage-dependent Ca2+ influx, are affected. The toxin-formed ion channels may act as a pathway for loss of fluid and electrolytes. Although most of the diarrhea-causing toxins have been reported to act via cation and anion channel formation, the properties of these channels have not been well studied, and the available biophysical properties that are needed for the characterization of these channels are inadequate.

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

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

Interactions between Na,K-ATPase alpha-subunit ATP-binding domains

The reaction mechanism of the Na,K-ATPase is thought to involve a number of ligand-induced conformational changes. The specific amino acid residues responsible for binding many of the important ligands have been identified; however, details of the specific conformational changes produced by ligand binding are largely undescribed. The experiments described in this paper begin to identify interactions between domains of the Na,K-ATPase alpha-subunit that depend on the presence of particular ligands. The major cytoplasmic loop (between TM4 and TM5), which we have previously shown contains the ATP-binding domain, was overexpressed in bacteria either with a His(6) tag or as a fusion protein with glutathione S-transferase. We have observed that these polypeptides associate in the presence of MgATP. Incubation with [gamma-(32)P]ATP under conditions that result in phosphorylation of the full-length Na,K-ATPase did not result in (32)P incorporation into either the His(6) tag or glutathione S-transferase fusion proteins. The MgATP-induced association was strongly inhibited by prior modification of the fusion proteins with fluorescein isothiocyanate or by simultaneous incubation with 10 microm eosin, indicating that the effect of MgATP is due to interactions within the nucleotide-binding domain. These data are consistent with Na,K-ATPase associating within cells via interactions in the nucleotide-binding domains. Although any functional significance of these associations for ion transport remains unresolved, they may play a role in cell function and in modulating interactions between the Na,K-ATPase and other proteins.

Specific lipid requirements of membrane proteins--a putative bottleneck in heterologous expression

Membrane proteins are mostly protein-lipid complexes. For more than 30 examples of membrane proteins from prokaryotes, yeast, plant and mammals, the importance of phospholipids and sterols for optimal activity is documented. All crystallized membrane protein complexes show defined lipid-protein contacts. In addition, lipid requirements may also be transitory and necessary only for correct folding and intercellular transport. With respect to specific lipid requirements of membrane proteins, the phospholipid and glycolipid as well as the sterol content of the host cell chosen for heterologous expression should be carefully considered. The lipid composition of bacteria, archaea, yeasts, insects,Xenopus oocytes, and typical plant and mammalian cells are given in this review. A few examples of heterologous expression of membrane proteins, where problems of specific lipid requirements have been noticed or should be thought of, have been chosen.

Biochemistry of Na,K-ATPase

The Na,K-ATPase or sodium pump carries out the coupled extrusion and uptake of Na and K ions across the plasma membranes of cells of most higher eukaryotes. It is a member of the P-type ATPase superfamily. This heterodimeric integral membrane protein is composed of a 100-kDa alpha-subunit with ten transmembrane segments and a heavily glycosylated beta subunit of about 55 kDa, which is a type II membrane protein. Current ideas on how the protein achieves active transport are based on a fusion of results of transport physiology, protein chemistry, and heterologous expression of mutant proteins. Recently acquired high resolution structural information provides an important new avenue for a more complete understanding of this protein. In this review, the current status of knowledge of Na,K-ATPase is discussed, and areas where there is still considerable uncertainty are highlighted.

Molecular characterization of hCTR1, the human copper uptake protein

We have expressed hCTR1, the human copper transporter, in Sf9 cells using a baculovirus-mediated expression system, and we observed greatly enhanced copper uptake. Western blots showed that the protein is delivered to the plasma membrane, where it mediates saturable copper uptake with a K(m) of approximately 3.5 microm. We also expressed functional transporters where the N-linked glycosylation sites were substituted, and we provided evidence for the extracellular location of the amino terminus. Accessibility of amino-terminal FLAG epitope to antibody prior to permeabilization and of carboxyl-terminal FLAG only after permeabilization confirmed the extracellular location of the amino terminus and established the intracellular location of the carboxyl terminus. Tryptic digestion of hCTR1 occurred within the cytoplasmic loop and generated a 10-Da carboxyl-terminal peptide; cleavage was prevented by the presence of copper. hCTR1 mutants where Cys-161 and Cys-189, the two native cysteines, were replaced with serines also mediated copper uptake, indicating that neither cysteine residue was essential for transport. However, the mutants provided evidence that these residues may stabilize hCTR1 oligomerization. Western blots of hCTR1 in Sf9 cells showed expression levels 100-fold higher than in mammalian (HepG2) cells. The high level of functional expression and the low level of endogenous copper uptake will enable future structure-function analysis of this important protein.

Functional properties of the copper-transporting ATPase ATP7B (the Wilson's disease protein) expressed in insect cells

Copper-transporting ATPase ATP7B is essential for normal distribution of copper in human cells. Mutations in the ATP7B gene lead to copper accumulation in a number of tissues and to a severe multisystem disorder, known as Wilson's disease. Primary sequence analysis suggests that the copper-transporting ATPase ATP7B or the Wilson's disease protein (WNDP) belongs to the large family of cation-transporting P-type ATPases, however, the detailed characterization of its enzymatic properties has been lacking. Here, we developed a baculovirus-mediated expression system for WNDP, which permits direct and quantitative analysis of catalytic properties of this protein. Using this system, we provide experimental evidence that WNDP has functional properties characteristic of a P-type ATPase. It forms a phosphorylated intermediate, which is sensitive to hydroxylamine, basic pH, and treatments with ATP or ADP. ATP stimulates phosphorylation with an apparent K(m) of 0.95 +/- 0.25 microm; ADP promotes dephosphorylation with an apparent K(m) of 3.2 +/- 0.7 microm. Replacement of Asp(1027) with Ala in a conserved sequence motif DKTG abolishes phosphorylation in agreement with the proposed role of this residue as an acceptor of phosphate during the catalytic cycle. Catalytic phosphorylation of WNDP is inhibited by the copper chelator bathocuproine; copper reactivates the bathocuproine-treated WNDP in a specific and cooperative fashion confirming that copper is required for formation of the acylphosphate intermediate. These studies establish the key catalytic properties of the ATP7B copper-transporting ATPase and provide a foundation for quantitative analysis of its function in normal and diseased cells.