Assembly of the (Na+ + K+)-adenosine triphosphatase. Post-translational membrane integration of the alpha subunit

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.

Capillary endothelial Na(+), K(+), ATPase transporter homeostasis and a new theory for migraine pathophysiology

BACKGROUND

Cerebrospinal fluid sodium concentration ([Na(+)](csf)) increases during migraine, but the cause of the increase is not known.

OBJECTIVE

Analyze biochemical pathways that influence [Na(+)](csf) to identify mechanisms that are consistent with migraine.

METHOD

We reviewed sodium physiology and biochemistry publications for links to migraine and pain.

RESULTS

Increased capillary endothelial cell (CEC) Na(+), K(+), -ATPase transporter (NKAT) activity is probably the primary cause of increased [Na(+)](csf). Physiological fluctuations of all NKAT regulators in blood, many known to be involved in migraine, are monitored by receptors on the luminal wall of brain CECs; signals are then transduced to their abluminal NKATs that alter brain extracellular sodium ([Na(+)](e)) and potassium ([K(+)](e)).

CONCLUSIONS

We propose a theoretical mechanism for aura and migraine when NKAT activity shifts outside normal limits: (1) CEC NKAT activity below a lower limit increases [K(+)](e), facilitates cortical spreading depression, and causes aura; (2) CEC NKAT activity above an upper limit elevates [Na(+)](e), increases neuronal excitability, and causes migraine; (3) migraine-without-aura may arise from CEC NKAT over-activity without requiring a prior decrease in activity and its consequent spreading depression; (4) migraine triggers disturb, and treatments improve, CEC NKAT homeostasis; (5) CEC NKAT-induced regulation of neural and vasomotor excitability coordinates vascular and neuronal activities, and includes occasional pathology from CEC NKAT-induced apoptosis or cerebral infarction.

Mechanisms of proximal tubule sodium transport regulation that link extracellular fluid volume and blood pressure

One-hundred years ago, Starling articulated the interdependence of renal control of circulating blood volume and effective cardiac performance. During the past 25 years, the molecular mechanisms responsible for the interdependence of blood pressure (BP), extracellular fluid volume (ECFV), the renin-angiotensin system (RAS), and sympathetic nervous system (SNS) have begun to be revealed. These variables all converge on regulation of renal proximal tubule (PT) sodium transport. The PT reabsorbs two-thirds of the filtered Na(+) and volume at baseline. This fraction is decreased when BP or perfusion pressure is increased, during a high-salt diet (elevated ECFV), and during inhibition of the production of ANG II; conversely, this fraction is increased by ANG II, SNS activation, and a low-salt diet. These variables all regulate the distribution of the Na(+)/H(+) exchanger isoform 3 (NHE3) and the Na(+)-phosphate cotransporter (NaPi2), along the apical microvilli of the PT. Natriuretic stimuli provoke the dynamic redistribution of these transporters along with associated regulators, molecular motors, and cytoskeleton-associated proteins to the base of the microvilli. The lipid raft-associated NHE3 remains at the base, and the nonraft-associated NaPi2 is endocytosed, culminating in decreased Na(+) transport and increased PT flow rate. Antinatriuretic stimuli return the same transporters and regulators to the body of the microvilli associated with an increase in transport activity and decrease in PT flow rate. In summary, ECFV and BP homeostasis are, at least in part, maintained by continuous and acute redistribution of transporter complexes up and down the PT microvilli, which affect regulation of PT sodium reabsorption in response to fluctuations in ECFV, BP, SNS, and RAS.

Mimicking human bipolar ion dysregulation models mania in rats

Psychiatric diseases in general, and bipolar illness in particular, are difficult to model in animals since the subjective nature of the core symptoms appears to preclude objective observation of behavioral changes. An adequate animal model of a psychiatric condition must fulfill three core criteria: share pathophysiological characteristics of the human condition (face validity), have similar behavioral manifestations as the human disease (construct validity), and improve with medications that improve the symptoms seen in afflicted humans (predictive validity). The ouabain model for bipolar illness mimics a widely reproduced biologic abnormality in mania: reduced sodium pump activity. An intracerebroventricular (ICV) administration of 5microL 10(-3)M ouabain induces motoric hyperactivity preventable by lithium, carbamazepine, and haloperidol. ICV ouabain may also produce environmentally dependent hypoactivity. The model, however, has not yet been examined for other potential manic behavior in rats such as reduced need for sleep, increased sexual activity, or increased irritability. While additional characterization of the model is required, the ouabain model for bipolar illness is the only available animal model that fulfills the three criteria for an adequate animal model for bipolar illness.

Na,K-ATPase beta1-subunit increases the translation efficiency of the alpha1-subunit in MSV-MDCK cells

The Na,K-ATPase consists of an alpha- and beta-subunit. Moloney sarcoma virus-transformed MDCK cells (MSV-MDCK) express low levels of Na,K-ATPase beta(1)-subunit. Ectopic expression of Na,K-ATPase beta(1)-subunit in these cells increased the protein levels of the alpha(1)-subunit of Na,K-ATPase. This increase was not due to altered transcription of the alpha(1)-subunit gene or half-life of the alpha(1)-subunit protein because both alpha(1)-subunit mRNA levels and half-life of the alpha(1)-subunit protein were comparable in MSV-MDCK and beta(1)-subunit expressing MSV-MDCK cells. However, short pulse labeling revealed that the initial translation rate of the alpha(1)-subunit in beta(1)-subunit expressing MSV-MDCK cells was six- to sevenfold higher compared with MSV-MDCK cells. The increased translation was specific to alpha(1)-subunit because translation rates of occludin and beta-catenin, membrane and cytosolic proteins, respectively, were not altered. In vitro cotranslation/translocation experiments using rabbit reticulocyte lysate and rough microsomes revealed that the alpha(1)-subunit mRNA is more efficiently translated in the presence of beta(1)-subunit. Furthermore, sucrose density gradient analysis revealed significantly more alpha(1)-subunit transcript associated with the polysomal fraction in beta(1)-subunit expressing MSV-MDCK cells compared with MSV-MDCK cells, indicating that in mammalian cells the Na,K-ATPase beta(1)-subunit is involved in facilitating the translation of the alpha(1)-subunit mRNA in the endoplasmic reticulum.

Mutational and biochemical analysis of plasma membrane targeting mediated by the farnesylated, polybasic carboxy terminus of K-ras4B

Mutational analysis and in vitro assays of membrane association have been combined to investigate the mechanism of plasma membrane targeting mediated by the farnesylated, polybasic carboxy-terminal sequence of K-ras4B in mammalian cells. Fluorescence-microscopic localization of chimeric proteins linking the enhanced green fluorescent protein (EGFP) to the K-ras4B carboxy-terminal sequence, or to variant forms of this sequence, reveals that the normal structure of this targeting motif can be greatly altered without compromising plasma membrane-targeting activity so long as an overall strongly polybasic/amphiphilic character is retained. An EGFP/K-ras4B(171-188) chimeric protein was readily abstracted from isolated cell membranes by negatively charged lipid vesicles, and this abstraction was markedly enhanced by the anionic lipid-binding agent neomycin. Our results strongly favor a mechanism in which at the plasma membrane the carboxy-terminal sequence of K-ras4B associates not with a classical specific proteinaceous receptor but rather with nonspecific but highly anionic 'sites' formed at least in part by the membrane lipid bilayer. Our findings also suggest that the recently demonstrated prenylation-dependent trafficking of immature forms of K-ras4B through the endoplasmic reticulum [Choy et al. (1999) Cell 98, 69-80], while required for maturation of the protein, beyond this stage may not be essential to allow the ultimate delivery of the mature protein to the plasma membrane.

Alpha 2 isoform of the Na,K-adenosine triphosphatase is reduced in temporal cortex of bipolar individuals

BACKGROUND

The pathophysiology of bipolar illness has been associated with changes in transmembrane ion flux and redistribution of biologically active ions. The recent identification of multiple isoforms of Na,K-adenosine triphosphatase (ATPase) alpha and beta subunits raises the possibility of altered pump isoform expression.

METHODS

We determined Na,K-ATPase alpha subunit expression in postmortem temporal cortex gray matter from individuals suffering from bipolar disorder, schizoaffective disorder, schizophrenia, and matched normal controls. Quantification of isoform expression was accomplished via densitometric scanning of Western blots utilizing isoform-specific antibodies.

RESULTS

Bipolar individuals exhibited a significant reduction in the abundance of the alpha 2 isoform of Na,K-ATPase compared to normal controls. Schizophrenic and schizo-affective brains were not significantly different from normal controls.

CONCLUSION

These data suggest that previously observed abnormalities in regulation and distribution of ions in bipolar illness may be related to specific alpha 2 dysregulation.

Role of opioidergic and monoaminergic neurotransmission in the GnRH release mechanism of EBP-primed OVX rats

We examined the effect of intracerebroventricular (i.c.v.) administration of mu-opioid agonist, morphine, and its antagonist naloxone followed by morphine on the activities of monoamine-metabolizing enzymes, namely tyrosine hydroxylase (TH) and monoamine oxidase (MAO) along with adenosinetriphosphatase (Na+, K+ -ATPase), the enzyme responsible for the maintenance of ionic gradients across the membrane, in seven discrete regions of brain from estrogen- and progesterone-primed ovariectomized rats. TH activity decreased after morphine treatment in some areas such as the median eminence-arcuate region (ME-ARC), the amygdala, and the thalamus, showing statistically significant change. MAO activity increased in all the areas studied, but more appreciable change was observed in medial preoptic area (mPOA), the ME-ARC region, and the cortex. Pronounced increase in Na+, K+ -ATPase enzyme activity was observed after the drug treatment. Naloxone given prior to morphine injection resulted in recovery of the enzyme activities in most of the areas studied. Our study may provide insights into the precise opioidergic modulation of gonadotropin releasing hormone (GnRH) release mechanisms through the involvement of monoaminergic system, elucidating the basis of various neuronal dysfunctions and their management in opioid addicts.

Assembly of the chimeric Na+/K+-ATPase and H+/K+-ATPase beta-subunit with the Na+/K+-ATPase alpha-subunit

Two sets of chimeric beta-subunits were constructed from subunits of Torpedo californica Na+/K+-ATPase and pig gastric H+/K+-ATPase. Five unique restriction sites (SnaBI, EcoRV, MunI, SphI and EcoT22I) were created at equivalent positions of the respective cDNAs and were used as joining points for the construction. One set of chimeras (HxN series) was made by exchanging the 5' portion of the Na+/K+-ATPase beta-subunit cDNA with the corresponding portion of the H+/K+-ATPase beta-subunit cDNA at the respective joining point. Complementary constructs were also prepared (NxH series). In the HxN series, the chimera joined at the SnaBI site formed a stable trypsin resistant complex with the Na+/K+-ATPase alpha-subunit, which was functional with respect to ATP hydrolysis and pump current generation, although the activities were less than those of the complex with the Na+/K+-ATPase beta-subunit. Trypsin resistance decreased for the complex of the chimera joined at the EcoRV site. In the NxH series, the chimeras joined at the SnaBI site and the EcoRV site formed rather trypsin-resistant complexes, but the expressions of the alpha-subunits were below 50% of the control. The chimeras joined at the MunI, SphI and EcoT22I site formed complexes susceptible to tryptic digestion. None of the chimeras in the NxH series were functional. These results suggest that at least two regions of the Na+/K+-ATPase beta-subunit [SnaBI site(Tyr40) to EcoRV site(Ile89) and EcoT22I site(Cys176) to C-terminus)] are involved in stable assembly with the Na+/K+-ATPase alpha-subunit and that the cytoplasmic domain [N-terminus to SnaBI site(Tyr40)] is functionally replaceable with the corresponding domain of the H+/K+-ATPase beta-subunit.

Subcellular distribution of tissue kallikrein and Na,K-ATPase alpha-subunit in rat parotid striated duct cells

Intracellular protein distribution and sorting were examined in rat parotid striated duct cells, in which tissue kallikrein is apical, and Na,K-ATPase is basolateral. Electron-microscopic immunogold cytochemistry, with both polyclonal and monoclonal antibodies, demonstrated these enzymes at opposite poles of the cells and in distinct intracellular sites. Kallikrein was found within apical secretory granules, whereas Na,K-ATPase was present on basolateral cell membranes. In addition, kallikrein was localized throughout cisternae of all Golgi profiles, whereas Na,K-ATPase (alpha-subunit) was found only in small peripheral vesicles and/or lateral cisternal extensions of a basal subset of Golgi profiles. These differences in the subcellular distribution of the two marker antigens were most clearly seen with double immunogold labelling. Our results suggest that kallikrein, an apical, regulated secretory protein, and Na,K-ATPase, a basolateral, constitutively transported membrane protein, are segregated at (or prior to) the level of the Golgi apparatus rather than in the trans-Golgi network (TGN), as was expected.

Na(+)-K(+)-ATPase alpha 1- and beta 1-subunit degradation: evidence for multiple subunit specific rates

Na(+)-K(+)-ATPase is a heterodimeric plasma membrane protein consisting of an alpha-catalytic and a beta-glycoprotein subunit. Because these two subunits are derived from two separate genes, they may not be synthesized with stoichiometric equivalence. The aim of this study was to estimate relative rates of synthesis and degradation of nascent and mature Na(+)-K(+)-ATPase alpha- and beta-subunits to determine whether either of the nascent subunits accumulates in excess and, if so, the fate of the excess subunits. We studied a pig kidney cell line (LLC-PK1/Cl4) that expresses only alpha 1- and beta 1-subunits. Relative synthesis and degradation rates of nascent subunits were first estimated by pulsing cells for 10 min with [35S]methionine followed by chase periods of up to 120 min and by immunoprecipitation. We found that directly after labeling, beta-subunits were present in threefold excess over alpha-subunits and that nearly 50% of this beta-subunit pool was degraded by 60 min. Nascent alpha-subunits were not degraded during the chase period. In a second strategy to examine relative rates of nascent alpha- vs. beta-subunit accumulation, cells were pulsed for 5-60 min and immunoprecipitated directly (without chase). The rate of accumulation of labeled alpha was greater than that of beta between 5 and 60 min, consistent with the results of the pulse-chase strategy, demonstrating a significant component of degradation of beta during this period. Despite the very different degradation rates of newly synthesized alpha- vs. beta-subunits, the degradation rates of alpha- and beta-subunits beyond 4 h after synthesis were indistinguishable (t0.5 = 10-12 h).(ABSTRACT TRUNCATED AT 250 WORDS)

Na,K-ATPase in several tissues of the rat: tissue-specific expression of subunit mRNAs and enzyme activity

The relative contents of Na,K-ATPase subunit mRNAs in rat renal cortex, ventricular myocardium, skeletal muscle (hind limb), liver and brain (cerebrum) were measured. Expressed per unit DNA, mRNA alpha 1 content was approximately 2-fold greater in the kidney and brain as compared to either heart, skeletal muscle or liver. The hierarchy of mRNA alpha 2 expression was brain > skeletal muscle > heart, whereas mRNA alpha 3 was restricted to brain. Beta 1 subunit mRNA content in both kidney and brain exceeded the abundance of liver mRNA beta 1 by approximately 7-fold. In all tissues examined, the combined abundances of the alpha subunit mRNAs exceeded the content of mRNA beta 1. The hierarchy of Na,K-ATPase activity expressed per unit DNA was brain > kidney > skeletal muscle = heart > liver. The sum of mRNA alpha as well as mRNA beta 1 content, expressed per g of tissue, was highest in brain and kidney. A statistically significant correlation between mRNA beta 1 content and Na,K-ATPase activity was evident.

Tunicamycin reduces Na(+)-K(+)-pump expression in cultured skeletal muscle

The purpose of this study was to examine effects of tunicamycin (TM), which inhibits core glycosylation of the beta-subunit, on functional expression of the Na(+)-K+ pump in primary cultures of embryonic chick skeletal muscle. Measurements were made of specific-[3H]-ouabain binding, ouabain-sensitive 86Rb uptake, resting membrane potential (Em), and electrogenic pump contribution to Em (Ep) of single myotubes with intracellular microelectrodes. Growth of 4-6-day-old skeletal myotubes in the presence of TM (1 microgram/ml) for 21-24 hr reduced the number of Na(+)-K+ pumps to 60-90% of control. Na(+)-K+ pump activity, the level of resting Em and Ep were also reduced significantly by TM. In addition, TM completely blocked the hyperpolarization of Em induced in single myotubes by cooling to 10 degrees C and then re-warming to 37 degrees C. Effects of tunicamycin were compared with those of tetrodotoxin (TTX; 2 x 10(-7) M for 24 hr), which blocks voltage-dependent Na+ channels. TM produced significantly greater decreases in ouabain-binding and Em than did TTX, findings that indicate that reduced Na(+)-K+ pump expression was not exclusively secondary to decreased intracellular Na+, the primary regulator of pump synthesis in cultured muscle. Similarly, effects of TM were significantly greater than those of cycloheximide, which inhibits protein synthesis by 95%. These findings demonstrate that effects were not due to inhibition of protein synthesis. We conclude that glycosylation of the Na(+)-K+ pump beta-subunit is required for full physiological expression of pump activity in skeletal muscle.

Reversal of the effects of a low extracellular potassium concentration on the number and activity of Na+/K+ pumps in an Epstein-Barr virus-transformed human lymphocyte cell line

A reduction in the extracellular concentration of potassium to 0.5 mM (low K) in Epstein-Barr (EB) virus-transformed lymphocytes caused changes in the number and activity of Na+/K+ pumps in the cell membrane, with increases in the Bmax and apparent Kd of ouabain binding, and concomitant increases in the Vmax and apparent Km of potassium (rubidium) influx. However, recovery from the effects of low K occurred more quickly than the original up-regulation. Furthermore, there were differences in the time-courses of the separate rates of recovery of the Bmax and Kd of ouabain binding after the cells were returned to normal K, the rate of recovery of the Kd being quicker than that of the Bmax, which was biphasic, with slow and fast rates of recovery. Inhibition of protein synthesis by emetine caused an increase in the rate of recovery of the Bmax of ouabain binding, but no effect on the Kd, suggesting that the slow phase of recovery of the Bmax is attributable to the synthesis and insertion of new protein, while the rapid phase of recovery is independent of protein synthesis and may represent internalization. The results suggested that during up-regulation of pump number in response to low K about 40% of the newly inserted Na+/K+ pumps are normal and the rest are abnormal. The half-time of removal of the abnormal pumps from the cell membrane during recovery from low K stress was 2.8 hr and the half-time of internalization of the normal pumps was 4.3 hr.

Synthesis and translocation of Na(+)-K(+)-ATPase alpha- and beta-subunits to plasma membrane in MDCK cells

Synthesis and translocation of Na(+)-K(+)-ATPase alpha-catalytic and beta-glycoprotein subunits from intracellular membranes to the plasma membrane were studied in Madin-Darby canine kidney cells (MDCK-T) by combining the methods of pulse-chase labeling, subcellular fractionation on sorbitol gradients, and immunoprecipitation. Immunoprecipitation from homogenates revealed that radioactive methionine incorporated into beta-subunit was equal to that incorporated into alpha-subunit after 15 min of labeling. Because the ratio of total methionines in alpha- vs. beta-subunit is approximately 5:1, these results suggest that beta-subunit is synthesized in molar excess over alpha-subunit. Half of the newly synthesized beta-subunit, likely unassembled units, were degraded by 60 min after labeling, while alpha-subunits were stable through 120 min after synthesis, suggesting alpha may be limiting for alpha beta-assembly. By 120 min the ratio of counts incorporated into alpha vs. beta approached 5, which is predicted by a 1:1 ratio of alpha to beta. The sorbitol gradient resolved two major membrane samples: a mixture of endoplasmic reticulum and Golgi populations and a plasma membrane-enriched sample. Immature beta (beta i) could not be detected in the plasma membrane-enriched samples at levels greater than could be attributed to cross-contamination by intracellular membranes. Mature beta (beta m) became detectable after 30 min, and conversion of beta i to beta m was 90% complete at 120 min. A peak of labeled alpha-subunit appeared in the plasma membrane-enriched sample at 60 min, coincident with the appearance of labeled beta m-subunit in this sample, suggesting movement as alpha beta-heterodimers.

The functional role of the beta-subunit in the maturation and intracellular transport of Na,K-ATPase

The minimal functional enzyme unit of Na,K-ATPase consists of an alpha-beta complex. The alpha-subunit bears all functional domains of the enzyme and so far a regulatory role for the beta-subunit in the catalytic cycle has not been established. On the other hand, increasing experimental evidence suggests that the beta-subunit is an indispensable element for the structural and functional maturation of the enzyme as well as its intracellular transport to the plasma membrane. This brief review summarizes the experimental data supporting the hypothesis that assembly of the beta-subunit is needed for the alpha-subunit to acquire the correct, stable configuration necessary for the acquisition of functional properties and its exit from the ER.

The genes of Na,K-ATPase, a selfreview

The review is devoted to analysis of research carried out in the author's laboratory on structure-function relationships in genes coding for Na,K-ATPases. Also considered are problems related to molecular evolution of ion-transporting ATPases. This brief review is devoted to a fragment of research carried out in my laboratory, the Laboratory of Human Genes Structure and Function at the Shemyakin Institute of Bioorganic Chemistry, USSR Academy of Sciences. The area of the review may be named as structural-evolutionary analysis of functional anatomies of genes. The approach is fairly standard and its essence was formulated long ago: evolution decides 'to be or not to be' based on usefulness or lack of it. The elements of genes that are important for the gene function are retained in the course of evolution, and a comparison of genes having similar functions in different species should, hopefully, reveal different behavior of gene blocks, conservation of functionally significant blocks and variability of less significant or insignificant ones. An approach like this has been widely used in comparing proteins. However, a study of genes gives the investigator yet another tool of structural and evolutionary import: the exon structure may be relevant to the gene's evolutionary history, with exons corresponding to the functional domains (arguments for and against this fascinating hypothesis have been reviewed by Blake (Blake, 1985). However, even if the exon-domain correlation does not hold in the general case, a similarity in the exon-intron pattern of genes from different species is indicative of their common evolutionary origin and is enforcing the logic of variability analysis, provided, of course, that the compared genes have a common predecessor. A few years ago we employed this approach to analyze the functional structure of genes coding for subunits of bacterial DNA-dependent RNA polymerases and constructed functional maps of the enzyme. After that, a similar study of Na,K-ATPase genes to be reviewed here was started. The entire project became possible through collaboration with the lab of Dr. N. N. Modyanov, an eminent specialist in protein chemistry who had already accumulated considerable information on Na,K-ATPase from pig kidneys by that time. I would also like to stress that the work has been started on the initiative of the deceased Director of the Institute, Yu. A. Ovchinnikov. Since this is a self-review, I am asking my colleagues whose work will not be cited here to excuse me.(ABSTRACT TRUNCATED AT 400 WORDS)

Assembly of the alpha-subunit of Torpedo californica Na+/K(+)-ATPase with its pre-existing beta-subunit in Xenopus oocytes

The alpha- and beta-subunits of Torpedo californica Na+/K(+)-ATPase were expressed in turn in single oocytes by alternately microinjecting the specific mRNAs for the alpha- and beta-subunits. The mRNA first injected was degraded prior to the injection of the second mRNA by injecting the antisense oligonucleotide specific for the first mRNA. The pre-existing beta-subunit, which had been synthesized by injecting mRNA for the beta-subunit, could assemble with the alpha-subunit expressed later in the single oocytes and the resulting alpha beta complex acquired both ouabain-binding and Na+/K(+)-ATPase activities. On the other hand, formation of the alpha beta complex was not detected when the alpha-subunit was expressed first, followed by the beta-subunit. These data suggest that the beta-subunit acts as a receptor or a stabilizer for the alpha-subunit upon the biogenesis of Na+/K(+)-ATPase.

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

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

Human Na+,K+-ATPase genes. Beta-subunit gene family contains at least one gene and one pseudogene

The existence of a chromosome gene family containing at least one gene and one pseudogene was shown for the Na+,K+-ATPase beta-subunit. A partial structure of the beta 1-gene was determined, the coding part of which was completely homologous to cDNA of the Na+,K+-ATPase beta I-subunit from HeLa cells. The region encoding the putative protein transmembrane domain was shown to be bordered by two introns. The structure of a pseudogene (beta psi) was determined. This pseudogene is processed and contains multiple stop codons. Its homology to the beta I-subunit cDNA from HeLa cells is about 88%.

A role for the beta-subunit in the expression of functional Na+-K+-ATPase in Xenopus oocytes

In all cellular systems studied so far, the catalytic alpha- and the glycosylated beta-subunit of Na+-K+-ATPase are coordinately synthesized and are assembled into stoichiometric alpha, beta-complexes. In contrast to these data, in this study we show that the fully grown oocyte of Xenopus laevis synthesizes much less beta-subunit than alpha-subunit. The alpha-subunit produced in excess over the beta-subunit is membrane associated but highly trypsin sensitive and can be compared with the immature alpha-subunit population identified in epithelial cells immediately after synthesis (K. Geering, J. P. Kraehenbuhl, and B.C. Rossier, J. Cell Biol. 105: 2613-2619, 1987). The Xenopus oocyte thus turns out to be a unique system to study the functional role of the beta-subunit. Injection of beta-subunit-specific mRNA transcribed in vitro from a beta-cDNA clone (derived from Xenopus kidney, A6 cells) into oocytes results in translation of a glycosylated beta-subunit. The synthesis of this exogenous beta-subunit increases significantly the proportion of trypsin-resistant oocyte alpha-subunits able to perform cation-dependent conformational changes. In addition, 25-65% more ouabian binding sites are expressed at the plasma membrane in beta-mRNA-injected oocytes. In contrast, newly synthesized alpha-subunit translated after injection of size-fractionated mRNA enriched in alpha-mRNA remains trypsin sensitive as the oocyte alpha-subunit. These data suggest that association of the beta-subunit to the alpha-subunit provokes a structural rearrangement of the alpha-subunit that might be a first step toward the functional maturation of the Na+-K+-ATPase and its expression at the plasma membrane.

Polarized infrared absorption of Na+/K+-ATPase studied by attenuated total reflection spectroscopy

Na+/K+-ATPase can be isolated from the outer medulla of mammalian kidney in the form of flat membrane fragments containing the enzyme in a density of 10(3)-10(4) protein molecules per microm2 (Deguchi et al. (1977) J. Cell. Biol. 75, 619-634). In this paper we show that these membrane fragments can be bound to a germanium plate coated with a phospholipid bilayer. With this system infrared spectroscopic studies of the enzyme have been carried out using the technique of attenuated total reflection (ATR). At a coverage of the lipid surface corresponding to 30-40% of a monolayer of membrane fragments, characteristic infrared bands of the protein such as the amide I and II bands can be resolved. About 24% of the NH-groups of the peptide backbone are found to be resistant to proton/deuterium exchange within a time period of several days. Evidence for orientation of the protein with respect to the supporting lipid layer is obtained from experiments with polarized light, the largest polarization effects being associated with the -COO- band at 1400 cm-1. Experiments with aqueous media of different ionic composition indicate that the average orientation of transition moments changes when K+ in the medium is replaced by Tris+ or Na+.

The search for the endogenous digitalis: an alternative hypothesis

The universal presence of a binding site for cardiac glycosides on Na+-K+-ATPase has engendered speculation as to whether it also serves as a receptor for an endogenous digitalis-like hormone or autacoid. If such a hormone were to exist, it could play a role in sodium homeostasis and in the pathophysiology of primary hypertension and uremia. However, we believe that this hypothesis rests on unproven assumptions. Although typical of many toxins and drugs, binding to a single protein that acts as both its receptor and effector mechanism at the cell membrane, thereby directly affecting transmembrane ion flux, would be unusual for a hormone or autacoid. As an alternative hypothesis for the evolutionary conservation of the cardiac glycoside binding site, we suggest that its endogenous ligand may exist within the cell. After cotranslational insertion of the alpha- and beta-subunits into the membrane of the rough endoplasmic reticulum, Na+-K+-ATPase, like most integral membrane proteins, 1) must be targeted through a complex network of intracellular organelles to the correct plasmalemmal domain, 2) must be monitored for appropriate protein conformation and subunit assembly, and perhaps 3) could have its catalytic function regulated before insertion in the cell membrane. Because the lumina of the endoplasmic reticulum, Golgi, and other organelles and vesicles are topologically equivalent to the outside of the cell, all three functions could be subserved by an intraorganellar ligand for the cardiac glycoside binding site.

Isoforms of Na,K-ATPase in Artemia saline: I. Detection by FITC binding and time course

Partially purified Na,K-ATPase from whole nauplii at various stages of development, analyzed by SDS-PAGE, reveals a polydisperse beta and two alpha subunits (denoted alpha 1 and alpha 2). In the absence of Ca2+, ATP-inhibitable fluorescein isothiocyanate (FITC) labeling is restricted to the alpha subunit of this enzyme, even in crude naupliar homogenates. The intensity of the alpha-specific fluorescent signal (i.e., the sum of the yield from both alpha isoforms) is proportional to Na,K-ATPase activity during development. FITC-labeled subunits were detected at 8 hr of development prior to the detection of measurable Na,K-ATPase activity. The alpha 2/alpha 1 ratio changed from an initial value of 1.25 to a peak of 1.75 at 32 hr of development, then reverted to a ratio of 1.25 by 42 hr, and remained constant thereafter. Pulse chase studies with 35S-methionine indicated that the developmental increase in enzyme activity is coincident with amino acid incorporation into the alpha subunits, implying that enzyme synthesis is active during enzyme accumulation.

A monoclonal antibody against horse kidney (Na+ + K+)-ATPase inhibits sodium pump and E2K to E1 conversion of (Na+ + K+)-ATPase from outside of the cell membrane

Monoclonal antibodies against horse kidney outer medulla (Na+ + K+)-ATPase were prepared. One of these antibodies (M45-80), was identified as an IgM, recognized the alpha subunit of the enzyme. M45-80 had the following effects on horse kidney (Na+ + K+)-ATPase: (1) it inhibited the enzyme activity by 50% in 140 mM Na+ and by 80% in 8.3 mM Na+; (2) it increased the Na+ concentration necessary for half-maximal activation (K0.5 for Na+) from 12.0 to 57.6 mM, but did not affect K0.5 for K+; (3) it slightly increased the K+-dependent p-nitrophenylphosphatase (K-pNPPase) activity; (4) it inhibited phosphorylation of the enzyme with ATP by 30%, but did not affect the step of dephosphorylation; and (5) it enhanced the ouabain binding rate. These data are compatible with a stabilizing effect on the E2 form of (Na+ + K+)-ATPase. M45-80 was concluded to bind to the extracellular surface of the plasmamembrane, based on the following evidence: (1) M45-80 inhibited by 50% the ouabain-sensitive 86Rb+ uptake in human intact erythrocytes from outside of the cells; (2) the inhibition of (Na+ + K+)-ATPase activity in right-side-out vesicles of human erythrocytes was greater than that in inside-out vesicles; and (3) the fluorescence intensity due to FITC-labeled rabbit anti-mouse IgM that reacted with M45-80 bound to the right-side-out vesicles was much greater than that in the case of the inside-out vesicles.

Rates of synthesis and degradation of Na+-K+-ATPase during chronic ouabain treatment

Chronic inhibition of Na+-K+-ATPase activity in outer medullary kidney tubules has previously been demonstrated to elicit a 60% increase in activity, measured under maximal velocity (Vmax) conditions (J. Biol. Chem. 260: 12740-12743, 1985). To investigate the cellular mechanism of this response, we measured the rates of Na+-K+-ATPase synthesis and degradation over its full time course. A transient increase in the rate of synthesis occurred after 12 h of ouabain treatment. After 24-h treatment, the rate of synthesis returned to a level not different from control levels. The relative degradation rate after 24-h treatment, however, was markedly lower in ouabain-treated cells than in control cells. Thus the augmentation of the number of Na+-K+-ATPase sites, elicited by the transient increase in synthesis described, was maintained under steady-state conditions by a reduction in apparent degradation rate constant.

In vitro biosynthesis of the beta-subunit of the Na+/K+-ATPase in developing brine shrimp: glycosylation and membrane insertion

We demonstrate here translation, glycosylation, and membrane insertion of the beta-subunit of the Na+/K+-ATPase of the developing brine shrimp, Artemia, in a reticulocyte lysate translation system. The apparent molecular weight of the primary translation product as determined by SDS-PAGE is 33,000 +/- 1000 (n = 7). When microsomal membranes are present during the entire translation period, a new band with an apparent molecular weight of 37,000 +/- 1000 (n = 7) appears. This change in apparent molecular weight is due to the addition of about two N-linked oligosaccharides. The temporal relationship between protein synthesis and glycosylation have also been examined. Glycosylation and membrane insertion could be achieved if membranes were added after completion of about 70% of the peptide chain. However, glycosylation did not occur if membranes were added after the completion of translation of the beta-subunit. The beta-subunit was synthesized on membrane-bound polysomes, where about two N-linked oligosaccharides were added to the growing polypeptide chain. These studies demonstrate that in vitro translation systems will be useful for studying the biosynthesis of the beta-subunit of the brine shrimp, which is a good model system to examine the developmental regulation of the Na+/K+-ATPase.

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

The beta-subunit of dog kidney (Na+ + K+)-ATPase is a sialoglycoprotein and contains three potential N-glycosylation sites. In this study, the oligosaccharide chains of purified dog kidney beta-subunit were labeled with tritium by oxidation with sodium periodate or galactose oxidase followed by NaB3H4 reduction. The beta-subunit was extensively digested by trypsin and the radioactive peptides were purified by HPLC. The enzyme, glycopeptidase A, which catalyzes the removal of N-linked oligosaccharide chains and the conversion of the glycosylated Asn residue to Asp, was used to demonstrate that a number of purified beta-subunit tryptic peptides were glycosylated. Amino-acid analysis of these beta-subunit peptides following glycopeptidase-A treatment revealed the expected Asn to Asp conversion for Asn-157, Asn-192 and Asn-264, demonstrating that all three potential N-glycosylation sites of the dog kidney beta-subunit are glycosylated. In addition, amino-acid sequence data suggest that a disulfide bond exists between Cys-158 and Cys-174.

Expression of functional (Na+ + K+)-ATPase from cloned cDNAs

Functional (Na+ + K+)-ATPase is formed in Xenopus oocytes injected with alpha- and beta-subunit-specific mRNAs derived from cloned Torpedo californica cDNAs. Both the mRNAs are required for the expression of functional (Na+ + K+)-ATPase.

Molecular cloning and sequence analysis of the (Na+ + K+)-ATPase beta subunit from dog kidney

cDNA complementary to mRNA coding for the beta subunit of dog renal (Na+ + K+)-ATPase has been cloned into lambda gt11 and the nucleotide sequence of the DNA has been determined. The amino acid sequence of the beta subunit polypeptide has also been deduced from the DNA. The mature form of the dog kidney beta subunit contains 302 amino acids with three potential asparagine-linked attachment sites for carbohydrate. The initiation methionine is removed during processing of the polypeptide to its mature form. Although the beta subunit is an integral membrane protein there is no signal sequence for the polypeptide, and hydropathy analysis predicts that the beta subunit polypeptide spans the cell membrane only once. Secondary structure predictions and a model for the structure of the beta subunit are proposed. DNA sequencing of the 5' non-coding region of the mRNA revealed a 200 bp inverted repeat from the coding region. Blot hybridization of a fragment of the beta subunit cDNA identified a single mRNA species of 2.7 kb in dog kidney and several rat tissues. RNA from rat liver was deficient in mRNA that hybridized to the dog kidney beta subunit cDNA, although mRNA that hybridized to an alpha subunit cDNA was detected. RNA from a human hepatoma cell line, HepG2, however, contained comparable levels of mRNA for both the alpha and the beta subunits.

Pretranslational regulation of Na-K-ATPase in cultured canine kidney cells by low K+

Long-term upregulation of the sodium pump [Na-K-adenosine triphosphatase (Na-K-ATPase)] entails an increase in the number of enzyme molecules. We incubated Madin-Darby canine kidney (MDCK) cells in low K+ medium and studied the time course and magnitude of change in the relative abundance of the two Na-K-ATPase subunits (alpha and beta), in the synthesis rate of the subunits, and in the relative abundance of alpha- and beta-mRNA. When cells were incubated in medium containing 0.25 mM K+, intracellular Na+ increased from 25.2 +/- 0.9 (SE) mmol/l cell H2O to 69.8 +/- 9.6 at 4 h and 132 +/- 6 at 16 h. Cell K+ fell from 146 +/- 4 mmol/l cell H2O to 105 +/- 9 at 4 h and 42.3 +/- 4.7 at 16 h. The relative abundance of Na-K-ATPase subunits, measured with immunoblots of cell homogenates, increased such that after 24 h alpha was 1.71 +/- 0.33 and beta was 1.67 +/- 0.22 times control. After 8 h of K+ depletion, alpha-synthesis rate, measured by immunoprecipitation of pulse-labeled cells, increased to 2.30 +/- 0.50 and beta increased to 2.07 +/- 0.42 times control. The alpha- and beta-subunit mRNA abundance, measured by hybridizing alpha- and beta-cDNA probes to total RNA, increased within 30 min to 1.93 +/- 0.24 and 2.29 +/- 0.64 times control, respectively. We conclude that regulatory adjustments of Na-K-ATPase abundance involve an increase in translation after a rapid and coordinate increase in the concentrations of alpha- and beta-subunit mRNA.

Orientation of the beta subunit polypeptide of (Na+ + K+)ATPase in the cell membrane

Although the animal cell (Na+ + K+)-ATPase is composed of two polypeptide subunits, alpha and beta, very little is known about the beta subunit. In order to obtain information about the structure of this polypeptide, the beta subunit has been investigated using proteolytic fragmentation, chemical modification of carbohydrate residues, and immunoblot analysis. The sialic acid moieties on the oligosaccharide groups on the beta subunit of (Na+ + K+)-ATPase were labeled with NaB3H4 after oxidation by sodium periodate, or the penultimate galactose residues on the oligosaccharides were similarly labeled after removal of sialic acid with neuraminidase and oxidation by galactose oxidase. All of the carbohydrate residues of the protein are located on regions of the beta subunit that are found on the non-cytoplasmic surface of the membrane. Cleavage of the galactose oxidase-treated, NaB3H4-labeled beta subunit by chymotrypsin at an extracellular site produced labeled fragments of 40 and 18 kDa, indicating multiple glycosylation sites along the polypeptide. Neither the 40 kDa fragment nor the 18 kDa fragment was released from the membrane by chymotrypsin digestion alone, but after cleavage the 40 kDa fragment could be removed from the membrane by treatment with 0.1 M NaOH. This indicates that the 40 kDa fragment does not span the lipid bilayer. The 40 kDa fragment and the 18 kDa fragment are also linked by at least one disulfide bond. The 18 kDa fragment also contains all of the binding sites found on the (Na+ + K+)-ATPase for anti-beta subunit antibodies. Both the 40 kDa fragment and the 18 kDa fragment were also generated using papain or trypsin to cleave the beta subunit. These data indicate that the beta subunit of (Na+ + K+)-ATPase contains multiple sites of glycosylation, that it inserts into the cell membrane near only one end of the polypeptide, and that one region of the polypeptide is particularly sensitive to proteolytic cleavage relative to the rest of the polypeptide.