Cloning of the H,K-ATPase beta subunit. Tissue-specific expression, chromosomal assignment, and relationship to Na,K-ATPase beta subunits

The Physiology of the Gastric Parietal Cell

Parietal cells are responsible for gastric acid secretion, which aids in the digestion of food, absorption of minerals, and control of harmful bacteria. However, a fine balance of activators and inhibitors of parietal cell-mediated acid secretion is required to ensure proper digestion of food, while preventing damage to the gastric and duodenal mucosa. As a result, parietal cell secretion is highly regulated through numerous mechanisms including the vagus nerve, gastrin, histamine, ghrelin, somatostatin, glucagon-like peptide 1, and other agonists and antagonists. The tight regulation of parietal cells ensures the proper secretion of HCl. The H+-K+-ATPase enzyme expressed in parietal cells regulates the exchange of cytoplasmic H+ for extracellular K+. The H+ secreted into the gastric lumen by the H+-K+-ATPase combines with luminal Cl- to form gastric acid, HCl. Inhibition of the H+-K+-ATPase is the most efficacious method of preventing harmful gastric acid secretion. Proton pump inhibitors and potassium competitive acid blockers are widely used therapeutically to inhibit acid secretion. Stimulated delivery of the H+-K+-ATPase to the parietal cell apical surface requires the fusion of intracellular tubulovesicles with the overlying secretory canaliculus, a process that represents the most prominent example of apical membrane recycling. In addition to their unique ability to secrete gastric acid, parietal cells also play an important role in gastric mucosal homeostasis through the secretion of multiple growth factor molecules. The gastric parietal cell therefore plays multiple roles in gastric secretion and protection as well as coordination of physiological repair.

Gastric H+,K+-ATPase

The gastric H(+),K(+)-ATPase is responsible for gastric acid secretion. This ATPase is composed of two subunits, the catalytic α subunit and the structural β subunit. The α subunit with molecular mass of about 100 kDa has 10 transmembrane domains and is strongly associated with the β subunit with a single transmembrane segment and a peptide mass of 35 kDa. Its three-dimensional structure is based on homology modeling and site-directed mutagenesis resulting in a proton extrusion and K(+) reabsorption model. There are three conserved H3O(+)-binding sites in the middle of the membrane domain and H3O(+) secretion depends on a conformational change involving Lys(791) insertion into the second H3O(+) site enclosed by E795, E820, and D824 that allows export of protons at a concentration of 160 mM. K(+) countertransport involves binding to this site after the release of protons with retrograde displacement of Lys(791) and then K(+) transfer to E343 and exit to the cytoplasm. This ATPase is the major therapeutic target in treatment of acid-related diseases and there are several known luminal inhibitors allowing analysis of the luminal vestibule. One class contains the acid-activated covalent, thiophilic proton pump inhibitors, the most effective of current acid-suppressive drugs. Their binding sites and trypsinolysis allowed identification of all ten transmembrane segments of the ATPase. In addition, various K(+)-competitive inhibitors of the ATPase are being developed, with the advantage of complete and rapid inhibition of acid secretion independent of pump activity and allowing further refinement of the structure of the luminal vestibule of the E2 form of this ATPase.

Physiology and pathophysiology of potassium channels in gastrointestinal epithelia

Epithelial cells of the gastrointestinal tract are an important barrier between the "milieu interne" and the luminal content of the gut. They perform transport of nutrients, salts, and water, which is essential for the maintenance of body homeostasis. In these epithelia, a variety of K(+) channels are expressed, allowing adaptation to different needs. This review provides an overview of the current literature that has led to a better understanding of the multifaceted function of gastrointestinal K(+) channels, thereby shedding light on pathophysiological implications of impaired channel function. For instance, in gastric mucosa, K(+) channel function is a prerequisite for acid secretion of parietal cells. In epithelial cells of small intestine, K(+) channels provide the driving force for electrogenic transport processes across the plasma membrane, and they are involved in cell volume regulation. Fine tuning of salt and water transport and of K(+) homeostasis occurs in colonic epithelia cells, where K(+) channels are involved in secretory and reabsorptive processes. Furthermore, there is growing evidence for changes in epithelial K(+) channel expression during cell proliferation, differentiation, apoptosis, and, under pathological conditions, carcinogenesis. In the future, integrative approaches using functional and postgenomic/proteomic techniques will help us to gain comprehensive insights into the role of K(+) channels of the gastrointestinal tract.

No potassium, no acid: K+ channels and gastric acid secretion

The gastric H+-K+-ATPase pumps H+ into the lumen and takes up K+ in parallel. In the acid-producing parietal cells, luminal KCNE2/KCNQ1 K+ channels play a pivotal role in replenishing K+ in the luminal fluid. Inactivation of KCNE2/KCNQ1 channels abrogates gastric acid secretion and dramatically modifies the architecture of gastric mucosa.

Systematic expression profiling of the gastric H+/K+ ATPase in human tissue

OBJECTIVE

The potassium-competitive acid blockers (P-CABs), comprise a new, innovative group of competitive and reversible inhibitors of the gastric H+/K+ ATPase. Our aim was to identify sites of expression of the H+/K+ ATPase that are potential targets of these compounds by examining the expression profile of the gastric H+/K+ ATPase in the human body from a broad range of tissues.

MATERIAL AND METHODS

Expression profiling was done by quantitative mRNA analysis (TaqMan PCR). Tissues that were mRNA-positive for the alpha subunit were investigated further by Western blot and immunohistochemistry (IHC) for the presence of gastric H+/K+ ATPase protein.

RESULTS

In addition to the very high expression levels in the stomach, the adrenal gland, cerebellum and pancreas gave unexpectedly positive mRNA signals for the alpha subunit of gastric H +/K+ ATPase. However, they were negative for mRNA of the beta subunit, and Western blot and IHC were negative for alpha and beta subunit protein. Another group of tissues with low alpha subunit mRNA expression including the frontal cortex, cortex grey matter, testis, thymus and larynx submucosa were also found negative for both alpha and beta subunit protein. In contrast to mouse kidney, no gastric H+/K+ ATPase could be detected in human kidney.

CONCLUSIONS

We therefore conclude that the only organ in humans expressing significant levels of the P-CAB target gastric H+/K+ ATPase is the stomach.

Mutational study on the roles of disulfide bonds in the beta-subunit of gastric H+,K+-ATPase

The gastric proton pump, H(+),K(+)-ATPase, consists of the catalytic alpha-subunit and the non-catalytic beta-subunit. Correct assembly between the alpha- and beta-subunits is essential for the functional expression of H(+),K(+)-ATPase. The beta-subunit contains nine conserved cysteine residues; two are in the cytoplasmic domain, one in the transmembrane domain, and six in the ectodomain. The six cysteine residues in the ectodomain form three disulfide bonds. In this study, we replaced each of the cysteine residues of the beta-subunit with serine individually and in several combinations. The mutant beta-subunits were co-expressed with the alpha-subunit in human embryonic kidney 293 cells, and the role of each cysteine residue or disulfide bond in the alpha/beta assembly, stability, and cell surface delivery of the alpha- and beta-subunits and H(+),K(+)-ATPase activity was studied. Mutant beta-subunits with a replacement of the cytoplasmic and transmembrane cysteines preserved H(+),K(+)-ATPase activity. All the mutant beta-subunits with replacement(s) of the extracellular cysteines did not assemble with the alpha-subunit, resulting in loss of H(+),K(+)-ATPase activity. These mutants did not permit delivery of the alpha-subunit to the cell surface. Therefore, each of these disulfide bonds of the beta-subunit is essential for assembly with the alpha-subunit and expression of H(+),K(+)-ATPase activity as well as for cell surface delivery of the alpha-subunit.

Gastric H-K-ATPase and acid-resistant surface proteins

Despite the fact that mucus and bicarbonate are important macroscopic components of the gastric mucosal barrier, severe acidic and peptic conditions surely exist at the apical membrane of gastric glandular cells, and these membranes must have highly specialized adaptations to oppose external insults. Parietal cells abundantly express the heterodimeric, acid-pumping H-K-ATPase in their apical membranes. Its beta-subunit (HKbeta), a glycoprotein with >70% of its mass and all its oligosaccharides on the extracellular side, may play a protective role. Here, we show that the extracellular domain of HKbeta is highly resistant to trypsin in the native state (much more than that of the structurally related Na-K-ATPase beta-subunit) and requires denaturation to expose tryptic sites. Native HKbeta also resists other proteases, such as chymotrypsin and V8 protease, which hydrolyze at hydrophobic and anionic amino acids, respectively. Removal of terminal alpha-anomeric-linked galactose does not appreciably alter tryptic sensitivity of HKbeta. However, full deglycosylation makes HKbeta much more susceptible to all proteases tested, including pepsin at pH <2.0. We propose that 1) intrinsic folding of HKbeta, 2) bonding forces between subunits, and 3) oligosaccharides on HKbeta provide a luminal protein domain that resists gastric lytic conditions. Protein folding that protects susceptible charged amino acids and is maintained by disulfide bonding and hydrophilic oligosaccharides would provide a stable structure in the face of large pH changes. The H-K-ATPase is an obvious model, but other gastric luminally exposed proteins are likely to possess analogous protective specializations.

Development and characterization of conditionally immortalized gastric epithelial cell lines from transgenic rats harboring temperature-sensitive simian virus 40 large T-antigen gene

Conditionally immortalized gastric epithelial cell lines were established from transgenic rats harboring temperature-sensitive simian virus 40 (tsSV40) large T-antigen gene. Gastric mucosal cells and epithelial tissues isolated from the stomach of the transgenic rats were cultured at permissive temperature (33 degrees C), and proliferative cells were cloned by colony formation. Six cell lines (designated as RGE1-01, RGE1-02, RGE1-03, RGE1-21, RGE1-22 and RGE2-01) showing epithelial-like morphology have been established. All cells grew at 33 degrees C, but did not at nonpermissive temperature (39 degrees C). High expression level of large T-antigen in the nuclei was observed at 33 degrees C, whereas the expression level was gradually decreased in a time-dependent manner at 39 degrees C. These results suggest that the temperature-sensitive growth characteristics arise as a result of a function of the tsSV40 large T-antigen. None of the cell lines were transformed as judged by anchorage-independent growth assay. Immunocytochemical findings indicated that all cells expressed epithelial cell markers including cytoskeletal (cytokeratin and actin), basement membrane (laminin and collagen type IV) and junctional complex (ZO-1 and desmoplakin I+II) proteins at 33 degrees C. All cells expressed mRNA of cathepsin E, a pit cell marker. Moreover, transepithelial resistance was observed between apical and basolateral sides in the cells. RGE1-22 cells produced prostaglandin E(2). Levels of mRNA for cathepsin E, transepithelial resistance and prostaglandin E(2) were influenced by the nonpermissive temperature. Thus, these conditionally immortalized gastric cell lines which preserve some epithelial cell characteristics will provide a useful in vitro model of gastric epithelium.

The betam protein, a member of the X,K-ATPase beta-subunits family, is located intracellularly in pig skeletal muscle

The sequence of the pig cDNA encoding the muscle-specific betam-protein, a member of the X,K-ATPase beta-subunits family, was determined. Two alternatively spliced transcripts encoding polypeptide chains of 355 and 351 residues were identified. The tissue specificity of expression of betam and other X,K-ATPase beta-subunit genes was studied by RT-PCR performed on 24 tissues from newborn pigs. The betam expression was shown to be highly tissue-specific, being detected at the highest level in skeletal muscle, at a lower level in heart, and at much lower level in skin. The betam transcripts are more abundant in the tissues from the newborn than adult. Immunoblotting and deglycosylation shift assay indicated that skeletal muscle membranes of newborn pigs contain betam protein with an electrophoretic mobility and carbohydrate content very similar to that of human betam. Fractionation of membranes from both newborn and adult pig skeletal muscles by isopycnic centrifugation revealed that the majority of the betam protein is concentrated in the sarcoplasmic reticulum-containing fractions. This intracellular location is a unique property that distinguishes the betam protein from other members of the X,K-ATPase beta-subunit family.

Ion pumps in polarized cells: sorting and regulation of the Na+, K+- and H+, K+-ATPases

The physiologic function of an ion transport protein is determined, in part, by its subcellular localization and by the cellular mechanisms that modulate its activity. The Na(+),K(+)-ATPase and the H(+),K(+)-ATPases are closely related members of the P-type family of ion transporting ATPases. Despite their homology, these pumps are sorted to different domains in polarized epithelial cells, and their enzymatic activities are subject to distinct regulatory pathways. The molecular signals responsible for these properties have begun to be elucidated. It appears that a complex array of inter- and intramolecular interactions govern trafficking, distribution, and catalytic capacities of these proteins.

The cardiac K+ channel KCNQ1 is essential for gastric acid secretion

BACKGROUND & AIMS

Gastric H+ secretion via the H+/K+-adenosine triphosphatase is coupled to the uptake of K+. However, the molecular identity of luminal K+ channels enabling K+ recycling in parietal cells is unknown. This study was aimed to investigate these luminal K+ channels.

METHODS

Acid secretion was measured in vivo and in vitro; KCNQ1 protein localization was assessed by immunofluorescence, and acid-sensitivity of KCNQ1 by patch-clamp.

RESULTS

We identified KCNQ1, which is mutated in cardiac long QT syndrome, as a K+ channel located in tubulovesicles and apical membrane of parietal cells, where it colocalized with H+/K+-adenosine triphosphatase. Blockade of KCNQ1 current by 293B led to complete inhibition of acid secretion. The putative KCNQ1 subunits, KCNE2 and KCNE3, were abundant in human stomach; KCNE1, however, was absent. Coexpression of KCNE3/KCNQ1 in COS cells led to an acid-insensitive current; KCNE2/KCNQ1 was activated by low extracellular pH.

CONCLUSIONS

We identified KCNQ1 as the missing luminal K+ channel in parietal cells and characterized its crucial role in acid secretion. Because KCNE3 and KCNE2 are expressed in human stomach, one or both are candidates to coassemble with KCNQ1 in parietal cells. Thus, stomach- and subunit-specific inhibitors of KCNQ1 might offer new therapeutical perspectives for peptic ulcer disease.

Potassium transport in the mammalian collecting duct

The mammalian collecting duct plays a dominant role in regulating K(+) excretion by the nephron. The collecting duct exhibits axial and intrasegmental cell heterogeneity and is composed of at least two cell types: collecting duct cells (principal cells) and intercalated cells. Under normal circumstances, the collecting duct cell in the cortical collecting duct secretes K(+), whereas under K(+) depletion, the intercalated cell reabsorbs K(+). Assessment of the electrochemical driving forces and of membrane conductances for transcellular and paracellular electrolyte movement, the characterization of several ATPases, patch-clamp investigation, and cloning of the K(+) channel have provided important insights into the role of pumps and channels in those tubule cells that regulate K(+) secretion and reabsorption. This review summarizes K(+) transport properties in the mammalian collecting duct. Special emphasis is given to the mechanisms of how K(+) transport is regulated in the collecting duct.

Gastric proton pump is expressed in the inner ear and choroid plexus of the rat

Inner ear fluids and cerebrospinal fluid show remarkably stable ionic concentrations, particularly that of K(+) and H(+), but the mechanisms which control the homeostasis of these media are not well understood. We investigated a possible role of the gastric H, K-ATPase (gH,K-ATPase) pump in this control since this pump is known to be expressed in other tissues than gastric parietal cells. Here, we show by reverse transcription-polymerase chain reaction that the rat gH,K-ATPase alpha- and beta-subunits are expressed in the inner ear (lateral wall, organ of Corti and spiral ganglion cells), while only the alpha-subunit is expressed in the choroid plexus (CP). The presence of the alpha-subunit in the inner ear and CP was confirmed by immunoblotting. Immunohistochemistry localized this protein in the intermediate cells of the stria vascularis, in the spiral ligament and the spiral ganglion. gH,K-ATPase could be involved in the maintenance of H(+) and K(+) equilibria in cerebrospinal and labyrinthine fluids.

Interaction of alpha- and beta-subunits in native H-K-ATPase and cultured cells transfected with H-K-ATPase beta-subunit

The assembly of the beta-subunit of the gastric H-K-ATPase (HKbeta) with the alpha-subunit of the H-K-ATPase or the Na-K-ATPase (NaKalpha) was characterized with two anti-HKbeta monoclonal antibodies (MAbs). In fixed gastric oxyntic cells, in H-K-ATPase in vitro, and in Madin-Darby canine kidney (MDCK) cells transfected with HKbeta, MAb 2/2E6 was observed to bind to HKbeta only when interactions between alpha- and beta-subunits were disrupted by various denaturants. The epitope for MAb 2/2E6 was mapped to the tetrapeptide S(226)LHY(229) of the extracellular domain of HKbeta. The epitope for MAb 2G11 was mapped to the eight NH(2)-terminal amino acids of the cytoplasmic domain of HKbeta. In transfected MDCK cells, MAb 2G11 could immunoprecipitate HKbeta with alpha-subunits of the endogenous cell surface NaKalpha, as well as that from early in the biosynthetic pathway, whereas MAb 2/2E6 immunoprecipitated only a cohort of unassembled endoglycosidase H-sensitive HKbeta. In HKbeta-transfected LLC-PK(1) cells, significant immunofluorescent labeling of HKbeta at the cell surface could be detected without postfixation denaturation or in live cells, although a fraction of transfected HKbeta could also be coimmunoprecipitated with NaKalpha. Thus assembly of HKbeta with NaKalpha does not appear to be a stringent requirement for cell surface delivery of HKbeta in LLC-PK(1) cells but may be required in MDCK cells. In addition, endogenous posttranslational regulatory mechanisms to prevent hybrid alpha-beta heterodimer assembly appear to be compromised in transfected cultured renal epithelial cells. Finally, the extracellular epitope for assembly-sensitive MAb 2/2E6 may represent a region of HKbeta that is associated with alpha-beta interaction.

The roles of carbohydrate chains of the beta-subunit on the functional expression of gastric H(+),K(+)-ATPase

Gastric H(+),K(+)-ATPase consists of alpha and beta-subunits. The alpha-subunit is the catalytic subunit, and the beta-subunit is a glycoprotein stabilizing the alpha/beta complex in the membrane as a functional enzyme. There are seven putative N-glycosylation sites on the beta-subunit. In this study, we examined the roles of the carbohydrate chains of the beta-subunit by expressing the alpha-subunit together with the beta-subunit in which one, several, or all of the asparagine residues in the N-glycosylation sites were replaced by glutamine. Removing any one of seven carbohydrate chains from the beta-subunit retained the H(+),K(+)-ATPase activity. The effects of a series of progressive removals of carbohydrate chains on the H(+),K(+)-ATPase activity were cumulative, and removal of all carbohydrate chains resulted in the complete loss of H(+), K(+)-ATPase activity. Removal of any single carbohydrate chain did not affect the alpha/beta assembly; however, little alpha/beta assembly was observed after removal of all the carbohydrate chains from the beta-subunit. In contrast, removal of three carbohydrate chains inhibited the surface delivery of the beta-subunit and the alpha-subunit assembled with the beta-subunit, indicating that the surface delivery mechanism is more dependent on the carbohydrate chains than the expression of the H(+),K(+)-ATPase activity and alpha/beta assembly.

The N-linked oligosaccharides of the beta-subunit of rabbit gastric H,K-ATPase: site localization and identification of novel structures

The gastric H,K-ATPase is responsible for acid secretion by parietal cells. Its beta-subunit is a glycoprotein which is exposed to the harsh, acidic environment of the stomach. The location and structural features of the N-linked oligosaccharides were determined using matrix-assisted laser desorption ionization mass spectrometry (MALDI/MS) (in conjunction with mass composition analysis and exoglycosidase digestions), Edman degradation, and monosaccharide composition analysis. All seven N-linked sequons at positions 99, 103, 130, 146, 161, 193, and 222 were fully glycosylated. An unusual restricted array of oligosaccharides was observed at individual Asn residues. Asn99 was modified exclusively with oligomannosidic-type structures (Man6GlcNAc2-Man8GlcNAc2). Asn193 contained both oligomannosidic (Man5GlcNAc2-Man8GlcNAc2) and lactosamine-type structures, indicating significant "leakiness" in the pathway which converts oligomannose to lactosamine-type at a single glycosylation site. MALDI/MS with collision-induced dissociation was required to demonstrate that sequons separated by a single residue (99Asn-Ile-Ser-Asp-Asn-Arg-Thr105) were modified with only oligomannose and lactosamine structures, respectively. Analysis of the total oligosaccharide pool using MALDI/MS and exoglycosidase analysis revealed 24 lactosamine species (bi-, tri-, and tetraantennary structures), with all branches terminated in alpha-linked Gal residues, most possessing a single Fuc residue. Nine novel oligosaccharides contained multiple alpha-linked Gal residues per branch. Bi- and triantennary structures, with and without lactosamine repeats, were observed at Asn146 and Asn161. Tetraantennary structures with lactosamine repeats were found only at Asn130, and this site also contained most of the structures with multiple alpha-linked Gal residues per branch.

Cloning of rat Muc5AC mucin gene: comparison of its structure and tissue distribution to that of human and mouse homologues

The human mucin gene MUC5AC codes for a large mucin which has tandem repeat units and cysteine rich regions characteristic of several members of this class of glycoproteins. Human epithelia expressing the mucin include that of stomach, bronchus/trachea, endocervix and conjunctiva. We report here a 3.8 kb partial sequence of a rat homologue for the human MUC5AC gene and compare its tandem repeat sequence and cysteine rich domains to those of the human and mouse gene. Rat and mouse have the same number of amino acids (16) in their Muc5AC tandem repeat units and share 69% sequence similarity, whereas human MUC5AC has only 8 amino acids in its tandem repeat. In rat, the tandem repeat domain is flanked at its 3' end by a non-repeat region coding for 1142 amino acids. Four cysteine rich subdomains were identified in this region; one of these has 64% similarity to a corresponding region in human MUC5AC and 80% similarity to a mouse MUC5AC cysteine rich region. Southern blot analysis revealed cross hybridization of a probe for the rat cysteine rich region, to human, mouse, rabbit, and porcine genomic DNA; the rat tandem repeat probe hybridized with mouse and rabbit only. Unlike humans, rat expressed MUC5AC message detectable by Northern blot and in situ hybridization only in stomach epithelium and conjunctival goblet cells.

Identification of the mammalian Na,K-ATPase 3 subunit

We have isolated and characterized cDNA clones encoding the human and rat Na,K-ATPase beta3 subunit isoform. The human cDNA encodes a polypeptide of 279 amino acids that exhibits primary sequence and secondary structure similarities to Na,K-ATPase beta subunit isoforms. Sequence comparisons showed that the human beta3 subunit closely resembles the beta3 subunit of Xenopus laevis (59% amino acid identity) and is less similar to the human Na,K-ATPase beta1 and beta2 subunits (38% and 48% amino acid identity, respectively). By analyzing the segregation of restriction fragment length polymorphisms among recombinant inbred strains of mice, we localized the beta3 subunit gene to murine chromosome 7. Northern blot analysis revealed that the beta3 subunit gene encodes two transcripts that are expressed in a variety of rat tissues including testis, brain, kidney, lung, stomach, small intestine, colon, spleen, and liver. Identification of the mammalian beta3 subunit suggests an even greater potential for Na,K-ATPase isoenzyme diversity than previously realized.

The Beta-subunit of the rabbit H,K-ATPase:a glycoprotein with all terminal lactosamine units capped with alpha-linked galactose residues

The beta-subunit of the gastric H,K-ATPase is the most abundant glycoprotein in the tubulovesicular compartment of the acid-secreting parietal cells. The oligosaccharides of the beta-subunit have been shown to contain fucose, N-acetylglucosamine, mannose, galactose, and N-acetylgalactosamine. Previous studies have shown that the rabbit beta-subunit is devoid of N-acetylneuraminic acid. Here we report the structural features of the N-linked oligosaccharides of the beta-subunit from rabbit H,K-ATPase. We used glycosidase digestions and analysis by high-pH anion-exchange chromatography with pulsed amperometric detection and matrix-assisted laser desorption/ionization mass spectrometry to analyze the peptide-N4-(N-acetyl-beta-D-glucosaminyl)asparagine amidase (PNGase F)- and endo-beta-N-acetylglucosaminidase H (Endo H)-released oligosaccharides. The studies showed that the oligosaccharides of the beta-subunit are a mixture of both oligomannosidic and lactosamine-type structures. The high-mannose structures were identified as Man5Man8GlcNAc2 species. A striking finding was that all the branches of the lactosamine-type structures were terminated with Galalpha-->Galbeta-->GlcNAc extensions. All of the lactosamine-type structures were found to be core fucosylated and some of them contained one to three lactosamine repeats. We propose that a part of the adaptation of the gastric beta-subunit to the acidic environment of the stomach is through providing acid-stable terminal residues on the oligosaccharides.

Functional expression of gastric H+,K(+)-ATPase and site-directed mutagenesis of the putative cation binding site and catalytic center

Gastric H+,K(+)-ATPase was functionally expressed in the human kidney HEK293 cell line. The expressed enzyme catalyzed ouabain-resistant K(+)-dependent ATP hydrolysis. The K(+)-ATPase activity was inhibited by SCH 28090, a specific inhibitor of gastric proton pump, in a dose-dependent manner. By using this functional expression system in combination with site-directed mutagenesis, we investigated effects of mutations in the putative cation binding site and the catalytic center of the gastric H+,K(+)-ATPase. In Na+,K(+)-ATPase, the glutamic acid residue in the 4th transmembrane segment is regarded as one of the residues responsible for the K(+)-induced conformational change (Kuntzweiler, T. A., Wallick, E. T., Johnson, C. L., and Lingrel, J. B. (1995) J. Biol. Chem. 270, 2993-3000). When the corresponding glutamic acid (Glu-345) of H+,K(+)-ATPase was mutated to aspartic acid, lysine, or valine, the SCH 28080-sensitive K(+)-ATPase activity was abolished. However, when this residue was replaced by glutamine, about 50% of the activity was retained. This mutant showed a 10-fold lower affinity for K+ (Km = 2.6 mM) compared with the wild-type enzyme (Km = 0.24 mm). Thus, Glu-345 is important in determining the K+ affinity of H+,K(+)-ATPase. When the aspartic acid residue in the phosphorylation site was mutated to glutamic acid, this mutant showed no SCH 28080-sensitive K(+)-ATPase activity. Thus, amino acid replacement of the phosphorylation site is not tolerated and a stringent structure appears to be required for enzyme activity. When the lysine residue in the fluorescein isothiocyanate binding site (part of ATP binding site) was mutated to arginine, asparagine, or glutamic acid, the SCH 28080-sensitive K(+)-ATPase activity was eliminated. However, the mutant in which this residue was changed to glutamine had about 30% of the activity, suggesting that amino acid replacement of this site is tolerated to a certain extent.

Primary sequence, tissue specificity and mRNA expression of the Na(+),K (+) -ATPase β1 subunit in the European eel (Anguilla anguilla)

The entire amino acid coding sequence of the Na(+),K(+)-ATPase β1 isoform was cloned from the gill of the European eel (Anguilla anguilla) by a PCR based method. The amino acid sequence translated from the nucleotide sequence shared 61.4 and 56.2% homology respectively with previously published Na(+),K(+)-ATPase β1 isoform sequences from the clawed toad (Xenopus laevis) and the ray (Torpedo californica) an elasmobranch fish. The size of the Na(+),K(+)-ATPase β1 mRNA transcript in eel tissues was demonstrated to be 2.35 Kb. Detectable levels of Na(+),K(+)-ATPase β1 mRNA were found at some level in all tissues except liver and cardiac muscle. The level of branchial Na(+),K(+)-ATPase β1 mRNA was observed to increase after the adaptation of fresh water eels to normal or double concentration sea water.

The influence of beta subunit structure on the interaction of Na+/K(+)-ATPase complexes with Na+. A chimeric beta subunit reduces the Na+ dependence of phosphoenzyme formation from ATP

High-affinity ouabain binding to Na+/K(+)-ATPase (sodium- and potassium-transport adenosine triphosphatase (EC 3.6.1.37)) requires phosphorylation of the alpha subunit of the enzyme either by ATP or by inorganic phosphate. For the native enzyme (alpha/beta 1), the ATP-dependent reaction proceeds about 4-fold more slowly in the absence of Na+ than when saturating concentrations of Na+ are present. Hybrid pumps were formed from either the alpha 1 or the alpha 3 subunit isoforms of Na+/K(+)-ATPase and a chimeric beta subunit containing the transmembrane segment of the Na+/K(+)-ATPase beta 1 isoform and the external domain of the gastric H+/K(+)-ATPase beta subunit (alpha/NH beta 1 complexes). In the absence of Na+, these complexes show a rate of ATP-dependent ouabain binding from approximately 75-100% of the rate seen in the presence of Na+ depending on buffer conditions. Nonhydrolyzable nucleotides or treatment of ATP with apyrase abolishes ouabain binding, demonstrating that ouabain binding to alpha/NH beta 1 complexes requires phosphorylation of the protein. Buffer ions inhibit ouabain binding by alpha/NH beta 1 in the absence of Na+ rather than promote ouabain binding, indicating that they are not substituting for sodium ions in the phosphorylation reaction. The pH dependence of ATP-dependent ouabain binding in the presence or absence of Na+ is similar, suggesting that protons are probably not substituting for Na+. Hybrid alpha/NH beta 1 pumps also show slightly higher apparent affinities (2-3-fold) for ATP, Na+, and ouabain; however, these are not sufficient to account for the increase in ouabain binding in the absence of Na+. In contrast to phosphoenzyme formation and ouabain binding by alpha/NH beta 1 complexes in the absence of Na+, ATPase activity, measured as release of phosphate from ATP, requires Na+. These data suggest that the transition from E1P to E2P during the catalytic cycle does not occur when the sodium binding sites are not occupied. Thus, the chimeric beta subunit reduces or eliminates the role of Na+ in phosphoenzyme formation from ATP, but Na+ binding or release by the enzyme is still required for ATP hydrolysis and release of phosphate.

Structural interactions between alpha- and beta-subunits of the gastric H,K-ATPase

Structural and functional interactions between alpha- and beta-subunits of the H,K-ATPase were explored. The sensitivity to trypsinolysis of alpha-subunit was monitored by SDS-PAGE in control H,K-ATPase-enriched microsomes and in microsomes in which disulfide bonds of the beta-subunit were reduced using 2-mercaptoethanol (2-ME). Reduction of beta-subunit disulfide bonds increased the susceptibility of the alpha-subunit to tryptic digestion. Kinetics of trypsinolysis were also carried out in the presence of ligands known to bind with H,K-ATPase and favor a particular conformer state in the native enzyme. The time-course for release of tryptic peptides was monitored in protein stained gels and Western blots probed with monoclonal antibody alpha-H,K,12.18. In control preparations, where beta-subunit disulfides remained intact, trypsinolysis in the presence of ATP or K+ produced distinctive patterns of tryptic fragments, each characteristic of the conformational states induced by the respective ligand. For 2-ME-treated microsomes the altered alpha-subunit was unable to undergo ligand-induced conformational changes. The increased susceptibility of the alpha-subunit to trypsinization, the change in accessibility of tryptic cleavage sites and the inability of the alpha-subunit to undergo ligand-induced conformational changes after reduction of the beta-subunit disulfides suggest that the interactions between alpha- and beta-subunits are important for the conformational stability of the functional holoenzyme. A model localizing the most susceptible tryptic cleavage sites in control and 2-ME-reduced states is presented.

Chromosomal assignment of 11 loci in the rat by mouse-rat somatic hybrids and linkage

Eleven rat genes have been assigned to rat chromosomes by use of mouse x rat somatic hybrids and/or use of linkage to known chromosome markers. Among them, the genes for the inducible nitric oxide synthase (Nos2) and for a vasoactive intestinal peptide receptor (Vipr) are potential candidates for genetic regulation of blood pressure and were localized to rat Chromosomes (Chrs) 10 and 8 respectively. Genes for gastric H,K-ATPase alpha subunit (Atp4a), Class I alcohol dehydrogenase (Adh), and aldolase C (Aldoc) were localized to Chrs 1, 2, and 10 respectively, and thus provide more DNA markers for genetic mapping of quantitative trait loci for blood pressure on those chromosomes. Genes for alkaline phosphatase (Alp1) and cardiac AE-3 Cl-/HCO3- exchanger (Ae3) were both localized to Chr 9. Genes for glutamate dehydrogenase (Glud) and gastric H,K-ATPase beta subunit (Atp4b) were localized to Chr 16. The ornithine decarboxylase (Odc) gene and ornithine decarboxylase pseudogene (Odcp) were localized to Chrs 6 and 11 respectively.

In situ hybridization of mRNA for the gastric H+,K(+)-ATPase in rat oxyntic mucosa

The H+,K(+)-ATPase member of the phosphorylating ion motive ATPases is composed of two subunits, a large alpha-subunit composed of about 1030 amino acids and a smaller beta-subunit consisting of about 290 amino acids. By biochemical and immunological methods both subunits have been found in high abundance in the gastric parietal cell. In the present study in situ hybridization was used for localizing and comparing concentrations of the mRNA for the two subunits in the gastric epithelium. For this purpose 3H-labelled probes were preferred. Hybridization was detected only in the parietal cells. The older parietal cells in the bottom of the mucosa gave a weaker hybridization signal than the younger parietal cells closer to the surface. The margin of experimental ulcers, where the parietal cells are of low differentiation, showed very weak, if any, hybridization. The differences observed in hybridization densities may reflect differences in mRNA synthesis or stability. It is conceivable that older parietal cells, as well as parietal cells of low differentiation, produce relatively small amounts of H+,K(+)-ATPase.

Genes encoding the H,K-ATPase alpha and Na,K-ATPase alpha 3 subunits are linked on mouse chromosome 7 and human chromosome 19

We have used linkage analysis and fluorescence in situ hybridization to determine the chromosomal organization and location of the mouse (Atp4a) and human (ATP4A) genes encoding the H,K-ATPase alpha subunit. Linkage analysis in recombinant inbred (BXD) strains of mice localized Atp4a to mouse Chromosome (Chr) 7. Segregation of restriction fragment length polymorphisms in backcross progeny of Mus musculus x Mus spretus mating confirmed this assignment and indicates that Atp4a and Atp1a3 (gene encoding the murine Na,K-ATPase alpha 3 subunit) are linked and separated by a distance of approximately 2 cM. Analysis of the segregation of simple sequence repeats suggested the gene order centromere-D7Mit21-D7Mit57/Atp1a3-D7Mit72/Atp 4a. A human Chr 19-enriched cosmid library was screened with both H,K-ATPase alpha and Na,K-ATPase alpha 3 subunit cDNA probes to isolate the corresponding human genes (ATP4A and ATP1A3, respectively). Fluorescence in situ hybridization with gene-specific cosmid clones localized ATP4A to the q13.1 region, and proximal to ATP1A3, which maps to the q13.2 region, of Chr 19. These results indicate that ATP4A and ATP1A3 are linked in both the mouse and human genomes.

Gastric H(+)-K(+)-ATPase activity is inhibited by reduction of disulfide bonds in beta-subunit

H(+)-K(+)-ATPase activity of rabbit isolated gastric microsomes was irreversibly inactivated by reducing agents, such as 2-mercaptoethanol and dithiothreitol. Similar to what has been observed for Na(+)-K(+)-ATPase, high concentrations of reagents, at moderately elevated temperatures, were required to inactivate H(+)-K(+)-ATPase, suggesting relative inaccessibility of the responsible disulfide bonds. Resistance against inactivation was conferred by monovalent cation activators of K(+)-stimulated ATPase and p-nitro-phenylphosphatase. The effectiveness of K+ congeners in protecting the enzyme was similar in sequence (Tl+ greater than K+ greater than Rb+) and concentration to their respective affinities for stimulating enzymatic activity, suggesting that the K(+)-bound form of the enzyme is more resistant to reduction than the free enzyme. Furthermore, Na+ antagonized the protective effect of K+. Labeling studies using fluorescein-maleimide indicated that 60-70% of the cysteine residues in the beta-subunit are in the oxidized form. Coupled with primary sequence data, this suggests that three disulfide bonds are present in the native beta-subunit. In contrast, less than 10% of the cysteine residues in the alpha-subunit are in the oxidized form. Kinetic studies showed that the 2-mercaptoethanol-induced loss of H(+)-K(+)-ATPase activity was correlated with a reduction of disulfide groups in the beta-subunit, while there was no significant change in the alpha-subunit. We conclude that reduction of disulfide bonds irreversibly inhibits H(+)-K(+)-ATPase activity, binding of K+ to the enzyme confers a resistance to disulfide bond reduction, and the responsible disulfide bonds are present in the beta-subunit.(ABSTRACT TRUNCATED AT 250 WORDS)

Two-dimensional crystals of membrane-bound gastric H,K-ATPase

Two-dimensional crystallization of membrane-bound H,K-ATPase (EC 3.6.1.36) in vesicle preparations from parietal cells of hog gastric mucosa was induced by an imidazole buffer containing Mg2+ and VO3- ions. A continuous reorganization of the protein molecules started within a few hours by the formation of linear arrays. At later stages confluent two-dimensional crystals were formed. Electron microscopy and image processing showed that these were of a single tetragonal type. The asymmetric unit consisted of one pear-shaped protein domain corresponding to a H,K-ATPase protomer. Through stain-deficient contact regions four adjacent protein units were connected forming a tetrameric structure.

Sequence motif in control regions of the H+/K+ ATPase alpha and beta subunit genes recognized by gastric specific nuclear protein(s)

A nuclear protein(s) from rat or pig stomach recognized a conserved sequence in the 5'-upstream regions of the rat and human H+/K(+)-ATPase alpha subunit genes. A gel retardation assay suggested that part of the binding site was located in the TAATCAGCTG sequence. No nuclear proteins capable of the binding could be detected in other tissues of rat (liver, brain, kidney, spleen and lung) or pig liver. The sequence motif (GATAGC) located 5'-upstream of the beta-subunit gene also seemed to be recognized by the same protein, because the binding of nuclear protein to the sequence motifs in the alpha and beta subunits was mutually competitive. Considering the sense-strand sequence of the binding motif in the alpha-subunit gene, we conclude that (G/C)PuPu(G/C)NGAT(A/T)PuPy is a core sequence motif for the gastric specific DNA binding protein (PCSF, parietal cell specific factor).

Assembly of a hybrid from the alpha subunit of Na+/K(+)-ATPase and the beta subunit of H+/K(+)-ATPase

Messenger RNA for the alpha subunit of Torpedo californica Na+/K(+)-ATPase was injected into Xenopus oocytes together with that of the beta subunit of rabbit H+/K(+)-ATPase. The Na+/K(+)-ATPase alpha subunit was assembled in the microsomal membranes with the H+/K(+)-ATPase beta subunit, and became resistant to trypsin. These results suggest that the H+/K(+)-ATPase beta subunit facilitates the stable assembly of the Na+/K(+)-ATPase alpha subunit in microsomes.

Rat gastric H,K-ATPase beta-subunit gene: intron/exon organization, identification of multiple transcription initiation sites, and analysis of the 5'-flanking region

A rat genomic library was screened using a gastric H,K-ATPase beta-subunit cDNA probe, and two clones were identified. Restriction endonuclease mapping and Southern hybridization analyses indicated that each of these clones contains the entire H,K-ATPase beta-subunit gene. The nucleotide sequence was determined for the 8.75-kb transcription unit and 2.2 kb of the 5'-flanking region. The gene consists of seven exons and shows a high degree of similarity to the Na,K-ATPase beta 1-subunit gene. Primer extension and S1 nuclease protection analyses identified a major transcription initiation site 23 bases upstream of the translation start site and several minor transcription initiation sites located further upstream. The 5'-flanking region of the gene has two potential TATA sequences, each located 25-30 bases upstream of a transcription initiation site, and a number of potential promoter and regulatory elements. In addition, the 5'-flanking region contains nucleotide sequences that may regulate transcription through the formation of unusual DNA structures. These include a sequence that may form a triple helix and an adjacent sequence with the potential to form Z-DNA.

Evolution of the Na,K- and H,K-ATPase beta subunit gene family: structure of the murine Na,K-ATPase beta 2 subunit gene

We have cloned and characterized the mouse Na,K-ATPase beta 2 subunit gene (Atp1b2). The gene spans approximately 7 kb and is split into seven exons. The transcription initiation site has been mapped and consensus TATA and putative CAAT sequences have been found at positions -23 and -137, respectively. Discrete structural domains of the beta 2 subunit protein are encoded by separate exons: The intracellular amino-terminal and putative transmembrane domains are encoded by individual exons and the extracellular carboxyl-terminal domain is encoded by five exons. The exon/intron organization of the beta 2 subunit gene closely resembles that of the H,K-ATPase beta subunit gene, suggesting that these two genes evolved from a common evolutionary ancestor. Comparison of the promoter region of the mouse and rat beta 2 subunit genes reveals a remarkably high degree of homology within a 788-nucleotide segment immediately upstream of the transcription start site. This observation suggests that elements that serve to regulate the cell-specific expression of the beta 2 subunit gene are likely to be located within this conserved region.

Mammalian phosphorylating ion-motive ATPases

The pumps discussed in this review are three members of the phosphorylating class of ion transport ATPases. They are the Na(+)-K(+)-, Ca(2+)- and H(+)-K(+)-ATPases. Recent work on their topology, possible transport mechanisms, ion-binding sites and role of the different subunits found for the Na(+)-K(+)- and H(+)-K(+)-ATPases is presented, with a suggestion of a unifying 10-membrane segment model for the catalytic subunit of this class of enzyme.

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.