cDNA cloning and sequence determination of pig gastric (H+ + K+)-ATPase

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.

Evolutionary Analysis of the Lysine-Rich N-terminal Cytoplasmic Domains of the Gastric H+,K+-ATPase and the Na+,K+-ATPase

The catalytic α-subunits of both the Na⁺,K⁺-ATPase and the gastric H⁺,K⁺-ATPase possess lysine-rich N-termini which project into the cytoplasm. Due to conflicting experimental results, it is currently unclear whether the N-termini play a role in ion pump function or regulation, and, if they do, by what mechanism. Comparison of the lysine frequencies of the N-termini of both proteins with those of all of their extramembrane domains showed that the N-terminal lysine frequencies are far higher than one would expect simply from exposure to the aqueous solvent. The lysine frequency was found to vary significantly between different vertebrate classes, but this is due predominantly to a change in N-terminal length. As evidenced by a comparison between fish and mammals, an evolutionary trend towards an increase of the length of the N-terminus of the H⁺,K⁺-ATPase on going from an ancestral fish to mammals could be identified. This evolutionary trend supports the hypothesis that the N-terminus is important in ion pump function or regulation. In placental mammals, one of the lysines is replaced by serine (Ser-27), which is a target for protein kinase C. In most other animal species, a lysine occupies this position and hence no protein kinase C target is present. Interaction with protein kinase C is thus not the primary role of the lysine-rich N-terminus. The disordered structure of the N-terminus may, via increased flexibility, facilitate interaction with another binding partner, e.g. the surrounding membrane, or help to stabilise particular enzyme conformations via the increased entropy it produces.

Potassium-competitive acid blockers: Advanced therapeutic option for acid-related diseases

Acid-related diseases (ARDs), such as peptic ulcers and gastroesophageal reflux disease, represent a major health-care concern. Some major milestones in our understanding of gastric acid secretion and ARD treatment reached during the last 50years include 1) discovery of histamine H₂-receptors and development of H₂-receptor antagonists, 2) identification of H⁺,K⁺-ATPase as the parietal cell proton pump and development of proton pump inhibitors (PPIs), and 3) identification of Helicobacter pylori (H. pylori) as the major cause of peptic ulcers and development of effective eradication regimens. Although PPI treatments have been effective and successful, there are limitations to their efficacy and usage, i.e. short half-life, insufficient acid suppression, slow onset of action, and large variation in efficacy among patients due to CYP2C19 metabolism. Potassium-competitive acid blockers (P-CABs) inhibit H⁺,K⁺-ATPase in a reversible and K⁺-competitive manner, and exhibit almost complete inhibition of gastric acid secretion from the first dose. Many pharmaceutical companies have tried to develop P-CABs, but most of their clinical development has been discontinued due to safety concerns or a similar efficacy to PPIs. Revaprazan was developed in Korea and was the first P-CAB approved for sale. Vonoprazan, approved in 2014 in Japan, has a completely different chemical structure and higher pKa value compared to other P-CABs, and exhibits rapid onset of action and prolonged control of intragastric acidity. Vonoprazan is an effective treatment for ARDs that is especially effective in healing reflux esophagitis and for H. pylori eradication. P-CABs, such as vonoprazan, promise to further improve the management of ARDs.

Protonated form: the potent form of potassium-competitive acid blockers

Potassium-competitive acid blockers (P-CABs) are highly safe and active drugs targeting H+,K+-ATPase to cure acid-related gastric diseases. In this study, we for the first time investigate the interaction mechanism between the protonated form of P-CABs and human H+,K+-ATPase using homology modeling, molecular docking, molecular dynamics and binding free energy calculation methods. The results explain why P-CABs have higher activities with higher pKa values or at lower pH. With positive charge, the protonated forms of P-CABs have more competitive advantage to block potassium ion into luminal channel and to bind with H+,K+-ATPase via electrostatic interactions. The binding affinity of the protonated form is more favorable than that of the neutral P-CABs. In particular, Asp139 should be a very important binding site for the protonated form of P-CABs through hydrogen bonds and electrostatic interactions. These findings could promote the rational design of novel P-CABs.

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.

Pharmacology of proton pump inhibitors

The gastric H,K-ATPase is the primary target for the treatment of acid-related diseases. Proton pump inhibitors (PPIs) are weak bases composed of two moieties, a substituted pyridine with a primary pK(a) of about 4.0, which allows selective accumulation in the secretory canaliculus of the parietal cell, and a benzimidazole with a second pK(a) of about 1.0. PPIs are acid-activated prodrugs that convert to sulfenic acids or sulfenamides that react covalently with one or more cysteines accessible from the luminal surface of the ATPase. Because of covalent binding, their inhibitory effects last much longer than their plasma half-life. However, the short half-life of the drug in the blood and the requirement for acid activation impair their efficacy in acid suppression, particularly at night. PPIs with longer half-life promise to improve acid suppression. All PPIs give excellent healing of peptic ulcers and produce good results in reflux esophagitis. PPIs combined with antibiotics eradicate Helicobacter pylori.

The gastric HK-ATPase: structure, function, and inhibition

The gastric H,K-ATPase, a member of the P(2)-type ATPase family, is the integral membrane protein responsible for gastric acid secretion. It is an alpha,beta-heterodimeric enzyme that exchanges cytoplasmic hydronium with extracellular potassium. The catalytic alpha subunit has ten transmembrane segments with a cluster of intramembranal carboxylic amino acids located in the middle of the transmembrane segments TM4, TM5,TM6, and TM8. Comparison to the known structure of the SERCA pump, mutagenesis, and molecular modeling has identified these as constituents of the ion binding domain. The beta subunit has one transmembrane segment with N terminus in cytoplasmic region. The extracellular domain of the beta subunit contains six or seven N-linked glycosylation sites. N-glycosylation is important for the enzyme assembly, maturation, and sorting. The enzyme pumps acid by a series of conformational changes from an E(1) (ion site in) to an E(2) (ion site out) configuration following binding of MgATP and phosphorylation. Several experimental observations support the hypothesis that expulsion of the proton at 160 mM (pH 0.8) results from movement of lysine 791 into the ion binding site in the E(2)P configuration. Potassium access from the lumen depends on activation of a K and Cl conductance via a KCNQ1/KCNE2 complex and Clic6. K movement through the luminal channel in E(2)P is proposed to displace the lysine along with dephosphorylation to return the enzyme to the E(1) configuration. This enzyme is inhibited by the unique proton pump inhibitor class of drug, allowing therapy of acid-related diseases.

Molecular mechanisms in therapy of acid-related diseases

Inhibition of gastric acid secretion is the mainstay of the treatment of gastroesophageal reflux disease and peptic ulceration; therapies to inhibit acid are among the best-selling drugs worldwide. Highly effective agents targeting the histamine H2 receptor were first identified in the 1970s. These were followed by the development of irreversible inhibitors of the parietal cell hydrogen-potassium ATPase (the proton pump inhibitors) that inhibit acid secretion much more effectively. Reviewed here are the chemistry, biological targets and pharmacology of these drugs, with reference to their current and evolving clinical utilities. Future directions in the development of acid inhibitory drugs include modifications of current agents and the emergence of a novel class of agents, the acid pump antagonists.

Our research on proton pumping ATPases over three decades: their biochemistry, molecular biology and cell biology

ATP is synthesized by F-type proton-translocating ATPases (F-ATPases) coupled with an electrochemical proton gradient established by an electron transfer chain. This mechanism is ubiquitously found in mitochondria, chloroplasts and bacteria. Vacuolar-type ATPases (V-ATPases) are found in endomembrane organelles, including lysosomes, endosomes, synaptic vesicles, etc., of animal and plant cells. These two physiologically different proton pumps exhibit similarities in subunit assembly, catalysis and the coupling mechanism from chemistry to proton transport through subunit rotation. We mostly discuss our own studies on the two proton pumps over the last three decades, including ones on purification, kinetic analysis, rotational catalysis and the diverse roles of acidic luminal organelles. The diversity of organellar proton pumps and their stochastic fluctuation are the important concepts derived recently from our studies.

Thr-774 (transmembrane segment M5), Val-920 (M8), and Glu-954 (M9) are involved in Na+ transport, and Gln-923 (M8) is essential for Na,K-ATPase activity

The highly conserved amino acids of rat Na,K-ATPase, Thr-774 in the transmembrane helices M5, Val-920 and Gln-923 in M8, and Glu-953 and Glu-954 in M9, the side chains of which appear to be in close proximity, were mutated, and the resulting proteins, T774A, E953A/K, and E954A/K, V920E and Q923N/E/D/L, were expressed in HeLa cells. Ouabain-resistant cell lines were obtained from T774A, V920E, E953A, and E954A, whereas Q923N/E/D/L, E953K, and E954K could only be transiently expressed as fusion proteins with an enhanced green fluorescent protein. The apparent K0.5 values for Na+, as estimated by the Na+-dependent phosphoenzyme formation (K0.5(Na,EP)) or Na,K-ATPase activity (K(0.5)(Na,ATPase)), were increased by around 2 approximately 8-fold in the case of T774A, V920E, and E954A. The apparent K0.5 values for K+, as estimated by the Na,K-ATPase (K0.5(K,ATPase)) or p-nitrophenylphosphatase activity (K0.5(K,pNPPase)), were affected only slightly by the 3 mutations, except that V920E showed a 1.7-fold increase in the K0.5(K,ATPase). The apparent K0.5 values for ATP (K0.5(EP)), as estimated by phosphorylation (a high affinity ATP effect), were increased by 1.6 approximately 2.6-fold in the case of T774A, V920E, and E954A. Those estimated by Na,K-ATPase activity (K0.5(ATPase)) and ATP-induced inhibition (K(i,0.5)(pNPPase)) of K-pNPPase activity (low affinity ATP effects) were, respectively, increased by 1.8-fold and unchanged in the case of T774A but decreased by 2- and 4.8-fold in the case of V920E and were slightly changed and increased by 1.7-fold in the case of E954A. The E953A showed little significant change in the apparent affinities. These results suggest that Gln-923 in M8 is crucial for the active transport of Na+ and/or K+ across membranes and that the side chain oxygen atom of Thr-774 in M5, the methyl group(s) of Val-920 in M8, and the carboxyl oxygen(s) of Glu-954 in M9 mainly play some role in the transport of Na+ and also in the high and low affinity ATP effects rather than the transport of K+.

Proton pumping ATPases and diverse inside-acidic compartments

Proton-translocating ATPases are essential cellular energy converters that transduce the chemical energy of ATP hydrolysis into transmembrane proton electrochemical potential differences. The structures, catalytic mechanism, and cellular functions of three major classes of ATPases including the F-type, V-type, and P-type ATPase are discussed in this review. Physiological roles of the acidic organelles and compartments contained are also discussed.

Molecular and Cellular Regulation of the Gastric Proton Pump

The gastric H+, K+-ATPase is a proton pump that is responsible for gastric acid secretion and that actively transports protons and K+ ions in opposite directions to generate in excess of a million-fold gradient across the membrane under physiological conditions. This pump is also a target molecule of proton pump inhibitors which are used for the clinical treatment of hyperacidity. In this review, we wish to summarize the molecular regulation of this pump based on mutational studies, particularly those used for the identification of binding sites for cations and specific inhibitors. Recent reports by Toyoshima et al (2000, 2002) presented precise three-dimensional (3-D) structures of the sarcoplasmic reticulum (SR) Ca2+-ATPase, which belongs to the same family as the gastric H+, K+-ATPase. We have studied the structure-function relationships for the gastric H+, K+-ATPase using 3-D structures constructed by homology modeling of the related SR Ca2+-ATPase, which was used as a template molecule. We also discuss in this review, the regulation of cell surface expression and synthesis control of the gastric proton pump.

Molecular mimicry between Helicobacter pylori antigens and H+, K+ --adenosine triphosphatase in human gastric autoimmunity

Autoimmune gastritis and Helicobacter pylori–associated gastric atrophy develop through similar mechanisms involving the proton pump H⁺,K⁺–adenosine triphosphatase as autoantigen. Here, we report that H. pylori–infected patients with gastric autoimmunity harbor in vivo–activated gastric CD4⁺ T cells that recognize both H⁺,K⁺–adenosine triphosphatase and H. pylori antigens. We characterized the submolecular specificity of such gastric T cells and identified cross-reactive epitopes from nine H. pylori proteins. Cross-reactive H. pylori peptides induced T cell proliferation and expression of T helper type 1 functions. We suggest that in genetically susceptible individuals, H. pylori infection can activate cross-reactive gastric T cells leading to gastric autoimmunity via molecular mimicry.

Identification and regulation of the gastric H+/K+-ATPase in the rat heart

Previous reports indicate that H+/K+-adenosine triphosphatase (ATPase) might be expressed in the heart.

AIMS

The objectives of the present study were to explore the presence of H+/K+-ATPase protein and gene expression in the rat heart and to investigate whether the enzyme could contribute to potassium transport across the sarcolemma.

METHODS AND RESULTS

We performed reverse transcription-polymerase chain reaction (RT-PCR) on mRNA from myocardium and isolated cardiomyocytes using primers specific for the gastric H+/K+-ATPase alpha-subunit. The PCR products were sequenced and the predicted gastric H+/K+-ATPase sequence was verified. Western blots from myocardium detected a 34-kDa band and a 94-kDa band, indicating the beta-subunit and alpha-subunit of the gastric H+/K+-ATPase, respectively. Immunocytochemistry detected significant immunoreactivity of the beta-subunit in cardiomyocytes. H+/K+-ATPase-dependent potassium transport was assessed by 86Rb+-uptake in isolated cardiomyocytes. Both ouabain and the selective H+/K+-ATPase inhibitor Schering 28080 reduced 86Rb+-uptake at maximum specific inhibition, by 70 and 25%, respectively; the effects were additive. Competitive RT-PCR analysis indicated a significant upregulation of the myocardial H+/K+-ATPase in heart failure after myocardial infarction.

CONCLUSION

The gastric isoform of H+/K+-ATPase is expressed in rat cardiac myocytes, both at transcript and protein levels. Functional studies indicate that the enzyme could contribute to potassium and pHi regulation in cardiomyocytes.

Significance of lysine/glycine cluster structure in gastric H+,K+-ATPase

Gastric H+,K+-ATPase consists of alpha- and beta-subunits. The catalytic alpha-subunit contains a very unique structure consisting of lysine and glycine clusters, KKK(or KKKK)AG(G/R)GGGK-(K/R)K, in the amino-terminal cytoplasmic region. This structure is well conserved in all gastric H+,K+-ATPases from different animal species, and was postulated to be the site controlling the access of cations (or proton) to its binding site. In this report, we studied the role of this unique structure by expressing several H+,K+-ATPase mutants of the alpha-subunit together with the wild-type beta-subunit in HEK-293 cells. Even after replacing all the positively-charged amino acid residues (six lysines and one arginine) in the cluster with alanine or removing all the glycine residues in the cluster, the mutants preserved the H+,K+-ATPase activity, and showed similar affinity for ATP and K+ as well as similar pH profiles as those of wild-type H+,K+-ATPase, indicating that the cluster is not indispensable for H+,K+-ATPase activity and not directly involved in determination of the affinity for cation (proton).

The K(+) affinity of gastric H(+),K(+)-ATPase is affected by both lipid composition and the beta-subunit

It is generally assumed that negatively charged residues present in the alpha-subunit of gastric H(+),K(+)-ATPase are involved in K(+) binding and transport. Despite the fact that there is no difference between various species regarding these negatively charged residues, it was observed that the apparent K(+) affinity of the pig enzyme was much lower than that of the rat H(+),K(+)-ATPase. By determining the K(+)-stimulated dephosphorylation reaction of the phosphorylated intermediate K(0.5) values for K(+) of 0.12+/-0.01 and 1.73+/-0.03 mM were obtained (ratio 14.4) for the rat and the pig enzyme, respectively. To investigate the reason for the observed difference in K(+) sensitivity, both enzymes originating from the gastric mucosa were either reconstituted in a similar lipid environment or expressed in Sf9 cells. After reconstitution in K(+)-permeable phosphatidylcholine/cholesterol liposomes K(0.5) values for K(+) of 0.16+/-0.01 and 0.35+/-0.05 mM for the rat and pig enzyme respectively were measured (ratio 2.2). After expression in Sf9 cells the pig gastric H(+),K(+)-ATPase still showed a 4.1 times lower K(+) sensitivity than that of the rat enzyme. This means that the difference in K(+) sensitivity of the rat and pig gastric H(+), K(+)-ATPase is not only due to a different lipid composition but also to the structure of either the alpha- or beta-subunit. Expression of hybrid enzymes in Sf9 cells showed that the difference in K(+) sensitivity between the rat and pig gastric H(+),K(+)-ATPase is primarily due to differences in the beta-subunit.

H+,K+-ATPase

The H+,K+-ATPases comprise a group of integral membrane proteins that belong to the X+,K+-ATPase subfamily of P-type cation-transporting ATPases. Although these H+,K+-ATPase isoforms share approximately 60-70% amino acid identity, they exhibit discrete kinetic and pharmacological properties when expressed in heterologous systems. HK alpha2 has been categorized by its insensitivity to Sch-28080, an inhibitor of the gastric H+,K+-ATPase, and partial sensitivity to ouabain, an inhibitor of the Na+,K+-ATPase. This functional profile contrasts with the pharmacological sensitivities ascribed to HK alpha2 in transport studies in rat isolated medullary collecting ducts perfused in vitro and in mouse medullary collecting duct cell lines. HK alpha2 mRNA and protein abundance appears to be both tissue and site-specifically upregulated in response to chronic hypokalemia. This regulatory response has been localized to the outer and inner medulla. To reconcile these expressed sensitivities to those reported in vitro in isolated tubules and cells in culture, it would be necessary to invoke modification of the pharmacologic insensitivity of the colonic H+,K+-ATPase to Sch-28080. Although a 'unique' beta-subunit has been reported recently, this beta-subunit (beta(c)) is identical at the amino acid level to the recently cloned beta3-Na+,K+-ATPase. Moreover, while HK alpha2 can assemble indiscriminately with any X+,K+-ATPase beta-subunit, HK alpha2 has been reported to assemble stably with beta1-Na+,K+-ATPase in the renal medulla and in the distal colon. It remains conceivable that subunit assembly could be tissue specific and might respond to different physiological and pathophysiological stimuli. Furthermore, recent studies have suggested that the H+,K+-ATPase is both Na+-dependent and localized to the apical membrane in the distal colon. Therefore, future studies will need to resolve these discrepancies by determining if a unique, yet undiscovered H+,K+-ATPase isoform exists in kidney, or if post-translational modifications of the alpha- and/or beta-subunits could account for these functional diversities.

Glu-857 moderates K+-dependent stimulation and SCH 28080-dependent inhibition of the gastric H,K-ATPase

The rabbit H,K-ATPase alpha- and beta-subunits were transiently expressed in HEK293 T cells. The co-expression of the H,K-ATPase alpha- and beta-subunits was essential for the functional H,K-ATPase. The K+-stimulated H,K-ATPase activity of 0.82 +/- 0.2 micromol/mg/h saturated with a K0.5 (KCl) of 0.6 +/- 0.1 mM, whereas the 2-methyl-8-(phenylmethoxy)imidazo[1,2a]pyridine-3-acetonitrile (SCH 28080)-inhibited ATPase of 0.62 +/- 0.07 micromol/mg/h saturated with a Ki (SCH 28080) of 1.0 +/- 0.3 microM. Site mutations were introduced at the N,N-dicyclohexylcarbodiimide-reactive residue, Glu-857, to evaluate the role of this residue in ATPase function. Variations in the side chain size and charge of this residue did not inhibit the specific activity of the H,K-ATPase, but reversal of the side chain charge by substitution of Lys or Arg for Glu produced a reciprocal change in the sensitivity of the H,K-ATPase to K+ and SCH 28080. The K0.5 for K+stimulated ATPase was decreased to 0.2 +/-.05 and 0.2 +/-.03 mM, respectively, in Lys-857 and Arg-857 site mutants, whereas the Ki for SCH 28080-dependent inhibition was increased to 6.5 +/- 1.4 and 5.9 +/- 1.5 microM, respectively. The H,K-ATPase kinetics were unaffected by the introduction of Ala at this site, but Leu produced a modest reciprocal effect. These data indicate that Glu-857 is not an essential residue for cation-dependent activity but that the residue influences the kinetics of both K+ and SCH 28080-mediated functions. This finding suggests a possible role of this residue in the conformational equilibrium of the H,K-ATPase.

Effect of aging on gastrin receptor gene expression in rat stomach

Gastrin is a pivotal humoral factor which regulates gastric acid secretion through its receptors. There is no report, however, concerning the age-related changes of gastrin receptor gene expression in the stomach. Northern blot analysis and in situ hybridization were performed to clarify the changes of gastrin receptor expression during the aging. In situ hybridization clarified that gastrin receptor mRNA was expressed mainly in enterochromaffin-like (ECL) cells in adult rat gastric mucosa. With aging, gastrin receptor gene expression in the stomach increased with the concomitant increase in histidine decarboxylase mRNA. Since histidine decarboxylase is a marker of gastric ECL cells, the augmented gastrin receptor mRNA in aged rats may be caused by the increased ECL cells in gastric mucosa during the aging.

Mutational analysis of putative SCH 28080 binding sites of the gastric H+,K+-ATPase

A compound, SCH 28080 (2-methyl-8-(phenylmethoxy)imidazo [1,2-a]pyridine-3-acetonitrile), reversibly inhibits gastric and renal ouabain-insensitive H+,K+-ATPase, but not colonic ouabain-sensitive H+,K+-ATPase. By using the functional expression system and site-directed mutagenesis, we analyzed the putative binding sites of SCH 28080 in gastric H+,K+-ATPase alpha-subunit. It was previously reported that the binding site of SCH 28080, which is a K+-site inhibitor specific for gastric H+,K+-ATPase, was in the first extracellular loop between the first and second transmembrane segments of the alpha-subunit; Phe-126 and Asp-138 were putative binding sites. However, we found that all the mutants in the first extracellular loop including Phe-126 and Asp-138 retained H+, K+-ATPase activity and sensitivity to SCH 28080. Therefore, amino acid residues in the first extracellular loop are not directly involved in the SCH 28080 binding nor indispensable for the H+, K+-ATPase activity. Here we propose a candidate residue that is important for the binding with SCH 28080, Glu-822 in the sixth transmembrane segment. Mutations of Glu-822 to Asp and Ala (mutants termed E822D and E822A, respectively) decreased the ATPase activity to about 45% and 35% of the wild-type enzyme, respectively, while the mutations to Gln and Leu abolished the activity. Mutant E822A showed a significantly lower affinity for K+ than the wild-type enzyme, indicating that Glu-822 is involved in determining the affinity for K+. The sensitivity of mutant E822D to SCH 28080 was 8 times lower than that of the wild-type enzyme. The counterpart of Glu-822 in gastric H+,K+-ATPase is Asp in Na+,K+-ATPase and other colonic ouabain-sensitive H+,K+-ATPase, which are insensitive to SCH 28080. These results suggest that Glu-822 is one of important sites that bind with SCH 28080.

Developmental gene expression of gastrin receptor in rat stomach

Gastrin, which is present in fetal plasma, may have important roles in the development of gastric mucosa, since it is not only a potent stimulator of gastric acid secretion but also a growth promoting factor. Gastrin regulates various cellular functions via its receptors on cell membrane. Therefore, in order to elucidate a role for gastrin in the development of gastrointestinal system during gestation, Northern blot analysis was performed. The results of the study suggested that gastrin receptor is mainly present on parietal cells. Furthermore, proton pump and gastrin receptor gene expressions in parietal cells were strongly stimulated by the administration of exogenous gastrin. In conclusion, gastrin may be involved in the developmental change of parietal cells through its receptors.

Subunit interactions in the Na,K-ATPase explored with the yeast two-hybrid system

Subunit interactions of the alpha1- and beta1-subunits of the chicken Na,K-ATPase were explored with the yeast two-hybrid system. Gal4-fusion proteins containing domains of the alpha1- and beta1-subunits were designed for examining both intersubunit and intrasubunit protein-protein interactions. Regions of the alpha- and beta-subunits known to be involved in alpha-beta-subunit assembly were positive in two-hybrid assay, supporting the validity of the assays. A library of beta-subunit ectodomains with C-terminal truncations was screened to find the maximal truncation retaining an interaction with the alpha-subunit extracellular H7H8 loop (where H7 refers to the seventh membrane span, and so on). The maximal truncation removed all the cysteines involved in disulfide bridges, leaving only 63 amino acids of the beta-subunit ectodomain. Scanning alanine mutagenesis led to identification of an evolutionarily conserved sequence of four amino acids (SYGQ) in the extracellular H7H8 loop of the alpha-subunit that is crucial to alpha-beta-intersubunit interactions. Oligomerization studies with single domains failed to detect self-association of either of the two large cytosolic loops (H2H3 and H4H5) within the alpha-subunit. However, evidence was found for an interaction between these two cytoplasmic loops.

Three-dimensional structure of the porcine gastric H,K-ATPase from negatively stained crystals

A low-resolution three-dimensional model of membrane-bound H,K-ATPase from pig gastric mucosa has been reconstructed by electron microscopy and image processing of two-dimensional crystals in negative stain. The crystal formation is induced by magnesium and vanadate, which stabilize the E2 conformation of the enzyme. The unit cell, with a size of a = b = 123 A, gamma = 90 degrees, has tetragonal p4 symmetry. There are four separate alpha beta protomers within each unit cell. The high-contrast region is limited to the cytoplasmic part of the protein. The total volume of the observed asymmetric protein domain corresponds to a molecular mass of 80-90 kDa. It consists mainly of a large pear-shaped domain measuring 60 x 45 A2, with a height of 50 A as measured perpendicular to the membrane plane. A small stalk segment, 20 A in length, forms a connection to the transmembrane region.

Transmembrane carboxyl residues are essential for cation-dependent function in the gastric H,K-ATPase

The K+-dependent ATPase activity of the H,K-ATPase was irreversibly inhibited by the carboxyl activating reagent, dicyclohexylcarbodiimide (DCCD). The inhibition was first order and displayed a concentration dependence with the K0.5 (DCCD) = 0.65 +/- 0.04 mM. KCl protected 70% of the ATPase activity from DCCD-dependent inhibition in a concentration-dependent manner with a K0.5 (K+) = 0.58 +/- 0.1 mM KCl. DCCD modification selectively inhibited the K+-dependent rather than ATP-dependent partial reactions including eosin fluorescence responses and ligand-stabilized initial tryptic cleavage patterns of the membrane-associated enzyme. DCCD modification also inhibited the binding of 86Rb+ and the fluorescent responses of the K+-competitive, fluorescent inhibitor 1-(2-methylphenyl)-4-methylamino-6-methyl-2, 3-dihydropyrrolo[3,2-c]quinoline. [14C]DCCD was incorporated into the H,K-ATPase in a time course identical to that describing the inactivation of the K+-dependent ATPase activity of the H,K-ATPase. A component of the [14C]DCCD incorporated into the H,K-ATPase was K+-sensitive where K+ reduced the [14C]DCCD incorporated into the enzyme by 1.6 nmol of [14C]DCCD/mg of protein. Membrane-associated tryptic peptides resolved from the [14C]DCCD-modified H,K-ATPase exhibited various K+ sensitivities with peptides at 23, 9.6, 8.2, 7.1, and 6.1 kDa containing 10-78%, 23-52%, 24-36%, 2%, and 3-4% K+-sensitivity, respectively. The N-terminal sequence of the K+-sensitive, approximately 23- and 9.6-kDa peptides was LVNE857, a C-terminal fragment of the ATPase alpha-subunit. The mass of the smaller peptide limited the residue assignment to the transmembrane M7/M8 domains and an intervening extracytoplasmic loop. An N-terminal sequence, SD840IM, was obtained from a 3.3-kDa, [14C]DCCD-labeled peptide resolved from a V8 digest of the partially purified alpha-subunit. This mass was sufficient to include LVNE but would exclude M8 and the intervening loop between M7 and M8. Glu857 is a unique residue present in each of the proteolytic preparations of the H,K-ATPase modified by [14C]DCCD. These data provide functional evidence of the selective inactivation of the K+-dependent partial reactions of the H,K-ATPase and show that Glu857 located at the M7 boundary in the C terminus of the pump molecule is a significant site of DCCD modification. These data are interpreted to indicate that this carboxyl residue is important for cation binding function.

Mutations in the Caenorhabditis elegans Na,K-ATPase alpha-subunit gene, eat-6, disrupt excitable cell function

We have cloned a Na,K-ATPase alpha-subunit gene from Caenorhabditis elegans and discovered that it is identical to the gene eat-6, eat-6 mutations cause feeble contractions and slow, delayed relaxations of pharyngeal muscle. The resting membrane potential of eat-6 mutant pharynxes is consistently depolarized compared to wild-type. The action potentials are smaller, and the return to resting potential is slower. To explain these abnormalities, we propose that a reduction of Na,K-ATPase activity in eat-6 mutants leads to a reduction of the ion concentration gradients that power membrane potential changes.

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.

Immunocytochemical localization of H(+)-K(+)-ATPase in the rat colon

The presence and distribution of gastric-type H(+)-K(+)-ATPase were investigated in the rat colon using a monoclonal antibody raised against hog gastric H(+)-K(+)-ATPase. Rat stomach was used as positive control. Rat kidney and ileum, in both of which H(+)-K(+)-ATPase has been reported in the past, were also studied. In stomach, very strong staining was found confined to the parietal cells, and a strong band at M(r) approximately 94 kDa on the immunoblots. In colon a moderate staining was found in the supranuclear region of the epithelial cells, with similar intensity and distribution of staining of the surface and deep mucosa of the crypts, throughout the length of the colon. Another monoclonal antibody, specific to the 31 kDa subunit of H(+)-ATPase, used as a negative control, or omission of the primary antibody, resulted in lack of any staining in either colon or stomach. On immunoblots of homogenates of colonic mucosa, no specific band could be identified, either due to very low expression of the H(+)-K(+)-ATPase or loss of antigenicity of the epitope during the processing steps. No positive staining was observed in rat kidney and ileum, suggesting that they contain isoforms that are structurally different.

Dimerization of the gastric H+, K(+)-ATPase

When the gastric H+, K(+)-ATPase was solubilized by n-dodecyl beta-D-maltoside and electrophoresed in blue native-polyacrylamide gels (BN-PAGE), one major band at about 360 kDa was observed. Since this band was recognized by both monoclonal antibodies 1218 (anti-alpha) and wheat germ agglutinin (anti-beta), the H+, K(+)-ATPase in its native state exists in a dimeric (alpha beta)2 form. The site of interaction between the heterodimers was determined using Cu(2+)-phenanthroline cross-linking. The Cu(2+)-phenanthroline reagent reacted with the H+, K(2+)-ATPase activity. This cross-linking and enzyme inhibition were prevented by ATP. Cross-linking followed by N-ethylmaleimide blockade of maleimide-reactive SH groups, then reduction and fluorescein 5-maleimide labeling, then reduction and fluorescent tryptic peptide of about 6.5 kDa that had been cross-linked. Since its N-terminal amino acid is Val561, the peptide probably ends at Arg616 or Arg621 and Cys565 and/or Cys615 are probably within the region of closest contact between the two alpha-subunits.

Reversible phosphorylation of both Tyr7 and Tyr10 in the alpha-chain of pig stomach H+,K(+)-ATPase by a membrane-bound kinase and a phosphatase

When pig stomach membrane H+,K(+)-ATPase preparations were incubated with [gamma-32P]ATP and Mg2+ with vanadate, 32P was incorporated into the alpha-chain of H+,K(+)-ATPase to a steady-state level of approximately 0.7 mol of phosphotyrosine (Tyr(P))/mol of phosphoenzyme intermediates. The addition of a membrane H+,K(+)-ATPase preparation with Mg2+ accelerated the liberation of 32P from Tyr(P) residues in the alpha-chain. Mild tosylphenylalanyl chloromethyl ketone-trypsin treatment solubilized 32P-containing peptides from the alpha-chain almost completely. A reverse-phase column chromatography of the supernatant gave two peaks of 32P-peptide with similar total radioactivities. The amino acid sequence of both peaks was shown to be Gly-Lys-Ala-Glu-Asn-Tyr-Glu-Leu-Tyr-Gln--, which is consistent with the amino-terminal sequence of the alpha-chain of H+,K(+)-ATPase deduced from cDNA from pig stomach except that the initial Met was absent. The comparison of the recovery of amino acid from each Edman cycle showed that the phosphorylation of Tyr10 occurred preceding the phosphorylation of Tyr7. These data and others suggested the presence of a novel membrane-bound enzyme system to participate in reversible phosphorylation of both Tyr residues in the alpha-chain of H+,K(+)-ATPase.

The proton pump inhibitor, E3810, binds to the N-terminal half of the alpha-subunit of gastric H+,K(+)-ATPase

E3810 (2-([4-(3-methoxypropoxy)-3-methylpyridine-2-yl]methylsulphinyl )- 1H-benzimidazole sodium salt), an inhibitor of gastric proton pump (gastric H+,K(+)-ATPase), is activated in a luminal acidic environment of gastric glands and binds to a Cys residue of H+,K(+)-ATPase on its luminal side. It was found that bound E3810 is transformed into a strongly fluorescent compound by UV-light irradiation (excitation wavelength = 335 nm, emission wavelength = 470 nm). The location of Cys residue bound with E3810 in the alpha-subunit of hog gastric H+,K(+)-ATPase was estimated from the fluorescence labelling and limited tryptic digestion of the enzyme. Tryptic digestion in the presence of Mg-ATP produces N-terminal 67 kDa subfragment which contains the phosphorylation and fluorescein 5'-isothiocyanate binding sites and C-terminal 35 kDa subfragment. Trypsin digestion in the presence of KCl produces N-terminal 42 kDa and C-terminal 56 kDa subfragments. E3810 was found to bind to both N-terminal but not to any of two C-terminal subfragments. Taking the amino acid sequence and topology of this ATPase as well as the fact that the ratio of specific binding sites per alpha-subunit is one into consideration, the possibility that E3810 specifically binds to Cys322 residue of hog gastric H+,K(+)-ATPase is discussed.

Primary structure and functional expression of the mouse and frog alpha-subunit of the gastric H(+)-K(+)-ATPase

The H(+)-K(+)-ATPase of the gastric parietal cells is responsible for the acidification of the stomach lumen. This heterodimeric protein belongs to the family of cation-translocating P-type ATPases, which includes the closely related Na(+)-ATPase. We have cloned the alpha-subunit cDNA of the Xenopus and murine gastric H(+)-K(+)-ATPase (alpha H-K). We have expressed Xenopus and murine alpha H-K along with the previously cloned gastric H(+)-K(+)-ATPase beta-subunit of rabbit (beta H-K) in Xenopus oocytes by cRNA injection. An antibody directed against the beta H-K coimmunoprecipitates under nondenaturing conditions the alpha H-K of both species, demonstrating assembly of the alpha/beta complex. Additionally, we demonstrate the presence of K(+)-transporting H(+)-K(+)-ATPase in the plasma membrane of oocytes by 86Rb- uptake. The H(+)-K(+)-ATPase-mediated K+ uptake was inhibited by the gastric H(+)-K(+)-ATPase inhibitor Sch-28080, but not by ouabain, and shows K(+)-dependent activation (K1/2 approximately 2 mM). Furthermore, H(+)-K(+)-ATPase-expressing oocytes show a Sch-28080 inhibitable proton extrusion. Our data indicate that the expressed H(+)-K(+)-ATPase behaves functionally in oocytes as in the gastric gland.

Establishment of primary epithelial cell culture from elutriated rat gastric mucosal cells

Proliferating cells in the gastric mucosal epithelium were successfully enriched by counterflow elutriation in a medium-sized cell fraction. When inoculated on culture plates coated with E-C-L cell attachment matrix, these cells differentiated into mucus-producing cells after reaching confluence. Northern blot analysis did not detect any transcript of the proton pump, histidine decarboxylase, somatostatin, or pepsinogen I, indicating the absence of parietal, ECL, D, and chief cells in the confluent monolayer. These mucus-producing cell monolayers that respond to various growth factors may be a suitable model with which to investigate the function of gastric mucus cells in vitro.

Pharmacological aspects of acid secretion

The secretion of gastric acid is regulated both centrally and peripherally. The finding that H2-receptor antagonists are able to reduce or abolish acid secretion due to vagal, gastrinergic, and histaminergic stimulation shows that histamine plays a pivotal role in stimulation of the parietal cell. In the rat, the fundic histamine is released from the ECL cell, in response to gastrin, acetylcholine, or epinephrine, and histamine release is inhibited by somatostatin or by the H3-receptor ligand, R-alpha-methyl histamine. The parietal cell has a muscarinic, M3, receptor responsible for [Ca]i regulation. Blockade of muscarinic receptors by atropine can be as effective as H2-receptor blockade in controlling acid secretion. However, general effects on muscarinic receptors elsewhere produce significant side effects. The different receptor pathways converge to stimulate the gastric H+,K(+)-ATPase, the pump responsible for acid secretion by the stomach. This enzyme is an alpha,beta heterodimer, present in cytoplasmic membrane vesicles of the resting cell and in the canaliculus of the stimulated cell. It has been shown that acid secretion by the pump depends on provision of K+Cl- efflux pathway becoming associated with the pump. As secretion occurs only in the canaliculus, this K+Cl- pathway is activated only when the pump inserts into the canalicular membrane. Transport by the enzyme involves reciprocal conformational changes in the cytoplasmic and extracytoplasmic domain. These result in changes in sidedness and affinity for H3O+ and K+, enabling active H+ for K+ exchange. The acid pump inhibitors of the substituted benzimidazole class, such as omeprazole, are concentrated in the canaliculus of the secreting parietal cell and are activated there to form sulfenamides. The omeprazole sulfenamide, for example, reacts covalently with two cysteines in the extracytoplasmic loops between the fifth and sixth transmembrane and the seventh and eighth transmembrane segments of the alpha subunit of the H+,K(+)-ATPase, forming disulfide derivatives. This inhibits ATP hydrolysis and H+ transport, resulting in effective, long-lasting regulation of acid secretion. Therefore, this class of acid pump inhibitor is significantly more effective and faster acting than the H2 receptor antagonists. K+ competitive antagonists bind to the M1 and M2 transmembrane segments of the alpha subunit of the acid pump and also abolish ATPase activity. These drugs should also be able to reduce acid secretion more effectively than receptor antagonists and provide shorter acting but complete inhibition of acid secretion.

A conserved epitope on H+,K(+)-adenosine triphosphatase of parietal cells discerned by a murine gastritogenic T-cell clone

BACKGROUND/AIMS

H+,K(+)-adenosine triphosphatase (H+,K(+)-ATPase) of parietal cells is an organ-specific enzyme recognized by autoantibodies found in human and murine autoimmune gastritis (AIG). Murine AIG can be induced in BALB/c mice by thymectomy 3 days after birth and is a T cell-mediated disease. This study examined the specificity of T cells that cause AIG and the role of H+,K(+)-ATPase in this disease.

METHODS

From an AIG mouse, a gastritogenic T-cell clone (II-6) was established, and its reactivity to synthetic peptides of H+,K(+)-ATPase was tested.

RESULTS

II-6 cells are CD4+, V beta 14+, and interferon gamma producers. Adoptive transfer of II-6 cells to syngeneic nude mice resulted in AIG without the production of autoantibodies to parietal cells. The II-6 cells were responsive not only to murine but also to human and porcine parietal cells. Their proliferation was also induced by amino acids 891-905 (alpha 891) and 892-906 (alpha 892) of the alpha subunit of porcine and human H+,K(+)-ATPase, respectively.

CONCLUSIONS

The T-cell response to a single epitope of H+,K(+)-ATPase, the amino acid sequence of which is conserved among at least three mammals tested, is sufficient to cause AIG. Autoantibodies to parietal cells are not detected in these AIG mice.

Gastrin receptor genes are expressed in gastric parietal and enterochromaffin-like cells of Mastomys natalensis

Although gastric enterochromaffin-like (ECL) carcinoid tumors are known to develop in patients with long-standing hypergastrinemia, the expression of the gastrin receptor gene in ECL cells has not yet been demonstrated. Therefore, this study was designed to examine gastrin receptor gene expression in ECL cells. Mastomys gastric mucosal cells isolated by enzyme dispersion were separated into 10 fractions (F1-10) by centrifugal elutriation. Each fraction was examined histologically to determine whether they contained ECL and/or parietal cells and Northern blot analysis was used to confirm the presence of histidine decarboxylase and H+, K(+)-ATPase gene expression. ECL cells were found only in fractions 1 and 2, whereas parietal cells were detected in fractions 6-10. Gastrin receptor gene expression was demonstrated in both parietal cell-rich and ECL cell-rich fractions. In addition, the gastrin receptor cDNA sequences obtained from the two of the fractions (F1 and 8) were identical. These results suggest that gastrin receptor genes are expressed in ECL cells as well as in parietal cells and that these receptors are identical.

C-terminal topology of gastric H+,K(+)-ATPase

An antibody was prepared against a peptide corresponding to residues 1024-1034 (the putative C-terminus) of the alpha-subunit of hog gastric H+,K(+)-ATPase. The antibody bound to a 95 kDa band of H+,K(+)-ATPase that was solubilized in SDS, but not to that of Na+,K(+)-ATPase. It also bound to products of tryptic digestion that included C-terminal fragments of the H+,K(+)-ATPase alpha-subunit. The same amount of the antibody bound to both intact (tight) and lyophilized (leaky) inside-out gastric vesicles, indicating that its epitope is present on the cytosolic side of the vesicles. This finding was further confirmed by using fluorescence-immunolocalization techniques and streptolysin-O to permeabilize newt oxyntic cells. Stimulation of isolated newt oxyntic cells with dibutyryl cyclic AMP induces fusion of tubulovesicles with the apical membrane, so that the luminal domains of the H+,K(+)-ATPase alpha-subunit directly face the cell-suspension medium. The antibody did not bind to the stimulated intact cell, but bound to cells permeabilized with streptolysin-O, indicating that it binds from the cytoplasmic side to the C-terminus of the H+,K(+)-ATPase alpha-subunit in apical and tubulovesicular membrane, and also that the H+,K(+)-ATPase alpha-subunit has an even number of transmembrane domains.

High sensitivity to site directed mutagenesis of the peptide segment connecting phosphorylation and Ca2+ binding domains in the Ca2+ transport ATPase

Nine residues (Leu321, Lys329, Asn330, Val333, Arg334, Leu336, Pro337, Val339 and Glu340), within the peptide segment intervening between the catalytic domain and the Ca2+ binding domain of the sarcoplasmic reticulum (SERCA 1) ATPase, were individually mutated to Ala. The mutated proteins were recovered in the microsomal fraction of COS-1 cells following transient expression, and exhibited inhibition of Ca2+ uptake and ATPase hydrolytic activity, while forming discernable levels of phosphorylated intermediate. Mutation of Glu340 to Gln (rather than to Ala) was much less effective, suggesting that the functional consequence of the mutation is related to structural perturbation, rather than loss of the acidic side chain. The high sensitivity of this peptide segment to single mutations suggests that its structural integrity is required for functional linkage of the phosphorylation and Ca2+ binding domains.

Identification of the amino acids comprising a surface-exposed epitope within the nucleotide-binding domain of the Na+,K(+)-ATPase using a random peptide library

Monoclonal antibodies that bind native protein can generate considerable information about structure/function relationships, but identification of their epitopes can be problematic. Previously, monoclonal antibody M8-P1-A3 has been shown to bind to the catalytic (alpha) subunit of the Na+,K(+)-ATPase holoenzyme and the synthetic peptide sequence 496-HLLVMK*GAPER-506, which includes Lys 501 (K*), the major site for fluorescein-5'-isothiocyanate labeling of the Na+,K(+)-ATPase. This sequence region of alpha is proposed to comprise a portion of the enzyme's ATP binding domain (Taylor, W. R. & Green, N. W., 1989, Eur. J. Biochem. 179, 241-248). In this study we have determined M8-P1-A3's ability to recognize the alpha-subunit or homologous E1E2-ATPase proteins from different species and tissues in order to deduce the antibody's epitope. In addition the bacteriophage random peptide or "epitope" library, recently developed by Scott and Smith (1990, Science 249, 386-390) and Devlin et al. (Devlin, J. J., Panganiban, L. C., & Devlin, P. E., 1990, Science 249, 404-406), has served as a convenient technique to confirm the species-specificity mapping data and to determine the exact amino acid requirements for antibody binding. The M8-P1-A3 epitope was found to consist of the five amino acid 494-PRHLL-498 sequence stretch of alpha, with residues PRxLx being critical for antibody recognition.

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.

Functional expression of gastric H,K-ATPase using the baculovirus expression system

A novel approach to construct a single recombinant baculovirus expressing two protein subunits simultaneously by replacing polyhedrin as well as p10 coding sequences is described. The recombinant baculovirus expressed the alpha- as well as the beta-subunit of the gastric H,K-ATPase. Sf9 cells infected with this virus exhibited a K(+)- and SCH 28080-sensitive ATP-dependent phosphorylation capacity in purified Sf9 membranes similar to native H,K-ATPase. This activity was not present in control membranes containing only one of the two H,K-ATPase subunits. We therefore conclude that both subunits are essential for the phosphorylation capacity of H,K-ATPase.

Membrane topology and omeprazole labeling of the gastric H+,K(+)-adenosinetriphosphatase

The gastric H+,K(+)-ATPase is an alpha beta heterodimer with close homology to the Na+,K(+)-ATPase. Digestion of intact cytoplasmic-side-out vesicles at a trypsin to protein ratio of 1/4 removed most of the cytoplasmic protein, leaving membrane-spanning pairs in high yield. These were visualized on gels and poly(vinylidene difluoride) (PVDF) membranes by sodium dodecyl sulfate solubilization of the membrane-embedded segments and labeling of the cysteine residues with fluorescein maleimide prior to electrophoresis. The membrane-spanning residues of the alpha subunit were found between positions 104 and 162 (M1/M2), 291 and 358(M3/M4), 776 and 835 (M5/M6), and 853 and 946 (M7/M8). Although this method did not detect membrane retention of the hydrophobic sequences subsequent to position 946, it provided biochemical evidence for at least eight membrane segments in the catalytic subunit. Intact vesicles containing this enzyme transport acid in the presence of KCl, valinomycin, and MgATP. Omeprazole accumulates in these acidified vesicles and converts to a cationic sulfenamide. This forms disulfides with accessible cysteines. The reaction with this extracytoplasmic thiol reagent inhibits ATPase activity. Full inhibition was obtained with a stoichiometry of 2.2 mol of omeprazole bound/mg of protein. Only the alpha subunit was labeled. The cysteines reacting with omeprazole were defined by proteolytic cleavage of 3H- or 14C-omeprazole-labeled enzyme followed by peptide sequencing of fragments separated on tricine gradient gels and transferred to PVDF membranes. Tryptic digestion at a 1/40 trypsin to protein ratio in the presence of ligands that stabilize the E2P form of the enzyme produced two large fragments, one of 68 kDa stretching from Glu47 to probably Arg666 that contained minor labeling and the other of 333 kDa beginning at Ala671 and extending to probably Arg946 that contained greater than 85% of the label. Digestion of labeled vesicles at 1/75 or 1/4 trypsin to protein ratios gave radioactive patterns consistent with labeling at Cys813 and/or Cys822 and at Cys892 and/or Cys927 and/or Cys938. V8 protease digestion of the solubilized alpha subunit produced a fragment extending from Ser838 to possible Asp900 that was omeprazole-labeled, showing that Cys892 was labeled and Cys927 and Cys938 were not. Hence, omeprazole labels the H+,K(+)-ATPase at cysteines within the M5/M6 and M7/M8 regions of the alpha subunit, accounting for its inhibitory action in vivo and in vitro.

Higher plant Ca(2+)-ATPase: primary structure and regulation of mRNA abundance by salt

Calcium-dependent regulatory mechanisms participate in diverse developmentally, hormonally, and environmentally regulated processes, with the precise control of cytosolic Ca2+ concentration being critical to such mechanisms. In plant cells, P-type Ca(2+)-ATPases localized in the plasma membrane and the endoplasmic reticulum are thought to play a central role in regulating cytoplasmic Ca2+ concentrations. Ca(2+)-ATPase activity has been identified in isolated plant cell membranes, but the protein has not been characterized at the molecular level. We have isolated a partial-length cDNA (LCA1) and a complete genomic clone (gLCA13) encoding a putative endoplasmic reticulum-localized Ca(2+)-ATPase in tomato. The deduced amino acid sequence specifies a protein (Lycopersicon Ca(2+)-ATPase) of 1048 amino acids with a molecular mass of 116 kDa, eight probable transmembrane domains, and all of the highly conserved functional domains common to P-type cation-translocating ATPases. In addition, the protein shares approximately 50% amino acid sequence identify with animal sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPases but less than 30% identity with other P-type ATPases. Genomic DNA blot hybridization analysis indicates that the Lycopersicon Ca(2+)-ATPase is encoded by a single gene. RNA blot hybridization analysis indicates the presence of three transcript sizes in root tissue and a single, much less abundant, transcript in leaves. Lycopersicon Ca(2+)-ATPase mRNA levels increase dramatically upon a 1-day exposure to 50 mM NaCl. Thus this report describes the primary structure of a higher-plant Ca(2+)-ATPase and the regulation of its mRNA abundance by salt stress.