The amino acid sequence of an active site peptide from the H,K-ATPase of gastric mucosa

P-type ATPases: Many more enigmas left to solve

P-type ATPases constitute a large ancient super-family of primary active pumps that have diverse substrate specificities ranging from H+ to phospholipids. The significance of these enzymes in biology cannot be overstated. They are structurally related, and their catalytic cycles alternate between high- and low-affinity conformations that are induced by phosphorylation and dephosphorylation of a conserved aspartate residue. In the year 1988, all P-type sequences available by then were analyzed and five major families, P1 to P5, were identified. Since then, a large body of knowledge has accumulated concerning the structure, function, and physiological roles of members of these families, but only one additional family, P6 ATPases, has been identified. However, much is still left to be learned. For each family a few remaining enigmas are presented, with the intention that they will stimulate interest in continued research in the field. The review is by no way comprehensive and merely presents personal views with a focus on evolution.

The potency of eriosematin E from Eriosema chinense Vogel. against enteropathogenic Escherichia coli induced diarrhoea using preclinical and molecular docking studies

The main objective of the present study was to evaluate the potential of eriosematin E (ECM) isolated from the roots of Eriosema chinense against enteropathogenic Escherichia coli (EPEC) induced diarrhoea. ECM isolated from the bioactive chloroform fraction of E. chinense was subjected to antidiarrhoeal evaluation on rats against diarrhoea, induced by the oral suspension of EPEC. The study included evaluation of behavioral parameters for 6 h and up to 24 h of induction, followed by estimation of water content, the density of EPEC in stools and evaluation of various blood parameters. Further, the colonic and small intestinal tissues were subjected to biochemical estimations, antioxidant evaluation, determination of ion concentration, Na⁺/K⁺ -ATPase activity, pro-inflammatory cytokines assessment and histopathology. Finally, the impact of ECM on Na⁺/K⁺-ATPase was studied through molecular docking studies. Significant antidiarrhoeal potential of ECM was demonstrated at 5 and 10 mg/kg, p.o., however, ECM at 10 mg/kg, p.o. was found to be more effective, as confirmed through higher % protection, density of EPEC in stools and water content of stools. ECM also significantly increased the level of WBC, Hb, platelets and revealed restoration of altered antioxidants, pro-inflammatory cytokines (IL-1β and TNF-α) status and also reactivated the suppressed Na⁺/K⁺-ATPase activity, which was also confirmed through docking studies showing H-bonding of hydroxyl group of ECM with amino acids Asp 190, Asn 167 and Glu 169 thus, maintaining proper electrolyte balance and also prevented epithelial tissue damage. The overall effect of ECM may be attributed to the decline in the elevated level of cytokines, inhibition in nitric oxide production and reactivation of Na⁺/K⁺-ATPase activity resulting in reduced intestinal secretion.

Systematic comparison of molecular conformations of H+,K+-ATPase reveals an important contribution of the A-M2 linker for the luminal gating

Gastric H⁺,K⁺-ATPase, an ATP-driven proton pump responsible for gastric acidification, is a molecular target for anti-ulcer drugs. Here we show its cryo-electron microscopy (EM) structure in an E2P analog state, bound to magnesium fluoride (MgF), and its K⁺-competitive antagonist SCH28080, determined at 7 Å resolution by electron crystallography of two-dimensional crystals. Systematic comparison with other E2P-related cryo-EM structures revealed that the molecular conformation in the (SCH)E2·MgF state is remarkably distinguishable. Although the azimuthal position of the A domain of the (SCH)E2·MgF state is similar to that in the E2·AlF (aluminum fluoride) state, in which the transmembrane luminal gate is closed, the arrangement of transmembrane helices in the (SCH)E2·MgF state shows a luminal-open conformation imposed on by bound SCH28080 at its luminal cavity, based on observations of the structure in the SCH28080-bound E2·BeF (beryllium fluoride) state. The molecular conformation of the (SCH)E2·MgF state thus represents a mixed overall structure in which its cytoplasmic and luminal half appear to be independently modulated by a phosphate analog and an antagonist bound to the respective parts of the enzyme. Comparison of the molecular conformations revealed that the linker region connecting the A domain and the transmembrane helix 2 (A-M2 linker) mediates the regulation of luminal gating. The mechanistic rationale underlying luminal gating observed in H⁺,K⁺-ATPase is consistent with that observed in sarcoplasmic reticulum Ca²⁺-ATPase and other P-type ATPases and is most likely conserved for the P-type ATPase family in general.

HK-ATPase Expression in the Susceptible BALB/c and the Resistant DBA/2 Strains of Mice to Autoimmune Gastritis

Neonatal thymectomy (NTx) in mice induces a group of alterations in the immune system homeostasis that results in the development of a variety of organ-specific autoimmune diseases such as gastritis, thyroiditis, oophoritis and orchitis. Given the importance of self-antigen expression in thymus for the control of autoreactive cells and generation of regulatory cells, we have compared the expression of parietal cell antigen in two strains of mice with the same H-2: BALB/c (susceptible to develop gastritis after NTx) and DBA/2 (resistant). We detected mRNA of HK-ATPase alpha and beta chains in day 1 thymi of both strains. Fifty percent of BALB/c mice presented mRNA levels similar to DBA/2. However, lower mRNA levels were found in the remaining BALB/c mice that may correspond to those that would develop AIG after NTx. Since the presence of the antigen in periphery is also necessary for the induction of regulatory cells, we have compared both strains observing in day 1 stomachs from resistant DBA/2 strain, a significantly higher content of positive cells for HK-ATPase subunits than stomachs from susceptible BALB/c strain. Also, the presence of antinuclear Abs in NTx BALB/c mice makes this model a useful experimental system for analyzing the responsible mechanisms breaking the non-specific self-tolerance.

The glycine residues G551 and G1349 within the ATP-binding cassette signature motifs play critical roles in the activation and inhibition of cystic fibrosis transmembrane conductance regulator channels by phloxine B

The cystic fibrosis transmembrane conductance regulator (CFTR) protein contains a canonical ATP-binding cassette (ABC) signature motif, LSGGQ, in nucleotide binding domain 1 (NBD1) and a degenerate LSHGH in NBD2. Here, we studied the contribution of the conserved residues G551 and G1349 to the pharmacological modulation of CFTR chloride channels by phloxine B using iodide efflux and whole-cell patch clamp experiments performed on the following green fluorescent protein (GFP)-tagged CFTR: wild-type, delF508, G551D, G1349D, and G551D/G1349D double mutant. We found that phloxine B stimulates and inhibits channel activity of wild-type CFTR (Ks = 3.2 +/- 1.6 microM: , Ki = 38 +/- 1.4 microM: ) and delF508 CFTR (Ks = 3 +/- 1.8 microM: , Ki = 33 +/- 1 microM: ). However, CFTR channels with the LSGDQ mutated motif (mutation G551D) are activated (Ks = 2 +/- 1.13 microM: ) but not inhibited by phloxine B. Conversely, CFTR channels with the LSHDH mutated motif (mutation G1349D) are inhibited (Ki = 40 +/- 1.01 microM: ) but not activated by phloxine B. Finally, the double mutant G551D/G1349D CFTR failed to respond not only to phloxine B stimulation but also to phloxine B inhibition, confirming the importance of both amino acid locations. Similar results were obtained with genistein, and kinetic parameters were determined to compare the pharmacological effects of both agents. These data show that G551 and G1349 control the inhibition and activation of CFTR by these agents, suggesting functional nonequivalence of the signature motifs of NBD in the ABC transporter CFTR.

Correlation between the activities and the oligomeric forms of pig gastric H/K-ATPase

Membrane-bound H/K-ATPase was solubilized by octaethylene glycol dodecyl ether (C(12)E(8)) or n-octyl glucoside (nOG). H/K-ATPase activity and the distribution of protomeric and oligomeric components were evaluated by high-performance gel chromatography (HPGC) and by single-molecule detection using total internal reflection fluorescence microscopy (TIRFM). As evidenced by HPGC of the C(12)E(8)-solubilized enzyme, the distribution of oligomers was 12% higher oligomeric, 44% diprotomeric, and 44% protomeric species, although solubilization by C(12)E(8) reduced the H/K-ATPase activity to 1.8% of that of the membrane-bound enzyme. The electron microscopic images of the C(12)E(8)-solubilized enzyme showed the presence of protomers and a combination of two and more protomers. While the nOG-solubilized H/K-ATPase retained the same turnover number and 71% of the specific activity as that of the membrane-bound enzyme, 56% higher oligomeric, 34% diprotomeric, and 10% protomeric species were detected. TIRFM analysis of solubilized fluorescein 5'-isothiocyanate (FITC)-modified H/K-ATPase at Lys-518 of the alpha-chain showed a quantized photobleaching of the FITC fluorescence intensity. For the C(12)E(8)-solubilized FITC-enzyme, the fraction of each of the initial relative fluorescence intensity units of 4, 2, and 1 was, respectively, 5%, 44% and 51%. In the case of the nOG-solubilized FITC-enzyme, each fraction of 4 and 2 units was, respectively, 54% and 46% with no detectable 1 unit fraction. This represents the first direct observation of H/K-ATPase in aqueous solution. The correlation between the enzymatic activities and distribution of oligomeric forms of H/K-ATPase by HPGC and the observation of a single molecule of H/K-ATPase and others suggests that the tetraprotomeric form of H/K-ATPase molecules represents the functional species in the membrane.

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

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

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.

The cavity structure for docking the K(+)-competitive inhibitors in the gastric proton pump

2-Methyl-8-(phenylmethoxy)imidazo[1,2-a]pyridine-3-acetonitrile (SCH 28080) is a reversible inhibitor specific for the gastric proton pump. The inhibition pattern is competitive with K(+). Here we studied the binding sites of this inhibitor on the putative three-dimensional structure of the gastric proton pump alpha-subunit that was constructed by homology modeling based on the structure of sarcoplasmic reticulum Ca(2+) pump. Alanine and serine mutants of Tyr(801) located in the fifth transmembrane segment of the gastric proton pump alpha-subunit retained the (86)Rb transport and K(+)-dependent ATPase (K(+)-ATPase) activities. These mutants showed 60-80-times lower sensitivity to SCH 28080 than the wild type in the (86)Rb transport activity. The K(+)-ATPase activities of these mutants were not completely inhibited by SCH 28080. The sensitivity to SCH 28080 was dependent on the bulkiness of the side chain at this position. Therefore, the side chain of Tyr(801) is important for the interaction with this inhibitor. In the three-dimensional structure of the E(2) form (conformation with high affinity for K(+)) of the gastric proton pump, Tyr(801) faces a cavity surrounded by the first, fourth, fifth, sixth, and eighth transmembrane segments and fifth/sixth, seventh/eighth, and ninth/tenth loops. SCH 28080 can dock in this cavity. However, SCH 28080 cannot dock in the same location in the E(1) form (conformation with high affinity for proton) of the gastric proton pump due to the drastic rearrangement of the transmembrane helices between the E(1) and E(2) forms. These results support the idea that this cavity is the binding pocket of SCH 28080.

Quantity and quality control of gastric proton pump in the endoplasmic reticulum by ubiquitin/proteasome system

The gastric proton pump, H(+),K(+)-ATPase, consists of the catalytic alpha-subunit and the noncatalytic beta-subunit. These subunits are assembled in the endoplasmic reticulum (ER) and leave the ER to reach to the cell surface as a functional holoenzyme. We studied the quantity control mechanism of the H(+),K(+)-ATPase in the ER by using a heterologous expression system in human embryonic kidney 293 cells. The alpha-subunit in the alpha-expressing cells was degraded more rapidly than in the alpha+beta-expressing cells. It was stabilized, however, in the presence of a proteasome inhibitor, lactacystin. Polyubiquitination of the alpha-subunit was observed in the alpha-expressing cells as well as in the alpha+beta-expressing cells. The extent of polyubiquitination was higher in the former alpha-expressing cells especially in the presence of lactacystin. On the other hand, polyubiquitination of the beta-subunit was not observed in the absence and presence of lactacystin. When the alpha-subunit was coexpressed with a mutant beta-subunit that lacks alpha/beta assembly capacity, degradation of the alpha-subunit was accelerated in parallel with increased polyubiquitination of the alpha-subunit. These results indicate that the ubiquitin/proteasome system is involved in degradation of the unassembled alpha-subunits in the ER to control the cell surface expression of the functional alpha/beta holoenzymes.

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.

Preparation of Na,K-ATPase specifically modified on the anti-fluorescein antibody-inaccessible site by fluorescein 5'-isothiocyanate

Specific labeling is required for energy transfer measurements and to avoid artifacts in the use of fluorophores as reporter groups. Therefore, a method for specific modification by one of the most popular reagents for P-type ATPases (fluorescein 5'-isothiocyanate) has been developed. Sulfhydryl reagents protected against modification of cysteine residues, and treatment with dithiothreitol eliminated a slow doubling of the fluorescence of conventionally modified Na,K-ATPase upon dilution that is attributed to disappearance of self-energy transfer. Removal of nonspecifically bound fluorescein was also confirmed by titration of the modified Na, K-ATPase with anti-fluorescein antibody and by time resolution of the fluorescence change when the modified enzyme was mixed with Na(+) in a stopped-flow instrument. The only fluorescence change when specifically modified Na,K-ATPase was mixed with Na(+) was the signal from fluorescein at the antibody-inaccessible, substrate-protectable site that reports the conformational change in unphosphorylated enzyme. The magnitude of the fluorescence change reporting the conformational change increased from between 8 and 12% to between 25 and 30% without affecting the kinetic constants estimated from titrations with Na(+) and K(+). The method should be generally applicable to the preparation of specifically labeled P-type pumps for use in kinetic and equilibrium titrations or energy transfer measurements.

Chimeric domain analysis of the compatibility between H(+), K(+)-ATPase and Na(+),K(+)-ATPase beta-subunits for the functional expression of gastric H(+),K(+)-ATPase

Gastric H(+),K(+)-ATPase consists of alpha-subunit with 10 transmembrane domains and beta-subunit with a single transmembrane domain. We constructed cDNAs encoding chimeric beta-subunits between the gastric H(+),K(+)-ATPase and Na(+),K(+)-ATPase beta-subunits and co-transfected them with the H(+),K(+)-ATPase alpha-subunit cDNA in HEK-293 cells. A chimeric beta-subunit that consists of the cytoplasmic plus transmembrane domains of Na(+),K(+)-ATPase beta-subunit and the ectodomain of H(+),K(+)-ATPase beta-subunit assembled with the H(+),K(+)-ATPase alpha-subunit and expressed the K(+)-ATPase activity. Therefore, the whole cytoplasmic and transmembrane domains of H(+),K(+)-ATPase beta-subunit were replaced by those of Na(+),K(+)-ATPase beta-subunit without losing the enzyme activity. However, most parts of the ectodomain of H(+),K(+)-ATPase beta-subunit were not replaced by the corresponding domains of Na(+), K(+)-ATPase beta-subunit. Interestingly, the extracellular segment between Cys(152) and Cys(178), which contains the second disulfide bond, was exchangeable between H(+),K(+)-ATPase and Na(+), K(+)-ATPase, preserving the K(+)-ATPase activity intact. Furthermore, the K(+)-ATPase activity was preserved when the N-terminal first 4 amino acids ((67)DPYT(70)) in the ectodomain of H(+),K(+)-ATPase beta-subunit were replaced by the corresponding amino acids ((63)SDFE(66)) of Na(+),K(+)-ATPase beta-subunit. The ATPase activity was abolished, however, when 4 amino acids ((76)QLKS(79)) in the ectodomain of H(+),K(+)-ATPase beta-subunit were replaced by the counterpart ((72)RVAP(75)) of Na(+),K(+)-ATPase beta-subunit, indicating that this region is the most N-terminal one that discriminates the H(+),K(+)-ATPase beta-subunit from that of Na(+), K(+)-ATPase.

Cyanide suppression of inwardly rectifying K+ channels in guinea pig chromaffin cells involves dephosphorylation

Treatment of chromaffin cells with cyanide induced a gradual decrease in an inwardly rectifying K+ current (IIR), and washout of the mitochondrial inhibitor resulted in a rapid recovery of IIR. This diminution of IIR was reversed in a time-dependent manner by infusion of ATP or UTP, but not by that of GTP, ITP, or CTP. The restoration by ATP was not altered by addition to the pipette solution of 50 microM fluorescein 5-isothiocyanate, an inhibitor of various ATPases. A similar recovery of IIR occurred with injection of adenosine 5'-O-(3-thiotriphosphate) (ATP gamma S), but not of 5'-adenylylimidodiphosphate or alpha,beta-methyleneadenosine 5'-triphosphate. The ATP gamma S effect was biphasic, resulting in first a run-up of the current in ATP-depleted cells followed by a rundown of the current. This rundown was almost abolished by addition of guanosine 5'-O-(2-thiodiphosphate) to the ATP gamma S solution, suggesting the involvement of a G protein. Bath application of the protein kinase inhibitor 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine at 100 microM, but not N-(2-[methylamino]-ethyl)-5-isoquinolinesulfonamide, induced a reversible inhibition of IIR in the presence of pipette ATP, and the inhibition was diminished by 1 microM calyculin A, a phosphatase inhibitor. Bath application of 1 microM phorbol 12,13-dibutyrate did not affect IIR. It is concluded that cyanide suppresses inward rectifier K+ channel activity via dephosphorylation and that protein kinase C, adenosine 3',5'-cyclic monophosphate-dependent kinase, or guanosine 3',5'-cyclic monophosphate-dependent kinase is not involved in modulation of the channel.

Involvement of the M7/M8 extracellular loop of the sodium pump alpha subunit in ion transport. Structural and functional homology to P-loops of ion channels

Mutations were introduced in the motif 884DDRW887 from an extracellular peptide of the sodium pump alpha subunit localized between M7 and M8 membrane spans to investigate a possible role of this structure in ion recognition. A homologous sequence 399QDCW402 that occurs in the P-loops of Na+ channels was shown earlier to be important for ion gating. Mutant sodium pumps were expressed in yeast and subsequently investigated for their behavior toward ouabain, Na+, K+, and ATP. Native enzyme and D884A, D884R, D885A, D885E, or D885R mutants all bind ouabain in the presence of phosphate and Mg2+. The KD values determined from Scatchard analysis are in the range 5-8 nM for the native enzyme and the D884A, D885E, or D885A mutants, and 15.7 +/- 2.04 and 30.1 +/- 4.32 nM for mutants D884R and D885R, respectively. This ouabain binding is reduced in the presence of K+ in a similar way for both native or mutant sodium pumps with relative affinities (K0.5) for K+ ranging from 1.4 to 3.7 mM. Ouabain binding in the presence of 100 microM ATP is promoted by Na+ with K0.5 = 1.64 +/- 0.01 mM for the native enzyme and K0.5 = 8. 6 +/- 1.35 mM for the D884R mutant. The K0.5 values of the two enzymes for ATP are 0.66 +/- 0.16 microM and 1.1 +/- 0.12 microM, respectively. Ouabain binding as a function of Na+ concentration, on the other hand, is very low for the D885R mutant, even at an ATP concentration of 2 mM. Phosphate or eosin, however, are recognized by this mutant enzyme, so that a major conformational change within the ATP-binding site appears unlikely. The inability of the D885R mutant to bind ouabain in the presence of Na+ and ATP could be explained by assuming that the M7/M8 connecting extracellular loop, which also contains the mutated amino acids, is invaginated within the plane of the plasma membrane and possibly involved in acceptance and/or release of Na+ ions coming from cytosolic areas of the protein. In this case, the placement of an additional positive charge might repel Na+ ions and interrupt their flow, thus not allowing the enzyme to assume the proper conformational state for ouabain binding. Such invaginated hydrophilic protein structures, such as the P-loops of Na+ and K+ channels, are already known and have been shown to participate in ion conduction.

Biochemical properties and cytochemical localization of ouabain-insensitive, potassium-dependent p-nitrophenylphosphatase activity in rat atrial myocytes

Enzyme activity that represents ouabain-insensitive, potassium-dependent p-nitrophenylphosphatase (p-NPPase) was assessed in rat atrial myocytes by biochemical and cytochemical procedures. No activity was detected in parallel experiments with ventricular myocytes. Fixed tissues were incubated in a reaction medium containing Tricine buffer, p-nitrophenylphosphate (p-NPP), KCl, MgCl2, CaCl2, CeCl3. Triton X-100, levamisole, and ouabain. Final pH was adjusted to 7.5. Biochemical studies showed that accumulation of p-nitrophenol in the medium was increased proportionally in accordance with the amount of incubated tissue. This activity was optimal with incubation at pH 7.5 and in the presence of KCl. Approximately 70% of the enzyme was inhibited by 2 mM CeCl3. Electron microscopic observations revealed reaction product (RP) at sites of ouabain-insensitive, potassium-dependent p-NPPase activity as electron-dense precipitate localized at the inner surface of the plasma membrane and at the T-tubules of atrial myocytes. Control experiments indicated that the activity was strongly inhibited by sodium orthovanadate and was repressed by omeprazole and 1,3-dicyclohexylcarbodiimide. X-ray microanalysis confirmed the presence of cerium within the cytochemical RP. The ouabain-insensitive, K-dependent p-NPPase activity detected in the present study is considered to be an isoform of a P-type, H-transporting, K-dependent adenosine triphosphatase (H,K-ATPase).

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.

Is cysteine residue important in FITC-sensitive ATP-binding site of P-type ATPases? A commentary to the state of the art

Treatment of P-type ATPases (from mammalian sources) by fluorescein isothiocyanate (ITC) revealed the ITC label on a lysine residue that was than considered as essential for binding of ATP in the ATP-binding site of these enzymes. On the other hand, experiments with site directed mutagenesis excluded the presence of an essential Iysine residue that would be localized in the ATP binding sites of ATPases. Other previous studies, including those of ourselves, indicated that the primary site of isothiocyanate interaction may be the sulfhydryl group of a cysteine residue and this may be essential for binding of ATP. In addition considerable knowledge accumulated since yet also about the differences in stability of reaction product of isothiocyanates with SH- or NH2- groups. Based upon evaluation of the data available up to now, in present paper the following tentative roles for lysine and cysteine residues located in the ATP-binding site of P-type ATPases are proposed: The positively charged micro-domain of the lysine residue may probably attract the negatively charged phosphate moiety of the ATP molecule whereas the cysteine residue may probably be responsible for recognition and binding of ATP by creation of a proton bridge with the amino group in position 6 on the adenosine ring of ATP.

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.

Genetic analysis of the fluorescein isothiocyanate binding site of the yeast plasma membrane H(+)-ATPase

The highly conserved motif of Saccharomyces cerevisiae H(+)-ATPase 474KGAP has been proposed to participate in the formation of the phosphorylated intermediate during the catalytic cycle (Portillo, F., and Serrano, R. (1988) EMBO J. 7, 1793-1798). In addition, Lys-474 is the FITC binding site of the yeast enzyme (Portillo, F. and Serrano, R. (1989) Eur. J. Biochem. 186, 501-507). We have performed an intragenic suppressor analysis of the K474R mutation to identify the interacting regions involved in these functions. Random in vitro mutagenesis of the K474R allele resulted in seven suppressor (second-site) mutations. One mutation (V396I), located 18 residues away from the Asp-378 residue, which is phosphorylated during catalysis, is allele-specific. This provides genetic evidence of a direct interaction between the KGAP motif and the phosphorylation domain during the catalytic cycle. Three mutations (V484I, V484I/E485K, and E485K/E486K) are located near Lys-474 and may compense the structural alteration introduced by the K474R mutation. Two substitutions at the end of the predicted transmembrane stretch 2 (A165V and V169I/D170N) and another in the predicted ATP binding domain (P536L) may act as allele-nonspecific suppressors, as they are also able to suppress a mutation at the enzyme's carboxyl terminus.

The beta-aspartyl phosphate intermediate in a Leishmania donovani promastigote plasma membrane P-type ATPase

The phosphorylated intermediate of a plasma membrane P-type ATPase in Leishmania donovani has been further characterized. The formation of the phosphorylated intermediate is sensitive to several ATPase inhibitors including vanadate, dicyclohexyl carbodiimide (DCCD), N-ethylmaleimide (NEM), and fluorescein isothiocyanate (FITC). These inhibitors affect purified immunoprecipitated protein as well as total plasma membrane fractions. Oligomycin, an inhibitor of mitochondrial ATPases, and ouabain, an inhibitor of Na+/K(+)-ATPases, had no effect on the formation of the phosphorylated intermediate. The ATPase phosphoprotein was acid stable and dephosphorylated at alkaline pH, indicating the presence of the acyl phosphate chemical linkage. Analysis of the phosphorylated amino acid by reduction with sodium boro[3H]hydride identified the residue as aspartate, confirming the formation of a beta-aspartyl phosphate intermediate. These data indicate the presence of a 105 kDa P-type ATPase on L. donovani plasma membrane that is mechanistically similar to to other P-type enzymes of higher eukaryotes.

Chemical modification of the Ca(2+)-ATPase of rabbit skeletal muscle sarcoplasmic reticulum: identification of sites labeled with aryl isothiocyanates and thiol-directed conformational probes

The Ca(2+)-ATPase protein of rabbit skeletal muscle sarcoplasmic reticulum is a single polypeptide chain of 1001 amino-acid residues. Among these residues are 24 Cys, 9 of which have previously been shown to be accessible to one or more thiol-specific reagents. Many studies on the structure and function of this Ca(2+)-ATPase have made use of sulfhydryl-directed, conformationally-sensitive probes, but the labeling sites for these probes have been directly identified in only a few cases, causing uncertainty in the interpretation of results. In the present work, we have investigated the Ca(2+)-ATPase labeling sites for three thiol-directed spectroscopic probes: fluorescein 5'-maleimide (Fmal), 4-dimethylaminophenyl-azo phenyl-4'-maleimide (DABmal), and 4-dimethylaminophenylazophenyl-4'-iodoacetamide (DABIA). Labeled Ca(2+)-ATPase was digested exhaustively with trypsin, and labeled peptides were purified and sequenced in order to identify the labeled Cys residues. Our results do not support the widely held assumptions that Cys-344 and Cys-364 are the most reactive residues with maleimide-based reagents, while Cys-670 and Cys-674 react most rapidly with iodoacetamide derivatives. We found instead that Fmal reacted most rapidly with Cys-471, followed by Cys-364, and more slowly with Cys-498, -525, -614 and -636. DABmal reacted most rapidly with Cys-364, followed by Cys-614, and more slowly with Cys-471, -498, -636 and -670. Cys-344 was not labeled by either Fmal or DABmal. DABIA reacted with the same six Cys residues, including Cys-670, as were labeled with DABmal, but in much lower yield. There was no evidence for labeling of Cys-674 with DABIA. The high reactivity of Fmal, but not the more hydrophobic DABmal, with Cys-471 is of interest because of previous studies suggesting that the accessibility of Cys-471 is influenced by ATP and that fluorescein derivatives bind to a hydrophobic pocket in the ATP binding site. Another derivative, fluorescein-5'-isothiocyanate (FITC), is thought to label the catalytic site of the Ca(2+)-ATPase and has been widely used as a conformational probe in structure-function studies on this and related proteins. We reinvestigated the chemical modification of the Ca(2+)-ATPase by FITC and 4-dimethyl-aminophenyl-4'-isothiocyanate (DABITC). Incorporation of stoichiometric amounts of FITC resulted in a nearly complete loss of ATPase activity. Labeling and inactivation of the Ca(2+)-ATPase by FITC did not occur in the presence of ATP. DABITC was less reactive than FITC, and did not inactivate the Ca(2+)-ATPase to any significant extent.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Simultaneous binding of phosphate and TNP-ADP to FITC-modified NA+,K(+)-ATPase

Double-reciprocal plots of the rate of ATP hydrolysis by Na+,K(+)-ATPase versus ATP concentration are not linear, and may reflect either two distinct binding sites for ATP or a single ATP binding site whose affinity for the nucleotide alternates between high-affinity and low-affinity states. In order to determine whether multiple nucleotides or nucleotide analogs can bind simultaneously to Na,+,K(+)-ATPase, the effects of nucleotides on the hydrolysis of p-nitrophenyl phosphate and on the dephosphorylation rate of Na+,K(+)-ATPase modified by fluorescein 5'-isothiocyanate (FITC) were measured. FITC blocks the high-affinity binding site for ATP on the Na+K(+)-ATPase and inhibits ATP hydrolysis at ATP concentrations as high as 8.3 mM. The hydrolysis of p-nitrophenyl phosphate and phosphoenzyme formation from inorganic phosphate and Mg2+ were not affected by FITC modification. The p-nitrophenylphosphatase activity of unmodified Na+,K(+)-ATPase was stimulated by low concentrations of ATP (10-100 microM) and other nucleotides, and was inhibited at higher nucleotide concentrations. In contrast, there was no effect on p-nitrophenyl phosphate hydrolysis by FITC-modified Na,K(+)-ATPase at ATP concentrations less than 100 microM. The hydrolysis of p-nitrophenyl phosphate by FITC-modified Na+,K(+)-ATPase was inhibited at ATP concentrations greater than 100 microM. These observations demonstrate that the effects of ATP acting at high-affinity sites are absent in FITC-modified Na+,K(+)-ATPase but the effects of ATP acting at low-affinity sites are still observed. In unmodified Na+,K(+)-ATPase, the rate of dephosphorylation of the phosphoenzyme formed from inorganic phosphate and Mg2+ was inhibited by ATP.(ABSTRACT TRUNCATED AT 250 WORDS)

Site-directed antibodies as topographical probes of the gastric H,K-ATPase alpha-subunit

Gastric acid is secreted by an ATP-driven H+ and K+ exchanger (H,K-ATPase), an integral apical membrane protein of parietal cells. Although the primary structure of the enzyme is known, its higher order structure is uncertain. In order to acquire topographical probes of native, microsomal H,K-ATPase, synthetic peptides corresponding to the 17 amino-terminal (N-peptide) and 16 carboxyl-terminal (C-peptide) residues of pig gastric H,K-ATPase alpha-subunit were coupled to keyhole limpet hemocyanin (KLH). Rabbits were immunized with peptide-KLH conjugates and their sera were tested for specificity by enzyme-linked immunosorbent assay (ELISA), immunoblotting, and immunocytochemistry. All sera showed high ELISA reactivities with synthetic peptides, peptide-BSA conjugates, and microsomal H,K-ATPase adsorbed to microtiter wells (some titers greater than 1:10(4)). Immunoblots of H,K-ATPase resolved by SDS-PAGE showed both N-peptide and C-peptide antibodies reacting with a single 94 kDa band. All sera selectively stained parietal cells in pig gastric mucosal sections. Preimmune sera gave negative or weak signals in all assays. In competition ELISAs, N-peptide antibodies, but not C-peptide antibodies, were displaced from the corresponding bound synthetic peptides by added microsomal H,K-ATPase. One of the N-peptide antibodies inhibited H,K-ATPase activity by more than 50%; binding of this antibody was decreased when ATP or K+ were bound to the enzyme. These results indicate a cytoplasmically-oriented alpha-subunit N-terminus which may participate conformationally in the H,K-ATPase catalytic cycle, and suggest that antibodies against synthetic H,K-ATPase peptides are potentially useful probes of native microsomal H,K-ATPase topography.

Characterization of the P-type and V-type ATPases of cholinergic synaptic vesicles and coupling of nucleotide hydrolysis to acetylcholine transport

Both phosphointermediate- and vacuolar-type (P- and V-type, respectively) ATPase activities found in cholinergic synaptic vesicles isolated from electric organ are immunoprecipitated by a monoclonal antibody to the SV2 epitope characteristic of synaptic vesicles. The two activities can be distinguished by assay in the absence and presence of vanadate, an inhibitor of the P-type ATPase. Each ATPase has two overlapping activity maxima between pH 5.5 and 9.5 and is inhibited by fluoride and fluorescein isothiocyanate. The P-type ATPase hydrolyzes ATP and dATP best among common nucleotides, and activity is supported well by Mg2+, Mn2+, or Co2+ but not by Ca2+, Cd2+, or Zn2+. It is stimulated by hyposmotic lysis, detergent solubilization, and some mitochondrial uncouplers. Kinetic analysis revealed two Michaelis constants for MgATP of 28 microM and 3.1 mM, and the native enzyme is proposed to be a dimer of 110-kDa subunits. The V-type ATPase hydrolyzes all common nucleoside triphosphates, and Mg2+, Ca2+, Cd2+, Mn2+, and Zn2+ all support activity effectively. Active transport of acetylcholine (ACh) also is supported by various nucleoside triphosphates in the presence of Ca2+ or Mg2+, and the Km for MgATP is 170 microM. The V-type ATPase is stimulated by mitochondrial uncouplers, but only at concentrations significantly above those required to inhibit ACh active uptake. Kinetic analysis of the V-type ATPase revealed two Michaelis constants for MgATP of approximately 26 microM and 2.0 mM. The V-type ATPase and ACh active transport were inhibited by 84 and 160 pmol of bafilomycin A1/mg of vesicle protein, respectively, from which it is estimated that only one or two V-type ATPase proton pumps are present per synaptic vesicle. The presence of presumably contaminating Na+,K(+)-ATPase in the synaptic vesicle preparation is demonstrated.

Gastric H,K-ATPase topography: amino acids 888-907 are cytoplasmic

Gastric acidification is mediated by H,K-ATPase, an integral protein of apical membranes of gastric parietal cells. Hydropathy analysis of H,K-ATPase alpha subunit primary structure predicts eight transmembrane (TM) domains, while omeprazole-binding data were interpreted in terms of ten TM domains (Mercier et al. (1991) FASEB J. 5, A749). In the present study, tryptic hydrolysis of gastric mucosal microsomes gave a set of peptides which bound the monoclonal antibody HK 12.18, a highly specific probe of the H,K-ATPase. An antiserum against the C-terminus of H,K-ATPase alpha subunit bound the same peptides, and one smaller peptide. The binding data suggested a putative epitope for HK 12.18, and a 20-mer peptide encompassing this site was synthesized. This peptide bound directly to HK 12.18, displaced HK 12.18 from microsomal H,K-ATPase, and blocked HK 12.18 immunostaining of gastric parietal cells. In addition, intact gastric microsomes competitively inhibited binding of HK 12.18 to peptide-BSA conjugate. Taken together, these data place the HK 12.18 epitope between amino acids 888-907 and identify this domain as cytosolic. This result specifically excludes a pair of TM domains between the sixth and seventh TM alpha helices of the H,K-ATPase and supports a secondary structure model with eight TM domains.

The parietal cell autoantigens recognized in neonatal thymectomy-induced murine gastritis are the alpha and beta subunits of the gastric proton pump [corrected]

Murine autoimmune gastritis, induced by neonatal thymectomy, bears a striking similarity in pathology to the human autoimmune disease, pernicious anemia. Autoantibodies to parietal cells are found in both murine and human diseases. Monoclonal immunoglobulin G autoantibodies, obtained from neonatally thymectomized mice, have previously been shown to recognize two groups of gastric parietal cell antigens. In the present study, it is shown that two of these monoclonal autoantibodies, designated 1H9 and 2B6, are directed against the alpha subunit and beta subunit, respectively, of the gastric hydrogen-potassium-stimulated adenosine triphosphatase (H+,K(+)-ATPase; proton pump). Monoclonal antibody 1H9 showed reactivity by immunoblotting with a 95-kilodalton component of dog gastric tubulovesicular membranes and with a fusion protein containing the hydrophilic domain of the alpha subunit of the H+,K(+)-ATPase. Monoclonal antibody 2B6 reacted by immunoblotting with the 60-90-kilodalton glycoprotein (beta subunit) of the tomato lectin-purified dog H+,K(+)-ATPase and with the 60-90-kilodalton autoantigen purified with human parietal cell autoantibodies. Monoclonal antibody 2B6 also reacted with the deglycosylated 35-kilodalton core protein of the tomato lectin-purified 60-90-kilodalton beta subunit and of the purified 60-90-kilodalton autoantigen. Parietal cell autoantibody-positive sera from 20 mice with experimentally induced gastritis showed reactivity predominantly with the alpha and/or beta subunit of the gastric H+,K(+)-ATPase. Therefore, it is concluded that the major molecules targeted by parietal cell autoantibodies from mice with neonatal thymectomy-induced murine autoimmune gastritis and from humans with pernicious anemia are identical.

Temperature dependence of the rates of conformational changes reported by fluorescein 5'-isothiocyanate modification of H+,K(+)- and Na+,K(+)-ATPases

Stopped-flow fluorometry has been used to measure the forward and reverse rates of the conformational change from E1 to E2 in the fluorescein-modified proton and sodium pumps (1) as a function of Na+ and K+ concentrations to verify the proposed mechanism of ion interaction with the enzymes and (2) as a function of temperature to gain insight into the nature of the conformational transition. (1) The fluorescence changes caused by Na+ and K+ are consistent with rapid competitive binding of the two ions to the E1 conformations of the enzymes followed by rate-limiting transitions between E1K and E2K. (2) Reaction coordinate diagrams for the E1K to E2K transitions in the H,K-ATPase and Na,K-ATPase are qualitatively similar. Enthalpy barriers to reaction are partially compensated by increased entropy in the transition states. However, there are striking quantitative differences between the two enzymes. The E2K to E1K reaction of the H,K-ATPase is more than 2 orders of magnitude faster (tau 1/2 = 6 ms at 22 degrees C) than the reverse rate of the Na,K-ATPase transition (tau 1/2 = 1.6 s), explaining repeated failure to detect a K(+)-"occluded" form of the H,K-enzyme. The E2K conformer of the Na,K-ATPase is 3 orders of magnitude more stable than E1K, while the E1K and E2K conformations of the H,K-ATPase are nearly equivalent energetically.

Gastric H+,K(+)-ATPase as a therapeutic target in peptic ulcer disease

The presence of unbuffered acid appears to be an essential contributory factor in the pathogenesis of peptic ulcer disease. Treatment has concentrated therefore on the reduction of acidity, and the last decade has seen the widespread and effective use of H2 antagonists. They are, at low doses, more successful in improving the natural history of duodenal ulcer disease than of gastric or esophageal ulceration. The H2 receptor plays a central role in activation of parietal cell acid secretion, and antagonists at this receptor block most (but not all) of the acid secretion due to even gastrinergic or muscarinic (vagal) stimulation. In hypergastrinemic states such as Zollinger-Ellison syndrome, or where acid secretion has to be inhibited by more than 20% over a 24-hr period, such as for treatment of esophagitis, NSAID damage, or gastric ulcers, the dose and frequency of administration of the currently available antagonists must be increased to achieve reliable therapy. This has led to a search for an alternative target for acid inhibitory drugs, such as the gastric acid pump, the H+,K(+)-ATPase. This article focuses on the function of this ATPase and suggests that inhibition of this pump will provide a more efficacious means of reduction of acid secretion by the stomach, hence improving and simplifying therapy of acid related diseases.

Reversible inhibition of sodium and potassium-dependent adenosine triphosphatase by the pyridine derivative, AU-1421 during turnover cycle

A novel pyridine derivative, (Z)-5-methyl-2-[2-(1-naphthyl)ethenyl]-4-piperidonopyridine hydrochloride, AU-1421, was found to produce reversible inhibition of the dog kidney sodium and potassium ion-dependent adenosine triphosphatase [(Na,K)-ATPase] with I50 values of about 50 microM. The reversible inhibition was observed when the enzyme was added directly to the enzyme assay media in the presence of saturating concentrations of the enzyme ligands, Na+, K+, Mg2+ and ATP ("turnover conditions"). In the present study, we focused on the reversible inhibition without preincubation of the enzyme with AU-1421. This inhibition was competitive with respect to K+. The K(+)-pNPPase activity of the same preparation was also inhibited by AU-1421 with I50 values of about 90 microM, and this manner was also competitive with respect to K+. ATP enhanced the AU-1421 inhibition of (Na,K)-ATPase, suggesting that AU-1421 also bound to the enzyme-substrate complex. AU-1421 inhibition of (Na,K)-ATPase was not antagonized by ouabain, suggesting the difference of the binding sites between AU-1421 and ouabain. It is therefore proposed that AU-1421 reversibly interacts at or near the K+ site during turnover conditions.