Oligomeric regulation of gastric H+,K+-ATPase

Involvement of the H3O+-Lys-164 -Gln-161-Glu-345 charge transfer pathway in proton transport of gastric H+,K+-ATPase

Gastric H(+),K(+)-ATPase is shown to transport 2 mol of H(+)/mol of ATP hydrolysis in isolated hog gastric vesicles. We studied whether the H(+) transport mechanism is due to charge transfer and/or transfer of hydronium ion (H(3)O(+)). From transport of [(18)O]H(2)O, 1.8 mol of water molecule/mol of ATP hydrolysis was found to be transported. We performed a molecular dynamics simulation of the three-dimensional structure model of the H(+),K(+)-ATPase alpha-subunit at E(1) conformation. It predicts the presence of a charge transfer pathway from hydronium ion in cytosolic medium to Glu-345 in cation binding site 2 (H(3)O(+)-Lys-164 -Gln-161-Glu-345). No charge transport pathway was formed in mutant Q161L, E345L, and E345D. Alternative pathways (H(3)O(+)-Gln-161-Glu-345) in mutant K164L and (H(3)O(+)-Arg-105-Gln-161-Gln-345) in mutant E345Q were formed. The H(+),K(+)-ATPase activity in these mutants reflected the presence and absence of charge transfer pathways. We also found charge transfer from sites 2 to 1 via a water wire and a charge transfer pathway (H(3)O(+)-Asn-794 -Glu-797). These results suggest that protons are charge-transferred from the cytosolic side to H(2)O in sites 2 and 1, the H(2)O comes from cytosolic medium, and H(3)O(+) in the sites are transported into lumen during the conformational transition from E(1)PtoE(2)P.

Self-association of isolated large cytoplasmic domain of plasma membrane H+ -ATPase from Saccharomyces cerevisiae: role of the phosphorylation domain in a general dimeric model for P-ATPases

Large cytoplasmic domain (LCD) plasma membrane H+ -ATPase from S. cerevisiae was expressed as two fusion polypeptides in E. coli: a DNA sequence coding for Leu353-Ileu674 (LCDh), comprising both nucleotide (N) and phosphorylation (P) domains, and a DNA sequence coding for Leu353-Thr543 (LCDDeltah, lacking the C-terminus of P domain), were inserted in expression vectors pDEST-17, yielding the respective recombinant plasmids. Overexpressed fusion polypeptides were solubilized with 6 M urea and purified on affinity columns, and urea was removed by dialysis. Their predicted secondary structure contents were confirmed by CD spectra. In addition, both recombinant polypeptides exhibited high-affinity 2',3'-O-(2,4,6-trinitrophenyl)adenosine-5'-triphosphate (TNP-ATP) binding (Kd = 1.9 microM and 2.9 microM for LCDh and LCDDeltah, respectively), suggesting that they have native-like folding. The gel filtration profile (HPLC) of purified LCDh showed two main peaks, with molecular weights of 95 kDa and 39 kDa, compatible with dimeric and monomeric forms, respectively. However, a single elution peak was observed for purified LCDDeltah, with an estimated molecular weight of 29 kDa, as expected for a monomer. Together, these data suggest that LCDh exist in monomer-dimer equilibrium, and that the C-terminus of P domain is necessary for self-association. We propose that such association is due to interaction between vicinal P domains, which may be of functional relevance for H+ -ATPase in native membranes. We discuss a general dimeric model for P-ATPases with interacting P domains, based on published crystallography and cryo-electron microscopy evidence.

Characterization of the inhibitory activity of tenatoprazole on the gastric H+,K+ -ATPase in vitro and in vivo

Tenatoprazole is a prodrug of the proton pump inhibitor (PPI) class, which is converted to the active sulfenamide or sulfenic acid by acid in the secretory canaliculus of the stimulated parietal cell of the stomach. This active species binds to luminally accessible cysteines of the gastric H+,K+ -ATPase resulting in disulfide formation and acid secretion inhibition. Tenatoprazole binds at the catalytic subunit of the gastric acid pump with a stoichiometry of 2.6 nmol mg(-1) of the enzyme in vitro. In vivo, maximum binding of tenatoprazole was 2.9 nmol mg(-1) of the enzyme at 2 h after IV administration. The binding sites of tenatoprazole were in the TM5/6 region at Cys813 and Cys822 as shown by tryptic and thermolysin digestion of the ATPase labeled by tenatoprazole. Decay of tenatoprazole binding on the gastric H+,K+ -ATPase consisted of two components. One was relatively fast, with a half-life 3.9 h due to reversal of binding at cysteine 813, and the other was a plateau phase corresponding to ATPase turnover reflecting binding at cysteine 822 that also results in sustained inhibition in the presence of reducing agents in vitro. The stability of inhibition and the long plasma half-life of tenatoprazole should result in prolonged inhibition of acid secretion as compared to omeprazole. Further, the bioavailability of tenatoprazole was two-fold greater in the (S)-tenatoprazole sodium salt hydrate form as compared to the free form in dogs which is due to differences in the crystal structure and hydrophobic nature of the two forms.

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.

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.

Specific interaction between GroEL and denatured protein measured by compression-free force spectroscopy

We investigated the interaction between GroEL and a denatured protein from a mechanical point of view using an atomic force microscope. Pepsin was bound to an atomic force microscope probe and used at a neutral pH as an example of denatured proteins. To measure a specific and delicate interaction force, we obtained force curves without pressing the probe onto GroEL molecules spread on a mica surface. Approximately 40 pN of tensile force was observed for approximately 10 nm while pepsin was pulled away from the chaperonin after a brief contact. This length of force duration corresponding to the circumference of GroEL's interior cavity was shortened by the addition of ATP. The relation between the observed mechanical parameters and the chaperonin's refolding function is discussed.

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.

Alanine-scanning mutagenesis of the sixth transmembrane segment of gastric H+,K+-ATPase alpha-subunit

The sixth transmembrane (M6) segment of the catalytic subunit plays an important role in the ion recognition and transport in the type II P-type ATPase families. In this study, we singly mutated all amino acid residues in the M6 segment of gastric H(+),K(+)-ATPase alpha-subunit with alanine, expressed the mutants in HEK-293 cells, and studied the effects of the mutation on the functions of H(+),K(+)-ATPase; overall K(+)-stimulated ATPase, phosphorylation, and dephosphorylation. Four mutants, L819A, D826A, I827A, and L833A, completely lost the K(+)-ATPase activity. Mutant L819A was phosphorylated but hardly dephosphorylated in the presence of K(+), whereas mutants D826A, I827A, and L833A were not phosphorylated from ATP. We found that almost all of these amino acid residues, which are important for the function, are located on the same side of the alpha-helix of the M6 segment. In addition, we found that amino acids involved in the phosphorylation are located exclusively in the cytoplasmic half of the M6 segment and those involved in the K(+)-dependent dephosphorylation are in the luminal half. Several mutants such as I821A, L823A, T825A, and P829A partly retained the K(+)-ATPase activity accompanying the decrease in the rate of phosphorylation.

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).

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.

The nongastric H+-K+-ATPases: molecular and functional properties

The Na-K/H-K-ATPase gene family is divided in three subgroups including the Na-K-ATPases, mainly involved in whole body and cellular ion homeostasis, the gastric H-K-ATPase involved in gastric fluid acidification, and the newly described nongastric H-K-ATPases for which the identification of physiological roles is still in its infancy. The first member of this last subfamily was first identified in 1992, rapidly followed by the molecular cloning of several other members. The relationship between each member remains unclear. The functional properties of these H-K-ATPases have been studied after their ex vivo expression in various functional expression systems, including the Xenopus laevis oocyte, the insect Sf9 cell line, and the human HEK 293 cells. All these H-K-ATPase alpha-subunits appear to encode H-K-ATPases when exogenously expressed in such expression systems. Recent data suggest that these H-K-ATPases could also transport Na+ in exchange for K+, revealing a complex cation transport selectivity. Moreover, they display a unique pharmacological profile compared with the canonical Na-K-ATPases or the gastric H-K-ATPase. In addition to their molecular and functional characterizations, a major goal is to correlate the molecular expression of these cloned H-K-ATPases with the native K-ATPases activities described in vivo. This appears to be more complex than anticipated. The discrepancies between the functional data obtained by exogenous expression of the nongastric H-K-ATPases and the physiological data obtained in native organs could have several explanations as discussed in the present review. Extensive studies will be required in the future to better understand the physiological role of these H-K-ATPases, especially in disease processes including ionic or acid-base disorders.

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.

The phospholipid flippase activity of gastric vesicles

We found that isolated gastric vesicles contain a novel Mg2+-ATP-dependent phospholipid translocation (flippase) activity. Fluorescence analogue of phosphatidylcholine, 2-(12-(7-nitrobenz-2-oxa-1, 3-diazol-4-yl)amino)dodecanoyl-1-hexadecanoyl-sn-glycero-3- phosphocholine, was ATP-dependently translocated from the outer (cytosolic) to inner (luminal) leaflet of the lipid membrane bilayer of hog gastric vesicles. The translocation was saturable and depended on time and the ATP concentration (Km = 3.1 microM). The basal Mg2+-ATPase activity of gastric vesicles in the absence of K+ showed high (Km = 1.6 microM) and low (Km = 80 microM) affinities for ATP, indicating that the present flippase activity is driven mostly by the high affinity Mg2+-ATPase activity. It required Mg2+ but not K+. Verapamil, which is an inhibitor of mouse mdr2 phosphatidylcholine flippase, did not inhibit the present flippase activity. Isolated sarcoplasmic reticulum vesicles that contain Ca2+-ATPase did not show any flippase activity. Fluorescence analogues of phosphatidylserine and phosphatidylethanolamine were similarly translocated by the gastric flippase. These phospholipid flippase activities were inhibited by 2-methyl-8-(phenylmethoxy)imidazo[1,2-a]pyridine-3-acetonitrile (SCH 28080) (IC50 = 0.14-0.25 microM), a specific K+-ATPase inhibitor of gastric H+,K+-ATPase rich in gastric vesicles. IC50 value for the SCH 28080-inhibitable Mg2+-ATPase activity was about 0.13 microM, indicating that the phospholipid translocation was driven mostly by the SCH 28080-sensitive Mg2+-ATPase activity. Possible physiological roles of flippases were discussed in relation with the gastric acid secretory and cytoprotective mechanisms.