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

In situ measurement of pH in the secreting canaliculus of the gastric parietal cell and adjacent structures

The gastric H(+)/K(+)-ATPase is located within an infolding (secretory canaliculus) of the apical plasma membrane of gastric parietal cells. Our aim was to measure the pH values in the cytosol and canaliculus of the acid-secreting parietal cell and the adjacent gland lumen in situ. We used ultrafine double-barreled tip-sealed microelectrodes at high acceleration rates for intracellular and canalicular measurements. Immunohistochemical staining of the parietal cells was used to identify the track of the electrode and to estimate the position of the electrode tip at the time of the last intracellular measurement. En route to the deepest regions of the mucosa, where the average gland lumen pH was approximately 3, and on advancing in steps of 2 mum, the electrode entered the cytosol of the parietal cells, where the pH value was 7.4. Advancing the electrode further resulted, in several instances, in a sharp decrease in pH to an average value of 1.7 +/- 0.2, which we interpreted as the measurement within the canaliculus. When the electrode was advanced even further, the pH reading returned to the cytosolic value. From the difference in pH between the secreting canaliculus and the adjacent gland lumen, we concluded that the released acid was immediately buffered. Thus, the only cellular structure directly exposed to the highly acidic canalicular content is the apical membrane forming the canaliculus in the parietal cell.

The human non-gastric H,K-ATPase has a different cation specificity than the rat enzyme

The primary sequence of non-gastric H,K-ATPase differs much more between species than that of Na,K-ATPase or gastric H,K-ATPase. To investigate whether this causes species-dependent differences in enzymatic properties, we co-expressed the catalytic subunit of human non-gastric H,K-ATPase in Sf9 cells with the beta(1) subunit of rat Na,K-ATPase and compared its properties with those of the rat enzyme (Swarts et al., J. Biol. Chem. 280, 33115-33122, 2005). Maximal ATPase activity was obtained with NH(4)(+) as activating cation. The enzyme was also stimulated by Na(+), but in contrast to the rat enzyme, hardly by K(+). SCH 28080 inhibited the NH(4)(+)-stimulated activity of the human enzyme much more potently than that of the rat enzyme. The steady-state phosphorylation level of the human enzyme decreased with increasing pH, [K(+)], and [Na(+)] and nearly doubled in the presence of oligomycin. Oligomycin increased the sensitivity of the phosphorylated intermediate to ADP, demonstrating that it inhibited the conversion of E(1)P to E(2)P. All three cations stimulated the dephosphorylation rate dose-dependently. Our studies support a role of the human enzyme in H(+)/Na(+) and/or H(+)/NH(4)(+) transport but not in Na(+)/K(+) transport.

Conversion of the Low Affinity Ouabain-binding Site of Non-gastric H,K-ATPase into a High Affinity Binding Site by Substitution of Only Five Amino Acids

P-type ATPases of the IIC subfamily exhibit large differences in sensitivity toward ouabain. This allows a strategy in which ouabain-insensitive members of this subfamily are used as template for mutational elucidation of the ouabain-binding site. With this strategy, we recently identified seven amino acids in Na,K-ATPase that conferred high affinity ouabain binding to gastric H,K-ATPase (Qiu, L. Y., Krieger, E., Schaftenaar, G., Swarts, H. G. P., Willems, P. H. G. M., De Pont, J. J. H. H. M., and Koenderink, J. B. (2005) J. Biol. Chem. 280, 32349-32355). Because important, but identical, amino acids were not recognized in that study, here we used the non-gastric H,K-ATPase, which is rather ouabain-insensitive, as template. The catalytic subunit of this enzyme, in which several amino acids from Na,K-ATPase were incorporated, was expressed with the Na,K-ATPase beta1 subunit in Xenopus laevis oocytes. A chimera containing 14 amino acids, located in M4, M5, and M6, which are unique to Na,K-ATPase, displayed high affinity ouabain binding. Four of these residues, all located in M5, appeared dispensable for high affinity binding. Individual mutation of the remaining 10 residues to their non-gastric H,K-ATPase counterparts yielded five amino acids (Glu312,Gly319, Pro778, Leu795, and Cys802) whose mutation resulted in a loss of ouabain binding. In a final gain-of-function experiment, we introduced these five amino acids in different combinations in non-gastric H,K-ATPase and demonstrated that all five were essential for high affinity ouabain binding. The non-gastric H,K-ATPase with these five mutations had a similar apparent affinity for ouabain as the wild type Na,K-ATPase and showed a 2000 times increased affinity for ouabain in the NH4+-stimulated ATPase activity in membranes of transfected Sf9 cells.

Molecular identification of Sch28080-sensitive K-ATPase activities in the mouse kidney

Rat collecting ducts display either an ouabain-insensitive or an ouabain-sensitive K-ATPase activity inhibited by Sch28080 according as animals are fed a normal or a potassium-depleted diet (types I and III K-ATPase, respectively). Two isoforms of H,K-ATPase have been cloned from rat gastric mucosa and colon, respectively. Gastric and colonic H,K-ATPase are expressed in the kidney, suggesting that they might account for types I and III K-ATPases. However, this hypothesis is not fully supported by segmental expression of gastric and colonic H,K-ATPase along the rat collecting duct, as well as by comparison of the pharmacological properties of gastric and colonic H,K-ATPase expressed in Xenopus ovocyte and types I and III K-ATPases in rat collecting ducts. The aim of the present work is to address directly the molecular origin of types I and III K-ATPases in the mouse collecting duct by measuring K-ATPase activities in collecting ducts of wild-type mice and mice genetically deficient in either gastric or colonic H,K-ATPase fed either a regular or a potassium-depleted diet. Like the rat, mouse collecting ducts display type I or III K-ATPase activity when fed a regular or a potassium-depleted diet, respectively. Type I K-ATPase activity is detected in colonic H,K-ATPase-deficient mice but not in gastric H,K-ATPase-deficient animals. Conversely, type III K-ATPase activity disappears in colonic H,K-ATPase-deficient but not in gastric H,K-ATPase-deficient mice. In conclusion, types I and III K-ATPases measured in collecting ducts of normal and potassium-depleted mice reflect the functional expression of gastric and colonic H,K-ATPase, respectively.

The non-gastric H,K-ATPase is oligomycin-sensitive and can function as an H+,NH4(+)-ATPase

We used the baculovirus/Sf9 expression system to gain new information on the mechanistic properties of the rat non-gastric H,K-ATPase, an enzyme that is implicated in potassium homeostasis. The alpha2-subunit of this enzyme (HKalpha2) required a beta-subunit for ATPase activity thereby showing a clear preference for NaKbeta1 over NaKbeta3 and gastric HKbeta. NH4(+), K+, and Na+ maximally increased the activity of HKalpha2-NaKbeta1 to 24.0, 14.2, and 5.0 micromol P(i) x mg(-1) protein x h(-1), respectively. The enzyme was inhibited by relatively high concentrations of ouabain and SCH 28080, whereas it was potently inhibited by oligomycin. From the phosphorylation level in the presence of oligomycin and the maximal NH4(+)-stimulated ATPase activity, a turnover number of 20,000 min(-1) was determined. All three cations decreased the steady-state phosphorylation level and enhanced the dephosphorylation rate, disfavoring the hypothesis that Na+ can replace H+ as the activating cation. The potency with which vanadate inhibited the cation-activated enzyme decreased in the order K+ > NH4(+) > Na+, indicating that K+ is a stronger E2 promoter than NH4(+), whereas in the presence of Na+ the enzyme is in the E1 form. For K+ and NH4(+), the E2 to E1 conformational equilibrium correlated with their efficacy in the ATPase reaction, indicating that here the transition from E2 to E1 is rate-limiting. Conversely, the low maximal ATPase activity with Na+ is explained by a poor stimulatory effect on the dephosphorylation rate. These data show that NH4(+) can replace K+ with similar affinity but higher efficacy as an extracellular activating cation in rat nongastric H,K-ATPase.

Asn792 participates in the hydrogen bond network around the K+-binding pocket of gastric H,K-ATPase

Asn792 present in M5 of gastric H,K-ATPase is highly conserved within the P-type ATPase family. A direct role in K+ binding was postulated for Na,K-ATPase but was not found in a recent model for gastric H,K-ATPase (Koenderink, J. B., Swarts, H. G. P., Willems, P. H. G. M., Krieger, E., and De Pont, J. J. H. H. M. (2004) J. Biol. Chem. 279, 16417-16424). Therefore, its role in K+ binding and E1/E2 conformational equilibrium in gastric H,K-ATPase was studied by site-directed mutagenesis and expression in Sf9 cells. N792Q and N792A, but not N792D and N792E, had a markedly reduced K+ affinity in both the ATPase and dephosphorylation reactions. In addition, N792A shifted the conformational equilibrium to the E1 form. In double mutants, the effect of N792A on K+ sensitivity was overruled by either E820Q (K(+)-independent activity) or E343D (no dephosphorylation activity). Models were made for the mutants based on the E2 structure of Ca(2+)-ATPase. In the wild-type model the acid amide group of Asn792 has hydrogen bridges to Lys791, Ala339, and Val341. Comparison of the effects of the various mutants suggests that the hydrogen bridge between the carbonyl oxygen of Asn792 and the amino group of Lys791 is essential for the K+ sensitivity and the E2 preference of wild-type enzyme. Moreover, there was a high positive correlation (r = 0.98) between the in silico calculated energy difference of the E2 form (mutants versus wild type) and the experimentally measured IC50 values for vanadate, which reflects the direction of the E2<-->E1 conformational equilibrium. These data strongly support the validity of the model in which Asn792 participates in the hydrogen bond network around the K(+)-binding pocket.

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.

A conformation-specific interhelical salt bridge in the K+ binding site of gastric H,K-ATPase

Homology modeling of gastric H,K-ATPase based on the E2 model of sarcoplasmic reticulum Ca2+-ATPase (Toyoshima, C., and Nomura, H. (2002) Nature 392, 835-839) revealed the presence of a single high-affinity binding site for K+ and an E2 form-specific salt bridge between Glu820 (M6) and Lys791 (M5). In the E820Q mutant this salt bridge is no longer possible, and the head group of Lys791, together with a water molecule, fills the position of the K+ ion and apparently mimics the K+-filled cation binding pocket. This gives an explanation for the K+-independent ATPase activity and dephosphorylation step of the E820Q mutant (Swarts, H. G. P., Hermsen, H. P. H., Koenderink, J. B., Schuurmans Stekhoven, F. M. A. H., and De Pont, J. J. H. H. M. (1998) EMBO J. 17, 3029-3035) and, indirectly, for its E1 preference. The model is strongly supported by a series of reported mutagenesis studies on charged and polar amino acid residues in the membrane domain. To further test this model, Lys791 was mutated alone and in combination with other crucial residues. In the K791A mutant, the K+ affinity was markedly reduced without altering the E2 preference of the enzyme. The K791A mutation prevented, in contrast to the K791R mutation, the spontaneous dephosphorylation of the E820Q mutant as well as its conformational equilibrium change toward E1. This indicates that the salt bridge is essential for high-affinity K+ binding and the E2 preference of H,K-ATPase. Moreover, its breakage (E820Q) can generate a K+-insensitive activity and an E1 preference. In addition, the study gives a molecular explanation for the electroneutrality of H,K-ATPases.

Osteocyte control of bone formation via sclerostin, a novel BMP antagonist

There is an unmet medical need for anabolic treatments to restore lost bone. Human genetic bone disorders provide insight into bone regulatory processes. Sclerosteosis is a disease typified by high bone mass due to the loss of SOST expression. Sclerostin, the SOST gene protein product, competed with the type I and type II bone morphogenetic protein (BMP) receptors for binding to BMPs, decreased BMP signaling and suppressed mineralization of osteoblastic cells. SOST expression was detected in cultured osteoblasts and in mineralizing areas of the skeleton, but not in osteoclasts. Strong expression in osteocytes suggested that sclerostin expressed by these central regulatory cells mediates bone homeostasis. Transgenic mice overexpressing SOST exhibited low bone mass and decreased bone strength as the result of a significant reduction in osteoblast activity and subsequently, bone formation. Modulation of this osteocyte-derived negative signal is therapeutically relevant for disorders associated with bone loss.

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.

Transient association of the sarcoplasmic reticulum Ca2+ ATPase with the Na+/K+-ATPase and H+/K+-ATPase beta-subunits during its biogenesis in Xenopus oocytes

We examined the effect of the beta-subunits of the Na+/K+ and H+/K+ ATPases on the biogenesis of the sarcoplasmic reticulum (SR) Ca2+ ATPase in Xenopus oocytes. Oocytes were simultaneously injected with cRNAs for both the SR Ca2+ ATPase and the beta-subunit of the Na+/K+ or the H+/K+ ATPase. Immunoprecipitation with antiserum specific for the beta-subunit of the Na+/K+ or the H+/K+ ATPase yielded not only the respective beta-subunit but also the SR Ca2+ ATPase, indicating that the SR Ca2+ ATPase was associated with the beta-subunits of the Na+/K+ and the H+/K+ ATPases. Pulse-chase experiments revealed that the complex between the SR Ca2+ ATPase and the beta-subunit of the Na+/K+ ATPase was formed transiently and dissociated during the course of maturation. This is the first report that demonstrates the association of the SR Ca2+ ATPase with the beta-subunit of the Na+/K+ and H+/K+ ATPases.

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.

The E1/E2-preference of gastric H,K-ATPase mutants

Gastric H,K-ATPase has, in the absence of ATP and added ions, a preference for the E(2) conformation. Mutations in the cation-binding pocket often result in a preference for the E(1)-conformation. This can be paralleled by the occurrence of K(+)-independent ATPase activity. These two phenomena could be separated by combined mutagenesis of several residues in and around the cation-binding pocket. Models of the three-dimensional structure of H,K-ATPase visualize the relationship between the E(1)/E(2) preference and the structure.

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.

K(+)-independent gastric H(+),K(+)-atpase activity. Dissociation of K(+)-independent dephosphorylation and preference for the E1 conformation by combined mutagenesis of transmembrane glutamate residues

Several mutations of residues Glu(795) and Glu(820) present in M5 and M6 of the catalytic subunit of gastric H(+),K(+)-ATPase have resulted in a K(+)-independent, SCH 28080-sensitive ATPase activity, caused by a high spontaneous dephosphorylation rate. The mutants with this property also have a preference for the E(1) conformation. This paper investigates the question of whether these two phenomena are coupled. This possibility was studied by combining mutations in residue Glu(343), present in M4, with those in residues 795 and 820. When in combined mutants Glu and/or Gln residues were present at positions 343, 795, and 820, the residue at position 820 dominated the behavior: a Glu giving K(+)-activated ATPase activity and an E(2) preference and a Gln giving K(+)-independent ATPase activity and an E(1) preference. With an Asp at position 343, the enzyme could be phosphorylated, but the dephosphorylation was blocked, independent of the presence of either a Glu or a Gln at positions 795 and 820. However, in these mutants, the direction of the E(2) <--> E(1) equilibrium was still dominated by the 820 residue: a Glu giving E(2) and a Gln giving E(1). This indicates that the preference for the E(1) conformation of the E820Q mutation is independent of an active dephosphorylation process.

Chimeras of X+, K+-ATPases. The M1-M6 region of Na+, K+-ATPase is required for Na+-activated ATPase activity, whereas the M7-M10 region of H+, K+-ATPase is involved in K+ de-occlusion

In this study we reveal regions of Na(+),K(+)-ATPase and H(+),K(+)-ATPase that are involved in cation selectivity. A chimeric enzyme in which transmembrane hairpin M5-M6 of H(+),K(+)-ATPase was replaced by that of Na(+),K(+)-ATPase was phosphorylated in the absence of Na(+) and showed no K(+)-dependent reactions. Next, the part originating from Na(+),K(+)-ATPase was gradually increased in the N-terminal direction. We demonstrate that chimera HN16, containing the transmembrane segments one to six and intermediate loops of Na(+),K(+)-ATPase, harbors the amino acids responsible for Na(+) specificity. Compared with Na(+),K(+)-ATPase, this chimera displayed a similar apparent Na(+) affinity, a lower apparent K(+) affinity, a higher apparent ATP affinity, and a lower apparent vanadate affinity in the ATPase reaction. This indicates that the E(2)K form of this chimera is less stable than that of Na(+),K(+)-ATPase, suggesting that it, like H(+),K(+)-ATPase, de-occludes K(+) ions very rapidly. Comparison of the structures of these chimeras with those of the parent enzymes suggests that the C-terminal 187 amino acids and the beta-subunit are involved in K(+) occlusion. Accordingly, chimera HN16 is not only a chimeric enzyme in structure, but also in function. On one hand it possesses the Na(+)-stimulated ATPase reaction of Na(+),K(+)-ATPase, while on the other hand it has the K(+) occlusion properties of H(+),K(+)-ATPase.

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.

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

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

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 beta-subunits of Na+,K+-ATPase and gastric H+,K+-ATPase have a high preference for their own alpha-subunit and affect the K+ affinity of these enzymes

The alpha- and beta-subunits of Na+,K+-ATPase and H+,K+-ATPase were expressed in Sf9 cells in different combinations. Immunoprecipitation of the alpha-subunits resulted in coprecipitation of the accompanying beta-subunit independent of the type of beta-subunit. This indicates cross-assembly of the subunits of the different ATPases. The hybrid ATPase with the catalytic subunit of Na+,K+-ATPase and the beta-subunit of H+,K+-ATPase (NaKalphaHKbeta) showed an ATPase activity, which was only 12 +/- 4% of the activity of the Na+,K+-ATPase with its own beta-subunit. Likewise, the complementary hybrid ATPase with the catalytic subunit of H+,K+-ATPase and the beta-subunit of Na+,K+-ATPase (HKalphaNaKbeta) showed an ATPase activity which was 9 +/- 2% of that of the recombinant H+,K+-ATPase. In addition, the apparent K+ affinity of hybrid NaKalphaHKbeta was decreased, while the apparent K+ affinity of the opposite hybrid HKalphaNaKbeta was increased. The hybrid NaKalphaHKbeta could be phosphorylated by ATP to a level of 21 +/- 7% of that of Na+,K+-ATPase. These values, together with the ATPase activity gave turnover numbers for NaKalphabeta and NaKalphaHKbeta of 8800 +/- 310 min-1 and 4800 +/- 160 min-1, respectively. Measurements of phosphorylation of the HKalphaNaKbeta and HKalphabeta enzymes are consistent with a higher turnover of the former. These findings suggest a role of the beta-subunit in the catalytic turnover. In conclusion, although both Na+,K+-ATPase and H+,K+-ATPase have a high preference for their own beta-subunit, they can function with the beta-subunit of the other enzyme, in which case the K+ affinity and turnover number are modified.

Post-thymectomy autoimmune gastritis: fine specificity and pathogenicity of anti-H/K ATPase-reactive T cells

Thymectomy at day 3 of life (d3Tx) results in the development of organ-specific autoimmunity. We have recently shown that d3Tx BALB/c mice which develop autoimmune gastritis contain CD4+ T cells specific for the gastric parietal cell proton pump, H/K ATPase. Here, we demonstrate that freshly explanted gastric lymph node (LN) cells from d3Tx mice react significantly to the H/K ATPase alpha chain, but only marginally to the beta chain. Two H/K ATPase-reactive T cell lines were derived from the gastric LN of d3Tx mice. Both are CD4+, TCR alpha/beta-, and I-Ad restricted, and recognize distinct peptides from the H/K ATPase alpha chain. One cell line secretes Th1 and the other Th2 cytokines, but both are equally potent in inducing gastritis with distinct profiles of cellular infiltration in nu/nu recipient animals. Neither of the cell lines induced disease in normal BALB/c recipients and transfer of disease to nu/nu recipients was blocked by co-transfer of normal BALB/c spleen cells containing CD4+ CD25+ cells. Although CD4+ CD25+ T cells are thought to emigrate from the thymus after day 3 of life, they could be identified in LN of 2-day-old animals. The capacity of CD4+ CD25+ T cells to abrogate the pathogenic activity in vivo of both activated Th1/Th2 lines strongly suggests that this suppressor T cell population may have a therapeutic role in other models of established autoimmunity. The availability of well-characterized lines of autoantigen-specific T cells should greatly facilitate the analysis of the mechanism of action and target of the CD4+ CD25+ immunoregulatory cells.

Mutagenesis of glutamate 820 of the gastric H+,K+-ATPase alpha-subunit to aspartate decreases the apparent ATP affinity

Mutagenesis of Glu820, present in the catalytic subunit of gastric H+,K+-ATPase, into an Asp hardly affects K+-stimulated ATPase and K+-stimulated dephosphorylation of the enzyme. The ATP phosphorylation rate of the E820D mutant, however, is rather low and the apparent affinity for ATP in the phosphorylation process of this mutant is 2-3 times lower than that of the wild type enzyme. The reduction in the ATP phosphorylation rate of the E820D mutant has only an effect on the ATPase activity at low temperature. These findings suggest that Glu820 might play a role in H+ stimulation of the phosphorylation process.

Characterization of the human gonadotropin-releasing hormone receptor heterologously produced using the baculovirus/insect cell and the Semliki Forest virus systems

1. Two eukaryotic viral systems, the baculovirus/insect cell and the Semliki Forest virus systems, were tested for heterologous expression of human gonadotropin-releasing hormone receptor (GnRHR) cDNA. 2. An unmodified as well as a c-myc epitope-tagged human GnRH receptor was produced in two insect cell lines (Spodoptera frugiperda, Trichoplusia ni) after infection with the respective recombinant baculoviruses. In both insect cell lines, the receptor was identified by immunoblot analysis as a triplet of bands between 35 and 40 kDa. After deglycosylation of the receptor the molecular mass decreased to 35 kDa. The GnRH receptor was localized in membrane compartments within the infected insect cells. However, only in membranes of infected Trichoplusia ni insect cells could approximately 2000 receptors per cell be detected. 3. Production of the GnRH receptor in BHK cells using the Semliki Forest virus system resulted in approximately 50,000 receptors per cell. A maximal yield of 0.42 pmol/mg membrane protein was obtained 24 hr after electroporation of BHK cells with in vitro synthesized RNA. Binding of the antagonist [125I]Cetrorelix was saturable with a KD of 1.3 nM. The receptor produced in the BHK cells was further characterized by ligand displacement studies. The rank order of agonist and antagonist affinities was Cetrorelix > Triptorelin > Antide > GnRH.

Constitutive activation of gastric H+,K+-ATPase by a single mutation

In the reaction cycle of P-type ATPases, an acid-stable phosphorylated intermediate is formed which is present in an intracellularly located domain of the membrane-bound enzymes. In some of these ATPases, such as Na+,K+-ATPase and gastric H+, K+-ATPase, extracellular K+ ions stimulate the rate of dephosphorylation of this phosphorylated intermediate and so stimulate the ATPase activity. The mechanism by which extracellular K+ ions stimulate the dephosphorylation process is unresolved. Here we show that three mutants of gastric H+,K+-ATPase lacking a negative charge on residue 820, located in transmembrane segment six of the alpha-subunit, have a high SCH 28080-sensitive, but K+-insensitive ATPase activity. This high activity is caused by an increased 'spontaneous' rate of dephosphorylation of the phosphorylated intermediate. A mutant with an aspartic acid instead of a glutamic acid residue in position 820 showed hardly any ATPase activity in the absence of K+, but K+ ions stimulated ATPase activity and the dephosphorylation process. These findings indicate that the negative charge normally present on residue 820 inhibits the dephosphorylation process. K+ ions do not stimulate dephosphorylation of the phosphorylated intermediate directly, but act by neutralizing the inhibitory effect of a negative charge in the membrane.

The negative charge of glutamic acid-820 in the gastric H+,K+-ATPase alpha-subunit is essential for K+ activation of the enzyme activity

To investigate the role of Glu820, located in transmembrane domain M6 of the alpha-subunit of gastric H+,K+-ATPase, a number of mutants was prepared and expressed in Sf9 cells using a baculovirus encoding for both H+,K+-ATPase subunits. The wild-type enzyme and the E820D (Glu820-->Asp) mutant showed a similar biphasic activation by K+ on the ATPase activity (maximum at 1 mM). The mutant E820A had a markedly decreased K+ affinity (maximum at 40-100 mM). The other mutants, E820Q, E820N, E820L and E820K, showed no K+-activated ATPase activity at all, whereas all mutants formed a phosphorylated intermediate. After preincubation with K+ before phosphorylation mutant E820D showed a similar K+-sensitivity as the wild-type enzyme. The mutants E820N and E820Q had a 10-20 times lower sensitivity, whereas the other three mutants were hardly sensitive towards K+. Upon preincubation with 3-(cyanomethyl)-2-methyl-8-(phenylmethoxy) imidazo [1,2a]-pyridine (SCH28080), all mutants showed similar sensitivity for this drug as the wild-type enzyme, except mutant E820Q, which could only partly be inhibited, and mutant E820K, which was completely insensitive towards SCH28080. These experiments suggest that, with a relatively large residue at position 820, the binding of SCH28080 is obstructed. The various mutants showed a behaviour in K+-stimulated-dephosphorylation experiments similar to that for K+-activated-ATPase-activity measurements. These results indicate that K+ binding, and indirectly the transition to the E2 form, is only fully possible when a negatively charged residue is present at position 820 in the alpha-subunit.

CD4+CD25+ T Cells Inhibit Both the Induction and Effector Function of Autoreactive T Cells and Represent a Unique Lineage of Immunoregulatory Cells

Thymectomy of susceptible strains of mice on day 3 of life results in a spectrum of organ-specific autoimmunity that can be prevented by reconstitution of the thymectomized animals early in life with normal adult lymphocytes. The effectors and suppressors of autoimmunity in this model have been convincingly shown to be CD4+ T cells. It has been demonstrated recently that the regulatory CD4+ T cells that prevent disease coexpress CD25. We have further characterized the population of CD4+CD25+ immunoregulatory cells and demonstrated that they can suppress not only the induction of disease post-thymectomy, but can also efficiently suppress disease induced by cloned autoantigen-specific effector cells. Furthermore, the CD4+CD25+ T cells appear to be members of a unique lineage of regulatory T cells, as the induction of CD25 expression on a monospecific population of T cells derived from TCR transgenic SCID mice did not result in suppression of post-thymectomy autoimmunity. In addition, the TCR transgenic SCID mice were highly susceptible to autoimmune disease induced by the cloned line of autoantigen-specific effectors, while normal mice were relatively resistant. The capacity of the cloned line to transfer disease to nu/nu recipients could be inhibited by normal spleen cell populations containing CD4+CD25+ cells and by purified CD4+CD25+ cells. Although the target Ag(s) and mechanism of action of the CD4+CD25+ T cells remain to be determined, it is likely that they also play an important role in modulating other autoimmune diseases that are mediated by activation of “ignorant” self-reactive T cells present in the normal peripheral lymphocyte pool.

Glycosylation is essential for biosynthesis of functional gastric H+,K+-ATPase in insect cells

The role of N-linked glycosylation in the functional properties of gastric H+,K+-ATPase has been examined with tunicamycin and I-deoxymannojirimycin, inhibitors in glycoprotein biosynthesis and glycoprotein processing respectively. Tunicamycin completely abolished both K+-stimulated and 3-(cyanomethyl)-2-methyl-8-(phenylmethoxy)-imidazo[1,2a]pyridine (SCH 28080)-sensitive ATPase activity and SCH 28080-sensitive phosphorylation capacity. The expression level of both H+,K+-ATPase subunits remained unaffected. 1-Deoxymannojirimycin clearly affected the structure of the N-linked oligosaccharide moieties without affecting specific phosphorylation capacity. Purification of the functional recombinant enzyme from non-functional H+,K+-ATPase subunits coincided with purification of glycosylated beta-subunits and not of non-glycosylated beta-subunits. Transport of the H+,K+-ATPase beta-subunit to the plasma membrane but not its ability to assemble with the alpha-subunit dependent on N-glycosylation events. We conclude that the acquisition, but not the exact structure, of N-linked oligosaccharide moieties, is essential for biosynthesis of functional gastric H+,K+-ATPase in insect cells.

Role of negatively charged residues in the fifth and sixth transmembrane domains of the catalytic subunit of gastric H+,K+-ATPase

The role of six negatively charged residues located in or around the fifth and sixth transmembrane domain of the catalytic subunit of gastric H+,K+-ATPase, which are conserved in P-type ATPases, was investigated by site-directed mutagenesis of each of these residues. The acid residues were converted into their corresponding acid amides. Sf9 cells were used as the expression system using a baculovirus with coding sequences for the alpha- and beta-subunits of H+,K+-ATPase behind two different promoters. Both subunits of all mutants were expressed like the wild type enzyme in intracellular membranes of Sf9 cells as indicated by Western blotting experiments, an enzyme-linked immunosorbent assay, and confocal laser scan microscopy studies. The mutants D824N, E834Q, E837Q, and D839N showed no 3-(cyanomethyl)-2-methyl-8(phenylmethoxy)-imidazo[1, 2a]pyridine (SCH 28080)-sensitive ATP dependent phosphorylation capacity. Mutants E795Q and E820Q formed a phosphorylated intermediate, which, like the wild type enzyme, was hydroxylamine-sensitive, indicating that an acylphosphate was formed. Formation of the phosphorylated intermediate from the E795Q mutant was similarly inhibited by K+ (I50 = 0.4 mM) and SCH 28080 (I50 = 10 nM) as the wild type enzyme, when the membranes were preincubated with these ligands before phosphorylation. The dephosphorylation reaction was K+-sensitive, whereas ADP had hardly any effect. Formation of the phosphorylated intermediate of mutant E820Q was much less sensitive toward K+ (I50 = 4.5 mM) and SCH 28080 (I50 = 1.7 microM) than the wild type enzyme. The dephosphorylation reaction of this intermediate was not stimulated by either K+ or ADP. In contrast to the wild type enzyme and mutant E795Q, mutant E820Q did not show any K+-stimulated ATPase activity. These findings indicate that residue Glu820 might be involved in K+ binding and transition to the E2 form of gastric H+,K+-ATPase.

The rat distal colon P-ATPase alpha subunit encodes a ouabain-sensitive H+, K+-ATPase

The functional properties and the pharmacological profile of the recently cloned cDNA colonic P-ATPase alpha subunit (Crowson, M.S., and Shull, G.E. (1992) J. Biol. Chem. 267, 13740-13748) were investigated by using the Xenopus oocyte expression system. Xenopus oocytes were injected with alpha subunit cRNAs from Bufo marinus bladder or rat distal colon and/or with beta subunit cRNA from B. marinus bladder. Two days after injection, K+ uptake was measured by using 86 Rb+ as a K+ surrogate, and pH measurements were performed by means of ion-selective microelectrodes. Co-injection of alpha and beta subunit cRNAs led to a large increase in 86Rb+ uptake, an intracellular alkalinization, and an extracellular medium acidification, as compared to alpha or beta injection alone. These results indicate that the colonic P-ATPase alpha subunit, like the bladder alpha subunit, acts as a functional H+,K+-ATPase, and that co-expression of alpha and beta subunits is required for the function. External K+ activation of the 86Rb+ uptake had a K1/2 of approximately 440 microM for the bladder isoform (consistent with the previously reported value (Jaisser, F., Horisberger, J.D., Geering, K., and Rossier, B.C. (1993) J. Cell. Biol. 123, 1421-1431) and a K1/2 of approximately 730 microM for the colonic isoform. Sch28080 was ineffective to reduce 86Rb+ uptake whereas ouabain inhibited the activity expressed from rat colon alpha subunit with a Ki of 970 microM when measured at the Vmax of the enzyme. We conclude that, when expressed in Xenopus oocytes, the rat colon P-ATPase alpha subunit encodes a ouabain-sensitive H+,K+-ATPase.

Co-localization and functional coupling of creatine kinase B and gastric H+/K(+)-ATPase on the apical membrane and the tubulovesicular system of parietal cells

Immunogold labelling of creatine kinase B (BB-CK) and gastric H+/K(+)-ATPase in the parietal cells of the stomach revealed colocalization of these two enzymes on the apical membrane and the membranes of the tubulovesicular system. Upon fractionation of hog parietal cells, a specific fraction of the BB-CK proteins remained associated with the purified vesicles, in which gastric H+/K(+)-ATPase is highly enriched. The BB-CK present in this highly purified preparation was able to support pronounced H+/K(+)-ATPase activity in K(+)-loaded vesicles in the presence of phosphocreatine and ADP, although only low levels of ATP were measured. In contrast, when pyruvate kinase, phosphoenolpyruvate and ADP were used as an ATP-generating system to sustain similar levels of H+/K(+)-ATPase activity, ATP levels were more than 10-fold higher. Changing the experimental conditions such that ATP levels were the same for both systems resulted in significantly elevated H+/K(+)-ATPase activities in the BB-CK/phosphocreatine system in comparison with the pyruvate kinase/phosphoenolpyruvate system. These results indicate that gastric H+/K(+)-ATPase has preferential access to ATP generated by creatine kinase co-localized on the membranes of the vesicles.

Overexpression of integral membrane proteins for structural studies

Determination of the structure of integral membrane proteins is a challenging task that is essential to understand how fundamental biological processes (such as photosynthesis, respiration and solute translocation) function at the atomic level. Crystallisation of membrane proteins in 3D has led to the determination of four atomic resolution structures [photosynthetic reaction centres (Allenet al. 1987; Changet al. 1991; Deisenhofer & Michel, 1989; Ermleret al. 1994); porins (Cowanet al. 1992; Schirmeret al. 1995; Weisset al. 1991); prostaglandin H2synthase (Picotet al. 1994); light harvesting complex (McDermottet al. 1995)], and crystals of membrane proteins formed in the plane of the lipid bilayer (2D crystals) have produced two more structures [bacteriorhodopsin (Hendersonet al. 1990); light harvesting complex (Kühlbrandtet al. 1994)].

A cytoplasmic region of the Na,K-ATPase alpha-subunit is necessary for specific alpha/alpha association

While most structural studies of the Na,K-ATPase support a subunit stoichiometry of one alpha-subunit to one beta-subunit, the exact quaternary structure of the Na,K-ATPase and its relevance to enzyme function is the subject of much debate. Formation of a higher order enzyme complex is supported by our previous study demonstrating specific alpha/alpha interactions among the rat Na,K-ATPase isoforms (alpha 1, alpha 2, alpha 3), expressed in virally infected Sf-9 insect cells and among native alpha isoforms in rat brain (1). This detergent-resistant association was not observed in insect cells coexpressing the homologous gastric H,K-ATPase alpha-subunit, nor was it dependent on the coexpression of the beta-subunit. To delineate domains necessary for alpha/alpha assembly, a series of H,K-ATPase-Na, K-ATPase chimerase were constructed by combining the N-terminal, cytoplasmic midregion and C-terminal segments derived from the Na,K-ATPase (N) and the H,K-ATPase (H) alpha-polypeptides (HNN, HNH, NHH, NHN, and HHN). The alpha-subunit chimeras were coexpressed with the Na,K-ATPase alpha 1-subunit in Sf-9 cells using the baculovirus expression system. Specific and detergent-stable association is observed between the Na,K-ATPase alpha-subunit and the HNN and HNH chimeras, but not with the NHH, NHN, or HHN chimeras. Consistent with the Na,K-ATPase cytoplasmic domain as being necessary for alpha/alpha interactions, the full-length alpha-subunit stably associates with an alpha N-terminal deletion mutant (delta Gly2-Leu273), but not with an alpha cytoplasmic deletion mutant (delta Arg350-Pro785). In addition, the naturally occurring C-terminal truncated alpha 1 isoform, alpha 1T (delta Gly554 to C terminus), does not associated with the alpha 1-subunit in Sf-9 cells coexpressing both polypeptides. thus, a cytoplasmic region in the alpha-subunit (Gly554-Pro785) is necessary for specific alpha/alpha association. The same cytoplasmic region contains a strongly hydrophobic segment that, by analogy with oligomerization of water-soluble proteins, may form the interface of the extramembranous alpha/alpha contact site.

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

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

The alpha and beta subunits of the Na,K-ATPase can assemble at the plasma membrane into functional enzyme

Synthesis and assembly of most oligomeric plasma membrane proteins occurs in the ER. However, the role the ER plays in oligomerization is unknown. We have previously demonstrated that unassociated alpha and beta subunits of the Na,K-ATPase are targeted to the plasma membrane when individually expressed in baculovirus-infected Sf-9 cells. This unique property allows us to determine if assembly of these two polypeptides is restricted to the ER, or if it can also occur at the plasma membrane. To investigate the assembly of the Na,K-ATPase we have taken advantage of the ability of baculovirus-infected cells to fuse. Lowering the extracellular pH of the infected cells triggers an endogenously expressed viral protein to initiate plasma membrane fusion. When individual Sf-9 cells expressing either the Na,K-ATPase alpha or beta subunits are plated together and subjected to a mild acidic shock, they form large syncytia. In the newly continuous plasma membrane the separate alpha and beta polypeptides associate and assemble into functional Na,K-ATPase molecules. However, a hybrid ATPase molecule consisting of a Na,K-ATPase alpha subunit and a H,K-ATPase beta subunit, which efficiently assembles in the ER of coinfected cells, does not assemble at the plasma membrane of fused cells. When cells expressing the Na,K-ATPase alpha subunit are fused to cells coexpressing the Na,K-ATPase beta subunit and the H,K-ATPase beta subunit, the Na,K-ATPase alpha subunit selectively assembles with the Na,K-ATPase beta subunit. However, when cells are coinfected and expressing all three polypeptides, the Na,K-ATPase alpha subunit assembles with both beta subunits in the ER, in what appears to be a random fashion. These experiments demonstrate that assembly between some polypeptides is restricted to the ER, and suggests that the ability of the Na,K-ATPase alpha and beta subunits to leave the ER and assemble at the plasma membrane may represent a novel mechanism of regulation of activity.

Functional expression and purification of the ABC transporter complex associated with antigen processing (TAP) in insect cells

Using the baculovirus expression system the gene products of human tap1 and tap2 were over-expressed as wild-type as well as oligohistidine fusion proteins in Spodoptera frugiperda (Sf9) insect cells. Both gene products were co-expressed within the same cells and were found enriched in microsomal membranes. Immunoprecipitation and immobilized metal affinity chromatography revealed complex formation between TAP1 and TAP2. The expressed TAP complex was shown to be functional by peptide translocation into microsomes of Sf9 cells. Peptide transport strictly requires TAP1 and TAP2 as well as ATP. For the first time the functional expression of the human TAP complex in insect cells has been demonstrated, indicating that additional cofactors of a highly developed immune system are not essential for peptide transport across microsomal membranes.

Ca2+ signaling in Sf9 insect cells and the functional expression of a rat brain M5 muscarinic receptor

The purpose of the present study was to examine Ca2+ signaling mechanisms in Sf9 cells and to demonstrate expression and functional linkage of a mammalian receptor to changes in cytosolic free Ca2+ concentration ([Ca2+]i). Addition of p-octopamine (50 microM to fura 2-loaded Sf9 cells produced a small transient increase in [Ca2+]i from a basal level of 58 +/- 10 to 194 +/- 7.6 (SD) nM. The response to octopamine was inhibited by both cyproheptadine and chlorpromazine and was mimicked by clonidine. In contrast, [Ca2+]i did not change in response to dopamine (50 microM), substance P (50 nM), histamine (50 microM), ATP (50 microM), acetylcholine (10 or 100 microM), carbachol (10 or 100 microM), serotonin (50 microM), epinephrine (10 microM), or bradykinin (50 nM). The Ca(2+)-adenosinetriphosphatase inhibitors thapsigargin (200 nM) and 2,5-di-tert-butylhydroquinone (BHQ; 10 microM) increased [Ca2+]i to 307 +/- 13 and 137 +/- 20 nM, respectively. In contrast to BHQ, the response to thapsigargin was attenuated by La3+ or removal of extracellular Ca2+ and increased by elevation of extracellular Ca2+. These results suggest that thapsigargin but not BHQ stimulates Ca2+ influx. The rat brain muscarinic receptor (subtype M5) was incorporated into the baculovirus by homologous recombination. Addition of carbachol (100 microM) increased [Ca2+]i from 92.7 +/- 6.4 to 480 +/- 26 nM in Sf9 cells infected with recombinant virus containing the M5 receptor cDNA. The effect of carbachol on [Ca2+]i was concentration dependent with a 50% effective concentration of approximately 30 microM and was blocked by atropine (10 microM).(ABSTRACT TRUNCATED AT 250 WORDS)