Determination of the epitope for the inhibitory monoclonal antibody 5-B6 on the catalytic subunit of gastric Mg(2+)-dependent H(+)-transporting and K(+)-stimulated ATPase

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.

Structural similarities of Na,K-ATPase and SERCA, the Ca(2+)-ATPase of the sarcoplasmic reticulum

The crystal structure of SERCA1a (skeletal-muscle sarcoplasmic-reticulum/endoplasmic-reticulum Ca(2+)-ATPase) has recently been determined at 2.6 A (note 1 A = 0.1 nm) resolution [Toyoshima, Nakasako, Nomura and Ogawa (2000) Nature (London) 405, 647-655]. Other P-type ATPases are thought to share key features of the ATP hydrolysis site and a central core of transmembrane helices. Outside of these most-conserved segments, structural similarities are less certain, and predicted transmembrane topology differs between subclasses. In the present review the homologous regions of several representative P-type ATPases are aligned with the SERCA sequence and mapped on to the SERCA structure for comparison. Homology between SERCA and the Na,K-ATPase is more extensive than with any other ATPase, even PMCA, the Ca(2+)-ATPase of plasma membrane. Structural features of the Na,K-ATPase are projected on to the Ca(2+)-ATPase crystal structure to assess the likelihood that they share the same fold. Homology extends through all ten transmembrane spans, and most insertions and deletions are predicted to be at the surface. The locations of specific residues are examined, such as proteolytic cleavage sites, intramolecular cross-linking sites, and the binding sites of certain other proteins. On the whole, the similarity supports a shared fold, with some particular exceptions.

Location of a cytoplasmic epitope for monoclonal antibody HK 12.18 on H,K-ATPase alpha subunit

The enzyme responsible for gastric acidification is a heterodimeric (alpha and beta subunit) P-type ATPase, an integral protein of parietal cell apical membranes, which promotes electroneutral exchange of exoplasmic K(+) for cytoplasmic H(3)O(+). The molecular mechanisms of the catalytic exchange reaction are imperfectly understood, and await clarification of the precise topology of the enzyme with respect to the secretory membrane. Antibodies directed against H,K-ATPase subunits have been useful in confirming hydropathy plot predictions of HKalpha and HKbeta secondary structure. The monoclonal antibody HK 12.18, which labels gastric mucosal parietal cells by immunocytochemistry, and which binds to a single M(r) approximately 94,000 polypeptide by SDS-PAGE immunoblot of gastric microsomes, has been widely used as a specific marker of parietal cells in clinical and cell biological studies of acid secretion, and as a specific HKalpha probe in biochemical studies. However, the uncertain location of the HK 12.18 epitope has limited the antibody's usefulness as a topology probe. In this study, HK 12. 18 immune reactivity with native H,K-ATPase tryptic peptides, HKalpha cDNA fragments expressed in bacteria, and overlapping synthetic HKalpha tridecapeptides, was used to identify the HK 12.18 epitope as seven consecutive amino acids (Asp(682)-Met-Asp-Pro-Ser-Glu-Leu(688)) in the cytoplasmic middle third of HKalpha.

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.

Characterization of a protease-resistant domain of the cytosolic portion of sarcoplasmic reticulum Ca2+-ATPase. Nucleotide- and metal-binding sites

Treatment of rabbit sarcoplasmic reticulum Ca2+-ATPase with a variety of proteases, including elastase, proteinase K, and endoproteinases Asp-N and Glu-C, results in accumulation of soluble fragments starting close to the ATPase phosphorylation site Asp351 and ending in the Lys605-Arg615 region, well before the conserved sequences generally described as constituting the "hinge" region of this P-type ATPase (residues 670-760). These fragments, designated as p29/30, presumably originate from a relatively compact domain of the cytoplasmic head of the ATPase. They retain two structural characteristics of intact Ca2+-ATPase as follows: high sensitivity of peptidic bond Arg505-Ala506 to trypsin cleavage, and high reactivity of lysine residue Lys515 toward the fluorescent label fluorescein 5'-isothiocyanate. Regarding functional properties, these fragments retain the ability to bind nucleotides, although with reduced affinity compared with intact Ca2+-ATPase. The fragments also bind Nd3+ ions, leaving open the possibility that these fragments could contain the metal-binding site(s) responsible for the inhibitory effect of lanthanide ions on ATPase activity. The p29/30 soluble domain, like similar proteolytic fragments that can be obtained from other P-type ATPases, may be useful for obtaining three-dimensional structural information on the cytosolic portion of these ATPases, with or without bound nucleotides. From our findings we infer that a real hinge region with conformational flexibility is located at the C-terminal boundary of p29/30 (rather than in the conserved region of residues 670-760); we also propose that the ATP-binding cleft is mainly located within the p29/30 domain, with the phosphorylation site strategically located at the N-terminal border of this domain.

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

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

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.

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

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

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

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

The epitope for the inhibitory antibody M7-PB-E9 contains Ser-646 and Asp-652 of the sheep Na+,K(+)-ATPase alpha-subunit

The binding of monoclonal antibody M7-PB-E9 to the alpha-subunit of Na+,K(+)-ATPase partially inhibits enzyme activity (35%) in competition with ATP, while in the presence of magnesium it stimulates the rate of ouabain binding severalfold [Ball, W. J. (1984) Biochemistry 23, 2275-2281]. These effects have been shown to result from an antibody-induced shifting of the enzyme's E1 <==> E2 conformational equilibrium to the right that affects all enzyme-ligand interactions except that with Mg2+ [Abbott, A.J., & Ball, W.J. (1992) Biochemistry 31, 11236-11243]. In order to identify the location of the M7-PB-E9 epitope, proteolytic fragments of the lamb kidney enzyme were generated and the immunoreactive alpha fragments were identified by Western blot analyses. These studies revealed a 47-kDa tryptic fragment, which bound both M7-PB-E9 and a -COOH terminus specific antisera and NH2-terminal sequencing showed to originate at Ala-590. Digestion with Staphylococcus aureus V8 protease produced a 36-kDa -COOH-terminus fragment which originated at Gly-697 and did not contain the antibody epitope. Thus the intracellular sequence region Ala-590 to Gly-697 was shown to contain the antibody epitope. When M7-PB-E9's ability to recognize the alpha subunits from various species and tissues was determined and correlated with available sequencing data, only Ser-646 was present in the highly reactive lamb, pig, and avian kidney alpha 1 proteins and altered (Asn) in the poorly recognized Xenopus and rat kidney and Torpedo electroplax organ enzymes.(ABSTRACT TRUNCATED AT 250 WORDS)

Constraints on models for the folding of the Na,K-ATPase

We have attempted to bring together in graphic fashion the available evidence on the structure of the Na,K-ATPase and the H,K-ATPase. There appears to be much room for modification of the existing models for transmembrane folding. More sites on each side of the membrane need to be identified. Whether these will be antibody epitopes, sites of covalent modification, or tags inserted by mutagenesis is less important than that there be many of them and that each be verified by alternative approaches. If any single principle has emerged from the study of the topography of membrane proteins, it is that it is easy to reach conclusions too soon.

The colon carcinoma cell line Caco-2 contains an H+/K(+)-ATPase that contributes to intracellular pH regulation

The presence of an H+/K(+)-ATPase and its contribution to the regulation of intracellular pH (pHi) was investigated in Caco-2 cells. The H+/K(+)-ATPase was detected immunologically using the monoclonal antibody 5-B6, which was raised against hog gastric H+/K(+)-ATPase. Cell pH was determined using the pH-sensitive dye 2',7'-bis(carboxyethyl)-carboxyfluorescein. Control pHi, measured in HCO(3-)-free medium, was 7.62 +/- 0.03 (n = 27) when cells were cultured for 14 days and decreased to 7.40 +/- 0.03 (n = 18) after 35 days in culture. Recovery of pHi following a NH+4/NH3 pulse could be reduced by either 100 microM SCH 28080 or 1 mM amiloride, or by removing extracellular Na+. The inhibitory effects of SCH 28080 and amiloride were additive, demonstrating the involvement of a gastric-like H+/K(+)-ATPase and a Na+/H+ exchanger in regulating pHi. Recovery rates at pHi 6.8 were not significantly different in cells cultured for up to 21 days, but were significantly lower in cells cultured for 28 and 35 days. This decrease in recovery rate was due to a decrease in the SCH-28080-insensitive recovery, indicating a reduction of the relative importance of Na+/H+ exchange to the recovery. Recovery of pHi was also inhibited by 1 mM N-ethylmaleimide. However, it is unlikely that N-ethyl-maleimide inhibited a vacuolar type of H+-ATPase, since bafilomycin A1 had no effect on pHi recovery. In conclusion, Caco-2 cells contain a SCH-28080-sensitive mechanism for regulating pHi, which is most conveniently studied after 28 days in culture, when the relative contribution of a Na+/H+ exchanger to pHi regulation is decreased.