The N-linked oligosaccharides of the beta-subunit of rabbit gastric H,K-ATPase: site localization and identification of novel structures

Altered glycosylation in secreting cells of the gastric glands of aquaporin-4-deficient mice

Aquaporins (AQPs) are important for water transport in the gastrointestinal tract. Changes in their expression and/or localization could cause in disorders and be used as therapeutic targets. Aquaporin-4 (AQP4) is expressed predominantly on the basolateral membrane of the parietal cells in the corpus of the murine gastric glands. Although the secretion of gastric juice is not affected in AQP4-deficient knockout, we evaluated by light microscopy whether the lack of AQP4 affects the glycopatterns of secreting gastric cells. Wild type (WT) and AQP4-deficient knockout mice (KO) were fed a standard diet ad libitum before sacrifice. Segments of stomach corpus were collected, fixed in buffered formalin, and embedded in paraffin wax. Sections, 5-μm thick, were analyzed by histochemical methods (Periodic acid-Schiff, Alcian Blue pH 2.5), and binding of lectins specific to GalNAc (SBA, DBA), Gal (PNA) GlcNAc (WGA, GSAII) mannose and/or glucose (ConA), and fucose (UEA-I, AAA, LTA). Immunohistochemical methods such as anti-Muc6 for neck cells and anti- β- H+/K+-ATPase for parietal cells were also performed. Compared to WT mice, in the mucous cells of KO lower amounts of glycans with galactosyl/galactosaminylated, glycosyl/glycosaminylated, and fucosylated residues were observed; lower fucosylation resulted also in the parietal cells. The observed differences of KO in respect to WT could lead to severer pathological conditions. RESEARCH HIGHLIGHTS: Glycopatterns in gastric glands were compared between wild type (WT) and AQP4-deficient knockout (KO) mice by histochemical and lectin-binding methods. In the mucous cells of KO lower amounts of glycans with galactosyl/galactosaminylated, glycosyl/glycosaminylated and fucosylated residues were observed. In the parietal cells lower fucosylation also resulted. AQP4-deficiency affects glycosylation and could result in altered functionality and pathological conditions.

NEGATIVE ION MASS SPECTROMETRY FOR THE ANALYSIS OF N-LINKED GLYCANS

N-glycans from glycoproteins are complex, branched structures whose structural determination presents many analytical problems. Mass spectrometry, usually conducted in positive ion mode, often requires extensive sample manipulation, usually by derivatization such as permethylation, to provide the necessary structure-revealing fragment ions. The newer but, so far, lesser used negative ion techniques, on the contrary, provide a wealth of structural information not present in positive ion spectra that greatly simplify the analysis of these compounds and can usually be conducted without the need for derivatization. This review describes the use of negative ion mass spectrometry for the structural analysis of N-linked glycans and emphasises the many advantages that can be gained by this mode of operation. Biosynthesis and structures of the compounds are described followed by methods for release of the glycans from the protein. Methods for ionization are discussed with emphasis on matrix-assisted laser desorption/ionization (MALDI) and methods for producing negative ions from neutral compounds. Acidic glycans naturally give deprotonated species under most ionization conditions. Fragmentation of negative ions is discussed next with particular reference to those ions that are diagnostic for specific features such as the branching topology of the glycans and substitution positions of moieties such as fucose and sulfate, features that are often difficult to identify easily by conventional techniques such as positive ion fragmentation and exoglycosidase digestions. The advantages of negative over positive ions for this structural work are emphasised with an example of a series of glycans where all other methods failed to produce a structure. Fragmentation of derivatized glycans is discussed next, both with respect to derivatives at the reducing terminus of the molecules, and to methods for neutralization of the acidic groups on sialic acids to both stabilize them for MALDI analysis and to produce the diagnostic fragments seen with the neutral glycans. The use of ion mobility, combined with conventional mass spectrometry is described with emphasis on its use to extract clean glycan spectra both before and after fragmentation, to separate isomers and its use to extract additional information from separated fragment ions. A section on applications follows with examples of the identification of novel structures from lower organisms and tables listing the use of negative ions for structural identification of specific glycoproteins, glycans from viruses and uses in the biopharmaceutical industry and in medicine. The review concludes with a summary of the advantages and disadvantages of the technique. © 2020 John Wiley & Sons Ltd. Mass Spec Rev.

[Negative regulation of gastric proton pump by desialylation suggested by fluorescent imaging with the sialic acid-specific nanoprobe]

Gastric proton pump (H⁺,K⁺-ATPase) which is responsible for H⁺ secretion of gastric acid (HCl) in gastric parietal cells is the major therapeutic target for treatment of acid-related diseases. H⁺,K⁺-ATPase consists of two subunits, a catalytic α-subunit (αHK) and a glycosylated β-subunit (βHK). N-glycosylation of βHK is essential for trafficking and stability of αHK in apical membrane of gastric parietal cells. Terminal sialic acid residues on sugar chains have an important role in various cellular functions. Recently, we succeeded in visualizing the sialylation and desialylation dynamics of βHK using a fluorescence bioimaging nanoprobe consisting of biocompatible polymers conjugated with lectins for detecting sialic acid. In H⁺,K⁺-ATPase-expressing cell lines, rat gastric mucosa, and primary culture of rat gastric parietal cells, fluorescence imaging of sialic acid with the nanoprobe showed that sialylation of βHK is regulated by intragastric pH and that inhibition of gastric acid secretion induces desialylation of βHK. In biochemical and pharmacological studies, we revealed that enzyme activity of αHK is negatively regulated by desialylation of βHK. Our studies uncovered a novel negative-feedback mechanism of H⁺,K⁺-ATPase in which sialic acids of βHK positively regulates H⁺,K⁺-ATPase activity, and acidic pH decreases the pump activity by cleaving sialic acids of βHK. In this topic, we introduce the overview of our research using the bioimaging nanoprobe.

Gastric H+,K+-ATPase

The gastric H(+),K(+)-ATPase is responsible for gastric acid secretion. This ATPase is composed of two subunits, the catalytic α subunit and the structural β subunit. The α subunit with molecular mass of about 100 kDa has 10 transmembrane domains and is strongly associated with the β subunit with a single transmembrane segment and a peptide mass of 35 kDa. Its three-dimensional structure is based on homology modeling and site-directed mutagenesis resulting in a proton extrusion and K(+) reabsorption model. There are three conserved H3O(+)-binding sites in the middle of the membrane domain and H3O(+) secretion depends on a conformational change involving Lys(791) insertion into the second H3O(+) site enclosed by E795, E820, and D824 that allows export of protons at a concentration of 160 mM. K(+) countertransport involves binding to this site after the release of protons with retrograde displacement of Lys(791) and then K(+) transfer to E343 and exit to the cytoplasm. This ATPase is the major therapeutic target in treatment of acid-related diseases and there are several known luminal inhibitors allowing analysis of the luminal vestibule. One class contains the acid-activated covalent, thiophilic proton pump inhibitors, the most effective of current acid-suppressive drugs. Their binding sites and trypsinolysis allowed identification of all ten transmembrane segments of the ATPase. In addition, various K(+)-competitive inhibitors of the ATPase are being developed, with the advantage of complete and rapid inhibition of acid secretion independent of pump activity and allowing further refinement of the structure of the luminal vestibule of the E2 form of this ATPase.

Active detergent-solubilized H+,K+-ATPase is a monomer

The H(+),K(+)-ATPase pumps protons or hydronium ions and is responsible for the acidification of the gastric fluid. It is made up of an α-catalytic and a β-glycosylated subunit. The relation between cation translocation and the organization of the protein in the membrane are not well understood. We describe here how pure and functionally active pig gastric H(+),K(+)-ATPase with an apparent Stokes radius of 6.3 nm can be obtained after solubilization with the non-ionic detergent C(12)E(8), followed by exchange of C(12)E(8) with Tween 20 on a Superose 6 column. Mass spectroscopy indicates that the β-subunit bears an excess mass of 9 kDa attributable to glycosylation. From chemical analysis, there are 0.25 g of phospholipids and around 0.024 g of cholesterol bound per g of protein. Analytical ultracentrifugation shows one main complex, sedimenting at s(20,)(w) = 7.2 ± 0.1 S, together with minor amounts of irreversibly aggregated material. From these data, a buoyant molecular mass is calculated, corresponding to an H(+),K(+)-ATPase α,β-protomer of 147.3 kDa. Complementary sedimentation velocity with deuterated water gives a picture of an α,β-protomer with 0.9-1.4 g/g of bound detergent and lipids and a reasonable frictional ratio of 1.5, corresponding to a Stokes radius of 7.1 nm. An α(2),β(2) dimer is rejected by the data. Light scattering coupled to gel filtration confirms the monomeric state of solubilized H(+),K(+)-ATPase. Thus, α,β H(+),K(+)-ATPase is active at least in detergent and may plausibly function as a monomer, as has been established for other P-type ATPases, Ca(2+)-ATPase and Na(+),K(+)-ATPase.

Affinity enrichment and characterization of mucin core-1 type glycopeptides from bovine serum

The lack of consensus sequence, common core structure, and universal endoglycosidase for the release of O-linked oligosaccharides makes O-glycosylation more difficult to tackle than N-glycosylation. Structural elucidation by mass spectrometry is usually inconclusive as the CID spectra of most glycopeptides are dominated by carbohydrate-related fragments, preventing peptide identification. In addition, O-linked structures also undergo a gas-phase rearrangement reaction, which eliminates the sugar without leaving a telltale sign at its former attachment site. In the present study we report the enrichment and mass spectrometric analysis of proteins from bovine serum bearing Galbeta1-3GalNAcalpha (mucin core-1 type) structures and the analysis of O-linked glycopeptides utilizing electron transfer dissociation and high resolution, high mass accuracy precursor ion measurements. Electron transfer dissociation (ETD) analysis of intact glycopeptides provided sufficient information for the identification of several glycosylation sites. However, glycopeptides frequently feature precursor ions of low charge density (m/z > approximately 850) that will not undergo efficient ETD fragmentation. Exoglycosidase digestion was utilized to reduce the mass of the molecules while retaining their charge. ETD analysis of species modified by a single GalNAc at each site was significantly more successful in the characterization of multiply modified molecules. We report the unambiguous identification of 21 novel glycosylation sites. We also detail the limitations of the enrichment method as well as the ETD analysis.

Expression of H(+),K(+)-ATPase and glycopattern analysis in the gastric glands of Rana esculenta

A multidisciplinary study involving lectin histochemistry, IHC, immuno-lectin blotting, and immunogold was carried out to determine the distribution of sugar residues in the glycoproteins of Rana esculenta oxynticopeptic cells. We considered animals in two experimental conditions, fasting and fed. It is known that, in mammals, the tubulovesicular membranes are rich in proteins with several functions. The proton pump H(+),K(+)-ATPase, a heterodimeric complex with a catalytic alpha-subunit and a heavily glycosylated beta-subunit, responsible for acid secretion, is the most abundant. No data have been published regarding the localization and the structures of H(+),K(+)-ATPase in amphibians. In the water frog, the luminal membrane and tubulovesicular system of oxynticopeptic cells, which differ in morphology according to their functional stage, reacted with the primary gold-conjugated antibody against the H(+),K(+)-ATPase alpha-subunit. By lectin histochemistry and immunoblotting, in the oxynticopeptic cells of R. esculenta we detected the presence of N-linked glycans having fucosylated (poly)lactosamine chains, which could correspond to the oligosaccharide chains of the beta subunit. The latter are somewhat different from those described in mammals, and this is probably because of an adaptation to the different microenvironmental conditions in which the oxynticopeptic cells find themselves, in terms of their different habits and phylogeny.

Teaching glycoproteins with a classical paper: Knowledge and methods in the course of an exciting discovery: The story of discovering HK-ATPASE β-subunit

To integrate research into the teaching of glycoproteins, the story of discovering hydrogen-potassium ATPase (HK-ATPase) β subunit is presented in a way covering all the important teaching points. The interaction between the HK-ATPase α subunit and a glycoprotein of 60-80 kDa was demonstrated to support the existence of the β subunit. On revealing the strategies and experimental designs of this discovery, the knowledge of glycoproteins is delivered. The purpose of this effort was to facilitate the teaching of scientific thinking in the science classroom, to make the biochemistry classroom a more interesting place, and to attract uncertain minds into the career of biochemistry research.

An Essential Oligomannosidic Glycan Chain in the Catalytic Domain of Autotaxin, a Secreted Lysophospholipase-D

Autotaxin/NPP2, a secreted lysophospholipase-D, promotes cell proliferation, survival, and motility by generating the signaling molecule lysophosphatidic acid. Here we show that ectonucleotide pyrophosphatase/phosphodiesterase 2 (NPP2) is N-glycosylated on Asn-53, Asn-410, and Asn-524. Mutagenesis and deglycosylation experiments revealed that only the glycosylation of Asn-524 is essential for the expression of the catalytic and motility-stimulating activities of NPP2. The N-glycan on Asn-524 was identified as Man8/9GlcNAc2, which is rarely present on mature eukaryotic glycoproteins. Additional studies show that this Asn-524-linked glycan is not accessible to alpha-1,2-mannosidase, suggesting that its non-reducing termini are buried inside the folded protein. Consistent with a structural role for the Asn-524-linked glycan, only the mutation of Asn-524 augmented the sensitivity of NPP2 to proteolysis and increased its mobility during Blue Native PAGE. Asn-524 is phylogenetically conserved and maps to the catalytic domain of NPP2, but a structural model of this domain suggests that Asn-524 is remote from the catalytic site. Our study defines an essential role for the Asn-524-linked glycan chain of NPP2.

Pepsinogen and H,K-ATPase mediate acid secretion in gastric glands of Triturus carnifex (Amphibia, Caudata)

The gastric glands of Triturus carnifex (Amphibia, Caudata) have been examined by histochemical and immunohistochemical methods with particular regard to hydrochloric acid and pepsinogen secretion. Fundic glands consist of mucous neck cells, endocrine cells and oxynticopeptic cells producing both pepsinogen and hydrochloric acid. The neck cells showed an unexpected distribution pattern which was only observed in the oral fundus, and produced neutral mucins with glycosidic residues of GalNAc and Gal beta1,3GalNAc, and in this respect they differ from the neck cells of anuran amphibians. The secretion of pepsinogen and hydrochloric acid as demonstrated by immunolabelling with anti-H,K-ATPase and with anti-pepsinogen, respectively, seems not to vary significantly along the longitudinal axis of the stomach. The mechanism of gastric acid secretion seems to be mediated by an ATPase, having similar features to the mammalian gastric H,K-ATPase, and is localised in the luminal membrane and in the subapical cytoplasm of the oxynticopeptic cells. Unusually, the same cytoplasmic areas revealed binding specificity for the winged pea lectin (WPA) from Lotus tetragonolobus, even after beta elimination, indicating the presence of fucosyl residues in N-linked oligosaccharidic chains in glycoproteins of beta-H,K-ATPase subunits.

Characterization of Protein N‐Glycosylation

Although mass spectrometry (MS)-based protein identification is a straightforward task, the characterization of most posttranslational modifications still represents a challenge. N-glycosylation with its well known consensus sequence, common core structure, and "universally" active endoglycosidase seems to belong to the easier category. In this chapter, MS methods for the analysis of N-glycosylated proteins are reviewed. In particular, LC-MS analysis of glycoprotein digests is discussed in detail. The examples included in this chapter illustrate the improved detection sensitivities achieved during the last decade. The characterization of site heterogeneity and of site occupancy is addressed. Low-energy collision-induced dissociation (CID) fragmentation of N-linked glycopeptides and their sodium-adducts is also described.

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.

Cell biology of acid secretion by the parietal cell

Acid secretion by the gastric parietal cell is regulated by paracrine, endocrine, and neural pathways. The physiological stimuli include histamine, acetylcholine, and gastrin via their receptors located on the basolateral plasma membranes. Stimulation of acid secretion typically involves an initial elevation of intracellular calcium and/or cAMP followed by activation of a cAMP-dependent protein kinase cascade that triggers the translocation and insertion of the proton pump enzyme, H,K-ATPase, into the apical plasma membrane of parietal cells. Whereas the H,K-ATPase contains a plasma membrane targeting motif, the stimulation-mediated relocation of the H,K-ATPase from the cytoplasmic membrane compartment to the apical plasma membrane is mediated by a SNARE protein complex and its regulatory proteins. This review summarizes the progress made toward an understanding of the cell biology of gastric acid secretion. In particular we have reviewed the early signaling events following histaminergic and cholinergic activation, the identification of multiple factors participating in the trafficking and recycling of the proton pump, and the role of the cytoskeleton in supporting the apical pole remodeling, which appears to be necessary for active acid secretion by the parietal cell. Emphasis is placed on identifying protein factors that serve as effectors for the mechanistic changes associated with cellular activation and the secretory response.

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.

Gastric H-K-ATPase and acid-resistant surface proteins

Despite the fact that mucus and bicarbonate are important macroscopic components of the gastric mucosal barrier, severe acidic and peptic conditions surely exist at the apical membrane of gastric glandular cells, and these membranes must have highly specialized adaptations to oppose external insults. Parietal cells abundantly express the heterodimeric, acid-pumping H-K-ATPase in their apical membranes. Its beta-subunit (HKbeta), a glycoprotein with >70% of its mass and all its oligosaccharides on the extracellular side, may play a protective role. Here, we show that the extracellular domain of HKbeta is highly resistant to trypsin in the native state (much more than that of the structurally related Na-K-ATPase beta-subunit) and requires denaturation to expose tryptic sites. Native HKbeta also resists other proteases, such as chymotrypsin and V8 protease, which hydrolyze at hydrophobic and anionic amino acids, respectively. Removal of terminal alpha-anomeric-linked galactose does not appreciably alter tryptic sensitivity of HKbeta. However, full deglycosylation makes HKbeta much more susceptible to all proteases tested, including pepsin at pH <2.0. We propose that 1) intrinsic folding of HKbeta, 2) bonding forces between subunits, and 3) oligosaccharides on HKbeta provide a luminal protein domain that resists gastric lytic conditions. Protein folding that protects susceptible charged amino acids and is maintained by disulfide bonding and hydrophilic oligosaccharides would provide a stable structure in the face of large pH changes. The H-K-ATPase is an obvious model, but other gastric luminally exposed proteins are likely to possess analogous protective specializations.

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.

Matrix-assisted laser desorption/ionization mass spectrometry of carbohydrates

This review describes the application of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry to carbohydrate analysis and covers the period 1991-1998. The technique is particularly valuable for carbohydrates because it enables underivatised, as well as derivatised compounds to be examined. The various MALDI matrices that have been used for carbohydrate analysis are described, and the use of derivatization for improving mass spectral detection limits is also discussed. Methods for sample preparation and for extracting carbohydrates from biological media prior to mass spectrometric analysis are compared with emphasis on highly sensitive mass spectrometric methods. Quantitative aspects of MALDI are covered with respect to the relationship between signal strength and both mass and compound structure. The value of mass measurements by MALDI to provide a carbohydrate composition is stressed, together with the ability of the technique to provide fragmentation spectra. The use of in-source and post-source decay and collision-induced fragmentation in this context is described with emphasis on ions that provide information on the linkage and branching patterns of carbohydrates. The use of MALDI mass spectrometry, linked with exoglycosidase sequencing, is described for N-linked glycans derived from glycoproteins, and methods for the analysis of O-linked glycans are also covered. The review ends with a description of various applications of the technique to carbohydrates found as constituents of glycoproteins, bacterial glycolipids, sphingolipids, and glycolipid anchors.

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.

Species-specific distribution of alpha-galactosyl epitopes on the gastric H/K ATPase beta-subunit: relevance to the binding of human anti-parietal cell autoantibodies

The gastric H/K ATPase beta-subunit, an abundant glycoprotein of the secretory membranes of gastric parietal cells, is the major autoantigen recognized by human parietal cell autoantibodies in gastric autoimmunity. Our previous studies demonstrated that the human autoantibodies recognize the H/K ATPase beta-subunit from a number of species and that glycosylation of the beta-subunit with complex N-glycans is required for autoantibody binding. The N-glycans of the beta-subunit contain polylactosamine chains. The lactosamine chains of the rabbit beta-subunit are terminated with alpha-linked galactosyl residues (alpha-galactosyl epitope) (Tyagarajan et al., Biochemistry, 1996, 35, 3238-3246). Here we have investigated the expression of alpha-galactosyl epitopes on the H/K ATPase beta-subunit from a number of species. Using the alpha-galactosyl binding lectin, BS1-IB4, and naturally occurring anti-alpha-galactosyl antibodies, we have demonstrated that the rat H/K ATPase beta-subunit also contains terminal alpha-galactosyl residues, but not the beta-subunit from pig, dog, and mouse, indicating species-specific differences in the terminal saccharide sequences of the beta-subunit. We also investigated the potential contribution of the alpha-galactosyl epitopes to the binding by human sera. The reactivity of human pernicious anemia serum with gastric parietal cells could not be inhibited with saccharide inhibitors and, in addition, no binding was observed with normal human sera. We conclude that the H/K ATPase beta-subunit oligosaccharides from rabbit and rat are terminated with alpha-galactosyl epitopes, and although the presence of this epitope does not contribute to binding by human parietal cell autoantibodies at the concentrations routinely used, it is recommended that neither rat or rabbit stomachs be used for screening human sera.

On-target exoglycosidase digestions/MALDI-MS for determining the primary structures of carbohydrate chains

One method used to determine the primary sequence of oligosaccharides is to digest them with exoglycosidases and analyze the resulting digestion products by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). Previous research has demonstrated that these digestions can be performed on the MALDI target. However, the procedure requires the sample to be incubated at elevated temperatures, and complete digestion requires a few hours. We demonstrate new conditions that permit exoglycosidase digestions to be performed on the MALDI target at room temperature within 30 min. Oligosaccharide standards were digested with one or more exoglycosidases to show that the enzymes retain their activity and specificity under these new reaction conditions. Using this method, the primary sequences of carbohydrate chains can be determined in a relatively short amount of time.

Identification and cloning of rat galectin-2: expression is predominantly in epithelial cells of the stomach

A complementary DNA clone preferentially expressed in the gastrointestinal tract was obtained from a rat stomach library. The protein coded by the clone had a single carbohydrate recognition domain having conserved motifs for beta-galactoside binding and showed 67% amino acid identity with human galectin-2. The recombinant protein synthesized in Escherichia coli could bind to an asialofetuin column and was eluted with beta-galactopyranoside. From these observations, we named the protein rat galectin-2 coded by the cDNA. The rat galectin-2 was predominantly expressed in the epithelial cells of stomach. Thus this protein may form a mucin layer cross-linking with the beta-galactoside moiety of glycoproteins.

Identification of a gastritogenic epitope of the H/K ATPase beta-subunit

We have previously shown that autoimmune gastritis can be elicited in mice by immunization with the gastric parietal cell H/K ATPase alpha beta heterodimer and that tolerance specifically induced to the H/K ATPase beta-subunit protects mice from the development of gastritis. Here we have identified the immunodominant gastritogenic epitope of the H/K ATPase beta-subunit (H/Kbeta). Epitope mapping was carried out with a panel of 21 overlapping peptides that spanned the entire sequence of the gastric H/K ATPase beta-subunit. T cells from gastric H/K ATPase-immunized mice responded to only one of the overlapping peptides, namely H/Kbeta253-277. Furthermore, a single subcutaneous immunization of 6-week-old BALB/c mice with the ATPase beta-subunit peptides resulted in a T-cell response to only H/Kbeta253-277. Multiple immunization with the overlapping H/K ATPase peptides demonstrated that H/Kbeta253-277 was capable of inducing a mononuclear infiltrate specifically within the gastric mucosa. We conclude that H/Kbeta253-277 is the dominant gastritogenic epitope of the gastric H/K ATPase.

Glycobiology and proteomics: is mass spectrometry the Holy Grail?

One characteristic of glycoproteins is that they are separated by two-dimensional electrophoresis (2-D PAGE) into typical 'trains' of protein spots which separate on the basis of different isoelectric point (pI) and/or molecular mass. The pattern of these trains often varies in development and disease. While the isoforms differ both in the number of sites of glycosylation and the types of carbohydrate attached to the protein, classical methods of glycan analysis are insensitive at the levels typically separated by 2-D PAGE. Developments in mass spectrometry technologies have enabled the characterization of most of the oligosaccharide attributes to be determined on picomole amounts of protein. These techniques are beginning to allow the glycoform heterogeneity on 2-D separated glycoproteins to be analyzed.

Analyzing glycoproteins separated by two-dimensional gel electrophoresis

Two-dimensional (2-D) electrophoresis is the preferred method for separating the glycoforms of proteins. The isoforms usually present as 'trains' of spots in the first dimension and may also differ in molecular weight. The primary goal for analyzing the carbohydrate content of glycoprotein spots is to understand the 'rules' which govern the migration of glycoproteins in 2-D electrophoresis. These rules can then be used to produce predictive vectors to interpret changes in glycosylation patterns. Techniques for the analysis of oligosaccharides released from glycoproteins which have been electroblotted to PVDF membrane after one-dimensional (1-D) and 2-D preparative gel electrophoresis are described. The oligosaccharides are removed enzymatically (PNGase F of N-linked oligosaccharides) or chemically (beta-elimination of O-linked oligosaccharides) and separated by high performance anion exchange chromatography (HPAEC-PAD) and identified by electrospray ionization mass spectrometry (ESI-MS) or analyzed directly by ESI-MS. After enzymic removal of the N-linked oligosaccharides the protein spots can be further analyzed by Edman sequence tagging for identification and quantitation of the protein and by acid hydrolysis for monosaccharide analysis of the O-linked oligosaccharides. These approaches have been proved on 1-D PAGE electroblotted bovine fetuin and human glycophorin A and then used to analyze two abundant proteins which separate as glycoforms on 2-D PAGE preparative narrow range (pH 4.5-5.5) blots of human plasma: alpha2-HS glycoprotein (human fetuin) and alpha1-antitrypsin (alpha1-protease inhibitor). It is apparent that both the macroheterogeneity (site occupation) and microheterogeneity (diversity of structures) of the glycosylation contribute to the separation of protein isoforms in 2-D PAGE.

Regions of association between the alpha and the beta subunit of the gastric H,K-ATPase

A binding and a yeast two-hybrid analysis were carried out on the gastric H,K-ATPase to determine interactive regions of the extracytoplasmic domains of the alpha and beta subunits of this P type ATPase. Wheat germ agglutinin fractionation of fluorescein 5-maleimide-labeled tryptic fragments of detergent-solubilized H, K-ATPase showed that a fragment Leu855 to Arg922 of the alpha subunit was bound to the beta subunit. The yeast two-hybrid system showed that the region containing only a part of the seventh transmembrane segment, the loop, and part of the eighth transmembrane segment was capable of giving positive interaction signals with the ectodomain of the beta subunit. The sequence in the extracytoplasmic loop close to the eighth transmembrane segment, namely Arg898 to Thr928, was identified as being the site of interaction using this method. We deduced that the sequence Arg898 to Arg922 in the alpha subunit has strong interaction with the extracytoplasmic domain of the beta subunit. Again, using yeast two-hybrid analysis, two different sequences in the beta subunit Gln64 to Asn130 and Ala156 to Arg188 were identified as association domains in the extracytoplasmic sequence of the beta subunit. These data enable identification of major associative regions of the alpha-beta subunits of the H,K-ATPase.