Targeting ATP12A, a Nongastric Proton Pump α Subunit, for Idiopathic Pulmonary Fibrosis Treatment

Idiopathic pulmonary fibrosis (IPF) is a pathological condition of unknown etiology that results from injury to the lung and an ensuing fibrotic response that leads to the thickening of the alveolar walls and obliteration of the alveolar space. The pathogenesis is not clear, and there are currently no effective therapies for IPF. Small airway disease and mucus accumulation are prominent features in IPF lungs, similar to cystic fibrosis lung disease. The ATP12A gene encodes the α-subunit of the nongastric H+, K+-ATPase, which functions to acidify the airway surface fluid and impairs mucociliary transport function in patients with cystic fibrosis. It is hypothesized that the ATP12A protein may play a role in the pathogenesis of IPF. The authors' studies demonstrate that ATP12A protein is overexpressed in distal small airways from the lungs of patients with IPF compared with normal human lungs. In addition, overexpression of the ATP12A protein in mouse lungs worsened bleomycin induced experimental pulmonary fibrosis. This was prevented by a potassium competitive proton pump blocker, vonoprazan. These data support the concept that the ATP12A protein plays an important role in the pathogenesis of lung fibrosis. Inhibition of the ATP12A protein has potential as a novel therapeutic strategy in IPF treatment.

Eutherian-Specific Functions of BetaM Acquired through Atp1b4 Gene Co-Option in the Regulation of MyoD Expression

Vertebrate ATP1B4 genes represent a rare instance of orthologous gene co-option, resulting in radically different functions of the encoded BetaM proteins. In lower vertebrates, BetaM is a Na, K-ATPase β-subunit that is a component of ion pumps in the plasma membrane. In placental mammals, BetaM lost its ancestral role and, through structural alterations of the N-terminal domain, became a skeletal and cardiac muscle-specific protein of the inner nuclear membrane, highly expressed during late fetal and early postnatal development. We previously determined that BetaM directly interacts with the transcriptional co-regulator SKI-interacting protein (SKIP) and is implicated in the regulation of gene expression. This prompted us to investigate a potential role for BetaM in the regulation of muscle-specific gene expression in neonatal skeletal muscle and cultured C2C12 myoblasts. We found that BetaM can stimulate expression of the muscle regulatory factor (MRF), MyoD, independently of SKIP. BetaM binds to the distal regulatory region (DRR) of MyoD, promotes epigenetic changes associated with activation of transcription, and recruits the SWI/SNF chromatin remodeling subunit, BRG1. These results indicate that eutherian BetaM regulates muscle gene expression by promoting changes in chromatin structure. These evolutionarily acquired new functions of BetaM might be very essential and provide evolutionary advantages to placental mammals.

Microcystin-LR aerosol induces inflammatory responses in healthy human primary airway epithelium

Harmful algal blooms plague bodies of freshwater globally. These blooms are often composed of outgrowths of cyanobacteria capable of producing the heptapeptide Microcystin-LR (MC-LR) which is a well-known hepatotoxin. Recently, MC-LR has been detected in aerosols generated from lake water. However, the risk for human health effects due to MC-LR inhalation exposure have not been extensively investigated. In this study, we exposed a fully differentiated 3D human airway epithelium derived from 14 healthy donors to MC-LR-containing aerosol once a day for 3 days. Concentrations of MC-LR ranged from 100 pM to 1 µM. Although there were little to no detrimental alterations in measures of the airway epithelial function (i.e. cell survival, tissue integrity, mucociliary clearance, or cilia beating frequency), a distinct shift in the transcriptional activity was found. Genes related to inflammation were found to be upregulated such as C-C motif chemokine 5 (CCL5; log2FC = 0.57, p = 0.03) and C-C chemokine receptor type 7 (CCR7; log2FC = 0.84, p = 0.03). Functionally, conditioned media from MC-LR exposed airway epithelium was also found to have significant chemo-attractive properties for primary human neutrophils. Additionally, increases were found in the concentration of secreted chemokine proteins in the conditioned media such as CCL1 (log2FC = 5.07, p = 0.0001) and CCL5 (log2FC = 1.02, p = 0.046). These results suggest that MC-LR exposure to the human airway epithelium is capable of inducing an inflammatory response that may potentiate acute or chronic disease.

As We Drink and Breathe: Adverse Health Effects of Microcystins and Other Harmful Algal Bloom Toxins in the Liver, Gut, Lungs and Beyond

Freshwater harmful algal blooms (HABs) are increasing in number and severity worldwide. These HABs are chiefly composed of one or more species of cyanobacteria, also known as blue-green algae, such as Microcystis and Anabaena. Numerous HAB cyanobacterial species produce toxins (e.g., microcystin and anatoxin—collectively referred to as HAB toxins) that disrupt ecosystems, impact water and air quality, and deter recreation because they are harmful to both human and animal health. Exposure to these toxins can occur through ingestion, inhalation, or skin contact. Acute health effects of HAB toxins have been well documented and include symptoms such as nausea, vomiting, abdominal pain and diarrhea, headache, fever, and skin rashes. While these adverse effects typically increase with amount, duration, and frequency of exposure, susceptibility to HAB toxins may also be increased by the presence of comorbidities. The emerging science on potential long-term or chronic effects of HAB toxins with a particular emphasis on microcystins, especially in vulnerable populations such as those with pre-existing liver or gastrointestinal disease, is summarized herein. This review suggests additional research is needed to define at-risk populations who may be helped by preventative measures. Furthermore, studies are required to develop a mechanistic understanding of chronic, low-dose exposure to HAB toxins so that appropriate preventative, diagnostic, and therapeutic strategies can be created in a targeted fashion.

Properties of a cryptic lysyl oxidase from haloarchaeon Haloterrigena turkmenica

Background

Lysyl oxidases (LOX) have been extensively studied in mammals, whereas properties and functions of recently found homologues in prokaryotic genomes remain enigmatic.

Methods

LOX open reading frame was cloned from Haloterrigena turkmenica in an E. coli expression vector. Recombinant Haloterrigena turkmenica lysyl oxidase (HTU-LOX) proteins were purified using metal affinity chromatography under denaturing conditions followed by refolding. Amine oxidase activity has been measured fluorometrically as hydrogen peroxide release coupled with the oxidation of 10-acetyl-3,7-dihydroxyphenoxazine in the presence of horseradish peroxidase. Rabbit polyclonal antibodies were obtained and used in western blotting.

Results

Cultured H. turkmenica has no detectable amine oxidase activity. HTU-LOX may be expressed in E. coli with a high protein yield. The full-length protein gives no catalytic activity. For this reason, we hypothesized that the hydrophobic N-terminal region may interfere with proper folding and its removal may be beneficial. Indeed, truncated His-tagged HTU-LOX lacking the N-terminal hydrophobic signal peptide purified under denaturing conditions can be successfully refolded into an active enzyme, and a larger N-terminal truncation further increases the amine oxidase activity. Refolding is optimal in the presence of Cu²⁺ at pH 6.2 and is not sensitive to salt. HTU-LOX is sensitive to LOX inhibitor 3-aminopropionitrile. HTU-LOX deaminates usual substrates of mammalian LOX such as lysine-containing polypeptides and polymers. The major difference between HTU-LOX and mammalian LOX is a relaxed substrate specificity of the former. HTU-LOX readily oxidizes various primary amines including such compounds as taurine and glycine, benzylamine being a poor substrate. Of note, HTU-LOX is also active towards several aminoglycoside antibiotics and polymyxin. Western blotting indicates that epitopes for the anti-HTU-LOX polyclonal antibodies coincide with a high molecular weight protein in H. turkmenica cells.

Conclusion

H. turkmenica contains a lysyl oxidase gene that was heterologously expressed yielding an active recombinant enzyme with important biochemical features conserved between all known LOXes, for example, the sensitivity to 3-aminopropionitrile. However, the native function in the host appears to be cryptic.

Significance

This is the first report on some properties of a lysyl oxidase from Archaea and an interesting example of evolution of enzymatic properties after hypothetical horizontal transfers between distant taxa.

Proton pump inhibitors decrease eotaxin-3/CCL26 expression in patients with chronic rhinosinusitis with nasal polyps: Possible role of the nongastric H,K-ATPase

BACKGROUND

Chronic rhinosinusitis with nasal polyps (CRSwNP) is often characterized by tissue eosinophilia that is associated with poor prognosis. Recent findings that proton pump inhibitors (PPIs) directly modulate the expression of eotaxin-3, an eosinophil chemoattractant, in patients with eosinophilic diseases suggest therapeutic potential for PPIs in those with CRSwNP.

OBJECTIVE

We assessed the effect of type 2 mediators, particularly IL-13 and eotaxin-3, on tissue eosinophilia and disease severity in patients with chronic rhinosinusitis (CRS). Further investigation focused on PPI suppression of eotaxin-3 expression in vivo and in vitro, with exploration of underlying mechanisms.

METHODS

Type 2 mediator levels in nasal tissues and secretions were measured by using a multiplex immunoassay. Eotaxin-3 and other chemokines expressed in IL-13-stimulated human sinonasal epithelial cells (HNECs) and BEAS-2B cells with or without PPIs were assessed by using ELISA, Western blotting, real-time PCR, and intracellular pH imaging.

RESULTS

Nasal tissues and secretions from patients with CRSwNP had increased IL-13, eotaxin-2, and eotaxin-3 levels, and these were positively correlated with tissue eosinophil cationic protein levels and radiographic scores in patients with CRS (P < .05). IL-13 stimulation of HNECs and BEAS-2B cells dominantly induced eotaxin-3 expression, which was significantly inhibited by PPIs (P < .05). Patients with CRS taking PPIs also showed lower in vivo eotaxin-3 levels compared with those without PPIs (P < .05). Using intracellular pH imaging and altering extracellular K⁺, we found that IL-13 enhanced H⁺,K⁺-exchange, which was blocked by PPIs and the mechanistically unrelated H,K-ATPase inhibitor, SCH-28080. Furthermore, knockdown of ATP12A (gene for the nongastric H,K-ATPase) significantly attenuated IL-13-induced eotaxin-3 expression in HNECs. PPIs also had effects on accelerating IL-13-induced eotaxin-3 mRNA decay.

CONCLUSION

Our results demonstrated that PPIs reduce IL-13-induced eotaxin-3 expression by airway epithelial cells. Furthermore, mechanistic studies suggest that the nongastric H,K-ATPase is necessary for IL-13-mediated epithelial responses, and its inhibitors, including PPIs, might be of therapeutic value in patients with CRSwNP by reducing epithelial production of eotaxin-3.

Evolutionary diversification of the BetaM interactome acquired through co-option of the ATP1B4 gene in placental mammals

ATP1B4 genes represent a rare instance of orthologous vertebrate gene co-option that radically changed properties of the encoded BetaM proteins, which function as Na,K-ATPase subunits in lower vertebrates and birds. Eutherian BetaM has lost its ancestral function and became a muscle-specific resident of the inner nuclear membrane. Our earlier work implicated BetaM in regulation of gene expression through direct interaction with the transcriptional co-regulator SKIP. To gain insight into evolution of BetaM interactome we performed expanded screening of eutherian and avian cDNA libraries using yeast-two-hybrid and split-ubiquitin systems. The inventory of identified BetaM interactors includes lamina-associated protein LAP-1, myocyte nuclear envelope protein Syne1, BetaM itself, heme oxidases HMOX1 and HMOX2; transcription factor LZIP/CREB3, ERGIC3, PHF3, reticulocalbin-3, and β-sarcoglycan. No new interactions were found for chicken BetaM and human Na,K-ATPase β1, β2 and β3 isoforms, indicating the uniqueness of eutherian BetaM interactome. Analysis of truncated forms of BetaM indicates that residues 72-98 adjacent to the membrane in nucleoplasmic domain are important for the interaction with SKIP. These findings demonstrate that evolutionary alterations in structural and functional properties of eutherian BetaM proteins are associated with the increase in its interactome complexity.

[P4-ATP-ase Atp8b1/FIC1: structural properties and (patho)physiological functions]

P4-ATP-ases comprise an interesting family among P-type ATP-ases, since they are thought to play a major role in the transfer of phospholipids such as phosphatydylserine from the outer leaflet to the inner leaflet. Isoforms of P4-ATP-ases are partially interchangeable but peculiarities of tissue-specific expression of their genes, intracellular localization of proteins, as well as regulatory pathways lead to the fact that, on the organismal level, serious pathologies may develop in the presence of structural abnormalities in certain isoforms. Among P4-ATP-ases a special place is occupied by ATP8B1, for which several mutations are known that lead to serious hereditary diseases: two forms of congenital cholestasis (PFIC1 or Byler disease and benign recurrent intrahepatic cholestasis) with extraliver symptoms such as sensorineural hearing loss. The physiological function of the Atp8b1/FIC1 protein is known in general outline: it is responsible for transport of certain phospholipids (phosphatydylserine, cardiolipin) for the outer monolayer of the plasma membrane to the inner one. It is well known that perturbation of membrane asymmetry, caused by the lack of Atp8B1 activity, leads to death of hairy cells of the inner ear, dysfunction of bile acid transport in liver-cells that causes cirrhosis. It is also probable that insufficient activity of Atp8b1/FIC1 increases susceptibility to bacterial pneumonia.Regulatory pathways of Atp8b1/FIC1 activity in vivo remain to be insufficiently studied and this opens novel perspectives for research in this field that may allow better understanding of molecular processes behind the development of certain pathologies and to reveal novel therapeutical targets.

Postnatal regulation of X,K-ATPases in rat skin and conserved lateroapical polarization of Na,K-ATPase in vertebrate epidermis

Development of epidermis creates stratified epithelium with different sets of ion-transporting enzymes in its layers. We have characterized expression of Na,K- and H,K-ATPase α and β subunits and FXYD isoforms in rat skin. Maturation of rat skin from newborn to adult is associated with an increase in FXYD4 and a decrease of Na,K-ATPase α1-isoform, ATP1B4 and FXYD6 transcripts. Na,K-ATPase of rat epidermis is represented predominantly by α1 and β3 isoforms. Keratinization is associated with the loss of the Na,K-ATPase α-subunit and an enrichment of αng. Na,K-ATPase α1 is abundant in the innermost layer, stratum basale, where it is lacking in basal membranes, thus indicating lateroapical polarization of Na,K-ATPase. Immunocytochemical detection of Na,K-ATPase in Xenopus laevis skin shows that cellular and subcellular localization of the enzyme has a pattern highly similar to that of mammals: basolateral in glandular epithelium and lateroapical in epidermis.

Evolution of Na,K-ATPase beta m-subunit into a coregulator of transcription in placental mammals

Change in gene functions (gene cooption) is one of the key mechanisms of molecular evolution. Genes can acquire new functions via alteration in properties of encoded proteins and/or via changes in temporal or spatial regulation of expression. Here we demonstrate radical changes in the functions of orthologous ATP1B4 genes during evolution of vertebrates. Expression of ATP1B4 genes is brain-specific in teleost fishes, whereas it is predominantly muscle-specific in tetrapods. The encoded beta m-proteins in fish, amphibian, and avian species are beta-subunits of Na,K-ATPase located in the plasma membrane. In placental mammals beta m-proteins lost their ancestral functions, accumulate in nuclear membrane of perinatal myocytes, and associate with transcriptional coregulator Ski-interacting protein (SKIP). Through interaction with SKIP, eutherian beta m acquired new functions as exemplified by regulation of TGF-beta-responsive reporters and by augmentation of mRNA levels of Smad7, an inhibitor of TGF-beta signaling. Thus, orthologous vertebrate ATP1B4 genes represent an instance of gene cooption that created fundamental changes in the functional properties of the encoded proteins.

Role of homologous ASP334 and GLU319 in human non-gastric H,K- and Na,K-ATPases in cardiac glycoside binding

Cardiac steroids inhibit Na,K-ATPase and the related non-gastric H,K-ATPase, while they do not interact with gastric H,K-ATPase. Introducing an arginine, the residue present in the gastric H,K-ATPase, in the second extracellular loop at the corresponding position 334 in the human non-gastric H,K-ATPase (D334R mutation) rendered it completely resistant to 2mM ouabain. The corresponding mutation (E319R) in alpha1 Na,K-ATPase produced a approximately 2-fold increase of the ouabain IC(50) in the ouabain-resistant rat alpha1 Na,K-ATPase and a large decrease of the ouabain affinity of human alpha1 Na,K-ATPase, on the other hand this mutation had no effect on the affinity for the aglycone ouabagenin. These results provide a strong support for the orientation of ouabain in its biding site with its sugar moiety interacting directly with the second extracellular loop.

Characterization of hampin/MSL1 as a node in the nuclear interactome

Hampin, homolog of Drosophila MSL1, is a partner of histone acetyltransferase MYST1/MOF. Functions of these proteins remain poorly understood beyond their participation in chromatin remodeling complex MSL. In order to identify new proteins interacting with hampin, we screened a mouse cDNA library in yeast two-hybrid system with mouse hampin as bait and found five high-confidence interactors: MYST1, TPR proteins TTC4 and KIAA0103, NOP17 (homolog of a yeast nucleolar protein), and transcription factor GC BP. Subsequently, all these proteins were used as baits in library screenings and more new interactions were found: tumor suppressor RASSF1C and spliceosome component PRP3 for KIAA0103, ring finger RNF10 for RASSF1C, and RNA polymerase II regulator NELF-C for MYST1. The majority of the observed interactions was confirmed in vitro by pull-down of bacterially expressed proteins. Reconstruction of a fragment of mammalian interactome suggests that hampin may be linked to diverse regulatory processes in the nucleus.

Loss of acidification of anterior prostate fluids in Atp12a-null mutant mice indicates that nongastric H-K-ATPase functions as proton pump in vivo

The physiological functions of nongastric (colonic) H-K-ATPase (gene symbol Atp12a), unlike those of Na-K-ATPase and gastric H-K-ATPase, are poorly understood. It has been suggested that it pumps Na+ more efficiently than H+; however, so far, there is no direct evidence that it pumps H+ in vivo. Previously, we found that the nongastric H-K-ATPase alpha-subunit is expressed in apical membranes of rodent anterior prostate epithelium, in a complex with the Na-K-ATPase beta1-subunit. Here we report the effects of Atp12a gene ablation on polarization of the beta1-subunit and secretory function of the anterior prostate. In nongastric H-K-ATPase-deficient prostate, the Na-K-ATPase alpha-subunit resided exclusively in basolateral membranes; however, the beta1-subunit disappeared from apical membranes, demonstrating that beta1 is an authentic subunit of nongastric H-K-ATPase in vivo and that apical localization of beta1 in the prostate is completely dependent on its association with the nongastric H-K-ATPase alpha-subunit. A remarkable reduction in acidification of anterior prostate fluids was observed: pH 6.38 +/- 0.14 for wild-type mice and 6.96 +/- 0.10 for homozygous mutants. These results show that nongastric H-K-ATPase is required for acidification of luminal prostate fluids, thereby providing a strong in vivo correlate of previous functional expression studies demonstrating that it operates as a proton pump.

Identification of the β-subunit for nongastric H-K-ATPase in rat anterior prostate

The structural organization of nongastric H-K-ATPase, unlike that of closely related Na-K-ATPase and gastric H-K-ATPase, is not well characterized. Recently, we demonstrated that nongastric H-K-ATPase α-subunit (αng) is expressed in apical membranes of rodent prostate. Its highest level, as well as relative abundance, with respect to α1-isoform of Na-K-ATPase, was observed in anterior lobe. Here, we aimed to determine the subunit composition of nongastric H-K-ATPase through the detailed analysis of the expression of all known X-K-ATPase β-subunits in rat anterior prostate (AP). RT-PCR detects transcripts of β-subunits of Na-K-ATPase only. Measurement of absolute protein content of these three β-subunit isoforms, with the use of quantitative Western blotting of AP membrane proteins, indicates that the abundance order is β1> β3≫ β2. Immunohistochemical experiments demonstrate that β1is present predominantly in apical membranes, coinciding with αng, whereas β3is localized in the basolateral compartment, coinciding with α1. This is the first direct demonstration of the αng-β1colocalization in situ indicating that, in rat AP, αngassociates only with β1. The existence of αng-β1complex has been confirmed by immunoprecipitation experiments. These results indicate that β1-isoform functions as the authentic subunit of Na-K-ATPase and nongastric H-K-ATPase. Putatively, the intracellular polarization of X-K-ATPase isoforms depends on interaction with other proteins.

Accumulation of βm, a structural member of X,K-ATPase β-subunit family, in nuclear envelopes of perinatal myocytes

Recently discovered muscle-specific βmprotein is structurally closely related to the X,K-ATPase β-subunits. However, it has a number of unique properties such as predominant localization in intracellular stores and lack of association with known X,K-ATPase α-subunits on heterologous coexpression. In this study, the primary structure of mouse βmwas determined and developmental regulation of the gene (ATP1B4) was analyzed. The expression is first detected at day 14 of gestation, is sharply increased at day 16, and reaches its maximum at day 18. After birth, the expression quickly decreases and is hardly detectable in adult mice. A more detailed subcellular localization study was undertaken, and its results indicate that βmnot only is located in sarcoplasmic reticulum but is concentrated in nuclear envelopes of both prenatal and postnatal skeletal muscles. Immunohistochemical studies show that βmis specific to myocytes and, at the subcellular level, many nuclear envelopes are intensively labeled in both fetal and newborn skeletal muscles. Accordingly, βmis detected by immunoblotting in purified nuclei and nuclear membranes from neonatal skeletal muscles. On transfection of human rhabdomyosarcoma cell line RD, green fluorescent protein-tagged βmresides intracellularly with significant enrichment in nuclear envelopes, whereas βmwith transmembrane domain deleted localizes in both cytoplasm and nucleoplasm. Nuclear βmapparently is not in association with Na,K-ATPase because we never detected its α-subunit in myonuclear membranes. These results indicate that βmhas a specialized function in mammalian perinatal myocytes, different from functions of other X,K-ATPase β-subunits. The unique temporospatial distribution of βmprotein expression suggests its important role in development of growing skeletal muscle.

Expression and cellular localization of Na,K-ATPase isoforms in the rat ventral prostate

OBJECTIVE

To determine the expression and plasma membrane domain location of isoforms of Na,K-ATPase in the rat ventral prostate.

MATERIALS AND METHODS

Ventral prostate glands from adult male rats were dissected, cryosectioned (7 micro m) and attached to poly-l-lysine coated glass slides. The sections were then fixed in methanol and subjected to indirect immunofluorescence and immunoperoxidase procedures using a panel of well-characterized monoclonal and polyclonal antibodies raised against known Na,K-ATPase subunit isoforms. Immunofluorescence micrographs were digitally captured and analysed by image analysis software.

RESULTS

There was expression of Na,K-ATPase alpha1, beta1, beta2 and beta3 subunit isoforms in the lateral and basolateral plasma membrane domains of prostatic epithelial cells. The alpha1 isoform was abundant but there was no evidence of alpha2, alpha3 or gamma isoform expression in epithelial cells. The alpha3 isoform was not detected, but there was a relatively low level of alpha2 isoform expression in the smooth muscle and stroma.

CONCLUSION

Rat prostate Na,K-ATPase consists of alpha1/beta1, alpha1/beta2 and alpha1/beta3 isoenzymes. These isoform proteins were located in the lateral and basolateral plasma membrane domains of ventral prostatic epithelial cells. The distribution and subcellular localization of Na,K-ATPase is different in rodent and human prostate. Basolateral Na,K-ATPase probably contributes to the establishment of transepithelial ionic gradients that are a prerequisite for the uptake of metabolites by secondary active transport mechanisms and active citrate secretion.

Human nongastric H+-K+-ATPase: transport properties of ATP1al1 assembled with different beta-subunits

To investigate whether nongastric H+-K+-ATPases transport Na+ in exchange for K+ and whether different beta-isoforms influence their transport properties, we compared the functional properties of the catalytic subunit of human nongastric H+-K+-ATPase, ATP1al1 (AL1), and of the Na+-K+-ATPase alpha1-subunit (alpha1) expressed in Xenopus oocytes, with different beta-subunits. Our results show that betaHK and beta1-NK can produce functional AL1/beta complexes at the oocyte cell surface that, in contrast to alpha1/beta1 NK and alpha1/betaHK complexes, exhibit a similar apparent K+ affinity. Similar to Na+-K+-ATPase, AL1/beta complexes are able to decrease intracellular Na+ concentrations in Na+-loaded oocytes, and their K+ transport depends on intra- and extracellular Na+ concentrations. Finally, controlled trypsinolysis reveals that beta-isoforms influence the protease sensitivity of AL1 and alpha1 and that AL1/beta complexes, similar to the Na+-K+-ATPase, can undergo distinct K+-Na+- and ouabain-dependent conformational changes. These results provide new evidence that the human nongastric H+-K+-ATPase interacts with and transports Na+ in exchange for K+ and that beta-isoforms have a distinct effect on the overall structural integrity of AL1 but influence its transport properties less than those of the Na+-K+-ATPase alpha-subunit.

Betam, a structural member of the X,K-ATPase beta subunit family, resides in the ER and does not associate with any known X,K-ATPase alpha subunit

betam, a muscle-specific protein, is structurally closely related to the X,K-ATPase beta subunits, but its intrinsic function is not known. In this study, we have expressed betam in Xenopus oocytes and have investigated its biosynthesis and processing as well as its putative role as a chaperone of X,K-ATPase alpha subunits, as a regulator of sarcoplasmic reticulum Ca(2+)-ATPase (SERCA), or as a Ca(2+)-sensing protein. Our results show that betam is stably expressed in the endoplasmic reticulum (ER) in its core glycosylated, partially trimmed form. Both full-length betam, initiated at Met(1), and short betam species, initiated at Met(89), are detected in in vitro translations as well as in Xenopus oocytes. betam cannot associate with and stabilize Na,K-ATPase (NK), or gastric and nongastric H,K-ATPase (HK) alpha isoforms. betam neither assembles stably with SERCA nor is its trypsin sensitivity or electrophoretic mobility influenced by Ca(2+). A mutant, in which the distinctive Glu-rich regions in the betam N-terminus are deleted, remains stably expressed in the ER and can associate with, but not stabilize X,K-ATPase alpha subunits. On the other hand, a chimera in which the ectodomain of betam is replaced with that of beta1 NK associates efficiently with alpha NK isoforms and produces functional Na,K-pumps at the plasma membrane. In conclusion, our results indicate that betam exhibits a cellular location and functional role clearly distinct from the typical X,K-ATPase beta subunits.

Role of the self-association of beta subunits in the oligomeric structure of Na+/K+-ATPase

The two subunits of Na(+)/K(+)-ATPase that are essential for function are alpha and beta. Previous cross-linking studies on the oligomeric structure of the membrane-bound enzyme identified alpha,beta and alpha,alpha associations, but only the former and not the latter could be detected after solubilization. To study the possibility of direct beta,beta association, the purified membrane enzyme and a trypsin-digested enzyme that occludes cations and contains an essentially intact beta and fragments of alpha were subjected to oxidative cross-linking in the presence of Cu(2+)-phenanthroline. Resolution of products on polyacrylamide gels, N-terminal analysis and reactivity with anti-beta antibody showed that, in addition to previously identified products (e.g. alpha,alpha and alpha,beta dimers), a beta,beta dimer, most likely linked through intramembrane Cys(44) residues of two chains, is also formed. This dimer was also noted when digitonin-solubilized intact enzyme, and the trypsin-digested enzyme solubilized with digitonin or polyoxyethylene 10-laurylether were subjected to cross-linking, indicating that the detected beta,beta association was not due to random collisions. In the digested enzyme, K(+) but not Na(+) enhanced beta,beta dimer formation. The alternative cross-linking of beta-Cys(44) to a Cys residue of a transmembrane alpha-helix was antagonized specifically by K(+) or Na(+). The findings (i) indicate the role of beta,beta association in maintaining the minimum oligomeric structure of (alpha,beta)(2), (ii) provide further support for conformation-dependent flexibilities of the spatial relations of the transmembrane helices of alpha and beta and (iii) suggest the possibility of significant differences between the quaternary structures of the P-type ATPases that do and do not contain a beta subunit.

Nongastric H-K-ATPase in rodent prostate: lobe-specific expression and apical localization

The molecular basis of active ion transport in secretory glands such as the prostate is not well characterized. Rat nongastric H-K-ATPase is expressed at high levels in distal colon surface cell apical membranes and thus is referred to as "colonic." Here we show that the ATPase is expressed in rodent prostate complex in a lobe-specific manner. RT-PCR and Western blot analyses indicate that rat nongastric H-K-ATPase alpha-subunit (alpha(ng)) mRNA and protein are present in coagulating gland (anterior prostate) and lateral and dorsal prostate and absent from ventral lobe, whereas Na-K-ATPase alpha-subunit is present in all lobes. RT-PCR analysis shows that Na-K-ATPase alpha(4) and alpha(3) and gastric H-K-ATPase alpha-subunit are not present in significant amounts in all prostate lobes. Relatively low levels of Na-K-ATPase alpha(2) were found in lateral, dorsal, and anterior lobes. alpha(ng) protein expression is anteriodorsolateral: highest in coagulating gland, somewhat lower in dorsal lobe, and even lower in lateral lobe. Na-K-ATPase protein abundance has the reverse order: expression in ventral lobe is higher than in coagulating gland. alpha(ng) protein abundance is higher in coagulating gland than distal colon membranes. Immunohistochemistry shows that in rat and mouse coagulating gland epithelium alpha(ng) protein has an apical polarization and Na-K-ATPase alpha(1) is localized in basolateral membranes. The presence of nongastric H-K-ATPase in rodent prostate apical membranes may indicate its involvement in potassium concentration regulation in secretions of these glands.

The betam protein, a member of the X,K-ATPase beta-subunits family, is located intracellularly in pig skeletal muscle

The sequence of the pig cDNA encoding the muscle-specific betam-protein, a member of the X,K-ATPase beta-subunits family, was determined. Two alternatively spliced transcripts encoding polypeptide chains of 355 and 351 residues were identified. The tissue specificity of expression of betam and other X,K-ATPase beta-subunit genes was studied by RT-PCR performed on 24 tissues from newborn pigs. The betam expression was shown to be highly tissue-specific, being detected at the highest level in skeletal muscle, at a lower level in heart, and at much lower level in skin. The betam transcripts are more abundant in the tissues from the newborn than adult. Immunoblotting and deglycosylation shift assay indicated that skeletal muscle membranes of newborn pigs contain betam protein with an electrophoretic mobility and carbohydrate content very similar to that of human betam. Fractionation of membranes from both newborn and adult pig skeletal muscles by isopycnic centrifugation revealed that the majority of the betam protein is concentrated in the sarcoplasmic reticulum-containing fractions. This intracellular location is a unique property that distinguishes the betam protein from other members of the X,K-ATPase beta-subunit family.

Catalytic function of nongastric H,K-ATPase expressed in Sf-21 insect cells

We previously demonstrated that the alpha-subunit of human nongastric H,K-ATPase (Atp1al1) can assemble with the gastric H,K-ATPase beta-subunit (betaHK) into an active ion pump upon coexpression in Xenopus oocytes. To gain insight into enzymatic functions, we have analyzed the Atp1al1-betaHK complex using a baculovirus expression system. The efficient formation of the functional Atp1al1-betaHK complex in membranes of Sf-21 insect cells was obtained upon co-infection with recombinant baculoviruses expressing Atp1al1 and betaHK. Expression of either protein alone did not produce active ATPase. The effects of K(+), Na(+), pH, and ATP and inhibitors on ATPase activity of the recombinant Atp1al1-betaHK complex were analyzed. The Atp1al1-betaHK complex was shown to exhibit significant ATPase activity in nominally K(+)-free medium. The addition of K(+) stimulated the ATP hydrolysis up to 3-fold with K(m) approximately 116 microM K(+). The ATPase activity was moderately sensitive to ouabain and to SCH 28080 with apparent K(i) values in K(+)-free medium of approximately 64 microM and approximately 93 microM, respectively. Potassium exhibited strong antagonism toward both inhibitors. Assays of the ouabain-sensitive ATPase activity revealed inhibitory effects of Na(+) with the apparent K(i) of approximately 24 mM in the absence of added K(+) and with K(i) within the range of 60-70 mM in the presence of > or = 1 mM K(+). Thus, the human nongastric H,K-ATPase represented by the recombinant Atp1al1-betaHK complex exhibits enzymatic properties of K(+)-dependent ATPase sensitive to ouabain, SCH 28080, and Na(+). It differs from Na,K-ATPase in cation dependence and differs from gastric H,K-ATPase and Na,K-ATPase in sensitivity to inhibitors.

Immunochemical demonstration of a novel beta-subunit isoform of X, K-ATPase in human skeletal muscle

Recently we have identified mRNA encoding a hitherto unknown mammalian X,K-ATPase beta-subunit expressed predominantly in muscle tissue (Pestov, N. B. et al. (1999) FEBS Lett. 456, 243-248). Here we demonstrate the existence of the predicted protein, designated as beta(m) (beta(muscle)), in human adult skeletal muscle membranes using immunoblotting with beta(m)-specific antibodies generated against recombinant polypeptide formed by extramembrane beta(m) domains. The electrophoretic mobility of beta(m) was shown to be abnormally low due to the presence of Glu-rich sequences. In contrast to mature forms of other known X,K-ATPase beta-subunits, carbohydrate moiety of beta(m) is sensitive to endoglycosidase H and appears to be composed of short high-mannose or hybrid N-glycans. This finding argues in favor of an intracellular location of beta(m) in human skeletal muscle.

Intersubunit interactions in human X,K-ATPases: role of membrane domains M9 and M10 in the assembly process and association efficiency of human, nongastric H,K-ATPase alpha subunits (ATP1al1) with known beta subunits

Na,K- and H,K-ATPase (X,K-ATPase) alpha subunits need association with a beta subunit for their maturation, but the authentic beta subunit of nongastric H,K-ATPase alpha subunits has not been identified. To better define alpha-beta interactions in these ATPases, we coexpressed human, nongastric H,K-ATPase alpha (AL1) and Na,K-ATPase alpha1 (alpha1NK) as well as AL1-alpha1 and alpha1-AL1 chimeras, which contain exchanged M9 and M10 membrane domains, together with each of the known beta subunits in Xenopus oocytes and followed their resistance to cellular and proteolytic degradation and their ER exit. We show that all beta subunits (gastric betaHK, beta1NK, beta2NK, beta3NK, or Bufo bladder beta) can associate efficiently with alpha1NK, but only gastric betaHK, beta2NK, and Bufo bladder beta can form stably expressed AL1-beta complexes that can leave the ER. The trypsin resistance and the forces of subunit interaction, probed by detergent resistance, are lower for AL1-beta complexes than for alpha1NK-beta complexes. Furthermore, chimeric alpha1-AL1 can be stabilized by beta subunits, but alpha1-AL1-gastric betaHK complexes are retained in the ER. On the other hand, chimeric AL1-alpha1 cannot be stabilized by any beta subunit. In conclusion, these results indicate that (1) none of the known beta subunits is the real partner subunit of AL1 but an as yet unidentified, authentic beta should have structural features resembling gastric betaHK, beta2NK, or Bufo bladder beta and (2) beta-mediated maturation of alpha subunits is a multistep process which depends on the membrane insertion properties of alpha subunits as well as on several discrete events of intersubunit interactions.

Packing of the transmembrane helices of Na,K-ATPase: direct contact between beta-subunit and H8 segment of alpha-subunit revealed by oxidative cross-linking

Spatial relationships among the transmembrane (TM) segments of alpha- and beta-subunits of the Na,K-ATPase molecule have been investigated using oxidative induction of disulfide bonds. The catalytic alpha-subunit contains 10 TM alpha-helices (H1-H10) with 9 Cys residues located within or close to the membrane moiety. There is one Cys residue in the single TM segment of beta-subunit (Hbeta). Previously, the cross-linking products containing the beta-subunit and two fragments of alpha-subunit (the N-terminal containing H1-H2 helices and the C-terminal containing H7-H10 helices) have been identified in experiments with membrane-bound or detergent-solubilized preparations of the membrane moiety of trypsin-digested Na,K-ATPase [Sarvazyan, N. A., Modyanov, N. N., and Askari, A. (1995) J. Biol. Chem. 270, 26528-26532 and Sarvazyan, N. A., Ivanov, A., Modyanov, N. N., and Askari, A. (1997) J. Biol. Chem. 272, 7855-7858]. Here, we have shown that Cu(2+)-phenanthroline treatment of digitonin-solubilized preparation provides the most efficient formation of intersubunit cross-linked product that is predominantly a dimer of beta-subunit and a 22-kDa C-terminal alpha-fragment containing H7-H10 helices. This cross-linked product was isolated and subjected to CNBr cleavage. The resulting fragments were electrophoretically separated and sequenced. A 17-kDa peptide composed of Ile853-Met942 alpha-fragment and Ala5-Met56 beta-fragment was identified as a product of intersubunit disulfide cross-link between Cys44 of Hbeta and either Cys911 or Cys930, located in H8. This provides the first direct experimental evidence of the juxtaposition of Hbeta and H8 within the Na,K-ATPase molecule. The second detected cross-linked product was composed of alpha-fragments Lys947-Met963 and Tyr974-Tyr1016 linked by induced disulfide bridge between Cys964 (H9) and Cys983 (H10). The spatial proximity of these Cys residues defines the mutual orientation of H9 and H10 helices of alpha-subunit.

Transport and pharmacological properties of nine different human Na, K-ATPase isozymes

Na,K-ATPase plays a crucial role in cellular ion homeostasis and is the pharmacological receptor for digitalis in man. Nine different human Na,K-ATPase isozymes, composed of 3 alpha and beta isoforms, were expressed in Xenopus oocytes and were analyzed for their transport and pharmacological properties. According to ouabain binding and K(+)-activated pump current measurements, all human isozymes are functional but differ in their turnover rates depending on the alpha isoform. On the other hand, variations in external K(+) activation are determined by a cooperative interaction mechanism between alpha and beta isoforms with alpha2-beta2 complexes having the lowest apparent K(+) affinity. alpha Isoforms influence the apparent internal Na(+) affinity in the order alpha1 > alpha2 > alpha3 and the voltage dependence in the order alpha2 > alpha1 > alpha3. All human Na,K-ATPase isozymes have a similar, high affinity for ouabain. However, alpha2-beta isozymes exhibit more rapid ouabain association as well as dissociation rate constants than alpha1-beta and alpha3-beta isozymes. Finally, isoform-specific differences exist in the K(+)/ouabain antagonism which may protect alpha1 but not alpha2 or alpha3 from digitalis inhibition at physiological K(+) levels. In conclusion, our study reveals several new functional characteristics of human Na,K-ATPase isozymes which help to better understand their role in ion homeostasis in different tissues and in digitalis action and toxicity.

Identification of a novel gene of the X,K-ATPase beta-subunit family that is predominantly expressed in skeletal and heart muscles

We have identified the fifth member of the mammalian X,K-ATPase beta-subunit gene family. The human and rat genes are largely expressed in skeletal muscle and at a lower level in heart. The deduced human and rat proteins designated as beta(muscle) (beta(m)) consist of 357 and 356 amino acid residues, respectively, and exhibit 89% identity. The sequence homology of beta(m) proteins with known Na,K- and H,K-ATPase beta-subunits are 30.5-39.4%. Unlike other beta-subunits, putative beta(m) proteins have large N-terminal cytoplasmic domains containing long Glu-rich sequences. The data obtained indicate the existence of hitherto unknown X,K-ATPase (most probably Na,K-ATPase) isozymes in muscle cells.

Ouabain-sensitive H,K-ATPase: tissue-specific expression of the mammalian genes encoding the catalytic alpha subunit

Human ATP1AL1 and corresponding genes of other mammals encode the catalytic alpha subunit of a non-gastric ouabain-sensitive H,K-ATPases, the ion pump presumably involved in maintenance of potassium homeostasis. The tissue specificity of the expression of these genes in different species has not been analyzed in detail. Here we report comparative RT-PCR screening of mouse, rat, rabbit, human, and dog tissues. Significant expression levels were observed in the skin, kidney and distal colon of all species (with the exception of the human colon). Analysis of rat urogenital organs also revealed strong expression in coagulating and preputial glands. Relatively lower expression levels were detected in many other tissues including brain, placenta and lung. In rabbit brain the expression was found to be specific to choroid plexus and cortex. Prominent similarity of tissue-specific expression patterns indicates that animal and human non-gastric H,K-ATPases are indeed products of homologous genes. This is also consistent with the high sequence similarity of non-gastric H,K-ATPases (including partial sequences of hitherto unknown cDNAs for mouse and dog proteins).

Structural analysis of the products of chymotryptic cleavage of the E1 form of Na,K-ATPase alpha-subunit: identification of the N-terminal fragments containing the transmembrane H1-H2 domain

Chymotryptic cleavage of the Na,K-ATPase in NaCl medium abolishes ATPase activity and alters other functional parameters. The structure of this modified enzyme is uncertain since only one product of selective proteolysis, the 83-kDa fragment of the alpha-subunit (Ala267-C-terminus) has been identified previously. Here, we applied additional tryptic digestion followed by oxidative cross-linking to identify the products originating from the N-terminal part of the alpha-subunit. These fragments start at Ala72 or Thr74 and contain the transmembrane H1-H2 domain. Formation of cross-linked product between alpha-fragments containing H1-H2 and H7-H10 demonstrate that the structural integrity of the membrane moiety is preserved. We also determined that secondary cleavage of the 83-kDa fragment leads to the formation of C-terminal 48-kDa alpha-fragments with multiple N-termini at Ile582, Ser583, Met584 and Ile585.

Enzymatic properties of human Na,K-ATPase alpha1beta3 isozyme

Recent results of a wide-scale human cDNA sequencing project have identified a cDNA which encodes a hitherto unknown human protein sequence exhibiting structural similarities with beta-subunits of the Na,K- and H,K-ATPase family and with the amphibian Na,KATPase beta3-subunit, in particular. In this study the ability of the putative human beta3-subunit to assemble with the human alpha1-subunit in functionally active Na,KATPase was examined using the baculovirus expression system. The recombinant baculovirus simultaneously expressing both alpha1 and beta3 human proteins was produced using the dual-promoter transfer vector p2Bac. The expression of both human proteins in baculovirus-infected Sf-9 cell membranes detected with specific antibodies resulted in the formation of a catalytically competent alpha1beta3 ATPase complex. Characterization of the recombinant ATPase complex involved the analysis of Na+, K+, and ATP dependencies of enzyme activity and its sensitivity toward ouabain. Preparations of HeLa cell membranes containing alpha1beta1 isozyme of human Na,K-ATPase were used as control. The data obtained clearly demonstrated that alpha1beta3 ATPase exhibits enzymatic properties which are characteristic of Na, K-ATPase. The recombinant alpha1beta3 isozyme displayed significantly lower sensitivity to ouabain than native alpha1beta1. These findings indicate that the hitherto unknown alpha1beta3 isozyme of human Na,K-ATPase is likely to exist in vivo, thus suggesting further expansion of human Na,K-ATPase isozyme diversity. The present studies are the first in which heterologous expression has been used for the characterization of an isozyme of human Na, K-ATPase.

Ligand-sensitive interactions among the transmembrane helices of Na+/K+-ATPase

An extensively trypsin-digested Na+/K+-ATPase, which retains the ability to bind Na+, K+, and ouabain, consists of four fragments of the alpha-subunit that contain all 10 transmembrane alpha domains, and the beta-subunit, a fraction of which is cleaved at Arg142-Gly143. In previous studies, we solubilized this preparation with a detergent and mapped the relative positions of several transmembrane helices of the subunits by chemical cross-linking. To determine if these detected helix-helix proximities were representative of those existing in the bilayer prior to solubilization, we have now done similar studies on the membrane-bound preparation of the same digested enzyme. After oxidative sulfhydryl cross-linking catalyzed by Cu2+-phenanthroline, two prominent products were identified by their mobilities and the analyses of their N termini. One was a dimer of a 11-kDa alpha-fragment containing the H1-H2 helices and a 22-kDa alpha-fragment containing the H7-H10 helices. This dimer seemed to be the same as that obtained in the solubilized preparation. The other product was a trimer of the above two alpha-fragments and that fraction of beta whose extracellular domain was cleaved at Arg142-Gly143. This product was different from a similar one of the solubilized preparation in that the latter contained the predominant fraction of beta without the extracellular cleavage. The cross-linking reactions of the membrane preparation, but not those of the solubilized one, were hindered specifically by Na+, K+, and ouabain. These findings indicate that (a) the H1-H2 transmembrane helices of alpha are adjacent to some of its H7-H10 helices both in solubilized and membrane-bound states, (b) the alignment of the residues of the single transmembrane helix of beta with the interacting H1-H2 and H7-H10 helices of alpha is altered by detergent solubilization and by structural changes in the extracellular domain of beta, and (c) the three-dimensional packing of the interacting transmembrane helices of alpha and beta are regulated by the specific ligands of the enzyme.

Functional expression of the cDNA encoded by the human ATP1AL1 gene

The human ATP1AL1 gene encodes a protein expressed in brain, kidney, and skin and that is highly homologous to the recently cloned nongastric isoforms of H-K-adenosinetriphosphatase H-K-ATPase). We have generated polyclonal antibodies against the protein encoded by ATP1AL1 and used them to monitor the protein's expression and distribution in transfection studies. The protein was retained in the endplasmic reticulum when it was transiently expressed alone in COS cells. In COS cells cotransfected with ATP1AL1 plus gastric H-K-ATPase beta-subunit cDNAs (ATP1AL1-gH-K beta), both proteins reached the surface. Stably transfected lines of HEK 293 cells expressing both of these proteins demonstrate a 86Rb+ uptake activity sensitive to both 2-methyl,8-(phenylmeoxy)imidazo(1,2-a)pyridine 3-acetonitrile (SCH-28080) and ouabain (inhibitory constants of approximately 131 and 42 microM, respectively). Outward proton fluxes were measured in the same cells as the spontaneous intracellular pH (pHi) recovery in Cells loaded with a pH-sensitive dye [2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein] and subjected to acid loading through an NH4Cl pulse. The cells expressing both the ATP1AL1-encoded protein and the gastric H-K-ATPase beta-subunit possess a net acid extrusion activity that can be inhibited by 1 mM ouabain. Comparison of the 86Rb+ influx and proton efflux, however, does not support equal H+/Rb+ exchange mediated by this pump under the conditions of pHi-monitoring experiments. Moreover, whereas the acid extrusion activity mediated by the pump shows a marked pH dependence, the 86Rb+ uptake activity present in the cells expressing the ATP1AL1-gH-K beta complex cannot be stimulated by acute lowering of pHi. These data suggest that the ATP1AL1-encoded protein is the catalytic alpha-subunit of a human K(+)-dependent ATPase. The possible implications of the discrepancy between 86Rb+ uptake and pHi monitoring data are discussed.

Restoration of phosphorylation capacity to the dormant half of the alpha-subunits of Na+, K(+)-ATPase

Purified kidney Na+, K(+)-ATPase whose alpha-subunit is cleaved by chymotrypsin at Leu266-Ala267, loses ATPase activity but forms the phosphoenzyme intermediate (EP) from ATP. When EP formation was correlated with extent of alpha-cleavage in the course of proteolysis, total EP increased with time before it declined. The magnitude of this rise indicated doubling of the number of phosphorylation sites after cleavage. Together with previous findings, these data establish that half of the alpha-subunits of oligomeric membrane-bound enzyme are dormant and that interaction of the N-terminal domain of alpha-subunit with its phosphorylation domain causes this half-site reactivity. Evidently, disruption of this interaction by proteolysis abolishes overall activity while it opens access to phosphorylation sites of all alpha-subunits.

Genomic organization of the human ATP1AL1 gene encoding a ouabain-sensitive H,K-ATPase

The human ATP1AL1 gene belongs to the family of Na,K-ATPase and H,K-ATPase (X,K-ATPases) genes. It encodes a catalytic subunit of hitherto unknown human ouabain-sensitive H,K-ATPase that represents a novel third group of X,K-ATPases distinct from the known Na,K-ATPase and gastric H,K-ATPase. Cloning of the ATP1AL1 gene is described in this report. The exon-intron structure of ATP1AL1 was found to be very similar to that of related genes. It contains 23 exons and spans approximately 32 kb of genomic DNA. All ATP1AL1 exons and 12 of its 22 introns were entirely sequenced. A total of nine Alu repeats were identified in introns. The transcription initiation site was mapped 187 bp upstream of the ATG initiation codon by primer extension and S1 nuclease protection analyses of RNA from human skin and colon. Sequence analysis of the 5'-flanking region (1.48 kb) revealed numerous potential binding sites for transcription factors Sp1 and AP2 and one putative NF-kappa B binding site. The 0.85-kb region from position -484 (5'-flanking region) to position +369 (intron 1) meets the structural criteria of a CpG island. It is suggested that the ATP1AL1 gene contains two poly(A) addition sites that may function in a tissue-specific manner.

Intersubunit and intrasubunit contact regions of Na+/K(+)-ATPase revealed by controlled proteolysis and chemical cross-linking

To identify interfaces of alpha- and beta-subunits of Na+/K(+)-ATPase, and contact points between different regions of the same alpha-subunit, purified kidney enzyme preparations whose alpha-subunits were subjected to controlled proteolysis in different ways were solubilized with digitonin to disrupt intersubunit alpha,alpha-interactions, and oxidatively cross-linked. The following disulfide cross-linked products were identified by gel electrophoresis, staining with specific antibodies, and N-terminal analysis. 1) In the enzyme that was partially cleaved at Arg438-Ala439, the cross-linked products were an alpha,beta-dimer, a dimer of N-terminal and C-terminal alpha fragments, and a trimer of beta and the two alpha fragments. 2) From an extensively digested enzyme that contained the 22-kDa C-terminal and several smaller fragments of alpha, two cross-linked products were obtained. One was a dimer of the 22-kDa C-terminal peptide and an 11-kDa N-terminal peptide containing the first two intramembrane helices of alpha (H1-H2). The other was a trimer of beta, the 11-kDa, and the 22-kDa peptides. 3) The cross-linked products of a preparation partially cleaved at Leu266-Ala267 were an alpha,beta-dimer and a dimer of beta and the 83-kDa C-terminal fragment. Assuming the most likely 10-span model of alpha, these findings indicate that (a) the single intramembrane helix of beta is in contact with portions of H8-H10 intramembrane helices of alpha; and (b) there is close contact between N-terminal H1-H2 and C-terminal H8-H10 segments of alpha; with the most probable interacting helices being the H1,H10-pair and the H2,H8-pair.

Human ATP1AL1 gene encodes a ouabain-sensitive H-K-ATPase

The cDNA for ATP1AL1, the fifth member of the human Na-K-adenosinetriphosphatase (ATPase)/H-K-ATPase gene family, was recently cloned (A. V. Grishin, V. E. Sverdlov, M. B. Kostina, and N. N. Modyanov. FEBS Lett. 349: 144-150, 1994). The encoded protein (ATP1AL1) has all the primary structural features common to the catalytic alpha-subunit of ion-transporting P-type ATPases and is similar (63-64% identity) to the Na-K-ATPase alpha-subunit isoforms and the gastric H-K-ATPase alpha-subunit. In this study, ATP1AL1 was expressed in Xenopus laevis oocytes in combination with the beta-subunit of rabbit gastric H-K-ATPase. The functional properties of the stable alpha/beta-complex were studied by 86Rb+ uptake and demonstrated that ATP1AL1 is a novel human K(+)-dependent ATPase [apparent half-constant activation/(K1/2) for K+ approximately 375 microM)]. ATP1AL1-mediated inward K+ transport was inhibited by ouabain (inhibition constant approximately 13 microM) and was found to be inhibited by high concentrations of SCH-28080 (approximately 70% at 500 microM). ATP1AL1 expression resulted in the alkalinization of the oocytes' cytoplasm and ouabain-sensitive proton extrusion, as measured with pH-sensitive microelectrodes. These data argue that ATP1AL1 is the catalytic alpha-subunit of a human nongastric P-type ATPase capable of exchanging extracellular potassium for intracellular protons.

Determination of the sidedness of the carboxy-terminus of the Na+/K(+)ATPase alpha-subunit using lactoperoxidase iodination

The orientation of the carboxy-terminal pair of tyrosines of the Na+/K(+)-ATPase alpha-subunit with respect to the plane of the plasma membrane was determined. The approach was based on lactoperoxidase-catalysed radioiodination of the tyrosine residues accessible on the surface of the enzyme molecule in intact cells of a pig kidney embryonic cell line and those accessible in a broken plasma membrane fraction and in isolated membrane-bound Na+/K(+)-ATPase. The labeled alpha-subunit was isolated by SDS gel electrophoresis followed by electroblotting. Then the COOH-terminal amino acids were hydrolyzed by carboxypeptidases B and Y. Radioactivity and quantitative analysis of the protein and released amino acids showed that the COOH-terminal tyrosine residues of the alpha-subunit were only accessible to modification only when lactoperoxidase had access to the inner side of the plasma membrane. Therefore, the COOH-terminus of the Na+/K(+)-ATPase alpha-subunit is located on the cytoplasmic surface of the pump molecule and its polypeptide chain must have an even number of transmembrane segments.

Cloning and characterization of the entire cDNA encoded by ATP1AL1--a member of the human Na,K/H,K-ATPase gene family

The cDNA for ATP1AL1--the fifth member of the human Na,K-/H,K-ATPase gene family--was cloned and sequenced. The deduced primary ATP1AL1 translation product is 1,039 amino acids in length and has Mr of 114,543. The encoded protein has all of the structural features common to known catalytic subunits of P-type membrane ion-transporting ATPases and is equally distant (63-64% of identity) from the Na,K-ATPase isoforms and the gastric H,K-ATPase. The ATP1AL1 encoded protein was proposed to represent a new separate group within the family of human potassium-dependent ion pumps.

Sequence analysis of cystine-containing peptides in the Na+, K(+)-ATPase alpha subunit

Tryptic digest of the pig kidney Na+, K(+)-ATPase was subjected to HPLC for separating water-soluble fragments. Using ammetric titration with silver nitrate, disulfide-containing peptides were identified and demonstrated to contain s-s bonds with differential sensitivity to reduction. The amino acid sequences of cystine-containing peptides were determined by chemical modification of cysteine residues during the sequence analysis. Among the cystine-containing peptides, three fragments of the alpha-subunit polypeptide chain were identified: Cys452-Lys461, Ile507-Lys519, Val545-Phe558.

Location of disulfide bonds in the Na+, K(+)-ATPase alpha subunit

For localizing S-S bonds in the pig kidney Na+,K(+)-ATPase alpha subunit, cystine-containing peptides (V-1, VII-1, and VII-2), obtained in our previous study from the enzyme's tryptic digest, were analysed. Chemical modification of the cystine-containing peptides performed at cysteine residues involved successive alkylation, first with radioactive iodoacetic acid and then with ABD-F in the absence and presence of a reducing agent, respectively. Cysteinyl peptides were isolated by HPLC, their amino acid sequences determined, and two disulfide bonds: Cys452-Cys456 and Cys511-Cys549 were localized by identification of fluorescent cysteine residues.

Application of three-dimensional molecular hydrophobicity potential to the analysis of spatial organization of membrane domains in proteins. III. Modeling of intramembrane moiety of Na+, K(+)-ATPase

The most probable interlocation of transmembrane alpha-helices of Na+, K(+)-ATPase has been calculated by a computer-aided molecular simulation approach in the framework of models with eight and 10 helical peptides for the alpha-subunit. The method is based on the concept of three-dimensional molecular hydrophobicity potential (MHP) and provides valuable description of spatial hydrophobic properties of membrane-spanning segments as well as helix-helix packing interactions inside the membrane. Resulting model of the arrangement of intramembrane domain agrees with recent results on hydrophobic photolabeling of an intramembrane part of the beta-subunit and the sixth transmembrane segment of the alpha-subunit. It is also consistent with current ideas on hydrophobic organization of integral membrane proteins. Possible topology of a cation-binding site is discussed.

Analysis of disulfide bonds in the Na+,K(+)-ATPase alpha-subunit

Ammetric titration with silver nitrate revealed the presence in pig kidney Na+,K(+)-ATPase of five disulfide bonds and twenty free cysteine residues, most of which are masked. Complete alkylation of all of free SH groups was found possible only after preliminary digestion of the membrane-bound Na+,K(+)-ATPase. A fraction of disulfide-containing peptides involving three fragments of the alpha-subunit polypeptide chain, namely: Cys452-Lys461, Ile507-Lys519, Val545-Phe558, has been isolated from the tryptic digest alkylated with 4-vinylpyridine. Reduction of S-S bonds with beta-mercaptoethanol and alkylation of the released cysteine residues with radiolabeled iodoacetic acid indicated that three above fragments contained cysteine residues that are involved in the formation of two disulfide bonds.

Application of three-dimensional molecular hydrophobicity potential to the analysis of spatial organization of membrane protein domains. II. Optimization of hydrophobic contacts in transmembrane hairpin structures of Na+, K(+)-ATPase

A method of packing of transmembrane hairpin helices in proteins is described. The procedure is based on the optimization of hydrophobic contacts calculated using the three-dimensional (3D) molecular hydrophobicity potential technique. To verify the validity of the computational scheme, we calculated relative orientations of membrane-spanning peptides in pairs L2-L3, M2-M3, and M4-M5 from L- and M-subunits of the photoreaction center of Rhodopseudomonas viridis and compared the predicted structures with those derived from atomic coordinates. The results of computer modeling agree with the X-ray data. We applied the approach proposed to study possible interhelical interactions in transmembrane hairpin structures of Na+, K(+)-ATPase.

Application of three-dimensional molecular hydrophobicity potential to the analysis of spatial organization of membrane domains in proteins: I. Hydrophobic properties of transmembrane segments of Na+, K(+)-ATPase

A new computer-aided molecular modeling approach based on the concept of three-dimensional (3D) molecular hydrophobicity potential has been developed to calculate the spatial organization of intramembrane domains in proteins. The method has been tested by calculating the arrangement of membrane-spanning segments in the photoreaction center of Rhodopseudomonas viridis and comparing the results obtained with those derived from the X-ray data. We have applied this computational procedure to the analysis of interhelical packing in membrane moiety of Na+, K(+)-ATPase. The work consists of three parts. In Part I, 3D distributions of electrostatic and molecular hydrophobicity potentials on the surfaces of transmembrane helical peptides were computed and visualized. The hydrophobic and electrostatic properties of helices are discussed from the point of view of their possible arrangement within the protein molecule. Interlocation of helical segments connected with short extramembrane loops found by means of optimization of their hydrophobic/hydrophilic contacts is considered in Part II. The most probable 3D model of packing of helical peptides in the membrane domain of Na+, K(+)-ATPase is discussed in the final part of the work.

Na,K-ATPase extracellular surface probed with a monoclonal antibody that enhances ouabain binding

The Na,K-stimulated ATPase is inhibited by extracellular cardiac glycosides, which bind to the enzyme's alpha subunit. We used a monoclonal antibody, VG4, as a probe of the extracellular surface. The antibody was specific for Na,K-ATPase and bound to intact cells. The epitope was mapped to the first extracellular loop (H1-H2) of alpha, using a combination of techniques including trypsinolysis, N-terminal sequence of a fragment containing the determinant, and analysis of the effects of species-specific sequence differences. The antibody inhibited Na,K-ATPase activity under certain circumstances, indicating that the H1-H2 loop participates in conformational changes that are transmitted to the active site. Mutations in the H1-H2 loop have been shown by others to affect ouabain affinity. Ouabain and the antibody acted synergistically to inhibit the enzyme, which seemingly supported the hypothesis that the H1-H2 loop is an essential part of the cardiac glycoside binding site. Direct measurements of the binding of [3H]ouabain, however, indicated that VG4 enhanced rather than inhibited binding, presumably by promoting favorable conformation changes. The data suggest the possibility that the cardiac glycoside binding site may be intramembrane rather than extracellular.

A monoclonal antibody recognizes an epitope in the first extracellular loop of the plasma membrane Ca2+ pump

A monoclonal antibody against the human erythrocyte Ca2+ pump (1E4) reacted with the enzyme in intact erythrocytes. Using trypsinized preparations of the pump the antibody only reacted with the N-terminal fragments of 33.5 and 35 kDa. The fragments span from the N terminus (35 kDa) or from residue 19 (33.5 kDa) to residue 314 of the hPMCA4 isoform of the pump. Exhaustive degradation with a number of agents produced smaller peptides which reacted with the antibody. Sequencing analysis on two chymotryptic fragments of 8.8 and 13.5 kDa identified the epitope in an approximately 80-residue domain beginning with Gly-81. Two peptides corresponding to the putative extramembrane portions of this region of the pump were synthesized. The antibody reacted with one of them, spanning residues Phe-121 to Gly-152 and containing the first putative external loop of the pump. Peptides corresponding to overlapping portions of this peptide were synthesized, leading to the location of the epitope in a 13-residue sequence (Glu-130 to Glu-142) in the first predicted extracellular loop (Verma, A. K., Filoteo, A. G., Stanford, D. R., Wieben, E. D., Strehler, E. E., Fischer, R., Heim, R., Vogel, G., Mathews, S., Strehler-Page, M-A., James, P., Vorherr, T., Krebs, J., Penniston, J. T., and Carafoli, E. (1988) J. Biol. Chem. 263, 14152-14159).