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Fedosova NU, Habeck M, Nissen P. Structure and Function of Na,K-ATPase-The Sodium-Potassium Pump. Compr Physiol 2021; 12:2659-2679. [PMID: 34964112 DOI: 10.1002/cphy.c200018] [Citation(s) in RCA: 19] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
Na,K-ATPase is an ubiquitous enzyme actively transporting Na-ions out of the cell in exchange for K-ions, thereby maintaining their concentration gradients across the cell membrane. Since its discovery more than six decades ago the Na-pump has been studied extensively and its vital physiological role in essentially every cell has been established. This article aims at providing an overview of well-established biochemical properties with a focus on Na,K-ATPase isoforms, its transport mechanism and principle conformations, inhibitors, and insights gained from crystal structures. © 2021 American Physiological Society. Compr Physiol 11:1-21, 2021.
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Affiliation(s)
| | - Michael Habeck
- Department of Molecular Biology and Genetics, Danish Research Institute of Translational Neuroscience - DANDRITE, Nordic EMBL Partnership for Molecular Medicine, Aarhus University, Aarhus, Denmark
| | - Poul Nissen
- Department of Molecular Biology and Genetics, Danish Research Institute of Translational Neuroscience - DANDRITE, Nordic EMBL Partnership for Molecular Medicine, Aarhus University, Aarhus, Denmark
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2
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Moreno C, Yano S, Bezanilla F, Latorre R, Holmgren M. Transient Electrical Currents Mediated by the Na +/K +-ATPase: A Tour from Basic Biophysics to Human Diseases. Biophys J 2020; 119:236-242. [PMID: 32579966 PMCID: PMC7376075 DOI: 10.1016/j.bpj.2020.06.006] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/07/2020] [Revised: 04/06/2020] [Accepted: 06/03/2020] [Indexed: 01/14/2023] Open
Abstract
The Na+/K+-ATPase is a chemical molecular machine responsible for the movement of Na+ and K+ ions across the cell membrane. These ions are moved against their electrochemical gradients, so the protein uses the free energy of ATP hydrolysis to transport them. In fact, the Na+/K+-ATPase is the single largest consumer of energy in most cells. In each pump cycle, the protein sequentially exports 3Na+ out of the cell, then imports 2K+ into the cell at an approximate rate of 200 cycles/s. In each half cycle of the transport process, there is a state in which ions are stably trapped within the permeation pathway of the protein by internal and external gates in their closed states. These gates are required to open alternately; otherwise, passive ion diffusion would be a wasteful end of the cell's energy. Once one of these gates open, ions diffuse from their binding sites to the accessible milieu, which involves moving through part of the electrical field across the membrane. Consequently, ions generate transient electrical currents first discovered more than 30 years ago. They have been studied in a variety of preparations, including native and heterologous expression systems. Here, we review three decades' worth of work using these transient electrical signals to understand the kinetic transitions of the movement of Na+ and K+ ions through the Na+/K+-ATPase and propose the significance that this work might have to the understanding of the dysfunction of human pump orthologs responsible for some newly discovered neurological pathologies.
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Affiliation(s)
- Cristina Moreno
- Molecular Neurophysiology Section, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland
| | - Sho Yano
- Medical Genetics and Genomic Medicine Training Program, National Human Genome Research Institute, National Institutes of Health, Bethesda, Maryland
| | - Francisco Bezanilla
- Department of Biochemistry and Molecular Biology, University of Chicago, Gordon Center for Integrative Sciences, Chicago, Illinois
| | - Ramon Latorre
- Centro Interdisciplinario de Neurociencia de Valparaíso, Universidad de Valparaíso, Valparaíso, Chile
| | - Miguel Holmgren
- Molecular Neurophysiology Section, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland.
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Cowgill J, Chanda B. The contribution of voltage clamp fluorometry to the understanding of channel and transporter mechanisms. J Gen Physiol 2019; 151:1163-1172. [PMID: 31431491 PMCID: PMC6785729 DOI: 10.1085/jgp.201912372] [Citation(s) in RCA: 18] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023] Open
Abstract
Cowgill and Chanda discuss the importance of voltage clamp fluorometry to the functional interpretation of ion channel and transporter structures. Key advances in single particle cryo-EM methods in the past decade have ushered in a resolution revolution in modern biology. The structures of many ion channels and transporters that were previously recalcitrant to crystallography have now been solved. Yet, despite having atomistic models of many complexes, some in multiple conformations, it has been challenging to glean mechanistic insight from these structures. To some extent this reflects our inability to unambiguously assign a given structure to a particular physiological state. One approach that may allow us to bridge this gap between structure and function is voltage clamp fluorometry (VCF). Using this technique, dynamic conformational changes can be measured while simultaneously monitoring the functional state of the channel or transporter. Many of the important papers that have used VCF to probe the gating mechanisms of channels and transporters have been published in the Journal of General Physiology. In this review, we provide an overview of the development of VCF and discuss some of the key problems that have been addressed using this approach. We end with a brief discussion of the outlook for this technique in the era of high-resolution structures.
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Affiliation(s)
- John Cowgill
- Graduate Program in Biophysics, University of Wisconsin, Madison, WI.,Department of Neuroscience, University of Wisconsin, Madison, WI
| | - Baron Chanda
- Department of Neuroscience, University of Wisconsin, Madison, WI .,Department of Biomolecular Chemistry, University of Wisconsin, Madison, WI
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Dempski RE. Voltage Clamp Fluorometry of P-Type ATPases. Methods Mol Biol 2016; 1377:281-291. [PMID: 26695040 PMCID: PMC4717471 DOI: 10.1007/978-1-4939-3179-8_25] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/05/2023]
Abstract
Voltage clamp fluorometry has become a powerful tool to compare partial reactions of P-type ATPases such as the Na(+),K(+)-ATPase and H(+),K(+)-ATPase with conformational dynamics of these ion pumps. Here, we describe the methodology to heterologously express membrane proteins in X. laevis oocytes and site-specifically label these proteins with one or more fluorophores. Fluorescence changes are measured simultaneously with current measurements under two-electrode voltage clamp conditions.
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Affiliation(s)
- Robert E Dempski
- Department of Chemistry and Biochemistry, Worcester Polytechnic Institute, 100 Institute Road, Worcester, MA, 01609, USA.
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Zhu W, Varga Z, Silva JR. Molecular motions that shape the cardiac action potential: Insights from voltage clamp fluorometry. PROGRESS IN BIOPHYSICS AND MOLECULAR BIOLOGY 2015; 120:3-17. [PMID: 26724572 DOI: 10.1016/j.pbiomolbio.2015.12.003] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/16/2015] [Revised: 11/11/2015] [Accepted: 12/16/2015] [Indexed: 01/04/2023]
Abstract
Very recently, voltage-clamp fluorometry (VCF) protocols have been developed to observe the membrane proteins responsible for carrying the ventricular ionic currents that form the action potential (AP), including those carried by the cardiac Na(+) channel, NaV1.5, the L-type Ca(2+) channel, CaV1.2, the Na(+)/K(+) ATPase, and the rapid and slow components of the delayed rectifier, KV11.1 and KV7.1. This development is significant, because VCF enables simultaneous observation of ionic current kinetics with conformational changes occurring within specific channel domains. The ability gained from VCF, to connect nanoscale molecular movement to ion channel function has revealed how the voltage-sensing domains (VSDs) control ion flux through channel pores, mechanisms of post-translational regulation and the molecular pathology of inherited mutations. In the future, we expect that this data will be of great use for the creation of multi-scale computational AP models that explicitly represent ion channel conformations, connecting molecular, cell and tissue electrophysiology. Here, we review the VCF protocol, recent results, and discuss potential future developments, including potential use of these experimental findings to create novel computational models.
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Affiliation(s)
- Wandi Zhu
- Department of Biomedical Engineering, Washington University in St. Louis, St. Louis, MO, USA
| | - Zoltan Varga
- MTA-DE-NAP B Ion Channel Structure-Function Research Group, RCMM, University of Debrecen, Debrecen, Hungary
| | - Jonathan R Silva
- Department of Biomedical Engineering, Washington University in St. Louis, St. Louis, MO, USA.
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Dürr KL, Tavraz NN, Spiller S, Friedrich T. Measuring cation transport by Na,K- and H,K-ATPase in Xenopus oocytes by atomic absorption spectrophotometry: an alternative to radioisotope assays. J Vis Exp 2013:e50201. [PMID: 23462593 DOI: 10.3791/50201] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/31/2022] Open
Abstract
Whereas cation transport by the electrogenic membrane transporter Na(+),K(+)-ATPase can be measured by electrophysiology, the electroneutrally operating gastric H(+),K(+)-ATPase is more difficult to investigate. Many transport assays utilize radioisotopes to achieve a sufficient signal-to-noise ratio, however, the necessary security measures impose severe restrictions regarding human exposure or assay design. Furthermore, ion transport across cell membranes is critically influenced by the membrane potential, which is not straightforwardly controlled in cell culture or in proteoliposome preparations. Here, we make use of the outstanding sensitivity of atomic absorption spectrophotometry (AAS) towards trace amounts of chemical elements to measure Rb(+) or Li(+) transport by Na(+),K(+)- or gastric H(+),K(+)-ATPase in single cells. Using Xenopus oocytes as expression system, we determine the amount of Rb(+) (Li(+)) transported into the cells by measuring samples of single-oocyte homogenates in an AAS device equipped with a transversely heated graphite atomizer (THGA) furnace, which is loaded from an autosampler. Since the background of unspecific Rb(+) uptake into control oocytes or during application of ATPase-specific inhibitors is very small, it is possible to implement complex kinetic assay schemes involving a large number of experimental conditions simultaneously, or to compare the transport capacity and kinetics of site-specifically mutated transporters with high precision. Furthermore, since cation uptake is determined on single cells, the flux experiments can be carried out in combination with two-electrode voltage-clamping (TEVC) to achieve accurate control of the membrane potential and current. This allowed e.g. to quantitatively determine the 3Na(+)/2K(+) transport stoichiometry of the Na(+),K(+)-ATPase and enabled for the first time to investigate the voltage dependence of cation transport by the electroneutrally operating gastric H(+),K(+)-ATPase. In principle, the assay is not limited to K(+)-transporting membrane proteins, but it may work equally well to address the activity of heavy or transition metal transporters, or uptake of chemical elements by endocytotic processes.
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Richards R, Dempski RE. Re-introduction of transmembrane serine residues reduce the minimum pore diameter of channelrhodopsin-2. PLoS One 2012; 7:e50018. [PMID: 23185520 PMCID: PMC3501483 DOI: 10.1371/journal.pone.0050018] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/31/2012] [Accepted: 10/17/2012] [Indexed: 11/18/2022] Open
Abstract
Channelrhodopsin-2 (ChR2) is a microbial-type rhodopsin found in the green algae Chlamydomonas reinhardtii. Under physiological conditions, ChR2 is an inwardly rectifying cation channel that permeates a wide range of mono- and divalent cations. Although this protein shares a high sequence homology with other microbial-type rhodopsins, which are ion pumps, ChR2 is an ion channel. A sequence alignment of ChR2 with bacteriorhodopsin, a proton pump, reveals that ChR2 lacks specific motifs and residues, such as serine and threonine, known to contribute to non-covalent interactions within transmembrane domains. We hypothesized that reintroduction of the eight transmembrane serine residues present in bacteriorhodopsin, but not in ChR2, will restrict the conformational flexibility and reduce the pore diameter of ChR2. In this work, eight single serine mutations were created at homologous positions in ChR2. Additionally, an endogenous transmembrane serine was replaced with alanine. We measured kinetics, changes in reversal potential, and permeability ratios in different alkali metal solutions using two-electrode voltage clamp. Applying excluded volume theory, we calculated the minimum pore diameter of ChR2 constructs. An analysis of the results from our experiments show that reintroducing serine residues into the transmembrane domain of ChR2 can restrict the minimum pore diameter through inter- and intrahelical hydrogen bonds while the removal of a transmembrane serine results in a larger pore diameter. Therefore, multiple positions along the intracellular side of the transmembrane domains contribute to the cation permeability of ChR2.
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Affiliation(s)
- Ryan Richards
- Department of Chemistry and Biochemistry, Worcester Polytechnic Institute, Worcester, Massachusetts, United States of America
| | - Robert E. Dempski
- Department of Chemistry and Biochemistry, Worcester Polytechnic Institute, Worcester, Massachusetts, United States of America
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Dach I, Olesen C, Signor L, Nissen P, le Maire M, Møller JV, Ebel C. Active detergent-solubilized H+,K+-ATPase is a monomer. J Biol Chem 2012; 287:41963-78. [PMID: 23055529 DOI: 10.1074/jbc.m112.398768] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022] Open
Abstract
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.
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Affiliation(s)
- Ingrid Dach
- Center for Membrane Pumps in Cells and Diseases, Danish Research Foundation, DK-8000 Aarhus, Denmark
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9
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Dürr KL, Tavraz NN, Friedrich T. Control of gastric H,K-ATPase activity by cations, voltage and intracellular pH analyzed by voltage clamp fluorometry in Xenopus oocytes. PLoS One 2012; 7:e33645. [PMID: 22448261 PMCID: PMC3308979 DOI: 10.1371/journal.pone.0033645] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/09/2011] [Accepted: 02/14/2012] [Indexed: 11/24/2022] Open
Abstract
Whereas electrogenic partial reactions of the Na,K-ATPase have been studied in depth, much less is known about the influence of the membrane potential on the electroneutrally operating gastric H,K-ATPase. In this work, we investigated site-specifically fluorescence-labeled H,K-ATPase expressed in Xenopus oocytes by voltage clamp fluorometry to monitor the voltage-dependent distribution between E1P and E2P states and measured Rb+ uptake under various ionic and pH conditions. The steady-state E1P/E2P distribution, as indicated by the voltage-dependent fluorescence amplitudes and the Rb+ uptake activity were highly sensitive to small changes in intracellular pH, whereas even large extracellular pH changes affected neither the E1P/E2P distribution nor transport activity. Notably, intracellular acidification by approximately 0.5 pH units shifted V0.5, the voltage, at which the E1P/E2P ratio is 50∶50, by −100 mV. This was paralleled by an approximately two-fold acceleration of the forward rate constant of the E1P→E2P transition and a similar increase in the rate of steady-state cation transport. The temperature dependence of Rb+ uptake yielded an activation energy of ∼90 kJ/mol, suggesting that ion transport is rate-limited by a major conformational transition. The pronounced sensitivity towards intracellular pH suggests that proton uptake from the cytoplasmic side controls the level of phosphoenzyme entering the E1P→E2P conformational transition, thus limiting ion transport of the gastric H,K-ATPase. These findings highlight the significance of cellular mechanisms contributing to increased proton availability in the cytoplasm of gastric parietal cells. Furthermore, we show that extracellular Na+ profoundly alters the voltage-dependent E1P/E2P distribution indicating that Na+ ions can act as surrogates for protons regarding the E2P→E1P transition. The complexity of the intra- and extracellular cation effects can be rationalized by a kinetic model suggesting that cations reach the binding sites through a rather high-field intra- and a rather low-field extracellular access channel, with fractional electrical distances of ∼0.5 and ∼0.2, respectively.
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Affiliation(s)
| | | | - Thomas Friedrich
- Institute of Chemistry, Technical University of Berlin, Berlin, Germany
- * E-mail:
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Wenzlau JM, Gardner TJ, Frisch LM, Davidson HW, Hutton JC. Development of a novel autoantibody assay for autoimmune gastritis in type 1 diabetic individuals. Diabetes Metab Res Rev 2011; 27:887-90. [PMID: 22069279 PMCID: PMC3812798 DOI: 10.1002/dmrr.1267] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/17/2022]
Abstract
BACKGROUND Autoimmune atrophic body gastritis (ABG) and pernicious anaemia are prototypical, organ-specific autoimmune diseases whose prevalence in the general population is 2.0 vs 2 and 0.15-1%, respectively. The incidence of disease increases with age and is frequently associated with other autoimmune disorders such as type 1 diabetes mellitus (T1DM). Early diagnosis of ABG/pernicious anaemia is essential for the prevention and/or treatment before manifestations of chronic disease become irreversible. Parietal cell autoantibody detection via enzyme-linked immunosorbent assay is currently the most widely used biomarker of disease with diagnosis confirmed by subsequent immunohistochemistry via biopsy. METHODS To improve the assay we designed a specific, molecularly defined radioimmunoprecipitation assay for early detection of ABG, targeting its major antigen, the gastric H+/K+ ATPase 4A subunit ATP4A. RESULTS The major antigenic domain in ATP4A was tested against a panel of sera from new onset patients with T1DM which tested positive for the gold standard T1DM autoantibodies (IAA, IA2A, GAD65A, and ZnT8A). Significant immunoreactivity to ATP4A was measured (25%) while 6% of first-degree relatives of subjects with T1DM who were sero-negative for T1DM autoantigens were positive for ATP4A autoantibodies. ATP4A antibody prevalence increased with age of onset of T1DM, which is atypical of other T1DM autoantibodies. Immunoreactivity to ATP4A, unlike that of T1DM antigens, demonstrates a significant gender bias in newly diagnosed individuals with T1DM. CONCLUSION Although the utility of the assay as a biomarker for T1DM is likely limited, it may serve as an improved indicator of ABG.
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Affiliation(s)
- J M Wenzlau
- Barbara Davis Center for Childhood Diabetes, University of Colorado Denver, Aurora, CO 80045, USA.
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Dürr KL, Seuffert I, Friedrich T. Deceleration of the E1P-E2P transition and ion transport by mutation of potentially salt bridge-forming residues Lys-791 and Glu-820 in gastric H+/K+-ATPase. J Biol Chem 2010; 285:39366-79. [PMID: 20921224 DOI: 10.1074/jbc.m110.133470] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022] Open
Abstract
A lysine residue within the highly conserved center of the fifth transmembrane segment in P(IIC)-type ATPase α-subunits is uniquely found in H,K-ATPases instead of a serine in all Na,K-ATPase isoforms. Because previous studies suggested a prominent role of this residue in determining the electrogenicity of non-gastric H,K-ATPase and in pK(a) modulation of the proton-translocating residues in the gastric H,K-ATPases as well, we investigated its functional significance for ion transport by expressing several Lys-791 variants of the gastric H,K-ATPase in Xenopus oocytes. Although the mutant proteins were all detected at the cell surface, none of the investigated mutants displayed any measurable K(+)-induced stationary currents. In Rb(+) uptake measurements, replacement of Lys-791 by Arg, Ala, Ser, and Glu substantially impaired transport activity and reduced the sensitivity toward the E(2)-specific inhibitor SCH28080. Furthermore, voltage clamp fluorometry using a reporter site in the TM5/TM6 loop for labeling with tetra-methylrhodamine-6-maleimide revealed markedly changed fluorescence signals. All four investigated mutants exhibited a strong shift toward the E(1)P state, in agreement with their reduced SCH28080 sensitivity, and an about 5-10-fold decreased forward rate constant of the E(1)P ↔ E(2)P conformational transition, thus explaining the E(1)P shift and the reduced Rb(+) transport activity. When Glu-820 in TM6 adjacent to Lys-791 was replaced by non-charged or positively charged amino acids, severe effects on fluorescence signals and Rb(+) transport were also observed, whereas substitution by aspartate was less disturbing. These results suggest that formation of an E(2)P-stabilizing interhelical salt bridge is essential to prevent futile proton exchange cycles of H(+) pumping P-type ATPases.
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Affiliation(s)
- Katharina L Dürr
- Technical University of Berlin, Institute of Chemistry, Secr. PC 14, Strasse des 17. Juni 135, D-10623 Berlin, Germany.
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Habeck M, Cirri E, Katz A, Karlish SJ, Apell HJ. Investigation of Electrogenic Partial Reactions in Detergent-Solubilized Na,K-ATPase. Biochemistry 2009; 48:9147-55. [DOI: 10.1021/bi901148k] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Michael Habeck
- Department of Biology, University of Konstanz, 78464 Konstanz, Germany
| | - Erica Cirri
- Department of Biology, University of Konstanz, 78464 Konstanz, Germany
| | - Adriana Katz
- Department of Biological Chemistry, Weizmann Institute of Science, Rehovot 76100, Israel
| | - Steven J. Karlish
- Department of Biological Chemistry, Weizmann Institute of Science, Rehovot 76100, Israel
| | - Hans-Jürgen Apell
- Department of Biology, University of Konstanz, 78464 Konstanz, Germany
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Dürr KL, Abe K, Tavraz NN, Friedrich T. E2P state stabilization by the N-terminal tail of the H,K-ATPase beta-subunit is critical for efficient proton pumping under in vivo conditions. J Biol Chem 2009; 284:20147-54. [PMID: 19491099 DOI: 10.1074/jbc.m109.005769] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The catalytic alpha-subunits of Na,K- and H,K-ATPase require an accessory beta-subunit for proper folding, maturation, and plasma membrane delivery but also for cation transport. To investigate the functional significance of the beta-N terminus of the gastric H,K-ATPase in vivo, several N-terminally truncated beta-variants were expressed in Xenopus oocytes, together with the S806C alpha-subunit variant. Upon labeling with the reporter fluorophore tetramethylrho da mine-6-maleimide, this construct can be used to determine the voltage-dependent distribution between E(1)P/E(2)P states. Whereas the E(1)P/E(2)P conformational equilibrium was unaffected for the shorter N-terminal deletions betaDelta4 and betaDelta8, we observed significant shifts toward E(1)P for the two larger deletions betaDelta13 and betaDelta29. Moreover, the reduced DeltaF/F ratios of betaDelta13 and betaDelta29 indicated an increased reverse reaction via E(2)P --> E(1)P + ADP --> E(1) + ATP, because cell surface expression was completely unaffected. This interpretation is supported by the reduced sensitivity of the mutants toward the E(2)P-specific inhibitor SCH28080, which becomes especially apparent at high concentrations (100 microm). Despite unaltered apparent Rb(+) affinities, the maximal Rb(+) uptake of these mutants was also significantly lowered. Considering the two putative interaction sites between the beta-N terminus and alpha-subunit revealed by the recent cryo-EM structure, the N-terminal tail of the H,K-ATPase beta-subunit may stabilize the pump in the E(2)P conformation, thereby increasing the efficiency of proton release against the million-fold proton gradient of the stomach lumen. Finally, we demonstrate that a similar truncation of the beta-N terminus of the closely related Na,K-ATPase does not affect the E(1)P/E(2)P distribution or pump activity, indicating that the E(2)P-stabilizing effect by the beta-N terminus is apparently a unique property of the H,K-ATPase.
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Affiliation(s)
- Katharina L Dürr
- Institute of Chemistry, Technical University of Berlin, D-10623 Berlin, Germany.
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14
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Voltage clamp fluorometry: Combining fluorescence and electrophysiological methods to examine the structure–function of the Na+/K+-ATPase. BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 2009; 1787:714-20. [DOI: 10.1016/j.bbabio.2009.03.021] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/04/2009] [Revised: 03/29/2009] [Accepted: 03/30/2009] [Indexed: 11/23/2022]
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