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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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Displacement of the Na +/K + pump's transmembrane domains demonstrates conserved conformational changes in P-type 2 ATPases. Proc Natl Acad Sci U S A 2021; 118:2019317118. [PMID: 33597302 DOI: 10.1073/pnas.2019317118] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/21/2023] Open
Abstract
Cellular survival requires the ion gradients built by the Na+/K+ pump, an ATPase that alternates between two major conformations (E1 and E2). Here we use state-specific engineered-disulfide cross-linking to demonstrate that transmembrane segment 2 (M2) of the pump's α-subunit moves in directions that are inconsistent with distances observed in existing crystal structures of the Na+/K+ pump in E1 and E2. We characterize this movement with voltage-clamp fluorometry in single-cysteine mutants. Most mutants in the M1-M2 loop produced state-dependent fluorescence changes upon labeling with tetramethylrhodamine-6-maleimide (TMRM), which were due to quenching by multiple endogenous tryptophans. To avoid complications arising from multiple potential quenchers, we analyzed quenching of TMRM conjugated to R977C (in the static M9-M10 loop) by tryptophans introduced, one at a time, in M1-M2. This approach showed that tryptophans introduced in M2 quench TMRM only in E2, with D126W and L130W on the same helix producing the largest fluorescence changes. These observations indicate that M2 moves outward as Na+ is deoccluded from the E1 conformation, a mechanism consistent with cross-linking results and with proposals for other P-type 2 ATPases.
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3
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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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4
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A Model for the Homotypic Interaction between Na +,K +-ATPase β 1 Subunits Reveals the Role of Extracellular Residues 221-229 in Its Ig-Like Domain. Int J Mol Sci 2019; 20:ijms20184538. [PMID: 31540261 PMCID: PMC6770782 DOI: 10.3390/ijms20184538] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/09/2019] [Revised: 08/13/2019] [Accepted: 08/16/2019] [Indexed: 12/15/2022] Open
Abstract
The Na+, K+-ATPase transports Na+ and K+ across the membrane of all animal cells. In addition to its ion transporting function, the Na+, K+-ATPase acts as a homotypic epithelial cell adhesion molecule via its β1 subunit. The extracellular region of the Na+, K+-ATPase β1 subunit includes a single globular immunoglobulin-like domain. We performed Molecular Dynamics simulations of the ectodomain of the β1 subunit and a refined protein-protein docking prediction. Our results show that the β1 subunit Ig-like domain maintains an independent structure and dimerizes in an antiparallel fashion. Analysis of the putative interface identified segment Lys221-Tyr229. We generated triple mutations on YFP-β1 subunit fusion proteins to assess the contribution of these residues. CHO fibroblasts transfected with mutant β1 subunits showed a significantly decreased cell-cell adhesion. Association of β1 subunits in vitro was also reduced, as determined by pull-down assays. Altogether, we conclude that two Na+, K+-ATPase molecules recognize each other by a large interface spanning residues 221–229 and 198–207 on their β1 subunits.
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5
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External Ion Access in the Na/K Pump: Kinetics of Na +, K +, and Quaternary Amine Interaction. Biophys J 2019; 115:361-374. [PMID: 30021111 DOI: 10.1016/j.bpj.2018.06.007] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/01/2018] [Revised: 05/18/2018] [Accepted: 06/06/2018] [Indexed: 11/23/2022] Open
Abstract
Na/K pumps build essential ion gradients across the plasmalemma of animal cells by coupling the extrusion of three Na+, with the import of two K+ and the hydrolysis of one ATP molecule. The mechanisms of selectivity and competition between Na+, K+, and inhibitory amines remain unclear. We measured the effects of external tetrapropylammonium (TPA+) and ethylenediamine (EDA2+) on three different Na/K pump transport modes in voltage-clamped Xenopus oocytes: 1) outward pump current (IP), 2) passive inward H+ current at negative voltages without Na+ or K+ (IH), and 3) transient charge movement reporting the voltage-dependent extracellular binding/release of Na+ (QNa). Both amines competed with K+ to inhibit IP. TPA+ inhibited IH without competing with H+, whereas EDA2+ did not alter IH at pH 7.6. TPA+ competed with Na+ in QNa measurements, reducing Na+-apparent affinity, evidenced by a ∼-75 mV shift in the charge-voltage curve (at 20 mM TPA+) without reduction of the total charge moved (Qtot). In contrast, EDA2+ and K+ did not compete with Na+ to inhibit QNa; both reduced Qtot without decreasing Na+-apparent affinity. EDA2+ (15 mM) right-shifted the charge-voltage curve by ∼+50 mV. Simultaneous occlusion of EDA2+ and Na+ by an E2P conformation unable to reach E1P was demonstrated by voltage-clamp fluorometry. Trypsinolysis experiments showed that EDA2+-bound pumps are much more proteolysis-resistant than Na+-, K+-, or TPA+-bound pumps, therefore uncovering unique EDA2+-bound conformations. K+ effects on QNa and IH were also evaluated in pumps inhibited with beryllium fluoride, a phosphate mimic. K+ reduced Qtot without shifting the charge-voltage curve, indicating noncompetitive effects, and partially inhibited IH to the same extent as TPA+ in non-beryllium-fluorinated pumps. These results demonstrate that K+ interacts with beryllium-fluorinated pumps inducing conformational changes that alter QNa and IH, suggesting that there are two external access pathways for proton transport by IH.
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6
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Nakajo K. Gating modulation of the KCNQ1 channel by KCNE proteins studied by voltage-clamp fluorometry. Biophys Physicobiol 2019; 16:121-126. [PMID: 31236320 PMCID: PMC6587909 DOI: 10.2142/biophysico.16.0_121] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/24/2019] [Accepted: 05/01/2019] [Indexed: 01/22/2023] Open
Abstract
The KCNQ1 channel is a voltage-dependent potassium channel and is ubiquitously expressed throughout the human body including the heart, lung, kidney, pancreas, intestine and inner ear. Gating properties of the KCNQ1 channel are modulated by KCNE auxiliary subunits. For example, the KCNQ1-KCNE1 channel produces a slowly-activating potassium current, while KCNE3 makes KCNQ1 a voltage-independent, constitutively open channel. Thus, physiological functions of KCNQ1 channels are greatly dependent on the type of KCNE protein that is co-expressed in that organ. It has long been debated how the similar single transmembrane KCNE proteins produce quite different gating behaviors. Recent applications of voltage-clamp fluorometry (VCF) for the KCNQ1 channel have shed light on this question. The VCF is a quite sensitive method to detect structural changes of membrane proteins and is especially suitable for tracking the voltage sensor domains of voltage-gated ion channels. In this short review, I will introduce how the VCF technique can be applied to detect structural changes and what have been revealed by the recent VCF applications to the gating modulation of KCNQ1 channels by KCNE proteins.
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Affiliation(s)
- Koichi Nakajo
- Division of Integrative Physiology, Department of Physiology, Jichi Medical University, Shimotsuke, Tochigi 329-0498, Japan
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7
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Vilchis-Nestor CA, Roldán ML, Leonardi A, Navea JG, Padilla-Benavides T, Shoshani L. Ouabain Enhances Cell-Cell Adhesion Mediated by β 1 Subunits of the Na +,K +-ATPase in CHO Fibroblasts. Int J Mol Sci 2019; 20:E2111. [PMID: 31035668 PMCID: PMC6539428 DOI: 10.3390/ijms20092111] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/16/2019] [Revised: 04/26/2019] [Accepted: 04/26/2019] [Indexed: 12/19/2022] Open
Abstract
Adhesion is a crucial characteristic of epithelial cells to form barriers to pathogens and toxic substances from the environment. Epithelial cells attach to each other using intercellular junctions on the lateral membrane, including tight and adherent junctions, as well as the Na+,K+-ATPase. Our group has shown that non-adherent chinese hamster ovary (CHO) cells transfected with the canine β1 subunit become adhesive, and those homotypic interactions amongst β1 subunits of the Na+,K+-ATPase occur between neighboring epithelial cells. Ouabain, a cardiotonic steroid, binds to the α subunit of the Na+,K+-ATPase, inhibits the pump activity and induces the detachment of epithelial cells when used at concentrations above 300 nM. At nanomolar non-inhibiting concentrations, ouabain affects the adhesive properties of epithelial cells by inducing the expression of cell adhesion molecules through the activation of signaling pathways associated with the α subunit. In this study, we investigated whether the adhesion between β1 subunits was also affected by ouabain. We used CHO fibroblasts stably expressing the β1 subunit of the Na+,K+-ATPase (CHO β1), and studied the effect of ouabain on cell adhesion. Aggregation assays showed that ouabain increased the adhesion between CHO β1 cells. Immunofluorescence and biotinylation assays showed that ouabain (50 nM) increases the expression of the β1 subunit of the Na+,K+-ATPase at the cell membrane. We also examined the effect of ouabain on the activation of signaling pathways in CHO β1 cells, and their subsequent effect on cell adhesion. We found that cSrc is activated by ouabain and, therefore, that it likely regulates the adhesive properties of CHO β1 cells. Collectively, our findings suggest that the β1 subunit adhesion is modulated by the expression levels of the Na+,K+-ATPase at the plasma membrane, which is regulated by ouabain.
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Affiliation(s)
- Claudia Andrea Vilchis-Nestor
- Department of Physiology Biophysics and Neurosciences, Center for Research and Advanced Studies, Cinvestav-Ipn, CDMX 07360, Mexico.
- Department of Biochemistry and Molecular Pharmacology, University of Massachusetts Medical School, Worcester, MA 01605, USA.
| | - María Luisa Roldán
- Department of Physiology Biophysics and Neurosciences, Center for Research and Advanced Studies, Cinvestav-Ipn, CDMX 07360, Mexico.
| | - Angelina Leonardi
- Department of Chemistry, Skidmore College, 815 North Broadway, Saratoga Springs, NY 12866, USA.
| | - Juan G Navea
- Department of Chemistry, Skidmore College, 815 North Broadway, Saratoga Springs, NY 12866, USA.
| | - Teresita Padilla-Benavides
- Department of Biochemistry and Molecular Pharmacology, University of Massachusetts Medical School, Worcester, MA 01605, USA.
| | - Liora Shoshani
- Department of Physiology Biophysics and Neurosciences, Center for Research and Advanced Studies, Cinvestav-Ipn, CDMX 07360, Mexico.
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8
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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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9
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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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10
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Garcia A, Eljack ND, Sani MA, Separovic F, Rasmussen HH, Kopec W, Khandelia H, Cornelius F, Clarke RJ. Membrane accessibility of glutathione. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2015; 1848:2430-6. [PMID: 26232559 DOI: 10.1016/j.bbamem.2015.07.016] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/24/2015] [Revised: 07/23/2015] [Accepted: 07/28/2015] [Indexed: 10/23/2022]
Abstract
Regulation of the ion pumping activity of the Na+,K+-ATPase is crucial to the survival of animal cells. Recent evidence has suggested that the activity of the enzyme could be controlled by glutathionylation of cysteine residue 45 of the β-subunit. Crystal structures so far available indicate that this cysteine is in a transmembrane domain of the protein. Here we have analysed via fluorescence and NMR spectroscopy as well as molecular dynamics simulations whether glutathione is able to penetrate into the interior of a lipid membrane. No evidence for any penetration of glutathione into the membrane was found. Therefore, the most likely mechanism whereby the cysteine residue could become glutathionylated is via a loosening of the α-β subunit association, creating a hydrophilic passageway between them to allow access of glutathione to the cysteine residue. By such a mechanism, glutathionylation of the protein would be expected to anchor the modified cysteine residue in a hydrophilic environment, inhibiting further motion of the β-subunit during the enzyme's catalytic cycle and suppressing enzymatic activity, as has been experimentally observed. The results obtained, therefore, suggest a possible structural mechanism of how the Na+,K+-ATPase could be regulated by glutathione.
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Affiliation(s)
- Alvaro Garcia
- School of Chemistry, University of Sydney, Sydney, New South Wales 2006, Australia
| | - Nasma D Eljack
- School of Chemistry, University of Sydney, Sydney, New South Wales 2006, Australia
| | - Marc-Antoine Sani
- School of Chemistry, Bio21 Institute, University of Melbourne, Melbourne, Victoria 3010, Australia
| | - Frances Separovic
- School of Chemistry, Bio21 Institute, University of Melbourne, Melbourne, Victoria 3010, Australia
| | - Helge H Rasmussen
- Department of Cardiology, Royal North Shore Hospital, Sydney, New South Wales 2065, Australia; Kolling Institute, University of Sydney, Sydney, New South Wales 2065, Australia
| | - Wojciech Kopec
- Center for BioMembrane Physics, University of Southern Denmark, Odense M5230, Denmark
| | - Himanshu Khandelia
- Center for BioMembrane Physics, University of Southern Denmark, Odense M5230, Denmark
| | - Flemming Cornelius
- Department of Biomedicine, University of Aarhus, DK-8000 Aarhus C, Denmark
| | - Ronald J Clarke
- School of Chemistry, University of Sydney, Sydney, New South Wales 2006, Australia.
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11
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Structure and mechanism of ATP-dependent phospholipid transporters. Biochim Biophys Acta Gen Subj 2014; 1850:461-75. [PMID: 24746984 DOI: 10.1016/j.bbagen.2014.04.008] [Citation(s) in RCA: 55] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/08/2014] [Revised: 04/04/2014] [Accepted: 04/07/2014] [Indexed: 01/09/2023]
Abstract
BACKGROUND ATP-binding cassette (ABC) transporters and P4-ATPases are two large and seemingly unrelated families of primary active pumps involved in moving phospholipids from one leaflet of a biological membrane to the other. SCOPE OF REVIEW This review aims to identify common mechanistic features in the way phospholipid flipping is carried out by two evolutionarily unrelated families of transporters. MAJOR CONCLUSIONS Both protein families hydrolyze ATP, although they employ different mechanisms to use it, and have a comparable size with twelve transmembrane segments in the functional unit. Further, despite differences in overall architecture, both appear to operate by an alternating access mechanism and during transport they might allow access of phospholipids to the internal part of the transmembrane domain. The latter feature is obvious for ABC transporters, but phospholipids and other hydrophobic molecules have also been found embedded in P-type ATPase crystal structures. Taken together, in two diverse groups of pumps, nature appears to have evolved quite similar ways of flipping phospholipids. GENERAL SIGNIFICANCE Our understanding of the structural basis for phospholipid flipping is still limited but it seems plausible that a general mechanism for phospholipid flipping exists in nature. This article is part of a Special Issue entitled Structural biochemistry and biophysics of membrane proteins.
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12
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Havlíková M, Huličiak M, Bazgier V, Berka K, Kubala M. Fluorone dyes have binding sites on both cytoplasmic and extracellular domains of Na,K-ATPase. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2012; 1828:568-76. [PMID: 23142565 DOI: 10.1016/j.bbamem.2012.10.029] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/28/2012] [Revised: 10/24/2012] [Accepted: 10/31/2012] [Indexed: 11/30/2022]
Abstract
Combination of fluorescence techniques and molecular docking was used to monitor interaction of Na,K-ATPase and its large cytoplasmic loop connecting fourth and fifth transmembrane helices (C45) with fluorone dyes (i.e. eosin Y, 5(6)-carboxyeosin, rose bengal, fluorescein, and erythrosine B). Our data suggested that there are at least two binding sites for all used fluorone dyes, except of 5(6)-carboxyeosin. The first binding site is located on C45 loop, and it is sensitive to the presence of nucleotide. The other site is located on the extracellular part of the enzyme, and it is sensitive to the presence of Na(+) or K(+) ions. The molecular docking revealed that in the open conformation of C45 loop (which is obtained in the presence of ATP) all used fluorone dyes occupy position directly inside the ATP-binding pocket, while in the closed conformation (i.e. in the absence of any ligand) they are located only near the ATP-binding site depending on their different sizes. On the extracellular part of the protein, the molecular docking predicts two possible binding sites with similar binding energy near Asp897(α) or Gln69(β). The former was identified as a part of interaction site between α- and β-subunits, the latter is in contact with conserved FXYD sequence of the γ-subunit. Our findings provide structural explanation for numerous older studies, which were performed with fluorone dyes before the high-resolution structures were known. Further, fluorone dyes seem to be good probes for monitoring of intersubunit interactions influenced by Na(+) and K(+) binding.
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Affiliation(s)
- Marika Havlíková
- Department of Biophysics, Faculty of Science, Palacký University in Olomouc, tř. 17. listopadu 12, 771 46 Olomouc, Czech Republic.
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13
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Allosteric property of the (Na++K+)-ATPase β1 subunit. Biochem Biophys Res Commun 2011; 415:479-84. [DOI: 10.1016/j.bbrc.2011.10.098] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/18/2011] [Accepted: 10/21/2011] [Indexed: 11/20/2022]
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14
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Richards R, Dempski RE. Examining the conformational dynamics of membrane proteins in situ with site-directed fluorescence labeling. J Vis Exp 2011:2627. [PMID: 21673634 PMCID: PMC3197104 DOI: 10.3791/2627] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022] Open
Abstract
Two electrode voltage clamp electrophysiology (TEVC) is a powerful tool to investigate the mechanism of ion transport1 for a wide variety of membrane proteins including ion channels2, ion pumps3, and transporters4. Recent developments have combined site-specific fluorophore labeling alongside TEVC to concurrently examine the conformational dynamics at specific residues and function of these proteins on the surface of single cells. We will describe a method to study the conformational dynamics of membrane proteins by simultaneously monitoring fluorescence and current changes using voltage-clamp fluorometry. This approach can be used to examine the molecular motion of membrane proteins site-specifically following cysteine replacement and site-directed fluorophore labeling5,6. Furthermore, this method provides an approach to determine distance constraints between specific residues7,8. This is achieved by selectively attaching donor and acceptor fluorophores to two mutated cysteine residues of interest. In brief, these experiments are performed following functional expression of the desired protein on the surface of Xenopus leavis oocytes. The large surface area of these oocytes enables facile functional measurements and a robust fluorescence signal5. It is also possible to readily change the extracellular conditions such as pH, ligand or cations/anions, which can provide further information on the mechanism of membrane proteins4. Finally, recent developments have also enabled the manipulation of select internal ions following co-expression with a second protein9. Our protocol is described in multiple parts. First, cysteine scanning mutagenesis proceeded by fluorophore labeling is completed at residues located at the interface of the transmembrane and extracellular domains. Subsequent experiments are designed to identify residues which demonstrate large changes in fluorescence intensity (<5%)3 upon a conformational change of the protein. Second, these changes in fluorescence intensity are compared to the kinetic parameters of the membrane protein in order to correlate the conformational dynamics to the function of the protein10. This enables a rigorous biophysical analysis of the molecular motion of the target protein. Lastly, two residues of the holoenzyme can be labeled with a donor and acceptor fluorophore in order to determine distance constraints using donor photodestruction methods. It is also possible to monitor the relative movement of protein subunits following labeling with a donor and acceptor fluorophore.
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Affiliation(s)
- Ryan Richards
- Department of Chemistry and Biochemistry, Worcester Polytechnic Institute
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15
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Lenoir G, Williamson P, Puts CF, Holthuis JCM. Cdc50p plays a vital role in the ATPase reaction cycle of the putative aminophospholipid transporter Drs2p. J Biol Chem 2009; 284:17956-67. [PMID: 19411703 PMCID: PMC2709398 DOI: 10.1074/jbc.m109.013722] [Citation(s) in RCA: 102] [Impact Index Per Article: 6.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/11/2009] [Revised: 04/27/2009] [Indexed: 11/06/2022] Open
Abstract
Members of the P(4) subfamily of P-type ATPases are believed to catalyze transport of phospholipids across cellular bilayers. However, most P-type ATPases pump small cations or metal ions, and atomic structures revealed a transport mechanism that is conserved throughout the family. Hence, a challenging problem is to understand how this mechanism is adapted in P(4)-ATPases to flip phospholipids. P(4)-ATPases form heteromeric complexes with Cdc50 proteins. The primary role of these additional polypeptides is unknown. Here, we show that the affinity of yeast P(4)-ATPase Drs2p for its Cdc50-binding partner fluctuates during the transport cycle, with the strongest interaction occurring at a point where the enzyme is loaded with phospholipid ligand. We also find that specific interactions with Cdc50p are required to render the ATPase competent for phosphorylation at the catalytically important aspartate residue. Our data indicate that Cdc50 proteins are integral components of the P(4)-ATPase transport machinery. Thus, acquisition of these subunits may have been a crucial step in the evolution of flippases from a family of cation pumps.
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Affiliation(s)
- Guillaume Lenoir
- From the Department of Membrane Enzymology, Bijvoet Center and Institute of Biomembranes, Utrecht University, 3584 CH Utrecht, The Netherlands and
| | - Patrick Williamson
- the Department of Biology, Amherst College, Amherst, Massachusetts 010022
| | - Catheleyne F. Puts
- From the Department of Membrane Enzymology, Bijvoet Center and Institute of Biomembranes, Utrecht University, 3584 CH Utrecht, The Netherlands and
| | - Joost C. M. Holthuis
- From the Department of Membrane Enzymology, Bijvoet Center and Institute of Biomembranes, Utrecht University, 3584 CH Utrecht, The Netherlands and
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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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17
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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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18
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Geys SA, Bamberg E, Dempski RE. Ligand-Dependent Effects on the Conformational Equilibrium of the Na+,K+-ATPase As Monitored by Voltage Clamp Fluorometry. Biophys J 2009; 96:4561-70. [DOI: 10.1016/j.bpj.2009.03.002] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/14/2008] [Revised: 03/01/2009] [Accepted: 03/06/2009] [Indexed: 11/29/2022] Open
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19
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Abstract
PURPOSE OF REVIEW Na,K-ATPase is an oligomeric protein composed of alpha subunits, beta subunits and FXYD proteins. The catalytic alpha subunit hydrolyzes ATP and transports the cations. Increasing experimental evidence suggest that beta subunits and FXYD proteins essentially contribute to the variable physiological needs of Na,K-ATPase function in different tissues. RECENT FINDINGS Beta subunits have a crucial role in the structural and functional maturation of Na,K-ATPase and modulate its transport properties. The chaperone function of the beta subunit is essential, for example, in the formation of tight junctions and cell polarity. Recent studies suggest that beta subunits also have inherent functions, which are independent of Na,K-ATPase activity and which may be involved in cell-cell adhesiveness and in suppression of cell motility. As for FXYD proteins, they modulate Na,K-ATPase activity in a tissue-specific way, in some cases in close cooperation with posttranslational modifications such as phosphorylation. SUMMARY A better understanding of the multiple functional roles of the accessory subunits of Na,K-ATPase is crucial to appraise their influence on physiological processes and their implication in pathophysiological states.
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20
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Pressley TA. The stoichiometry of the Na-K pump: one plus one doesn't equal one. Am J Physiol Renal Physiol 2008; 295:F1313. [PMID: 18768585 DOI: 10.1152/ajprenal.90512.2008] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
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21
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Clifford RJ, Kaplan JH. beta-Subunit overexpression alters the stoicheometry of assembled Na-K-ATPase subunits in MDCK cells. Am J Physiol Renal Physiol 2008; 295:F1314-23. [PMID: 18701620 DOI: 10.1152/ajprenal.90406.2008] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022] Open
Abstract
In eukaryotic cells, the apparent maintenance of 1:1 stoicheometry between the Na-K-ATPase alpha- and beta-subunits led us to question whether this was alterable and thus if some form of regulation was involved. We have examined the consequences of overexpressing Na-K-ATPase beta1-subunits using Madin-Darby canine kidney (MDCK) cells expressing flag-tagged beta1-subunits (beta1flag) or Myc-tagged beta1-subunits (beta1myc) under the control of a tetracycline-dependent promoter. The induction of beta1flag subunit synthesis in MDCK cells, which increases beta1-subunit expression at the plasma membrane by more than twofold, while maintaining stable alpha1 expression levels, revealed that all mature beta1-subunits associate with alpha1-subunits, and no evidence of "free" beta1-subunits was obtained. Consequently, the ratio of assembled beta1- to alpha1-subunits is significantly increased when "extra" beta-subunits are expressed. An increased beta1/alpha1 stoicheometry is also observed in cells treated with tunicamycin, suggesting that the protein-protein interactions involved in these complexes are not dependent on glycosylation. Confocal images of cocultured beta1myc-expressing and beta1flag-expressing MDCK cells show colocalization of beta1myc and beta1flag subunits at the lateral membranes of neighboring cells, suggesting the occurrence of intercellular interactions between the beta-subunits. Immunoprecipitation using MDCK cells constitutively expressing beta1myc and tetracycline-regulated beta1flag subunits confirmed beta-beta-subunit interactions. These results demonstrate that the equimolar ratio of assembled beta1/alpha1-subunits of the Na-K-ATPase in kidney cells is not fixed by the inherent properties of the interacting subunits. It is likely that cellular mechanisms are present that regulate the individual Na-K-ATPase subunit abundance.
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Affiliation(s)
- Rebecca J Clifford
- Department of Biochemistry and Molecular Genetics, University of Illinois at Chicago, Chicago, IL 60607-7170, USA
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22
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Dürr KL, Tavraz NN, Zimmermann D, Bamberg E, Friedrich T. Characterization of Na,K-ATPase and H,K-ATPase Enzymes with Glycosylation-Deficient β-Subunit Variants by Voltage-Clamp Fluorometry in Xenopus Oocytes. Biochemistry 2008; 47:4288-97. [DOI: 10.1021/bi800092k] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Katharina L. Dürr
- Max Volmer Laboratory for Biophysical Chemistry, Institute of Chemistry, Technical University of Berlin, Secr. PC 14, Strasse des 17. Juni 135, D-10623 Berlin, Germany, Department of Biophysical Chemistry, Max Planck Institute of Biophysics, Max-von-Laue-Strasse 3, D-60438 Frankfurt/Main, Germany, and Chemical and Pharmaceutical Sciences Department, Johann Wolfgang Goethe University Frankfurt, Max-von-Laue-Strasse 1, 7-9, D-60439 Frankfurt/Main, Germany
| | - Neslihan N. Tavraz
- Max Volmer Laboratory for Biophysical Chemistry, Institute of Chemistry, Technical University of Berlin, Secr. PC 14, Strasse des 17. Juni 135, D-10623 Berlin, Germany, Department of Biophysical Chemistry, Max Planck Institute of Biophysics, Max-von-Laue-Strasse 3, D-60438 Frankfurt/Main, Germany, and Chemical and Pharmaceutical Sciences Department, Johann Wolfgang Goethe University Frankfurt, Max-von-Laue-Strasse 1, 7-9, D-60439 Frankfurt/Main, Germany
| | - Dirk Zimmermann
- Max Volmer Laboratory for Biophysical Chemistry, Institute of Chemistry, Technical University of Berlin, Secr. PC 14, Strasse des 17. Juni 135, D-10623 Berlin, Germany, Department of Biophysical Chemistry, Max Planck Institute of Biophysics, Max-von-Laue-Strasse 3, D-60438 Frankfurt/Main, Germany, and Chemical and Pharmaceutical Sciences Department, Johann Wolfgang Goethe University Frankfurt, Max-von-Laue-Strasse 1, 7-9, D-60439 Frankfurt/Main, Germany
| | - Ernst Bamberg
- Max Volmer Laboratory for Biophysical Chemistry, Institute of Chemistry, Technical University of Berlin, Secr. PC 14, Strasse des 17. Juni 135, D-10623 Berlin, Germany, Department of Biophysical Chemistry, Max Planck Institute of Biophysics, Max-von-Laue-Strasse 3, D-60438 Frankfurt/Main, Germany, and Chemical and Pharmaceutical Sciences Department, Johann Wolfgang Goethe University Frankfurt, Max-von-Laue-Strasse 1, 7-9, D-60439 Frankfurt/Main, Germany
| | - Thomas Friedrich
- Max Volmer Laboratory for Biophysical Chemistry, Institute of Chemistry, Technical University of Berlin, Secr. PC 14, Strasse des 17. Juni 135, D-10623 Berlin, Germany, Department of Biophysical Chemistry, Max Planck Institute of Biophysics, Max-von-Laue-Strasse 3, D-60438 Frankfurt/Main, Germany, and Chemical and Pharmaceutical Sciences Department, Johann Wolfgang Goethe University Frankfurt, Max-von-Laue-Strasse 1, 7-9, D-60439 Frankfurt/Main, Germany
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Dempski RE, Hartung K, Friedrich T, Bamberg E. Fluorometric measurements of intermolecular distances between the alpha- and beta-subunits of the Na+/K+-ATPase. J Biol Chem 2006; 281:36338-46. [PMID: 16980302 DOI: 10.1074/jbc.m604788200] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The Na+/K+-ATPase maintains the physiological Na+ and K+ gradients across the plasma membrane in most animal cells. The functional unit of the ion pump is comprised of two mandatory subunits including the alpha-subunit, which mediates ATP hydrolysis and ion translocation, as well as the beta-subunit, which acts as a chaperone to promote proper membrane insertion and trafficking in the plasma membrane. To examine the conformational dynamics between the alpha- and beta-subunits of the Na+/K+-ATPase during ion transport, we have used fluorescence resonance energy transfer, under voltage clamp conditions on Xenopus laevis oocytes, to differentiate between two models that have been proposed for the relative orientation of the alpha- and beta-subunits. These experiments were performed by measuring the time constant of irreversible donor fluorophore destruction with fluorescein-5-maleimide as the donor fluorophore and in the presence or absence of tetramethylrhodamine-6-maleimide as the acceptor fluorophore following labeling on the M3-M4 or M5-M6 loop of the alpha-subunit and the beta-subunit. We have also used fluorescence resonance energy transfer to investigate the relative movement between the two subunits as the ion pump shuttles between the two main conformational states (E1 and E2) as described by the Albers-Post scheme. The results from this study have identified a model for the orientation of the beta-subunit in relation to the alpha-subunit and suggest that the alpha- and beta-subunits move toward each other during the E2 to E1 conformational transition.
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Affiliation(s)
- Robert E Dempski
- Department of Biophysical Chemistry, Max Planck Institute of Biophysics, D-60438 Frankfurt am Main, Germany
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