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El Salamouni NS, Buckley BJ, Lee R, Ranson M, Kelso MJ, Yu H. Ion Transport and Inhibitor Binding by Human NHE1: Insights from Molecular Dynamics Simulations and Free Energy Calculations. J Phys Chem B 2024; 128:440-450. [PMID: 38185879 DOI: 10.1021/acs.jpcb.3c05863] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/09/2024]
Abstract
The human Na+/H+ exchanger (NHE1) plays a crucial role in maintaining intracellular pH by regulating the electroneutral exchange of a single intracellular H+ for one extracellular Na+ across the plasma membrane. Understanding the molecular mechanisms governing ion transport and the binding of inhibitors is of importance in the development of anticancer therapeutics targeting NHE1. In this context, we performed molecular dynamics (MD) simulations based on the recent cryo-electron microscopy (cryo-EM) structures of outward- and inward-facing conformations of NHE1. These simulations allowed us to explore the dynamics of the protein, examine the ion-translocation pore, and confirm that Asp267 is the ion-binding residue. Our free energy calculations did not show a significant difference between Na+ and K+ binding at the ion-binding site. Consequently, Na+ over K+ selectivity cannot be solely explained by differences in ion binding. Our MD simulations involving NHE1 inhibitors (cariporide and amiloride analogues) maintained stable interactions with Asp267 and Glu346. Our study highlights the importance of the salt bridge between the positively charged acylguanidine moiety and Asp267, which appears to play a role in the competitive inhibitory mechanism for this class of inhibitors. Our computational study provides a detailed mechanistic interpretation of experimental data and serves the basis of future structure-based inhibitor design.
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Affiliation(s)
- Nehad S El Salamouni
- School of Chemistry and Molecular Bioscience, University of Wollongong, Wollongong, NSW 2522, Australia
- Molecular Horizons, University of Wollongong, Wollongong, NSW 2522, Australia
| | - Benjamin J Buckley
- School of Chemistry and Molecular Bioscience, University of Wollongong, Wollongong, NSW 2522, Australia
- Molecular Horizons, University of Wollongong, Wollongong, NSW 2522, Australia
| | - Richmond Lee
- School of Chemistry and Molecular Bioscience, University of Wollongong, Wollongong, NSW 2522, Australia
- Molecular Horizons, University of Wollongong, Wollongong, NSW 2522, Australia
| | - Marie Ranson
- School of Chemistry and Molecular Bioscience, University of Wollongong, Wollongong, NSW 2522, Australia
- Molecular Horizons, University of Wollongong, Wollongong, NSW 2522, Australia
| | - Michael J Kelso
- School of Chemistry and Molecular Bioscience, University of Wollongong, Wollongong, NSW 2522, Australia
- Molecular Horizons, University of Wollongong, Wollongong, NSW 2522, Australia
| | - Haibo Yu
- School of Chemistry and Molecular Bioscience, University of Wollongong, Wollongong, NSW 2522, Australia
- Molecular Horizons, University of Wollongong, Wollongong, NSW 2522, Australia
- ARC Centre of Excellence in Quantum Biotechnology, University of Wollongong, Wollongong, NSW 2522, Australia
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2
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Amino Acids 785, 787 of the Na +/H + Exchanger Cytoplasmic Tail Modulate Protein Activity and Tail Conformation. Int J Mol Sci 2021; 22:ijms222111349. [PMID: 34768780 PMCID: PMC8583816 DOI: 10.3390/ijms222111349] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/20/2021] [Revised: 10/14/2021] [Accepted: 10/18/2021] [Indexed: 11/30/2022] Open
Abstract
The mammalian Na+/H+ exchanger isoform 1 (NHE1) is a plasma membrane protein ubiquitously present in humans. It regulates intracellular pH by removing an intracellular proton in exchange for an extracellular sodium. It consists of a 500 amino acid membrane domain plus a 315 amino acid, regulatory cytosolic tail. Here, we investigated the effect of mutation of two amino acids of the regulatory tail, Ser785 and Ser787, that were similar in location and context to two amino acids of the Arabidopsis Na+/H+ exchanger SOS1. Mutation of these two amino acids to either Ala or phosphomimetic Glu did not affect surface targeting but led to a slight reduction in the level of protein expressed. The activity of the NHE1 protein was reduced in the phosphomimetic mutations and the effect was due to a decrease in Vmax activity. The Ser to Glu mutations also caused a change in the apparent molecular weight of both the full-length protein and of the cytosolic tail of NHE1. A conformational change in this region was indicated by differential trypsin sensitivity. We also found that a peptide containing amino acids 783–790 bound to several more proximal regions of the NHE1 tail in in vitro protein interaction experiments. The results are the first characterization of these two amino acids and show that they have significant effects on enzyme kinetics and the structure of the NHE1 protein.
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3
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Acidic residues of extracellular loop 3 of the Na +/H + exchanger type 1 are important in cation transport. Mol Cell Biochem 2020; 468:13-20. [PMID: 32130622 DOI: 10.1007/s11010-020-03707-9] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/23/2019] [Accepted: 02/21/2020] [Indexed: 12/11/2022]
Abstract
Mammalian Na+/H+ exchanger type I isoform (NHE1) is a ubiquitously expressed membrane protein that regulates intracellular pH (pHi) by removing one intracellular proton in exchange for one extracellular sodium ion. Abnormal activity of the protein occurs in cardiovascular disease and breast cancer. The purpose of this study is to examine the role of negatively charged amino acids of extracellular loop 3 (EL3) in the activity of the NHE protein. We mutated glutamic acid 217 and aspartic acid 226 to alanine, and to glutamine and asparagine, respectively. We examined effects on expression levels, cell surface targeting and activity of NHE1, and also characterized affinity for extracellular sodium and lithium ions. Individual mutation of these amino acids had little effect on protein function. However, mutation of both these amino acids together impaired transport, decreasing the Vmax for both Na+ and Li+ ions. We suggested that amino acids E217 and D226 form part of a negatively charged coordination sphere, which facilitates cation transport in the NHE1 protein.
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4
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Amino Acids 563-566 of the Na +/H + Exchanger Isoform 1 C-Terminal Cytosolic Tail Prevent Protein Degradation and Stabilize Protein Expression and Activity. Int J Mol Sci 2020; 21:ijms21051737. [PMID: 32138345 PMCID: PMC7084640 DOI: 10.3390/ijms21051737] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/21/2020] [Revised: 02/28/2020] [Accepted: 02/29/2020] [Indexed: 12/28/2022] Open
Abstract
Isoform one of the mammalian Na+/H+ exchanger is a plasma membrane protein that is ubiquitously present in humans. It regulates intracellular pH through the removal of one intracellular proton in exchange for a single extracellular sodium. It consists of a 500 amino acid membrane domain plus a 315 amino acid, C-terminal tail. We examined amino acids of the C-terminal tail that are important in the targeting and activity of the protein. A previous study demonstrated that stop codon polymorphisms can result in decreased activity, expression, targeting and enhanced protein degradation. Here, we determine elements that are critical in these anomalies. A series of progressive deletions of the C-terminal tail demonstrated a progressive decrease in activity and targeting, though these remained until a final drop off with the deletion of amino acids 563–566. The deletion of the 562LIAGERS568 sequence or the alteration to the 562LAAAARS568 sequence caused the decreased protein expression, aberrant targeting, reduced activity and enhanced degradation of the Na+/H+ exchanger (NHE1) protein. The 562LIAGERS568 sequence bound to other regions of the C-terminal cytosolic domain. We suggest this region is necessary for the activity, targeting, stability, and expression of the NHE1 protein. The results define a new sequence that is important in maintenance of NHE1 protein levels and activity.
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Pedersen SF, Counillon L. The SLC9A-C Mammalian Na +/H + Exchanger Family: Molecules, Mechanisms, and Physiology. Physiol Rev 2019; 99:2015-2113. [PMID: 31507243 DOI: 10.1152/physrev.00028.2018] [Citation(s) in RCA: 123] [Impact Index Per Article: 20.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/06/2023] Open
Abstract
Na+/H+ exchangers play pivotal roles in the control of cell and tissue pH by mediating the electroneutral exchange of Na+ and H+ across cellular membranes. They belong to an ancient family of highly evolutionarily conserved proteins, and they play essential physiological roles in all phyla. In this review, we focus on the mammalian Na+/H+ exchangers (NHEs), the solute carrier (SLC) 9 family. This family of electroneutral transporters constitutes three branches: SLC9A, -B, and -C. Within these, each isoform exhibits distinct tissue expression profiles, regulation, and physiological roles. Some of these transporters are highly studied, with hundreds of original articles, and some are still only rudimentarily understood. In this review, we present and discuss the pioneering original work as well as the current state-of-the-art research on mammalian NHEs. We aim to provide the reader with a comprehensive view of core knowledge and recent insights into each family member, from gene organization over protein structure and regulation to physiological and pathophysiological roles. Particular attention is given to the integrated physiology of NHEs in the main organ systems. We provide several novel analyses and useful overviews, and we pinpoint main remaining enigmas, which we hope will inspire novel research on these highly versatile proteins.
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Affiliation(s)
- S F Pedersen
- Section for Cell Biology and Physiology, Department of Biology, University of Copenhagen, Copenhagen, Denmark; and Université Côte d'Azur, CNRS, Laboratoire de Physiomédecine Moléculaire, LP2M, France, and Laboratories of Excellence Ion Channel Science and Therapeutics, Nice, France
| | - L Counillon
- Section for Cell Biology and Physiology, Department of Biology, University of Copenhagen, Copenhagen, Denmark; and Université Côte d'Azur, CNRS, Laboratoire de Physiomédecine Moléculaire, LP2M, France, and Laboratories of Excellence Ion Channel Science and Therapeutics, Nice, France
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6
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Dutta D, Ullah A, Bibi S, Fliegel L. Functional Analysis of Conserved Transmembrane Charged Residues and a Yeast Specific Extracellular Loop of the Plasma Membrane Na +/H + Antiporter of Schizosaccharomyces pombe. Sci Rep 2019; 9:6191. [PMID: 30996236 PMCID: PMC6470128 DOI: 10.1038/s41598-019-42658-0] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/31/2018] [Accepted: 04/01/2019] [Indexed: 01/20/2023] Open
Abstract
The Na+/H+ exchanger of the plasma membrane of S. pombe (SpNHE1) removes excess intracellular sodium in exchange for an extracellular proton. We examined the functional role of acidic amino acids of a yeast specific periplasmic extracellular loop 6 (EL6) and of Glu74 and Arg77 of transmembrane segment 3. Glu74 and Arg77 are conserved in yeast species while Glu74 is conserved throughout various phyla. The mutation E74A caused a minor effect, while mutation R77A had a larger effect on the ability of SpNHE1 to confer salt tolerance. Mutation of both residues to Ala or Glu also eliminated the ability to confer salt tolerance. Arg341 and Arg342 were also necessary for SpNHE1 transport in S. pombe. Deletion of 3 out of 4 acidic residues (Asp389, Glu390, Glu392, Glu397) of EL6 did not greatly affect SpNHE1 function while deletion of all did. Replacement of EL6 with a segment from the plant Na+/H+ exchanger SOS1 also did not affect function. We suggest that EL6 forms part of a cation coordination sphere, attracting cations for transport but that the region is not highly specific for the location of acidic charges. Overall, we identified a number of polar amino acids important in SpNHE1 function.
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Affiliation(s)
- Debajyoti Dutta
- Department of Biochemistry, University of Alberta, Edmonton, Alberta, T6G 2H7, Canada
| | - Asad Ullah
- Department of Biochemistry, University of Alberta, Edmonton, Alberta, T6G 2H7, Canada
| | - Sana Bibi
- Department of Biochemistry, University of Alberta, Edmonton, Alberta, T6G 2H7, Canada
| | - Larry Fliegel
- Department of Biochemistry, University of Alberta, Edmonton, Alberta, T6G 2H7, Canada.
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7
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Dutta D, Fliegel L. Molecular modeling and inhibitor docking analysis of the Na +/H + exchanger isoform one 1. Biochem Cell Biol 2018; 97:333-343. [PMID: 30058365 DOI: 10.1139/bcb-2018-0158] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022] Open
Abstract
Na+/H+ exchanger isoform one (NHE1) is a mammalian plasma membrane protein that removes intracellular protons, thereby elevating intracellular pH (pHi). NHE1 uses the energy of allowing an extracellular sodium down its gradient into cells to remove one intracellular proton. The ubiquitous protein has several important physiological and pathological influences on mammalian cells as a result of its activity. The three-dimensional structure of human NHE1 (hNHE1) is not known. Here, we modeled NHE1 based on the structure of MjNhaP1 of Methanocaldoccocus jannaschii in combination with biochemical surface accessibility data. hNHE1 contained 12 transmembrane segments including a characteristic Na+/H+ antiporter fold of two transmembrane segments with a helix - extended region - helix conformation crossing each other within the membrane. Amino acids 363-410 mapped principally to the extracellular surface as an extracellular loop (EL5). A large preponderance of amino acids shown to be surface accessible by biochemical experiments mapped near to, or on, the extracellular surface. Docking of Na+/H+ exchanger inhibitors to the extracellular surface suggested that inhibitor binding on an extracellular site is made up from several amino acids of different regions of the protein. The results present a novel testable, three-dimensional model illustrating NHE1 structure and accounting for experimental biochemical data.
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Affiliation(s)
- Debajyoti Dutta
- Department of Biochemistry, University of Alberta, Edmonton, AB T6G 2H7, Canada.,Department of Biochemistry, University of Alberta, Edmonton, AB T6G 2H7, Canada
| | - Larry Fliegel
- Department of Biochemistry, University of Alberta, Edmonton, AB T6G 2H7, Canada.,Department of Biochemistry, University of Alberta, Edmonton, AB T6G 2H7, Canada
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8
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Dutta D, Fliegel L. Structure and function of yeast and fungal Na + /H + antiporters. IUBMB Life 2017; 70:23-31. [PMID: 29219228 DOI: 10.1002/iub.1701] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/26/2017] [Accepted: 11/21/2017] [Indexed: 12/22/2022]
Abstract
Sodium proton antiporters (or sodium proton exchangers [NHEs]) are a critical family of membrane proteins that exchange sodium for protons across cell membranes. In yeast and plants, their primary function is to keep the sodium concentration low inside the cytoplasm. One class of NHE constitutively expressed in yeast is the plasma membrane Na+ /H+ antiporter, and another class is expressed on the endosomal/vacuolar membrane. At present, four bacterial plasma membrane antiporter structures are known and nuclear magnetic resonance structures are available for the membrane spanning transmembrane helices of mammalian and yeast NHEs. Additionally, a vast amount of mutational data are available on the role of individual amino acids and critical motifs involved in transport. We combine this information to obtain a more detailed picture of the yeast NHE plasma membrane protein and review mechanisms of transport, conserved motifs, unique residues important in function, and regulation of these proteins. The Na+ /H+ antiporter of Schizosaccharomyces pombe, SpNHE1, is an interesting model protein in an easy to study system and is representative of fungal Na+ /H+ antiporters. © IUBMB Life, 70(1):23-31, 2018.
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Affiliation(s)
- Debajyoti Dutta
- Department of Biochemistry, University of Alberta, Edmonton, AB, Canada
| | - Larry Fliegel
- Department of Biochemistry, University of Alberta, Edmonton, AB, Canada
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9
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Dutta D, Shin K, Rainey JK, Fliegel L. Transmembrane Segment XI of the Na +/H + Antiporter of S. pombe is a Critical Part of the Ion Translocation Pore. Sci Rep 2017; 7:12793. [PMID: 29038548 PMCID: PMC5643542 DOI: 10.1038/s41598-017-12701-z] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/29/2017] [Accepted: 09/18/2017] [Indexed: 01/01/2023] Open
Abstract
The Na+/H+ exchanger of the plasma membrane of S. pombe (SpNHE1) removes intracellular sodium in exchange for an extracellular proton. We examined the structure and functional role of amino acids 360–393 of putative transmembrane (TM) segment XI of SpNHE1. Structural analysis suggested that it had a helical propensity over amino acids 360–368, an extended region from 369–378 and was helical over amino acids 379–386. TM XI was sensitive to side chain alterations. Mutation of eight amino acids to alanine resulted in loss of one or both of LiCl or NaCl tolerance when re-introduced into SpNHE1 deficient S. pombe. Mutation of seven other amino acids had minor effects. Analysis of structure and functional mutations suggested that Glu361 may be involved in cation coordination on the cytoplasmic face of the protein with a negative charge in this position being important. His367, Ile371 and Gly372 were important in function. Ile371 may have important hydrophobic interactions with other residues and Gly372 may be important in maintaining an extended conformation. Several residues from Val377 to Leu384 are important in function possibly involved in hydrophobic interactions with other amino acids. We suggest that TM XI forms part of the ion translocation core of this Na+/H+ exchanger.
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Affiliation(s)
- Debajyoti Dutta
- Department of Biochemistry, University of Alberta, Edmonton, Alberta, T6G 2H7, Canada
| | - Kyungsoo Shin
- Department of Biochemistry & Molecular Biology, Dalhousie University, Halifax, Nova Scotia, B3H 4R2, Canada
| | - Jan K Rainey
- Department of Biochemistry & Molecular Biology, Dalhousie University, Halifax, Nova Scotia, B3H 4R2, Canada.,Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, B3H 4R2, Canada
| | - Larry Fliegel
- Department of Biochemistry, University of Alberta, Edmonton, Alberta, T6G 2H7, Canada.
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10
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Li X, Augustine A, Chen S, Fliegel L. Stop Codon Polymorphisms in the Human SLC9A1 Gene Disrupt or Compromise Na+/H+ Exchanger Function. PLoS One 2016; 11:e0162902. [PMID: 27636896 PMCID: PMC5026351 DOI: 10.1371/journal.pone.0162902] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/17/2016] [Accepted: 08/30/2016] [Indexed: 11/18/2022] Open
Abstract
The NHE1 isoform of the mammalian Na+/H+ exchanger is a ubiquitous plasma membrane protein that regulates intracellular pH in mammalian cells by removing one intracellular proton in exchange for one extracellular sodium. Deletion of the NHE1 gene (SLC9A1) affects the growth and motor ability of mice and humans but mutations and polymorphisms of the gene are only beginning to be characterized. NHE1 has a cytosolic C-terminal regulatory tail of approximately 315 amino acids and a 500 amino acid membrane domain. We examined the functional effects of three human stop codon mutations at amino acids 321, 449 and 735 in comparison with a mutant that had a shortened tail region (543 stop codon). The short mutants, 321, 449 and 543 stop codon mutant proteins, lost NHE1 activity and expression, and did not target to the plasma membrane. Protein for these short mutants was more rapidly degraded than the wild type and 735 ending proteins. The 735 terminating mutant, with the membrane domain and much of the cytosolic tail, had reduced protein expression and activity. The results demonstrate that early stop codon polymorphisms have significant and deleterious effects on the activity of the SLC9A1 protein product. The 735-NHE1 mutant, without the last 80 amino acids, had more minor defects. Surprisingly, retention of a proximal 43 amino acids adjacent to the membrane domain did little to maintain NHE1 expression, targeting and activity.
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Affiliation(s)
- Xiuju Li
- Department of Biochemistry, University Alberta, Edmonton, AB T6G 2H7, Canada
| | - Aruna Augustine
- Department of Biochemistry, University Alberta, Edmonton, AB T6G 2H7, Canada
| | - Shuo Chen
- Department of Biochemistry, University Alberta, Edmonton, AB T6G 2H7, Canada
| | - Larry Fliegel
- Department of Biochemistry, University Alberta, Edmonton, AB T6G 2H7, Canada
- * E-mail:
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11
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Expression and characterization of the SOS1 Arabidopsis salt tolerance protein. Mol Cell Biochem 2016; 415:133-43. [DOI: 10.1007/s11010-016-2685-2] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/08/2015] [Accepted: 03/12/2016] [Indexed: 10/22/2022]
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12
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Counillon L, Bouret Y, Marchiq I, Pouysségur J. Na(+)/H(+) antiporter (NHE1) and lactate/H(+) symporters (MCTs) in pH homeostasis and cancer metabolism. BIOCHIMICA ET BIOPHYSICA ACTA-MOLECULAR CELL RESEARCH 2016; 1863:2465-80. [PMID: 26944480 DOI: 10.1016/j.bbamcr.2016.02.018] [Citation(s) in RCA: 89] [Impact Index Per Article: 9.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/25/2015] [Revised: 02/19/2016] [Accepted: 02/22/2016] [Indexed: 12/17/2022]
Abstract
The Na(+)/H(+)-exchanger NHE1 and the monocarboxylate transporters MCT1 and MCT4 are crucial for intracellular pH regulation, particularly under active metabolism. NHE1, a reversible antiporter, uses the energy provided by the Na(+) gradient to expel H(+) ions generated in the cytosol. The reversible H(+)/lactate(-) symporters MCT1 and 4 cotransport lactate and proton, leading to the net extrusion of lactic acid in glycolytic tumors. In the first two sections of this article we review important features and remaining questions on the structure, biochemical function and cellular roles of these transporters. We then use a fully-coupled mathematical model to simulate their relative contribution to pH regulation in response to lactate production, as it occurs in highly hypoxic and glycolytic tumor cells. This article is part of a Special Issue entitled: Mitochondrial Channels edited by Pierre Sonveaux, Pierre Maechler and Jean-Claude Martinou.
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Affiliation(s)
- Laurent Counillon
- University of Nice-Sophia Antipolis, LP2M UMR7370, Faculty of Medicine, 28 Avenue Valombrose, 06107 Nice France; Laboratories of Excellence Ion Channel Science and Therapeutics, France.
| | - Yann Bouret
- University of Nice-Sophia Antipolis, LPMC UMR 7336, 28 Avenue Valrose, 06108 Nice, France
| | - Ibtissam Marchiq
- IRCAN, Centre A. Lacassagne, University of Nice-Sophia Antipolis, 33 Avenue Valombrose, 06107 Nice, France
| | - Jacques Pouysségur
- IRCAN, Centre A. Lacassagne, University of Nice-Sophia Antipolis, 33 Avenue Valombrose, 06107 Nice, France; Centre Scientifique de Monaco (CSM), 8, Quai Antoine 1er, Monaco.
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13
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Sodium-Proton (Na+/H+) Antiporters: Properties and Roles in Health and Disease. Met Ions Life Sci 2016; 16:391-458. [DOI: 10.1007/978-3-319-21756-7_12] [Citation(s) in RCA: 56] [Impact Index Per Article: 6.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/22/2023]
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14
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Liu Y, Basu A, Li X, Fliegel L. Topological analysis of the Na+/H+ exchanger. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2015. [DOI: 10.1016/j.bbamem.2015.07.011] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/23/2022]
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15
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Jinadasa T, Josephson CB, Boucher A, Orlowski J. Determinants of Cation Permeation and Drug Sensitivity in Predicted Transmembrane Helix 9 and Adjoining Exofacial Re-entrant Loop 5 of Na+/H+ Exchanger NHE1. J Biol Chem 2015; 290:18173-18186. [PMID: 26063808 DOI: 10.1074/jbc.m115.642199] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/29/2015] [Indexed: 12/25/2022] Open
Abstract
Mammalian Na(+)/H(+) exchangers (NHEs) regulate numerous physiological processes and are involved in the pathogenesis of several diseases, including tissue ischemia and reperfusion injuries, cardiac hypertrophy and failure, and cancer progression. Hence, NHEs are being targeted for pharmaceutical-based clinical therapies, but pertinent information regarding the structural elements involved in cation translocation and drug binding remains incomplete. Molecular manipulations of the prototypical NHE1 isoform have implicated several predicted membrane-spanning (M) helices, most notably M4, M9, and M11, as important determinants of cation permeation and drug sensitivity. Here, we have used substituted-cysteine accessibility mutagenesis and thiol-modifying methanethiosulfonate (MTS) reagents to further probe the involvement of evolutionarily conserved sites within M9 (residues 342-363) and the adjacent exofacial re-entrant loop 5 between M9 and M10 (EL5; residues 364-415) of a cysteine-less variant of rat NHE1 on its kinetic and pharmacological properties. MTS treatment significantly reduced the activity of mutants containing substitutions within M9 (H353C, S355C, and G356C) and EL5 (G403C and S405C). In the absence of MTS, mutants S355C, G403C, and S405C showed modest to significant decreases in their apparent affinities for Na(+) o and/or H(+) i. In addition, mutations Y370C and E395C within EL5, whereas failing to confer sensitivity to MTS, nevertheless, reduced the affinity for Na(+) o, but not for H(+) i. The Y370C mutant also exhibited higher affinity for ethylisopropylamiloride, a competitive antagonist of Na(+) o transport. Collectively, these results further implicate helix M9 and EL5 of NHE1 as important elements involved in cation transport and inhibitor sensitivity, which may inform rational drug design.
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Affiliation(s)
- Tushare Jinadasa
- Department of Physiology, McGill University, Montréal, Québec H3G 1Y6
| | - Colin B Josephson
- Department of Physiology, McGill University, Montréal, Québec H3G 1Y6; Division of Clinical Neurosciences, University of Calgary Foothills Medical Centre, Calgary, Alberta T2N 2T9, Canada
| | - Annie Boucher
- Department of Physiology, McGill University, Montréal, Québec H3G 1Y6
| | - John Orlowski
- Department of Physiology, McGill University, Montréal, Québec H3G 1Y6.
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16
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Li X, Fliegel L. A novel human mutation in the SLC9A1 gene results in abolition of Na+/H+ exchanger activity. PLoS One 2015; 10:e0119453. [PMID: 25760855 PMCID: PMC4356549 DOI: 10.1371/journal.pone.0119453] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/02/2014] [Accepted: 01/06/2015] [Indexed: 11/19/2022] Open
Abstract
The SLC9A1 gene, the Na+/H+ exchanger isoform 1 is the principal plasma membrane Na+/H+ exchanger of mammalian cells and functions by exchanging one intracellular proton for one extracellular sodium. The human protein is 815 amino acids in length. Five hundred N-terminal amino acids make up the transport domain of the protein and are believed to form 12 transmembrane segments. Recently, a genetic mutation of the Na+/H+ exchanger isoform 1, N266H, was discovered in a human patient through exome sequencing. We examined the effect of this mutation on expression, targeting and activity of the Na+/H+ exchanger. Mutant N266H protein was expressed in AP-1 cells, which lack their endogenous Na+/H+ exchanger protein. Targeting of the mutant protein to the cell surface was normal and expression levels were only slightly reduced relative to the wild type protein. However, the N266H mutant protein had no detectable Na+/H+ exchanger activity. A histidine residue at this location may disrupt the cation binding site or the pore of the Na+/H+ exchanger protein.
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Affiliation(s)
- Xiuju Li
- Department of Biochemistry, University of Alberta, Edmonton, Alberta, Canada
| | - Larry Fliegel
- Department of Biochemistry, University of Alberta, Edmonton, Alberta, Canada
- * E-mail:
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17
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Li X, Ma Y, Fliegel L. Functional role of arginine 425 in the mammalian Na+/H+ exchanger. Biochem Cell Biol 2014; 92:541-6. [DOI: 10.1139/bcb-2014-0070] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Na+/H+ exchanger isoform 1 (NHE1) is the principal plasma membrane Na+/H+ exchanger of mammalian cells and functions by exchanging one intracellular proton for one extracellular sodium ion. Critical transmembrane segments of Na+/H+ exchangers have discontinuous transmembrane helices, which result in a dipole within the membrane. Amino acid R425 has been suggested to play an important role in neutralizing one such helix dipole. To investigate this hypothesis, R425 was mutated to alanine, glutamine, histidine, or lysine and the mutant NHE1 proteins were expressed and characterized in NHE1-deficient cells. The R425A and R425E mutants exhibited complete loss of expression of mature, fully glycosylated NHE1, reduced expression overall, and greatly reduced cell surface targeting. The cell surface targeting, expression, and activity of the R425H and R425K mutant proteins were also impaired, though residual NHE1 activity remained. When reduced targeting and expression were accounted for, the R425H and R425K mutant proteins had activity similar to that of the wild-type protein. The results suggest that R425 is critical for NHE1 expression, targeting, and activity and that replacement with another basic residue can rescue activity. The findings are consistent with a role for R425 in both neutralizing a helix dipole and maintaining NHE1 structure and function.
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Affiliation(s)
- Xiuju Li
- Department of Biochemistry, University of Alberta, 347 Medical Sciences Building, Edmonton, AB T6G 2H7, Canada
| | - Yike Ma
- Department of Biochemistry, University of Alberta, 347 Medical Sciences Building, Edmonton, AB T6G 2H7, Canada
| | - Larry Fliegel
- Department of Biochemistry, University of Alberta, 347 Medical Sciences Building, Edmonton, AB T6G 2H7, Canada
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Alves C, Ma Y, Li X, Fliegel L. Characterization of human mutations in phosphorylatable amino acids of the cytosolic regulatory tail of SLC9A1. Biochem Cell Biol 2014; 92:524-9. [PMID: 25162926 DOI: 10.1139/bcb-2014-0071] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The NHE1 isoform of the mammalian Na(+)/H(+) exchanger is a ubiquitous plasma membrane protein that regulates intracellular pH in cells by removing one intracellular proton in exchange for one extracellular sodium. Genetic defects in NHE1 have been shown to affect the growth and motor ability of mice, but mutations in humans have not been studied. NHE1 has a cytosolic C-terminal regulatory domain of approximately 300 amino acids. We investigated the functional effects of two human mutations found in the regulatory phosphorylatable amino acids Ser(703) and Ser(771). A Ser703Pro mutant protein had essentially the same activity, expression, and targeting as the wild type NHE1 protein. In contrast, the Ser771Pro protein had reduced activity and expression of NHE1 protein, though cell surface targeting was normal. In dual pulse assays the Ser771Pro mutant was not further activated by sustained intracellular acidosis but displayed an unusual activation by brief pulses of acidosis. The results demonstrate that the Ser771Pro human genetic mutation has significant and detrimental physiological effects on the activity of the NHE1 protein, SLC9A1.
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Affiliation(s)
- Claudia Alves
- Department of Biochemistry, University Alberta, Edmonton, AB T6G 2H7, Canada
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Alves C, Lee BL, Sykes BD, Fliegel L. Structural and Functional Analysis of the Transmembrane Segment Pair VI and VII of the NHE1 Isoform of the Na+/H+ Exchanger. Biochemistry 2014; 53:3658-70. [DOI: 10.1021/bi500392y] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- Claudia Alves
- Department of Biochemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2H7
| | - Brian L. Lee
- Department of Biochemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2H7
| | - Brian D. Sykes
- Department of Biochemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2H7
| | - Larry Fliegel
- Department of Biochemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2H7
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Kemp G, Fliegel L, Young HS. Membrane transport piece by piece: production of transmembrane peptides for structural and functional studies. CURRENT PROTOCOLS IN PROTEIN SCIENCE 2014; 75:29.8.1-29.8.28. [PMID: 24510677 DOI: 10.1002/0471140864.ps2908s75] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
Membrane proteins are involved in all cellular processes from signaling cascades to nutrient uptake and waste disposal. Because of these essential functions, many membrane proteins are recognized as important, yet elusive, clinical targets. Recent advances in structural biology have answered many questions about how membrane proteins function, yet one of the major bottlenecks remains the ability to obtain sufficient quantities of pure and homogeneous protein. This is particularly true for human membrane proteins, where novel expression strategies and structural techniques are needed to better characterize their function and therapeutic potential. One way to approach this challenge is to determine the structure of smaller pieces of membrane proteins that can be assembled into models of the complete protein. This unit describes the rationale for working with single or multiple transmembrane segments and provides a description of strategies and methods to express and purify them for structural and functional studies using a maltose binding protein (MBP) fusion. The bulk of the unit outlines a detailed methodology and justification for producing these peptides under native-like conditions.
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Affiliation(s)
- Grant Kemp
- Department of Biochemistry, Faculty of Medicine and Dentistry, University of Alberta, Edmonton, Alberta, Canada
| | - Larry Fliegel
- Department of Biochemistry, Faculty of Medicine and Dentistry, University of Alberta, Edmonton, Alberta, Canada
| | - Howard S Young
- Department of Biochemistry, Faculty of Medicine and Dentistry, University of Alberta, Edmonton, Alberta, Canada.,National Institute for Nanotechnology, University of Alberta, Edmonton, Alberta, Canada
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21
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Hendus-Altenburger R, Kragelund BB, Pedersen SF. Structural dynamics and regulation of the mammalian SLC9A family of Na⁺/H⁺ exchangers. CURRENT TOPICS IN MEMBRANES 2014; 73:69-148. [PMID: 24745981 DOI: 10.1016/b978-0-12-800223-0.00002-5] [Citation(s) in RCA: 62] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/19/2022]
Abstract
Mammalian Na⁺/H⁺ exchangers of the SLC9A family are widely expressed and involved in numerous essential physiological processes. Their primary function is to mediate the 1:1 exchange of Na⁺ for H⁺ across the membrane in which they reside, and they play central roles in regulation of body, cellular, and organellar pH. Their function is tightly regulated through mechanisms involving interactions with multiple protein and lipid-binding partners, phosphorylations, and other posttranslational modifications. Biochemical and mutational analyses indicate that the SLC9As have a short intracellular N-terminus, 12 transmembrane (TM) helices necessary and sufficient for ion transport, and a C-terminal cytoplasmic tail region with essential regulatory roles. No high-resolution structures of the SLC9As exist; however, models based on crystal structures of the bacterial NhaAs support the 12 TM organization and suggest that TMIV and XI may form a central part of the ion-translocation pathway, whereas pH sensing may involve TMII, TMIX, and several intracellular loops. Similar to most ion transporters studied, SLC9As likely exist as coupled dimers in the membrane, and this appears to be important for the well-studied cooperativity of H⁺ binding. The aim of this work is to summarize and critically discuss the currently available evidence on the structural dynamics, regulation, and binding partner interactions of SLC9As, focusing in particular on the most widely studied isoform, SLC9A1/NHE1. Further, novel bioinformatic and structural analyses are provided that to some extent challenge the existing paradigm on how ions are transported by mammalian SLC9As.
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Affiliation(s)
- Ruth Hendus-Altenburger
- Section for Biomolecular Sciences, Department of Biology, University of Copenhagen, Copenhagen, Denmark; Section for Cell and Developmental Biology, Department of Biology, University of Copenhagen, Copenhagen, Denmark
| | - Birthe B Kragelund
- Section for Biomolecular Sciences, Department of Biology, University of Copenhagen, Copenhagen, Denmark
| | - Stine Falsig Pedersen
- Section for Cell and Developmental Biology, Department of Biology, University of Copenhagen, Copenhagen, Denmark.
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Abstract
Tightly coupled exchange of Na(+) for H(+) occurs across the surface membrane of virtually all living cells. For years, the underlying molecular entity was unknown and the full physiological significance of the exchange process was not appreciated, but much knowledge has been gained in the last two decades. We now realize that, unlike most of the other transporters that specialize in supporting one specific function, Na(+)/H(+) exchangers (NHE) participate in a remarkable assortment of physiological processes, ranging from pH homeostasis and epithelial salt transport, to systemic and cellular volume regulation. In parallel, we have learned a great deal about the biochemistry and molecular biology of Na(+)/H(+) exchange. Indeed, it has now become apparent that exchange is mediated not by one, but by a diverse family of related yet distinct carriers (antiporters) sometimes present in different cell types and located in various intracellular compartments. Each one of these has unique structural features that dictate its functional role and mode of regulation. The biological relevance of Na(+)/H(+) exchange is emphasized by its evolutionary conservation; analogous exchangers are present from bacteria to man. Because of its wide distribution and versatile function, Na(+)/H(+) exchange has attracted an enormous amount of interest and therefore generated a vast literature. The vastness and complexity of the field has been compounded by the multiplicity of NHE isoforms. For reasons of space and in the spirit of this series, this overview is restricted to the family of mammalian NHEs.
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Affiliation(s)
- John Orlowski
- Department of Physiology, McGill University, Montreal, Canada
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Rösner HI, Kragelund BB. Structure and dynamic properties of membrane proteins using NMR. Compr Physiol 2013; 2:1491-539. [PMID: 23798308 DOI: 10.1002/cphy.c110036] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Abstract
Integral membrane proteins are one of the most challenging groups of macromolecules despite their apparent conformational simplicity. They manage and drive transport, circulate information, and participate in cellular movements via interactions with other proteins and through intricate conformational changes. Their structural and functional decoding is challenging and has imposed demanding experimental development. Solution nuclear magnetic resonance (NMR) spectroscopy is one of the techniques providing the capacity to make a significant difference in the deciphering of the membrane protein structure-function paradigm. The method has evolved dramatically during the last decade resulting in a plethora of new experiments leading to a significant increase in the scientific repertoire for studying membrane proteins. Besides solving the three-dimensional structures using state-of-the-art approaches, a large variety of developments of well-established techniques are available providing insight into membrane protein flexibility, dynamics, and interactions. Inspired by the speed of development in the application of new strategies, by invention of methods to measure solvent accessibility and describe low-populated states, this review seeks to introduce the vast possibilities solution NMR can offer to the study of membrane protein structure-function analyses with special focus on applicability.
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Affiliation(s)
- Heike I Rösner
- Structural Biology and NMR Laboratory, Department of Biology, University of Copenhagen, Copenhagen, Denmark
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24
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Structural and functional insights into the cardiac Na+/H+ exchanger. J Mol Cell Cardiol 2013; 61:60-7. [DOI: 10.1016/j.yjmcc.2012.11.019] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 11/14/2012] [Revised: 11/26/2012] [Accepted: 11/28/2012] [Indexed: 11/19/2022]
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25
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Ullah A, Kemp G, Lee B, Alves C, Young H, Sykes BD, Fliegel L. Structural and functional analysis of transmembrane segment IV of the salt tolerance protein Sod2. J Biol Chem 2013; 288:24609-24. [PMID: 23836910 DOI: 10.1074/jbc.m113.483065] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/24/2023] Open
Abstract
Sod2 is the plasma membrane Na(+)/H(+) exchanger of the fission yeast Schizosaccharomyces pombe. It provides salt tolerance by removing excess intracellular sodium (or lithium) in exchange for protons. We examined the role of amino acid residues of transmembrane segment IV (TM IV) ((126)FPQINFLGSLLIAGCITSTDPVLSALI(152)) in activity by using alanine scanning mutagenesis and examining salt tolerance in sod2-deficient S. pombe. Two amino acids were critical for function. Mutations T144A and V147A resulted in defective proteins that did not confer salt tolerance when reintroduced into S. pombe. Sod2 protein with other alanine mutations in TM IV had little or no effect. T144D and T144K mutant proteins were inactive; however, a T144S protein was functional and provided lithium, but not sodium, tolerance and transport. Analysis of sensitivity to trypsin indicated that the mutations caused a conformational change in the Sod2 protein. We expressed and purified TM IV (amino acids 125-154). NMR analysis yielded a model with two helical regions (amino acids 128-142 and 147-154) separated by an unwound region (amino acids 143-146). Molecular modeling of the entire Sod2 protein suggested that TM IV has a structure similar to that deduced by NMR analysis and an overall structure similar to that of Escherichia coli NhaA. TM IV of Sod2 has similarities to TM V of the Zygosaccharomyces rouxii Na(+)/H(+) exchanger and TM VI of isoform 1 of mammalian Na(+)/H(+) exchanger. TM IV of Sod2 is critical to transport and may be involved in cation binding or conformational changes of the protein.
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Affiliation(s)
- Asad Ullah
- Department of Biochemistry, University of Alberta, Edmonton, Alberta T6G 2H7, Canada
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Structural changes in the C-terminal regulatory region of the Na⁺/H⁺ exchanger mediate phosphorylation induced regulation. J Mol Cell Cardiol 2013; 61:153-63. [PMID: 23602949 DOI: 10.1016/j.yjmcc.2013.04.007] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 12/03/2012] [Revised: 03/21/2013] [Accepted: 04/09/2013] [Indexed: 11/23/2022]
Abstract
The Na(+)/H(+) exchanger isoform 1 (NHE1) is a plasma membrane pH regulatory protein that removes one intracellular H(+) in exchange for an extracellular Na(+). NHE1 is regulated by phosphorylation of the cytoplasmic regulatory region and amino acids Ser(770) and Ser(771) of the regulatory domain are necessary for activation by sustained intracellular acidosis. The phosphomimetic mutations (S770D/S771D) resulted in an activated form of the protein. Immunoprecipitation of full length NHE1 protein showed that the phosphomimetic mutant had increased sensitivity to digestion with trypsin indicating a conformational change. Tryptic digestion of purified C-terminal regulatory region showed that the S770D/S771D mutation altered sensitivity to trypsin digestion. Wild type and phosphomimetic purified C-terminal region (577-815) of human NHE1 were compared and tryptophan fluorescence indicated that there were pH-dependent differences in the conformation of the proteins. Native polyacrylamide gel electrophoresis demonstrated that the phosphomimetic mutant had a more compact structure. Bottom-up hydrogen/deuterium exchange mass spectrometry demonstrated that a peptide fragment containing the phosphomimetic mutations became strongly stabilized relative to the wild type protein. Overall, the results suggested that phosphorylation of S770/S771 changes the conformation of the C-terminal regulatory region in a pH-dependent manner, resulting in a more compact region that affects NHE1 activity. This article is part of a Special Issue entitled "Na(+) Regulation in Cardiac Myocytes".
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Kang C, Li Q. Solution NMR study of integral membrane proteins. Curr Opin Chem Biol 2011; 15:560-9. [DOI: 10.1016/j.cbpa.2011.05.025] [Citation(s) in RCA: 66] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/20/2011] [Revised: 05/12/2011] [Accepted: 05/23/2011] [Indexed: 11/29/2022]
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Elevated expression of activated Na(+)/H(+) exchanger protein induces hypertrophy in isolated rat neonatal ventricular cardiomyocytes. Mol Cell Biochem 2011; 358:179-87. [PMID: 21720766 DOI: 10.1007/s11010-011-0933-z] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/04/2011] [Accepted: 06/21/2011] [Indexed: 10/18/2022]
Abstract
The plasma membrane protein the Na(+)/H(+) exchanger isoform1 (NHE1) has been implicated in various cardiac pathologies including ischemia/reperfusion damage to the myocardium and cardiac hypertrophy. Levels of NHE1 protein and activity are elevated in cardiac disease; however, the mechanism by which these factors contribute to the accompanying hypertrophy in the myocardium is still not clear. To investigate the mechanism of NHE1-induced hypertrophy in the myocardium we constructed two adenoviral vectors expressing either wild type NHE1 protein or a constitutively active NHE1 protein. Infection of neonatal rat ventricular cardiomyocytes (NRVM) resulted in elevated expression of both wild type NHE1 or constitutively active NHE1. Only expression of activated NHE1 protein resulted in an increase in cell size and in an increase in protein synthesis in isolated cardiomyocyte cells. The results demonstrate that expression of activated NHE1 promotes cardiac hypertrophy in isolated cardiac cells and that simple elevation of levels of wild type NHE1 protein does not have a significant hypertrophic effect in NRVM. The results suggest that regulation of NHE1 activity is a critical direct effector of the hypertrophic effect induced in the myocardium by the NHE1 protein.
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Structural and functional analysis of critical amino acids in TMVI of the NHE1 isoform of the Na+/H+ exchanger. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2011; 1808:2327-35. [PMID: 21600870 DOI: 10.1016/j.bbamem.2011.05.004] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/27/2011] [Revised: 04/28/2011] [Accepted: 05/05/2011] [Indexed: 01/16/2023]
Abstract
The mammalian Na(+)/H(+) exchanger isoform 1 (NHE1) resides on the plasma membrane and exchanges one intracellular H(+) for one extracellular Na(+). It maintains intracellular pH and regulates cell volume, and cell functions including growth and cell differentiation. Previous structural and functional studies on TMVI revealed several amino acids that are potentially pore lining. We examined these and other critical residues by site-directed mutagenesis substituting Asn227→Ala, Asp, Arg; Ile233→Ala; Leu243→Ala; Glu247→Asp, Gln; Glu248→Asp, Gln. Mutant NHE1 proteins were characterized in AP-1 cells, which do not express endogenous NHE1. All the TMVI critical amino acids were highly sensitive to substitution and changes often lead to a dysfunctional protein. Mutations of Asn227→Ala, Asp, Arg; Ile233→Ala; Leu243→Ala; Glu247→Asp; Glu248→Gln yielded significant reduction in NHE1 activity. Mutants of Asn227 demonstrated defects in protein expression, targeting and activity. Substituting Asn227→Arg and Ile233→Ala decreased the surface localization and expression of NHE1 respectively. The pore lining amino acids Ile233 and Leu243 were both essential for activity. Glu247 was not essential, but the size of the residue at this location was important while the charge on residue Glu248 was more critical to NHE1 function. Limited trypsin digestion on Leu243→Ala and Glu248→Gln revealed that they had increased susceptibility to proteolytic attack, indicating an alteration in protein conformation. Modeling of TMVI with TMXI suggests that these TM segments form part of the critical fold of NHE1 with Ile233 and Leu465 of TMXI forming a critical part of the extracellular facing ion conductance pathway.
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Qureshi T, Goto NK. Contemporary methods in structure determination of membrane proteins by solution NMR. Top Curr Chem (Cham) 2011; 326:123-85. [PMID: 22160391 DOI: 10.1007/128_2011_306] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/02/2023]
Abstract
Integral membrane proteins are vital to life, being responsible for information and material exchange between a cell and its environment. Although high-resolution structural information is needed to understand how these functions are achieved, membrane proteins remain an under-represented subset of the protein structure databank. Solution NMR is increasingly demonstrating its ability to help address this knowledge shortfall, with the development of a diverse array of techniques to counter the challenges presented by membrane proteins. Here we document the advances that are helping to define solution NMR as an effective tool for membrane protein structure determination. Developments introduced over the last decade in the production of isotope-labeled samples, reconstitution of these samples into the growing selection of NMR-compatible membrane-mimetic systems, and the approaches used for the acquisition and application of structural restraints from these complexes are reviewed.
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Affiliation(s)
- Tabussom Qureshi
- Department of Chemistry, University of Ottawa, Ottawa, ON, Canada
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