1
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Manori B, Da'adoosh B, Haitin Y, Giladi M. Identification of a magnesium-binding site at the primary allosteric calcium sensor of the sodium-calcium exchanger: Implications for physiological regulation. Protein Sci 2024; 33:e5114. [PMID: 38989557 DOI: 10.1002/pro.5114] [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] [Received: 05/16/2024] [Revised: 06/14/2024] [Accepted: 06/29/2024] [Indexed: 07/12/2024]
Abstract
Sodium-calcium exchanger (NCX) proteins are ubiquitously expressed and play a pivotal role in cellular calcium homeostasis by mediating uphill calcium efflux across the cell membrane. Intracellular calcium allosterically regulates the exchange activity by binding to two cytoplasmic calcium-binding domains, CBD1 and CBD2. However, the calcium-binding affinities of these domains are seemingly inadequate to sense physiological calcium oscillations. Previously, magnesium binding to either domain was shown to tune their affinity for calcium, bringing it into the physiological range. However, while the magnesium-binding site of CBD2 was identified, the identity of the CBD1 magnesium site remains elusive. Here, using molecular dynamics in combination with differential scanning fluorimetry and mutational analysis, we pinpoint the magnesium-binding site in CBD1. Specifically, among four calcium-binding sites (Ca1-Ca4) in this domain, only Ca1 can accommodate magnesium with an affinity similar to its free intracellular concentration. Moreover, our results provide mechanistic insights into the modulation of the regulatory calcium affinity by magnesium, which allows an adequate NCX activity level throughout varying physiological needs.
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Affiliation(s)
- Bar Manori
- Department of Physiology and Pharmacology, Faculty of Medical and Health Sciences, Tel Aviv University, Tel Aviv, Israel
| | - Benny Da'adoosh
- Blavatnic Center for Drug Discovery, Tel Aviv University, Tel Aviv, Israel
| | - Yoni Haitin
- Department of Physiology and Pharmacology, Faculty of Medical and Health Sciences, Tel Aviv University, Tel Aviv, Israel
- Sagol School of Neuroscience, Tel Aviv University, Tel Aviv, Israel
| | - Moshe Giladi
- Department of Physiology and Pharmacology, Faculty of Medical and Health Sciences, Tel Aviv University, Tel Aviv, Israel
- Tel Aviv Sourasky Medical Center, Tel Aviv, Israel
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2
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Gök C, Fuller W. Rise of palmitoylation: A new trick to tune NCX1 activity. BIOCHIMICA ET BIOPHYSICA ACTA. MOLECULAR CELL RESEARCH 2024; 1871:119719. [PMID: 38574822 DOI: 10.1016/j.bbamcr.2024.119719] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/22/2023] [Revised: 03/11/2024] [Accepted: 03/27/2024] [Indexed: 04/06/2024]
Abstract
The cardiac Na+/Ca2+ Exchanger (NCX1) controls transmembrane calcium flux in numerous tissues. The only reversible post-translational modification established to regulate NCX1 is palmitoylation, which alters the ability of the exchanger to inactivate. Palmitoylation creates a binding site for the endogenous XIP domain, a region of the NCX1 intracellular loop established to inactivate NCX1. The binding site created by NCX1 palmitoylation sensitizes the transporter to XIP. Herein we summarize our recent knowledge on NCX1 palmitoylation and its association with cardiac pathologies, and discuss these findings in the light of the recent cryo-EM structures of human NCX1.
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Affiliation(s)
- Caglar Gök
- School of Cardiovascular and Metabolic Health (SCMH), Sir James Black Building, University of Glasgow, Glasgow G12 8QQ, United Kingdom.
| | - William Fuller
- School of Cardiovascular and Metabolic Health (SCMH), Sir James Black Building, University of Glasgow, Glasgow G12 8QQ, United Kingdom
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3
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Proton-modulated interactions of ions with transport sites of prokaryotic and eukaryotic NCX prototypes. Cell Calcium 2021; 99:102476. [PMID: 34564055 DOI: 10.1016/j.ceca.2021.102476] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/06/2021] [Revised: 09/13/2021] [Accepted: 09/16/2021] [Indexed: 11/23/2022]
Abstract
The cytosolic pH decline from 7.2 to 6.9 results in 90% inactivation of mammalian Na+/Ca2+ exchangers (NCXs) due to protons interactions with regulatory and transport domains ("proton block"). Remarkably, the pH titration curves of mammalian and prokaryotic NCXs significantly differ, even after excluding the allosteric effects through regulatory domains. This is fascinating since "only" three (out of twelve) ion-coordinating residues (T50S, E213D, and D240N) differ between the archaeal NCX_Mj and mammalian NCXs although they contain either three or two carboxylates, respectively. To resolve the underlying mechanisms of pH-dependent regulation, the ion-coordinating residues of NCX_Mj were mutated to imitate the ion ligation arrays of mammalian NCXs; the mutational effects were tested on the ion binding/transport by using ion-flux assays and two-dimensional infrared (2D IR) spectroscopy. Our analyses revealed that two deprotonated carboxylates ligate 3Na+ or 1Ca2+ in NCX prototypes with three or two carboxylates. The Na+/Ca2+ exchange rates of NCX_Mj reach saturation at pH 5.0, whereas the Na+/Ca2+ exchange rates of the cardiac NCX1.1 gradually increase even at alkaline pHs. The T50S replacement in NCX_Mj "recapitulates" the pH titration curves of mammalian NCX by instigating an alkaline shift. Proteolytic shaving of regulatory CBD domains activates NCX1.1, although the normalized pH-titration curves are comparable in trypsin treated and untreated NCX1.1. Thus, the T50S-dependent alkaline shift sets a dynamic range for "proton block" function at physiological pH, whereas the CBDs (and other regulatory modes) modulate incremental changes in the transport rates rather than affect the shape of pH dependent curves.
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4
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Scranton K, John S, Escobar A, Goldhaber JI, Ottolia M. Modulation of the cardiac Na +-Ca 2+ exchanger by cytoplasmic protons: Molecular mechanisms and physiological implications. Cell Calcium 2019; 87:102140. [PMID: 32070924 DOI: 10.1016/j.ceca.2019.102140] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/27/2019] [Revised: 12/06/2019] [Accepted: 12/07/2019] [Indexed: 01/31/2023]
Abstract
A precise temporal and spatial control of intracellular Ca2+ concentration is essential for a coordinated contraction of the heart. Following contraction, cardiac cells need to rapidly remove intracellular Ca2+ to allow for relaxation. This task is performed by two transporters: the plasma membrane Na+-Ca2+ exchanger (NCX) and the sarcoplasmic reticulum (SR) Ca2+-ATPase (SERCA). NCX extrudes Ca2+ from the cell, balancing the Ca2+entering the cytoplasm during systole through L-type Ca2+ channels. In parallel, following SR Ca2+ release, SERCA activity replenishes the SR, reuptaking Ca2+ from the cytoplasm. The activity of the mammalian exchanger is fine-tuned by numerous ionic allosteric regulatory mechanisms. Micromolar concentrations of cytoplasmic Ca2+ potentiate NCX activity, while an increase in intracellular Na+ levels inhibits NCX via a mechanism known as Na+-dependent inactivation. Protons are also powerful inhibitors of NCX activity. By regulating NCX activity, Ca2+, Na+ and H+ couple cell metabolism to Ca2+ homeostasis and therefore cardiac contractility. This review summarizes the recent progress towards the understanding of the molecular mechanisms underlying the ionic regulation of the cardiac NCX with special emphasis on pH modulation and its physiological impact on the heart.
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Affiliation(s)
- Kyle Scranton
- Department of Anesthesiology and Perioperative Medicine, Division of Molecular Medicine, UCLA, Los Angeles, CA 90095, USA
| | - Scott John
- Department of Medicine (Cardiology), UCLA, Los Angeles, CA 90095, USA; Cardiovascular Research Laboratory, UCLA, Los Angeles, CA 90095, USA
| | - Ariel Escobar
- Department of Bioengineering, School of Engineering, UC Merced, Merced, CA 95343, USA
| | - Joshua I Goldhaber
- Smidt Heart Institute, Cedars Sinai Medical Center, Los Angeles, CA 90048, USA
| | - Michela Ottolia
- Department of Anesthesiology and Perioperative Medicine, Division of Molecular Medicine, UCLA, Los Angeles, CA 90095, USA; Cardiovascular Research Laboratory, UCLA, Los Angeles, CA 90095, USA.
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5
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Doliba NM, Babsky AM, Osbakken MD. The Role of Sodium in Diabetic Cardiomyopathy. Front Physiol 2018; 9:1473. [PMID: 30405433 PMCID: PMC6207851 DOI: 10.3389/fphys.2018.01473] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/19/2018] [Accepted: 09/28/2018] [Indexed: 12/11/2022] Open
Abstract
Cardiovascular complications are the major cause of mortality and morbidity in diabetic patients. The changes in myocardial structure and function associated with diabetes are collectively called diabetic cardiomyopathy. Numerous molecular mechanisms have been proposed that could contribute to the development of diabetic cardiomyopathy and have been studied in various animal models of type 1 or type 2 diabetes. The current review focuses on the role of sodium (Na+) in diabetic cardiomyopathy and provides unique data on the linkage between Na+ flux and energy metabolism, studied with non-invasive 23Na, and 31P-NMR spectroscopy, polarography, and mass spectroscopy. 23Na NMR studies allow determination of the intracellular and extracellular Na+ pools by splitting the total Na+ peak into two resonances after the addition of a shift reagent to the perfusate. Using this technology, we found that intracellular Na+ is approximately two times higher in diabetic cardiomyocytes than in control possibly due to combined changes in the activity of Na+–K+ pump, Na+/H+ exchanger 1 (NHE1) and Na+-glucose cotransporter. We hypothesized that the increase in Na+ activates the mitochondrial membrane Na+/Ca2+ exchanger, which leads to a loss of intramitochondrial Ca2+, with a subsequent alteration in mitochondrial bioenergetics and function. Using isolated mitochondria, we showed that the addition of Na+ (1–10 mM) led to a dose-dependent decrease in oxidative phosphorylation and that this effect was reversed by providing extramitochondrial Ca2+ or by inhibiting the mitochondrial Na+/Ca2+ exchanger with diltiazem. Similar experiments with 31P-NMR in isolated superfused mitochondria embedded in agarose beads showed that Na+ (3–30 mM) led to significantly decreased ATP levels and that this effect was stronger in diabetic rats. These data suggest that in diabetic cardiomyocytes, increased Na+ leads to abnormalities in oxidative phosphorylation and a subsequent decrease in ATP levels. In support of these data, using 31P-NMR, we showed that the baseline β-ATP and phosphocreatine (PCr) were lower in diabetic cardiomyocytes than in control, suggesting that diabetic cardiomyocytes have depressed bioenergetic function. Thus, both altered intracellular Na+ levels and bioenergetics and their interactions may significantly contribute to the pathology of diabetic cardiomyopathy.
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Affiliation(s)
- Nicolai M Doliba
- Department of Biochemistry and Biophysics, Institute for Diabetes, Obesity and Metabolism, School of Medicine, University of Pennsylvania, Philadelphia, PA, United States
| | - Andriy M Babsky
- Department of Biophysics and Bioinformatics, Ivan Franko National University of Lviv, Lviv, Ukraine
| | - Mary D Osbakken
- School of Biomedical Engineering, Science and Health Systems, Drexel University, Philadelphia, PA, United States
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John S, Kim B, Olcese R, Goldhaber JI, Ottolia M. Molecular determinants of pH regulation in the cardiac Na +-Ca 2+ exchanger. J Gen Physiol 2018; 150:245-257. [PMID: 29301861 PMCID: PMC5806679 DOI: 10.1085/jgp.201611693] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/09/2016] [Revised: 05/25/2017] [Accepted: 11/29/2017] [Indexed: 11/20/2022] Open
Abstract
The cardiac Na+-Ca2+ exchanger (NCX) plays a critical role in the heart by extruding Ca2+ after each contraction and thus regulates cardiac contractility. The activity of NCX is strongly inhibited by cytosolic protons, which suggests that intracellular acidification will have important effects on heart contractility. However, the mechanisms underlying this inhibition remain elusive. It has been suggested that pH regulation originates from the competitive binding of protons to two Ca2+-binding domains within the large cytoplasmic loop of NCX and requires inactivation by intracellular Na+ to fully develop. By combining mutagenesis and electrophysiology, we demonstrate that NCX pH modulation is an allosteric mechanism distinct from Na+ and Ca2+ regulation, and we show that cytoplasmic Na+ can affect the sensitivity of NCX to protons. We further identify two histidines (His 124 and His 165) that are important for NCX proton sensitivity and show that His 165 plays the dominant role. Our results reveal a complex interplay between the different allosteric mechanisms that regulate the activity of NCX. Because of the central role of NCX in cardiac function, these findings are important for our understanding of heart pathophysiology.
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Affiliation(s)
- Scott John
- Department of Medicine and Cardiology, David Geffen School of Medicine at UCLA, Los Angeles, CA
| | - Brian Kim
- Cedars-Sinai Heart Institute, Los Angeles, CA
| | - Riccardo Olcese
- Department of Anesthesiology and Perioperative Medicine, Division of Molecular Medicine, David Geffen School of Medicine at UCLA, Los Angeles, CA.,Department of Physiology, David Geffen School of Medicine at UCLA, Los Angeles, CA
| | - Joshua I Goldhaber
- Cedars-Sinai Heart Institute, Los Angeles, CA.,Division of Applied Cell Biology and Physiology, Cedars-Sinai Medical Center, Los Angeles, CA
| | - Michela Ottolia
- Department of Anesthesiology and Perioperative Medicine, Division of Molecular Medicine, David Geffen School of Medicine at UCLA, Los Angeles, CA
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7
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Plain F, Congreve SD, Yee RSZ, Kennedy J, Howie J, Kuo CW, Fraser NJ, Fuller W. An amphipathic α-helix directs palmitoylation of the large intracellular loop of the sodium/calcium exchanger. J Biol Chem 2017; 292:10745-10752. [PMID: 28432123 PMCID: PMC5481580 DOI: 10.1074/jbc.m116.773945] [Citation(s) in RCA: 34] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/22/2016] [Revised: 04/19/2017] [Indexed: 11/06/2022] Open
Abstract
The electrogenic sodium/calcium exchanger (NCX) mediates bidirectional calcium transport controlled by the transmembrane sodium gradient. NCX inactivation occurs in the absence of phosphatidylinositol 4,5-bisphosphate and is facilitated by palmitoylation of a single cysteine at position 739 within the large intracellular loop of NCX. The aim of this investigation was to identify the structural determinants of NCX1 palmitoylation. Full-length NCX1 (FL-NCX1) and a YFP fusion protein of the NCX1 large intracellular loop (YFP-NCX1) were expressed in HEK cells. Single amino acid changes around Cys-739 in FL-NCX1 and deletions on the N-terminal side of Cys-739 in YFP-NCX1 did not affect NCX1 palmitoylation, with the exception of the rare human polymorphism S738F, which enhanced FL-NCX1 palmitoylation, and D741A, which modestly reduced it. In contrast, deletion of a 21-amino acid segment enriched in aromatic amino acids on the C-terminal side of Cys-739 abolished YFP-NCX1 palmitoylation. We hypothesized that this segment forms an amphipathic α-helix whose properties facilitate Cys-739 palmitoylation. Introduction of negatively charged amino acids to the hydrophobic face or of helix-breaking prolines impaired palmitoylation of both YFP-NCX1 and FL-NCX1. Alanine mutations on the hydrophilic face of the helix significantly reduced FL-NCX1 palmitoylation. Of note, when the helix-containing segment was introduced adjacent to cysteines that are not normally palmitoylated, they became palmitoylation sites. In conclusion, we have identified an amphipathic α-helix in the NCX1 large intracellular loop that controls NCX1 palmitoylation. NCX1 palmitoylation is governed by a distal secondary structure element rather than by local primary sequence.
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Affiliation(s)
- Fiona Plain
- From the Division of Molecular and Clinical Medicine, School of Medicine, Ninewells Hospital, University of Dundee, Dundee DD1 9SY, Scotland, United Kingdom
| | - Samitha Dilini Congreve
- From the Division of Molecular and Clinical Medicine, School of Medicine, Ninewells Hospital, University of Dundee, Dundee DD1 9SY, Scotland, United Kingdom
| | - Rachel Sue Zhen Yee
- From the Division of Molecular and Clinical Medicine, School of Medicine, Ninewells Hospital, University of Dundee, Dundee DD1 9SY, Scotland, United Kingdom
| | - Jennifer Kennedy
- From the Division of Molecular and Clinical Medicine, School of Medicine, Ninewells Hospital, University of Dundee, Dundee DD1 9SY, Scotland, United Kingdom
| | - Jacqueline Howie
- From the Division of Molecular and Clinical Medicine, School of Medicine, Ninewells Hospital, University of Dundee, Dundee DD1 9SY, Scotland, United Kingdom
| | - Chien-Wen Kuo
- From the Division of Molecular and Clinical Medicine, School of Medicine, Ninewells Hospital, University of Dundee, Dundee DD1 9SY, Scotland, United Kingdom
| | - Niall J Fraser
- From the Division of Molecular and Clinical Medicine, School of Medicine, Ninewells Hospital, University of Dundee, Dundee DD1 9SY, Scotland, United Kingdom
| | - William Fuller
- From the Division of Molecular and Clinical Medicine, School of Medicine, Ninewells Hospital, University of Dundee, Dundee DD1 9SY, Scotland, United Kingdom
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8
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Kinetic and equilibrium properties of regulatory Ca2+-binding domains in sodium–calcium exchangers 2 and 3. Cell Calcium 2016; 59:181-8. [DOI: 10.1016/j.ceca.2016.01.008] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/07/2015] [Revised: 01/14/2016] [Accepted: 01/23/2016] [Indexed: 12/21/2022]
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9
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Structure-dynamic determinants governing a mode of regulatory response and propagation of allosteric signal in splice variants of Na+/Ca2+ exchange (NCX) proteins. Biochem J 2015; 465:489-501. [DOI: 10.1042/bj20141036] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
Ca2+ binding to CBD1 (calcium-binding domain 1) and CBD2 regulates Na+/Ca2+ exchangers (NCX). In the present study, we demonstrate that Ca2+ binding rigidifies the main chain of CBD2, but not of CBD1, in a splice variant-dependent manner. The dynamic differences account for variant-dependent responses to Ca2+.
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10
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Towards Understanding the Role of the Na+-Ca2+ Exchanger Isoform 3. Rev Physiol Biochem Pharmacol 2015; 168:31-57. [DOI: 10.1007/112_2015_23] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
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11
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Khananshvili D. Sodium-calcium exchangers (NCX): molecular hallmarks underlying the tissue-specific and systemic functions. Pflugers Arch 2013; 466:43-60. [PMID: 24281864 DOI: 10.1007/s00424-013-1405-y] [Citation(s) in RCA: 88] [Impact Index Per Article: 8.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/24/2013] [Revised: 11/06/2013] [Accepted: 11/09/2013] [Indexed: 12/19/2022]
Abstract
NCX proteins explore the electrochemical gradient of Na(+) to mediate Ca(2+)-fluxes in exchange with Na(+) either in the Ca(2+)-efflux (forward) or Ca(2+)-influx (reverse) mode, whereas the directionality depends on ionic concentrations and membrane potential. Mammalian NCX variants (NCX1-3) and their splice variants are expressed in a tissue-specific manner to modulate the heartbeat rate and contractile force, the brain's long-term potentiation and learning, blood pressure, renal Ca(2+) reabsorption, the immune response, neurotransmitter and insulin secretion, apoptosis and proliferation, mitochondrial bioenergetics, etc. Although the forward mode of NCX represents a major physiological module, a transient reversal of NCX may contribute to EC-coupling, vascular constriction, and synaptic transmission. Notably, the reverse mode of NCX becomes predominant in pathological settings. Since the expression levels of NCX variants are disease-related, the selective pharmacological targeting of tissue-specific NCX variants could be beneficial, thereby representing a challenge. Recent structural and biophysical studies revealed a common module for decoding the Ca(2+)-induced allosteric signal in eukaryotic NCX variants, although the phenotype variances in response to regulatory Ca(2+) remain unclear. The breakthrough discovery of the archaebacterial NCX structure may serve as a template for eukaryotic NCX, although the turnover rates of the transport cycle may differ ~10(3)-fold among NCX variants to fulfill the physiological demands for the Ca(2+) flux rates. Further elucidation of ion-transport and regulatory mechanisms may lead to selective pharmacological targeting of NCX variants under disease conditions.
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Affiliation(s)
- Daniel Khananshvili
- Department of Physiology and Pharmacology, Sackler School of Medicine, Tel-Aviv University, Ramat-Aviv, Tel-Aviv, 69978, Israel,
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12
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Khananshvili D. The SLC8 gene family of sodium-calcium exchangers (NCX) - structure, function, and regulation in health and disease. Mol Aspects Med 2013; 34:220-35. [PMID: 23506867 DOI: 10.1016/j.mam.2012.07.003] [Citation(s) in RCA: 153] [Impact Index Per Article: 13.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/13/2011] [Accepted: 03/08/2012] [Indexed: 01/12/2023]
Abstract
The SLC8 gene family encoding Na(+)/Ca(2+) exchangers (NCX) belongs to the CaCA (Ca(2+)/Cation Antiporter) superfamily. Three mammalian genes (SLC8A1, SLC8A2, and SLC8A3) and their splice variants are expressed in a tissue-specific manner to mediate Ca(2+)-fluxes across the cell-membrane and thus, significantly contribute to regulation of Ca(2+)-dependent events in many cell types. A long-wanted mitochondrial Na(+)/Ca(2+) exchanger has been recently identified as NCLX protein, representing a gene product of SLC8B1. Distinct NCX isoform/splice variants contribute to excitation-contraction coupling, long-term potentiation of the brain and learning, blood pressure regulation, immune response, neurotransmitter and insulin secretion, mitochondrial bioenergetics, etc. Altered expression and regulation of NCX proteins contribute to distorted Ca(2+)-homeostasis in heart failure, arrhythmia, cerebral ischemia, hypertension, diabetes, renal Ca(2+) reabsorption, muscle dystrophy, etc. Recently, high-resolution X-ray structures of Ca(2+)-binding regulatory domains of eukaryotic NCX and of full-size prokaryotic NCX have become available and the dynamic properties have been analyzed by advanced biophysical approaches. Molecular silencing/overexpression of NCX in cellular systems and organ-specific KO mouse models provided useful information on the contribution of distinct NCX variants to cellular and systemic functions under various pathophysiological conditions. Selective inhibition or activation of predefined NCX variants in specific diseases might have clinical relevance, although this breakthrough has not yet been realized. A better understanding of the underlying molecular mechanisms as well as the development of in vitro procedures for high-throughput screening of "drug-like" compounds may lead to selective pharmacological targeting of NCX variants.
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Affiliation(s)
- Daniel Khananshvili
- Department of Physiology and Pharmacology, Sackler School of Medicine, Tel-Aviv University, Ramat-Aviv, Tel-Aviv 69978, Israel.
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13
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Giladi M, Hiller R, Hirsch JA, Khananshvili D. Population shift underlies Ca2+-induced regulatory transitions in the sodium-calcium exchanger (NCX). J Biol Chem 2013; 288:23141-9. [PMID: 23798674 DOI: 10.1074/jbc.m113.471698] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022] Open
Abstract
In eukaryotic Na(+)/Ca(2+) exchangers (NCX) the Ca(2+) binding CBD1 and CBD2 domains form a two-domain regulatory tandem (CBD12). An allosteric Ca(2+) sensor (Ca3-Ca4 sites) is located on CBD1, whereas CBD2 contains a splice-variant segment. Recently, a Ca(2+)-driven interdomain switch has been described, albeit how it couples Ca(2+) binding with signal propagation remains unclear. To resolve the dynamic features of Ca(2+)-induced conformational transitions we analyze here distinct splice variants and mutants of isolated CBD12 at varying temperatures by using small angle x-ray scattering (SAXS) and equilibrium (45)Ca(2+) binding assays. The ensemble optimization method SAXS analysis demonstrates that the apo and Mg(2+)-bound forms of CBD12 are highly flexible, whereas Ca(2+) binding to the Ca3-Ca4 sites results in a population shift of conformational landscape to more rigidified states. Population shift occurs even under conditions in which no effect of Ca(2+) is observed on the globally derived Dmax (maximal interatomic distance), although under comparable conditions a normal [Ca(2+)]-dependent allosteric regulation occurs. Low affinity sites (Ca1-Ca2) of CBD1 do not contribute to Ca(2+)-induced population shift, but the occupancy of these sites by 1 mM Mg(2+) shifts the Ca(2+) affinity (Kd) at the neighboring Ca3-Ca4 sites from ∼ 50 nM to ∼ 200 nM and thus, keeps the primary Ca(2+) sensor (Ca3-Ca4 sites) within a physiological range. Thus, Ca(2+) binding to the Ca3-Ca4 sites results in a population shift, where more constraint conformational states become highly populated at dynamic equilibrium in the absence of global conformational transitions in CBD alignment.
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Affiliation(s)
- Moshe Giladi
- Department of Physiology and Pharmacology, Sackler School of Medicine, Tel-Aviv University, Ramat-Aviv 69978, Israel
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14
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Giladi M, Khananshvili D. Molecular determinants of allosteric regulation in NCX proteins. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2013; 961:35-48. [PMID: 23224868 DOI: 10.1007/978-1-4614-4756-6_4] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
Abstract
Allosteric activation of NCX involves the binding of cytosolic Ca(2+) to regulatory domains CBD1 and CBD2. Previous studies with isolated CBD12 and full-size NCX identified synergistic interactions between the two CBD domains that modify the affinity and kinetic properties of Ca(2+) sensing, although it remains unclear how the Ca(2+)-binding signal is decoded and propagates to transmembrane domains. Biophysical analyses (X-ray, SAXS, and stopped-flow techniques) of isolated preparations of CBD1, CBD2, and CBD12 have shown that Ca(2+) binding to Ca3-Ca4 sites of CBD1 results in interdomain tethering of CBDs through specific amino acids on CBD1 (Asp499 and Asp500) and CBD2 (Arg532 and Asp565). Mutant analyses of isolated CBDs suggest that the two-domain interface governs Ca(2+)-driven conformational alignment of CBDs, resulting in slow dissociation of Ca(2+) from CBD12, and thus, it mediates Ca(2+)-induced conformational transitions associated with allosteric signal transmission. Specifically, occupation of Ca3-Ca4 sites by Ca(2+) induces disorder-to-order transition owing to charge neutralization and coordination, thereby constraining CBD conformational freedom, rigidifying the NCX1 f-loop, and triggering allosteric signal transmission to the membrane domain. The newly found interdomain switch is highly conserved among NCX isoform/splice variants, although some additional structural motifs may shape the regulatory specificity of NCX variants.
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Affiliation(s)
- Moshe Giladi
- Department of Physiology and Pharmacology, Tel-Aviv University, Ramat-Aviv, Tel-Aviv, Israel
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15
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Giladi M, Friedberg I, Fang X, Hiller R, Wang YX, Khananshvili D. G503 is obligatory for coupling of regulatory domains in NCX proteins. Biochemistry 2012; 51:7313-20. [PMID: 22924554 DOI: 10.1021/bi300739z] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
In multidomain proteins, interdomain linkers allow an efficient transfer of regulatory information, although it is unclear how the information encoded in the linker structure coins dynamic coupling. Allosteric regulation of NCX proteins involves Ca(2+)-driven tethering of regulatory CBD1 and CBD2 (through a salt bridge network) accompanied by alignment of CBDs and Ca(2+) occlusion at the interface of the two CBDs. Here we investigated "alanine-walk" substitutions in the CBD1-CBD2 linker (501-HAGIFT-506) and found that among all linker residues, only G503 is obligatory for Ca(2+)-induced reorientations of CBDs and slow dissociation of occluded Ca(2+). Moreover, swapping between positions A502 and G503 in the CBD1-CBD2 linker results in a complete loss of slow dissociation of occluded Ca(2+), meaning that dynamic coupling of CBDs requires an exact pose of glycine at position 503. Therefore, accumulating data revealed that position 503 occupied by glycine is absolutely required for Ca(2+)-driven tethering of CBDs, which in turn limits the linker's flexibility and, thus, restricts CBD movements. Because G503 is extremely well conserved in eukaryotic NCX proteins, the information encoded in G503 is essential for dynamic coupling of the two-domain CBD tandem and, thus, for propagation of the allosteric signal.
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Affiliation(s)
- Moshe Giladi
- Department of Physiology and Pharmacology, Sackler School of Medicine, Tel-Aviv University, Ramat-Aviv 69978, Israel
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16
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Abstract
The binding of Ca(2+) to two adjacent Ca(2+)-binding domains, CBD1 and CBD2, regulates ion transport in the Na(+)/Ca(2+) exchanger. As sensors for intracellular Ca(2+), the CBDs form electrostatic switches that induce the conformational changes required to initiate and sustain Na(+)/Ca(2+) exchange. Depending on the presence of a few key residues in the Ca(2+)-binding sites, zero to four Ca(2+) ions can bind with affinities between 0.1 to 20 μm. Importantly, variability in CBD2 as a consequence of alternative splicing modulates not only the number and affinities of the Ca(2+)-binding sites in CBD2 but also the Ca(2+) affinities in CBD1.
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Affiliation(s)
- Mark Hilge
- Center for Cellular Imaging and NanoAnalytics (C-CINA), Biozentrum, University Basel, CH-4058 Basel, Switzerland.
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17
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Giladi M, Sasson Y, Fang X, Hiller R, Buki T, Wang YX, Hirsch JA, Khananshvili D. A common Ca2+-driven interdomain module governs eukaryotic NCX regulation. PLoS One 2012; 7:e39985. [PMID: 22768191 PMCID: PMC3386913 DOI: 10.1371/journal.pone.0039985] [Citation(s) in RCA: 30] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/14/2012] [Accepted: 05/30/2012] [Indexed: 11/19/2022] Open
Abstract
Na(+)/Ca(2+) exchanger (NCX) proteins mediate Ca(2+)-fluxes across the cell membrane to maintain Ca(2+) homeostasis in many cell types. Eukaryotic NCX contains Ca(2+)-binding regulatory domains, CBD1 and CBD2. Ca(2+) binding to a primary sensor (Ca3-Ca4 sites) on CBD1 activates mammalian NCXs, whereas CALX, a Drosophila NCX ortholog, displays an inhibitory response to regulatory Ca(2+). To further elucidate the underlying regulatory mechanisms, we determined the 2.7 Å crystal structure of mammalian CBD12-E454K, a two-domain construct that retains wild-type properties. In conjunction with stopped-flow kinetics and SAXS (small-angle X-ray scattering) analyses of CBD12 mutants, we show that Ca(2+) binding to Ca3-Ca4 sites tethers the domains via a network of interdomain salt-bridges. This Ca(2+)-driven interdomain switch controls slow dissociation of "occluded" Ca(2+) from the primary sensor and thus dictates Ca(2+) sensing dynamics. In the Ca(2+)-bound conformation, the interdomain angle of CBD12 is very similar in NCX and CALX, meaning that the interdomain distances cannot account for regulatory diversity in NCX and CALX. Since the two-domain interface is nearly identical among eukaryotic NCXs, including CALX, we suggest that the Ca(2+)-driven interdomain switch described here represents a general mechanism for initial conduction of regulatory signals in NCX variants.
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Affiliation(s)
- Moshe Giladi
- Department of Physiology and Pharmacology, Sackler School of Medicine, Tel-Aviv University, Ramat-Aviv, Tel-Aviv, Israel
| | - Yehezkel Sasson
- Department of Biochemistry and Molecular Biology, Faculty of Life Sciences, Tel-Aviv University, Ramat-Aviv, Tel-Aviv, Israel
| | - Xianyang Fang
- Protein–Nucleic Acid Interaction Section, Structural Biophysics Laboratory, National Cancer Institute at Frederick, National Institutes of Health, Frederick, Maryland, United States of America
| | - Reuben Hiller
- Department of Physiology and Pharmacology, Sackler School of Medicine, Tel-Aviv University, Ramat-Aviv, Tel-Aviv, Israel
| | - Tal Buki
- Department of Physiology and Pharmacology, Sackler School of Medicine, Tel-Aviv University, Ramat-Aviv, Tel-Aviv, Israel
| | - Yun-Xing Wang
- Protein–Nucleic Acid Interaction Section, Structural Biophysics Laboratory, National Cancer Institute at Frederick, National Institutes of Health, Frederick, Maryland, United States of America
| | - Joel A. Hirsch
- Department of Biochemistry and Molecular Biology, Faculty of Life Sciences, Tel-Aviv University, Ramat-Aviv, Tel-Aviv, Israel
- * E-mail: (DK); (JAH)
| | - Daniel Khananshvili
- Department of Physiology and Pharmacology, Sackler School of Medicine, Tel-Aviv University, Ramat-Aviv, Tel-Aviv, Israel
- * E-mail: (DK); (JAH)
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18
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Giladi M, Bohbot H, Buki T, Schulze DH, Hiller R, Khananshvili D. Dynamic features of allosteric Ca2+ sensor in tissue-specific NCX variants. Cell Calcium 2012; 51:478-85. [PMID: 22571864 DOI: 10.1016/j.ceca.2012.04.007] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/13/2012] [Revised: 04/10/2012] [Accepted: 04/12/2012] [Indexed: 11/19/2022]
Abstract
The Na(+)-Ca(2+) exchanger (NCX) mediated Ca(2+) fluxes are essential for handling Ca(2+) homeostasis in many cell-types. Eukaryotic NCX variants contain regulatory CBD1 and CBD2 domains, whereas in distinct variants the Ca(2+) binding to Ca3-Ca4 sites of CBD1 results either in sustained activation, inhibition or no effect. CBD2 contains an alternatively spliced segment, which is expressed in a tissue-specific manner although its impact on allosteric regulation remains unclear. Recent studies revealed that the Ca(2+) binding to Ca3-Ca4 sites results in interdomain tethering of CBDs, which rigidifies CBDs movements with accompanied slow dissociation of "occluded" Ca(2+). Here we investigate the effects of CBD2 variants on Ca(2+) occlusion in the two-domain construct (CBD12). Mutational studies revealed that both sites (Ca3 and Ca4) contribute to Ca(2+) occlusion, whereas after dissociation of the first Ca(2+) ion the second Ca(2+) ion becomes occluded. This mechanism is common for the brain, kidney and cardiac splice variants of CBD12, although the occluded Ca(2+) exhibits 20-50-fold difference in off-rates among the tested variants. Therefore, the spliced exons on CBD2 affect the rate-limiting step of the occluded Ca(2+) dissociation at the primary regulatory sensor to shape dynamic features of allosteric regulation in NCX variants.
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Affiliation(s)
- Moshe Giladi
- Department of Physiology and Pharmacology, Sackler School of Medicine, Tel-Aviv University, Ramat-Aviv, Israel
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19
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Structural and dynamic aspects of Ca2+ and Mg2+ binding of the regulatory domains of the Na+/Ca2+ exchanger. Biochem Soc Trans 2012; 40:409-14. [DOI: 10.1042/bst20110742] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
Intracellular Ca2+ regulates the activity of the NCX (Na+/Ca2+ exchanger) through binding to the cytosolic CBD (Ca2+-binding domain) 1 and CBD2. In vitro studies of the structure and dynamics of CBD1 and CBD2, as well as studies of their kinetics and thermodynamics of Ca2+ binding, greatly enhanced our understanding of NCX regulation. We describe the fold of the CBDs in relation to other known structures and review Ca2+ binding of the different CBD variants from a structural perspective. We also report on new findings concerning Mg2+ binding to the CBDs and finally we discuss recent results on CBD1–CBD2 interdomain interactions.
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20
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Salinas RK, Bruschweiler-Li L, Johnson E, Brüschweiler R. Ca2+ binding alters the interdomain flexibility between the two cytoplasmic calcium-binding domains in the Na+/Ca2+ exchanger. J Biol Chem 2011; 286:32123-31. [PMID: 21778234 DOI: 10.1074/jbc.m111.249268] [Citation(s) in RCA: 31] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The Na(+)/Ca(2+) exchanger (NCX) is a membrane protein, which catalyzes the counter transport of Na(+) and Ca(2+) ions across the plasma membrane, playing a key role in the maintenance of the intracellular Ca(2+) homeostasis in various cell types. NCX consists of a transmembrane part and a large intracellular loop. The activation of the NCX transport function requires the binding of Ca(2+) to two tandem C2 domains, CBD1 and CBD2, which are an integral part of the exchanger's intracellular loop. Although high-resolution structures of individual CBD1 and CBD2 are available, their interdomain structure and dynamics and the atomic level mechanism of allosteric Ca(2+)-regulation remains unknown. Here, we use solution NMR spectroscopy to study the interdomain dynamics of CBD12, a 32 kDa construct that contains both the CBD1 and CBD2 domains connected by a short linker. Analysis of NMR residual dipolar couplings shows that CBD12 assumes on average an elongated shape both in the absence and in the presence of Ca(2+). NMR (15)N relaxation data of the Apo state indicate that the two domains sample a wide range of relative arrangements on the nanosecond time scale. These arrangements comprise significantly non-linear interdomain orientations. Binding of Ca(2+) to CBD1 significantly restricts the interdomain flexibility, stabilizing a more rigid elongated conformation. These findings suggest a molecular mechanism for the role of CBD12 in the function of NCX.
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Affiliation(s)
- Roberto K Salinas
- Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306-4390, USA
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21
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Boyman L, Hagen BM, Giladi M, Hiller R, Lederer WJ, Khananshvili D. Proton-sensing Ca2+ binding domains regulate the cardiac Na+/Ca2+ exchanger. J Biol Chem 2011; 286:28811-28820. [PMID: 21680748 DOI: 10.1074/jbc.m110.214106] [Citation(s) in RCA: 48] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The cardiac Na(+)/Ca(2+) exchanger (NCX) regulates cellular [Ca(2+)](i) and plays a central role in health and disease, but its molecular regulation is poorly understood. Here we report on how protons affect this electrogenic transporter by modulating two critically important NCX C(2) regulatory domains, Ca(2+) binding domain-1 (CBD1) and CBD2. The NCX transport rate in intact cardiac ventricular myocytes was measured as a membrane current, I(NCX), whereas [H(+)](i) was varied using an ammonium chloride "rebound" method at constant extracellular pH 7.4. At pH(i) = 7.2 and [Ca(2+)](i) < 120 nM, I(NCX) was less than 4% that of its maximally Ca(2+)-activated value. I(NCX) increases steeply at [Ca(2+)](i) between 130-150 nM with a Hill coefficient (n(H)) of 8.0 ± 0.7 and K(0.5) = 310 ± 5 nM. At pH(i) = 6.87, the threshold of Ca(2+)-dependent activation of I(NCX) was shifted to much higher [Ca(2+)](i) (600-700 nM), and the relationship was similarly steep (n(H) = 8.0±0.8) with K(0.5) = 1042 ± 15 nM. The V(max) of Ca(2+)-dependent activation of I(NCX) was not significantly altered by low pH(i). The Ca(2+) affinities for CBD1 (0.39 ± 0.06 μM) and CBD2 (K(d) = 18.4 ± 6 μM) were exquisitely sensitive to [H(+)], decreasing 1.3-2.3-fold as pH(i) decreased from 7.2 to 6.9. This work reveals for the first time that NCX can be switched off by physiologically relevant intracellular acidification and that this depends on the competitive binding of protons to its C(2) regulatory domains CBD1 and CBD2.
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Affiliation(s)
- Liron Boyman
- Department of Physiology and Pharmacology, Sackler School of Medicine, Tel Aviv University, Ramat-Aviv 69978, Israel and; Laboratory of Molecular Cardiology, Center for Biomedical Engineering and Technology, University of Maryland, Baltimore, Maryland 21201
| | - Brian M Hagen
- Laboratory of Molecular Cardiology, Center for Biomedical Engineering and Technology, University of Maryland, Baltimore, Maryland 21201
| | - Moshe Giladi
- Department of Physiology and Pharmacology, Sackler School of Medicine, Tel Aviv University, Ramat-Aviv 69978, Israel and
| | - Reuben Hiller
- Department of Physiology and Pharmacology, Sackler School of Medicine, Tel Aviv University, Ramat-Aviv 69978, Israel and
| | - W Jonathan Lederer
- Laboratory of Molecular Cardiology, Center for Biomedical Engineering and Technology, University of Maryland, Baltimore, Maryland 21201
| | - Daniel Khananshvili
- Department of Physiology and Pharmacology, Sackler School of Medicine, Tel Aviv University, Ramat-Aviv 69978, Israel and.
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22
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Ca2+-dependent structural rearrangements within Na+-Ca2+ exchanger dimers. Proc Natl Acad Sci U S A 2011; 108:1699-704. [PMID: 21209335 DOI: 10.1073/pnas.1016114108] [Citation(s) in RCA: 58] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Cytoplasmic Ca(2+) is known to regulate Na(+)-Ca(2+) exchanger (NCX) activity by binding to two adjacent Ca(2+)-binding domains (CBD1 and CBD2) located in the large intracellular loop between transmembrane segments 5 and 6. We investigated Ca(2+)-dependent movements as changes in FRET between exchanger proteins tagged with CFP or YFP at position 266 within the large cytoplasmic loop. Data indicate that the exchanger assembles as a dimer in the plasma membrane. Addition of Ca(2+) decreases the distance between the cytoplasmic loops of NCX pairs. The Ca(2+)-dependent movements detected between paired NCXs were abolished by mutating the Ca(2+) coordination sites in CBD1 (D421A, E451A, and D500V), whereas disruption of the primary Ca(2+) coordination site in CBD2 (E516L) had no effect. Thus, the Ca(2+)-induced conformational changes of NCX dimers arise from the movement of CBD1. FRET studies of CBD1, CBD2, and CBD1-CBD2 peptides displayed Ca(2+)-dependent movements with different apparent affinities. CBD1-CBD2 showed a Ca(2+)-dependent phenotype mirroring full-length NCX but distinct from both CBD1 and CBD2.
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23
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Giladi M, Boyman L, Mikhasenko H, Hiller R, Khananshvili D. Essential role of the CBD1-CBD2 linker in slow dissociation of Ca2+ from the regulatory two-domain tandem of NCX1. J Biol Chem 2010; 285:28117-25. [PMID: 20587421 PMCID: PMC2934676 DOI: 10.1074/jbc.m110.127001] [Citation(s) in RCA: 40] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/25/2010] [Revised: 06/28/2010] [Indexed: 11/06/2022] Open
Abstract
In NCX proteins CBD1 and CBD2 domains are connected through a short linker (3 or 4 amino acids) forming a regulatory tandem (CBD12). Only three of the six CBD12 Ca(2+)-binding sites contribute to NCX regulation. Two of them are located on CBD1 (K(d) = approximately 0.2 microM), and one is on CBD2 (K(d) = approximately 5 microM). Here we analyze how the intrinsic properties of individual regulatory sites are affected by linker-dependent interactions in CBD12 (AD splice variant). The three sites of CBD12 and CBD1 + CBD2 have comparable K(d) values but differ dramatically in their Ca(2+) dissociation kinetics. CBD12 exhibits multiphasic kinetics for the dissociation of three Ca(2+) ions (k(r) = 280 s(-1), k(f) = 7 s(-1), and k(s) = 0.4 s(-1)), whereas the dissociation of two Ca(2+) ions from CBD1 (k(f) = 16 s(-1)) and one Ca(2+) ion from CBD2 (k(r) = 125 s(-1)) is monophasic. Insertion of seven alanines into the linker (CBD12-7Ala) abolishes slow dissociation of Ca(2+), whereas the kinetic and equilibrium properties of three Ca(2+) sites of CBD12-7Ala and CBD1 + CBD2 are similar. Therefore, the linker-dependent interactions in CBD12 decelerate the Ca(2+) on/off kinetics at a specific CBD1 site by 50-80-fold, thereby representing Ca(2+) "occlusion" at CBD12. Notably, the kinetic and equilibrium properties of the remaining two sites of CBD12 are "linker-independent," so their intrinsic properties are preserved in CBD12. In conclusion, the dynamic properties of three sites are specifically modified, conserved, diversified, and integrated by the linker in CBD12, thereby generating a wide range dynamic sensor.
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Affiliation(s)
- Moshe Giladi
- From the Department of Physiology and Pharmacology, Sackler School of Medicine, Tel-Aviv University, Ramat-Aviv 69978, Israel
| | - Liron Boyman
- From the Department of Physiology and Pharmacology, Sackler School of Medicine, Tel-Aviv University, Ramat-Aviv 69978, Israel
| | - Helen Mikhasenko
- From the Department of Physiology and Pharmacology, Sackler School of Medicine, Tel-Aviv University, Ramat-Aviv 69978, Israel
| | - Reuben Hiller
- From the Department of Physiology and Pharmacology, Sackler School of Medicine, Tel-Aviv University, Ramat-Aviv 69978, Israel
| | - Daniel Khananshvili
- From the Department of Physiology and Pharmacology, Sackler School of Medicine, Tel-Aviv University, Ramat-Aviv 69978, Israel
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24
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Ottolia M, Nicoll DA, Philipson KD. Roles of two Ca2+-binding domains in regulation of the cardiac Na+-Ca2+ exchanger. J Biol Chem 2009; 284:32735-41. [PMID: 19801651 DOI: 10.1074/jbc.m109.055434] [Citation(s) in RCA: 64] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
We expressed full-length Na(+)-Ca(2+) exchangers (NCXs) with mutations in two Ca(2+)-binding domains (CBD1 and CBD2) to determine the roles of the CBDs in Ca(2+)-dependent regulation of NCX. CBD1 has four Ca(2+)-binding sites, and mutation of residues Asp(421) and Glu(451), which primarily coordinate Ca(2+) at sites 1 and 2, had little effect on regulation of NCX by Ca(2+). In contrast, mutations at residues Glu(385), Asp(446), Asp(447), and Asp(500), which coordinate Ca(2+) at sites 3 and 4 of CBD1, resulted in a drastic decrease in the apparent affinity of peak exchange current for regulatory Ca(2+). Another mutant, M7, with 7 key residues of CBD1 replaced, showed a further decrease in apparent Ca(2+) affinity but retained regulation, confirming a contribution of CBD2 to Ca(2+) regulation. Addition of the mutation K585E (located in CBD2) into the M7 background induced a marked increase in Ca(2+) affinity for both steady-state and peak currents. Also, we have shown previously that the CBD2 mutations E516L and E683V have no Ca(2+)-dependent regulation. We now demonstrate that introduction of a positive charge at these locations rescues Ca(2+)-dependent regulation. Finally, our data demonstrate that deletion of the unstructured loops between beta-strands F and G of both CBDs does not alter the regulation of the exchanger by Ca(2+), indicating that these segments are not important in regulation. Thus, CBD1 and CBD2 have distinct roles in Ca(2+)-dependent regulation of NCX. CBD1 determines the affinity of NCX for regulatory Ca(2+), although CBD2 is also necessary for Ca(2+)-dependent regulation.
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Affiliation(s)
- Michela Ottolia
- Department of Physiology and the Cardiovascular Research Laboratories, David Geffen School of Medicine, UCLA, Los Angeles, California 90095-1760, USA.
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