1
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Multiple sub-state structures of SERCA2b reveal conformational overlap at transition steps during the catalytic cycle. Cell Rep 2022; 41:111760. [PMID: 36476867 DOI: 10.1016/j.celrep.2022.111760] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/12/2022] [Revised: 09/06/2022] [Accepted: 11/09/2022] [Indexed: 12/12/2022] Open
Abstract
Sarco/endoplasmic reticulum Ca2+ ATPase (SERCA) pumps Ca2+ into the endoplasmic reticulum (ER). Herein, we present cryo-electron microscopy (EM) structures of three intermediates of SERCA2b: Ca2+-bound phosphorylated (E1P·2Ca2+) and Ca2+-unbound dephosphorylated (E2·Pi) intermediates and another between the E2P and E2·Pi states. Our cryo-EM analysis demonstrates that the E1P·2Ca2+ state exists in low abundance and preferentially transitions to an E2P-like structure by releasing Ca2+ and that the Ca2+ release gate subsequently undergoes stepwise closure during the dephosphorylation processes. Importantly, each intermediate adopts multiple sub-state structures including those like the next one in the catalytic series, indicating conformational overlap at transition steps, as further substantiated by atomistic molecular dynamic simulations of SERCA2b in a lipid bilayer. The present findings provide insight into how enzymes accelerate catalytic cycles.
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2
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Zhang Y, Inaba K. Structural basis of the conformational and functional regulation of human SERCA2b, the ubiquitous endoplasmic reticulum calcium pump. Bioessays 2022; 44:e2200052. [PMID: 35560336 DOI: 10.1002/bies.202200052] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/06/2022] [Revised: 04/29/2022] [Accepted: 04/29/2022] [Indexed: 11/06/2022]
Abstract
Sarco/endoplasmic reticulum Ca2+ ATPase 2b (SERCA2b), a member of the SERCA family, is expressed ubiquitously and transports Ca2+ into the sarco/endoplasmic reticulum using the energy provided by ATP binding and hydrolysis. The crystal structure of SERCA2b in its Ca2+ - and ATP-bound (E1∙2Ca2+ -ATP) state and cryo-electron microscopy (cryo-EM) structures of the protein in its E1∙2Ca2+ -ATP and Ca2+ -unbound phosphorylated (E2P) states have provided essential insights into how the overall conformation and ATPase activity of SERCA2b is regulated by the transmembrane helix 11 and the subsequent luminal extension loop, both of which are specific to this isoform. More recently, our cryo-EM analysis has revealed that SERCA2b likely adopts open and closed conformations of the cytosolic domains in the Ca2+ -bound but ATP-free (E1∙2Ca2+ ) state, and that the closed conformation represents a state immediately prior to ATP binding. This review article summarizes the unique mechanisms underlying the conformational and functional regulation of SERCA2b.
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Affiliation(s)
- Yuxia Zhang
- Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, Miyagi, Japan
| | - Kenji Inaba
- Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, Miyagi, Japan.,Department of Chemistry, Graduate School of Science, Tohoku University, Sendai, Miyagi, Japan.,Department of Molecular and Chemical Life Sciences, Graduate School of Life Sciences, Tohoku University, Sendai, Miyagi, Japan.,Core Research for Evolutional Science and Technology (CREST), Japan Agency for Medical Research and Development (AMED), Japan
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3
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Raguimova ON, Aguayo-Ortiz R, Robia SL, Espinoza-Fonseca LM. Dynamics-Driven Allostery Underlies Ca 2+-Mediated Release of SERCA Inhibition by Phospholamban. Biophys J 2020; 119:1917-1926. [PMID: 33069270 PMCID: PMC7677127 DOI: 10.1016/j.bpj.2020.09.014] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/01/2020] [Revised: 09/17/2020] [Accepted: 09/18/2020] [Indexed: 12/20/2022] Open
Abstract
Sarcoplasmic reticulum (SR) Ca2+-ATPase (SERCA) and phospholamban (PLB) are essential for intracellular Ca2+ transport in myocytes. Ca2+-dependent activation of SERCA-PLB provides a control function that regulates cytosolic and SR Ca2+ levels. Although experimental and computational studies alone have led to a greater insight into SERCA-PLB regulation, the structural mechanisms for Ca2+ binding reversing inhibition of the complex remain poorly understood. Therefore, we have performed atomistic simulations totaling 32.7 μs and cell-based intramolecular fluorescence resonance energy transfer (FRET) experiments to determine structural changes of PLB-bound SERCA in response to binding of a single Ca2+ ion. Complementary MD simulations and FRET experiments showed that open-to-closed transitions in the structure of the headpiece underlie PLB inhibition of SERCA, and binding of a single Ca2+ ion is sufficient to shift the protein population toward a structurally closed structure of the complex. Closure is accompanied by functional interactions between the N-domain β5-β6 loop and the A-domain and the displacement of the catalytic phosphorylation domain toward a competent structure. We propose that reversal of SERCA-PLB inhibition is achieved by stringing together its controlling modules (A-domain and loop Nβ5-β6) with catalytic elements (P-domain) to regulate function during intracellular Ca2+ signaling. We conclude that binding of a single Ca2+ is a critical mediator of allosteric signaling that dictates structural changes and motions that relieve SERCA inhibition by PLB. Understanding allosteric regulation is of paramount importance to guide therapeutic modulation of SERCA and other evolutionarily related ion-motive ATPases.
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Affiliation(s)
- Olga N Raguimova
- Department of Cell and Molecular Physiology, Stritch School of Medicine, Loyola University Chicago, Maywood, Illinois
| | - Rodrigo Aguayo-Ortiz
- Center for Arrhythmia Research, Department of Internal Medicine, Division of Cardiovascular Medicine, University of Michigan, Ann Arbor, Michigan
| | - Seth L Robia
- Department of Cell and Molecular Physiology, Stritch School of Medicine, Loyola University Chicago, Maywood, Illinois
| | - L Michel Espinoza-Fonseca
- Center for Arrhythmia Research, Department of Internal Medicine, Division of Cardiovascular Medicine, University of Michigan, Ann Arbor, Michigan.
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4
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Aguayo-Ortiz R, Espinoza-Fonseca LM. Atomistic Structure and Dynamics of the Ca 2+-ATPase Bound to Phosphorylated Phospholamban. Int J Mol Sci 2020; 21:ijms21197261. [PMID: 33019581 PMCID: PMC7583845 DOI: 10.3390/ijms21197261] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/14/2020] [Revised: 09/29/2020] [Accepted: 09/29/2020] [Indexed: 01/22/2023] Open
Abstract
Sarcoplasmic reticulum Ca2+-ATPase (SERCA) and phospholamban (PLB) are essential components of the cardiac Ca2+ transport machinery. PLB phosphorylation at residue Ser16 (pSer16) enhances SERCA activity in the heart via an unknown structural mechanism. Here, we report a fully atomistic model of SERCA bound to phosphorylated PLB and study its structural dynamics on the microsecond time scale using all-atom molecular dynamics simulations in an explicit lipid bilayer and water environment. The unstructured N-terminal phosphorylation domain of PLB samples different orientations and covers a broad area of the cytosolic domain of SERCA but forms a stable complex mediated by pSer16 interactions with a binding site formed by SERCA residues Arg324/Lys328. PLB phosphorylation does not affect the interaction between the transmembrane regions of the two proteins; however, pSer16 stabilizes a disordered structure of the N-terminal phosphorylation domain that releases key inhibitory contacts between SERCA and PLB. We found that PLB phosphorylation is sufficient to guide the structural transitions of the cytosolic headpiece that are required to produce a competent structure of SERCA. We conclude that PLB phosphorylation serves as an allosteric molecular switch that releases inhibitory contacts and strings together the catalytic elements required for SERCA activation. This atomistic model represents a vivid atomic-resolution visualization of SERCA bound to phosphorylated PLB and provides previously inaccessible insights into the structural mechanism by which PLB phosphorylation releases SERCA inhibition in the heart.
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Affiliation(s)
- Rodrigo Aguayo-Ortiz
- Center for Arrhythmia Research, Department of Internal Medicine, Division of Cardiovascular Medicine, University of Michigan, Ann Arbor, MI 48109, USA;
- Departamento de Fisicoquímica, Universidad Nacional Autónoma de México, Mexico City 04510, Mexico
| | - L. Michel Espinoza-Fonseca
- Center for Arrhythmia Research, Department of Internal Medicine, Division of Cardiovascular Medicine, University of Michigan, Ann Arbor, MI 48109, USA;
- Correspondence: ; Tel.: +1-734-998-7500
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5
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Aguayo-Ortiz R, Espinoza-Fonseca LM. Linking Biochemical and Structural States of SERCA: Achievements, Challenges, and New Opportunities. Int J Mol Sci 2020; 21:ijms21114146. [PMID: 32532023 PMCID: PMC7313052 DOI: 10.3390/ijms21114146] [Citation(s) in RCA: 33] [Impact Index Per Article: 8.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/22/2020] [Revised: 06/05/2020] [Accepted: 06/08/2020] [Indexed: 02/07/2023] Open
Abstract
Sarcoendoplasmic reticulum calcium ATPase (SERCA), a member of the P-type ATPase family of ion and lipid pumps, is responsible for the active transport of Ca2+ from the cytoplasm into the sarcoplasmic reticulum lumen of muscle cells, into the endoplasmic reticulum (ER) of non-muscle cells. X-ray crystallography has proven to be an invaluable tool in understanding the structural changes of SERCA, and more than 70 SERCA crystal structures representing major biochemical states (defined by bound ligand) have been deposited in the Protein Data Bank. Consequently, SERCA is one of the best characterized components of the calcium transport machinery in the cell. Emerging approaches in the field, including spectroscopy and molecular simulation, now help integrate and interpret this rich structural information to understand the conformational transitions of SERCA that occur during activation, inhibition, and regulation. In this review, we provide an overview of the crystal structures of SERCA, focusing on identifying metrics that facilitate structure-based categorization of major steps along the catalytic cycle. We examine the integration of crystallographic data with different biophysical approaches and computational methods to link biochemical and structural states of SERCA that are populated in the cell. Finally, we discuss the challenges and new opportunities in the field, including structural elucidation of functionally important and novel regulatory complexes of SERCA, understanding the structural basis of functional divergence among homologous SERCA regulators, and bridging the gap between basic and translational research directed toward therapeutic modulation of SERCA.
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6
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Structural dynamics of P-type ATPase ion pumps. Biochem Soc Trans 2020; 47:1247-1257. [PMID: 31671180 DOI: 10.1042/bst20190124] [Citation(s) in RCA: 44] [Impact Index Per Article: 11.0] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/05/2019] [Revised: 09/06/2019] [Accepted: 09/16/2019] [Indexed: 02/04/2023]
Abstract
P-type ATPases transport ions across biological membranes against concentration gradients and are essential for all cells. They use the energy from ATP hydrolysis to propel large intramolecular movements, which drive vectorial transport of ions. Tight coordination of the motions of the pump is required to couple the two spatially distant processes of ion binding and ATP hydrolysis. Here, we review our current understanding of the structural dynamics of P-type ATPases, focusing primarily on Ca2+ pumps. We integrate different types of information that report on structural dynamics, primarily time-resolved fluorescence experiments including single-molecule Förster resonance energy transfer and molecular dynamics simulations, and interpret them in the framework provided by the numerous crystal structures of sarco/endoplasmic reticulum Ca2+-ATPase. We discuss the challenges in characterizing the dynamics of membrane pumps, and the likely impact of new technologies on the field.
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7
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Britzolaki A, Saurine J, Klocke B, Pitychoutis PM. A Role for SERCA Pumps in the Neurobiology of Neuropsychiatric and Neurodegenerative Disorders. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2020; 1131:131-161. [PMID: 31646509 DOI: 10.1007/978-3-030-12457-1_6] [Citation(s) in RCA: 30] [Impact Index Per Article: 7.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
Abstract
Calcium (Ca2+) is a fundamental regulator of cell fate and intracellular Ca2+ homeostasis is crucial for proper function of the nerve cells. Given the complexity of neurons, a constellation of mechanisms finely tunes the intracellular Ca2+ signaling. We are focusing on the sarco/endoplasmic reticulum (SR/ER) calcium (Ca2+)-ATPase (SERCA) pump, an integral ER protein. SERCA's well established role is to preserve low cytosolic Ca2+ levels ([Ca2+]cyt), by pumping free Ca2+ ions into the ER lumen, utilizing ATP hydrolysis. The SERCA pumps are encoded by three distinct genes, SERCA1-3, resulting in 12 known protein isoforms, with tissue-dependent expression patterns. Despite the well-established structure and function of the SERCA pumps, their role in the central nervous system is not clear yet. Interestingly, SERCA-mediated Ca2+ dyshomeostasis has been associated with neuropathological conditions, such as bipolar disorder, schizophrenia, Parkinson's disease and Alzheimer's disease. We summarize here current evidence suggesting a role for SERCA in the neurobiology of neuropsychiatric and neurodegenerative disorders, thus highlighting the importance of this pump in brain physiology and pathophysiology.
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Affiliation(s)
- Aikaterini Britzolaki
- Department of Biology & Center for Tissue Regeneration and Engineering at Dayton (TREND), University of Dayton, Dayton, OH, USA
| | - Joseph Saurine
- Department of Biology & Center for Tissue Regeneration and Engineering at Dayton (TREND), University of Dayton, Dayton, OH, USA
| | - Benjamin Klocke
- Department of Biology & Center for Tissue Regeneration and Engineering at Dayton (TREND), University of Dayton, Dayton, OH, USA
| | - Pothitos M Pitychoutis
- Department of Biology & Center for Tissue Regeneration and Engineering at Dayton (TREND), University of Dayton, Dayton, OH, USA.
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8
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Orellana L. Large-Scale Conformational Changes and Protein Function: Breaking the in silico Barrier. Front Mol Biosci 2019; 6:117. [PMID: 31750315 PMCID: PMC6848229 DOI: 10.3389/fmolb.2019.00117] [Citation(s) in RCA: 54] [Impact Index Per Article: 10.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/20/2019] [Accepted: 10/14/2019] [Indexed: 12/16/2022] Open
Abstract
Large-scale conformational changes are essential to link protein structures with their function at the cell and organism scale, but have been elusive both experimentally and computationally. Over the past few years developments in cryo-electron microscopy and crystallography techniques have started to reveal multiple snapshots of increasingly large and flexible systems, deemed impossible only short time ago. As structural information accumulates, theoretical methods become central to understand how different conformers interconvert to mediate biological function. Here we briefly survey current in silico methods to tackle large conformational changes, reviewing recent examples of cross-validation of experiments and computational predictions, which show how the integration of different scale simulations with biological information is already starting to break the barriers between the in silico, in vitro, and in vivo worlds, shedding new light onto complex biological problems inaccessible so far.
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Affiliation(s)
- Laura Orellana
- Institutionen för Biokemi och Biofysik, Stockholms Universitet, Stockholm, Sweden.,Science for Life Laboratory, Solna, Sweden
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9
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Bovo E, Nikolaienko R, Bhayani S, Kahn D, Cao Q, Martin JL, Kuo IY, Robia SL, Zima AV. Novel approach for quantification of endoplasmic reticulum Ca 2+ transport. Am J Physiol Heart Circ Physiol 2019; 316:H1323-H1331. [PMID: 30901276 PMCID: PMC6620677 DOI: 10.1152/ajpheart.00031.2019] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 01/22/2019] [Revised: 03/01/2019] [Accepted: 03/04/2019] [Indexed: 01/14/2023]
Abstract
The type 2a sarco-/endoplasmic reticulum Ca2+-ATPase (SERCA2a) plays a key role in Ca2+ regulation in the heart. However, available techniques to study SERCA function are either cell destructive or lack sensitivity. The goal of this study was to develop an approach to selectively measure SERCA2a function in the cellular environment. The genetically encoded Ca2+ sensor R-CEPIA1er was used to measure the concentration of Ca2+ in the lumen of the endoplasmic reticulum (ER) ([Ca2+]ER) in HEK293 cells expressing human SERCA2a. Coexpression of the ER Ca2+ release channel ryanodine receptor (RyR2) created a Ca2+ release/reuptake system that mimicked aspects of cardiac myocyte Ca2+ handling. SERCA2a function was quantified from the rate of [Ca2+]ER refilling after ER Ca2+ depletion; then, ER Ca2+ leak was measured after SERCA inhibition. ER Ca2+ uptake and leak were analyzed as a function of [Ca2+]ER to determine maximum ER Ca2+ uptake rate and maximum ER Ca2+ load. The sensitivity of this assay was validated by analyzing effects of SERCA inhibitors, [ATP]/[ADP], oxidative stress, phospholamban, and a loss-of-function SERCA2a mutation. In addition, the feasibility of using R-CEPIA1er to study SERCA2a in a native system was evaluated by using in vivo gene delivery to express R-CEPIA1er in mouse hearts. After ventricular myocyte isolation, the same methodology used in HEK293 cells was applied to study endogenous SERCA2a. In conclusion, this new approach can be used as a sensitive screening tool to study the effect of different drugs, posttranslational modifications, and mutations on SERCA function. NEW & NOTEWORTHY The aim of this study was to develop a sensitive approach to selectively measure sarco-/endoplasmic reticulum Ca2+-ATPase (SERCA) function in the cellular environment. The newly developed Ca2+ sensor R-CEPIA1er was used to successfully analyze Ca2+ uptake mediated by recombinant and native cardiac SERCA. These results demonstrate that this new approach can be used as a powerful tool to study new mechanisms of Ca2+ pump regulation.
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Affiliation(s)
- Elisa Bovo
- Department of Cell and Molecular Physiology, Loyola University Chicago , Chicago, Illinois
| | - Roman Nikolaienko
- Department of Cell and Molecular Physiology, Loyola University Chicago , Chicago, Illinois
| | - Siddharth Bhayani
- Department of Cell and Molecular Physiology, Loyola University Chicago , Chicago, Illinois
| | - Daniel Kahn
- Department of Cell and Molecular Physiology, Loyola University Chicago , Chicago, Illinois
| | - Quan Cao
- Department of Cell and Molecular Physiology, Loyola University Chicago , Chicago, Illinois
| | - Jody L Martin
- Department of Physiology and Biophysics, University of Illinois at Chicago , Chicago, Illinois
| | - Ivana Y Kuo
- Department of Cell and Molecular Physiology, Loyola University Chicago , Chicago, Illinois
| | - Seth L Robia
- Department of Cell and Molecular Physiology, Loyola University Chicago , Chicago, Illinois
| | - Aleksey V Zima
- Department of Cell and Molecular Physiology, Loyola University Chicago , Chicago, Illinois
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10
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Sun B, Stewart BD, Kucharski AN, Kekenes-Huskey PM. Thermodynamics of Cation Binding to the Sarcoendoplasmic Reticulum Calcium ATPase Pump and Impacts on Enzyme Function. J Chem Theory Comput 2019; 15:2692-2705. [PMID: 30807147 DOI: 10.1021/acs.jctc.8b01312] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022]
Abstract
Sarcoendoplasmic reticulum Ca2+-ATPase (SERCA) is a transmembrane pump that plays an important role in transporting calcium into the sarcoplasmic reticulum (SR). While calcium (Ca2+) binds SERCA with micromolar affinity, magnesium (Mg2+) and potassium (K+) also compete with Ca2+ binding. However, the molecular bases for these competing ions' influence on the SERCA function and the selectivity of the pump for Ca2+ are not well-established. We therefore used in silico methods to resolve molecular determinants of cation binding in the canonical site I and II Ca2+ binding sites via (1) triplicate molecular dynamics (MD) simulations of Mg2+, Ca2+, and K+-bound SERCA, (2) mean spherical approximation (MSA) theory to score the affinity and selectivity of cation binding to the MD-resolved structures, and (3) state models of SERCA turnover informed from MSA-derived affinity data. Our key findings are that (a) coordination at sites I and II is optimized for Ca2+ and to a lesser extent for Mg2+ and K+, as determined by MD-derived cation-amino acid oxygen and bound water configurations, (b) the impaired coordination and high desolvation cost for Mg2+ precludes favorable Mg2+ binding relative to Ca2+, while K+ has limited capacity to bind site I, and (c) Mg2+ most likely acts as inhibitor and K+ as intermediate in SERCA's reaction cycle, based on a best-fit state model of SERCA turnover. These findings provide a quantitative basis for SERCA function that leverages molecular-scale thermodynamic data and rationalizes enzyme activity across broad ranges of K+, Ca2+, and Mg2+ concentrations.
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Affiliation(s)
- Bin Sun
- Department of Chemistry , University of Kentucky , 505 Rose Street, Chemistry-Physics Building , Lexington , Kentucky 40506 , United States
| | - Bradley D Stewart
- Department of Chemistry , University of Kentucky , 505 Rose Street, Chemistry-Physics Building , Lexington , Kentucky 40506 , United States
| | - Amir N Kucharski
- Department of Chemistry , University of Kentucky , 505 Rose Street, Chemistry-Physics Building , Lexington , Kentucky 40506 , United States
| | - Peter M Kekenes-Huskey
- Department of Chemistry , University of Kentucky , 505 Rose Street, Chemistry-Physics Building , Lexington , Kentucky 40506 , United States.,Department of Chemical and Materials Engineering , University of Kentucky , 177 F. Paul Anderson Tower , Lexington , Kentucky 40506 , United States
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11
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Rui H, Das A, Nakamoto R, Roux B. Proton Countertransport and Coupled Gating in the Sarcoplasmic Reticulum Calcium Pump. J Mol Biol 2018; 430:5050-5065. [PMID: 30539761 DOI: 10.1016/j.jmb.2018.10.014] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/21/2018] [Revised: 09/25/2018] [Accepted: 10/20/2018] [Indexed: 10/28/2022]
Abstract
The calcium pump of the sarcoplasmic reticulum (SERCA) is an ATP-driven active transporter of Ca2+ ions that functions via an "alternating-access" cycle mechanism. In each cycle, SERCA transports two Ca2+ ions toward the lumen of the sarcoplasmic reticulum and two to three protons to the cytoplasm. How the latter conformational transition is coupled to cytoplasmic release of protons remains poorly understood. The present computational study shows how the mechanism of proton countertransport is coupled to the alternating access gating process in SERCA. Molecular dynamics simulation trajectories are generated starting from a series of configurations taken along the E2 to E1 transition pathway determined by the string method with swarms-of-trajectories. Simulations of different protonation configurations at the binding sites reveal how deprotonation events affect the opening of the cytoplasmic gate. The results show that there is a strong coupling between the chronological order of deprotonation, the entry of water molecules into the TM region, and the opening of the cytoplasmic gate. Deprotonation of E309 and E771 is sequential with E309 being the first to lose the proton. The deprotonation promotes the opening of the cytoplasmic gate but leads to a productive gating transition only if it occurs after the transmembrane domain has reached an intermediate conformation. Deprotonation of E309 and E771 is unproductive when it occurs too early because it causes the re-opening of the luminal gate.
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Affiliation(s)
- Huan Rui
- Department of Biochemistry and Molecular Biology, The University of Chicago, 929 East 57th Street, Chicago, IL 60637, USA
| | - Avisek Das
- Department of Biochemistry and Molecular Biology, The University of Chicago, 929 East 57th Street, Chicago, IL 60637, USA
| | - Robert Nakamoto
- Department of Molecular Physiology and Biological Physics, University of Virginia School of Medicine, PO Box 800886, 480 Ray C. Hunt Drive, Charlottesville, VA 22908, USA
| | - Benoît Roux
- Department of Biochemistry and Molecular Biology, The University of Chicago, 929 East 57th Street, Chicago, IL 60637, USA.
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12
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Schaaf TM, Li A, Grant BD, Peterson K, Yuen S, Bawaskar P, Kleinboehl E, Li J, Thomas DD, Gillispie GD. Red-Shifted FRET Biosensors for High-Throughput Fluorescence Lifetime Screening. BIOSENSORS 2018; 8:E99. [PMID: 30352972 PMCID: PMC6315989 DOI: 10.3390/bios8040099] [Citation(s) in RCA: 29] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 09/20/2018] [Revised: 10/16/2018] [Accepted: 10/19/2018] [Indexed: 12/19/2022]
Abstract
We have developed fluorescence resonance energy transfer (FRET) biosensors with red-shifted fluorescent proteins (FP), yielding improved characteristics for time-resolved (lifetime) fluorescence measurements. In comparison to biosensors with green and red FRET pairs (GFP/RFP), FPs that emit at longer wavelengths (orange and maroon, OFP/MFP) increased the FRET efficiency, dynamic range, and signal-to-background of high-throughput screening (HTS). OFP and MFP were fused to specific sites on the human cardiac calcium pump (SERCA2a) for detection of structural changes due to small-molecule effectors. When coupled with a recently improved HTS fluorescence lifetime microplate reader, this red-shifted FRET biosensor enabled high-precision nanosecond-resolved fluorescence decay measurements from microliter sample volumes at three minute read times per 1536-well-plate. Pilot screens with a library of small-molecules demonstrate that the OFP/MFP FRET sensor substantially improves HTS assay quality. These high-content FRET methods detect minute FRET changes with high precision, as needed to elucidate novel structural mechanisms from small-molecule or peptide regulators discovered through our ongoing HTS efforts. FRET sensors that emit at longer wavelengths are highly attractive to the FRET biosensor community for drug discovery and structural interrogation of new therapeutic targets.
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Affiliation(s)
- Tory M Schaaf
- Department of Biochemistry, Molecular Biology and Biophysics, University of Minnesota, Minneapolis, MN 55455, USA.
| | - Ang Li
- Department of Biochemistry, Molecular Biology and Biophysics, University of Minnesota, Minneapolis, MN 55455, USA.
| | | | - Kurt Peterson
- Fluorescence Innovations Inc., Minneapolis, MN 55455, USA.
| | - Samantha Yuen
- Department of Biochemistry, Molecular Biology and Biophysics, University of Minnesota, Minneapolis, MN 55455, USA.
| | - Prachi Bawaskar
- Department of Biochemistry, Molecular Biology and Biophysics, University of Minnesota, Minneapolis, MN 55455, USA.
| | - Evan Kleinboehl
- Department of Biochemistry, Molecular Biology and Biophysics, University of Minnesota, Minneapolis, MN 55455, USA.
| | - Ji Li
- Department of Biochemistry, Molecular Biology and Biophysics, University of Minnesota, Minneapolis, MN 55455, USA.
| | - David D Thomas
- Department of Biochemistry, Molecular Biology and Biophysics, University of Minnesota, Minneapolis, MN 55455, USA.
- Photonic Pharma LLC, Minneapolis, MN 55410, USA.
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13
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Raguimova ON, Smolin N, Bovo E, Bhayani S, Autry JM, Zima AV, Robia SL. Redistribution of SERCA calcium pump conformers during intracellular calcium signaling. J Biol Chem 2018; 293:10843-10856. [PMID: 29764938 PMCID: PMC6052202 DOI: 10.1074/jbc.ra118.002472] [Citation(s) in RCA: 21] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/15/2018] [Revised: 05/01/2018] [Indexed: 11/06/2022] Open
Abstract
The conformational changes of a calcium transport ATPase were investigated with molecular dynamics (MD) simulations as well as fluorescence resonance energy transfer (FRET) measurements to determine the significance of a discrete structural element for regulation of the conformational dynamics of the transport cycle. Previous MD simulations indicated that a loop in the cytosolic domain of the SERCA calcium transporter facilitates an open-to-closed structural transition. To investigate the significance of this structural element, we performed additional MD simulations and new biophysical measurements of SERCA structure and function. Rationally designed in silico mutations of three acidic residues of the loop decreased SERCA domain-domain contacts and increased domain-domain separation distances. Principal component analysis of MD simulations suggested decreased sampling of compact conformations upon N-loop mutagenesis. Deficits in headpiece structural dynamics were also detected by measuring intramolecular FRET of a Cer-YFP-SERCA construct (2-color SERCA). Compared with WT, the mutated 2-color SERCA shows a partial FRET response to calcium, whereas retaining full responsiveness to the inhibitor thapsigargin. Functional measurements showed that the mutated transporter still hydrolyzes ATP and transports calcium, but that maximal enzyme activity is reduced while maintaining similar calcium affinity. In live cells, calcium elevations resulted in concomitant FRET changes as the population of WT 2-color SERCA molecules redistributed among intermediates of the transport cycle. Our results provide novel insights on how the population of SERCA pumps responds to dynamic changes in intracellular calcium.
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Affiliation(s)
- Olga N Raguimova
- From the Department of Cell and Molecular Physiology, Loyola University Chicago, Maywood, Illinois 60153 and
| | - Nikolai Smolin
- From the Department of Cell and Molecular Physiology, Loyola University Chicago, Maywood, Illinois 60153 and
| | - Elisa Bovo
- From the Department of Cell and Molecular Physiology, Loyola University Chicago, Maywood, Illinois 60153 and
| | - Siddharth Bhayani
- From the Department of Cell and Molecular Physiology, Loyola University Chicago, Maywood, Illinois 60153 and
| | - Joseph M Autry
- the Department of Biochemistry, Molecular Biology, and Biophysics, University of Minnesota, Minneapolis, Minnesota 55455
| | - Aleksey V Zima
- From the Department of Cell and Molecular Physiology, Loyola University Chicago, Maywood, Illinois 60153 and
| | - Seth L Robia
- From the Department of Cell and Molecular Physiology, Loyola University Chicago, Maywood, Illinois 60153 and
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14
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Fernández-de Gortari E, Espinoza-Fonseca LM. Structural basis for relief of phospholamban-mediated inhibition of the sarcoplasmic reticulum Ca 2+-ATPase at saturating Ca 2+ conditions. J Biol Chem 2018; 293:12405-12414. [PMID: 29934304 DOI: 10.1074/jbc.ra118.003752] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/27/2018] [Revised: 06/21/2018] [Indexed: 11/06/2022] Open
Abstract
Sarcoplasmic reticulum Ca2+-ATPase (SERCA) is critical for cardiac Ca2+ transport. Reversal of phospholamban (PLB)-mediated SERCA inhibition by saturating Ca2+ conditions operates as a physiological rheostat to reactivate SERCA function in the absence of PLB phosphorylation. Here, we performed extensive atomistic molecular dynamics simulations to probe the structural mechanism of this process. Simulation of the inhibitory complex at superphysiological Ca2+ concentrations ([Ca2+] = 10 mm) revealed that Ca2+ ions interact primarily with SERCA and the lipid headgroups, but not with PLB's cytosolic domain or the cytosolic side of the SERCA-PLB interface. At this [Ca2+], a single Ca2+ ion was translocated from the cytosol to the transmembrane transport sites. We used this Ca2+-bound complex as an initial structure to simulate the effects of saturating Ca2+ at physiological conditions ([Ca2+]total ≈ 400 μm). At these conditions, ∼30% of the Ca2+-bound complexes exhibited structural features consistent with an inhibited state. However, in ∼70% of the Ca2+-bound complexes, Ca2+ moved to transport site I, recruited Glu771 and Asp800, and disrupted key inhibitory contacts involving the conserved PLB residue Asn34 Structural analysis showed that Ca2+ induces only local changes in interresidue inhibitory interactions, but does not induce repositioning or changes in PLB structural dynamics. Upon relief of SERCA inhibition, Ca2+ binding produced a site I configuration sufficient for subsequent SERCA activation. We propose that at saturating [Ca2+] and in the absence of PLB phosphorylation, binding of a single Ca2+ ion in the transport sites rapidly shifts the equilibrium toward a noninhibited SERCA-PLB complex.
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Affiliation(s)
- Eli Fernández-de Gortari
- From the Center for Arrhythmia Research, Department of Internal Medicine, Division of Cardiovascular Medicine, University of Michigan, Ann Arbor, Michigan 48109
| | - L Michel Espinoza-Fonseca
- From the Center for Arrhythmia Research, Department of Internal Medicine, Division of Cardiovascular Medicine, University of Michigan, Ann Arbor, Michigan 48109
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15
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Wu W, Shao X, Zhao J, Wu M. Controllable Photodynamic Therapy Implemented by Regulating Singlet Oxygen Efficiency. ADVANCED SCIENCE (WEINHEIM, BADEN-WURTTEMBERG, GERMANY) 2017; 4:1700113. [PMID: 28725533 PMCID: PMC5515253 DOI: 10.1002/advs.201700113] [Citation(s) in RCA: 94] [Impact Index Per Article: 13.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/06/2017] [Revised: 04/08/2017] [Indexed: 05/25/2023]
Abstract
With singlet oxygen (1O2) as the active agent, photodynamic therapy (PDT) is a promising technique for the treatment of various tumors and cancers. But it is hampered by the poor selectivity of most traditional photosensitizers (PS). In this review, we present a summary of controllable PDT implemented by regulating singlet oxygen efficiency. Herein, various controllable PDT strategies based on different initiating conditions (such as pH, light, H2O2 and so on) have been summarized and introduced. More importantly, the action mechanisms of controllable PDT strategies, such as photoinduced electron transfer (PET), fluorescence resonance energy transfer (FRET), intramolecular charge transfer (ICT) and some physical/chemical means (e.g. captivity and release), are described as a key point in the article. This review provide a general overview of designing novel PS or strategies for effective and controllable PDT.
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Affiliation(s)
- Wenting Wu
- State Key Laboratory of Heavy Oil ProcessingChina University of PetroleumQingdao266580China
- State Key Laboratory of Fine ChemicalsSchool of Chemical EngineeringDalian University of TechnologyDalian116024P. R. China
| | - Xiaodong Shao
- State Key Laboratory of Heavy Oil ProcessingChina University of PetroleumQingdao266580China
| | - Jianzhang Zhao
- State Key Laboratory of Fine ChemicalsSchool of Chemical EngineeringDalian University of TechnologyDalian116024P. R. China
| | - Mingbo Wu
- State Key Laboratory of Heavy Oil ProcessingChina University of PetroleumQingdao266580China
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16
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Fernández-de Gortari E, Espinoza-Fonseca LM. Preexisting domain motions underlie protonation-dependent structural transitions of the P-type Ca 2+-ATPase. Phys Chem Chem Phys 2017; 19:10153-10162. [PMID: 28374038 PMCID: PMC5472844 DOI: 10.1039/c7cp00243b] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022]
Abstract
We have performed microsecond molecular dynamics (MD) simulations to determine the mechanism for protonation-dependent structural transitions of the sarco/endoplasmic reticulum Ca2+-ATPase (SERCA), one of the most prominent members of the large P-type ATPase superfamily that transports ions across biological membranes. The release of two H+ from the transport sites activates SERCA by inducing a structural transition between low (E2) and high (E1) Ca2+-affinity states (E2-to-E1 transition), but the structural mechanism by which transport site deprotonation facilitates this transition is unknown. We performed microsecond all-atom MD simulations to determine the effects of transport site protonation on the structural dynamics of the E2 state in solution. We found that the protonated E2 state has structural characteristics that are similar to those observed in crystal structures of E2. Upon deprotonation, a single Na+ ion rapidly (<10 ns) binds to the transmembrane transport sites and induces a kink in M5, disrupts the M3-M5 interface, and increases the mobility of the M3/A-M3 linker. Principal component analysis showed that counter-rotation of the cytosolic N-A domains about the membrane normal axis, which is the primary motion driving the E2-to-E1 transition, is present in both protonated and deprotonated E2 states; however, protonation-dependent structural changes in the transmembrane domain control the hierarchical organization and amplitude of this motion. We propose that preexisting rigid-body domain motions underlie structural transitions of SERCA, where the functionally important directionality is preserved while transport site protonation controls the dominance and amplitude of motion to shift the equilibrium between the E1 and E2 states. We conclude that ligand-induced modulation of preexisting domain motions is likely a common theme in structural transitions of the P-type ATPase superfamily.
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Affiliation(s)
- Eli Fernández-de Gortari
- Department of Biochemistry, Molecular Biology and Biophysics, University of Minnesota, Minneapolis, MN 55455, USA.
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17
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Schaaf TM, Peterson KC, Grant BD, Bawaskar P, Yuen S, Li J, Muretta JM, Gillispie GD, Thomas DD. High-Throughput Spectral and Lifetime-Based FRET Screening in Living Cells to Identify Small-Molecule Effectors of SERCA. SLAS DISCOVERY : ADVANCING LIFE SCIENCES R & D 2017; 22:262-273. [PMID: 27899691 PMCID: PMC5323330 DOI: 10.1177/1087057116680151] [Citation(s) in RCA: 37] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/10/2023]
Abstract
A robust high-throughput screening (HTS) strategy has been developed to discover small-molecule effectors targeting the sarco/endoplasmic reticulum calcium ATPase (SERCA), based on a fluorescence microplate reader that records both the nanosecond decay waveform (lifetime mode) and the complete emission spectrum (spectral mode), with high precision and speed. This spectral unmixing plate reader (SUPR) was used to screen libraries of small molecules with a fluorescence resonance energy transfer (FRET) biosensor expressed in living cells. Ligand binding was detected by FRET associated with structural rearrangements of green fluorescent protein (GFP, donor) and red fluorescent protein (RFP, acceptor) fused to the cardiac-specific SERCA2a isoform. The results demonstrate accurate quantitation of FRET along with high precision of hit identification. Fluorescence lifetime analysis resolved SERCA's distinct structural states, providing a method to classify small-molecule chemotypes on the basis of their structural effect on the target. The spectral analysis was also applied to flag interference by fluorescent compounds. FRET hits were further evaluated for functional effects on SERCA's ATPase activity via both a coupled-enzyme assay and a FRET-based calcium sensor. Concentration-response curves indicated excellent correlation between FRET and function. These complementary spectral and lifetime FRET detection methods offer an attractive combination of precision, speed, and resolution for HTS.
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Affiliation(s)
- Tory M. Schaaf
- Department of Biochemistry, Molecular Biology and Biophysics, University of Minnesota, Minneapolis, Minnesota 55455
| | | | | | - Prachi Bawaskar
- Department of Biochemistry, Molecular Biology and Biophysics, University of Minnesota, Minneapolis, Minnesota 55455
| | - Samantha Yuen
- Department of Biochemistry, Molecular Biology and Biophysics, University of Minnesota, Minneapolis, Minnesota 55455
| | - Ji Li
- Department of Biochemistry, Molecular Biology and Biophysics, University of Minnesota, Minneapolis, Minnesota 55455
| | - Joseph M. Muretta
- Department of Biochemistry, Molecular Biology and Biophysics, University of Minnesota, Minneapolis, Minnesota 55455
| | | | - David D. Thomas
- Department of Biochemistry, Molecular Biology and Biophysics, University of Minnesota, Minneapolis, Minnesota 55455
- Photonic Pharma LLC, Minneapolis, MN 55410
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18
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Prediction and validation of protein intermediate states from structurally rich ensembles and coarse-grained simulations. Nat Commun 2016; 7:12575. [PMID: 27578633 PMCID: PMC5013691 DOI: 10.1038/ncomms12575] [Citation(s) in RCA: 50] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/26/2016] [Accepted: 07/13/2016] [Indexed: 12/28/2022] Open
Abstract
Protein conformational changes are at the heart of cell functions, from signalling to ion transport. However, the transient nature of the intermediates along transition pathways hampers their experimental detection, making the underlying mechanisms elusive. Here we retrieve dynamic information on the actual transition routes from principal component analysis (PCA) of structurally-rich ensembles and, in combination with coarse-grained simulations, explore the conformational landscapes of five well-studied proteins. Modelling them as elastic networks in a hybrid elastic-network Brownian dynamics simulation (eBDIMS), we generate trajectories connecting stable end-states that spontaneously sample the crystallographic motions, predicting the structures of known intermediates along the paths. We also show that the explored non-linear routes can delimit the lowest energy passages between end-states sampled by atomistic molecular dynamics. The integrative methodology presented here provides a powerful framework to extract and expand dynamic pathway information from the Protein Data Bank, as well as to validate sampling methods in general. Protein conformational changes are key to a wide range of cellular functions but remain difficult to access experimentally. Here the authors describe eBDIMS, a novel approach to predict intermediates observed in structural transition pathways from experimental ensembles.
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Dimerization of human uridine diphosphate glucuronosyltransferase allozymes 1A1 and 1A9 alters their quercetin glucuronidation activities. Sci Rep 2016; 6:23763. [PMID: 27025983 PMCID: PMC4837415 DOI: 10.1038/srep23763] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/05/2016] [Accepted: 02/26/2016] [Indexed: 11/08/2022] Open
Abstract
Uridine diphosphate glucuronosyltransferase 1A (UGT1A) is a major phase II drug-metabolism enzyme superfamily involved in the glucuronidation of endobiotics and xenobiotics in humans. Many polymorphisms in UGT1A genes are reported to inhibit or decrease UGT1A activity. In this study, two UGT1A1 allozymes, UGT1A1 wild-type and a splice mutant, as well as UGT1A9 wild-type and its three UGT1A9 allozymes, UGT1A9*2(C3Y), UGT1A9*3(M33T), and UGT1A9*5(D256N) were single- or double-expressed in a Bac-to-Bac expression system. Dimerization of UGT1A1 or UGT1A9 allozymes was observed via fluorescence resonance energy transfer (FRET) and co-immunoprecipitation analysis. SNPs of UGT1A altered the ability of protein-protein interaction, resulting in differential FRET efficiencies and donor-acceptor r distances. Dimerization changed the chemical regioselectivity, substrate-binding affinity, and enzymatic activity of UGT1A1 and UGT1A9 in glucuronidation of quercetin. These findings provide molecular insights into the consequences of homozygous and heterozygous UGT1A1 and UGT1A9 allozymes expression on quercetin glucuronidation.
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20
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Iram SH, Gruber SJ, Raguimova ON, Thomas DD, Robia SL. ATP-Binding Cassette Transporter Structure Changes Detected by Intramolecular Fluorescence Energy Transfer for High-Throughput Screening. Mol Pharmacol 2015; 88:84-94. [PMID: 25924616 DOI: 10.1124/mol.114.096792] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/20/2014] [Accepted: 04/29/2015] [Indexed: 11/22/2022] Open
Abstract
Multidrug resistance protein 1 (MRP1) actively transports a wide variety of drugs out of cells. To quantify MRP1 structural dynamics, we engineered a "two-color MRP1" construct by fusing green fluorescent protein (GFP) and TagRFP to MRP1 nucleotide-binding domains NBD1 and NBD2, respectively. The recombinant MRP1 protein expressed and trafficked normally to the plasma membrane. Two-color MRP1 transport activity was normal, as shown by vesicular transport of [(3)H]17β-estradiol-17-β-(D-glucuronide) and doxorubicin efflux in AAV-293 cells. We quantified fluorescence resonance energy transfer (FRET) from GFP to TagRFP as an index of NBD conformational changes. Our results show that ATP binding induces a large-amplitude conformational change that brings the NBDs into closer proximity. FRET was further increased by substrate in the presence of ATP but not by substrate alone. The data suggest that substrate binding is required to achieve a fully closed and compact structure. ATP analogs bind MRP1 with reduced apparent affinity, inducing a partially closed conformation. The results demonstrate the utility of the two-color MRP1 construct for investigating ATP-binding cassette transporter structural dynamics, and it holds great promise for high-throughput screening of chemical libraries for unknown activators, inhibitors, or transportable substrates of MRP1.
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Affiliation(s)
- Surtaj H Iram
- Department of Cell and Molecular Physiology (S.H.I., O.N.R., S.L.R.), Cardiovascular Research Institute (O.N.R., S.L.R.), Stritch School of Medicine, Loyola University Chicago, Maywood, Illinois; and Department of Biochemistry, Molecular Biology, and Biophysics, University of Minnesota, Minneapolis, Minnesota (S.J.G., D.D.T); and Department of Chemistry and Biochemistry, South Dakota State University, Brookings, South Dakota (S.H.I.)
| | - Simon J Gruber
- Department of Cell and Molecular Physiology (S.H.I., O.N.R., S.L.R.), Cardiovascular Research Institute (O.N.R., S.L.R.), Stritch School of Medicine, Loyola University Chicago, Maywood, Illinois; and Department of Biochemistry, Molecular Biology, and Biophysics, University of Minnesota, Minneapolis, Minnesota (S.J.G., D.D.T); and Department of Chemistry and Biochemistry, South Dakota State University, Brookings, South Dakota (S.H.I.)
| | - Olga N Raguimova
- Department of Cell and Molecular Physiology (S.H.I., O.N.R., S.L.R.), Cardiovascular Research Institute (O.N.R., S.L.R.), Stritch School of Medicine, Loyola University Chicago, Maywood, Illinois; and Department of Biochemistry, Molecular Biology, and Biophysics, University of Minnesota, Minneapolis, Minnesota (S.J.G., D.D.T); and Department of Chemistry and Biochemistry, South Dakota State University, Brookings, South Dakota (S.H.I.)
| | - David D Thomas
- Department of Cell and Molecular Physiology (S.H.I., O.N.R., S.L.R.), Cardiovascular Research Institute (O.N.R., S.L.R.), Stritch School of Medicine, Loyola University Chicago, Maywood, Illinois; and Department of Biochemistry, Molecular Biology, and Biophysics, University of Minnesota, Minneapolis, Minnesota (S.J.G., D.D.T); and Department of Chemistry and Biochemistry, South Dakota State University, Brookings, South Dakota (S.H.I.)
| | - Seth L Robia
- Department of Cell and Molecular Physiology (S.H.I., O.N.R., S.L.R.), Cardiovascular Research Institute (O.N.R., S.L.R.), Stritch School of Medicine, Loyola University Chicago, Maywood, Illinois; and Department of Biochemistry, Molecular Biology, and Biophysics, University of Minnesota, Minneapolis, Minnesota (S.J.G., D.D.T); and Department of Chemistry and Biochemistry, South Dakota State University, Brookings, South Dakota (S.H.I.)
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