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Tahk MJ, Torp J, Ali MAS, Fishman D, Parts L, Grätz L, Müller C, Keller M, Veiksina S, Laasfeld T, Rinken A. Live-cell microscopy or fluorescence anisotropy with budded baculoviruses-which way to go with measuring ligand binding to M 4 muscarinic receptors? Open Biol 2022; 12:220019. [PMID: 35674179 PMCID: PMC9175271 DOI: 10.1098/rsob.220019] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/19/2022] [Accepted: 04/27/2022] [Indexed: 01/04/2023] Open
Abstract
M4 muscarinic acetylcholine receptor is a G protein-coupled receptor (GPCR) that has been associated with alcohol and cocaine abuse, Alzheimer's disease, and schizophrenia which makes it an interesting drug target. For many GPCRs, the high-affinity fluorescence ligands have expanded the options for high-throughput screening of drug candidates and serve as useful tools in fundamental receptor research. Here, we explored two TAMRA-labelled fluorescence ligands, UR-MK342 and UR-CG072, for development of assays for studying ligand-binding properties to M4 receptor. Using budded baculovirus particles as M4 receptor preparation and fluorescence anisotropy method, we measured the affinities and binding kinetics of both fluorescence ligands. Using the fluorescence ligands as reporter probes, the binding affinities of unlabelled ligands could be determined. Based on these results, we took a step towards a more natural system and developed a method using live CHO-K1-hM4R cells and automated fluorescence microscopy suitable for the routine determination of unlabelled ligand affinities. For quantitative image analysis, we developed random forest and deep learning-based pipelines for cell segmentation. The pipelines were integrated into the user-friendly open-source Aparecium software. Both image analysis methods were suitable for measuring fluorescence ligand saturation binding and kinetics as well as for screening binding affinities of unlabelled ligands.
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Affiliation(s)
- Maris-Johanna Tahk
- Institute of Chemistry, University of Tartu, Ravila 14a, 50411 Tartu, Estonia
| | - Jane Torp
- Institute of Chemistry, University of Tartu, Ravila 14a, 50411 Tartu, Estonia
| | - Mohammed A. S. Ali
- Department of Computer Science, University of Tartu, Narva Street 20, 51009 Tartu, Estonia
| | - Dmytro Fishman
- Department of Computer Science, University of Tartu, Narva Street 20, 51009 Tartu, Estonia
| | - Leopold Parts
- Department of Computer Science, University of Tartu, Narva Street 20, 51009 Tartu, Estonia
- Wellcome Sanger Institute, Wellcome Genome Campus, Hinxton, Cambridgeshire, UK
| | - Lukas Grätz
- Institute of Pharmacy, Faculty of Chemistry and Pharmacy, University of Regensburg, Universitätsstrasse 31, 93053 Regensburg, Germany
| | - Christoph Müller
- Institute of Pharmacy, Faculty of Chemistry and Pharmacy, University of Regensburg, Universitätsstrasse 31, 93053 Regensburg, Germany
| | - Max Keller
- Institute of Pharmacy, Faculty of Chemistry and Pharmacy, University of Regensburg, Universitätsstrasse 31, 93053 Regensburg, Germany
| | - Santa Veiksina
- Institute of Chemistry, University of Tartu, Ravila 14a, 50411 Tartu, Estonia
| | - Tõnis Laasfeld
- Institute of Chemistry, University of Tartu, Ravila 14a, 50411 Tartu, Estonia
- Department of Computer Science, University of Tartu, Narva Street 20, 51009 Tartu, Estonia
| | - Ago Rinken
- Institute of Chemistry, University of Tartu, Ravila 14a, 50411 Tartu, Estonia
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2
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Veiksina S, Tahk MJ, Laasfeld T, Link R, Kopanchuk S, Rinken A. Fluorescence Anisotropy-Based Assay for Characterization of Ligand Binding Dynamics to GPCRs: The Case of Cy3B-Labeled Ligands Binding to MC 4 Receptors in Budded Baculoviruses. Methods Mol Biol 2021; 2268:119-136. [PMID: 34085265 DOI: 10.1007/978-1-0716-1221-7_8] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Abstract
During the past decade, fluorescence methods have become valuable tools for characterizing ligand binding to G protein-coupled receptors (GPCRs). However, only a few of the assays enable studying wild-type receptors and monitor the ligand binding in real time. One of the approaches that is inherently suitable for this purpose is the fluorescence anisotropy (FA) assay. In the FA assay, the change of ligand's rotational freedom connected with its binding to the receptor can be monitored with a conventional fluorescence plate reader equipped with suitable optical filters. To achieve the high receptor concentration required for the assay and the low autofluorescence levels essential for reliable results, budded baculoviruses that display GPCRs on their surfaces can be used. The monitoring process generates a substantial amount of kinetic data, which is usually stored as a proprietary file format limiting the flexibility of data analysis. To solve this problem, we propose the use of the data curation software Aparecium ( http://gpcr.ut.ee/aparecium.html ), which integrates experimental data with metadata in a Minimum Information for Data Analysis in Systems Biology (MIDAS) format. Aparecium enables data export to different software packages for fitting to suitable kinetic or equilibrium models. A combination of the FA assay with the novel data analysis strategy is suitable for screening new active compounds, but also for modeling complex systems of ligand binding to GPCRs. We present the proposed approach using different fluorescent probes and assay types to characterize ligand binding to melanocortin 4 (MC4) receptor.
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Affiliation(s)
- Santa Veiksina
- Institute of Chemistry, University of Tartu, Tartu, Estonia
| | | | - Tõnis Laasfeld
- Institute of Chemistry, University of Tartu, Tartu, Estonia
| | - Reet Link
- Institute of Chemistry, University of Tartu, Tartu, Estonia
| | | | - Ago Rinken
- Institute of Chemistry, University of Tartu, Tartu, Estonia.
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3
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BRET- and fluorescence anisotropy-based assays for real-time monitoring of ligand binding to M 2 muscarinic acetylcholine receptors. BIOCHIMICA ET BIOPHYSICA ACTA-MOLECULAR CELL RESEARCH 2020; 1868:118930. [PMID: 33347921 DOI: 10.1016/j.bbamcr.2020.118930] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Subscribe] [Scholar Register] [Received: 09/08/2020] [Revised: 11/26/2020] [Accepted: 12/11/2020] [Indexed: 12/14/2022]
Abstract
BRET and fluorescence anisotropy (FA) are two fluorescence-based techniques used for the characterization of ligand binding to G protein-coupled receptors (GPCRs) and both allow monitoring of ligand binding in real time. In this study, we present the first direct comparison of BRET-based and FA-based binding assays using the human M2 muscarinic acetylcholine receptor (M2R) and two TAMRA (5-carboxytetramethylrhodamine)-labeled fluorescent ligands as a model system. The determined fluorescent ligand affinities from both assays were in good agreement with results obtained from radioligand competition binding experiments. The assays yielded real-time kinetic binding data revealing differences in the mechanism of binding for the investigated fluorescent probes. Furthermore, the investigation of various unlabeled M2R ligands yielded pharmacological profiles in accordance with earlier reported data. Taken together, this study showed that BRET- and FA-based binding assays represent valuable alternatives to radioactivity-based methods for screening purposes and for a precise characterization of binding kinetics supporting the exploration of binding mechanisms.
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Allikalt A, Laasfeld T, Ilisson M, Kopanchuk S, Rinken A. Quantitative analysis of fluorescent ligand binding to dopamine D 3 receptors using live-cell microscopy. FEBS J 2020; 288:1514-1532. [PMID: 32783364 DOI: 10.1111/febs.15519] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/23/2020] [Revised: 07/14/2020] [Accepted: 07/30/2020] [Indexed: 12/29/2022]
Abstract
Dopamine receptors are G protein-coupled receptors that have several essential functions in the central nervous system. A better understanding of the regulatory mechanisms of ligand binding to the receptor may open new possibilities to affect the downstream signal transduction pathways. The majority of the available ligand binding assays use either membrane preparations, cell suspensions, or genetically modified receptors, which may give at least partially incorrect understanding of ligand binding. In this study, we implemented an assay combining fluorescence and bright-field microscopy to measure ligand binding to dopamine D3 receptors in live mammalian cells. For membrane fluorescence intensity quantification from microscopy images, we developed a machine learning-based user-friendly software membrane tools and incorporated it into a data management software aparecium that has been previously developed in our workgroup. For the experiments, a fluorescent ligand NAPS-Cy3B was synthesized by conjugating a dopaminergic antagonist N-(p-aminophenethyl)spiperone with a fluorophore Cy3B. The subnanomolar affinity of NAPS-Cy3B makes it a suitable ligand for the characterization of D3 receptors in live HEK293 cells. Using a microplate compatible automated widefield fluorescence microscope, together with the membrane tools software, enables the detection and quantification of ligand binding with a high-throughput. The live cell assay is suitable for the characterization of fluorescent ligand binding and also in the competition experiments for the screening of novel unlabeled dopaminergic ligands. We propose that this simple yet more native-like approach is feasible in GPCR research, as it enables the detection of ligand binding in an environment containing more components involved in the signal transduction cascade.
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Affiliation(s)
- Anni Allikalt
- Institute of Chemistry, University of Tartu, Estonia.,Department of Chemistry and Pharmacy, Medicinal Chemistry, Friedrich-Alexander University Erlangen-Nürnberg, Germany
| | | | | | | | - Ago Rinken
- Institute of Chemistry, University of Tartu, Estonia
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5
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Miljus T, Sykes DA, Harwood CR, Vuckovic Z, Veprintsev DB. GPCR Solubilization and Quality Control. Methods Mol Biol 2020; 2127:105-127. [PMID: 32112318 DOI: 10.1007/978-1-0716-0373-4_8] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
G protein-coupled receptors (GPCRs) are versatile membrane proteins involved in the regulation of many physiological processes and pathological conditions, making them interesting pharmacological targets. In order to study their structure and function, GPCRs are traditionally extracted from membranes using detergents. However, due to their hydrophobic nature, intrinsic instability in aqueous solutions, and their denaturing effects, the isolation of properly folded and functional GPCRs is not trivial. Therefore, it is of crucial importance to solubilize receptors under mild conditions and control the sample quality subsequently. Here we describe widely used methods for small-scale GPCR solubilization, followed by quality control based on fluorescence size-exclusion chromatography, SDS-PAGE, temperature-induced protein unfolding (CPM dye binding) and fluorescent ligand binding assay. These methods can easily be used to assess the thermostability and functionality of a GPCR sample exposed to different conditions, such as the use of various detergents, addition of lipids and ligands, making them valuable for obtaining an optimal sample quality for structural and functional studies.
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Affiliation(s)
- Tamara Miljus
- Laboratory of Biomolecular Research, Paul Scherrer Institute, Villigen PSI, Switzerland
- Department of Biology, ETH Zürich, Zürich, Switzerland
- Centre of Membrane Proteins and Receptors (COMPARE), University of Birmingham and University of Nottingham, Midlands, UK
- Institute of Metabolism and Systems Research, College of Medical and Dental Sciences, University of Birmingham, Birmingham, UK
| | - David A Sykes
- Centre of Membrane Proteins and Receptors (COMPARE), University of Birmingham and University of Nottingham, Midlands, UK
- Division of Physiology, Pharmacology & Neuroscience, School of Life Sciences, University of Nottingham, Nottingham, UK
| | - Clare R Harwood
- Centre of Membrane Proteins and Receptors (COMPARE), University of Birmingham and University of Nottingham, Midlands, UK
- Division of Physiology, Pharmacology & Neuroscience, School of Life Sciences, University of Nottingham, Nottingham, UK
| | - Ziva Vuckovic
- Laboratory of Biomolecular Research, Paul Scherrer Institute, Villigen PSI, Switzerland
- Department of Biology, ETH Zürich, Zürich, Switzerland
- Drug Discovery Biology, Monash Institute of Pharmaceutical Sciences, Monash University, Parkville, VIC, Australia
| | - Dmitry B Veprintsev
- Laboratory of Biomolecular Research, Paul Scherrer Institute, Villigen PSI, Switzerland.
- Department of Biology, ETH Zürich, Zürich, Switzerland.
- Centre of Membrane Proteins and Receptors (COMPARE), University of Birmingham and University of Nottingham, Midlands, UK.
- Division of Physiology, Pharmacology & Neuroscience, School of Life Sciences, University of Nottingham, Nottingham, UK.
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6
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Link R, Veiksina S, Tahk MJ, Laasfeld T, Paiste P, Kopanchuk S, Rinken A. The constitutive activity of melanocortin-4 receptors in cAMP pathway is allosterically modulated by zinc and copper ions. J Neurochem 2019; 153:346-361. [PMID: 31792980 DOI: 10.1111/jnc.14933] [Citation(s) in RCA: 13] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/16/2019] [Revised: 11/06/2019] [Accepted: 11/29/2019] [Indexed: 01/25/2023]
Abstract
Melanocortin-4 receptors (MC4 R) are unique among G-protein-coupled receptors (GPCRs) as they have endogenous ligands that can exhibit inverse agonistic properties in the case of elevated basal activity. It is known that the constitutive activity of GPCRs strongly affects the ligand-dependent physiological responses, but little is known about these regulatory mechanisms. Since several metal ions have been shown to be important modulators of the signal transduction of GPCRs, we hypothesized that metal ions regulate the basal activity of MC4 Rs. Implementation of a fluorescence anisotropy assay and novel redshifted fluorescent peptides enabled kinetic characterization of ligand binding to MC4 R expressed on budded baculoviruses. We show that Ca2+ is required for high-affinity ligand binding, but Zn2+ and Cu2+ in the presence of Ca2+ behave as negative allosteric modulators of ligand binding to MC4 R. FRET-based cAMP biosensor was used to measure the activation of MC4 R stably expressed in CHO-K1 cells. At low micromolar concentrations, Zn2+ caused MC4 R-dependent activation of the cAMP pathway, whereas Cu2+ reduced the activity of MC4 R even below the basal level. These findings indicate that at physiologically relevant concentrations can Zn2+ and Cu2+ function as MC4 R agonists or inverse agonists, respectively. This means that depending on the level of constitutive activity induced by Zn2+ ions, the pharmacological effect of orthosteric ligands of MC4 R can be switched from a partial to an inverse agonist. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. More information about the Open Science badges can be found at https://cos.io/our-services/open-science-badges/.
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Affiliation(s)
- Reet Link
- Institute of Chemistry, University of Tartu, Tartu, Estonia
| | - Santa Veiksina
- Institute of Chemistry, University of Tartu, Tartu, Estonia
| | | | - Tõnis Laasfeld
- Institute of Chemistry, University of Tartu, Tartu, Estonia
| | - Päärn Paiste
- Department of Geology, University of Tartu, Tartu, Estonia
| | | | - Ago Rinken
- Institute of Chemistry, University of Tartu, Tartu, Estonia
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7
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Viirlaid E, Ilisson M, Kopanchuk S, Mäeorg U, Rinken A, Rinken T. Immunoassay for rapid on-site detection of glyphosate herbicide. ENVIRONMENTAL MONITORING AND ASSESSMENT 2019; 191:507. [PMID: 31342281 DOI: 10.1007/s10661-019-7657-z] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/16/2019] [Accepted: 07/15/2019] [Indexed: 06/10/2023]
Abstract
Glyphosate is the most widespread herbicide and its global use is steadily increasing. Although glyphosate is considered to have low toxicity, its wide application has raised concerns about its effects on human health. The extensive use of glyphosate has risen a need of its continuous monitoring in drinking and surface waters to assure in accordance with the set standards. Within the present study, we have developed a novel assay for the on-site detection of glyphosate by combining flow-through technology with the high specificity of immunorecognition. The proposed biosensing system was based on the detection of fluorescence signal generated by the quantitative replacement of glyphosate in antigen-antibody complex with IgY-type anti-glyphosate antibodies on microbeads by synthetic 5-carboxytetramethylrhodamine (5-TAMRA) conjugated glyphosate. The working range of this assay was in low millimolar range and the time required for glyphosate detection around 0.5 h. The applicability of the immunoassay for glyphosate detection in surface water was tested and the biosensor results were validated with high-performance liquid chromatography.
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Affiliation(s)
- E Viirlaid
- Institute of Chemistry, University of Tartu, Ravila 14A, 50411, Tartu, Estonia.
| | - M Ilisson
- Institute of Chemistry, University of Tartu, Ravila 14A, 50411, Tartu, Estonia
| | - S Kopanchuk
- Institute of Chemistry, University of Tartu, Ravila 14A, 50411, Tartu, Estonia
| | - U Mäeorg
- Institute of Chemistry, University of Tartu, Ravila 14A, 50411, Tartu, Estonia
| | - A Rinken
- Institute of Chemistry, University of Tartu, Ravila 14A, 50411, Tartu, Estonia
| | - T Rinken
- Institute of Chemistry, University of Tartu, Ravila 14A, 50411, Tartu, Estonia
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8
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Allikalt A, Kopanchuk S, Rinken A. Implementation of fluorescence anisotropy-based assay for the characterization of ligand binding to dopamine D1 receptors. Eur J Pharmacol 2018; 839:40-46. [DOI: 10.1016/j.ejphar.2018.09.008] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/05/2018] [Revised: 09/04/2018] [Accepted: 09/05/2018] [Indexed: 01/11/2023]
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9
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Wang S, Zhang Y, Zhang L, Zhang M, Tian C. Conformational change of E. coli sulfurtransferase YgaP upon SCN− in intact native membrane revealed by fluorescence lifetime and anisotropy. CHINESE CHEM LETT 2018. [DOI: 10.1016/j.cclet.2018.08.002] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/28/2022]
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10
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Assays with Detection of Fluorescence Anisotropy: Challenges and Possibilities for Characterizing Ligand Binding to GPCRs. Trends Pharmacol Sci 2018; 39:187-199. [DOI: 10.1016/j.tips.2017.10.004] [Citation(s) in RCA: 28] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/31/2017] [Revised: 10/10/2017] [Accepted: 10/10/2017] [Indexed: 01/24/2023]
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11
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Allikalt A, Rinken A. Budded baculovirus particles as a source of membrane proteins for radioligand binding assay: The case of dopamine D 1 receptor. J Pharmacol Toxicol Methods 2017; 86:81-86. [DOI: 10.1016/j.vascn.2017.04.004] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/14/2016] [Revised: 03/16/2017] [Accepted: 04/08/2017] [Indexed: 01/27/2023]
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12
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Bernetti M, Cavalli A, Mollica L. Protein-ligand (un)binding kinetics as a new paradigm for drug discovery at the crossroad between experiments and modelling. MEDCHEMCOMM 2017; 8:534-550. [PMID: 30108770 PMCID: PMC6072069 DOI: 10.1039/c6md00581k] [Citation(s) in RCA: 59] [Impact Index Per Article: 8.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/18/2016] [Accepted: 01/25/2017] [Indexed: 12/14/2022]
Abstract
In the last three decades, protein and nucleic acid structure determination and comprehension of the mechanisms, leading to their physiological and pathological functions, have become a cornerstone of biomedical sciences. A deep understanding of the principles governing the fates of cells and tissue at the molecular level has been gained over the years, offering a solid basis for the rational design of drugs aimed at the pharmacological treatment of numerous diseases. Historically, affinity indicators (i.e. Kd and IC50/EC50) have been assumed to be valid indicators of the in vivo efficacy of a drug. However, recent studies pointed out that the kinetics of the drug-receptor binding process could be as important or even more important than affinity in determining the drug efficacy. This eventually led to a growing interest in the characterisation and prediction of the rate constants of protein-ligand association and dissociation. For instance, a drug with a longer residence time can kinetically select a given receptor over another, even if the affinity for both receptors is comparable, thus increasing its therapeutic index. Therefore, understanding the molecular features underlying binding and unbinding processes is of central interest towards the rational control of drug binding kinetics. In this review, we report the theoretical framework behind protein-ligand association and highlight the latest advances in the experimental and computational approaches exploited to investigate the binding kinetics.
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Affiliation(s)
- M Bernetti
- Department of Pharmacy and Biotechnology , University of Bologna , via Belmeloro 6 , 40126 Bologna , Italy
- CompuNet , Istituto Italiano di Tecnologia , via Morego 30 , 16163 Genova , Italy .
| | - A Cavalli
- Department of Pharmacy and Biotechnology , University of Bologna , via Belmeloro 6 , 40126 Bologna , Italy
- CompuNet , Istituto Italiano di Tecnologia , via Morego 30 , 16163 Genova , Italy .
| | - L Mollica
- CompuNet , Istituto Italiano di Tecnologia , via Morego 30 , 16163 Genova , Italy .
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13
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Link R, Veiksina S, Rinken A, Kopanchuk S. Characterization of ligand binding to melanocortin 4 receptors using fluorescent peptides with improved kinetic properties. Eur J Pharmacol 2017; 799:58-66. [DOI: 10.1016/j.ejphar.2017.01.040] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/15/2016] [Revised: 01/25/2017] [Accepted: 01/25/2017] [Indexed: 12/24/2022]
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14
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Dynamics of ligand binding to GPCR: Residence time of melanocortins and its modulation. Pharmacol Res 2016; 113:747-753. [DOI: 10.1016/j.phrs.2016.05.030] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 04/18/2016] [Revised: 05/22/2016] [Accepted: 05/27/2016] [Indexed: 01/06/2023]
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15
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Stoddart LA, White CW, Nguyen K, Hill SJ, Pfleger KDG. Fluorescence- and bioluminescence-based approaches to study GPCR ligand binding. Br J Pharmacol 2016; 173:3028-37. [PMID: 26317175 PMCID: PMC5125978 DOI: 10.1111/bph.13316] [Citation(s) in RCA: 86] [Impact Index Per Article: 10.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/04/2015] [Revised: 08/01/2015] [Accepted: 08/20/2015] [Indexed: 01/15/2023] Open
Abstract
Ligand binding is a vital component of any pharmacologist's toolbox and allows the detailed investigation of how a molecule binds to its receptor. These studies enable the experimental determination of binding affinity of labelled and unlabelled compounds through kinetic, saturation (Kd ) and competition (Ki ) binding assays. Traditionally, these studies have used molecules labelled with radioisotopes; however, more recently, fluorescent ligands have been developed for this purpose. This review will briefly cover receptor ligand binding theory and then discuss the use of fluorescent ligands with some of the different technologies currently employed to examine ligand binding. Fluorescent ligands can be used for direct measurement of receptor-associated fluorescence using confocal microscopy and flow cytometry as well as in assays such as fluorescence polarization, where ligand binding is monitored by changes in the free rotation when a fluorescent ligand is bound to a receptor. Additionally, fluorescent ligands can act as donors or acceptors for fluorescence resonance energy transfer (FRET) with the development of assays based on FRET and time-resolved FRET (TR-FRET). Finally, we have recently developed a novel bioluminescence resonance energy transfer (BRET) ligand binding assay utilizing a small (19 kDa), super-bright luciferase subunit (NanoLuc) from a deep sea shrimp. In combination with fluorescent ligands, measurement of RET now provides an array of methodologies to study ligand binding. While each method has its own advantages and drawbacks, binding studies using fluorescent ligands are now a viable alternative to the use of radioligands. Linked Articles This article is part of a themed section on Molecular Pharmacology of G Protein-Coupled Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v173.20/issuetoc.
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Affiliation(s)
- Leigh A Stoddart
- Cell Signalling Research Group, School of Life Sciences, University of Nottingham, Nottingham, UK
| | - Carl W White
- Molecular Endocrinology and Pharmacology, Harry Perkins Institute of Medical Research, Nedlands, WA, Australia
- Centre for Medical Research, The University of Western Australia, Crawley, WA, Australia
| | - Kim Nguyen
- Molecular Endocrinology and Pharmacology, Harry Perkins Institute of Medical Research, Nedlands, WA, Australia
- Centre for Medical Research, The University of Western Australia, Crawley, WA, Australia
| | - Stephen J Hill
- Cell Signalling Research Group, School of Life Sciences, University of Nottingham, Nottingham, UK
- Molecular Endocrinology and Pharmacology, Harry Perkins Institute of Medical Research, Nedlands, WA, Australia
- Centre for Medical Research, The University of Western Australia, Crawley, WA, Australia
| | - Kevin D G Pfleger
- Molecular Endocrinology and Pharmacology, Harry Perkins Institute of Medical Research, Nedlands, WA, Australia.
- Centre for Medical Research, The University of Western Australia, Crawley, WA, Australia.
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16
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Martin JN, Ball LM, Solomon TL, Dewald AH, Criss AK, Columbus L. Neisserial Opa Protein-CEACAM Interactions: Competition for Receptors as a Means of Bacterial Invasion and Pathogenesis. Biochemistry 2016; 55:4286-94. [PMID: 27442026 DOI: 10.1021/acs.biochem.6b00124] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
Carcino-embryonic antigen-like cellular adhesion molecules (CEACAMs), members of the immunoglobulin superfamily, are responsible for cell-cell interactions and cellular signaling events. Extracellular interactions with CEACAMs have the potential to induce phagocytosis, as is the case with pathogenic Neisseria bacteria. Pathogenic Neisseria species express opacity-associated (Opa) proteins, which interact with a subset of CEACAMs on human cells, and initiate the engulfment of the bacterium. We demonstrate that recombinant Opa proteins reconstituted into liposomes retain the ability to recognize and interact with CEACAMs in vitro but do not maintain receptor specificity compared to that of Opa proteins natively expressed by Neisseria gonorrhoeae. We report that two Opa proteins interact with CEACAMs with nanomolar affinity, and we hypothesize that this high affinity is necessary to compete with the native CEACAM homo- and heterotypic interactions in the host. Understanding the mechanisms of Opa protein-receptor recognition and engulfment enhances our understanding of Neisserial pathogenesis. Additionally, these mechanisms provide insight into how human cells that are typically nonphagocytic can utilize CEACAM receptors to internalize exogenous matter, with implications for the targeted delivery of therapeutics and development of imaging agents.
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Affiliation(s)
- Jennifer N Martin
- Department of Chemistry and ‡Department of Microbiology, Immunology, and Cancer Biology, University of Virginia , Charlottesville, Virginia 22903, United States
| | - Louise M Ball
- Department of Chemistry and ‡Department of Microbiology, Immunology, and Cancer Biology, University of Virginia , Charlottesville, Virginia 22903, United States
| | - Tsega L Solomon
- Department of Chemistry and ‡Department of Microbiology, Immunology, and Cancer Biology, University of Virginia , Charlottesville, Virginia 22903, United States
| | - Alison H Dewald
- Department of Chemistry and ‡Department of Microbiology, Immunology, and Cancer Biology, University of Virginia , Charlottesville, Virginia 22903, United States
| | - Alison K Criss
- Department of Chemistry and ‡Department of Microbiology, Immunology, and Cancer Biology, University of Virginia , Charlottesville, Virginia 22903, United States
| | - Linda Columbus
- Department of Chemistry and ‡Department of Microbiology, Immunology, and Cancer Biology, University of Virginia , Charlottesville, Virginia 22903, United States
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17
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Link R, Kopanchuk S, Rinken A. Novel peptides for kinetic studies of ligand binding to melanocortin-4 receptors using fluorescence anisotropy. SPRINGERPLUS 2015. [PMCID: PMC4796485 DOI: 10.1186/2193-1801-4-s1-p23] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Download PDF] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
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Hoffmann C, Castro M, Rinken A, Leurs R, Hill SJ, Vischer HF. Ligand Residence Time at G-protein–Coupled Receptors—Why We Should Take Our Time To Study It. Mol Pharmacol 2015; 88:552-60. [DOI: 10.1124/mol.115.099671] [Citation(s) in RCA: 62] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/28/2015] [Accepted: 07/07/2015] [Indexed: 12/15/2022] Open
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Veiksina S, Kopanchuk S, Rinken A. Allosteric modulation of peptide ligand binding to Neuropeptide Y receptor Y1 revealed by fluorescence-based assay. SPRINGERPLUS 2015. [PMCID: PMC4796559 DOI: 10.1186/2193-1801-4-s1-p52] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Download PDF] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
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20
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Veiksina S, Kopanchuk S, Mazina O, Link R, Lille A, Rinken A. Homogeneous fluorescence anisotropy-based assay for characterization of ligand binding dynamics to GPCRs in budded baculoviruses: the case of Cy3B-NDP-α-MSH binding to MC4 receptors. Methods Mol Biol 2015; 1272:37-50. [PMID: 25563175 DOI: 10.1007/978-1-4939-2336-6_3] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/04/2023]
Abstract
Despite the availability of numerous conceptually different approaches for the characterization of ligand-receptor interactions, there remains a great requirement for complementary methods that are suitable for kinetic studies, especially for the characterization of membrane protein systems and G protein-coupled receptors (GPCRs) in particular. One of the potential approaches that inherently fits well for this purpose is fluorescence anisotropy (FA), a method that allows continuous monitoring of ligand binding processes and characterization of ligand binding dynamics. However, significant changes in FA signal of fluorescently labeled ligands can be detected only if the ratio of bound to free fluorescent ligand portions is altered, which means that receptor and ligand concentrations have to be comparable. As most of the GPCRs are normally present at relatively low concentrations in native tissues and conventional receptor preparations from overexpressed systems often generate high background levels due to significant autofluorescence, receptor preparations with sufficiently high receptor concentrations have become a critical requirement for successful FA assay performance. We propose that budded baculoviruses that display GPCRs on their surfaces can be used as a receptor source in FA assays. Here, we describe the experimental setup of this homogeneous budded baculovirus/FA-based assay system for investigation of receptor-ligand interactions and a novel strategy for FA kinetic data analysis that is taking into account the effect of nonspecific interactions and the depletion of the fluorescent ligand during the binding reaction. The developed budded baculovirus/FA-based assay system brings the experimental data to a level that could solve complex models of ligand-receptor interactions and become a valuable tool for the screening of pharmacologically active compounds. Melanocortin 4 (MC4) receptors and the fluorescent ligand Cy3B-NDP-α-MSH were used as the model system.
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Affiliation(s)
- Santa Veiksina
- Institute of Chemistry, University of Tartu, Ravila 14a, 50411, Tartu, Estonia
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Liu J, Yu J, Chen J, Yang R, Shih K. Signal-amplification and real-time fluorescence anisotropy detection of apyrase by carbon nanoparticle. MATERIALS SCIENCE & ENGINEERING. C, MATERIALS FOR BIOLOGICAL APPLICATIONS 2014; 38:206-11. [PMID: 24656370 DOI: 10.1016/j.msec.2014.02.001] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Subscribe] [Scholar Register] [Received: 07/17/2013] [Revised: 01/23/2014] [Accepted: 02/04/2014] [Indexed: 11/19/2022]
Abstract
Carbon nanomaterial combined with aptamer has been developed as an efficient bioanalytical method in sensor design. Herein, depending on carbon nanoparticle (cCNP)-enhanced fluorescence anisotropy (FA), a novel aptamer-based sensor (aptasensor) enabling signal-amplification and real-time detection of apyrase is reported. The foundation of our sensor design based on ATP-aptamer(P) can be adsorbed on the surface of cCNPs, resulting in the increase of FA due to the mass of cCNPs, and P-ATP complex has weak binding ability to cCNPs with minimal change of FA. Apyrase, being an integral membrane protein, can hydrolyze ATP and make P-ATP complex disassemble, and thus lead to the increasing of FA. Therefore, this approach is demonstrated to be a novel candidate for the detection of apyrase, with high sensitivity and selectivity. The linear dynamic range for the concentrations of apyrase is between 0.1 and 0.5 U/μL along with a detection limit of 0.05 U/μL. Furthermore, these results indicated that our design is a flexible and sensitive method for biomolecule analysis, which makes it promising for practical biomolecule analyses.
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Affiliation(s)
- Jinhua Liu
- College of Chemistry and Life Science, Zhejiang Normal University, Jinhua 321004, People's Republic of China; College of Geography and Environmental Science, Zhejiang Normal University, Jinhua 321004, People's Republic of China; Department of Civil Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, Hong Kong SAR, People's Republic of China.
| | - Jing Yu
- College of Chemistry and Life Science, Zhejiang Normal University, Jinhua 321004, People's Republic of China; College of Geography and Environmental Science, Zhejiang Normal University, Jinhua 321004, People's Republic of China
| | - Jianrong Chen
- College of Chemistry and Life Science, Zhejiang Normal University, Jinhua 321004, People's Republic of China; College of Geography and Environmental Science, Zhejiang Normal University, Jinhua 321004, People's Republic of China.
| | - Ronghua Yang
- State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University, Changsha 410082, People's Republic of China
| | - Kaimin Shih
- Department of Civil Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, Hong Kong SAR, People's Republic of China.
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Tõntson L, Kopanchuk S, Rinken A. Characterization of 5-HT₁A receptors and their complexes with G-proteins in budded baculovirus particles using fluorescence anisotropy of Bodipy-FL-NAN-190. Neurochem Int 2014; 67:32-8. [PMID: 24508405 DOI: 10.1016/j.neuint.2014.01.012] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/18/2013] [Revised: 01/16/2014] [Accepted: 01/23/2014] [Indexed: 12/25/2022]
Abstract
Bodipy-FL-NAN-190 was found to be well suited for characterization of ligand binding to 5-HT1A receptors expressed in budded baculovirus particles, as binding is accompanied by large increases in fluorescence intensity and anisotropy. This ligand appears to bind rapidly (t1/2,ass<1 min), reversibly (t1/2,diss∼6 min) and has high affinity (Kd=0.30 ± 0.13 nM). This fluorescence anisotropy assay based on Bodipy-FL-NAN-190 binding to baculovirus particles was also a suitable assay system for the pharmacological characterization of non-labelled serotonergic ligands, as well as being sensitive to the presence of G-proteins and guanine nucleotides. Coexpression of αi subunits of human G-proteins in baculovirus particles resulted in the appearance of significantly greater proportion of nucleotide sensitive high affinity agonist binding sites. There were no significant differences between αi1 and αi3 subtypes, while ligand binding in the presence of αi2 had higher sensitivity to GDP and Mn(2+).
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Affiliation(s)
- Lauri Tõntson
- University of Tartu, Institute of Chemistry, Ravila 14a, 50411 Tartu, Estonia
| | - Sergei Kopanchuk
- University of Tartu, Institute of Chemistry, Ravila 14a, 50411 Tartu, Estonia; Competence Centre on Reproductive Medicine & Biology, Tiigi 61b, 50410 Tartu, Estonia
| | - Ago Rinken
- University of Tartu, Institute of Chemistry, Ravila 14a, 50411 Tartu, Estonia; Competence Centre on Reproductive Medicine & Biology, Tiigi 61b, 50410 Tartu, Estonia.
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Veiksina S, Kopanchuk S, Rinken A. Budded baculoviruses as a tool for a homogeneous fluorescence anisotropy-based assay of ligand binding to G protein-coupled receptors: The case of melanocortin 4 receptors. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2014; 1838:372-81. [DOI: 10.1016/j.bbamem.2013.09.015] [Citation(s) in RCA: 32] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/19/2013] [Revised: 09/19/2013] [Accepted: 09/24/2013] [Indexed: 12/01/2022]
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Tõntson L, Kopanchuk S, Rinken A. Biarsenical ligands bind to endogenous G-protein α-subunits and enable allosteric sensing of nucleotide binding. BMC BIOCHEMISTRY 2013; 14:37. [PMID: 24344803 PMCID: PMC3878488 DOI: 10.1186/1471-2091-14-37] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 06/14/2013] [Accepted: 12/13/2013] [Indexed: 11/13/2022]
Abstract
BACKGROUND Heterotrimeric G-proteins relay extracellular signals to intracellular effector proteins. Multiple methods have been developed to monitor their activity; including labeled nucleotides and biosensors based on genetically engineered G-proteins. Here we describe a method for monitoring unlabeled nucleotide binding to endogenous G-proteins α-subunits in a homogeneous assay based on the interaction of 4',5'-bis(1,2,3-dithioarsolan-2-yl)-2',7'-difluorofluorescein (F2FlAsH) with G-protein α-subunits. RESULTS The biarsenic fluorescent ligand F2FlAsH binds to various wild-type G-protein α-subunits (αi1, αi2, αi3, αslong, αsshort, αolf, αq, α13) via high affinity As-cysteine interactions. This allosteric label enables real time monitoring of the nucleotide bound states of α-subunits via changes in fluorescence anisotropy and intensity of their F2FlAsH-complexes. We have found that different α-subunits displayed different signal amplitudes when interacting with F2FlAsH, being more sensitive to nucleotide binding to αi, αs, αolf and αq than to α13. Addition of nucleotides to F2FlAsH-labeled α-subunits caused concentration-dependent effects on their fluorescence anisotropy. pEC50 values of studied nucleotides depended on the subtype of the α-subunit and were from 5.7 to 8.2 for GTPγS, from 5.4 to 8.1 for GppNHp and from 4.8 to 8.2 for GDP and lastly up to 5.9 for GMP. While GDP and GMP increased the fluorescence anisotropy of F2FlAsH complexes with αi-subunits, they had the opposite effect on the other αβγM complexes studied. CONCLUSIONS Biarsenical ligands interact allosterically with endogenous G-protein α-subunits in a nucleotide-sensitive manner, so the presence or absence of guanine nucleotides has an effect on the fluorescence anisotropy, intensity and lifetime of F2FlAsH-G-protein complexes.
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Affiliation(s)
- Lauri Tõntson
- University of Tartu, Institute of Chemistry, Ravila 14a, 50411, Tartu, Estonia
| | - Sergei Kopanchuk
- University of Tartu, Institute of Chemistry, Ravila 14a, 50411, Tartu, Estonia
- Competence Centre on Reproductive Medicine & Biology, Tiigi 61b, 50410, Tartu, Estonia
| | - Ago Rinken
- University of Tartu, Institute of Chemistry, Ravila 14a, 50411, Tartu, Estonia
- Competence Centre on Reproductive Medicine & Biology, Tiigi 61b, 50410, Tartu, Estonia
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Application of Baculovirus Technology for Studies of G Protein-Coupled Receptor Signaling. ACTA ACUST UNITED AC 2013. [DOI: 10.1007/978-1-4614-7675-7_23] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/20/2023]
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Yu Y, Liu Y, Zhen SJ, Huang CZ. A graphene oxide enhanced fluorescence anisotropy strategy for DNAzyme-based assay of metal ions. Chem Commun (Camb) 2013; 49:1942-4. [DOI: 10.1039/c3cc38129c] [Citation(s) in RCA: 77] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
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Leach BI, Kuntimaddi A, Schmidt CR, Cierpicki T, Johnson SA, Bushweller JH. Leukemia fusion target AF9 is an intrinsically disordered transcriptional regulator that recruits multiple partners via coupled folding and binding. Structure 2012; 21:176-183. [PMID: 23260655 DOI: 10.1016/j.str.2012.11.011] [Citation(s) in RCA: 79] [Impact Index Per Article: 6.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/12/2012] [Revised: 10/19/2012] [Accepted: 11/13/2012] [Indexed: 01/31/2023]
Abstract
Mixed lineage leukemia (MLL) fusion proteins cause oncogenic transformation of hematopoietic cells by constitutive recruitment of elongation factors to HOX promoters, resulting in overexpression of target genes. The structural basis of transactivation by MLL fusion partners remains undetermined. We show that the ANC1 homology domain (AHD) of AF9, one of the most common MLL translocation partners, is intrinsically disordered and recruits multiple transcription factors through coupled folding and binding. We determined the structure of the AF9 AHD in complex with the elongation factor AF4 and show that aliphatic residues, which are conserved in each of the AF9 binding partners, form an integral part of the hydrophobic core of the complex. Nuclear magnetic resonance relaxation measurements show that AF9 retains significant dynamic behavior which may facilitate exchange between disordered partners. We propose that AF9 functions as a signaling hub that regulates transcription through dynamic recruitment of cofactors in normal hematopoiesis and in acute leukemia.
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Affiliation(s)
- Benjamin I Leach
- Department of Molecular Physiology and Biological Physics, University of Virginia, Charlottesville, VA 22908, USA
| | - Aravinda Kuntimaddi
- Department of Molecular Physiology and Biological Physics, University of Virginia, Charlottesville, VA 22908, USA
| | - Charles R Schmidt
- Department of Molecular Physiology and Biological Physics, University of Virginia, Charlottesville, VA 22908, USA
| | - Tomasz Cierpicki
- Department of Molecular Physiology and Biological Physics, University of Virginia, Charlottesville, VA 22908, USA
| | - Stephanie A Johnson
- Department of Chemistry, University of Virginia, Charlottesville, VA 22904, USA
| | - John H Bushweller
- Department of Molecular Physiology and Biological Physics, University of Virginia, Charlottesville, VA 22908, USA; Department of Chemistry, University of Virginia, Charlottesville, VA 22904, USA.
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Pallicer JM, Krämer SD. Evaluation of fluorescence anisotropy to assess drug–lipid membrane partitioning. J Pharm Biomed Anal 2012; 71:219-27. [DOI: 10.1016/j.jpba.2012.08.009] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/26/2012] [Revised: 08/05/2012] [Accepted: 08/08/2012] [Indexed: 01/17/2023]
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Mazina O, Reinart-Okugbeni R, Kopanchuk S, Rinken A. BacMam system for FRET-based cAMP sensor expression in studies of melanocortin MC1 receptor activation. ACTA ACUST UNITED AC 2012; 17:1096-101. [PMID: 22674933 DOI: 10.1177/1087057112449862] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
Cyclic adenosine monophosphate (cAMP) is a second messenger of many G-protein-coupled receptors (GPCRs) and a useful readout molecule to estimate the biological activity of various GPCR-specific agents. Here we report the development and use of a Förster resonance energy transfer (FRET) biosensor for cAMP (Epac2-camps) combined with a baculovirus-based BacMam transduction system. The constructed BacMam-Epac2-camps viral transduction system is a simple and robust tool for ligand screening at the second-messenger level in a variety of mammalian cell lines. The level of biosensor protein expression can easily be adjusted in a dose-dependent manner depending on the multiplicity of viral infection. For setting up the assay, we used a B16F10 murine melanoma cell line with endogenous expression of melanocortin-1 receptor (MC(1)R). The receptor activation was characterized by a set of MC(1)R full and partial agonists. Bivalent ions Ca(2+) as well as Mg(2+) modulated ligand potencies, whereas the effect was ligand and ion specific. Results obtained for MC(1)R indicate that the BacMam-Epac2-camps system may also be applicable for studying the activation of other GPCRs and may be implemented in routine analysis as well as in high-throughput screening.
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Affiliation(s)
- Olga Mazina
- University of Tartu, Institute of Chemistry, Tartu, Estonia
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Tõntson L, Babina A, Võsumaa T, Kopanchuk S, Rinken A. Characterization of heterotrimeric nucleotide-depleted Gα(i)-proteins by Bodipy-FL-GTPγS fluorescence anisotropy. Arch Biochem Biophys 2012; 524:93-8. [PMID: 22659491 DOI: 10.1016/j.abb.2012.05.017] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/12/2012] [Revised: 05/18/2012] [Accepted: 05/21/2012] [Indexed: 10/28/2022]
Abstract
Recombinant heterotrimeric G-protein α(i1), α(i2) and α(i3) subunits were purified in GDP-depleting conditions by affinity chromatography using StrepII-tagged β₁γ₂ subunits. Real-time monitoring of fluorescence anisotropy of Bodipy-FL-GTPγS was used for characterization of nucleotide binding properties and inactivation of the purified proteins. All GDP-depleted α(i) were unstable at room temperature and therefore nucleotide binding could be characterized only in a nonequilibrium state. In comparison to Mg²⁺, Mn²⁺ inhibited nucleotide binding to all α(i)-heterotrimers studied and accelerated nucleotide release. Mn²⁺ had stabilizing effect on the nucleotide free state of the α(i1) subunit, whereas both Mn²⁺ as well as G-protein activation by mastoparan destabilized the α(i2) subunit.
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Affiliation(s)
- Lauri Tõntson
- University of Tartu, Institute of Chemistry, Ravila 14a, 50411 Tartu, Estonia
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