1
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Wolf P, Gavins G, Beck‐Sickinger AG, Seitz O. Strategies for Site-Specific Labeling of Receptor Proteins on the Surfaces of Living Cells by Using Genetically Encoded Peptide Tags. Chembiochem 2021; 22:1717-1732. [PMID: 33428317 PMCID: PMC8248378 DOI: 10.1002/cbic.202000797] [Citation(s) in RCA: 12] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/27/2020] [Revised: 01/08/2021] [Indexed: 12/14/2022]
Abstract
Fluorescence microscopy imaging enables receptor proteins to be investigated within their biological context. A key challenge is to site-specifically incorporate reporter moieties into proteins without interfering with biological functions or cellular networks. Small peptide tags offer the opportunity to combine inducible labeling with small tag sizes that avoid receptor perturbation. Herein, we review the current state of live-cell labeling of peptide-tagged cell-surface proteins. Considering their importance as targets in medicinal chemistry, we focus on membrane receptors such as G protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs). We discuss peptide tags that i) are subject to enzyme-mediated modification reactions, ii) guide the complementation of reporter proteins, iii) form coiled-coil complexes, and iv) interact with metal complexes. Given our own contributions in the field, we place emphasis on peptide-templated labeling chemistry.
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Affiliation(s)
- Philipp Wolf
- Faculty of Life SciencesInstitute of BiochemistryLeipzig UniversityBrüderstrasse 3404103LeipzigGermany
| | - Georgina Gavins
- Faculty of Mathematics and Natural SciencesDepartment of ChemistryHumboldt-Universität zu BerlinBrook-Taylor-Str. 212489BerlinGermany
| | - Annette G. Beck‐Sickinger
- Faculty of Life SciencesInstitute of BiochemistryLeipzig UniversityBrüderstrasse 3404103LeipzigGermany
| | - Oliver Seitz
- Faculty of Mathematics and Natural SciencesDepartment of ChemistryHumboldt-Universität zu BerlinBrook-Taylor-Str. 212489BerlinGermany
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2
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Ward RJ, Pediani JD, Marsango S, Jolly R, Stoneman MR, Biener G, Handel TM, Raicu V, Milligan G. Chemokine receptor CXCR4 oligomerization is disrupted selectively by the antagonist ligand IT1t. J Biol Chem 2021; 296:100139. [PMID: 33268380 PMCID: PMC7949023 DOI: 10.1074/jbc.ra120.016612] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/29/2020] [Revised: 11/29/2020] [Accepted: 12/01/2020] [Indexed: 12/15/2022] Open
Abstract
CXCR4, a member of the family of chemokine-activated G protein-coupled receptors, is widely expressed in immune response cells. It is involved in both cancer development and progression as well as viral infection, notably by HIV-1. A variety of methods, including structural information, have suggested that the receptor may exist as a dimer or an oligomer. However, the mechanistic details surrounding receptor oligomerization and its potential dynamic regulation remain unclear. Using both biochemical and biophysical means, we confirm that CXCR4 can exist as a mixture of monomers, dimers, and higher-order oligomers in cell membranes and show that oligomeric structure becomes more complex as receptor expression levels increase. Mutations of CXCR4 residues located at a putative dimerization interface result in monomerization of the receptor. Additionally, binding of the CXCR4 antagonist IT1t-a small drug-like isothiourea derivative-rapidly destabilizes the oligomeric structure, whereas AMD3100, another well-characterized CXCR4 antagonist, does not. Although a mutation that regulates constitutive activity of CXCR4 also results in monomerization of the receptor, binding of IT1t to this variant promotes receptor dimerization. These results provide novel insights into the basal organization of CXCR4 and how antagonist ligands of different chemotypes differentially regulate its oligomerization state.
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Affiliation(s)
- Richard J Ward
- Centre for Translational Pharmacology, Institute of Molecular, Cell and Systems Biology, College of Medical, Veterinary and Life Sciences, University of Glasgow, Glasgow, Scotland, United Kingdom
| | - John D Pediani
- Centre for Translational Pharmacology, Institute of Molecular, Cell and Systems Biology, College of Medical, Veterinary and Life Sciences, University of Glasgow, Glasgow, Scotland, United Kingdom
| | - Sara Marsango
- Centre for Translational Pharmacology, Institute of Molecular, Cell and Systems Biology, College of Medical, Veterinary and Life Sciences, University of Glasgow, Glasgow, Scotland, United Kingdom
| | - Richard Jolly
- Centre for Translational Pharmacology, Institute of Molecular, Cell and Systems Biology, College of Medical, Veterinary and Life Sciences, University of Glasgow, Glasgow, Scotland, United Kingdom
| | - Michael R Stoneman
- Physics Department, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin, USA
| | - Gabriel Biener
- Physics Department, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin, USA
| | - Tracy M Handel
- Skaggs School of Pharmacy and Pharmaceutical Sciences, University of California, San Diego, La Jolla, California, USA
| | - Valerică Raicu
- Physics Department, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin, USA
| | - Graeme Milligan
- Centre for Translational Pharmacology, Institute of Molecular, Cell and Systems Biology, College of Medical, Veterinary and Life Sciences, University of Glasgow, Glasgow, Scotland, United Kingdom.
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3
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DeNies MS, Rosselli-Murai LK, Schnell S, Liu AP. Clathrin Heavy Chain Knockdown Impacts CXCR4 Signaling and Post-translational Modification. Front Cell Dev Biol 2019; 7:77. [PMID: 31139626 PMCID: PMC6518350 DOI: 10.3389/fcell.2019.00077] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/15/2018] [Accepted: 04/23/2019] [Indexed: 01/22/2023] Open
Abstract
Recent research has implicated endocytic pathways as important regulators of receptor signaling. However, the role of endocytosis in regulating chemokine CXC receptor 4 (CXCR4) signaling remains largely unknown. In the present work we systematically investigate the impact of clathrin knockdown on CXCR4 internalization, signaling, and receptor post-translational modification. Inhibition of clathrin-mediated endocytosis (CME) significantly reduced CXCR4 internalization. In contrast to other receptors, clathrin knockdown increased CXCL12-dependent ERK1/2 signaling. Simultaneous inhibition of CME and lipid raft disruption abrogated this increase in ERK1/2 phosphorylation suggesting that endocytic pathway compensation can influence signaling outcomes. Interestingly, using an antibody sensitive to CXCR4 post-translational modification, we also found that our ability to detect CXCR4 was drastically reduced upon clathrin knockdown. We hypothesize that this effect was due to differences in receptor post-translational modification as total CXCR4 protein and mRNA levels were unchanged. Lastly, we show that clathrin knockdown reduced CXCL12-dependent cell migration irrespective of an observed increase in ERK1/2 phosphorylation. Altogether, this work supports a complex model by which modulation of endocytosis affects not only receptor signaling and internalization but also receptor post-translational modification.
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Affiliation(s)
- Maxwell S DeNies
- Cellular and Molecular Biology Graduate Program, University of Michigan Medical School, Ann Arbor, MI, United States
| | | | - Santiago Schnell
- Cellular and Molecular Biology Graduate Program, University of Michigan Medical School, Ann Arbor, MI, United States.,Department of Molecular and Integrative Physiology, University of Michigan Medical School, Ann Arbor, MI, United States.,Department of Computational Medicine and Bioinformatics, University of Michigan Medical School, Ann Arbor, MI, United States
| | - Allen P Liu
- Cellular and Molecular Biology Graduate Program, University of Michigan Medical School, Ann Arbor, MI, United States.,Department of Mechanical Engineering, University of Michigan, Ann Arbor, MI, United States.,Department of Biomedical Engineering, University of Michigan, Ann Arbor, MI, United States.,Department of Biophysics, University of Michigan, Ann Arbor, MI, United States
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4
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Gahbauer S, Pluhackova K, Böckmann RA. Closely related, yet unique: Distinct homo- and heterodimerization patterns of G protein coupled chemokine receptors and their fine-tuning by cholesterol. PLoS Comput Biol 2018; 14:e1006062. [PMID: 29529028 PMCID: PMC5864085 DOI: 10.1371/journal.pcbi.1006062] [Citation(s) in RCA: 29] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/07/2017] [Revised: 03/22/2018] [Accepted: 02/28/2018] [Indexed: 12/21/2022] Open
Abstract
Chemokine receptors, a subclass of G protein coupled receptors (GPCRs), play essential roles in the human immune system, they are involved in cancer metastasis as well as in HIV-infection. A plethora of studies show that homo- and heterodimers or even higher order oligomers of the chemokine receptors CXCR4, CCR5, and CCR2 modulate receptor function. In addition, membrane cholesterol affects chemokine receptor activity. However, structural information about homo- and heterodimers formed by chemokine receptors and their interplay with cholesterol is limited. Here, we report homo- and heterodimer configurations of the chemokine receptors CXCR4, CCR5, and CCR2 at atomistic detail, as obtained from thousands of molecular dynamics simulations. The observed homodimerization patterns were similar for the closely related CC chemokine receptors, yet they differed significantly between the CC receptors and CXCR4. Despite their high sequence identity, cholesterol modulated the CC homodimer interfaces in a subtype-specific manner. Chemokine receptor heterodimers display distinct dimerization patterns for CXCR4/CCR5 and CXCR4/CCR2. Furthermore, associations between CXCR4 and CCR5 reveal an increased cholesterol-sensitivity as compared to CXCR4/CCR2 heterodimerization patterns. This work provides a first comprehensive structural overview over the complex interaction network between chemokine receptors and indicates how heterodimerization and the interaction with the membrane environment diversifies the function of closely related GPCRs.
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MESH Headings
- Animals
- Chemokines/metabolism
- Cholesterol/metabolism
- Computer Simulation
- Dimerization
- Humans
- Molecular Dynamics Simulation
- Receptors, CCR2/chemistry
- Receptors, CCR2/metabolism
- Receptors, CCR2/ultrastructure
- Receptors, CCR5/chemistry
- Receptors, CCR5/metabolism
- Receptors, CCR5/ultrastructure
- Receptors, CXCR4/chemistry
- Receptors, CXCR4/metabolism
- Receptors, CXCR4/ultrastructure
- Receptors, Chemokine/chemistry
- Receptors, Chemokine/genetics
- Receptors, G-Protein-Coupled/genetics
- Signal Transduction
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Affiliation(s)
- Stefan Gahbauer
- Computational Biology, Department of Biology, Friedrich-Alexander University Erlangen-Nürnberg, Erlangen, Germany
| | - Kristyna Pluhackova
- Computational Biology, Department of Biology, Friedrich-Alexander University Erlangen-Nürnberg, Erlangen, Germany
| | - Rainer A. Böckmann
- Computational Biology, Department of Biology, Friedrich-Alexander University Erlangen-Nürnberg, Erlangen, Germany
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5
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Liu AP, Botelho RJ, Antonescu CN. The big and intricate dreams of little organelles: Embracing complexity in the study of membrane traffic. Traffic 2017; 18:567-579. [DOI: 10.1111/tra.12497] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/30/2017] [Revised: 05/30/2017] [Accepted: 05/30/2017] [Indexed: 12/16/2022]
Affiliation(s)
- Allen P. Liu
- Department of Mechanical Engineering University of Michigan Ann Arbor Michigan
- Department of Biomedical Engineering University of Michigan Ann Arbor Michigan
- Cellular and Molecular Biology Program University of Michigan Ann Arbor Michigan
- Biophysics Program University of Michigan Ann Arbor Michigan
| | - Roberto J. Botelho
- The Graduate Program in Molecular Science and Department of Chemistry and Biology Ryerson University Toronto Canada
| | - Costin N. Antonescu
- The Graduate Program in Molecular Science and Department of Chemistry and Biology Ryerson University Toronto Canada
- Keenan Research Centre for Biomedical Science St. Michael's Hospital Toronto Canada
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6
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Evidence for the heterotetrameric structure of the adenosine A2A-dopamine D2 receptor complex. Biochem Soc Trans 2016; 44:595-600. [PMID: 27068975 DOI: 10.1042/bst20150276] [Citation(s) in RCA: 26] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/04/2016] [Indexed: 11/17/2022]
Abstract
Heteromers of G-protein-coupled receptors (GPCRs) have emerged as potential novel targets for drug development. Accumulating evidence indicates that GPCRs can form homodimers and heteromers, with homodimers being the predominant species and oligomeric receptors being formed as multiples of dimers. Recently, heterotetrameric structures have been proposed for dopamine D1receptor (D1R)-dopamine D3receptor (D3R) and adenosine A2Areceptor (A2AR)-dopamine D2receptor (D2R) heteromers. The structural model proposed for these complexes is a heteromer constituted by two receptor homodimers. The existence of GPCR homodimers and heteromers provides a structural basis for inter-protomer allosteric mechanisms that might account for a multiplicity of unique pharmacological properties. In this review, we focus on the A2AR-D2R heterotetramer as an example of an oligomeric structure that is key in the modulation of striatal neuronal function. We also review the interfaces involved in this and other recently reported heteromers of GPCRs. Furthermore, we discuss several published studies showing theex vivoexpression of A2AR-D2R heteromers. The ability of A2AR agonists to decrease the affinity of D2R agonists has been reported and, on the basis of this interaction, A2AR antagonists have been proposed as potential drugs for the treatment of Parkinson's disease. The heterotetrameric structure of the A2AR-D2R complex offers a novel model that can provide new clues about how to adjust the drug dosage to the expected levels of endogenous adenosine.
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7
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Tian H, Fürstenberg A, Huber T. Labeling and Single-Molecule Methods To Monitor G Protein-Coupled Receptor Dynamics. Chem Rev 2016; 117:186-245. [DOI: 10.1021/acs.chemrev.6b00084] [Citation(s) in RCA: 84] [Impact Index Per Article: 10.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/14/2023]
Affiliation(s)
- He Tian
- Laboratory of Chemical Biology
and Signal Transduction, The Rockefeller University, 1230 York
Avenue, New York, New York 10065, United States
| | - Alexandre Fürstenberg
- Laboratory of Chemical Biology
and Signal Transduction, The Rockefeller University, 1230 York
Avenue, New York, New York 10065, United States
| | - Thomas Huber
- Laboratory of Chemical Biology
and Signal Transduction, The Rockefeller University, 1230 York
Avenue, New York, New York 10065, United States
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8
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Liebick M, Schläger C, Oppermann M. Analysis of Chemokine Receptor Trafficking by Site-Specific Biotinylation. PLoS One 2016; 11:e0157502. [PMID: 27310579 PMCID: PMC4911081 DOI: 10.1371/journal.pone.0157502] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/04/2016] [Accepted: 05/31/2016] [Indexed: 12/17/2022] Open
Abstract
Chemokine receptors undergo internalization and desensitization in response to ligand activation. Internalized receptors are either preferentially directed towards recycling pathways (e.g. CCR5) or sorted for proteasomal degradation (e.g. CXCR4). Here we describe a method for the analysis of receptor internalization and recycling based on specific Bir A-mediated biotinylation of an acceptor peptide coupled to the receptor, which allows a more detailed analysis of receptor trafficking compared to classical antibody-based detection methods. Studies on constitutive internalization of the chemokine receptors CXCR4 (12.1% ± 0.99% receptor internalization/h) and CCR5 (13.7% ± 0.68%/h) reveals modulation of these processes by inverse (TAK779; 10.9% ± 0.95%/h) or partial agonists (Met-CCL5; 15.6% ± 0.5%/h). These results suggest an actively driven internalization process. We also demonstrate the advantages of specific biotinylation compared to classical antibody detection during agonist-induced receptor internalization, which may be used for immunofluorescence analysis as well. Site-specific biotinylation may be applicable to studies on trafficking of transmembrane proteins, in general.
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MESH Headings
- Amides/pharmacology
- Animals
- Antibodies, Monoclonal/biosynthesis
- Antibodies, Monoclonal/chemistry
- Antibodies, Monoclonal/isolation & purification
- Basophils/cytology
- Basophils/drug effects
- Basophils/metabolism
- Biotin/chemistry
- Biotin/metabolism
- Biotinylation
- CCR5 Receptor Antagonists/pharmacology
- Carbon-Nitrogen Ligases/genetics
- Carbon-Nitrogen Ligases/metabolism
- Cell Line, Tumor
- Chemokine CCL5/pharmacology
- Escherichia coli Proteins/genetics
- Escherichia coli Proteins/metabolism
- Gene Expression
- Genetic Vectors/chemistry
- Genetic Vectors/metabolism
- Mice
- Protein Transport/drug effects
- Quaternary Ammonium Compounds/pharmacology
- Rats
- Receptors, CXCR4/antagonists & inhibitors
- Receptors, CXCR4/genetics
- Receptors, CXCR4/metabolism
- Receptors, CXCR5/antagonists & inhibitors
- Receptors, CXCR5/genetics
- Receptors, CXCR5/metabolism
- Recombinant Fusion Proteins/genetics
- Recombinant Fusion Proteins/metabolism
- Repressor Proteins/genetics
- Repressor Proteins/metabolism
- Transfection
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Affiliation(s)
- Marcel Liebick
- Department of Cellular and Molecular Immunology, University of Göttingen, Göttingen, Niedersachsen, Germany
| | - Christian Schläger
- Department of Cellular and Molecular Immunology, University of Göttingen, Göttingen, Niedersachsen, Germany
| | - Martin Oppermann
- Department of Cellular and Molecular Immunology, University of Göttingen, Göttingen, Niedersachsen, Germany
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9
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Lotze J, Reinhardt U, Seitz O, Beck-Sickinger AG. Peptide-tags for site-specific protein labelling in vitro and in vivo. MOLECULAR BIOSYSTEMS 2016; 12:1731-45. [DOI: 10.1039/c6mb00023a] [Citation(s) in RCA: 129] [Impact Index Per Article: 16.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/20/2022]
Abstract
Peptide-tag based labelling can be achieved by (i) enzymes (ii) recognition of metal ions or small molecules and (iii) peptide–peptide interactions and enables site-specific protein visualization to investigate protein localization and trafficking.
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Affiliation(s)
- Jonathan Lotze
- Institut für Biochemie
- Universität Leipzig
- D-04103 Leipzig
- Germany
| | - Ulrike Reinhardt
- Institut für Chemie
- Humboldt-Universität zu Berlin
- D-12489 Berlin
- Germany
| | - Oliver Seitz
- Institut für Chemie
- Humboldt-Universität zu Berlin
- D-12489 Berlin
- Germany
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10
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Lajko M, Haddad AF, Robinson CA, Connolly SA. Using proximity biotinylation to detect herpesvirus entry glycoprotein interactions: Limitations for integral membrane glycoproteins. J Virol Methods 2015; 221:81-9. [PMID: 25958131 DOI: 10.1016/j.jviromet.2015.04.031] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/03/2014] [Revised: 04/02/2015] [Accepted: 04/28/2015] [Indexed: 01/23/2023]
Abstract
Herpesvirus entry into cells requires coordinated interactions among several viral transmembrane glycoproteins. Viral glycoproteins bind to receptors and interact with other glycoproteins to trigger virus-cell membrane fusion. Details of these glycoprotein interactions are not well understood because they are likely transient and/or low affinity. Proximity biotinylation is a promising protein-protein interaction assay that can capture transient interactions in live cells. One protein is linked to a biotin ligase and a second protein is linked to a short specific acceptor peptide (AP). If the two proteins interact, the ligase will biotinylate the AP, without requiring a sustained interaction. To examine herpesvirus glycoprotein interactions, the ligase and AP were linked to herpes simplex virus 1 (HSV1) gD and Epstein Barr virus (EBV) gB. Interactions between monomers of these oligomeric proteins (homotypic interactions) served as positive controls to demonstrate assay sensitivity. Heterotypic combinations served as negative controls to determine assay specificity, since HSV1 gD and EBV gB do not interact functionally. Positive controls showed strong biotinylation, indicating that viral glycoprotein proximity can be detected. Unexpectedly, the negative controls also showed biotinylation. These results demonstrate the special circumstances that must be considered when examining interactions among glycosylated proteins that are constrained within a membrane.
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Affiliation(s)
- Michelle Lajko
- DePaul University, Department of Biological Sciences, Chicago, IL, USA
| | | | | | - Sarah A Connolly
- DePaul University, Department of Biological Sciences, Chicago, IL, USA; DePaul University, Department of Health Sciences, Chicago, IL, USA.
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11
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Naganathan S, Ray-Saha S, Park M, Tian H, Sakmar TP, Huber T. Multiplex detection of functional G protein-coupled receptors harboring site-specifically modified unnatural amino acids. Biochemistry 2015; 54:776-86. [PMID: 25524496 PMCID: PMC4310623 DOI: 10.1021/bi501267x] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
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We developed a strategy for identifying
positions in G protein-coupled
receptors that are amenable to bioorthogonal modification with a peptide
epitope tag under cell culturing conditions. We introduced the unnatural
amino acid p-azido-l-phenylalanine (azF)
into human CC chemokine receptor 5 (CCR5) at site-specific amber codon
mutations. We then used strain-promoted azide–alkyne [3+2]
cycloaddition to label the azF-CCR5 variants with a FLAG peptide epitope-conjugated
aza-dibenzocyclooctyne (DBCO) reagent. A microtiter plate-based sandwich
fluorophore-linked immunosorbent assay was used to probe simultaneously
the FLAG epitope and the receptor using infrared dye-conjugated antibodies
so that the extent of DBCO incorporation, corresponding nominally
to labeling efficiency, could be quantified ratiometrically. The extent
of incorporation of DBCO at the various sites was evaluated in the
context of a recent crystal structure of maraviroc-bound CCR5. We
observed that labeling efficiency varied dramatically depending on
the topological location of the azF in CCR5. Interestingly, position
109 in transmembrane helix 3, located in a hydrophobic cavity on the
extracellular side of the receptor, was labeled most efficiently.
Because the bioorthogonal labeling and detection strategy described
might be used to introduce a variety of different peptide epitopes
or fluorophores into engineered expressed receptors, it might prove
to be useful for a wide range of applications, including single-molecule
detection studies of receptor trafficking and signaling mechanism.
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Affiliation(s)
- Saranga Naganathan
- Laboratory of Chemical Biology & Signal Transduction, The Rockefeller University , New York, New York 10065, United States
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12
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Stoichiometry and geometry of the CXC chemokine receptor 4 complex with CXC ligand 12: molecular modeling and experimental validation. Proc Natl Acad Sci U S A 2014; 111:E5363-72. [PMID: 25468967 DOI: 10.1073/pnas.1417037111] [Citation(s) in RCA: 59] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022] Open
Abstract
Chemokines and their receptors regulate cell migration during development, immune system function, and in inflammatory diseases, making them important therapeutic targets. Nevertheless, the structural basis of receptor:chemokine interaction is poorly understood. Adding to the complexity of the problem is the persistently dimeric behavior of receptors observed in cell-based studies, which in combination with structural and mutagenesis data, suggest several possibilities for receptor:chemokine complex stoichiometry. In this study, a combination of computational, functional, and biophysical approaches was used to elucidate the stoichiometry and geometry of the interaction between the CXC-type chemokine receptor 4 (CXCR4) and its ligand CXCL12. First, relevance and feasibility of a 2:1 stoichiometry hypothesis was probed using functional complementation experiments with multiple pairs of complementary nonfunctional CXCR4 mutants. Next, the importance of dimers of WT CXCR4 was explored using the strategy of dimer dilution, where WT receptor dimerization is disrupted by increasing expression of nonfunctional CXCR4 mutants. The results of these experiments were supportive of a 1:1 stoichiometry, although the latter could not simultaneously reconcile existing structural and mutagenesis data. To resolve the contradiction, cysteine trapping experiments were used to derive residue proximity constraints that enabled construction of a validated 1:1 receptor:chemokine model, consistent with the paradigmatic two-site hypothesis of receptor activation. The observation of a 1:1 stoichiometry is in line with accumulating evidence supporting monomers as minimal functional units of G protein-coupled receptors, and suggests transmission of conformational changes across the dimer interface as the most probable mechanism of altered signaling by receptor heterodimers.
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