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Fusion with Promiscuous Gα 16 Subunit Reveals Signaling Bias at Muscarinic Receptors. Int J Mol Sci 2021; 22:ijms221810089. [PMID: 34576254 PMCID: PMC8469978 DOI: 10.3390/ijms221810089] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/18/2021] [Revised: 09/06/2021] [Accepted: 09/14/2021] [Indexed: 11/25/2022] Open
Abstract
A complex evaluation of agonist bias at G-protein coupled receptors at the level of G-protein classes and isoforms including non-preferential ones is essential for advanced agonist screening and drug development. Molecular crosstalk in downstream signaling and a lack of sufficiently sensitive and selective methods to study direct coupling with G-protein of interest complicates this analysis. We performed binding and functional analysis of 11 structurally different agonists on prepared fusion proteins of individual subtypes of muscarinic receptors and non-canonical promiscuous α-subunit of G16 protein to study agonist bias. We have demonstrated that fusion of muscarinic receptors with Gα16 limits access of other competitive Gα subunits to the receptor, and thus enables us to study activation of Gα16 mediated pathway more specifically. Our data demonstrated agonist-specific activation of G16 pathway among individual subtypes of muscarinic receptors and revealed signaling bias of oxotremorine towards Gα16 pathway at the M2 receptor and at the same time impaired Gα16 signaling of iperoxo at M5 receptors. Our data have shown that fusion proteins of muscarinic receptors with α-subunit of G-proteins can serve as a suitable tool for studying agonist bias, especially at non-preferential pathways.
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Robichaux WG, Oner SS, Lanier SM, Blumer JB. Direct Coupling of a Seven-Transmembrane-Span Receptor to a Gαi G-Protein Regulatory Motif Complex. Mol Pharmacol 2015; 88:231-7. [PMID: 25972449 DOI: 10.1124/mol.115.097741] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/08/2015] [Accepted: 05/12/2015] [Indexed: 12/29/2022] Open
Abstract
Group II activator of G-protein signaling (AGS) proteins contain one or more G-protein regulatory motifs (GPR), which serve as docking sites for GαiGDP independent of Gβγ and stabilize the GDP-bound conformation of Gαi, acting as guanine nucleotide dissociation inhibitors. The GαGPR interaction is regulated by seven-transmembrane-spanning (7TM) receptors in the intact cell as determined by bioluminescence resonance energy transfer (BRET). It is hypothesized that a 7TM receptor directly couples to the GαGPR complex in a manner analogous to receptor coupling to the Gαβγ heterotrimer. As an initial approach to test this hypothesis, we used BRET to examine 7TM receptor-mediated regulation of GαGPR in the intact cell when Gαi2 yellow fluorescent protein (YFP) was tethered to the carboxyl terminus of the α2A adrenergic receptor (α2AAR-Gαi2YFP). AGS3- and AGS4-Renilla luciferase (Rluc) exhibited robust BRET with the tethered GαiYFP, and this interaction was regulated by receptor activation localizing the regulation to the receptor microenvironment. Agonist regulation of the receptor-Gαi-GPR complex was also confirmed by coimmunoprecipitation and cell fractionation. The tethered Gαi2 was rendered pertussis toxin-insensitive by a C352I mutation, and receptor coupling to endogenous Gαi/oβγ was subsequently eliminated by cell treatment with pertussis toxin (PT). Basal and agonist-induced regulation of α2AAR-Gαi2YFP(C352I):AGS3Rluc and α2AAR-Gαi2YFP(C352I):AGS4Rluc BRET was not altered by PT treatment or Gβγ antagonists. Thus, the localized regulation of GαGPR by receptor activation appears independent of endogenous Gαi/oβγ, suggesting that GαiAGS3 and GαiAGS4 directly sense agonist-induced conformational changes in the receptor, as is the case for 7TM receptor coupling to the Gαβγ heterotrimer. The direct coupling of a receptor to the GαiGPR complex provides an unexpected platform for signal propagation with broad implications.
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Affiliation(s)
- William G Robichaux
- Department of Cell and Molecular Pharmacology and Experimental Therapeutics (W.G.R., S.S.O., S.M.L., J.B.B.) and Department of Neurosciences (J.B.B.), Medical University of South Carolina, Charleston, South Carolina
| | - Sukru S Oner
- Department of Cell and Molecular Pharmacology and Experimental Therapeutics (W.G.R., S.S.O., S.M.L., J.B.B.) and Department of Neurosciences (J.B.B.), Medical University of South Carolina, Charleston, South Carolina
| | - Stephen M Lanier
- Department of Cell and Molecular Pharmacology and Experimental Therapeutics (W.G.R., S.S.O., S.M.L., J.B.B.) and Department of Neurosciences (J.B.B.), Medical University of South Carolina, Charleston, South Carolina
| | - Joe B Blumer
- Department of Cell and Molecular Pharmacology and Experimental Therapeutics (W.G.R., S.S.O., S.M.L., J.B.B.) and Department of Neurosciences (J.B.B.), Medical University of South Carolina, Charleston, South Carolina
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Shen F, Cheng L, Douglas AE, Riobo NA, Manning DR. Smoothened is a fully competent activator of the heterotrimeric G protein G(i). Mol Pharmacol 2013; 83:691-7. [PMID: 23292797 DOI: 10.1124/mol.112.082511] [Citation(s) in RCA: 57] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/10/2023] Open
Abstract
Smoothened (Smo) is a 7-transmembrane protein essential to the activation of Gli transcription factors (Gli) by hedgehog morphogens. The structure of Smo implies interactions with heterotrimeric G proteins, but the degree to which G proteins participate in the actions of hedgehogs remains controversial. We posit that the G(i) family of G proteins provides to hedgehogs the ability to expand well beyond the bounds of Gli. In this regard, we evaluate here the efficacy of Smo as it relates to the activation of G(i), by comparing Smo with the 5-hydroxytryptamine(1A) (5-HT(1A)) receptor, a quintessential G(i)-coupled receptor. We find that with use of [(35)S]guanosine 5'-(3-O-thio)triphosphate, first, with forms of G(i) endogenous to human embryonic kidney (HEK)-293 cells made to express epitope-tagged receptors and, second, with individual forms of Gα(i) fused to the C terminus of each receptor, Smo is equivalent to the 5-HT(1A) receptor in the assay as it relates to capacity to activate G(i). This finding is true regardless of subtype of G(i) (e.g., G(i2), G(o), and G(z)) tested. We also find that Smo endogenous to HEK-293 cells, ostensibly through inhibition of adenylyl cyclase, decreases intracellular levels of cAMP. The results indicate that Smo is a receptor that can engage not only Gli but also other more immediate effectors.
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Affiliation(s)
- Feng Shen
- Department of Pharmacology, Perelman School of Medicine, University of Pennsylvania, 3610 Hamilton Walk, Philadelphia, Pennsylvania 19104-6084, USA
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Sutor S, Heilmann J, Seifert R. Impact of fusion to Gα(i2) and co-expression with RGS proteins on pharmacological properties of human cannabinoid receptors CB₁R and CB₂R. ACTA ACUST UNITED AC 2011; 63:1043-55. [PMID: 21718288 DOI: 10.1111/j.2042-7158.2011.01307.x] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
OBJECTIVES G protein coupled receptor (GPCR)-Gα fusion proteins are often employed to investigate receptor/G protein interaction. In this study, the impact of Gα fusion proteins on pharmacology of CBRs, both mediating signals through Gα(i) proteins, were investigated. Gα(i2) was fused to the C-terminus of the CBRs or co-expressed with non-fused Gα(i2) in Sf9 cells, always together with Gβ₁γ₂. Furthermore, the impact of RGS proteins on CBR signaling in combination with the CBR fusion approach was examined, using RGS4 and RGS19 as paradigms. METHODS CBR ligands were characterized in the steady-state GTPase assay and pharmacological properties of ligands in the different test systems were correlated. KEY FINDINGS Fusion of CBRs to Gα(i2) enhanced the maximal stimulatory effects of ligands compared to the co-expression system, especially for CB₂R. RGS4, but not RGS19, behaved as a GTPase-activating protein at CBRs in the Gα(i2) co-expression and fusion system. Fusion of GPCR, most prominently CB₂R, to Gα(i2) , and co-expression with RGS4 altered the pharmacological properties of ligands. CONCLUSIONS Our data suggest that fusion of CB₂R to Gα(i2) and co-expression with RGS4 impedes with conformational changes. Moreover, our results support the concept of ligand-specific receptor conformations. Finally, this paper describes the most sensitive CBR test system currently available.
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Affiliation(s)
- Sarah Sutor
- Department of Pharmaceutical Biology, Institute of Pharmacy, University of Regensburg, Regensburg Institute of Pharmacology, Medical School of Hannover, Hannover, Germany
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Ma AWS, Dong JY, Ma D, Wells JW. Cleavage-resistant fusion proteins of the M(2) muscarinic receptor and Gα(i1). Homotropic and heterotropic effects in the binding of ligands. Biochim Biophys Acta Gen Subj 2011; 1810:592-602. [PMID: 21397664 DOI: 10.1016/j.bbagen.2011.03.003] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/27/2010] [Revised: 01/28/2011] [Accepted: 03/02/2011] [Indexed: 11/30/2022]
Abstract
BACKGROUND G protein-coupled receptors fused to a Gα-subunit are functionally similar to their unfused counterparts. They offer an intriguing view into the nature of the receptor-G protein complex, but their usefulness depends upon the stability of the fusion. METHODS Fusion proteins of the M(2) muscarinic receptor and the α-subunit of G(i1) were expressed in CHO and Sf9 cells, extracted in digitonin-cholate, and examined for their binding properties and their electrophoretic mobility on western blots. RESULTS Receptor fused to native α(i1) underwent proteolysis near the point of fusion to release a fragment with the mobility of α(i1). The cleavage was prevented by truncation of the α-subunit at position 18. Binding of the agonist oxotremorine-M to the stable fusion protein from Sf9 cells was biphasic, and guanylylimidodiphosphate promoted an apparent interconversion of sites from higher to lower affinity. With receptor from CHO cells, the apparent capacity for N-[(3)H]methylscopolamine was 60% of that for [(3)H]quinuclidinylbenzilate; binding at saturating concentrations of the latter was inhibited in a noncompetitive manner at low concentrations of unlabeled N-methylscopolamine. CONCLUSIONS A stable fusion protein of the M(2) receptor and truncated α(i1) resembles the native receptor-G protein complex with respect to the guanyl nucleotide-sensitive binding of agonists and the noncompetitive binding of antagonists. GENERAL SIGNIFICANCE Release of the α-subunit is likely to occur with other such fusion proteins, rendering the data ambiguous or misleading. The properties of a chemically stable fusion protein support the notion that signaling proceeds via a stable multimeric complex of receptor and G protein.
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Affiliation(s)
- Amy W-S Ma
- Department of Pharmaceutical Sciences, University of Toronto, Toronto, Ontario, Canada
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Abstract
In many cases, the coexpression of GPCRs with G-proteins and/or regulators of G-protein signaling (RGS-proteins) allows a successful reconstitution of high-affinity agonist binding and functional responses. However, in some cases, coexpressed GPCRs and G-proteins interact inefficiently, resulting in weak [³⁵S]GTPγS- and steady-state GTPase assay signals. This may be, for example, caused by a rapid dissociation of the G-protein from the plasma membrane, as has been reported for Gα(s). Moreover, for a detailed characterization of GPCR/G-protein interactions, it may be required to work with a defined GPCR/G-protein stoichiometry and to avoid cross-interaction with endogenous G-proteins. Cross-talk to endogenous G-proteins has been shown to play a role in some mammalian expression systems. These problems can be addressed by the generation of GPCR-Gα fusion proteins and their expression in Sf9 insect cells. When the C-terminus of the receptor is fused to the N-terminus of the G-protein, a 1:1 stoichiometry of both proteins is achieved. In addition, the close proximity of GPCR and G-protein in fusion proteins leads to enhanced interaction efficiency, resulting in increased functional signals. This approach can also be extended to fusion proteins of GPCRs with RGS-proteins, specifically when steady-state GTP hydrolysis is used as read-out. GPCR-RGS fusion proteins optimize the interaction of RGS-proteins with coexpressed Gα subunits, since the location of the RGS-protein is close to the site of receptor-mediated G-protein activation. Moreover, in contrast to coexpression systems, GPCR-Gα and GPCR-RGS fusion proteins provide a possibility to imitate physiologically occurring interactions, for example, the precoupling of receptors and G-proteins or the formation of complexes between GPCRs, G-proteins and RGS-proteins (transducisomes). In this chapter, we describe the technique for the generation of fusion proteins and show the application of this approach for the characterization of constitutively active receptors.
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Marino SF. High-level production and characterization of a G-protein coupled receptor signaling complex. FEBS J 2009; 276:4515-28. [PMID: 19645726 DOI: 10.1111/j.1742-4658.2009.07158.x] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Elucidation of the molecular details of signal transduction through G-protein coupled receptors (GPCRs) awaits the solution of high-resolution structures of the receptor species involved in passing the extracellular information across the plasma membrane. The critical challenge in this effort is the production of sufficient quantities of active and homogeneous receptor species amenable to crystallization screening. We describe here the high-level expression in mammalian cells and characterization of a fusion complex between the kappa opioid receptor and its cognate G-protein alpha subunit, G alpha(i1). Optimization of growth conditions resulted in the highest level of active binding sites reported to date for either opioid receptors or GPCR-G alpha fusions. In cells, the kappa opioid receptor was stabilized against proteolysis in the context of the fusion protein and was competent to bind both agonists and antagonists. Coupling of the kappa opioid receptor with the G alpha subunit was demonstrated by changes in agonist affinity in the presence of guanine nucleotides and by agonist-induced increases in the rate of guanine nucleotide hydrolysis. In addition to representing a physiologically relevant signaling complex, the additional hydrophilic surface area provided by the G-protein may enhance the chances of producing well-diffracting crystals from the purified complex.
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Affiliation(s)
- Stephen F Marino
- Department of Molecular Membrane Biology, Max Planck Institute of Biophysics, Frankfurt am Main, Germany.
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Kaya AI, Ugur O, Oner SS, Bastepe M, Onaran HO. Coupling of beta2-adrenoceptors to XLalphas and Galphas: a new insight into ligand-induced G protein activation. J Pharmacol Exp Ther 2009; 329:350-9. [PMID: 19144685 PMCID: PMC2670595 DOI: 10.1124/jpet.108.149989] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/17/2008] [Accepted: 01/12/2009] [Indexed: 11/22/2022] Open
Abstract
Galpha(s) and extra-large Galpha(s) (XLalpha(s)) can both transduce receptor activation into intracellular cAMP generation. It is unknown, however, whether these two GNAS-locus products display distinct properties with respect to receptor coupling. Here, we show that XLalpha(s) couples to the beta2-adrenoceptor more efficiently than Galpha(s). In transfected human embryonic kidney 293 cells and mouse embryonic fibroblasts null for both Galpha(s) and XLalpha(s) (2B2 cells), basal cAMP accumulation mediated by XLalpha(s) was higher than that mediated by Galpha(s). Inverse agonist treatment reduced Galpha(s)-mediated basal activity, whereas its effect was markedly blunted on XLalpha(s)-mediated basal activity. Rank order of ligand efficacies regarding cAMP accumulation was the same when the receptor was coupled to XLalpha(s) or Galpha(s). However, ligand-induced and XLalpha(s)-mediated cAMP generation was higher than that mediated by Galpha(s). The relatively high efficiency of XLalpha(s)-mediated cAMP generation was conditional, disappearing with increased level of receptor expression or increased efficacy of ligand. Full-agonist responses in XLalpha(s)- and Galpha(s)-expressing cells were comparable even at low receptor levels, whereas partial agonist responses became comparable only when the receptor expression was increased (>3 pmol/mg). Radioligand binding studies showed that the high-affinity component in agonist binding to beta2-adrenoceptor was more pronounced in cells expressing XLalpha(s) than those expressing Galpha(s). We discuss these findings in the framework of current receptor-G protein activation models and offer an extended ternary complex model that can fully explain our observations.
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Affiliation(s)
- A I Kaya
- Ankara University Biotechnology Institute, Tandogan, Ankara, Turkey
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9
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Abstract
Opioid efficacy on mu-receptor may be influenced by various Gi/o-G-protein subunits interacting with intracellular face of receptor. Pertussis toxin-insensitive Galphai1 and Galphai2 subunits tethered with mu-receptor were stably transfected into AtT20 cells to (i) determine coupling of different alpha-subunits on opioid efficacy, and (ii) determine coupling to downstream effectors, for example, calcium and potassium channels. After pertussis toxin, stimulation of [35S]GTP-gamma-S incorporation persisted. Both constructs were able to couple to native calcium and potassium channels, with endomorphins 1 and 2 equally effective. However, pertussis toxin abolished opioid actions on calcium and potassium channels suggesting strong coupling to endogenous G-proteins, and that differences in coupling efficacy to Galphai1 and Galphai2 previously observed are restricted to initial step of signaling cascade.
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Novi F, Millan MJ, Corsini GU, Maggio R. Partial agonist actions of aripiprazole and the candidate antipsychotics S33592, bifeprunox, N-desmethylclozapine and preclamol at dopamine D2Lreceptors are modified by co-transfection of D3receptors: potential role of heterodimer formation. J Neurochem 2007; 102:1410-24. [PMID: 17532788 DOI: 10.1111/j.1471-4159.2007.04660.x] [Citation(s) in RCA: 36] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Aripiprazole and the candidate antipsychotics, S33592, bifeprunox, N-desmethylclozapine (NDMC) and preclamol, are partial agonists at D(2) receptors. Herein, we examined their actions at D(2L) and D(3) receptors expressed separately or together in COS-7 cells. In D(2L) receptor-expressing cells co-transfected with (D(3) receptor-insensitive) chimeric adenylate cyclase-V/VI, drugs reduced forskolin-stimulated cAMP production by approximately 20% versus quinpirole (48%). Further, quinpirole-induced inhibition was blunted by aripiprazole and S33592, confirming partial agonist properties. In cells co-transfected with equal amounts of D(2L)and D(3) receptors (1 : 1), efficacies of aripiprazole and S33592 were attenuated. Further, in cells co-transfected with D(2L) and an excess of D(3) receptors (1 : 3), aripiprazole and S33592 were completely inactive, and they abolished the actions of quinpirole. Likewise, bifeprunox, NDMC and preclamol lost agonist properties in cells co-transfected with D(2L)and D(3) receptors. Accordingly, at split D(2trunk)/D(3tail) and D(3trunk)/D(2tail) chimeras, agonist actions of quinpirole were blocked by aripiprazole and S33592 that, like bifeprunox, NDMC and preclamol, were inactive alone. Conversely, when a 12 amino acid sequence in the third intracellular loop of D(3) receptors was replaced by the homologous sequence of D(2L) receptors, aripiprazole, S33592, bifeprunox, NDMC and preclamol inhibited cAMP formation by approximately 20% versus quinpirole (42%). Moreover, at D(2L) receptor-expressing cells co-transfected with modified D(3i3(D2)) receptors, drugs behaved as partial agonists. To summarize, low efficacy agonist actions of aripiprazole, S33592, bifeprunox, NDMC and preclamol at D(2L) receptors are abrogated upon co-expression of D(3) receptors, probably due to physical association and weakened coupling efficacy. These findings have implications for the functional profiles of antipsychotics.
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Affiliation(s)
- Francesca Novi
- Department of Neuroscience, University of Pisa, Pisa, Italy
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Suga H, Haga T. Ligand screening system using fusion proteins of G protein-coupled receptors with G protein alpha subunits. Neurochem Int 2007; 51:140-64. [PMID: 17659814 DOI: 10.1016/j.neuint.2007.06.006] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/15/2007] [Revised: 06/07/2007] [Accepted: 06/08/2007] [Indexed: 01/04/2023]
Abstract
G protein-coupled receptors (GPCRs) constitute one of the largest families of genes in the human genome, and are the largest targets for drug development. Although a large number of GPCR genes have recently been identified, ligands have not yet been identified for many of them. Various assay systems have been employed to identify ligands for orphan GPCRs, but there is still no simple and general method to screen for ligands of such GPCRs, particularly of G(i)-coupled receptors. We have examined whether fusion proteins of GPCRs with G protein alpha subunit (Galpha) could be utilized for ligand screening and showed that the fusion proteins provide an effective method for the purpose. This article focuses on the followings: (1) characterization of GPCR genes and GPCRs, (2) identification of ligands for orphan GPCRs, (3) characterization of GPCR-Galpha fusion proteins, and (4) identification of ligands for orphan GPCRs using GPCR-Galpha fusion proteins.
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Affiliation(s)
- Hinako Suga
- Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA.
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12
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Abstract
G protein-coupled receptor (GPCR)-Galpha fusion proteins were first characterized more than 10 years ago as a strategy for studying receptor-G protein signaling. A large number of studies have used this approach to characterize receptor coupling to members of the Gs, Gi, and Gq families of Galpha subunits, but this strategy has not been widely used to study Galpha12 and Galpha13. As described in the article by Zhang et al. in this issue of Molecular Pharmacology (p. 1433) characterization of the signaling properties of thromboxane A2 receptor (TPalpha) -Galpha12 and -Galpha13 fusion constructs demonstrates the applicability of this strategy to members of this unique family of Galpha subunits, and how this strategy can be used to resolve otherwise difficult problems of receptor pharmacology associated with these proteins. The general strategy of making receptor-Galpha fusion constructs has wide applicability to a number of research problems, but there are perhaps also "hidden messages" in how different receptor-Galpha subunit fusion pairs behave.
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Affiliation(s)
- John D Hildebrandt
- Department of Cell and Molecular Pharmacology, Medical University of South Carolina, 173 Ashley Ave., 303BSB, Charleston, SC 29425, USA.
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Ramsay D, Carr IC, Pediani J, Lopez-Gimenez JF, Thurlow R, Fidock M, Milligan G. High-affinity interactions between human alpha1A-adrenoceptor C-terminal splice variants produce homo- and heterodimers but do not generate the alpha1L-adrenoceptor. Mol Pharmacol 2004; 66:228-39. [PMID: 15266013 DOI: 10.1124/mol.66.2.228] [Citation(s) in RCA: 51] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022] Open
Abstract
Using combinations of bioluminescence resonance energy transfer, time-resolved fluorescence resonance energy transfer and the functional complementation of pairs of inactive receptor-G protein fusion proteins, the human alpha(1A-1)-adrenoceptor was shown to form homodimeric/oligomeric complexes when expressed in human embryonic kidney (HEK) 293 cells. Saturation bioluminescence resonance energy transfer studies indicated the alpha(1A-1)-adrenoceptor homodimer interactions to be high affinity and some 75 times greater than interactions between the alpha(1A-1)-adrenoceptor and the delta opioid peptide receptor. Only a fraction of the alpha(1A-1)-adrenoceptors was at the plasma membrane of HEK293 cells at steady state. However, dimers of alpha(1A-1)-adrenoceptors were also present in intracellular membranes, and the dimer status of those delivered to the cell surface was unaffected by the presence of agonist. Splice variation can generate at least three forms of the human alpha(1A-1)-adrenoceptor with differences limited to the C-terminal tail. Each of the alpha(1A-1), alpha(1A-2a), and alpha(1A-3a)-adrenoceptor splice variants formed homodimers/oligomers, and all combinations of these splice variants were able to generate heterodimeric/oligomeric interactions. Despite the coexpression of these splice variants in human tissues that possess the pharmacologically defined alpha(1L)-adrenoceptor binding site, coexpression of any pair in HEK293 cells failed to generate ligand binding characteristic of the alpha(1L)-adrenoceptor.
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Affiliation(s)
- Douglas Ramsay
- Division of Biochemistry and Molecular Biology, Institute of Biomedical and Life Sciences, University of Glasgow, Scotland, United Kingdom
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14
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Carrillo JJ, Pediani J, Milligan G. Dimers of class A G protein-coupled receptors function via agonist-mediated trans-activation of associated G proteins. J Biol Chem 2003; 278:42578-87. [PMID: 12920117 DOI: 10.1074/jbc.m306165200] [Citation(s) in RCA: 91] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The histamine H1 receptor and the alpha1b-adrenoreceptor are G protein-coupled receptors that elevate intracellular [Ca2+] via activation of Gq/G11. Assessed by co-immunoprecipitation and time-resolved fluorescence resonance energy transfer they both exist as homo-dimers. The addition of the G protein G11alpha to the C terminus of these receptors did not prevent dimerization. Agonists produced a large stimulation of guanosine 5'-3-O-([35S]thio)triphosphate ([35S]GTPgammaS) binding to receptor-G protein fusions containing wild type forms of both polypeptides. For both receptors this was abolished by incorporation of G208AG11alpha into the fusions. Mutation of a highly conserved leucine in intracellular loop 2 of each receptor also eliminated agonist function but not binding. Co-expression of the two non-functional but complementary fusion constructs reconstituted agonist-mediated binding of [35S]GTPgammaS in membranes of HEK293 cells and elevation of [Ca2+]i in mouse embryo fibroblasts lacking both Gq and G11. Co-expression of the histamine H1 receptor- and the alpha1b-adrenoreceptor-G11alpha fusions allowed detection of functional hetero-dimeric complexes, whereas co-expression of histamine H1 receptor-G11alpha with increasing amounts of L151Dalpha1b-adrenoreceptor resulted in decreasing levels of histamine-stimulated [35S]GTPgammaS binding. Co-expression of the alpha1b-adrenoreceptor with a fusion protein incorporating the N-terminal domain and transmembrane helix 1 of the alpha1b-adrenoreceptor and G11alpha did not result in agonist activation of the G protein but did indicate a role for transmembrane helix 1 in dimerization. These data demonstrate that dimers of these class A receptors function via trans-activation of associated G proteins.
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MESH Headings
- Amino Acid Substitution
- Animals
- Cell Line
- Dimerization
- GTP-Binding Protein alpha Subunits, Gq-G11/genetics
- GTP-Binding Protein alpha Subunits, Gq-G11/metabolism
- GTP-Binding Proteins/biosynthesis
- GTP-Binding Proteins/genetics
- GTP-Binding Proteins/metabolism
- Guanosine Triphosphate/metabolism
- Humans
- Mice
- Receptors, Adrenergic, alpha-1/genetics
- Receptors, Adrenergic, alpha-1/physiology
- Receptors, G-Protein-Coupled/agonists
- Receptors, G-Protein-Coupled/genetics
- Receptors, G-Protein-Coupled/physiology
- Receptors, Histamine H1
- Recombinant Fusion Proteins/genetics
- Recombinant Fusion Proteins/metabolism
- Transcriptional Activation
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Affiliation(s)
- Juan J Carrillo
- Molecular Pharmacology Group, Division of Biochemistry and Molecular Biology, Institute of Biomedical and Life Sciences, University of Glasgow, Glasgow G12 8QQ, Scotland, U.K
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15
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Hermans E. Biochemical and pharmacological control of the multiplicity of coupling at G-protein-coupled receptors. Pharmacol Ther 2003; 99:25-44. [PMID: 12804697 DOI: 10.1016/s0163-7258(03)00051-2] [Citation(s) in RCA: 221] [Impact Index Per Article: 10.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
Abstract
For decades, it has been generally proposed that a given receptor always interacts with a particular GTP-binding protein (G-protein) or with multiple G-proteins within one family. However, for several G-protein-coupled receptors (GPCR), it now becomes generally accepted that simultaneous functional coupling with distinct unrelated G-proteins can be observed, leading to the activation of multiple intracellular effectors with distinct efficacies and/or potencies. Multiplicity in G-protein coupling is frequently observed in artificial expression systems where high densities of receptors are obtained, raising the question of whether such complex signalling reveals artefactual promiscuous coupling or is a genuine property of GPCRs. Multiple biochemical and pharmacological evidence in favour of an intrinsic property of GPCRs were obtained in recent studies. Thus, there are now many examples showing that the coupling to multiple signalling pathways is dependent on the agonist used (agonist trafficking of receptor signals). In addition, the different couplings were demonstrated to involve distinct molecular determinants of the receptor and to show distinct desensitisation kinetics. Such multiplicity of signalling at the level of G-protein coupling leads to a further complexity in the functional response to agonist stimulation of one of the most elaborate cellular transmission systems. Indeed, the physiological relevance of such versatility in signalling associated with a single receptor requires the existence of critical mechanisms of dynamic regulation of the expression, the compartmentalisation, and the activity of the signalling partners. This review aims at summarising the different studies that support the concept of multiplicity of G-protein coupling. The physiological and pharmacological relevance of this coupling promiscuity will be discussed.
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Affiliation(s)
- Emmanuel Hermans
- Laboratoire de Pharmacologie Expérimentale, Université Catholique de Louvain, FARL 54.10, Avenue Hippocrate 54, B-1200 Brussels, Belgium.
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16
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Gazi L, Wurch T, Lopéz-Giménez JF, Pauwels PJ, Strange PG. Pharmacological analysis of a dopamine D(2Short):G(alphao) fusion protein expressed in Sf9 cells. FEBS Lett 2003; 545:155-60. [PMID: 12804767 DOI: 10.1016/s0014-5793(03)00520-9] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
A dopamine D(2Short) receptor:G(alphao) fusion protein was expressed in Sf9 cells using the baculovirus expression system. [(3)H]Spiperone bound to D(2Short):G(alphao) with a pK(d) approximately 10. Dopamine stimulated the binding of [(35)S]guanosine-5'-O-(3-thio)triphosphate (GTPgammaS) to D(2Short):G(alphao) expressed with Gbeta(1)gamma(2) (E(max)>460%; pEC(50) 5.43+/-0.06). Most of the putative D(2) antagonists behaved as inverse agonists (suppressing basal [(35)S]GTPgammaS binding) at D(2Short):G(alphao)/Gbeta(1)gamma(2) although (-)-sulpiride and ziprasidone were neutral antagonists. Competition of [(3)H]spiperone binding by dopamine and 10,11-dihydroxy-N-n-propylnorapomorphine revealed two binding sites of different affinities, even in the presence of GTP (100 micro M). The D(2Short):G(alphao) fusion protein is therefore a good model for characterising D(2) receptors.
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Affiliation(s)
- Lucien Gazi
- School of Animal and Microbial Sciences, University of Reading, Whiteknights, RG6 6AJ, Reading, UK
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17
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Molinari P, Ambrosio C, Riitano D, Sbraccia M, Grò MC, Costa T. Promiscuous coupling at receptor-Galpha fusion proteins. The receptor of one covalent complex interacts with the alpha-subunit of another. J Biol Chem 2003; 278:15778-88. [PMID: 12598520 DOI: 10.1074/jbc.m300731200] [Citation(s) in RCA: 43] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Fusion proteins between heptahelical receptors (GPCR) and G protein alpha-subunits show enhanced signaling efficiency in transfected cells. This is believed to be the result of molecular proximity, because the interaction between linked modules of one protein chain, if not constrained by structure, should be strongly favored compared with the same in which partners react as free species. To test this assumption we made a series of fusion proteins (type 1 and 4 opioid receptors with G(o) and beta(2) adrenergic and dopamine 1 receptors with G(sL)) and some mutated analogs carrying different tags and defective GPCR or Galpha subunits. Using cotransfection experiments with readout protocols able to distinguish activation at fused and non-fused alpha-subunits, we found that both the GPCR and the Galpha limb of one fusion protein can freely interact with non-fused proteins and the tethered partners of a neighboring fusion complex. Moreover, a bulky polyanionic inhibitor can suppress with identical potency receptor-Galpha interaction, either when occurring between latched domains of a fused system or separate elements of distinct molecules, indicating that the binding surfaces are equally accessible in both cases. These data demonstrate that there is no entropy drive from the linked condition of fusion proteins and suggest that their signaling may result from the GPCR of one complex interacting with the alpha-subunit of another. Moreover, the enhanced coupling efficiency commonly observed for fusion proteins is not due to the receptor tether, but to the transmembrane helix that anchors Galpha to the membrane.
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Affiliation(s)
- Paola Molinari
- Department of Pharmacology, Istituto Superiore di Sanità, Viale Regina Elena 299, 00161 Rome, Italy
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18
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Bourova L, Kostrnova A, Hejnova L, Moravcova Z, Moon HE, Novotny J, Milligan G, Svoboda P. delta-Opioid receptors exhibit high efficiency when activating trimeric G proteins in membrane domains. J Neurochem 2003; 85:34-49. [PMID: 12641725 DOI: 10.1046/j.1471-4159.2003.01667.x] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Low-density membrane fragments (domains) were separated from the bulk of plasma membranes of human embryonic kidney (HEK)293 cells expressing a delta-opioid (DOP) receptor-Gi1alpha fusion protein by drastic homogenization and flotation on equilibrium sucrose density gradients. The functional activity of trimeric G proteins and capacity of the DOP receptor to stimulate both the fusion protein-linked Gi1alpha and endogenous pertussis-toxin sensitive G proteins was measured as d-Ala2, d-Leu5-enkephalin stimulated high-affinity GTPase or guanosine-5'-[gamma-35S]triphosphate ([35S]GTPgammaS) binding. The maximum d-Ala2-d-Leu5 enkephalin (DADLE)-stimulated GTPase was two times higher in low-density membrane fragments than in bulk of plasma membranes; 58 and 27 pmol/mg/min, respectively. The same difference was obtained for [35S]GTPgammaS binding. Contrarily, the low-density domains contained no more than half the DOP receptor binding sites (Bmax = 6.6 pmol/mg versus 13.6 pmol/mg). Thus, when corrected for expression levels of the receptor, low-density domains exhibited four times higher agonist-stimulated GTPase and [35S]GTPgammaS binding than the bulk plasma membranes. The regulator of G protein signaling RGS1, enhanced further the G protein functional activity but did not remove the difference between domain-bound and plasma membrane pools of G protein. The potency of the agonist in functional studies and the affinity of specific [3H]DADLE binding to the receptor were, however, the same in both types of membranes - EC50 = 4.5 +/- 0.1 x 10(-8) and 3.2 +/- 1.4 x 10(-8) m for GTPase; Kd = 1.2 +/- 0.1 and 1.3 +/- 0.1 nm for [3H]DADLE radioligand binding assay. Similar results were obtained when sodium bicarbonate was used for alkaline isolation of membrane domains. By contrast, detergent-insensitive membrane domains isolated following treatment of cells with Triton X100 exhibited no DADLE-stimulated GTPase or GTPgammaS binding. Functional coupling between the DOP receptor and cognate G proteins was also blocked by high-energy ultrasound and repeated freezing-thawing. Our data indicate, for the first time, that membrane domains isolated using 'detergent-free' procedures exhibit higher efficiency of coupling between a G protein-coupled receptor and its corresponding G protein(s) than bulk plasma membranes. Detergent-extraction diminishes these interactions, even when the receptor and G proteins are physically tethered together.
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Affiliation(s)
- Lenka Bourova
- Institute of Physiology, Academy of Sciences of the Czech Republic, Prague, Czech Republic
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19
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Bertaso F, Ward RJ, Viard P, Milligan G, Dolphin AC. Mechanism of action of Gq to inhibit G beta gamma modulation of CaV2.2 calcium channels: probed by the use of receptor-G alpha tandems. Mol Pharmacol 2003; 63:832-43. [PMID: 12644584 DOI: 10.1124/mol.63.4.832] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The stable interaction of a G-protein coupled receptor and a particular partner G-protein was made possible by creating tandems between the alpha(2A) adrenergic receptor (alpha(2A)-R) and pertussis toxin-resistant mutants of different G alpha subunits of heterotrimeric G-proteins. Both alpha(2A)-R-G alpha(o) and alpha(2A)-R-G alpha(i) proved able to reconstitute agonist-induced voltage-dependent inhibition of N-type calcium channels (Ca(V)2.2) similar to the wild-type alpha(2A)-R when expressed in COS-7 cells. The interaction of G(q) with the G(i/o) signaling pathways was studied by expressing either G alpha(q) or a chimeric construct based on G alpha(q) containing the last five amino acids of G alpha(z), which is activated by alpha(2A)-R. It was found that G alpha(qz5) activated by the wild-type alpha(2A)-R inhibited Ca(V)2.2 currents in a voltage-independent fashion. Furthermore, G alpha(qz5) counteracted the voltage-dependent inhibition resulting from alpha(2A)-R-G alpha(o) activation. We subsequently investigated the basis for the behavior of G alpha(qz5). Our evidence suggests that this occurs as a result of a downstream effect of activation of G alpha(qz5) because it was blocked by C-terminal construct of phospholipase C beta 1. Furthermore it is likely to occur in part via protein kinase C (PKC) activation, because the PKC activator phorbol dibutyrate mimicked the effects of G alpha(qz5) in alpha(2A)-R-G alpha(o)-transfected cells. Conversely, cells expressing both alpha(2A)-R-G alpha(o) and G alpha(qz5) exhibited a partial restoration of voltage-dependent inhibition in the presence of the PKC inhibitor bisindolylmaleimide I (GF 109203X). The potential sites of phosphorylation are discussed.
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Affiliation(s)
- Federica Bertaso
- Department of Pharmacology, University College London, London, United Kingdom
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20
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Hommers LG, Lohse MJ, Bünemann M. Regulation of the inward rectifying properties of G-protein-activated inwardly rectifying K+ (GIRK) channels by Gbeta gamma subunits. J Biol Chem 2003; 278:1037-43. [PMID: 12403784 DOI: 10.1074/jbc.m205325200] [Citation(s) in RCA: 40] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Gbetagamma subunits are known to bind to and activate G-protein-activated inwardly rectifying K(+) channels (GIRK) by regulating their open probability and bursting behavior. Studying G-protein regulation of either native GIRK (I(KACh)) channels in feline atrial myocytes or heterologously expressed GIRK1/4 channels in Chinese hamster ovary cells and HEK 293 cells uncovered a novel Gbetagamma subunit mediated regulation of the inwardly rectifying properties of these channels. I(KACh) activated by submaximal concentrations of acetylcholine exhibited a approximately 2.5-fold stronger inward rectification than I(KACh) activated by saturating concentrations of acetylcholine. Similarly, the inward rectification of currents through GIRK1/4 channels expressed in HEK cells was substantially weakened upon maximal stimulation with co-expressed Gbetagamma subunits. Analysis of the outward current block underlying inward rectification demonstrated that the fraction of instantaneously blocked channels was reduced when Gbetagamma was over-expressed. The Gbetagamma induced weakening of inward rectification was associated with reduced potencies for Ba(2+) and Cs(+) to block channels from the extracellular side. Based on these results we propose that saturation of the channel with Gbetagamma leads to a conformational change within the pore of the channel that reduced the potency of extracellular cations to block the pore and increased the fraction of channels inert to a pore block in outward direction.
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Affiliation(s)
- Leif G Hommers
- University of Würzburg, Department of Pharmacology & Toxicology, Versbacherstrasse 9, Germany
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21
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Welsby PJ, Carr IC, Wilkinson G, Milligan G. Regulation of the avidity of ternary complexes containing the human 5-HT(1A) receptor by mutation of a receptor contact site on the interacting G protein alpha subunit. Br J Pharmacol 2002; 137:345-52. [PMID: 12237254 PMCID: PMC1573502 DOI: 10.1038/sj.bjp.0704880] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/30/2002] [Revised: 07/10/2002] [Accepted: 07/19/2002] [Indexed: 11/08/2022] Open
Abstract
1 Fusion proteins were constructed between the human 5-HT(1A) receptor and pertussis toxin-resistant forms of both G(i1)alpha and G(o1)alpha mutated at residue(351) from cysteine to either glycine or isoleucine. Each of these was expressed stably in HEK293 cells. 2 Increasing concentrations of GDP inhibited binding of the agonist [(3)H]-8-OH-DPAT but not the antagonist [(3)H]-MPPF to each construct. 3 The IC(50) for GDP was greater for constructs containing isoleucine at residue(351) of the G proteins compared to those with glycine at this position. 4 The G protein antagonist suramin had similar effects to GDP on the binding of [(3)H]-8-OH-DPAT. 5 The proportion of 5-HT(1A) receptor binding sites detected by [(3)H]-MPPF that displayed high affinity for 8-OH-DPAT was significantly greater when the interacting G protein contained isoleucine rather than glycine at residue(351). 6 The 5-HT(1A) receptor displayed similar avidity of interaction with G(i1)alpha and G(o1)alpha. 7 These results indicate that a higher avidity ternary complex is formed between 8-OH-DPAT, the 5-HT(1A) receptor and G proteins when isoleucine rather than glycine is located at residue(351) of the interacting G protein.
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Affiliation(s)
- Philip J Welsby
- Molecular Pharmacology Group, Division of Biochemistry and Molecular Biology, Institute of Biomedical and Life Sciences, University of Glasgow, Glasgow G12 8QQ
| | - I Craig Carr
- Molecular Pharmacology Group, Division of Biochemistry and Molecular Biology, Institute of Biomedical and Life Sciences, University of Glasgow, Glasgow G12 8QQ
| | - Graeme Wilkinson
- Molecular Pharmacology and Biochemistry, Department of Enabling Science and Technology, Astra-Zeneca, Alderley Park, Macclesfield, Cheshire SK10 4TG
| | - Graeme Milligan
- Molecular Pharmacology Group, Division of Biochemistry and Molecular Biology, Institute of Biomedical and Life Sciences, University of Glasgow, Glasgow G12 8QQ
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22
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Carrillo JJ, Stevens PA, Milligan G. Measurement of agonist-dependent and -independent signal initiation of alpha(1b)-adrenoceptor mutants by direct analysis of guanine nucleotide exchange on the G protein galpha(11). J Pharmacol Exp Ther 2002; 302:1080-8. [PMID: 12183666 DOI: 10.1124/jpet.102.035501] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Immunoprecipitation of a fusion protein between the alpha(1b)-adrenoceptor and Galpha(11) following a [(35)S]GTPgammaS [guanosine-5'-O-(3-thio)triphosphate] binding assay resulted in incorporation of low levels of nucleotide. The agonist phenylephrine increased incorporation some 30-fold. Agonist-induced binding represented 1.0 mol of [(35)S]GTPgammaS/mol of fusion protein. This was to the G protein linked to the receptor rather than endogenous Galpha(q)/Galpha(11) as a fusion protein containing the alpha(1b)-adrenoceptor and a form of Galpha(11) (G(208)A) unable to exchange guanine nucleotides effectively, bound [(35)S]GTPgammaS very poorly. Fusion proteins between A(293)E, D(142)A, and 3CAM mutants of the alpha(1b)-adrenoceptor and Galpha(11) bound substantially greater levels of [(35)S]GTPgammaS in the absence of agonist than the fusion incorporating the wild-type receptor. Constitutive binding of the nucleotide induced by these mutants was only 20% of the level achieved by phenylephrine. These mutant receptors thus do not provide an accurate mimic of the agonist-occupied state. Phentolamine reduced the binding of [(35)S]GTPgammaS and acted as a partial inverse agonist for each of the constitutively active mutants. [(35)S]GTPgammaS binding to Galpha(11) was elevated by phenylephrine in both wild-type and constitutively active mutant forms of the fusion proteins, but agonist potency and binding affinity were 50 times higher for the fusions containing the mutated receptors. These studies provide the first direct demonstration of the capacity of constitutively active mutants of a receptor to stimulate guanine nucleotide exchange on the alpha subunit of a G(q) family G protein and defines a strategy potentially suitable for any receptor that couples to these G proteins.
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Affiliation(s)
- Juan J Carrillo
- Molecular Pharmacology Group, Division of Biochemistry and Molecular Biology, University of Glasgow, Glasgow G12 8QQ, Scotland, United Kingdom
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23
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Martin RS, Reynen PH, Calixto JJ, Reyes CL, Chang TK, Dietrich PS, Bonhaus DW, MacLennan SJ. Pharmacological comparison of a recombinant CB1 cannabinoid receptor with its G(alpha 16) fusion product. JOURNAL OF BIOMOLECULAR SCREENING 2002; 7:281-9. [PMID: 12097191 DOI: 10.1177/108705710200700312] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
The pharmacology of G protein-coupled receptors is widely accepted to depend on the G protein subunit to which the agonist-stimulated receptor couples. In order to investigate whether CB(1) agonist-mediated signal transduction via an engineered G(alpha 16) system is different than that of the G(i/o) coupling normally preferred by the CB(1) receptor, we transfected the human recombinant CB(1) receptor (hCB(1)) or a fusion protein comprising the hCB(1) receptor and G(alpha 16) (hCB(1)-G(alpha 16)) into HEK293 cells. From competition binding studies, the rank order of ligand affinities at the hCB(1)-G(alpha 16) fusion protein was found to be similar to that for hCB(1): HU 210 > CP 55,940 > or = SR 141716A > WIN 55212-2 > anandamide > JWH 015. Agonists increased [(35)S]GTP gamma S binding or inhibited forskolin-stimulated cAMP, presumably by coupling to G(i/o), in cells expressing hCB(1) but not hCB(1)-G(alpha 16). However, an analogous rank order of potencies was observed for these agonists in their ability to evoke increases in intracellular calcium concentration in cells expressing hCB(1)-G(alpha 16) but not hCB(1). These data demonstrate that ligand affinities for the hCB(1) receptor are not affected by fusion to the G(alpha 16) subunit. Furthermore, there is essentially no difference in the function of the hCB(1) receptor when coupled to G(i/o) or G (alpha 16).
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Affiliation(s)
- Renée S Martin
- CNS Therapy Area, Neurobiology Unit, Roche Bioscience, Palo Alto, CA 94304, USA.
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24
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Massotte D, Brillet K, Kieffer B, Milligan G. Agonists activate Gi1 alpha or Gi2 alpha fused to the human mu opioid receptor differently. J Neurochem 2002; 81:1372-82. [PMID: 12068084 DOI: 10.1046/j.1471-4159.2002.00946.x] [Citation(s) in RCA: 41] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
As preferential coupling of opioid receptor to various inhibitory Galpha subunits is still under debate, we have investigated the selectivity of the human mu opioid receptor fused to a pertussis toxin insensitive C351I Gi1 alpha or C352I Gi2 alpha in stably transfected HEK 293 cells. Overall agonist binding affinities were increased for both fusion constructs when compared to the wild type receptor. [35 S]GTPgammaS binding was performed on pertussis toxin treated cells to monitor coupling efficiency of the fusion constructs. Upon agonist addition hMOR-C351I Gi1 a exhibited an activation profile similar to the non-fused receptor while hMOR-C352I Gi2 alpha was poorly activated. Interestingly no correlation could be drawn between agonist binding affinity and efficacy. Upon agonist addition, forskolin-stimulated cAMP production, as measured using a reporter gene assay, was inhibited by signals transduced via the fused Gi1 alpha and Gi2 alpha mainly. In contrast both fusion constructs were able to initiate ERK-MAPK phosphorylation via coupling to endogenous G proteins only. In conclusion our data indicate that hMOR couples more efficiently to Gi1 alpha than Gi2 alpha and that the coupling efficacy is clearly agonist-dependent.
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Affiliation(s)
- Dominique Massotte
- Département des Récepteurs et Protéines Membranaires, CNRS UPR 9050, Ecole Supérieure de Biotechnologie de Strasbourg, Illkirch-Graffenstaden, France.
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Abstract
Efficacy has been defined in receptor pharmacology as a proportionality factor denoting the amount of physiological response a given ligand imparts to a biological system for a given amount of receptor occupancy. While first defined in terms of response, the concept can be expanded to a wide variety of G protein-coupled receptor (GPCR) behaviors, which includes pleiotropic interaction with multiple G proteins, internalization, oligomerization, desensitization, and interaction with membrane auxilliary proteins. Thus, there can be numerous types of efficacy, and different ligands can have a range of efficacies for different receptor behaviors. This review discusses the use of the efficacy concept in GPCR models based on the thermodynamic linkage theory and also in terms of the protein ensemble theory, in which macroaffinity of ligands for an ensemble of receptor microstates produces a new ligand-bound ensemble. The pharmacological characteristics of the ligand emerge from the intersection of the ligand-bound ensemble with the various ensembles defining pharmacological receptor behaviors. Receptor behaviors discussed are activation of G proteins; ability to be phosphorylated, desensitized, and internalized; formation of dimers and oligomers; and the interaction with auxiliary membrane and cytosolic proteins. The concepts of ligand-specific receptor conformation and conditional efficacy are also discussed in the context of ligand control of physiological response.
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Affiliation(s)
- Terry Kenakin
- GlaxoSmithKline Research and Development, Research Triangle Park, North Carolina 27709, USA.
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26
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Milligan G. Construction and analysis of function of G protein-coupled receptor-G protein fusion proteins. Methods Enzymol 2002; 343:260-73. [PMID: 11665572 DOI: 10.1016/s0076-6879(02)43141-2] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Affiliation(s)
- Graeme Milligan
- Molecular Pharmacology Group, Division of Biochemistry and Molecular Biology, Institute of Biomedical and Life Sciences, University of Glasgow, Glasgow G12 8QQ, Scotland, United Kingdom
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27
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Ward RJ, Milligan G. Reciprocal mutations of highly conserved residues in transmembrane helices 2 and 7 of the alpha(2A)-adrenoceptor restore agonist activation of G(i1)alpha. Cell Signal 2002; 14:139-44. [PMID: 11781138 DOI: 10.1016/s0898-6568(01)00218-2] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/01/2022]
Abstract
Fusion proteins were constructed between the alpha(2A)-adrenoceptor and the alpha-subunit of the G-protein G(i1). Mutation of the highly conserved Asp(79) in transmembrane (TM) helix 2 of the receptor to Asn reduced the capacity of agonists to activate G(i1)alpha by 95% without altering [3H]antagonist or agonist ligand-binding affinity. A reciprocal mutation in TM helix 7 (Asn(422)Asp) was without effect on signalling effectiveness. Combination of these two mutations overcame the effect of the Asp(79)Asp mutation. By examining alterations in this helix 2-helix 7 microdomain, we further demonstrate the utility of receptor-G-protein fusion proteins to quantitate mutational effects on receptor-G-protein interactions and information transfer.
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Affiliation(s)
- Richard J Ward
- Molecular Pharmacology Group, Division of Biochemistry and Molecular Biology, Institute of Biomedical and Life Sciences, University of Glasgow, Glasgow G12 8QQ, Scotland, UK
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28
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Abstract
At present, the drug-discovery process centres on ligands that either block or produce physiological responses. However, there are therapeutic uses for ligands that do neither of these things, but which still affect receptors in other ways. This review discusses the intimate relationship between the affinity of a ligand for its receptor, and the probability that the binding of the ligand will produce some change in the receptor, resulting in efficacy. This, in turn, argues that ligands that have affinity should be tested more broadly, for a wider range of efficacies, to detect hidden therapeutic activities.
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Affiliation(s)
- Terry Kenakin
- GlaxoSmithKline Research and Development, 5 Moore Drive, Research Triangle Park, North Carolina 27709, USA.
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29
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Nasman J, Kukkonen JP, Ammoun S, Akerman KE. Role of G-protein availability in differential signaling by alpha 2-adrenoceptors. Biochem Pharmacol 2001; 62:913-22. [PMID: 11543726 DOI: 10.1016/s0006-2952(01)00730-4] [Citation(s) in RCA: 38] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/01/2022]
Abstract
The impact of G-protein expression on the coupling specificity of the human alpha(2B)-adrenergic receptor (alpha(2B)-AR) was studied in Sf9 cells. The alpha(2B)-AR was shown to activate both coexpressed G(s)- and G(i)-proteins in a [(35)S]GTPgammaS binding assay. Noradrenaline and the synthetic agonist UK14,304 were equally potent and efficacious in stimulating G(i) activation. At the effector level (adenylyl cyclase), both ligands stimulated cAMP production. In the presence of forskolin, the effects of the agonists were more complex. Noradrenaline stimulated cAMP production, while UK14,304 showed a biphasic concentration-response curve with inhibition of stimulated cAMP production at low agonist concentrations and further stimulation at high agonist concentrations. G(s) coexpression caused a monophasic stimulatory response with both ligands. Coexpression with G(i) resulted in a biphasic concentration-response curve for noradrenaline and a monophasic inhibition with UK14,304. Experiments with a panel of agonists demonstrated that the more efficacious an agonist is in stimulating cAMP production, the weaker is its ability to couple to inhibition of cAMP accumulation via exogenous G(i). To be able to explain the mechanistic consequences of dual G-protein coupling described above, we developed a mathematical model based on the hypothesis that an agonist induces different conformations of the receptor having different affinity for different G-proteins. The model reproduced the profiles seen in the concentration-response curves with G(s) and G(i) coexpression. The model predicts that the affinity of the receptor conformation for G-proteins as well as the availability of G-proteins will determine the ultimate response of the receptor.
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Affiliation(s)
- J Nasman
- Department of Physiology, Division of Cell Physiology, Uppsala University, BMC, Box 572, SE-75123, Uppsala, Sweden.
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30
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Lim WK, Myung CS, Garrison JC, Neubig RR. Receptor-G protein gamma specificity: gamma11 shows unique potency for A(1) adenosine and 5-HT(1A) receptors. Biochemistry 2001; 40:10532-41. [PMID: 11523995 DOI: 10.1021/bi010950c] [Citation(s) in RCA: 43] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
G protein coupled receptors activate signal transducing guanine nucleotide-binding proteins (G proteins), which consist of an alpha subunit and a betagamma dimer. Whole cell studies have reported that receptors signal through specific betagamma subtypes. Membrane reconstitution studies with the adenosine A(1) and alpha(2A) adrenergic receptors have reached a similar conclusion. We aimed to test the generality of this finding by comparing the gamma subtype specificity for four G(i)-coupled receptors: alpha(2A) adrenergic; A1 adenosine (A(1)-R); 5-hydroxytryptamine(1A) (5-HT(1A)-R); mu opioid. Membranes were reconstituted with Galpha(i)(1) and five gamma subtypes (dimerized to beta1). Using a sensitive alpha-betagamma binding assay, we show that all recombinant betagamma (except beta1gamma1) had comparable affinity for alpha(i)(1). Using high affinity agonist binding as a measure of receptor-G protein coupling, betagamma-containing gamma11 was the most potent for A(1)-R and 5-HT(1A)-R (p < 0.05, one way ANOVA) while gamma7 was most potent for the other two receptors. gamma11 was 3-8-fold more potent for the A(1)-R than were the other gamma subtypes. Also, gamma11 was 2-8-fold more potent for A(1)-R than at the other receptors, suggesting a unique coupling specificity of the A(1)-R for gamma11. In contrast, the discrimination by receptors for the other betagamma subtypes (beta1 and gamma1, gamma2, gamma7, and gamma10) was limited (2-3-fold). Thus the exquisite betagamma specificity of individual receptors reported in whole cell studies may depend on in vivo mechanisms beyond direct receptor recognition of betagamma subtypes.
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MESH Headings
- Animals
- CHO Cells
- Cell Line
- Cricetinae
- GTP-Binding Protein alpha Subunits, Gi-Go/metabolism
- Heterotrimeric GTP-Binding Proteins/agonists
- Heterotrimeric GTP-Binding Proteins/metabolism
- Receptors, Adrenergic, alpha-2/metabolism
- Receptors, Opioid, mu/metabolism
- Receptors, Purinergic P1/metabolism
- Receptors, Serotonin/metabolism
- Receptors, Serotonin, 5-HT1
- Swine
- Tumor Cells, Cultured
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Affiliation(s)
- W K Lim
- Department of Pharmacology, The University of Michigan, Ann Arbor, Michigan 48109-0632, USA
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31
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Wurch T, Pauwels PJ. Analytical pharmacology of G protein-coupled receptors by stoichiometric expression of the receptor and G(alpha) protein subunits. J Pharmacol Toxicol Methods 2001; 45:3-16. [PMID: 11489661 DOI: 10.1016/s1056-8719(01)00126-5] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
Abstract
The description of a new family of recombinant proteins, which are constructed by the covalent fusion of the cDNA encoding a G protein-coupled receptor with that of a G(alpha) protein subunit, has recently been introduced as an original strategy to explore receptor pharmacology under defined experimental conditions. As such, a controlled 1:1 stoichiometry of receptor and G(alpha) protein expression can be achieved, as well as a forced spatial proximity to each other. Fusion proteins have been revealed as active at the receptor ligand binding level and functional at the G(alpha) protein and effector level. Insights on analytical pharmacological data are discussed for wild-type and mutant receptors interacting with a given G(alpha) protein subunit and different subtypes of either wild-type or mutant G(alpha) proteins activated by a single receptor subtype. A possible alteration of the receptor:G(alpha) protein selectivity may occur due either to the spatial proximity of both protein partners or to a constraint receptor state unable to accommodate to different G(alpha) protein states. Coactivation of endogenous G(alpha) proteins in host cells expressing a fusion protein has also been observed, but depends mainly on the coupling efficiency of the receptor and G(alpha) protein engaged in the fusion process. The ligand's apparent intrinsic activity has been shown to be either enhanced, attenuated, or unmodified when the functional responses of a fusion protein are compared to the coexpression of both fusion protein partners.
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Affiliation(s)
- T Wurch
- Department of Cellular and Molecular Biology, Centre de Recherche Pierre Fabre, 17, Avenue Jean Moulin, 81106 Castres Cédex, France
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32
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Abstract
Fourteen different receptor subtypes might be regarded as a diversity that is sufficient to accommodate the wide-ranging physiological roles of 5-hydroxytryptamine (5-HT). However, it is becoming clear that, for 5-HT as for other neurotransmitters, the concept of a receptor as a gatekeeper for a specific cellular process or event is too restrictive. Multiple receptor-mediated biochemical cascades can be activated in cells in response to an agonist by a number of mechanisms. Whereas it is well established that different agonists do not necessarily elicit the same magnitude of response, they probably also select between various possible signal transduction pathways. Receptor signalling may be diverse via a single receptor subtype as a consequence of specific agonist-receptor-G protein interactions. 5-HT receptors are even more heterogeneous when one considers that the amino acid sequence of these receptor subtypes may vary from individual to individual, and that there is an increasing number of receptor isoforms due to alternative splicing and RNA editing of 5-HT receptor transcripts. Activation, in particular constitutive, agonist-independent activation, of some of these receptor isoforms has been reported to be altered. This implies that ligands with similar binding affinities may display different pharmacological properties (partial agonist, antagonist, or inverse agonist) versus these receptor isoforms, depending on their activation state. Therefore, intervention with receptor ligands to modify hampered neurotransmission pathways is a difficult task, and one needs to consider the growing evidence of diversity in G protein-coupled receptor signalling.
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Affiliation(s)
- P J Pauwels
- Centre de Recherche Pierre Fabre, Department of Cellular and Molecular Biology, 81106, Castres Cédex, France.
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33
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Stanasila L, Lim WK, Neubig RR, Pattus F. Coupling efficacy and selectivity of the human mu-opioid receptor expressed as receptor-Galpha fusion proteins in Escherichia coli. J Neurochem 2000; 75:1190-9. [PMID: 10936202 DOI: 10.1046/j.1471-4159.2000.0751190.x] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Two constructs encoding the human micro-opioid receptor (hMOR) fused at its C terminus to either one of two Galpha subunits, Galpha(o1) (hMOR-Galpha(o1)) and Galpha(i2) (hMOR-Galpha(i2)), were expressed in Escherichia coli at levels suitable for pharmacological studies (0.4-0.5 pmol/mg). Receptors fused to Galpha(o1) or to Galpha(i2) maintained high-affinity binding of the antagonist diprenorphine. Affinities of the micro-selective agonists morphine, [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]enkephalin (DAMGO), and endomorphins as well as their potencies and intrinsic activities in stimulating guanosine 5'-O-(3-[(35)S]thiotriphosphate) ([(35)S]GTPgammaS) binding were assessed in the presence of added purified Gbetagamma subunits. Both fusion proteins displayed high-affinity agonist binding and agonist-stimulated [(35)S]GTPgammaS binding. In the presence of Gbetagamma dimers, the affinities of DAMGO and endomorphin-1 and -2 were higher at hMOR-Galpha(i2) than at hMOR-Galpha(o1), whereas morphine displayed similar affinities at the two chimeras. Potencies of the four agonists in stimulating [(35)S]GTPgammaS binding at hMOR-Galpha(o1) were similar, whereas at hMOR-Galpha(i2), endomorphin-1 and morphine were more potent than DAMGO and endomorphin-2. The intrinsic activities of the four agonists at the two fusion constructs were similar. The results confirm hMOR coupling to Galpha(o1) and Galpha(i2) and support the hypothesis of the existence of multiple receptor conformational states, depending on the nature of the G protein to which it is coupled.
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Affiliation(s)
- L Stanasila
- Département des Récepteurs et Protéines Membranaires, UPR 9050 CNRS, ESBS, Illkirch, France.
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34
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Cavalli A, Druey KM, Milligan G. The regulator of G protein signaling RGS4 selectively enhances alpha 2A-adreoreceptor stimulation of the GTPase activity of Go1alpha and Gi2alpha. J Biol Chem 2000; 275:23693-9. [PMID: 10807934 DOI: 10.1074/jbc.m910395199] [Citation(s) in RCA: 56] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Agonist-stimulated high affinity GTPase activity of fusion proteins between the alpha(2A)-adrenoreceptor and the alpha subunits of forms of the G proteins G(i1), G(i2), G(i3), and G(o1), modified to render them insensitive to the action of pertussis toxin, was measured following transient expression in COS-7 cells. Addition of a recombinant regulator of G protein signaling protein, RGS4, did not significantly affect basal GTPase activity nor agonist stimulation of the fusion proteins containing Galpha(i1) and Galpha(i3) but markedly enhanced agonist-stimulation of the proteins containing Galpha(i2) and Galpha(o1.) The effect of RGS4 on the alpha(2A)-adrenoreceptor-Galpha(o1) fusion protein was concentration-dependent with EC(50) of 30 +/- 3 nm and the potency of the receptor agonist UK14304 was reduced 3-fold by 100 nm RGS4. Equivalent reconstitution with Asn(88)-Ser RGS4 failed to enhance agonist function on the alpha(2A)-adrenoreceptor-Galpha(o1) or alpha(2A)-adrenoreceptor-Galpha(i2) fusion proteins. Enzyme kinetic analysis of the GTPase activity of the alpha(2A)-adrenoreceptor-Galpha(o1) and alpha(2A)-adrenoreceptor-Galpha(i2) fusion proteins demonstrated that RGS4 both substantially increased GTPase V(max) and significantly increased K(m) of the fusion proteins for GTP. The increase in K(m) for GTP was dependent upon RGS4 amount and is consistent with previously proposed mechanisms of RGS function. Agonist-stimulated GTPase turnover number in the presence of 100 nm RGS4 was substantially higher for alpha(2A)-adrenoreceptor-Galpha(o1) than for alpha(2A)-adrenoreceptor-Galpha(i2). These studies demonstrate that although RGS4 has been described as a generic stimulator of the GTPase activity of G(i)-family G proteins, selectivity of this interaction and quantitative variation in its function can be monitored in the presence of receptor activation of the G proteins.
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Affiliation(s)
- A Cavalli
- Molecular Pharmacology Group, Division of Biochemistry and Molecular Biology, Institute of Biomedical and Life Sciences, University of Glasgow, Glasgow G12 8QQ, Scotland, United Kingdom
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35
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Unson CG, Wu CR, Sakmar TP, Merrifield RB. Selective stabilization of the high affinity binding conformation of glucagon receptor by the long splice variant of Galpha(s). J Biol Chem 2000; 275:21631-8. [PMID: 10791965 DOI: 10.1074/jbc.m002093200] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
To analyze functional differences in the interactions of the glucagon receptor (GR) with the two predominant splice variants of Galpha(s), GR was covalently linked to the short and the long forms Galpha(s)-S and Galpha(s)-L to produce the fusion proteins GR-Galpha(s)-S and GR-Galpha(s)-L. GR-Galpha(s)-S bound glucagon with an affinity similar to that of GR, while GR-Galpha(s)-L showed a 10-fold higher affinity for glucagon. In the presence of GTPgammaS, GR-Galpha(s)-L reverted to the low affinity glucagon binding conformation. Both GR-Galpha(s)-L and GR-Galpha(s)-S were constitutively active, causing elevated basal levels of cAMP even in the absence of glucagon. A mutant GR that failed to activate G(s) (G23D1R) was fused to Galpha(s)-L. G23D1R-Galpha(s)-L bound glucagon with high affinity, but failed to elevate cAMP levels, suggesting that the mechanisms of GR-mediated Galpha(s)-L activation and Galpha(s)-L-induced high affinity glucagon binding are independent. Both GR-Galpha(s)-S and GR-Galpha(s)-L bound the antagonist desHis(1)[Nle(9),Ala(11),Ala(16)]glucagon amide with affinities similar to GR. The antagonist displayed partial agonist activity with GR-Galpha(s)-L, but not with GR-Galpha(s)-S. Therefore, the partial agonist activity of the antagonist observed in intact cells appears to be due to GRs coupled to Galpha(s)-L. We conclude that Galpha(s)-S and Galpha(s)-L interact differently with GR and that specific coupling of GR to Galpha(s)-L may account for GTP-sensitive high affinity glucagon binding.
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Affiliation(s)
- C G Unson
- Rockefeller University and the Howard Hughes Medical Institute, Laboratory of Molecular Biology and Biochemistry, Rockefeller University, New York, New York 10021, USA.
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36
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Rauly I, Ailhaud M, Wurch T, Pauwels PJ. alpha(2A)-adrenoceptor: G(alphai1) protein-mediated pertussis toxin-resistant attenuation of G(s) coupling to the cyclic AMP pathway. Biochem Pharmacol 2000; 59:1531-8. [PMID: 10799649 DOI: 10.1016/s0006-2952(00)00276-8] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/16/2022]
Abstract
Fusion proteins were constructed between a recombinant human alpha(2A)-adrenoceptor and either a rat wild-type G(alphai1) or putative pertussis toxin-resistant form of the G(alphai1) protein (G(alphai1)Cys(351)Gly). [(3)H]2-[2-(2-Methoxy-1, 4-benzodioxanyl)]imidazoline hydrochloride (RX 821002) saturation binding experiments demonstrated that both fusion proteins were expressed at a similar level as the alpha(2A)-adrenoceptor co-expressed with either a wild-type G(alphai1) or mutant G(alphai1)Cys(351)Gly protein in COS-7 cells, and displayed a ligand binding profile similar to that for the alpha(2A)-adrenoceptor protein. In alpha(2A)-adrenoceptor-transfected COS-7 cells, 5-bromo-6-(2-imidazolin-2-yl-amino) quinoxaline tartrate (brimonidine, 10 microM) induced stimulation (151 +/- 28%) of adenosine 3',5'-cyclic monophosphate (cAMP) formation which was prevented by cholera toxin treatment, demonstrating a direct coupling of the alpha(2A)-adrenoceptor to an endogenous G(alphas) protein in COS-7 cells. Expression of either the wild-type G(alphai1) or mutant G(alphai1)Cys(351)Gly protein in co-expression or fusion with the alpha(2A)-adrenoceptor in COS-7 cells suppressed the brimonidine-induced stimulation of cAMP formation, both in the presence and absence of pertussis toxin pretreatment. Hence, the G(alphai1) protein apparently blocks the G(s)-coupled alpha(2A)-adrenoceptor-mediated pathway in a pertussis toxin-non-sensitive way.
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Affiliation(s)
- I Rauly
- Department of Cellular and Molecular Biology, Centre de Recherche Pierre Fabre, 81106, Castres, France.
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37
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Vorobiov D, Bera AK, Keren-Raifman T, Barzilai R, Dascal N. Coupling of the muscarinic m2 receptor to G protein-activated K(+) channels via Galpha(z) and a receptor-Galpha(z) fusion protein. Fusion between the receptor and Galpha(z) eliminates catalytic (collision) coupling. J Biol Chem 2000; 275:4166-70. [PMID: 10660578 DOI: 10.1074/jbc.275.6.4166] [Citation(s) in RCA: 46] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023] Open
Abstract
G protein-activated K(+) channel (GIRK), which is activated by the G(betagamma) subunit of heterotrimeric G proteins, and muscarinic m2 receptor (m2R) were coexpressed in Xenopus oocytes. Acetylcholine evoked a K(+) current, I(ACh), via the endogenous pertussis toxin (PTX)-sensitive G(i/o) proteins. Activation of I(ACh) was accelerated by increasing the expression of m2R, suggesting a collision coupling mechanism in which one receptor catalytically activates several G proteins. Coexpression of the alpha subunit of the PTX-insensitive G protein G(z), Galpha(z), induced a slowly activating PTX-insensitive I(ACh), whose activation kinetics were also compatible with the collision coupling mechanism. When GIRK was coexpressed with an m2R x Galpha(z) fusion protein (tandem), in which the C terminus of m2R was tethered to the N terminus of Galpha(z), part of I(ACh) was still eliminated by PTX. Thus, the m2R of the tandem activates the tethered Galpha(z) but also the nontethered G(i/o) proteins. After PTX treatment, the speed of activation of the m2R x Galpha(z)-mediated response did not depend on the expression level of m2R x Galpha(z) and was faster than when m2R and Galpha(z) were coexpressed as separate proteins. These results demonstrate that fusing the receptor and the Galpha strengthens their coupling, support the collision-coupling mechanism between m2R and the G proteins, and suggest a noncatalytic (stoichiometric) coupling between the G protein and GIRK in this model system.
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Affiliation(s)
- D Vorobiov
- Department of Physiology and Pharmacology, Sackler School of Medicine, Tel Aviv University, Ramat Aviv 69978, Israel
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38
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Abstract
Production of chimeric DNAs in which the 5' end of G-protein alpha-subunits are linked directly to the 3' tail of a G-protein-coupled receptor has recently offered an unusual strategy to explore the detailed pharmacology of receptor-G-protein interactions. Expression of such fusion proteins ensures a 1:1 stoichiometry of receptor and G-protein expression and their proximity to each other. The capacity of such fusion proteins to be regarded as agonist-activated GTPases that allow simple enzyme kinetics to be applied to issues of ligand efficacy will be considered. In addition, the effects of point mutations, in both receptors and G proteins, on ligand function are particularly amenable to the types of robust quantitative analyses that can be produced using such fusion proteins.
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Affiliation(s)
- G Milligan
- Molecular Pharmacology Group, Division of Biochemistry and Molecular Biology, Institute of Biomedical and Life Sciences, University of Glasgow, Glasgow, UK G12 8QQ.
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39
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Wang Y, Windh RT, Chen CA, Manning DR. N-Myristoylation and betagamma play roles beyond anchorage in the palmitoylation of the G protein alpha(o) subunit. J Biol Chem 1999; 274:37435-42. [PMID: 10601316 DOI: 10.1074/jbc.274.52.37435] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Many of the alpha subunits of heterotrimeric GTP-binding regulatory proteins (G proteins) are palmitoylated, a modification proposed to play a key role in the stable anchorage of the subunits to the plasma membrane. Palmitoylation of alpha subunits from the G(i) family is preceded by N-myristoylation, which alone or together with betagamma probably supports a reversible interaction of the alpha subunit with membrane as a prerequisite to the eventual incorporation of palmitate. Previous studies have not addressed, however, the question of whether membrane association alone, carried out through N-myristoylation, interaction with betagamma, or other events, is sufficient for palmitoylation. We report here for alpha(o) that it is not. We found that N-myristoylation is required for palmitoylation at least in part because it supports events subsequent to membrane attachment. Mutants of alpha(o) designed to target the subunit to membrane without an N-myristoyl group are unable to be palmitoylated as evaluated by incorporation of [(3)H]palmitate. Mutants of alpha(o) unable to interact normally with betagamma yet still attach to membrane demonstrate that betagamma, in contrast, is not required for palmitoylation. betagamma becomes necessary, however, when the N-myristoyl group is absent. Our results suggest that N-myristoylation and betagamma, while almost certainly relevant to the reversible interaction of alpha(o) with membrane, also play at least partly overlapping, post-anchorage roles in palmitoylation.
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Affiliation(s)
- Y Wang
- Department of Pharmacology, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104-6084, USA
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40
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Ward RJ, Milligan G. An Asp79Asn mutation of the alpha2A-adrenoceptor interferes equally with agonist activation of individual Gialpha-family G protein subtypes. FEBS Lett 1999; 462:459-63. [PMID: 10622745 DOI: 10.1016/s0014-5793(99)01581-1] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
The quantitative effects of an Asp79Asn mutation in the porcine alpha2A-adrenoceptor on adrenaline-mediated stimulation of the alpha subunit of individual members of the Gi family of G proteins were assessed by measuring GTP turnover number for fusion proteins between the wild type or mutated receptor and pertussis toxin-resistant forms of each of Gi1, Gi2 and Gi3. In each case the receptor mutation limited activation of the G protein to 8-14% of that produced by the wild type receptor. Previous demonstration that in a single cell this mutation selectively interferes with alpha2A-adrenoceptor regulation of distinct effector end points transduced by Gi family members must therefore reflect differential requirements for amplification or the cellular location of individual, co-expressed, G proteins.
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Affiliation(s)
- R J Ward
- Division of Biochemistry and Molecular Biology, Institute of Biomedical and Life Sciences, University of Glasgow, UK
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41
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Abstract
Agonist-bound G-protein-coupled receptors (GPCRs) facilitate GDP-GTP exchange on their cognate G proteins. The binding properties of GPCRs are adequately described by the ternary complex model. However, in this article a more realistic (steady-state) model, which is necessary to describe the catalytic effect of agonist-bound receptors on G-protein activation, will be discussed. This model predicts that agonist potency and efficacy might vary from tissue to tissue, depending on the G-protein concentration and can be extended to explain why an agonist's ability to increase the receptor's affinity for empty G proteins (in the absence of GTP) is related to the agonist's efficacy.
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Affiliation(s)
- I Waelbroeck
- Laboratoire de Chimie Biologique et de la Nutrition, Faculté de Médecine de l'Université Libre de Bruxelles, Bât. G/E, CP 611, 808 Route de Lennik, B - 1070 Brussels, Belgium.
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42
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Bevan N, Palmer T, Drmota T, Wise A, Coote J, Milligan G, Rees S. Functional analysis of a human A(1) adenosine receptor/green fluorescent protein/G(i1)alpha fusion protein following stable expression in CHO cells. FEBS Lett 1999; 462:61-5. [PMID: 10580092 DOI: 10.1016/s0014-5793(99)01467-2] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
Abstract
Fusion proteins between the human A(1) adenosine receptor and the pertussis toxin resistant (Cys351Gly) mutant of the G-protein alpha subunit G(i1)alpha (A1/Gi), and between the human A(1) adenosine receptor, the Aequorea victoria green fluorescent protein (GFP) and Cys351Gly G(i1)alpha (A1/GFP/Gi), were expressed in CHO cells. The agonist NECA caused a stimulation of [(35)S]GTPgammaS binding at both fusion proteins with similar concentration dependence as at the native receptor. However in the presence of pertussis toxin NECA stimulation of [(35)S]GTPgammaS binding was only seen at the A1/GFP/Gi fusion protein. The regulation of the adenylyl cyclase and MAP kinase effector systems by both fusion proteins was attenuated following pertussis toxin treatment. These studies demonstrate for the first time the characterisation of a fusion protein between a G-protein coupled receptor, GFP and a G-protein alpha subunit.
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Affiliation(s)
- N Bevan
- Biological Chemistry, Glaxo Wellcome Research and Development, Gunnels Wood Road, Stevenage, UK.
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43
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Wenzel-Seifert K, Arthur JM, Liu HY, Seifert R. Quantitative analysis of formyl peptide receptor coupling to g(i)alpha(1), g(i)alpha(2), and g(i)alpha(3). J Biol Chem 1999; 274:33259-66. [PMID: 10559200 DOI: 10.1074/jbc.274.47.33259] [Citation(s) in RCA: 72] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The human formyl peptide receptor (FPR) is a prototypical G(i) protein-coupled receptor, but little is known about quantitative aspects of FPR-G(i) protein coupling. To address this issue, we fused the FPR to G(i)alpha(1), G(i)alpha(2), and G(i)alpha(3) and expressed the fusion proteins in Sf9 insect cells. Fusion of a receptor to Galpha ensures a defined 1:1 stoichiometry of the signaling partners. By analyzing high affinity agonist binding, the kinetics of agonist- and inverse agonist-regulated guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) binding and GTP hydrolysis and photolabeling of Galpha, we demonstrate highly efficient coupling of the FPR to fused G(i)alpha(1), G(i)alpha(2), and G(i)alpha(3) without cross-talk of the receptor to insect cell G proteins. The FPR displayed high constitutive activity when coupled to all three G(i)alpha isoforms. The K(d) values of high affinity agonist binding were approximately 100-fold lower than the EC(50) (concentration that gives half-maximal stimulation) values of agonist for GTPase activation. Based on the B(max) values of agonist saturation binding and ligand-regulated GTPgammaS binding, it was previously proposed that the FPR activates G proteins catalytically, i.e. one FPR activates several G(i) proteins. Analysis of agonist saturation binding, ligand-regulated GTPgammaS saturation binding and quantitative immunoblotting with membranes expressing FPR-G(i)alpha fusion proteins and nonfused FPR now reveals that FPR agonist binding greatly underestimates the actual FPR expression level. Our data show the following: (i) the FPR couples to G(i)alpha(1), G(i)alpha(2), and G(i)alpha(3) with similar efficiency; (ii) the FPR can exist in a state of low agonist affinity that couples efficiently to G proteins; and (iii) in contrast to the previously held view, the FPR appears to activate G(i) proteins linearly and not catalytically.
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Affiliation(s)
- K Wenzel-Seifert
- Higuchi Biosciences Center, The University of Kansas, Lawrence, Kansas 66045-2505, USA.
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44
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Abstract
The G-protein coupled receptors form a large and diverse multi-gene superfamily with many important physiological functions. As such, they have become important targets in pharmaceutical research. Molecular modelling and site-directed mutagenesis have played an important role in our increasing understanding of the structural basis of drug action at these receptors. Aspects of this understanding, how these techniques can be used within a drug-design programme, and remaining challenges for the future are reviewed.
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MESH Headings
- Binding Sites
- Combinatorial Chemistry Techniques
- Drug Design
- GTP-Binding Proteins/chemistry
- Ligands
- Models, Molecular
- Molecular Structure
- Mutagenesis, Site-Directed
- Receptor, Angiotensin, Type 1
- Receptor, Angiotensin, Type 2
- Receptors, Adrenergic, beta-2/chemistry
- Receptors, Angiotensin/chemistry
- Receptors, Cell Surface/chemistry
- Receptors, Cell Surface/classification
- Receptors, Cell Surface/genetics
- Receptors, G-Protein-Coupled
- Saccharomyces cerevisiae Proteins
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Affiliation(s)
- D R Flower
- Department of Physical Sciences, ASTRA Charnwood, Bakewell Rd, Loughborough, Leicestershire, UK.
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45
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Waldhoer M, Wise A, Milligan G, Freissmuth M, Nanoff C. Kinetics of ternary complex formation with fusion proteins composed of the A(1)-adenosine receptor and G protein alpha-subunits. J Biol Chem 1999; 274:30571-9. [PMID: 10521440 DOI: 10.1074/jbc.274.43.30571] [Citation(s) in RCA: 41] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
High affinity agonist binding to G protein-coupled receptors depends on the formation of a ternary complex between agonist, receptor, and G protein. This process is too slow to be accounted for by a simple diffusion-controlled mechanism. We have tested if the interaction between activated receptor and G protein is rate-limiting by fusing the coding sequence of the human A(1)-adenosine receptor to that of Galpha(i-1) (A(1)/Galpha(i-1)) and of Galpha(o) (A(1)/Galpha(o)). Fusion proteins of the expected molecular mass were detected following transfection of HEK293 cells. Ternary complex formation was monitored by determining the kinetics for binding of the high affinity agonist (-)-N(6)-3[(125)I](iodo-4-hydroxyphenylisopropyl)adenosine; these were similar in the wild-type receptor and the fusion proteins over the temperature range of 10 to 30 degrees C. Agonist dissociation may be limited by the stability of the ternary complex. This assumption was tested by creating fusion proteins in which the Cys(351) of Galpha(i-1) was replaced with glycine (A(1)/Galpha(i-1)C351G) or isoleucine (A(1)/Galpha(i-1)C351I) to lower the affinity of the receptor for the G protein. In these mutated fusion proteins, the dissociation rate of the ternary complex was accelerated; in contrast, the rate of the forward reaction was not affected. We therefore conclude that (i) receptor activation per se rather than its interaction with the G protein is rate-limiting in ternary complex formation; (ii) the stability of the ternary complex is determined by the dissociation rate of the G protein. These features provide for a kinetic proofreading mechanism that sustains the fidelity of receptor-G protein coupling.
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Affiliation(s)
- M Waldhoer
- Institute of Pharmacology, University of Vienna, Währinger Strasse 13a, A-1090 Vienna, Austria
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46
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Seifert R, Wenzel-Seifert K, Kobilka BK. GPCR-Galpha fusion proteins: molecular analysis of receptor-G-protein coupling. Trends Pharmacol Sci 1999; 20:383-9. [PMID: 10462762 DOI: 10.1016/s0165-6147(99)01368-1] [Citation(s) in RCA: 100] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Abstract
The efficiency of interactions between G-protein-coupled receptors (GPCRs) and heterotrimeric guanine nucleotide-binding proteins (G proteins) is greatly influenced by the absolute and relative densities of these proteins in the plasma membrane. The study of these interactions has been facilitated by the use of GPCR-Galpha fusion proteins, which are formed by the fusion of GPCR to Galpha. These fusion proteins ensure a defined 1:1 stoichiometry of GPCR to Galpha and force the physical proximity of the signalling partners. Thus, fusion of GPCR to Galpha enhances coupling efficiency can be used to study aspects of receptor-G-protein coupling that could not otherwise be examined by co-expressing GPCRs and G proteins as separate proteins. The results of studies that have made use of GPCR-Galpha fusion proteins will be discussed in this article, along with the strengths and limitations of this approach.
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Affiliation(s)
- R Seifert
- Department of Pharmacology and Toxicology, The University of Kansas, 5001 Malott Hall, Lawrence, KS 66045, USA.
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47
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Seifert R, Gether U, Wenzel-Seifert K, Kobilka BK. Effects of guanine, inosine, and xanthine nucleotides on beta(2)-adrenergic receptor/G(s) interactions: evidence for multiple receptor conformations. Mol Pharmacol 1999; 56:348-58. [PMID: 10419554 DOI: 10.1124/mol.56.2.348] [Citation(s) in RCA: 93] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023] Open
Abstract
The aim of our study was to examine the effects of different purine nucleotides [GTP, ITP, and xanthosine 5'-triphosphate (XTP)] on receptor/G protein coupling. As a model system, we used a fusion protein of the beta(2)-adrenergic receptor and the alpha subunit of the G protein G(s). GTP was more potent and efficient than ITP and XTP at inhibiting ternary complex formation and supporting adenylyl cyclase (AC) activation. We also studied the effects of several beta(2)-adrenergic receptor ligands on nucleotide hydrolysis and on AC activity in the presence of GTP, ITP, and XTP. The efficacy of agonists at promoting GTP hydrolysis correlated well with the efficacy of agonists for stimulating AC in the presence of GTP. This was, however, not the case for ITP hydrolysis and AC activity in the presence of ITP. The efficacy of ligands at stimulating AC in the presence of XTP differed considerably from the efficacies of ligands in the presence of GTP and ITP, and there was no evidence for receptor-regulated XTP hydrolysis. Our findings support the concept of multiple ligand-specific receptor conformations and demonstrate the usefulness of purine nucleotides as tools to study conformational states of receptors.
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MESH Headings
- Adenosine Triphosphate/pharmacology
- Adenylyl Cyclases/metabolism
- Adrenergic beta-Agonists/pharmacology
- Adrenergic beta-Antagonists/pharmacology
- Animals
- Cell Membrane/drug effects
- Cell Membrane/metabolism
- Cells, Cultured
- GTP Phosphohydrolases/metabolism
- GTP-Binding Protein alpha Subunits, Gs/chemistry
- GTP-Binding Protein alpha Subunits, Gs/genetics
- GTP-Binding Protein alpha Subunits, Gs/metabolism
- Guanosine Triphosphate/pharmacology
- Hydrolysis
- Inosine Triphosphate/pharmacology
- Insecta
- Isoproterenol/pharmacology
- Kinetics
- Ligands
- Propanolamines/pharmacology
- Protein Binding/drug effects
- Protein Conformation
- Purine Nucleotides/pharmacology
- Pyrophosphatases/metabolism
- Receptors, Adrenergic, beta-2/chemistry
- Receptors, Adrenergic, beta-2/genetics
- Receptors, Adrenergic, beta-2/metabolism
- Recombinant Fusion Proteins/metabolism
- Ribonucleotides/pharmacology
- Inosine Triphosphatase
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Affiliation(s)
- R Seifert
- Howard Hughes Medical Institute, Stanford University Medical School, Stanford, California, USA
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48
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Jackson VN, Bahia DS, Milligan G. Modulation of relative intrinsic activity of agonists at the alpha-2A adrenoceptor by mutation of residue 351 of G protein gi1alpha. Mol Pharmacol 1999; 55:195-201. [PMID: 9927608 DOI: 10.1124/mol.55.2.195] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Compared with epinephrine, the relative intrinsic activity of a series of partial agonists to activate fusion proteins between the porcine alpha-2A adrenoceptor and the alpha-subunit of Gi1 was reduced after a single-point mutation (Cys351Gly) in the G protein. Although UK14304 was close to a full agonist at the fusion construct containing wild-type (Cys351)Gi1alpha, it was a partial agonist at that containing Gly351Gi1alpha. Moreover, although clonidine functioned as a good partial agonist to activate the fusion protein containing Cys351Gi1alpha, it was essentially an antagonist at the Gly351Gi1alpha-containing fusion protein. By contrast, incorporation of Ile351Gi1alpha into the fusion protein resulted in all partial agonists displaying higher intrinsic activity relative to epinephrine to activate this fusion protein than the one containing the wild-type G protein sequence. This is the first demonstration that the relative intrinsic activity of a series of agonists can be modified by a point mutation in a G protein rather than a receptor and indicates that the nature of a key contact site between a G protein and a receptor can selectively regulate partial agonist function. We provide a model for this based on the hydrophobicity of a key receptor-G protein alpha-subunit interaction interface.
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Affiliation(s)
- V N Jackson
- Molecular Pharmacology Group, Division of Biochemistry and Molecular Biology, Institute of Biomedical and Life Sciences, University of Glasgow, Glasgow, Scotland, United Kingdom
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Colquhoun D. Binding, gating, affinity and efficacy: the interpretation of structure-activity relationships for agonists and of the effects of mutating receptors. Br J Pharmacol 1998; 125:924-47. [PMID: 9846630 PMCID: PMC1565672 DOI: 10.1038/sj.bjp.0702164] [Citation(s) in RCA: 727] [Impact Index Per Article: 28.0] [Reference Citation Analysis] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/13/2023] Open
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50
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Sautel M, Milligan G. Loss of activation of Gs but not Gi following expression of an alpha2A-adrenoceptor-Gi1alpha fusion protein. FEBS Lett 1998; 436:46-50. [PMID: 9771891 DOI: 10.1016/s0014-5793(98)01094-1] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/09/2023]
Abstract
Both the porcine alpha2A-adrenoceptor and a fusion protein between this receptor and a pertussis toxin-resistant form of Gila were stably expressed in Rat-1 fibroblasts. The agonist UK14304 mediated a biphasic regulation of adenylyl cyclase activity via the isolated receptor with inhibition of the enzyme activity at low concentrations of the compound which was subsequently reversed at higher concentrations. By contrast, stimulation of the fusion protein with this agonist could only produce inhibition of enzyme activity. This inhibition was produced by activation of endogenous Gi rather than the fused alpha subunit of Gi1, as pertussis toxin treatment obliterated inhibitory regulation of adenylyl cyclase via the fusion construct. Pertussis toxin treatment potentiated stimulation of adenylyl cyclase via the isolated receptor but such treatment was unable to uncover capacity of the fusion protein to produce such an effect.
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MESH Headings
- Adenylate Cyclase Toxin
- Adenylyl Cyclases/drug effects
- Adenylyl Cyclases/metabolism
- Adrenergic alpha-Agonists/pharmacology
- Animals
- Binding Sites
- Brimonidine Tartrate
- Cholera Toxin/pharmacology
- Colforsin/pharmacology
- Dose-Response Relationship, Drug
- GTP-Binding Protein alpha Subunits, Gi-Go/genetics
- GTP-Binding Protein alpha Subunits, Gi-Go/metabolism
- GTP-Binding Protein alpha Subunits, Gs/genetics
- GTP-Binding Protein alpha Subunits, Gs/metabolism
- Oxymetazoline/pharmacology
- Pertussis Toxin
- Quinoxalines/pharmacology
- Rats
- Receptors, Adrenergic, alpha-2/drug effects
- Receptors, Adrenergic, alpha-2/genetics
- Receptors, Adrenergic, alpha-2/metabolism
- Recombinant Fusion Proteins/drug effects
- Recombinant Fusion Proteins/genetics
- Recombinant Fusion Proteins/metabolism
- Virulence Factors, Bordetella/pharmacology
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Affiliation(s)
- M Sautel
- Division of Biochemistry and Molecular Biology, Institute of Biomedical and Life Sciences, University of Glasgow, UK
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