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Bondar A, Lazar J. Optical sensors of heterotrimeric G protein signaling. FEBS J 2020; 288:2570-2584. [DOI: 10.1111/febs.15655] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/20/2020] [Revised: 11/26/2020] [Accepted: 12/03/2020] [Indexed: 01/14/2023]
Affiliation(s)
- Alexey Bondar
- Center for Nanobiology and Structural Biology Institute of Microbiology of the Czech Academy of Sciences Nove Hrady Czech Republic
- Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences Prague Czech Republic
- Faculty of Science University of South Bohemia Ceske Budejovice Czech Republic
| | - Josef Lazar
- Center for Nanobiology and Structural Biology Institute of Microbiology of the Czech Academy of Sciences Nove Hrady Czech Republic
- Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences Prague Czech Republic
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Bondar A, Lazar J. The G protein G i1 exhibits basal coupling but not preassembly with G protein-coupled receptors. J Biol Chem 2017; 292:9690-9698. [PMID: 28438833 DOI: 10.1074/jbc.m116.768127] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/21/2016] [Revised: 04/21/2017] [Indexed: 11/06/2022] Open
Abstract
The Gi/o protein family transduces signals from a diverse group of G protein-coupled receptors (GPCRs). The observed specificity of Gi/o-GPCR coupling and the high rate of Gi/o signal transduction have been hypothesized to be enabled by the existence of stable associates between Gi/o proteins and their cognate GPCRs in the inactive state (Gi/o-GPCR preassembly). To test this hypothesis, we applied the recently developed technique of two-photon polarization microscopy (2PPM) to Gαi1 subunits labeled with fluorescent proteins and four GPCRs: the α2A-adrenergic receptor, GABAB, cannabinoid receptor type 1 (CB1R), and dopamine receptor type 2. Our experiments with non-dissociating mutants of fluorescently labeled Gαi1 subunits (exhibiting impaired dissociation from activated GPCRs) showed that 2PPM is capable of detecting GPCR-G protein interactions. 2PPM experiments with non-mutated fluorescently labeled Gαi1 subunits and α2A-adrenergic receptor, GABAB, or dopamine receptor type 2 receptors did not reveal any interaction between the Gi1 protein and the non-stimulated GPCRs. In contrast, non-stimulated CB1R exhibited an interaction with the Gi1 protein. Further experiments revealed that this interaction is caused solely by CB1R basal activity; no preassembly between CB1R and the Gi1 protein could be observed. Our results demonstrate that four diverse GPCRs do not preassemble with non-active Gi1 However, we also show that basal GPCR activity allows interactions between non-stimulated GPCRs and Gi1 (basal coupling). These findings suggest that Gi1 interacts only with active GPCRs and that the well known high speed of GPCR signal transduction does not require preassembly between G proteins and GPCRs.
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Affiliation(s)
- Alexey Bondar
- From the Center for Nanobiology and Structural Biology, Institute of Microbiology, Academy of Sciences of the Czech Republic, 37333 Nove Hrady,
| | - Josef Lazar
- From the Center for Nanobiology and Structural Biology, Institute of Microbiology, Academy of Sciences of the Czech Republic, 37333 Nove Hrady.,the Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, 16610 Prague, and.,the Faculty of Science, University of South Bohemia, 37005 Ceske Budejovice, Czech Republic
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Yakubovich D, Berlin S, Kahanovitch U, Rubinstein M, Farhy-Tselnicker I, Styr B, Keren-Raifman T, Dessauer CW, Dascal N. A Quantitative Model of the GIRK1/2 Channel Reveals That Its Basal and Evoked Activities Are Controlled by Unequal Stoichiometry of Gα and Gβγ. PLoS Comput Biol 2015; 11:e1004598. [PMID: 26544551 PMCID: PMC4636287 DOI: 10.1371/journal.pcbi.1004598] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/27/2015] [Accepted: 10/13/2015] [Indexed: 12/02/2022] Open
Abstract
G protein-gated K+ channels (GIRK; Kir3), activated by Gβγ subunits derived from Gi/o proteins, regulate heartbeat and neuronal excitability and plasticity. Both neurotransmitter-evoked (Ievoked) and neurotransmitter-independent basal (Ibasal) GIRK activities are physiologically important, but mechanisms of Ibasal and its relation to Ievoked are unclear. We have previously shown for heterologously expressed neuronal GIRK1/2, and now show for native GIRK in hippocampal neurons, that Ibasal and Ievoked are interrelated: the extent of activation by neurotransmitter (activation index, Ra) is inversely related to Ibasal. To unveil the underlying mechanisms, we have developed a quantitative model of GIRK1/2 function. We characterized single-channel and macroscopic GIRK1/2 currents, and surface densities of GIRK1/2 and Gβγ expressed in Xenopus oocytes. Based on experimental results, we constructed a mathematical model of GIRK1/2 activity under steady-state conditions before and after activation by neurotransmitter. Our model accurately recapitulates Ibasal and Ievoked in Xenopus oocytes, HEK293 cells and hippocampal neurons; correctly predicts the dose-dependent activation of GIRK1/2 by coexpressed Gβγ and fully accounts for the inverse Ibasal-Ra correlation. Modeling indicates that, under all conditions and at different channel expression levels, between 3 and 4 Gβγ dimers are available for each GIRK1/2 channel. In contrast, available Gαi/o decreases from ~2 to less than one Gα per channel as GIRK1/2's density increases. The persistent Gβγ/channel (but not Gα/channel) ratio support a strong association of GIRK1/2 with Gβγ, consistent with recruitment to the cell surface of Gβγ, but not Gα, by GIRK1/2. Our analysis suggests a maximal stoichiometry of 4 Gβγ but only 2 Gαi/o per one GIRK1/2 channel. The unique, unequal association of GIRK1/2 with G protein subunits, and the cooperative nature of GIRK gating by Gβγ, underlie the complex pattern of basal and agonist-evoked activities and allow GIRK1/2 to act as a sensitive bidirectional detector of both Gβγ and Gα. Many neurotransmitters and hormones inhibit the electric activity of excitable cells (such as cardiac cells and neurons) by activating a K+ channel, GIRK (G protein-gated Inwardly Rectifying K+ channel). GIRK channels also possess constitutive “basal” activity which contributes to regulation of neuronal and cardiac excitability and certain disorders, but the mechanism of this activity and its interrelation with the neurotransmitter-evoked activity are poorly understood. In this work we show that key features of basal and neurotransmitter-evoked activities are similar in cultured hippocampal neurons and in two model systems (mammalian HEK293 cells and Xenopus oocytes). Using experimental data of the neuronal GIRK1/2 channel function upon changes in GIRK and G protein concentrations, we constructed a mathematical model that quantitatively accounts for basal and evoked activity, and for the inverse correlation between the two. Our analysis suggests a novel and unexpected mechanism of interaction of GIRK1/2 with the G protein subunits, where the tetrameric GIRK channel can assemble with 4 molecules of the Gβγ subunits but only 2 molecules of Gα. GIRK is a prototypical effector of Gβγ, and the unequal stoichiometry of interaction with G protein subunits may have general implications for G protein signaling.
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Affiliation(s)
- Daniel Yakubovich
- Department of Physiology and Pharmacology and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv, Israel
| | - Shai Berlin
- Department of Physiology and Pharmacology and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv, Israel
| | - Uri Kahanovitch
- Department of Physiology and Pharmacology and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv, Israel
| | - Moran Rubinstein
- Department of Physiology and Pharmacology and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv, Israel
| | - Isabella Farhy-Tselnicker
- Department of Physiology and Pharmacology and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv, Israel
| | - Boaz Styr
- Department of Physiology and Pharmacology and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv, Israel
| | - Tal Keren-Raifman
- Department of Physiology and Pharmacology and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv, Israel
| | - Carmen W. Dessauer
- Department of Integrative Biology and Pharmacology, University of Texas Health Science Center, Houston, Texas, United States of America
| | - Nathan Dascal
- Department of Physiology and Pharmacology and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv, Israel
- * E-mail:
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Bondar A, Lazar J. Dissociated GαGTP and Gβγ protein subunits are the major activated form of heterotrimeric Gi/o proteins. J Biol Chem 2013; 289:1271-81. [PMID: 24307173 DOI: 10.1074/jbc.m113.493643] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Although most heterotrimeric G proteins are thought to dissociate into Gα and Gβγ subunits upon activation, the evidence in the Gi/o family has long been inconsistent and contradictory. The Gi/o protein family mediates inhibition of cAMP production and regulates the activity of ion channels. On the basis of experimental evidence, both heterotrimer dissociation and rearrangement have been postulated as crucial steps of Gi/o protein activation and signal transduction. We have now investigated the process of Gi/o activation in living cells directly by two-photon polarization microscopy and indirectly by observations of G protein-coupled receptor kinase-derived polypeptides. Our observations of existing fluorescently labeled and non-modified Gαi/o constructs indicate that the molecular mechanism of Gαi/o activation is affected by the presence and localization of the fluorescent label. All investigated non-labeled, non-modified Gi/o complexes dissociate extensively upon activation. The dissociated subunits can activate downstream effectors and are thus likely to be the major activated Gi/o form. Constructs of Gαi/o subunits fluorescently labeled at the N terminus (GAP43-CFP-Gαi/o) seem to faithfully reproduce the behavior of the non-modified Gαi/o subunits. Gαi constructs labeled within the helical domain (Gαi-L91-YFP) largely do not dissociate upon activation, yet still activate downstream effectors, suggesting that the dissociation seen in non-modified Gαi/o proteins is not required for downstream signaling. Our results appear to reconcile disparate published data and settle a long running dispute.
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Affiliation(s)
- Alexey Bondar
- From the Faculty of Science, University of South Bohemia, Branisovska 31a, 37005 Ceske Budejovice, Czech Republic and
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5
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Berlin S, Tsemakhovich VA, Castel R, Ivanina T, Dessauer CW, Keren-Raifman T, Dascal N. Two distinct aspects of coupling between Gα(i) protein and G protein-activated K+ channel (GIRK) revealed by fluorescently labeled Gα(i3) protein subunits. J Biol Chem 2011; 286:33223-35. [PMID: 21795707 DOI: 10.1074/jbc.m111.271056] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
G protein-activated K(+) channels (Kir3 or GIRK) are activated by direct interaction with Gβγ. Gα is essential for specific signaling and regulates basal activity of GIRK (I(basal)) and kinetics of the response elicited by activation by G protein-coupled receptors (I(evoked)). These regulations are believed to occur within a GIRK-Gα-Gβγ signaling complex. Fluorescent energy resonance transfer (FRET) studies showed strong GIRK-Gβγ interactions but yielded controversial results regarding the GIRK-Gα(i/o) interaction. We investigated the mechanisms of regulation of GIRK by Gα(i/o) using wild-type Gα(i3) (Gα(i3)WT) and Gα(i3) labeled at three different positions with fluorescent proteins, CFP or YFP (xFP). Gα(i3)xFP proteins bound the cytosolic domain of GIRK1 and interacted with Gβγ in a guanine nucleotide-dependent manner. However, only an N-terminally labeled, myristoylated Gα(i3)xFP (Gα(i3)NT) closely mimicked all aspects of Gα(i3)WT regulation except for a weaker regulation of I(basal). Gα(i3) labeled with YFP within the Gα helical domain preserved regulation of I(basal) but failed to restore fast I(evoked). Titrated expression of Gα(i3)NT and Gα(i3)WT confirmed that regulation of I(basal) and of the kinetics of I(evoked) of GIRK1/2 are independent functions of Gα(i). FRET and direct biochemical measurements indicated much stronger interaction between GIRK1 and Gβγ than between GIRK1 and Gα(i3). Thus, Gα(i/o)βγ heterotrimer may be attached to GIRK primarily via Gβγ within the signaling complex. Our findings support the notion that Gα(i/o) actively regulates GIRK. Although regulation of I(basal) is a function of Gα(i)(GDP), our new findings indicate that regulation of kinetics of I(evoked) is mediated by Gα(i)(GTP).
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Affiliation(s)
- Shai Berlin
- Department of Physiology and Pharmacology, Sackler School of Medicine, Tel Aviv University, Tel Aviv 69978, Israel
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6
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Two-photon polarization microscopy reveals protein structure and function. Nat Methods 2011; 8:684-90. [PMID: 21725301 DOI: 10.1038/nmeth.1643] [Citation(s) in RCA: 50] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/13/2010] [Accepted: 05/17/2011] [Indexed: 11/08/2022]
Abstract
Membrane proteins are a large, diverse group of proteins, serving a multitude of cellular functions. They are difficult to study because of their requirement of a lipid membrane for function. Here we show that two-photon polarization microscopy can take advantage of the cell membrane requirement to yield insights into membrane protein structure and function, in living cells and organisms. The technique allows sensitive imaging of G-protein activation, changes in intracellular calcium concentration and other processes, and is not limited to membrane proteins. Conveniently, many suitable probes for two-photon polarization microscopy already exist.
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7
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Jaskolski F, Mayo-Martin B, Jane D, Henley JM. Dynamin-dependent membrane drift recruits AMPA receptors to dendritic spines. J Biol Chem 2009; 284:12491-503. [PMID: 19269965 DOI: 10.1074/jbc.m808401200] [Citation(s) in RCA: 55] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022] Open
Abstract
The surface expression and localization of AMPA receptors (AMPARs) at dendritic spines are tightly controlled to regulate synaptic transmission. Here we show that de novo exocytosis of the GluR2 AMPAR subunit occurs at the dendritic shaft and that new AMPARs diffuse into spines by lateral diffusion in the membrane. However, membrane topology restricts this lateral diffusion. We therefore investigated which mechanisms recruit AMPARs to spines from the shaft and demonstrated that inhibition of dynamin GTPase activity reduced lateral diffusion of membrane-anchored green fluorescent protein and super-ecliptic pHluorin (SEP)-GluR2 into spines. In addition, the activation of synaptic N-methyl-d-aspartate (NMDA) receptors enhanced lateral diffusion of SEP-GluR2 and increased the number of endogenous AMPARs in spines. The NMDA-invoked effects were prevented by dynamin inhibition, suggesting that activity-dependent dynamin-mediated endocytosis within spines generates a net inward membrane drift that overrides lateral diffusion barriers to enhance membrane protein delivery into spines. These results provide a novel mechanistic explanation of how AMPARs and other membrane proteins are recruited to spines by synaptic activity.
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Affiliation(s)
- Frédéric Jaskolski
- Department of Anatomy and Physiology, Medical Research Council Centre for Synaptic Plasticity, School of Medical Sciences, University of Bristol, Bristol BS8 1TD, United Kingdom
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Abankwa D, Vogel H. A FRET map of membrane anchors suggests distinct microdomains of heterotrimeric G proteins. J Cell Sci 2007; 120:2953-62. [PMID: 17690305 DOI: 10.1242/jcs.001404] [Citation(s) in RCA: 44] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
The standard model of heterotrimeric G protein signaling postulates a dissociation of Gα and Gβγ subunits after activation. We hypothesized that the different combination of lipid-modifications on Gα and Gαβγ subunits directs them into different microdomains. By characterizing rapidly and at high sensitivity 38 fluorescence resonance energy transfer (FRET) pairs of heterotrimeric-G-protein constructs, we defined their microdomains in relation to each other, free from the constraints of the raft/non-raft dualism. We estimated that in a cell ∼30% of these membrane-anchored proteins are mostly clustered in 3400-16,200 copies of 30-nm microdomains. We found that the membrane anchors of Gα and Gαβγ subunits of both the Gi/o and Gq family co-cluster differently with microdomain markers. Moreover, anchors of the Gαi/o and Gαq subunits co-clustered only weakly, whereas constructs that contained the anchors of the corresponding heterotrimers co-clustered considerably, suggesting the existence of at least three types of microdomain. Finally, FRET experiments with full-length heterotrimeric G proteins confirmed that the inactive, heterotrimerized Gα subunit is in microdomains shared by heterotrimers from different subclasses, from where it displaces upon activation into a membrane-anchor- and subclass-specific microdomain.
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Affiliation(s)
- Daniel Abankwa
- Ecole Polytechnique Fédérale de Lausanne (EPFL), Institut des Sciences et Ingénierie Chimiques, CH-1015 Lausanne, Switzerland.
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Renner U, Glebov K, Lang T, Papusheva E, Balakrishnan S, Keller B, Richter DW, Jahn R, Ponimaskin E. Localization of the mouse 5-hydroxytryptamine(1A) receptor in lipid microdomains depends on its palmitoylation and is involved in receptor-mediated signaling. Mol Pharmacol 2007; 72:502-13. [PMID: 17540717 DOI: 10.1124/mol.107.037085] [Citation(s) in RCA: 59] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
In the present study, we have used wild-type and palmitoylation-deficient mouse 5-hydroxytryptamine(1A) receptor (5-HT1A) receptors fused to the yellow fluorescent protein- and the cyan fluorescent protein (CFP)-tagged alpha(i3) subunit of heterotrimeric G-protein to study spatiotemporal distribution of the 5-HT1A-mediated signaling in living cells. We also addressed the question on the molecular mechanisms by which receptor palmitoylation may regulate communication between receptors and G(i)-proteins. Our data demonstrate that activation of the 5-HT1A receptor caused a partial release of Galpha(i) protein into the cytoplasm and that this translocation is accompanied by a significant increase of the intracellular Ca(2+) concentration. In contrast, acylation-deficient 5-HT1A mutants failed to reproduce both Galpha(i3)-CFP relocation and changes in [Ca(2+)](i) upon agonist stimulation. By using gradient centrifugation and copatching assays, we also demonstrate that a significant fraction of the 5-HT1A receptor resides in membrane rafts, whereas the yield of the palmitoylation-deficient receptor in these membrane microdomains is reduced considerably. Our results suggest that receptor palmitoylation serves as a targeting signal responsible for the retention of the 5-HT1A receptor in membrane rafts. More importantly, the raft localization of the 5-HT1A receptor seems to be involved in receptor-mediated signaling.
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Affiliation(s)
- Ute Renner
- Abteilung Neuro- und Sinnesphysiologie, Physiologisches Institut, Universität Göttingen, Göttingen, Germany
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10
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Perez JB, Segura JM, Abankwa D, Piguet J, Martinez KL, Vogel H. Monitoring the diffusion of single heterotrimeric G proteins in supported cell-membrane sheets reveals their partitioning into microdomains. J Mol Biol 2006; 363:918-30. [PMID: 16996083 DOI: 10.1016/j.jmb.2006.08.036] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/26/2006] [Revised: 07/07/2006] [Accepted: 08/14/2006] [Indexed: 11/23/2022]
Abstract
Supported cell-membrane sheets are promising in vitro systems to investigate the properties of membranes with native protein/lipid composition, in particular their sub-compartmentalization and the differential localization of proteins associated to them. While such studies are usually performed using static microscopy techniques, we demonstrate here the potential offered by dynamic diffusion measurements. Whereas the overall fluidity of the lipid bilayer was preserved, the preparation of the membrane sheets led to the selective immobilization of extracellular and transmembrane (TM) glycosylated proteins and the anchored proteins/lipids associated with them. Taking advantage of this, we investigated the association of the G protein Gq with TM proteins, in particular G-protein coupled receptors (GPCRs), by monitoring the changes in diffusion occurring after preparation of the supported membranes. Two fluorescently tagged Galphaq proteins were constructed, which remained either mostly monomeric in the plasma membrane or associated with Gbetagamma in heterotrimers. While both constructs diffused similarly in living cells, the preparation of the supported membranes led to the selective immobilization of the heterotrimers with minimal changes of the diffusion of the monomeric Galphaq. The diverse mobility of monomeric and heterotrimeric Galphaq was a result of their different lipid anchors as demonstrated by monitoring the diffusion of the corresponding anchors alone. We propose that the immobilization of the heterotrimer was caused by its partitioning inside membrane microdomains surrounding GPCRs.
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Affiliation(s)
- Jean-Baptiste Perez
- Ecole Polytechnique Fédérale de Lausanne (EPFL), Institut des Sciences et Ingénierie Chimiques, CH-1015 Lausanne, Switzerland
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Wiser O, Qian X, Ehlers M, Ja WW, Roberts RW, Reuveny E, Jan YN, Jan LY. Modulation of basal and receptor-induced GIRK potassium channel activity and neuronal excitability by the mammalian PINS homolog LGN. Neuron 2006; 50:561-73. [PMID: 16701207 DOI: 10.1016/j.neuron.2006.03.046] [Citation(s) in RCA: 46] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/20/2004] [Revised: 10/19/2005] [Accepted: 03/28/2006] [Indexed: 12/24/2022]
Abstract
G protein-activated inwardly rectifying potassium (GIRK) channels mediate slow synaptic inhibition and control neuronal excitability. It is unknown whether GIRK channels are subject to regulation by guanine dissociation inhibitor (GDI) proteins like LGN, a mammalian homolog of Drosophila Partner of Inscuteable (mPINS). Here we report that LGN increases basal GIRK current but reduces GIRK activation by metabotropic transmitter receptors coupled to Gi or Go, but not Gs. Moreover, expression of its N-terminal, TPR-containing protein interaction domains mimics the effects of LGN in mammalian cells, probably by releasing sequestered endogenous LGN. In hippocampal neurons, expression of LGN, or LGN fragments that mimic or enhance LGN activity, hyperpolarizes the resting potential due to increased basal GIRK activity and reduces excitability. Using Lenti virus for LGN RNAi to reduce endogenous LGN levels in hippocampal neurons, we further show an essential role of LGN for maintaining basal GIRK channel activity and for harnessing neuronal excitability.
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Affiliation(s)
- Ofer Wiser
- Howard Hughes Medical Institute, Department of Physiology, University of California, San Francisco, 1550 4th Street, San Francisco, California 94143, USA
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Abstract
There is accumulating evidence that regulators of G-protein signalling (RGS) can have roles in signal transduction that are not related to GAP activity. Furthermore, RGSs have much more selective effects in vivo than might be anticipated from their behaviour in in vitro assays. I discuss the molecular mechanisms by which these phenomena might be explained including specific interactions between the RGS and G-protein coupled receptor, G-protein and effector.
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Affiliation(s)
- Andrew Tinker
- BHF Laboratories and Department of Medicine, University College London, 5 University Street, London WC1E 6JJ, UK.
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13
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Nobles M, Benians A, Tinker A. Heterotrimeric G proteins precouple with G protein-coupled receptors in living cells. Proc Natl Acad Sci U S A 2005; 102:18706-11. [PMID: 16352729 PMCID: PMC1317907 DOI: 10.1073/pnas.0504778102] [Citation(s) in RCA: 171] [Impact Index Per Article: 9.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Using fluorescence resonance energy transfer (FRET) microscopy, we investigate how heterotrimeric G proteins interact with G protein-coupled receptors (GPCRs). In the absence of receptor activation, the alpha2A adrenergic and muscarinic M4 receptors are present on the cell membrane as dimers. Furthermore, there is an interaction between the G protein subunits alpha o, beta1, and gamma2 and a number of GPCRs including M4, alpha2A, the adenosine A1 receptor, and the dopamine D2 receptor under resting conditions. The interaction between GPCRs and Galpha proteins shows specificity: there is interaction between the alpha2A receptor and Go, but little interaction between the alpha2A receptor and Gs. In contrast, the predominantly Gs-coupled prostacyclin receptor interacted with Gs, but there was little interaction between the prostacyclin receptor and Go. Inverse agonists did not change the FRET ratio, whereas the addition of agonist resulted in a modest fall. Our work suggests that GPCR dimers and the G protein heterotrimer are present at the cell membrane in the resting state in a pentameric complex.
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Affiliation(s)
- Muriel Nobles
- British Heart Foundation Laboratories and Department of Medicine, University College London, 5 University Street, London WC1E 6JJ, United Kingdom
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Benians A, Nobles M, Hosny S, Tinker A. Regulators of G-protein signaling form a quaternary complex with the agonist, receptor, and G-protein. A novel explanation for the acceleration of signaling activation kinetics. J Biol Chem 2005; 280:13383-94. [PMID: 15677457 DOI: 10.1074/jbc.m410163200] [Citation(s) in RCA: 63] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Regulators of G-protein signaling (RGS) proteins modulate signaling through heterotrimeric G-proteins. They act to enhance the intrinsic GTPase activity of the Galpha subunit but paradoxically have also been shown to enhance receptor-stimulated activation. To study this paradox, we used a G-protein gated K+ channel to report the dynamics of the G-protein cycle and fluorescence resonance energy transfer techniques with cyan and yellow fluorescent protein-tagged proteins to report physical interaction. Our data show that the acceleration of the activation kinetics is dissociated from deactivation kinetics and dependent on receptor and RGS type, G-protein isoform, and RGS expression levels. By using fluorescently tagged proteins, fluorescence resonance energy transfer microscopy showed a stable physical interaction between the G-protein alpha subunit and RGS (RGS8 and RGS7) that is independent of the functional state of the G-protein. RGS8 does not directly interact with G-protein-coupled receptors. Our data show participation of the RGS in the ternary complex between agonist-receptor and G-protein to form a "quaternary complex." Thus we propose a novel model for the action of RGS proteins in the G-protein cycle in which the RGS protein appears to enhance the "kinetic efficacy" of the ternary complex, by direct association with the G-protein alpha subunit.
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Affiliation(s)
- Amy Benians
- BHF Laboratories and Department of Medicine, University College London, Room 420, 4th Floor, 5 University Street, London WC1E 6JJ, United Kingdom
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Milligan G. Applications of bioluminescence- and fluorescence resonance energy transfer to drug discovery at G protein-coupled receptors. Eur J Pharm Sci 2004; 21:397-405. [PMID: 14998570 DOI: 10.1016/j.ejps.2003.11.010] [Citation(s) in RCA: 95] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/18/2003] [Revised: 11/13/2003] [Accepted: 11/13/2003] [Indexed: 11/22/2022]
Abstract
Bioluminescence (BRET)- and fluorescence resonance energy transfer (FRET) techniques have become integral approaches in studies of protein-protein interactions in living cells. They rely on non-radiative transfer of energy between donor and acceptor species that can be appended to the proteins of interest. These techniques display exquisite dependence on distance and orientation between the energy transfer partners. This means they are well suited to measure both small conformational changes in response to ligand binding between partner proteins that remain within a complex or more extensive translocations of proteins between cellular compartments that occur in response to cellular challenge. Introduction of both energy donor and acceptor into a single polypeptide can also allow the detection of ligand-induced conformational switches in monomeric proteins in the millisecond time scale. Many of these approaches are amenable to high throughput screening and the drug discovery process. G protein-coupled receptors (GPCRs) represent a key drug target class. Specific applications of resonance energy transfer techniques to the identification of ligands for this class of protein are highlighted to illustrate general principles.
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Affiliation(s)
- Graeme Milligan
- Molecular Pharmacology Group, Division of Biochemistry and Molecular Biology, University of Glasgow, Glasgow G12 8QQ, Scotland, UK.
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Quinn KV, Giblin JP, Tinker A. Multisite phosphorylation mechanism for protein kinase A activation of the smooth muscle ATP-sensitive K+ channel. Circ Res 2004; 94:1359-66. [PMID: 15087422 DOI: 10.1161/01.res.0000128513.34817.c4] [Citation(s) in RCA: 65] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
The activation of ATP-sensitive K+ channels by protein kinase A in vascular smooth muscle is an important component of the action of vasodilators. In this study, we examine the molecular mechanisms of regulation of the cloned equivalent of this channel comprising the sulfonylurea receptor 2B and the inward rectifier 6.1 subunit (SUR2B/Kir6.1). Specifically, we focus on whether the channel is directly phosphorylated and the sites at which this occurs in the protein complex. We identify one site in Kir6.1 (S385) and two sites in SUR2B (T633 and S1465) using a combination of biochemical and functional assays. Our work supports a model in which multiple sites in the channel complex have to be phosphorylated before activation occurs.
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Affiliation(s)
- Kathryn V Quinn
- British Heart Foundation Laboratories and the Department of Medicine, University College London, UK
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17
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Leaney JL. Contribution of Kir3.1, Kir3.2A and Kir3.2C subunits to native G protein-gated inwardly rectifying potassium currents in cultured hippocampal neurons. Eur J Neurosci 2003; 18:2110-8. [PMID: 14622172 DOI: 10.1046/j.1460-9568.2003.02933.x] [Citation(s) in RCA: 45] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
G protein-gated inwardly rectifying potassium (GIRK) channels are found in neurons, atrial myocytes and neuroendocrine cells. A characteristic feature is their activation by stimulation of Gi/o-coupled receptors. In central neurons, for example, they are activated by adenosine and GABA and, as such, they play an important role in neurotransmitter-mediated regulation of membrane excitability. The channels are tetrameric assemblies of Kir3.x subunits (Kir3.1-3.4 plus splice variants). In this study I have attempted to identify the channel subunits which contribute to the native GIRK current recorded from primary cultured rat hippocampal pyramidal neurons. Reverse transcriptase-polymerase chain reaction revealed the expression of mRNA for Kir3.1, 3.2A, 3.2C and 3.3 subunits and confocal immunofluorescence microscopy was used to investigate their expression patterns. Diffuse staining was observed on both cell somata and dendrites for Kir3.1 and Kir3.2A yet that for Kir3.2C was weaker and punctate. Whole-cell patch clamp recordings were used to record GIRK currents from hippocampal pyramidal neurons which were identified on the basis of inward rectification, dependence of reversal potential on external potassium concentration and sensitivity to tertiapin. The GIRK currents were enhanced by the stimulation of a number of Gi/o-coupled receptors and were inhibited by pertussis toxin. In order to ascertain which Kir3.x subunits were responsible for the native GIRK current I compared the properties with those of the cloned Kir3.1 + 3.2A and Kir3.1 + 3.2C channels heterologously expressed in HEK293 cells.
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MESH Headings
- Adenosine/pharmacology
- Analgesics/pharmacology
- Animals
- Animals, Newborn
- Baclofen/pharmacology
- Benzoxazines
- Blotting, Northern
- Calcium Channel Blockers/pharmacology
- Carbachol/pharmacology
- Cell Cycle Proteins/metabolism
- Cells, Cultured
- Cholinergic Agonists/pharmacology
- Dose-Response Relationship, Drug
- Drug Interactions
- Embryo, Mammalian
- Free Radical Scavengers/pharmacology
- G Protein-Coupled Inwardly-Rectifying Potassium Channels
- GABA Agonists/pharmacology
- GTP-Binding Proteins/physiology
- Hippocampus/cytology
- Hormones/pharmacology
- Humans
- Kidney
- Membrane Potentials/drug effects
- Microscopy, Confocal/instrumentation
- Microscopy, Confocal/methods
- Morpholines/pharmacology
- Naphthalenes/pharmacology
- Neurons/drug effects
- Neurons/physiology
- Patch-Clamp Techniques/methods
- Pertussis Toxin/pharmacology
- Potassium Channels/metabolism
- Potassium Channels/physiology
- Potassium Channels, Inwardly Rectifying/classification
- Potassium Channels, Inwardly Rectifying/physiology
- Protein Subunits/physiology
- RNA, Messenger/biosynthesis
- Rats
- Rats, Sprague-Dawley
- Receptors, G-Protein-Coupled/metabolism
- Reverse Transcriptase Polymerase Chain Reaction/methods
- Serotonin/pharmacology
- Somatostatin/pharmacology
- Time Factors
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Affiliation(s)
- Joanne L Leaney
- Department of Medicine, UCL, The Rayne Building, 5 University Street, London WC1E 6JJ, UK.
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18
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Abstract
High-content assays rely on the imaging of cellular events. They can be used to monitor the activation of G-protein-coupled receptors (or other receptors), their internalization into the cell, or alterations in their amount. In addition, multiplexed assays can provide further information about the characteristics of the receptor. Recent improvements in throughput using high-content screening platforms means that such assays are now an integral element of functional analysis in the drug discovery process.
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Affiliation(s)
- Graeme 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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19
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Benians A, Leaney JL, Tinker A. Agonist unbinding from receptor dictates the nature of deactivation kinetics of G protein-gated K+ channels. Proc Natl Acad Sci U S A 2003; 100:6239-44. [PMID: 12719528 PMCID: PMC156356 DOI: 10.1073/pnas.1037595100] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
G protein-gated inwardly rectifying K(+) (Kir) channels are found in neurones, atrial myocytes, and endocrine cells and are involved in generating late inhibitory postsynaptic potentials, slowing the heart rate and inhibiting hormone release. They are activated by G protein-coupled receptors (GPCRs) via the inhibitory family of G protein, G(i/o), in a membrane-delimited fashion by the direct binding of Gbetagamma dimers to the channel complex. In this study we are concerned with the kinetics of deactivation of the cloned neuronal G protein-gated K(+) channel, Kir3.1 + 3.2A, after stimulation of a number of GPCRs. Termination of the channel activity on agonist removal is thought to solely depend on the intrinsic hydrolysis rate of the G protein alpha subunit. In this study we present data that illustrate a more complex behavior. We hypothesize that there are two processes that account for channel deactivation: agonist unbinding from the GPCR and GTP hydrolysis by the G protein alpha subunit. With some combinations of agonist/GPCR, the rate of agonist unbinding is slow and rate-limiting, and deactivation kinetics are not modulated by regulators of G protein-signaling proteins. In another group, channel deactivation is generally faster and limited by the hydrolysis rate of the G protein alpha subunit. G protein isoform and interaction with G protein-signaling proteins play a significant role with this group of GPCRs.
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Affiliation(s)
- Amy Benians
- Department of Medicine, Centre for Clinical Pharmacology and British Heart Foundation Laboratories, University College London, Room 420, 4th Floor, 5 University Street, United Kingdom
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20
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Benians A, Leaney JL, Milligan G, Tinker A. The dynamics of formation and action of the ternary complex revealed in living cells using a G-protein-gated K+ channel as a biosensor. J Biol Chem 2003; 278:10851-8. [PMID: 12529316 DOI: 10.1074/jbc.m212299200] [Citation(s) in RCA: 38] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Traditionally the consequences of activation of G-protein-coupled receptors (GPCRs) by an agonist are studied using biochemical assays. In this study we use live cells and take advantage of a G-protein-gated inwardly rectifying potassium channel (Kir3.1+3.2A) that is activated by the direct binding of Gbetagamma subunit to the channel complex to report, in real-time, using the patch clamp technique the activity of the "ternary complex" of agonist/receptor/G-protein. This analysis is further facilitated by the use of pertussis toxin-resistant fluorescent and non-fluorescent Galpha(i/o) subunits and a series of HEK293 cell lines stably expressing both channel and receptors (including the adenosine A(1) receptor, the adrenergic alpha(2A) receptor, the dopamine D(2S) receptor, the M4 muscarinic receptor, and the dimeric GABA-B(1b/2) receptor). We systematically analyzed the contribution of the various inputs to the observed kinetic response of channel activation. Our studies indicate that the combination of agonist, GPCR, and G-protein isoform uniquely specify the behavior of these channels and thus support the importance of the whole ternary complex at a kinetic level.
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Affiliation(s)
- Amy Benians
- Centre for Clinical Pharmacology, The BHF Laboratories, Department of Medicine, University College London, United Kingdom
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