Recovery of oligomers and cooperativity when monomers of the M2 muscarinic cholinergic receptor are reconstituted into phospholipid vesicles

Allostery of atypical modulators at oligomeric G protein-coupled receptors

Many G protein-coupled receptors (GPCRs) are therapeutic targets, with most drugs acting at the orthosteric site. Some GPCRs also possess allosteric sites, which have become a focus of drug discovery. In the M2 muscarinic receptor, allosteric modulators regulate the binding and functional effects of orthosteric ligands through a mix of conformational changes, steric hindrance and electrostatic repulsion transmitted within and between the constituent protomers of an oligomer. Tacrine has been called an atypical modulator because it exhibits positive cooperativity, as revealed by Hill coefficients greater than 1 in its negative allosteric effect on binding and response. Radioligand binding and molecular dynamics simulations were used to probe the mechanism of that modulation in monomers and oligomers of wild-type and mutant M2 receptors. Tacrine is not atypical at monomers, which indicates that its atypical effects are a property of the receptor in its oligomeric state. These results illustrate that oligomerization of the M2 receptor has functional consequences.

Shaky ground - The nature of metastable GPCR signalling complexes

How G protein-coupled receptors (GPCR) interact with one another remains an area of active investigation. Obligate dimers of class C GPCRs such as metabotropic GABA and glutamate receptors are well accepted, although whether this is a general feature of other GPCRs is still strongly debated. In this review, we focus on the idea that GPCR dimers and oligomers are better imagined as parts of larger metastable signalling complexes. We discuss the nature of functional oligomeric entities, their stabilities and kinetic features and how structural and functional asymmetries of such metastable entities might have implications for drug discovery. This article is part of the Special Issue entitled 'Receptor heteromers and their allosteric receptor-receptor interactions'.

Ligand-Induced Coupling between Oligomers of the M2 Receptor and the Gi1 Protein in Live Cells

Uncertainty over the mechanism of signaling via G protein-coupled receptors (GPCRs) relates in part to questions regarding their supramolecular structure. GPCRs and heterotrimeric G proteins are known to couple as monomers under various conditions. Many GPCRs form oligomers under many of the same conditions, however, and the biological role of those complexes is unclear. We have used dual-color fluorescence correlation spectroscopy to identify oligomers of the M₂ muscarinic receptor and of G_i1 in purified preparations and live Chinese hamster ovary cells. Measurements on differently tagged receptors (i.e., eGFP-M₂ and mCherry-M₂) and G proteins (i.e., eGFP-Gα_i1β₁γ₂ and mCherry-Gα_i1β₁γ₂) detected significant cross-correlations between the two fluorophores in each case, both in detergent micelles and in live cells, indicating that both the receptor and G_i1 can exist as homo-oligomers. Oligomerization of differently tagged G_i1 decreased upon the activation of co-expressed wild-type M₂ receptor by an agonist. Measurements on a tagged M₂ receptor (M₂-mCherry) and eGFP-Gα_i1β₁γ₂ co-expressed in live cells detected cross-correlations only in the presence of an agonist, which therefore promoted coupling of the receptor and the G protein. The effect of the agonist was retained when a fluorophore-tagged receptor lacking the orthosteric site (i.e., M₂(D103A)-mCherry) was co-expressed with the wild-type receptor and eGFP-Gα_i1β₁γ₂, indicating that the ligand acted via an oligomeric receptor. Our results point to a model in which an agonist promotes transient coupling of otherwise independent oligomers of the M₂ receptor on the one hand and of G_i1 on the other and that an activated complex leads to a reduction in the oligomeric size of the G protein. They suggest that GPCR-mediated signaling proceeds, at least in part, via oligomers.

The Dynamics of GPCR Oligomerization and Their Functional Consequences

The functional importance of G protein-coupled receptor (GPCR) oligomerization remains controversial. Although obligate dimers of class C GPCRs are well accepted, the generalizability of this phenomenon is still strongly debated with respect to other classes of GPCRs. In this review, we focus on understanding the organization and dynamics between receptor equivalents and their signaling partners in oligomeric receptor complexes, with a view toward integrating disparate viewpoints into a unified understanding. We discuss the nature of functional oligomeric entities, and how asymmetries in receptor structure and function created by oligomers might have implications for receptor function as allosteric machines and for future drug discovery.

An update on the physiological and therapeutic relevance of GPCR oligomers

The traditional view on GPCRs held that they function as single monomeric units composed of identical subunits. This notion was overturned by the discovery that GPCRs can form homo- and hetero-oligomers, some of which are obligatory, and can further assemble into receptor mosaics consisting of three or more protomers. Oligomerisation exerts significant impacts on receptor function and physiology, offering a platform for the diversification of receptor signalling, pharmacology, regulation, crosstalk, internalization and trafficking. Given their involvement in the modulation of crucial physiological processes, heteromers could constitute important therapeutic targets for a wide range of diseases, including schizophrenia, Parkinson's disease, substance abuse or obesity. This review aims at depicting the current developments in GPCR oligomerisation research, documenting various class A, B and C GPCR heteromers detected in vitro and in vivo using biochemical and biophysical approaches, as well as recently identified higher-order oligomeric complexes. It explores the current understanding of dimerization dynamics and the possible interaction interfaces that drive oligomerisation. Most importantly, it provides an inventory of the wide range of physiological processes and pathophysiological conditions to which GPCR oligomers contribute, surveying some of the oligomers that constitute potential drug targets. Finally, it delineates the efforts to develop novel classes of ligands that specifically target and tether to receptor oligomers instead of a single monomeric entity, thus ameliorating their ability to modulate GPCR function.

Allosteric modulation in monomers and oligomers of a G protein-coupled receptor

The M₂ muscarinic receptor is the prototypic model of allostery in GPCRs, yet the molecular and the supramolecular determinants of such effects are unknown. Monomers and oligomers of the M₂ muscarinic receptor therefore have been compared to identify those allosteric properties that are gained in oligomers. Allosteric interactions were monitored by means of a FRET-based sensor of conformation at the allosteric site and in pharmacological assays involving mutants engineered to preclude intramolecular effects. Electrostatic, steric, and conformational determinants of allostery at the atomic level were examined in molecular dynamics simulations. Allosteric effects in monomers were exclusively negative and derived primarily from intramolecular electrostatic repulsion between the allosteric and orthosteric ligands. Allosteric effects in oligomers could be positive or negative, depending upon the allosteric-orthosteric pair, and they arose from interactions within and between the constituent protomers. The complex behavior of oligomers is characteristic of muscarinic receptors in myocardial preparations.

DOI:http://dx.doi.org/10.7554/eLife.11685.001

Proteins called G protein-coupled receptors (GPCRs) are found on the surface of cells throughout the body. Hormones or other signal molecules – collectively known as ligands – from outside the cell can bind to the receptors to activate them. This causes a change in the structure of the receptor, which triggers a signal inside the cell to alter the cell’s behavior. GPCRs are known to form clusters of two or more receptor units, but it is not known if these clusters have unique properties or what role they play in cells.

Many drugs can bind to GPCRs and most of them block the activity of the receptors by taking the place of the natural ligand. Another way to alter the activity of a GPCR is with so-called 'allosteric' drugs. These bind to different sites on the receptor than the natural ligands do and can inhibit or enhance binding of the ligands by altering the shape of the receptor.

Shivnaraine et al. investigated how a type of GPCR called muscarinic cholinergic receptors interact within clusters. This involved developing a method to track the receptor in mammalian cells using a fluorescent sensor that detects changes in the allosteric site. The experiments show that two or more GPCRs need to interact for the receptors to respond to allosteric drugs in a manner that reflects the normal effect of the drugs on the body. This result is unexpected in light of the assumption that individual receptor molecules act independently. Shivnaraine et al.’s findings indicate that the clusters may play a role in the normal behavior of GPCRs in cells. A future challenge is to understand exactly how the GPCRs interact with each other.

DOI:http://dx.doi.org/10.7554/eLife.11685.002

Molecular Insights into Metabotropic Glutamate Receptor Allosteric Modulation

The metabotropic glutamate (mGlu) receptors are a group of eight family C G protein-coupled receptors that are expressed throughout the central nervous system (CNS) and periphery. Within the CNS the different subtypes are found in neurons, both pre- and/or postsynaptically, where they mediate modulatory roles and in glial cells. The mGlu receptor family provides attractive targets for numerous psychiatric and neurologic disorders, with the majority of discovery programs focused on targeting allosteric sites, with allosteric ligands now available for all mGlu receptor subtypes. However, the development of allosteric ligands remains challenging. Biased modulation, probe dependence, and molecular switches all contribute to the complex molecular pharmacology exhibited by mGlu receptor allosteric ligands. In recent years we have made significant progress in our understanding of this molecular complexity coupled with an increased understanding of the structural basis of mGlu allosteric modulation.

The molecular basis of oligomeric organization of the human M3 muscarinic acetylcholine receptor

G protein-coupled receptors, including the M3 muscarinic acetylcholine receptor, can form homo-oligomers. However, the basis of these interactions and the overall organizational structure of such oligomers are poorly understood. Combinations of site-directed mutagenesis and homogenous time-resolved fluorescence resonance energy transfer studies that assessed interactions between receptor protomers at the surface of transfected cells indicated important contributions of regions of transmembrane domains I, IV, V, VI, and VII as well as intracellular helix VIII to the overall organization. Molecular modeling studies based on both these results and an X-ray structure of the inactive state of the M3 receptor bound by the antagonist/inverse agonist tiotropium were then employed. The results could be accommodated fully by models in which a proportion of the cell surface M3 receptor population is a tetramer with rhombic, but not linear, orientation. This is consistent with previous studies based on spectrally resolved, multiphoton fluorescence resonance energy transfer. Modeling studies furthermore suggest an important role for molecules of cholesterol at the dimer + dimer interface of the tetramer, which is consistent with the presence of cholesterol at key locations in many G protein-coupled receptor crystal structures. Mutants that displayed disrupted quaternary organization were often poorly expressed and showed immature N-glycosylation. Sustained treatment of cells expressing such mutants with the muscarinic receptor inverse agonist atropine increased cellular levels and restored both cell surface delivery and quaternary organization to many of the mutants. These observations suggest that organization as a tetramer may occur before plasma membrane delivery and may be a key step in cellular quality control assessment.

Constitutive phospholipid scramblase activity of a G protein-coupled receptor

Opsin, the rhodopsin apoprotein, was recently shown to be an ATP-independent flippase (or scramblase) that equilibrates phospholipids across photoreceptor disc membranes in mammalian retina, a process required for disc homeostasis. Here we show that scrambling is a constitutive activity of rhodopsin, distinct from its light-sensing function. Upon reconstitution into vesicles, discrete conformational states of the protein (rhodopsin, a metarhodopsin II-mimic, and two forms of opsin) facilitated rapid (>10,000 phospholipids per protein per second) scrambling of phospholipid probes. Our results indicate that the large conformational changes involved in converting rhodopsin to metarhodopsin II are not required for scrambling, and that the lipid translocation pathway either lies near the protein surface or involves membrane packing defects in the vicinity of the protein. Additionally, we demonstrate that β₂-adrenergic and adenosine A_2A receptors scramble lipids, suggesting that rhodopsin-like G protein-coupled receptors may play an unexpected moonlighting role in re-modeling cell membranes.

Coupling of g proteins to reconstituted monomers and tetramers of the M2 muscarinic receptor

G protein-coupled receptors can be reconstituted as monomers in nanodiscs and as tetramers in liposomes. When reconstituted with G proteins, both forms enable an allosteric interaction between agonists and guanylyl nucleotides. Both forms, therefore, are candidates for the complex that controls signaling at the level of the receptor. To identify the biologically relevant form, reconstituted monomers and tetramers of the purified M2 muscarinic receptor were compared with muscarinic receptors in sarcolemmal membranes for the effect of guanosine 5'-[β,γ-imido]triphosphate (GMP-PNP) on the inhibition of N-[(3)H]methylscopolamine by the agonist oxotremorine-M. With monomers, a stepwise increase in the concentration of GMP-PNP effected a lateral, rightward shift in the semilogarithmic binding profile (i.e. a progressive decrease in the apparent affinity of oxotremorine-M). With tetramers and receptors in sarcolemmal membranes, GMP-PNP effected a vertical, upward shift (i.e. an apparent redistribution of sites from a state of high affinity to one of low affinity with no change in affinity per se). The data were analyzed in terms of a mechanistic scheme based on a ligand-regulated equilibrium between uncoupled and G protein-coupled receptors (the "ternary complex model"). The model predicts a rightward shift in the presence of GMP-PNP and could not account for the effects at tetramers in vesicles or receptors in sarcolemmal membranes. Monomers present a special case of the model in which agonists and guanylyl nucleotides interact within a complex that is both constitutive and stable. The results favor oligomers of the M2 receptor over monomers as the biologically relevant state for coupling to G proteins.

The prevalence, maintenance, and relevance of G protein-coupled receptor oligomerization

Over the past decade, ideas and experimental support for the hypothesis that G protein-coupled receptors may exist as dimeric or oligomeric complexes moved initially from heresy to orthodoxy, to the current situation in which the capacity of such receptors to interact is generally accepted but the prevalence, maintenance, and relevance of such interactions to both pharmacology and function remain unclear. A vast body of data obtained following transfection of cultured cells is still to be translated to native systems and, even where this has been attempted, results often remain controversial and contradictory. This review will consider approaches that are currently being applied and why these might be challenging to interpret, and will suggest means to overcome these limitations.

Heterotropic cooperativity within and between protomers of an oligomeric M(2) muscarinic receptor

At least four allosteric sites have been found to mediate the dose-dependent effects of gallamine on the binding of [(3)H]quinuclidinylbenzilate (QNB) and N-[(3)H]methylscopolamine (NMS) to M(2) muscarinic receptors in membranes and solubilized preparations from porcine atria, CHO cells, and Sf9 cells. The rate of dissociation of [(3)H]QNB was affected in a bell-shaped manner with at least one Hill coefficient (n(H)) greater than 1, indicating that at least three allosteric sites are involved. The level of binding of [(3)H]QNB was decreased in a biphasic manner, revealing at least two allosteric sites; binding of [(3)H]NMS was affected in a triphasic, serpentine manner, revealing at least three sites, and values of n(H) >1 pointed to at least four sites. Several lines of evidence indicate that all effects of gallamine were allosteric in nature and could be observed at equilibrium. The rates of equilibration and dissociation suggest that the receptor was predominately oligomeric, and the heterogeneity revealed by gallamine can be attributed to differences in its affinity for the constituent protomers of a tetramer. Those differences appear to arise from inter- and intramolecular cooperativity between gallamine and the radioligand.

The functional size of GPCRs - monomers, dimers or tetramers?

In almost 16 years since the word "dimer" was used in a publication to describe the organization of G protein-coupled receptors (GPCRs), a large number of studies have since weighed in on this notion. Are native, functional GPCRs monomers, dimers or as some would suggest even higher order structures? Here, we review some of the latest evidence regarding the organization of these receptors in both homo- and hetero-oligomeric formats, with a particular focus on β-adrenergic receptors. This is particularly important for understanding the allosteric nature of receptor/receptor interactions. It is likely that, over the course of evolution, mechanisms have come into play using all of the possible variations in receptor/receptor stoichiometry, depending on the cell and the physiological context in question. Finally, we provide some data that suggests that higher order structures of GPCRs, as with dimers themselves are probably assembled in the ER.

Cleavage-resistant fusion proteins of the M(2) muscarinic receptor and Gα(i1). Homotropic and heterotropic effects in the binding of ligands

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.

Imaging-based approaches to understanding g protein-coupled receptor signalling complexes

In the last 10 years, imaging assays based on resonance energy transfer (RET) and protein fragment complementation have made it possible to study interactions between components of G protein-coupled receptor (GPCR) signalling complexes in living cells under physiological conditions. Here, we consider the history of such approaches, the current tools available and how they have changed our understanding of GPCR signalling. We also discuss some theoretical and methodological issues important when combining the different types of assay.

Scalable rule-based modelling of allosteric proteins and biochemical networks

Much of the complexity of biochemical networks comes from the information-processing abilities of allosteric proteins, be they receptors, ion-channels, signalling molecules or transcription factors. An allosteric protein can be uniquely regulated by each combination of input molecules that it binds. This “regulatory complexity” causes a combinatorial increase in the number of parameters required to fit experimental data as the number of protein interactions increases. It therefore challenges the creation, updating, and re-use of biochemical models. Here, we propose a rule-based modelling framework that exploits the intrinsic modularity of protein structure to address regulatory complexity. Rather than treating proteins as “black boxes”, we model their hierarchical structure and, as conformational changes, internal dynamics. By modelling the regulation of allosteric proteins through these conformational changes, we often decrease the number of parameters required to fit data, and so reduce over-fitting and improve the predictive power of a model. Our method is thermodynamically grounded, imposes detailed balance, and also includes molecular cross-talk and the background activity of enzymes. We use our Allosteric Network Compiler to examine how allostery can facilitate macromolecular assembly and how competitive ligands can change the observed cooperativity of an allosteric protein. We also develop a parsimonious model of G protein-coupled receptors that explains functional selectivity and can predict the rank order of potency of agonists acting through a receptor. Our methodology should provide a basis for scalable, modular and executable modelling of biochemical networks in systems and synthetic biology.

The complexity of biochemical networks challenges our ability to create quantitative and predictive models of cellular responses to extracellular changes. In these networks, the regulation of allosteric receptors and proteins by multiple drugs or endogenous ligands introduces “regulatory complexity” because a large number of parameters is required to describe such interactions. Protein interactions also give rise to “combinatorial complexity” by generating large numbers of protein complexes and covalent modification states. To address these twin problems, we propose a modelling framework that combines a modular description of protein structure and function with a rule-based description of protein interactions. We define the input-output function of an allosteric protein through its thermodynamic properties and structural components. We show that our “biomolecule-centric” methodology, in contrast to ad hoc approaches that emphasize the regulatory logic of interactions, can reduce the number of parameters required to model experimental observations. We also demonstrate how the application of our framework gives insights into the assembly of macromolecular complexes and increases the predictive power of a standard model of G protein-coupled receptors. These benefits are possible in many systems, given the ubiquity of allostery in biochemical networks. Our research delineates a fundamental relationship between allostery, modularity, and complexity in biochemical networks.

Oligomeric size of the m2 muscarinic receptor in live cells as determined by quantitative fluorescence resonance energy transfer

Fluorescence resonance energy transfer (FRET), measured by fluorescence intensity-based microscopy and fluorescence lifetime imaging, has been used to estimate the size of oligomers formed by the M(2) muscarinic cholinergic receptor. The approach is based on the relationship between the apparent FRET efficiency within an oligomer of specified size (n) and the pairwise FRET efficiency between a single donor and a single acceptor (E). The M(2) receptor was fused at the N terminus to enhanced green or yellow fluorescent protein and expressed in Chinese hamster ovary cells. Emission spectra were analyzed by spectral deconvolution, and apparent efficiencies were estimated by donor-dequenching and acceptor-sensitized emission at different ratios of enhanced yellow fluorescent protein-M(2) receptor to enhanced green fluorescent protein-M(2) receptor. The data were interpreted in terms of a model that considers all combinations of donor and acceptor within a specified oligomer to obtain fitted values of E as follows: n = 2, 0.495 +/- 0.019; n = 4, 0.202 +/- 0.010; n = 6, 0.128 +/- 0.006; n = 8, 0.093 +/- 0.005. The pairwise FRET efficiency determined independently by fluorescence lifetime imaging was 0.20-0.24, identifying the M(2) receptor as a tetramer. The strategy described here yields an explicit estimate of oligomeric size on the basis of fluorescence properties alone. Its broader application could resolve the general question of whether G protein-coupled receptors exist as dimers or larger oligomers. The size of an oligomer has functional implications, and such information can be expected to contribute to an understanding of the signaling process.

Ligand regulation of the quaternary organization of cell surface M3 muscarinic acetylcholine receptors analyzed by fluorescence resonance energy transfer (FRET) imaging and homogeneous time-resolved FRET

Flp-In(TM) T-REx(TM) 293 cells expressing a wild type human M(3) muscarinic acetylcholine receptor construct constitutively and able to express a receptor activated solely by synthetic ligand (RASSL) form of this receptor on demand maintained response to the muscarinic agonist carbachol but developed response to clozapine N-oxide only upon induction of the RASSL. The two constructs co-localized at the plasma membrane and generated strong ratiometric fluorescence resonance energy transfer (FRET) signals consistent with direct physical interactions. Increasing levels of induction of the FRET donor RASSL did not alter wild type receptor FRET-acceptor levels substantially. However, ratiometric FRET was modulated in a bell-shaped fashion with maximal levels of the donor resulting in decreased FRET. Carbachol, but not the antagonist atropine, significantly reduced the FRET signal. Cell surface homogeneous time-resolved FRET, based on SNAP-tag technology and employing wild type and RASSL forms of the human M(3) receptor expressed stably in Flp-In(TM) TREx(TM) 293 cells, also identified cell surface dimeric/oligomeric complexes. Now, however, signals were enhanced by appropriate selective agonists. At the wild type receptor, large increases in FRET signal to carbachol and acetylcholine were concentration-dependent with EC(50) values consistent with the relative affinities of the two ligands. These studies confirm the capacity of the human M(3) muscarinic acetylcholine receptor to exist as dimeric/oligomeric complexes at the surface of cells and demonstrate that the organization of such complexes can be modified by ligand binding. However, conclusions as to the effect of ligands on such complexes may depend on the approach used.

Formation and dissociation of M1 muscarinic receptor dimers seen by total internal reflection fluorescence imaging of single molecules

G-protein-coupled receptors (GPCRs) are the largest family of transmembrane signaling proteins in the human genome. Events in the GPCR signaling cascade have been well characterized, but the receptor composition and its membrane distribution are still generally unknown. Although there is evidence that some members of the GPCR superfamily exist as constitutive dimers or higher oligomers, interpretation of the results has been disputed, and recent studies indicate that monomeric GPCRs may also be functional. Because there is controversy within the field, to address the issue we have used total internal reflection fluorescence microscopy (TIRFM) in living cells to visualize thousands of individual molecules of a model GPCR, the M(1) muscarinic acetylcholine receptor. By tracking the position of individual receptors over time, their mobility, clustering, and dimerization kinetics could be directly determined with a resolution of approximately 30 ms and approximately 20 nm. In isolated CHO cells, receptors are randomly distributed over the plasma membrane. At any given time, approximately 30% of the receptor molecules exist as dimers, and we found no evidence for higher oligomers. Two-color TIRFM established the dynamic nature of dimer formation with M(1) receptors undergoing interconversion between monomers and dimers on the timescale of seconds.

Co-operativity in agonist binding at the D2 dopamine receptor: evidence from agonist dissociation kinetics

There is much evidence to suggest that G protein coupled receptors exist as oligomers but the relevance to their function is unclear. We have, therefore, examined the binding of the radiolabelled agonist [(3)H]NPA to membranes of CHO cells expressing the D(2) dopamine receptor in dissociation rate experiments. When [(3)H]NPA dissociation was started by dilution, the dissociation rate in the absence of sodium ions was unaffected by addition of the antagonist/inverse agonist (+)-butaclamol, but was accelerated by addition of agonists e.g. dopamine, suggesting that the receptor was not behaving as a monomer with a single binding site. The very low efficacy partial agonist, aripiprazole provided an intermediate level of acceleration of dissociation. [(3)H]NPA dissociation experiments started by addition of ligands without dilution gave a similar pattern. [(3)H]NPA dissociation could also be accelerated by GTP. Dissociation of [(3)H]NPA in the presence of GTP and dopamine provided a greater acceleration than for either modulator alone, suggesting synergistic effects related to receptor/G protein interaction. When [(3)H]NPA dissociation experiments were performed in the presence of sodium ions, dissociation was faster than in their absence but the rate still depended on the ligand present in the assay. Overall the data cannot be explained by a ternary complex model and are consistent with an oligomeric receptor in which binding of [(3)H]NPA, as an example of an agonist ligand, can be modulated co-operatively by ligands binding elsewhere in the oligomer. Interactions with G proteins also occurs providing further modulation of [(3)H]NPA binding. Both agonists and G proteins are proposed to modulate the oligomer by switching high affinity agonist binding sites to low affinity sites.

Ligand-regulated oligomerization of beta(2)-adrenoceptors in a model lipid bilayer

The β₂-adrenoceptor (β₂AR) was one of the first Family A G protein-coupled receptors (GPCRs) shown to form oligomers in cellular membranes, yet we still know little about the number and arrangement of protomers in oligomers, the influence of ligands on the organization or stability of oligomers, or the requirement for other proteins to promote oligomerization. We used fluorescence resonance energy transfer (FRET) to characterize the oligomerization of purified β₂AR site-specifically labelled at three different positions with fluorophores and reconstituted into a model lipid bilayer. Our results suggest that the β₂AR is predominantly tetrameric following reconstitution into phospholipid vesicles. Agonists and antagonists have little effect on the relative orientation of protomers in oligomeric complexes. In contrast, binding of inverse agonists leads to significant increases in FRET efficiencies for most labelling pairs, suggesting that this class of ligand promotes tighter packing of protomers and/or the formation of more complex oligomers by reducing conformational fluctuations in individual protomers. The results provide new structural insights into β₂AR oligomerization and suggest a possible mechanism for the functional effects of inverse agonists.

A division of labor: asymmetric roles for GPCR subunits in receptor dimers

The functional architecture of dimeric or oligomeric GPCR signaling remains incompletely understood. Using a clever combination of receptor-G protein fusions and various receptor mutations, new research provides a glimpse into how oligomers might be arranged with respect to the G proteins they interact with.

GPCR homomers and heteromers: a better choice as targets for drug development than GPCR monomers?

G protein-coupled receptors (GPCR) are targeted by many therapeutic drugs marketed to fight against a variety of diseases. Selection of novel lead compounds are based on pharmacological parameters obtained assuming that GPCR are monomers. However, many GPCR are expressed as dimers/oligomers. Therefore, drug development may consider GPCR as homo- and hetero-oligomers. A two-state dimer receptor model is now available to understand GPCR operation and to interpret data obtained from drugs interacting with dimers, and even from mixtures of monomers and dimers. Heteromers are distinct entities and therefore a given drug is expected to have different affinities and different efficacies depending on the heteromer. All these concepts would lead to broaden the therapeutic potential of drugs targeting GPCRs, including receptor heteromer-selective drugs with a lower incidence of side effects, or to identify novel pharmacological profiles using cell models expressing receptor heteromers.

Modelling the interdependence between the stoichiometry of receptor oligomerization and ligand binding for a coexisting dimer/tetramer receptor system

Many G protein-coupled receptors have been shown to exist as oligomers, but the oligomerization state and the effects of this on receptor function are unclear. For some G protein-coupled receptors, in ligand binding assays, different radioligands provide different maximal binding capacities. Here we have developed mathematical models for co-expressed dimeric and tetrameric species of receptors. We have considered models where the dimers and tetramers are in equilibrium and where they do not interconvert and we have also considered the potential influence of the ligands on the degree of oligomerization. By analogy with agonist efficacy, we have considered ligands that promote, inhibit or have no effect on oligomerization. Cell surface receptor expression and the intrinsic capacity of receptors to oligomerize are quantitative parameters of the equations. The models can account for differences in the maximal binding capacities of radioligands in different preparations of receptors and provide a conceptual framework for simulation and data fitting in complex oligomeric receptor situations.

Pirenzepine promotes the dimerization of muscarinic M1 receptors through a three-step binding process

Ligand binding to G protein-coupled receptors is a complex process that involves sequential receptor conformational changes, ligand translocation, and possibly ligand-induced receptor oligomerization. Binding events at muscarinic acetylcholine receptors are usually interpreted from radioligand binding studies in terms of two-step ligand-induced receptor isomerization. We report here, using a combination of fluorescence approaches, on the molecular mechanisms for Bodipy-pirenzepine binding to enhanced green fluorescent protein (EGFP)-fused muscarinic M1 receptors in living cells. Real time monitoring, under steady-state conditions, of the strong fluorescence energy transfer signal elicited by this interaction permitted a fine kinetic description of the binding process. Time-resolved fluorescence measurements allowed us to identify discrete EGFP lifetime species and to follow their redistribution upon ligand binding. Fluorescence correlation spectroscopy, with EGFP brightness analysis, showed that EGFP-fused muscarinic M1 receptors predominate as monomers in the absence of ligand and dimerize upon pirenzepine binding. Finally, all these experimental data could be quantitatively reconciled into a three-step mechanism, with four identified receptor conformational states. Fast ligand binding to a peripheral receptor site initiates a sequence of conformational changes that allows the ligand to access to inner regions of the protein and drives ligand-receptor complexes toward a high affinity dimeric state.

The neuropeptide Y (NPY) Y2 receptors are largely dimeric in the kidney, but monomeric in the forebrain

The neuropeptide Y(NPY) Y2 receptors are detected largely as dimers in the clonal expressions in CHO cells and in particulates from rabbit kidney cortex. However, in two areas of the forebrain (rat or rabbit piriform cortex and hypothalamus), these receptors are found mainly as monomers. Evidence is presented that this difference relates to large levels of G proteins containing the Gi alpha -subunit in the forebrain areas. The predominant monomeric status of these Y2 receptors should also be physiologically linked to large synaptic inputs of the agonist NPY. The rabbit kidney and the human CHO cell-expressed Y2 dimers are converted by agonists to monomers in vitro at a similar rate in the presence of divalent cations.

Heterooligomers of the muscarinic receptor and G proteins purified from porcine atria

Muscarinic receptor extracted from porcine atria in digitonin-cholate copurified with Galpha(o), Galpha(i1-3), and caveolins. The presence of complexes was confirmed by coimmunoprecipitation of the receptor, alpha-subunits, and caveolins in various combinations. Homooligomers of alpha(i2) were detected on Western blots, and heterooligomers of alpha(i2) and alpha(o) were identified by coimmunoprecipitation; thus, a complex may contain at least two alpha-subunits. Other combinations of alpha-subunit were not detected. The ratio of total alpha-subunit to receptor was near 1, as measured by [(35)S]GTPgammaS and the antagonist [(3)H]quinuclidinylbenzilate, and the binding of [(35)S]GTPgammaS was manifestly biphasic. The ratio of alpha(o) to alpha(i1,2) also was near 1, as determined from the intensity of Western blots. Cardiac muscarinic receptors therefore can be purified as a mixture of complexes that contain caveolins and oligomers of alpha-subunit, some of which are heteromeric. Each complex would appear to contain equal numbers of alpha-subunit and the receptor.

Binding of orthosteric ligands to the allosteric site of the M(2) muscarinic cholinergic receptor

The M(2) muscarinic receptor has two topographically distinct sites: the orthosteric site and an allosteric site recognized by compounds such as gallamine. It also can exhibit cooperative effects in the binding of orthosteric ligands, presumably to the orthosteric sites within an oligomer. Such effects would be difficult to interpret, however, if those ligands also bound to the allosteric site. Monomers of the hemagglutinin (HA)- and FLAG-tagged human M(2) receptor therefore have been purified from coinfected Sf9 cells and examined for any effect of the antagonist N-methyl scopolamine or the agonist oxotremorine-M on the rate at which N-[(3)H]methyl scopolamine dissociates from the orthosteric site (k(obsd)). The predominantly monomeric status was confirmed by coimmunoprecipitation and by cross-linking with bis(sulfosuccinimidyl)suberate. Both N-methyl scopolamine and oxotremorine-M acted in a cooperative manner to decrease k(obsd) by 4.5- and 9.1-fold, respectively; the corresponding estimates of affinity (log K(L)) are -2.55 +/- 0.13 and -2.29 +/- 0.14. Gallamine and the allosteric ligand obidoxime decreased k(obsd) by more than 100-fold (log K(L) = -4.12 +/- 0.04) and by only 1.1-fold (log K(L) = -1.73 +/- 0.91), respectively. Obidoxime reversed the effect of N-methyl scopolamine, oxotremorine-M, and gallamine in a manner that could be described by a model in which all four ligands compete for a common allosteric site. Ligands generally assumed to be exclusively orthosteric therefore can act at the allosteric site of the M(2) receptor, albeit at comparatively high concentrations.