The G protein-coupled receptor rhodopsin in the native membrane

Efficient coupling of transducin to monomeric rhodopsin in a phospholipid bilayer

G protein-coupled receptors (GPCRs) are seven transmembrane domain proteins that transduce extracellular signals across the plasma membrane and couple to the heterotrimeric family of G proteins. Like most intrinsic membrane proteins, GPCRs are capable of oligomerization, the function of which has only been established for a few different receptor systems. One challenge in understanding the function of oligomers relates to the inability to separate monomeric and oligomeric receptor complexes in membrane environments. Here we report the reconstitution of bovine rhodopsin, a GPCR expressed in the retina, into an apolipoprotein A-I phospholipid particle, derived from high density lipoprotein (HDL). We demonstrate that rhodopsin, when incorporated into these 10 nm reconstituted HDL (rHDL) particles, is monomeric and functional. Rhodopsin.rHDL maintains the appropriate spectral properties with respect to photoactivation and formation of the active form, metarhodopsin II. Additionally, the kinetics of metarhodopsin II decay is similar between rhodopsin in native membranes and rhodopsin in rHDL particles. Photoactivation of monomeric rhodopsin.rHDL also results in the rapid activation of transducin, at a rate that is comparable with that found in native rod outer segments and 20-fold faster than rhodopsin in detergent micelles. These data suggest that monomeric rhodopsin is the minimal functional unit in G protein activation and that oligomerization is not absolutely required for this process.

Differential distribution of proteins and lipids in detergent-resistant and detergent-soluble domains in rod outer segment plasma membranes and disks

Membrane heterogeneity plays a significant role in regulating signal transduction and other cellular activities. We examined the protein and lipid components associated with the detergent-resistant membrane (DRM) fractions from retinal rod outer segment (ROS) disk and plasma membrane-enriched preparations. Proteomics and correlative western blot analysis revealed the presence of alpha and beta subunits of the rod cGMP-gated ion channel and glucose transporter type 1, among other proteins. The glucose transporter was present exclusively in ROS plasma membrane (not disks) and was highly enriched in DRMs, as was the cGMP-gated channel beta-subunit. In contrast, the majority of rod opsin and ATP-binding cassette transporter A4 was localized to detergent-soluble domains in disks. As expected, the cholesterol : fatty acid mole ratio was higher in DRMs than in the corresponding parent membranes (disk and plasma membranes, respectively) and was also higher in disks compared to plasma membranes. Furthermore, the ratio of saturated : polyunsaturated fatty acids was also higher in DRMs compared to their respective parent membranes (disk and plasma membranes). These results confirm that DRMs prepared from both disks and plasma membranes are enriched in cholesterol and in saturated fatty acids compared to their parent membranes. The dominant fatty acids in DRMs were 16 : 0 and 18 : 0; 22 : 6n3 and 18 : 1 levels were threefold higher and twofold lower, respectively, in disk-derived DRMs compared to plasma membrane-derived DRMs. We estimate, based on fatty acid recovery that DRMs account for only approximately 8% of disks and approximately 12% of ROS plasma membrane.

Noladin ether, a putative endocannabinoid, inhibits mu-opioid receptor activation via CB2 cannabinoid receptors

We examined the occurrence of possible changes in mRNA expression and the functional activity of opioid receptors after acute in vivo and in vitro treatment with the putative endogenous cannabinoid noladin ether. While noladin ether (NE) demonstrates agonist activity at CB1 cannabinoid receptors, recent data indicate that NE acts as a full agonist at CB2 cannabinoid receptors too. Considering the functional interactions between opioids and cannabinoids, it is of interest to examine whether NE affects the opioid system. To that end, we studied the influence of NE on mu-opioid receptor (MOR) mRNA expression and MOR mediated G-protein signaling. We used real-time PCR and [35S]GTPgammaS binding assays to examine the changes of MOR mRNA levels and the capability of the mu-opioid agonist peptide ([D-Ala2,(NMe)Phe4,Gly5-ol]enkephalin (DAMGO) in activating regulatory G-proteins via MORs in forebrain membrane fractions of wild-type (w.t., CB1+/+) and CB1 receptor deficient transgenic mice (knockout, CB1-/-). We found, that the expression of MOR mRNAs significantly decreased both in CB1+/+ and CB1-/- forebrain after a single injection of NE at 1 mg/kg when compared to control. Consequently, MOR-mediated signaling is attenuated after acute in vivo treatment with NE in both CB1+/+ and CB1-/- mice. Inhibition on MOR mediated activation is observed after in vitro NE administration as well. Radioligand binding competition studies showed that the noticed effect of NE on MOR signaling is not mediated through MORs. Both in vivo and in vitro attenuations of NE can be antagonized by the CB2 selective antagonist SR144528. Taken together, our data suggest that the NE caused pronounced decrease in the activity of MOR is mediated via CB2 cannabinoid receptors.

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

FLAG- and HA-tagged M2 muscarinic receptors from coinfected Sf9 cells have been purified in digitonin-cholate and reconstituted into phospholipid vesicles. The purified receptor was predominantly monomeric: it showed no detectable coimmunoprecipitation; it migrated as a monomer during electrophoresis before or after cross-linking with bis(sulfosuccinimidyl)suberate; and it bound agonists and antagonists in a manner indicative of identical and mutually independent sites. Receptor cross-linked after reconstitution or after reconstitution and subsequent solubilization in digitonin-cholate migrated almost exclusively as a tetramer. The binding properties of the reconstituted receptor mimicked those reported previously for cardiac muscarinic receptors. The apparent capacity for N-[3H]methylscopolamine (NMS) was only 60% of that for [3H]quinuclidinylbenzilate (QNB), yet binding at saturating concentrations of [3H]QNB was inhibited fully and in a noncompetitive manner at comparatively low concentrations of unlabeled NMS. Reconstitution of the receptor with a saturating quantity of functional G proteins led to the appearance of three classes of sites for the agonist oxotremorine-M in assays with [3H]QNB; GMP-PNP caused an apparent interconversion from highest to lowest affinity and the concomitant emergence of a fourth class of intermediate affinity. All of the data can be described quantitatively in terms of cooperativity among four interacting sites, presumably within a tetramer; the effect of GMP-PNP can be accommodated as a shift in the distribution of tetramers between two states that differ in their cooperative properties. Monomers of the M2 receptor therefore can be assembled into tetramers with binding properties that closely resemble those of the muscarinic receptor in myocardial preparations.

Agonist occupancy of a single monomeric element is sufficient to cause internalization of the dimeric beta2-adrenoceptor

A range of studies have indicated that many rhodopsin-like, family A G protein-coupled receptors, including the beta(2)-adrenoceptor, exist and probably function as dimers. It is less clear if receptors internalize as dimers and if agonist occupancy of only one element of a dimer is sufficient to cause internalization of a receptor dimer into the cell. We have used a chemogenomic approach to demonstrate that this is the case. Following expression of the wild type beta(2)-adrenoceptor, isoprenaline but not 1-(3''4'-dihydroxyphenyl)-3-methyl-1-butanone, which does not have significant affinity for the wild type receptor, caused receptor internalization. By contrast, 1-(3'4'-dihydroxyphenyl)-3-methyl-1-butanone, but not isoprenaline that does not have high affinity for the mutated receptor, caused internalization of Asp(113)Serbeta(2)-adrenoceptor. Following co-expression of wild type and Asp(113)Serbeta(2)-adrenoceptors each of isoprenaline and 1-(3'4'-dihydroxyphenyl)-3-methyl-1-butanone caused the co-internalization of both of these two forms of the receptor. Co-expressed wild type and Asp(113)Serbeta(2)-adrenoceptors were able to be co-immunoprecipitated and 1-(3'4'-dihydroxyphenyl)-3-methyl-1-butanone produced internalization of the wild type receptor that was not prevented by the beta-adrenoceptor antagonist propranolol that binds with high affinity only to the wild type receptor. These results demonstrate that agonist occupancy of either single binding site of the beta(2)-adrenoceptor dimer is sufficient to cause internalization of the dimer and that antagonist occupation of one of the two ligand binding sites is unable to prevent agonist-mediated internalization.

G protein-coupled receptor dimerisation: Molecular basis and relevance to function

The belief that G protein-coupled receptors exist and function as monomeric, non-interacting species has been largely supplanted in recent years by evidence, derived from a range of approaches, that indicate they can form dimers and/or higher-order oligomeric complexes. Key roles for receptor homo-dimerisation include effective quality control of protein folding prior to plasma membrane delivery and interactions with hetero-trimeric G proteins. Growing evidence has also indicated the potential for many co-expressed G protein-coupled receptors to form hetero-dimers/oligomers. The relevance of this to physiology and function is only beginning to be unravelled but may offer great potential for more selective therapeutic intervention.

Modeling of the complex between transducin and photoactivated rhodopsin, a prototypical G-protein-coupled receptor

Obtaining a reliable 3D model for the complex formed by photoactivated rhodopsin (R*) and its G-protein, transducin (Gtalphabetagamma), would significantly benefit the entire field of structural biology of G-protein-coupled receptors (GPCRs). In this study, we have performed extensive configurational sampling for the isolated C-terminal fragment of the alpha-subunit of transducin, Gtalpha 340-350, within cavities of photoactivated rhodopsin formed by different energetically feasible conformations of the intracellular loops. Our results suggested a new 3D model of the rhodopsin-transducin complex that fully satisfied all available experimental data on site-directed mutagenesis of rhodopsin and Gtalphabetagamma as well as data from disulfide-linking experiments. Importantly, the experimental data were not used as a priori constraints in model building. We performed a thorough comparison of existing computational models of the rhodopsin-transducin complex with each other and with current experimental data. It was found that different models suggest interactions with different molecules in the rhodopsin oligomer, that providing valuable guidance in design of specific novel experimental studies of the R*-Gtalphabetagamma complex. Finally, we demonstrated that the isolated Gtalpha 340-350 fragment does not necessarily bind rhodopsin in the same binding mode as the same segment in intact Gtalpha.

Transducin activation by nanoscale lipid bilayers containing one and two rhodopsins

Nanodiscs are nanometer scale planar membranes of controlled size that are rendered soluble in aqueous solution via an encircling amphipathic membrane scaffold protein "belt" (Bayburt, T. H., Grinkova, Y. V., and Sligar, S. G. (2002) Nano. Lett. 2, 853-856). Integral membrane proteins can be self-assembled into the Nanodisc bilayer with defined stoichiometry, which allows an unprecedented opportunity to investigate the nature of the oligomerization state of a G-protein-coupled receptor and its coupling to heterotrimeric G-proteins. We generated Nanodiscs having one and two rhodopsins present in the 10-nm-diameter lipid bilayer domain. Efficient transducin activation and isolation of a high affinity transducin-metarhodopsin II complex was demonstrated for a monodisperse and monomeric receptor. A population of Nanodiscs containing two rhodopsins was generated using an increased ratio of receptor to membrane scaffold protein in the self-assembly mixture. The two-rhodopsin population was isolated and purified by density gradient centrifugation. Interestingly, in this case, only one of the two receptors present in the Nanodisc was able to form a stable metarhodopsin II-G-protein complex. Thus there is clear evidence that a monomeric rhodopsin is capable of full coupling to transducin. Importantly, presumably due to steric interactions, it appears that only a single receptor in the Nanodiscs containing two rhodopsins can interact with G-protein. These results have important implications for the stoichiometry of receptor-G-protein coupling and cross talk in signaling pathways.

Opsin oligomerization in a heterologous cell system

Using bioluminescence resonance energy transfer (BRET) we studied opsin oligomerization in heterologous expression systems and quantitatively assessed its oligomerization state. BRET2 saturation and competition experiments were performed with live COS-7 cells expressing Rluc-and GFP2-tagged receptor constructs. BRET2 saturation curves obtained were hyperbolic, and the calculated oligomerization state (N = 1 for dimers) suggested that opsin (N = 1.34 +/- 0.25) forms higher oligomers. Very high BRET2 values obtained for the opsin homo-dimer pair indicated a large energy transfer efficiency (E) and for cases where E >> 0.1 a modified saturation curve was proposed. The existence of homo-dimer complexes was additionally supported by competition assay results and was also observed in HEK-293 cells. Furthermore, evidence was provided for homo-and hetero-dimerization of family A (beta2-adrenergic) and B (gastric inhibitory polypeptide, GIP) receptors. In summary, these experiments demonstrate homo-and hetero-dimerization for opsin, beta 2-adrenergic, and GIP receptors.

International Union of Basic and Clinical Pharmacology. LXVII. Recommendations for the Recognition and Nomenclature of G Protein-Coupled Receptor Heteromultimers

G protein-coupled receptors (GPCRs) have long been considered to be monomeric membrane proteins. Although numerous recent studies have indicated that GPCRs can form multimeric complexes, the functional and pharmacological consequences of this phenomenon have remained elusive. With the discovery that the functional GABA(B) receptor is an obligate heterodimer and with the use of energy transfer technologies, it is now accepted that GPCRs can form heteromultimers. In some cases, specific properties of such heteromers not shared by their respective homomers have been reported. Although in most cases these properties have only been observed in heterologous expression systems, there are a few reports describing data consistent with such heteromultimeric GPCR complexes also existing in native tissues. The present article illustrates well-documented examples of such native multimeric complexes, lists a number of recommendations for recognition and acceptance of such multimeric receptors, and gives recommendations for their nomenclature.

Stabilizing effect of Zn2+ in native bovine rhodopsin

Single-molecule force spectroscopy (SMFS) is a powerful tool to dissect molecular interactions that govern the stability and function of proteins. We applied SMFS to understand the effect of Zn2+ on the molecular interactions underlying the structure of rhodopsin. Force-distance curves obtained from SMFS assays revealed the strength and location of molecular interactions that stabilize structural segments within this receptor. The inclusion of ZnCl2 in SMFS assay buffer increased the stability of most structural segments. This effect was not mimicked by CaCl2, CdCl2, or CoCl2. Thus, Zn2+ stabilizes the structure of rhodopsin in a specific manner.

Crystal packing analysis of Rhodopsin crystals

Oligomerization has been proposed as one of several mechanisms to regulate the activity of G protein-coupled receptors (GPCRs), but little is known about the structure of GPCR oligomers. Crystallographic analyses of two new crystal forms of rhodopsin reveal an interaction surface which may be involved in the formation of functional dimers or oligomers. New crystallization conditions lead to the formation of two crystal forms with similar rhodopsin-rhodopsin interactions, but changes in the crystal lattice are induced by the addition of different surfactant additives. However, the intermolecular interactions between rhodopsin molecules in these crystal structures may reflect the contacts necessary for the maintenance of dimers or oligomers in rod outer segment membranes. Similar contacts may assist in the formation of dimers or oligomers in other GPCRs as well. These new dimers are compared with other models proposed by crystallography or EM and AFM studies. The inter-monomer surface contacts are different for each model, but several of these models coincide in implicating helix I, II, and H-8 as contributors to the main contact surface stabilizing the dimers.

G protein-coupled receptor rhodopsin

The rhodopsin crystal structure provides a structural basis for understanding the function of this and other G protein-coupled receptors (GPCRs). The major structural motifs observed for rhodopsin are expected to carry over to other GPCRs, and the mechanism of transformation of the receptor from inactive to active forms is thus likely conserved. Moreover, the high expression level of rhodopsin in the retina, its specific localization in the internal disks of the photoreceptor structures [termed rod outer segments (ROS)], and the lack of other highly abundant membrane proteins allow rhodopsin to be examined in the native disk membranes by a number of methods. The results of these investigations provide evidence of the propensity of rhodopsin and, most likely, other GPCRs to dimerize, a property that may be pertinent to their function.

Transmembrane segment peptides can disrupt cholecystokinin receptor oligomerization without affecting receptor function

Oligomerization of the G protein-coupled cholecystokinin (CCK) receptor has been demonstrated, but its molecular basis and functional importance are not clear. We now examine contributions of transmembrane (TM) segments to oligomerization of this receptor using a peptide competitive inhibition strategy. Oligomerization of CCK receptors tagged at the carboxyl terminus with Renilla luciferase or yellow fluorescent protein was quantified using bioluminescence resonance energy transfer (BRET). Synthetic peptides representing TM I, II, V, VI, and VII of the CCK receptor were utilized as competitors. Of these, only TM VI and VII peptides disrupted receptor BRET. Control studies established that the beta2-adrenergic receptor TM VI peptide that disrupts oligomerization of that receptor had no effect on CCK receptor BRET. Notably, disruption of CCK receptor oligomerization had no effect on agonist binding, biological activity, or receptor internalization. To gain insight into the face of TM VI contributing to oligomerization, we utilized analogous peptides with alanines in positions 315, 319, and 323 (interhelical face) or 317, 321, and 325 (external lipid-exposed face). The Ala317,321,325 peptide eliminated the disruptive effect on CCK receptor BRET, whereas the other mutant peptide behaved like wild-type TM VI. This suggests that the lipid-exposed face of the CCK receptor TM VI most contributes to oligomerization and supports external contact dimerization of helical bundles, rather than domain-swapped dimerization. Fluorescent CCK receptor mutants with residues 317, 321, and 325 replaced with alanines were also prepared and failed to yield significant resonance transfer signals using either BRET or a morphological FRET assay, further supporting this interpretation.

The alpha1b-adrenoceptor exists as a higher-order oligomer: effective oligomerization is required for receptor maturation, surface delivery, and function

Approaches to identify G protein-coupled receptor oligomers rather than dimers have been lacking. Using concatamers of fluorescent proteins, we established conditions to monitor sequential three-color fluorescence resonance energy transfer (3-FRET) and used these to detect oligomeric complexes of the alpha(1b)-adrenoceptor in single living cells. Mutation of putative key hydrophobic residues in transmembrane domains I and IV resulted in substantial reduction of sequential 3-FRET and was associated with lack of protein maturation, prevention of plasma membrane delivery, and elimination of signaling function. Although these mutations prevented cell surface delivery, bimolecular fluorescence complementation studies indicated that they did not ablate protein-protein interactions and confirmed endoplasmic reticulum/Golgi retention of the transmembrane domain I plus transmembrane domain IV mutated receptor. The transmembrane domain I plus transmembrane domain IV mutated receptor was a "dominant-negative" in blocking cell surface delivery of the wild-type receptor. Mutations only in transmembrane domain I did not result in a reduction in 3-FRET, whereas restricting mutation to transmembrane domain IV did result in reduced 3-FRET. Mutations in either transmembrane domain I or transmembrane domain IV, however, were sufficient to eliminate cell surface delivery. Terminal N-glycosylation is insufficient to determine cell surface delivery because both transmembrane domain I and transmembrane domain IV mutants matured as effectively as the wild-type receptor. These data indicate that the alpha(1b)-adrenoceptor is able to form oligomeric rather than only simple dimeric complexes and that disruption of effective oligomerization by introducing mutations into transmembrane domain IV has profound consequences for cell surface delivery and function.

Assembly and structural characterization of an authentic complex between human follicle stimulating hormone and a hormone-binding ectodomain of its receptor

Follicle stimulating hormone (FSH) is secreted from the pituitary gland to regulate reproduction in vertebrates. FSH signals through a G-protein coupled receptor (FSHR) on the target cell surface. We describe here the strategy to produce a soluble FSH-FSHR complex that involves the co-secretion of a truncated FSHR ectodomain (FSHR(HB)) and a covalently linked FSHalphabeta heterodimer from baculovirus-infected insect cells. FSH binds to FSHR(HB) with a high affinity comparable to that for the full-length receptor. The crystal structure of the FSH-FSHR(HB) complex provides explanations for the high affinity and specificity of FSH interaction with FSHR, and it shows an unexpected dimerization of these complexes. Here we also compare the crystal structure with theoretical models of the FSH-FSHR-binding mode. We conclude that the FSH-FSHR(HB) structure gives an authentic representation of FSH binding to intact FSHR.

The role of GPCR dimerisation/oligomerisation in receptor signalling

A wide range of techniques have been employed to examine the quaternary structure of G-protein-coupled receptors (GPCRs). Although it is well established that homo-dimerisation is common, recent studies have sought to explore the physical basis of these interactions and the role of dimerisation in signal transduction. Growing evidence hints at the existence of higher-order organisation of individual GPCRs and the potential for hetero-dimerisation between pairs of co-expressed GPCRs. Here we consider how both homo-dimerisation/oligomerisation and hetero-dimerisation can regulate signal transduction through GPCRs and the potential consequences of this for function of therapeutic medicines that target GPCRs. Hetero-dimerisation is not the sole means by which co-expressed GPCRs may regulate the function of one another. Heterologous desensitisation may be at least as important and we also consider if this can be the basis for physiological antagonism between pairs of co-expressed GPCRs. Although there may be exceptions (Meyer et al. 2006), a great deal of recent evidence has indicated that most G-protein-coupled receptors (GPCRs) do not exist as monomers but rather as dimers or, potentially, within higher-order oligomers (Milligan 2004b; Park et al. 2004). Support for such models has been provided by a range of studies employing different approaches, including co-immunoprecipitation of differentially epitope-tagged but co-expressed forms of the same GPCR, co-operativity in ligand binding and a variety of resonance energy transfer techniques (Milligan and Bouvier 2005). Only for the photon receptor rhodopsin has the organisational structure of a GPCR been studied in situ. The application of atomic force microscopy to murine rod outer segment discs indicated that rhodopsin is organised in a series of parallel arrays of dimers (Liang et al. 2003) and based on this, molecular models were constructed to try to define and interpret regions of contact between the monomers (Fotiadis et al. 2004). Only for relatively few other GPCRs are details of the molecular basis of dimerisation available but within this limited data set, recent studies on the dopamine D2 receptor suggest a means by which information on the binding of an agonist can be transmitted between the two elements of the dimer via the dimer interface (Guo et al. 2005). Although the availability of cDNAs encoding molecularly defined GPCRs has allowed high-throughput screening for ligands that modulate GPCR function, this is performed almost exclusively in heterologous cell lines transfected to express only the specific GPCR of interest. Given that the human genome contains some 400-450 genes encoding non-chemosensory GPCRs, it is clear that any individual cell of the body may express a considerable number of GPCRs. Interactions between these, either via hetero-dimerisation, via heterologous desensitisation or via the integration of downstream signals can potentially alter the pharmacology, sensitivity and function of receptor agonists and hence produce varied responses. In this article, we will use specific examples to consider the role of homo-dimerisation/oligomerisation in GPCR function and whether either direct hetero-dimerisation or heterologous desensitisation between pairs of co-expressed GPCRs affects the function of the receptor pairs.

Curvature and hydrophobic forces drive oligomerization and modulate activity of rhodopsin in membranes

G protein-coupled receptors (GPCRs) are essential components of cellular signaling pathways. They are the targets of many current pharmaceuticals and are postulated to dimerize or oligomerize in cellular membranes in conjunction with their functional mechanisms. We demonstrate using fluorescence resonance energy transfer how association of rhodopsin occurs by long-range lipid-protein interactions due to geometrical forces, yielding greater receptor crowding. Constitutive association of rhodopsin is promoted by a reduction in membrane thickness (hydrophobic mismatch), but also by an increase in protein/lipid molar ratio, showing the importance of interactions extending well beyond a single annulus of boundary lipids. The fluorescence data correlate with the pK(a) for the MI-to-MII transition of rhodopsin, where deprotonation of the retinylidene Schiff base occurs in conjunction with helical movements leading to activation of the photoreceptor. A more dispersed membrane environment optimizes formation of the MII conformation that results in visual function. A flexible surface model explains both the dispersal and activation of rhodopsin in terms of bilayer curvature deformation (strain) and hydrophobic solvation energy. The bilayer stress is related to the lateral pressure profile in terms of the spontaneous curvature and associated bending rigidity. Transduction of the strain energy (frustration) of the bilayer drives protein oligomerization and conformational changes in a coupled manner. Our findings illuminate the physical principles of membrane protein association due to chemically nonspecific interactions in fluid lipid bilayers. Moreover, they yield a conceptual framework for understanding how the tightly regulated lipid compositions of cellular membranes influence their protein-mediated functions.

Crystal structure of a photoactivated deprotonated intermediate of rhodopsin

The changes that lead to activation of G protein-coupled receptors have not been elucidated at the structural level. In this work we report the crystal structures of both ground state and a photoactivated deprotonated intermediate of bovine rhodopsin at a resolution of 4.15 A. In the photoactivated state, the Schiff base linking the chromophore and Lys-296 becomes deprotonated, reminiscent of the G protein-activating state, metarhodopsin II. The structures reveal that the changes that accompany photoactivation are smaller than previously predicted for the metarhodopsin II state and include changes on the cytoplasmic surface of rhodopsin that possibly enable the coupling to its cognate G protein, transducin. Furthermore, rhodopsin forms a potentially physiologically relevant dimer interface that involves helices I, II, and 8, and when taken with the prior work that implicates helices IV and V as the physiological dimer interface may account for one of the interfaces of the oligomeric structure of rhodopsin seen in the membrane by atomic force microscopy. The activation and oligomerization models likely extend to the majority of other G protein-coupled receptors.

Oligomeric structure of the alpha1b-adrenoceptor: comparisons with rhodopsin

The structural basis of the quaternary organization of rhodopsin has recently been explored and modeled. Because information obtained from studying rhodopsin has frequently been directly applicable to other G protein-coupled receptors we wished to ascertain if dimeric and/or oligomeric forms of the alpha(1b)-adrenoceptor could be observed and if so whether rhodopsin might provide insights into the quaternary structure of this receptor. Co-immunoprecipitation and both conventional and time-resolved fluorescence resonance energy transfer studies demonstrated quaternary structure of the alpha(1b)-adrenoceptor and, in concert with the reconstitution of fragments of this receptor, provided information on the molecular basis of these interactions. Development of three color fluorescence resonance energy transfer (FRET) allowed the imaging of alpha(1b)-adrenoceptor oligomers in single living cells. Mutation of hydrophobic residues in transmembrane domains I and IV of the receptor resulted in marked reduction in three color FRET suggesting an alteration in oligomeric organization and potential similarities with rhodopsin. The mutated alpha(1b)-adrenoceptor was unable to reach the cell surface, did not become terminally N-glycosylated and was unable to signal.

Comparison of the retinitis pigmentosa mutations in rhodopsin with a functional map of the C5a receptor

We compare the known retinitis pigmentosa (RP) mutations in rhodopsin with mutational data obtained for the complement factor 5a receptor (C5aR), a member of the rhodopsin-like family of G protein-coupled receptors (GPCRs). We have performed genetic analyses that define residues that are required for C5aR folding and function. The cognate residues in rhodopsin are not preferentially mutated in RP, suggesting that the predominant molecular defect in RP involves more than simple misfolding or inactivation. Energy calculations are performed to elucidate the structural effects of the RP mutations. Many of these mutations specifically disrupt the environment of the retinal prosthetic group of rhodopsin, and these do not correspond to essential residues in C5aR. This may be because a retinal group is present in rhodopsin but not in C5aR. Another subset of RP mutations is more generally important for receptor structure, and these mutations correlate with essential residues of C5aR.

Nanoimaging for protein misfolding and related diseases

Misfolding and aggregation of proteins is a common thread linking a number of important human health problems. The misfolded and aggregated proteins are inducers of cellular stress and activators of immunity in neurodegenerative diseases. They might possess clear cytotoxic properties, being responsible for the dysfunction and loss of cells in the affected organs. Despite the crucial importance of protein misfolding and abnormal interactions, very little is currently known about the molecular mechanism underlying these processes. Factors that lead to protein misfolding and aggregation in vitro are poorly understood, not to mention the complexities involved in the formation of protein nanoparticles with different morphologies (e.g., the nanopores) in vivo. A better understanding of the molecular mechanisms of misfolding and aggregation might facilitate development of the rational approaches to prevent pathologies mediated by protein misfolding. The conventional tools currently available to researchers can only provide an averaged picture of a living system, whereas much of the subtle or short-lived information is lost. We believe that the existing and emerging nanotools might help solving these problems by opening the entirely novel pathways for the development of early diagnostic and therapeutic approaches. This article summarizes recent advances of the nanoscience in detection and characterization of misfolded protein conformations. Based on these findings, we outline our view on the nanoscience development towards identification intracellular nanomachines and/or multicomponent complexes critically involved in protein misfolding.

Serotonin 5-HT2C receptor homodimer biogenesis in the endoplasmic reticulum: real-time visualization with confocal fluorescence resonance energy transfer

Dimerization is a common property of G-protein-coupled receptors (GPCR). While the formation of GPCR dimers/oligomers has been reported to play important roles in regulating receptor expression, ligand binding, and second messenger activation, less is known about how and where GPCR dimerization occurs. The present study was performed to identify the precise cellular compartment in which class A GPCR dimer/oligomer biogenesis occurs. We addressed this issue using confocal microscopy and fluorescence resonance energy transfer (FRET) to monitor GPCR proximity within discrete intracellular compartments of intact living cells. Time-lapse confocal imaging was used to follow CFP- and YFP-tagged serotonin 5-HT2C receptors during biosynthesis in the endoplasmic reticulum (ER), trafficking through the Golgi apparatus and subsequent expression on the plasma membrane. Real-time monitoring of FRET between CFP- and YFP-tagged 5-HT2C receptors was performed by acceptor photobleaching within discrete regions of the ER, Golgi, and plasma membrane. The FRET signal was dependent on the ratio of CFP- to YFP-tagged 5-HT2C receptors expressed in each region and was independent of receptor expression level, as predicted for proteins in a non-random, clustered distribution. FRET efficiencies measured in the ER, Golgi, and plasma membrane were similar. These experiments provide direct evidence for homodimerization/oligomerization of class A GPCR in the ER and Golgi of intact living cells, and suggest that dimer/oligomer formation is a naturally occurring step in 5-HT2C receptor maturation and processing.

Single-molecule studies of membrane proteins

Characterizing membrane proteins with single-molecule techniques provides structural and functional insights that cannot be obtained with conventional approaches. Recent studies show that atomic force microscopy (AFM) in the context of a 'lab on a tip' enables the measurement of multiple parameters of membrane proteins. This multifunctional tool can be applied to probe the oligomeric states and conformational changes of membrane protein assemblies in their native environment. The ability to determine diverse properties at high spatial resolution facilitates the mapping of structural flexibilities, electrostatic potentials and electric currents. By using the AFM tip as tweezer, it is possible to characterize unfolding and refolding pathways of single proteins and the location of their molecular interactions. These interactions dictate the stability of the protein and might be modulated by ligands that alter the protein's functional state.

G-protein-coupled receptor heterodimers: pharmacology, function and relevance to drug discovery

The growing recognition that members of the rhodopsin-like family A G-protein-coupled receptors (GPCRs) exist and function as dimers or higher-order oligomers, and that GPCR hetero-dimers and -oligomers are present in physiological tissues, offers novel opportunities for drug discovery. Differential pharmacology, function and regulation of GPCR hetero-dimers and -oligomers suggest means to selectively target GPCRs in different tissues and hint that the mechanism of function of several pharmacological agents might be different in vivo than anticipated from simple ligand-screening programmes that rely on heterologous expression of a single GPCR.

Autosomal recessive retinitis pigmentosa and E150K mutation in the opsin gene

Retinitis pigmentosa (RP) is a heterogeneous group of hereditary disorders of the retina caused by mutation in genes of the photoreceptor proteins with an autosomal dominant (adRP), autosomal recessive (arRP), or X-linked pattern of inheritance. Although there are over 100 identified mutations in the opsin gene associated with RP, only a few of them are inherited with the arRP pattern. E150K is the first reported missense mutation associated with arRP. This opsin mutation is located in the second cytoplasmic loop of this G protein-coupled receptor. E150K opsin expressed in HEK293 cells and reconstituted with 11-cis-retinal displayed an absorption spectrum similar to the wild type (WT) counterpart and activated G protein transducin slightly faster than WT receptor. However, the majority of E150K opsin showed a higher apparent molecular mass in SDS-PAGE and was resistant to endoglycosidase H deglycosidase. Instead of being transported to the plasma membrane, E150K opsin is partially colocalized with the cis/medial Golgi compartment markers such as GM130 and Vti1b but not with the trans-Golgi network. In contrast to the endoplasmic reticulum-retained adRP mutant, P23H opsin, Golgi-retained E150K opsin did not influence the proper transport of the WT opsin when coexpressed in HEK293 cells. This result is consistent with the recessive pattern of inheritance of this mutation. Thus, our study reveals a novel molecular mechanism for retinal degeneration that results from deficient export of opsin from the Golgi apparatus.

mu opioid and CB1 cannabinoid receptor interactions: reciprocal inhibition of receptor signaling and neuritogenesis

Several studies have described functional interactions between opioid and cannabinoid receptors; the underlying mechanism(s) have not been well explored. One possible mechanism is direct receptor-receptor interactions, as has been demonstrated for a number of G-protein-coupled receptors. In order to investigate interactions between opioid and cannabinoid receptors, we epitope tagged mu, delta and kappa opioid receptors with Renilla luciferase and CB1 cannabinoid or CCR5 chemokine receptors with yellow fluorescent protein and examined the extent of substrate hydrolysis induced bioluminescence resonance energy transfer (BRET) signal. We find that coexpression of opioid receptors with cannabinoid receptors, but not with chemokine receptors, leads to a significant increase in the level of BRET signal, suggesting that the opioid-cannabinoid interactions are receptor specific. In order to examine the implications of these interactions to signaling, we used GTPgammaS binding and mitogen-activated protein kinase (MAPK) phosphorylation assays and examined the effect of receptor activation on signaling. We find that the mu receptor-mediated signaling is attenuated by the CB1 receptor agonist; this effect is reciprocal and is seen in heterologous cells and endogenous tissue expressing both receptors. In order to explore the physiological consequences of this interaction, we examined the effect of receptor activation on the extent of Src and STAT3 phosphorylation and neuritogenesis in Neuro-2A cells. We find that the simultaneous activation of mu opioid and CB1 cannabinoid receptors leads to a significant attenuation of the response seen upon activation of individual receptors, implicating a role for receptor-receptor interactions in modulating neuritogenesis.

The opioid ligand binding of human μ-opioid receptor is modulated by novel splice variants of the receptor

The pharmacological actions of morphine and morphine-like drugs, such as heroin, mediate primarily through the mu-opioid receptor (MOR). It has been proposed that the functional diversity of MOR may be related to alternative splicing of the MOR gene. Although a number of MOR mRNA splice variants have been reported, their biological function has been controversial. In this study, two novel splice variants of the human MOR gene were discovered. Splice variants 1 and 2 (here called the SV1 and SV2) retain different portions of intron I. In vitro translation of SV1 and SV2 produced proteins with the predicted molecular weights. The splice variant proteins were identical to the wild-type MOR-1 up to the first transmembrane domains, but were different after the first intracellular loop domains. SV1 and SV2 of hMOR were present in human neuroblastoma NMB cells and human whole brain confirmed by RT-PCR. In a receptor binding assay, cells expressing the SV1 and SV2 do not exhibit binding to [(3)H]diprenorphine. The formations of MOR.SV1 and MOR.SV2 heterodimers were demonstrated by co-immunoprecipitation and bioluminescence resonance energy transfer between MOR and splice variants. Co-transfection of MOR-GFP and SV-DsRed gene showed that MOR and SV protein co-localized at the cytoplasmic membrane. In NMB cells expressing human MOR gene, transfection of SV1 or SV2 reduced binding activity of the endogenous MOR. These data support a potential role of SV1 and SV2 proteins as possible biological modulator of human mu-opioid receptor.

Structure of the rhodopsin dimer: a working model for G-protein-coupled receptors

G-protein-coupled receptors (GPCRs) participate in virtually all physiological processes. They constitute the largest and most structurally conserved family of signaling molecules. Several class C GPCRs have been shown to exist as dimers in their active form and growing evidence indicates that many, if not all, class A receptors also form dimers and/or higher-order oligomers. High-resolution crystal structures are available only for the detergent-solubilized light receptor rhodopsin (Rho), the archetypal class A GPCR. In addition, Rho is the only GPCR for which the presumed higher-order oligomeric state has been demonstrated, by imaging native disk membranes using atomic force microscopy (AFM). Based on these data and the X-ray structure, an atomic model of Rho dimers has been proposed, a model that is currently scrutinized in various ways. AFM has also been used to measure the forces required to unfold single Rho molecules, thereby revealing which residues are responsible for Rho's stability. Recent functional analyses of fractions from solubilized disk membranes revealed that higher-order Rho oligomers are the most active species. These and other results have enhanced our understanding of GPCR structure and function.

Computational prediction of homodimerization of the A3 adenosine receptor

Increasing evidence suggests that G protein-coupled receptors form functional dimers or larger oligomeric complexes through homo- or heterodimerization, and that various transmembrane (TM) domains contribute dimerization interfaces. In this study, monomeric receptor structures - either the monomeric crystallographic structure of bovine rhodopsin or an A(3) adenosine receptor (AR) homology model - were dimerized by computational methods assuming various TM contact regions, optimized, and compared. The semi-empirical oligomeric structure of mouse rhodopsin studied in a native disc membrane with atomic force microscopy was used to establish the distance between monomers in the initial dimeric models. Among eight variations of symmetrical homodimers of bovine rhodopsin, the favored dimeric assembly closely resembled the semi-empirical model, in which TM domains 4 and 5 were the contact site, thus validating this approach. We used similar methods to generate eight homodimers of the A(3)AR and found the favored dimeric interface similarly to be TM4-5. By this method, dimeric variations - TM1-2, TM2-3, TM2-4, TM3-4, TM4-5, TM5-6, TM6-7, and TM7-1 - were constructed with the SYBYL 7.0 program by using a novel "fit-centroids-normal" method. Fitting atoms considered one of eight TM-TM centroids or seven-TM centroids, two centroids of each monomer, and a normal atom passing through the plane containing all centroids. Following molecular dynamics, the most energetically favorable contact modes were identified. In addition to TM4-5, which was favored in both rhodopsin and A(3)AR dimeric models, TM1-2 dimers in which helices 8 also contacted each other were judged favorable. The largest contact surface area between the monomers among the various homodimers, determined by van der Waals calculation with the MOLCAD surface program, was for the TM4-5 dimer. This contact surface also showed a high degree of shape complementarity. In addition, the TM4-5 dimers made by this theoretical method were more stable than the semi-empirically determined dimer.

Equilibrium between Metarhodopsin-I and Metarhodopsin-II Is Dependent on the Conformation of the Third Cytoplasmic Loop

Rhodopsin is a G-protein-coupled receptor (GPCR) that is the light detector in the rod cells of the eye. Rhodopsin is the best understood member of the large GPCR superfamily and is the only GPCR for which atomic resolution structures have been determined. However, these structures are for the inactive, dark-adapted form. Characterization of the conformational changes in rhodopsin caused by light-induced activation is of wide importance, because the metarhodopsin-II photoproduct is analogous to the agonist-occupied conformation of other GPCRs, and metarhodopsin-I may be similar to antagonist-occupied GPCR conformations. In this work we characterize the interaction of antibody K42-41L with the metarhodopsin photoproducts. K42-41L is shown to inhibit formation of metarhodopsin-II while it stabilizes the metarhodopsin-I state. Thus, K42-41L recognizes an epitope accessible in dark-adapted rhodopsin and metarhodopsin-I that is lost upon formation of metarhodopsin-II. Previous work has shown that the peptide TGALQERSK is able to mimic the K42-41L epitope, and we have now determined the structure of the K42-41L-peptide complex. The structure demonstrates a central role for elements of the rhodopsin C3 loop, particularly Gln238 and Glu239, in the interaction with K42-41L. Geometric constraints taken from the antibody-bound peptide were used to model the epitope on the rhodopsin surface. The resulting model suggests that K42-41L locks the C3 loop into an extended conformation that is intermediate between two compact conformations seen in crystal structures of dark-adapted rhodopsin. Together, the structural and functional data strongly suggest that the equilibrium between metarhodopsin-I and metarhodopsin-II is dependent upon the conformation of the C3 loop. The biological implications of this model and its possible relations to dimeric and multimeric complexes of rhodopsin are discussed.

Rhodopsin self-associates in asolectin liposomes

We show that the photoreceptor rhodopsin (Rh) can exist in the membrane as a dimer or multimer using luminescence resonance energy transfer and FRET methods. Our approach looked for interactions between Rh molecules reconstituted into asolectin liposomes. The low receptor density used in the measurements ensured minimal receptor crowding and artifactual association. The fluorescently labeled Rh molecules were fully functional, as measured by their ability to activate the G protein transducin. The luminescence resonance energy transfer measurements revealed a distance of 47-50 Angstroms between Rh molecules. The measured efficiency of FRET between receptors was close to the theoretical maximum possible, indicating nearly quantitative Rh-Rh association. Together, these results provide compelling evidence that Rh spontaneously self-associates in membranes.

Functional and structural characterization of rhodopsin oligomers

A major question in G protein-coupled receptor signaling concerns the quaternary structure required for signal transduction. Do these transmembrane receptors function as monomers, dimers, or larger oligomers? We have investigated the oligomeric state of the model G protein-coupled receptor rhodopsin (Rho), which absorbs light and initiates a phototransduction-signaling cascade that forms the basis of vision. In this study, different forms of Rho were isolated using gel filtration techniques in mild detergents, including n-dodecyl-beta-D-maltoside, n-tetradecyl-beta-D-maltoside, and n-hexadecyl-beta-D-maltoside. The quaternary structure of isolated Rho was determined by transmission electron microscopy, demonstrating that in micelles containing n-dodecyl-beta-D-maltoside, Rho exists as a mixture of monomers and dimers whereas in n-tetradecyl-beta-D-maltoside and n-hexadecyl-beta-D-maltoside Rho forms higher ordered structures. Especially in n-hexadecyl-beta-D-maltoside, most of the particles are present in tightly packed rows of dimers. The oligomerization of Rho seems to be important for interaction with its cognate G protein, transducin. Although the activated Rho (Meta II) monomer or dimers are capable of activating the G protein, transducin, the activation process is much faster when Rho exists as organized dimers. Our studies provide direct comparisons between signaling properties of Meta II in different quaternary complexes.

Opsin is present as dimers in COS1 cells: identification of amino acids at the dimeric interface

Rhodopsin in the disk membranes of rod outer segments serves as the dim-light photoreceptor and is a prototypic member of a G protein-coupled receptor family. Electron and atomic-force microscopy indicate that rhodopsin is present as dimers in the native membranes. Here, we have expressed the protein, opsin, in COS1 cells and have studied its molecular state by using FRET and by intermolecular cross-linking after site-directed cysteine mutagenesis. To observe FRET, the ends of the genes corresponding to the N termini of the cyan or yellow fluorescent proteins were fused to the ends of the genes corresponding to the C terminus of the opsin and the resulting fused genes were expressed in COS1 cells. The emission spectra in situ of the expressed proteins were recorded, and FRET was then calculated. The result indicated intermolecular interaction between opsin molecules in COS1 cells. To identify the amino acids involved in the interaction, those predicted by molecular modeling to be at the dimer interface were mutated one at a time to cysteine, and dimer formation was measured by the rate of disulfide bond formation in the presence of cupric orthophenanthroline. The mutants W175C and Y206C formed the dimers most rapidly, showing that the two amino acids were at the dimer interface.

Detecting molecular interactions that stabilize native bovine rhodopsin

Using single-molecule force spectroscopy we probed molecular interactions within native bovine rhodopsin and discovered structural segments of well-defined mechanical stability. Highly conserved residues among G protein-coupled receptors were located at the interior of individual structural segments, suggesting a dual role for these segments in rhodopsin. Firstly, structural segments stabilize secondary structure elements of the native protein, and secondly, they position and hold the highly conserved residues at functionally important environments. Two main classes of force curves were observed. One class corresponded to the unfolding of rhodopsin with the highly conserved Cys110-Cys187 disulfide bond remaining intact and the other class corresponded to the unfolding of the entire rhodopsin polypeptide chain. In the absence of the Cys110-Cys187 bond, the nature of certain molecular interactions within folded rhodopsin was altered. These changes highlight the structural importance of this disulfide bond and may form the basis of dysfunctions associated with its absence.

Inhibition of Serotonin 5-Hydroxytryptamine2C Receptor Function through Heterodimerization

Although dimerization appears to be a common property of G-protein-coupled receptors (GPCRs), it remains unclear whether a GPCR dimer binds one or two molecules of ligand and whether ligand binding results in activation of one or two G-proteins when measured using functional assays in intact living cells. Previously, we demonstrated that serotonin 5-hydroxytryptamine2C (5-HT2C) receptors form homodimers (Herrick-Davis, K., Grinde, E., and Mazurkiewicz, J. (2004) Biochemistry 43, 13963-13971). In the present study, an inactive 5-HT(2C) receptor was created and coexpressed with wild-type 5-HT2C receptors to determine whether dimerization regulates receptor function and to determine the ligand/dimer/G-protein stoichiometry in living cells. Mutagenesis of Ser138 to Arg (S138R) produced a 5-HT2C receptor incapable of binding ligand or stimulating inositol phosphate (IP) signaling. Confocal fluorescence imaging revealed plasma membrane expression of yellow fluorescent protein-tagged S138R receptors. Expression of wild-type 5-HT2C receptors in an S138R-expressing stable cell line had no effect on ligand binding to wild-type 5-HT2C receptors, but inhibited basal and 5-HT-stimulated IP signaling as well as constitutive and 5-HT-stimulated endocytosis of wild-type 5-HT2C receptors. M1 muscarinic receptor activation of IP production was normal in the S138R-expressing cells. Heterodimerization of S138R with wild-type 5-HT2C receptors was visualized in living cells using confocal fluorescence resonance energy transfer (FRET). FRET was dependent on the donor/acceptor ratio and independent of the receptor expression level. Therefore, inactive 5-HT2C receptors inhibit wild-type 5-HT2C receptor function by forming nonfunctional heterodimers expressed on the plasma membrane. These results are consistent with a model in which one GPCR dimer binds two molecules of ligand and one G-protein and indicate that dimerization is essential for 5-HT receptor function.

Design and synthesis of specific probes for human 5-HT4 receptor dimerization studies

Recently, human 5-HT4 receptors have been demonstrated to form constitutive dimers in living cells. To evaluate the role of dimerization on the 5-HT4 receptor function, we investigated the conception and the synthesis of bivalent molecules able to influence the dimerization process. Their conception is based on a model of the 5-HT4 receptor dimer derived from protein/protein docking experiments. These bivalent ligands are constituted by two ML10302 units, a specific 5-HT4 ligand, linked through a spacer of different sizes and natures. These synthesized bivalent ligands were evaluated in binding assays and cyclic AMP production on the 5-HT4(e/g) receptor isoform stably transfected in C6 glial cells. Our data showed that bivalent ligands conserved a similar affinity compared to the basal ML10302 unit. Nevertheless, according to the nature and the size of the spacer, the pharmacological profile of ML10302 is more or less conserved. In view of the interest of bivalent ligands for investigating the GPCR dimerization process, these 5-HT4 specific bivalent ligands constitute valuable pharmacological tools for the study of 5-HT4 receptor dimerization.

Imaging and detecting molecular interactions of single transmembrane proteins

Single-molecule atomic force microscopy (AFM) provides novel ways to characterize structure-function relationships of native membrane proteins. High-resolution AFM-topographs allow observing substructures of single membrane proteins at sub-nanometer resolution as well as their conformational changes, oligomeric state, molecular dynamics and assembly. Complementary to AFM imaging, single-molecule force spectroscopy experiments allow detecting molecular interactions established within and between membrane proteins. The sensitivity of this method makes it possible to detect the interactions that stabilize secondary structures such as transmembrane alpha-helices, polypeptide loops and segments within. Changes in temperature or protein-protein assembly do not change the position of stable structural segments, but influence their stability established by collective molecular interactions. Such changes alter the probability of proteins to choose a certain unfolding pathway. Recent examples have elucidated unfolding and refolding pathways of membrane proteins as well as their energy landscapes. We review current and future potential of these approaches to reveal insights into membrane protein structure, function, and unfolding as we recognize that they could help answering key questions in the molecular basis of certain neuro-pathological dysfunctions.

G-protein-coupled receptors in drug discovery: nanosizing using cell-free technologies and molecular biology approaches

Signal transduction by G-protein-coupled receptors (GPCRs) underpins a multitude of physiological processes. Ligand recognition by the receptor leads to activation of a generic molecular switch involving heterotrimeric G-proteins and guanine nucleotides. Signal transduction has been studied extensively with both cell-based systems and assays comprising isolated signaling components. Interest and commercial investment in GPCRs in areas such as drug targets, orphan receptors, high throughput screening, biosensors, and so on will focus greater attention on assay development to allow for miniaturization, ultra-high throughput and, eventually, microarray/biochip assay formats. Although cell-based assays are adequate for many GPCRs, it is likely that these formats will limit the development of higher density GPCR assay platforms mandatory for other applications. Stable, robust, cell-free signaling assemblies comprising receptor and appropriate molecular switching components will form the basis of future GPCR assay platforms adaptable for such applications as microarrays. The authors review current cell-free GPCR assay technologies and molecular biological approaches for construction of novel, functional GPCR assays.

Monomeric G-protein-coupled receptor as a functional unit

Rhodopsin, the first purified G-protein-coupled receptor (GPCR), was characterized as a functional monomer 30 year ago, but dimerization of GPCRs recently became the new paradigm of signal transduction. It has even been claimed, on the basis of recent biophysical and biochemical studies, that this new concept could be extended to higher-order oligomerization. Here this view is challenged. The new studies of rhodopsin and other simple (class 1a) GPCRs solubilized in detergent are re-assessed and are compared to the earlier classical studies of rhodopsin and other membrane proteins solubilized in detergent. The new studies are found to strengthen rather than invalidate the conclusions of the early ones and to support a monomeric model for rhodopsin and other class 1a GPCRs. A molecular model is proposed for the functional coupling of a rhodopsin monomeric unit with a G-protein heterotrimer. This model should be valid even for GPCRs that exist as structural dimers.

Methods to monitor the quaternary structure of G protein-coupled receptors

A wide range of approaches has been applied to examine the quaternary structure of G protein-coupled receptors, the basis of such protein-protein interactions and how such interactions might modulate the pharmacology and function of these receptors. These include co-immunoprecipitation, various adaptations of resonance energy transfer techniques, functional complementation studies and the analysis of ligand-binding data. Each of the available techniques has limitations that restrict interpretation of the data. However, taken together, they provide a coherent body of evidence indicating that many, if not all, G protein-coupled receptors exist and function as dimer/oligomers. Herein we assess the widely applied techniques and discuss the relative benefits and limitations of these approaches.

Unfolding and extraction of a transmembrane alpha-helical peptide: dynamic force spectroscopy and molecular dynamics simulations

An atomic force microscope (AFM) was used to visualize CWALP(19)23 peptides ((+)H(3)N-ACAGAWWLALALALALALALWWA-COO(-)) inserted in gel-phase DPPC and DSPC bilayers. The peptides assemble in stable linear structures and domains. A model for the organization of the peptides is given from AFM images and a 20 ns molecular dynamics (MD) simulation. Gold-coated AFM cantilevers were used to extract single peptides from the bilayer through covalent bonding to the cystein residue. Experimental and simulated force curves show two distinct force maxima. In the simulations these two maxima correspond to the extraction of the two pairs of tryptophan residues from the membrane. Unfolding of the peptide precedes extraction of the second distal set of tryptophans. To probe the energies involved, AFM force curves were obtained from 10 to 10(4) nm/s and MD force curves were simulated with 10(8)-10(11) nm/s pulling velocities (V). The velocity relationship with the force, F, was fitted to two fluctuation adhesive potential models. The first assumes the pulling produces a constant bias in the potential and predicts an F approximately ln (V) relationship. The second takes into account the ramped bias that the linker feels as it is being driven out of the adhesion complex and scales as F approximately (ln V)2/3.

The CXCR1 and CXCR2 Receptors Form Constitutive Homo- and Heterodimers Selectively and with Equal Apparent Affinities

Both homo- and heterodimeric interactions between the CXCR1 and CXCR2 chemokine receptors were observed following co-expression of forms of these receptors in HEK293 cells using assays, including co-immunoprecipitation, single cell imaging of fluorescence resonance energy transfer, cell surface time-resolved fluorescence resonance energy transfer, and bioluminescence resonance energy transfer. These interactions were constitutive and unaffected by the presence of the agonist interleukin 8 and selective as no significant interactions were noted between either the CXCR1 or CXCR2 receptor and the alpha(1A)-adrenoreceptor. Saturation bioluminescence resonance energy transfer indicated that heteromeric interactions between CXCR1 and CXCR2 were of similar affinity as the corresponding homomeric interactions. A novel endoplasmic reticulum trapping strategy demonstrated that these interactions were initiated during protein synthesis and maturation and prior to cell surface delivery. These studies indicated that CXCR1-CXCR2 heterodimers are as likely to form in cells co-expressing these two chemokine receptors as the corresponding homodimers and stand in contrast to previous studies indicating an inability of the CXCR1 receptor to homodimerize or to interact with the CXCR2 receptor (Trettel, F., Di Bartolomeo, S., Lauro, C., Catalano, M., Ciotti, M. T., and Limatola, C. (2003) J. Biol. Chem. 278, 40980-40988).

Oligomerization of the fifth transmembrane domain from the adenosine A2A receptor

The human adenosine A2A receptor (A(2A)R) belongs to one of the largest family of membrane proteins, the G-protein coupled receptors (GPCRs), characterized by seven transmembrane (TM) helices. Little is known about the determinants of their structures, folding, assembly, activation mechanisms, and oligomeric states. Previous studies in our group showed that peptides corresponding to all seven TM domains form stable helical structures in detergent micelles and lipid vesicles. However, the peptides behave differently; TM5 is the only peptide to have a ratio [theta]222/[theta]208 obtained by circular dichroism (CD) spectroscopy>1. This finding suggested to us that TM5 might self-associate. In the present study, we investigate the unique properties of the TM5 domain. We performed detailed analyses of TM5 peptide behavior in membrane-mimetic environments using CD spectroscopy, fluorescence spectroscopy and Förster resonance energy transfer, and gel electrophoresis. We find that TM5 peptide has the ability to self-associate to form oligomeric structures in various hydrophobic milieus and that these oligomers are highly resistant to temperature and chemical denaturation. We also find that mutation of the full-length A(2A)R at position M193, which is located in the fifth TM domain, noticeably alters A(2A)R monomer: dimer ratio as observed on SDS-PAGE. Our results suggest that parallel association of TM5 dimers may play a role in the known adenosine A2A receptor dimerization. This study represents the first evidence of an individual GPCR transmembrane domain self-association.

Effect of packing density on rhodopsin stability and function in polyunsaturated membranes

Rod outer segment disk membranes are densely packed with rhodopsin. The recent notion of raft or microdomain structures in disk membranes suggests that the local density of rhodopsin in disk membranes could be much higher than the average density corresponding to the lipid/protein ratio. Little is known about the effect of high packing density of rhodopsin on the structure and function of rhodopsin and lipid membranes. Here we examined the role of rhodopsin packing density on membrane dynamic properties, membrane acyl chain packing, and the structural stability and function of rhodopsin using a combination of biophysical and biochemical techniques. We reconstituted rhodopsin into large unilamellar vesicles consisting of polyunsaturated 18:0,22:6n3PC, which approximates the polyunsaturated nature of phospholipids in disk membranes, with rhodopsin/lipid ratios ranging from 1:422 to 1:40. Our results showed that increased rhodopsin packing density led to reduced membrane dynamics revealed by the fluorescent probe 1,6-diphenyl-1,3,5-hexatriene, increased phospholipid acyl chain packing, and reduced rhodopsin activation, yet it had minimal impact on the structural stability of rhodopsin. These observations imply that densely packed rhodopsin may impede the diffusion and conformational changes of rhodopsin, which could reduce the speed of visual transduction.

Dimerization of chemokine receptors and its functional consequences

It became clear over the recent years that most, if not all, G protein-coupled receptors (GPCR) are able to form dimers or higher order oligomers. Chemokine receptors make no exception to this new rule and both homo- and heterodimerization were demonstrated for CC and CXC receptors. Functional analyses demonstrated negative binding cooperativity between the two subunits of a dimer. The consequence is that only one chemokine can bind with high affinity onto a receptor dimer. In the context of receptor activation, this implies that the motions of helical domains triggered by the binding of agonists induce correlated changes in the other protomer. The impact of the chemokine dimerization process in terms of co-receptor function and drug development is discussed.

Functional complementation and the analysis of opioid receptor homodimerization

Complementation of function after coexpression of pairs of nonfunctional G protein-coupled receptors that contain distinct inactivating mutations supports the hypothesis that such receptors exist as dimers. Chimeras between members of the metabotropic glutamate receptor-like family have been particularly useful because the N-terminal ligand binding and heptahelical transmembrane elements can be considered distinct domains. To examine the utility of a related approach for opioid receptors, fusion proteins were generated in which a pertussis toxin-resistant (Cys351Ile) variant of the G protein Gi1alpha was linked to the C-terminal tails of the delta opioid peptide (DOP), kappa opioid peptide, and mu opioid peptide receptors. Each was functional as measured by agonist stimulation of guanosine 5'-([gamma-35S]thio)triphosphate ([35S]GTPgammaS) binding in Gialpha immunoprecipitates from membranes of pertussis toxin-treated HEK293 cells. Agonist function was eliminated either by fusion of the receptors to Gi1alphaGly202Ala,Cys351Ile or mutation of a pair of conserved Val residues in intracellular loop 2 of each receptor. Coexpression, but not simple mixing, of the two inactive fusion proteins reconstituted agonist-loading of [35S]GTPgammaS for each receptor. At equimolar amounts, reconstitution of DOP receptor function was more extensive than kappa or mu opioid receptor. Reconstitution of DOP function required two intact receptors and was not achieved by provision of extra Gi1alphaCys351Ile membrane anchored by linkage to DOP transmembrane domain 1. Inactive forms of all G protein alpha subunits can be produced by mutations equivalent to Gi1alphaGly202Ala. Because the amino acids modified in the opioid receptors are highly conserved in most rhodopsin-like receptors, this approach should be widely applicable to study the existence and molecular basis of receptor dimerization.