Opsin oligomerization in a heterologous cell system

Computational Modeling and Characterization of Peptides Derived from Nanobody Complementary-Determining Region 2 (CDR2) Targeting Active-State Conformation of the β2-Adrenergic Receptor (β2AR)

This study assessed the suitability of the complementarity-determining region 2 (CDR2) of the nanobody (Nb) as a template for the derivation of nanobody-derived peptides (NDPs) targeting active-state β2-adrenergic receptor (β2AR) conformation. Sequences of conformationally selective Nbs favoring the agonist-occupied β2AR were initially analyzed by the informational spectrum method (ISM). The derived NDPs in complex with β2AR were subjected to protein-peptide docking, molecular dynamics (MD) simulations, and metadynamics-based free-energy binding calculations. Computational analyses identified a 25-amino-acid-long CDR2-NDP of Nb71, designated P4, which exhibited the following binding free-energy for the formation of the β2AR:P4 complex (ΔG = -6.8 ± 0.8 kcal/mol or a Ki = 16.5 μM at 310 K) and mapped the β2AR:P4 amino acid interaction network. In vitro characterization showed that P4 (i) can cross the plasma membrane, (ii) reduces the maximum isoproterenol-induced cAMP level by approximately 40% and the isoproterenol potency by up to 20-fold at micromolar concentration, (iii) has a very low affinity to interact with unstimulated β2AR in the cAMP assay, and (iv) cannot reduce the efficacy and potency of the isoproterenol-mediated β2AR/β-arrestin-2 interaction in the BRET2-based recruitment assay. In summary, the CDR2-NDP, P4, binds preferentially to agonist-activated β2AR and disrupts Gαs-mediated signaling.

Computational design and characterization of nanobody-derived peptides that stabilize the active conformation of the β2-adrenergic receptor (β2-AR)

This study aimed to design and functionally characterize peptide mimetics of the nanobody (Nb) related to the β₂-adrenergic receptor (β₂-AR) (nanobody-derived peptide, NDP). We postulated that the computationally derived and optimized complementarity-determining region 3 (CDR3) of Nb is sufficient for its interaction with receptor. Sequence-related Nb-families preferring the agonist-bound active conformation of β₂-AR were analysed using the informational spectrum method (ISM) and β₂-AR:NDP complexes studied using protein-peptide docking and molecular dynamics (MD) simulations in conjunction with metadynamics calculations of free energy binding. The selected NDP of Nb71, designated P3, was 17 amino acids long and included CDR3. Metadynamics calculations yielded a binding free energy for the β₂-AR:P3 complex of ΔG = (-7.23 ± 0.04) kcal/mol, or a Kd of (7.9 ± 0.5) μM, for T = 310 K. In vitro circular dichroism (CD) spectropolarimetry and microscale thermophoresis (MST) data provided additional evidence for P3 interaction with agonist-activated β₂-AR, which displayed ~10-fold higher affinity for P3 than the unstimulated receptor (MST-derived EC₅₀ of 3.57 µM vs. 58.22 µM), while its ability to inhibit the agonist-induced interaction of β₂-AR with β-arrestin 2 was less evident. In summary, theoretical and experimental evidence indicated that P3 preferentially binds agonist-activated β₂-AR.

Functional characterization of β2-adrenergic and insulin receptor heteromers

This study aimed to functionally characterize β₂-adrenergic (β₂AR) and insulin receptor (IR) heteromers in regard to β-arrestin 2 (βarr2) recruitment and cAMP signaling and to examine the involvement of the cytoplasmic portion of the IR β chain in heteromerization with β₂AR. Evidence for β₂AR:IR:βarr2 complex formation and the specificity of the IR:βarr2 interaction was first provided by bioinfomatics analysis. Receptor-heteromer investigation technology (HIT) then provided functional evidence of β₂AR:IR heterodimerization by showing isoproterenol-induced but not insulin-induced GFP²-βarr2 recruitment to the β₂AR:IR complex; the IR:βarr2 interaction was found to only be constitutive. The constitutive IR:βarr2 BRET signal (BRET_const) was significantly smaller in cells coexpressing IR-RLuc8 and a GFP²-βarr2 1-185 mutant lacking the proposed IR binding domain. β₂AR:IR heteromerization also influenced the pharmacological phenotype of β₂AR, i.e., its efficacy in recruiting βarr2 and activating cAMP signaling. Evidence suggesting involvement of the cytoplasmic portion of the IR β chain in the interaction with β₂AR was provided by BRET² saturation and HIT assays using an IR 1-1271 stop mutant lacking the IR C-terminal tail region. For the complex consisting of IR 1-1271-RLuc8:β₂AR-GFP², saturation was not reached, most likely reflecting random collisions between IR 1-1271 and β₂AR. Furthermore, in the HIT assay, no substantial agonist-induced increase in the BRET² signal was detected that would be indicative of βarr2 recruitment to the IR 1-1271:β₂AR heteromer. Complementary 3D visualization of β₂AR:IR provided supporting evidence for stability of the heterotetramer complex and identified amino acid residues involved in β₂AR:IR heteromerization. This article is part of the Special Issue entitled 'Receptor heteromers and their allosteric receptor-receptor interactions'.

Bidirectional allosteric interactions between cannabinoid receptor 1 (CB1) and dopamine receptor 2 long (D2L) heterotetramers

Type 1 cannabinoid (CB₁) and dopamine 2 long form (D_2L) receptors can physically interact to form heteromers that display unique pharmacology in vitro compared to homomeric complexes. Co-expression of CB₁ and D_2L and co-application of CB₁ and D₂ agonists increases cAMP levels while administration of either agonist alone decreases cAMP levels. To understand the observed co-agonist response, our first goal of the current study was to define the stoichiometry of CB₁/D_2L/Gα protein complexes. Using bioluminescence resonance energy transfer 2 (BRET²), we confirmed that, CB₁ homodimers, D_2L homodimers, and CB₁/D_2L heteromers are formed. By using sequential resonance energy transfer 2 (SRET²) combined with bimolecular fluorescence complementation (BiFC), we were able to demonstrate that CB₁/D_2L form heterotetramers consisting of CB₁ and D_2L homodimers. We demonstrated that CB₁/D_2L heterotetramers are coupled to at least two Gα proteins. The second aim of the study was to investigate allosteric effects of a D_2L agonist (quinpirole) on CB₁ receptor function and to investigate the effects of a CB₁ agonist [arachidonyl-2-chloroethylamide (ACEA)] on D_2L receptor function within CB₁/D_2L heterotetramers. Treating cells co-expressing CB₁ and D_2L with both ACEA and quinpirole switched CB₁ and D_2L receptor coupling and signaling from Gα_i to Gα_s proteins, enhanced β-arrestin1 recruitment and receptor co-internalization. The concept of bidirectional allosteric interaction within CB₁/D₂ heterotetramers has important implications for understanding the activity of receptor complexes in native tissues and under pathological conditions.

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.

Molecular Pharmacology of the Incretin Receptors

The incretin hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are important regulators of insulin and glucagon secretion as well as lipid metabolism and appetite. These biological functions make their respective receptors (GIPR and GLP-1R) attractive targets in the treatment of both type 2 diabetes mellitus (T2DM) and obesity. The use of these native peptides in the treatment of these conditions is limited by their short half-lives. However, long-acting GLP-1R agonists and inhibitors of the enzyme that rapidly inactivates GIP and GLP-1 (dipeptidyl peptidase IV) are in clinical use. Although there is a loss of response to both hormones in T2DM, this effect appears to be more pronounced for GIP. This has made targeting GIPR less successful than GLP-1R. Furthermore, results demonstrating that GIPR knockout mice were resistant to diet-induced obesity suggested that GIPR antagonists may prove to be useful therapeutics. More recently, molecules that activate both receptors have shown promise in terms of glycemic and body weight control. This review focused on recent advances in the understanding of the signaling mechanisms and regulation of these two clinically important receptors.

Oligomerization of Family B GPCRs: Exploration in Inter-Family Oligomer Formation

G-protein-coupled receptors (GPCRs) are classified into A to F subfamilies in which only families A, B, and C are present in mammals. Some of these GPCRs were found to form higher ordered structures such as oligomers with the discovery of interacting receptors in the form of homomers or heteromers. The importance of these oligomers on regulating receptor functions has recently been an intense research focus. It has been proposed that receptor oligomer formation has impact on its physiological importance on receptor trafficking, signaling, ligand-related regulation, and also is related to certain diseases. The present body of knowledge, however, comprises mainly intra-family oligomers formation and their consequences. Inter-family oligomers are recognized but there is limited information. This article aims to provide a current view regarding inter-family GPCR oligomerization in the subfamilies A, B, and C found in mammals.

Demonstration of a direct interaction between β2-adrenergic receptor and insulin receptor by BRET and bioinformatics

Glucose metabolism is under the cooperative regulation of both insulin receptor (IR) and β₂-adrenergic receptor (β₂AR), which represent the receptor tyrosine kinases (RTKs) and seven transmembrane receptors (7TMRs), respectively. Studies demonstrating cross-talk between these two receptors and their endogenous coexpression have suggested their possible interactions. To evaluate the effect of IR and prospective heteromerization on β₂AR properties, we showed that IR coexpression had no effect on the ligand binding properties of β₂AR; however, IR reduced β₂AR surface expression and accelerated its internalization. Additionally, both receptors displayed a similar distribution pattern with a high degree of colocalization. To test the possible direct interaction between β₂AR and IR, we employed quantitative BRET² saturation and competition assays. Saturation assay data suggested constitutive β₂AR and IR homo- and heteromerization. Calculated acceptor/donor (AD₅₀) values as a measure of the relative affinity for homo- and heteromer formation differed among the heteromers that could not be explained by a simple dimer model. In heterologous competition assays, a transient increase in the BRET² signal with a subsequent hyperbolical decrease was observed, suggesting higher-order heteromer formation. To complement the BRET² data, we employed the informational spectrum method (ISM), a virtual spectroscopy method to investigate protein-protein interactions. Computational peptide scanning of β₂AR and IR identified intracellular domains encompassing residues at the end of the 7^th TM domain and C-terminal tail of β₂AR and a cytoplasmic part of the IR β chain as prospective interaction domains. ISM further suggested a high probability of heteromer formation and homodimers as basic units engaged in heteromerization. In summary, our data suggest direct interaction and higher-order β₂AR:IR oligomer formation, likely comprising heteromers of homodimers.

Evidence for aggregation of protein kinase CK2 in the cell: a novel strategy for studying CK2 holoenzyme interaction by BRET(2)

Protein kinase CK2 is a ubiquitous pro-survival kinase whose substrate targets are involved in various cellular processes. Crystal structure analysis confirmed constitutive activity of the kinase, yet CK2 activity regulation in the cell is still obscure. In-vitro studies suggest autoinhibitory aggregation of the hetero-tetrameric CK2 holoenzyme as a basis for CK2 regulation. In this study, we applied bioluminescent resonance energy transfer (BRET) technology to investigate CK2 holoenzyme aggregation in living cells. We designed a BRET(2) pair consisting of the fusion proteins CK2α-Rluc8 and CK2α-GFP(2). This BRET(2) sensor reported specific interaction of CK2 holoenzyme complexes. Furthermore, the BRET(2) sensor was applied to study modulators of CK2 aggregation. We found that CK2 aggregation is not static and can be influenced by the CK2-binding protein alpha subunit of the heterotrimeric G-protein that stimulates adenylyl cyclase (Gαs) and the polycationic compound polylysine. Gαs, but not the CK2 substrate β-arrestin2, decreased the BRET(2) signal by up to 50%. Likewise polylysine, but not the CK2 inhibitor DRB, decreased the signal in a dose-dependent manner up to 50%. For the first time, we present direct experimental evidence for CK2 holoenzyme aggregates in the cell. Our data suggest that CK2 activity may be controlled by holoenzyme aggregation, to our knowledge a novel mechanism for protein kinase regulation. Moreover, the BRET(2) sensor used in our study is a novel tool for studying CK2 regulation by aggregation and pharmacological screening for novel allosteric CK2 effectors.

Homo- and hetero-oligomerization of β2-adrenergic receptor in receptor trafficking, signaling pathways and receptor pharmacology

The β2-adrenergic receptor (β2AR) is the prototypic member of G protein-coupled receptors (GPCRs) involved in the production of physiological responses to adrenaline and noradrenaline. Research done in the past few years vastly demonstrated that β2AR can form homo- and hetero-oligomers. Despite the fact that currently this phenomenon is widely accepted, the spread and relevance of β2AR oligomerization are still a matter of debate. This review considers the progress achieved in the field of β2AR oligomerization with focus on the implications of the receptor-receptor interactions to β2AR trafficking, pharmacology and downstream signal transduction pathways.

BRET Biosensor Analysis of Receptor Tyrosine Kinase Functionality

Bioluminescence resonance energy transfer (BRET) is an improved version of earlier resonance energy transfer technologies used for the analysis of biomolecular protein interaction. BRET analysis can be applied to many transmembrane receptor classes, however the majority of the early published literature on BRET has focused on G protein-coupled receptor (GPCR) research. In contrast, there is limited scientific literature using BRET to investigate receptor tyrosine kinase (RTK) activity. This limited investigation is surprising as RTKs often employ dimerization as a key factor in their activation, as well as being important therapeutic targets in medicine, especially in the cases of cancer, diabetes, neurodegenerative, and respiratory conditions. In this review, we consider an array of studies pertinent to RTKs and other non-GPCR receptor protein-protein signaling interactions; more specifically we discuss receptor-protein interactions involved in the transmission of signaling communication. We have provided an overview of functional BRET studies associated with the RTK superfamily involving: neurotrophic receptors [e.g., tropomyosin-related kinase (Trk) and p75 neurotrophin receptor (p75NTR)]; insulinotropic receptors [e.g., insulin receptor (IR) and insulin-like growth factor receptor (IGFR)] and growth factor receptors [e.g., ErbB receptors including the EGFR, the fibroblast growth factor receptor (FGFR), the vascular endothelial growth factor receptor (VEGFR) and the c-kit and platelet-derived growth factor receptor (PDGFR)]. In addition, we review BRET-mediated studies of other tyrosine kinase-associated receptors including cytokine receptors, i.e., leptin receptor (OB-R) and the growth hormone receptor (GHR). It is clear even from the relatively sparse experimental RTK BRET evidence that there is tremendous potential for this technological application for the functional investigation of RTK biology.

The Pseudo signal peptide of the corticotropin-releasing factor receptor type 2A prevents receptor oligomerization

N-terminal signal peptides mediate the interaction of native proteins with the translocon complex of the endoplasmic reticulum membrane and are cleaved off during early protein biogenesis. The corticotropin-releasing factor receptor type 2a (CRF(2(a))R) possesses an N-terminal pseudo signal peptide, which represents a so far unique domain within the large protein family of G protein-coupled receptors (GPCRs). In contrast to a conventional signal peptide, the pseudo signal peptide remains uncleaved and consequently forms a hydrophobic extension at the N terminus of the receptor. The functional consequence of the presence of the pseudo signal peptide is not understood. Here, we have analyzed the significance of this domain for receptor dimerization/oligomerization in detail. To this end, we took the CRF(2(a))R and the homologous corticotropin-releasing factor receptor type 1 (CRF(1)R) possessing a conventional cleaved signal peptide and conducted signal peptide exchange experiments. Using single cell and single molecule imaging methods (fluorescence resonance energy transfer and fluorescence cross-correlation spectroscopy, respectively) as well as biochemical experiments, we obtained two novel findings; we could show that (i) the CRF(2(a))R is expressed exclusively as a monomer, and (ii) the presence of the pseudo signal peptide prevents its oligomerization. Thus, we have identified a novel functional domain within the GPCR protein family, which plays a role in receptor oligomerization and which may be useful to study the functional significance of this process in general.

Regulation of GIP and GLP1 receptor cell surface expression by N-glycosylation and receptor heteromerization

In response to a meal, Glucose-dependent Insulinotropic Polypeptide (GIP) and Glucagon-like Peptide-1 (GLP-1) are released from gut endocrine cells into the circulation and interact with their cognate G-protein coupled receptors (GPCRs). Receptor activation results in tissue-selective pleiotropic responses that include augmentation of glucose-induced insulin secretion from pancreatic beta cells. N-glycosylation and receptor oligomerization are co-translational processes that are thought to regulate the exit of functional GPCRs from the ER and their maintenance at the plasma membrane. Despite the importance of these regulatory processes, their impact on functional expression of GIP and GLP-1 receptors has not been well studied. Like many family B GPCRs, both the GIP and GLP-1 receptors possess a large extracellular N-terminus with multiple consensus sites for Asn-linked (N)-glycosylation. Here, we show that each of these Asn residues is glycosylated when either human receptor is expressed in Chinese hamster ovary cells. N-glycosylation enhances cell surface expression and function in parallel but exerts stronger control over the GIP receptor than the GLP-1 receptor. N-glycosylation mainly lengthens receptor half-life by reducing degradation in the endoplasmic reticulum. N-glycosylation is also required for expression of the GIP receptor at the plasma membrane and efficient GIP potentiation of glucose-induced insulin secretion from the INS-1 pancreatic beta cell line. Functional expression of a GIP receptor mutant lacking N-glycosylation is rescued by co-expressed wild type GLP1 receptor, which, together with data obtained using Bioluminescence Resonance Energy Transfer, suggests formation of a GIP-GLP1 receptor heteromer.

Mathematical models for quantitative assessment of bioluminescence resonance energy transfer: application to seven transmembrane receptors oligomerization

The idea that seven transmembrane receptors (7TMRs; also designated G-protein coupled receptors, GPCRs) might form dimers or higher order oligomeric complexes was formulated more than 20 years ago and has been intensively studied since then. In the last decade, bioluminescence resonance energy transfer (BRET) has been one of the most frequently used biophysical methods for studying 7TMRs oligomerization. This technique enables monitoring physical interactions between protein partners in living cells fused to donor and acceptor moieties. It relies on non-radiative transfer of energy between donor and acceptor, depending on their intermolecular distance (1-10 nm) and relative orientation. Results derived from BRET-based techniques are very persuasive; however, they need appropriate controls and critical interpretation. To overcome concerns about the specificity of BRET-derived results, a set of experiments has been proposed, including negative control with a non-interacting receptor or protein, BRET dilution, saturation, and competition assays. This article presents the theoretical background behind BRET assays, then outlines mathematical models for quantitative interpretation of BRET saturation and competition assay results, gives examples of their utilization and discusses the possibilities of quantitative analysis of data generated with other RET-based techniques.

Receptor oligomerization in family B1 of G-protein-coupled receptors: focus on BRET investigations and the link between GPCR oligomerization and binding cooperativity

The superfamily of the seven transmembrane G-protein-coupled receptors (7TM/GPCRs) is the largest family of membrane-associated receptors. GPCRs are involved in the pathophysiology of numerous human diseases, and they constitute an estimated 30-40% of all drug targets. During the last two decades, GPCR oligomerization has been extensively studied using methods like bioluminescence resonance energy transfer (BRET) and today, receptor-receptor interactions within the GPCR superfamily is a well-established phenomenon. Evidence of the impact of GPCR oligomerization on, e.g., ligand binding, receptor expression, and signal transduction indicates the physiological and pharmacological importance of these receptor interactions. In contrast to the larger and more thoroughly studied GPCR subfamilies A and C, the B1 subfamily is small and comprises only 15 members, including, e.g., the secretin receptor, the glucagon receptor, and the receptors for parathyroid hormone (PTHR1 and PTHR2). The dysregulation of several family B1 receptors is involved in diseases, such as diabetes, chronic inflammation, and osteoporosis which underlines the pathophysiological importance of this GPCR subfamily. In spite of this, investigation of family B1 receptor oligomerization and especially its pharmacological importance is still at an early stage. Even though GPCR oligomerization is a well-established phenomenon, there is a need for more investigations providing a direct link between these interactions and receptor functionality in family B1 GPCRs. One example of the functional effects of GPCR oligomerization is the facilitation of allosterism including cooperativity in ligand binding to GPCRs. Here, we review the currently available data on family B1 GPCR homo- and heteromerization, mainly based on BRET investigations. Furthermore, we cover the functional influence of oligomerization on ligand binding as well as the link between oligomerization and binding cooperativity.

Beta-arrestin-based Bret2 screening assay for the "non"-beta-arrestin binding CB1 receptor

CB1 receptor (CB1R) antagonists have been demonstrated to be effective in treating obesity and related disorders. This study has been focused on establishing a beta-arrestin 2-based screening assay for the CB1R using BRET2 technology. When the existing BRET2 screening platform was applied to the CB1R, the authors discovered that the receptor interacted weakly with beta-arrestin 2, resulting in unsatisfactory assay performance. To enhance the beta-arrestin binding capacity, they replaced the C-terminal tail of the CB1R with tails from either the V2 or BRS3 receptors, both of which interact strongly with beta-arrestin 2. Using this chimeric approach, the authors screened a small compound library and identified 21 antagonist and inverse agonist hits with IC50 and EC50 values ranging from 0.3 nM to 7.5 microM. Both primary and secondary screening were performed with Z'>0.5, suggesting that the assay is a robust and cost-effective alternative to existing cell-based assays.

Negative cooperativity in H2 relaxin binding to a dimeric relaxin family peptide receptor 1

H2 relaxin, a member of the insulin superfamily, binds to the G-protein-coupled receptor RXFP1 (relaxin family peptide 1), a receptor that belongs to the leucine-rich repeat (LRR)-containing subgroup (LGRs) of class A GPCRs. We recently demonstrated negative cooperativity in INSL3 binding to RXFP2 and showed that this subgroup of GPCRs functions as constitutive dimers. In this work, we investigated whether the binding of H2 relaxin to RXFP1 also shows negative cooperativity, and whether this receptor functions as a dimer using BRET(2). Both binding and dissociation were temperature dependent, and the pH optimum for binding was pH 7.0. Our results showed that RXFP1 is a constitutive dimer with negative cooperativity in ligand binding, that dimerization occurs through the 7TM domain, and that the ectodomain has a stabilizing effect on this interaction. Dimerization and negative cooperativity appear to be general properties of LGRs involved in reproduction as well as other GPCRs.

Direct comparison of bioluminescence-based resonance energy transfer methods for monitoring of proteolytic cleavage

Bioluminescence resonance energy transfer (BRET) is a powerful tool for the study of protein-protein interactions and conformational changes within proteins. Two common implementations of BRET are BRET(1) with Renilla luciferase (RLuc) and coelenterazine h (CLZ, lambda(em) approximately 475 nm) and BRET(2) with the substrate coelenterazine 400a (CLZ400A substrate, lambda(em)=395 nm) as the respective donors. For BRET(1) the acceptor is yellow fluorescent protein (YFP) (lambda(em) approximately 535 nm), a mutant of green fluorescent protein (GFP), and for BRET(2) it is GFP(2) (lambda(em) approximately 515 nm). It is not clear from previous studies which of these systems has superior signal-to-background characteristics. Here we directly compared BRET(1) and BRET(2) by placing two different protease-specific cleavage sequences between the donor and acceptor domains. The intact proteins simulate protein-protein association. Proteolytic cleavage of the peptide linker simulates protein dissociation and can be detected as a change in the BRET ratios. Complete cleavage of its target sequence by thrombin changed the BRET(2) ratio by a factor of 28.9+/-0.2 (relative standard deviation [RSD], n=3) and changed the BRET(1) ratio by a factor of 3.05+/-0.07. Complete cleavage of a caspase-3 target sequence resulted in the BRET ratio changes by factors of 15.45+/-0.08 for BRET(2) and 2.00+/-0.04 for BRET(1). The BRET(2) assay for thrombin was 2.9 times more sensitive compared with the BRET(1) version. Calculated detection limits (blank signal+3sigma(b), where sigma(b)=standard deviation [SD] of blank signal) were 53 pM (0.002 U) thrombin with BRET(1) and 15 pM (0.0005 U) thrombin with BRET(2). The results presented here suggest that BRET(2) is a more suitable system than BRET(1) for studying protein-protein interactions and as a potential sensor for monitoring protease activity.

A day in the life of a G protein-coupled receptor: the contribution to function of G protein-coupled receptor dimerization

G protein-coupled receptors are one of the most actively studied families of proteins. However, despite the ubiquity of protein dimerization and oligomerization as a structural and functional motif in biology, until the last decade they were generally considered as monomeric, non-interacting polypeptides. For the metabotropic glutamate-like group of G protein-coupled receptors, it is now firmly established that they exist and function as dimers or, potentially, even within higher-order structures. Despite some evidence continuing to support the view that rhodopsin-like G protein-coupled receptors are predominantly monomers, many recent studies are consistent with the dimerization/oligomerization of such receptors. Key roles suggested for dimerization of G protein-coupled receptors include control of protein maturation and cell surface delivery and providing the correct framework for interactions with both hetero-trimeric G proteins and arrestins to allow signal generation and its termination. As G protein-coupled receptors are the most targeted group of proteins for the development of therapeutic small molecule medicines, recent indications that hetero-dimerization between co-expressed G protein-coupled receptors may be a common process offers the potential for the development of more selective and tissue restricted medicines. However, many of the key experiments have, so far, been limited to model cell systems. Priorities for the future include the generation of tools and reagents able to identify unequivocally potential G protein-coupled receptor hetero-dimers in native tissues and detailed analyses of the influence of hetero-dimerization on receptor function and pharmacology.

Cooperative binding of insulin-like Peptide 3 to a dimeric relaxin family peptide receptor 2

Insulin-like peptide 3 (INSL3) binds to a G protein-coupled receptor (GPCR) called relaxin family peptide receptor 2 (RXFP2). RXFP2 belongs to the leucine-rich repeat-containing subgroup (LGR) of class A GPCRs. Negative cooperativity has recently been demonstrated in other members of the LGR subgroup. In this work, the kinetics of INSL3 binding to HEK293 cells stably transfected with RXFP2 (HEK293-RXFP2) have been investigated in detail to study whether negative cooperativity occurs and whether this receptor functions as a dimer. Our results show that negative cooperativity is present and that INSL3-RXFP2 binding shows both similarities and differences with insulin binding to the insulin receptor. A dose-response curve for the negative cooperativity of INSL3 binding had a reverse bell shape reminiscent of that seen for the negative cooperativity of insulin binding to its receptor. This suggests that binding of INSL3 may happen in a trans rather than in a cis way in a receptor dimer. Bioluminescence resonance energy transfer (BRET(2)) experiments confirmed that RXFP2 forms constitutive homodimers. Heterodimerization between RXFP2 and RXFP1 was also observed.