One‐third of the ∼400 nonodorant G protein‐coupled receptors (GPCRs) are still orphans. Although a considerable number of these receptors are likely to transduce cellular signals in response to ligands that remain to be identified, they may also have ligand‐independent functions. Several members of the GPCR family have been shown to modulate the function of other receptors through heterodimerization. We show that GPR50, an orphan GPCR, heterodimerizes constitutively and specifically with MT1 and MT2 melatonin receptors, using biochemical and biophysical approaches in intact cells. Whereas the association between GPR50 and MT2 did not modify MT2 function, GPR50 abolished high‐affinity agonist binding and G protein coupling to the MT1 protomer engaged in the heterodimer. Deletion of the large C‐terminal tail of GPR50 suppressed the inhibitory effect of GPR50 on MT1 without affecting heterodimerization, indicating that this domain regulates the interaction of regulatory proteins to MT1. Pairing orphan GPCRs to potential heterodimerization partners might be of clinical importance and may become a general strategy to better understand the function of orphan GPCRs.

G protein-coupled receptor (GPCR) oligomerization is a growing concept that has emerged from several studies suggesting that GPCRs can form both homo- and heterodimers. Using both coimmunoprecipitation and bioluminescence resonance energy transfer (BRET) approaches, we established that the vasopressin V1a, V2, and the oxytocin receptors exist as homo- and hetero-dimers in transfected human embryonic kidney 293T cells. Each receptor protomer had a similar propensity to form homo- and heterodimers, indicating that their relative expression levels may determine the homo-/heterodimer ratio. The finding that immature forms of the receptor can be immunoprecipitated as homo- and heterodimers and the detection by BRET of such oligomer in endoplasmic reticulum-enriched fractions suggest that the oligomerization processes take place early during biosynthesis. Treatment with agonists or antagonists did not modify the BRET among any of the vasopressin and oxytocin receptor pairs studied, indicating that the dimerization state of the receptors is not regulated by ligand binding once they have reached the cell surface. Taken together, these results strongly support the notion that GPCR dimerization is a constitutive process.

Several G protein-coupled receptors have been shown to exist as homo-and hetero-oligomeric complexes in living cells. However, the link between ligand-induced receptor activation and its oligomerization state as well as the proportion of the total receptor population that can engage in oligomeric complexes remain open questions. Here, the closely related human MT1 and MT2 melatonin receptors (MT1R, MT2R) were used to address these issues. Bioluminescence resonance energy transfer (BRET) experiments in living HEK 293 cells revealed that these receptors form homo- and hetero-oligomers. Constitutive energy transfer was observed for all receptor combinations at physiological expression levels and could be detected in single cell BRET experiments. Inhibition of the energy transfer by dilution of the BRET partners identified MT1R and MT2R dimers as the predominant receptor species, and this oligomerization state did not change upon agonist and antagonist binding. Agonists, neutral antagonists, and inverse agonists all promoted increases in BRET values for MT2R but not for MT1R homodimers in living cells and isolated plasma membranes. This indicates that no correlation could be inferred between the receptor activation state and the dimerization state of the receptor. This also suggests that ligand-promoted BRET increases represent specific ligand-induced conformational changes of pre-existing dimers rather then increased dimerization. The observation that ligands favored the energy transfer within the hetero-oligomer from MT1R to MT2R but not in the reverse orientation, from MT2R to MT1R, supports this view.

G protein–coupled receptors (GPCRs) compose one of the largest families of membrane proteins involved in intracellular signaling. They are involved in numerous physiological and pathological processes and are prime candidates for drug development. Over the past decade, an increasing number of studies have reported heteromerization between GPCRs. Many investigations in heterologous systems have provided important indications of potential novel pharmacology; however, the physiological relevance of these findings has yet to be established with endogenous receptors in native tissues. In this review, we focus on family A GPCRs and describe the techniques and criteria to assess their heteromerization. We conclude that advances in approaches to study receptor complex functionality in heterologous systems, coupled with techniques that enable specific examination of native receptor heteromers in vivo, are likely to establish GPCR heteromers as novel therapeutic targets.

Heterodimerization has been documented for several members of the G protein-coupled receptor (GPCR) superfamily, including the closely related MT(1) and MT(2) melatonin receptors. However, the relative abundance of hetero-versus homodimers and the specific properties, which can be attributed to each form, are difficult to determine. Using a bioluminescence resonance energy transfer (BRET) donor saturation assay, we show that half-maximal MT(1)/MT(2) heterodimer formation is reached for expression levels as low as approximately 4000 receptors per cell. The relative propensity of MT(1) homodimer and MT(1)/MT(2) heterodimer formation are similar, whereas that for the MT(2) homodimer formation is 3- to 4-fold lower. These data indicate that both the relative expression level of each receptor isoform and the affinities between monomers may determine the actual proportion of homo- and heterodimers. The specific interaction of ligands with the MT(1)/MT(2) heterodimer was studied using a BRET-based assay as a readout for the conformational changes of the heterodimer. An MT(1)/MT(2) heterodimer-specific profile and ligands selective for the MT(1)/MT(2) heterodimer compared with the MT(2) homodimer could be identified. Classic radioligand binding and BRET studies suggest that heterodimers contain two functional ligand binding sites that maintain their respective selectivity for MT(1) and MT(2) ligands. Occupation of either binding site is sufficient to induce a conformational change within the heterodimer. Taken together, these results show that the probability of GPCR heterodimer formation may be equal to or even higher than that of the corresponding homodimers and that specific properties of heterodimers can be revealed using a BRET-based ligand/receptor interaction assay.