G protein-coupled receptors (GPCRs) are among the most promising drug targets. They often form homo- and heterodimers with allosteric cross-talk between receptor entities, which contributes to fine tuning of transmembrane signaling. Specifically controlling the activity of GPCR dimers with ligands is a good approach to clarify their physiological roles and to validate them as drug targets. Here, we examined the mode of action of positive allosteric modulators (PAMs) that bind at the interface of the transmembrane domains of the heterodimeric GABAB receptor. Our site-directed mutagenesis results show that mutations of this interface impact the function of the three PAM tested. The data support the inference that they act at the active interface between both transmembrane domains, the binding site involving residues of the TM6s of the GABAB1 and the GABAB2 subunit. Importantly, the agonist activity of these PAMs involves a key region in the central core of the GABAB2 transmembrane domain, which also controls the constitutive activity of the GABAB receptor. This region corresponds to the sodium ion binding site in class A GPCRs that controls the basal state of the receptors. Overall, these data reveal the possibility of developing allosteric compounds able to specifically modulate the activity of GPCR homo- and heterodimers by acting at their transmembrane interface.

The glucagon-like peptide 1 receptor (GLP1R) is a major drug target with several agonists being prescribed in patients with type 2 diabetes (T2D) and obesity. The impact of genetic variability of the GLP1R gene on receptor function and its association with metabolic traits are unclear. Here, functional profiling of 59 rare and one common GLP1R variant across four signaling pathways reveals an unexpected diversity of phenotypes ranging from defective cell surface expression to complete or pathway-specific gain- and loss-of-functions. Defective insulin secretion of loss-of-function GLP1R variants was rescued by allosteric GLP1R ligands or high exendin-4 concentrations in INS-1 823/3 cells. Genetic association studies in 200K participants from UK Biobank show that impaired GLP1R cell surface expression contributes to impaired glucose control and increased adiposity with increased HbA1c, BMI and diastolic blood pressure. This study defines impaired GLP-1R cell surface expression as a risk factor for T2D- and obesity-associated traits.

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Despite advances in characterizing the structures and functions of G protein-coupled receptors (GPCRs), our understanding of GPCR activation and signaling is still limited by the lack of information on conformational dynamics. It is particularly challenging to study the dynamics of GPCR complexes with their signaling partners because of their transient nature and low stability. Here, by combining cross-linking mass spectrometry (CLMS) with integrative structure modeling, we map the conformational ensemble of an activated GPCR-G protein complex at near-atomic resolution. The integrative structures describe heterogeneous conformations for a high number of potential alternative active states of the GLP-1 receptor−G s complex. These structures show marked differences from the previously determined cryo-EM structure, especially at the receptor−G s interface and in the interior of the G s heterotrimer. Alaninescanning mutagenesis coupled with pharmacological assays validates the functional significance of 24 interface residue contacts only observed in the integrative structures, yet absent in the cryo-EM structure. Through the integration of spatial connectivity data from CLMS with structure modeling, our study provides a new approach that is generalizable to characterizing the conformational dynamics of GPCR signaling complexes.

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