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Family C G-protein-coupled receptors (GPCRs) operate as obligate dimers with extracellular domains that recognize small ligands, leading to G-protein activation on the transmembrane (TM) domains of these receptors by an unknown mechanism 1 . Here we show structures of homodimers of the family C metabotropic glutamate receptor 2 (mGlu2) in distinct functional states and in complex with heterotrimeric G i . Upon activation of the extracellular domain, the two transmembrane domains undergo extensive rearrangement in relative orientation to establish an asymmetric TM6–TM6 interface that promotes conformational changes in the cytoplasmic domain of one protomer. Nucleotide-bound G i can be observed pre-coupled to inactive mGlu2, but its transition to the nucleotide-free form seems to depend on establishing the active-state TM6–TM6 interface. In contrast to family A and B GPCRs, G-protein coupling does not involve the cytoplasmic opening of TM6 but is facilitated through the coordination of intracellular loops 2 and 3, as well as a critical contribution from the C terminus of the receptor. The findings highlight the synergy of global and local conformational transitions to facilitate a new mode of G-protein activation. Cryo-electron microscopy structures show that metabotropic glutamate receptor 2 forms a dimer to which only one G protein is coupled, revealing the basis for asymmetric signal transduction.

Adhesion G-protein-coupled receptors (aGPCRs) are characterized by the presence of auto-proteolysing extracellular regions that are involved in cell–cell and cell–extracellular matrix interactions 1 . Self cleavage within the aGPCR auto-proteolysis-inducing (GAIN) domain produces two protomers—N-terminal and C-terminal fragments—that remain non-covalently attached after receptors reach the cell surface 1 . Upon dissociation of the N-terminal fragment, the C-terminus of the GAIN domain acts as a tethered agonist (TA) peptide to activate the seven-transmembrane domain with a mechanism that has been poorly understood 2 – 5 . Here we provide cryo-electron microscopy snapshots of two distinct members of the aGPCR family, GPR56 (also known as ADGRG1) and latrophilin 3 (LPHN3 (also known as ADGRL3)). Low-resolution maps of the receptors in their N-terminal fragment-bound state indicate that the GAIN domain projects flexibly towards the extracellular space, keeping the encrypted TA peptide away from the seven-transmembrane domain. High-resolution structures of GPR56 and LPHN3 in their active, G-protein-coupled states, reveal that after dissociation of the extracellular region, the decrypted TA peptides engage the seven-transmembrane domain core with a notable conservation of interactions that also involve extracellular loop 2. TA binding stabilizes breaks in the middle of transmembrane helices 6 and 7 that facilitate aGPCR coupling and activation of heterotrimeric G proteins. Collectively, these results enable us to propose a general model for aGPCR activation. Cryo-electron microscopy structures of GPR56 and latrophilin 3 show how the released tethered agonist peptide interacts with the transmembrane domain, suggesting a model for the activation mechanism of adhesion G-protein-coupled receptors.

The calcium-sensing receptor (CaSR) is a family C G-protein-coupled receptor 1 (GPCR) that has a central role in regulating systemic calcium homeostasis 2 , 3 . Here we use cryo-electron microscopy and functional assays to investigate the activation of human CaSR embedded in lipid nanodiscs and its coupling to functional G i versus G q proteins in the presence and absence of the calcimimetic drug cinacalcet. High-resolution structures show that both G i and G q drive additional conformational changes in the activated CaSR dimer to stabilize a more extensive asymmetric interface of the seven-transmembrane domain (7TM) that involves key protein–lipid interactions. Selective G i and G q coupling by the receptor is achieved through substantial rearrangements of intracellular loop 2 and the C terminus, which contribute differentially towards the binding of the two G-protein subtypes, resulting in distinct CaSR–G-protein interfaces. The structures also reveal that natural polyamines target multiple sites on CaSR to enhance receptor activation by zipping negatively charged regions between two protomers. Furthermore, we find that the amino acid l -tryptophan, a well-known ligand of CaSR extracellular domains, occupies the 7TM bundle of the G-protein-coupled protomer at the same location as cinacalcet and other allosteric modulators. Together, these results provide a framework for G-protein activation and selectivity by CaSR, as well as its allosteric modulation by endogenous and exogenous ligands. Cryo-electron microscopy structures of the human calcium-sensing receptor in complex with G i and G q proteins reveal how this receptor activates distinct G protein subtypes and how its function is modulated by a variety of ligands.

Cryogenic electron microscopy (cryo-EM) has widened the field of structure-based drug discovery by allowing for routine determination of membrane protein structures previously intractable. Despite representing one of the largest classes of therapeutic targets, most inactive-state G protein-coupled receptors (GPCRs) have remained inaccessible for cryo-EM because their small size and membrane-embedded nature impedes projection alignment for high-resolution map reconstructions. Here we demonstrate that the same single-chain camelid antibody (nanobody) recognizing a grafted intracellular loop can be used to obtain cryo-EM structures of inactive-state GPCRs at resolutions comparable or better than those obtained by X-ray crystallography. Using this approach, we obtained structures of neurotensin 1 receptor bound to antagonist SR48692, μ-opioid receptor bound to alvimopan, apo somatostatin receptor 2 and histamine receptor 2 bound to famotidine. We expect this rapid, straightforward approach to facilitate the broad exploration of GPCR inactive states without the need for extensive engineering and crystallization. Cryo-EM has facilitated structural studies of membrane proteins, but inactive GPCRs have remained inaccessible due to their small size. Robertson et al. demonstrate a common nanobody-based approach to streamline the determination of such structures.

Stimulation of the metabotropic GABA B receptor by γ-aminobutyric acid (GABA) results in prolonged inhibition of neurotransmission, which is central to brain physiology 1 . GABA B belongs to family C of the G-protein-coupled receptors, which operate as dimers to transform synaptic neurotransmitter signals into a cellular response through the binding and activation of heterotrimeric G proteins 2 , 3 . However, GABA B is unique in its function as an obligate heterodimer in which agonist binding and G-protein activation take place on distinct subunits 4 , 5 . Here we present cryo-electron microscopy structures of heterodimeric and homodimeric full-length GABA B receptors. Complemented by cellular signalling assays and atomistic simulations, these structures reveal that extracellular loop 2 (ECL2) of GABA B has an essential role in relaying structural transitions by ordering the linker that connects the extracellular ligand-binding domain to the transmembrane region. Furthermore, the ECL2 of each of the subunits of GABA B caps and interacts with the hydrophilic head of a phospholipid that occupies the extracellular half of the transmembrane domain, thereby providing a potentially crucial link between ligand binding and the receptor core that engages G proteins. These results provide a starting framework through which to decipher the mechanistic modes of signal transduction mediated by GABA B dimers, and have important implications for rational drug design that targets these receptors. Cryo-electron microscopy structures of heterodimeric and homodimeric full-length GABA B receptors, combined with cellular signalling assays, shed light on the mechanisms that underpin signal transduction mediated by these receptors.

G-protein-coupled receptors (GPCRs) activate heterotrimeric G proteins by stimulating guanine nucleotide exchange in the Gα subunit 1 . To visualize this mechanism, we developed a time-resolved cryo-EM approach that examines the progression of ensembles of pre-steady-state intermediates of a GPCR–G-protein complex. By monitoring the transitions of the stimulatory G s protein in complex with the β 2 -adrenergic receptor at short sequential time points after GTP addition, we identified the conformational trajectory underlying G-protein activation and functional dissociation from the receptor. Twenty structures generated from sequential overlapping particle subsets along this trajectory, compared to control structures, provide a high-resolution description of the order of main events driving G-protein activation in response to GTP binding. Structural changes propagate from the nucleotide-binding pocket and extend through the GTPase domain, enacting alterations to Gα switch regions and the α5 helix that weaken the G-protein–receptor interface. Molecular dynamics simulations with late structures in the cryo-EM trajectory support that enhanced ordering of GTP on closure of the α-helical domain against the nucleotide-bound Ras-homology domain correlates with α5 helix destabilization and eventual dissociation of the G protein from the GPCR. These findings also highlight the potential of time-resolved cryo-EM as a tool for mechanistic dissection of GPCR signalling events. Time-resolved cryo-EM is used to capture structural transitions during G-protein activation stimulated by a G-protein-coupled receptor.

G-protein-coupled receptors (GPCRs) activate heterotrimeric G proteins by promoting guanine nucleotide exchange. Here, we investigate the coupling of G proteins with GPCRs and describe the events that ultimately lead to the ejection of GDP from its binding pocket in the Gα subunit, the rate-limiting step during G-protein activation. Using molecular dynamics simulations, we investigate the temporal progression of structural rearrangements of GDP-bound G s protein (G s ·GDP; hereafter G s GDP ) upon coupling to the β 2 -adrenergic receptor (β 2 AR) in atomic detail. The binding of G s GDP to the β 2 AR is followed by long-range allosteric effects that significantly reduce the energy needed for GDP release: the opening of α1-αF helices, the displacement of the αG helix and the opening of the α-helical domain. Signal propagation to the G s occurs through an extended receptor interface, including a lysine-rich motif at the intracellular end of a kinked transmembrane helix 6, which was confirmed by site-directed mutagenesis and functional assays. From this β 2 AR–G s GDP intermediate, G s undergoes an in-plane rotation along the receptor axis to approach the β 2 AR–G s empty state. The simulations shed light on how the structural elements at the receptor–G-protein interface may interact to transmit the signal over 30 Å to the nucleotide-binding site. Our analysis extends the current limited view of nucleotide-free snapshots to include additional states and structural features responsible for signaling and G-protein coupling specificity. Using molecular dynamics simulations and functional assays, authors track the structural changes in heterotrimeric G proteins in response to receptor coupling that lead to the ejection of GDP, the rate-limiting step during G-protein activation.

Distinct ligands for the same G-protein coupled receptor (GPCR) activate intracellular signaling partners to varying extents, but the molecular mechanisms driving these differences remains elusive. Hypothesizing that such differences in signaling efficacy may be captured structurally in intermediate states under non-equilibrium conditions, we implemented a time-resolved (TR) cryo-EM approach to visualize the GTP-induced activation of the Gαiβγ heterotrimer by the μ-opioid receptor (MOR) bound to three ligands displaying partial, full, or super-agonism on the receptor. We resolved ensembles of conformational states along the G-protein activation pathway, including a previously unobserved intermediate state that enabled us to visualize receptor dynamics as a function of bound ligand. The results demonstrate ligand-dependent differences in state occupancy and conformational stability, with higher ligand efficacy correlating with increased dynamics of the receptor's transmembrane (TM) helices 5 and 6. Furthermore, we identify key mechanistic differences in the GTP-induced activation of Gi compared to Gs that likely underlie their distinct activation kinetics. Corroborated by molecular dynamics (MD) simulations, these findings provide a dynamic structural landscape of GPCR-G-protein interactions for ligands of different efficacy and suggest partial agonists may produce a 'kinetic trap' during G-protein activation.

G protein-coupled receptors (GPCRs) activate heterotrimeric G proteins by stimulating the exchange of guanine nucleotide in the Gα subunit. To visualize this mechanism, we developed a time-resolved cryo-EM approach that examines the progression of ensembles of pre-steady-state intermediates of a GPCR-G protein complex. Using variability analysis to monitor the transitions of the stimulatory Gs protein in complex with the β -adrenergic receptor (β AR) at short sequential time points after GTP addition, we identified the conformational trajectory underlying G protein activation and functional dissociation from the receptor. Twenty transition structures generated from sequential overlapping particle subsets along this trajectory, compared to control structures, provide a high-resolution description of the order of events driving G protein activation upon GTP binding. Structural changes propagate from the nucleotide-binding pocket and extend through the GTPase domain, enacting alterations to Gα Switch regions and the α5 helix that weaken the G protein-receptor interface. Molecular dynamics (MD) simulations with late structures in the cryo-EM trajectory support that enhanced ordering of GTP upon closure of the alpha-helical domain (AHD) against the nucleotide-bound Ras-homology domain (RHD) correlates with irreversible α5 helix destabilization and eventual dissociation of the G protein from the GPCR. These findings also highlight the potential of time-resolved cryo-EM as a tool for mechanistic dissection of GPCR signaling events.

The calcium-sensing receptor (CaSR) is a family C G-protein-coupled receptor (GPCR) that has a central role in regulating systemic calcium homeostasis . Here we use cryo-electron microscopy and functional assays to investigate the activation of human CaSR embedded in lipid nanodiscs and its coupling to functional G versus G proteins in the presence and absence of the calcimimetic drug cinacalcet. High-resolution structures show that both G and G drive additional conformational changes in the activated CaSR dimer to stabilize a more extensive asymmetric interface of the seven-transmembrane domain (7TM) that involves key protein-lipid interactions. Selective G and G coupling by the receptor is achieved through substantial rearrangements of intracellular loop 2 and the C terminus, which contribute differentially towards the binding of the two G-protein subtypes, resulting in distinct CaSR-G-protein interfaces. The structures also reveal that natural polyamines target multiple sites on CaSR to enhance receptor activation by zipping negatively charged regions between two protomers. Furthermore, we find that the amino acid L-tryptophan, a well-known ligand of CaSR extracellular domains, occupies the 7TM bundle of the G-protein-coupled protomer at the same location as cinacalcet and other allosteric modulators. Together, these results provide a framework for G-protein activation and selectivity by CaSR, as well as its allosteric modulation by endogenous and exogenous ligands.