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G-protein-coupled receptor (GPCR)-mediated signal transduction is central to human physiology and disease intervention, yet the molecular mechanisms responsible for ligand-dependent signalling responses remain poorly understood. In class A GPCRs, receptor activation and G-protein coupling entail outward movements of transmembrane helix 6 (TM6). Here, using single-molecule fluorescence resonance energy transfer imaging, we examine TM6 movements in the β 2 adrenergic receptor (β 2 AR) upon exposure to orthosteric ligands with different efficacies, in the absence and presence of the G s heterotrimer. We show that partial and full agonists differentially affect TM6 motions to regulate the rate at which GDP-bound β 2 AR–G s complexes are formed and the efficiency of nucleotide exchange leading to G s activation. These data also reveal transient nucleotide-bound β 2 AR–G s species that are distinct from known structures, and provide single-molecule perspectives on the allosteric link between ligand- and nucleotide-binding pockets that shed new light on the G-protein activation mechanism. Single-molecule FRET imaging provides insights into the allosteric link between the ligand-binding and G-protein nucleotide-binding pockets of the β 2 adrenergic receptor (β 2 AR) and improved understanding of the G-protein activation mechanism. Monitoring G-protein activation by a GPCR G-protein-coupled receptor (GPCR)-mediated signal transduction is central to human physiology and disease, and understanding the molecular basis of ligand efficacy downstream of receptor activation is important for therapeutic development. For the GPCR β 2 adrenergic receptor (β 2 AR), receptor activation and coupling to the G protein G s involve outward movements of the receptor transmembrane helix 6 (TM6). Here, Scott Blanchard and colleagues apply single-molecule fluorescence resonance energy transfer (smFRET) imaging methods to directly monitor movements of TM6 in β 2 AR bound to a range of ligands with distinct efficacy profiles. They find that partial and full agonists affect TM6 motions in an efficacy-dependent manner. These motions differentially regulate the rate at which β 2 AR couples with GDP-bound G s and the efficiency of nucleotide exchange leading to G s activation. The work provides single-molecule insight into the allosteric link between the ligand- and G-protein-nucleotide-binding pockets of the receptor and improved understanding of the G-protein activation mechanism.

Camelid single-domain antibody fragments (‘nanobodies’) provide the remarkable specificity of antibodies within a single 15-kDa immunoglobulin VHH domain. This unique feature has enabled applications ranging from use as biochemical tools to therapeutic agents. Nanobodies have emerged as especially useful tools in protein structural biology, facilitating studies of conformationally dynamic proteins such as G-protein-coupled receptors (GPCRs). Nearly all nanobodies available to date have been obtained by animal immunization, a bottleneck restricting many applications of this technology. To solve this problem, we report a fully in vitro platform for nanobody discovery based on yeast surface display. We provide a blueprint for identifying nanobodies, demonstrate the utility of the library by crystallizing a nanobody with its antigen, and most importantly, we utilize the platform to discover conformationally selective nanobodies to two distinct human GPCRs. To facilitate broad deployment of this platform, the library and associated protocols are freely available for nonprofit research.

Neurotensin receptor 1 (NTSR1) is a G-protein-coupled receptor (GPCR) that engages multiple subtypes of G protein, and is involved in the regulation of blood pressure, body temperature, weight and the response to pain. Here we present structures of human NTSR1 in complex with the agonist JMV449 and the heterotrimeric G i1 protein, at a resolution of 3 Å. We identify two conformations: a canonical-state complex that is similar to recently reported GPCR–G i/o complexes (in which the nucleotide-binding pocket adopts more flexible conformations that may facilitate nucleotide exchange), and a non-canonical state in which the G protein is rotated by about 45 degrees relative to the receptor and exhibits a more rigid nucleotide-binding pocket. In the non-canonical state, NTSR1 exhibits features of both active and inactive conformations, which suggests that the structure may represent an intermediate form along the activation pathway of G proteins. This structural information, complemented by molecular dynamics simulations and functional studies, provides insights into the complex process of G-protein activation. Cryo-electron microscopy structures of human neurotensin receptor 1 in complex with G i1 protein and the agonist JMV449 reveal a non-canonical state that may represent an intermediate form in G-protein activation.

Single-particle cryo-electron microscopy (cryo-EM) has recently enabled high-resolution structure determination of numerous biological macromolecular complexes. Despite this progress, the application of high-resolution cryo-EM to G protein coupled receptors (GPCRs) in complex with heterotrimeric G proteins remains challenging, owning to both the relative small size and the limited stability of these assemblies. Here we describe the development of antibody fragments that bind and stabilize GPCR-G protein complexes for the application of high-resolution cryo-EM. One antibody in particular, mAb16, stabilizes GPCR/G-protein complexes by recognizing an interface between Gα and Gβγ subunits in the heterotrimer, and confers resistance to GTPγS-triggered dissociation. The unique recognition mode of this antibody makes it possible to transfer its binding and stabilizing effect to other G-protein subtypes through minimal protein engineering. This antibody fragment is thus a broadly applicable tool for structural studies of GPCR/G-protein complexes. The determination of high resolution structures of G protein coupled receptors (GPCRs) in complex with heterotrimeric G proteins is challenging. Here authors develop an antibody fragment, mAB16, which stabilizes GPCR/G-protein complexes and facilitates the application of high resolution cryo-EM.

G protein-coupled receptors (GPCRs) relay diverse extracellular signals into cells by catalyzing nucleotide release from heterotrimeric G proteins, but the mechanism underlying this quintessential molecular signaling event has remained unclear. Here we use atomic-level simulations to elucidate the nucleotide-release mechanism. We find that the G protein a subunit Ras and helical domains—previously observed to separate widely upon receptor binding to expose the nucleotide-binding site—separate spontaneously and frequently even in the absence of a receptor. Domain separation is necessary but not sufficient for rapid nucleotide release. Rather, receptors catalyze nucleotide release by favoring an internal structural rearrangement of the Ras domain that weakens its nucleotide affinity. We use double electron-electron resonance spectroscopy and protein engineering to confirm predictions of our computationally determined mechanism.

Here we describe the cryo-electron microscopy structure of the human histamine 2 receptor (H 2 R) in an active conformation with bound histamine and in complex with G s heterotrimeric protein at an overall resolution of 3.4 Å. The complex was generated by cotranslational insertion of the receptor into preformed nanodisc membranes using cell-free synthesis in E . coli lysates. Structural comparison with the inactive conformation of H 2 R and the inactive and G q -coupled active state of H 1 R together with structure-guided functional experiments reveal molecular insights into the specificity of ligand binding and G protein coupling for this receptor family. We demonstrate lipid-modulated folding of cell-free synthesized H 2 R, its agonist-dependent internalization and its interaction with endogenously synthesized H 1 R and H 2 R in HEK293 cells by applying a recently developed nanotransfer technique. The study describes the molecular structure of the human histamine 2 receptor in active conformation and in complex with G s heterotrimer, synthesized in a cell-free system and co-translationally inserted into preformed nanodiscs.

Both designed and natural anion-conducting channelrhodopsins (dACRs and nACRs, respectively) have been widely applied in optogenetics (enabling selective inhibition of target-cell activity during animal behaviour studies), but each class exhibits performance limitations, underscoring trade-offs in channel structure-function relationships. Therefore, molecular and structural insights into dACRs and nACRs will be critical not only for understanding the fundamental mechanisms of these light-gated anion channels, but also to create next-generation optogenetic tools. Here we report crystal structures of the dACR iC++, along with spectroscopic, electrophysiological and computational analyses that provide unexpected insights into pH dependence, substrate recognition, channel gating and ion selectivity of both dACRs and nACRs. These results enabled us to create an anion-conducting channelrhodopsin integrating the key features of large photocurrent and fast kinetics alongside exclusive anion selectivity. Crystal structures and molecular simulations of the designed anion-conducting channelrhodopsin iC++ provide molecular insights that enable structure-based design of channelrhodopsins with desirable properties for use as optogenetic tools.

G-protein-coupled receptors (GPCRs) relay numerous extracellular signals by triggering intracellular signaling through coupling with G proteins and arrestins. Recent breakthroughs in the structural determination of GPCRs and GPCR–transducer complexes represent important steps toward deciphering GPCR signal transduction at a molecular level. A full understanding of the molecular basis of GPCR-mediated signaling requires elucidation of the dynamics of receptors and their transducer complexes as well as their energy landscapes and conformational transition rates. Here, we summarize current insights into the structural plasticity of GPCR–G-protein and GPCR–arrestin complexes that underlies the regulation of the receptor’s intracellular signaling profile.

Helicobacter pylori is cause of chronic gastritis, duodenal ulcer and gastric carcinoma in humans. L-proline is a preferred energy source of the microaerophilic bacterium. Previous analyses revealed that HpputP and HpputA, the genes that are predicted to play a central role in proline metabolism as they encode for the proline transporter and proline dehydrogenase, respectively, are essential for stomach colonization. Here, the molecular basis of proline transport in H. pylori by HpPutP was investigated experimentally for the first time. Measuring radiolabeled substrate transport in H. pylori and E. coli heterologously expressing HpputP as well as in proteoliposomes reconstituted with HpPutP, we demonstrate that the observed proline transport in H. pylori is mediated by HpPutP. HpPutP is specific and exhibits a high affinity for L-proline. Notably, L-proline transport is exclusively dependent on Na(+) as coupling ion, i.e., Na(+)/L-proline symport, reminiscent to the properties of PutP of E. coli even though H. pylori lives in a more acidic environment. Homology model-based structural comparisons and substitution analyses identified amino acids crucial for function. HpPutP-catalyzed proline uptake was efficiently inhibited by the known proline analogs 3,4-dehydro-D,L-proline and L-azetidine-2-carboxylic acid.