Structures of 70 unique G protein-coupled receptors (GPCRs) have been determined, with over 370 structures in total bound to different ligands and the receptors in various conformational states. ...Structure-based drug design has been applied to an increasing number of GPCR targets over the past decade and now a few of these drug candidates have entered clinical trials. Given the length of time required for a drug to reach the market, there are no documented examples of licensed drugs being developed with the aid of a structure, but this is likely to change as current efforts come to fruition.
The promise of the intensive structural investigations of GPCRs over the past decade was that they would improve and expedite the design of drugs targeting this important therapeutic class of proteins. Tate & colleagues examine whether this strategy has paid off.
Adenosine receptors and β-adrenoceptors are G-protein-coupled receptors (GPCRs) that activate intracellular G proteins on binding the agonists adenosine or noradrenaline, respectively. GPCRs have ...similar structures consisting of seven transmembrane helices that contain well-conserved sequence motifs, indicating that they are probably activated by a common mechanism. Recent structures of β-adrenoceptors highlight residues in transmembrane region 5 that initially bind specifically to agonists rather than to antagonists, indicating that these residues have an important role in agonist-induced activation of receptors. Here we present two crystal structures of the thermostabilized human adenosine A(2A) receptor (A(2A)R-GL31) bound to its endogenous agonist adenosine and the synthetic agonist NECA. The structures represent an intermediate conformation between the inactive and active states, because they share all the features of GPCRs that are thought to be in a fully activated state, except that the cytoplasmic end of transmembrane helix 6 partially occludes the G-protein-binding site. The adenine substituent of the agonists binds in a similar fashion to the chemically related region of the inverse agonist ZM241385 (ref. 8). Both agonists contain a ribose group, not found in ZM241385, which extends deep into the ligand-binding pocket where it makes polar interactions with conserved residues in H7 (Ser 277(7.42) and His 278(7.43); superscripts refer to Ballesteros-Weinstein numbering) and non-polar interactions with residues in H3. In contrast, the inverse agonist ZM241385 does not interact with any of these residues and comparison with the agonist-bound structures indicates that ZM241385 sterically prevents the conformational change in H5 and therefore it acts as an inverse agonist. Comparison of the agonist-bound structures of A(2A)R with the agonist-bound structures of β-adrenoceptors indicates that the contraction of the ligand-binding pocket caused by the inward motion of helices 3, 5 and 7 may be a common feature in the activation of all GPCRs.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Electron cryo-microscopy (cryo-EM) has revolutionized structure determination of membrane proteins and holds great potential for structure-based drug discovery. Here we discuss the potential of ...cryo-EM in the rational design of therapeutics for membrane proteins compared to X-ray crystallography. We also detail recent progress in the field of drug receptors, focusing on cryo-EM of two protein families with established therapeutic value, the γ-aminobutyric acid A receptors (GABA
A
Rs) and G protein-coupled receptors (GPCRs). GABA
A
Rs are pentameric ion channels, and cryo-EM structures of physiological heteromeric receptors in a lipid environment have uncovered the molecular basis of receptor modulation by drugs such as diazepam. The structures of ten GPCR-G protein complexes from three different classes of GPCRs have now been determined by cryo-EM. These structures give detailed insights into molecular interactions with drugs, GPCR-G protein selectivity, how accessory membrane proteins alter receptor-ligand pharmacology, and the mechanism by which HIV uses GPCRs to enter host cells.
G protein–coupled receptors (GPCRs) are key signaling proteins that regulate nearly every aspect of cell function. Studies of GPCRs have benefited greatly from the development of molecular tools to ...monitor receptor activation and downstream signaling. Here, we show that mini G proteins are robust probes that can be used in a variety of assay formats to report GPCR activity in living cells. Mini G (mG) proteins are engineered GTPase domains of Gα subunits that were developed for structural studies of active-state GPCRs. Confocal imaging revealed that mG proteins fused to fluorescent proteins were located diffusely in the cytoplasm and translocated to sites of receptor activation at the cell surface and at intracellular organelles. Bioluminescence resonance energy transfer (BRET) assays with mG proteins fused to either a fluorescent protein or luciferase reported agonist, superagonist, and inverse agonist activities. Variants of mG proteins (mGs, mGsi, mGsq, and mG12) corresponding to the four families of Gα subunits displayed appropriate coupling to their cognate GPCRs, allowing quantitative profiling of subtype-specific coupling to individual receptors. BRET between luciferase–mG fusion proteins and fluorescent markers indicated the presence of active GPCRs at the plasma membrane, Golgi apparatus, and endosomes. Complementation assays with fragments of NanoLuc luciferase fused to GPCRs and mG proteins reported constitutive receptor activity and agonist-induced activation with up to 20-fold increases in luminescence. We conclude that mG proteins are versatile tools for studying GPCR activation and coupling specificity in cells and should be useful for discovering and characterizing G protein subtype–biased ligands.
The β1-adrenoceptor (β1AR) is the site of action of beta blockers used in the treatment of cardiac-related illnesses. Two beta blockers, carvedilol and bucindolol, show distinctive activities ...compared to other beta blockers and have been proposed as treatments tailored to the Arg/Gly3898.56 polymorphism of the human β1AR. Both carvedilol and bucindolol are classified as biased agonists, because they stimulate G protein-independent signaling, while acting as either inverse or partial agonists of the G protein pathway. We have determined the crystal structures of a thermostabilized avian β1AR mutant bound to bucindolol and to carvedilol at 3.2 and 2.3 Å resolution, respectively. In comparison to other beta blockers, bucindolol and carvedilol interact with additional residues, in extracellular loop 2 and transmembrane helix 7, which may promote G protein-independent signaling. The structures also suggest that there may be a structural explanation for the pharmacological differences arising from the Arg/Gly3898.56 polymorphism.
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► Structures of β1AR bound to the biased agonists bucindololol and carvedilol ► The biased agonists form unique contacts with β1AR not seen with other antagonists ► The structures explain the pharmacological differences in the Arg389Gly polymorphism
G-protein-coupled receptors (GPCRs) are medically important membrane proteins that are targeted by over 30% of small molecule drugs. At the time of writing, 15 unique GPCR structures have been ...determined, with 77 structures deposited in the PDB database, which offers new opportunities for drug development and for understanding the molecular mechanisms of GPCR activation. Many different factors have contributed to this success, but if there is one single factor that can be singled out as the foundation for producing well-diffracting GPCR crystals, it is the stabilisation of the detergent-solubilised receptor–ligand complex. This review will focus predominantly on one of the successful strategies for the stabilisation of GPCRs, namely the thermostabilisation of GPCRs using systematic mutagenesis coupled with thermostability assays. Structures of thermostabilised GPCRs bound to a wide variety of ligands have been determined, which has led to an understanding of ligand specificity; why some ligands act as agonists as opposed to partial or inverse agonists; and the structural basis for receptor activation.
Mini-G proteins are the engineered GTPase domains of Gα subunits. They couple to GPCRs and recapitulate the increase in agonist affinity observed upon coupling of a native heterotrimeric G protein. ...Given the small size and stability of mini-G proteins, and their ease of expression and purification, they are ideal for biophysical studies of GPCRs in their fully active state. The first mini-G protein developed was mini-Gs. Here we extend the family of mini-G proteins to include mini-Golf, mini-Gi1, mini-Go1 and the chimeras mini-Gs/q and mini-Gs/i. The mini-G proteins were shown to couple to relevant GPCRs and to form stable complexes with purified receptors that could be purified by size exclusion chromatography. Agonist-bound GPCRs coupled to a mini-G protein showed higher thermal stability compared to the agonist-bound receptor alone. Fusion of GFP at the N-terminus of mini-G proteins allowed receptor coupling to be monitored by fluorescence-detection size exclusion chromatography (FSEC) and, in a separate assay, the affinity of mini-G protein binding to detergent-solubilised receptors was determined. This work provides the foundation for the development of any mini-G protein and, ultimately, for the structure determination of GPCRs in a fully active state.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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•Active state structures of a μOR–nanobody complex determined to 2.3Å resolution.•The first GPCR–arrestin structure defines the arrestin–opsin interface.•Structure determined of A2AR ...coupled to an engineered G protein.•Inactive, intermediate-active and active structures have now been solved for A2AR.•G protein and arrestin complexes provide insight into GPCR coupling specificity.
G protein-coupled receptors (GPCRs) regulate cellular signalling through heterotrimeric G proteins and arrestins in response to an array of extracellular stimuli. Structure determination of GPCRs in an active conformation bound to intracellular signalling proteins has proved to be highly challenging. Nonetheless, three new structures of GPCRs in an active state have been published during the last year, namely the adenosine A2A receptor (A2AR) bound to an engineered G protein, opsin bound to visual arrestin and the μ opioid receptor (μOR) bound to a G protein-mimicking nanobody. These structures have provided novel insight into the sequence of events leading to GPCR activation, and have highlighted both similarities and differences in the structure of the interface between GPCRs and different signalling proteins.
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