Structural insights into the activation of metabotropic glutamate receptors.

Metabotropic glutamate receptors are family C G-protein-coupled receptors. They form obligate dimers and possess extracellular ligand-binding Venus flytrap domains, which are linked by cysteine-rich domains to their 7-transmembrane domains. Spectroscopic studies show that signalling is a dynamic process, in which large-scale conformational changes underlie the transmission of signals from the extracellular Venus flytraps to the G protein-coupling domains-the 7-transmembrane domains-in the membrane. Here, using a combination of X-ray crystallography, cryo-electron microscopy and signalling studies, we present a structural framework for the activation mechanism of metabotropic glutamate receptor subtype 5. Our results show that agonist binding at the Venus flytraps leads to a compaction of the intersubunit dimer interface, thereby bringing the cysteine-rich domains into close proximity. Interactions between the cysteine-rich domains and the second extracellular loops of the receptor enable the rigid-body repositioning of the 7-transmembrane domains, which come into contact with each other to initiate signalling.

Author Correction: Structural insights into the activation of metabotropic glutamate receptors.

The surname of author Toon Laeremans was misspelled 'Laermans'. This error has been corrected online.

Structural determinants of dual incretin receptor agonism by tirzepatide.

SignificanceTirzepatide is a dual agonist of the glucose-dependent insulinotropic polypeptide receptor (GIPR) and the glucagon-like peptide-1 receptor (GLP-1R), which are incretin receptors that regulate carbohydrate metabolism. This investigational agent has proven superior to selective GLP-1R agonists in clinical trials in subjects with type 2 diabetes mellitus. Intriguingly, although tirzepatide closely resembles native GIP in how it activates the GIPR, it differs markedly from GLP-1 in its activation of the GLP-1R, resulting in less agonist-induced receptor desensitization. We report how cryogenic electron microscopy and molecular dynamics simulations inform the structural basis for the unique pharmacology of tirzepatide. These studies reveal the extent to which fatty acid modification, combined with amino acid sequence, determines the mode of action of a multireceptor agonist.

Cryo-EM structure of the activated GLP-1 receptor in complex with a G protein.

Glucagon-like peptide 1 (GLP-1) is a hormone with essential roles in regulating insulin secretion, carbohydrate metabolism and appetite. GLP-1 effects are mediated through binding to the GLP-1 receptor (GLP-1R), a class B G-protein-coupled receptor (GPCR) that signals primarily through the stimulatory G protein Gs. Class B GPCRs are important therapeutic targets; however, our understanding of their mechanism of action is limited by the lack of structural information on activated and full-length receptors. Here we report the cryo-electron microscopy structure of the peptide-activated GLP-1R-Gs complex at near atomic resolution. The peptide is clasped between the N-terminal domain and the transmembrane core of the receptor, and further stabilized by extracellular loops. Conformational changes in the transmembrane domain result in a sharp kink in the middle of transmembrane helix 6, which pivots its intracellular half outward to accommodate the α5-helix of the Ras-like domain of Gs. These results provide a structural framework for understanding class B GPCR activation through hormone binding.

Structural basis for GLP-1 receptor activation by LY3502970, an orally active nonpeptide agonist.

Glucagon-like peptide-1 receptor (GLP-1R) agonists are efficacious antidiabetic medications that work by enhancing glucose-dependent insulin secretion and improving energy balance. Currently approved GLP-1R agonists are peptide based, and it has proven difficult to obtain small-molecule activators possessing optimal pharmaceutical properties. We report the discovery and mechanism of action of LY3502970 (OWL833), a nonpeptide GLP-1R agonist. LY3502970 is a partial agonist, biased toward G protein activation over ß-arrestin recruitment at the GLP-1R. The molecule is highly potent and selective against other class B G protein-coupled receptors (GPCRs) with a pharmacokinetic profile favorable for oral administration. A high-resolution structure of LY3502970 in complex with active-state GLP-1R revealed a unique binding pocket in the upper helical bundle where the compound is bound by the extracellular domain (ECD), extracellular loop 2, and transmembrane helices 1, 2, 3, and 7. This mechanism creates a distinct receptor conformation that may explain the partial agonism and biased signaling of the compound. Further, interaction between LY3502970 and the primate-specific Trp33 of the ECD informs species selective activity for the molecule. In efficacy studies, oral administration of LY3502970 resulted in glucose lowering in humanized GLP-1R transgenic mice and insulinotropic and hypophagic effects in nonhuman primates, demonstrating an effect size in both models comparable to injectable exenatide. Together, this work determined the molecular basis for the activity of an oral agent being developed for the treatment of type 2 diabetes mellitus, offering insights into the activation of class B GPCRs by nonpeptide ligands.

Structural insights into probe-dependent positive allosterism of the GLP-1 receptor.

Drugs that promote the association of protein complexes are an emerging therapeutic strategy. We report discovery of a G protein-coupled receptor (GPCR) ligand that stabilizes an active state conformation by cooperatively binding both the receptor and orthosteric ligand, thereby acting as a 'molecular glue'. LSN3160440 is a positive allosteric modulator of the GLP-1R optimized to increase the affinity and efficacy of GLP-1(9-36), a proteolytic product of GLP-1(7-36). The compound enhances insulin secretion in a glucose-, ligand- and GLP-1R-dependent manner. Cryo-electron microscopy determined the structure of the GLP-1R bound to LSN3160440 in complex with GLP-1 and heterotrimeric Gs. The modulator binds high in the helical bundle at an interface between TM1 and TM2, allowing access to the peptide ligand. Pharmacological characterization showed strong probe dependence of LSN3160440 for GLP-1(9-36) versus oxyntomodulin that is driven by a single residue. Our findings expand protein-protein modulation drug discovery to uncompetitive, active state stabilizers for peptide hormone receptors.

Crystal structures of the M1 and M4 muscarinic acetylcholine receptors.

Muscarinic M1-M5 acetylcholine receptors are G-protein-coupled receptors that regulate many vital functions of the central and peripheral nervous systems. In particular, the M1 and M4 receptor subtypes have emerged as attractive drug targets for treatments of neurological disorders, such as Alzheimer's disease and schizophrenia, but the high conservation of the acetylcholine-binding pocket has spurred current research into targeting allosteric sites on these receptors. Here we report the crystal structures of the M1 and M4 muscarinic receptors bound to the inverse agonist, tiotropium. Comparison of these structures with each other, as well as with the previously reported M2 and M3 receptor structures, reveals differences in the orthosteric and allosteric binding sites that contribute to a role in drug selectivity at this important receptor family. We also report identification of a cluster of residues that form a network linking the orthosteric and allosteric sites of the M4 receptor, which provides new insight into how allosteric modulation may be transmitted between the two spatially distinct domains.

Crystal structure of the adenosine A2A receptor bound to an antagonist reveals a potential allosteric pocket.

The adenosine A2A receptor (A2AR) has long been implicated in cardiovascular disorders. As more selective A2AR ligands are being identified, its roles in other disorders, such as Parkinson's disease, are starting to emerge, and A2AR antagonists are important drug candidates for nondopaminergic anti-Parkinson treatment. Here we report the crystal structure of A2A receptor bound to compound 1 (Cmpd-1), a novel A2AR/N-methyl d-aspartate receptor subtype 2B (NR2B) dual antagonist and potential anti-Parkinson candidate compound, at 3.5 Å resolution. The A2A receptor with a cytochrome b562-RIL (BRIL) fusion (A2AR-BRIL) in the intracellular loop 3 (ICL3) was crystallized in detergent micelles using vapor-phase diffusion. Whereas A2AR-BRIL bound to the antagonist ZM241385 has previously been crystallized in lipidic cubic phase (LCP), structural differences in the Cmpd-1-bound A2AR-BRIL prevented formation of the lattice observed with the ZM241385-bound receptor. The crystals grew with a type II crystal lattice in contrast to the typical type I packing seen from membrane protein structures crystallized in LCP. Cmpd-1 binds in a position that overlaps with the native ligand adenosine, but its methoxyphenyl group extends to an exosite not previously observed in other A2AR structures. Structural analysis revealed that Cmpd-1 binding results in the unique conformations of two tyrosine residues, Tyr91.35 and Tyr2717.36, which are critical for the formation of the exosite. The structure reveals insights into antagonist binding that are not observed in other A2AR structures, highlighting flexibility in the binding pocket that may facilitate the development of A2AR-selective compounds for the treatment of Parkinson's disease.

Structural and biochemical studies of human lysine methyltransferase Smyd3 reveal the important functional roles of its post-SET and TPR domains and the regulation of its activity by DNA binding.

The SET- and MYND-domain containing (Smyd) proteins constitute a special subfamily of the SET-containing lysine methyltransferases. Here we present the structure of full-length human Smyd3 in complex with S-adenosyl-L-homocysteine at 2.8 Å resolution. Smyd3 affords the first example that other region(s) besides the SET domain and its flanking regions participate in the formation of the active site. Structural analysis shows that the previously uncharacterized C-terminal domain of Smyd3 contains a tetratrico-peptide repeat (TPR) domain which together with the SET and post-SET domains forms a deep, narrow substrate binding pocket. Our data demonstrate the important roles of both TPR and post-SET domains in the histone lysine methyltransferase (HKMT) activity of Smyd3, and show that the hydroxyl group of Tyr239 is critical for the enzymatic activity. The characteristic MYND domain is located nearby to the substrate binding pocket and exhibits a largely positively charged surface. Further biochemical assays show that DNA binding of Smyd3 can stimulate its HKMT activity and the process may be mediated via the MYND domain through direct DNA binding.

Crystal structure of dopamine D1 receptor in complex with G protein and a non-catechol agonist.

Dopamine D1 receptor (D1R) is an important drug target implicated in many psychiatric and neurological disorders. Selective agonism of D1R are sought to be the therapeutic strategy for these disorders. Most selective D1R agonists share a dopamine-like catechol moiety in their molecular structure, and their therapeutic potential is therefore limited by poor pharmacological properties in vivo. Recently, a class of non-catechol D1R selective agonists with a distinct scaffold and pharmacological properties were reported. Here, we report the crystal structure of D1R in complex with stimulatory G protein (Gs) and a non-catechol agonist Compound 1 at 3.8 Å resolution. The structure reveals the ligand bound to D1R in an extended conformation, spanning from the orthosteric site to extracellular loop 2 (ECL2). Structural analysis reveals that the unique features of D1R ligand binding pocket explains the remarkable selectivity of this scaffold for D1R over other aminergic receptors, and sheds light on the mechanism for D1R activation by the non-catechol agonist.

Structure of human MRG15 chromo domain and its binding to Lys36-methylated histone H3.

Human MRG15 is a transcription factor that plays a vital role in embryonic development, cell proliferation and cellular senescence. It comprises a putative chromo domain in the N-terminal part that has been shown to participate in chromatin remodeling and transcription regulation. We report here the crystal structure of human MRG15 chromo domain at 2.2 A resolution. The MRG15 chromo domain consists of a beta-barrel and a long alpha-helix and assumes a structure more similar to the Drosophila MOF chromo barrel domain than the typical HP1/Pc chromo domains. The beta-barrel core contains a hydrophobic pocket formed by three conserved aromatic residues Tyr26, Tyr46 and Trp49 as a potential binding site for a modified residue of histone tail. However, the binding groove for the histone tail seen in the HP1/Pc chromo domains is pre-occupied by an extra beta-strand. In vitro binding assay results indicate that the MRG15 chromo domain can bind to methylated Lys36, but not methylated Lys4, Lys9 and Lys27 of histone H3. These data together suggest that the MRG15 chromo domain may function as an adaptor module which can bind to a modified histone H3 in a mode different from that of the HP1/Pc chromo domains.

Molecular basis of the interaction of Saccharomyces cerevisiae Eaf3 chromo domain with methylated H3K36.

Eaf3 is a component of both NuA4 histone acetyltransferase and Rpd3S histone deacetylase complexes in Saccharomyces cerevisiae. It is involved in the regulation of the global pattern of histone acetylation that distinguishes promoters from coding regions. Eaf3 contains a chromo domain at the N terminus that can bind to methylated Lys-36 of histone H3 (H3K36). We report here the crystal structures of the Eaf3 chromo domain in two truncation forms. Unlike the typical HP1 and Polycomb chromo domains, which contain a large groove to bind the modified histone tail, the Eaf3 chromo domain assumes an autoinhibited chromo barrel domain similar to the human MRG15 chromo domain. Compared with other chromo domains, the Eaf3 chromo domain contains a unique 38-residue insertion that folds into two short beta-strands and a long flexible loop to flank the beta-barrel core. Both isothermal titration calorimetry and surface plasmon resonance studies indicate that the interaction between the Eaf3 chromo domain and the trimethylated H3K36 peptide is relatively weak, with a K(D) of approximately 10(-4) m. NMR titration studies demonstrate that the methylated H3K36 peptide is bound to the cleft formed by the C-terminal alpha-helix and the beta-barrel core. Site-directed mutagenesis study and in vitro binding assay results show that the conserved aromatic residues Tyr-23, Tyr-81, Trp-84, and Trp-88, which form a hydrophobic pocket at one end of the beta-barrel, are essential for the binding of the methylated H3K36. These results reveal the molecular mechanism of the recognition and binding of the methylated H3K36 by Eaf3 and provide new insights into the functional roles of the Eaf3 chromo domain.