Pil Seok Chae

Crystal structure of the β2 adrenergic receptor-Gs protein complex.

G protein-coupled receptors (GPCRs) are responsible for the majority of cellular responses to hormones and neurotransmitters as well as the senses of sight, olfaction and taste. The paradigm of GPCR signalling is the activation of a heterotrimeric GTP binding protein (G protein) by an agonist-occupied receptor. The β2 adrenergic receptor (β2AR) activation of Gs, the stimulatory G protein for adenylyl cyclase, has long been a model system for GPCR signalling. Here we present the crystal structure of the active state ternary complex composed of agonist-occupied monomeric β2AR and nucleotide-free Gs heterotrimer. The principal interactions between the β2AR and Gs involve the amino- and carboxy-terminal α-helices of Gs, with conformational changes propagating to the nucleotide-binding pocket. The largest conformational changes in the β2AR include a 14 Å outward movement at the cytoplasmic end of transmembrane segment 6 (TM6) and an α-helical extension of the cytoplasmic end of TM5. The most surprising observation is a major displacement of the α-helical domain of Gαs relative to the Ras-like GTPase domain. This crystal structure represents the first high-resolution view of transmembrane signalling by a GPCR.

Structure of a nanobody-stabilized active state of the β2 adrenoceptor

G protein coupled receptors (GPCRs) exhibit a spectrum of functional behaviours in response to natural and synthetic ligands. Recent crystal structures provide insights into inactive states of several GPCRs. Efforts to obtain an agonist-bound active-state GPCR structure have proven difficult due to the inherent instability of this state in the absence of a G protein. We generated a camelid antibody fragment (nanobody) to the human β2 adrenergic receptor (β2AR) that exhibits G protein-like behaviour, and obtained an agonist-bound, active-state crystal structure of the receptor-nanobody complex. Comparison with the inactive β2AR structure reveals subtle changes in the binding pocket; however, these small changes are associated with an 11 Å outward movement of the cytoplasmic end of transmembrane segment 6, and rearrangements of transmembrane segments 5 and 7 that are remarkably similar to those observed in opsin, an active form of rhodopsin. This structure provides insights into the process of agonist binding and activation.

Structure and function of an irreversible agonist-β 2 adrenoceptor complex

G-protein-coupled receptors (GPCRs) are eukaryotic integral membrane proteins that modulate biological function by initiating cellular signalling in response to chemically diverse agonists. Despite recent progress in the structural biology of GPCRs, the molecular basis for agonist binding and allosteric modulation of these proteins is poorly understood. Structural knowledge of agonist-bound states is essential for deciphering the mechanism of receptor activation, and for structure-guided design and optimization of ligands. However, the crystallization of agonist-bound GPCRs has been hampered by modest affinities and rapid off-rates of available agonists. Using the inactive structure of the human β2 adrenergic receptor (β2AR) as a guide, we designed a β2AR agonist that can be covalently tethered to a specific site on the receptor through a disulphide bond. The covalent β2AR-agonist complex forms efficiently, and is capable of activating a heterotrimeric G protein. We crystallized a covalent agonist-bound β2AR–T4L fusion protein in lipid bilayers through the use of the lipidic mesophase method, and determined its structure at 3.5 Å resolution. A comparison to the inactive structure and an antibody-stabilized active structure (companion paper) shows how binding events at both the extracellular and intracellular surfaces are required to stabilize an active conformation of the receptor. The structures are in agreement with long-timescale (up to 30 μs) molecular dynamics simulations showing that an agonist-bound active conformation spontaneously relaxes to an inactive-like conformation in the absence of a G protein or stabilizing antibody.

Structure and dynamics of the M3 muscarinic acetylcholine receptor

Acetylcholine, the first neurotransmitter to be identified, exerts many of its physiological actions via activation of a family of G-protein-coupled receptors (GPCRs) known as muscarinic acetylcholine receptors (mAChRs). Although the five mAChR subtypes (M1–M5) share a high degree of sequence homology, they show pronounced differences in G-protein coupling preference and the physiological responses they mediate. Unfortunately, despite decades of effort, no therapeutic agents endowed with clear mAChR subtype selectivity have been developed to exploit these differences. We describe here the structure of the Gq/11-coupled M3 mAChR (‘M3 receptor’, from rat) bound to the bronchodilator drug tiotropium and identify the binding mode for this clinically important drug. This structure, together with that of the Gi/o-coupled M2 receptor, offers possibilities for the design of mAChR subtype-selective ligands. Importantly, the M3 receptor structure allows a structural comparison between two members of a mammalian GPCR subfamily displaying different G-protein coupling selectivities. Furthermore, molecular dynamics simulations suggest that tiotropium binds transiently to an allosteric site en route to the binding pocket of both receptors. These simulations offer a structural view of an allosteric binding mode for an orthosteric GPCR ligand and provide additional opportunities for the design of ligands with different affinities or binding kinetics for different mAChR subtypes. Our findings not only offer insights into the structure and function of one of the most important GPCR families, but may also facilitate the design of improved therapeutics targeting these critical receptors.

Maltose-neopentyl glycol (MNG) amphiphiles for solubilization, stabilization and crystallization of membrane proteins

The understanding of integral membrane protein (IMP) structure and function is hampered by the difficulty of handling these proteins. Aqueous solubilization, necessary for many types of biophysical analysis, generally requires a detergent to shield the large lipophilic surfaces of native IMPs. Many proteins remain difficult to study owing to a lack of suitable detergents. We introduce a class of amphiphiles, each built around a central quaternary carbon atom derived from neopentyl glycol, with hydrophilic groups derived from maltose. Representatives of this maltose–neopentyl glycol (MNG) amphiphile family show favorable behavior relative to conventional detergents, as manifested in multiple membrane protein systems, leading to enhanced structural stability and successful crystallization. MNG amphiphiles are promising tools for membrane protein science because of the ease with which they may be prepared and the facility with which their structures may be varied.

Conformational changes in the G protein Gs induced by the β2 adrenergic receptor.

G protein-coupled receptors represent the largest family of membrane receptors that instigate signalling through nucleotide exchange on heterotrimeric G proteins. Nucleotide exchange, or more precisely, GDP dissociation from the G protein α-subunit, is the key step towards G protein activation and initiation of downstream signalling cascades. Despite a wealth of biochemical and biophysical studies on inactive and active conformations of several heterotrimeric G proteins, the molecular underpinnings of G protein activation remain elusive. To characterize this mechanism, we applied peptide amide hydrogen–deuterium exchange mass spectrometry to probe changes in the structure of the heterotrimeric bovine G protein, Gs (the stimulatory G protein for adenylyl cyclase) on formation of a complex with agonist-bound human β2 adrenergic receptor (β2AR). Here we report structural links between the receptor-binding surface and the nucleotide-binding pocket of Gs that undergo higher levels of hydrogen–deuterium exchange than would be predicted from the crystal structure of the β2AR–Gs complex. Together with X-ray crystallographic and electron microscopic data of the β2AR–Gs complex (from refs 2, 3), we provide a rationale for a mechanism of nucleotide exchange, whereby the receptor perturbs the structure of the amino-terminal region of the α-subunit of Gs and consequently alters the ‘P-loop’ that binds the β-phosphate in GDP. As with the Ras family of small-molecular-weight G proteins, P-loop stabilization and β-phosphate coordination are key determinants of GDP (and GTP) binding affinity.

Tandem Facial Amphiphiles for Membrane Protein Stabilization

We describe a new type of synthetic amphiphile that is intended to support biochemical characterization of intrinsic membrane proteins. Members of this new family displayed favorable behavior with four of five membrane proteins tested, and these amphiphiles formed relatively small micelles.

A New Class of Amphiphiles Bearing Rigid Hydrophobic Groups for Solubilization and Stabilization of Membrane Proteins

Integral membrane proteins (IMPs) are crucial cellular components, mediating the transfer of material and signals between the environment and the cytoplasm, or between different cellular compartments. Structural and functional analysis of IMPs is important; more than half of current pharmaceutical agents target proteins in this class. [1] IMP characterization is often challenging, and sometimes impossible, because of difficulties associated with handling these macromolecules.[2] IMPs in the native state display large hydrophobic surfaces, which are not compatible with an aqueous environment; therefore, detergents are required to extract IMPs from the lipid bilayer and to maintain the native state of the protein in solution.[3] Nonionic detergents, such as dodecyl-β-D-maltoside (DDM) and octyl-β-D-glucoside (OG), are generally preferred for these applications. Despite the comparatively mild nature of DDM, OG and related detergents, many membrane proteins denature and/or aggregate upon solubilization with these agents.[4]

Structural flexibility of the G alpha s alpha-helical domain in the beta2-adrenoceptor Gs complex.

The active-state complex between an agonist-bound receptor and a guanine nucleotide-free G protein represents the fundamental signaling assembly for the majority of hormone and neurotransmitter signaling. We applied single-particle electron microscopy (EM) analysis to examine the architecture of agonist-occupied β2-adrenoceptor (β2AR) in complex with the heterotrimeric G protein Gs (Gαsβγ). EM 2D averages and 3D reconstructions of the detergent-solubilized complex reveal an overall architecture that is in very good agreement with the crystal structure of the active-state ternary complex. Strikingly however, the α-helical domain of Gαs appears highly flexible in the absence of nucleotide. In contrast, the presence of the pyrophosphate mimic foscarnet (phosphonoformate), and also the presence of GDP, favor the stabilization of the α-helical domain on the Ras-like domain of Gαs. Molecular modeling of the α-helical domain in the 3D EM maps suggests that in its stabilized form it assumes a conformation reminiscent to the one observed in the crystal structure of Gαs-GTPγS. These data argue that the α-helical domain undergoes a nucleotide-dependent transition from a flexible to a conformationally stabilized state.

Glycotripod Amphiphiles for Solubilization and Stabilization of a Membrane-Protein Superassembly: Importance of Branching in the Hydrophilic Portion

Intrinsic membrane proteins must usually be extracted from the native membrane with the aid of synthetic amphiphiles and then stabilized in a soluble form before detailed structural and functional characterization is possible. We describe new amphiphiles with unusual architectures that are useful for extraction and stabilization of photosynthetic protein superassemblies from bacterial membranes. Our results suggest that incorporating branch points in both the hydrophilic and lipophilic portions can lead to favorable amphiphile behavior.