G protein-coupled receptor

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A Mu-opioid G-protein-coupled receptor with its agonist
A Mu-opioid G-protein-coupled receptor with its agonist
Figure 1. The seven transmembrane α-helix structure of a G-protein-coupled receptor.
Figure 1. The seven transmembrane α-helix structure of a G-protein-coupled receptor.

G-protein-coupled receptors (GPCRs), also known as seven transmembrane receptors, 7TM receptors, heptahelical receptors, and G-protein-linked receptors (GPLR), are a large protein family of transmembrane receptors that sense molecules outside the cell and activate inside signal transduction pathways and, ultimately, cellular responses. G-protein-coupled receptors are found only in eukaryotes, including yeast, plants, choanoflagellates,[1] and animals. The ligands that bind and activate these receptors include light-sensitive compounds, odors, pheromones, hormones, and neurotransmitters, and vary in size from small molecules to peptides to large proteins. G-protein-coupled receptors are involved in many diseases, but are also the target of around half of all modern medicinal drugs.[2]

Contents

GPCRs can be grouped into 6 classes based on sequence homology and functional similarity:[3][4][5][6]

The very large rhodopsin A group has been further subdivided into 19 subgroups (A1-A19).[7] More recently, an alternative classification system called GRAFS (Glutamate, Rhodopsin, Adhesion, Frizzled, Taste2, Secretin) has been proposed.[8]

GPCRs are involved in a wide variety of physiological processes. Some examples of their physiological roles include:

  1. the visual sense: the opsins use a photoisomerization reaction to translate electromagnetic radiation into cellular signals. Rhodopsin, for example, uses the conversion of 11-cis-retinal to all-trans-retinal for this purpose
  2. the sense of smell: receptors of the olfactory epithelium bind odorants (olfactory receptors) and pheromones (vomeronasal receptors)
  3. behavioral and mood regulation: receptors in the mammalian brain bind several different neurotransmitters, including serotonin, dopamine, GABA, and glutamate
  4. regulation of immune system activity and inflammation: chemokine receptors bind ligands that mediate intercellular communication between cells of the immune system; receptors such as histamine receptors bind inflammatory mediators and engage target cell types in the inflammatory response
  5. autonomic nervous system transmission: both the sympathetic and parasympathetic nervous systems are regulated by GPCR pathways, responsible for control of many automatic functions of the body such as blood pressure, heart rate, and digestive processes
  6. cell density sensing: A novel GPCR role in regulating cell density sensing.
Figure 2. G-protein-coupled receptor mechanism.
Figure 2. G-protein-coupled receptor mechanism.


GPCRs are integral membrane proteins that possess seven membrane-spanning domains or transmembrane helices (Figure 1). The extracellular parts of the receptor can be glycosylated. These extracellular loops also contain two highly-conserved cysteine residues that form disulfide bonds to stabilize the receptor structure. Some seven transmembrane helix proteins (such as channelrhodopsin) that resemble GPCRs may contain different functional groups, such as entire ion channels, within their protein.

Early structural models for GPCRs were based on their weak analogy to bacteriorhodopsin for which a structure had been determined by both electron diffraction (PDB 2BRD, 1AT9)[9][10] and X ray-based crystallography (1AP9).[11] In 2000, the first crystal structure of a mammalian GPCR, that of bovine rhodopsin (1F88), was solved.[12] While the main feature, the seven transmembrane helices, is conserved, the relative orientation of the helices differ significantly from that of bacteriorhodopsin. In 2007, the first structure of a human GPCR was solved (2R4R, 2R4S).[13] This was followed immediately by a higher resolution structure of the same receptor (2RH1).[14][15] This human β2-adrenergic receptor GPCR structure, proved to be highly similar to the bovine rhodopsin in terms of the relative orientation of the seven transmembrane helices. However the conformation of the second extracellular loop is entirely different between the two structures. Since this loop constitutes the "lid" that covers the top of the ligand binding site, this conformational difference highlights the difficulties in constructing homology models of other GPCRs based only on the rhodopsin structure.

G protein-coupled receptor are activated by an external signal in the form of a ligand or other signal mediator. This creates a conformational change in the receptor, causing activation of a G protein. Further effect depends on the type of G protein.

GPCRs include receptors for sensory signal mediators (e.g., light and olfactory stimulatory molecules); adenosine, bombesin, bradykinin, endothelin, y-aminobutyric acid (GABA), hepatocyte growth factor, melanocortins, neuropeptide Y, opioid peptides, opsins, somatostatin, tachykinins, vasoactive intestinal polypeptide family, and vasopressin; biogenic amines (e.g., dopamine, epinephrine, norepinephrine, histamine, glutamate (metabotropic effect), glucagon, acetylcholine (muscarinic effect), and serotonin); chemokines; lipid mediators of inflammation (e.g., prostaglandins, prostanoids, platelet-activating factor, and leukotrienes); and peptide hormones (e.g., calcitonin, C5a anaphylatoxin, follicle-stimulating hormone (FSH), gonadotropic-releasing hormone (GnRH), neurokinin, thyrotropin-releasing hormone (TRH), and oxytocin). GPCRs that act as receptors for stimuli that have yet to be identified are known as orphan receptors.

Whereas, in other types of receptors that have been studied, ligands bind externally to the membrane, the ligands of GPCRs typically bind within the transmembrane domain.

The transduction of the signal through the membrane by the receptor is not completely understood. It is known that the inactive G protein is bound to the receptor in its inactive state. Once the ligand is recognized, the receptor shifts conformation and thus mechanically activates the G protein, which detaches from the receptor. The receptor can now either activate another G protein or switch back to its inactive state. This is an overly simplistic explanation, but suffices to convey the overall set of events.

It is believed that a receptor molecule exists in a conformational equilibrium between active and inactive biophysical states.[16] The binding of ligands to the receptor may shift the equilibrium toward the active receptor states.[17] Three types of ligands exist: agonists are ligands that shift the equilibrium in favour of active states; inverse agonists are ligands that shift the equilibrium in favour of inactive states; and neutral antagonists are ligands that do not affect the equilibrium. It is not yet known how exactly the active and inactive states differ from each other.

See also: G protein

If a receptor in an active state encounters a G protein, it may activate it (Figure 2, blue protein in part B). Some evidence suggests that receptors and G proteins are actually pre-coupled. For example, binding of G proteins to receptors affects the receptor's affinity for ligands. Activated G proteins are bound to GTP.

Further signal transduction depends on the type of G protein. The enzyme adenylate cyclase (Figure 2, green protein in panel C) is an example of a cellular protein that can be regulated by a G protein, in this case the G protein Gs. Adenylate cyclase activity is activated when it binds to a subunit of the activated G protein (Figure 2, Panel D). Activation of adenylate cyclase ends when the G protein returns to the GDP-bound state (Figure 2, panels E and A).

In the late 1990s, evidence began accumulating to suggest that some GPCRs are able to signal without G proteins. The ERK2 mitogen-activated protein kinase, a key signal transduction mediator downstream of receptor activation in many pathways, has been shown to be activated in response to cAMP-mediated receptor activation in the slime mold D. discoideum despite the absence of the associated G protein α- and β-subunits.

In mammalian cells, the much-studied β2-adrenoceptor has been demonstrated to activate the ERK2 pathway after arrestin-mediated uncoupling of G-protein-mediated signaling. It therefore seems likely that some mechanisms previously believed to be purely related to receptor desensitisation are actually examples of receptors switching their signaling pathway rather than simply being switched off.

In kidney cells, the bradykinin B2 receptor has been shown to interact directly with a protein tyrosine phosphatase. The presence of a tyrosine-phosphorylated ITIM (immunoreceptor tyrosine-based inhibitory motif) sequence in the B2 receptor is necessary to mediate this interaction and subsequently the antiproliferative effect of bradykinin.[18]

GPCRs become desensitized when exposed to their ligand for a prolonged period of time. There are two recognized forms of desensitization: 1) homologous desensitization, in which the activated GPCR is downregulated; and 2) heterologous desensitization, wherein the activated GPCR causes downregulation of a different GPCR. The key reaction of this downregulation is the phosphorylation of the intracellular (or cytoplasmic) receptor domain by protein kinases.

Cyclic AMP-dependent protein kinases (protein kinase A) are activated by the signal chain coming from the G protein (that was activated by the receptor) via adenylate cyclase and cyclic AMP (cAMP). In a feedback mechanism, these activated kinases phosphorylate the receptor. The longer the receptor remains active, the more kinases are activated, the more receptors are phosphorylated.

The G protein-coupled receptor kinases (GRKs) are protein kinases that phosphorylate only active GPCRs.

Phosphorylation of the receptor can have two consequences:

  1. Translocation: The receptor is, along with the part of the membrane it is embedded in, brought to the inside of the cell, where it is dephosphorylated and then brought back. This mechanism is used to regulate long-term exposure, for example, to a hormone.
  2. Arrestin linking: The phosphorylated receptor can be linked to arrestin molecules that prevent it from binding (and activating) G proteins, effectively switching it off for a short period of time. This mechanism is used, for example, with rhodopsin in retina cells to compensate for exposure to bright light. In many cases, arrestin binding to the receptor is a prerequisite for translocation.

It is generally accepted that G-protein-coupled receptors can form homo- and/or heterodimers and possibly more complex oligomeric structures, and indeed heterodimerization has been shown to be essential for the function of receptors such as the metabotropic GABA(B) receptors. However, it is presently unproven that true heterodimers exist. Present biochemical and physical techniques lack the resolution to differentiate between distinct homodimers assembled into an oligomer or true 1:1 heterodimers. It is also unclear what the functional significance of oligomerization might be, although it is thought that the phenomenon may contribute to the pharmacological heterogeneity of GPCRs in a manner not previously anticipated. This is an actively-studied area in GPCR research.

GCR2 is a G-protein-coupled receptor for the plant hormone abscisic acid that has been identified in Arabidopsis thaliana. Another putative receptor is GCR1 for which no ligand has been identified yet.[19]

A novel GPCR containing a lipid kinase domain has recently been identified in Dictyostelium that regulates cell density sensing.[20]

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v • d • e
Membrane proteins, receptors: transmembrane receptors
Asialoglycoprotein receptor - Autoreceptors - Biogenic amine receptor - Eicosanoid receptor (Prostaglandin receptor) - G protein-coupled receptor - Immune receptor - N-Acetylglucosamine receptor - Neuropilins - Neurotransmitter receptor - Olfactory receptor - Protease-activated receptor - Purinergic receptor - Transferrin receptor
v • d • e
Transmembrane receptor: G protein-coupled receptors
Class A: Rhodopsin like Acetylcholine (M1, M2, M3, M4, M5) - Adrenergic (α1 (A, B, D), α2 (A, B, C), β1, β2, β3) - Adrenomedullin - Anaphylatoxin (C3a, C5a) - Angiotensin (1, 2) - Apelin - Bile acid - Bombesin (BRS3, GRPR, NMBR) - Bradykinin (B1, B2) - Cannabinoid (CB1, CB2) - Chemokine - Cholecystokinin (A, B) - Dopamine (D1, D2, D3, D4, D5) - Eicosanoid (CysLT (1, 2), LTB4 (1, 2), FPRL1, OXE, Prostaglandin ((DP (1, 2), EP (1, 2, 3, 4), PGF, Prostacyclin, Thromboxane) - EBI2 - Endothelin (A, B) - Estrogen - Formyl peptide (1, L1, L2) - Free fatty acid (1, 2, 3, 4) - FSH - Galanin (1, 2, 3) - Gonadotropin-releasing hormone (1, 2) - GPR (1, 3, 4, 6, 12, 15, 17, 18, 19, 20, 21, 22, 23, 25, 26, 27, 31, 32, 33, 34, 35, 37, 39, 42, 44, 45, 50, 52, 55, 61, 62, 63, 65, 68, 75, 77, 78, 79, 82, 83, 84, 85, 87, 88, 92, 101, 103, 119, 120, 132, 135, 139, 141, 142, 146, 148, 149, 150, 151, 152, 153, 160, 161, 162, 171, 172, 173, 174, 176, 182) - Ghrelin - Histamine (H1, H2, H3, H4) - Kisspeptin - Luteinizing hormone/choriogonadotropin - Lysophospholipid (1, 2, 3, 4, 5, 6, 7, 8) - MAS (1, 1L, D, E, F, G, X1, X2, X3, X4) - Melanocortin (1, 2, 3, 4, 5) - MCHR (1, 2) - Melatonin (1A, 1B)- Motilin - Neuromedin (B, U (1, 2)) - Neuropeptide (B/W (1, 2), FF (1, 2), S, Y (1, 2, 4, 5)) - Neurotensin (1, 2) - Opioid (Delta, Kappa, Mu, Nociceptin, but not Sigma) - Olfactory - Opsin (3, 4, 5, 1LW, 1MW, 1SW, RGR, RRH) - Orexin (1, 2) - Oxytocin - Oxoglutarate - PAF - Prokineticin (1, 2) - Prolactin-releasing peptide - Protease-activated (1, 2, 3, 4) - Purinergics (Adenosine (A1, A2a, A2b, A3), P2Y, (1, 2, 4, 5, 6, 8, 9, 10, 11, 12, 13, 14)) - Relaxin (1, 2, 3, 4) - Somatostatin (1, 2, 3, 4, 5) - Serotonin, all but 5-HT3 (5-HT1 (A, B, D, E, F), 5-HT2 (A, B, C), 5-HT (4, 5A, 6, 7)) - SREB - Succinate - TAAR (1, 2, 3, 5, 6, 8, 9) - Tachykinin (1, 2, 3) - Thyrotropin - Thyrotropin-releasing hormone - Urotensin-II - Vasopressin (1A, 1B, 2)
Class B: Secretin like Brain-specific angiogenesis inhibitor (1, 2, 3) - Cadherin (1, 2, 3) - Calcitonin - CD97 - Corticotropin-releasing hormone (1, 2) - EMR (1, 2, 3) - Glucagon (GR, GIPR, GLP1R, GLP2R) - Growth hormone releasing hormone - PACAPR1- GPR (56, 64, 97, 98, 110, 111, 112, 113, 114, 115, 116, 123, 124, 125, 126, 128, 133, 143, 144, 157) - Latrophilin (1, 2, 3, ELTD1) - Parathyroid hormone (1, 2) - Secretin - Vasoactive intestinal peptide (1, 2)
Class C: Metabotropic glutamate / pheromone Calcium-sensing receptor - GABA B (1, 2) - Glutamate receptor (Metabotropic glutamate (1, 2, 3, 4, 5, 6, 7, 8)) - GPRC6A - GPR (156, 158, 179) - RAIG (1, 2, 3, 4) - Taste receptors (TAS1R (1, 2, 3) TAS2R (1, 3, 4, 5, 8, 9, 10, 12, 13, 14, 16, 38, 39, 40, 41, 43, 44, 45, 46, 47, 48, 49, 50))
Frizzled / Smoothened Frizzled (1, 2, 3, 4, 5, 6, 7, 8, 9, 10) - Smoothened

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