G-proteins coupled receptors

G-proteins are molecular timers that couple transmembrane receptor activation to downstream members of the pathway (Fig. 9-5). They are called G-proteins because they are intimately involved with the nucleotide, GTP. Before activation, the G-protein is hanging around in its GDP form. When the activated receptor finds its G-protein it activates it by increasing the rate of exchange between GTP and GDP. Once activated, the G-protein interacts with downstream effectors and can activate 

The Ras pathway is shown here. Ras is a G-protein that couples signaling from growth factors. The activated receptor is a GNRP that increases the exchange of GDP for GTP and activates the G-protein. Ras GAP inactivates the G-protein. The downstream signal for activated Ras is eventually the mitogen-activated protein kinase pathway (MAPK). 

Most G proteins activate events that lead to an increase in second messenger concentrations (Ca or cAMP) weTl talk about that later. 

There are different kinds of G-proteins. The trimeric G-proteins are used mainly with transmembrane receptors. They have three subunits, a, P, and 7. Tdie a subunit is the GTPase part, but it s kept in its GDP form when it is bound to the p-y subunits. When GTP is bound, the a-subunit is released as an activated G-protein. The activated G-proteins can stimulate some downstream enzyme (these are called stimulatory G-proteins, or Gf). However, some G-proteins may be inhibitory in their active form—they re called Gj. Whether the target protein is stimulated or inhibited will depend on the type of G-protein. 

There are also monomeric G-proteins. Just like the trimeric G-pro-teins, they are involved as signal relays and timers. The Ras superfamily relays signals from receptor tyrosine kinases to downstream elements that eventually regulate transcription. Rho and Rac relay signals from cell-surface receptors to the cytoskeleton, while Rab regulates intracellular transport of vesicles. Regardless of what they do, they use the timer mechanism provided by the G-protein. Three-letter acronyms (TLA), such as Ras, Rho, and Rab, are difficult to remember, sometimes even when you know what the letters stand for. Unfortunately, there s nothing you can do about this except to memorize them. 

Orphan Receptors Matching Drugs to TargetswithoutKnowing Normal FuNOTION 

The way you select which orphan receptor to work on first is that you look at the distribution of the receptor using a probe and in situ hybridization (ISH). If it is found in the hippocampus and you are interested in memory, you may start with this one. Homology to known GPCRs may help determine function, but anatomical distribution strongly suggests function. If the receptor is sitting in the hypothalamus in the paraventricular nucleus (PVN), then—based on your knowledge of 

126 pqj. example, if it is homologous to, say, the melanocortin-3 receptor, it would suggest an exploitable role that could really be worked on. 

The endogenous agonists and antagonists may be very large molecules, say a peptide that is in specific cases, 11, 30, 36, or 41 amino acids long, or as large as trombin protein that also activates a GPCR Just like the small molecules noradrenaline, histamine, and so on. 

However, we might still expect to discover low-molecular-weight agonists and antagonists to act pharmacologically as orally administered drugs. 

FIGURE 5.3 Model of a typical G-protein-coupled receptor 

As their name suggests, GPCRs bind G-proteins. A complete G-protein is a heterotri-mer of three separate proteins, consisting of a-, /3-, and -y-subunits. The a-subunit binds either guanosine diphosphate (GDP) or triphosphate (GTP). The affinity for GDP/GTP is the origin of the name G-protein. The G-protein heterotrimer binds GPCRs on both the C-terminal tail and the linker between the fifth and sixth transmembrane helices on the intracellular face of the receptor.12 

Activation of receptors can also be mediated by proteolytic cleavage of the extracellular domain of the receptor by proteases like thrombin. For these protease-activated receptors, a proteolytically produced peptide functions as the activating ligand. 

In addition, physical stimuli such as light signals are registered and converted into intracellular signals by G protein-coupled receptors they are also involved in perception of taste and smell. 

Subunit tissue Examples of receptors Effector protein, function 

Muscarinic acetylcholine receptors (several t5 es) Catecholamine receptors Serotonin receptors 5-HTj 245 GABAg receptor 

Phosphorylation at Ser/Thr or Tyr residues of the cytosolic domain may lead to inactivation or activation of the receptor and thus weaken or strengthen signal transmission. In this way, Ser/Thr-phosphorylation is used in the process of internalization of receptors, in order to remove the receptor from circulation after it has been activated (see 5.3.4). Furthermore, Ser/Thr phosphorylation can be used as a switch for coupling a given receptor to different G subunits. Protein kinase A mediated phosphorylation of the P-adrenergic receptor has been shown to switch coupling of the receptor from Gs to Gi and initiate a new set of signaling events (Daaka et al., 1997). 

The cytosohc domain thus carries sequences important for short-term or long-term regulation of receptor activity. 

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The G-proteins are heterotrimers made of three families of subunits, a, P, and y, which can interact specifically with discrete regions on G-protein-coupled receptors. This includes most receptors for neurotransmitters and polypeptide hormones (see Neuroregulators). G-protein-coupled receptors also embrace the odorant receptor family and the rhodopsin-linked visual cascade. 

A variety of cellular and viral proteins contain fatty acids covalently bound via ester linkages to the side chains of cysteine and sometimes to serine or threonine residues within a polypeptide chain (Figure 9.18). This type of fatty acyl chain linkage has a broader fatty acid specificity than A myristoylation. Myristate, palmitate, stearate, and oleate can all be esterified in this way, with the Cjg and Cjg chain lengths being most commonly found. Proteins anchored to membranes via fatty acyl thioesters include G-protein-coupled receptors, the surface glycoproteins of several viruses, and the transferrin receptor protein. 

Kenakin, T. P. (1997). Differences between natural and recombinant G-protein coupled receptor systems with varying receptor/G-protein stoichiometry. Trends Pharmacol. Sci. 18 456-464. 

The ternary complex model followed by the extended ternary complex model were devised to describe the action of drugs on G-protein-coupled receptors. 

One target type for which the molecular mechanism of efficacy has been partly elucidated is the G-protein-coupled receptor (GPCR). It is known that activation of GPCRs leads to an interaction of the receptor with separate membrane G-proteins to cause dissociation of the G-protein subunits and subsequent activation of effectors (see Chapter 2). For the purposes of binding, this process can lead to an aberration in the binding reaction as perceived in experimental binding studies. Specifically, the activation of the receptor with subsequent binding of that... 

Christopoulos, A. (2000). Quantification of allosteric interactions at G-protein coupled receptors using radioligand assays. In Current protocol in pharmacology, edited by Enna, S. J., pp. 1.22.21-1.22.40. Wiley and Sons, New York. 

Lazareno, S., and Birdsall, N. J. M. (1995). Detection, quantitation, and verification of allosteric interactions of agents with labeled and unlabeled ligands at G protein-coupled receptors Interactions of strychnine and acetylcholine at muscarinic receptors. Mol. Pharmacol. 48 362-378. 

Wildoer, M., Fong, J., Jones, R. M., Lunzer, M. M., Sharma, S. K., Kostensis, E., Portoghese, P. S., and Whistler, J. L. (2005). A heterodimer-selective agonist shows in vivo relevance of G-protein coupled receptor dimmers. Proc Natl. Acad. Sci. USA 102 9050-9055. 

Heptahelical receptors, another name for 7 TM receptors or G-protein-coupled receptors. It refers to the motif of the helices of the protein crossing the cell membrane seven times to form intracellular and extracellular domains. 

See G-protein = coupled receptors historical studies of, 2-4 operational model of, 45—47, 46f,... 

Adenosine is produced by many tissues, mainly as a byproduct of ATP breakdown. It is released from neurons, glia and other cells, possibly through the operation of the membrane transport system. Its rate of production varies with the functional state of the tissue and it may play a role as an autocrine or paracrine mediator (e.g. controlling blood flow). The uptake of adenosine is blocked by dipyridamole, which has vasodilatory effects. The effects of adenosine are mediated by a group of G protein-coupled receptors (the Gi/o-coupled Ai- and A3 receptors, and the Gs-coupled A2a-/A2B receptors). Ai receptors can mediate vasoconstriction, block of cardiac atrioventricular conduction and reduction of force of contraction, bronchoconstriction, and inhibition of neurotransmitter release. A2 receptors mediate vasodilatation and are involved in the stimulation of nociceptive afferent neurons. A3 receptors mediate the release of mediators from mast cells. Methylxanthines (e.g. caffeine) function as antagonists of Ai and A2 receptors. Adenosine itself is used to terminate supraventricular tachycardia by intravenous bolus injection. 

Extracellular adenosine acts through a class of G protein-coupled receptors (GPCRs), defined across mammalian species as Ab A2a, A2B, and A3ARs (adenosine receptors). Adenosine has a cytoprotective role in the body, both in the periphery and in the central nervous system. Following binding of adenosine, or another naturally occurring agonist, the receptor... 

Adrenaline (epinephrine) is a catecholamine, which is released as a neurotransmitter from neurons in the central nervous system and as a hormone from chromaffin cells of the adrenal gland. Adrenaline is required for increased metabolic and cardiovascular demand during stress. Its cellular actions are mediated via plasma membrane bound G-protein-coupled receptors. 

The biological actions of adrenaline and noradrenaline are mediated via nine different G-protein-coupled receptors, which are located in the plasma membrane of neuronal and nonneuronal target cells. These recqrtors are divided into two different groups, a-adrenergic receptors and P-adrenergic recqrtors (see P-adrenergic system). 

Cotecchia S et al (2004) Structural determinants involved in the activation and regulation of G protein-coupled receptors lessons from the ai -adrenergic receptor sub-types. Biol Cell 96 327-333... 

Recent studies indicate that - like many other receptors - G-protein-coupled receptors may form dimers, either homodimers or dimers with another type of receptor. The role of dimer formation in the cell surface expression of receptors and in their signalling and the resultant pharmacology are currently under intensive investigation [1]. 

Bulenger S, Marullo S, Bouvier M (2005) Emerging role of homo- and heterodimerization in G-protein-coupled receptor biosynthesis and maturation. Trends Pharmacol Sci 26 131-137... 

Opioids act on heptahelical G-protein-coupled receptors. Three types of opioid receptors (p, 8, k) have been cloned. Additional subtypes (e.g., pl3 p2, 81 82), possibly resulting from gene polymorphisms, splice variants or alternative processing have been proposed. Opioid receptors are localized and can be activated... 

G-protein-coupled Receptors Dopamine System Adenosine Receptors Chemokine Receptors... 

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