The G-Protein-Coupled Receptors

Protein-Ligand Interactions From Molecular Recognition to Drug Design. 

Copyright 2003 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim 

---

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... 

Fredriksson R, Lagerstrom MC, Lundin LG et al (2003) The G-protein-coupled receptors in Hie human genome form five main families. Phylogenetic analysis, paralo-gon groups, and fingerprints. Mol Pharmacol 63 1256-1272... 

Sensory receptors that structurally and functionally belong to the G protein coupled receptor superfamily. Olfactory receptors are a large GPCR family with >300 members in human that are expressed in neurons of the nasal olfactory epithelium where they sense mostly volatile olfactory molecule. The overall number of olfactory receptors differs widely between species and an expansion of different recqrtors is in particular obvious in species that depend on their olfactory sense for survival. 

Bjarnadottir TK, Gloriam DE, Hellstrand SH et al (2006) Comprehensive repertoire and phylogenetic analysis of the G protein-coupled receptors in human and mouse. Genomics 88 263-273... 

Yang LV, Radu CG, Roy M et al (2007) Vascular abnormalities in mice deficient for the G Protein-coupled receptor GPR4 that functions as a pH sensor. Mol Cell Biol 27 1334-1347... 

Bohn LM, Dykstra LA, Lefkowitz RJ, Caron MG, Barak LS (2004) Relative opioid efficacy is determined by the complements of the G protein-coupled receptor desensitization machinery. Mol Pharmacol 66 106-112... 

The ion channel receptors are relatively simple in functional terms because the primary response to receptor activation is generated by the ion channel which is an integral part of the protein. Therefore, no accessory proteins are needed to observe the response to nicotinic AChR activation and the full functioning of the receptor can be observed by isolating and purifying the protein biochemically and reconstituting the protein in an artificial lipid membrane. In contrast, the G-protein-coupled receptors require both G-proteins and those elements such as phospholipase-C illustrated in Fig. 3.1, in order to observe the response to receptor activation (in this case a rise in intracellular calcium concentration resulting from the action of IP3 on intracellular calcium stores). 

Figure 3.5 shows diagrammatic illustrations of the transmembrane topology of the G-protein-coupled receptor families. Three main families have been identified ... 

By far the most studied family of the G-protein-coupled receptors are the rhodopsin-like receptors. These are also the largest group of receptors in number as they include receptors not only for the monoamines, nucleotides, neuropeptides and peptide hormones, but they also include the odorant receptors which number several hundreds of related receptors. These receptors have short N-termini, a conserved disulphide bridge between the TM2-TM3 and TM4—TM5 extracellular domains, and variable-length C-termini. In some cases the C-terminus is myristolyated which by tying the C-terminus to the cell membrane generates a fourth intracellular loop. 

The apparent reliance of enzyme activation on phosphorylation and intracellular Ca + gives a clue as to how the rate of 5-HT synthesis might be coupled to its impulse-evoked release. Certainly, the impulse-induced increase in intracellular Ca +, and/or activation of the G protein-coupled receptors that govern synthesis of cAMP, could modify the activity of tryptophan hydroxylase. Indeed, this could explain why activation of either somal 5-HTia autoreceptors in the Raphe nuclei (which depress the firing rate of 5-HT neurons) or terminal 5-HTib autoreceptors (which depress 5-HT release) can reduce the production of cAMP and attenuate 5-HT synthesis. 

Forster R, Emrich T, Kremmer E, Lipp M. Expression of the G-protein-coupled receptor BLR1 defines mature, recirculating B cells and a subset of T-helper memory cells. Blood 1994 84 830-40. 

Vassilatis DK, Hohmann JG, Zeng H, et al. The G protein-coupled receptor repertoires of human and mouse. Proc Natl Acad Sci U S A 2003 100(8) 4903 1908. 

Marchese A, Benovic JL. Agonist-promoted ubiquitination of the G protein-coupled receptor CXCR4 mediates lysosomal sorting. J Biol Chem 2001 276(49) 4550SM5512. 

Marchese A, Raiborg C, Santini F, Keen JH, Stenmark H, Benovic JL. The E3 ubiquitin ligase AIP4 mediates ubiquitination and sorting of the G protein-coupled receptor CXCR4. Dev Cell 2003 5(5) 709-722. 

This predicts the constitutive activity of the G-protein-coupled receptor. Note the dependence on the effective concentration of the G-protein. 

Most of the G-protein-coupled receptors are homologous with rhodopsin however, other quantitatively minor families as well as some individual receptors do not share any of the structural features common to the rhodopsin family (Figure 2.3). The most dominant of these are the glucagon/VIP/caldtonin receptor family, or family B (which has approximately 65 members), and the metabotropic glutamate receptor family, or family C (which has approximately 15 members), as well as the frizzled/smoothened family of receptors. Thus, the only structural feature that all G-protein-coupled receptors have in common is the seven-transmembrane helical bundle. Nevertheless, most non-rhodopsin-like receptors do have certain minor structural features in common with the rhodopsin-like receptors — for example, a disulfide bridge between the top of TM-III and the middle of extracellular loop-3, and a cluster of basic residues located just below TM-VI. 

FIGURE 2.3 The three main families of mammalian G-protein-coupled 7TM receptors in mammals. No obvious sequence identity is found between the rhodopsin-like family A, the glucagon/VIP/calcitonin family B, and the metabotropic glutamate/chemosensor family C of G-protein-coupled 7TM receptors, with the exception of the disulfide bridge between the top of TM-III and the middle of extracellular loop-2 (see Figure 2.2). Similarly, no apparent sequence identity exists among members of these three families and, for example the 7TM bitter taste receptors, the V1R pheromone receptors, and the 7TM frizzled proteins, which all are either known or believed to be G-protein-coupled receptors. Bacteriorhodopsins, which are not G-protein-coupled proteins but proton pumps, are totally different in respect to amino-acid sequence but have a seven-helical bundle arranged rather similarly to that for the G-protein-coupled receptors. 

Filardo EJ (2002) Epidermal growth factor receptor (EGFR) transactivation by estrogen via the G-protein-coupled receptor, GPR30 a novel signaling pathway with potential significance for breast cancer. J Steroid Biochem Mol Biol 80 231... 

---