G Protein-Coupled Receptor Allosterism and Complexing

Abstract

G protein-coupled receptors (GPCRs) represent the largest family of cell-surface receptors. These receptors are natural allosteric proteins because agonist-mediated signaling by GPCRs requires a conformational change in the receptor protein transmitted between two topographically distinct binding sites, one for the agonist and another for the G protein. It is now becoming increasingly recognized, however, that the agonist-bound GPCR can also form ternary complexes with other ligands or "accessory" proteins and display altered binding and/or signaling properties in relation to the binary agonist-receptor complex. Allosteric sites on GPCRs represent novel drug targets because allosteric modulators possess a number of theoretical advantages over classic orthosteric ligands, such as a ceiling level to the allosteric effect and a potential for greater GPCR subtype-selectivity. Because of the noncompetitive nature of allosteric phenomena, the detection and quantification of such effects often relies on a combination of equilibrium binding, nonequilibrium kinetic, and functional signaling assays. This review discusses the development and properties of allosteric receptor models for GPCRs and the detection and quantification of allosteric effects. Moreover, we provide an overview of the current knowledge regarding the location of possible allosteric sites on GPCRs and candidate endogenous allosteric modulators. Finally, we discuss the potential for allosteric effects arising from the formation of GPCR oligomers or GPCRs complexed with accessory cellular proteins. It is proposed that the study of allosteric phenomena will become of progressively greater import to the drug discovery process due to the advent of newer and more sensitive GPCR screening technologies.

The L293 residue in transmembrane domain 2 of the 5-HT3A receptor is a molecular determinant of allosteric modulation by 5-hydroxyindole

“…Therefore an enhanced agonist affinity might contribute to 5-HI potentiation in the wild-type receptor. However, the observation of 5-HI potentiation of the response activated by a maximally efficacious agonist concentration indicates that a process other than increased agonist affinity is also involved since an increase in affinity alone should not enhance maximal agonist efficacy (Christopoulos & Kenakin, 2002). An increase in channel conductance could also produce an increase in peak current amplitude.…”

“…Furthermore, our previous studies suggest that there is little correlation between the physicochemical properties of the amino acid residues substituted at L293 (15′) and alcohol action (Hu et al, 2006). It is believed that allosteric agents act by modulating receptor activity through conformational changes in the receptor transmitted from the allosteric to the orthosteric site and/or to the coupling sites (Christopoulos & Kenakin, 2002). It is possible that L293 is a key residue in regions linking the allosteric site of 5-HI binding to agonist binding and/or channel gating.…”

Rapid-mix flow cytometry measurements of subsecond regulation of G protein-coupled receptor ternary complex dynamics by guanine nucleotides

“…Interestingly our kinetics for the dissociation of FPR-GFP or wild type FPR from Gα i in the detergent solubilized system are much faster than was reported in cells from experiments involving FRET between β 2 AR and Gα s [19]. We do not know whether this reflects differences between the intact cells and detergent, the difference between specific receptors and G proteins, the possibility that their measurement has contributions from a higher order topographical redistribution, or the possibility that ligand dissociation and rebinding [44][45][46] is rate-limiting in their measurement of RG dissociation.…”

Designing allosteric peptide ligands targeting a globular protein

“…This is generally consistent with the postulated empirical and opportunistic character of allosteric site discovery in proteins. 97 …”

“…. ..'' 97 The studies of temperature dependent isotherms of peptide activation-binding of GAL (Figures 8A-8C; Figure 9), although consistent with our hypothesis of hydrophobic eigenmode matched binding, must be considered prelimi- FIGURE 9 The plots of the temperature dependence of the negative Gibbs free energies, ÀDG (in cal/mol), computed from the affinity constants, Ka's, of the monoclonal antibody, mAB, and peptide-GAL binding, such that DG ¼ À2.3RT ln Ka. The negative free energies are consistent with observed spontaneous binding.…”