Ligand interaction with family C receptors

The VFTMs of family C GPCRs are responsible for agonist recognition. This is well illustrated by the ability of such modules to bind ligands even when produced as an isolated soluble protein. The crystal structure of the VFTM of the mGlu1 metabotropic glutamate receptor subtype has been solved with and without bound messenger (glutamate) and antagonist (a-methyl-4-carboxyphenylglycine). In fact, these receptors constitute the first GPCRs for which we have precise structural information about their binding domain. Similar to bacterial periplasmic-binding proteins, the VFTM of the mGlu1 receptor contains two lobes separated by a cleft where ligands bind. In the absence of ligand, the VFTM appears to oscillate between an 'open' and a 'closed' conformation, but the 'open' conformation predominates (Figure 100). Both conformations also exist in the presence of bound agonist, but now the 'closed' conformation is stabilized by the agonist. Hence, after binding of the messenger or related agonist, the VFTM is likely to 'close', trapping the messenger between both lobes in somewhat the same way the carnivorous Venus Fly trap plant

Figure 100 The 'Venus Flytrap' model for family C receptor activation. The agonist binds first to the N-terminal VFTM, the module 'closes' and is then presented to the extracellular loops of the receptor.

does to catch insects. Competitive mGluR antagonists appear to bind within the same binding pocket, but contrary to agonists, they are able to prevent the closure of the VFTM. In fact, its correct closure can only take place if the ligand 'fits' in the closed conformation. This explains why drugs that are structurally related to agonists, but which contain additional or misplaced substituents are able to act as antagonists.

Although the closure of the VFTM represents an important step, it is not sufficient to confer family C GPCR activation. Based on the observation that these receptors act as dimers, it has now been proposed that agonist binding to (and closure of) one VFTM provokes a change in the relative orientation of both VFTMs and that this somehow produces a conformational change in the 7TM domain, ultimately resulting in G protein binding and activation (see also Section 4.4). The participation of the 7TM domain in family C GPCR activation allows ligands that specifically bind to this domain either to potentiate or to inhibit (Figure 101) the action of VFTM-binding agonists. Such ligands are non-competitive and they are termed 'positive allosteric modulators' and 'negative allosteric modulators', respectively (see Section 4.15). These compounds do not need to activate the receptor by themselves. Instead, they may act by modulating the potency and/or efficacy of the VFTM-binding agonists. To become a positive allosteric modulator, the compound has to stabilize the VTFM-7TM domain interaction and, hence, the closed state of the VFTM. On the other hand, negative allosteric modulators may destabilize the VTFM-7TM domain interaction and/or prevent conformational changes within the 7TM domain that are necessary for receptor activation without affecting agonist binding to the VTFMs per se (Hulme et al., 1999; Urwyler et al, 2001).

Figure 101 Family C receptor antagonists may (A) bind to the N-terminal domain of their receptors and so prevent the binding of agonists (competitive antagonism) or (B) bind within the central cleft formed by the 7TM domains (non-competitive antagonism). This will not necessarily affect agonist binding, but it will prevent conformational changes within the transmembrane region of the receptor that are necessary for its activation.

Figure 101 Family C receptor antagonists may (A) bind to the N-terminal domain of their receptors and so prevent the binding of agonists (competitive antagonism) or (B) bind within the central cleft formed by the 7TM domains (non-competitive antagonism). This will not necessarily affect agonist binding, but it will prevent conformational changes within the transmembrane region of the receptor that are necessary for its activation.

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