Ligand interaction with family B receptors

Except for the recognized activation of frizzled receptors by Wnts (350-360 amino acid proteins) soluble extracellular ligands for LNB-7TM and smoothened receptors appear to be more elusive or may even be inexistent. In contrast, all members of the secretin/glucagon/VIP receptor family are stimulated by known peptide hormones and neuropeptides. These have a relatively high molecular weight (e.g. 27 amino acids for human secretin, 29 for pancreatic glucagon, 37 for calcitonin gene-related peptide and 41 for corticotropin-releasing factor).

Mutation studies have provided considerable information concerning the interaction between the family B1 peptide hormone receptors and their ligands. For example, it was shown for the glucagon receptor that:

• All seven TM domains of the receptor are needed for its proper folding and processing.

• Receptor glycosylation is not essential for binding or activation.

• The extracellular amino terminus is required for ligand binding.

• The 206-219 segment of the first extracellular loop is important for ligand binding and activation but the terminal portion of the intracellular carboxyl terminus is not.

In general, as illustrated for the glucagon-like protein 1 (GLP1) receptor (Figure 99), it seems that the large amino terminus of these receptors plays a key role for the binding

Figure 99 Schematic representation of the glucagon like protein 1 (GLP1) receptor. Residues that are important for binding are shown: they are present in the N-terminal portion, extracellular loops and TM helices. Reproduced from Frimurer, T. M. and Bywater, P., Structure of the integral membrane domain of the GLP1 receptor. Proteins 35, 375-386 (1999), by permission of John Wiley & Sons Ltd.

Figure 99 Schematic representation of the glucagon like protein 1 (GLP1) receptor. Residues that are important for binding are shown: they are present in the N-terminal portion, extracellular loops and TM helices. Reproduced from Frimurer, T. M. and Bywater, P., Structure of the integral membrane domain of the GLP1 receptor. Proteins 35, 375-386 (1999), by permission of John Wiley & Sons Ltd.

of the peptide ligands. However, additional interactions involving the extracellular loops and specific TM domains appear also to be required for binding and activation.

The existence of topographically distinct binding domains at these receptors allows both ends of their cognate peptide ligands to display different, but complementary functions. For example, CRF receptors (CRF1R and CRF2R) use their amino terminus to anchor the N-terminal part of their different peptide ligands. This plays an important role with respect to their receptor subtype selectivity. On the other hand, binding of the C-terminal part of these ligands to specific receptor TM domains is necessary to produce receptor activation.

While the peptide hormone receptors have distinct binding domains for peptide ligands, small (i.e. synthetic) molecules are thought to mainly bind to the TM region. The occurrence of topographically distinct binding sites for the peptide agonists and small molecule antagonists paves the way to allosteric interactions and thereby associated 'non-competitive' antagonism (see Section 4.15). This may invalidate the classical methods of analyzing antagonist-receptor interactions.

Of note is that many family B GPCRs have been shown to interact with other membrane-associated proteins and for the calcitonin receptor-like receptor (CRLR), this has clearly been demonstrated to affect their migration to the cell plasma membrane as well as their pharmacological profile/identity (see Section 4.8). Whereas the heterodimer between CRLR and a type 1 RAMP (a receptor activity modifying protein with a single TM domain) generates a CGRP receptor, CRLR-RAMP2 or -RAMP3 complexes generate an adrenomedullin receptor (Figure 152). This example clearly illustrates that GPCRs do not necessarily act on their own. Instead, their ligand binding and other properties may be highly sensitive to the presence of certain proteins in their surroundings.

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