Localization of the domains involved in ligand binding and activation of the glucose-dependent insulinotropic polypeptide receptor

The receptors for the two structurally related insulinotropic hormones Glucose-dependent Insulinotropic Polypeptide (GIP) and Glucagon-Like Peptide-1 (GLP-1) share approximately 40% sequence identity and demonstrate complete specificity for their endogenous ligands, while utilizing similar second messenger pathways. In the current study chimeric GIP-GLP-1 receptors were prepared, and the effect of domain-exchange on ligand binding and adenylyl cyclase activation examined. A chimera (CH-2) consisting of the first 132 amino acids of the external N-terminal (NT) domain bound I-125-GIP with high affinity (27.77 +/- 11.85 nM). However, for receptor coupling to cAMP production it was necessary to extend the NT into the first transmembrane (TM-1) region (CH-3: IC50 = 9.04 +/- 1.07 nM; EC50 = 17.1 +/- 3.5 nM). A chimera which included part of TM-3 (CH-4) demonstrated binding and signalling (IC50 = 8.33 +/- 0.14 nM; EC50 = 467.5 +/- 173.6 pM) similar to the wild type receptor (IC50 = 1.33 +/- 0.19 nM; EC50 = 497.3 +/- 211.7 pM). Surprisingly constructs CH-2 and CH-3, while devoid of detectable I-125-GLP-1 binding, were capable of eliciting GLP-1-specific cAMP production (EC50S CH-2 = 81.4 +/- 19.6 nM; CH-3 = 5.99 +/- 0.68 nM) suggesting that receptor activation is not completely dependent on, high affinity receptor binding. These data clearly demonstrate that the NT domain of the GTP receptor acts as the ligand-specific binding domain and that the first transmembrane domain is important for receptor activation.