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Title: Investigating Amino Acid Residues that Contribute to the Constitutive Activity of the Glucose-Dependent Insulinotropic Polypeptide
Authors: Lobna Adi 
Supervisor: Prof. Yunus Luqmani
Keywords: glucagon receptor;constitutive activity of GIPR
Issue Date: 2016
Publisher:  Kuwait university - college of graduate studies
Abstract: incretin receptors, glucagon-like peptide-1 receptor (GLP-1R) and glucose-dependent insulinotropic polypeptide receptor (GIPR) are members of the glucagon receptor family of class B (secretin) G-protein coupled receptors (GPCRs). At comparable levels, GIPR displays significantly higher ligand-independent (constitutive) activity than GLP-1R. A naturally existing E354Q polymorphism in human GIPR is characterized with significantly lowered constitutive activity. In addition, substituting the third extracellular loop (ECL3) of the glucagon receptor (GCGR) with that of GLP-1R produces a chimeric receptor with significantly higher constitutive activity. GCGR could adopt both a closed (inactive) conformation and an open (active) conformation stabilized by peptide ligand binding. The aim of this study was to identify the amino acid residues that contribute to the constitutive activity of GIPR. A proline (P) at position 359 was substituted with a phenylalanine (F) and the ECL3 of GIPR was mutated to that of the GLP-1R. A glutamate (E) at position 354 was substituted with glutamine (Q) to generate the E354Q mutant. These mutants were transfected into HEK 293 cells and characterized using Cre Luciferase and Receptor-Surface Expression assays. Exchanging the ECL3 of GIPR with that of GLP-1R created a model that was consistent with the notion that the extracellular domain (ECD) – ECL3 interaction in GIPR plays a major role in maintaining the receptor’s closed (inactive) conformation. In addition, it was found that the P359F mutation significantly compromised the effect of the ECL3 mutation on the basal activity of GIPR. Interestingly, GIP was more potent at the E354Q mutant than it was on the WT GIPR which was also in agreement with a recent study that demonstrated that the equivalent mutation in GLP-1R (E364Q) resulted in a receptor with increased affinity for GLP-1. This suggests that the increased potency of GIP at GIPR E354Q was related to an increase in affinity for GIP.
Appears in Programs:1100 Pharmaceutical Sciences

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