Abstract
Elevated glucagon levels and increased hepatic glucagon receptor (GCGR) signaling contribute to hyperglycemia in type 2 diabetes. We have identified a monoclonal antibody that inhibits GCGR, a class B G-protein coupled receptor (GPCR), through a unique allosteric mechanism. Receptor inhibition is mediated by the binding of this antibody to two distinct sites that lie outside of the glucagon binding cleft. One site consists of a patch of residues that are surface-exposed on the face of the extracellular domain (ECD) opposite the ligand-binding cleft, whereas the second binding site consists of residues in the αA helix of the ECD. A docking model suggests that the antibody does not occlude the ligand-binding cleft. We solved the crystal structure of GCGR ECD containing a naturally occurring G40S mutation and found a shift in the register of the αA helix that prevents antibody binding. We also found that alterations in the αA helix impact the normal function of GCGR. We present a model for the allosteric inhibition of GCGR by a monoclonal antibody that may form the basis for the development of allosteric modulators for the treatment of diabetes and other class B GPCR-related diseases.
Highlights
Allosteric regulators of G-protein coupled receptor (GPCR) provide unique pharmacological properties
We present a model for the allosteric inhibition of glucagon receptor (GCGR) by a monoclonal antibody that may form the basis for the development of allosteric modulators for the treatment of diabetes and other class B GPCR-related diseases
All of the inhibitory antibodies that we have studied target the extracellular domain (ECD) of GCGR, but these can differ in their molecular pharmacology
Summary
Allosteric regulators of GPCRs provide unique pharmacological properties. Results: The mechanism of allosteric inhibition of the glucagon receptor by an antibody, which is uniquely sensitive to a naturally occurring G40S mutation, is detailed. Based on studies with the glucagon receptor, we recently proposed a model of receptor activation in which the ECD binds and presents glucagon to the transmembrane core for receptor activation and undergoes a conformational change upon ligand binding that relieves inhibition of the receptor by the ECD This negative regulation of the receptor by the ECD is mediated by an interaction between the ECD and extracellular loop 3 of the transmembrane ␣-helical bundle, an activity uncovered through the characterization of an inverse agonist of GCGR [6]. The data presented here provide a rationale for blocking the activity of a class B GPCR through an allosteric mechanism
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