Journal Article

Extra-helical binding site of a glucagon receptor antagonist

Errey, James C., Teobald, Iryna, Southall, Stacey M., Bortolato, Andrea, Marshall, Fiona H., Doré, Andrew S., Andrews, Stephen P., Robertson, Nathan J., Brown, Alastair J. H., Weir, Malcolm, Lamb, Daniel, Jazayeri, Ali, Baig, Asma H., Cooke, Robert M., Krishnamurthy, Harini, Koglin, Markus

2016

Overview

The X-ray crystal structure of the transmembrane portion of the human glucagon receptor, a class B G-protein-coupled receptor (GPCR), is solved in the presence of the antagonist MK-0893, with potential implications for the development of therapeutics that target other class B GPCRs. Binding of glucagon receptor antagonists The glucagon receptor is a class B G-protein-coupled receptor (GPCR) that binds to the glucagon peptide to trigger the release of glucose from the liver. This GPCR is a potential drug target for type 2 diabetes. These authors have solved an X-ray crystal structure of the transmembrane portion of the human glucagon receptor in the presence of MK-0893, an antagonist. The compound was found at a previously unknown allosteric site inside the lipid bilayer, where it 'pins' one of the seven transmembrane helices in an inactive conformation. It may be possible to develop new potential therapeutics that target the allosteric site on this, and potentially other, class B GPCRs. Glucagon is a 29-amino-acid peptide released from the α-cells of the islet of Langerhans, which has a key role in glucose homeostasis 1 . Glucagon action is transduced by the class B G-protein-coupled glucagon receptor (GCGR), which is located on liver, kidney, intestinal smooth muscle, brain, adipose tissue, heart and pancreas cells, and this receptor has been considered an important drug target in the treatment of diabetes. Administration of recently identified small-molecule GCGR antagonists in patients with type 2 diabetes results in a substantial reduction of fasting and postprandial glucose concentrations 2 . Although an X-ray structure of the transmembrane domain of the GCGR 3 has previously been solved, the ligand (NNC0640) was not resolved. Here we report the 2.5 Å structure of human GCGR in complex with the antagonist MK-0893 (ref. 4 ), which is found to bind to an allosteric site outside the seven transmembrane (7TM) helical bundle in a position between TM6 and TM7 extending into the lipid bilayer. Mutagenesis of key residues identified in the X-ray structure confirms their role in the binding of MK-0893 to the receptor. The unexpected position of the binding site for MK-0893, which is structurally similar to other GCGR antagonists, suggests that glucagon activation of the receptor is prevented by restriction of the outward helical movement of TM6 required for G-protein coupling. Structural knowledge of class B receptors is limited, with only one other ligand-binding site defined—for the corticotropin-releasing hormone receptor 1 (CRF 1 R)—which was located deep within the 7TM bundle 5 . We describe a completely novel allosteric binding site for class B receptors, providing an opportunity for structure-based drug design for this receptor class and furthering our understanding of the mechanisms of activation of these receptors.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 631/535/1266 / 631/92/612/194 / Adipose tissue / Allosteric Site - drug effects / Antagonist drugs / beta-Alanine - analogs & derivatives / beta-Alanine - chemistry / beta-Alanine - metabolism / beta-Alanine - pharmacology / Binding sites / Binding sites (Biochemistry) / Crystallography, X-Ray / Glucagon / Glucagon - metabolism / Glucagon - pharmacology / Humanities and Social Sciences / Humans / letter / Ligands / Lipid Bilayers - chemistry / Lipid Bilayers - metabolism / Models, Molecular / multidisciplinary / Mutation / Observations / Peptides / Physiological aspects / Protein Conformation - drug effects / Proteins / Pyrazoles - chemistry / Pyrazoles - metabolism / Pyrazoles - pharmacology / Receptors, Corticotropin-Releasing Hormone - chemistry / Receptors, Corticotropin-Releasing Hormone - metabolism / Receptors, Glucagon - antagonists & inhibitors / Receptors, Glucagon - chemistry / Receptors, Glucagon - classification / Receptors, Glucagon - metabolism / Science