Crystal structure of human endothelin ET_B receptor in complex with peptide inverse agonist IRL2500

Endothelin receptors (ET_A and ET_B) are G-protein-coupled receptors activated by endothelin-1 and are involved in blood pressure regulation. IRL2500 is a peptide-mimetic of the C-terminal tripeptide of endothelin-1, and has been characterized as a potent ET_B-selective antagonist, which has preventive effects against brain edema. Here, we report the crystal structure of the human ET_B receptor in complex with IRL2500 at 2.7 Å-resolution. The structure revealed the different binding modes between IRL2500 and endothelin-1, and provides structural insights into its ET_B-selectivity. Notably, the biphenyl group of IRL2500 penetrates into the transmembrane core proximal to D^2.50, thus stabilizing the inactive conformation. Using the newly-established constitutively active mutant, we clearly demonstrate that IRL2500 functions as an inverse agonist for the ET_B receptor. The current findings will expand the chemical space of ETR antagonists and facilitate the design of inverse agonists for other class A GPCRs.