Understanding the Molecular Mechanism of Endothelin ETA Receptor Selecting Isopeptides Endothelin-1 and -3.

Endothelin-1 and -3 (ET1 and ET3) are potent vasoconstricting isopeptides that are involved in the pathophysiology of various diseases, such as cardiovascular and renal diseases. The two ETs exert their effects by binding to two G-protein-coupled receptors (GPCRs), the endothelin ETA receptor (ETAR) and the endothelin ETB receptor (ETBR). ETAR and ETBR reveal different preferences in the recognition of the two ETs: The binding affinity of ETAR with ET1 is much higher than that with ET3, while the binding affinities of ETBR with the two ETs are similar. Recently, the structures of ETBR with ET1 and ET3 were determined. These crystal structures provide detailed descriptions of how ETBR interacts with ET1 and ET3. However, the knowledge of how ETAR recognizes the two isopeptides is still lacking. Based on the structure of ETBR in complex with ET1, the structures of ETAR with ET1 and ET3 were modeled. Then, molecular dynamics simulations were performed to study how ETAR discriminates the two ETs. Simulation results demonstrate that ET1 has greater binding free energy with ETAR than ET3, which is in agreement with the binding affinity data of ETAR. By interaction energy analysis, the key residues of ETAR that discriminate between ET1 and ET3 are identified. Structural and dynamical analyses indicate that, compared with ET3, ET1 is more stable in binding with ETAR. Furthermore, the orientation change of W319 in the conserved CWxP motif that plays an important role in the signaling function of GPCRs was observed. Through the orientation change of this residue, the difference in the orthosteric pocket volume resulting from the binding of ET1 and ET3 on the extracellular side of ETAR leads to a conformational difference of TM6 on the intracellular side of ETAR. This study elucidates the molecular mechanism of how ETAR selects the isopeptides ET1 and ET3.