Mechanism of Tethered Agonist Binding to an Adhesion G-Protein-Coupled Receptor

Adhesion G-protein-coupled receptors (ADGRs) contain a GPCR autoproteolysis-inducing (GAIN) domain that is proximal to the receptor N-terminus and undergoes autoproteolysis at a highly conserved GPCR proteolysis site to generate the N-terminal fragment and transmembrane C-terminal fragment. Dissociation of the N-terminal fragment reveals a peptide tethered agonist (TA) that is responsible for the activation of the ADGRs. The inactive complete ADGRs contain the encrypted TA that assumes a β-strand configuration within the GAIN domain, which is markedly different from the U-shaped α-helical configuration of TA in the active cryo-EM structures of ADGRs. However, how the TA dramatically changes its configuration and binds to the ADGR C-terminal fragment remains unknown. In this study, we have performed all-atom enhanced sampling simulations using a novel Peptide Gaussian-accelerated Molecular Dynamics (Pep-GaMD) method for the binding of TA to ADGRD1. The Pep-GaMD simulations captured spontaneous binding of the TA into the orthosteric pocket of ADGRD1 and its large conformational transition from the extended β-strand to the U-shaped α-helical configuration. We were able to identify important low-energy conformations of the TA in the binding pathway, as well as different active and inactive states of ADGRD1, in the presence and absence of the G_s-protein. Therefore, our Pep-GaMD simulations have revealed the dynamic mechanism of the TA binding to an ADGR, which will facilitate the rational design of the peptide regulators of ADGRs.