Structural clarity is brought to adhesion G protein-coupled receptor tethered agonism

An elusive problem in the adhesion G protein-coupled receptor (AGPCR) field is full understanding of the activation mechanisms of the 33-member receptor class. With the recent solution of active-state structures of nearly one quarter of AGPCRs, clarity has been brought to how AGPCRs are activated in response to endogenous full agonists. AGPCRs are self-activated via a tethered peptide agonist (TA) that transitions from a concealed or encrypted location to a decrypted state that binds to a typical GPCR orthosteric binding pocket. Here, we summarize the key milestones that led to the discovery of the AGPCR TA activation mechanism and discuss how extracellular shear forces may initiate TA decryption in physiological contexts. We compare the new active-state AGPCR structures and note that the orthosteric site-engaged TAs adopt a remarkably similar partial alpha-helical hook-like conformation, despite divergence of overall receptor similarity. Further, we contrast the TA-bound AGPCR structures to a partially active AGPCR structure to highlight the transitions AGPCRs may undergo during activation. Finally, we provide commentary on the validity of alternative AGPCR activation mechanisms.