Mechanism of G-Protein Coupled Receptor Activation

Abstract

G protein coupled receptors (GPCRs) are seven-helix transmembane proteins that constitute the largest family of membrane proteins in the human genome. GPCRs are responsible for the majority of signal transduction events involving hormones and neurotransmitters across the cell membrane. Activating ligands (agonists) promote binding to and nucleotide exchange by a heterotrimeric G-protein, which dissociates and interacts with downstream effectors. Recent crystal structures have revealed inactive states of several GPCRs, but efforts to obtain an agonist-bound active-state GPCR structure have proven difficult due to the inherent instability of this state in the absence of a G protein. We generated a camelid antibody fragment (nanobody) to the human beta2-adrenergic receptor that exhibits G protein-like behavior, and obtained an agonist-bound, active-state crystal structure of the receptor-nanobody complex at 3.5 Angstrom resolution. Comparison with the inactive beta2-adrenergic receptor structure reveals subtle changes in the binding pocket, but these small changes are associated with an 11 Angstrom outward movement of the cytoplasmic end of transmembrane segment 6, and rearrangements of transmembrane segments 5 and 7 that are remarkably similar to those observed in opsin, an active form of rhodopsin. This structure provides insights into the process of agonist binding and activation of GPCRs.