ABSTRACT
G protein coupled receptors (GPCRs) exhibit a spectrum of functional behaviours in response to natural and synthetic ligands. Recent crystal structures provide insights into inactive states of several GPCRs. 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 ß(2) adrenergic receptor (ß(2)AR) that exhibits G protein-like behaviour, and obtained an agonist-bound, active-state crystal structure of the receptor-nanobody complex. Comparison with the inactive ß(2)AR structure reveals subtle changes in the binding pocket; however, these small changes are associated with an 11 Å 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.
Subject(s)
Adrenergic beta-2 Receptor Agonists/chemistry , Adrenergic beta-2 Receptor Agonists/pharmacology , Immunoglobulin Fragments/chemistry , Immunoglobulin Fragments/immunology , Nanostructures/chemistry , Receptors, Adrenergic, beta-2/chemistry , Receptors, Adrenergic, beta-2/metabolism , Adrenergic beta-2 Receptor Agonists/immunology , Adrenergic beta-2 Receptor Agonists/metabolism , Animals , Binding Sites , Camelids, New World , Crystallography, X-Ray , Drug Inverse Agonism , Humans , Immunoglobulin Fragments/metabolism , Immunoglobulin Fragments/pharmacology , Ligands , Models, Molecular , Movement/drug effects , Opsins/agonists , Opsins/chemistry , Opsins/metabolism , Propanolamines/chemistry , Propanolamines/metabolism , Propanolamines/pharmacology , Protein Conformation/drug effects , Protein Stability/drug effects , Viral Proteins/chemistry , Viral Proteins/metabolismABSTRACT
The activation mechanism of G protein-coupled receptors has presented a puzzle that finally may be close to solution. These receptors have a relatively simple architecture consisting of seven transmembrane helices that contain just a handful of highly conserved amino acids, yet they respond to light and a range of chemically diverse ligands. Recent NMR structural studies on the active metarhodopsin II intermediate of the visual receptor rhodopsin, along with the recent crystal structure of the apoprotein opsin, have revealed multiple structural elements or 'switches' that must be simultaneously triggered to achieve full activation. The confluence of several required structural changes is an example of "coincidence counting", which is often used by nature to regulate biological processes. In ligand-activated G protein-coupled receptors, the presence of multiple switches may provide an explanation for the differences between full, partial and inverse agonists.
