G protein-coupled receptors comprise the largest family of eukaryotic signal transduction proteins that communicate across the membrane. We report the crystal structure of a human β 2 -adrenergic receptor-T4 lysozyme fusion protein bound to the partial inverse agonist carazolol at 2.4 Å resolution. The structure provides a high-resolution view of a human G protein-coupled receptor bound to a diffusible ligand. Ligand-binding site accessibility is enabled by the second extracellular loop which is held out of the binding cavity by a pair of closely spaced disulfide bridges and a short helical segment within the loop. Cholesterol, a necessary component for crystallization, mediates an intriguing parallel association of receptor molecules in the crystal lattice. Although the location of carazolol in the β 2 -adrenergic receptor is very similar to that of retinal in rhodopsin, structural differences in the ligand binding site and other regions highlight the challenges in using rhodopsin as a template model for this large receptor family.* To whom correspondence should be addressed: stevens@scripps.edu; kobilka@stanford.edu $ These authors contributed equally Author Contributions: RCS and BKK independently pushed the GPCR structural biology projects for more than 15 years. BKK managed the protein design, production and purification. RCS managed novel crystallization and data collection methods development and experiments. VC developed novel methods for, and performed LCP crystallization, LCP crystal mounting, LCP data collection, model refinement, analyzed the results, and was involved in manuscript preparation. DMR supplied protein materials for all crystallization trials, grew and collected data from the bicelle crystals, collected, processed and refined the 3.5 Å LCP structure, refined the 2.4 Å structure, analyzed the results, and was involved in manuscript preparation. MAH designed the blind crystal screening protocol and collected the 2.4 Å data set, processed the 2.4 Å data, solved the structure by MR at 3.5 Å and 2.4 Å resolution, wrote the initial draft of the manuscript and created all figures. SGFR assisted with the final stages of β 2 AR-T4L purification. FST expressed β 2 AR-T4L in insect cells and, together with TSK, performed the initial stage of β 2 AR purification. HJC assisted with the refinement. PK assisted in developing novel methods to screen the transparent crystals, data collection, refinement, and was involved in manuscript preparation. WIW assisted with low resolution data collection and processing, solved the β 2 AR-T4L molecular replacement problem at 3.5 Å, participated in the 2.4 Å refinement process, and participated in structure analysis and manuscript preparation. BKK additionally assisted with β 2 AR-T4L purification, β 2 AR-T4L 3.5 Å synchrotron data collection, structure analysis and manuscript preparation. BKK and DMR designed the β 2 AR-T4L fusion protein strategy. RCS additionally assisted with β 2 AR-T4L crystallization, 2.4 Å data collection, structure solution, refinem...

G protein coupled receptors (GPCRs) exhibit a spectrum of functional behaviors 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 β2 adrenergic receptor (β2AR) that exhibits G protein-like behavior, and obtained an agonist-bound, active-state crystal structure of the receptor-nanobody complex. Comparison with the inactive β2AR structure reveals subtle changes in the binding pocket; however, these small changes are associated with an 11Å 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.

Structural analysis of G-protein-coupled receptors (GPCRs) for hormones and neurotransmitters has been hindered by their low natural abundance, inherent structural flexibility, and instability in detergent solutions. Here we report a structure of the human beta2 adrenoceptor (beta2AR), which was crystallized in a lipid environment when bound to an inverse agonist and in complex with a Fab that binds to the third intracellular loop. Diffraction data were obtained by high-brilliance microcrystallography and the structure determined at 3.4 A/3.7 A resolution. The cytoplasmic ends of the beta2AR transmembrane segments and the connecting loops are well resolved, whereas the extracellular regions of the beta2AR are not seen. The beta2AR structure differs from rhodopsin in having weaker interactions between the cytoplasmic ends of transmembrane (TM)3 and TM6, involving the conserved E/DRY sequences. These differences may be responsible for the relatively high basal activity and structural instability of the beta2AR, and contribute to the challenges in obtaining diffraction-quality crystals of non-rhodopsin GPCRs.

The beta2-adrenergic receptor (beta2AR) is a well-studied prototype for heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs) that respond to diffusible hormones and neurotransmitters. To overcome the structural flexibility of the beta2AR and to facilitate its crystallization, we engineered a beta2AR fusion protein in which T4 lysozyme (T4L) replaces most of the third intracellular loop of the GPCR ("beta2AR-T4L") and showed that this protein retains near-native pharmacologic properties. Analysis of adrenergic receptor ligand-binding mutants within the context of the reported high-resolution structure of beta2AR-T4L provides insights into inverse-agonist binding and the structural changes required to accommodate catecholamine agonists. Amino acids known to regulate receptor function are linked through packing interactions and a network of hydrogen bonds, suggesting a conformational pathway from the ligand-binding pocket to regions that interact with G proteins.