Crystal structures of the β₂-adrenergic receptor

G protein coupled receptors (GPCRs) constitute the largest family of membrane proteins in the human genome, and are responsible for the majority of signal transduction events involving hormones and neurotransmitters across the cell membrane. GPCRs that bind to diffusible ligands have low natural abundance, are relatively unstable in detergents, and display basal G protein activation even in the absence of ligands. To overcome these problems two approaches were taken to obtain crystal structures of the β₂-adrenergic receptor (β₂AR), a well-characterized GPCR that binds catecholamine hormones. The receptor was bound to the partial inverse agonist carazolol and co-crystallized with a Fab made to a three-dimensional epitope formed by the third intracellular loop (ICL3), or by replacement of ICL3 with T4 lysozyme. Small crystals were obtained in lipid bicelles (β₂AR-Fab) or lipidic cubic phase (β₂AR-T4 lysozyme), and diffraction data were obtained using microfocus technology. The structures provide insights into the basal activity of the receptor, the structural features that enable binding of diffusible ligands, and the coupling between ligand binding and G-protein activation.