GPCR Engineering Yields High-Resolution Structural Insights into β ₂ -Adrenergic Receptor Function

The β ₂ -adrenergic receptor (β ₂ AR) 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 β ₂ AR and to facilitate its crystallization, we engineered a β ₂ AR fusion protein in which T4 lysozyme (T4L) replaces most of the third intracellular loop of the GPCR (“β ₂ AR-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 β ₂ AR-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.