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G-protein-coupled receptors (GPCRs) are remarkably versatile signaling molecules.
Members of this large family of membrane proteins respond to structurally diverse
ligands and mediate most transmembrane signal transduction in response to hormones
and neurotransmitters, and in response to the senses of sight, smell and taste. Individual
GPCRs can signal through several G-protein subtypes and through G-protein-independent
pathways, often in a ligand-specific manner. This functional plasticity can be attributed
to structural flexibility of GPCRs and the ability of ligands to induce or to stabilize
ligand-specific conformations. Here, we review what has been learned about the dynamic
nature of the structure and mechanism of GPCR activation, primarily focusing on spectroscopic
studies of purified human beta2 adrenergic receptor.