Recording Kinetics of Adrenergic Receptor Activation in Live Cells

G protein-coupled receptors (GPCR) biosensors have been recently developed that measure intramolecular conformational changes by recording fluorescence resonance energy transfer (FRET) between fluorescent protein tags introduced at two intracellular sites in the receptor. This technique allows the spatial and temporal recording of agonist (full, partial, and inverse)-induced receptor conformational changes in live cells in real time. This review discusses the kinetics of receptor activation, the direct measurement of ligand efficacy at the level of the receptor, and how ligand efficacy can be modulated by receptor heteromers.