The interaction of activated G protein‐coupled receptors with G proteins is a key event in signal transduction. Here, using a fluorescence resonance energy transfer (FRET)‐based assay, we measure directly and in living cells the interaction of YFP‐labeled α2A‐adrenergic receptors with CFP‐labeled G proteins. Upon agonist stimulation, a small, concentration‐dependent increase in FRET was observed. No specific basal FRET was detected in the absence of agonist. Kinetics of the onset of receptor/G protein interaction were <100 ms and depended on expression levels of Gα. Simultaneously recorded G protein‐regulated inwardly rectifying K+ channel currents revealed a maximal current response already at agonist concentrations producing submaximal FRET amplitudes. By analyzing FRET signals in the presence of a Gα mutant, which dissociates more slowly from activated receptors, it was demonstrated that only a fraction of wild‐type G proteins interacts with the activated receptor at any time. Our data suggest that α2A‐adrenergic receptors and G proteins interact by rapid collision coupling and indicate that there is no significant precoupling between these receptors and G proteins.

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G-protein-coupled receptors (GPCRs) are the largest group of cell surface receptors. They are stimulated by a variety of stimuli and signal to different classes of effectors, including several types of ion channels and second messenger-generating enzymes. Recent technical advances, most importantly in the optical recording with energy transfer techniques--fluorescence and bioluminescence resonance energy transfer, FRET and BRET--, have permitted a detailed kinetic analysis of the individual steps of the signalling chain, ranging from ligand binding to the production of second messengers in intact cells. The transfer of information, which is initiated by ligand binding, triggers a signalling cascade that displays various rate-controlling steps at different levels. This review summarizes recent findings illustrating the speed and the complexity of this signalling system.