Dynamics of receptor/G protein coupling in living cells

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Abstract
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.