Abundance and stability of complexes containing inactive G protein‐coupled receptors and G proteins

G protein‐coupled receptors (GPCRs) interact directly with heterotrimeric G proteins to transduce physiological signals. Early studies of this interaction concluded that GPCRs (R) and G proteins (G) collide with each other randomly after receptor activation and that R‐G complexes are transient. More recent studies have suggested that inactive R and G are preassembled (precoupled) as stable R‐G complexes. Here we examine the stability of complexes formed between cyan fluorescent protein‐labeled α 2A ‐adrenoreceptors (C‐α2ARs) and G proteins in cells using fluorescence recovery after photobleaching (FRAP). Labeled G proteins diffused in the plasma membrane with equal mobility in the absence and presence of immobile C‐α2ARs. Immobile C‐α2ARs activated labeled G proteins, demonstrating functional coupling without stable physical association. In contrast, a stable R‐G interaction was detected when G proteins were deprived of nucleotides and C‐α2ARs were active, as predicted by the ternary complex model. Overexpression of regulator of G protein signaling 4 (RGS4) accelerated the onset of effector activation but did not detectably alter the interaction between C‐α2ARs and G proteins. We conclude that at most a small fraction of C‐α2ARs and G proteins exist as R‐G complexes at any moment.—Qin, K., Sethi, P. R., Lambert, N. A. Abundance and stability of complexes containing inactive G protein‐coupled receptors and G proteins. FASEB J. 22, 2920–2927 (2008)