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

Graduate Program in Neuroscience and Department of Pharmacology and Toxicology, Medical College of Georgia, Augusta, Georgia, USA

¹Correspondence: Department of Pharmacology and Toxicology, Medical College of Georgia, Augusta, GA 30809, USA. E-mail: nlambert{at}mcg.edu

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 _2A-adrenoreceptors (C-2ARs) 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-2ARs. Immobile C-2ARs 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-2ARs 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-2ARs and G proteins. We conclude that at most a small fraction of C-2ARs 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.

Key Words: FRAP • heterotrimers • precoupling • collision

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