G protein–coupled receptors (GPCRs) transmit signals by forming active-state complexes with heterotrimeric G proteins. It has been suggested that some GPCRs also assemble with G proteins before ligand-induced activation and that inactive-state preassembly facilitates rapid and specific G protein activation. However, no mechanism of preassembly has been described, and no functional consequences of preassembly have been demonstrated. Here we show that M3 muscarinic acetylcholine receptors (M3R) form inactive-state complexes with Gq heterotrimers in intact cells. The M3R C terminus is sufficient, and a six-amino-acid polybasic sequence distal to helix 8 (565KKKRRK570) is necessary for preassembly with Gq. Replacing this sequence with six alanine residues prevents preassembly, slows the rate of Gq activation and decreases steady-state agonist sensitivity. That other Gq-coupled receptors possess similar polybasic regions and also preassemble with Gq suggests that these GPCRs may use a common preassembly mechanism to facilitate activation of Gq heterotrimers.
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We thank G. Digby and S. Kuravi for invaluable assistance. This work was supported by US National Institutes of Health grants GM076167 (G.W.), GM096762 and GM078319 (N.A.L.).
The authors declare no competing financial interests.
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Qin, K., Dong, C., Wu, G. et al. Inactive-state preassembly of Gq-coupled receptors and Gq heterotrimers. Nat Chem Biol 7, 740–747 (2011). https://doi.org/10.1038/nchembio.642
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