title: Inactive-state preassembly of G(q)-coupled receptors and G(q) heterotrimers.
abstract: 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 M(3) muscarinic acetylcholine receptors (M3R) form inactive-state complexes with G(q) heterotrimers in intact cells. The M3R C terminus is sufficient, and a six-amino-acid polybasic sequence distal to helix 8 ((565)KKKRRK(570)) is necessary for preassembly with G(q). Replacing this sequence with six alanine residues prevents preassembly, slows the rate of G(q) activation and decreases steady-state agonist sensitivity. That other G(q)-coupled receptors possess similar polybasic regions and also preassemble with G(q) suggests that these GPCRs may use a common preassembly mechanism to facilitate activation of G(q) heterotrimers.
authors.affiliation: Department of Pharmacology and Toxicology, Georgia Health Sciences University, Augusta, Georgia, USA.
authors.forename: Kou
authors.initials: K
authors.lastname: Qin
authors.forename: Chunmin
authors.initials: C
authors.lastname: Dong
authors.forename: Guangyu
authors.initials: G
authors.lastname: Wu
authors.forename: Nevin A
authors.initials: NA
authors.lastname: Lambert
mesh_terms: D000595:Amino Acid Sequence; D002478:Cells, Cultured; D043802:GTP-Binding Protein alpha Subunits, Gq-G11 / Q000737:chemistry* / Q000378:metabolism*; D057809:HEK293 Cells; D006801:Humans; D008969:Molecular Sequence Data; D011485:Protein Binding; D020836:Protein Structure, Quaternary; D043587:Receptor, Muscarinic M3 / Q000737:chemistry* / Q000378:metabolism*; D015398:Signal Transduction; D013997:Time Factors
chemical_list: D043587:Receptor, Muscarinic M3; D043802:GTP-Binding Protein alpha Subunits, Gq-G11