title: Use of a disulfide cross-linking strategy to study muscarinic receptor structure and mechanisms of activation.
abstract: To gain insight into the molecular architecture of the cytoplasmic surface of G protein-coupled receptors, we have developed a disulfide cross-linking strategy using the m3 muscarinic receptor as a model system. To facilitate the interpretation of disulfide cross-linking data, we initially generated a mutant m3 muscarinic receptor (referred to as m3'(3C)-Xa) in which most native Cys residues had been deleted or substituted with Ala or Ser (remaining Cys residues Cys-140, Cys-220, and Cys-532) and in which the central portion of the third intracellular loop had been replaced with a factor Xa cleavage site. Radioligand binding and second messenger assays showed that the m3'(3C)-Xa mutant receptor was fully functional. In the next step, pairs of Cys residues were reintroduced into the m3'(3C)-Xa construct, thus generating 10 double Cys mutant receptors. All 10 mutant receptors contained a Cys residue at position 169 at the beginning of the second intracellular loop and a second Cys within the C-terminal portion of the third intracellular loop, at positions 484-493. Radioligand binding studies and phosphatidylinositol assays indicated that all double Cys mutant receptors were properly folded. Membrane lysates prepared from COS-7 cells transfected with the different mutant receptor constructs were incubated with factor Xa protease and the oxidizing agent Cu(II)-(1,10-phenanthroline)3, and the formation of intramolecular disulfide bonds between juxtaposed Cys residues was monitored by using a combined immunoprecipitation/immunoblotting strategy. To our surprise, efficient disulfide cross-linking was observed with 8 of the 10 double Cys mutant receptors studied (Cys-169/Cys-484 to Cys-491), suggesting that the intracellular m3 receptor surface is characterized by pronounced backbone fluctuations. Moreover, [35S]guanosine 5'-3-O-(thio)triphosphate binding assays indicated that the formation of intramolecular disulfide cross-links prevented or strongly inhibited receptor-mediated G protein activation, suggesting that the highly dynamic character of the cytoplasmic receptor surface is a prerequisite for efficient receptor-G protein interactions. This is the first study using a disulfide mapping strategy to examine the three-dimensional structure of a hormone-activated G protein-coupled receptor.
authors.affiliation: Laboratory of Bioorganic Chemistry, NIDDK, National Institutes of Health, Bethesda, Maryland 20892, USA.
authors.forename: F Y
authors.initials: FY
authors.lastname: Zeng
authors.forename: A
authors.initials: A
authors.lastname: Hopp
authors.forename: A
authors.initials: A
authors.lastname: Soldner
authors.forename: J
authors.initials: J
authors.lastname: Wess
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D019556:COS Cells; D004220:Disulfides / Q000378:metabolism*; D015951:Factor Xa / Q000378:metabolism; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D051381:Rats; D043587:Receptor, Muscarinic M3; D011976:Receptors, Muscarinic / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D013329:Structure-Activity Relationship
chemical_list: D004220:Disulfides; D043587:Receptor, Muscarinic M3; D011976:Receptors, Muscarinic; D015951:Factor Xa