Ligand-specific changes in M3 muscarinic acetylcholine receptor structure detected by a disulfide scanning strategy.
G protein-coupled receptor (GPCR) function can be modulated by different classes of ligands including full and inverse agonists. At present, little is known about the conformational changes that agonist ligands induce in their target GPCRs. In this study, we employed an in situ disulfide cross-linking strategy to monitor ligand-induced structural changes in a series of cysteine (Cys)-substituted mutant M 3 muscarinic acetylcholine receptors. One of our goals was to study whether the cytoplasmic end of transmembrane domain V (TM V), a region known to be critically involved in receptor/G protein coupling, undergoes a major conformational change, similar to the adjacent region of TM VI. Another goal was to determine and compare the disulfide cross-linking patterns observed after treatment of the different mutant receptors with full versus inverse muscarinic agonists. Specifically, we generated 20 double Cys mutant M 3 receptors harboring one Cys substitution within the cytoplasmic end of TM V (L249-I253) and a second one within the cytoplasmic end of TM VI (A489-L492). These receptors were transiently expressed in COS-7 cells and subsequently characterized in pharmacological and disulfide cross-linking studies. Our cross-linking data, in conjunction with a three-dimensional model of the M 3 muscarinic receptor, indicate that M 3 receptor activation does not trigger major structural disturbances within the cytoplasmic segment of TM V, in contrast to the pronounced structural changes predicted to occur at the cytoplasmic end of TM VI. We also demonstrated that full and inverse muscarinic agonists had distinct effects on the efficiency of disulfide bond formation in specific double Cys mutant M 3 receptors. The present study provides novel information about the dynamic changes that accompany M 3 receptor activation and how the receptor conformations induced (or stabilized) by full versus inverse muscarinic agonists differ from each other at the molecular level. Because all class I GPCRs are predicted to share a similar transmembrane topology, the conclusions drawn from the present study should be of broad general relevance.
Li
Jian Hua
JH
Molecular Signaling Section, Laboratory of Bioorganic Chemistry, National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), 8 Center Drive, Bethesda, Maryland 20892-0810, USA.
Hamdan
Fadi F
FF
Kim
Soo-Kyung
SK
Jacobson
Kenneth A
KA
Zhang
Xiaohong
X
Han
Sung-Jun
SJ
Wess
Jürgen
J
D000595:Amino Acid Sequence; D000818:Animals; D015153:Blotting, Western; D019556:COS Cells; D002217:Carbachol / Q000737:chemistry / Q000378:metabolism; D002522:Chlorocebus aethiops; D004220:Disulfides / Q000737:chemistry* / Q000378:metabolism; D006868:Hydrolysis; D008024:Ligands; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009154:Mutation; D010716:Phosphatidylinositols / Q000737:chemistry / Q000378:metabolism; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D043587:Receptor, Muscarinic M3 / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism
D004220:Disulfides; D008024:Ligands; D010716:Phosphatidylinositols; D043587:Receptor, Muscarinic M3; D002217:Carbachol