Identification of a ligand-dependent switch within a muscarinic receptor.
G-protein-coupled receptors spontaneously switch between active and inactive conformations. Agonists stabilize the active conformation, whereas antagonists stabilize the inactive conformation. In a systematic search for residues that participate in receptor function, several regions of the m5 muscarinic receptor were randomly mutated and tested for their functional properties. Mutations spanning one face of transmembrane 6 (TM6) were found to induce high levels of receptor activity in the absence of agonists (constitutive activity). The same face of TM6 contained several residues crucial for receptor activation by agonists and one residue identified as a contact site for both agonists and antagonists. In addition, one mutation induced agonist-like responses from the receptor when exposed to classical antagonists. These results suggest that TM6 is a switch that defines the activation state of the receptor, and that ligand interactions with TM6 stabilize the receptor in either an active or an inactive conformation.
ACADIA Pharmaceuticals, Inc., San Diego, California 92121, USA. tspalding@acadia-pharm.com
T A
TA
Spalding
E S
ES
Burstein
S C
SC
Henderson
K R
KR
Ducote
M R
MR
Brann
D016475:3T3 Cells; D000595:Amino Acid Sequence; D000818:Animals; D001285:Atropine / Q000378:metabolism; D001665:Binding Sites; D002217:Carbachol / Q000378:metabolism; D008024:Ligands; D051379:Mice; D008969:Molecular Sequence Data; D018721:Muscarinic Agonists / Q000378:metabolism; D018727:Muscarinic Antagonists / Q000378:metabolism; D016297:Mutagenesis, Site-Directed; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D011976:Receptors, Muscarinic / Q000235:genetics / Q000378:metabolism*
D008024:Ligands; D018721:Muscarinic Agonists; D018727:Muscarinic Antagonists; D011976:Receptors, Muscarinic; D001285:Atropine; D002217:Carbachol