IDENTIFICATION OF A LIGAND-DEPENDENT SWITCH WITHIN A MUSCARINIC RECEPTOR

G-protein-coupled receptors spontaneously switch between active and in active conformations. Agonists stabilize the active conformation, wher eas antagonists stabilize the inactive conformation. In a systematic s earch for residues that participate in receptor function, several regi ons of the m5 muscarinic receptor were randomly mutated and tested for their functional properties. Mutations spanning one face of transmemb rane 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 an tagonists. In addition, one mutation induced agonist-like responses fr om the receptor when exposed to classical antagonists. These results s uggest that TMB is a switch that defines the activation state of the r eceptor, and that ligand interactions with TM6 stabilize the receptor in either an active or an inactive conformation.