AGONISTS INDUCE CONFORMATIONAL-CHANGES IN TRANSMEMBRANE DOMAIN-III AND DOMAIN-VI OF THE BETA(2) ADRENOCEPTOR

Agonist binding to G protein-coupled receptors is believed to promote a conformational change that leads to the formation of the active rece ptor state, However, the character of this conformational change which provides the important link between agonist binding and G protein cou pling is not known, Here we report evidence that agonist binding to th e beta(2) adrenoceptor induces a conformational change around (125)Cys in transmembrane domain (TM) III and around (285)Cys in TM VI, A seri es of mutant beta(2) adrenoceptors with a limited number of cysteines available for chemical derivatization were purified, site-selectively labeled with the conformationally sensitive, cysteine-reactive fluorop hore IANBD and analyzed by fluorescence spectroscopy, Like the wild-ty pe receptor, mutant receptors containing (125)Cys and/or (285)Cys show ed an agonist-induced decrease in fluorescence, while no agonist-induc ed response was observed in a receptor where these two cysteines were mutated, These data suggest that IANBD bound to (125)Cys and (285)Cys are exposed to a more polar environment upon agonist binding, and indi cate that movements of transmembrane segments III and VI are involved in activation of G protein-coupled receptors.