FRET-based monitoring of conformational change of the β2 adrenergic receptor in living cells

The β2 adrenergic receptor (β2AR) is a G protein-coupled receptor that is selective to epinephrine. We demonstrate herein monitoring of an agonist-induced conformational change of β2AR in living cells. The monitoring method is based on fluorescence resonance energy transfer from a cyan fluorescent protein (CFP) to a biarsenical fluorophore, FlAsH, attached to the C-terminus, and the third intracellular loop (ICL3), respectively. Recombinant β2ARs exhibited agonist-induced increases in the FlAsH/CFP emission ratio, indicating that the ICL3 approached the C-terminus upon activation. Since the emission ratio changes were on a time scale of seconds, the conformational change of β2AR in living cells was more rapid than that of purified β2AR measured in vitro. Interestingly, the direction of the emission ratio change of β2AR was opposite to that of the norepinephrine-responsive α2A adrenergic receptor reported recently. It was suggested that this discrepancy corresponds directly to the diametric biological functions, i.e., the activation or inactivation of adenylyl cyclase.