Endothelin receptor dimers evaluated by FRET, ligand binding, and calcium mobilization

Endothelin-1 (ET-1) mediates physiological responses via endothelin A (ET(A)) and B (ET(B)) receptors, which may form homo- and heterodimers with unknown function. Here, we investigated ET-receptor dimerization using fluorescence resonance energy transfer (FRET) between receptors tagged with CFP (donor) and receptors tagged with tetracysteine-FlAsH (fluorescein arsenical hairpin) (acceptor) expressed in HEK293 cells. FRET efficiencies were 15%, 22%, and 27% for ET(A)/ET(A), ET(B)/ET(B), and ET(A)/ET(B), respectively, and dimerization was further supported by coimmunoprecipitation. For all dimer pairs, the natural but nonselective ligand ET-1 rapidly (50%, but did not detectably reduce coimmunoprecipitation. ET-1 stimulated a transient increase in intracellular Ca(2+) ([Ca(2+)](i)) lasting 1-2 min for both homodimer pairs, and these ET-1 actions on FRET and [Ca(2+)](i) elevation were blocked by the appropriate subtype-selective antagonist. In contrast, ET(A)/ET(B) heterodimers mediated a sustained [Ca(2+)](i) increase lasting >10 min, and required a combination of ET(A) and ET(B) antagonists to block the observed FRET and [Ca(2+)](i) responses. The sensitive CFP/FlAsH FRET assay used here provides new insights into endothelin-receptor dimer function, and represents a unique approach to characterize G-protein-coupled receptor oligomers, including their pharmacology.