Fluorescence resonance energy transfer analysis reveals the existence of endothelin-A and endothelin-B receptor homodimers

Endothelin-1 is a potent vasoactive peptide that exerts its action via endothelia-A (ETA) and endothelin-B (ETB) receptors. Both types of receptor belong to the rhodopsin-like (class A) G protein-coupled receptors. Since there is an increasing body of evidence that G protein-coupled receptors function as homo- and heterodimers, we sought to prove whether endothelin receptor subtypes also exist as homodimers. To this end we generated plasmids encoding fusion proteins of the endothelin-A and endothelin-B receptor with cyan (ETA.CFP, ETB.CFP) or yellow fluorescent protein (ETA.YFP, ETB.YFP), suitable for fluorescence resonance energy transfer experiments. HEK293 cells transiently co-expressing either ETA.CFP and ETA.YFP or ETB.CFP and ETB.YFP showed significant fluorescence resonance energy transfer. The calculated efficiencies of fluorescence resonance energy transfer were between 12 and 18% for both endothelin-A and endothelin-B receptor homodimers. When cells co-expressing ETA.CFP and ETA.YFP or ETB.CFP and ETB.YFP were treated with endothelin-1 for 30 minutes at 37degreesC, no change in fluorescence resonance energy transfer efficiencies was found. Similarly, the endothelin-A receptor-selective antagonist, BQ123, and the endothelin-B receptor-selective antagonist, BQ788, did not change the fluorescence resonance energy transfer efficiencies of ETA.CFP/ ETA.YFP and ETB.CFP/ETB.YFP homodimers, respectively. The data demonstrate that endothelin-A and endothelin-B receptors exist as constitutive homodimers, which are not altered in the presence of agonists or antagonists.