Cloning and characterization of an endothelin-3 specific receptor (ETC receptor) from Xenopus laevis dermal melanophores.

We report here the presence of a receptor specific for endothelin-3 (termed ETc receptor or ETcR) on Xenopus laevis dermal melanophores. Activation of ETcR causes the dispersion of the pigment granules within the melanophores. The EC50 for ET-3 to induce the pigment dispersion is 24 +/- 7 nM, compared to greater than 10 microM for both ET-1 and -2. This effect desensitizes in a manner that is dependent on both time and the concentration of ET-3 used to stimulate the cells. A cDNA encoding for ETcR was isolated by a polymerase chain reaction-mediated DNA amplification strategy using degenerate oligonucleotides prepared based on conserved regions of other known G-protein-coupled receptor sequences and by the subsequent screening of a frog melanophore cDNA library. The cloned cDNA consists of 2,240 nucleotides, with an open reading frame coding for 444 amino acids containing an initial 20-amino acid signal sequence. The predicted mature peptide consists of 424 amino acids with a heptahelical structure common to the G-protein-coupled receptor surperfamily. Its deduced amino acid sequence is 47 and 52% identical to ETA and ETB receptors, respectively, while ETA and ETB are 48% identical to each other. Expression of cDNA in HeLa cells, which do not contain endothelin receptors, enables the cells to specifically bind [125I]ET-3. Competition binding experiments performed on HeLa cells transiently expressing pETc show that the apparent Ki values for ET-3 and ET-1 to displace [125I]ET-3 are 45.5 +/- 16 and 114 +/- 22 nM, respectively.