CHARACTERIZATION OF ENDOTHELIN RECEPTORS ETA AND ETB EXPRESSED IN COS CELLS

The cloned cDNA genes for endothelin receptors ET(A) and ET(B) were expressed in COS cells, and the binding characteristics of the two receptors with three isopeptide ligands (ET-1, ET-2, and ET-3) were examined in detail. The results indicated that the stability of receptor-ET-1 complexes formed with ET(A) and ET(B) were significantly different from each other, while their affinities to ET-1 were similar. The preformed ET(A)-ET-1 complex readily dissociated upon SDS-PAGE, as did many of the other receptors so far studied, while the ET(B)-ET-1 complex survived SDS-PAGE when it was run at low temperature (approximately 4-degrees-C). Clear differences in stability were also shown in comparative studies of acid treatment of the two types of complexes. Only the ET(B)-ET-1 complex was resistant to acid treatment (0.2 M acetic acid, 0.5 M NaCl), and its 50 kDa monomeric receptor-ligand complex remained intact. The ET(B)-ET-1 complex (50 kDa) formed at 4-degrees-C on the surface of COS cells, however, was susceptible to limited proteolysis at 37-degrees-C that reduced the molecular size of the complex to a distinct 35 kDa. No such size reduction was observed with the preformed ET(A)-ET-1 complex. The overall structure of two endothelin receptors, as deduced from the sequence of cloned cDNAs, is similar in many respects. However, the present findings demonstrate distinct differences in the biochemical nature of the two receptors, which suggest their distinct biological functions.