Mechanisms of endothelin receptor subtype-specific targeting to distinct intracellular trafficking pathways

We recently reported that the endothelin (ET) receptor subtypes ET(A) and ET(B) are targeted to distinct intracellular destinations upon agonist stimulation (Bremnes, T., Paasche, J. D., Mehlum, A., Sandberg, C., Bremnes, B., and Attramadal, H. (2000) J. Biol. Chem. 275, 17596-17604). The ET(A) receptor was shown to follow the recycling route of transferrin, whereas ET(B) is targeted to lysosomes for degradation. In the present study we have investigated the mechanisms of ET receptor subtype-specific targeting to distinct intracellular trafficking pathways. Truncation mutants of the ET(A) and ET(B) receptors with deletions of the cytoplasmic carboxyl-terminal tail distal to the palmitoylation site were found to mediate inositol phosphate accumulation and to internalize upon agonist stimulation, although internalization occurred at a slower rate as compared with the wild-type receptors. However, the truncated ET(A) receptor was no longer able to undergo recycling. Rather, both truncation mutants were recognized by beta-arrestin for recruitment to endocytosis and were sorted to lysosomes by a dynamin-dependent internalization pathway. Furthermore, studies of chimeric ET(A) and ET(B) receptors where the cytoplasmic tail of ET(A) was swapped with the corresponding domain of ET(B), and vice versa, revealed that the cytoplasmic tail of ET(B) is required for efficient lysosomal sorting and that signals for targeting to recycling reside in the cytoplasmic tail of the ET(A) receptor.