Subtype-specific sorting of the ETA endothelin receptor by a novel endocytic recycling signal for G protein-coupled receptors

We have previously reported that endocytic sorting of ETA endothelin receptors to the recycling pathway is dependent on a signal residing in the cytoplasmic carboxyl- terminal region. The aim of the present work was to characterize the carboxyl-terminal recycling motif of the ETA receptor. Assay of truncation mutants of the ETA receptor with increasing deletions of the carboxyl- terminal tail revealed that amino acids 390 to 406 contained information critical for the ability of the receptor to recycle. This peptide sequence displayed significant sequence similarity to several protein segments confirmed by X-ray crystallography to adopt antiparallel beta-strand structures (beta-finger). One of these segments was the beta-finger motif of neuronal nitric-oxide synthase reported to function as an internal PDZ (postsynaptic density-95/disc-large/zona occludens) domain-binding ligand. Based on these findings, the three-dimensional structure of the recycling motif of ETA receptor was predicted to attain a beta-finger conformation acting as an internal PDZ ligand. Site-directed mutagenesis at residues that would be crucial to the structural integrity of the putative beta-finger conformation or PDZ ligand function prevented recycling of the ETA receptor. Analysis of more than 300 G protein-coupled receptors (GPCRs) identified 35 different human GPCRs with carboxyl-terminal sequence patterns that fulfilled the structural criteria of an internal PDZ ligand. Among these are several receptors reported to follow a recycling pathway. In conclusion, recycling of ETA receptor is mediated by a motif with the structural characteristics of an internal PDZ ligand. This structural motif may represent a more general principle of endocytic sorting of GPCRs.