Involvement of a cytoplasmic-tail serine cluster in urotensin II receptor internalization

Most G-protein-coupled receptors that undergo agonist-dependent internalization require the presence of specific cytoplasmic-tail residues to initiate interactions with proteins of the endocytic machinery. Here we show that the UT receptor (urotensin II receptor) undergoes internalization, and that specific serine residues of the receptor's cytoplasmic tail participate in this process. We first observed a time-dependent increase in internalization of the UT receptor expressed in COS-7 cells following binding of the agonist urotensin II. This sequestration was significantly reduced in the presence of sucrose, demonstrating that the agonist-activated UT receptor is internalized in part by clathrin-coated pits. Moreover, the sequestered receptor was co-localized in endocytic vesicles with beta-arrestin1 and beta-arrestin2. To assess whether specific regions of the receptor's cytoplasmic tail were involved in internalization, five UT receptor mutants were constructed. In four constructs the receptor's cytoplasmic tail was truncated at various positions (UTDelta367, UTDelta363, UTDelta350 and UTDelta336), and in the other four adjacent serine residues at positions 364-367 were replaced by Ala (Mut4S). Each mutant, except UTDelta367, demonstrated a significantly reduced internalization rate, thereby revealing the importance of specific serine residues within the cytoplasmic tail of the UT receptor for its ability to be internalized efficiently.