Ubiquitination-independent trafficking of G protein-coupled receptors to lysosomes

Ubiquitination of cytoplasmic lysine residues can target G protein-coupled receptors (GPCRs) to proteasomes and has recently been shown to also be required for sorting of certain GPCRs to lysosomes following ligand-induced endocytosis. We addressed the generality of this mechanism by examining regulated proteolysis of the murine delta opioid receptor (DOR) expressed in human embryonic kidney 293 cells, a well characterized model system in which receptors are sorted to lysosomes. Incubation of cells in the presence of the highly specific proteasome inhibitor lactacystin did not detectably affect ligand-induced proteolysis of DOR but significantly delayed ligand-induced proteolysis of epidermal growth factor receptors. Mutation of all cytoplasmic lysine residues in DOR, creating a mutant opioid receptor that is unable to be ubiquitinated, did not detectably inhibit either ligand-induced endocytosis or proteolytic degradation of endocytosed receptors. Furthermore, the lysine-mutated DOR, like its wild type counterpart, colocalized extensively with lysosomes after ligand-induced endocytosis. These results demonstrate that ubiquitination of DOR is not required either for its ligand-induced endocytosis or for postendocytic trafficking to lysosomes.