Lysosomal down-regulation of the mu opioid receptor is opposed by the Retromer complex

A critical homeostatic mechanism for regulating G protein-coupled receptor (GPCR) activity is agonist-induced GPCR endocytosis and trafficking to the lysosome for proteolytic down-regulation. The mu opioid receptor (MOR) is a notable example of this type of cellular regulation, where prolonged exposure to high-efficacy opioid drugs causes MOR to traffic to the lysosome. Here, we used functional genomics to identify cellular proteins that control MOR lysosomal down-regulation. We found that the central regulator of MOR postendocytic trafficking is the Retromer complex, which rescues MOR from opioid-induced down-regulation by promoting MOR recycling from endosomes to the plasma membrane. Critically, MOR accesses the Retromer recycling pathway through its noncanonical bileucine recycling motif, and this mechanism controls how MOR is regulated following chronic exposure to opioid drugs. Additionally, we show that this bileucine pathway for Retromer-based recycling is present in other classes of membrane proteins including the glucose transporter GLUT4.