A kinase-regulated PDZ-domain interaction controls endocytic sorting of the β2-adrenergic receptor

A fundamental question in cell biology is how membrane proteins are sorted in the endocytic pathway. The sorting of internalized β2-adrenergic receptors between recycling endosomes and lysosomes is responsible for opposite effects on signal transduction and is regulated by physiological stimuli1,2. Here we describe a mechanism that controls this sorting operation, which is mediated by a family of conserved protein-interaction modules called PDZ domains3. The phosphoprotein EBP50 (for ezrin–radixin–moesin(ERM)-binding phosphoprotein-50)4 binds to the cytoplasmic tail of the β2-adrenergic receptor through a PDZ domain and to the cortical actin cytoskeleton through an ERM-binding domain. Disrupting the interaction of EBP50 with either domain or depolymerization of the actin cytoskeleton itself causes missorting of endocytosed β2-adrenergic receptors but does not affect the recycling of transferrin receptors. A serine residue at position 411 in the tail of the β2-adrenergic receptor is a substrate for phosphorylation by GRK-5 (for G-protein-coupled-receptor kinase-5) (ref. 5) and is required for interaction with EBP50 and for proper recycling of the receptor. Our results identify a new role for PDZ-domain-mediated protein interactions and for the actin cytoskeleton in endocytic sorting, and suggest a mechanism by which GRK-mediated phosphorylation could regulate membrane trafficking of G-protein-coupled receptors after endocytosis.