Downregulation of a GPCR by β-Arrestin2-Mediated Switch from an Endosomal to a TGN Recycling Pathway

Abstract

Glucose-dependent insulinotropic polypeptide (GIP) is an incretin hormone involved in nutrient homeostasis. GIP receptor (GIPR) is constitutively internalized and returned to the plasma membrane, atypical behavior for a G-protein-coupled receptor (GPCR). GIP promotes GIPR downregulation from the plasmamembrane by inhibiting recycling without affecting internalization. This transient desensitization is achieved by altered intracellular trafficking of activated GIPR. GIP stimulation induces a switch in GIPR recycling from a rapid endosomal to a slow trans-Golgi network (TGN) pathway. GPCR kinases and β-arrestin2 are required for this switch in recycling. A coding sequence variant of GIPR, which hasbeen associated with metabolic alterations, has altered post-activation trafficking characterized byenhanced downregulation and prolonged desensitization. Downregulation of the variant requires β-arrestin2targeting to the TGN but is independent ofGPCR kinases. The single amino acid substitution in the variant biases the receptor to promote GIP-stimulated β-arrestin2 recruitment without receptor phosphorylation, thereby enhancing downregulation.