Multi-faceted roles of β-arrestins in G protein-coupled receptor endocytosis.

The internalization of G protein-coupled receptors (GPCRs) is a key process limiting cell surface receptor activity, and possibly promoting their intracellular signaling. Dysregulation of this process has been implicated in various diseases. This process is known to result from the recruitment of β-arrestins (βarrs) to the active receptors, although some receptors do not require this for unclear reasons. Here, we clarify the importance of βarrs in the internalization of 60 different GPCRs. We show that βarrs are essential for agonist-induced internalization for only a third of the receptors, half of the GPCRs being partially dependent on βarrs or even independent for endocytosis. Furthermore, we develop several Förster resonance energy transfer (FRET)-based molecular interaction assays to elucidate the molecular mechanism of internalization of the glucagon-like peptide-1 receptor (GLP-1R). We show that GLP-1R internalizes mainly by a βarr-independent mechanism involving the direct binding of the clathrin adaptor protein complex-2 to the receptor. However, when this adaptor protein complex-2-dependent mechanism is affected, the GLP-1R internalization becomes βarr-dependent. It shows that multiple mechanisms regulate the internalization of a specific receptor. Our data highlight the various processes used to control cell surface GPCRs, and open novel possibilities to specifically control their signaling processes.