Novel insights into the role of arrestin‐2 in endosomal sorting of CXCR4

The chemokine receptor CXCR4, a G protein‐coupled receptor (GPCR), and its cognate ligand stromal cell‐derived factor 1α (SDF‐1α; a.k.a. CXCL12) form an important receptor/chemokine signaling axis in normal mammalian physiology and disease. Activated CXCR4 is targeted for lysosomal degradation via a ubiquitin‐dependent mechanism that involves the endosomal sorting complex required for transport (ESCRT) machinery and arrestin‐2. However, how arrestin‐2 integrates with the ESCRT machinery to target CXCR4 into the degradative pathway remains unknown. Here, we show that arrestin‐2 interacts with ESCRT‐0 to regulate CXCR4 entry into the degradative pathway. ESCRT‐0, a complex consisting of signal‐transducing adaptor molecule (STAM) and hepatocyte growth factor‐regulated tyrosine kinase substrate (HRS), is predicted to initially recognize ubiquitinated CXCR4 on endosomes where sorting into the degradative pathway occurs. Our data reveal that STAM‐1, but not related STAM‐2, interacts directly with arrestin‐2 and together colocalize with CXCR4 on early endosomes. Depletion of STAM‐1 by RNA interference and selective disruption of the arrestin‐2/STAM‐1 interaction accelerates agonist promoted degradation of CXCR4, suggesting that STAM‐1 via its interaction with arrestin‐2 negatively regulates CXCR4 endosomal sorting. Interestingly, disruption of this interaction also blocks agonist promoted ubiquitination of HRS, but not CXCR4 and STAM‐1 ubiquitination, suggesting that arrestin‐2 via its interaction with STAM‐1 mediates ubiquitination of HRS. We propose a model whereby arrestin‐2 initially recruits CXCR4 into the degradative pathway and it subsequently interacts with ESCRT‐0 to regulate its sorting function thereby controlling the amount of CXCR4 that is degraded. Research was supported by NIGMS (AM) and the American Heart Association (RM).