Agonist-promoted ubiquitination of the G protein-coupled receptor CXCR4 mediates lysosomal sorting

Ligand-induced trafficking plays an important role in the physiologic regul ation of many G protein-coupled receptors (GPCRs). Although numerous GPCRs are sorted to a degradative pathway upon prolonged stimulation, the molecul ar events leading to degradation are poorly understood. Here we report that the human immunodeficiency virus co-receptor CXCR4 undergoes rapid agonist -promoted degradation by a process involving endocytosis via clathrin-coate d pits and subsequent sorting to lysosomes. Studies analyzing the sorting o f various CXCR4 mutants revealed the presence of a degradation motif (SSLKI LSKGK) in the carboxyl terminus of CXCR4. The first two serines as well as the dileucine motif were critical for agonist-induced endocytosis, whereas all three serines but not the dileucine were important in mediating degrada tion. Mutation of the three lysine residues had no effect on CXCR4 endocyto sis yet completely inhibited receptor degradation. Because lysine residues represent potential sites of ubiquitination, we also examined the ubiquitin ation of CXCR4. Interestingly, CXCR4 was shown to undergo rapid agonist-pro moted ubiquitination that was attenuated by mutation of the lysine residues within the degradation motif. These studies implicate a specific role for ubiquitination in sorting endocytosed GPCRs to lysosomes.