Atypical regulation of G protein-coupled receptor intracellular trafficking by ubiquitination.

G protein-coupled receptor (GPCR) signaling is precisely regulated. After activation, GPCRs are desensitized, internalized and either recycled to the cell surface or sorted to lysosomes for degradation. The main route for GPCR lysosomal sorting requires ubiquitination and the endosomal-sorting complex required for transport (ESCRT). Four distinct ESCRT adaptor protein complexes act sequentially to bind and sort ubiquitinated cargo to lysosomes. Several studies now indicate that alternate pathways exist for GPCR lysosomal sorting that require only some components of the ESCRT and autophagy machinery. While direct GPCR ubiquitination is not required for alternate lysosomal sorting, new evidence suggests that ubiquitin may function indirectly to modulate adaptor protein activity. Here, we discuss the atypical regulation of GPCR lysosomal sorting by ubiquitination.