ALIX Regulates the Ubiquitin-Independent Lysosomal Sorting of the P2Y(1) Purinergic Receptor via a YPX(3)L Motif

Endocytic sorting and lysosomal degradation are integral to the regulation of G protein-coupled receptor (GPCR) function. Upon ligand binding, classical GPCRs are activated, internalized and recycled or sorted to lysosomes for degradation, a process that requires receptor ubiquitination. However, recent studies have demonstrated that numerous GPCRs are sorted to lysosomes independent of receptor ubiquitination. Here, we describe an ubiquitin-independent lysosomal sorting pathway for the purinergic GPCR P2Y(1). After activation, P2Y(1) sorts to lysosomes for degradation independent of direct ubiquitination that is mediated by a YPX(3)L motif within the second intracellular loop that serves as a binding site for the adaptor protein ALIX. Depletion of ALIX or site-directed mutation of the YPX(3)L motif inhibits P2Y(1) sorting into the lumen of multivesicular endosomes/lysosomes and degradation. These findings confirm the function of YPX(3)L motifs as lysosomal targeting sequences for GPCRs and demonstrate that ALIX mediates the ubiquitin-independent degradation of certain GPCRs.