Following agonist induced endocytosis, G protein-coupled receptors (GPCRs) may either be resensitized and recycled back to the plasma membrane or targeted to the lysosomes for degradation. It has been recently shown that the cellular fate of the mu (MOR) and delta (DOR) opioid receptors and also the D1 and D2 dopamine receptors, which are all members of the GPCR superfamily and implicated in addiction, can be controlled by a specific protein interaction of the C-termini with GASP (GPCR-associated sorting protein) (1,2). In particular, GPCRs that interact with GASP appear to be targeted to the degradative pathway, and those receptors that do not interact with GASP, recycle. The fundamental question is what determines whether a receptor recycles or degrades? In the present study, the postendocytic sorting and potential GASP binding was investigated in Human Embryonic Kidney (HEK) 293 cells stably expressing either the D3, D4 or D5 dopamine receptors. The GST pulldown assays revealed that recombinant GASP-1 produced by in vitro translation bound with much higher affinity to GST fusion proteins containing the C-tails of the D2-like receptors (D2, D3 and D4) than to the D1-like (D1 and D5) or GST alone. A similar profile was seen with GASP-2 binding. Confocal microscopy revealed that D2-like receptors are poorly endocytosed when compared to D1-like receptors, however some internalization does occur in response to agonist. Biotin protection assays revealed that D3 was extensively proteolyzed after prolonged agonist treatment (10µM dopamine, 180mins) as were the D4 isoforms D4.2 and D4.7. In addition, transfection of siRNA oligonucleotides corresponding to regions in either GASP-1 or GASP-2 subsequently down-regulated the expression of either protein in a time dependent manner. Therefore, the affect of down-regulation of GASP on receptor trafficking will also be studied.