Interactions of GIPC with Dopamine D 2 , D 3 but not D 4 Receptors Define a Novel Mode of Regulation of G Protein-coupled Receptors

The C-terminus domain of G protein-coupled receptors confers a functional cytoplasmic interface involved in protein association. By screening a rat brain cDNA library using the yeast two-hybrid system with the C-terminus domain of the dopamine D 3 receptor (D 3 R) as bait, we characterized a new interaction with the PDZ domain-containing protein, GIPC (GAIP interacting protein, C terminus). This interaction was specific for the dopamine D 2 receptor (D 2 R) and D 3 R, but not for the dopamine D 4 receptor (D 4 R) subtype. Pull-down and affinity chromatography assays confirmed this interaction with recombinant and endogenous proteins. Both GIPC mRNA and protein are widely expressed in rat brain and together with the D 3 R in neurons of the islands of Calleja at plasma membranes and in vesicles. GIPC reduced D 3 R signaling, cointernalized with D 2 R and D 3 R, and sequestered receptors in sorting vesicles to prevent their lysosomal degradation. Through its dimerization, GIPC acts as a selective scaffold protein to assist receptor functions. Our results suggest a novel function for GIPC in the maintenance, trafficking, and signaling of GPCRs.