Characterization of functional roles of DRY motif in the 2nd intracellular loop of dopamine D2 and D3 receptors

Dopamine D2R and D3R (D2R, D3R) show very high sequence homology and employ virtually identical signaling pathways even though D2R is 2 similar to 5 times more active. Among the structural motifs identified, a triplet sequence, Asp-Arg-Tyr (DRY motif), plays critical roles in the determination of receptor conformations for signaling and intracellular trafficking of G protein-coupled receptors by forming intramolecular interactions. Thus, it is possible that different signaling efficiencies of D2R and D3R might be caused by the receptor activation levels stabilized by their own DRY motifs. In this study, the Arg and Asp residues of D2R and D3R were mutated, and resulting changes in their signaling and intracellular trafficking properties were comparatively studied. Mutation of the Arg residues of D2R and D3R abolished their signaling but differently affected their intracellular localizations. The wildtype and R132H-D2R were expressed mainly on the plasma membrane. On the other hand, compared with the wildtype D3R, a substantial amount of R128H-D3R was localized intracellularly. The expression of receptor proteins on the plasma membrane and their signaling efficiencies were more drastically affected by the mutation of the Asp residue of D3R than D2R. Therefore, it was concluded that the different levels of conformational strain exerted by the DRY motif might partly determine the quantitative differences in the signaling efficiencies between D2R and D3R.