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

Dopamine D 2 R and D 3 R (D 2 R, D 3 R) show very high sequence homology and employ virtually identical signaling pathways even though D 2 R is 2 ∼ 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 D 2 R and D 3 R might be caused by the receptor activation levels stabilized by their own DRY motifs. In this study, the Arg and Asp residues of D 2 R and D 3 R were mutated, and resulting changes in their signaling and intracellular trafficking properties were comparatively studied. Mutation of the Arg residues of D 2 R and D 3 R abolished their signaling but differently affected their intracellular localizations. The wildtype and R132H-D 2 R were expressed mainly on the plasma membrane. On the other hand, compared with the wildtype D 3 R, a substantial amount of R128H-D 3 R 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 D 3 R than D 2 R. 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 D 2 R and D 3 R.