G protein coupling and ligand selectivity of the D2L and D3 dopamine receptors.

The human dopamine D(2L) receptor couples promiscuously to multiple members of the Galpha(i/o) subfamily. Despite the high homology of the D(2L) and D(3) receptors, the G protein coupling specificity of the human D(3) receptor is less clearly characterized. The primary aim of this study, then, was the parallel characterization of the G protein coupling specificity of the D(2L) and D(3) receptors. By using both receptor-G protein fusion proteins and stable cell lines in which pertussis toxin-resistant mutants of individual Galpha(i)-family G proteins were expressed in an inducible fashion, we demonstrated highly selective coupling of the D(3) receptor to Galpha(o1). Furthermore, by using the fusion proteins to ensure identical stoichiometry of receptor to G protein for each pairing, a range of ligands displayed higher potency and, for partial agonists, higher efficacy at the D(3) receptor when coupled to Galpha(o1) compared with the D(2L) receptor. The second aim of this study was to investigate the molecular basis of the above differential G protein coupling specificity. The importance of a 12-amino acid sequence from the C-terminal end of the third intracellular loop of the D(2L) receptor in providing promiscuity in G protein coupling was demonstrated using a chimeric D(3)/D(2) receptor in which the equivalent region of the D(3) receptor was exchanged for this sequence. This chimera displayed D(3)-like affinity for [(3)H]spiperone and potency for agonists but gained D(2)-like ability to couple to each of Galpha(i1-3) as well as Galpha(o1).