Chimeric dopamine D2/angiotensin AT1 receptors: role of the length of third intracellular loop of D2 receptors in conferring specificity of receptor binding and G-protein coupling.

AIM: To define roles of the third intracellular loop (IL3) length of G-protein
coupled receptors in conferring the specificity for receptor binding and
G-protein coupling.
METHODS: By polymerase chain reaction (PCR), the IL3 of D2 receptor was replaced
with the counter part of AT1 receptor which has the shortest loop among all
G-protein coupled receptors. D2/AT1 receptor cDNA was then stably transfected
into Chinese hamster ovary cells and a clone with high level expression was
obtained for receptor binding and agonist-induced phosphatidylinositols (PI)
turnover experiments.
RESULTS: Comparing to the D2 receptor, D2/AT1 chimeric receptor had lower
affinities for all D2 receptor antagonists tested (spiperone, haloperidol,
(+)-butaclamol, chlopromazine, clozapine, trifluoperdazine) and different
affinity profiles to agonists (apomorphine, dopamine, quinpirole, bromocriptine).
But the chimeric receptor failed to couple to G-protein and subsequent
stimulation of PI turnover.
CONCLUSION: The length of IL3 of D2 receptor participates defining recpetor
binding sites conformation, and structure beyond IL3 may affect receptor
G-protein coupling.