Cell Surface Expression of α1D-Adrenergic Receptors Is Controlled by Heterodimerization with α1B-Adrenergic Receptors*

α1-Adrenergic receptors (ARs) belong to the large Class I G protein-coupled receptor superfamily and comprise three subtypes (α1A, α1B, and α1D). Previous work with heterologously expressed C-terminal green fluorescent protein (GFP)-tagged α1-ARs showed that α1A- and α1B-ARs localize to the plasma membrane, whereas α1D-ARs accumulate intracellularly. We recently showed that α1D- and α1B-ARs form heterodimers, whereas α1D- and α1A-ARs do not. Here, we examined the role of heterodimerization in regulating α1D-AR localization using both confocal imaging of GFP- or CFP-tagged α1-ARs and a luminometer-based surface expression assay in HEK293 cells. Co-expression with α1B-ARs caused α1D-ARs to quantitatively translocate to the cell surface, but co-expression with α1A-ARs did not. Truncation of the α1B-AR extracellular N terminus or intracellular C terminus had no effect on surface expression of α1D-ARs, suggesting primary involvement of the hydrophobic core. Co-transfection with an uncoupled mutant α1B-AR (Δ12α1B) increased both α1D-AR surface expression and coupling to norepinephrine-stimulated Ca2+ mobilization. Finally, GFP-tagged α1D-ARs were not detected on the cell surface when expressed in rat aortic smooth muscle cells that express no endogenous ARs, but were almost exclusively localized on the surface when expressed in DDT1MF-2 cells, which express endogenous α1B-ARs. These studies demonstrate that α1B/α1D-AR heterodimerization controls surface expression and functional coupling of α1D-ARs, the N- and C-terminal domains are not involved in this interaction, and that α1B-AR G protein coupling is not required. These observations may be relevant to many other Class I G protein-coupled receptors, where the functional consequences of heterodimerization are still poorly understood.