Heterodimerization with β2-adrenergic receptors promotes surface expression and functional activity of α1D-adrenergic receptors

The alpha(1D)-adrenergic receptor (alpha(1D)-AR) is a G protein-coupled receptor (GPCR) that is poorly trafficked to the cell surface and largely nonfunctional when heterologously expressed by itself in a variety of cell types. We screened a library of approximately 30 other group I GPCRs in a quantitative luminometer assay for the ability to promote alpha(1D)-AR cell surface expression. Strikingly, these screens revealed only two receptors capable of inducing robust increases in the amount of alpha(1D)-AR at the cell surface: alpha(1B)-AR and beta(2)-AR. Confocal imaging confirmed that coexpression with beta(2)-AR resulted in translocation of alpha(1D)-AR from intracellular sites to the plasma membrane. Additionally, coimmunoprecipitation studies demonstrated that alpha(1D)-AR and beta(2)-AR specifically interact to form heterodimers when coexpressed in HEK-293 cells. Ligand binding studies revealed an increase in total alpha(1D)-AR binding sites upon coexpression with beta(2)-AR, but no apparent effect on the pharmacological properties of the receptors. In functional studies, coexpression with beta(2)-AR significantly enhanced the coupling of alpha(1D)-AR to norepinephrine-stimulated Ca2+ mobilization. Heterodimerization of beta(2)-AR with alpha(1D)-AR also conferred the ability of alpha(1D)-AR to cointernalize upon beta(2)-AR agonist stimulation, revealing a novel mechanism by which these different adrenergic receptor subtypes may regulate each other's activity. These findings demonstrate that the selective association of alpha(1D)-AR with other receptors is crucial for receptor surface expression and function and also shed light on a novel mechanism of cross talk between alpha(1)- and beta(2)-ARs that is mediated through heterodimerization and cross-internalization.