D ₂-like dopamine and β-adrenergic receptors form a signaling complex that integrates G _s- and G _i-mediated regulation of adenylyl cyclase

β-Adrenergic receptors (βAR) and D ₂-like dopamine receptors (which include D ₂-, D ₃- and D ₄-dopamine receptors) activate G _s and G _i, the stimulatory and inhibitory heterotrimeric G proteins, respectively, which in turn regulate the activity of adenylyl cyclase (AC). β ₂-Adrenergic receptors (β ₂AR) and D ₄-dopamine receptors (D ₄DR) co-immunoprecipitated when co-expressed in HEK 293 cells, suggesting the existence of a signaling complex containing both receptors. In order to determine if these receptors are closely associated with each other, and with other components involved in G protein-mediated signal transduction, β ₂AR, D ₄DR, G protein subunits (Gα _i1 and the Gβ ₁γ ₂ heterodimer) and AC were tagged so that bioluminescence resonance energy transfer (BRET) could be used to monitor their interactions. All of the tagged proteins retained biological function. For the first time, FlAsH-labeled proteins were used in BRET experiments as fluorescent acceptors for the energy transferred from Renilla luciferase-tagged donor proteins. Our experiments revealed that β ₂AR, D ₄DR, G proteins and AC were closely associated in a functional signaling complex in cellulo. Furthermore, BRET experiments indicated that although activation of G _i caused a conformational change within the heterotrimeric protein, it did not cause the Gβγ heterodimer to dissociate from the Gα _i1 subunit. Evidence for the presence of a signaling complex in vivo was obtained by purifying βAR from detergent extracts of mouse brain with alprenolol-Sepharose and showing that the precipitate also contained both D ₂-like dopamine receptors and AC.