Giα and Gβ subunits both define selectivity of G protein activation by α2-adrenergic receptors

Previous studies of the specificity of receptor interactions with G protein subunits in living cells have relied on measurements of second messengers or other downstream responses. We have examined the selectivity of interactions between alpha 2-adrenergic receptors (alpha 2R) and various combinations of Gi alpha and G beta subunit isoforms by measuring changes in FRET between Gi alpha-yellow fluorescent protein and cyan fluorescent protein-G beta chimeras in HeLa cells. All combinations of Gi alpha 1, -2, or -3 with G beta 1, -2, or -4 were activated to some degree by endogenous alpha 2Rs as judged by agonist-dependent decreases in FRET. The degree of G protein activation is determined by the combination of Gia and G beta subunits rather than by the identity of an individual subunit. RT-PCR analysis and small interfering RNA knockdown of alpha 2R subtypes, followed by quantification of radiolabeled antagonist binding, demonstrated that HeLa cells express alpha 2a- and alpha 2b-adrenergic receptor isoforms in a 2:1 ratio. Increasing receptor number by overexpression of the alpha 2aR subtype minimized the differences among coupling preferences for Gia and G beta isoforms. The molecular properties of each Gi alpha, G beta, and alpha 2-adrenergic receptor subtype influence signaling efficiency for the a2-adrenergic receptor-mediated signaling pathway.