1. Howard Hughes Medical Institute, Departments of Medicine and Biochemistry, Box 3821, Duke University Medical Center, Durham, North Carolina 27710, USA

Correspondence to: Robert J. Lefkowitz¹ Correspondence and requests for materials should be addressed to R.J.L. (e-mail: Email: lefko001@mc.duke.edu).

Abstract

Many of the G-protein-coupled receptors for hormones that bind to the cell surface can signal to the interior of the cell through several different classes of G protein^{1, 2, 3, 4}. For example, although most of the actions of the prototype ₂-adrenergic receptor are mediated through G_s proteins and the cyclic-AMP-dependent protein kinase (PKA) system^{5, 6}, -adrenergic receptors can also couple to G_i proteins^{1, 2}. Here we investigate the mechanism that controls the specificity of this coupling. We show that in HEK293 cells, stimulation of mitogen-activated protein (MAP) kinase by the ₂-adrenergic receptor is mediated by the subunits of pertussis-toxin-sensitive G proteins through a pathway involving the non-receptor tyrosine kinase c-Src and the G protein Ras. Activation of this pathway by the ₂-adrenergic receptor requires that the receptor be phosphorylated by PKA because it is blocked by H-89, an inhibitor of PKA. Additionally, a mutant of the receptor, which lacks the sites normally phosphorylated by PKA, can activate adenylyl cyclase⁵, the enzyme that generates cAMP, but not MAP kinase. Our results demonstrate that a mechanism previously shown to mediate uncoupling of the ₂-adrenergic receptor from G_s and thus heterologous desensitization⁷ (PKA-mediated receptor phosphorylation), also serves to 'switch' coupling of this receptor from G_s to G_i and initiate a new set of signalling events.