Insulin-like growth factor receptor-1 stimulates phosphorylation of the β₂-adrenergic receptor in vivo on sites distinct from those phosphorylated in response to insulin

G-protein-linked receptors have been shown to be substrates for growth factor receptors with intrinsic tyrosine kinase activity typified by the ability of insulin to both phosphorylate tyrosyl residues in the C terminus of and to counter-regulate the action of the β₂-adrenergic receptor (Karoor, V., Baltensperger, K., Paul, H., Czech, M. P., and Malbon, C. C. (1995) J. Biol. Chem. 270, 25305-25308). Insulin-like growth factor-1 (IGF- 1), another member of the growth factor family operating via receptors with intrinsic tyrosine kinase, is shown in the present work to stimulate in vivo the phosphorylation of the β₂-adrenergic receptor. Analysis of tryptic digests prepared from phosphorylated β₂-adrenergic receptors of IGF-1- treated, metabolically labeled smooth muscle cells was performed using reversed-phase high performance liquid chromatography, two-dimensional peptide mapping, and matrix-assisted laser desorption/ionization time-of- flight mass spectrometry. The results of these separate analyses reveal that IGF-1 stimulates phosphorylation predominantly on tyrosyl residues Y132/141 of the second intracellular loop of the β₂-adrenergic receptor rather than the C-terminal region targeted by the activated insulin receptor (Y350/354, Y364), although both growth factors block β-adrenergic agonist action. These data demonstrate selective phosphorylation of a G-protein-linked receptor by receptor tyrosine kinases for insulin and IGF-1 mapping to spatially distinct regions of this heptihelical membrane receptor.