Mass spectrometric analysis of agonist effects on posttranslational modifications of the β-2 adrenoceptor in mammalian cells

Posttranslational modifications (PTMs) of the beta-2 adrenoceptor (B2AR) play a fundamental role in receptor regulation by agonists. We have examined the effects of several agonists on net levels of B2AR palmitoylation and phosphorylation using epitope tagging in stably transfected human embryonal kidney (HEK) 293 cells, immunoaffinity purification, and mass spectrometry combined with the method of stable isotope labeling by amino acids in cell culture (SILAC). Palmitoylation of Cys341 was confirmed and did not change delectably after 30 min exposure of cells to saturating concentrations of dopamine, epinephrine, or isoproterenol. However, all of these agonists produced a marked increase in net phosphorylation. Phosphorylation of the third cytoplasmic loop was increased to a similar degree by all three agonists, whereas differences between agonists were observed in net phosphorylation of the carboxyl-terminal cytoplasmic domain (isoproterenol similar to epinephrine >> dopamine). Interestingly, agonist-induced phosphorylation of the carboxyl-terminal cytoplasmic domain was observed exclusively in a proximal portion (between residues 339-369). None of the agonists produced detectable phosphorylation in a distal portion of the cytoplasmic tail, which contains all sites of agonist-induced phosphorylation identified previously by in vitro reconstitution. These results provide insight to agonist-dependent regulation of the B2AR in intact cells, suggest the existence of significant differences in regulatory phospborylation events occurring between in vitro and in vivo conditions, and outline a general analytical approach to investigate regulated PTM of receptors in mammalian cells.