A Ser/Thr cluster within the C-terminal domain of the rat prostaglandin receptor EP3alpha is essential for agonist-induced phosphorylation, desensitization and internalization

Two isoforms of the rat prostaglandin E(2) receptor, rEP3alpha-R and rEP3beta-R, differ only in their C-terminal domain. To analyze the function of the rEP3-R C-terminal domain in agonist induced desensitization, a cluster of Ser/Thr residues in the C-terminal domain of the rEP3alpha-R was mutated to Ala and both isoforms and the receptor mutant (rEP3alpha-ST341-349A-R) were stably expressed in HEK293 cells. All rEP3-R receptors showed a similar ligand-binding profile. They were functionally coupled to Gi and reduced forskolin-induced cAMP-formation. Repeated exposure of cells expressing the rEP3alpha-R isoform to PGE(2) reduced the agonist induced inhibition of forskolin-stimulated cAMP-formation by 50% and led to internalization of the receptor to intracellular endocytotic vesicles. By contrast, Gi-response as well as plasma membrane localization of the rEP3beta-R and the rEP3alpha-ST341-349A-R were not affected by prior agonist-stimulation. Agonist-stimulation of HEK293-rEP3alpha-R cells induced a time- and dose-dependent phosphorylation of the receptor most likely by G protein-coupled receptor kinases and not by protein kinase A or protein kinase C. By contrast, upon agonist-stimulation the rEP3beta-R was not phosphorylated and the rEP3alpha-ST341-349A-R was phosphorylated only weakly. These results led to the hypothesis that agonist-induced desensitization of the rEP3alpha-R isoform is mediated most likely by a GRK-dependent phosphorylation of Ser/Thr residues 341-349. Phosphorylation then initiates uncoupling of the receptor from Gi protein and receptor internalization.