Agents that elevate cyclic AMP induce receptor phosphorylation primarily at serine 331 in HEK 293 cells overexpressing human thromboxane receptor α

Abstract

Human embryonic kidney (HEK) 293 cells stably transfected with the His-tagged thromboxane receptor α (TPα) were used to study the phosphorylation and desensitization of the receptor induced by prostaglandin E 1 (PGE 1) or forskolin. These agents are known to increase the intracellular level of cyclic AMP (cAMP) and activate cAMP-dependent protein kinase (PKA). Pretreatment of cells with either agent significantly attenuated Ca 2+ release induced by the agonist [1 S-[1α,2α( Z),3β(1 E,3 S),4α]]-7-[3-[3-hydroxy-4-(4-indophenoxy)-1-butenyl]-7-oxabicyclo[2,2,1]hept-2-yl]-5-heptenoic acid (I-BOP). These agents also induced concentration-dependent phosphorylation of TPα as demonstrated by increased 32 P -labeling of the receptor from cells prelabeled with 32 P i . To facilitate the identification of the intracellular domains involved in phosphorylation, glutathione S-transferase (GST)-intracellular domain fusion proteins were used as substrates for purified PKA. It was found that only the C-terminal tail fusion protein could serve as a substrate for PKA. To identify the specific serine/threonine residues in the C-terminal tail that are involved in phosphorylation, various alanine mutants of these residues were checked for their ability to serve as substrates. Ser-331 was found to be involved in PKA-mediated phosphorylation. The S331A mutant receptor overexpressed in HEK 293 cells was not phosphorylated significantly following stimulation by PGE 1 or forskolin, indicating that Ser-331 was the major site of phosphorylation. Furthermore, cells overexpressing the mutant receptor became responsive to I-BOP-induced Ca 2+ mobilization even after pretreatment with PGE 1 or forskolin. These results indicate that Ser-331 is the primary site responsible for the phosphorylation and desensitization of the human TPα induced by agents that activate PKA.