The long cytoplasmic carboxyl terminus of the prostaglandin E(2) receptor EP(4) subtype is essential for agonist-induced desensitization

The 488-amino acid human prostaglandin E(2) receptor EP(4) subtype, which couples to stimulation of adenylyl cyclase, shares the major structural features of G protein-coupled receptors, having seven putative transmembrane domains, an extracellular amino terminus, and a cytoplasmic carboxyl terminus. The latter is composed of 156 amino acids and contains 38 serine and threonine residues, which are potential phosphorylation sites. The carboxyl terminus may be important in receptor function; in some receptors, truncation of the cytoplasmic tail abolishes desensitization. In others, truncation leads to constitutive activity, and in other receptors, truncation has no effect on receptor function. To investigate the role of the long cytoplasmic tail of the EP(4) receptor, we constructed a mutant EP(4) that lacks the last 138 amino acids at the carboxyl terminus, including 36 serine and threonine residues. The truncated EP(4) receptor was stably expressed in Chinese hamster ovary cells at levels comparable to that of the wild-type receptor and exhibited a K-d value for [H-3]PGE(2) binding similar to that of the wild-type receptor. PGE(2)-mediated adenylyl cyclase activity as a function of PGE(2) concentration was similar in cells expressing the wild-type and truncated EP(4) receptors. Neither the wild-type receptor nor the truncated form showed any constitutive activity. However, the wild-type EP(4) receptor underwent PGE(2)-induced desensitization fully within 15-20 min, whereas the truncated EP(4) receptor, lacking 36 of the 38 carboxyl-terminal serines and threonines, displayed a sustained activation. Despite the continuous presence of PGE(2), the rate of cAMP synthesis via stimulation of the truncated receptor remained constant over greater than or equal to 20 min. These findings suggest that the cytoplasmic tail of EP(4) plays an important role in agonist-induced desensitization.