The Long Cytoplasmic Carboxyl Terminus of the Prostaglandin E2 Receptor EP4 Subtype Is Essential for Agonist-Induced Desensitization

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Abstract
The 488-amino acid human prostaglandin E2 receptor EP4 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 EP4 receptor, we constructed a mutant EP4 that lacks the last 138 amino acids at the carboxyl terminus, including 36 serine and threonine residues. The truncated EP4 receptor was stably expressed in Chinese hamster ovary cells at levels comparable to that of the wild-type receptor and exhibited aKd value for [3H]PGE2 binding similar to that of the wild-type receptor. PGE2-mediated adenylyl cyclase activity as a function of PGE2 concentration was similar in cells expressing the wild-type and truncated EP4 receptors. Neither the wild-type receptor nor the truncated form showed any constitutive activity. However, the wild-type EP4 receptor underwent PGE2-induced desensitization fully within 15–20 min, whereas the truncated EP4 receptor, lacking 36 of the 38 carboxyl-terminal serines and threonines, displayed a sustained activation. Despite the continuous presence of PGE2, the rate of cAMP synthesis via stimulation of the truncated receptor remained constant over ≥20 min. These findings suggest that the cytoplasmic tail of EP4 plays an important role in agonist-induced desensitization.