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, wh ich couples to stimulation of adenylyl cyclase, shares the major struc tural features of G protein-coupled receptors, having seven putative t ransmembrane domains, an extracellular amino terminus, and a cytoplasm ic carboxyl terminus. The latter is composed of 156 amino acids and co ntains 38 serine and threonine residues, which are potential phosphory lation sites. The carboxyl terminus may be important in receptor funct ion; in some receptors, truncation of the cytoplasmic tail abolishes d esensitization. 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) rec eptor, we constructed a mutant EP(4) that lacks the last 138 amino aci ds at the carboxyl terminus, including 36 serine and threonine residue s. The truncated EP(4) receptor was stably expressed in Chinese hamste r ovary cells at levels comparable to that of the wild-type receptor a nd exhibited a K-d value for [H-3]PGE(2) binding similar to that of th e wild-type receptor. PGE(2)-mediated adenylyl cyclase activity as a f unction of PGE(2) concentration was similar in cells expressing the wi ld-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 full y within 15-20 min, whereas the truncated EP(4) receptor, lacking 36 o f the 38 carboxyl-terminal serines and threonines, displayed a sustain ed activation. Despite the continuous presence of PGE(2), the rate of cAMP synthesis via stimulation of the truncated receptor remained cons tant over greater than or equal to 20 min. These findings suggest that the cytoplasmic tail of EP(4) plays an important role in agonist-indu ced desensitization.