We recently demonstrated that two exclusively Gi-coupled isoforms of the mouse EP3 receptor, EP3α and β, with different carboxyl-terminal tails, differed in agonist-independent constitutive Gi activity, and the carboxyl-terminal tail-truncated receptor showed full constitutive activity (Hasegawa, H., Negishi, M., and Ichikawa, A. (1996) J. Biol. Chem. 271, 1857–1860). Here we further examined Gi and Gs activities of the third isoform, EP3γ, coupled to both Gi and Gs. The EP3γ receptor showed mostly full constitutive Gi activity and agonist-dependent Gs activity. The truncated receptor also showed agonist-dependent Gs activity, but the level was lower than that of the EP3γ receptor. Thus, the carboxyl-terminal tail would differentially regulate Gi and Gs activities of the EP3 receptor.
