Third isoform of the prostaglandin‐E‐receptor EP 3 subtype with different C‐terminal tail coupling to both stimulation and inhibition of adenylate cyclase

A functional cDNA clone for a third isoform of the mouse prostaglandin‐E‐receptor EP 3 subtype, derived by alternative RNA splicing, named the EP 3γ receptor, was obtained in addition to those for the two other isoforms, EP 3α and EP 3β . The three isoforms are only different in the amino acid sequence of the putative cytoplasmic carboxy‐terminal tail. When expressed, EP 3γ shows identical ligand‐binding properties to these of the other isoforms. The EP 3 ‐selective agonist, M&B 28767, increased the basal cAMP level and inhibited the forskolin‐induced increase in the cAMP level in EP 3γ , while it decreased both the basal and forskolin‐elevated cAMP levels in EP 3α and EP 3β . The M&B 28767‐stimulated GTPase activity consisted of pertussis‐toxin‐sensitive and cholera‐toxin‐sensitive portions in the EP 3γ ‐expressing cell membrane, suggested that EP 3γ is coupled to both guanine nucleotide‐binding inhibitory and stimulatory proteins. These results indicate that EP 3γ is coupled to both stimulation and inhibition of adenylate cyclase, but that EP 3α and EP 3β are exclusively coupled to inhibition of adenylate cyclase. Thus, alternative splicing produces a third isoform with a different carboxy‐terminal tail, which differs from the other two isoforms in the specificity of coupling to a signal‐transduction pathway.