3RD ISOFORM OF THE PROSTAGLANDIN-E-RECEPTOR EP3 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 EP3 subtype, derived by alternative RNA splicing, named the EP3gamma receptor, was obtained in addition to those for the two other isoforms, EP3alpha and EP3beta. The three isoforms are only different in the amino acid sequence of the putative cytoplasmic carboxy-terminal tail. When expressed, EP3gamma shows identical ligand-binding properties to these of the other isoforms. The EP3-selective agonist, M&B 28767, increased the basal cAMP level and inhibited the forskolin-induced increase in the cAMP level in EP3gamma, while it decreased both the basal and forskolin-elevated cAMP levels in EP3alpha and EP3beta. The M&B 28767-stimulated GTPase activity consisted of pertussis-toxin-sensitive and cholera-toxin-sensitive portions in the EP3gamma-expressing cell membrane, suggested that EP3gamma is coupled to both guanine nucleotide-binding inhibitory and stimulatory proteins. These results indicate that EP3gamma is coupled to both stimulation and inhibition of adenylate cyclase, but that EP3alpha and EP3beta 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.