Prostaglandin (PG) E2 binds to PGE receptor EP3 subtype and induces Gi activity. To assess the role of the interaction of the carboxylic acid group of agonists and its putative binding site, Arg-309 in the seventh transmembrane domain of EP3alpha receptor, in receptor activation, we have mutated the positively charged Arg-309 to the polar but uncharged Gln (EP3alpha-R309Q) and Asn (EP3alpha-R309N), and to the non-polar Leu (EP3alpha-R309L). Wild-type, EP3alpha-R309Q and EP3alpha-R309N receptors showed high-affinity binding for PGE2, but the EP3alpha-R309L receptor showed very-low-affinity binding. Guanosine 5'-[gamma-thio]triphosphate increased the PGE2 binding to the wild-type receptor, decreased the binding to EP3alpha-R309Q and EP3alpha-R309N receptors, but did not affect that to the EP3alpha-R309L receptor. Furthermore we examined the Gi activities of two types of EP3 agonist, TEI-3356 with a negatively charged carboxylic acid, and TEI-4343, a methyl ester of TEI-3356 with an uncharged but polar group, towards those receptors. Both agonists inhibited the forskolin-stimulated cAMP formation in wild-type, EP3alpha-R309Q and EP3alpha-R309N receptors in the same concentration-dependent manner, but the agonists showed a very low inhibition of EP3alpha-R309L receptor. These findings demonstrate that the hydrogen-bonding interaction of EP3 agonists and residue 309 in the seventh transmembrane domain of the EP3alpha receptor is sufficient for the functional activation of the EP3alpha receptor.