Identification by site-directed mutagenesis of amino acids contributing to ligand-binding specificity or signal transduction properties of the human FP prostanoid receptor

Prostanoid receptors belong to the class of heptahelical plasma membrane receptors. For the five prostanoids, eight receptor subtypes have been identified. They display an overall sequence similarity of roughly 30%. Based on sequence comparison, single amino acids in different subtypes of different species have previously been identified by site-directed mutagenesis or in hybrid receptors that appear to be essential for ligand binding or G-protein coupling. Based on this information, a series of mutants of the human FP receptor was generated and characterized in ligand-binding and second-messenger-formation studies. It was found that mutation of His-81 to Ala in transmembrane domain 2 and of Arg-291 to Leu in transmembrane domain 7, which are putative interaction partners for the prostanoid9s carboxyl group, abolished ligand binding. Mutants in which Ser-263 in transmembrane domain 6 or Asp-300 in transmembrane domain 7 had been replaced by Ala or Gln, respectively, no longer discriminated between prostaglandins PGF 2α and PGD 2 . Thus distortion of the topology of transmembrane domains 6 and 7 appears to interfere with the cyclopentane ring selectivity of the receptor. PGF 2α -induced inositol formation was strongly reduced in the mutant Asp-300Gln, inferring a role for this residue in agonist-induced G-protein activation.