title: Agonist recognition by proteinase-activated receptor 2 and thrombin receptor. Importance of extracellular loop interactions for receptor function.
abstract: Thrombin receptor and proteinase-activated receptor 2 (PAR2) define a family of G protein-coupled receptors that are activated by a novel proteolytic mechanism. Specific cleavage of their amino-terminal exodomains unmasks a new amino terminus which then serves as a tethered ligand, docking intramolecularly to the body of the receptor to effect signaling. Identification of the docking interactions between tethered ligand domain and receptor is critical for understanding transmembrane signaling by these receptors. Synthetic "agonist peptides" that mimic the tethered ligand domains of thrombin receptor and PAR2 act as agonists at their respective receptors. Toward defining the docking interactions which mediate receptor activation, we determined the specificity of the thrombin receptor and PAR2 for their respective agonist peptides and used receptor chimeras to identify the receptor domains responsible for such specificity. PAR2 responded to both thrombin receptor and PAR2 agonist peptides. In contrast, thrombin receptor was selective for its own agonist peptide. Substitution of the extracellular face of PAR2, its amino-terminal exodomain and three extracellular loops, for the cognate thrombin receptor structures yielded a chimeric receptor with PAR2-like agonist specificity. Substitution of individual extracellular domains revealed that the primary determinant of agonist specificity was extracellular loop 2. Strikingly, substitution of either the amino-terminal exodomain or third extracellular loop alone caused marked loss of receptor function, but the double substitution yielded a functional receptor. Thus, the extracellular domains of these G protein-coupled receptors are more than simply passive links between transmembrane domains. They participate in agonist recognition and must interact, directly or indirectly, for proper receptor function.
authors.affiliation: Cardiovascular Research Institute, Department of Medicine, and Daiichi Research Center, University of California, San Francisco, California 94143, USA.
authors.forename: D J
authors.initials: DJ
authors.lastname: Lerner
authors.forename: M
authors.initials: M
authors.lastname: Chen
authors.forename: T
authors.initials: T
authors.lastname: Tram
authors.forename: S R
authors.initials: SR
authors.lastname: Coughlin
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D001665:Binding Sites; D017931:DNA Primers; D007668:Kidney / Q000378:metabolism*; D007700:Kinetics; D051379:Mice; D008961:Models, Structural; D008969:Molecular Sequence Data; D009842:Oligopeptides / Q000494:pharmacology*; D010446:Peptide Fragments / Q000494:pharmacology*; D016133:Polymerase Chain Reaction; D017433:Protein Structure, Secondary; D044464:Receptor, PAR-2; D011956:Receptors, Cell Surface / Q000819:agonists* / Q000737:chemistry*; D018179:Receptors, Thrombin / Q000819:agonists* / Q000737:chemistry*; D011993:Recombinant Fusion Proteins / Q000819:agonists / Q000737:chemistry
chemical_list: D017931:DNA Primers; D009842:Oligopeptides; D010446:Peptide Fragments; D044464:Receptor, PAR-2; D011956:Receptors, Cell Surface; D018179:Receptors, Thrombin; D011993:Recombinant Fusion Proteins; C099570:seryl-leucyl-isoleucyl-glycyl-arginyl-leucine; C082835:thrombin receptor peptide (42-47)