title: Alanine exchanges of polar amino acids in the transmembrane domains of a platelet-activating factor receptor generate both constitutively active and inactive mutants.
abstract: To determine ligand-binding sites of a platelet-activating factor (PAF) receptor, alanine-scanning mutagenesis was carried out. All 23 polar amino acids in the putative 7-transmembrane (TM) domains of a guinea pig PAF receptor were individually replaced with alanine. The ligand-binding properties of mutant receptors were determined after transient expression in COS-7 cells. Mutants in TM II (N58A, D63A), TM III (N100A, T101A, S104A) and TM VII (D289A) displayed higher PAF-binding affinities than seen with the wild-type receptor. In contrast, mutants in TM V (H188A), TM VI (H248A, H249A, Q252A), and TM VII (Q276A, T278A) showed lower affinities. Representative mutants were then stably expressed in Chinese hamster ovary cells to observe PAF-induced cellular signals (arachidonate release, phosphatidylinositol hydrolysis, adenylyl cyclase inhibition). An N100A mutant with the highest affinity was constitutively active and was responsive to lyso-PAF, an inactive derivative of PAF. One nanomolar PAF induced no signals in low affinity mutants, an EC50 value for the wild-type receptor. Three histidines (His-188, His-248, His-249) might form a binding pocket for the phosphate group of PAF, since zinc effectively inhibited ligand binding. Based on these results, a three-dimensional molecular model of PAF and its receptor was generated using bacteriorhodopsin as a reference protein.
authors.affiliation: Department of Biochemistry, Faculty of Medicine, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113, Japan.
authors.forename: I
authors.initials: I
authors.lastname: Ishii
authors.forename: T
authors.initials: T
authors.lastname: Izumi
authors.forename: H
authors.initials: H
authors.lastname: Tsukamoto
authors.forename: H
authors.initials: H
authors.lastname: Umeyama
authors.forename: M
authors.initials: M
authors.lastname: Ui
authors.forename: T
authors.initials: T
authors.lastname: Shimizu
mesh_terms: D000409:Alanine / Q000737:chemistry*; D000595:Amino Acid Sequence; D000596:Amino Acids / Q000737:chemistry*; D000818:Animals; D016466:CHO Cells; D019556:COS Cells; D006224:Cricetinae; D008565:Membrane Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D010972:Platelet Activating Factor / Q000378:metabolism; D010980:Platelet Membrane Glycoproteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D011485:Protein Binding; D011956:Receptors, Cell Surface; D043562:Receptors, G-Protein-Coupled; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D015398:Signal Transduction; D015032:Zinc / Q000378:metabolism
chemical_list: D000596:Amino Acids; D008565:Membrane Proteins; D010972:Platelet Activating Factor; D010980:Platelet Membrane Glycoproteins; D011956:Receptors, Cell Surface; D043562:Receptors, G-Protein-Coupled; D011994:Recombinant Proteins; C039351:platelet activating factor receptor; D015032:Zinc; D000409:Alanine