title: Modification of the function of pertussis toxin substrate GTP-binding protein by cholera toxin-catalyzed ADP-ribosylation.
abstract: The alpha-subunit of Gi-2, in addition to that of Gs (GTP-binding proteins involved in adenylate cyclase inhibition and stimulation, respectively) was ADP-ribosylated by cholera toxin in HL-60 cell membranes when a chemotactic receptor was stimulated by formyl-Met-Leu-Phe (fMLP), and the sites modified by cholera and pertussis toxins on the alpha-subunit of Gi-2 were different (Iiri, T., Tohkin, M., Morishima, N., Ohoka, Y., Ui, M., and Katada, T. (1989) J. Biol. Chem. 264, 21394-21400). In order to investigate how the functions of Gi-2 were modified by cholera toxin, the ADP-ribosylated and unmodified proteins were purified from HL-60 cell membranes that had been incubated in the presence and absence of cholera toxin, respectively. The modified Gi-2 displayed unique properties as follows. 1) The ADP-ribosylated alpha-subunit had a more acidic pI than the unmodified one, leading to a partial resolution of the modified Gir2 trimer from the unmodified protein by an anion column chromatography. 2) When the purified proteins were incubated with [gamma-32P]GTP, the radioactivity was more greatly retained in the modified Gi-2 than in the unmodified protein. 3) The actual catalytic rate (kcat) of GTP hydrolysis was, indeed, markedly inhibited by cholera toxin-induced modification. 4) There was an increase in the apparent affinity of Gi-2 for GDP by cholera toxin-induced modification. 5) The modified Gi-2 exhibited a low substrate activity for pertussis toxin-catalyzed ADP-ribosylation. 6) A high-affinity fMLP binding to HL-60 cell membranes was more effectively reconstituted with the ADP-ribosylated Gi-2 than with the unmodified protein. These results suggested that the agonist-fMLP receptor complex was effectively coupled with the ADP-ribosylated Gi-2, resulting in the GTP-bound form, and that the hydrolysis of GTP on the modified alpha-subunit was selectively attenuated. Thus, cholera toxin ADP-ribosylated Gi-2 appeared to be not only a less sensitive pertussis toxin substrate but also an efficient signal transducer between receptors and effectors.
authors.affiliation: Department of Life Science, Tokyo Institute of Technology, Japan.
authors.forename: T
authors.initials: T
authors.lastname: Iiri
authors.forename: Y
authors.initials: Y
authors.lastname: Ohoka
authors.forename: M
authors.initials: M
authors.lastname: Ui
authors.forename: T
authors.initials: T
authors.lastname: Katada
mesh_terms: D000246:Adenosine Diphosphate Ribose / Q000378:metabolism*; D037361:Adenylate Cyclase Toxin; D001667:Binding, Competitive; D015153:Blotting, Western; D002384:Catalysis; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D002772:Cholera Toxin / Q000378:metabolism*; D002853:Chromatography, Liquid; D015180:Electrophoresis, Gel, Two-Dimensional; D004591:Electrophoresis, Polyacrylamide Gel; D019204:GTP-Binding Proteins / Q000378:metabolism*; D006150:Guanine Nucleotides / Q000378:metabolism; D006868:Hydrolysis; D037342:Pertussis Toxin; D010566:Virulence Factors, Bordetella / Q000378:metabolism*
chemical_list: D037361:Adenylate Cyclase Toxin; D006150:Guanine Nucleotides; D010566:Virulence Factors, Bordetella; D000246:Adenosine Diphosphate Ribose; D002772:Cholera Toxin; D037342:Pertussis Toxin; D019204:GTP-Binding Proteins