title: Gi2-mediated activation of the MAP kinase cascade.
abstract: The Gi class of heterotrimeric G proteins has been implicated in transmitting mitogenic signals from a variety of seven-transmembrane domain receptors. In addition, the alpha subunit of Gi2 (alpha i2) is oncogenic when mutated to a constitutively active form (gip2). The mechanism by which Gi2 stimulates cellular proliferation is unknown, but is believed to involve activation of the mitogen-activated protein kinase (MAPK) signaling cascade. To study Gi2 activation of the cascade, we transiently expressed a mutant, pertussis toxin (PTX)-resistant alpha i2 in Chinese hamster ovary cells. After PTX treatment of these cells, Gi-coupled receptors specifically activated PTX-resistant Gi2 without activating other Gi proteins. Receptor-mediated activation of Gi2 led to activation of MAPK and its upstream activator, MAPK/ERK-activating kinase (MEK). Activation of MAPK and MEK by Gi2 was blocked by expression of a dominant-negative mutant of Ras. Gi2 activation did not, however, detectably increase the proportion of Ras protein in the GTP-bound form. Additional experiments suggest that Gi2 stimulates the MAPK pathway, at least in part, by mechanisms that involve release of its beta gamma subunit, as well as activation of phosphatidylinositol-3 kinase.
authors.affiliation: Department of Cellular and Molecular Pharmacology, University of California, San Francisco 94143, USA.
authors.forename: A M
authors.initials: AM
authors.lastname: Pace
authors.forename: M
authors.initials: M
authors.lastname: Faure
authors.forename: H R
authors.initials: HR
authors.lastname: Bourne
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D016466:CHO Cells; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000378:metabolism*; D006224:Cricetinae; D000242:Cyclic AMP / Q000378:metabolism; D004789:Enzyme Activation; D000939:Epitopes / Q000032:analysis; D019204:GTP-Binding Proteins / Q000378:metabolism*; D015870:Gene Expression; D006153:Guanosine Diphosphate / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000378:metabolism; D007700:Kinetics; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism*; D008969:Molecular Sequence Data; D016296:Mutagenesis; D009419:Nerve Tissue Proteins / Q000378:metabolism*; D037342:Pertussis Toxin; D051381:Rats; D011994:Recombinant Proteins / Q000378:metabolism; D015398:Signal Transduction; D014162:Transfection; D010566:Virulence Factors, Bordetella / Q000494:pharmacology; D018631:ras Proteins / Q000096:biosynthesis / Q000378:metabolism
chemical_list: D000939:Epitopes; D009419:Nerve Tissue Proteins; D011994:Recombinant Proteins; D010566:Virulence Factors, Bordetella; D006153:Guanosine Diphosphate; D006160:Guanosine Triphosphate; D000242:Cyclic AMP; D037342:Pertussis Toxin; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D020928:Mitogen-Activated Protein Kinases; D019204:GTP-Binding Proteins; D018631:ras Proteins