title: Membrane-targeting potentiates guanine nucleotide exchange factor CDC25 and SOS1 activation of Ras transforming activity.
abstract: Growth factor-triggered activation of Ras proteins is believed to be mediated by guanine nucleotide exchange factors (CDC25/GRF and SOS1/2) that promote formation of the active Ras GTP-bound state. Although the mechanism(s) of guanine nucleotide exchange factor regulation is unclear, recent studies suggest that translocation of SOS1 to the plasma membrane, where Ras is located, might be responsible for Ras activation. To evaluate this model, we generated constructs that encode the catalytic domains of human CDC25 or mouse SOS1, either alone (designated cCDC25 and cSOS1, respectively) or terminating in the carboxyl-terminal CAAX membrane-targeting sequence from K-Ras4B (designated cCDC25-CAAX and cSOS1-CAAX, respectively; in CAAX, C is Cys, A is an aliphatic amino acid, and X is Ser or Met). We then compared the transforming potential of cCDC25 and cSOS1 with their membrane-targeted counterparts. We observed that addition of the Ras plasma membrane-targeting sequence to the catalytic domains of CDC25 and SOS1 greatly enhanced their focus-forming activity (10- to 50-fold) in NIH 3T3 transfection assays. Similarly, we observed that the membrane-targeted versions showed a 5- to 10-fold enhanced ability to induce transcriptional activation from the Ets/AP-1 Ras-responsive element. Furthermore, whereas cells that stably expressed cCDC25 or cSOS1 exhibited the same morphologies as untransformed NIH 3T3 cells, cells expressing cCDC25-CAAX or cSOS1-CAAX displayed transformed morphologies that were indistinguishable from the elongated and refractile morphology of oncogenic Ras-transformed cells. Thus, these results suggest that membrane translocation alone is sufficient to potentiate guanine nucleotide exchange factor activation of Ras.
authors.affiliation: Department of Pharmacology, School of Medicine, University of North Carolina at Chapel Hill.
authors.forename: L A
authors.initials: LA
authors.lastname: Quilliam
authors.forename: S Y
authors.initials: SY
authors.lastname: Huff
authors.forename: K M
authors.initials: KM
authors.lastname: Rabun
authors.forename: W
authors.initials: W
authors.lastname: Wei
authors.forename: W
authors.initials: W
authors.lastname: Park
authors.forename: D
authors.initials: D
authors.lastname: Broek
authors.forename: C J
authors.initials: CJ
authors.lastname: Der
mesh_terms: D016475:3T3 Cells; D000595:Amino Acid Sequence; D000818:Animals; D002451:Cell Compartmentation; D002462:Cell Membrane / Q000378:metabolism; D002471:Cell Transformation, Neoplastic; D005656:Fungal Proteins / Q000378:metabolism*; D019204:GTP-Binding Proteins / Q000378:metabolism*; D020662:Guanine Nucleotide Exchange Factors; D051379:Mice; D008969:Molecular Sequence Data; D010749:Phosphoprotein Phosphatases / Q000378:metabolism*; D011506:Proteins / Q000378:metabolism; D016283:Proto-Oncogene Proteins p21(ras) / Q000378:metabolism*; D012097:Repressor Proteins / Q000378:metabolism*; D020837:SOS1 Protein; D013329:Structure-Activity Relationship; D020663:ras Guanine Nucleotide Exchange Factors; D020666:ras-GRF1
chemical_list: D005656:Fungal Proteins; D020662:Guanine Nucleotide Exchange Factors; D011506:Proteins; D012097:Repressor Proteins; D020837:SOS1 Protein; D020663:ras Guanine Nucleotide Exchange Factors; D020666:ras-GRF1; D010749:Phosphoprotein Phosphatases; D019204:GTP-Binding Proteins; D016283:Proto-Oncogene Proteins p21(ras)