title: Inositol phospholipids regulate the guanine-nucleotide-exchange factor Tiam1 by facilitating its binding to the plasma membrane and regulating GDP/GTP exchange on Rac1.
abstract: Binding of the Rac1-specific guanine-nucleotide-exchange factor, Tiam1, to the plasma membrane requires the N-terminal pleckstrin homology domain. In the present study, we show that membrane-association is mediated by binding of PtdIns(4,5)P(2) to the pleckstrin homology domain. Moreover, in 1321N1 astrocytoma cells, translocation of Tiam1 to the cytosol, following receptor-mediated stimulation of PtdIns(4,5)P(2) breakdown, correlates with decreased Rac1-GTP levels, indicating that membrane-association is required for GDP/GTP exchange on Rac1. In addition, we show that platelet-derived growth factor activates Rac1 in vivo by increasing PtdIns(3,4,5)P(3) concentrations, rather than the closely related lipid, PtdIns(3,4)P(2). Finally, the data demonstrate that PtdIns(4,5)P(2) and PtdIns(3,4,5)P(3) bind to the same pleckstrin homology domain in Tiam1 and that soluble inositol phosphates appear to compete with lipids for this binding. Together, these novel observations provide strong evidence that distinct phosphoinositides regulate different functions of this enzyme, indicating that local concentrations of signalling lipids and the levels of cytosolic inositol phosphates will play crucial roles in determining its activity in vivo.
authors.affiliation: Division of Signal Transduction, School of Life Sciences, University of Dundee, Dundee, Scotland DD1 5EH, UK.
authors.forename: Ian N
authors.initials: IN
authors.lastname: Fleming
authors.forename: Ian H
authors.initials: IH
authors.lastname: Batty
authors.forename: Alan R
authors.initials: AR
authors.lastname: Prescott
authors.forename: Alex
authors.initials: A
authors.lastname: Gray
authors.forename: Gursant S
authors.initials: GS
authors.lastname: Kular
authors.forename: Hazel
authors.initials: H
authors.lastname: Stewart
authors.forename: C Peter
authors.initials: CP
authors.lastname: Downes
mesh_terms: D000730:Androstadienes / Q000494:pharmacology; D045744:Cell Line, Tumor; D002462:Cell Membrane / Q000378:metabolism; D003600:Cytosol / Q000378:metabolism; D020662:Guanine Nucleotide Exchange Factors / Q000378:metabolism*; D006153:Guanosine Diphosphate / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000378:metabolism; D006801:Humans; D010446:Peptide Fragments / Q000378:metabolism; D019269:Phosphatidylinositol 4,5-Diphosphate / Q000378:metabolism; D018129:Phosphatidylinositol Phosphates / Q000378:metabolism* / Q000502:physiology; D000081082:Phosphoinositide-3 Kinase Inhibitors; D010982:Platelet-Derived Growth Factor / Q000494:pharmacology; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D021381:Protein Transport; D011506:Proteins / Q000378:metabolism*; D018160:Receptors, Cytoplasmic and Nuclear / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D000075684:T-Lymphoma Invasion and Metastasis-inducing Protein 1; D013917:Thrombin / Q000378:metabolism; D000077191:Wortmannin; D020830:rac1 GTP-Binding Protein / Q000378:metabolism*
chemical_list: D000730:Androstadienes; D020662:Guanine Nucleotide Exchange Factors; D010446:Peptide Fragments; D019269:Phosphatidylinositol 4,5-Diphosphate; D018129:Phosphatidylinositol Phosphates; D000081082:Phosphoinositide-3 Kinase Inhibitors; D010982:Platelet-Derived Growth Factor; D011506:Proteins; D018160:Receptors, Cytoplasmic and Nuclear; D011993:Recombinant Fusion Proteins; D000075684:T-Lymphoma Invasion and Metastasis-inducing Protein 1; C087334:TIAM1 protein, human; C060974:phosphatidylinositol 3,4,5-triphosphate; C040985:phosphatidylinositol receptors; C082835:thrombin receptor peptide (42-47); D006153:Guanosine Diphosphate; D006160:Guanosine Triphosphate; D013917:Thrombin; D020830:rac1 GTP-Binding Protein; D000077191:Wortmannin