title: Identification of a GDI displacement factor that releases endosomal Rab GTPases from Rab-GDI.
abstract: Prenylated Rab GTPases occur in the cytosol in their GDP-bound conformations bound to a cytosolic protein termed GDP-dissociation inhibitor (GDI). Rab-GDI complexes represent a pool of active, recycling Rab proteins that can deliver Rabs to specific and distinct membrane-bound compartments. Rab delivery to cellular membranes involves release of GDI, and the membrane-associated Rab protein then exchanges its bound GDP for GTP. We report here the identification of a novel, membrane-associated protein factor that can release prenylated Rab proteins from GDI. This GDI-displacement factor (GDF) is not a guanine nucleotide exchange factor because it did not influence the intrinsic rates of nucleotide exchange by Rabs 5, 7 or 9. Rather, GDF caused the release of each of these endosomal Rabs from GDI, permitting them to exchange nucleotide at their intrinsic rates. GDF displayed the greatest catalytic rate enhancement on Rab9-GDI complexes. However, catalytic rate enhancement paralleled the potency of GDI in blocking nucleotide exchange: GDI was shown to be most potent in blocking nucleotide exchange by Rab9. The failure of GDF to act on Rab1-GDI complexes suggests that it may be specific for endosomal Rab proteins. This novel, membrane-associated activity may be part of the machinery used to localize Rabs to their correct intracellular compartments.
authors.affiliation: Department of Biochemistry, Stanford University School of Medicine, CA 94305-5307, USA.
authors.forename: A B
authors.initials: AB
authors.lastname: Dirac-Svejstrup
authors.forename: T
authors.initials: T
authors.lastname: Sumizawa
authors.forename: S R
authors.initials: SR
authors.lastname: Pfeffer
mesh_terms: D000818:Animals; D001921:Brain / Q000378:metabolism; D002417:Cattle; D004591:Electrophoresis, Polyacrylamide Gel; D011992:Endosomes / Q000737:chemistry / Q000201:enzymology; D020558:GTP Phosphohydrolases / Q000378:metabolism; D019204:GTP-Binding Proteins / Q000378:metabolism*; D020730:Guanine Nucleotide Dissociation Inhibitors; D006153:Guanosine Diphosphate / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000378:metabolism; D006801:Humans; D008565:Membrane Proteins / Q000302:isolation & purification / Q000378:metabolism / Q000494:pharmacology; D017368:Protein Prenylation / Q000502:physiology; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D020691:rab GTP-Binding Proteins; D020696:rab5 GTP-Binding Proteins; D000090802:rab7 GTP-Binding Proteins
chemical_list: C086796:GDP dissociation inhibitor 1; D020730:Guanine Nucleotide Dissociation Inhibitors; D008565:Membrane Proteins; D011994:Recombinant Proteins; D000090802:rab7 GTP-Binding Proteins; D006153:Guanosine Diphosphate; D006160:Guanosine Triphosphate; D020558:GTP Phosphohydrolases; D019204:GTP-Binding Proteins; C079856:RAB9A protein, human; D020691:rab GTP-Binding Proteins; D020696:rab5 GTP-Binding Proteins