title: Role of the guanine nucleotide exchange factor Ost in negative regulation of receptor endocytosis by the small GTPase Rac1.
abstract: The Rho family of GTPases has been implicated in the regulation of intracellular vesicle trafficking. Here, we investigated the mechanism underlying the negative regulation of clathrin-mediated endocytosis of cell surface receptors mediated by the Rho family protein Rac1. Contrary to previous reports, only the activated mutant of Rac1, but not other Rho family members including RhoA and Cdc42, suppressed internalization of the transferrin receptor. On the other hand, down-regulation of Rac1 expression by RNA interference resulted in enhanced receptor internalization, suggesting that endogenous Rac1 in fact functions as a negative regulator. We identified a guanine nucleotide exchange factor splice variant designated Ost-III, which contains a unique C-terminal region including an Src homology 3 domain, as a regulator of Rac1 involved in the inhibition of receptor endocytosis. In contrast, other splice variants Ost-I and Ost-II exerted virtually no effect on receptor endocytosis. We also examined subcellular localization of synaptojanin 2, a putative Rac1 effector implicated in negative regulation of receptor endocytosis. Each Ost splice variant induced distinct subcellular localization of synaptojanin 2, depending on Rac1 activation. Furthermore, we isolated gamma-aminobutyric acid type A receptor-associated protein (GABARAP) as a protein that binds to the C-terminal region of Ost-III. When ectopically expressed, GABARAP was co-localized with Ost-III and potently suppressed the Ost-III-dependent Rac1 activation and the inhibition of receptor endocytosis. Lipid modification of GABARAP was necessary for the suppression of Ost-III. These results are discussed in terms of subcellular region-specific regulation of the Rac1-dependent signaling pathway that negatively regulates clathrin-mediated endocytosis.
authors.affiliation: Division of Molecular Biology, Department of Biochemistry and Molecular Biology, Kobe University Graduate School of Medicine, Kobe 650-0017, Japan.
authors.forename: Katsuaki
authors.initials: K
authors.lastname: Ieguchi
authors.forename: Shuji
authors.initials: S
authors.lastname: Ueda
authors.forename: Tohru
authors.initials: T
authors.lastname: Kataoka
authors.forename: Takaya
authors.initials: T
authors.lastname: Satoh
mesh_terms: D048868:Adaptor Proteins, Signal Transducing / Q000378:metabolism; D051017:Apoptosis Regulatory Proteins; D002966:Clathrin / Q000378:metabolism; D004705:Endocytosis; D015971:Gene Expression Regulation, Enzymologic; D020662:Guanine Nucleotide Exchange Factors / Q000737:chemistry* / Q000502:physiology*; D006367:HeLa Cells; D006801:Humans; D008055:Lipids / Q000737:chemistry; D008869:Microtubule-Associated Proteins / Q000378:metabolism; D008954:Models, Biological; D017434:Protein Structure, Tertiary; D011990:Receptors, Transferrin / Q000378:metabolism; D064067:Rho Guanine Nucleotide Exchange Factors; D015398:Signal Transduction; D020764:cdc42 GTP-Binding Protein / Q000378:metabolism; D020830:rac1 GTP-Binding Protein / Q000737:chemistry / Q000502:physiology*; D020742:rhoA GTP-Binding Protein / Q000378:metabolism
chemical_list: D048868:Adaptor Proteins, Signal Transducing; D051017:Apoptosis Regulatory Proteins; D002966:Clathrin; C116741:GABARAP protein, human; D020662:Guanine Nucleotide Exchange Factors; D008055:Lipids; C577400:MCF2L protein, human; D008869:Microtubule-Associated Proteins; C107953:RAC1 protein, human; D011990:Receptors, Transferrin; D064067:Rho Guanine Nucleotide Exchange Factors; D020764:cdc42 GTP-Binding Protein; D020830:rac1 GTP-Binding Protein; D020742:rhoA GTP-Binding Protein