title: The roles of toc34 and toc75 in targeting the toc159 preprotein receptor to chloroplasts.
abstract: The Toc complex at the outer envelope of chloroplasts initiates the import of nuclear-encoded preproteins from the cytosol into the organelle. The core of the Toc complex is composed of two receptor GTPases, Toc159 and Toc34, as well as Toc75, a beta-barrel membrane channel. Toc159 is equally distributed between a soluble cytoplasmic form and a membrane-inserted form, suggesting that assembly of the Toc complex is dynamic. In the present study, we used the Arabidopsis thaliana orthologs of Toc159 and Toc34, atToc159 and atToc33, respectively, to investigate the requirements for assembly of the trimeric Toc complex. In addition to its intrinsic GTPase activity, we demonstrate that integration of atToc159 into the Toc complex requires atToc33 GTPase activity. Additionally, we show that the interaction of the two GTPase domains stimulates association of the membrane anchor of atToc159 with the translocon. Finally, we employ reconstituted proteoliposomes to demonstrate that proper insertion of the receptor requires both Toc75 and Toc34. Collectively these data suggest that Toc34 and Toc75 act sequentially to mediate docking and insertion of Toc159 resulting in assembly of the functional translocon.
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Massachusetts, Amherst, Massachusetts 01003, USA.
authors.forename: Tanya R
authors.initials: TR
authors.lastname: Wallas
authors.forename: Matthew D
authors.initials: MD
authors.lastname: Smith
authors.forename: Sobeida
authors.initials: S
authors.lastname: Sanchez-Nieto
authors.forename: Danny J
authors.initials: DJ
authors.lastname: Schnell
mesh_terms: D017360:Arabidopsis / Q000737:chemistry; D029681:Arabidopsis Proteins; D001665:Binding Sites; D002462:Cell Membrane / Q000737:chemistry / Q000378:metabolism; D002736:Chloroplasts / Q000378:metabolism*; D003600:Cytosol / Q000378:metabolism; D020558:GTP Phosphohydrolases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D006160:Guanosine Triphosphate / Q000378:metabolism; D008081:Liposomes / Q000737:chemistry; D046911:Macromolecular Substances; D008565:Membrane Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism* / Q000502:physiology*; D016296:Mutagenesis; D010940:Plant Proteins / Q000378:metabolism*; D011498:Protein Precursors / Q000737:chemistry / Q000235:genetics / Q000378:metabolism* / Q000502:physiology*; D011994:Recombinant Proteins
chemical_list: D029681:Arabidopsis Proteins; D008081:Liposomes; D046911:Macromolecular Substances; D008565:Membrane Proteins; C103933:OEP75 protein precursor, plant; D010940:Plant Proteins; D011498:Protein Precursors; D011994:Recombinant Proteins; C403444:TOC159 protein, Arabidopsis; C491798:TOC34 protein, Arabidopsis; C491244:TOC75 protein, Arabidopsis; C112475:Toc34 protein, plant; D006160:Guanosine Triphosphate; D020558:GTP Phosphohydrolases