title: Dimerization of Toc-GTPases at the chloroplast protein import machinery.
abstract: Import of chloroplast precursor proteins is controlled by the coordinate action of two homologous GTPases, Toc159 and Toc33, located at the cytosol-outer membrane interface. Recent studies in Arabidopsis showed that the cytosolic form of the precursor binding protein Toc159 is targeted to its receptor at the import machinery, Toc33, via heterodimerization of their GTP-binding domains. Toc33 may also form GDP-bound homodimers, as suggested by the crystal structure of its pea ortholog. Moreover, the structural data suggested that arginine 130 (Arg130) of Arabidopsis Toc33 may function as a GTPase-activating "arginine-finger" at the other monomer in the Toc33 dimer. Here, we demonstrate that Arg130 of Toc33 does not function as an Arginine-finger. A mutant, Toc33-R130A, binds and hydrolyzes GTP like the wild type. However, we demonstrate that Arg130 is involved in both homodimerization of Toc33 and in heterodimerization with the GTP-binding domain of Toc159. The dependence of Toc33 homodimerization on Arg130 is mutual, requiring the presence of Arg130 at both monomers. As the GTPase is not activated by dimerization, it may be activated independently at either monomer, possibly even before dimerization. Independent regulation of GTPase activity may serve to coordinate the interactions of the GTPases during the import of proteins into the chloroplast.
authors.affiliation: Laboratoire de Physiologie Végétale, Institut de Botanique, Université de Neuchâtel, Rue Emile-Argand 11, 2007 Neuchâtel, Switzerland.
authors.forename: Petra
authors.initials: P
authors.lastname: Weibel
authors.forename: Andreas
authors.initials: A
authors.lastname: Hiltbrunner
authors.forename: Lukas
authors.initials: L
authors.lastname: Brand
authors.forename: Felix
authors.initials: F
authors.lastname: Kessler
mesh_terms: D000595:Amino Acid Sequence; D029681:Arabidopsis Proteins; D002384:Catalysis; D002736:Chloroplasts / Q000378:metabolism*; D019281:Dimerization; D020558:GTP Phosphohydrolases / Q000737:chemistry* / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000378:metabolism; D006868:Hydrolysis; D008565:Membrane Proteins / Q000737:chemistry* / Q000378:metabolism; D008969:Molecular Sequence Data; D010940:Plant Proteins / Q000737:chemistry* / Q000378:metabolism; D021381:Protein Transport
chemical_list: D029681:Arabidopsis Proteins; D008565:Membrane Proteins; D010940:Plant Proteins; C403444:TOC159 protein, Arabidopsis; C114786:Toc33 protein, Arabidopsis; D006160:Guanosine Triphosphate; D020558:GTP Phosphohydrolases