title: Crystal structure of pea Toc34, a novel GTPase of the chloroplast protein translocon.
abstract: Toc34, a 34-kDa integral membrane protein, is a member of the Toc (translocon at the outer-envelope membrane of chloroplasts) complex, which associates with precursor proteins during protein transport across the chloroplast outer membrane. Here we report the 2.0 A resolution crystal structure of the cytosolic part of pea Toc34 in complex with GDP and Mg2+. In the crystal, Toc34 molecules exist as dimers with features resembling those found in a small GTPase in complex with a GTPase activating protein (GAP). However, gel filtration experiments revealed that dimeric and monomeric forms of Toc34 coexisted in phosphate saline buffer solution at pH 7.2. Mutation of Arg 128, an essential residue for dimerization, to an Ala residue led to the formation of an exclusively monomeric species whose GTPase activity is significantly reduced compared to that of wild type Toc34. These results, together with a number of structural features unique to Toc34, suggest that each monomer acts as a GAP on the other interacting monomer.
authors.affiliation: Institute of Molecular Biology, Academia Sinica, Taipei, Taiwan 115, ROC.
authors.forename: Yuh-Ju
authors.initials: YJ
authors.lastname: Sun
authors.forename: Farhad
authors.initials: F
authors.lastname: Forouhar
authors.forename: Hsou-min
authors.initials: HM
authors.lastname: Li Hm
authors.forename: Shuh-Long
authors.initials: SL
authors.lastname: Tu
authors.forename: Yi-Hong
authors.initials: YH
authors.lastname: Yeh
authors.forename: Sen
authors.initials: S
authors.lastname: Kao
authors.forename: Hui-Lin
authors.initials: HL
authors.lastname: Shr
authors.forename: Chia-Cheng
authors.initials: CC
authors.lastname: Chou
authors.forename: Chinpan
authors.initials: C
authors.lastname: Chen
authors.forename: Chwan-Deng
authors.initials: CD
authors.lastname: Hsiao
mesh_terms: D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D001120:Arginine / Q000235:genetics / Q000378:metabolism; D001665:Binding Sites; D002736:Chloroplasts / Q000737:chemistry / Q000378:metabolism*; D002850:Chromatography, Gel; D018360:Crystallography, X-Ray; D003600:Cytosol / Q000201:enzymology; D019281:Dimerization; D020558:GTP Phosphohydrolases / Q000737:chemistry* / Q000378:metabolism; D020690:GTPase-Activating Proteins / Q000737:chemistry / Q000378:metabolism; D006153:Guanosine Diphosphate / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000378:metabolism; D008274:Magnesium / Q000378:metabolism; D008565:Membrane Proteins / Q000737:chemistry* / Q000378:metabolism; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009154:Mutation / Q000235:genetics; D018532:Pisum sativum / Q000201:enzymology* / Q000378:metabolism; D010940:Plant Proteins / Q000737:chemistry* / Q000378:metabolism; D011485:Protein Binding; D011487:Protein Conformation; D021381:Protein Transport; D016415:Sequence Alignment
chemical_list: D020690:GTPase-Activating Proteins; D008565:Membrane Proteins; D010940:Plant Proteins; C112475:Toc34 protein, plant; D006153:Guanosine Diphosphate; D006160:Guanosine Triphosphate; D001120:Arginine; D020558:GTP Phosphohydrolases; D008274:Magnesium