title: Toc receptor dimerization participates in the initiation of membrane translocation during protein import into chloroplasts.
abstract: The post-translational import of nucleus-encoded preproteins into chloroplasts occurs through multimeric translocons in the outer (Toc) and inner (Tic) membranes. The high fidelity of the protein import process is maintained by specific recognition of the transit peptide of preproteins by the coordinate activities of two homologous GTPase Toc receptors, Toc34 and Toc159. Structural and biochemical studies suggest that dimerization of the Toc receptors functions as a component of the mechanism to control access of preproteins to the membrane translocation channel of the translocon. We show that specific mutations that disrupted receptor dimerization in vitro reduced the rate of protein import in transgenic Arabidopsis compared with the wild type receptor. The mutations did not affect the GTPase activities of the receptors. Interestingly, these mutations did not decrease the initial preprotein binding at the receptors, but they reduced the efficiency of the transition from preprotein binding to membrane translocation. These data indicate that dimerization of receptors has a direct role in protein import and support a hypothesis in which receptor-receptor interactions participate in the initiation of membrane translocation of chloroplast preproteins as part of the molecular mechanism of GTP-regulated protein import.
authors.affiliation: Department of Biochemistry and Molecular Biology and Program in Plant Biology, University of Massachusetts, Amherst, Massachusetts 01003, USA.
authors.forename: Jeonghwa
authors.initials: J
authors.lastname: Lee
authors.forename: Fei
authors.initials: F
authors.lastname: Wang
authors.forename: Danny J
authors.initials: DJ
authors.lastname: Schnell
mesh_terms: D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D002736:Chloroplasts / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D019281:Dimerization; D020558:GTP Phosphohydrolases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D008565:Membrane Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D008969:Molecular Sequence Data; D009154:Mutation; D011485:Protein Binding; D021381:Protein Transport; D016415:Sequence Alignment
chemical_list: D029681:Arabidopsis Proteins; D008565:Membrane Proteins; C403444:TOC159 protein, Arabidopsis; C491798:TOC34 protein, Arabidopsis; C114786:Toc33 protein, Arabidopsis; D020558:GTP Phosphohydrolases