title: In vivo analysis of the role of atTic20 in protein import into chloroplasts.
abstract: The import of nucleus-encoded preproteins into plastids requires the coordinated activities of membrane protein complexes that facilitate the translocation of polypeptides across the envelope double membrane. Tic20 was identified previously as a component of the import machinery of the inner envelope membrane by covalent cross-linking studies with trapped preprotein import intermediates. To investigate the role of Tic20 in preprotein import, we altered the expression of the Arabidopsis Tic20 ortholog (atTic20) by antisense expression. Several antisense lines exhibited pronounced chloroplast defects exemplified by pale leaves, reduced accumulation of plastid proteins, and significant growth defects. The severity of the phenotypes correlated directly with the reduction in levels of atTic20 expression. In vitro import studies with plastids isolated from control and antisense plants indicated that the antisense plastids are defective specifically in protein translocation across the inner envelope membrane. These data suggest that Tic20 functions as a component of the protein-conducting channel at the inner envelope membrane.
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Massachusetts, Amherst, Massachusetts 01003, USA.
authors.forename: Xuejun
authors.initials: X
authors.lastname: Chen
authors.forename: Matthew D
authors.initials: MD
authors.lastname: Smith
authors.forename: Lynda
authors.initials: L
authors.lastname: Fitzpatrick
authors.forename: Danny J
authors.initials: DJ
authors.lastname: Schnell
mesh_terms: D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000235:genetics* / Q000378:metabolism; D029681:Arabidopsis Proteins / Q000235:genetics* / Q000276:immunology / Q000378:metabolism; D001692:Biological Transport; D002734:Chlorophyll / Q000378:metabolism; D002736:Chloroplasts / Q000378:metabolism* / Q000648:ultrastructure; D016373:DNA, Antisense / Q000235:genetics; D020869:Gene Expression Profiling; D015151:Immunoblotting; D026901:Membrane Transport Proteins / Q000235:genetics* / Q000276:immunology / Q000378:metabolism; D008854:Microscopy, Electron; D008969:Molecular Sequence Data; D010641:Phenotype; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism; D030821:Plants, Genetically Modified; D021381:Protein Transport; D017386:Sequence Homology, Amino Acid
chemical_list: D029681:Arabidopsis Proteins; D016373:DNA, Antisense; D026901:Membrane Transport Proteins; D010940:Plant Proteins; C457764:Tic20 protein, Arabidopsis; D002734:Chlorophyll