title: The localization of Tic20 proteins in Arabidopsis thaliana is not restricted to the inner envelope membrane of chloroplasts.
abstract: Tic20 is a central, membrane-embedded component of the precursor protein translocon of the inner envelope of chloroplasts (TIC). In Arabidopsis thaliana, four different isoforms of Tic20 exist. They are annotated as atTic20-I, -II, -IV and -V and form two distinct phylogenetic subfamilies in embryophyta. Consistent with atTic20-I being the only essential isoform for chloroplast development, we show that the protein is exclusively targeted to the chloroplasts inner envelope. The same result is observed for atTic20-II. In contrast, atTic20-V is localized in thylakoids and atTic20-IV dually localizes to chloroplasts and mitochondria. These results together with the previously established expression profiles explain the recently described phenotypes of Tic20 knockout plants and point towards a functional diversification of these proteins within the family. For all Tic20 proteins a 4-helix topology is proposed irrespective of the targeted membrane, which in part could be confirmed in vivo by application of a self-assembling GFP-based topology approach. By the same approach we show that the inner envelope localized Tic20 proteins expose their C-termini to the chloroplast stroma. This localization would be consistent with the positive inside rule considering a stromal translocation intermediate as discussed.
authors.affiliation: Department of Biosciences, Molecular Cell Biology of Plants, Johann-Wolfgang-Goethe University Frankfurt, Max-von-Laue Strasse 9, Frankfurt am Main, Germany.
authors.forename: Anu B
authors.initials: AB
authors.lastname: Machettira
authors.forename: Lucia E
authors.initials: LE
authors.lastname: Gross
authors.forename: Maik S
authors.initials: MS
authors.lastname: Sommer
authors.forename: Benjamin L
authors.initials: BL
authors.lastname: Weis
authors.forename: Gisela
authors.initials: G
authors.lastname: Englich
authors.forename: Joanna
authors.initials: J
authors.lastname: Tripp
authors.forename: Enrico
authors.initials: E
authors.lastname: Schleiff
mesh_terms: D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000378:metabolism* / Q000648:ultrastructure; D029681:Arabidopsis Proteins / Q000032:analysis / Q000737:chemistry / Q000378:metabolism*; D002458:Cell Fractionation; D002736:Chloroplasts / Q000378:metabolism* / Q000648:ultrastructure; D007425:Intracellular Membranes / Q000378:metabolism*; D026901:Membrane Transport Proteins / Q000032:analysis / Q000737:chemistry / Q000378:metabolism*; D008969:Molecular Sequence Data; D020033:Protein Isoforms / Q000032:analysis / Q000737:chemistry / Q000378:metabolism; D016415:Sequence Alignment
chemical_list: D029681:Arabidopsis Proteins; D026901:Membrane Transport Proteins; D020033:Protein Isoforms; C457764:Tic20 protein, Arabidopsis