title: Toc64/OEP64 is not essential for the efficient import of proteins into chloroplasts in Arabidopsis thaliana.
abstract: Toc64/OEP64 was identified biochemically in pea as a putative component of the chloroplast protein import apparatus. In Arabidopsis, three paralogous genes (atTOC64-III, atTOC64-V and atTOC64-I) encode Toc64-related proteins, and these have been reported to localize in chloroplasts, mitochondria and the cytosol, respectively. To assess the role of the atToc64-III protein in chloroplast protein import in an in vivo context, we identified and characterized Arabidopsis knockout mutants. The absence of detectable defects in toc64-III single mutants raised the possibility of redundancy, and prompted us to also identify toc64-V and toc64-I mutants, cross them to toc64-III, and generate double- and triple-mutant combinations. The toc64 mutants were analysed carefully with respect to a variety of criteria, including chlorophyll accumulation, photosynthetic performance, organellar ultrastructure and chloroplast protein accumulation. In each case, the mutant plants were indistinguishable from wild type. Furthermore, the efficiency of chloroplast protein import was not affected by the toc64 mutations, even when a putative substrate of the atToc64-III protein (wheatgerm-translated precursor of the 33 kDa subunit of the oxygen-evolving complex, OE33) was examined. Moreover, under various stress conditions (high light, osmotic stress and cold), the toc64 triple-mutant plants were not significantly different from wild type. These results demonstrate that Toc64/OEP64 is not essential for the efficient import of proteins into chloroplasts in Arabidopsis, and draw into question the functional significance of this component.
authors.affiliation: Department of Plant and Environmental Sciences, Göteborg University, Box 461, SE-405 30 Göteborg, Sweden.
authors.forename: Henrik
authors.initials: H
authors.lastname: Aronsson
authors.forename: Patrik
authors.initials: P
authors.lastname: Boij
authors.forename: Ramesh
authors.initials: R
authors.lastname: Patel
authors.forename: Anthony
authors.initials: A
authors.lastname: Wardle
authors.forename: Mats
authors.initials: M
authors.lastname: Töpel
authors.forename: Paul
authors.initials: P
authors.lastname: Jarvis
mesh_terms: D017360:Arabidopsis / Q000378:metabolism / Q000502:physiology*; D029681:Arabidopsis Proteins / Q000235:genetics / Q000502:physiology*; D001483:Base Sequence; D002736:Chloroplasts / Q000378:metabolism*; D017931:DNA Primers; D004269:DNA, Bacterial / Q000235:genetics; D008565:Membrane Proteins / Q000235:genetics / Q000502:physiology*; D046529:Microscopy, Electron, Transmission; D016254:Mutagenesis, Insertional; D010802:Phylogeny; D021381:Protein Transport; D020133:Reverse Transcriptase Polymerase Chain Reaction
chemical_list: D029681:Arabidopsis Proteins; D017931:DNA Primers; D004269:DNA, Bacterial; D008565:Membrane Proteins; C036483:T-DNA; C488374:Toc64 protein, Arabidopsis