| 1. |
- Aronsson, Henrik, 1971, et al.
(författare)
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Toc64/OEP64 is not essential for the efficient import of proteins into chloroplasts in Arabidopsis thaliana
- 2007
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Ingår i: Plant Journal. - 0960-7412. ; 52:1, s. 53-68
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Tidskriftsartikel (refereegranskat)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.
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| 2. |
- Boij, Patrik, 1978
(författare)
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Characterization of cholorplast protein import in Arabidopsis thaliana with emphasis on Toc64 and Tic55
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Although the chloroplast possesses a genome of its own most genes have been transferred during the evolution to the nucleus to acquire more transcriptional control, and couple the activities of the wh... merole cell to events at the chloroplast. It is estimated that approximately 95 % of all proteins found in the chloroplast are transcribed from nuclear genes. This means that the proteins translated on ribosomes in the cytosol must subsequently enter the chloroplast by some means. The majority of proteins destined to the chloroplast carry an N-terminal address tag known as the transit peptide that directs the protein to its proper location. This transit peptide is recognized by receptors at the chloroplast outer envelope membrane. These receptors are part of the TOC/TIC (Translocon at the Outer/Inner envelope membrane of Chloroplast) complex mediating chloroplast protein import, which also consists of membrane spanning channels allowing the protein to enter the chloroplast stroma. The process is not spontaneous and requires energy in the form of GTP and ATP. The stepwise translocation process permits a high degree of regulation and control over the translocation process at the inner and outer envelope membrane. Translocation occurs in a concerted manner making sure transport is a unidirectional process. The number of identified Toc/Tic components in the import machinery is currently around 20. In the last year chloroplast protein import has been the subject of much research and the major components and their functions have been characterized. Nevertheless, some of the components still have unclear functions and in some cases the proposed function is supported by a relatively small amount of experimental data. The Toc64 protein, first discovered in pea (Pisum sativum), was proposed to function as a receptor in the chloroplast outer envelope membrane and to interact with the transit peptide of incoming preproteins together with molecular chaperones. In addition it was later proposed that Toc64 recruits other components in the intermembrane space and that the receptor function may only be valid for certain incoming preproteins. In Arabidopsis thaliana three homologs (referred to as Toc64-III, Toc64-V and Toc64-I where the roman number indicates chromosome location) for the pea Toc64 exists. We examined a triple toc64-III/V/I mutant and compared it to wild-type plants. In all aspects measured the mutant plants were indistinguishable from wild-type plants. Furthermore, import of various preproteins was not affected by the mutation. These findings indicate that Toc64 is not vital for chloroplast protein import in Arabidopsis. Toc64-I did however display a subtle phenotype that could possibly be attributed to altered auxin levels but further analysis of the mutant plants is required. The Tic55 protein of the inner envelope membrane has been proposed to function as a redox sensing component linking chloroplast protein import to the energetic state of the cell. However this claim is largely based on assumptions and an experiment conducted with diethylpyrocarbonate (DEPC), a chemical suggested to specifically target the Tic55 component. In Arabidopsis the specific effect of DEPC could not be repeated. It can not be ruled out that DEPC acted on multiple targets since an effect was observed that was not mutant specific. We therefore conclude that Tic55 is not vital for chloroplast protein import and that if Tic55 acts as a redox sensor it is probably of minor importance. We also examined the Tic55 homolog Protochlorophyllide (Pchlide)-dependent Translocon Component of 52 kDa (PTC52). PTC52 is believed to constitute a separate translocon that specifically import the NADPH:Pchlide oxidoreductase A (pPORA) protein and in this way form a substrate dependent pathway. Mutating PTC52 did not have an effect on import of various preproteins and more importantly the import of pPORA was not affected. Thus, there is no evidence for a substrate dependent pathway in Arabidopsis. Finally, a proteomic investigation of chloroplasts from the tic55-II and toc64-III/V/I mutants supported their roles as non-essential components of the chloroplast import machinery since only a handful proteins were significantly up/down-regulated. Further experiments are required to fully elucidate the exact roles of Toc64 and Tic55 in Arabidopsis.
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| 3. |
- Boij, Patrik, 1978, et al.
(författare)
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In vivo Studies on the Roles of Tic55-Related Proteins in Chloroplast Protein Import in Arabidopsis thaliana
- 2009
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Ingår i: Molecular Plant. ; 2:6, s. 1397-1409
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Tidskriftsartikel (refereegranskat)abstract
- The Tic55 (Translocon at the inner envelope membrane of chloroplasts, 55 kDa) protein was identified in pea as a putative regulator, possibly linking chloroplast protein import to the redox state of the photosynthetic machinery. Two Tic55 homologs have been proposed to exist in Arabidopsis: atTic55-II and AtPTC52 (Protochlorophyllide-dependent Translocon Component, 52 kDa; has also been called atTic55-IV). Our phylogenetic analysis shows that atTic55-II is an ortholog of psTic55 from pea (Pisum sativum), and that AtPTC52 is a more distant homolog of the two. AtPTC52 was included in this study to rule out possible functional links between the proteins in Arabidopsis. No detectable mutant phenotypes were found in two independent T-DNA knockout mutant plant lines for each Arabidopsis protein, when compared with wildtype: visible appearance, chlorophyll content, photosynthetic performance, and chloroplast protein import, for example, were all normal. Both wild-type and tic55-II mutant chloroplasts exhibited deficient protein import when treated with diethylpyrocarbonate, indicating that Tic55 is not the sole target of this reagent in relation to protein import. Furthermore, ptc52 mutant chloroplasts were not defective with respect to pPORA import, which was previously reported to involve PTC52 in barley. Thus, we conclude that atTic55-II and AtPTC52 are not strictly required for functional protein import in Arabidopsis.
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