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Zhang Z, Fan Y, Xiong J, Guo X, Hu K, Wang Z, Gao J, Wen J, Yi B, Shen J, Ma C, Fu T, Xia S, Tu J. Two young genes reshape a novel interaction network in Brassica napus. THE NEW PHYTOLOGIST 2020; 225:530-545. [PMID: 31407340 DOI: 10.1111/nph.16113] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/24/2019] [Accepted: 08/06/2019] [Indexed: 06/10/2023]
Abstract
New genes often drive the evolution of gene interaction networks. In Brassica napus, the widely used genic male sterile breeding system 7365ABC is controlled by two young genes, Bnams4b and BnaMs3. However, the interaction mechanism of these two young genes remains unclear. Here, we confirmed that Bnams4b interacts with the nuclear localised E3 ligase BRUTUS (BTS). Ectopic expression of AtBRUTUS (AtBTS) and comparison between Bnams4b -transgenic Arabidopsis and bts mutants suggested that Bnams4b may drive translocation of BTS to cause various toxic defects. BnaMs3 gained an exclusive interaction with the plastid outer-membrane translocon Toc33 compared with Bnams3 and AtTic40, and specifically compensated for the toxic effects of Bnams4b . Heat shock treatment also rescued the sterile phenotype, and high temperature suppressed the interaction between Bnams4b and BTS in yeast. Furthermore, the ubiquitin system and TOC (translocon at the outer envelope membrane of chloroplasts) component accumulation were affected in Bnams4b -transgenic Arabidopsis plants. Taken together, these results indicate that new chimeric Bnams4b carries BTS from nucleus to chloroplast, which may disrupt the normal ubiquitin-proteasome system to cause toxic effects, and these defects can be compensated by BnaMs3-Toc33 interaction or environmental heat shock. It reveals a scenario in which two population-specific coevolved young genes reshape a novel interaction network in plants.
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Affiliation(s)
- Zhiqiang Zhang
- National Key Laboratory of Crop Genetic Improvement, National Center of Rapeseed Improvement, Huazhong Agricultural University, Wuhan, 430070, China
| | - Yu Fan
- National Key Laboratory of Crop Genetic Improvement, National Center of Rapeseed Improvement, Huazhong Agricultural University, Wuhan, 430070, China
| | - Jie Xiong
- National Key Laboratory of Crop Genetic Improvement, National Center of Rapeseed Improvement, Huazhong Agricultural University, Wuhan, 430070, China
| | - Xiang Guo
- National Key Laboratory of Crop Genetic Improvement, National Center of Rapeseed Improvement, Huazhong Agricultural University, Wuhan, 430070, China
| | - Kaining Hu
- National Key Laboratory of Crop Genetic Improvement, National Center of Rapeseed Improvement, Huazhong Agricultural University, Wuhan, 430070, China
| | - Zhixin Wang
- National Key Laboratory of Crop Genetic Improvement, National Center of Rapeseed Improvement, Huazhong Agricultural University, Wuhan, 430070, China
| | - Jie Gao
- National Key Laboratory of Crop Genetic Improvement, National Center of Rapeseed Improvement, Huazhong Agricultural University, Wuhan, 430070, China
| | - Jing Wen
- National Key Laboratory of Crop Genetic Improvement, National Center of Rapeseed Improvement, Huazhong Agricultural University, Wuhan, 430070, China
| | - Bin Yi
- National Key Laboratory of Crop Genetic Improvement, National Center of Rapeseed Improvement, Huazhong Agricultural University, Wuhan, 430070, China
| | - Jinxiong Shen
- National Key Laboratory of Crop Genetic Improvement, National Center of Rapeseed Improvement, Huazhong Agricultural University, Wuhan, 430070, China
| | - Chaozhi Ma
- National Key Laboratory of Crop Genetic Improvement, National Center of Rapeseed Improvement, Huazhong Agricultural University, Wuhan, 430070, China
| | - Tingdong Fu
- National Key Laboratory of Crop Genetic Improvement, National Center of Rapeseed Improvement, Huazhong Agricultural University, Wuhan, 430070, China
| | - Shengqian Xia
- National Key Laboratory of Crop Genetic Improvement, National Center of Rapeseed Improvement, Huazhong Agricultural University, Wuhan, 430070, China
| | - Jinxing Tu
- National Key Laboratory of Crop Genetic Improvement, National Center of Rapeseed Improvement, Huazhong Agricultural University, Wuhan, 430070, China
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Bölter B, Soll J. Once upon a Time - Chloroplast Protein Import Research from Infancy to Future Challenges. MOLECULAR PLANT 2016; 9:798-812. [PMID: 27142186 DOI: 10.1016/j.molp.2016.04.014] [Citation(s) in RCA: 29] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/15/2015] [Revised: 04/25/2016] [Accepted: 04/27/2016] [Indexed: 05/08/2023]
Abstract
Protein import into chloroplasts has been a focus of research for several decades. The first publications dealing with this fascinating topic appeared in the 1970s. From the initial realization that many plastid proteins are being encoded for in the nucleus and require transport into their target organelle to the identification of import components in the cytosol, chloroplast envelopes, and stroma, as well as elucidation of some mechanistic details, more fascinating aspects are still being unraveled. With this overview, we present a survey of the beginnings of chloroplast protein import research, the first steps on this winding road, and end with a glimpse into the future.
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Affiliation(s)
- Bettina Bölter
- Department Biologie I-Botanik, Ludwig-Maximilians-Universität, Großhaderner Straße 2-4, 82152 Planegg-Martinsried, Germany; Munich Center for Integrated Protein Science CiPSM, Ludwig-Maximilians-Universität, Feodor-Lynen-Strasse 25, 81377 Munich, Germany.
| | - Jürgen Soll
- Department Biologie I-Botanik, Ludwig-Maximilians-Universität, Großhaderner Straße 2-4, 82152 Planegg-Martinsried, Germany; Munich Center for Integrated Protein Science CiPSM, Ludwig-Maximilians-Universität, Feodor-Lynen-Strasse 25, 81377 Munich, Germany
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The TIC complex uncovered: The alternative view on the molecular mechanism of protein translocation across the inner envelope membrane of chloroplasts. BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 2015; 1847:957-67. [PMID: 25689609 DOI: 10.1016/j.bbabio.2015.02.011] [Citation(s) in RCA: 69] [Impact Index Per Article: 7.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/10/2014] [Revised: 01/19/2015] [Accepted: 02/07/2015] [Indexed: 12/29/2022]
Abstract
Chloroplasts must import thousands of nuclear-encoded preproteins synthesized in the cytosol through two successive protein translocons at the outer and inner envelope membranes, termed TOC and TIC, respectively, to fulfill their complex physiological roles. The molecular identity of the TIC translocon had long remained controversial; two proteins, namely Tic20 and Tic110, had been proposed to be central to protein translocation across the inner envelope membrane. Tic40 also had long been considered to be another central player in this process. However, recently, a novel 1-megadalton complex consisting of Tic20, Tic56, Tic100, and Tic214 was identified at the chloroplast inner membrane of Arabidopsis and was demonstrated to constitute a general TIC translocon which functions in concert with the well-characterized TOC translocon. On the other hand, direct interaction between this novel TIC transport system and Tic110 or Tic40 was hardly observed. Consequently, the molecular model for protein translocation across the inner envelope membrane of chloroplasts might need to be extensively revised. In this review article, I intend to propose such alternative view regarding the TIC transport system in contradistinction to the classical view. I also would emphasize importance of reevaluation of previous works in terms of with what methods these classical Tic proteins such as Tic110 or Tic40 were picked up as TIC constituents at the very beginning as well as what actual evidence there were to support their direct and specific involvement in chloroplast protein import. This article is part of a Special Issue entitled: Chloroplast Biogenesis.
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Hardré H, Kuhn L, Albrieux C, Jouhet J, Michaud M, Seigneurin-Berny D, Falconet D, Block MA, Maréchal E. The selective biotin tagging and thermolysin proteolysis of chloroplast outer envelope proteins reveals information on protein topology and association into complexes. FRONTIERS IN PLANT SCIENCE 2014; 5:203. [PMID: 24999344 PMCID: PMC4064156 DOI: 10.3389/fpls.2014.00203] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/14/2014] [Accepted: 04/25/2014] [Indexed: 05/27/2023]
Abstract
The understanding of chloroplast function requires the precise localization of proteins in each of its sub-compartments. High-sensitivity mass spectrometry has allowed the inventory of proteins in thylakoid, stroma, and envelope fractions. Concerning membrane association, proteins can be either integral or peripheral or even soluble proteins bound transiently to a membrane complex. We sought a method providing information at the surface of the outer envelope membrane (OEM), based on specific tagging with biotin or proteolysis using thermolysin, a non-membrane permeable protease. To evaluate this method, envelope, thylakoid, and stroma proteins were separated by two-dimensional electrophoresis and analyzed by immunostaining and mass spectrometry. A short selection of proteins associated to the chloroplast envelope fraction was checked after superficial treatments of intact chloroplasts. We showed that this method could allow the characterization of OEM embedded proteins facing the cytosol, as well as peripheral and soluble proteins associated via tight or lose interactions. Some stromal proteins were associated with biotinylated spots and analyzes are still needed to determine whether polypeptides were tagged prior import or if they co-migrated with OEM proteins. This method also suggests that some proteins associated with the inner envelope membrane (IEM) might need the integrity of a trans-envelope (IEM-OEM) protein complex (e.g., division ring-forming components) or at least an intact OEM partner. Following this evaluation, proteomic analyzes should be refined and the putative role of inter-membrane space components stabilizing trans-envelope complexes demonstrated. For future comprehensive studies, perspectives include the dynamic analyses of OEM proteins and IEM-OEM complexes in various physiological contexts and using virtually any other purified membrane organelle.
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Affiliation(s)
- Hélène Hardré
- Laboratoire de Physiologie Cellulaire et Végétale, UMR 5168 CNRS-CEA-INRA-Université Grenoble Alpes, iRTSVCEA Grenoble, Grenoble, France
| | - Lauriane Kuhn
- Laboratoire de Biologie à Grande Echelle, iRTSVCEA Grenoble, Grenoble, France
| | - Catherine Albrieux
- Laboratoire de Physiologie Cellulaire et Végétale, UMR 5168 CNRS-CEA-INRA-Université Grenoble Alpes, iRTSVCEA Grenoble, Grenoble, France
| | - Juliette Jouhet
- Laboratoire de Physiologie Cellulaire et Végétale, UMR 5168 CNRS-CEA-INRA-Université Grenoble Alpes, iRTSVCEA Grenoble, Grenoble, France
| | - Morgane Michaud
- Laboratoire de Physiologie Cellulaire et Végétale, UMR 5168 CNRS-CEA-INRA-Université Grenoble Alpes, iRTSVCEA Grenoble, Grenoble, France
| | - Daphné Seigneurin-Berny
- Laboratoire de Physiologie Cellulaire et Végétale, UMR 5168 CNRS-CEA-INRA-Université Grenoble Alpes, iRTSVCEA Grenoble, Grenoble, France
| | - Denis Falconet
- Laboratoire de Physiologie Cellulaire et Végétale, UMR 5168 CNRS-CEA-INRA-Université Grenoble Alpes, iRTSVCEA Grenoble, Grenoble, France
| | - Maryse A. Block
- Laboratoire de Physiologie Cellulaire et Végétale, UMR 5168 CNRS-CEA-INRA-Université Grenoble Alpes, iRTSVCEA Grenoble, Grenoble, France
| | - Eric Maréchal
- Laboratoire de Physiologie Cellulaire et Végétale, UMR 5168 CNRS-CEA-INRA-Université Grenoble Alpes, iRTSVCEA Grenoble, Grenoble, France
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Shi LX, Theg SM. The chloroplast protein import system: from algae to trees. BIOCHIMICA ET BIOPHYSICA ACTA-MOLECULAR CELL RESEARCH 2012; 1833:314-31. [PMID: 23063942 DOI: 10.1016/j.bbamcr.2012.10.002] [Citation(s) in RCA: 138] [Impact Index Per Article: 11.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/02/2012] [Revised: 09/07/2012] [Accepted: 10/01/2012] [Indexed: 01/15/2023]
Abstract
Chloroplasts are essential organelles in the cells of plants and algae. The functions of these specialized plastids are largely dependent on the ~3000 proteins residing in the organelle. Although chloroplasts are capable of a limited amount of semiautonomous protein synthesis - their genomes encode ~100 proteins - they must import more than 95% of their proteins after synthesis in the cytosol. Imported proteins generally possess an N-terminal extension termed a transit peptide. The importing translocons are made up of two complexes in the outer and inner envelope membranes, the so-called Toc and Tic machineries, respectively. The Toc complex contains two precursor receptors, Toc159 and Toc34, a protein channel, Toc75, and a peripheral component, Toc64/OEP64. The Tic complex consists of as many as eight components, namely Tic22, Tic110, Tic40, Tic20, Tic21 Tic62, Tic55 and Tic32. This general Toc/Tic import pathway, worked out largely in pea chloroplasts, appears to operate in chloroplasts in all green plants, albeit with significant modifications. Sub-complexes of the Toc and Tic machineries are proposed to exist to satisfy different substrate-, tissue-, cell- and developmental requirements. In this review, we summarize our understanding of the functions of Toc and Tic components, comparing these components of the import machinery in green algae through trees. We emphasize recent findings that point to growing complexities of chloroplast protein import process, and use the evolutionary relationships between proteins of different species in an attempt to define the essential core translocon components and those more likely to be responsible for regulation. This article is part of a Special Issue entitled: Protein Import and Quality Control in Mitochondria and Plastids.
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Affiliation(s)
- Lan-Xin Shi
- Department of Plant Biology, University of California-Davis, One Shields Avenue, Davis, CA 95616, USA.
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6
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Abstract
Plastids are a heterogeneous family of organelles found ubiquitously in plants and algal cells. Most prominent are the chloroplasts, which carry out such essential processes as photosynthesis and the biosynthesis of fatty acids as well as of amino acids. As mitochondria, chloroplasts are derived from a single endosymbiotic event. They are believed to have evolved from an ancient cyanobacterium, which was engulfed by an early eukaryotic ancestor. During evolution the plastid genome has been greatly reduced and most of the genes have been transferred to the host nucleus. Consequently, more than 98% of all plastid proteins are translated on cytosolic ribosomes. They have to be posttranslationally targeted to and imported into the organelle. Targeting is assisted by cytosolic proteins which interact with proteins destined for plastids and thereby keep them in an import competent state. After reaching the target organelle, many proteins have to conquer the barrier of the chloroplast outer and inner envelope. This process is mediated by complex molecular machines in the outer (Toc complex) and inner (Tic complex) envelope of chloroplasts, respectively. Most proteins destined for the compartments inside the chloroplast contain a cleavable N-terminal transit peptide, whereas most of the outer envelope components insert into the membrane without such a targeting peptide.
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Affiliation(s)
- Penelope Strittmatter
- Department Biologie I-Botanik, Ludwig-Maximilians-Universität, Planegg-Martinsried and Munich Center for Integrated Protein Science, CiPSM, Ludwig-Maximilians- Universität, Munich, Germany
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Shi LX, Theg SM. A stromal heat shock protein 70 system functions in protein import into chloroplasts in the moss Physcomitrella patens. THE PLANT CELL 2010; 22:205-20. [PMID: 20061551 PMCID: PMC2828695 DOI: 10.1105/tpc.109.071464] [Citation(s) in RCA: 98] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/11/2023]
Abstract
Heat shock protein 70s (Hsp70s) are encoded by a multigene family and are located in different cellular compartments. They have broad-ranging functions, including involvement in protein trafficking, prevention of protein aggregation, and assistance in protein folding. Hsp70s work together with their cochaperones, J domain proteins and nucleotide exchange factors (e.g., GrpEs), in a functional cycle of substrate binding and release accompanied by ATP hydrolysis. We have taken advantage of the gene targeting capability of the moss Physcomitrella patens to investigate the functions of chloroplast Hsp70s. We identified four Hsp70 genes and two GrpE cochaperone homolog genes (CGE) in moss that encode chloroplast proteins. Disruption of one of the Hsp70 genes, that for Hsp70-2, caused lethality, and protein import into heat-shocked chloroplasts isolated from temperature-sensitive hsp70-2 mutants was appreciably impaired. Whereas the double cge null mutant was not viable, we recovered a cge1 null/cge2 knock down mutant in which Hsp70-2 was upregulated. Chloroplasts isolated from this mutant demonstrated a defect in protein import. In addition, two different precursors staged as early import intermediates could be immunoprecipitated with an Hsp70-2-specific antibody. This immunoprecipitate also contained Hsp93 and Tic40, indicating that it represents a precursor still in the Toc/Tic translocon. Together, these data indicate that a stromal Hsp70 system plays a crucial role in protein import into chloroplasts.
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Ahsan N, Komatsu S. Comparative analyses of the proteomes of leaves and flowers at various stages of development reveal organ-specific functional differentiation of proteins in soybean. Proteomics 2009; 9:4889-907. [PMID: 19862761 DOI: 10.1002/pmic.200900308] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/12/2009] [Accepted: 08/10/2009] [Indexed: 12/19/2022]
Abstract
The functional differentiation of protein networks in individual organs and tissues of soybean at various developmental stages was investigated by proteomic approach. Protein extraction by Mg/NP-40 buffer followed by alkaline phenol-based method was optimized for proteomic analysis. Proteome analyses of leaves at various developmental stages showed 26 differentially expressed proteins, wherein proteins in translocon at the outer/inner envelope membrane of chloroplast protein-transport machineries increased significantly at the first trifoliate. Immunoblot analysis showed chaperonin-60 expressed abundantly in young leaves, whereas HSP 70 and ATP-synthase beta were constitutively expressed in all tissues. The net photosynthesis rate and chlorophyll content showed an age-dependent correlation in leaves. These results suggest that proteins involved in carbon assimilation, folding and assembly, and energy may work synchronously and show a linear correlation to photosynthesis at developmental stages of leaves. Comparison of flower bud and flower proteome reveals 29 differentially expressed proteins, wherein proteins involved in mitochondrial protein transport and assembly, secondary metabolism, and pollen-tube growth were up-regulated during flower development. Together, these results suggest that during developmental stages, each type of tissue is associated with a specific group of proteins; wherein proteins involved in energy, sugar metabolism, and folding, assembly, and destination may play pivotal roles in the maturation process of each organ or tissue.
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Affiliation(s)
- Nagib Ahsan
- National Institute of Crop Science, Tsukuba, Japan
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Aronsson H, Jarvis P. The Chloroplast Protein Import Apparatus, Its Components, and Their Roles. PLANT CELL MONOGRAPHS 2008. [DOI: 10.1007/978-3-540-68696-5_3] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/02/2023]
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10
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The Chloroplast Protein Import Apparatus, Its Components, and Their Roles. PLANT CELL MONOGRAPHS 2008. [DOI: 10.1007/7089_2008_40] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/16/2023]
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Bédard J, Kubis S, Bimanadham S, Jarvis P. Functional similarity between the chloroplast translocon component, Tic40, and the human co-chaperone, Hsp70-interacting protein (Hip). J Biol Chem 2007; 282:21404-14. [PMID: 17535810 DOI: 10.1074/jbc.m611545200] [Citation(s) in RCA: 53] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Tic40 is a component of the protein import apparatus of the inner envelope of chloroplasts, but its role in the import mechanism has not been clearly defined. The C terminus of Tic40 shares weak similarity with the C-terminal Sti1 domains of the mammalian Hsp70-interacting protein (Hip) and Hsp70/Hsp90-organizing protein (Hop) co-chaperones. Additionally, Tic40 may possess a tetratricopeptide repeat (TPR) protein-protein interaction domain, another characteristic feature of Hip/Hop co-chaperones. To investigate the functional importance of different parts of the Tic40 protein and to determine whether the homology between Tic40 and co-chaperones is functionally significant, different Tic40 deletion and Tic40:Hip fusion constructs were generated and assessed for complementation activity in the Arabidopsis Tic40 knock-out mutant, tic40. Interestingly, all Tic40 deletion constructs failed to complement tic40, indicating that each part removed is essential for Tic40 function; these included a construct lacking the Sti1-like domain (DeltaSti1), a second lacking a central region, including the putative TPR domain (DeltaTPR), and a third lacking the predicted transmembrane anchor region. Moreover, the DeltaSti1 and DeltaTPR constructs caused strong dominant-negative, albino phenotypes in tic40 transformants, indicating that the truncated Tic40 proteins interfere with the residual chloroplast protein import that occurs in tic40 plants. Remarkably, the Tic40:Hip fusion constructs showed that the Sti1 domain of human Hip is functionally equivalent to the Sti1-like region of Tic40, strongly suggesting a co-chaperone role for the Tic40 protein. Supporting this notion, yeast two-hybrid and bimolecular fluorescence complementation assays demonstrated the in vivo interaction of Tic40 with Tic110, a protein believed to recruit stromal chaperones to protein import sites.
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Affiliation(s)
- Jocelyn Bédard
- Department of Biology, University of Leicester, University Road, Leicester LE1 7RH, United Kingdom
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12
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Karakasis K, Taylor D, Ko K. Uncovering a Link between a Plastid Translocon Component and Rhomboid Proteases Using Yeast Mitochondria-Based Assays. ACTA ACUST UNITED AC 2007; 48:655-61. [PMID: 17327256 DOI: 10.1093/pcp/pcm031] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
Rhomboid proteases are present in bacteria, insects, yeasts, parasites, mammals and plants. These proteases are part of the regulated intramembrane proteolysis mechanism for controlling processes such as development, stress response, lipid metabolism and mitochondrial membrane remodeling. Specific rhomboid protease substrates linked to these processes have been identified from insects to mammals, but not for plants. Identification of a link is a key step for elucidating the role of each rhomboid protease. Here, using a yeast mitochondria-based approach, we report evidence of a potential link between a plastid translocon component and organellar rhomboid proteases. This identification expands the types of processes involving regulated intramembrane proteolysis potentially to include at least one aspect of plastid protein transport.
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Affiliation(s)
- Katherine Karakasis
- Department of Biology, Queen's University, Kingston, Ontario, Canada K7L 3N6
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14
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Abstract
Plastids are a heterogeneous family of organelles found ubiquitously in plant and algal cells. Most prominent are the chloroplasts, which carry out such essential processes as photosynthesis and the biosynthesis of fatty acids as well as of amino acids. As mitochondria, chloroplasts derived from a single endosymbiotic event. They are believed to have evolved from an ancient cyanobacterium, which was engulfed by an early eukaryotic ancestor. During evolution the plastid genome has been greatly reduced and most of the genes have been transferred to the host nucleus. Consequently, more than 98% of all plastid proteins are translated on cytosolic ribosomes. They have to be posttranslationally targeted to and imported into the organelle. Targeting is assisted by cytosolic proteins, which interact with proteins destined for plastids and thereby keep them in an import-competent state. After reaching the target organelle, many proteins have to conquer the barrier of the chloroplast outer and inner envelopes. This process is mediated by complex molecular machines in the outer (Toc complex) and inner (Tic complex) envelope of chloroplasts, respectively. Most proteins destined for compartments inside the chloroplast contain a cleavable N-terminal transit peptide, whereas most of the outer envelope components insert into the membrane without such a targeting peptide.
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Affiliation(s)
- Friederike Hörmann
- Department Biologie I, Ludwig-Maximilians-Universität, Menzingerst, Germany
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15
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Bédard J, Jarvis P. Recognition and envelope translocation of chloroplast preproteins. JOURNAL OF EXPERIMENTAL BOTANY 2005; 56:2287-320. [PMID: 16087701 DOI: 10.1093/jxb/eri243] [Citation(s) in RCA: 68] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/03/2023]
Abstract
Plastids are a diverse group of plant organelles that perform essential functions including important steps in many biosynthetic pathways. Chloroplasts are the best characterized type of plastid, and constitute the site of oxygenic photosynthesis in plants, a process essential to all higher life forms. It is well established that the majority (>90%) of chloroplast proteins are nucleus-encoded and must be post-translationally imported into these envelope-bound compartments. Most nucleus-encoded chloroplast proteins are translated in precursor form on cytosolic ribosomes, targeted to the chloroplast surface, and then imported across the double-membrane envelope by translocons in the outer and inner envelope membranes of the chloroplast, termed TOC and TIC, respectively. Recently, significant progress has been made in our understanding of how proteins are targeted to the chloroplast surface and translocated across the chloroplast envelope into the stroma. Evidence suggesting the existence of multiple import pathways at the outer envelope membrane for different classes of precursor proteins has been presented. These pathways appear to utilize similar TOC complexes equipped with different combinations of homologous GTPase receptors, providing preprotein recognition specificity.
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Affiliation(s)
- Jocelyn Bédard
- Department of Biology, University of Leicester, University Road, Leicester LE1 7RH, UK
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Becker T, Qbadou S, Jelic M, Schleiff E. Let's talk about...chloroplast import. PLANT BIOLOGY (STUTTGART, GERMANY) 2005; 7:1-14. [PMID: 15666210 DOI: 10.1055/s-2004-830447] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/24/2023]
Affiliation(s)
- T Becker
- Botanisches Institut, Ludwig-Maximilian-Universität München, Menzinger Strasse 67, 80368 München, Germany
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Ko K, Taylor D, Argenton P, Innes J, Pedram B, Seibert F, Granell A, Ko Z. Evidence That the Plastid Translocon Tic40 Components Possess Modulating Capabilities. J Biol Chem 2005; 280:215-24. [PMID: 15513916 DOI: 10.1074/jbc.m410088200] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The transport of proteins into the plastid is a process that faces changing cellular needs such as the situation found in different plant organs or developing tissues. The plastid translocon must therefore be responsive to the changing cell environment to deliver efficiently different arrays of structurally diverse proteins. Although the Tic40-related envelope proteins appear to be translocon components designed to address the varying needs of protein translocation, details of their involvement remain elusive. This study was thus designed to combine plant-based experiments and yeast mitochondrion-based approaches for unveiling clues related to how the Tic40 components may behave during the protein translocation process. The main findings related to how Tic40 proteins may work are: 1) natural fluctuations are apparent in developing tissues, in different organs of the same plant, and in different species; 2) transgenic Arabidopsis seedlings can tolerate functionally a wide range of variations in Tic40 levels, from partial suppression to excessive production; 3) the Tic40 proteins themselves exhibit configurational changes in their association with yeast mitochondria in response to different carbon sources; 4) the presence of Tic40 proteins in yeast mitochondria influences regulatory aspects of the mitochondrial translocon; and 5) the Tic40 proteins associate with mitochondrial translocon components involved in regulatory-like events. The combined data provide evidence that Tic40 proteins possess modulating capabilities.
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Affiliation(s)
- Kenton Ko
- Department of Biology, Queen's University, Kingston, Ontario K7L 3N6, Canada.
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Abstract
DNA helicases are molecular 'motor' enzymes that use the energy of NTP hydrolysis to separate transiently energetically stable duplex DNA into single strands. They are therefore essential in nearly all DNA metabolic transactions. They act as essential molecular tools for the cellular machinery. Since the discovery of the first DNA helicase in Escherichia coli in 1976, several have been isolated from both prokaryotic and eukaryotic systems. DNA helicases generally bind to ssDNA or ssDNA/dsDNA junctions and translocate mainly unidirectionally along the bound strand and disrupt the hydrogen bonds between the duplexes. Most helicases contain conserved motifs which act as an engine to drive DNA unwinding. Crystal structures have revealed an underlying common structural fold for their function. These structures suggest the role of the helicase motifs in catalytic function and offer clues as to how these proteins can translocate and unwind DNA. The genes containing helicase motifs may have evolved from a common ancestor. In this review we cover the conserved motifs, structural information, mechanism of DNA unwinding and translocation, and functional aspects of DNA helicases.
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Affiliation(s)
- Narendra Tuteja
- International Centre for Genetic Engineering and Biotechnology (ICGEB), New Delhi, India.
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19
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Chou ML, Fitzpatrick LM, Tu SL, Budziszewski G, Potter-Lewis S, Akita M, Levin JZ, Keegstra K, Li HM. Tic40, a membrane-anchored co-chaperone homolog in the chloroplast protein translocon. EMBO J 2003; 22:2970-80. [PMID: 12805212 PMCID: PMC162133 DOI: 10.1093/emboj/cdg281] [Citation(s) in RCA: 146] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
Abstract
The function of Tic40 during chloroplast protein import was investigated. Tic40 is an inner envelope membrane protein with a large hydrophilic domain located in the stroma. Arabidopsis null mutants of the atTic40 gene were very pale green and grew slowly but were not seedling lethal. Isolated mutant chloroplasts imported precursor proteins at a lower rate than wild-type chloroplasts. Mutant chloroplasts were normal in allowing binding of precursor proteins. However, during subsequent translocation across the inner membrane, fewer precursors were translocated and more precursors were released from the mutant chloroplasts. Cross-linking experiments demonstrated that Tic40 was part of the translocon complex and functioned at the same stage of import as Tic110 and Hsp93, a member of the Hsp100 family of molecular chaperones. Tertiary structure prediction and immunological studies indicated that the C-terminal portion of Tic40 contains a TPR domain followed by a domain with sequence similarity to co-chaperones Sti1p/Hop and Hip. We propose that Tic40 functions as a co-chaperone in the stromal chaperone complex that facilitates protein translocation across the inner membrane.
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Affiliation(s)
- Ming-Lun Chou
- Institute of Molecular Biology, Academia Sinica, Taipei 11529, Taiwan
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20
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Kroth PG. Protein transport into secondary plastids and the evolution of primary and secondary plastids. INTERNATIONAL REVIEW OF CYTOLOGY 2003; 221:191-255. [PMID: 12455749 DOI: 10.1016/s0074-7696(02)21013-x] [Citation(s) in RCA: 49] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/14/2022]
Abstract
Chloroplasts are key organelles in algae and plants due to their photosynthetic abilities. They are thought to have evolved from prokaryotic cyanobacteria taken up by a eukaryotic host cell in a process termed primary endocytobiosis. In addition, a variety of organisms have evolved by subsequent secondary endocytobioses, in which a heterotrophic host cell engulfed a eukaryotic alga. Both processes dramatically enhanced the complexity of the resulting cells. Since the first version of the endosymbiotic theory was proposed more than 100 years ago, morphological, physiological, biochemical, and molecular data have been collected substantiating the emerging picture about the origin and the relationship of individual organisms with different primary or secondary chloroplast types. Depending on their origin, plastids in different lineages may have two, three, or four envelope membranes. The evolutionary success of endocytobioses depends, among other factors, on the specific exchange of molecules between the host and endosymbiont. This raises questions concerning how targeting of nucleus-encoded proteins into the different plastid types occurs and how these processes may have developed. Most studies of protein translocation into plastids have been performed on primary plastids, but in recent years more complex protein-translocation systems of secondary plastids have been investigated. Analyses of transport systems in different algal lineages with secondary plastids reveal that during evolution existing translocation machineries were recycled or recombined rather than being developed de novo. This review deals with current knowledge about the evolution and function of primary and secondary plastids and the respective protein-targeting systems.
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Affiliation(s)
- Peter G Kroth
- Department of Biology, University of Konstanz, 78457 Konstanz, Germany
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21
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Miras S, Salvi D, Ferro M, Grunwald D, Garin J, Joyard J, Rolland N. Non-canonical transit peptide for import into the chloroplast. J Biol Chem 2002; 277:47770-8. [PMID: 12368288 DOI: 10.1074/jbc.m207477200] [Citation(s) in RCA: 134] [Impact Index Per Article: 6.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The large majority of plastid proteins are nuclear-encoded and, thus, must be imported within these organelles. Unlike most of the outer envelope proteins, targeting of proteins to all other plastid compartments (inner envelope membrane, stroma, and thylakoid) is strictly dependent on the presence of a cleavable transit sequence in the precursor N-terminal region. In this paper, we describe the identification of a new envelope protein component (ceQORH) and demonstrate that its subcellular localization is limited to the inner membrane of the chloroplast envelope. Immunopurification, microsequencing of the natural envelope protein and cloning of the corresponding full-length cDNA demonstrated that this protein is not processed in the N-terminal region during its targeting to the inner envelope membrane. Transient expression experiments in plant cells were performed with truncated forms of the ceQORH protein fused to the green fluorescent protein. These experiments suggest that neither the N-terminal nor the C-terminal are essential for chloroplastic localization of the ceQORH protein. These observations are discussed in the frame of the endosymbiotic theory of chloroplast evolution and suggest that a domain of the ceQORH bacterial ancestor may have evolved so as to exclude the general requirement of an N-terminal plastid transit sequence.
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MESH Headings
- Amino Acid Sequence
- Arabidopsis/metabolism
- Base Sequence
- Biological Transport
- Blotting, Western
- Cell Membrane/metabolism
- Chloroplasts/metabolism
- Cloning, Molecular
- DNA, Complementary/metabolism
- Detergents/pharmacology
- Electrophoresis, Polyacrylamide Gel
- Escherichia coli/metabolism
- Gene Library
- Genes, Reporter
- Green Fluorescent Proteins
- Luminescent Proteins/metabolism
- Membrane Proteins/chemistry
- Membrane Proteins/metabolism
- Microscopy, Fluorescence
- Models, Biological
- Models, Molecular
- Molecular Sequence Data
- Oxidation-Reduction
- Peptides/chemistry
- Plasmids/metabolism
- Plastids/chemistry
- Protein Binding
- Protein Structure, Tertiary
- Recombinant Fusion Proteins/metabolism
- Sequence Homology, Amino Acid
- Spinacia oleracea/metabolism
- Time Factors
- Nicotiana/metabolism
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Affiliation(s)
- Stéphane Miras
- Laboratoire de Physiologie Cellulaire Végétale, UMR-5019 CNRS/CEA/Université Joseph Fourier, Grenoble, France
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22
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Vothknecht UC, Soll J. Chloroplast quest: a journey from the cytosol into the chloroplast and beyond. Rev Physiol Biochem Pharmacol 2002; 145:181-222. [PMID: 12224527 DOI: 10.1007/bfb0116432] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/26/2023]
Abstract
Chloroplasts are characteristic organelles of plants and algae and the site of oxygenic photosynthesis. They are surrounded by a double membrane and possess an internal membrane system, the thylakoids, on which the photosynthetic machinery is located. They originated more than 1.2 billion years ago from an endosymbiotic event between an already photosynthetic ancestor of present day cyanobacteria and a mitochondriate host cell. During the transformation of the internalized cyanobacterium into a cell organelle most of the genetic information of the endosymbiot got lost or was transferred into the nucleus of the host. Chloroplast proteins encoded by nuclear genes are synthesized on cytoplasmic ribosomes and have to be relocated into the organelle. This is achieved by a proteinaceous import machinery in the outer and inner envelope of the chloroplasts. Proteins destined for the thylakoid membrane and the thylakoid lumen are further translocated by several different pathways into or across this membrane. The subject of this review is the quest of nuclear encoded chloroplast proteins into the organelle and to their final suborganellar location.
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Affiliation(s)
- Ute C Vothknecht
- Botanisches Institut, Ludwig-Maximilian-Universität München, Menzinger Str. 67, D-80638 München, Germany
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23
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Abstract
The vast majority of chloroplast proteins are synthesized in precursor form on cytosolic ribosomes. Chloroplast precursor proteins have cleavable, N-terminal targeting signals called transit peptides. Transit peptides direct precursor proteins to the chloroplast in an organelle-specific way. They can be phosphorylated by a cytosolic protein kinase, and this leads to the formation of a cytosolic guidance complex. The guidance complex--comprising precursor, hsp70 and 14-3-3 proteins, as well as several unidentified components--docks at the outer envelope membrane. Translocation of precursor proteins across the envelope is achieved by the joint action of molecular machines called Toc (translocon at the outer envelope membrane of chloroplasts) and Tic (translocon at the inner envelope membrane of chloroplasts), respectively. The action of the Toc/Tic apparatus requires the hydrolysis of ATP and GTP at different levels, indicating energetic requirements and regulatory properties of the import process. The main subunits of the Toc and Tic complexes have been identified and characterized in vivo, in organello and in vitro. Phylogenetic evidence suggests that several translocon subunits are of cyanobacterial origin, indicating that today's import machinery was built around a prokaryotic core.
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Affiliation(s)
- Paul Jarvis
- Department of Biology, University of Leicester, UK.
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24
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Heins L, Mehrle A, Hemmler R, Wagner R, Küchler M, Hörmann F, Sveshnikov D, Soll J. The preprotein conducting channel at the inner envelope membrane of plastids. EMBO J 2002; 21:2616-25. [PMID: 12032074 PMCID: PMC126020 DOI: 10.1093/emboj/21.11.2616] [Citation(s) in RCA: 107] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/16/2001] [Revised: 03/18/2002] [Accepted: 03/27/2002] [Indexed: 11/12/2022] Open
Abstract
The preprotein translocation at the inner envelope membrane of chloroplasts so far involves five proteins: Tic110, Tic55, Tic40, Tic22 and Tic20. The molecular function of these proteins has not yet been established. Here, we demonstrate that Tic110 constitutes a central part of the preprotein translocation pore. Dependent on the presence of intact Tic110, radiolabelled preprotein specifically interacts with isolated inner envelope vesicles as well as with purified, recombinant Tic110 reconstituted into liposomes. Circular dichroism analysis reveals that Tic110 consists mainly of beta-sheets, a structure typically found in pore proteins. In planar lipid bilayers, recombinant Tic110 forms a cation-selective high-conductance channel with a calculated inner pore opening of 1.7 nm. Purified transit peptide causes strong flickering and a voltage-dependent block of the channel. Moreover, at the inner envelope membrane, a peptide-sensitive channel is described that shows properties basically identical to the channel formed by recombinant Tic110. We conclude that Tic110 has a distinct preprotein binding site and functions as a preprotein translocation pore at the inner envelope membrane.
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Affiliation(s)
- Lisa Heins
- Botanisches Institut, Universität Kiel, Am Botanischen Garten 1-9, D-24118 Kiel, Germany
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25
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Abstract
The vast majority of chloroplast proteins are synthesized in precursor form on cytosolic ribosomes. Chloroplast precursor proteins have cleavable, N-terminal targeting signals called transit peptides. Transit peptides direct precursor proteins to the chloroplast in an organelle-specific way. They can be phosphorylated by a cytosolic protein kinase, and this leads to the formation of a cytosolic guidance complex. The guidance complex--comprising precursor, hsp70 and 14-3-3 proteins, as well as several unidentified components--docks at the outer envelope membrane. Translocation of precursor proteins across the envelope is achieved by the joint action of molecular machines called Toc (translocon at the outer envelope membrane of chloroplasts) and Tic (translocon at the inner envelope membrane of chloroplasts), respectively. The action of the Toc/Tic apparatus requires the hydrolysis of ATP and GTP at different levels, indicating energetic requirements and regulatory properties of the import process. The main subunits of the Toc and Tic complexes have been identified and characterized in vivo, in organello and in vitro. Phylogenetic evidence suggests that several translocon subunits are of cyanobacterial origin, indicating that today's import machinery was built around a prokaryotic core.
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Affiliation(s)
- P Jarvis
- Department of Biology, University of Leicester, UK.
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26
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Budziszewski GJ, Lewis SP, Glover LW, Reineke J, Jones G, Ziemnik LS, Lonowski J, Nyfeler B, Aux G, Zhou Q, McElver J, Patton DA, Martienssen R, Grossniklaus U, Ma H, Law M, Levin JZ. Arabidopsis genes essential for seedling viability: isolation of insertional mutants and molecular cloning. Genetics 2001; 159:1765-78. [PMID: 11779813 PMCID: PMC1461917 DOI: 10.1093/genetics/159.4.1765] [Citation(s) in RCA: 100] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
We have undertaken a large-scale genetic screen to identify genes with a seedling-lethal mutant phenotype. From screening approximately 38,000 insertional mutant lines, we identified >500 seedling-lethal mutants, completed cosegregation analysis of the insertion and the lethal phenotype for >200 mutants, molecularly characterized 54 mutants, and provided a detailed description for 22 of them. Most of the seedling-lethal mutants seem to affect chloroplast function because they display altered pigmentation and affect genes encoding proteins predicted to have chloroplast localization. Although a high level of functional redundancy in Arabidopsis might be expected because 65% of genes are members of gene families, we found that 41% of the essential genes found in this study are members of Arabidopsis gene families. In addition, we isolated several interesting classes of mutants and genes. We found three mutants in the recently discovered nonmevalonate isoprenoid biosynthetic pathway and mutants disrupting genes similar to Tic40 and tatC, which are likely to be involved in chloroplast protein translocation. Finally, we directly compared T-DNA and Ac/Ds transposon mutagenesis methods in Arabidopsis on a genome scale. In each population, we found only about one-third of the insertion mutations cosegregated with a mutant phenotype.
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Affiliation(s)
- G J Budziszewski
- Syngenta Biotechnology, Inc., Research Triangle Park, North Carolina 27709, USA
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27
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Jackson-Constan D, Keegstra K. Arabidopsis genes encoding components of the chloroplastic protein import apparatus. PLANT PHYSIOLOGY 2001; 125:1567-76. [PMID: 11299338 PMCID: PMC88814 DOI: 10.1104/pp.125.4.1567] [Citation(s) in RCA: 78] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/08/2000] [Revised: 01/05/2001] [Accepted: 01/23/2001] [Indexed: 05/17/2023]
Abstract
The process of protein import into plastids has been studied extensively using isolated pea (Pisum sativum) chloroplasts. As a consequence, virtually all of the known components of the proteinaceous apparatus that mediates import were originally cloned from pea. With the recent completion of the Arabidopsis genome sequencing project, it is now possible to identify putative homologs of the import components in this species. Our analysis has revealed that Arabidopsis homologs with high sequence similarity exist for all of the pea import complex subunits, making Arabidopsis a valid model for further study of this system. Multiple homologs can be identified for over one-half of the components. In all but one case it is known that more than one of the putative isoforms for a particular subunit are expressed. Thus, it is possible that multiple types of import complexes are present within the same cell, each having a unique affinity for different chloroplastic precursor proteins, depending upon the exact mix of isoforms it contains. Sequence analysis of the putative Arabidopsis homologs for the chloroplast protein import apparatus has revealed many questions concerning subunit function and evolution. It should now be possible to use the genetic tools available in Arabidopsis, including the generation of knockout mutants and antisense technology, to address these questions and learn more about the molecular functions of each of the components during the import process.
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Affiliation(s)
- D Jackson-Constan
- Department of Energy Plant Research Laboratory, Michigan State University, East Lansing, Michigan 48824-1312, USA
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28
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Abstract
Plastids originated from an endosymbiotic event between an early eukaryotic host cell and an ancestor of today's cyanobacteria. During the events by which the engulfed endosymbiont was transformed into a permanent organelle, many genes were transferred from the plastidal genome to the nucleus of the host cell. Proteins encoded by these genes are synthesised in the cytosol and subsequently translocated into the plastid. Therefore they contain an N-terminal cleavable transit sequence that is necessary for translocation. The sequence is plastid-specific, thus preventing mistargeting into other organelles. Receptors embedded into the outer envelope of the plastid recognise the transit sequences, and precursor proteins are translocated into the chloroplast by a proteinaceous import machinery located in both the outer and inner envelopes. Inside the stroma the transit sequences are cleaved off and the proteins are further routed to their final locations within the plastid.
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Affiliation(s)
- U C Vothknecht
- Botanisches Institut der Christian-Albrechts-Universität Kiel, Germany
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29
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Sohrt K, Soll J. Toc64, a new component of the protein translocon of chloroplasts. J Cell Biol 2000; 148:1213-21. [PMID: 10725334 PMCID: PMC2174300 DOI: 10.1083/jcb.148.6.1213] [Citation(s) in RCA: 114] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/13/1999] [Accepted: 02/15/2000] [Indexed: 11/22/2022] Open
Abstract
A subunit of the preprotein translocon of the outer envelope of chloroplasts (Toc complex) of 64 kD is described, Toc64. Toc64 copurifies on sucrose density gradients with the isolated Toc complex. Furthermore, it can be cross-linked in intact chloroplasts to a high molecular weight complex containing both Toc and Tic subunits and a precursor protein. The 0 A cross-linker CuCl(2) yields the reversible formation of disulfide bridge(s) between Toc64 and the established Toc complex subunits in purified outer envelope membranes. Toc64 contains three tetratricopeptide repeat motifs that are exposed at the chloroplast cytosol interface. We propose that Toc64 functions early in preprotein translocation, maybe as a docking protein for cytosolic cofactors of the protein import into chloroplasts.
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Affiliation(s)
- Karen Sohrt
- Botanisches Institut, Christian-Albrechts-Universität Kiel, D-24118 Kiel, Germany
| | - Jürgen Soll
- Botanisches Institut, Christian-Albrechts-Universität Kiel, D-24118 Kiel, Germany
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30
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Stahl T, Glockmann C, Soll J, Heins L. Tic40, a new "old" subunit of the chloroplast protein import translocon. J Biol Chem 1999; 274:37467-72. [PMID: 10601321 DOI: 10.1074/jbc.274.52.37467] [Citation(s) in RCA: 99] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The protein import translocon at the inner envelope of chloroplasts (Tic complex) is a heteroligomeric multisubunit complex. Here, we describe Tic40 from pea as a new component of this complex. Tic40 from pea is a homologue of a protein described earlier from Brassica napus as Cim/Com44 or the Toc36 subunit of the translocon at the outer envelope of chloroplasts, respectively (Wu, C., Seibert, F. S., and Ko, K. (1994) J. Biol. Chem. 269, 32264-32271; Ko, K., Budd, D., Wu, C., Seibert, F., Kourtz, L., and Ko, Z. W. (1995) J. Biol. Chem. 270, 28601-28608; Pang, P., Meathrel, K., and Ko, K. (1997) J. Biol. Chem. 272, 25623-25627). Tic40 can be covalently connected to Tic110 by the formation of a disulfide bridge under oxidizing conditions, indicating its close physical proximity to an established translocon component. The Tic40 protein is synthesized in the cytosol as a precursor with an N-terminal cleavable chloroplast targeting signal and imported into the organelle via the general import pathway. Immunoblotting and immunogold-labeling studies exclusively confine Tic40 to the chloroplastic inner envelope, in which it is anchored by a single putative transmembrane span.
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Affiliation(s)
- T Stahl
- Botanisches Institut, Universität Kiel, 24118 Kiel, Germany
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31
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Ko K, Ko ZW. In vitro targeting of the Toc36 component of the chloroplast envelope protein import apparatus involves a complex set of information. BIOCHIMICA ET BIOPHYSICA ACTA 1999; 1421:198-206. [PMID: 10561484 DOI: 10.1016/s0005-2736(99)00126-1] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/17/2022]
Abstract
Toc36 is a family of 44-kDa envelope polypeptides previously identified as components of the chloroplast protein import apparatus. Toc36 exists as multiple outer and inner envelope membrane forms. One member, Toc36B (formerly Bce44B), is targeted to the envelope without the typical maturation event. Targeting and assembly into the envelope is thus likely to involve a complex interplay of indigenous signals. These signals were examined by testing the effects of truncations and chimeric fusions on the targeting of Toc36B. The targeting ability of Toc36B appeared unaffected by carboxyl truncations of up to 80% of the protein, but was abolished by N-terminal deletions. The N-terminal 39 residues of Toc36B conferred the same targeting profile to mouse dihydrofolate reductase as that displayed by unaltered Toc36B. However, removal of 18 residues from the carboxyl end of the N-terminal 39-amino acid segment abolished targeting to the chloroplast. Additional information in the remaining Toc36B segment was also apparent based on the import results of chimeric fusions between the transit peptide of the small subunit of ribulose-1,5-bisphosphate carboxylase and Toc36B. The targeting of Toc36B to various destinations in the chloroplast envelope appears to be influenced by information from at least two segments of the protein.
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Affiliation(s)
- K Ko
- Department of Biology, Queen's University, Kingston, Ont., Canada.
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32
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Abstract
Chloroplasts are believed to have originated from a photosynthetic, prokaryotic ancestor. As the result of endosymbiotic evolution, most of the genes of the endocytobiont were displaced to the host nucleus. Today's chloroplasts must import most of their proteins from the cytosol as precursors. Oligomeric protein complexes in the chloroplast outer and inner envelope membranes are responsible for the specific recognition and membrane translocation of precursor proteins. The translocon at the outer membrane of chloroplasts and the inner membrane of chloroplasts act jointly during the import process. Several translocon subunits have been partially characterized in their molecular structure and function. Initial evidence indicates the prokaryotic origin of some chloroplast translocon components.
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Affiliation(s)
- T May
- Botanisches Institut der Universität Kiel, Germany
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33
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Abstract
Chloroplasts have evolved an elaborate system of membrane and soluble subcompartments to organize and regulate photosynthesis and essential aspects of amino acid and lipid metabolism. The biogenesis and maintenance of organellar architecture rely on protein subunits encoded by both nuclear and plastid genomes. Import of nuclear-encoded proteins is mediated by interactions between the intrinsic N-terminal transit sequence of the nuclear-encoded preprotein and a common import machinery at the chloroplast envelope. Recent investigations have shown that there are two unique membrane-bound translocation systems, in the outer and inner envelope membranes, which physically associate during import to transport preproteins from the cytoplasm to the internal stromal compartment. This review discusses current understanding of these translocation systems and models for the way in which they might function.
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Affiliation(s)
- X Chen
- Dept of Biological Sciences, Rutgers, The State University of New Jersey, 101 Warren Street, Newark, NJ 07102, USA
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34
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Bölter B, Soll J, Schulz A, Hinnah S, Wagner R. Origin of a chloroplast protein importer. Proc Natl Acad Sci U S A 1998; 95:15831-6. [PMID: 9861056 PMCID: PMC28130 DOI: 10.1073/pnas.95.26.15831] [Citation(s) in RCA: 132] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/24/1998] [Indexed: 11/18/2022] Open
Abstract
During evolution, chloroplasts have relinquished the majority of their genes to the nucleus. The products of transferred genes are imported into the organelle with the help of an import machinery that is distributed across the inner and outer plastid membranes. The evolutionary origin of this machinery is puzzling because, in the putative predecessors, the cyanobacteria, the outer two membranes, the plasma membrane, and the lipopolysaccharide layer lack a functionally similar protein import system. A 75-kDa protein-conducting channel in the outer envelope of pea chloroplasts, Toc75, shares approximately 22% amino acid identity to a similarly sized protein, designated SynToc75, encoded in the Synechocystis PCC6803 genome. Here we show that SynToc75 is located in the outer membrane (lipopolysaccharide layer) of Synechocystis PCC6803 and that SynToc75 forms a voltage-gated, high conductance channel with a high affinity for polyamines and peptides in reconstituted liposomes. These findings suggest that a component of the chloroplast protein import system, Toc75, was recruited from a preexisting channel-forming protein of the cyanobacterial outer membrane. Furthermore, the presence of a protein in the chloroplastic outer envelope homologous to a cyanobacterial protein provides support for the prokaryotic nature of this chloroplastic membrane.
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Affiliation(s)
- B Bölter
- Botanisches Institut, Universität Kiel, Am Botanischen Garten 1-9, D-24118 Kiel, Germany
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35
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Bölter B, May T, Soll J. A protein import receptor in pea chloroplasts, Toc86, is only a proteolytic fragment of a larger polypeptide. FEBS Lett 1998; 441:59-62. [PMID: 9877165 DOI: 10.1016/s0014-5793(98)01525-7] [Citation(s) in RCA: 81] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Abstract
The protein import complex of the chloroplastic outer envelope (Toc-complex) contains a prominent subunit of 86 kDa molecular weight (Toc86). Toc86 was identified as a putative precursor receptor. The Arabidopsis genome sequencing project indicates that Toc86 represents only a proteolytic fragment of a larger polypeptide of 160 kDa. The 160-kDa protein, which we name Toc160, is only present in significant amounts in pea chloroplasts isolated under stringent conditions. The capacity of chloroplasts to import an in vitro translated precursor protein correlates well with the integrity of Toc160. We conclude that Toc160 is still a bonafide subunit of the protein import machinery of chloroplasts.
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Affiliation(s)
- B Bölter
- Botanisches Institut, Universität Kiel, Germany
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36
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Abstract
▪ Abstract The assembly of the photosynthetic apparatus at the thylakoid begins with the targeting of proteins from their site of synthesis in the cytoplasm or stroma to the thylakoid membrane. Plastid-encoded proteins are targeted directly to the thylakoid during or after synthesis on plastid ribosomes. Nuclear-encoded proteins undergo a two-step targeting process requiring posttranslational import into the organelle from the cytoplasm and subsequent targeting to the thylakoid membrane. Recent investigations have revealed a single general import machinery at the envelope that mediates the direct transport of preproteins from the cytoplasm to the stroma. In contrast, at least four distinct pathways exist for the targeting of proteins to the thylakoid membrane. At least two of these systems are homologous to translocation systems that operate in bacteria and at the endoplasmic reticulum, indicating that elements of the targeting mechanisms have been conserved from the original prokaryotic endosymbiont.
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Affiliation(s)
- Danny J. Schnell
- Department of Biological Sciences, Rutgers, The State University of New Jersey, Newark, New Jersey 07102; e-mail:
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37
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Abstract
Chloroplast biogenesis requires the translocation of proteins across the outer and inner envelopes. The membrane components of this transport machinery completely differ from those of other organelles, but recently homologues of some of the components have been detected in prokaryotes.
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Affiliation(s)
- L Heins
- Institut für Botanik, Universität Kiel, Germany
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Caliebe A, Grimm R, Kaiser G, Lübeck J, Soll J, Heins L. The chloroplastic protein import machinery contains a Rieske-type iron-sulfur cluster and a mononuclear iron-binding protein. EMBO J 1997; 16:7342-50. [PMID: 9405363 PMCID: PMC1170334 DOI: 10.1093/emboj/16.24.7342] [Citation(s) in RCA: 143] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
Transport of precursor proteins across the chloroplastic envelope membranes requires the interaction of protein translocons localized in both the outer and inner envelope membranes. Analysis by blue native gel electrophoresis revealed that the translocon of the inner envelope membranes consisted of at least six proteins with molecular weights of 36, 45, 52, 60, 100 and 110 kDa, respectively. Tic110 and ClpC, identified as components of the protein import apparatus of the inner envelope membrane, were prominent constituents of this complex. The amino acid sequence of the 52 kDa protein, deduced from the cDNA, contains a predicted Rieske-type iron-sulfur cluster and a mononuclear iron-binding site. Diethylpyrocarbonate, a Rieske-type protein-modifying reagent, inhibits the translocation of precursor protein across the inner envelope membrane, whereas binding of the precursor to the outer envelope membrane is still possible. In another independent experimental approach, the 52 kDa protein could be co-purified with a trapped precursor protein in association with the chloroplast protein translocon subunits Toc86, Toc75, Toc34 and Tic110. Together, these results strongly suggest that the 52 kDa protein, named Tic55 due to its calculated molecular weight, is a member of the chloroplastic inner envelope protein translocon.
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Affiliation(s)
- A Caliebe
- Botanisches Institut, Christian-Albrechts-Universität, Am Botanischen Garten 1-9, D-24118 Kiel, Germany
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Pang P, Meathrel K, Ko K. A component of the chloroplast protein import apparatus functions in bacteria. J Biol Chem 1997; 272:25623-7. [PMID: 9325283 DOI: 10.1074/jbc.272.41.25623] [Citation(s) in RCA: 24] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
Toc36 is a family of 44-kDa envelope polypeptides previously identified as components of the chloroplast protein import apparatus by virtue of their close physical proximity to translocating proteins. An indication of their function thus remains at large. A heterologous in vivo approach for studying the function of Toc36 was developed in this study by introducing a member of Toc36 into E. coli to assess its effect on bacterial protein translocation. The presence of Toc36 enhances the translocation of two bacterial periplasmic proteins in a manner resembling the chloroplast system. Translocation of the two bacterial periplasmic proteins was less sensitive to sodium azide, resembling more the azide-insensitive nature of the chloroplast protein import process. Mutated Toc36 proteins were not capable of causing the same effect as that observed for unaltered Toc36. Toc36 was also capable of complementing bacterial strains with temperature-sensitive secA mutations that affected protein translocation. The combined results provide evidence that Toc36 plays a central role in the chloroplast protein translocation process.
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Affiliation(s)
- P Pang
- Department of Biology, Queen's University, Kingston, Ontario, Canada K7L 3N6
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Schnell DJ, Blobel G, Keegstra K, Kessler F, Ko K, Soll J. A consensus nomenclature for the protein-import components of the chloroplast envelope. Trends Cell Biol 1997; 7:303-4. [PMID: 17708964 DOI: 10.1016/s0962-8924(97)01111-2] [Citation(s) in RCA: 108] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
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Akita M, Nielsen E, Keegstra K. Identification of protein transport complexes in the chloroplastic envelope membranes via chemical cross-linking. J Cell Biol 1997; 136:983-94. [PMID: 9060464 PMCID: PMC2132478 DOI: 10.1083/jcb.136.5.983] [Citation(s) in RCA: 165] [Impact Index Per Article: 6.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/07/1996] [Revised: 01/09/1997] [Indexed: 02/03/2023] Open
Abstract
Transport of cytoplasmically synthesized proteins into chloroplasts uses an import machinery present in the envelope membranes. To identify the components of this machinery and to begin to examine how these components interact during transport, chemical cross-linking was performed on intact chloroplasts containing precursor proteins trapped at a particular stage of transport by ATP limitation. Large cross-linked complexes were observed using three different reversible homobifunctional cross-linkers. Three outer envelope membrane proteins (OEP86, OEP75, and OEP34) and one inner envelope membrane protein (IEP110), previously reported to be involved in protein import, were identified as components of these complexes. In addition to these membrane proteins, a stromal member of the hsp100 family, ClpC, was also present in the complexes. We propose that ClpC functions as a molecular chaperone, cooperating with other components to accomplish the transport of precursor proteins into chloroplasts. We also propose that each envelope membrane contains distinct translocation complexes and that a portion of these interact to form contact sites even in the absence of precursor proteins.
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Affiliation(s)
- M Akita
- MSU-DOE Plant Research Laboratory, Michigan State University, East Lansing 48824-1312, USA
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Kouranov A, Schnell DJ. Protein translocation at the envelope and thylakoid membranes of chloroplasts. J Biol Chem 1996; 271:31009-12. [PMID: 8940090 DOI: 10.1074/jbc.271.49.31009] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023] Open
Affiliation(s)
- A Kouranov
- Department of Biological Sciences, Rutgers, The State University of New Jersey, Newark, New Jersey 07102, USA
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