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López B, Izquierdo Y, Cascón T, Zamarreño ÁM, García-Mina JM, Pulido P, Castresana C. Mutant noxy8 exposes functional specificities between the chloroplast chaperones CLPC1 and CLPC2 in the response to organelle stress and plant defence. PLANT, CELL & ENVIRONMENT 2024; 47:2336-2350. [PMID: 38500380 DOI: 10.1111/pce.14882] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/03/2023] [Revised: 02/20/2024] [Accepted: 03/01/2024] [Indexed: 03/20/2024]
Abstract
Chloroplast function is essential for growth, development, and plant adaptation to stress. Organelle stress and plant defence responses were examined here using noxy8 (nonresponding to oxylipins 8) from a series of Arabidopsis mutants. The noxy8 mutation was located at the CLPC2 gene, encoding a chloroplast chaperone of the protease complex CLP. Although its CLPC1 paralogue is considered to generate redundancy, our data reveal significant differences distinguishing CLPC2 and CLPC1 functions. As such, clpc1 mutants displayed a major defect in housekeeping chloroplast proteostasis, leading to a pronounced reduction in growth and pigment levels, enhanced accumulation of chloroplast and cytosol chaperones, and resistance to fosmidomycin. Conversely, clpc2 mutants showed severe susceptibility to lincomycin inhibition of chloroplast translation and resistance to Antimycin A inhibition of mitochondrial respiration. In the response to Pseudomonas syringae pv. tomato, clpc2 but not clpc1 mutants were resistant to bacterial infection, showing higher salicylic acid levels, defence gene expression and 9-LOX pathway activation. Our findings suggest CLPC2 and CLPC1 functional specificity, with a preferential involvement of CLPC1 in housekeeping processes and of CLPC2 in stress responses.
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Affiliation(s)
- Bran López
- Centro Nacional de Biotecnología (CNB-CSIC), Cantoblanco, Madrid, Spain
| | - Yovanny Izquierdo
- Centro Nacional de Biotecnología (CNB-CSIC), Cantoblanco, Madrid, Spain
| | - Tomás Cascón
- Centro Nacional de Biotecnología (CNB-CSIC), Cantoblanco, Madrid, Spain
| | - Ángel M Zamarreño
- Department of Environmental Biology, Bioma Institute, University of Navarra, Navarra, Spain
| | - José M García-Mina
- Department of Environmental Biology, Bioma Institute, University of Navarra, Navarra, Spain
| | - Pablo Pulido
- Centro Nacional de Biotecnología (CNB-CSIC), Cantoblanco, Madrid, Spain
| | - Carmen Castresana
- Centro Nacional de Biotecnología (CNB-CSIC), Cantoblanco, Madrid, Spain
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2
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Mishra LS, Cook SD, Kushwah S, Isaksson H, Straub IR, Abele M, Mishra S, Ludwig C, Libby E, Funk C. Overexpression of the plastidial pseudo-protease AtFtsHi3 enhances drought tolerance while sustaining plant growth. PHYSIOLOGIA PLANTARUM 2024; 176:e14370. [PMID: 38818570 DOI: 10.1111/ppl.14370] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/31/2024] [Revised: 04/18/2024] [Accepted: 04/25/2024] [Indexed: 06/01/2024]
Abstract
With climate change, droughts are expected to be more frequent and severe, severely impacting plant biomass and quality. Here, we show that overexpressing the Arabidopsis gene AtFtsHi3 (FtsHi3OE) enhances drought-tolerant phenotypes without compromising plant growth. AtFtsHi3 encodes a chloroplast envelope pseudo-protease; knock-down mutants (ftshi3-1) are found to be drought tolerant but exhibit stunted growth. Altered AtFtsHi3 expression therefore leads to drought tolerance, while only diminished expression of this gene leads to growth retardation. To understand the underlying mechanisms of the enhanced drought tolerance, we compared the proteomes of ftshi3-1 and pFtsHi3-FtsHi3OE (pFtsHi3-OE) to wild-type plants under well-watered and drought conditions. Drought-related processes like osmotic stress, water transport, and abscisic acid response were enriched in pFtsHi3-OE and ftshi3-1 mutants following their enhanced drought response compared to wild-type. The knock-down mutant ftshi3-1 showed an increased abundance of HSP90, HSP93, and TIC110 proteins, hinting at a potential downstream role of AtFtsHi3 in chloroplast pre-protein import. Mathematical modeling was performed to understand how variation in the transcript abundance of AtFtsHi3 can, on the one hand, lead to drought tolerance in both overexpression and knock-down lines, yet, on the other hand, affect plant growth so differently. The results led us to hypothesize that AtFtsHi3 may form complexes with at least two other protease subunits, either as homo- or heteromeric structures. Enriched amounts of AtFtsH7/9, AtFtsH11, AtFtsH12, and AtFtsHi4 in ftshi3-1 suggest a possible compensation mechanism for these proteases in the hexamer.
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Affiliation(s)
| | - Sam D Cook
- Department of Chemistry, Umeå University, Umeå, Sweden
| | | | - Hanna Isaksson
- Department of Mathematics and Mathematical Statistics, Integrated Science Lab (Icelab), Umeå University, Umeå, Sweden
- IceLab, Umeå University, Umeå, Sweden
| | - Isabella R Straub
- Bavarian Center for Biomolecular Mass Spectrometry (BayBioMS), School of Life Sciences Weihenstephan, Technical University of Munich (TUM), Freising, Germany
| | - Miriam Abele
- Bavarian Center for Biomolecular Mass Spectrometry (BayBioMS), School of Life Sciences Weihenstephan, Technical University of Munich (TUM), Freising, Germany
| | - Sanatkumar Mishra
- Umeå Plant Science Centre, Department of Forest Genetics and Plant Physiology, Swedish University of Agricultural Sciences, Umeå, Sweden
| | - Christina Ludwig
- Bavarian Center for Biomolecular Mass Spectrometry (BayBioMS), School of Life Sciences Weihenstephan, Technical University of Munich (TUM), Freising, Germany
| | - Eric Libby
- Department of Mathematics and Mathematical Statistics, Integrated Science Lab (Icelab), Umeå University, Umeå, Sweden
- IceLab, Umeå University, Umeå, Sweden
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3
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An H, Ke X, Li L, Liu Y, Yuan S, Wang Q, Hou X, Zhao J. ALBINO EMBRYO AND SEEDLING is required for RNA splicing and chloroplast homeostasis in Arabidopsis. PLANT PHYSIOLOGY 2023; 193:483-501. [PMID: 37311175 DOI: 10.1093/plphys/kiad341] [Citation(s) in RCA: 2] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/05/2023] [Revised: 05/03/2023] [Accepted: 05/07/2023] [Indexed: 06/15/2023]
Abstract
Pentatricopeptide repeat (PPR) proteins form a large protein family and have diverse functions in plant development. Here, we identified an ALBINO EMBRYO AND SEEDLING (AES) gene that encodes a P-type PPR protein expressed in various tissues, especially the young leaves of Arabidopsis (Arabidopsis thaliana). Its null mutant aes exhibited a collapsed chloroplast membrane system, reduced pigment content and photosynthetic activity, decreased transcript levels of PEP (plastid-encoded polymerase)-dependent chloroplast genes, and defective RNA splicing. Further work revealed that AES could directly bind to psbB-psbT, psbH-petB, rps8-rpl36, clpP, ycf3, and ndhA in vivo and in vitro and that the splicing efficiencies of these genes and the expression levels of ycf3, ndhA, and cis-tron psbB-psbT-psbH-petB-petD decreased dramatically, leading to defective PSI, PSII, and Cyt b6f in aes. Moreover, AES could be transported into the chloroplast stroma via the TOC-TIC channel with the assistance of Tic110 and cpSRP54 and may recruit HCF244, SOT1, and CAF1 to participate in the target RNA process. These findings suggested that AES is an essential protein for the assembly of photosynthetic complexes, providing insights into the splicing of psbB operon (psbB-psbT-psbH-petB-petD), ycf3, and ndhA, as well as maintaining chloroplast homeostasis.
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Affiliation(s)
- Hongqiang An
- State Key Laboratory of Hybrid Rice, College of Life Sciences, Wuhan University, 430072 Wuhan, China
| | - Xiaolong Ke
- State Key Laboratory of Hybrid Rice, College of Life Sciences, Wuhan University, 430072 Wuhan, China
| | - Lu Li
- State Key Laboratory of Hybrid Rice, College of Life Sciences, Wuhan University, 430072 Wuhan, China
| | - Yantong Liu
- State Key Laboratory of Hybrid Rice, College of Life Sciences, Wuhan University, 430072 Wuhan, China
| | - Sihui Yuan
- State Key Laboratory of Hybrid Rice, College of Life Sciences, Wuhan University, 430072 Wuhan, China
| | - Qiuyu Wang
- State Key Laboratory of Hybrid Rice, College of Life Sciences, Wuhan University, 430072 Wuhan, China
| | - Xin Hou
- State Key Laboratory of Hybrid Rice, College of Life Sciences, Wuhan University, 430072 Wuhan, China
| | - Jie Zhao
- State Key Laboratory of Hybrid Rice, College of Life Sciences, Wuhan University, 430072 Wuhan, China
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4
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Sun Y, Li J, Zhang L, Lin R. Regulation of chloroplast protein degradation. J Genet Genomics 2023:S1673-8527(23)00049-8. [PMID: 36863685 DOI: 10.1016/j.jgg.2023.02.010] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/24/2022] [Revised: 02/02/2023] [Accepted: 02/14/2023] [Indexed: 03/04/2023]
Abstract
Chloroplasts are unique organelles that not only provide sites for photosynthesis and many metabolic processes, but also are sensitive to various environmental stresses. Chloroplast proteins are encoded by genes from both nuclear and chloroplast genomes. During chloroplast development and responses to stresses, the robust protein quality control systems are essential for regulation of protein homeostasis and the integrity of chloroplast proteome. In this review, we summarize the regulatory mechanisms of chloroplast protein degradation refer to protease system, ubiquitin-proteasome system, and the chloroplast autophagy. These mechanisms symbiotically play a vital role in chloroplast development and photosynthesis under both normal or stress conditions.
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Affiliation(s)
- Yang Sun
- State Key Laboratory of Crop Stress Adaptation and Improvement, School of Life Sciences, Henan University, Kaifeng, Henan 475001, China
| | - Jialong Li
- Key Laboratory of Photobiology, Institute of Botany, Chinese Academy of Sciences, Beijing 100093, China
| | - Lixin Zhang
- State Key Laboratory of Crop Stress Adaptation and Improvement, School of Life Sciences, Henan University, Kaifeng, Henan 475001, China.
| | - Rongcheng Lin
- Key Laboratory of Photobiology, Institute of Botany, Chinese Academy of Sciences, Beijing 100093, China.
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5
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Gao LL, Hong ZH, Wang Y, Wu GZ. Chloroplast proteostasis: A story of birth, life, and death. PLANT COMMUNICATIONS 2023; 4:100424. [PMID: 35964157 PMCID: PMC9860172 DOI: 10.1016/j.xplc.2022.100424] [Citation(s) in RCA: 6] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/16/2022] [Revised: 08/02/2022] [Accepted: 08/10/2022] [Indexed: 06/02/2023]
Abstract
Protein homeostasis (proteostasis) is a dynamic balance of protein synthesis and degradation. Because of the endosymbiotic origin of chloroplasts and the massive transfer of their genetic information to the nucleus of the host cell, many protein complexes in the chloroplasts are constituted from subunits encoded by both genomes. Hence, the proper function of chloroplasts relies on the coordinated expression of chloroplast- and nucleus-encoded genes. The biogenesis and maintenance of chloroplast proteostasis are dependent on synthesis of chloroplast-encoded proteins, import of nucleus-encoded chloroplast proteins from the cytosol, and clearance of damaged or otherwise undesired "old" proteins. This review focuses on the regulation of chloroplast proteostasis, its interaction with proteostasis of the cytosol, and its retrograde control over nuclear gene expression. We also discuss significant issues and perspectives for future studies and potential applications for improving the photosynthetic performance and stress tolerance of crops.
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Affiliation(s)
- Lin-Lin Gao
- Shanghai Collaborative Innovation Center of Agri-Seeds, School of Agriculture and Biology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China; Joint Center for Single Cell Biology, School of Agriculture and Biology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China
| | - Zheng-Hui Hong
- Shanghai Collaborative Innovation Center of Agri-Seeds, School of Agriculture and Biology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China; Joint Center for Single Cell Biology, School of Agriculture and Biology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China
| | - Yinsong Wang
- Shanghai Collaborative Innovation Center of Agri-Seeds, School of Agriculture and Biology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China; Joint Center for Single Cell Biology, School of Agriculture and Biology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China
| | - Guo-Zhang Wu
- Shanghai Collaborative Innovation Center of Agri-Seeds, School of Agriculture and Biology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China; Joint Center for Single Cell Biology, School of Agriculture and Biology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China.
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6
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Chloroplasts Protein Quality Control and Turnover: A Multitude of Mechanisms. Int J Mol Sci 2022; 23:ijms23147760. [PMID: 35887108 PMCID: PMC9319218 DOI: 10.3390/ijms23147760] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/23/2022] [Revised: 07/11/2022] [Accepted: 07/12/2022] [Indexed: 11/16/2022] Open
Abstract
As the organelle of photosynthesis and other important metabolic pathways, chloroplasts contain up to 70% of leaf proteins with uniquely complex processes in synthesis, import, assembly, and turnover. Maintaining functional protein homeostasis in chloroplasts is vitally important for the fitness and survival of plants. Research over the past several decades has revealed a multitude of mechanisms that play important roles in chloroplast protein quality control and turnover under normal and stress conditions. These mechanisms include: (i) endosymbiotically-derived proteases and associated proteins that play a vital role in maintaining protein homeostasis inside the chloroplasts, (ii) the ubiquitin-dependent turnover of unimported chloroplast precursor proteins to prevent their accumulation in the cytosol, (iii) chloroplast-associated degradation of the chloroplast outer-membrane translocon proteins for the regulation of chloroplast protein import, (iv) chloroplast unfolded protein response triggered by accumulated unfolded and misfolded proteins inside the chloroplasts, and (v) vesicle-mediated degradation of chloroplast components in the vacuole. Here, we provide a comprehensive review of these diverse mechanisms of chloroplast protein quality control and turnover and discuss important questions that remain to be addressed in order to better understand and improve important chloroplast functions.
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7
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Li M, Guo G, Pidon H, Melzer M, Prina AR, Börner T, Stein N. ATP-Dependent Clp Protease Subunit C1, HvClpC1, Is a Strong Candidate Gene for Barley Variegation Mutant luteostrians as Revealed by Genetic Mapping and Genomic Re-sequencing. FRONTIERS IN PLANT SCIENCE 2021; 12:664085. [PMID: 33936155 PMCID: PMC8086601 DOI: 10.3389/fpls.2021.664085] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 02/09/2021] [Accepted: 03/29/2021] [Indexed: 06/12/2023]
Abstract
Implementation of next-generation sequencing in forward genetic screens greatly accelerated gene discovery in species with larger genomes, including many crop plants. In barley, extensive mutant collections are available, however, the causative mutations for many of the genes remains largely unknown. Here we demonstrate how a combination of low-resolution genetic mapping, whole-genome resequencing and comparative functional analyses provides a promising path toward candidate identification of genes involved in plastid biology and/or photosynthesis, even if genes are located in recombination poor regions of the genome. As a proof of concept, we simulated the prediction of a candidate gene for the recently cloned variegation mutant albostrians (HvAST/HvCMF7) and adopted the approach for suggesting HvClpC1 as candidate gene for the yellow-green variegation mutant luteostrians.
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Affiliation(s)
- Mingjiu Li
- Leibniz Institute of Plant Genetics and Crop Plant Research (IPK), Seeland, Germany
| | - Ganggang Guo
- Institute of Crop Sciences, Chinese Academy of Agricultural Sciences, Beijing, China
| | - Hélène Pidon
- Leibniz Institute of Plant Genetics and Crop Plant Research (IPK), Seeland, Germany
| | - Michael Melzer
- Leibniz Institute of Plant Genetics and Crop Plant Research (IPK), Seeland, Germany
| | - Alberto R. Prina
- Institute of Genetics ‘Ewald A. Favret’ (IGEAF), INTA CICVyA/Argentina, Hurlingham, Buenos Aires, Argentina
| | - Thomas Börner
- Molecular Genetics, Institute of Biology, Humboldt University, Berlin, Germany
| | - Nils Stein
- Leibniz Institute of Plant Genetics and Crop Plant Research (IPK), Seeland, Germany
- Center for Integrated Breeding Research (CiBreed), Department of Crop Sciences, Georg-August-University, Göttingen, Germany
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8
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Yuan H, Pawlowski EG, Yang Y, Sun T, Thannhauser TW, Mazourek M, Schnell D, Li L. Arabidopsis ORANGE protein regulates plastid pre-protein import through interacting with Tic proteins. JOURNAL OF EXPERIMENTAL BOTANY 2021; 72:1059-1072. [PMID: 33165598 DOI: 10.1093/jxb/eraa528] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/28/2020] [Accepted: 10/30/2020] [Indexed: 05/19/2023]
Abstract
Chloroplast-targeted proteins are actively imported into chloroplasts via the machinery spanning the double-layered membranes of chloroplasts. While the key translocons at the outer (TOC) and inner (TIC) membranes of chloroplasts are defined, proteins that interact with the core components to facilitate pre-protein import are continuously being discovered. A DnaJ-like chaperone ORANGE (OR) protein is known to regulate carotenoid biosynthesis as well as plastid biogenesis and development. In this study, we found that OR physically interacts with several Tic proteins including Tic20, Tic40, and Tic110 in the classic TIC core complex of the chloroplast import machinery. Knocking out or and its homolog or-like greatly affects the import efficiency of some photosynthetic and non-photosynthetic pre-proteins. Consistent with the direct interactions of OR with Tic proteins, the binding efficiency assay revealed that the effect of OR occurs at translocation at the inner envelope membrane (i.e. at the TIC complex). OR is able to reduce the Tic40 protein turnover rate through its chaperone activity. Moreover, OR was found to interfere with the interaction between Tic40 and Tic110, and reduces the binding of pre-proteins to Tic110 in aiding their release for translocation and processing. Our findings suggest that OR plays a new and regulatory role in stabilizing key translocons and in facilitating the late stage of plastid pre-protein translocation to regulate plastid pre-protein import.
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Affiliation(s)
- Hui Yuan
- Robert W. Holley Center for Agriculture and Health, USDA-ARS, Cornell University, Ithaca, NY, USA
- Plant Breeding and Genetics Section, School of Integrative Plant Science, Cornell University, Ithaca, NY, USA
| | - Emily G Pawlowski
- Department of Plant Biology, Michigan State University, East Lansing, MI, USA
| | - Yong Yang
- Robert W. Holley Center for Agriculture and Health, USDA-ARS, Cornell University, Ithaca, NY, USA
| | - Tianhu Sun
- Robert W. Holley Center for Agriculture and Health, USDA-ARS, Cornell University, Ithaca, NY, USA
- Plant Breeding and Genetics Section, School of Integrative Plant Science, Cornell University, Ithaca, NY, USA
| | - Theodore W Thannhauser
- Robert W. Holley Center for Agriculture and Health, USDA-ARS, Cornell University, Ithaca, NY, USA
| | - Michael Mazourek
- Plant Breeding and Genetics Section, School of Integrative Plant Science, Cornell University, Ithaca, NY, USA
| | - Danny Schnell
- Department of Plant Biology, Michigan State University, East Lansing, MI, USA
| | - Li Li
- Robert W. Holley Center for Agriculture and Health, USDA-ARS, Cornell University, Ithaca, NY, USA
- Plant Breeding and Genetics Section, School of Integrative Plant Science, Cornell University, Ithaca, NY, USA
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9
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Recombinant expression, purification and SAXS analysis of Arabidopsis thaliana ClpC1. Int J Biol Macromol 2020; 167:1273-1280. [PMID: 33189753 DOI: 10.1016/j.ijbiomac.2020.11.081] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/05/2020] [Revised: 11/10/2020] [Accepted: 11/11/2020] [Indexed: 11/23/2022]
Abstract
Caseinolytic protease-associated chaperones (Clp chaperones) are HSP100 proteins belonging to the family of ATPases having diverse cellular functions, and they occur in various organisms ranging from bacteria to plants and mammals. Most Clp chaperones have a hexameric organization and associate with tetradecameric Clp proteases to recognize and unfold protein substrates that get degraded within the cellular milieu. Vascular plants have a diverse family of Clp chaperones compared to other organisms; wherein, the chloroplasts of Arabidopsis thaliana alone contain four distinct Clp chaperones, such as ClpC1, ClpC2, ClpD, and ClpB3. The paralogs AtClpC1 and AtClpC2 are more than 90% identical, though the extent of functional overlap between the two is not clear. Moreover, in vitro characterization reports are available only for AtClpC2, as AtClpC1 could not be expressed in recombinant form in the past. Herein, using a bacterial expression system, we have successfully expressed and purified AtClpC1 with a short N-terminal truncation, employing a three-step chromatographic purification strategy. We show that AtClpC1 exists as a hexamer in the presence of ATP and MgCl2, as known for other functional Clp chaperones. Further, our SAXS analyses provide a low-resolution envelope structure for the hexameric AtClpC1, which very well fits a ClpC hexamer model.
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10
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Structural and Functional Heat Stress Responses of Chloroplasts of Arabidopsis thaliana. Genes (Basel) 2020; 11:genes11060650. [PMID: 32545654 PMCID: PMC7349189 DOI: 10.3390/genes11060650] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/18/2020] [Revised: 06/04/2020] [Accepted: 06/08/2020] [Indexed: 11/17/2022] Open
Abstract
Temperature elevations constitute a major threat to plant performance. In recent years, much was learned about the general molecular mode of heat stress reaction of plants. The current research focuses on the integration of the knowledge into more global networks, including the reactions of cellular compartments. For instance, chloroplast function is central for plant growth and survival, and the performance of chloroplasts is tightly linked to the general status of the cell and vice versa. We examined the changes in photosynthesis, chloroplast morphology and proteomic composition posed in Arabidopsis thaliana chloroplasts after a single or repetitive heat stress treatment over a period of two weeks. We observed that the acclimation is potent in the case of repetitive application of heat stress, while a single stress results in lasting alterations. Moreover, the physiological capacity and its adjustment are dependent on the efficiency of the protein translocation process as judged from the analysis of mutants of the two receptor units of the chloroplast translocon, TOC64, and TOC33. In response to repetitive heat stress, plants without TOC33 accumulate Hsp70 proteins and plants without TOC64 have a higher content of proteins involved in thylakoid structure determination when compared to wild-type plants.
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11
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Protein import into chloroplasts and its regulation by the ubiquitin-proteasome system. Biochem Soc Trans 2020; 48:71-82. [PMID: 31922184 PMCID: PMC7054747 DOI: 10.1042/bst20190274] [Citation(s) in RCA: 27] [Impact Index Per Article: 6.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/15/2019] [Revised: 12/19/2019] [Accepted: 12/23/2019] [Indexed: 02/08/2023]
Abstract
Chloroplasts are photosynthetic plant organelles descended from a bacterial ancestor. The vast majority of chloroplast proteins are synthesized in the cytosol and then imported into the chloroplast post-translationally. Translocation complexes exist in the organelle's outer and inner envelope membranes (termed TOC and TIC, respectively) to facilitate protein import. These systems recognize chloroplast precursor proteins and mediate their import in an energy-dependent manner. However, many unanswered questions remain regarding mechanistic details of the import process and the participation and functions of individual components; for example, the cytosolic events that mediate protein delivery to chloroplasts, the composition of the TIC apparatus, and the nature of the protein import motor all require resolution. The flux of proteins through TOC and TIC varies greatly throughout development and in response to specific environmental cues. The import process is, therefore, tightly regulated, and it has emerged that the ubiquitin-proteasome system (UPS) plays a key role in this regard, acting at several different steps in the process. The UPS is involved in: the selective degradation of transcription factors that co-ordinate the expression of chloroplast precursor proteins; the removal of unimported chloroplast precursor proteins in the cytosol; the inhibition of chloroplast biogenesis pre-germination; and the reconfiguration of the TOC apparatus in response to developmental and environmental signals in a process termed chloroplast-associated protein degradation. In this review, we highlight recent advances in our understanding of protein import into chloroplasts and how this process is regulated by the UPS.
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12
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Li HM, Schnell D, Theg SM. Protein Import Motors in Chloroplasts: On the Role of Chaperones. THE PLANT CELL 2020; 32:536-542. [PMID: 31932485 PMCID: PMC7054032 DOI: 10.1105/tpc.19.00300] [Citation(s) in RCA: 9] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/25/2019] [Revised: 08/01/2019] [Accepted: 01/08/2020] [Indexed: 05/09/2023]
Affiliation(s)
- Hsou-Min Li
- Institute of Molecular Biology Academia Sinica Taipei 11529, Taiwan
| | - Danny Schnell
- Department of Plant Biology Michigan State University East Lansing, Michigan 48824
| | - Steven M Theg
- Department of Plant Biology University of California Davis, California 95616
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Ali MS, Baek KH. Co-Suppression of NbClpC1 and NbClpC2, Encoding Clp Protease Chaperons, Elicits Significant Changes in the Metabolic Profile of Nicotiana benthamiana. PLANTS 2020; 9:plants9020259. [PMID: 32085404 PMCID: PMC7076384 DOI: 10.3390/plants9020259] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 01/18/2020] [Revised: 02/10/2020] [Accepted: 02/15/2020] [Indexed: 12/30/2022]
Abstract
Metabolites in plants are the products of cellular metabolic processes, and their differential amount can be regarded as the final responses of plants to genetic, epigenetic, or environmental stresses. The Clp protease complex, composed of the chaperonic parts and degradation proteases, is the major degradation system for proteins in plastids. ClpC1 and ClpC2 are the two chaperonic proteins for the Clp protease complex and share more than 90% nucleotide and amino acid sequence similarities. In this study, we employed virus-induced gene silencing to simultaneously suppress the expression of ClpC1 and ClpC2 in Nicotiana benthamiana (NbClpC1/C2). The co-suppression of NbClpC1/C2 in N. benthamiana resulted in aberrant development, with severely chlorotic leaves and stunted growth. A comparison of the control and NbClpC1/C2 co-suppressed N. benthamiana metabolomes revealed a total of 152 metabolites identified by capillary electrophoresis time-of-flight mass spectrometry. The co-suppression of NbClpC1/C2 significantly altered the levels of metabolites in glycolysis, the tricarboxylic acid cycle, the pentose phosphate pathway, and the purine biosynthetic pathway, as well as polyamine and antioxidant metabolites. Our results show that the simultaneous suppression of ClpC1 and ClpC2 leads to aberrant morphological changes in chloroplasts and that these changes are related to changes in the contents of major metabolites acting in cellular metabolism and biosynthetic pathways.
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14
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Chloroplast Signaling Gates Thermotolerance in Arabidopsis. Cell Rep 2019; 22:1657-1665. [PMID: 29444421 PMCID: PMC5847188 DOI: 10.1016/j.celrep.2018.01.054] [Citation(s) in RCA: 55] [Impact Index Per Article: 11.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/17/2017] [Revised: 11/17/2017] [Accepted: 01/18/2018] [Indexed: 11/23/2022] Open
Abstract
Temperature is a key environmental variable influencing plant growth and survival. Protection against high temperature stress in eukaryotes is coordinated by heat shock factors (HSFs), transcription factors that activate the expression of protective chaperones such as HEAT SHOCK PROTEIN 70 (HSP70); however, the pathway by which temperature is sensed and integrated with other environmental signals into adaptive responses is not well understood. Plants are exposed to considerable diurnal variation in temperature, and we have found that there is diurnal variation in thermotolerance in Arabidopsis thaliana, with maximal thermotolerance coinciding with higher HSP70 expression during the day. In a forward genetic screen, we identified a key role for the chloroplast in controlling this response, suggesting that light-induced chloroplast signaling plays a key role. Consistent with this, we are able to globally activate binding of HSFA1a to its targets by altering redox status in planta independently of a heat shock. There is a diurnal pattern of basal thermotolerance in Arabidopsis Thermotolerance correlates with diurnal expression patterns of heat-associated genes Chloroplast mutants have greater heat shock gene expression and thermotolerance A chloroplast generated light signal gates HSFA1 and heat shock gene expression
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15
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de Luna-Valdez LA, Villaseñor-Salmerón CI, Cordoba E, Vera-Estrella R, León-Mejía P, Guevara-García AA. Functional analysis of the Chloroplast GrpE (CGE) proteins from Arabidopsis thaliana. PLANT PHYSIOLOGY AND BIOCHEMISTRY : PPB 2019; 139:293-306. [PMID: 30927692 DOI: 10.1016/j.plaphy.2019.03.027] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/08/2018] [Revised: 02/26/2019] [Accepted: 03/17/2019] [Indexed: 05/20/2023]
Abstract
The function of proteins depends on specific partners that regulate protein folding, degradation and protein-protein interactions, such partners are the chaperones and cochaperones. In chloroplasts, proteins belonging to several families of chaperones have been identified: chaperonins (Cpn60s), Hsp90s (Hsp90-5/Hsp90C), Hsp100s (Hsp93/ClpC) and Hsp70s (cpHsc70s). Several lines of evidence have demonstrated that cpHsc70 chaperones are involved in molecular processes like protein import, protein folding and oligomer formation that impact important physiological aspects in plants such as thermotolerance and thylakoid biogenesis. Despite the vast amount of data existing around the function of cpHcp70s chaperones, very little attention has been paid to the roles of DnaJ and GrpE cochaperones in the chloroplast. In this study, we performed a phylogenetic analysis of the chloroplastic GrpE (CGE) proteins from 71 species. Based on their phylogenetic relationships and on a motif enrichment analysis, we propose a classification system for land plants' CGEs, which include two independent groups with specific primary structure traits. Furthermore, using in vivo assays we determined that the two CGEs from A. thaliana (AtCGEs) complement the mutant phenotype displayed by a knockout E. coli strain defective in the bacterial grpE gene. Moreover, we determined in planta that the two AtCGEs are bona fide chloroplastic proteins, which form the essential homodimers needed to establish direct physical interactions with the cpHsc70-1 chaperone. Finally, we found evidence suggesting that AtCGE1 is involved in specific physiological phenomena in A. thaliana, such as the chloroplastic response to heat stress, and the correct oligomerization of the photosynthesis-related LHCII complex.
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Affiliation(s)
- L A de Luna-Valdez
- Departamento de Biología Molecular de Plantas, Instituto de Biotecnología, Universidad Nacional Autónoma de México, Av. Universidad 2001, Col. Chamilpa, Cuernavaca, Morelos, CP 62210, Mexico.
| | - C I Villaseñor-Salmerón
- Departamento de Biología Molecular de Plantas, Instituto de Biotecnología, Universidad Nacional Autónoma de México, Av. Universidad 2001, Col. Chamilpa, Cuernavaca, Morelos, CP 62210, Mexico.
| | - E Cordoba
- Departamento de Biología Molecular de Plantas, Instituto de Biotecnología, Universidad Nacional Autónoma de México, Av. Universidad 2001, Col. Chamilpa, Cuernavaca, Morelos, CP 62210, Mexico.
| | - R Vera-Estrella
- Departamento de Biología Molecular de Plantas, Instituto de Biotecnología, Universidad Nacional Autónoma de México, Av. Universidad 2001, Col. Chamilpa, Cuernavaca, Morelos, CP 62210, Mexico.
| | - P León-Mejía
- Departamento de Biología Molecular de Plantas, Instituto de Biotecnología, Universidad Nacional Autónoma de México, Av. Universidad 2001, Col. Chamilpa, Cuernavaca, Morelos, CP 62210, Mexico.
| | - A A Guevara-García
- Departamento de Biología Molecular de Plantas, Instituto de Biotecnología, Universidad Nacional Autónoma de México, Av. Universidad 2001, Col. Chamilpa, Cuernavaca, Morelos, CP 62210, Mexico.
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16
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Rodriguez-Concepcion M, D'Andrea L, Pulido P. Control of plastidial metabolism by the Clp protease complex. JOURNAL OF EXPERIMENTAL BOTANY 2019; 70:2049-2058. [PMID: 30576524 DOI: 10.1093/jxb/ery441] [Citation(s) in RCA: 20] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/07/2018] [Accepted: 11/29/2018] [Indexed: 05/23/2023]
Abstract
Plant metabolism is strongly dependent on plastids. Besides hosting the photosynthetic machinery, these endosymbiotic organelles synthesize starch, fatty acids, amino acids, nucleotides, tetrapyrroles, and isoprenoids. Virtually all enzymes involved in plastid-localized metabolic pathways are encoded by the nuclear genome and imported into plastids. Once there, protein quality control systems ensure proper folding of the mature forms and remove irreversibly damaged proteins. The Clp protease is the main machinery for protein degradation in the plastid stroma. Recent work has unveiled an increasing number of client proteins of this proteolytic complex in plants. Notably, a substantial proportion of these substrates are required for normal chloroplast metabolism, including enzymes involved in the production of essential tetrapyrroles and isoprenoids such as chlorophylls and carotenoids. The Clp protease complex acts in coordination with nuclear-encoded plastidial chaperones for the control of both enzyme levels and proper folding (i.e. activity). This communication involves a retrograde signaling pathway, similarly to the unfolded protein response previously characterized in mitochondria and endoplasmic reticulum. Coordinated Clp protease and chaperone activities appear to further influence other plastid processes, such as the differentiation of chloroplasts into carotenoid-accumulating chromoplasts during fruit ripening.
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Affiliation(s)
| | - Lucio D'Andrea
- Centre for Research in Agricultural Genomics (CRAG) CSIC-IRTA-UAB-UB, Barcelona, Spain
| | - Pablo Pulido
- Centre for Research in Agricultural Genomics (CRAG) CSIC-IRTA-UAB-UB, Barcelona, Spain
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17
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Kikuchi S, Asakura Y, Imai M, Nakahira Y, Kotani Y, Hashiguchi Y, Nakai Y, Takafuji K, Bédard J, Hirabayashi-Ishioka Y, Mori H, Shiina T, Nakai M. A Ycf2-FtsHi Heteromeric AAA-ATPase Complex Is Required for Chloroplast Protein Import. THE PLANT CELL 2018; 30:2677-2703. [PMID: 30309901 PMCID: PMC6305978 DOI: 10.1105/tpc.18.00357] [Citation(s) in RCA: 100] [Impact Index Per Article: 16.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/07/2018] [Revised: 08/16/2018] [Accepted: 10/01/2018] [Indexed: 05/20/2023]
Abstract
Chloroplasts import thousands of nucleus-encoded preproteins synthesized in the cytosol through the TOC and TIC translocons on the outer and inner envelope membranes, respectively. Preprotein translocation across the inner membrane requires ATP; however, the import motor has remained unclear. Here, we report that a 2-MD heteromeric AAA-ATPase complex associates with the TIC complex and functions as the import motor, directly interacting with various translocating preproteins. This 2-MD complex consists of a protein encoded by the previously enigmatic chloroplast gene ycf2 and five related nuclear-encoded FtsH-like proteins, namely, FtsHi1, FtsHi2, FtsHi4, FtsHi5, and FtsH12. These components are each essential for plant viability and retain the AAA-type ATPase domain, but only FtsH12 contains the zinc binding active site generally conserved among FtsH-type metalloproteases. Furthermore, even the FtsH12 zinc binding site is dispensable for its essential function. Phylogenetic analyses suggest that all AAA-type members of the Ycf2/FtsHi complex including Ycf2 evolved from the chloroplast-encoded membrane-bound AAA-protease FtsH of the ancestral endosymbiont. The Ycf2/FtsHi complex also contains an NAD-malate dehydrogenase, a proposed key enzyme for ATP production in chloroplasts in darkness or in nonphotosynthetic plastids. These findings advance our understanding of this ATP-driven protein translocation system that is unique to the green lineage of photosynthetic eukaryotes.
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Affiliation(s)
- Shingo Kikuchi
- Institute for Protein Research, Osaka University, Osaka 565-0871, Japan
| | - Yukari Asakura
- Institute for Protein Research, Osaka University, Osaka 565-0871, Japan
| | - Midori Imai
- Institute for Protein Research, Osaka University, Osaka 565-0871, Japan
| | - Yoichi Nakahira
- College of Agriculture, Ibaraki University, Ami-cho, Inashiki, Ibaraki 300-0393, Japan
| | - Yoshiko Kotani
- Institute for Protein Research, Osaka University, Osaka 565-0871, Japan
| | - Yasuyuki Hashiguchi
- Department of Biology, Osaka Medical College, Takatsuki, Osaka 569-8686, Japan
| | - Yumi Nakai
- Department of Biochemistry, Osaka Medical College, Takatsuki, Osaka 569-8686, Japan
| | - Kazuaki Takafuji
- CoMIT Omics Center, Graduate School of Medicine, Osaka University, Osaka 565-0871, Japan
| | - Jocelyn Bédard
- Institute for Protein Research, Osaka University, Osaka 565-0871, Japan
| | | | - Hitoshi Mori
- Graduate School of Bioagricultural Sciences, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8601, Japan
| | - Takashi Shiina
- School of Human Environment Science, Kyoto Prefectural University, Sakyo-ku, Kyoto 606-8522, Japan
| | - Masato Nakai
- Institute for Protein Research, Osaka University, Osaka 565-0871, Japan
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18
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Ji Y, Chen P, Chen J, Pennerman KK, Liang X, Yan H, Zhou S, Feng G, Wang C, Yin G, Zhang X, Hu Y, Huang L. Combinations of Small RNA, RNA, and Degradome Sequencing Uncovers the Expression Pattern of microRNA⁻mRNA Pairs Adapting to Drought Stress in Leaf and Root of Dactylis glomerata L. Int J Mol Sci 2018; 19:E3114. [PMID: 30314311 PMCID: PMC6213654 DOI: 10.3390/ijms19103114] [Citation(s) in RCA: 18] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/25/2018] [Revised: 10/05/2018] [Accepted: 10/08/2018] [Indexed: 11/17/2022] Open
Abstract
Drought stress is a global problem, and the lack of water is a key factor that leads to agricultural shortages. MicroRNAs play a crucial role in the plant drought stress response; however, the microRNAs and their targets involved in drought response have not been well elucidated. In the present study, we used Illumina platform (https://www.illumina.com/) and combined data from miRNA, RNA, and degradome sequencing to explore the drought- and organ-specific miRNAs in orchardgrass (Dactylis glomerata L.) leaf and root. We aimed to find potential miRNA⁻mRNA regulation patterns responding to drought conditions. In total, 519 (486 conserved and 33 novel) miRNAs were identified, of which, 41 miRNAs had significant differential expression among the comparisons (p < 0.05). We also identified 55,366 unigenes by RNA-Seq, where 12,535 unigenes were differently expressed. Finally, our degradome analysis revealed that 5950 transcripts were targeted by 487 miRNAs. A correlation analysis identified that miRNA ata-miR164c-3p and its target heat shock protein family A (HSP70) member 5 gene comp59407_c0 (BIPE3) may be essential in organ-specific plant drought stress response and/or adaptation in orchardgrass. Additionally, Gene ontology (GO) and Kyoto encyclopedia of genes and genomes (KEGG) analyses found that "antigen processing and presentation" was the most enriched downregulated pathway in adaptation to drought conditions. Taken together, we explored the genes and miRNAs that may be involved in drought adaptation of orchardgrass and identified how they may be regulated. These results serve as a valuable genetic resource for future studies focusing on how plants adapted to drought conditions.
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Affiliation(s)
- Yang Ji
- Sichuan Animal Science Academy, Chengdu 610066, China.
| | - Peilin Chen
- Department of Grassland Science, Faculty of Animal Science and Technology, Sichuan Agricultural University, Chengdu 611130, China.
| | - Jing Chen
- Department of Grassland Science, Faculty of Animal Science and Technology, Sichuan Agricultural University, Chengdu 611130, China.
| | - Kayla K Pennerman
- Department of Plant Biology, Rutgers, The State University of New Jersey, New Brunswick, NJ 08901, USA.
| | - Xiaoyu Liang
- Sichuan Animal Science Academy, Chengdu 610066, China.
| | - Haidong Yan
- Department of Horticulture, Virginia Tech, Blacksburg, VA 24061, USA.
| | - Sifan Zhou
- Department of Grassland Science, Faculty of Animal Science and Technology, Sichuan Agricultural University, Chengdu 611130, China.
| | - Guangyan Feng
- Department of Grassland Science, Faculty of Animal Science and Technology, Sichuan Agricultural University, Chengdu 611130, China.
| | - Chengran Wang
- Department of Grassland Science, Faculty of Animal Science and Technology, Sichuan Agricultural University, Chengdu 611130, China.
| | - Guohua Yin
- Department of Plant Biology, Rutgers, The State University of New Jersey, New Brunswick, NJ 08901, USA.
| | - Xinquan Zhang
- Department of Grassland Science, Faculty of Animal Science and Technology, Sichuan Agricultural University, Chengdu 611130, China.
| | - Yuanbin Hu
- Sichuan Animal Science Academy, Chengdu 610066, China.
| | - Linkai Huang
- Department of Grassland Science, Faculty of Animal Science and Technology, Sichuan Agricultural University, Chengdu 611130, China.
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19
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Zhang S, Zhang H, Xia Y, Xiong L. The caseinolytic protease complex component CLPC1 in Arabidopsis maintains proteome and RNA homeostasis in chloroplasts. BMC PLANT BIOLOGY 2018; 18:192. [PMID: 30208840 PMCID: PMC6136230 DOI: 10.1186/s12870-018-1396-0] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/01/2018] [Accepted: 08/27/2018] [Indexed: 05/18/2023]
Abstract
BACKGROUND Homeostasis of the proteome is critical to the development of chloroplasts and also affects the expression of certain nuclear genes. CLPC1 facilitates the translocation of chloroplast pre-proteins and mediates protein degradation. RESULTS We found that proteins involved in photosynthesis are dramatically decreased in their abundance in the clpc1 mutant, whereas many proteins involved in chloroplast transcription and translation were increased in the mutant. Expression of the full-length CLPC1 protein, but not of the N-terminus-deleted CLPC1 (ΔN), in the clpc1 mutant background restored the normal levels of most of these proteins. Interestingly, the ΔN complementation line could also restore some proteins affected by the mutation to normal levels. We also found that that the clpc1 mutation profoundly affects transcript levels of chloroplast genes. Sense transcripts of many chloroplast genes are up-regulated in the clpc1 mutant. The level of SVR7, a PPR protein, was affected by the clpc1 mutation. We showed that SVR7 might be a target of CLPC1 as CLPC1-SVR7 interaction was detected through co-immunoprecipitation. CONCLUSION Our study indicates that in addition to its role in maintaining proteome homeostasis, CLPC1 and likely the CLP proteasome complex also play a role in transcriptome homeostasis through its functions in maintaining proteome homeostasis.
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Affiliation(s)
- Shoudong Zhang
- Department of Biology, Hong Kong Baptist University, Kowloon Tong, Hong Kong SAR, China
- Division of Biological and Environmental Sciences and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal, 23955-6900 Saudi Arabia
- Centre for Soybean Research, Partner State Key Laboratory of Agrobiotechnology and School of Life Sciences, The Chinese University of Hong Kong, Shatin, Hong Kong, Special Administrative Region China
| | - Huoming Zhang
- Core labs, King Abdullah University of Science and Technology (KAUST), Thuwal, 23955-6900 Saudi Arabia
| | - Yiji Xia
- Department of Biology, Hong Kong Baptist University, Kowloon Tong, Hong Kong SAR, China
- Partner State Key Laboratory of Environmental and Biological Analysis, Hong Kong Baptist University, Shatin, Hong Kong SAR, China
- Partner State Key Laboratory of Agrobiotechnology, Chinese University of Hong Kong, Shatin, Hong Kong, SAR, China
| | - Liming Xiong
- Division of Biological and Environmental Sciences and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal, 23955-6900 Saudi Arabia
- Texas A&M AgriLife Research Center, Dallas, TX 75252 USA
- Department of Horticultural Sciences, Texas A&M University, College Station, TX 77843 USA
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20
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Richardson LGL, Small EL, Inoue H, Schnell DJ. Molecular Topology of the Transit Peptide during Chloroplast Protein Import. THE PLANT CELL 2018; 30:1789-1806. [PMID: 29991536 PMCID: PMC6139696 DOI: 10.1105/tpc.18.00172] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/22/2018] [Revised: 06/26/2018] [Accepted: 07/06/2018] [Indexed: 05/04/2023]
Abstract
Chloroplast protein import is directed by the interaction of the targeting signal (transit peptide) of nucleus-encoded preproteins with translocons at the outer (TOC) and inner (TIC) chloroplast envelope membranes. Studies of the energetics and determinants of transit peptide binding have led to the hypothesis that import occurs through sequential recognition of transit peptides by components of TOC and TIC during protein import. To test this hypothesis, we employed a site-specific cross-linking approach to map transit peptide topology in relation to TOC-TIC components at specific stages of import in Arabidopsis thaliana and pea (Pisum sativum). We demonstrate that the transit peptide is in contact with Tic20 at the inner envelope in addition to TOC complex components at the earliest stages of chloroplast binding. Low levels of ATP hydrolysis catalyze the commitment of the preprotein to import by promoting further penetration across the envelope membranes and stabilizing the association of the preprotein with TOC-TIC. GTP hydrolysis at the TOC receptors serves as a checkpoint to regulate the ATP-dependent commitment of the preprotein to import and is not essential to drive preprotein import. Our results demonstrate the close cooperativity of the TOC and TIC machinery at each stage of transit peptide recognition and membrane translocation during protein import.
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Affiliation(s)
- Lynn G L Richardson
- Department of Plant Biology, Michigan State University, East Lansing, Michigan 48824
| | - Eliana L Small
- Department of Plant Biology, Michigan State University, East Lansing, Michigan 48824
| | - Hitoshi Inoue
- Department of Plant Biology, Michigan State University, East Lansing, Michigan 48824
| | - Danny J Schnell
- Department of Plant Biology, Michigan State University, East Lansing, Michigan 48824
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21
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Nakai M. New Perspectives on Chloroplast Protein Import. PLANT & CELL PHYSIOLOGY 2018; 59:1111-1119. [PMID: 29684214 DOI: 10.1093/pcp/pcy083] [Citation(s) in RCA: 52] [Impact Index Per Article: 8.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/27/2018] [Accepted: 04/13/2018] [Indexed: 05/21/2023]
Abstract
Virtually all chloroplasts in extant photosynthetic eukaryotes derive from a single endosymbiotic event that probably occurred more than a billion years ago between a host eukaryotic cell and a cyanobacterium-like ancestor. Many endosymbiont genes were subsequently transferred to the host nuclear genome, concomitant with the establishment of a system for protein transport through the chloroplast double-membrane envelope. Presently, 2,000-3,000 different nucleus-encoded chloroplast proteins must be imported into the chloroplast following their synthesis in the cytosol. The TOC (translocon at the outer envelope membrane of chloroplasts) and TIC (translocon at the inner envelope membrane of chloroplasts) complexes are protein translocation machineries at the outer and inner envelope membranes, respectively, that facilitate this chloroplast protein import with the aid of a TIC-associated ATP-driven import motor. All the essential components of this protein import system seemed to have been identified through biochemical analyses and subsequent genetic studies that initiated in the late 1990s. However, in 2013, the Nakai group reported a novel inner envelope membrane TIC complex, for which a novel ATP-driven import motor associated with this TIC complex is likely to exist. In this mini review, I will summarize these recent discoveries together with new, or reanalyzed, data presented by other groups in recent years. Whereas the precise concurrent view of chloroplast protein import is still a matter of some debate, it is anticipated that the entire TOC/TIC/ATP motor system, including any novel components, will be conclusively established in the next decade. Such findings may lead to an extensively revised view of the evolution and molecular mechanisms of chloroplast protein import.
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Affiliation(s)
- Masato Nakai
- Institute for Protein Research, Osaka University, 3-2 Yamadaoka, Suita, Osaka, 565-0871 Japan
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22
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Wu GZ, Chalvin C, Hoelscher M, Meyer EH, Wu XN, Bock R. Control of Retrograde Signaling by Rapid Turnover of GENOMES UNCOUPLED1. PLANT PHYSIOLOGY 2018; 176:2472-2495. [PMID: 29367233 PMCID: PMC5841721 DOI: 10.1104/pp.18.00009] [Citation(s) in RCA: 32] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/09/2018] [Accepted: 01/17/2018] [Indexed: 05/18/2023]
Abstract
The exchange of signals between cellular compartments coordinates development and differentiation, modulates metabolic pathways, and triggers responses to environmental conditions. The proposed central regulator of plastid-to-nucleus retrograde signaling, GENOMES UNCOUPLED1 (GUN1), is present at very low levels, which has hampered the discovery of its precise molecular function. Here, we show that the Arabidopsis (Arabidopsis thaliana) GUN1 protein accumulates to detectable levels only at very early stages of leaf development, where it functions in the regulation of chloroplast biogenesis. GUN1 mRNA is present at high levels in all tissues, but GUN1 protein undergoes rapid degradation (with an estimated half-life of ∼4 h) in all tissues where chloroplast biogenesis has been completed. The rapid turnover of GUN1 is controlled mainly by the chaperone ClpC1, suggesting degradation of GUN1 by the Clp protease. Degradation of GUN1 slows under stress conditions that alter retrograde signaling, thus ensuring that the plant has sufficient GUN1 protein. We also find that the pentatricopeptide repeat motifs of GUN1 are important determinants of GUN1 stability. Moreover, overexpression of GUN1 causes an early flowering phenotype, suggesting a function of GUN1 in developmental phase transitions beyond chloroplast biogenesis. Taken together, our results provide new insight into the regulation of GUN1 by proteolytic degradation, uncover its function in early chloroplast biogenesis, and suggest a role in developmental phase transitions.
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Affiliation(s)
- Guo-Zhang Wu
- Max-Planck-Institut für Molekulare Pflanzenphysiologie, D-14476 Potsdam-Golm, Germany
| | - Camille Chalvin
- Max-Planck-Institut für Molekulare Pflanzenphysiologie, D-14476 Potsdam-Golm, Germany
| | - Matthijs Hoelscher
- Max-Planck-Institut für Molekulare Pflanzenphysiologie, D-14476 Potsdam-Golm, Germany
| | - Etienne H Meyer
- Max-Planck-Institut für Molekulare Pflanzenphysiologie, D-14476 Potsdam-Golm, Germany
| | - Xu Na Wu
- Department of Plant Systems Biology, University of Hohenheim, 70599 Stuttgart, Germany
| | - Ralph Bock
- Max-Planck-Institut für Molekulare Pflanzenphysiologie, D-14476 Potsdam-Golm, Germany
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23
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Moreno JC, Martínez-Jaime S, Schwartzmann J, Karcher D, Tillich M, Graf A, Bock R. Temporal Proteomics of Inducible RNAi Lines of Clp Protease Subunits Identifies Putative Protease Substrates. PLANT PHYSIOLOGY 2018; 176:1485-1508. [PMID: 29229697 PMCID: PMC5813558 DOI: 10.1104/pp.17.01635] [Citation(s) in RCA: 22] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/13/2017] [Accepted: 12/07/2017] [Indexed: 05/20/2023]
Abstract
The Clp protease in the chloroplasts of plant cells is a large complex composed of at least 13 nucleus-encoded subunits and one plastid-encoded subunit, which are arranged in several ring-like structures. The proteolytic P-ring and the structurally similar R-ring form the core complex that contains the proteolytic chamber. Chaperones of the HSP100 family help with substrate unfolding, and additional accessory proteins are believed to assist with Clp complex assembly and/or to promote complex stability. Although the structure and function of the Clp protease have been studied in great detail in both bacteria and chloroplasts, the identification of bona fide protease substrates has been very challenging. Knockout mutants of genes for protease subunits are of limited value, due to their often pleiotropic phenotypes and the difficulties with distinguishing primary effects (i.e. overaccumulation of proteins that represent genuine protease substrates) from secondary effects (proteins overaccumulating for other reasons). Here, we have developed a new strategy for the identification of candidate substrates of plant proteases. By combining ethanol-inducible knockdown of protease subunits with time-resolved analysis of changes in the proteome, proteins that respond immediately to reduced protease activity can be identified. In this way, secondary effects are minimized and putative protease substrates can be identified. We have applied this strategy to the Clp protease complex of tobacco (Nicotiana tabacum) and identified a set of chloroplast proteins that are likely degraded by Clp. These include several metabolic enzymes but also a small number of proteins involved in photosynthesis.
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Affiliation(s)
- Juan C Moreno
- Max-Planck-Institut für Molekulare Pflanzenphysiologie, D-14476 Potsdam-Golm, Germany
| | - Silvia Martínez-Jaime
- Max-Planck-Institut für Molekulare Pflanzenphysiologie, D-14476 Potsdam-Golm, Germany
| | - Joram Schwartzmann
- Max-Planck-Institut für Molekulare Pflanzenphysiologie, D-14476 Potsdam-Golm, Germany
| | - Daniel Karcher
- Max-Planck-Institut für Molekulare Pflanzenphysiologie, D-14476 Potsdam-Golm, Germany
| | - Michael Tillich
- Max-Planck-Institut für Molekulare Pflanzenphysiologie, D-14476 Potsdam-Golm, Germany
| | - Alexander Graf
- Max-Planck-Institut für Molekulare Pflanzenphysiologie, D-14476 Potsdam-Golm, Germany
| | - Ralph Bock
- Max-Planck-Institut für Molekulare Pflanzenphysiologie, D-14476 Potsdam-Golm, Germany
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24
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Welsch R, Zhou X, Yuan H, Álvarez D, Sun T, Schlossarek D, Yang Y, Shen G, Zhang H, Rodriguez-Concepcion M, Thannhauser TW, Li L. Clp Protease and OR Directly Control the Proteostasis of Phytoene Synthase, the Crucial Enzyme for Carotenoid Biosynthesis in Arabidopsis. MOLECULAR PLANT 2018; 11:149-162. [PMID: 29155321 DOI: 10.1016/j.molp.2017.11.003] [Citation(s) in RCA: 74] [Impact Index Per Article: 12.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/09/2017] [Revised: 10/01/2017] [Accepted: 11/10/2017] [Indexed: 05/17/2023]
Abstract
Phytoene synthase (PSY) is the crucial plastidial enzyme in the carotenoid biosynthetic pathway. However, its post-translational regulation remains elusive. Likewise, Clp protease constitutes a central part of the plastid protease network, but its substrates for degradation are not well known. In this study, we report that PSY is a substrate of the Clp protease. PSY was uncovered to physically interact with various Clp protease subunits (i.e., ClpS1, ClpC1, and ClpD). High levels of PSY and several other carotenogenic enzyme proteins overaccumulate in the clpc1, clpp4, and clpr1-2 mutants. The overaccumulated PSY was found to be partially enzymatically active. Impairment of Clp activity in clpc1 results in a reduced rate of PSY protein turnover, further supporting the role of Clp protease in degrading PSY protein. On the other hand, the ORANGE (OR) protein, a major post-translational regulator of PSY with holdase chaperone activity, enhances PSY protein stability and increases the enzymatically active proportion of PSY in clpc1, counterbalancing Clp-mediated proteolysis in maintaining PSY protein homeostasis. Collectively, these findings provide novel insights into the quality control of plastid-localized proteins and establish a hitherto unidentified post-translational regulatory mechanism of carotenogenic enzymes in modulating carotenoid biosynthesis in plants.
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Affiliation(s)
- Ralf Welsch
- University of Freiburg, Faculty of Biology II, 79104 Freiburg, Germany.
| | - Xiangjun Zhou
- Robert W. Holley Center for Agriculture and Health, USDA-ARS, Cornell University, Ithaca, NY 14853, USA; Plant Breeding and Genetics Section, School of Integrative Plant Science, Cornell University, Ithaca, NY 14853, USA
| | - Hui Yuan
- Robert W. Holley Center for Agriculture and Health, USDA-ARS, Cornell University, Ithaca, NY 14853, USA; Plant Breeding and Genetics Section, School of Integrative Plant Science, Cornell University, Ithaca, NY 14853, USA
| | - Daniel Álvarez
- University of Freiburg, Faculty of Biology II, 79104 Freiburg, Germany
| | - Tianhu Sun
- Robert W. Holley Center for Agriculture and Health, USDA-ARS, Cornell University, Ithaca, NY 14853, USA; Plant Breeding and Genetics Section, School of Integrative Plant Science, Cornell University, Ithaca, NY 14853, USA
| | | | - Yong Yang
- Robert W. Holley Center for Agriculture and Health, USDA-ARS, Cornell University, Ithaca, NY 14853, USA
| | - Guoxin Shen
- Zhejiang Academy of Agricultural Sciences, Hangzhou 310021, China
| | - Hong Zhang
- Department of Biological Sciences, Texas Tech University, Lubbock, TX 79409, USA
| | - Manuel Rodriguez-Concepcion
- Centre for Research in Agricultural Genomics (CRAG) CSIC-IRTA-UAB-UB, Campus UAB Bellaterra, Barcelona, Spain
| | - Theodore W Thannhauser
- Robert W. Holley Center for Agriculture and Health, USDA-ARS, Cornell University, Ithaca, NY 14853, USA
| | - Li Li
- Robert W. Holley Center for Agriculture and Health, USDA-ARS, Cornell University, Ithaca, NY 14853, USA; Plant Breeding and Genetics Section, School of Integrative Plant Science, Cornell University, Ithaca, NY 14853, USA.
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25
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Abstract
The plastids, including chloroplasts, are a group of interrelated organelles that confer photoautotrophic growth and the unique metabolic capabilities that are characteristic of plant systems. Plastid biogenesis relies on the expression, import, and assembly of thousands of nuclear encoded preproteins. Plastid proteomes undergo rapid remodeling in response to developmental and environmental signals to generate functionally distinct plastid types in specific cells and tissues. In this review, we will highlight the central role of the plastid protein import system in regulating and coordinating the import of functionally related sets of preproteins that are required for plastid-type transitions and maintenance.
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Bédard J, Trösch R, Wu F, Ling Q, Flores-Pérez Ú, Töpel M, Nawaz F, Jarvis P. Suppressors of the Chloroplast Protein Import Mutant tic40 Reveal a Genetic Link between Protein Import and Thylakoid Biogenesis. THE PLANT CELL 2017; 29:1726-1747. [PMID: 28684427 PMCID: PMC5559741 DOI: 10.1105/tpc.16.00962] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/04/2017] [Revised: 06/02/2017] [Accepted: 07/02/2017] [Indexed: 05/23/2023]
Abstract
To extend our understanding of chloroplast protein import and the role played by the import machinery component Tic40, we performed a genetic screen for suppressors of chlorotic tic40 knockout mutant Arabidopsis thaliana plants. As a result, two suppressor of tic40 loci, stic1 and stic2, were identified and characterized. The stic1 locus corresponds to the gene ALBINO4 (ALB4), which encodes a paralog of the well-known thylakoid protein targeting factor ALB3. The stic2 locus identified a previously unknown stromal protein that interacts physically with both ALB4 and ALB3. Genetic studies showed that ALB4 and STIC2 act together in a common pathway that also involves cpSRP54 and cpFtsY. Thus, we conclude that ALB4 and STIC2 both participate in thylakoid protein targeting, potentially for a specific subset of thylakoidal proteins, and that this targeting pathway becomes disadvantageous to the plant in the absence of Tic40. As the stic1 and stic2 mutants both suppressed tic40 specifically (other TIC-related mutants were not suppressed), we hypothesize that Tic40 is a multifunctional protein that, in addition to its originally described role in protein import, is able to influence downstream processes leading to thylakoid biogenesis.
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Affiliation(s)
- Jocelyn Bédard
- Department of Plant Sciences, University of Oxford, Oxford OX1 3RB, United Kingdom
- Department of Biology, University of Leicester, Leicester LE1 7RH, United Kingdom
| | - Raphael Trösch
- Department of Biology, University of Leicester, Leicester LE1 7RH, United Kingdom
| | - Feijie Wu
- Department of Biology, University of Leicester, Leicester LE1 7RH, United Kingdom
| | - Qihua Ling
- Department of Plant Sciences, University of Oxford, Oxford OX1 3RB, United Kingdom
- Department of Biology, University of Leicester, Leicester LE1 7RH, United Kingdom
| | - Úrsula Flores-Pérez
- Department of Plant Sciences, University of Oxford, Oxford OX1 3RB, United Kingdom
- Department of Biology, University of Leicester, Leicester LE1 7RH, United Kingdom
| | - Mats Töpel
- Department of Biology, University of Leicester, Leicester LE1 7RH, United Kingdom
| | - Fahim Nawaz
- Department of Plant Sciences, University of Oxford, Oxford OX1 3RB, United Kingdom
| | - Paul Jarvis
- Department of Plant Sciences, University of Oxford, Oxford OX1 3RB, United Kingdom
- Department of Biology, University of Leicester, Leicester LE1 7RH, United Kingdom
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27
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Pulido P, Llamas E, Rodriguez-Concepcion M. Both Hsp70 chaperone and Clp protease plastidial systems are required for protection against oxidative stress. PLANT SIGNALING & BEHAVIOR 2017; 12:e1290039. [PMID: 28277974 PMCID: PMC5399908 DOI: 10.1080/15592324.2017.1290039] [Citation(s) in RCA: 23] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/15/2016] [Revised: 01/26/2017] [Accepted: 01/27/2017] [Indexed: 05/25/2023]
Abstract
Environmental stress conditions such as high light, extreme temperatures, salinity or drought trigger oxidative stress and eventually protein misfolding in plants. In chloroplasts, chaperone systems refold proteins after stress, while proteases degrade misfolded and aggregated proteins that cannot be refolded. We observed that reduced activity of chloroplast Hsp70 chaperone or Clp protease systems both prevented growth of Arabidopsis thaliana seedlings after treatment with the oxidative agent methyl viologen. Besides showing a role for these particular protein quality control components on the protection against oxidative stress, we provide evidence supporting the existence of a yet undiscovered pathway for Clp-mediated degradation of the damaged proteins.
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Affiliation(s)
- Pablo Pulido
- Centre for Research in Agricultural Genomics (CRAG) CSIC-IRTA-UAB-UB, Campus UAB Bellaterra, Barcelona, Spain
| | - Ernesto Llamas
- Centre for Research in Agricultural Genomics (CRAG) CSIC-IRTA-UAB-UB, Campus UAB Bellaterra, Barcelona, Spain
| | - Manuel Rodriguez-Concepcion
- Centre for Research in Agricultural Genomics (CRAG) CSIC-IRTA-UAB-UB, Campus UAB Bellaterra, Barcelona, Spain
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28
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Nishimura K, Kato Y, Sakamoto W. Essentials of Proteolytic Machineries in Chloroplasts. MOLECULAR PLANT 2017; 10:4-19. [PMID: 27585878 DOI: 10.1016/j.molp.2016.08.005] [Citation(s) in RCA: 66] [Impact Index Per Article: 9.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/23/2016] [Revised: 08/17/2016] [Accepted: 08/21/2016] [Indexed: 05/22/2023]
Abstract
Plastids are unique organelles that can alter their structure and function in response to environmental and developmental stimuli. Chloroplasts are one type of plastid and are the sites for various metabolic processes, including photosynthesis. For optimal photosynthetic activity, the chloroplast proteome must be properly shaped and maintained through regulated proteolysis and protein quality control mechanisms. Enzymatic functions and activities are conferred by protein maturation processes involving consecutive proteolytic reactions. Protein abundances are optimized by the balanced protein synthesis and degradation, which is depending on the metabolic status. Malfunctioning proteins are promptly degraded. Twenty chloroplast proteolytic machineries have been characterized to date. Specifically, processing peptidases and energy-driven processive proteases are the major players in chloroplast proteome biogenesis, remodeling, and maintenance. Recently identified putative proteases are potential regulators of photosynthetic functions. Here we provide an updated, comprehensive overview of chloroplast protein degradation machineries and discuss their importance for photosynthesis. Wherever possible, we also provide structural insights into chloroplast proteases that implement regulated proteolysis of substrate proteins/peptides.
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Affiliation(s)
- Kenji Nishimura
- Institute of Plant Science and Resources, Okayama University, 2-20-1 Chuo, Kurashiki, Okayama 710-0046, Japan
| | - Yusuke Kato
- Institute of Plant Science and Resources, Okayama University, 2-20-1 Chuo, Kurashiki, Okayama 710-0046, Japan
| | - Wataru Sakamoto
- Institute of Plant Science and Resources, Okayama University, 2-20-1 Chuo, Kurashiki, Okayama 710-0046, Japan.
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29
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Abstract
Chloroplasts are structurally complex organelles containing ~2000-3000 proteins. They are delimited by a double membrane system or envelope, have an inner aqueous compartment called the stroma, and possess a second internal membrane system called the thylakoids. Thus, determining the suborganellar location of a chloroplast protein is vital to understanding or verifying its function. One way in which protein localization can be addressed is through fractionation. Here we present two rapid and simple methods that may be applied sequentially on the same day: (a) The isolation of intact chloroplasts from Arabidopsis thaliana plants that may be used directly (e.g., for functional studies such as protein import analysis), or for further processing as follows; (b) separation of isolated chloroplasts into three suborganellar fractions (envelope membranes, a soluble fraction containing stromal proteins, and the thylakoids). These methods are routinely used in our laboratory, and they provide a good yield of isolated chloroplasts and suborganellar fractions that can be used for various downstream applications.
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Nishimura K, Kato Y, Sakamoto W. Chloroplast Proteases: Updates on Proteolysis within and across Suborganellar Compartments. PLANT PHYSIOLOGY 2016; 171:2280-93. [PMID: 27288365 PMCID: PMC4972267 DOI: 10.1104/pp.16.00330] [Citation(s) in RCA: 88] [Impact Index Per Article: 11.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/01/2016] [Indexed: 05/08/2023]
Abstract
Chloroplasts originated from the endosymbiosis of ancestral cyanobacteria and maintain transcription and translation machineries for around 100 proteins. Most endosymbiont genes, however, have been transferred to the host nucleus, and the majority of the chloroplast proteome is composed of nucleus-encoded proteins that are biosynthesized in the cytosol and then imported into chloroplasts. How chloroplasts and the nucleus communicate to control the plastid proteome remains an important question. Protein-degrading machineries play key roles in chloroplast proteome biogenesis, remodeling, and maintenance. Research in the past few decades has revealed more than 20 chloroplast proteases, which are localized to specific suborganellar locations. In particular, two energy-dependent processive proteases of bacterial origin, Clp and FtsH, are central to protein homeostasis. Processing endopeptidases such as stromal processing peptidase and thylakoidal processing peptidase are involved in the maturation of precursor proteins imported into chloroplasts by cleaving off the amino-terminal transit peptides. Presequence peptidases and organellar oligopeptidase subsequently degrade the cleaved targeting peptides. Recent findings have indicated that not only intraplastidic but also extraplastidic processive protein-degrading systems participate in the regulation and quality control of protein translocation across the envelopes. In this review, we summarize current knowledge of the major chloroplast proteases in terms of type, suborganellar localization, and diversification. We present details of these degradation processes as case studies according to suborganellar compartment (envelope, stroma, and thylakoids). Key questions and future directions in this field are discussed.
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Affiliation(s)
- Kenji Nishimura
- Institute of Plant Science and Resources, Okayama University, Kurashiki, Okayama 710-0046, Japan
| | - Yusuke Kato
- Institute of Plant Science and Resources, Okayama University, Kurashiki, Okayama 710-0046, Japan
| | - Wataru Sakamoto
- Institute of Plant Science and Resources, Okayama University, Kurashiki, Okayama 710-0046, Japan
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31
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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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32
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Tadini L, Pesaresi P, Kleine T, Rossi F, Guljamow A, Sommer F, Mühlhaus T, Schroda M, Masiero S, Pribil M, Rothbart M, Hedtke B, Grimm B, Leister D. GUN1 Controls Accumulation of the Plastid Ribosomal Protein S1 at the Protein Level and Interacts with Proteins Involved in Plastid Protein Homeostasis. PLANT PHYSIOLOGY 2016; 170:1817-30. [PMID: 26823545 PMCID: PMC4775149 DOI: 10.1104/pp.15.02033] [Citation(s) in RCA: 84] [Impact Index Per Article: 10.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/04/2016] [Accepted: 01/27/2016] [Indexed: 05/03/2023]
Abstract
Developmental or metabolic changes in chloroplasts can have profound effects on the rest of the plant cell. Such intracellular responses are associated with signals that originate in chloroplasts and convey information on their physiological status to the nucleus, which leads to large-scale changes in gene expression (retrograde signaling). A screen designed to identify components of retrograde signaling resulted in the discovery of the so-called genomes uncoupled (gun) mutants. Genetic evidence suggests that the chloroplast protein GUN1 integrates signals derived from perturbations in plastid redox state, plastid gene expression, and tetrapyrrole biosynthesis (TPB) in Arabidopsis (Arabidopsis thaliana) seedlings, exerting biogenic control of chloroplast functions. However, the molecular mechanism by which GUN1 integrates retrograde signaling in the chloroplast is unclear. Here we show that GUN1 also operates in adult plants, contributing to operational control of chloroplasts. The gun1 mutation genetically interacts with mutations of genes for the chloroplast ribosomal proteins S1 (PRPS1) and L11. Analysis of gun1 prps1 lines indicates that GUN1 controls PRPS1 accumulation at the protein level. The GUN1 protein physically interacts with proteins involved in chloroplast protein homeostasis based on coimmunoprecipitation experiments. Furthermore, yeast two-hybrid and bimolecular fluorescence complementation experiments suggest that GUN1 might transiently interact with several TPB enzymes, including Mg-chelatase subunit D (CHLD) and two other TPB enzymes known to activate retrograde signaling. Moreover, the association of PRPS1 and CHLD with protein complexes is modulated by GUN1. These findings allow us to speculate that retrograde signaling might involve GUN1-dependent formation of protein complexes.
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Affiliation(s)
- Luca Tadini
- Department Biology I, Ludwig-Maximilians-Universität München, D-82152 Planegg-Martinsried, Germany (L.T., T.K., A.G., M.P., D.L.);Department of Biosciences, University of Milan, I-20133 Milano, Italy (P.P., F.R., S.M.);Department of Biology, Technische Universität Kaiserslautern, D-67663 Kaiserslautern, Germany (F.S., T.M., M.S.);Institute of Biology, Humboldt-University of Berlin, D-10115 Berlin, Germany (M.R., B.H., B.G.); andCopenhagen Plant Science Center, University of Copenhagen, 1871 Frederiksberg C, Denmark (D.L.)
| | - Paolo Pesaresi
- Department Biology I, Ludwig-Maximilians-Universität München, D-82152 Planegg-Martinsried, Germany (L.T., T.K., A.G., M.P., D.L.);Department of Biosciences, University of Milan, I-20133 Milano, Italy (P.P., F.R., S.M.);Department of Biology, Technische Universität Kaiserslautern, D-67663 Kaiserslautern, Germany (F.S., T.M., M.S.);Institute of Biology, Humboldt-University of Berlin, D-10115 Berlin, Germany (M.R., B.H., B.G.); andCopenhagen Plant Science Center, University of Copenhagen, 1871 Frederiksberg C, Denmark (D.L.)
| | - Tatjana Kleine
- Department Biology I, Ludwig-Maximilians-Universität München, D-82152 Planegg-Martinsried, Germany (L.T., T.K., A.G., M.P., D.L.);Department of Biosciences, University of Milan, I-20133 Milano, Italy (P.P., F.R., S.M.);Department of Biology, Technische Universität Kaiserslautern, D-67663 Kaiserslautern, Germany (F.S., T.M., M.S.);Institute of Biology, Humboldt-University of Berlin, D-10115 Berlin, Germany (M.R., B.H., B.G.); andCopenhagen Plant Science Center, University of Copenhagen, 1871 Frederiksberg C, Denmark (D.L.)
| | - Fabio Rossi
- Department Biology I, Ludwig-Maximilians-Universität München, D-82152 Planegg-Martinsried, Germany (L.T., T.K., A.G., M.P., D.L.);Department of Biosciences, University of Milan, I-20133 Milano, Italy (P.P., F.R., S.M.);Department of Biology, Technische Universität Kaiserslautern, D-67663 Kaiserslautern, Germany (F.S., T.M., M.S.);Institute of Biology, Humboldt-University of Berlin, D-10115 Berlin, Germany (M.R., B.H., B.G.); andCopenhagen Plant Science Center, University of Copenhagen, 1871 Frederiksberg C, Denmark (D.L.)
| | - Arthur Guljamow
- Department Biology I, Ludwig-Maximilians-Universität München, D-82152 Planegg-Martinsried, Germany (L.T., T.K., A.G., M.P., D.L.);Department of Biosciences, University of Milan, I-20133 Milano, Italy (P.P., F.R., S.M.);Department of Biology, Technische Universität Kaiserslautern, D-67663 Kaiserslautern, Germany (F.S., T.M., M.S.);Institute of Biology, Humboldt-University of Berlin, D-10115 Berlin, Germany (M.R., B.H., B.G.); andCopenhagen Plant Science Center, University of Copenhagen, 1871 Frederiksberg C, Denmark (D.L.)
| | - Frederik Sommer
- Department Biology I, Ludwig-Maximilians-Universität München, D-82152 Planegg-Martinsried, Germany (L.T., T.K., A.G., M.P., D.L.);Department of Biosciences, University of Milan, I-20133 Milano, Italy (P.P., F.R., S.M.);Department of Biology, Technische Universität Kaiserslautern, D-67663 Kaiserslautern, Germany (F.S., T.M., M.S.);Institute of Biology, Humboldt-University of Berlin, D-10115 Berlin, Germany (M.R., B.H., B.G.); andCopenhagen Plant Science Center, University of Copenhagen, 1871 Frederiksberg C, Denmark (D.L.)
| | - Timo Mühlhaus
- Department Biology I, Ludwig-Maximilians-Universität München, D-82152 Planegg-Martinsried, Germany (L.T., T.K., A.G., M.P., D.L.);Department of Biosciences, University of Milan, I-20133 Milano, Italy (P.P., F.R., S.M.);Department of Biology, Technische Universität Kaiserslautern, D-67663 Kaiserslautern, Germany (F.S., T.M., M.S.);Institute of Biology, Humboldt-University of Berlin, D-10115 Berlin, Germany (M.R., B.H., B.G.); andCopenhagen Plant Science Center, University of Copenhagen, 1871 Frederiksberg C, Denmark (D.L.)
| | - Michael Schroda
- Department Biology I, Ludwig-Maximilians-Universität München, D-82152 Planegg-Martinsried, Germany (L.T., T.K., A.G., M.P., D.L.);Department of Biosciences, University of Milan, I-20133 Milano, Italy (P.P., F.R., S.M.);Department of Biology, Technische Universität Kaiserslautern, D-67663 Kaiserslautern, Germany (F.S., T.M., M.S.);Institute of Biology, Humboldt-University of Berlin, D-10115 Berlin, Germany (M.R., B.H., B.G.); andCopenhagen Plant Science Center, University of Copenhagen, 1871 Frederiksberg C, Denmark (D.L.)
| | - Simona Masiero
- Department Biology I, Ludwig-Maximilians-Universität München, D-82152 Planegg-Martinsried, Germany (L.T., T.K., A.G., M.P., D.L.);Department of Biosciences, University of Milan, I-20133 Milano, Italy (P.P., F.R., S.M.);Department of Biology, Technische Universität Kaiserslautern, D-67663 Kaiserslautern, Germany (F.S., T.M., M.S.);Institute of Biology, Humboldt-University of Berlin, D-10115 Berlin, Germany (M.R., B.H., B.G.); andCopenhagen Plant Science Center, University of Copenhagen, 1871 Frederiksberg C, Denmark (D.L.)
| | - Mathias Pribil
- Department Biology I, Ludwig-Maximilians-Universität München, D-82152 Planegg-Martinsried, Germany (L.T., T.K., A.G., M.P., D.L.);Department of Biosciences, University of Milan, I-20133 Milano, Italy (P.P., F.R., S.M.);Department of Biology, Technische Universität Kaiserslautern, D-67663 Kaiserslautern, Germany (F.S., T.M., M.S.);Institute of Biology, Humboldt-University of Berlin, D-10115 Berlin, Germany (M.R., B.H., B.G.); andCopenhagen Plant Science Center, University of Copenhagen, 1871 Frederiksberg C, Denmark (D.L.)
| | - Maxi Rothbart
- Department Biology I, Ludwig-Maximilians-Universität München, D-82152 Planegg-Martinsried, Germany (L.T., T.K., A.G., M.P., D.L.);Department of Biosciences, University of Milan, I-20133 Milano, Italy (P.P., F.R., S.M.);Department of Biology, Technische Universität Kaiserslautern, D-67663 Kaiserslautern, Germany (F.S., T.M., M.S.);Institute of Biology, Humboldt-University of Berlin, D-10115 Berlin, Germany (M.R., B.H., B.G.); andCopenhagen Plant Science Center, University of Copenhagen, 1871 Frederiksberg C, Denmark (D.L.)
| | - Boris Hedtke
- Department Biology I, Ludwig-Maximilians-Universität München, D-82152 Planegg-Martinsried, Germany (L.T., T.K., A.G., M.P., D.L.);Department of Biosciences, University of Milan, I-20133 Milano, Italy (P.P., F.R., S.M.);Department of Biology, Technische Universität Kaiserslautern, D-67663 Kaiserslautern, Germany (F.S., T.M., M.S.);Institute of Biology, Humboldt-University of Berlin, D-10115 Berlin, Germany (M.R., B.H., B.G.); andCopenhagen Plant Science Center, University of Copenhagen, 1871 Frederiksberg C, Denmark (D.L.)
| | - Bernhard Grimm
- Department Biology I, Ludwig-Maximilians-Universität München, D-82152 Planegg-Martinsried, Germany (L.T., T.K., A.G., M.P., D.L.);Department of Biosciences, University of Milan, I-20133 Milano, Italy (P.P., F.R., S.M.);Department of Biology, Technische Universität Kaiserslautern, D-67663 Kaiserslautern, Germany (F.S., T.M., M.S.);Institute of Biology, Humboldt-University of Berlin, D-10115 Berlin, Germany (M.R., B.H., B.G.); andCopenhagen Plant Science Center, University of Copenhagen, 1871 Frederiksberg C, Denmark (D.L.)
| | - Dario Leister
- Department Biology I, Ludwig-Maximilians-Universität München, D-82152 Planegg-Martinsried, Germany (L.T., T.K., A.G., M.P., D.L.);Department of Biosciences, University of Milan, I-20133 Milano, Italy (P.P., F.R., S.M.);Department of Biology, Technische Universität Kaiserslautern, D-67663 Kaiserslautern, Germany (F.S., T.M., M.S.);Institute of Biology, Humboldt-University of Berlin, D-10115 Berlin, Germany (M.R., B.H., B.G.); andCopenhagen Plant Science Center, University of Copenhagen, 1871 Frederiksberg C, Denmark (D.L.)
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33
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Bionda T, Gross LE, Becker T, Papasotiriou DG, Leisegang MS, Karas M, Schleiff E. Eukaryotic Hsp70 chaperones in the intermembrane space of chloroplasts. PLANTA 2016; 243:733-47. [PMID: 26669598 DOI: 10.1007/s00425-015-2440-z] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/15/2015] [Accepted: 11/27/2015] [Indexed: 06/05/2023]
Abstract
MAIN CONCLUSION Multiple eukaryotic Hsp70 typically localized in the cytoplasm are also distributed to the intermembrane space of chloroplasts and might thereby represent the missing link in energizing protein translocation. Protein translocation into organelles is a central cellular process that is tightly regulated. It depends on signals within the preprotein and on molecular machines catalyzing the process. Molecular chaperones participate in transport and translocation of preproteins into organelles to control folding and to provide energy for the individual steps. While most of the processes are explored and the components are identified, the transfer of preproteins into and across the intermembrane space of chloroplasts is not yet understood. The existence of an energy source in this compartment is discussed, because the required transit peptide length for successful translocation into chloroplasts is shorter than that found for mitochondria where energy is provided exclusively by matrix chaperones. Furthermore, a cytosolic-type Hsp70 homologue was proposed as component of the chloroplast translocon in the intermembrane space energizing the initial translocation. The molecular identity of such intermembrane space localized Hsp70 remained unknown, which led to a controversy concerning its existence. We identified multiple cytosolic Hsp70s by mass spectrometry on isolated, thermolysin-treated Medicago sativa chloroplasts. The localization of these Hsp70s of M. sativa or Arabidopsis thaliana in the intermembrane space was confirmed by a self-assembly GFP-based in vivo system. The localization of cytosolic Hsp70s in the stroma of chloroplasts or different mitochondrial compartments could not be observed. Similarly, we could not identify any cytosolic Hsp90 in the intermembrane space of chloroplast. With respect to our results we discuss the possible targeting and function of the Hsp70 found in the intermembrane space.
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Affiliation(s)
- Tihana Bionda
- Molecular Cell Biology of Plants, Goethe University, Max von Laue Str. 9, 60438, Frankfurt, Germany
- Institute of Biochemistry II, Goethe University, Theodor-Stern-Kai 7, 60590, Frankfurt, Germany
| | - Lucia E Gross
- Molecular Cell Biology of Plants, Goethe University, Max von Laue Str. 9, 60438, Frankfurt, Germany
| | - Thomas Becker
- Molecular Cell Biology of Plants, Goethe University, Max von Laue Str. 9, 60438, Frankfurt, Germany
- Biochemistry and Molecular Biology, ZBMZ, and BIOSS Centre for Biological Signalling Studies, Albert-Ludwigs-University Freiburg, Stefan-Meier-Str. 17, 79104, Freiburg, Germany
| | - Dimitrios G Papasotiriou
- Pharmaceutical Chemistry, Goethe University, Max von Laue Str. 9, 60438, Frankfurt, Germany
- Syngenta Ltd., Jealott's Hill International Research Centre, Bracknell, Berkshire, RG42 6EY, UK
| | - Matthias S Leisegang
- Molecular Cell Biology of Plants, Goethe University, Max von Laue Str. 9, 60438, Frankfurt, Germany
- Institute for Cardiovascular Physiology, Goethe University, Theodor-Stern-Kai 7, 60590, Frankfurt, Germany
| | - Michael Karas
- Pharmaceutical Chemistry, Goethe University, Max von Laue Str. 9, 60438, Frankfurt, Germany
| | - Enrico Schleiff
- Molecular Cell Biology of Plants, Goethe University, Max von Laue Str. 9, 60438, Frankfurt, Germany.
- Molecular Cell Biology of Plants, Cluster of Excellence Frankfurt, Goethe University, Max von Laue Str. 9, 60438, Frankfurt, Germany.
- Buchmann Institut for Molecular Life Sciences, Max von Laue Str. 9, 60438, Frankfurt, Germany.
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Huang PK, Chan PT, Su PH, Chen LJ, Li HM. Chloroplast Hsp93 Directly Binds to Transit Peptides at an Early Stage of the Preprotein Import Process. PLANT PHYSIOLOGY 2016; 170:857-66. [PMID: 26676256 PMCID: PMC4734592 DOI: 10.1104/pp.15.01830] [Citation(s) in RCA: 21] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/23/2015] [Accepted: 12/16/2015] [Indexed: 05/20/2023]
Abstract
Three stromal chaperone ATPases, cpHsc70, Hsp90C, and Hsp93, are present in the chloroplast translocon, but none has been shown to directly bind preproteins in vivo during import, so it remains unclear whether any function as a preprotein-translocating motor and whether they have different functions during the import process. Here, using protein crosslinking followed by ionic detergent solubilization, we show that Hsp93 directly binds to the transit peptides of various preproteins undergoing active import into chloroplasts. Hsp93 also binds to the mature region of a preprotein. A time course study of import, followed by coimmunoprecipitation experiments, confirmed that Hsp93 is present in the same complexes as preproteins at an early stage when preproteins are being processed to the mature size. In contrast, cpHsc70 is present in the same complexes as preproteins at both the early stage and a later stage after the transit peptide has been removed, suggesting that cpHsc70, but not Hsp93, is important in translocating processed mature proteins across the envelope.
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Affiliation(s)
- Po-Kai Huang
- Institute of Molecular Biology, Academia Sinica, Nankang, Taipei 11529, Taiwan
| | - Po-Ting Chan
- Institute of Molecular Biology, Academia Sinica, Nankang, Taipei 11529, Taiwan
| | - Pai-Hsiang Su
- Institute of Molecular Biology, Academia Sinica, Nankang, Taipei 11529, Taiwan
| | - Lih-Jen Chen
- Institute of Molecular Biology, Academia Sinica, Nankang, Taipei 11529, Taiwan
| | - Hsou-min Li
- Institute of Molecular Biology, Academia Sinica, Nankang, Taipei 11529, Taiwan
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Pulido P, Llamas E, Llorente B, Ventura S, Wright LP, Rodríguez-Concepción M. Specific Hsp100 Chaperones Determine the Fate of the First Enzyme of the Plastidial Isoprenoid Pathway for Either Refolding or Degradation by the Stromal Clp Protease in Arabidopsis. PLoS Genet 2016; 12:e1005824. [PMID: 26815787 PMCID: PMC4729485 DOI: 10.1371/journal.pgen.1005824] [Citation(s) in RCA: 79] [Impact Index Per Article: 9.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/02/2015] [Accepted: 01/04/2016] [Indexed: 01/08/2023] Open
Abstract
The lifespan and activity of proteins depend on protein quality control systems formed by chaperones and proteases that ensure correct protein folding and prevent the formation of toxic aggregates. We previously found that the Arabidopsis thaliana J-protein J20 delivers inactive (misfolded) forms of the plastidial enzyme deoxyxylulose 5-phosphate synthase (DXS) to the Hsp70 chaperone for either proper folding or degradation. Here we show that the fate of Hsp70-bound DXS depends on pathways involving specific Hsp100 chaperones. Analysis of individual mutants for the four Hsp100 chaperones present in Arabidopsis chloroplasts showed increased levels of DXS proteins (but not transcripts) only in those defective in ClpC1 or ClpB3. However, the accumulated enzyme was active in the clpc1 mutant but inactive in clpb3 plants. Genetic evidence indicated that ClpC chaperones might be required for the unfolding of J20-delivered DXS protein coupled to degradation by the Clp protease. By contrast, biochemical and genetic approaches confirmed that Hsp70 and ClpB3 chaperones interact to collaborate in the refolding and activation of DXS. We conclude that specific J-proteins and Hsp100 chaperones act together with Hsp70 to recognize and deliver DXS to either reactivation (via ClpB3) or removal (via ClpC1) depending on the physiological status of the plastid. In this paper we report a relatively simple mechanism by which plant chloroplasts deal with inactive forms of DXS, the main rate-determining enzyme for the production of plastidial isoprenoids relevant for photosynthesis and development. We provide evidence supporting that particular members of the Hsp100 chaperone family contribute to either refold or degrade inactive DXS proteins specifically recognized by the J-protein adaptor J20 and delivered to Hsp70 chaperones. Our results also unveil a J-protein-based mechanism for substrate delivery to the Clp complex, the main protease in the chloroplast stroma. Together, this work allows a better understanding of how chloroplasts get rid of damaged DXS (and potentially other proteins), which should contribute to take more informed decisions in future approaches aimed to manipulate the levels of plastidial metabolites of interest (including vitamins, biofuels, or drugs against cancer and malaria) in crop plants.
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Affiliation(s)
- Pablo Pulido
- Centre for Research in Agricultural Genomics (CRAG) CSIC-IRTA-UAB-UB, Campus UAB Bellaterra, Barcelona, Spain
| | - Ernesto Llamas
- Centre for Research in Agricultural Genomics (CRAG) CSIC-IRTA-UAB-UB, Campus UAB Bellaterra, Barcelona, Spain
| | - Briardo Llorente
- Centre for Research in Agricultural Genomics (CRAG) CSIC-IRTA-UAB-UB, Campus UAB Bellaterra, Barcelona, Spain
| | - Salvador Ventura
- Institut de Biotecnologia i Biomedicina and Departament de Bioquímica i Biologia Molecular, Universitat Autònoma de Barcelona, Campus UAB Bellaterra, Barcelona, Spain
| | | | - Manuel Rodríguez-Concepción
- Centre for Research in Agricultural Genomics (CRAG) CSIC-IRTA-UAB-UB, Campus UAB Bellaterra, Barcelona, Spain
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Muthusamy SK, Dalal M, Chinnusamy V, Bansal KC. Differential Regulation of Genes Coding for Organelle and Cytosolic ClpATPases under Biotic and Abiotic Stresses in Wheat. FRONTIERS IN PLANT SCIENCE 2016; 7:929. [PMID: 27446158 PMCID: PMC4923199 DOI: 10.3389/fpls.2016.00929] [Citation(s) in RCA: 21] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/18/2015] [Accepted: 06/10/2016] [Indexed: 05/06/2023]
Abstract
A sub-group of class I Caseinolytic proteases (Clps) function as molecular chaperone and confer thermotolerance to plants. We identified class I Clp family consisting of five ClpB/HSP100, two ClpC, and two ClpD genes from bread wheat. Phylogenetic analysis showed that these genes were highly conserved across grass genomes. Subcellular localization prediction revealed that TaClpC and TaClpD subgroup proteins and TaClpB1 proteins are potentially targeted to chloroplast, while TaClpB5 to mitochondria, and TaClpB2, TaClpB3, and TaClpB4 to cytoplasm. Spatio-temporal expression pattern analysis revealed that four TaClpB and TaClpD2 genes are expressed in majority of all tissues and developmental stages of wheat. Real-time RT-PCR analysis of expression levels of Clp genes in seven wheat genotypes under different abiotic stresses revealed that genes coding for the cytosolic Clps namely TaClpB2 and TaClpB3 were upregulated under heat, salt and oxidative stress but were downregulated by cold stress in most genotypes. In contrast, genes coding for the chloroplastic Clps TaClpC1, TaClpC2, and TaClpD1 genes were significantly upregulated by mainly by cold stress in most genotypes, while TaClpD2 gene was upregulated >2 fold by salt stress in DBW16. The TaClpB5 gene coding for mitochondrial Clp was upregulated in all genotypes under heat, salt and oxidative stresses. In addition, we found that biotic stresses also upregulated TaClpB4 and TaClpD1. Among biotic stresses, Tilletia caries induced TaClpB2, TaClpB3, TaClpC1, and TaClpD1. Differential expression pattern under different abiotic and biotic stresses and predicted differential cellular localization of Clps suggest their non-redundant organelle and stress-specific roles. Our results also suggest the potential role of Clps in cold, salt and biotic stress responses in addition to the previously established role in thermotolerance of wheat.
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Affiliation(s)
- Senthilkumar K. Muthusamy
- ICAR-National Research Centre on Plant BiotechnologyNew Delhi, India
- Division of Crop Improvement, ICAR-Indian Institute of Wheat and Barley ResearchKarnal, India
| | - Monika Dalal
- ICAR-National Research Centre on Plant BiotechnologyNew Delhi, India
| | - Viswanathan Chinnusamy
- Division of Plant Physiology, ICAR-Indian Agricultural Research InstituteNew Delhi, India
| | - Kailash C. Bansal
- ICAR-National Research Centre on Plant BiotechnologyNew Delhi, India
- ICAR-National Bureau of Plant Genetic ResourcesNew Delhi, India
- *Correspondence: Kailash C. Bansal
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Flores-Pérez Ú, Bédard J, Tanabe N, Lymperopoulos P, Clarke AK, Jarvis P. Functional Analysis of the Hsp93/ClpC Chaperone at the Chloroplast Envelope. PLANT PHYSIOLOGY 2016; 170:147-62. [PMID: 26586836 PMCID: PMC4704595 DOI: 10.1104/pp.15.01538] [Citation(s) in RCA: 27] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/29/2015] [Accepted: 11/16/2015] [Indexed: 05/20/2023]
Abstract
The Hsp100-type chaperone Hsp93/ClpC has crucial roles in chloroplast biogenesis. In addition to its role in proteolysis in the stroma, biochemical and genetic evidence led to the hypothesis that this chaperone collaborates with the inner envelope TIC complex to power preprotein import. Recently, it was suggested that Hsp93, working together with the Clp proteolytic core, can confer a protein quality control mechanism at the envelope. Thus, the role of envelope-localized Hsp93, and the mechanism by which it participates in protein import, remain unclear. To analyze the function of Hsp93 in protein import independently of its ClpP association, we created a mutant of Hsp93 affecting its ClpP-binding motif (PBM) (Hsp93[P-]), which is essential for the chaperone's interaction with the Clp proteolytic core. The Hsp93[P-] construct was ineffective at complementing the pale-yellow phenotype of hsp93 Arabidopsis (Arabidopsis thaliana) mutants, indicating that the PBM is essential for Hsp93 function. As expected, the PBM mutation negatively affected the degradation activity of the stromal Clp protease. The mutation also disrupted association of Hsp93 with the Clp proteolytic core at the envelope, without affecting the envelope localization of Hsp93 itself or its association with the TIC machinery, which we demonstrate to be mediated by a direct interaction with Tic110. Nonetheless, Hsp93[P-] expression did not detectably improve the protein import efficiency of hsp93 mutant chloroplasts. Thus, our results do not support the proposed function of Hsp93 in protein import propulsion, but are more consistent with the notion of Hsp93 performing a quality control role at the point of import.
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Affiliation(s)
- Úrsula Flores-Pérez
- Department of Plant Sciences, University of Oxford, Oxford OX1 3RB, United Kingdom (U.F.-P., J.B., P.J.) and Department of Biological and Environmental Sciences, Gothenburg University, 405 30 Gothenburg, Sweden (N.T., P.L., A.K.C.)
| | - Jocelyn Bédard
- Department of Plant Sciences, University of Oxford, Oxford OX1 3RB, United Kingdom (U.F.-P., J.B., P.J.) and Department of Biological and Environmental Sciences, Gothenburg University, 405 30 Gothenburg, Sweden (N.T., P.L., A.K.C.)
| | - Noriaki Tanabe
- Department of Plant Sciences, University of Oxford, Oxford OX1 3RB, United Kingdom (U.F.-P., J.B., P.J.) and Department of Biological and Environmental Sciences, Gothenburg University, 405 30 Gothenburg, Sweden (N.T., P.L., A.K.C.)
| | - Panagiotis Lymperopoulos
- Department of Plant Sciences, University of Oxford, Oxford OX1 3RB, United Kingdom (U.F.-P., J.B., P.J.) and Department of Biological and Environmental Sciences, Gothenburg University, 405 30 Gothenburg, Sweden (N.T., P.L., A.K.C.)
| | - Adrian K Clarke
- Department of Plant Sciences, University of Oxford, Oxford OX1 3RB, United Kingdom (U.F.-P., J.B., P.J.) and Department of Biological and Environmental Sciences, Gothenburg University, 405 30 Gothenburg, Sweden (N.T., P.L., A.K.C.)
| | - Paul Jarvis
- Department of Plant Sciences, University of Oxford, Oxford OX1 3RB, United Kingdom (U.F.-P., J.B., P.J.) and Department of Biological and Environmental Sciences, Gothenburg University, 405 30 Gothenburg, Sweden (N.T., P.L., A.K.C.)
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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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Trösch R, Mühlhaus T, Schroda M, Willmund F. ATP-dependent molecular chaperones in plastids--More complex than expected. BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 2015; 1847:872-88. [PMID: 25596449 DOI: 10.1016/j.bbabio.2015.01.002] [Citation(s) in RCA: 51] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [Key Words] [Subscribe] [Scholar Register] [Received: 10/31/2014] [Revised: 01/03/2015] [Accepted: 01/08/2015] [Indexed: 11/27/2022]
Abstract
Plastids are a class of essential plant cell organelles comprising photosynthetic chloroplasts of green tissues, starch-storing amyloplasts of roots and tubers or the colorful pigment-storing chromoplasts of petals and fruits. They express a few genes encoded on their organellar genome, called plastome, but import most of their proteins from the cytosol. The import into plastids, the folding of freshly-translated or imported proteins, the degradation or renaturation of denatured and entangled proteins, and the quality-control of newly folded proteins all require the action of molecular chaperones. Members of all four major families of ATP-dependent molecular chaperones (chaperonin/Cpn60, Hsp70, Hsp90 and Hsp100 families) have been identified in plastids from unicellular algae to higher plants. This review aims not only at giving an overview of the most current insights into the general and conserved functions of these plastid chaperones, but also into their specific plastid functions. Given that chloroplasts harbor an extreme environment that cycles between reduced and oxidized states, that has to deal with reactive oxygen species and is highly reactive to environmental and developmental signals, it can be presumed that plastid chaperones have evolved a plethora of specific functions some of which are just about to be discovered. Here, the most urgent questions that remain unsolved are discussed, and guidance for future research on plastid chaperones is given. This article is part of a Special Issue entitled: Chloroplast Biogenesis.
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Affiliation(s)
- Raphael Trösch
- TU Kaiserslautern, Molecular Biotechnology & Systems Biology, Paul-Ehrlich-Straße 23, 67663 Kaiserslautern, Germany; HU Berlin, Institute of Biology, Chausseestraße 117, 10115 Berlin, Germany; TU Kaiserslautern, Molecular Genetics of Eukaryotes, Paul-Ehrlich-Straße 23, 67663 Kaiserslautern, Germany.
| | - Timo Mühlhaus
- TU Kaiserslautern, Molecular Biotechnology & Systems Biology, Paul-Ehrlich-Straße 23, 67663 Kaiserslautern, Germany.
| | - Michael Schroda
- TU Kaiserslautern, Molecular Biotechnology & Systems Biology, Paul-Ehrlich-Straße 23, 67663 Kaiserslautern, Germany.
| | - Felix Willmund
- TU Kaiserslautern, Molecular Genetics of Eukaryotes, Paul-Ehrlich-Straße 23, 67663 Kaiserslautern, Germany.
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Tapken W, Kim J, Nishimura K, van Wijk KJ, Pilon M. The Clp protease system is required for copper ion-dependent turnover of the PAA2/HMA8 copper transporter in chloroplasts. THE NEW PHYTOLOGIST 2015; 205:511-7. [PMID: 25262970 DOI: 10.1111/nph.13093] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/08/2014] [Accepted: 09/09/2014] [Indexed: 05/26/2023]
Abstract
The distribution of essential metal ions over subcellular compartments for use as cofactors requires control of membrane transporters. PAA2/HMA8 is a copper-transporting P1B -type ATPase in the thylakoid membrane, required for the maturation of plastocyanin. When copper is highly available to the plant this transporter is degraded, which implies the action of a protease. In order to identify the proteolytic machinery responsible for PAA2/HMA8 turnover in Arabidopsis, mutant lines defective in five different chloroplast protease systems were analyzed. Plants defective in the chloroplast caseinolytic protease (Clp) system were specifically impaired in PAA2/HMA8 protein turnover on media containing elevated copper concentrations. However, the abundance of a core Clp component was not directly affected by copper. Furthermore, the expression and activity of both cytosolic and chloroplast-localized superoxide dismutases (SODs), which are known to be dependent on copper, were not altered in the clp mutants, indicating that the loss of PAA2/HMA8 turnover in these lines was not caused by a lack of stromal copper. The results suggest that copper excess in the stroma triggers selection of the thylakoid-localized PAA2 transporter for degradation by the Clp protease, but not several other chloroplast proteases, and support a novel role for this proteolytic system in cellular copper homeostasis.
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Affiliation(s)
- Wiebke Tapken
- Biology Department, Colorado State University, Fort Collins, CO, 80523, USA
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Organization, function and substrates of the essential Clp protease system in plastids. BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 2014; 1847:915-30. [PMID: 25482260 DOI: 10.1016/j.bbabio.2014.11.012] [Citation(s) in RCA: 128] [Impact Index Per Article: 12.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/31/2014] [Accepted: 11/20/2014] [Indexed: 01/21/2023]
Abstract
Intra-plastid proteolysis is essential in plastid biogenesis, differentiation and plastid protein homeostasis (proteostasis). We provide a comprehensive review of the Clp protease system present in all plastid types and we draw lessons from structural and functional information of bacterial Clp systems. The Clp system plays a central role in plastid development and function, through selective removal of miss-folded, aggregated, or otherwise unwanted proteins. The Clp system consists of a tetradecameric proteolytic core with catalytically active ClpP and inactive ClpR subunits, hexameric ATP-dependent chaperones (ClpC,D) and adaptor protein(s) (ClpS1) enhancing delivery of subsets of substrates. Many structural and functional features of the plastid Clp system are now understood though extensive reverse genetics analysis combined with biochemical analysis, as well as large scale quantitative proteomics for loss-of-function mutants of Clp core, chaperone and ClpS1 subunits. Evolutionary diversification of Clp system across non-photosynthetic and photosynthetic prokaryotes and organelles is illustrated. Multiple substrates have been suggested based on their direct interaction with the ClpS1 adaptor or screening of different loss-of-function protease mutants. The main challenge is now to determine degradation signals (degrons) in Clp substrates and substrate delivery mechanisms, as well as functional interactions of Clp with other plastid proteases. This article is part of a Special Issue entitled: Chloroplast Biogenesis.
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Hemme D, Veyel D, Mühlhaus T, Sommer F, Jüppner J, Unger AK, Sandmann M, Fehrle I, Schönfelder S, Steup M, Geimer S, Kopka J, Giavalisco P, Schroda M. Systems-wide analysis of acclimation responses to long-term heat stress and recovery in the photosynthetic model organism Chlamydomonas reinhardtii. THE PLANT CELL 2014; 26:4270-97. [PMID: 25415976 PMCID: PMC4277220 DOI: 10.1105/tpc.114.130997] [Citation(s) in RCA: 77] [Impact Index Per Article: 7.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/12/2014] [Revised: 10/13/2014] [Accepted: 10/24/2014] [Indexed: 05/19/2023]
Abstract
We applied a top-down systems biology approach to understand how Chlamydomonas reinhardtii acclimates to long-term heat stress (HS) and recovers from it. For this, we shifted cells from 25 to 42°C for 24 h and back to 25°C for ≥8 h and monitored abundances of 1856 proteins/protein groups, 99 polar and 185 lipophilic metabolites, and cytological and photosynthesis parameters. Our data indicate that acclimation of Chlamydomonas to long-term HS consists of a temporally ordered, orchestrated implementation of response elements at various system levels. These comprise (1) cell cycle arrest; (2) catabolism of larger molecules to generate compounds with roles in stress protection; (3) accumulation of molecular chaperones to restore protein homeostasis together with compatible solutes; (4) redirection of photosynthetic energy and reducing power from the Calvin cycle to the de novo synthesis of saturated fatty acids to replace polyunsaturated ones in membrane lipids, which are deposited in lipid bodies; and (5) when sinks for photosynthetic energy and reducing power are depleted, resumption of Calvin cycle activity associated with increased photorespiration, accumulation of reactive oxygen species scavengers, and throttling of linear electron flow by antenna uncoupling. During recovery from HS, cells appear to focus on processes allowing rapid resumption of growth rather than restoring pre-HS conditions.
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Affiliation(s)
- Dorothea Hemme
- Molekulare Biotechnologie and Systembiologie, TU Kaiserslautern, D-67663 Kaiserslautern, Germany Max-Planck-Institut für Molekulare Pflanzenphysiologie, D-14476 Potsdam-Golm, Germany
| | - Daniel Veyel
- Molekulare Biotechnologie and Systembiologie, TU Kaiserslautern, D-67663 Kaiserslautern, Germany Max-Planck-Institut für Molekulare Pflanzenphysiologie, D-14476 Potsdam-Golm, Germany
| | - Timo Mühlhaus
- Molekulare Biotechnologie and Systembiologie, TU Kaiserslautern, D-67663 Kaiserslautern, Germany Max-Planck-Institut für Molekulare Pflanzenphysiologie, D-14476 Potsdam-Golm, Germany
| | - Frederik Sommer
- Molekulare Biotechnologie and Systembiologie, TU Kaiserslautern, D-67663 Kaiserslautern, Germany Max-Planck-Institut für Molekulare Pflanzenphysiologie, D-14476 Potsdam-Golm, Germany
| | - Jessica Jüppner
- Max-Planck-Institut für Molekulare Pflanzenphysiologie, D-14476 Potsdam-Golm, Germany
| | - Ann-Katrin Unger
- Zellbiologie/Elektronenmikroskopie, Universität Bayreuth, D-95440 Bayreuth, Germany
| | - Michael Sandmann
- Institut für Biochemie und Biologie, Universität Potsdam, D-14476 Potsdam-Golm, Germany
| | - Ines Fehrle
- Max-Planck-Institut für Molekulare Pflanzenphysiologie, D-14476 Potsdam-Golm, Germany
| | - Stephanie Schönfelder
- Max-Planck-Institut für Molekulare Pflanzenphysiologie, D-14476 Potsdam-Golm, Germany
| | - Martin Steup
- Institut für Biochemie und Biologie, Universität Potsdam, D-14476 Potsdam-Golm, Germany
| | - Stefan Geimer
- Zellbiologie/Elektronenmikroskopie, Universität Bayreuth, D-95440 Bayreuth, Germany
| | - Joachim Kopka
- Max-Planck-Institut für Molekulare Pflanzenphysiologie, D-14476 Potsdam-Golm, Germany
| | - Patrick Giavalisco
- Max-Planck-Institut für Molekulare Pflanzenphysiologie, D-14476 Potsdam-Golm, Germany
| | - Michael Schroda
- Molekulare Biotechnologie and Systembiologie, TU Kaiserslautern, D-67663 Kaiserslautern, Germany Max-Planck-Institut für Molekulare Pflanzenphysiologie, D-14476 Potsdam-Golm, Germany
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Paila YD, Richardson LGL, Schnell DJ. New insights into the mechanism of chloroplast protein import and its integration with protein quality control, organelle biogenesis and development. J Mol Biol 2014; 427:1038-1060. [PMID: 25174336 DOI: 10.1016/j.jmb.2014.08.016] [Citation(s) in RCA: 92] [Impact Index Per Article: 9.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/23/2014] [Revised: 08/20/2014] [Accepted: 08/23/2014] [Indexed: 01/04/2023]
Abstract
The translocons at the outer (TOC) and the inner (TIC) envelope membranes of chloroplasts mediate the targeting and import of several thousand nucleus-encoded preproteins that are required for organelle biogenesis and homeostasis. The cytosolic events in preprotein targeting remain largely unknown, although cytoplasmic chaperones have been proposed to facilitate delivery to the TOC complex. Preprotein recognition is mediated by the TOC GTPase receptors Toc159 and Toc34. The receptors constitute a GTP-regulated switch, which initiates membrane translocation via Toc75, a member of the Omp85 (outer membrane protein 85)/TpsB (two-partner secretion system B) family of bacterial, plastid and mitochondrial β-barrel outer membrane proteins. The TOC receptor systems have diversified to recognize distinct sets of preproteins, thereby maximizing the efficiency of targeting in response to changes in gene expression during developmental and physiological events that impact organelle function. The TOC complex interacts with the TIC translocon to allow simultaneous translocation of preproteins across the envelope. Both the two inner membrane complexes, the Tic110 and 1 MDa complexes, have been implicated as constituents of the TIC translocon, and it remains to be determined how they interact to form the TIC channel and assemble the import-associated chaperone network in the stroma that drives import across the envelope membranes. This review will focus on recent developments in our understanding of the mechanisms and diversity of the TOC-TIC systems. Our goal is to incorporate these recent studies with previous work and present updated or revised models for the function of TOC-TIC in protein import.
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Affiliation(s)
- Yamuna D Paila
- Department of Biochemistry and Molecular Biology, Life Sciences Laboratories Room N431, 240 Thatcher Rd, University of Massachusetts, Amherst MA 01003-9364, USA
| | - Lynn G L Richardson
- Department of Biochemistry and Molecular Biology, Life Sciences Laboratories Room N431, 240 Thatcher Rd, University of Massachusetts, Amherst MA 01003-9364, USA
| | - Danny J Schnell
- Department of Biochemistry and Molecular Biology, Life Sciences Laboratories Room N431, 240 Thatcher Rd, University of Massachusetts, Amherst MA 01003-9364, USA
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Colombo CV, Ceccarelli EA, Rosano GL. Characterization of the accessory protein ClpT1 from Arabidopsis thaliana: oligomerization status and interaction with Hsp100 chaperones. BMC PLANT BIOLOGY 2014; 14:228. [PMID: 25149061 PMCID: PMC4243950 DOI: 10.1186/s12870-014-0228-0] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/28/2014] [Accepted: 08/18/2014] [Indexed: 05/26/2023]
Abstract
BACKGROUND The caseinolytic protease (Clp) is crucial for chloroplast biogenesis and proteostasis. The Arabidopsis Clp consists of two heptameric rings (P and R rings) assembled from nine distinct subunits. Hsp100 chaperones (ClpC1/2 and ClpD) are believed to dock to the axial pores of Clp and then transfer unfolded polypeptides destined to degradation. The adaptor proteins ClpT1 and 2 attach to the protease, apparently blocking the chaperone binding sites. This competition was suggested to regulate Clp activity. Also, monomerization of ClpT1 from dimers in the stroma triggers P and R rings association. So, oligomerization status of ClpT1 seems to control the assembly of the Clp protease. RESULTS In this work, ClpT1 was obtained in a recombinant form and purified. In solution, it mostly consists of monomers while dimers represent a small fraction of the population. Enrichment of the dimer fraction could only be achieved by stabilization with a crosslinker reagent. We demonstrate that ClpT1 specifically interacts with the Hsp100 chaperones ClpC2 and ClpD. In addition, ClpT1 stimulates the ATPase activity of ClpD by more than 50% when both are present in a 1:1 molar ratio. Outside this optimal proportion, the stimulatory effect of ClpT1 on the ATPase activity of ClpD declines. CONCLUSIONS The accessory protein ClpT1 behaves as a monomer in solution. It interacts with the chloroplastic Hsp100 chaperones ClpC2 and ClpD and tightly modulates the ATPase activity of the latter. Our results provide new experimental evidence that may contribute to revise and expand the existing models that were proposed to explain the roles of this poorly understood regulatory protein.
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Affiliation(s)
- Clara V Colombo
- Instituto de Biología Molecular y Celular de Rosario (IBR), CONICET, Facultad de Ciencias Bioquímicas y Farmacéuticas, Universidad Nacional de Rosario, Esmeralda y Ocampo, Rosario, Argentina
| | - Eduardo A Ceccarelli
- Instituto de Biología Molecular y Celular de Rosario (IBR), CONICET, Facultad de Ciencias Bioquímicas y Farmacéuticas, Universidad Nacional de Rosario, Esmeralda y Ocampo, Rosario, Argentina
| | - Germán L Rosano
- Instituto de Biología Molecular y Celular de Rosario (IBR), CONICET, Facultad de Ciencias Bioquímicas y Farmacéuticas, Universidad Nacional de Rosario, Esmeralda y Ocampo, Rosario, Argentina
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Putarjunan A, Liu X, Nolan T, Yu F, Rodermel S. Understanding chloroplast biogenesis using second-site suppressors of immutans and var2. PHOTOSYNTHESIS RESEARCH 2013; 116:437-53. [PMID: 23703455 DOI: 10.1007/s11120-013-9855-9] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/15/2013] [Accepted: 05/15/2013] [Indexed: 05/07/2023]
Abstract
Chloroplast biogenesis is an essential light-dependent process involving the differentiation of photosynthetically competent chloroplasts from precursors that include undifferentiated proplastids in leaf meristems, as well as etioplasts in dark-grown seedlings. The mechanisms that govern these developmental processes are poorly understood, but entail the coordinated expression of nuclear and plastid genes. This coordination is achieved, in part, by signals generated in response to the metabolic and developmental state of the plastid that regulate the transcription of nuclear genes for photosynthetic proteins (retrograde signaling). Variegation mutants are powerful tools to understand pathways of chloroplast biogenesis, and over the years our lab has focused on immutans (im) and variegated2 (var2), two nuclear gene-induced variegations of Arabidopsis. im and var2 are among the best-characterized chloroplast biogenesis mutants, and they define the genes for plastid terminal oxidase (PTOX) and the AtFtsH2 subunit of the thylakoid FtsH metalloprotease complex, respectively. To gain insight into the function of these proteins, forward and reverse genetic approaches have been used to identify second-site suppressors of im and var2 that replace or bypass the need for PTOX and AtFtsH2 during chloroplast development. In this review, we provide a brief update of im and var2 and the functions of PTOX and AtFtsH2. We then summarize information about second-site suppressors of im and var2 that have been identified to date, and describe how they have provided insight into mechanisms of photosynthesis and pathways of chloroplast development.
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Affiliation(s)
- Aarthi Putarjunan
- Department of Genetics, Development and Cell Biology, Iowa State University, Ames, IA, 50011, USA
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