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Christian R, Labbancz J, Usadel B, Dhingra A. Understanding protein import in diverse non-green plastids. Front Genet 2023; 14:969931. [PMID: 37007964 PMCID: PMC10063809 DOI: 10.3389/fgene.2023.969931] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/15/2022] [Accepted: 02/24/2023] [Indexed: 03/19/2023] Open
Abstract
The spectacular diversity of plastids in non-green organs such as flowers, fruits, roots, tubers, and senescing leaves represents a Universe of metabolic processes in higher plants that remain to be completely characterized. The endosymbiosis of the plastid and the subsequent export of the ancestral cyanobacterial genome to the nuclear genome, and adaptation of the plants to all types of environments has resulted in the emergence of diverse and a highly orchestrated metabolism across the plant kingdom that is entirely reliant on a complex protein import and translocation system. The TOC and TIC translocons, critical for importing nuclear-encoded proteins into the plastid stroma, remain poorly resolved, especially in the case of TIC. From the stroma, three core pathways (cpTat, cpSec, and cpSRP) may localize imported proteins to the thylakoid. Non-canonical routes only utilizing TOC also exist for the insertion of many inner and outer membrane proteins, or in the case of some modified proteins, a vesicular import route. Understanding this complex protein import system is further compounded by the highly heterogeneous nature of transit peptides, and the varying transit peptide specificity of plastids depending on species and the developmental and trophic stage of the plant organs. Computational tools provide an increasingly sophisticated means of predicting protein import into highly diverse non-green plastids across higher plants, which need to be validated using proteomics and metabolic approaches. The myriad plastid functions enable higher plants to interact and respond to all kinds of environments. Unraveling the diversity of non-green plastid functions across the higher plants has the potential to provide knowledge that will help in developing climate resilient crops.
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Affiliation(s)
- Ryan Christian
- Department of Horticulture, Washington State University, Pullman, WA, United States
| | - June Labbancz
- Department of Horticulture, Washington State University, Pullman, WA, United States
- Department of Horticultural Sciences, Texas A&M University, College Station, TX, United States
| | | | - Amit Dhingra
- Department of Horticulture, Washington State University, Pullman, WA, United States
- Department of Horticultural Sciences, Texas A&M University, College Station, TX, United States
- *Correspondence: Amit Dhingra,
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2
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Barth MA, Soll J, Akbaş Ş. Prokaryotic and eukaryotic traits support the biological role of the chloroplast outer envelope. BIOCHIMICA ET BIOPHYSICA ACTA. MOLECULAR CELL RESEARCH 2022; 1869:119224. [PMID: 35120999 DOI: 10.1016/j.bbamcr.2022.119224] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/07/2021] [Revised: 01/22/2022] [Accepted: 01/26/2022] [Indexed: 06/14/2023]
Abstract
The plastid outer envelope (OE) is a mixture of components inherited from their prokaryotic ancestor like galactolipids, carotenoids and porin type ion channels supplemented with eukaryotic inventions to make the endosymbiotic process successful as well as to control plastid biogenesis and differentiation. In this review we wanted to highlight the importance of the OE proteins and its evolutionary origin. For a long time, the OE was thought to be a diffusion barrier only, but with the recent discoveries of all kinds of different proteins in the OE it has been shown that the OE can modulate various functions within the cell. The phenotypic changes show that channels like the outer envelope proteins OEP40, OEP16 or JASSY have a pronounced ion selectivity that cannot be replaced by other ion channels present in the OE. Eukaryotic additions, like the GTPase receptors Toc33 and Toc159 or the ubiquitin proteasome system for chloroplast protein quality control, round up the profile of the OE.
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Affiliation(s)
- Melanie Anette Barth
- Department Biologie 1, Botanik, Ludwig-Maximilians-Universität München, Großhaderner Str. 2-4, 82152 Planegg-Martinsried, Germany
| | - Jürgen Soll
- Department Biologie 1, Botanik, Ludwig-Maximilians-Universität München, Großhaderner Str. 2-4, 82152 Planegg-Martinsried, Germany.
| | - Şebnem Akbaş
- Department Biologie 1, Botanik, Ludwig-Maximilians-Universität München, Großhaderner Str. 2-4, 82152 Planegg-Martinsried, Germany
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3
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Wiesemann K, Simm S, Mirus O, Ladig R, Schleiff E. Regulation of two GTPases Toc159 and Toc34 in the translocon of the outer envelope of chloroplasts. BIOCHIMICA ET BIOPHYSICA ACTA. PROTEINS AND PROTEOMICS 2019; 1867:627-636. [PMID: 30611779 DOI: 10.1016/j.bbapap.2019.01.002] [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: 07/31/2018] [Revised: 12/20/2018] [Accepted: 01/02/2019] [Indexed: 01/03/2023]
Abstract
The GTPases Toc159 and Toc34 of the translocon of the outer envelope of chloroplasts (TOC) are involved in recognition and transfer of precursor proteins at the cytosolic face of the organelle. Both proteins engage multiple interactions within the translocon during the translocation process, including dimeric states of their G-domains. The units of the Toc34 homodimer are involved in the recognition of the transit peptide representing the translocation signal of precursor proteins. This substrate recognition is part of the regulation of the GTPase cycle of Toc34. The Toc159 monomer and the Toc34 homodimer recognize the transit peptide of the small subunit of Rubisco at the N- and at the C-terminal region, respectively. Analysis of the transit peptide interaction by crosslinking shows that the heterodimer between both G-domains binds pSSU most efficiently. While substrate recognition by Toc34 homodimer was shown to regulate nucleotide exchange, we provide evidence that the high activation energy of the GTPase Toc159 is lowered by substrate recognition. The nucleotide affinity of Toc34G homodimer and Toc159G monomer are distinct, Toc34G homodimer recognizes GDP and Toc159G GTP with highest affinity. Moreover, the analysis of the nucleotide association rates of the monomeric and dimeric receptor units suggests that the heterodimer has an arrangement distinct from the homodimer of Toc34. Based on the biochemical parameters determined we propose a model for the order of events at the cytosolic side of TOC. The molecular processes described by this hypothesis range from transit peptide recognition to perception of the substrate by the translocation channel.
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Affiliation(s)
- Katharina Wiesemann
- Department of Molecular Cell Biology of Plants, Goethe University, Max-von-Laue Str. 9, D-60438 Frankfurt, Germany
| | - Stefan Simm
- Department of Molecular Cell Biology of Plants, Goethe University, Max-von-Laue Str. 9, D-60438 Frankfurt, Germany; Frankfurt Institute for Advanced Studies, Ruth-Moufang-Straße 1, D-60438 Frankfurt, Germany
| | - Oliver Mirus
- Department of Molecular Cell Biology of Plants, Goethe University, Max-von-Laue Str. 9, D-60438 Frankfurt, Germany
| | - Roman Ladig
- Department of Molecular Cell Biology of Plants, Goethe University, Max-von-Laue Str. 9, D-60438 Frankfurt, Germany; Cluster of Excellence Frankfurt, Goethe University, D-60438 Frankfurt, Germany
| | - Enrico Schleiff
- Department of Molecular Cell Biology of Plants, Goethe University, Max-von-Laue Str. 9, D-60438 Frankfurt, Germany; Frankfurt Institute for Advanced Studies, Ruth-Moufang-Straße 1, D-60438 Frankfurt, Germany; Cluster of Excellence Frankfurt, Goethe University, D-60438 Frankfurt, Germany; Buchmann Institute for Molecular Life Sciences, Goethe University, Max-von-Laue Str. 15, D-60438 Frankfurt, Germany.
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4
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Kano Y, Gebregiworgis T, Marshall CB, Radulovich N, Poon BPK, St-Germain J, Cook JD, Valencia-Sama I, Grant BMM, Herrera SG, Miao J, Raught B, Irwin MS, Lee JE, Yeh JJ, Zhang ZY, Tsao MS, Ikura M, Ohh M. Tyrosyl phosphorylation of KRAS stalls GTPase cycle via alteration of switch I and II conformation. Nat Commun 2019; 10:224. [PMID: 30644389 PMCID: PMC6333830 DOI: 10.1038/s41467-018-08115-8] [Citation(s) in RCA: 66] [Impact Index Per Article: 13.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/27/2018] [Accepted: 12/17/2018] [Indexed: 12/27/2022] Open
Abstract
Deregulation of the RAS GTPase cycle due to mutations in the three RAS genes is commonly associated with cancer development. Protein tyrosine phosphatase SHP2 promotes RAF-to-MAPK signaling pathway and is an essential factor in RAS-driven oncogenesis. Despite the emergence of SHP2 inhibitors for the treatment of cancers harbouring mutant KRAS, the mechanism underlying SHP2 activation of KRAS signaling remains unclear. Here we report tyrosyl-phosphorylation of endogenous RAS and demonstrate that KRAS phosphorylation via Src on Tyr32 and Tyr64 alters the conformation of switch I and II regions, which stalls multiple steps of the GTPase cycle and impairs binding to effectors. In contrast, SHP2 dephosphorylates KRAS, a process that is required to maintain dynamic canonical KRAS GTPase cycle. Notably, Src- and SHP2-mediated regulation of KRAS activity extends to oncogenic KRAS and the inhibition of SHP2 disrupts the phosphorylation cycle, shifting the equilibrium of the GTPase cycle towards the stalled ‘dark state’. Deregulation of the RAS GTPase cycle due to mutations in RAS genes is commonly associated with cancer development. Here authors use NMR and mass spectrometry to shows that KRAS phosphorylation via Src alters the conformation of switch I and II regions and thereby impacts the GTPase cycle.
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Affiliation(s)
- Yoshihito Kano
- Department of Laboratory Medicine and Pathobiology, University of Toronto, 661 University Avenue, Toronto, ON, M5G 1M1, Canada.,Department of Biochemistry, University of Toronto, 661 University Avenue, Toronto, ON, M5G 1M1, Canada
| | - Teklab Gebregiworgis
- Princess Margaret Cancer Centre, University Health Network and Department of Medical Biophysics, University of Toronto, 101 College Street, Toronto, ON, M5G 1L7, Canada
| | - Christopher B Marshall
- Princess Margaret Cancer Centre, University Health Network and Department of Medical Biophysics, University of Toronto, 101 College Street, Toronto, ON, M5G 1L7, Canada
| | - Nikolina Radulovich
- Princess Margaret Cancer Centre, University Health Network and Department of Pathology, University of Toronto, Toronto, ON, M5G 1L7, Canada
| | - Betty P K Poon
- Department of Laboratory Medicine and Pathobiology, University of Toronto, 661 University Avenue, Toronto, ON, M5G 1M1, Canada
| | - Jonathan St-Germain
- Princess Margaret Cancer Centre, University Health Network and Department of Medical Biophysics, University of Toronto, 101 College Street, Toronto, ON, M5G 1L7, Canada
| | - Jonathan D Cook
- Department of Laboratory Medicine and Pathobiology, University of Toronto, 661 University Avenue, Toronto, ON, M5G 1M1, Canada
| | - Ivette Valencia-Sama
- Department of Laboratory Medicine and Pathobiology, University of Toronto, 661 University Avenue, Toronto, ON, M5G 1M1, Canada.,Peter Gilgan Centre for Research and Learning, The Hospital for Sick Children, 686 Bay Street, Toronto, ON, 5G OA4, Canada
| | - Benjamin M M Grant
- Princess Margaret Cancer Centre, University Health Network and Department of Medical Biophysics, University of Toronto, 101 College Street, Toronto, ON, M5G 1L7, Canada
| | - Silvia Gabriela Herrera
- Lineberger Comprehensive Cancer Center, University of North Carolina, Chapel Hill, NC, 27599, USA
| | - Jinmin Miao
- Department of Medicinal Chemistry and Molecular Pharmacology, Center for Cancer Research and Institute for Drug Discovery, Purdue University, 720 Clinic Drive, West Lafayette, IN, 47907, USA
| | - Brian Raught
- Princess Margaret Cancer Centre, University Health Network and Department of Medical Biophysics, University of Toronto, 101 College Street, Toronto, ON, M5G 1L7, Canada
| | - Meredith S Irwin
- Peter Gilgan Centre for Research and Learning, The Hospital for Sick Children, 686 Bay Street, Toronto, ON, 5G OA4, Canada
| | - Jeffrey E Lee
- Department of Laboratory Medicine and Pathobiology, University of Toronto, 661 University Avenue, Toronto, ON, M5G 1M1, Canada
| | - Jen Jen Yeh
- Lineberger Comprehensive Cancer Center, University of North Carolina, Chapel Hill, NC, 27599, USA.,Department of Surgery, University of North Carolina, Chapel Hill, NC, 27599, USA.,Department of Pharmacology, University of North Carolina, Chapel Hill, NC, 27599, USA
| | - Zhong-Yin Zhang
- Department of Medicinal Chemistry and Molecular Pharmacology, Center for Cancer Research and Institute for Drug Discovery, Purdue University, 720 Clinic Drive, West Lafayette, IN, 47907, USA
| | - Ming-Sound Tsao
- Princess Margaret Cancer Centre, University Health Network and Department of Pathology, University of Toronto, Toronto, ON, M5G 1L7, Canada
| | - Mitsuhiko Ikura
- Princess Margaret Cancer Centre, University Health Network and Department of Medical Biophysics, University of Toronto, 101 College Street, Toronto, ON, M5G 1L7, Canada
| | - Michael Ohh
- Department of Laboratory Medicine and Pathobiology, University of Toronto, 661 University Avenue, Toronto, ON, M5G 1M1, Canada. .,Department of Biochemistry, University of Toronto, 661 University Avenue, Toronto, ON, M5G 1M1, Canada.
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5
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Reinbothe S, Bartsch S, Rossig C, Davis MY, Yuan S, Reinbothe C, Gray J. A Protochlorophyllide (Pchlide) a Oxygenase for Plant Viability. FRONTIERS IN PLANT SCIENCE 2019; 10:593. [PMID: 31156665 PMCID: PMC6530659 DOI: 10.3389/fpls.2019.00593] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/29/2018] [Accepted: 04/24/2019] [Indexed: 05/19/2023]
Abstract
Higher plants contain a small, 5-member family of Rieske non-heme oxygenases that comprise the inner plastid envelope protein TIC55, phaeophorbide a oxygenasee (PAO), chlorophyllide a oxygenase (CAO), choline monooxygenase, and a 52 kDa protein (PTC52) associated with the precursor NADPH:protochlorophyllide (Pchlide) oxidoreductase A (pPORA) A translocon (PTC). Some of these chloroplast proteins have documented roles in chlorophyll biosynthesis (CAO) and degradation (PAO and TIC55), whereas the function of PTC52 remains unresolved. Biochemical evidence provided here identifies PTC52 as Pchlide a oxygenase of the inner plastid envelope linking Pchlide b synthesis to pPORA import. Protochlorophyllide b is the preferred substrate of PORA and its lack no longer allows pPORA import. The Pchlide b-dependent import pathway of pPORA thus operates in etiolated seedlings and is switched off during greening. Using dexamethasone-induced RNA interference (RNAi) we tested if PTC52 is involved in controlling both, pPORA import and Pchlide homeostasis in planta. As shown here, RNAi plants deprived of PTC52 transcript and PTC52 protein were unable to import pPORA and died as a result of excess Pchlide a accumulation causing singlet oxygen formation during greening. In genetic studies, no homozygous ptc52 knock-out mutants could be obtained presumably as a result of embryo lethality, suggesting a role for PTC52 in the initial greening of plant embryos. Phylogenetic studies identified PTC52-like genes amongst unicellular photosynthetic bacteria and higher plants, suggesting that the biochemical function associated with PTC52 may have an ancient evolutionary origin. PTC52 also harbors conserved motifs with bacterial oxygenases such as the terminal oxygenase component of 3-ketosteroid 9-alpha-hydroxylase (KshA) from Rhodococcus rhodochrous. 3D-modeling of PTC52 structure permitted the prediction of amino acid residues that contribute to the substrate specificity of this enzyme. In vitro-mutagenesis was used to test the predicted PTC52 model and provide insights into the reaction mechanism of this Rieske non-heme oxygenase.
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Affiliation(s)
- Steffen Reinbothe
- Laboratoire de Génétique Moléculaire des Plantes and Biologie Environnementale et Systémique (BEeSy), Université Grenoble Alpes, Grenoble, France
- *Correspondence: Steffen Reinbothe, John Gray,
| | - Sandra Bartsch
- Laboratoire de Génétique Moléculaire des Plantes and Biologie Environnementale et Systémique (BEeSy), Université Grenoble Alpes, Grenoble, France
| | - Claudia Rossig
- Laboratoire de Génétique Moléculaire des Plantes and Biologie Environnementale et Systémique (BEeSy), Université Grenoble Alpes, Grenoble, France
| | | | - Shu Yuan
- College of Resources, Sichuan Agricultural University, Chengdu, China
| | - Christiane Reinbothe
- Laboratoire de Génétique Moléculaire des Plantes and Biologie Environnementale et Systémique (BEeSy), Université Grenoble Alpes, Grenoble, France
| | - John Gray
- Department of Biological Sciences, The University of Toledo, Toledo, OH, United States
- *Correspondence: Steffen Reinbothe, John Gray,
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6
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Plant mitochondrial protein import: the ins and outs. Biochem J 2018; 475:2191-2208. [PMID: 30018142 DOI: 10.1042/bcj20170521] [Citation(s) in RCA: 24] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/24/2018] [Revised: 06/19/2018] [Accepted: 06/21/2018] [Indexed: 01/29/2023]
Abstract
The majority of the mitochondrial proteome, required to fulfil its diverse range of functions, is cytosolically synthesised and translocated via specialised machinery. The dedicated translocases, receptors, and associated proteins have been characterised in great detail in yeast over the last several decades, yet many of the mechanisms that regulate these processes in higher eukaryotes are still unknown. In this review, we highlight the current knowledge of mitochondrial protein import in plants. Despite the fact that the mechanisms of mitochondrial protein import have remained conserved across species, many unique features have arisen in plants to encompass the developmental, tissue-specific, and stress-responsive regulation in planta. An understanding of unique features and mechanisms in plants provides us with a unique insight into the regulation of mitochondrial biogenesis in higher eukaryotes.
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7
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Zufferey M, Montandon C, Douet V, Demarsy E, Agne B, Baginsky S, Kessler F. The novel chloroplast outer membrane kinase KOC1 is a required component of the plastid protein import machinery. J Biol Chem 2017; 292:6952-6964. [PMID: 28283569 DOI: 10.1074/jbc.m117.776468] [Citation(s) in RCA: 15] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/12/2017] [Revised: 02/28/2017] [Indexed: 11/06/2022] Open
Abstract
The biogenesis and maintenance of cell organelles such as mitochondria and chloroplasts require the import of many proteins from the cytosol, a process that is controlled by phosphorylation. In the case of chloroplasts, the import of hundreds of different proteins depends on translocons at the outer and inner chloroplast membrane (TOC and TIC, respectively) complexes. The essential protein TOC159 functions thereby as an import receptor. It has an N-terminal acidic (A-) domain that extends into the cytosol, controls receptor specificity, and is highly phosphorylated in vivo However, kinases that phosphorylate the TOC159 A-domain to enable protein import have remained elusive. Here, using co-purification with TOC159 from Arabidopsis, we discovered a novel component of the chloroplast import machinery, the regulatory kinase at the outer chloroplast membrane 1 (KOC1). We found that KOC1 is an integral membrane protein facing the cytosol and stably associates with TOC. Moreover, KOC1 phosphorylated the A-domain of TOC159 in vitro, and in mutant koc1 chloroplasts, preprotein import efficiency was diminished. koc1 Arabidopsis seedlings had reduced survival rates after transfer from the dark to the light in which protein import into plastids is required to rapidly complete chloroplast biogenesis. In summary, our data indicate that KOC1 is a functional component of the TOC machinery that phosphorylates import receptors, supports preprotein import, and contributes to efficient chloroplast biogenesis.
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Affiliation(s)
- Mónica Zufferey
- From the Laboratory of Plant Physiology, University of Neuchâtel, 2000 Neuchâtel, Switzerland
| | - Cyrille Montandon
- the College of Agriculture and Life Sciences, Cornell University, Ithaca, New York 14853
| | - Véronique Douet
- From the Laboratory of Plant Physiology, University of Neuchâtel, 2000 Neuchâtel, Switzerland
| | - Emilie Demarsy
- the Department of Botany and Plant Biology, University of Geneva, 1211 Geneva, Switzerland, and
| | - Birgit Agne
- Institut für Biochemie und Biotechnologie, Martin-Luther-Universität Halle-Wittenberg, 06099 Halle (Saale), Germany
| | - Sacha Baginsky
- Institut für Biochemie und Biotechnologie, Martin-Luther-Universität Halle-Wittenberg, 06099 Halle (Saale), Germany
| | - Felix Kessler
- From the Laboratory of Plant Physiology, University of Neuchâtel, 2000 Neuchâtel, Switzerland,
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9
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Holbrook K, Subramanian C, Chotewutmontri P, Reddick LE, Wright S, Zhang H, Moncrief L, Bruce BD. Functional Analysis of Semi-conserved Transit Peptide Motifs and Mechanistic Implications in Precursor Targeting and Recognition. MOLECULAR PLANT 2016; 9:1286-1301. [PMID: 27378725 DOI: 10.1016/j.molp.2016.06.004] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/16/2016] [Revised: 05/13/2016] [Accepted: 06/07/2016] [Indexed: 05/17/2023]
Abstract
Over 95% of plastid proteins are nuclear-encoded as their precursors containing an N-terminal extension known as the transit peptide (TP). Although highly variable, TPs direct the precursors through a conserved, posttranslational mechanism involving translocons in the outer (TOC) and inner envelope (TOC). The organelle import specificity is mediated by one or more components of the Toc complex. However, the high TP diversity creates a paradox on how the sequences can be specifically recognized. An emerging model of TP design is that they contain multiple loosely conserved motifs that are recognized at different steps in the targeting and transport process. Bioinformatics has demonstrated that many TPs contain semi-conserved physicochemical motifs, termed FGLK. In order to characterize FGLK motifs in TP recognition and import, we have analyzed two well-studied TPs from the precursor of RuBisCO small subunit (SStp) and ferredoxin (Fdtp). Both SStp and Fdtp contain two FGLK motifs. Analysis of large set mutations (∼85) in these two motifs using in vitro, in organello, and in vivo approaches support a model in which the FGLK domains mediate interaction with TOC34 and possibly other TOC components. In vivo import analysis suggests that multiple FGLK motifs are functionally redundant. Furthermore, we discuss how FGLK motifs are required for efficient precursor protein import and how these elements may permit a convergent function of this highly variable class of targeting sequences.
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Affiliation(s)
- Kristen Holbrook
- Department of Biochemistry, Cellular and Molecular Biology, University of Tennessee, Knoxville, TN 37996, USA
| | - Chitra Subramanian
- Graduate Program in Plant Physiology and Genetics, University of Tennessee, Knoxville, TN 37996, USA
| | | | - L Evan Reddick
- Department of Biochemistry, Cellular and Molecular Biology, University of Tennessee, Knoxville, TN 37996, USA
| | - Sarah Wright
- Department of Botany, University of Tennessee, Knoxville, TN 37996, USA
| | - Huixia Zhang
- Department of Biochemistry, Cellular and Molecular Biology, University of Tennessee, Knoxville, TN 37996, USA
| | - Lily Moncrief
- Department of Biochemistry, Cellular and Molecular Biology, University of Tennessee, Knoxville, TN 37996, USA
| | - Barry D Bruce
- Department of Biochemistry, Cellular and Molecular Biology, University of Tennessee, Knoxville, TN 37996, USA; Graduate Program in Plant Physiology and Genetics, University of Tennessee, Knoxville, TN 37996, USA; Graduate School of Genome Science and Technology, University of Tennessee, Knoxville, TN 37996, USA.
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10
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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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11
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Ling Q, Jarvis P. Functions of plastid protein import and the ubiquitin-proteasome system in plastid development. BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 2015; 1847:939-48. [PMID: 25762164 DOI: 10.1016/j.bbabio.2015.02.017] [Citation(s) in RCA: 24] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/15/2014] [Revised: 02/18/2015] [Accepted: 02/26/2015] [Indexed: 02/05/2023]
Abstract
Plastids, such as chloroplasts, are widely distributed endosymbiotic organelles in plants and algae. Apart from their well-known functions in photosynthesis, they have roles in processes as diverse as signal sensing, fruit ripening, and seed development. As most plastid proteins are produced in the cytosol, plastids have developed dedicated translocon machineries for protein import, comprising the TOC (translocon at the outer envelope membrane of chloroplasts) and TIC (translocon at the inner envelope membrane of chloroplasts) complexes. Multiple lines of evidence reveal that protein import via the TOC complex is actively regulated, based on the specific interplay between distinct receptor isoforms and diverse client proteins. In this review, we summarize recent advances in our understanding of protein import regulation, particularly in relation to control by the ubiquitin-proteasome system (UPS), and how such regulation changes plastid development. The diversity of plastid import receptors (and of corresponding preprotein substrates) has a determining role in plastid differentiation and interconversion. The controllable turnover of TOC components by the UPS influences the developmental fate of plastids, which is fundamentally linked to plant development. Understanding the mechanisms by which plastid protein import is controlled is critical to the development of breakthrough approaches to increase the yield, quality and stress tolerance of important crop plants, which are highly dependent on plastid development. This article is part of a Special Issue entitled: Chloroplast Biogenesis.
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Affiliation(s)
- Qihua Ling
- Department of Plant Sciences, University of Oxford, South Parks Road, Oxford OX1 3RB, UK
| | - Paul Jarvis
- Department of Plant Sciences, University of Oxford, South Parks Road, Oxford OX1 3RB, UK.
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12
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Demarsy E, Lakshmanan AM, Kessler F. Border control: selectivity of chloroplast protein import and regulation at the TOC-complex. FRONTIERS IN PLANT SCIENCE 2014; 5:483. [PMID: 25278954 PMCID: PMC4166117 DOI: 10.3389/fpls.2014.00483] [Citation(s) in RCA: 27] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/17/2014] [Accepted: 09/01/2014] [Indexed: 05/25/2023]
Abstract
Plants have evolved complex and sophisticated molecular mechanisms to regulate their development and adapt to their surrounding environment. Particularly the development of their specific organelles, chloroplasts and other plastid-types, is finely tuned in accordance with the metabolic needs of the cell. The normal development and functioning of plastids require import of particular subsets of nuclear encoded proteins. Most preproteins contain a cleavable sequence at their N terminal (transit peptide) serving as a signal for targeting to the organelle and recognition by the translocation machinery TOC-TIC (translocon of outer membrane complex-translocon of inner membrane complex) spanning the dual membrane envelope. The plastid proteome needs constant remodeling in response to developmental and environmental factors. Therefore selective regulation of preprotein import plays a crucial role in plant development. In this review we describe the diversity of transit peptides and TOC receptor complexes, and summarize the current knowledge and potential directions for future research concerning regulation of the different Toc isoforms.
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Affiliation(s)
| | | | - Felix Kessler
- *Correspondence: Felix Kessler, Laboratory of Plant Physiology, Université de Neuchâtel, UniMail, Rue Emile Argand 11, 2000 Neuchâtel, Switzerland e-mail:
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13
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Src promotes GTPase activity of Ras via tyrosine 32 phosphorylation. Proc Natl Acad Sci U S A 2014; 111:E3785-94. [PMID: 25157176 DOI: 10.1073/pnas.1406559111] [Citation(s) in RCA: 74] [Impact Index Per Article: 7.4] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Mutations in Ras GTPase and various other components of the Ras signaling pathways are among the most common genetic alterations in human cancers and also have been identified in several familial developmental syndromes. Over the past few decades it has become clear that the activity or the oncogenic potential of Ras is dependent on the nonreceptor tyrosine kinase Src to promote the Ras/Raf/MAPK pathway essential for proliferation, differentiation, and survival of eukaryotic cells. However, no direct relationship between Ras and Src has been established. We show here that Src binds to and phosphorylates GTP-, but not GDP-, loaded Ras on a conserved Y32 residue within the switch I region in vitro and that in vivo, Ras-Y32 phosphorylation markedly reduces the binding to effector Raf and concomitantly increases binding to GTPase-activating proteins and the rate of GTP hydrolysis. These results suggest that, in the context of predetermined crystallographic structures, Ras-Y32 serves as an Src-dependent keystone regulatory residue that modulates Ras GTPase activity and ensures unidirectionality to the Ras GTPase cycle.
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14
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Lumme C, Altan-Martin H, Dastvan R, Sommer MS, Oreb M, Schuetz D, Hellenkamp B, Mirus O, Kretschmer J, Lyubenova S, Kügel W, Medelnik JP, Dehmer M, Michaelis J, Prisner TF, Hugel T, Schleiff E. Nucleotides and substrates trigger the dynamics of the Toc34 GTPase homodimer involved in chloroplast preprotein translocation. Structure 2014; 22:526-38. [PMID: 24631462 DOI: 10.1016/j.str.2014.02.004] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/19/2013] [Revised: 01/29/2014] [Accepted: 02/01/2014] [Indexed: 12/13/2022]
Abstract
GTPases are molecular switches that control numerous crucial cellular processes. Unlike bona fide GTPases, which are regulated by intramolecular structural transitions, the less well studied GAD-GTPases are activated by nucleotide-dependent dimerization. A member of this family is the translocase of the outer envelope membrane of chloroplast Toc34 involved in regulation of preprotein import. The GTPase cycle of Toc34 is considered a major circuit of translocation regulation. Contrary to expectations, previous studies yielded only marginal structural changes of dimeric Toc34 in response to different nucleotide loads. Referencing PELDOR and FRET single-molecule and bulk experiments, we describe a nucleotide-dependent transition of the dimer flexibility from a tight GDP- to a flexible GTP-loaded state. Substrate binding induces an opening of the GDP-loaded dimer. Thus, the structural dynamics of bona fide GTPases induced by GTP hydrolysis is replaced by substrate-dependent dimer flexibility, which likely represents a general regulatory mode for dimerizing GTPases.
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Affiliation(s)
- Christina Lumme
- Physics Department E22 and IMETUM, Technical University Munich, 85748 Garching, Germany
| | - Hasret Altan-Martin
- Institute of Molecular Cell Biology of Plants, Goethe University, 60438 Frankfurt, Germany
| | - Reza Dastvan
- Institute of Physical and Theoretical Chemistry, Goethe University, 60438 Frankfurt, Germany; Cluster of Excellence "Macromolecular Complexes", Goethe University, 60438 Frankfurt, Germany; Center for Biomolecular Magnetic Resonance, Goethe University, 60438 Frankfurt, Germany; Department of Molecular Physiology & Biophysics, Vanderbilt University, 741 Light Hall, 2215 Garland Avenue, Nashville, TN 37232, USA
| | - Maik S Sommer
- Institute of Molecular Cell Biology of Plants, Goethe University, 60438 Frankfurt, Germany
| | - Mislav Oreb
- Physics Department E22 and IMETUM, Technical University Munich, 85748 Garching, Germany
| | - Denise Schuetz
- Institute of Physical and Theoretical Chemistry, Goethe University, 60438 Frankfurt, Germany; Cluster of Excellence "Macromolecular Complexes", Goethe University, 60438 Frankfurt, Germany; Center for Biomolecular Magnetic Resonance, Goethe University, 60438 Frankfurt, Germany
| | - Björn Hellenkamp
- Physics Department E22 and IMETUM, Technical University Munich, 85748 Garching, Germany
| | - Oliver Mirus
- Institute of Molecular Cell Biology of Plants, Goethe University, 60438 Frankfurt, Germany
| | - Jens Kretschmer
- Institute of Molecular Cell Biology of Plants, Goethe University, 60438 Frankfurt, Germany
| | - Sevdalina Lyubenova
- Institute of Physical and Theoretical Chemistry, Goethe University, 60438 Frankfurt, Germany; Cluster of Excellence "Macromolecular Complexes", Goethe University, 60438 Frankfurt, Germany; Center for Biomolecular Magnetic Resonance, Goethe University, 60438 Frankfurt, Germany
| | | | - Jan P Medelnik
- Institute of Molecular Cell Biology of Plants, Goethe University, 60438 Frankfurt, Germany
| | - Manuela Dehmer
- Institute of Molecular Cell Biology of Plants, Goethe University, 60438 Frankfurt, Germany
| | | | - Thomas F Prisner
- Institute of Physical and Theoretical Chemistry, Goethe University, 60438 Frankfurt, Germany; Cluster of Excellence "Macromolecular Complexes", Goethe University, 60438 Frankfurt, Germany; Center for Biomolecular Magnetic Resonance, Goethe University, 60438 Frankfurt, Germany
| | - Thorsten Hugel
- Physics Department E22 and IMETUM, Technical University Munich, 85748 Garching, Germany
| | - Enrico Schleiff
- Institute of Molecular Cell Biology of Plants, Goethe University, 60438 Frankfurt, Germany; Cluster of Excellence "Macromolecular Complexes", Goethe University, 60438 Frankfurt, Germany; Center for Membrane Proteomics, Goethe University, 60438 Frankfurt, Germany.
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15
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Shi LX, Theg SM. The chloroplast protein import system: from algae to trees. BIOCHIMICA ET BIOPHYSICA ACTA-MOLECULAR CELL RESEARCH 2012; 1833:314-31. [PMID: 23063942 DOI: 10.1016/j.bbamcr.2012.10.002] [Citation(s) in RCA: 138] [Impact Index Per Article: 11.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/02/2012] [Revised: 09/07/2012] [Accepted: 10/01/2012] [Indexed: 01/15/2023]
Abstract
Chloroplasts are essential organelles in the cells of plants and algae. The functions of these specialized plastids are largely dependent on the ~3000 proteins residing in the organelle. Although chloroplasts are capable of a limited amount of semiautonomous protein synthesis - their genomes encode ~100 proteins - they must import more than 95% of their proteins after synthesis in the cytosol. Imported proteins generally possess an N-terminal extension termed a transit peptide. The importing translocons are made up of two complexes in the outer and inner envelope membranes, the so-called Toc and Tic machineries, respectively. The Toc complex contains two precursor receptors, Toc159 and Toc34, a protein channel, Toc75, and a peripheral component, Toc64/OEP64. The Tic complex consists of as many as eight components, namely Tic22, Tic110, Tic40, Tic20, Tic21 Tic62, Tic55 and Tic32. This general Toc/Tic import pathway, worked out largely in pea chloroplasts, appears to operate in chloroplasts in all green plants, albeit with significant modifications. Sub-complexes of the Toc and Tic machineries are proposed to exist to satisfy different substrate-, tissue-, cell- and developmental requirements. In this review, we summarize our understanding of the functions of Toc and Tic components, comparing these components of the import machinery in green algae through trees. We emphasize recent findings that point to growing complexities of chloroplast protein import process, and use the evolutionary relationships between proteins of different species in an attempt to define the essential core translocon components and those more likely to be responsible for regulation. This article is part of a Special Issue entitled: Protein Import and Quality Control in Mitochondria and Plastids.
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Affiliation(s)
- Lan-Xin Shi
- Department of Plant Biology, University of California-Davis, One Shields Avenue, Davis, CA 95616, USA.
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16
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Substrate binding disrupts dimerization and induces nucleotide exchange of the chloroplast GTPase Toc33. Biochem J 2011; 436:313-9. [PMID: 21434866 DOI: 10.1042/bj20110246] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
GTPases act as molecular switches to control many cellular processes, including signalling, protein translation and targeting. Switch activity can be regulated by external effector proteins or intrinsic properties, such as dimerization. The recognition and translocation of pre-proteins into chloroplasts [via the TOC/TIC (translocator at the outer envelope membrane of chloroplasts/inner envelope membrane of chloroplasts)] is controlled by two homologous receptor GTPases, Toc33 and Toc159, whose reversible dimerization is proposed to regulate translocation of incoming proteins in a GTP-dependent manner. Toc33 is a homodimerizing GTPase. Functional analysis suggests that homodimerization is a key step in the translocation process, the molecular functions of which, as well as the elements regulating this event, are largely unknown. In the present study, we show that homodimerization reduces the rate of nucleotide exchange, which is consistent with the observed orientation of the monomers in the crystal structure. Pre-protein binding induces a dissociation of the Toc33 homodimer and results in the exchange of GDP for GTP. Thus homodimerization does not serve to activate the GTPase activity as discussed many times previously, but to control the nucleotide-loading state. We discuss this novel regulatory mode and its impact on the current models of protein import into the chloroplast.
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17
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Aronsson H, Combe J, Patel R, Agne B, Martin M, Kessler F, Jarvis P. Nucleotide binding and dimerization at the chloroplast pre-protein import receptor, atToc33, are not essential in vivo but do increase import efficiency. THE PLANT JOURNAL : FOR CELL AND MOLECULAR BIOLOGY 2010; 63:297-311. [PMID: 20444229 DOI: 10.1111/j.1365-313x.2010.04242.x] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/08/2023]
Abstract
The atToc33 protein is one of several pre-protein import receptors in the outer envelope of Arabidopsis chloroplasts. It is a GTPase with motifs characteristic of such proteins, and its loss in the plastid protein import 1 (ppi1) mutant interferes with the import of photosynthesis-related pre-proteins, causing a chlorotic phenotype in mutant plants. To assess the significance of GTPase cycling by atToc33, we generated several atToc33 point mutants with predicted effects on GTP binding (K49R, S50N and S50N/S51N), GTP hydrolysis (G45R, G45V, Q68A and N101A), both binding and hydrolysis (G45R/K49N/S50R), and dimerization or the functional interaction between dimeric partners (R125A, R130A and R130K). First, a selection of these mutants was assessed in vitro, or in yeast, to confirm that the mutations have the desired effects: in relation to nucleotide binding and dimerization, the mutants behaved as expected. Then, activities of selected mutants were tested in vivo, by assessing for complementation of ppi1 in transgenic plants. Remarkably, all tested mutants mediated high levels of complementation: complemented plants were similar to the wild type in growth rate, chlorophyll accumulation, photosynthetic performance, and chloroplast ultrastructure. Protein import into mutant chloroplasts was also complemented to >50% of the wild-type level. Overall, the data indicate that neither nucleotide binding nor dimerization at atToc33 is essential for chloroplast import (in plants that continue to express the other TOC receptors in native form), although both processes do increase import efficiency. Absence of atToc33 GTPase activity might somehow be compensated for by that of the Toc159 receptors. However, overexpression of atToc33 (or its close relative, atToc34) in Toc159-deficient plants did not mediate complementation, indicating that the receptors do not share functional redundancy in the conventional sense.
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Affiliation(s)
- Henrik Aronsson
- Department of Plant and Environmental Sciences, University of Gothenburg, Box 461, SE-405 30 Gothenburg, Sweden
| | - Jonathan Combe
- Department of Biology, University of Leicester, University Road, Leicester LE1 7RH, UK
| | - Ramesh Patel
- Department of Biology, University of Leicester, University Road, Leicester LE1 7RH, UK
| | - Birgit Agne
- Laboratoire de Physiologie Végétale, Université de Neuchâtel, Rue Emile-Argand 11, 2007 Neuchâtel, Switzerland
| | - Meryll Martin
- Laboratoire de Physiologie Végétale, Université de Neuchâtel, Rue Emile-Argand 11, 2007 Neuchâtel, Switzerland
| | - Felix Kessler
- Laboratoire de Physiologie Végétale, Université de Neuchâtel, Rue Emile-Argand 11, 2007 Neuchâtel, Switzerland
| | - Paul Jarvis
- Department of Biology, University of Leicester, University Road, Leicester LE1 7RH, UK
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18
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Agne B, Andrès C, Montandon C, Christ B, Ertan A, Jung F, Infanger S, Bischof S, Baginsky S, Kessler F. The acidic A-domain of Arabidopsis TOC159 occurs as a hyperphosphorylated protein. PLANT PHYSIOLOGY 2010; 153:1016-30. [PMID: 20457805 PMCID: PMC2899928 DOI: 10.1104/pp.110.158048] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/18/2010] [Accepted: 05/06/2010] [Indexed: 05/17/2023]
Abstract
The translocon at the outer membrane of the chloroplast assists the import of a large class of preproteins with amino-terminal transit sequences. The preprotein receptors Toc159 and Toc33 in Arabidopsis (Arabidopsis thaliana) are specific for the accumulation of abundant photosynthetic proteins. The receptors are homologous GTPases known to be regulated by phosphorylation within their GTP-binding domains. In addition to the central GTP-binding domain, Toc159 has an acidic N-terminal domain (A-domain) and a C-terminal membrane-anchoring domain (M-domain). The A-domain of Toc159 is dispensable for its in vivo activity in Arabidopsis and prone to degradation in pea (Pisum sativum). Therefore, it has been suggested to have a regulatory function. Here, we show that in Arabidopsis, the A-domain is not simply degraded but that it accumulates as a soluble, phosphorylated protein separated from Toc159. However, the physiological relevance of this process is unclear. The data show that the A-domain of Toc159 as well as those of its homologs Toc132 and Toc120 are targets of a casein kinase 2-like activity.
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19
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Shang Y, Yan L, Liu ZQ, Cao Z, Mei C, Xin Q, Wu FQ, Wang XF, Du SY, Jiang T, Zhang XF, Zhao R, Sun HL, Liu R, Yu YT, Zhang DP. The Mg-chelatase H subunit of Arabidopsis antagonizes a group of WRKY transcription repressors to relieve ABA-responsive genes of inhibition. THE PLANT CELL 2010; 22:1909-35. [PMID: 20543028 PMCID: PMC2910980 DOI: 10.1105/tpc.110.073874] [Citation(s) in RCA: 375] [Impact Index Per Article: 26.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/06/2010] [Revised: 04/10/2010] [Accepted: 05/25/2010] [Indexed: 05/17/2023]
Abstract
The phytohormone abscisic acid (ABA) plays a vital role in plant development and response to environmental challenges, but the complex networks of ABA signaling pathways are poorly understood. We previously reported that a chloroplast protein, the magnesium-protoporphyrin IX chelatase H subunit (CHLH/ABAR), functions as a receptor for ABA in Arabidopsis thaliana. Here, we report that ABAR spans the chloroplast envelope and that the cytosolic C terminus of ABAR interacts with a group of WRKY transcription factors (WRKY40, WRKY18, and WRKY60) that function as negative regulators of ABA signaling in seed germination and postgermination growth. WRKY40, a central negative regulator, inhibits expression of ABA-responsive genes, such as ABI5. In response to a high level of ABA signal that recruits WRKY40 from the nucleus to the cytosol and promotes ABAR-WRKY40 interaction, ABAR relieves the ABI5 gene of inhibition by repressing WRKY40 expression. These findings describe a unique ABA signaling pathway from the early signaling events to downstream gene expression.
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Affiliation(s)
- Yi Shang
- Protein Science Laboratory of the Ministry of Education, School of Life Sciences, Tsinghua University, Beijing 100084, China
| | - Lu Yan
- Protein Science Laboratory of the Ministry of Education, School of Life Sciences, Tsinghua University, Beijing 100084, China
- College of Biological Sciences, China Agricultural University, Beijing 100094, China
| | - Zhi-Qiang Liu
- Protein Science Laboratory of the Ministry of Education, School of Life Sciences, Tsinghua University, Beijing 100084, China
- College of Biological Sciences, China Agricultural University, Beijing 100094, China
| | - Zheng Cao
- Protein Science Laboratory of the Ministry of Education, School of Life Sciences, Tsinghua University, Beijing 100084, China
- College of Biological Sciences, China Agricultural University, Beijing 100094, China
| | - Chao Mei
- Protein Science Laboratory of the Ministry of Education, School of Life Sciences, Tsinghua University, Beijing 100084, China
| | - Qi Xin
- Protein Science Laboratory of the Ministry of Education, School of Life Sciences, Tsinghua University, Beijing 100084, China
- College of Biological Sciences, China Agricultural University, Beijing 100094, China
| | - Fu-Qing Wu
- Protein Science Laboratory of the Ministry of Education, School of Life Sciences, Tsinghua University, Beijing 100084, China
- College of Biological Sciences, China Agricultural University, Beijing 100094, China
| | - Xiao-Fang Wang
- Protein Science Laboratory of the Ministry of Education, School of Life Sciences, Tsinghua University, Beijing 100084, China
- College of Biological Sciences, China Agricultural University, Beijing 100094, China
| | - Shu-Yuan Du
- Protein Science Laboratory of the Ministry of Education, School of Life Sciences, Tsinghua University, Beijing 100084, China
| | - Tao Jiang
- Protein Science Laboratory of the Ministry of Education, School of Life Sciences, Tsinghua University, Beijing 100084, China
- College of Biological Sciences, China Agricultural University, Beijing 100094, China
| | - Xiao-Feng Zhang
- Protein Science Laboratory of the Ministry of Education, School of Life Sciences, Tsinghua University, Beijing 100084, China
| | - Rui Zhao
- Protein Science Laboratory of the Ministry of Education, School of Life Sciences, Tsinghua University, Beijing 100084, China
- College of Biological Sciences, China Agricultural University, Beijing 100094, China
| | - Hai-Li Sun
- Protein Science Laboratory of the Ministry of Education, School of Life Sciences, Tsinghua University, Beijing 100084, China
- College of Biological Sciences, China Agricultural University, Beijing 100094, China
| | - Rui Liu
- Protein Science Laboratory of the Ministry of Education, School of Life Sciences, Tsinghua University, Beijing 100084, China
- College of Biological Sciences, China Agricultural University, Beijing 100094, China
| | - Yong-Tao Yu
- Protein Science Laboratory of the Ministry of Education, School of Life Sciences, Tsinghua University, Beijing 100084, China
| | - Da-Peng Zhang
- Protein Science Laboratory of the Ministry of Education, School of Life Sciences, Tsinghua University, Beijing 100084, China
- Address correspondence to
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20
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Andrès C, Agne B, Kessler F. The TOC complex: preprotein gateway to the chloroplast. BIOCHIMICA ET BIOPHYSICA ACTA 2010; 1803:715-23. [PMID: 20226817 DOI: 10.1016/j.bbamcr.2010.03.004] [Citation(s) in RCA: 49] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/28/2009] [Revised: 02/26/2010] [Accepted: 03/03/2010] [Indexed: 01/22/2023]
Abstract
Photosynthetic eukaryotes strongly depend on chloroplast metabolic pathways. Most if not all involve nuclear encoded proteins. These are synthesized as cytosolic preproteins with N-terminal, cleavable targeting sequences (transit peptide). Preproteins are imported by a major pathway composed of two proteins complexes: TOC and TIC (Translocon of the Outer and Inner membranes of the Chloroplasts, respectively). These selectively recognize the preproteins and facilitate their transport across the chloroplast envelope. The TOC core complex consists of three types of components, each belonging to a small family: Toc34, Toc75 and Toc159. Toc34 and Toc159 isoforms represent a subfamily of the GTPase superfamily. The members of the Toc34 and Toc159 subfamily act as GTP-dependent receptors at the chloroplast surface and distinct members of each occur in defined, substrate-specific TOC complexes. Toc75, a member of the Omp85 family, is conserved from prokaryotes and functions as the unique protein-conducting channel at the outer membrane. In this review we will describe the current state of knowledge regarding the composition and function of the TOC complex.
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Affiliation(s)
- Charles Andrès
- Institut de Biologie, Université de Neuchâtel, CH-2009 Neuchâtel, Switzerland
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21
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Lee J, Wang F, Schnell DJ. Toc receptor dimerization participates in the initiation of membrane translocation during protein import into chloroplasts. J Biol Chem 2009; 284:31130-41. [PMID: 19744928 PMCID: PMC2781512 DOI: 10.1074/jbc.m109.053751] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/07/2009] [Revised: 09/09/2009] [Indexed: 01/24/2023] Open
Abstract
The post-translational import of nucleus-encoded preproteins into chloroplasts occurs through multimeric translocons in the outer (Toc) and inner (Tic) membranes. The high fidelity of the protein import process is maintained by specific recognition of the transit peptide of preproteins by the coordinate activities of two homologous GTPase Toc receptors, Toc34 and Toc159. Structural and biochemical studies suggest that dimerization of the Toc receptors functions as a component of the mechanism to control access of preproteins to the membrane translocation channel of the translocon. We show that specific mutations that disrupted receptor dimerization in vitro reduced the rate of protein import in transgenic Arabidopsis compared with the wild type receptor. The mutations did not affect the GTPase activities of the receptors. Interestingly, these mutations did not decrease the initial preprotein binding at the receptors, but they reduced the efficiency of the transition from preprotein binding to membrane translocation. These data indicate that dimerization of receptors has a direct role in protein import and support a hypothesis in which receptor-receptor interactions participate in the initiation of membrane translocation of chloroplast preproteins as part of the molecular mechanism of GTP-regulated protein import.
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Affiliation(s)
- Jeonghwa Lee
- From the Department of Biochemistry and Molecular Biology and Program in Plant Biology, University of Massachusetts, Amherst, Massachusetts 01003 and
| | - Fei Wang
- the Department of Molecular and Cellular Biology, Harvard University, Cambridge, Massachusetts 02138
| | - Danny J. Schnell
- From the Department of Biochemistry and Molecular Biology and Program in Plant Biology, University of Massachusetts, Amherst, Massachusetts 01003 and
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22
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Abstract
Abstract
Protein transport, especially into different cellular compartments, is a highly coordinated and regulated process. The molecular machineries which carry out these transport processes are highly complex in structure, function, and regulation. In the case of chloroplasts, thousands of protein molecules have been estimated to be transported across the double-membrane bound envelope per minute. In this brief review, we summarize current knowledge about the molecular interplay during precursor protein import into chloroplasts, focusing on the initial events at the outer envelope.
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23
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Aronsson H, Jarvis P. The Chloroplast Protein Import Apparatus, Its Components, and Their Roles. PLANT CELL MONOGRAPHS 2008. [DOI: 10.1007/978-3-540-68696-5_3] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/02/2023]
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24
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Bionda T, Koenig P, Oreb M, Tews I, Schleiff E. pH Sensitivity of the GTPase Toc33 as a Regulatory Circuit for Protein Translocation into Chloroplasts. ACTA ACUST UNITED AC 2008; 49:1917-21. [DOI: 10.1093/pcp/pcn171] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022]
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25
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Wang F, Agne B, Kessler F, Schnell DJ. The role of GTP binding and hydrolysis at the atToc159 preprotein receptor during protein import into chloroplasts. ACTA ACUST UNITED AC 2008; 183:87-99. [PMID: 18824565 PMCID: PMC2557045 DOI: 10.1083/jcb.200803034] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/18/2023]
Abstract
The majority of nucleus-encoded chloroplast proteins are targeted to the organelle by direct binding to two membrane-bound GTPase receptors, Toc34 and Toc159. The GTPase activities of the receptors are implicated in two key import activities, preprotein binding and driving membrane translocation, but their precise functions have not been defined. We use a combination of in vivo and in vitro approaches to study the role of the Toc159 receptor in the import reaction. We show that atToc159-A864R, a receptor with reduced GTPase activity, can fully complement a lethal insertion mutation in the ATTOC159 gene. Surprisingly, the atToc159-A864R receptor increases the rate of protein import relative to wild-type receptor in isolated chloroplasts by stabilizing the formation of a GTP-dependent preprotein binding intermediate. These data favor a model in which the atToc159 receptor acts as part of a GTP-regulated switch for preprotein recognition at the TOC translocon.
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Affiliation(s)
- Fei Wang
- Department of Biochemistry and Molecular Biology and Program in Molecular and Cellular Biology, University of Massachusetts, Amherst, MA 01003, USA
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26
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Abstract
Plastids are a diverse group of essential organelles in plants that include chloroplasts. The biogenesis and maintenance of these organelles relies on the import of thousands of nucleus-encoded proteins. The complexity of plastid structure has resulted in the evolution of at least four general import pathways that target proteins into and across the double membrane of the plastid envelope. Several of these pathways can be further divided into specialty pathways that mediate and regulate the import of specific classes of proteins. The co-ordination of import by these specialized pathways with changes in gene expression is critical for plastid and plant development. Moreover, protein import is acutely regulated in response to physiological and metabolic changes within the cell. In the present review we summarize the current knowledge of the mechanism of import via these pathways and highlight the regulatory mechanisms that integrate the plastid protein-trafficking pathways with the developmental and metabolic state of the plant.
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27
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Abstract
Most chloroplast proteins are encoded in the nucleus and synthesized on free, cytosolic ribosomes in precursor form. Each precursor has an amino-terminal extension called a transit peptide, which directs the protein through a post-translational targeting pathway and is removed upon arrival inside the organelle. This 'protein import' process is mediated by the coordinate action of two multiprotein complexes, one in each of the envelope membranes: the TOC and TIC (Translocon at the Outer/ Inner envelope membrane of Chloroplasts) machines. Many components of these complexes have been identified biochemically in pea; these include transit peptide receptors, channel proteins, and molecular chaperones. Intriguingly, the Arabidopsis genome encodes multiple, homologous genes for receptor components of the TOC complex. Careful analysis indicated that the different receptor isoforms operate in different import pathways with distinct precursor recognition specificities. These 'substrate-specific' import pathways might play a role in the differentiation of different plastid types, and/or act to prevent deleterious competition effects between abundant and nonabundant precursors. Until recently, all proteins destined for internal chloroplast compartments were thought to possess a cleavable transit peptide, and to engage the TOC/TIC machinery. New studies using proteomics and other approaches have revealed that this is far from true. Remarkably, a significant number of chloroplast proteins are transported via a pathway that involves the endoplasmic reticulum and Golgi apparatus. Other recent reports have elucidated an intriguing array of protein targeting routes leading to the envelope membranes themselves.
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Affiliation(s)
- Paul Jarvis
- Department of Biology, University of Leicester, Leicester LE1 7RH, UK
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28
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Koenig P, Oreb M, Rippe K, Muhle-Goll C, Sinning I, Schleiff E, Tews I. On the significance of Toc-GTPase homodimers. J Biol Chem 2008; 283:23104-12. [PMID: 18541539 DOI: 10.1074/jbc.m710576200] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Precursor protein translocation across the outer chloroplast membrane depends on the action of the Toc complex, containing GTPases as recognizing receptor components. The G domains of the GTPases are known to dimerize. In the dimeric conformation an arginine contacts the phosphate moieties of bound nucleotide in trans. Kinetic studies suggested that the arginine in itself does not act as an arginine finger of a reciprocal GTPase-activating protein (GAP). Here we investigate the specific function of the residue in two GTPase homologues. Arginine to alanine replacement variants have significantly reduced affinities for dimerization compared with wild-type GTPases. The amino acid exchange does not impact on the overall fold and nucleotide binding, as seen in the monomeric x-ray crystallographic structure of the Arabidopsis Toc33 arginine-alanine replacement variant at 2.0A. We probed the catalytic center with the transition state analogue GDP/AlF(x) using NMR and analytical ultracentrifugation. AlF(x) binding depends on the arginine, suggesting the residue can play a role in catalysis despite the non-GAP nature of the homodimer. Two non-exclusive functional models are discussed: 1) the coGAP hypothesis, in which an additional factor activates the GTPase in homodimeric form; and 2) the switch hypothesis, in which a protein, presumably the large Toc159 GTPase, exchanges with one of the homodimeric subunits, leading to activation.
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Affiliation(s)
- Patrick Koenig
- Heidelberg University Biochemistry Center, Im Neuenheimer Feld 328, 69120 Heidelberg, Germany
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Oreb M, Höfle A, Mirus O, Schleiff E. Phosphorylation regulates the assembly of chloroplast import machinery. JOURNAL OF EXPERIMENTAL BOTANY 2008; 59:2309-16. [PMID: 18487635 PMCID: PMC2423650 DOI: 10.1093/jxb/ern095] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/29/2008] [Revised: 03/06/2008] [Accepted: 03/10/2008] [Indexed: 05/24/2023]
Abstract
Chloroplast function depends on the translocation of cytosolically synthesized precursor proteins into the organelle. The recognition and transfer of most precursor proteins across the outer membrane depend on a membrane inserted complex. Two receptor components of this complex, Toc34 and Toc159, are GTPases, which can be phosphorylated by kinases present in the hosting membrane. However, the physiological function of phosphorylation is not yet understood in detail. It is demonstrated that both receptors are phosphorylated within their G-domains. In vitro, the phosphorylation of Toc34 disrupts both homo- and heterodimerization of the G-domains as determined using a phospho-mimicking mutant. In endogenous membranes this mutation or phosphorylation of the wild-type receptor disturbs the association of Toc34, but not of Toc159 with the translocation pore. Therefore, phosphorylation serves as an inhibitor for the association of Toc34 with other components of the complex and phosphorylation can now be discussed as a mechanism to exchange different isoforms of Toc34 within this ensemble.
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Affiliation(s)
| | | | | | - Enrico Schleiff
- Present address and to whom correspondence should be sent: Molecular Plant Sciences, Biocenter, N 200, 3. OG, Max-von-Laue-Str. 9, D-60438 Frankfurt/Main, Germany. E-mail:
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30
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Koenig P, Oreb M, Höfle A, Kaltofen S, Rippe K, Sinning I, Schleiff E, Tews I. The GTPase cycle of the chloroplast import receptors Toc33/Toc34: implications from monomeric and dimeric structures. Structure 2008; 16:585-96. [PMID: 18400179 DOI: 10.1016/j.str.2008.01.008] [Citation(s) in RCA: 45] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/17/2007] [Revised: 01/18/2008] [Accepted: 01/22/2008] [Indexed: 11/18/2022]
Abstract
Transport of precursor proteins across chloroplast membranes involves the GTPases Toc33/34 and Toc159 at the outer chloroplast envelope. The small GTPase Toc33/34 can homodimerize, but the regulation of this interaction has remained elusive. We show that dimerization is independent of nucleotide loading state, based on crystal structures of dimeric Pisum sativum Toc34 and monomeric Arabidopsis thaliana Toc33. An arginine residue is--in the dimer--positioned to resemble a GAP arginine finger. However, GTPase activation by dimerization is sparse and active site features do not explain catalysis, suggesting that the homodimer requires an additional factor as coGAP. Access to the catalytic center and an unusual switch I movement in the dimeric structure support this finding. Potential binding sites for interactions within the Toc translocon or with precursor proteins can be derived from the structures.
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Affiliation(s)
- Patrick Koenig
- Heidelberg University Biochemistry Center, Im Neuenheimer Feld 328, 69120 Heidelberg, Germany
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31
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Inoue H, Akita M. Three sets of translocation intermediates are formed during the early stage of protein import into chloroplasts. J Biol Chem 2008; 283:7491-502. [PMID: 18199746 DOI: 10.1074/jbc.m709571200] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
During the early stage of protein import into chloroplasts, precursor proteins synthesized in the cytosol irreversibly bind to chloroplasts to form the early translocation intermediate under stringent energy conditions. Many efforts have been made to identify the components involved in protein import by analyzing the early intermediate. However, the state of the precursor within the intermediate has not been well investigated so far. In this study, an attempt was made to evaluate the extent of translocation of the precursor by determining the state of the precursor in the early intermediate under various conditions and analyzing the fragments generated by limited proteolysis of the precursors docked to chloroplasts. Our results indicate that three different sets of early intermediate are formed based on temperature and the hydrolysis of GTP/ATP. These have been identified based on the size of proteolytic fragments of the precursor as "energy-dependent association," "insertion," and "penetration" states. These findings suggest two individual ATP-hydrolyzing steps during the early stage of protein import, one of which is temperature-sensitive. Our results also demonstrate that translocation through the outer envelope membrane is mainly dependent on internal ATP.
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Affiliation(s)
- Hitoshi Inoue
- United Graduate School of Agricultural Science, Ehime University, Matsuyama, Ehime 790-8566, Japan
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32
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The Chloroplast Protein Import Apparatus, Its Components, and Their Roles. PLANT CELL MONOGRAPHS 2008. [DOI: 10.1007/7089_2008_40] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/16/2023]
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33
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Oreb M, Tews I, Schleiff E. Policing Tic 'n' Toc, the doorway to chloroplasts. Trends Cell Biol 2008; 18:19-27. [PMID: 18068366 DOI: 10.1016/j.tcb.2007.10.002] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/08/2007] [Revised: 10/08/2007] [Accepted: 10/22/2007] [Indexed: 10/22/2022]
Abstract
The organization of eukaryotic cells into different membrane-enclosed compartments requires an ordered and regulated system for targeting and translocating proteins synthesized in the cytosol across organellar membranes. Protein translocation through integral membrane proteinaceous complexes shares common principles in different organelles, whereas molecular mechanisms and energy requirements are diverse. Translocation into mitochondria and plastids requires most proteins to cross two membranes, and translocation must be regulated to accommodate environmental or metabolic changes. In the last decade, the first ideas were formulated about the regulation of protein translocation into chloroplasts, thereby laying the foundation for this field. Here, we describe recent models for the regulation of translocation by precursor protein phosphorylation, receptor dimerization, redox sensing and calcium signaling. We suggest how these mechanisms might fit within the regulatory framework for the entry of proteins into chloroplasts.
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Affiliation(s)
- Mislav Oreb
- LMU München, Cluster of Excellence CIPS, Department of Biology I, Menziger Str. 67, 80638 München, Germany
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34
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Oreb M, Zoryan M, Vojta A, Maier UG, Eichacker LA, Schleiff E. Phospho-mimicry mutant of atToc33 affects early development of Arabidopsis thaliana. FEBS Lett 2007; 581:5945-51. [DOI: 10.1016/j.febslet.2007.11.071] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/14/2007] [Accepted: 11/22/2007] [Indexed: 11/17/2022]
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35
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Qbadou S, Becker T, Bionda T, Reger K, Ruprecht M, Soll J, Schleiff E. Toc64 - A Preprotein-receptor at the Outer Membrane with Bipartide Function. J Mol Biol 2007; 367:1330-46. [PMID: 17306301 DOI: 10.1016/j.jmb.2007.01.047] [Citation(s) in RCA: 61] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/28/2006] [Revised: 01/15/2007] [Accepted: 01/17/2007] [Indexed: 11/25/2022]
Abstract
Protein translocation across membranes is assisted by translocation machineries present in the membrane targeted by the precursor proteins. Translocon subunits can be functionally divided into receptor proteins warranting the specificity of this machine and a translocation channel. At the outer envelope of chloroplasts two sets of receptor proteins regulate protein translocation facing the cytosol or acting in the intermembrane space. One, Toc64 is a receptor of the translocon at the outer envelope of chloroplasts (Toc complex) with dual function. Toc64 recognizes Hsp90 delivered precursor proteins via a cytosolic exposed domain containing three tetratrico-peptide repeat motifs and as demonstrated in here, Toc64 functions also as a major component of a complex facing the intermembrane space. The latter complex is composed of an Hsp70 localized in the intermembrane space, its interaction partner Toc12, a J-domain containing protein and the intermembrane space protein Tic22. We analyzed the intermembrane space domain of Toc64. This domain is involved in preprotein recognition and association with the Toc-complex independent of the cytosolic domain of the Toc64 receptor. Therefore, Toc64 is involved in preprotein translocation across the outer envelope at both sites of the membrane.
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Affiliation(s)
- Soumya Qbadou
- Botanik, LMU München, Menzinger Str. 67, 80638 München, Germany
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36
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Li HM, Kesavulu MM, Su PH, Yeh YH, Hsiao CD. Toc GTPases. J Biomed Sci 2007; 14:505-8. [PMID: 17394099 DOI: 10.1007/s11373-007-9166-2] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/31/2007] [Accepted: 03/08/2007] [Indexed: 10/23/2022] Open
Abstract
Chloroplasts import more than 90% of their protein constituents from the cytosol. The import is mediated by translocon complexes located in the chloroplast envelope and the stroma. This review focuses on the two GTPases in the Toc (translocon at the outer envelope membrane of chloroplasts) complex. Hypotheses are presented about gating across the outer membrane and the possible functional states of the GTPases.
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Affiliation(s)
- Hsou-Min Li
- Institute of Molecular Biology, Academia Sinica, Taipei, 115, Taiwan, ROC
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37
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Reddick LE, Vaughn MD, Wright SJ, Campbell IM, Bruce BD. In vitro comparative kinetic analysis of the chloroplast Toc GTPases. J Biol Chem 2007; 282:11410-26. [PMID: 17261588 DOI: 10.1074/jbc.m609491200] [Citation(s) in RCA: 39] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
A unique aspect of protein transport into plastids is the coordinate involvement of two GTPases in the translocon of the outer chloroplast membrane (Toc). There are two subfamilies in Arabidopsis, the small GTPases (Toc33 and Toc34) and the large acidic GTPases (Toc90, Toc120, Toc132, and Toc159). In chloroplasts, Toc34 and Toc159 are implicated in precursor binding, yet mechanistic details are poorly understood. How the GTPase cycle is modulated by precursor binding is complex and in need of careful dissection. To this end, we have developed novel in vitro assays to quantitate nucleotide binding and hydrolysis of the Toc GTPases. Here we present the first systematic kinetic characterization of four Toc GTPases (cytosolic domains of atToc33, atToc34, psToc34, and the GTPase domain of atToc159) to permit their direct comparison. We report the KM, Vmax, and Ea values for GTP hydrolysis and the Kd value for nucleotide binding for each protein. We demonstrate that GTP hydrolysis by psToc34 is stimulated by chloroplast transit peptides; however, this activity is not stimulated by homodimerization and is abolished by the R133A mutation. Furthermore, we show peptide stimulation of hydrolytic rates are not because of accelerated nucleotide exchange, indicating that transit peptides function as GTPase-activating proteins and not guanine nucleotide exchange factors in modulating the activity of psToc34. Finally, by using the psToc34 structure, we have developed molecular models for atToc33, atToc34, and atToc159G. By combining these models with the measured enzymatic properties of the Toc GTPases, we provide new insights of how the chloroplast protein import cycle may be regulated.
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Affiliation(s)
- L Evan Reddick
- Department of Biochemistry, Cellular and Molecular Biology, University of Tennessee, Knoxville 37996, USA
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38
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39
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Abstract
Plastids are a heterogeneous family of organelles found ubiquitously in plant and algal cells. Most prominent are the chloroplasts, which carry out such essential processes as photosynthesis and the biosynthesis of fatty acids as well as of amino acids. As mitochondria, chloroplasts derived from a single endosymbiotic event. They are believed to have evolved from an ancient cyanobacterium, which was engulfed by an early eukaryotic ancestor. During evolution the plastid genome has been greatly reduced and most of the genes have been transferred to the host nucleus. Consequently, more than 98% of all plastid proteins are translated on cytosolic ribosomes. They have to be posttranslationally targeted to and imported into the organelle. Targeting is assisted by cytosolic proteins, which interact with proteins destined for plastids and thereby keep them in an import-competent state. After reaching the target organelle, many proteins have to conquer the barrier of the chloroplast outer and inner envelopes. This process is mediated by complex molecular machines in the outer (Toc complex) and inner (Tic complex) envelope of chloroplasts, respectively. Most proteins destined for compartments inside the chloroplast contain a cleavable N-terminal transit peptide, whereas most of the outer envelope components insert into the membrane without such a targeting peptide.
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Affiliation(s)
- Friederike Hörmann
- Department Biologie I, Ludwig-Maximilians-Universität, Menzingerst, Germany
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40
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Kessler F, Schnell DJ. The function and diversity of plastid protein import pathways: a multilane GTPase highway into plastids. Traffic 2006; 7:248-57. [PMID: 16497220 DOI: 10.1111/j.1600-0854.2005.00382.x] [Citation(s) in RCA: 91] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
The photosynthetic chloroplast is the hallmark organelle of green plants. During the endosymbiotic evolution of chloroplasts, the vast majority of genes from the original cyanobacterial endosymbiont were transferred to the host cell nucleus. Chloroplast biogenesis therefore requires the import of nucleus-encoded proteins from their site of synthesis in the cytosol. The majority of proteins are imported by the activity of Toc and Tic complexes located within the chloroplast envelope. In addition to chloroplasts, plants have evolved additional, non-photosynthetic plastid types that are essential components of all cells. Recent studies indicate that the biogenesis of various plastid types relies on distinct but homologous Toc-Tic import pathways that have specialized in the import of specific classes of substrates. These different import pathways appear to be necessary to balance the essential physiological role of plastids in cellular metabolism with the demands of cellular differentiation and plant development.
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Affiliation(s)
- Felix Kessler
- Laboratoire de Physiologie Végétale, Institut de Botanique, Université de Neuchâtel, Rue Emile-Argand 11, 2007 Neuchâtel, Switzerland.
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41
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Smith MD. Protein import into chloroplasts: an ever-evolving storyThis review is one of a selection of papers published in the Special Issue on Plant Cell Biology. ACTA ACUST UNITED AC 2006. [DOI: 10.1139/b06-050] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Chloroplasts are but one type of a diverse group of essential organelles that distinguish plant cells and house many critical biochemical pathways, including photosynthesis. The biogenesis of plastids is essential to plant growth and development and relies on the targeting and import of thousands of nuclear-encoded proteins from the cytoplasm. The import of the vast majority of these proteins is dependent on translocons located in the outer and inner envelope membranes of the chloroplast, termed the Toc and Tic complexes, respectively. The core components of the Toc and Tic complexes have been identified within the last 12 years; however, the precise functions of many components are still being elucidated, and new components are still being identified. In Arabidopsis thaliana (and other species), many of the components are encoded by more than one gene, and it appears that the isoforms differentially associate with structurally distinct import complexes. Furthermore, it appears that these complexes represent functionally distinct targeting pathways, and the regulation of import by these separate pathways may play a role in the differentiation and specific functions of distinct plastid types during plant growth and development. This review summarizes these recent discoveries and emphasizes the mechanisms of differential Toc complex assembly and substrate recognition.
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Affiliation(s)
- Matthew D. Smith
- Department of Biology, Wilfrid Laurier University, Waterloo, ON N2L 3C5, Canada (e-mail: )
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42
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Gutensohn M, Fan E, Frielingsdorf S, Hanner P, Hou B, Hust B, Klösgen RB. Toc, Tic, Tat et al.: structure and function of protein transport machineries in chloroplasts. JOURNAL OF PLANT PHYSIOLOGY 2006; 163:333-47. [PMID: 16386331 DOI: 10.1016/j.jplph.2005.11.009] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/30/2005] [Accepted: 11/13/2005] [Indexed: 05/05/2023]
Abstract
The chloroplast is an organelle of prokaryotic origin that is situated in an eukaryotic cellular environment. As a result of this formerly endosymbiotic situation, the chloroplast houses a unique set of protein transport machineries. Among those are evolutionarily young transport pathways which are responsible for the import of the nuclear-encoded proteins into the organelle as well as ancient pathways operating in the 'export' of proteins from the stroma (the former cyanobacterial cytosol) across the thylakoid membrane into the thylakoid lumen. In this review, we have tried to address the main features of these various transport pathways.
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Affiliation(s)
- Michael Gutensohn
- Institut für Pflanzenphysiologie, Martin-Luther-Universität Halle-Wittenberg, Weinbergweg 10, D-06120 Halle (Saale), Germany
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43
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Aronsson H, Combe J, Patel R, Jarvis P. In vivo assessment of the significance of phosphorylation of theArabidopsischloroplast protein import receptor, atToc33. FEBS Lett 2005; 580:649-55. [PMID: 16412428 DOI: 10.1016/j.febslet.2005.12.055] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/30/2005] [Revised: 12/15/2005] [Accepted: 12/15/2005] [Indexed: 11/18/2022]
Abstract
atToc33 is a transit peptide receptor of the chloroplast outer envelope membrane, and possesses GTPase activity. In vitro, its transit peptide- and GTP-binding properties are abrogated by its phosphorylation at serine 181, which was proposed to represent an important regulatory mechanism. We mutated S181 to alanine (to prevent phosphorylation), and to aspartate and glutamate (to mimic the effects of phosphoserine), and expressed all three proteins in ppi1 (atToc33 knockout) plants using the native promoter. The mutants complemented ppi1 with equal efficiency in respect of all criteria tested, including protein import efficiency and light stress tolerance. The data suggest that atToc33 phosphorylation may not play an important role in vivo.
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Affiliation(s)
- Henrik Aronsson
- Department of Plant and Environmental Sciences, Göteborg University, Box 461, SE-405 30 Göteborg, Sweden
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44
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Bédard J, Jarvis P. Recognition and envelope translocation of chloroplast preproteins. JOURNAL OF EXPERIMENTAL BOTANY 2005; 56:2287-320. [PMID: 16087701 DOI: 10.1093/jxb/eri243] [Citation(s) in RCA: 68] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/03/2023]
Abstract
Plastids are a diverse group of plant organelles that perform essential functions including important steps in many biosynthetic pathways. Chloroplasts are the best characterized type of plastid, and constitute the site of oxygenic photosynthesis in plants, a process essential to all higher life forms. It is well established that the majority (>90%) of chloroplast proteins are nucleus-encoded and must be post-translationally imported into these envelope-bound compartments. Most nucleus-encoded chloroplast proteins are translated in precursor form on cytosolic ribosomes, targeted to the chloroplast surface, and then imported across the double-membrane envelope by translocons in the outer and inner envelope membranes of the chloroplast, termed TOC and TIC, respectively. Recently, significant progress has been made in our understanding of how proteins are targeted to the chloroplast surface and translocated across the chloroplast envelope into the stroma. Evidence suggesting the existence of multiple import pathways at the outer envelope membrane for different classes of precursor proteins has been presented. These pathways appear to utilize similar TOC complexes equipped with different combinations of homologous GTPase receptors, providing preprotein recognition specificity.
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Affiliation(s)
- Jocelyn Bédard
- Department of Biology, University of Leicester, University Road, Leicester LE1 7RH, UK
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45
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Becker T, Qbadou S, Jelic M, Schleiff E. Let's talk about...chloroplast import. PLANT BIOLOGY (STUTTGART, GERMANY) 2005; 7:1-14. [PMID: 15666210 DOI: 10.1055/s-2004-830447] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/24/2023]
Affiliation(s)
- T Becker
- Botanisches Institut, Ludwig-Maximilian-Universität München, Menzinger Strasse 67, 80368 München, Germany
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46
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Becker T, Hritz J, Vogel M, Caliebe A, Bukau B, Soll J, Schleiff E. Toc12, a novel subunit of the intermembrane space preprotein translocon of chloroplasts. Mol Biol Cell 2004; 15:5130-44. [PMID: 15317846 PMCID: PMC524789 DOI: 10.1091/mbc.e04-05-0405] [Citation(s) in RCA: 82] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/17/2004] [Accepted: 08/09/2004] [Indexed: 11/11/2022] Open
Abstract
Translocation of proteins across membranes is essential for the biogenesis of each cell and is achieved by proteinaceous complexes. We analyzed the translocation complex of the intermembrane space from chloroplasts and identified a 12-kDa protein associated with the Toc machinery. Toc12 is an outer envelope protein exposing a soluble domain into the intermembrane space. Toc12 contains a J-domain and stimulates the ATPase activity of DnaK. The conformational stability and the ability to stimulate Hsp70 are dependent on a disulfide bridge within the loop region of the J-domain, suggesting a redox-regulated activation of the chaperone. Toc12 is associated with Toc64 and Tic22. Its J-domain recruits the Hsp70 of outer envelope membrane to the intermembrane space translocon and facilitates its interaction to the preprotein.
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Affiliation(s)
- Thomas Becker
- Botanisches Institut, LMU München, 80638 Munich, Germany
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47
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Gutensohn M, Pahnke S, Kolukisaoglu U, Schulz B, Schierhorn A, Voigt A, Hust B, Rollwitz I, Stöckel J, Geimer S, Albrecht V, Flügge UI, Klösgen RB. Characterization of a T-DNA insertion mutant for the protein import receptor atToc33 from chloroplasts. Mol Genet Genomics 2004; 272:379-96. [PMID: 15517392 DOI: 10.1007/s00438-004-1068-7] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/22/2004] [Accepted: 09/11/2004] [Indexed: 11/26/2022]
Abstract
In Arabidopsis thaliana, the Toc34 receptor component of the chloroplast import machinery is encoded by two independent but highly homologous genes, atToc33 and atToc34. We have isolated a T-DNA insertion mutant of atToc33 which is characterized by a pale phenotype, due to reductions in the levels of photosynthetic pigments, and alterations in protein composition. The latter involve not only chloroplast proteins but also some cytosolic polypeptides, including 14-3-3 proteins which, among other functions, have been proposed to be cytosolic targeting factors for nucleus-encoded chloroplast proteins. Within the chloroplast, many, though not all, proteins of the photosynthetic apparatus, as well as proteins not directly involved in photosynthesis, are found in significantly reduced amounts in the mutant. However, the accumulation of other chloroplast proteins is unaffected. This suggests that the atToc33 receptor is responsible for the import of a specific subset of nucleus-encoded chloroplast proteins. Supporting evidence for this conclusion was obtained by antisense repression of the atToc34 gene in the atToc33 mutant, which results in an exacerbation of the phenotype.
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Affiliation(s)
- M Gutensohn
- Institut für Pflanzenphysiologie, Martin-Luther-Universität Halle-Wittenberg, Weinbergweg 10, 06120 Halle, Germany.
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48
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Ivanova Y, Smith MD, Chen K, Schnell DJ. Members of the Toc159 import receptor family represent distinct pathways for protein targeting to plastids. Mol Biol Cell 2004; 15:3379-92. [PMID: 15090618 PMCID: PMC452591 DOI: 10.1091/mbc.e03-12-0923] [Citation(s) in RCA: 160] [Impact Index Per Article: 8.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/23/2003] [Revised: 03/18/2004] [Accepted: 03/31/2004] [Indexed: 11/11/2022] Open
Abstract
Plastids represent a diverse group of organelles that perform essential metabolic and signaling functions within all plant cells. The differentiation of specific plastid types relies on the import of selective sets of proteins from among the approximately 2500 nucleus-encoded plastid proteins. The Toc159 family of GTPases mediates the initial targeting of proteins to plastids. In Arabidopsis thaliana, the Toc159 family consists of four genes: atTOC159, atTOC132, atTOC120, and atTOC90. In vivo analysis of atToc159 function indicates that it is required specifically for the import of proteins necessary for chloroplast biogenesis. In this report, we demonstrate that atToc120 and atToc132 represent a structurally and functionally unique subclass of protein import receptors. Unlike atToc159, mutants lacking both atToc120 and atToc132 are inviable. Furthermore, atToc120 and atToc132 exhibit preprotein binding properties that are distinct from atToc159. These data indicate that the different members of the Toc159 family represent distinct pathways for protein targeting to plastids and are consistent with the hypothesis that separate pathways have evolved to ensure balanced import of essential proteins during plastid development.
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Affiliation(s)
- Yordanka Ivanova
- Department of Biochemistry and Molecular Biology, University of Massachusetts, Amherst, Massachusetts 01003, USA
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49
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Abstract
The fidelity of the numerous intracellular protein-trafficking pathways to different organelles is dictated by the interactions between the intrinsic targeting signals of substrate proteins and specific receptors that deliver the substrate to the proper organelle. Recent studies of protein targeting to chloroplasts suggest a novel mechanism in which GTP-dependent substrate recognition is coupled to a GTP-driven motor that initiates the translocation of proteins into the organelle.
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Affiliation(s)
- Felix Kessler
- Laboratoire de Physiologie Végétale, Institute de Botanique, Université de Neuchâtel, Rue Emile-Argand 11, 2007 Neuchâtel, Switzerland.
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50
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Smith MD, Rounds CM, Wang F, Chen K, Afitlhile M, Schnell DJ. atToc159 is a selective transit peptide receptor for the import of nucleus-encoded chloroplast proteins. J Cell Biol 2004; 165:323-34. [PMID: 15138290 PMCID: PMC2172197 DOI: 10.1083/jcb.200311074] [Citation(s) in RCA: 125] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/14/2003] [Accepted: 03/26/2004] [Indexed: 11/24/2022] Open
Abstract
The members of the Toc159 family of GTPases act as the primary receptors for the import of nucleus-encoded preproteins into plastids. Toc159, the most abundant member of this family in chloroplasts, is required for chloroplast biogenesis (Bauer, J., K. Chen, A. Hiltbunner, E. Wehrli, M. Eugster, D. Schnell, and F. Kessler. 2000. Nature. 403:203-207) and has been shown to covalently cross-link to bound preproteins at the chloroplast surface (Ma, Y., A. Kouranov, S. LaSala, and D.J. Schnell. 1996. J. Cell Biol. 134:1-13; Perry, S.E., and K. Keegstra. 1994. Plant Cell. 6:93-105). These reports led to the hypothesis that Toc159 functions as a selective import receptor for preproteins that are required for chloroplast development. In this report, we provide evidence that Toc159 is required for the import of several highly expressed photosynthetic preproteins in vivo. Furthermore, we demonstrate that the cytoplasmic and recombinant forms of soluble Toc159 bind directly and selectively to the transit peptides of these representative photosynthetic preproteins, but not representative constitutively expressed plastid preproteins. These data support the function of Toc159 as a selective import receptor for the targeting of a set of preproteins required for chloroplast biogenesis.
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Affiliation(s)
- Matthew D Smith
- Department of Biochemistry and Molecular Biology and Program in Plant Biology, 820 LGRC, University of Massachusetts, Amherst, MA 01003, USA
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