1
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Aw YTV, Seidi A, Hayward JA, Lee J, Makota FV, Rug M, van Dooren GG. A key cytosolic iron-sulfur cluster synthesis protein localizes to the mitochondrion of Toxoplasma gondii. Mol Microbiol 2020; 115:968-985. [PMID: 33222310 DOI: 10.1111/mmi.14651] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/30/2020] [Revised: 11/16/2020] [Accepted: 11/18/2020] [Indexed: 12/21/2022]
Abstract
Iron-sulfur (Fe-S) clusters are prosthetic groups on proteins that function in a range of enzymatic and electron transfer reactions. Fe-S cluster synthesis is essential for the survival of all eukaryotes. Independent Fe-S cluster biosynthesis pathways occur in the mitochondrion, plastid, and cytosolic compartments of eukaryotic cells. Little is known about the cytosolic Fe-S cluster biosynthesis in apicomplexan parasites, the causative agents of diseases such as malaria and toxoplasmosis. NBP35 serves as a key scaffold protein on which cytosolic Fe-S clusters assemble, and has a cytosolic localization in most eukaryotes studied thus far. Unexpectedly, we found that the NBP35 homolog of the apicomplexan Toxoplasma gondii (TgNBP35) localizes to the outer mitochondrial membrane, with mitochondrial targeting mediated by an N-terminal transmembrane domain. We demonstrate that TgNBP35 is critical for parasite proliferation, but that, despite its mitochondrial localization, it is not required for Fe-S cluster synthesis in the mitochondrion. Instead, we establish that TgNBP35 is important for the biogenesis of cytosolic Fe-S proteins. Our data are consistent with TgNBP35 playing a central and specific role in cytosolic Fe-S cluster biosynthesis, and imply that the assembly of cytosolic Fe-S clusters occurs on the cytosolic face of the outer mitochondrial membrane in these parasites.
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Affiliation(s)
- Yi Tong Vincent Aw
- Research School of Biology, Australian National University, Canberra, ACT, Australia
| | - Azadeh Seidi
- Research School of Biology, Australian National University, Canberra, ACT, Australia
| | - Jenni A Hayward
- Research School of Biology, Australian National University, Canberra, ACT, Australia
| | - Jiwon Lee
- Centre for Advanced Microscopy, Australian National University, Canberra, ACT, Australia
| | - F Victor Makota
- Research School of Biology, Australian National University, Canberra, ACT, Australia
| | - Melanie Rug
- Centre for Advanced Microscopy, Australian National University, Canberra, ACT, Australia
| | - Giel G van Dooren
- Research School of Biology, Australian National University, Canberra, ACT, Australia
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2
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Park J, Lee SY, Jeong H, Kang MG, Van Haute L, Minczuk M, Seo JK, Jun Y, Myung K, Rhee HW, Lee C. The structure of human EXD2 reveals a chimeric 3' to 5' exonuclease domain that discriminates substrates via metal coordination. Nucleic Acids Res 2020; 47:7078-7093. [PMID: 31127291 PMCID: PMC6648332 DOI: 10.1093/nar/gkz454] [Citation(s) in RCA: 12] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/17/2019] [Revised: 05/07/2019] [Accepted: 05/10/2019] [Indexed: 12/24/2022] Open
Abstract
EXD2 (3′-5′ exonuclease domain-containing protein 2) is an essential protein with a conserved DEDDy superfamily 3′-5′ exonuclease domain. Recent research suggests that EXD2 has two potential functions: as a component of the DNA double-strand break repair machinery and as a ribonuclease for the regulation of mitochondrial translation. Herein, electron microscope imaging analysis and proximity labeling revealed that EXD2 is anchored to the mitochondrial outer membrane through a conserved N-terminal transmembrane domain, while the C-terminal region is cytosolic. Crystal structures of the exonuclease domain in complex with Mn2+/Mg2+ revealed a domain-swapped dimer in which the central α5−α7 helices are mutually crossed over, resulting in chimeric active sites. Additionally, the C-terminal segments absent in other DnaQ family exonucleases enclose the central chimeric active sites. Combined structural and biochemical analyses demonstrated that the unusual dimeric organization stabilizes the active site, facilitates discrimination between DNA and RNA substrates based on divalent cation coordination and generates a positively charged groove that binds substrates.
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Affiliation(s)
- Jumi Park
- Department of Biological Sciences, School of Life Sciences, Ulsan National Institute of Science and Technology, 50 UNIST-gil, Ulsan 44919, Republic of Korea.,Cell Logistics Research Center, Gwangju Institute of Science and Technology, Gwangju 61005, Republic of Korea
| | - Song-Yi Lee
- Department of Chemistry, Ulsan National Institute of Science and Technology, 50 UNIST-gil, Ulsan 44919, Republic of Korea.,Department of Chemistry, Seoul National University, Seoul 08826, Republic of Korea
| | - Hanbin Jeong
- Department of Biological Sciences, School of Life Sciences, Ulsan National Institute of Science and Technology, 50 UNIST-gil, Ulsan 44919, Republic of Korea.,Cell Logistics Research Center, Gwangju Institute of Science and Technology, Gwangju 61005, Republic of Korea
| | - Myeong-Gyun Kang
- Department of Chemistry, Ulsan National Institute of Science and Technology, 50 UNIST-gil, Ulsan 44919, Republic of Korea
| | - Lindsey Van Haute
- MRC Mitochondrial Biology Unit, University of Cambridge, Cambridge CB2 0XY, UK
| | - Michal Minczuk
- MRC Mitochondrial Biology Unit, University of Cambridge, Cambridge CB2 0XY, UK
| | - Jeong Kon Seo
- UNIST Central Research Facilities (UCRF), Ulsan National Institute of Science and Technology, 50 UNIST-gil, Ulsan 44919, Republic of Korea
| | - Youngsoo Jun
- School of Life Sciences, Gwangju Institute of Science and Technology, Gwangju 61005, Republic of Korea.,Cell Logistics Research Center, Gwangju Institute of Science and Technology, Gwangju 61005, Republic of Korea
| | - Kyungjae Myung
- Department of Biological Sciences, School of Life Sciences, Ulsan National Institute of Science and Technology, 50 UNIST-gil, Ulsan 44919, Republic of Korea.,Center for Genomic Integrity, Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea
| | - Hyun-Woo Rhee
- Department of Chemistry, Seoul National University, Seoul 08826, Republic of Korea
| | - Changwook Lee
- Department of Biological Sciences, School of Life Sciences, Ulsan National Institute of Science and Technology, 50 UNIST-gil, Ulsan 44919, Republic of Korea.,Cell Logistics Research Center, Gwangju Institute of Science and Technology, Gwangju 61005, Republic of Korea.,Center for Genomic Integrity, Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea
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3
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Bruggisser J, Käser S, Mani J, Schneider A. Biogenesis of a Mitochondrial Outer Membrane Protein in Trypanosoma brucei: TARGETING SIGNAL AND DEPENDENCE ON A UNIQUE BIOGENESIS FACTOR. J Biol Chem 2017; 292:3400-3410. [PMID: 28100781 DOI: 10.1074/jbc.m116.755983] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/29/2016] [Revised: 01/16/2017] [Indexed: 11/06/2022] Open
Abstract
The mitochondrial outer membrane (OM) contains single and multiple membrane-spanning proteins that need to contain signals that ensure correct targeting and insertion into the OM. The biogenesis of such proteins has so far essentially only been studied in yeast and related organisms. Here we show that POMP10, an OM protein of the early diverging protozoan Trypanosoma brucei, is signal-anchored. Transgenic cells expressing variants of POMP10 fused to GFP demonstrate that the N-terminal membrane-spanning domain flanked by a few positively charged or neutral residues is both necessary and sufficient for mitochondrial targeting. Carbonate extraction experiments indicate that although the presence of neutral instead of positively charged residues did not interfere with POMP10 localization, it weakened its interaction with the OM. Expression of GFP-tagged POMP10 in inducible RNAi cell lines shows that its mitochondrial localization depends on pATOM36 but does not require Sam50 or ATOM40, the trypanosomal analogue of the Tom40 import pore. pATOM36 is a kinetoplastid-specific OM protein that has previously been implicated in the assembly of OM proteins and in mitochondrial DNA inheritance. In summary, our results show that although the features of the targeting signal in signal-anchored proteins are widely conserved, the protein machinery that mediates their biogenesis is not.
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Affiliation(s)
- Julia Bruggisser
- Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland
| | - Sandro Käser
- Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland
| | - Jan Mani
- Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland
| | - André Schneider
- Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland.
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4
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Kroll K, Shekhova E, Mattern DJ, Thywissen A, Jacobsen ID, Strassburger M, Heinekamp T, Shelest E, Brakhage AA, Kniemeyer O. The hypoxia-induced dehydrogenase HorA is required for coenzyme Q10 biosynthesis, azole sensitivity and virulence ofAspergillus fumigatus. Mol Microbiol 2016; 101:92-108. [DOI: 10.1111/mmi.13377] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 03/14/2016] [Indexed: 12/30/2022]
Affiliation(s)
- Kristin Kroll
- Department of Molecular and Applied Microbiology; Leibniz Institute for Natural Product Research and Infection Biology - Hans Knöll Institute (HKI); Jena Germany
| | - Elena Shekhova
- Department of Molecular and Applied Microbiology; Leibniz Institute for Natural Product Research and Infection Biology - Hans Knöll Institute (HKI); Jena Germany
| | - Derek J. Mattern
- Department of Molecular and Applied Microbiology; Leibniz Institute for Natural Product Research and Infection Biology - Hans Knöll Institute (HKI); Jena Germany
| | - Andreas Thywissen
- Department of Molecular and Applied Microbiology; Leibniz Institute for Natural Product Research and Infection Biology - Hans Knöll Institute (HKI); Jena Germany
| | - Ilse D. Jacobsen
- Research Group Microbial Immunology, Leibniz Institute for Natural Product Research and Infection Biology - Hans Knöll Institute (HKI), Jena, and Friedrich Schiller University Jena; Jena Germany
| | - Maria Strassburger
- Department of Molecular and Applied Microbiology; Leibniz Institute for Natural Product Research and Infection Biology - Hans Knöll Institute (HKI); Jena Germany
- Transfer Group Anti-Infectives, Leibniz Institute for Natural Product Research and Infection Biology (HKI); Jena Germany
| | - Thorsten Heinekamp
- Department of Molecular and Applied Microbiology; Leibniz Institute for Natural Product Research and Infection Biology - Hans Knöll Institute (HKI); Jena Germany
- Department of Microbiology and Molecular Biology; Institute of Microbiology, Friedrich Schiller University; Jena Germany
| | - Ekaterina Shelest
- Research Group Systems Biology and Bioinformatics, Leibniz Institute for Natural Product Research and Infection Biology - Hans Knöll Institute (HKI), Jena, and Friedrich Schiller University Jena; Jena Germany
| | - Axel A. Brakhage
- Department of Molecular and Applied Microbiology; Leibniz Institute for Natural Product Research and Infection Biology - Hans Knöll Institute (HKI); Jena Germany
- Department of Microbiology and Molecular Biology; Institute of Microbiology, Friedrich Schiller University; Jena Germany
| | - Olaf Kniemeyer
- Department of Molecular and Applied Microbiology; Leibniz Institute for Natural Product Research and Infection Biology - Hans Knöll Institute (HKI); Jena Germany
- Department of Microbiology and Molecular Biology; Institute of Microbiology, Friedrich Schiller University; Jena Germany
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5
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Distribution and apoptotic function of outer membrane proteins depend on mitochondrial fusion. Mol Cell 2014; 54:870-8. [PMID: 24813948 DOI: 10.1016/j.molcel.2014.03.048] [Citation(s) in RCA: 43] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/09/2012] [Revised: 02/06/2014] [Accepted: 03/25/2014] [Indexed: 12/20/2022]
Abstract
Cells deficient in mitochondrial fusion have been shown to have defects linked to the exchange of inner membrane and matrix components. Because outer-mitochondrial membrane (OMM) constituents insert directly from the cytoplasm, a role for fusion in their intermitochondrial transfer was unanticipated. Here, we show that fibroblasts lacking the GTPases responsible for OMM fusion, mitofusins 1 and 2 (MFN1 and MFN2), display more heterogeneous distribution of OMM proteins. Proteins with different modes of OMM association display varying degrees of heterogeneity in Mfn1/2(-/-) cells and different kinetics of transfer during fusion in fusion-competent cells. Proapoptotic Bak exhibits marked heterogeneity, which is normalized upon expression of MFN2. Bak is critical for Bid-induced OMM permeabilization and cytochrome c release, and Mfn1/2(-/-) cells show dysregulation of Bid-dependent apoptotic signaling. Bid sensitivity of Bak-deficient mitochondria is regained upon fusion with Bak-containing mitochondria. Thus, OMM protein distribution depends on mitochondrial fusion and is a locus of apoptotic dysfunction in conditions of fusion deficiency.
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6
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Krumpe K, Rapaport D. An assay to monitor the membrane integration of single-span proteins. Methods Mol Biol 2013; 1033:301-306. [PMID: 23996185 DOI: 10.1007/978-1-62703-487-6_19] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/02/2023]
Abstract
In vitro import experiments with isolated organelles are a powerful tool for investigation of the biogenesis of proteins. A key issue in such experiments is an assay to distinguish between correctly and incorrectly imported proteins. Here we describe an assay to monitor in vitro the proper membrane integration of single-span proteins. In this assay non-imported proteins are distinguished from correctly imported protein species by labelling of unprotected cysteine residues and a resulting migration shift in SDS-PAGE.
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Affiliation(s)
- Katrin Krumpe
- Interfaculty Institute of Biochemistry, University of Tübingen, Tübingen, Germany
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7
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Klein JM, Busch JD, Potting C, Baker MJ, Langer T, Schwarz G. The mitochondrial amidoxime-reducing component (mARC1) is a novel signal-anchored protein of the outer mitochondrial membrane. J Biol Chem 2012; 287:42795-803. [PMID: 23086957 DOI: 10.1074/jbc.m112.419424] [Citation(s) in RCA: 57] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/23/2023] Open
Abstract
The mitochondrial amidoxime-reducing component (mARC) was recently discovered as the fifth eukaryotic molybdenum cofactor-containing enzyme. The human genome encodes two mARC proteins, mARC1 and mARC2, sharing significant homologies with respect to sequence and function. Whereas mARC2 was identified as a mitochondrial enzyme, the subcellular localization of mARC1 has remained uncharacterized, although the similarity of both proteins suggested identical subcellular localizations. In addition, neither mARC1 nor mARC2 could be attributed unambiguously to one of the four mitochondrial subcompartments. Accordingly, mechanisms triggering the subcellular distribution of both enzymes have been unexplored so far. Here, we shed light on the subcellular localization of mARC1 and demonstrate that it is integrated into the outer mitochondrial membrane. The C-terminal catalytic domain of the protein remains exposed to the cytosol and confers an N((in))-C((out)) membrane orientation of mARC1. This localization is triggered by the N terminus of the enzyme, being composed of a weak N-terminal mitochondrial targeting signal and a downstream transmembrane helix. We demonstrate the transmembrane domain of mARC1 to be sufficient for mitochondrial targeting and the N-terminal targeting signal to function as a supportive receptor for the outer mitochondrial membrane. According to its localization and targeting mechanism, we classify mARC1 as a novel signal-anchored mitochondrial protein. During mitochondrial import, mARC1 is not processed, and membrane integration proceeds membrane potential independently but requires external ATP, which finally results in the assembly of mARC1 into high oligomeric protein complexes.
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Affiliation(s)
- Julian M Klein
- Institute of Biochemistry, Department of Chemistry and Center for Molecular Medicine, University of Cologne, 50674 Cologne, Germany
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8
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Membrane integration of a mitochondrial signal-anchored protein does not require additional proteinaceous factors. Biochem J 2012; 442:381-9. [DOI: 10.1042/bj20111363] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
The MOM (mitochondrial outer membrane) contains SA (signal-anchored) proteins that bear at their N-terminus a single hydrophobic segment that serves as both a mitochondrial targeting signal and an anchor at the membrane. These proteins, like the vast majority of mitochondrial proteins, are encoded in the nucleus and have to be imported into the organelle. Currently, the mechanisms by which they are targeted to and inserted into the OM (outer membrane) are unclear. To shed light on these issues, we employed a recombinant version of the SA protein OM45 and a synthetic peptide corresponding to its signal-anchor segment. Both forms are associated with isolated mitochondria independently of cytosolic factors. Interaction with mitochondria was diminished when a mutated form of the signal-anchor was employed. We demonstrate that the signal-anchor peptide acquires an α-helical structure in a lipid environment and adopted a TM (transmembrane) topology within artificial lipid bilayers. Moreover, the peptide's affinity to artificial membranes with OM-like lipid composition was much higher than that of membranes with ER (endoplasmic reticulum)-like lipid composition. Collectively, our results suggest that SA proteins are specifically inserted into the MOM by a process that is not dependent on additional proteins, but is rather facilitated by the distinct lipid composition of this membrane.
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9
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Lackey SWK, Wideman JG, Kennedy EK, Go NE, Nargang FE. The Neurospora crassa TOB complex: analysis of the topology and function of Tob38 and Tob37. PLoS One 2011; 6:e25650. [PMID: 21980517 PMCID: PMC3182244 DOI: 10.1371/journal.pone.0025650] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/25/2011] [Accepted: 09/07/2011] [Indexed: 11/18/2022] Open
Abstract
The TOB or SAM complex is responsible for assembling several proteins into the mitochondrial outer membrane, including all β-barrel proteins. We have identified several forms of the complex in Neurospora crassa. One form contains Tob55, Tob38, and Tob37; another contains these three subunits plus the Mdm10 protein; while additional complexes contain only Tob55. As previously shown for Tob55, both Tob37 and Tob38 are essential for viability of the organism. Mitochondria deficient in Tob37 or Tob38 have reduced ability to assemble β-barrel proteins. The function of two hydrophobic domains in the C-terminal region of the Tob37 protein was investigated. Mutant Tob37 proteins lacking either or both of these regions are able to restore viability to cells lacking the protein. One of the domains was found to anchor the protein to the outer mitochondrial membrane but was not necessary for targeting or association of the protein with mitochondria. Examination of the import properties of mitochondria containing Tob37 with deletions of the hydrophobic domains reveals that the topology of Tob37 may be important for interactions between specific classes of β-barrel precursors and the TOB complex.
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Affiliation(s)
| | - Jeremy G. Wideman
- Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada
| | - Erin K. Kennedy
- Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada
| | - Nancy E. Go
- Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada
| | - Frank E. Nargang
- Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada
- * E-mail:
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10
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Danne JC, Waller RF. Analysis of dinoflagellate mitochondrial protein sorting signals indicates a highly stable protein targeting system across eukaryotic diversity. J Mol Biol 2011; 408:643-53. [PMID: 21376056 DOI: 10.1016/j.jmb.2011.02.057] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/22/2010] [Revised: 02/21/2011] [Accepted: 02/24/2011] [Indexed: 11/17/2022]
Abstract
Protein targeting into mitochondria from the cytoplasm is fundamental to the cell biology of all eukaryotes. Our understanding of this process is heavily biased towards "model" organisms, such as animals and fungi, and it is less clear how conserved this process is throughout diverse eukaryotes. In this study, we have surveyed mitochondrial protein sorting signals from a representative of the dinoflagellate algae. Dinoflagellates are a phylum belonging to the group Alveolata, which also includes apicomplexan parasites and ciliates. We generated 46 mitochondrial gene sequences from the dinoflagellate Karlodinium micrum and analysed these for mitochondrial sorting signals. Most of the sequences contain predicted N-terminal peptide extensions that conform to mitochondrial targeting peptides from animals and fungi in terms of length, amino acid composition, and propensity to form amphipathic α-helices. The remainder lack predicted mitochondrial targeting peptides and represent carrier proteins of the inner mitochondrial membrane that have internal targeting signals in model eukaryotes. We tested for functional conservation of the dinoflagellate mitochondrial sorting signals by expressing K. micrum mitochondrial proteins in the fungus Saccharomyces cerevisiae. Both the N-terminal and internal targeting signals were sufficiently conserved to operate in this distantly related system. This study indicates that the character of mitochondrial sorting signals was well established prior to the radiation of major eukaryotic lineages and has shown remarkable conservation during long periods of evolution.
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Affiliation(s)
- Jillian C Danne
- School of Botany, University of Melbourne, Victoria 3010, Australia
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11
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Dukanovic J, Rapaport D. Multiple pathways in the integration of proteins into the mitochondrial outer membrane. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2011; 1808:971-80. [DOI: 10.1016/j.bbamem.2010.06.021] [Citation(s) in RCA: 92] [Impact Index Per Article: 7.1] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/06/2010] [Revised: 06/22/2010] [Accepted: 06/23/2010] [Indexed: 11/25/2022]
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12
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Rucktäschel R, Halbach A, Girzalsky W, Rottensteiner H, Erdmann R. De novo synthesis of peroxisomes upon mitochondrial targeting of Pex3p. Eur J Cell Biol 2010; 89:947-54. [PMID: 20655617 DOI: 10.1016/j.ejcb.2010.06.012] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022] Open
Abstract
Peroxisomes can form either by growth and division of pre-existing peroxisomes or by de novo synthesis from the endoplasmic reticulum. Pex3p is the key component for both pathways and its targeting to the ER is thought to initiate the de novo formation of peroxisomes. Here, we addressed the question whether Pex3p also can induce peroxisome formation from mitochondrial membranes. Pex3p was targeted to mitochondria by fusion with the mitochondrial targeting signal of Tom20p. The Tom20p-Pex3p-fusion protein was expressed in Pex3p-deficient cells, which are characterized by the lack of peroxisomal membranes. De novo formation of import-competent peroxisomes was observed upon expression of the mitochondrial Pex3p in the mutant cells. This de novo synthesis is independent of the GTPases Vps1p and Dnm1p, two proteins required for peroxisome fission. We conclude that natural or artificial targeting of Pex3p to any endomembrane may initiate peroxisome formation and that also Pex3p-containing mitochondria can serve as source for the de novo synthesis of peroxisomes.
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Affiliation(s)
- Robert Rucktäschel
- Institut für Physiologische Chemie, Abteilung für Systembiochemie, Ruhr-Universität Bochum, Universitätsstrasse 150, 44780 Bochum, Germany
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13
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Hwang YT, McCartney AW, Gidda SK, Mullen RT. Localization of the Carnation Italian ringspot virus replication protein p36 to the mitochondrial outer membrane is mediated by an internal targeting signal and the TOM complex. BMC Cell Biol 2008; 9:54. [PMID: 18811953 PMCID: PMC2573885 DOI: 10.1186/1471-2121-9-54] [Citation(s) in RCA: 43] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/14/2008] [Accepted: 09/23/2008] [Indexed: 11/24/2022] Open
Abstract
BACKGROUND Carnation Italian ringspot virus (CIRV) is a positive-strand RNA virus that causes massive structural alterations of mitochondria in infected host cells, the most conspicuous being the formation of numerous internal vesicles/spherules that are derived from the mitochondrial outer membrane and serve as the sites for viral RNA replication. While the membrane-bound components of the CIRV replication complex, including a 36-kD RNA-binding protein (p36), are known to be essential for these changes in mitochondrial morphology and are relatively well characterized in terms of their roles in nascent viral RNA synthesis, how these proteins are specifically targeted and inserted into mitochondria is poorly defined. RESULTS Here we report on the molecular signal responsible for sorting p36 to the mitochondrial outer membrane. Using a combination of gain-of-function assays with portions of p36 fused to reporter proteins and domain-swapping assays with p36 and another closely-related viral RNA-binding protein, p33, that sorts specifically to the peroxisomal boundary membrane, we show that the mitochondrial targeting information in p36 resides within its two transmembrane domains (TMDs) and intervening hydrophilic loop sequence. Comprehensive mutational analysis of these regions in p36 revealed that the primary targeting determinants are the moderate hydrophobicity of both TMDs and the positively-charged face of an amphipathic helix within the intervening loop sequence. We show also using bimolecular fluorescence complementation (BiFC) that p36 interacts with certain components of the translocase complex in the mitochondrial outer membrane (TOM), but not with the sorting and assembly machinery (SAM). CONCLUSION Our results provide insight to how viruses, such as CIRV, exploit specific host-cell protein sorting pathways to facilitate their replication. The characterization of the targeting and insertion of p36 into the mitochondrial outer membrane also sheds light on the mechanisms involved in sorting of host-cell membrane proteins to mitochondria, a process that has been largely unexplored in plants.
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Affiliation(s)
- Yeen Ting Hwang
- Department of Molecular and Cellular Biology, University of Guelph, Guelph, Ontario, N1G 2W1, Canada
| | - Andrew W McCartney
- Department of Molecular and Cellular Biology, University of Guelph, Guelph, Ontario, N1G 2W1, Canada
- JD Irving, Limited, Woodlands Division, 1350 Regent Street, Fredericton, New Brunswick, E3C 2G6, Canada
| | - Satinder K Gidda
- Department of Molecular and Cellular Biology, University of Guelph, Guelph, Ontario, N1G 2W1, Canada
| | - Robert T Mullen
- Department of Molecular and Cellular Biology, University of Guelph, Guelph, Ontario, N1G 2W1, Canada
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14
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Millen JI, Pierson J, Kvam E, Olsen LJ, Goldfarb DS. The luminal N-terminus of yeast Nvj1 is an inner nuclear membrane anchor. Traffic 2008; 9:1653-64. [PMID: 18694438 DOI: 10.1111/j.1600-0854.2008.00789.x] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
The endoplasmic reticulum (ER) in Saccharomyces cerevisiae is largely divided between perinuclear and cortical compartments. Yeast Nvj1 localizes exclusively to small patches on the perinuclear ER where it interacts with Vac8 in the vacuole membrane to form nucleus-vacuole (NV) junctions. Three regions of Nvj1 mediate the biogenesis of NV junctions. A membrane-spanning domain targets the protein to the ER. The C-terminus binds Vac8 in the vacuole membrane, which induces the clustering of both proteins into NV junctions. The luminal N-terminus is required for strict perinuclear localization. Three-dimensional cryo-electron tomography reveals that Nvj1 clamps the separation between the two nuclear membranes to half the width of bulk nuclear envelope. The N-terminus contains a hydrophobic sequence bracketed by basic residues that resembles outer mitochondrial membrane signal-anchors. The hydrophobic sequence can be scrambled or reversed without affecting function. Mutations that reduce the hydrophobicity of the core sequence or affect the distribution of basic residues cause mislocalization to the cortical ER. We conclude that the N-terminus of Nvj1 is a retention sequence that bridges the perinuclear lumen and inserts into the inner nuclear membrane.
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Affiliation(s)
- Jonathan I Millen
- Department of Biology, University of Rochester, Rochester, NY 14627, USA
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15
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Winter L, Abrahamsberg C, Wiche G. Plectin isoform 1b mediates mitochondrion-intermediate filament network linkage and controls organelle shape. ACTA ACUST UNITED AC 2008; 181:903-11. [PMID: 18541706 PMCID: PMC2426950 DOI: 10.1083/jcb.200710151] [Citation(s) in RCA: 88] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Plectin is a versatile intermediate filament (IF)–bound cytolinker protein with a variety of differentially spliced isoforms accounting for its multiple functions. One particular isoform, plectin 1b (P1b), remains associated with mitochondria after biochemical fractionation of fibroblasts and cells expressing exogenous P1b. Here, we determined that P1b is inserted into the outer mitochondrial membrane with the exon 1b–encoded N-terminal sequence serving as a mitochondrial targeting and anchoring signal. To study P1b-related mitochondrial functions, we generated mice that selectively lack this isoform but express all others. In primary fibroblasts and myoblasts derived from these mice, we observe a substantial elongation of mitochondrial networks, whereas other mitochondrial properties remain largely unaffected. Normal morphology of mitochondria could be restored by isoform-specific overexpression of P1b in P1b-deficient as well as plectin-null cells. We propose a model where P1b both forms a mitochondrial signaling platform and affects organelle shape and network formation by tethering mitochondria to IFs.
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Affiliation(s)
- Lilli Winter
- Department of Molecular Cell Biology, Max F. Perutz Laboratories, University of Vienna, 1030 Vienna, Austria
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Walther DM, Rapaport D. Biogenesis of mitochondrial outer membrane proteins. BIOCHIMICA ET BIOPHYSICA ACTA-MOLECULAR CELL RESEARCH 2008; 1793:42-51. [PMID: 18501716 DOI: 10.1016/j.bbamcr.2008.04.013] [Citation(s) in RCA: 94] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/29/2008] [Revised: 04/15/2008] [Accepted: 04/25/2008] [Indexed: 11/29/2022]
Abstract
Mitochondria are surrounded by two distinct membranes: the outer and the inner membrane. The mitochondrial outer membrane mediates numerous interactions between the mitochondrial metabolic and genetic systems and the rest of the eukaryotic cell. Proteins of this membrane are nuclear-encoded and synthesized as precursor proteins in the cytosol. They are targeted to the mitochondria and inserted into their target membrane via various pathways. This review summarizes our current knowledge of the sorting signals for this specific targeting and describes the mechanisms by which the mitochondrial import machineries recognize precursor proteins, mediate their membrane integration and facilitate assembly into functional complexes.
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Affiliation(s)
- Dirk M Walther
- Interfakultäres Institut für Biochemie, Hoppe-Seyler-Str. 4, University of Tübingen, 72076 Tübingen, Germany
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17
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Meineke B, Engl G, Kemper C, Vasiljev-Neumeyer A, Paulitschke H, Rapaport D. The outer membrane form of the mitochondrial protein Mcr1 follows a TOM-independent membrane insertion pathway. FEBS Lett 2008; 582:855-60. [DOI: 10.1016/j.febslet.2008.02.009] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/02/2007] [Revised: 01/31/2008] [Accepted: 02/06/2008] [Indexed: 10/22/2022]
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18
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Chapter 5 New Insights into the Mechanism of Precursor Protein Insertion into the Mitochondrial Membranes. INTERNATIONAL REVIEW OF CELL AND MOLECULAR BIOLOGY 2008; 268:147-90. [DOI: 10.1016/s1937-6448(08)00805-8] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 04/10/2023]
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19
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Becker T, Pfannschmidt S, Guiard B, Stojanovski D, Milenkovic D, Kutik S, Pfanner N, Meisinger C, Wiedemann N. Biogenesis of the Mitochondrial TOM Complex. J Biol Chem 2008; 283:120-127. [DOI: 10.1074/jbc.m706997200] [Citation(s) in RCA: 119] [Impact Index Per Article: 7.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
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20
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Paterson JK, Shukla S, Black CM, Tachiwada T, Garfield S, Wincovitch S, Ernst DN, Agadir A, Li X, Ambudkar SV, Szakacs G, Akiyama SI, Gottesman MM. Human ABCB6 Localizes to Both the Outer Mitochondrial Membrane and the Plasma Membrane. Biochemistry 2007; 46:9443-52. [PMID: 17661442 DOI: 10.1021/bi700015m] [Citation(s) in RCA: 96] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Expression of the ATP-binding cassette transporter ABCB6 has been associated with multiple cellular functions, including resistance to several cytotoxic agents, iron homeostasis, and porphyrin transport. To further elucidate its physiological function and/or role in drug resistance, we determined the subcellular location of ABCB6. Using three novel ABCB6-specific antibodies, Western blot analysis of cells expressing cDNA-derived or endogenous ABCB6 revealed two distinct molecular weight forms. Confocal microscopy indicates that the protein localizes to both mitochondria and the plasma membrane. Differential centrifugation revealed that the lower molecular weight form predominantly resides in the mitochondria, while the larger protein form is more abundant in the plasma membrane. Preliminary studies indicate that ABCB6 is functionally relevant in the plasma membrane, where its expression prevents the accumulation of specific porphyrins in the cell. Digitonin solubilization of mitochondria demonstrated that ABCB6 is present in the outer mitochondrial membrane, while back-titration assays with the ABCB6-specific antibodies reveal that the nucleotide binding domain of ABCB6 is cytoplasmic. These studies are the first to demonstrate that ABCB6 exists in two molecular weight forms, is localized to both the outer mitochondrial membrane and the plasma membrane, and plays a functional role in the plasma membrane.
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Affiliation(s)
- Jill K Paterson
- Laboratory of Cell Biology, Center for Cancer Research, National Cancer Institute, NIH, DHHS, Bethesda, Maryland 20892-4256, USA
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21
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Habib SJ, Neupert W, Rapaport D. Analysis and prediction of mitochondrial targeting signals. Methods Cell Biol 2007; 80:761-81. [PMID: 17445721 DOI: 10.1016/s0091-679x(06)80035-x] [Citation(s) in RCA: 46] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Affiliation(s)
- Shukry J Habib
- Institut für Physiologische Chemie, Universität München, D-81377 Munich, Germany
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22
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23
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Baldwin AJ, Inoue K. The most C-terminal tri-glycine segment within the polyglycine stretch of the pea Toc75 transit peptide plays a critical role for targeting the protein to the chloroplast outer envelope membrane. FEBS J 2006; 273:1547-55. [PMID: 16689940 DOI: 10.1111/j.1742-4658.2006.05175.x] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
The protein translocation channel at the outer envelope membrane of chloroplasts (Toc75) is synthesized as a larger precursor with an N-terminal transit peptide. Within the transit peptide of the pea Toc75, a major portion of the 10 amino acid long stretch that contains nine glycine residues was shown to be necessary for directing the protein to the chloroplast outer membrane in vitro. In order to get insights into the mechanism by which the polyglycine stretch mediates correct targeting, we divided it into three tri-glycine segments and examined the importance of each domain in targeting specificity in vitro. Replacement of the most C-terminal segment with alanine residues resulted in mistargeting the protein to the stroma, while exchange of either of the other two tri-glycine regions had no effect on correct targeting. Furthermore, simultaneous replacement of the N-terminal and middle tri-glycine segments with alanine repeats did not cause mistargeting of the protein as much as those of the N- and C-terminal, or the middle and C-terminal segments. These results indicate that the most C-terminal tri-glycine segment is important for correct targeting. Exchanging this portion with a repeat of leucine or glutamic acid also caused missorting of Toc75 to the stroma. By contrast, its replacement with repeats of asparagine, aspartic acid, serine, and proline did not largely affect correct targeting. These data suggest that relatively compact and nonhydrophobic side chains in this particular region play a crucial role in correct sorting of Toc75.
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Affiliation(s)
- Amy J Baldwin
- Department of Plant Sciences, College of Agricultural and Environmental Sciences, University of California, CA 95616, USA
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24
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Colombo S, Longhi R, Alcaro S, Ortuso F, Sprocati T, Flora A, Borgese N. N-myristoylation determines dual targeting of mammalian NADH-cytochrome b5 reductase to ER and mitochondrial outer membranes by a mechanism of kinetic partitioning. ACTA ACUST UNITED AC 2005; 168:735-45. [PMID: 15738266 PMCID: PMC2171821 DOI: 10.1083/jcb.200407082] [Citation(s) in RCA: 63] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Mammalian NADH-cytochrome b(5) reductase (b5R) is an N-myristoylated protein that is dually targeted to ER and mitochondrial outer membranes. The N-linked myristate is not required for anchorage to membranes because a stretch of hydrophobic amino acids close to the NH2 terminus guarantees a tight interaction of the protein with the phospholipid bilayer. Instead, the fatty acid is required for targeting of b5R to mitochondria because a nonmyristoylated mutant is exclusively localized to the ER. Here, we have investigated the mechanism by which N-linked myristate affects b5R targeting. We find that myristoylation interferes with interaction of the nascent chain with signal recognition particle, so that a portion of the nascent chains escapes from cotranslational integration into the ER and can be post-translationally targeted to the mitochondrial outer membrane. Thus, competition between two cotranslational events, binding of signal recognition particle and modification by N-myristoylation, determines the site of translation and the localization of b5R.
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Affiliation(s)
- Sara Colombo
- Consiglio Nazionale delle Ricerche Institute of Neuroscience, Cellular and Molecular Pharmacology Section and Department of Medical Pharmacology, University of Milan, 20129 Milan, Italy
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25
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Ahting U, Waizenegger T, Neupert W, Rapaport D. Signal-anchored Proteins Follow a Unique Insertion Pathway into the Outer Membrane of Mitochondria. J Biol Chem 2005; 280:48-53. [PMID: 15501820 DOI: 10.1074/jbc.m410905200] [Citation(s) in RCA: 62] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Signal-anchored proteins are a class of mitochondrial outer membrane proteins that expose a hydrophilic domain to the cytosol and are anchored to the membrane by a single transmembrane domain in the N-terminal region. Like the vast majority of mitochondrial proteins, signal-anchored proteins are synthesized on cytosolic ribosomes and are subsequently imported into the organelle. We have studied the mechanisms by which precursors of these proteins are recognized by the mitochondria and are inserted into the outer membrane. The import of signal-anchored proteins was found to be independent of the known import receptors, Tom20 and Tom70, but to require the major Tom component, Tom40. In contrast to precursors destined to internal compartments of mitochondria and those of outer membrane beta-barrel proteins, precursors of signal-anchored proteins appear not to be inserted via the general import pore. Taken together, we propose a novel pathway for insertion of these proteins into the outer membrane of mitochondria.
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Affiliation(s)
- Uwe Ahting
- Institut für Physiologische Chemie der Universität München, D-81377 Munich, Germany
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26
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Taylor RD, Pfanner N. The protein import and assembly machinery of the mitochondrial outer membrane. BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 2004; 1658:37-43. [PMID: 15282172 DOI: 10.1016/j.bbabio.2004.04.017] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/18/2004] [Accepted: 04/19/2004] [Indexed: 12/21/2022]
Abstract
The process of mitochondrial protein import has been studied for many years. Despite this attention, many processes associated with mitochondrial biogenesis are poorly understood. Insight into one of these processes, assembly of beta-barrel proteins into the mitochondrial outer membrane, will be discussed. This review focuses on recent data that suggest that assembly of beta-barrel proteins into the outer mitochondrial membrane is dependent on a newly identified protein complex termed the sorting and assembly machinery (SAM complex). Members of the SAM complex have been identified in both eukaryotic and prokaryotic organisms, suggesting that the process of beta-barrel assembly into membranes has been conserved through evolution.
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Affiliation(s)
- Rebecca D Taylor
- Institut für Biochemie und Molekularbiologie, Universität Freiburg, Hermann-Herder-Strasse 7, D-79104 Freiburg, Germany
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27
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Guo YX, Chan SW, Kwang J. Membrane association of greasy grouper nervous necrosis virus protein A and characterization of its mitochondrial localization targeting signal. J Virol 2004; 78:6498-508. [PMID: 15163743 PMCID: PMC416515 DOI: 10.1128/jvi.78.12.6498-6508.2004] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Localization of RNA replication to intracellular membranes is a universal feature of positive-strand RNA viruses. The betanodavirus greasy grouper (Epinephelus tauvina) nervous necrosis virus (GGNNV) is a positive-RNA virus with one of the smallest genomes among RNA viruses replicating in fish cells. To understand the localization of GGNNV replication complexes, we generated polyclonal antisera against protein A, the GGNNV RNA-dependent RNA polymerase. Protein A was detected at 5 h postinfection in infected sea bass cells. Biochemical fractionation experiments revealed that GGNNV protein A sedimented with intracellular membranes upon treatment with an alkaline pH and a high salt concentration, indicating that GGNNV protein A is tightly associated with intracellular membranes in infected cells. Confocal immunofluorescence microscopy and bromo-UTP incorporation studies identified mitochondria as the intracellular site of protein A localization and viral RNA synthesis. In addition, protein A fused with green fluorescent protein (GFP) was detected in the mitochondria in transfected cells and was demonstrated to be tightly associated with intracellular membranes by biochemical fractionation analysis and membrane flotation assays, indicating that protein A alone was sufficient for mitochondrial localization in the absence of RNA replication, nonstructural protein B, or capsid proteins. Three sequence analysis programs showed two regions of hydrophobic amino acid residues, amino acids 153 to 173 and 229 to 249, to be transmembrane domains (TMD) that might contain a membrane association domain. Membrane fraction analysis showed that the major domain is N-terminal amino acids 215 to 255, containing the predicted TMD from amino acids 229 to 249. Using GFP as the reporter by systematically introducing deletions of these two regions in the constructs, we further confirmed that the N-terminal amino acids 215 to 255 of protein A function as a mitochondrial targeting signal.
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Affiliation(s)
- Yan Xiang Guo
- Animal Health Biotechnology Unit, Temasek Life Sciences Laboratory, 1 Research Link, The National University of Singapore, Singapore 117604
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28
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Rapaport D. Finding the right organelle. Targeting signals in mitochondrial outer-membrane proteins. EMBO Rep 2004; 4:948-52. [PMID: 14528265 PMCID: PMC1326395 DOI: 10.1038/sj.embor.embor937] [Citation(s) in RCA: 162] [Impact Index Per Article: 8.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/27/2003] [Accepted: 07/16/2003] [Indexed: 12/19/2022] Open
Abstract
The mitochondrial outer membrane contains a diverse set of proteins that includes enzymes, components of the preprotein translocation machinery, pore-forming proteins, regulators of programmed cell death, and those that control the morphology of the organelle. All these proteins, like the vast majority of mitochondrial proteins, are encoded in the nucleus, so they are synthesized in the cytosol and contain signals that are essential for their subsequent import into mitochondria. This review summarizes our current knowledge of the signals that target mitochondrial outer-membrane proteins to their correct intracellular location. In addition, the mechanisms by which these signals are decoded by the mitochondria are discussed.
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Affiliation(s)
- Doron Rapaport
- Institut für Physiologische Chemie der Universität München, Butenandtstrasse 5, D-81377 Munich, Germany.
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29
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Nakamura Y, Suzuki H, Sakaguchi M, Mihara K. Targeting and Assembly of Rat Mitochondrial Translocase of Outer Membrane 22 (TOM22) into the TOM Complex. J Biol Chem 2004; 279:21223-32. [PMID: 14985332 DOI: 10.1074/jbc.m314156200] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Tom22 is a preprotein receptor and organizer of the mitochondrial outer membrane translocase complex (TOM complex). Rat Tom22 (rTOM22) is a 142-residue protein, embedded in the outer membrane through the internal transmembrane domain (TMD) with 82 N-terminal residues in the cytosol and 41 C-terminal residues in the intermembrane space. We analyzed the signals that target rTOM22 to the mitochondrial outer membrane and assembly into the TOM complex in cultured mammalian cells. Deletions or mutations were systematically introduced into the molecule, and the intracellular localization of the mutant constructs in HeLa cells was examined by confocal microscopy and cell fractionation. Their assembly into the TOM complex was also examined using blue native gel electrophoresis. These experiments revealed three separate structural elements: a cytoplasmic 10-residue segment with an acidic alpha-helical structure located 30 residues upstream of the TMD (the import sequence), TMD with an appropriate hydrophobicity, and a 20-residue C-terminal segment located 22 residues downstream of the TMD (C-tail signal). The import sequence and TMD were both essential for targeting and integration into the TOM complex, whereas the C-tail signal affected the import efficiency. The import sequence combined with foreign TMD functioned as a mitochondrial targeting and anchor signal but failed to integrate the construct into the TOM complex. Thus, the mitochondrial-targeting and TOM integration signal could be discriminated. A yeast two-hybrid assay revealed that the import sequence interacted with two intramolecular elements, the TMD and C-tail signal, and that it also interacted with the import receptor Tom20.
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Affiliation(s)
- Yasuhiko Nakamura
- Department of Molecular Biology, Graduate School of Medical Science, Kyushu University, Fukuoka 812-0054, Japan
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30
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Heimlich G, McKinnon AD, Bernardo K, Brdiczka D, Reed JC, Kain R, Krönke M, Jürgensmeier JM. Bax-induced cytochrome c release from mitochondria depends on alpha-helices-5 and -6. Biochem J 2004; 378:247-55. [PMID: 14614769 PMCID: PMC1223939 DOI: 10.1042/bj20031152] [Citation(s) in RCA: 78] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/30/2003] [Revised: 10/31/2003] [Accepted: 11/13/2003] [Indexed: 01/21/2023]
Abstract
The pro-apoptotic protein Bax plays a key role in the mitochondrial signalling pathway. Upon induction of apoptosis, Bax undergoes a conformational change and translocates to mitochondrial membranes, where it inserts and mediates the release of cytochrome c from the intermembrane space into the cytosol. However, the domains of Bax that are essential for the induction of cytochrome c release are still elusive. Therefore various Bax deletion mutants were generated and expressed in Escherichia coli. The proteins were then purified in order to delineate the function of the transmembrane domain, the BH3 (Bcl-2 homology 3) domain and the putative pore-forming alpha-helices-5 and -6. These proteins were used to analyse the mechanism of Bax-induced cytochrome c release from mitochondria. None of the Bax proteins caused cytochrome c release merely through physical perturbation of the mitochondrial outer membrane. The alpha-helices-5 and -6 of Bax were shown to mediate the insertion of the protein into mitochondrial membranes and to be essential for the cytochrome c -releasing activity of Bax. In contrast, neither the transmembrane domain nor a functional BH3 domain is required for the Bax-mediated release of cytochrome c from mitochondria.
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Affiliation(s)
- Gerd Heimlich
- Institute for Medical Microbiology, Immunology and Hygiene, University of Köln, 50935 Köln, Germany
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31
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Waizenegger T, Stan T, Neupert W, Rapaport D. Signal-anchor domains of proteins of the outer membrane of mitochondria: structural and functional characteristics. J Biol Chem 2003; 278:42064-71. [PMID: 12917440 DOI: 10.1074/jbc.m305736200] [Citation(s) in RCA: 63] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
We have studied the topogenesis of a class of mitochondrial outer membrane proteins that expose a hydrophilic domain to the cytosol and are anchored to the membrane by a single transmembrane domain in the N-terminal region. To determine the role of these latter sequences in the targeting and insertion of such proteins we took two approaches. First, a functional complementation assay was used to define the structural elements that together with the anchor domain make up the topogenic signal. Moderate hydrophobicity of the transmembrane domain was found to be the most important requirement. Variants with a scrambled sequence of the membrane-spanning segment were only partially functional suggesting that specificity in the amino acid sequence is also of considerable importance. A net positive charge at both flanking regions of the transmembrane domain contributes to the efficiency of targeting and membrane integration but is not an essential structural feature of this signal. Second, chimeras of Tom20, Tom70, and OM45 were generated that contained the cytosolic domain of Tom20 or Tom70 and the anchor domain of one of the other members of the class. These hybrid proteins were able to rescue the growth of cells lacking Tom20 or Tom70. Thus, anchor domains of outer membrane proteins are functionally interchangeable. They play only a minor role in the specific function of these proteins, but have a decisive role in topogenic signaling.
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Affiliation(s)
- Thomas Waizenegger
- Institut für Physiologische Chemie der Universität München, Butenandtstrasse 5, Haus B, D-81377 Munich, Germany
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32
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Horie C, Suzuki H, Sakaguchi M, Mihara K. Targeting and assembly of mitochondrial tail-anchored protein Tom5 to the TOM complex depend on a signal distinct from that of tail-anchored proteins dispersed in the membrane. J Biol Chem 2003; 278:41462-71. [PMID: 12896971 DOI: 10.1074/jbc.m307047200] [Citation(s) in RCA: 53] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Mitochondrial outer membrane proteins are synthesized without a cleavable presequence but instead contain segments responsible for mitochondrial targeting and membrane integration within the molecule: the transmembrane segment (TMS) and N- or C-terminal flanking segment. We analyzed targeting and integration of Tom5, a C-tail anchor protein associated with the preprotein translocase of the outer membrane, to the yeast mitochondrial outer membrane in vivo using green fluorescent protein as the reporter and compared the signal with other signals for proteins dispersed in the membrane. The functional assembly of Tom5 into the TOM complex was assessed by blue native PAGE and complementation of temperature-sensitive deltatom5 cells. Correct targeting and assembly required (i). an appropriate length TMS rather than hydrophobicity, (ii). a proline residue located at correct position in the TMS and specific residues near the proline, and (iii). that, in contrast to proteins dispersed in the outer membrane, the positive C-terminal segment was dispensable. Based on these findings, we constructed green fluorescent protein fusions with a C-terminal TMS in which the deduced sequences (minimum: Ser-Pro-Met) were inserted at an appropriate position within artificial Leu-Ala repeats. They were targeted to mitochondria and complemented the temperature-sensitive growth phenotype of deltatom5 yeast cells. The membrane-targeting mechanism of Tom5 appears to be distinct from that for proteins that are dispersed in the outer membrane.
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Affiliation(s)
- Chika Horie
- Department of Molecular Biology, Graduate School of Medical Sciences, Kyushu University, Fukuoka 812-8582, Japan
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33
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Inoue K, Keegstra K. A polyglycine stretch is necessary for proper targeting of the protein translocation channel precursor to the outer envelope membrane of chloroplasts. THE PLANT JOURNAL : FOR CELL AND MOLECULAR BIOLOGY 2003; 34:661-9. [PMID: 12787247 DOI: 10.1046/j.1365-313x.2003.01755.x] [Citation(s) in RCA: 60] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/20/2023]
Abstract
Toc75 is a protein translocation channel in the outer envelope membrane of chloroplasts and its presence is essential for the biogenesis of the organelles. Toc75 is the only protein identified so far in the outer membrane of chloroplasts or mitochondria that is synthesized as a larger precursor, preToc75, with a bipartite transit peptide. Its N-terminus targets the protein to the stroma and is removed by the stromal processing peptidase, whereas its C-terminus mediates envelope targeting and is removed by a yet unknown peptidase. Several conserved domains have been identified in the C-terminal portion of the preToc75 transit peptide from six plant species. We evaluated their importance in the biogenesis of Toc75 by means of deletion or site-directed mutagenesis, followed by import experiments using isolated chlroplasts. Among the conserved domains, a polyglycine stretch was found to be necessary for envelope targeting. Substitution of this domain with other stretches of a single amino acid such as alanine caused mistargeting of the protein into the stroma, indicating an important role for this domain. Furthermore, a glutamate at +2 and two alanine residues at -3 and -1 to the second cleavage site were found to be important for processing. A potential mechanism for the biogenesis of Toc75 is discussed.
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Affiliation(s)
- Kentaro Inoue
- MSU-DOE Plant Research Laboratory, Michigan State University, East Lansing, MI 48824, USA.
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34
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Takida S, Wedegaertner PB. Heterotrimer formation, together with isoprenylation, is required for plasma membrane targeting of Gbetagamma. J Biol Chem 2003; 278:17284-90. [PMID: 12609996 DOI: 10.1074/jbc.m213239200] [Citation(s) in RCA: 79] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Nascent beta and gamma subunits of heterotrimeric G proteins need to be targeted to the cytoplasmic face of the plasma membrane (PM) in order to transmit signals. We show that beta(1)gamma(2) is poorly targeted to the PM and predominantly localized to endoplasmic reticulum (ER) membranes when expressed in HEK293 cells, but co-expression of a G protein alpha subunit allows strong PM localization of the beta(1)gamma(2). Furthermore, C-terminal isoprenylation of the gamma subunit is necessary but not sufficient for PM localization of beta(1)gamma(2). Isoprenylation of gamma(2) and localization of beta(1)gamma(2) to the ER occurs independently of alpha expression. Efficient PM localization of beta(1)gamma(2) in the absence of co-expressed alpha is observed when a site for palmitoylation, a putative second membrane targeting signal, is introduced into gamma(2). When a mutant of alpha(s) is targeted to mitochondria, beta(1)gamma(2) follows, consistent with an important role for alpha in promoting subcellular localization of betagamma. Furthermore, we directly demonstrate the requirement for alpha by showing that disruption of heterotrimer formation by the introduction of alpha binding mutations into beta(1) impedes PM targeting of beta(1)gamma(2). The results indicate that two membrane targeting signals, lipid modification and alpha binding, make concerted contributions to PM localization of betagamma.
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Affiliation(s)
- Satoshi Takida
- Department of Microbiology and Immunology, Kimmel Cancer Center, Thomas Jefferson University, Philadelphia, Pennsylvania 19107, USA
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35
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Bauer MF, Hofmann S, Neupert W. Import of mitochondrial proteins. INTERNATIONAL REVIEW OF NEUROBIOLOGY 2003; 53:57-90. [PMID: 12512337 DOI: 10.1016/s0074-7742(02)53004-x] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/23/2022]
Affiliation(s)
- Matthias F Bauer
- Institute of Clinical Chemistry, Molecular Diagnostics and Mitochondrial Genetics and Diabetes Research Group, Academic Hospital Munich-Schwabing Kölner Platz, D-80804 München, Germany
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36
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Cartron PF, Priault M, Oliver L, Meflah K, Manon S, Vallette FM. The N-terminal end of Bax contains a mitochondrial-targeting signal. J Biol Chem 2003; 278:11633-41. [PMID: 12529375 DOI: 10.1074/jbc.m208955200] [Citation(s) in RCA: 95] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The translocation of Bax alpha, a pro-apoptotic member of the BCL-2 family from the cytosol to mitochondria, is a central event of the apoptotic program. We report here that the N-terminal (NT) end of Bax alpha, which contains its first alpha helix (Eta alpha 1), is a functional mitochondrial-addressing signal both in mammals and in yeast. Similar results were obtained with a newly described variant of Bax called Bax psi, which lacks the first 20 amino acids of Bax alpha and is constitutively associated with mitochondria. Deletion of Eta alpha 1 impairs the binding of Bax psi to mitochondria, whereas a fusion of the N terminus of Bax alpha, which contains Eta alpha 1 with a cytosolic protein, results in the binding of the chimeric proteins to mitochondria both in a cell-free assay and in vitro. More importantly, the mitochondria-bound chimeric proteins inhibit the interaction of Bax psi with mitochondria as well as Bax-apoptogenic properties. The mutations of the Eta alpha 1, which inhibit Bax alpha and Bax psi translocation to mitochondria, also block the subsequent activation of the execution phase of apoptosis. Conversely, a deletion of the C terminus does not appear to influence Bax alpha and Bax psi mitochondrial addressing. Taken together, our results suggest that Bax is targeted to mitochondria by its NT and thus through a pathway that is unique for a member of the BCL-2 family.
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Legros F, Lombès A, Frachon P, Rojo M. Mitochondrial fusion in human cells is efficient, requires the inner membrane potential, and is mediated by mitofusins. Mol Biol Cell 2002; 13:4343-54. [PMID: 12475957 PMCID: PMC138638 DOI: 10.1091/mbc.e02-06-0330] [Citation(s) in RCA: 500] [Impact Index Per Article: 22.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/10/2002] [Revised: 08/07/2002] [Accepted: 08/23/2002] [Indexed: 11/11/2022] Open
Abstract
Mitochondrial fusion remains a largely unknown process despite its observation by live microscopy and the identification of few implicated proteins. Using green and red fluorescent proteins targeted to the mitochondrial matrix, we show that mitochondrial fusion in human cells is efficient and achieves complete mixing of matrix contents within 12 h. This process is maintained in the absence of a functional respiratory chain, despite disruption of microtubules or after significant reduction of cellular ATP levels. In contrast, mitochondrial fusion is completely inhibited by protonophores that dissipate the inner membrane potential. This inhibition, which results in rapid fragmentation of mitochondrial filaments, is reversible: small and punctate mitochondria fuse to reform elongated and interconnected ones upon withdrawal of protonophores. Expression of wild-type or dominant-negative dynamin-related protein 1 showed that fragmentation is due to dynamin-related protein 1-mediated mitochondrial division. On the other hand, expression of mitofusin 1 (Mfn1), one of the human Fzo homologues, increased mitochondrial length and interconnectivity. This process, but not Mfn1 targeting, was dependent on the inner membrane potential, indicating that overexpressed Mfn1 stimulates fusion. These results show that human mitochondria represent a single cellular compartment whose exchanges and interconnectivity are dynamically regulated by the balance between continuous fusion and fission reactions.
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Affiliation(s)
- Frédéric Legros
- Institut National de la Santé et de la Recharche Médicale U523, Institut de Myologie, Groupe Hospitalier Pitié-Salpêtrière, 75651 Paris, France
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38
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Weber-Lotfi F, Dietrich A, Russo M, Rubino L. Mitochondrial targeting and membrane anchoring of a viral replicase in plant and yeast cells. J Virol 2002; 76:10485-96. [PMID: 12239325 PMCID: PMC136569 DOI: 10.1128/jvi.76.20.10485-10496.2002] [Citation(s) in RCA: 72] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/18/2002] [Accepted: 06/17/2002] [Indexed: 02/08/2023] Open
Abstract
Replication of the Carnation Italian ringspot virus genomic RNA in plant cells occurs in multivesicular bodies which develop from the mitochondrial outer membrane during infection. ORF1 in the viral genome encodes a 36-kDa protein, while ORF2 codes for the 95-kDa replicase by readthrough of the ORF1 stop codon. We have shown previously that the N-terminal part of ORF1 contains the information leading to vesiculation of mitochondria and that the 36-kDa protein localizes to mitochondria. Using infection, in vivo expression of green fluorescent protein fusions in plant and yeast cells, and in vitro mitochondrial integration assays, we demonstrate here that both the 36-kDa protein and the complete replicase are targeted to mitochondria and anchor to the outer membrane with the N terminus and C terminus on the cytosolic side. Analysis of deletion mutants indicated that the anchor sequence is likely to correspond approximately to amino acids 84 to 196, containing two transmembrane domains. No evidence for a matrix-targeting presequence was found, and the data suggest that membrane insertion of the viral proteins is mediated by an import receptor-independent signal-anchor mechanism relying on the two transmembrane segments and multiple recognition signals present in the N-terminal part of ORF1.
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Affiliation(s)
- Frédérique Weber-Lotfi
- Institut de Biologie Moléculaire des Plantes du CNRS, Université Louis Pasteur, 12 rue du Général Zimmer, 67084 Strasbourg Cedex, France
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39
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Horie C, Suzuki H, Sakaguchi M, Mihara K. Characterization of signal that directs C-tail-anchored proteins to mammalian mitochondrial outer membrane. Mol Biol Cell 2002; 13:1615-25. [PMID: 12006657 PMCID: PMC111131 DOI: 10.1091/mbc.01-12-0570] [Citation(s) in RCA: 122] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022] Open
Abstract
We analyzed the signal that directs the outer membrane protein with the C-terminal transmembrane segment (TMS) to mammalian mitochondria by using yeast Tom5 as a model and green fluorescent protein as a reporter. Deletions or mutations were systematically introduced into the TMS or the flanking regions and their intracellular localization in COS-7 cells was examined using confocal microscopy and cell fractionation. 1) Three basic amino acid residues within the C-terminal five-residue segment (C-segment) contained the information required for mitochondrial-targeting. Reduction of the net positive charge in this segment decreased mitochondrial specificity, and the mutants were distributed throughout the intracellular membranes. 2) Elongation of the TMS interfered with the function of the C-segment and the mutants were delivered to the intracellular membranes. 3) Separation of the TMS and C-segment by linker insertion severely impaired mitochondrial targeting function, leading to mislocalization to the cytoplasm. 4) Mutations or small deletions in the region of the TMS flanking the C-segment also impaired the mitochondrial targeting. Therefore, the moderate length of the TMS, the positive charges in the C-segment, and the distance between or context of the TMS and C-segment are critical for the targeting signal. The structural characteristics of the signal thus defined were also confirmed with mammalian C-tail-anchored protein OMP25.
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Affiliation(s)
- Chika Horie
- Department of Molecular Biology, Graduate School of Medical Sciences, Kyushu University, Fukuoka 812-8582, Japan
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40
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Abstract
The translocase at the outer membrane of mitochondria (TOM complex) mediates the initial steps of the import of preproteins into the organelle, which are essential for mitochondrial biogenesis and, therefore, for eukaryotic cell viability. The TOM complex is a multisubunit molecular machine with a dynamic structure. The biogenesis of TOM is of special interest because the complex is required for its own biogenesis. This article describes the mechanisms by which Tom components are targeted to the mitochondria and inserted into the outer membrane. The assembly of newly synthesized subunits into the functional TOM complex might occur via assembly intermediates that are in equilibrium with the mature complex.
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Affiliation(s)
- Doron Rapaport
- Institut für Physiologische Chemie der Universität München Butenandtstr. 5, Haus B D-81377, München, Germany.
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41
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Kira Y, Sato EF, Inoue M. Association of Cu,Zn-type superoxide dismutase with mitochondria and peroxisomes. Arch Biochem Biophys 2002; 399:96-102. [PMID: 11883908 DOI: 10.1006/abbi.2001.2738] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The subcellular localization of Cu,Zn-type superoxide dismutase (Cu,Zn-SOD) was investigated in rat tissues and cultured human fibroblasts. Subcellular fractionation, Nycodenz gradient centrifugation, and immunoblot analysis using specific antibodies showed that Cu,Zn-SOD was localized in cytosol, mitochondria, and peroxisomes of rat liver and brain. Treatment of highly purified mitochondria from rat liver with either Chaps or Triton X-100 released the bound Cu,Zn-SOD into supernatant fraction. Depolarization of mitochondria by inorganic phosphate and Ca(2+) released both Cu,Zn-SOD and cytochrome c from mitochondria. Digitonin also released Cu,Zn-SOD but not cytochrome c from mitochondria. Confocal immunofluorescence microscopy revealed that anti-Cu,Zn-SOD antibody in cultured human fibroblasts was found to colocalize with antibodies to Mn-SOD and PMP-70, markers of mitochondria and peroxisomes, respectively. Incubation of human Cu,Zn-SOD with purified mitochondria resulted in their association. These results indicate that Cu,Zn-SOD associates with mitochondria and peroxisomes in various cell types such as those in brain, liver, and skin.
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Affiliation(s)
- Yukimi Kira
- Department of Biochemistry and Molecular Pathology, Osaka City University Medical School, 1-4-3 Asahimachi, Abeno, Osaka, 545-8585, Japan
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42
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Deber CM, Liu LP, Wang C, Goto NK, Reithmeier RA. The hydrophobicity threshold for peptide insertion into membranes. CURRENT TOPICS IN MEMBRANES 2002. [DOI: 10.1016/s1063-5823(02)52018-4] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/18/2023]
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43
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Huh KW, Siddiqui A. Hepatitis B virus X protein: a multifunctional viral regulator. J Gastroenterol 2001; 1:349-59. [PMID: 16120289 DOI: 10.1016/s1567-7249(01)00040-x] [Citation(s) in RCA: 47] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 09/05/2001] [Revised: 11/01/2001] [Accepted: 11/05/2001] [Indexed: 01/27/2023]
Abstract
Hepatitis B Virus (HBV) infection is one of the major causes of hepatocellular carcinoma (HCC). X protein (HBx) has been suspected to be oncogenic, although the precise role(s) remain uncertain. HBx is a multifunctional viral regulator that modulates transcription, cell responses to genotoxic stress, protein degradation, and signaling pathways. These modulations affect viral replication and viral proliferation, directly or indirectly. HBx also affects cell cycle checkpoints, cell death, and carcinogenesis. This article presents an overview of the progress in HBx research over the past several years.
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Affiliation(s)
- Kyung-Won Huh
- Department of Microbiology, University of Colorado Health Sciences Center, Denver, Colorado, OH, USA
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44
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Rapaport D, Taylor RD, Käser M, Langer T, Neupert W, Nargang FE. Structural requirements of Tom40 for assembly into preexisting TOM complexes of mitochondria. Mol Biol Cell 2001; 12:1189-98. [PMID: 11359915 PMCID: PMC34577 DOI: 10.1091/mbc.12.5.1189] [Citation(s) in RCA: 41] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022] Open
Abstract
Tom40 is the major subunit of the translocase of the outer mitochondrial membrane (the TOM complex). To study the assembly pathway of Tom40, we have followed the integration of the protein into the TOM complex in vitro and in vivo using wild-type and altered versions of the Neurospora crassa Tom40 protein. Upon import into isolated mitochondria, Tom40 precursor proteins lacking the first 20 or the first 40 amino acid residues were assembled as the wild-type protein. In contrast, a Tom40 precursor lacking residues 41 to 60, which contains a highly conserved region of the protein, was arrested at an intermediate stage of assembly. We constructed mutant versions of Tom40 affecting this region and transformed the genes into a sheltered heterokaryon containing a tom40 null nucleus. Homokaryotic strains expressing the mutant Tom40 proteins had growth rate defects and were deficient in their ability to form conidia. Analysis of the TOM complex in these strains by blue native gel electrophoresis revealed alterations in electrophoretic mobility and a tendency to lose Tom40 subunits from the complex. Thus, both in vitro and in vivo studies implicate residues 41 to 60 as containing a sequence required for proper assembly/stability of Tom40 into the TOM complex. Finally, we found that TOM complexes in the mitochondrial outer membrane were capable of exchanging subunits in vitro. A model is proposed for the integration of Tom40 subunits into the TOM complex.
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Affiliation(s)
- D Rapaport
- Department of Biochemistry, Hebrew University-Hadassah Medical School, Jerusalem 91120, Israel
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45
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Rebrin I, Geha RM, Chen K, Shih JC. Effects of carboxyl-terminal truncations on the activity and solubility of human monoamine oxidase B. J Biol Chem 2001; 276:29499-506. [PMID: 11371556 DOI: 10.1074/jbc.m100431200] [Citation(s) in RCA: 43] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Monoamine oxidase is an outer mitochondrial membrane protein that catalyzes the deamination of a number of neurotransmitters and dietary amines. To determine the roles of the carboxyl-terminal amino acids on the activity and solubility of human monoamine oxidase (MAO B), 10 sequential mutants were made with stop codons at amino acid positions 511, 504, 498, 492, 486, 481, 476, 467, 417, and 397, respectively. All truncated mutants were expressed in Sf21 insect cells using baculovirus, and the enzyme kinetic parameters were determined. Truncations at amino acid positions 511, 504, and 498 slightly decreased MAO B catalytic activity and had no significant changes on deprenyl inhibition. Further deletions up to amino acid 417 decreased the specific activity 10--100-fold without significant changes of the K(m) for phenylethylamine or dopamine or the IC(50) for deprenyl and clorgyline. The truncation mutant C397, which lacks covalently attached FAD, was inactive. Progressive carboxyl-terminal truncations up to position 481 were correlated with increased solubility of MAO B mutants. 47% of the activity of the truncated C481 was found in the 105,000 x g supernatant in the absence of detergent. However, further truncated mutants, C476, C467, and C417, remained associated with the membrane fraction. In contrast to crude homogenate, the water-soluble C481 mutant was rapidly inactivated at 4 degrees C and 37 degrees C, which indicates that the membrane environment is required for the stability of MAO B. Expression of the green fluorescent protein-MAO B C481 fusion protein revealed that this mutant was located in the cytoplasm, whereas its counterpart in MAO A, truncated mutant C490, was located on the mitochondria. These results suggest that the carboxyl-terminal amino acid residues 417--520 of MAO B are not directly involved in the active site but are required for maintaining the appropriate conformation and interaction with the outer mitochondrial membrane. The different solubilities of the various carboxyl-terminal truncation mutants indicate that the interaction of MAO B with mitochondrial membrane is not simply anchoring through the carboxyl-terminal hydrophobic tail. Further, our results suggest that the carboxyl-terminal of MAO A and B plays different roles in mitochondrial attachment.
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Affiliation(s)
- I Rebrin
- Department of Molecular Pharmacology and Toxicology, School of Pharmacy, University of Southern California, Los Angeles, California 90089, USA
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46
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Cohen I, Guillerault F, Girard J, Prip-Buus C. The N-terminal domain of rat liver carnitine palmitoyltransferase 1 contains an internal mitochondrial import signal and residues essential for folding of its C-terminal catalytic domain. J Biol Chem 2001; 276:5403-11. [PMID: 11087756 DOI: 10.1074/jbc.m009555200] [Citation(s) in RCA: 21] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
We have previously shown that the first 147 N-terminal residues of the rat liver carnitine palmitoyltransferase 1 (CPT1), encompassing its two transmembrane (TM) segments, specify both mitochondrial targeting and anchorage at the outer mitochondrial membrane (OMM). In the present study, we have identified the precise import sequence in this polytopic OMM protein. In vitro import studies with fusion and deletion CPT1 proteins demonstrated that none of its TM segments behave as a signal anchor sequence. Analysis of the regions flanking the TM segments revealed that residues 123-147, located immediately downstream of TM2, function as a noncleavable, matrix-targeting signal. They specify mitochondrial targeting, whereas the hydrophobic TM segment(s) acts as a stop-transfer sequence that stops and anchors the translocating CPT1 into the OMM. Heterologous expression in Saccharomyces cerevisiae of several deleted CPT1 proteins not only confirms the validity of the "stop-transfer" import model but also indicates that residues 1-82 of CPT1 contain a putative microsomal targeting signal whose cellular significance awaits further investigation. Finally, we identified a highly folded core within the C-terminal domain of CPT1 that is hidden in the entire protein by its cytosolic N-terminal residues. Functional analysis of the deleted CPT1 proteins indicates that this folded C-terminal core, which may belong to the catalytic domain of CPT1, requires TM2 for its correct folding achievement and is in close proximity to residues 1-47.
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Affiliation(s)
- I Cohen
- Endocrinologie, Métabolisme et Développement, CNRS-UPR 1524, 9 Rue J. Hetzel, 92190 Meudon, France
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47
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Kanaji S, Iwahashi J, Kida Y, Sakaguchi M, Mihara K. Characterization of the signal that directs Tom20 to the mitochondrial outer membrane. J Cell Biol 2000; 151:277-88. [PMID: 11038175 PMCID: PMC2192658 DOI: 10.1083/jcb.151.2.277] [Citation(s) in RCA: 165] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/01/2022] Open
Abstract
Tom20 is a major receptor of the mitochondrial preprotein translocation system and is bound to the outer membrane through the NH(2)-terminal transmembrane domain (TMD) in an Nin-Ccyt orientation. We analyzed the mitochondria-targeting signal of rat Tom20 (rTom20) in COS-7 cells, using green fluorescent protein (GFP) as the reporter by systematically introducing deletions or mutations into the TMD or the flanking regions. Moderate TMD hydrophobicity and a net positive charge within five residues of the COOH-terminal flanking region were both critical for mitochondria targeting. Constructs without net positive charges within the flanking region, as well as those with high TMD hydrophobicity, were targeted to the ER-Golgi compartments. Intracellular localization of rTom20-GFP fusions, determined by fluorescence microscopy, was further verified by cell fractionation. The signal recognition particle (SRP)-induced translation arrest and photo-cross-linking demonstrated that SRP recognized the TMD of rTom20-GFP, but with reduced affinity, while the positive charge at the COOH-terminal flanking segment inhibited the translation arrest. The mitochondria-targeting signal identified in vivo also functioned in the in vitro system. We conclude that NH(2)-terminal TMD with a moderate hydrophobicity and a net positive charge in the COOH-terminal flanking region function as the mitochondria-targeting signal of the outer membrane proteins, evading SRP-dependent ER targeting.
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Affiliation(s)
- S Kanaji
- Department of Molecular Biology, Graduate School of Medical Science, Kyushu University, Fukuoka 812-8582, Japan
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48
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Schleiff E, McBride H. The central matrix loop drives import of uncoupling protein 1 into mitochondria. J Cell Sci 2000; 113 ( Pt 12):2267-72. [PMID: 10825298 DOI: 10.1242/jcs.113.12.2267] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
The uncoupling protein (UCP1) is a carrier protein of the inner mitochondrial membrane spanning the bilayer six times. It does not contain a typical amino-terminal targeting signal and the mechanism of targeting and insertion is unknown. Here we focus on the biogenesis of UCP1 by analysing the import signals contained within the three repeated units of the protein. The amino-terminal third of the protein can mediate insertion into the outer membrane and therefore acts as artificial targeting signal when fused to DHFR. However, in the context of full-length UCP, the targeting information contained within the first repeated unit is not sufficient to trigger insertion into the outer membrane. Deletion of either the first or third repeated unit from UCP1 did not reduce import into the inner membrane and bound to the outer membrane receptor protein hTom20 with the characteristics of full-length UCP1. Deletion of the second repeat of UCP1 completely abolished all import into the mitochondria. Consistent with this, the central repeat alone was efficiently imported to the inner membrane and bound hTom20 with the characteristics of UCP1. We conclude that the site for binding hTom20 is within the central repeat and that this domain contains the complete targeting signal for directing UCP1 to the inner membrane.
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Affiliation(s)
- E Schleiff
- Department of Biochemistry, McGill University Montreal, Montreal, Canada, H3G 1Y6.
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49
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Cohen I, Girard J, Prip-Buus C. Biogenesis of the rat liver mitochondrial carnitine palmitoyltransferase I. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2000; 466:1-16. [PMID: 10709623 DOI: 10.1007/0-306-46818-2_1] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/15/2023]
Affiliation(s)
- I Cohen
- Endocrinologie, Métabolisme et Développement CNRS UPR1524, Meudon, France
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50
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Abstract
Most mitochondrial proteins are synthesized in the cytosol as preproteins with a cleavable presequence and are delivered to the import receptors on the mitochondria by cytoplasmic import factors. The proteins are then imported to the intramitochondrial compartments by the import systems of the outer and inner membranes, TOM and TIM. Mitochondrial outer membrane proteins are synthesized without a cleavable presequence and most of them contain hydrophobic transmembrane domains, which, in conjunction with the flanking segments, function as the mitochondria import signals. Some of the proteins are inserted into the outer membrane by the TOM machinery; the import signal probably arrests further translocation and is released from the translocation channel to the lipid bilayer. The other proteins are inserted into the membrane by a novel pathway independent of the TOM machinery. This article reviews recent developments in the biogenesis of mitochondrial outer membrane proteins.
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Affiliation(s)
- K Mihara
- Department of Molecular Biology, Graduate School of Medical Science, Kyushu University, Fukuoka, Fukuoka 812-8582, Japan.
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