101
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Abstract
Different RNA species are exported from the nucleus by distinct mechanisms. Among the different RNAs, mRNAs and major spliceosomal U snRNAs share several structural similarities, yet they are exported by distinct factors. We previously showed that U1 snRNAs behaved like an mRNA in nuclear export if various approximately 300-nucleotide fragments were inserted in a central position. Here we show that this export switch is dependent on the length of the insertion but independent of its position, indicating unequivocally that this switch is indeed the result of RNA length. We also show that intronless mRNAs can be progressively converted to use the U snRNA export pathway if the mRNAs are progressively shortened by deletion. In addition, immunoprecipitation experiments show that the protein composition of export RNPs is influenced by RNA length. These findings indicate that RNA length is one of the key determinants of the choice of RNA export pathway. Based on these results and previous observations, a unified model of how an RNA is committed to a specific export pathway is proposed.
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Affiliation(s)
- Kaoru Masuyama
- Institute for Virus Research, Kyoto University, Sakyo-ku, Kyoto 606-8507, Japan
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102
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O'Hagan HM, Ljungman M. Efficient NES-dependent protein nuclear export requires ongoing synthesis and export of mRNAs. Exp Cell Res 2004; 297:548-59. [PMID: 15212955 DOI: 10.1016/j.yexcr.2004.03.051] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/08/2004] [Revised: 03/20/2004] [Indexed: 11/25/2022]
Abstract
The mechanisms regulating nuclear export of proteins are not fully understood. To investigate whether the efficiency of protein nuclear export may depend on ongoing RNA synthesis and/or mRNA nuclear export, we used a microinjection approach with a fluorescent reporter protein containing a nuclear export signal (NES) and scored protein export in human fibroblasts under conditions when the synthesis or export of mRNAs was inhibited. We show that inhibition of transcription significantly attenuated generic NES-dependent nuclear export. Furthermore, digestion of endogenous nuclear RNAs by co-microinjection of RNAse A inhibited NES-dependent nuclear export. Finally, nuclear export of the NES reporter protein was significantly inhibited in cells in which nuclear export of mRNAs had been specifically blocked by microinjection of anti-TAP antibodies or by expression of a dominant negative form of NUP160. These results demonstrate a novel role for ongoing synthesis and export of mRNAs in NES-dependent protein nuclear export.
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MESH Headings
- ATP-Binding Cassette Transporters
- Active Transport, Cell Nucleus/drug effects
- Cell Nucleus/drug effects
- Cell Nucleus/metabolism
- Cells, Cultured
- Cytoplasm/drug effects
- Cytoplasm/metabolism
- Dose-Response Relationship, Drug
- Fibroblasts/cytology
- Fibroblasts/metabolism
- Fluorescent Dyes
- Glutathione Transferase/metabolism
- Green Fluorescent Proteins
- Histocompatibility Antigens Class I/metabolism
- Humans
- Karyopherins/metabolism
- Luminescent Proteins
- Microinjections
- Models, Biological
- Nuclear Localization Signals/metabolism
- Nuclear Pore Complex Proteins/metabolism
- Nuclear Proteins/metabolism
- RNA, Messenger/antagonists & inhibitors
- RNA, Messenger/biosynthesis
- RNA, Messenger/drug effects
- RNA, Messenger/metabolism
- Receptors, Cytoplasmic and Nuclear
- Recombinant Proteins/metabolism
- Rhodamines
- Ribonuclease, Pancreatic/pharmacology
- Transcription, Genetic/drug effects
- Exportin 1 Protein
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Affiliation(s)
- Heather M O'Hagan
- Department of Radiation Oncology, University of Michigan Comprehensive Cancer Center, Ann Arbor, MI 48109, USA
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103
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Léger-Silvestre I, Milkereit P, Ferreira-Cerca S, Saveanu C, Rousselle JC, Choesmel V, Guinefoleau C, Gas N, Gleizes PE. The ribosomal protein Rps15p is required for nuclear exit of the 40S subunit precursors in yeast. EMBO J 2004; 23:2336-47. [PMID: 15167894 PMCID: PMC423291 DOI: 10.1038/sj.emboj.7600252] [Citation(s) in RCA: 90] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/17/2004] [Accepted: 05/04/2004] [Indexed: 11/09/2022] Open
Abstract
We have conducted a genetic screen in order to identify ribosomal proteins of Saccharomyces cerevisiae involved in nuclear export of the small subunit precursors. This has led us to distinguish Rps15p as a protein dispensable for maturation of the pre-40S particles, but whose assembly into the pre-ribosomes is a prerequisite to their nuclear exit. Upon depletion of Rps15p, 20S pre-rRNA is released from the nucleolus and retained in the nucleus, without alteration of the pre-rRNA early cleavages. In contrast, Rps18p, which contacts Rps15p in the small subunit, is required upstream for pre-rRNA processing at site A2. Most pre-40S specific factors are correctly associated with the intermediate particles accumulating in the nucleus upon Rps15p depletion, except the late-binding proteins Tsr1p and Rio2p. Here we show that these two proteins are dispensable for nuclear exit; instead, they participate in 20S pre-rRNA processing in the cytoplasm. We conclude that, during the final maturation steps in the nucleus, incorporation of the ribosomal protein Rps15p is specifically required to render the pre-40S particles competent for translocation to the cytoplasm.
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Affiliation(s)
- Isabelle Léger-Silvestre
- Laboratoire de Biologie Moléculaire Eucaryote, CNRS and Université Paul Sabatier, Toulouse, France
| | - Philipp Milkereit
- Laboratoire de Biologie Moléculaire Eucaryote, CNRS and Université Paul Sabatier, Toulouse, France
| | - Sébastien Ferreira-Cerca
- Laboratoire de Biologie Moléculaire Eucaryote, CNRS and Université Paul Sabatier, Toulouse, France
| | - Cosmin Saveanu
- Génétique des Interactions Macromoléculaires, Institut Pasteur, Paris, France
| | | | - Valérie Choesmel
- Laboratoire de Biologie Moléculaire Eucaryote, CNRS and Université Paul Sabatier, Toulouse, France
| | - Cécile Guinefoleau
- Laboratoire de Biologie Moléculaire Eucaryote, CNRS and Université Paul Sabatier, Toulouse, France
| | - Nicole Gas
- Laboratoire de Biologie Moléculaire Eucaryote, CNRS and Université Paul Sabatier, Toulouse, France
| | - Pierre-Emmanuel Gleizes
- Laboratoire de Biologie Moléculaire Eucaryote, CNRS and Université Paul Sabatier, Toulouse, France
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104
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Dlakić M, Tollervey D. The Noc proteins involved in ribosome synthesis and export contain divergent HEAT repeats. RNA (NEW YORK, N.Y.) 2004; 10:351-4. [PMID: 14970380 PMCID: PMC1370930 DOI: 10.1261/rna.5184704] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/05/2023]
Abstract
The Noc1-4p proteins were previously reported to be involved in intranuclear and nucleocytoplasmic transport of pre-ribosomes. Using fold recognition and structural modeling, we show that Noc1-4p are largely comprised of alpha-helical repeats similar to HEAT repeats. Because other HEAT-repeat proteins play key roles in transport processes, this finding provides a plausible mechanistic explanation for the function of the Noc proteins.
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105
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Willis IM, Desai N, Upadhya R. Signaling repression of transcription by RNA polymerase III in yeast. PROGRESS IN NUCLEIC ACID RESEARCH AND MOLECULAR BIOLOGY 2004; 77:323-53. [PMID: 15196897 DOI: 10.1016/s0079-6603(04)77009-9] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/16/2023]
Affiliation(s)
- Ian M Willis
- Department of Biochemistry, Albert Einstein College of Medicine, 1300 Morris Park Avenue, Bronx, NY 10461 USA
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106
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Pian JP, Huang TL, Tsai PC, Shi JP, Cu H, Pan BT. A 32 kDa protein?whose phosphorylation correlates with oncogenic Ras-induced cell cycle arrest in activatedXenopus egg extracts?is identified as ribosomal protein S6. J Cell Physiol 2004; 201:305-19. [PMID: 15334665 DOI: 10.1002/jcp.20069] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022]
Abstract
Oncogenic Ras induces cell-cycle arrest in mammalian cells and in fertilized Xenopus eggs. How oncogenic Ras induces cell-cycle arrest remains unclear. We previously showed that oncogenic Ras induces cell-cycle arrest in activated Xenopus egg extracts (cycling extracts) and that the induced cell-cycle arrest correlates with hyperphosphorylation of a 32 kDa protein. However, the identity of the 32 kDa protein was not known. By using a sucrose density-gradient centrifugation, Triton X-100-acetic acid-urea (TAU)-gel electrophoresis, composite agarose-polyacrylamide gel electrophoresis (CAPAGE), SDS-PAGE, and partial tryptic peptide sequence analysis, the 32 kDa protein has now been identified as S6, a 40S subunit ribosomal protein. Hence, our results indicate that the oncogenic Ras-induced cell-cycle arrest is correlated with hyperphosphorylation of S6, suggesting that phosphorylation of S6 plays an important role in the induced cell-cycle arrest. It has been shown that conditional deletion of gene encoding S6 in mammalian cells prevents proliferation, demonstrating the importance of S6 in cell proliferation. The exact role S6 plays in cell proliferation is unclear. However, phosphorylation of S6 has been implicated in the regulation of protein synthesis. Thus, our results are consistent with the concept that oncogenic Ras induces S6 phosphorylation to influence protein synthesis, thereby contributing to the cell-cycle arrest. In addition, our results also demonstrate that composite agarose-polyacrylamide gel electrophoresis is suitable for the separation of large molecular complexes.
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Affiliation(s)
- Jerry Pinghwa Pian
- Graduate Center for Nutritional Sciences, University of Kentucky Medical Center, 800 Rose Street, Lexington, Kentucky 40536, USA
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107
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Dimario PJ. Cell and Molecular Biology of Nucleolar Assembly and Disassembly. INTERNATIONAL REVIEW OF CYTOLOGY 2004; 239:99-178. [PMID: 15464853 DOI: 10.1016/s0074-7696(04)39003-0] [Citation(s) in RCA: 48] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/23/2022]
Abstract
Nucleoli disassemble in prophase of the metazoan mitotic cycle, and they begin their reassembly (nucleologenesis) in late anaphase?early telophase. Nucleolar disassembly and reassembly were obvious to the early cytologists of the eighteenth and nineteenth centuries, and although this has lead to a plethora of literature describing these events, our understanding of the molecular mechanisms regulating nucleolar assembly and disassembly has expanded immensely just within the last 10-15 years. We briefly survey the findings of nineteenth-century cytologists on nucleolar assembly and disassembly, followed by the work of Heitz and McClintock on nucleolar organizers. A primer review of nucleolar structure and functions precedes detailed descriptions of modern molecular and microscopic studies of nucleolar assembly and disassembly. Nucleologenesis is concurrent with the reinitiation of rDNA transcription in telophase. The perichromosomal sheath, prenucleolar bodies, and nucleolar-derived foci serve as repositories for nucleolar processing components used in the previous interphase. Disassembly of the perichromosomal sheath along with the dynamic movements and compositional changes of the prenucleolar bodies and nucleolus-derived foci coincide with reactivation of rDNA synthesis within the chromosomal nucleolar organizers during telophase. Nucleologenesis is considered in various model organisms to provide breadth to our understanding. Nucleolar disassembly occurs at the onset of mitosis primarily as a result of the mitosis-specific phosphorylation of Pol I transcription factors and processing components. Although we have learned much regarding nucleolar assembly and disassembly, many questions still remain, and these questions are as vibrant for us today as early questions were for nineteenth- and early twentieth-century cytologists.
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Affiliation(s)
- Patrick J Dimario
- Department of Biological Sciences, Louisiana State University, Baton Rouge, Louisiana 70803-1715, USA
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108
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Neumann S, Petfalski E, Brügger B, Großhans H, Wieland F, Tollervey D, Hurt E. Formation and nuclear export of tRNA, rRNA and mRNA is regulated by the ubiquitin ligase Rsp5p. EMBO Rep 2003; 4:1156-62. [PMID: 14608372 PMCID: PMC1326418 DOI: 10.1038/sj.embor.7400026] [Citation(s) in RCA: 60] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/29/2003] [Revised: 09/15/2003] [Accepted: 09/22/2003] [Indexed: 11/08/2022] Open
Abstract
The yeast ubiquitin-protein ligase Rsp5p regulates processes as diverse as polII transcription and endocytosis. Here, we identify Rsp5p in a screen for tRNA export (tex) mutants. The tex23-1/rsp5-3 mutant, which is complemented by RSP5, not only shows a strong nuclear accumulation of tRNAs at the restrictive temperature, but also is severely impaired in the nuclear export of mRNAs and 60S pre-ribosomal subunits. In contrast, nuclear localization sequence (NLS)-mediated nuclear protein import is unaffected in this mutant. Strikingly, the nuclear RNA export defects seen in the rsp5-3 strain are accompanied by a dramatic inhibition of both rRNA and tRNA processing, a combination of phenotypes that has not been reported for any previously characterized mutation in yeast. These data implicate ubiquitination as a mechanism coordinating the major nuclear RNA biogenesis pathways.
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Affiliation(s)
- Silvia Neumann
- Biochemie-Zentrum Heidelberg (BZH),
Im Neuenheimer Feld 328, D-69120
Heidelberg, Germany
| | - Elisabeth Petfalski
- Wellcome Trust Centre for Cell Biology,
University of Edinburgh, Edinburgh EH9 3JR,
UK
| | - Britta Brügger
- Biochemie-Zentrum Heidelberg (BZH),
Im Neuenheimer Feld 328, D-69120
Heidelberg, Germany
| | - Helge Großhans
- Biochemie-Zentrum Heidelberg (BZH),
Im Neuenheimer Feld 328, D-69120
Heidelberg, Germany
| | - Felix Wieland
- Biochemie-Zentrum Heidelberg (BZH),
Im Neuenheimer Feld 328, D-69120
Heidelberg, Germany
| | - David Tollervey
- Wellcome Trust Centre for Cell Biology,
University of Edinburgh, Edinburgh EH9 3JR,
UK
| | - Ed Hurt
- Biochemie-Zentrum Heidelberg (BZH),
Im Neuenheimer Feld 328, D-69120
Heidelberg, Germany
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109
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Stüven T, Hartmann E, Görlich D. Exportin 6: a novel nuclear export receptor that is specific for profilin.actin complexes. EMBO J 2003; 22:5928-40. [PMID: 14592989 PMCID: PMC275422 DOI: 10.1093/emboj/cdg565] [Citation(s) in RCA: 245] [Impact Index Per Article: 11.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/24/2003] [Revised: 09/12/2003] [Accepted: 09/15/2003] [Indexed: 11/12/2022] Open
Abstract
Active macromolecular transport between the nucleus and cytoplasm proceeds through nuclear pore complexes and is mostly mediated by transport receptors of the importin beta-superfamily. Here we identify exportin 6 (Exp6) as a novel family member from higher eukaryotes and show that it mediates nuclear export of profilin.actin complexes. Exp6 appears to contact primarily actin, but the interaction is greatly enhanced by the presence of profilin. Profilin thus functions not only as the nucleotide exchange factor for actin, but can also be regarded as a cofactor of actin export and hence as a suppressor of actin polymerization in the nucleus. Even though human and Drosophila Exp6 share only approximately 20% identical amino acid residues, their function in profilin.actin export is conserved. A knock-down of Drosophila Exp6 by RNA interference abolishes nuclear exclusion of actin and results in the appearance of nuclear actin paracrystals. In contrast to a previous report, we found no indications of a major and direct role for CRM1 in actin export from mammalian or insect nuclei.
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Affiliation(s)
- Theis Stüven
- ZMBH, INF 282, D-69120 Heidelberg and Universität Lübeck, Ratzeburger Allee 160, D-23538 Lübeck, Germany
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110
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Cushman I, Stenoien D, Moore MS. The dynamic association of RCC1 with chromatin is modulated by Ran-dependent nuclear transport. Mol Biol Cell 2003; 15:245-55. [PMID: 14565978 PMCID: PMC307544 DOI: 10.1091/mbc.e03-06-0409] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022] Open
Abstract
Regulator of chromosome condensation (RCC1) binding to chromatin is highly dynamic, as determined by fluorescence recovery after photobleaching analysis of GFP-RCC1 in stably transfected tsBN2 cells. Microinjection of wild-type or Q69L Ran markedly slowed the mobility of GFP-RCC1, whereas T24N Ran (defective in nucleotide loading) decreased it further still. We found significant alterations in the mobility of intranuclear GFP-RCC1 after treatment with agents that disrupt different Ran-dependent nuclear export pathways. Leptomycin B, which inhibits Crm1/RanGTP-dependent nuclear export, significantly increased the mobility of RCC1 as did high levels of actinomycin D (to inhibit RNA polymerases I, II, and III) or alpha-amanitin (to inhibit RNA polymerases II and III) as well as energy depletion. Inhibition of just mRNA transcription, however, had no affect on GFP-RCC1 mobility consistent with mRNA export being a Ran-independent process. In permeabilized cells, cytosol and GTP were required for the efficient release of GFP-RCC1 from chromatin. Recombinant Ran would not substitute for cytosol, and high levels of supplemental Ran inhibited the cytosol-stimulated release. Thus, RCC1 release from chromatin in vitro requires a factor(s) distinct from, or in addition to, Ran and seems linked in vivo to the availability of Ran-dependent transport cargo.
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Affiliation(s)
- Ian Cushman
- Interdepartmental Program in Cell and Molecular Biology, Baylor College of Medicine, Houston, Texas 77030, USA
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111
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Carter CC, Izadpanah R, Bridge E. Evaluating the role of CRM1-mediated export for adenovirus gene expression. Virology 2003; 315:224-33. [PMID: 14592774 DOI: 10.1016/s0042-6822(03)00526-9] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/01/2022]
Abstract
A complex of the Adenovirus (Ad) early region 1b 55-kDa (E1b-55kDa) and early region 4 ORF6 34-kDa (E4-34kDa) proteins promotes viral late gene expression. E1b-55kDa and E4-34kDa have leucine-rich nuclear export signals (NESs) similar to that of HIV Rev. It was proposed that E1b-55kDa and/or E4-34kDa might promote the export of Ad late mRNA via their Rev-like NESs, and the transport receptor CRM1. We treated infected cells with the cytotoxin leptomycin B to inhibit CRM1-mediated export; treatment initially delays the onset of late gene expression, but this activity completely recovers as the late phase progresses. We find that the E1b-55kDa NES is not required to promote late gene expression. Previous results showed that E4-34kDa-mediated late gene expression does not require an intact NES (J. Virol. 74 (2000), 6684-6688). Our results indicate that these Ad regulatory proteins promote late gene expression without intact NESs or active CRM1.
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Affiliation(s)
- Christoph C Carter
- Department of Microbiology, 32 Pearson Hall, Miami University, Oxford, OH 45056, USA
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112
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Merkle T. Nucleo-cytoplasmic partitioning of proteins in plants: implications for the regulation of environmental and developmental signalling. Curr Genet 2003; 44:231-60. [PMID: 14523572 DOI: 10.1007/s00294-003-0444-x] [Citation(s) in RCA: 62] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/29/2003] [Revised: 08/21/2003] [Accepted: 09/01/2003] [Indexed: 12/21/2022]
Abstract
Considerable progress has been made in the past few years in characterising Arabidopsis nuclear transport receptors and in elucidating plant signal transduction pathways that employ nucleo-cytoplasmic partitioning of a member of the signal transduction chain. This review briefly introduces the major principles of nuclear transport of macromolecules across the nuclear envelope and the proteins involved, as they have been described in vertebrates and yeast. Proteins of the plant nuclear transport machinery that have been identified to date are discussed, the focus being on Importin beta-like nuclear transport receptors. Finally, the importance of nucleo-cytoplasmic partitioning as a regulatory tool for signalling is highlighted, and different plant signal transduction pathways that make use of this regulatory potential are presented.
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Affiliation(s)
- Thomas Merkle
- Institute of Biology II, Cell Biology, University of Freiburg, Schänzlestrasse 1, 79104 Freiburg, Germany.
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113
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Basu U, Si K, Deng H, Maitra U. Phosphorylation of mammalian eukaryotic translation initiation factor 6 and its Saccharomyces cerevisiae homologue Tif6p: evidence that phosphorylation of Tif6p regulates its nucleocytoplasmic distribution and is required for yeast cell growth. Mol Cell Biol 2003; 23:6187-99. [PMID: 12917340 PMCID: PMC180954 DOI: 10.1128/mcb.23.17.6187-6199.2003] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/16/2003] [Accepted: 06/06/2003] [Indexed: 11/20/2022] Open
Abstract
The synthesis of 60S ribosomal subunits in Saccharomyces cerevisiae requires Tif6p, the yeast homologue of mammalian eukaryotic translation initiation factor 6 (eIF6). In the present work, we have isolated a protein kinase from rabbit reticulocyte lysates on the basis of its ability to phosphorylate recombinant human eIF6. Mass spectrometric analysis as well as antigenic properties of the purified kinase identified it as casein kinase I. The site of in vitro phosphorylation, which is highly conserved from yeast to mammals, was identified as the serine residues at positions 174 (major site) and 175 (minor site). The homologous yeast protein Tif6p was also phosphorylated in vivo in yeast cells. Mutation of Tif6p at serine-174 to alanine reduced phosphorylation drastically and caused loss of cell growth and viability. When both Ser-174 and Ser-175 were mutated to alanine, phosphorylation of Tif6p was completely abolished. Furthermore, while wild-type Tif6p was distributed both in nuclei and the cytoplasm of yeast cells, the mutant Tif6p (with Ser174Ala and Ser175Ala) became a constitutively nuclear protein. These results suggest that phosphorylatable Ser-174 and Ser-175 play a critical role in the nuclear export of Tif6p.
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Affiliation(s)
- Uttiya Basu
- Department of Developmental and Molecular Biology, Albert Einstein College of Medicine of Yeshiva University, Bronx, New York 10461, USA
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114
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Abstract
Ribosome synthesis is a highly complex and coordinated process that occurs not only in the nucleolus but also in the nucleoplasm and the cytoplasm of eukaryotic cells. Based on the protein composition of several ribosomal subunit precursors recently characterized in yeast, a total of more than 170 factors are predicted to participate in ribosome biogenesis and the list is still growing. So far the majority of ribosomal factors have been implicated in RNA maturation (nucleotide modification and processing). Recent advances gave insight into the process of ribosome export and assembly. Proteomic approaches have provided the first indications for a ribosome assembly pathway in eukaryotes and confirmed the dynamic character of the whole process.
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115
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Thomas F, Kutay U. Biogenesis and nuclear export of ribosomal subunits in higher eukaryotes depend on the CRM1 export pathway. J Cell Sci 2003; 116:2409-19. [PMID: 12724356 DOI: 10.1242/jcs.00464] [Citation(s) in RCA: 137] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
The production of ribosomes constitutes a major biosynthetic task for cells. Eukaryotic small and large ribosomal subunits are assembled in the nucleolus and independently exported to the cytoplasm. Most nuclear export pathways require RanGTP-binding export receptors. We analyzed the role of CRM1, the export receptor for leucine-rich nuclear export signals (NES), in the biogenesis of ribosomal subunits in vertebrate cells. Inhibition of the CRM1 export pathway led to a defect in nuclear export of both 40S and 60S subunits in HeLa cells. Moreover, the export of newly made ribosomal subunits in Xenopus oocytes was efficiently and specifically competed by BSA-NES conjugates. The CRM1 dependence of 60S subunit export suggested a conserved function for NMD3, a factor proposed to be a 60S subunit export adaptor in yeast. Indeed, we observed that nuclear export of human NMD3 (hNMD3) is sensitive to leptomycin B (LMB), which inactivates CRM1. It had, however, not yet been demonstrated that Nmd3 can interact with CRM1. Using purified recombinant proteins we have shown here that hNMD3 binds to CRM1 directly, in a RanGTP-dependent manner, by way of a C-terminal NES sequence. Our results suggest that the functions of CRM1 and NMD3 in ribosomal subunit export are conserved from yeast to higher eukaryotes.
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Affiliation(s)
- Franziska Thomas
- Swiss Federal Institute of Technology (ETH) Zürich, Institute of Biochemistry, HPM F11.1, Switzerland
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116
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Trotta CR, Lund E, Kahan L, Johnson AW, Dahlberg JE. Coordinated nuclear export of 60S ribosomal subunits and NMD3 in vertebrates. EMBO J 2003; 22:2841-51. [PMID: 12773398 PMCID: PMC156746 DOI: 10.1093/emboj/cdg249] [Citation(s) in RCA: 101] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/02/2002] [Revised: 03/26/2003] [Accepted: 03/26/2003] [Indexed: 11/14/2022] Open
Abstract
60S and 40S ribosomal subunits are assembled in the nucleolus and exported from the nucleus to the cytoplasm independently of each other. We show that in vertebrate cells, transport of both subunits requires the export receptor CRM1 and Ran.GTP. Export of 60S subunits is coupled with that of the nucleo- cytoplasmic shuttling protein NMD3. Human NMD3 (hNMD3) contains a CRM-1-dependent leucine-rich nuclear export signal (NES) and a complex, dispersed nuclear localization signal (NLS), the basic region of which is also required for nucleolar accumulation. When present in Xenopus oocytes, both wild-type and export-defective mutant hNMD3 proteins bind to newly made nuclear 60S pre-export particles at a late step of subunit maturation. The export-defective hNMD3, but not the wild-type protein, inhibits export of 60S subunits from oocyte nuclei. These results indicate that the NES mutant protein competes with endogenous wild-type frog NMD3 for binding to nascent 60S subunits, thereby preventing their export. We propose that NMD3 acts as an adaptor for CRM1-Ran.GTP-mediated 60S subunit export, by a mechanism that is conserved from vertebrates to yeast.
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Affiliation(s)
- Christopher R Trotta
- Department of Biomolecular Chemistry, University of Wisconsin Medical School, 1300 University Avenue, Madison, WI 53706, USA
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117
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Abstract
Eukaryotic ribosomes are assembled in the nucleolus before export to the cytoplasm. Ribosome formation is a highly dynamic and coordinated multistep process, which requires synthesis, processing and modification of pre-rRNAs, assembly with ribosomal proteins and transient interaction of numerous non-ribosomal factors with the evolving pre-ribosomal particles. In the past two years, exciting insights into the sequential events occurring during pre-ribosome formation have been obtained, thanks largely to the advances in proteomic analyses. We now have a first biochemical map of the earliest 90S pre-ribosomes and of their daughter pre-40S and pre-60S ribosomal subunits along their path from the nucleolus to the cytoplasm. The future challenge will be to assign functions to the more than 150 non-ribosomal factors that transiently associate with the developing pre-ribosomes.
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Affiliation(s)
- Herbert Tschochner
- Biochemie-Zentrum Heidelberg (BZH), Im Neuenheimer Feld 328, D-69120 Heidelberg, Germany.
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118
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Metzler DE, Metzler CM, Sauke DJ. Ribosomes and the Synthesis of Proteins. Biochemistry 2001. [DOI: 10.1016/b978-012492543-4/50032-5] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
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