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Labokha AA, Fassati A. Viruses challenge selectivity barrier of nuclear pores. Viruses 2013; 5:2410-23. [PMID: 24084236 PMCID: PMC3814595 DOI: 10.3390/v5102410] [Citation(s) in RCA: 15] [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: 08/20/2013] [Revised: 09/24/2013] [Accepted: 09/25/2013] [Indexed: 01/04/2023] Open
Abstract
Exchange between the nucleus and the cytoplasm occurs through nuclear pore complexes (NPCs) embedded in the double membrane of the nuclear envelope. NPC permeability barrier restricts the entry of inert molecules larger than 5 nm in diameter but allows facilitated entry of selected cargos, whose size can reach up to 39 nm. The translocation of large molecules is facilitated by nuclear transport receptors (NTRs) that have affinity to proteins of NPC permeability barrier. Viruses that enter the nucleus replicate evolved strategies to overcome this barrier. In this review, we will discuss the functional principles of NPC barrier and nuclear transport machinery, as well as the various strategies viruses use to cross the selective barrier of NPCs.
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Affiliation(s)
- Aksana A. Labokha
- Authors to whom correspondence should be addressed; E-Mails: (A.A.L.); (A.F.); Tel.: (+44(0)2031082141); Fax: (+44(0)2031082123); Tel.: (+44(0)2031082138); Fax: (+44(0)2031082123)
| | - Ariberto Fassati
- Authors to whom correspondence should be addressed; E-Mails: (A.A.L.); (A.F.); Tel.: (+44(0)2031082141); Fax: (+44(0)2031082123); Tel.: (+44(0)2031082138); Fax: (+44(0)2031082123)
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Abstract
Influenza A virus vRNP nuclear export is CRM1-dependent. Ran-binding protein 3 (RanBP3) is a Ran-interacting protein that is best known for its role as a cofactor of CRM1-mediated cargo nuclear export. In this study, we investigated the role of RanBP3 during the influenza A virus life cycle. We found that RanBP3 was phosphorylated at Ser58 in the early and late phases of infection. Knockdown of RanBP3 expression led to vRNP nuclear retention, suggesting that RanBP3 is involved in vRNP nuclear export. Moreover, we demonstrated that the function of RanBP3 during vRNP nuclear export is regulated by phosphorylation at Ser58, and that RanBP3 phosphorylation is modulated by both PI3K/Akt and Ras/ERK/RSK pathways in the late phase of viral infection.
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Affiliation(s)
- Rey Predicala
- Vaccine and Infectious Disease Organization, University of Saskatchewan, Saskatoon, Saskatchewan, S7N 5E3, Canada
| | - Yan Zhou
- Vaccine and Infectious Disease Organization, University of Saskatchewan, Saskatoon, Saskatchewan, S7N 5E3, Canada
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53
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Choreography of importin-α/CAS complex assembly and disassembly at nuclear pores. Proc Natl Acad Sci U S A 2013; 110:E1584-93. [PMID: 23569239 DOI: 10.1073/pnas.1220610110] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Nuclear pore complexes (NPCs) mediate the exchange of macromolecules between the cytoplasm and the nucleoplasm. Soluble nuclear transport receptors bind signal-dependent cargos to form transport complexes that diffuse through the NPC and are then disassembled. Although transport receptors enable the NPC's permeability barrier to be overcome, directionality is established by complex assembly and disassembly. Here, we delineate the choreography of importin-α/CAS complex assembly and disassembly in permeabilized cells, using single-molecule fluorescence resonance energy transfer and particle tracking. Monitoring interaction sequences in intact NPCs ensures spatiotemporal preservation of structures and interactions critical for activity in vivo. We show that key interactions between components are reversible, multiple outcomes are often possible, and the assembly and disassembly of complexes are precisely controlled to occur at the appropriate place and time. Importin-α mutants that impair interactions during nuclear import were used together with cytoplasmic Ran GTPase-activating factors to demonstrate that importin-α/CAS complexes form in the nuclear basket region, at the termination of protein import, and disassembly of importin-α/CAS complexes after export occurs in the cytoplasmic filament region of the NPC. Mathematical models derived from our data emphasize the intimate connection between transport and the coordinated assembly and disassembly of importin-α/CAS complexes for generating productive transport cycles.
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54
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Structural basis for the nuclear export activity of Importin13. EMBO J 2013; 32:899-913. [PMID: 23435562 DOI: 10.1038/emboj.2013.29] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/07/2012] [Accepted: 01/28/2013] [Indexed: 02/05/2023] Open
Abstract
Importin13 (Imp13) is a bidirectional karyopherin that can mediate both import and export of cargoes. Imp13 recognizes several import cargoes, which include the exon junction complex components Mago-Y14 and the E2 SUMO-conjugating enzyme Ubc9, and one known export cargo, the translation initiation factor 1A (eIF1A). To understand how Imp13 can perform double duty, we determined the 3.6-Å crystal structure of Imp13 in complex with RanGTP and with eIF1A. eIF1A binds at the inner surface of the Imp13 C-terminal arch adjacent and concomitantly to RanGTP illustrating how eIF1A can be exported by Imp13. Moreover, the 3.0-Å structure of Imp13 in its unbound state reveals the existence of an open conformation in the cytoplasm that explains export cargo release and completes the export branch of the Imp13 pathway. Finally, we demonstrate that Imp13 is able to bind and export eIF1A in vivo and that its function is essential.
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55
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Sumoylation regulates Kap114-mediated nuclear transport. EMBO J 2012; 31:2461-72. [PMID: 22562154 DOI: 10.1038/emboj.2012.102] [Citation(s) in RCA: 29] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/23/2011] [Accepted: 03/28/2012] [Indexed: 11/08/2022] Open
Abstract
The nuclear import receptor Kap114 carries transcription factors and other cargos across nuclear pores into the nucleus. Here we show that yeast Kap114 is modified by SUMO (small ubiquitin-related modifier) and that sumoylation is required for Kap114-mediated nuclear import. Among the four known SUMO-specific E3 ligases in yeast, Mms21 is the preferred E3 enzyme responsible for the covalent attachment of SUMO to the Kap114 protein. Kap114 is sumoylated on lysine residue 909, which is part of a ΨKxD/E sumoylation consensus motif. Kap114 containing a lysine-to-arginine point mutation at position 909 mislocalizes to the nucleus and is defective in promoting nuclear import. Similarly, mutants defective in sumoylation or desumoylation specifically accumulate Kap114 in the nucleus and are blocked in import of Kap114 cargos. Ran-GTP is not sufficient to disassemble Kap114/cargo complexes, which necessitates additional cargo release mechanisms in the nucleus. Remarkably, sumoylation of Kap114 greatly stimulates cargo dissociation in vitro. We propose that sumoylation occurs at the site of Kap114 cargo function and that SUMO is a cargo release factor involved in intranuclear targeting.
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McGuire AT, Mangroo D. Cex1p facilitates Rna1p-mediated dissociation of the Los1p-tRNA-Gsp1p-GTP export complex. Traffic 2011; 13:234-56. [PMID: 22008473 DOI: 10.1111/j.1600-0854.2011.01304.x] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/16/2011] [Revised: 10/17/2011] [Accepted: 10/17/2011] [Indexed: 01/17/2023]
Abstract
Nuclear tRNA export plays an essential role in key cellular processes such as regulation of protein synthesis, cell cycle progression, response to nutrient availability and DNA damage and development. Like other nuclear export processes, assembly of the nuclear tRNA export complex in the nucleus is dependent on Ran-GTP/Gsp1p-GTP, and dissociation of the export receptor-tRNA-Ran-GTP/Gsp1p-GTP complex in the cytoplasm requires RanBP1/Yrb1p and RanGAP/Rna1p to activate the GTPase activity of Ran-GTP/Gsp1p-GTP. The Saccharomyces cerevisiae Cex1p and Human Scyl1 have also been proposed to participate in unloading of the tRNA export receptors at the cytoplasmic face of the nuclear pore complex (NPC). Here, we provide evidence suggesting that Cex1p is required for activation of the GTPase activity of Gsp1p and dissociation of the receptor-tRNA-Gsp1p export complex in S. cerevisiae. The data suggest that Cex1p recruits Rna1p from the cytoplasm to the NPC and facilitates Rna1p activation of the GTPase activity of Gsp1p by enabling Rna1p to gain access to Gsp1p-GTP bound to the export receptor tRNA complex. It is possible that this tRNA unloading mechanism is conserved in evolutionarily diverse organisms and that other Gsp1p-GTP-dependent export processes use a pathway-specific component to recruit Rna1p to the NPC.
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Affiliation(s)
- Andrew T McGuire
- Department of Molecular and Cellular Biology, University of Guelph, Guelph, Ontario, N1G 2W1, Canada
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57
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Goryaynov A, Ma J, Yang W. Single-molecule studies of nucleocytoplasmic transport: from one dimension to three dimensions. Integr Biol (Camb) 2011; 4:10-21. [PMID: 22020388 DOI: 10.1039/c1ib00041a] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
Abstract
In eukaryotic cells, the bidirectional trafficking of proteins and genetic materials across the double-membrane nuclear envelope is mediated by nuclear pore complexes (NPCs). A highly selective barrier formed by the phenylalanine-glycine (FG)-nucleoporin (Nup) in the NPC allows for two transport modes: passive diffusion and transport receptor-facilitated translocation. Strict regulation of nucleocytoplasmic transport is crucial for cell survival, differentiation, growth and other essential activities. However, due to the limited knowledge of the native configuration of the FG-Nup barrier and the interactions between the transiting molecules and the barrier in the NPC, the precise nucleocytoplasmic transport mechanism remains unresolved. To refine the transport mechanism, single-molecule fluorescence microscopy methods have been employed to obtain the transport kinetics of individual fluorescent molecules through the NPC and to map the interactions between transiting molecules and the FG-Nup barrier. Important characteristics of nucleocytoplasmic transport, such as transport time, transport efficiency and spatial distribution of single transiting molecules in the NPC, have been obtained that could not be measured by either ensemble average methods or conventional electron microscopy. In this critical review, we discuss the development of various single-molecule techniques and their application to nucleocytoplasmic transport in vitro and in vivo. In particular, we highlight a recent advance from one-dimensional to three-dimensional single-molecule characterization of transport through the NPC and present a comprehensive understanding of the nucleocytoplasmic transport mechanism obtained by this new technical development (105 references).
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Affiliation(s)
- Alexander Goryaynov
- Department of Biological Sciences, Center for Photochemical Sciences, Bowling Green State University, Bowling Green, OH 43403, USA
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58
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Ran-dependent nuclear export mediators: a structural perspective. EMBO J 2011; 30:3457-74. [PMID: 21878989 DOI: 10.1038/emboj.2011.287] [Citation(s) in RCA: 159] [Impact Index Per Article: 12.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/01/2011] [Accepted: 07/22/2011] [Indexed: 12/25/2022] Open
Abstract
Nuclear export is an essential eukaryotic activity. It proceeds through nuclear pore complexes (NPCs) and is mediated by soluble receptors that shuttle between nucleus and cytoplasm. RanGTPase-dependent export mediators (exportins) constitute the largest class of these carriers and are functionally highly versatile. All of these exportins load their substrates in response to RanGTP binding in the nucleus and traverse NPCs as ternary RanGTP-exportin-cargo complexes to the cytoplasm, where GTP hydrolysis leads to export complex disassembly. The different exportins vary greatly in their substrate range. Recent structural studies of both protein- and RNA-specific exporters have illuminated how exportins bind their cargoes, how Ran triggers cargo loading and how export complexes are disassembled in the cytoplasm. Here, we review the current state of knowledge and highlight emerging principles as well as prevailing questions.
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59
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Fox AM, Ciziene D, McLaughlin SH, Stewart M. Electrostatic interactions involving the extreme C terminus of nuclear export factor CRM1 modulate its affinity for cargo. J Biol Chem 2011; 286:29325-29335. [PMID: 21708948 PMCID: PMC3190738 DOI: 10.1074/jbc.m111.245092] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/30/2011] [Revised: 06/17/2011] [Indexed: 12/11/2022] Open
Abstract
The toroid-shaped nuclear protein export factor CRM1 is constructed from 21 tandem HEAT repeats, each of which contains an inner (B) and outer (A) α-helix joined by loops. Proteins targeted for export have a nuclear export signal (NES) that binds between the A-helices of HEAT repeats 11 and 12 on the outer surface of CRM1. RanGTP binding increases the affinity of CRM1 for NESs. In the absence of RanGTP, the CRM1 C-terminal helix, together with the HEAT repeat 9 loop, modulates its affinity for NESs. Here we show that there is an electrostatic interaction between acidic residues at the extreme distal tip of the C-terminal helix and basic residues on the HEAT repeat 12 B-helix that lies on the inner surface of CRM1 beneath the NES binding site. Small angle x-ray scattering indicates that the increased affinity for NESs generated by mutations in the C-terminal helix is not associated with large scale changes in CRM1 conformation, consistent with the modulation of NES affinity being mediated by a local change in CRM1 near the NES binding site. These data also suggest that in the absence of RanGTP, the C-terminal helix lies across the CRM1 toroid in a position similar to that seen in the CRM1-Snurportin crystal structure. By creating local changes that stabilize the NES binding site in its closed conformation and thereby reducing the affinity of CRM1 for NESs, the C-terminal helix and HEAT 9 loop facilitate release of NES-containing cargo in the cytoplasm and also inhibit their return to the nucleus.
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Affiliation(s)
- Abigail M Fox
- Medical Research Council Laboratory of Molecular Biology, Cambridge, United Kingdom
| | - Danguole Ciziene
- Medical Research Council Laboratory of Molecular Biology, Cambridge, United Kingdom
| | - Stephen H McLaughlin
- Medical Research Council Laboratory of Molecular Biology, Cambridge, United Kingdom
| | - Murray Stewart
- Medical Research Council Laboratory of Molecular Biology, Cambridge, United Kingdom.
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60
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Wang X, Xu X, Ma Z, Huo Y, Xiao Z, Li Y, Wang Y. Dynamic mechanisms for pre-miRNA binding and export by Exportin-5. RNA (NEW YORK, N.Y.) 2011; 17:1511-28. [PMID: 21712399 PMCID: PMC3153975 DOI: 10.1261/rna.2732611] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/16/2023]
Abstract
The biogenesis and function of mature microRNAs (miRNAs) is dependent on the nuclear export of miRNA precursors (pre-miRNA) by Exportin-5 (Exp5). To characterize the molecular mechanisms of how pre-miRNA is recognized and transported by Exp5, we have performed 21 molecular dynamic (MD) simulations of RNA-bound Exp5 (Exp5-RanGTP-premiRNA, Exp5-RanGDP-premiRNA, Exp5-premiRNA), RNA-unbound Exp5 (Exp5-RanGTP, Exp5-RanGDP, apo-Exp5), and pre-miRNA. Our simulations with standard MD, steered molecular dynamics (SMD), and energy analysis have shown that (1) Free Exp5 undergoes extensive opening motion, and in this way facilitates the RanGTP binding. (2) RanGTP efficiently regulates the association/dissociation of pre-miRNA to its complex by inducing conformational changes in the HEAT-repeat helix stacking of Exp5. (3) The GTP hydrolysis prevents Ran from rebinding to Exp5 by regulating the hydrophobic interfaces and salt bridges between Ran and Exp5. (4) The transition from the A'-form to the A-form of the pre-miRNA modulates the structural complementarities between the protein and the pre-miRNA, thus promoting efficient assembly of the complex. (5) The base-flipping process (from the closed to the fully flipped state) of the 2-nt 3' overhang is a prerequisite for the pre-miRNA recognition by Exp5, which occurs in a sequence-independent manner as evidenced by the fact that different 2-nt 3' overhangs bind to Exp5 in essentially the same way. And finally, a plausible mechanism of the pre-miRNA export cycle has been proposed explaining how the protein-protein and protein-RNA interactions are coordinated in physiological conditions.
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Affiliation(s)
- Xia Wang
- Center of Bioinformatics, Northwest A&F University, Yangling 712100, Shaanxi, China
- College of Life Sciences, Northwest A&F University, Yangling 712100, Shaanxi, China
| | - Xue Xu
- Center of Bioinformatics, Northwest A&F University, Yangling 712100, Shaanxi, China
- College of Life Sciences, Northwest A&F University, Yangling 712100, Shaanxi, China
| | - Zhi Ma
- Center of Bioinformatics, Northwest A&F University, Yangling 712100, Shaanxi, China
- College of Life Sciences, Northwest A&F University, Yangling 712100, Shaanxi, China
| | - Yingqiu Huo
- College of Information Engineering, Northwest A&F University, Yangling 712100, Shaanxi, China
| | - Zhengtao Xiao
- Center of Bioinformatics, Northwest A&F University, Yangling 712100, Shaanxi, China
- College of Life Sciences, Northwest A&F University, Yangling 712100, Shaanxi, China
| | - Yan Li
- Department of Materials Science and Chemical Engineering, Dalian University of Technology, Dalian 116023, Liaoning, China
| | - Yonghua Wang
- Center of Bioinformatics, Northwest A&F University, Yangling 712100, Shaanxi, China
- College of Life Sciences, Northwest A&F University, Yangling 712100, Shaanxi, China
- Corresponding author.E-mail
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61
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Soderholm JF, Bird SL, Kalab P, Sampathkumar Y, Hasegawa K, Uehara-Bingen M, Weis K, Heald R. Importazole, a small molecule inhibitor of the transport receptor importin-β. ACS Chem Biol 2011; 6:700-8. [PMID: 21469738 PMCID: PMC3137676 DOI: 10.1021/cb2000296] [Citation(s) in RCA: 180] [Impact Index Per Article: 13.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/06/2023]
Abstract
During interphase, the transport receptor importin-β carries cargoes into the nucleus, where RanGTP releases them. A similar mechanism operates in mitosis to generate a gradient of active spindle assembly factors around mitotic chromosomes. Importin-β and RanGTP have been implicated in additional cellular processes, but the precise roles of the Ran/importin-β pathway throughout the cell cycle remain poorly understood. We implemented a FRET-based, high-throughput small molecule screen for compounds that interfere with the interaction between RanGTP and importin-β and identified importazole, a 2,4-diaminoquinazoline. Importazole specifically blocks importin-β-mediated nuclear import both in Xenopus egg extracts and cultured cells, without disrupting transportin-mediated nuclear import or CRM1-mediated nuclear export. When added during mitosis, importazole impairs the release of an importin-β cargo FRET probe and causes both predicted and novel defects in spindle assembly. Together, these results indicate that importazole specifically inhibits the function of importin-β, likely by altering its interaction with RanGTP. Importazole is a valuable tool to evaluate the function of the importin-β/RanGTP pathway at specific stages during the cell cycle.
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Affiliation(s)
- Jonathan F Soderholm
- The Department of Molecular and Cell Biology, University of California, Berkeley, MC 3200 LSA, Berkeley, California 94720-3200, United States
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Calcium regulation of nucleocytoplasmic transport. Protein Cell 2011; 2:291-302. [PMID: 21528351 DOI: 10.1007/s13238-011-1038-x] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/15/2011] [Accepted: 03/27/2011] [Indexed: 10/18/2022] Open
Abstract
Bidirectional trafficking of macromolecules between the cytoplasm and the nucleus is mediated by the nuclear pore complexes (NPCs) embedded in the nuclear envelope (NE) of eukaryotic cell. The NPC functions as the sole pathway to allow for the passive diffusion of small molecules and the facilitated translocation of larger molecules. Evidence shows that these two transport modes and the conformation of NPC can be regulated by calcium stored in the lumen of nuclear envelope and endoplasmic reticulum. However, the mechanism of calcium regulation remains poorly understood. In this review, we integrate data on the observations of calciumregulated structure and function of the NPC over the past years. Furthermore, we highlight challenges in the measurements of dynamic conformational changes and transient transport kinetics in the NPC. Finally, an innovative imaging approach, single-molecule superresolution fluorescence microscopy, is introduced and expected to provide more insights into the mechanism of calcium-regulated nucleocytoplasmic transport.
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63
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Abstract
Roles of the GTPase Ran in cell life and division rely on a largely conserved mechanism, i.e. Ran's ability to interact with transport vectors. Modes of control of downstream factors, however, are diversified at particular times of the cell cycle. Specificity and fine-tuning emerge most clearly during mitosis. In the present article, we focus on the distinction between global mitotic control by the chromosomal Ran gradient and specific spatial and temporal control operated by localized Ran network members at sites of the mitotic apparatus in human cells.
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64
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Nagai M, Moriyama T, Mehmood R, Tokuhiro K, Ikawa M, Okabe M, Tanaka H, Yoneda Y. Mice lacking Ran binding protein 1 are viable and show male infertility. FEBS Lett 2011; 585:791-6. [PMID: 21310149 DOI: 10.1016/j.febslet.2011.02.002] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/12/2010] [Revised: 01/31/2011] [Accepted: 02/01/2011] [Indexed: 11/29/2022]
Abstract
The small GTPase Ran plays important roles in multiple aspects of cellular function. Maximal RanGAP activity is achieved with the aid of RanBP1 and/or presumably of RanBP2. Here, we show that RanBP1-knockout mice are unexpectedly viable, and exhibit male infertility due to a spermatogenesis arrest, presumably caused by down-regulation of RanBP2 during spermatogenesis. Indeed, siRNA-mediated depletion of RanBP2 caused severe cell death only in RanBP1-deficient MEFs, indicating that simultaneous depletion of RanBP1 and RanBP2 severely affects normal cell viability. Collectively, we conclude that the dramatic decrease in "RanBP" activity impairs germ cell viability and affects spermatogenesis decisively in RanBP1-knockout mice.
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Affiliation(s)
- Masahiro Nagai
- Biomolecular Dynamics Laboratory, Department of Frontier Biosciences, Graduate School of Frontier Biosciences, Osaka University, 1-3 Yamada-oka, Suita, Osaka 565-0871, Japan
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65
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Importin β-type nuclear transport receptors have distinct binding affinities for Ran-GTP. Biochem Biophys Res Commun 2011; 406:383-8. [PMID: 21329658 DOI: 10.1016/j.bbrc.2011.02.051] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/26/2011] [Accepted: 02/11/2011] [Indexed: 01/25/2023]
Abstract
Cargos destined to enter or leave the cell nucleus are typically transported by receptors of the importin β family to pass the nuclear pore complex. The yeast Saccharomyces cerevisiae comprises 14 members of this protein family, which can be divided in importins and exportins. The Ran GTPase regulates the association and dissociation of receptors and cargos as well as the transport direction through the nuclear pore. All receptors bind to Ran exclusively in its GTP-bound state and this event is restricted to the nuclear compartment. We determined the Ran-GTP binding properties of all yeast transport receptors by biosensor measurements and observed that the affinity of importins for Ran-GTP differs significantly. The dissociation constants range from 230 pM to 270 nM, which is mostly based on a variability of the off-rate constants. The divergent affinity of importins for Ran-GTP suggests the existence of a novel mode of nucleocytoplasmic transport regulation. Furthermore, the cellular concentration of β-receptors and of other Ran-binding proteins was determined. We found that the number of β-receptors altogether about equals the amounts of yeast Ran, but Ran-GTP is not limiting in the nucleus. The implications of our results for nucleocytoplasmic transport mechanisms are discussed.
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66
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Merkle T. Nucleo-cytoplasmic transport of proteins and RNA in plants. PLANT CELL REPORTS 2011; 30:153-76. [PMID: 20960203 PMCID: PMC3020307 DOI: 10.1007/s00299-010-0928-3] [Citation(s) in RCA: 53] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/30/2010] [Accepted: 09/30/2010] [Indexed: 05/19/2023]
Abstract
Transport of macromolecules between the nucleus and the cytoplasm is an essential necessity in eukaryotic cells, since the nuclear envelope separates transcription from translation. In the past few years, an increasing number of components of the plant nuclear transport machinery have been characterised. This progress, although far from being completed, confirmed that the general characteristics of nuclear transport are conserved between plants and other organisms. However, plant-specific components were also identified. Interestingly, several mutants in genes encoding components of the plant nuclear transport machinery were investigated, revealing differential sensitivity of plant-specific pathways to impaired nuclear transport. These findings attracted attention towards plant-specific cargoes that are transported over the nuclear envelope, unravelling connections between nuclear transport and components of signalling and developmental pathways. The current state of research in plants is summarised in comparison to yeast and vertebrate systems, and special emphasis is given to plant nuclear transport mutants.
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Affiliation(s)
- Thomas Merkle
- Faculty of Biology, Institute for Genome Research and Systems Biology, University of Bielefeld, 33594 Bielefeld, Germany.
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67
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Comparative genomics of proteins involved in RNA nucleocytoplasmic export. BMC Evol Biol 2011; 11:7. [PMID: 21223572 PMCID: PMC3032688 DOI: 10.1186/1471-2148-11-7] [Citation(s) in RCA: 29] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/15/2009] [Accepted: 01/11/2011] [Indexed: 12/18/2022] Open
Abstract
Background The establishment of the nuclear membrane resulted in the physical separation of transcription and translation, and presented early eukaryotes with a formidable challenge: how to shuttle RNA from the nucleus to the locus of protein synthesis. In prokaryotes, mRNA is translated as it is being synthesized, whereas in eukaryotes mRNA is synthesized and processed in the nucleus, and it is then exported to the cytoplasm. In metazoa and fungi, the different RNA species are exported from the nucleus by specialized pathways. For example, tRNA is exported by exportin-t in a RanGTP-dependent fashion. By contrast, mRNAs are associated to ribonucleoproteins (RNPs) and exported by an essential shuttling complex (TAP-p15 in human, Mex67-mtr2 in yeast) that transports them through the nuclear pore. The different RNA export pathways appear to be well conserved among members of Opisthokonta, the eukaryotic supergroup that includes Fungi and Metazoa. However, it is not known whether RNA export in the other eukaryotic supergroups follows the same export routes as in opisthokonts. Methods Our objective was to reconstruct the evolutionary history of the different RNA export pathways across eukaryotes. To do so, we screened an array of eukaryotic genomes for the presence of homologs of the proteins involved in RNA export in Metazoa and Fungi, using human and yeast proteins as queries. Results Our genomic comparisons indicate that the basic components of the RanGTP-dependent RNA pathways are conserved across eukaryotes, and thus we infer that these are traceable to the last eukaryotic common ancestor (LECA). On the other hand, several of the proteins involved in RanGTP-independent mRNA export pathways are less conserved, which would suggest that they represent innovations that appeared later in the evolution of eukaryotes. Conclusions Our analyses suggest that the LECA possessed the basic components of the different RNA export mechanisms found today in opisthokonts, and that these mechanisms became more specialized throughout eukaryotic evolution.
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68
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Kaláb P, Solc P, Motlík J. The role of RanGTP gradient in vertebrate oocyte maturation. Results Probl Cell Differ 2011; 53:235-67. [PMID: 21630149 DOI: 10.1007/978-3-642-19065-0_12] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022]
Abstract
The maturation of vertebrate oocyte into haploid gamete, the egg, consists of two specialized asymmetric cell divisions with no intervening S-phase. Ran GTPase has an essential role in relaying the active role of chromosomes in their own segregation by the meiotic process. In addition to its conserved role as a key regulator of macromolecular transport between nucleus and cytoplasm, Ran has important functions during cell division, including in mitotic spindle assembly and in the assembly of nuclear envelope at the exit from mitosis. The cellular functions of Ran are mediated by RanGTP interactions with nuclear transport receptors (NTRs) related to importin β and depend on the existence of chromosome-centered RanGTP gradient. Live imaging with FRET biosensors indeed revealed the existence of RanGTP gradient throughout mouse oocyte maturation. NTR-dependent transport of cell cycle regulators including cyclin B1, Wee2, and Cdc25B between the oocyte cytoplasm and germinal vesicle (GV) is required for normal resumption of meiosis. After GVBD in mouse oocytes, RanGTP gradient is required for timely meiosis I (MI) spindle assembly and provides long-range signal directing egg cortex differentiation. However, RanGTP gradient is not required for MI spindle migration and may be dispensable for MI spindle function in chromosome segregation. In contrast, MII spindle assembly and function in maturing mouse and Xenopus laevis eggs depend on RanGTP gradient, similar to X. laevis MII-derived egg extracts.
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Affiliation(s)
- Petr Kaláb
- Laboratory of Cellular and Molecular Biology, National Cancer Institute, NIH, Bethesda, MD 20892-4256, USA.
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69
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Jamali T, Jamali Y, Mehrbod M, Mofrad MRK. Nuclear pore complex: biochemistry and biophysics of nucleocytoplasmic transport in health and disease. INTERNATIONAL REVIEW OF CELL AND MOLECULAR BIOLOGY 2011; 287:233-86. [PMID: 21414590 DOI: 10.1016/b978-0-12-386043-9.00006-2] [Citation(s) in RCA: 75] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/24/2023]
Abstract
Nuclear pore complexes (NPCs) are the gateways connecting the nucleoplasm and cytoplasm. This structures are composed of over 30 different proteins and 60-125 MDa of mass depending on type of species. NPCs are bilateral pathways that selectively control the passage of macromolecules into and out of the nucleus. Molecules smaller than 40 kDa diffuse through the NPC passively while larger molecules require facilitated transport provided by their attachment to karyopherins. Kinetic studies have shown that approximately 1000 translocations occur per second per NPC. Maintaining its high selectivity while allowing for rapid translocation makes the NPC an efficient chemical nanomachine. In this review, we approach the NPC function via a structural viewpoint. Putting together different pieces of this puzzle, this chapter confers an overall insight into what molecular processes are engaged in import/export of active cargos across the NPC and how different transporters regulate nucleocytoplasmic transport. In the end, the correlation of several diseases and disorders with the NPC structural defects and dysfunctions is discussed.
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Affiliation(s)
- T Jamali
- Department of Bioengineering, University of California, Berkeley, California, USA
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70
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Tu LC, Musser SM. Single molecule studies of nucleocytoplasmic transport. BIOCHIMICA ET BIOPHYSICA ACTA-MOLECULAR CELL RESEARCH 2010; 1813:1607-18. [PMID: 21167872 DOI: 10.1016/j.bbamcr.2010.12.011] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/01/2010] [Revised: 10/18/2010] [Accepted: 12/08/2010] [Indexed: 01/09/2023]
Abstract
Molecular traffic between the cytoplasm and the nucleoplasm of eukaryotic cells is mediated by nuclear pore complexes (NPCs). Hundreds, if not thousands, of molecules interact with and transit through each NPC every second. The pore is blocked by a permeability barrier, which consists of a network of intrinsically unfolded polypeptides containing thousands of phenylalanine-glycine (FG) repeat motifs. This FG-network rejects larger molecules and admits smaller molecules or cargos bound to nuclear transport receptors (NTRs). For a cargo transport complex, minimally consisting of a cargo molecule plus an NTR, access to the permeability barrier is provided by interactions between the NTR and the FG repeat motifs. Numerous models have been postulated to explain the controlled accessibility and the transport characteristics of the FG-network, but the amorphous, flexible nature of this structure has hindered characterization. A relatively recent development is the ability to monitor the real-time movement of single molecules through individual NPCs via single molecule fluorescence (SMF) microscopy. A major advantage of this approach is that it can be used to continuously monitor a series of specific molecular interactions in an active pore with millisecond time resolution, which therefore allows one to distinguish between kinetic and thermodynamic control. Novel insights and prospects for the future are outlined in this review. This article is part of a Special Issue entitled: Regulation of Signaling and Cellular Fate through Modulation of Nuclear Protein Import.
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Affiliation(s)
- Li-Chun Tu
- Department of Molecular and Cellular Medicine, College of Medicine, Texas A&M Health Science Center, College Station, TX 77843, USA
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71
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Lott K, Cingolani G. The importin β binding domain as a master regulator of nucleocytoplasmic transport. BIOCHIMICA ET BIOPHYSICA ACTA-MOLECULAR CELL RESEARCH 2010; 1813:1578-92. [PMID: 21029753 DOI: 10.1016/j.bbamcr.2010.10.012] [Citation(s) in RCA: 136] [Impact Index Per Article: 9.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/10/2010] [Revised: 10/11/2010] [Accepted: 10/19/2010] [Indexed: 12/16/2022]
Abstract
Specific and efficient recognition of import cargoes is essential to ensure nucleocytoplasmic transport. To this end, the prototypical karyopherin importin β associates with import cargoes directly or, more commonly, through import adaptors, such as importin α and snurportin. Adaptor proteins bind the nuclear localization sequence (NLS) of import cargoes while recruiting importin β via an N-terminal importin β binding (IBB) domain. The use of adaptors greatly expands and amplifies the repertoire of cellular cargoes that importin β can efficiently import into the cell nucleus and allows for fine regulation of nuclear import. Accordingly, the IBB domain is a dedicated NLS, unique to adaptor proteins that functions as a molecular liaison between importin β and import cargoes. This review provides an overview of the molecular role played by the IBB domain in orchestrating nucleocytoplasmic transport. Recent work has determined that the IBB domain has specialized functions at every step of the import and export pathway. Unexpectedly, this stretch of ~40 amino acids plays an essential role in regulating processes such as formation of the import complex, docking and translocation through the nuclear pore complex (NPC), release of import cargoes into the cell nucleus and finally recycling of import adaptors and importin β into the cytoplasm. Thus, the IBB domain is a master regulator of nucleocytoplasmic transport, whose complex molecular function is only recently beginning to emerge. This article is part of a Special Issue entitled: Regulation of Signaling and Cellular Fate through Modulation of Nuclear Protein Import.
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Affiliation(s)
- Kaylen Lott
- Department of Biochemistry and Molecular Biology, Thomas Jefferson University, 233 South 10th Street, Philadelphia, PA 19107, USA
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72
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Hintersteiner M, Ambrus G, Bednenko J, Schmied M, Knox AJS, Meisner NC, Gstach H, Seifert JM, Singer EL, Gerace L, Auer M. Identification of a small molecule inhibitor of importin β mediated nuclear import by confocal on-bead screening of tagged one-bead one-compound libraries. ACS Chem Biol 2010; 5:967-79. [PMID: 20677820 DOI: 10.1021/cb100094k] [Citation(s) in RCA: 43] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
Abstract
In eukaryotic cells, proteins and RNAs are transported between the nucleus and the cytoplasm by nuclear import and export receptors. Over the past decade, small molecules that inhibit the nuclear export receptor CRM1 have been identified, most notably leptomycin B. However, up to now no small molecule inhibitors of nuclear import have been described. Here we have used our automated confocal nanoscanning and bead picking method (CONA) for on-bead screening of a one-bead one-compound library to identify the first such import inhibitor, karyostatin 1A. Karyostatin 1A binds importin β with high nanomolar affinity and specifically inhibits importin α/β mediated nuclear import at low micromolar concentrations in vitro and in living cells, without perturbing transportin mediated nuclear import or CRM1 mediated nuclear export. Surface plasmon resonance binding experiments suggest that karyostatin 1A acts by disrupting the interaction between importin β and the GTPase Ran. As a selective inhibitor of the importin α/β import pathway, karyostatin 1A will provide a valuable tool for future studies of nucleocytoplasmic trafficking.
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Affiliation(s)
- Martin Hintersteiner
- The University of Edinburgh, School of Biological Sciences (CSE) and School of Biomedical Sciences (CMVM), CH Waddington Building, 3.07, The King’s Buildings, Mayfield Road, Edinburgh EH9 3JD, U.K
| | - Géza Ambrus
- Department of Cell Biology, The Scripps Research Institute, 10550 N. Torrey Pines Road, La Jolla, California 92037
| | - Janna Bednenko
- Department of Cell Biology, The Scripps Research Institute, 10550 N. Torrey Pines Road, La Jolla, California 92037
| | | | - Andrew J. S. Knox
- The University of Edinburgh, School of Biological Sciences (CSE) and School of Biomedical Sciences (CMVM), CH Waddington Building, 3.07, The King’s Buildings, Mayfield Road, Edinburgh EH9 3JD, U.K
| | | | | | | | - Eric L. Singer
- Department of Cell Biology, The Scripps Research Institute, 10550 N. Torrey Pines Road, La Jolla, California 92037
| | - Larry Gerace
- Department of Cell Biology, The Scripps Research Institute, 10550 N. Torrey Pines Road, La Jolla, California 92037
| | - Manfred Auer
- The University of Edinburgh, School of Biological Sciences (CSE) and School of Biomedical Sciences (CMVM), CH Waddington Building, 3.07, The King’s Buildings, Mayfield Road, Edinburgh EH9 3JD, U.K
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73
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Ciciarello M, Roscioli E, Di Fiore B, Di Francesco L, Sobrero F, Bernard D, Mangiacasale R, Harel A, Schininà ME, Lavia P. Nuclear reformation after mitosis requires downregulation of the Ran GTPase effector RanBP1 in mammalian cells. Chromosoma 2010; 119:651-68. [PMID: 20658144 DOI: 10.1007/s00412-010-0286-5] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/14/2010] [Revised: 06/28/2010] [Accepted: 06/30/2010] [Indexed: 11/26/2022]
Abstract
The GTPase Ran regulates nucleocytoplasmic transport in interphase and spindle organisation in mitosis via effectors of the importin beta superfamily. Ran-binding protein 1 (RanBP1) regulates guanine nucleotide turnover on Ran, as well as its interactions with effectors. Unlike other Ran network members that are steadily expressed, RanBP1 abundance is modulated during the mammalian cell cycle, peaking in mitosis and declining at mitotic exit. Here, we show that RanBP1 downregulation takes place in mid to late telophase, concomitant with the reformation of nuclei. Mild RanBP1 overexpression in murine cells causes RanBP1 to persist in late mitosis and hinders a set of events underlying the telophase to interphase transition, including chromatin decondensation, nuclear expansion and nuclear lamina reorganisation. Moreover, the reorganisation of nuclear pores fails associated with defective nuclear relocalisation of NLS cargoes. Co-expression of importin beta, together with RanBP1, however mitigates these defects. Thus, RanBP1 downregulation is required for nuclear reorganisation pathways operated by importin beta after mitosis.
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Affiliation(s)
- Marilena Ciciarello
- CNR National Research Council, Institute of Molecular Biology and Pathology, c/o Sapienza University of Rome, Rome, 00185, Italy
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74
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Hilliard M, Frohnert C, Spillner C, Marcone S, Nath A, Lampe T, Fitzgerald DJ, Kehlenbach RH. The anti-inflammatory prostaglandin 15-deoxy-delta(12,14)-PGJ2 inhibits CRM1-dependent nuclear protein export. J Biol Chem 2010; 285:22202-10. [PMID: 20457605 PMCID: PMC2903415 DOI: 10.1074/jbc.m110.131821] [Citation(s) in RCA: 42] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/09/2010] [Revised: 05/06/2010] [Indexed: 12/30/2022] Open
Abstract
The signaling molecule 15-deoxy-Delta(12,14)-prostaglandin J(2) (15d-PGJ(2)) has been described as the "anti-inflammatory prostaglandin." Here we show that substrates of the nuclear export receptor CRM1 accumulate in the nucleus in the presence of 15d-PGJ(2), identifying this prostaglandin as a regulator of CRM1-dependent nuclear protein export that can be produced endogenously. Like leptomycin B (LMB), an established fungal CRM1-inhibitor, 15d-PGJ(2) reacts with a conserved cysteine residue in the CRM1 sequence. This covalent modification prevents the formation of nuclear export complexes. Cells that are transfected with mutant CRM1 (C528S) are resistant to the inhibitory effects of LMB and 15d-PGJ(2), demonstrating that the same single amino acid is targeted by the two compounds. Inhibition of the CRM1 pathway by endogenously produced prostaglandin and/or exogenously applied 15d-PGJ(2) may contribute to its anti-inflammatory, anti-proliferative, and anti-viral effects.
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Affiliation(s)
- Mark Hilliard
- From the UCD Conway Institute, Belfield, Dublin 4, Ireland and
| | - Cornelia Frohnert
- the Department of Biochemistry I, Faculty of Medicine, Georg-August-University of Göttingen, Humboldtallee 23, 37073 Göttingen, Germany
| | - Christiane Spillner
- the Department of Biochemistry I, Faculty of Medicine, Georg-August-University of Göttingen, Humboldtallee 23, 37073 Göttingen, Germany
| | - Simone Marcone
- From the UCD Conway Institute, Belfield, Dublin 4, Ireland and
| | - Annegret Nath
- the Department of Biochemistry I, Faculty of Medicine, Georg-August-University of Göttingen, Humboldtallee 23, 37073 Göttingen, Germany
| | - Tina Lampe
- the Department of Biochemistry I, Faculty of Medicine, Georg-August-University of Göttingen, Humboldtallee 23, 37073 Göttingen, Germany
| | | | - Ralph H. Kehlenbach
- the Department of Biochemistry I, Faculty of Medicine, Georg-August-University of Göttingen, Humboldtallee 23, 37073 Göttingen, Germany
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75
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Kaláb P, Soderholm J. The design of Förster (fluorescence) resonance energy transfer (FRET)-based molecular sensors for Ran GTPase. Methods 2010; 51:220-32. [PMID: 20096786 DOI: 10.1016/j.ymeth.2010.01.022] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/19/2010] [Accepted: 01/19/2010] [Indexed: 01/01/2023] Open
Abstract
The application of FRET-based molecular biosensors provided confirmation of the central model of Ran GTPase function and led to important new insights into its physiological role. In many fields of cell biology, methods employing FRET are a standard approach that is becoming increasingly accessible due to advances in instrumentation and available fluorophores. However, the optimal design of a FRET sensor remains to be the cornerstone of any successful FRET application. Utilizing the recent literature on FRET applications and our studies on Ran, we outline the basic considerations involved in designing molecular FRET sensors. We point to several broadly applicable principles that were used in many different FRET sensors that can detect a wide range of molecular events. Using the FRET sensors for Ran that we created as examples, we then focus on the practical aspects of FRET assays. We describe the preparation of a bipartite FRET sensor consisting of ECFP-Ran and EYFP-importin beta and its validation as a reporter for FRET-based high throughput screening in small molecule libraries. Finally, we review the design and optimization of monomolecular FRET sensors that monitor the RanGTP-RanBP1 interaction, and of sensors detecting the RanGTP-regulated importin beta cargo release.
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Affiliation(s)
- Petr Kaláb
- National Cancer Institute, NIH, Bethesda, MD 20892-4256, USA.
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76
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Cardarelli F, Bizzarri R, Serresi M, Albertazzi L, Beltram F. Probing nuclear localization signal-importin alpha binding equilibria in living cells. J Biol Chem 2009; 284:36638-36646. [PMID: 19858191 DOI: 10.1074/jbc.m109.036699] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The regulated process of protein import into the nucleus of a eukaryotic cell is mediated by specific nuclear localization signals (NLSs) that are recognized by protein-import receptors. In this study, we present fluorescence-based methods to quantitatively address the physicochemical details of NLS recognition by the receptor protein importin alpha (Impalpha) in living cells. First, by combining fluorescence recovery after photobleaching measurements and protein-concentration calibration, we quantitatively define nuclear import saturability and afford an affinity value for NLS-Impalpha binding. Second, by fluorescence lifetime imaging microscopy, we directly monitor the occurrence of NLS-Impalpha interaction and measure its effective dissociation constant (K(D)) in the actual cellular environment. Our kinetic and thermodynamic analyses independently indicate that the subsaturation of Impalpha with the expressed NLS cargo regulates nuclear import rates in living cells, in contrast to what can be predicted on the basis of available in vitro data. Finally, our experiments also provide evidence for the regulation of nuclear import mediated by the intrasteric importin beta-binding domain of Impalpha and yield the first estimate of its autoinhibition energy in living cells.
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Affiliation(s)
- Francesco Cardarelli
- National Enterprise for nanoScience and nanoTechnology, Scuola Normale Superiore, and Consiglio Nazionale delle Ricerche-Istituto Nazionale per la Fisica della Materia, Piazza S. Silvestro 12, 56126 Pisa, Italy.
| | - Ranieri Bizzarri
- National Enterprise for nanoScience and nanoTechnology, Scuola Normale Superiore, and Consiglio Nazionale delle Ricerche-Istituto Nazionale per la Fisica della Materia, Piazza S. Silvestro 12, 56126 Pisa, Italy; IIT@NEST, Center for Nanotechnology Innovation, Piazza S. Silvestro 12, 56126 Pisa, Italy
| | - Michela Serresi
- National Enterprise for nanoScience and nanoTechnology, Scuola Normale Superiore, and Consiglio Nazionale delle Ricerche-Istituto Nazionale per la Fisica della Materia, Piazza S. Silvestro 12, 56126 Pisa, Italy; IIT@NEST, Center for Nanotechnology Innovation, Piazza S. Silvestro 12, 56126 Pisa, Italy
| | - Lorenzo Albertazzi
- National Enterprise for nanoScience and nanoTechnology, Scuola Normale Superiore, and Consiglio Nazionale delle Ricerche-Istituto Nazionale per la Fisica della Materia, Piazza S. Silvestro 12, 56126 Pisa, Italy; IIT@NEST, Center for Nanotechnology Innovation, Piazza S. Silvestro 12, 56126 Pisa, Italy
| | - Fabio Beltram
- National Enterprise for nanoScience and nanoTechnology, Scuola Normale Superiore, and Consiglio Nazionale delle Ricerche-Istituto Nazionale per la Fisica della Materia, Piazza S. Silvestro 12, 56126 Pisa, Italy; IIT@NEST, Center for Nanotechnology Innovation, Piazza S. Silvestro 12, 56126 Pisa, Italy
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77
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Frankel MB, Knoll LJ. The ins and outs of nuclear trafficking: unusual aspects in apicomplexan parasites. DNA Cell Biol 2009; 28:277-84. [PMID: 19348590 DOI: 10.1089/dna.2009.0853] [Citation(s) in RCA: 30] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
Apicomplexa is a phylum within the kingdom Protista that contains some of the most significant threats to public health. One of the members of this phylum, Toxoplasma gondii, is amenable to molecular genetic analyses allowing for the identification of factors critical for colonization and disease. A pathway found to be important for T. gondii pathogenesis is the Ran network of nuclear trafficking. Bioinformatics analysis of apicomplexan genomes shows that while Ran is well conserved, the key regulators of Ran--Regulator of Chromosome Condensation 1 and Ran GTPase activating protein--are either highly divergent or absent. Likewise, several import and export receptor molecules that are crucial for nuclear transport are either not present or have experienced genetic drift such that they are no longer recognizable by bioinformatics tools. In this minireview we describe the basics of nuclear trafficking and compare components within apicomplexans to defined systems in humans and yeast. A detailed analysis of the nuclear trafficking network in these eukaryotes is required to understand how this potentially unique cellular biological pathway contributes to host-parasite interactions.
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Affiliation(s)
- Matthew B Frankel
- Department of Medical Microbiology and Immunology, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA
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78
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Gontan C, Güttler T, Engelen E, Demmers J, Fornerod M, Grosveld FG, Tibboel D, Görlich D, Poot RA, Rottier RJ. Exportin 4 mediates a novel nuclear import pathway for Sox family transcription factors. Dev Biol 2009. [PMID: 19349578 DOI: 10.1016/j.ydbio.2008.05.306] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022]
Abstract
SRY and other Sox-type transcription factors are important developmental regulators with various implications in human disease. In this study, we identified Exp4 (exportin 4) as an interaction partner of Sox2 in mouse embryonic stem cells and neural progenitors. We show that, besides its established function in nuclear export, Exp4 acts as a bona fide nuclear import receptor for Sox2 and SRY. Thus, Exp4 is an example of a nuclear transport receptor carrying distinct cargoes into different directions. In contrast to a published study, we observed that the import activity of Imp-alpha (importin-a) isoforms toward Sox2 is negligible. Instead, we found that Imp9 and the Imp-beta/7 heterodimer mediate nuclear import of Sox2 in parallel to Exp4. Import signals for the three pathways overlap and include conserved residues in the Sox2 high-mobility group (HMG) box domain that are also critical for DNA binding. This suggests that nuclear import of Sox proteins is facilitated by several parallel import pathways.
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Affiliation(s)
- Cristina Gontan
- Department of Pediatric Surgery, Erasmus Medical Center, Rotterdam, Netherlands
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79
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Gontan C, Güttler T, Engelen E, Demmers J, Fornerod M, Grosveld FG, Tibboel D, Görlich D, Poot RA, Rottier RJ. Exportin 4 mediates a novel nuclear import pathway for Sox family transcription factors. ACTA ACUST UNITED AC 2009; 185:27-34. [PMID: 19349578 PMCID: PMC2700522 DOI: 10.1083/jcb.200810106] [Citation(s) in RCA: 65] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023]
Abstract
SRY and other Sox-type transcription factors are important developmental regulators with various implications in human disease. In this study, we identified Exp4 (exportin 4) as an interaction partner of Sox2 in mouse embryonic stem cells and neural progenitors. We show that, besides its established function in nuclear export, Exp4 acts as a bona fide nuclear import receptor for Sox2 and SRY. Thus, Exp4 is an example of a nuclear transport receptor carrying distinct cargoes into different directions. In contrast to a published study, we observed that the import activity of Imp-α (importin-a) isoforms toward Sox2 is negligible. Instead, we found that Imp9 and the Imp-β/7 heterodimer mediate nuclear import of Sox2 in parallel to Exp4. Import signals for the three pathways overlap and include conserved residues in the Sox2 high-mobility group (HMG) box domain that are also critical for DNA binding. This suggests that nuclear import of Sox proteins is facilitated by several parallel import pathways.
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Affiliation(s)
- Cristina Gontan
- Department of Pediatric Surgery, Erasmus Medical Center, Rotterdam, Netherlands
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80
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Joining the dots: Production, processing and targeting of U snRNP to nuclear bodies. BIOCHIMICA ET BIOPHYSICA ACTA-MOLECULAR CELL RESEARCH 2008; 1783:2137-44. [DOI: 10.1016/j.bbamcr.2008.07.025] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/08/2008] [Revised: 07/22/2008] [Accepted: 07/23/2008] [Indexed: 11/20/2022]
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81
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Zachariae U, Grubmüller H. Importin-beta: structural and dynamic determinants of a molecular spring. Structure 2008; 16:906-15. [PMID: 18547523 DOI: 10.1016/j.str.2008.03.007] [Citation(s) in RCA: 47] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/19/2007] [Revised: 03/06/2008] [Accepted: 03/06/2008] [Indexed: 11/18/2022]
Abstract
The beta-karyopherin/RanGTP system constitutes the largest known family of cellular cargo transporters. The flexibility of the karyopherin transport receptors is the key to their versatility in binding cargoes of different shape and size. Despite strong binding of the Ran complex, the comparably low energy associated with GTP hydrolysis suffices to drive dissociation and fuel the transport cycle. Here, we elucidate the drastic structural dynamics of the prototypic karyopherin, importin-beta, and show that its flexibility also solves this energetic puzzle. Our nonequilibrium atomistic simulations reveal fast conformational changes, validated by small-angle X-ray scattering data, and unusually large structural fluctuations. The characteristic dynamic patterns of importin-beta and the observed unfolding pathway of the IBB domain suggest a cooperative mechanism of importin-beta function in the nucleus. We propose a molecular model in which the stored energy and structural dynamics account for an exchange pathway that explains the high observed rates of nucleocytoplasmic transport. Karyopherins utilize a mechanism of entropy/enthalpy control that might be a general feature of highly flexible proteins involved in protein-protein interactions.
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Affiliation(s)
- Ulrich Zachariae
- Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, 37077 Göttingen, Germany
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82
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Kap95p binding induces the switch loops of RanGDP to adopt the GTP-bound conformation: implications for nuclear import complex assembly dynamics. J Mol Biol 2008; 383:772-82. [PMID: 18708071 DOI: 10.1016/j.jmb.2008.07.090] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/25/2008] [Revised: 07/31/2008] [Accepted: 07/31/2008] [Indexed: 11/20/2022]
Abstract
The asymmetric distribution of the nucleotide-bound state of Ran across the nuclear envelope is crucial for determining the directionality of nuclear transport. In the nucleus, Ran is primarily in the guanosine 5'-triphosphate (GTP)-bound state, whereas in the cytoplasm, Ran is primarily guanosine 5'-diphosphate (GDP)-bound. Conformational changes within the Ran switch I and switch II loops are thought to modulate its affinity for importin-beta. Here, we show that RanGDP and importin-beta form a stable complex with a micromolar dissociation constant. This complex can be dissociated by importin-beta binding partners such as importin-alpha. Surprisingly, the crystal structure of the Kap95p-RanGDP complex shows that Kap95p induces the switch I and II regions of RanGDP to adopt a conformation that resembles that of the GTP-bound form. The structure of the complex provides insights into the structural basis for the gradation of affinities regulating nuclear protein transport.
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83
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Single-molecule measurements of importin alpha/cargo complex dissociation at the nuclear pore. Proc Natl Acad Sci U S A 2008; 105:8613-8. [PMID: 18562297 DOI: 10.1073/pnas.0710867105] [Citation(s) in RCA: 49] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/25/2022] Open
Abstract
Macromolecules are transported between the cytoplasm and the nucleoplasm of eukaryotic cells through nuclear pore complexes (NPCs). Large (more than approximately 40 kDa) transport cargoes imported into the nucleus typically form a complex with at least one soluble transport cofactor of the importin (Imp) beta superfamily. Many cargoes require an accessory cofactor, Imp alpha, which binds to Imp beta and to the nuclear localization sequence on the cargo. We previously reported the use of narrow-field epifluorescence microscopy to directly monitor cargoes in transit through NPCs in permeabilized cells. We now report an expanded approach in which single-molecule fluorescence resonance energy transfer (FRET) is used to detect the disassembly of Imp alpha/cargo complexes as they transit through NPCs. We found that CAS, the recycling cofactor for Imp alpha, and RanGTP are essential for this dissociation process. After Imp alpha/cargo complex dissociation, most Imp alpha and cargo molecules entered the nucleoplasm. In contrast, the majority of Imp alpha/cargo complexes that did not dissociate at the NPC in the presence of CAS and RanGTP returned to the cytoplasm. These data are consistent with a model in which Imp alpha/cargo complexes are dissociated on the nucleoplasmic side of the NPC, and this dissociation requires both CAS and RanGTP.
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84
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Engelsma D, Valle N, Fish A, Salomé N, Almendral JM, Fornerod M. A supraphysiological nuclear export signal is required for parvovirus nuclear export. Mol Biol Cell 2008; 19:2544-52. [PMID: 18385513 PMCID: PMC2397317 DOI: 10.1091/mbc.e08-01-0009] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/08/2008] [Revised: 03/04/2008] [Accepted: 03/24/2008] [Indexed: 01/07/2023] Open
Abstract
CRM1 exports proteins that carry a short leucine-rich peptide signal, the nuclear export signal (NES), from the nucleus. Regular NESs must have low affinity for CRM1 to function optimally. We previously generated artificial NESs with higher affinities for CRM1, termed supraphysiological NESs. Here we identify a supraphysiological NES in an endogenous protein, the NS2 protein of parvovirus Minute Virus of Mice (MVM). NS2 interacts with CRM1 without the requirement of RanGTP, whereas addition of RanGTP renders the complex highly stable. Mutation of a single hydrophobic residue that inactivates regular NESs lowers the affinity of the NS2 NES for CRM1 from supraphysiological to regular. Mutant MVM harboring this regular NES is compromised in viral nuclear export and productivity. In virus-infected mouse fibroblasts we observe colocalization of NS2, CRM1 and mature virions, which is dependent on the supraphysiological NS2 NES. We conclude that supraphysiological NESs exist in nature and that the supraphysiological NS2 NES has a critical role in active nuclear export of mature MVM particles before cell lysis.
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Affiliation(s)
| | - Noelia Valle
- Centro de Biología Molecular “Severo Ochoa” (Consejo Superior de Investigaciones Científicas-Universidad Autónoma de Madrid), 28049 Cantoblanco, Madrid, Spain; and
| | - Alexander Fish
- Molecular Carcinogenesis, The Netherlands Cancer Institute, 1066 CX Amsterdam, The Netherlands
| | - Nathalie Salomé
- Infection and Cancer Program, Division F010 and INSERM U701, Deutsches Krebsforschungszentrum, D-69120 Heidelberg, Germany
| | - José M. Almendral
- Centro de Biología Molecular “Severo Ochoa” (Consejo Superior de Investigaciones Científicas-Universidad Autónoma de Madrid), 28049 Cantoblanco, Madrid, Spain; and
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85
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Abstract
The small nuclear GTPase Ran controls the directionality of macromolecular transport between the nucleus and the cytoplasm. Ran also has important roles during mitosis, when the nucleus is dramatically reorganized to allow chromosome segregation. Ran directs the assembly of the mitotic spindle, nuclear-envelope dynamics and the timing of cell-cycle transitions. The mechanisms that underlie these functions provide insights into the spatial and temporal coordination of the changes that occur in intracellular organization during the cell-division cycle.
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Affiliation(s)
- Paul R Clarke
- Biomedical Research Centre, Ninewells Hospital and Medical School, University of Dundee, Dundee, DD1 9SY, Scotland, UK.
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86
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Abstract
The GTPase Ran has a key role in nuclear import and export, mitotic spindle assembly and nuclear envelope formation. The cycling of Ran between its GTP- and GDP-bound forms is catalyzed by the chromatin-bound guanine nucleotide exchange factor RCC1 and the cytoplasmic Ran GTPase-activating protein RanGAP. The result is an intracellular concentration gradient of RanGTP that equips eukaryotic cells with a ;genome-positioning system' (GPS). The binding of RanGTP to nuclear transport receptors (NTRs) of the importin beta superfamily mediates the effects of the gradient and generates further downstream gradients, which have been elucidated by fluorescence resonance energy transfer (FRET) imaging and computational modeling. The Ran-dependent GPS spatially directs many functions required for genome segregation by the mitotic spindle during mitosis. Through exportin 1, RanGTP recruits essential centrosome and kinetochore components, whereas the RanGTP-induced release of spindle assembly factors (SAFs) from importins activates SAFs to nucleate, bind and organize nascent spindle microtubules. Although a considerable fraction of cytoplasmic SAFs is active and RanGTP induces only partial further activation near chromatin, bipolar spindle assembly is robustly induced by cooperativity and positive-feedback mechanisms within the network of Ran-activated SAFs. The RanGTP gradient is conserved, although its roles vary among different cell types and species, and much remains to be learned regarding its functions.
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Affiliation(s)
- Petr Kalab
- Laboratory of Cell and Molecular Biology, National Cancer Institute, Bethesda, MD 20892-4256, USA.
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87
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Schulte R, Talamas J, Doucet C, Hetzer MW. Single bead affinity detection (SINBAD) for the analysis of protein-protein interactions. PLoS One 2008; 3:e2061. [PMID: 18446240 PMCID: PMC2329591 DOI: 10.1371/journal.pone.0002061] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/05/2008] [Accepted: 03/12/2008] [Indexed: 11/18/2022] Open
Abstract
We present a miniaturized pull-down method for the detection of protein-protein interactions using standard affinity chromatography reagents. Binding events between different proteins, which are color-coded with quantum dots (QDs), are visualized on single affinity chromatography beads by fluorescence microscopy. The use of QDs for single molecule detection allows the simultaneous analysis of multiple protein-protein binding events and reduces the amount of time and material needed to perform a pull-down experiment.
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Affiliation(s)
- Roberta Schulte
- Molecular and Cell Biology Laboratory, Salk Institute for Biological Studies, La Jolla, California, United States of America
| | - Jessica Talamas
- Molecular and Cell Biology Laboratory, Salk Institute for Biological Studies, La Jolla, California, United States of America
| | - Christine Doucet
- Molecular and Cell Biology Laboratory, Salk Institute for Biological Studies, La Jolla, California, United States of America
| | - Martin W. Hetzer
- Molecular and Cell Biology Laboratory, Salk Institute for Biological Studies, La Jolla, California, United States of America
- * E-mail:
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88
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Hutten S, Flotho A, Melchior F, Kehlenbach RH. The Nup358-RanGAP complex is required for efficient importin alpha/beta-dependent nuclear import. Mol Biol Cell 2008; 19:2300-10. [PMID: 18305100 DOI: 10.1091/mbc.e07-12-1279] [Citation(s) in RCA: 110] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022] Open
Abstract
In vertebrate cells, the nucleoporin Nup358/RanBP2 is a major component of the filaments that emanate from the nuclear pore complex into the cytoplasm. Nup358 forms a complex with SUMOylated RanGAP1, the GTPase activating protein for Ran. RanGAP1 plays a pivotal role in the establishment of a RanGTP gradient across the nuclear envelope and, hence, in the majority of nucleocytoplasmic transport pathways. Here, we investigate the roles of the Nup358-RanGAP1 complex and of soluble RanGAP1 in nuclear protein transport, combining in vivo and in vitro approaches. Depletion of Nup358 by RNA interference led to a clear reduction of importin alpha/beta-dependent nuclear import of various reporter proteins. In vitro, transport could be partially restored by the addition of importin beta, RanBP1, and/or RanGAP1 to the transport reaction. In intact Nup358-depleted cells, overexpression of importin beta strongly stimulated nuclear import, demonstrating that the transport receptor is the most rate-limiting factor at reduced Nup358-concentrations. As an alternative approach, we used antibody-inhibition experiments. Antibodies against RanGAP1 inhibited the enzymatic activity of soluble and nuclear pore-associated RanGAP1, as well as nuclear import and export. Although export could be fully restored by soluble RanGAP, import was only partially rescued. Together, these data suggest a dual function of the Nup358-RanGAP1 complex as a coordinator of importin beta recycling and reformation of novel import complexes.
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Affiliation(s)
- Saskia Hutten
- Department of Biochemistry I, Faculty of Medicine, Georg-August University of Göttingen, 37073, Göttingen, Germany
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89
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Engelsma D, Rodriguez JA, Fish A, Giaccone G, Fornerod M. Homodimerization Antagonizes Nuclear Export of Survivin. Traffic 2007; 8:1495-502. [PMID: 17714426 DOI: 10.1111/j.1600-0854.2007.00629.x] [Citation(s) in RCA: 38] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
Abstract
Survivin plays separate roles during different phases of the cell cycle. In mitosis, Survivin is a key regulator of cell division, while in interphase, Survivin is able to protect cells from apoptosis. Survivin shuttles between nucleus and cytoplasm under the influence of one or more nuclear export signals (NESs). Paradoxically, our data show that Survivin poorly binds CRM1 in vitro because hydrophobic residues of the NES are occupied in homodimer contacts. We show that NES-preserving dimerization mutants behave as monomers in solution, show dramatically increased CRM1 binding and are more efficiently exported in vivo than wild-type Survivin. These data indicate that Survivin contains a monomer-specific NES and that dimerization modulates cytoplasmic access of the protein. Our findings have implications for both the mitotic and interphase roles of survivin.
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Affiliation(s)
- Dieuwke Engelsma
- Department of Tumor Biology, The Netherlands Cancer Institute, Plesmanlaan 121, 1066 CX Amsterdam, The Netherlands
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90
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Riddick G, Macara IG. The adapter importin-alpha provides flexible control of nuclear import at the expense of efficiency. Mol Syst Biol 2007; 3:118. [PMID: 17551513 PMCID: PMC1911202 DOI: 10.1038/msb4100160] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/31/2006] [Accepted: 04/25/2007] [Indexed: 11/08/2022] Open
Abstract
Although there exists a large family of nuclear transport receptors (Karyopherins), the majority of known import cargoes use an adapter protein, Importin-alpha (Impalpha), which links the cargo to a karyopherin, Importin-beta (Impbeta). The reason for the existence of transport adapters is unknown. One hypothesis is that, as Impalpha re-export is coupled to GTP hydrolysis, it can drive a higher concentration of nuclear cargo than could be achieved by direct cargo binding to Importin-beta. However, computer simulations predicted the opposite outcome, and showed that direct transport is faster than adapter-mediated transport. These predictions were validated experimentally. The data, together with previous analyses of nuclear protein import, suggest that the use of adapters such as importin-alpha provides the cell with increased dynamic range for control of nuclear import rates, but at the expense of efficiency.
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Affiliation(s)
- Greg Riddick
- Departments of Biochemistry and Microbiology, University of Virginia, Charlottesville, VA, USA
| | - Ian G Macara
- Departments of Biochemistry and Microbiology, University of Virginia, Charlottesville, VA, USA
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91
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Sarić M, Zhao X, Körner C, Nowak C, Kuhlmann J, Vetter IR. Structural and biochemical characterization of the Importin-beta.Ran.GTP.RanBD1 complex. FEBS Lett 2007; 581:1369-76. [PMID: 17359978 DOI: 10.1016/j.febslet.2007.02.067] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/12/2007] [Revised: 02/20/2007] [Accepted: 02/27/2007] [Indexed: 10/23/2022]
Abstract
Here we present the crystal structure of Importin-beta(1-462).Ran.GTP.RanBD1DeltaN as solved by molecular replacement. HPLC dissociation measurements on this complex show, that the N-terminus of RanBD may be involved in the release of the hydrolysis- and dissociation-block of Ran by Transportin/Importin-beta. We could identify a pair of amino acids which - upon mutation - weaken the interaction between Ran and Importin-beta specifically to allow dissociation without RanBD. These findings support the hypothesis that a ternary complex of Importin-beta.Ran.GTP.RanBD exists in the final step of the export of Importin-beta from the nucleus and that interaction of the N-terminus of RanBD with Ran plays a crucial role in disassembly of this complex.
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Affiliation(s)
- Marc Sarić
- Max-Planck-Institute for Molecular Physiology, Department Structural Biology, Otto-Hahn-Str. 11, D-44227 Dortmund, Germany
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92
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Abstract
The nuclear import of proteins through nuclear pore complexes (NPCs) illustrates how a complex biological function can be generated by a spatially and temporally organized cycle of interactions between cargoes, carriers and the Ran GTPase. Recent work has given considerable insight into this process, especially about how interactions are coordinated and the basis for the molecular recognition that underlies the process. Although considerable progress has been made in identifying and characterizing the molecular interactions in the soluble phase that drive the nuclear protein import cycle, understanding the precise mechanism of translocation through NPCs remains a major challenge.
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Affiliation(s)
- Murray Stewart
- MRC Laboratory of Molecular Biology, Hills Road, Cambridge CB2 2QH, UK.
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93
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Abstract
In eukaryotic cells, segregation of DNA replication and RNA biogenesis in the nucleus and protein synthesis in the cytoplasm poses the requirement of transporting thousands of macromolecules between the two cellular compartments. Transport between nucleus and cytoplasm is mediated by soluble receptors that recognize specific cargoes and carry them through the nuclear pore complex (NPC), the sole gateway between the two compartments at interphase. Nucleocytoplasmic transport is specific not only in terms of cargo recognition, but also in terms of directionality, with nuclear proteins imported into the nucleus and RNAs exported from it. How is directionality achieved? How can the receptors be both specific and versatile in recognizing a multitude of cargoes? And how can their interaction with NPCs allow fast translocation? We describe the molecular mechanisms underlying nucleocytoplasmic transport as they have been revealed by structural studies of the receptors and regulators in different steps of transport cycles.
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Affiliation(s)
- Atlanta Cook
- European Molecular Biology Laboratory, D-69117 Heidelberg, Germany.
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94
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Paradise A, Levin MK, Korza G, Carson JH. Significant proportions of nuclear transport proteins with reduced intracellular mobilities resolved by fluorescence correlation spectroscopy. J Mol Biol 2006; 365:50-65. [PMID: 17056062 PMCID: PMC1831836 DOI: 10.1016/j.jmb.2006.09.089] [Citation(s) in RCA: 42] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/03/2006] [Revised: 09/26/2006] [Accepted: 09/27/2006] [Indexed: 11/20/2022]
Abstract
Nuclear transport requires freely diffusing nuclear transport proteins to facilitate movement of cargo molecules through the nuclear pore. We analyzed dynamic properties of importin alpha, importin beta, Ran and NTF2 in nucleus, cytoplasm and at the nuclear pore of neuroblastoma cells using fluorescence correlation spectroscopy. Mobile components were quantified by global fitting of autocorrelation data from multiple cells. Immobile components were quantified by analysis of photobleaching kinetics. Wild-type Ran was compared to various mutant Ran proteins to identify components representing GTP or GDP forms of Ran. Untreated cells were compared to cells treated with nocodazole or latrunculin to identify components associated with cytoskeletal elements. The results indicate that freely diffusing importin alpha, importin beta, Ran and NTF2 are in dynamic equilibrium with larger pools associated with immobile binding partners such as microtubules in the cytoplasm. These findings suggest that formation of freely diffusing nuclear transport intermediates is in competition with binding to immobile partners. Variation in concentrations of freely diffusing nuclear transport intermediates among cells indicates that the nuclear transport system is sufficiently robust to function over a wide range of conditions.
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Affiliation(s)
- Allison Paradise
- Department of Molecular Microbial and Structural Biology, University of Connecticut Health Center, 263 Farmington Avenue, Farmington, CT 06030, USA
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95
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Abstract
Although many components and reaction steps necessary for bidirectional transport across the nuclear envelope (NE) have been characterized, the mechanism and control of cargo migration through nuclear pore complexes (NPCs) remain poorly understood. Single-molecule fluorescence microscopy was used to track the movement of cargos before, during, and after their interactions with NPCs. At low importin β concentrations, about half of the signal-dependent cargos that interacted with an NPC were translocated across the NE, indicating a nuclear import efficiency of ∼50%. At high importin β concentrations, the import efficiency increased to ∼80% and the transit speed increased approximately sevenfold. The transit speed and import efficiency of a signal-independent cargo was also increased by high importin β concentrations. These results demonstrate that maximum nucleocytoplasmic transport velocities can be modulated by at least ∼10-fold by the importin β concentration and therefore suggest a potential mechanism for regulating the speed of cargo traffic across the NE.
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Affiliation(s)
- Weidong Yang
- Department of Molecular and Cellular Medicine, The Texas A&M University System Health Science Center, College Station, TX 77843, USA
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96
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Hutten S, Kehlenbach RH. Nup214 is required for CRM1-dependent nuclear protein export in vivo. Mol Cell Biol 2006; 26:6772-85. [PMID: 16943420 PMCID: PMC1592874 DOI: 10.1128/mcb.00342-06] [Citation(s) in RCA: 118] [Impact Index Per Article: 6.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/24/2006] [Revised: 03/24/2006] [Accepted: 06/30/2006] [Indexed: 11/20/2022] Open
Abstract
Nucleoporins mediate transport of macromolecules across the nuclear pore complex, yet the function of many individual nucleoporins is largely unresolved. To address this question, we depleted cells of the cytoplasmic nucleoporins Nup214/CAN and Nup358/RanBP2 by RNA interference. Depletion of Nup214 resulted in codepletion of its binding partner, Nup88. Nuclear pore complexes assembled in the absence of Nup214/Nup88 or Nup358 were fully functional in nuclear protein import, whereas nuclear mRNA export was slightly impaired. Depletion of Nup358 had only a minor effect on nuclear protein export. In contrast, depletion of Nup214/Nup88 led to strongly reduced CRM1-mediated export of the shuttling transcription factor NFAT as well as a human immunodeficiency virus-Rev derivative. A specific role of Nup214 in protein export is furthered by the biochemical properties of a high-affinity complex containing Nup214, CRM1, RanGTP, and an export cargo. Our results show that the Nup214/Nup88 complex is required for efficient CRM1-mediated transport, supporting a model involving a high-affinity binding site for CRM1 at Nup214 in the terminal steps of export.
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Affiliation(s)
- Saskia Hutten
- Universität Göttingen, Zentrum für Biochemie und Molekulare Zellbiologie, Humboldtallee 23, 37073 Göttingen, Germany
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97
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Arnold M, Nath A, Hauber J, Kehlenbach RH. Multiple importins function as nuclear transport receptors for the Rev protein of human immunodeficiency virus type 1. J Biol Chem 2006; 281:20883-20890. [PMID: 16704975 DOI: 10.1074/jbc.m602189200] [Citation(s) in RCA: 71] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The Rev protein of human immunodeficiency virus type 1 is an RNA-binding protein that is required for nuclear export of unspliced and partially spliced viral mRNAs. Nuclear import of human immunodeficiency virus type 1 Rev has been suggested to depend on the classic nuclear transport receptor importin beta, but not on the adapter protein importin alpha. We now show that, similar to importin alpha, Rev is able to dissociate RanGTP from recycling importin beta, a reaction that leads to the formation of a novel import complex. Besides importin beta, the transport receptors transportin, importin 5, and importin 7 specifically interact with Rev and promote its nuclear import in digitonin-permeabilized cells. A single arginine-rich nuclear localization sequence of Rev is required for interaction with all importins tested so far. In contrast to the importin beta-binding domain of importin alpha, Rev interacts with an N-terminal fragment of importin beta. Transportin contains two independent binding sites for Rev. Hence, the mode of interaction of importin beta and transportin with Rev is clearly distinct from that with their classic import cargoes. Taken together, the viral protein takes advantage of multiple cellular transport pathways for its nuclear accumulation.
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Affiliation(s)
- Marc Arnold
- Universität Heidelberg, Abteilung Virologie, Im Neuenheimer Feld 324, 69120 Heidelberg, Germany; Universität Göttingen, Zentrum für Biochemie und Molekulare Zellbiologie, Humboldtallee 23, 37073 Göttingen, Germany
| | - Annegret Nath
- Universität Göttingen, Zentrum für Biochemie und Molekulare Zellbiologie, Humboldtallee 23, 37073 Göttingen, Germany
| | - Joachim Hauber
- Heinrich Pette Institute for Experimental Virology and Immunology, Martinistrasse 52, 20251 Hamburg, Germany
| | - Ralph H Kehlenbach
- Universität Heidelberg, Abteilung Virologie, Im Neuenheimer Feld 324, 69120 Heidelberg, Germany; Universität Göttingen, Zentrum für Biochemie und Molekulare Zellbiologie, Humboldtallee 23, 37073 Göttingen, Germany.
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98
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Takeda E, Hieda M, Katahira J, Yoneda Y. Phosphorylation of RanGAP1 stabilizes its interaction with Ran and RanBP1. Cell Struct Funct 2006; 30:69-80. [PMID: 16428860 DOI: 10.1247/csf.30.69] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022] Open
Abstract
Ran is a nuclear Ras-like GTPase that is required for various nuclear events including the bi-directional transport of proteins and ribonucleoproteins through the nuclear pore complex, spindle formation, and reassembly of the nuclear envelope. One of the key regulators of Ran is RanGAP1, a Ran specific GTPase activating protein. The question of whether a mechanism exists for controlling nucleocytoplasmic transport through the regulation of RanGAP1 activity continues to be debated. Here we show that RanGAP1 is phosphorylated in vivo and in vitro. Serine-358 (358S) was identified as the major phosphorylation site, by MALDI-TOF-MS spectrometry. Site directed mutagenesis at this position abolished the phosphorylation. Experiments using purified recombinant kinase and specific inhibitors such as DRB and apigenin strongly suggest that casein kinase II (CK2) is the responsible kinase. Although the phosphorylation of 358S of RanGAP1 did not significantly alter its GAP activity, the phosphorylated wild type RanGAP1, but not a mutant harboring a mutation at the phosphorylation site 358S, efficiently formed a stable ternary complex with Ran and RanBP1 in vivo, suggesting that the 358S phosphorylation of RanGAP1 affects the Ran system.
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Affiliation(s)
- Eri Takeda
- Department of Cell Biology and Neuroscience, Graduate School of Medicine, Osaka University, 2-2 Yamadaoka, Suita, Osaka 565-0871, Japan
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99
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Silverman-Gavrila RV, Wilde A. Ran is required before metaphase for spindle assembly and chromosome alignment and after metaphase for chromosome segregation and spindle midbody organization. Mol Biol Cell 2006; 17:2069-80. [PMID: 16481399 PMCID: PMC1415283 DOI: 10.1091/mbc.e05-10-0991] [Citation(s) in RCA: 38] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022] Open
Abstract
The Ran pathway has been shown to have a role in spindle assembly. However, the extent of the role of the Ran pathway in mitosis in vivo is unclear. We report that perturbation of the Ran pathway disrupted multiple steps of mitosis in syncytial Drosophila embryos and uncovered new mitotic processes that are regulated by Ran. During the onset of mitosis, the Ran pathway is required for the production, organization, and targeting of centrosomally nucleated microtubules to chromosomes. However, the role of Ran is not restricted to microtubule organization, because Ran is also required for the alignment of chromosomes at the metaphase plate. In addition, the Ran pathway is required for postmetaphase events, including chromosome segregation and the assembly of the microtubule midbody. The Ran pathway mediates these mitotic events, in part, by facilitating the correct targeting of the kinase Aurora A and the kinesins KLP61F and KLP3A to spindles.
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100
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Caudron M, Bunt G, Bastiaens P, Karsenti E. Spatial coordination of spindle assembly by chromosome-mediated signaling gradients. Science 2005; 309:1373-6. [PMID: 16123300 DOI: 10.1126/science.1115964] [Citation(s) in RCA: 243] [Impact Index Per Article: 12.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/02/2022]
Abstract
During cell division, chromosomes are distributed to daughter cells by the mitotic spindle. This system requires spatial cues to reproducibly self-organize. We report that such cues are provided by chromosome-mediated interaction gradients between the small guanosine triphosphatase (GTPase) Ran and importin-beta. This produces activity gradients that determine the spatial distribution of microtubule nucleation and stabilization around chromosomes and that are essential for the self-organization of microtubules into a bipolar spindle.
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Affiliation(s)
- Maïwen Caudron
- Cell Biology and Biophysics Department, European Molecular Biology Laboratory (EMBL), EMBL, 69117 Heidelberg, Germany
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