901
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Braun IC, Rohrbach E, Schmitt C, Izaurralde E. TAP binds to the constitutive transport element (CTE) through a novel RNA-binding motif that is sufficient to promote CTE-dependent RNA export from the nucleus. EMBO J 1999; 18:1953-65. [PMID: 10202158 PMCID: PMC1171280 DOI: 10.1093/emboj/18.7.1953] [Citation(s) in RCA: 156] [Impact Index Per Article: 6.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
The constitutive transport element (CTE) of the simian type D retroviruses overcomes nuclear retention and allows nuclear export of unspliced viral RNAs by recruiting TAP, a host factor which is thought to be required for export of cellular mRNAs. In this report, we show that the first 372 amino acid residues of TAP, comprising a stretch of leucine-rich repeats, are both necessary and sufficient for binding to the CTE RNA and promoting its export to the cytoplasm. Moreover, like the full-length protein, this domain migrates to the cytoplasm upon nuclear co-injection with the CTE RNA. Together, these results indicate that the CTE-binding domain includes the signals for nuclear export. We also describe a derivative of TAP that bears a triple amino acid substitution within the CTE-binding domain and substantially reduces the export of mRNAs from the nucleus. This provides further evidence for a role for TAP in this process. Thus, the CTE-binding domain of TAP defines a novel RNA-binding motif which has dual functions, both recognizing the CTE RNA and interacting with other components of the nuclear transport machinery.
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Affiliation(s)
- I C Braun
- University of Geneva, Department of Molecular Biology, 30 quai Ernest-Ansermet, 1211 Geneva 4, Switzerland
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902
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Nakielny S, Shaikh S, Burke B, Dreyfuss G. Nup153 is an M9-containing mobile nucleoporin with a novel Ran-binding domain. EMBO J 1999; 18:1982-95. [PMID: 10202161 PMCID: PMC1171283 DOI: 10.1093/emboj/18.7.1982] [Citation(s) in RCA: 178] [Impact Index Per Article: 7.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
We employed a phage display system to search for proteins that interact with transportin 1 (TRN1), the import receptor for shuttling hnRNP proteins with an M9 nuclear localization sequence (NLS), and identified a short region within the N-terminus of the nucleoporin Nup153 which binds TRN1. Nup153 is located at the nucleoplasmic face of the nuclear pore complex (NPC), in the distal basket structure, and functions in mRNA export. We show that this Nup153 TRN1-interacting region is an M9 NLS. We found that both import and export receptors interact with several regions of Nup153, in a RanGTP-regulated fashion. RanGTP dissociates Nup153-import receptor complexes, but is required for Nup153-export receptor interactions. We also show that Nup153 is a RanGDP-binding protein, and that the interaction is mediated by the zinc finger region of Nup153. This represents a novel Ran-binding domain, which we term the zinc finger Ran-binding motif. We provide evidence that Nup153 shuttles between the nuclear and cytoplasmic faces of the NPC. The presence of an M9 shuttling domain in Nup153, together with its ability to move within the NPC and to interact with export receptors, suggests that this nucleoporin is a mobile component of the pore which carries export cargos towards the cytoplasm.
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Affiliation(s)
- S Nakielny
- Howard Hughes Medical Institute and Department of Biochemistry and Biophysics, University of Pennsylvania School of Medicine, Philadelphia, PA 19104-6148, USA
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903
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Yokoya F, Imamoto N, Tachibana T, Yoneda Y. beta-catenin can be transported into the nucleus in a Ran-unassisted manner. Mol Biol Cell 1999; 10:1119-31. [PMID: 10198061 PMCID: PMC25239 DOI: 10.1091/mbc.10.4.1119] [Citation(s) in RCA: 194] [Impact Index Per Article: 7.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022] Open
Abstract
The nuclear accumulation of beta-catenin plays an important role in the Wingless/Wnt signaling pathway. This study describes an examination of the nuclear import of beta-catenin in living mammalian cells and in vitro semi-intact cells. When injected into the cell cytoplasm, beta-catenin rapidly migrated into the nucleus in a temperature-dependent and wheat germ agglutinin-sensitive manner. In the cell-free import assay, beta-catenin rapidly migrates into the nucleus without the exogenous addition of cytosol, Ran, or ATP/GTP. Cytoplasmic injection of mutant Ran defective in its GTP hydrolysis did not prevent beta-catenin import. Studies using tsBN2, a temperature-sensitive mutant cell line that possesses a point mutation in the RCC1 gene, showed that the import of beta-catenin is insensitive to nuclear Ran-GTP depletion. These results show that beta-catenin possesses the ability to constitutively translocate through the nuclear pores in a manner similar to importin beta in a Ran-unassisted manner. We further showed that beta-catenin also rapidly exits the nucleus in homokaryons, suggesting that the regulation of nuclear levels of beta-catenin involves both nuclear import and export of this molecule.
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Affiliation(s)
- F Yokoya
- Department of Anatomy and Cell Biology, Osaka University Medical School, Osaka 565-0871, Japan.
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904
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Politz JC, Tuft RA, Pederson T, Singer RH. Movement of nuclear poly(A) RNA throughout the interchromatin space in living cells. Curr Biol 1999; 9:285-91. [PMID: 10209094 DOI: 10.1016/s0960-9822(99)80136-5] [Citation(s) in RCA: 155] [Impact Index Per Article: 6.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
BACKGROUND Messenger RNA (mRNA) is transcribed and processed in the nucleus of eucaryotic cells and then exported to the cytoplasm through nuclear pores. It is not known whether the movement of mRNA from its site of synthesis to the nuclear pore is directed or random. Directed movement would suggest that there is an energy-requiring step in addition to the step required for active transport through the pore, whereas random movement would indicate that mRNAs can make their way to the nuclear envelope by diffusion. RESULTS We devised a method to visualize movement of endogenous polymerase II transcripts in the nuclei of living cells. Oligo(dT) labeled with chemically masked (caged) fluorescein was allowed to penetrate cells and hybridize to nuclear poly(A) RNA. Laser spot photolysis then uncaged the oligo(dT) at a given intranuclear site and the resultant fluorescent, hybridized oligo(dT) was tracked using high-speed imaging microscopy. Poly(A) RNA moved away from the uncaging spot in all directions with a mean square displacement that varied linearly with time, and the same apparent diffusion coefficient was measured for the movement at both 37 degrees C and 23 degrees C. These properties are characteristic of a random diffusive process. High resolution three-dimensional imaging of live cells containing both Hoechst-labeled chromosomes and uncaged oligo(dT) showed that, excluding nucleoli, the poly(A) RNA could access most, if not all, of the non-chromosomal space in the nucleus. CONCLUSIONS Poly(A) RNA can move freely throughout the interchromatin space of the nucleus with properties characteristic of diffusion.
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Affiliation(s)
- J C Politz
- Department of Biochemistry and Molecular Biology, Biomedical Imaging Center, University of Massachusetts Medical School, Worcester, Massachusetts 01605, USA.
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905
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Fontoura BM, Blobel G, Matunis MJ. A conserved biogenesis pathway for nucleoporins: proteolytic processing of a 186-kilodalton precursor generates Nup98 and the novel nucleoporin, Nup96. J Cell Biol 1999; 144:1097-112. [PMID: 10087256 PMCID: PMC2150585 DOI: 10.1083/jcb.144.6.1097] [Citation(s) in RCA: 195] [Impact Index Per Article: 7.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/21/2023] Open
Abstract
The mammalian nuclear pore complex (NPC) is comprised of approximately 50 unique proteins, collectively known as nucleoporins. Through fractionation of rat liver nuclei, we have isolated >30 potentially novel nucleoporins and have begun a systematic characterization of these proteins. Here, we present the characterization of Nup96, a novel nucleoporin with a predicted molecular mass of 96 kD. Nup96 is generated through an unusual biogenesis pathway that involves synthesis of a 186-kD precursor protein. Proteolytic cleavage of the precursor yields two nucleoporins: Nup98, a previously characterized GLFG-repeat containing nucleoporin, and Nup96. Mutational and functional analyses demonstrate that both the Nup98-Nup96 precursor and the previously characterized Nup98 (synthesized independently from an alternatively spliced mRNA) are proteolytically cleaved in vivo. This biogenesis pathway for Nup98 and Nup96 is evolutionarily conserved, as the putative Saccharomyces cerevisiae homologues, N-Nup145p and C-Nup145p, are also produced through proteolytic cleavage of a precursor protein. Using immunoelectron microscopy, Nup96 was localized to the nucleoplasmic side of the NPC, at or near the nucleoplasmic basket. The correct targeting of both Nup96 and Nup98 to the nucleoplasmic side of the NPC was found to be dependent on proteolytic cleavage, suggesting that the cleavage process may regulate NPC assembly. Finally, by biochemical fractionation, a complex containing Nup96, Nup107, and at least two Sec13- related proteins was identified, revealing that a major sub-complex of the NPC is conserved between yeast and mammals.
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Affiliation(s)
- B M Fontoura
- Laboratory of Cell Biology, Howard Hughes Medical Institute, The Rockefeller University, New York 10021, USA
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906
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Kose S, Imamoto N, Tachibana T, Yoshida M, Yoneda Y. beta-subunit of nuclear pore-targeting complex (importin-beta) can be exported from the nucleus in a Ran-independent manner. J Biol Chem 1999; 274:3946-52. [PMID: 9933584 DOI: 10.1074/jbc.274.7.3946] [Citation(s) in RCA: 50] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The nuclear export of importin-alpha is mediated by CAS, which is related to importin-beta, whereas the mechanism for the export of importin-beta remains unclear. In this study, we demonstrate that the nuclear export of importin-beta is mediated by the nuclear pore complex-binding domain of this molecule. Insensitivity to leptomycin B indicates that its export is not mediated by a leucine-rich nuclear export signal-specific receptor, CRM1. Furthermore, the nuclear export of importin-beta was not inhibited by co-injection with a GTPase-deficient Ran mutant (G19V). The cell line tsBN2 contains a temperature-sensitive point mutation in the RCC1 gene, which encodes a guanine nucleotide exchange factor of Ran. At the nonpermissive temperature, importin-beta was exported from the nucleus of these cells, even when RanGAP1, a GTPase-activating protein for Ran, was co-injected. These results not only provide support for the view that Ran-dependent GTP hydrolysis is not required for the nuclear export of importin-beta but also indicate that nuclear RanGTP is not essential for its export. As a result, we propose that importin-beta can be recycled from the nucleus alone in a Ran-independent manner.
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Affiliation(s)
- S Kose
- Department of Anatomy and Cell Biology, Osaka University Medical School, 2-2 Yamada-oka, Suita, Osaka 565-0871, Japan
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907
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Truant R, Cullen BR. The arginine-rich domains present in human immunodeficiency virus type 1 Tat and Rev function as direct importin beta-dependent nuclear localization signals. Mol Cell Biol 1999; 19:1210-7. [PMID: 9891055 PMCID: PMC116050 DOI: 10.1128/mcb.19.2.1210] [Citation(s) in RCA: 321] [Impact Index Per Article: 12.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Protein nuclear import is generally mediated by basic nuclear localization signals (NLSs) that serve as targets for the importin alpha (Imp alpha) NLS receptor. Imp alpha is in turn bound by importin beta (Imp beta), which targets the resultant protein complex to the nucleus. Here, we report that the arginine-rich NLS sequences present in the human immunodeficiency virus type 1 regulatory proteins Tat and Rev fail to interact with Imp alpha and instead bind directly to Imp beta. Using in vitro nuclear import assays, we demonstrate that Imp alpha is entirely dispensable for Tat and Rev nuclear import. In contrast, Imp beta proved both sufficient and necessary, in that other beta-like import factors, such as transportin, were unable to support Tat or Rev nuclear import. Using in vitro competition assays, it was demonstrated that the target sites on Imp beta for Imp alpha, Tat, and Rev binding either are identical or at least overlap. The interaction of Tat and Rev with Imp beta is also similar to Imp alpha binding in that it is inhibited by RanGTP but not RanGDP, a finding that may in part explain why the interaction of the Rev nuclear RNA export factor with target RNA species is efficient in the cell nucleus yet is released in the cytoplasm. Together, these studies define a novel class of arginine-rich NLS sequences that are direct targets for Imp beta and that therefore function independently of Imp alpha.
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Affiliation(s)
- R Truant
- Howard Hughes Medical Institute and Department of Genetics, Duke University Medical Center, Durham, North Carolina 27710, USA
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908
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Palmeri D, Malim MH. Importin beta can mediate the nuclear import of an arginine-rich nuclear localization signal in the absence of importin alpha. Mol Cell Biol 1999; 19:1218-25. [PMID: 9891056 PMCID: PMC116051 DOI: 10.1128/mcb.19.2.1218] [Citation(s) in RCA: 173] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/17/1998] [Accepted: 10/27/1998] [Indexed: 11/20/2022] Open
Abstract
The import of proteins into the nucleus is dependent on cis-acting targeting sequences, nuclear localization signals (NLSs), and members of the nuclear transport receptor (importin-beta-like) superfamily. The most extensively characterized import pathway, often termed the classical pathway, is utilized by many basic-type (lysine-rich) NLSs and requires an additional component, importin alpha, to serve as a bridge between the NLS and the import receptor importin beta. More recently, it has become clear that a variety of proteins enter the nucleus via alternative import receptors and that their NLSs bind directly to those receptors. By using the digitonin-permeabilized cell system for protein import in vitro, we have defined the import pathway for the Rex protein of human T-cell leukemia virus type 1. Interestingly, the arginine-rich NLS of Rex uses importin beta for import but does so by a mechanism that is importin alpha independent. Based on the ability of the Rex NLS to inhibit the import of the lysine-rich NLS of T antigen and of both NLSs to be inhibited by the domain of importin alpha that binds importin beta (the IBB domain), we infer that the Rex NLS interacts with importin beta directly. In addition, and in keeping with other receptor-mediated nuclear import pathways, Rex import is dependent on the integrity of the Ran GTPase cycle. Based on these results, we suggest that importin beta can mediate the nuclear import of arginine-rich NLSs directly, or lysine-rich NLSs through the action of importin alpha.
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Affiliation(s)
- D Palmeri
- Cell and Molecular Biology Graduate Group, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104-6148, USA
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909
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Abstract
The protein Mei2 performs at least two functions required in fission yeast for the switch from mitotic to meiotic cell cycles. One of these functions also requires meiRNA. It appears that meiRNA targets Mei2 to the nucleus, where it can promote the first meiotic division.
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Affiliation(s)
- M Ohno
- European Molecular Biology Laboratory, Meyerhofstrasse 1 D-69117, Heidelberg, Germany
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910
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Abstract
Two crystal structures of an IkappaB-NFkappaB complex have recently been determined. The structures show in detail how IkappaB controls the subcellular localization and activity of the eukaryotic transcription factor NFkappaB.
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Affiliation(s)
- P Cramer
- European Molecular Biology Laboratory (EMBL), Grenoble Outstation, c/o ILL, BP 156, F-38042 Grenoble Cedex 9, France
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911
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Izaurralde E, Kann M, Panté N, Sodeik B, Hohn T. Viruses, microorganisms and scientists meet the nuclear pore. Leysin, VD, Switzerland, February 26-March 1, 1998. EMBO J 1999; 18:289-96. [PMID: 9889185 PMCID: PMC1171123 DOI: 10.1093/emboj/18.2.289] [Citation(s) in RCA: 22] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
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912
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Englmeier L, Olivo JC, Mattaj IW. Receptor-mediated substrate translocation through the nuclear pore complex without nucleotide triphosphate hydrolysis. Curr Biol 1999; 9:30-41. [PMID: 9889120 DOI: 10.1016/s0960-9822(99)80044-x] [Citation(s) in RCA: 129] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/17/2022]
Abstract
BACKGROUND The transport of macromolecules between the nucleus and cytoplasm is an energy-dependent process. Substrates are translocated across the nuclear envelope through nuclear pore complexes (NPCs). Translocation requires nucleocytoplasmic transport receptors of the importin beta family, which interact both with the NPC and, either directly or via an adaptor, with the transport substrate. Although certain receptors have recently been shown to cross the NPC in an energy-independent manner, translocation of substrate-receptor complexes through the NPC has generally been regarded as an energy-requiring step. RESULTS We describe an in vitro system that is based on permeabilised cells and supports nuclear export mediated by leucine-rich nuclear export signals. In this system, export is dependent on exogenous CRM1/Exportin1 - a nuclear export receptor - the GTPase Ran and nucleotide triphosphates (NTPs), and is further stimulated by Ran-binding protein 1 (RanBP1) and nuclear transport factor 2 (NTF2). Unexpectedly, non-hydrolysable NTP analogues completely satisfy the NTP requirements for a single-round of CRM1-mediated translocation of protein substrates across the NPC. Similarly, single transportin-mediated nuclear protein import events are shown not to require hydrolysable NTPs and to occur in the absence of the Ran GTPase. CONCLUSIONS Our data show that, contrary to expectation and prior conclusions, the translocation of substrate-receptor complexes across the NPC in either direction occurs in the absence of NTP hydrolysis and is thus energy independent. The energy needed to drive substrate transport against a concentration gradient is supplied at the step of receptor recycling in the cytoplasm.
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Affiliation(s)
- L Englmeier
- European Molecular Biology Laboratory, Meyerhofstrasse, 1 D-69117, Heidelberg, Germany
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913
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Ribbeck K, Kutay U, Paraskeva E, Görlich D. The translocation of transportin-cargo complexes through nuclear pores is independent of both Ran and energy. Curr Biol 1999; 9:47-50. [PMID: 9889126 DOI: 10.1016/s0960-9822(99)80046-3] [Citation(s) in RCA: 103] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/19/2022]
Abstract
Active transport between nucleus and cytoplasm proceeds through nuclear pore complexes (NPCs) and is mediated largely by shuttling transport receptors that use direct RanGTP binding to coordinate loading and unloading of cargo [1] [2] [3] [4]. Import receptors such as importin beta or transportin bind their substrates at low RanGTP levels in the cytoplasm and release them upon encountering RanGTP in the nucleus, where a high RanGTP concentration is predicted. This substrate release is, in the case of import by the importin alpha/beta heterodimer, coupled directly to importin beta release from the NPCs. If the importin beta -RanGTP interaction is prevented, import intermediates arrest at the nuclear side of the NPCs [5] [6]. This arrest makes it difficult to probe directly the Ran and energy requirements of the actual translocation from the cytoplasmic to the nuclear side of the NPC, which immediately precedes substrate release. Here, we have shown that in the case of transportin, dissociation of transportin-substrate complexes is uncoupled from transportin release from NPCs. This allowed us to dissect the requirements of translocation through the NPC, substrate release and transportin recycling. Surprisingly, translocation of transportin-substrate complexes into the nucleus requires neither Ran nor nucleoside triphosphates (NTPs). It is only nuclear RanGTP, not GTP hydrolysis, that is needed for dissociation of transportin-substrate complexes and for re-export of transportin to the cytoplasm. GTP hydrolysis is apparently required only to restore the import competence of the re-exported transportin and, thus, for multiple rounds of transportin-dependent import. In addition, we provide evidence that at least one type of substrate can also complete NPC passage mediated by importin beta independently of Ran and energy.
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Affiliation(s)
- K Ribbeck
- Zentrum für Molekulare Biologie, der Universität Heidelberg, Im Neuenheimer, Feld 282 69120, Heidelberg, Germany
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914
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915
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Affiliation(s)
- S L Wolin
- Departments of Cell Biology and Molecular Biophysics and Biochemistry, Howard Hughes Medical Institute, Yale University School of Medicine, New Haven, Connecticut 06536-0812,
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916
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Marelli M, Aitchison JD, Wozniak RW. Specific binding of the karyopherin Kap121p to a subunit of the nuclear pore complex containing Nup53p, Nup59p, and Nup170p. J Cell Biol 1998; 143:1813-30. [PMID: 9864357 PMCID: PMC2175238 DOI: 10.1083/jcb.143.7.1813] [Citation(s) in RCA: 137] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
We have identified a specific karyopherin docking complex within the yeast nuclear pore complex (NPC) that contains two novel, structurally related nucleoporins, Nup53p and Nup59p, and the NPC core protein Nup170p. This complex was affinity purified from cells expressing a functional Nup53p-protein A chimera. The localization of Nup53p, Nup59p, and Nup170p within the NPC by immunoelectron microscopy suggests that the Nup53p-containing complex is positioned on both the cytoplasmic and nucleoplasmic faces of the NPC core. In association with the isolated complex, we have also identified the nuclear transport factor Kap121p (Pse1p). Using in vitro binding assays, we showed that each of the nucleoporins interacts with one another. However, the association of Kap121p with the complex is mediated by its interaction with Nup53p. Moreover, Kap121p is the only beta-type karyopherin that binds Nup53p suggesting that Nup53p acts as a specific Kap121p docking site. Kap121p can be released from Nup53p by the GTP bound form of the small GTPase Ran. The physiological relevance of the interaction between Nup53p and Kap121p was further underscored by the observation that NUP53 mutations alter the subcellular distribution of Kap121p and the Kap121p- mediated import of a ribosomal L25 reporter protein. Interestingly, Nup53p is specifically phosphorylated during mitosis. This phenomenon is correlated with a transient decrease in perinuclear-associated Kap121p.
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Affiliation(s)
- M Marelli
- Department of Cell Biology, University of Alberta, Edmonton, Alberta, Canada T6G 2H7
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917
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Schwoebel ED, Talcott B, Cushman I, Moore MS. Ran-dependent signal-mediated nuclear import does not require GTP hydrolysis by Ran. J Biol Chem 1998; 273:35170-5. [PMID: 9857054 DOI: 10.1074/jbc.273.52.35170] [Citation(s) in RCA: 93] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Nuclear import of classical nuclear localization sequence-containing proteins involves the assembly of an import complex at the cytoplasmic face of the nuclear pore complex (NPC) followed by movement of this complex through the NPC and release of the import substrate into the nuclear interior. This process has historically been thought to require nucleotide hydrolysis as a source of energy. We found, using hydrolysis-resistant GTP analogs and a mutant Ran unable to hydrolyze GTP, that transport of classical nuclear localization sequence containing substrate through the NPC and release of the substrate into the nucleus did not require hydrolysis of GTP by Ran. In fact, for movement of this type of import substrate into the nuclear interior we did not observe a requirement for hydrolysis of any nucleotide triphosphate. We did, however, find that a pool of free GTP (or its structural equivalent) must be added, probably because the GDP Ran that is added must be converted to GTP Ran during the import process. We found that a requirement for GTP hydrolysis can be restored to an import mixture consisting of recombinant import factors by the addition of RCC1, the Ran guanine nucleotide exchange factor.
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Affiliation(s)
- E D Schwoebel
- Department of Cell Biology, Baylor College of Medicine, Houston, Texas 77030, USA
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918
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Arts GJ, Kuersten S, Romby P, Ehresmann B, Mattaj IW. The role of exportin-t in selective nuclear export of mature tRNAs. EMBO J 1998; 17:7430-41. [PMID: 9857198 PMCID: PMC1171087 DOI: 10.1093/emboj/17.24.7430] [Citation(s) in RCA: 181] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
Exportin-t (Xpo-t) is a vertebrate nuclear export receptor for tRNAs that binds tRNA cooperatively with GTP-loaded Ran. Xpo-t antibodies are shown to efficiently block tRNA export from Xenopus oocyte nuclei suggesting that it is responsible for at least the majority of tRNA export in these cells. We examine the mechanism by which Xpo-t-RanGTP specifically exports mature tRNAs rather than other forms of nuclear RNA, including tRNA precursors. Chemical and enzymatic footprinting together with phosphate modification interference reveals an extensive interaction between the backbone of the TPsiC and acceptor arms of tRNAPhe and Xpo-t-RanGTP. Analysis of mutant or precursor tRNA forms demonstrates that, aside from these recognition elements, accurate 5' and 3' end-processing of tRNA affects Xpo-t-RanGTP interaction and nuclear export, while aminoacylation is not essential. Intron-containing, end-processed, pre-tRNAs can be bound by Xpo-t-RanGTP and are rapidly exported from the nucleus if Xpo-t is present in excess. These results suggest that at least two mechanisms are involved in discrimination of pre-tRNAs and mature tRNAs prior to nuclear export.
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Affiliation(s)
- G J Arts
- European Molecular Biology Laboratory, Meyerhofstrasse 1, D-69117 Heidelberg, Germany
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919
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Askjaer P, Jensen TH, Nilsson J, Englmeier L, Kjems J. The specificity of the CRM1-Rev nuclear export signal interaction is mediated by RanGTP. J Biol Chem 1998; 273:33414-22. [PMID: 9837918 DOI: 10.1074/jbc.273.50.33414] [Citation(s) in RCA: 169] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Nuclear export of intron-containing human immunodeficiency virus type 1 (HIV-1) RNA is mediated by the viral Rev protein that contains both an RNA binding domain specific for the viral Rev response element (RRE) and a nuclear export signal (NES). The cellular CRM1 (Exportin1) protein functions as a nuclear export receptor for proteins carrying a Rev-like NES in a process that also requires the GTP bound form of the Ran GTPase. Using purified recombinant factors, we show by co-precipitation, gel mobility shift and protein footprinting assays that full-length Rev protein interacts directly with CRM1 in vitro independently of both the integrity of the characteristic leucine residues of the NES and the presence of the cytotoxin leptomycin B (LMB). Addition of RanGTP induces the formation of an RRE-Rev-CRM1-RanGTP complex that is sensitive to LMB, NES mutations, and Ran being charged with GTP. Within this complex, CRM1 is readily cross-linked to Cys89 near the NES of Rev. By protein footprinting, we demonstrate that the NES of Rev and two regions in CRM1 become inaccessible to endoproteinases upon binding suggesting that these regions are involved in protein-protein interactions. Our data are consistent with a model in which CRM1 is the nuclear export receptor for the Rev-RRE ribonucleoprotein complex and that RanGTP binds to a preformed Rev-CRM1 complex and specifies a functional interaction with the NES.
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Affiliation(s)
- P Askjaer
- Department of Molecular and Structural Biology, University of Aarhus, C. F. Mollers Allé, Building 130, DK-8000 Aarhus C, Denmark
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920
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Yamada M, Tachibana T, Imamoto N, Yoneda Y. Nuclear transport factor p10/NTF2 functions as a Ran-GDP dissociation inhibitor (Ran-GDI). Curr Biol 1998; 8:1339-42. [PMID: 9843686 DOI: 10.1016/s0960-9822(07)00566-0] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022]
Abstract
The cytosolic nuclear transport factor p10/NTF2 is required for the translocation of karyophilic molecules through nuclear pores [1] [2] [3], and the small GTPase Ran is a key regulator of protein transport between the nucleus and cytoplasm [4] [5]. It has been reported that p10/NTF2 interacts directly and specifically with Ran-GDP but not with Ran-GTP [6]. The precise role(s) of p10/NTF2 in the Ran GTP/GDP cycle are thus far unclear, however. In this study, we show that mammalian p10/NTF2 dramatically inhibits the dissociation of [3H]GDP from Ran and the binding of [35S]GTPgammaS to Ran following the dissociation of non-radioactive GDP by RCC1, the only known mammalian guanine nucleotide exchange factor for Ran (Ran-GEF) [7]. In contrast, the dissociation of [35S]GTP gamma S from Ran, which was also catalyzed by RCC1, was not affected by p10/NTF2. Furthermore, the activities of wild-type p10/NTF2 and the mutant forms M84T and D92G in an assay of nuclear protein import in a digitonin-permeabilized cell-free system correlated with their level of inhibition of the dissociation of nucleotide from Ran-GDP. These results suggest that p10/NTF2 acts as a GDP dissociation inhibitor for Ran (Ran-GDI), thereby coordinating the Ran-dependent reactions that underlie nuclear protein import.
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Affiliation(s)
- M Yamada
- Department of Anatomy and Cell Biology Osaka University Medical School 2-2 Yamada-oka Suita Osaka 565-0871 Japan
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921
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Truant R, Fridell RA, Benson ER, Herold A, Cullen BR. Nucleocytoplasmic shuttling by protein nuclear import factors. Eur J Cell Biol 1998; 77:269-75. [PMID: 9930651 DOI: 10.1016/s0171-9335(98)80085-x] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022] Open
Abstract
Protein nuclear import factors are not, in general, believed to function in the nuclear export of macromolecules and their reutilization therefore requires their recycling from the nucleus to the cytoplasm. Two possible mechanisms for recycling have been proposed. On the one hand, protein import factors such as importin beta and transportin (Trn) could continuously shuttle between cytoplasm and nucleoplasm. On the other hand, these proteins could penetrate into the nucleus only as far as the inner surface of the nuclear pore complex and then directly return to the cytoplasm. In this manuscript, we have used microinjection analysis in human cells, and in vitro nuclear assays, to demonstrate that importin beta, transportin and importin alpha are all nucleocytoplasmic shuttle proteins that efficiently enter and exit the cell nucleoplasm. In the case of transportin, we have mapped sequences required for nucleocytoplasmic shuttling to the carboxy-terminal 270 amino acids of this 890 amino acid import factor, thus demonstrating that nuclear export is independent of the amino-terminal Ran-binding domain of Trn. We further show that Trn shuttling is independent of nuclear RNA transcription. Overall, these data suggest that nucleocytoplasmic shuttling is likely to be a general attribute of protein nuclear import factors.
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Affiliation(s)
- R Truant
- Howard Hughes Medical Institute, Durham, NC 27710, USA
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922
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Cordes VC, Hase ME, Müller L. Molecular segments of protein Tpr that confer nuclear targeting and association with the nuclear pore complex. Exp Cell Res 1998; 245:43-56. [PMID: 9828100 DOI: 10.1006/excr.1998.4246] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Tpr is a 267-kDa protein of unknown function recently identified as a constitutive component of the nuclear pore complex (NPC2)-attached intranuclear filaments. Secondary structure predictions suggest that the protein is divided into a large, coiled-coil forming aminoterminal domain and a shorter, highly acidic carboxyterminal domain. To identify which of Tpr's molecular segments determine its specific intranuclear localization, we have constructed expression vectors encoding various Tpr deletion mutants as well as chimeric combinations of Tpr sequences with the soluble cytoplasmic protein pyruvate kinase. Following transfection of cultured mammalian cells, we have identified a short region within Tpr's carboxyterminal domain that is essential and sufficient to mediate nuclear import of Tpr and which can also confer nuclear accumulation of pyruvate kinase. Tpr deletion mutants that contain this nuclear targeting segment, but lack the aminoterminal domain, appear evenly dispersed throughout the nucleus without any noticeable association to the NPC. In contrast, the aminoterminal domain lacking the carboxyterminal region remains located within the cytoplasm, forming aggregate-like structures not associated with the nuclear envelope. However, when tagged to Tpr's short nuclear targeting segment or to the nuclear localization signal of the SV40 large T protein, the aminoterminal domain is imported into the nucleus, where it then associates with the NPC. This association is mediated by shorter molecular segments within the aminoterminal domain which contain clusters of heptad repeats, whereas other regions are dispensable. This assignment of different topogenetic properties to distinct molecular segments of Tpr will now allow the design of future experiments to study the protein's structural properties further and determine its actual function.
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Affiliation(s)
- V C Cordes
- Medical Nobel Institute, Karolinska Institutet, Stockholm, S-17177, Sweden.
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923
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Abstract
Importin beta family transport receptors shuttle between the nucleus and the cytoplasm and mediate transport of macromolecules through nuclear pore complexes (NPCs). The interactions between these receptors and their cargoes are regulated by binding RanGTP; all receptors probably exit the nucleus complexed with RanGTP, and so should deplete RanGTP continuously from the nucleus. We describe here the development of an in vitro system to study how nuclear Ran is replenished. Nuclear import of Ran does not rely on simple diffusion as Ran's small size would permit, but instead is stimulated by soluble transport factors. This facilitated import is specific for cytoplasmic RanGDP and employs nuclear transport factor 2 (NTF2) as the actual carrier. NTF2 binds RanGDP initially to NPCs and probably also mediates translocation of the NTF2-RanGDP complex to the nuclear side of the NPCs. A direct NTF2-RanGDP interaction is crucial for this process, since point mutations that disturb the RanGDP-NTF2 interaction also interfere with Ran import. The subsequent nuclear accumulation of Ran also requires GTP, but not GTP hydrolysis. The release of Ran from NTF2 into the nucleus, and thus the directionality of Ran import, probably involves nucleotide exchange to generate RanGTP, for which NTF2 has no detectable affinity, followed by binding of the RanGTP to an importin beta family transport receptor.
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Affiliation(s)
- K Ribbeck
- Zentrum für Molekulare Biologie der Universität Heidelberg, Im Neuenheimer Feld 282, 69120 Heidelberg, Germany
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924
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Ostareck-Lederer A, Ostareck DH, Hentze MW. Cytoplasmic regulatory functions of the KH-domain proteins hnRNPs K and E1/E2. Trends Biochem Sci 1998; 23:409-11. [PMID: 9852755 DOI: 10.1016/s0968-0004(98)01301-2] [Citation(s) in RCA: 149] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Affiliation(s)
- A Ostareck-Lederer
- Gene Expression Programme, European Molecular Biology Laboratory, Heidelberg, Germany.
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925
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Bogerd HP, Echarri A, Ross TM, Cullen BR. Inhibition of human immunodeficiency virus Rev and human T-cell leukemia virus Rex function, but not Mason-Pfizer monkey virus constitutive transport element activity, by a mutant human nucleoporin targeted to Crm1. J Virol 1998; 72:8627-35. [PMID: 9765402 PMCID: PMC110274 DOI: 10.1128/jvi.72.11.8627-8635.1998] [Citation(s) in RCA: 141] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/11/1998] [Accepted: 07/21/1998] [Indexed: 12/31/2022] Open
Abstract
The hypothesis that the cellular protein Crm1 mediates human immunodeficiency virus type 1 (HIV-1) Rev-dependent nuclear export posits that Crm1 can directly interact both with the Rev nuclear export signal (NES) and with cellular nucleoporins. Here, we demonstrate that Crm1 is indeed able to interact with active but not defective forms of the HIV-1 Rev NES and of NESs found in other retroviral nuclear export factors. In addition, we demonstrate that Crm1 can bind the Rev NES when Rev is assembled onto the Rev response element RNA target and that Crm1, like Rev, is a nucleocytoplasmic shuttle protein. Crm1 also specifically binds the Rev NES in vitro, although this latter interaction is detectable only in the presence of added Ran . GTP. Overexpression of a truncated, defective form of the nucleoporin Nup214/CAN, termed DeltaCAN, that retains Crm1 binding ability resulted in the effective inhibition of HIV-1 Rev or human T-cell leukemia virus Rex-dependent gene expression. In contrast, DeltaCAN had no significant affect on Mason-Pfizer monkey virus constitutive transport element (MPMV CTE)-dependent nuclear RNA export or on the expression of RNAs dependent on the cellular mRNA export pathway. As a result, DeltaCAN specifically blocked late, but not early, HIV-1 gene expression in HIV-1-infected cells. These data strongly validate Crm1 as a cellular cofactor for HIV-1 Rev and demonstrate that the MPMV CTE nuclear RNA export pathway uses a distinct, Crm1-independent mechanism. In addition, these data identify a novel and highly potent inhibitor of leucine-rich NES-dependent nuclear export.
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Affiliation(s)
- H P Bogerd
- Howard Hughes Medical Institute, Duke University Medical Center, Durham, North Carolina 27710, USA
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926
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Affiliation(s)
- F Stutz
- Institut de Microbiologie, Centre Hospitalier Universitaire Vaudois, 1011 Lausanne, Switzerland.
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927
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Herold A, Truant R, Wiegand H, Cullen BR. Determination of the functional domain organization of the importin alpha nuclear import factor. J Cell Biol 1998; 143:309-18. [PMID: 9786944 PMCID: PMC2132842 DOI: 10.1083/jcb.143.2.309] [Citation(s) in RCA: 103] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/09/1998] [Revised: 09/09/1998] [Indexed: 12/22/2022] Open
Abstract
Although importin alpha (Imp alpha) has been shown to act as the receptor for basic nuclear localization signals (NLSs) and to mediate their recruitment to the importin beta nuclear import factor, little is known about the functional domains present in Imp alpha, with the exception that importin beta binding is known to map close to the Imp alpha NH2 terminus. Here, we demonstrate that sequences essential for binding to the CAS nuclear export factor are located near the Imp alpha COOH terminus and include a critical acidic motif. Although point mutations introduced into this acidic motif inactivated both CAS binding and Imp alpha nuclear export, a putative leucine-rich nuclear export signal proved to be neither necessary nor sufficient for Imp alpha nuclear export. Analysis of sequences within Imp alpha that bind to the SV-40 T antigen NLS or to the similar LEF-1 NLS revealed that both NLSs interact with a subset of the eight degenerate armadillo (Arm) repeats that form the central part of Imp alpha. However, these two NLS-binding sites showed only minimal overlap, thus suggesting that the degeneracy of the Arm repeat region of Imp alpha may serve to facilitate binding to similar but nonidentical basic NLSs. Importantly, the SV-40 T NLS proved able to specifically inhibit the interaction of Imp alpha with CAS in vitro, thus explaining why the SV-40 T NLS is unable to also function as a nuclear export signal.
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Affiliation(s)
- A Herold
- Department of Genetics, Duke University Medical Center, Durham, North Carolina 27710, USA
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928
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Jiang CJ, Imamoto N, Matsuki R, Yoneda Y, Yamamoto N. In vitro characterization of rice importin beta1: molecular interaction with nuclear transport factors and mediation of nuclear protein import. FEBS Lett 1998; 437:127-30. [PMID: 9804185 DOI: 10.1016/s0014-5793(98)01207-1] [Citation(s) in RCA: 23] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
We recently isolated two cDNAs encoding importin 3 homologues (rice importin beta1 and beta2), the first such homologues identified in plants. To address the function of rice importin beta1 in the process of nuclear import of proteins, we carried out in vitro binding and nuclear import assays. Recombinant protein of rice importin beta1 assembled a complex (PTAC) with rice importin alpha1 and NLS protein, and also bound to the nuclear envelope of tobacco BY-2 cells. Ran-GTP, but not Ran-GDP, interacted with rice importin beta1 and dissociated the heterodimer formed between rice importin alpha1 and rice importin beta1. An in vitro nuclear import assay using digitonin-permeabilized HeLa cells revealed that rice importin beta1 can mediate nuclear envelope docking of NLS proteins and their subsequent translocation into the nucleus. These data strongly suggest that rice importin beta1 functions as a component of the NLS receptor in plant cells.
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Affiliation(s)
- C J Jiang
- Plant Physiology Department, National Institute of Agrobiological Resources, Tsukuba, Ibaraki, Japan
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929
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Santama N, Krijnse-Locker J, Griffiths G, Noda Y, Hirokawa N, Dotti CG. KIF2beta, a new kinesin superfamily protein in non-neuronal cells, is associated with lysosomes and may be implicated in their centrifugal translocation. EMBO J 1998; 17:5855-67. [PMID: 9774330 PMCID: PMC1170913 DOI: 10.1093/emboj/17.20.5855] [Citation(s) in RCA: 61] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
Lysosomes concentrate juxtanuclearly in the region around the microtubule-organizing center by interaction with microtubules. Different experimental and physiological conditions can induce these organelles to move to the cell periphery by a mechanism implying a plus-end-directed microtubule-motor protein (a kinesin-like motor). The responsible kinesin-superfamily protein, however, is unknown. We have identified a new mouse isoform of the kinesin superfamily, KIF2beta, an alternatively spliced isoform of the known, neuronal kinesin, KIF2. Developmental expression pattern and cell-type analysis in vivo and in vitro reveal that KIF2beta is abundant at early developmental stages of the hippocampus but is then downregulated in differentiated neuronal cells, and it is mainly or uniquely expressed in non-neuronal cells while KIF2 remains exclusively neuronal. Electron microscopy of mouse fibroblasts and immunofluorescence of KIF2beta-transiently-transfected fibroblasts show KIF2 and KIF2beta primarily associated with lysosomes, and this association can be disrupted by detergent treatment. In KIF2beta-overexpressing cells, lysosomes (labeled with anti-lysosome-associated membrane protein-1) become abnormally large and peripherally located at some distance from their usual perinuclear positions. Overexpression of KIF2 or KIF2beta does not change the size or distribution of early, late and recycling endosomes nor does overexpression of different kinesin superfamily proteins result in changes in lysosome size or positioning. These results implicate KIF2beta as a motor responsible for the peripheral translocation of lysosomes.
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Affiliation(s)
- N Santama
- Cell Biology Programme, European Molecular Biology Laboratory, Heidelberg D-69012, Germany
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930
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MESH Headings
- Amino Acid Sequence
- Animals
- Base Sequence
- Biological Transport, Active
- Carrier Proteins/metabolism
- Cell Nucleus/metabolism
- Cell Nucleus/virology
- Gene Products, rev/genetics
- Gene Products, rev/metabolism
- Genes, rev
- HIV-1/genetics
- HIV-1/physiology
- Humans
- Karyopherins
- Models, Biological
- Molecular Sequence Data
- Nuclear Proteins/metabolism
- Nucleic Acid Conformation
- RNA Splicing
- RNA, Viral/chemistry
- RNA, Viral/genetics
- RNA, Viral/metabolism
- Receptors, Cytoplasmic and Nuclear
- Retroviridae/genetics
- Retroviridae/physiology
- Virus Replication
- rev Gene Products, Human Immunodeficiency Virus
- Exportin 1 Protein
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Affiliation(s)
- B R Cullen
- Department of Genetics, Duke University Medical Center, Durham, North Carolina, 27710, USA.
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931
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Jiang CJ, Imamoto N, Matsuki R, Yoneda Y, Yamamoto N. Functional characterization of a plant importin alpha homologue. Nuclear localization signal (NLS)-selective binding and mediation of nuclear import of nls proteins in vitro. J Biol Chem 1998; 273:24083-7. [PMID: 9727027 DOI: 10.1074/jbc.273.37.24083] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Nuclear import of most nuclear proteins is initiated by recognition of the nuclear localization signal (NLS) by importin alpha. We recently isolated an importin alpha homologue from rice (rice importin alpha1) and demonstrated that transcription of the gene is down-regulated by light in rice leaves. To address the function of rice importin alpha1 in the process of nuclear import of proteins, we performed in vitro binding and nuclear import assays. The rice importin alpha1 showed specific binding to fusion proteins containing either monopartite or bipartite NLSs, but not to a fusion protein containing a Matalpha-2-type NLS, suggesting that there exists selective binding of rice importin alpha1 to different plant NLSs. The rice importin alpha1 is also capable of forming a complex with mouse importin beta and NLS protein in vitro. An in vitro nuclear import assay using permeabilized HeLa cells revealed that rice importin alpha1, in conjunction with other vertebrate transport factors, mediates the nuclear envelope docking of NLS proteins and their subsequent translocation into the nucleus. These data provide strong, direct evidence suggesting that rice importin alpha1 functions as a component of the NLS receptor in plant cells.
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Affiliation(s)
- C J Jiang
- National Institute of Agrobiological Resources, 2-1-2 Kannondai, Tsukuba, Ibaraki 305-8602, Japan
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932
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Jäkel S, Görlich D. Importin beta, transportin, RanBP5 and RanBP7 mediate nuclear import of ribosomal proteins in mammalian cells. EMBO J 1998; 17:4491-502. [PMID: 9687515 PMCID: PMC1170780 DOI: 10.1093/emboj/17.15.4491] [Citation(s) in RCA: 405] [Impact Index Per Article: 15.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
The assembly of eukaryotic ribosomal subunits takes place in the nucleolus and requires nuclear import of ribosomal proteins. We have studied this import in a mammalian system and found that the classical nuclear import pathway using the importin alpha/beta heterodimer apparently plays only a minor role. Instead, at least four importin beta-like transport receptors, namely importin beta itself, transportin, RanBP5 and RanBP7, directly bind and import ribosomal proteins. We found that the ribosomal proteins L23a, S7 and L5 can each be imported alternatively by any of the four receptors. We have studied rpL23a in detail and identified a very basic region to which each of the four import receptors bind avidly. This domain might be considered as an archetypal import signal that evolved before import receptors diverged in evolution. The presence of distinct binding sites for rpL23a and the M9 import signal in transportin, and for rpL23a and importin alpha in importin beta might explain how a single receptor can recognize very different import signals.
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Affiliation(s)
- S Jäkel
- Zentrum für Molekulare Biologie der Universität Heidelberg, Germany
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933
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Gant TM, Goldberg MW, Allen TD. Nuclear envelope and nuclear pore assembly: analysis of assembly intermediates by electron microscopy. Curr Opin Cell Biol 1998; 10:409-15. [PMID: 9640543 DOI: 10.1016/s0955-0674(98)80018-5] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
At mitosis, the nucleus of higher eukaryotic cells disassemblies into components which subsequently reform functional nuclear envelopes in the two daughter cells. The molecular mechanisms underlying this remarkable morphological reorganization are the focus of active investigation. Recent electron microscopy techniques have provided intriguing glimpses of intermediate structures in both nuclear envelope and nuclear pore complex reassembly.
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Affiliation(s)
- T M Gant
- Department of Cell Biology and Anatomy, Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA.
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934
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Abstract
Significant and exciting advances in the field of RNA and protein export have been made recently, due in large part to discovery of the roles played by Ran, a small, soluble GTPase present in both the nucleus and cytoplasm of all eukaryotic cells. Ran is thought to be primarily bound to GTP in the nucleus and to GDP in the cytoplasm, as a result of the assymetric distribution of factors that interact with Ran to promote guanine nucleotide exchange (in the nucleus) and GTP hydrolysis (in the cytoplasm). A key function of the nuclear Ran.GTP is to support formation of complexes containing an export receptor (an exportin) and cargos such as RNAs, RNPs or proteins that are destined for export. In the cytoplasm, removal of the Ran.GTP from the complex results in its destabilization and release of the export cargo. Although Ran.GTP is required for formation of the export complex, GTP hydrolysis does not appear to be necessary for translocation through the nuclear pore complex or cytoplasmic release. Nevertheless, the GTPase of Ran does appear to be required in as yet unidentified intranuclear steps prior to export of some, but not all, RNAs.
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Affiliation(s)
- J E Dahlberg
- Department of Biomolecular Chemistry, University of Wisconsin-Madison, WI 53706, USA.
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935
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Affiliation(s)
- B R Cullen
- Howard Hughes Medical Institute and Department of Genetics, Duke University Medical Center, Durham, North Carolina 27710, USA
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936
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Affiliation(s)
- D Görlich
- Zentrum für Molekulare Biologie der Universität Heidelberg, Heidelberg, Germany.
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937
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Abstract
BACKGROUND Transport of macromolecules between the nucleus and cytoplasm of eukaryotic cells is mediated by nuclear import and export receptors. The receptors identified to date are members of a family of Ran GTPase-binding proteins whose founding member is importin-beta. Interaction between these receptors and their cargo is regulated by the GTP-bound form of Ran. Export complexes form and import complexes disassemble on binding of RanGTP to the receptor. Yeast Los 1 p is a member of the importin-beta family with a poorly defined role in tRNA production. RESULTS A human member of the importin-beta family that is distantly related to Los 1 p (21% identity) has been characterized. The protein shuttled between the nucleus and cytoplasm and interacts with tRNA in a RanGTP-dependent manner. Injection of the protein into the nuclei of Xenopus oocytes resulted in a specific stimulation of the export of tRNA from the nucleus and in relief of the competitive inhibition of tRNA export caused by the introduction of saturating amounts of nuclear tRNA. CONCLUSIONS The human protein has the functional properties expected of a transport receptor that mediates export of tRNA from the nucleus. We therefore name the protein Exportin(tRNA).
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Affiliation(s)
- G J Arts
- European Molecular Biology Laboratory, Heidelberg, Germany
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