scholarly journals Selective Disruption of Nuclear Import by a Functional Mutant Nuclear Transport Carrier

2000 ◽  
Vol 151 (2) ◽  
pp. 321-332 ◽  
Author(s):  
Cynthia M. Lane ◽  
Ian Cushman ◽  
Mary Shannon Moore
Keyword(s):  
Nuclear Import ◽  
Binding Sites ◽  
Mammalian Cells ◽  
Nuclear Transport ◽  
Dominant Negative ◽  
Nuclear Localization Sequence ◽  
Nuclear Pore ◽  
Binding Studies ◽  
Transport Carrier ◽  
Docking Sites

p10/NTF2 is a nuclear transport carrier that mediates the uptake of cytoplasmic RanGDP into the nucleus. We constructed a point mutant of p10, D23A, that exhibited unexpected behavior both in digitonin-permeabilized and microinjected mammalian cells. D23A p10 was markedly more efficient than wild-type (wt) p10 at supporting Ran import, but simultaneously acted as a dominant-negative inhibitor of classical nuclear localization sequence (cNLS)-mediated nuclear import supported by karyopherins (Kaps) α and β1. Binding studies indicated that these two nuclear transport carriers of different classes, p10 and Kap-β1, compete for identical and/or overlapping binding sites at the nuclear pore complex (NPC) and that D23A p10 has an increased affinity relative to wt p10 and Kap-β1 for these shared binding sites. Because of this increased affinity, D23A p10 is able to import its own cargo (RanGDP) more efficiently than wt p10, but Kap-β1 can no longer compete efficiently for shared NPC docking sites, thus the import of cNLS cargo is inhibited. The competition of different nuclear carriers for shared NPC docking sites observed here predicts a dynamic equilibrium between multiple nuclear transport pathways inside the cell that could be easily shifted by a transient modification of one of the carriers.

scholarly journals Importin β contains a COOH-terminal nucleoporin binding region important for nuclear transport

2003 ◽  
Vol 162 (3) ◽  
pp. 391-401 ◽  
Author(s):  
Janna Bednenko ◽  
Gino Cingolani ◽  
Larry Gerace
Keyword(s):  
Nuclear Pore Complex ◽  
Nuclear Import ◽  
Binding Sites ◽  
Nuclear Transport ◽  
Terminal Region ◽  
Nuclear Pore ◽  
Binding Region ◽  
Similar Extent ◽  
Importin Α ◽  
Import Receptor

Proteins containing a classical NLS are transported into the nucleus by the import receptor importin β, which binds to cargoes via the adaptor importin α. The import complex is translocated through the nuclear pore complex by interactions of importin β with a series of nucleoporins. Previous studies have defined a nucleoporin binding region in the NH2-terminal half of importin β. Here we report the identification of a second nucleoporin binding region in its COOH-terminal half. Although the affinity of the COOH-terminal region for nucleoporins is dramatically weaker than that of the NH2-terminal region, sets of mutations that perturb the nucleoporin binding of either region reduce the nuclear import activity of importin β to a similar extent (∼50%). An importin β mutant with a combination of mutations in the NH2- and COOH-terminal regions is completely inactive for nuclear import. Thus, importin β possesses two nucleoporin binding sites, both of which are important for its nuclear import function.

scholarly journals Major Binding Sites for the Nuclear Import Receptor Are the Internal Nucleoporin Nup153 and the Adjacent Nuclear Filament Protein Tpr

1998 ◽  
Vol 141 (1) ◽  
pp. 31-49 ◽  
Author(s):  
Sundeep Shah ◽  
Stuart Tugendreich ◽  
Douglass Forbes
Keyword(s):  
Nuclear Import ◽  
Binding Sites ◽  
Nuclear Pore ◽  
Binding Studies ◽  
Nuclear Pores ◽  
Major Question ◽  
Egg Extracts ◽  
Importin Α ◽  
Xenopus Egg ◽  
Filament Protein

A major question in nuclear import concerns the identity of the nucleoporin(s) that interact with the nuclear localization sequences (NLS) receptor and its cargo as they traverse the nuclear pore. Ligand blotting and solution binding studies of isolated proteins have attempted to gain clues to the identities of these nucleoporins, but the studies have from necessity probed binding events far from an in vivo context. Here we have asked what binding events occur in the more physiological context of a Xenopus egg extract, which contains nuclear pore subcomplexes in an assembly competent state. We have then assessed our conclusions in the context of assembled nuclear pores themselves. We have used immunoprecipitation to identify physiologically relevant complexes of nucleoporins and importin subunits. In parallel, we have demonstrated that it is possible to obtain immunofluorescence localization of nucleoporins to subregions of the nuclear pore and its associated structures. By immunoprecipitation, we find the nucleoporin Nup153 and the pore-associated filament protein Tpr, previously shown to reside at distinct sites on the intranuclear side of assembled pores, are each in stable subcomplexes with importin α and β in Xenopus egg extracts. Importin subunits are not in stable complexes with nucleoporins Nup62, Nup93, Nup98, or Nup214/CAN, either in egg extracts or in extracts of assembled nuclear pores. In characterizing the Nup153 complex, we find that Nup153 can bind to a complete import complex containing importin α, β, and an NLS substrate, consistent with an involvement of this nucleoporin in a terminal step of nuclear import. Importin β binds directly to Nup153 and in vitro can do so at multiple sites in the Nup153 FXFG repeat region. Tpr, which has no FXFG repeats, binds to importin β and to importin α/β heterodimers, but only to those that do not carry an NLS substrate. That the complex of Tpr with importin β is fundamentally different from that of Nup153 is additionally demonstrated by the finding that recombinant β or β45–462 fragment freely exchanges with the endogenous importin β/Nup153 complex, but cannot displace endogenous importin β from a Tpr complex. However, the GTP analogue GMP-PNP is able to disassemble both Nup153– and Tpr–importin β complexes. Importantly, analysis of extracts of isolated nuclei indicates that Nup153– and Tpr–importin β complexes exist in assembled nuclear pores. Thus, Nup153 and Tpr are major physiological binding sites for importin β. Models for the roles of these interactions are discussed.

scholarly journals O-GlcNAc modification of nuclear pore complexes accelerates bi-directional transport

2020 ◽  
Author(s):  
Tae Yeon Yoo ◽  
Timothy J Mitchison
Keyword(s):  
Nuclear Import ◽  
Nuclear Transport ◽  
Super Resolution ◽  
Nucleocytoplasmic Transport ◽  
Nuclear Pore ◽  
Permeability Barrier ◽  
Nuclear Pore Complexes ◽  
Facilitated Diffusion ◽  
Import And Export ◽  
Pore Complexes

AbstractMacromolecular transport across the nuclear envelope depends on facilitated diffusion through nuclear pore complexes (NPCs). The interior of NPCs contains a permeability barrier made of phenylalanine-glycine (FG) repeat domains that selectively facilitates the permeation of cargoes bound to nuclear transport receptors (NTRs). FG repeats in NPC are a major site of O-linked N-acetylglucosamine (O-GlcNAc) modification, but the functional role of this modification in nucleocytoplasmic transport is unclear. We developed high-throughput assays based on optogenetic probes to quantify the kinetics of nuclear import and export in living human cells. We found that increasing O-GlcNAc modification of the NPC accelerated NTR-facilitated nucleocytoplasmic transport of proteins in both directions, and decreasing modification slowed transport. Super-resolution imaging revealed strong enrichment of O-GlcNAc at the FG-repeat barrier. O-GlcNAc modification also accelerated passive permeation of a small, inert protein through NPCs. We conclude that O-GlcNAc modification accelerates nucleocytoplasmic transport by enhancing the non-specific permeability the FG-repeat barrier, perhaps by steric inhibition of interactions between FG repeats.SummaryNuclear pore complexes mediate nuclear transport and are highly modified with O-linked N-acetylglucosamine (O-GlcNAc) on FG repeat domains. Using a new quantitative live-cell imaging assay, Yoo and Mitchison demonstrate acceleration of nuclear import and export by O-GlcNAc modification.

scholarly journals Nuclear Import of IκBα Is Accomplished by a Ran-Independent Transport Pathway

2000 ◽  
Vol 20 (5) ◽  
pp. 1571-1582 ◽  
Author(s):  
Shrikesh Sachdev ◽  
Sriparna Bagchi ◽  
Donna D. Zhang ◽  
Angela C. Mings ◽  
Mark Hannink
Keyword(s):  
Nuclear Import ◽  
Nuclear Export ◽  
Dominant Negative ◽  
Dominant Negative Mutant ◽  
Nuclear Pore ◽  
Transport Pathway ◽  
Repeat Domain ◽  
Nuclear Export Receptor

ABSTRACT The inhibitor of kappa B alpha (IκBα) protein is able to shuttle between the cytoplasm and the nucleus. We have utilized a combination of in vivo and in vitro approaches to provide mechanistic insight into nucleocytoplasmic shuttling by IκBα. IκBα contains multiple functional domains that contribute to shuttling of IκBα between the cytoplasm and the nucleus. Nuclear import of IκBα is mediated by the central ankyrin repeat domain. Similar to previously described nuclear import pathways, nuclear import of IκBα is temperature and ATP dependent and is blocked by a dominant-negative mutant of importin β. However, in contrast to classical nuclear import pathways, nuclear import of IκBα is independent of soluble cytosolic factors and is not blocked by the dominant-negative RanQ69L protein. Nuclear export of IκBα is mediated by an N-terminal nuclear export sequence. Nuclear export of IκBα requires the CRM1 nuclear export receptor and is blocked by the dominant-negative RanQ69L protein. Our results are consistent with a model in which nuclear import of IκBα is mediated through direct interactions with components of the nuclear pore complex, while nuclear export of IκBα is mediated via a CRM1-dependent pathway.

scholarly journals Crystal structure of MICU2 and comparison with MICU1 reveal insights into the uniporter gating mechanism

2019 ◽  
Vol 116 (9) ◽  
pp. 3546-3555 ◽  
Author(s):  
Kimberli J. Kamer ◽  
Wei Jiang ◽  
Virendar K. Kaushik ◽  
Vamsi K. Mootha ◽  
Zenon Grabarek
Keyword(s):  
Binding Sites ◽  
Mammalian Cells ◽  
Dominant Negative ◽  
Dominant Negative Effect ◽  
Functional Coupling ◽  
Gating Mechanism ◽  
Ef Hand ◽  
Negative Effect ◽  
Oligomeric Complex

The mitochondrial uniporter is a Ca2+-channel complex resident within the organelle’s inner membrane. In mammalian cells the uniporter’s activity is regulated by Ca2+ due to concerted action of MICU1 and MICU2, two paralogous, but functionally distinct, EF-hand Ca2+-binding proteins. Here we present the X-ray structure of the apo form of Mus musculus MICU2 at 2.5-Å resolution. The core structure of MICU2 is very similar to that of MICU1. It consists of two lobes, each containing one canonical Ca2+-binding EF-hand (EF1, EF4) and one structural EF-hand (EF2, EF3). Two molecules of MICU2 form a symmetrical dimer stabilized by highly conserved hydrophobic contacts between exposed residues of EF1 of one monomer and EF3 of another. Similar interactions stabilize MICU1 dimers, allowing exchange between homo- and heterodimers. The tight EF1–EF3 interface likely accounts for the structural and functional coupling between the Ca2+-binding sites in MICU1, MICU2, and their complex that leads to the previously reported Ca2+-binding cooperativity and dominant negative effect of mutation of the Ca2+-binding sites in either protein. The N- and C-terminal segments of the two proteins are distinctly different. In MICU2 the C-terminal helix is significantly longer than in MICU1, and it adopts a more rigid structure. MICU2’s C-terminal helix is dispensable in vitro for its interaction with MICU1 but required for MICU2’s function in cells. We propose that in the MICU1–MICU2 oligomeric complex the C-terminal helices of both proteins form a central semiautonomous assembly which contributes to the gating mechanism of the uniporter.

The nuclear pore complex, nuclear transport, and apoptosisThis paper is one of a selection of papers published in this Special Issue, entitled The Nucleus: A Cell Within A Cell.

2006 ◽  
Vol 84 (3-4) ◽  
pp. 279-286 ◽  
Author(s):  
Birthe Fahrenkrog
Keyword(s):  
Nuclear Pore Complex ◽  
Nuclear Import ◽  
Morphological Changes ◽  
Nuclear Transport ◽  
Nuclear Pore ◽  
Simple Fact ◽  
Cell Death Program ◽  
A Cell ◽  
Cellular Compartments ◽  
Selection Of

The nuclear pore complex (NPC) is the sole gateway between the nucleus and the cytoplasm of interphase eukaryotic cells, and it mediates all trafficking between these 2 cellular compartments. As such, the NPC and nuclear transport play central roles in translocating death signals from the cell membrane to the nucleus where they initiate biochemical and morphological changes occurring during apoptosis. Recent findings suggest that the correlation between the NPC, nuclear transport, and apoptosis goes beyond the simple fact that NPCs mediate nuclear transport of key players involved in the cell death program. In this context, the accessibility of key regulators of apoptosis appears to be highly modulated by nuclear transport (e.g., impaired nuclear import might be an apoptotic trigger). In this review, recent findings concerning the unexpected tight link between NPCs, nuclear transport, and apoptosis will be presented and critically discussed.

scholarly journals Baculovirus Infection of Nondividing Mammalian Cells: Mechanisms of Entry and Nuclear Transport of Capsids

2001 ◽  
Vol 75 (2) ◽  
pp. 961-970 ◽  
Author(s):  
Nico-Dirk van Loo ◽  
Elisabetta Fortunati ◽  
Erich Ehlert ◽  
Martijn Rabelink ◽  
Frank Grosveld ◽  
...  
Keyword(s):  
Mammalian Cells ◽  
Fluorescent Protein ◽  
Nuclear Transport ◽  
Microscopic Analysis ◽  
Cell Types ◽  
Nuclear Pore ◽  
Fusion Event ◽  
Electron Microscopic ◽  
Cytoplasmic Transport ◽  
Baculovirus Infection

ABSTRACT We have studied the infection pathway of Autographa californica multinuclear polyhedrosis virus (baculovirus) in mammalian cells. By titration with a baculovirus containing a green fluorescent protein cassette, we found that several, but not all, mammalian cell types can be infected efficiently. In contrast to previous suggestions, our data show that the asialoglycoprotein receptor is not required for efficient infection. We demonstrate for the first time that this baculovirus can infect nondividing mammalian cells, which implies that the baculovirus is able to transport its genome across the nuclear membrane of mammalian cells. Our data further show that the virus enters via endocytosis, followed by an acid-induced fusion event, which releases the nucleocapsid into the cytoplasm. Cytochalasin D strongly reduces the infection efficiency but not the delivery of nucleocapsids to the cytoplasm, suggesting involvement of actin filaments in cytoplasmic transport of the capsids. Electron microscopic analysis shows the cigar-shaped nucleocapsids located at nuclear pores of nondividing cells. Under these conditions, we observed the viral genome, major capsid protein, and electron-dense capsids inside the nucleus. This suggests that the nucleocapsid is transported through the nuclear pore. This mode of transport seems different from viruses with large spherical capsids, such as herpes simplex virus and adenovirus, which are disassembled before nuclear transport of the genome. The implications for the application of baculovirus or its capsid proteins in gene therapy are discussed.

scholarly journals Nuclear Import of Cdk/Cyclin Complexes: Identification of Distinct Mechanisms for Import of Cdk2/Cyclin E and Cdc2/Cyclin B1

1999 ◽  
Vol 144 (2) ◽  
pp. 213-224 ◽  
Author(s):  
Jonathan D. Moore ◽  
Jing Yang ◽  
Ray Truant ◽  
Sally Kornbluth
Keyword(s):  
Nuclear Localization ◽  
Nuclear Import ◽  
Mammalian Cells ◽  
Molecular Mechanisms ◽  
Cyclin E ◽  
Cyclin B1 ◽  
Transport Processes ◽  
Nuclear Localization Sequence ◽  
Nuclear Targeting ◽  
Importin Α

Reversible phosphorylation of nuclear proteins is required for both DNA replication and entry into mitosis. Consequently, most cyclin-dependent kinase (Cdk)/cyclin complexes are localized to the nucleus when active. Although our understanding of nuclear transport processes has been greatly enhanced by the recent identification of nuclear targeting sequences and soluble nuclear import factors with which they interact, the mechanisms used to target Cdk/cyclin complexes to the nucleus remain obscure; this is in part because these proteins lack obvious nuclear localization sequences. To elucidate the molecular mechanisms responsible for Cdk/cyclin transport, we examined nuclear import of fluorescent Cdk2/cyclin E and Cdc2/cyclin B1 complexes in digitonin-permeabilized mammalian cells and also examined potential physical interactions between these Cdks, cyclins, and soluble import factors. We found that the nuclear import machinery recognizes these Cdk/cyclin complexes through direct interactions with the cyclin component. Surprisingly, cyclins E and B1 are imported into nuclei via distinct mechanisms. Cyclin E behaves like a classical basic nuclear localization sequence–containing protein, binding to the α adaptor subunit of the importin-α/β heterodimer. In contrast, cyclin B1 is imported via a direct interaction with a site in the NH2 terminus of importin-β that is distinct from that used to bind importin-α.

scholarly journals GTP hydrolysis by Ran occurs at the nuclear pore complex in an early step of protein import.

1995 ◽  
Vol 131 (3) ◽  
pp. 571-581 ◽  
Author(s):  
F Melchior ◽  
T Guan ◽  
N Yokoyama ◽  
T Nishimoto ◽  
L Gerace
Keyword(s):  
Nuclear Pore Complex ◽  
Nuclear Import ◽  
Protein Import ◽  
Small Gtpase ◽  
Nuclear Pore ◽  
Binding Studies ◽  
Early Step ◽  
Permeabilized Cells ◽  
Complex Protein ◽  
Ran Gtp

Mediated import of proteins into the nucleus involves multiple cytosolic factors, including the small GTPase Ran. Whether Ran functions by interacting with other cytosolic proteins or components of the nuclear pore complex has been unclear. Furthermore, the precise transport step where Ran acts has not been determined. To address these questions, we have analyzed the binding interactions of Ran using permeabilized cells and isolated nuclear envelopes. By light and electron microscope immunolocalization, we have found that Ran accumulates specifically at the cytoplasmic surface of the nuclear pore complex when nuclear import in permeabilized cells is inhibited by nonhydrolyzable analogs of GTP. Ran associates with a peripheral pore complex region that is similar to the area where transport ligands accumulate by depletion of ATP, which arrests an early step of transport. Binding studies with isolated nuclear envelopes in the absence of added cytosol indicate that Ran-GTP directly interacts with a pore complex protein. Using blot overlay techniques, we detected a single prominent polypeptide of isolated nuclear envelopes that binds Ran-GTP. This corresponds to the 358-kD protein RanBP2, a Ran binding pore complex protein recently identified by two-hybrid screening. Thus, RanBP2 is likely to constitute the Ran-GTP-binding site detected at the cytoplasmic periphery of the pore complex. These data support a model in which initial ligand binding to the nuclear pore complex occurs at or near RanBP2, and that hydrolysis of GTP by Ran at this site serves to define commitment to the nuclear import pathway.

scholarly journals The cytoplasmic filaments of the nuclear pore complex are dispensable for selective nuclear protein import

2002 ◽  
Vol 158 (1) ◽  
pp. 63-77 ◽  
Author(s):  
Tobias C. Walther ◽  
Helen S. Pickersgill ◽  
Volker C. Cordes ◽  
Martin W. Goldberg ◽  
Terry D. Allen ◽  
...  
Keyword(s):  
Nuclear Pore Complex ◽  
Nuclear Import ◽  
Protein Import ◽  
Nuclear Localization Sequence ◽  
Nuclear Pore ◽  
Slight Reduction ◽  
Cytoplasmic Filaments ◽  
Localization Sequence ◽  
Importin Α

The nuclear pore complex (NPC) mediates bidirectional macromolecular traffic between the nucleus and cytoplasm in eukaryotic cells. Eight filaments project from the NPC into the cytoplasm and are proposed to function in nuclear import. We investigated the localization and function of two nucleoporins on the cytoplasmic face of the NPC, CAN/Nup214 and RanBP2/Nup358. Consistent with previous data, RanBP2 was localized at the cytoplasmic filaments. In contrast, CAN was localized near the cytoplasmic coaxial ring. Unexpectedly, extensive blocking of RanBP2 with gold-conjugated antibodies failed to inhibit nuclear import. Therefore, RanBP2-deficient NPCs were generated by in vitro nuclear assembly in RanBP2-depleted Xenopus egg extracts. NPCs were formed that lacked cytoplasmic filaments, but that retained CAN. These nuclei efficiently imported nuclear localization sequence (NLS) or M9 substrates. NPCs lacking CAN retained RanBP2 and cytoplasmic filaments, and showed a minor NLS import defect. NPCs deficient in both CAN and RanBP2 displayed no cytoplasmic filaments and had a strikingly immature cytoplasmic appearance. However, they showed only a slight reduction in NLS-mediated import, no change in M9-mediated import, and were normal in growth and DNA replication. We conclude that RanBP2 is the major nucleoporin component of the cytoplasmic filaments of the NPC, and that these filaments do not have an essential role in importin α/β– or transportin-dependent import.

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