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Solà Colom M, Fu Z, Gunkel P, Güttler T, Trakhanov S, Srinivasan V, Gregor K, Pleiner T, Görlich D. A checkpoint function for Nup98 in nuclear pore formation suggested by novel inhibitory nanobodies. EMBO J 2024:10.1038/s44318-024-00081-w. [PMID: 38649536 DOI: 10.1038/s44318-024-00081-w] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/12/2023] [Revised: 03/11/2024] [Accepted: 03/13/2024] [Indexed: 04/25/2024] Open
Abstract
Nuclear pore complex (NPC) biogenesis is a still enigmatic example of protein self-assembly. We now introduce several cross-reacting anti-Nup nanobodies for imaging intact nuclear pore complexes from frog to human. We also report a simplified assay that directly tracks postmitotic NPC assembly with added fluorophore-labeled anti-Nup nanobodies. During interphase, NPCs are inserted into a pre-existing nuclear envelope. Monitoring this process is challenging because newly assembled NPCs are indistinguishable from pre-existing ones. We overcame this problem by inserting Xenopus-derived NPCs into human nuclear envelopes and using frog-specific anti-Nup nanobodies for detection. We further asked whether anti-Nup nanobodies could serve as NPC assembly inhibitors. Using a selection strategy against conserved epitopes, we obtained anti-Nup93, Nup98, and Nup155 nanobodies that block Nup-Nup interfaces and arrest NPC assembly. We solved structures of nanobody-target complexes and identified roles for the Nup93 α-solenoid domain in recruiting Nup358 and the Nup214·88·62 complex, as well as for Nup155 and the Nup98 autoproteolytic domain in NPC scaffold assembly. The latter suggests a checkpoint linking pore formation to the assembly of the Nup98-dominated permeability barrier.
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Affiliation(s)
- Mireia Solà Colom
- Department of Cellular Logistics, Max Planck Institute for Multidisciplinary Sciences, Göttingen, Germany
- AI Proteins, 20 Overland St., Boston, MA, USA
| | - Zhenglin Fu
- Department of Cellular Logistics, Max Planck Institute for Multidisciplinary Sciences, Göttingen, Germany
| | - Philip Gunkel
- Department of Cellular Logistics, Max Planck Institute for Multidisciplinary Sciences, Göttingen, Germany
| | - Thomas Güttler
- Department of Cellular Logistics, Max Planck Institute for Multidisciplinary Sciences, Göttingen, Germany
- Octapharma Biopharmaceuticals, Im Neuenheimer Feld 590, 69120, Heidelberg, Germany
| | - Sergei Trakhanov
- Department of Cellular Logistics, Max Planck Institute for Multidisciplinary Sciences, Göttingen, Germany
| | - Vasundara Srinivasan
- Department of Cellular Logistics, Max Planck Institute for Multidisciplinary Sciences, Göttingen, Germany
- Department of Chemistry, Institute of Biochemistry and Molecular Biology, Universität Hamburg, Hamburg, Germany
| | - Kathrin Gregor
- Department of Cellular Logistics, Max Planck Institute for Multidisciplinary Sciences, Göttingen, Germany
| | - Tino Pleiner
- Department of Cellular Logistics, Max Planck Institute for Multidisciplinary Sciences, Göttingen, Germany
- Department of Molecular and Cellular Physiology, Stanford University School of Medicine, Stanford, CA, USA
| | - Dirk Görlich
- Department of Cellular Logistics, Max Planck Institute for Multidisciplinary Sciences, Göttingen, Germany.
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Stevens TA, Tomaleri GP, Hazu M, Wei S, Nguyen VN, DeKalb C, Voorhees RM, Pleiner T. A nanobody-based strategy for rapid and scalable purification of human protein complexes. Nat Protoc 2024; 19:127-158. [PMID: 37974029 DOI: 10.1038/s41596-023-00904-w] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/03/2023] [Accepted: 08/18/2023] [Indexed: 11/19/2023]
Abstract
The isolation of proteins in high yield and purity is a major bottleneck for the analysis of their three-dimensional structure, function and interactome. Here, we present a streamlined workflow for the rapid production of proteins or protein complexes using lentiviral transduction of human suspension cells, combined with highly specific nanobody-mediated purification and proteolytic elution. Application of the method requires prior generation of a plasmid coding for a protein of interest (POI) fused to an N- or C-terminal GFP or ALFA peptide tag using a lentiviral plasmid toolkit we have designed. The plasmid is then used to generate human suspension cell lines stably expressing the tagged fusion protein by lentiviral transduction. By leveraging the picomolar affinity of the GFP and ALFA nanobodies for their respective tags, the POI can be specifically captured from the resulting cell lysate even when expressed at low levels and under a variety of conditions, including detergents and mild denaturants. Finally, rapid and specific elution of the POI (in its tagged or untagged form) under native conditions is achieved within minutes at 4 °C, using the engineered SUMO protease SENPEuB. We demonstrate the wide applicability of the method by purifying multiple challenging soluble and membrane protein complexes to high purity from human cells. Our strategy is also directly compatible with many widely used GFP-expression plasmids, cell lines and transgenic model organisms. Finally, our method is faster than alternative approaches, requiring only 8 d from plasmid to purified protein, and results in substantially improved yields and purity.
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Affiliation(s)
- Taylor Anthony Stevens
- Division of Biology and Biological Engineering, California Institute of Technology, Pasadena, CA, USA
| | - Giovani Pinton Tomaleri
- Division of Biology and Biological Engineering, California Institute of Technology, Pasadena, CA, USA
| | - Masami Hazu
- Division of Biology and Biological Engineering, California Institute of Technology, Pasadena, CA, USA
| | - Sophia Wei
- Division of Biology and Biological Engineering, California Institute of Technology, Pasadena, CA, USA
| | - Vy N Nguyen
- Division of Biology and Biological Engineering, California Institute of Technology, Pasadena, CA, USA
| | - Charlene DeKalb
- Division of Biology and Biological Engineering, California Institute of Technology, Pasadena, CA, USA
| | - Rebecca M Voorhees
- Division of Biology and Biological Engineering, California Institute of Technology, Pasadena, CA, USA.
| | - Tino Pleiner
- Division of Biology and Biological Engineering, California Institute of Technology, Pasadena, CA, USA.
- Department of Molecular and Cellular Physiology, Stanford University School of Medicine, Stanford, CA, USA.
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Pleiner T, Hazu M, Pinton Tomaleri G, Nguyen VN, Januszyk K, Voorhees RM. A selectivity filter in the ER membrane protein complex limits protein misinsertion at the ER. J Cell Biol 2023; 222:e202212007. [PMID: 37199759 PMCID: PMC10200711 DOI: 10.1083/jcb.202212007] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/04/2022] [Revised: 03/31/2023] [Accepted: 04/20/2023] [Indexed: 05/19/2023] Open
Abstract
Tail-anchored (TA) proteins play essential roles in mammalian cells, and their accurate localization is critical for proteostasis. Biophysical similarities lead to mistargeting of mitochondrial TA proteins to the ER, where they are delivered to the insertase, the ER membrane protein complex (EMC). Leveraging an improved structural model of the human EMC, we used mutagenesis and site-specific crosslinking to map the path of a TA protein from its cytosolic capture by methionine-rich loops to its membrane insertion through a hydrophilic vestibule. Positively charged residues at the entrance to the vestibule function as a selectivity filter that uses charge-repulsion to reject mitochondrial TA proteins. Similarly, this selectivity filter retains the positively charged soluble domains of multipass substrates in the cytosol, thereby ensuring they adopt the correct topology and enforcing the "positive-inside" rule. Substrate discrimination by the EMC provides a biochemical explanation for one role of charge in TA protein sorting and protects compartment integrity by limiting protein misinsertion.
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Affiliation(s)
- Tino Pleiner
- Division of Biology and Biological Engineering, California Institute of Technology, Pasadena, CA, USA
| | - Masami Hazu
- Division of Biology and Biological Engineering, California Institute of Technology, Pasadena, CA, USA
| | - Giovani Pinton Tomaleri
- Division of Biology and Biological Engineering, California Institute of Technology, Pasadena, CA, USA
| | - Vy N. Nguyen
- Division of Biology and Biological Engineering, California Institute of Technology, Pasadena, CA, USA
| | - Kurt Januszyk
- Division of Biology and Biological Engineering, California Institute of Technology, Pasadena, CA, USA
| | - Rebecca M. Voorhees
- Division of Biology and Biological Engineering, California Institute of Technology, Pasadena, CA, USA
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Pleiner T, Hazu M, Tomaleri GP, Januszyk K, Oania RS, Sweredoski MJ, Moradian A, Guna A, Voorhees RM. WNK1 is an assembly factor for the human ER membrane protein complex. Mol Cell 2021; 81:2693-2704.e12. [PMID: 33964204 DOI: 10.1016/j.molcel.2021.04.013] [Citation(s) in RCA: 21] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/10/2020] [Revised: 03/02/2021] [Accepted: 04/14/2021] [Indexed: 12/13/2022]
Abstract
The assembly of nascent proteins into multi-subunit complexes is a tightly regulated process that must occur at high fidelity to maintain cellular homeostasis. The ER membrane protein complex (EMC) is an essential insertase that requires seven membrane-spanning and two soluble cytosolic subunits to function. Here, we show that the kinase with no lysine 1 (WNK1), known for its role in hypertension and neuropathy, functions as an assembly factor for the human EMC. WNK1 uses a conserved amphipathic helix to stabilize the soluble subunit, EMC2, by binding to the EMC2-8 interface. Shielding this hydrophobic surface prevents promiscuous interactions of unassembled EMC2 and directly competes for binding of E3 ubiquitin ligases, permitting assembly. Depletion of WNK1 thus destabilizes both the EMC and its membrane protein clients. This work describes an unexpected role for WNK1 in protein biogenesis and defines the general requirements of an assembly factor that will apply across the proteome.
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Affiliation(s)
- Tino Pleiner
- Division of Biology and Biological Engineering, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA
| | - Masami Hazu
- Division of Biology and Biological Engineering, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA
| | - Giovani Pinton Tomaleri
- Division of Biology and Biological Engineering, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA
| | - Kurt Januszyk
- Division of Biology and Biological Engineering, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA
| | - Robert S Oania
- Division of Biology and Biological Engineering, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA
| | - Michael J Sweredoski
- Division of Biology and Biological Engineering, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA
| | - Annie Moradian
- Division of Biology and Biological Engineering, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA
| | - Alina Guna
- Division of Biology and Biological Engineering, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA
| | - Rebecca M Voorhees
- Division of Biology and Biological Engineering, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA.
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Pleiner T, Tomaleri GP, Januszyk K, Inglis AJ, Hazu M, Voorhees RM. Structural basis for membrane insertion by the human ER membrane protein complex. Science 2020; 369:433-436. [PMID: 32439656 DOI: 10.1126/science.abb5008] [Citation(s) in RCA: 96] [Impact Index Per Article: 24.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/02/2020] [Accepted: 05/12/2020] [Indexed: 12/23/2022]
Abstract
A defining step in the biogenesis of a membrane protein is the insertion of its hydrophobic transmembrane helices into the lipid bilayer. The nine-subunit endoplasmic reticulum (ER) membrane protein complex (EMC) is a conserved co- and posttranslational insertase at the ER. We determined the structure of the human EMC in a lipid nanodisc to an overall resolution of 3.4 angstroms by cryo-electron microscopy, permitting building of a nearly complete atomic model. We used structure-guided mutagenesis to demonstrate that substrate insertion requires a methionine-rich cytosolic loop and occurs via an enclosed hydrophilic vestibule within the membrane formed by the subunits EMC3 and EMC6. We propose that the EMC uses local membrane thinning and a positively charged patch to decrease the energetic barrier for insertion into the bilayer.
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Affiliation(s)
- Tino Pleiner
- Division of Biology and Biological Engineering, California Institute of Technology, 1200 E. California Ave., Pasadena, CA 91125, USA
| | - Giovani Pinton Tomaleri
- Division of Biology and Biological Engineering, California Institute of Technology, 1200 E. California Ave., Pasadena, CA 91125, USA
| | - Kurt Januszyk
- Division of Biology and Biological Engineering, California Institute of Technology, 1200 E. California Ave., Pasadena, CA 91125, USA
| | - Alison J Inglis
- Division of Biology and Biological Engineering, California Institute of Technology, 1200 E. California Ave., Pasadena, CA 91125, USA
| | - Masami Hazu
- Division of Biology and Biological Engineering, California Institute of Technology, 1200 E. California Ave., Pasadena, CA 91125, USA
| | - Rebecca M Voorhees
- Division of Biology and Biological Engineering, California Institute of Technology, 1200 E. California Ave., Pasadena, CA 91125, USA.
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Aksu M, Pleiner T, Karaca S, Kappert C, Dehne HJ, Seibel K, Urlaub H, Bohnsack MT, Görlich D. Xpo7 is a broad-spectrum exportin and a nuclear import receptor. J Cell Biol 2018; 217:2329-2340. [PMID: 29748336 PMCID: PMC6028547 DOI: 10.1083/jcb.201712013] [Citation(s) in RCA: 29] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/04/2017] [Revised: 04/09/2018] [Accepted: 04/24/2018] [Indexed: 11/28/2022] Open
Abstract
Exportins bind cargo molecules in a RanGTP-dependent manner inside nuclei and transport them through nuclear pores to the cytoplasm. CRM1/Xpo1 is the best-characterized exportin because specific inhibitors such as leptomycin B allow straightforward cargo validations in vivo. The analysis of other exportins lagged far behind, foremost because no such inhibitors had been available for them. In this study, we explored the cargo spectrum of exportin 7/Xpo7 in depth and identified not only ∼200 potential export cargoes but also, surprisingly, ∼30 nuclear import substrates. Moreover, we developed anti-Xpo7 nanobodies that acutely block Xpo7 function when transfected into cultured cells. The inhibition is pathway specific, mislocalizes export cargoes of Xpo7 to the nucleus and import substrates to the cytoplasm, and allowed validation of numerous tested cargo candidates. This establishes Xpo7 as a broad-spectrum bidirectional transporter and paves the way for a much deeper analysis of exportin and importin function in the future.
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Affiliation(s)
- Metin Aksu
- Department of Cellular Logistics, Max Planck Institute for Biophysical Chemistry, Göttingen, Germany
| | - Tino Pleiner
- Department of Cellular Logistics, Max Planck Institute for Biophysical Chemistry, Göttingen, Germany
| | - Samir Karaca
- Bioanalytical Mass Spectrometry Group, Max Planck Institute for Biophysical Chemistry, Göttingen, Germany
| | - Christin Kappert
- Department of Cellular Logistics, Max Planck Institute for Biophysical Chemistry, Göttingen, Germany
| | - Heinz-Jürgen Dehne
- Department of Cellular Logistics, Max Planck Institute for Biophysical Chemistry, Göttingen, Germany
| | - Katharina Seibel
- Department of Cellular Logistics, Max Planck Institute for Biophysical Chemistry, Göttingen, Germany
| | - Henning Urlaub
- Bioanalytical Mass Spectrometry Group, Max Planck Institute for Biophysical Chemistry, Göttingen, Germany
- Institute for Clinical Chemistry, University Medical Center Göttingen, Göttingen, Germany
| | - Markus T Bohnsack
- Institute for Molecular Biology, University Medical Center Göttingen, Göttingen, Germany
| | - Dirk Görlich
- Department of Cellular Logistics, Max Planck Institute for Biophysical Chemistry, Göttingen, Germany
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Pleiner T, Bates M, Görlich D. A toolbox of anti-mouse and anti-rabbit IgG secondary nanobodies. J Cell Biol 2017; 217:1143-1154. [PMID: 29263082 PMCID: PMC5839796 DOI: 10.1083/jcb.201709115] [Citation(s) in RCA: 79] [Impact Index Per Article: 11.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/28/2017] [Revised: 11/29/2017] [Accepted: 12/07/2017] [Indexed: 01/09/2023] Open
Abstract
Pleiner, Bates, and Görlich introduce anti–mouse and anti–rabbit IgG nanobodies that can be produced in E. coli and fused to reporters or labeled fluorescently to create bright and specific detection reagents with unique advantages over conventional polyclonal secondary antibodies. Polyclonal anti–immunoglobulin G (anti-IgG) secondary antibodies are essential tools for many molecular biology techniques and diagnostic tests. Their animal-based production is, however, a major ethical problem. Here, we introduce a sustainable alternative, namely nanobodies against all mouse IgG subclasses and rabbit IgG. They can be produced at large scale in Escherichia coli and could thus make secondary antibody production in animals obsolete. Their recombinant nature allows fusion with affinity tags or reporter enzymes as well as efficient maleimide chemistry for fluorophore coupling. We demonstrate their superior performance in Western blotting, in both peroxidase- and fluorophore-linked form. Their site-specific labeling with multiple fluorophores creates bright imaging reagents for confocal and superresolution microscopy with much smaller label displacement than traditional secondary antibodies. They also enable simpler and faster immunostaining protocols, and allow multitarget localization with primary IgGs from the same species and of the same class.
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Affiliation(s)
- Tino Pleiner
- Department of Cellular Logistics, Max Planck Institute for Biophysical Chemistry, Göttingen, Germany
| | - Mark Bates
- Department of NanoBiophotonics, Max Planck Institute for Biophysical Chemistry, Göttingen, Germany
| | - Dirk Görlich
- Department of Cellular Logistics, Max Planck Institute for Biophysical Chemistry, Göttingen, Germany
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Pleiner T, Bates M, Trakhanov S, Lee CT, Schliep JE, Chug H, Böhning M, Stark H, Urlaub H, Görlich D. Correction: Nanobodies: site-specific labeling for super-resolution imaging, rapid epitope-mapping and native protein complex isolation. eLife 2016; 5. [PMID: 26982220 PMCID: PMC4805537 DOI: 10.7554/elife.15597] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/26/2016] [Accepted: 02/26/2016] [Indexed: 11/15/2022] Open
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Pleiner T, Bates M, Trakhanov S, Lee CT, Schliep JE, Chug H, Böhning M, Stark H, Urlaub H, Görlich D. Nanobodies: site-specific labeling for super-resolution imaging, rapid epitope-mapping and native protein complex isolation. eLife 2015; 4:e11349. [PMID: 26633879 PMCID: PMC4755751 DOI: 10.7554/elife.11349] [Citation(s) in RCA: 136] [Impact Index Per Article: 15.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/03/2015] [Accepted: 12/02/2015] [Indexed: 12/23/2022] Open
Abstract
Nanobodies are single-domain antibodies of camelid origin. We generated nanobodies against the vertebrate nuclear pore complex (NPC) and used them in STORM imaging to locate individual NPC proteins with <2 nm epitope-label displacement. For this, we introduced cysteines at specific positions in the nanobody sequence and labeled the resulting proteins with fluorophore-maleimides. As nanobodies are normally stabilized by disulfide-bonded cysteines, this appears counterintuitive. Yet, our analysis showed that this caused no folding problems. Compared to traditional NHS ester-labeling of lysines, the cysteine-maleimide strategy resulted in far less background in fluorescence imaging, it better preserved epitope recognition and it is site-specific. We also devised a rapid epitope-mapping strategy, which relies on crosslinking mass spectrometry and the introduced ectopic cysteines. Finally, we used different anti-nucleoporin nanobodies to purify the major NPC building blocks – each in a single step, with native elution and, as demonstrated, in excellent quality for structural analysis by electron microscopy. The presented strategies are applicable to any nanobody and nanobody-target.
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Affiliation(s)
- Tino Pleiner
- Department of Cellular Logistics, Max Planck Institute for Biophysical Chemistry, Göttingen, Germany
| | - Mark Bates
- Department of NanoBiophotonics, Max Planck Institute for Biophysical Chemistry, Göttingen, Germany
| | - Sergei Trakhanov
- Department of Cellular Logistics, Max Planck Institute for Biophysical Chemistry, Göttingen, Germany
| | - Chung-Tien Lee
- Bioanalytical Mass Spectrometry, Max Planck Institute for Biophysical Chemistry, Göttingen, Germany.,Bioanalytics, Institute for Clinical Chemistry, University Medical Center Göttingen, Göttingen, Germany
| | - Jan Erik Schliep
- 3D Electron Cryo-Microscopy Group, Max Planck Institute for Biophysical Chemistry, Göttingen, Germany
| | - Hema Chug
- Department of Cellular Logistics, Max Planck Institute for Biophysical Chemistry, Göttingen, Germany
| | - Marc Böhning
- Department of Cellular Logistics, Max Planck Institute for Biophysical Chemistry, Göttingen, Germany
| | - Holger Stark
- 3D Electron Cryo-Microscopy Group, Max Planck Institute for Biophysical Chemistry, Göttingen, Germany
| | - Henning Urlaub
- Bioanalytical Mass Spectrometry, Max Planck Institute for Biophysical Chemistry, Göttingen, Germany.,Bioanalytics, Institute for Clinical Chemistry, University Medical Center Göttingen, Göttingen, Germany
| | - Dirk Görlich
- Department of Cellular Logistics, Max Planck Institute for Biophysical Chemistry, Göttingen, Germany
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Chug H, Trakhanov S, Hülsmann BB, Pleiner T, Görlich D. Crystal structure of the metazoan Nup62•Nup58•Nup54 nucleoporin complex. Science 2015; 350:106-10. [DOI: 10.1126/science.aac7420] [Citation(s) in RCA: 87] [Impact Index Per Article: 9.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/05/2015] [Accepted: 08/06/2015] [Indexed: 02/02/2023]
Abstract
Nuclear pore complexes (NPCs) conduct nucleocytoplasmic transport and gain transport selectivity through nucleoporin FG domains. Here, we report a structural analysis of the FG Nup62•58•54 complex, which is a crucial component of the transport system. It comprises a ≈13 nanometer-long trimerization interface with an unusual 2W3F coil, a canonical heterotrimeric coiled coil, and a kink that enforces a compact six-helix bundle. Nup54 also contains a ferredoxin-like domain. We further identified a heterotrimeric Nup93-binding module for NPC anchorage. The quaternary structure alternations in the Nup62 complex, which were previously proposed to trigger a general gating of the NPC, are incompatible with the trimer structure. We suggest that the highly elongated Nup62 complex projects barrier-forming FG repeats far into the central NPC channel, supporting a barrier that guards the entire cross section.
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