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Schirmeisen K, Naiman K, Fréon K, Besse L, Chakraborty S, Saada AA, Carr AM, Kramarz K, Lambert SAE. SUMO protease and proteasome recruitment at the nuclear periphery differently affect replication dynamics at arrested forks. Nucleic Acids Res 2024; 52:8286-8302. [PMID: 38917328 DOI: 10.1093/nar/gkae526] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/28/2023] [Revised: 05/02/2024] [Accepted: 06/10/2024] [Indexed: 06/27/2024] Open
Abstract
Nuclear pore complexes (NPCs) have emerged as genome organizers, defining a particular nuclear compartment enriched for SUMO protease and proteasome activities, and act as docking sites for the repair of DNA damage. In fission yeast, the anchorage of perturbed replication forks to NPCs is an integral part of the recombination-dependent replication restart mechanism (RDR) that resumes DNA synthesis at terminally dysfunctional forks. By mapping DNA polymerase usage, we report that SUMO protease Ulp1-associated NPCs ensure efficient initiation of restarted DNA synthesis, whereas proteasome-associated NPCs sustain the progression of restarted DNA polymerase. In contrast to Ulp1-dependent events, this last function is not alleviated by preventing SUMO chain formation. By analyzing the role of the nuclear basket, the nucleoplasmic extension of the NPC, we reveal that the activities of Ulp1 and the proteasome cannot compensate for each other and affect the dynamics of RDR in distinct ways. Our work probes two distinct mechanisms by which the NPC environment ensures optimal RDR, both controlled by different NPC components.
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Affiliation(s)
- Kamila Schirmeisen
- Institut Curie, Université PSL, CNRS UMR3348, 91400 Orsay, France
- Université Paris-Saclay, CNRS UMR3348, 91400 Orsay, France
| | - Karel Naiman
- INSERM U1068, CNRS UMR7258, Aix Marseille Univ U105, Institut Paoli-Calmettes, CRCM, Marseille, France
- Genome Damage and Stability Centre, School of Life Sciences, University of Sussex, Falmer BN1 9RQ, UK
| | - Karine Fréon
- Institut Curie, Université PSL, CNRS UMR3348, 91400 Orsay, France
- Université Paris-Saclay, CNRS UMR3348, 91400 Orsay, France
| | - Laetitia Besse
- Institut Curie, Université PSL, CNRS UAR2016, Inserm US43, Université Paris-Saclay, Multimodal Imaging Center, 91400 Orsay, France
| | - Shrena Chakraborty
- Institut Curie, Université PSL, CNRS UMR3348, 91400 Orsay, France
- Université Paris-Saclay, CNRS UMR3348, 91400 Orsay, France
| | - Anissia Ait Saada
- Institut Curie, Université PSL, CNRS UMR3348, 91400 Orsay, France
- Université Paris-Saclay, CNRS UMR3348, 91400 Orsay, France
| | - Antony M Carr
- Genome Damage and Stability Centre, School of Life Sciences, University of Sussex, Falmer BN1 9RQ, UK
| | - Karol Kramarz
- Academic Excellence Hub - Research Centre for DNA Repair and Replication, Faculty of Biological Sciences, University of Wroclaw, 50-328 Wroclaw, Poland
| | - Sarah A E Lambert
- Institut Curie, Université PSL, CNRS UMR3348, 91400 Orsay, France
- Université Paris-Saclay, CNRS UMR3348, 91400 Orsay, France
- Equipe Labellisée Ligue Nationale Contre le cancer, France
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2
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Deng X, Yao Q, Horvath A, Jiang Z, Zhao J, Fischer T, Sugiyama T. The fission yeast ortholog of Coilin, Mug174, forms Cajal body-like nuclear condensates and is essential for cellular quiescence. Nucleic Acids Res 2024:gkae463. [PMID: 38828770 DOI: 10.1093/nar/gkae463] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/02/2023] [Revised: 05/08/2024] [Accepted: 05/16/2024] [Indexed: 06/05/2024] Open
Abstract
The Cajal body, a nuclear condensate, is crucial for ribonucleoprotein assembly, including small nuclear RNPs (snRNPs). While Coilin has been identified as an integral component of Cajal bodies, its exact function remains unclear. Moreover, no Coilin ortholog has been found in unicellular organisms to date. This study unveils Mug174 (Meiosis-upregulated gene 174) as the Coilin ortholog in the fission yeast Schizosaccharomyces pombe. Mug174 forms phase-separated condensates in vitro and is often associated with the nucleolus and the cleavage body in vivo. The generation of Mug174 foci relies on the trimethylguanosine (TMG) synthase Tgs1. Moreover, Mug174 interacts with Tgs1 and U snRNAs. Deletion of the mug174+ gene in S. pombe causes diverse pleiotropic phenotypes, encompassing defects in vegetative growth, meiosis, pre-mRNA splicing, TMG capping of U snRNAs, and chromosome segregation. In addition, we identified weak homology between Mug174 and human Coilin. Notably, human Coilin expressed in fission yeast colocalizes with Mug174. Critically, Mug174 is indispensable for the maintenance of and transition from cellular quiescence. These findings highlight the Coilin ortholog in fission yeast and suggest that the Cajal body is implicated in cellular quiescence, thereby preventing human diseases.
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Affiliation(s)
- Xiaoling Deng
- School of Life Science and Technology, ShanghaiTech University, Shanghai 201210, China
| | - Qinglian Yao
- School of Life Science and Technology, ShanghaiTech University, Shanghai 201210, China
| | - Attila Horvath
- The John Curtin School of Medical Research, The Australian National University, Canberra 2601, Australia
| | - Ziling Jiang
- School of Life Science and Technology, ShanghaiTech University, Shanghai 201210, China
| | - Junjie Zhao
- School of Life Science and Technology, ShanghaiTech University, Shanghai 201210, China
| | - Tamás Fischer
- The John Curtin School of Medical Research, The Australian National University, Canberra 2601, Australia
| | - Tomoyasu Sugiyama
- School of Life Science and Technology, ShanghaiTech University, Shanghai 201210, China
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3
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Huang P, Zhang X, Cheng Z, Wang X, Miao Y, Huang G, Fu YF, Feng X. The nuclear pore Y-complex functions as a platform for transcriptional regulation of FLOWERING LOCUS C in Arabidopsis. THE PLANT CELL 2024; 36:346-366. [PMID: 37877462 PMCID: PMC10827314 DOI: 10.1093/plcell/koad271] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/17/2023] [Revised: 09/22/2023] [Accepted: 09/27/2023] [Indexed: 10/26/2023]
Abstract
The nuclear pore complex (NPC) has multiple functions beyond the nucleo-cytoplasmic transport of large molecules. Subnuclear compartmentalization of chromatin is critical for gene expression in animals and yeast. However, the mechanism by which the NPC regulates gene expression is poorly understood in plants. Here we report that the Y-complex (Nup107-160 complex, a subcomplex of the NPC) self-maintains its nucleoporin homeostasis and modulates FLOWERING LOCUS C (FLC) transcription via changing histone modifications at this locus. We show that Y-complex nucleoporins are intimately associated with FLC chromatin through their interactions with histone H2A at the nuclear membrane. Fluorescence in situ hybridization assays revealed that Nup96, a Y-complex nucleoporin, enhances FLC positioning at the nuclear periphery. Nup96 interacted with HISTONE DEACETYLASE 6 (HDA6), a key repressor of FLC expression via histone modification, at the nuclear membrane to attenuate HDA6-catalyzed deposition at the FLC locus and change histone modifications. Moreover, we demonstrate that Y-complex nucleoporins interact with RNA polymerase II to increase its occupancy at the FLC locus, facilitating transcription. Collectively, our findings identify an attractive mechanism for the Y-complex in regulating FLC expression via tethering the locus at the nuclear periphery and altering its histone modification.
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Affiliation(s)
- Penghui Huang
- Zhejiang Lab, Research Institute of Intelligent Computing, Hangzhou 310012, China
- MARA Key Laboratory of Soybean Biology (Beijing), State Key Laboratory of Crop Gene Resources and Breeding, Institute of Crop Sciences, Chinese Academy of Agricultural Sciences, Beijing 100081, China
| | - Xiaomei Zhang
- MARA Key Laboratory of Soybean Biology (Beijing), State Key Laboratory of Crop Gene Resources and Breeding, Institute of Crop Sciences, Chinese Academy of Agricultural Sciences, Beijing 100081, China
| | - Zhiyuan Cheng
- CAS Key Laboratory of Soybean Molecular Design Breeding, Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Changchun 130102, China
| | - Xu Wang
- Peking University Institute of Advanced Agricultural Sciences, Shandong Laboratory of Advanced Agricultural Sciences at Weifang, Weifang, Shandong 261325, China
| | - Yuchen Miao
- State Key Laboratory of Crop Stress Adaptation and Improvement, School of Life Sciences, Henan University, Kaifeng 475004, China
| | - Guowen Huang
- Department of Biological Sciences and Chemical Engineering, Hunan University of Science and Engineering, Yongzhou 425100, Hunan, China
| | - Yong-Fu Fu
- MARA Key Laboratory of Soybean Biology (Beijing), State Key Laboratory of Crop Gene Resources and Breeding, Institute of Crop Sciences, Chinese Academy of Agricultural Sciences, Beijing 100081, China
| | - Xianzhong Feng
- Zhejiang Lab, Research Institute of Intelligent Computing, Hangzhou 310012, China
- CAS Key Laboratory of Soybean Molecular Design Breeding, Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Changchun 130102, China
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4
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The Nuclear Pore Complex: Birth, Life, and Death of a Cellular Behemoth. Cells 2022; 11:cells11091456. [PMID: 35563762 PMCID: PMC9100368 DOI: 10.3390/cells11091456] [Citation(s) in RCA: 29] [Impact Index Per Article: 14.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/31/2022] [Revised: 04/22/2022] [Accepted: 04/23/2022] [Indexed: 02/01/2023] Open
Abstract
Nuclear pore complexes (NPCs) are the only transport channels that cross the nuclear envelope. Constructed from ~500–1000 nucleoporin proteins each, they are among the largest macromolecular assemblies in eukaryotic cells. Thanks to advances in structural analysis approaches, the construction principles and architecture of the NPC have recently been revealed at submolecular resolution. Although the overall structure and inventory of nucleoporins are conserved, NPCs exhibit significant compositional and functional plasticity even within single cells and surprising variability in their assembly pathways. Once assembled, NPCs remain seemingly unexchangeable in post-mitotic cells. There are a number of as yet unresolved questions about how the versatility of NPC assembly and composition is established, how cells monitor the functional state of NPCs or how they could be renewed. Here, we review current progress in our understanding of the key aspects of NPC architecture and lifecycle.
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Varberg JM, Unruh JR, Bestul AJ, Khan AA, Jaspersen SL. Quantitative analysis of nuclear pore complex organization in Schizosaccharomyces pombe. Life Sci Alliance 2022; 5:5/7/e202201423. [PMID: 35354597 PMCID: PMC8967992 DOI: 10.26508/lsa.202201423] [Citation(s) in RCA: 10] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/23/2022] [Revised: 03/11/2022] [Accepted: 03/14/2022] [Indexed: 01/06/2023] Open
Abstract
Characterization of nuclear pores in Schizosaccharomyces pombe identifies regions of heterogeneous NPC density and composition and shows that NPCs are excluded near the spindle pole body by Lem2-mediated centromere tethering. The number, distribution, and composition of nuclear pore complexes (NPCs) in the nuclear envelope varies between cell types and changes during cellular differentiation and in disease. To understand how NPC density and organization are controlled, we analyzed the NPC number and distribution in the fission yeast Schizosaccharomyces pombe using structured illumination microscopy. The small size of yeast nuclei, genetic features of fungi, and our robust image analysis pipeline allowed us to study NPCs in intact nuclei under multiple conditions. Our data revealed that NPC density is maintained across a wide range of nuclear sizes. Regions of reduced NPC density are observed over the nucleolus and surrounding the spindle pole body (SPB). Lem2-mediated tethering of the centromeres to the SPB is required to maintain NPC exclusion near SPBs. These findings provide a quantitative understanding of NPC number and distribution in S. pombe and show that interactions between the centromere and the nuclear envelope influences local NPC distribution.
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Affiliation(s)
| | - Jay R Unruh
- Stowers Institute for Medical Research, Kansas City, MO, USA
| | - Andrew J Bestul
- Stowers Institute for Medical Research, Kansas City, MO, USA
| | - Azqa A Khan
- Stowers Institute for Medical Research, Kansas City, MO, USA
| | - Sue L Jaspersen
- Stowers Institute for Medical Research, Kansas City, MO, USA .,Department of Molecular and Integrative Physiology, University of Kansas Medical Center, Kansas City, KS, USA
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Gallardo P, Real-Calderón P, Flor-Parra I, Salas-Pino S, Daga RR. Acute Heat Stress Leads to Reversible Aggregation of Nuclear Proteins into Nucleolar Rings in Fission Yeast. Cell Rep 2020; 33:108377. [DOI: 10.1016/j.celrep.2020.108377] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/08/2020] [Revised: 08/24/2020] [Accepted: 10/21/2020] [Indexed: 12/15/2022] Open
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7
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Expósito-Serrano M, Sánchez-Molina A, Gallardo P, Salas-Pino S, Daga RR. Selective Nuclear Pore Complex Removal Drives Nuclear Envelope Division in Fission Yeast. Curr Biol 2020; 30:3212-3222.e2. [DOI: 10.1016/j.cub.2020.05.066] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/02/2020] [Revised: 04/24/2020] [Accepted: 05/20/2020] [Indexed: 01/09/2023]
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8
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Álvarez-Herrera C, Maisanaba S, Repetto G. Investigation of mechanisms of toxicity and exclusion by transporters of the preservatives triclosan and propylparaben using batteries of Schizosaccharomyces pombe strains. ENVIRONMENTAL RESEARCH 2020; 183:108983. [PMID: 31810592 DOI: 10.1016/j.envres.2019.108983] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/23/2019] [Revised: 11/27/2019] [Accepted: 11/29/2019] [Indexed: 06/10/2023]
Abstract
Triclosan (TCS) and propylparaben (PPB) are antimicrobials widely used. They present many similarities in their applications and also in their human and environmental health risks. In order to investigate the mechanisms of toxic action and the efflux pumps involved in their detoxication, we used a strategy with batteries of Schizosaccharomyces pombe yeast strains, either defective in cell signalling, in detoxification pumps, or in cell surveillance mechanisms. Yeast were exposed up to 20 h in solid medium or in liquid medium in 96-well plates. The mechanisms of action investigated were spindle defects (mph1), stress (pmk1), DNA interference (rad3) or diverse effects (MDR-sup). The efflux pumps investigated were Bfr1, Pmd1, Mfs1 and Caf5 or the Pap1 transcription factor. Here we show that TCS was 75 times more toxic than PPB in the wild type fission yeast. More oxidative stress and less protection by exclusion pumps were observed for TCS than for PPB. The cytotoxicity produced by TCS decreased from bfr1>mfs1>pmd1 > pap1 and caf5A deficient strains. In contrast, cytotoxic concentrations of PPB caused only a mild stress. The protection provided for PPB by the transporters was more marked than for TCS, decreasing from Pmd1, Caf5, Mfs1 and Bfr1. Furthermore, microtubule and DNA interferences were revealed for PPB, according to the cytotoxicity of mph1 and rad3 defective cells, respectively. As both compounds present complex adverse effects at concentrations close to exposure, and their combination clearly causes a strong potentiation, more exhaustive controls and regulations in their use should be considered.
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Affiliation(s)
| | - Sara Maisanaba
- Area of Toxicology, Universidad Pablo de Olavide, 41013, Sevilla, Spain.
| | - Guillermo Repetto
- Area of Toxicology, Universidad Pablo de Olavide, 41013, Sevilla, Spain
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9
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Chik JK, Moiseeva V, Goel PK, Meinen BA, Koldewey P, An S, Mellone BG, Subramanian L, Cho US. Structures of CENP-C cupin domains at regional centromeres reveal unique patterns of dimerization and recruitment functions for the inner pocket. J Biol Chem 2019; 294:14119-14134. [PMID: 31366733 DOI: 10.1074/jbc.ra119.008464] [Citation(s) in RCA: 13] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/15/2019] [Revised: 07/26/2019] [Indexed: 01/05/2023] Open
Abstract
The successful assembly and regulation of the kinetochore are critical for the equal and accurate segregation of genetic material during the cell cycle. CENP-C (centromere protein C), a conserved inner kinetochore component, has been broadly characterized as a scaffolding protein and is required for the recruitment of multiple kinetochore proteins to the centromere. At its C terminus, CENP-C harbors a conserved cupin domain that has an established role in protein dimerization. Although the crystal structure of the Saccharomyces cerevisiae Mif2CENP-C cupin domain has been determined, centromeric organization and kinetochore composition vary greatly between S. cerevisiae (point centromere) and other eukaryotes (regional centromere). Therefore, whether the structural and functional role of the cupin domain is conserved throughout evolution requires investigation. Here, we report the crystal structures of the Schizosaccharomyces pombe and Drosophila melanogaster CENP-C cupin domains at 2.52 and 1.81 Å resolutions, respectively. Although the central jelly roll architecture is conserved among the three determined CENP-C cupin domain structures, the cupin domains from organisms with regional centromeres contain additional structural features that aid in dimerization. Moreover, we found that the S. pombe Cnp3CENP-C jelly roll fold harbors an inner binding pocket that is used to recruit the meiosis-specific protein Moa1. In summary, our results unveil the evolutionarily conserved and unique features of the CENP-C cupin domain and uncover the mechanism by which it functions as a recruitment factor.
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Affiliation(s)
- Jennifer K Chik
- Department of Biological Chemistry, University of Michigan Medical School, Ann Arbor, Michigan 48109.,Program in Cellular and Molecular Biology, University of Michigan Medical School, Ann Arbor, Michigan 48109
| | - Vera Moiseeva
- School of Biological and Chemical Sciences, Queen Mary University of London, London E1 4NS, United Kingdom
| | - Pavitra K Goel
- School of Biological and Chemical Sciences, Queen Mary University of London, London E1 4NS, United Kingdom
| | - Ben A Meinen
- Department of Molecular, Cellular, and Developmental Biology, University of Michigan, Ann Arbor, Michigan 48109
| | - Philipp Koldewey
- Department of Molecular, Cellular, and Developmental Biology, University of Michigan, Ann Arbor, Michigan 48109
| | - Sojin An
- Department of Biological Chemistry, University of Michigan Medical School, Ann Arbor, Michigan 48109
| | - Barbara G Mellone
- Institute for Systems Genomics, University of Connecticut, Storrs, Connecticut 06269
| | - Lakxmi Subramanian
- School of Biological and Chemical Sciences, Queen Mary University of London, London E1 4NS, United Kingdom
| | - Uhn-Soo Cho
- Department of Biological Chemistry, University of Michigan Medical School, Ann Arbor, Michigan 48109
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10
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Asymmetrical localization of Nup107-160 subcomplex components within the nuclear pore complex in fission yeast. PLoS Genet 2019; 15:e1008061. [PMID: 31170156 PMCID: PMC6553703 DOI: 10.1371/journal.pgen.1008061] [Citation(s) in RCA: 20] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/19/2018] [Accepted: 03/01/2019] [Indexed: 01/05/2023] Open
Abstract
The nuclear pore complex (NPC) forms a gateway for nucleocytoplasmic transport. The outer ring protein complex of the NPC (the Nup107-160 subcomplex in humans) is a key component for building the NPC. Nup107-160 subcomplexes are believed to be symmetrically localized on the nuclear and cytoplasmic sides of the NPC. However, in S. pombe immunoelectron and fluorescence microscopic analyses revealed that the homologous components of the human Nup107-160 subcomplex had an asymmetrical localization: constituent proteins spNup132 and spNup107 were present only on the nuclear side (designated the spNup132 subcomplex), while spNup131, spNup120, spNup85, spNup96, spNup37, spEly5 and spSeh1 were localized only on the cytoplasmic side (designated the spNup120 subcomplex), suggesting the complex was split into two pieces at the interface between spNup96 and spNup107. This contrasts with the symmetrical localization reported in other organisms. Fusion of spNup96 (cytoplasmic localization) with spNup107 (nuclear localization) caused cytoplasmic relocalization of spNup107. In this strain, half of the spNup132 proteins, which interact with spNup107, changed their localization to the cytoplasmic side of the NPC, leading to defects in mitotic and meiotic progression similar to an spNup132 deletion strain. These observations suggest the asymmetrical localization of the outer ring spNup132 and spNup120 subcomplexes of the NPC is necessary for normal cell cycle progression in fission yeast. The nuclear pore complexes (NPCs) form gateways to transport intracellular molecules between the nucleus and the cytoplasm across the nuclear envelope. The Nup107-160 subcomplex, that forms nuclear and cytoplasmic outer rings, is a key complex responsible for building the NPC by symmetrical localization on the nuclear and cytoplasmic sides of the nuclear pore. This structural characteristic was found in various organisms including humans and budding yeasts, and therefore believed to be common among “all” eukaryotes. However, in this paper, we revealed an asymmetrical localization of the homologous components of the human Nup107-160 subcomplex in fission yeast by immunoelectron and fluorescence microscopic analyses: in this organism, the Nup107-160 subcomplex is split into two pieces, and each of the split pieces is differentially distributed to the nuclear and cytoplasmic side of the NPC: one piece is only in the nuclear side while the other piece is only in the cytoplasmic side. This contrasts with the symmetrical localization reported in other organisms. In addition, we confirmed that the asymmetrical configuration of the outer ring structure is necessary for cell cycle progression in fission yeast. This study provides notions of diverse structures and functions of NPCs evolved in eukaryotes.
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Abstract
This study compares the native structures of cytosolic and nuclear proteasomes, visualized directly within cells. The assembly states and functional states of proteasomes in each compartment were similar, indicating comparable levels of proteolytic activity per proteasome. Nuclear proteasomes were tethered to two different sites at the nuclear pore complex (NPC): the inner nuclear membrane and the NPC basket. Structural analysis revealed mechanistic details of the two tethering interactions. These results present direct evidence that proteasomes bind at NPCs, establishing a cellular hub for protein degradation at the gateway between the nucleus and cytoplasm. This work demonstrates how cryo-electron tomography can reveal biological mechanisms by directly observing the interactions between molecular complexes within the native cellular environment. The partitioning of cellular components between the nucleus and cytoplasm is the defining feature of eukaryotic life. The nuclear pore complex (NPC) selectively gates the transport of macromolecules between these compartments, but it is unknown whether surveillance mechanisms exist to reinforce this function. By leveraging in situ cryo-electron tomography to image the native cellular environment of Chlamydomonas reinhardtii, we observed that nuclear 26S proteasomes crowd around NPCs. Through a combination of subtomogram averaging and nanometer-precision localization, we identified two classes of proteasomes tethered via their Rpn9 subunits to two specific NPC locations: binding sites on the NPC basket that reflect its eightfold symmetry and more abundant binding sites at the inner nuclear membrane that encircle the NPC. These basket-tethered and membrane-tethered proteasomes, which have similar substrate-processing state frequencies as proteasomes elsewhere in the cell, are ideally positioned to regulate transcription and perform quality control of both soluble and membrane proteins transiting the NPC.
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