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Zhu Z, Becam I, Tovey CA, Elfarkouchi A, Yen EC, Bernard F, Guichet A, Conduit PT. Multifaceted modes of γ-tubulin complex recruitment and microtubule nucleation at mitotic centrosomes. J Cell Biol 2023; 222:e202212043. [PMID: 37698931 PMCID: PMC10497398 DOI: 10.1083/jcb.202212043] [Citation(s) in RCA: 2] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/09/2022] [Revised: 05/18/2023] [Accepted: 07/24/2023] [Indexed: 09/14/2023] Open
Abstract
Microtubule nucleation is mediated by γ-tubulin ring complexes (γ-TuRCs). In most eukaryotes, a GCP4/5/4/6 "core" complex promotes γ-tubulin small complex (γ-TuSC) association to generate cytosolic γ-TuRCs. Unlike γ-TuSCs, however, this core complex is non-essential in various species and absent from budding yeasts. In Drosophila, Spindle defective-2 (Spd-2) and Centrosomin (Cnn) redundantly recruit γ-tubulin complexes to mitotic centrosomes. Here, we show that Spd-2 recruits γ-TuRCs formed via the GCP4/5/4/6 core, but Cnn can recruit γ-TuSCs directly via its well-conserved CM1 domain, similar to its homologs in budding yeast. When centrosomes fail to recruit γ-tubulin complexes, they still nucleate microtubules via the TOG domain protein Mini-spindles (Msps), but these microtubules have different dynamic properties. Our data, therefore, help explain the dispensability of the GCP4/5/4/6 core and highlight the robustness of centrosomes as microtubule organizing centers. They also suggest that the dynamic properties of microtubules are influenced by how they are nucleated.
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Affiliation(s)
- Zihan Zhu
- Department of Zoology, University of Cambridge, Cambridge, UK
| | - Isabelle Becam
- Université Paris Cité, CNRS, Institut Jacques Monod, Paris, France
| | - Corinne A. Tovey
- Université Paris Cité, CNRS, Institut Jacques Monod, Paris, France
| | - Abir Elfarkouchi
- Université Paris Cité, CNRS, Institut Jacques Monod, Paris, France
| | - Eugenie C. Yen
- Department of Zoology, University of Cambridge, Cambridge, UK
| | - Fred Bernard
- Université Paris Cité, CNRS, Institut Jacques Monod, Paris, France
| | - Antoine Guichet
- Université Paris Cité, CNRS, Institut Jacques Monod, Paris, France
| | - Paul T. Conduit
- Department of Zoology, University of Cambridge, Cambridge, UK
- Université Paris Cité, CNRS, Institut Jacques Monod, Paris, France
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2
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Abstract
As one of four filament types, microtubules are a core component of the cytoskeleton and are essential for cell function. Yet how microtubules are nucleated from their building blocks, the αβ-tubulin heterodimer, has remained a fundamental open question since the discovery of tubulin 50 years ago. Recent structural studies have shed light on how γ-tubulin and the γ-tubulin complex proteins (GCPs) GCP2 to GCP6 form the γ-tubulin ring complex (γ-TuRC). In parallel, functional and single-molecule studies have informed on how the γ-TuRC nucleates microtubules in real time, how this process is regulated in the cell and how it compares to other modes of nucleation. Another recent surprise has been the identification of a second essential nucleation factor, which turns out to be the well-characterized microtubule polymerase XMAP215 (also known as CKAP5, a homolog of chTOG, Stu2 and Alp14). This discovery helps to explain why the observed nucleation activity of the γ-TuRC in vitro is relatively low. Taken together, research in recent years has afforded important insight into how microtubules are made in the cell and provides a basis for an exciting era in the cytoskeleton field.
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Affiliation(s)
- Akanksha Thawani
- California Institute for Quantitative Biosciences (QB3), University of California, Berkeley, Berkeley, CA 94720, USA
| | - Sabine Petry
- Department of Molecular Biology, Princeton University, Princeton, NJ 08544, USA
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3
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Thawani A, Rale MJ, Coudray N, Bhabha G, Stone HA, Shaevitz JW, Petry S. The transition state and regulation of γ-TuRC-mediated microtubule nucleation revealed by single molecule microscopy. eLife 2020; 9:e54253. [PMID: 32538784 PMCID: PMC7338055 DOI: 10.7554/elife.54253] [Citation(s) in RCA: 34] [Impact Index Per Article: 8.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/07/2019] [Accepted: 06/15/2020] [Indexed: 12/31/2022] Open
Abstract
Determining how microtubules (MTs) are nucleated is essential for understanding how the cytoskeleton assembles. While the MT nucleator, γ-tubulin ring complex (γ-TuRC) has been identified, precisely how γ-TuRC nucleates a MT remains poorly understood. Here, we developed a single molecule assay to directly visualize nucleation of a MT from purified Xenopus laevis γ-TuRC. We reveal a high γ-/αβ-tubulin affinity, which facilitates assembly of a MT from γ-TuRC. Whereas spontaneous nucleation requires assembly of 8 αβ-tubulins, nucleation from γ-TuRC occurs efficiently with a cooperativity of 4 αβ-tubulin dimers. This is distinct from pre-assembled MT seeds, where a single dimer is sufficient to initiate growth. A computational model predicts our kinetic measurements and reveals the rate-limiting transition where laterally associated αβ-tubulins drive γ-TuRC into a closed conformation. NME7, TPX2, and the putative activation domain of CDK5RAP2 h γ-TuRC-mediated nucleation, while XMAP215 drastically increases the nucleation efficiency by strengthening the longitudinal γ-/αβ-tubulin interaction.
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Affiliation(s)
- Akanksha Thawani
- Department of Chemical and Biological Engineering, Princeton UniversityPrincetonUnited States
| | - Michael J Rale
- Department of Molecular Biology, Princeton UniversityPrincetonUnited States
| | - Nicolas Coudray
- Department of Cell Biology, New York University School of MedicineNew YorkUnited States
| | - Gira Bhabha
- Department of Cell Biology, New York University School of MedicineNew YorkUnited States
| | - Howard A Stone
- Department of Mechanical and Aerospace Engineering, Princeton UniversityPrincetonUnited States
| | - Joshua W Shaevitz
- Lewis-Sigler Institute for Integrative GenomicsPrincetonUnited States
- Department of Physics, Princeton UniversityPrincetonUnited States
| | - Sabine Petry
- Department of Molecular Biology, Princeton UniversityPrincetonUnited States
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4
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Verma V, Maresca TJ. Direct observation of branching MT nucleation in living animal cells. J Cell Biol 2019; 218:2829-2840. [PMID: 31340987 PMCID: PMC6719462 DOI: 10.1083/jcb.201904114] [Citation(s) in RCA: 23] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/18/2019] [Revised: 06/07/2019] [Accepted: 07/01/2019] [Indexed: 02/07/2023] Open
Abstract
Branching microtubule nucleation by its molecular mediators has never been directly observed in animal cells. By imaging augmin, γ-TuRC, and microtubules with high spatiotemporal resolution, Verma and Maresca quantitatively define the sequential steps of augmin-mediated branching microtubule nucleation in dividing Drosophila cells. Centrosome-mediated microtubule (MT) nucleation has been well characterized; however, numerous noncentrosomal MT nucleation mechanisms exist. The branching MT nucleation pathway envisages that the γ-tubulin ring complex (γ-TuRC) is recruited to MTs by the augmin complex to initiate nucleation of new MTs. While the pathway is well conserved at a molecular and functional level, branching MT nucleation by core constituents has never been directly observed in animal cells. Here, multicolor TIRF microscopy was applied to visualize and quantitatively define the entire process of branching MT nucleation in dividing Drosophila cells during anaphase. The steps of a stereotypical branching nucleation event entailed augmin binding to a mother MT and recruitment of γ-TuRC after 15 s, followed by nucleation 16 s later of a daughter MT at a 36° branch angle. Daughters typically remained attached throughout their ∼40-s lifetime unless the mother depolymerized past the branch point. Assembly of branched MT arrays, which did not require Drosophila TPX2, enhanced localized RhoA activation during cytokinesis.
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Affiliation(s)
- Vikash Verma
- Biology Department, University of Massachusetts, Amherst, MA
| | - Thomas J Maresca
- Biology Department, University of Massachusetts, Amherst, MA .,Molecular and Cellular Biology Graduate Program, University of Massachusetts, Amherst, MA
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5
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The XMAP215 Ortholog Alp14 Promotes Microtubule Nucleation in Fission Yeast. Curr Biol 2018; 28:1681-1691.e4. [PMID: 29779879 DOI: 10.1016/j.cub.2018.04.008] [Citation(s) in RCA: 56] [Impact Index Per Article: 9.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/28/2017] [Revised: 01/19/2018] [Accepted: 04/03/2018] [Indexed: 12/18/2022]
Abstract
The organization and number of microtubules (MTs) in a cell depend on the proper regulation of MT nucleation. Currently, the mechanism of nucleation is the most poorly understood aspect of MT dynamics. XMAP215/chTOG/Alp14/Stu2 proteins are MT polymerases that stimulate MT polymerization at MT plus ends by binding and releasing tubulin dimers. Although these proteins also localize to MT organizing centers and have nucleating activity in vitro, it is not yet clear whether these proteins participate in MT nucleation in vivo. Here, we demonstrate that in the fission yeast Schizosaccharomyces pombe, the XMAP215 ortholog Alp14 is critical for efficient MT nucleation in vivo. In multiple assays, loss of Alp14 function led to reduced nucleation rate and numbers of interphase MT bundles. Conversely, activation of Alp14 led to increased nucleation frequency. Alp14 associated with Mto1 and γ-tubulin complex components, and artificially targeting Alp14 to the γ-tubulin ring complexes (γ-TuRCs) stimulated nucleation. In imaging individual nucleation events, we found that Alp14 transiently associated with a γ-tubulin particle shortly before the appearance of a new MT. The transforming acidic coiled-coil (TACC) ortholog Alp7 mediated the localization of Alp14 at nucleation sites but not plus ends, and was required for efficient nucleation but not for MT polymerization. Our findings provide the strongest evidence to date that Alp14 serves as a critical MT nucleation factor in vivo. We suggest a model in which Alp14 associates with the γ-tubulin complex in an Alp7-dependent manner to facilitate the assembly or stabilization of the nascent MT.
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6
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Thawani A, Kadzik RS, Petry S. XMAP215 is a microtubule nucleation factor that functions synergistically with the γ-tubulin ring complex. Nat Cell Biol 2018; 20:575-585. [PMID: 29695792 PMCID: PMC5926803 DOI: 10.1038/s41556-018-0091-6] [Citation(s) in RCA: 103] [Impact Index Per Article: 17.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/16/2017] [Accepted: 03/20/2018] [Indexed: 12/16/2022]
Abstract
How microtubules (MTs) are generated in the cell is a major question in understanding how the cytoskeleton is assembled. For several decades, γ-tubulin has been accepted as the universal MT nucleator of the cell. Although there is evidence that γ-tubulin complexes are not the sole MT nucleators, identification of other nucleation factors has proven difficult. Here, we report that the well-characterized MT polymerase XMAP215 (chTOG/Msps/Stu2p/Alp14/Dis1 homologue) is essential for MT nucleation in Xenopus egg extracts. The concentration of XMAP215 determines the extent of MT nucleation. Even though XMAP215 and the γ-tubulin ring complex (γ-TuRC) possess minimal nucleation activity individually, together, these factors synergistically stimulate MT nucleation in vitro. The amino-terminal TOG domains 1-5 of XMAP215 bind to αβ-tubulin and promote MT polymerization, whereas the conserved carboxy terminus is required for efficient MT nucleation and directly binds to γ-tubulin. In summary, XMAP215 and γ-TuRC together function as the principal nucleation module that generates MTs in cells.
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Affiliation(s)
- Akanksha Thawani
- Department of Chemical and Biological Engineering, Princeton University, Princeton, NJ, USA
| | - Rachel S Kadzik
- Department of Molecular Biology, Princeton University, Princeton, NJ, USA
| | - Sabine Petry
- Department of Molecular Biology, Princeton University, Princeton, NJ, USA.
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7
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Microtubule-Organizing Centers: Towards a Minimal Parts List. Trends Cell Biol 2017; 28:176-187. [PMID: 29173799 DOI: 10.1016/j.tcb.2017.10.005] [Citation(s) in RCA: 52] [Impact Index Per Article: 7.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/14/2017] [Revised: 10/20/2017] [Accepted: 10/23/2017] [Indexed: 12/17/2022]
Abstract
Despite decades of molecular analysis of the centrosome, an important microtubule-organizing center (MTOC) of animal cells, the molecular basis of microtubule organization remains obscure. A major challenge is the sheer complexity of the interplay of the hundreds of proteins that constitute the centrosome. However, this complexity owes not only to the centrosome's role as a MTOC but also to the requirements of its duplication cycle and to various other functions such as the formation of cilia, the integration of various signaling pathways, and the organization of actin filaments. Thus, rather than using the parts lists to reconstruct the centrosome, we propose to identify the subset of proteins minimally needed to assemble a MTOC and to study this process at non-centrosomal sites.
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8
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Roostalu J, Surrey T. Microtubule nucleation: beyond the template. Nat Rev Mol Cell Biol 2017; 18:702-710. [PMID: 28831203 DOI: 10.1038/nrm.2017.75] [Citation(s) in RCA: 119] [Impact Index Per Article: 17.0] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/17/2022]
Abstract
Microtubules are cytoskeletal filaments central to a wide range of essential cellular functions in eukaryotic cells. Consequently, cells need to exert tight control over when, where and how many microtubules are being made. Whereas the regulation of microtubule dynamics is well studied, the molecular mechanisms of microtubule nucleation are still poorly understood. Next to the established master template of nucleation, the γ-tubulin ring complex, other microtubule-associated proteins that affect microtubule dynamic properties have recently been found to contribute to nucleation. It has begun to emerge that the nucleation efficiency is controlled not only by template activity but also by, either additionally or alternatively, the stabilization of the nascent microtubule 'nucleus'. This suggests a simple conceptual framework for the mechanisms regulating microtubule nucleation in cells.
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Affiliation(s)
| | - Thomas Surrey
- The Francis Crick Institute, 1 Midland Road, London NW1 1AT, UK
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9
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Kapoor TM. Metaphase Spindle Assembly. BIOLOGY 2017; 6:biology6010008. [PMID: 28165376 PMCID: PMC5372001 DOI: 10.3390/biology6010008] [Citation(s) in RCA: 30] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 10/03/2016] [Revised: 01/17/2017] [Accepted: 01/19/2017] [Indexed: 01/31/2023]
Abstract
A microtubule-based bipolar spindle is required for error-free chromosome segregation during cell division. In this review I discuss the molecular mechanisms required for the assembly of this dynamic micrometer-scale structure in animal cells.
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Affiliation(s)
- Tarun M Kapoor
- Laboratory of Chemistry and Cell Biology, the Rockefeller University, New York, NY 10065, USA.
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10
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Ritter A, Kreis NN, Louwen F, Wordeman L, Yuan J. Molecular insight into the regulation and function of MCAK. Crit Rev Biochem Mol Biol 2016; 51:228-45. [DOI: 10.1080/10409238.2016.1178705] [Citation(s) in RCA: 21] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022]
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11
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King M, Petry S. Visualizing and Analyzing Branching Microtubule Nucleation Using Meiotic Xenopus Egg Extracts and TIRF Microscopy. Methods Mol Biol 2016; 1413:77-85. [PMID: 27193844 DOI: 10.1007/978-1-4939-3542-0_6] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
Abstract
Mitotic and meiotic spindles consist primarily of microtubules, which originate from centrosomes and within the vicinity of chromatin. Indirect evidence suggested that microtubules also originate throughout the spindle, but the high microtubule density within the spindle precludes the direct observation of this phenomenon. By using meiotic Xenopus laevis egg extract and employing total internal reflection (TIRF) microscopy, microtubule nucleation from preexisting microtubules could be demonstrated and analyzed. Branching microtubule nucleation is an ideal mechanism to assemble and maintain a mitotic spindle, because microtubule numbers are amplified while preserving their polarity. Here, we describe the assays that made these findings possible and the experiments that helped identify the key molecular players involved.
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Affiliation(s)
- Matthew King
- Department of Molecular Biology, Princeton University, Washington Road, Princeton, NJ, 08544, USA
| | - Sabine Petry
- Department of Molecular Biology, Princeton University, Washington Road, Princeton, NJ, 08544, USA.
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12
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Roostalu J, Cade NI, Surrey T. Complementary activities of TPX2 and chTOG constitute an efficient importin-regulated microtubule nucleation module. Nat Cell Biol 2015; 17:1422-34. [PMID: 26414402 PMCID: PMC4826748 DOI: 10.1038/ncb3241] [Citation(s) in RCA: 115] [Impact Index Per Article: 12.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/22/2014] [Accepted: 08/21/2015] [Indexed: 01/28/2023]
Abstract
Spindle assembly and function require precise control of microtubule nucleation and dynamics. The chromatin-driven spindle assembly pathway exerts such control locally in the vicinity of chromosomes. One of the key targets of this pathway is TPX2. The molecular mechanism of how TPX2 stimulates microtubule nucleation is not understood. Using microscopy-based dynamic in vitro reconstitution assays with purified proteins, we find that human TPX2 directly stabilizes growing microtubule ends and stimulates microtubule nucleation by stabilizing early microtubule nucleation intermediates. Human microtubule polymerase chTOG (XMAP215/Msps/Stu2p/Dis1/Alp14 homologue) only weakly promotes nucleation, but acts synergistically with TPX2. Hence, a combination of distinct and complementary activities is sufficient for efficient microtubule formation in vitro. Importins control the efficiency of the microtubule nucleation by selectively blocking the interaction of TPX2 with microtubule nucleation intermediates. This in vitro reconstitution reveals the molecular mechanism of regulated microtubule formation by a minimal nucleation module essential for chromatin-dependent microtubule nucleation in cells.
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Affiliation(s)
- Johanna Roostalu
- The Francis Crick Institute, 44 Lincoln’s Inn Fields, London WC2A 3LY, United Kingdom
| | - Nicholas I. Cade
- The Francis Crick Institute, 44 Lincoln’s Inn Fields, London WC2A 3LY, United Kingdom
| | - Thomas Surrey
- The Francis Crick Institute, 44 Lincoln’s Inn Fields, London WC2A 3LY, United Kingdom
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14
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Jambhekar A, Emerman AB, Schweidenback CTH, Blower MD. RNA stimulates Aurora B kinase activity during mitosis. PLoS One 2014; 9:e100748. [PMID: 24968351 PMCID: PMC4072698 DOI: 10.1371/journal.pone.0100748] [Citation(s) in RCA: 42] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/17/2014] [Accepted: 05/22/2014] [Indexed: 11/19/2022] Open
Abstract
Accurate chromosome segregation is essential for cell viability. The mitotic spindle is crucial for chromosome segregation, but much remains unknown about factors that regulate spindle assembly. Recent work implicates RNA in promoting proper spindle assembly independently of mRNA translation; however, the mechanism by which RNA performs this function is currently unknown. Here, we show that RNA regulates both the localization and catalytic activity of the mitotic kinase, Aurora-B (AurB), which is present in a ribonucleoprotein (RNP) complex with many mRNAs. Interestingly, AurB kinase activity is reduced in Xenopus egg extracts treated with RNase, and its activity is stimulated in vitro by RNA binding. Spindle assembly defects following RNase-treatment are partially rescued by inhibiting MCAK, a microtubule depolymerase that is inactivated by AurB-dependent phosphorylation. These findings implicate AurB as an important RNA-dependent spindle assembly factor, and demonstrate a translation-independent role for RNA in stimulating AurB.
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Affiliation(s)
- Ashwini Jambhekar
- Department of Molecular Biology, Massachusetts General Hospital, Boston, Massachusetts, United States of America
- Department of Genetics, Harvard Medical School, Boston, Massachusetts, United States of America
| | - Amy B. Emerman
- Department of Molecular Biology, Massachusetts General Hospital, Boston, Massachusetts, United States of America
- Department of Genetics, Harvard Medical School, Boston, Massachusetts, United States of America
| | - Caterina T. H. Schweidenback
- Department of Molecular Biology, Massachusetts General Hospital, Boston, Massachusetts, United States of America
- Department of Genetics, Harvard Medical School, Boston, Massachusetts, United States of America
| | - Michael D. Blower
- Department of Molecular Biology, Massachusetts General Hospital, Boston, Massachusetts, United States of America
- Department of Genetics, Harvard Medical School, Boston, Massachusetts, United States of America
- * E-mail:
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Syred HM, Welburn J, Rappsilber J, Ohkura H. Cell cycle regulation of microtubule interactomes: multi-layered regulation is critical for the interphase/mitosis transition. Mol Cell Proteomics 2013; 12:3135-47. [PMID: 23893837 PMCID: PMC3820929 DOI: 10.1074/mcp.m113.028563] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/20/2013] [Revised: 07/25/2013] [Indexed: 12/17/2022] Open
Abstract
Microtubules dramatically change their dynamics and organization at the entry into mitosis. Although this change is mediated by microtubule-associated proteins (MAPs), how MAPs themselves are regulated is not well understood. Here we used an integrated multi-level approach to establish the framework and biological significance of MAP regulation critical for the interphase/mitosis transition. Firstly, we applied quantitative proteomics to determine global cell cycle changes in the profiles of MAPs in human and Drosophila cells. This uncovered a wide range of cell cycle regulations of MAPs previously unidentified. Secondly, systematic studies of human kinesins highlighted an overlooked aspect of kinesins: most mitotic kinesins suppress their affinity to microtubules or reduce their protein levels in interphase in combination with nuclear localization. Thirdly, in-depth analysis of a novel Drosophila MAP (Mink) revealed that the suppression of the microtubule affinity of this mitotic MAP in combination with nuclear localization is essential for microtubule organization in interphase, and phosphorylation of Mink is needed for kinetochore-microtubule attachment in mitosis. Thus, this first comprehensive analysis of MAP regulation for the interphase/mitosis transition advances our understanding of kinesin biology and reveals the prevalence and importance of multi-layered MAP regulation.
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Affiliation(s)
- Heather M. Syred
- From the ‡Wellcome Trust Centre for Cell Biology, School of Biological Sciences, The University of Edinburgh, Edinburgh EH9 3JR, United Kingdom
| | - Julie Welburn
- From the ‡Wellcome Trust Centre for Cell Biology, School of Biological Sciences, The University of Edinburgh, Edinburgh EH9 3JR, United Kingdom
| | - Juri Rappsilber
- From the ‡Wellcome Trust Centre for Cell Biology, School of Biological Sciences, The University of Edinburgh, Edinburgh EH9 3JR, United Kingdom
| | - Hiroyuki Ohkura
- From the ‡Wellcome Trust Centre for Cell Biology, School of Biological Sciences, The University of Edinburgh, Edinburgh EH9 3JR, United Kingdom
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16
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Upregulation of the TPX2 gene is associated with enhanced tumor malignance of esophageal squamous cell carcinoma. Biomed Pharmacother 2013; 67:751-5. [DOI: 10.1016/j.biopha.2013.04.004] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/18/2013] [Accepted: 04/19/2013] [Indexed: 11/21/2022] Open
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17
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XMAP215 activity sets spindle length by controlling the total mass of spindle microtubules. Nat Cell Biol 2013; 15:1116-22. [PMID: 23974040 DOI: 10.1038/ncb2834] [Citation(s) in RCA: 92] [Impact Index Per Article: 8.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/31/2012] [Accepted: 07/24/2013] [Indexed: 12/30/2022]
Abstract
Metaphase spindles are microtubule-based structures that use a multitude of proteins to modulate their morphology and function. Today, we understand many details of microtubule assembly, the role of microtubule-associated proteins, and the action of molecular motors. Ultimately, the challenge remains to understand how the collective behaviour of these nanometre-scale processes gives rise to a properly sized spindle on the micrometre scale. By systematically engineering the enzymatic activity of XMAP215, a processive microtubule polymerase, we show that Xenopus laevis spindle length increases linearly with microtubule growth velocity, whereas other parameters of spindle organization, such as microtubule density, lifetime and spindle shape, remain constant. We further show that mass balance can be used to link the global property of spindle size to individual microtubule dynamic parameters. We propose that spindle length is set by a balance of non-uniform nucleation and global microtubule disassembly in a liquid-crystal-like arrangement of microtubules.
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18
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Branching microtubule nucleation in Xenopus egg extracts mediated by augmin and TPX2. Cell 2013; 152:768-77. [PMID: 23415226 DOI: 10.1016/j.cell.2012.12.044] [Citation(s) in RCA: 256] [Impact Index Per Article: 23.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/13/2012] [Revised: 12/17/2012] [Accepted: 12/21/2012] [Indexed: 11/24/2022]
Abstract
The microtubules that comprise mitotic spindles in animal cells are nucleated at centrosomes and by spindle assembly factors that are activated in the vicinity of chromatin. Indirect evidence has suggested that microtubules also might be nucleated from pre-existing microtubules throughout the spindle, but this process has not been observed directly. Here, we demonstrate microtubule nucleation from the sides of existing microtubules in meiotic Xenopus egg extracts. Daughter microtubules grow at a low branch angle and with the same polarity as mother filaments. Branching microtubule nucleation requires γ-tubulin and augmin and is stimulated by factors previously implicated in chromatin-stimulated nucleation, guanosine triphosphate(GTP)-bound Ran and its effector, TPX2. Because of the rapid amplification of microtubule numbers and the preservation of microtubule polarity, microtubule-dependent microtubule nucleation is well suited for spindle assembly and maintenance.
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19
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Remy MH, Merdes A, Gregory-Pauron L. Assembly of Gamma-Tubulin Ring Complexes. PROGRESS IN MOLECULAR BIOLOGY AND TRANSLATIONAL SCIENCE 2013; 117:511-30. [DOI: 10.1016/b978-0-12-386931-9.00019-2] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/17/2023]
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20
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Interplay Between Spindle Architecture and Function. INTERNATIONAL REVIEW OF CELL AND MOLECULAR BIOLOGY 2013; 306:83-125. [DOI: 10.1016/b978-0-12-407694-5.00003-1] [Citation(s) in RCA: 63] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/24/2022]
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21
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Meunier S, Vernos I. Microtubule assembly during mitosis - from distinct origins to distinct functions? J Cell Sci 2012; 125:2805-14. [PMID: 22736044 DOI: 10.1242/jcs.092429] [Citation(s) in RCA: 84] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022] Open
Abstract
The mitotic spindle is structurally and functionally defined by its main component, the microtubules (MTs). The MTs making up the spindle have various functions, organization and dynamics: astral MTs emanate from the centrosome and reach the cell cortex, and thus have a major role in spindle positioning; interpolar MTs are the main constituent of the spindle and are key for the establishment of spindle bipolarity, chromosome congression and central spindle assembly; and kinetochore-fibers are MT bundles that connect the kinetochores with the spindle poles and segregate the sister chromatids during anaphase. The duplicated centrosomes were long thought to be the origin of all of these MTs. However, in the last decade, a number of studies have contributed to the identification of non-centrosomal pathways that drive MT assembly in dividing cells. These pathways are now known to be essential for successful spindle assembly and to participate in various processes such as K-fiber formation and central spindle assembly. In this Commentary, we review the recent advances in the field and discuss how different MT assembly pathways might cooperate to successfully form the mitotic spindle.
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Affiliation(s)
- Sylvain Meunier
- Microtubule Function and Cell Division group, Cell and Developmental Biology Program, Centre for Genomic Regulation (CRG) and UPF, Dr. Aiguader 88, 08003 Barcelona, Spain
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Teixidó-Travesa N, Roig J, Lüders J. The where, when and how of microtubule nucleation – one ring to rule them all. J Cell Sci 2012; 125:4445-56. [DOI: 10.1242/jcs.106971] [Citation(s) in RCA: 131] [Impact Index Per Article: 10.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/17/2022] Open
Abstract
The function of microtubules depends on their arrangement into highly ordered arrays. Spatio-temporal control over the formation of new microtubules and regulation of their properties are central to the organization of these arrays. The nucleation of new microtubules requires γ-tubulin, an essential protein that assembles into multi-subunit complexes and is found in all eukaryotic organisms. However, the way in which γ-tubulin complexes are regulated and how this affects nucleation and, potentially, microtubule behavior, is poorly understood. γ-tubulin has been found in complexes of various sizes but several lines of evidence suggest that only large, ring-shaped complexes function as efficient microtubule nucleators. Human γ-tubulin ring complexes (γTuRCs) are composed of γ-tubulin and the γ-tubulin complex components (GCPs) 2, 3, 4, 5 and 6, which are members of a conserved protein family. Recent work has identified additional unrelated γTuRC subunits, as well as a large number of more transient γTuRC interactors. In this Commentary, we discuss the regulation of γTuRC-dependent microtubule nucleation as a key mechanism of microtubule organization. Specifically, we focus on the regulatory roles of the γTuRC subunits and interactors and present an overview of other mechanisms that regulate γTuRC-dependent microtubule nucleation and organization.
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Sanhaji M, Friel CT, Wordeman L, Louwen F, Yuan J. Mitotic centromere-associated kinesin (MCAK): a potential cancer drug target. Oncotarget 2011; 2:935-47. [PMID: 22249213 PMCID: PMC3282097 DOI: 10.18632/oncotarget.416] [Citation(s) in RCA: 52] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/31/2011] [Accepted: 12/31/2011] [Indexed: 11/25/2022] Open
Abstract
The inability to faithfully segregate chromosomes in mitosis results in chromosome instability, a hallmark of solid tumors. Disruption of microtubule dynamics contributes highly to mitotic chromosome instability. The kinesin-13 family is critical in the regulation of microtubule dynamics and the best characterized member of the family, the mitotic centromere-associated kinesin (MCAK), has recently been attracting enormous attention. MCAK regulates microtubule dynamics as a potent depolymerizer of microtubules by removing tubulin subunits from the polymer end. This depolymerizing activity plays pivotal roles in spindle formation, in correcting erroneous attachments of microtubule-kinetochore and in chromosome movement. Thus, the accurate regulation of MCAK is important for ensuring the faithful segregation of chromosomes in mitosis and for safeguarding chromosome stability. In this review we summarize recent data concerning the regulation of MCAK by mitotic kinases, Aurora A/B, Polo-like kinase 1 and cyclin-dependent kinase 1. We propose a molecular model of the regulation of MCAK by these mitotic kinases and relevant phosphatases throughout mitosis. An ever-increasing quantity of data indicates that MCAK is aberrantly regulated in cancer cells. This deregulation is linked to increased malignance, invasiveness, metastasis and drug resistance, most probably due to increased chromosomal instability and remodeling of the microtubule cytoskeleton in cancer cells. Most interestingly, recent observations suggest that MCAK could be a novel molecular target for cancer therapy, as a new cancer antigen or as a mitotic regulator. This collection of new data indicates that MCAK could be a new star in the cancer research sky due to its critical roles in the control of genome stability and the cytoskeleton. Further investigations are required to dissect the fine details of the regulation of MCAK throughout mitosis and its involvements in oncogenesis.
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Affiliation(s)
- Mourad Sanhaji
- Department of Gynecology and Obstetrics, School of Medicine, J. W. Goethe-University, Frankfurt, Germany
| | - Claire T. Friel
- School of Biomedical Sciences, University of Nottingham, Medical School, Queen's Medical Centre, Nottingham, UK
| | - Linda Wordeman
- Department of Physiology and Biophysics, University of Washington, Seattle, WA 98195
- Center for Cell Dynamics, Friday Harbor, Laboratories, Friday Harbor, WA 98250, USA
| | - Frank Louwen
- Department of Gynecology and Obstetrics, School of Medicine, J. W. Goethe-University, Frankfurt, Germany
| | - Juping Yuan
- Department of Gynecology and Obstetrics, School of Medicine, J. W. Goethe-University, Frankfurt, Germany
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24
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Identification of a TPX2-like microtubule-associated protein in Drosophila. PLoS One 2011; 6:e28120. [PMID: 22140519 PMCID: PMC3227607 DOI: 10.1371/journal.pone.0028120] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/21/2011] [Accepted: 11/01/2011] [Indexed: 12/31/2022] Open
Abstract
Chromosome segregation during mitosis and meiosis relies on the spindle and the functions of numerous microtubule-associated proteins (MAPs). One of the best-studied spindle MAPs is the highly conserved TPX2, which has been reported to have characteristic intracellular dynamics and molecular activities, such as nuclear localisation in interphase, poleward movement in the metaphase spindle, microtubule nucleation, microtubule stabilisation, microtubule bundling, Aurora A kinase activation, kinesin-5 binding, and kinesin-12 recruitment. This protein has been shown to be essential for spindle formation in every cell type analysed so far. However, as yet, TPX2 homologues have not been found in the Drosophila genome. In this study, I found that the Drosophila protein Ssp1/Mei-38 has significant homology to TPX2. Sequence conservation was limited to the putative spindle microtubule-associated region of TPX2, and intriguingly, D-TPX2 (Ssp1/Mei-38) lacks Aurora A- and kinesin-5-binding domains, which are highly conserved in other animal and plant species, including many insects such as ants and bees. D-TPX2 uniformly localised to kinetochore microtubule-enriched regions of the metaphase spindle in the S2 cell line, and it had microtubule binding and bundling activities in vitro. In comparison with other systems, the contribution of D-TPX2 to cell division seems to be minor; live cell imaging of microtubules and chromosomes after RNAi knockdown identified significant delay in chromosome congression in only 18% of the cells. Thus, while this conserved spindle protein is present in Drosophila, other mechanisms may largely compensate for its spindle assembly and chromosome segregation functions.
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25
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Augmin promotes meiotic spindle formation and bipolarity in Xenopus egg extracts. Proc Natl Acad Sci U S A 2011; 108:14473-8. [PMID: 21844347 DOI: 10.1073/pnas.1110412108] [Citation(s) in RCA: 71] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Female meiotic spindles in many organisms form in the absence of centrosomes, the organelle typically associated with microtubule (MT) nucleation. Previous studies have proposed that these meiotic spindles arise from RanGTP-mediated MT nucleation in the vicinity of chromatin; however, whether this process is sufficient for spindle formation is unknown. Here, we investigated whether a recently proposed spindle-based MT nucleation pathway that involves augmin, an 8-subunit protein complex, also contributes to spindle morphogenesis. We used an assay system in which hundreds of meiotic spindles can be observed forming around chromatin-coated beads after introduction of Xenopus egg extracts. Spindles forming in augmin-depleted extracts showed reduced rates of MT formation and were predominantly multipolar, revealing a function of augmin in stabilizing the bipolar shape of the acentrosomal meiotic spindle. Our studies also have uncovered an apparent augmin-independent MT nucleation process from acentrosomal poles, which becomes increasingly active over time and appears to partially rescue the spindle defects that arise from augmin depletion. Our studies reveal that spatially and temporally distinct MT generation pathways from chromatin, spindle MTs, and acentrosomal poles all contribute to robust bipolar spindle formation in meiotic extracts.
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Groen AC, Coughlin M, Mitchison TJ. Microtubule assembly in meiotic extract requires glycogen. Mol Biol Cell 2011; 22:3139-51. [PMID: 21737678 PMCID: PMC3164461 DOI: 10.1091/mbc.e11-02-0158] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022] Open
Abstract
We identified a clarified extract containing the soluble factors for microtubule assembly. We found that microtubule assembly does not require ribosomes, mitochondria, or membranes. Our clarified extracts will provide a powerful tool for activity-based biochemical fractionations for microtubule assembly. The assembly of microtubules during mitosis requires many identified components, such as γ-tubulin ring complex (γ-TuRC), components of the Ran pathway (e.g., TPX2, HuRP, and Rae1), and XMAP215/chTOG. However, it is far from clear how these factors function together or whether more factors exist. In this study, we used biochemistry to attempt to identify active microtubule nucleation protein complexes from Xenopus meiotic egg extracts. Unexpectedly, we found both microtubule assembly and bipolar spindle assembly required glycogen, which acted both as a crowding agent and as metabolic source glucose. By also reconstituting microtubule assembly in clarified extracts, we showed microtubule assembly does not require ribosomes, mitochondria, or membranes. Our clarified extracts will provide a powerful tool for activity-based biochemical fractionations for microtubule assembly.
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Affiliation(s)
- Aaron C Groen
- Department of Systems Biology, Harvard Medical School, Boston, MA 02115, USA.
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27
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Sironi L, Solon J, Conrad C, Mayer TU, Brunner D, Ellenberg J. Automatic quantification of microtubule dynamics enables RNAi-screening of new mitotic spindle regulators. Cytoskeleton (Hoboken) 2011; 68:266-78. [DOI: 10.1002/cm.20510] [Citation(s) in RCA: 36] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/02/2010] [Revised: 02/03/2011] [Accepted: 03/02/2011] [Indexed: 11/08/2022]
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Abstract
The mitotic spindle accurately segregates genetic instructions by moving chromosomes to spindle poles (anaphase A) and separating the poles (anaphase B) so that, in general, the chromosomes and poles are positioned near the centers of the nascent daughter cell products of each cell division. Because the size of different types of dividing cells, and thus the spacing of their daughter cell centers, can vary significantly, the length of the metaphase or postanaphase B spindle often scales with cell size. However, significant exceptions to this scaling rule occur, revealing the existence of cell size–independent, spindle-associated mechanisms of spindle length control. The control of spindle length reflects the action of mitotic force-generating mechanisms, and its study may illuminate general principles by which cells regulate the size of internal structures. Here we review molecules and mechanisms that control spindle length, how these mechanisms are deployed in different systems, and some quantitative models that describe the control of spindle length.
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Affiliation(s)
- Gohta Goshima
- Division of Biological Science, Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan.
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29
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Vanden Bosch A, Raemaekers T, Denayer S, Torrekens S, Smets N, Moermans K, Dewerchin M, Carmeliet P, Carmeliet G. NuSAP is essential for chromatin-induced spindle formation during early embryogenesis. J Cell Sci 2010; 123:3244-55. [DOI: 10.1242/jcs.063875] [Citation(s) in RCA: 51] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022] Open
Abstract
Mitotic spindle assembly is mediated by two processes: a centrosomal and a chromosomal pathway. RanGTP regulates the latter process by releasing microtubule-associated proteins from inhibitory complexes. NuSAP, a microtubule- and DNA-binding protein, is a target of RanGTP and promotes the formation of microtubules near chromosomes. However, the contribution of NuSAP to cell proliferation in vivo is unknown. Here, we demonstrate that the expression of NuSAP highly correlates with cell proliferation during embryogenesis and adult life, making it a reliable marker of proliferating cells. Additionally, we show that NuSAP deficiency in mice leads to early embryonic lethality. Spindle assembly in NuSAP-deficient cells is highly inefficient and chromosomes remain dispersed in the mitotic cytoplasm. As a result of sustained spindle checkpoint activity, the cells are unable to progress through mitosis, eventually leading to caspase activation and apoptotic cell death. Together, our findings demonstrate that NuSAP is essential for proliferation of embryonic cells and, simultaneously, they underscore the importance of chromatin-induced spindle assembly.
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Affiliation(s)
- An Vanden Bosch
- Laboratory of Experimental Medicine and Endocrinology, Katholieke Universiteit Leuven, 3000 Leuven, Belgium
| | - Tim Raemaekers
- Laboratory of Membrane Trafficking, Katholieke Universiteit Leuven, 3000 Leuven, Belgium
| | - Sarah Denayer
- Laboratory of Molecular Endocrinology, Katholieke Universiteit Leuven, 3000 Leuven, Belgium
| | - Sophie Torrekens
- Laboratory of Experimental Medicine and Endocrinology, Katholieke Universiteit Leuven, 3000 Leuven, Belgium
| | - Nico Smets
- Laboratory of Experimental Medicine and Endocrinology, Katholieke Universiteit Leuven, 3000 Leuven, Belgium
| | - Karen Moermans
- Laboratory of Experimental Medicine and Endocrinology, Katholieke Universiteit Leuven, 3000 Leuven, Belgium
| | - Mieke Dewerchin
- Vesalius Research Center (VRC), Katholieke Universiteit Leuven, 3000 Leuven, Belgium
- Vesalius Research Center (VRC), VIB, B-3000 Leuven, Belgium
| | - Peter Carmeliet
- Vesalius Research Center (VRC), Katholieke Universiteit Leuven, 3000 Leuven, Belgium
- Vesalius Research Center (VRC), VIB, B-3000 Leuven, Belgium
| | - Geert Carmeliet
- Laboratory of Experimental Medicine and Endocrinology, Katholieke Universiteit Leuven, 3000 Leuven, Belgium
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30
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Vogt E, Sanhaji M, Klein W, Seidel T, Wordeman L, Eichenlaub-Ritter U. MCAK is present at centromeres, midspindle and chiasmata and involved in silencing of the spindle assembly checkpoint in mammalian oocytes. Mol Hum Reprod 2010; 16:665-84. [PMID: 20406800 DOI: 10.1093/molehr/gaq025] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022] Open
Abstract
Mitotic centromere-associated kinesin (MCAK) is an ATP-dependent microtubule (MT) depolymerase regulated by Aurora kinase (AURK) phosphorylation and implicated in resolution of improper MT attachments in mitosis. Distribution of MCAK was studied in oocyte maturation by anti-MCAK antibody, anti-tubulin antibody, anti-AURKB antibody and anti-centromere antibody (ACA) and by the expression of MCAK-enhanced green fluorescent protein fusion protein in maturing mouse oocytes. Function was assessed by knockdown of MCAK and Mad2, by inhibiting AURK or the proteasome, by live imaging with polarization microscope and by chromosomal analysis. The results show that MCAK is transiently recruited to the nucleus and transits to spindle poles, ACA-positive domains and chiasmata at prometaphase I. At metaphase I and II, it is present at centrosomes and centromeres next to AURKB and checkpoint proteins Mad2 and BubR1. It is retained at centromeres at telophase I and also at the midbody. Knockdown of MCAK causes a delay in chromosome congression but does not prevent bipolar spindle assembly. MCAK knockdown also induces a meiosis I arrest, which is overcome by knockdown of Mad2 resulting in chiasma resolution, chromosome separation, formation of aberrant meiosis II spindles and increased hypoploidy. In conclusion, MCAK appears to possess a unique distribution and function in oocyte maturation. It is required for meiotic progression from meiosis I to meiosis II associated with silencing of the spindle assembly checkpoint. Alterations in abundance and activity of MCAK, as implicated in aged oocytes, may therefore contribute to the loss of control of cell cycle and chromosome behaviour, thus increasing risk for errors in chromosome segregation and aneuploidy.
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Affiliation(s)
- E Vogt
- Faculty of Biology, Gene Technology/Microbiology, University Bielefeld, Bielefeld, Germany
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31
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Giesecke A, Stewart M. Novel binding of the mitotic regulator TPX2 (target protein for Xenopus kinesin-like protein 2) to importin-alpha. J Biol Chem 2010; 285:17628-35. [PMID: 20335181 PMCID: PMC2878527 DOI: 10.1074/jbc.m110.102343] [Citation(s) in RCA: 71] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Several aspects of mitotic spindle assembly are orchestrated by the Ran GTPase through its modulation of the interaction between spindle assembly factors and importin-α. One such factor is TPX2 that promotes microtubule assembly in the vicinity of chromosomes. TPX2 is inhibited when bound to importin-α, which occurs when the latter is bound to importin-β. The importin-α:β interaction is disrupted by the high RanGTP concentration near the chromosomes, releasing TPX2. In more distal regions, where Ran is predominantly GDP-bound, TPX2 remains bound to importin-α and so is inhibited. Here we use a combination of structural and biochemical methods to define the basis for TPX2 binding to importin-α. A 2.2 Å resolution crystal structure shows that the primary nuclear localization signal (284KRKH287) of TPX2, which has been shown to be crucial for inhibition, binds to the minor NLS-binding site on importin-α. This atypical interaction pattern was confirmed using complementary binding studies that employed importin-α variants in which binding to either the major or minor NLS-binding site was impaired, together with competition assays using the SV40 monopartite NLS that binds primarily to the major site. The different way in which TPX2 binds to importin-α could account for much of the selectivity necessary during mitosis because this would reduce the competition for binding to importin-α from other NLS-containing proteins.
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Affiliation(s)
- Astrid Giesecke
- Medical Research Council Laboratory of Molecular Biology, Cambridge CB2 0QH, United Kingdom
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32
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Ma N, Tulu US, Ferenz NP, Fagerstrom C, Wilde A, Wadsworth P. Poleward transport of TPX2 in the mammalian mitotic spindle requires dynein, Eg5, and microtubule flux. Mol Biol Cell 2010; 21:979-88. [PMID: 20110350 PMCID: PMC2836978 DOI: 10.1091/mbc.e09-07-0601] [Citation(s) in RCA: 63] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023] Open
Abstract
TPX2 is a spindle assembly factor that is required for MT assembly near chromosomes. Using photoactivation of fluorescence, we report that TPX2 is transported poleward in the half-spindle. Poleward transport of TPX2 is sensitive to inhibition of dynein or Eg5, and to suppression of MT flux. TPX2 is a Ran-regulated spindle assembly factor that is required for kinetochore fiber formation and activation of the mitotic kinase Aurora A. TPX2 is enriched near spindle poles and is required near kinetochores, suggesting that it undergoes dynamic relocalization throughout mitosis. Using photoactivation, we measured the movement of PA-GFP-TPX2 in the mitotic spindle. TPX2 moves poleward in the half-spindle and is static in the interzone and near spindle poles. Poleward transport of TPX2 is sensitive to inhibition of dynein or Eg5 and to suppression of microtubule flux with nocodazole or antibodies to Kif2a. Poleward transport requires the C terminus of TPX2, a domain that interacts with Eg5. Overexpression of TPX2 lacking this domain induced excessive microtubule formation near kinetochores, defects in spindle assembly and blocked mitotic progression. Our data support a model in which poleward transport of TPX2 down-regulates its microtubule nucleating activity near kinetochores and links microtubules generated at kinetochores to dynein for incorporation into the spindle.
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Affiliation(s)
- Nan Ma
- University of Massachusetts, Amherst, MA 01003, USA
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33
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Needleman DJ, Groen A, Ohi R, Maresca T, Mirny L, Mitchison T. Fast microtubule dynamics in meiotic spindles measured by single molecule imaging: evidence that the spindle environment does not stabilize microtubules. Mol Biol Cell 2009; 21:323-33. [PMID: 19940016 PMCID: PMC2808228 DOI: 10.1091/mbc.e09-09-0816] [Citation(s) in RCA: 76] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/21/2023] Open
Abstract
Metaphase spindles are steady-state ensembles of microtubules. We used single molecule imaging to measure tubulin dynamics in spindles and non-spindle assemblies in Xenopus egg extract. Our results argue that the high density of microtubules in spindles is caused by local enhancement of nucleation, and not by local stabilization. Metaphase spindles are steady-state ensembles of microtubules that turn over rapidly and slide poleward in some systems. Since the discovery of dynamic instability in the mid-1980s, models for spindle morphogenesis have proposed that microtubules are stabilized by the spindle environment. We used single molecule imaging to measure tubulin turnover in spindles, and nonspindle assemblies, in Xenopus laevis egg extracts. We observed many events where tubulin molecules spend only a few seconds in polymer and thus are difficult to reconcile with standard models of polymerization dynamics. Our data can be quantitatively explained by a simple, phenomenological model—with only one adjustable parameter—in which the growing and shrinking of microtubule ends is approximated as a biased random walk. Microtubule turnover kinetics did not vary with position in the spindle and were the same in spindles and nonspindle ensembles nucleated by Tetrahymena pellicles. These results argue that the high density of microtubules in spindles compared with bulk cytoplasm is caused by local enhancement of nucleation and not by local stabilization. It follows that the key to understanding spindle morphogenesis will be to elucidate how nucleation is spatially controlled.
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Affiliation(s)
- Daniel J Needleman
- School of Engineering and Applied Sciences, Molecular and Cellular Biology, Harvard University, Cambridge, MA 02138, USA.
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34
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Allard JF, Wasteneys GO, Cytrynbaum EN. Mechanisms of self-organization of cortical microtubules in plants revealed by computational simulations. Mol Biol Cell 2009; 21:278-86. [PMID: 19910489 PMCID: PMC2808237 DOI: 10.1091/mbc.e09-07-0579] [Citation(s) in RCA: 76] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/20/2023] Open
Abstract
Microtubules confined to the two-dimensional cortex of elongating plant cells must form a parallel yet dispersed array transverse to the elongation axis for proper cell wall expansion. Some of these microtubules exhibit free minus-ends, leading to migration at the cortex by hybrid treadmilling. Collisions between microtubules can result in plus-end entrainment ("zippering") or rapid depolymerization. Here, we present a computational model of cortical microtubule organization. We find that plus-end entrainment leads to self-organization of microtubules into parallel arrays, whereas catastrophe-inducing collisions do not. Catastrophe-inducing boundaries (e.g., upper and lower cross-walls) can tune the orientation of an ordered array to a direction transverse to elongation. We also find that changes in dynamic instability parameters, such as in mor1-1 mutants, can impede self-organization, in agreement with experimental data. Increased entrainment, as seen in clasp-1 mutants, conserves self-organization, but delays its onset and fails to demonstrate increased ordering. We find that branched nucleation at acute angles off existing microtubules results in distinctive sparse arrays and infer either that microtubule-independent or coparallel nucleation must dominate. Our simulations lead to several testable predictions, including the effects of reduced microtubule severing in katanin mutants.
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Affiliation(s)
- Jun F Allard
- Institute of Applied Mathematics and Department of Botany, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z2
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