151
|
Orjalo AV, Arnaoutov A, Shen Z, Boyarchuk Y, Zeitlin SG, Fontoura B, Briggs S, Dasso M, Forbes DJ. The Nup107-160 nucleoporin complex is required for correct bipolar spindle assembly. Mol Biol Cell 2006; 17:3806-18. [PMID: 16807356 PMCID: PMC1593160 DOI: 10.1091/mbc.e05-11-1061] [Citation(s) in RCA: 118] [Impact Index Per Article: 6.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022] Open
Abstract
The Nup107-160 complex is a critical subunit of the nuclear pore. This complex localizes to kinetochores in mitotic mammalian cells, where its function is unknown. To examine Nup107-160 complex recruitment to kinetochores, we stained human cells with antisera to four complex components. Each antibody stained not only kinetochores but also prometaphase spindle poles and proximal spindle fibers, mirroring the dual prometaphase localization of the spindle checkpoint proteins Mad1, Mad2, Bub3, and Cdc20. Indeed, expanded crescents of the Nup107-160 complex encircled unattached kinetochores, similar to the hyperaccumulation observed of dynamic outer kinetochore checkpoint proteins and motors at unattached kinetochores. In mitotic Xenopus egg extracts, the Nup107-160 complex localized throughout reconstituted spindles. When the Nup107-160 complex was depleted from extracts, the spindle checkpoint remained intact, but spindle assembly was rendered strikingly defective. Microtubule nucleation around sperm centrosomes seemed normal, but the microtubules quickly disassembled, leaving largely unattached sperm chromatin. Notably, Ran-GTP caused normal assembly of microtubule asters in depleted extracts, indicating that this defect was upstream of Ran or independent of it. We conclude that the Nup107-160 complex is dynamic in mitosis and that it promotes spindle assembly in a manner that is distinct from its functions at interphase nuclear pores.
Collapse
Affiliation(s)
- Arturo V Orjalo
- Sections of Cell and Developmental Biology, Division of Biological Sciences, University of California-San Diego Medical School, La Jolla, CA 92093-0347, USA
| | | | | | | | | | | | | | | | | |
Collapse
|
152
|
Wong J, Fang G. HURP controls spindle dynamics to promote proper interkinetochore tension and efficient kinetochore capture. ACTA ACUST UNITED AC 2006; 173:879-91. [PMID: 16769820 PMCID: PMC2063914 DOI: 10.1083/jcb.200511132] [Citation(s) in RCA: 140] [Impact Index Per Article: 7.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/08/2023]
Abstract
Through a functional genomic screen for mitotic regulators, we identified hepatoma up-regulated protein (HURP) as a protein that is required for chromosome congression and alignment. In HURP-depleted cells, the persistence of unaligned chromosomes and the reduction of tension across sister kinetochores on aligned chromosomes resulted in the activation of the spindle checkpoint. Although these defects transiently delayed mitotic progression, HeLa cells initiated anaphase without resolution of these deficiencies. This bypass of the checkpoint arrest provides a tumor-specific mechanism for chromosome missegregation and genomic instability. Mechanistically, HURP colocalized with the mitotic spindle in a concentration gradient increasing toward the chromosomes. HURP binds directly to microtubules in vitro and enhances their polymerization. In vivo, HURP stabilizes mitotic microtubules, promotes microtubule polymerization and bipolar spindle formation, and decreases the turnover rate of the mitotic spindle. Thus, HURP controls spindle stability and dynamics to achieve efficient kinetochore capture at prometaphase, timely chromosome congression to the metaphase plate, and proper interkinetochore tension for anaphase initiation.
Collapse
Affiliation(s)
- Jim Wong
- Department of Biological Sciences, Stanford University, Stanford, CA 94305, USA
| | | |
Collapse
|
153
|
Tulu US, Fagerstrom C, Ferenz NP, Wadsworth P. Molecular requirements for kinetochore-associated microtubule formation in mammalian cells. Curr Biol 2006; 16:536-41. [PMID: 16527751 PMCID: PMC1500889 DOI: 10.1016/j.cub.2006.01.060] [Citation(s) in RCA: 165] [Impact Index Per Article: 9.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/20/2005] [Revised: 01/17/2006] [Accepted: 01/20/2006] [Indexed: 11/30/2022]
Abstract
In centrosome-containing cells, microtubules nucleated at centrosomes are thought to play a major role in spindle assembly. In addition, microtubule formation at kinetochores has also been observed, most recently under physiological conditions in live cells. The relative contributions of microtubule formation at kinetochores and centrosomes to spindle assembly, and their molecular requirements, remain incompletely understood. Using mammalian cells released from nocodazole-induced disassembly, we observed microtubule formation at centrosomes and at Bub1-positive sites on chromosomes. Kinetochore-associated microtubules rapidly coalesced into pole-like structures in a dynein-dependent manner. Microinjection of excess importin-beta or depletion of the Ran-dependent spindle assembly factor, TPX2, blocked kinetochore-associated microtubule formation, enhanced centrosome-associated microtubule formation, but did not prevent chromosome capture by centrosomal microtubules. Depletion of the chromosome passenger protein, survivin, reduced microtubule formation at kinetochores in an MCAK-dependent manner. Microtubule formation in cells depleted of Bub1 or Nuf2 was indistinguishable from that in controls. Our data demonstrate that microtubule assembly at centrosomes and kinetochores is kinetically distinct and differentially regulated. The presence of microtubules at kinetochores provides a mechanism to reconcile the time required for spindle assembly in vivo with that observed in computer simulations of search and capture.
Collapse
Affiliation(s)
- U Serdar Tulu
- Department of Biology and Program in Molecular and Cellular Biology, University of Massachusetts, Amherst, Amherst, Massachusetts 01003, USA
| | | | | | | |
Collapse
|
154
|
Tsai MY, Wang S, Heidinger JM, Shumaker DK, Adam SA, Goldman RD, Zheng Y. A mitotic lamin B matrix induced by RanGTP required for spindle assembly. Science 2006; 311:1887-93. [PMID: 16543417 DOI: 10.1126/science.1122771] [Citation(s) in RCA: 234] [Impact Index Per Article: 13.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/02/2022]
Abstract
Mitotic spindle morphogenesis is a series of highly coordinated movements that lead to chromosome segregation and cytokinesis. We report that the intermediate filament protein lamin B, a component of the interphase nuclear lamina, functions in spindle assembly. Lamin B assembled into a matrix-like network in mitosis through a process that depended on the presence of the guanosine triphosphate-bound form of the small guanosine triphosphatase Ran. Depletion of lamin B resulted in defects in spindle assembly. Dominant negative mutant lamin B proteins that disrupt lamin B assembly in interphase nuclei also disrupted spindle assembly in mitosis. Furthermore, lamin B was essential for the formation of the mitotic matrix that tethers a number of spindle assembly factors. We propose that lamin B is a structural component of the long-sought-after spindle matrix that promotes microtubule assembly and organization in mitosis.
Collapse
Affiliation(s)
- Ming-Ying Tsai
- Department of Embryology, Carnegie Institution of Washington and Howard Hughes Medical Institute, Baltimore, MD 21218, USA
| | | | | | | | | | | | | |
Collapse
|
155
|
Miura K, Yoshinobu K, Imaizumi T, Haruna K, Miyamoto Y, Yoneda Y, Nakagata N, Araki M, Miyakawa T, Yamamura KI, Araki K. Impaired expression of importin/karyopherin β1 leads to post-implantation lethality. Biochem Biophys Res Commun 2006; 341:132-8. [PMID: 16414015 DOI: 10.1016/j.bbrc.2005.12.151] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/30/2005] [Accepted: 12/24/2005] [Indexed: 01/23/2023]
Abstract
Importin beta1 (Impbeta)/karyopherin beta1 (Kpnb1) mediates the nuclear import of a large variety of substrates. This study aimed to investigate the requirement for the Kpnb1 gene in mouse development, using a gene trap line, B6-CB-Ayu8108(GtgeoIMEG) (Ayu8108(geo)), in which the trap vector was inserted into the promoter region of the Kpnb1 gene, but in reverse orientation of the Kpnb1 gene. Ayu8108(geo/geo) homozygous embryos could develop to the blastocyst stage, but died before embryonic day 5.5, and expression of the Kpnb1 gene in homozygous blastocysts was undetectable. We also replaced the betageo gene with Impbeta cDNA through Cre-mediated recombination to rescue Impbeta expression. Homozygous mice for the rescued allele Ayu8108(Impbeta/Impbeta) were born and developed normally. These results demonstrated that the cause of post-implantation lethality of Ayu8108(geo/geo) homozygous embryos was impaired expression of the Kpnb1 gene, indicating indispensable roles of Impbeta1 in early development of mice.
Collapse
Affiliation(s)
- Katsutaka Miura
- Institute of Molecular Embryology and Genetics, Kumamoto University, Japan
| | | | | | | | | | | | | | | | | | | | | |
Collapse
|
156
|
Bastiaens P, Caudron M, Niethammer P, Karsenti E. Gradients in the self-organization of the mitotic spindle. Trends Cell Biol 2006; 16:125-34. [PMID: 16478663 DOI: 10.1016/j.tcb.2006.01.005] [Citation(s) in RCA: 94] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/14/2005] [Revised: 12/05/2005] [Accepted: 01/24/2006] [Indexed: 01/08/2023]
Abstract
Recent evidence points at a role of protein interaction gradients around chromatin in mitotic spindle morphogenesis in large eukaryotic cells. Here, we explain how gradients can arise over distances of tens of microns around supramolecular structures from mixtures of soluble molecules. We discuss how coupled sets of such reaction diffusion processes generate the spatial information that determines the local dynamics of microtubules required to form a bipolar spindle. We argue that such reaction diffusion processes are involved in the self-organization of supramolecular structures in the cell.
Collapse
Affiliation(s)
- Philippe Bastiaens
- Cell Biology and Biophysics Program, EMBL, Meyerhofstrasse 1, D-69117 Heidelberg, Germany
| | | | | | | |
Collapse
|
157
|
Abstract
The International Symposium on Ran and the Cell Cycle was held on October 1-4, 2005, at the Awaji Island Resort near Osaka, to celebrate the career and scientific achievements of Professor Takeharu Nishimoto. One hundred of his former lab members, collaborators and other scientific colleagues from around the world attended the symposium organized by Mary Dasso (National Institutes of Health) and Yoshihiro Yoneda (Osaka University). The program was divided into sessions on cell cycle and chromosomes, nuclear import and export of proteins and RNA, nuclear envelope and the nuclear pore complex, and RCC1 and chromatin. Dr. Nishimoto's retirement from Kyushu University is a perfect time to look back at the history of Ran and RCC1, assess the current state of the field, and discuss the challenges that remain in order to unravel the complexities of the Ran GTPase system.
Collapse
Affiliation(s)
- Shelley Sazer
- Verna and Marrs Mclean Department of Biochemistry, Baylor College of Medicine, Houston, Texas 77030, USA.
| |
Collapse
|
158
|
Gorjánácz M, Török I, Pomozi I, Garab G, Szlanka T, Kiss I, Mechler BM. Domains of Importin-alpha2 required for ring canal assembly during Drosophila oogenesis. J Struct Biol 2006; 154:27-41. [PMID: 16458020 DOI: 10.1016/j.jsb.2005.12.007] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/20/2005] [Revised: 11/10/2005] [Accepted: 12/07/2005] [Indexed: 10/25/2022]
Abstract
Null-mutation in Drosophila importin-alpha2, such as the deficiency imp-alpha2(D14), causes recessive female sterility with the formation of dumpless eggs. In imp-alpha2(D14) the transfer of nurse cell components to the oocyte is interrupted and the Kelch protein, an oligomeric ring canal actin organizer, is normally produced but fails to associate with the ring canals resulting in their occlusion. To define domains regulating Kelch deposition on ring canals we performed site-directed mutagenesis on protein binding domains and putative phosphorylation sites of Imp-alpha2. Phenotypic analysis of the mutant transgenes in imp-alpha2(D14) revealed that mutations affecting the Imp-beta binding-domain, the dimerization domain, and specific serine residues of putative phosphorylation sites led to a normal or nearly normal oogenesis but arrested early embryonic development, whereas mutations in the nuclear localization signal (NLS) and CAS/exportin binding domains resulted in ring canal occlusion and a drastic nuclear accumulation of the mutant proteins. Deletion of the Imp-beta binding domain also gave rise to a nuclear localization of the mutant protein, which partially retained its function in ring canal assembly. Thus, we propose that mutations in NLS and CAS binding domains affect the deposition of Kelch onto the ring canals and prevent the association of Imp-alpha2 with a negative regulator of Kelch function.
Collapse
Affiliation(s)
- Mátyás Gorjánácz
- Institute of Genetics, Biological Research Center of the Hungarian Academy of Sciences, Szeged, Hungary
| | | | | | | | | | | | | |
Collapse
|
159
|
Cullen CF, Brittle AL, Ito T, Ohkura H. The conserved kinase NHK-1 is essential for mitotic progression and unifying acentrosomal meiotic spindles in Drosophila melanogaster. ACTA ACUST UNITED AC 2006; 171:593-602. [PMID: 16301329 PMCID: PMC2171570 DOI: 10.1083/jcb.200508127] [Citation(s) in RCA: 60] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023]
Abstract
Conventional centrosomes are absent from the spindle in female meiosis in many species, but it is not clear how multiple chromosomes form one shared bipolar spindle without centrosomes. We identified a female sterile mutant in which each bivalent chromosome often forms a separate bipolar metaphase I spindle. Unlike wild type, prophase I chromosomes fail to form a single compact structure within the oocyte nucleus, although the integrity of metaphase I chromosomes appears to be normal. Molecular analysis indicates that the mutant is defective in the conserved kinase nucleosomal histone kinase-1 (NHK-1). Isolation of further alleles and RNA interference in S2 cells demonstrated that NHK-1 is also required for mitotic progression. NHK-1 itself is phosphorylated in mitosis and female meiosis, suggesting that this kinase is part of the regulatory system coordinating progression of mitosis and meiosis.
Collapse
Affiliation(s)
- C Fiona Cullen
- Wellcome Trust Centre for Cell Biology, Institute of Cell Biology, School of Biological Sciences, The University of Edinburgh, Edinburgh EH9 3JR, Scotland, UK
| | | | | | | |
Collapse
|
160
|
Tokai-Nishizumi N, Ohsugi M, Suzuki E, Yamamoto T. The chromokinesin Kid is required for maintenance of proper metaphase spindle size. Mol Biol Cell 2005; 16:5455-63. [PMID: 16176979 PMCID: PMC1266440 DOI: 10.1091/mbc.e05-03-0244] [Citation(s) in RCA: 66] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022] Open
Abstract
The human chromokinesin Kid/kinesin-10, a plus end-directed microtubule (MT)-based motor with both microtubule- and DNA-binding domains, is required for proper chromosome alignment at the metaphase plate. Here, we performed RNA interference experiments to deplete endogenous Kid from HeLa cells and confirmed defects in metaphase chromosome arm alignment in Kid-depleted cells. In addition, we noted a shortening of the spindle length, resulting in a pole-to-pole distance only 80% of wild type. The spindle microtubule-bundles with which Kid normally colocalize became less robust. Rescue of the two Kid deficiency phenotypes-imprecise chromosome alignment at metaphase and shortened spindles- exhibited distinct requirements. Mutants lacking either the DNA-binding domain or the MT motor ATPase failed to rescue the former defect, whereas rescue of the shortened spindle phenotype required neither activity. Kid also exhibits microtubule bundling activity in vitro, and rescue of the shortened spindle phenotype and the bundling activity displayed similar domain requirements, except that rescue required a coiled-coil domain not needed for bundling. These results suggest that distinct from its role in chromosome movement, Kid contributes to spindle morphogenesis by mediating spindle microtubules stabilization.
Collapse
Affiliation(s)
- Noriko Tokai-Nishizumi
- Division of Oncology, Department of Cancer Biology, Institute of Medical Science, University of Tokyo, Tokyo 108-8639, Japan
| | | | | | | |
Collapse
|
161
|
Mirouse V, Dastugue B, Couderc JL. The Drosophila Toucan protein is a new mitotic microtubule-associated protein required for spindle microtubule stability. Genes Cells 2005; 10:37-46. [PMID: 15670212 DOI: 10.1111/j.1365-2443.2004.00813.x] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Mitotic spindle dynamics are highly dependent on proteins that interact with microtubules to influence their organization or stability. Here, we show that the Drosophila Toucan protein interacts directly with microtubules. Its localization to the microtubule network when it is expressed in mammalian cells and its direct interaction with microtubules in vitro are dependent on its central basic domain. Moreover, Toc expression in mammalian cells strongly protects microtubules from depolymerization. By using in vivo inducible RNAi in syncytial embryos, we generated a dose-sensitive loss of function of toucan, demonstrating that this technique is an efficient method for inactivating a maternal transcript. This enabled us to accurately characterize several new mitotic defects from the early to the late phases of mitosis, depending on Toucan depletion level. Toucan is required for metaphase spindle formation and centrosome anchoring to the poles. Then, during anaphase, Toc depletion affects kinetochore microtubules and therefore chromosome segregation. Toc is also necessary for central spindle formation by the interpolar microtubules. In contrast, astral microtubules are not disturbed by Toc depletion. Taken together, our results show that Toucan is a microtubule-associated protein specifically required for the stability of spindle microtubules throughout mitosis.
Collapse
Affiliation(s)
- Vincent Mirouse
- Institut National de la Santé et de la Recherche Médicale UMR384, Laboratoire de Biochimie, UFR Médecine, 28, place Henri Dunant, 63001 Clermont-Fd, France
| | | | | |
Collapse
|
162
|
Salman H, Abu-Arish A, Oliel S, Loyter A, Klafter J, Granek R, Elbaum M. Nuclear localization signal peptides induce molecular delivery along microtubules. Biophys J 2005; 89:2134-45. [PMID: 16040740 PMCID: PMC1366715 DOI: 10.1529/biophysj.105.060160] [Citation(s) in RCA: 71] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022] Open
Abstract
Many essential processes in eukaryotic cells depend on regulated molecular exchange between its two major compartments, the cytoplasm and the nucleus. In general, nuclear import of macromolecular complexes is dependent on specific peptide signals and their recognition by receptors that mediate translocation through the nuclear pores. Here we address the question of how protein products bearing such nuclear localization signals arrive at the nuclear membrane before import, i.e., by simple diffusion or perhaps with assistance of cytoskeletal elements or cytoskeleton-associated motor proteins. Using direct single-particle tracking and detailed statistical analysis, we show that the presence of nuclear localization signals invokes active transport along microtubules in a cell-free Xenopus egg extract. Chemical and antibody inhibition of minus-end directed cytoplasmic dynein blocks this active movement. In the intact cell, where microtubules project radially from the centrosome, such an interaction would effectively deliver nuclear-targeted cargo to the nuclear envelope in preparation for import.
Collapse
Affiliation(s)
- Hanna Salman
- Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel
| | | | | | | | | | | | | |
Collapse
|
163
|
Edick MJ, Cheng C, Yang W, Cheok M, Wilkinson MR, Pei D, Evans WE, Kun LE, Pui CH, Relling MV. Lymphoid gene expression as a predictor of risk of secondary brain tumors. Genes Chromosomes Cancer 2005; 42:107-16. [PMID: 15543619 DOI: 10.1002/gcc.20121] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
Gene expression profiles are tissue-specific but may also reflect germ-line-driven expression patterns across tissue types. Previously, using a targeted pharmacologic approach, we identified germ-line polymorphisms in a single gene (thiopurine methyltransferase) associated with the risk of irradiation- and chemotherapy-induced secondary brain tumors in children with acute lymphoblastic leukemia (ALL). To identify additional candidate genetic risk factors, in identically treated patients, we compared the gene expression profiles of diagnostic ALL blasts of those who did develop irradiation-associated brain tumors (n = 9) with the profiles from those who did not (n = 33). Weighted rank regression was used to identify 33 probe sets associated with the time-dependent development of brain tumors; k-means clustering (k = 2) identified 2 groups that differed significantly in cumulative incidence of brain tumors (P = 0.012). Permutation analysis was used to estimate the probability (P = 0.18) of obtaining 2 such clusters by chance. Linear discriminant analysis (time-independent categorization of outcome) was used to identify 70 probe sets whose expression differentiated between the 2 groups of patients. Permutation analyses (n = 1,000) was used to estimate the probability of selecting these probe sets by chance (P = 0.055). Five probe sets were in common between the time-independent and time-dependent methods. The distinguishing genes are involved in neural growth (FGFR1) and in nuclear trafficking (HNRPL, KPNB1). These data suggest that gene expression profiling from accessible tissues may identify targets involved in therapy-related malignancies in unrelated tissues.
Collapse
Affiliation(s)
- Mathew J Edick
- Department of Pharmaceutical Sciences, St. Jude Children's Research Hospital, University of Tennessee, Memphis, Tennessee, USA
| | | | | | | | | | | | | | | | | | | |
Collapse
|
164
|
|
165
|
Blower MD, Nachury M, Heald R, Weis K. A Rae1-containing ribonucleoprotein complex is required for mitotic spindle assembly. Cell 2005; 121:223-34. [PMID: 15851029 DOI: 10.1016/j.cell.2005.02.016] [Citation(s) in RCA: 214] [Impact Index Per Article: 11.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/04/2004] [Revised: 12/23/2004] [Accepted: 02/10/2005] [Indexed: 11/18/2022]
Abstract
Centrosome-independent microtubule polymerization around chromosomes has been shown to require a local gradient of RanGTP, which discharges mitotic cargoes from the nuclear import receptor importin beta. Here, we have used an activity-based assay in Xenopus egg extracts to purify the mRNA export protein Rae1 as a spindle assembly factor regulated by this pathway. Rae1 is a microtubule-associated protein that binds directly to importin beta. Depletion of Rae1 from extracts or cells severely inhibits mitotic spindle assembly. A purified Rae1 complex stabilizes microtubules in egg extracts in a RanGTP/importin beta-regulated manner. Interestingly, Rae1 exists in a large ribonucleoprotein complex, which requires RNA for its activity to control microtubule dynamics in vitro. Furthermore, we provide evidence that RNA associates with the mitotic spindle and that it plays a direct, translation-independent role in spindle assembly. Our studies reveal an unexpected function for RNA in spindle morphogenesis.
Collapse
Affiliation(s)
- Michael D Blower
- Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, California 94720, USA
| | | | | | | |
Collapse
|
166
|
Maiato H, DeLuca J, Salmon ED, Earnshaw WC. The dynamic kinetochore-microtubule interface. J Cell Sci 2005; 117:5461-77. [PMID: 15509863 DOI: 10.1242/jcs.01536] [Citation(s) in RCA: 282] [Impact Index Per Article: 14.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022] Open
Abstract
The kinetochore is a control module that both powers and regulates chromosome segregation in mitosis and meiosis. The kinetochore-microtubule interface is remarkably fluid, with the microtubules growing and shrinking at their point of attachment to the kinetochore. Furthermore, the kinetochore itself is highly dynamic, its makeup changing as cells enter mitosis and as it encounters microtubules. Active kinetochores have yet to be isolated or reconstituted, and so the structure remains enigmatic. Nonetheless, recent advances in genetic, bioinformatic and imaging technology mean we are now beginning to understand how kinetochores assemble, bind to microtubules and release them when the connections made are inappropriate, and also how they influence microtubule behaviour. Recent work has begun to elucidate a pathway of kinetochore assembly in animal cells; the work has revealed that many kinetochore components are highly dynamic and that some cycle between kinetochores and spindle poles along microtubules. Further studies of the kinetochore-microtubule interface are illuminating: (1) the role of the Ndc80 complex and components of the Ran-GTPase system in microtubule attachment, force generation and microtubule-dependent inactivation of kinetochore spindle checkpoint activity; (2) the role of chromosomal passenger proteins in the correction of kinetochore attachment errors; and (3) the function of microtubule plus-end tracking proteins, motor depolymerases and other proteins in kinetochore movement on microtubules and movement coupled to microtubule poleward flux.
Collapse
Affiliation(s)
- Helder Maiato
- Laboratory of Cell Regulation, NYSDH-Division of Molecular Medicine, Wadsworth Center, Empire State Plaza, PO Box 509, Albany, NY 12201-0509, USA
| | | | | | | |
Collapse
|
167
|
Jeong SY, Rose A, Joseph J, Dasso M, Meier I. Plant-specific mitotic targeting of RanGAP requires a functional WPP domain. THE PLANT JOURNAL : FOR CELL AND MOLECULAR BIOLOGY 2005; 42:270-82. [PMID: 15807788 DOI: 10.1111/j.1365-313x.2005.02368.x] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/19/2023]
Abstract
The small GTPase Ran is involved in nucleocytoplasmic transport, spindle formation, nuclear envelope (NE) formation, and cell-cycle control. In vertebrates, these functions are controlled by a three-dimensional gradient of Ran-GTP to Ran-GDP, established by the spatial separation of Ran GTPase-activating protein (RanGAP) and the Ran guanine nucleotide exchange factor RCC1. While this spatial separation is established by the NE during interphase, it is orchestrated during mitosis by association of RCC1 with the chromosomes and RanGAP with the spindle and kinetochores. SUMOylation of vertebrate RanGAP1 is required for NE, spindle, and centromere association. Arabidopsis RanGAP1 (AtRanGAP1) lacks the SUMOylated C-terminal domain of vertebrate RanGAP, but contains a plant-specific N-terminal domain (WPP domain), which is necessary and sufficient for its targeting to the NE in interphase. Here we show that the human and plant RanGAP-targeting domains are kingdom specific. AtRanGAP1 has a mitotic trafficking pattern uniquely different from that of vertebrate RanGAP, which includes targeting to the outward-growing rim of the cell plate. The WPP domain is necessary and sufficient for this targeting. Point mutations in conserved residues of the WPP domain also abolish targeting to the nuclear rim and the cell plate, suggesting that the same mechanism is involved in both targeting events. These results indicate that plant and animal RanGAPs undergo different migration patterns during cell division, which require their kingdom-specific targeting domains.
Collapse
Affiliation(s)
- Sun Yong Jeong
- Plant Biotechnology Center and Department of Plant Cellular and Molecular Biology, The Ohio State University, Columbus, OH 43210, USA
| | | | | | | | | |
Collapse
|
168
|
Abstract
Microtubules are dynamic polymers required for many aspects of eukaryotic cell function. The interphase microtubule network is essential for intracellular transport, organization, and cell polarization, whereas the mitotic spindle is required for chromosome segregation and cell division. Studies in different areas such as cell migration, mitosis, and asymmetric cell division have shown that Ran, Rho, and heterotrimeric G proteins regulate many aspects of microtubule functions. This review surveys how G protein-signaling coordinates microtubule polymerization and organization with specific cellular activities.
Collapse
Affiliation(s)
- Yixian Zheng
- Department of Embryology, Carnegie Institution of Washington and Howard Hughes Medical Institute, Baltimore, MD 21210, USA.
| |
Collapse
|
169
|
Abstract
Animal spermatids and primary oocytes initially have typical centrosomes comprising pairs of centrioles and pericentriolar fibrous centrosomal proteins. These somatic cell-like centrosomes are partially or completely degenerated during gametogenesis. Centrosome reduction during spermiogenesis comprises attenuation of microtubule nucleation function, loss of pericentriolar material, and centriole degeneration. Centrosome reduction during oogenesis is due to complete degeneration of centrioles, which leads to dispersal of the pericentriolar centrosomal proteins, loss of replicating capacity of the spindle poles, and switching to acentrosomal mode of spindle organization. Oocyte centrosome reduction plays an important role in preventing parthenogenetic embryogenesis and balancing centrosome number in the embryonic cells.
Collapse
|
170
|
Abstract
In a eukaryotic cell, the nuclear envelope (NE) separates genetic information from the environment of biosynthesis and metabolism. Transfer of macromolecules across the NE involves the nuclear pores--large multisubunit protein complexes--and machinery that facilitates rapid, directional, and selective transport. While core elements of the transport process are conserved between kingdoms, different solutions to similar problems have also evolved. Although the structure and composition of the yeast and mammalian nuclear pore have been unraveled recently, the plant nuclear pore remains largely enigmatic. Like any other process, nucleocytoplasmic transport can be regulated. Several examples from plants are discussed that promise insights into the regulation of signaling pathways. While controlling the partitioning of cellular components, the nuclear envelope also presents an obstacle to viruses and transforming agents that need access to the genome, and different mechanisms have evolved to overcome this obstacle. Finally, the recent recognition of the importance of small RNAs for gene regulation emphasizes the need to understand small RNA nuclear export and the levels of its regulation. This review attempts to wed our molecular-mechanistic understanding of nucleocytoplasmic trafficking drawn from all model systems with the intriguing examples of regulated nucleocytoplasmic partitioning in plants.
Collapse
Affiliation(s)
- Iris Meier
- Plant Biotechnology Center and Department of Plant Cellular and Molecular Biology, The Ohio State University, Columbus, Ohio 43210, USA
| |
Collapse
|
171
|
Abstract
Importin beta, once thought to be exclusively a nuclear transport receptor, is emerging as a global regulator of diverse cellular functions. Importin beta acts positively in multiple interphase roles: in nuclear import, as a chaperone for highly charged nuclear proteins, and as a potential motor adaptor for movement along microtubules. In contrast, importin beta plays a negative regulatory role in mitotic spindle assembly, centrosome dynamics, nuclear membrane formation, and nuclear pore assembly. In most of these, importin beta is counteracted by its regulator, Ran-GTP. In light of this, the recent discovery of Ran's involvement in spindle checkpoint control suggested a potential new arena for importin beta action, although it is also possible that one of importin beta's relatives, the karyopherin family of proteins, manages this checkpoint. Lastly, importin beta plays a role in transducing damage signals from the axons of injured neurons back to the cell body.
Collapse
Affiliation(s)
- Amnon Harel
- Section of Cell and Developmental Biology, Division of Biological Sciences 0347, University of California, San Diego, 9500 Gilman Drive, Room 2124A, Pacific Hall, La Jolla, CA 92093, USA
| | | |
Collapse
|
172
|
Abstract
Recent work has provided new insights into the mechanism of spindle assembly. Growing evidence supports a model in which the small GTPase Ran plays a central role in this process. Here, we examine the evidence for the existence of a RanGTP gradient around mitotic chromosomes and some controversial data on the role that chromosomes play in spindle assembly. We review the current knowledge on the Ran downstream targets for spindle assembly and we focus on the multiple roles of TPX2, one of the targets of RanGTP during cell division.
Collapse
Affiliation(s)
- Oliver J Gruss
- Cell Biology and Biophysics Program, European Molecular Biology Laboratory, Meyerhofstrasse 1, Heidelberg 69117, Germany.
| | | |
Collapse
|
173
|
Ciciarello M, Mangiacasale R, Thibier C, Guarguaglini G, Marchetti E, Di Fiore B, Lavia P. Importin beta is transported to spindle poles during mitosis and regulates Ran-dependent spindle assembly factors in mammalian cells. J Cell Sci 2004; 117:6511-22. [PMID: 15572412 DOI: 10.1242/jcs.01569] [Citation(s) in RCA: 69] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023] Open
Abstract
Spatial control is a key issue in cell division. The Ran GTPase regulates several fundamental processes for cell life, largely acting through importin molecules. The best understood of these is protein import through the nuclear envelope in interphase, but roles in mitotic spindle assembly are also established. In mammalian cells, in which centrosomes are major spindle organizers, a link is emerging between the Ran network, centrosomes and spindle poles. Here, we show that, after nuclear envelope breakdown, importin beta is transported to the spindle poles in mammalian cells. This localization is temporally regulated from prometaphase until anaphase, when importin beta dissociates from poles and is recruited back around reforming nuclei. Importin beta sediments with mitotic microtubules in vitro and its accumulation at poles requires microtubule integrity and dynamics in vivo. Furthermore, RNA interference-dependent inactivation of TPX2, the major Ran-dependent spindle organizer, abolishes importin beta accumulation at poles. Importin beta has a functional role in spindle pole organization, because overexpression yields mitotic spindles with abnormal, fragmented poles. Coexpression of TPX2 with importin beta mitigates these abnormalities. Together, these results indicate that the balance between importins and spindle regulators of the TPX2 type is crucial for spindle formation. Targeting of TPX2/importin-beta complexes to poles is a key aspect in Ran-dependent control of the mitotic apparatus in mammalian cells.
Collapse
Affiliation(s)
- Marilena Ciciarello
- Institute of Molecular Biology and Pathology, Section of Genetics, CNR Consiglio Nazionale delle Ricerche, Via degli Apuli 4, Rome 00185, Italy
| | | | | | | | | | | | | |
Collapse
|
174
|
Albertson R, Chabu C, Sheehan A, Doe CQ. Scribble protein domain mapping reveals a multistep localization mechanism and domains necessary for establishing cortical polarity. J Cell Sci 2004; 117:6061-70. [PMID: 15536119 DOI: 10.1242/jcs.01525] [Citation(s) in RCA: 103] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023] Open
Abstract
The Drosophila tumor suppressor protein Scribble is required for epithelial polarity, neuroblast polarity, neuroblast spindle asymmetry and limiting cell proliferation. It is a member of the newly described LAP protein family, containing 16 leucine rich repeats (LRRs), four PDZ domains and an extensive carboxyl-terminal (CT) domain. LRR and PDZ domains mediate protein-protein interactions, but little is know about their function within LAP family proteins. We have determined the role of the LRR, PDZ and CT domains for Scribble localization in neuroblasts and epithelia, and for Scribble function in neuroblasts. We found that the LRR and PDZ domains are both required for proper targeting of Scribble to septate junctions in epithelia; that the LRR domain is necessary and sufficient for cortical localization in mitotic neuroblasts, and that the PDZ2 domain is required for efficient cortical and apical localization of Scribble in neuroblasts. In addition, we show that the LRR domain is sufficient to target Miranda protein to the neuroblast cortex, but that LRR+PDZ will exclude Miranda from the cortex. Our results highlight the importance of both LRR and PDZ domains for the proper localization and function of Scribble in neuroblasts.
Collapse
Affiliation(s)
- Roger Albertson
- Institute of Molecular Biology, Institute of Neuroscience, Howard Hughes Medical Institute, University of Oregon 1254, Eugene, OR 97403, USA
| | | | | | | |
Collapse
|
175
|
Mosammaparast N, Pemberton LF. Karyopherins: from nuclear-transport mediators to nuclear-function regulators. Trends Cell Biol 2004; 14:547-56. [PMID: 15450977 DOI: 10.1016/j.tcb.2004.09.004] [Citation(s) in RCA: 258] [Impact Index Per Article: 12.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
Abstract
The karyopherin beta (or importin beta) family comprises soluble transport factors that mediate the movement of proteins and RNAs between the nucleus and cytoplasm. Recent studies have extended the role of karyopherins to regulating assembly of the nuclear pore complex (NPC), assembly of the nuclear envelope, mitosis and replication. New data also address how karyopherins specifically recognize and transport many distinct cargoes and traverse the NPC. These data raise the possibility that, although there might be a universal mechanism for nuclear transport, specific interactions between karyopherins and components of the NPC might function to regulate differentially the ability of the different karyopherins to cross the NPC.
Collapse
Affiliation(s)
- Nima Mosammaparast
- Center for Cell Signaling and Department of Microbiology, University of Virginia, Charlottesville, VA 22908, USA
| | | |
Collapse
|
176
|
Bertos NR, Gilquin B, Chan GKT, Yen TJ, Khochbin S, Yang XJ. Role of the tetradecapeptide repeat domain of human histone deacetylase 6 in cytoplasmic retention. J Biol Chem 2004; 279:48246-54. [PMID: 15347674 DOI: 10.1074/jbc.m408583200] [Citation(s) in RCA: 115] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Histone deacetylase 6 (HDAC6) contains tandem catalytic domains and a ubiquitin-binding zinc finger and displays deacetylase activity toward acetylated microtubules. Here we show that unlike its orthologs from Caenorhabditis elegans, Drosophila, and mouse, human HDAC6 possesses a tetradecapeptide repeat domain located between the second deacetylase domain and the C-terminal ubiquitin-binding motif. Related to this structural difference, the cytoplasmic localization of human, but not murine, HDAC6 is resistant to treatment with leptomycin B (LMB). Although it is dispensable for the deacetylase and ubiquitin binding activities of human HDAC6, the tetradecapeptide repeat domain displays acetyl-microtubule targeting ability. Moreover, it forms a unique structure and is required for the LMB-resistant cytoplasmic localization of human HDAC6. Besides the tetradecapeptide repeat domain, human HDAC6 possesses two LMB-sensitive nuclear export signals and a nuclear localization signal. These results thus indicate that the cytoplasmic localization for murine and human HDAC6 proteins is differentially regulated and suggest that the tetradecapeptide repeat domain serves as an important sequence element to stably retain human HDAC6 in the cytoplasm.
Collapse
Affiliation(s)
- Nicholas R Bertos
- Molecular Oncology Group, Department of Medicine, McGill University Health Centre, Montreal, Quebec H3A 1A1, Canada
| | | | | | | | | | | |
Collapse
|
177
|
Sampath SC, Ohi R, Leismann O, Salic A, Pozniakovski A, Funabiki H. The chromosomal passenger complex is required for chromatin-induced microtubule stabilization and spindle assembly. Cell 2004; 118:187-202. [PMID: 15260989 DOI: 10.1016/j.cell.2004.06.026] [Citation(s) in RCA: 323] [Impact Index Per Article: 16.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/24/2004] [Revised: 06/08/2004] [Accepted: 06/11/2004] [Indexed: 11/16/2022]
Abstract
In cells lacking centrosomes, such as those found in female meiosis, chromosomes must nucleate and stabilize microtubules in order to form a bipolar spindle. Here we report the identification of Dasra A and Dasra B, two new components of the vertebrate chromosomal passenger complex containing Incenp, Survivin, and the kinase Aurora B, and demonstrate that this complex is required for chromatin-induced microtubule stabilization and spindle formation. The failure of microtubule stabilization caused by depletion of the chromosomal passenger complex was rescued by codepletion of the microtubule-depolymerizing kinesin MCAK, whose activity is negatively regulated by Aurora B. By contrast, we present evidence that the Ran-GTP pathway of chromatin-induced microtubule nucleation does not require the chromosomal passenger complex, indicating that the mechanisms of microtubule assembly by these two pathways are distinct. We propose that the chromosomal passenger complex regulates local MCAK activity to permit spindle formation via stabilization of chromatin-associated microtubules.
Collapse
Affiliation(s)
- Srinath C Sampath
- Laboratory of Chromosome and Cell Biology, The Rockefeller University, New York, NY 10021, USA
| | | | | | | | | | | |
Collapse
|
178
|
Abstract
In all eukaryotes, morphogenesis of the microtubule cytoskeleton into a bipolar spindle is required for the faithful transmission of the genome to the two daughter cells during division. This process is facilitated by the intrinsic polarity and dynamic properties of microtubules and involves many proteins that modulate microtubule organization and stability. Recent work has begun to uncover the molecular mechanisms behind these dynamic events. Here we describe current models and discuss some of the complex repertoire of factors required for spindle assembly and chromosome segregation.
Collapse
Affiliation(s)
- Sharat Gadde
- Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, California 94720-3200, USA
| | | |
Collapse
|
179
|
Taimen P, Parvinen M, Osborn M, Kallajoki M. NuMA in rat testis—Evidence for roles in proliferative activity and meiotic cell division. Exp Cell Res 2004; 298:512-20. [PMID: 15265698 DOI: 10.1016/j.yexcr.2004.05.002] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/13/2004] [Revised: 04/13/2004] [Indexed: 10/26/2022]
Abstract
NuMA is a well-characterized organizer of the mitotic spindle, which is believed to play a structural role in interphase nucleus. We studied the expression of NuMA in rat seminiferous epithelium in detail. Different stages of the cycle of the seminiferous epithelium were identified using transillumination. Corresponding areas were microdissected and analysed using immunofluorescence, immunohistochemistry, or immunoblotting. NuMA was expressed in Sertoli cells, proliferating type A and B spermatogonia, and early spermatids but it was absent in late spermatids and mature spermatozoa. Interestingly, NuMA-positive primary spermatocytes lost their nuclear NuMA at the beginning of long-lasting prophase of the first meiotic division. A strong expression was again observed at the end of the prophase and finally, a redistribution of NuMA into pole regions of the meiotic spindle was observed in first and second meiotic divisions. In immunoblotting, a single 250-kDa protein present in all stages of the rat seminiferous epithelial cycle was detected. Our results show that NuMA is not essential for the organization of nuclear structure in all cell types and suggest that its presence is more likely connected to the proliferation phase of the cells. They also suggest that NuMA may play an important role in meiotic cell division.
Collapse
Affiliation(s)
- Pekka Taimen
- Department of Anatomy, University of Turku, FIN-20520 Turku, Finland.
| | | | | | | |
Collapse
|
180
|
Shi WY, Skeath JB. The Drosophila RCC1 homolog, Bj1, regulates nucleocytoplasmic transport and neural differentiation during Drosophila development. Dev Biol 2004; 270:106-21. [PMID: 15136144 DOI: 10.1016/j.ydbio.2004.02.011] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/07/2003] [Revised: 02/10/2004] [Accepted: 02/12/2004] [Indexed: 11/18/2022]
Abstract
The Bj1 gene encodes the Drosophila homolog of RCC1, the guanine-nucleotide exchange factor for RanGTPase. Here, we provide the first phenotypic characterization of a RCC1 homolog in a developmental model system. We identified Bj1 (dRCC1) in a genetic screen to identify mutations that alter central nervous system development. We find that zygotic dRCC1 mutant embryos exhibit specific defects in the development and differentiation of lateral CNS neurons although cell division and the cell cycle appear grossly normal. dRCC1 mutant nerve cords contain abnormally large cells with compartmentalized nuclei and exhibit increased transcription in the lateral CNS. As RCC1 is an important component of the nucleocytoplasmic transport machinery, we find that dRCC1 function is required for nuclear import of nuclear localization signal sequence (NLS)-carrying cargo molecules. Finally, we show that dRCC1 is required for cell proliferation and/or survival during germline, eye and wing development and that dRCC1 appears to facilitate apoptosis.
Collapse
Affiliation(s)
- Wei-Yang Shi
- Program in Developmental Biology, Washington University School of Medicine, St. Louis, MO 63110, USA
| | | |
Collapse
|
181
|
Zimmerman WC, Sillibourne J, Rosa J, Doxsey SJ. Mitosis-specific anchoring of gamma tubulin complexes by pericentrin controls spindle organization and mitotic entry. Mol Biol Cell 2004; 15:3642-57. [PMID: 15146056 PMCID: PMC491825 DOI: 10.1091/mbc.e03-11-0796] [Citation(s) in RCA: 227] [Impact Index Per Article: 11.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022] Open
Abstract
Microtubule nucleation is the best known function of centrosomes. Centrosomal microtubule nucleation is mediated primarily by gamma tubulin ring complexes (gamma TuRCs). However, little is known about the molecules that anchor these complexes to centrosomes. In this study, we show that the centrosomal coiled-coil protein pericentrin anchors gamma TuRCs at spindle poles through an interaction with gamma tubulin complex proteins 2 and 3 (GCP2/3). Pericentrin silencing by small interfering RNAs in somatic cells disrupted gamma tubulin localization and spindle organization in mitosis but had no effect on gamma tubulin localization or microtubule organization in interphase cells. Similarly, overexpression of the GCP2/3 binding domain of pericentrin disrupted the endogenous pericentrin-gamma TuRC interaction and perturbed astral microtubules and spindle bipolarity. When added to Xenopus mitotic extracts, this domain uncoupled gamma TuRCs from centrosomes, inhibited microtubule aster assembly, and induced rapid disassembly of preassembled asters. All phenotypes were significantly reduced in a pericentrin mutant with diminished GCP2/3 binding and were specific for mitotic centrosomal asters as we observed little effect on interphase asters or on asters assembled by the Ran-mediated centrosome-independent pathway. Additionally, pericentrin silencing or overexpression induced G2/antephase arrest followed by apoptosis in many but not all cell types. We conclude that pericentrin anchoring of gamma tubulin complexes at centrosomes in mitotic cells is required for proper spindle organization and that loss of this anchoring mechanism elicits a checkpoint response that prevents mitotic entry and triggers apoptotic cell death.
Collapse
Affiliation(s)
- Wendy C Zimmerman
- Department of Molecular Medicine, University of Massachusetts Medical School, Worcester, Massachusetts 01605, USA
| | | | | | | |
Collapse
|
182
|
Li HY, Zheng Y. Phosphorylation of RCC1 in mitosis is essential for producing a high RanGTP concentration on chromosomes and for spindle assembly in mammalian cells. Genes Dev 2004; 18:512-27. [PMID: 15014043 PMCID: PMC374234 DOI: 10.1101/gad.1177304] [Citation(s) in RCA: 106] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022]
Abstract
Spindle assembly is subject to the regulatory controls of both the cell-cycle machinery and the Ran-signaling pathway. An important question is how the two regulatory pathways communicate with each other to achieve coordinated regulation in mitosis. We show here that Cdc2 kinase phosphorylates the serines located in or near the nuclear localization signal (NLS) of human RCC1, the nucleotide exchange factor for Ran. This phosphorylation is necessary for RCC1 to generate RanGTP on mitotic chromosomes in mammalian cells, which in turn is required for spindle assembly and chromosome segregation. Moreover, phosphorylation of the NLS of RCC1 is required to prevent the binding of importin alpha and beta to RCC1, thereby allowing RCC1 to couple RanGTP production to chromosome binding. These findings reveal that the cell-cycle machinery directly regulates the Ran-signaling pathway by placing a high RanGTP concentration on the mitotic chromosome in mammalian cells.
Collapse
Affiliation(s)
- Hoi-Yeung Li
- Department of Embryology, Howard Hughes Medical Institute, Carnegie Institution of Washington, Baltimore, Maryland 21210, USA
| | | |
Collapse
|
183
|
Ou Y, Rattner JB. The Centrosome in Higher Organisms: Structure, Composition, and Duplication. INTERNATIONAL REVIEW OF CYTOLOGY 2004; 238:119-82. [PMID: 15364198 DOI: 10.1016/s0074-7696(04)38003-4] [Citation(s) in RCA: 42] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/23/2022]
Abstract
The centrosome found in higher organisms is an organelle with a complex and dynamic architecture and composition. This organelle not only functions as a microtubule-organizing center, but also is integrated with or impacts a number of cellular processes. Defects associated with this organelle have been linked to a variety of human diseases including several forms of cancer. Here we review the emerging picture of how the structure, composition, duplication, and function of the centrosome found in higher organisms are interrelated.
Collapse
Affiliation(s)
- Young Ou
- Department of Cell Biology and Anatomy, University of Calgary 3330 Hospital Drive NW, Calgary, Alberta, Canada
| | | |
Collapse
|
184
|
Fant X, Merdes A, Haren L. Cell and molecular biology of spindle poles and NuMA. INTERNATIONAL REVIEW OF CYTOLOGY 2004; 238:1-57. [PMID: 15364196 DOI: 10.1016/s0074-7696(04)38001-0] [Citation(s) in RCA: 49] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/02/2022]
Abstract
Mitotic and meiotic cells contain a bipolar spindle apparatus of microtubules and associated proteins. To arrange microtubules into focused spindle poles, different mechanisms are used by various organisms. Principally, two major pathways have been characterized: nucleation and anchorage of microtubules at preexisting centers such as centrosomes or spindle pole bodies, or microtubule growth off the surface of chromosomes, followed by sorting and focusing into spindle poles. These two mechanisms can even be found in cells of the same organism: whereas most somatic animal cells utilize the centrosome as an organizing center for spindle microtubules, female meiotic cells build an acentriolar spindle apparatus. Most interestingly, the molecular components that drive acentriolar spindle pole formation are also present in cells containing centrosomes. They include microtubule-dependent motor proteins and a variety of structural proteins that regulate microtubule orientation, anchoring, and stability. The first of these spindle pole proteins, NuMA, had already been identified more than 20 years ago. In addition, several new proteins have been characterized more recently. This review discusses their role during spindle formation and their regulation in the cell cycle.
Collapse
Affiliation(s)
- Xavier Fant
- Wellcome Trust Centre for Cell Biology, University of Edinburgh, King's Buildings, Edinburgh EH9 3JR, United Kingdom
| | | | | |
Collapse
|
185
|
Trieselmann N, Armstrong S, Rauw J, Wilde A. Ran modulates spindle assembly by regulating a subset of TPX2 and Kid activities including Aurora A activation. J Cell Sci 2003; 116:4791-8. [PMID: 14600264 DOI: 10.1242/jcs.00798] [Citation(s) in RCA: 95] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Ran, a GTPase in the Ras superfamily, is proposed to be a spatial regulator of microtubule spindle assembly by maintaining key spindle assembly factors in an active state close to chromatin. RanGTP is hypothesized to maintain the spindle assembly factors in the active state by binding to importin β, part of the nuclear transport receptor complex, thereby preventing the inhibitory binding of the nuclear transport receptors to spindle assembly factors. To directly test this hypothesis, two putative downstream targets of the Ran spindle assembly pathway, TPX2, a protein required for correct spindle assembly and Kid, a chromokinesin involved in chromosome arm orientation on the spindle, were analyzed to determine if their direct binding to nuclear transport receptors inhibited their function. In the amino-terminal domain of TPX2 we identified nuclear targeting information, microtubule-binding and Aurora A binding activities. Nuclear transport receptor binding to TPX2 inhibited Aurora A binding activity but not the microtubule-binding activity of TPX2. Inhibition of the interaction between TPX2 and Aurora A prevented Aurora A activation and recruitment to microtubules. In addition we identified nuclear targeting information in both the amino-terminal microtubule-binding domain and the carboxy-terminal DNA binding domain of Kid. However, the binding of nuclear transport receptors to Kid only inhibited the microtubule-binding activity of Kid. Therefore, by regulating a subset of TPX2 and Kid activities, Ran modulates at least two processes involved in spindle assembly.
Collapse
Affiliation(s)
- Nadia Trieselmann
- Department of Medical Genetics and Microbiology, University of Toronto, 1 King's College Circle, Toronto, Ontario, Canada, M5S 1A8
| | | | | | | |
Collapse
|
186
|
Mason DA, Máthé E, Fleming RJ, Goldfarb DS. The Drosophila melanogaster importin α3 Locus Encodes an Essential Gene Required for the Development of Both Larval and Adult Tissues. Genetics 2003; 165:1943-58. [PMID: 14704178 PMCID: PMC1462915 DOI: 10.1093/genetics/165.4.1943] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
Abstract
The nuclear transport of classical nuclear localization signal (cNLS)-containing proteins is mediated by the cNLS receptor importin α. The conventional importin α gene family in metazoan animals is composed of three clades that are conserved between flies and mammals and are referred to here as α1, α2, and α3. In contrast, plants and fungi contain only α1 genes. In this study we report that Drosophila importin α3 is required for the development of both larval and adult tissues. Importin α3 mutant flies die around the transition from first to second instar larvae, and homozygous importin α3 mutant eyes are defective. The transition to second instar larvae was rescued with importin α1, α2, or α3 transgenes, indicating that Importin α3 is normally required at this stage for an activity shared by all three importin α's. In contrast, an α3-specific biochemical activity(s) of Importin α3 is probably required for development to adults and photoreceptor cell development, since only an importin α3 transgene rescued these processes. These results are consistent with the view that the importin α's have both overlapping and distinct functions and that their role in animal development involves the spatial and temporal control of their expression.
Collapse
Affiliation(s)
- D Adam Mason
- Department of Biology, University of Rochester, Rochester, New York 14627, USA
| | | | | | | |
Collapse
|
187
|
Merkle T. Nucleo-cytoplasmic partitioning of proteins in plants: implications for the regulation of environmental and developmental signalling. Curr Genet 2003; 44:231-60. [PMID: 14523572 DOI: 10.1007/s00294-003-0444-x] [Citation(s) in RCA: 62] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/29/2003] [Revised: 08/21/2003] [Accepted: 09/01/2003] [Indexed: 12/21/2022]
Abstract
Considerable progress has been made in the past few years in characterising Arabidopsis nuclear transport receptors and in elucidating plant signal transduction pathways that employ nucleo-cytoplasmic partitioning of a member of the signal transduction chain. This review briefly introduces the major principles of nuclear transport of macromolecules across the nuclear envelope and the proteins involved, as they have been described in vertebrates and yeast. Proteins of the plant nuclear transport machinery that have been identified to date are discussed, the focus being on Importin beta-like nuclear transport receptors. Finally, the importance of nucleo-cytoplasmic partitioning as a regulatory tool for signalling is highlighted, and different plant signal transduction pathways that make use of this regulatory potential are presented.
Collapse
Affiliation(s)
- Thomas Merkle
- Institute of Biology II, Cell Biology, University of Freiburg, Schänzlestrasse 1, 79104 Freiburg, Germany.
| |
Collapse
|
188
|
Ems-McClung SC, Zheng Y, Walczak CE. Importin alpha/beta and Ran-GTP regulate XCTK2 microtubule binding through a bipartite nuclear localization signal. Mol Biol Cell 2003; 15:46-57. [PMID: 13679510 PMCID: PMC307526 DOI: 10.1091/mbc.e03-07-0454] [Citation(s) in RCA: 111] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022] Open
Abstract
The small GTPase Ran is essential for spindle assembly. Ran is proposed to act through its nuclear import receptors importin alpha and/or importin beta to control the sequestration of proteins necessary for spindle assembly. To date, the molecular mechanisms by which the Ran pathway functions remain unclear. Using purified proteins, we have reconstituted Ran-regulated microtubule binding of the C-terminal kinesin XCTK2, a kinesin important for spindle assembly. We show that the tail of XCTK2 binds to microtubules and that this binding is inhibited in the presence of importin alpha and beta (alpha/beta) and restored by addition of Ran-GTP. The bipartite nuclear localization signal (NLS) in the tail of XCTK2 is essential to this process, because mutation of the NLS abolishes importin alpha/beta-mediated regulation of XCTK2 microtubule binding. Our data show that importin alpha/beta directly regulates the activity of XCTK2 and that one of the molecular mechanisms of Ran-regulated spindle assembly is identical to that used in classical NLS-driven nuclear transport.
Collapse
|
189
|
Raemaekers T, Ribbeck K, Beaudouin J, Annaert W, Van Camp M, Stockmans I, Smets N, Bouillon R, Ellenberg J, Carmeliet G. NuSAP, a novel microtubule-associated protein involved in mitotic spindle organization. J Cell Biol 2003; 162:1017-29. [PMID: 12963707 PMCID: PMC2172854 DOI: 10.1083/jcb.200302129] [Citation(s) in RCA: 169] [Impact Index Per Article: 8.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Here, we report on the identification of nucleolar spindle-associated protein (NuSAP), a novel 55-kD vertebrate protein with selective expression in proliferating cells. Its mRNA and protein levels peak at the transition of G2 to mitosis and abruptly decline after cell division. Microscopic analysis of both fixed and live mammalian cells showed that NuSAP is primarily nucleolar in interphase, and localizes prominently to central spindle microtubules during mitosis. Direct interaction of NuSAP with microtubules was demonstrated in vitro. Overexpression of NuSAP caused profound bundling of cytoplasmic microtubules in interphase cells, and this relied on a COOH-terminal microtubule-binding domain. In contrast, depletion of NuSAP by RNA interference resulted in aberrant mitotic spindles, defective chromosome segregation, and cytokinesis. In addition, many NuSAP-depleted interphase cells had deformed nuclei. Both overexpression and knockdown of NuSAP impaired cell proliferation. These results suggest a crucial role for NuSAP in spindle microtubule organization.
Collapse
Affiliation(s)
- Tim Raemaekers
- Laboratory for Experimental Medicine and Endocrinology, Katholieke Universiteit Leuven, Leuven, Belgium
| | | | | | | | | | | | | | | | | | | |
Collapse
|
190
|
Di Fiore B, Ciciarello M, Mangiacasale R, Palena A, Tassin AM, Cundari E, Lavia P. Mammalian RanBP1 regulates centrosome cohesion during mitosis. J Cell Sci 2003; 116:3399-411. [PMID: 12840069 DOI: 10.1242/jcs.00624] [Citation(s) in RCA: 64] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
The Ran GTPase plays a central function in control of nucleo-cytoplasmic transport in interphase. Mitotic roles of Ran have also been firmly established in Xenopus oocyte extracts. In this system, Ran-GTP, or the RCC1 exchange factor for Ran, drive spindle assembly by regulating the availability of 'aster-promoting activities'. In previous studies to assess whether the Ran network also influences mitosis in mammalian cells, we found that overexpression of Ran-binding protein 1 (RanBP1), a major effector of Ran, induces multipolar spindles. We now show that these abnormal spindles are generated through loss of cohesion in mitotic centrosomes. Specifically, RanBP1 excess induces splitting of mother and daughter centrioles at spindle poles; the resulting split centrioles can individually organize functional microtubule arrays, giving rise to functional spindle poles. RanBP1-dependent centrosome splitting is specifically induced in mitosis and requires microtubule integrity and Eg5 activity. In addition, we have identified a fraction of RanBP1 at the centrosome. These data indicate that overexpressed RanBP1 interferes with crucial factor(s) that control structural and dynamic features of centrosomes during mitosis and contribute to uncover novel mitotic functions downstream of the Ran network.
Collapse
Affiliation(s)
- Barbara Di Fiore
- CNR Institute of Molecular Biology and Pathology, Section of Genetics, c/o University 'La Sapienza', 00185 Rome, Italy
| | | | | | | | | | | | | |
Collapse
|
191
|
Harel A, Chan RC, Lachish-Zalait A, Zimmerman E, Elbaum M, Forbes DJ. Importin beta negatively regulates nuclear membrane fusion and nuclear pore complex assembly. Mol Biol Cell 2003; 14:4387-96. [PMID: 14551248 PMCID: PMC266759 DOI: 10.1091/mbc.e03-05-0275] [Citation(s) in RCA: 119] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022] Open
Abstract
Assembly of a eukaryotic nucleus involves three distinct events: membrane recruitment, fusion to form a double nuclear membrane, and nuclear pore complex (NPC) assembly. We report that importin beta negatively regulates two of these events, membrane fusion and NPC assembly. When excess importin beta is added to a full Xenopus nuclear reconstitution reaction, vesicles are recruited to chromatin but their fusion is blocked. The importin beta down-regulation of membrane fusion is Ran-GTP reversible. Indeed, excess RanGTP (RanQ69L) alone stimulates excessive membrane fusion, leading to intranuclear membrane tubules and cytoplasmic annulate lamellae-like structures. We propose that a precise balance of importin beta to Ran is required to create a correct double nuclear membrane and simultaneously to repress undesirable fusion events. Interestingly, truncated importin beta 45-462 allows membrane fusion but produces nuclei lacking any NPCs. This reveals distinct importin beta-regulation of NPC assembly. Excess full-length importin beta and beta 45-462 act similarly when added to prefused nuclear intermediates, i.e., both block NPC assembly. The importin beta NPC block, which maps downstream of GTPgammaS and BAPTA-sensitive steps in NPC assembly, is reversible by cytosol. Remarkably, it is not reversible by 25 microM RanGTP, a concentration that easily reverses fusion inhibition. This report, using a full reconstitution system and natural chromatin substrates, significantly expands the repertoire of importin beta. Its roles now encompass negative regulation of two of the major events of nuclear assembly: membrane fusion and NPC assembly.
Collapse
Affiliation(s)
- Amnon Harel
- Section of Cell and Developmental Biology, Division of Biological Sciences 0347, University of California, San Diego, La Jolla, California 92093-0347, USA
| | | | | | | | | | | |
Collapse
|
192
|
Keryer G, Di Fiore B, Celati C, Lechtreck KF, Mogensen M, Delouvee A, Lavia P, Bornens M, Tassin AM. Part of Ran is associated with AKAP450 at the centrosome: involvement in microtubule-organizing activity. Mol Biol Cell 2003; 14:4260-71. [PMID: 14517334 PMCID: PMC207017 DOI: 10.1091/mbc.e02-11-0773] [Citation(s) in RCA: 102] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/30/2023] Open
Abstract
The small Ran GTPase, a key regulator of nucleocytoplasmic transport, is also involved in microtubule assembly and nuclear membrane formation. Herein, we show by immunofluorescence, immunoelectron microscopy, and biochemical analysis that a fraction of Ran is tightly associated with the centrosome throughout the cell cycle. Ran interaction with the centrosome is mediated by the centrosomal matrix A kinase anchoring protein (AKAP450). Accordingly, when AKAP450 is delocalized from the centrosome, Ran is also delocalized, and as a consequence, microtubule regrowth or anchoring is altered, despite the persisting association of gamma-tubulin with the centrosome. Moreover, Ran is recruited to Xenopus sperm centrosome during its activation for microtubule nucleation. We also demonstrate that centrosomal proteins such as centrin and pericentrin, but not gamma-tubulin, AKAP450, or ninein, undertake a nucleocytoplasmic exchange as they concentrate in the nucleus upon export inhibition by leptomycin B. Together, these results suggest a challenging possibility, namely, that centrosome activity could depend upon nucleocytoplasmic exchange of centrosomal proteins and local Ran-dependent concentration at the centrosome.
Collapse
Affiliation(s)
- Guy Keryer
- Institut Curie, Section Recherche, Unité Mixte Recherche 144-Centre National de la Recherche Scientifique, 75248 Paris, France
| | | | | | | | | | | | | | | | | |
Collapse
|
193
|
Tirián L, Timinszky G, Szabad J. P446L-importin-beta inhibits nuclear envelope assembly by sequestering nuclear envelope assembly factors to the microtubules. Eur J Cell Biol 2003; 82:351-9. [PMID: 12924630 DOI: 10.1078/0171-9335-00324] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The P446L mutant Drosophila importin-beta (P446L-imp-beta) has been reported to prohibit--in dominant negative fashion--nuclear envelope (NE) assembly. Along elucidating the mode of action of P446L-imp-beta we studied in vitro NE assembly on Sepharose beads. While Drosophila embryo extracts support NE assembly over Sepharose beads coated with Ran, NE assembly does not take place in extracts supplied with exogenous P446L-imp-beta. A NE also forms over importin-beta-coated beads. Surprisingly, when immobilized to Sepharose beads P446L-imp-beta as efficiently recruits NE vesicles as normal importin-beta. The discrepancy in behavior of cytoplasmic and bead-bound P446L-imp-beta appears to be related to icreased--as compared to normal importin-beta--microtubule (MT) binding ability of P446L-imp-beta. While wild-type importin-beta is able to bind MTs and the binding decreases upon RanGTP interaction, P446L-imp-beta cannot be removed from the MTs by RanGTP. P446L-imp-beta, like normal importin-beta, binds some types of the nucleoporins that have been known to be required for NE assembly at the end of mitosis. It appears that the inhibitory effect of P446L-imp-beta on NE assembly is caused by sequestering some of the nucleoporins required for NE assembly to the MTs.
Collapse
Affiliation(s)
- László Tirián
- The University of Szeged, Faculty of Medicine, Department of Biology, Szeged, Hungary
| | | | | |
Collapse
|
194
|
Abstract
Spindle microtubules interact with mitotic chromosomes, binding to their kinetochores to generate forces that are important for accurate chromosome segregation. Motor enzymes localized both at kinetochores and spindle poles help to form the biologically significant attachments between spindle fibers and their cargo, but microtubule-associated proteins without motor activity contribute to these junctions in important ways. This review examines the molecules necessary for chromosome-microtubule interaction in a range of well-studied organisms, using biological diversity to identify the factors that are essential for organized chromosome movement. We conclude that microtubule dynamics and the proteins that control them are likely to be more important for mitosis than the current enthusiasm for motor enzymes would suggest.
Collapse
Affiliation(s)
- J Richard McIntosh
- Department of Molecular Cellular and Developmental Biology, University of Colorado, Boulder 80309-0347, USA.
| | | | | |
Collapse
|
195
|
Radisavljevic ZM, González-Flecha B, Manasija-Radisavljevic Z. Signaling through Cdk2, importin-alpha and NuMA is required for H2O2-induced mitosis in primary type II pneumocytes. BIOCHIMICA ET BIOPHYSICA ACTA 2003; 1640:163-70. [PMID: 12729926 DOI: 10.1016/s0167-4889(03)00044-2] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
Abstract
Proliferation of alveolar type II pneumocytes, the multipotent stem cells of the alveoli, has been implicated in the development of lung adenocarcinoma. Hydrogen peroxide (H(2)O(2)), a potent promoter of signaling cascades, can mediate the transmission of many intracellular signals including those involved in cell proliferation. In this study using rat primary type II pneumocytes, we demonstrate that H(2)O(2) significantly increases mitosis through a pathway that includes cyclin-dependent kinase 2 (Cdk2); importin-alpha, a nuclear trafficking regulator; and nuclear mitotic apparatus protein (NuMA), an essential component in mitotic spindle pole formation. Upon H(2)O(2) treatment, Cdk2 is phosphorylated at position thr-160 leading to increases in importin-alpha and NuMA protein levels and resulting in a significant increase of G(2)/M phase in a roscovatine-dependent manner. Type II pneumocytes transfected with NuMA cDNA also show significant increases in G(2)/M phase, NuMA, Cdk2 thr-160 and importin-alpha expression. These effects were prevented by catalase. These results demonstrate that H(2)O(2) orchestrates a complex signaling network regulating S phase entry, nuclear trafficking and spindle pole formation through activation of Cdk2, importin-alpha, and NuMA. This pathway is essential for H(2)O(2)-induced mitosis in type II pneumocytes.
Collapse
Affiliation(s)
- Ziv Manasija Radisavljevic
- Department of Environmental Health, Physiology Program, Harvard University, School of Public Health, 665 Huntington Avenue, Boston, MA 02115, USA.
| | | | | |
Collapse
|
196
|
Ovechkina Y, Maddox P, Oakley CE, Xiang X, Osmani SA, Salmon ED, Oakley BR. Spindle formation in Aspergillus is coupled to tubulin movement into the nucleus. Mol Biol Cell 2003; 14:2192-200. [PMID: 12802085 PMCID: PMC165107 DOI: 10.1091/mbc.e02-10-0641] [Citation(s) in RCA: 47] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022] Open
Abstract
In many important organisms, including many algae and most fungi, the nuclear envelope does not disassemble during mitosis. This fact raises the possibility that mitotic onset and/or exit might be regulated, in part, by movement of important mitotic proteins into and out of the nucleoplasm. We have used two methods to determine whether tubulin levels in the nucleoplasm are regulated in the fungus Aspergillus nidulans. First, we have used benomyl to disassemble microtubules and create a pool of free tubulin that can be readily observed by immunofluorescence. We find that tubulin is substantially excluded from interphase nuclei, but is present in mitotic nuclei. Second, we have observed a green fluorescent protein/alpha-tubulin fusion in living cells by time-lapse spinning-disk confocal microscopy. We find that tubulin is excluded from interphase nuclei, enters the nucleus seconds before the mitotic spindle begins to form, and is removed from the nucleoplasm during the M-to-G1 transition. Our data indicate that regulation of intranuclear tubulin levels plays an important, perhaps essential, role in the control of mitotic spindle formation in A. nidulans. They suggest that regulation of protein movement into the nucleoplasm may be important for regulating mitotic onset in organisms with intranuclear mitosis.
Collapse
Affiliation(s)
- Yulia Ovechkina
- Department of Molecular Genetics, The Ohio State University, Columbus 43210, USA
| | | | | | | | | | | | | |
Collapse
|
197
|
Schatz CA, Santarella R, Hoenger A, Karsenti E, Mattaj IW, Gruss OJ, Carazo-Salas RE. Importin alpha-regulated nucleation of microtubules by TPX2. EMBO J 2003; 22:2060-70. [PMID: 12727873 PMCID: PMC156067 DOI: 10.1093/emboj/cdg195] [Citation(s) in RCA: 151] [Impact Index Per Article: 7.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022] Open
Abstract
The importin alpha-regulated microtubule-associated protein TPX2 is known to be critical for meiotic and mitotic spindle formation in vertebrates, but its detailed mechanism of action and regulation is not understood. Here, the site of interaction on TPX2 for importin alpha is mapped. A TPX2 mutant that cannot bind importin alpha is constitutively active in the induction of microtubule-containing aster-like structures in Xenopus egg extract, demonstrating that no other importin alpha or RanGTPase target is required to mediate microtubule assembly in this system. Further, recombinant TPX2 is shown to induce the formation and bundling of microtubules in dilute solutions of pure tubulin. In this purified system, importin alpha prevents TPX2-induced microtubule formation, but not TPX2-tubulin interaction or microtubule bundling. This demonstrates that TPX2 has more than one mode of interaction with tubulin and that only one of these types of interaction is abolished by importin alpha. The data suggest that the critical early function in spindle formation regulated by importin alpha is TPX2-mediated microtubule nucleation.
Collapse
Affiliation(s)
- Christoph A Schatz
- European Molecular Biology Laboratory, Meyerhofstrasse 1, D-69117 Heidelberg, Germany
| | | | | | | | | | | | | |
Collapse
|
198
|
Kim SH, Roux SJ. An Arabidopsis Ran-binding protein, AtRanBP1c, is a co-activator of Ran GTPase-activating protein and requires the C-terminus for its cytoplasmic localization. PLANTA 2003; 216:1047-1052. [PMID: 12687374 DOI: 10.1007/s00425-002-0959-2] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/30/2002] [Accepted: 11/16/2002] [Indexed: 05/24/2023]
Abstract
Ran-binding proteins (RanBPs) are a group of proteins that bind to Ran (Ras-related nuclear small GTP-binding protein), and thus either control the GTP/GDP-bound states of Ran or help couple the Ran GTPase cycle to a cellular process. AtRanBP1c is a Ran-binding protein from Arabidopsis thaliana (L.) Heynh. that was recently shown to be critically involved in the regulation of auxin-induced mitotic progression [S.-H. Kim et al. (2001) Plant Cell 13:2619-2630]. Here we report that AtRanBP1c inhibits the EDTA-induced release of GTP from Ran and serves as a co-activator of Ran-GTPase-activating protein (RanGAP) in vitro. Transient expression of AtRanBP1c fused to a beta-glucuronidase (GUS) reporter reveals that the protein localizes primarily to the cytosol. Neither the N- nor C-terminus of AtRanBP1c, which flank the Ran-binding domain (RanBD), is necessary for the binding of PsRan1-GTP to the protein, but both are needed for the cytosolic localization of GUS-fused AtRanBP1c. These findings, together with a previous report that AtRanBP1c is critically involved in root growth and development, imply that the promotion of GTP hydrolysis by the Ran/RanGAP/AtRanBP1c complex in the cytoplasm, and the resulting concentration gradient of Ran-GDP to Ran-GTP across the nuclear membrane could be important in the regulation of auxin-induced mitotic progression in root tips of A. thaliana.
Collapse
Affiliation(s)
- Soo-Hwan Kim
- Section of Molecular Cell and Developmental Biology, University of Texas, Austin, TX 78712, USA
| | | |
Collapse
|
199
|
Abstract
Here, we report the first evidence that the Ran GTPase cycle is required for nuclear pore complex (NPC) assembly. Using a genetic approach, factors required for NPC assembly were identified in Saccharomyces cerevisiae. Four mutant complementation groups were characterized that correspond to respective mutations in genes encoding Ran (gsp1), and essential Ran regulatory factors Ran GTPase-activating protein (rna1), Ran guanine nucleotide exchange factor (prp20), and the RanGDP import factor (ntf2). All the mutants showed temperature-dependent mislocalization of green fluorescence protein (GFP)-tagged nucleoporins (nups) and the pore-membrane protein Pom152. A decrease in GFP fluorescence associated with the nuclear envelope was observed along with an increase in the diffuse, cytoplasmic signal with GFP foci. The defects did not affect the stability of existing NPCs, and nup mislocalization was dependent on de novo protein synthesis and continued cell growth. Electron microscopy analysis revealed striking membrane perturbations and the accumulation of vesicles in arrested mutants. Using both biochemical fractionation and immunoelectron microscopy methods, these vesicles were shown to contain nups. We propose a model wherein a Ran-mediated vesicular fusion step is required for NPC assembly into intact nuclear envelopes.
Collapse
Affiliation(s)
- Kathryn J Ryan
- Department of Cell and Developmental Biology, Vanderbilt University Medical Center, Nashville, TN 37232, USA
| | | | | |
Collapse
|
200
|
Görlich D, Seewald MJ, Ribbeck K. Characterization of Ran-driven cargo transport and the RanGTPase system by kinetic measurements and computer simulation. EMBO J 2003; 22:1088-100. [PMID: 12606574 PMCID: PMC150346 DOI: 10.1093/emboj/cdg113] [Citation(s) in RCA: 177] [Impact Index Per Article: 8.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
Here, we analyse the RanGTPase system and its coupling to receptor-mediated nuclear transport. Our simulations predict nuclear RanGTP levels in HeLa cells to be very sensitive towards the cellular energy charge and to exceed the cytoplasmic concentration approximately 1000-fold. The steepness of the RanGTP gradient appears limited by both the cytoplasmic RanGAP concentration and the imperfect retention of nuclear RanGTP by nuclear pore complexes (NPCs), but not by the nucleotide exchange activity of RCC1. Neither RanBP1 nor the NPC localization of RanGAP has a significant direct impact on the RanGTP gradient. NTF2-mediated import of Ran appears to be the bottleneck for maximal capacity of Ran-driven nuclear transport. We show that unidirectional nuclear transport can be faithfully simulated without the assumption of a vectorial NPC passage; transport receptors only need to reversibly cross NPCs and switch their affinity for cargo in response to the RanGTP gradient. A significant RanGTP gradient after nuclear envelope (NE) breakdown can apparently exist only in large cytoplasm. This indicates that RanGTP gradients can provide positional information for mitotic spindle and NE assembly in early embryonic cells, but hardly any in small somatic cells.
Collapse
Affiliation(s)
- Dirk Görlich
- ZMBH, INF 282, 69120 Heidelberg and
MPI für Molekulare Physiologie, Otto-Hahn-Str. 11, 44227 Dortmund, Germany Present address: EMBL, Meyerhofstraße 1, Heidelberg, Germany Corresponding author e-mail:
| | | | - Katharina Ribbeck
- ZMBH, INF 282, 69120 Heidelberg and
MPI für Molekulare Physiologie, Otto-Hahn-Str. 11, 44227 Dortmund, Germany Present address: EMBL, Meyerhofstraße 1, Heidelberg, Germany Corresponding author e-mail:
| |
Collapse
|