Role of importin-beta in coupling Ran to downstream targets in microtubule assembly

Ran-GTP assembles a specialized spindle structure for accurate chromosome segregation in medaka early embryos

Despite drastic cellular changes during cleavage, a mitotic spindle assembles in each blastomere to accurately segregate duplicated chromosomes. Mechanisms of mitotic spindle assembly have been extensively studied using small somatic cells. However, mechanisms of spindle assembly in large vertebrate embryos remain little understood. Here, we establish functional assay systems in medaka (Oryzias latipes) embryos by combining CRISPR knock-in with auxin-inducible degron technology. Live imaging reveals several unexpected features of microtubule organization and centrosome positioning that achieve rapid, accurate cleavage. Importantly, Ran-GTP assembles a dense microtubule network at the metaphase spindle center that is essential for chromosome segregation in early embryos. This unique spindle structure is remodeled into a typical short, somatic-like spindle after blastula stages, when Ran-GTP becomes dispensable for chromosome segregation. We propose that despite the presence of centrosomes, the chromosome-derived Ran-GTP pathway has essential roles in functional spindle assembly in large, rapidly dividing vertebrate early embryos, similar to acentrosomal spindle assembly in oocytes.

Mechanisms underlying spindle assembly and robustness

The microtubule-based spindle orchestrates chromosome segregation during cell division. Following more than a century of study, many components and pathways contributing to spindle assembly have been described, but how the spindle robustly assembles remains incompletely understood. This process involves the self-organization of a large number of molecular parts - up to hundreds of thousands in vertebrate cells - whose local interactions give rise to a cellular-scale structure with emergent architecture, mechanics and function. In this Review, we discuss key concepts in our understanding of spindle assembly, focusing on recent advances and the new approaches that enabled them. We describe the pathways that generate the microtubule framework of the spindle by driving microtubule nucleation in a spatially controlled fashion and present recent insights regarding the organization of individual microtubules into structural modules. Finally, we discuss the emergent properties of the spindle that enable robust chromosome segregation.

HURP localization in metaphase is the result of a multi-step process requiring its phosphorylation at Ser627 residue

Faithful chromosome segregation during cell division requires accurate mitotic spindle formation. As mitosis occurs rapidly within the cell cycle, the proteins involved in mitotic spindle assembly undergo rapid changes, including their interactions with other proteins. The proper localization of the HURP protein on the kinetochore fibers, in close proximity to chromosomes, is crucial for ensuring accurate congression and segregation of chromosomes. In this study, we employ photoactivation and FRAP experiments to investigate the impact of alterations in microtubule flux and phosphorylation of HURP at the Ser627 residue on its dynamics. Furthermore, through immunoprecipitations assays, we demonstrate the interactions of HURP with various proteins, such as TPX2, Aurora A, Eg5, Dynein, Kif5B, and Importin β, in mammalian cells during mitosis. We also find that phosphorylation of HURP at Ser627 regulates its interaction with these partners during mitosis. Our findings suggest that HURP participates in at least two distinct complexes during metaphase to ensure its proper localization in close proximity to chromosomes, thereby promoting the bundling and stabilization of kinetochore fibers.

Importin α/β promote Kif18B microtubule association and enhance microtubule destabilization activity

Tight regulation of microtubule (MT) dynamics is necessary for proper spindle assembly and chromosome segregation. The MT destabilizing Kinesin-8, Kif18B, controls astral MT dynamics and spindle positioning. Kif18B interacts with importin α/β as well as with the plus-tip tracking protein EB1, but how these associations modulate Kif18B is not known. We mapped the key binding sites on Kif18B, made residue-specific mutations, and assessed their impact on Kif18B function. Blocking EB1 interaction disrupted Kif18B MT plus-end accumulation and inhibited its ability to control MT length on monopolar spindles in cells. Blocking importin α/β interaction disrupted Kif18B localization without affecting aster size. In vitro, importin α/β increased Kif18B MT association by increasing the on-rate and decreasing the off-rate from MTs, which stimulated MT destabilization. In contrast, EB1 promoted MT destabilization without increasing lattice binding in vitro, which suggests that EB1 and importin α/β have distinct roles in the regulation of Kif18B-mediated MT destabilization. We propose that importin α/β spatially modulate Kif18B association with MTs to facilitate its MT destabilization activity. Our results suggest that Ran regulation is important not only to control molecular motor function near chromatin but also to provide a spatial control mechanism to modulate MT binding of nuclear localization signal-containing spindle assembly factors.

Ran GTPase and Its Importance in Cellular Signaling and Malignant Phenotype

Ran is a member of the Ras superfamily of proteins, which primarily regulates nucleocytoplasmic trafficking and mediates mitosis by regulating spindle formation and nuclear envelope (NE) reassembly. Therefore, Ran is an integral cell fate determinant. It has been demonstrated that aberrant Ran expression in cancer is a result of upstream dysregulation of the expression of various factors, such as osteopontin (OPN), and aberrant activation of various signaling pathways, including the extracellular-regulated kinase/mitogen-activated protein kinase (ERK/MEK) and phosphatidylinositol 3-kinase/Protein kinase B (PI3K/Akt) pathways. In vitro, Ran overexpression has severe effects on the cell phenotype, altering proliferation, adhesion, colony density, and invasion. Therefore, Ran overexpression has been identified in numerous types of cancer and has been shown to correlate with tumor grade and the degree of metastasis present in various cancers. The increased malignancy and invasiveness have been attributed to multiple mechanisms. Increased dependence on Ran for spindle formation and mitosis is a consequence of the upregulation of these pathways and the ensuing overexpression of Ran, which increases cellular dependence on Ran for survival. This increases the sensitivity of cells to changes in Ran concentration, with ablation being associated with aneuploidy, cell cycle arrest, and ultimately, cell death. It has also been demonstrated that Ran dysregulation influences nucleocytoplasmic transport, leading to transcription factor misallocation. Consequently, patients with tumors that overexpress Ran have been shown to have a higher malignancy rate and a shorter survival time compared to their counterparts.

Aurora A phosphorylates Ndel1 to reduce the levels of Mad1 and NuMA at spindle poles

Dynein inactivates the spindle assembly checkpoint (SAC) by transporting checkpoint proteins away from kinetochores toward spindle poles in a process known as "stripping." We find that inhibition of Aurora A kinase, which is localized to spindle poles, enables the accumulation of the spindle checkpoint activator Mad1 at poles where it is normally absent. Aurora kinases phosphorylate the dynein activator NudE neurodevelopment protein 1 like 1 (Ndel1) on Ser285 and Mad1 accumulates at poles when Ndel1 is replaced by a nonphosphorylatable mutant in human cells. The pole focusing protein NuMA, transported to poles by dynein, also accumulates at poles in cells harboring a mutant Ndel1. Phosphorylation of Ndel1 on Ser285 is required for robust spindle checkpoint activity and regulates the poles of asters in Xenopus extracts. Our data suggest that dynein/SAC complexes that are generated at kinetochores and then transported directionally toward poles on microtubules are inhibited by Aurora A before they reach spindle poles. These data suggest that Aurora A generates a spatial signal at spindle poles that controls dynein transport and spindle function.

Suppression of classical nuclear import pathway by importazole and ivermectin inhibits rotavirus replication

BACKGROUND

Rotavirus is the foremost cause of acute gastroenteritis among infants in resource-poor countries, causing severe morbidity and mortality. The currently available rotavirus vaccines are effective in reducing severity of the disease but not the infection rates, thus antivirals as an adjunct therapy are needed to reduce the morbidity in children. Viruses rely on host cellular machinery for nearly every step of the replication cycle. Therefore, targeting host factors that are indispensable for virus replication could be a promising strategy.

OBJECTIVES

To assess the therapeutic potential of ivermectin and importazole against rotaviruses.

METHODS

Antirotaviral activity of importazole and ivermectin was measured against various rotavirus strains (RV-SA11, RV-Wa, RV-A5-13, RV-EW) in vitro and in vivo by quantifying viral protein expression by western blot, analysing viroplasm formation by confocal microscopy, and measuring virus yield by plaque assay.

RESULTS

Importin-β1 and Ran were found to be induced during rotavirus infection. Knocking down importin-β1 severely impaired rotavirus replication, suggesting a critical role for importin-β1 in the rotavirus life cycle. In vitro studies revealed that treatment of ivermectin and importazole resulted in reduced synthesis of viral proteins, diminished production of infectious virus particles, and decrease in viroplasm-positive cells. Mechanistic study proved that both drugs perform antirotavirus activity by inhibiting the function of importin-β1. In vivo investigations in mice also confirmed the antirotavirus potential of importazole and ivermectin at non-toxic doses. Treatments of rotavirus-infected mice with either drug resulted in diminished shedding of viral particles in the stool sample, reduced expression of viral protein in the small intestine and restoration of damaged intestinal villi comapared to untreated infected mice.

CONCLUSIONS

The study highlights the potential of importazole and ivermectin as antirotavirus therapeutics.

A celebration of the 25th anniversary of chromatin-mediated spindle assembly

Formation of a bipolar spindle is required for the faithful segregation of chromosomes during cell division. Twenty-five years ago, a transformative insight into how bipolarity is achieved was provided by Rebecca Heald, Eric Karsenti, and colleagues in their landmark publication characterizing a chromatin-mediated spindle assembly pathway in which centrosomes and kinetochores were dispensable. The discovery revealed that bipolar spindle assembly is a self-organizing process where microtubules, which possess an intrinsic polarity, polymerize around chromatin and become sorted by mitotic motors into a bipolar structure. On the 25^th anniversary of this seminal paper, we discuss what was known before, what we have learned since, and what may lie ahead in understanding the bipolar spindle.

Flavors of Non-Random Meiotic Segregation of Autosomes and Sex Chromosomes

Segregation of chromosomes is a multistep process occurring both at mitosis and meiosis to ensure that daughter cells receive a complete set of genetic information. Critical components in the chromosome segregation include centromeres, kinetochores, components of sister chromatid and homologous chromosomes cohesion, microtubule organizing centres, and spindles. Based on the cytological work in the grasshopper Brachystola, it has been accepted for decades that segregation of homologs at meiosis is fundamentally random. This ensures that alleles on chromosomes have equal chance to be transmitted to progeny. At the same time mechanisms of meiotic drive and an increasing number of other examples of non-random segregation of autosomes and sex chromosomes provide insights into the underlying mechanisms of chromosome segregation but also question the textbook dogma of random chromosome segregation. Recent advances provide a better understanding of meiotic drive as a prominent force where cellular and chromosomal changes allow autosomes to bias their segregation. Less understood are mechanisms explaining observations that autosomal heteromorphism may cause biased segregation and regulate alternating segregation of multiple sex chromosome systems or translocation heterozygotes as an extreme case of non-random segregation. We speculate that molecular and cytological mechanisms of non-random segregation might be common in these cases and that there might be a continuous transition between random and non-random segregation which may play a role in the evolution of sexually antagonistic genes and sex chromosome evolution.

Exportins can inhibit major mitotic assembly events in vitro: membrane fusion, nuclear pore formation, and spindle assembly

Xenopus

egg extracts are a powerful in vitro tool for studying complex biological processes, including nuclear reconstitution, nuclear membrane and pore assembly, and spindle assembly. Extracts have been further used to demonstrate a moonlighting regulatory role for nuclear import receptors or importins on these cell cycle assembly events. Here we show that exportins can also play a role in these events. Addition of Crm1, Exportin-t, or Exportin-5 decreased nuclear pore assembly in vitro. RanQ69L-GTP, a constitutively active form of RanGTP, ameliorated inhibition. Both Crm1 and Exportin-t inhibited fusion of nuclear membranes, again counteracted by RanQ69L-GTP. In mitotic extracts, Crm1 and Exportin-t negatively impacted spindle assembly. Pulldowns from the extracts using Crm1- or Exportin-t-beads revealed nucleoporins known to be essential for both nuclear pore and spindle assembly, with RanQ69L-GTP decreasing a subset of these target interactions. This study suggests a model where exportins, like importins, can regulate major mitotic assembly events.

Integrative Cell Type-Specific Multi-Omics Approaches Reveal Impaired Programs of Glial Cell Differentiation in Mouse Culture Models of DM1

Myotonic dystrophy type 1 (DM1) is a neuromuscular disorder caused by a non-coding CTG repeat expansion in the DMPK gene. This mutation generates a toxic CUG RNA that interferes with the RNA processing of target genes in multiple tissues. Despite debilitating neurological impairment, the pathophysiological cascade of molecular and cellular events in the central nervous system (CNS) has been less extensively characterized than the molecular pathogenesis of muscle/cardiac dysfunction. Particularly, the contribution of different cell types to DM1 brain disease is not clearly understood. We first used transcriptomics to compare the impact of expanded CUG RNA on the transcriptome of primary neurons, astrocytes and oligodendrocytes derived from DMSXL mice, a transgenic model of DM1. RNA sequencing revealed more frequent expression and splicing changes in glia than neuronal cells. In particular, primary DMSXL oligodendrocytes showed the highest number of transcripts differentially expressed, while DMSXL astrocytes displayed the most severe splicing dysregulation. Interestingly, the expression and splicing defects of DMSXL glia recreated molecular signatures suggestive of impaired cell differentiation: while DMSXL oligodendrocytes failed to upregulate a subset of genes that are naturally activated during the oligodendroglia differentiation, a significant proportion of missplicing events in DMSXL oligodendrocytes and astrocytes increased the expression of RNA isoforms typical of precursor cell stages. Together these data suggest that expanded CUG RNA in glial cells affects preferentially differentiation-regulated molecular events. This hypothesis was corroborated by gene ontology (GO) analyses, which revealed an enrichment for biological processes and cellular components with critical roles during cell differentiation. Finally, we combined exon ontology with phosphoproteomics and cell imaging to explore the functional impact of CUG-associated spliceopathy on downstream protein metabolism. Changes in phosphorylation, protein isoform expression and intracellular localization in DMSXL astrocytes demonstrate the far-reaching impact of the DM1 repeat expansion on cell metabolism. Our multi-omics approaches provide insight into the mechanisms of CUG RNA toxicity in the CNS with cell type resolution, and support the priority for future research on non-neuronal mechanisms and proteomic changes in DM1 brain disease.

The Nuclear Mitotic Apparatus (NuMA) Protein: A Key Player for Nuclear Formation, Spindle Assembly, and Spindle Positioning

The nuclear mitotic apparatus (NuMA) protein is well conserved in vertebrates, and dynamically changes its subcellular localization from the interphase nucleus to the mitotic/meiotic spindle poles and the mitotic cell cortex. At these locations, NuMA acts as a key structural hub in nuclear formation, spindle assembly, and mitotic spindle positioning, respectively. To achieve its variable functions, NuMA interacts with multiple factors, including DNA, microtubules, the plasma membrane, importins, and cytoplasmic dynein. The binding of NuMA to dynein via its N-terminal domain drives spindle pole focusing and spindle positioning, while multiple interactions through its C-terminal region define its subcellular localizations and functions. In addition, NuMA can self-assemble into high-ordered structures which likely contribute to spindle positioning and nuclear formation. In this review, we summarize recent advances in NuMA’s domains, functions and regulations, with a focus on human NuMA, to understand how and why vertebrate NuMA participates in these functions in comparison with invertebrate NuMA-related proteins.

More than a zip code: global modulation of cellular function by nuclear localization signals

Extensive structural and functional studies have been carried out in the field of nucleocytoplasmic transport. Nuclear transport factors, such as Importin-α/-β, recognize nuclear localization signals (NLSs) on cargo, and together with the small GTPase Ran, facilitate their nuclear localization. However, it is now emerging that binding of nuclear transport factors to NLSs not only mediates nuclear transport but also contributes to a variety of cellular functions in eukaryotes. Here, we describe recent advances that reveal how NLSs facilitate diverse cellular functions beyond nuclear transport activity. We review separately NLS-mediated regulatory mechanisms at different levels of biological organization, including (a) assembly of higher-order structures; (b) cellular organelle dynamics; and (c) modulation of cellular stress responses and viral infections. Finally, we provide mechanistic insights into how NLSs can regulate such a broad range of functions via their structural and biochemical properties.

RanGTP induces an effector gradient of XCTK2 and importin α/β for spindle microtubule cross-linking

High RanGTP around chromatin is important for governing spindle assembly during meiosis and mitosis by releasing the inhibitory effects of importin α/β. Here we examine how the Ran gradient regulates Kinesin-14 function to control spindle organization. We show that Xenopus Kinesin-14, XCTK2, and importin α/β form an effector gradient that is highest at the poles and diminishes toward the chromatin, which is opposite the RanGTP gradient. Importin α and β preferentially inhibit XCTK2 antiparallel microtubule cross-linking and sliding by decreasing the microtubule affinity of the XCTK2 tail domain. This change in microtubule affinity enables RanGTP to target endogenous XCTK2 to the spindle. We propose that these combined actions of the Ran pathway are critical to promote Kinesin-14 parallel microtubule cross-linking to help focus spindle poles for efficient bipolar spindle assembly. Furthermore, our work illustrates that RanGTP regulation in the spindle is not simply a switch, but rather generates effector gradients where importins α and β gradually tune the activities of spindle assembly factors.

Complementary functions for the Ran gradient during division

The Ran pathway has a well-described function in nucleocytoplasmic transport, where active Ran dissociates importin/karyopherin-bound cargo containing a nuclear localization signal (NLS) in the nucleus. As cells enter mitosis, the nuclear envelope breaks down and a gradient of active Ran forms where levels are highest near chromatin. This gradient plays a crucial role in regulating mitotic spindle assembly, where active Ran binds to and releases importins from NLS-containing spindle assembly factors. An emerging theme is that the Ran gradient also regulates the actomyosin cortex for processes including polar body extrusion during meiosis, and cytokinesis. For these events, active Ran could play an inhibitory role, where importin-binding may help promote or stabilize a conformation or interaction that favours the recruitment and function of cortical regulators. For either spindle assembly or cortical polarity, the gradient of active Ran determines the extent of importin-binding, the effects of which could vary for different proteins.

NuMA assemblies organize microtubule asters to establish spindle bipolarity in acentrosomal human cells

In most animal cells, mitotic spindle formation is mediated by coordination of centrosomal and acentrosomal pathways. At the onset of mitosis, centrosomes promote spindle bipolarization. However, the mechanism through which the acentrosomal pathways facilitate the establishment of spindle bipolarity in early mitosis is not completely understood. In this study, we show the critical roles of nuclear mitotic apparatus protein (NuMA) in the generation of spindle bipolarity in acentrosomal human cells. In acentrosomal human cells, we found that small microtubule asters containing NuMA formed at the time of nuclear envelope breakdown. In addition, these asters were assembled by dynein and the clustering activity of NuMA. Subsequently, NuMA organized the radial array of microtubules, which incorporates Eg5, and thus facilitated spindle bipolarization. Importantly, in cells with centrosomes, we also found that NuMA promoted the initial step of spindle bipolarization in early mitosis. Overall, these data suggest that canonical centrosomal and NuMA-mediated acentrosomal pathways redundantly promote spindle bipolarity in human cells.

Phosphorylation of importin-α1 by CDK1-cyclin B1 controls mitotic spindle assembly

Importin-α serves as an adaptor linking importin-β to proteins carrying a nuclear localization sequence (NLS). During interphase, this interaction enables nuclear protein import, while in mitosis it regulates spindle assembly factors (SAFs) and controls microtubule nucleation, stabilization and spindle function. Here, we show that human importin-α1 is regulated during the cell cycle and is phosphorylated at two sites (threonine 9 and serine 62) during mitosis by the major mitotic protein kinase CDK1-cyclin B. Mutational analysis indicates that the mitotic phosphorylation of importin-α1 inhibits its binding to importin-β and promotes the release of TPX2 and KIFC1, which are then targeted like importin-β to the spindle. Loss of importin-α1 or expression of a non-phosphorylated mutant of importin-α1 results in the formation of shortened spindles with reduced microtubule density and induces a prolonged metaphase, whereas phosphorylation-mimicking mutants are functional in mitosis. We propose that phosphorylation of importin-α1 is a general mechanism for the spatial and temporal control of mitotic spindle assembly by CDK1-cyclin B1 that acts through the release of SAFs such as TPX2 and KIFC1 from inhibitory complexes that restrict spindle assembly.

Protein phase separation in mitosis

Through phase separation, some proteins form liquid-like condensates or droplets which can flow, fuse, and even deform when pressure is applied. In some cases, the condensates 'mature' to form gel or solid-like structure. Recent studies suggest that the liquid-like condensates form the structural basis for several membrane-less subcellular organelles such as stress granules and other subcellular structures. Here, we review and discuss studies that implicate protein phase separation in the function of the spindle apparatus and centrosomes.

The Biology of the Nuclear Envelope and Its Implications in Cancer Biology

The formation of the nuclear envelope and the subsequent compartmentalization of the genome is a defining feature of eukaryotes. Traditionally, the nuclear envelope was purely viewed as a physical barrier to preserve genetic material in eukaryotic cells. However, in the last few decades, it has been revealed to be a critical cellular component in controlling gene expression and has been implicated in several human diseases. In cancer, the relevance of the cell nucleus was first reported in the mid-1800s when an altered nuclear morphology was observed in tumor cells. This review aims to give a current and comprehensive view of the role of the nuclear envelope on cancer first by recapitulating the changes of the nuclear envelope during cell division, second, by reviewing the role of the nuclear envelope in cell cycle regulation, signaling, and the regulation of the genome, and finally, by addressing the nuclear envelope link to cell migration and metastasis and its use in cancer prognosis.

Reconstituting Nuclear and Chromosome Dynamics Using Xenopus Extracts

Extracts prepared from the eggs of frogs, particularly Xenopus species, have provided critical material for seminal studies of nuclear and chromosome dynamics over several decades. Their usefulness for these types of analyses lies in several important characteristics: stockpiled nuclear components, absence of endogenous DNA, and intact and functioning signaling networks. These factors have allowed detailed molecular analyses of many aspects of chromosome biology, including DNA replication, checkpoint signaling, epigenetic control, and chromosome condensation, cohesion, and segregation. In this introduction, the preparation and application of Xenopus egg extracts for the study of chromosomes and chromatin are described in detail.

Animal Female Meiosis: The Challenges of Eliminating Centrosomes

Sexual reproduction requires the generation of gametes, which are highly specialised for fertilisation. Female reproductive cells, oocytes, grow up to large sizes when they accumulate energy stocks and store proteins as well as mRNAs to enable rapid cell divisions after fertilisation. At the same time, metazoan oocytes eliminate their centrosomes, i.e., major microtubule-organizing centres (MTOCs), during or right after the long growth phases. Centrosome elimination poses two key questions: first, how can the centrosome be re-established after fertilisation? In general, metazoan oocytes exploit sperm components, i.e., the basal body of the sperm flagellum, as a platform to reinitiate centrosome production. Second, how do most metazoan oocytes manage to build up meiotic spindles without centrosomes? Oocytes have evolved mechanisms to assemble bipolar spindles solely around their chromosomes without the guidance of pre-formed MTOCs. Female animal meiosis involves microtubule nucleation and organisation into bipolar microtubule arrays in regulated self-assembly under the control of the Ran system and nuclear transport receptors. This review summarises our current understanding of the molecular mechanism underlying self-assembly of meiotic spindles, its spatio-temporal regulation, and the key players governing this process in animal oocytes.

Meeting the meiotic challenge: Specializations in mammalian oocyte spindle formation

Oocytes uniquely accumulate cytoplasmic constituents to support early embryogenesis. This unique specialization is accompanied by acquisition of a large size and by execution of asymmetric meiotic divisions that preserve precious ooplasm through the expulsion of minimal size polar bodies. While often taken for granted, these basic features of oogenesis necessitate unique specializations of the meiotic apparatus. These include a chromatin-sourced RanGTP gradient that restricts spindle size by defining a spatial domain where meiotic spindles form, acentriolar centrosomes that rely on microtubule organizing centers to form spindle poles, and an actin-based mechanism for asymmetric spindle positioning. Additionally, localized protein synthesis to support spindle formation is achieved in the spindle forming region, whilst protein synthesis is reduced elsewhere in the ooplasm. This is achieved through enrichment of spindle-related mRNAs in the spindle forming region combined with local PLK1-mediated phosphorylation and inactivation of the translational repressor EIF4EBP1. This allows PLK1 to function as an important regulatory nexus through which endogenous and exogenous signals can impact spindle formation and function, and highlights the important role that PLK1 may have in maintaining oocyte quality and fertility.

Regulation of mitotic spindle assembly factor NuMA by Importin-β

Ran-guanosine triphosphatase orchestrates mitotic spindle assembly by modulation of the interaction between Importin-α/-β and spindle assembly factors (SAFs). The inhibition of SAFs performed by importins needs to be done without much sequestration from abundant nuclear localization signal (NLS) -containing proteins. However, the molecular mechanisms that determine NLS-binding selectivity and that inhibit activity of Importin-β-regulated SAFs (e.g., nuclear mitotic apparatus protein [NuMA]) remain undefined. Here, we present a crystal structure of the Importin-α-NuMA C terminus complex showing a novel binding pattern that accounts for selective NLS recognition. We demonstrate that, in the presence of Importin-α, Importin-β inhibits the microtubule-binding function of NuMA. Further, we have identified a high-affinity microtubule-binding region that lies carboxyl-terminal to the NLS, which is sterically masked by Importin-β on being bound by Importin-α. Our study provides mechanistic evidence of how Importin-α/-β regulates the NuMA functioning required for assembly of higher-order microtubule structures, further illuminating how Ran-governed transport factors regulate diverse SAFs and accommodate various cell demands.

14-3-3 regulation of Ncd reveals a new mechanism for targeting proteins to the spindle in oocytes

The meiotic spindle is formed without centrosomes in a large volume of oocytes. Local activation of crucial spindle proteins around chromosomes is important for formation and maintenance of a bipolar spindle in oocytes. We found that phosphodocking 14-3-3 proteins stabilize spindle bipolarity in Drosophila melanogaster oocytes. A critical 14-3-3 target is the minus end-directed motor Ncd (human HSET; kinesin-14), which has well-documented roles in stabilizing a bipolar spindle in oocytes. Phospho docking by 14-3-3 inhibits the microtubule binding activity of the nonmotor Ncd tail. Further phosphorylation by Aurora B kinase can release Ncd from this inhibitory effect of 14-3-3. As Aurora B localizes to chromosomes and spindles, 14-3-3 facilitates specific association of Ncd with spindle microtubules by preventing Ncd from binding to nonspindle microtubules in oocytes. Therefore, 14-3-3 translates a spatial cue provided by Aurora B to target Ncd selectively to the spindle within the large volume of oocytes.

Bridging length scales to measure polymer assembly

Time-resolvable quantitative measurements of polymer concentration are very useful to elucidate protein polymerization pathways. There are numerous techniques to measure polymer concentrations in purified protein solutions, but few are applicable in vivo. Here we develop a methodology combining microscopy and spectroscopy to overcome the limitations of both approaches for measuring polymer concentration in cells and cell extracts. This technique is based on quantifying the relationship between microscopy and spectroscopy measurements at many locations. We apply this methodology to measure microtubule assembly in tissue culture cells and Xenopus egg extracts using two-photon microscopy with FLIM measurements of FRET. We find that the relationship between FRET and two-photon intensity quantitatively agrees with predictions. Furthermore, FRET and intensity measurements change as expected with changes in acquisition time, labeling ratios, and polymer concentration. Taken together, these results demonstrate that this approach can quantitatively measure microtubule assembly in complex environments. This methodology should be broadly useful for studying microtubule nucleation and assembly pathways of other polymers.

Ran-dependent TPX2 activation promotes acentrosomal microtubule nucleation in neurons

The microtubule (MT) cytoskeleton is essential for the formation of morphologically appropriate neurons. The existence of the acentrosomal MT organizing center in neurons has been proposed but its identity remained elusive. Here we provide evidence showing that TPX2 is an important component of this acentrosomal MT organizing center. First, neurite elongation is compromised in TPX2-depleted neurons. In addition, TPX2 localizes to the centrosome and along the neurite shaft bound to MTs. Depleting TPX2 decreases MT formation frequency specifically at the tip and the base of the neurite, and these correlate precisely with the regions where active GTP-bound Ran proteins are enriched. Furthermore, overexpressing the downstream effector of Ran, importin, compromises MT formation and neuronal morphogenesis. Finally, applying a Ran-importin signaling interfering compound phenocopies the effect of TPX2 depletion on MT dynamics. Together, these data suggest a model in which Ran-dependent TPX2 activation promotes acentrosomal MT nucleation in neurons.

Metaphase Spindle Assembly

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.

Symmetry Does not Come for Free: Cellular Mechanisms to Achieve a Symmetric Cell Division

During mitosis cells can divide symmetrically to proliferate or asymmetrically to generate tissue diversity. While the mechanisms that ensure asymmetric cell division have been extensively studied, it is often assumed that a symmetric cell division is the default outcome of mitosis. Recent studies, however, imply that the symmetric nature of cell division is actively controlled, as they reveal numerous mechanisms that ensure the formation of equal-sized daughter cells as cells progress through cell division. Here we review our current knowledge of these mechanisms and highlight possible key questions in the field.

Haspin kinase regulates microtubule-organizing center clustering and stability through Aurora kinase C in mouse oocytes

Meiotic oocytes lack classic centrosomes and, therefore, bipolar spindle assembly depends on clustering of acentriolar microtubule-organizing centers (MTOCs) into two poles. However, the molecular mechanism regulating MTOC assembly into two poles is not fully understood. The kinase haspin (also known as GSG2) is required to regulate Aurora kinase C (AURKC) localization at chromosomes during meiosis I. Here, we show that inhibition of haspin perturbed MTOC clustering into two poles and the stability of the clustered MTOCs. Furthermore, we show that AURKC localizes to MTOCs in mouse oocytes. Inhibition of haspin perturbed the localization of AURKC at MTOCs, and overexpression of AURKC rescued the MTOC-clustering defects in haspin-inhibited oocytes. Taken together, our data uncover a role for haspin as a regulator of bipolar spindle assembly by regulating AURKC function at acentriolar MTOCs in oocytes.

The deubiquitinating enzyme complex BRISC is required for proper mitotic spindle assembly in mammalian cells

The microtubule-associated protein BRISC regulates the interaction of NuMA with dynein and importin-β by removing K63-linked polyubiquitin chains from NuMA, thereby promoting proper bipolar spindle assembly.

Deubiquitinating enzymes (DUBs) negatively regulate protein ubiquitination and play an important role in diverse physiological processes, including mitotic division. The BRCC36 isopeptidase complex (BRISC) is a DUB that is specific for lysine 63–linked ubiquitin hydrolysis; however, its biological function remains largely undefined. Here, we identify a critical role for BRISC in the control of mitotic spindle assembly in cultured mammalian cells. BRISC is a microtubule (MT)-associated protein complex that predominantly localizes to the minus ends of K-fibers and spindle poles and directly binds to MTs; importantly, BRISC promotes the assembly of functional bipolar spindle by deubiquitinating the essential spindle assembly factor nuclear mitotic apparatus (NuMA). The deubiquitination of NuMA regulates its interaction with dynein and importin-β, which are required for its function in spindle assembly. Collectively, these results uncover BRISC as an important regulator of the mitotic spindle assembly and cell division, and have important implications for the development of anticancer drugs targeting BRISC.

Phosphorylation of β-Tubulin by the Down Syndrome Kinase, Minibrain/DYRK1a, Regulates Microtubule Dynamics and Dendrite Morphogenesis

Dendritic arborization patterns are consistent anatomical correlates of genetic disorders such as Down syndrome (DS) and autism spectrum disorders (ASDs). In a screen for abnormal dendrite development, we identified Minibrain (MNB)/DYRK1a, a kinase implicated in DS and ASDs, as a regulator of the microtubule cytoskeleton. We show that MNB is necessary to establish the length and cytoskeletal composition of terminal dendrites by controlling microtubule growth. Altering MNB levels disrupts dendrite morphology and perturbs neuronal electrophysiological activity, resulting in larval mechanosensation defects. Using in vivo and in vitro approaches, we uncover a molecular pathway whereby direct phosphorylation of β-tubulin by MNB inhibits tubulin polymerization, a function that is conserved for mammalian DYRK1a. Our results demonstrate that phosphoregulation of microtubule dynamics by MNB/DYRK1a is critical for dendritic patterning and neuronal function, revealing a previously unidentified mode of posttranslational microtubule regulation in neurons and uncovering a conserved pathway for a DS- and ASD-associated kinase.

Mechanisms of Mitotic Spindle Assembly

Life depends on cell proliferation and the accurate segregation of chromosomes, which are mediated by the microtubule (MT)-based mitotic spindle and ∼200 essential MT-associated proteins. Yet, a mechanistic understanding of how the mitotic spindle is assembled and achieves chromosome segregation is still missing. This is mostly due to the density of MTs in the spindle, which presumably precludes their direct observation. Recent insight has been gained into the molecular building plan of the metaphase spindle using bulk and single-molecule measurements combined with computational modeling. MT nucleation was uncovered as a key principle of spindle assembly, and mechanistic details about MT nucleation pathways and their coordination are starting to be revealed. Lastly, advances in studying spindle assembly can be applied to address the molecular mechanisms of how the spindle segregates chromosomes.

Targeting nuclear transporters in cancer: Diagnostic, prognostic and therapeutic potential

The Karyopherin superfamily is a major class of soluble transport receptors consisting of both import and export proteins. The trafficking of proteins involved in transcription, cell signalling and cell cycle regulation among other functions across the nuclear membrane is essential for normal cellular functioning. However, in cancer cells, the altered expression or localization of nuclear transporters as well as the disruption of endogenous nuclear transport inhibitors are some ways in which the Karyopherin proteins are dysregulated. The value of nuclear transporters in the diagnosis, prognosis and treatment of cancer is currently being elucidated with recent studies highlighting their potential as biomarkers and therapeutic targets.

Ahead of the Curve: New Insights into Microtubule Dynamics

Microtubule dynamics are fundamental for many aspects of cell physiology, but their mechanistic underpinnings remain unclear despite 40 years of intense research. In recent years, the continued union of reconstitution biochemistry, structural biology, and modeling has yielded important discoveries that deepen our understanding of microtubule dynamics. These studies, which we review here, underscore the importance of GTP hydrolysis-induced changes in tubulin structure as microtubules assemble, and highlight the fact that each aspect of microtubule behavior is the output of complex, multi-step processes. Although this body of work moves us closer to appreciating the key features of microtubule biochemistry that drive dynamic instability, the divide between our understanding of microtubules in isolation versus within the cellular milieu remains vast. Bridging this gap will serve as fertile grounds of cytoskeleton-focused research for many years to come.

NuMA Phosphorylation by Aurora-A Orchestrates Spindle Orientation

Spindle positioning is essential for tissue morphogenesis and homeostasis. The signaling network synchronizing spindle placement with mitotic progression relies on timely recruitment at the cell cortex of NuMA:LGN:Gαi complexes, in which NuMA acts as a receptor for the microtubule motor Dynein. To study the implication of Aurora-A in spindle orientation, we developed protocols for the partial inhibition of its activity. Under these conditions, in metaphase NuMA and Dynein accumulate abnormally at the spindle poles and do not reach the cortex, while the cortical distribution of LGN remains unperturbed. FRAP experiments revealed that Aurora-A governs the dynamic exchange between the cytoplasmic and the spindle pole-localized pools of NuMA. We show that Aurora-A phosphorylates directly the C terminus of NuMA on three Ser residues, of which Ser1969 determines the dynamic behavior and the spindle orientation functions of NuMA. Most interestingly, we identify a new microtubule-binding domain of NuMA, which does not overlap with the LGN-binding motif. Our study demonstrates that in metaphase the direct phosphorylation of NuMA by Aurora-A controls its cortical enrichment, and that this is the major event underlying the spindle orientation functions of Aurora-A in transformed and non-transformed cells in culture. Phosphorylation of NuMA by Aurora-A does not affect its affinity for microtubules or for LGN but rather determines the mobility of the protein at the spindle poles. The finding that NuMA can associate concomitantly with LGN and microtubules suggests that its microtubule-binding activity contributes to anchor Dynein-loaded microtubule +TIPs at cortical sites with LGN.

The RanGTP Pathway: From Nucleo-Cytoplasmic Transport to Spindle Assembly and Beyond

The small GTPase Ran regulates the interaction of transport receptors with a number of cellular cargo proteins. The high affinity binding of the GTP-bound form of Ran to import receptors promotes cargo release, whereas its binding to export receptors stabilizes their interaction with the cargo. This basic mechanism linked to the asymmetric distribution of the two nucleotide-bound forms of Ran between the nucleus and the cytoplasm generates a switch like mechanism controlling nucleo-cytoplasmic transport. Since 1999, we have known that after nuclear envelope breakdown (NEBD) Ran and the above transport receptors also provide a local control over the activity of factors driving spindle assembly and regulating other aspects of cell division. The identification and functional characterization of RanGTP mitotic targets is providing novel insights into mechanisms essential for cell division. Here we review our current knowledge on the RanGTP system and its regulation and we focus on the recent advances made through the characterization of its mitotic targets. We then briefly review the novel functions of the pathway that were recently described. Altogether, the RanGTP system has moonlighting functions exerting a spatial control over protein interactions that drive specific functions depending on the cellular context.

The GTPase RAN regulates multiple steps of the centrosome life cycle

Growing lines of evidence implicate the small GTPase RAN, its regulators and effectors--predominantly, nuclear transport receptors--in practically all aspects of centrosome biology in mammalian cells. These include duplication licensing, cohesion, positioning, and microtubule-nucleation capacity. RAN cooperates with the protein nuclear export vector exportin 1/CRM1 to recruit scaffolding proteins containing nuclear export sequences that play roles in the structural organization of centrosomes. Together, they also limit centrosome reduplication by regulating the localization of key "licensing" proteins during the centrosome duplication cycle. In parallel, RAN also regulates the capacity of centrosomes to nucleate and organize functional microtubules, and this predominanlty involves importin vectors: many factors regulating microtubule nucleation or function harbor nuclear localization sequences that interact with importin molecules and such interaction inhibits their activity. Active RANGTP binding to importin molecules removes the inhibition and releases microtubule regulatory factors in the free productive form. A dynamic scenario emerges, in which RAN is pivotal in linking spatiotemporal control of centrosome regulators to the cell cycle machinery.

Differentially Expressed Genes and Signature Pathways of Human Prostate Cancer

Genomic technologies including microarrays and next-generation sequencing have enabled the generation of molecular signatures of prostate cancer. Lists of differentially expressed genes between malignant and non-malignant states are thought to be fertile sources of putative prostate cancer biomarkers. However such lists of differentially expressed genes can be highly variable for multiple reasons. As such, looking at differential expression in the context of gene sets and pathways has been more robust. Using next-generation genome sequencing data from The Cancer Genome Atlas, differential gene expression between age- and stage- matched human prostate tumors and non-malignant samples was assessed and used to craft a pathway signature of prostate cancer. Up- and down-regulated genes were assigned to pathways composed of curated groups of related genes from multiple databases. The significance of these pathways was then evaluated according to the number of differentially expressed genes found in the pathway and their position within the pathway using Gene Set Enrichment Analysis and Signaling Pathway Impact Analysis. The “transforming growth factor-beta signaling” and “Ran regulation of mitotic spindle formation” pathways were strongly associated with prostate cancer. Several other significant pathways confirm reported findings from microarray data that suggest actin cytoskeleton regulation, cell cycle, mitogen-activated protein kinase signaling, and calcium signaling are also altered in prostate cancer. Thus we have demonstrated feasibility of pathway analysis and identified an underexplored area (Ran) for investigation in prostate cancer pathogenesis.

Changing expression and subcellular distribution of karyopherins during murine oogenesis

Mammalian oocyte growth and development is driven by a strict program of gene expression that relies on the timely presence of transcriptional regulators via nuclear pores. By targeting specific cargos for nucleo-cytoplasmic transport, karyopherin (KPN) proteins are key to the relocation of essential transcription factors and chromatin-remodelling factors into and out of the nucleus. Using multiple complementary techniques, here we establish that KPNA genes and proteins are dynamically expressed and relocalised throughout mouse oogenesis and folliculogenesis. Of the KPNAs examined (Kpna1, Kpna2, Kpna3, Kpna4, Kpna6, Kpna7, Kpnb1, Ipo5 and Xpo1), all were expressed in the embryonic ovary with up-regulation of protein levels concomitant with meiotic entry for KPNA2, accompanied by the redistribution of the cellular localisation of KPNA2 and XPO1. In contrast, postnatal folliculogenesis revealed significant up-regulation of Kpna1, Kpna2, Kpna4, Kpna6 and Ipo5 and down-regulation of Kpnb1, Kpna7 and Xpo1 at the primordial to primary follicle transition. KPNAs exhibited different localisation patterns in both oocytes and granulosa cells during folliculogenesis, with three KPNAs – KPNA1, KPNA2 and IPO5 – displaying marked enrichment in the nucleus by antral follicle stage. Remarkably, varied subcellular expression profiles were also identified in isolated pre-ovulatory oocytes with KPNAs KPNA2, KPNB1 and IPO5 detected in the cytoplasm and at the nuclear rim and XPO1 in cytoplasmic aggregates. Intriguingly, meiotic spindle staining was also observed for KPNB1 and XPO1 in meiosis II eggs, implying roles for KPNAs outside of nucleo-cytoplasmic transport. Thus, we propose that KPNAs, by targeting specific cargoes, are likely to be key regulators of oocyte development.

The Ran Pathway in Drosophila melanogaster Mitosis

Over the last two decades, the small GTPase Ran has emerged as a central regulator of both mitosis and meiosis, particularly in the generation, maintenance, and regulation of the microtubule (MT)-based bipolar spindle. Ran-regulated pathways in mitosis bear many similarities to the well-characterized functions of Ran in nuclear transport and, as with transport, the majority of these mitotic effects are mediated through affecting the physical interaction between karyopherins and Spindle Assembly Factors (SAFs)—a loose term describing proteins or protein complexes involved in spindle assembly through promoting nucleation, stabilization, and/or depolymerization of MTs, through anchoring MTs to specific structures such as centrosomes, chromatin or kinetochores, or through sliding MTs along each other to generate the force required to achieve bipolarity. As such, the Ran-mediated pathway represents a crucial functional module within the wider spindle assembly landscape. Research into mitosis using the model organism Drosophila melanogaster has contributed substantially to our understanding of centrosome and spindle function. However, in comparison to mammalian systems, very little is known about the contribution of Ran-mediated pathways in Drosophila mitosis. This article sets out to summarize our understanding of the roles of the Ran pathway components in Drosophila mitosis, focusing on the syncytial blastoderm embryo, arguing that it can provide important insights into the conserved functions on Ran during spindle formation.

The nucleoporin MEL-28 promotes RanGTP-dependent γ-tubulin recruitment and microtubule nucleation in mitotic spindle formation

The GTP-bound form of the Ran GTPase (RanGTP), produced around chromosomes, drives nuclear envelope and nuclear pore complex (NPC) re-assembly after mitosis. The nucleoporin MEL-28/ELYS binds chromatin in a RanGTP-regulated manner and acts to seed NPC assembly. Here we show that, upon mitotic NPC disassembly, MEL-28 dissociates from chromatin and re-localizes to spindle microtubules and kinetochores. MEL-28 directly binds microtubules in a RanGTP-regulated way via its C-terminal chromatin-binding domain. Using Xenopus egg extracts, we demonstrate that MEL-28 is essential for RanGTP-dependent microtubule nucleation and spindle assembly, independent of its function in NPC assembly. Specifically, MEL-28 interacts with the γ-tubulin ring complex and recruits it to microtubule nucleation sites. Our data identify MEL-28 as a RanGTP target that functions throughout the cell cycle. Its cell cycle-dependent binding to chromatin or microtubules discriminates MEL-28 functions in interphase and mitosis, and ensures that spindle assembly occurs only after NPC breakdown.

Polyglutamylated Tubulin Binding Protein C1orf96/CSAP Is Involved in Microtubule Stabilization in Mitotic Spindles

The centrosome-associated C1orf96/Centriole, Cilia and Spindle-Associated Protein (CSAP) targets polyglutamylated tubulin in mitotic microtubules (MTs). Loss of CSAP causes critical defects in brain development; however, it is unclear how CSAP association with MTs affects mitosis progression. In this study, we explored the molecular mechanisms of the interaction of CSAP with mitotic spindles. Loss of CSAP caused MT instability in mitotic spindles and resulted in mislocalization of Nuclear protein that associates with the Mitotic Apparatus (NuMA), with defective MT dynamics. Thus, CSAP overload in the spindles caused extensive MT stabilization and recruitment of NuMA. Moreover, MT stabilization by CSAP led to high levels of polyglutamylation on MTs. MT depolymerization by cold or nocodazole treatment was inhibited by CSAP binding. Live-cell imaging analysis suggested that CSAP-dependent MT-stabilization led to centrosome-free MT aster formation immediately upon nuclear envelope breakdown without γ-tubulin. We therefore propose that CSAP associates with MTs around centrosomes to stabilize MTs during mitosis, ensuring proper bipolar spindle formation and maintenance.

RanGTP aids anaphase entry through Ubr5-mediated protein turnover

Cells sense the overall chromosome congression state in metaphase to promote anaphase entry using RanGTP, which induces the reduction of two spindle assembly checkpoint proteins, BuGZ and Bub3, via the E3 ligase Ubr5.

RanGTP is known to regulate the spindle assembly checkpoint (SAC), but the underlying molecular mechanism is unclear. BuGZ stabilizes SAC protein Bub3 through direct interaction and facilitates its mitotic function. Here we show that RanGTP promotes the turnover of BuGZ and Bub3 in metaphase, which in turn facilitates metaphase-to-anaphase transition. BuGZ and Bub3 interact with either importin-β or an E3 ubiquitin ligase, Ubr5. RanGTP promotes the dissociation of importin-β from BuGZ and Bub3 in metaphase. This results in increased binding of BuGZ and Bub3 to Ubr5, leading to ubiquitination and subsequent turnover of both proteins. We propose that elevated metaphase RanGTP levels use Ubr5 to couple overall chromosome congression to SAC silencing.

Spatial gradients controlling spindle assembly

The mitotic spindle is the macromolecular machine utilized to accurately segregate chromosomes in cells. How this self-organized structure assembles is a key aspect of understanding spindle morphogenesis. In the present review, we focus on understanding mechanisms of spindle self-assembly and address how subcellular signalling gradients, such as Ran-GTP and Aurora B, contribute to spindle organization and function.

Spatiotemporal Regulation of Nuclear Transport Machinery and Microtubule Organization

Spindle microtubules capture and segregate chromosomes and, therefore, their assembly is an essential event in mitosis. To carry out their mission, many key players for microtubule formation need to be strictly orchestrated. Particularly, proteins that assemble the spindle need to be translocated at appropriate sites during mitosis. A small GTPase (hydrolase enzyme of guanosine triphosphate), Ran, controls this translocation. Ran plays many roles in many cellular events: nucleocytoplasmic shuttling through the nuclear envelope, assembly of the mitotic spindle, and reorganization of the nuclear envelope at the mitotic exit. Although these events are seemingly distinct, recent studies demonstrate that the mechanisms underlying these phenomena are substantially the same as explained by molecular interplay of the master regulator Ran, the transport factor importin, and its cargo proteins. Our review focuses on how the transport machinery regulates mitotic progression of cells. We summarize translocation mechanisms governed by Ran and its regulatory proteins, and particularly focus on Ran-GTP targets in fission yeast that promote spindle formation. We also discuss the coordination of the spatial and temporal regulation of proteins from the viewpoint of transport machinery. We propose that the transport machinery is an essential key that couples the spatial and temporal events in cells.

Reprint of "Nuclear transport factors: global regulation of mitosis"

The unexpected repurposing of nuclear transport proteins from their function in interphase to an equally vital and very different set of functions in mitosis was very surprising. The multi-talented cast when first revealed included the import receptors, importin alpha and beta, the small regulatory GTPase RanGTP, and a subset of nuclear pore proteins. In this review, we report that recent years have revealed new discoveries in each area of this expanding story in vertebrates: (a) The cast of nuclear import receptors playing a role in mitotic spindle regulation has expanded: both transportin, a nuclear import receptor, and Crm1/Xpo1, an export receptor, are involved in different aspects of spindle assembly. Importin beta and transportin also regulate nuclear envelope and pore assembly. (b) The role of nucleoporins has grown to include recruiting the key microtubule nucleator – the γ-TuRC complex – and the exportin Crm1 to the mitotic kinetochores of humans. Together they nucleate microtubule formation from the kinetochores toward the centrosomes. (c) New research finds that the original importin beta/RanGTP team have been further co-opted by evolution to help regulate other cellular and organismal activities, ranging from the actual positioning of the spindle within the cell perimeter, to regulation of a newly discovered spindle microtubule branching activity, to regulation of the interaction of microtubule structures with specific actin structures. (d) Lastly, because of the multitudinous roles of karyopherins throughout the cell cycle, a recent large push toward testing their potential as chemotherapeutic targets has begun to yield burgeoning progress in the clinic.

GM130 Regulates Golgi-Derived Spindle Assembly by Activating TPX2 and Capturing Microtubules

Spindle assembly requires the coordinated action of multiple cellular structures to nucleate and organize microtubules in a precise spatiotemporal manner. Among them, the contributions of centrosomes, chromosomes, and microtubules have been well studied, yet the involvement of membrane-bound organelles remains largely elusive. Here, we provide mechanistic evidence for a membrane-based, Golgi-derived microtubule assembly pathway in mitosis. Upon mitotic entry, the Golgi matrix protein GM130 interacts with importin α via a classical nuclear localization signal that recruits importin α to the Golgi membranes. Sequestration of importin α by GM130 liberates the spindle assembly factor TPX2, which activates Aurora-A kinase and stimulates local microtubule nucleation. Upon filament assembly, nascent microtubules are further captured by GM130, thus linking Golgi membranes to the spindle. Our results reveal an active role for the Golgi in regulating spindle formation to ensure faithful organelle inheritance.