Nucleocytoplasmic transport of Alp7/TACC organizes spatiotemporal microtubule formation in fission yeast

Human Cytomegalovirus Exploits TACC3 To Control Microtubule Dynamics and Late Stages of Infection

While it is well established that microtubules (MTs) facilitate various stages of virus replication, how viruses actively control MT dynamics and functions remains less well understood. Recent work has begun to reveal how several viruses exploit End-Binding (EB) proteins and their associated microtubule plus-end tracking proteins (+TIPs), in particular to enable loading of viral particles onto MTs for retrograde transport during early stages of infection. Distinct from other viruses studied to date, at mid- to late stages of its unusually protracted replication cycle, human cytomegalovirus (HCMV) increases the expression of all three EB family members. This occurs coincident with the formation of a unique structure, termed the assembly compartment (AC), which serves as a Golgi-derived MT organizing center. Together, the AC and distinct EB proteins enable HCMV to increase the formation of dynamic and acetylated microtubule subsets to regulate distinct aspects of the viral replication cycle. Here, we reveal that HCMV also exploits EB-independent +TIP pathways by specifically increasing the expression of transforming acidic coiled coil protein 3 (TACC3) to recruit the MT polymerase, chTOG, from initial sites of MT nucleation in the AC out into the cytosol, thereby increasing dynamic MT growth. Preventing TACC3 increases or depleting chTOG impaired MT polymerization, resulting in defects in early versus late endosome organization in and around the AC as well as defects in viral trafficking and spread. Our findings provide the first example of a virus that actively exploits EB-independent +TIP pathways to regulate MT dynamics and control late stages of virus replication. IMPORTANCE Diverse viruses rely on host cell microtubule networks to transport viral particles within the dense cytoplasmic environment and to control the broader architecture of the cell to facilitate their replication. However, precisely how viruses regulate the dynamic behavior and function of microtubule filaments remains poorly defined. We recently showed that the assembly compartment (AC) formed by human cytomegalovirus (HCMV) acts as a Golgi-derived microtubule organizing center. Here, we show that at mid- to late stages of infection, HCMV increases the expression of transforming acidic coiled coil protein 3 (TACC3) to control the localization of the microtubule polymerase, chTOG. This, in turn, enables HCMV to generate dynamic microtubule subsets that organize endocytic vesicles in and around the AC and facilitate the transport of new viral particles released into the cytosol. Our findings reveal the first instance of viral targeting of TACC3 to control microtubule dynamics and virus spread.

Simplification of nutritional conditions in transformation procedures for genome editing with the CRISPR/Cas9 system for fission yeast

CRISPR/Cas9 is a powerful tool for genome editing. Several studies have been conducted to take the benefit of the versatile tool in the fission yeast Schizosaccharomyces pombe. However, the protocols for the CRISPR/Cas9 system proposed in previous studies are complicated in culture conditions compared to traditional genome editing methods. In this study, we introduced vectors for expression of sgRNA as well as Cas9, which employ natMX6 and bsdMX6 dominant selection markers. Using these materials, we examined nutritional conditions of cell cultures and found that nitrogen depletion introduced in previous methods does not affect the efficiency of genome editing. We found that bsdMX6-based plasmids enable us to skip any recovery steps before plating onto medium containing blasticidin S, unlike other antibiotic resistance selection markers. We thus propose easier transformation procedures with natMX6 and particularly bsdMX6 markers. We also simulate prescreening of mutants by genotyping with DNA endonucleases or proofreading PCR instead of relying on existing knowledge of mutant phenotypes. These materials and methods assist easy construction of S. pombe strains using CRISPR/Cas9, thereby accelerating seamless introduction of CRISPR/Cas9 to S. pombe researchers.

TACC3 Regulates Microtubule Plus-End Dynamics and Cargo Transport in Interphase Cells

End-binding proteins (EBs) are widely viewed as master regulators of microtubule dynamics and function. Here, we show that while EB1 mediates the dynamic microtubule capture of herpes simplex virus type 1 (HSV-1) in fibroblasts, in neuronal cells, infection occurs independently of EBs through stable microtubules. Prompted by this, we find that transforming acid coiled-coil protein 3 (TACC3), widely studied in mitotic spindle formation, regulates the cytoplasmic localization of the microtubule polymerizing factor chTOG and influences microtubule plus-end dynamics during interphase to control infection in distinct cell types. Furthermore, perturbing TACC3 function in neuronal cells resulted in the formation of disorganized stable, detyrosinated microtubule networks and changes in cellular morphology, as well as impaired trafficking of both HSV-1 and transferrin. These trafficking defects in TACC3-depleted cells were reversed by the depletion of kinesin-1 heavy chains. As such, TACC3 is a critical regulator of interphase microtubule dynamics and stability that influences kinesin-1-based cargo trafficking.

While EB proteins are widely studied as master regulators of microtubule plus-end dynamics, Furey et al. report EB-independent regulation of microtubule arrays and cargo trafficking by the transforming acid coiled-coil-containing protein, TACC3. By controlling the formation of detyrosinated stable microtubule networks, TACC3 influences kinesin-1-based sorting of both host and pathogenic cargoes.

Two XMAP215/TOG Microtubule Polymerases, Alp14 and Dis1, Play Non-Exchangeable, Distinct Roles in Microtubule Organisation in Fission Yeast

Proper bipolar spindle assembly underlies accurate chromosome segregation. A cohort of microtubule-associated proteins orchestrates spindle microtubule formation in a spatiotemporally coordinated manner. Among them, the conserved XMAP215/TOG family of microtubule polymerase plays a central role in spindle assembly. In fission yeast, two XMAP215/TOG members, Alp14 and Dis1, share essential roles in cell viability; however how these two proteins functionally collaborate remains undetermined. Here we show the functional interplay and specification of Alp14 and Dis1. Creation of new mutant alleles of alp14, which display temperature sensitivity in the absence of Dis1, enabled us to conduct detailed analyses of a double mutant. We have found that simultaneous inactivation of Alp14 and Dis1 results in early mitotic arrest with very short, fragile spindles. Intriguingly, these cells often undergo spindle collapse, leading to a lethal “cut” phenotype. By implementing an artificial targeting system, we have shown that Alp14 and Dis1 are not functionally exchangeable and as such are not merely redundant paralogues. Interestingly, while Alp14 promotes microtubule nucleation, Dis1 does not. Our results uncover that the intrinsic specification, not the spatial regulation, between Alp14 and Dis1 underlies the collaborative actions of these two XMAP215/TOG members in mitotic progression, spindle integrity and genome stability.

Role of nucleocytoplasmic transport in interphase microtubule organization in fission yeast

The proper organization of microtubules is essential for many cellular functions. Microtubule organization and reorganization are highly regulated during the cell cycle, but the underlying mechanisms remain elusive. Here we characterized unusual interphase microtubule organization in fission yeast nuclear export mutant crm1-124. The mutant cells have an intranuclear microtubule bundle during interphase that pushes the nuclear envelope to assume a protruding morphology. We showed that the formation of this protruding microtubule bundle requires the nuclear accumulation of two microtubule-associated proteins (MAPs), Alp14/TOG and Mal3/EB1. Interestingly, the forced accumulation of Alp14 in the nucleus of wild type cells is sufficient to form the intranuclear microtubule bundle. Furthermore, the frequency of the intranuclear microtubule formation by Alp14 accumulated in the nucleus is prominently increased by a reduction in the nucleation activity of interphase cytoplasmic microtubules. We propose that properly regulated nucleocytoplasmic transport and maintained activity of cytoplasmic microtubule nucleation during interphase are important for the proper organization of interphase cytoplasmic microtubules.

Exportin Crm1 is repurposed as a docking protein to generate microtubule organizing centers at the nuclear pore

Non-centrosomal microtubule organizing centers (MTOCs) are important for microtubule organization in many cell types. In fission yeast Schizosaccharomyces pombe, the protein Mto1, together with partner protein Mto2 (Mto1/2 complex), recruits the γ-tubulin complex to multiple non-centrosomal MTOCs, including the nuclear envelope (NE). Here, we develop a comparative-interactome mass spectrometry approach to determine how Mto1 localizes to the NE. Surprisingly, we find that Mto1, a constitutively cytoplasmic protein, docks at nuclear pore complexes (NPCs), via interaction with exportin Crm1 and cytoplasmic FG-nucleoporin Nup146. Although Mto1 is not a nuclear export cargo, it binds Crm1 via a nuclear export signal-like sequence, and docking requires both Ran in the GTP-bound state and Nup146 FG repeats. In addition to determining the mechanism of MTOC formation at the NE, our results reveal a novel role for Crm1 and the nuclear export machinery in the stable docking of a cytoplasmic protein complex at NPCs.

A microtubule polymerase cooperates with the kinesin-6 motor and a microtubule cross-linker to promote bipolar spindle assembly in the absence of kinesin-5 and kinesin-14 in fission yeast

Kinesin-5 is required for bipolar spindle assembly; yet in the absence of kinesins-5 and -14, cells can form spindles. In fission yeast, three distinct pathways compensate for their loss. Microtubule polymerase, kinesin-6, and microtubule cross-linker execute individual roles in concert at different mitotic stages in place of the two kinesins.

Accurate chromosome segregation relies on the bipolar mitotic spindle. In many eukaryotes, spindle formation is driven by the plus-end–directed motor kinesin-5 that generates outward force to establish spindle bipolarity. Its inhibition leads to the emergence of monopolar spindles with mitotic arrest. Intriguingly, simultaneous inactivation of the minus-end–directed motor kinesin-14 restores spindle bipolarity in many systems. Here we show that in fission yeast, three independent pathways contribute to spindle bipolarity in the absence of kinesin-5/Cut7 and kinesin-14/Pkl1. One is kinesin-6/Klp9 that engages with spindle elongation once short bipolar spindles assemble. Klp9 also ensures the medial positioning of anaphase spindles to prevent unequal chromosome segregation. Another is the Alp7/TACC-Alp14/TOG microtubule polymerase complex. Temperature-sensitive alp7cut7pkl1 mutants are arrested with either monopolar or very short spindles. Forced targeting of Alp14 to the spindle pole body is sufficient to render alp7cut7pkl1 triply deleted cells viable and promote spindle assembly, indicating that Alp14-mediated microtubule polymerization from the nuclear face of the spindle pole body could generate outward force in place of Cut7 during early mitosis. The third pathway involves the Ase1/PRC1 microtubule cross-linker that stabilizes antiparallel microtubules. Our study, therefore, unveils multifaceted interplay among kinesin-dependent and -independent pathways leading to mitotic bipolar spindle assembly.

Alp7/TACC-Alp14/TOG generates long-lived, fast-growing MTs by an unconventional mechanism

Alp14 is a TOG-family microtubule polymerase from S. pombe that tracks plus ends and accelerates their growth. To interrogate its mechanism, we reconstituted dynamically unstable single isoform S. pombe microtubules with full length Alp14/TOG and Alp7, the TACC-family binding partner of Alp14. We find that Alp14 can drive microtubule plus end growth at GTP-tubulin concentrations at least 10-fold below the usual critical concentration, at the expense of increased catastrophe. This reveals Alp14 to be a highly unusual enzyme that biases the equilibrium for the reaction that it catalyses. Alp7/TACC enhances the effectiveness of Alp14, by increasing its occupancy. Consistent with this, we show in live cells that Alp7 deletion produces very similar MT dynamics defects to Alp14 deletion. The ability of Alp7/14 to accelerate and bias GTP-tubulin exchange at microtubule plus ends allows it to generate long-lived, fast-growing microtubules at very low cellular free tubulin concentrations.

Differentiating the roles of microtubule-associated proteins at meiotic kinetochores during chromosome segregation

Meiosis is a specialised cell division process for generating gametes. In contrast to mitosis, meiosis involves recombination followed by two consecutive rounds of cell division, meiosis I and II. A vast field of research has been devoted to understanding the differences between mitotic and meiotic cell divisions from the viewpoint of chromosome behaviour. For faithful inheritance of paternal and maternal genetic information to offspring, two events are indispensable: meiotic recombination, which generates a physical link between homologous chromosomes, and reductional segregation, in which homologous chromosomes move towards opposite poles, thereby halving the ploidy. The cytoskeleton and its regulators play specialised roles in meiosis to accomplish these divisions. Recent studies have shown that microtubule-associated proteins (MAPs), including tumour overexpressed gene (TOG), play unique roles during meiosis. Furthermore, the conserved mitotic protein kinase Polo modulates MAP localisation in meiosis I. As Polo is a well-known regulator of reductional segregation in meiosis, the evidence suggests that Polo constitutes a plausible link between meiosis-specific MAP functions and reductional segregation. Here, we review the latest findings on how the localisation and regulation of MAPs in meiosis differ from those in mitosis, and we discuss conservation of the system between yeast and higher eukaryotes.

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.

Mto2 multisite phosphorylation inactivates non-spindle microtubule nucleation complexes during mitosis

Microtubule nucleation is highly regulated during the eukaryotic cell cycle, but the underlying molecular mechanisms are largely unknown. During mitosis in fission yeast Schizosaccharomyces pombe, cytoplasmic microtubule nucleation ceases simultaneously with intranuclear mitotic spindle assembly. Cytoplasmic nucleation depends on the Mto1/2 complex, which binds and activates the γ-tubulin complex and also recruits the γ-tubulin complex to both centrosomal (spindle pole body) and non-centrosomal sites. Here we show that the Mto1/2 complex disassembles during mitosis, coincident with hyperphosphorylation of Mto2 protein. By mapping and mutating multiple Mto2 phosphorylation sites, we generate mto2-phosphomutant strains with enhanced Mto1/2 complex stability, interaction with the γ-tubulin complex and microtubule nucleation activity. A mutant with 24 phosphorylation sites mutated to alanine, mto2[24A], retains interphase-like behaviour even in mitotic cells. This provides a molecular-level understanding of how phosphorylation ‘switches off' microtubule nucleation complexes during the cell cycle and, more broadly, illuminates mechanisms regulating non-centrosomal microtubule nucleation.

In S. pombe, cytoplasmic microtubule nucleation, which depends on the Mto1/2 complex, ceases during mitosis. Here, Borek et al., show that multisite phosphorylation of Mto1/2 during mitosis disassembles the Mto1/2 complex and prevents microtubule nucleation activity.

Role of Ran-regulated nuclear-cytoplasmic trafficking of pVHL in the regulation of microtubular stability-mediated HIF-1α in hypoxic cardiomyocytes

Our previous study suggested that microtubule network alteration affects the process of glycolysis in cardiomyocytes (CMs) via the regulation of hypoxia-inducible factor (HIF)-1α during the early stages of hypoxia. However, little is known regarding the underlying mechanisms of microtubule network alteration-induced changes of HIF-1α. The von Hippel–Lindau tumor suppressor protein (pVHL) has been shown to mediate the ubiquitination of HIF-1α in the nuclear compartment prior to HIF-1α exportation to the cytoplasm, and pVHL dynamic nuclear-cytoplasmic trafficking is indicated to be involved in the process of HIF-1α degradation. In this study, by administering different microtubule-stabilizing and -depolymerizing interventions, we demonstrated that microtubule stabilization promoted pVHL nuclear export and drove the translocation of pVHL to the cytoplasm, while microtubule disruption prevented pVHL nuclear export in hypoxic CMs. Moreover, the ratio between nuclear and cytoplasmic pVHL was associated with HIF-1α regulation. Importantly, microtubule network alteration also affected the subcellular localization of Ran, which was involved in the regulation of pVHL nuclear-cytoplasmic trafficking. The above results suggest that the subcellular translocation of pVHL plays an important role in microtubular structure alteration-induced HIF-1α regulation. Interestingly, Ran is involved in the process of pVHL nuclear-cytoplasmic trafficking following microtubule network alteration in hypoxic CMs.

Optimization of the analogue-sensitive Cdc2/Cdk1 mutant by in vivo selection eliminates physiological limitations to its use in cell cycle analysis

Analogue-sensitive (as) mutants of kinases are widely used to selectively inhibit a single kinase with few off-target effects. The analogue-sensitive mutant cdc2-as of fission yeast (Schizosaccharomyces pombe) is a powerful tool to study the cell cycle, but the strain displays meiotic defects, and is sensitive to high and low temperature even in the absence of ATP-analogue inhibitors. This has limited the use of the strain for use in these settings. Here, we used in vivo selection for intragenic suppressor mutations of cdc2-as that restore full function in the absence of ATP-analogues. The cdc2-asM17 underwent meiosis and produced viable spores to a similar degree to the wild-type strain. The suppressor mutation also rescued the sensitivity of the cdc2-as strain to high and low temperature, genotoxins and an anti-microtubule drug. We have used cdc2-asM17 to show that Cdc2 activity is required to maintain the activity of the spindle assembly checkpoint. Furthermore, we also demonstrate that maintenance of the Shugoshin Sgo1 at meiotic centromeres does not require Cdc2 activity, whereas localization of the kinase aurora does. The modified cdc2-asM17 allele can be thus used to analyse many aspects of cell-cycle-related events in fission yeast.

Alp7/TACC recruits kinesin-8-PP1 to the Ndc80 kinetochore protein for timely mitotic progression and chromosome movement

Upon establishment of proper kinetochore–microtubule attachment, the spindle assembly checkpoint (SAC) must be silenced to allow onset of anaphase, which is when sister chromatids segregate equally to two daughter cells. However, how proper kinetochore–microtubule attachment leads to timely anaphase onset remains elusive. Furthermore, the molecular mechanisms of chromosome movement during anaphase A remain unclear. In this study, we show that the fission yeast Alp7/TACC protein recruits a protein complex consisting of the kinesin-8 (Klp5–Klp6) and protein phosphatase 1 (PP1) to the kinetochore upon kinetochore–microtubule attachment. Accumulation of this complex at the kinetochore, on the one hand, facilitates SAC inactivation through PP1, and, on the other hand, accelerates polewards chromosome movement driven by the Klp5–Klp6 motor. We identified an alp7 mutant that had specific defects in binding to the Klp5–Klp6–PP1 complex but with normal localisation to the microtubule and kinetochore. Consistent with our proposition, this mutant shows delayed anaphase onset and decelerated chromosome movement during anaphase A. We propose that the recruitment of kinesin-8–PP1 to the kinetochore through Alp7/TACC interaction plays a crucial role in regulation of timely mitotic progression and chromosome movement during anaphase A.

The kinetochore protein Kis1/Eic1/Mis19 ensures the integrity of mitotic spindles through maintenance of kinetochore factors Mis6/CENP-I and CENP-A

Microtubules play multiple roles in a wide range of cellular phenomena, including cell polarity establishment and chromosome segregation. A number of microtubule regulators have been identified, including microtubule-associated proteins and kinases, and knowledge of these factors has contributed to our molecular understanding of microtubule regulation of each relevant cellular process. The known regulators, however, are insufficient to explain how those processes are linked to one another, underscoring the need to identify additional regulators. To find such novel mechanisms and microtubule regulators, we performed a screen that combined genetics and microscopy for fission yeast mutants defective in microtubule organization. We isolated approximately 900 mutants showing defects in either microtubule organization or the nuclear envelope, and these mutants were classified into 12 categories. We particularly focused on one mutant, kis1, which displayed spindle defects in early mitosis. The kis1 mutant frequently failed to assemble a normal bipolar spindle. The responsible gene encoded a kinetochore protein, Mis19 (also known as Eic1), which localized to the interface of kinetochores and spindle poles. We also found that the inner kinetochore proteins Mis6/CENP-I and Cnp1/CENP-A were delocalized from kinetochores in the kis1 cells and that kinetochore-microtubule attachment was defective. Another mutant, mis6, also displayed similar spindle defects. We conclude that Kis1 is required for inner kinetochore organization, through which Kis1 ensures kinetochore-microtubule attachment and spindle integrity. Thus, we propose an unexpected relationship between inner kinetochore organization and spindle integrity.

Targeting Alp7/TACC to the spindle pole body is essential for mitotic spindle assembly in fission yeast

The conserved TACC protein family localises to the centrosome (the spindle pole body, SPB in fungi) and mitotic spindles, thereby playing a crucial role in bipolar spindle assembly. However, it remains elusive how TACC proteins are recruited to the centrosome/SPB. Here, using fission yeast Alp7/TACC, we have determined clustered five amino acid residues within the TACC domain required for SPB localisation. Critically, these sequences are essential for the functions of Alp7, including proper spindle formation and mitotic progression. Moreover, we have identified pericentrin-like Pcp1 as a loading factor to the mitotic SPB, although Pcp1 is not a sole platform.

CDK-dependent phosphorylation of Alp7-Alp14 (TACC-TOG) promotes its nuclear accumulation and spindle microtubule assembly

As cells transition from interphase to mitosis, the microtubule cytoskeleton is reorganized to form the mitotic spindle. In the closed mitosis of fission yeast, a microtubule-associated protein complex, Alp7-Alp14 (transforming acidic coiled-coil-tumor overexpressed gene), enters the nucleus upon mitotic entry and promotes spindle formation. However, how the complex is controlled to accumulate in the nucleus only during mitosis remains elusive. Here we demonstrate that Alp7-Alp14 is excluded from the nucleus during interphase using the nuclear export signal in Alp14 but is accumulated in the nucleus during mitosis through phosphorylation of Alp7 by the cyclin-dependent kinase (CDK). Five phosphorylation sites reside around the nuclear localization signal of Alp7, and the phosphodeficient alp7-5A mutant fails to accumulate in the nucleus during mitosis and exhibits partial spindle defects. Thus our results reveal one way that CDK regulates spindle assembly at mitotic entry: CDK phosphorylates the Alp7-Alp14 complex to localize it to the nucleus.

Transforming acidic coiled-coil proteins (TACCs) in human cancer

Fine-tuned regulation of the centrosome/microtubule dynamics during mitosis is essential for faithful cell division. Thus, it is not surprising that deregulations in this dynamic network can contribute to genomic instability and tumorigenesis. Indeed, centrosome loss or amplification, spindle multipolarity and aneuploidy are often found in a majority of human malignancies, suggesting that defects in centrosome and associated microtubules may be directly or indirectly linked to cancer. Therefore, future research to identify and characterize genes required for the normal centrosome function and microtubule dynamics may help us gain insight into the complexity of cancer, and further provide new avenues for prognostic, diagnostics and therapeutic interventions. Members of the transforming acidic coiled-coil proteins (TACCs) family are emerging as important players of centrosome and microtubule-associated functions. Growing evidence indicates that TACCs are involved in the progression of certain solid tumors. Here, we will discuss our current understanding of the biological function of TACCs, their relevance to human cancer and possible implications for cancer management.

The internal loop of fission yeast Ndc80 binds Alp7/TACC-Alp14/TOG and ensures proper chromosome attachment

The Ndc80 outer kinetochore complex plays a critical role in kinetochore-microtubule attachment, yet our understanding of the mechanism by which this complex interacts with spindle microtubules for timely and accurate chromosome segregation remains limited. Here we address this issue using an ndc80 mutant (ndc80-NH12) from fission yeast that contains a point mutation within a ubiquitous internal loop. This mutant is normal for assembly of the Ndc80 complex and bipolar spindle formation yet defective in proper end-on attachment to the spindle microtubule, with chromosome alignment defects and missegregation happening later during mitosis. We find that ndc80-NH12 exhibits impaired localization of the microtubule-associated protein complex Alp7/transforming acidic coiled coil (TACC)-Alp14/tumor-overexpressed gene (TOG) to the mitotic kinetochore. Consistently, wild-type Ndc80 binds these two proteins, whereas the Ndc80-NH12 mutant protein displays a substantial reduction of interaction. Crucially, forced targeting of Alp7-Alp14 to the outer kinetochore rescues ndc80-NH12-mutant phenotypes. The loop was previously shown to bind Dis1/TOG, by which it ensures initial chromosome capture during early mitosis. Strikingly, ndc80-NH12 is normal in Dis1 localization. Genetic results indicate that the loop recruits Dis1/TOG and Alp7/TACC-Alp14/TOG independently. Our work therefore establishes that the Ndc80 loop plays sequential roles in spindle-kinetochore attachment by connecting the Ndc80 complex to Dis1/TOG and Alp7/TACC-Alp14/TOG.

Spindle pole body components are reorganized during fission yeast meiosis

We show that spindle pole body (SPB) remodeling during meiosis in fission yeast is essential for meiosis. Many SPB components disappear during meiotic prophase and return to the SPBs at meiosis I onset. We found novel functions for Polo kinase/Plo1 and centrin/Cdc31 in the meiotic reorganization of SPB components.

During meiosis, the centrosome/spindle pole body (SPB) must be regulated in a manner distinct from that of mitosis to achieve a specialized cell division that will produce gametes. In this paper, we demonstrate that several SPB components are localized to SPBs in a meiosis-specific manner in the fission yeast Schizosaccharomyces pombe. SPB components, such as Cut12, Pcp1, and Spo15, which stay on the SPB during the mitotic cell cycle, disassociate from the SPB during meiotic prophase and then return to the SPB immediately before the onset of meiosis I. Interestingly, the polo kinase Plo1, which normally localizes to the SPB during mitosis, is excluded from them in meiotic prophase, when meiosis-specific, horse-tail nuclear movement occurs. We found that exclusion of Plo1 during this period was essential to properly remodel SPBs, because artificial targeting of Plo1 to SPBs resulted in an overduplication of SPBs. We also found that the centrin Cdc31 was required for meiotic SPB remodeling. Thus Plo1 and a centrin play central roles in the meiotic SPB remodeling, which is essential for generating the proper number of meiotic SPBs and, thereby provide unique characteristics to meiotic divisions.

Localized accumulation of tubulin during semi-open mitosis in the Caenorhabditis elegans embryo

The assembly of microtubules inside the cell is controlled both spatially and temporally. During mitosis, microtubule assembly must be activated locally at the nascent spindle region for mitotic spindle assembly to occur efficiently. In this paper, we report that mitotic spindle components, such as free tubulin subunits, accumulated in the nascent spindle region, independent of spindle formation in the Caenorhabditis elegans embryo. This accumulation coincided with nuclear envelope permeabilization, suggesting that permeabilization might trigger the accumulation. When permeabilization was induced earlier by knockdown of lamin, tubulin also accumulated earlier. The boundaries of the region of accumulation coincided with the remnant nuclear envelope, which remains after nuclear envelope breakdown in cells that undergo semi-open mitosis, such as those of C. elegans. Ran, a small GTPase protein, was required for tubulin accumulation. Fluorescence recovery after photobleaching analysis revealed that the accumulation was accompanied by an increase in the immobile fraction of free tubulin inside the remnant nuclear envelope. We propose that this newly identified mechanism of accumulation of free tubulin-and probably of other molecules-at the nascent spindle region contributes to efficient assembly of the mitotic spindle in the C. elegans embryo.

Space shuttling in the cell: nucleocytoplasmic transport and microtubule organization during the cell cycle

Microtubules form a multifunctional filamentous structure essential for the cell. In interphase, microtubules form networks in the cytoplasm and play pivotal roles in cell polarity and intracellular transport of various biomolecules. In mitosis, microtubules dramatically change their morphology to assemble the mitotic spindle, thereby pulling the chromosomes toward the spindle poles. One long-standing question is how microtubules are reorganized upon mitotic entry. Yeast cells undergo closed mitosis, in which the nuclear envelope persists, whereas higher eukaryotes undergo open mitosis, in which the nuclear envelope breaks down. Microtubule reorganization must be controlled by selective localization of microtubule-assembly factors. Recent findings in fission yeast indicate that several microtubule-associated proteins (MAPs) shuttle between the cytoplasm and the nucleus through regulation by Ran GTPase, the universal organizer of nucleocytoplasmic transport. Furthermore, the synergistic interplay of Ran and cyclin-dependent kinase (CDK) induces the critical spatiotemporal shift of modes in microtubule assembly from cytoplasmic arrays to nuclear spindles. A MAP complex Alp7/TACC-Alp14/TOG undergoes nucleocytoplasmic shuttling in interphase, whereas it is retained in the mitotic nucleus through a decrease of its nuclear export by CDK. Our understanding of how microtubules are reorganized during the cell cycle is beginning to emerge.