CLIP-170 facilitates the formation of kinetochore-microtubule attachments

α-tubulin detyrosination fine-tunes kinetochore-microtubule attachments

Post-translational cycles of α-tubulin detyrosination and tyrosination generate microtubule diversity, the cellular functions of which remain largely unknown. Here we show that α-tubulin detyrosination regulates kinetochore-microtubule attachments to ensure normal chromosome oscillations and timely anaphase onset during mitosis. Remarkably, detyrosinated α-tubulin levels near kinetochore microtubule plus-ends depend on the direction of chromosome motion during metaphase. Proteomic analyses unveil that the KNL-1/MIS12/NDC80 (KMN) network that forms the core microtubule-binding site at kinetochores and the microtubule-rescue protein CLASP2 are enriched on tyrosinated and detyrosinated microtubules during mitosis, respectively. α-tubulin detyrosination enhances CLASP2 binding and NDC80 complex diffusion along the microtubule lattice in vitro. Rescue experiments overexpressing NDC80, including variants with slower microtubule diffusion, suggest a functional interplay with α-tubulin detyrosination for the establishment of a labile kinetochore-microtubule interface. These results offer a mechanistic explanation for how different detyrosinated α-tubulin levels near kinetochore microtubule plus-ends fine-tune load-bearing attachments to both growing and shrinking microtubules.

The substrate quality of CK2 target sites has a determinant role on their function and evolution

Most biological processes are regulated by signaling modules that bind to short linear motifs. For protein kinases, substrates may have full or only partial matches to the kinase recognition motif, a property known as "substrate quality." However, it is not clear whether differences in substrate quality represent neutral variation or if they have functional consequences. We examine this question for the kinase CK2, which has many fundamental functions. We show that optimal CK2 sites are phosphorylated at maximal stoichiometries and found in many conditions, whereas minimal substrates are more weakly phosphorylated and have regulatory functions. Optimal CK2 sites tend to be more conserved, and substrate quality is often tuned by selection. For intermediate sites, increases or decreases in substrate quality may be deleterious, as we demonstrate for a CK2 substrate at the kinetochore. The results together suggest a strong role for substrate quality in phosphosite function and evolution. A record of this paper's transparent peer review process is included in the supplemental information.

The motor domain of the kinesin Kip2 promotes microtubule polymerization at microtubule tips

Kinesins are microtubule-dependent motor proteins, some of which moonlight as microtubule polymerases, such as the yeast protein Kip2. Here, we show that the CLIP-170 ortholog Bik1 stabilizes Kip2 at microtubule ends where the motor domain of Kip2 promotes microtubule polymerization. Live-cell imaging and mathematical estimation of Kip2 dynamics reveal that disrupting the Kip2-Bik1 interaction aborts Kip2 dwelling at microtubule ends and abrogates its microtubule polymerization activity. Structural modeling and biochemical experiments identify a patch of positively charged residues that enables the motor domain to bind free tubulin dimers alternatively to the microtubule shaft. Neutralizing this patch abolished the ability of Kip2 to promote microtubule growth both in vivo and in vitro without affecting its ability to walk along microtubules. Our studies suggest that Kip2 utilizes Bik1 as a cofactor to track microtubule tips, where its motor domain then recruits free tubulin and catalyzes microtubule assembly.

Coordination between the Ndc80 complex and dynein is essential for microtubule plus-end capture by kinetochores during early mitosis

Mitotic kinetochores are initially captured by dynamic microtubules via a "search-and-capture" mechanism. The microtubule motor, dynein, is critical for kinetochore capture as it has been shown to transport microtubule-attached chromosomes toward the spindle pole during prometaphase. The microtubule-binding nuclear division cycle 80 (Ndc80) complex that is recruited to kinetochores in prophase is known to play a central role in forming kinetochore-microtubule (kMT) attachments in metaphase. It is not yet clear, however, how Ndc80 contributes to initial kMT capture during prometaphase. Here, by combining CRISPR/Cas9-mediated knockout and RNAi technology with assays specific to study kMT capture, we show that mitotic cells lacking Ndc80 exhibit substantial defects in this function during prometaphase. Rescue experiments show that Ndc80 mutants deficient in microtubule-binding are unable to execute proper kMT capture. While cells inhibited of dynein alone are predominantly able to make initial kMT attachments, cells co-depleted of Ndc80 and dynein show severe defects in kMT capture. Further, we use an in vitro total internal reflection fluorescence microscopy assay to reconstitute microtubule capture events, which suggest that Ndc80 and dynein coordinate with each other for microtubule plus-end capture and that the phosphorylation status of Ndc80 is critical for productive kMT capture. A novel interaction between Ndc80 and dynein that we identify in prometaphase extracts might be critical for efficient plus-end capture. Thus, our studies, for the first time, identify a distinct event in the formation of initial kMT attachments, which is directly mediated by Ndc80 and in coordination with dynein is required for efficient kMT capture and chromosome alignment.

Mitotic and chromosomal effects induced for biosynthesized nanoparticles from three mediators on Allium cepa root cells

The genotoxicity of biogenic silver nanoparticles (AgNPs) obtained from three microbial mediators was assessed using the Allium cepa assay. Three clusters were differentiated for the highest frequency of end points of clastogenicity (stick-ends, fragments and bridges), end points of missegregation (C-metaphases and disorder anaphases), and lowest frequency of all the end points. In these clusters, the treatments were grouped respectively as I) positive control (GSF); II) silver nanoparticles form Aspergillus niger (AgNPs-An); and III) silver nanoparticles from both Cryptococcus laurentii (AgNPs-Cl) and Rhodotorula glutinis (AgNPs-Rg), Ag + , and negative control (NC). These results were in according to the principal component analisys (PCA) where treatments were associated to each component of the genotoxic effects. The statistical comparative analysis of the mitotic index (IM) and the abnormal mitosis frequency (AM) indicated that both GSF and AgNPsAn induce significant genotoxic effect. Low genotoxic effects were attributed to AgNPs-Cl and AgNPs-Rg, but mitogenic stimuli, similar to that obtained by the silver ions Ag + , were observed. Results suggested that different features of biogenic nanoparticles such as composition, size, and coating may be involved in the different cytological responses of the meristematic cells.

Inhibition of polar actin assembly by astral microtubules is required for cytokinesis

During cytokinesis, the actin cytoskeleton is partitioned into two spatially distinct actin isoform specific networks: a β-actin network that generates the equatorial contractile ring, and a γ-actin network that localizes to the cell cortex. Here we demonstrate that the opposing regulation of the β- and γ-actin networks is required for successful cytokinesis. While activation of the formin DIAPH3 at the cytokinetic furrow underlies β-actin filament production, we show that the γ-actin network is specifically depleted at the cell poles through the localized deactivation of the formin DIAPH1. During anaphase, CLIP170 is delivered by astral microtubules and displaces IQGAP1 from DIAPH1, leading to formin autoinhibition, a decrease in cortical stiffness and localized membrane blebbing. The contemporaneous production of a β-actin contractile ring at the cell equator and loss of γ-actin from the poles is required to generate a stable cytokinetic furrow and for the completion of cell division.

During cell division, the actin cytoskeletal network at both the equatorial contractile ring and cell cortex are known to play a role, but the regulation of γ-actin during cytokinesis is less well understood. Here, the authors show that recruitment of β-actin to the contractile ring and loss of γ-actin from the cell poles is required for completion of cell division.

Stress-induced phosphorylation of CLIP-170 by JNK promotes microtubule rescue

The stress-induced c-Jun N-terminal kinase (JNK) controls microtubule dynamics by enhancing both microtubule growth and rescues. Here, we show that upon cell stress, JNK directly phosphorylates the microtubule rescue factor CLIP-170 in its microtubule-binding domain to increase its rescue-promoting activity. Phosphomimetic versions of CLIP-170 enhance its ability to promote rescue events in vitro and in cells. Furthermore, while phosphomimetic mutations do not alter CLIP-170’s capability to form comets at growing microtubule ends, both phosphomimetic mutations and JNK activation increase the occurrence of CLIP-170 remnants on the microtubule lattice at the rear of comets. As the CLIP-170 remnants, which are potential sites of microtubule rescue, display a shorter lifetime when CLIP-170 is phosphorylated, we propose that instead of acting at the time of rescue occurrence, CLIP-170 would rather contribute in preparing the microtubule lattice for future rescues at these predetermined sites.

Arsenic trioxide disturbs the LIS1/NDEL1/dynein microtubule dynamic complex by disrupting the CLIP170 zinc finger in head and neck cancer

Cancer mortality is mainly caused by metastasis, which requires dynamic remodeling of cytoskeletal components such as microtubules. Targeting microtubules presents a promising antimetastatic strategy that could prevent cancer spreading and recurrence. It is known that arsenic trioxide (ATO) is able to inhibit the migration and invasion of solid malignant tumors, but its exact molecular mechanism remains unclear. Here, we report a novel molecular target and antimetastatic mechanism of ATO in head and neck squamous cell carcinoma (HNSCC). We found that cytoplasmic linker protein 170 (CLIP170) was overexpressed in HNSCC tissues and cells compared to normal controls. ATO at non-cytotoxic level (1 μM) inhibited the migration and invasion of HNSCC cells by displacing zinc in the zinc finger motif of CLIP170, which is a key protein that controls microtubule dynamics. The antimetastatic effects of ATO were equivalent to those of siRNA-mediated CLIP170 knockdown. Furthermore, ATO dysregulated microtubule polymerization via the CLIP170/LIS1/NDEL1/dynein signaling pathway in Cal27 cells as a functional consequence of CLIP170 zinc finger disruption. The effect was partially reversed by zinc supplementation. Taken together, these findings reveal that CLIP170 is a novel molecular target of ATO and demonstrate the capability and underlying mechanisms of ATO as a potential antimetastatic agent for HNSCC treatment.

The Global Phosphorylation Landscape of SARS-CoV-2 Infection

The causative agent of the coronavirus disease 2019 (COVID-19) pandemic, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has infected millions and killed hundreds of thousands of people worldwide, highlighting an urgent need to develop antiviral therapies. Here we present a quantitative mass spectrometry-based phosphoproteomics survey of SARS-CoV-2 infection in Vero E6 cells, revealing dramatic rewiring of phosphorylation on host and viral proteins. SARS-CoV-2 infection promoted casein kinase II (CK2) and p38 MAPK activation, production of diverse cytokines, and shutdown of mitotic kinases, resulting in cell cycle arrest. Infection also stimulated a marked induction of CK2-containing filopodial protrusions possessing budding viral particles. Eighty-seven drugs and compounds were identified by mapping global phosphorylation profiles to dysregulated kinases and pathways. We found pharmacologic inhibition of the p38, CK2, CDK, AXL, and PIKFYVE kinases to possess antiviral efficacy, representing potential COVID-19 therapies.

Crowning the Kinetochore: The Fibrous Corona in Chromosome Segregation

The kinetochore is at the heart of chromosome segregation in mitosis and meiosis. Rather than a static linker complex for chromatin and spindle microtubules, it is highly dynamic in composition, size, and shape. While known for decades that it can expand and grow a fibrous meshwork known as the corona, it was until recently unclear what constitutes this 'crown' and what its relevance is for kinetochore function. Here, we highlight recent discoveries in fibrous corona biology, and place them in the context of the processes that orchestrate high-fidelity chromosome segregation.

A Journey through the Cytoskeleton with Protein Kinase CK2

Substrate pleiotropicity, a very acidic phosphorylation consensus sequence, and an apparent uncontrolled activity, are the main features of CK2, a Ser/Thr protein kinase that is required for a plethora of cell functions. Not surprisingly, CK2 appears to affect cytoskeletal structures and correlated functions such as cell shape, mechanical integrity, cell movement and division. This review outlines our current knowledge of how CK2 regulates cytoskeletal structures, and discusses involved pathways and molecular mechanisms.

Splicing factors Sf3A2 and Prp31 have direct roles in mitotic chromosome segregation

Several studies have shown that RNAi-mediated depletion of splicing factors (SFs) results in mitotic abnormalities. However, it is currently unclear whether these abnormalities reflect defective splicing of specific pre-mRNAs or a direct role of the SFs in mitosis. Here, we show that two highly conserved SFs, Sf3A2 and Prp31, are required for chromosome segregation in both Drosophila and human cells. Injections of anti-Sf3A2 and anti-Prp31 antibodies into Drosophila embryos disrupt mitotic division within 1 min, arguing strongly against a splicing-related mitotic function of these factors. We demonstrate that both SFs bind spindle microtubules (MTs) and the Ndc80 complex, which in Sf3A2- and Prp31-depleted cells is not tightly associated with the kinetochores; in HeLa cells the Ndc80/HEC1-SF interaction is restricted to the M phase. These results indicate that Sf3A2 and Prp31 directly regulate interactions among kinetochores, spindle microtubules and the Ndc80 complex in both Drosophila and human cells.

Lateral attachment of kinetochores to microtubules is enriched in prometaphase rosette and facilitates chromosome alignment and bi-orientation establishment

Faithful chromosome segregation is ensured by the establishment of bi-orientation; the attachment of sister kinetochores to the end of microtubules extending from opposite spindle poles. In addition, kinetochores can also attach to lateral surfaces of microtubules; called lateral attachment, which plays a role in chromosome capture and transport. However, molecular basis and biological significance of lateral attachment are not fully understood. We have addressed these questions by focusing on the prometaphase rosette, a typical chromosome configuration in early prometaphase. We found that kinetochores form uniform lateral attachments in the prometaphase rosette. Many transient kinetochore components are maximally enriched, in an Aurora B activity-dependent manner, when the prometaphase rosette is formed. We revealed that rosette formation is driven by rapid poleward motion of dynein, but can occur even in its absence, through slow kinetochore movements caused by microtubule depolymerization that is supposedly dependent on kinetochore tethering at microtubule ends by CENP-E. We also found that chromosome connection to microtubules is extensively lost when lateral attachment is perturbed in cells defective in end-on attachment. Our findings demonstrate that lateral attachment is an important intermediate in bi-orientation establishment and chromosome alignment, playing a crucial role in incorporating chromosomes into the nascent spindle.

Cytoplasmic Linker Protein CLIP170 Negatively Regulates TLR4 Signaling by Targeting the TLR Adaptor Protein TIRAP

Cytoplasmic linker protein 170 (CLIP170) is a CAP-Gly domain-containing protein that is associated with the plus end of growing microtubules and implicated in various cellular processes, including the regulation of microtubule dynamics, cell migration, and intracellular transport. Our studies revealed a previously unrecognized property and role of CLIP170. We identified CLIP170 as one of the interacting partners of Brucella effector protein TcpB that negatively regulates TLR2 and TLR4 signaling. In this study, we demonstrate that CLIP170 interacts with the TLR2 and TLR4 adaptor protein TIRAP. Furthermore, our studies revealed that CLIP170 induces ubiquitination and subsequent degradation of TIRAP to negatively regulate TLR4-mediated proinflammatory responses. Overexpression of CLIP170 in mouse macrophages suppressed the LPS-induced expression of IL-6 and TNF-α whereas silencing of endogenous CLIP170 potentiated the levels of proinflammatory cytokines. In vivo silencing of CLIP170 in C57BL/6 mice by CLIP170-specific small interfering RNA enhanced LPS-induced IL-6 and TNF-α expression. Furthermore, we found that LPS modulates the expression of CLIP170 in mouse macrophages. Overall, our experimental data suggest that CLIP170 serves as an intrinsic negative regulator of TLR4 signaling that targets TIRAP.

Plk1 bound to Bub1 contributes to spindle assembly checkpoint activity during mitosis

For faithful chromosome segregation, the formation of stable kinetochore-microtubule attachment and its monitoring by the spindle assembly checkpoint (SAC) are coordinately regulated by mechanisms that are currently ill-defined. Here, we show that polo-like kinase 1 (Plk1), which is instrumental in forming stable kinetochore-microtubule attachments, is also involved in the maintenance of SAC activity by binding to Bub1, but not by binding to CLASP2 or CLIP-170. The effect of Plk1 on the SAC was found to be mediated through phosphorylation of Mps1, an essential kinase for the SAC, as well as through phosphorylation of the MELT repeats in Knl1. Bub1 acts as a platform for assembling other SAC components on the phosphorylated MELT repeats. We propose that Bub1-bound Plk1 is important for the maintenance of SAC activity by supporting Bub1 localization to kinetochores in prometaphase, a time when the kinetochore Mps1 level is reduced, until the formation of stable kinetochore-microtubule attachment is completed. Our study reveals an intricate mechanism for coordinating the formation of stable kinetochore-microtubule attachment and SAC activity.

The role of MACF1 in nervous system development and maintenance

Microtubule-actin crosslinking factor 1 (MACF1), also known as actin crosslinking factor 7 (ACF7), is essential for proper modulation of actin and microtubule cytoskeletal networks. Most MACF1 isoforms are expressed broadly in the body, but some are exclusively found in the nervous system. Consequentially, MACF1 is integrally involved in multiple neural processes during development and in adulthood, including neurite outgrowth and neuronal migration. Furthermore, MACF1 participates in several signaling pathways, including the Wnt/β-catenin and GSK-3 signaling pathways, which regulate key cellular processes, such as proliferation and cell migration. Genetic mutation or dysregulation of the MACF1 gene has been associated with neurodevelopmental and neurodegenerative diseases, specifically schizophrenia and Parkinson's disease. MACF1 may also play a part in neuromuscular disorders and have a neuroprotective role in the optic nerve. In this review, the authors seek to synthesize recent findings relating to the roles of MACF1 within the nervous system and explore potential novel functions of MACF1 not yet examined.

Spindle assembly checkpoint signaling and sister chromatid cohesion are disrupted by HPV E6-mediated transformation

Aneuploidy, a condition that results from unequal partitioning of chromosomes during mitosis, is a hallmark of many cancers, including those caused by human papillomaviruses (HPVs). E6 and E7 are the primary transforming proteins in HPV that drive tumor progression. In this study, we stably expressed E6 and E7 in noncancerous RPE1 cells and analyzed the specific mitotic defects that contribute to aneuploidy in each cell line. We find that E6 expression results in multiple chromosomes associated with one or both spindle poles, causing a significant mitotic delay. In most cells, the misaligned chromosomes eventually migrated to the spindle equator, leading to mitotic exit. In some cells, however, mitotic exit occurred in the presence of pole-associated chromosomes. We determined that this premature mitotic exit is due to defects in spindle assembly checkpoint (SAC) signaling, such that cells are unable to maintain a prolonged mitotic arrest in the presence of unaligned chromosomes. This SAC defect is caused in part by a loss of kinetochore-associated Mad2 in E6-expressing cells. Our results demonstrate that E6-expressing cells exhibit previously unappreciated mitotic defects that likely contribute to HPV-mediated cancer progression.

Mechanisms of Chromosome Congression during Mitosis

Chromosome congression during prometaphase culminates with the establishment of a metaphase plate, a hallmark of mitosis in metazoans. Classical views resulting from more than 100 years of research on this topic have attempted to explain chromosome congression based on the balance between opposing pulling and/or pushing forces that reach an equilibrium near the spindle equator. However, in mammalian cells, chromosome bi-orientation and force balance at kinetochores are not required for chromosome congression, whereas the mechanisms of chromosome congression are not necessarily involved in the maintenance of chromosome alignment after congression. Thus, chromosome congression and maintenance of alignment are determined by different principles. Moreover, it is now clear that not all chromosomes use the same mechanism for congressing to the spindle equator. Those chromosomes that are favorably positioned between both poles when the nuclear envelope breaks down use the so-called "direct congression" pathway in which chromosomes align after bi-orientation and the establishment of end-on kinetochore-microtubule attachments. This favors the balanced action of kinetochore pulling forces and polar ejection forces along chromosome arms that drive chromosome oscillatory movements during and after congression. The other pathway, which we call "peripheral congression", is independent of end-on kinetochore microtubule-attachments and relies on the dominant and coordinated action of the kinetochore motors Dynein and Centromere Protein E (CENP-E) that mediate the lateral transport of peripheral chromosomes along microtubules, first towards the poles and subsequently towards the equator. How the opposite polarities of kinetochore motors are regulated in space and time to drive congression of peripheral chromosomes only now starts to be understood. This appears to be regulated by position-dependent phosphorylation of both Dynein and CENP-E and by spindle microtubule diversity by means of tubulin post-translational modifications. This so-called "tubulin code" might work as a navigation system that selectively guides kinetochore motors with opposite polarities along specific spindle microtubule populations, ultimately leading to the congression of peripheral chromosomes. We propose an integrated model of chromosome congression in mammalian cells that depends essentially on the following parameters: (1) chromosome position relative to the spindle poles after nuclear envelope breakdown; (2) establishment of stable end-on kinetochore-microtubule attachments and bi-orientation; (3) coordination between kinetochore- and arm-associated motors; and (4) spatial signatures associated with post-translational modifications of specific spindle microtubule populations. The physiological consequences of abnormal chromosome congression, as well as the therapeutic potential of inhibiting chromosome congression are also discussed.

Phosphorylation of EB1 regulates the recruitment of CLIP-170 and p150glued to the plus ends of astral microtubules

Phosphorylation of end-binding protein 1 (EB1), a key member of microtubule plus end-tracking proteins (+TIPs), by apoptosis signal-regulating kinase 1 (ASK1) has been demonstrated to promote the stability of astral microtubules during mitosis by stimulating the binding of EB1 to microtubule plus ends. However, the roles of other members of the +TIPs family in ASK1/EB1-mediated regulation of astral microtubules are unknown. Herein, we show that ASK1-mediated phosphorylation of EB1 enhances the localization of cytoplasmic linker protein 170 (CLIP-170) and p150glued to the plus ends of astral microtubules. Depletion of ASK1 or expression of phospho-deficient or phospho-mimetic EB1 mutants results in changes in the levels of plus-end localized CLIP-170 or p150glued. Mechanistic studies reveal that EB1 phosphorylation promotes its interactions with CLIP-170 and p150glued, thereby recruiting these +TIPs to microtubules. Structural analysis suggests that serine-40 is the primary phosphorylation site on EB1 that exerts these effects. Together, these findings provide novel insight into the molecular mechanisms that regulate the interactions of EB1 with other +TIPs.

EB1 regulates attachment of Ska1 with microtubules by forming extended structures on the microtubule lattice

Kinetochore couples chromosome movement to dynamic microtubules, a process that is fundamental to mitosis in all eukaryotes but poorly understood. In vertebrates, spindle-kinetochore-associated (Ska1–3) protein complex plays an important role in this process. However, the proteins that stabilize Ska-mediated kinetochore-microtubule attachment remain unknown. Here we show that microtubule plus-end tracking protein EB1 facilitates Ska localization on microtubules in vertebrate cells. EB1 depletion results in a significant reduction of Ska1 recruitment onto microtubules and defects in mitotic chromosome alignment, which is also reflected in computational modelling. Biochemical experiments reveal that EB1 interacts with Ska1, facilitates Ska1-microtubule attachment and together stabilizes microtubules. Structural studies reveal that EB1 either with Ska1 or Ska complex forms extended structures on microtubule lattice. Results indicate that EB1 promotes Ska association with K-fibres and facilitates kinetochore-microtubule attachment. They also implicate that in vertebrates, chromosome coupling to dynamic microtubules could be mediated through EB1-Ska extended structures.

Ska1 is a kinetochore-localised protein that couples kinetochore movement to microtubule (MT) depolymerisation. Here Thomas et al. show that the MT +TIP binding protein EB1 recruits Ska1 to the MT-kinetochore interface and stabilises the interaction between Ska1 and MTs.

Chromosome misalignments induce spindle-positioning defects

Cortical pulling forces on astral microtubules are essential to position the spindle. These forces are generated by cortical dynein, a minus-end directed motor. Previously, another dynein regulator termed Spindly was proposed to regulate dynein-dependent spindle positioning. However, the mechanism of how Spindly regulates spindle positioning has remained elusive. Here, we find that the misalignment of chromosomes caused by Spindly depletion is directly provoking spindle misorientation. Chromosome misalignments induced by CLIP-170 or CENP-E depletion or by noscapine treatment are similarly accompanied by severe spindle-positioning defects. We find that cortical LGN is actively displaced from the cortex when misaligned chromosomes are in close proximity. Preventing the KT recruitment of Plk1 by the depletion of PBIP1 rescues cortical LGN enrichment near misaligned chromosomes and re-establishes proper spindle orientation. Hence, KT-enriched Plk1 is responsible for the negative regulation of cortical LGN localization. In summary, we uncovered a compelling molecular link between chromosome alignment and spindle orientation defects, both of which are implicated in tumorigenesis.

CLIP-170 tethers kinetochores to microtubule plus ends against poleward force by dynein for stable kinetochore-microtubule attachment

The cytoplasmic linker protein (CLIP)-170 localizes to kinetochores and is suggested to function in stable attachment of kinetochores to microtubule ends. Here we show that defects in kinetochore-microtubule attachment and chromosome alignment in CLIP-170-depleted cells were rescued by co-depletion of p150glued, a dynactin subunit required for kinetochore localization of CLIP-170. CLIP-170 recruited p150glued to microtubule ends. Kinetochore localization at microtubule ends was perturbed by CLIP-170 depletion, which was rescued by co-depleting p150glued. Our results imply that CLIP-170 tethers kinetochores to microtubule ends against the dynein-mediated poleward force to slide kinetochores along microtubules, facilitating the stable kinetochore attachment to microtubules.

A protein-tagging system for signal amplification in gene expression and fluorescence imaging

Signals in many biological processes can be amplified by recruiting multiple copies of regulatory proteins to a site of action. Harnessing this principle, we have developed a protein scaffold, a repeating peptide array termed SunTag, which can recruit multiple copies of an antibody-fusion protein. We show that the SunTag can recruit up to 24 copies of GFP, thereby enabling long-term imaging of single protein molecules in living cells. We also use the SunTag to create a potent synthetic transcription factor by recruiting multiple copies of a transcriptional activation domain to a nuclease-deficient CRISPR/Cas9 protein and demonstrate strong activation of endogenous gene expression and re-engineered cell behavior with this system. Thus, the SunTag provides a versatile platform for multimerizing proteins on a target protein scaffold and is likely to have many applications in imaging and controlling biological outputs.

Microtubule plus-end tracking proteins and their roles in cell division

Microtubules are cellular components that are required for a variety of essential processes such as cell motility, mitosis, and intracellular transport. This is possible because of the inherent dynamic properties of microtubules. Many of these properties are tightly regulated by a number of microtubule plus-end-binding proteins or +TIPs. These proteins recognize the distal end of microtubules and are thus in the right context to control microtubule dynamics. In this review, we address how microtubule dynamics are regulated by different +TIP families, focusing on how functionally diverse +TIPs spatially and temporally regulate microtubule dynamics during animal cell division.

CLIP-170 recruits PLK1 to kinetochores during early mitosis for chromosome alignment

The cytoplasmic linker protein (CLIP)-170, an outer kinetochore protein, has a role in kinetochore-microtubule attachment and chromosome alignment during mitosis. However, the mechanism by which CLIP-170 is involved in chromosome alignment is not known. Here, we show that CLIP-170 colocalizes with Polo-like kinase 1 (PLK1) at kinetochores during early mitosis. Depletion of CLIP-170 results in a significant reduction in PLK1 recruitment to kinetochores and causes kinetochore-fiber (K-fiber) instability and defects in chromosome alignment at the metaphase plate. These phenotypes are dependent on the phosphorylation of CLIP-170 at a CDK1-dependent site, T287, as ectopic expression of wild-type CLIP-170, but not the expression of a non-phosphorylatable mutant, CLIP-170-T287A, restores PLK1 localization at kinetochores and rescues K-fiber stability and chromosome alignment in CLIP-170-depleted cells. These data suggest that CLIP-170 acts as a novel recruiter and spatial regulator of PLK1 at kinetochores during early mitosis, promoting K-fiber stability and chromosome alignment for error-free chromosome segregation.

A defect in the CLIP1 gene (CLIP-170) can cause autosomal recessive intellectual disability

In the context of a comprehensive research project, investigating novel autosomal recessive intellectual disability (ARID) genes, linkage analysis based on autozygosity mapping helped identify an intellectual disability locus on Chr.12q24, in an Iranian family (LOD score = 3.7). Next-generation sequencing (NGS) following exon enrichment in this novel interval, detected a nonsense mutation (p.Q1010*) in the CLIP1 gene. CLIP1 encodes a member of microtubule (MT) plus-end tracking proteins, which specifically associates with the ends of growing MTs. These proteins regulate MT dynamic behavior and are important for MT-mediated transport over the length of axons and dendrites. As such, CLIP1 may have a role in neuronal development. We studied lymphoblastoid and skin fibroblast cell lines established from healthy and affected patients. RT-PCR and western blot analyses showed the absence of CLIP1 transcript and protein in lymphoblastoid cells derived from affected patients. Furthermore, immunofluorescence analyses showed MT plus-end staining only in fibroblasts containing the wild-type (and not the mutant) CLIP1 protein. Collectively, our data suggest that defects in CLIP1 may lead to ARID.

Plk1 phosphorylates CLIP-170 and regulates its binding to microtubules for chromosome alignment

The microtubule (MT) cytoskeleton is essential for cellular morphogenesis, cell migration, and cell division. MT organization is primarily mediated by a variety of MT-associated proteins. Among these proteins, plus-end-tracking proteins (+TIPs) are evolutionarily conserved factors that selectively accumulate at growing MT plus ends. Cytoplasmic linker protein (CLIP)-170 is a +TIP that associates with diverse proteins to determine the behavior of MT ends and their linkage to intracellular structures, including mitotic chromosomes. However, how CLIP-170 activity is spatially and temporally controlled is largely unknown. Here, we show that phosphorylation at Ser312 in the third serine-rich region of CLIP-170 is increased during mitosis. Polo-like kinase 1 (Plk1) is responsible for this phosphorylation during the mitotic phase of dividing cells. In vitro analysis using a purified CLIP-170 N-terminal fragment showed that phosphorylation by Plk1 diminishes CLIP-170 binding to the MT ends and lattice without affecting binding to EB3. Furthermore, we demonstrate that during mitosis, stable kinetochore/MT attachment and subsequent chromosome alignment require CLIP-170 and a proper phosphorylation/dephosphorylation cycle at Ser312. We propose that CLIP-170 phosphorylation by Plk1 regulates proper chromosome alignment by modulating the interaction between CLIP-170 and MTs in mitotic cells and that CLIP-170 activity is stringently controlled by its phosphorylation state, which depends on the cellular context.

Regulatory mechanisms of kinetochore-microtubule interaction in mitosis

Interaction of microtubules with kinetochores is fundamental to chromosome segregation. Kinetochores initially associate with lateral surfaces of microtubules and subsequently become attached to microtubule ends. During these interactions, kinetochores can move by sliding along microtubules or by moving together with depolymerizing microtubule ends. The interplay between kinetochores and microtubules leads to the establishment of bi-orientation, which is the attachment of sister kinetochores to microtubules from opposite spindle poles, and subsequent chromosome segregation. Molecular mechanisms underlying these processes have been intensively studied over the past 10 years. Emerging evidence suggests that the KNL1-Mis12-Ndc80 (KMN) network plays a central role in connecting kinetochores to microtubules, which is under fine regulation by a mitotic kinase, Aurora B. However, a growing number of additional molecules are being shown to be involved in the kinetochore-microtubule interaction. Here I overview the current range of regulatory mechanisms of the kinetochore-microtubule interaction, and discuss how these multiple molecules contribute cooperatively to allow faithful chromosome segregation.

Microtubule plus-ends within a mitotic cell are 'moving platforms' with anchoring, signalling and force-coupling roles

The microtubule polymer grows and shrinks predominantly from one of its ends called the 'plus-end'. Plus-end regulation during interphase is well understood. However, mitotic regulation of plus-ends is only beginning to be understood in mammalian cells. During mitosis, the plus-ends are tethered to specialized microtubule capture sites. At these sites, plus-end-binding proteins are loaded and unloaded in a regulated fashion. Proper tethering of plus-ends to specialized sites is important so that the microtubule is able to translate its growth and shrinkage into pushing and pulling forces that move bulky subcellular structures. We discuss recent advances on how mitotic plus-ends are tethered to distinct subcellular sites and how plus-end-bound proteins can modulate the forces that move subcellular structures. Using end binding 1 (EB1) as a prototype plus-end-binding protein, we highlight the complex network of plus-end-binding proteins and their regulation through phosphorylation. Finally, we develop a speculative 'moving platform' model that illustrates the plus-end's role in distinguishing correct versus incorrect microtubule interactions.

Cdk1 and Plk1 mediate a CLASP2 phospho-switch that stabilizes kinetochore-microtubule attachments

Accurate chromosome segregation during mitosis relies on a dynamic kinetochore (KT)-microtubule (MT) interface that switches from a labile to a stable condition in response to correct MT attachments. This transition is essential to satisfy the spindle-assembly checkpoint (SAC) and couple MT-generated force with chromosome movements, but the underlying regulatory mechanism remains unclear. In this study, we show that during mitosis the MT- and KT-associated protein CLASP2 is progressively and distinctively phosphorylated by Cdk1 and Plk1 kinases, concomitant with the establishment of KT-MT attachments. CLASP2 S1234 was phosphorylated by Cdk1, which primed CLASP2 for association with Plk1. Plk1 recruitment to KTs was enhanced by CLASP2 phosphorylation on S1234. This was specifically required to stabilize KT-MT attachments important for chromosome alignment and to coordinate KT and non-KT MT dynamics necessary to maintain spindle bipolarity. CLASP2 C-terminal phosphorylation by Plk1 was also required for chromosome alignment and timely satisfaction of the SAC. We propose that Cdk1 and Plk1 mediate a fine CLASP2 "phospho-switch" that temporally regulates KT-MT attachment stability.

The cholangiocyte marker, BD. 1, forms a stable complex with CLIP170 and shares an identity with eIF3a, a multifunctional subunit of the eIF3 initiation complex

We have previously described the generation of a monoclonal antibody recognizing a novel cholangiocyte marker, designated BD.1, that is expressed by fetal and adult rat cholangiocytes but not hepatocytes or the hepatic progenitor cells known as oval cells. In the present report, we have undertaken a comprehensive examination of BD.1 expressed by long-term cultures of bile duct epithelial cells (BDEC) and prostate epithelial cells (PEC). We show that with continued passage, the levels of BD.1 expressed by BDEC and PEC drop significantly, a decrease that is temporally associated with transition from a diploid to an aneuploid karyotype. Cell cycle analysis revealed cell cycle dependent expression of BD.1 characterized by decreased BD.1 levels within the first 10 h after release from serum starvation followed by reacquisition as cells entered S phase. MAb BD.1 recognized a 170 kDa protein in Western blots and showed strong reactivity with a 170 kDa band in blots prepared from phosphoproteins isolated by metal affinity chromatography. Analysis by mass spectrometry of tryptic peptides generated from BD.1 purified by continuous elution electrophoresis identified the plus end microtubule-binding protein, CLIP170, in the fraction reactive with MAb BD.1. Double immunofluorescence with MAb BD.1 and a MAb specific for CLIP170 showed that both were reactive with intrahepatic bile ducts. However, overexpression or siRNA knockdown of CLIP170 in 293T cells did not significantly alter BD.1 levels, indicating that CLIP170 and BD.1 were distinct, co-migrating proteins. Immunoprecipitation analysis with MAb BD.1 and anti-CLIP170 antibodies showed that under microtubule depolymerizing conditions the two proteins could be co-precipitated with both antibodies, leading us to conclude they were capable of forming stable complexes. Two different protocols were devised to enrich for the CLIP170 binding protein recognized by MAb BD.1. Analysis of tryptic peptides by LC-ESI-MS/MS identified BD.1 as eIF3a, the largest subunit of the elongation initiation factor 3 (eIF3) complex. This identity was confirmed by the simultaneous knockdown of both BD.1 and eIF3a by eIF3a-specific siRNAs and by the strong reactivity of MAb BD.1 with the 170 kDa protein immunoprecipitated with the anti-eIF3a antibody, 5H10. Based on these findings, we concluded that the BD.1 antigen was identical to eIF3a, a multifunctional subunit of the eIf3 complex shown here to associate with microtubules through its interactions with CLIP170.

Multiple domains of human CLASP contribute to microtubule dynamics and organization in vitro and in Xenopus egg extracts

Cytoplasmic linker associated proteins (CLASPs) comprise a class of microtubule (MT) plus end-binding proteins (+TIPs) that contribute to the dynamics and organization of MTs during many cellular processes, among them mitosis. Human CLASP proteins contain multiple MT-binding domains, including tumor over-expressed gene (TOG) domains, and a Ser-x-Ile-Pro (SxIP) motif known to target some +TIPs though interaction with end-binding protein 1 (EB1). However, how individual domains contribute to CLASP function is poorly understood. We generated full-length recombinant human CLASP1 and a series of truncation mutants and found that two N-terminal TOG domains make the strongest contribution to MT polymerization and bundling, but also identified a third TOG domain that further contributes to CLASP activity. Plus end tracking by CLASP requires the SxIP motif and interaction with EB1. The C-terminal coiled-coil domain mediates dimerization and association with many other factors, including the kinetochore motor centromere protein E (CENP-E), and the chromokinesin Xkid. Only the full-length protein was able to rescue spindle assembly in Xenopus egg extracts depleted of endogenous CLASP. Deletion of the C-terminal domain caused aberrant MT polymerization and dramatic spindle phenotypes, even with small amounts of added protein, indicating that proper localization of CLASP activity is essential to control MT polymerization during mitosis. © 2012 Wiley Periodicals, Inc

Microtubule-binding protein CLIP-170 is a mediator of paclitaxel sensitivity

CLIP-170 is a microtubule-binding protein and participates in diverse microtubule-associated cellular activities by regulating microtubule dynamics. Here we provide evidence that CLIP-170 is a mediator of the sensitivity of the anti-microtubule drug paclitaxel in breast cancer. In vitro cell proliferation assays reveal that CLIP-170 expression in breast cancer cell lines correlates with their sensitivity to paclitaxel. In addition, CLIP-170 expression in clinical samples of breast cancer correlates with the pathological response of tumours to paclitaxel-containing chemotherapy. Mitotic index and caspase-3 activity analyses reveal that CLIP-170 increases the abilities of paclitaxel to block cell cycle progression at mitosis and to induce apoptosis in breast cancer cells. By microtubule sedimentation assay and binding affinity analysis, we further find that CLIP-170 promotes paclitaxel binding to microtubules. In vitro tubulin polymerization assay shows that CLIP-170 enhances the activity of paclitaxel to promote microtubule assembly. These results demonstrate that CLIP-170 mediates paclitaxel sensitivity in breast cancer via a microtubule-dependent mechanism.

A complex of Kif18b and MCAK promotes microtubule depolymerization and is negatively regulated by Aurora kinases

INTRODUCTION

Spindle assembly requires tight control of microtubule (MT) dynamics. This is dependent on a variety of MT binding proteins and their upstream regulators. The Aurora kinases have several well-described functions during cell division, but it remains unclear whether they control global spindle microtubule dynamics.

RESULTS

Here, we find that simultaneous inhibition of Aurora A and B results in a dramatic decrease in spindle MT stability, and we identify the uncharacterized kinesin-8 Kif18b as a mediator of this effect. In interphase, Kif18b is nuclear, but upon nuclear envelope breakdown, Kif18b binds to astral MT plus ends through an interaction with EB1. Surprisingly, Kif18b also binds to the kinesin-13 motor MCAK, and this interaction is required for robust MT depolymerization. Furthermore, the Kif18b-MCAK interaction is negatively regulated by Aurora kinases through phosphorylation of MCAK, indicating that Aurora kinases regulate MT plus-end stability in mitosis through control of Kif18b-MCAK complex formation.

CONCLUSION

Together, these results uncover a novel role for Aurora kinases in regulating spindle MT dynamics through Kif18b-MCAK and suggest that the Kif18b-MCAK complex constitutes the major MT plus-end depolymerizing activity in mitotic cells.

Localized Aurora B activity spatially controls non-kinetochore microtubules during spindle assembly

Efficient spindle assembly involves the generation of spatial cues around chromosomes that locally stabilize microtubule (MT) plus-ends. In addition to the small GTPase Ran, there is evidence that Aurora B kinase might also generate a spatial cue around chromosomes but direct proof for this is still lacking. Here, we find that the Aurora B substrate MCAK localizes to MT plus-ends throughout the mitotic spindle, but its accumulation is strongly reduced on MT plus-ends near chromatin, suggesting that a signal emanating from chromosomes negatively regulates MCAK plus-end binding. Indeed, we show that Aurora B is the kinase responsible for producing this chromosome-derived signal. These results are the first to visualize spatially restricted Aurora B kinase activity around chromosomes on an endogenous substrate and explain how Aurora B could spatially control the dynamics of non-kinetochore MTs during spindle assembly.

A novel microtubule-modulating noscapinoid triggers apoptosis by inducing spindle multipolarity via centrosome amplification and declustering

We have previously shown that a non-toxic noscapinoid, EM011 binds tubulin without altering its monomer/polymer ratio. EM011 is more active than the parent molecule, noscapine, in inducing G2/M arrest, inhibiting cellular proliferation and tumor growth in various human xenograft models. However, the mechanisms of mitotic-block and subsequent cell death have remained elusive. Here, we show that EM011-induced attenuation of microtubule dynamics was associated with impaired association of microtubule plus-end tracking proteins, such as EB1 and CLIP-170. EM011 treatment then led to the formation of multipolar spindles containing 'real' centrioles indicating drug-induced centrosome amplification and persistent centrosome declustering. Centrosome amplification was accompanied by an upregulation of Aurora A and Plk4 protein levels, as well as a surge in the kinase activity of Aurora A, suggesting a deregulation of the centrosome duplication cycle. Cell-cycle phase-specific experiments showed that the 'cytotoxicity-window' of the drug encompasses the late S-G2 period. Drug-treatment, excluding S-phase, not only resulted in lower sub-G1 population but also attenuated centrosome amplification and spindle multipolarity, suggesting that drug-induced centrosome amplification is essential for maximal cell death. Subsequent to a robust mitotic arrest, EM011-treated cells displayed diverse cellular fates suggesting a high degree of intraline variation. Some 'apoptosis-evasive' cells underwent aberrant cytokinesis to generate rampant aneuploidy that perhaps contributed to drug-induced cell death. These data indicate that spindle multipolarity induction by means of centrosome amplification has an exciting chemotherapeutic potential that merits further investigation.

CAMP (C13orf8, ZNF828) is a novel regulator of kinetochore-microtubule attachment

Proper attachment of microtubules to kinetochores is essential for accurate chromosome segregation. Here, we report a novel protein involved in kinetochore-microtubule attachment, chromosome alignment-maintaining phosphoprotein (CAMP) (C13orf8, ZNF828). CAMP is a zinc-finger protein containing three characteristic repeat motifs termed the WK, SPE, and FPE motifs. CAMP localizes to chromosomes and the spindle including kinetochores, and undergoes CDK1-dependent phosphorylation at multiple sites during mitosis. CAMP-depleted cells showed severe chromosome misalignment, which was associated with the poor resistance of K-fibres to the tension exerted upon establishment of sister kinetochore bi-orientation. We found that the FPE region, which is responsible for spindle and kinetochore localization, is essential for proper chromosome alignment. The C-terminal region containing the zinc-finger domains negatively regulates chromosome alignment, and phosphorylation in the FPE region counteracts this regulation. Kinetochore localization of CENP-E and CENP-F was affected by CAMP depletion, and by expressing CAMP mutants that cannot functionally rescue CAMP depletion, placing CENP-E and CENP-F as downstream effectors of CAMP. These data suggest that CAMP is required for maintaining kinetochore-microtubule attachment during bi-orientation.

The substrates of Plk1, beyond the functions in mitosis

Polo-like kinase 1 (Plk1) is a key regulator of cell division in eukaryotic cells. In this short review, we briefly summarized the well-established functions modulated by Plk1 during mitosis. Beyond mitosis, we focused mainly on the unexpected processes in which Plk1 emerges as a critical player, including microtubule dynamics, DNA replication, chromosome dynamics, p53 regulation, and recovery from the G2 DNA-damage checkpoint. Our discussion is mainly based on the critical substrates targeted by Plk1 during these cellular events and the functional significance associated with each phosphorylation event.

Phosphorylation of CLIP-170 by Plk1 and CK2 promotes timely formation of kinetochore-microtubule attachments

CLIP-170 is implicated in the formation of kinetochore-microtubule attachments through direct interaction with the dynein/dynactin complex. However, whether this important function of CLIP-170 is regulated by phosphorylation is unknown. Herein, we have identified polo-like kinase 1 (Plk1) and casein kinase 2 (CK2) as two kinases of CLIP-170 and mapped S195 and S1318 as their respective phosphorylation sites. We showed that a CK2 unphosphorylatable mutant lost its ability to bind to dynactin and to localize to kinetochores during prometaphase, indicating that the CK2 phosphorylation of CLIP-170 is involved in its dynactin-mediated kinetochore localization. Furthermore, we provide evidence that Plk1 phosphorylation of CLIP-170 at S195 enhances its association with CK2. Finally, we detected defects in the formation of kinetochore fibres in cells expressing the CLIP-S195A and -S1318A, but not the CLIP-S195E and -S1318D, confirming that Plk1- and CK2-associated phosphorylations of CLIP-170 are involved in the timely formation of kinetochore-microtubule attachments in mitosis.

Polo-like kinase 1 phosphorylation of p150Glued facilitates nuclear envelope breakdown during prophase

Nuclear envelope breakdown (NEBD) is an essential step during the G2/M transition in higher eukaryotic cells. Increasing evidence supports the notion that both microtubules and microtubule-associated motor proteins are critical regulators of NEBD. Although it has been described that p150(Glued), the major component of the dynein/dynactin complex, localizes in the nuclear envelope during prophase, the exact role of p150(Glued) and its regulation during NEBD are largely elusive. Polo-like kinase 1 (Plk1), the best characterized Ser/Thr kinase, is involved in mitotic entry in several systems; however, the targets of Plk1 during NEBD are unknown. Herein, we show that in mammalian cells both Plk1 and p150(Glued) regulate NEBD and that Plk1 interacts with and phosphoryates p150(Glued) during NEBD at prophase. Using various approaches, we showed that Plk1 phosphorylates p150(Glued) at Ser-179 and that the pS179 epitope is generated at the nuclear envelope of prophase cells. Significantly, Plk1-mediated phosphorylation of p150(Glued) at Ser-179 positively regulates its accumulation at the nuclear envelope during prophase. Finally, we found that cells expressing the Plk1-unphosphorylatable mutant (p150(Glued)-S179A) arrest at G2, as indicated by reduced NEBD, increased levels of cyclin B and phospho-H3, but a decreased level of Cdc2 kinase activity. Taking these data together, we conclude that Plk1 phosphorylation of p150(Glued) might be one major pathway of NEBD regulation.

Finding the middle ground: how kinetochores power chromosome congression

Genomic stability requires error-free chromosome segregation during mitosis. Chromosome congression to the spindle equator precedes chromosome segregation in anaphase and is a hallmark of metazoan mitosis. Here we review the current knowledge and concepts on the processes that underlie chromosome congression, including initial attachment to spindle microtubules, biorientation, and movements, from the perspective of the kinetochore.

Phosphorylation controls autoinhibition of cytoplasmic linker protein-170

CLIP-170 conformational changes are regulated by phosphorylation on S³⁰⁹ and S³¹¹ residues resulting in diminished binding of CLIP-170 for growing MT ends and p150Glued.

Cytoplasmic linker protein (CLIP)-170 is a microtubule (MT) plus-end-tracking protein that regulates MT dynamics and links MT plus ends to different intracellular structures. We have shown previously that intramolecular association between the N and C termini results in autoinhibition of CLIP-170, thus altering its binding to MTs and the dynactin subunit p150^Glued (J. Cell Biol. 2004: 166, 1003–1014). In this study, we demonstrate that conformational changes in CLIP-170 are regulated by phosphorylation that enhances the affinity between the N- and C-terminal domains. By using site-directed mutagenesis and phosphoproteomic analysis, we mapped the phosphorylation sites in the third serine-rich region of CLIP-170. A phosphorylation-deficient mutant of CLIP-170 displays an “open” conformation and a higher binding affinity for growing MT ends and p150^Glued as compared with nonmutated protein, whereas a phosphomimetic mutant confined to the “folded back” conformation shows decreased MT association and does not interact with p150^Glued. We conclude that phosphorylation regulates CLIP-170 conformational changes resulting in its autoinhibition.

Tip1/CLIP-170 protein is required for correct chromosome poleward movement in fission yeast

The plus-end microtubule binding proteins (+TIPs) play an important role in the regulation of microtubule stability and cell polarity during interphase. In S. pombe, the CLIP-170 like protein Tip1, together with the kinesin Tea2, moves along the microtubules towards their plus ends. Tip1 also requires the EB1 homolog Mal3 to localize to the microtubule tips. Given the requirement for Tip1 for microtubule stability, we have investigated its role during spindle morphogenesis and chromosome movement. Loss of Tip1 affects metaphase plate formation and leads to the activation of the spindle assembly checkpoint. In the absence of Tip1 we also observed the appearance of lagging chromosomes, which do not influence the normal rate of spindle elongation. Our results suggest that S. pombe Tip1/CLIP170 is directly or indirectly required for correct chromosome poleward movement independently of Mal3/EB1.

Bicaudal D2, dynein, and kinesin-1 associate with nuclear pore complexes and regulate centrosome and nuclear positioning during mitotic entry

Mammalian Bicaudal D2 is the missing molecular link between cytoplasmic motor proteins and the nucleus during nuclear positioning prior to the onset of mitosis.

BICD2 is one of the two mammalian homologues of the Drosophila Bicaudal D, an evolutionarily conserved adaptor between microtubule motors and their cargo that was previously shown to link vesicles and mRNP complexes to the dynein motor. Here, we identified a G2-specific role for BICD2 in the relative positioning of the nucleus and centrosomes in dividing cells. By combining mass spectrometry, biochemical and cell biological approaches, we show that the nuclear pore complex (NPC) component RanBP2 directly binds to BICD2 and recruits it to NPCs specifically in G2 phase of the cell cycle. BICD2, in turn, recruits dynein-dynactin to NPCs and as such is needed to keep centrosomes closely tethered to the nucleus prior to mitotic entry. When dynein function is suppressed by RNA interference-mediated depletion or antibody microinjection, centrosomes and nuclei are actively pushed apart in late G2 and we show that this is due to the action of kinesin-1. Surprisingly, depletion of BICD2 inhibits both dynein and kinesin-1-dependent movements of the nucleus and cytoplasmic NPCs, demonstrating that BICD2 is needed not only for the dynein function at the nuclear pores but also for the antagonistic activity of kinesin-1. Our study demonstrates that the nucleus is subject to opposing activities of dynein and kinesin-1 motors and that BICD2 contributes to nuclear and centrosomal positioning prior to mitotic entry through regulation of both dynein and kinesin-1.

Bidirectional microtubule-based transport is responsible for the positioning of a large variety of cellular organelles, but the molecular mechanisms underlying the recruitment of microtubule-based motors to their cargoes and their activation remain poorly understood. In particular, the molecular players involved in the important processes of nuclear and centrosomal positioning prior to the onset of cell division are not known. In this study we focus on the function of one of the mammalian homologues of Drosophila Bicaudal D, an adaptor for the microtubule minus-end-directed dynein-dynactin motor complex. Previously, Drosophila Bicaudal D and its mammalian homologues were shown to act as linkers between the dynein motor and mRNP complexes or secretory vesicles. Here, we identify a new cargo for mammalian Bicaudal D2 (BICD2)–the nucleus. We show that BICD2 specifically binds to nuclear pore complexes in cells in G2 phase of the cell division cycle. We also show that this interaction is required for G2-specific recruitment of dynein to the nuclear envelope and thus for proper positioning of the nucleus relative to centrosomes prior to the onset of mitosis. Further, our findings demonstrate that the motor protein kinesin-1 opposes dynein's activity during this process and requires BICD2 for its activity. Our study therefore reveals BICD2 as the critical molecular adaptor that allows molecular motors to regulate nuclear and centrosomal positioning before cell division.

Cyclin G-associated kinase promotes microtubule outgrowth from chromosomes during spindle assembly

During mitosis, all chromosomes must attach to microtubules of the mitotic spindle to ensure correct chromosome segregation. Microtubule attachment occurs at specialized structures at the centromeric region of chromosomes, called kinetochores. These kinetochores can generate microtubule attachments through capture of centrosome-derived microtubules, but in addition, they can generate microtubules themselves, which are subsequently integrated with centrosome-derived microtubules to form the mitotic spindle. Here, we have performed a large scale RNAi screen and identify cyclin G-associated kinase (GAK) as a novel regulator of microtubule generation at kinetochores/chromatin. This function of GAK requires its C-terminal J-domain, which is essential for clathrin recycling from endocytic vesicles. Consistently, cells lacking GAK show strongly reduced levels of clathrin on the mitotic spindle, and reduction of clathrin levels also inhibits microtubule generation at kinetochores/chromosomes. Finally, we present evidence that association of clathrin with the spindle is promoted by a signal coming from the chromosomes. These results identify a role for GAK and clathrin in microtubule outgrowth from kinetochores/chromosomes and suggest that GAK acts through clathrin to control microtubule outgrowth around chromosomes.

Probing interactions between CLIP-170, EB1, and microtubules

Cytoplasmic linker protein 170 (CLIP-170) is a microtubule (MT) plus-end tracking protein (+TIP) that dynamically localizes to the MT plus end and regulates MT dynamics. The mechanisms of these activities remain unclear because the CLIP-170-MT interaction is poorly understood, and even less is known about how CLIP-170 and other +TIPs act together as a network. CLIP-170 binds to the acidic C-terminal tail of alpha-tubulin. However, the observation that CLIP-170 has two CAP-Gly (cytoskeleton-associated protein glycine-rich) motifs and multiple serine-rich regions suggests that a single CLIP-170 molecule has multiple tubulin binding sites, and that these sites might bind to multiple parts of the tubulin dimer. Using a combination of chemical cross-linking and mass spectrometry, we find that CLIP-170 binds to both alpha-tubulin and beta-tubulin, and that binding is not limited to the acidic C-terminal tails. We provide evidence that these additional binding sites include the H12 helices of both alpha-tubulin and beta-tubulin and are significant for CLIP-170 activity. Previous work has shown that CLIP-170 binds to end-binding protein 1 (EB1) via the EB1 C-terminus, which mimics the acidic C-terminal tail of tubulin. We find that CLIP-170 can utilize its multiple tubulin binding sites to bind to EB1 and MT simultaneously. These observations help to explain how CLIP-170 can nucleate MTs and alter MT dynamics, and they contribute to understanding the significance and properties of the +TIP network.