Mammalian RanBP1 regulates centrosome cohesion during mitosis

Abnormal centrosome amplification in cells through the targeting of Ran-binding protein-1 by the human T cell leukemia virus type-1 Tax oncoprotein

Human T cell leukemia virus type-1 (HTLV-1) is an oncogenic retrovirus etiologically causal of adult T cell leukemia. The virus encodes a Tax oncoprotein that functions in transcriptional regulation, cell cycle control, and transformation. Because adult T cell leukemia like many other human cancers is a disease of genomic instability with frequent gains and losses of chromosomes, to understand this disease it is important to comprehend how HTLV-1 engenders aneuploidy in host cells. In this regard, loss of cell cycle checkpoints permits tolerance of aneuploidy but does not explain how aneuploidy is created. We show here that HTLV-1 Tax causes abnormal centrosome fragmentation in the mitotic phase of the cell cycle. We report that Tax directly binds Ran and Ran-binding protein-1, locates to centrosomes/spindle poles, and causes supernumerary centrosomes.

Rootletin interacts with C-Nap1 and may function as a physical linker between the pair of centrioles/basal bodies in cells

Rootletin, a major structural component of the ciliary rootlet, is located at the basal bodies and centrosomes in ciliated and nonciliated cells, respectively. Here we investigated its potential role in the linkage of basal bodies/centrioles and the mechanism involved in such linkages. We show that rootletin interacts with C-Nap1, a protein restricted at the ends of centrioles and functioning in centrosome cohesion in interphase cells. Their interaction in vivo is supported by their colocalization at the basal bodies/centrioles and coordinated association with the centrioles during the cell cycle. Ultrastructural examinations demonstrate that rootletin fibers connect the basal bodies in ciliated cells and are present both at the ends of and in between the pair of centrioles in nonciliated cells. The latter finding stands in contrast with C-Nap1, which is present only at the ends of the centrioles. Transient expression of C-Nap1 fragments dissociated rootletin fibers from the centrioles, resulting in centrosome separation in interphase. Overexpression of rootletin in cells caused multinucleation, micronucleation, and irregularity of nuclear shape and size, indicative of defects in chromosome separation. These data suggest that rootletin may function as a physical linker between the pair of basal bodies/centrioles by binding to C-Nap1.

Dissociation of nuclear import cargo complexes by the protein Ran: a fluorescence correlation spectroscopy study

In nucleated cells, proteins designed for nuclear import form complexes with soluble nuclear transport receptors prior to translocation across the nuclear envelope. The directionality of transport is due to the asymmetric distribution of the protein Ran, which dissociates import cargo complexes only in its nuclear RanGTP form. Using fluorescence correlation spectroscopy, we have studied the stability of cargo complexes in solution in the presence and in the absence of RanGTP. We find that RanGTP has a higher affinity for the major import receptor, the importin alpha/beta heterodimer, when importin alpha does not carry a cargo, suggesting that some nuclear transport targets might be preferentially released.

Loading and unloading: orchestrating centrosome duplication and spindle assembly by Ran/Crm1

The cell cycle is an intricate process of DNA replication and cell division that concludes with the formation of two genetically equivalent daughter cells. In this progression, the centrosome is duplicated once and only once during the G1/S transition to produce the bipolar spindle and ensure proper chromosome segregation. The presence of multiple centrosomes in cancer cells suggests that this process is mis-regulated during carcinogenesis. This suggests that certain factors exist that license the progression of centrosome duplication and serve to inhibit further duplications during a single cell cycle. Recent studies suggest that the Ran/Crm1 complex not only regulates nucleocytoplasmic transport but is also independently involved in mitotic spindle assembly. Factors that are capable of interacting with Ran/Crm1 through their nuclear export sequences, such as cyclins/cdks, p53 and Brca1/2, may potentially function as centrosome checkpoints akin to the G1/S and G2/M checkpoints of the cell cycle. Our recent findings indicate that nucleophosmin, a protein whose trafficking is mediated by the Ran/Crm1 network, is one of such checkpoint factors for maintaining proper centrosome duplication. We propose that Ran/Crm1 may act as a 'loading dock' to coordinate various checkpoint factors in regulating the fidelity of centrosome duplication during cell cycle progression, and the disruption of these processes may lead to genomic instability and an acceleration of oncogenesis.

Cell cycle-dependent localization and possible roles of the small GTPase Ran in mouse oocyte maturation, fertilization and early cleavage

The small GTPase Ran controls numerous cellular processes of the mitotic cell cycle. In this experiment, we investigated the localization and possible roles of Ran during mouse oocyte meiotic maturation, fertilization and early cleavage by using confocal laser scanning microscopy, antibody microinjection and microtubule disturbance. The results showed that Ran was localized mainly in the nucleus (except for the nucleolus) in the oocyte, zygote and early embryo. At pro-metaphase of meiosis I, Ran distributed throughout the cell, but predominantly concentrated around the condensed chromosomes. During the completion of meiosis I and meiosis II, it concentrated to the meiotic spindle microtubules except for the midbody region. After sperm penetration, Ran dispersed with the extrusion of the second polar body and gradually concentrated in the male and female pronuclei thereafter. Ran was also observed to exist diffusely in the cytoplasm in prophase; it concentrated at the mitotic spindle, and migrated to the nucleus during early cleavage. Ran’s concentration around the spindle disappeared when microtubule assembly was inhibited by colchicine, while it was concentrated around the chromosomes after microtubule stabilization with taxol treatment. Ran did not display any role in cytokinesis during division when pseudo-cleavage of germinal vesicle-intact oocytes was induced. Anti-Ran antibody microinjection decreased the germinal vesicle breakdown and the first polar body extrusion, and distorted spindle organization and chromosome alignment. Our results indicate that Ran has a cell cycle-dependent localization and may have regulatory roles in cell cycle progression and microtubule organization in mouse oocytes, fertilized eggs and early embryos.

Centrosome-targeting region of CG-NAP causes centrosome amplification by recruiting cyclin E-cdk2 complex

Centrosome duplication occurs once per cell cycle and is thought to be triggered by cyclin E-cdk2. However, it is largely unknown how the duplication is regulated. Here, we found that the expression of the centrosome-targeting region of CG-NAP (centrosome and Golgi-localized PKN-associated protein), which we designate as CG-NAP/D, increased the number of centrosomes in Chinese hamster ovary (CHO)-K1 cells. The amplified centrosomes co-localized with centrosome markers gamma-tubulin, centrin-2 and kendrin as well as endogenous CG-NAP. When CG-NAP/D was dislocated from centrosomes by deleting the centrosome-targeting domain or by fusing with a membrane-targeting sequence, centrosome amplification was suppressed. CG-NAP/D interacted with exogenously expressed cyclin E, which co-localized at centrosomes. The immunoprecipitates of CG-NAP/D exhibited histone H1 kinase activity, suggesting the co-immunoprecipitation of active cyclin-cdk complexes. Furthermore, centrosome fractions prepared from cells expressing CG-NAP/D contained increased amount of cdk2 compared with those from control cells. Centrosome amplification by CG-NAP/D was suppressed by co-expression of a mutant cyclin E unable to interact with cdk2. These results suggest that CG-NAP/D causes centrosome amplification by anchoring excess amount of cyclin E-cdk2 to centrosomes and, possibly, CG-NAP participates in centrosome duplication by recruiting cyclin E-cdk2 to centrosomes in normal cell cycle.

Temporal and spatial control of nucleophosmin by the Ran-Crm1 complex in centrosome duplication

Centrosome duplication is tightly controlled during faithful cell division, and unnecessary reduplication can lead to supernumerary centrosomes and multipolar spindles that are associated with most human cancer cells. In addition to nucleocytoplasmic transport, the Ran-Crm1 network is involved in regulating centrosome duplication to ensure the formation of a bipolar spindle. Here, we discover that nucleophosmin (NPM) may be a Ran-Crm1 substrate that controls centrosome duplication. NPM contains a functional nuclear export signal (NES) that is responsible for both its nucleocytoplasmic shuttling and its association with centrosomes, which are Ran-Crm1-dependent as they are sensitive to Crm1-specific nuclear export inhibition, either by leptomycin B (LMB) or by the expression of a Ran-binding protein, RanBP1. Notably, LMB treatment induces premature centrosome duplication in quiescent cells, which coincides with NPM dissociation from centrosomes. Moreover, deficiency of NPM by RNA interference results in supernumerary centrosomes, which can be reversed by reintroducing wild-type but not NES-mutated NPM. Mutation of a potential proline-dependent kinase phosphorylation site at residue 95, from threonine to aspartic acid (T95D) within the NES motif, abolishes NPM association and inhibition of centrosome duplication. Our results are consistent with the hypothesis that the Ran-Crm1 complex may promote a local enrichment of NPM on centrosomes, thereby preventing centrosome reduplication.

Importin beta: conducting a much larger cellular symphony

Importin beta, once thought to be exclusively a nuclear transport receptor, is emerging as a global regulator of diverse cellular functions. Importin beta acts positively in multiple interphase roles: in nuclear import, as a chaperone for highly charged nuclear proteins, and as a potential motor adaptor for movement along microtubules. In contrast, importin beta plays a negative regulatory role in mitotic spindle assembly, centrosome dynamics, nuclear membrane formation, and nuclear pore assembly. In most of these, importin beta is counteracted by its regulator, Ran-GTP. In light of this, the recent discovery of Ran's involvement in spindle checkpoint control suggested a potential new arena for importin beta action, although it is also possible that one of importin beta's relatives, the karyopherin family of proteins, manages this checkpoint. Lastly, importin beta plays a role in transducing damage signals from the axons of injured neurons back to the cell body.

Importin beta is transported to spindle poles during mitosis and regulates Ran-dependent spindle assembly factors in mammalian cells

Spatial control is a key issue in cell division. The Ran GTPase regulates several fundamental processes for cell life, largely acting through importin molecules. The best understood of these is protein import through the nuclear envelope in interphase, but roles in mitotic spindle assembly are also established. In mammalian cells, in which centrosomes are major spindle organizers, a link is emerging between the Ran network, centrosomes and spindle poles. Here, we show that, after nuclear envelope breakdown, importin beta is transported to the spindle poles in mammalian cells. This localization is temporally regulated from prometaphase until anaphase, when importin beta dissociates from poles and is recruited back around reforming nuclei. Importin beta sediments with mitotic microtubules in vitro and its accumulation at poles requires microtubule integrity and dynamics in vivo. Furthermore, RNA interference-dependent inactivation of TPX2, the major Ran-dependent spindle organizer, abolishes importin beta accumulation at poles. Importin beta has a functional role in spindle pole organization, because overexpression yields mitotic spindles with abnormal, fragmented poles. Coexpression of TPX2 with importin beta mitigates these abnormalities. Together, these results indicate that the balance between importins and spindle regulators of the TPX2 type is crucial for spindle formation. Targeting of TPX2/importin-beta complexes to poles is a key aspect in Ran-dependent control of the mitotic apparatus in mammalian cells.

E pluribus unum: towards a universal mechanism for spindle assembly

Recent data have revealed that the mitotic spindle might form by centrosome-independent mechanisms, even in centrosome-containing cells. This suggests that spindle assembly might proceed by a generally conserved acentrosomal mechanism in all higher eukaryotes, regardless of the presence of centrosomes. Thus, what is the role of centrosomes in mitosis? We propose that these organelles are needed to generate radial arrays of microtubules that integrate preassembled (by centrosome-independent mechanisms) spindle components into a common spindle and orientate the spindle within malleable animal cells.

The RanGAP1-RanBP2 complex is essential for microtubule-kinetochore interactions in vivo

RanGAP1 is the activating protein for the Ran GTPase. Vertebrate RanGAP1 is conjugated to a small ubiquitin-like protein, SUMO-1. This modification promotes association of RanGAP1 with the interphase nuclear pore complex (NPC) through binding to the nucleoporin RanBP2, also known as Nup358. During mitosis, RanGAP1 is concentrated at kinetochores in a microtubule- (MT) and SUMO-1-dependent fashion. RanBP2 is also abundantly found on kinetochores in mitosis. Here we show that ablation of proteins required for MT-kinetochore attachment (Hec1/Ndc80, Nuf2 ) disrupts RanGAP1 and RanBP2 targeting to kinetochores. No similar disruption was observed after ablation of proteins nonessential for MT-kinetochore interactions (CENP-I, Bub1, CENP-E ). Acquisition of RanGAP1 and RanBP2 by kinetochores is temporally correlated in untreated cells with MT attachment. These patterns of accumulation suggest a loading mechanism wherein the RanGAP1-RanBP2 complex may be transferred along the MT onto the kinetochore. Depletion of RanBP2 caused mislocalization of RanGAP1, Mad1, Mad2, CENP-E, and CENP-F, as well as loss of cold-stable kinetochore-MT interactions and accumulation of mitotic cells with multipolar spindles and unaligned chromosomes. Taken together, our observations indicate that RanBP2 and RanGAP1 are targeted as a single complex that is both regulated by and essential for stable kinetochore-MT association.

TOGp, the human homolog of XMAP215/Dis1, is required for centrosome integrity, spindle pole organization, and bipolar spindle assembly

The XMAP215/Dis1 MAP family is thought to regulate microtubule plus-end assembly in part by antagonizing the catastrophe-promoting function of kin I kinesins, yet XMAP215/Dis1 proteins localize to centrosomes. We probed the mitotic function of TOGp (human homolog of XMAP215/Dis1) using siRNA. Cells lacking TOGp assembled multipolar spindles, confirming results of Gergely et al. (2003. Genes Dev. 17, 336-341). Eg5 motor activity was necessary to maintain the multipolar morphology. Depletion of TOGp decreased microtubule length and density in the spindle by approximately 20%. Depletion of MCAK, a kin I kinesin, increased MT lengths and density by approximately 20%, but did not disrupt spindle morphology. Mitotic cells lacking both TOGp and MCAK formed bipolar and monopolar spindles, indicating that TOGp and MCAK contribute to spindle bipolarity, without major effects on MT stability. TOGp localized to centrosomes in the absence of MTs and depletion of TOGp resulted in centrosome fragmentation. TOGp depletion also disrupted MT minus-end focus at the spindle poles, detected by localizations of NuMA and the p150 component of dynactin. The major functions of TOGp during mitosis are to focus MT minus ends at spindle poles, maintain centrosome integrity, and contribute to spindle bipolarity.

The Centrosome in Higher Organisms: Structure, Composition, and Duplication

The centrosome found in higher organisms is an organelle with a complex and dynamic architecture and composition. This organelle not only functions as a microtubule-organizing center, but also is integrated with or impacts a number of cellular processes. Defects associated with this organelle have been linked to a variety of human diseases including several forms of cancer. Here we review the emerging picture of how the structure, composition, duplication, and function of the centrosome found in higher organisms are interrelated.

Part of Ran is associated with AKAP450 at the centrosome: involvement in microtubule-organizing activity

The small Ran GTPase, a key regulator of nucleocytoplasmic transport, is also involved in microtubule assembly and nuclear membrane formation. Herein, we show by immunofluorescence, immunoelectron microscopy, and biochemical analysis that a fraction of Ran is tightly associated with the centrosome throughout the cell cycle. Ran interaction with the centrosome is mediated by the centrosomal matrix A kinase anchoring protein (AKAP450). Accordingly, when AKAP450 is delocalized from the centrosome, Ran is also delocalized, and as a consequence, microtubule regrowth or anchoring is altered, despite the persisting association of gamma-tubulin with the centrosome. Moreover, Ran is recruited to Xenopus sperm centrosome during its activation for microtubule nucleation. We also demonstrate that centrosomal proteins such as centrin and pericentrin, but not gamma-tubulin, AKAP450, or ninein, undertake a nucleocytoplasmic exchange as they concentrate in the nucleus upon export inhibition by leptomycin B. Together, these results suggest a challenging possibility, namely, that centrosome activity could depend upon nucleocytoplasmic exchange of centrosomal proteins and local Ran-dependent concentration at the centrosome.