Live analysis of free centrosomes in normal and aphidicolin-treated Drosophila embryos

In a number of embryonic systems, centrosomes that have lost their association with the nuclear envelope and spindle maintain their ability to duplicate and induce astral microtubules. To identify additional activities of free centrosomes, we monitored astral microtubule dynamics by injecting living syncytial Drosophila embryos with fluorescently labeled tubulin. Our recordings follow multiple rounds of free centrosome duplication and separation during the cortical division. The rate and distance of free sister centrosome separation corresponds well with the initial phase of associated centrosome separation. However, the later phase of separation observed for centrosomes associated with a spindle (anaphase B) does not occur. Free centrosome separation regularly occurs on a plane parallel to the plasma membrane. While previous work demonstrated that centrosomes influence cytoskeletal dynamics, this observation suggests that the cortical cytoskeleton regulates the orientation of centrosome separation. Although free centrosomes do not form spindles, they display relatively normal cell cycle-dependent modulations of their astral microtubules. In addition, free centrosome duplication, separation, and modulation of microtubule dynamics often occur in synchrony with neighboring associated centrosomes. These observations suggest that free centrosomes respond normally to local nuclear division signals. Disruption of the cortical nuclear divisions with aphidicolin supports this conclusion; large numbers of abnormal nuclei recede into the interior while their centrosomes remain on the cortex. Following individual free centrosomes through multiple focal planes for 45 min after the injection of aphidicolin reveals that they do not undergo normal modulation of their astral dynamics nor do they undergo multiple rounds of duplication and separation. We conclude that in the absence of normally dividing cortical nuclei many centrosome activities are disrupted and centrosome duplication is extensively delayed. This indicates the presence of a feedback mechanism that creates a dependency relationship between the cortical nuclear cycles and the centrosome cycles.     

A model for cleavage plane determination in early amphibian and fish embryos

Current models for cleavage plane determination propose that metaphase spindles are positioned and oriented by interactions of their astral microtubules with the cellular cortex, followed by cleavage in the plane of the metaphase plate [1, 2]. We show that in early frog and fish embryos, where cells are unusually large, astral microtubules in metaphase are too short to position and orient the spindle. Rather, the preceding interphase aster centers and orients a pair of centrosomes prior to nuclear envelope breakdown, and the spindle assembles between these prepositioned centrosomes. Interphase asters center and orient centrosomes with dynein-mediated pulling forces. These forces act before astral microtubules contact the cortex; thus, dynein must pull from sites in the cytoplasm, not the cell cortex as is usually proposed for smaller cells. Aster shape is determined by interactions of the expanding periphery with the cell cortex or with an interaction zone that forms between sister-asters in telophase. We propose a model to explain cleavage plane geometry in which the length of astral microtubules is limited by interaction with these boundaries, causing length asymmetries. Dynein anchored in the cytoplasm then generates length-dependent pulling forces, which move and orient centrosomes.     

The centrosomin protein is required for centrosome assembly and function during cleavage in Drosophila.

Centrosomin is a 150 kDa centrosomal protein of Drosophila melanogaster. To study the function of Centrosomin in the centrosome, we have recovered mutations that are viable but male and female sterile (cnnmfs). We have shown that these alleles (1, 2, 3, 7, 8 and hk21) induce a maternal effect on early embryogenesis and result in the accumulation of low or undetectable levels of Centrosomin in the centrosomes of cleavage stage embryos. Hemizygous cnn females produce embryos that show dramatic defects in chromosome segregation and spindle organization during the syncytial cleavage divisions. In these embryos the syncytial divisions proceed as far as the twelfth cycle, and embryos fail to cellularize. Aberrant divisions and nuclear fusions occur in the early cycles of the nuclear divisions, and become more prominent at later stages. Giant nuclei are seen in late stage embryos. The spindles that form in mutant embryos exhibit multiple anomalies. There is a high occurrence of apparently linked spindles that share poles, indicating that Centrosomin is required for the proper spacing and separation of mitotic spindles within the syncytium. Spindle poles in the mutants contain little or no detectable amounts of the centrosomal proteins CP60, CP190 and (gamma)-tubulin and late stage embryos often do not have astral microtubules at their spindle poles. Spindle morphology and centrosomal composition suggest that the primary cause of these division defects in mutant embryos is centrosomal malfunction. These results suggest that Centrosomin is required for the assembly and function of centrosomes during the syncytial cleavage divisions.     

Cyclins A and B associate with chromatin and the polar regions of spindles, respectively, and do not undergo complete degradation at anaphase in syncytial Drosophila embryos

Maternally contributed cyclin A and B proteins are initially distributed uniformly throughout the syncytial Drosophila embryo. As dividing nuclei migrate to the cortex of the embryo, the A and B cyclins become concentrated in surface layers extending to depths of approximately 30-40 microns and 5-10 microns, respectively. The initiation of nuclear envelope breakdown, spindle formation, and the initial congression of the centromeric regions of the chromosomes onto the metaphase plate all take place within the surface layer occupied by cyclin B on the apical side of the blastoderm nuclei. Cyclin B is seen mainly, but not exclusively, in the vicinity of microtubules throughout the mitotic cycle. It is most conspicuous around the centrosomes. Cyclin A is present at its highest concentrations throughout the cytoplasm during the interphase periods of the blastoderm cycles, although weak punctate staining can also be detected in the nucleus. It associates with the condensing chromosomes during prophase, segregates into daughter nuclei in association with chromosomes during anaphase, to redistribute into the cytoplasm after telophase. In contrast to the cycles following cellularization, neither cyclin is completely degraded upon the metaphase-anaphase transition.     

Antagonistic microtubule-sliding motors position mitotic centrosomes in Drosophila early embryos

The positioning of centrosomes, or microtubule-organizing centres, within cells plays a critical part in animal development. Here we show that, in Drosophila embryos undergoing mitosis, the positioning of centrosomes within bipolar spindles and between daughter nuclei is determined by a balance of opposing forces generated by a bipolar kinesin motor, KLP61F, that is directed to microtubule plus ends, and a carboxy-terminal kinesin motor, Ncd, that is directed towards microtubule minus ends. This activity maintains the spacing between separated centrosomes during prometaphase and metaphase, and repositions centrosomes and daughter nuclei during late anaphase and telophase. Surprisingly, we do not observe a function for KLP61F in the initial separation of centrosomes during prophase. Our data indicate that KLP61F and Ncd may function by crosslinking and sliding antiparallel spindle microtubules in relation to one another, allowing KLP61F to push centrosomes apart and Ncd to pull them together.     

Microtubule nucleating sites in higher plant cells identified by an auto-antibody against pericentriolar material

Human scleroderma serum 5051, which is known to recognize the amorphous pericentriolar microtubule organizing center material of a variety of vertebrate cells, was found to immunostain spindle poles of meristematic higher plants from pre-prophase to late anaphase. Subsequently, during cytokinesis, staining was redistributed around the reforming telophase nuclei, but was not evident in the cytokinetic phragmoplast. At the transition between telophase and interphase, before the typical cortical interphase microtubule array was established, short microtubules radiated from the nucleus and in such cells the material recognized by 5051 was located around the daughter nuclei and not the cortex. These observations have led us to propose that the perinuclear region, or the nuclear surface, may function as a nucleation center for both spindle and interphase microtubules in higher plant cells.     

The centrosome cycle in syncytial Drosophila embryos analyzed by energy filtering transmission electron microscopy.

We have investigated the centrosome cycle in Drosophila syncytial embryos at the ultrastructural level by using a transmission electron microscope equipped with an electron energy filtering device (Omega filter). This new technique allows the study of uncontrasted thick sections with a high resolution. We have been able to characterize two classes of filamentous structures in the centrosomal apparatus that were not detectable on ultrathin sections. These new filamentous structures are: 1) a very orderly lattice that connects the two centrioles during mitosis; and 2) a fibrogranular connection between the centrosome and the nuclear envelope. The intercentriolar linkage could be involved in the precise timing of separation of the centrioles during late anaphase. The centrosome-nuclear envelope connection probably prevents the loss of centrosomes in this syncytial environment, and ensures the proper migration of the centrosomes along the surface of the nucleus. This connection may also couple the nuclei to the cytoskeleton, thus allowing their migration and their anchorage to the cortex at the blastoderm stage. This thick section analysis has also allowed us to precisely reconstitute the centrosome cycle. From their separation at telophase and throughout most of interphase, centrosomes are composed of a single centriole. We conclude that in the early Drosophila embryo there is an unusual delay between the separation of the parent centrioles and their duplication. This leaves a surprisingly short time to assemble a daughter centriole.     

Microtubule arrays present during the syncytial and cellular blastoderm stages of the early Drosophila embryo

The organization of microtubules within the surface caps of Drosophila embryos is described for the mitotic cycles of the syncytial blastoderm stage (particularly cycle 10), and for the subsequent cellularization process. Tubulin was labelled with the well characterized monoclonal antibody YL 1/2 (Kilmartin et al., J cell biol 93 (1982) 576). Each surface cap was found to contain an array of microtubules running around the nucleus. The microtubules originated at prominent centrosomes located close to the apical surface of each cap nucleus. During mitosis the spindle microtubules stained strongly for tubulin. A novel finding was that the spindle microtubules of the interzone region appeared to reduce their connections with the centrosomes at the end of anaphase. The spindle remnant remained in position during telophase but then became smaller in size, disappearing by interphase. At this phase of the cell cycle duplication of the aster centrosomes occurred. The cellular blastoderm stage was marked by a change in the main axis of microtubule orientation. The centrosomes of each cap separated somewhat and formed initiation centres for the development of a well developed basket of microtubules around each nucleus, but now perpendicular to the surface. The microtubule baskets were seen to extend in parallel with nuclear elongation, but not in concert with growth of the cell membranes, which extended some way beneath the bases of the nuclei.     

Spindle Dynamics and the Role of γ-Tubulin in Early Caenorhabditis elegans Embryos

gamma-Tubulin is a ubiquitous and highly conserved component of centrosomes in eukaryotic cells. Genetic and biochemical studies have demonstrated that gamma-tubulin functions as part of a complex to nucleate microtubule polymerization from centrosomes. We show that, as in other organisms, Caenorhabditis elegans gamma-tubulin is concentrated in centrosomes. To study centrosome dynamics in embryos, we generated transgenic worms that express GFP::gamma-tubulin or GFP::beta-tubulin in the maternal germ line and early embryos. Multiphoton microscopy of embryos produced by these worms revealed the time course of daughter centrosome appearance and growth and the differential behavior of centrosomes destined for germ line and somatic blastomeres. To study the role of gamma-tubulin in nucleation and organization of spindle microtubules, we used RNA interference (RNAi) to deplete C. elegans embryos of gamma-tubulin. gamma-Tubulin (RNAi) embryos failed in chromosome segregation, but surprisingly, they contained extensive microtubule arrays. Moderately affected embryos contained bipolar spindles with dense and long astral microtubule arrays but with poorly organized kinetochore and interpolar microtubules. Severely affected embryos contained collapsed spindles with numerous long astral microtubules. Our results suggest that gamma-tubulin is not absolutely required for microtubule nucleation in C. elegans but is required for the normal organization and function of kinetochore and interpolar microtubules.     

ULTRASTRUCTURE AND TIME COURSE OF MITOSIS IN THE FUNGUS FUSARIUM OXYSPORUM

Mitosis in Fusarium oxysporum Schlect. was studied by light and electron microscopy. The average times required for the stages of mitosis, as determined from measurements made on living nuclei, were as follows: prophase, 70 sec; metaphase, 120 sec; anaphase, 13 sec; and telophase, 125 sec, for a total of 5.5 min. New postfixation procedures were developed specifically to preserve the fine-structure of the mitotic apparatus. Electron microscopy of mitotic nuclei revealed a fibrillo-granular, extranuclear Spindle Pole Body (SPB) at each pole of the intranuclear, microtubular spindles. Metaphase chromosomes were attached to spindle microtubules via kinetochores, which were found near the spindle poles at telophase. The still-intact, original nuclear envelope constricted around the incipient daughter nuclei during telophase.     

Drosophila APC2 and Armadillo participate in tethering mitotic spindles to cortical actin

Proper positioning of mitotic spindles ensures equal allocation of chromosomes to daughter cells. This often involves interactions between spindle and astral microtubules and cortical actin. In yeast and Caenorhabditis elegans, some of the protein machinery that connects spindles and cortex has been identified but, in most animal cells, this process remains mysterious. Here, we report that the tumour suppressor homologue APC2 and its binding partner Armadillo both play roles in spindle anchoring during the syncytial mitoses of early Drosophila embryos. Armadillo, alpha-catenin and APC2 all localize to sites of cortical spindle attachment. APC2-Armadillo complexes often localize with interphase microtubules. Zeste-white 3 kinase, which can phosphorylate Armadillo and APC, is also crucial for spindle positioning and regulates the localization of APC2-Armadillo complexes. Together, these data suggest that APC2, Armadillo and alpha-catenin provide an important link between spindles and cortical actin, and that this link is regulated by Zeste-white 3 kinase.     

Microtubule dynamics during infection-related morphogenesis of Colletotrichum lagenarium.

Using a green fluorescent protein (GFP)-tubulin fusion protein, we have investigated the dynamic rearrangement of microtubules during appressorium formation of Colletotrichum lagenarium. Two alpha-tubulin genes of C. lagenarium were isolated, and GFP-alpha-tubulin protein was expressed in this fungus. The strain expressing the fusion protein formed fluorescent filaments that were disrupted by a microtubule-depolymerizing drug, benomyl, demonstrating successful visualization of microtubules. In preincubated conidia, GFP-labeled interphase microtubules, showing random orientation, were observed. At conidial germination, microtubules oriented toward a germination site. At nuclear division, when germ tubes had formed appressoria, mitotic spindles appeared inside conidia followed by disassembly of interphase microtubules. Remarkably, time-lapse views showed that interphase microtubules contact a microtubule-associated center at the cell cortex of conidia that is different from a nuclear spindle pole body (SPB) before their disassembly. Duplicated nuclear SPBs separately moved toward conidium and appressorium accompanied by astral microtubule formation. Benomyl treatment caused movement of both daughter nuclei into 70% of appressoria and affected appressorium morphogenesis. In conidia elongating hyphae without appressoria, microtubules showed polar elongation which is distinct from their random orientation inside appressoria.     

daughterless-abo-like, a Drosophila maternal-effect mutation that exhibits abnormal centrosome separation during the late blastoderm divisions

daughterless-abo-like (dal) is a maternal-effect semilethal mutation in Drosophila. The nuclear divisions of embryos derived from homozygous dal females are normal through nuclear cycle 10. However, during nuclear cycles 11, 12 and 13, a total of about half of the nuclei in each embryo either fail to divide or fuse with a neighboring nucleus during telophase. These abnormal nuclei eventually sink into the interior of the embryo, leaving their centrosomes behind on the surface. The loss of about one-half of the peripheral nuclei into the interior of the embryo results in these embryos cellularizing during nuclear cycle 14 with about one-half the normal number of cells. Surprisingly, many of these embryos develop a nearly normal larval cuticle and 8% develop to adulthood. Observations of live embryos doubly injected with tubulin and histones that have been fluorescently labeled allows nuclear and centrosomal behavior to be directly followed as the embryo develops. We find that the abnormal nuclei arise from nuclei whose centrosomes have failed to separate normally in the previous interphase. These incompletely separated centrosomes can cause a non-functional spindle to form, leading to a nuclear division failure. Alternatively, they can form an abnormal spindle with a centrosome from a neighboring nucleus, causing two nuclei to share a common spindle pole. Such nuclei with a shared centrosome will undergo telophase fusions, unequal divisions, or division failures later in mitosis. These findings have helped us to understand the function of the centrosome in the Drosophila embryo.     

DNA damage leads to a Cyclin A-dependent delay in metaphase-anaphase transition in the Drosophila gastrula

BACKGROUND: In response to DNA damage, fission yeast, mammalian cells, and cells of the Drosophila gastrula inhibit Cdk1 to delay the entry into mitosis. In contrast, budding yeast delays metaphase-anaphase transition by stabilization of an anaphase inhibitor, Pds1p. A variation of the second response is seen in Drosophila cleavage embryos; when nuclei enter mitosis with damaged DNA, centrosomes lose gamma-tubulin, spindles lose astral microtubules, chromosomes fail to reach a metaphase configuration, and interphase resumes without an intervening anaphase. The resulting polyploid nuclei are eliminated. RESULTS: The cells of the Drosophila gastrula can also delay metaphase-anaphase transition in response to DNA damage. This delay accompanies the stabilization of Cyclin A, a known inhibitor of sister chromosome separation in Drosophila. Unlike in cleavage embryos, gamma-tubulin remains at the spindle poles, and anaphase always occurs after the delay. Cyclin A mutants fail to delay metaphase-anaphase transition after irradiation and show an increased frequency of chromosome breakage in the subsequent anaphase. CONCLUSIONS: DNA damage delays metaphase-anaphase transition in Drosophila by stabilizing Cyclin A. This delay may normally serve to preserve chromosomal integrity during segregation. To our knowledge this is the first report of a metazoan metaphase-anaphase transition being delayed in response to DNA damage. Though mitotic progression is modulated in response to DNA damage in both cleaving and gastruating embryos of Drosophila, different mechanisms operate. These differences are discussed in the context of differential cell cycle regulation in cleavage and gastrula stages.     

Downregulation of protein 4.1R, a mature centriole protein, disrupts centrosomes, alters cell cycle progression, and perturbs mitotic spindles and anaphase

Centrosomes nucleate and organize interphase microtubules and are instrumental in mitotic bipolar spindle assembly, ensuring orderly cell cycle progression with accurate chromosome segregation. We report that the multifunctional structural protein 4.1R localizes at centrosomes to distal/subdistal regions of mature centrioles in a cell cycle-dependent pattern. Significantly, 4.1R-specific depletion mediated by RNA interference perturbs subdistal appendage proteins ninein and outer dense fiber 2/cenexin at mature centrosomes and concomitantly reduces interphase microtubule anchoring and organization. 4.1R depletion causes G(1) accumulation in p53-proficient cells, similar to depletion of many other proteins that compromise centrosome integrity. In p53-deficient cells, 4.1R depletion delays S phase, but aberrant ninein distribution is not dependent on the S-phase delay. In 4.1R-depleted mitotic cells, efficient centrosome separation is reduced, resulting in monopolar spindle formation. Multipolar spindles and bipolar spindles with misaligned chromatin are also induced by 4.1R depletion. Notably, all types of defective spindles have mislocalized NuMA (nuclear mitotic apparatus protein), a 4.1R binding partner essential for spindle pole focusing. These disruptions contribute to lagging chromosomes and aberrant microtubule bridges during anaphase/telophase. Our data provide functional evidence that 4.1R makes crucial contributions to the structural integrity of centrosomes and mitotic spindles which normally enable mitosis and anaphase to proceed with the coordinated precision required to avoid pathological events.     

Localization of Pavarotti-KLP in living Drosophila embryos suggests roles in reorganizing the cortical cytoskeleton during the mitotic cycle

Pav-KLP is the Drosophila member of the MKLP1 family essential for cytokinesis. In the syncytial blastoderm embryo, GFP-Pav-KLP cyclically associates with astral, spindle, and midzone microtubules and also to actomyosin pseudocleavage furrows. As the embryo cellularizes, GFP-Pav-KLP also localizes to the leading edge of the furrows that form cells. In mononucleate cells, nuclear localization of GFP-Pav-KLP is mediated through NLS elements in its C-terminal domain. Mutants in these elements that delocalize Pav-KLP to the cytoplasm in interphase do not affect cell division. In mitotic cells, one population of wild-type GFP-Pav-KLP associates with the spindle and concentrates in the midzone at anaphase B. A second is at the cell cortex on mitotic entry and later concentrates in the region of the cleavage furrow. An ATP binding mutant does not localize to the cortex and spindle midzone but accumulates on spindle pole microtubules to which actin is recruited. This leads either to failure of the cleavage furrow to form or later defects in which daughter cells remain connected by a microtubule bridge. Together, this suggests Pav-KLP transports elements of the actomyosin cytoskeleton to plus ends of astral microtubules in the equatorial region of the cell to permit cleavage ring formation.     

Cell division and nuclear movement in the saccoderm desmidNetrium interruptus

Summary During anaphase in thisNetrium, the reforming daughter nuclei hardly pause at the poles before they elongate and rapidly and smoothly move along the daughter cells in one of the grooves in the chloroplast. Ahead of each nucleus is a pointed mass of cytoplasm that is distinctly striated; straight, mobile strands of cytoplasm emanate from this region ahead of the nucleus. When the nucleus reaches the large vacuole that divides the two chloroplasts, it steadily slides over to the chloroplast surface distal to the cleavage furrow. It then stops moving and slowly expands into the normal interphase morphology.Under the electron microscope, the chromosome-to-pole distance does not decrease much during anaphase (i.e., anaphase A is minimal) and so the half spindles remain about the same length by telophase. The poles of the open spindle are initially broad and contain typical spindle microtubules (MTs). These persist intact during anaphase and become focused upon a discrete Organizing Centre as the daughter nuclei reform. These MTs become a cone-shaped array that creates the pointed cytoplasmic mass ahead of the moving nucleus in live cells. Thus, this placoderm desmid behaves very likeClosterium during division and shows the lack of anaphase A, and the transformation of the telophase spindle into a MT-based motility system, now characteristic of many members of the Zygnematales.Abbreviations MT microtubule - MTOC microtubule organizing centre Dedicated to the memory of Professor Oswald Kiermayer     

Drosophila parthenogenesis: a tool to decipher centrosomal vs acentrosomal spindle assembly pathways

Development of unfertilized eggs in the parthenogenetic strain K23-O-im of Drosophila mercatorum requires the stochastic interactions of self-assembled centrosomes with the female chromatin. In a portion of the unfertilized eggs that do not assemble centrosomes, microtubules organize a bipolar anastral mitotic spindle around the chromatin like the one formed during the first female meiosis, suggesting that similar pathways may be operative. In the cytoplasm of eggs in which centrosomes do form, monastral and biastral spindles are found. Analysis by laser scanning confocal microscopy suggests that these spindles are derived from the stochastic interaction of astral microtubules directly with kinetochore regions or indirectly with kinetochore microtubules. Our findings are consistent with the idea that mitotic spindle assembly requires both acentrosomal and centrosomal pathways, strengthening the hypothesis that astral microtubules can dictate the organization of the spindle by capturing kinetochore microtubules.     

Asymmetric division in mouse oocytes: with or without Mos

In both vertebrates and invertebrates, meiotic divisions in oocytes are typically asymmetric, resulting in the formation of a large oocyte and small polar bodies. The size difference between the daughter cells is usually a consequence of asymmetric positioning of the spindle before cytokinesis. Spindle movements are often related to interactions between the cell cortex and the spindle asters [1,2]. The spindles of mammalian oocytes are, however, typically devoid of astral microtubules, which normally connect the spindle to the cortex, suggesting that another mechanism is responsible for the unequal divisions in these oocytes. We observed the formation of the first polar body in wild-type oocytes and oocytes derived from c-Mos knockout mice [3]. In wild-type oocytes, the meiotic spindle formed in the centre of the cell and migrated to the cortex just before polar-body extrusion. The spindle did not elongate during anaphase. In mos-/- oocytes, the spindle formed centrally but did not migrate, although an asymmetric division still took place. In these oocytes, the spindle elongated during anaphase and the pole closest to the cortex moved while the other remained in place. Thus, a compensation mechanism exists in mouse oocytes and formation of the first polar body can be achieved in two ways: either after migration of the spindle to the cortex in wild-type oocytes, or after elongation, without migration, of the first meiotic spindle in mos-/- oocytes.     

ZYG-9, A Caenorhabditis elegans Protein Required for Microtubule Organization and Function, Is a Component of Meiotic and Mitotic Spindle Poles

We describe the molecular characterization of zyg-9, a maternally acting gene essential for microtubule organization and function in early Caenorhabditis elegans embryos. Defects in zyg-9 mutants suggest that the zyg-9 product functions in the organization of the meiotic spindle and the formation of long microtubules. One-cell zyg-9 embryos exhibit both meiotic and mitotic spindle defects. Meiotic spindles are disorganized, pronuclear migration fails, and the mitotic apparatus forms at the posterior, orients incorrectly, and contains unusually short microtubules. We find that zyg-9 encodes a component of the meiotic and mitotic spindle poles. In addition to the strong staining of spindle poles, we consistently detect staining in the region of the kinetochore microtubules at metaphase and early anaphase in mitotic spindles. The ZYG-9 signal at the mitotic centrosomes is not reduced by nocodazole treatment, indicating that ZYG-9 localization to the mitotic centrosomes is not dependent upon long astral microtubules. Interestingly, in embryos lacking an organized meiotic spindle, produced either by nocodazole treatment or mutations in the mei-1 gene, ZYG-9 forms a halo around the meiotic chromosomes. The protein sequence shows partial similarity to a small set of proteins that also localize to spindle poles, suggesting a common activity of the proteins.