The kinetochore of higher eucaryotes: a molecular view

Comparison of the effects of BPA and BPAF on oocyte spindle assembly and polar body release in mice

Bisphenol AF (BPAF), a homolog of bisphenol A (BPA), is a widely used environmental chemical that has adverse effects on reproduction. The aim of this study was to analyse the effects of BPA and BPAF exposure on oocyte maturation in vitro. Oocytes were cultured in the presence of BPA or BPAF (2, 20, 50 or 100 μg/ml) for 18 h. At concentrations of 50 and 100 μg/ml, BPA and BPAF inhibited oocyte maturation, with BPAF treatment causing a sharp decrease in the number of oocytes reaching maturity. Oocytes were exposed to BPA or BPAF at 2 μg/ml and cultured for different durations (6, 9, 12, 15 or 18 h). Both BPAF and BPA caused a cell cycle delay under these conditions. Oocytes cultured in the presence of BPA or BPAF (50 μg/ml) for 21 h were tested for the localization of α-tubulin and MAD2 using immunofluorescence. High concentrations of BPAF induced cell cycle arrest through the activation of the spindle assembly checkpoint. After 12 h of culture in BPAF (50 μg/ml), oocytes were transferred to control medium for 9 h. Only 63.3% oocytes treated in this manner progressed to metaphase II (MII). Oocytes exposed to high doses of BPA experienced a cell cycle delay, but managed to progress to MII when the culture period was prolonged. In addition, MAD2 was localized in the cytoplasm of these oocytes. In conclusion, both BPAF and BPA exposure affected oocyte maturation, however BPAF and BPA have differential effects on SAC activity.

The Drosophila kinesin-13, KLP59D, impacts Pacman- and Flux-based chromosome movement

Chromosome movements are linked to the active depolymerization of spindle microtubule (MT) ends. Here we identify the kinesin-13 family member, KLP59D, as a novel and uniquely important regulator of spindle MT dynamics and chromosome motility in Drosophila somatic cells. During prometaphase and metaphase, depletion of KLP59D, which targets to centrosomes and outer kinetochores, suppresses the depolymerization of spindle pole-associated MT minus ends, thereby inhibiting poleward tubulin Flux. Subsequently, during anaphase, loss of KLP59D strongly attenuates chromatid-to-pole motion by suppressing the depolymerization of both minus and plus ends of kinetochore-associated MTs. The mechanism of KLP59D's impact on spindle MT plus and minus ends appears to differ. Our data support a model in which KLP59D directly depolymerizes kinetochore-associated plus ends during anaphase, but influences minus ends indirectly by localizing the pole-associated MT depolymerase KLP10A. Finally, electron microscopy indicates that, unlike the other Drosophila kinesin-13s, KLP59D is largely incapable of oligomerizing into MT-associated rings in vitro, suggesting that such structures are not a requisite feature of kinetochore-based MT disassembly and chromosome movements.

Meiotic maturation failure induced by DICER1 deficiency is derived from primary oocyte ooplasm

RNA interference (RNAi) has diverse functions across cellular processes, including a role in the development of the mammalian oocyte. Mouse primary oocytes deficient in the key RNAi enzyme DICER1 exhibit pronounced defects in chromosome congression and spindle formation during meiotic maturation. The cause of this meiotic maturation failure is unknown. In this study, observations of chromosomes and spindle microtubules during prometaphase in DICER1-deficient oocytes indicate that chromosome congression and spindle formation are overtly normal. Spindle breakdown and chromosome displacement occur after the metaphase plate has formed, during the metaphase to anaphase transition. We hypothesised that this defect could be attributed to either RNAi-mediated regulation of nuclear factors, such as the regulation of centromere chromatin assembly, or the regulation of mRNA expression within the cytoplasm. By transplanting germinal vesicles between DICER1-deficient and wild-type primary oocytes, we show that, unexpectedly, the meiotic failure is not caused by a deficiency derived from the germinal vesicle component. Instead, we reveal that the ooplasm of primary oocytes contains DICER1-dependent factors that are crucial for chromosome segregation and meiotic maturation.

Targeting mitosis for anti-cancer therapy

Basic research that has focused on achieving a mechanistic understanding of mitosis has provided unprecedented molecular and biochemical insights into this highly complex phase of the cell cycle. The discovery process has uncovered an ever-expanding list of novel proteins that orchestrate and coordinate spindle formation and chromosome dynamics during mitosis. That many of these proteins appear to function solely in mitosis makes them ideal targets for the development of mitosis-specific cancer drugs. The clinical successes seen with anti-microtubule drugs such as taxanes and the vinca alkaloids have also encouraged the development of drugs that specifically target mitosis. Drugs that selectively inhibit mitotic kinesins involved in spindle and kinetochore functions, as well as kinases that regulate these activities, are currently in various stages of clinical trials. Our increased understanding of mitosis has also revealed that this process is targeted by inhibitors of farnesyl transferase, histone deacetylase, and Hsp90. Although these drugs were originally designed to block cell proliferation by inhibiting signaling pathways and altering gene expression, it is clear now that these drugs can also directly interfere with the mitotic process. The increased attention to mitosis as a chemotherapeutic target has also raised an important issue regarding the cellular determinants that specify drug sensitivity. One likely contribution is the mitotic checkpoint, a failsafe mechanism that delays mitotic exit so that cells whose chromosomes are not properly attached to the spindle have extra time to correct their errors. As the biochemical activity of the mitotic checkpoint is finite, cells cannot indefinitely sustain the delay, as in cases where cells are treated with anti-mitotic drugs. When the mitotic checkpoint activity is eventually lost, cells will exit mitosis and become aneuploid. While many of the aneuploid cells may die because of massive chromosome imbalance, survivors that continue to proliferate will no doubt be selected. This is clearly an undesirable outcome, thus efforts to obtain fundamental insights into why some cells that arrest in mitosis die without exiting mitosis will be exceedingly important in enhancing our understanding of the drug sensitivity of cancer cells.

Cytoplasmic dynein participates in meiotic checkpoint inactivation in mouse oocytes by transporting cytoplasmic mitotic arrest-deficient (Mad) proteins from kinetochores to spindle poles

The present study was designed to investigate the localization and function of cytoplasmic dynein (dynein) during mouse oocyte meiosis and its relationship with two major spindle checkpoint proteins, mitotic arrest-deficient (Mad) 1 and Mad2. Oocytes at various stages during the first meiosis were fixed and immunostained for dynein, Mad1, Mad2, kinetochores, microtubules, and chromosomes. Some oocytes were treated with nocodazole before examination. Anti-dynein antibody was injected into the oocytes at germinal vesicle (GV) stage before the examination of its effects on meiotic progression or Mad1 and Mad2 localization. Results showed that dynein was present in the oocytes at various stages from GV to metaphase II and the locations of Mad1 and Mad2 were associated with dynein’s movement. Both Mad1 and Mad2 had two existing states: one existed in the cytoplasm (cytoplasmic Mad1 or cytoplasmic Mad2), which did not bind to kinetochores, while the other bound to kinetochores (kinetochore Mad1 or kinetochore Mad2). The equilibrium between the two states varied during meiosis and/or in response to the changes of the connection between microtubules and kinetochores. Cytoplasmic Mad1 and Mad2 recruited to chromosomes when the connection between microtubules and chromosomes was destroyed. Inhibition of dynein interferes with cytoplasmic Mad1 and Mad2 transportation from chromosomes to spindle poles, thus inhibits checkpoint silence and delays anaphase onset. These results indicate that dynein may play a role in spindle checkpoint inactivation.

Inhibition of protein kinase C zeta blocks the attachment of stable microtubules to kinetochores leading to abnormal chromosome alignment

The attachment of spindle microtubules to kinetochores is crucial for accurate segregation of chromosomes to daughter cells during mitosis. While a growing number of proteins involving this step are being identified, its molecular mechanisms are still not clear. Here we show that protein kinase C zeta (PKCzeta) is localized at the mitotic spindle during mitosis and plays a role in stable kinetochore-microtubule attachment. Striking staining for PKCzeta was observed at the mitotic spindle and spindle poles in cells at prometaphase and metaphase. PKCzeta molecules at these stages were phosphorylated at Thr-410, as detected by a phosphospecific antibody. PKCzeta was also detected at the spindle midzone and the midbody during anaphase and telophase, respectively, and PKCzeta at these stages was no longer phosphorylated at Thr-410. The polarity determinants Par3 and Par6, which are known to associate with PKCzeta, were also localized to the spindles and spindle poles at prometaphase and metaphase. Knockdown of PKCzeta by RNA interference affected normal chromosome alignment leading to generation of cells with aberrant nuclei. A specific PKCzeta inhibitor strongly blocked the formation of cold-sensitive stable kinetochore microtubules, and thus prevented microtubule-kinetochore attachment. Treatment of cells with the PKCzeta inhibitor also dislocated the minus-end directed motor protein dynein from kinetochores, but not the mitotic checkpoint proteins Mad2 and CENP-E. Prolonged exposure to the PKCzeta inhibitor eventually resulted in cell death. These results suggest a critical role of PKCzeta in spindle microtubule-kinetochore attachment and subsequent chromosomal separation.

Mitotic Microtubule Development and Histone H3 Phosphorylation in the Holocentric Chromosomes of Rhynchospora Tenuis (Cyperaceae)

In the present work we report the phosphorylation pattern of histone H3 and the development of microtubular structures using immunostaining techniques, in mitosis of Rhynchospora tenuis (2n = 4), a Cyperaceae with holocentric chromosomes. The main features of the holocentric chromosomes of R. tenuis coincide with those of other species namely: the absence of primary constriction in prometaphase and metaphase, and the parallel separation of sister chromatids at anaphase. Additionaly, we observed a highly conserved chromosome positioning at anaphase and early telophase sister nuclei. Four microtubule arrangements were distinguished during the root tip cell cycle. Interphase cells showed a cortical microtubule arrangement that progressively forms the characteristic pre-prophase band. At prometaphase the microtubules were homogeneously distributed around the nuclear envelope. Metaphase cells displayed the spindle arrangement with kinetochore microtubules attached throughout the entire chromosome extension. At anaphase kinetochoric microtubules become progressively shorter, whereas bundles of interzonal microtubules became increasingly broader and denser. At late telophase the microtubules were observed equatorially extended beyond the sister nuclei and reaching the cell wall. Immunolabelling with an antibody against phosphorylated histone H3 revealed the four chromosomes labelled throughout their entire extension at metaphase and anaphase. Apparently, the holocentric chromosomes of R. tenuis function as an extended centromeric region both in terms of cohesion and H3 phosphorylation.

The simple ultrastructure of the maize kinetochore fits a two-domain model

Light microscope observations suggest there are two discrete biochemical domains in the plant kinetochore, an inner domain containing structural proteins, and an outer domain containing proteins involved in motility. We analyzed the ultrastructure of maize meiotic kinetochores following high pressure freezing and freeze substitution, a method that provides excellent sample preservation. Data from meiosis II support previous descriptions of plant kinetochores as diffuse, nearly invisible domains, sometimes nesting in a cup of darkly staining chromatin. The ultrastructure is similar in meiosis I but there are two sister kinetochores that each protrude away from the chromosome and form their own distinct kinetochore fibers. Microtubules terminate within kinetochores where their ends are splayed in a cone-shaped configuration suggestive of microtubule disassembly. We could not detect any significant substructure within the kinetochore proper. We suggest that the diffuse structure classically defined as the kinetochore represents only the outer domain of a two-domain organelle. The inner domain, known to contain chromatin-binding proteins, probably extends into the electron-dense chromatin of the primary constriction.

Silencing mitosin induces misaligned chromosomes, premature chromosome decondensation before anaphase onset, and mitotic cell death

Mitosin (also named CENP-F) is a large human nuclear protein transiently associated with the outer kinetochore plate in M phase. Using RNA interference and fluorescence microscopy, we showed that mitosin depletion attenuated chromosome congression and led to metaphase arrest with misaligned polar chromosomes whose kinetochores showed few cold-stable microtubules. Kinetochores of fully aligned chromosomes often failed to show orientation in the direction of the spindle long axis. Moreover, tension across their sister kinetochores was decreased by 53% on average. These phenotypes collectively imply defects in motor functions in mitosin-depleted cells and are similar to those of CENP-E depletion. Consistently, the intensities of CENP-E and cytoplasmic dynein and dynactin, which are motors controlling microtubule attachment and chromosome movement, were reduced at the kinetochore in a microtubule-dependent manner. In addition, after being arrested in pseudometaphase for approximately 2 h, mitosin-depleted cells died before anaphase initiation through apoptosis. The dying cells exhibited progressive chromosome arm decondensation, while the centromeres were still associated with spindles. Mitosin is therefore essential for full chromosome alignment, possibly by promoting proper kinetochore attachments through modulating CENP-E and dynein functions. Its depletion also prematurely triggers chromosome decondensation, a process that normally occurs from telophase for the nucleus reassembly, thus resulting in apoptosis.

Hsp70 protects mitotic cells against heat-induced centrosome damage and division abnormalities

The effect of heat shock on centrosomes has been mainly studied in interphase cells. Centrosomes play a key role in proper segregation of DNA during mitosis. However, the direct effect and consequences of heat shock on mitotic cells and a possible cellular defense system against proteotoxic stress during mitosis have not been described in detail. Here, we show that mild heat shock, applied during mitosis, causes loss of dynamitin/p50 antibody staining from centrosomes and kinetochores. In addition, it induces division errors in most cells and in the remaining cells progression through mitosis is delayed. Expression of heat shock protein (Hsp)70 protects against most heat-induced division abnormalities. On heat shock, Hsp70 is rapidly recruited to mitotic centrosomes and normal progression through mitosis is observed immediately after release of Hsp70 from centrosomes. In addition, Hsp70 expression coincides with restoration of dynamitin/p50 antibody staining at centrosomes but not at kinetochores. Our data show that during mitosis, centrosomes are particularly affected resulting in abnormal mitosis. Hsp70 is sufficient to protect against most division abnormalities, demonstrating the involvement of Hsp70 in a repair mechanism of heat-damaged mitotic centrosomes.

HCP-4/CENP-C promotes the prophase timing of centromere resolution by enabling the centromere association of HCP-6 in Caenorhabditis elegans

Prior to microtubule capture, sister centromeres resolve from one another, coming to rest on opposite surfaces of the condensing chromosome. Subsequent assembly of sister kinetochores at each sister centromere generates a geometry favorable for equal levels of segregation of chromatids. The holocentric chromosomes of Caenorhabditis elegans are uniquely suited for the study of centromere resolution and subsequent kinetochore assembly. In C. elegans, only two proteins have been identified as being necessary for centromere resolution, the kinase AIR-2 (prophase only) and the centromere protein HCP-4/CENP-C. Here we found that the loss of proteins involved in chromosome cohesion bypassed the requirement for HCP-4/CENP-C but not for AIR-2. Interestingly, the loss of cohesin proteins also restored the localization of HCP-6 to the kinetochore. The loss of the condensin II protein HCP-6 or MIX-1/SMC2 impaired centromere resolution. Furthermore, the loss of HCP-6 or MIX-1/SMC2 resulted in no centromere resolution when either nocodazole or RNA interference (RNAi) of the kinetochore protein KNL-1 perturbed spindle-kinetochore interactions. This result suggests that normal prophase centromere resolution is mediated by condensin II proteins, which are actively recruited to sister centromeres to mediate the process of resolution.

Roles of polo-like kinase 1 in the assembly of functional mitotic spindles

BACKGROUND

The stable association of chromosomes with both poles of the mitotic spindle (biorientation) depends on spindle pulling forces. These forces create tension across sister kinetochores and are thought to stabilize microtubule-kinetochore interactions and to silence the spindle checkpoint. Polo-like kinase 1 (Plk1) has been implicated in regulating centrosome maturation, mitotic entry, sister chromatid cohesion, the anaphase-promoting complex/cyclosome (APC/C), and cytokinesis, but it is unknown if Plk1 controls chromosome biorientation.

RESULTS

We have analyzed Plk1 functions in synchronized mammalian cells by RNA interference (RNAi). Plk1-depleted cells enter mitosis after a short delay, accumulate in a preanaphase state, and subsequently often die by apoptosis. Spindles in Plk1-depleted cells lack focused poles and are not associated with centrosomes. Chromosomes attach to these spindles, but the checkpoint proteins Mad2, BubR1, and CENP-E are enriched at many kinetochores. When Plk1-depleted cells are treated with the Aurora B inhibitor Hesperadin, which silences the spindle checkpoint by stabilizing microtubule-kinetochore interactions, cells degrade APC/C substrates and exit mitosis without chromosome segregation and cytokinesis. Experiments with monopolar spindles that are induced by the kinesin inhibitor Monastrol indicate that Plk1 is required for the assembly of spindles that are able to generate poleward pulling forces.

CONCLUSIONS

Our results imply that Plk1 is not essential for mitotic entry and APC/C activation but is required for proper spindle assembly and function. In Plk1-depleted cells spindles may not be able to create enough tension across sister kinetochores to stabilize microtubule-kinetochore interactions and to silence the spindle checkpoint.

Microtubule-associated proteins and their essential roles during mitosis

Microtubules play essential roles during mitosis, including chromosome capture, congression, and segregation. In addition, microtubules are also required for successful cytokinesis. At the heart of these processes is the ability of microtubules to do work, a property that derives from their intrinsic dynamic behavior. However, if microtubule dynamics were not properly regulated, it is certain that microtubules alone could not accomplish any of these tasks. In vivo, the regulation of microtubule dynamics is the responsibility of microtubule-associated proteins. Among these, we can distinguish several classes according to their function: (1) promotion and stabilization of microtubule polymerization, (2) destabilization or severance of microtubules, (3) functioning as linkers between various structures, or (4) motility-related functions. Here we discuss how the various properties of microtubule-associated proteins can be used to assemble an efficient mitotic apparatus capable of ensuring the bona fide transmission of the genetic information in animal cells.

Localization of Mitotic Arrest Deficient 1 (MAD1) in Mouse Oocytes During the First Meiosis and Its Functions as a Spindle Checkpoint Protein1

The present study was designed to investigate the localization of mitotic arrest deficient 1 (MAD1) in mouse oocytes during meiotic maturation and its relationship with kinetochores, chromosomes, and microtubules. Oocytes at various stages during the first meiosis were fixed and immunostained for MAD1, kinetochores, microtubules, and chromosomes. The stained oocytes were examined by confocal microscopy. Some oocytes were treated with nocodazole or Taxol before examination. The anti-MAD1 antibody was injected into the oocytes at the germinal vesicle (GV) stage for examination of chromosome alignment and spindle formation. It was found that MAD1 was present in the oocytes from the GV to prometaphase I stages around the nuclei. When the oocytes reached the metaphase I (M-I) to metaphase II (M-II) stages, MAD1 was mainly localized at the spindle poles. However, MAD1 relocated to the vicinity of the chromosomes when spindles were disassembled by nocodazole or cooling, and the relocated MAD1 moved back to the spindle poles during spindle recovery. Taxol treatment did not affect the MAD1 localization. Although anti-MAD1 antibody injection did not affect nuclear maturation, significantly higher proportions of injected oocytes had misaligned chromosomes when the oocytes reached the M-I to M-II stages. The results of the present study indicate that MAD1 is present in mouse oocytes at all stages during the first meiosis and that it participates in spindle checkpoint during meiosis. However, MAD1 could not check misaligned chromosomes during spindle recovery after the spindles were destroyed by drug or cooling, which caused some chromosomes to scatter in the oocytes.

Functional complementation of human centromere protein A (CENP-A) by Cse4p from Saccharomyces cerevisiae

We have employed a novel in vivo approach to study the structure and function of the eukaryotic kinetochore multiprotein complex. RNA interference (RNAi) was used to block the synthesis of centromere protein A (CENP-A) and Clip-170 in human cells. By coexpression, homologous kinetochore proteins from Saccharomyces cerevisiae were then tested for the ability to complement the RNAi-induced phenotypes. Cse4p, the budding yeast CENP-A homolog, was specifically incorporated into kinetochore nucleosomes and was able to complement RNAi-induced cell cycle arrest in CENP-A-depleted human cells. Thus, Cse4p can structurally and functionally substitute for CENP-A, strongly suggesting that the basic features of centromeric chromatin are conserved between yeast and mammals. Bik1p, the budding yeast homolog of human CLIP-170, also specifically localized to kinetochores during mitosis, but Bik1p did not rescue CLIP-170 depletion-induced cell cycle arrest. Generally, the newly developed in vivo complementation assay provides a powerful new tool for studying the function and evolutionary conservation of multiprotein complexes from yeast to humans.

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.

Centromere protein b-null mice display decreasing reproductive performance through successive generations of breeding due to diminishing endometrial glands

Centromere protein B is a highly conserved constitutive protein found at centromeres. Gene knockout studies in mice have unexpectedly identified Cenpb as a candidate gene involved in uterine function. The present study further explores the role of Cenpb in mice by intermating Cenpb-null mice over several generations. Breeding studies and analysis of uterine tissue indicate that Cenpb-null mice lose reproductive fitness over a number of generations due to a significant reduction in endometrial glands. These results suggest that Cenpb may play an important function in the short- and long-term maintenance of uterine integrity.

Gene silencing of CENP-E by small interfering RNA in HeLa cells leads to missegregation of chromosomes after a mitotic delay

Centromeric protein-E (CENP-E) is a kinesin-like motor protein required for chromosome congression at prometaphase. Functional perturbation of CENP-E by various methods results in a consistent phenotype, i.e., unaligned chromosomes during mitosis. One unresolved question from previous studies is whether cells complete mitosis or sustain mitotic arrest in the presence of unaligned chromosomes. Using RNA interference and video-microscopy, we analyzed the dynamic process of mitotic progression of HeLa(H2B)-GFP cells lacking CENP-E. Our results demonstrate that these cells initiated anaphase after a delayed mitotic progression due to the presence of unaligned chromosomes. In some dividing cells, unaligned chromosomes are present during anaphase, causing nondisjunction of some sister chromatids producing aneuploid daughter cells. Unlike in Xenopus extract, the loss of CENP-E in HeLa cells does not impair gross checkpoint activation because cells were arrested in mitosis in response to microtubule-interfering agents. However, the lack of CENP-E at kinetochores reduced the hyperphosphorylation of BubR1 checkpoint protein during mitosis, which may explain the loss of sensitivity of a cell to a few unaligned chromosomes in the absence of CENP-E. We also found that presynchronization with nocodazole sensitizes cells to the depletion of CENP-E, leading to more unaligned chromosomes, longer arrest, and cell death.

Intra-oocyte localization of MAD2 and its relationship with kinetochores, microtubules, and chromosomes in rat oocytes during meiosis

The present study was designed to investigate subcellular localization of MAD2 in rat oocytes during meiotic maturation and its relationship with kinetochores, chromosomes, and microtubules. Oocytes at germinal vesicle (GV), prometaphase I (ProM-I), metaphase I (M-I), anaphase I (A-I), telophase I (T-I), and metaphase II (M-II) were fixed and immunostained for MAD2, kinetochores, microtubules and chromosomes. The stained oocytes were examined by confocal microscopy. Some oocytes from GV to M-II stages were treated by a microtubule disassembly drug, nocodazole, or treated by a microtubule stabilizer, Taxol, before examination. Anti-MAD2 antibody was also injected into the oocytes at GV stage and the injected oocytes were cultured for 6 h for examination of chromosome alignment and spindle formation. It was found that MAD2 was at the kinetochores in the oocytes at GV and ProM-I stages. Once the oocytes reached M-I stage in which an intact spindle was formed and all chromosomes were aligned at the equator of the spindle, MAD2 disappeared. However, when oocytes from GV to M-II stages were treated by nocodazole, spindles were destroyed and MAD2 was observed in all treated oocytes. When nocodazole-treated oocytes at M-I and M-II stages were washed and cultured for spindle recovery, it was found that, once the relationship between microtubules and chromosomes was established, MAD2 disappeared in the oocytes even though some chromosomes were not aligned at the equator of the spindle. On the other hand, when oocytes were treated with Taxol, MAD2 localization was not changed and was the same as that in the control. However, immunoblotting of MAD2 indicated that MAD2 was present in the oocytes at all stages; nocodazole and Taxol treatment did not influence the quantity of MAD2 in the cytoplasm. Significantly higher proportions of anti-MAD2 antibody-injected oocytes proceeded to premature A-I stage and more oocytes had misaligned chromosomes in the spindles. The present study indicates that MAD2 is a spindle checkpoint protein in rat oocytes during meiosis. When the spindle was destroyed by nocodazole, MAD2 was reactivated in the oocytes to overlook the attachment between chromosomes and microtubules. However, in this case, MAD2 could not check unaligned chromosomes in the recovered spindles, suggesting that a normal chromosome alignment is maintained only in the oocytes without any microtubule damages during maturation.

Structure, function, and regulation of budding yeast kinetochores

Kinetochores are multiprotein complexes that assemble on centromeric DNA and mediate the attachment and movement of chromosomes along the microtubules (MTs) of the mitotic spindle. This review focuses on the simplest eukaryotic centromeres and kinetochores, those found in the budding yeast Saccharomyces cerevisiae. Research on kinetochore function and chromosome segregation is focused on four questions of general significance: what specifies the location of centromeres? What are the protein components of kinetochores, and how do they assemble a MT attachment site? How do MT attachments generate force? How do cells sense the state of attachment via the spindle assembly checkpoint?

Regulation of KinI kinesin ATPase activity by binding to the microtubule lattice

KinI kinesins are important in regulating the complex dynamics of the microtubule cytoskeleton. They are unusual in that they depolymerize, rather than move along microtubules. To determine the attributes of KinIs that distinguish them from translocating kinesins, we examined the ATPase activity, microtubule affinity, and three-dimensional microtubule-bound structure of a minimal KinI motor domain. Together, the kinetic, affinity, and structural data lead to the conclusion that on binding to the microtubule lattice, KinIs release ADP and enter a stable, low-affinity, regulated state, from which they do not readily progress through the ATPase cycle. This state may favor detachment, or diffusion of the KinI to its site of action, the microtubule ends. Unlike conventional translocating kinesins, which are microtubule lattice–stimulated ATPases, it seems that with KinIs, nucleotide-mediated modulation of tubulin affinity is only possible when it is coupled to protofilament deformation. This provides an elegant mechanistic basis for their unique depolymerizing activity.

Mitosin/CENP-F is a conserved kinetochore protein subjected to cytoplasmic dynein-mediated poleward transport

Mitosin/CENP-F is a human nuclear protein transiently associated with the outer kinetochore plate in M phase and is involved in M phase progression. LEK1 and CMF1, which are its murine and chicken orthologs, however, are implicated in muscle differentiation and reportedly not distributed at kinetochores. We therefore conducted several assays to clarify this issue. The typical centromere staining patterns were observed in mitotic cells from both human primary culture and murine, canine, and mink cell lines. A C-terminal portion of LEK1 also conferred centromere localization. Our analysis further suggests conserved kinetochore localization of mammalian mitosin orthologs. Moreover, mitosin was associated preferentially with kinetochores of unaligned chromosomes. It was also constantly transported from kinetochores to spindle poles by cytoplasmic dynein. These properties resemble those of other kinetochore proteins important for the spindle checkpoint, thus implying a role of mitosin in this checkpoint. Therefore, mitosin family may serve as multifunctional proteins involved in both mitosis and differentiation.

The condensin complex is required for proper spindle assembly and chromosome segregation in Xenopus egg extracts

Chromosome condensation is required for the physical resolution and segregation of sister chromatids during cell division, but the precise role of higher order chromatin structure in mitotic chromosome functions is unclear. Here, we address the role of the major condensation machinery, the condensin complex, in spindle assembly and function in Xenopus laevis egg extracts. Immunodepletion of condensin inhibited microtubule growth and organization around chromosomes, reducing the percentage of sperm nuclei capable of forming spindles, and causing dramatic defects in anaphase chromosome segregation. Although the motor CENP-E was recruited to kinetochores pulled poleward during anaphase, the disorganized chromosome mass was not resolved. Inhibition of condensin function during anaphase also inhibited chromosome segregation, indicating its continuous requirement. Spindle assembly around DNA-coated beads in the absence of kinetochores was also impaired upon condensin inhibition. These results support an important role for condensin in establishing chromosomal architecture necessary for proper spindle assembly and chromosome segregation.

Transient CENP-E-like kinetochore proteins in plants

Derived from candidate sequences of a barley EST database two proteins with homology to the coiled coil region of the human kinetochore protein (KP) CENP-E were generated and classified as centromere protein E-like 1 and 2 (Cpell and Cpe12). Specific antibodies produced against recombinant Cpe11 and Cpe12 proteins labeled the centromere on mitotic chromosomes of barley and field bean and recognized specifically proteins from nuclear/chromosomal protein extracts on immunoblots. No function was predicted for homologues of Cpe11 within the databases for Arabidopsis and rice genomes. However, the centromeric location of Cpe11 and Cpe12 suggests they may have a function within the kinetochore. Plant homologues to barley Cpe12 are N-type kinesins, suggesting that Cpe12 is functionally homologous to human CENP-E.

Chromosome-microtubule interactions during mitosis

Spindle microtubules interact with mitotic chromosomes, binding to their kinetochores to generate forces that are important for accurate chromosome segregation. Motor enzymes localized both at kinetochores and spindle poles help to form the biologically significant attachments between spindle fibers and their cargo, but microtubule-associated proteins without motor activity contribute to these junctions in important ways. This review examines the molecules necessary for chromosome-microtubule interaction in a range of well-studied organisms, using biological diversity to identify the factors that are essential for organized chromosome movement. We conclude that microtubule dynamics and the proteins that control them are likely to be more important for mitosis than the current enthusiasm for motor enzymes would suggest.

Cell division

In creating the mitotic spindle and the contractile ring, natural selection has engineered fascinating precision machines whose movements depend upon forces generated by ensembles of cytoskeletal proteins. These machines segregate chromosomes and divide the cell with high fidelity. Current research on the mechanisms and regulation of spindle morphogenesis, chromosome motility and cytokinesis emphasizes how ensembles of dynamic cytoskeletal polymers and multiple motors cooperate to generate the forces that guide the cell through mitosis and cytokinesis.

Mitosis through the microscope: advances in seeing inside live dividing cells

The most visually spectacular events in the life of a cell occur when it divides. This is especially true in higher eukaryotes, where the size and geometry of cells allow the division process to be followed through a microscope with considerable clarity. In these organisms, the membrane surrounding the nucleus breaks down after the replicated DNA has condensed to form discrete chromosomes. Several new structures are then assembled to separate the chromosomes and partition the cytoplasm into two separate cells.

Human CENP-I specifies localization of CENP-F, MAD1 and MAD2 to kinetochores and is essential for mitosis

The kinetochore, a macromolecular complex located at the centromere of chromosomes, provides essential functions for accurate chromosome segregation. Kinetochores contain checkpoint proteins that monitor attachments between the kinetochore and microtubules to ensure that cells do not exit mitosis in the presence of unaligned chromosomes. Here we report that human CENP-I, a constitutive protein of the kinetochore that shares limited similarity with Mis6 of Schizosaccharomyces pombe, is required for the localization of CENP-F and the checkpoint proteins MAD1 and MAD2 to kinetochores. Depletion of CENP-I from kinetochores causes the cell cycle to delay in G2. Although monopolar chromosomes in CENP-I-depleted cells fail to establish bipolar connections, the cells are unable to arrest in mitosis. These cells are transiently delayed in mitosis in a MAD2-dependent manner, even though their kinetochores are depleted of MAD2. The delay is extended considerably when the number of unattached kinetochores is increased. This suggests that no single unattached kinetochore in CENP-I-depleted cells can arrest mitosis. The collective output from many unattached kinetochores is required to reach a threshold signal of 'wait for anaphase' to sustain a prolonged mitotic arrest.

EB1 targets to kinetochores with attached, polymerizing microtubules

Microtubule polymerization dynamics at kinetochores is coupled to chromosome movements, but its regulation there is poorly understood. The plus end tracking protein EB1 is required both for regulating microtubule dynamics and for maintaining a euploid genome. To address the role of EB1 in aneuploidy, we visualized its targeting in mitotic PtK1 cells. Fluorescent EB1, which localized to polymerizing ends of astral and spindle microtubules, was used to track their polymerization. EB1 also associated with a subset of attached kinetochores in late prometaphase and metaphase, and rarely in anaphase. Localization occurred in a narrow crescent, concave toward the centromere, consistent with targeting to the microtubule plus end-kinetochore interface. EB1 did not localize to kinetochores lacking attached kinetochore microtubules in prophase or early prometaphase, or upon nocodazole treatment. By time lapse, EB1 specifically targeted to kinetochores moving antipoleward, coupled to microtubule plus end polymerization, and not during plus end depolymerization. It localized independently of spindle bipolarity, the spindle checkpoint, and dynein/dynactin function. EB1 is the first protein whose targeting reflects kinetochore directionality, unlike other plus end tracking proteins that show enhanced kinetochore binding in the absence of microtubules. Our results suggest EB1 may modulate kinetochore microtubule polymerization and/or attachment.

Cohesin defects lead to premature sister chromatid separation, kinetochore dysfunction, and spindle-assembly checkpoint activation

Scc1/Mcd1 is a component of the cohesin complex that plays an essential role in sister chromatid cohesion in eukaryote cells. Knockout experiments of this gene have been described in budding yeast, fission yeast, and chicken cells, but no study has been reported on human Scc1 thus far. In this study, we found that an N-terminally truncated human Scc1 shows a dominant-negative effect, and we examined the phenotypes of human cells defective in Scc1 function. Scc1 defects led to failure of sister chromatid cohesion in both interphase and mitotic cells. Interestingly, four chromatids derived from two homologues occupied four distinct territories in the nucleus in chromosome painting experiments. In mitotic Scc1-defective cells, chromatids were disjoined with normal condensation, and the spindle-assembly checkpoint was activated. We also found that, although the disjoined kinetochore (half-kinetochore) in Scc1-defective cells contains CENP-A, -B, -C, and -E normally, it apparently does not establish the kinetochore-microtubule association. These results indicate that Scc1 is essential for the association of kinetochores with microtubules.

Mad2 and BubR1 function in a single checkpoint pathway that responds to a loss of tension

The spindle checkpoint monitors microtubule attachment and tension at kinetochores to ensure proper chromosome segregation. Previously, PtK1 cells in hypothermic conditions (23 degrees C) were shown to have a pronounced mitotic delay, despite having normal numbers of kinetochore microtubules. At 23 degrees C, we found that PtK1 cells remained in metaphase for an average of 101 min, compared with 21 min for cells at 37 degrees C. The metaphase delay at 23 degrees C was abrogated by injection of Mad2 inhibitors, showing that Mad2 and the spindle checkpoint were responsible for the prolonged metaphase. Live cell imaging showed that kinetochore Mad2 became undetectable soon after chromosome congression. Measurements of the stretch between sister kinetochores at metaphase found a 24% decrease in tension at 23 degrees C, and metaphase kinetochores at 23 degrees C exhibited higher levels of 3F3/2, Bub1, and BubR1 compared with 37 degrees C. Microinjection of anti-BubR1 antibody abolished the metaphase delay at 23 degrees C, indicating that the higher kinetochore levels of BubR1 may contribute to the delay. Disrupting both Mad2 and BubR1 function induced anaphase with the same timing as single inhibitions, suggesting that these checkpoint genes function in the same pathway. We conclude that reduced tension at kinetochores with a full complement of kinetochore microtubules induces a checkpoint dependent metaphase delay associated with elevated amounts of kinetochore 3F3/2, Bub1, and BubR1 labeling.

The spindle checkpoint: structural insights into dynamic signalling

Chromosome segregation is a complex and astonishingly accurate process whose inner working is beginning to be understood at the molecular level. The spindle checkpoint plays a key role in ensuring the fidelity of this process. It monitors the interactions between chromosomes and microtubules, and delays mitotic progression to allow extra time to correct defects. Here, we review and integrate findings on the dynamics of checkpoint proteins at kinetochores with structural information about signalling complexes.

A simple, mechanistic model for directional instability during mitotic chromosome movements

During mitosis, chromosomes become attached to microtubules that emanate from the two spindle poles. Thereafter, a chromosome moves along these microtubule "tracks" as it executes a series of movements that bring it to the spindle equator. After the onset of anaphase, the sister chromatids separate and move to opposite spindle poles. These movements are often characterized by "directional instability" (a series of runs with approximately constant speed, punctuated by sudden reversals in the direction of movement). To understand mitosis, it is critical to describe the physical mechanisms that underlie the coordination of the forces that drive directional instability. We propose a simple mechanistic model that describes the origin of the forces that move chromosomes and the coordination of these forces to produce directional instability. The model demonstrates that forces, speeds, and direction of motion associated with prometaphase through anaphase chromosome movements can be predicted from the molecular kinetics of interactions between dynamic microtubules and arrays of microtubule binding sites that are linked to the chromosome by compliant elements.

LIS1, CLIP-170's key to the dynein/dynactin pathway

CLIP-170 is a plus-end tracking protein which may act as an anticatastrophe factor. It has been proposed to mediate the association of dynein/dynactin to microtubule (MT) plus ends, and it also binds to kinetochores in a dynein/dynactin-dependent fashion, both via its C-terminal domain. This domain contains two zinc finger motifs (proximal and distal), which are hypothesized to mediate protein-protein interactions. LIS1, a protein implicated in brain development, acts in several processes mediated by the dynein/dynactin pathway by interacting with dynein and other proteins. Here we demonstrate colocalization and direct interaction between CLIP-170 and LIS1. In mammalian cells, LIS1 recruitment to kinetochores is dynein/dynactin dependent, and recruitment there of CLIP-170 is dependent on its site of binding to LIS1, located in the distal zinc finger motif. Overexpression of CLIP-170 results in a zinc finger-dependent localization of a phospho-LIS1 isoform and dynactin to MT bundles, raising the possibility that CLIP-170 and LIS1 regulate dynein/dynactin binding to MTs. This work suggests that LIS1 is a regulated adapter between CLIP-170 and cytoplasmic dynein at sites involved in cargo-MT loading, and/or in the control of MT dynamics.

Polyploids require Bik1 for kinetochore-microtubule attachment

The attachment of kinetochores to spindle microtubules (MTs) is essential for maintaining constant ploidy in eukaryotic cells. Here, biochemical and imaging data is presented demonstrating that the budding yeast CLIP-170 orthologue Bik1is a component of the kinetochore-MT binding interface. Strikingly, Bik1 is not required for viability in haploid cells, but becomes essential in polyploids. The ploidy-specific requirement for BIK1 enabled us to characterize BIK1 without eliminating nonhomologous genes, providing a new approach to circumventing the overlapping function that is a common feature of the cytoskeleton. In polyploid cells, Bik1 is required before anaphase to maintain kinetochore separation and therefore contributes to the force that opposes the elastic recoil of attached sister chromatids. The role of Bik1 in kinetochore separation appears to be independent of the role of Bik1 in regulating MT dynamics. The finding that a protein involved in kinetochore-MT attachment is required for the viability of polyploids has potential implications for cancer therapeutics.

Cytoplasmic dynein/dynactin drives kinetochore protein transport to the spindle poles and has a role in mitotic spindle checkpoint inactivation

We discovered that many proteins located in the kinetochore outer domain, but not the inner core, are depleted from kinetochores and accumulate at spindle poles when ATP production is suppressed in PtK1 cells, and that microtubule depolymerization inhibits this process. These proteins include the microtubule motors CENP-E and cytoplasmic dynein, and proteins involved with the mitotic spindle checkpoint, Mad2, Bub1R, and the 3F3/2 phosphoantigen. Depletion of these components did not disrupt kinetochore outer domain structure or alter metaphase kinetochore microtubule number. Inhibition of dynein/dynactin activity by microinjection in prometaphase with purified p50 "dynamitin" protein or concentrated 70.1 anti-dynein antibody blocked outer domain protein transport to the spindle poles, prevented Mad2 depletion from kinetochores despite normal kinetochore microtubule numbers, reduced metaphase kinetochore tension by 40%, and induced a mitotic block at metaphase. Dynein/dynactin inhibition did not block chromosome congression to the spindle equator in prometaphase, or segregation to the poles in anaphase when the spindle checkpoint was inactivated by microinjection with Mad2 antibodies. Thus, a major function of dynein/dynactin in mitosis is in a kinetochore disassembly pathway that contributes to inactivation of the spindle checkpoint.

Sequence organization of barley centromeres

By sequencing, fingerprinting and in situ hybridization of a centromere-specific large insert clone (BAC 7), the sequence organization of centromeric DNA of barley could be elucidated. Within 23 kb, three copies of the Ty3/gypsy-like retroelement cereba were present. Two elements of approximately 7 kb, arranged in tandem, include long terminal repeats (LTRs) (approximately 1 kb) similar to the rice centromeric retrotransposon RIRE 7 and to the cereal centromeric sequence family, the primer binding site, the complete polygene flanked by untranslated regions, as well as a polypurine tract 5' of the downstream LTR. The high density (approximately 200 elements/centromere) and completeness of cereba elements and the absence of internally deleted elements and solo LTRs from the BAC 7 insert represent unique features of the barley centromeres as compared to those of other cereals. Obviously, the conserved cereba elements together with barley-specific G+C-rich satellite sequences constitute the major components of centromeric DNA in this species.

Kinetochore function: the complications of becoming attached

Kinetochores are specialized protein complexes assembled on centromeric DNA that connect to spindle microtubules and mediate proper chromosome segregation during cell division. Recent results have implicated specific kinetochore proteins in microtubule attachment.

Evolutionary conservation between budding yeast and human kinetochores

Accurate chromosome segregation during mitosis requires the correct assembly of kinetochores--complexes of centromeric DNA and proteins that link chromosomes to spindle microtubules. Studies on the kinetochore of the budding yeast Saccharomyces cerevisiae have revealed functionally novel components of the kinetochore and its regulatory complexes, some of which are highly conserved in humans.

Microtubule-dependent changes in assembly of microtubule motor proteins and mitotic spindle checkpoint proteins at PtK1 kinetochores

The ability of kinetochores to recruit microtubules, generate force, and activate the mitotic spindle checkpoint may all depend on microtubule- and/or tension-dependent changes in kinetochore assembly. With the use of quantitative digital imaging and immunofluorescence microscopy of PtK1 tissue cells, we find that the outer domain of the kinetochore, but not the CREST-stained inner core, exhibits three microtubule-dependent assembly states, not directly dependent on tension. First, prometaphase kinetochores with few or no kinetochore microtubules have abundant punctate or oblate fluorescence morphology when stained for outer domain motor proteins CENP-E and cytoplasmic dynein and checkpoint proteins BubR1 and Mad2. Second, microtubule depolymerization induces expansion of the kinetochore outer domain into crescent and ring morphologies around the centromere. This expansion may enhance recruitment of kinetochore microtubules, and occurs with more than a 20- to 100-fold increase in dynein and relatively little change in CENP-E, BubR1, and Mad2 in comparison to prometaphase kinetochores. Crescents disappear and dynein decreases substantially upon microtubule reassembly. Third, when kinetochores acquire their full metaphase complement of kinetochore microtubules, levels of CENP-E, dynein, and BubR1 decrease by three- to sixfold in comparison to unattached prometaphase kinetochores, but remain detectable. In contrast, Mad2 decreases by 100-fold and becomes undetectable, consistent with Mad2 being a key factor for the "wait-anaphase" signal produced by unattached kinetochores. Like previously found for Mad2, the average amounts of CENP-E, dynein, or BubR1 at metaphase kinetochores did not change with the loss of tension induced by taxol stabilization of microtubules.

HIM-10 is required for kinetochore structure and function on Caenorhabditis elegans holocentric chromosomes

Macromolecular structures called kinetochores attach and move chromosomes within the spindle during chromosome segregation. Using electron microscopy, we identified a structure on the holocentric mitotic and meiotic chromosomes of Caenorhabditis elegans that resembles the mammalian kinetochore. This structure faces the poles on mitotic chromosomes but encircles meiotic chromosomes. Worm kinetochores require the evolutionarily conserved HIM-10 protein for their structure and function. HIM-10 localizes to the kinetochores and mediates attachment of chromosomes to the spindle. Depletion of HIM-10 disrupts kinetochore structure, causes a failure of bipolar spindle attachment, and results in chromosome nondisjunction. HIM-10 is related to the Nuf2 kinetochore proteins conserved from yeast to humans. Thus, the extended kinetochores characteristic of C. elegans holocentric chromosomes provide a guide to the structure, molecular architecture, and function of conventional kinetochores.

HCP-4, a CENP-C-like protein in Caenorhabditis elegans, is required for resolution of sister centromeres

The centromere plays a critical role in the segregation of chromosomes during mitosis. In mammals, sister centromeres are resolved from one another in the G2 phase of the cell cycle. During prophase, chromosomes condense with sister centromeres oriented in a back to back configuration enabling only one chromatid to be captured by each half spindle. To study this process, we identified a centromere protein (CENP)-C-like protein, holocentric protein (HCP)-4, in Caenorhabditis elegans based on sequence identity, loss of function phenotype, and centromeric localization. HCP-4 is found in the cytoplasm during interphase, but is nuclear localized in mitosis, where it localizes specifically to the centromere. The localization of HCP-4 to the centromere is dependent on the centromeric histone HCP-3; in addition, HCP-3 and HCP-4 are both required for localization of a CENP-F-like protein, HCP-1, indicating an ordered assembly pathway. Loss of HCP-4 expression by RNA-mediated interference resulted in a failure to generate resolution of sister centromeres on chromosomes, suggesting that HCP-4 is required for sister centromere resolution. These chromosomes also failed to form a functional kinetochore. Thus, the CENP-C-like protein HCP-4 is essential for both resolution sister centromeres and attachment to the mitotic spindle.

Functional analysis of kinetochore assembly in Caenorhabditis elegans

In all eukaryotes, segregation of mitotic chromosomes requires their interaction with spindle microtubules. To dissect this interaction, we use live and fixed assays in the one-cell stage Caenorhabditis elegans embryo. We compare the consequences of depleting homologues of the centromeric histone CENP-A, the kinetochore structural component CENP-C, and the chromosomal passenger protein INCENP. Depletion of either CeCENP-A or CeCENP-C results in an identical "kinetochore null" phenotype, characterized by complete failure of mitotic chromosome segregation as well as failure to recruit other kinetochore components and to assemble a mechanically stable spindle. The similarity of their depletion phenotypes, combined with a requirement for CeCENP-A to localize CeCENP-C but not vice versa, suggest that a key step in kinetochore assembly is the recruitment of CENP-C by CENP-A-containing chromatin. Parallel analysis of CeINCENP-depleted embryos revealed mitotic chromosome segregation defects different from those observed in the absence of CeCENP-A/C. Defects are observed before and during anaphase, but the chromatin separates into two equivalently sized masses. Mechanically stable spindles assemble that show defects later in anaphase and telophase. Furthermore, kinetochore assembly and the recruitment of CeINCENP to chromosomes are independent. These results suggest distinct roles for the kinetochore and the chromosomal passengers in mitotic chromosome segregation.

Merotelic kinetochore orientation is a major mechanism of aneuploidy in mitotic mammalian tissue cells

In mitotic cells, an error in chromosome segregation occurs when a chromosome is left near the spindle equator after anaphase onset (lagging chromosome). In PtK1 cells, we found 1.16% of untreated anaphase cells exhibiting lagging chromosomes at the spindle equator, and this percentage was enhanced to 17.55% after a mitotic block with 2 microM nocodazole. A lagging chromosome seen during anaphase in control or nocodazole-treated cells was found by confocal immunofluorescence microscopy to be a single chromatid with its kinetochore attached to kinetochore microtubule bundles extending toward opposite poles. This merotelic orientation was verified by electron microscopy. The single kinetochores of lagging chromosomes in anaphase were stretched laterally (1.2--5.6-fold) in the directions of their kinetochore microtubules, indicating that they were not able to achieve anaphase poleward movement because of pulling forces toward opposite poles. They also had inactivated mitotic spindle checkpoint activities since they did not label with either Mad2 or 3F3/2 antibodies. Thus, for mammalian cultured cells, kinetochore merotelic orientation is a major mechanism of aneuploidy not detected by the mitotic spindle checkpoint. The expanded and curved crescent morphology exhibited by kinetochores during nocodazole treatment may promote the high incidence of kinetochore merotelic orientation that occurs after nocodazole washout.

A solid foundation: functional specialization of centromeric chromatin

Centromeres provide a distinctive mechanical function for the chromosomes as the site of kinetochore assembly and force generation in mitosis and meiosis. Recent studies show that a unique form of chromatin, based on the histone-H3-like protein CENP-A and homologues, provides a conserved foundation for this mechanical chromatin domain. CENP-A plays a role in templating kinetochore assembly and may be a central element in the epigenetic maintenance of centromere identity. Cohesion at the centromere, intimately linked to kinetochore assembly, is required for integrating spindle forces exerted across the centromere and for establishing the bipolar geometry of sister kinetochores.

Budding yeast chromosome structure and dynamics during mitosis

Using green fluorescent protein probes and rapid acquisition of high-resolution fluorescence images, sister centromeres in budding yeast are found to be separated and oscillate between spindle poles before anaphase B spindle elongation. The rates of movement during these oscillations are similar to those of microtubule plus end dynamics. The degree of preanaphase separation varies widely, with infrequent centromere reassociations observed before anaphase. Centromeres are in a metaphase-like conformation, whereas chromosome arms are neither aligned nor separated before anaphase. Upon spindle elongation, centromere to pole movement (anaphase A) was synchronous for all centromeres and occurred coincident with or immediately after spindle pole separation (anaphase B). Chromatin proximal to the centromere is stretched poleward before and during anaphase onset. The stretched chromatin was observed to segregate to the spindle pole bodies at rates greater than centromere to pole movement, indicative of rapid elastic recoil between the chromosome arm and the centromere. These results indicate that the elastic properties of DNA play an as of yet undiscovered role in the poleward movement of chromosome arms.