A chromatin-associated kinesin-related protein required for normal mitotic chromosome segregation in Drosophila

BUB1/KIF14 complex promotes anaplastic thyroid carcinoma progression by inducing chromosome instability

Chromosome instability (CIN) is a common contributor driving the formation and progression of anaplastic thyroid cancer (ATC), but its mechanism remains unclear. The BUB1 mitotic checkpoint serine/threonine kinase (BUB1) is responsible for the alignment of mitotic chromosomes, which has not been thoroughly studied in ATC. Our research demonstrated that BUB1 was remarkably upregulated and closely related to worse progression-free survival. Knockdown of BUB1 attenuated cell viability, invasion, migration and induced cell cycle arrests, whereas overexpression of BUB1 promoted the cell cycle progression of papillary thyroid cancer cells. BUB1 knockdown remarkably repressed tumour growth and tumour formation of nude mice with ATC xenografts and suppressed tumour metastasis in a zebrafish xenograft model. Inhibition of BUB1 by its inhibitor BAY-1816032 also exhibited considerable anti-tumour activity. Further studies showed that enforced expression of BUB1 evoked CIN in ATC cells. BUB1 induced CIN through phosphorylation of KIF14 at serine1292 (Ser1292 ). Overexpression of the KIF14ΔSer1292 mutant was unable to facilitate the aggressiveness of ATC cells when compared with that of the wild type. Collectively, these findings demonstrate that the BUB1/KIF14 complex drives the aggressiveness of ATC by inducing CIN.

Acentrosomal spindles assemble from branching microtubule nucleation near chromosomes in Xenopus laevis egg extract

Microtubules are generated at centrosomes, chromosomes, and within spindles during cell division. Whereas microtubule nucleation at the centrosome is well characterized, much remains unknown about where, when, and how microtubules are nucleated at chromosomes. To address these questions, we reconstitute microtubule nucleation from purified chromosomes in meiotic Xenopus egg extract and find that chromosomes alone can form spindles. We visualize microtubule nucleation near chromosomes using total internal reflection fluorescence microscopy to find that this occurs through branching microtubule nucleation. By inhibiting molecular motors, we find that the organization of the resultant polar branched networks is consistent with a theoretical model where the effectors for branching nucleation are released by chromosomes, forming a concentration gradient that spatially biases branching microtbule nucleation. In the presence of motors, these branched networks are ultimately organized into functional spindles, where the number of emergent spindle poles scales with the number of chromosomes and total chromatin area.

SILAC-based quantitative proteomics and microscopy analysis of cancer cells treated with the N-glycolyl GM3-specific anti-tumor antibody 14F7

Cancer immunotherapy represents a promising approach to specifically target and treat cancer. The most common mechanisms by which monoclonal antibodies kill cells include antibody-dependent cell-mediated cytotoxicity, complement-dependent cytotoxicity and apoptosis, but also other mechanisms have been described. 14F7 is an antibody raised against the tumor-associated antigen NeuGc GM3, which was previously reported to kill cancer cells without inducing apoptotic pathways. The antibody was reported to induce giant membrane lesions in tumor cells, with apparent changes in the cytoskeleton. Here, we investigated the effect of humanized 14F7 on HeLa cells using stable isotope labeling with amino acids in cell culture (SILAC) in combination with LC-MS and live cell imaging. 14F7 did not kill the HeLa cells, however, it caused altered protein expression (MS data are available via ProteomeXchange with identifier PXD024320). Several cytoskeletal and nucleic-acid binding proteins were found to be strongly down-regulated in response to antibody treatment, suggesting how 14F7 may induce membrane lesions in cells that contain higher amounts of NeuGc GM3. The altered expression profile identified in this study thus contributes to an improved understanding of the unusual killing mechanism of 14F7.

The Drosophila Forkhead/Fox transcription factor Jumeau mediates specific cardiac progenitor cell divisions by regulating expression of the kinesin Nebbish

Forkhead (Fkh/Fox) domain transcription factors (TFs) mediate multiple cardiogenic processes in both mammals and Drosophila. We showed previously that the Drosophila Fox gene jumeau (jumu) controls three categories of cardiac progenitor cell division—asymmetric, symmetric, and cell division at an earlier stage—by regulating Polo kinase activity, and mediates the latter two categories in concert with the TF Myb. Those observations raised the question of whether other jumu-regulated genes also mediate all three categories of cardiac progenitor cell division or a subset thereof. By comparing microarray-based expression profiles of wild-type and jumu loss-of-function mesodermal cells, we identified nebbish (neb), a kinesin-encoding gene activated by jumu. Phenotypic analysis shows that neb is required for only two categories of jumu-regulated cardiac progenitor cell division: symmetric and cell division at an earlier stage. Synergistic genetic interactions between neb, jumu, Myb, and polo and the rescue of jumu mutations by ectopic cardiac mesoderm-specific expression of neb demonstrate that neb is an integral component of a jumu-regulated subnetwork mediating cardiac progenitor cell divisions. Our results emphasize the central role of Fox TFs in cardiogenesis and illustrate how a single TF can utilize different combinations of other regulators and downstream effectors to control distinct developmental processes.

Loss-of-function mutations in KIF14 cause severe microcephaly and kidney development defects in humans and zebrafish

Mutations in KIF14 have previously been associated with either severe, isolated or syndromic microcephaly with renal hypodysplasia (RHD). Syndromic microcephaly-RHD was strongly reminiscent of clinical ciliopathies, relating to defects of the primary cilium, a signalling organelle present on the surface of many quiescent cells. KIF14 encodes a mitotic kinesin, which plays a key role at the midbody during cytokinesis and has not previously been shown to be involved in cilia-related functions. Here, we analysed four families with fetuses presenting with the syndromic form and harbouring biallelic variants in KIF14. Our functional analyses showed that the identified variants severely impact the activity of KIF14 and likely correspond to loss-of-function mutations. Analysis in human fetal tissues further revealed the accumulation of KIF14-positive midbody remnants in the lumen of ureteric bud tips indicating a shared function of KIF14 during brain and kidney development. Subsequently, analysis of a kif14 mutant zebrafish line showed a conserved role for this mitotic kinesin. Interestingly, ciliopathy-associated phenotypes were also present in mutant embryos, supporting a potential direct or indirect role for KIF14 at cilia. However, our in vitro and in vivo analyses did not provide evidence of a direct role for KIF14 in ciliogenesis and suggested that loss of kif14 causes ciliopathy-like phenotypes through an accumulation of mitotic cells in ciliated tissues. Altogether, our results demonstrate that KIF14 mutations result in a severe syndrome associating microcephaly and RHD through its conserved function in cytokinesis during kidney and brain development.

KIF14 promotes cell proliferation via activation of Akt and is directly targeted by miR-200c in colorectal cancer

As a mitotic kinesin, kinesin family member 14 (KIF14) has been reported to serve oncogenic roles in a variety of malignancies; however, its functional role and regulatory mechanisms in colorectal cancer (CRC) remain unclear. In the present study, KIF14 was observed to be markedly overexpressed in CRC, and this upregulation was associated with tumor size and marker of proliferation Ki-67 immunostaining scores. Gain- and loss-of-function experiments were applied to identify the function of KIF14 in CRC progression. In vitro and in vivo assays revealed that KIF14 promoted CRC cell proliferation and accelerated the cell cycle via activation of protein kinase B. In addition, the present study investigated the potential mechanisms underlying KIF14 overexpression in CRC. Bioinformatics analyses and validation experiments, including reverse transcription-quantitative polymerase chain reaction, western blotting and a Dual-Luciferase reporter assay, demonstrated that, in addition to genomic amplification and transcriptional activation, KIF14 was regulated by microRNA (miR)-200c at the post-transcriptional level. Rescue experiments further demonstrated that decreased miR-200c expression could facilitate KIF14 to exert its pro-proliferative role. The expression of miR-200c was negatively correlated with KIF14 in CRC specimens. Collectively, the findings of the present study demonstrated the oncogenic role of KIF14 in colorectal tumorigenesis, and also revealed a complexity of regulatory mechanisms mediating KIF14 overexpression, which may provide insight for developing novel treatments for patients with CRC.

Up-regulation of KIF14 is a predictor of poor survival and a novel prognostic biomarker of chemoresistance to paclitaxel treatment in cervical cancer

KIF14 may serve as a predictor of poor survival and a novel prognostic biomarker of chemoresistance to paclitaxel treatment in cervical cancer.

Kinesin family member 14 (KIF14) is a member of kinesin family proteins which have been found to be dysregulated in various cancer types. However, the expression of KIF14 and its potential prognostic significance have not been investigated in cervical cancer. Real-time PCR was performed to assess the expression levels of KIF14 in 47 pairs of cervical cancer tissues and their matched normal tissues from patients who had not been exposed to chemotherapy as well as tissue samples from 57 cervical cancer patients who are sensitive to paclitaxel treatment and 53 patients who are resistant. The association between KIF14 expression levels in tissue and clinicopathological features or chemosensitivity was examined. Kaplan–Meier analysis and Cox proportional hazards model were applied to assess the correlation between KIF14 expression levels and overall survival (OS) of cervical cancer patients. KIF14 expression levels were significantly increased in cervical cancer tissues compared with matched non-cancerous tissues and it was higher in tissues of patients who are chemoresistant compared with those who are chemosensitive. KIF14 expression was positively associated with high tumour stage (P=0.0044), lymph node metastasis (P=0.0034) and chemoresistance (P<0.0001). Kaplan–Meier analysis showed that high KIF14 expression levels predicted poor survival in patients with (P=0.0024) or without (P=0.0028) paclitaxel treatment. Multivariate analysis revealed that KIF14 was an independent prognostic factor for OS. Our study suggests that KIF14 may serve as a predictor of poor survival and a novel prognostic biomarker of chemoresistance to paclitaxel treatment in cervical cancer.

The Ran Pathway in Drosophila melanogaster Mitosis

Over the last two decades, the small GTPase Ran has emerged as a central regulator of both mitosis and meiosis, particularly in the generation, maintenance, and regulation of the microtubule (MT)-based bipolar spindle. Ran-regulated pathways in mitosis bear many similarities to the well-characterized functions of Ran in nuclear transport and, as with transport, the majority of these mitotic effects are mediated through affecting the physical interaction between karyopherins and Spindle Assembly Factors (SAFs)—a loose term describing proteins or protein complexes involved in spindle assembly through promoting nucleation, stabilization, and/or depolymerization of MTs, through anchoring MTs to specific structures such as centrosomes, chromatin or kinetochores, or through sliding MTs along each other to generate the force required to achieve bipolarity. As such, the Ran-mediated pathway represents a crucial functional module within the wider spindle assembly landscape. Research into mitosis using the model organism Drosophila melanogaster has contributed substantially to our understanding of centrosome and spindle function. However, in comparison to mammalian systems, very little is known about the contribution of Ran-mediated pathways in Drosophila mitosis. This article sets out to summarize our understanding of the roles of the Ran pathway components in Drosophila mitosis, focusing on the syncytial blastoderm embryo, arguing that it can provide important insights into the conserved functions on Ran during spindle formation.

The motor protein KIF14 inhibits tumor growth and cancer metastasis in lung adenocarcinoma

The motor protein kinesin superfamily proteins (KIFs) are involved in cancer progression. The depletion of one of the KIFs, KIF14, might delay the metaphase-to-anaphase transition, resulting in a binucleated status, which enhances tumor progression; however, the exact correlation between KIF14 and cancer progression remains ambiguous. In this study, using loss of heterozygosity and array comparative genomic hybridization analyses, we observed a 30% loss in the regions surrounding KIF14 on chromosome 1q in lung adenocarcinomas. In addition, the protein expression levels of KIF14 in 122 lung adenocarcinomas also indicated that approximately 30% of adenocarcinomas showed KIF14 down-regulation compared with the expression in the bronchial epithelial cells of adjacent normal counterparts. In addition, the reduced expression of KIF14 mRNA or proteins was correlated with poor overall survival (P = 0.0158 and <0.0001, respectively), and the protein levels were also inversely correlated with metastasis (P<0.0001). The overexpression of KIF14 in lung adenocarcinoma cells inhibited anchorage-independent growth in vitro and xenograft tumor growth in vivo. The overexpression and silencing of KIF14 also inhibited or enhanced cancer cell migration, invasion and adhesion to the extracellular matrix proteins laminin and collagen IV. Furthermore, we detected the adhesion molecules cadherin 11 (CDH11) and melanoma cell adhesion molecule (MCAM) as cargo on KIF14. The overexpression and silencing of KIF14 enhanced or reduced the recruitment of CDH11 in the membrane fraction, suggesting that KIF14 might act through recruiting adhesion molecules to the cell membrane and modulating cell adhesive, migratory and invasive properties. Thus, KIF14 might inhibit tumor growth and cancer metastasis in lung adenocarcinomas.

Sustained elongation of sperm tail promoted by local remodeling of giant mitochondria in Drosophila

BACKGROUND

Sperm length in Drosophilidae varies from a few hundred microns to 6 cm as a result of evolutionary selection. In postcopulatory competition, longer sperm have an advantage in positioning their head closer to the egg. Sperm cell elongation can proceed in the absence of an axoneme, suggesting that a mechanism besides intraflagellar transport emerged to sustain it.

RESULTS

Here we report that sperm elongation in Drosophila melanogaster is driven by the interdependent extension of giant mitochondria and microtubule array that is formed around the mitochondrial surface. In primary cultures of elongating spermatids, we demonstrated that the mitochondrial integrity and local dynamics of microtubules at the tail tip region are essential for uniaxial elongation of the sperm tail. Mitochondria-microtubule linker protein Milton accumulated on mitochondria near the tail tip and is required for the sliding movement of microtubules. Disruption of Milton and its associated protein dMiro, and of potential microtubule crosslinkers Nebbish and Fascetto, caused strong elongation defects, indicating that mitochondria-microtubule association and microtubule crosslinking are required for spermatid tail elongation.

CONCLUSIONS

Mitochondria play unexpected roles in sperm tail elongation in Drosophila by providing a structural platform for microtubule reorganization to support the robust elongation taking place at the tip of the very long sperm tail. The identification of mitochondria as an organizer of cytoskeletal dynamics extends our understanding of mechanisms of cell morphogenesis.

CAML loss causes anaphase failure and chromosome missegregation

Calcium modulating cyclophilin ligand (CAML) is a ubiquitously expressed cytoplasmic protein that is implicated in the EGFR and LCK signaling pathways and required for early embryonic and thymocyte development. To further define the critical biological functions of CAML at the cellular level, we generated CAML-deleted mouse embryonic fibroblasts (MEFs) using an in vitro Cre-loxP mediated conditional knockout system. We found that CAML(-/-) MEFs have severely impaired proliferation and a strong reduction of normal anaphases. The primary mitotic defect of CAML(-/-) MEFs is that duplicated chromosomes fail to segregate in anaphase, resulting in nuclear bisection by the cleavage furrow as cells decondense their DNA and exit mitosis, highly reminiscent of the "cut" phenotype in fission yeast. This phenotype is due to spindle dysfunction rather than inability to resolve physical connections between sister chromatids. Furthermore, CAML(-/-) MEFs display defects often seen in cells with mitotic checkpoint gene deficiencies, including lagging and misaligned chromosomes and chromatin bridges. Consistent with this, we found that CAML(-/-) MEFs have a modestly weakened spindle assembly checkpoint (SAC) and increased aneuploidy. Thus, our data identify CAML as a novel chromosomal instability gene and suggest that CAML protein acts as a key regulator of mitotic spindle function and a modulator of SAC maintenance.

Mitotic spindle dynamics in Drosophila

Mitosis, the process by which the replicated chromosomes are segregated equally into daughter cells, has been studied for over a century. Drosophila melanogaster is an ideal organism for this research. Drosophila embryos are well suited to image mitosis, because during cycles 10-13 nuclei divide rapidly at the surface of the embryo, but mitotic cells during larval stages and spermatocytes are also used for the study of mitosis. Drosophila can be easily maintained, many mutant stocks exist, and transgenic flies expressing mutated or fluorescently labeled proteins can be made. In addition, the genome has been completed and RNA interference can be used in Drosophila tissue culture cells. Here, we review our current understanding of spindle dynamics, looking at the experiments and quantitative modeling on which it is based. Many molecular players in the Drosophila mitotic spindle are similar to those in mammalian spindles, so findings in Drosophila can be extended to other organisms.

KIF14 mRNA expression is a predictor of grade and outcome in breast cancer

Gain of chromosome 1q is a hallmark of breast cancer, and likely reflects oncogene amplification. We previously identified mitotic kinesin KIF14 (kinesin family member 14) as an overexpressed candidate oncogene in the 1q31.3-1q32.1 minimal region of genomic gain in breast cancer cell lines. KIF14 also showed high expression in other cancers, notably an association with survival in lung tumors. We now report KIF14 expression in 99 primary breast tumors and 10 normal breast controls. Measured by real-time RT-PCR, KIF14 was overexpressed 10-fold on average in tumors relative to normals (t test p = 0.000054); expression increased with grade (ANOVA p = 0.000006). Infiltrating ductal carcinomas had higher KIF14 levels than lobular (p = 0.017), and estrogen receptor (ER) negative tumors had higher KIF14 levels than ER positive tumors (t test p = 0.030). KIF14 expression correlated positively with Ki-67 mRNA level (Spearman r = 0.692, p = 0.000001), fraction of positive nodes (r = 0.227, p = 0.024) and percent invasive cells (r = 0.360, p = 0.0002), and negatively with percent fatty stroma (r = -0.258, p = 0.010) and percent normal epithelium (r = -0.291, p = 0.003). KIF14 expression is thus tumor-specific and increased in more aggressive tumors. Indeed, KIF14 expression predicted overall survival (univariate Cox p = 0.010), with an odds ratio of 3.60 (1.37-9.48), in 50 tumors with available outcome data. KIF14 overexpression also predicted decreased disease-free survival (log-rank p = 0.049). These findings are the first evidence of association between expression of a mitotic kinesin and prognostic variables in breast cancer.

RNA interference-mediated silencing of mitotic kinesin KIF14 disrupts cell cycle progression and induces cytokinesis failure

KIF14 is a microtubule motor protein whose elevated expression is associated with poor-prognosis breast cancer. Here we demonstrate KIF14 accumulation in mitotic cells, where it associated with developing spindle poles and spindle microtubules. Cells at later stages of mitosis were characterized by the concentration of KIF14 at the midbody. Time-lapse microscopy revealed that strong RNA interference (RNAi)-mediated silencing of KIF14 induced cytokinesis failure, causing several rounds of endoreduplication and resulting in multinucleated cells. Additionally, less efficacious KIF14-specific short interfering RNAs (siRNAs) induced multiple phenotypes, all of which resulted in acute apoptosis. Our data demonstrate the ability of siRNA-mediated silencing to generate epiallelic hypomorphs associated with KIF14 depletion. Furthermore, the link we observed between siRNA efficacy and phenotypic outcome indicates that distinct stages during cell cycle progression are disrupted by the differential modulation of KIF14 expression.

Model of chromosome motility in Drosophila embryos: adaptation of a general mechanism for rapid mitosis

During mitosis, ensembles of dynamic MTs and motors exert forces that coordinate chromosome segregation. Typically, chromosomes align at the metaphase spindle equator where they oscillate along the pole-pole axis before disjoining and moving poleward during anaphase A, but spindles in different cell types display differences in MT dynamicity, in the amplitude of chromosome oscillations and in rates of chromatid-to-pole motion. Drosophila embryonic mitotic spindles, for example, display remarkably dynamic MTs, barely detectable metaphase chromosome oscillations, and a rapid rate of "flux-pacman-dependent" anaphase chromatid-to-pole motility. Here we develop a force-balance model that describes Drosophila embryo chromosome motility in terms of a balance of forces acting on kinetochores and kMTs that is generated by multiple polymer ratchets and mitotic motors coupled to tension-dependent kMT dynamics. The model shows that i), multiple MTs displaying high dynamic instability can drive steady and rapid chromosome motion; ii), chromosome motility during metaphase and anaphase A can be described by a single mechanism; iii), high kinetochore dynein activity is deployed to dampen metaphase oscillations, to augment the basic flux-pacman mechanism, and to drive rapid anaphase A; iv), modulation of the MT rescue frequency by the kinetochore-associated kinesin-13 depolymerase promotes metaphase chromosome oscillations; and v), this basic mechanism can be adapted to a broad range of spindles.

Microtubule movements on the arms of mitotic chromosomes: polar ejection forces quantified in vitro

During mitosis, "polar ejection forces" (PEFs) are hypothesized to direct prometaphase chromosome movements by pushing chromosome arms toward the spindle equator. PEFs are postulated to be caused by (i) plus-end-directed microtubule (MT)-based motor proteins on the chromosome arms, namely chromokinesins, and (ii) the polymerization of spindle MTs into the chromosome. However, the exact role of PEFs is unclear, because little is known about their magnitude or their forms (e.g., impulsive vs. sustained, etc.). In this study, we employ optical tweezers to bring about the lateral interaction between chromosome arms and MTs in vitro to directly measure the speed and force of the PEFs developed on chromosome arms. We find that forces are unidirectional and frequently exceed 1 pN, with maximum forces of 2-3 pN and peak velocities of 63 +/- 41 nm/s; the movements are ATP-dependent and exhibit a characteristic noncontinuous motion that includes displacements of >50 nm, stalls, and backwards slippage of the MT even under low loads. We perform experiments using antibodies to the chromokinesins Kid and KIF4 that identify Kid as the principal force-producing agent for PEFs. At first glance, this motor activity appears surprisingly weak and erratic, but it explains how PEFs can guide chromosome movements without severely deforming or damaging the local chromosome structure.

KIF14 is a candidate oncogene in the 1q minimal region of genomic gain in multiple cancers

Gain of chromosome 1q31-1q32 is seen in >50% of retinoblastoma and is common in other tumors. To define the minimal 1q region of gain, we determined genomic copy number by quantitative multiplex PCR of 14 sequence tagged sites (STSs) spanning 1q25.3-1q41. The most frequently gained STS at 1q32.1 (71%; 39 of 55 retinoblastoma) defined a 3.06 Mbp minimal region of gain between flanking markers, containing 14 genes. Of these, only KIF14, a putative chromokinesin, was overexpressed in various cancers by real-time RT-PCR. KIF14 mRNA was expressed in 20/22 retinoblastoma samples 100-1000-fold higher than in retina (t-test P=0.00002); cell lines (n=10) had higher levels than tumors (n=12) (P=0.009). KIF14 protein was overexpressed in retinoblastoma tumors and breast cancer cell lines by immunoblot. KIF14 was expressed in 4/4 breast cancer cell lines 31-92-fold higher than in normal breast tissue, in 5/5 medulloblastoma cell lines 22-79-fold higher than in fetal brain, and in 10/22 primary lung tumors 3-34-fold higher than in normal lung. Patients with lung tumors that overexpress KIF14 showed a trend toward decreased survival. KIF14 may thus be important in oncogenesis, and has promise as a prognostic indicator and therapeutic target.

Functional analysis of human microtubule-based motor proteins, the kinesins and dyneins, in mitosis/cytokinesis using RNA interference

Microtubule (MT)-based motor proteins, kinesins and dyneins, play important roles in multiple cellular processes including cell division. In this study, we describe the generation and use of an Escherichia coli RNase III-prepared human kinesin/dynein esiRNA library to systematically analyze the functions of all human kinesin/dynein MT motor proteins. Our results indicate that at least 12 kinesins are involved in mitosis and cytokinesis. Eg5 (a member of the kinesin-5 family), Kif2A (a member of the kinesin-13 family), and KifC1 (a member of the kinesin-14 family) are crucial for spindle formation; KifC1, MCAK (a member of the kinesin-13 family), CENP-E (a member of the kinesin-7 family), Kif14 (a member of the kinesin-3 family), Kif18 (a member of the kinesin-8 family), and Kid (a member of the kinesin-10 family) are required for chromosome congression and alignment; Kif4A and Kif4B (members of the kinesin-4 family) have roles in anaphase spindle dynamics; and Kif4A, Kif4B, MKLP1, and MKLP2 (members of the kinesin-6 family) are essential for cytokinesis. Using immunofluorescence analysis, time-lapse microscopy, and rescue experiments, we investigate the roles of these 12 kinesins in detail.

Functional role of centrosomes in spindle assembly and organization

The centrosome is the main MT organizing center in animal cells, and has traditionally been regarded as essential for organization of the bipolar spindle that facilitates chromosome segregation during mitosis. Centrosomes are associated with the poles of the mitotic spindle, and several cell types require these organelles for spindle formation. However, most plant cells and some female meiotic systems get along without this organelle, and centrosome-independent spindle assembly has now been identified within some centrosome containing cells. How can such observations, which point to mutually incompatible conclusions regarding the requirement of centrosomes in spindle formation, be interpreted? With emphasis on the functional role of centrosomes, this article summarizes the current models of spindle formation, and outlines how observations obtained from spindle assembly assays in vitro may reconcile conflicting opinions about the mechanism of spindle assembly. It is further described how Drosophila mutants are used to address the functional interrelationships between individual centrosomal proteins and spindle formation in vivo.

Spindle mechanics and dynamics during mitosis in Drosophila

Drosophila melanogaster is an excellent model for studying mitosis. Syncytial embryos are amenable to time-lapse imaging of hundreds of synchronously dividing spindles, allowing the quantitation of spindle and chromosome dynamics with unprecedented fidelity. Other Drosophila cell types, including neuroblasts, cultured cells, spermatocytes and oocytes, contain spindles that differ in their design, providing cells amenable to different types of experiments and allowing identification of common core mechanisms. The function of mitotic proteins can be studied using mutants, inhibitor microinjection and RNA interference (RNAi) to identify the full inventory of mitotic proteins encoded by the genome. Here, we review recent advances in understanding how ensembles of mitotic proteins coordinate spindle assembly and chromosome motion in this system.

The roles of microtubule-based motor proteins in mitosis: comprehensive RNAi analysis in the Drosophila S2 cell line

Kinesins and dyneins play important roles during cell division. Using RNA interference (RNAi) to deplete individual (or combinations of) motors followed by immunofluorescence and time-lapse microscopy, we have examined the mitotic functions of cytoplasmic dynein and all 25 kinesins in Drosophila S2 cells. We show that four kinesins are involved in bipolar spindle assembly, four kinesins are involved in metaphase chromosome alignment, dynein plays a role in the metaphase-to-anaphase transition, and one kinesin is needed for cytokinesis. Functional redundancy and alternative pathways for completing mitosis were observed for many single RNAi knockdowns, and failure to complete mitosis was observed for only three kinesins. As an example, inhibition of two microtubule-depolymerizing kinesins initially produced monopolar spindles with abnormally long microtubules, but cells eventually formed bipolar spindles by an acentrosomal pole-focusing mechanism. From our phenotypic data, we construct a model for the distinct roles of molecular motors during mitosis in a single metazoan cell type.

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.

KRP3A and KRP3B: candidate motors in spermatid maturation in the seminiferous epithelium

We have identified KRP3, a novel kinesin-related protein expressed in the mammalian testis, and have examined the tissue distribution and subcellular localization of isoforms of this protein. Isolation of KRP3 clones, using the head domain identified in a previous PCR screen as probe, identified at least two KRP3 isoforms in the rat. We have isolated coding sequences of two highly related cDNAs from the rat testis that we have termed KRP3A and KRP3B (kinesin-related protein 3, A and B). Both cDNAs code for predicted polypeptides with the three-domain structure typical of kinesin superfamily members; namely a conserved motor domain, a region capable of forming a limited coiled-coil secondary structure, and a globular tail domain. Although almost identical in their head and stalk domains, these motors diverge in their tail domains. This group of motors is found in many tissues and cell types. The KRP3B motor contains DNA-binding motifs and an RCC1 (regulator of chromosome condensation 1) consensus sequence in its tail domain. Despite this similarity, KRP3B is not associated with the same structures as RCC1. Instead, KRP3 isoforms localize with the nuclei of developing spermatids, and their immunolocalization in the testis overlaps with that of the small GTPase Ran. Like Ran, KRP3 motors are associated in a polarized fashion with the nucleus of maturing spermatids at various stages of elongation. Our findings suggest a possible role for KRP3 motor isoforms in spermatid maturation mediated by possible interaction with the Ran GTPase.

The FHA domain

The forkhead-associated (FHA) domain is a small protein module recently shown to recognize phosphothreonine epitopes on proteins. It is present in a diverse range of proteins in eukaryotic cells, such as kinases, phosphatases, kinesins, transcription factors, RNA-binding proteins, and metabolic enzymes. It is also found in a number of bacterial proteins. This suggests that FHA domain-mediated phospho-dependent assembly of protein complexes is an ancient and widespread regulatory mechanism.

Drosophila male meiosis as a model system for the study of cytokinesis in animal cells

Drosophila male meiosis offers unique opportunities for mutational dissection of cytokinesis. This system allows easy and unambiguos identification of mutants defective in cytokinesis through the examination of spermatid morphology. Moreover, cytokinesis defects and protein immunostaining can be analyzed with exquisite cytological resolution because of the large size of meiotic spindles. In the past few years several mutations have been isolated that disrupt meiotic cytokinesis in Drosophila males. These mutations specify genes required for the assembly, proper constriction or disassembly of the contractile ring. Molecular characterization of these genes has identified essential components of the cytokinetic machinery, highlighting the role of the central spindle during cytokinesis. This structure appears to be both necessary and sufficient for signaling cytokinesis. In addition, many data indicate that the central spindle microtubules cooperatively interact with elements of the actomyosin contractile ring, so that impairment of either of these structures prevents the formation of the other.

Map position and expression of the genes in the 38 region of Drosophila

With the completion of the Drosophila genome sequence, an important next step is to extract its biological information by systematic functional analysis of genes. We have produced a high-resolution genetic map of cytological region 38 of Drosophila using 41 deficiency stocks that provide a total of 54 breakpoints within the region. Of a total of 45 independent P-element lines that mapped by in situ hybridization to the region, 14 targeted 7 complementation groups within the 38 region. Additional EMS, X-ray, and spontaneous mutations define a total of 17 complementation groups. Because these two pools partially overlap, the completed analysis revealed 21 distinct complementation groups defined by point mutations. Seven additional functions were defined by trans-heterozygous combinations of deficiencies, resulting in a total of 28 distinct functions. We further produced a developmental expression profile for the 760 kb from 38B to 38E. Of 135 transcription units predicted by GENSCAN, 22 have at least partial homology to mobile genetic elements such as transposons and retroviruses and 17 correspond to previously characterized genes. We analyzed the developmental expression pattern of the remaining genes using poly(A)(+) RNA from ovaries, early and late embryos, larvae, males, and females. We discuss the correlation between GENSCAN predictions and experimentally confirmed transcription units, the high number of male-specific transcripts, and the alignment of the genetic and physical maps in cytological region 38.

All kinesin superfamily protein, KIF, genes in mouse and human

Intracellular transport is essential for morphogenesis and functioning of the cell. The kinesin superfamily proteins (KIFs) have been shown to transport membranous organelles and protein complexes in a microtubule- and ATP-dependent manner. More than 30 KIFs have been reported in mice. However, the nomenclature of KIFs has not been clearly established, resulting in various designations and redundant names for a single KIF. Here, we report the identification and classification of all KIFs in mouse and human genome transcripts. Previously unidentified murine KIFs were found by a PCR-based search. The identification of all KIFs was confirmed by a database search of the total human genome. As a result, there are a total of 45 KIFs. The nomenclature of all KIFs is presented. To understand the function of KIFs in intracellular transport in a single tissue, we focused on the brain. The expression of 38 KIFs was detected in brain tissue by Northern blotting or PCR using cDNA. The brain, mainly composed of highly differentiated and polarized cells such as neurons and glia, requires a highly complex intracellular transport system as indicated by the increased number of KIFs for their sophisticated functions. It is becoming increasingly clear that the cell uses a number of KIFs and tightly controls the direction, destination, and velocity of transportation of various important functional molecules, including mRNA. This report will set the foundation of KIF and intracellular transport research.

Regulation of molecular motor proteins

Motor proteins in the kinesin, dynein, and myosin superfamilies are tightly regulated to perform multiple functions in the cell requiring force generation. Although motor proteins within families are diverse in sequence and structure, there are general mechanisms by which they are regulated. We first discuss the regulation of the subset of kinesin family members for which such information exists, and then address general mechanisms of kinesin family regulation. We review what is known about the regulation of axonemal and cytoplasmic dyneins. Recent work on cytoplasmic dynein has revealed the existence of multiple isoforms for each dynein chain, making the study of dynein regulation more complicated than previously realized. Finally, we discuss the regulation of myosins known to be involved in membrane trafficking. Myosins and kinesins may be evolutionarily related, and there are common themes of regulation between these two classes of motors.

Kinesin subfamily UNC104 contains a FHA domain: boundaries and physicochemical characterization

By sequence analysis we show that the U104 domain found in the UNC104 subfamily of kinesins is a forkhead homology-associated domain (FHA). A combination of limited proteolysis, mass spectroscopy, and physicochemical analysis define this domain as a genuine autonomously folding domain. Our data show that the FHA domain is shorter than previously reported since the C-terminal alpha-helix is not part of its minimum core. Key amino acids postulated to recognize phosphorylated residues are conserved. These data suggest that the kinesin FHA domains are functional domains involved in protein-protein interactions regulated by phosphorylation.

Microtubule-based transport systems in neurons: the roles of kinesins and dyneins

The large size and extreme polarization of neurons is crucial to their ability to communicate at long distances and to form the complex cellular networks of the nervous system. The size, shape, and compartmentalization of these specialized cells must be generated and supported by the cytoskeletal systems of intracellular transport. One of the major systems is the microtubule-based transport system along which kinesin and dynein motor proteins generate force and drive the traffic of many cellular components. This review describes our current understanding of the functions of kinesins and dyneins and how these motor proteins may be harnessed to generate some of the unique properties of neuronal cells.

GAKIN, a novel kinesin-like protein associates with the human homologue of the Drosophila discs large tumor suppressor in T lymphocytes

Reorganization of the cortical cytoskeleton is a hallmark of T lymphocyte activation. Upon binding to antigen presenting cells, the T cells rapidly undergo cytoskeletal re-organization thus forming a cap at the cell-cell contact site leading to receptor clustering, protein segregation, and cellular polarization. Previously, we reported cloning of the human lymphocyte homologue of the Drosophila Discs Large tumor suppressor protein (hDlg). Here we show that a novel protein termed GAKIN binds to the guanylate kinase-like domain of hDlg. Affinity protein purification, peptide sequencing, and cloning of GAKIN cDNA from Jurkat J77 lymphocytes identified GAKIN as a novel member of the kinesin superfamily of motor proteins. GAKIN mRNA is ubiquitously expressed, and the predicted amino acid sequence shares significant sequence similarity with the Drosophila kinesin-73 motor protein. GAKIN sequence contains a motor domain at the NH(2) terminus, a central stalk domain, and a putative microtubule-interacting sequence called the CAP-Gly domain at the COOH terminus. Among the MAGUK superfamily of proteins examined, GAKIN binds to the guanylate kinase-like domain of PSD-95 but not of p55. The hDlg and GAKIN are localized mainly in the cytoplasm of resting T lymphocytes, however, upon CD2 receptor cross-linking the hDlg can translocate to the lymphocyte cap. We propose that the GAKIN-hDlg interaction lays the foundation for a general paradigm of coupling MAGUKs to the microtubule-based cytoskeleton, and that this interaction may be functionally important for the intracellular trafficking of MAGUKs and associated protein complexes in vivo.

Microtubule motors in mitosis

The mitotic spindle uses microtubule-based motor proteins to assemble itself and to segregate sister chromatids. It is becoming clear that motors invoke several distinct mechanisms to generate the forces that drive mitosis. Moreover, in carrying out its function, the spindle appears to pass through a series of transient steady-state structures, each established by a delicate balance of forces generated by multiple complementary and antagonistic motors. Transitions from one steady state to the next can occur when a change in the activity of a subset of mitotic motors tips the balance.

The Xenopus chromokinesin Xkid is essential for metaphase chromosome alignment and must be degraded to allow anaphase chromosome movement

At anaphase, the linkage betweeh sister chromatids is dissolved and the separated sisters move toward opposite poles of the spindle. We developed a method to purify metaphase and anaphase chromosomes from frog egg extracts and identified proteins that leave chromosomes at anaphase using a new form of expression screening. This approach identified Xkid, a Xenopus homolog of human Kid (kinesin-like DNA binding protein) as a protein that is degraded in anaphase by ubiquitin-mediated proteolysis. Immunodepleting Xkid from egg extracts prevented normal chromosome alignment on the metaphase spindle. Adding a mild excess of wild-type or nondegradable Xkid to egg extracts prevented the separated chromosomes from moving toward the poles. We propose that Xkid provides the metaphase force that pushes chromosome arms toward the equator of the spindle and that its destruction is needed for anaphase chromosome movement.

Xkid, a chromokinesin required for chromosome alignment on the metaphase plate

Metaphase chromosome alignment is a key step of animal cell mitosis. The molecular mechanism leading to this equatorial positioning is still not fully understood. Forces exerted at kinetochores and on chromosome arms drive chromosome movements that culminate in their alignment on the metaphase plate. In this paper, we show that Xkid, a kinesin-like protein localized on chromosome arms, plays an essential role in metaphase chromosome alignment and in its maintenance. We propose that Xkid is responsible for the polar ejection forces acting on chromosome arms. Our results show that these forces are essential to ensure that kinetochores and chromosome arms align on a narrow equatorial plate during metaphase, a prerequisite for proper chromosome segregation.

A kinesin-related protein, KRP(180), positions prometaphase spindle poles during early sea urchin embryonic cell division

We have investigated the intracellular roles of an Xklp2-related kinesin motor, KRP(180), in positioning spindle poles during early sea urchin embryonic cell division using quantitative, real-time analysis. Immunolocalization reveals that KRP(180) concentrates on microtubules in the central spindle, but is absent from centrosomes. Microinjection of inhibitory antibodies and dominant negative constructs suggest that KRP(180) is not required for the initial separation of spindle poles, but instead functions to transiently position spindle poles specifically during prometaphase.

The road less traveled: emerging principles of kinesin motor utilization

Proteins of the kinesin superfamily utilize a conserved catalytic motor domain to generate movements in a wide variety of cellular processes. In this review, we discuss the rapid expansion in our understanding of how eukaryotic cells take advantage of these proteins to generate force and movement in diverse functional contexts. We summarize several recent examples revealing that the simplest view of a kinesin motor protein binding to and translocating a cargo along a microtubule track is inadequate. In fact, this paradigm captures only a small subset of the many ways in which cells harness force production of the generation of intracellular movements and functions. We also highlight several situations where the catalytic kinesin motor domain may not be used to generate movement, but instead may be used in other biochemical and functional contexts. Finally, we review some recent ideas about kinesin motor regulation, redundancy, and cargo attachment strategies.

Microtubule motors in spindle and chromosome motility

Many of the kinesin microtubule motor proteins discovered during the past 8-9 years have roles in spindle assembly and function or chromosome movement during meiosis or mitosis. The discovery of kinesin motor proteins with a clear involvement in spindle and chromosome motility, together with recent evidence that cytoplasmic dynein plays a role in chromosome distribution, has attracted great interest. The identification of microtubule motors that function in chromosome distribution represents a major advance in understanding the forces that underlie chromosome and spindle movements during cell division.

Motor and cargo interactions

The movements of intracellular cargo along microtubules within cells are often saltatory or of short duration. Further, calculations of the fraction of membrane vesicles that are moving at any period, indicate that active motor complexes are rare. From observations of normal vesicle traffic in cells, there appears to be position-dependent activation of motors and a balance of traffic in the inward and outward directions. In-vitro binding of motors to cargo is observed under many conditions but motility is not. Multi-component complexes appear to be involved in producing active organelle movements by a graded activation system that is highly localized in the cell. The basis of the activation of motility of the organelle motor complexes is still unknown but phosphorylation has been implicated in many systems. In the case of the motor-binding protein, kinectin, it has been linked to active organelle movements powered by conventional kinesin. From the coiled-coil structure of kinectin and the coiled-coil tail of kinesin, it is postulated that a coiled-coil assembly is responsible for the binding interaction. Many other cargoes are transported but the control of transport will be customized for each function, such as axonemal rafts or cytoskeletal complexes. Each function will have to be analyzed separately and motor activity will need to be integrated into the specific aspects of the function.

The vertebrate cell kinetochore and its roles during mitosis

A replicated chromosome possesses two discrete, complex, dynamic, macromolecular assemblies, known as kinetochores, that are positioned on opposite sides of the primary constriction of the chromosome. Here, the authors review how kinetochores control chromosome segregation during mitosis in vertebrates. They attach the chromosome to the opposing spindle poles by trapping the dynamic plus-ends of microtubules growing from the poles. They then produce much of the force for chromosome poleward motion, regulate when this force is applied, and act as a site for microtubule assembly and disassembly. Finally, they control the metaphase-anaphase transition by inhibiting chromatid separation until the chromatids are properly attached.