Phosphatase-regulated recruitment of the spindle- and kinetochore-associated (Ska) complex to kinetochores

HIV-1 Vif disrupts phosphatase feedback regulation at the kinetochore, leading to a pronounced pseudo-metaphase arrest

The human immunodeficiency virus type 1 (HIV-1) Virion Infectivity Factor (Vif) targets and degrades cellular APOBEC3 proteins, key regulators of intrinsic and innate antiretroviral immune responses, thereby facilitating HIV-1 infection. While Vif's role in degrading APOBEC3G is well-studied, Vif is also known to cause cell cycle arrest but the detailed nature of Vif's effects on the cell cycle has yet to be delineated. In this study, we employed high-temporal single-cell live imaging and super-resolution microscopy to monitor individual cells during Vif-induced cell cycle arrest. Our findings reveal that Vif does not affect the G2/M boundary as previously thought. Instead, Vif triggers a unique and robust pseudo-metaphase arrest, which is markedly distinct from the mild prometaphase arrest induced by the HIV-1 accessory protein, Vpr, known for modulating the cell cycle. During Vif-mediated arrest, chromosomes align properly to form a metaphase plate but later disassemble, resulting in polar chromosomes. Notably, unlike Vpr, Vif significantly reduces the levels of both Phosphatase 1 (PP1) and 2 (PP2) at kinetochores, which are key regulators of chromosome-microtubule interactions. These results reveal a novel function of Vif in kinetochore regulation that governs the spatial organization of chromosomes during mitosis.

SKA3 targeted therapies in cancer precision surgery: bridging bench discoveries to clinical applications - review article

Spindle and kinetochore-associated complex subunit 3 (SKA3) is a microtubule-binding subcomplex of the outer kinetochore, which plays a vital role in proper chromosomal segregation and cell division. Recently, SKA3 have been demonstrated its oncogenic role of tumorigenesis and development in cancers. In this review, the authors comprehensively deciphered SKA3 in human cancer from various aspects, including bibliometrics, pan-cancer analysis, and narrative summary. The authors also provided the top 10 predicted drugs targeting SKA3. The authors proposed that SKA3 was a potential target and brought new therapeutic opportunities for cancer patients.

Prognostic and immunological values of SKA3 for overall survival in lung adenocarcinoma and its RNA binding protein involved mechanisms

This article aimed to investigate the correlations among SKA3 expression and prognosis, clinical relevance, tumor immunity, and RNA-binding protein (RBP)-involved mechanisms for overall survival (OS) in lung adenocarcinoma (LUAD). To explore the SKA3 expression level in LUAD by analyzing the genomic data as well as related clinical characteristics from the database of TCGA. Nomogram and gene set enrichment analysis (GSEA) were applied, respectively, to evaluate the performance of SKA3 in LUAD. Correlations between SKA3 and immunity and RBP-involved mechanisms were also performed. SKA3 had a higher expression level in LUAD samples than in adjacent normal lung samples, with shorter survival times in the high-SKA3-expressed LUAD subgroup (P < 0.05). qRT-PCR results remained consistent (P < 0.05). Uni-/multivariate Cox analyses revealed that SKA3 could have independent prognostic ability for LUAD (both P < 0.05). The nomogram model constructed with clinical pathological parameters and SKA3 expression levels predicted OS rates for LUAD and GSEA revealed SKA3-related pathways. In aspects of tumor immunity, SKA3 was significantly involved with tumor neoantigen burden, tumor mutational burden, immune cell pathways, and immune checkpoint inhibitor (ICI) molecules (all P < 0.05). The CellMiner database also found significant correlations between SKA3 and the antitumor drug sensitivity of chemotherapy, fenretinide, and PX-316. Besides, a total of nine LncRNA/RBP/SKA3 networks were revealed in LUAD for their RBP-involved mechanisms. SKA3 could serve as a potential biomarker for OS prognosis and immunotherapy in LUAD. LncRNA/RBP/SKA3 networks were identified in LUAD for their RBP-involved mechanisms, paving the way for further experimental verifications.

Immunological role and prognostic value of the SKA family in pan-cancer analysis

Background

The spindle and kinetochore associated (SKA) complex, which plays important roles in proper chromosome segregation during mitosis by maintaining the stabilization of kinetochore-spindle microtubule attachment during mitosis, has recently been reported to exert regulatory effects on the initiation and progression of various human cancer types. Nevertheless, the prognostic significance and immune infiltration of the SKA family across cancers have not been well elucidated.

Methods

Using data from three large public datasets, including The Cancer Genome Atlas, Genotype-Tissue Expression, and Gene Expression Omnibus databases, a novel scoring system (termed the SKA score) was developed to quantify the SKA family level across cancers. We then evaluated the prognostic impact of the SKA score on survival and assessed the effect of the SKA score on immunotherapy at the pan-cancer level using multiomics bioinformatic analyses. The correlation of the SKA score and the tumor microenvironment (TME) was also explored in depth. Potential small molecular compounds and chemotherapeutic agents were assessed by CTRP and GDSC analyses. Immunohistochemistry was performed to verify the expression of the SKA family genes.

Results

Our results demonstrated a close correlation between the SKA score and tumor development and prognosis in multiple cancers. The SKA score was positively related to cell cycle pathways and DNA replication across cancers, such as E2F targets, the G2M checkpoint, MYC targets V1/V2, mitotic spindles and DNA repair. Additionally, the SKA score was negatively related to the infiltration of various immune cells with antitumor effects in the TME. In addition, the potential value of the SKA score was identified to predict immunotherapy response for melanoma and bladder cancer. We also demonstrated a correlation between SKA1/2/3 and the response to drug treatment across cancers and the promising potential of the SKA complex and its genes as therapeutic targets in cancer. Immunohistochemistry demonstrated that the expression differences of SKA1/2/3 were significant between the breast cancer group and the paracancerous group.

Conclusion

The SKA score plays a critical role in 33 cancer types and is highly related to tumor prognosis. Patients with elevated SKA scores have a clear immunosuppressive TME. The SKA score may serve as a predictor for patients receiving anti-PD-1/L1 therapy.

SKA3 is a prognostic biomarker and associated with immune infiltration in bladder cancer

BACKGROUND

Spindle and kinetochore‑associated complex subunit 3 (SKA3) has recently been considered a key regulator of carcinogenesis. However, the connection between SKA3 and immune cell infiltration remains unknown.

METHODS

The current study investigated the expression mode, prognostic effect, and functional role of SKA3 in different tumors, particularly bladder cancer using numerous databases, comprising TIMER, GEPIA, HPA, UALCAN, PrognoScan, and Kaplan-Meier Plotter. Differentially expressed gene and enrichment analyses were implemented on SKA3 using R packages "edgR" and "clusterProfiler". Immunohistochemistry was further used to validate the expression of SKA3 gene in bladder cancer. Following that, the relevance of SKA3 expression to immune infiltration level in bladder cancer was evaluated using TIMER.

RESULTS

Overall, the level of SKA3 expression in tumor tissue significantly increased than in normal tissue. In bladder cancer and other tumors, patients with high SKA3 expression levels had worse overall survival (OS) (p = 0.016), disease-specific survival (DSS) (p = 0.00004), and disease-free survival (DFS) (p = 0.032). Additionally, the major molecular functions for SKA3 included nuclear division, mitotic nuclear division, mitotic sister chromatid segregation, humoral immune response, and cell chemotaxis. Additionally, SKA3 expression was found to be positively associated with enhanced M2 macrophage and T helper (Th) 2 cell infiltration in bladder cancer.

CONCLUSIONS

Our study implies that SKA3 contributes to M2 macrophage and Th2 cell polarization by acting as an oncogene in bladder cancer. SKA3 might be a novel biomarker for evaluating prognosis and immune infiltration in bladder cancer.

Overexpression of SKA Complex Is Associated With Poor Prognosis in Gliomas

The spindle and kinetochore-associated complex is composed of three members: SKA1, SKA2, and SKA3. It is necessary for stabilizing spindle microtubules attaching to kinetochore (KT) in the middle stage of mitosis. The SKA complex is associated with poor prognosis in several human cancers. However, the role of SKA complex in rare malignant diseases, such as gliomas, has not been fully investigated. We investigated several databases, including Oncomine, UALCAN, and cBioPortal to explore the expression profile and prognostic significance of SKA complex in patients with gliomas. Gene ontology and Kyoto Encyclopedia of Genes and Genome pathways were used to analyze the potential enriched pathways. The genes co-expressed with SKA complex were identified and used for developing a protein-protein interaction (PPI) network using the STRING database. We found a significant overexpression of the mRNA levels of SKA1, SKA2, and SKA3 in patients with glioma patients. Higher expression of SKA1 and SKA3, but not SKA2, was significantly correlated with shorter overall survival of patients with glioma. In glioma, SKA complex was found to be involved in nuclear division, chromosome segregation, and DNA replication. The results of PPI network identified 10 hub genes (CCNB2, UBE2C, BUB1B, TPX2, CCNA2, CCNB1, MELK, TOP2A, PBK, and KIF11), all of which were overexpressed and negatively associated with prognosis of patients with glioma. In conclusion, our study sheds new insights into the biological role and prognostic significance of SKA complex in glioma.

Transcript levels of spindle and kinetochore-associated complex 1/3 as prognostic biomarkers correlated with immune infiltrates in hepatocellular carcinoma

The spindle and kinetochore-associated protein complex (Ska) is an essential component in chromosome segregation. It comprises three proteins (Ska1, Ska2, and Ska3) with theorized roles in chromosomal instability and tumor development, and its overexpression has been widely reported in a variety of tumors. However, the prognostic significance and immune infiltration of Ska proteins in hepatocellular carcinoma (HCC) are not completely understood. The bioinformatics tools Oncomine, UALCAN, gene expression profiling interactive analysis 2 (GEPIA2), cBioPortal, GeneMANIA, Metascape, and TIMER were used to analyze differential expression, prognostic value, genetic alteration, and immune cell infiltration of the Ska protein complex in HCC patients. We found that the mRNA expression of the Ska complex was markedly upregulated in HCC. High expression of the Ska complex is closely correlated with tumor stage, patient race, tumor grade, and TP53 mutation status. In addition, high expression of the Ska complex was significantly correlated with poor disease-free survival, while the high expression levels of Ska1 and Ska3 were associated with shorter overall survival. The biological functions of the Ska complex in HCC primarily involve the amplification of signals from kinetochores, the mitotic spindle, and (via a MAD2 invasive signal) unattached kinetochores. Furthermore, the expression of the complex was positively correlated with tumor-infiltrating cells. These results may provide new insights into the development of immunotherapeutic targets and prognostic biomarkers for HCC.

Spindle and kinetochore‑associated complex subunit 3 accelerates breast cancer cell proliferation and invasion through the regulation of Akt/Wnt/β-catenin signaling

PURPOSE

Spindle and kinetochore‑associated complex subunit 3 (SKA3) has recently been identified as a novel regulator of carcinogenesis in multiple types of cancers. However, the function and potential regulatory mechanisms of SKA3 in breast cancer remain poorly understood. The present study was designed to gain a detailed relevance of SKA3 in breast cancer.

METHODS

Expression of SKA3 in breast cancer was examined via real-time quantitative PCR, western blotting and immunohistochemistry analysis. Malignant behaviors of breast cancer cells were investigated via cell counting kit-8, cell apoptosis, and transwell invasion assays. The activity of Wnt/β-catenin signaling was monitored via luciferase reporter assay. The tumorigenicity of breast cancer cells in vivo was assessed via xenograft tumor assay.

RESULTS

SKA3 expression was elevated in breast cancer tissue and was correlated with shorter survival rates in breast cancer patients. Knockdown of SKA3 caused marked reductions in cellular proliferation and invasion in breast cancer cells, whereas SKA3 overexpression accelerated proliferation and invasion. Knockdown of SKA3 resulted in decreased Akt and glycogen synthase kinase-3β phosphorylation, and decreased expression of active β-catenin, which lead to the inactivation of Wnt/β-catenin signaling. Inhibition of Akt significantly reversed the SKA3 overexpression-induced activation of Wnt/β-catenin signaling. Inhibition of Wnt/β-catenin signaling markedly abrogated SKA3 overexpression-induced tumor-promotion effects, while re-activation of Wnt/β-catenin signaling significantly reversed SKA3 knockdown-mediated tumor-inhibition effects. Knockdown of SKA3 resulted in a significant decrease in breast cancer tumor formation in vivo.

CONCLUSIONS

SKA3 accelerates proliferation and invasion in breast cancer through the modulation of Akt/Wnt/β-catenin signaling.

Multisite phosphorylation determines the formation of Ska-Ndc80 macro-complexes that are essential for chromosome segregation during mitosis

The human Ska complex (Ska) localizing to both spindle microtubules and kinetochores is essential for proper chromosome segregation during mitosis. Although several mechanisms have been proposed to explain how Ska is recruited to kinetochores, it is still not fully understood. By analyzing Ska3 phosphorylation, we identified six critical Cdk1 sites, including the previously identified Thr358 and Thr360. Mutations of these sites to phospho-deficient alanine (6A) in cells completely abolished Ska3 localization to kinetochores and Ska functions in chromosome segregation. In vitro, Cdk1 phosphorylation on Ska enhanced WT, not phospho-deficient 6A, binding to Ndc80C. Strikingly, the phosphomimetic Ska 6D complex formed a stable macro-complex with Ndc80C, but Ska WT failed to do so. These results suggest that multisite Cdk1 phosphorylation-enabled Ska–Ndc80 binding is decisive for Ska localization to kinetochores and its functions. Moreover, we found that Ska decrease at kinetochores triggered by the microtubule-depolymerizing drug nocodazole is independent of Aurora B but can be overridden by Ska3 overexpression, suggestive of a role of spindle microtubules in promoting Ska kinetochore recruitment. Thus, based on the current and previous results, we propose that multisite Cdk1 phosphorylation is critical for the formation of Ska–Ndc80 macro-complexes that are essential for chromosome segregation.

ZWINT: A potential therapeutic biomarker in patients with glioblastoma correlates with cell proliferation and invasion

Glioblastoma (GBM) is the most aggressive primary intracranial tumor in adults. Chemoradiotherapy resistance and recurrence after surgery are the main malignant progression factors, leading to a high mortality rate. Therefore, the exploration of novel biomarkers and molecular mechanisms of GBM is urgent. Differentially expressed genes (DEGs) of GBM were screened in a TCGA dataset. Homo sapiens ZW10 interacting kinetochore protein (ZWINT) was found to be upregulated in GBM, which was confirmed by immunohistochemical staining of a tissue microarray. Gene Ontology (GO) annotation and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis were performed using the Database for Annotation, Visualization and Integrated Discovery (DAVID) database. A protein-protein interaction (PPI) network was established by the STRING database, and hub genes were visualized by Cytoscape. The correlation results were verified with the GSE15824 dataset. Bioinformatic analysis confirmed that ZWINT was significantly positively correlated with kinetochore protein NDC80 homolog (NDC80), serine/threonine-protein kinase PLK1 (PLK1) and spindle and kinetochore associated complex subunit 1 (SKA1) and together are involved in regulating mitosis and the cell cycle of GBM. ZWINT expression was knocked down in U251 and U87 MG GBM cells by lentiviral vectors carrying a small hairpin RNA (shRNA) targeting ZWINT. The effect of ZWINT silencing on cell proliferation, invasion and apoptosis was determined by the Celigo assay, MTT assay, Transwell assay, flow cytometry and caspase-3/7 assay in vitro. A subcutaneous xenograft tumor model was established to explore the influence of ZWINT knockdown on GBM growth in vivo. Our preliminary study demonstrated that ZWINT knockdown effectively inhibited proliferation and invasion and induced apoptosis of GBM cells and notably suppressed GBM growth in vivo. Therefore, we speculate that ZWINT may be a potential therapeutic biomarker for GBM, with NDC80 and PLK1 conjointly involved in regulating cell division and the mitotic cell cycle.

SKA1/2/3 serves as a biomarker for poor prognosis in human lung adenocarcinoma

Background

Spindle and kinetochore associated complex subunit 1/2/3 (SKA1/2/3), which stabilized spindle microtubules attaching to kinetochore (KT) in the middle stage of mitosis, were dysregulated, and closely related to prognosis in several malignant tumors. Nevertheless, the potential clinical value of SKA1/2/3, especially in terms of prognosis and development of NSCLC, had not been fully elucidated.

Methods

ONCOMINE, GEPIA, UALCAN, TCGA, STRING and other databases were used to analyze the expression of SKA1/2/3 in patients with lung adenocarcinoma (LUAD) and its clinical value, and to explore the possible regulatory mechanism of SKA in the occurrence and development of LUAD.

Results

In patients with LUAD, SKA1/2/3 mRNA expression level was significantly up-regulated, and AUC was 0.9558, 0.7034 and 0.9775, respectively. Increased SKA 1/2/3 expression was associated with smoking, tissue typing, and poor prognosis in LUAD patients. Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genome (KEGG) showed that SKA1/2/3 was mainly enriched in DNA replication, cell cycle, homologous recombination, p53 signaling pathway, etc. Hub genes in protein-protein interactions are CDK1, BUB1, CCNA2, CDC20, CCNB2, CCNB1, BUB1B, AURKB, TOP2A and MAD2L1. Hub gene expression in LUAD is increased, and its increased expression is related to poor prognosis of LUAD patients. Finally, the expression of SKA1/2/3 and its correlation with clinicopathological features were verified in 30 clinical LUAD samples.

Conclusions

SKA1/2/3 may serve as a potential prognostic biomarker and target for LUAD. In addition, SKA 1/2/3 may affect the prognosis of LUAD through DNA replication, cell cycle, homologous recombination and p53 signaling pathway.

Molecular determinants of the Ska-Ndc80 interaction and their influence on microtubule tracking and force-coupling

Errorless chromosome segregation requires load-bearing attachments of the plus ends of spindle microtubules to chromosome structures named kinetochores. How these end-on kinetochore attachments are established following initial lateral contacts with the microtubule lattice is poorly understood. Two microtubule-binding complexes, the Ndc80 and Ska complexes, are important for efficient end-on coupling and may function as a unit in this process, but precise conditions for their interaction are unknown. Here, we report that the Ska-Ndc80 interaction is phosphorylation-dependent and does not require microtubules, applied force, or several previously identified functional determinants including the Ndc80-loop and the Ndc80-tail. Both the Ndc80-tail, which we reveal to be essential for microtubule end-tracking, and Ndc80-bound Ska stabilize microtubule ends in a stalled conformation. Modulation of force-coupling efficiency demonstrates that the duration of stalled microtubule disassembly predicts whether a microtubule is stabilized and rescued by the kinetochore, likely reflecting a structural transition of the microtubule end.

Kinetochore Recruitment of the Spindle and Kinetochore-Associated (Ska) Complex Is Regulated by Centrosomal PP2A in Caenorhabditis elegans

During mitosis, kinetochore-microtubule interactions ensure that chromosomes are accurately segregated to daughter cells. RSA-1 (regulator of spindle assembly-1) is a regulatory B″ subunit of protein phosphatase 2A that was previously proposed to modulate microtubule dynamics during spindle assembly. We have identified a genetic interaction between the centrosomal protein, RSA-1, and the spindle- and kinetochore-associated (Ska) complex in Caenorhabditis elegans In a forward genetic screen for suppressors of rsa-1(or598) embryonic lethality, we identified mutations in ska-1 and ska-3 Loss of SKA-1 and SKA-3, as well as components of the KMN (KNL-1/MIS-12/NDC-80) complex and the microtubule end-binding protein EBP-2, all suppressed the embryonic lethality of rsa-1(or598) These suppressors also disrupted the intracellular localization of the Ska complex, revealing a network of proteins that influence Ska function during mitosis. In rsa-1(or598) embryos, SKA-1 is excessively and prematurely recruited to kinetochores during spindle assembly, but SKA-1 levels return to normal just prior to anaphase onset. Loss of the TPX2 homolog, TPXL-1, also resulted in overrecruitment of SKA-1 to the kinetochores and this correlated with the loss of Aurora A kinase on the spindle microtubules. We propose that rsa-1 regulates the kinetochore localization of the Ska complex, with spindle-associated Aurora A acting as a potential mediator. These data reveal a novel mechanism of protein phosphatase 2A function during mitosis involving a centrosome-based regulatory mechanism for Ska complex recruitment to the kinetochore.

SKA3 promotes cell proliferation and migration in cervical cancer by activating the PI3K/Akt signaling pathway

Background

Cervical cancer (CC) is one of the most common cancers among females worldwide. Spindle and kinetochore-associated complex subunit 3 (SKA3), located on chromosome 13q, was identified as a novel gene involved in promoting malignant transformation in cancers. However, the function and underlying mechanisms of SKA3 in CC remain unknown. Using the Oncomine database, we found that expression of SKA3 mRNA is higher in CC tissues than in normal tissues and is linked with poor prognosis.

Methods

In our study, immunohistochemistry showed increased expression of SKA3 in CC tissues. The effect of SKA3 on cell proliferation and migration was evaluated by CCK8, clone formation, Transwell and wound-healing assays in HeLa and SiHa cells with stable SKA3 overexpression and knockdown. In addition, we established a xenograft tumor model in vivo.

Results

SKA3 overexpression promoted cell proliferation and migration and accelerated tumor growth. We further identified that SKA3 is involved in regulating cell cycle progression and the PI3K/Akt signaling pathway via RNA-sequencing (RNA-Seq) and gene set enrichment analyses. Western blotting results revealed that SKA3 overexpression increased levels of p-Akt, cyclin E2, CDK2, cyclin D1, CDK4, E2F1 and p-Rb in HeLa cells. Additionally, the use of an Akt inhibitor (GSK690693) significantly reversed the cell proliferation capacity induced by SKA3 overexpression in HeLa cells.

Conclusions

We suggest that SKA3 overexpression contributes to CC cell growth and migration by promoting cell cycle progression and activating the PI3K-Akt signaling pathway, which may provide potential novel therapeutic targets for CC treatment.

SKA2/FAM33A: A novel gene implicated in cell cycle, tumorigenesis, and psychiatric disorders

SKA2 (spindle and KT associated 2), also referred to as FAM33A (family with sequence similarity 33, member A), is a recently identified gene involved in cell cycle regulation, and growing evidence is implicating its roles in tumorigenesis and psychiatric disorders. It has been demonstrated that SKA2, along with its coworkers SKA1 and SKA3, constitutes the SKA complex which plays a critical role in the maintenance of the metaphase plate and/or spindle checkpoint silencing during mitosis. SKA2 is over-expressed both in cancer cell lines and clinical samples including small cell lung cancer and breast cancer, whereas downregulation of SKA2 is associated with depression and suicidal ideation. The expression of SKA2 is regulated by transcription factors including NF-κΒ and CREB, miRNAs as well as DNA methylation. In this review, we provide an overview of studies that reveal SKA2 gene and protein characteristics as well as physiological function, with a special focus on its transcription regulatory mechanisms, and also provide a summary regarding the translational opportunity of the SKA2 gene as a clinical biomarker for cancers and psychiatric disorders.

The kinetochore-microtubule interface at a glance

Accurate chromosome segregation critically depends on the formation of attachments between microtubule polymers and each sister chromatid. The kinetochore is the macromolecular complex that assembles at the centromere of each chromosome during mitosis and serves as the link between the DNA and the microtubules. In this Cell Science at a Glance article and accompanying poster, we discuss the activities and molecular players that are involved in generating kinetochore-microtubule attachments, including the initial stages of lateral kinetochore-microtubule interactions and maturation to stabilized end-on attachments. We additionally explore the features that contribute to the ability of the kinetochore to track with dynamic microtubules. Finally, we examine the contributions of microtubule-associated proteins to the organization and stabilization of the mitotic spindle and the control of microtubule dynamics.

Kinase and Phosphatase Cross-Talk at the Kinetochore

Multiple kinases and phosphatases act on the kinetochore to control chromosome segregation: Aurora B, Mps1, Bub1, Plk1, Cdk1, PP1, and PP2A-B56, have all been shown to regulate both kinetochore-microtubule attachments and the spindle assembly checkpoint. Given that so many kinases and phosphatases converge onto two key mitotic processes, it is perhaps not surprising to learn that they are, quite literally, entangled in cross-talk. Inhibition of any one of these enzymes produces secondary effects on all the others, which results in a complicated picture that is very difficult to interpret. This review aims to clarify this picture by first collating the direct effects of each enzyme into one overarching schematic of regulation at the Knl1/Mis12/Ndc80 (KMN) network (a major signaling hub at the outer kinetochore). This schematic will then be used to discuss the implications of the cross-talk that connects these enzymes; both in terms of why it may be needed to produce the right type of kinetochore signals and why it nevertheless complicates our interpretations about which enzymes control what processes. Finally, some general experimental approaches will be discussed that could help to characterize kinetochore signaling by dissociating the direct from indirect effect of kinase or phosphatase inhibition in vivo. Together, this review should provide a framework to help understand how a network of kinases and phosphatases cooperate to regulate two key mitotic processes.

Multitasking Ska in Chromosome Segregation: Its Distinct Pools Might Specify Various Functions

The human spindle and kinetochore associated (Ska) complex is required for proper mitotic progression. Extensive studies have demonstrated its important functions in both stable kinetochore-microtubule interactions and spindle checkpoint silencing. We suggest a model to explain how various Ska functions might be fulfilled by distinct pools of Ska at kinetochores. The Ndc80-loop pool of Ska is recruited by the Ndc80 loop, or together with some of its flanking sequences, and the recruitment is also dependent on Cdk1-mediated Ska3 phosphorylation. This pool seems to play a more important role in silencing the spindle checkpoint than stabilizing kinetochore-microtubule interactions. In contrast, the Ndc80-N-terminus pool of Ska is recruited by the N-terminal domains of Ndc80 and appears to be more important for stabilizing kinetochore-microtubule interactions. Here, we review and discuss the evidence that supports this model and suggest further experiments to test the functioning mechanisms of the Ska complex.