The C terminus of mitosin is essential for its nuclear localization, centromere/kinetochore targeting, and dimerization

Centromere Protein F in Tumor Biology: Cancer's Achilles Heel

BACKGROUND

Centromere protein F (CENP-F) is an important nuclear matrix protein that regulates mitosis and the cell cycle, and plays a crucial role in recruiting spindle checkpoint proteins to maintain the accuracy of chromosome segregation. Studies have shown that CENP-F is closely involved in the pathogenesis of various diseases, particularly in the development and progression of malignant tumors, where it exhibits significant oncogenic activity.

OBJECTIVE

This review aims to systematically summarize the molecular structure, subcellular localization, expression regulation, intracellular transport mechanisms, biological functions, and carcinogenic mechanisms of CENP-F, as well as explore its potential value in cancer diagnosis and therapy.

METHODS

A comprehensive review and analysis of domestic and international research literature related to CENP-F were conducted, focusing on its role in tumorigenesis, development, and as a therapeutic target.

RESULTS

CENP-F acts as an oncogene and can maintain or promote the malignant phenotype of tumor cells through multiple mechanisms, including regulating signaling pathways related to cell proliferation and apoptosis, promoting metabolic reprogramming, angiogenesis, and tumor cell invasion and metastasis. Additionally, it plays an important role in the immune microenvironment and drug resistance regulation.

CONCLUSION

CENP-F plays a key, multidimensional role in tumor biology and is a promising therapeutic target for cancer treatment. Further exploration of the core pathways through which CENP-F regulates tumorigenesis and its potential for clinical translation is needed.

Neotelomeres and Telomere-Spanning Chromosomal Arm Fusions in Cancer Genomes Revealed by Long-Read Sequencing

Alterations in the structure and location of telomeres are key events in cancer genome evolution. However, previous genomic approaches, unable to span long telomeric repeat arrays, could not characterize the nature of these alterations. Here, we applied both long-read and short-read genome sequencing to assess telomere repeat-containing structures in cancers and cancer cell lines. Using long-read genome sequences that span telomeric repeat arrays, we defined four types of telomere repeat variations in cancer cells: neotelomeres where telomere addition heals chromosome breaks, chromosomal arm fusions spanning telomere repeats, fusions of neotelomeres, and peri-centromeric fusions with adjoined telomere and centromere repeats. Analysis of lung adenocarcinoma genome sequences identified somatic neotelomere and telomere-spanning fusion alterations. These results provide a framework for systematic study of telomeric repeat arrays in cancer genomes, that could serve as a model for understanding the somatic evolution of other repetitive genomic elements.

Crowning the Kinetochore: The Fibrous Corona in Chromosome Segregation

The kinetochore is at the heart of chromosome segregation in mitosis and meiosis. Rather than a static linker complex for chromatin and spindle microtubules, it is highly dynamic in composition, size, and shape. While known for decades that it can expand and grow a fibrous meshwork known as the corona, it was until recently unclear what constitutes this 'crown' and what its relevance is for kinetochore function. Here, we highlight recent discoveries in fibrous corona biology, and place them in the context of the processes that orchestrate high-fidelity chromosome segregation.

CENP-F stabilizes kinetochore-microtubule attachments and limits dynein stripping of corona cargoes

Accurate chromosome segregation demands efficient capture of microtubules by kinetochores and their conversion to stable bioriented attachments that can congress and then segregate chromosomes. An early event is the shedding of the outermost fibrous corona layer of the kinetochore following microtubule attachment. Centromere protein F (CENP-F) is part of the corona, contains two microtubule-binding domains, and physically associates with dynein motor regulators. Here, we have combined CRISPR gene editing and engineered separation-of-function mutants to define how CENP-F contributes to kinetochore function. We show that the two microtubule-binding domains make distinct contributions to attachment stability and force transduction but are dispensable for chromosome congression. We further identify a specialized domain that functions to limit the dynein-mediated stripping of corona cargoes through a direct interaction with Nde1. This antagonistic activity is crucial for maintaining the required corona composition and ensuring efficient kinetochore biorientation.

The Mitotic Apparatus and Kinetochores in Microcephaly and Neurodevelopmental Diseases

Regulators of mitotic division, when dysfunctional or expressed in a deregulated manner (over- or underexpressed) in somatic cells, cause chromosome instability, which is a predisposing condition to cancer that is associated with unrestricted proliferation. Genes encoding mitotic regulators are growingly implicated in neurodevelopmental diseases. Here, we briefly summarize existing knowledge on how microcephaly-related mitotic genes operate in the control of chromosome segregation during mitosis in somatic cells, with a special focus on the role of kinetochore factors. Then, we review evidence implicating mitotic apparatus- and kinetochore-resident factors in the origin of congenital microcephaly. We discuss data emerging from these works, which suggest a critical role of correct mitotic division in controlling neuronal cell proliferation and shaping the architecture of the central nervous system.

Regulation of Cenp-F localization to nuclear pores and kinetochores

In metazoans, the assembly of kinetochores on centrometric chromatin and the dismantling of nuclear pore complexes are processes that have to be tightly coordinated to ensure the proper assembly of the mitotic spindle and a successful mitosis. It is therefore noteworthy that these two macromolecular assemblies share a subset of constituents. One of these multifaceted components is Cenp-F, a protein implicated in cancer and developmental pathologies. During the cell cycle, Cenp-F localizes in multiple cellular structures including the nuclear envelope in late G2/early prophase and kinetochores throughout mitosis. We recently characterized the molecular determinants of Cenp-F interaction with Nup133, a structural nuclear pore constituent. In parallel with two other independent studies, we further elucidated the mechanisms governing Cenp-F kinetochore recruitment that mainly relies on its interaction with Bub1, with redundant contribution of Cenp-E upon acute microtubule depolymerisation. Here we synthesize the current literature regarding the dual location of Cenp-F at nuclear pores and kinetochores and extend our discussion to the regulation of these NPC and kinetochore localizations by mitotic kinase and spindle microtubules.

The kinetochore proteins CENP-E and CENP-F directly and specifically interact with distinct BUB mitotic checkpoint Ser/Thr kinases

The segregation of chromosomes during cell division relies on the function of the kinetochores, protein complexes that physically connect chromosomes with microtubules of the spindle. The metazoan proteins, centromere protein E (CENP-E) and CENP-F, are components of a fibrous layer of mitotic kinetochores named the corona. Several of their features suggest that CENP-E and CENP-F are paralogs: they are very large (comprising ∼2700 and 3200 residues, respectively), contain abundant predicted coiled-coil structures, are C-terminally prenylated, and are endowed with microtubule-binding sites at their termini. Moreover, CENP-E contains an ATP-hydrolyzing motor domain that promotes microtubule plus end–directed motion. Here, we show that both CENP-E and CENP-F are recruited to mitotic kinetochores independently of the main corona constituent, the Rod/Zwilch/ZW10 (RZZ) complex. We identified specific interactions of CENP-F and CENP-E with budding uninhibited by benzimidazole 1 (BUB1) and BUB1-related (BUBR1) mitotic checkpoint Ser/Thr kinases, respectively, paralogous proteins involved in mitotic checkpoint control and chromosome alignment. Whereas BUBR1 was dispensable for kinetochore localization of CENP-E, BUB1 was stringently required for CENP-F localization. Through biochemical reconstitution, we demonstrated that the CENP-E/BUBR1 and CENP-F/BUB1 interactions are direct and require similar determinants, a dimeric coiled-coil in CENP-E or CENP-F and a kinase domain in BUBR1 or BUB1. Our findings are consistent with the existence of structurally similar BUB1/CENP-F and BUBR1/CENP-E complexes, supporting the notion that CENP-E and CENP-F are evolutionarily related.

Identification of Cyclin-dependent Kinase 1 Specific Phosphorylation Sites by an In Vitro Kinase Assay

Cyclin-dependent kinase 1 (Cdk1) is a master controller for the cell cycle in all eukaryotes and phosphorylates an estimated 8 - 13% of the proteome; however, the number of identified targets for Cdk1, particularly in human cells is still low. The identification of Cdk1-specific phosphorylation sites is important, as they provide mechanistic insights into how Cdk1 controls the cell cycle. Cell cycle regulation is critical for faithful chromosome segregation, and defects in this complicated process lead to chromosomal aberrations and cancer. Here, we describe an in vitro kinase assay that is used to identify Cdk1-specific phosphorylation sites. In this assay, a purified protein is phosphorylated in vitro by commercially available human Cdk1/cyclin B. Successful phosphorylation is confirmed by SDS-PAGE, and phosphorylation sites are subsequently identified by mass spectrometry. We also describe purification protocols that yield highly pure and homogeneous protein preparations suitable for the kinase assay, and a binding assay for the functional verification of the identified phosphorylation sites, which probes the interaction between a classical nuclear localization signal (cNLS) and its nuclear transport receptor karyopherin α. To aid with experimental design, we review approaches for the prediction of Cdk1-specific phosphorylation sites from protein sequences. Together these protocols present a very powerful approach that yields Cdk1-specific phosphorylation sites and enables mechanistic studies into how Cdk1 controls the cell cycle. Since this method relies on purified proteins, it can be applied to any model organism and yields reliable results, especially when combined with cell functional studies.

Disentangling the molecular determinants for Cenp-F localization to nuclear pores and kinetochores

Cenp-F is a multifaceted protein implicated in cancer and developmental pathologies. The Cenp-F C-terminal region contains overlapping binding sites for numerous proteins that contribute to its functions throughout the cell cycle. Here, we focus on the nuclear pore protein Nup133 that interacts with Cenp-F both at nuclear pores in prophase and at kinetochores in mitosis, and on the kinase Bub1, known to contribute to Cenp-F targeting to kinetochores. By combining in silico structural modeling and yeast two-hybrid assays, we generate an interaction model between a conserved helix within the Nup133 β-propeller and a short leucine zipper-containing dimeric segment of Cenp-F. We thereby create mutants affecting the Nup133/Cenp-F interface and show that they prevent Cenp-F localization to the nuclear envelope, but not to kinetochores. Conversely, a point mutation within an adjacent leucine zipper affecting the kinetochore targeting of Cenp-F KT-core domain impairs its interaction with Bub1, but not with Nup133, identifying Bub1 as the direct KT-core binding partner of Cenp-F. Finally, we show that Cenp-E redundantly contributes together with Bub1 to the recruitment of Cenp-F to kinetochores.

Mechanism for G2 phase-specific nuclear export of the kinetochore protein CENP-F

Centromere protein F (CENP-F) is a component of the kinetochore and a regulator of cell cycle progression. CENP-F recruits the dynein transport machinery and orchestrates several cell cycle-specific transport events, including transport of the nucleus, mitochondria and chromosomes. A key regulatory step for several of these functions is likely the G2 phase-specific export of CENP-F from the nucleus to the cytosol, where the cytoplasmic dynein transport machinery resides; however, the molecular mechanism of this process is elusive. Here, we have identified 3 phosphorylation sites within the bipartite classical nuclear localization signal (cNLS) of CENP-F. These sites are specific for cyclin-dependent kinase 1 (Cdk1), which is active in G2 phase. Phosphomimetic mutations of these residues strongly diminish the interaction of the CENP-F cNLS with its nuclear transport receptor karyopherin α. These mutations also diminish nuclear localization of the CENP-F cNLS in cells. Notably, the cNLS is phosphorylated in the -1 position, which is important to orient the adjacent major motif for binding into its pocket on karyopherin α. We propose that localization of CENP-F is regulated by a cNLS, and a nuclear export pathway, resulting in nuclear localization during most of interphase. In G2 phase, the cNLS is weakened by phosphorylation through Cdk1, likely resulting in nuclear export of CENP-F via the still active nuclear export pathway. Once CENP-F resides in the cytosol, it can engage in pathways that are important for cell cycle progression, kinetochore assembly and the faithful segregation of chromosomes into daughter cells.

Strømme Syndrome Is a Ciliary Disorder Caused by Mutations in CENPF

Strømme syndrome was first described by Strømme et al. (1993) in siblings presenting with "apple peel" type intestinal atresia, ocular anomalies and microcephaly. The etiology remains unknown to date. We describe the long-term clinical follow-up data for the original pair of siblings as well as two previously unreported siblings with a severe phenotype overlapping that of the Strømme syndrome including fetal autopsy results. Using family-based whole-exome sequencing, we identified truncating mutations in the centrosome gene CENPF in the two nonconsanguineous Caucasian sibling pairs. Compound heterozygous inheritance was confirmed in both families. Recently, mutations in this gene were shown to cause a fetal lethal phenotype, the phenotype and functional data being compatible with a human ciliopathy [Waters et al., 2015]. We show for the first time that Strømme syndrome is an autosomal-recessive disease caused by mutations in CENPF that can result in a wide phenotypic spectrum.

Centromere protein F includes two sites that couple efficiently to depolymerizing microtubules

Both N- and C-terminal microtubule (MT)-binding domains of CENP-F can follow depolymerizing MT ends while bearing a significant load, and the N-terminal domain prefers binding to curled oligomers of tubulin relative to MT walls by approximately fivefold, suggesting that CENP-F may play a role in the firm bonds that form between kinetochores and the flared plus ends of dynamic MTs.

Firm attachments between kinetochores and dynamic spindle microtubules (MTs) are important for accurate chromosome segregation. Centromere protein F (CENP-F) has been shown to include two MT-binding domains, so it may participate in this key mitotic process. Here, we show that the N-terminal MT-binding domain of CENP-F prefers curled oligomers of tubulin relative to MT walls by approximately fivefold, suggesting that it may contribute to the firm bonds between kinetochores and the flared plus ends of dynamic MTs. A polypeptide from CENP-F’s C terminus also bound MTs, and either protein fragment diffused on a stable MT wall. They also followed the ends of dynamic MTs as they shortened. When either fragment was coupled to a microbead, the force it could transduce from a shortening MT averaged 3–5 pN but could exceed 10 pN, identifying CENP-F as a highly effective coupler to shortening MTs.

A novel role of farnesylation in targeting a mitotic checkpoint protein, human Spindly, to kinetochores

The mitotic checkpoint protein Spindly is farnesylated in vivo and this modification is required for its interaction with the RZZ complex and its localization to kinetochores.

Kinetochore (KT) localization of mitotic checkpoint proteins is essential for their function during mitosis. hSpindly KT localization is dependent on the RZZ complex and hSpindly recruits the dynein–dynactin complex to KTs during mitosis, but the mechanism of hSpindly KT recruitment is unknown. Through domain-mapping studies we characterized the KT localization domain of hSpindly and discovered it undergoes farnesylation at the C-terminal cysteine residue. The N-terminal 293 residues of hSpindly are dispensable for its KT localization. Inhibition of farnesylation using a farnesyl transferase inhibitor (FTI) abrogated hSpindly KT localization without affecting RZZ complex, CENP-E, and CENP-F KT localization. We showed that hSpindly is farnesylated in vivo and farnesylation is essential for its interaction with the RZZ complex and hence KT localization. FTI treatment and hSpindly knockdown displayed the same mitotic phenotypes, indicating that hSpindly is a key FTI target in mitosis. Our data show a novel role of lipidation in targeting a checkpoint protein to KTs through protein–protein interaction.

A regulatory effect of INMAP on centromere proteins: antisense INMAP induces CENP-B variation and centromeric halo

CENP-B is a highly conserved protein that facilitates the assembly of specific centromere structures both in interphase nuclei and on mitotic chromosomes. INMAP is a conserved protein that localizes at nucleus in interphase cells and at mitotic apparatus in mitotic cells. Our previous results showed that INMAP over-expression leads to spindle defects, mitotic arrest and formation of polycentrosomal and multinuclear cells, indicating that INMAP may modulate the function of (a) key protein(s) in mitotic apparatus. In this study, we demonstrate that INMAP interacts with CENP-B and promotes cleavage of the N-terminal DNA binding domain from CENP-B. The cleaved CENP-B cannot associate with centromeres and thus lose its centromere-related functions. Consistent with these results, CENP-B in INMAP knockdown cells becomes more diffused around kinetochores. Although INMAP knockdown cells do not exhibit gross defects in mitotic spindle formation, these cells go through mitosis, especially prophase and metaphase, with different relative timing, indicating subtle abnormality. These results identify INMAP as a model regulator of CENP-B and support the notion that INMAP regulates mitosis through modulating CENP-B-mediated centromere organization.

Correlation between centromere protein-F autoantibodies and cancer analyzed by enzyme-linked immunosorbent assay

Background

Centromere protein-F (CENP-F) is a large nuclear protein of 367 kDa, which is involved in multiple mitosis-related events such as proper assembly of the kinetochores, stabilization of heterochromatin, chromosome alignment and mitotic checkpoint signaling. Several studies have shown a correlation between CENP-F and cancer, e.g. the expression of CENP-F has been described to be upregulated in cancer cells. Furthermore, several studies have described a significant correlation between the expression of autoantibodies to CENP-F and cancer.

Methods

Autoantibodies to CENP-F were detected in a small number of samples during routine indirect immunofluorescence (IIF) analysis for anti-nuclear antibodies (ANA) using HEp-2 cells as substrate. Using overlapping synthetic peptides covering a predicted structural maintenance of chromosomes (SMC) domain, we developed an enzyme-linked immunosorbent assay (ELISA) for detection of CENP-F antibodies.

Results

Analyzing the reactivity of the sera positive in IIF for CENP-F antibodies to overlapping CENP-F peptides, we showed that autoantibodies to several peptides correlate with the presence of antibodies to CENP-F and a diagnosis of cancer, as increased CENP-F antibody expression specific for malignant cancer patients to five peptides was found (A9, A12, A14, A16, A27). These antibodies to CENP-F in clinical samples submitted for ANA analysis were found to have a positive predictive value for cancer of 50%. Furthermore, the expression of cancer-correlated CENP-F antibodies seemed to increase as a function of time from diagnosis.

Conclusion

These results conform to previous findings that approximately 50% of those patients clinically tested for ANA analyses who express CENP-F antibodies are diagnosed with cancer, confirming that these antibodies may function as circulating tumor markers. Thus, a peptide-based CENP-F ELISA focused on the SMC domain may aid in identifying individuals with a potential cancer.

The microtubule binding properties of CENP-E's C-terminus and CENP-F

CENP-E (centromere protein E) and CENP-F (centromere protein F), also known as mitosin, are large, multi-functional proteins associated with the outer kinetochore. CENP-E features a well-characterized kinesin motor domain at its N-terminus and a second microtubule-binding domain at its C-terminus of unknown function. CENP-F is important for the formation of proper kinetochore-microtubule attachment and, similar to CENP-E, contains two microtubule-binding domains at its termini. While the importance of these proteins is known, the details of their interactions with microtubules have not yet been investigated. We have biochemically and structurally characterized the microtubule-binding properties of the amino- and carboxyl-terminal domains of CENP-F as well as the carboxyl-terminal (non-kinesin) domain of CENP-E. CENP-E's C-terminus and CENP-F's N-terminus bind microtubules with similar affinity to the well-characterized Ndc80 complex, while CENP-F's C-terminus shows much lower affinity. Electron microscopy analysis reveals that all of these domains engage the microtubule surface in a disordered manner, suggesting that these factors have no favored binding geometry and may allow for initial side-on attachments early in mitosis.

Slk19 clusters kinetochores and facilitates chromosome bipolar attachment

Yeast kinetochore protein Slk19 is required for kinetochore clustering, and nocodazole exposure to slk19 mutant cells causes impaired kinetochore capture and delayed chromosome bipolar attachment after nocodazole washout.

In all eukaryotic cells, DNA is packaged into multiple chromosomes that are linked to microtubules through a large protein complex called a kinetochore. Previous data show that the kinetochores are clustered together during most of the cell cycle, but the mechanism and the biological significance of kinetochore clustering are unknown. As a kinetochore protein in budding yeast, the role of Slk19 in the stability of the anaphase spindle has been well studied, but its function in chromosome segregation has remained elusive. Here we show that Slk19 is required for kinetochore clustering when yeast cells are treated with the microtubule-depolymerizing agent nocodazole. We further find that slk19Δ mutant cells exhibit delayed kinetochore capture and chromosome bipolar attachment after the disruption of the kinetochore–microtubule interaction by nocodazole, which is likely attributed to defective kinetochore clustering. In addition, we show that Slk19 interacts with itself, suggesting that the dimerization of Slk19 may mediate the interaction between kinetochores for clustering. Therefore Slk19 likely acts as kinetochore glue that clusters kinetochores to facilitate efficient and faithful chromosome segregation.

The kinetochore protein Cenp-F is a potential novel target for zoledronic acid in breast cancer cells

The anti-resorptive agent zoledronic acid inhibits key enzymes in the mevalonate pathway, disrupting post-translational modification and thereby correct protein localization and function. Inhibition of prenylation may also be responsible for the reported anti-tumour effects of zoledronic acid, but the specific molecular targets have not been identified. Cenp-F/mitosin, a kinetochore-associated protein involved in the correct separation of chromosomes during mitosis, has been shown to undergo post-translational prenylation and may therefore be a novel target contributing to the anti-tumour effects of zoledronic acid. We investigated whether zoledronic acid causes loss of Cenp-F from the kinetochore in breast cancer cells, to determine if the reported anti-tumour effects may be mediated by impairing correct chromosome separation. MDA-MB-436, MDA-MB-231 and MCF-7 breast cancer cells and MCF-10A non-malignant breast epithelial cells were treated with zoledronic acid in vitro, and the effect on Cenp-F localization was analysed by immunoflourescence microscopy. Zoledronic acid caused loss of Cenp-F from the kinetochore, accompanied by an increase in the number of cells in pro-, /prometa- and metaphase in all of the cancer cell lines. There was also a significant increase in the number of lagging chromosomes in mitotic cells. The effects of zoledronic acid could be reversed by inclusion of an intermediary of the mevalonate pathway, showing that the loss of Cenp-F from the kinetochore was caused by the inhibition of farnesylation. In contrast, no effect was seen on Cenp-F in non-malignant MCF-10A cells. This is the first report showing a specific effect of zoledronic acid on a protein involved in the regulation of chromosome segregation, identifying Cenp-F as a potential new molecular target for NBPs in tumour cells.

The Plk1-dependent phosphoproteome of the early mitotic spindle

Polo-like kinases regulate many aspects of mitotic and meiotic progression from yeast to man. In early mitosis, mammalian Polo-like kinase 1 (Plk1) controls centrosome maturation, spindle assembly, and microtubule attachment to kinetochores. However, despite the essential and diverse functions of Plk1, the full range of Plk1 substrates remains to be explored. To investigate the Plk1-dependent phosphoproteome of the human mitotic spindle, we combined stable isotope labeling by amino acids in cell culture with Plk1 inactivation or depletion followed by spindle isolation and mass spectrometry. Our study identified 358 unique Plk1-dependent phosphorylation sites on spindle proteins, including novel substrates, illustrating the complexity of the Plk1-dependent signaling network. Over 100 sites were validated by in vitro phosphorylation of peptide arrays, resulting in a broadening of the Plk1 consensus motif. Collectively, our data provide a rich source of information on Plk1-dependent phosphorylation, Plk1 docking to substrates, the influence of phosphorylation on protein localization, and the functional interaction between Plk1 and Aurora A on the early mitotic spindle.

Involvement of Cenp-F in interphase chromatin organization possibly through association with DNA-dependent protein kinase

Cenp-F (also named mitosin) is a 350-kDa human kinetochore protein important for the mitotic progression. It is also a nuclear matrix protein in interphase cells. Here, we showed that overexpression of N-terminal deletion mutants of Cenp-F containing the C-terminal 112 residues induced chromatin condensation into numerous aggregates of varying sizes in interphase nucleus, colocalizing with the exogenous proteins. In situ hybridization using whole chromosome painting probes indicated that the chromatin aggregates were not prematurely condensed individual chromosomes. Neither were they due to apoptosis. We provided evidence showing association of Cenp-F with certain regions of interphase chromatin fibers. Cenp-F associated with the DNA-dependent protein kinase (DNA-PK), a trimeric protein complex critical for genome homeostasis. Moreover, the DNA-PK association activity of Cenp-F mutants correlated with their ability to induce chromatin aggregation. These results imply a role of Cenp-F in organization of interphase chromatin through association and possibly regulation of DNA-PK.

Bub1 and CENP-F can contribute to Kaposi's sarcoma-associated herpesvirus genome persistence by targeting LANA to kinetochores

The latency-associated nuclear antigen (LANA) encoded by Kaposi's sarcoma-associated herpesvirus (KSHV) is critical for segregation of viral episomes to progeny nuclei and allows for maintenance of the viral genome in newly divided daughter cells. LANA binds to KSHV terminal repeat (TR) DNA and simultaneously associates with chromatin-bound cellular proteins. This process tethers the viral episomes to host chromosomes. However, the mechanism of tethering is complex and involves multiple protein-protein interactions. Our previous proteomics studies which showed the association of LANA with centromeric protein F (CENP-F) prompted us to further study whether LANA targets centromeric proteins for persistence of KSHV episomes during cell division. Here we show that LANA colocalized with CENP-F as speckles, some of which are paired at centromeric regions of a subset of chromosomes in KSHV-infected JSC-1 cells. We also confirm that both the amino and carboxy termini of LANA can bind to CENP-F. Moreover, LANA associated with another kinetochore protein, Bub1 (budding uninhibited by benzimidazole 1), which is known to form a complex with CENP-F. Importantly, we demonstrated the dynamic association of LANA and Bub1/CENP-F and the colocalization between Bub1, LANA, and the KSHV episome tethered to the host chromosome using fluorescence in situ hybridization (FISH). Knockdown of Bub1 expression by lentivirus-delivered short hairpin RNA (shRNA) dramatically reduced the number of KSHV genome copies, whereas no dramatic effect was seen with CENP-F knockdown. Therefore, the interaction between LANA and the kinetochore proteins CENP-F and Bub1 is important for KSHV genome tethering and its segregation to new daughter cells, with Bub1 potentially playing a more critical role in the long-term persistence of the viral genome in the infected cell.

Murine CENP-F regulates centrosomal microtubule nucleation and interacts with Hook2 at the centrosome

The microtubule (MT) network is essential in a broad spectrum of cellular functions. Many studies have linked CENP-F to MT-based activities as disruption of this protein leads to major changes in MT structure and function. Still, the basis of CENP-F regulation of the MT network remains elusive. Here, our studies reveal a novel and critical localization and role for CENP-F at the centrosome, the major MT organizing center (MTOC) of the cell. Using a yeast two-hybrid screen, we identify Hook2, a linker protein that is essential for regulation of the MT network at the centrosome, as a binding partner of CENP-F. With recently developed immunochemical reagents, we confirm this interaction and reveal the novel localization of CENP-F at the centrosome. Importantly, in this first report of CENP-F(-/-) cells, we demonstrate that ablation of CENP-F protein function eliminates MT repolymerization after standard nocodazole treatment. This inhibition of MT regrowth is centrosome specific because MT repolymerization is readily observed from the Golgi in CENP-F(-/-) cells. The centrosome-specific function of CENP-F in the regulation of MT growth is confirmed by expression of truncated CENP-F containing only the Hook2-binding domain. Furthermore, analysis of partially reconstituted MTOC asters in cells that escape complete repolymerization block shows that disruption of CENP-F function impacts MT nucleation and anchoring rather than promoting catastrophe. Our study reveals a major new localization and function of CENP-F at the centrosome that is likely to impact a broad array of MT-based actions in the cell.

Single nucleotide polymorphisms in chromosomal instability genes and risk and clinical outcome of breast cancer: a Swedish prospective case-control study

Chromosomal instability (CIN) is a major characteristic of many cancers. We investigated whether putatively functional single nucleotide polymorphisms (SNPs) in genes related to CIN (CENPF, ESPL1, NEK2, PTTG1, ZWILCH, ZWINT) affect breast cancer (BC) risk and clinical outcome in a Swedish cohort of 749 incident BC cases with detailed clinical data and up to 15 years of follow-up and 1493 matched controls. As a main observation, carriers of the A allele of the CENPF SNP rs438034 had a worse BC-specific survival compared to the wild type genotype GG carriers (hazard ratio (HR) 2.65, 95% confidence interval (CI) 1.19-5.90), although they were less likely to have regional lymph node metastases (odds ratio (OR) 0.71, 95% CI 0.51-1.01) and tumours of stage II-IV (OR 0.73, 95% CI 0.54-0.99). As there is increasing evidence that CENPF is associated with poor prognosis in patients with primary BC, further independent studies are needed to clarify the importance of genetic variation in the CENPF gene in the clinic.

CMF1-Rb interaction promotes myogenesis in avian skeletal myoblasts

CMF1 protein is expressed in developing striated muscle before the expression of contractile proteins, and depletion of CMF1 in myoblasts results in inability to express muscle-specific proteins. Previous studies of CMF1 identify a functional Rb-binding domain, which is conserved in the murine and human homologues. Here, we show that CMF1 binds Rb family members, while a CMF1 protein with deletion of the Rb-binding domain (Rb-del CMF1) does not. Myogenic cell lines over-expressing Rb-del CMF1 proliferate normally, but exhibit markedly impaired differentiation, including dramatically reduced contractile proteins gene expression and failure to fuse into myotubes. Furthermore, by quantitative real-time polymerase chain reaction, MyoD and Myf5 mRNA levels are comparable to wild-type, while myogenin and contractile protein mRNA levels are significantly attenuated. These data demonstrate that CMF1 regulates myocyte differentiation by interaction with Rb family members to induce expression of myogenic regulatory factors.

Murine CENPF interacts with syntaxin 4 in the regulation of vesicular transport

Syntaxin 4 is a component of the SNARE complex that regulates membrane docking and fusion. Using a yeast two-hybrid screen, we identify a novel interaction between syntaxin 4 and cytoplasmic murine CENPF, a protein previously demonstrated to associate with the microtubule network and SNAP-25. The binding domain for syntaxin 4 in CENPF was defined by yeast two-hybrid assay and co-immunoprecipitation. Confocal analyses in cell culture reveal a high degree of colocalization between endogenously expressed proteins in interphase cells. Additionally, the endogenous SNARE proteins can be isolated as a complex with CENPF in immunoprecipitation experiments. Further analyses demonstrate that murine CENPF and syntaxin 4 colocalize with components of plasma membrane recycling: SNAP-25 and VAMP2. Depletion of endogenous CENPF disrupts GLUT4 trafficking whereas expression of a dominant-negative form of CENPF inhibits cell coupling. Taken together, these studies demonstrate that CENPF provides a direct link between proteins of the SNARE system and the microtubule network and indicate a diverse role for murine CENPF in vesicular transport.

Synthetic lethal interactions identify phenotypic "interologs" of the spindle assembly checkpoint components

Here, we report genetic interactions with mdf-1(gk2)/MAD1 in Caenorhabditis elegans. Nine are evolutionarily conserved or phenotypic "interologs" and two are novel enhancers, hcp-1 and bub-3. We show that HCP-1 and HCP-2, the two CENP-F-related proteins, recently implicated in the spindle assembly checkpoint (SAC) function, do not have identical functions, since hcp-1(RNAi), but not hcp-2(RNAi), enhances the lethality of the SAC mutants.

Conserved C-terminal domains of mCenp-F (LEK1) regulate subcellular localization and mitotic checkpoint delay

Centromeric Protein-F (Cenp-F) family members have been identified in organisms from yeast to human. Cenp-F proteins are a component of kinetochores during mitosis, bind to the Rb family of tumor suppressors, and have regulatory effects on the cell cycle and differentiation; however, their role in these processes has not been resolved. Here, we provide evidence that the role of murine Cenp-F (mCenp-F, also known as LEK1) remains largely conserved and that the domains within the C-terminus collectively function to regulate the G2/M cell cycle checkpoint. Overexpression of the C-terminal domain of mCenp-F decreases DNA synthesis. Analyses of deletion mutants of mCenp-F reveal that the complete C-terminal domain is required to delay cell cycle progression at G2/M. Signal transduction pathway profiling experiments indicate that the mCenp-F-mediated cell cycle delay does not involve transcriptional activity of key cell cycle regulators such as Rb, E2F, p53, or Myc. However, endogenous mCenp-F colocalizes with pRb and p107, which demonstrates in vivo protein-protein interaction during cell division. These observations suggest that the domains of the C-terminus of mCenp-F have a conserved function in control of mitotic progression through protein-protein interaction with pocket proteins, thus providing a direct connection between cell cycle regulation and mitotic progression.

The human Nup107-160 nuclear pore subcomplex contributes to proper kinetochore functions

We previously demonstrated that a fraction of the human Nup107-160 nuclear pore subcomplex is recruited to kinetochores at the onset of mitosis. However, the molecular determinants for its kinetochore targeting and the functional significance of this localization were not investigated. Here, we show that the Nup107-160 complex interacts with CENP-F, but that CENP-F only moderately contributes to its targeting to kinetochores. In addition, we show that the recruitment of the Nup107-160 complex to kinetochores mainly depends on the Ndc80 complex. We further demonstrate that efficient depletion of the Nup107-160 complex from kinetochores, achieved either by combining siRNAs targeting several of its subunits excluding Seh1, or by depleting Seh1 alone, induces a mitotic delay. Further analysis of Seh1-depleted cells revealed impaired chromosome congression, reduced kinetochore tension and kinetochore-microtubule attachment defects. Finally, we show that the presence of the Nup107-160 complex at kinetochores is required for the recruitment of Crm1 and RanGAP1-RanBP2 to these structures. Together, our data thus provide the first molecular clues underlying the function of the human Nup107-160 complex at kinetochores.

Specific deletion of CMF1 nuclear localization domain causes incomplete cell cycle withdrawal and impaired differentiation in avian skeletal myoblasts

CMF1 is a protein expressed in embryonic striated muscle with onset of expression preceding that of contractile proteins. Disruption of CMF1 in myoblasts disrupts muscle-specific protein expression. Preliminary studies indicate both nuclear and cytoplasmic distribution of CMF1 protein, suggesting functional roles in both cellular compartments. Here we examine the nuclear function of CMF1, using a newly characterized antibody generated against the CMF1 nuclear localization domain and a CMF1 nuclear localization domain-deleted stable myocyte line. The antibody demonstrates nuclear distribution of the CMF1 protein both in vivo and in cell lines, with clustering of CMF1 protein around chromatin during mitosis. In more differentiated myocytes, the protein shifts to the cytoplasm. The CMF1 NLS-deleted cell lines have markedly impaired capacity to differentiate. Specifically, these cells express less contractile protein than wild-type or full-length CMF1 stably transfected cells, and do not fuse properly into multinucleate syncytia with linear nuclear alignment. In response to low serum medium, a signal to differentiate, CMF1 NLS-deleted cells enter G0, but continue to express proliferation markers and will reenter the cell cycle when stimulated by restoring growth medium. These data suggest that CMF1 is involved in regulation the transition from proliferation to differentiation in embryonic muscle.

CENP-F is a novel microtubule-binding protein that is essential for kinetochore attachments and affects the duration of the mitotic checkpoint delay

Centromeric protein F (CENP-F) is a 367-kDa human kinetochore protein that was identified a decade ago, but its function was only recently revealed by studies that used small interfering RNA to deplete the protein from cells. All studies showed that CENP-F is important for chromosome alignment, but these studies differed as to whether CENP-F is important to the mitotic checkpoint. We report here that CENP-F is essential for cells to sustain a prolonged mitotic delay in response to unattached kinetochores. Cells depleted of CENP-F exit mitosis in the presence of defective kinetochore attachments resulting from treatment with nocodazole, or the depletion of kinetochore proteins CENP-E and hSgo1. Kinetochores depleted of CENP-F exhibited a reduction in the amounts of the mitotic checkpoint proteins Mad1, Mad2, hBUBR1, hBUB1, and hMps1. We postulate that CENP-F is not an essential component of the mitotic checkpoint but facilitates the duration of the mitotic delay. Separately, we show that CENP-F is a novel microtubule-binding protein that possesses two microtubule-binding domains at opposite ends of the molecule. The C-terminal microtubule-binding domain was found to stimulate microtubule polymerization in vitro. These activities provide a biochemical explanation for how CENP-F contributes to kinetochore attachments in vivo.

Cenp-F (mitosin) is more than a mitotic marker

Cenp-F (mitosin) is a large coiled-coil protein whose function has remained obscure since its identification a decade ago. It has been suggested that the protein plays a role in the kinetochore-mediated mitotic functions but until recently there was little evidence to support this postulation. Recent results from five laboratories have given insights on how Cenp-F may participate in the regulation of cell division. In this mini-review, we will summarize the current data regarding the mitotic tasks of Cenp-F as well as discuss how it is used as a proliferation marker of malignant cell growth in the clinic. Also, the protein's post-translational modification by farnesylation and potential contribution to cell cycle effects of farnesyl transferase inhibitors will be addressed.

Mitosin/CENP-F in mitosis, transcriptional control, and differentiation

Mitosin/CENP-F is a large nuclear/kinetochore protein containing multiple leucine zipper motifs potentially for protein interactions. Its expression levels and subcellular localization patterns are regulated in a cell cycle-dependent manner. Recently, accumulating lines of evidence have suggested it a multifunctional protein involved in mitotic control, microtubule dynamics, transcriptional regulation, and muscle cell differentiation. Consistently, it is shown to interact directly with a variety of proteins including CENP-E, NudE/Nudel, ATF4, and Rb. Here we review the current progress and discuss possible mechanisms through which mitosin may function.

Silencing Cenp-F weakens centromeric cohesion, prevents chromosome alignment and activates the spindle checkpoint

Cenp-F is an unusual kinetochore protein in that it localizes to the nuclear matrix in interphase and the nuclear envelope at the G2/M transition; it is farnesylated and rapidly degraded after mitosis. We have recently shown that farnesylation of Cenp-F is required for G2/M progression, its localization to kinetochores, and its degradation. However, the role Cenp-F plays in mitosis has remained enigmatic. Here we show that, following repression of Cenp-F by RNA interference (RNAi), the processes of metaphase chromosome alignment, anaphase chromosome segregation and cytokinesis all fail. Although kinetochores attach to microtubules in Cenp-F-deficient cells, the oscillatory movements that normally occur following K-fibre formation are severely dampened. Consistently, inter-kinetochore distances are reduced. In addition, merotelic associations are observed, suggesting that whereas kinetochores can attach microtubules in the absence of Cenp-F, resolving inappropriate interactions is inhibited. Repression of Cenp-F does not appear to compromise the spindle checkpoint. Rather, the chromosome alignment defect induced by Cenp-F RNA interference is accompanied by a prolonged mitosis, indicating checkpoint activation. Indeed, the prolonged mitosis induced by Cenp-F RNAi is dependent on the spindle checkpoint kinase BubR1. Surprisingly, chromosomes in Cenp-F-deficient cells frequently show a premature loss of chromatid cohesion. Thus, in addition to regulating kinetochore-microtubule interactions, Cenp-F might be required to protect centromeric cohesion prior to anaphase commitment. Intriguingly, whereas most of the sister-less kinetochores cluster near the spindle poles, some align at the spindle equator, possibly through merotelic or lateral orientations.

LEK1 protein expression in normal and dysregulated cardiomyocyte mitosis

A defining characteristic of embryonic cells is their ability to divide rapidly, even in tissues such as cardiac muscle, which cannot divide once fully differentiated. This suggests that regulators of cell division differ in embryonic and differentiated cells. LEK1 is a member of an emerging family of proteins with diverse functions but shared structural domains, including numerous leucine zippers, a nuclear localization site, and a functional Rb-binding domain. LEK1 is expressed ubiquitously in the developing mouse embryo from the earliest stages of differentiation through birth. It is absent in adult tissues, even those that maintain active cell division. We hypothesize that LEK1 is a regulator of mitosis restricted to the developing embryo and early neonate. Here, using BrdU incorporation, we show that LEK1 protein downregulation in cardiac myocytes correlates directly with cessation of DNA synthesis between neonatal days 6 and 10. In contrast, in an immortalized cardiac cell line (HL1 cells), both BrdU incorporation and LEK1 protein expression persist, and actively dividing cells express LEK1. However, BrdU incorporation can be decreased in these cells by treatment with a morpholino targeting LEK1 mRNA. These data suggest a role for LEK1 in regulating the normal embryonic cardiomyocyte cell cycle and in promoting continued mitosis in transformed, abnormally dividing cardiomyocytes.

Unstable microtubule capture at kinetochores depleted of the centromere-associated protein CENP-F

Centromere protein F (CENP-F) (or mitosin) accumulates to become an abundant nuclear protein in G2, assembles at kinetochores in late G2, remains kinetochore-bound until anaphase, and is degraded at the end of mitosis. Here we show that the absence of nuclear CENP-F does not affect cell cycle progression in S and G2. In a subset of CENP-F depleted cells, kinetochore assembly fails completely, thereby provoking massive chromosome mis-segregation. In contrast, the majority of CENP-F depleted cells exhibit a strong mitotic delay with reduced tension between kinetochores of aligned, bi-oriented sister chromatids and decreased stability of kinetochore microtubules. These latter kinetochores generate mitotic checkpoint signaling when unattached, recruiting maximum levels of Mad2. Use of YFP-marked Mad1 reveals that throughout the mitotic delay some aligned, CENP-F depleted kinetochores continuously recruit Mad1. Others rebind YFP-Mad1 intermittently so as to produce 'twinkling', demonstrating cycles of mitotic checkpoint reactivation and silencing and a crucial role for CENP-F in efficient assembly of a stable microtubule-kinetochore interface.

Immunohistochemical detection of cdc2 is useful in predicting survival in patients with mantle cell lymphoma

Recent cDNA microarray studies have reported the prognostic value of several genes in mantle cell lymphoma patients. We aimed to validate the prognostic significance of three of these genes: alpha-tubulin, cdc2, and CENP-F. The protein expression of alpha-tubulin, cdc2, and CENP-F was assessed using immunohistochemistry. Their immunoreactivity in 48 formalin-fixed/paraffin-embedded mantle cell lymphoma tumors was determined by estimating the percentage of positive cells. These results were correlated with the expression of proliferation marker Ki67 and survival. Of these 48 mantle cell lymphoma patients, 41 were men and seven were women. The median age at time of diagnosis was 64.5 years, and the overall median survival was 40 months. In benign lymph nodes, the expression of cdc2 and alpha-tubulin was restricted to the germinal centers; mantle zones were negative. Expression of CENP-F was more uniformly distributed. In mantle cell lymphoma, Ki67 significantly correlated with all three markers (P<0.05, Spearman), but only Ki67 (>50%) and cdc2 (>25%) significantly correlated with shorter survival (P<0.0006, Spearman). Of several clinical parameters examined, international prognostic index of >or=2 correlated with worse clinical outcome, and high clinical stage (ie 4 vs <or=3) showed a trend for shorter survival. The prognostic significance of cdc2 and Ki67 was independent of international prognostic index and clinical stage. We have validated the prognostic value of cdc2, and confirmed that of Ki67, in a cohort of mantle cell lymphoma patients. Immunohistochemical detection of cdc2 and Ki67 may be a useful and simple method in evaluating the prognosis of mantle cell lymphoma patients.

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.

Cytoplasmic LEK1 is a regulator of microtubule function through its interaction with the LIS1 pathway

LIS1 and nuclear distribution gene E (NudE) are partner proteins in a conserved pathway regulating the function of dynein and microtubules. Here, we present data that cytoplasmic LEK1 (cytLEK1), a large protein containing a spectrin repeat and multiple leucine zippers, is a component of this pathway through its direct interaction with NudE, as determined by a yeast two-hybrid screen. We identified the binding domains in each molecule, and coimmunoprecipitation and colocalization studies confirmed the specificity of the interaction between cytLEK1 and NudE. Confocal deconvolution analysis revealed that cytLEK1 exhibits colocalization with endogenous NudE and with the known NudE binding partners, LIS1 and dynein. By localizing the NudE-binding domain of cytLEK1 to a small domain within the molecule, we were able to disrupt cytLEK1 function by using a dominant negative approach in addition to LEK1 knockdown and, thus, examine the role of the cytLEK1-NudE interaction in cells. Consistent with a defect in the LIS1 pathway, disruption of cytLEK1 function resulted in alteration of microtubule organization and cellular shape. The microtubule network of cells became tightly focused around the nucleus and resulted in a rounded cell shape. Additionally, cells exhibited a severe inability to repolymerize their microtubule networks after nocodazole challenge. Taken together, our studies revealed that cytLEK1 is essential for cellular functions regulated by the LIS1 pathway.

Mitosin/CENP-F as a negative regulator of activating transcription factor-4

Mitosin/CENP-F is a human nuclear matrix protein with multiple leucine zipper motifs. Its accumulation in S-G2 phases and transient kinetochore localization in mitosis suggest a multifunctional protein for cell proliferation. Moreover, its murine and avian orthologs are implicated in myocyte differentiation. Here we report its interaction with activating transcription factor-4 (ATF4), a ubiquitous basic leucine zipper transcription factor important for proliferation, differentiation, and stress response. The C-terminal portion of mitosin between residues 2488 and 3113 bound to ATF4 through two distinct domains, one of which was a leucine zipper motif. Mitosin mutants containing these domains were able to either supershift or disrupt the ATF4-DNA complex. On the other hand, ATF4, but not ATF1-3 or ATF6, interacted with mitosin through a region containing the basic leucine zipper motif. Moreover, overexpression of full-length mitosin repressed the transactivation activity of ATF4 in dual luciferase-based reporter assays, while knocking down mitosin expression manifested the opposite effects. These findings suggest mitosin to be a negative regulator of ATF4 in interphase through direct interaction.

A spindle checkpoint functions during mitosis in the early Caenorhabditis elegans embryo

During mitosis, chromosome segregation is regulated by a spindle checkpoint mechanism. This checkpoint delays anaphase until all kinetochores are captured by microtubules from both spindle poles, chromosomes congress to the metaphase plate, and the tension between kinetochores and their attached microtubules is properly sensed. Although the spindle checkpoint can be activated in many different cell types, the role of this regulatory mechanism in rapidly dividing embryonic animal cells has remained controversial. Here, using time-lapse imaging of live embryonic cells, we show that chemical or mutational disruption of the mitotic spindle in early Caenorhabditis elegans embryos delays progression through mitosis. By reducing the function of conserved checkpoint genes in mutant embryos with defective mitotic spindles, we show that these delays require the spindle checkpoint. In the absence of a functional checkpoint, more severe defects in chromosome segregation are observed in mutants with abnormal mitotic spindles. We also show that the conserved kinesin CeMCAK, the CENP-F-related proteins HCP-1 and HCP-2, and the core kinetochore protein CeCENP-C all are required for this checkpoint. Our analysis indicates that spindle checkpoint mechanisms are functional in the rapidly dividing cells of an early animal embryo and that this checkpoint can prevent chromosome segregation defects during mitosis.

LEK1 is a potential inhibitor of pocket protein-mediated cellular processes

LEK1, a member of the LEK family of proteins, is ubiquitously expressed in developing murine tissues. Our current studies are aimed at identifying the role of LEK1 during cell growth and differentiation. Little is known about the function of LEK proteins. Recent studies in our laboratory have focused on the characterization of the LEK1 atypical Rb-binding domain that is conserved among all LEK proteins. Our findings suggest that LEK1 potentially functions as a universal regulator of pocket protein activity. Pocket proteins exhibit distinct expression patterns during development and function to regulate cell cycle, apoptosis, and tissue-specific gene expression. We show that LEK1 interacts with all three pocket proteins, p107, p130, and pRb. Additionally, this interaction occurs specifically between the LEK1 Rb-binding motif and the "pocket domain" of Rb proteins responsible for Rb association with other targets. Analyses of the effects of disruption of LEK1 protein expression by morpholino oligomers demonstrate that LEK1 depletion decreases cell proliferation, disrupts cell cycle progression, and induces apoptosis. Given its expression in developing cells, its association with pocket proteins, and its effects on proliferation, cell cycle, and viability of cells, we suggest that LEK1 functions in a similar manner to phosphorylation to disrupt association of Rb proteins with appropriate binding targets. Thus, the LEK1/Rb interaction serves to retain cells in a pre-differentiative, actively proliferative state despite the presence of Rb proteins during development. Our data suggest that LEK1 is unique among LEK family members in that it specifically functions during murine development to regulate the activity of Rb proteins during cell division and proliferation. Furthermore, we discuss the distinct possibility that a yet unidentified splice variant of the closely related human CENP-F, serves a similar function to LEK1 in humans.

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.

Farnesylation of Cenp-F is required for G2/M progression and degradation after mitosis

Farnesyl transferase inhibitors induce G2/M cell cycle delays that cannot be explained by inhibition of the Ras GTPase. Recently, the kinetochore protein Cenp-F has been shown to be farnesylated. Here, we show that ectopic expression of the kinetochore targeting domain of Cenp-F delays progression through G2/M. Significantly, this is dependent on the CAAX farnesylation motif. We also show that localisation of Cenp-F to the nuclear envelope at G2/M and kinetochores in prometaphase is dependent both on its CAAX motif and farnesyl transferase activity. Strikingly, farnesyl transferase activity is also required for Cenp-F degradation after mitosis. Thus, these observations suggest that farnesylation of Cenp-F is required not only for its localisation to the nuclear envelope and kinetochores but also for timely progression through G2/M and its degradation after mitosis. In addition, these observations raise the possibility that the anti-proliferative effects induced by farnesyl transferase inhibitors may be due to inhibition of Cenp-F function and/or turnover.

Retinoblastoma protein partners

Studies of the retinoblastoma gene (Rb) have shown that its protein product (pRb) acts to restrict cell proliferation, inhibit apoptosis, and promote cell differentiation. The frequent mutation of the Rb gene, and the functional inactivation of pRb in tumor cells, have spurred interest in the mechanism of pRb action. Recently, much attention has focused on pRb's role in the regulation of the E2F transcription factor. However, biochemical studies have suggested that E2F is only one of many pRb-targets and, to date, at least 110 cellular proteins have been reported to associate with pRb. The plethora of pRb-binding proteins raises several important questions. How many functions does pRb possess, which of these functions are important for development, and which contribute to tumor suppression? The goal of this review is to summarize the current literature of pRb-associated proteins.

Mitosin and DNA topoisomerase IIalpha: two novel proliferation markers in the prognostication of diffuse astrocytoma patient survival

The expression of two novel proliferation-associated markers, mitosin and topoisomerase IIalpha (Topo IIalpha), was evaluated immunohistochemically in consecutive paraffin sections from 60 diffuse astrocytomas (grades 2 to 4) in relation to clinicopathologic parameters, proliferating cell nuclear antigen (PCNA) and Ki-67 (MIB-1) expression and survival. The percentage of mitosin and Topo IIalpha-positive cells (LI) increased with grade and Ki-67 LI, but could not discriminate between grade 3 on the one hand and grades 2 or 4 on the other hand. In 51% of cases, Ki-67 LI exceeded Topo IIalpha LI, especially within grade 4. Topo IIalpha and mitosin expression was adversely related to overall and disease-free survival in the entire cohort and in grades 2/3. However, only Topo IIalpha LI affected disease-free survival in grade 4 tumors. Multivariate analysis selected only mitosin LI along with the age of the patient, as the independent parameters predicting overall survival, whereas Topo IIalpha emerged as the single independent predictor of disease-free survival. It is concluded that the proliferative potential of astrocytomas, as measured by mitosin and Topo IIalpha immunostaining, conveys useful prognostic information, in addition to that obtained by standard clinicopathologic parameters.

CENP-F gene amplification and overexpression in head and neck squamous cell carcinomas

BACKGROUND

Antibodies against cancer-related genes have been detected in human cancers including head and neck cancers. High titers of c-Myc autoantibodies have been linked to gene amplification and tumor progression. Centromere protein-F (CENP-F) autoantibodies have been detected in patients with various cancers, suggesting similar gene alteration.

METHODS

CENP-F and c-MYC amplification was assessed in 72 head and neck squamous cell carcinoma (HNSCC) patients. Tumor and matched mucosa from 22 patients were analyzed for CENP-F mRNA levels by RT-PCR.

RESULTS

The larynx was the site most altered by amplification of either gene. CENP-F and c-MYC were amplified in 11% and 17% of the tumors, respectively. Coamplification was found in 7% of the tumors, most of which showed regional node involvement. CENP-F mRNA was overexpressed in 36% of tumors, and 23% of paired mucosa.

CONCLUSION

Our results provide the first evidence that CENP-F gene is amplified and overexpressed in HNSCC. No correlation was noted between CENP-F amplification and clinicopathologic parameters. However, CENP-F overexpression correlated with nodal metastasis.

Analysis of CMF1 reveals a bone morphogenetic protein-independent component of the cardiomyogenic pathway

Disruption of the CMF1 function in anterior mesoderm inhibits cardiac myogenesis in avian embryos. In the present study, we show that CMF1 is a member of an emerging family of proteins that includes centromeric protein-F, mitosin, and LEK1. These proteins are characterized by their large size (350 kDa), dynamic subcellular distribution, and potential functions in cell division and differentiation. The current data suggest that CMF1 is a unique member of this family by virtue of its restricted protein expression and variant subcellular distribution. Immunochemical analysis demonstrates that CMF1 protein is expressed in cardiogenic cells prior to the activation of cardiac structural gene products. In addition, we show that expression of CMF1 is not dependent on the bone morphogenetic protein (BMP) signaling pathway during development. Still, CMF1 cannot direct cardiomyogenesis in the absence of such factors as NKX-2.5. Taken with our previous data, this study suggests that CMF1 is a BMP-independent component of the cardiomyogenic pathway.

C-terminal domain of the mitotic apparatus protein p62 targets the protein to the nucleolus during interphase

Mitotic apparatuses from sea urchin embryos contain a protein (p62), previously shown to be required for mitotic progression. This protein localizes to the mitotic apparatus during cell division in urchin embryos and mammalian tissue culture cells. We show here by immunofluorescence that p62 is localized to the nucleus of mammalian cells during interphase and is highly concentrated in nucleoli. In addition, a fusion protein composed of full-length p62 and green fluorescent protein also localizes to nucleoli when expressed in COS-7 cells in culture. Analysis of the primary sequence of p62 reveals three distinct domains of the protein based on amino acid charge distribution: the acidic N-terminal domain, the basic C-terminal domain, and the central, M-domain, which contains alternating subdomains of clusters of acidic and basic residues. To identify the domain important for nucleolar localization during interphase, specific domains of p62 alone, or in combination with each other or with beta-galactosidase were fused to green fluorescent protein. Following confirmation of the fusion constructs by sequence analysis, the constructs were expressed in mammalian cells, expression was confirmed by immunoblotting, and the fusion proteins were localized via fluorescence microscopy. The data demonstrate that the C-terminal domain of p62 is both necessary and sufficient for the nuclear localization and nucleolar binding of p62 that is observed during interphase.

The kinetochore of higher eucaryotes: a molecular view

This review summarizes results concerning the molecular nature of the higher eucaryotic kinetochore. The first major section of this review includes kinetochore proteins whose general functions remain to be determined, precluding their entry into a discrete functional category. Many of the proteins in this section, however, are likely to be involved in kinetochore formation or structure. The second major section is concerned with how microtubule motor proteins function to cause chromosome movement. The microtubule motors dynein, CENP-E, and MCAK have all been observed at the kinetochore. While their precise functions are not well understood, all three are implicated in chromosome movement during mitosis. Finally, the last section deals with kinetochore components that play a role in the spindle checkpoint; a checkpoint that delays mitosis until all kinetochores have attached to the mitotic spindle. Brief reviews of kinetochore morphology and of an important technical breakthrough that enabled the molecular dissection of the kinetochore are also included.

Sequence determinants of nuclear localization in the alpha and beta isoforms of human topoisomerase II

The alpha and beta isoforms of DNA topoisomerase II (topo II) are targets for several widely used chemotherapeutic agents, and resistance to some of these drugs may be associated with reduced nuclear localization of the alpha isoform. Human topo IIalpha contains a strong bipartite nuclear localization signal (NLS) sequence between amino acids 1454 and 1497 (alphaNLS(1454-1497)). In the present study, we show that human topo IIalpha tagged with green fluorescence protein is still detectable in the nucleus when alphaNLS(1454-1497) has been disrupted. Seven additional regions in topo IIalpha containing overlapping potential bipartite NLSs were evaluated for their nuclear targeting abilities using a beta-galactosidase reporter system. A moderately functional NLS was identified between amino acids 1259 and 1296. When human topo IIbeta was examined in a similar fashion, it was found to contain two strongly functional sequences betaNLS(1522-1548) and betaNLS(1538-1573) in the region of topo IIbeta comparable to the region in topo IIalpha that contains the strongly functional alphaNLS(1454-1497). The third, betaNLS(1294-1332), although weaker than the other two beta sequences, is significantly stronger than the analogous alphaNLS(1259-1296). Differences in the NLS sequences of human topo II isoforms may contribute to their differences in subnuclear localization.