NuSAP,a novel microtubule-associated protein involved in mitotic spindle organization

Here, we report on the identification of nucleolar spindle-associated protein (NuSAP), a novel 55-kD vertebrate protein with selective expression in proliferating cells. Its mRNA and protein levels peak at the transition of G2 to mitosis and abruptly decline after cell division. Microscopic analysis of both fixed and live mammalian cells showed that NuSAP is primarily nucleolar in interphase, and localizes prominently to central spindle microtubules during mitosis. Direct interaction of NuSAP with microtubules was demonstrated in vitro. Overexpression of NuSAP caused profound bundling of cytoplasmic microtubules in interphase cells, and this relied on a COOH-terminal microtubule-binding domain. In contrast, depletion of NuSAP by RNA interference resulted in aberrant mitotic spindles, defective chromosome segregation, and cytokinesis. In addition, many NuSAP-depleted interphase cells had deformed nuclei. Both overexpression and knockdown of NuSAP impaired cell proliferation. These results suggest a crucial role for NuSAP in spindle microtubule organization.     

Involvement of a type 1 protein phosphatase encoded by bws1+ in fission yeast mitotic control

Fission yeast cdc25+ and wee1+ interact genetically with cdc2+ in the regulation of cell division, respectively as a mitotic activator and inhibitor. cdc25+ is normally essential for mitosis, but this requirement is alleviated in a loss-of-function wee1 mutant background. A plasmid-borne sequence, other than wee1+, that causes a cdc25ts wee1- double mutant to revert to a temperature-sensitive cdc phenotype has been isolated. The gene carried by this plasmid is called bws1+ (for bypass of wee suppression). bws1+ also bypasses the ability of alleles of cdc2 that confer a wee phenotype (cdc2w) to suppress loss-of-function cdc25 mutants. The nucleotide sequence of bws1+ shows that the predicted protein shares 81% amino acid identity with the catalytic subunit of mammalian type 1 protein phosphatase. Thus a genetic screen that might have yielded a protein kinase (wee1+) uncovered a phosphatase that also appears to be involved in the pathway of mitotic control.     

Eps1, a novel PDI-related protein involved in ER quality control in yeast.

PMA1 is an essential gene encoding the yeast plasma membrane [H(+)]ATPase. A pma1-D378N mutant has a dominant-negative effect on cell growth because both newly synthesized mutant and wild-type Pma1 molecules are retained and degraded in the endoplasmic reticulum (ER). Like other substrates for ER-associated degradation, Pma1-D378N is stabilized in mutants defective in components of the ubiquitination machinery. A genetic selection was performed for eps (ER-retained pma1 suppressing) mutants in which the growth defect caused by the D378N allele is suppressed. In an eps1 mutant, both mutant and wild-type Pma1 molecules are allowed to travel to the plasma membrane; however, normal retention of resident ER proteins Shr3 and Kar2 is not perturbed. Eps1 is a novel membrane protein belonging to the protein disulfide isomerase (PDI) family, and Eps1 co-localizes with Pma1-D378N in the ER. In the absence of Pma1-D378N, ER export of wild-type Pma1 is not affected by eps1 deletion, but export of the plasma membrane protein Gas1 is delayed. Because Eps1 is required for retention and degradation of Pma1-D378N, we propose a model in which Eps1 acts as a novel membrane-bound chaperone in ER quality control.     

The fission yeast dis3+ gene encodes a 110-kDa essential protein implicated in mitotic control.

The fission yeast mutant dis3-54 is defective in mitosis and fails in chromosome disjunction. Its phenotype is similar to that of dis2-11, a mutant with a mutation in the type 1 protein phosphatase gene. We cloned the dis3+ gene by transformation. Nucleotide sequencing predicts a coding region of 970 amino acids interrupted by a 164-bp intron at the 65th codon. The predicted dis3+ protein shares a weak but significant similarity with the budding yeast SSD1 or SRK1 gene product, the gene for which is a suppressor for the absence of a protein phosphatase SIT4 gene or the BCY1 regulatory subunit of cyclic AMP-dependent protein kinase. Anti-dis3 antibodies recognized the 110-kDa dis3+ gene product, which is part of a 250- to 350-kDa oligomer and is enriched in the nucleus. The cellular localization of the dis3+ protein is reminiscent of that of the dis2+ protein, but these two proteins do not form a complex. A type 1 protein phosphatase activity in the dis3-54 mutant extracts is apparently not affected. The dis3+ gene is essential for growth; gene disruptant cells do not germinate and fail in cell division. Increased dis3+ gene dosage reverses the Ts+ phenotype of a cdc25 wee1 strain, as does increased type 1 protein phosphatase gene dosage. Double mutant dis3 dis2 is lethal even at the permissive temperature, suggesting that the dis2+ and dis3+ genes may be functionally overlapped. The role of the dis3+ gene product in mitosis is unknown, but this gene product may be directly or indirectly involved in the regulation of mitosis.     

Centrophilin: a novel mitotic spindle protein involved in microtubule nucleation

A novel protein has been identified which may serve a key function in nucleating spindle microtubule growth in mitosis. This protein, called centrophilin, is sequentially relocated from the centromeres to the centrosomes to the midbody in a manner dependent on the mitotic phase. Centrophilin was initially detected by immunofluorescence with a monoclonal, primate-specific antibody (2D3) raised against kinetochore-enriched chromosome extract from HeLa cells (Valdivia, M. M., and B. R. Brinkley. 1985. J. Cell Biol. 101:1124-1134). Centrophilin forms prominent crescents at the poles of the metaphase spindle, gradually diminishes during anaphase, and bands the equatorial ends of midbody microtubules in telophase. The formation and breakdown of the spindle and midbody correlates in time and space with the aggregation and disaggregation of centrophilin foci. Immunogold EM reveals that centrophilin is a major component of pericentriolar material in metaphase. During recovery from microtubule inhibition, centrophilin foci act as nucleation sites for the assembly of spindle tubules. The 2D3 probe recognizes two high molecular mass polypeptides, 180 and 210 kD, on immunoblots of whole HeLa cell extract. Taken together, these data and the available literature on microtubule dynamics point inevitably to a singular model for control of spindle tubule turnover.     

Five novel elements involved in the regulation of mitosis in fission yeast

Summary Five new elements of the mitotic control in the fission yeast Schizosaccharomyces pombe were isolated from gene libraries as multicopy suppressors of the conditional lethal phenotype of win1-1 weel ^ts cdc25^ts triple mutant strains. These genes were designated wisl ⁺ –wis5⁺for win suppressing, and do not correspond to winl ⁺ or any of the previously characterised mitotic control genes. None of the wis genes is capable of suppressing the cdc phenotype of cdc25 ^ts strains, suggesting that their effect is not simply to reverse the effect of loss of cdc25 function. wisl ⁺ has been previously reported to encode a putative serine/threonine protein kinase that acts as a dosage-dependent inducer of mitosis. wis4 ⁺ appears to be a specific suppressor of the winl-1 mutation. wis2 ⁺ and wis3 ⁺ are capable of suppressing a wide range of cdc phenotypes arising from the combination of various mutations with wee1 ^ts and cdc25 ^ts, suggesting that the wis2 ⁺ and wis3 ⁺ products may interact with elements central to the mitotic control.     

Fission yeast pkl1 is a kinesin-related protein involved in mitotic spindle function.

We have used anti-peptide antibodies raised against highly conserved regions of the kinesin motor domain to identify kinesin-related proteins in the fission yeast Schizosaccharomyces pombe. Here we report the identification of a new kinesin-related protein, which we have named pkl1. Sequence homology and domain organization place pkl1 in the Kar3/ncd subfamily of kinesin-related proteins. Bacterially expressed pkl1 fusion proteins display microtubule-stimulated ATPase activity, nucleotide-sensitive binding, and bundling of microtubules. Immunofluorescence studies with affinity-purified antibodies indicate that the pkl1 protein localizes to the nucleus and the mitotic spindle. Pkl1 null mutants are viable but have increased sensitivity to microtubule-disrupting drugs. Disruption of pkl1+ suppresses mutations in another kinesin-related protein, cut7, which is known to act in the spindle. Overexpression of pkl1 to very high levels causes a similar phenotype to that seen in cut7 mutants: V-shaped and star-shaped microtubule structures are observed, which we interpret to be spindles with unseparated spindle poles. These observations suggest that pkl1 and cut7 provide opposing forces in the spindle. We propose that pkl1 functions as a microtubule-dependent motor that is involved in microtubule organization in the mitotic spindle.     

A mitotic role for a novel fission yeast protein kinase dsk1 with cell cycle stage dependent phosphorylation and localization.

The fission yeast dsk1+ gene, a multicopy suppressor for cold-sensitive dis1 mutants, encodes a novel 61-kd protein kinase. It is a phosphoprotein, and phosphoserine is the major phosphorylated amino acid. Hyperphosphorylation of dsk1 causes a mobility shift, resulting in two dsk1-specific protein bands. The phosphorylation pattern is strikingly altered when cell cycle progression is delayed or arrested. The slowly migrating phosphorylated form is prominent in mitotically arrested cells, and the fast migrating form is enriched in interphase-arrested cells. dsk1 is a protein kinase. It auto-phosphorylates as well as phosphorylates myelin basic protein (MBP). Phosphotyrosine as well as phosphoserine/threonine were found in autophosphorylation, but no tyrosine phosphorylation occurs when MBP was used as the substrate. The dsk1 immunoprecipitates from mitotically arrested cells have a several-fold higher kinase activity than that from wild type. The haploid gene disruptant is viable, indicating that the dsk1+ gene is non-essential for viability. High dosage of dsk1+, however, strongly delays the G2/M progression. Immunofluorescence microscopy using anti-dsk1 antibody shows that localization pattern of dsk1 protein strikingly alters depending on cell cycle stages. In G2-arrested cells, dsk1 locates in the cytoplasm, whereas in mitotically arrested cells, nuclear stain is intense. In wild-type cells, nuclear stain is seen only in mitotic cells. Hence dsk1 protein may play an important role in mitotic control by altering cellular location, degree of phosphorylation and kinase activity. We discuss possible roles of dsk1 kinase as an add-on regulator in mitosis.     

cdc25+ functions as an inducer in the mitotic control of fission yeast

In the fission yeast S. pombe the cdc25+ gene function is required to initiate mitosis. We have cloned the cdc25+ gene and have found that increased cdc25+ expression causes mitosis to initiate at a reduced cell size. This shows that cdc25+ functions as a dosage-dependent inducer in mitotic control, the first such mitotic control element to be specifically identified. DNA sequencing of the cdc25+ gene has shown that it can encode a protein of MW 67,000. Evidence is described showing that cdc25+ functions to counteract the activity of the mitotic inhibitor wee1+, and indicating that both mitotic control elements act independently to regulate the initiation of mitosis.     

Isolation and characterization of a novel F-box protein Pof10 in fission yeast

The SCF complex is a type of ubiquitin-protein ligase (E3) that consists of invariable components, including Skp1, Cdc53/Cul1, and Rbx1, as well as variable components known as F-box proteins. Using a yeast two-hybrid system, we isolated six proteins that interact with Schizosaccharomyces pombe Skp1. Among them, Pof10 is a novel F-box protein consisting of 662 amino acids, harboring the F-box domain required for the binding to Skp1 and followed by four WD40 repeats. Overexpression of Pof10 in fission yeast resulted in loss of viability with marked morphological changes that are similar to those in pop1 mutant yeast. Coexpression of Skp1 with Pof10 prevented the lethality, suggesting that the lethality from Pof10 overexpression results from the sequestration of Skp1 from other F-box proteins including Pop1. Whereas most F-box proteins show rapid turnover, Pof10 has a remarkably long half-life in vivo and has been shown to be localized predominantly in cytoplasm. These results suggest that the stable F-box protein Pof10 might target abundant cytoplasmic proteins for degradation in fission yeast.     

The roles of fission yeast ase1 in mitotic cell division, meiotic nuclear oscillation, and cytokinesis checkpoint signaling

The Ase1/Prc1 proteins constitute a conserved microtubule-associated protein family that is implicated in central spindle formation and cytokinesis. Here we characterize a role for fission yeast Ase1. Ase1 localizes to microtubule overlapping zones and displays dynamic alterations of localization during the cell cycle. In particular, its spindle localization during metaphase is reduced substantially, followed by robust appearance at the spindle midzone in anaphase. ase1 deletions are viable but defective in nuclear and septum positioning and completion of cytokinesis, which leads to diploidization and chromosome loss. Time-lapse imaging shows that elongating spindles collapse abruptly in the middle of anaphase B. Either absence or overproduction of Ase1 results in profound defects on microtubule bundling in an opposed manner, indicating that Ase1 is a dose-dependent microtubule-bundling factor. In contrast microtubule nucleating activities are not noticeably compromised in ase1 mutants. During meiosis astral microtubules are not bundled and oscillatory nuclear movement is impaired significantly. The Aurora kinase does not correctly localize to central spindles in the absence of Ase1. Finally Ase1 acts as a regulatory component in the cytokinesis checkpoint that operates to inhibit nuclear division when the cytokinesis apparatus is perturbed. Ase1, therefore, couples anaphase completion with cytokinesis upon cell division.     

Expression of a novel 90-kDa protein, Lsd90, involved in the metabolism of very long-chain fatty acid-containing phospholipids in a mitosis-defective fission yeast mutant

The fission yeast lsd1/fas2 strain carries a temperature-sensitive mutation of the fatty-acid-synthase alpha-subunit, exhibiting an aberrant mitosis lsd phenotype, with accumulation of very-long-chain fatty-acid-containing phospholipid (VLCFA-PL). A novel 90-kDa protein, Lsd90 (SPBC16E9.16c), was found to be newly expressed in small particle-like structures in lsd1/fas2 cells under restrictive conditions. Two mismatches leading to a double frame shift were found between the sequences of the lsd90(+) gene registered in the genomic database and the sequences determined experimentally at the amino acid, cDNA and genomic DNA levels. Unexpectedly, overexpression and disruption of the lsd90(+) gene in either lsd1/fas2 or wild-type cells did not affect either cell growth or expression of the lsd phenotype. The amounts of VLCFA-PL that accumulated in lsd90-overexpressing lsd1/fas2 cells were significantly lower than those in lsd1/fas2 cells, suggesting the involvement of Lsd90 in the metabolism of VLCFA-PL.     

Dnt1 acts as a mitotic inhibitor of the spindle checkpoint protein dma1 in fission yeast

The Schizosaccharomyces pombe checkpoint protein Dma1 couples mitotic progression with cytokinesis and is important in delaying mitotic exit and cytokinesis when kinetochores are not properly attached to the mitotic spindle. Dma1 is a ubiquitin ligase and potential functional relative of the human tumor suppressor Chfr. Dma1 delays mitotic exit and cytokinesis by ubiquitinating a scaffold protein (Sid4) of the septation initiation network, which, in turn, antagonizes the ability of the Polo-like kinase Plo1 to promote cell division. Here we identify Dnt1 as a Dma1-binding protein. Several lines of evidence indicate that Dnt1 inhibits Dma1 function during metaphase. First, Dnt1 interacts preferentially with Dma1 during metaphase. Second, Dma1 ubiquitin ligase activity and Sid4 ubiquitination are elevated in dnt1 cells. Third, the enhanced mitotic defects in dnt1Δ plo1 double mutants are partially rescued by deletion of dma1(+), suggesting that the defects in dnt1 plo1 double mutants are attributable to excess Dma1 activity. Taken together, these data show that Dnt1 acts to restrain Dma1 activity in early mitosis to allow normal mitotic progression.     

Pyp3 PTPase acts as a mitotic inducer in fission yeast.

The p34cdc2 M-phase kinase is regulated by inhibitory phosphorylation of Tyr15, largely through the actions of the p107wee1 tyrosine kinase and p80cdc25 protein tyrosine phosphatase (PTPase). In this study we demonstrate that a second PTPase, encoded by pyp3, also contributes to tyrosyl dephosphorylation of p34cdc2. Pyp3 was identified as a high copy suppressor of a cdc25- mutation. The pyp3 gene encodes a 33 kDa PTPase that is more closely related to human PTP1B and fission yeast pyp1 and pyp2 PTPases than to cdc25. Pyp3 does not share an essential overlapping function with pyp1 or pyp2. We demonstrate that disruption of pyp3 causes a mitotic delay that is greatly exacerbated in cells that are partially defective for cdc25 function and that pyp3 function is essential in cdc25-disruption wee1- strains. Pyp3 PTPase effectively dephosphorylates and activates the p34cdc2 kinase in vitro. We conclude that the pyp3 PTPase acts cooperatively with p80cdc25 to dephosphorylate Tyr15 of p34cdc2.     

Radiation-induced mitotic delay: a genetic characterization in the fission yeast.

Radiation-induced mitotic delay is under investigation in the fission yeast, Schizosaccharomyces pombe. A large range of cell cycle- and radiation-sensitive mutants of this yeast is available to facilitate this effort. Through an examination of such mutants it has been shown that the X-ray transition point and the p34cdc2 execution point are coincident; wee1- strains are not delayed by irradiation; and the radiation-sensitive mutants rad1-1, rad3-136, rad9-192, and rad17-W are not delayed by radiation or by inhibitors of DNA synthesis, including hydroxyurea. A model is proposed: Damaged DNA generates a signal to delay mitosis which is carried by the products of the rad genes to activate the tyrosine kinase p110wee1. This in turn inactivates the serine/threonine kinase p34cdc2, thereby blocking entry to mitosis. Unreplicated DNA also initiates a signal to delay mitosis which is carried by these same rad genes but, as indicated in the literature, transmission to p34cdc2 does not require p110wee1. The delay-deficient rad mutants may possess some properties of tumor suppressor genes, with implications for mutagenesis and oncogenesis.     

The fission yeast ptr1+ gene involved in nuclear mRNA export encodes a putative ubiquitin ligase

Fission yeast ptr1-1 is one of the mRNA transport mutants that accumulate poly(A)+ RNA in the nuclei at the nonpermissive temperature. We found that the ptr1+ gene encodes a homolog of Saccharomyces cerevisiae Tom1p, a hect type ubiquitin ligase. In ptr1-1, a conserved amino acid in the hect domain of Ptr1p is mutated. The ptr1+ gene is essential for growth and its mutation did not affect nuclear protein export. A ptr1-1 rae1-167 double mutant showed a synthetic effect on a growth defect, indicating that Ptr1p functionally interacts with an essential mRNA export factor Rae1p. We also isolated a multi-copy suppressor for ptr1-1 and found that it is the mpd2+ gene isolated as a multi-copy suppressor of cdc7-PD1.     

Characterization of the ptr5+ gene involved in nuclear mRNA export in fission yeast

Lung cancer is the leading cause of cancer related deaths worldwide. It is necessary to better understand the molecular mechanisms involved in lung cancer in order to develop more effective therapeutics for the treatment of this disease. Recent reports have shown that Wnt signaling pathway is important in a number of cancer types including lung cancer. However, the role of Frizzled-8 (Fzd-8), one of the Frizzled family of receptors for the Wnt ligands, in lung cancer still remains to be elucidated. Here in this study we showed that Fzd-8 was over-expressed in human lung cancer tissue samples and cell lines. To investigate the functional importance of the Fzd-8 over-expression in lung cancer, we used shRNA to knock down Fzd-8 mRNA in lung cancer cells expressing the gene. We observed that Fzd-8 shRNA inhibited cell proliferation along with decreased activity of Wnt pathway in vitro, and also significantly suppressed A549 xenograft model in vivo (p<0.05). Furthermore, we found that knocking down Fzd-8 by shRNA sensitized the lung cancer cells to chemotherapy Taxotere. These data suggest that Fzd-8 is a putative therapeutic target for human lung cancer and over-expression of Fzd-8 may be important for aberrant Wnt activation in lung cancer.