BIMA, a TPR-containing protein required for mitosis, localizes to the spindle pole body in Aspergillus nidulans

The MYB-like protein MylA contributes to conidiogenesis and conidial germination in Aspergillus nidulans

Myeloblastosis (MYB)-like proteins are a family of highly conserved transcription factors in animals, plants, and fungi and are involved in the regulation of mRNA expression of genes. In this study, we identified and characterized one MYB-like protein in the model organism Aspergillus nidulans. We screened the mRNA levels of genes encoding MYB-like proteins containing two MYB repeats in conidia and found that the mRNA levels of four genes including flbD, cicD, and two uncharacterized genes, were high in conidia. To investigate the roles of two uncharacterized genes, AN4618 and AN10944, deletion mutants for each gene were generated. Our results revealed that AN4618 was required for fungal development. Therefore, we further investigated the role of AN4618, named as mylA, encoding the MYB-like protein containing two MYB repeats. Functional studies revealed that MylA was essential for normal fungal growth and development. Phenotypic and transcriptomic analyses demonstrated that deletion of mylA affected stress tolerance, cell wall integrity, and long-term viability in A. nidulans conidia. In addition, the germination rate of the mylA deletion mutant conidia was decreased compared with that of the wild-type conidia. Overall, this study suggests that MylA is critical for appropriate development, conidial maturation, dormancy, and germination in A. nidulans.

Gamma-tubulin regulates the anaphase-promoting complex/cyclosome during interphase

Activation of the APC/C requires microtubule-nucleating independent aspects of γ-tubulin function.

A cold-sensitive γ-tubulin allele of Aspergillus nidulans, mipAD159, causes defects in mitotic and cell cycle regulation at restrictive temperatures that are apparently independent of microtubule nucleation defects. Time-lapse microscopy of fluorescently tagged mitotic regulatory proteins reveals that cyclin B, cyclin-dependent kinase 1, and the Ancdc14 phosphatase fail to accumulate in a subset of nuclei at restrictive temperatures. These nuclei are permanently removed from the cell cycle, whereas other nuclei, in the same multinucleate cell, cycle normally, accumulating and degrading these proteins. After each mitosis, additional daughter nuclei fail to accumulate these proteins, resulting in an increase in noncycling nuclei over time and consequent inhibition of growth. Extensive analyses reveal that these noncycling nuclei result from a nuclear autonomous, microtubule-independent failure of inactivation of the anaphase-promoting complex/cyclosome. Thus, γ-tubulin functions to regulate this key mitotic and cell cycle regulatory complex.

Cdk1 phosphorylation sites on Cdc27 are required for correct chromosomal localisation and APC/C function in syncytial Drosophila embryos

Anaphase-promoting complex or cyclosome (APC/C) controls the metaphase-to-anaphase transition and mitosis exit by triggering the degradation of key cell cycle regulators such as securin and B-type cyclins. However, little is known about the functions of individual APC/C subunits and how they might regulate APC/C activity in space and time. Here, we report that two potential Cdk1 kinase phosphorylation sites are required for the chromosomal localisation of GFP::Cdc27 during mitosis. Either or both of the highly conserved proline residues in the Cdk1 phosphorylation consensus sequence motifs were mutated to alanine (Cdc27 P304A or P456A). The singly mutated fusion proteins, GFP::Cdc27P304A and GFP::Cdc27P456A, can still localise to mitotic chromosomes in a manner identical to wild-type GFP::Cdc27 and are functional in that they can rescue the phenotype of the cdc27L7123 mutant in vivo. However, when both of the Cdk1 phosphorylation sequence motifs were mutated, the resulting GFP::Cdc27P304A,P456A construct was not localised to the chromosomes during mitosis and was no longer functional, as it failed to rescue mutant phenotypes of the cdc27L7123 gene. High levels of cyclin B and cyclin A were detected in mutant third instar larvae brain samples compared with its wild-type control. These results show for the first time that the two potential Cdk1 phosphorylation sites on Drosophila Cdc27 are required for its chromosomal localisation during mitosis and imply that these localisations specific to Cdc27 are crucial for APC/C functions.

Cytoplasmic dynein's mitotic spindle pole localization requires a functional anaphase-promoting complex, gamma-tubulin, and NUDF/LIS1 in Aspergillus nidulans

In Aspergillus nidulans, cytoplasmic dynein and NUDF/LIS1 are found at the spindle poles during mitosis, but they seem to be targeted to this location via different mechanisms. The spindle pole localization of cytoplasmic dynein requires the function of the anaphase-promoting complex (APC), whereas that of NUDF does not. Moreover, although NUDF's localization to the spindle poles does not require a fully functional dynein motor, the function of NUDF is important for cytoplasmic dynein's targeting to the spindle poles. Interestingly, a gamma-tubulin mutation, mipAR63, nearly eliminates the localization of cytoplasmic dynein to the spindle poles, but it has no apparent effect on NUDF's spindle pole localization. Live cell analysis of the mipAR63 mutant revealed a defect in chromosome separation accompanied by unscheduled spindle elongation before the completion of anaphase A, suggesting that gamma-tubulin may recruit regulatory proteins to the spindle poles for mitotic progression. In A. nidulans, dynein is not apparently required for mitotic progression. In the presence of a low amount of benomyl, a microtubule-depolymerizing agent, however, a dynein mutant diploid strain exhibits a more pronounced chromosome loss phenotype than the control, indicating that cytoplasmic dynein plays a role in chromosome segregation.

Screening for microtubule-disrupting antifungal agents by using a mitotic-arrest mutant of Aspergillus nidulans and novel action of phenylalanine derivatives accompanying tubulin loss

The microtubule, which is one of the major targets of anthelmintics, anticancer drugs, and fungicides, is composed mainly of alpha- and beta-tubulins. We focused on a unique characteristic of an Aspergillus nidulans benA33 mutant to screen for microtubule-disrupting antifungal agents. This mutant, which has a beta-tubulin with a mutation of a single amino acid, undergoes mitotic arrest due to the formation of hyperstable microtubules at 37 degrees C. The heat sensitivity of the mutant is remedied by some antimicrotubule agents. We found that an agar plate assay with the mutant was able to distinguish three types of microtubule inhibitors. The growth recovery zones of the mutant were formed around paper disks containing microtubule inhibitors, including four benzimidazoles, ansamitocin P-3, griseofulvin, and rhizoxin, on the agar plate at 37 degrees C. Nocodazole, thiabendazole, and griseofulvin reversed the mitotic arrest of the mutant and promoted its hyphal growth. Ansamitocin P-3 and rhizoxin showed growth recovery zones around the growth-inhibitory zones. Benomyl and carbendazim also reversed mitotic arrest but produced weaker growth recovery than the aforementioned drugs. Other microtubule inhibitors, such as colchicine, Colcemid, paclitaxel, podophyllotoxin, TN-16, vinblastine, and vincristine, as well as some cytoskeletal inhibitors tested, did not show such activity. In our screening, we newly identified two mycotoxins, citrinin and patulin, two sesquiterpene dialdehydes, polygodial and warburganal, and four phenylalanine derivatives, arphamenine A, L-2,5-dihydrophenylalanine (DHPA), N-tosyl-L-phenylalanine chloromethylketone, and N-carbobenzoxy-L-phenylalanine chloromethyl ketone. In a wild-type strain of A. nidulans, DHPA caused selective losses of microtubules, as determined by fluorescence microscopy, and of both alpha- and beta-tubulins, as determined by Western blot analysis. This screening method involving the benA33 mutant of A. nidulans is useful, convenient, and highly selective. The phenylalanine derivatives tested are of a novel type of microtubule-disrupting antifungal agents, producing an accompanying loss of tubulins, and are different from well-known tubulin inhibitors affecting the assembly of tubulin dimers into microtubules.

The Arabidopsis anaphase-promoting complex or cyclosome: molecular and genetic characterization of the APC2 subunit

In yeast and animals, the anaphase-promoting complex or cyclosome (APC/C) is an essential ubiquitin protein ligase that regulates mitotic progression and exit by controlling the stability of cell cycle regulatory proteins, such as securin and the mitotic cyclins. In plants, the function, regulation, and substrates of the APC/C are poorly understood. To gain more insight into the roles of the plant APC/C, we characterized at the molecular level one of its subunits, APC2, which is encoded by a single-copy gene in Arabidopsis. We show that the Arabidopsis gene is able to partially complement a budding yeast apc2 ts mutant. By yeast two-hybrid assays, we demonstrate an interaction of APC2 with two other APC/C subunits: APC11 and APC8/CDC23. A reverse-genetic approach identified Arabidopsis plants carrying T-DNA insertions in the APC2 gene. apc2 null mutants are impaired in female megagametogenesis and accumulate a cyclin-beta-glucuronidase reporter protein but do not display metaphase arrest, as observed in other systems. The APC2 gene is expressed in various plant organs and does not seem to be cell cycle regulated. Finally, we report intriguing differences in APC2 protein subcellular localization compared with that in other systems. Our observations support a conserved function of the APC/C in plants but a different mode of regulation.

Identification of essential genes in the human fungal pathogen Aspergillus fumigatus by transposon mutagenesis

The opportunistic pathogen Aspergillus fumigatus is the most frequent cause of deadly airborne fungal infections in developed countries. In order to identify novel antifungal-drug targets, we investigated the genome of A. fumigatus for genes that are necessary for efficient fungal growth. An artificial A. fumigatus diploid strain with one copy of an engineered impala160 transposon from Fusarium oxysporum integrated into its genome was used to generate a library of diploid strains by random in vivo transposon mutagenesis. Among 2,386 heterozygous diploid strains screened by parasexual genetics, 1.2% had a copy of the transposable element integrated into a locus essential for A. fumigatus growth. Comparison of genomic sequences flanking impala160 in these mutants with that of the genome of A. fumigatus allowed the characterization of 20 previously uncharacterized A. fumigatus genes. Among these, homologues of genes essential for Saccharomyces cerevisiae growth have been identified, as well as genes that do not have homologues in other fungal species. These results confirm that heterologous transposition using the transposable element impala is a powerful tool for functional genomics in ascomycota, and they pave the way for defining the complete set of essential genes in A. fumigatus, the first step toward target-based development of new antifungal drugs.

The dynamic localisation of the Drosophila APC/C: evidence for the existence of multiple complexes that perform distinct functions and are differentially localised

In Drosophila cells, the destruction of cyclin B is spatially regulated. In cellularised embryos, cyclin B is initially degraded on the mitotic spindle and is then degraded in the cytoplasm. In syncytial embryos,only the spindle-associated cyclin B is degraded at the end of mitosis. The anaphase promoting complex/cyclosome (APC/C) targets cyclin B for destruction,but its subcellular localisation remains controversial. We constructed GFP fusions of two core APC/C subunits, Cdc16 and Cdc27. These fusion proteins were incorporated into the endogenous APC/C and were largely localised in the cytoplasm during interphase in living syncytial embryos. Both fusion proteins rapidly accumulated in the nucleus prior to nuclear envelope breakdown but only weakly associated with mitotic spindles throughout mitosis. Thus, the global activation of a spatially restricted APC/C cannot explain the spatially regulated destruction of cyclin B. Instead, different subpopulations of the APC/C must be activated at different times to degrade cyclin B. Surprisingly,we noticed that GFP-Cdc27 associated with mitotic chromosomes, whereas GFP-Cdc16 did not. Moreover, reducing the levels of Cdc16 or Cdc27 by >90%in tissue culture cells led to a transient mitotic arrest that was both biochemically and morphologically distinct. Taken together, our results raise the intriguing possibility that there could be multiple forms of the APC/C that are differentially localised and perform distinct functions.

Identification of the human crooked neck gene by comparative gene identification

Drosophila melanogaster serves as a useful model organism for functional genomic studies, and its genome project was recently completed. We previously described a comparative-gene-identification approach to assist human ortholog gene identification that involves applying an entire proteome as an alignment template. Analysis of the available 14100 Drosophila protein sequences revealed that 37% of them (5228 sequences) might lead to discoveries of novel human genes. Upon further database interrogations, we found several putative full-length human gene transcripts, including the human crooked neck (crn) gene. Based on sequence gap-closure experiments using reverse transcriptase-polymerase chain reaction as well as bioinformatic analysis, we found that the assembled human cDNA contig of crooked neck gene was at least 3903 base pairs in length with alternative splicing variations which encoded mainly for a 687-amino-acid residue protein. The human crooked neck gene was located on chromosome 20 with at least 15 exons. The unique features of the 16 copies of the tetratrico peptide repeat (TPR) motif were conserved in the yeast, fly and human crooked neck orthologous proteins, which were important for spliceosome assembly in cells.

Mitotic histone H3 phosphorylation by the NIMA kinase in Aspergillus nidulans

Phosphorylation of histone H3 serine 10 correlates with chromosome condensation and is required for normal chromosome segregation in Tetrahymena. This phosphorylation is dependent upon activation of the NIMA kinase in Aspergillus nidulans. NIMA expression also induces Ser-10 phosphorylation inappropriately in S phase-arrested cells and in the absence of NIMX(cdc2) activity. At mitosis, NIMA becomes enriched on chromatin and subsequently localizes to the mitotic spindle and spindle pole bodies. The chromatin-like localization of NIMA early in mitosis is tightly correlated with histone H3 phosphorylation. Finally, NIMA can phosphorylate histone H3 Ser-10 in vitro, suggesting that NIMA is a mitotic histone H3 kinase, perhaps helping to explain how NIMA promotes chromatin condensation in A. nidulans and when expressed in other eukaryotes.

The anaphase-promoting complex: new subunits and regulators

Ubiquitin-mediated proteolysis of cell cycle regulators is a crucial process during the cell cycle. The anaphase-promoting complex (APC) is a large, multiprotein complex whose E3-ubiquitin ligase activity is required for the ubiquitination of mitotic cyclins and other regulatory proteins that are targeted for destruction during cell division. The recent identification of new APC subunits and regulatory proteins has begun to reveal some of the intricate mechanisms that govern APC regulation. One mechanism is the use of specificity factors to impose temporal control over substrate degradation. A second mechanism is the APC-mediated proteolysis of specific APC regulators. Finally, components of both the APC and the SCF E3 ubiquitin-ligase complex contain several conserved sequence motifs, including WD-40 repeats and cullin homology domains, which suggest that both complexes may use a similar mechanism for substrate ubiquitination.

Fission yeast APC/cyclosome subunits, Cut20/Apc4 and Cut23/Apc8, in regulating metaphase-anaphase progression and cellular stress responses

BACKGROUND

The 20S cyclosome/APC complex promotes metaphase-anaphase transition by ubiquitinating its specific substrates such as mitotic cyclins and anaphase inhibitor Cut2/Pds1/securin. The complex has been shown to contain more than 10 proteins in budding yeast and frog. In fission yeast, however, only five (Cut4, Cut9, Nuc2, Apc10, Hcn1) have been identified.

RESULTS

More than five hundred temperature-sensitive mutants were screened for identifying those defective in mitotic anaphase. Fifty-five showed the cut (cell untimely torn) phenotype or metaphase-arrest phenotypes, 27 of them locating at new loci. Their extracts were run in sucrose gradient centrifugation, and four showed alterations in the sedimentation profiles. The gene products of cut20+ and cut23+ were thus identified. Phenotypes of cut20-100 mutant highly resemble cut4-533 in many ways: they are hypersensitive to canavanine and CdCl2, and suppressed by PKA-inactivating regulators, cAMP-dependent phosphodiesterase and PKA regulatory subunits. Cut20 interacts closely with Cut4 in the assembly process of cyclosome. But cut20 mutant differs from cut4, as a novel gene stw1+ suppresses cut20 mutant but not cut4. cut23-194 mutant cells are sterile and blocked at metaphase, but does not show sensitivity to the stress and cAMP. TPR repeat-containing Cut23 may not be the stable component of APC/cyclosome, and its level significantly fluctuates during cell cycle. Cut23 may be ubiquitinated and degraded in a cell cycle dependent fashion.

CONCLUSIONS

We identified two new subunits of fission yeast cyclosome/APC complex. Our observations indicate that cyclosome components are divided into several subgroups with distinctly different roles.

Mitosis in filamentous fungi: how we got where we are

This review traces the principal advances in the study of mitosis in filamentous fungi from its beginnings near the end of the 19(th) century to the present day. Meiosis and mitosis had been accurately described and illustrated by the second decade of the present century and were known to closely resemble nuclear divisions in higher eukaryotes. This information was effectively lost in the mid-1950s, and the essential features of mitosis were then rediscovered from about the mid-1960s to the mid-1970s. Interest in the forces that separate chromatids and spindle poles during fungal mitosis followed closely on the heels of detailed descriptions of the mitotic apparatus in vivo and ultrastructurally during this and the following decade. About the same time, fundamental studies of the structure of fungal chromatin and biochemical characterization of fungal tubulin were being carried out. These cytological and biochemical studies set the stage for a surge of renewed interest in fungal mitosis that was issued in by the age of molecular biology. Filamentous fungi have provided model studies of the cytology and genetics of mitosis, including important advances in the study of mitotic forces, microtubule-associated motor proteins, and mitotic regulatory mechanisms.

Subunits and substrates of the anaphase-promoting complex

The initiation of anaphase and exit from mitosis depend on a ubiquitination complex called the anaphase-promoting complex (APC) or cyclosome. The APC is composed of more than 10 constitutive subunits and associates with additional regulatory factors in mitosis and during the G1 phase of the cell cycle. At the metaphase-anaphase transition the APC ubiquitinates proteins such as Pds1 in budding yeast and Cut2 in fission yeast whose subsequent degradation by the 26S proteasome is essential for the initiation of sister chromatid separation. Later in anaphase and telophase the APC promotes the inactivation of the mitotic cyclin-dependent protein kinase 1 by ubiquitinating its activating subunit cyclin B. The APC also mediates the ubiquitin-dependent proteolysis of several other mitotic regulators, including other protein kinases, APC activators, spindle-associated proteins, and inhibitors of DNA replication.

Regulation of the anaphase-promoting complex/cyclosome by bimAAPC3 and proteolysis of NIMA

Surprisingly, although highly temperature-sensitive, the bimA1(APC3) anaphase-promoting complex/cyclosome (APC/C) mutation does not cause arrest of mitotic exit. Instead, rapid inactivation of bimA1(APC3) is shown to promote repeating oscillations of chromosome condensation and decondensation, activation and inactivation of NIMA and p34(cdc2) kinases, and accumulation and degradation of NIMA, which all coordinately cycle multiple times without causing nuclear division. These bimA1(APC3)-induced cell cycle oscillations require active NIMA, because a nimA5 + bimA1(APC3) double mutant arrests in a mitotic state with very high p34(cdc2) H1 kinase activity. NIMA protein instability during S phase and G2 was also found to be controlled by the APC/C. The bimA1(APC3) mutation therefore first inactivates the APC/C but then allows its activation in a cyclic manner; these cycles depend on NIMA. We hypothesize that bimAAPC3 could be part of a cell cycle clock mechanism that is reset after inactivation of bimA1(APC3). The bimA1(APC3) mutation may also make the APC/C resistant to activation by mitotic substrates of the APC/C, such as cyclin B, Polo, and NIMA, causing mitotic delay. Once these regulators accumulate, they activate the APC/C, and cells exit from mitosis, which then allows this cycle to repeat. The data indicate that bimAAPC3 regulates the APC/C in a NIMA-dependent manner.

Structural requirements for in vivo myosin I function in Aspergillus nidulans

We have investigated the minimal requirements of the tail region for myosin I function in vivo using the filamentous fungus Aspergillus nidulans. The CL3 strain (McGoldrick, C. A., Gruver, C., and May, G. S. (1995) J. Cell Biol. 128, 577-587) was transformed with a variety of myoA constructs containing mutations in the IQ, TH-1-like, SH3, and proline-rich domains by frameshift or in-frame deletions of the tail domains. The resulting strains contained wild type myoA driven by the alcA promoter and a mutant myoA driven by its endogenous promoter. This strategy allowed for selective expression of the wild type and/or mutant form of MYOA by the choice of growth medium. Proper septation and hyphal branching were found to be dependent on the interaction of the IQ motifs with calmodulin, as well as, the presence of its proline-rich domain. Additionally, a single proline-rich motif was sufficient for nearly wild type MYOA function. Most surprisingly, the SH3 domain was not essential for MYOA function. These studies expand our previous knowledge of the function of MYOA to include roles in hyphal morphogenesis, septal wall formation, and cell polarity, laying the groundwork for more detailed investigations on the function of the various tail domains in MYOA.

A role for NIMA in the nuclear localization of cyclin B in Aspergillus nidulans

NIMA promotes entry into mitosis in late G2 by some mechanism that is after activation of the Aspergillus nidulans G2 cyclin-dependent kinase, NIMXCDC2/NIMECyclin B. Here we present two independent lines of evidence which indicate that this mechanism involves control of NIMXCDC2/NIMECyclin B localization. First, we found that NIMECyclin B localized to the nucleus and the nucleus-associated organelle, the spindle pole body, in a NIMA-dependent manner. Analysis of cells from asynchronous cultures, synchronous cultures, and cultures arrested in S or G2 showed that NIMECyclin B was predominantly nuclear during interphase, with maximal nuclear accumulation in late G2. NIMXCDC2 colocalized with NIMECyclin B in G2 cells. Although inactivation of NIMA using either the nimA1 or nimA5 temperature-sensitive mutations blocked cells in G2, NIMXCDC2/NIMECyclin B localization was predominantly cytoplasmic rather than nuclear. Second, we found that nimA interacts genetically with sonA, which is a homologue of the yeast nucleocytoplasmic transporter GLE2/RAE1. Mutations in sonA were identified as allele-specific suppressors of nimA1. The sonA1 suppressor alleviated the nuclear division and NIMECyclin B localization defects of nimA1 cells without markedly increasing NIMXCDC2 or NIMA kinase activity. These results indicate that NIMA promotes the nuclear localization of the NIMXCDC2/ NIMECyclin B complex, by a process involving SONA. This mechanism may be involved in coordinating the functions of NIMXCDC2 and NIMA in the regulation of mitosis.

The fission yeast microtubule cytoskeleton

The Schizosaccharomyces pombe genome sequencing project (http://www.sanger.ac.uk/Projects/S_pombe/) is nearly complete, and this is likely to generate interest in fission yeast as a model system beyond its traditional strongholds in the study of the cell cycle and sexual differentiation. In many fields S. pombe will offer a useful complement to the more widely studied Saccharomyces cerevisiae, but in some areas the impact of S. pombe may well rival or exceed that of this budding yeast in terms of relevance to higher systems. Because of the considerable differences from the S. cerevisiae microtubule cytoskeleton, studying microtubules in S. pombe is likely to enhance the contribution of model systems to our understanding of the principles and practices of microtubule organisation in eukaryotes in general.

BIMAAPC3, a component of the Aspergillus anaphase promoting complex/cyclosome, is required for a G2 checkpoint blocking entry into mitosis in the absence of NIMA function

Temperature sensitive (ts) nimA mutants of Aspergillus nidulans arrest at a unique point in G2 which is post activation of CDC2. Here we show that this G2 arrest is due to loss of nimA function and that it is dependent on BIMAAPC3, a component of the anaphase promoting complex/cyclosome (APC/C). Whereas nimA single mutants arrested in G2 with decondensed chromatin and interphase microtubule arrays, nimA, bimAAPC3 double mutants arrested growth with condensed chromatin and aster-like microtubule arrays. nimA, bimAAPC3 double mutants entered mitosis with kinetics similar to bimAAPC3 single mutants and wild-type cells, indicating a checkpoint-like role for BIMAAPC3 in G2. Even cells which had been depleted for NIMA protein and which contained insignificant levels of NIMA kinase activity entered mitosis on inactivation of bimAAPC3. BIMAAPC3 was present in a >25S complex containing BIMEAPC1, and bimAAPC3 mutants were sensitive to elevated CYCLIN B expression, consistent with BIMAAPC3 being a component of the APC/C. Inactivation of bimAAPC3 had little affect on the steady state levels of the B-type cyclin, NIMECyclin B. Our results indicate that BIMAAPC3, and most likely the APC/C itself, is activated in G2 in nimA mutants. We propose that APC/C activation is part of a novel, late G2 checkpoint, which responds to a defective process or structure in nimA mutants, and which prevents inappropriate entry into mitosis.

Motoring along the hyphae: molecular motors and the fungal cytoskeleton

This review discusses molecular motors that use the microfilament and microtubule cytoskeletal systems in filamentous fungi. There has been an explosion in our knowledge of kinesins over the past year, because of the integration of genetic and biochemical data. The recognition of possible interactions between septation genes and cytokinesis has also advanced our understanding of microfilament-based cytoskeletal systems. We review recent findings on microfilament motors, including conventional and unconventional myosins, and the microtubule motors of the kinesin family and cytoplasmic dynein. The roles that these molecules play in hyphal morphogenesis and organelle transport provide an insight into cytoskeletal-based transport systems.

Regulation of the G1 phase of the cell cycle by periodic stabilization and degradation of the p25rum1 CDK inhibitor

In fission yeast, the cyclin-dependent kinase (CDK) inhibitor p25(rum1) is a key regulator of progression through the G1 phase of the cell cycle. We show here that p25(rum1) protein levels are sharply periodic. p25(rum1) begins to accumulate at anaphase, persists in G1 and is destroyed during S phase. p25(rum1 )is stabilized and polyubiquitinated in a mutant defective in the 26S proteasome, suggesting that its degradation normally occurs through the ubiquitin-dependent 26S proteasome pathway. Phosphorylation of p25(rum1 )by cdc2-cyclin complexes at residues T58 and T62 is important to target the protein for degradation. Mutation of one or both of these residues to alanine causes stabilization of p25(rum1) and induces a cell cycle delay in G1 and polyploidization due to occasional re-initiation of DNA replication before mitosis. The CDK-cyclin complex cdc2-cig1, which is insensitive to p25(rum1 )inhibition, seems to be the main kinase that phosphorylates p25(rum1). Phosphorylation of p25(rum1) in S phase and G2 serves as the trigger for p25(rum1) proteolysis. Thus, periodic accumulation and degradation of the CDK inhibitor p25(rum1 )in G1 plays a role in setting a threshold of cyclin levels important in determining the length of the pre-Start G1 phase and in ensuring the correct order of cell cycle events.

A subunit of the anaphase-promoting complex is a centromere-associated protein in mammalian cells

Sister chromatids in early mitotic cells are held together mainly by interactions between centromeres. The separation of sister chromatids at the transition between the metaphase and the anaphase stages of mitosis depends on the anaphase-promoting complex (APC), a 20S ubiquitin-ligase complex that targets proteins for destruction. A subunit of the APC, called APC-alpha in Xenopus (and whose homologs are APC-1, Cut4, BIME, and Tsg24), has recently been identified and shown to be required for entry into anaphase. We now show that the mammalian APC-alpha homolog, Tsg24, is a centromere-associated protein. While this protein is detected only during the prophase to the anaphase stages of mitosis in Chinese hamster cells, it is constitutively associated with the centromeres in murine cells. We show that there are two forms of this protein in mammalian cells, a soluble form associated with other components of the APC and a centromere-bound form. We also show that both the Tsg24 protein and the Cdc27 protein, another APC component, are bound to isolated mitotic chromosomes. These results therefore support a model in which the APC by ubiquitination of a centromere protein regulates the sister chromatid separation process.

Mitosis in vertebrate somatic cells with two spindles: implications for the metaphase/anaphase transition checkpoint and cleavage

During mitosis an inhibitory activity associated with unattached kinetochores prevents PtK1 cells from entering anaphase until all kinetochores become attached to the spindle. To gain a better understanding of how unattached kinetochores block the metaphase/anaphase transition we followed mitosis in PtK1 cells containing two independent spindles in a common cytoplasm. We found that unattached kinetochores on one spindle did not block anaphase onset in a neighboring mature metaphase spindle 20 microm away that lacked unattached kinetochores. As in cells containing a single spindle, anaphase onset occurred in the mature spindles x = 24 min after the last kinetochore attached regardless of whether the adjacent immature spindle contained one or more unattached kinetochores. These findings reveal that the inhibitory activity associated with an unattached kinetochore is functionally limited to the vicinity of the spindle containing the unattached kinetochore. We also found that once a mature spindle entered anaphase the neighboring spindle also entered anaphase x = 9 min later regardless of whether it contained monooriented chromosomes. Thus, anaphase onset in the mature spindle catalyzes a "start anaphase" reaction that spreads globally throughout the cytoplasm and overrides the inhibitory signal produced by unattached kinetochores in an adjacent spindle. Finally, we found that cleavage furrows often formed between the two independent spindles. This reveals that the presence of chromosomes and/or a spindle between two centrosomes is not a prerequisite for cleavage in vertebrate somatic cells.

Regulation of CDK/cyclin complexes during the cell cycle

The eukaryotic cell cycle is regulated by the temporal activation of different cyclin-dependent kinase (CDK)/cyclin complexes. Whilst the level of the catalytic subunit of the complex, the CDK, remains relatively constant through the cycle, the level of the cyclin subunit generally oscillates. Cyclins are synthesized, bind and activate the CDK and are then destroyed. In this review, we summarize the current knowledge of the regulation of the cell cycle by CDK/cyclin complexes with special emphasis on new developments in cyclin biosynthesis and destruction, the structural analysis of the CDK/cyclin complexes and the role of a set of inhibitors of CDK/cyclin complexes that are important for the coordination of the different stages of the cell cycle.

How proteolysis drives the cell cycle

Oscillations in the activity of cyclin-dependent kinases (CDKs) promote progression through the eukaryotic cell cycle. This review examines how proteolysis regulates CDK activity-by degrading CDK activators or inhibitors-and also how proteolysis may directly trigger the transition from metaphase to anaphase. Proteolysis during the cell cycle is mediated by two distinct ubiquitin-conjugation pathways. One pathway, requiring CDC34, initiates DNA replication by degrading a CDK inhibitor. The second pathway, involving a large protein complex called the anaphase-promoting complex or cyclosome, initiates chromosome segregation and exit from mitosis by degrading anaphase inhibitors and mitotic cyclins. Proteolysis therefore drives cell cycle progression not only by regulating CDK activity, but by directly influencing chromosome and spindle dynamics.

The role of controlled proteolysis in cell-cycle regulation

Cyclins and cyclin-dependent kinases are key regulators of the cell cycle. The binding of different cyclins, required to activate the catalytically inactive cyclin-dependent kinases, determines the substrate specificity of the enzymes. Cyclin-dependent-kinase inhibitors have an adverse effect, blocking the catalytic activity of cyclin-activated cyclin-dependent kinases. The cell cycle is a cyclic process of successive transient activation or inactivation of cyclin-dependent kinases by association with different cyclin regulatory subunits or cyclin-dependent kinase inhibitors. As the concentration of cyclin-dependent kinases is fairly constant during the cell cycle and exceeds the total amount of cyclins present in the cell, the exchange of regulatory subunits is determined by the availability of the different cyclins. Transcriptional control of cyclin gene expression is the most decisive factor determining the total amount of different cyclins synthesized. The actual concentration of a cyclin, however, is always the result of an equilibrium between the rates of its synthesis and degradation. While cyclin gene expression has long been known to be cell-cycle controlled, the idea of the rapid destruction of cyclins or cyclin-dependent-kinase inhibitors as an equally important factor contributing to the progress of the cell cycle is more recent. The role of controlled proteolysis in the regulation of cell cycle is discussed in this review. Two general features of this regulation are worth mentioning: cyclin-dependent kinases activated by different cyclin regulatory subunits have a central role both in the transcriptional regulation of their own genes and in the regulated, selective destruction of cyclins or cyclin-dependent kinase inhibitors; transcriptional regulation of cyclin gene expression ensures fine-tuned, continuous changes, and controlled proteolysis generates abrupt, irreversible transitions. The progress of the cell cycle is based on a delicate balance of the these mutual, but opposite regulations.

Cell cycle regulation in Aspergillus by two protein kinases

Great progress has recently been made in our understanding of the regulation of the eukaryotic cell cycle, and the central role of cyclin-dependent kinases is now clear. In Aspergillus nidulans it has been established that a second class of cell-cycle-regulated protein kinases, typified by NIMA (encoded by the nimA gene), is also required for cell cycle progression into mitosis. Indeed, both p34cdc2/cyclin B and NIMA have to be correctly activated before mitosis can be initiated in this species, and p34cdc2/cyclin B plays a role in the mitosis-specific activation of NIMA. In addition, both kinases have to be proteolytically destroyed before mitosis can be completed. NIMA-related kinases may also regulate the cell cycle in other eukaryotes, as expression of NIMA can promote mitotic events in yeast, frog or human cells. Moreover, dominant-negative versions of NIMA can adversely affect the progression of human cells into mitosis, as they do in A. nidulans. The ability of NIMA to influence mitotic regulation in human and frog cells strongly suggests the existence of a NIMA pathway of mitotic regulation in higher eukaryotes. A growing number of NIMA-related kinases have been isolated from organisms ranging from fungi to humans, and some of these kinases are also cell-cycle-regulated. How NIMA-related kinases and cyclin-dependent kinases act in concert to promote cell cycle transitions is just beginning to be understood. This understanding is the key to a full knowledge of cell cycle regulation.

The self-destructive personality of a cell cycle in transition

The transition from G1 to S phase, sister chromatid separation in anaphase, and the exit from mitosis are driven by the destruction of cell cycle regulatory proteins by distinct ubiquitin-dependent proteolytic pathways. The components and targets of these key degradation pathways are now becoming clear. Genetic and biochemical dissections of these extremely specific and well regulated destruction pathways are providing fundamental insights into the mechanisms of control of the cell division cycle.

The Aspergillus nidulans bimE (blocked-in-mitosis) gene encodes multiple cell cycle functions involved in mitotic checkpoint control and mitosis

The bimE (blocked-in-mitosis) gene appears to function as a negative mitotic regulator because the recessive bimE7 mutation can override certain interphase-arresting treatments and mutations, causing abnormal induction of mitosis. We have further investigated the role of bimE in cell cycle checkpoint control by: (1) coordinately measuring mitotic induction and DNA content of bimE7 mutant cells; and (2) analyzing epistasis relationships between bimE7 and 16 different nim mutations. A combination of cytological and flow cytometric techniques was used to show that bimE7 cells at restrictive temperature (44 degrees C) undergo a normal, although somewhat slower cell cycle prior to mitotic arrest. Most bimE7 cells were fully reversible from restrictive temperature arrest, indicating that they are able to enter mitosis normally, and therefore require bimE function in order to finish mitosis. Furthermore, epistasis studies between bimE7 and mutations in cdc2 pathway components revealed that the induction of mitosis caused by inactivation of bimE requires functional p34cdc2 kinase, and that mitotic induction by bimE7 depends upon several other nim genes whose functions are not yet known. The involvement of bimE in S phase function and mitotic checkpoint control was suggested by three lines of evidence. First, at restrictive temperature the bimE7 mutation slowed the cell cycle by delaying the onset or execution of S phase. Second, at permissive temperature (30 degrees C) the bimE7 mutation conferred enhanced sensitivity to the DNA synthesis inhibitor hydroxyurea. Finally, the checkpoint linking M phase to the completion of S phase was abolished when bimE7 was combined with two nim mutations that cause arrest in G1 or S phase. A model for bimE function based on these findings is presented.

Characterization and localization of the cytoplasmic dynein heavy chain in Aspergillus nidulans

Migration of nuclei throughout the mycelium is essential for the growth and differentiation of filamentous fungi. In Aspergillus nidulans, the nudA gene, which is involved in nuclear migration, encodes a cytoplasmic dynein heavy chain. In this paper we use antibodies to characterize the Aspergillus cytoplasmic dynein heavy chain (ACDHC) and to show that the ACDHC is concentrated at the growing tip of the fungal mycelium. We demonstrate that four temperature-sensitive mutations in the nudA gene result in a striking decrease in ACDHC protein. Cytoplasmic dynein has been implicated in nuclear division in animal cells. Because the temperature-sensitive nudA mutants are able to grow slowly with occasional nuclei found in the mycelium and are able to undergo nuclear division, we have created a deletion/disruption nudA mutation and a tightly downregulated nudA mutation. These mutants exhibit a phenotype very similar to that of the temperature-sensitive nudA mutants with respect to growth, nuclear distribution, and nuclear division. This suggests that there are redundant backup motor proteins for both nuclear migration and nuclear division.

Extragenic suppressors of nudC3, a mutation that blocks nuclear migration in Aspergillus nidulans

Nuclear migration plays an important role in the growth and development of many organisms including the filamentous fungus Aspergillus nidulans. We have cloned three genes from A. nidulans, nudA, nudC, and nudF, in which mutations affect nuclear migration. The nudA gene encodes the heavy chain of cytoplasmic dynein. The nudC gene encodes a 22-kD protein. The nudF gene was identified as an extra copy suppressor of the temperature sensitive (ts-) nudC3 mutation. The nudC3 mutation substantially decreases the intracellular concentration of the nudF protein at restrictive temperature. This is restored toward the normal level by an extra copy of nudF. To identify other genes whose products interact directly or indirectly with the NUDC protein, we have isolated a set of extragenic suppressors showed them to represent nine different genes, designated sncA-sncI (for suppressor of nudC). sncA-sncH were either dominant or semidominant in diploids homozygous for nudC3 and heterozygous for the snc mutations. All of the suppressors reversed the ts- phenotype of nudC3 by restoring the intracellular concentration of the NUDF protein.

p93dis1, which is required for sister chromatid separation, is a novel microtubule and spindle pole body-associating protein phosphorylated at the Cdc2 target sites

Fission yeast cold-sensitive (cs) dis1 mutants are defective in sister chromatid separation. The dis1+ gene was isolated by chromosome walking. The null mutant showed the same phenotype as that of cs mutants. The dis1+ gene product was identified as a novel 93-kD protein, and its localization was determined by use of anti-dis1 antibodies and green fluorescent protein (GFP) tagged to the carboxyl end of p93dis1. The tagged p93dis1 in living cells localizes along cytoplasmic microtubule arrays in interphase and the elongating anaphase spindle in mitosis, but association with the short metaphase spindle microtubules is strikingly reduced. In the spindle, the tagged p93dis1 is enriched at the spindle pole bodies (SPBs). Time-lapse video images of single cells support the localization shift of p93dis1 to the SPBs in metaphase and spindle microtubules in anaphase. The carboxy-terminal fragment, which is essential for Dis1 function, accumulates around the mitotic SPB. We propose that these localization shifts of p93dis1 in mitosis facilitates sister chromatid separation by affecting SPB and anaphase spindle function.

Frontier questions about sister chromatid separation in anaphase

Sister chromatid separation in anaphase is an important event in the cell's transmission of genetic information to a descendent. It has been investigated from different aspects: cell cycle regulation, spindle and chromosome dynamics within the three-dimensional cell architecture, transmission fidelity control and cellular signaling. Integrated studies directed toward unified understanding are possible using multidisciplinary methods with model organisms. Ubiquitin-dependent proteolysis, protein dephosphorylation, an unknown function by the TPR repeat proteins, chromosome transport by microtubule-based motors and DNA topological change by DNA topoisomerase II are all necessary for progression from metaphase to anaphase. Chromosome condensation, mitotic kinetochore function and spindle formation require a larger number of proteins, which are prerequisites for successful sister chromatid separation. Factors that help to retain sister chromatid connection after replication and prevent premature separation remain to be determined. Although sister chromatid separation occurs in anaphase, gene functions in other cell cycle stages also ensure the progression of correct chromatid separation.

A 20S complex containing CDC27 and CDC16 catalyzes the mitosis-specific conjugation of ubiquitin to cyclin B

Cyclin B is degraded at the onset of anaphase by a ubiquitin-dependent proteolytic system. We have fractionated mitotic Xenopus egg extracts to identify components required for this process. We find that UBC4 and at least one other ubiquitin-conjugating enzyme can support cyclin B ubiquitination. The mitotic specificity of cyclin ubiquitination is determined by a 20S complex that contains homologs of budding yeast CDC16 and CDC27. Because these proteins are required for anaphase in yeast and mammalian cells, we refer to this complex as the anaphase-promoting complex (APC). CDC27 antibodies deplete APC activity, while immunopurified CDC27 complexes are sufficient to complement either interphase extracts or a mixture of recombinant UBC4 and the ubiquitin-activating enzyme E1. These results suggest that APC functions as a regulated ubiquitin-protein ligase that targets cyclin B for destruction in mitosis.

CDC27Hs colocalizes with CDC16Hs to the centrosome and mitotic spindle and is essential for the metaphase to anaphase transition

We have isolated cDNAs and raised antibodies corresponding to the human homologs of the S. cerevisiae CDC27 and CDC16 proteins, which are tetratrico peptide repeat (TPR)-containing proteins essential for mitosis in budding yeast. We find that the CDC27Hs and CDC16Hs proteins colocalize to the centrosome at all stages of the mammalian cell cycle, and to the mitotic spindle. Injection of affinity-purified anti-CDC27Hs antibodies into logarithmically growing HeLa cells causes a highly reproducible cell cycle arrest in metaphase with apparently normal spindle structure. We conclude that CDC27 and CDC16 are evolutionarily conserved components of the centrosome and mitotic spindle that control the onset of postmetaphase events during mitosis.

Fission yeast TPR-family protein nuc2 is required for G1-arrest upon nitrogen starvation and is an inhibitor of septum formation

Fission yeast nuc2+ gene encodes a protein of a tetratricopeptide repeat (TPR) family which is conserved throughout evolution. We previously showed that nuc2 is required for exit from the mitotic metaphase. In this study, we present evidence which shows that nuc2 has two additional roles in the cell cycle. We showed that the nuc2 mutant is sterile even at the permissive temperature and septation occurs in the absence of chromosome separation at the restrictive temperature. The nuc2 mutant fails to arrest at the G1 phase upon nitrogen starvation at the permissive temperature which is a prerequisite for conjugation. Upon starvation, however, the nuc2 mutant ceased division normally and induced starvation-dependent gene expression. Therefore, the nuc2 mutant is deficient only for failure to block DNA replication upon starvation. At the lower restrictive temperature, the nuc2 mutant showed a ‘cut’ phenotype where septation and cytokinesis takes place without the completion of mitosis. Ectopic overexpression of the nuc2+ gene caused multiple rounds of S and M phases in the complete absence of septum formation. We propose that nuc2 is a novel cell cycle regulator essential for three events; firstly for exit from mitosis, secondly for DNA replication restraint under nutrient starvation and thirdly for inhibition of septation and cytokinesis until the completion of mitosis.

NudF, a nuclear migration gene in Aspergillus nidulans, is similar to the human LIS-1 gene required for neuronal migration

During a study of the genetics of nuclear migration in the filamentous fungus Aspergillus nidulans, we cloned a gene, nudF, which is required for nuclear migration during vegetative growth as well as development. The NUDF protein level is controlled by another protein NUDC, and extra copies of the nudF gene can suppress the nudC3 mutation. nudF encodes a protein with 42% sequence identity to the human LIS-1 (Miller-Dieker lissencephaly-1) gene, which is required for proper neuronal migration during brain development. This strong similarity suggests that the LIS-1 gene product may have a function similar to that of NUDF and supports previous findings to suggest that nuclear migration may play a role in neuronal migration.

DBF8, an essential gene required for efficient chromosome segregation in Saccharomyces cerevisiae

To investigate chromosome segregation in Saccharomyces cerevisiae, we examined a collection of temperature-sensitive mutants that arrest as large-budded cells at restrictive temperatures (L. H. Johnston and A. P. Thomas, Mol. Gen. Genet. 186:439-444, 1982). We characterized dbf8, a mutation that causes cells to arrest with a 2c DNA content and a short spindle. DBF8 maps to chromosome IX near the centromere, and it encodes a 36-kDa protein that is essential for viability at all temperatures. Mutational analysis reveals that three dbf8 alleles are nonsense mutations affecting the carboxy-terminal third of the encoded protein. Since all of these mutations confer temperature sensitivity, it appears that the carboxyl-terminal third of the protein is essential only at a restrictive temperature. In support of this conclusion, an insertion of URA3 at the same position also confers a temperature-sensitive phenotype. Although they show no evidence of DNA damage, dbf8 mutants exhibit increased rates of chromosome loss and nondisjunction even at a permissive temperature. Taken together, our data suggest that Dbf8p plays an essential role in chromosome segregation.

DBF8, an essential gene required for efficient chromosome segregation in Saccharomyces cerevisiae

To investigate chromosome segregation in Saccharomyces cerevisiae, we examined a collection of temperature-sensitive mutants that arrest as large-budded cells at restrictive temperatures (L. H. Johnston and A. P. Thomas, Mol. Gen. Genet. 186:439-444, 1982). We characterized dbf8, a mutation that causes cells to arrest with a 2c DNA content and a short spindle. DBF8 maps to chromosome IX near the centromere, and it encodes a 36-kDa protein that is essential for viability at all temperatures. Mutational analysis reveals that three dbf8 alleles are nonsense mutations affecting the carboxy-terminal third of the encoded protein. Since all of these mutations confer temperature sensitivity, it appears that the carboxyl-terminal third of the protein is essential only at a restrictive temperature. In support of this conclusion, an insertion of URA3 at the same position also confers a temperature-sensitive phenotype. Although they show no evidence of DNA damage, dbf8 mutants exhibit increased rates of chromosome loss and nondisjunction even at a permissive temperature. Taken together, our data suggest that Dbf8p plays an essential role in chromosome segregation.

Genetic and biochemical approaches to spindle function and chromosome segregation in eukaryotic microorganisms

The past year saw the molecular characterization of components of the Saccharomyces cerevisiae kinetochore and spindle pole body. In Schizosaccharomyces pombe, new cytological methods have been described for detection of centromeric DNA by light microscopy and probable kinetochores by electron microscopy.

The calcium-binding protein cell division cycle 31 of Saccharomyces cerevisiae is a component of the half bridge of the spindle pole body

Cdc31 mutants of Saccharomyces cerevisiae arrest at the nonpermissive temperature with large buds, G2 DNA content and, a single, abnormally large spindle pole body (SPB) (Byers, B. 1981. Molecular Genetics in Yeast. Alfred Benzon Symposium. 16:119-133). In this report, we show that the CDC31 gene product is essential for cell viability. We demonstrate that purified CDC31 protein binds Ca2+ and that this binding is highly specific. Taken together, three lines of evidence indicate that CDC31 is a component of the SPB. First, CDC31 cofractionates with enriched preparations of SPBs. Second, immunofluorescence staining indicates that CDC31 colocalizes with a known SPB component. Third, immunoelectron microscopy with whole cells and with isolated SPBs reveals that CDC31 is localized to the half bridge of the SPB, which lies immediately adjacent to the SPB plaques. CDC31 was detected mainly at the cytoplasmic side of the half bridge and, therefore, defines a further substructure of the SPB. We suggest that CDC31 is a member of a family of calcium-binding, centrosome-associated proteins from a phylogenetically diverse group of organisms.