Genetic analysis of the mitotic spindle

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.

Lesions in many different spindle components activate the spindle checkpoint in the budding yeast Saccharomyces cerevisiae

The spindle checkpoint arrests cells in mitosis in response to defects in the assembly of the mitotic spindle or errors in chromosome alignment. We determined which spindle defects the checkpoint can detect by examining the interaction of mutations that compromise the checkpoint (mad1, mad2, and mad3) with those that damage various structural components of the spindle. Defects in microtubule polymerization, spindle pole body duplication, microtubule motors, and kinetochore components all activate the MAD-dependent checkpoint. In contrast, the cell cycle arrest caused by mutations that induce DNA damage (cdc13), inactivate the cyclin proteolysis machinery (cdc16 and cdc23), or arrest cells in anaphase (cdc15) is independent of the spindle checkpoint.

Antagonistic microtubule-sliding motors position mitotic centrosomes in Drosophila early embryos

The positioning of centrosomes, or microtubule-organizing centres, within cells plays a critical part in animal development. Here we show that, in Drosophila embryos undergoing mitosis, the positioning of centrosomes within bipolar spindles and between daughter nuclei is determined by a balance of opposing forces generated by a bipolar kinesin motor, KLP61F, that is directed to microtubule plus ends, and a carboxy-terminal kinesin motor, Ncd, that is directed towards microtubule minus ends. This activity maintains the spacing between separated centrosomes during prometaphase and metaphase, and repositions centrosomes and daughter nuclei during late anaphase and telophase. Surprisingly, we do not observe a function for KLP61F in the initial separation of centrosomes during prophase. Our data indicate that KLP61F and Ncd may function by crosslinking and sliding antiparallel spindle microtubules in relation to one another, allowing KLP61F to push centrosomes apart and Ncd to pull them together.

The growth-related, translationally controlled protein P23 has properties of a tubulin binding protein and associates transiently with microtubules during the cell cycle

The translationally controlled protein P23 was discovered by the early induction of its rate of synthesis after mitogenic stimulation of mouse fibroblasts. P23 is expressed in almost all mammalian tissues and it is highly conserved between animals, plants and yeast. Based on its amino acid sequence, P23 cannot be attributed to any known protein family, and its cellular function remains to be elucidated. Here, we present evidence that P23 has properties of a tubulin binding protein that associates with microtubules in a cell cycle-dependent manner. (1) P23 is a cytoplasmic protein that occurs in complexes of 100–150 kDa, and part of P23 can be immunoprecipitated from HeLa cell extracts with anti-tubulin antibodies. (2) In immunolocalisation experiments we find P23 associated with microtubules during G1, S, G2 and early M phase of the cell cycle. At metaphase, P23 is also bound to the mitotic spindle, and it is detached from the spindle during metaphase-anaphase transition. (3) A GST-P23 fusion protein interacts with alpha- and beta-tubulin, and recombinant P23 binds to taxol-stabilised microtubules in vitro. The tubulin binding domain of P23 was identified by mutational analysis; it shows similarity to part of the tubulin binding domain of the microtubule-associated protein MAP-1B. (4) Overexpression of P23 results in cell growth retardation and in alterations of cell morphology. Moreover, elevation of P23 levels leads to microtubule rearrangements and to an increase in microtubule mass and stability.

Motile properties of the kinesin-related Cin8p spindle motor extracted from Saccharomyces cerevisiae cells

We have developed microtubule binding and motility assays for Cin8p, a kinesin-related mitotic spindle motor protein from Saccharomyces cerevisiae. The methods examine Cin8p rapidly purified from crude yeast cell extracts. We created a recombinant form of CIN8 that fused the biotin carrying polypeptide from yeast pyruvate carboxylase to the carboxyl terminus of Cin8p. This form was biotinated in yeast cells and provided Cin8p activity in vivo. Avidin-coated glass surfaces were used to specifically bind biotinated Cin8p from crude extracts. Microtubules bound to the Cin8p-coated surfaces and moved at 3.4 +/- 0.5 micrometer/min in the presence of ATP. Force production by Cin8p was directed toward the plus ends of microtubules. A mutation affecting the microtubule-binding site within the motor domain (cin8-F467A) decreased Cin8p's ability to bind microtubules to the glass surface by >10-fold, but reduced gliding velocity by only 35%. The cin8-3 mutant form, affecting the alpha2 helix of the motor domain, caused a moderate defect in microtubule binding, but motility was severely affected. cin8-F467A cells, but not cin8-3 cells, were greatly impaired in bipolar spindle forming ability. We conclude that microtubule binding by Cin8p is more important than motility for proper spindle formation.

Kinetochores and the checkpoint mechanism that monitors for defects in the chromosome segregation machinery

Whether we consider the division of the simplest unicellular organisms into two daughter cells or the generation of haploid gametes by the most complex eukaryotes, no two processes secure the continuance of life more than the proper replication and segregation of the genetic material. The cell cycle, marked in part by the periodic rise and fall of cyclin-dependent kinase (CDK) activities, is the means by which these two processes are separated. DNA damage and mistakes in chromosome segregation are costly, so nature has further devised elaborate checkpoint mechanisms that halt cell cycle progression, allowing time for repairs or corrections. In this article, we review the mitotic checkpoint mechanism that responds to defects in the chromosome segregation machinery and arrests cells in mitosis prior to anaphase onset. At opposite ends of this pathway are the kinetochore, where many checkpoint proteins reside, and the anaphase-promoting complex (APC), the metaphase-to-interphase transition regulator. Throughout this review we focus on budding yeast but reference parallel processes found in other organisms.

MCM21 and MCM22, two novel genes of the yeast Saccharomyces cerevisiae are required for chromosome transmission

The minichromosome maintenance genes, MCM21 and MCM22, have been cloned and are shown to code for the ORFs YDR318W and YJR135C respectively. Mutations in these genes caused a decrease in the stability of the minichromosome. This decrease in stability was associated with an increase in the copy number of the minichromosome in cells carrying it. Small circular dicentric plasmids were maintained relatively stably and structurally intact in the mutants compared with the wild-type strain. In the latter, such plasmids were mitotically unstable and, upon recovery, showed frequent rearrangements of their DNA. A centromere offered less obstruction to transcription in mutant cells than in the wild type, showing that both these mutants had a more relaxed kinetochore assembly. The mutant strains showed elevated rates of chromosome loss but not those of recombination. Both the mutations caused the cells to display a higher sensitivity towards the anti-mitotic drug benomyl. All these observations suggest that MCM21 and MCM22 are important for chromosome segregation with a potential role in kinetochore function. These genes are non-essential, as their deletions from chromosomes did not cause loss of cell viability. However, exponentially growing mutant cells carrying the deletion of the MCM21 gene had a significant population of large-budded cells with a single nucleus at the neck. Furthermore, the DNA content of these cells showed a shift towards 2N, suggesting a temporary pause of cells in G2 or in an early phase of mitosis. The mcm21 and mcm22 mutations do not show synthetic lethality or any further enhancement of growth defects, implying that they could be carrying out non-overlapping functions in chromosome segregation.

SPC72: a spindle pole component required for spindle orientation in the yeast Saccharomyces cerevisiae

The monoclonal antibody 78H6 recognises an 85 kDa component of the yeast spindle pole body. Here we identify and characterise this component as Spc72p, the product of YAL047C. The sequence of SPC72 contains potential coiled-coil domains; its overexpression induced formation of large polymers that were strictly localised at the outer plaque and at the bridge of the spindle pole body. Immunoelectron microscopy confirmed that Spc72p was a component of these polymers. SPC72 was found to be non-essential for cell growth, but its deletion resulted in abnormal spindle positioning, aberrant nuclear migration and defective mating capability. Precisely, deletion of SPC72 resulted in a decreased number of astral microtubules: early in the cell cycle only few were detectable, and these were unattached to the spindle pole body in small-budded cells. Later in the cell cycle few, if any, remained, and they were unable to align the spindle properly. We conclude that Spc72p is not absolutely required for nucleation per se, but is needed for normal abundance and stability of astral microtubules.

A cytoplasmic dynein required for mitotic aster formation in vivo

An astral pulling force helps to elongate the mitotic spindle in the filamentous ascomycete, Nectria haematococca. Evidence is mounting that dynein is required for the formation of mitotic spindles and asters. Obviously, this would be an important mitotic function of dynein, since it would be a prerequisite for astral force to be applied to a spindle pole. Missing from the evidence for such a role of dynein in aster formation, however, has been a dynein mutant lacking mitotic asters. To determine whether or not cytoplasmic dynein is involved in mitotic aster formation in N. haematococca, a dynein-deficient mutant was made. Immunocytochemistry visualized few or no mitotic astral microtubules in the mutant cells, and studies of living cells confirmed the veracity of this result by revealing the absence of mitotic aster functions in vivo: intra-astral motility of membranous organelles was not apparent; the rate and extent of spindle elongation during anaphase B were reduced; and spindle pole body separation almost stopped when the anaphase B spindle in the mutant was cut by a laser microbeam, demonstrating unequivocally that no astral pulling force was present. These unique results not only provide a demonstration that cytoplasmic dynein is required for the formation of mitotic asters in N. haematococca; they also represent the first report of mitotic phenotypes in a dynein mutant of any filamentous fungus and the first cytoplasmic dynein mutant of any organism whose mitotic phenotypes demonstrate the requirement of cytoplasmic dynein for aster formation in vivo.

Mitotic centromere-associated kinesin is important for anaphase chromosome segregation

Mitotic centromere-associated kinesin (MCAK) is recruited to the centromere at prophase and remains centromere associated until after telophase. MCAK is a homodimer that is encoded by a single gene and has no associated subunits. A motorless version of MCAK that binds centromeres but not microtubules disrupts chromosome segregation during anaphase. Antisense-induced depletion of MCAK results in the same defect. MCAK overexpression induces centromere-independent bundling and eventual loss of spindle microtubule polymer suggesting that centromere-associated bundling and/or depolymerization activity is required for anaphase. Live cell imaging indicates that MCAK may be required to coordinate the onset of sister centromere separation.

Mechanism of DNA segregation in prokaryotes: replicon pairing by parC of plasmid R1

Prokaryotic chromosomes and plasmids encode partitioning systems that are required for DNA segregation at cell division. The systems are thought to be functionally analogous to eukaryotic centromeres and to play a general role in DNA segregation. The parA system of plasmid R1 encodes two proteins ParM and ParR, and a cis-acting centromere-like site denoted parC. The ParR protein binds to parC in vivo and in vitro. The ParM protein is an ATPase that interacts with ParR specifically bound to parC. Using electron microscopy, we show here that parC mediates efficient pairing of plasmid molecules. The pairing requires binding of ParR to parC and is stimulated by the ParM ATPase. The ParM mediated stimulation of plasmid pairing is dependent on ATP hydrolysis by ParM. Using a ligation kinetics assay, we find that ParR stimulates ligation of parC-containing DNA fragments. The rate-of-ligation was increased by wild type ParM protein but not by mutant ParM protein deficient in the ATPase activity. Thus, two independent assays show that parC mediates pairing of plasmid molecules in vitro. These results are consistent with the proposal that replicon pairing is part of the mechanism of DNA segregation in prokaryotes.

The aberrant positioning of nuclei and the microtubular cytoskeleton in Saccharomyces cerevisiae due to improper actin function

An excentric position of the nuclei, random orientation of mitoses, and multinuclear budding cells were identified in part of a population of temperature-sensitive (ts) Saccharomyces cerevisiae actin mutants at the permissive temperature of 23 degrees C by fluorescence and electron microscopy. The phenotype resembled that of mutants in beta-tubulin, dynein, JNM1, NUM1, ACT3, ACT5, myosins, profilin, tropomyosin 1, SLA2 and other genes. The question was addressed whether the cause was (i) defects in cell polarity in some ts actin mutants, manifested by lack of asymmetry of actin cortical patches, or (ii) lack of cytoplasmic or astral microtubules. The results indicated that in the cells with the nuclear defects, actin cortical patches showed the normal asymmetric distribution typical of undisturbed polarity. Cytoplasmic astral and spindle microtubules were also preserved. The principal difference found between the wild-type and actin mutant cells was in actin cables, which in the actin mutants were developed insufficiently. It is suggested that actin cables serve as a 'suspensory apparatus' and/or 'intracellular corridor', predetermining: the location of the nucleus in the central position in interphase; the axis of nuclear movement to the bud neck before mitosis; the direction of the elongating nucleus during mitosis; and the motion of each nucleus from an excentric to a central position during cytokinesis, in cooperation with the above-mentioned and other gene products.

Chromosome dynamics: yeast pulls it apart

Sister chromatid cohesion and chromosome condensation are both essential to the successful completion of mitosis. The recent identification and characterization of the yeast Mcd1p/Scc1p protein reveals a previously unsuspected mechanistic link between these processes.

MOB1, an essential yeast gene required for completion of mitosis and maintenance of ploidy

Mob1p is an essential Saccharomyces cerevisiae protein, identified from a two-hybrid screen, that binds Mps1p, a protein kinase essential for spindle pole body duplication and mitotic checkpoint regulation. Mob1p contains no known structural motifs; however MOB1 is a member of a conserved gene family and shares sequence similarity with a nonessential yeast gene, MOB2. Mob1p is a phosphoprotein in vivo and a substrate for the Mps1p kinase in vitro. Conditional alleles of MOB1 cause a late nuclear division arrest at restrictive temperature. MOB1 exhibits genetic interaction with three other yeast genes required for the completion of mitosis, LTE1, CDC5, and CDC15 (the latter two encode essential protein kinases). Most haploid mutant mob1 strains also display a complete increase in ploidy at permissive temperature. The mechanism for the increase in ploidy may occur through MPS1 function. One mob1 strain, which maintains stable haploidy at both permissive and restrictive temperature, diploidizes at permissive temperature when combined with the mps1-1 mutation. Strains containing mob2Delta also display a complete increase in ploidy when combined with the mps1-1 mutation. Perhaps in addition to, or as part of, its essential function in late mitosis, MOB1 is required for a cell cycle reset function necessary for the initiation of the spindle pole body duplication.

Mal3, the fission yeast homologue of the human APC-interacting protein EB-1 is required for microtubule integrity and the maintenance of cell form

Through a screen designed to isolate novel fission yeast genes required for chromosome segregation, we have identified mal3+. The mal3-1 mutation decreased the transmission fidelity of a nonessential minichromosome and altered sensitivity to microtubule-destabilizing drugs. Sequence analysis revealed that the 35-kD Mal3 is a member of an evolutionary conserved protein family. Its human counterpart EB-1 was identified in an interaction screen with the tumour suppressor protein APC. EB-1 was able to substitute for the complete loss of the mal3+ gene product suggesting that the two proteins might have similar functions. Cells containing a mal3 null allele were viable but showed a variety of phenotypes, including impaired control of cell shape. A fusion protein of Mal3 with the Aequorea victoria green fluorescent protein led to in vivo visualization of both cytoplasmic and mitotic microtubule structures indicating association of Mal3 with microtubules. The absence of Mal3 protein led to abnormally short, often faint cytoplasmic microtubules as seen by indirect antitubulin immunofluorescence. While loss of the mal3+ gene product had no gross effect on mitotic spindle morphology, overexpression of mal3+ compromised spindle formation and function and led to severe growth inhibition and abnormal cell morphology. We propose that Mal3 plays a role in regulating the integrity of microtubules possibly by influencing their stability.

Saccharomyces cerevisiae genes required in the absence of the CIN8-encoded spindle motor act in functionally diverse mitotic pathways

Kinesin-related Cin8p is the most important spindle-pole-separating motor in Saccharomyces cerevisiae but is not essential for cell viability. We identified 20 genes whose products are specifically required by cell deficient for Cin8p. All are associated with mitotic roles and represent at least four different functional pathways. These include genes whose products act in two spindle motor pathways that overlap in function with Cin8p, the kinesin-related Kip1p pathway and the cytoplasmic dynein pathway. In addition, genes required for mitotic spindle checkpoint function and for normal microtubule stability were recovered. Mutant alleles of eight genes caused phenotypes similar to dyn1 (encodes the dynein heavy chain), including a spindle-positioning defect. We provide evidence that the products of these genes function in concept with dynein. Among the dynein pathway gene products, we found homologues of the cytoplasmic dynein intermediate chain, the p150Glued subunit of the dynactin complex, and human LIS-1, required for normal brain development. These findings illustrate the complex cellular interactions exhibited by Cin8p, a member of a conserved spindle motor family.