The fission yeast microtubule cytoskeleton

Molecular and structural characterization of the spindle pole bodies in the fission yeast Schizosaccharomyces japonicus var japonicus

The structure and localization of the microtubule organization centres (MTOCs) of the fission yeast Schizosaccharomyces japonicus var. japonicus were examined by fluorescence microscopy and electron microscopy. Spindle pole bodies (SPBs), which are the fungal equivalent of centrosomes, of Sz. japonicus were visualized by immunofluorescent staining using a monoclonal anti-gamma-tubulin antibody. The behaviour of the SPBs during the cell cycle mostly coincided with previous reports on the most widely used fission yeast Schizosaccharomyces pombe. We cloned the gamma-tubulin gene from Sz. japonicus by PCR using redundant sets of primers corresponding to conserved regions of known gamma-tubulins. The predicted amino acid sequence of Sz. japonicus gamma-tubulin was most similar to the Sz. pombe gamma-tubulin. Under the electron microscope, the SPBs of Sz. japonicus were detected as electron-dense multilayered structures located just outside the nuclear envelope. The SPBs of Sz. japonicus were composed of three electron-dense layers and were surrounded by fuzzy material. Each layer showed structural changes according to the progression of the cell cycle. In mitotic cells, the SPBs were located on the fenestrae of the nuclear envelopes through which the mitotic spindle microtubules ran into the nucleoplasm. Our results show that Sz. japonicus is a very potent and attractive organism for the investigation of the microtubule nucleation system and morphogenesis in yeasts. The Accession No. for the nucleotide sequence of the Sz. japonicus gtb1(+) gene is AF159163.

A fourth component of the fission yeast gamma-tubulin complex, Alp16, is required for cytoplasmic microtubule integrity and becomes indispensable when gamma-tubulin function is compromised

gamma-Tubulin functions as a multiprotein complex, called the gamma-tubulin complex (gamma-TuC), and composes the microtubule organizing center (MTOC). Fission yeast Alp4 and Alp6 are homologues of two conserved gamma-TuC proteins, hGCP2 and hGCP3, respectively. We isolated a novel gene, alp16(+), as a multicopy suppressor of temperature-sensitive alp6-719 mutants. alp16(+) encodes a 759-amino-acid protein with two conserved regions found in all other members of gamma-TuC components. In addition, Alp16 contains an additional motif, which shows homology to hGCP6/Xgrip210. Gene disruption shows that alp16(+) is not essential for cell viability. However, alp16 deletion displays abnormally long cytoplasmic microtubules, which curve around the cell tip. Furthermore, alp16-deleted mutants are hypersensitive to microtubule-depolymerizing drugs and synthetically lethal with either temperature-sensitive alp4-225, alp4-1891, or alp6-719 mutants. Overproduction of Alp16 is lethal, with defective phenotypes very similar to loss of Alp4 or Alp6. Alp16 localizes to the spindle pole body throughout the cell cycle and to the equatorial MTOC at postanaphase. Alp16 coimmunoprecipitates with gamma-tubulin and cosediments with the gamma-TuC in a large complex (>20 S). Alp16 is, however, not required for the formation of this large complex. We discuss evolutional conservation and divergence of structure and function of the gamma-TuC between yeast and higher eukaryotes.

Roles of fission yeast tea1p in the localization of polarity factors and in organizing the microtubular cytoskeleton

The cylindrical shape of the fission yeast cell is generated by linear polarized growth from its cell ends. Using immunofluorescence and live imaging microscopy, we have investigated the roles of the cell end marker tea1p in generating linear polarized growth. We found that tea1p is primarily transported on plus ends of microtubules from the vicinity of the nucleus to the cell ends, and that its movement near the nucleus is independent of the kinesin tea2p. Deletion analysis identified a coiled-coil domain in tea1p essential for its retention at cell ends, and demonstrated that tea1p exerts different functions dependent on its location. On the tips of microtubules, tea1p prevents the curling of microtubules around the cell ends, whereas it is required for maintaining linear cell growth and for retention of polarity factors such as the Dyrk kinase pom1p, the CLIP170-like tip1p, and tea2p at the cell ends. We propose that tea1p has roles in organizing the microtubule cytoskeleton on the tips of microtubules, and in the retention of factors at the cell ends necessary for the cell to grow in a straight line.

Different mechanisms of cell polarisation in vegetative and shmooing growth in fission yeast

Schizosaccharomyces pombe cells have two polarised growth modes:an intrinsic vegetative growth mode, determined by an internal positioning mechanism and an extrinsic shmooing growth mode, activated by external pheromone. We have analysed the role of the cell end marker Tea1p, the CLIP170 like protein Tip1p, the kinesin like protein Tea2p and the Dyrk-like kinase Pom1p, during the switch between the two growth patterns, with the intention of studying the switch away from the vegetative growth mode. In vegetative growth these morphological factors are concentrated at cell ends, whereas during shmooing growth they are delocalised from the cell ends. In the absence of Tea1p, Tip1p and Tea2p, vegetative cells display microtubule and cell polarisation defects, but shmooing cells are indistinguishable from wild-type and shmoo more readily. These results suggest that Tea1p, Tip1p and Tea2p are not required for polarised growth during shmooing, but form part of the intrinsic vegetative growth mode that needs to be dismantled before cells can generate an extrinsic growth patterns. In contrast, Pom1p appears to have a role in the initial stages of the switch to the shmooing growth mode.

F-actin ring formation and the role of F-actin cables in the fission yeastSchizosaccharomyces pombe

Cells of the fission yeast Schizosaccharomyces pombe divide by the contraction of the F-actin ring formed at the medial region of the cell. We investigated the process of F-actin ring formation in detail using optical sectioning and three-dimensional reconstruction fluorescence microscopy. In wild-type cells, formation of an aster-like structure composed of F-actin cables and accumulation of F-actin cables were recognized at the medial cortex of the cell during prophase to metaphase. The formation of the aster-like structure seemed to initiate from branching of the longitudinal F-actin cables at a site near the spindle pole bodies, which had been duplicated but not yet separated. A single cable extended from the aster and encircled the cell at the equator to form a primary F-actin ring during metaphase. During anaphase,the accumulated F-actin cables were linked to the primary F-actin ring, and then all of these structures seemed to be packed to form the F-actin ring. These observations suggest that formation of the aster-like structure and the accumulation of the F-actin cables at the medial region of the cell during metaphase may be required to initiate the F-actin ring formation. In the nda3 mutant, which has a mutation in ß-tubulin and has been thought to be arrested at prophase, an F-actin ring with accumulated F-actin cables similar to that of anaphase wild-type cells was formed at a restrictive temperature. Immediately after shifting to a permissive temperature, this structure changed into a tightly packed ring. This suggests that the F-actin ring formation progresses beyond prophase in the nda3 cells once the cells enter prophase. We further examined F-actin structures in both cdc12 and cdc15 early cytokinesis mutants. As a result,Cdc12 seemed to be required for the primary F-actin ring formation during prophase, whereas Cdc15 may be involved in both packing the F-actin cables to form the F-actin ring and rearrangement of the F-actin after anaphase. In spg1, cdc7 and sid2 septum initiation mutants, the F-actin ring seemed to be formed in order.

Kinesinsklp5+ andklp6+ are required for normal chromosome movement in mitosis

Proper mitotic chromosome segregation requires dynamic interactions between spindle microtubules and kinetochores. Here we demonstrate that two related fission yeast kinesins, klp5+ and klp6+, are required for normal chromosome segregation in mitosis. Null mutants frequently lack a normal metaphase chromosome alignment. Chromosome pairs move back and forth along the spindle for an extended period prior to sister chromatid separation, a phenotype reminiscent of the loss of CENP-E in metazoans. Ultimately, sister chromatids segregate, regardless of chromosome position along the spindle, and viable daughter cells are usually produced. The initiation of anaphase B is sometimes delayed, but the rate of spindle elongation is similar to wildtype. Despite a delay, anaphase B often begins before anaphase A is completed. The klp5Δ and klp6Δ null mutants are synthetically lethal with a deletion of the spindle assembly checkpoint gene, bub1+, several mutants in components of the anaphase promoting complex, and a cold sensitive allele of the kinetochore and microtubule-binding protein, Dis1p. Klp5p-GFP and Klp6p-GFP localize to kinetochores from prophase to the onset of anaphase A, but relocalize to the spindle midzone during anaphase B. These data indicate that Klp5p and Klp6p are kinetochore kinesins required for normal chromosome movement in prometaphase.

MTOC formation during mitotic exit in fission yeast

Microtubules polymerise from nucleation templates containing γ tubulin. These templates are generally concentrated in discrete structures called microtubule organising centres (MTOCs). In Schizosaccharomyces pombe, an equatorial MTOC (EMTOC) forms mid-way through anaphase B and then disassembles during the final stages of cell separation. We show that the EMTOC was generated by recruiting γ tubulin to the equatorial F-actin ring before it constricted to cleave the cell in two during cytokinesis. The EMTOC was not a continuous ring. It had a variable structure ranging from a horseshoe to a number of short bars. EMTOC integrity depended upon the integrity of the F-actin but not the microtubule cytoskeleton. EMTOC assembly required the activity of both the septation-inducing network (SIN) that regulates the onset of cytokinesis and the anaphase-promoting complex. Activation of the SIN in interphase cells induced F-actin ring formation and contraction and the synthesis of the primary septum but did not promote EMTOC assembly. In contrast, overproduction of the polo-like kinase, Plo1, which also induced multiple rounds of septation in interphase cells, induced EMTOC formation. Thus, the network governing EMTOC formation shared many of the regulatory elements that control cytokinesis but was more complex and revealed an additional function for Plo1 during mitotic exit.

Two related kinesins, klp5+ and klp6+, foster microtubule disassembly and are required for meiosis in fission yeast

The kinesin superfamily of microtubule motor proteins is important in many cellular processes, including mitosis and meiosis, vesicle transport, and the establishment and maintenance of cell polarity. We have characterized two related kinesins in fission yeast, klp5+ and klp6+,, that are amino-terminal motors of the KIP3 subfamily. Analysis of null mutants demonstrates that neither klp5+ nor klp6+, individually or together, is essential for vegetative growth, although these mutants have altered microtubule behavior. klp5Delta and klp6Delta are resistant to high concentrations of the microtubule poison thiabendazole and have abnormally long cytoplasmic microtubules that can curl around the ends of the cell. This phenotype is greatly enhanced in the cell cycle mutant cdc25-22, leading to a bent, asymmetric cell morphology as cells elongate during cell cycle arrest. Klp5p-GFP and Klp6p-GFP both localize to cytoplasmic microtubules throughout the cell cycle and to spindles in mitosis, but their localizations are not interdependent. During the meiotic phase of the life cycle, both of these kinesins are essential. Spore viability is low in homozygous crosses of either null mutant. Heterozygous crosses of klp5Delta with klp6Delta have an intermediate viability, suggesting cooperation between these proteins in meiosis.

pkl1(+)and klp2(+): Two kinesins of the Kar3 subfamily in fission yeast perform different functions in both mitosis and meiosis

We have identified Klp2p, a new kinesin-like protein (KLP) of the KAR3 subfamily in fission yeast. The motor domain of this protein is 61% identical and 71% similar to Pkl1p, another fission yeast KAR3 protein, yet the two enzymes are different in behavior and function. Pkl1p is nuclear throughout the cell cycle, whereas Klp2p is cytoplasmic during interphase. During mitosis Klp2p enters the nucleus where it forms about six chromatin-associated dots. In metaphase-arrested cells these migrate back and forth across the nucleus. During early anaphase they segregate with the chromosomes into two sets of about three, fade, and are replaced by other dots that form on the spindle interzone. Neither klp2(+) nor pkl1(+) is essential, and the double deletion is also wild type for both vegetative and sexual reproduction. Each deletion rescues different alleles of cut7(ts), a KLP that contributes to spindle formation and elongation. When either or both deletions are combined with a dynein deletion, vegetative growth is normal, but sexual reproduction fails: klp2 Delta,dhc1-d1 in karyogamy, pkl1 Delta,dhc1-d1 in multiple phases of meiosis, and the triple deletion in both. Deletion of Klp2p elongates a metaphase-arrested spindle, but pkl1 Delta shortens it. The anaphase spindle of klp2 Delta becomes longer than the cell, leading it to curl around the cell's ends. Apparently, Klp2p promotes spindle disassembly and contributes to the behavior of mitotic chromosomes.

Roles of the fission yeast formin for3p in cell polarity, actin cable formation and symmetric cell division

BACKGROUND

Both symmetric and asymmetric cell divisions are required for the generation of appropriate cell lineages during development. Wild-type Schizosaccharomyces pombe cells divide in a symmetric fashion to produce two similar rod-shaped daughter cells. Formins are proteins with conserved roles in cell polarity, cytokinesis, and the regulation of actin and microtubule cytoskeletons.

RESULTS

Here, we identify and characterize a new S. pombe formin, for3p. for3 Delta mutant cells divide in an asymmetric manner; a mother cell divides medially to produce one daughter cell that develops into a monopolar cell and one daughter that develops into a bipolar cell. Both daughter cells recapitulate similar asymmetric lineages themselves. Inheritance of the bipolar pattern correlates with inheritance of the recent birth scar, not with asymmetry in the spindle pole bodies. for3 Delta mutants lack interphase actin cables and have delocalized actin patch and myo52p (type V myosin) distributions. for3 Delta cells have normal microtubule dynamics and cortical interactions but have defects in microtubule organization and increased numbers of microtubule bundles. for3p-GFP is localized at both cell tips in an actin-dependent manner and at the cell division site.

CONCLUSIONS

for3p is a cell polarity factor required for interphase actin cable formation and microtubule organization. The for3 Delta phenotype suggests that cells are able to grow in a polarized manner even in the absence of functional actin cables and polarized distribution of actin patches. for3p and possibly actin cables are part of a regulatory network that ensures that cell divisions are symmetric.

The mitotic spindle: a self-made machine

The mitotic spindle is a highly dynamic molecular machine composed of tubulin, motors, and other molecules. It assembles around the chromosomes and distributes the duplicated genome to the daughter cells during mitosis. The biochemical and physical principles that govern the assembly of this machine are still unclear. However, accumulated discoveries indicate that chromosomes play a key role. Apparently, they generate a local cytoplasmic state that supports the nucleation and growth of microtubules. Then soluble and chromosome-associated molecular motors sort them into a bipolar array. The emerging picture is that spindle assembly is governed by a combination of modular principles and that their relative contribution may vary in different cell types and in various organisms.

Microtubules and actin cytoskeleton in Cryptococcus neoformans compared with ascomycetous budding and fission yeasts

Actin cytoskeleton and microtubules were studied in a human fungal pathogen, the basidiomycetous yeast Cryptococcus neoformans (haploid phase of Filobasidiella neoformans), during its asexual reproduction by budding using fluorescence and electron microscopy. Staining with rhodamine-conjugated phalloidin revealed an F-actin cytoskeleton consisting of cortical patches, cables and cytokinetic ring. F-actin patches accumulated at the regions of cell wall growth, i. e. in sterigma, bud and septum. In mother cells evenly distributed F-actin patches were joined to F-actin cables, which were directed to the growing sterigma and bud. Some F-actin cables were associated with the cell nucleus. The F-actin cytokinetic ring was located in the bud neck, where the septum originated. Antitubulin TAT1 antibody revealed a microtubular cytoskeleton consisting of cytoplasmic and spindle microtubules. In interphase cells cytoplasmic microtubules pointed to the growing sterigma and bud. As the nucleus was translocated to the bud for mitosis, the cytoplasmic microtubules disassembled and were replaced by a short intranuclear spindle. Astral microtubules then emanated from the spindle poles. Elongation of the mitotic spindle from bud to mother cell preceded nuclear division, followed by cytokinesis (septum formation in the bud neck). Electron microscopy of ultrathin sections of chemically fixed and freeze-substituted cells revealed filamentous bundles directed to the cell cortex. The bundles corresponded in width to the actin microfilament cables. At the bud neck numerous ribosomes accumulated before septum synthesis. We conclude: (i) the topology of F-actin patches, cables and rings in C. neoformans resembles ascomycetous budding yeast Saccharomyces, while the arrangement of interphase and mitotic microtubules resembles ascomycetous fission yeast Schizosaccharomyces. The organization of the cytoskeleton of the mitotic nucleus, however, is characteristic of basidiomycetous yeasts. (ii) A specific feature of C. neoformans was the formation of a cylindrical sterigma, characterized by invasion of F-actin cables and microtubules, followed by accumulation of F-actin patches around its terminal region resulting in development of an isodiametrical bud.

A journey into space

The fission yeast, Schizosaccharomyces pombe, has been used as a model eukaryote to study processes such as the cell cycle and cell morphology. In this single-celled organism, growing in a straight line and maintaining the nucleus in the centre of the cell depend on intracellular positional information. Microtubules and microtubular transport are important for generating positional information within the fission yeast cell, and these molecular mechanisms are also probably relevant for generating positional information in other eukaryotic cells.

Cell polarity: a tale of two Ts

The microtubule cytoskeleton plays an important role in cell polarity. Central to this process in fission yeast is tea1p, a marker of polarized cell growth that is delivered to the cell surface in a microtubule-dependent fashion. Recent studies suggest that the actin-binding protein bud6p may be a tea1p effector.

Role of bud6p and tea1p in the interaction between actin and microtubules for the establishment of cell polarity in fission yeast

BACKGROUND

In many cell types, microtubules are thought to direct the spatial distribution of F-actin in cell polarity. Schizosaccharomyces pombe cells exhibit a regulated program of polarized cell growth: after cell division, they grow first in a monopolar manner at the old end, and in G2 phase, initiate growth at the previous cell division site (the new end). The role of microtubule ends in cell polarity is highlighted by the finding that the cell polarity factor, tea1p, is present on microtubule plus ends and cell tips [1].

RESULTS

Here, we characterize S. pombe bud6p/fat1p, a homolog of S. cerevisiae Bud6/Aip3. bud6Delta mutant cells have a specific defect in the efficient initiation of growth at the new end and like tea1Delta cells, form T-shaped cells in a cdc11 background. Bud6-GFP localizes to both cell tips and the cytokinesis ring. Maintenance of cell tip localization is dependent upon actin but not microtubules. Bud6-GFP localization is tea1p dependent, and tea1p localization is not bud6p dependent. tea1Delta and bud6Delta cells generally grow in a monopolar manner but exhibit different growth patterns. tea1(Delta)bud6Delta mutants resemble tea1Delta mutants. Tea1p and bud6p coimmunoprecipitate and comigrate in large complexes.

CONCLUSIONS

Our studies show that tea1p (a microtubule end-associated factor) and bud6p (an actin-associated factor) function in a common pathway, with bud6p downstream of tea1p. To our knowledge, bud6p is the first protein shown to interact physically with tea1p. These studies delineate a pathway for how microtubule plus ends function to polarize the actin cytoskeleton through actin-associated polarity factors.

Cytokinesis in prokaryotes and eukaryotes: common principles and different solutions

Cytokinesis requires duplication of cellular structures followed by bipolarization of the predivisional cell. As a common principle, this applies to prokaryotes as well as eukaryotes. With respect to eukaryotes, the discussion has focused mainly on Saccharomyces cerevisiae and on Schizosaccharomyces pombe. Escherichia coli and to a lesser extent Bacillus subtilis have been used as prokaryotic examples. To establish a bipolar cell, duplication of a eukaryotic origin of DNA replication as well as its genome is not sufficient. Duplication of the microtubule-organizing center is required as a prelude to mitosis, and it is here that the dynamic cytoskeleton with all its associated proteins comes to the fore. In prokaryotes, a cytoskeleton that pervades the cytoplasm appears to be absent. DNA replication and the concomitant DNA segregation seem to occur without help from extensive cytosolic supramacromolecular assemblies but with help from the elongating cellular envelope. Prokaryotic cytokinesis proceeds through a contracting ring, which has a roughly 100-fold-smaller circumference than its eukaryotic counterpart. Although the ring contains proteins that can be considered as predecessors of actin, tubulin, and microtubule-associated proteins, its macromolecular composition is essentially different.

Establishment of a cellular axis in fission yeast

Recent studies in fission yeast Schizosaccharomyces pombe reveal how cells establish a cellular axis that specifies domains as the functional 'ends' and 'middle' of the cell. During interphase, dynamic microtubules position the nucleus at the middle of the cell and orientate microtubule 'plus' ends towards the ends of the cell. At the cell ends, the microtubule plus ends might establish a zone of polarized cell growth and actin assembly by depositing factors such as Tea1p. At the cell middle, the nucleus might specify the position of the actin contractile ring and the future cell division site by positioning cytokinesis factors such as Mid1p.

A mechanism for nuclear positioning in fission yeast based on microtubule pushing

The correct positioning of the nucleus is often important in defining the spatial organization of the cell, for example, in determining the cell division plane. In interphase Schizosaccharomyces pombe cells, the nucleus is positioned in the middle of the cylindrical cell in an active microtubule (MT)-dependent process. Here, we used green fluorescent protein markers to examine the dynamics of MTs, spindle pole body, and the nuclear envelope in living cells. We find that interphase MTs are organized in three to four antiparallel MT bundles arranged along the long axis of the cell, with MT plus ends facing both the cell tips and minus ends near the middle of the cell. The MT bundles are organized from medial MT-organizing centers that may function as nuclear attachment sites. When MTs grow to the cell tips, they exert transient forces produced by plus end MT polymerization that push the nucleus. After an average of 1.5 min of growth at the cell tip, MT plus ends exhibit catastrophe and shrink back to the nuclear region before growing back to the cell tip. Computer modeling suggests that a balance of these pushing MT forces can provide a mechanism to position the nucleus at the middle of the cell.

Microtubules in the fungal pathogen Ustilago maydis are highly dynamic and determine cell polarity

Many fungal pathogens undergo a yeast-hyphal transition during their pathogenic development that requires rearrangement of the cytoskeleton, followed by directed membrane traffic towards the growth region. The role of microtubules and their dynamic behavior during this process is not well understood. Here we set out to elucidate the organization, cellular role and in vivo dynamics of microtubules in the dimorphic phytopathogen Ustilago maydis. Hyphae and unbudded yeast-like cells of U. maydis contain bundles of spindle pole body-independent microtubules. At the onset of bud formation two spherical tubulin structures focus microtubules towards the growth region, suggesting that they support polar growth in G(2), while spindle pole body-nucleated astral microtubules participate in nuclear migration in M and early G(1). Conditional mutants of an essential alpha-tubulin gene from U. maydis, tub1, confirmed a role for interphase microtubules in determination of cell polarity and growth. Observation of GFP-Tub1 fusion protein revealed that spindle pole body-independent and astral microtubules are dynamic, with elongation and shrinkage rates comparable to those found in vertebrate systems. In addition, very fast depolymerization was measured within microtubule bundles. Unexpectedly, interphase microtubules underwent bending and rapid translocations within the cell, suggesting that unknown motor activities participate in microtubule organization in U. maydis. Movies available on-line: http://www.biologists.com/JCS/movies/jcs1792.html

Live analysis of lagging chromosomes during anaphase and their effect on spindle elongation rate in fission yeast

The fission yeast Schizosaccharomyces pombe is widely used as a model system for studies of the cell cycle and chromosome biology. To enhance these studies we have fused GFP to the chromodomain protein Swi6p, thus allowing nuclear and chromosome behaviour to be followed in living cells using time-lapse fluorescence microscopy. Like endogenous Swi6p, GFP-Swi6p localises to the nucleus and is concentrated at the heterochromatic centromeres and telomeres. The nucleus is highly dynamic during interphase: the clustered centromeres, in particular, are highly mobile. By expressing GFP-(α)2-tubulin and GFP-Swi6p in the same cells we observe that the clustered centromeres move in concert with the cytoplasmic microtubules, which is likely to reflect their association with the spindle pole body. Drug treatment indicates that this movement is dependent on intact cytoplasmic microtubules. We have also used GFP-Swi6p to investigate the properties of lagging chromosomes observed in mutants with defects in chromosome segregation. Lagging chromosomes display a variety of behaviours on anaphase spindles, most surprisingly, chromosomes appear to initiate microtubule interactions and move to the poles late in anaphase B. Interestingly, in cells displaying lagging chromosomes, the rate of spindle elongation is slowed by a factor of two. This suggests that cells are able to sense the presence of a lagging chromosome and slow anaphase B in order to allow it extra time to reach the pole. However, this mechanism is not dependent on the spindle checkpoint proteins Bub1p or Dma1p, raising the possibility that a novel checkpoint mechanism operates to retard spindle elongation if lagging chromosomes are detected. An alternative model is also discussed in which single defective kinetochores on lagging chromatids are able to interact simultaneously with microtubules emanating from both poles and affect spindle dynamics by counteracting the spindle elongation force.

A conserved interaction between Moe1 and Mal3 is important for proper spindle formation in Schizosaccharomyces pombe

Moe1 is a conserved fission yeast protein that negatively affects microtubule stability/assembly. We conducted a two-hybrid screen to search for Moe1-binding proteins and isolated Mal3, a homologue of human EB1. We show that Moe1 and Mal3 expressed in bacteria form a complex and that Moe1 and Mal3 expressed in fission yeast cosediment with microtubules. Deletion of either moe1 or mal3 does not result in lethality; however, deletion of both moe1 and mal3 leads to cell death in the cold. The resulting cells appear to die of chromosome missegregation, which correlates with the presence of abnormal spindles. We investigated the cause for the formation of monopolar spindles and found that only one of the two spindle pole bodies (SPBs) contains gamma-tubulin, although both SPBs appear to be equal in size and properly inserted in the nuclear membrane. Moreover, the moe1 mal3 double null mutant in the cold contains abnormally short and abundant interphase microtubule bundles. These data suggest that Moe1 and Mal3 play a role in maintaining proper microtubule dynamics/integrity and distribution of gamma-tubulin to the SPBs during mitosis. Finally, we show that human Moe1 and EB1 can each rescue the phenotype of the moe1 mal3 double null mutant and form a complex, suggesting that these proteins are part of a well-conserved mechanism for regulating spindle functioning.

The fission yeast ran GTPase is required for microtubule integrity

The microtubule cytoskeleton plays a pivotal role in cytoplasmic organization, cell division, and the correct transmission of genetic information. In a screen designed to identify fission yeast genes required for chromosome segregation, we identified a strain that carries a point mutation in the SpRan GTPase. Ran is an evolutionarily conserved eukaryotic GTPase that directly participates in nucleocytoplasmic transport and whose loss affects many biological processes. Recently a transport-independent effect of Ran on spindle formation in vitro was demonstrated, but the in vivo relevance of these findings was unclear. Here, we report the characterization of a Schizosaccharomyces pombe Ran GTPase partial loss of function mutant in which nucleocytoplasmic protein transport is normal, but the microtubule cytoskeleton is defective, resulting in chromosome missegregation and abnormal cell shape. These abnormalities are exacerbated by microtubule destabilizing drugs, by loss of the spindle checkpoint protein Mph1p, and by mutations in the spindle pole body component Cut11p, indicating that SpRan influences microtubule integrity. As the SpRan mutant phenotype can be partially suppressed by the presence of extra Mal3p, we suggest that SpRan plays a role in microtubule stability.

Tea2p is a kinesin-like protein required to generate polarized growth in fission yeast

Cytoplasmic microtubules are critical for establishing and maintaining cell shape and polarity. Our investigations of kinesin-like proteins (klps) and morphological mutants in the fission yeast Schizosaccharomyces pombe have identified a kinesin-like gene, tea2(+), that is required for cells to generate proper polarized growth. Cells deleted for this gene are often bent during exponential growth and initiate growth from improper sites as they exit stationary phase. They have a reduced cytoplasmic microtubule network and display severe morphological defects in genetic backgrounds that produce long cells. The tip-specific marker, Tea1p, is mislocalized in both tea2-1 and tea2Delta cells, indicating that Tea2p function is necessary for proper localization of Tea1p. Tea2p is localized to the tips of the cell and in a punctate pattern within the cell, often coincident with the ends of cytoplasmic microtubules. These results suggest that this kinesin promotes microtubule growth, possibly through interactions with the microtubule end, and that it is important for establishing and maintaining polarized growth along the long axis of the cell.

CLIP170-like tip1p spatially organizes microtubular dynamics in fission yeast

Rod-shaped fission yeast cells grow in a polarized manner, and unlike budding yeast, the correct positioning of the growth sites at cell ends requires interphase microtubules. Here we describe a microtubule guidance mechanism that orients microtubules in the intracellular space along the long axis of the cell, guiding them to their target region at the cell ends. This mechanism involves tip1p, a CLIP170-like protein that localizes to distal tips of cytoplasmic microtubules. In the absence of tip1p, microtubular catastrophe is no longer restricted to cell ends but occurs when microtubules reach any region of the cellular cortex. Thus, tip1p enables microtubules to discriminate different cortical regions and regulates their dynamics accordingly.

New concepts in fission yeast morphogenesis

The ability to generate spatial form is a fundamental characteristic of all living organisms, which has been much studied by successive generations of developmental biologists. In recent years increasing numbers of cell biologists have turned their attention to the mechanisms by which cells generate their spatial form. These include the mechanisms that position components in different places within the cell, that specify the position of these components, and that generate the overall shape of these components. These problems are entirely analogous to those studied by developmental biologists, although usually at the level of the whole organism, organ or tissue. Because the organization of all cells is basically similar, it is possible that the concepts and the underlying molecular mechanisms of cell morphogenesis may be highly conserved. In this article we consider the generation of spatial form within the fission yeast cell, focusing on emerging new concepts, which may be applicable to the morphogenesis of other cells.

Dynamics of interphase microtubules in Schizosaccharomyces pombe

BACKGROUND

Microtubules in interphase Schizosaccharomyces pombe are essential for maintaining the linear growth habit of these cells. The dynamics of assembly and disassembly of these microtubules are so far uncharacterised.

RESULTS

Live cell confocal imaging of alpha1 tubulin tagged with enhanced green fluorescent protein revealed longitudinally oriented, dynamically unstable interphase microtubule assemblies (IMAs). The IMAs were uniformly bright along their length apart from a zone of approximately doubly intense fluorescence commonly present close to their centres. The ends of each IMA switched from growth ( approximately 3.0 microm/min) to shrinkage ( approximately 4.5 microm/min) at 1.0 events per minute and from shrinkage to growth at 1.9 events per minute, and the two ends were equivalently dynamic, suggesting equivalent structure. We accordingly propose a symmetrical model for microtubule packing within the IMAs, in which microtubules are plus ends out and overlap close to the equator of the cell. IMAs may contain multiple copies of this motif; if so, then within each IMA end, the microtubule ends must synchronise catastrophe and rescue. When both ends of an IMA lodge in the hemispherical cell ends, the IMAs start to bend under compression and their overall growth rate is inhibited about twofold. Similar microtubule dynamics were observed in cells ranging in size from half to twice normal length. Patterned photobleaching indicated no detectable treadmilling or microtubule sliding during interphase.

CONCLUSIONS

The consequence of the mechanisms described is continuous recruitment of microtubule ends to the ends of growing cells, supporting microtubule-based transport into the cell ends and qualitatively accounting for the essential role for microtubules in directing linear cell growth in S. pombe.

Microtubule and actin-dependent movement of the formin cdc12p in fission yeast

Although a number of gene products involved in cytokinesis have been identified, still little is known about how these proteins are localized to the proper site and assembled into a ring structure. How is the plane of cell division is positioned in the cell? Schizosaccharomyces pombe are simple rod-shaped eukaryotic cells that divide by medial fission using a medial contractile ring. S. pombe cdc12p encodes a member of the formin gene family, proteins with conserved roles in cytokinesis and actin organization. cdc12p is required specifically for the formation of the medial ring and is located in this ring during mitosis. Time-lapse microscopy of cells expressing GFP-cdc12p protein fusions reveals that during interphase, S. pombe cdc12p is present in a discrete, motile cytoplasmic particle that moves using both actin and microtubules. At the onset of mitosis, the spot moves to the future site of cell division and spreads out into a ring. These studies demonstrate that a cytokinesis factor may travel on both microtubule and actin networks to the site of contractile ring assembly. These findings suggest a potential mechanism for how the mitotic spindle positions the cell division plane in animal cells.

A mutation in gamma-tubulin alters microtubule dynamics and organization and is synthetically lethal with the kinesin-like protein pkl1p

Mitotic segregation of chromosomes requires spindle pole functions for microtubule nucleation, minus end organization, and regulation of dynamics. gamma-Tubulin is essential for nucleation, and we now extend its role to these latter processes. We have characterized a mutation in gamma-tubulin that results in cold-sensitive mitotic arrest with an elongated bipolar spindle but impaired anaphase A. At 30 degrees C cytoplasmic microtubule arrays are abnormal and bundle into single larger arrays. Three-dimensional time-lapse video microscopy reveals that microtubule dynamics are altered. Localization of the mutant gamma-tubulin is like the wild-type protein. Prediction of gamma-tubulin structure indicates that non-alpha/beta-tubulin protein-protein interactions could be affected. The kinesin-like protein (klp) Pkl1p localizes to the spindle poles and spindle and is essential for viability of the gamma-tubulin mutant and in multicopy for normal cell morphology at 30 degrees C. Localization and function of Pkl1p in the mutant appear unaltered, consistent with a redundant function for this protein in wild type. Our data indicate a broader role for gamma-tubulin at spindle poles in regulating aspects of microtubule dynamics and organization. We propose that Pkl1p rescues an impaired function of gamma-tubulin that involves non-tubulin protein-protein interactions, presumably with a second motor, MAP, or MTOC component.

The cellular roles of molecular motors in fungi

Motors are molecular machines that move their cargo along F-actin or microtubules. Fungal representatives of myosin, kinesin and dynein motors support many cellular processes including polar growth, cell division and mitosis. Recent progress in understanding their cellular roles has revealed common principles. However, it has become obvious that fungi have also developed diverse strategies to cope with long-distance organelle transport.

Role of cell shape in determination of the division plane in Schizosaccharomyces pombe: random orientation of septa in spherical cells

The establishment of growth polarity in Schizosaccharomyces pombe cells is a combined function of the cytoplasmic cytoskeleton and the shape of the cell wall inherited from the mother cell. The septum that divides the cylindrical cell into two siblings is formed midway between the growing poles and perpendicularly to the axis that connects them. Since the daughter cells also extend at their ends and form their septa at right angles to the longitudinal axis, their septal (division) planes lie parallel to those of the mother cell. To gain a better understanding of how this regularity is ensured, we investigated septation in spherical cells that do not inherit morphologically predetermined cell ends to establish poles for growth. We studied four mutants (defining four novel genes), over 95% of whose cells displayed a completely spherical morphology and a deficiency in mating and showed a random distribution of cytoplasmic microtubules, Tea1p, and F-actin, indicating that the cytoplasmic cytoskeleton was poorly polarized or apolar. Septum positioning was examined by visualizing septa and division scars by calcofluor staining and by the analysis of electron microscopic images. Freeze-substitution, freeze-etching, and scanning electron microscopy were used. We found that the elongated bipolar shape is not essential for the determination of a division plane that can separate the postmitotic nuclei. However, it seems to be necessary for the maintenance of the parallel orientation of septa over the generations. In the spherical cells, the division scars and septa usually lie at angles to each other on the cell surface. We hypothesize that the shape of the cell indirectly affects the positioning of the septum by directing the extension of the spindle.

A fission yeast kinesin affects Golgi membrane recycling

We report here an in vivo study of kinesin heavy chain (KHC) functions in yeast. We have identified in Schizosaccharomyces pombe a kinesin motor gene, klp3(+), which has the highest homology to the Neurospora crassa KHC. Using indirect immunofluorescence, HA epitope-tagged Klp3 protein is cytoplasmic and appears as one to a few distinct patches that are coincident with microtubules. The klp3 null allele is viable. In klp3 deleted cells, ER, Golgi and mitochondrial distribution appear normal. Mitochondrial distribution in S. pombe is known to be microtubule-associated. We show that latrunculin A does not cause mitochondria to aggregate, suggesting that mitochondrial distribution in fission yeast, unlike budding yeast, is not dependent upon actin-based processes. Neither latrunculin A nor thiabendazole affects ER or Golgi distribution. We also used the vital dye FM4-64 to visualize the internalization of the dye and its transport to vacuoles in fission yeast in the presence and absence of Klp3. We observed no significant difference between the wild-type and Klp3 null cells in either the dynamics of endocytosis or the distribution and fusion of vacuoles. The drug brefeldin A causes Golgi-to-ER recycling in wild-type fission yeast cells. Although recycling of Golgi to ER after brefeldin A treatment occurs in klp3 null cells, recycling is defective and the distribution pattern we see is different from that observed in the wild-type strain. We conclude that Klp3 plays a role in BFA-induced membrane transport. The nucleotide sequence of S. pombe klp3(+) was submitted to GenBank under Accession No. AF154055.

Sid2p, a spindle pole body kinase that regulates the onset of cytokinesis

The fission yeast Schizosaccharomyces pombe divides by medial fission through the use of an actomyosin contractile ring. Precisely at the end of anaphase, the ring begins to constrict and the septum forms. Proper coordination of cell division with mitosis is crucial to ensure proper segregation of chromosomes to daughter cells. The Sid2p kinase is one of several proteins that function as part of a novel signaling pathway required for initiation of medial ring constriction and septation. Here, we show that Sid2p is a component of the spindle pole body at all stages of the cell cycle and localizes transiently to the cell division site during medial ring constriction and septation. A medial ring and an intact microtubule cytoskeleton are required for the localization of Sid2p to the division site. We have established an in vitro assay for measuring Sid2p kinase activity, and found that Sid2p kinase activity peaks during medial ring constriction and septation. Both Sid2p localization to the division site and activity depend on the function of all of the other septation initiation genes: cdc7, cdc11, cdc14, sid1, spg1, and sid4. Thus, Sid2p, a component of the spindle pole body, by virtue of its transient localization to the division site, appears to determine the timing of ring constriction and septum delivery in response to activating signals from other Sid gene products.

Movement of a cytokinesis factor cdc12p to the site of cell division

A key question in cytokinesis is how the plane of cell division is positioned within the cell. Although a number of cytokinesis factors involved in formation of the actomyosin contractile ring have been identified, little is known about how these factors are localized and assembled at the cell-division site. Cells of the fission yeast Schizosaccharomyces pombe divide using a medial actomyosin ring that assembles in early mitosis [1]. The S. pombe cdc12 gene encodes a formin, a member of a family of proteins that have functions in cytokinesis and cell polarity and that may bind Rho/Cdc42 GTPases, profilin and other actin-associated proteins [1] [2] [3] [4]. The cdc12 protein (cdc12p) is required specifically for medial-ring assembly during cytokinesis and is a component of this ring [2] [5]. In this study, cdc12p was found, during interphase, in a discrete, motile cytoplasmic spot that moved to the future site of cell division at the onset of mitosis. Three lines of evidence indicated that this cdc12p spot moved on both actin and microtubule networks: movement required either actin or microtubules; the spot was associated with actin and microtubule structures; and individual spots were seen to move along both microtubule and non-microtubule tracks. These findings demonstrate that a cytokinesis factor may travel on both microtubule and actin networks to the future site of cell division.

Sto1p, a fission yeast protein similar to tubulin folding cofactor E, plays an essential role in mitotic microtubule assembly

The proper functioning of microtubules depends crucially on the availability of polymerizable alpha/beta tubulin dimers. Their production occurs concomitant with the folding of the tubulin polypeptides and is accomplished in part by proteins known as Cofactors A through E. In the fission yeast, Schizosaccharomyces pombe, this tubulin folding pathway is essential. We have taken advantage of the excellent cytology available in S. pombe to examine the phenotypic consequences of a deletion of sto1(+), a gene that encodes a protein similar to Cofactor E, which is required for the folding of alpha-tubulin. The interphase microtubule cytoskeleton in sto1-delta cells is severely disrupted, and as cells enter mitosis their spindles fail to form. After a transient arrest with condensed chromosomes, the cells exit mitosis and resume DNA synthesis, whereupon they septate abnormally and die. Overexpression of Spo1p is toxic to cells carrying a cold-sensitive allele of the alpha- but not the beta-tubulin gene, consistent with the suggestion that this protein plays a role like that of Cofactor E. Unlike its presumptive partner Cofactor D (Alp1p), however, Sto1p does not localize to microtubules but is found throughout the cell. Overexpression of Sto1p has no toxic effects in wild-type cells, suggesting that it is unable to disrupt alpha/beta tubulin dimers in vivo.

The novel murine calmodulin-binding protein Sha1 disrupts mitotic spindle and replication checkpoint functions in fission yeast

Entry into mitosis is normally blocked in eukaryotic cells that have not completed replicative DNA synthesis; this ‘S-M’ checkpoint control is fundamental to the maintenance of genomic integrity. Mutants of the fission yeast Schizosaccharomyces pombe defective in the S-M checkpoint fail to arrest the cell cycle when DNA replication is inhibited and hence attempt mitosis and cell division with unreplicated chromosomes, resulting in the ‘cut’ phenotype. In an attempt to identify conserved molecules involved in the S-M checkpoint we have screened a regulatable murine cDNA library in S. pombe and have identified cDNAs that induce the cut phenotype in cells arrested in S phase by hydroxyurea. One such cDNA encodes a novel protein with multiple calmodulin-binding motifs that, in addition to its effects on the S-M checkpoint, perturbed mitotic spindle functions, although spindle pole duplication was apparently normal. Both aspects of the phenotype induced by this cDNA product, which we term Sha1 (for spindle and hydroxyurea checkpoint abnormal), were suppressed by simultaneous overexpression of calmodulin. Sha1 is structurally related to the product of the Drosophila gene abnormal spindle (asp). These data suggest that calmodulin-binding protein(s) are important in the co-ordination of mitotic spindle functions with mitotic entry in fission yeast, and probably also in multicellular eukaryotes.

On growth and form: control of cell morphogenesis in fission yeast

In the past year, we have gained considerable insight into the process of cell morphogenesis and the establishment of positional information in fission yeast. The highlights include a better understanding of the role of the microtubule cytoskeleton in the control of cell shape, as well as the identification of novel genes essential for the establishment of cell polarity and for the positioning of the site of cell division.

Conjugation in S. pombe: identification of a microtubule-organising centre, a requirement for microtubules and a role for Mad2

During the G1 phase of the cell cycle, cells of the fission yeast Schizosaccharomyces pombe can be induced to mate by nitrogen starvation and the presence of mating pheromones. Polarised growth towards cells of the opposite mating type (P or M) leads to the formation of a projection tip and, upon contact, localised cell wall degradation results in conjugation and cell fusion [1]. Here, we have investigated the role of microtubules in this process. We describe a previously unidentified microtubule-organising centre (MTOC) that forms at projection tips upon cell-to-cell contact, before cells fuse. Treatment of mating cells with the microtubule-destabilising drug thiabendazole (TBZ) showed that microtubule integrity was required for mating at two distinct stages: during projection tip formation and cell fusion. Projection tip formation requires filamentous (F) actin function [2] and microtubules are required for the localisation of F actin to the projection tip. We also identify a role during mating for Mad2--a mitotic checkpoint protein that is required in all eukaryotes to maintain the mitotic state in response to microtubule depolymerisation [3]. S. pombe mad2 mutant cells were compromised in their ability to mate upon removal of TBZ, indicating that in fission yeast, in the absence of microtubules, Mad2 is also required to maintain mating competence.

The yeast cytoskeleton: the closer We look, the more We See

May, K. M., and Hyams, J. S. 1998. The yeast cytoskeleton: The closer we look, the more we see. Copyright 1998 Academic Press.