Cellular events during sexual development from amoeba to plasmodium in the slime mould Physarum polycephalum

Time-lapse cinematography and immunofluorescence microscopy were used to study cellular events during amoebal fusions and sexual plasmodium development in Physarum polycephalum. Amoebal fusions occurred frequently in mixtures of strains heteroallelic or homoallelic for the mating-type locus matA, but plasmodia developed only in the matA-heteroallelic cultures. These observations confirmed that matA controls development of fusion cells rather than cell fusion. Analysis of cell pedigrees showed that, in both types of culture, amoebae fused at any stage of the cell cycle except mitosis. In matA-heteroallelic fusion cells, nuclear fusion occurred in interphase about 2 h after cell fusion; interphase nuclear fusion did not occur in matA-homoallelic fusion cells. The diploid zygote, formed by nuclear fusion in matA-heteroallelic fusion cells, entered an extended period of cell growth which ended in the formation of a binucleate plasmodium by mitosis without cytokinesis. In contrast, no extension to the cell cycle was observed in matA-homoallelic fusion cells and mitosis was always accompanied by cytokinesis. In matA-homoallelic cultures, many of the binucleate fusion cells split apart without mitosis, regenerating pairs of uninucleate amoebae; in the remaining fusion cells, the nuclei entered mitosis synchronously and spindle fusion sometimes occurred, giving rise to a variety of products. Immunofluorescence microscopy showed that matA-heteroallelic fusion cells possessed two amoebal microtubule organizing centres, and that most zygotes possessed only one; amoebal microtubule organization was lost gradually over several cell cycles. In matA-homoallelic cultures, all the cells retained amoebal microtubule organization.     

Variable pathways for developmental changes of mitosis and cytokinesis in Physarum polycephalum

The development of a uninucleate ameba into a multinucleate, syncytial plasmodium in myxomycetes involves a change from the open, astral mitosis of the ameba to the intranuclear, anastral mitosis of the plasmodium, and the omission of cytokinesis from the cell cycle. We describe immunofluorescence microscopic studies of the amebal-plasmodial transition (APT) in Physarum polycephalum. We demonstrate that the reorganization of mitotic spindles commences in uninucleate cells after commitment to plasmodium formation, is completed by the binucleate stage, and occurs via different routes in individual developing cells. Most uninucleate developing cells formed mitotic spindles characteristic either of amebae or of plasmodia. However, chimeric mitotic figures exhibiting features of both amebal and plasmodial mitoses, and a novel star microtubular array were also observed. The loss of the ameba-specific alpha 3-tubulin and the accumulation of the plasmodium-specific beta 2-tubulin isotypes during development were not sufficient to explain the changes in the organization of mitotic spindles. The majority of uninucleate developing cells undergoing astral mitoses (amebal and chimeric) exhibited cytokinetic furrows, whereas cells with the anastral plasmodial mitosis exhibited no furrows. Thus, the transition from astral to anastral mitosis during the APT could be sufficient for the omission of cytokinesis from the cell cycle. However, astral mitosis may not ensure cytokinesis: some cells undergoing amebal or chimeric mitosis contained unilateral cytokinetic furrows or no furrow at all. These cells would, most probably, fail to divide. We suggest that a uninucleate committed cell undergoing amebal or chimeric mitosis can either divide or else form a binucleate cell. In contrast, a uninucleate cell with a mitotic spindle of the plasmodial type gives rise only to a binucleate cells. Further, the decision to enter mitosis after commitment to the APT is independent of the developmental changes in the organization of the mitotic spindle and cytokinesis.     

Analysis of development and growth in a mutant of Physarum polycephalum with defective cytokinesis

Cultures of amoebae of the mutant strain ATS23 isolated from strain CLd of Physarum polycephalum contain multinucleate cells and cells with increased nuclear DNA content. Plasmodia derived from ATS23 clones show abnormal morphology and defective sporulation. All abnormalities are enhanced by high incubation temperature (31 °C). Genetic analysis suggested that all the abnormalities were caused by a single mutation, denoted hts-23. The kinetics of plasmodium formation were followed in cultures of apogamic amoebae carrying hts-23 and hts⁺ (wild type) respectively. Results indicated that, relative to wild type, hts-23 did not increase the rate of plasmodium formation. There was evidence that, in both mutant and wild-type strains, commitment to plasmodium development occurred in uninucleate cells. Analysis of cell pedigrees by time-lapse cinematography indicated that the primary abnormal event in cultures of hts-23 amoebae was failure of cytokinesis; an apparently complete cleavage furrow was formed but cell separation failed, resulting in a binucleate cell. This event occurred randomly in pedigrees in which the majority of divisions were completed normally; its frequency increased during incubation at 31 °C. All other abnormalities in hts-23 amoebal cultures could be attributed to this primary event, assuming that DNA synthesis continued in the absence of cytokinesis and that the binucleate cells underwent the amoebal type of “open” mitosis, allowing the possibility of spindle fusion. This implies that the acquisition of “closed” mitosis is an essential early step in plasmodium development.     

Immunofluorescence and inhibitor studies on creatine kinase and mitosis

Immunofluorescence microscopy of mitotiv PtKl cells with antibody against M-line creatine kinase from chicken breast muscle revealed spindle staining largely identical with that of antibody against tubulin. Treatment with epoxycreatine, a creatine analog and covalent creatine kinase inhibitor, allowed the study of creatine kinase function in these non-muscle cells. Cells fixed at interphase following epoxycreatine treatment maintained actin stress fibers and creatine kinase localization on intermediate filaments. Treated prometaphase cells lacked spindles as observed by immunofluorescence microscopy, but displayed instead a central region of diffuse immunofluorescent staining surrounded by condensed chromosomes. Those observed in metaphase or anaphase appeared normal by phase-contrast microscopy, but lacked immunofluorescent staining. Living epoxycreatine-treated cells were observed by phase-contrast microscopy to form spindles and progress through mitosis following an extended prometaphase.     

Microtubule rearrangements during mitosis in multinucleate cells

The peroxidase-antiperoxidase (PAP) method for the detection of polymerized tubulin has been used to study the microtubule rearrangements during mitosis in PtK1 and HeLa multinucleate cells obtained by polyethyleneglycol (PEG)-mediated fusion. We demonstrate here that the transition of the microtubular cytoskeleton from interphase to mitosis is an inducible event and independent of the factor(s) responsible for chromatin condensation and nuclear envelope breakdown. However, for the induction of the microtubule rearrangements nuclear envelope breakdown is required. At midprophase, cytoskeletal microtubule rearrangements start for multinucleate PtK1 cells, whereas in HeLa cells such changes are delayed, and a more abrupt transition is observed here. After complete nuclear envelope breakdown (prometaphase) mitotic asters and spindles but no cytoplasmic (interphase) microtubuli can be observed in both systems. Metaphase is characterized by an interaction between the different mitotic poles which show the form of bipolar spindles, but individual separated mitotic poles far removed from the chromatin can also be seen.     

Isolation of a Schizosaccharomyces pombe rad21ts mutant that is aberrant in chromosome segregation,microtubule function,DNA repair and sensitive to hydroxyurea: possible involvement of Rad21 in ubiquitin-mediated proteolysis.

The fission yeast DNA repair gene rad21+ is essential for cell growth. To investigate the function essential for cell proliferation, we have isolated a temperature-sensitive mutant of the rad21+ gene. The mutant, rad21-K1, showed abnormal mitosis at the nonpermissive temperature. Some cells contained abnormal nuclear structures, such as condensed chromosomes with short spindles, or chromosomes stretched or unequally separated by elongating spindles. Other cells exhibited the displaced nucleus or a cut-like phenotype. Similar abnormalities were observed when the Rad21 protein was depleted from cells. We therefore concluded that Rad21 is essential for proper segregation of chromosomes. Moreover, the rad21-K1 mutant is sensitive not only to UV and gamma-ray irradiation but to thiabendazole and hydroxyurea, indicating that Rad21 plays important roles in microtubule function, DNA repair, and S phase function. The relation to the microtubule function was further confirmed by the fact that rad21+ genetically interacts with tubulin genes, nda2+ and nda3+. Finally, the growth of the rad21-K1 mutant was inhibited at the permissive temperature by introduction of another mutation in the cut9+ gene, coding for a component of the 20S cyclosome/anaphase promoting complex, which is involved in ubiquitin-mediated proteolysis. The results suggest that these diverse functions of Rad21 may be facilitated through ubiquitin-mediated proteolysis.     

The microtubule-destabilizing kinesin XKCM1 regulates microtubule dynamic instability in cells

The dynamic activities of cellular microtubules (MTs) are tightly regulated by a balance between MT-stabilizing and -destabilizing proteins. Studies in Xenopus egg extracts have shown that the major MT destabilizer during interphase and mitosis is the kinesin-related protein XKCM1, which depolymerizes MT ends in an ATP-dependent manner. Herein, we examine the effects of both overexpression and inhibition of XKCM1 on the regulation of MT dynamics in vertebrate somatic cells. We found that XKCM1 is a MT-destabilizing enzyme in PtK2 cells and that XKCM1 modulates cellular MT dynamics. Our results indicate that perturbation of XKCM1 levels alters the catastrophe frequency and the rescue frequency of cellular MTs. In addition, we found that overexpression of XKCM1 or inhibition of KCM1 during mitosis leads to the formation of aberrant spindles and a mitotic delay. The predominant spindle defects from excess XKCM1 included monoastral and monopolar spindles, as well as small prometaphase-like spindles with improper chromosomal attachments. Inhibition of KCM1 during mitosis led to prometaphase spindles with excessively long MTs and spindles with partially separated poles and a radial MT array. These results show that KCM1 plays a critical role in regulating both interphase and mitotic MT dynamics in mammalian cells.     

Protein kinase D1 inhibition interferes with mitosis progression

Protein kinase D1 (PKD1) plays a vital role in signal transduction, cell proliferation, membrane trafficking, and cancer; however, the majority of the studies up to date had centered primarily on PKD1 functions in interphase, very little is known about its role during cell division. We previously demonstrated that during mitosis PKD1 is activated and associated with centrosomes, spindles, and midbodies. However, these observations did not address whether PKD1 was associated with mitosis regulation. Accordingly, we used rapidly acting PKD-specific inhibitors to examine the contribution of PKD1 the sequence of events in mitosis. We found that although PKD1 overexpression did not affect mitosis progression, suppression of its catalytic activity by two structurally unrelated inhibitors (kb NB 142-70 and CRT 0066101) induced a significant delay in metaphase to anaphase transition time. PKD1 inhibition during mitosis also produced the appearance of abnormal spindles, defects in chromosome alignment, and segregation as well as apoptosis. Thus, these observations indicate that PKD1 activity is associated with mitosis regulation.     

Growth and development in relation to the cell cycle inPhysarum polycephalum

Summary In strain CL ofPhysarum polycephalum, multinucleate, haploid plasmodia form within clones of uninucleate, haploid amoebae. Analysis of plasmodium development, using time-lapse cinematography, shows that binucleate cells arise from uninucleate cells, by mitosis without cytokinesis. Either one or both daughter cells, from an apparently normal amoebal division, can enter an extended cell cycle (28.7 hours compared to the 11.8 hours for vegetative amoebae) that ends in the formation of a binucleate cell. This long cycle is accompanied by extra growth; cells that become binucleate are twice as big as amoebae at the time of mitosis. Nuclear size also increases during the extended cell cycle: flow cytometric analysis indicates that this is not associated with an increase over the haploid DNA content. During the extended cell cycle uninucleate cells lose the ability to transform into flagellated cells and also become irreversibly committed to plasmodium development. It is shown that commitment occurs a maximum of 13.5 hours before binucleate cell formation and that loss of ability to flagellate precedes commitment by 3–5 hours. Plasmodia develop from binucleate cells by cell fusions and synchronous mitoses without cytokinesis.Abbreviations CL Colonia Leicester - DSDM Dilute semi-defined medium - FKB Formalin killed bacterial suspension - IMT Intermitotic time - LIA Liver infusion agar - SBS Standard bacterial suspension - SDM Semi-defined medium     

Centrosome and spindle pole microtubules are main targets of a fluorescent taxoid inducing cell death

Microtubules offer a very large local concentration of binding sites for cytotoxic taxoids or for hypothetical endogenous regulators. Several compounds from diverse sources stabilize microtubules and arrest cell division similarly to the antitumour drug Taxol. We have investigated the subcellular location of the Taxol binding sites, employing a fluorescent taxoid (FLUTAX) that reversibly interacts with the Taxol binding sites of microtubules and induces cellular effects similar to Taxol. The specific binding of FLUTAX to a fraction of the available cellular binding sites effectively inhibits division of cultured human tumour cells at G(2)/M, and triggers apoptotic death. The loci of reversible binding, directly imaged in intact U937 cells treated with cytotoxic doses of fluorescent taxoid are the centrosomes, with a few associated microtubules in interphase cells, and the spindle pole microtubules in mitotic cells, instead of uniformly labelling the microtubule cytoskeleton. Cytoskeletal lesions induced and visualized with FLUTAX consist of microtubule bundles and abnormal mitoses, including monopolar spindles and highly fluorescent multipolar spindles. The multiple asters and monopolar spindles mark arrested U937 leukaemia and OVCAR-3 ovarian carcinoma cells on their path to apoptosis (as well as K562, HeLa, and MCF-7 cells). Depending on the FLUTAX treatment, OVCAR-3 cells died from abnormal mitosis or from a subsequent interphase with double chromatin content and lobulated nuclei (micronuclei), indicating impairment of centrosome separation. Fragmented centrosomes could be observed in FLUTAX-treated non-transformed 3T3.A31 cells, which developed micronuclei but were resistant to apoptosis. These results strongly suggest that centrosomal impairment by taxoid binding during interphase, in addition to the suppression of the kinetochore microtubule dynamics in the mitotic spindle, is a primary cause of cell cycle de-regulation and cell death.     

The Ascogenous Hyphae of Pyronema confluens

The ascogenous hyphae arise from the oogonium, opposite groupsof nuclei, as minute, enucleate papillae. Nuclei pass into themsingly, rarely two at a time, and a knob-like swelling is formed,containing several nuclei and later growing out into one ormore branches. The nuclei are in single file in the branchesand irregularly arranged in the bulbous base. There are frequentlytwo nuclei in a leading position at the tip of the young branch,but the nuclei may become more evenly spaced as the hypha elongates.The nuclei undergo a simultaneous mitosis. The spindles of thedividing nuclei in the branches are not parallel and this is,therefore, not a conjugate division. Walls are formed as ingrowingrings across the spindles so that the ascogenous hypha, whenseptate, has a uninucleate end cell followed by one, or usuallymore, binucleate cells and a basal bulb containing a variablenumber of nuclei. Croziers are formed as lateral, hooked outgrowths from the binucleatecells. After a simultaneous mitosis of the two nuclei a uninucleateend cell, a binucleate penultimate cell, and a uninucleate stalkcell are formed. Thus, the division in the crozier and thatin the ascogenous hypha are alike. The binucleate cell of the crozier may proliferate to form anothercrozier, or it may form an ascus after the fusion of its twonuclei. The stalk and terminal cell of the crozier may anastomoseand grow out to form a lateral crozier. The chromosome number in the mitosis in the ascogenous hyphais twelve and there are twelve bivalents at the first divisionof meiosis in the ascus. The effect of increasing the illumination of the cultures withan electric lamp in addition to diffuse daylight is to ensurethe further development of all early formed sexual organs, tomake the ascogenous hyphae develop rapidly, to make the lattershort and curved in form with few binucleate cells, and to increasethe tendency towards a period of erect proliferation beforethe formation of the asci and lateral proliferation begin. The bearing of the results on current theories of sexualityin the Ascomycetes is discussed.     

The Golgi protein GM130 regulates centrosome morphology and function

The Golgi apparatus (GA) of mammalian cells is positioned in the vicinity of the centrosome, the major microtubule organizing center of the cell. The significance of this physical proximity for organelle function and cell cycle progression is only beginning to being understood. We have identified a novel function for the GA protein, GM130, in the regulation of centrosome morphology, position and function during interphase. RNA interference-mediated depletion of GM130 from five human cell lines revealed abnormal interphase centrosomes that were mispositioned and defective with respect to microtubule organization and cell migration. When GM130-depleted cells entered mitosis, they formed multipolar spindles, arrested in metaphase, and died. We also detected aberrant centrosomes during interphase and multipolar spindles during mitosis in ldlG cells, which do not contain detectable GM130. Although GA proteins have been described to regulate mitotic centrosomes and spindle formation, this is the first report of a role for a GA protein in the regulation of centrosomes during interphase.     

Microtubule patterns during meiosis in two higher plant species

Summary A monoclonal antibody to higher plant tubulin was used to trace microtubule (MT) structures by immunofluorescence throughout mitosis and meiosis in two angiosperms,Lycopersicon esculentum andOrnithogalum virens. Root tip cells showed stage specific MT patterns typical of higher plant cells. These included parallel cortical interphase arrays oriented perpendicular to the long axis of the cell, preprophase band MTs in late interphase through prophase, barrelshaped spindles, and finally phragmoplasts. Pollen mother cell divisions exhibited randomly oriented cortical MT arrays in prophase I, pointed spindles during karyokinesis, and elongate phragmoplasts. A preprophase band was not observed in either meiotic division. MT initiation sites were seen as broad zones associated with the nuclear envelope.     

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

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

Distribution of a matrix component of the midbody during the cell cycle in Chinese hamster ovary cells

Monoclonal antibodies were raised against isolated spindles of CHO (Chinese hamster ovary) cells to probe for molecular components specific to the mitotic apparatus. One of the antibodies, CHO1, recognized an antigen localized to the midbody during mitosis. Immunofluorescence staining of metaphase cells showed that although the total spindle area was labeled faintly, the antigen corresponding to CHO1 was preferentially localized in the equatorial region of the spindle. With the progression of mitosis, the antigen was further organized into discrete short lines along the spindle axis, and eventually condensed into a bright fluorescent dot at the midzone of the intercellular bridge between two daughter cells. Parallel immunostaining of tubulin showed that the CHO1-stained area corresponded to the dark region where microtubules are entrapped by the amorphous dense matrix components and possibly blocked from binding to tubulin antibody. Immunoblot analysis indicated that CHO1 recognized two polypeptides of mol wt 95,000 and 105,000. The immunoreaction was always stronger in preparations of isolated midbodies than in mitotic spindle fractions. The protein doublet was retained in the particulate matrix fraction after Sarkosyl extraction (Mullins, J. M., and J. R. McIntosh. 1982. J. Cell Biol. 94:654-661), suggesting that CHO1 antigen is indeed a component of the dense matrix. In addition to the equatorial region of spindles and midbodies, CHO1 also stained interphase centrosomes, and nuclei in a speckled pattern that was cell cycle-dependent. Thus, the midbody appears to share either common molecular component(s) or a similar epitope with interphase centrosomes and nuclei.     

Tubulin composition and microtubule nucleation of a griseofulvin-resistant Chinese hamster ovary cell mutant with abnormal spindles

A griseofulvin-resistant Chinese hamster ovary (CHO) mutant (Grs-2) which has an altered beta-tubulin subunit as well as wild-type beta-tubulin is temperature-sensitive (ts) for growth at 40.5 degrees C. This growth defect appears to result from the formation of abnormal mitotic spindles at the non-permissive temperature (Abraham, I et al., J cell biol 97 (1983) 1055) [19]. Light microscopy of spindles isolated from mutant cells cultured at the permissive temperature showed a typical bipolar morphology, whereas spindles isolated at the non-permissive temperature were multipolar. In order to study the role of tubulin in spindle formation, we analyzed the tubulin composition of the multipolar spindles. Two-dimensional gels and immunoblotting analysis of one-dimensional electrophoretic gels stained with monoclonal anti-Chinese hamster brain beta-tubulin antibody revealed that both mutant and wild-type beta-tubulins were present in similar proportions in both bipolar spindles at 37 degrees C and multipolar spindles at 40.5 degrees C. The ratio between wild-type and mutant tubulin in spindles was also found to be the same as in the cytoplasmic microtubule network in interphase cells, providing evidence that the mutant beta-tubulin appeared to be incorporated in a similar manner into both interphase and mitotic microtubule structures. In vitro microtubule polymerization onto centrosomes prepared from mutant Grs-2 demonstrated that 80% of the sites for microtubule nucleation were without centrioles, suggesting fragmentation of pericentriolar material away from centrioles. This may be one of the causes of multipolar spindle formation in the mutant cells. These results, therefore, suggest that abnormal formation of spindles in mutant cells is due not to the presence of the mutant tubulin per se, but to the abnormal behavior of this mutant tubulin in the cellular environment during mitosis or abnormal interaction with other components in the spindle at 40.5 degrees C.     

Localization of the nuclear matrix protein mitotin in mouse cells with a mitotic or endomitotic cell cycle.

Immunofluorescence staining was used to study the precise subcellular distribution of the nuclear matrix antigen, mitotin, in mouse cells characterized by either a mitotic or an endomitotic organization of the cell cycle. In mitotically dividing cells, mitotin showed a speckled distribution within interphase nuclei. In addition, some interphase cells exhibited a weak, focused signal adjacent to the nucleus, reflecting a possible staining of the centrosome region. Using digital contrast-enhanced immunofluorescence microscopy, a distinct association of mitotin to the centrosome, pole microtubules, and midbody could be revealed in cells at different stages of mitosis. In parallel, trophoblast giant cells characterized by an endomitotic cell cycle were derived from blastocyst outgrowths and analyzed likewise. In all giant cells examined so far, mitotin was restricted to the nuclear compartment alone, although different patterns of intranuclear staining could be detected. The present study provides further information about the precise localization of mitotin in mitotic cells, especially during mitosis. In view of the results, the staining pattern observed in endomitotic cells may allow for a better understanding of the origin and the organization of the endomitotic cell cycle.     

Localization of gamma-tubulin in interphase and mitotic cells of a unicellular eukaryote, Giardia intestinalis

Giardia intestinalis, a bi-nucleated amitochondrial flagellate, possesses a complex cytoskeleton based on several microtubular systems (flagella, adhesive disk, median body, funis, mitotic spindles). MTOCs of the individual systems have not been fully defined. By using monoclonal antibodies against a conserved synthetic peptide from the C-terminus of human gamma-tubulin we investigated occurrence and distribution of gamma-tubulin in interphase and mitotic Giardia cells. On the immunoblots of Giardia cytoskeletal extracts the antibodies bound to a single polypeptide of approximately 50 kDa. Immunostaining of the interphase cell demonstrated gamma-tubulin as four bright spots at the basis of four out of eight flagella. Gamma-tubulin label was associated with perikinetosomal areas of the ventral and posterolateral pairs of flagella which are formed de novo during cell division. Basal body regions of the anterolateral and caudal pairs of flagella which persist during the division and are integrated into the flagellar systems of the daughter cells did not show gamma-tubulin staining. At early mitosis, gamma-tubulin spots disappeared reappearing again at late mitosis in accord with reorientation of parent flagella and reorganization of flagellar apparatus during cell division. The antibody-detectable gamma-tubulin epitope was absent at the poles of both mitotic spindles. Albendazole-treated Giardia, in which spindle assembly was completely inhibited, showed the same gamma-tubulin staining pattern thus confirming that the fluorescent label is exclusively located in the basal body regions. Our results point to a role of gamma-tubulin in nucleation of microtubules of newly formed flagella and indicate unusual mitotic spindle assembly. Moreover, the demonstration of gamma-tubulin in Giardia shows ubiquity of this protein through the evolutionary history of eukaryotes.     

Nuclear and microtubular cycles in heterophasic multinuclearTriticum root-tip cells induced by caffeine

Summary Nuclear and microtubular cycles were studied in large heterophasic multinuclear cells induced in root tips ofTriticum turgidum by caffeine treatment. Multinuclear cells and cells with polyploid nuclei exhibited various configurations of multiple and complex preprophase microtubule (Mt) bands (PPBs), including helical ones. The developmental stages of PPBs in some heterophasic cells did not comply with the cell cycle stages of the associated nuclei, a fact indicating that these events are not directly controlled by the associated nuclei. The heterophasic cells exhibited asynchronous nuclei at different stages of mitosis. In cells displaying prophase and interphase nuclei, the prophase spindle was either absent or developed around both of them or developed around the prophase nuclei earlier than around the interphase ones. During prometaphase-metaphase of the advanced nuclei the lagging interphase nuclei were induced to form prematurely condensed chromosomes (PCCs) along with spindle formation around them. These observations suggest that the mitotic transition in heterophasic cells is delayed but is ultimately achieved due to the effect of the advanced nuclei, which induces a premature mitotic entry of the lagging nuclei. Although kinetochore Mt bundles were found associated with PCCs, their metaphase and anaphase spindles were abnormal resulting in abnormal or abortive anaphases. In some heterophasic cells, metaphase-anaphase transition did not take place simultaneously in different chromosome groups, signifying that the cells do not exit from the mitotic state after anaphase initiation of the advanced nuclei. Asynchronous pace of mitosis of different chromosome groups was also observed during anaphase and telophase. Implications of these observations in understanding plant cell cycle regulation are discussed.Abbreviations cdk cyclin dependent kinase - Mt microtubule - PCC prematurely condensed chromosome - PPB preprophase band     

Mutations in alpha- and beta-tubulin affect spindle formation in Chinese hamster ovary cells

Two Chinese hamster ovary cell lines with mutated beta-tubulins (Grs-2 and Cmd-4) and one that has a mutation in alpha-tubulin (Tax-1) are temperature sensitive for growth at 40.5 degrees C. To determine the functional defect in these mutant cells at the nonpermissive temperature, they were characterized with respect to cell cycle parameters and microtubule organization and function after relatively short periods at 40.5 degrees C. At the nonpermissive temperature all the mutants had normal appearing cytoplasmic microtubules. Premature chromosome condensation analysis failed to show any discrete step in the interphase cell cycle in which these mutants are arrested. These cells, however, show several defects at the nonpermissive temperature that appear related to the function of microtubules during mitosis. Time-lapse studies showed that mitosis was lengthened in the three mutant lines at 40.5 degrees C as compared with the wild-type cells at this temperature, resulting in a higher proportion of cells in mitosis after temperature shift. There was also a large increase in multinucleated cells in mutant populations after incubation at the nonpermissive temperature. Immunofluorescent studies using a monoclonal anti--alpha-tubulin antibody showed that the mutant cells had a high proportion of abnormal spindles at the nonpermissive temperature. The two altered beta-tubulins and the altered alpha-tubulin all were found to cause a similar phenotype at the high temperature that results in mitotic delay, defective cytokinesis, multinucleation, and ultimately, cell death. We conclude that spindle formation is the limiting microtubule function in these mutant cell lines at the nonpermissive temperature and that these cell lines will be of value for the study of the precise role of tubulin in mammalian spindle formation.