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Vol. 18, Issue 6, 2216-2225, June 2007
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*Molecular, Cellular, and Developmental Biology Department, University of Colorado at Boulder, Boulder, CO 80309; Institute of General Pathology and Pathophysiology, Moscow 125315, Russia; and National Research Center for Hematology, Moscow 125167, Russia
Submitted November 6, 2006;
Revised February 27, 2006;
Accepted March 22, 2007
Monitoring Editor: Kerry Bloom
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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; McIntosh et al., 2002; Maiato et al., 2004; Tanaka et al., 2005b). Subsequent movement away from that pole, i.e., congression to the metaphase plate, is effected by several spindle forces that push chromosome to the spindle midzone. Such forces can be generated by chromokinesins, which bind to chromosome arms, and by kinetochore-localized motors (Schaar et al., 1997; Goshima and Vale, 2003; Mazumdar and Misteli, 2005; Zhu et al., 2005). For example, the plus end–directed, kinetochore motor CENP-E contributes to the congression of a mono-oriented, pole-proximal chromosome by sliding its unattached kinetochore toward the plus ends of the nearby microtubules (MTs; Kapoor et al., 2006). Such movements bring this kinetochore closer to the previously distant pole, presumably increasing its chances of binding MTs that grow from that pole. Alternatively, congression can be brought about by attachment of one of the pole-proximal sister kinetochores to MTs that grow from the distant pole (Nicklas, 1997). In this case congression is presumably driven by minus end–directed motors pulling the distal kinetochore toward the pole it faces (Savoian et al., 2000; Sharp et al., 2000, Schmidt et al., 2005).
The fission yeast, Schizosaccharomyces pombe, is a powerful system for the study of mitotic processes because of its excellent genetics and cytology and because the chromosome motions in this cell resemble those of higher eukaryotes. Its fully sequenced genome includes genes for three minus end–directed, MT-dependent motors that could contribute to chromosome congression: two members of the kinesin 14 family (Pkl1p and Klp2p, Pidoux et al., 1996; Troxell et al., 2001) and dynein (Yamamoto et al., 1999). The motor domains of Pkl1p and Klp2p are similar in sequence, both to each other and to the Saccharomyces cerevisiae motor Kar3p (Meluh and Rose, 1990). Klp2p resides in the cytoplasm during interphase, where it contributes to MTs sliding and cell polarity (Carazo-Salas et al., 2005). Early in mitosis it moves to the kinetochores (Troxell et al., 2001), where it aids the chromosome poleward motion in prometaphase by promoting processive kinetochore MT shortening (Grishchuk and McIntosh, 2006). The Kar3p motor of budding yeast has also been shown to contribute to mitotic kinetochore-MT attachments and movements (Tanaka et al., 2005a; Tytell and Sorger, 2006).
The second S. pombe kinesin 14, Pkl1p, is localized to the nucleus throughout the cell cycle; in mitosis it is found on the spindle and spindle pole bodies (SPBs; Pidoux et al., 1996). Although its importance for normal cell division has been established, the exact role of this motor is still unknown. Pkl1p enrichment at the poles and its genetic interactions with 
-tubulin suggest that it may play some role in SPB functioning (Paluh et al., 2000). The third S. pombe minus end–directed motor, dynein (Dhc1p), drives nuclear oscillations during fission yeast meiosis (Yamamoto et al., 1999). A direct study of chromosome poleward motion during mitotic prometaphase has revealed little or no abnormality when either Dhc1p or Pkl1p was absent (Grishchuk and McIntosh, 2006). Late in mitotic division dynein and Pkl1p may be involved in localizing the checkpoint protein Mad2p to the spindle midzone (Mayer et al., 2006).
At the beginning of mitosis, the kinetochores of three fission yeast chromosomes are located immediately adjacent to the duplicated SPBs (Funabiki et al., 1993). This close juxtaposition may aid in the normal establishment of MT links between kinetochores and poles before SPB separation. Indeed, as soon as the spindle begins elongation, the kinetochores are already stretched between the separating poles. Such early kinetochore-SPB interactions might explain why S. pombe is viable and therefore often succeeds in chromosome biorientation, even in the absence of all three minus end–directed motors (Troxell et al., 2001).
To explore the possible contributions of minus end–directed motors to chromosome congression and biorientation, we have studied these processes in prometaphase cells in which both sister kinetochores of chromosome 2 have become associated with one of the two, already separated spindle poles. This situation was achieved by using strains that carried the cold-sensitive allele of 
-tubulin, nda3-KM311. Restrictive temperatures disrupt the MTs of these cells, providing a reversible arrest in early mitosis (Hiraoka et al., 1984; Kanbe et al., 1990). During such a block, the kinetochores of S. pombe frequently lose their attachment to the SPBs and diffuse away. When these cells are warmed to permissive temperatures, MTs form and move the kinetochores back to the poles (Supplementary Figure 1). During the retrieval, the kinetochores attach via MTs only to the pole to which they move (Grishchuk and McIntosh, 2006), so their subsequent biorientation requires the establishment of de novo MT links with the opposite pole. Here, we use a combination of electron tomography and fluorescent imaging of live and fixed cells to examine the congression and biorientation of the retrieved, pole-proximal kinetochores. Our results show that although the minus end–directed motors in S. pombe are not essential for these processes, Pkl1p and dynein contribute independently to their efficacy. We conclude that these motors are required not for chromosome motion during congression but for the normal organization and/or function of the spindle poles and their associated MTs.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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). Cells carrying the nda3-KM311 mutation and various motor deletions (Table 1) were synchronized in mitosis by 7–8-h incubation at 18°C. DNA was visualized with DAPI. The kinetochore marker Mis12 was visualized with anti-hemagglutinin (HA) antibodies in the strain nda3-KM311 mis12-HA LEU2 leu1 ura4-D18 h–. Missegregation of chromosome 2 was analyzed for different motor deletions in the genetic background nda3-KM311 mis12-HA LEU2 cen2-GFP leu1 ura4 h–. Anti-green fluorescent protein (GFP) antibodies for cen2-GFP immunofluorescence were kindly provided by P. Silver and J. Kahana (Harvard University, Cambridge, MA). Mad2-GFP signal was scored in cells fixed in 4% paraformaldehyde and stained with DAPI. Spindle poles were visualized with anti-Sad1 antibody (kindly provided by I. Hagan, University of Manchester, United Kingdom); mini-chromosome loss was measured as described (Halverson et al., 1997; Fedyanina et al., 2006). Other reagents were as described in (Grishchuk and McIntosh, 2006). Virtually all experiments were carried out using two or more independent isolates of strains with identical genotypes.
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). Spindle poles were visualized using a chimera of GFP with Pcp1p, the S. pombe homolog of budding yeast Spc110p (Flory et al., 2002). The nda3 cells carrying both constructs were processed as described (Grishchuk and McIntosh, 2006). Images were acquired every 15 s as stacks of 7–12 planes (0.25-µm step, 150-ms exposure for each plane). In our microscope the temperature of the 100x oil immersion objective and of the water-jacket around the high NA oil condenser increased from the initial 14°C up to 32°C within 2 min, as measured by the temperature of the immersion oil on the objective lens. The Supplementary Videos show the averaged, depixelated XY-projections manually aligned to minimize the image drift brought about by the abrupt temperature change.
Electron Microscopy
Strains were grown at 18°C, as described above, then shifted to 32°C for 2–15 min, quickly collected by filtration, high-pressure frozen, and processed as previously described (Ding et al., 1997; Grishchuk and McIntosh, 2006). Tomograms were computed for each of two orthogonal tilt axes and then aligned and combined (Mastronarde, 1997). Spindle MTs and SPBs were modeled using the IMOD program (Kremer et al., 1996). Structural features of SPBs and MT ends were analyzed by extracting a slice of image data 1-voxel thick, using the Slicer feature of IMOD to adjust the position and orientation of the plane to obtain different views (O'Toole et al., 1999).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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; Troxell et al., 2001). In our test system, however, SPB separation in the absence of these motors was almost undelayed and occurred at the normal rate, as determined by 4D fluorescence microscopy of live cells (Figure 1A). A similar conclusion was drawn by examining the kinetics of SPB separation using fluorescence imaging of kinetochores and poles in cells fixed at different times after the temperature shift (Figure 1B).
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and dhc1 cells (Figure 1C). Interestingly, however, among the kinetochores that congressed successfully, there were no statistically significant differences in the times required for either biorientation or metaphase (Figure 1A). Thus, the absence of Pkl1p or dynein motors led to two distinct populations of cells: one in which the kinetochores failed in these processes, and another in which kinetochores congressed and bioriented in a manner that appeared perfectly normal. For example, after their initial poleward movement, the labeled kinetochores in some pkl1 cells gradually rotated around the pole and moved toward a midspindle position, just as in virtually all control and klp2 cells (Supplementary Videos 1 and 2). Kinetochore movement to the midspindle position was also not abolished in cells that lacked all three motors (Klp2p, Pkl1p and dynein; Supplementary Video 3), showing that minus end–directed motor activity is not essential for chromosome congression in S. pombe. Thus, some result of Pkl1p or dynein motor deletion makes normal congression and biorientation less likely, but the minus end–directed motors are not required for these processes.
When Chromosomes Fail to Congress and Biorient in the Absence of Pkl1p, Anaphase Initiation Is Not Delayed
Previous analysis of the mitotic progression in pkl1 nda3 cells revealed a slight advancement in the average time of anaphase initiation (Grishchuk and McIntosh, 2006). To examine the consequences of such precocious mitotic exit we measured the viability of this and other motor deletion strains during the synchronization procedure. The strains lacking pkl1+ had noticeably lower viability than all other strains examined, even when grown at 32°C (permissive conditions for the mutant tubulin; Figure 1D). There was a further decrease in cell survival after incubation at restrictive temperature relative to survival before the arrest (48%). The precocious anaphase initiation, combined with the decreased viability of pkl1 cells, suggested that the deletion of Pkl1p abrogated a mitotic checkpoint. This effect was unlikely to have resulted from problems with spindle formation, because as described above, the rates of SPB separation were statistically indistinguishable in all these strains.
In all strains we have examined, there were occasionally kinetochores that failed to biorient within the duration of the experiment (up to 40 min). In control cells, or those lacking Klp2p or dynein, the inability of a chromosome to leave the pole with which it first associated was accompanied by a delay in anaphase onset (Supplementary Video 4). However, in pkl1 cells spindle elongation was frequently not delayed: in 4 of 5 cells with pole-proximal kinetochores anaphase spindle elongation began 10.5 ± 2.4 min after kinetochore retrieval, even though the tagged kinetochore failed to congress and biorient (Supplementary Video 5). This time is identical to the duration of metaphase in control cells (10.1 ± 2.1 min). Because sample size from live cell imaging is limited, we analyzed the missegregation frequency of chromosome 2 in fixed cell cultures by scoring the percent of late anaphase cells in which both cen2 signals were in one daughter nucleus (Figure 1E). In pkl1 cells chromosome 2 missegregated more frequently than in control cells or in those with deletions of the klp2 or dhc1 genes (Figure 1F). Apparently, the premature mitotic exit seen in fixed cultures of pkl1 cells, the decreased cell viability, and the elevated chromosome missegregation all resulted from the inability of pole-proximal kinetochores to block anaphase spindle elongation in the absence of Pkl1p motor.
Pkl1p Does Not Play a Direct Role in the Mad2-dependent Checkpoint Pathway
One possibility for how a deletion in a minus end–directed motor could lead to abnormal mitotic checkpoint control is that this motor is involved in the transport or regulation of an important checkpoint component. At least two kinetochore-localized motors, CENP-E and dynein, are known to contribute directly to checkpoint inactivation, in the latter case by removing Mad2 from the bioriented kinetochores (Mao et al., 2005; Howell et al., 2001). If Pkl1p too were involved in checkpoint inactivation, cells deleted for this gene would be expected to arrest in mitosis and delay anaphase initiation, a phenotype that is opposite to the one observed. To further examine the checkpoint deficiency in pkl1 cells, we determined the localization of Mad2 protein in these cells synchronized via the nda3-KM311 mutation. When these cells were subjected to our customary low-temperature mitotic arrest, which depolymerizes essentially all of the cell's MTs (Grishchuk and McIntosh, 2006), their condensed chromosomes often contained 2–3 bright fluorescent spots of Mad2-GFP, indicating a normal recruitment of this checkpoint component to the unattached kinetochores (Figure 2A; Ikui et al., 2002). After a shift to the permissive temperature, the pkl1 nda3-KM311 mad2-GFP cells exited mitosis slightly faster than control cells, and the percentage of cells with Mad2-GFP bright dots also decreased fast (Figure 2, B and C). Thus, Pkl1p plays no detectable role in deactivating the checkpoint via the reduction of Mad2 levels during prometaphase.
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; Garcia et al., 2002; Saitoh et al., 2005), and unlike a previous report (Mayer et al., 2006), this pattern was unaffected by the absence of Pkl1p (Figure 2D). Furthermore, only 1 of 17 anaphase cells with unequal daughter nuclei (such as in Figure 1E) exhibited a relatively bright Mad2-GFP signal in the larger nucleus. Thus, despite apparently faulty chromosome attachments, the kinetochores of the missegregating, pole-proximal chromosome have normal, reduced levels of Mad2, which is likely to explain their failure to inhibit anaphase initiation. Consistent with the lack of evidence for the checkpoint activation in these cells, growth of the pkl1 cells was unaffected by the deletion of either mad2 or bub1 genes.
The Increase in Chromosome Loss in pkl1 Correlates with a Disorganized SPB
To seek other explanations for the anaphase initiation in pkl1 cells with pole-proximal chromosomes, we have used electron tomography (ET) to examine structural features of the mitotic spindles formed in the absence of Pkl1p. Cells with deletions in the minus end–directed motor enzymes were synchronized using the nda3-KM311 mutation and were high-pressure frozen several minutes after their release from the cold temperature block (see Materials and Methods). The reconstructed spindles in pkl1 and dhc1 cells looked fairly normal (based on 3 and 5 full reconstructions, respectively), but the morphology of spindle poles in pkl1 cells was distinctly abnormal. Fission yeast SPBs are normally prominent, plaque-like structures that enter a fenestra in the nuclear envelope during mitosis (Ding et al., 1997). They are associated with a diffuse "bridge" on the cytoplasmic side of the nuclear envelope (Figure 3, A and B; Uzawa et al., 2004). Figure, 3, C and C', shows slices from a tomogram of a pkl1 nda3 pole whose overall structure looks normal. However, the second pole in this cell did not have a plaque, although both poles had a normal-looking "bridge" (Figure 3, D and D').
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nda3 cell with an early prometaphase spindle, one of the poles appeared normal, but the other again lacked the normal, plaque-like structure (Figure 4, A and B). The area near this pole was occupied by a well-stained, structure-less material, and spindle MTs originated from beside this area at a small, abnormal evagination of the nuclear envelope (Supplementary Video 6). A similar configuration was found in a cell of the same genotype that contained a longer spindle (Figure 4, C and D); here the evagination was much larger (Supplementary Video 7). Again, MTs emanated from the diffuse, envelope-associated area that lacked normal pole morphology; the resulting spindle MTs appeared less focused and bundled than normal.
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-tubulin (Paluh et al., 2000), Pkl1p is unlikely to be involved directly in either MT nucleation or in the anchoring of MT ends at the pole. Instead, this motor appears to contribute to the maintenance of overall pole structure. If a pole becomes prone to partial fragmentation in the absence of Pkl1p, the resulting ectopic areas of MT nucleation could promote abnormal biorientation to that pole alone. It is likely that such an attachment could escape the checkpoint machinery (Figure 4E; see Discussion).
Absence of Dynein Leads to Rare Defects in Biorientation and Chromosome Segregation, as well as to a Less Efficient MT Bundling at the SPB
Although intracellular protein levels of dynein heavy chain are low and the localization of this protein in mitosis is still unknown, the expression of the dhc1+ gene is readily detectable in vegetative cells (Supplementary Figure 3). Using the above assays, we established that dhc1 cells had normal kinetics of mitotic progression (Figure 1, A and B) and normal levels of both viability and chromosome 2 missegregation (Figure 1, D–F). However, examination of biorientation in live dhc1 nda3 cells revealed some subtle but detectable abnormalities (Figure 1C). For example, the pole-proximal kinetochore in dhc1 cells frequently moved away from the pole, often not toward the opposite pole, so up to three fluorescent spots could be resolved simultaneously (Supplementary Video 8). In contrast, in cells of other genotypes, in which the kinetochore was at the pole for an extended time, the cen2-GFP signal remained closely associated with the pole (Figure 5A; Supplementary Video 4). In two dhc1 cells that failed in congression, anaphase eventually started (>34 min after kinetochore retrieval in Supplementary Video 9), but unlike in pkl1 cells, one of the sister kinetochores lagged transiently behind the moving pole (Figure 5B).
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cells showed normal overall morphology of the spindle and SPBs. Only 1 of 5 fully reconstructed spindles had a distinctly abnormal feature (Figure 5D), which we have not encountered in 12 additional, full-spindle reconstructions from cells of other genotypes. The pole-associated MTs on one side of the spindle were separated into two bundles (arrow). Moreover, the minus ends of the MTs in the interdigitating core bundle were farther from the SPB, and the second bundle was turned sideways (Supplementary Video 10). Such splitting of the MT bundles could have made a looser, less cohesive kinetochore-SPB association, accounting for the abnormal motions of the pole-proximal kinetochores seen in live dhc1 cells.
Dynein and Pkl1p Contribute to Chromosome Biorientation via Different Pathways
Deletion of the pkl1+ gene is known to suppress the temperature sensitivity of growth in cut7ts cells (kinesin 5) in allele-specific manner (Pidoux et al., 1996, Troxell et al., 2001). To examine whether Pkl1p and dynein promote chromosome biorientation via the same pathway, we have carried out the analysis of tetrads from genetic crosses between dhc1 and the temperature-sensitive strains cut7–21 and cut7–24; these two alleles are suppressed in the strongest and weakest manner, respectively, by the pkl1+ and klp2+ deletions (Troxell et al., 2001). Temperature sensitivity (25–36°C range) of both, cut7–21 and cut7–24 strains, was unchanged in the dhc1 background, suggesting that dynein and Pkl1p work in different mitotic pathways.
Because chromosome congression and biorientation were partially impeded by the absence of either dynein or Pkl1p, we then asked if there was an enhanced defect in their double deletion. The rate of cen2 missegregation in this strain was identical to that seen in pkl1 alone (Figure 1F). Live imaging of the double mutant cells revealed characteristics of each single mutant: premature anaphase in the absence of biorientation (pkl1 phenotype), and a loose kinetochore-pole association as seen by kinetochore lagging in anaphase (dhc1 phenotype; Figure 5C). Similar additive behavior was also seen in dhc1 pkl1 klp2 cells. For example, after the kinetochore was retrieved in this triple deletion strain (Supplementary Video 11), the spindle began elongation even though this kinetochore had failed to congress and biorient. These results suggest that dynein and Pkl1p contribute to chromosome biorientation by distinct and perhaps nonredundant mechanisms.
The above observations were carried out in the presence of a conditional allele of -tubulin, so we also examined roles of the minus end–directed motors in chromosome segregation by measuring the rate of a mini-chromosome loss in S. pombe cells carrying the wild-type nda3 gene (strains listed in Table 1). Consistent with the above conclusions, mini-chromosome loss was unaffected by the klp2+ deletion, and it was increased fourfold in the absence of dynein: 1.2 ± 0.2 x 10–3 losses per division in dhc1 cells versus 3.1 ± 1 x 10–4 in control cells (Figure 6). Deletion of Pkl1p led to the highest rate of the mini-chromosome loss among all single motor deletions: this strain showed a 
25-fold increase relative to wild-type cells. Thus, the minus end–directed motors Pkl1p, and to a lesser degree dynein, contribute to the accuracy of chromosome segregation independently of the nda3-KM311 mutation. Furthermore, because the mini-chromosome was lost only slightly more frequently in pkl1 dhc1 cells than in pkl1 cells, these two motors are likely to contribute independently to the accuracy of chromosome segregation. Surprisingly, there was a twofold increase in the mini-chromosome loss in pkl1 klp2 cells relative to the pkl1 cells alone (Figure 6), although our direct measurement of cen2 missegregation has revealed no interaction between these mutations (Figure 1F). It appears that Pkl1p and Klp2p might share some important mitotic function, other than in kinetochore congression and biorientation.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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). Together, these results strongly suggest that congression in S. pombe does not rely exclusively on minus end–directed motility. This conclusion is consistent with a conspicuous absence of long spindle MTs (other than those of the interpolar spindle bundle) that end in the vicinity of the opposite pole, where pole-proximal kinetochores are seen via live imaging (based on 17 full ET reconstructions of the prometaphase spindles). Such MTs would be expected to be common if their attachment to a pole-proximal kinetochore were necessary for its congression. Thus, it appears likely that chromosome congression in S. pombe is largely promoted by some other forces, such as those that push the kinetochores toward the metaphase plate (e.g., by a chromokinesin or plus end–directed, kinetochore-associated kinesins). Our study has also revealed that successful chromosome biorientation in S. pombe depends on the normal organization and functioning of the spindle poles. In particular, two minus end–directed motors, dynein and Pkl1p, contribute independently to normal pole and spindle structure.
The distinct, albeit subtle abnormalities in spindle organization in dhc1 cell (Figure 5D; Supplementary Video 10), suggest that dynein contributes to spindle organization by promoting MT bundling and by anchoring MT minus ends. Such an interpretation is consistent with a previously described role for dynein in other cell types (Heald et al., 1996; Gaglio et al., 1997; Walczak et al., 1998; Goshima and Vale, 2003; Goshima et al., 2005; Morales-Mulia and Scholey, 2005). If dynein works in S. pombe to transport kinetochore fibers to the pole, the less tight association of the kinetochore fibers with a pole that would result from its absence might explain the more mobile behavior of pole-associated kinetochores (Figure 5A; Supplementary Video 8), as well as the lagging of chromosomes during anaphase (Figure 5, B and C; Supplementary Video 9), and an occasional failure in chromosome biorientation (Figures 1C and 6).
The importance of the pole's normal functions in achieving accurate kinetochore biorientation is further highlighted by the phenotype of the Pkl1p deletion. Early in mitosis the spindle poles should slide apart, while situated in fenestrae within the nuclear envelope. It is likely that during their separation, the SPBs experience forces (while interacting with each other and the kinetochores) that push and pull their different parts in various directions. In the absence of Pkl1p such activities may result in the disorganization of polar material. Likewise, the regions of nuclear envelope that become distorted in this mutant may have responded to normal mitotic forces whose action could not be contained in the absence of this kinesin-14. We propose that Pkl1p, which localizes to the spindle and its poles (Pidoux et al., 1996), plays a role in maintaining the coherence of the spindle pole material, analogues to that suggested or implied for other members of this family, e.g., Ncd in Drosophila (Goshima et al., 2005) and KifC1/HSET and CHO2 in mammalian cells (Kuriyama et al., 1995; Gordon et al., 2001; Chakravarty et al., 2004; Zhu et al., 2005). The finding that in three fully reconstructed pkl1 nda3 spindles only one of the two poles was markedly disrupted suggests that either the mother or the daughter pole is more prone to fragmentation in this genetic background. This supposition could not be tested by EM tomography, because we were unable to find duplicated but unseparated poles in our preparations, in spite of a serious search for them.
Although an apparently normal, bipolar spindle can still form in the absence of Pkl1p and distant kinetochores are retrieved normally, the poles in these cells are not completely functional; an improperly attached kinetochore can escape detection by the checkpoint system. Previous studies of S. pombe have shown that kinetochores that are not attached to MTs are readily detected by the checkpoint mechanism, so long as they are in the same nucleus as the metaphase spindle (Grishchuk and McIntosh, 2006). Anaphase spindle elongation was inhibited by such unattached chromosomes in control cells, as well as in those with other motor deletions, including pkl1. Consistently, the recruitment of the Mad2 checkpoint protein to the unattached kinetochores appeared to be normal, even in the absence of Pkl1p (Figure 2). In contrast, after the chromosome had been pulled to one of the poles in pkl1 cells, Mad2 left the kinetochores; after the normal duration for metaphase the anaphase spindle began to elongate in the absence of biorientation. Although our data do not exclude the possibility that Pkl1p is involved in activation of some other important checkpoint protein, the defects we have observed in the organization of pkl1 SPBs provide an alternative explanation for such checkpoint deficiency. Indeed, if the pole is fragmented, two sister kinetochores might become syntelically attached but stably stretched between different parts of the same pole (red ovals on Figure 4E), thus avoiding detection by the checkpoint machinery.
With an increasing number of motor deletions, the details of mitotic progression change; cells carrying deletions in all three minus end–directed motors assume phenotypic features of each of the single deletion strains. Although this leads to more variability in each mitotic feature, such as kinetochore kinematics, the duration of different mitotic phases, spindle length dynamics, and the elevated rate of mini-chromosome loss, the overall parameters of mitotic progression in populations of cells with different genotypes remained surprisingly unperturbed in the absence of the minus end–directed motors. The fine deviations from normal that are sometimes seen are hard to generalize (e.g., compare control and triple deletion cells in Supplementary Videos 1 and 3), but cells lacking the Pkl1p motor are an exception. These cells are measurably less viable (Figure 1D) and fail in accurate chromosome segregation more frequently (Figures 1F and 6), most likely as a direct consequence of their checkpoint failure. We therefore conclude that none of the major kinetochore motions during kinetochore congression and biorientation in S. pombe is driven exclusively by minus end–directed motors; instead, these motors improve the quality and stability of essential spindle components, such as the centrosome. Minus end–directed motor enzymes appear not to be at the root of minus end–directed, spindle-mediated motility, but their presence improves the quality and reliability of such motions. As such, they confer a significant adaptive value to chromosome segregation, whose essence is the fidelity of the final product, not the speed or economy of action.
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ACKNOWLEDGMENTS |
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Footnotes |
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The online version of this article contains supplemental material at MBC Online (http://www.molbiolcell.org). 
Address correspondence to: Ekaterina L. Grishchuk (Katya{at}colorado.edu)
Abbreviations used: DAPI, 4',6'-diamidino-2-phenylindole dihydrochloride; EM, electron microscopy; ET, electron tomography; GFP, green fluorescent protein; MT, microtubule; SPB, spindle pole body.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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) and after the arrest (
). After growth at 32°C, the number of colonies was counted, and viability was expressed as a percent of the expected number of colonies, based on the number of cells plated. Numbers above the bars are the percent of cells surviving after the cold block, expressed relative to those surviving before the block. (E) The sister kinetochores of chromosome 2 (green) were sometimes found in only one of two late anaphase nuclei (2:0 segregation, arrow). DNA is stained with DAPI (blue), and all three kinetochores are stained in red using an antibody to the kinetochore-specific protein Mis12-HAp. Cen2-GFP marker was visualized using an anti-GFP. Cells boundaries are drawn for clarity. (F) The percent of late anaphase cells with 2:0 segregation of cen2 in different genetic backgrounds. In control cultures the percent of such cells was higher than expected from the reported frequency of chromosome loss in wild-type cells (Bodi et al., 1991
