Metallothionein as a clonable tag for protein localization by electron microscopy of cells

A benign, clonable tag for the localization of proteins by electron microscopy of cells would be valuable, especially if it provided labelling with high signal-to-noise ratio and good spatial resolution. Here we explore the use of metallothionein as such a localization marker. We have achieved good success with desmin labelled in vitro and with a component of the yeast spindle pole body labelled in cells. Heavy metals added after fixation and embedding or during the process of freeze-substitution fixation provide readily visible signals with no concern that the heavy atoms are affecting the behaviour of the protein in its physiological environment. However, our methods did not work with protein components of the nuclear pore complex, suggesting that this approach is not yet universally applicable. We provide a full description of our optimal labelling conditions and other conditions tried, hoping that our work will allow others to label their own proteins of interest and/or improve on the methods we have defined.

Effects of caffeine on session ratings of perceived exertion

This study examined effects of caffeine on session ratings of perceived exertion (RPE) following 30 min constant-load cycling. Individuals (n = 15) of varying aerobic fitness completed a [Formula: see text] max trial and two 30 min cycling bouts (double-blind, counterbalanced) following ingestion of 6 mL/kg of caffeine or matched placebo. RPE overall, legs and breathing were estimated every 5 min and session RPE was estimated 30 min post-exercise using the OMNI pictorial scale. Session RPE for caffeine and placebo trails were compared using paired t test. Between-trial comparisons of HR, RPE overall, RPE legs and RPE breathing were analyzed using an independent 2 (trial) × 6 (time point) repeated measures analysis of variance (ANOVA) for each dependent variable. Caffeine resulted in a significantly lower session RPE (p < 0.05) for caffeine (6.1 ± 2.2) versus placebo (6.8 ± 2.1). Acute perceptual responses were significantly lower for caffeine for RPE overall (15, 20, 25, and 30 min), RPE breathing (15, 20, 25, and 30 min) and RPE legs (20 and 30 min). Survey responses post-exercise revealed greater feelings of nervousness, tremors, restlessness and stomach distress following caffeine versus placebo. Blunted acute RPE and survey responses suggest participants responded to caffeine ingestion. Caffeine decreased acute RPE during exercise which could partially account for lower session RPE responses. However, decreased session RPE could also reveal a latent analgesic affect of caffeine extending into recovery. Extending the understanding of session RPE could benefit coaches in avoiding overtraining when adjusting training programs.

The distribution of spindle microtubules during mitosis in cultured human cells

WI-38 and HeLa cells in mitosis have been selected from fixed monolayer cultures and serially sectioned for electron microscopy. Sections perpendicular to the spindle axis permit counting of the number of microtubules at each position on the spindle axis and hence the preparation of tubule distribution profiles. Errors intrinsic to this method are discussed. The changes in the tubule distributions from one mitotic stage to another provide evidence concerning the behavior of the spindle tubules during mitosis. The ratio of the number of tubules passing the chromosomes on the metaphase plate to the maximum number in each half spindle is about 1/2. This ratio changes little in early anaphase, and then decreases in late anaphase at about the same time that a zone of increased tubule number develops at the middle of the interzone. The region where the stem bodies form contains about 3/2 the number of tubules seen elsewhere in the interzone. This ratio is almost constant as the mid-body forms in telophase and then increases to 2/1 in early interphase before the final stages of cytokinesis occur.

Fabrication and characterization of a carbon nanotube-based nanoknife

We demonstrate the fabrication and testing of a prototype microtome knife based on a multiwalled carbon nanotube (MWCNT) for cutting approximately 100 nm thick slices of frozen-hydrated biological samples. A piezoelectric-based 3D manipulator was used inside a scanning electron microscope (SEM) to select and position individual MWCNTs, which were subsequently welded in place using electron beam-induced deposition. The knife is built on a pair of tungsten needles with provision to adjust the distance between the needle tips, accommodating various lengths of MWCNTs. We performed experiments to test the mechanical strength of a MWCNT in the completed device using an atomic force microscope tip. An increasing force was applied at the mid-point of the nanotube until failure occurred, which was observed in situ in the SEM. The maximum breaking force was approximately (8 x 10(-7)) N which corresponds well with the typical microtome cutting forces reported in the literature. In situ cutting experiments were performed on a cell biological embedding plastic (epoxy) by pushing it against the nanotube. Initial experiments show indentation marks on the epoxy surface. Quantitative analysis is currently limited by the surface asperities, which have the same dimensions as the nanotube.

Force production by depolymerizing microtubules: a theoretical study

Chromosome movement during mitosis is powered in part by energy released through the depolymerization of kinetochore microtubules (MTs). Strong but indirect evidence suggests the existence of a specialized coupling between kinetochores and MT plus ends that enables this transduction of chemical energy into mechanical work. Analysis of this phenomenon is important for learning how energy is stored within the MT lattice, how it is transduced, and how efficient the process can be, given coupling devices of different designs. Here we use a recently developed molecular-mechanical model of MTs to examine the mechanism of disassembly dependent force generation. Our approach is based on changes in tubulin dimer conformation that occur during MT disassembly. We find that all of the energy of polymerization-associated GTP hydrolysis can be stored as deformations of the longitudinal bonds between tubulin dimers, and its optimal use does not require the weakening of lateral bonds between dimers. Maximum utilization of this stored energy and, hence, the generation of the strongest possible force, is achieved by a protofilament power-stroke mechanism, so long as the coupling device does not restrict full dissociation of the lateral bonds between tubulin dimers.

Mcl1p is a polymerase alpha replication accessory factor important for S-phase DNA damage survival

Mcl1p is an essential fission yeast chromatin-binding protein that belongs to a family of highly conserved eukaryotic proteins important for sister chromatid cohesion. The essential function is believed to result from its role as a Pol1p (polymerase alpha) accessory protein, a conclusion based primarily on analogy to Ctf4p's interaction with Pol1p. In this study, we show that Mcl1p also binds to Pol1p with high affinity for the N terminus of Pol1p during S phase and DNA damage. Characterization of an inducible allele of mcl1+, (nmt41)mcl1-MH, shows that altered expression levels of Mcl1p lead to sensitivity to DNA-damaging agents and synthetic lethality with the replication checkpoint mutations rad3Delta, rqh1Delta, and hsk1-1312. Further, we find that the overexpression of the S-phase checkpoint kinase, Cds1, or the loss of Hsk1 kinase activity can disrupt Mcl1p's interaction with chromatin and Pol1p during replication arrest with hydroxyurea. We take these data to mean that Mcl1p is a dynamic component of the polymerase alpha complex during replication and is important for the replication stress response in fission yeast.

The use of filter membranes for high-pressure freezing of cell monolayers

Rapid freezing of cells and tissues, followed by freeze-substitution fixation and plastic embedding, has become a highly reliable method for preparing samples for imaging in the electron microscope. High-pressure freezing is an efficient means of immobilizing suspensions of yeasts, thick pellets of mammalian cells, or small (< 0.5 mm) pieces of plant or animal tissue. Monolayers of cultured mammalian cells that are too thick for efficient immobilization by other modes of rapid freezing have also been successfully preserved by this method. Monolayer cultures are often important because they can be imaged by light microscopy (LM) both before and after their preparation for electron microscopy (EM). Additionally, some monolayer cultures serve as model systems for physiological processes, so it is important that cells under study can grow on a substrate that is both physiologically appropriate and convenient for EM processing. Here we describe a reliable method for preparing mammalian cell monolayers (PtK1 and polarized MDCK) for EM. Our protocol results in good preservation of cellular ultrastructure, it is a useful companion to studies of cell physioloy and, with some limitation, is suitable for correlative LM and EM.

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.

The domain structure of centromeres is conserved from fission yeast to humans

The centromeric DNA of fission yeast is arranged with a central core flanked by repeated sequences. The centromere-associated proteins, Mis6p and Cnp1p (SpCENP-A), associate exclusively with central core DNA, whereas the Swi6 protein binds the surrounding repeats. Here, electron microscopy and immunofluorescence light microscopy reveal that the central core and flanking regions occupy distinct positions within a heterochromatic domain. An "anchor" structure containing the Ndc80 protein resides between this heterochromatic domain and the spindle pole body. The organization of centromere-associated proteins in fission yeast is reminiscent of the multilayered structures of human kinetochores, indicating that such domain structure is conserved in eukaryotes.

Structural evidence for multiple transport mechanisms through the Golgi in the pancreatic beta-cell line, HIT-T15

Accurate data on the three-dimensional architecture of the Golgi is prerequisite for evaluating the mechanisms of transit through this organelle. Here we detail the structure of the Golgi ribbon within part of an insulin-secreting cell in three dimensions at approximately 6 nm resolution. Rapid freezing, freeze-substitution and electron tomography were employed. The Golgi in this region is composed of seven cisternae. The cis-most element is structurally intermediate between the endoplasmic reticulum (ER)-Golgi intermediate compartment (ERGIC) and the cis-most cisterna characterized in three dimensions at high resolution in a normal rat kidney cell [Ladinsky, Mastronarde, McIntosh, Howell and Staehelin (1999) J. Cell Biol. 144, 1135-1149]. There are three trans-cisternae that demonstrate morphological and functional variation. The membrane surface areas and volumes of these elements decrease from cis to trans. The two trans-most cisternae are dissociated from the stack and are fragmented by tubulation. ER closely adheres to and inserts between individual trans-cisternae. Many of the 2119 small, clathrin-negative vesicles that are in close proximity to the Golgi fill the region where trans-cisternae have moved out of register with the ribbon. These data provide evidence that cisternal progression/maturation, trafficking via membrane tubules and vesicle-mediated transport act in concert in the same region of the Golgi ribbon, and suggest an important role for the ER in regulating membrane dynamics at the trans-Golgi.

Organellar relationships in the Golgi region of the pancreatic beta cell line, HIT-T15, visualized by high resolution electron tomography

The positional relationships among all of the visible organelles in a densely packed region of cytoplasm from an insulin secreting, cultured mammalian cell have been analyzed in three dimensions (3-D) at approximately 6 nm resolution. Part of a fast frozen/freeze-substituted HIT-T15 cell that included a large portion of the Golgi ribbon was reconstructed in 3-D by electron tomography. The reconstructed volume (3.1 x 3.2 x 1.2 microm(3)) allowed sites of interaction between organelles, and between microtubules and organellar membranes, to be accurately defined in 3-D and quantitatively analyzed by spatial density analyses. Our data confirm that the Golgi in an interphase mammalian cell is a single, ribbon-like organelle composed of stacks of flattened cisternae punctuated by openings of various sizes [Rambourg, A., Clermont, Y., & Hermo, L. (1979) Am. J. Anat. 154, 455-476]. The data also show that the endoplasmic reticulum (ER) is a single continuous compartment that forms close contacts with mitochondria, multiple trans Golgi cisternae, and compartments of the endo-lysosomal system. This ER traverses the Golgi ribbon from one side to the other via cisternal openings. Microtubules form close, non-random associations with the cis Golgi, the ER, and endo-lysosomal compartments. Despite the dense packing of organelles in this Golgi region, approximately 66% of the reconstructed volume is calculated to represent cytoplasmic matrix. We relate the intimacy of structural associations between organelles in the Golgi region, as quantified by spatial density analyses, to biochemical mechanisms for membrane trafficking and organellar communication in mammalian cells.

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.

Slk19p is a centromere protein that functions to stabilize mitotic spindles

We have identified a novel centromere-associated gene product from Saccharomyces cerevisiae that plays a role in spindle assembly and stability. Strains with a deletion of SLK19 (synthetic lethal Kar3p gene) exhibit abnormally short mitotic spindles, increased numbers of astral microtubules, and require the presence of the kinesin motor Kar3p for viability. When cells are deprived of both Slk19p and Kar3p, rapid spindle breakdown and mitotic arrest is observed. A functional fusion of Slk19p to green fluorescent protein (GFP) localizes to kinetochores and, during anaphase, to the spindle midzone, whereas Kar3p-GFP was found at the nuclear side of the spindle pole body. Thus, these proteins seem to play overlapping roles in stabilizing spindle structure while acting from opposite ends of the microtubules.

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.

A cytoplasmic dynein heavy chain is required for oscillatory nuclear movement of meiotic prophase and efficient meiotic recombination in fission yeast

Meiotic recombination requires pairing of homologous chromosomes, the mechanisms of which remain largely unknown. When pairing occurs during meiotic prophase in fission yeast, the nucleus oscillates between the cell poles driven by astral microtubules. During these oscillations, the telomeres are clustered at the spindle pole body (SPB), located at the leading edge of the moving nucleus and the rest of each chromosome dangles behind. Here, we show that the oscillatory nuclear movement of meiotic prophase is dependent on cytoplasmic dynein. We have cloned the gene encoding a cytoplasmic dynein heavy chain of fission yeast. Most of the cells disrupted for the gene show no gross defect during mitosis and complete meiosis to form four viable spores, but they lack the nuclear movements of meiotic prophase. Thus, the dynein heavy chain is required for these oscillatory movements. Consistent with its essential role in such nuclear movement, dynein heavy chain tagged with green fluorescent protein (GFP) is localized at astral microtubules and the SPB during the movements. In dynein-disrupted cells, meiotic recombination is significantly reduced, indicating that the dynein function is also required for efficient meiotic recombination. In accordance with the reduced recombination, which leads to reduced crossing over, chromosome missegregation is increased in the mutant. Moreover, both the formation of a single cluster of centromeres and the colocalization of homologous regions on a pair of homologous chromosomes are significantly inhibited in the mutant. These results strongly suggest that the dynein-driven nuclear movements of meiotic prophase are necessary for efficient pairing of homologous chromosomes in fission yeast, which in turn promotes efficient meiotic recombination.

High-voltage electron tomography of spindle pole bodies and early mitotic spindles in the yeast Saccharomyces cerevisiae

The spindle pole body (SPB) is the major microtubule-organizing center of budding yeast and is the functional equivalent of the centrosome in higher eukaryotic cells. We used fast-frozen, freeze-substituted cells in conjunction with high-voltage electron tomography to study the fine structure of the SPB and the events of early spindle formation. Individual structures were imaged at 5-10 nm resolution in three dimensions, significantly better than can be achieved by serial section electron microscopy. The SPB is organized in distinct but coupled layers, two of which show ordered two-dimensional packing. The SPB central plaque is anchored in the nuclear envelope with hook-like structures. The minus ends of nuclear microtubules (MTs) are capped and are tethered to the SPB inner plaque, whereas the majority of MT plus ends show a distinct flaring. Unbudded cells containing a single SPB retain 16 MTs, enough to attach to each of the expected 16 chromosomes. Their median length is approximately 150 nm. MTs growing from duplicated but not separated SPBs have a median length of approximately 130 nm and interdigitate over the bridge that connects the SPBs. As a bipolar spindle is formed, the median MT length increases to approximately 300 nm and then decreases to approximately 30 nm in late anaphase. Three-dimensional models confirm that there is no conventional metaphase and that anaphase A occurs. These studies complement and extend what is known about the three-dimensional structure of the yeast mitotic spindle and further our understanding of the organization of the SPB in intact cells.

Life cycles of yeast spindle pole bodies: getting microtubules into a closed nucleus

The spindle pole body (SPB) is the principal microtubule organizing center of budding and fission yeast. We have examined SPBs and their associated microtubules from both organisms, using electron microscopy and three-dimensional reconstruction techniques, to identify the structural changes that accompany progression through the cell cycle. In this report, we compare these changes in the two kinds of yeasts and present a model for how microtubules get into a closed nucleus.

Golgi structure in three dimensions: functional insights from the normal rat kidney cell

Three-dimensional reconstructions of portions of the Golgi complex from cryofixed, freeze-substituted normal rat kidney cells have been made by dual-axis, high-voltage EM tomography at approximately 7-nm resolution. The reconstruction shown here ( approximately 1 x 1 x 4 microm3) contains two stacks of seven cisternae separated by a noncompact region across which bridges connect some cisternae at equivalent levels, but none at nonequivalent levels. The rest of the noncompact region is filled with both vesicles and polymorphic membranous elements. All cisternae are fenestrated and display coated buds. They all have about the same surface area, but they differ in volume by as much as 50%. The trans-most cisterna produces exclusively clathrin-coated buds, whereas the others display only nonclathrin coated buds. This finding challenges traditional views of where sorting occurs within the Golgi complex. Tubules with budding profiles extend from the margins of both cis and trans cisternae. They pass beyond neighboring cisternae, suggesting that these tubules contribute to traffic to and/or from the Golgi. Vesicle-filled "wells" open to both the cis and lateral sides of the stacks. The stacks of cisternae are positioned between two types of ER, cis and trans. The cis ER lies adjacent to the ER-Golgi intermediate compartment, which consists of discrete polymorphic membranous elements layered in front of the cis-most Golgi cisterna. The extensive trans ER forms close contacts with the two trans-most cisternae; this apposition may permit direct transfer of lipids between ER and Golgi membranes. Within 0.2 microm of the cisternae studied, there are 394 vesicles (8 clathrin coated, 190 nonclathrin coated, and 196 noncoated), indicating considerable vesicular traffic in this Golgi region. Our data place structural constraints on models of trafficking to, through, and from the Golgi complex.

Effects of the myosin inhibitor 2,3-butanedione monoxime on the physiology of fission yeast

F-actin and associated myosins are thought to take part in a wide range of cellular processes, like motility and contraction, polarized growth, and secretion. The reagent 2,3-butanedione monoxime (BDM) is a well characterized inhibitor of the contraction of vertebrate muscle that reversibly affects myosin function and influences the intracellular concentration of Ca2+. Here we describe the influence of BDM on growth and division of the fission yeast Schizosaccharomyces pombe. At concentrations from 1-30 mM, BDM gradually inhibited formation and growth of S. pombe colonies on agar plates, with a lethal effect at > or = 15 mM. In strains of S. pombe that were blocked by elevated temperature from entry into mitosis, drug treatment reversibly decreased microtubule-independent tip growth and septation, with an IC50 value around 12 mM; nuclear division, on the other hand, was essentially unaffected by up to 15 mM BDM. At 30 mM BDM the secretion of invertase, which required both F-actin and microtubules, was decreased to the same extent as that seen when cytochalasin D was used to disrupt F-actin. However, the actin cytoskeleton was insensitive to up to 10 mM BDM, while the actin patches lost their polar distribution at 20-30 mM BDM. Cells treated with 5-20 mM BDM for 3 hours and then high pressure frozen did not show an accumulation of secretory vesicles. However, 10 mM BDM treatment disorganized the fungal cell wall, resulting in some unusually thick parts lying next to regions were the wall was almost absent. These defects could be rescued by incubating the cells in inhibitors of glucanases. Osmolytic stabilization with sorbitol rescued the effect of 15 mM BDM on colony survival, indicating that the secretion of wall components and/or wall-modifying enzymes may be the principal reason for cell death caused by BDM. Our results are consistent with the hypothesis that BDM influences actin-dependent processes in fission yeast and that actomyosin-dependent motility contributes to the secretory process of tip growth.

Localization of the 26S proteasome during mitosis and meiosis in fission yeast

The 26S proteasome is a large multisubunit complex involved in degrading both cytoplasmic and nuclear proteins. We have investigated the localization of this complex in the fission yeast, Schizosaccharomyces pombe. Immunofluorescence microscopy shows a striking localization pattern whereby the proteasome is found predominantly at the nuclear periphery, both in interphase and throughout mitosis. Electron microscopic analysis revealed a concentration of label near the inner side of the nuclear envelope. The localization of green fluorescent protein (GFP)-tagged 26S proteasomes was analyzed in live cells during mitosis and meiosis. Throughout mitosis the proteasome remained predominantly at the nuclear periphery. During meiosis the proteasome was found to undergo dramatic changes in its localization. Throughout the first meiotic division, the signal is more dispersed over the nucleus. During meiosis II, there was a dramatic re-localization, and the signal became restricted to the area between the separating DNA until the end of meiosis when the signal dispersed before returning to the nuclear periphery during spore formation. These findings strongly imply that the nuclear periphery is a major site of protein degradation in fission yeast both in interphase and throughout mitosis. Furthermore they raise interesting questions as to the spatial organization of protein degradation during meiosis.

The dynamic behavior of individual microtubules associated with chromosomes in vitro

Mitotic movements of chromosomes are usually coupled to the elongation and shortening of the microtubules to which they are bound. The lengths of kinetochore-associated microtubules change by incorporation or loss of tubulin subunits, principally at their chromosome-bound ends. We have reproduced aspects of this phenomenon in vitro, using a real-time assay that displays directly the movements of individual chromosome-associated microtubules as they elongate and shorten. Chromosomes isolated from cultured Chinese hamster ovary cells were adhered to coverslips and then allowed to bind labeled microtubules. In the presence of tubulin and GTP, these microtubules could grow at their chromosome-bound ends, causing the labeled segments to move away from the chromosomes, even in the absence of ATP. Sometimes a microtubule would switch to shortening, causing the direction of movement to change abruptly. The link between a microtubule and a chromosome was mechanically strong; 15 pN of tension was generally insufficient to detach a microtubule, even though it could add subunits at the kinetochore-microtubule junction. The behavior of the microtubules in vitro was regulated by the chromosomes to which they were bound; the frequency of transitions from polymerization to depolymerization was decreased, and the speed of depolymerization-coupled movement toward chromosomes was only one-fifth the rate of shortening for microtubules free in solution. Our results are consistent with a model in which each microtubule interacts with an increasing number of chromosome-associated binding sites as it approaches the kinetochore.

cut11(+): A gene required for cell cycle-dependent spindle pole body anchoring in the nuclear envelope and bipolar spindle formation in Schizosaccharomyces pombe

The "cut" mutants of Schizosaccharomyces pombe are defective in spindle formation and/or chromosome segregation, but they proceed through the cell cycle, resulting in lethality. Analysis of temperature-sensitive alleles of cut11(+) suggests that this gene is required for the formation of a functional bipolar spindle. Defective spindle structure was revealed with fluorescent probes for tubulin and DNA. Three-dimensional reconstruction of mutant spindles by serial sectioning and electron microscopy showed that the spindle pole bodies (SPBs) either failed to complete normal duplication or were free floating in the nucleoplasm. Localization of Cut11p tagged with the green fluorescent protein showed punctate nuclear envelope staining throughout the cell cycle and SPBs staining from early prophase to mid anaphase. This SPB localization correlates with the time in the cell cycle when SPBs are inserted into the nuclear envelope. Immunoelectron microscopy confirmed the localization of Cut11p to mitotic SPBs and nuclear pore complexes. Cloning and sequencing showed that cut11(+) encodes a novel protein with seven putative membrane-spanning domains and homology to the Saccharomyces cerevisiae gene NDC1. These data suggest that Cut11p associates with nuclear pore complexes and mitotic SPBs as an anchor in the nuclear envelope; this role is essential for mitosis.

Activation of the MKK/ERK pathway during somatic cell mitosis: direct interactions of active ERK with kinetochores and regulation of the mitotic 3F3/2 phosphoantigen

The mitogen-activated protein (MAP) kinase pathway, which includes extracellular signal-regulated protein kinases 1 and 2 (ERK1, ERK2) and MAP kinase kinases 1 and 2 (MKK1, MKK2), is well-known to be required for cell cycle progression from G1 to S phase, but its role in somatic cell mitosis has not been clearly established. We have examined the regulation of ERK and MKK in mammalian cells during mitosis using antibodies selective for active phosphorylated forms of these enzymes. In NIH 3T3 cells, both ERK and MKK are activated within the nucleus during early prophase; they localize to spindle poles between prophase and anaphase, and to the midbody during cytokinesis. During metaphase, active ERK is localized in the chromosome periphery, in contrast to active MKK, which shows clear chromosome exclusion. Prophase activation and spindle pole localization of active ERK and MKK are also observed in PtK1 cells. Discrete localization of active ERK at kinetochores is apparent by early prophase and during prometaphase with decreased staining on chromosomes aligned at the metaphase plate. The kinetochores of chromosomes displaced from the metaphase plate, or in microtubule-disrupted cells, still react strongly with the active ERK antibody. This pattern resembles that reported for the 3F3/2 monoclonal antibody, which recognizes a phosphoepitope that disappears with kinetochore attachment to the spindles, and has been implicated in the mitotic checkpoint for anaphase onset (Gorbsky and Ricketts, 1993. J. Cell Biol. 122:1311-1321). The 3F3/2 reactivity of kinetochores on isolated chromosomes decreases after dephosphorylation with protein phosphatase, and then increases after subsequent phosphorylation by purified active ERK or active MKK. These results suggest that the MAP kinase pathway has multiple functions during mitosis, helping to promote mitotic entry as well as targeting proteins that mediate mitotic progression in response to kinetochore attachment.

Kinesin from the plant pathogenic fungus Ustilago maydis is involved in vacuole formation and cytoplasmic migration

A gene encoding the heavy chain of conventional kinesin (kin2) has recently been identified in the dimorphic fungus Ustilago maydis (Lehmler et al., 1997). From the phenotype of kin2 null-mutants it was concluded that Kin2 might be involved in vesicle traffic towards the tip. However, this model did not explain why kin2-null mutant hyphae were unable to create empty cell compartments that are normally left behind the growing tip cell. Here we present a re-investigation of the function of Kin2 in hyphae and sporidia. We provide evidence that suggests a different and unexpected role of this kinesin motor in hyphal growth of Ustilago maydis. In addition, Kin2 was partially purified from U. maydis and in vitro properties were investigated. Isolated kinesin supported in vitro microtubule gliding at speeds of up to 1.8 micron/second, and showed motility properties and hydrodynamic behavior similar to those described for kinesin from N. crassa. It appears to be the product of the kin2 gene. Compared with wild-type sporidia, the kin2-null mutant sporidia grew normally but were defective in accumulation of Lucifer Yellow in their vacuoles, which were smaller than normal and often misplaced. The dikaryotic hyphae, produced by the fusion of two kin2-null sporidia, showed tip growth, but unlike wild-type hyphae, these structures lacked the large, basal vacuole and contain significantly more 200–400 nm vesicles scattered over the hole hypha. This defect was accompanied by a failure to generate regular empty cell compartments that are left behind in wild-type tip cells as the hyphae grow longer. These results suggest that Kin2 is a microtubule-dependent motor enzyme which is involved in the formation of vacuoles. The accumulation of these vacuoles at the basal end of the tip cell might be crucial for the formation of the empty sections and supports cytoplasmic migration during the growth of dikaryotic hyphae.

A screen for genes involved in the anaphase proteolytic pathway identifies tsm1(+), a novel Schizosaccharomyces pombe gene important for microtubule integrity

The growth of several mitotic mutants of Schizosaccharomyces pombe, including nuc2-663, is inhibited by the protease inhibitor N-Tosyl-L-Phenylalanine Chloromethyl Ketone (TPCK). Because nuc2(+) encodes a presumptive component of the Anaphase Promoting Complex, which is required for the ubiquitin-dependent proteolysis of certain proteins during exit from mitosis, we have used sensitivity to TPCK as a criterion by which to search for novel S. pombe mutants defective in the anaphase-promoting pathway. In a genetic screen for temperature-sensitive mitotic mutants that were also sensitive to TPCK at a permissive temperature, we isolated three tsm (TPCK-sensitive mitotic) strains. Two of these are alleles of cut1(+), but tsm1-512 maps to a novel genetic location. The tsm1-512 mutation leads to delayed nuclear division at restrictive temperatures, apparently as a result of an impaired ability to form a metaphase spindle. After shift of early G2 cells to 36 degrees, tsm1-512 arrests transiently in the second mitotic division and then exits mitosis, as judged by spindle elongation and septation. The chromosomes, however, often fail to segregate properly. Genetic interactions between tsm1-512 and components of the anaphase proteolytic pathway suggest a functional involvement of the Tsm1 protein in this pathway.

The spindle pole body of Schizosaccharomyces pombe enters and leaves the nuclear envelope as the cell cycle proceeds

The cycle of spindle pole body (SPB) duplication, differentiation, and segregation in Schizosaccharomyces pombe is different from that in some other yeasts. Like the centrosome of vertebrate cells, the SPB of S. pombe spends most of interphase in the cytoplasm, immediately next to the nuclear envelope. Some gamma-tubulin is localized on the SPB, suggesting that it plays a role in the organization of interphase microtubules (MTs), and serial sections demonstrate that some interphase MTs end on or very near to the SPB. gamma-Tubulin is also found on osmiophilic material that lies near the inner surface of the nuclear envelope, immediately adjacent to the SPB, even though there are no MTs in the interphase nucleus. Apparently, the MT initiation activities of gamma-tubulin in S. pombe are regulated. The SPB duplicates in the cytoplasm during late G2 phase, and the two resulting structures are connected by a darkly staining bridge until the mitotic spindle forms. As the cell enters mitosis, the nuclear envelope invaginates beside the SPB, forming a pocket of cytoplasm that accumulates dark amorphous material. The nuclear envelope then opens to form a fenestra, and the duplicated SPB settles into it. Each part of the SPB initiates intranuclear MTs, and then the two structures separate to lie in distinct fenestrae as a bipolar spindle forms. Through metaphase, the SPBs remain in their fenestrae, bound to the polar ends of spindle MTs; at about this time, a small bundle of cytoplasmic MTs forms in association with each SPB. These MTs are situated with one end near to, but not on, the SPBs, and they project into the cytoplasm at an orientation that is oblique to the simple axis. As anaphase proceeds, the nuclear fenestrae close, and the SPBs are extruded back into the cytoplasm. These observations define new fields of enquiry about the control of SPB duplication and the dynamics of the nuclear envelope.

Three-dimensional analysis and ultrastructural design of mitotic spindles from the cdc20 mutant of Saccharomyces cerevisiae

The three-dimensional organization of mitotic microtubules in a mutant strain of Saccharomyces cerevisiae has been studied by computer-assisted serial reconstruction. At the nonpermissive temperature, cdc20 cells arrested with a spindle length of approximately 2.5 microns. These spindles contained a mean of 81 microtubules (range, 56-100) compared with 23 in wild-type spindles of comparable length. This increase in spindle microtubule number resulted in a total polymer length up to four times that of wild-type spindles. The spindle pole bodies in the cdc20 cells were approximately 2.3 times the size of wild-type, thereby accommodating the abnormally large number of spindle microtubules. The cdc20 spindles contained a large number of interpolar microtubules organized in a "core bundle." A neighbor density analysis of this bundle at the spindle midzone showed a preferred spacing of approximately 35 nm center-to-center between microtubules of opposite polarity. Although this is evidence of specific interaction between antiparallel microtubules, mutant spindles were less ordered than the spindle of wild-type cells. The number of noncore microtubules was significantly higher than that reported for wild-type, and these microtubules did not display a characteristic metaphase configuration. cdc20 spindles showed significantly more cross-bridges between spindle microtubules than were seen in the wild type. The cross-bridge density was highest between antiparallel microtubules. These data suggest that spindle microtubules are stabilized in cdc20 cells and that the CDC20 gene product may be involved in cell cycle processes that promote spindle microtubule disassembly.

Patterning of functional antibodies and other proteins by photolithography of silane monolayers

We have demonstrated the assembly of two-dimensional patterns of functional antibodies on a surface. In particular, we have selectively adsorbed micrometer-scale regions of biotinylated immunoglobulin that exhibit specific antigen binding after adsorption. The advantage of this technique is its potential adaptability to adsorbing arbitrary proteins in tightly packed monolayers while retaining functionality. The procedure begins with the formation of a self-assembled monolayer of n-octadecyltrimethoxysilane (OTMS) on a silicon dioxide surface. This monolayer can then be selectively removed by UV photolithography. Under appropriate solution conditions, the OTMS regions will adsorb a monolayer of bovine serum albumin (BSA), while the silicon dioxide regions where the OTMS has been removed by UV light will adsorb less than 2% of a monolayer, thus creating high contrast patterned adsorption of BSA. The attachment of the molecule biotin to the BSA allows the pattern to be replicated in a layer of streptavidin, which bonds to the biotinylated BSA and in turn will bond an additional layer of an arbitrary biotinylated protein. In our test case, functionality of the biotinylated goat antibodies raised against mouse immunoglobulin was demonstrated by the specific binding of fluorescently labeled mouse IgG.

Mammalian cells express three distinct dynein heavy chains that are localized to different cytoplasmic organelles

We describe two dynein heavy chain (DHC)-like polypeptides (DHCs 2 and 3) that are distinct from the heavy chain of conventional cytoplasmic dynein (DHC1) but are expressed in a variety of mammalian cells that lack axonemes. DHC2 is a distant member of the "cytoplasmic" branch of the dynein phylogenetic tree, while DHC3 shares more sequence similarity with dynein-like polypeptides that have been thought to be axonemal. Each cytoplasmic dynein is associated with distinct cellular organelles. DHC2 is localized predominantly to the Golgi apparatus. Moreover, the Golgi disperses upon microinjection of antibodies to DHC2, suggesting that this motor is involved in establishing proper Golgi organization. DCH3 is associated with as yet unidentified structures that may represent transport intermediates between two or more cytoplasmic compartments. Apparently, specific cytoplasmic dyneins, like individual members of the kinesin superfamily, play unique roles in the traffic of cytomembranes.

Computer visualization of three-dimensional image data using IMOD

We have developed a computer software package, IMOD, as a tool for analyzing and viewing three-dimensional biological image data. IMOD is useful for studying and modeling data from tomographic, serial section, and optical section reconstructions. The software allows image data to be visualized by several different methods. Models of the image data can be visualized by volume or contour surface rendering and can yield quantitative information.

Analysis of MAP 4 function in living cells using green fluorescent protein (GFP) chimeras

MAP 4 is a ubiquitous microtubule-associated protein thought to play a role in the polymerization and stability of microtubules in interphase and mitotic cells. We have analyzed the behavior of protein domains of MAP 4 in vivo using chimeras constructed from these polypeptides and the green fluorescent protein (GFP). GFP-MAP 4 localizes to microtubules; this is confirmed by colocalization of GFP-MAP 4 with microtubules that have incorporated microinjected rhodamine-tubulin, and by loss of localized fluorescence after treatment of cells with anti-microtubule agents. Different subdomains of MAP 4 have distinct effects on microtubule organization and dynamics. The entire basic domain of MAP 4 reorganizes microtubules into bundles and stabilizes these arrays against depolymerization with nocodazole. Within the basic domain, the PGGG repeats, which are conserved with MAP 2 and tau, have a weak affinity for microtubules and are dispensable for microtubule binding, whereas the MAP 4-unique PSP region can function independently in binding. The projection domain shows no microtubule localization, but does modulate the association of various binding subdomains with microtubules. The acidic carboxy terminus of MAP 4 strongly affects the microtubule binding characteristics of the other domains, despite constituting less than 6% of the protein. These data show that MAP 4 association with microtubules is modulated by sequences both within and outside the basic domain. Further, our work demonstrates that GFP chimeras will allow an in vivo analysis of the effects of MAPs and their variants on microtubule dynamics in real time.

Three-dimensional ultrastructural analysis of the Saccharomyces cerevisiae mitotic spindle

The three dimensional organization of microtubules in mitotic spindles of the yeast Saccharomyces cerevisiae has been determined by computer-aided reconstruction from electron micrographs of serially cross-sectioned spindles. Fifteen spindles ranging in length from 0.6-9.4 microns have been analyzed. Ordered microtubule packing is absent in spindles up to 0.8 micron, but the total number of microtubules is sufficient to allow one microtubule per kinetochore with a few additional microtubules that may form an interpolar spindle. An obvious bundle of about eight interpolar microtubules was found in spindles 1.3-1.6 microns long, and we suggest that the approximately 32 remaining microtubules act as kinetochore fibers. The relative lengths of the microtubules in these spindles suggest that they may be in an early stage of anaphase, even though these spindles are all situated in the mother cell, not in the isthmus between mother and bud. None of the reconstructed spindles exhibited the uniform populations of kinetochore microtubules characteristic of metaphase. Long spindles (2.7-9.4 microns), presumably in anaphase B, contained short remnants of a few presumed kinetochore microtubules clustered near the poles and a few long microtubules extending from each pole toward the spindle midplane, where they interdigitated with their counterparts from the other pole. Interpretation of these reconstructed spindles offers some insights into the mechanisms of mitosis in this yeast.

Minus-end-directed motion of kinesin-coated microspheres driven by microtubule depolymerization

Dynamic changes in microtubule (MT) length have long been thought to contribute to intracellular motility. Both the polymerization and depolymerization of tubulin have been shown to do work in vitro, but the biochemical complexity of objects moved, such as chromosomes, has complicated the identification of proteins that couple MT dynamics with motility. Work with MTs grown from and tethered to pellicles of lysed Tetrahymena has shown that disassembly-dependent movement of chromosomes in vitro can be inhibited with antibodies against the motor domain of kinesin. To study proteins that can function in disassembly-dependent motion, we have refined this motility assay, replacing chromosomes with protein-coated latex microspheres. We report here the ability of several enzymes, including kinesin, to support in vitro motility of latex microspheres on disassembling MTs (Fig. 1a). The polarity of kinesin's motor activity can be reversed by MT disassembly and interactions between a motor and a MT end can either slow or speed the rate of tubulin depolymerization.

Antibodies to the kinesin motor domain and CENP-E inhibit microtubule depolymerization-dependent motion of chromosomes in vitro

Chromosomes can move with the ends of depolymerizing microtubules (MTs) in vitro, even in the absence of nucleotide triphosphates (Coue, M., V. A. Lombillo, and J. R. McIntosh. 1991. J. Cell Biol. 112:1165-1175.) Here, we describe an immunological investigation of the proteins important for this form of motility. Affinity-purified polyclonal antibodies to kinesin exert a severe inhibitory effect on depolymerization-dependent chromosome motion. These antibodies predominantly recognize a polypeptide of M(r) approximately 250 kD on immunoblots of CHO chromosomes and stain kinetochores as well as some vesicles that are in the chromosome preparation. Antibodies to CENP-E, a kinetochore-associated kinesin-like protein, also recognize a 250-kD electrophoretic component, but they stain only the kinetochroe region of isolated chromosomes. Polyclonal antibodies that recognize specific domains of the CENP-E polypeptide affect MT disassembly-dependent chromosome motion in different ways; antibodies to the head or tail portions slow motility threefold, while those raised against the neck region stop motion completely. Analogous antibodies that block conventional, ATP-dependent motility of cytoplasmic dynein (Vaisberg, G., M. P. Koonce, and J. R. McIntosh. 1993. J. Cell Biol. 123:849-858) have no effect on disassembly-dependent chromosome motion, even though they bind to kinetochores. These observations suggest that CENP-E helps couple chromosomes to depolymerizing MTs. A similar coupling activity may allow spindle MTs to remain kinetochore-bound while their lengths change during both prometaphase and anaphase A.

Molecular characterization of a cytoplasmic dynein from Dictyostelium

Cytoplasmic dynein is a high molecular weight, microtubule-based mechanochemical ATPase that is believed to provide motive force for a number of intracellular motilities, including transport of membrane-bound organelles. Cytoplasmic dynein also localizes to the mitotic spindles of some organisms and to the kinetochore regions of some condensed chromosomes, where it may play an active role in spindle assembly, spindle position, and/or chromosome movement during cell division. Despite active research efforts from a number of laboratories, little detail is yet available about dynein-based cellular activities. This paper describes our efforts to characterize cytoplasmic dynein from Dictyostelium and to use this protist as a molecular genetic factory to probe structure-function relationships of this molecule.

HVEM tomography of the trans-Golgi network: structural insights and identification of a lace-like vesicle coat

High voltage electron microscopy and computer axial tomography have been used to study the 3-D structure of trans-Golgi cisternae and trans-Golgi networks (TGNs) in NRK cells. Both structures were specifically labeled by photoconversion of a fluorescent analogue of ceramide using a modification of the techique of Pagano et al. (J. Cell Biol. 1991. 113: 1267-1279). Regions of the Golgi ribbon in fixed, stained cells were cut in 250-nm sections and analyzed by tilt series microscopy and subsequent tomographic reconstruction. Resolution of the reconstructions ranged from 6 to 10 nm. The size and structure of the TGN varied considerably throughout the Golgi ribbon; all reconstructions were made from regions with pronounced TGN. Most regions analyzed contained multiple (2-4) Golgi cisternae that stain with ceramide. These "peel off" from the closely stacked cisternae and are continuous at their ends with tubules that contribute to the TGN. Most vesicular profiles visualized in the TGN are connected to TGN tubules. The budding of vesicles appears to occur synchronously along the length of a TGN tubule. Two distinct coats were visualized on budding vesicles: clathrin cages and a novel, lace-like structure. Individual TGN tubules produce vesicles of only one coat type. These observations lead to the following predictions: (a) sorting of molecules must occur prior to the formation of TGN tubules; (b) vesicle formation takes place almost synchronously along a given TGN tubule; and (c) lace-like coats form an exocytic vesicles.

A cytoplasmic dynein motor in Drosophila: identification and localization during embryogenesis

We have characterized a cytoplasmic dynein motor isoform that is present in extracts of Drosophila embryos. A prominent high molecular weight (HMW) polypeptide (&gt; 400 kDa) is enriched in microtubules prepared from nucleotide-depleted embryonic extracts. Based on its ATP-sensitive microtubule binding activity, 20 S sedimentation coefficient, sensitivity to UV-vanadate and nucleotide specificity, the HMW polypeptide resembles cytoplasmic dyneins prepared from other organisms. The Drosophila cytoplasmic dynein acts as a minus-end motor that promotes microtubule translocation in vitro. A polyclonal antibody raised against the dynein heavy chain polypeptide was used to localize the dynein antigen in whole-mount preparations of embryos by immunofluorescence microscopy. These studies show that the dynein motor is associated with microtubules throughout embryogenesis, including mitotic spindle microtubules and microtubules of the embryonic nervous system.

Interpolar spindle microtubules in PTK cells

Spindle microtubules (MTs) in PtK1 cells, fixed at stages from metaphase to telophase, have been reconstructed using serial sections, electron microscopy, and computer image processing. We have studied the class of MTs that form an interdigitating system connecting the two spindle poles (interpolar MTs or ipMTs) and their relationship to the spindle MTs that attach to kinetochores (kMTs). Viewed in cross section, the ipMTs cluster with antiparallel near neighbors throughout mitosis; this bundling becomes much more pronounced as anaphase proceeds. While the minus ends of most kMTs are near the poles, those of the ipMTs are spread over half of the spindle length, with at least 50% lying > 1.5 microns from the poles. Longitudinal views of the ipMT bundles demonstrate a major rearrangement of their plus ends between mid- and late anaphase B. However, the minus ends of these MTs do not move appreciably farther from the spindle midplane, suggesting that sliding of these MTs contributes little to anaphase B. The minus ends of ipMTs are markedly clustered in the bundles of kMTs throughout anaphase A. These ends lie close to kMTs much more frequently than would be expected by chance, suggesting a specific interaction. As sister kinetochores separate and kMTs shorten, the minus ends of the kMTs remain associated with the spindle poles, but the minus ends of many ipMTs are released from the kMT bundles, allowing the spindle pole and the kMTs to move away from the ipMTs as the spindle elongates.

Cytoplasmic dynein plays a role in mammalian mitotic spindle formation

The formation and functioning of a mitotic spindle depends not only on the assembly/disassembly of microtubules but also on the action of motor enzymes. Cytoplasmic dynein has been localized to spindles, but whether or how it functions in mitotic processes is not yet known. We have cloned and expressed DNA fragments that encode the putative ATP-hydrolytic sites of the cytoplasmic dynein heavy chain from HeLa cells and from Dictyostelium. Monospecific antibodies have been raised to the resulting polypeptides, and these inhibit dynein motor activity in vitro. Their injection into mitotic mammalian cells blocks the formation of spindles in prophase or during recovery from nocodazole treatment at later stages of mitosis. Cells become arrested with unseparated centrosomes and form monopolar spindles. The injected antibodies have no detectable effect on chromosome attachment to a bipolar spindle or on motions during anaphase. These data suggest that cytoplasmic dynein plays a unique and important role in the initial events of bipolar spindle formation, while any later roles that it may play are redundant. Possible mechanisms of dynein's involvement in mitosis are discussed.

High voltage cryomicroscopy of human blood platelets

We have analyzed the ultrastructure of human blood platelets using ultrarapid freezing techniques followed by either freeze-substitution fixation or direct imaging of frozen hydrated specimens in a high voltage electron microscope. Freeze substitution preserved a regularity in the arrangement of microtubules when compared with chemically fixed specimens, as identified by a preferred center-to-center distance of 30 nm. Platelets imaged in the frozen hydrated state during different stages of adhesion and spreading were rich in cytoplasmic detail and revealed novel features not previously reported. Different granule populations were distinguished in the unstained samples, and the morphology of the cells was dramatically different from that of dried cells. Membrane channels formed as the cells spread, increasing in size and number as the number of granules in the cell decreased, suggesting a fusion of granule membranes with one another or an additional membrane system to release their contents. The data support the idea that direct cryoimaging is an optimum method for characterizing rapid structural changes of labile organelles in whole cells.

Three-dimensional reconstruction and analysis of mitotic spindles from the yeast, Schizosaccharomyces pombe

Mitotic spindles of Schizosaccharomyces pombe have been studied by EM, using serial cross sections to reconstruct 12 spindles from cells that were ultrarapidly frozen and fixed by freeze substitution. The resulting distributions of microtubules (MTs) have been analyzed by computer. Short spindles contain two kinds of MTs: continuous ones that run from pole to pole and MTs that originate at one pole and end in the body of the spindle. Among the latter there are three pairs of MT bundles that end on fibrous, darkly staining structures that we interpret as kinetochores. The number of MTs ending at each putative kinetochore ranges from two to four; all kinetochore-associated MTs disappear as the spindle elongates from 3-6 microns. At this and greater spindle lengths, there are no continuous MTs, only polar MTs that interdigitate at the spindle midzone, but the spindle continues to elongate. An analysis of the density of neighboring MTs at the midzone of long spindles shows that their most common spacing is approximately 40 nm, center to center, and that there is a preferred angular separation of 90 degrees. Only hints of such square-packing are found at the midzone of short spindles, and near the poles there is no apparent order at any mitotic stage. Our data suggest that the kinetochore MTs (KMTs) do not interact directly with nonkinetochore MTs, but that interdigitating MTs from the two spindle poles do interact to form a mechanically stable bundle that connects the poles. As the spindle elongates, the number of MTs decreases while the mean length of the MTs that remain increases. We conclude that the chromosomes of S. pombe become attached to the spindle by kinetochore MTs, that these MTs disappear as the chromosomes segregate, that increased separation of daughter nuclei is accompanied by a sliding apart of anti-parallel MTs, and that the mitotic processes of S. pombe are much like those in other eukaryotic cells.