Incorporation of Paramecium axonemal tubulin into higher plant cells reveals functional sites of microtubule assembly

The dissection of meiotic chromosome movement in mice using an in vivo electroporation technique

During meiosis, the rapid movement of telomeres along the nuclear envelope (NE) facilitates pairing/synapsis of homologous chromosomes. In mammals, the mechanical properties of chromosome movement and the cytoskeletal structures responsible for it remain poorly understood. Here, applying an in vivo electroporation (EP) technique in live mouse testis, we achieved the quick visualization of telomere, chromosome axis and microtubule organizing center (MTOC) movements. For the first time, we defined prophase sub-stages of live spermatocytes morphologically according to ^GFP-TRF1 and ^GFP-SCP3 signals. We show that rapid telomere movement and subsequent nuclear rotation persist from leptotene/zygotene to pachytene, and then decline in diplotene stage concomitant with the liberation of SUN1 from telomeres. Further, during bouquet stage, telomeres are constrained near the MTOC, resulting in the transient suppression of telomere mobility and nuclear rotation. MTs are responsible for these movements by forming cable-like structures on the NE, and, probably, by facilitating the rail-tacking movements of telomeres on the MT cables. In contrast, actin regulates the oscillatory changes in nuclear shape. Our data provide the mechanical scheme for meiotic chromosome movement throughout prophase I in mammals.

Meiosis is a special type of cell division for gametogenesis, errors in which cause several genetic disorders such as infertility and Down syndrome. In meiotic prophase I, chromosomes are tethered to the nuclear envelope (NE) through telomeres, and move rapidly along the NE to get homologs aligned and juxtaposed. Following homologous recombination and synapsis, the bivalent chromosome structure is established, which promotes genetic varieties, and also ensures accurate chromosome segregation in following anaphase I. Although there have been extensive studies addressing meiotic chromosome dynamics in yeast and worms, the same in mammalian meiosis remains largely elusive. Here, we utilized an in vivo electroporation (EP) technique to visualize chromosome movement in live mouse spermatocytes. We, for the first time, define the meiotic sub-stages in live cells based on telomeres and chromosome axis morphologies, and reveal chromosome movements regulated in a stage-specific manner. Putting the live-observations together with our cytological observations in fixed cells, we propose that meiotic chromosome movements in mammals are mediated by the rail-tracking movement of telomeres along the MT cables surrounding the meiotic nucleus.

Microtubule initiation from the nuclear surface controls cortical microtubule growth polarity and orientation in Arabidopsis thaliana

The nuclear envelope in plant cells has long been known to be a microtubule organizing center (MTOC), but its influence on microtubule organization in the cell cortex has been unclear. Here we show that nuclear MTOC activity favors the formation of longitudinal cortical microtubule (CMT) arrays. We used green fluorescent protein (GFP)-tagged gamma tubulin-complex protein 2 (GCP2) to identify nuclear MTOC activity and GFP-tagged End-Binding Protein 1b (EB1b) to track microtubule growth directions. We found that microtubules initiate from nuclei and enter the cortex in two directions along the long axis of the cell, creating bipolar longitudinal CMT arrays. Such arrays were observed in all cell types showing nuclear MTOC activity, including root hairs, recently divided cells in root tips, and the leaf epidermis. In order to confirm the causal nature of nuclei in bipolar array formation, we displaced nuclei by centrifugation, which generated a corresponding shift in the bipolarity split point. We also found that bipolar CMT arrays were associated with bidirectional trafficking of vesicular components to cell ends. Together, these findings reveal a conserved function of plant nuclear MTOCs and centrosomes/spindle pole bodies in animals and fungi, wherein all structures serve to establish polarities in microtubule growth.

The rise and fall of the phragmoplast microtubule array

The cytokinetic apparatus, the phragmoplast, contains a bipolar microtubule (MT) framework that has the MT plus ends concentrated at or near the division site. This anti-parallel MT array provides tracks for the transport of Golgi-derived vesicles toward the plus ends so that materials enclosed are subsequently deposited at the division site. Here we will discuss a proposed model of the centrifugal expansion of the phragmoplast that takes place concomitantly with the assembly of the cell plate, the ultimate product of vesicle fusion. The expansion is a result of continuous MT assembly at the phragmoplast periphery while the MTs toward the center of the phragmoplast are disassembled. These events are the result of MT-dependent MT polymerization, bundling of anti-parallel MTs coming from opposite sides of the division plane that occurs selectively at the phragmoplast periphery, positioning of the plus ends of cross-linked MTs at or near the division site by establishing a minimal MT-overlapping zone, and debundling of anti-parallel MTs that is triggered by phosphorylation of MT-associated proteins. The debundled MTs are disassembled at last by factors including the MT severing enzyme katanin.

Interaction of antiparallel microtubules in the phragmoplast is mediated by the microtubule-associated protein MAP65-3 in Arabidopsis

In plant cells, microtubules (MTs) in the cytokinetic apparatus phragmoplast exhibit an antiparallel array and transport Golgi-derived vesicles toward MT plus ends located at or near the division site. By transmission electron microscopy, we observed that certain antiparallel phragmoplast MTs overlapped and were bridged by electron-dense materials in Arabidopsis thaliana. Robust MT polymerization, reported by fluorescently tagged End Binding1c (EB1c), took place in the phragmoplast midline. The engagement of antiparallel MTs in the central spindle and phragmoplast was largely abolished in mutant cells lacking the MT-associated protein, MAP65-3. We found that endogenous MAP65-3 was selectively detected on the middle segments of the central spindle MTs at late anaphase. When MTs exhibited a bipolar appearance with their plus ends placed in the middle, MAP65-3 exclusively decorated the phragmoplast midline. A bacterially expressed MAP65-3 protein was able to establish the interdigitation of MTs in vitro. MAP65-3 interacted with antiparallel microtubules before motor Kinesin-12 did during the establishment of the phragmoplast MT array. Thus, MAP65-3 selectively cross-linked interdigitating MTs (IMTs) to allow antiparallel MTs to be closely engaged in the phragmoplast. Although the presence of IMTs was not essential for vesicle trafficking, they were required for the phragmoplast-specific motors Kinesin-12 and Phragmoplast-Associated Kinesin-Related Protein2 to interact with MT plus ends. In conclusion, we suggest that the phragmoplast contains IMTs and highly dynamic noninterdigitating MTs, which work in concert to bring about cytokinesis in plant cells.

Two Arabidopsis phragmoplast-associated kinesins play a critical role in cytokinesis during male gametogenesis

In plant cells, cytokinesis is brought about by the phragmoplast. The phragmoplast has a dynamic microtubule array of two mirrored sets of microtubules, which are aligned perpendicularly to the division plane with their plus ends located at the division site. It is not well understood how the phragmoplast microtubule array is organized. In Arabidopsis thaliana, two homologous microtubule motor kinesins, PAKRP1/Kinesin-12A and PAKRP1L/Kinesin-12B, localize exclusively at the juxtaposing plus ends of the antiparallel microtubules in the middle region of the phragmoplast. When either kinesin was knocked out by T-DNA insertions, mutant plants did not show a noticeable defect. However, in the absence of both kinesins, postmeiotic development of the male gametophyte was severely inhibited. In dividing microspores of the double mutant, microtubules often became disorganized following chromatid segregation and failed to form an antiparallel microtubule array between reforming nuclei. Consequently, the first postmeiotic cytokinesis was abolished without the formation of a cell plate, which led to failures in the birth of the generative cell and, subsequently, the sperm. Thus, our results indicate that Kinesin-12A and Kinesin-12B jointly play a critical role in the organization of phragmoplast microtubules during cytokinesis in the microspore that is essential for cell plate formation. Furthermore, we conclude that Kinesin-12 members serve as dynamic linkers of the plus ends of antiparallel microtubules in the phragmoplast.

Interactions of tobacco microtubule-associated protein MAP65-1b with microtubules

Tobacco microtubule associated protein (MAP65) (NtMAP65s) constitute a family of microtubule-associated proteins with apparent molecular weight around 65 kDa that collectively induce microtubule bundling and promote microtubule assembly in vitro. They are associated with most of the tobacco microtubule arrays in situ. Recently, three NtMAP65s belonging to the NtMAP65-1 subfamily have been cloned. Here we investigated in vitro the biochemical properties of one member of this family, the tobacco NtMAP65-1b. We demonstrated that recombinant NtMAP65-1b is a microtubule-binding and a microtubule-bundling protein. NtMAP65-1b has no effect on microtubule polymerization rate and binds microtubules with an estimated equilibrium constant of dissociation (K(d)) of 0.57 micro m. Binding of NtMAP65-1b to microtubules occurs through the carboxy-terminus of tubulin, as NtMAP65-1b was no longer able to bind subtilisin-digested tubulin. In vitro, NtMAP65-1b stabilizes microtubules against depolymerization induced by cold, but not against katanin-induced destabilization. The biological implications of these results are discussed.

Disruption of actin microfilaments causes cortical microtubule disorganization and extra-phragmoplast formation at M/G1 interface in synchronized tobacco cells

The roles of actin microfilaments (MFs) in the organization of microtubules (MTs) at the M/G1 interface were investigated in transgenic tobacco BY-2 cells stably expressing a GFP-tubulin fusion protein, using the MF-disrupting agent, Bistheonellide A (BA). When MFs were disrupted by BA treatment, cortical MTs (CMTs) did not become reorganized even 3 h after phragmoplast collapse, whereas non-treated cells completed CMT reorganization within 1 h. Furthermore, in the absence of MFs, the tubulin proteins did not show appropriate recruitment but remained at the site where the phragmoplast had existed, or extra-phragmoplasts were organized. These extra-phragmoplasts could functionally form extra-cell plates. This is the first observation of the formation of multiple cell plates during one nuclear division, and of phragmoplast generation irrespective of the position of the mitotic spindle or nuclei. The significance of these observations on the role of MFs at the M/G1 interface is discussed.

Origin of cortical microtubules organized at M/G1 interface: recruitment of tubulin from phragmoplast to nascent microtubules

The origin of cortical microtubules (CMTs) was investigated in transgenic BY-2 cells stably expressing a GFP (green fluorescent protein) -tubulin fusion protein (BY-GT16). In a previous study, we found that CMTs were initially organized in the perinuclear regions but then elongated to reach the cell cortex where they formed bright spots, and that the appearance of parallel MTs from the bright spots was followed by the appearance of transverse MTs (Kumagai et al., Plant Cell Physiol. 42, 723-732, 2001). In this study, we investigated the migration of tubulin to the reorganization sites of CMTs at the M/G1 interface. After synchronization of the BY-GT16 cells by aphidicolin, the localization of GFP-tubulin was monitored and analyzed by deconvolution microscopy. GFP-tubulin was found to accumulate on the nuclear surface near the cell plate at the final stage of phragmoplast collapse. Subsequently, GFP-tubulin accumulated again on the nuclear surface opposite the cell plate, where nascent MTs elongated to the cell cortex. The significance of these observations on the mode of CMT organization is discussed.

The architecture of microtubular network and Golgi orientation in osteoclasts--major differences between avian and mammalian species

In the present study, we analyze multinuclear osteoclasts obtained from several avian and mammalian species and describe the reorganization of their microtubular architecture and Golgi complex orientation during osteoclast differentiation and activation for bone resorption. In nonresorbing quail and chicken multinuclear osteoclasts, microtubules radiate from multiple centrosomal microtubule-organizing centers (MTOCs), whose number is equal to the number of nuclei. However, centrosomal MTOCs disappear at the time of cell activation for bone resorption and the Golgi membranes redistribute to circumscribe nuclei. In contrast to avian osteoclasts, both resorbing and nonresorbing rat, rabbit, and human osteoclasts have no or few centrosomal MTOCs. Instead, after cold-induced depolymerization, regrowing microtubules nucleate from the perinuclear area where immunofluoresce and immunoelectron scanning microscopy reveal pericentriolar matrix protein pericentrin associated with vimentin filaments. Furthermore, the circumnuclear reorganization of MTOCs and the Golgi is a result of mammalian osteoclast maturation and occur before any resorptive activity of the mononuclear osteoclasts and their fusion into multinucleated cells. Our results show that unlike previously suggested, the nuclear surfaces of mammalian osteoclasts act as the microtubule anchoring sites similarly to nuclear surfaces in multinucleated myotubes and suggest the role of perinuclear intermediate filament network in orchestrating the microtubular cytoskeleton.

Reorganization and polarization of the meiotic bouquet-stage cell can be uncoupled from telomere clustering

Striking cellular reorganizations mark homologous pairing during meiotic prophase. We address the interdependence of chromosomal and cellular polarization during meiotic telomere clustering, the defining feature of the bouquet stage, by examining nuclear positioning and microtubule and nuclear pore reorganization. Polarization of meiotic cellular architecture was coincident with telomere clustering: microtubules were focused on the nuclear surface opposite the telomere cluster, the nucleus was positioned eccentrically in the cell such that the telomeres faced the direction of nuclear displacement and nuclear pores were clustered in a single region of the nuclear surface opposite the telomeres. Treatment of pre-bouquet stage cells with colchicine inhibited telomere clustering. Asymmetric nuclear positioning and nuclear pore clustering were normal in the presence of unclustered telomeres resulting from colchicine treatment. Nuclear pores were positioned normally with respect to the cell cortex in the absence of telomere clustering, indicating that telomere positioning is not required for polarization. This work provides evidence of meiotic cell polarization and suggests that telomeres may be positioned relative to an asymmetry present in the cell at the time of bouquet formation.

TMBP200, a microtubule bundling polypeptide isolated from telophase tobacco BY-2 cells is a MOR1 homologue

Bundles of microtubules and cross-bridges between microtubules in the bundles have been observed in phragmoplasts, but proteins responsible for forming the cross-bridges have not been identified. We isolated TMBP200, a novel microtubule bundling polypeptide with an estimated relative molecular mass of about 200,000 from telophase tobacco BY-2 cells. Ultrastructural observation of microtubules bundled by purified TMBP200 in vitro revealed that TMBP200 forms cross-bridges between microtubules. The structure of the bundles and lengths of the cross-bridges were quite similar to those observed in phragmoplasts, suggesting that TMBP200 participates in the formation of microtubule bundles in phragmoplasts. The cDNA encoding TMBP200 was cloned and the deduced amino acid sequence showed homology to a class of microtubule-associated proteins including Xenopus XMAP215, human TOGp and Arabidopsis MOR1.

Expansion of the phragmoplast during plant cytokinesis: a MAPK pathway may MAP it out

Plant cytokinesis involves the formation of a cell plate. This is accomplished with the help of the phragmoplast, a plant-specific cytokinetic apparatus that consists of microtubules and microfilaments. During centrifugal growth of the cell plate, the phragmoplast expands to keep its microtubules at the leading edge of the cell plate. Recent studies have revealed potential regulators of phragmoplast microtubule dynamics and the involvement of a mitogen-activated protein kinase cascade in the control of phragmoplast expansion. These studies provide new insights into the molecular mechanisms of plant cytokinesis.

A novel plant kinesin-related protein specifically associates with the phragmoplast organelles

In higher plants, the formation of the cell plate during cytokinesis requires coordinated microtubule (MT) reorganization and vesicle transport in the phragmoplast. MT-based kinesin motors are important players in both processes. To understand the mechanisms underlying plant cytokinesis, we have identified AtPAKRP2 (for Arabidopsis thaliana phragmoplast-associated kinesin-related protein 2). AtPAKRP2 is an ungrouped N-terminal motor kinesin. It first appeared in a punctate pattern among interzonal MTs during late anaphase. When the phragmoplast MT array appeared in a mirror pair, AtPAKRP2 became more concentrated near the division site, and additional signal could be detected elsewhere in the phragmoplast. In contrast, the previously identified AtPAKRP1 protein is associated specifically with bundles of MTs in the phragmoplast at or near their plus ends. Localization of the tobacco homolog(s) of AtPAKRP2 was altered by treatment of brefeldin A in BY-2 cells. We discuss the possibility that AtPAKRP1 plays a role in establishing and/or maintaining the phragmoplast MT array, and AtPAKRP2 may contribute to the transport of Golgi-derived vesicles in the phragmoplast.

A novel plant kinesin-related protein specifically associates with the phragmoplast organelles

In higher plants, the formation of the cell plate during cytokinesis requires coordinated microtubule (MT) reorganization and vesicle transport in the phragmoplast. MT-based kinesin motors are important players in both processes. To understand the mechanisms underlying plant cytokinesis, we have identified AtPAKRP2 (for Arabidopsis thaliana phragmoplast-associated kinesin-related protein 2). AtPAKRP2 is an ungrouped N-terminal motor kinesin. It first appeared in a punctate pattern among interzonal MTs during late anaphase. When the phragmoplast MT array appeared in a mirror pair, AtPAKRP2 became more concentrated near the division site, and additional signal could be detected elsewhere in the phragmoplast. In contrast, the previously identified AtPAKRP1 protein is associated specifically with bundles of MTs in the phragmoplast at or near their plus ends. Localization of the tobacco homolog(s) of AtPAKRP2 was altered by treatment of brefeldin A in BY-2 cells. We discuss the possibility that AtPAKRP1 plays a role in establishing and/or maintaining the phragmoplast MT array, and AtPAKRP2 may contribute to the transport of Golgi-derived vesicles in the phragmoplast.

Isolation of cortical MTs from tobacco BY-2 cells

We isolated the cortical microtubules (CMTs) from tobacco BY-2 cells to identify their components. By centrifugation of protoplasts homogenized in the presence of taxol, a MT-stabilizing reagent, in a density gradient of Percoll, we obtained membranous vesicles to which MTs forming a sheet-like bundle were attached. Rhodamine-conjugated Ricinus communis agglutinin I (RCA-I), a lectin that bound to the surface of protoplasts, stained these vesicles, indicating that they were plasma membrane (PM) vesicles that retained CMTs. CMTs were released by solubilization of PM vesicles with Triton X-100. A sheet-like array of CMTs was retained even after solubilization of PM vesicles. Immunoblot analysis of the isolated CMTs demonstrated the presence of tubulin, actin, the 65 kDa microtubule-associated protein (MAP) and a 130 kDa RCA-I binding protein. Purification of the isolated CMTs by the temperature dependent disassembly-reassembly cycling method revealed four polypeptides, 190, 120, 85 and 65 kDa, co-assembling with CMTs.

Minus end-directed kinesin-like motor protein, Kcbp, localizes to anaphase spindle poles in Haemanthus endosperm

Microtubule-based motor proteins assemble and reorganize acentrosomal mitotic and meiotic spindles in animal cells. The functions of motor proteins in acentrosomal plant spindles are unknown. The cellulosic cell wall and relative small size of most plant cells precludes accurate detection of the spatial distribution of motors in mitosis. Large cell size and absence of a cellulosic cell wall in Haemanthus endosperm make these cells ideally suited for studies of the spatial distribution of motor proteins during cell division. Immunolocalization of a kinesin-like calmodulin-binding protein (KCBP) in Haemanthus endosperm revealed its mitotic distribution. KCBP appears first in association with the prophase spindle. Highly concentrated within the cores of individual kinetochore fibers, KCBP decorates microtubules of kinetochore-fibers through metaphase. By mid-anaphase (when a barrel-shaped spindle becomes convergent), the protein redistributes and accumulates at the spindle polar regions. In telophase, KCBP relocates toward the phragmoplast and cell plate. These data suggest a role for KCBP in anaphase spindle microtubule convergence, which assures coherence of kinetochore-fibers within each sister chromosome group. Increasing coherence of kinetochore-fibers prevents splitting within each sister chromosome group and formation of multinucleated cells.

Early stages of spindle formation and independence of chromosome and microtubule cycles in Haemanthus endosperm

We analyzed transformation of the interphase microtubular cytoskeleton into the prophase spindle and followed the pattern of spindle axis determination. Microtubules in endosperm of the higher plant Haemanthus (Scadoxus) were stained by the immunogold and immunogold silver-enhanced methods. Basic structural units involved in spindle morphogenesis were "microtubule converging centers." We emphasized the importance of relative independence of chromosomal and microtubular cycles, and the influence of these cycles on the progress of mitosis. Cells with moderately desynchronized cycles were functional, but extreme desynchronization led to aberrant mitosis. There were three distinct phases of spindle development. The first one comprised interphase and early to mid-prophase. During this phase, the interphase microtubule meshwork radiating from the nuclear surface into the cytoplasm rearranged and formed a dense microtubule cage around the nucleus. The second phase comprised mid to late prophase, and resulted in the formation of normal (bipolar) or transitory aberrant (apolar or multipolar) prophase spindles. The third phase comprised late prophase with prometaphase. The onset of prometaphase was accompanied by a rapid association of microtubule converging centers with kinetochores. In this stage aberrant spindles transformed invariably into bipolar ones. Lateral association of a few bipolar kinetochore fibers at early prometaphase established the core of the bipolar spindle and its alignment. We concluded that (1) spindle formation is a largely independent microtubular process modified by the chromosomal/kinetochore cycle; and (2) the initial polarity of the spindle is established by microtubule converging centers, which are a functional substitute of the centrosome/MTOC. We believe that the dynamics of microtubule converging centers is an expression of microtubule self-organization driven by motor proteins as proposed by Mitchison [1992: Philos. Trans. R. Soc. Lond. B. 336:99].

Higher plant cells: gamma-tubulin and microtubule nucleation in the absence of centrosomes

The assembly of the higher plant cytoskeleton poses several fundamental questions. Since different microtubule arrays are successively assembled during the cell cycle in the absence of centrosomes, we can ask how these arrays are assembled and spatially organized. Two hypotheses are under debate. Either multiple nucleation sites are responsible for the assembly and organization of microtubule arrays or microtubule nucleation takes place at one site, the nuclear surface. In the latter case, microtubule nucleation and organization would be two distinct but coregulated processes. During recent years, novel approaches have provided entirely new insights to understand the assembly and dynamics of the plant cytoskeleton. In the present review, we summarize advances made in microscopy and in molecular biology which lead to novel hypotheses and open up new fields of investigation. From the results obtained, it is clear that the higher plant cell is a powerful model system to investigate cytoskeletal organization in acentrosomal eukaryotic cells.

Characterization of microsome-associated tobacco BY-2 centrins

Centrin - higher plants - MTOCs - microtubules nucleation In most eukaryotic cells, the Ca(2+)-binding protein centrin is associated with structured microtubule-organizing centers (MTOCs) such as centrosomes. In these cells, centrin either forms centrosome-associated contractile fibers, or is involved in centrosome biogenesis. Our aim was to investigate the functions of centrin in higher plant cells which do not contain centrosome-like MTOCs. We have cloned two tobacco BY-2 centrin cDNAs and we show that higher plant centrins define a phylogenetic group of proteins distinct from centrosome-associated centrins. In addition, tobacco centrins were found primarily associated with microsomes and did not colocalize with gamma-tubulin, a known MTOC marker. While the overall level of centrin did not vary during the cell cycle, centrin was prominently detected at the cell plate during telophase. Our results suggest that in tobacco, the major portion of centrin is not MTOC-associated and could be involved in the formation of the cell plate during cytokinesis.

ASYMMETRIC CELL DIVISION IN PLANTS

Asymmetric cell divisions generate cells with different fates. In plants, where cells do not move relative to another cell, the specification and orientation of these divisions is an important mechanism to generate the overall cellular pattern during development. This review summarizes our knowledge of selected cases of asymmetric cell division in plants, in the context of recent insights into mechanisms underlying this process in bacteria, algae, yeast, and animals.

Microtubule-organizing centers and nucleating sites in land plants

Microtubule-organizing centers (MTOCs) are morphologically diverse cellular sites involved in the nucleation and organization of microtubules (MTs). These structures are synonymous with the centrosome in mammalian cells. In most land plant cells, however, no such structures are observed and some have argued that plant cells may not have MTOCs. This review summarizes a number of experimental approaches toward the elucidation of those subcellular sites involved in microtubule nucleation and organization. In lower land plants, structurally well-defined MTOCs are present, such as the blepharoplast, multilayered structure, and polar organizer. In higher plants, much of the nucleation and organization of MTs occurs on the nuclear envelope or other endomembranes, such as the plasmalemma and smooth (tubular) endoplasmic reticulum. In some instances, one endomembrane may serve as a site of nucleation whereas others serve as the site of organization. Structural and motor microtubule-associated proteins also appear to be involved in MT nucleation and organization. Immunochemical evidence indicates that at least several of the proteins found in mammalian centrosomes, gamma-tubulin, centrin, pericentrin, and polypeptides recognized by the monoclonal antibodies MPM-2, 6C6, and C9 also recognize putative lower land plant MTOCs, indicating shared mechanisms of nucleation/organization in plants and animals. The most recent data from tubulin incorporation in vivo, mutants with altered MT organization, and molecular studies indicate the potential of these research tools in investigation of MTOCs in plants.

TKRP125, a kinesin-related protein involved in the centrosome-independent organization of the cytokinetic apparatus in tobacco BY-2 cells

Analysis of a cDNA for a 125 kDa polypeptide, previously isolated from phragmoplasts of tobacco BY-2 cells as a candidate for a plus end-directed microtubule motor, revealed this polypeptide to be a novel member of the kinesin superfamily. We named this protein TKRP125 (tobacco kinesin-related polypeptide of 125 kDa). The strong similarity between TKRP125 and members of the bimC subfamily in terms of the amino acid sequence of the amino-terminal motor domain indicated that TKRP125 belonged to the bimC subfamily. An antibody against a short peptide from the motor domain of TKRP125 inhibited the GTP- or ATP-dependent translocation of phragmoplast microtubules in membrane-permeabilized BY-2 cells, suggesting a role for TKRP125 in microtubule translocation, which is considered to be involved in the formation and/or maintenance of the bipolar structure of the phragmoplast. The expression of TKRP125 was found to be cell cycle-dependent. TKRP125 was not present in cells at the G1 phase. It began to appear at the S phase and accumulated during the G2 phase. The distribution of TKRP125 changed as the arrangement of microtubules changed with the progression of the cell cycle. TKRP125 was distributed along cortical microtubules during the S phase and along microtubules in the preprophase band and perinuclear microtubules in premitotic cells. It was also present in the nucleus in premitotic cells. In cells in M phase, TKRP125 was distributed along spindle microtubules. It accumulated at the equatorial plane of the spindle as the spindle elongated. In cytokinetic cells, TKRP125 was colocalized with phragmoplast microtubules. These observations suggest the possible involvement of TKRP125 in the cell cycle-dependent changes in arrays of microtubules, including the organization of the phragmoplast, and in the movement of chromosomes in anaphase cells.

Nuclear components with microtubule-organizing properties in multicellular eukaryotes: functional and evolutionary considerations

The nucleus and the microtubular cytoskeleton of eukaryotic cells appear to be structurally and functionally interrelated. Together they constitute a "cell body". One of the most important components of this body is a primary microtubule-organizing center (MTOC-I) located on or near the nuclear surface and composed of material that, in addition to constitutive centrosomal material, also comprises some nuclear matrix components. The MTOC-I shares a continuity with the mitotic spindle and, in animal cells, with the centrosome also. Secondary microtubule-organizing centers (MTOC-IIs) are a special feature of walled plant cells and are found at the plasma membrane where they organize arrays of cortical MTs that are essential for ordered cell wall synthesis and hence for cellular morphogenesis. MTOC-IIs are held to be similar in origin to the MTOC-I, but their material has been translocated to the cell periphery, perhaps by MTs organized and radiating from the MTOC-I. Many intranuclear, matrix-related components have been identified to participate in MT organization during mitosis and cytokinesis; some of them also seem to be related to the condensation and decondensation of chromatin during the mitotic chromosome cycle.

Cell model systems in plant cytoskeleton studies

Cytoskeletons play an essential role in cellular functions in both animal and plant cells. In studies of the molecular mechanisms of their functions, a variety of cell model systems, mainly of animal cells, have yielded much information. With plant cells, cell model systems have mostly been restricted to studies on the mechanism of cytoplasmic streaming. Recently, however, there have been several reports of studies employing plant cell model systems to investigate plant cytoskeletons that have revealed new concepts about their structure and functions. To promote and support a general understanding of cell model systems, this review attempts to categorize them, present currently known information on the structure and function of plant cytoskeletons, and offer a possible role of cell model systems in future studies of plant cytoskeletons.

The force for poleward chromosome motion in Haemanthus cells acts along the length of the chromosome during metaphase but only at the kinetochore during anaphase

The force for poleward chromosome motion during mitosis is thought to act, in all higher organisms, exclusively through the kinetochore. We have used time-lapse. video-enhanced, differential interference contrast light microscopy to determine the behavior of kinetochore-free "acentric" chromosome fragments and "monocentric" chromosomes containing one kinetochore, created at various stages of mitosis in living higher plant (Haemanthus) cells by laser microsurgery. Acentric fragments and monocentric chromosomes generated during spindle formation and metaphase both moved towards the closest spindle pole at a rate (approximately 1.0 microm/min) similar to the poleward motion of anaphase chromosomes. This poleward transport of chromosome fragments ceased near the onset of anaphase and was replaced. near midanaphase, by another force that now transported the fragments to the spindle equator at 1.5-2.0 microm/min. These fragments then remained near the spindle midzone until phragmoplast development, at which time they were again transported randomly poleward but now at approximately 3 microm/min. This behavior of acentric chromosome fragments on anastral plant spindles differs from that reported for the astral spindles of vertebrate cells, and demonstrates that in forming plant spindles, a force for poleward chromosome motion is generated independent of the kinetochore. The data further suggest that the three stages of non-kinetochore chromosome transport we observed are all mediated by the spindle microtubules. Finally, our findings reveal that there are fundamental differences between the transport properties of forming mitotic spindles in plants and vertebrates.

The centrosome in animal cells and its functional homologs in plant and yeast cells

The centrosome is the principal microtubule-organizing center in mammalian cells. Until recently, the centrosome could only be studied at the ultrastructural level and defined as a functional entity. However, during the past decade a number of clever experimental strategies have been used to identify numerous molecular components of the centrosome. The identification of biochemical subunits of the centrosome complex has allowed the centrosome to be investigated in much more detail, resulting in important advances being made in our understanding of microtubule nucleation events, spindle formation, the assembly and replication of the centrosome, and the nature of the microtubule-organizing centers in plant cells and lower eukaryotes. The next several years should see additional rapid progress in our understanding of the microtubule cytoskeleton as investigators begin to assign functions to the centrosome proteins that have already been reported and as additional centrosome components are discovered.

Microtubule organization and the distribution of gamma-tubulin in spermatogenesis of a beetle, Tenebrio molitor (Tenebrionidae, Coleoptera, Insecta)

The present study focuses on the restructuring of the microtubule (MT) cytoskeleton and microtubule-organizing centres (MTOCs) throughout spermatogenesis of a darkling beetle, Tenebrio molitor (Tenebrionidae, Coleoptera, Insecta). To this end, serial ultrathin sections through male germ cells were studied using transmission electron microscopy. Additionally, spindles and young spermatids were isolated from testes under MT-stabilizing conditions and doubly labeled with antibodies against beta- and gamma-tubulin. The latter is a tubulin isoform detected in MTOCs of a wide variety of species. The observations suggest that microtubules may be nucleated from sites with and without high gamma-tubulin content and that these sites do not necessarily possess canonical centrosomes. In a prominent cytoplasmic MT system of primary spermatocytes in prophase, microtubule nucleation apparently occurs in the absence of immunologically detectable gamma-tubulin. At the poles of meiotic spindles, MTs are directly inserted into gamma-tubulin-containing material and this connection is considered responsible for their nucleation. The interzone spindle MTs of telophase cells contain gamma-tubulin and this may confer stability to them. Finally, manchette MTs of spermatids originate in the vicinity of the acrosome precursor but are not inserted into this body. The acrosome precursor is surrounded by a membrane and is clearly detected by the antibody against gamma-tubulin.

Phragmoplast of the green alga Spirogyra is functionally distinct from the higher plant phragmoplast

Cytokinesis in the green alga Spirogyra (Zygnemataceae) is characterized by centripetal growth of a septum, which impinges on a persistent, centrifugally expanding telophase spindle, leading to a phragmoplast-like structure of potential phylogenetic significance (Fowke, L. C., and J. D. Pickett-Heaps. 1969. J. Phycol. 5:273-281). Combining fluorescent tagging of the cytoskeleton in situ and video-enhanced differential interference contrast microscopy of live cells, the process of cytokinesis was investigated with emphasis on cytoskeletal reorganization and concomitant redistribution of organelles. Based on a sequence of cytoskeletal arrangements and the effects of cytoskeletal inhibitors thereon, cytokinetic progression could be divided into three functional stages with respect to the contribution of microfilaments (MFs) and microtubules (MTs): (1) Initiation: in early prophase, a cross wall initial was formed independently of MFs and MTs at the presumptive site of wall growth. (2) Septum ingrowth: numerous organelles accumulated at the cross wall initial concomitant with reorganization of the extensive peripheral interphase MF array into a distinct circumferential MF array. This array guided the ingrowing septum until it contacted the expanding interzonal MT array. (3) Cross wall closure: MFs at the growing edge of the septum coaligned with and extended along the interzonal MTs toward the daughter nuclei. Thus, actin-based transportation of small organelles during this third stage occurred, in part, along a scaffold previously deployed in space by MTs. Displacement of the nuclei-associated interzonal MT array by centrifugation and depolymerization of the phragmoplast-like structure showed that the success of cytokinesis at the third stage depends on the interaction of both MF and MT cytoskeletons. Important features of the phragmoplast-like structure in Spirogyra were different from the higher plant phragmoplast: in particular, MFs were responsible for the positioning of organelles at the fusion site, contrary to the proposed role of MTs in the higher plant phragmoplast.

Cytokinesis in tobacco BY-2 and root tip cells: a new model of cell plate formation in higher plants

Cell plate formation in tobacco root tips and synchronized dividing suspension cultured tobacco BY-2 cells was examined using cryofixation and immunocytochemical methods. Due to the much improved preservation of the cells, many new structural intermediates have been resolved, which has led to a new model of cell plate formation in higher plants. Our electron micrographs demonstrate that cell plate formation consists of the following stages: (1) the arrival of Golgi-derived vesicles in the equatorial plane, (2) the formation of thin (20 +/- 6 nm) tubes that grow out of individual vesicles and fuse with others giving rise to a continuous, interwoven, tubulo-vesicular network, (3) the consolidation of the tubulo-vesicular network into an interwoven smooth tubular network rich in callose and then into a fenestrated plate-like structure, (4) the formation of hundreds of finger-like projections at the margins of the cell plate that fuse with the parent cell membrane, and (5) cell plate maturation that includes closing of the plate fenestrae and cellulose synthesis. Although this is a temporal chain of events, a developing cell plate may be simultaneously involved in all of these stages because cell plate formation starts in the cell center and then progresses centrifugally towards the cell periphery. The "leading edge" of the expanding cell plate is associated with the phragmoplast microtubule domain that becomes concentrically displaced during this process. Thus, cell plate formation can be summarized into two phases: first the formation of a membrane network in association with the phragmoplast microtubule domain; second, cell wall assembly within this network after displacement of the microtubules. The phragmoplast microtubules end in a filamentous matrix that encompasses the delicate tubulo-vesicular networks but not the tubular networks and fenestrated plates. Clathrin-coated buds/vesicles and multivesicular bodies are also typical features of the network stages of cell plate formation, suggesting that excess membrane material may be recycled in a selective manner. Immunolabeling data indicate that callose is the predominant lumenal component of forming cell plates and that it forms a coat-like structure on the membrane surface. We postulate that callose both helps to mechanically stabilize the early delicate membrane networks of forming cell plates, and to create a spreading force that widens the tubules and converts them into plate-like structures. Cellulose is first detected in the late smooth tubular network stage and its appearance seems to coincide with the flattening and stiffening of the cell plate.

Cell cycle dependent distribution of a centrosomal antigen at the perinuclear MTOC or at the kinetochores of higher plant cells

Compelling evidence has been obtained in favour of the idea that the nuclear surface of higher plant cells is a microtubule-nucleating and/or organizing site (MTOC), in the absence of defined centrosomes. How these plant MTOC proteins are redistributed and function during the progression of the cell cycle remains entirely unknown. Using a monoclonal antibody (mAb 6C6) raised against isolated calf thymus centrosomes and showing apparent reaction with the plant nuclear surface, we followed the targeted antigen distribution during mitosis and meiosis of higher plants. Immunoblot analysis of protein fractions from Allium root meristematic cell extracts probed with mAb 6C6 reveals a polypeptide of an apparent Mr of 78000. In calf centrosome extracts, a polypeptide of comparable molecular mass is found in addition to a major antigen of Mr 180000 after mAb 6C6 immunoblotting. During mitotic initiation, the plant antigen is prominent on the periphery of the prophase nucleus. When the nuclear envelope breaks down, the antigen suddenly becomes associated with the centromere-kinetochores until late anaphase. In telophase, when the nuclear envelope is being reconstructed, it is no longer detected at the kinetochores but is solely associated again with the nuclear surface. This antigen displays a unique spatial and temporal distribution, which may reflect the pathway of plant protein(s) between the nuclear surface and the kinetochores under cell cycle control. So far, such processes have not been described in higher plant cells. These observations shed light on the putative activity of the plant kinetochore as a protein transporter.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolated Plant Nuclei Nucleate Microtubule Assembly: The Nuclear Surface in Higher Plants Has Centrosome-like Activity

In most eukaryotic cells, microtubules (MTs) are assembled at identified nucleating sites, such as centrosomes or spindle pole bodies. Higher plant cells do not possess such centrosome-like structures. Thus, the fundamental issues of where and how the intracellular plant MTs are nucleated remain highly debatable. A large body of evidence indicates that plant MTs emerge from the nuclear periphery. In this study, we developed an in vitro assay in which isolated maize nuclei nucleate MT assembly at a tubulin concentration (14 [mu]M of neurotubulin) that is not efficient for spontaneous MT assembly. No MT-stabilizing agents, such as taxol or dimethyl sulfoxide, were used. Our model provides evidence that the nuclear surface functions as a MT-nucleating site in higher plant cells. A monoclonal antibody raised against a pericentriolar antigen immunostained the surface of isolated nuclei, and a 100-kD polypeptide in 4 M urea-treated nuclear extracts was detected.

Microtubule converging centers and reorganization of the interphase cytoskeleton and the mitotic spindle in higher plant Haemanthus

We analyzed the distribution and orientation of transitory microtubule structures, microtubule converging centers, during interphase and mitosis in endosperm of the higher plant Haemanthus. In interphase the pointed tips of microtubule converging centers are associated with the nuclear envelope. Their orientation gradually reverses during prophase, and the tips tend to point away from the nucleus. From prometaphase through early telophase, microtubule converging centers are present predominantly in the cytoplasm at the polar region. They are either "free" or associated with chromosomes or microtubule bundles. In late telophase, pointed tips of microtubule converging centers are again associated with the reconstructed nuclear envelope and, additionally, they often appear in the phragmoplast area. The orientation of microtubule converging centers seems to be directly correlated to the previously determined microtubule polarity, with the converging tip being minus and the diverging one, plus. Elevated temperature (35 degrees-37 degrees C) enhances the number of microtubule converging centers in the cytoplasm and at the nuclear envelope. This is especially pronounced during the telophase-interphase transition and in some interphase cells, indicating temperature and stage dependence. Our data imply that microtubule converging centers bind together MT minus ends and, thus, control the predominant direction of elongation and shortening of microtubule arrays. We argue that these configurations are instrumental during the reorganization of interphase cytoskeleton and mitotic spindle in Haemanthus endosperm.

Identification and preliminary characterization of a 65 kDa higher-plant microtubule-associated protein

Microtubules in plant cells, as in animal cells, are dynamic structures. However, our lack of knowledge about the constituents of microtubules in plant cells has prevented us from understanding the mechanisms that control microtubule dynamics. To characterize some of these constituents, a cytoplasmic extract was prepared from evacuolated protoplasts (miniprotoplasts) of tobacco BY-2 cells, and microtubules were assembled in the presence of taxol and disassembled by cold treatment in the presence of Ca2+ and a high concentration of NaCl. SDS-PAGE analysis of triple-cycled microtubule protein revealed the presence of 120 kDa, 110 kDa and a group of 60–65 kDa polypeptides in addition to tubulin. Since these polypeptides had copolymerized with tubulin, through the three cycles of assembly and disassembly, and they bundle microtubules, we tentatively identified the three polypeptides as microtubule-associated proteins (MAPs). To characterize these factors further, triple-cycled microtubule protein was fractionated by Mono-Q anion-exchange chromatography and the microtubule-bundling activity of each fraction was examined. Fractions having microtubule-bundling activity contained only the 65 kDa MAP, an indication that the 65 kDa MAP is responsible for the bundling of microtubules. Purified 65 kDa MAP formed cross-bridge structures between adjacent microtubules in vitro. Polyclonal antibodies were raised in mice against the 65 kDa MAP. Immunofluorescence microscopy revealed that the 65 kDa MAP colocalized with microtubules in BY-2 cells throughout the cell cycle. Western blotting analysis of extracts from several species of plants suggested that the 65 kDa MAP and/or related peptides are widely distributed in the plant kingdom.

A gamma-tubulin-related protein associated with the microtubule arrays of higher plants in a cell cycle-dependent manner

An antibody specific for a conserved gamma-tubulin peptide identifies a plant polypeptide of 58 kDa. gamma-Tubulin antibody affinity purified from this polypeptide recognizes the centrosome in mammalian cells. Using immunofluorescence microscopy, we determined the distribution of this gamma-tubulin-related polypeptide during the complex changes in microtubule arrays that occur throughout the plant cell cycle. We report a punctate association of gamma-tubulin-related polypeptide with the cortical microtubule array and the preprophase band. As cells enter prophase, gamma-tubulin-related polypeptide accumulates around the nucleus and forms a polar cap from which early spindle microtubules radiate. During metaphase and anaphase, gamma-tubulin-related polypeptide preferentially associates with kinetochore fibers and eventually accumulates at the poles. In telophase, localization occurs over the phragmoplast. gamma-Tubulin-related polypeptide appears to be excluded from the plus ends of microtubules at the metaphase plate and cell plate. Its distribution during the cell cycle may be significant in light of differences in the behavior and organization of plant microtubules. The identification of gamma-tubulin-related polypeptide could help characterize microtubule organizing centers in these organisms.

Microtubule and F-actin dynamics at the division site in living Tradescantia stamen hair cells

We have visualised F-actin and microtubules in living Tradescantia virginiana stamen hair cells by confocal laser scanning microscopy after microinjecting rhodamine-phalloidin or carboxyfluorescein-labelled brain tubulin. We monitored these components of the cytoskeleton as the cells prepared for division at preprophase and progressed through mitosis to cytokinesis. Reorganisation of the interphase cortical cytoskeleton results in preprophase bands of both F-actin and microtubules that coexist in the cell cortex, centred on the site at which the future cell plate will fuse with the parent cell wall. The preprophase band of microtubules is formed from microtubules that polymerise and incorporate tubulin during prophase. The preprophase band of actin may form either by reorganisation of pre-existing filaments or by de novo polymerisation. Both cytoskeletal components disappear from the future division site approximately five minutes prior to the breakdown of the nuclear envelope. Cortical microtubules are undetectable throughout mitosis and cytokinesis, whereas cortical F-actin remains abundant, although it is notably excluded from the division site. The phragmoplast, containing both F-actin and microtubules, expands towards the cortical actin exclusion-zone through a region that has no detectable microtubules or F-actin. The phragmoplast comes to rest in the predefined region of the cortex that is devoid of F-actin. It is proposed that cortical F-actin may act as a “negative” template which could position the phragmoplast and cell plate correctly. This is the first in vivo documentation of F- actin dynamics at the division site in living plant cells.

Control of division plane in normal and griseofulvin-treated microsporocytes of Magnolia

Meiotic cytokinesis in microsporocytes of Magnolia is an unusual form of the simultaneous type; phragmoplast expansion is not accompanied by a cell plate, wall deposition is centripetal, and infurrowing of the cytoplasm after first meiosis results in semicells connected by an isthmus. Dyad domains are further defined by interaction of extensive radial systems of microtubules emanating from the daughter nuclei and by a band of organelles polarized in the equatorial region. After second meiosis, phragmoplasts are organized in the interzonal regions between the sister nuclei in each semicell and also at the interfaces of microtubules forming secondary interzonals between non-sister nuclei. Wall deposition is not initiated until after phragmoplasts expand to the cell periphery and fuse in the isthmus. Centripetal wall deposition in boundaries of spore domains marked by radial arrays of microtubules results in simultaneous quadripartitioning of the microsporocyte into a tetrad of microspores. Treatment of microsporocytes with griseofulvin resulted in atypically placed nuclei and supernumerary nuclei. Abnormalities could be traced to displaced spindles and to spindles with multiple poles. Drug-induced multinucleate coenocytes were able to organize microtubules and initiate cytokinesis in altered patterns. The data suggest that spindle alignment and aggregation of spindle poles are two components of spatial control that are operative in determining the normal arrangement of nuclei, and that the final placement of walls is a function of the postmeiotic nuclear-based radial arrays of microtubules which define spore domains.

Patterns of microtubule organization in two polyhedral cell types in the gametophyte of the liverwort Marchantia paleacea Bert

The typical scale cells (TSCs) of Marchantia paleacea Bert, contain a well-developed cortical microtubule (Mt) cytoskeleton, particularly below the anticlinal walls and also display complete but broad preprophase-prophase Mt bands (PMBs). In contrast, the cortical cytoskeleton of the inner thallus cells (ITCs) is less developed than that of TSCs and the PMBs are incomplete. The latter consist of one to four separate Mt bundles which lie on the cytokinetic plane, but do not form a complete Mt ring. In both cell types PMB formation precedes or keeps pace with the activation of the polar Mt-organizing centres (MTOCs) and nuclear shaping. The Mts in the PMBs are more numerous and densely packed at the cell edges than on the cell face. The polar MTOCs persist up to late prophase-prometaphase. Afterwards, the spindle Mts are focused on several minipoles, where endoplasmic reticulum is localized. In postcytokinetic cells the cortical Mts first reappear on the daughter wall surface. Our findings suggest that: (a) The formation of complete or incomplete PMBs in TSCs and ITCs of M. paleacea is related to differences in the development of the interphase cortical Mt arrays, (b) The cell edges are able to form or at least arrange the Mts of the PMB. (c) Tight mature PMBs like those found in flowering plant cells are not formed in the cells examined in the present study. (d) The final orientation of the cell plate is controlled by the PMB cortical zone. (e) The cytoplasm abutting on the postcytokinetic daughter wall has the ability to assemble cortical Mts.