Dynamic Organization of Plant Microtubules at the Three Distinct Transition Points During the Cell Cycle Progression of Synchronized Tobacco BY-2 Cells

The distribution of TPX2 in dividing leaf cells of the fern Asplenium nidus

Plant cell division requires the dynamic organisation of several microtubule arrays. The mechanisms of regulation of the above arrays are under rigorous research. Among several factors that are involved in plant microtubule dynamics, the Targeting Protein for Xklp2 (TPX2) has been found to play a role in spindle organisation, in combination with Aurora kinases, in dividing cells of angiosperms. Microtubule organisation in dividing cells of ferns exhibits certain peculiarities. Accordingly, the presence and distribution of a TPX2 homologue might be helpful in understanding the patterns and regulatory mechanisms of microtubule arrays in this plant group. In this study, a putative TPX2 homologue was identified using Western blotting in the fern Asplenium nidus. It was found, using immunostaining and CLSM, that it is co-localised with perinuclear preprophase microtubules and the prophase spindle, and follows the microtubule pattern during metaphase/anaphase and telophase. During cytokinesis, while in angiosperms TPX2 is degraded, in A. nidus the TPX2 signal persists, co-localising with the phragmoplast. In early post-cytokinetic cells, a TPX2 signal is present on the nuclear surface facing the daughter cell wall and, thereafter it is co-localised with the fern-specific microtubule aggregation that lines the new wall, which is possibly involved in cortical microtubule assembly.

Radial microtubule organization by histone H1 on nuclei of cultured tobacco BY-2 cells

In acentriolar higher plant cells, the surface of the nucleus acts as a microtubule-organizing center, substituting for the centrosome. However, the protein factors responsible for this microtubule organization are unknown. The nuclear surfaces of cultured tobacco BY-2 cells possess particles that generate microtubules. We attempted to isolate the proteins in these particles to determine their role in microtubule organization. When incubated with plant or mammalian tubulin, some, but not all, of the isolated nuclei generated abundant microtubules radially from their surfaces. The substance to induce the formation of radial microtubules was confirmed by SDS-PAGE to be a protein with apparent molecular mass of 38 kDa. Partial analysis of the amino acid sequences of the peptide fragments suggested it was a histone H1-related protein. Cloning and cDNA sequence analysis confirmed this and revealed that when the recombinant protein was incubated with tubulin, it could organize microtubules as well as the 38-kDa protein. Histone H1 and tubulin formed complexes immediately, even on ice, and then clusters of these structures were formed. These clusters generated radial microtubules. This microtubule-organizing property was confined to histone H1; all other core histones failed to act as organizers. On immunoblot analysis, rabbit antibodies raised against the 38-kDa protein cross-reacted with histone H1 proteins from tobacco BY-2 cells. These antibodies virtually abolished the ability of the nucleus to organize radial microtubules. Indirect immunofluorescence showed that the antigen was distributed at the nuclear plasm and particularly at nuclear periphery independently from DNA.

Microtubule-associated proteins in higher plants

A variety of microtubule-associated proteins (MAPs) have been reported in higher plants. Microtubule (MT) polymerization starts from the gamma-tubulin complex (gammaTuC), a component of the MT nucleation site. MAP200/MOR1 and katanin regulate the length of the MT by promoting the dynamic instability of MTs and cutting MTs, respectively. In construction of different MT structures, MTs are bundled or are associated with other components--actin filaments, the plasma membrane, and organelles. The MAP65 family and some of kinesin family are important in bundling MTs. MT plus-end-tracking proteins (+TIPs) including end-binding protein 1 (EB1), Arabidopsis thaliana kinesin 5 (ATK5), and SPIRAL 1 (SPR1) localize to the plus end of MTs. It has been suggested that +TIPs are involved in binding of MT to other structures. Phospholipase D (PLD) is a possible candidate responsible for binding of MTs to the plasma membrane. Many candidates have been reported as actin-binding MAPs, for example calponin-homology domain (KCH) family kinesin, kinesin-like calmodulin-binding protein (KCBP), and MAP190. RNA distribution and translation depends on MT structures, and several RNA-related MAPs have been reported. This article gives an overview of predicted roles of these MAPs in higher plants.

Microtubule-dependent microtubule nucleation in plant cells

Regulation of microtubule nucleation sites is an essential step in microtubule organization. Cortical microtubule arrays in green plant cells at inter-phase are organized in a distinct manner--the array is formed in the absence of previously recognized organelles for microtubule nucleation, for example the centrosome and spindle pole body. Microtubules in the cortical array were recently found to be nucleated as branches on pre-existing microtubules via recruitment of cytosolic gamma-tubulin. In this review we briefly summarize the mechanism of microtubule-dependent microtubule nucleation and discuss a possible role of this mechanism in other cellular processes and their evolution.

Higher plant RecA-like protein is homologous to RadA

A novel RecA-like protein, differing from Dmc1 and Rad51, was characterized in Oryza sativa L. cv. Nipponbare. Because the protein is homologous to bacterial RadA, the gene was designated OsRadA. The open reading frame was predicted to encode a 66kDa protein of 619 amino acid residues and was found in plants but not animals or yeast. OsRadA showed D-loop and single-stranded DNA-dependent ATPase activities. Gene expression was found to be high in meristematic tissues, and was localized in the nucleus. An RNAi mutant of Arabidopsis thaliana RadA (AtRadA) was sensitive to mutagenic agents such as UV and MMC, suggesting that RadA functions in DNA repair.

Characterization of plant Aurora kinases during mitosis

The Aurora kinase family is a well-characterized serine/threonine protein kinase family that regulates different processes of mitotic events. Although functions of animal and yeast Aurora kinases have been analyzed, plant aurora kinases were not identified and characterized. We identified three Aurora kinase orthologs in Arabidopsis thaliana and designated these as AtAUR1, AtAUR2, and AtAUR3. These AtAURs could phosphorylate serine 10 in histone H3, in vitro. Dynamic analyses of GFP-fused AtAUR proteins revealed that AtAUR1 and AtAUR2 localized at the nuclear membrane in interphase and located in mitotic spindles during cell division. AtAUR1 also localized in the cell plates. AtAUR3 showed dot-like distribution on condensed chromosomes at prophase and then localized at the metaphase plate. At late anaphase, AtAUR3 is evenly localized on chromosomes. The localization of AtAUR3 during mitosis is very similar to that of phosphorylated histone H3. Interestingly, an overexpression of AtAUR3 induces disassembly of spindle microtubules and alteration of orientation of cell division. Our results indicate that plant Aurora kinases have different characters from that of Aurora kinases of other eukaryotes.

Decision of spindle poles and division plane by double preprophase bands in a BY-2 cell line expressing GFP-tubulin

The preprophase band (PPB) of microtubules is thought to be involved in deciding the future division site. In this study, we investigated the effects of double PPBs on spindle formation and the directional decision of cytokinesis by using transgenic BY-2 cells expressing green fluorescent protein (GFP)-tubulin. At prophase, most of the cells with double PPBs formed multipolar spindles, whereas all cells with single PPBs formed normal bipolar spindles, clearly implicating the PPB in deciding the spindle poles. At metaphase, however, both cell types possessed the bipolar spindles, indicating the existence of correctional mechanism(s) at prometaphase. From prometaphase to metaphase, the spindles in double PPB cells altered their directions to become oblique to the cell-elongating axis, and these orientations were maintained in the phragmoplast and resulted in the oblique division planes. These oblique cell plates decreased when actin microfilaments were disrupted, and double actin-depleted zones (ADZs) appeared where the double PPBs had existed. These results suggest that the information necessary for proper cytokinesis may be transferred from the PPBs to the ADZs, even in the case of the double PPBs.

Inhibition of proteasome by MG-132 treatment causes extra phragmoplast formation and cortical microtubule disorganization during M/G1 transition in synchronized tobacco cells

The 26S proteasome plays essential roles in cell cycle progression in various types of cell. We previously reported that the inhibition of 26S proteasome activities by a proteasome inhibitor, MG-132, exclusively caused cell cycle arrest in synchronized tobacco BY-2 cells. Here we report a further observation of 26S proteasome involvement during M/G1 transition utilizing a transgenetic BY-2 cell line that stably expresses a GFP-alpha-tubulin fusion protein (BY-GT16). Interestingly, MG-132 treatment caused the arrest of cell cycle progression prior to entering the G1 phase. Indeed, phragmoplast-like structures were formed and cortical microtubules were not organized after the collapse of the original phragmoplasts. Additionally, actin microfilaments showed irregular rearrangements when further incubated with MG-132 and as the phragmoplast-like structures developed. Since these phragmoplast-like structures had a similar configuration and ability to form cell plates to that of the original phragmoplasts, we designated these phragmoplast-like structures as extra phragmoplasts. Furthermore, we showed that a tobacco kinesin-related polypeptide of 125 kDa (TKRP125) localized in the extra phragmoplasts and that its protein level remained unchanged during MG-132 treatment. We propose that TKRP125 might be one of the possible targets of the ubiquitin-proteasome degradation pathway during M/G1 transition.

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.

Rice UV-damaged DNA binding protein homologues are most abundant in proliferating tissues

Ultraviolet-damaged DNA binding protein (UV-DDB) is an important factor involved in DNA repair. To study the role of UV-DDB, we attempted to obtain the cDNA and the protein of a plant UV-DDB. We succeeded in isolating both genes for UV-DDB subunits from rice (Oryza sativa cv. Nipponbare), designated as OsUV-DDB1 and OsUV-DDB2. OsUV-DDB2 (65 kDa) was much larger than human UV-DDB2, but immunoprecipitation and gel mobility shift assay suggested that OsUV-DDB2 is a plant counterpart of UV-DDB2. The transcripts were expressed in proliferating tissues such as the meristem, but were detected at only low levels in the mature leaves, although the leaves are strongly exposed to UV. These transcripts were induced in the meristem after UV-irradiation. The expression levels of OsUV-DDB were significantly reduced when cell proliferation was temporarily halted. These results indicated that the level of OsUV-DDB expression is correlated with cell proliferation, and its expression may be required mostly for DNA repair in DNA replication.

Gamma-tubulin distribution during cortical microtubule reorganization at the M/G1 interface in tobacco BY-2 cells

Cortical microtubules are considered to regulate the direction of cellulose microfibril deposition. Despite their significant role in determining cell morphology, cortical microtubules completely disappear from the cell cortex during M phase and become reorganized at G1 phase. The mechanism by which these microtubules become properly formed again is, however, still unclear. We have proposed that the origin of cortical microtubules is on the daughter nuclear surface, but further cortical microtubule reorganization occurs at the cell cortex. Hence it is probable that the locations of microtubule organizing centers (MTOCs) are actively changing. However, the actual MTOC sites of cortical microtubules were not clearly determined. In this paper, we have examined the distribution of gamma-tubulin, one of the key molecules of MTOCs in various organisms, during cortical microtubule reorganization using both immunofluorescence and a GFP reporter system. Using a monoclonal antibody (clone G9) that recognizes highly conserved residues in y-tubulin, y-tubulin was found to be constitutively expressed and to be clearly localized to microtubule structures, such as the preprophase bands, spindles, and phragmoplasts, specific to each cell cycle stage. This distribution pattern was confirmed by the GFP reporter system. During cortical microtubule reorganization at the M to G1 transition phase, gamma-tubulin first accumulated at the daughter nuclear surfaces, and then seemed to spread onto the cell cortex along with microtubules elongating from the daughter nuclei. Based on the results, it was confirmed that daughter nuclear surfaces acted as origins of cortical microtubules, and that further reorganization occurred on the cell cortex.

Dynamic changes and the role of the cytoskeleton during the cell cycle in higher plant cells

In higher plant cells microtubules (MTs) show dynamic structural changes during cell cycle progression and play significant roles in cell morphogenesis. The cortical MT (CMT), preprophase band (PPB), and phragmoplast, all of which are plant-specific MT structures, can be observed during interphase, from the late G2 phase to prophase, and from anaphase to telophase, respectively. The CMT controls cell shape, either irreversibly or reversibly, by orientating cellulose microfibril (CMF) deposition in the cell wall; the PPB is involved in determining the site of division; and the phragmoplast forms the cell plate at cytokinesis. The appearance and disappearance of these MT structures during the cell cycle have been extensively studied by immunofluorescence microscopy using highly synchronized tobacco BY-2 cells. Indeed, these studies, together with visualization of MT dynamics in living plant cells using the green fluorescent protein, have revealed much about the modes of MT structural organization, for example, of CMTs at the M/G1 interphase. The microfilaments which also show dynamic changes during the cell cycle, being similar to MTs at particular stages and different at other stages, appear to play roles in supporting MTs. In this article, we summarize our ongoing research and that of related studies of the structure and function of the plant cytoskeleton during cell cycle progression.

Dynamic organization of vacuolar and microtubule structures during cell cycle progression in synchronized tobacco BY-2 cells

In higher plant cells, vacuoles show considerable diversity in their shapes and functions. The roles of vacuoles in the storage, osmoregulation, digestion and secretory pathway are well established; however, their functions in cell morphogenesis and cell division are still unclear. To observe the dynamic changes of vacuoles in living plant cells, we attempted to visualize the vacuolar membrane (VM) by pulse-labeling tobacco BY-2 cells with a styryl fluorescent dye, FM4-64. By time-sequence observations using confocal laser scanning microscopy (CLSM), we could follow the dynamics of vacuolar structures throughout the cell cycle in living higher plant cells. We also confirmed the dynamic changes of VM structures by the observation using transgenic BY-2 cells expressing GFP-AtVam3p fusion protein (BY-GV). Furthermore, by using transgenic BY-2 cells that stably express a GFP-tubulin fusion protein [BY-GT16, Kumagai et al. (2001) Plant Cell Physiol. 42: 723], we could study the relationship between the dynamics of vacuoles and microtubules. From these observations, we identified, for the first time, some remarkable events: (1) at the late G(2) phase, tubular structures of the vacuolar membrane developed in the central region of the cell, probably in the premitotic cytoplasmic band (phragmosome), surrounding the mitotic apparatus; (2) from anaphase to telophase, these tubular structures invaded the region of the phragmoplast within which the cell plate was being formed; (3) at the early G(1) phase, some of the tubular structures expanded rapidly between the cell plate and daughter nuclei, and subsequently developed into large vacuoles at interphase.

Cell-cycle dependent dynamic change of 26S proteasome distribution in tobacco BY-2 cells

The 26S proteasome is known to play pivotal roles in cell-cycle progression in various eukaryotic cells; however, little is known about its role in higher plants. Here we report that the subcellular distribution of the 26S proteasome is dynamically changed in a cell-cycle dependent manner in tobacco BY-2 cells as determined by immunostaining with anti-Rpn10 (a regulatory PA700 subunit) and anti-20S catalytic proteasome antibodies. The 26S proteasome was found to localize not only in nuclear envelopes and mitotic spindles but also in preprophase bands (PPBs) and phragmoplasts appearing in G(2) and M phases, respectively. MG132, a proteasome inhibitor, exclusively caused cell-cycle arrest not only at the metaphase but also the early stage of PPB formation at the G(2) phase and the collapse of the phragmoplast, which seems to be closely related to proteasome distribution in the cells.

Synchronous Arabidopsis suspension cultures for analysis of cell-cycle gene activity

Synchronized suspension cultures are powerful tools in plant cell-cycle studies. However, few Arabidopsis cell cultures are available, and synchrony extending over several sequential phases of the cell cycle has not been reported. Here we describe the first useful synchrony in Arabidopsis, achieved by selecting the rapidly dividing Arabidopsis cell suspensions MM1 and MM2d. Synchrony may be achieved either by removing and re-supplying sucrose to the growth media or by applying an aphidicolin block/release. Synchronization with aphidicolin produced up to 80% S-phase cells and up to 92% G2 cells, together with clear separation of different cell-cycle phases. These synchronization procedures can be used for analysis of gene expression and protein activity. We show that representatives of three CDK gene classes of Arabidopsis (CDKA, CDKB1 and CDKB2) show differential expression timing, and that three CDK inhibitor genes show strikingly different expression patterns during cell-cycle re-entry. We propose that ICK2 (KRP2) may have a specific role in this process.

Leucine aminopeptidase during meiotic development

We found a leucine aminopeptidase (LAP; EC 3.4.11.1) to be abundant in meiotic prophase tissue of a basidiomycete, Coprinus cinereus. After direct purification of the aminopeptidase component from meiocytes, we cloned the gene by degenerate PCR using partial amino-acid sequences of the purified enzyme and 5' and 3' RACE. It was homologous to the eukaryotic leucine aminopeptidase gene. The recombinant protein possesses the characteristic activities of a Coprinus leucine aminopeptidase (CoLAP) with a molecular mass of 52.4 kDa, and forms a homohexamer. Northern blot and spatial distribution analysis by immunohistochemical staining indicated CoLAP to be abundant in meiotic prophase cells and the supporting cells around meiocytes, but scarce in mycelium cells. Interestingly, from zygotene to pachytene, CoLAP was mostly present in supporting cells around meiocytes, but from diplotene onwards, it was plentiful in meiocytes themselves, suggesting that its expression is required to control some of the biochemical events at meiotic prophase. Moreover, the strong expression of CoLAP mRNA immediately after treatment with methyl methanesulfonate in mycelium implies that CoLAP has a role in somatic DNA repair.

Proliferating cell nuclear antigen from a basidiomycete, Coprinus cinereus. Alternative truncation and expression in meiosis

The primary purpose of the present study was to investigate whether DNA replication at meiotic prophase also requires replication factors, especially proliferating cell nuclear antigen (PCNA). We cloned PCNA cDNAs (CoPCNA) from a cDNA library made from basidia of the basidiomycete, Coprinus cinereus. Interestingly, although CoPCNA is a single-copy gene in the genome, two different PCNA cDNA species were isolated using degenerate primers and a meiotic cDNA library, and were designated as CoPCNA-alpha and CoPCNA-beta. CoPCNA-beta was made by truncating at specific sites in CoPCNA-alpha mRNA, 5'-AAGAAGGAGAAG-3' and 5'-GAAGAGGAAGAA-3'. Both of these sequences were present in exon IV in the genomic sequence, and interestingly the former was the same as the inverse sequence of the latter. CoPCNA-alpha was 107 amino acids larger than human PCNA, and so the 107 amino-acid sequence was inserted in a loop, the so-called D2E2 loop, in human PCNA. Northern blotting analysis indicated that CoPCNA was expressed not only at premeiotic S but also at the meiotic prophase stages such as leptotene and early zygotene, just before and when karyogamy occurs and the homologous chromosomes pair. Western blotting analysis using anti-(CoPCNA-alpha) Ig revealed that at least two CoPCNA mRNAs before and after truncation were translated at the meiotic prophase as CoPCNA-alpha and CoPCNA-beta.

Characterization of plant proliferating cell nuclear antigen (PCNA) and flap endonuclease-1 (FEN-1), and their distribution in mitotic and meiotic cell cycles

The biochemical and cell cycle-dependent properties of proliferating cell nuclear antigen (OsPCNA) and flap endonuclease-1 (OsFEN-1) were characterized from rice (Oryza sativa). OsPCNA was physically associated with OsFEN-1 and increased the flap-endonuclease activity of OsFEN-1 by 2.5-fold. Northern and Western blotting analysis revealed that OsPCNA and OsFEN-1 were present in meristematic tissues such as cultured cells, shoot apical meristem and root apical meristem. No expression was detected in the mature leaves, although they were exposed to UV. Both of these proteins were localized in the nuclei of the interphase cells including G1, S and G2, and in the nuclear region at telophase. The distribution patterns of plant PCNA and FEN-1 in meiotic cell progression were investigated using microsporocytes of lily (Lilium longiflorum cv. Hinomoto). During the leptotene to pachytene stages, PCNA and FEN-1 were localized in the nuclear region. The florescence gradually disappeared from diplotene to metaphase I. Interestingly, signals for PCNA formed 10-20 intense spots at leptotene. The number of spots decreased to 1-5 at zygotene and finally to 1 at pachytene. The roles of OsPCNA and OsFEN-1 in mitotic and meiotic cell cycles are discussed.

Mammalian Bax-induced plant cell death can be down-regulated by overexpression of Arabidopsis Bax Inhibitor-1 (AtBI-1)

We recently isolated the AtBI-1 (Arabidopsis Bax Inhibitor-1) gene, the expression of which suppressed Bax-induced cell death in yeast. To determine whether the same is true in the plant system, transgenic Arabidopsis plants overexpressing Bax protein under a dexamethasone (DEX)-inducible promoter were generated. On DEX treatment, such transgenic plants exhibited marked cell death at the whole-plant level, cell shrinkage, membranous destruction, and other apoptotic phenotypes. Transgenic Bax plants were retransformed with a vector containing the AtBI-1 gene (tagged with green fluorescent protein) under the control of the cauliflower mosaic virus 35S promoter. Plants expressing both Bax and AtBI-1 were able to maintain growth on DEX-treatment by sustaining intracellular integrity. Thus, we present here direct genetic evidence that the plant antiapoptotic protein AtBI-1 is biologically active in suppressing the mammalian Bax action in planta.

Two types of replication protein A 70 kDa subunit in rice, Oryza sativa: molecular cloning, characterization, and cellular & tissue distribution

Replication protein A (RPA), which is comprised of three subunits, is an important factor involved in DNA replication, repair, and transcription. We isolated and characterized 70 and 32 kDa subunits of RPA from rice (Oryza sativa cv. Nipponbare) termed OsRPA70a and OsRPA32. OsRPA70a shows a low level of homology with OsRPA1 which was isolated from deepwater rice (Oryza sativa cv. Pin Gaew 56), previously. We also succeeded to isolate OsRPA70b which is homologue to OsRPA1 from Oryza sativa cv. Nipponbare. OsRPA70a shows only 33.8% sequence identity with OsRPA70b, indicating that two different types of 70 kDa RPA subunits are present in Oryza sativa cv. Nipponbare. These subunits showed differences in their expression patterns among tissues. The transcripts of OsRPA70a and OsRPA32 were expressed strongly in proliferating tissues such as root tips and young leaves that contain root apical meristem and marginal meristem, respectively, and weakly in the mature leaves which have no proliferating tissues. On the other hand, OsRPA70b was expressed mostly in the proliferating tissues. The roles of these molecules in plant DNA replication and DNA repair are discussed.

Fate of nascent microtubules organized at the M/G1 interface, as visualized by synchronized tobacco BY-2 cells stably expressing GFP-tubulin: time-sequence observations of the reorganization of cortical microtubules in living plant cells

Transgenic BY-2 cells stably expressing a GFP (green fluorescent protein)-tubulin fusion protein (BY-GT16) were subcultured in a modified Linsmaier and Skoog medium. The BY-GT16 cells could be synchronized by aphidicolin and the dynamics of their microtubules (MTs) were monitored by the confocal laser scanning microscopy (CLSM). We have succeeded in investigating the mode of reorganization of cortical MTs at the M/G1 interface. The cortical MTs were initially organized in the perinuclear regions and then they elongated to reach the cell cortex, forming the bright spots there. Subsequently, the first cortical MTs rapidly elongated from the spots and they were oriented parallel to the long axis towards the distal end of the cells. Around the time when the tips of the parallel MTs reached the distal end, the formation of transverse cortical MTs followed in the cortex near the division site, as we had previously suggested [Hasezawa and Nagata (1991) Bot. Acta 104: 206, Nagata et al. (1994) Planta 193: 567]. It was confirmed in independent observations that the appearance of the parallel MTs was followed by the appearance of the transverse MTs in each cell. We found that the transverse MTs spread through the whole cell cortex within about 20-30 min, while the parallel MTs disappeared. The significance of these observations on the mode of cortical MT organization is discussed.

Isotype-specific changes in the amount of beta-tubulin RNA in synchronized tobacco BY2 cells

The 3'-ends of the beta-tubulin cDNA were amplified from tobacco BY2 polyA+ RNA. According to the differences in the predicted amino acid sequence at the extreme C-terminal, they were grouped into three different isotypes, NTB1 in which "EEGDYYEEDEEDLNEA", NTB2 in which "EEEYYEDEEEA QED" and NTB3 in which "DECEYEEEEEYDHEGN" follows the conservative "YQQYQDATAD" sequence. Using unique 3'-untranslated regions as probes, changes in the RNA levels of each beta-tubulin isotype were determined by dot-blot hybridization. The levels exhibited characteristic rhythms in the cell cycle. NTB1 RNA was highest in S phase in comparison to NTB2 RNA level which was highest in late G2. On the other hand, NTB3 RNA level was highest in early G2.

Role of cortical microtubules in the orientation of cellulose microfibril deposition in higher-plant cells

Cortical microtubules (MTs) have been implicated in the morphogenesis of plant cells by regulating the orientation of newly deposited cellulose microfibrils (CMFs). However, the role of MTs in oriented CMF deposition is still unclear. We have investigated the mechanism of CMF deposition with cultured tobacco protoplasts derived from taxol-treated BY-2 cells (taxol protoplasts). The BY-2 protoplasts regenerated patches of beta-l,3-glucan (callose) and fibrils of beta-l,4-glucan (cellulose). Taxol protoplasts possessed the same ordered MT arrays as material cells and regenerated CMFs with patterns almost coincidental with MTs. Electron microscopy revealed that, on the surface of cultured taxol protoplasts, each CMF bundle appeared to be deposited on each cortical MT. These results suggest that MTs may attach directly to the cellulose-synthesizing complexes, by some form of linkage, and regulate the movement of these complexes in higher-plant 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.

Isolation of a novel RNA-binding protein and its association with a large ribonucleoprotein particle present in the nucleoplasm of tobacco cells

A cDNA encoding a protein with a consensus sequence-type RNA-binding domain (CS-RBD) has been isolated from a Nicotiana sylvestris cDNA library. The deduced protein (designated 'RZ-1') contains CS-RBD in its N-terminal half, arginine/aspartic acid repeats in its center and a glycine-rich-C-terminal region in which a zinc finger motif of the CCHC type is present. The corresponding gene appears to be expressed constitutively in all tobacco organs. Immunocytochemical assays revealed that RZ-1 is localized in the nucleoplasm of tobacco cultured cells. Glycerol gradient fractionation of tobacco nuclear lysates showed that RZ-1 is associated with a large ribonucleoprotein particle of around 60 S in size. Nucleic acid-binding assays indicated that RZ-1 binds preferentially to poly (G) and both the CS-RBD and glycine-rich region are necessary for its binding activity. A possible role of RZ-1 is discussed.

Cell cycle-dependent accumulation of a kinesin-like protein, KatB/C in synchronized tobacco BY-2 cells

Immunoblot analysis with antibodies prepared against highly purified recombinant truncated kinesin-like proteins, KatB(5-249) and KatC(207-754), encoded by the katB and katC genes of Arabidopsis thaliana revealed the presence of a kinesin-like polypeptide, termed KatB/C, in cultured tobacco BY-2 cells. The KatB/C polypeptide cosedimented with microtubules in the presence of a nonhydrolyzable ATP analogue and was released from microtubules in the presence of ATP, both of which are characteristics of kinesin proteins. The amount of KatB/C polypeptide in synchronous BY-2 cells increased during M phase of the cell cycle. Microtubule-based structures present in cells at M phase, such as the spindle and phragmoplast, may be the site of action of the KatB/C protein.

Sequencing and characterization of the kinesin-related genes katB and katC of Arabidopsis thaliana

Complementary DNAs of two kinesin-related genes, katB and katC, were isolated from Arabidopsis thaliana and sequenced. The carboxyl-terminal regions of the polypeptides encoded by these genes, especially the presumptive ATP-binding and microtubule-binding domains, share significant sequence homology with the mechanochemical motor domain of the kinesin heavy chain. The predicted secondary structures of KatB and KatC proteins include a large globular domain in the carboxyl-terminal region and a small globular domain in the amino-terminal region that are separated by a long alpha-helical coiled-coil with heptad repeats. A truncated KatC polypeptide (KatC(207-754)), which includes the carboxyl-terminal region of KatC, was expressed in Escherichia coli and was shown to possess microtubule-stimulated ATPase activity and to bind to microtubules in an ATP-sensitive manner, both of which are characteristics of kinesin and kinesin-like proteins.

Microtubule-organizing centers in higher plants

Compelling evidence has recently been obtained for the idea that the nuclear surface of higher plant cells has a microtubule-nucleating function under cell cycle dependent control. Whatever the final organization of microtubules, they all seem to originate in the nuclear periphery. The spatial distribution of microtubules is a secondary process involving actin and microtubule-associated proteins, and other morphogenetic controls. How microtubules are nucleated and/or reorganized at the spindle poles during mitosis remains unknown.

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.