Dynamic recruitment of Cdc2 to specific microtubule structures during mitosis

Spindle Assembly and Mitosis in Plants

In contrast to well-studied fungal and animal cells, plant cells assemble bipolar spindles that exhibit a great deal of plasticity in the absence of structurally defined microtubule-organizing centers like the centrosome. While plants employ some evolutionarily conserved proteins to regulate spindle morphogenesis and remodeling, many essential spindle assembly factors found in vertebrates are either missing or not required for producing the plant bipolar microtubule array. Plants also produce proteins distantly related to their fungal and animal counterparts to regulate critical events such as the spindle assembly checkpoint. Plant spindle assembly initiates with microtubule nucleation on the nuclear envelope followed by bipolarization into the prophase spindle. After nuclear envelope breakdown, kinetochore fibers are assembled and unified into the spindle apparatus with convergent poles. Of note, compared to fungal and animal systems, relatively little is known about how plant cells remodel the spindle microtubule array during anaphase. Uncovering mitotic functions of novel proteins for spindle assembly in plants will illuminate both common and divergent mechanisms employed by different eukaryotic organisms to segregate genetic materials.

Genome-wide identification and expression profiling of thes MAPK, MAPKK, and MAPKKK gene families in Ophiocordyceps sinensis

Mitogen-activated protein kinase (MAPK) cascades have a universal cell signaling mechanism in eukaryotes. A typical MAPK signal transduction module comprises three kinds of sequentially phosphorylated protein kinases: MAPK, Mitogen-activated protein kinase kinase (MAPKK), and Mitogen-activated protein kinase kinase kinase (MAPKKK). However, little is known regarding the genes involved in MAPK cascades in Ophiocordyceps sinensis. Nine genes (three MAPK, three MAPKK, and three MAPKKK) were identified in this study. The MAPK, MAPKK, and MAPKKK genes were divided into three subfamilies, according to the phylogenetic analysis. TEY and TGY represented the activation domains of the MAPKs; the corresponding domains in MAPKKs were SDIWS and SDVWS, and those in the MAPKKs were GSVFYWMAPEV and GTPMYMSPEV. Transcription data analysis and quantitative real-time polymerase chain reaction showed that the MAPK cascade was related to the growth of the fruiting body. This is the first study to report a genome-wide identification of the MAPK, MAPKK, and MAPKK gene families in O. sinensis.

Connections between the Cell Cycle and the DNA Damage Response in Plants

Due to their sessile lifestyle, plants are especially exposed to various stresses, including genotoxic stress, which results in altered genome integrity. Upon the detection of DNA damage, distinct cellular responses lead to cell cycle arrest and the induction of DNA repair mechanisms. Interestingly, it has been shown that some cell cycle regulators are not only required for meristem activity and plant development but are also key to cope with the occurrence of DNA lesions. In this review, we first summarize some important regulatory steps of the plant cell cycle and present a brief overview of the DNA damage response (DDR) mechanisms. Then, the role played by some cell cycle regulators at the interface between the cell cycle and DNA damage responses is discussed more specifically.

CDKD-dependent activation of CDKA;1 controls microtubule dynamics and cytokinesis during meiosis

Precise control of cytoskeleton dynamics and its tight coordination with chromosomal events are key to cell division. This is exemplified by formation of the spindle and execution of cytokinesis after nuclear division. Here, we reveal that the central cell cycle regulator CYCLIN DEPENDENT KINASE A;1 (CDKA;1), the Arabidopsis homologue of Cdk1 and Cdk2, partially in conjunction with CYCLIN B3;1 (CYCB3;1), is a key regulator of the microtubule cytoskeleton in meiosis. For full CDKA;1 activity, the function of three redundantly acting CDK-activating kinases (CAKs), CDKD;1, CDKD;2, and CDKD;3, is necessary. Progressive loss of these genes in combination with a weak loss-of-function mutant in CDKA;1 allowed a fine-grained dissection of the requirement of cell-cycle kinase activity for meiosis. Notably, a moderate reduction of CDKA;1 activity converts the simultaneous cytokinesis in Arabidopsis, i.e., one cytokinesis separating all four meiotic products concurrently into two successive cytokineses with cell wall formation after the first and second meiotic division, as found in many monocotyledonous species.

γ-Tubulin has a conserved intrinsic property of self-polymerization into double stranded filaments and fibrillar networks

γ-Tubulin is essential for microtubule nucleation and also plays less understood roles in nuclear and cell-cycle-related functions. High abundancy of γ-tubulin in acentrosomal Arabidopsis cells facilitated purification and biochemical characterization of large molecular species of γ-tubulin. TEM, fluorescence, and atomic force microscopy of purified high molecular γ-tubulin forms revealed the presence of linear filaments with a double protofilament substructure, filament bundles and aggregates. Filament formation from highly purified γ-tubulin free of γ-tubulin complex proteins (GCPs) was demonstrated for both plant and human γ-tubulin. Moreover, γ-tubulin associated with porcine brain microtubules formed oligomers. Experimental evidence on the intrinsic ability of γ-tubulin to oligomerize/polymerize was supported by conservation of α- and β-tubulin interfaces for longitudinal and lateral interactions for γ-tubulins. STED (stimulated emission depletion) microscopy of Arabidopsis cells revealed fine, short γ-tubulin fibrillar structures enriched on mitotic microtubular arrays that accumulated at polar regions of acentrosomal spindles and the outer nuclear envelope before mitosis, and were also present in nuclei. Fine fibrillar structures of γ-tubulin representing assemblies of higher order were localized in cell-cycle-dependent manner at sites of dispersed γ-tubulin location in acentrosomal plant cells as well as at sites of local γ-tubulin enrichment after drug treatment. Our findings that γ-tubulin preserves the capability of prokaryotic tubulins to self-organize into filaments assembling by lateral interaction into bundles/clusters help understanding of the relationship between structure and multiple cellular functions of this protein species and suggest that besides microtubule nucleation and organization, γ-tubulin may also have scaffolding or sequestration functions.

Interregulation of CDKA/CDK1 and the Plant-Specific Cyclin-Dependent Kinase CDKB in Control of the Chlamydomonas Cell Cycle

The cyclin-dependent kinase CDK1 is essential for mitosis in fungi and animals. Plant genomes contain the CDK1 ortholog CDKA and a plant kingdom-specific relative, CDKB. The green alga Chlamydomonas reinhardtii has a long G1 growth period followed by rapid cycles of DNA replication and cell division. We show that null alleles of CDKA extend the growth period prior to the first division cycle and modestly extend the subsequent division cycles, but do not prevent cell division, indicating at most a minor role for the CDK1 ortholog in mitosis in Chlamydomonas. A null allele of cyclin A has a similar though less extreme phenotype. In contrast, both CDKB and cyclin B are essential for mitosis. CDK kinase activity measurements imply that the predominant in vivo complexes are probably cyclin A-CDKA and cyclin B-CDKB. We propose a negative feedback loop: CDKA activates cyclin B-CDKB. Cyclin B-CDKB in turn promotes mitotic entry and inactivates cyclin A-CDKA. Cyclin A-CDKA and cyclin B-CDKB may redundantly promote DNA replication. We show that the anaphase-promoting complex is required for inactivation of both CDKA and CDKB and is essential for anaphase. These results are consistent with findings in Arabidopsis thaliana and may delineate the core of plant kingdom cell cycle control that, compared with the well-studied yeast and animal systems, exhibits deep conservation in some respects and striking divergence in others.

Phragmoplast microtubule dynamics - a game of zones

Plant morphogenesis relies on the accurate positioning of the partition (cell plate) between dividing cells during cytokinesis. The cell plate is synthetized by a specialized structure called the phragmoplast, which consists of microtubules, actin filaments, membrane compartments and associated proteins. The phragmoplast forms between daughter nuclei during the transition from anaphase to telophase. As cells are commonly larger than the originally formed phragmoplast, the construction of the cell plate requires phragmoplast expansion. This expansion depends on microtubule polymerization at the phragmoplast forefront (leading zone) and loss at the back (lagging zone). Leading and lagging zones sandwich the 'transition' zone. A population of stable microtubules in the transition zone facilitates transport of building materials to the midzone where the cell plate assembly takes place. Whereas microtubules undergo dynamic instability in all zones, the overall balance appears to be shifted towards depolymerization in the lagging zone. Polymerization of microtubules behind the lagging zone has not been reported to date, suggesting that microtubule loss there is irreversible. In this Review, we discuss: (1) the regulation of microtubule dynamics in the phragmoplast zones during expansion; (2) mechanisms of the midzone establishment and initiation of cell plate biogenesis; and (3) signaling in the phragmoplast.

Systemic control of cell division and endoreduplication by NAA and BAP by modulating CDKs in root tip cells of Allium cepa

Molecular mechanism regulated by auxin and cytokinin during endoreduplication, cell division, and elongation process is studied by using Allium cepa roots as a model system. The activity of CDK genes modulated by auxin and cytokinin during cell division, elongation, and endoreduplication process is explained in this research work. To study the significance of auxin and cytokinin in the management of cell division and endoreduplication process in plant meristematic cells at molecular level endoreduplication was developed in root tips of Allium cepa by giving colchicine treatment. There were inhibition of vegetative growth, formation of c-tumor at root tip, and development of endoreduplicated cells after colchicine treatment. This c-tumor was further treated with NAA and BAP to reinitiate vegetative growth in roots. BAP gave positive response in reinitiation of vegetative growth of roots from center of c-tumor. However, NAA gave negative response in reinitiation of vegetative growth of roots from c-tumor. Further, CDKs gene expression analysis from normal, endoreduplicated, and phytohormone (NAA or BAP) treated root tip was done and remarkable changes in transcription level of CDK genes in normal, endoreduplicated, and phytohormones treated cells were observed.

Overexpressed TPX2 causes ectopic formation of microtubular arrays in the nuclei of acentrosomal plant cells

TPX2 performs multiple roles in microtubule organization. Previously, it was shown that plant AtTPX2 binds AtAurora1 kinase and colocalizes with microtubules in a cell cycle-specific manner. To elucidate the function of TPX2 further, this work analysed Arabidopsis cells overexpressing AtTPX2-GFP. Distinct arrays of bundled microtubules, decorated with AtTPX2-GFP, were formed in the vicinity of the nuclear envelope and in the nuclei of overexpressing cells. The microtubular arrays showed reduced sensitivity to anti-microtubular drugs. TPX2-mediated formation of nuclear/perinuclear microtubular arrays was not specific for the transition to mitosis and occurred independently of Aurora kinase. The fibres were not observed in cells with detectable programmed cell death and, in this respect, they differed from TPX2-dependent microtubular assemblies functioning in mammalian apoptosis. Colocalization and co-purification data confirmed the interaction of importin with AtTPX2-GFP. In cells with nuclear foci of overexpressed AtTPX2-GFP, strong nuclear signals for Ran and importin diminished when microtubular arrays were assembled. This observation suggests that TPX2-mediated microtubule formation might be triggered by a Ran cycle. Collectively, the data suggest that in the acentrosomal plant cell, in conjunction with importin, overexpressed AtTPX2 reinforces microtubule formation in the vicinity of chromatin and the nuclear envelope.

Universal rules for division plane selection in plants

Coordinated cell divisions and cell expansion are the key processes that command growth in all organisms. The orientation of cell divisions and the direction of cell expansion are critical for normal development. Symmetric divisions contribute to proliferation and growth, while asymmetric divisions initiate pattern formation and differentiation. In plants these processes are of particular importance since their cells are encased in cellulosic walls that determine their shape and lock their position within tissues and organs. Several recent studies have analyzed the relationship between cell shape and patterns of symmetric cell division in diverse organisms and employed biophysical and mathematical considerations to develop computer simulations that have allowed accurate prediction of cell division patterns. From these studies, a picture emerges that diverse biological systems follow simple universal rules of geometry to select their division planes and that the microtubule cytoskeleton takes a major part in sensing the geometric information and translates this information into a specific division outcome. In plant cells, the division plane is selected before mitosis, and spatial information of the division plane is preserved throughout division by the presence of reference molecules at a distinct region of the plasma membrane, the cortical division zone. The recruitment of these division zone markers occurs multiple times by several mechanisms, suggesting that the cortical division zone is a highly dynamic region.

A nodulin/glutamine synthetase-like fusion protein is implicated in the regulation of root morphogenesis and in signalling triggered by flagellin

The nodulin/glutamine synthetase-like protein (NodGS) that we identified proteomically in Arabidopsis thaliana is a fusion protein composed of an N-terminal amidohydrolase domain that shares homology with nodulins and a C-terminal domain of prokaryotic glutamine synthetase type I. The protein is homologous to the FluG protein, a morphogenetic factor in fungi. Although genes encoding NodGS homologues are present in many plant genomes, their products have not yet been characterized. The Arabidopsis NodGS was present in an oligomeric form of ~700-kDa, mainly in the cytosol, and to a lesser extent in the microsomal membrane fraction. The oligomeric NodGS was incorporated into large heterogeneous protein complexes >700 kDa and partially co-immunoprecipitated with γ-tubulin. In situ and in vivo microscopic analyses revealed a NodGS signal in the cytoplasm, with endomembranes, particularly in the perinuclear area. NodGS had no detectable glutamine synthetase activity. Downregulation of NodGS by RNAi resulted in plants with a short main root, reduced meristematic activity and disrupted development of the root cap. Y2H analysis and publicly available microarray data indicated a role for NodGS in biotic stress signalling. We found that flagellin enhanced the expression of the NodGS protein, which was then preferentially localized in the nuclear periphery. Our results point to a role for NodGS in root morphogenesis and microbial elicitation. These data might help in understanding the family of NodGS/FluG-like fusion genes that are widespread in prokaryotes, fungi and plants.

Plant organelle proteomics: collaborating for optimal cell function

Organelle proteomics describes the study of proteins present in organelle at a particular instance during the whole period of their life cycle in a cell. Organelles are specialized membrane bound structures within a cell that function by interacting with cytosolic and luminal soluble proteins making the protein composition of each organelle dynamic. Depending on organism, the total number of organelles within a cell varies, indicating their evolution with respect to protein number and function. For example, one of the striking differences between plant and animal cells is the plastids in plants. Organelles have their own proteins, and few organelles like mitochondria and chloroplast have their own genome to synthesize proteins for specific function and also require nuclear-encoded proteins. Enormous work has been performed on animal organelle proteomics. However, plant organelle proteomics has seen limited work mainly due to: (i) inter-plant and inter-tissue complexity, (ii) difficulties in isolation of subcellular compartments, and (iii) their enrichment and purity. Despite these concerns, the field of organelle proteomics is growing in plants, such as Arabidopsis, rice and maize. The available data are beginning to help better understand organelles and their distinct and/or overlapping functions in different plant tissues, organs or cell types, and more importantly, how protein components of organelles behave during development and with surrounding environments. Studies on organelles have provided a few good reviews, but none of them are comprehensive. Here, we present a comprehensive review on plant organelle proteomics starting from the significance of organelle in cells, to organelle isolation, to protein identification and to biology and beyond. To put together such a systematic, in-depth review and to translate acquired knowledge in a proper and adequate form, we join minds to provide discussion and viewpoints on the collaborative nature of organelles in cell, their proper function and evolution.

Morphological analysis of CDC2 and glycogen synthase kinase 3β phosphorylation as markers of g2 → m transition in glioma

G2 → M transition is a strategic target for glioma chemotherapy. Key players in G2 → M transition include CDC2 and glycogen synthase kinase 3β (GSK3β), which are highly regulated by posttranslational phosphorylation. This report is a morphological analysis of CDC2 and GSK3β phosphorylation using immunohistochemistry in gliomas with different biological properties. GBM showed a 2.8-fold and 5.6-fold increase in number of cells positive for pThr161CDC2 and a 4.2- and 6.9-fold increase in number of cells positive for pTyr15CDC2 relative to oligodendroglioma and ependymoma, respectively. Elevated labeling for inhibited phospho-CDC2 (pTyr15CDC) correlates with elevated levels of phosphorylated glycogen synthase kinase 3β (GSK3β). 71% of the GBM cases showed intermediate to high intensity staining for pSer9SGK3β 53% of oligodendroglioma, and 73% of ependymoma showed low intensity staining. CDC2 gene amplification correlates with increased survival in glioblastoma multiforme (GBM) and astrocytoma WHO grades II-III, but not in oligodendroglioma WHO grades II-III.

Nucleolar localization of RNA binding proteins induced by actinomycin D and heat shock in Trypanosoma cruzi

In this work we show that under Actinomycin D (ActD) treatment, several RNA Binding Proteins (RBPs) involved in mRNA metabolism are relocalized into the nucleolus in Trypanosoma cruzi as a specific stress response. ATP depletion as well as kinase inhibition markedly reduced the nucleolar localization response, suggesting that an energy-dependent transport modulated by the phosphorylation status of the parasite might be required. Deletion analyses in one of such proteins, TcSR62, showed that a domain bearing basic amino acids located in the COOH terminal region was sufficient to promote its nucleolar relocalization. Interestingly, we showed that in addition to RBPs, poly(A)+ RNA is also accumulated into the nucleolus in response to ActD treatment. Finally, we found out that nucleolar relocalization of RBPs is also triggered by severe heat shock in a reversible way. Together, these results suggest that the nucleolus of an early divergent eukaryote is either able to sequester key factors related to mRNA metabolism in response to transcriptional stress or behaves as a RBP processing center, arguing in favour to the hypothesis that the non-traditional features of the nucleolus could be acquired early during evolution.

Regulatory dephosphorylation of CDK at G₂/M in plants: yeast mitotic phosphatase cdc25 induces cytokinin-like effects in transgenic tobacco morphogenesis

BACKGROUND

During the last three decades, the cell cycle and its control by cyclin-dependent kinases (CDKs) have been extensively studied in eukaryotes. This endeavour has produced an overall picture that basic mechanisms seem to be largely conserved among all eukaryotes. The intricate regulation of CDK activities includes, among others, CDK activation by CDC25 phosphatase at G₂/M. In plants, however, studies of this regulation have lagged behind as a plant Cdc25 homologue or other unrelated phosphatase active at G₂/M have not yet been identified.

SCOPE

Failure to identify a plant mitotic CDK activatory phosphatase led to characterization of the effects of alien cdc25 gene expression in plants. Tobacco, expressing the Schizosaccharomyces pombe mitotic activator gene, Spcdc25, exhibited morphological, developmental and biochemical changes when compared with wild type (WT) and, importantly, increased CDK dephosphorylation at G₂/M. Besides changes in leaf shape, internode length and root development, in day-neutral tobacco there was dramatically earlier onset of flowering with a disturbed acropetal floral capacity gradient typical of WT. In vitro, de novo organ formation revealed substantially earlier and more abundant formation of shoot primordia on Spcdc25 tobacco stem segments grown on shoot-inducing media when compared with WT. Moreover, in contrast to WT, stem segments from transgenic plants formed shoots even without application of exogenous growth regulator. Spcdc25-expressing BY-2 cells exhibited a reduced mitotic cell size due to a shortening of the G₂ phase together with high activity of cyclin-dependent kinase, NtCDKB1, in early S-phase, S/G₂ and early M-phase. Spcdc25-expressing tobacco ('Samsun') cell suspension cultures showed a clustered, more circular, cell phenotype compared with chains of elongated WT cells, and increased content of starch and soluble sugars. Taken together, Spcdc25 expression had cytokinin-like effects on the characteristics studied, although determination of endogenous cytokinin levels revealed a dramatic decrease in Spcdc25 transgenics.

CONCLUSIONS

The data gained using the plants expressing yeast mitotic activator, Spcdc25, clearly argue for the existence and importance of activatory dephosphorylation at G₂/M transition and its interaction with cytokinin signalling in plants. The observed cytokinin-like effects of Spcdc25 expression are consistent with the concept of interaction between cell cycle regulators and phytohormones during plant development. The G₂/M control of the plant cell cycle, however, remains an elusive issue as doubts persist about the mode of activatory dephosphorylation, which in other eukaryotes is provided by Cdc25 phosphatase serving as a final all-or-nothing mitosis regulator.

Interaction of a 14-3-3 protein with the plant microtubule-associated protein EDE1

BACKGROUND AND AIMS

The cell cycle-regulated protein ENDOSPERM DEFECTIVE 1 (EDE1) is a novel plant microtubule-associated protein essential for plant cell division and for microtubule organization in endosperm. EDE1 is only present on microtubules at mitosis and its expression is highly cell cycle regulated both at the protein and the transcript levels.

METHODS

To search for EDE1-interacting proteins, a yeast two-hybrid screen was used in which EDE1 was fused with GAL4 DNA binding domain and expressed in a yeast strain that was then mated with a strain carrying a cDNA library fused to the GAL4 transactivation domain. Candidate interacting proteins were identified and confirmed in vitro.

KEY RESULTS

14-3-3 upsilon was isolated several times from the library screen. In in vitro tests, it also interacted with EDE1: 14-3-3 upsilon most strongly associates with EDE1 in its free form, but also weakly when EDE1 is bound to microtubules. This study shows that EDE1 is a cyclin-dependent kinase substrate but that phosphorylation is not required for interaction with 14-3-3 upsilon.

CONCLUSIONS

The results suggest that 14-3-3 proteins may play a role in cytoskeletal organization of plant cells. The potential role of this interaction in the dynamics of EDE1 during the cell cycle is discussed.

Systematic localization of the Arabidopsis core cell cycle proteins reveals novel cell division complexes

Cell division depends on the correct localization of the cyclin-dependent kinases that are regulated by phosphorylation, cyclin proteolysis, and protein-protein interactions. Although immunological assays can define cell cycle protein abundance and localization, they are not suitable for detecting the dynamic rearrangements of molecular components during cell division. Here, we applied an in vivo approach to trace the subcellular localization of 60 Arabidopsis (Arabidopsis thaliana) core cell cycle proteins fused to green fluorescent proteins during cell division in tobacco (Nicotiana tabacum) and Arabidopsis. Several cell cycle proteins showed a dynamic association with mitotic structures, such as condensed chromosomes and the preprophase band in both species, suggesting a strong conservation of targeting mechanisms. Furthermore, colocalized proteins were shown to bind in vivo, strengthening their localization-function connection. Thus, we identified unknown spatiotemporal territories where functional cell cycle protein interactions are most likely to occur.

Arabidopsis TONNEAU1 proteins are essential for preprophase band formation and interact with centrin

Plant cells have specific microtubule structures involved in cell division and elongation. The tonneau1 (ton1) mutant of Arabidopsis thaliana displays drastic defects in morphogenesis, positioning of division planes, and cellular organization. These are primarily caused by dysfunction of the cortical cytoskeleton and absence of the preprophase band of microtubules. Characterization of the ton1 insertional mutant reveals complex chromosomal rearrangements leading to simultaneous disruption of two highly similar genes in tandem, TON1a and TON1b. TON1 proteins are conserved in land plants and share sequence motifs with human centrosomal proteins. The TON1 protein associates with soluble and microsomal fractions of Arabidopsis cells, and a green fluorescent protein-TON1 fusion labels cortical cytoskeletal structures, including the preprophase band and the interphase cortical array. A yeast two-hybrid screen identified Arabidopsis centrin as a potential TON1 partner. This interaction was confirmed both in vitro and in plant cells. The similarity of TON1 with centrosomal proteins and its interaction with centrin, another key component of microtubule organizing centers, suggests that functions involved in the organization of microtubule arrays by the centrosome were conserved across the evolutionary divergence between plants and animals.

Tobacco cells transformed with the fission yeast Spcdc25 mitotic inducer display growth and morphological characteristics as well as starch and sugar status evocable by cytokinin application

In plants, the G2/M control of cell cycle remains an elusive issue as doubts persist about activatory dephosphorylation--in other eukaryotes provided by CDC25 phosphatase and serving as a final all-or-nothing mitosis regulator. We report on the effects of tobacco (Nicotiana tabacum L., cv. Samsun) transformation with fission yeast (Schizosaccharomyces pombe) cdc25 (Spcdc25) on cell characteristics. Transformed cell suspension cultures showed higher dry mass accumulation during the exponential phase and clustered more circular cell phenotypes compared to chains of elongated WT cells. Similar cell parameters, as in the transformants, can be induced in WT by cytokinins. Spcdc25 cells, after cytokinin treatment, showed giant cell clusters and growth inhibition. In addition, Spcdc25 expression led to altered carbohydrate status: increased starch and soluble sugars with higher sucrose:hexoses ratio, inducible in WT by cytokinin treatment. Taken together, the Spcdc25 transformation had a cytokinin-like effect on studied characteristics. However, endogenous cytokinin determination revealed markedly lower cytokinin levels in Spcdc25 transformants. This indicates that the cells sense Spcdc25 expression as an increased cytokinin availability, manifested by changed cell morphology, and in consequence decrease endogenous cytokinin levels. Clearly, the results on cell growth and morphology are consistent with the model of G2/M control including cytokinin-regulated activatory dephosphorylation. Nevertheless, no clear link is obvious between Spcdc25 transformation and carbohydrate status and thus the observed cytokinin-like effect on carbohydrate levels poses a problem. Hence, we propose that Spcdc25-induced higher CDK(s) activity at G2/M generates a signal-modifying carbohydrate metabolism to meet high energy and C demands of forthcoming cell division.

Demarcation of the cortical division zone in dividing plant cells

Somatic cytokinesis in higher plants involves, besides the actual construction of a new cell wall, also the determination of a division zone. Several proteins have been shown to play a part in the mechanism that somatic plant cells use to control the positioning of the new cell wall. Plant cells determine the division zone at an early stage of cell division and use a transient microtubular structure, the preprophase band (PPB), during this process. The PPB is formed at the division zone, leaving behind a mark that during cytokinesis is utilized by the phragmoplast to guide the expanding cell plate toward the correct cortical insertion site. This review discusses old and new observations with regard to mechanisms implicated in the orientation of cell division and determination of a cortical division zone.

Arabidopsis cyclin-dependent kinase inhibitors are nuclear-localized and show different localization patterns within the nucleoplasm

The Arabidopsis genome contains seven cyclin-dependent kinase (CDK) inhibitors (ICK for inhibitor/interactor with cyclin-dependent kinase) which share a small conserved C-terminal domain responsible for the CDK-inhibition activity by these proteins. Different ICK/KRPs have been shown to have unique expression patterns within tissues, organs and during the cell cycle. Previous studies have shown that overexpressing one of the ICK/KRPs inhibits CDK activity, cell division, and profoundly affects plant growth and development. In this study, we investigated the subcellular localization of the seven Arabidopsis ICK proteins and domains responsible for this localization. Using transgenic expression in Arabidopsis plants and transient expression in tobacco leaf cells, all ICK/KRPs fused to green fluorescent protein (GFP) were localized to the nucleus, suggesting that the nucleus is the cellular compartment for the plant CDK inhibitors to function. While ICK2/KRP2, ICK4/KRP6, and ICK5/KRP7 were localized to the nucleoplasm in a homogeneous manner, ICK1/KRP1, ICK3/KRP5, ICK6/KRP3, and ICK7/KRP4 showed a punctate pattern of localization. A small motif conserved amongst the latter group of ICK/KRPs is required to confer this subcellular pattern as deletion of this motif from ICK7/KRP4 resulted in a shift from a punctate to a homogeneous pattern of localization. While a single nuclear localization signal (NLS) is responsible for the nuclear localization of ICK2/KRP2, multiple mechanisms for nuclear localization are suggested to exist for the other six ICK/KRPs since deletion mutants lacking predicted NLS motifs and the conserved C-terminal domain are still localized in the nucleus.

Insights into nuclear organization in plants as revealed by the dynamic distribution of Arabidopsis SR splicing factors

Serine/arginine-rich (SR) proteins are splicing regulators that share a modular structure consisting of one or two N-terminal RNA recognition motif domains and a C-terminal RS-rich domain. We investigated the dynamic localization of the Arabidopsis thaliana SR protein RSZp22, which, as we showed previously, distributes in predominant speckle-like structures and in the nucleolus. To determine the role of RSZp22 diverse domains in its nucleolar distribution, we investigated the subnuclear localization of domain-deleted mutant proteins. Our results suggest that the nucleolar localization of RSZp22 does not depend on a single targeting signal but likely involves different domains/motifs. Photobleaching experiments demonstrated the unrestricted dynamics of RSZp22 between nuclear compartments. Selective inhibitor experiments of ongoing cellular phosphorylation influenced the rates of exchange of RSZp22 between the different nuclear territories, indicating that SR protein mobility is dependent on the phosphorylation state of the cell. Furthermore, based on a leptomycin B- and fluorescence loss in photobleaching-based sensitive assay, we suggest that RSZp22 is a nucleocytoplasmic shuttling protein. Finally, with electron microscopy, we confirmed that RSp31, a plant-specific SR protein, is dynamically distributed in nucleolar cap-like structures upon phosphorylation inhibition. Our findings emphasize the high mobility of Arabidopsis SR splicing factors and provide insights into the dynamic relationships between the different nuclear compartments.

Molecular control of nuclear and subnuclear targeting of the plant CDK inhibitor ICK1 and ICK1-mediated nuclear transport of CDKA

ICK1 is the first member of a family of plant cyclin-dependent kinase (CDK) inhibitors. It has been shown that ICK1 is localized in the nuclei of transgenic Arabidopsis plants. Since cellular localization is important for the functions of cell cycle regulators, a comprehensive analysis was undertaken to identify specific sequences regulating the cellular localization of ICK1. Deletion and site-specific mutants fused to the green fluorescent protein (GFP) were used in transgenic Arabidopsis plants and transfected tobacco cells. Surprisingly, three separate sequences in the N-terminal, central and C-terminal regions of ICK1 could independently confer nuclear localization of the GFP fusion proteins. The central nuclear localization signal NLS(ICK1) could transport the much larger GUS (beta-glucuronidase)-GFP fusion protein into nuclei, while the other two sequences were unable to. These results suggest that NLS(ICK1) is a strong NLS that actively transports the fusion protein into nuclei, while the other two sequences are either a weaker NLS or confer the nuclear localization of GFP indirectly. It was further observed that the N-terminal sequence specifies a punctate pattern of subnuclear localization, while the C-terminal sequence suppresses it. Furthermore, co-expression of ICK1 and Arabidopsis CDKA, tagged with different GFP variants, showed that ICK1 could mediate the transport of CDKA into nuclei while a mutant ICK1(1-162) that does not interact with CDKA lost this ability. These results illustrate how the nuclear localization of ICK1 is regulated and also suggest a possible role of ICK1 in regulating the cellular distribution of CDKA.

Control of the AtMAP65-1 interaction with microtubules through the cell cycle

Cell division depends on the fine control of both microtubule dynamics and microtubule organisation. The microtubule bundling protein MAP65 is a ;midzone MAP' essential for the integrity of the anaphase spindle and cell division. Arabidopsis thaliana MAP65-1 (AtMAP65-1) binds and bundles microtubules by forming 25 nm cross-bridges. Moreover, as AtMAP65-1 bundles microtubules in interphase, anaphase and telophase but does not bind microtubules in prophase or metaphase, its activity through the cell cycle must be under tight control. Here we show that AtMAP65-1 is hyperphosphorylated during prometaphase and metaphase and that CDK and MAPK are involved in this phosphorylation. This phosphorylation inhibits AtMAP65-1 activity. Expression of non-phosphorylatable AtMAP65-1 has a negative effect on mitotic progression resulting in excessive accumulation of microtubules in the metaphase spindle midzone causing a delay in mitosis. We conclude that normal metaphase spindle organisation and the transition to anaphase is dependent on inactivation of AtMAP65-1.

Analysis of the subcellular localization, function, and proteolytic control of the Arabidopsis cyclin-dependent kinase inhibitor ICK1/KRP1

Recent studies have shown that cyclin-dependent kinase (CDK) inhibitors can have a tremendous impact on cell cycle progression in plants. In animals, CDK inhibitors are tightly regulated, especially by posttranslational mechanisms of which control of nuclear access and regulation of protein turnover are particularly important. Here we address the posttranslational regulation of INHIBITOR/INTERACTOR OF CDK 1 (ICK1)/KIP RELATED PROTEIN 1 (KRP1), an Arabidopsis (Arabidopsis thaliana) CDK inhibitor. We show that ICK1/KRP1 exerts its function in the nucleus and its presence in the nucleus is controlled by multiple nuclear localization signals as well as by nuclear export. In addition, we show that ICK1/KRP1 localizes to different subnuclear domains, i.e. in the nucleoplasm and to the chromocenters, hinting at specific actions within the nuclear compartment. Localization to the chromocenters is mediated by an N-terminal domain, in addition we find that this domain may be involved in cyclin binding. Further we demonstrate that ICK1/KRP1 is an unstable protein and degraded by the 26S proteasome in the nucleus. This degradation is mediated by at least two domains indicating the presence of at least two different pathways impinging on ICK1/KRP1 protein stability.

Arabidopsis PASTICCINO2 is an antiphosphatase involved in regulation of cyclin-dependent kinase A

PASTICCINO2 (PAS2), a member of the protein Tyr phosphatase-like family, is conserved among all eukaryotes and is characterized by a mutated catalytic site. The cellular functions of the Tyr phosphatase-like proteins are still unknown, even if they are essential in yeast and mammals. Here, we demonstrate that PAS2 interacts with a cyclin-dependent kinase (CDK) that is phosphorylated on Tyr and not with its unphosphorylated isoform. Phosphorylation of the conserved regulatory Tyr-15 is involved in the binding of CDK to PAS2. Loss of the PAS2 function dephosphorylated Arabidopsis thaliana CDKA;1 and upregulated its kinase activity. In accordance with its role as a negative regulator of the cell cycle, overexpression of PAS2 slowed down cell division in suspension cell cultures at the G2-to-M transition and early mitosis and inhibited Arabidopsis seedling growth. The latter was accompanied by altered leaf development and accelerated cotyledon senescence. PAS2 was localized in the cytoplasm of dividing cells but moved into the nucleus upon cell differentiation, suggesting that the balance between cell division and differentiation is regulated through the interaction between CDKA;1 and the antiphosphatase PAS2.

Gamma-tubulin is essential for acentrosomal microtubule nucleation and coordination of late mitotic events in Arabidopsis

Gamma-tubulin is required for microtubule (MT) nucleation at MT organizing centers such as centrosomes or spindle pole bodies, but little is known about its noncentrosomal functions. We conditionally downregulated gamma-tubulin by inducible expression of RNA interference (RNAi) constructs in Arabidopsis thaliana. Almost complete RNAi depletion of gamma-tubulin led to the absence of MTs and was lethal at the cotyledon stage. After induction of RNAi expression, gamma-tubulin was gradually depleted from both cytoplasmic and microsomal fractions. In RNAi plants with partial loss of gamma-tubulin, MT recovery after drug-induced depolymerization was impaired. Similarly, immunodepletion of gamma-tubulin from Arabidopsis extracts severely compromised in vitro polymerization of MTs. Reduction of gamma-tubulin protein levels led to randomization and bundling of cortical MTs. This finding indicates that MT-bound gamma-tubulin is part of a cortical template guiding the microtubular network and is essential for MT nucleation. Furthermore, we found that cells with decreased levels of gamma-tubulin could progress through mitosis, but cytokinesis was strongly affected. Stepwise diminution of gamma-tubulin allowed us to reveal roles for MT nucleation in plant development, such as organization of cell files, anisotropic and polar tip growth, and stomatal patterning. Some of these functions of gamma-tubulin might be independent of MT nucleation.

Midbodies and phragmoplasts: analogous structures involved in cytokinesis

Cytokinesis is an event common to all organisms that involves the precise coordination of independent pathways involved in cell-cycle regulation and microtubule, membrane, actin and organelle dynamics. In animal cells, the spindle midzone/midbody with associated endo-membrane system are required for late cytokinesis events, including furrow ingression and scission. In plants, cytokinesis is mediated by the phragmoplast, an array of microtubules, actin filaments and associated molecules that act as a framework for the future cell wall. In this article (which is part of the Cytokinesis series), we discuss recent studies that highlight the increasing number of similarities in the components and function of the spindle midzone/midbody in animals and the phragmoplast in plants, suggesting that they might be analogous structures.

Narrowing of the preprophase microtubule band is not required for cell division plane determination in cultured plant cells

In most higher-plant cells, cortical microtubules form a tightly focused preprophase band (PPB) that disappears with the onset of prometaphase, but whose location defines the future location of the cell plate at the end of cytokinesis. It is unclear whether the PPB microtubules themselves designate the precise area where the cell plate will insert, or rather if these microtubules are responding to a hierarchical signal(s). Here we show that narrowing of the microtubules within the PPB zone is not necessary for proper division plane determination. In cultured tobacco BY-2 cells in which PPB microtubules are depolymerized, the phragmoplast can still accurately locate and insert at the proper site. The data do not support a role for PPB microtubule narrowing in focusing the signal that is used later by the phragmoplast to position the cell plate; rather, proper phragmoplast positioning is more likely a consequence of a non-microtubule positional element. Although the PPB microtubules do not directly mark the division site, we show that they are required for accurate spindle positioning, an activity that presets the future growth trajectory of the phragmoplast and is necessary for insuring high-fidelity cell plate positioning.

Atypical regulation of a green lineage-specific B-type cyclin-dependent kinase

Cyclin-dependent kinases (CDKs) are the main regulators of cell cycle progression in eukaryotes. The role and regulation of canonical CDKs, such as the yeast (Saccharomyces cerevisiae) Cdc2 or plant CDKA, have been extensively characterized. However, the function of the plant-specific CDKB is not as well understood. Besides being involved in cell cycle control, Arabidopsis (Arabidopsis thaliana) CDKB would integrate developmental processes to cell cycle progression. We investigated the role of CDKB in Ostreococcus (Ostreococcus tauri), a unicellular green algae with a minimal set of cell cycle genes. In this primitive alga, at the basis of the green lineage, CDKB has integrated two levels of regulations: It is regulated by Tyr phosphorylation like cdc2/CDKA and at the level of synthesis-like B-type CDKs. Furthermore, Ostreococcus CDKB/cyclin B accounts for the main peak of mitotic activity, and CDKB is able to rescue a yeast cdc28(ts) mutant. By contrast, Ostreococcus CDKA is not regulated by Tyr phosphorylation, and it exhibits a low and steady-state activity from DNA replication to exit of mitosis. This suggests that from a major role in the control of mitosis in green algae, CDKB has evolved in higher plants to assume other functions outside the cell cycle.

CycD1, a putative G1 cyclin from Antirrhinum majus, accelerates the cell cycle in cultured tobacco BY-2 cells by enhancing both G1/S entry and progression through S and G2 phases

A putative G1 cyclin gene, Antma;CycD1;1 (CycD1), from Antirrhinum majus is known to be expressed throughout the cell cycle in the meristem and other actively proliferating cells. To test its role in cell cycle progression, we examined the effect of CycD1 expression in the tobacco (Nicotiana tabacum) cell suspension culture BY-2. Green fluorescent protein:CycD1 is located in the nucleus throughout interphase. Using epitope-tagged CycD1, we show that it interacts in vivo with CDKA, a cyclin dependent protein kinase that acts at both the G1/S and the G2/M boundaries. We examined the effect of induced expression at different stages of the cell cycle. Expression in G0 cells accelerated entry into both S-phase and mitosis, whereas expression during S-phase accelerated entry into mitosis. Consistent with acceleration of both transitions, the CycD1-associated cyclin dependent kinase can phosphorylate both histone H1 and Rb proteins. The expression of cyclinD1 led to the early activation of total CDK activity, consistent with accelerated cell cycle progression. Continuous expression of CycD1 led to moderate increases in growth rate. Therefore, in contrast with animal D cyclins, CycD1 can promote both G0/G1/S and S/G2/M progression. This indicates that D cyclin function may have diverged between plants and animals.

AtSGP1, AtSGP2 and MAP4Kα are nucleolar plant proteins that can complement fission yeast mutants lacking a functional SIN pathway

In the fission yeast Schizosaccharomyces pombe, the onset of septum formation is signalled via the septation initiation network (SIN) involving several protein kinases and a GTPase. Arabidopsis thaliana and Brassica napus proteins homologous to fission yeast spg1p (AtSGP1, AtSGP2), cdc7p (AtMAP3Kϵ1, AtMAP3Kϵ2, BnMAP3Kϵ1) and sid1p (AtMAP4Kα1, AtMAP4Kα2, BnMAP4Kα2) exhibit a significant similarity. The plant proteins AtSGP1/2 and BnMAP4Kα2 are able to complement the S. pombe mutant proteins spg1-B8 and sid1-239, respectively and to induce mutisepta when overexpressed in wild-type yeast. Yeast two-hybrid assays demonstrated interactions both between plant proteins and between plant and yeast proteins of the SIN pathway. However, the primary structure of the proteins as well as the partial complementation of yeast mutants indicates that plant homologous proteins and their yeast counterparts have diverged during evolution. Real-time RT-PCR studies demonstrated plant SIN-related gene expression in all organs tested and a co-expression pattern during the cell cycle, with a higher accumulation at G2-M. During interphase, the plant SIN-related proteins were found to co-localise predominantly in the nucleolus of the plant cells, as shown by fusions to green fluorescent protein. These data suggest the existence of a plant SIN-related pathway.

Large-scale identification of tubulin-binding proteins provides insight on subcellular trafficking, metabolic channeling, and signaling in plant cells

Microtubules play an essential role in the growth and development of plants and are known to be involved in regulating many cellular processes ranging from translation to signaling. In this article, we describe the proteomic characterization of Arabidopsis tubulin-binding proteins that were purified using tubulin affinity chromatography. Microtubule co-sedimentation assays indicated that most, if not all, of the proteins in the tubulin-binding protein fraction possessed microtubule-binding activity. Two-dimensional gel electrophoresis of the tubulin-binding protein fraction was performed, and 86 protein spots were excised and analyzed for protein identification. A total of 122 proteins were identified with high confidence using LC-MS/MS. These proteins were grouped into six categories based on their predicted functions: microtubule-associated proteins, translation factors, RNA-binding proteins, signaling proteins, metabolic enzymes, and proteins with other functions. Almost one-half of the proteins identified in this fraction were related to proteins that have previously been reported to interact with microtubules. This study represents the first large-scale proteomic identification of eukaryotic cytoskeleton-binding proteins, and provides insight on subcellular trafficking, metabolic channeling, and signaling in plant cells.

A plant-specific subclass of C-terminal kinesins contains a conserved a-type cyclin-dependent kinase site implicated in folding and dimerization

Cyclin-dependent kinases (CDKs) control cell cycle progression through timely coordinated phosphorylation events. Two kinesin-like proteins that interact with CDKA;1 were identified and designated KCA1 and KCA2. They are 81% identical and have a similar three-partite domain organization. The N-terminal domain contains an ATP and microtubule-binding site typical for kinesin motors. A green fluorescent protein (GFP) fusion of the N-terminal domain of KCA1 decorated microtubules in Bright Yellow-2 cells, demonstrating microtubule-binding activity. During cytokinesis the full-length GFP-fusion protein accumulated at the midline of young and mature expanding phragmoplasts. Two-hybrid analysis and coimmunoprecipitation experiments showed that coiled-coil structures of the central stalk were responsible for homo- and heterodimerization of KCA1 and KCA2. By western-blot analysis, high molecular mass KCA molecules were detected in extracts from Bright Yellow-2 cells overproducing the full-length GFP fusion. Treatment of these cultures with the phosphatase inhibitor vanadate caused an accumulation of these KCA molecules. In addition to dimerization, interactions within the C-terminally located tail domain were revealed, indicating that the tail could fold onto itself. The tail domains of KCA1 and KCA2 contained two adjacent putative CDKA;1 phosphorylation sites, one of which is conserved in KCA homologs from other plant species. Site-directed mutagenesis of the conserved phosphorylation sites in KCA1 resulted in a reduced binding with CDKA;1 and abolished intramolecular tail interactions. The data show that phosphorylation of the CDKA;1 site provokes a conformational change in the structure of KCA with implications in folding and dimerization.

Expression of a nondegradable cyclin B1 affects plant development and leads to endomitosis by inhibiting the formation of a phragmoplast

In plants after the disassembly of mitotic spindle, a specific cytokinetic structure called the phragmoplast is built, and after cytokinesis, microtubules populate the cell cortex in an organized orientation that determines cell elongation and shape. Here, we show that impaired cyclin B1 degradation, resulting from a mutation within its destruction box, leads to an isodiametric shape of epidermal cells in leaves, stems, and roots and retarded growth of seedlings. Microtubules in these misshaped cells are grossly disorganized, focused around the nucleus, whereas they were entirely missing or abnormally organized along the cell cortex. A high percentage of cells expressing nondestructible cyclin B1 had doubled DNA content as a result of undergoing endomitosis. During anaphase the cytokinesis-specific syntaxin KNOLLE could still localize to the midplane of cell division, whereas NPK1-activating kinesin-like protein 1, a cytokinetic kinesin-related protein, was unable to do so, and instead of the formation of a phragmoplast, the midzone microtubules persisted between the separated nuclei, which eventually fused. In summary, our results show that the timely degradation of mitotic cyclins in plants is required for the reorganization of mitotic microtubules to the phragmoplast and for proper cytokinesis. Subsequently, the presence of nondegradable cyclin B1 leads to a failure in organizing properly the cortical microtubules that determine cell elongation and shape.

Arabidopsis kinome: after the casting

Arabidopsis thaliana is used as a favourite experimental organism for many aspects of plant biology. We capitalized on the recently available Arabidopsis genome sequence and predicted proteome, to draw up a genome-scale protein serine/threonine kinase (PSTK) inventory. The PSTKs represent about 4% of the A. thaliana proteome. In this study, we provide a description of the content and diversity of the non-receptor PSTKs. These kinases have crucial functions in sensing, mediating and coordinating cellular responses to an extensive range of stimuli. A total of 369 predicted non receptor PSTKs were detailed: the Raf superfamily, the CMGC, CaMK, AGC and STE families, as well as a few small clades and orphan sequences. An extensive relationship analysis of these kinases allows us to classify the proteins in superfamilies, families, sub-families and groups. The classification provides a better knowledge of the characteristics shared by the different clades. We focused on the MAP kinase module elements, with particular attention to their docking sites for protein-protein interaction and their biological function. The large number of A. thaliana genes encoding kinases might have been achieved through successive rounds of gene and genome duplications. The evolution towards an increasing gene number suggests that functional redundancy plays an important role in plant genetic robustness.

Alteration of microtubule dynamic instability during preprophase band formation revealed by yellow fluorescent protein-CLIP170 microtubule plus-end labeling

At the onset of mitosis, plant cells form a microtubular preprophase band that defines the plane of cell division, but the mechanism of its formation remains a mystery. Here, we describe the use of mammalian yellow fluorescent protein-tagged CLIP170 to visualize the dynamic plus ends of plant microtubules in transfected cowpea protoplasts and in stably transformed and dividing tobacco Bright Yellow 2 cells. Using plus-end labeling, we observed dynamic instability in different microtubular conformations in live plant cells. The interphase plant microtubules grow at 5 micro m/min, shrink at 20 micro m/min, and display catastrophe and rescue frequencies of 0.02 and 0.08 events/s, respectively, exhibiting faster turnover than their mammalian counterparts. Strikingly, during preprophase band formation, the growth rate and catastrophe frequency of plant microtubules double, whereas the shrinkage rate and rescue frequency remain unchanged, making microtubules shorter and more dynamic. Using these novel insights and four-dimensional time-lapse imaging data, we propose a model that can explain the mechanism by which changes in microtubule dynamic instability drive the dramatic rearrangements of microtubules during preprophase band and spindle formation in plant cells.

The Arabidopsis lue1 mutant defines a katanin p60 ortholog involved in hormonal control of microtubule orientation during cell growth

The lue1 mutant was previously isolated in a bio-imaging screen for Arabidopsis mutants exhibiting inappropriate regulation of an AtGA20ox1 promoter-luciferase reporter fusion. Here we show that lue1 is allelic to fra2, bot1 and erh3, and encodes a truncated katanin-like microtubule-severing protein (AtKSS). Complementation of lue1 with the wild-type AtKSS gene restored both wild-type stature and luciferase reporter levels. Hormonal responses of lue1 to ethylene and gibberellins revealed inappropriate cortical microtubule reorientation during cell growth. Moreover, a fusion between the AtKSS protein and GFP decorated cortical microtubules. A yeast two-hybrid screen with AtKSS as the bait identified proteins related to those involved in microtubule processing, including a katanin p80 subunit and a kinesin ortholog. These results indicate that AtKSS is involved in microtubule dynamics in response to plant hormones.

Plant gamma-tubulin interacts with alphabeta-tubulin dimers and forms membrane-associated complexes

gamma-Tubulin is assumed to participate in microtubule nucleation in acentrosomal plant cells, but the underlying molecular mechanisms are still unknown. Here, we show that gamma-tubulin is present in protein complexes of various sizes and different subcellular locations in Arabidopsis and fava bean. Immunoprecipitation experiments revealed an association of gamma-tubulin with alphabeta-tubulin dimers. gamma-Tubulin cosedimented with microtubules polymerized in vitro and localized along their whole length. Large gamma-tubulin complexes resistant to salt treatment were found to be associated with a high-speed microsomal fraction. Blue native electrophoresis of detergent-solubilized microsomes showed that the molecular mass of the complexes was >1 MD. Large gamma-tubulin complexes were active in microtubule nucleation, but nucleation activity was not observed for the smaller complexes. Punctate gamma-tubulin staining was associated with microtubule arrays, accumulated with short kinetochore microtubules interacting in polar regions with membranes, and localized in the vicinity of nuclei and in the area of cell plate formation. Our results indicate that the association of gamma-tubulin complexes with dynamic membranes might ensure the flexibility of noncentrosomal microtubule nucleation. Moreover, the presence of other molecular forms of gamma-tubulin suggests additional roles for this protein species in microtubule organization.

Sub-cellular localisation of GFP-tagged tobacco mitotic cyclins during the cell cycle and after spindle checkpoint activation

We have previously shown that the tobacco cyclin B1;1 protein accumulates during the G2 phase of the cell cycle and is subsequently destroyed during mitosis. Here, we investigated the sub-cellular localisation of two different B1-types and one A3-type cyclin during the cell cycle by using confocal imaging and differential interference contrast (DIC) microscopy. The cyclins were visualised as GFP-tagged fusion proteins in living tobacco cells. Both B1-type cyclins were found in the cytoplasm and in the nucleus during G2 but when cells entered into prophase, both cyclins became associated with condensing chromatin and remained on chromosomes until metaphase. As cells exited metaphase, the B1-type cyclins became degraded, as shown by time-lapse images. A stable variant of cyclin B1;1-GFP fusion protein, in which the destruction box had been mutated, maintained its association with the nuclear material at later phases of mitosis such as anaphase and telophase. Furthermore, we demonstrated that cyclin B1;1 protein is stabilised in metaphase-arrested cells after microtubule destabilising drug treatments. In contrast to the B1-type cyclins, the cyclin A3;1 was found exclusively in the nucleus in interphase cells and disappeared earlier than the cyclin B1 proteins during mitosis.