A conserved role for kinesin-5 in plant mitosis

Functions of the Arabidopsis kinesin superfamily of microtubule-based motor proteins

Plants possess a large number of microtubule-based kinesin motor proteins. While the kinesin-2, 3, 9, and 11 families are absent from land plants, the kinesin-7 and 14 families are greatly expanded. In addition, some kinesins are specifically present only in land plants. The distinctive inventory of plant kinesins suggests that kinesins have evolved to perform specialized functions in plants. Plants assemble unique microtubule arrays during their cell cycle, including the interphase cortical microtubule array, preprophase band, anastral spindle and phragmoplast. In this review, we explore the functions of plant kinesins from a microtubule array viewpoint, focusing mainly on Arabidopsis kinesins. We emphasize the conserved and novel functions of plant kinesins in the organization and function of the different microtubule arrays.

Functions of the Arabidopsis kinesin superfamily of microtubule-based motor proteins

Plants possess a large number of microtubule-based kinesin motor proteins. While the kinesin-2, 3, 9, and 11 families are absent from land plants, the kinesin-7 and 14 families are greatly expanded. In addition, some kinesins are specifically present only in land plants. The distinctive inventory of plant kinesins suggests that kinesins have evolved to perform specialized functions in plants. Plants assemble unique microtubule arrays during their cell cycle, including the interphase cortical microtubule array, preprophase band, anastral spindle and phragmoplast. In this review, we explore the functions of plant kinesins from a microtubule array viewpoint, focusing mainly on Arabidopsis kinesins. We emphasize the conserved and novel functions of plant kinesins in the organization and function of the different microtubule arrays.

The novel functions of kinesin motor proteins in plants

Kinesin superfamily proteins are important microtubule-based motor proteins with a kinesin motor domain that is conserved among all eukaryotic organisms. They are responsible for unidirectionally transporting various cargoes, including membranous organelles, protein complexes, and mRNAs. They also play critical roles in mitosis, morphogenesis, and signal transduction. Most kinesins in plants are evolutionarily divergent from their counterparts in animals and fungi. The mitotic kinesins in the plant kinesin-5 and kinesin-14 subfamilies appear to be similar to those in fungi and animals. However, others with nonmotor sequences are unique to plants. The kinesins affect microtubule organization, organelle distribution, vesicle transport, and cellulose microfibril order. Ultimately, plant kinesins contribute directly or indirectly to cell division and cell growth in various tissues. Here, we review a novel function of kinesins with transcription activation activity in regulating gibberellin biosynthesis and cell growth. These findings will open exciting new areas of kinesin research.

Dynamic reorganization of Eg5 in the mammalian spindle throughout mitosis requires dynein and TPX2

Kinesin-5 is an essential mitotic motor. However, how its spatial-temporal distribution is regulated in mitosis remains poorly understood. We expressed localization and affinity purification-tagged Eg5 from a mouse bacterial artificial chromosome (this construct was called mEg5) and found its distribution to be tightly regulated throughout mitosis. Fluorescence recovery after photobleaching analysis showed rapid Eg5 turnover throughout mitosis, which cannot be accounted for by microtubule turnover. Total internal reflection fluorescence microscopy and high-resolution, single-particle tracking revealed that mEg5 punctae on both astral and midzone microtubules rapidly bind and unbind. mEg5 punctae on midzone microtubules moved transiently both toward and away from spindle poles. In contrast, mEg5 punctae on astral microtubules moved transiently toward microtubule minus ends during early mitosis but switched to plus end-directed motion during anaphase. These observations explain the poleward accumulation of Eg5 in early mitosis and its redistribution in anaphase. Inhibition of dynein blocked mEg5 movement on astral microtubules, whereas depletion of the Eg5-binding protein TPX2 resulted in plus end-directed mEg5 movement. However, motion of Eg5 on midzone microtubules was not altered. Our results reveal differential and precise spatial and temporal regulation of Eg5 in the spindle mediated by dynein and TPX2.

Phylogenetic analysis of the Kinesin superfamily from physcomitrella

Kinesins are an ancient superfamily of microtubule dependent motors. They participate in an extensive and diverse list of essential cellular functions, including mitosis, cytokinesis, cell polarization, cell elongation, flagellar development, and intracellular transport. Based on phylogenetic relationships, the kinesin superfamily has been subdivided into 14 families, which are represented in most eukaryotic phyla. The functions of these families are sometimes conserved between species, but important variations in function across species have been observed. Plants possess most kinesin families including a few plant specific families. With the availability of an ever increasing number of genome sequences from plants, it is important to document the complete complement of kinesins present in a given organism. This will help develop a molecular framework to explore the function of each family using genetics, biochemistry, and cell biology. The moss Physcomitrella patens has emerged as a powerful model organism to study gene function in plants, which makes it a key candidate to explore complex gene families, such as the kinesin superfamily. Here we report a detailed phylogenetic characterization of the 71 kinesins of the kinesin superfamily in Physcomitrella. We found a remarkable conservation of families and subfamily classes with Arabidopsis, which is important for future comparative analysis of function. Some of the families, such as kinesins 14s are composed of fewer members in moss, while other families, such as the kinesin 12s are greatly expanded. To improve the comparison between species, and to simplify communication between research groups, we propose a classification of subfamilies based on our phylogenetic analysis.

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

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

Brittle Culm 12, a dual-targeting kinesin-4 protein, controls cell-cycle progression and wall properties in rice

Kinesins are encoded by a large gene family involved in many basic processes of plant development. However, the number of functionally identified kinesins in rice is very limited. Here, we report the functional characterization of Brittle Culm12 (BC12), a gene encoding a kinesin-4 protein. bc12 mutants display dwarfism resulting from a significant reduction in cell number and brittleness due to an alteration in cellulose microfibril orientation and wall composition. BC12 is expressed mainly in tissues undergoing cell division and secondary wall thickening. In vitro biochemical analyses verified BC12 as an authentic motor protein. This protein was present in both the nucleus and cytoplasm and associated with microtubule arrays during cell division. Mitotic microtubule array comparison, flow cytometric analysis and expression assays of cyclin-dependent kinase (CDK) complexes in root-tip cells showed that cell-cycle progression is affected in bc12 mutants. BC12 is very probably regulated by CDKA;3 based on yeast two-hybrid and microarray data. Therefore, BC12 functions as a dual-targeting kinesin protein and is implicated in cell-cycle progression, cellulose microfibril deposition and wall composition in the monocot plant rice.

A conditional mutation in Arabidopsis thaliana separase induces chromosome non-disjunction, aberrant morphogenesis and cyclin B1;1 stability

The caspase family protease, separase, is required at anaphase onset to cleave the cohesin complex, which joins sister chromatids. However, among eukaryotes, separases have acquired novel functions. Here, we show that Arabidopsis thaliana radially swollen 4 (rsw4), a temperature-sensitive mutant isolated previously on the basis of root swelling, harbors a mutation in At4g22970, the A. thaliana separase. Loss of separase function in rsw4 at the restrictive temperature is indicated by the widespread failure of replicated chromosomes to disjoin. Surprisingly, rsw4 has neither pronounced cell cycle arrest nor anomalous spindle formation, which occur in other eukaryotes upon loss of separase activity. However, rsw4 roots have disorganized cortical microtubules and accumulate the mitosis-specific cyclin, cyclin B1;1, excessive levels of which have been associated with altered microtubules and morphology. Cyclin B1;1 also accumulates in certain backgrounds in response to DNA damage, but we find no evidence for aberrant responses to DNA damage in rsw4. Our characterization of rsw4 leads us to hypothesize that plant separase, in addition to cleaving cohesin, regulates cyclin B1;1, with profound ramifications for morphogenesis.

Mitotic functions of kinesin-5

In all eukaryotic cells, molecular motor proteins play essential roles in spindle assembly and function. The homotetrameric kinesin-5 motors in particular generate outward forces that establish and maintain spindle bipolarity and contribute to microtubule flux. Cell-cycle dependent phosphorylation of kinesin-5 motors regulates their localization to the mitotic spindle. Analysis of live cells further shows that kinesin-5 motors are highly dynamic in the spindle. Understanding the interactions of kinesin-5 motors with microtubules and other spindle proteins is likely to broaden the documented roles of kinesin-5 motors during cell division.

Microtubule dynamics in plant cells

This chapter describes some of the choices and unavoidable compromises to be made when studying microtubule dynamics in plant cells. The choice of species still depends very much on the ability to produce transgenic plants and most work has been done in the relatively small cells of Arabidopsis plants or in tobacco BY-2 suspension cells. Fluorescence-tagged microtubule proteins have been used to label entire microtubules, or their plus ends, but there are still few minus-end markers for these acentrosomal cells. Pragmatic decisions have to be made about probes, balancing the efficacy of microtubule labeling against a tendency to overstabilize and bundle the microtubules and even induce helical plant growth. A key limitation in visualizing plant microtubules is the ability to keep plants alive for long periods under the microscope and we describe a biochamber that allows for plant cell growth and development while allowing gas exchange and reducing evaporation. Another major difficulty is the limited fluorescence lifetime and we describe imaging strategies to reduce photobleaching in long-term imaging. We also discuss methods of measuring microtubule dynamics, with emphasis on the behavior of plant-specific microtubule arrays.

Rapid screening for temperature-sensitive alleles in plants

We developed a simple and fast method to identify temperature-sensitive alleles of essential plant genes. We used primary and tertiary structure information to identify residues in the core of the protein of interest. These residues were mutated and tested for temperature sensitivity, taking advantage of the exceptionally rapid 1-week complementation assay in the moss Physcomitrella patens. As test molecules, we selected the actin-binding proteins profilin and actin-depolymerizing factor, because they are essential and their loss-of-function phenotype can be fully rescued. Screening a small number of candidate mutants, we successfully identified temperature-sensitive alleles of both profilin and actin-depolymerizing factor. Plants harboring these alleles grew well at the permissive temperature of 20 degrees C to 25 degrees C but showed a complete loss of function at the restrictive temperature of 32 degrees C. Notably, the profilin mutation identified in the moss gene can be transferred to profilins from other plant species, also rendering them temperature sensitive. The ability to routinely generate temperature-sensitive alleles of essential plant proteins provides a powerful tool for the study of gene function in plants.

Dual functions of Nicotiana benthamiana Rae1 in interphase and mitosis

Rae1 performs multiple functions in animal systems, acting in interphase as an mRNA export factor and during mitosis as a mitotic checkpoint and spindle assembly regulator. In this study we characterized multiple functions of Rae1 in plants. Virus-induced gene silencing of Nicotiana benthamiana Rae1, NbRae1, which encodes a protein with four WD40 repeats, resulted in growth arrest and abnormal leaf development. NbRae1 was mainly associated with the nuclear envelope during interphase, and NbRae1 deficiency caused accumulation of poly(A) RNA in the nuclei of leaf cells, suggesting defective mRNA export. In the shoot apex, depletion of NbRae1 led to reduced mitotic activities, accompanied by reduced cyclin-dependent kinase (CDK) activity and decreased expression of cyclin B1, CDKB1-1, and histones H3 and H4. The secondary growth of stem vasculature was also inhibited, indicating reduced cambial activities. Differentiated leaf cells of NbRae1-silenced plants exhibited elevated ploidy levels. Immunolabeling in BY-2 cells showed that NbRae1 protein localized to mitotic microtubules and the cell plate-forming zone during mitosis, and recombinant NbRae1 directly bound to microtubules in vitro. Inhibition of NbRae1 expression in BY-2 cells using a beta-estradiol-inducible RNAi system resulted in severe defects in spindle organization and chromosome alignment and segregation, which correlated with delays in cell cycle progression. Together, these results suggest that NbRae1 plays a dual role in mRNA export in interphase and in spindle assembly in mitosis.

Localization and function of Kinesin-5-like proteins during assembly and maintenance of mitotic spindles in Silvetia compressa

Background

Kinesin-5 (Eg-5) motor proteins are essential for maintenance of spindle bipolarity in animals. The roles of Kinesin-5 proteins in other systems, such as Arabidopsis, Dictyostelium, and sea urchin are more varied. We are studying Kinesin-5-like proteins during early development in the brown alga Silvetia compressa. Previously, this motor was shown to be needed to assemble a bipolar spindle, similar to animals. This report builds on those findings by investigating the localization of the motor and probing its function in spindle maintenance.

Findings

Anti-Eg5 antibodies were used to investigate localization of Kinesin-5-like proteins in brown algal zygotes. In interphase zygotes, localization was predominantly within the nucleus. As zygotes entered mitosis, these motor proteins strongly associated with spindle poles and, to a lesser degree, with the polar microtubule arrays and the spindle midzone. In order to address whether Kinesin-5-like proteins are required to maintain spindle bipolarity, we applied monastrol to synchronized zygotes containing bipolar spindles. Monastrol is a cell-permeable chemical inhibitor of the Kinesin-5 class of molecular motors. We found that inhibition of motor function in pre-formed spindles induced the formation of multipolar spindles and short bipolar spindles.

Conclusion

Based upon these localization and inhibitor studies, we conclude that Kinesin-5-like motors in brown algae are more similar to the motors of animals than those of plants or protists. However, Kinesin-5-like proteins in S. compressa serve novel roles in spindle formation and maintenance not observed in animals.

The parallel lives of microtubules and cellulose microfibrils

A major breakthrough was the recent discovery that cellulose synthases really do move along the plasma membrane upon tracks provided by the underlying cortical microtubules. It emphasized the cytoplasmic contribution to cell wall organization. A growing number of microtubule-associated proteins has been identified and shown to affect the way that microtubules are ordered, with downstream effects on the pattern of growth. The dynamic properties of microtubules turn out to be key in understanding the behaviour of the global array and good progress has been made in deciphering the rules by which the array is self-organized.

MOR1, the Arabidopsis thaliana homologue of Xenopus MAP215, promotes rapid growth and shrinkage, and suppresses the pausing of microtubules in vivo

MOR1, the Arabidopsis thaliana homologue of the Xenopus microtubule-associated protein MAP215, is required for spatial organization of the acentrosomal microtubule arrays of plant cells. To determine how loss of MOR1 function affects microtubule dynamics, we compared various parameters of microtubule dynamics in the temperature-sensitive mor1-1 mutant at its permissive and restrictive temperatures, 21 degrees C and 31 degrees C, respectively. Dynamic events were tracked in live cells expressing either GFP-tagged beta-tubulin or the plus end tracking EB1. Microtubule growth and shrinkage velocities were both dramatically reduced in mor1-1 at 31 degrees C and the incidence and duration of pause events increased. Interestingly, the association of EB1 with microtubule plus ends was reduced in mor1-1 whereas side wall binding increased, suggesting that MOR1 influences the association of EB1 with microtubules either by modulating microtubule plus end structure or by interacting with EB1. Although mor1-1 microtubules grew and shrank more slowly than wild-type microtubules at 21 degrees C, the incidence of pause was not altered, suggesting that pause events, which occur more frequently at 31 degrees C, have a major detrimental role in the spatial organization of cortical microtubules. Extensive increases in microtubule dynamics in wild-type cells when shifted from 21 degrees C to 31 degrees C underline the importance of careful temperature control in live cell imaging.

Kinesin-5 is not essential for mitotic spindle elongation in Dictyostelium

The proper assembly and operation of the mitotic spindle is essential to ensure the accurate segregation of chromosomes and to position the cytokinetic furrow during cell division in eukaryotes. Not only are dynamic microtubules required but also the concerted actions of multiple motor proteins are necessary to effect spindle pole separation, chromosome alignment, chromatid segregation, and spindle elongation. Although a number of motor proteins are known to play a role in mitosis, there remains a limited understanding of their full range of functions and the details by which they interact with other spindle components. The kinesin-5 (BimC/Eg5) family of motors is largely considered essential to drive spindle pole separation during the initial and latter stages of mitosis. We have deleted the gene encoding the kinesin-5 member in Dictyostelium, (kif13), and find that, in sharp contrast with results found in vertebrate, fly, and yeast organisms, kif13(-) cells continue to grow at rates indistinguishable from wild type. Phenotype analysis reveals a slight increase in spindle elongation rates in the absence of Kif13. More importantly, there is a dramatic, premature separation of spindle halves in kif13(-) cells, suggesting a novel role of this motor in maintaining spindle integrity at the terminal stages of division.

Kinesins are indispensable for interdigitation of phragmoplast microtubules in the moss Physcomitrella patens

Microtubules form arrays with parallel and antiparallel bundles and function in various cellular processes, including subcellular transport and cell division. The antiparallel bundles in phragmoplasts, plant-unique microtubule arrays, are mostly unexplored and potentially offer new cellular insights. Here, we report that the Physcomitrella patens kinesins KINID1a and KINID1b (for kinesin for interdigitated microtubules 1a and 1b), which are specific to land plants and orthologous to Arabidopsis thaliana PAKRP2, are novel factors indispensable for the generation of interdigitated antiparallel microtubules in the phragmoplasts of the moss P. patens. KINID1a and KINID1b are predominantly localized to the putative interdigitated parts of antiparallel microtubules. This interdigitation disappeared in double-deletion mutants of both genes, indicating that both KINID1a and 1b are indispensable for interdigitation of the antiparallel microtubule array. Furthermore, cell plates formed by these phragmoplasts did not reach the plasma membrane in approximately 20% of the mutant cells examined. We observed that in the double-deletion mutant lines, chloroplasts remained between the plasma membrane and the expanding margins of the cell plate, while chloroplasts were absent from the margins of the cell plates in the wild type. This suggests that the kinesins, the antiparallel microtubule bundles with interdigitation, or both are necessary for proper progression of cell wall expansion.

Arabidopsis mutants and the network of microtubule-associated functions

In early eukaryotes, the microtubule system was engaged in mitosis, intracellular transport, and flagellum-based motility. In the plant lineage, the evolution of a multicellular body involved the conservation of some core functions, the loss of others, and the elaboration of new microtubule functions associated with the multicellular plant habit. This diversification is reflected by the presence of both conserved (animal/fungi-like) and novel (plant-like) sequences encoding microtubule-related functions in the Arabidopsis genome. The collection of microtubule mutants has grown rapidly over recent years. These mutants present a wide range of phenotypes, consistent with the hypothesis of a functional diversification of the microtubule system. In this review, we focus on mutant analysis and, in particular, discuss double mutant analysis as a valuable tool for pinpointing pathways of gene function. A future challenge will be to define the complete network of genetic and physical interactions of microtubule function in plants. In addition to reviewing recent progress in the functional analysis of the 'MAPome', we present an online database of Arabidopsis mutants impaired in microtubule functions.

Emerging molecular mechanisms that power and regulate the anastral mitotic spindle of flowering plants

Flowering plants, lacking centrosomes as well as dynein, assemble their mitotic spindle via a pathway that is distinct visually and molecularly from that of animals and yeast. The molecular components underlying mitotic spindle assembly and function in plants are beginning to be discovered. Here, we review recent evidence suggesting the preprophase band in plants functions analogously to the centrosome in animals in establishing spindle bipolarity, and we review recent progress characterizing the roles of specific motor proteins in plant mitosis. Loss of function of certain minus-end-directed KIN-14 motor proteins causes a broadening of the spindle pole; whereas, loss of function of a KIN-5 causes the formation of monopolar spindles, resembling those formed when the homologous motor protein (e.g., Eg5) is knocked out in animal cells. We present a phylogeny of the kinesin-5 motor domain, which shows deep divergence among plant sequences, highlighting possibilities for specialization. Finally, we review information concerning the roles of selected structural proteins at mitosis as well as recent findings concerning regulation of M-phase in plants. Insight into the mitotic spindle will be obtained through continued comparison of mitotic mechanisms in a diversity of cells.

Functional divergence of the duplicated AtKIN14a and AtKIN14b genes: critical roles in Arabidopsis meiosis and gametophyte development

Gene duplication is important for gene family evolution, allowing for functional divergence and innovation. In flowering plants, duplicated genes are widely observed, and functional redundancy of closely related duplicates has been reported, but few cases of functional divergence of close duplicates have been described. Here, we show that the Arabidopsis AtKIN14a and AtKIN14b genes encoding highly similar kinesins are two of the most closely related Arabidopsis paralogs, which were formed by a duplication event that occurred after the split of Arabidopsis and poplar. In addition, AtKIN14a and AtKIN14b exhibit varying degrees of coding sequence divergence. Further genetic studies of plants carrying atkin14a and/or atkin14b mutations indicate that, although these two genes have similar functions, there is clear evidence for functional divergence. Although both genes are important for male and female meiosis, AtKIN14a plays a more critical role in male meiosis than AtKIN14b. Moreover, either one of these two genes is necessary and sufficient for gametophyte development, indicating that they are redundant for this function. Therefore, AtKIN14a and AtKIN14b together play important roles in controlling plant reproductive development. Our results suggest that the AtKIN14a and AtKIN14b genes have retained similar functions in gametophyte development and female meiosis, but have evolved partially distinct functions in male meiosis, with AtKIN14a playing a more substantive role.

MAP65-3 microtubule-associated protein is essential for nematode-induced giant cell ontogenesis in Arabidopsis

The infection of plants by obligate parasitic nematodes constitutes an interesting model for investigating plant cytoskeleton functions. Root knot nematodes have evolved the ability to manipulate host functions to their own advantage by redifferentiating root cells into multinucleate and hypertrophied feeding cells. These giant cells result from repeated rounds of karyokinesis without cell division. Detailed functional analyses demonstrated that Arabidopsis thaliana Microtubule-Associated Protein65-3 (MAP65-3) was essential for giant cell ontogenesis and that cytokinesis was initiated but not completed in giant cells. In developing giant cells, MAP65-3 was associated with a novel kind of cell plate-the giant cell mini cell plate-that separates daughter nuclei. In the absence of functional MAP65-3, giant cells developed but failed to fully differentiate and were eventually destroyed. These defects in giant cells impaired the maturation of nematode larvae. Thus, MAP65-3 is essential for giant cell development during root knot nematode infection. Subcellular localization of MAP65-3 and analysis of microtubule organization in the dyc283 T-DNA map65-3 mutant demonstrated that MAP65-3 played a critical role in organizing the mitotic microtubule array during both early and late mitosis in all plant organs. Here, we propose a model for the role of MAP65-3 in giant cell ontogenesis.