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

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.

Roles for kinesin and myosin during cytokinesis

Cytokinesis in higher plants involves the phragmoplast, a complex cytoplasmic structure that consists of microtubules (MTs), microfilaments (MFs) and membrane elements. Both MTs and MFs are essential for cell plate formation, although it is not clear which motor proteins are involved. Some candidate processes for motor proteins include transport of Golgi vesicles to the plane of the cell plate and the spatiotemporal organization of the cytoskeletal elements in order to achieve proper deposition and alignment of the cell plate. We have focused on the kinesin-like calmodulin binding protein (KCBP) and, more broadly, on myosins. Using an antibody that constitutively activates KCBP, we find that this MT motor, which is minus-end directed, contributes to the organization of the spindle and phragmoplast MTs. It does not participate in vesicle transport; rather, because of the orientation of the phragmoplast MTs, it is supposed that plus-end kinesins fill this role. Myosins, on the other hand, based on their inhibition with 2,3-butanedione monoxime and 1-(5-iodonaphthalene-1-sulphonyl)-1H-hexahydro-1,4-diazepine (ML-7), are associated with the process of post-mitotic spindle/phragmoplast alignment and with late lateral expansion of the cell plate. They are also not the principal motors involved in vesicle transport.

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

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

Expansion of the cell plate in plant cytokinesis requires a kinesin-like protein/MAPKKK complex

The tobacco mitogen-activated protein kinase kinase kinase NPK1 regulates lateral expansion of the cell plate at cytokinesis. Here, we show that the kinesin-like proteins NACK1 and NACK2 act as activators of NPK1. Biochemical analysis suggests that direct binding of NACK1 to NPK1 stimulates kinase activity. NACK1 is accumulated specifically in M phase and colocalized with NPK1 at the phragmoplast equator. Overexpression of a truncated NACK1 protein that lacks the motor domain disrupts NPK1 concentration at the phragmoplast equator and cell plate formation. Incomplete cytokinesis is also observed when expression of NACK1 and NACK2 is repressed by virus-induced gene silencing and in embryonic cells from Arabidopsis mutants in which a NACK1 ortholog is disrupted. Thus, we conclude that expansion of the cell plate requires NACK1/2 to regulate the activity and localization of NPK1.

The Arabidopsis HINKEL gene encodes a kinesin-related protein involved in cytokinesis and is expressed in a cell cycle-dependent manner

Plant cytokinesis starts in the center of the division plane, with vesicle fusion generating a new membrane compartment, the cell plate, that subsequently expands laterally by continuous fusion of newly arriving vesicles to its margin. Targeted delivery of vesicles is assisted by the dynamic reorganization of a plant-specific cytoskeletal array, the phragmoplast, from a solid cylinder into an expanding ring-shaped structure. This lateral translocation is brought about by depolymerization of microtubules in the center, giving way to the expanding cell plate, and polymerization of microtubules along the edge. Whereas several components are known to mediate cytokinetic vesicle fusion [8-10], no gene function involved in phragmoplast dynamics has been identified by mutation. Mutations in the Arabidopsis HINKEL gene cause cytokinesis defects, such as enlarged cells with incomplete cell walls and multiple nuclei. Proper targeting of the cytokinesis-specific syntaxin KNOLLE [8] and lateral expansion of the phragmoplast are not affected. However, the phragmoplast microtubules appear to persist in the center, where vesicle fusion should result in cell plate formation. Molecular analysis reveals that the HINKEL gene encodes a plant-specific kinesin-related protein with a putative N-terminal motor domain and is expressed in a cell cycle-dependent manner similar to the KNOLLE gene. Our results suggest that HINKEL plays a role in the reorganization of phragmoplast microtubules during cell plate formation.

Golgi secretion is not required for marking the preprophase band site in cultured tobacco cells

The preprophase band predicts the future cell division site. However, the mechanism of how a transient preprophase band fulfils this function is unknown. We have investigated the possibility that Golgi secretion might be involved in marking the preprophase band site. Observations on living BY-2 cells labeled for microtubules and Golgi stacks indicated an increased Golgi stack frequency at the preprophase band site. However, inhibition of Golgi secretion by brefeldin A during preprophase band formation did not prevent accurate phragmoplast fusion, and subsequent cell plate formation, at the preprophase band site. The results show that Golgi secretion does not mark the preprophase band site and thus does not play an active role in determination of the cell division site.

Molecular aspects of microtubule dynamics in plants

Microtubules are highly dynamic structures that play a major role in a wide range of processes, including cell morphogenesis, cell division, intracellular transport and signaling. The recent identification in plants of proteins involved in microtubule organization has begun to reveal how cytoskeleton dynamics are controlled.

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

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

"Caged cytoskeletons": a rapid method for the isolation of microtubule-associated proteins from synchronized plant suspension cells

In the cytoskeleton method for isolating microtubule-associated proteins MAP65, DcKRP120-1 and DcKRP120-2, carrot cells are first converted to protoplasts but this method cannot be used to isolate mitotic MAPs as mitotic synchrony is eroded during lengthy cellulase treatment. Anti-microtubule cycle blocks would also be unsuitable. We report here a method for overcoming these problems. Cellulase degradation of tobacco BY-2 cells for only several minutes allows extraction of detergent-soluble proteins, leaving synchronized "caged cytoskeletons" for depolymerization and enabling affinity purification of MAPs on neurotubules. This rapid and simple method should be of general utility: it can be bulked up, avoids anti-microtubule blocks, and is applicable to other cell suspensions. The effectiveness of the caged cytoskeleton method is demonstrated by comparing known MAPs (the 65 kDa structural MAPs and the kinesin-related protein, TKRP125) in synchronized cells taken at the mitotic peak with those in unsynchronized cells.

Kinesins in the Arabidopsis genome: a comparative analysis among eukaryotes

BACKGROUND

Kinesins constitute a superfamily of microtubule motor proteins that are found in eukaryotic organisms. Members of the kinesin superfamily perform many diverse cellular functions such as transport of vesicles and organelles, spindle formation and elongation, chromosome segregation, microtubule dynamics and morphogenesis. Only a few kinesins have been characterized in plants including Arabidopsis thaliana. Because of the diverse cellular functions in which kinesins are involved, the number, types and characteristics of kinesins present in the Arabidopsis genome would provide valuable information for many researchers.

RESULTS

Here we have analyzed the recently completed Arabidopsis genome sequence and identified sixty-one kinesin genes in the Arabidopsis genome. Among the five completed eukaryotic genomes the Arabidopsis genome has the highest percentage of kinesin genes. Further analyses of the kinesin gene products have resulted in identification of several interesting domains in Arabidopsis kinesins that provide clues in understanding their functions. A phylogenetic analysis of all Arabidopsis kinesin motor domain sequences with 113 motor domain sequences from other organisms has revealed that Arabidopsis has seven of the nine recognized subfamilies of kinesins whereas some kinesins do not fall into any known family.

CONCLUSION

There are groups of Arabidopsis kinesins that are not present in yeast, Caenorhabditis elegans and Drosophila melanogaster that may, therefore, represent new subfamilies specific to plants. The domains present in different kinesins may provide clues about their functions in cellular processes. The comparative analysis presented here provides a framework for future functional studies with Arabidopsis kinesins.

CYTOKINESIS AND BUILDING OF THE CELL PLATE IN PLANTS

Cytokinesis in plant cells is more complex than in animals, as it involves building a cell plate as the final step in generating two cells. The cell plate is built in the center of phragmoplast by fusion of Golgi-derived vesicles. This step imposes an architectural problem where ballooning of the fused structures has to be avoided to create a plate instead. This is apparently achieved by squeezing the vesicles into dumbbell-shaped vesicle-tubule-vesicle (VTV) structures with the help of phragmoplastin, a homolog of dynamin. These structures are fused at their ends in a star-shaped body creating a tubulovesicular "honeycomb-like" structure sandwiched between the positive ends of the phragmoplast microtubules. This review summarizes our current understanding of various mechanisms involved in budding-off of Golgi vesicles, delivery and fusion of vesicles to initiate cell plate, and the synthesis of polysaccharides at the forming cell plate. Little is known about the molecular mechanisms involved in determining the site, direction, and the point of attachment of the growing cell plate with the parental cell wall. These gaps may be filled soon, as many genes that have been identified by mutations are analyzed and functions of their products are deciphered.

The protein kinases AtMAP3Kepsilon1 and BnMAP3Kepsilon1 are functional homologues of S. pombe cdc7p and may be involved in cell division

We identified an Arabidopsis thaliana gene, AtMAP3Kepsilon1, and a Brassica napus cDNA, BnMAP3Kepsilon1, encoding functional protein serine/threonine kinases closely related to cdc7p and Cdc15p from Schizosaccharomyces pombe and Saccharomyces cerevisiae, respectively. This is the first report of cdc7-related genes in non-fungal eukaryotes; no such genes have as yet been identified in Metazoans. The B. napus protein is able to partially complement a cdc7 loss of function mutation in S. pombe. RT-PCR and in situ hybridisation revealed that the A. thaliana and B. napus genes are expressed in both the sporophytic and the gametophytic tissues of the respective plant species and revealed further that expression is highest in dividing cells. Moreover, AtMAP3Kepsilon1 gene expression is cell cycle-regulated, with higher expression in G2-M phases. Our results strongly suggest that the plant cdc7p-related protein kinases are involved in a signal transduction pathway similar to the SIN pathway, which positively regulates cytokinesis in S. pombe.

Molecular motors and their functions in plants

Molecular motors that hydrolyze ATP and use the derived energy to generate force are involved in a variety of diverse cellular functions. Genetic, biochemical, and cellular localization data have implicated motors in a variety of functions such as vesicle and organelle transport, cytoskeleton dynamics, morphogenesis, polarized growth, cell movements, spindle formation, chromosome movement, nuclear fusion, and signal transduction. In non-plant systems three families of molecular motors (kinesins, dyneins, and myosins) have been well characterized. These motors use microtubules (in the case of kinesines and dyneins) or actin filaments (in the case of myosins) as tracks to transport cargo materials intracellularly. During the last decade tremendous progress has been made in understanding the structure and function of various motors in animals. These studies are yielding interesting insights into the functions of molecular motors and the origin of different families of motors. Furthermore, the paradigm that motors bind cargo and move along cytoskeletal tracks does not explain the functions of some of the motors. Relatively little is known about the molecular motors and their roles in plants. In recent years, by using biochemical, cell biological, molecular, and genetic approaches a few molecular motors have been isolated and characterized from plants. These studies indicate that some of the motors in plants have novel features and regulatory mechanisms. The role of molecular motors in plant cell division, cell expansion, cytoplasmic streaming, cell-to-cell communication, membrane trafficking, and morphogenesis is beginning to be understood. Analyses of the Arabidopsis genome sequence database (51% of genome) with conserved motor domains of kinesin and myosin families indicates the presence of a large number (about 40) of molecular motors and the functions of many of these motors remain to be discovered. It is likely that many more motors with novel regulatory mechanisms that perform plant-specific functions are yet to be discovered. Although the identification of motors in plants, especially in Arabidopsis, is progressing at a rapid pace because of the ongoing plant genome sequencing projects, only a few plant motors have been characterized in any detail. Elucidation of function and regulation of this multitude of motors in a given species is going to be a challenging and exciting area of research in plant cell biology. Structural features of some plant motors suggest calcium, through calmodulin, is likely to play a key role in regulating the function of both microtubule- and actin-based motors in plants.

Regulation of molecular motor proteins

Motor proteins in the kinesin, dynein, and myosin superfamilies are tightly regulated to perform multiple functions in the cell requiring force generation. Although motor proteins within families are diverse in sequence and structure, there are general mechanisms by which they are regulated. We first discuss the regulation of the subset of kinesin family members for which such information exists, and then address general mechanisms of kinesin family regulation. We review what is known about the regulation of axonemal and cytoplasmic dyneins. Recent work on cytoplasmic dynein has revealed the existence of multiple isoforms for each dynein chain, making the study of dynein regulation more complicated than previously realized. Finally, we discuss the regulation of myosins known to be involved in membrane trafficking. Myosins and kinesins may be evolutionarily related, and there are common themes of regulation between these two classes of motors.

Boursin, a sea urchin bimC kinesin protein, plays a role in anaphase and cytokinesis

We have isolated and characterized Boursin, a kinesin-related protein of the bimC family, from Paracentrotus lividus sea urchin eggs. Boursin is expressed at high levels in eggs and embryos during early cleavage stages. Boursin was found to be associated with different parts of the mitotic spindle from early prophase to telophase. Expression of a form of the protein predicted to act as a dominant negative mutant caused severe defects in cell division and resulted in the formation of embryos with polyploid and multiastral blastomeres. Immunofluorescence analysis indicated that these defects did not arise from failure in either centrosome separation or bipolar spindle formation. Time-lapse observations showed rather that these perturbations in cell division resulted from abnormal anaphase and failure to complete cytokinesis. These phenotypes differ from the phenotype described following perturbation of the function of bimC family members in other organisms. Our study has thus uncovered roles for a bimC kinesin in late stages of cell division.

Isolation of cortical MTs from tobacco BY-2 cells

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

The NPK1 mitogen-activated protein kinase kinase kinase is a regulator of cell-plate formation in plant cytokinesis

Mitogen-activated protein kinase (MAPK) cascades play important roles not only in the transduction of extracellular signals but in the progression of the cell cycle. However, evidence for their role in cytokinesis is limited. Here, we show that a tobacco MAPK kinase kinase (MAPKKK), designated NPK1, is required for cytokinesis. The activity of NPK1 increases in the late M phase of the tobacco cell cycle. During expansion of a new cross-wall (cell plate) toward the cell cortex, NPK1 is consistently localized to the equatorial zone of the phragmoplast, the cytokinetic apparatus where the cell plate is formed. Expression of a kinase-negative mutant of NPK1 results in the generation of multinucleate cells with incomplete cell plates. Phragmoplasts can be formed, but its expansion toward the cell cortex is also blocked. Thus, our results indicate that the NPK1 MAPKKK is essential for the formation of the cell plate, especially for its lateral growth.

Cyclin/Cdk complexes: their involvement in cell cycle progression and mitotic division

DNA replication and mitosis are dependent on the activity of cyclin-dependent protein kinase (CDK) enzymes, which are heterodimers of a catalytic subunit with a cyclin subunit. Cyclin binding to specific individual proteins is thought to provide potential substrates to Cdk. Protein binding by cyclins is assessed in terms of its mechanisms and biological significance, using evidence from diverse organisms including substrate specificity in animal Cdk enzymes containing D-, A-, and B-type cyclins and extensive cyclin gene manipulations in yeasts. Assembly of protein complexes with cyclin/Cdk is noted and the capacity of the cyclin-dependent kinase subunit Cks, in such complex, to extend the range of Cdk substrates is documented and discussed in terms of cell cycle regulation. Cell cycle progression involves changing abundance of individual cyclins, due to changing rates of their transcription or proteolysis, with consequent changes in the substrates of CDK through the cell cycle. Some overlap of the functions of individual cyclins in vivo has been identified by cyclin deletions and is suggested to follow a pattern in which cyclins can commonly complete functions initiated by the preceding cyclins well enough to preserve viability as groups of cyclins are removed by proteolysis. Cyclin accumulation is particularly important in terminating the G1 phase, when it raises CDK activity and starts events leading to DNA replication. It is suggested that plants share this mechanism. The distribution of cyclins and Cdk in maize root tip cells during mitosis and cytokinesis indicates the presence of Cdk1 (Cdc2a) and cyclin CycB1zm;2 at the mature and disassembling preprophase band and the presence of CycB1zm;2 at condensing and condensed chromosomes. Both observations correlate with the earlier-reported capacity of injected metaphase cyclin/CDK to accelerate preprophase band disassembly and chromosome condensation and with observations of the location of Cdk and cyclins in other laboratories. Additionally CycB1zm;2 is seen at the nuclear envelope during its breakdown, which correlates with an acceleration of the process by injected metaphase cyclin B/CDK. A phenomenon possibly unique to the plant kingdom is the persistence of mitotic cyclins after anaphase. Participation of cyclins in cytokinesis is indicated by the concentration of the mitotic cyclin CycA1;zm;1 at the phragmoplast. It is suggested that cyclins have a general function of spatially focusing Cdk activity and that in the plant cell the concentrations of cyclins are important mediators of CDK activity at the cytoskeleton, chromosomes, spindle, nuclear envelope, and phragmoplast.

Microtubule-associated proteins in plants--why we need a MAP

Plants have four main microtubule assemblies. Three are involved in arranging when and where the cell wall is laid down and have no direct homologues in animals. Microtubule-associated proteins are important components of these assemblies, and we are now starting to uncover what these proteins are and how they might work.

Identification of a novel plant-specific kinesin-like protein that is highly expressed in interphase tobacco BY-2 cells

Through reverse transcription-polymerase chain reaction and Northern blot analysis, we identified TBK5, a novel plant-specific kinesin-like protein (KLP) that is highly expressed in interphase tobacco BY-2 cells. TBK5 mRNA was present at a high level throughout the growth cycle, even in cells that had entered the stationary phase, where cell proliferation had ceased. However, transcripts for five other tobacco KLPs that we have identified were preferentially expressed in mitotic cells, and either not or only slightly accumulated in cells that had entered the stationary phase. Thus, TBK5 appears to be a KLP whose cellular function most closely relates to the cortical array of microtubules that plays a key role in plant cell morphogenesis. The predicted structure of TBK5 is characterized by a central motor domain that is phylogenetically distant from those of other reported KLPs, coiled-coil domains located on both sides of the motor domain, and a basic C-terminal domain. In addition, TBK5 has a putative neck domain which is closely related to the neck domain of KLPs with C-terminal motor domains, previously shown to control the direction of KLP movement towards the minus ends. Antibodies against truncated TBK5 recognized a polypeptide with a molecular mass of 74 kDa in cytoplasmic extracts of interphase cells, and this polypeptide cosedimented with microtubules assembled in the cytoplasmic extracts. The 74 kDa polypeptide corresponding to TBK5 dissociated from microtubules with high concentrations of NaCl but was not dissociated by MgATP. We hypothesize that TBK5 functions in the regulation of the arrangement of cortical microtubules.

Two kinesin-related proteins associated with the cold-stable cytoskeleton of carrot cells: characterization of a novel kinesin, DcKRP120-2

We have previously described the biochemical isolation of 65 kDa and 120 kDa microtubule-associated proteins from carrot cytoskeletons. The 65 kDa MAPs have subsequently been shown to be structural MAPs that reconstitute 30 nm cross-bridges of the kind that maintain cortical microtubules in parallel groups. By exploiting its avid binding to microtubules, we have now devised a method for isolating MAP120 from protoplast extracts, and shown that it has properties of a kinesin-related protein. MAP120 segregates with the cold stable pool of microtubules in carrot cytoskeletons, whilst the 65 kDa MAPs are also associated with the cold-sensitive microtubules. On gradient gels, MAP120 resolves as two kinesin-like bands. We report the isolation of a carrot cDNA, DcKRP120-2, corresponding to a novel kinesin of the BimC class known to move to the plus ends of microtubules. Antibodies raised against specific expressed sequences recognize the upper band, while the lower band is recognized by antibodies to the tobacco kinesin-related protein, TKRP125. We have also isolated a partial genomic carrot DNA, DcKRP120-1, homologous to the motor region of tobacco TKRP125. Immunofluorescence of the two proteins produces different staining patterns. Anti-TKRP125 labels the cortical microtubules and the pre-prophase band, but anti-DcKRP120-2 does so only weakly. Both clearly stain the spindle and the phragmoplast, but in a proportion of cells anti-DcKRP120-2 strongly decorates the phragmoplast mid-line where the plus ends of the microtubules overlap. We discuss the potential roles of these proteins during the microtubule cycle.

A new class of microtubule-associated proteins in plants

In plants there are three microtubule arrays involved in cellular morphogenesis that have no equivalent in animal cells. In animals, microtubules are decorated by another class of proteins - the structural MAPS - which serve to stabilize microtubules and assist in their organization. The best-studied members of this class in plants are the MAP-65 proteins that can be purified together with plant microtubules after several cycles of polymerization and depolymerization. Here we identify three similar MAP-65 complementary DNAs representing a small gene family named NtMAP65-1, which encode a new set of proteins, collectively called NtMAP65-1. We show that NtMAP65-1 protein localizes to areas of overlapping microtubules, indicating that it may function in the behaviour of antiparallel microtubules in the mitotic spindle and the cytokinetic phragmoplast.

A novel calcium/calmodulin-regulated kinesin-like protein is highly conserved between monocots and dicots

Recently, a novel kinesin-like protein (KCBP) that is regulated by Ca2+/calmodulin was isolated from dicot plants. A homolog of KCBP has not been reported in monocots. To determine if this motor protein is present in phylogenetically divergent flowering plants, Arabidopsis KCBP cDNA was used as a probe to screen a genomic library of maize, an evolutionarily divergent species. This screening resulted in isolation of a KCBP homolog. Comparison of the predicted amino acid sequence of the KCBP from maize (ZmKCBP), a monocot, with the previously reported KCBP sequences from dicot species showed that the amino acid sequence, domain organization, and gene structure are highly conserved between monocots and dicots. The C-terminal region of ZmKCBP, containing the motor domain and the calmodulin-binding domain, and the N-terminal tail, with a myosin tail homology region (MyTH4) and talin-like region, showed strong sequence similarity to the KCBP homolog from dicots. However, the coiled-coil region is less conserved between monocots and dicots. The ZmKCBP gene contained 22 exons and 21 introns. The location of 19 of the 21 introns of ZmKCBP is also conserved. The ZmKCBP protein is encoded by a single gene and expressed in all tissues. Affinity-purified antibody to the calmodulin-binding domain of Arabidopsis KCBP detected a protein in both the soluble and the microsomal fractions. The C-terminal region of ZmKCBP, containing the motor and calmodulin-binding domains, bound calmodulin in the presence of calcium and failed to bind in the presence of EGTA. The ZmKCBP, along with other KCBPs from dicots, was grouped into a distinct group in the C-terminal subfamily of kinesin-like proteins. These data suggest that the KCBP is ubiquitous and highly conserved in all flowering plants and the origin of KCBP predated the divergence of monocots and dicots.

Identification of a phragmoplast-associated kinesin-related protein in higher plants

The phragmoplast executes cytokinesis in higher plants. The major components of the phragmoplast are microtubules, which are arranged in two mirror-image arrays perpendicular to the division plane [1]. The plus ends of these microtubules are located near the site of the future cell plate. Golgi-derived vesicles are transported along microtubules towards the plus ends to deliver materials bound for the cell plate [2] [3]. During cell division, rapid microtubule reorganization in the phragmoplast requires the orchestrated activities of microtubule motor proteins such as kinesins. We isolated an Arabidopsis cDNA clone of a gene encoding an amino-terminal motor kinesin, AtPAKRP1, and have determined the partial sequence of its rice homolog. Immunofluorescence experiments with two sets of specific antibodies revealed consistent localization of AtPAKRP1 and its homolog in Arabidopsis and rice cells undergoing anaphase, telophase and cytokinesis. AtPAKRP1 started to accumulate along microtubules towards the spindle midzone during late anaphase. Once the phragmoplast microtubule array was established, AtPAKRP1 conspicuously localized to microtubules near the future cell plate. Our results provide evidence that AtPAKRP1 is a hitherto unknown motor that may take part in the establishment and/or maintenance of the phragmoplast microtubule array.

Ca2+/calmodulin regulation of the Arabidopsis kinesin-like calmodulin-binding protein

The kinesin family motor protein KCBP (kinesin-like calmodulin binding protein) was identified during a screen for Arabidopsis calmodulin-binding proteins [Reddy, et al., 1996b: J. Biol Chem. 271:7052-7060]. KCBP contains a C-terminal motor domain and is unique among kinesin motors in that it has a calmodulin-binding site. We expressed the KCBP motor domain in Escherichia coli and examined its microtubule (MT) binding and ATPase activity. KCBP bound MTs in an ATP-dependent manner and exhibited MT-stimulated ATPase activity. Ca2+/ calmodulin inhibited binding of KCBP to MTs under conditions that normally favor tight motor-MT interactions, and the extent of inhibition was dependent on the concentration of calcium and calmodulin. Ca2+/calmodulin did not affect KCBP's basal ATPase activity, but reduced the motor's MT-stimulated ATPase activity. The substantial reduction in affinity of KCBP for MTs in the presence of Ca2+/calmodulin suggests that Ca2+/calmodulin may modulate the activity of KCBP in vivo by regulating the motor's association with MTs. KCBP is the first MT-dependent motor protein found to be regulated by direct binding of Ca2+/calmodulin to its motor subunit.

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

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

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

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

CYTOSKELETAL PERSPECTIVES ON ROOT GROWTH AND MORPHOGENESIS

Growth and development of all plant cells and organs relies on a fully functional cytoskeleton comprised principally of microtubules and microfilaments. These two polymeric macromolecules, because of their location within the cell, confer structure upon, and convey information to, the peripheral regions of the cytoplasm where much of cellular growth is controlled and the formation of cellular identity takes place. Other ancillary molecules, such as motor proteins, are also important in assisting the cytoskeleton to participate in this front-line work of cellular development. Roots provide not only a ready source of cells for fundamental analyses of the cytoskeleton, but the formative zone at their apices also provides a locale whereby experimental studies can be made of how the cytoskeleton permits cells to communicate between themselves and to cooperate with growth-regulating information supplied from the apoplasm.

Mitotic motors in Saccharomyces cerevisiae

The budding yeast Saccharomyces cerevisiae provides a unique opportunity for study of the microtubule-based motor proteins that participate in mitotic spindle function. The genome of Saccharomyces encodes a relatively small and genetically tractable set of microtubule-based motor proteins. The single cytoplasmic dynein and five of the six kinesin-related proteins encoded have been implicated in mitotic spindle function. Each motor protein is unique in amino acid sequence. On account of functional overlap, no single motor is uniquely required for cell viability, however. The ability to create and analyze multiple mutants has allowed experimental dissection of the roles performed by each mitotic motor. Some of the motors operate within the nucleus to assemble and elongate the bipolar spindle (kinesin-related Cin8p, Kip1p, Kip3p and Kar3p). Others operate on the cytoplasmic microtubules to effect spindle and nuclear positioning within the cell (dynein and kinesin-related Kip2p, Kip3p and Kar3p). The six motors apparently contribute three fundamental activities to spindle function: motility, microtubule cross-linking and regulation of microtubule dynamics.

Division decisions and the spatial regulation of cytokinesis

Cytokinesis in plant cells in accomplished when a membranous cell plate is guided to a pre-established division site. The orientation of the new wall establishes the starting position of a cell in a growing tissue, but the impact of this position on future development varies. Recently, proteins have been identified that participate in forming, stabilizing and guiding the cell plate to the correct division site. Mutations that affect cytokinesis with varying impacts on plant development are providing information about the mechanics of cytokinesis and also about how the division site is selected.

Bundling of microtubules by motor and tail domains of a kinesin-like calmodulin-binding protein from Arabidopsis: regulation by Ca(2+)/Calmodulin

Kinesin-like calmodulin-binding protein (KCBP), a novel kinesin-like protein from plants, is unique among kinesins and kinesin-like proteins in having a calmodulin-binding domain adjacent to its motor domain. KCBP localizes to mitotic microtubule (MT) arrays including the preprophase band, the spindle apparatus, and the phragmoplast, suggesting a role for KCBP in establishing these MT arrays by bundling MTs. To determine if KCBP bundles MTs, we expressed C-terminal motor and N-terminal tail domains of KCBP, and used the purified proteins in MT bundling assays. The 1.5 C protein with the motor and calmodulin-binding domains induced MT bundling. The 1.5 C-induced bundles were dissociated in the presence of Ca(2+)/calmodulin. Similar results were obtained with a 1.4 C protein, which lacks much of the coiled-coil region present in 1.5 C protein and does not form dimers. The N-terminal tail of KCBP, which contains an ATP-independent MT binding site, is also capable of bundling MTs. These results, together with the KCBP localization data, suggest the involvement of KCBP in establishing mitotic MT arrays during different stages of cell division and that Ca(2+)/calmodulin regulates the formation of these MT arrays.

Regulation of cell division and the cytoskeleton by mitogen-activated protein kinases in higher plants

The microtubule-associated protein 2 kinase (MAP2-kinase), now better known as mitogen-activated protein kinase (MAPK), was initially discovered in association with the cytoskeleton, and was later also implicated in cell division. The importance of mitogenic stimulation in plant development roused interest in finding the plant homologues of MAPKs. However, data on plant MAPKs in cell division are rather sparse and fragmentary. Therefore we place the available information on cell cycle control of MAPKs in plants into a broader context. We discuss four aspects of cell division control: cell proliferation and the G1/S-phase transition, G2-phase and mitosis, cytokinesis, and cytoskeletal reorganisation. Future work will reveal to what extent plants use signalling pathways that are similar or different to those of animal or yeast cells in regulating cell divisions.

The MAP kinase cascade that includes MAPKKK-related protein kinase NPK1 controls a mitotic proces in plant cells

The tobacco NPK1 cDNA was the first-isolated plant cDNA for a homolog of mitogen-activated protein kinase kinase kinases (MAPKKKs). The kinase domain of the NPK1 protein can replace the functions of MAPKKKs in yeasts, while the amino acid sequence of the kinase-unrelated region does not have any homology to those of MAPKKKs from other organisms. Transcription of the NPK1 gene takes place in meristematic tissues or immature organs in a tobacco plant. During a tobacco cell cycle, transcriptional and translational products of NPK1 are present from S to M phase and decrease after the M phase. Expression of the NACK1 gene, which is predicted to encode a novel kinesin-like microtubule-based motor protein capable of activating NPK1, is specific to M phase, suggesting that activation of NPK1 occurs in M phase. Characterization of cDNAs for a MAPKK and a MAPK which can act downstream of NPK1 makes a proposition that the MAP kinase pathway involving NPK1 regulates a mitotic process associated with microtubules.

Cytoskeleton in plant development

The plant cytoskeleton has crucial functions in a number of cellular processes that are essential for cell morphogenesis, organogenesis and development. These functions have been intensively investigated using single cell model systems. With the recent characterization of plant mutants that show aberrant organogenesis resulting from primary defects in cytoskeletal organization, an integrated understanding of the importance of the cytoskeleton for plant development has begun to emerge. Newly established techniques that allow the non-destructive visualization of microtubules or actin filaments in living plant cells and organs will further advance this understanding.

Divide and conquer: cytokinesis in plant cells

Plant cells divide in two by constructing a new cell wall (cell plate) between daughter nuclei after mitosis. Golgi-derived vesicles are transported to the equator of a cytoskeletal structure called a phragmoplast, where they fuse together to form the cell plate. Orientation of new cell walls involves actindependent guidance of phragmoplasts and associated cell plates to cortical sites established prior to mitosis. Recent work has provided new insights into how actin filaments and other proteins in the phragmoplast and cell plate contribute to cytokinesis. Newly discovered mutations have identified a variety of genes required for cytokinesis or its spatial regulation.

Characterization of katD, a kinesin-like protein gene specifically expressed in floral tissues of Arabidopsis thaliana

Kinesin and kinesin-like proteins (KLPs) are microtubule-based motor proteins that play important roles in organelle transport. Based on the homology to these proteins, a katD cDNA has now been isolated from a library prepared from flowers of Arabidopsis thaliana ecotype Columbia. Sequence analysis of the katD cDNA revealed an open reading frame of 2691bp [corrected], encoding a protein of 987 amino acids. Comparison of the nucleotide sequences of katD genomic and cDNA clones revealed the presence of 18 introns, 17 of which conform to the GU-AG rule. The central region of the KatD polypeptide exhibits substantial amino acid sequence homology to the motor domain of kinesin heavy chains, although the motor domain of KatD appears to be phylogenetically distant from those of other KLPs in plants. The amino-terminal region of KatD shares marked sequence similarity with the calponin homology domain, whereas the approximately 240-residue carboxyl-terminal region shows no significant homology to other known proteins. The predicted secondary structure of KatD revealed the lack of an alpha-helical coiled coil structure typical of kinesin heavy chains, suggesting that KatD may function as a monomeric motor. A recombinant truncated KatD protein containing the putative motor domain was shown both to bind to mammalian microtubules in a manner dependent on a non-hydrolyzable ATP analog, and to possess microtubule-dependent ATPase activity. Immunoblot and Northern blot analyses showed that both KatD protein and mRNA are expressed specifically in floral tissues. These results suggest that the structurally distinct KatD protein functions as a floral tissue-specific motor protein.

Cytokinesis in eukaryotes: a mechanistic comparison

Cytokinesis is a crucial but poorly understood process of cell proliferation. Recently, molecular genetic analyses of fungal cytokinesis have led to an appreciation of contractile mechanisms in simple eukaryotes, and studies in animal and plant cells have led to new insights into the role of microtubules in the cleavage process. These findings suggest that fundamental mechanisms of cytokinesis may be highly conserved among eukaryotic organisms.

Cytokinesis in flowering plants: cellular process and developmental integration

In phragmoplast-assisted cytokinesis of somatic cells, vesicle fusion generates a cell plate that matures into a new cell wall and its flanking plasma membranes. Insight into this dynamic process has been gained in the past few years and additional molecular components of the basic machinery of cytokinesis have been identified. Specialized modes of cytokinesis occur in meiosis and gametophyte development, and recent studies indicate that they are genetically distinct from somatic cytokinesis.

Treatment of Vicia faba root tip cells with specific inhibitors to cyclin-dependent kinases leads to abnormal spindle formation

Many events during cell division are triggered by an evolutionary conserved regulator, the cyclin-dependent kinase (Cdk). Here we used two novel drugs, the purine analogues bohemine and roscovitine, to study the role of Cdks in cell cycle progression and microtubule organisation in Vicia faba root tip cells. Both drugs inhibited the activity of immunopurified Vicia faba and alfalfa Cdc2-kinase. The transcript levels of an A- and B-type cyclin, as well as of the cdc2 genes, declined in treated root tips, while the mRNA level of a D-type cyclin gene was not affected. An observed transient arrest at the G1/S and G2/M regulatory points indicated that inhibition of the Cdc2-kinase had an effect on both transitions. In contrast to the regular bipolar spindle in untreated cell, in drug-treated metaphase cells abnormally short and dense kinetochore microtubule fibres were observed. These microtubules were randomly arranged in the vicinity of the kinetochores and connected the chromosomes. Thus, the chromosomes were not aligned on the metaphase plate but were arranged in a circle, with kinetochores pointing inwards and chromosome arms pointing outwards. gamma-Tubulin, which plays a role in microtubule nucleation, also localised to the centre of the monopolar spindle. The observed abnormalities in mitosis, after inhibition of Cdc2-kinase by specific drugs, suggest a role for this enzyme in regulating some of the steps leading to a bipolar spindle structure.

A cell cycle regulated MAP kinase with a possible role in cytokinesis in tobacco cells

Mitogen-activated protein (MAP) kinases have been demonstrated to have a role in meiosis but their involvement in mitotic events is less clear. Using a peptide antibody raised against the tobacco MAP kinase p43(Ntf6) and extracts from synchronized tobacco cell suspension cultures, we show that this kinase is activated specifically during mitosis. Entry into mitosis appears to be necessary for the activation of the kinase, which occurs as a post-translational event. The activation of the kinase occurs in late anaphase/early telophase. The p43(Ntf6) protein shows a transient localization to the cell plate in anaphase cells, in the middle of the two microtubule arrays characteristic of the phragmoplast, a plant-specific structure involved in laying down the new cell wall. The combined data support a role for the MAP kinase p43(Ntf6) in cytokinesis in tobacco cells.

Microtubule-organizing centers and nucleating sites in land plants

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

Mutations in the bimC box of Cut7 indicate divergence of regulation within the bimC family of kinesin related proteins

Members of the bimC family of kinesin related proteins (KRPs) play vital roles in the formation and function of the mitotic spindle. Although they share little amino acid homology outside the highly conserved microtubule motor domain, several family members do contain a ‘bimC box’, a sequence motif around a p34(cdc2) consensus phosphorylation site in their carboxy-terminal ‘tail’ region. One family member, Eg5, requires phosphorylation at this site for association with the mitotic spindle. We show that mutations in the Schizosaccharomyces pombe cut7+ gene that change the bimC box p34(cdc2) consensus phosphorylation site at position 1,011 and a neighbouring MAP kinase consensus phosphorylation site at position 1,020 to non-phosphorylatable residues did not affect the ability of S. pombe cut7 genes to complement temperature sensitive cut7 mutants. Phosphorylation site mutants expressed as fusions to green fluorescent protein associated with the mitotic spindle with a localisation indistinguishable from similarly expressed wild-type Cut7. Cells in which cut7.T1011A replaced the genomic copy of cut7+ were viable and formed normal spindles. Deletion of the entire carboxy-terminal tail region did not affect the ability of Cut7 to associate with the mitotic spindle but did inhibit normal spindle formation. Thus, unlike Eg5, neither the p34(cdc2) consensus phosphorylation site in the bimC box nor the entire tail region of Cut7 are required for association with the mitotic spindle.

Cytoskeletal control of polar growth in plant cells

There are two quite different modes of polar cell expansion in plant cells, namely, diffuse growth and tip growth. The direction of diffuse growth is determined by the orientation of cellulose microfibrils in the cell wall, which in turn are aligned by microtubules in the cell cortex. The orientation of the cortical microtubule array changes in response to developmental and environmental signals, and recent evidence indicates that microtubule disassembly/reassembly and microtubule translocation participate in reorientation of the array. Tip growth, in contrast, is governed mainly by F-actin, which has several putative forms and functions in elongating cells. Longitudinal cables are involved in vesicle transport to the expanding apical dome and, in some tip growers, a subapical ring of F-actin may participate in wall-membrane adhesions. The structure and function of F-actin within the apical dome may be variable, ranging from a dense meshwork to sparse single filaments. The presence of multiple F-actin structures in elongating tips suggests extensive regulation of this cytoskeletal array.

Structural organization of a gene encoding a novel calmodulin-binding kinesin-like protein from Arabidopsis

Kinesin-like calmodulin-binding protein (KCBP) is a recently identified microtubule motor protein that appears to be unique to plants. Here we report isolation and sequence analysis of a gene encoding Arabidopsis KCBP. KCBP gene contains 21 exons and 20 introns. All exons except exon 3 are short (94-272 nt). Exons 1-9 code for the globular tail region whereas the coiled-coil region is coded by exons 10-15. The conserved motor domain is coded by exons 16-20. Calmodulin-binding domain that is present in the C-terminal region of the protein and unique to KCBP is coded by the last exon. The size of introns ranged from 71 (intron 17) to 320 (intron 19) nucleotides. As in most plant introns, the content of AT is very high in all introns (up to 76%). Phylogenetic analysis of KCBP using motor domain sequence grouped KCBP with other known C-terminal microtubule motor proteins. However, Arabidopsis KCBP together with its homologs from potato and tobacco constitute a distinct group within the C-terminal subfamily of motors which is consistent with structural and functional features of KCBP.

The Arabidopsis KNOLLE protein is a cytokinesis-specific syntaxin

In higher plant cytokinesis, plasma membrane and cell wall originate by vesicle fusion in the plane of cell division. The Arabidopsis KNOLLE gene, which is required for cytokinesis, encodes a protein related to vesicle-docking syntaxins. We have raised specific rabbit antiserum against purified recombinant KNOLLE protein to show biochemically and by immunoelectron microscopy that KNOLLE protein is membrane associated. Using immunofluorescence microscopy, KNOLLE protein was found to be specifically expressed during mitosis and, unlike the plasma membrane H+-ATPase, to localize to the plane of division during cytokinesis. Arabidopsis dynamin-like protein ADL1 accumulates at the plane of cell plate formation in knolle mutant cells as in wild-type cells, suggesting that cytokinetic vesicle traffic is not affected. Furthermore, electron microscopic analysis indicates that vesicle fusion is impaired. KNOLLE protein was detected in mitotically dividing cells of various parts of the developing plant, including seedling root, inflorescence meristem, floral meristems and ovules, and the cellularizing endosperm, but not during cytokinesis after the male second meiotic division. Thus, KNOLLE is the first syntaxin-like protein that appears to be involved specifically in cytokinetic vesicle fusion.

The mechanism and control of cytokinesis

Cytokinesis is under active investigation in each of the dominant experimental model systems. During 1996 and 1997, several developments necessitated the reassessment of the prevailing model for cytokinesis. In addition, the inventory of proteins required for cytokinesis has grown considerably. However, a molecular understanding of cytokinesis still remains elusive.