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

Two microtubule-associated proteins of the Arabidopsis MAP65 family function differently on microtubules

The organization and dynamics of microtubules are regulated by microtubule-associated proteins, or MAPs. In Arabidopsis (Arabidopsis thaliana), nine genes encode proteins of the evolutionarily conserved MAP65 family. We proposed that different MAP65s might have distinct roles in the interaction with microtubules. In this study, two AtMAP65 proteins, AtMAP65-1 and AtMAP65-6, were chosen to test this hypothesis in vitro. Although both fusion proteins were able to cosediment with microtubules in vitro, different properties on tubulin polymerization and microtubule bundling were observed. AtMAP65-1 was able to promote tubulin polymerization, enhance microtubule nucleation, and decrease the critical concentration for tubulin polymerization. It also induced the formation of large microtubule bundles by forming cross-bridges between microtubules evenly along the whole length of microtubules. In the presence of AtMAP65-1, microtubule bundles were more resistant to cold and dilution treatments. AtMAP65-6, however, demonstrated no activity in promoting tubulin polymerization and stabilizing preformed microtubules. AtMAP65-6 induced microtubules to form a mesh-like network with individual microtubules. Cross-bridge-like interactions were only found at regional sites between microtubules. The microtubule network induced by AtMAP65-6 was more resistant to high concentration of NaCl than the bundles induced by AtMAP65-1. Purified monospecific anti-AtMAP65-6 antibodies revealed that AtMAP65-6 was associated with mitochondria in Arabidopsis cells. It was concluded that these two MAP65 proteins were targeted to distinct sites, thus performing distinct functions in Arabidopsis cells.

Identification of a novel family of 70 kDa microtubule-associated proteins in Arabidopsis cells

Most plant microtubule-associated proteins (MAPs) have homologues across the phylogenetic spectrum. To find potential plant-specific MAPs that will have evaded bioinformatic searches we devised a low stringency method for isolating proteins from an Arabidopsis cell suspension on endogenous taxol-microtubules. By tryptic peptide mass fingerprinting we identified 55 proteins that were enriched on taxol-microtubules. Amongst a range of known MAPs, such as kinesins, MAP65 isoforms and MOR1, we detected 'unknown' 70 kDa proteins that belong to a family of five closely related Arabidopsis proteins having no known homologues amongst non-plant organisms. To verify that AtMAP70-1 associates with microtubules in vivo, it was expressed as a GFP fusion. This confirmed that the protein decorates all four microtubule arrays in both transiently infected Arabidopsis and stably transformed tobacco BY-2 suspension cells. Microtubule-directed drugs perturbed the localization of AtMAP70-1 but cytochalasin D did not. AtMAP70-1 contains four predicted coiled-coil domains and truncation studies identified a central domain that targets the fusion protein to microtubules in vivo. This study therefore introduces a novel family of plant-specific proteins that interact with microtubules.

Identification and characterization of plasma membrane proteins that bind to microtubules in pollen tubes and generative cells of tobacco

The organization and function of microtubules in plant cells are important in many developmental stages. Connections between microtubules and the endomembrane system of plant cells have been discovered by microscopy, but the molecular characteristics of these relationships are mostly unknown except for a few cases. Using two antibodies raised against microtubule-associated proteins (MAPs) from maize, we have identified two polypeptides that share properties of the MAP family in the pollen tube of Nicotiana tabacum. The two polypeptides (with an apparent Mr of 161 and 90 kDa) bind efficiently to animal and plant microtubules and are found in association with the cellular membranes of the pollen tube, from which they can be solubilized with a zwitterionic detergent. One of these proteins has been purified and shown to promote the assembly of tubulin and, to a lesser extent, the bundling of microtubules. Subcellular fractionation indicated that the two proteins are associated with the plasma membrane compartment. The two proteins are found to co-localize in situ with cortical microtubules in the vegetative cytoplasm of tobacco pollen tubes; co-localization is also evident in the generative cell. According to these data, both the 161 and 90 kDa polypeptides are likely to mediate the interactions between the plasma membrane and microtubules in pollen tubes. In addition, functional data indicate that these MAP-like proteins take part in the process of microtubule assembly and reorganization occurring during cell growth. The evidence that both proteins associate with different cellular compartments also suggests a broad-spectrum role in mediating the dynamic relationships between microtubules and plant cell membranes.

In vivo dynamics and differential microtubule-binding activities of MAP65 proteins

Plant cells produce different microtubule arrays that are essential for cell division and morphogenesis without equivalent in other eukaryotes. Microtubule-associated proteins influence the behavior of microtubules that is presumed to culminate into transitions from one array to another. We analyzed the microtubule-binding properties of three Arabidopsis (Arabidopsis thaliana) members, AtMAP65-1, AtMAP65-4, and AtMAP65-5, in live cells using laser scanning confocal microscopy. Depending on the overall organization of the cortical array, AtMAP65-1-GFP (green fluorescent protein) and AtMAP65-5-GFP associated with a subset of microtubules. In cells containing both coaligned and oblique microtubules, AtMAP65-1-GFP and AtMAP65-5-GFP tended to be associated with the coaligned microtubules. Cortical microtubules labeled with AtMAP65-1-GFP and AtMAP65-5-GFP appeared as thick bundles and showed more resistance to microtubule-destabilizing drugs. The polymerization rates of AtMAP65-1-GFP and AtMAP65-5-GFP microtubules were similar to those of tubulin-GFP marked microtubules but were different from AtEB1a-GFP, a microtubule plus-end-binding EB1-like protein that stimulated polymerization. By contrast, depolymerization rates of AtMAP65-1-GFP- and AtMAP65-5-GFP-labeled microtubules were reduced. AtMAP65-1-GFP associated with polymerizing microtubules within a bundle, and with fixed microtubule termini, suggesting that AtMAP65-1's function is to bundle and stabilize adjacent microtubules of the cortex. Polymerization within a bundle took place in either direction so that bundling occurred between parallel or antiparallel aligned microtubules. AtMAP65-4-GFP did not label cortical microtubules or the preprophase band, despite continuous expression driven by the 35S promoter, and its subcellular localization was restricted to microtubules that rearranged to form a spindle and the polar sides of the spindle proper. The expression of AtMAP65-4 peaked at mitosis, in agreement with a function related to spindle formation, whereas AtMAP65-1 and AtMAP65-5 were expressed throughout the cell cycle.

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.

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

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

The plant microtubule-associated protein AtMAP65-3/PLE is essential for cytokinetic phragmoplast function

Directional cell expansion in interphase and nuclear and cell division in M-phase are mediated by four microtubule arrays, three of which are unique to plants: the interphase array, the preprophase band, and the phragmoplast. The plant microtubule-associated protein MAP65 has been identified as a key structural component in these arrays. The Arabidopsis genome has nine MAP65 genes, and here we show that one, AtMAP65-3/PLE, locates only to the mitotic arrays and is essential for cytokinesis. The Arabidopsis pleiade (ple) alleles are single recessive mutations, and we show that these mutations are in the AtMAP65-3 gene. Moreover, these mutations cause C-terminal truncations that abolish microtubule binding. In the ple mutants the anaphase spindle is normal, and the cytokinetic phragmoplast can form but is distorted; not only is it wider, but the midline, the region where oppositely oriented microtubules overlap, is unusually expanded. Here we present data that demonstrate an essential role for AtMAP65-3/PLE in cytokinesis in plant cells.

Microtubules and the shape of plants to come

Plants control the direction of cell expansion as a way of shaping growth. Since their discovery in plants 40 years ago, microtubules have been suspected of forming a template that helps to regulate the direction of growth. The detailed mechanism, however, has been elusive, especially as plants lack a microtubule-organizing centre. Developmental mutants are now beginning to show how microtubules are organized and how this affects plant morphology.

Re-organisation of the cytoskeleton during developmental programmed cell death in Picea abies embryos

Cell and tissue patterning in plant embryo development is well documented. Moreover, it has recently been shown that successful embryogenesis is reliant on programmed cell death (PCD). The cytoskeleton governs cell morphogenesis. However, surprisingly little is known about the role of the cytoskeleton in plant embryogenesis and associated PCD. We have used the gymnosperm, Picea abies, somatic embryogenesis model system to address this question. Formation of the apical-basal embryonic pattern in P. abies proceeds through the establishment of three major cell types: the meristematic cells of the embryonal mass on one pole and the terminally differentiated suspensor cells on the other, separated by the embryonal tube cells. The organisation of microtubules and F-actin changes successively from the embryonal mass towards the distal end of the embryo suspensor. The microtubule arrays appear normal in the embryonal mass cells, but the microtubule network is partially disorganised in the embryonal tube cells and the microtubules disrupted in the suspensor cells. In the same embryos, the microtubule-associated protein, MAP-65, is bound only to organised microtubules. In contrast, in a developmentally arrested cell line, which is incapable of normal embryonic pattern formation, MAP-65 does not bind the cortical microtubules and we suggest that this is a criterion for proembryogenic masses (PEMs) to passage into early embryogeny. In embryos, the organisation of F-actin gradually changes from a fine network in the embryonal mass cells to thick cables in the suspensor cells in which the microtubule network is completely degraded. F-actin de-polymerisation drugs abolish normal embryonic pattern formation and associated PCD in the suspensor, strongly suggesting that the actin network is vital in this PCD pathway.

Identification of a MAP65 isoform involved in directional expansion of plant cells

MAP65 comprises a multigene family specific to plants. To see which isoform is utilised for the unique mechanism of cell expansion, uncomplicated by division structures, carrot cells were deprived of auxin whereupon they stopped dividing and elongated instead. During elongation, a MAP65 protein triplet reduced to a single band. Mass spectrometric analysis demonstrated that this corresponded to a single carrot cDNA; it also corresponded to the major protein previously shown to form filamentous cross-bridges between microtubules in vitro. This MAP65 isoform is concluded to have a major role in establishing the parallel microtubule arrays characteristic of cells undergoing directional expansion.

Microtubule organization in the green kingdom: chaos or self-order?

Plant microtubule arrays differ fundamentally from their animal, fungal and protistan counterparts. These differences largely reflect the requirements of plant composite polymer cell walls and probably also relate to the acquisition of chloroplasts. Plant microtubules are usually dispersed and lack conspicuous organizing centres. The key to understanding this dispersed nature is the identification of proteins that interact with and regulate the spatial and dynamic properties of microtubules. Over the past decade, a number of these proteins have been uncovered, including numerous kinesin-related proteins and a 65 kDa class of structural microtubule-associated proteins that appear to be unique to plants. Mutational analysis has identified MOR1, a probable stabilizer of microtubules that is a homologue of the TOGp-XMAP215 class of high-molecular-weight microtubule-associated proteins, and a katanin p60 subunit homologue implicated in the severing of microtubules. The identification of these two proteins provides new insights into the mechanisms controlling microtubule assembly and dynamics, particularly in the dispersed cortical array found in highly polarized plant cells.

Identification of a rice RNA- and microtubule-binding protein as the multifunctional protein, a peroxisomal enzyme involved in the beta -oxidation of fatty acids

The control of subcellular mRNA localization and translation is often mediated by protein factors that are directly or indirectly associated with the cytoskeleton. We report the identification and characterization of a rice seed protein that possesses both RNA and microtubule binding activities. In vitro UV cross-linking assays indicated that this protein binds to all mRNA sequences tested, although there was evidence for preferential binding to RNAs that contained A-C nucleotide sequence motifs. The protein was purified to homogeneity using a two-step procedure, and amino acid sequencing identified it as the multifunctional protein (MFP), a peroxisomal enzyme known to possess a number of activities involved in the beta-oxidation of fatty acids. The recombinant version of this rice MFP binds to RNA in UV cross-linking and gel mobility shift experiments, co-sediments specifically with microtubules, and possesses at least two enzymatic activities involved in peroxisomal fatty acid beta-oxidation. Taken together these data suggest that MFP has an important role in mRNA physiology in the cytoplasm, perhaps in regulating the localization or translation of mRNAs through an interaction with microtubules, in addition to its peroxisomal function.

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.

"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.

Plant microtubule-associated proteins: the HEAT is off in temperature-sensitive mor1

Microtubules perform essential functions in plant cells and govern, with other cytoskeletal elements, cell division, formation of cell walls and morphogenesis. For microtubules to perform their roles in the cell their organization and dynamics must be regulated and microtubule-associated proteins bear the main responsibility for these activities. We are just beginning to identify these plant microtubule-regulating proteins. Biochemical, molecular and genetic procedures have identified plant homologues of known microtubule-associated proteins, such as kinesins, katanin and XMAP215, and novel classes of plant microtubule-associated proteins, such as MAP65 and MAP190. Showing how these proteins coordinate the microtubule cytoskeleton in vivo is now the challenge. The recent identification and characterization of the Arabidopsis thaliana microtubule organization mutant, mor1, begins to address this challenge and here we highlight the significance of this work.

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.

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.

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.

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 65-kDa carrot microtubule-associated protein forms regularly arranged filamentous cross-bridges between microtubules

In plants, cortical microtubules (MTs) occur in characteristically parallel groups maintained up to one microtubule diameter apart by fine filamentous cross-bridges. However, none of the plant microtubule-associated proteins (MAPs) so far purified accounts for the observed separation between MTs in cells. We previously isolated from carrot cytoskeletons a MAP fraction including 120- and 65-kDa MAPs and have now separated the 65-kDa carrot MAP by sucrose density centrifugation. MAP65 does not induce tubulin polymerization but induces the formation of bundles of parallel MTs in a nucleotide-insensitive manner. The bundling effect is inhibited by porcine MAP2, but, unlike MAP2, MAP65 is heat-labile. In the electron microscope, MAP65 appears as filamentous cross-bridges, maintaining an intermicrotubule spacing of 25-30 nm. Microdensitometer-computer correlation analysis reveals that the cross-bridges are regularly spaced, showing a regular axial spacing that is compatible with a symmetrical helical superlattice for 13 protofilament MTs. Because MAP65 maintains in vitro the inter-MT spacing observed in plants and is shown to decorate cortical MTs, it is proposed that this MAP is important for the organization of the cortical array in vivo.

In situ synthesis of beta-glucan microfibrils on tobacco plasma membrane sheets

A major concern in plant morphogenesis is whether cortical microtubules are responsible for the arrangement and action of beta-glucan synthases in the plasma membrane. We prepared isolated plasma membrane sheets with cortical microtubules attached and tested whether beta-glucan synthases penetrated through the membrane to form microfibrils and whether these synthases moved in the fluid membrane along the cortical microtubules. This technique enabled us to examine synthesis of beta-glucan as a fiber with a two-dimensional structure. The synthesis of beta-glucan microfibrils was directed in arrays by cortical microtubules at many loci on the membrane sheets. The microfibrils were mainly arranged along the microtubules, but the distribution of microfibrils was not always parallel to that of the microtubules. The rate of beta-glucan elongation as determined directly on the exoplasmic surface was 620 nm per min. When the assembly of microtubules was disrupted by treatment with propyzamide, the beta-glucans were not deposited in arrays but in masses. This finding shows that the arrayed cortical microtubules are not required for beta-glucan synthesis but are required for the formation of arranged microfibrils on the membrane sheet.

Suppression of endogenous alpha and beta tubulin synthesis in transgenic maize calli overexpressing alpha and beta tubulins

Maize Black Mexican Sweetcorn cells have been transformed with constructs containing alpha and beta tubulin coding sequences either singly or together. It is shown that recovery of stable maize transformants is dependent on the co-expression of transfected alpha and beta tubulin in the same lines, indicating that plant cells cannot tolerate an imbalance in the ratio of alpha tubulin to beta tubulin within the cytoplasm. The co-expression of transfected alpha and beta tubulin in maize cells results in an increase in the overall tubulin content (approximately threefold). The transfected alpha and beta tubulins are incorporated into cortical, spindle and phragmoplast microtubule arrays indicating that they are functional. Furthermore, the co-expression of the transfected alpha and beta tubulins results in the suppression of endogenous alpha and beta tubulin synthesis. This suppression increases both with the strength of the promoter in the constructs and with the number of copies of the transgenes inserted into the maize genome. The implications for the post-transcriptional and post-translational regulation of tubulin synthesis in plant cells are discussed.

Herbicide resistance caused by spontaneous mutation of the cytoskeletal protein tubulin

The dinitroaniline herbicides (such as trifluralin and oryzalin) have been developed for the selective control of weeds in arable crops. However, prolonged use of these chemicals has resulted in the selection of resistant biotypes of goosegrass, a major weed. These herbicides bind to the plant tubulin protein but not to mammalian tubulin. Here we show that the major alpha-tubulin gene of the resistant biotype has three base changes within the coding sequence. These base changes swap cytosine and thymine, most likely as the result of the spontaneous deamination of methylated cytosine. One of these base changes causes an amino-acid change in the protein: normal threonine at position 239 is changed to isoleucine. This position is close to the site of interaction between tubulin dimers in the microtubule protofilament. We show that the mutated gene is the cause of the herbicide resistance by using it to transform maize and confer resistance to dinitroaniline herbicides. Our results provide a molecular explanation for the resistance of goosegrass to dinitroanaline herbicides, a phenomenon that has arisen, and been selected for, as a result of repeated exposure to this class of herbicide.

Functional interactions among cytoskeleton, membranes, and cell wall in the pollen tube of flowering plants

The pollen tube is a cellular system that plays a fundamental role during the process of fertilization in higher plants. Because it is so important, the pollen tube has been subjected to intensive studies with the aim of understanding its biology. The pollen tube represents a fascinating model for studying interactions between the internal cytoskeletal machinery, the membrane system, and the cell wall. These compartments, often studied as independent units, show several molecular interactions and can influence the structure and organization of each other. The way the cell wall is constructed, the dynamics of the endomembrane system, and functions of the cytoskeleton suggest that these compartments are a molecular "continuum," which represents a link between the extracellular environment and the pollen tube cytoplasm. Several experimental approaches have been used to understand how these interactions may translate the pollen-pistil interactions into differential processes of pollen tube growth.

A 60-kDa plant microtubule-associated protein promotes the growth and stabilization of neurotubules in vitro

The search for microtubule-associated proteins (MAPs) in plants is relatively recent. In particular, the "classical MAPs," which stimulate the polymerization and stabilization of microtubules, have only been examined in heterogeneous fractions. As a first step in dissecting the role of individual MAPs, we have chromatographically purified a single 60-kDa protein from a carrot MAP fraction and analyzed its effects on tubulin assembly. MAP60 promoted the formation of long, morphologically regular brain microtubules in vitro, an effect inhibited by preincubation of the MAP with affinity-purified antibodies against this protein. MAP60 also increased the stability of microtubules to dilution and significantly enhanced cold stability to the normally cold-sensitive neurotubules. These in vitro properties are consistent with a role for MAP60 in regulating the turnover/assembly of dynamic plant microtubules in vivo.

Cell model systems in plant cytoskeleton studies

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

Cold-stable and cold-adapted microtubules

Most mammalian microtubules disassemble at low temperature, but some are cold stable. This probably has little to do with a need for cold-stable microtubules, but reflects that certain populations of microtubules must be stabilized for specific functions. There are several routes by which to achieve cold stability. Factors that interact with microtubules, such as microtubule-associated proteins, STOPs (stable tubule only polypeptides), histones, and possibly capping factors, are involved. Specific tubulin isotypes and posttranslational modifications might also be of importance. More permanent stable microtubules can be achieved by bundling factors, associations to membranes, as well as by assembly of microtubule doublets and triplets. This is, however, not the explanation for cold adaptation of microtubules from poikilothermic animals, that is, animals that must have all their microtubules adapted to low temperatures. All evidence so far suggests that cold adaptation is intrinsic to the tubulins, but it is unknown whether it depends on different amino acid sequences or posttranslational modifications.

Antibody against phosphorylated proteins (MPM-2) recognizes mitotic microtubules in endosperm cells of higher plant Haemanthus

In diverse cell types, monoclonal antibody MPM-2 recognizes a class of phosphorylated proteins related to microtubule organizing centers and abundant during mitosis. We have used this antibody in an attempt to identify the spatial and temporal localization of putative microtubule organizing centers in endosperm cells of the higher plant Haemanthus. Our results show that MPM-2 recognized epitope is present in interphase cells and enriched in mitotic cells. In interphase the antibody usually stains cytoplasmic granules. During the interphase-prophase transition immunoreactive material appears in the nucleus, at the nuclear envelope, and in association with microtubules. Concomitantly, we observed an increase of immunoreactivity of the cytoplasm. During mitosis the phosphorproteins recognized by MPM-2 are detected in the cytoplasm, in association with microtubules of the spindle, the phragmoplast, and in the newly-formed cell plate. After completion of mitosis, only the cell plate and cytoplasmic granules are MPM-2 positive. Extraction of the cells with Triton X-100 prior to fixation removes staining of the cytoplasm by MPM-2. The detergent resistant immunoreactive material remains associated with surrounding the nucleus microtubules of the prophase spindle, the core of kinetochore fibers, and the phragmoplast. In the phragmoplast, however, segments of microtubules which are distal to the cell plate are depleted of MPM-2. These data demonstrate that microtubule arrays of endosperm cells are phosphorylated during mitosis. Thus, similar to animal cells, interphase and mitotic microtubules of higher plants have different properties. Additionally, the localization of detergent resistant MPM-2 antigen points to the difference in microtubule nucleation/organization between higher plant and animal cells.

Characterization of a 100-kDa heat-stable microtubule-associated protein from higher plants

In higher-plant cells, the different cell-cycle-dependent microtubule arrays are involved in a wide range of activities including chromosome segregation, cell-plate formation and cellulose microfibril distribution and orientation. A wealth of data, obtained using animal cells, has indicated that the differential stability and function of microtubules during cell-cycle and/or differentiation could be primarily regulated by selective microtubule-associated proteins (MAP). Compared to animal MAP, our knowledge of plant MAP is so far very limited. In this study, we have identified a maize heat-stable protein with apparent molecular mass 100 kDa (P-100) which binds to taxol-stabilized neurotubules and copolymerizes in vitro with purified neural tubulin. Moreover, P-100 cross-reacts with affinity-purified tau antibodies like a maize 83-kDa putative MAP described previously [Vantard, M., Schellenbaum, P., Fellous, A. & Lambert, A. M. (1991) Biochemistry 30, 9334-9340]. Polyclonal antibodies directed against P-100 were obtained and indicated that this protein is found in diverse higher-plant cultured cells suggesting the ubiquitous nature of this protein. P-100 can be phosphorylated in vitro by protein kinases present in a maize cytosol extract. Together, our data suggest that P-100 could be a higher plant MAP.

The plant cytoskeleton

Particles that can nucleate microtubules in vitro have been isolated from higher plant cells. Observations of living cells injected with fluorescent probes have improved our understanding of plant cytoskeleton dynamics. Despite growing recognition of the need for biochemical studies on cytoskeleton-associated proteins, little progress has been made in this field in the past year, although plant lamins have been isolated and partially characterized.