Microtubules and microtubule-associated proteins

Mechanism of scaffolding-directed virus assembly suggested by comparison of scaffolding-containing and scaffolding-lacking P22 procapsids

Assembly of certain classes of bacterial and animal viruses requires the transient presence of molecules known as scaffolding proteins, which are essential for the assembly of the precursor procapsid. To assemble a procapsid of the proper size, each viral coat subunit must adopt the correct quasiequivalent conformation from several possible choices, depending upon the T number of the capsid. In the absence of scaffolding protein, the viral coat proteins form aberrantly shaped and incorrectly sized capsids that cannot package DNA. Although scaffolding proteins do not form icosahedral cores within procapsids, an icosahedrally ordered coat/scaffolding interaction could explain how scaffolding can cause conformational differences between coat subunits. To identify the interaction sites of scaffolding protein with the bacteriophage P22 coat protein lattice, we have determined electron cryomicroscopy structures of scaffolding-containing and scaffolding-lacking procapsids. The resulting difference maps suggest specific interactions of scaffolding protein with only four of the seven quasiequivalent coat protein conformations in the T = 7 P22 procapsid lattice, supporting the idea that the conformational switching of a coat subunit is regulated by the type of interactions it undergoes with the scaffolding protein. Based on these results, we propose a model for P22 procapsid assembly that involves alternating steps in which first coat, then scaffolding subunits form self-interactions that promote the addition of the other protein. Together, the coat and scaffolding provide overlapping sets of binding interactions that drive the formation of the procapsid.

The growth-related, translationally controlled protein P23 has properties of a tubulin binding protein and associates transiently with microtubules during the cell cycle

The translationally controlled protein P23 was discovered by the early induction of its rate of synthesis after mitogenic stimulation of mouse fibroblasts. P23 is expressed in almost all mammalian tissues and it is highly conserved between animals, plants and yeast. Based on its amino acid sequence, P23 cannot be attributed to any known protein family, and its cellular function remains to be elucidated. Here, we present evidence that P23 has properties of a tubulin binding protein that associates with microtubules in a cell cycle-dependent manner. (1) P23 is a cytoplasmic protein that occurs in complexes of 100–150 kDa, and part of P23 can be immunoprecipitated from HeLa cell extracts with anti-tubulin antibodies. (2) In immunolocalisation experiments we find P23 associated with microtubules during G1, S, G2 and early M phase of the cell cycle. At metaphase, P23 is also bound to the mitotic spindle, and it is detached from the spindle during metaphase-anaphase transition. (3) A GST-P23 fusion protein interacts with alpha- and beta-tubulin, and recombinant P23 binds to taxol-stabilised microtubules in vitro. The tubulin binding domain of P23 was identified by mutational analysis; it shows similarity to part of the tubulin binding domain of the microtubule-associated protein MAP-1B. (4) Overexpression of P23 results in cell growth retardation and in alterations of cell morphology. Moreover, elevation of P23 levels leads to microtubule rearrangements and to an increase in microtubule mass and stability.

Confocal microscopy and 3-D reconstruction of the cytoskeleton of Xenopus oocytes

Xenopus oocytes contain a complex cytoskeleton composed of three filament systems: (1) microtubules, composed of tubulin and at least three different microtubule-associated proteins (XMAPs); (2) microfilaments composed of actin and associated proteins; and (3) intermediate filaments, composed of keratins. For the past several years, we have used confocal immunofluorescence microscopy to characterize the organization of the oocyte cytoskeleton throughout the course of oogenesis. Together with computer-assisted reconstruction of the oocyte in three dimensions, confocal microscopy gives an unprecedented view of the assembly and reorganization of the cytoskeleton during oocyte growth and differentiation. Results of these studies, combined with the effects of cytoskeletal inhibitors, suggest that organization of the cytoskeleton in Xenopus oocytes is dependent upon a hierarchy of interactions between microtubules, microfilaments, and keratin filaments. This article presents a gallery of confocal images and 3-D reconstructions depicting the assembly and organization of the oocyte cytoskeleton during stages 0-VI of oogenesis, a discussion of the mechanisms that might regulate cytoskeletal organization during oogenesis, and speculates on the potential roles of the oocyte cytoskeleton during oogenesis and axis formation.

Structural characterization by tandem mass spectrometry of the posttranslational polyglycylation of tubulin

Polyglycylation is a posttranslational modification specific to tubulin. This modification was originally identified in highly stable microtubules from Paramecium cilia. As many as 34 posttranslationally added glycine residues have been located in the C-terminal domains of Paramecium alpha- and beta-tubulin. In this study, post source decay matrix-assisted laser desorption/ionization mass spectrometry (PSD MALDI MS) and electrospray ionization on a hybrid quadrupole orthogonal time-of-flight tandem mass spectrometer (ESI Q-TOF MS/MS) were both used to demonstrate that a single molecule of beta-tubulin, from either dynamic cytoplasmic microtubules or stable axonemal microtubules, can be glycylated on each of the last four C-terminal glutamate residues Glu437, Glu438, Glu439, and Glu441 in the sequence 427DATAEEEGEFEEEGEQ442. In both dynamic and stable microtubules the most abundant beta-tubulin isoform contains six posttranslationally added glycine residues: two glycine residues on both Glu437 and Glu438 and one glycine residue on both Glu439 and Glu441. The number and relative abundance of glycylated isoforms of beta-tubulin in both cytoplasmic and axonemal microtubules were compared by MALDI MS.1 The abundance of the major glycylated isoforms in axonemal tubulin decreases regularly with glycylation levels from 6 to 19 whereas it drops abruptly in cytoplasmic tubulin with glycylation levels from 6 to 9. However, the polyglycine chains are similarly distributed on the four C-terminal glutamate residues of cytoplasmic and axonemal tubulin. The polyglycylation results in bulky C-terminal domains with negatively charged surfaces, all surrounding the microtubular structure.

Mutations at phosphorylation sites of Xenopus microtubule-associated protein 4 affect its microtubule-binding ability and chromosome movement during mitosis

Microtubule-associated proteins (MAPs) bind to and stabilize microtubules (MTs) both in vitro and in vivo and are thought to regulate MT dynamics during the cell cycle. It is known that p220, a major MAP of Xenopus, is phosphorylated by p34(cdc2) kinase as well as MAP kinase in mitotic cells, and that the phosphorylated p220 loses its MT-binding and -stabilizing abilities in vitro. We cloned a full-length cDNA encoding p220, which identified p220 as a Xenopus homologue of MAP4 (XMAP4). To examine the physiological relevance of XMAP4 phosphorylation in vivo, Xenopus A6 cells were transfected with cDNAs encoding wild-type or various XMAP4 mutants fused with a green fluorescent protein. Mutations of serine and threonine residues at p34(cdc2) kinase-specific phosphorylation sites to alanine interfered with mitosis-associated reduction in MT affinity of XMAP4, and their overexpression affected chromosome movement during anaphase A. These findings indicated that phosphorylation of XMAP4 (probably by p34(cdc2) kinase) is responsible for the decrease in its MT-binding and -stabilizing abilities during mitosis, which are important for chromosome movement during anaphase A.

Microtubule-dependent plus- and minus end-directed motilities are competing processes for nuclear targeting of adenovirus

Adenovirus (Ad) enters target cells by receptor-mediated endocytosis, escapes to the cytosol, and then delivers its DNA genome into the nucleus. Here we analyzed the trafficking of fluorophore-tagged viruses in HeLa and TC7 cells by time-lapse microscopy. Our results show that native or taxol-stabilized microtubules (MTs) support alternating minus- and plus end-directed movements of cytosolic virus with elementary speeds up to 2.6 micrometer/s. No directed movement was observed in nocodazole-treated cells. Switching between plus- and minus end-directed elementary speeds at frequencies up to 1 Hz was observed in the periphery and near the MT organizing center (MTOC) after recovery from nocodazole treatment. MT-dependent motilities allowed virus accumulation near the MTOC at population speeds of 1-10 micrometer/min, depending on the cell type. Overexpression of p50/dynamitin, which is known to affect dynein-dependent minus end-directed vesicular transport, significantly reduced the extent and the frequency of minus end-directed migration of cytosolic virus, and increased the frequency, but not the extent of plus end-directed motility. The data imply that a single cytosolic Ad particle engages with two types of MT-dependent motor activities, the minus end- directed cytoplasmic dynein and an unknown plus end- directed activity.

Upregulation and redistribution of E-MAP-115 (epithelial microtubule-associated protein of 115 kDa) in terminally differentiating keratinocytes is coincident with the formation of intercellular contacts

Microtubules are involved in the positioning and movement of organelles and vesicles and therefore play fundamental roles in cell polarization and differentiation. Their organization and properties are cell-type specific and are controlled by microtubule-associated proteins (MAP). E-MAP-115 (epithelial microtubule-associated protein of 115 kDa) has been identified as a microtubule-stabilizing protein predominantly expressed in epithelial cells. We have used human skin and primary keratinocytes as a model to assess a putative function of E-MAP-115 in stabilizing and reorganizing the microtubule network during epithelial cell differentiation. Immunolabeling of skin sections indicated that E-MAP-115 is predominantly expressed in the suprabasal layers of the normal epidermis and, in agreement with this observation, is relatively abundant in squamous cell carcinomas but barely detectable in basal cell carcinomas. In primary keratinocytes whose terminal differentiation was induced by increasing the Ca2+ concentration of the medium, E-MAP-115 expression significantly increased during the first day, as observed by northern and western blot analysis. Parallel immunofluorescence studies showed an early redistribution of E-MAP-115 from microtubules with a paranuclear localization to cortical microtubules organized in spike-like bundles facing intercellular contacts. This phenomenon is transient and can be reversed by Ca2+ depletion. Treatment of cells with cytoskeleton-active drugs after raising the Ca2+ concentration indicated that E-MAP-115 is associated with a subset of stable microtubules and that the cortical localization of these microtubules is dependent on other microtubules but not on strong interactions with the actin cytoskeleton or the plasma membrane. The mechanism whereby E-MAP-115 would redistribute to and stabilize cortical microtubules used for the polarized transport of vesicles towards the plasma membrane, where important reorganizations take place upon stratification, is discussed.

Role of microtubules in the organization of the Golgi complex

The Golgi complex of mammalian cells is composed of cisternal stacks that function in processing and sorting of membrane and luminal proteins during transport from the site of synthesis in the endoplasmic reticulum to lysosomes, secretory vacuoles, and the cell surface. Even though exceptions are found, the Golgi stacks are usually arranged as an interconnected network in the region around the centrosome, the major organizing center for cytoplasmic microtubules. A close relation thus exists between Golgi elements and microtubules (especially the stable subpopulation enriched in detyrosinated and acetylated tubulin). After drug-induced disruption of microtubules, the Golgi stacks are disconnected from each other, partly broken up, dispersed in the cytoplasm, and redistributed to endoplasmic reticulum exit sites. Despite this, intracellular protein traffic is only moderately disturbed. Following removal of the drugs, scattered Golgi elements move along reassembling microtubules back to the centrosomal region and reunite into a continuous system. The microtubule-dependent motor proteins cytoplasmic dynein and kinesin bind to Golgi membranes and have been implicated in vesicular transport to and from the Golgi complex. Microinjection of dynein heavy chain antibodies causes dispersal of the Golgi complex, and the Golgi complex of cells lacking cytoplasmic dynein is likewise spread throughout the cytoplasm. In a similar manner, kinesin antibodies have been found to inhibit Golgi-to-endoplasmic reticulum transport in brefeldin A-treated cells and scattering of Golgi elements along remaining microtubules in cells exposed to a low concentration of nocodazole. The molecular mechanisms in the interaction between microtubules and membranes are, however, incompletely understood. During mitosis, the Golgi complex is extensively reorganized in order to ensure an equal partitioning of this single-copy organelle between the daughter cells. Mitosis-promoting factor, a complex of cdc2 kinase and cyclin B, is a key regulator of this and other events in the induction of cell division. Cytoplasmic microtubules depolymerize in prophase and as a result thereof, the Golgi stacks become smaller, disengage from each other, and take up a perinuclear distribution. The mitotic spindle is thereafter put together, aligns the chromosomes in the metaphase plate, and eventually pulls the sister chromatids apart in anaphase. In parallel, the Golgi stacks are broken down into clusters of vesicles and tubules and movement of protein along the exocytic and endocytic pathways is inhibited. Using a cell-free system, it has been established that the fragmentation of the Golgi stacks is due to a continued budding of transport vesicles and a concomitant inhibition of the fusion of the vesicles with their target membranes. In telophase and after cytokinesis, a Golgi complex made up of interconnected cisternal stacks is recreated in each daughter cell and intracellular protein traffic is resumed. This restoration of a normal interphase morphology and function is dependent on reassembly of a radiating array of cytoplasmic microtubules along which vesicles can be carried and on reactivation of the machinery for membrane fusion.

XMAP230 is required for the organization of cortical microtubules and patterning of the dorsoventral axis in fertilized Xenopus eggs

The dorsoventral axis of Xenopus embryos is specified by a rotation of the egg cortex relative to the underlying yolky cytoplasm. This cortical rotation, which occurs during the first cell cycle after fertilization, is dependent upon an array of parallel microtubules in the subcortical cytoplasm. We have used confocal immunofluorescent microscopy and microinjection of affinity-purified anti-XMAP230 antibody to address the role of XMAP230, one of three high-molecular-weight microtubule-associated proteins (MAPs) in Xenopus eggs, in the assembly and organization of the cortical microtubule array and specification of the dorsoventral axis. Confocal immunofluorescence microscopy revealed that XMAP230 was associated with cortical microtubules shortly after their appearance in the subcortical cytoplasm. XMAP230 staining became more prominent as microtubules were aligned and bundled during the cortical rotation. Loss of XMAP230 appeared to precede disassembly of cortical microtubules at the end of the first cell cycle. Deeper within the cytoplasm, XMAP230 was associated with microtubules early in the assembly of the sperm aster. However, later in the first cell cycle, XMAP230 was associated with microtubules (MTs) of the first mitotic spindle, spindle asters, and the cortical MTs, but not with microtubule remnants of the sperm aster. Microinjection of anti-XMAP230 antibody locally disrupted the assembly and organization of microtubules in the cortex of activated or fertilized eggs and resulted in defects in the dorsoventral patterning of embryos. These results indicate that the assembly and/or organization of cortical microtubules in fertilized Xenopus eggs and subsequent specification of the dorsoventral axis are dependent upon XMAP230.

Dynamic localization of CLIP-170 to microtubule plus ends is coupled to microtubule assembly

CLIP-170 is a cytoplasmic linker protein that localizes to plus ends of microtubules in vivo. In this study, we have characterized the microtubule-binding properties of CLIP-170, to understand the mechanism of its plus end targeting. We show that the NH2-terminal microtubule-interacting domain of CLIP-170 alone localizes to microtubule plus ends when transfected into cells. Association of CLIP-170 with newly-formed microtubules was observed in cells microinjected with biotinylated tubulin, used as a tracer for growing microtubules. Using in vitro assays, association of CLIP-170 with recently polymerized tubulin is also seen. Cross-linking and sedimentation velocity experiments suggest association of CLIP-170 with nonpolymerized tubulin. We conclude from these experiments that the microtubule end targeting of CLIP-170 is closely linked to tubulin polymerization.

The microtubule-associated protein tau cross-links to two distinct sites on each alpha and beta tubulin monomer via separate domains

The interaction between tubulin subunits and microtubule-associated proteins (MAPs) such as tau is fundamental for microtubule structure and function. Previous work has suggested that the "microtubule binding domain" of tau (composed of three or four imperfect 18-amino acid repeats, separated by 13- or 14-amino acid inter-repeat regions) can bind to the C-terminal ends of both alpha and beta tubulin monomers. Here, using covalent cross-linking strategies, we demonstrate that there are two distinct tau cross-linking sites (designated as "C-terminal" and "internal") on each alpha and beta tubulin monomer. The C-terminal tau cross-linking site is located within the 12 C-terminal amino acids of both alpha and beta tubulin, while the internal tau cross-linking site is located within the C-terminal one-third of alpha and beta tubulin but not within the last 12 amino acids. In addition, we show that tau cross-links to the C-terminal site via its repeat 1 and/or the R1-R2 inter-repeat. The cross-linking of tau to the internal site is mediated by some subset of its other repeat units. Integrating these and earlier data with the 3.7 A resolution model of the alphabeta tubulin dimer recently presented by E. Nogales et al. [(1998), Nature 391, 199-203], we propose a new model for the tau-microtubule interaction.

Overexpression of tau protein inhibits kinesin-dependent trafficking of vesicles, mitochondria, and endoplasmic reticulum: implications for Alzheimer's disease

The neuronal microtubule-associated protein tau plays an important role in establishing cell polarity by stabilizing axonal microtubules that serve as tracks for motor-protein-driven transport processes. To investigate the role of tau in intracellular transport, we studied the effects of tau expression in stably transfected CHO cells and differentiated neuroblastoma N2a cells. Tau causes a change in cell shape, retards cell growth, and dramatically alters the distribution of various organelles, known to be transported via microtubule-dependent motor proteins. Mitochondria fail to be transported to peripheral cell compartments and cluster in the vicinity of the microtubule-organizing center. The endoplasmic reticulum becomes less dense and no longer extends to the cell periphery. In differentiated N2a cells, the overexpression of tau leads to the disappearance of mitochondria from the neurites. These effects are caused by tau's binding to microtubules and slowing down intracellular transport by preferential impairment of plus-end-directed transport mediated by kinesin-like motor proteins. Since in Alzheimer's disease tau protein is elevated and mislocalized, these observations point to a possible cause for the gradual degeneration of neurons.

Cytosolic free calcium and the cytoskeleton in the control of leukocyte chemotaxis

In response to a chemotactic gradient, leukocytes extravasate and chemotax toward the site of pathogen invasion. Although fundamental in the control of many leukocyte functions, the role of cytosolic free Ca2+ in chemotaxis is unclear and has been the subject of debate. Before becoming motile, the cell assumes a polarized morphology, as a result of modulation of the cytoskeleton by G protein and kinase activation. This morphology may be reinforced during chemotaxis by the intracellular redistribution of Ca2+ stores, cytoskeletal constituents, and chemoattractant receptors. Restricted subcellular distributions of signaling molecules, such as Ca2+, Ca2+/calmodulin, diacylglycerol, and protein kinase C, may also play a role in some types of leukocyte. Chemotaxis is an essential function of most cells at some stage during their development, and a deeper understanding of the molecular signaling and structural components involved will enable rational design of therapeutic strategies in a wide variety of diseases.

Involvement of precerebellar nuclei in Pick's disease

Pick's disease is a progressive degenerative disorder of the human brain which involves not only numerous areas of the cerebral cortex but also a characteristic set of subcortical nuclei. The disorder is associated with the formation of abnormal and hyperphosphorylated tau protein, which occurs in only a few susceptible neuronal types and leads to major cytoskeletal alterations. Preferentially affected by the destructive process are small nerve cells of both cortical areas and subcortical nuclei. Immunoreactions for abnormally phosphorylated tau protein permit identification of the alterations in their entirety. In an initial step in their development, patches of a nonargyrophilic material appear, irregularly filling both the somata and neurites of afflicted cells. The abnormal material is then partially converted into condensed spindle-shaped or spherical structures, which gradually become significantly argyrophilic. Globose argyrophilic Pick bodies eventually appear within the somata, and small Pick neurites of variable sizes and shapes develop in varicose expansions of the dendritic processes. Silver staining reveals only a fraction of the abnormal material and is adequate only for diagnostic purposes, while immunostaining of the abnormal tau protein discloses the complete neuropathological picture. The present study points to a conspicuous affliction of specific precerebellar nuclei in Pick's disease. Immunoreactive punctae, probably corresponding to terminal synaptic boutons of afferent fibers, appear at sites in the inferior olive receiving intense input from the cerebral cortex. The brunt of the changes, however, are borne by the pontine gray, the arcuate nucleus, the pontobulbar body, and the paramedian reticular nucleus. Altered areas show immunoreactive punctae and an abundance of small immunoreactive nerve cells partially containing Pick bodies and Pick neurites. Again, a feature common to all the affected nuclei is that they receive major input from the cerebral cortex, while other precerebellar nuclei with preponderant input from the spinal cord and/or other noncortical sources remain unscathed or exhibit only sparse involvement. The lesional pattern which develops in specific precerebellar nuclei is interpreted to be a partial reflection of the cortical involvement of Pick's disease.

Zomepirac acyl glucuronide covalently modifies tubulin in vitro and in vivo and inhibits its assembly in an in vitro system

Drugs possessing a carboxylate functional group usually form acyl glucuronides as major metabolites. These electrophilic metabolites can undergo several spontaneous reactions, including covalent adduct formation with proteins. The present study examined whether covalent adducts were formed with microtubular protein (MTP, 85%, alpha/beta-tubulin) and whether this influenced its ability to assemble into microtubules. Bovine brain microtubular protein (MTP) was purified by assembly-disassembly cycles and incubated with the nonsteroidal anti-inflammatory drug (NSAID) zomepirac (ZP), its acyl glucuronide (ZAG) and rearrangement isomers (iso-ZAG) at various concentrations for 2 h at room temperature and pH 7.5. Assembly was monitored by change in turbidity (increase in absorbance at 340 nm). Both ZAG and iso-ZAG caused dose-dependent inhibition of assembly (50% inhibition at about 1 mM), while ZP caused modest inhibition (< 50% inhibition at 4 mM). In a slightly different system, incubation of performed microtubules with 4 mM ZAG caused about 35% inhibition of reassembly ability, while modification of MTP under similar conditions resulted in about 85% reduction of assembly ability. Immunoblotting with a ZP antiserum showed that ZAG and iso-ZAG covalently modified MTP in a dose-dependent manner, while ZP itself caused no modification. Tubulin and many minor proteins comprising MTP were modified. ZP-modified tubulin was shown to be present in the cytosol of livers from rats dosed twice daily for 3 days with ZP at 50 mg/kg, using a sandwich ELISA with ZP and tubulin antisera. Whether any perturbation of microtubule assembly occurs in vivo as a result of this in vivo modification is currently under investigation.

Regulation of parathyroid hormone-related protein expression in a canine squamous carcinoma cell line by colchicine

The regulation of parathyroid hormone-related protein expression by colchicine, vinblastine, nocodazole, taxol, transforming growth factor-beta1 (TGFbeta1), and epidermal growth factor (EGF) was investigated in a canine squamous carcinoma cell line (SCC 2/88 cells). SCC 2/88 cells were stably transfected with a human P2/P3 PTHrP promoter-luciferase reporter gene construct and gene expression was measured after chemical treatments. The greatest increase in reporter gene expression was observed after colchicine treatment and small increases occurred after treatment with vinblastine, taxol, TGFbeta1, or EGF. Nocodazole had no significant effect on reporter gene expression. Colchicine also increased PTHrP steady state mRNA expression and PTHrP secretion by SCC 2/88 cells. These results demonstrated that PTHrP production was increased in SCC 2/88 cells by colchicine and suggested that factors or events during mitosis are capable of stimulating PTHrP production. An increase in PTHrP production during mitosis of malignant epithelial cells may be important in the pathogenesis of humoral hypercalcemia of malignancy.

Protein phosphatase 1 is targeted to microtubules by the microtubule-associated protein Tau

Phosphorylation has been implicated in the regulation of microtubule (MT) stability and function by controlling the interactions between MTs and MT-associated proteins. We found previously that protein phosphatase inhibitors selectively break down stable MTs, suggesting that protein phosphatases may be involved in regulating MT stability. To identify the protein phosphatases involved, we examined purified calf brain MTs and found a protein phosphatase activity that copurified with MTs to constant stoichiometry. Western blot analysis and inhibitor profiles demonstrated that the MT-associated phosphatase was a type 1 protein phosphatase (PP1), which we named PP1MT. Recombinant PP1 catalytic subunit (PP1c) did not bind to MTs, whereas PP1MT did bind, suggesting the presence of proteins that target PP1 to MTs. By Sepharose CL-6B chromatography, the phosphatase activity of PP1MT eluted as a large protein complex of approximately 400 kDa. High salt (2 M NaCl) treatment followed by CL-6B chromatography dissociated PP1MT into PP1c and the MT-targeting subunit(s). The MT-targeting subunit was shown to be the MT-associated protein tau by PP1 blot overlays and other assays. Also, recombinant tau reconstituted the binding of PP1c to MTs. These results identify PP1 as the first tau binding protein and suggest that tau is a novel PP1-targeting subunit.

XMAP230 is required for the assembly and organization of acetylated microtubules and spindles in Xenopus oocytes and eggs

We used affinity-purified polyclonal antibodies to characterize the distribution and function of XMAP230, a heat-stable microtubule-associated protein isolated from Xenopus eggs, during oogenesis. Immunoblots revealed that XMAP230 was present throughout oogenesis and early development, but was most abundant in late stage oocytes, eggs, and early embryos. Immunofluorescence microscopy revealed that XMAP230 was associated with microtubules in oogonia, post-mitotic stage 0 oocytes, early stage I oocytes, and during stage IV-VI of oogenesis. However, staining of microtubules by anti-XMAP230 was not detectable during late stage I through stage III. In stage VI oocytes, anti-XMAP230 stained a large subset of microtubules that were also stained with monoclonal antibodies specific for acetylated (α)-tubulin. During oocyte maturation, XMAP230 was associated with the transient microtubule array that serves as the precursor of the first meiotic spindle, as well as both first and second meiotic spindles. The extensive array of cytoplasmic microtubules present throughout maturation was not detectably stained by anti-XMAP230. Microinjection of anti-XMAP230 locally disrupted the organization and acetylation of microtubules in stage VI oocytes, and reduced the re-acetylation of microtubules during recovery from cold-induced microtubule disassembly. Subsequent maturation of oocytes injected with anti-XMAP230 resulted in defects in the assembly of the transient microtubules array and first meiotic spindle. These observations suggest that XMAP230 is required for the stabilization and organization of cytoplasmic and spindle microtubules in Xenopus oocytes and eggs.

Mechanisms of nuclear positioning

The mechanisms underlying two types of microtubule-dependent nuclear positioning are discussed. ‘MTOC-dependent nuclear positioning’ occurs when a nucleus is tightly associated with a microtubule organizing center (MTOC). ‘Nuclear tracking along microtubules’ is analogous to the motor-driven motility of other organelles and occurs when the nucleus lacks an associated MTOC. These two basic types of microtubule-dependent nuclear positioning may cooperate in many proliferating animal cells to achieve proper nuclear positioning. Microtubule polymerization and dynamics, motor proteins, MAPs and specialized sites such as cortical anchors function to control nuclear movements within cells.

MAPs, MARKs and microtubule dynamics

Microtubules (MTs) serve as tracks for cellular transport, and regulate cell shape and polarity. Rapid transitions between stable and dynamic forms of MTs are central to these processes. This dynamic instability is regulated by a number of cellular factors, including the structural MT-associated proteins (MAPs), which in turn are regulated by phosphorylation. MT-affinity-regulating kinases (MARKs) are novel mammalian serine/threonine kinases that phosphorylate the tubulin-binding domain of MAPs and thereby cause their detachment from MTs and increased MT dynamics. Molecular cloning of MARKs revealed a family of four closely related protein kinases that share homology with genes from the nematode Caenorhabditis elegans and fission yeast that are involved in the generation of cell shape and polarity. Hence, MARKs might play a role in the regulation of MT stability during morphogenesis.

The distribution of murine 115-kDa epithelial microtubule-associated protein (E-MAP-115) during embryogenesis and in adult organs suggests a role in epithelial polarization and differentiation

In interphase cells microtubules play fundamental roles in the intracellular distribution and movement of organelles and vesicles and thereby contribute to cellular polarization and differentiation. The organization of microtubules varies with the cell type and is presumably controlled by tissue-specific microtubule-associated proteins (MAPs). The 115-kDa epithelial MAP (E-MAP-115) has been identified as a microtubule-stabilizing protein predominantly expressed in cell lines of epithelial origin. To assess a putative function of E-MAP-115 in epithelial morphogenesis in vivo, we have cloned the cDNA encoding the murine protein and studied the cellular distribution of E-MAP-115 mRNA and protein during murine embryogenesis and in adult organs. Analysis of the predicted amino acid sequence of murine E-MAP-115 revealed 81% sequence identity with its human homolog, the best-conserved part of the protein being the microtubule-binding site. Our data indicate that E-MAP-115 is expressed in several epithelia from 9.5 days of embryogenesis onwards and that its expression levels increase during development. From 14.5 days onwards, E-MAP-115 mRNA is found in some neuronal cells as well. In adult organs, E-MAP-115 is most abundant in epithelial cells of kidney tubules, in absorptive cells of the intestine and is widely distributed in the testis. E-MAP-115 expression correlates with the differentiation of certain epithelial cell types: in the adult intestine, for example, E-MAP-115 mRNA and protein are more abundant in the differentiating than in the proliferative cell compartment. Moreover, E-MAP-115 expression clearly correlates with the degree of cellular apicobasal polarity. In the developing kidney, E-MAP-115 mRNA is detected in the cuboidal cells of S-shaped bodies, of primitive tubules and glomerula, whereas, E-MAP-115 mRNA and protein are absent from mature podocytes which have lost their initial apico-basal polarity. The pattern of distribution of E-MAP-115 in vivo is so far unique for a MAP. Taken together, our results provide support for a role of E-MAP-115 in reorganizing the microtubule cytoskeleton during epithelial cell polarization and differentiation.

Dendritic changes in Alzheimer's disease and factors that may underlie these changes

It seems likely that the Alzheimer disease (AD)-related dendritic changes addressed in this article are induced by two principally different processes. One process is linked to the plastic response associated with deafferentation, that is, long-lasting transneuronally induced regressive changes in dendritic geometry and structure. The other process is associated with severe alterations of the dendritic- and perikaryal cytoskeleton as seen in neurons with the neurofibrillary pathology of AD, that is, the formation of paired helical filaments formed by hyperphosphorylated microtubule-associated protein tau. As the development of dendritic and cytoskeletal abnormalities are at least mediated by alterations in signal transduction, this article also reviews changes in signal pathways in AD. We also discuss transgenic approaches developed to model and understand cytoskeletal abnormalities.

The role of nucleation in patterning microtubule networks

Control of microtubule nucleation is important for many microtubule dependent processes in cells. Traditionally, research has focused on nucleation of microtubules from centrosomes. However, it is clear that microtubules can nucleate from non-centrosome dependent sites. In this review we discuss the consequences of non-centrosome dependent microtubule nucleation for formation of microtubule patterns, concentrating on the assembly of mitotic spindles.

Rapid assembly of Alzheimer-like paired helical filaments from microtubule-associated protein tau monitored by fluorescence in solution

Alzheimer's disease is characterized by the progressive deposition of two types of fibers in the affected brains, the amyloid fibers (consisting of the Abeta peptide, generating the amyloid plaques) and paired helical filaments (PHFs, made up of tau protein, forming the neurofibrillary tangles). While the principles of amyloid aggregation are known in some detail, the investigation of PHF assembly has been hampered by the low efficiency of tau aggregation, the requirement of high protein concentrations, and the lack of suitable detection methods. Here we report a quantitative assay system that permits monitoring of the assembly of PHFs in real time by the fluorescence of dyes such as thioflavine S or T. Using this assay, we evaluated parameters that influence the efficiency of filament formation. Disulfide-linked dimers of tau constructs representing the repeat domain assemble into PHFs most efficiently, but other tau isoforms or constructs form bona fide PHFs as well. The rate of assembly is greatly enhanced by polyanions such as RNA, heparin, and notably polyglutamate which resembles the acidic tail of tubulin. The assembly is optimal at pH approximately 6 and low ionic strengths (<50 mM) and increases steeply with temperatures above 30 degreesC, indicating that it is an entropy-driven process.

STOP proteins are responsible for the high degree of microtubule stabilization observed in neuronal cells

Neuronal differentiation and function require extensive stabilization of the microtubule cytoskeleton. Neurons contain a large proportion of microtubules that resist the cold and depolymerizing drugs and exhibit slow subunit turnover. The origin of this stabilization is unclear. Here we have examined the role of STOP, a calmodulin-regulated protein previously isolated from cold-stable brain microtubules. We find that neuronal cells express increasing levels of STOP and of STOP variants during differentiation. These STOP proteins are associated with a large proportion of microtubules in neuronal cells, and are concentrated on cold-stable, drug-resistant, and long-lived polymers. STOP inhibition abolishes microtubule cold and drug stability in established neurites and impairs neurite formation. Thus, STOP proteins are responsible for microtubule stabilization in neurons, and are apparently required for normal neurite formation.

A novel taxol-induced vimentin phosphorylation and stabilization revealed by studies on stable microtubules and vimentin intermediate filaments

To understand how protein phosphorylation modulates cytoskeletal organization, we used immunofluorescence microscopy to examine the effects of okadaic acid, a serine/threonine protein phosphatase inhibitor, and taxol, a microtubule-stabilizing agent, on stable (acetylated and detyrosinated) microtubules, vimentin intermediate filaments and other cytoskeletal elements in CV-1 cells. Okadaic acid caused major changes in both stable microtubules and vimentin intermediate filaments, but through independent mechanisms. At 300 nM, okadaic acid caused apparent fragmentation and loss of stable microtubules which was not prevented by prior exposure to K252a. In contrast, major reorganization of vimentin intermediate filaments elicited at 750 nM okadaic acid was prevented by prior exposure to K252a. Taxol pretreatment blocked the effects of okadaic acid on stable microtubules and vimentin intermediate filaments. Recent reports have revealed that taxol can activate cellular signal transduction pathways in addition to its known ability to promote microtubule stabilization, so the possibility that taxol-induced resistance of vimentin intermediate filaments to okadaic acid was through a microtubule-independent mechanism involving direct phosphorylation of intermediate filament proteins was explored. Vimentin immunoprecipitation from cytoskeletal extracts from 32P-labeled cells revealed that taxol (4 microM, 1 or 2 hours) caused about a 2-fold increase in vimentin phosphorylation. This phosphorylation was recovered exclusively in cytoskeletal vimentin, in contrast to the increased phosphorylation of soluble and cytoskeletal vimentin caused by exposure to 750 nM okadaic acid. Phosphorylation of soluble and cytoskeletal vimentin from cells exposed to taxol (4 microM, 1 hour) then okadaic acid (750 nM, 1 hour) was comparable to taxol-treatment alone. These findings demonstrate a novel new activity of taxol, induction of vimentin phosphorylation, that may impact on vimentin organization and stability.

Expression of Reticulomyxa filosa alpha- and beta-tubulins in Escherichia coli yields soluble and partially correctly folded material

Tubulins are highly conserved multidomain proteins that have to interact with eukaryotic chaperonins to gain their correct three-dimensional conformation. The prokaryotic chaperonin system of GroEL/ES is able to generate intermediate folding states but not natively folded tubulin. To create a system for studying these folding intermediates, tubulins from the giant amoeba Reticulomyxa filosa (alpha 2- and beta 2-tubulin) were expressed in Escherichia coli singly or in tandem. In all cases, soluble tubulin was generated in amounts of 5-10 mg/l culture. This is the first reported expression of soluble tubulin in bacterial cells. Of particular interest was the observation that upon coexpression with R. filosa beta 2-tubulin, proteolytic degradation of alpha 2-tubulin was reduced and more full-length product remained intact. This observation points to a specific interaction of alpha 2- and beta 2-tubulins in the E. coli cell. The sites of interaction are most probably the same that are responsible for the binding of native alpha 2- and beta 2-tubulin. The established expression system therefore seems well suited for further studies concerning the folding of tubulins.

Role of chromosomes in assembly of meiotic and mitotic spindles

The assembly of a mitotic spindle requires the interaction of microtubules with chromosomes. As a cell enters mitosis, long microtubules are converted to short ones, as microtubules become unstable. Dynamic microtubules are then stabilised by chromosomes, forming a bipolar spindle. In this review, we discuss the different roles of kinetochores and chromosome arms during spindle assembly. Kinetochores, required for proper chromosomes segregation, capture microtubules and maintain attachment. Chromosome arms greatly enhance microtubule stability, and alone can be sufficient for spindle assembly.

Epithelial and fibroblastoid cells contain numerous cell-type specific putative microtubule-regulating proteins, among which are ezrin and fodrin

Upon cell junction formation, the microtubules of polarizing epithelial cells become reorganized by unknown signaling mechanisms and regulating proteins. Microtubule-associated (MAPs) and other types of proteins are likely to be involved in this process, but most of these are unknown. Such proteins are called here collectively microtubule-regulating proteins (MRPs). As a first step towards their characterization, we used co-sedimentation of cytosolic proteins of MDCK cells and A72, a dog fibroblastoid line, with an excess of taxol-stabilized MTs, to obtain a cell fraction enriched in putative MRPs ("MRPs"). Additional tests have led to the inventory of around 40 "MRPs" among the 80 proteins present in the microtubule pellet. We also found that "MRPs" are recovered in higher amounts from MDCK cytosol, and that half of these are cell-type specific. These results corroborate data from yeast cells and insect eggs, and show that in mammalian somatic cells too, a large number of proteins seems to be involved in microtubule regulation, and that different cell types use a specific set of MRPs. "MRPs" found in both cell types are the intermediate chain of cytoplasmic dynein, Arp1, the major subunit of the dynactin complex, and CLIP-170. Two MDCK-specific "MRPs" were identified as the actin-binding proteins ezrin and alpha-fodrin. These results are discussed with regard to a possible involvement of ezrin and fodrin in morphogenetic interactions of microtubules with the membrane cytoskeleton in polarizing epithelia upon junction formation.

Effect of lead on cytoskeletal proteins expressed in E14 mesencephalic primary cultures

Several lines of evidence indicated that Pb exposure in vivo and in vitro altered neurite morphology in central and peripheral neurons. The present report shows that neurite length in mesencephalic primary cultures, consisting of neurons and glia, was decreased by Pb exposure when serum factors, presumably essential for glial functions, were absent in the culture medium. We studied whether a serum factor might control the mechanisms involved in the uptake and accumulation of Pb and its effect on cytoskeleton proteins. The total amount of Pb taken up in cell cultures was measured by atomic absorption spectroscopy and appeared to be down-regulated by a non-albumin-like serum component. In presence of serum, Pb exposure failed to alter cytoskeletal proteins. Instead, in serum-free neurobasal medium, Pb uptake failed to reach saturation within 6 h. Western blot analysis showed that the tau, 280 kDa MAP-2b, 70 kDa MAP-2c and GAP-43 protein bands were decreased 24 h after a 3 h exposure to 3 or 6 microM Pb in absence of serum. However, if cultures were maintained in serum-containing media after a 3 h Pb exposure without serum, the immunoblots did not differ from those of controls. It can be inferred that a serum factor prevents cytoskeletal protein alterations by Pb. In serum free medium, Pb that is primarily scavenged by the metallothionein I/II isoforms present in glial cells, may bind to thiol residues of proteins involved in either oxidative stress response or transcriptional regulation of cytoskeletal proteins.

Partitioning of cytoplasmic organelles during mitosis with special reference to the Golgi complex

During mitosis, not only the genetic material stored in the nucleus but also the constituents of the cytoplasm should be equally partitioned between the daughter cells. For this sake, the dividing cell goes through an extensive structural reorganization and transport along the endocytic and exocytic pathways is temporarily arrested. Early in prophase, the radiating array of cytoplasmic microtubules disassembles and the membrane systems of the secretory apparatus start to split up. In metaphase, the nuclear envelope fragments and the condensing chromosomes associate with the forming mitotic spindle. The cisternal and tubular elements of the endoplasmic reticulum and the Golgi complex break down into small vesicles, presumably as the result of an imbalance between vesicle budding and fusion. In anaphase, the two sets of chromosomes are pulled apart and a cleavage furrow forms halfway between the spindle poles. Since most organelles occur in multiple and widely dispersed copies at this stage, they will be evenly distributed between the daughter cells. During telophase and cytokinesis, the preceding fragmentation process is reversed. A nuclear envelope reappears around the chromosomes and cytoplasmic microtubules reassemble. The endoplasmic reticulum is rebuilt as a continuous system of flattened cisternae and tubules. Stacks of Golgi cisternae arise from small vesicles and are rearranged in an interconnected network. In parallel, the biosynthetic functions of the cell are normalized and intracellular membrane traffic is resumed.

Ganglioside GM1 alters neuronal morphology by modulating the association of MAP2 with microtubules and actin filaments

In previous studies, we demonstrated that the exogenous ganglioside GM1 increased the complexity of the microtubular network and level of tubulin, selectively changed the distribution of microtubule associated protein-2 (MAP2) immunoreactivity from the perikarya to distal neuritic processes and increased immunogold label of MAP2 in the subplasmalemmal cytoplasm, neuritic filopodia and growth cones of Neuro-2a neuroblastoma cells. Since these areas are rich in actin filaments, our data suggested that MAP2 may be associated with microfilaments in the early stages of ganglioside-induced neuritogenesis. To determine if GM1 alters neuronal morphology by facilitating the interaction of actin and MAP2, we examined the immunolocalization of these two proteins with confocal and electron microscopy. We found that along with the redistribution of MAP2 from perikaryal to neuritic regions, there was parallel redistribution of actin. The uniform subplasmalemmal actin meshwork was disrupted in areas of processes and filopodia with a redistribution of actin to these areas in close association with MAP2. Our present results suggest that gangliosides enhance neuritogenesis by redistributing actin as well as MAP2 to processes and filopodia thereby facilitating their interaction. The association of MAP2 with actin filaments is likely to be an early step in ganglioside-mediated filopodia formation.

Essential role of tubulin-folding cofactor D in microtubule assembly and its association with microtubules in fission yeast

The main structural components of microtubules are alpha- and beta-tubulins. A group of proteins called cofactors are crucial in the formation of assembly-competent tubulin molecules in vitro. Whilst an in vitro role is emerging for these cofactors, their biological functions in vivo remain to be established. In order to understand the fundamental mechanisms that determine cell polarity, we have screened for fission yeast mutants with altered polarity. Here we show that alp1+ encodes a homologue of cofactor D and executes a function essential for cell viability. A temperature-sensitive alp1 mutant shows a variety of defects including abnormal mitoses, loss of microtubule structures, displacement of the nucleus, altered growth polarity and asymmetrical cell division. Overexpression of Alp1 is lethal in wild-type cells, resulting in altered cell shape, but is rescued by co-overexpression of beta-tubulin. Alp1 co-localizes with microtubules, both interphase arrays and mitotic spindles. Furthermore, Alp1 binds to and co-sediments with taxol (paclitaxel)-stabilized porcine microtubules. Our results suggest that, in addition to a function in the folding of beta-tubulin, cofactor D may play a vital role in microtubule-dependent processes as a microtubule-associated protein.

Molecular motors and their role in membrane traffic

Recent investigations support a role for the vesicle motor proteins (kinesin, cytoplasmic dynein, and myosin) in numerous membrane trafficking events including endocytosis and transcytosis. Kinesin and cytoplasmic dynein are responsible for movement of membrane vesicles along cellular microtubules to and from cellular membrane compartments, while certain members of the myosin family also appear to drive membrane vesicles along actin filaments to and from membrane compartments. In this review, our current understanding of the role of these vesicle motors in membrane trafficking is highlighted. Future areas of interest which may be able to make use of these vesicle motors as potential targets for drug delivery are also discussed.

Regulation of microtubule dynamics by extracellular signals: cAMP-dependent protein kinase switches off the activity of oncoprotein 18 in intact cells

Oncoprotein 18 (Op18, also termed p19, 19K, metablastin, stathmin, and prosolin) is a recently identified regulator of microtubule (MT) dynamics. Op18 is a target for both cell cycle and cell surface receptor-coupled kinase systems, and phosphorylation of Op18 on specific combinations of sites has been shown to switch off its MT-destabilizing activity. Here we show that induced expression of the catalytic subunit of cAMP-dependent protein kinase (PKA) results in a dramatic increase in cellular MT polymer content concomitant with phosphorylation and partial degradation of Op18. That PKA may regulate the MT system by downregulation of Op18 activity was evaluated by a genetic system allowing conditional co-expression of PKA and a series of kinase target site-deficient mutants of Op18. The results show that phosphorylation of Op18 on two specific sites, Ser-16 and Ser-63, is necessary and sufficient for PKA to switch off Op18 activity in intact cells. The regulatory importance of dual phosphorylation on Ser-16 and Ser-63 of Op18 was reproduced by in vitro assays. These results suggest a simple model where PKA phosphorylation downregulates the MT-destabilizing activity of Op18, which in turn promotes increased tubulin polymerization. Hence, the present study shows that Op18 has the potential to regulate the MT system in response to external signals such as cAMP-linked agonists.

Structure of the alpha beta tubulin dimer by electron crystallography

The alphabeta tubulin heterodimer is the structural subunit of microtubules, which are cytoskeletal elements that are essential for intracellular transport and cell division in all eukaryotes. Each tubulin monomer binds a guanine nucleotide, which is nonexchangeable when it is bound in the alpha subunit, or N site, and exchangeable when bound in the beta subunit, or E site. The alpha- and beta-tubulins share 40% amino-acid sequence identity, both exist in several isotype forms, and both undergo a variety of posttranslational modifications. Limited sequence homology has been found with the proteins FtsZ and Misato, which are involved in cell division in bacteria and Drosophila, respectively. Here we present an atomic model of the alphabeta tubulin dimer fitted to a 3.7-A density map obtained by electron crystallography of zinc-induced tubulin sheets. The structures of alpha- and beta-tubulin are basically identical: each monomer is formed by a core of two beta-sheets surrounded by alpha-helices. The monomer structure is very compact, but can be divided into three functional domains: the amino-terminal domain containing the nucleotide-binding region, an intermediate domain containing the Taxol-binding site, and the carboxy-terminal domain, which probably constitutes the binding surface for motor proteins.

Angiotensin II and NGF differentially influence microtubule proteins in PC12W cells: role of the AT2 receptor

Angiotensin AT2 receptors have been shown to play a role in cell differentiation characterized by neurite outgrowth in neuronal cells of different origin. To further investigate AT2 receptor-mediated events leading to neurite formation, we examined the effect of AT2 receptor stimulation on the microtubule components, beta-tubulin, MAP1B and MAP2, by Western blot analysis and immunofluorescence in quiescent and nerve growth factor (NGF)-differentiated PC12W cells. These proteins are involved in neurite extension and neuronal maturation. Whereas NGF (0.5, 10, and 50 ng/ml) up-regulated these proteins after 3 days of stimulation, angiotensin II (ANG II; 10(-7) M) induced a different pattern. In quiescent PC12W cells, AT2 receptor stimulation up-regulated polymerized beta-tubulin and MAP2 but down-regulated MAP1B protein levels. In PC12W cells, differentiated by NGF (0.5 ng/ml), ANG II elevated polymerized beta-tubulin and reduced MAP1B. All ANG II effects were abolished by the AT2 receptor antagonist PD123177 (10(-5) M) but not affected by the AT1 receptor antagonist losartan (10(-5) M). These results implicate a specific role of AT2 receptors in cell differentiation and nerve regeneration via regulation of the cytoskeleton.

Atypical microtubule organization in undifferentiated human colon cancer cells

We previously reported that undifferentiated colonic cancer HT-29 cells, unlike the differentiated ones, exhibit unusual organelle distributions and atypical vesicle trafficking patterns, which are microtubule-independent and microfilament-dependent. In the present study, we have analyzed the microtubule network in both phenotypes, using confocal microscopy, and determined the expression levels of some microtubule-associated proteins by quantitative immunoblotting. Differentiated cells exhibited the microtubular organization of polarized epithelial cells. Non-polarized undifferentiated cells presented an atypical microtubule organization as microtubules were localized mainly at the cell 'top'. Immunoblot analysis indicated the absence or low content of several structural and motor microtubule-associated proteins in undifferentiated cells, compared to differentiated cells. This may explain in part their atypical microtubular organization. This study agrees with a crucial role for microfilaments in the intracellular organization of undifferentiated HT-29 cancer cells, while differentiated HT-29 cells exhibit intracellular organization similar to that of normal enterocytic cells, although they are also tumoral.

A major 170 kDa protein associated with bovine adrenal medulla microtubules: a member of the centrosomin family?

Microtubules isolated from bovine adrenal medulla cells contain a major 170 kDa protein (p170). p170 is heat-labile and is associated with microtubules in an ATP-insensitive manner. This protein was purified to near homogeneity using FPLC. A preparation containing purified p170 caused bundling of microtubules. By microsequencing of p170, two polypeptides were identified which appeared to be identical to a recently sequenced p167 centrosomin-related protein. Polyclonal affinity-purified anti-p170 antibody was found to immunostain microtubules and to recognize the 170 kDa polypeptide in culture cells. We suggest that p170 is a new member of a centrosomin family and is a new structural protein associated with microtubules in some cell types.

Coupling cell division and cell death to microtubule dynamics

The mitotic spindle is a self-organizing structure that is constructed primarily from microtubules. Among the most important spindle microtubules are those that bind to kinetochores and form the fibers along which chromosomes move. Chemotherapeutics such as taxol and the vinca alkaloids perturb kinetochore-microtubule attachment and disrupt chromosome segregation. This activates a checkpoint pathway that delays cell cycle progression and induces programmed cell death. Recent work has identified at least four mammalian spindle assembly checkpoint proteins.

Reduction of microtubule catastrophe events by LIS1, platelet-activating factor acetylhydrolase subunit

Forming the structure of the human brain involves extensive neuronal migration, a process dependent on cytoskeletal rearrangement. Neuronal migration is believed to be disrupted in patients exhibiting the developmental brain malformation lissencephaly. Previous studies have shown that LIS1, the defective gene found in patients with lissencephaly, is a subunit of the platelet-activating factor acetylhydrolase. Our results indicated that LIS1 has an additional function. By interacting with tubulin it suppresses microtubule dynamics. We detected LIS1 interaction with microtubules by immunostaining and co-assembly. LIS1-tubulin interactions were assayed by co-immunoprecipitation and by surface plasmon resonance changes. Microtubule dynamic measurements in vitro indicated that physiological concentrations of LIS1 indeed reduced microtubule catastrophe events, thereby resulting in a net increase in the maximum length of the microtubules. Furthermore, the LIS1 protein concentration in the brain, measured by quantitative Western blots, is high and is approximately one-fifth of the concentration of brain tubulin. Our new findings show that LIS1 is a protein exhibiting several cellular interactions, and the interaction with the cytoskeleton may prove to be the mode of transducing a signal generated by platelet-activating factor. We postulate that the LIS1-cytoskeletal interaction is important for neuronal migration, a process that is defective in lissencephaly patients.

XMAP310: a Xenopus rescue-promoting factor localized to the mitotic spindle

To understand the role of microtubule-associated proteins (MAPs) in the regulation of microtubule (MT) dynamics we have characterized MAPs prepared from Xenopus laevis eggs (Andersen, S.S.L., B. Buendia, J.E. Domínguez, A. Sawyer, and E. Karsenti. 1994. J. Cell Biol. 127:1289-1299). Here we report on the purification and characterization of a 310-kD MAP (XMAP310) that localizes to the nucleus in interphase and to mitotic spindle MTs in mitosis. XMAP310 is present in eggs, oocytes, a Xenopus tissue culture cell line, testis, and brain. We have purified XMAP310 to homogeneity from egg extracts. The purified protein cross-links pure MTs. Analysis of the effect of this protein on MT dynamics by time-lapse video microscopy has shown that it increases the rescue frequency 5-10-fold and decreases the shrinkage rate twofold. It has no effect on the growth rate or the catastrophe frequency. Microsequencing data suggest that XMAP230 and XMAP310 are novel MAPs. Although the three Xenopus MAPs characterized so far, XMAP215 (Vasquez, R.J., D.L. Gard, and L. Cassimeris. 1994. J. Cell Biol. 127:985-993), XMAP230, and XMAP310 are localized to the mitotic spindle, they have distinct effects on MT dynamics. While XMAP215 promotes rapid MT growth, XMAP230 decreases the catastrophe frequency and XMAP310 increases the rescue frequency. This may have important implications for the regulation of MT dynamics during spindle morphogenesis and chromosome segregation.

Domains of neuronal microtubule-associated proteins and flexural rigidity of microtubules

Microtubules are flexible polymers whose mechanical properties are an important factor in the determination of cell architecture and function. It has been proposed that the two most prominent neuronal microtubule-associated proteins (MAPs), tau and MAP2, whose microtubule binding regions are largely homologous, make an important contribution to the formation and maintenance of neuronal processes, putatively by increasing the rigidity of microtubules. Using optical tweezers to manipulate single microtubules, we have measured their flexural rigidity in the presence of various constructs of tau and MAP2c. The results show a three- or fourfold increase of microtubule rigidity in the presence of wild-type tau or MAP2c, respectively. Unexpectedly, even low concentrations of MAPs promote a substantial increase in microtubule rigidity. Thus at approximately 20% saturation with full-length tau, a microtubule exhibits >80% of the rigidity observed at near saturating concentrations. Several different constructs of tau or MAP2 were used to determine the relative contribution of certain subdomains in the microtubule-binding region. All constructs tested increase microtubule rigidity, albeit to different extents. Thus, the repeat domains alone increase microtubule rigidity only marginally, whereas the domains flanking the repeats make a significant contribution. Overall, there is an excellent correlation between the strength of binding of a MAP construct to microtubules (as represented by its dissociation constant Kd) and the increase in microtubule rigidity. These findings demonstrate that neuronal MAPs as well as constructs derived from them increase microtubule rigidity, and that the changes in rigidity observed with different constructs correlate well with other biochemical and physiological parameters.

Chinese hamster protein homologous to human putative protein kinase KIAA0204 is associated with nuclei, microtubules and centrosomes in CHO-K1 cells

Monoclonal antibody raised against a preparation of loach fish sperm centrosomes was used for screening of cDNA expressing library of Chinese hamster CHO-K1 cells. Two positive clones appeared to encode 628 amino acid protein fragment that was 72% identical to human KIAA0204 protein, i.e. putative protein kinase. Polyclonal antibodies raised against products of cDNA expression in E. coli recognized 210-kDa polypeptide in CHO-K1 cells and immunostained nuclear speckles, centrosomes and microtubules in these cells. The 210-kDa polypeptide (named MAK-L) co-sedimented with exogenous microtubules. Thus, one more protein kinase seems to be associated with the microtubule network in vertebrate cells.

The third member of the Tetrahymena CCT subunit gene family, TpCCT alpha, encodes a component of the hetero-oligomeric chaperonin complex

The sequence of a third member of the Tetrahymena pyriformis chaperonin CCT ('chaperonin containing TCP1') subunit gene family is presented. This gene, designated TpCCT alpha, is the orthologue of the mouse chaperonin gene TCP1/CCT alpha. To characterize the CCT complex in this ciliate, we have produced polyclonal antibodies against synthetic peptides based on C-terminal sequences deduced from the primary sequences of the TpCCT alpha, TpCCT gamma and TpCCT eta subunits. We have also used polyclonal antibodies produced against recombinant yeast CCT alpha and CCT beta subunits. Using these antibodies, we show that Tetrahymena cells contain a hetero-oligomeric CCT chaperonin comprising at least seven distinct subunits. Three of these were assigned to specific TpCCT genes, whereas a fourth was recognized by the polyclonal antibody against yeast CCT beta, suggesting that this gene is also present in the ciliate. The CCT complex also contains other unidentified proteins that were recognized by the polyclonal antibody UM-1, raised against the putative ATP binding domain of the chaperonin proteins. TpCCT alpha gene expression was shown in exponentially growing cells and cells regenerating their cilia for different periods to have a similar pattern to the previously identified genes TpCCT gamma and TpCCT eta, and also to tubulin genes.

Concentration-dependent stimulation and inhibition of growth cone behavior and neurite elongation by protein kinase inhibitors KT5926 and K-252a

We examined the concentration- and time-dependent effects of two related protein kinase inhibitors, KT5926 and K-252a, on neurite formation and nerve growth cone migration of chick embryo sensory neurons. The effects of these drugs on neurite formation over an 18-h period were dissimilar. KT5926 stimulated neurite formation at concentrations between 100 and 500 nM and inhibited neurite formation at 5 microM. K-252a had no stimulatory effects on neurite formation, and it inhibited neurite formation at concentrations above 50 nM. This difference may occur because K-252a inhibits activation of the nerve growth factor receptor trk A, while KT5926 does not inhibit trk A. Both drugs, however, had similar immediate effects on growth cone migration. Growth cone migration and lamellipodial spreading were rapidly stimulated by 500 nM concentrations of KT5926 and K-252a. At 2 microM levels of either drug, growth cone spreading was still stimulated, but growth cone migration was inhibited by both drugs. These results show that changes in protein phosphorylation/dephosphorylation can rapidly regulate the cellular machinery that is responsible for driving growth cone migration and neurite elongation. The different effects of 2 microM concentrations of either KT5926 or K-252a on growth cone spreading versus migration suggests that the actin-dependent protrusive motility of the growth cone leading margin is regulated differently by changes in protein phosphorylation and dephosphorylation than the cytoskeletal mechanism that drives neurite elongation.

Microtubule-associated proteins (MAPs) in the peripheral nervous system during development and regeneration

In this article, we have described the structure and distribution of the various variants of the microtubule-associated proteins (MAPs), tau, MAP2, MAP1A, and MAP1B, that are expressed in the dorsal root ganglion (DRG) and spinal cord during development and regeneration. We have summarized the data on their gene structure and compared the sequence of the major transcripts encoding these MAPs that are expressed in the brain, the spinal cord, and the DRG. Finally, we have surveyed the studies that used a variety of experimental approaches (e.g., antisense inhibition, transgenic knockouts, and expression in neuronal and nonneuronal cells) to understand the functional significance of MAPs heterogeneity and differences observed between the central nervous system (CNS) and the peripheral nervous system (PNS) both during development and regeneration.