Differential interaction of synthetic peptides from the carboxyl-terminal regulatory domain of tubulin with microtubule-associated proteins

A new tau dephosphorylation-targeting chimera for the treatment of tauopathies

Abnormal accumulation of hyperphosphorylated tau protein plays a pivotal role in a collection of neurodegenerative diseases named tauopathies, including Alzheimer's disease (AD). We have recently conceptualized the design of hetero-bifunctional chimeras for selectively promoting the proximity between tau and phosphatase, thus specifically facilitating tau dephosphorylation and removal. Here, we sought to optimize the construction of tau dephosphorylating-targeting chimera (DEPTAC) and obtained a new chimera D14, which had high efficiency in reducing tau phosphorylation both in cell and tauopathy mouse models, while showing limited cytotoxicity. Moreover, D14 ameliorated neurodegeneration in primary cultured hippocampal neurons treated with toxic tau-K18 fragments, and improved cognitive functions of tauopathy mice. These results suggested D14 as a cost-effective drug candidate for the treatment of tauopathies.

Generation of tau dephosphorylation-targeting chimeras for the treatment of Alzheimer's disease and related tauopathies

Abnormal hyperphosphorylation and accumulation of tau protein play a pivotal role in neurodegeneration in Alzheimer's disease (AD) and many other tauopathies. Selective elimination of hyperphosphorylated tau is promising for the therapy of these diseases. We have conceptualized a strategy, named dephosphorylation-targeting chimeras (DEPTACs), for specifically hijacking phosphatases to tau to debilitate its hyperphosphorylation. Here, we conducted the step-by-step optimization of each constituent motif to generate DEPTACs with reasonable effectiveness in facilitating the dephosphorylation and subsequent clearance of pathological tau. Specifically, for one of the selected chimeras, D16, we demonstrated its significant efficiency in rescuing the neurodegeneration caused by neurotoxic K18-tau seeds in vitro. Moreover, intravenous administration of D16 also alleviated tau pathologies in the brain and improved memory deficits in AD mice. These results suggested DEPTACs as targeted modulators of tau phosphorylation, which hold therapeutic potential for AD and other tauopathies.

Scrutinizing the Therapeutic Potential of PROTACs in the Management of Alzheimer's Disease

Finding an effective cure for Alzheimer's disease has eluded scientists despite intense research. The disease is a cause of suffering for millions of people worldwide and is characterized by dementia accompanied by cognitive and motor deficits, ultimately culminating in the death of the patient. The course of the disease progression has various underlying contributing pathways, with the first and foremost factor being the development and accumulation of aberrant and misfolded proteins exhibiting neurotoxic functions. The impairment of cellular clearance mechanisms adds to their accumulation, resulting in neuronal death. This is where the PROteolysis TArgeting Chimera (PROTAC) technology comes into play, bringing the UPS degradation machinery in the proximity of the target protein for initiating its degradation and clearing abnormal protein debris with unparalleled precision demonstrating an edge over traditional protein inhibitors in many respects. The technology is widely explored in cancer research and utilized in the treatment of various tumors and malignancies, and is now being applied in treating AD. This review explores the application of PROTAC technology in developing lead compounds for managing this deadly disease along with detailing the pieces of evidence justifying its utility and efficacy.

Glucose metabolism and AD: evidence for a potential diabetes type 3

BACKGROUND

Alzheimer's disease is the most prevalent cause of dementia in the elderly. Neuronal death and synaptic dysfunctions are considered the main hallmarks of this disease. The latter could be directly associated to an impaired metabolism. In particular, glucose metabolism impairment has demonstrated to be a key regulatory element in the onset and progression of AD, which is why nowadays AD is considered the type 3 diabetes.

METHODS

We provide a thread regarding the influence of glucose metabolism in AD from three different perspectives: (i) as a regulator of the energy source, (ii) through several metabolic alterations, such as insulin resistance, that modify peripheral signaling pathways that influence activation of the immune system (e.g., insulin resistance, diabetes, etc.), and (iii) as modulators of various key post-translational modifications for protein aggregation, for example, influence on tau hyperphosphorylation and other important modifications, which determine its self-aggregating behavior and hence Alzheimer's pathogenesis.

CONCLUSIONS

In this revision, we observed a 3 edge-action in which glucose metabolism impairment is acting in the progression of AD: as blockade of energy source (e.g., mitochondrial dysfunction), through metabolic dysregulation and post-translational modifications in key proteins, such as tau. Therefore, the latter would sustain the current hypothesis that AD is, in fact, the novel diabetes type 3.

Molecular Evolution of Tubulins in Diatoms

Microtubules are formed by α- and β-tubulin heterodimers nucleated with γ-tubulin. Tubulins are conserved eukaryotic proteins. Previously, it was shown that microtubules are involved in diatom silica frustule morphogenesis. Diatom frustules are varied, and their morphology is species-specific. Despite the attractiveness of the problem of elucidating the molecular mechanisms of genetically programmed morphogenesis, the structure and evolution of diatom tubulins have not been studied previously. Based on available genomic and transcriptome data, we analyzed the phylogeny of the predicted amino acid sequences of diatom α-, β- and γ-tubulins and identified five groups for α-tubulins, six for β-tubulins and four for γ-tubulins. We identified characteristic amino acids of each of these groups and also analyzed possible posttranslational modification sites of diatom tubulins. According to our results, we assumed what changes occurred in the diatom tubulin structures during their evolution. We also identified which tubulin groups are inherent in large diatom taxa. The similarity between the evolution of diatom tubulins and the evolution of diatoms suggests that molecular changes in α-, β- and γ-tubulins could be one of the factors in the formation of a high morphological diversity of diatoms.

Effect of Strain Rate on Single Tau, Dimerized Tau and Tau-Microtubule Interface: A Molecular Dynamics Simulation Study

Microtubule-associated protein (MAP) tau is a cross-linking molecule that provides structural stability to axonal microtubules (MT). It is considered a potential biomarker for Alzheimer's disease (AD), dementia, and other neurological disorders. It is also a signature protein for Traumatic Brain Injury (TBI) assessment. In the case of TBI, extreme dynamic mechanical energies can be felt by the axonal cytoskeletal members. As such, fundamental understandings of the responses of single tau protein, polymerized tau protein, and tau-microtubule interfaces under high-rate mechanical forces are important. This study attempts to determine the high-strain rate mechanical behavior of single tau, dimerized tau, and tau-MT interface using molecular dynamics (MD) simulation. The results show that a single tau protein is a highly stretchable soft polymer. During deformation, first, it significantly unfolds against van der Waals and electrostatic bonds. Then it stretches against strong covalent bonds. We found that tau acts as a viscoelastic material, and its stiffness increases with the strain rate. The unfolding stiffness can be ~50-500 MPa, while pure stretching stiffness can be >2 GPa. The dimerized tau model exhibits similar behavior under similar strain rates, and tau sliding from another tau is not observed until it is stretched to >7 times of original length, depending on the strain rate. The tau-MT interface simulations show that very high strain and strain rates are required to separate tau from MT suggesting Tau-MT bonding is stronger than MT subunit bonding between themselves. The dimerized tau-MT interface simulations suggest that tau-tau bonding is stronger than tau-MT bonding. In summary, this study focuses on the structural response of individual cytoskeletal components, namely microtubule (MT) and tau protein. Furthermore, we consider not only the individual response of a component, but also their interaction with each other (such as tau with tau or tau with MT). This study will eventually pave the way to build a bottom-up multiscale brain model and analyze TBI more comprehensively.

Independent tubulin binding and polymerization by the proline-rich region of Tau is regulated by Tau's N-terminal domain

Tau is an intrinsically disordered, microtubule-associated protein that has a role in regulating microtubule dynamics. Despite intensive research, the molecular mechanisms of Tau-mediated microtubule polymerization are poorly understood. Here we used single-molecule fluorescence to investigate the role of Tau's N-terminal domain (NTD) and proline-rich region (PRR) in regulating interactions of Tau with soluble tubulin. We assayed both full-length Tau isoforms and truncated variants for their ability to bind soluble tubulin and stimulate microtubule polymerization. We found that Tau's PRR is an independent tubulin-binding domain that has tubulin polymerization capacity. In contrast to the relatively weak interactions with tubulin mediated by sites distributed throughout Tau's microtubule-binding region (MTBR), resulting in heterogeneous Tau: tubulin complexes, the PRR bound tubulin tightly and stoichiometrically. Moreover, we demonstrate that interactions between the PRR and MTBR are reduced by the NTD through a conserved conformational ensemble. On the basis of these results, we propose that Tau's PRR can serve as a core tubulin-binding domain, whereas the MTBR enhances polymerization capacity by increasing the local tubulin concentration. Moreover, the NTD appears to negatively regulate tubulin-binding interactions of both of these domains. The findings of our study draw attention to a central role of the PRR in Tau function and provide mechanistic insight into Tau-mediated polymerization of tubulin.

Dynamical decoration of stabilized-microtubules by Tau-proteins

Tau is a microtubule-associated protein that regulates axonal transport, stabilizes and spatially organizes microtubules in parallel networks. The Tau-microtubule pair is crucial for maintaining the architecture and integrity of axons. Therefore, it is essential to understand how these two entities interact to ensure and modulate the normal axonal functions. Based on evidence from several published experiments, we have developed a two-dimensional model that describes the interaction between a population of Tau proteins and a stabilized microtubule at the scale of the tubulin dimers (binding sites) as an adsorption-desorption dynamical process in which Tau can bind on the microtubule outer surface via two distinct modes: a longitudinal (along a protofilament) and lateral (across adjacent protofilaments) modes. Such a process yields a dynamical distribution of Tau molecules on the microtubule surface referred to as microtubule decoration that we have characterized at the equilibrium using two observables: the total microtubule surface coverage with Tau's and the distribution of nearest neighbors Tau's. Using both analytical and numerical approaches, we have derived expressions and computed these observables as a function of key parameters controlling the binding reaction: the stoichiometries of the Taus in the two binding modes, the associated dissociation constants and the ratio of the Tau concentration to that of microtubule tubulin dimers.

Microtubule Dynamics in Neuronal Development, Plasticity, and Neurodegeneration

Neurons are the basic information-processing units of the nervous system. In fulfilling their task, they establish a structural polarity with an axon that can be over a meter long and dendrites with a complex arbor, which can harbor ten-thousands of spines. Microtubules and their associated proteins play important roles during the development of neuronal morphology, the plasticity of neurons, and neurodegenerative processes. They are dynamic structures, which can quickly adapt to changes in the environment and establish a structural scaffold with high local variations in composition and stability. This review presents a comprehensive overview about the role of microtubules and their dynamic behavior during the formation and maturation of processes and spines in the healthy brain, during aging and under neurodegenerative conditions. The review ends with a discussion of microtubule-targeted therapies as a perspective for the supportive treatment of neurodegenerative disorders.

Collapsin response mediator proteins (CRMPs) are a new class of microtubule-associated protein (MAP) that selectively interacts with assembled microtubules via a taxol-sensitive binding interaction

Collapsin response mediator proteins are ubiquitously expressed from multiple genes (CRMPs 1-5) and play important roles in dividing cells and during semaphorin 3A (Sema3A) signaling. Nonetheless, their mode of action remains opaque. Here we carried out in vivo and in vitro assays that demonstrate that CRMPs are a new class of microtubule-associated protein (MAP). In experiments with CRMP1 or CRMP2 and their derivatives, only the C-terminal region (residues 490-572) mediated microtubule binding. The in vivo microtubule association of CRMPs was abolished by taxol or epothilone B, which is highly unusual. CRMP2-depleted cells exhibited destabilized anaphase astral microtubules and altered spindle position. In a cell-based assay, all CRMPs stabilized interphase microtubules against nocodazole-mediated depolymerization, with CRMP1 being the most potent. Remarkably, a 82-residue C-terminal region of CRMP1 or CRMP2, unrelated to other microtubule binding motifs, is sufficient to stabilize microtubules. In cells, we demonstrate that glycogen synthase kinase-3β (GSK3β) inhibition potentiates this activity. Thus, CRMPs are a new class of MAP that binds through a unique motif, but in common with others such as Tau, is antagonized by GSK3β. This regulation is consistent with such kinases being critical for the Sema3A (collapsin) pathway. These findings have implications for cancer and neurodegeneration.

The C terminus of tubulin, a versatile partner for cationic molecules: binding of Tau, polyamines, and calcium

The C-terminal region of tubulin is involved in multiple aspects of the regulation of microtubule assembly. To elucidate the molecular mechanisms of this regulation, we study here, using different approaches, the interaction of Tau, spermine, and calcium, three representative partners of the tubulin C-terminal region, with a peptide composed of the last 42 residues of α1a-tubulin. The results show that their binding involves overlapping amino acid stretches in the C-terminal tubulin region: amino acid residues 421-441 for Tau, 430-432 and 444-451 for spermine, and 421-443 for calcium. Isothermal titration calorimetry, NMR, and cosedimentation experiments show that Tau and spermine have similar micromolar binding affinities, whereas their binding stoichiometry differs (C-terminal tubulin peptide/spermine stoichiometry 1:2, and C-terminal tubulin peptide/Tau stoichiometry 8:1). Interestingly, calcium, known as a negative regulator of microtubule assembly, can compete with the binding of Tau and spermine with the C-terminal domain of tubulin and with the positive effect of these two partners on microtubule assembly in vitro. This observation opens up the possibility that calcium may participate in the regulation of microtubule assembly in vivo through direct (still unknown) or indirect mechanism (displacement of microtubule partners). The functional importance of this part of tubulin was also underlined by the observation that an α-tubulin mutant deleted from the last 23 amino acid residues does not incorporate properly into the microtubule network of HeLa cells. Together, these results provide a structural basis for a better understanding of the complex interactions and putative competition of tubulin cationic partners with the C-terminal region of tubulin.

Prion protein region 23-32 interacts with tubulin and inhibits microtubule assembly

In previous studies we have demonstrated that prion protein (PrP) binds directly to tubulin and this interaction leads to the inhibition of microtubule formation by inducement of tubulin oligomerization. This report is aimed at mapping the regions of PrP and tubulin involved in the interaction and identification of PrP domains responsible for tubulin oligomerization. Preliminary studies focused our attention to the N-terminal flexible part of PrP encompassing residues 23-110. Using a panel of deletion mutants of PrP, we identified two microtubule-binding motifs at both ends of this part of the molecule. We found that residues 23-32 constitute a major site of interaction, whereas residues 101-110 represent a weak binding site. The crucial role of the 23-32 sequence in the interaction with tubulin was confirmed employing chymotryptic fragments of PrP. Surprisingly, the octarepeat region linking the above motifs plays only a supporting role in the interaction. The binding of Cu(2+) to PrP did not affect the interaction. We also demonstrate that PrP deletion mutants lacking residues 23-32 exhibit very low efficiency in the inducement of tubulin oligomerization. Moreover, a synthetic peptide corresponding to this sequence, but not that identical with fragment 101-110, mimics the effects of the full-length protein on tubulin oligomerization and microtubule assembly. At the cellular level, peptide composed of the PrP motive 23-30 and signal sequence (1-22) disrupted the microtubular cytoskeleton. Using tryptic and chymotryptic fragments of alpha- and beta-tubulin, we mapped the docking sites for PrP within the C-terminal domains constituting the outer surface of microtubule.

Complementary dimerization of microtubule-associated tau protein: Implications for microtubule bundling and tau-mediated pathogenesis

Tau is an intrinsically unstructured microtubule (MT)-associated protein capable of binding to and organizing MTs into evenly spaced parallel assemblies known as "MT bundles." How tau achieves MT bundling is enigmatic because each tau molecule possesses only one MT-binding region. To dissect this complex behavior, we have used a surface forces apparatus to measure the interaction forces of the six CNS tau isoforms when bound to mica substrates in vitro. Two types of measurements were performed for each isoform: symmetric configuration experiments measured the interactions between two tau-coated mica surfaces, whereas "asymmetric" experiments examined tau-coated surfaces interacting with a smooth bare mica surface. Depending on the configuration (of which there were 12), the forces were weakly adhesive, strongly adhesive, or purely repulsive. The equilibrium spacing was determined mainly by the length of the tau projection domain, in contrast to the adhesion force/energy, which was determined by the number of repeats in the MT-binding region. Taken together, the data are incompatible with tau acting as a monomer; rather, they indicate that two tau molecules associate in an antiparallel configuration held together by an electrostatic "zipper" of complementary salt bridges composed of the N-terminal and central regions of each tau monomer, with the C-terminal MT-binding regions extending outward from each end of the dimeric backbone. This tau dimer determines the length and strength of the linker holding two MTs together and could be the fundamental structural unit of tau, underlying both its normal and pathological action.

TTLL7 is a mammalian beta-tubulin polyglutamylase required for growth of MAP2-positive neurites

Microtubules form a cytoskeletal framework that influences cell shape and provides structural support for the cell. Microtubules in the nervous system undergo a unique post-translational modification, polyglutamylation of the C termini of their tubulin subunits. The mammalian enzymes that perform beta-tubulin polyglutamylation as well as their physiological functions in the neuronal tissue remain elusive. We report identification of a mammalian polyglutamylase with specificity for beta-tubulin as well as its distribution and function in neurite growth. To identify putative tubulin polyglutamylases, we searched tubulin tyrosine ligase-like (TTLL) proteins for those predominantly expressed in the nervous system. Of 13 TTLL proteins, TTLL7 was transcribed at the highest level in the nervous system. Recombinant TTLL7 catalyzed tubulin polyglutamylation with high preference to beta-tubulin in vitro. When expressed in HEK293T cells, TTLL7 demonstrated specificity for beta-tubulin and not for alpha-tubulin or nucleosome assembly protein 1. Consistent with these findings, knockdown of TTLL7 in a primary culture of superior cervical ganglion neurons caused a loss of polyglutamylated beta-tubulin. Following stimulation of PC12 cells with nerve growth factor to differentiate, the level of TTLL7 increased concomitantly with polyglutamylation of beta-tubulin. Short interference RNA-mediated knockdown of TTLL7 repressed nerve growth factor-stimulated MAP (microtubule-associated protein) 2-positive neurite growth in PC12 cells. Consistent with having a role in the growth of MAP2-positive neurites, TTLL7 accumulated within a MAP2-enriched somatodendritic portion of superior cervical ganglion, as did polyglutamylated beta-tubulin. Anti-TTLL7 antibody revealed that TTLL7 was distributed in a somatodendritic compartment in the mouse brain. These findings indicate that TTLL7 is a beta-tubulin polyglutamylase and is required for the growth of MAP2-positive neurites in PC12 cells.

Solution structure of microtubule-associated protein light chain 3 and identification of its functional subdomains

Microtubule-associated protein (MAP) light chain 3 (LC3) is a human homologue of yeast Apg8/Aut7/Cvt5 (Atg8), which is essential for autophagy. MAP-LC3 is cleaved by a cysteine protease to produce LC3-I, which is located in cytosolic fraction. LC3-I, in turn, is converted to LC3-II through the actions of E1- and E2-like enzymes. LC3-II is covalently attached to phosphatidylethanolamine on its C terminus, and it binds tightly to autophagosome membranes. We determined the solution structure of LC3-I and found that it is divided into N- and C-terminal subdomains. Additional analysis using a photochemically induced dynamic nuclear polarization technique also showed that the N-terminal subdomain of LC3-I makes contact with the surface of the C-terminal subdomain and that LC3-I adopts a single compact conformation in solution. Moreover, the addition of dodecylphosphocholine into the LC3-I solution induced chemical shift perturbations primarily in the C-terminal subdomain, which implies that the two subdomains have different sensitivities to dodecylphosphocholine micelles. On the other hand, deletion of the N-terminal subdomain abolished binding of tubulin and microtubules. Thus, we showed that two subdomains of the LC3-I structure have distinct functions, suggesting that MAP-LC3 can act as an adaptor protein between microtubules and autophagosomes.

Interaction of cellular tubulin with Sendai virus M protein regulates transcription of viral genome

Cellular tubulin has been shown to activate in vitro transcription with Sendai virus (SeV) particles. In this study, the molecular basis for the transcriptional activation by tubulin was investigated. We showed that tubulin dissociates viral matrix (M) protein, which acts as a negative regulator for transcription, from viral ribonucleoprotein (RNP) consisting of L, P, N proteins, and the genome RNA. Both alpha and beta subunits of human tubulin, which were expressed as GST fusion proteins, were found to stimulate viral mRNA synthesis similar to native alpha/beta-heterodimer tubulin. Pull-down assay using GST-tubulin subunits demonstrated that M protein is released from the RNP as a complex with each tubulin subunit. In vitro-binding analyses revealed that M protein directly interacts with tubulin as well as microtubules. These findings suggest that interaction of M protein with tubulin may have an important role in the regulation of SeV transcription.

Tubulin, actin, and tau protein interactions and the study of their macromolecular assemblies

The intracellular polymerization of cytoskeletal proteins into their supramolecular assemblies raises many questions regarding the regulatory patterns that control this process. Binding experiments using the ELISA solid phase system, together with protein assembly assays and electron microscopical studies provided clues on the protein-protein associations in the polymerization of tubulin and actin networks. In vitro reconstitution experiments of these cytoskeletal filaments using purified tau, tubulin, and actin proteins were carried out. Tau protein association with tubulin immobilized in a solid phase support system was inhibited by actin monomer, and a higher inhibition was attained in the presence of preassembled actin filaments. Conversely, tubulin and assembled microtubules strongly inhibited tau interaction with actin in the solid phase system. Actin filaments decreased the extent of in vitro tau-induced tubulin assembly. Studies on the morphological aspects of microtubules and actin filaments coexisting in vitro, revealed the association between both cytoskeletal filaments, and in some cases, the presence of fine filamentous structures bridging these polymers. Immunogold studies showed the association of tau along polymerized microtubules and actin filaments, even though a preferential localization of labeled tau with microtubules was revealed. The studies provide further evidence for the involvement of tau protein in modulating the interactions of microtubules and actin polymers in the organization of the cytsokeletal network.

Nonsaturable binding indicates clustering of tau on the microtubule surface in a paired helical filament-like conformation

Tau protein modulates microtubule dynamics and forms insoluble aggregates in Alzheimer's disease. Because there is a discrepancy between reported affinities of Tau to microtubules, we determined the interaction over a wide concentration range using a sensitive enzyme-linked immunosorbent assay. We found that the interaction is biphasic and not monophasic as assumed earlier. The first binding phase is typical for identical and noninteracting binding sites, with dissociation constants around 0.1 micrometer and stoichiometries around 0.2 Tau/tubulin dimer. Surprisingly, the second phase is nonsaturable and shows a nearly linear increase in bound Tau versus free Tau for free Tau concentrations higher than 2 micrometer. The slope is proportional to the microtubule concentration. From this we define an overloading parameter with values around 50 micrometer. The influence of Tau isoform, phosphorylation, and dimerization on both phases was investigated. Remarkably the overloading of Tau on microtubules leads to a thioflavin S fluorescence increase reminiscent of that seen with Tau aggregated into Alzheimer paired helical filaments. Because polyanions stimulate Tau aggregation and because the C-terminal domain of tubulin is polyanionic, we suggest that an early conformational change in Tau leading to paired helical filament aggregation occurs right on the microtubule surface.

Characterization of an alpha-tubulin gene of Cryptosporidium parvum

A gene encoding an alpha-tubulin of Cryptosporidium parvum was isolated and characterized. It had no introns, and encoded a 441-amino acid protein whose predicted ORF represented a typical alpha-tubulin protein with a MW of 50.5 kDa. This tubulin had an amino acid sequence similarity with Apicomplexa Plasmodium falciparum and Toxoplasma gondii higher than 88% and shared a number of conserved motifs.

The carboxy-terminal sequence Asp427-Glu432 of beta-tubulin plays an important function in axonemal motility

Flagellar motility is the result of specific interactions between axonemal microtubular proteins and the dynein motors. Tubulin, the main component of microtubule, is a very polymorphic protein resulting from the expression of several isogenes and from the existence of various post-translational modifications. In order to characterize tubulin isoforms and tubulin domains that are important for flagellar movement, we prepared monoclonal antibodies against axonemal proteins from whole sea-urchin sperm tails. The monoclonal antibodies obtained were screened for their potency to inhibit demembranated-reactivated sperm models and for their monospecific immunoreactivity on immunoblot. Among the different antibodies we obtained, D66 reacted specifically with a subset of beta-tubulin isoforms. Limited proteolysis, HPLC, peptide sequencing, mass spectroscopy and immunoblotting experiments indicated that D66 recognized an epitope localized in the primary sequence Gln423-Glu435 of the C-terminal domain of Lytechinus pictus beta2-tubulin, and that this sequence belongs to class IVb. The use of synthetic peptides and immunoblotting analysis further narrowed the amino acids important for antibody recognition to Asp427-Glu432. Because the primary effect of this antibody on sperm motility is to decrease the flagellar beat frequency, we suggest that this sequence is involved in the tubulin-dynein head interaction.

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.

Morphological transformation of liposomes caused by assembly of encapsulated tubulin and determination of shape by microtubule-associated proteins (MAPs)

To examine the role of cytoskeletons in cellular morphogenesis, we generated liposomes encapsulating tubulin, with or without microtubule-associated proteins (MAPs), and observed their transformation using dark-field microscopy. When tubulin was polymerized with MAPs in liposomes, liposomes were transformed into a "bipolar" shape with a central sphere and two tubular membrane protrusions that aligned in a straight line. On the other hand, when pure tubulin was polymerized in liposomes without MAPs, they initially transformed into a bipolar shape but subsequently re-transformed into a "monopolar" shape, i.e. a sphere with only one straight tubular portion. This re-transformation occurred in two ways: first, by shortening of one of the tubular portions due to microtubule disassembly; or second, by fluctuation of the central sphere toward one of the ends without shortening of the tube portion. MAPs prevented this re-transformation, and their role in stabilizing the shape of transformed liposomes was studied by the co-sedimentation method. The results show that MAPs, particularly MAP1 and MAP2, mediate binding between microtubules and the liposomal membrane. However, MAP2 by itself did not bind to liposomes, but was able to stabilize bipolar liposomes. This stabilization is caused not only by direct links between microtubules and liposomes, but also by prevention of Brownian motion of microtubules through an increase in friction.

Brassinolides promote the expression of a new Cicer arietinum beta-tubulin gene involved in the epicotyl elongation

A cDNA clone, CanTUB, encoding a putative beta-tubulin protein was isolated from a cDNA library constructed from 5-day old chickpea (Cicer arietinum) epicotyls. Analysis of its deduced amino acid sequence showed all the typical structural motifs of plant beta-tubulins. Putative sequences for autoregulation and tubulin mRNA stability, GTP and Ca2+/MAPs (microtubule-associated proteins) binding sites were present. Southern blot analysis of chickpea genomic DNA revealed that there are multiple beta-tubulin genes. The level of expression of beta-tubulin genes was correlated with the rate of growth in either seedlings and adult plants. The transcript levels of beta-tubulin genes were higher in actively elongating tissues such as etiolated epicotyls, roots and stem tissues of adult plants. Brassinolide-induced growth in chickpea epicotyls was accompanied by promotion of the expression of the gene coding for beta-tubulin.

Human natural resistance-associated macrophage protein is a new type of microtubule-associated protein

Natural resistance-associated macrophage protein 1 (NRAMP1) is a putative membrane protein that dominates natural resistance to infection. An NRAMP1-glutathione S-transferase fusion protein was used to test the ability of the NRAMP1 NH2-terminal domain to bind to taxol-stabilized microtubules. Co-sedimentation analysis showed that the fusion protein binds to microtubules. Although the NH2-terminal domain of the NRAMP1 molecule has structural homology with the Pro-rich region of microtubule-associated protein 4 (MAP4), the presence of the MAP4 microtubule-binding domain fragment had little effect on the binding of the fusion protein to microtubules.

The interaction of Mip-90 with microtubules and actin filaments in human fibroblasts

The novel microtubule-interacting protein Mip-90 was originally isolated from HeLa cells by using affinity columns of agarose derivatized with peptides from the C-terminal regulatory domain on beta-tubulin. Biochemical and immunocytochemical data have suggested that the association of Mip-90 with the microtubule system contributes to its cellular organization. Here we report the interaction patterns of Mip-90 with microtubules and actin filaments in interphase human fibroblasts. A polyclonal monospecific antibody against Mip-90 was used for immunofluorescence microscopy analysis to compare the distribution patterns of this protein with tubulin and actin. A detailed observation of fibroblasts revealed the colocalization of Mip-90 with microtubules and actin filaments. These studies were complemented with experiments using cytoskeleton-disrupting drugs which showed that colocalization patterns of Mip-90 with microtubules and actin filaments requires the integrity of these cytoskeletal components. Interestingly, a colocalization of Mip-90 with actin at the leading edge of fibroblasts grown under subconfluency was observed, suggesting that Mip-90 could play a role in actin organization, particularly at this cellular domain. Mip-90 interaction with actin polymers was further supported in vitro by cosedimentation and immunoprecipitation experiments. The cosedimentation analysis indicated that Mip-90 bound to actin filaments with an association constant Ka = 1 x 10(6) M-1, while an stoichiometry Mip-90/actin of 1:12 mol/mol was calculated. Western blots of the immunoprecipitates revealed that Mip-90 associated to both actin and tubulin in fibroblasts extracts. These studies indicate that Mip-90, described as a microtubule-interacting protein, also bears the capacity to interact with the microfilament network, suggesting that it may play a role in modulating the interactions between these cytoskeletal filaments in nonneuronal cells.

The association of tau-like proteins with vimentin filaments in cultured cells

There is increasing evidence that the different polymers that constitute the cytoskeleton are interconnected to form a three-dimensional network. The macromolecular interaction patterns that stabilize this network and its intrinsic dynamics are the basis for numerous cellular processes. Within this context, in vitro studies have pointed to the existence of specific associations between microtubules, microfilaments, and intermediate filaments. It has also been postulated that microtubule-associated proteins (MAPs) are directly involved in mediating these interactions. The interactions of tau with vimentin filaments, and its relationships with other filaments of the cytoskeletal network, were analyzed in SW-13 adenocarcinoma cells, through an integrated approach that included biochemical and immunological studies. This cell line has the advantage of presenting a wild-type clone (vim+) and a mutant clone (vim-) which is deficient in vimentin expression. We analyzed the cellular roles of tau, focusing on its interactions with vimentin filaments, within the context of its functional aspects in the organization of the cytoskeletal network. Cosedimentation experiments of microtubular protein with vimentin in cell extracts enriched in intermediate filaments, combined with studies on the direct interaction of tau with nitrocellulose-bound vimentin and analysis of tau binding to vimentin immobilized in single-strand DNA affinity columns, indicate that tau interacts with the vimentin network. These studies were confirmed by a quantitative analysis of the immunofluorescence patterns of cytoskeleton-associated tubulin, tau, and vimentin using flow cytometry. In this regard, a decrease in the levels of tau associated to the cytoskeletal network in the vim- cell mutant compared with the wild-type clones was observed. However, immunofluorescence data on SW-13 cells suggest that the absence of a structured network of vimentin in the mutant vim- cells does not affect the cytoplasmic organization formed by microtubules and actin filaments, when compared with the wild-type vim+ cells. These studies suggest that tau associates with vimentin filaments and that these interactions may play a structural role in cells containing these filaments.

Microtubule stabilization in pressure overload cardiac hypertrophy

Increased microtubule density, for which microtubule stabilization is one potential mechanism, causes contractile dysfunction in cardiac hypertrophy. After microtubule assembly, alpha-tubulin undergoes two, likely sequential, time-dependent posttranslational changes: reversible carboxy-terminal detyrosination (Tyr-tubulin left and right arrow Glu-tubulin) and then irreversible deglutamination (Glu-tubulin --> Delta2-tubulin), such that Glu- and Delta2-tubulin are markers for long-lived, stable microtubules. Therefore, we generated antibodies for Tyr-, Glu-, and Delta2-tubulin and used them for staining of right and left ventricular cardiocytes from control cats and cats with right ventricular hypertrophy. Tyr- tubulin microtubule staining was equal in right and left ventricular cardiocytes of control cats, but Glu-tubulin and Delta2-tubulin staining were insignificant, i.e., the microtubules were labile. However, Glu- and Delta2-tubulin were conspicuous in microtubules of right ventricular cardiocytes from pressure overloaded cats, i.e., the microtubules were stable. This finding was confirmed in terms of increased microtubule drug and cold stability in the hypertrophied cells. In further studies, we found an increase in a microtubule binding protein, microtubule-associated protein 4, on both mRNA and protein levels in pressure-hypertrophied myocardium. Thus, microtubule stabilization, likely facilitated by binding of a microtubule-associated protein, may be a mechanism for the increased microtubule density characteristic of pressure overload cardiac hypertrophy.

Isolation and characterization of a gene encoding alpha-tubulin from Candida albicans

A gene encoding the alpha-tubulin of Candida albicans has been cloned and characterized. Nucleotide sequence analysis reveals the presence of an intron within the structural gene and predicts the synthesis of a polypeptide of 448 amino acid residues. Comparison of nucleotide and amino acid sequences with the Saccharomyces cerevisiae alpha-tubulin encoding genes shows a 75% homology and about 92% similarity respectively. In contrast to S. cerevisiae, C. albicans appears to possess only one gene for alpha-tubulin which is able to functionally complement a S. cerevisiae cold-sensitive tub1 mutant.

Interaction of kinesin motor domains with alpha- and beta-tubulin subunits at a tau-independent binding site. Regulation by polyglutamylation

Interaction of rat kinesin and Drosophila nonclaret disjunctional motor domains with tubulin was studied by a blot overlay assay. Either plus-end or minus-end-directed motor domain binds at the same extent to both alpha- and beta-tubulin subunits, suggesting that kinesin binding is an intrinsic property of each tubulin subunit and that motor directionality cannot be related to a preferential interaction with a given tubulin subunit. Binding features of dimeric versus monomeric rat kinesin heads suggest that dimerization could drive conformational changes to enhance binding to tubulin. Competition experiments have indicated that kinesin interacts with tubulin at a Tau-independent binding site. Complementary experiments have shown that kinesin does not interact with the same efficiency with the different tubulin isoforms. Masking the polyglutamyl chains with a specific monoclonal antibody leads to a complete inhibition of kinesin binding. These results are consistent with a model in which polyglutamylation of tubulin regulates kinesin binding through progressive conformational changes of the whole carboxyl-terminal domain of tubulin as a function of the polyglutamyl chain length, thus modulating the affinity of tubulin for kinesin and Tau as well. These results indicate that microtubules, through tubulin polymorphism, do have the ability to control microtubule-associated protein binding.

Basis for increased microtubules in pressure-hypertrophied cardiocytes

BACKGROUND

We have shown the levels of the sarcomere and the cardiocyte that a persistent increase in microtubule density accounts to a remarkable degree for the contractile dysfunction seen in pressure-overload right ventricular hypertrophy. In the present study, we have asked whether these linked phenotypic and contractile abnormalities are an immediate and direct effect of load input into the cardiocyte or instead a concomitant of hypertrophic growth in response to pressure overloading.

METHODS AND RESULTS

The feline right ventricle was pressure-overloaded by pulmonary artery banding. The quantity of microtubules was estimated from immunoblots and immunofluorescent micrographs, and their mechanical effects were assessed by measuring sarcomere motion during microtubule depolymerization. The biogenesis of microtubules was estimated from Northern and Western blot analyses of tubulin mRNAs and proteins. These measurements were made in control cats and in operated cats during and after the completion of right ventricular hypertrophy; the left ventricle from each heart served as a normally loaded same-animal control. We have shown that the alterations in microtubule density and sarcomere mechanics are not an immediate consequence of pressure overloading but instead appear in parallel with the load-induced increase in cardiac mass. Of potential mechanistic importance, both these changes and increases in tubulin poly A+ mRNA and protein coexist indefinitely after a new, higher steady state of right ventricular mass is reached.

CONCLUSIONS

Because we find persistent increases both in microtubules and in their biosynthetic precursors in pressure-hypertrophied myocardium, the mechanisms for this cytoskeletal abnormality must be sought through studies of the control both of microtubule stability and of tubulin synthesis.

Syk, activated by cross-linking the B-cell antigen receptor, localizes to the cytosol where it interacts with and phosphorylates alpha-tubulin on tyrosine

Syk (p72syk) is a 72-kDa, nonreceptor, protein-tyrosine kinase that becomes tyrosine-phosphorylated and activated in B lymphocytes following aggregation of the B-cell antigen receptor. To explore the subcellular location of activated Syk, anti-IgM-activated B-cells were fractionated into soluble and particulate fractions by ultracentrifugation. Activated and tyrosine-phosphorylated Syk was found predominantly in the soluble fraction and was not associated with components of the antigen receptor. Similarly, the activated forms of Syk and its homolog, ZAP-70, were found in soluble fractions prepared from pervanadate-treated Jurkat T-cells. A 54-kDa protein that co-immunoprecipitated with Syk from the soluble fraction of activated B-cells was identified by peptide mapping as alpha-tubulin. alpha-Tubulin was an excellent in vitro substrate for Syk and was phosphorylated on a single tyrosine present within an acidic stretch of amino acids located near the carboxyl terminus. alpha-Tubulin was phosphorylated on tyrosine in intact cells following aggregation of the B-cell antigen receptor in a reaction that was inhibited by the Syk-selective inhibitor, piceatannol. Thus, once activated, Syk releases from the aggregated antigen receptor complex and is free to associate with and phosphorylate soluble proteins including alpha-tubulin.

Subpopulations of tau interact with microtubules and actin filaments in various cell types

It has been demonstrated that microtubule-associated proteins (MAPs) interact with tubulin in vitro and in vivo. However, there is no clear evidence on the possible roles of the interactions of MAPs in vivo with other cytoskeletal components in maintaining the integrity of the cell architecture. To address this question we extracted the neuronal cytoskeleton from brain cells and studied the selective dissociation of specific molecular isospecies of tau protein under various experimental conditions. Tau, and in some cases MPA-2, were analysed by the use of anti-idiotypic antibodies that recognize epitopes on their tubulin binding sites. Fractions of microtubule-bound tau isoforms were extracted with 0.35 M NaCl or after the addition of nocodazole to allow microtubule depolymerization. Protein eluted with this inhibitor contained most of the assembled tubulin dimer pool and part of the remaining tau and MAP-2. When the remaining cytoskeletal pellet was treated with cytochalasin D to allow depolymerization of actin filaments, only tau isoforms were extracted. Immunoprecipitation studies along with immunolocalization experiments in cell lines containing tau-like components supported the findings on the roles of tau isospecies as linkers between tubulin in the microtubular structure with actin filaments. Interestingly, in certain types of cells, antibody-reactive tau isospecies were detected by immunofluorescence with a discrete distribution pattern along actin filaments, which was affected by cytochalasin disruption of the actin filament network. These results suggest the possible in vivo roles of subsets of tau protein in modulating the interactions between microtubules and actin filaments.

A stomatin-like protein necessary for mechanosensation in C. elegans

The mec-2 gene is required for the function of a set of six touch receptor neurons in the nematode Caenorhabditis elegans; mec-2 mutants, which are touch-insensitive, have touch cells that appear morphologically normal. Gene interaction studies suggest that mec-2 positively regulates the activity of the putative mechanosensory transduction channel (and the present paper), comprised in part of proteins encoded by the two degenerin genes mec-4 and mec-10 The central region of the mec-2 protein (MEC-2) is very similar to stomatin, an integral membrane protein (band 7.2b) in human red blood cells that is thought to regulate cation conductance. MEC-2-LacZ fusions are distributed along the touch receptor axons. This axonal distribution, which is mediated by the mec-2-specific amino terminus, is disrupted by mutations in mec-12, an alpha-tubulin gene needed for touch cell function. Our results indicate that MEC-2 links the mechanosensory channel and the microtubule cytoskeleton of the touch receptor neurons. Such linkage provides the basis for a mechanism of mechanosensation whereby microtubule displacement leads to channel opening.

Beta tubulin of bull sperm is polyglycylated

Carboxy-terminal fragments of alpha and beta tubulin from bull sperm were isolated and characterized by automated sequencing and mass spectrometry. About 60% of sperm alpha tubulin is polyglycylated. The lateral chain, which can reach 13 residues in length, is covalently attached via an isopeptide bond. The fully detyrosinated sperm alpha tubulin lacks polyglycylation. Thus mammalian sperm microtubules differ from the ciliary axonemal microtubules of the protozoan Paramecium for which others have documented a complete polyglycylation of both alpha and beta tubulin.

A 205 kDa protein from non-neuronal cells in culture contains tubulin binding epitopes

Microtubule-associated proteins (MAPs) interact with tubulin in vitro and in vivo. Despite that there is a large amount of information on the roles of these proteins in neurons, the data on non-neuronal MAPs or MAPs-related proteins is scarce. There is an increasing number of microtubule-interacting proteins that have been identified in different cultured cell lines, and some of them share common functional epitopes with the most well-known MAPs, MAP-2 and tau. In a search for tubulin-interacting proteins in non-neuronal cells we identified a 205 kDa protein in the monkey kidney Vero cells in culture, on the basis of immunological studies and affinity chromatography. This protein interacts with the C-terminal moiety of beta-tubulin and cosediments with taxol assembled microtubules, but it was not recovered after successive cycles of assembly and disassembly. The presence of neuronal MAPs such as MAP-1, MAP-2 and tau was not detected in these cells. Interestingly, the studies showed that the 205 kDa protein contained a tubulin binding motif which was recognized by site-directed antibodies that also tag tubulin binding epitopes on MAP-2 and tau. This characteristic led us to designate this protein as MBD-205, a component that shares binding domains with these MAPs, rather than as a marker of the MAPs family. On the other hand, immunofluorescence experiments using site-specific antibodies, i.e. MAP-reacting monoclonal anti-idiotypic reagent MTB6.22 and a polyclonal antibody to the second tau repeat, revealed a MBD-205 co-localization with membrane structures and microtubule-organizing centers in Vero cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Stabilization and bundling of subtilisin-treated microtubules induced by microtubule associated proteins

The acidic carboxy-terminal regions of alpha- and beta-tubulin subunits are currently thought to be centrally involved in microtubule stability and in microtubule association with a variety of proteins (MAPs) such as MAP2 and tau proteins. Here, pure tubulin microtubules were exposed to subtilisin to produce polymers composed of cleaved tubulin subunits lacking carboxy termini. Polymer exposure to subtilisin was achieved in buffer conditions compatible with further tests of microtubule stability. Microtubules composed of normal alpha-tubulin and cleaved beta-tubulin were indistinguishable from control microtubules with regard to resistance to dilution-induced disassembly, to cold temperature-induced disassembly and to Ca(2+)-induced disassembly. Microtubules composed of cleaved alpha- and beta-tubulins showed normal sensitivity to dilution-induced disassembly and to low temperature-induced disassembly, but marked resistance to Ca(2+)-induced disassembly. Polymers composed of normal alpha-tubulin and cleaved beta-tubulin or of cleaved alpha- and beta-tubulins were stabilized in the presence of added MAP2, myelin basic protein and histone H1. Cleavage of tubulin carboxy termini greatly potentiated microtubule stabilization by tau proteins. We show that this potentiation of polymer stabilization can be ascribed to tau-induced microtubule bundling. In our working conditions, such bundling upon association with tau proteins occurred only in the case of microtubules composed of cleaved alpha- and beta-tubulins and triggered apparent microtubule cross-stabilization among the bundled polymers. These results, as well as immunofluorescence analysis, which directly showed interactions between subtilisin-treated microtubules and MAPs, suggest that the carboxy termini of alpha- and beta-tubulins are not primarily involved in the binding of MAPs onto microtubules. However, interactions between tubulin carboxy termini and MAPs remain possible and might be involved in the regulation of MAP-induced microtubule bundling.

Class I and IVa beta-tubulin isotypes expressed in adult mouse brain are glutamylated

Several types of post-translational modifications contribute to the high level of tubulin heterogeneity in the brain. An important modification is glutamylation of the major brain-specific isotypes, such as class Ia/b of alpha-tubulin and classes II and III of beta-tubulin. Here we describe experiments to determine if additional, minor tubulin isotypes, expressed in adult mouse brain, could also be glutamylated. Purified tubulin from adult mouse brain was cleaved with thermolysin. Proteolytically released carboxy-terminal peptides of both alpha- and beta-tubulin were isolated by sequential anion exchange and reverse-phase column-chromatography. Anionic peptides were then characterized by amino acid sequencing and mass spectrometry. We show that brain-specific class IVa and constitutive class I beta-tubulin isotypes can be glutamylated, at Glu434 and Glu441, respectively.

Energy transfer studies of the distances between the colchicine, ruthenium red, and bisANS binding sites on calf brain tubulin

Fluorescence energy transfer experiments were performed in order to measure the spatial separation between the colchine and Ruthenium Red binding sites, the high-affinity bisANS and Ruthenium Red sites, and the allocolchicine and high-affinity bisANS sites on calf brain tubulin. Energy transfer was observed between both colchicine and allocolchicine and Ruthenium Red, resulting in a distance of 40-45 A between these sites on the tubulin molecule. No detectable energy transfer could be observed when allocolchicine was used as fluorescence donor and bisANS as acceptor or when bisANS was used as donor and Ruthenium Red as acceptor. This indicates that the distance of separation between the allocolchicine and bisANS sites is greater than 50 A, while that between the bisANS and Ruthenium Red sites is greater than 72 A. On the basis of these and previous distance measurements (Ward & Timasheff, 1988), two triangles of binding sites have been defined (colchicine-bisANS-E-site and colchicine-bisANS-Ruthenium Red). Since the dihedral angle between them is not known, a schematic model has been drawn with all the sites located in a single plane.(ABSTRACT TRUNCATED AT 250 WORDS)

Cloning, genomic organization and transcription of the Entamoeba histolytica alpha-tubulin-encoding gene

We have isolated and characterized an Entamoeba histolytica alpha-tubulin (alpha Tub)-encoding gene (Eh alpha tub). A 700-bp DNA fragment was amplified by PCR using primers derived from consensus alpha- and beta-Tub amino acid (aa) sequences from different organisms and E. histolytica DNA as the template. These PCR fragments were used to screen both genomic DNA and cDNA libraries in order to isolate an Eh alpha tub structural gene. Two overlapping clones containing the complete alpha tub ORF (1392 bp) were isolated from the genome and cDNA libraries. The deduced aa sequence of Eh alpha Tub has 55.5, 50 and 52% identity to Plasmodium falciparum alpha Tub 2, Saccharomyces cerevisiae alpha Tub 2 and human alpha Tub, respectively. Interestingly, the predicted Eh alpha Tub protein lacks a poly-acidic motif at its C terminus which is involved in Tub polymerization and microtubule-associated protein binding in other organisms. This fact may indicate a difference in tubule assembly in this organism and could provide a potential key for the development of therapeutic agents. According to Southern blot experiments and the sequences of several clones, at least two non-adjacent copies of alpha tub are present in the E. histolytica genome. A 1.5-kb transcript corresponding to the alpha tub mRNA was detected in mRNA from asynchronous E. histolytica trophozoites.

Carboxyl terminal sequences of beta-tubulin involved in the interaction of HMW-MAPs. Studies using site-specific antibodies

After the finding of the involvement of the C-terminal moieties of tubulin subunits in the interaction of MAPs, different studies have focused on the substructure of the binding domains for the different MAPs. Current biochemical evidence point to the role of a low-homology sequence between alpha and beta-subunits within the conserved region of the C-terminal domain of tubulin, in the binding of MAP-2 and tau. Another line of studies indicates that a site for interaction of the high molecular weight MAPs is located in the variable region defined by the glutamic-rich C-terminus of beta-tubulin. Here, we report the usefulness of idiotypic site-directed antibodies, produced by immunization with peptides from different beta-tubulin isoforms, to study both MAP-1 and MAP-2 binding sites on tubulin. On the basis of these results with site-specific antibodies along with previous structural information (Cross et al., 1991, Biochemistry 30: 4362-4366), we propose the role of consensus sequences, from the invariant beta-tubulin C-terminal domain in the binding of MAP-2 and from the variable domain in the interactions of MAP-1 and MAP-2.

Purification and characterization of the high molecule weight microtubule associated proteins from neonatal rat brain

The changes in the levels of microtubule-associated proteins (MAPs) during advanced embryonic stages, neonatal and adult organisms reflect the importance of these cytoskeletal proteins in relation to the morphogenesis of the central nervous system. MAP-1B is found in prenatal brains and it appears to have the highest levels in neonatal rat brains, being a developmentally-regulated protein. In this research, a fast procedure to isolate MAP-1B, as well as MAP-2 and MAP-3 from neonatal rat brains was designed, based on the differential capacity of poly L-aspartic acid to release MAPs during temperature-dependent cycles of microtubule assembly in the absence of taxol. The high molecular weight MAP-1B was recovered in the warm supernatants after microtubular protein polymerization in the presence of low concentrations of polyaspartic acid. Instead, MAP-2 and a 180 kDa protein with characteristics of MAP-3 remained associated to the polymer after the assembly. Further purification of MAP-1B was attained after phosphocellulose chromatography. Isolation of MAP-2 isoforms together with MAP-3 was achieved on the basis of their selective interactions with calmodulin-agarose affinity columns. In addition, MAP-2 and MAP-3 were also purified on the basis of their capacities to interact with the tubulin peptide beta-II (422-434) derivatized on an Affigel matrix. However, MAP-1B did not interact with the beta-II tubulin fragment, but it showed interaction with the Affigel-conjugated beta-I (431-444) tubulin peptide. The different MAPs components were characterized by western blots using specific monoclonal antibodies.(ABSTRACT TRUNCATED AT 250 WORDS)

A beta-tubulin gene of Naegleria encodes a carboxy-terminal tyrosine. Aromatic amino acids are conserved at carboxy termini

A gene that directs the programmed synthesis of flagellar beta-tubulin during the rapid differentiation of Naegleria gruberi from amoebae to flagellates has been cloned and sequenced. The intronless gene is one of 8 to 10 similar but non-identical genes that are dispersed in the genome. beta-Tubulin mRNA homologous to this gene family is expressed transiently during differentiation, and has not been detected in amoebae. The encoded beta-tubulin is strongly conserved, with features that closely resemble the beta-tubulins of diverse organisms, especially organisms that, like Naegleria, use tubulin to assemble flagellar axonemes. In most sequenced alpha-tubulins, the encoded carboxy-terminal amino acid is tyrosine, which undergoes post-translational removal and readdition, conserved processes of unknown function. In N. gruberi, unusually, the terminus of alpha-tubulin is encoded as glutamine while that of beta-tubulin is tyrosine. The presence of these divergent termini on subunits of a conserved tubulin provoked us to re-examine aromatic amino acids at the termini of alpha- and beta-tubulins. Although evolution has tinkered extensively with the carboxy-terminal domains of tubulin subunits, we find an unexpected conservation. In every organism or cell type for which both tubulin subunits have been sequenced, except the ciliate Stylonychia lemnae, at least one tubulin subunit of some or all tubulin heterodimers terminates in an aromatic amino acid, either tyrosine or phenylalanine. This remarkable conservation of carboxy-terminal aromatic amino acids suggests that these residues serve some crucial function.

Cellular microtubules heterogeneous in their content of microtubule-associated protein 4 (MAP4)

Previous immunolocalization studies using many primate cultured cell lines demonstrated that a microtubule-associated protein of M(r) approximately 210,000 which is now called MAP4, is present along the length of microtubules in interphase and mitotic cells [Bulinski and Borisy (1980) J. Cell Biol. 87:802-808; DeBrabander et al. (1981) J. Cell Biol. 91:438-455]. Since MAP4 has been implicated as a microtubule stabilizer, we asked whether all classes of microtubules possess an equal complement of MAP4. We have reexamined the cellular distribution of MAP4, using both conventional double-label immunofluorescence and an antibody blocking technique [Schulze and Kirschner (1987) J. Cell Biol. 104:277-288] to highlight microtubules lacking, or depleted in, MAP4. These techniques have revealed that thin processes extending from monkey kidney cells (TC-7), and those made by human neuroblastoma cells (IMR-32) in response to retinoic acid, are often deficient in MAP4 immunoreactivity. Since both types of cellular processes contain stable microtubules, which are enriched in detyrosinated (Glu) tubulin, we tested the ability of MAP4 to bind to microtubules made from pure Glu and pure tyrosinated (Tyr) tubulin in vitro. MAP4 bound to both types of microtubules, and the similar saturation level of MAP4 binding to Glu and Tyr microtubules suggested that differential binding to these forms of tubulin does not contribute directly to a mechanism for segregation of MAP4 on microtubules in vivo. In TC-7 cells, we also observed MAP4-depletion on single microtubules, distal regions of broad cytoplasmic extensions, and midbodies of dividing cells. MAP4 depletion may reflect recent, rapid growth of microtubules to which MAP4 has not yet bound, or the presence of other MAPs that may compete with MAP4 for binding sites on the MT. We suggest that different levels of MAP4 on microtubules may directly modulate microtubule dynamics within single cells, as well as other microtubule functions such as those involving microtubule motor activity.

Assembly and bundling of marginal band microtubule protein: role of tau

Microtubule protein extracted from dogfish erythrocyte cytoskeletons by disassembly of marginal bands at low temperature formed linear microtubule (MT) bundles upon reassembly at 22 degrees C. The bundles, which were readily visible by video-enhanced phase contrast or DIC microscopy, increased in length and thickness with time. At steady state after 1 hour, most bundles were 6-11 microns in length and 2-5 MTs in thickness. No inter-MT cross-bridges were visible by negative staining. The bundles exhibited mechanical stability in flow as well as flexibility, in this respect resembling native marginal bands. As analyzed by SDS-PAGE and immunoblotting, our standard extraction conditions yielded MT protein preparations and bundles containing tau protein but not high molecular weight MAPs such as MAP-2 or syncolin. In addition, late fractions of MT protein obtained by gel filtration were devoid of high molecular weight proteins but still produced MT bundles. The marginal band tau was salt-extractable and heat-stable, bound antibodies to mammalian brain tau, and formed aggregates upon desalting. Antibodies to tau blocked MT assembly, but both assembly and bundling occurred in the presence of antibodies to actin or syncolin. The MTs were "unbundled" by subtilisin or by high salt (0.5-1 M KCl or NaCl), consistent with tau involvement in bundling. High salt extracts retained bundling activity, and salt-induced unbundling was reversible with desalting. However, reversibility was observed only after salt-induced MT disassembly had occurred. Reconstitution experiments showed that addition of marginal band tau to preassembled MTs did not produce bundles, whereas tau presence during MT reassembly did yield bundles. Thus, in this system, tau appears to play a role in both MT assembly and bundling, serving in the latter function as a coassembly factor.