Phosphorylation of microtubule-associated proteins MAP2 and MAP4 by the protein kinase p110mark. Phosphorylation sites and regulation of microtubule dynamics

The phosphorylation of microtubule-associated proteins (MAPs) is thought to be a key factor in the regulation of microtubule stability. We have shown recently that a novel protein kinase, termed p110 microtubule-affinity regulating kinase ("MARK"), phosphorylates microtubule-associated protein tau at the KXGS motifs in the region of internal repeats and causes the detachment of tau from microtubules (Drewes, G., Trinczek, B., Illenberger, S., Biernat, J., Schmitt-Ulms, G., Meyer, H.E., Mandelkow, E.-M., and Mandelkow, E. (1995) J. Biol. Chem. 270, 7679-7688). Here we show that p110mark phosphorylates analogous KXGS sites in the microtubule binding domains of the neuronal MAP2 and the ubiquitous MAP4. Phosphorylation in vitro leads to the dissociation of MAP2 and MAP4 from microtubules and to a pronounced increase in dynamic instability. Thus, the phosphorylation of the repeated motifs in the microtubule binding domains of MAPs by p110mark might provide a mechanism for the regulation of microtubule dynamics in cells.

A muscle-specific variant of microtubule-associated protein 4 (MAP4) is required in myogenesis

Microtubule-associated protein 4 (MAP4) transcripts vary in different mouse tissues, with striated muscle (skeletal and cardiac) expressing 8- and 9-kb transcripts preferentially to the more widely distributed 5.5- and 6.5-kb transcripts (West, R. W., Tenbarge, K. M. and Olmsted, J. B. (1991). J. Biol. Chem. 266, 21886–21896). Cloning of the sequence unique to the muscle transcripts demonstrated that these mRNAs vary from the more ubiquitous ones by a single 3.2-kb coding region insertion within the projection domain of MAP4. During differentiation of the myogenic cell line, C2C12, muscle-specific MAP4 transcripts appear within 24 hours of growth in differentiation medium, and a larger MAP4 isotype (350 X 10(3) Mr) accumulates to high levels by 48 hours of differentiation. In situ hybridization analyses of transcript distribution in mouse embryos demonstrated that muscle-specific transcripts appear early in myogenesis. To block the expression of the muscle-specific MAP4, stable lines of C2C12 were generated bearing an antisense construct with the muscle-specific MAP4 sequence. Myoblast growth was unaffected whereas myotube formation was severely perturbed. Fusion occurred in the absence of the muscle MAP4 isotype, but the multinucleate syncytia were short and apolar, microtubules were disorganized and normal anisotropic myofibrils were absent. The patterns of expression of the muscle-specific transcripts and the antisense experiments indicated that this unique structural form of MAP4 plays a critical role in the formation and maintenance of muscle.

Analysis of MAP 4 function in living cells using green fluorescent protein (GFP) chimeras

MAP 4 is a ubiquitous microtubule-associated protein thought to play a role in the polymerization and stability of microtubules in interphase and mitotic cells. We have analyzed the behavior of protein domains of MAP 4 in vivo using chimeras constructed from these polypeptides and the green fluorescent protein (GFP). GFP-MAP 4 localizes to microtubules; this is confirmed by colocalization of GFP-MAP 4 with microtubules that have incorporated microinjected rhodamine-tubulin, and by loss of localized fluorescence after treatment of cells with anti-microtubule agents. Different subdomains of MAP 4 have distinct effects on microtubule organization and dynamics. The entire basic domain of MAP 4 reorganizes microtubules into bundles and stabilizes these arrays against depolymerization with nocodazole. Within the basic domain, the PGGG repeats, which are conserved with MAP 2 and tau, have a weak affinity for microtubules and are dispensable for microtubule binding, whereas the MAP 4-unique PSP region can function independently in binding. The projection domain shows no microtubule localization, but does modulate the association of various binding subdomains with microtubules. The acidic carboxy terminus of MAP 4 strongly affects the microtubule binding characteristics of the other domains, despite constituting less than 6% of the protein. These data show that MAP 4 association with microtubules is modulated by sequences both within and outside the basic domain. Further, our work demonstrates that GFP chimeras will allow an in vivo analysis of the effects of MAPs and their variants on microtubule dynamics in real time.

Stable expression of heterologous microtubule-associated proteins (MAPs) in Chinese hamster ovary cells: evidence for differing roles of MAPs in microtubule organization

To study the effects of microtubule-associated proteins (MAPs) on in vivo microtubule assembly, cDNAs containing the complete coding sequences of a Drosophila 205-kD heat stable MAP, human MAP 4, and human tau were stably transfected into CHO cells. Constitutive expression of the transfected genes was low in most cases and had no obvious effects on the viability of the transfected cell lines. High levels of expression, as judged by Western blots, immunofluorescence, and Northern blots, could be induced by treating cells with sodium butyrate. High levels of MAPs were maintained for at least 24-48 h after removal of the sodium butyrate. Immunofluorescence analysis indicated that all three MAPs bound to cellular microtubules, but only the transfected tau caused a rearrangement of microtubules into bundles. Despite high levels of expression of these exogenous MAPs and the bundling of microtubules in cells expressing tau, transfected cells had normal levels of assembled and unassembled tubulin. With the exception of the tau-induced bundles, microtubules in transfected cells showed the same sensitivity as control cells to microtubule depolymerization by Colcemid. Further, all three MAPs were ineffective in reversing the taxol-dependent phenotype of a CHO mutant cell line. The absence of a quantitative effect of any of these heterologous proteins on the assembly of tubulin suggests that these MAPs may have different roles in vivo from those inferred previously from in vitro experiments.

Mouse microtubule-associated protein 4 (MAP4) transcript diversity generated by alternative polyadenylation

Mouse microtubule-associated protein 4 (MAP4) is a protein that co-locates with microtubules in vivo. It is encoded by a single-copy gene that expresses multiple transcripts in most cell types [West et al., J. Biol. Chem. 266 (1991) 21886-21896]. This report describes the identification of two distinct 3'-untranslated regions (UTR) for MAP4 transcripts. The 3'-UTRs of the transcripts are identical up to the site of polyadenylation of the shorter mRNA. The longer transcript contains an additional 775 nucleotides after the first polyadenylation site. Both poly(A) tails follow the canonical polyadenylation site motif, AAUAAA. These data show that two different UTRs arise as a result of alternative polyadenylation site usage. Northern blots of RNA from different tissues probed with coding sequence show hybridization to the common 5.5- and 6.5-kb transcripts, whereas blots probed with sequence unique to the longer 3'-UTR show hybridization only to the 6.5-kb band. Both transcripts are found within the same cell type. In addition, muscle contains additional transcripts of 8 and 9 kb, of which only the 9-kb transcript hybridizes to the longer 3'-UTR probe.

A model for microtubule-associated protein 4 structure. Domains defined by comparisons of human, mouse, and bovine sequences

cDNAs encoding human and mouse microtubule-associated protein 4 (MAP 4) were isolated. MAP 4 is encoded by a single gene. Multiple MAP 4 mRNAs are transcribed that are differentially expressed among mouse tissues. Open reading frames for the human and mouse MAP 4 clones indicate three distinct regions consisting of related sequences with different motifs. Approximately 30% of the protein is tandem related repeats of approximately 14 amino acids. Another region contains clusters of serine and proline. Four 18-mer repeats characteristic of the microtubule-binding domains of MAP 2 and tau are located at the carboxyl-terminal portion of MAP 4. Amino acid sequence analysis revealed that human and mouse MAP 4 are homologs of the bovine 190-kDa MAP/MAP U (Aizawa, H., Emori, Y., Murofushi, H., Kawasakai, H., Sakai, H., and Suzuki, K. (1990) J. Biol. Chem. 265, 13849-13855). Mouse and human MAP 4 and the bovine 190-kDa MAP are approximately 75% similar, indicating that these proteins are all members of the same class. Domains with extremely high conservation (greater than or equal to 88%) are: 1) the extreme amino terminus; 2) a proline-rich region between the KDM and S,P domains; 3) the microtubule-binding domain; and 4) the extreme carboxyl terminus.

Non-motor microtubule-associated proteins

Cloning of primary sequences has generated information on the structures of the non-motor microtubule-associated proteins and their relationship to one another. Questions about how classes of microtubule-associated proteins interact are starting to be addressed in vitro and, in vivo, tests of function are being pursued using a variety of cellular and molecular biological strategies.

Cell cycle-dependent changes in the dynamics of MAP 2 and MAP 4 in cultured cells

To examine the behavior of microtubule-associated proteins (MAPs) in living cells, MAP 4 and MAP 2 have been derivatized with 6-iodoacetamido-fluorescein, and the distribution of microinjected MAP has been analyzed using a low light level video system and fluorescence redistribution after photobleaching. Within 1 min following microinjection of fluoresceinated MAP 4 or MAP 2, fluorescent microtubule arrays were visible in interphase or mitotic PtK1 cells. After cold treatment of fluorescent MAP 2-containing cells (3 h, 4 degrees C), microtubule fluorescence disappeared, and the only fluorescence above background was located at the centrosomes; microtubule patterns returned upon warming. Loss of microtubule immunofluorescence after nocodozole treatment was similar in MAP-injected and control cells, suggesting that injected fluorescein-labeled MAP 2 did not stabilize microtubules. The dynamics of the MAPs were examined further by FRAP. FRAP analysis of interphase cells demonstrated that MAP 2 redistributed with half-times slightly longer (60 +/- 25 s) than those for MAP 4 (44 +/- 20 s), but both types of MAPs bound to microtubules in vivo exchanged with soluble MAPs at rates exceeding the rate of tubulin turnover. These data imply that microtubules in interphase cells are assembled with constantly exchanging populations of MAP. Metaphase cells at 37 degrees C or 26 degrees C showed similar mean redistribution half-times for both MAP 2 and MAP 4; these were 3-4 fold faster than the interphase rates (MAP 2, t1/2 = 14 +/- 6 s; MAP 4, t1/2 = 17 +/- 5 s). The extent of recovery of spindle fluorescence in MAP-injected cells was to 84-94% at either 26 or 37 degrees C. Although most metaphase tubulin, like the MAPs, turns over rapidly and completely under physiologic conditions, published work shows either reduced rates or extents of turnover at 26 degrees C, suggesting that the fast mitotic MAP exchange is not simply because of fast tubulin turnover. Exchange of MAP 4 bound to telophase midbodies occurred with dynamics comparable to those seen in metaphase spindles (t1/2 = approximately 27 s) whereas midbody tubulin exchange was slow (greater than 300 s). These data demonstrate that the rate of MAP exchange on microtubules is a function of time in the cell cycle.

Microtubule-associated protein 4 antibody: a new marker for astroglia and oligodendroglia

An antibody to a 240,000 dalton microtubule-associated protein, microtubule-associated protein 4, was used to illustrate the distribution of this protein in semi-thin sections of the central nervous system. Immunofluorescence microscopy indicated that microtubule-associated protein 4 was restricted to non-neuronal elements of the brain and spinal cord. Astrocytes, oligodendrocytes and "specialized" glia, including tanycytes, Bergmann glia and Muller cells, contained microtubule-associated protein 4. This distribution of microtubule-associated protein 4 in neural tissue in distinct from that described for the other major brain microtubule-associated proteins, microtubule-associated protein 1 and microtubule-associated protein 2. The reactivity of MAP 4 antibody with these glia demonstrates the antigenic relatedness of these cells and further distinguishes glia from other elements of the nervous system.

MAP 4: a microtubule-associated protein specific for a subset of tissue microtubules

The cytological distribution of microtubule-associated protein 4 (MAP 4) (L. M. Parysek, C. F. Asnes, J. B. Olmsted, 1984, J. Cell Biol., 99:1309-1315) in mouse tissues has been examined. Adjacent 0.5-0.9-micron sections of polyethylene glycol-embedded tissues were incubated with affinity-purified MAP 4 or tubulin antibodies, and the immunofluorescent images were compared. Tubulin antibody labeling showed distinct microtubules in all tissues examined. MAP 4 antibody also labeled microtubule-like patterns, but the extent of MAP 4 reactivity was cell type-specific within each tissue. MAP 4 antibody labeled microtubules in vascular elements of all tissues and in other cells considered to have supportive functions, including Sertoli cells in the testis and glial elements in the nervous system. Microtubule patterns were also observed in cardiac, smooth, and skeletal (eye) muscle, podocytes in kidney, Kuppfer cells in liver, and spermatid manchettes. The only MAP 4-positive cells in which the pattern was not microtubule-like were the principal cells of the collecting ducts in kidney cortex, in which diffuse fluorescence was seen. MAP 4 antibody did not react with microtubule-rich neuronal elements of the central and peripheral nervous system, skeletal muscle from anterior thigh, liver parenchymal cells, columnar epithelial cells of the small intestine, and absorptive cells of the tubular component of the nephron. These observations indicate that MAP 4 may be associated with only certain kinds of cell functions as demonstrated by the preferential distribution with microtubules of defined cell types.

MAP 4: occurrence in mouse tissues

A polyclonal antiserum to a microtubule-associated protein (MAP) from mouse neuroblastoma cells (MAP 4) was used to examine the distribution of this protein in mouse tissues. Immunoblots of neuroblastoma cell microtubule protein preparations demonstrated that the antiserum reacted with a triplet of proteins at 215,000-240,000 mol wt. Antibodies affinity purified from any of the bands showed cross-reaction with the other bands, indicating these polypeptides were all immunologically related. Antibodies specific to MAP 4 decorated microtubules in cultured murine cells fixed with glutaraldehyde, and diffuse staining was seen following treatment of cells with nocodazole. The antiserum reacted with MAP 4 in extracts of brain, heart, liver, and lung from adult mouse; the triplet in brain was more closely spaced than in the other tissues or neuroblastoma cells. In kidney, spleen, and stomach, only a single band (band 4) was labeled; this band was immunologically related to the triplet and was also present in all tissues positive for the triplet. Skeletal muscle, sperm, and peripheral blood contained no reactive polypeptides. After taxol-induced polymerization, the MAP 4 triplet was preferentially associated with the microtubule pellet whereas band 4 remained in the supernatant. These data indicate that there is tissue specificity in the distribution of MAP 4, and that some tissues contain a polypeptide related to MAP 4 (band 4) that does not bind to microtubules in vitro.

Human anticentromere antibodies: distribution, characterization of antigens, and effect on microtubule organization

Properties of human anticentromere autoantibodies were analyzed. In intact cells or isolated cell fractions, these sera stain the centromeres of mitotic chromosomes and discrete speckles (prekinetochores) in nuclei. Staining is also retained in matrix preparations from nuclei or chromosomes. Immunoprecipitation or immunoblotting demonstrates protein antigens of 14, 20, 23, and 34 kd in HeLa nuclei and chromosomes; immunoprecipitates of nuclei also contain a protein of 15.5 kd. Matrix preparations contain only the 20, 23, and 34 kd species. Absorption of the anticentromere serum with any one of the four nuclear antigens immobilized on nitrocellulose is sufficient to eliminate centromere staining. Using a lysed cell model for microtubule nucleation, anticentromere sera are shown to inhibit specifically the organization of microtubules at the kinetochore.

A rapid procedure for preparing fluorescein-labeled specific antibodies from whole antiserum: its use in analyzing cytoskeletal architecture

A rapid method for the direct conjugation of affinity-purified antibodies with fluorescein (termed DCAPA) is described. This procedure involves the immobilization of antibodies as antigen-antibody complexes on nitrocellulose blots, and subsequently the bound antibodies are reacted with fluorescein isothiocyanate. An enriched sample of smooth muscle tropomysin transferred to nitrocellulose paper by the Western blotting procedure has been used as the affinity medium for purification of specific tropomyosin antibody from whole rabbit antiserum. Direct conjugation of the antibody with fluorescein was carried out following the binding of antibody to antigen. Direct conjugation and affinity purification of antibodies directed against tropomyosin was accomplished in 2-3 d using an enriched tropomyosin sample and whole antiserum directed against tropomyosin. The immunofluorescence images obtained with this procedure exhibit distinct advantages with regard to background fluorescence and overall specificity of antibody binding. The usefulness of this direct conjugation method in various experimental protocols is discussed.

Interaction of methylmercury with microtubules in cultured cells and in vitro

The effects of methylmercury (MeHg) on cytoplasmic microtubules in cultured fibroblasts and on the in vitro polymerization of microtubules were examined. MeHg caused disruption of cellular microtubules in a concentration- and time-dependent manner. Addition of the metal-chelating agent, dimercaptosuccinic acid (DMSA), both prevented and reversed the effect of MeHg. Comparisons of the cellular levels of mercury and microtubule integrity indicated that microtubules dissociated at levels higher than 0.6 microgram Hg/mg protein. In vitro polymerization was also directly inhibited by MeHg; this effect was prevented by the addition of DMSA.

A conserved histone variant enriched in nucleoli of mammalian cells

An antibody has been raised to Tetrahymena histone variant hv1 that specifically stains the macronucleus, but not the micronucleus, of Tetrahymena. This antiserum also stains small punctate regions in nucleoli of several mammalian cell lines. These observations suggest that this histone variant has been highly conserved in evolution and may be associated with transcribed sequences.

Immunolocalization of von Willebrand protein in Weibel-Palade bodies of human endothelial cells

Immunofluorescence staining of cultured human umbilical vein endothelial cells has shown the presence of von Willebrand protein in the perinuclear region, in small rodlike structures through the cytoplasm, and on filaments of the extracellular matrix. Nonendothelial cells showed no staining with anti-von Willebrand protein antiserum. At the light microscope level, immunoperoxidase treatment of endothelial cells revealed the same pattern and antibody specificity as the fluorescence staining. Thin sections of the peroxidase-stained cells showed decorated filaments close to the substratum and also specific deposits in the endoplasmic reticulum and Weibel-Palade bodies. Control antisera against other selected proteins in endothelial cells failed to stain the Weibel-Palade bodies. These data suggest that the Weibel-Palade bodies of endothelial cells are storage and/or processing organelles for von Willebrand protein.

Affinity purification of antibodies from diazotized paper blots of heterogeneous protein samples

A method is described for the affinity purification of antibodies using protein samples that have been electrophoretically transferred to diazotized paper. Using differentiated neuroblastoma cells as the protein sample and a heterogeneous anti-microtubule protein serum, antibodies were isolated that specifically bound only to tubulin on blots and that stained microtubule networks in cells. The general utility of this method for various types of applications is discussed.

Tubulin pools in differentiating neuroblastoma cells

The distribution of tubulin in soluble, reversibly stabilized (assembled) and insoluble forms has been determined in neuroblastoma cells undergoing microtubule-dependent neurite elongation. Procedures were developed to obtain reproducible tubulin fractions and to assay total tubulin. Radioimmunoassays showed that both differentiated and nondifferentiated cell contained approximately 4 pg of tubulin per cell, of which 3-10% was in an insoluble, particulate form. The amount of tubulin assembled in differentiated cells was four to five times greater than in nondifferentiated cells, constituting 48-63% and 11-16% of the total tubulin pool in the respective cell types. Calculation of the concentration of soluble tubulin indifferentiated cells (approximately 0.8 mg/ml) and nondifferentiated cells (approximately 1.6 mg/ml) indicates that a critical concentration of subunits probably does not limit the induction of microtubule formation during neurite elongation.

A microtubule-associated protein specific to differentiated neuroblastoma cells

A modified procedure is reported that enables microtubule assembly to occur in extracts of differentiated neuroblastoma cells. Of the proteins that co-assemble through five successive cycles, only two, with molecular weights of 215,000 and 71,000, are retained in constant ratio to tubulin. The 215,000-dalton protein is quantitatively sedimented with microtubules during later cycles of assembly, whereas the 71,000-dalton protein is distributed between assembling and nonassembling fractions. Similar procedures do not induce assembly for microtubules from nondifferentiated neuroblastoma cells. Two-dimensional gel analyses indicate that the differentiated cell extracts contain the 215,000-dalton protein. In contrast, gels of extracts from nondifferentiated cells show no protein in the equivalent region. These data suggest that the 215,000-dalton protein is a microtubule-associated protein that may play a role in microtubule-dependent neurite differentiation.

The quantitation of tubulin in neuroblastoma cells by radioimmunoassay

The amount of tubulin in a clone of neuroblastoma cells (Nb2a-AB-1) which undergoes microtubule-dependent neurite elongation has been determined. This clone undergoes shape differentiation without cessation of cell division, and both differentiated and nondifferentiated cells have the same cell cycle parameters. To quantitate tubulin pools, a radioimmunoassay was developed with a rabbit serum raised to sodium dodecyl sulfate-treated Tetrahymena axonemal tubulin. Using purified hog brain tubulin as labeled tracer and competitor, competition curves covering the range of 0.1 to 100 microgram/ml were obtained. Curves obtained using purified mouse brain or neuroblastoma tubulin as competitors were similar to those obtained with hog brain tubulin. The levels of competition obtained with hog tubulin, partially purified tubulin, or fixed microtubules were identical, indicating that the polymerization state of tubulin had no effect on measurements. Postmitochondrial supernatants derived either from cells grown in suspension culture (rounded morphology) or monolayer culture (neurite-bearing morphology) contained equivalent amounts of tubulin (3.6 +/- 0.5 pg/cell and 3.1 +/- 0.2 pg/cell, respectively); tubulin constituted 2.7% of the postmitochondrial supernatant protein in either cell type. These data indicated that cells utilizing microtubules primarily for mitosis or for cytoskeletal functions and mitosis show no net change in the total soluble tubulin pool.

Ionic and nucleotide requirements for microtubule polymerization in vitro

The ionic and nucleotide requirements for the in vitro polymerization of microtubules from purified brain tubulin have been characterized by viscometry. Protein was purified by successive cycles of a temperature dependent assembly-diassembly scheme. Maximal polymerization occurred at a concentration of 0.1 M Pipes (piperazine-N,N'-bis(2-ethanesulfonic acid)); increasing ionic strength by addition of NaCl to samples prepared in lower buffer concentrations did not result in an equivalent level of polymerization. Both Na-+ and K-+ inhibited microtubule formation at levels greater than 240 mM, withmaximal assembly occurring at physiological concentrations of 150 mM. Maximal extent of assembly occurred at pH 6.8 and optimal rate at pH 6.6. Inhibition of polymerization was half-maximal at added calcium concentrations of 1.0 mM and magnesium concentrations of 10.0 mM. EGTA (ethylene glycol bis(beta-aminoethyl ether)tetraacetic acid), which chelates Ca-2+, had no effect on polymerization over a concentration range of 0.01-10.0 mM. In contrast, EDTA (ethylenediaminetetraacetic acid), which chelates both Mg-2+ and Ca-2+, inhibited assemble half-maximally at 0.25 mM and totally at 2.0 mM. As determined from experiments using Mg-2+-EDTA buffers, magnesium was required for polymerization. Magnesium promoted the maximal extent of assembly at substoichiometric levels relative to tubulin, but was maximal for both rate and extent at stoichiometric concentrations. Elemental analyses indicated that approximately 1 mol of magnesium was tightly bound/mol of tubulin dimer. Viscosity development was dependent upon hydrolyzable nucleoside triphosphate, and stoichiometric levels of GTP were sufficient for maximal polymerization. The effect of magnesium in increasing the rate of GTP-dependent polymerization suggests that a Mg-2+-GTP complex is the substrate required for a step in assembly.

The ultrastructure of mouse neuroblastoma cells in tissue culture

Neuroblastoma cells grown in suspension culture are round and have no distinctive structural characteristics. However, cells transferred to substrates flatten, develop long neurites, and assume the morphology of normal neurons. The resemblance of monolayered neuroblastoma cells to normal neurons is amplified by treatment with hypertonic medium; under these conditions, cell division is inhibited and the neurites become long and differentiated. The treated cells contain clusters of clear vesicles, 400-600 A in diameter, which are morphologically indistinguishable from the synaptic vesicles of normal neurons. Specialized cell contacts are observed between the treated cells as well as between confluent cells grown in normal medium.

In vitro aggregation of cytoplasmic microtubule subunits

The colchicine-binding protein in procine-brain tissue is a dimer of molecular weight 110,000 that is believed to be the subunit of neuronal microtubules. Conditions are established under which the dimers aggregate with reproducible kinetics. This aggregation reaction, which is monitored by development of turbidity, has the following characteristics: (a) Colchicine inhibits development of turbidity; (b) the reaction inhibited by colchicine is reversed by long-wave ultraviolet irradiation; (c) the aggregation is temperature-dependent; (d) the reaction is nucleotide triphosphate-specific, being stimulated by 1 mM GTP; (e) the reaction appears to be specific for microtubule subunits since in the presence of other added proteins and in curde cell extracts, only microtubule subunits aggregate. On the basis of these criteria, we conclude that we have established an in vitro system for the aggregation of microtubule subunits that shares some of the properties characteristic of the in vivo assembly of cytoplasmic and spindle microtubules.

Nucleated assembly of microtubules in porcine brain extracts

Disk-type structures found in extracts of porcine brain tissue appear to be required for microtubule assembly in vitro. From the morphology of the disks and the dependence of microtubule assembly on the presence of these structures, we propose that the disks are nucleation centers for the polymerization of microtubule protein.

Comparison of the microtubule proteins of neuroblastoma cells, brain, and Chlamydomonas flagella

Intact A microtubules isolated from outer doublet microtubules of Chlamydomonas flagella contain two separable proteins (tubulins) that differ in molecular weight and in amino-acid composition. The microtubule protein isolated from brain or neuroblastoma cells also has two electrophoretically distinct tubulins. Although the two tubulins of brain and neuroblastoma cells are electrophoretically similar to each other, only one of these tubulins migrates with the flagellar tubulins. This is the first evidence that (a) isolated, morphologically intact, single microtubules from flagella contain at least two different tubulins, and (b) at least one of these tubulins differs from tubulins that are isolated from other sources.

Isolation of microtubule protein from cultured mouse neuroblastoma cells

The addition of vinblastine to high-speed supernatants derived from homogenates of cultured mouse neuroblastoma cells results in the formation of a precipitate which has been characterized as microtubule protein by the following criteria: colchicine-binding activity, molecular weight, amino acid composition, and electrophoretic mobility. The method therefore permits the rapid isolation of microtubule protein from crude supernatants of neuroblastoma cells.