Molecular characterization of high molecular weight microtubule-associated proteins: Some answers, many questions

Isolation of microtubules and microtubule proteins

This unit describes various protocols for the isolation and purification of the main constituents of microtubules, chiefly alpha- and beta-tubulin, and the most significant microtubule associated proteins (MAPs), specifically MAP1A, MAP1B, MAP2, and tau. We include a classical isolation method for soluble tubulin heterodimer as the first basic purification protocol. In addition, we show how to analyze the tubulin and MAPs obtained after a phosphocellulose chromatography purification procedure. This unit also details a powerful and simple method to determine the native state of the purified tubulin based on one-dimensional electrophoresis under nondenaturing conditions (UNIT 6.5). The last protocol describes the application of a new technique that allows visualizing the quality of polymerized microtubules based on atomic force microscopy (AFM).

Overexpression of the 77-kD echinoderm microtubule-associated protein (EMAP), a WD-40 repeat protein, in baculovirus-infected Sf9 cells

The purpose of this study was to test whether any assembly-promoting microtubule-associated protein (MAP) would bundle microtubules and induce process formation in recombinant baculovirus-infected Sf9 cells, in particular, whether a non-neural MAP from a normally rounded cell would produce cellular asymmetries. To carry out these experiments, we constructed a recombinant baculovirus that expressed the full-length 77-kD EMAP, an abundant MAP that localizes to the mitotic spindle of cleavage-stage sea urchin embryos and to the interphase array of microtubules in adult coelomocytes. Expression of EMAP in Sf9 cells had no detectable effect on cellular morphology, microtubule organization, or stability. These results indicate that process formation in Sf9 cells is MAP specific.

The Xenopus XMAP215 and its human homologue TOG proteins interact with cyclin B1 to target p34cdc2 to microtubules during mitosis

Cytoskeleton reorganization, leading to mitotic spindle formation, is an M-phase-specific event and is controlled by maturation promoting factor (MPF: p34cdc2-cyclinB1 complex). It has previously been demonstrated that the p34cdc2-cyclin B complex associates with mitotic spindle microtubules and that microtubule-associated proteins (MAPs), in particular MAP4, might be responsible for this interaction. In this study, we report that another ubiquitous MAP, TOG in human and its homologue in Xenopus XMAP215, associates also with p34cdc2 kinase and directs it to the microtubule cytoskeleton. Costaining of Xenopus cells with anti-TOGp and anti-cyclin B1 antibodies demonstrated colocalization in interphase cells and also with microtubules throughout the cell cycle. Cyclin B1, TOG/XMAP215, and p34cdc2 proteins were recovered in microtubule pellets isolated from Xenopus egg extracts and were eluted with the same ionic strength. Cosedimentation of cyclin B1 with in vitro polymerized microtubules was detected only in the presence of purified TOG protein. Using a recombinant C-terminal TOG fragment containing a Pro-rich region, we showed that this domain is sufficient to mediate cosedimentation of cyclin B1 with microtubules. Finally, we demonstrated interaction between TOG/XMAP215 and cyclin B1 by co-immunoprecipitation assays. As XMAP215 was shown to be the only identified assembly promoting MAP which increases the rapid turnover of microtubules, the TOG/XMAP215-cyclin B1 interaction may be important for regulation of microtubule dynamics at mitosis.

Mutual protection of microtubule-associated protein 2 (MAP2) and cyclic AMP-dependent protein kinase II against mu-calpain

Phosphorylation by adenosine-3',5'-cyclic monophosphate (cAMP)-dependent protein kinase (PKA), but not by Ca(++)-calmodulin-dependent protein kinase II (CaMK II), was shown earlier to protect microtubule-associated protein 2 (MAP2) from cleavage by m-calpain (Johnson and Foley: J Neurosci Res 34: 642-647, 1993). We reinvestigated this phenomenon with the physiologically more relevant mu-calpain and found a qualitatively similar but quantitatively different picture. We further demonstrate that 1) protection is biphasically dependent on the degree of phosphorylation; 2) Ca(++)-phospholipid-dependent protein kinase (PKC) has about the same effect as PKA; 3) the effects of kinases A and C are not additive; and 4) stripping of native MAP2 from its phosphate content (17.8 +/- 2.3 mol/mol) enhances calpainolysis 3.6-fold. A reciprocal effect between kinase A and MAP2 was found: the RII, but not the RI, regulatory subunit of kinase A, which was shown to bind specifically to MAP2, is protected by MAP2 from mu-calpain attack. It is suggested that the specific anchoring of kinase A-II on MAP2 may serve not only kinase targeting in the dendrites, but also protection from calpainolytic degradation.

Purification and characterization of ensconsin, a novel microtubule stabilizing protein

In previous studies (Bulinski and Borisy (1979). Proc. Nat. Acad. Sci. 76, 293–297; Weatherbee et al. (1980). Biochemistry 19, 4116–4123) a microtubule-associated protein (MAP) of M(r) approximately 125,000 was identified as a prominent MAP in HeLa cells. We set out to perform a biochemical characterization of this protein, and to determine its in vitro functions and in vivo distribution. We determined that, like the assembly-promoting MAPs, tau, MAP2 and MAP4, the 125 kDa MAP was both proteolytically sensitive and thermostable. An additional property of this MAP; namely, its unusually tight association with a calcium-insensitive population of MTs in the presence of taxol, was exploited in devising an efficient purification strategy. Because of the MAP's tenacious association with a stable population of MTs, and because it appeared to contribute to the stability of this population of MTs in vitro, we have named this protein ensconsin. We examined the binding of purified ensconsin to MTs; ensconsin exhibited binding that saturated its MT binding sites at an approximate molar ratio of 1:6 (ensconsin:tubulin). Unlike other MAPs characterized to date, ensconsin's binding to MTs was insensitive to moderate salt concentrations (< or = 0.6 M). We further characterized ensconsin in immunoblotting experiments using mouse polyclonal anti-ensconsin antibodies and antibodies reactive with previously described MAPs, such as high molecular mass tau isoforms, dynamin, STOP, CLIP-170 and kinesin. These experiments demonstrated that ensconsin is distinct from other proteins of similar M(r) that may be present in association with MTs. Immunofluorescence with anti-ensconsin antibodies demonstrated that ensconsin was detectable in association with most or all of the MTs of several lines of human epithelial, fibroblastic and muscle cells; its in vivo properties and distribution, especially in response to drug or other treatments of cells, were found to be different from those of MAP4, the predominant MAP found in these cell types. We conclude that ensconsin, a MAP found in a variety of human cells, is biochemically - and perhaps functionally - distinct from other MAPs present in non-neuronal cells.

Structure, regional and developmental expression of rat MAP2d, a MAP2 splice variant encoding four microtubule-binding domains

MAP2, a major component of microtubule polymers in neurons consists of high molecular weight (HMW) proteins MAP2a, MAP2b and a low molecular weight (LMW) MAP2c, expressed in the developing brain. These isoforms are produced from a single gene by alternative splicing and share identical C-termini encompassing 3 tandem repeats, critical in microtubule binding. We describe the structure, regional and developmental expression of a novel MAP2 splice variant, MAP2d, containing an insertion whose sequence is homologous to the three and four repeats of MAP2 and Tau respectively. This insertion is absent from the mRNAs encoding HMW MAP2. MAP2d mRNAs are expressed at higher levels than MAP2c in all adult nervous tissues of the rat, and are found at low levels in glial cell cultures when compared to primary cultures of cerebellar neurons. Splicing of the fourth repeat in mature Tau precedes that in MAP2d during rat brain development. The tardive expression of a four microtubule-binding domain LMW MAP2 suggests it could play in extended neurites a similar role as mature Tau in axons.

Microtubule organization and dynamics dependent on microtubule-associated proteins

High-resolution microscopic analysis has precisely revealed the control of microtubule dynamics by individual microtubule-associated proteins (MAPs) in vitro. Furthermore, transfection of MAP cDNA into fibroblasts and subsequent analysis using microinjection of caged fluorescein-labeled tubulin and photoactivation have enabled the function of MAPs in microtubule dynamics to be studied in detail in vivo. Systematic, quantitative studies using transfection of various kinds of MAP cDNA deletion mutants have demonstrated the complex mechanism for microtubule bundling in vivo, and have shown the involvement in microtubule bundling of both microtubule binding and projection regions of MAPs. A similar approach, combined with detailed structural analysis, has indicated clearly that differences in the amino-terminal projection region of MAPs can determine differential organization of MT bundles, and thus influence the characteristic organization of microtubule domains in dendrites and axons.

Coordinate regulation of tubulin and microtubule associated protein genes during development of hamster brain

The present study documents the patterns of mRNA expression for 5 different tubulin genes and 4 of the structural microtubule-associated protein (MAP) genes during normal development of hamster forebrain. Northern blotting in conjunction with densitometric analysis was used to study changes in the levels of the mRNAs for alpha 1-tubulin, classes beta I-, beta II-, beta III- and beta IV-tubulin, as well as the mRNAs for tau, MAP1A, MAP1B and MAP2, using total RNA isolated from hamster forebrain at various embryonic (E) and postnatal (P) stages. Densitometric analyses of the autoradiograms from the Northern blots revealed that each of the tubulin genes exhibited distinct developmental patterns of expression, several of which appeared to be temporally correlated with the expression of specific MAP mRNAs. The beta I-, beta II- and beta III-tubulin mRNAs increased rapidly between late embryonic stages to birth, reached peak levels early in the first postnatal week, and declined thereafter. alpha 1-Tubulin mRNA was easily detected during embryonic stages, rose to peak levels at P7-P9 and then gradually declined. A similar pattern was seen for tau mRNA. After the first postnatal week, the size of the tau mRNA also shifted to a slightly larger size, presumably due to differential splicing. beta IV-Tubulin mRNA levels did not become significant until very late in postnatal development (3-4 weeks). MAP2 mRNA expression was unusual in that peak levels were reached at two different stages of development--an initial peak occurred in the first postnatal week, followed by a decline, and then a second rise occurred during the third and fourth postnatal weeks. The expression of the beta IV-tubulin mRNA coincided temporally with the second peak in MAP2 mRNA expression. MAP1B mRNA abruptly reached high levels at birth, remained abundant during the first two postnatal weeks, and then decreased. In contrast, MAP1A mRNA levels were low in the initial postnatal interval and increased only at very late developmental stages. The findings of a temporal correspondence in expression of high levels of tau and MAP1B with beta I-, beta II-, beta III- and alpha 1-tubulin mRNAs suggest that this profile of gene expression is one that endows greater plasticity to the neuronal cytoskeleton. Conversely, the transition to increased expression of beta IV-tubulin, MAP1A, and a larger tau mRNA species defines a portion of the molecular pattern that underpins the increased stability of neuronal form during maturation.

Expression of high molecular weight tau in the central and peripheral nervous systems

Using a novel PCR approach, we have cloned a cDNA encoding the entire high molecular weight tau molecule from rat dorsal root ganglia. The resulting 2080 bp cDNA differs from low molecular weight rat brain tau by the insertion of a novel 762 bp region (exon 4a) between exons 4 and 5. This cDNA clone is identical in sequence with a high molecular weight tau (HMW) cDNA from rat PC12 tumor cells and is closely related to a HMW tau cDNA from mouse N115 tumor cells. In vitro transcription/translation produces a protein that migrates on SDS-PAGE with the same apparent molecular weight as HMW tau purified from rat sciatic nerve. The HMW tau protein is generated from an 8 kb mRNA, which can be detected by northern blots in peripheral ganglia, but not in brain. A more sensitive assay using PCR and Southern blot analysis demonstrates the presence of exon 4a in spinal cord and in retina. In combination with immunohistochemical studies of spinal cord, these data suggest that HMW tau, though primarily in the peripheral nervous system, is also expressed in limited areas of the central nervous system, although its presence cannot be detected in the cerebral cortices.

Estramustine and estrone analogs rapidly and reversibly inhibit deoxyribonucleic acid synthesis and alter morphology in cultured human glioblastoma cells

Estramustine is an estradiol-based agent that has been shown to accumulate in human glioma cells, resulting in a concentration-dependent alteration in cell size and shape within minutes and an inhibition of proliferation over 3 to 6 days. We evaluated human glioblastoma cultures with [3H]thymidine incorporation assays to determine estramustine's early effects on deoxyribonucleic acid synthesis in these tumors. Because estramustine shares a common structural motif with other antimicrotubule drugs, we synthesized four A-ring conjugates of estrone that contained a carbamate moiety but lacked nitrogen mustard. These analogs were examined by [3H]thymidine incorporation and compared with vinblastine. Greater than 70% inhibition of [3H]thymidine incorporation occurred within 1 hour of treatment with estramustine at 10(-5) mol/L, which increased to 80% inhibition at 4 hours. Ethyl carbamate JE208 was nearly as effective as estramustine in inhibiting deoxyribonucleic acid synthesis, and both were more effective than vinblastine. The inhibitory effects of estramustine and estrone analogs were reversible; vinblastine was not reversible. Although estramustine and JE208 induced similar antiproliferative and morphological changes in glioblastoma cells that persisted for at least 4 days, there was a modest recovery of morphology and thymidine incorporation with JE208 after prolonged treatment. The common findings with estramustine and JE208 suggest that these agents may have a similar mechanism of action and form the basis for the investigation of new agents that may rapidly and reversibly inhibit glioblastoma.

High affinity DNA-microtubule interactions: evidence for a conserved DNA-MAP interaction involving unusual high CsCl density repetitious DNA families

We have examined high affinity interactions of chick brain microtubule proteins with 35S labelled tracer DNAs from chick, mouse and D. melanogaster under equilibrium conditions by the nitrocellulose filter binding technique. Ternary reaction mixtures of the above two components and a third component, an excess of unlabelled competitor DNA from either E. coli., mouse, D. melanogaster or chick, were used to measure small fractions of DNA in each case (1-4%) bound to microtubule protein under high stringency- large competitor DNA concentration and 0.5 M NaCl. As seen in part previously (Marx, K.A. and Denial, T. (1985) in The Molecular Basis of Cancer, 172B, 65-75 (Rein, ed), A. Liss, N.Y.) the measured order of competitor DNA strengths was identical for all three tracer DNAs. That is: chick > mouse > D. melanogaster > E. coli competitor DNA. Since the homologous interaction, chick competitor DNA with chick brain microtubule protein, is always the strongest interaction measured, we interpret this as evidence for a conserved protein-DNA sequence interaction. 35S chick DNA tracer sequences, isolated from nitrocellulose filters following the stringent binding in the presence of 0.9 mM-1 E. coli. competitor DNA, was used in driven reassociation reactions with total chick driver DNA. This fraction was found to be significantly enriched in repetitive chick DNA sequences. Since we have observed a similar phenomenon in mouse, we then compared the stringent binding mouse sequences and showed that the bulk of these sequences did not cross-hybridize with total chick DNA. Finally, all three 35S tracer DNAs binding to nitrocellulose were isolated and sedimented to equilibrium on CsCl density gradients. The CsCl density distributions from all three DNAs showed significant (100-fold) enrichment in classical satellite DNAs as well as higher enrichment in two very unusual high CsCl density families of DNA (1.720-1.740 g/cm3; 1.750-1.765 g/cm3). These families are never observed as distinct bands in total DNA CsCl gradients, nor could we isolate them in purified tubulin control binding experiments. This apparently general phenomena may be identifying some of the sequence families involved in the high affinity microtubule interaction, which appears to be conserved in evolution.

Microtubule organization by cross-linking and bundling proteins

To understand microtubule function the factors regulating their spatial organization and their interaction with cellular organelles, including other microtubules, must be elucidated. Many proteins are implicated in these organizational events and the known consequences of their actions within the cell are increasing. For example, the function of microtubule bundles at the surfaces of polarized cells has recently received attention, as has the action in cortical rotation of a transient arrangement of microtubules found beneath the vegetal surface of fertilized frog eggs. The in vivo association of microtubules during early Xenopus oogenesis has added interest as microtubules bundled in cell-free extracts are protected against the action of a severing protein found in this animal. A 52 kDa F-actin bundling protein purified from Physarum polycephalum organizes microtubules and causes the cobundling of microtubules and microfilaments. These observations, in concert with others that are presented, emphasize the diversity within the family of microtubule cross-linking proteins. The challenge is to determine which proteins are relevant from a physiological perspective, to ascertain their molecular mechanisms of action and to describe how they affect cytoplasmic organization and cell function. To realize this objective, the proteins which cross-link and bundle microtubules must be investigated by techniques which reveal different but related aspects of their properties. Cloning and sequencing of genes for cross-linking proteins, their subcellular localization especially as microtubule-related changes in cell morphology are occurring and the application of genetic studies are necessary. Study of the neural MAP provides the best example of just how powerful current experimental approaches are and at the same time shows their limits. The neural MAP have long been noted for their enhancement of tubulin assembly and microtubule stability. Their spatial distribution has been studied during the morphogenesis of neural cells. Sequencing of cloned genes has revealed the functional domains of neural MAP including carboxy-terminal microtubule-binding sites. Similarities to microtubule binding proteins from other cell types stimulate interest in the neural MAP and further suggest their importance in microtubule organization. For example, MAP4 enjoys a wide cellular distribution and has microtubule-binding sequences very similar to those in the neural MAP. Moreover, the nontubulin proteins of marginal bands are immunologically related to neural MAP, indicating shared structural/functional domains. Even with these findings the mechanism by which neural MAP cross-link microtubules remains uncertain. Indeed, some researchers express doubt that microtubule cross-linking is actually a function of neural MAP in vivo.(ABSTRACT TRUNCATED AT 400 WORDS)

Chemically synthesized 182-235 segment of tau protein and analogue peptides are efficient in vitro microtubule assembly inducers of low apparent sequence specificity

A 54-amino acid peptide reproducing the first and second repeats and intervening spacer sequence of the tubulin binding motif (residues 182-235) of murine tau protein, and several congeners representing different degrees of sequence scrambling have been prepared by solid phase methods and fully characterized chemically. These double-repeat peptides have been shown to induce microtubule formation at concentrations about one order of magnitude lower than single-repeat controls, under conditions close to the critical concentration needed for tubulin self-assembly. On the other hand, partial loss of microtubule-inducing capacity was observed for peptides with primary structures increasingly disorganized with respect to the canonical peptide. These results call into question the assumption that a high degree of primary structure specificity is involved in the tau-tubulin interaction leading to in vitro microtubule formation.

Suppression of MAP2 in cultured cerebellar macroneurons inhibits minor neurite formation

We show here that antisense MAP2 oligonucleotides inhibit neurite outgrowth in cultured cerebellar macroneurons. Unlike control neurons, which first extend a lamellipodial veil followed by a consolidation phase during which the cells extend minor neurites, MAP2-suppressed cells persist with lamellipodia and later become rounded. The induction of microtubules containing tyrosinated tubulin, which parallels neurite outgrowth in control neurons, was blocked under antisense conditions. The small but significant increase in acetylated microtubules was not affected. In contrast, the suppression of tau, which selectively blocks axonal elongation, completely prevented the increase of acetylated microtubules, but did not modify the induction of labile microtubules. These results suggest that MAP2 and tau have different functions: the initial establishment of neurites depends upon MAP2, whereas further neurite elongation depends upon tau and microtubule stabilization.

Cloning, sequence and expression of a beta-tubulin-encoding gene in the homobasidiomycete Schizophyllum commune

The beta-tubulin (beta Tub)-encoding gene (tub-2) of Schizophyllum commune is the first tubulin gene isolated, cloned and sequenced from higher filamentous fungi (homobasidiomycetes). The S. commune tub-2 gene is organized into nine exons and eight introns. The introns vary from 48 to 107 nt in length, and are distributed throughout the gene. The tub-2 exons code for a protein of 445 amino acids (aa), which shows great homology with beta Tubs of filamentous ascomycetes, plants, and animals, but less homology with yeasts. The codon usage of tub-2 from S. commune is biased, as it is in most beta Tub-encoding genes of filamentous fungi. The S. commune beta Tub shows a conserved aa sequence in the C-terminal domain, which is suggested to interact with microtubule-associated proteins in animals. In contrast, the S. commune beta Tub deviates from most known beta Tubs by having a Cys165 residue, which might be significant for the insensitivity of S. commune haploid strains to the antimicrotubule drug, benomyl. In tub-2 of different haploid strains, sequence polymorphisms occur in the 5' and 3' flanking regions. The expression of tub-2 is high in young mycelium, which has a high number of extending apical cells, but decreases with the aging of the mycelium. No significant difference in the hybridization signal intensity for the tub-2 transcripts was recorded either during intercellular nuclear migration at early mating, or in mycelia with a mutation in the B mating-type gene.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential sorting of beta tubulin isotypes into colchicine-stable microtubules during neuronal and muscle differentiation of embryonal carcinoma cells

Pluripotent P19 embryonal carcinoma (EC) cells were differentiated along the neuronal and muscle pathways. Comparisons of class I, II, III, and IV beta tubulin isotypes in total and colchicine-stable microtubule (MT) arrays from uncommitted EC, neuronal, and muscle cells were made by immunoblotting and by indirect immunofluorescence microscopy. In undifferentiated EC cells the relative amounts of these four isotypes are the same in both the total and stable MT populations. Subcellular sorting of beta tubulin isotypes was demonstrated in both neuronal and muscle differentiated cells. During neuronal differentiation, class II beta tubulin is preferentially incorporated into the colchicine-stable MTs while class III beta tubulin is preferentially found in the colchicine-labile MTs. The subcellular sorting of class II into stable MTs correlates with the increased staining of MAP 1B, and with the expression of MAP 2C and tau. Although muscle differentiated cells express class II beta tubulin, stable MTs in these cells do not preferentially incorporate this isotype but instead show increased incorporation of class IV beta tubulin. Muscle cells do not show high levels of MAP 1B and do not express MAP 2C or tau. These results are consistent with the hypothesis that a subcellular sorting of tubulin isotypes is the result of a complex interaction between tubulin isotypes and MT-associated proteins.

Microtubule functions

Microtubules, with intermediate filaments and microfilaments, are the components of the cell skeleton which determinates the shape of a cell. Microtubules are involved in different functions including the assembly of mitotic spindle, in dividing cells, or axon extension, in neurons. In the first case, microtubules are highly dynamic, while in the second case microtubules are quite stable, suggesting that microtubule with different physical properties (stability) are involved in different functions. Thus, to understand the mechanisms of microtubule functions it is very important to understand microtubule dynamics. Historically, tubulin, the main component of microtubules, was first characterized as the major component of the mitotic spindle that binds to colchicine. Afterwards, it was found that tubulin is particularly more abundant in brain than in other tissues. Therefore, the roles of microtubules in mitosis, and in neurons, have been more extensively analyzed and, in this review, these roles will be discussed.

Different assembly properties of cod, bovine, and rat brain microtubules

Assembly properties of cod, bovine, and rat brain microtubules were compared. Estramustine phosphate, heparin, poly-L-aspartic acid, as well as NaCl, inhibited the assembly and disassembled both bovine and rat microtubules by inhibition of the binding between tubulin and MAPs. The assembly of cod brain microtubules was in contrast only marginally affected by these agents, in spite of a release of the MAPs. The results suggest that cod tubulin has a high intrinsic ability to assemble. This was confirmed by studies on phosphocellulose-purified cod tubulin, since the critical concentration for assembly was independent of the presence or absence of MAPs. The results show therefore that cod brain tubulin has, in contrast to bovine and rat brain tubulins, a high propensity to assembly under conditions which normally require the presence of MAPs. Even if cod MAPs, which have an unusual protein composition, were not needed for the assembly of cod microtubules, they were able to induce assembly of bovine brain tubulin. Both cod and bovine MAPs bound to cod microtubules, and bovine MAP1 and MAP2 bound to, and substituted at least the 400 kDa cod protein. This suggests that the tubulin-binding sites and the assembly-stimulatory ability of MAPs are common properties of MAPs from different species, independent of the tubulin assembly propensity.

MAP2: a sensitive cross-linker and adjustable spacer in dendritic architecture

Microtubule-associated protein 2 (MAP2), a long, filamentous molecule thought to cross-link dendritic cytoskeleton, is rich in PEST sequences, putative signals for rapid proteolytic degradation. It is suggested that MAP2 is indeed highly susceptible to protease, e.g. calpain, attack, which is needed for a plastic change, but actual breakdown depends on the regulation of protease(s). Phosphorylation is expected to make the molecule longer and rigid, similarly to what was observed with the related tau protein. Such a structural transition may provide a mechanism for the putative role of MAP2 in dendritic branching.

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.