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

Expression of Class II β-Tubulin by Proliferative Myoepithelial Cells in Canine Mammary Mixed Tumors

Benign mammary mixed tumors in dogs resemble human salivary pleomorphic adenomas with regard to their histogenesis, including the occurrence of cartilaginous or bony metaplasia as well as the expression pattern of cytoskeletal proteins in proliferative myoepithelial cells. Recently, a monoclonal antibody specific for class II β-tubulin has been developed. The epitope it recognizes was determined to be the hepta-peptide Glu-Glu-Glu-Glu-Gly-Glu-Asp, which is the common sequence found among the canine, rat, mouse, and human class II β-tubulin-specific regions. We carried out immunohistochemical studies on mammary mixed tumors obtained from three female dogs using this the monoclonal antibody. The antibody to class II β-tubulin reacted intensely with proliferative myoepithelial cells in canine mammary mixed tumors, whereas staining was barely detectable in normal myoepithelial cells surrounding alveoli and alveolar ducts within the tumor and adjacent normal tissue. Proliferative myoepithelial cells also expressed vimentin, but α-smooth muscle actin (αSMA) staining was barely detectable. Immunoblot analysis showed that class II β-tubulin and vimentin were expressed in myoepithelial cell lines prepared from the three mammary mixed tumors. On the other hand, only one cell line, which was negative for αSMA, produced cartilage-specific type II collagen. These results suggest that class II β-tubulin could be a new molecular marker of proliferating myoepithelial cells in canine mammary mixed tumors and that differential expression of cytoskeletal components is associated with cartilaginous metaplasia of proliferative myoepithelial cells in mixed mammary tumors.

Structural evidence for cooperative microtubule stabilization by Taxol and the endogenous dynamics regulator MAP4

Microtubules (MTs) composed of αβ-tubulin heterodimers are highly dynamic polymers, whose stability can be regulated by numerous endogenous and exogenous factors. Both the antimitotic drug Taxol and microtubule-associated proteins (MAPs) stabilize this dynamicity by binding to and altering the conformation of MTs. In the current study, amide hydrogen/deuterium exchange coupled with mass spectrometry (HDX-MS) was used to examine the structural and dynamic properties of the MT complex with the microtubule binding domain of MAP4 (MTB-MAP4) in the presence and absence of Taxol. The changes in the HDX levels indicate that MTB-MAP4 may bind to both the outside and the luminal surfaces of the MTs and that Taxol reduces both of these interactions. The MTB-MAP4 binding induces conformational rearrangements of α- and β-tubulin that promote an overall stabilization of MTs. Paradoxically, despite Taxol's negative effects on MAP4 interactions with the MTs, its binding to the MTB-MAP4-MT complex further reduces the overall deuterium incorporation, suggesting that a more stable complex is formed in the presence of the drug.

Isolation and sequence analysis of a beta-tubulin gene from arbuscular mycorrhizal fungi

A full-length beta-tubulin gene has been cloned and sequenced from Gigaspora gigantea and Glomus clarum, two arbuscular mycorrhizal fungi (AMF) species in the phylum Glomeromyota. The gene in both species is organized into five exons and four introns. Both genes are 94.9% similar and encode a 447 amino acid protein. In comparison with other fungal groups, the amino acid sequence is most similar to that of fungi in the Chytridiomycota. The codon usage of the gene in both AMF species is broad and biased in favor of an A or a T in the third position. The four introns varied in length from 87 to 168 bp for G. gigantea and from 90 to 136 bp for G. clarum. Of all fungi in which full-length sequences have been published, only AMF do not have an intron before codon 174. The introns positioned at codons 174 and 257 in AMF match the position of different introns in beta-tubulin genes of some Zygomycete, Basidiomycete, and Ascomycete fungi. The 5' and 3' splice site consensus sequences are similar to those found in introns of most fungi. Sequence analysis from single-strand conformation polymorphism analysis confirmed the presence of two beta-tubulin gene copies in G. clarum, but only one copy was evident in G. gigantea based on Southern hybridization analysis.

Beta IV tubulin is selectively expressed by oligodendrocytes in the central nervous system

Oligodendrocyte differentiation and myelination involve dramatic changes in cell signaling pathways, gene expression patterns, cell shape, and cytoskeletal organization. In a pilot study investigating CNS angiogenesis, oligodendrocytes were intensely labeled by antisera directed against the C-terminal of Tie-2, a 140-kDa transmembrane receptor for angiopoietin. Immunoprecipitation of rat brain proteins with Tie-2 C-terminal antisera, however, produced a single spot of approximately 55-kDa pI approximately 5 by two-dimensional (2D) electrophoresis, which was identified as beta-tubulin by mass spectrometry. Isotype-specific antibodies for beta(IV) tubulin selectively labeled oligodendrocytes. First detected in premyelinating oligodendrocytes, beta(IV) tubulin was abundant in myelinating oligodendrocyte perinuclear cytoplasm and processes extending to and along developing myelin internodes. Beta(IV) tubulin-positive MTs were diffusely distributed in oligodendrocyte perinuclear cytoplasm and not organized around the centrosome. Beta(IV) tubulin may play a role in establishing the oligodendrocyte MT network, which is essential for the transport of myelin proteins, lipids, and RNA during myelination.

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.

The best of all worlds or the best possible world? Developmental constraint in the evolution of beta-tubulin and the sperm tail axoneme

Through evolutionary history, some features of the phenotype show little variation. Stabilizing selection could produce this result, but the possibility also exists that a feature is conserved because it is developmentally constrained--only one or a few developmental mechanisms can produce that feature. We present experimental data documenting developmental constraint in the assembly of the motile sperm tail axoneme. The 9+2 microtubule architecture of the eukaryotic axoneme has been deeply conserved. We argue that the quality of motility supported by axonemes with this morphology explains their long conservation, rather than a developmental necessity for the 9+2 architecture. However, our functional tests in Drosophila spermatogenesis reveal considerable constraint in the coevolution of testis-specific beta-tubulin and the sperm tail axoneme. The evolution of testis beta-tubulins used in insect sperm tail axonemes is highly punctuated, indicating some pressure acting on their evolution. We provide a mechanistic explanation for their punctuated evolution by testing structure-function relationships between testis beta-tubulin and the motile axoneme in D. melanogaster. We discovered that a highly conserved sequence feature of beta-tubulins used in motile axonemes is needed to specify central pair formation. Second, our data suggest that cooperativity in the function of internal beta-tubulin amino acids is needed to support the long axonemes characteristic of Drosophila sperm tails. Thus, central pair formation constrains the evolution of the axoneme motif, and intramolecular cooperativity makes the evolution of the internal residues path dependent, which slows their evolution. Our results explain why a highly specialized beta-tubulin is needed to construct the Drosophila sperm tail axoneme. We conclude that these constraints have fixed testis-specific beta-tubulin identity in Drosophila.

Distribution of the class II beta-tubulin in developmental and adult rat tissues

During a screen of monoclonal antibodies raised against a cytoskeletal preparation of neonatal rat cerebrum, we have identified a monoclonal antibody, MAb58A, that is specific for the class II beta-tubulin isotype. Immunoscreening of a rat brain cDNA library using MAb58A yielded the cDNA retaining a class II-specific nucleotide sequence. The specificity of MAb58A to the class II beta-tubulin isotype was confirmed by immunoreactivity to synthetic peptides corresponding to isotype-specific sequence of class I, II, III, IVa, or IVb. Further, the results of an immunoassay against a series of overlapping octapeptides derived from a class II-specific region revealed that the antibody epitope was a heptapeptide that consists of Glu-Glu-Glu-Glu-Gly-Glu-Asp (EEEEGED). Immunoblot analysis revealed that the class II isotype represented a significant portion of beta-tubulin present in the adrenal gland, brain, and testis of adult rats. In fetal tissues, this isotype was detected in skeletal muscle, as well as in the brain. Immunohistochemically, MAb58A reacted predominantly with components of the developing rat nervous system, such as migrating neuroblasts, peripheral nerves and ganglion cells, and sensory organs. MAb58A-immunoreactivity was also found in developing skeletal and smooth muscle cells, chondrocytes, and vascular endothelia. In adults, MAb58A-immunoreactivity was remarkably diminished, but persisted in peripheral nerves and ganglion cells, chondrocytes, and capillary components. Together, our results demonstrate that MAb58A is specific for the class II beta-tubulin isotype, which may retain an embryonic nature in both neuronal and non-neuronal tissues.

Characterization of isotype-specific regions of five classes of canine beta-tubulin and their expression in several tissues and cell culture

The structure of isotype-specific regions of classes 1, II, III, IVa and IVb of canine beta-tubulin was characterized by 3'-RACE and the expression of these isotypes in canine tissues was examined by ribonuclease protection assay (RPA). Furthermore, a malignant mammary tumor-derived osteosarcoma-like cell line was established and the altered expression of beta-tubulin isotypes in taxol-resistant sublines was analyzed. The deduced amino acid sequences in isotype-specific regions corresponding to classes I, II and IVb were identical to those of humans and mice, but those in classes III and IVa showed slight differences among species. RPA revealed that classes I and IVb were widely distributed, but classes II, III and IVa were restricted to the brain. Because RPA could clearly distinguish the expression of class IVa from that of class IVb, it was thought to be more useful than northern blot for analysis of beta-tubulin isotype expression. In vitro, taxol-resistant sublines displayed a significant increase in class IVa as compared with taxol-sensitive cells, suggesting that altered expression of class IVa was associated with taxol resistance in these cell lines.

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.

Altered beta-tubulin isotype expression in paclitaxel-resistant human prostate carcinoma cells

To investigate the role of beta-tubulin isotype composition in resistance to paclitaxel, an anti-microtubule agent, human prostate carcinoma (DU-145) cells were intermittently exposed to increasing concentrations of paclitaxel. Cells that were selected and maintained at 10 nM paclitaxel (Pac-10) were fivefold resistant to the drug. Pac-10 cells accumulated radiolabelled paclitaxel to the same extent as DU-145 cells and were negative for MDR-1. Analysis of Pac-10 and DU-145 cells by flow cytometry showed similar cell cycle patterns. Immunofluorescent staining revealed an overall increase of alpha- and beta-tubulin levels in Pac-10 cells compared with DU-145 cells. Examination of beta-tubulin isotype composition revealed a significant increase in betaIII isotype in the resistant cells, both by immunofluorescence and by western blot analysis. Reverse transcription-polymerase chain reaction (RT-PCR) analysis of the isotypes confirmed the increase observed for the betaIII by exhibiting ninefold higher betaIII mRNA levels and also showed fivefold increase of the betaIVa transcript. In addition, analysis of paclitaxel-resistant cells that were selected at increasing levels of the drug (Pac 2, 4, 6, 8 and 10) exhibited a positive correlation between increasing betaIII levels and increasing resistance to paclitaxel. Increased expression of specific beta-tubulin isotypes and subsequent incorporation into microtubules may alter cellular microtubule dynamics, providing a defence against the anti-microtubule effects of paclitaxel and other tubulin-binding drugs.

Immunohistochemical analysis of beta-tubulin isotypes in human prostate carcinoma and benign prostatic hypertrophy

BACKGROUND

beta-tubulin, the intracellular target of several antimicrotubule agents, is encoded by at least six genes and exists as multiple isotypes with tissue-specific expression. Previous in vitro studies indicated that tubulin isotype composition may affect polymerization properties, dynamics, and sensitivity to drugs.

METHODS

To investigate the isotype composition of beta-tubulin in human prostate, tissues were collected from 26 patients after radical prostatectomy and sections were stained with isotype-specific antibodies.

RESULTS

beta IV tubulin is the predominant isotype in benign prostatic hyperplasia (BPH) and adenocarcinoma, showing significantly stronger immunohistochemical expression than beta II and beta III, particularly in Gleason's grade 3 and 4 cancers. Staining for the beta II isotype was invariably weak and often absent in BPH and normal glands. There was a marked increase in beta II isotype stain from BPH to cancer in 77% of the patients, suggesting that the expression of this isotype is related to malignant status.

CONCLUSIONS

The beta II tubulin isotype is a potential marker for prostate adenocarcinoma. The possibility that tumor beta-tubulin isotype composition may effect the response to antimicrotubule drug therapy in prostate cancer and other tumors merit investigation.

Immunological characterization of epitopes on tau of Alzheimer's type and chemically modified tau

The microtubule-associated protein tau is the main structural component of paired helical filaments (PHFs), which in turn are one of the major aberrant polymers found in Alzheimer's disease. Immunological studies were carried out using site-directed monoclonal and polyclonal antibodies that recognize tubulin binding epitopes on tau, to further understand the mechanisms of tau self-association into PHFs. Tau protein was subjected to either carbamoylation with potassium cyanate (KCNO) or glycation with glucose, and the immunoreactivity of the chemically-modified protein with these antibodies was compared with tau derived from paired helical filaments and with normal brain tau. The data on the immunoblot patterns of tau isoforms and the ELISA titration curves revealed significant differences between the modified tau and normal controls. However, the Western blot patterns of immunoreactive tau from the chemically-modified protein and from Alzheimer brains were similar. The data on the differences in the electrophoretic profiles and Western blots of normal brain tau as compared with solubilized paired helical filaments, insoluble tangles and tau proteins of the Alzheimer's type, provide new clues to understand the anomalous interactions of tau in Alzheimer's disease.

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.