Purification and characterization of tubulin from mung bean (Vigna radiata)

Tubulin has been purified from mung bean seedling by Zn²⁺-induced polymerization. Both α- and β-subunits of mung bean tubulin are different from those of brain tubulin in electrophoretic mobility, colchicine binding and peptide map. Heterogeneity of mung bean tubulin has also been documented suggesting diversification of tubulin despite its conserved nature in general.     

In vitro assembly of microtubules from tubulins of several higher plants

Tubulins were isolated by a combination of affinity (ethyl N-phenylcarbamate-Sepharose 4B) and ion exchange (DEAE-Sephacel) chromatography from several higher plants (mung bean, pea, whole pod bean, zucchini, cucumber seedlings and carrot suspension cultured cells). All these higher plant tubulins readily polymerized to microtubules in a polymerization medium containing GTP, Mg2+, EGTA, leupeptin and DMSO. Tubulins from mung bean, pea and whole pod bean showed identical behaviour on polyacrylamide gel electrophoresis but differed from carrot zucchini and cucumber tubulin. Consequently, tubulin of higher plants seems to have different molecular properties in different plant species.     

Identification of multiple tubulins in taxol microtubules purified from carrot suspension cells

Tubulin has been purified from carrot suspension cells by ion-exchange chromatography and assembled into microtubules in the presence of 20 microM taxol. One-dimensional SDS-PAGE suggested that the alpha band migrated faster than the beta band (as has been established for some lower eukaryotic tubulins) and this heterology with brain tubulins was confirmed by peptide mapping. When subjected to two-dimensional gel electrophoresis, the plant tubulins could be separated into multiple alpha and beta isotypes. Immunoblotting, using monoclonal anti-tubulins, confirmed that the tubulin isotypes identified in taxol microtubules represent all of the tubulins present in homogenates of unsynchronised log-phase carrot suspension cells. All identified tubulins are therefore assembly-competent under these conditions. Plant cells can contain four different microtubule arrays, but cells arrested in G0/G1 contain only cortical microtubule arrays; such cells, however, exhibit the same tubulin profile as non-synchronised cells, thereby showing no restriction in the number of subunits during this phase of the cell cycle.     

Tubulin, hybrid dimers, and tubulin S. Stepwise charge reduction and polymerization

Limited proteolysis of rat brain tubulin (alpha beta) by subtilisin cleaves a 1-2-kDa fragment from the carboxyl-terminal ends of both the alpha and beta subunits with a corresponding loss in negative charge of the proteins. The beta subunit is split much more rapidly (and exclusively at 5 degrees C), yielding a protein with cleaved beta and intact alpha subunit, called alpha beta s, which is of intermediate charge. Further proteolysis cleaves the carboxyl terminus of the alpha subunit leading, irreversibly, to the doubly cleaved product, named tubulin S, with a composition alpha s beta s. Both cleavage products are polymerization-competent and their polymers are resistant to 1 mM Ca2+- and 0.24 M NaCl-induced depolymerization. The two polymers differ in that the alpha beta s polymer is stable to cold, GDP, and podophyllotoxin, whereas tubulin S polymer is disassembled by these agents; moreover, alpha beta s forms ring-shaped polymers, whereas alpha s beta s forms filaments associated into bundles and sheets. Tubulin S co-polymerizes with native tubulin yielding a mixed product of intermediate stability. The presence of low mole fractions of tubulin S leads to a marked reduction in the critical concentration for polymerization of the mixture.     

Cytosolic chaperonin-containing t-complex polypeptide 1 changes the content of a particular subunit species concomitant with substrate binding and folding activities during the cell cycle.

The chaperonin-containing t-complex polypeptide 1 (CCT) is a cytosolic molecular chaperone composed of eight subunits that assists in the folding of actin, tubulin and other cytosolic proteins. We show here that the content of particular subunits of CCT within mammalian cells decreases concomitantly with the reduction of chaperone activity during cell cycle arrest at M phase. CCT recovers chaperone activity upon resumption of these subunits after release from M phase arrest or during arrest at S phase. The levels of alpha, delta and zeta-1 subunits decreased more rapidly than the other subunits during M phase arrest by colcemid treatment and recovered after release from the arrest. Gel filtration chromatography or native (nondenaturing) PAGE analysis followed by immunoblotting indicated that the alpha and delta subunit content in the 700- to 900-kDa CCT complex was appreciably lower in the M phase cells than in asynchronous cells. In vivo, the CCT complex of M-phase-arrested cells was found to bind lower amounts of tubulin than that of asynchronous cells. In vitro, the CCT complex of M phase-arrested cells was less active in binding and folding denatured actin than that of asynchronous cells. On the other hand, the CCT complex of asynchronous cells (a mixture of various phases of cell cycle) exhibited lower alpha and delta subunit content and lower chaperone activity than that of S-phase-arrested cells obtained by excess thymidine treatment. In addition, turnover (synthesis and degradation) rates of the alpha and delta subunits in vivo were more rapid than those of most other subunits. These results suggest that the content of alpha and delta subunits of CCT reduces from the complete active complex in S phase cells to incomplete inactive complex in M phase cells.     

A vacuolar ATPase and pyrophosphatase in Acetabularia acetabulum.

Vacuole-rich fractions were isolated from Acetabularia acetabulum by Ficoll step gradient centrifugation. The tonoplast-rich vesicles showed ATP-dependent and pyrophosphate-dependent H(+)-transport activities. ATP-dependent H(+)-transport and ATPase activity were both inhibited by the addition of a specific inhibitor of vacuolar ATPase, bafilomycin B1. A 66 kDa polypeptide present in the preparation cross-reacted with the anti-IgG fractions against the alpha and beta subunits of Halobacterium halobium ATPase and with the antibody against the A subunit (68 kDa subunit) of mung bean vacuolar ATPase. A 56 kDa polypeptide present in the vacuole preparation showed cross-reactivity with the antibody against the B subunit (57 kDa) of mung bean vacuolar ATPase but not with the anti-beta subunit of H. halobium ATPase. A 73 kDa polypeptide cross-reacted with the antibody against inorganic pyrophosphatase of mung bean vacuoles. These results suggest that vacuolar membrane of A. acetabulum equipped energy transducing systems similar to those found in other plant vacuoles.     

Characterization of Blue-Green Fluorescence in the Mesophyll of Sugar Beet (Beta vulgaris L.) Leaves Affected by Iron Deficiency

Protease C1, an enzyme from soybean (Glycine max [L.] Merrill cv Amsoy 71) seedling cotyledons, was previously determined to be the enzyme responsible for the initial degradation of the alpha' and alpha subunits, but not the beta subunit, of beta-conglycinin storage protein. The sizes of the proteolytic products generated by the action of protease C1 suggest that the cleavage sites on the alpha' and alpha subunits of beta-conglycinin may be located in their N-terminal domain, which is not found in the beta subunit of beta-conglycinin. To check this hypothesis, storage proteins from other plant species that are homologous to either the alpha'/alpha or the beta subunit of beta-conglycinin were tested as substrates. As expected, the convicilin from pea (Pisum sativum), a protein homologous to the alpha' and alpha subunits of beta-conglycinin, was digested by protease C1. The vicilins from pea as well as vicilins from adzuki bean (Vigna angularis), garden bean (Phaseolus vulgaris), black-eyed pea (Vigna unguiculata), and mung bean (Vigna radiata), storage proteins that are homologous to the beta subunit of soybean beta-conglycinin, were not degraded by protease C1. Degradation of soybean beta-conglycinin involves a sequential attack of the alpha subunit at multiple sites, culminating in the formation of a stable intermediate of 53.5 kD and a final product of 48.0 kD. The cleavage sites resulting in this formation of the intermediates and final product were determined by N-terminal analysis. These were compared to the known amino acid sequences of the three beta-conglycinin subunits. Results showed these two polypeptides to be generated by proteolysis of the alpha subunit at regions bearing long strings of acidic amino acid residues.     

Expression of a beta 1-related integrin by oligodendroglia in primary culture: evidence for a functional role in myelination

We have investigated the expression of integrins by rat oligodendroglia grown in primary culture and the functional role of these proteins in myelinogenesis. Immunochemical analysis, using antibodies to a number of alpha and beta integrin subunits, revealed that oligodendrocytes express only one detectable integrin receptor complex (alpha OL beta OL). This complex is immunoprecipitated by a polyclonal anti-human beta 1 integrin subunit antibody. In contrast, astrocytes, the other major glial cell type in brain, express multiple integrins including alpha 1 beta 1, alpha 3 beta 1, and alpha 5 beta 1 complexes that are immunologically and electrophoretically indistinguishable from integrins expressed by rat fibroblasts. The beta subunit of the oligodendrocyte integrin (beta OL) and rat fibroblast beta 1 have different electrophoretic mobilities in SDS-PAGE. However, the two beta subunits appear to be highly related based on immunological cross- reactivity and one-dimensional peptide mapping. After removal of N- linked carbohydrate chains, beta OL and beta 1 comigrated in SDS-PAGE and peptide maps of the two deglycosylated subunits were identical, suggesting differential glycosylation of beta 1 and beta OL accounts entirely for their size differences. The oligodendrocyte alpha subunit, alpha OL, was not immunoprecipitated by antibodies against well characterized alpha chains which are known to associate with beta 1 (alpha 3, alpha 4, and alpha 5). However, an antibody to alpha 8, a more recently identified integrin subunit, did precipitate two integrin subunits with electrophoretic mobilities in SDS-PAGE identical to alpha OL and beta OL. Functional studies indicated that disruption of oligodendrocyte adhesion to a glial-derived matrix by an RGD-containing synthetic peptide resulted in a substantial decrease in the level of mRNAs for several myelin components including myelin basic protein (MBP), proteolipid protein (PLP), and cyclic nucleotide phosphodiesterase (CNP). These results suggest that integrin-mediated adhesion of oligodendrocytes may trigger signal(s) that induce the expression of myelin genes and thus influence oligodendrocyte differentiation.     

Monoclonal antibodies specific to plant tubulin

Summary Tubulin was isolated from mung bean seedling by a combination of affinity (ethyl N-phenylcarbamate-Sepharose 4 B) and ion exchange (DEAE-Sephacel) chromatography. Using SDS-PAGE together with blotting with subunit-specific antitubulins, mung bean tubulin has been shown to consist of two -tubulin subunits, MBT₂ and MBT₃, of which MBT₃ is a minor component, and one -tubulin, MBT₁.Monoclonal antibodies were produced by fusing mouse myeloma cells and spleen cells from a Balb/c mouse immunized with mung bean tubulin. Antibody producing cell lines were identified by an ELISA assay and immunofluorescence microscopy and subsequently cloned by limiting dilution.The properties of monoclonal antibody (K₄E₇G₃) were examined by Western blot analysis and indirect immunofluorescence studies. K₄E₇G₃ reacts with MBT₂ and MBT₃ -tubulin subunits of mung bean tubulin, but not with MBT₁ -tubulin nor with the - and -subunits of sheep brain tubulin. Peptide fragments transferred onto nitrocellulose papers were treated with K₄E₇G₃ and with other monoclonal antibodies that are known to be specific to the -subunit of yeast tubulin and - or -subunit of mammalian brain tubulin. MBT₂ and MBT₃ are shown to be similar but not identical and are quite different from MBT₁ and the -subunit of sheep brain tubulin. K₄E₇G₃ reacts with peptide fragments in MBT₂ and MBT₃ that are not found in digests of brain tubulin, and that are either not reactive or only weakly reactive to the antibodies to yeast and brain -tubulin. It is concluded that K₄E₇G₃ and another monoclonal antibody, K₂D₇B₈, which has similar properties, are relatively specific for plant -tubulin.In indirect immunofluorescence studies on a wide range of plant cells, the epitopes recognised by these monoclonal antibodies are shown to be present in all types of microtubule array that were investigated. The spindle, preprophase band, phragmoplast and interphase microtubules were clearly observed in onion and mung bean root tip cells. Reactions with spindle microtubules ofFunaria spore mother cells and with the blepharoplast and flagella microtubules of fern spermatozoa are also seen. However, studies using several animal cell lines have shown that K₄E₇G₃ and K₂D₇B₈ do not give positive immunofluorescent localization of animal microtubules, correlating with the inability of K₄E₇G₃ to react with brain tubulin subunits on Western blot analysis.     

Differential proteolysis of the subunits of pyrophosphate-dependent 6-phosphofructo-1-phosphotransferase

Antibodies against the alpha (Mr 67,000) and beta (Mr 60,000) subunits of wheat seedling Fru-2,6-P2-stimulated pyrophosphate-dependent 6-phosphofructo-1-phosphotransferase (PFP) were used to probe the subunit structures of several partially purified plant PFPs after tryptic digestion. Antisera to the alpha and beta subunits of wheat seedling PFP cross-reacted with the corresponding alpha and beta subunits of PFP preparations from wheat germ, potato tubers, and lettuce leaves. With the mung bean PFP, both antisera reacted with a protein band of Mr 60,000. A protein band corresponding to the Mr 67,000 alpha subunit was not detected in the mung bean PFP preparation. Tryptic digestion of wheat seedling and potato tuber PFPs resulted in the preferential cleavage of the alpha subunit. The trypsinized PFP retained most of its Fru-2,6-P2-stimulated activity but not its basal activity. The proteolyzed enzyme also exhibited a 2-fold increase in Ka for Fru-2,6-P2. Studies with the mung bean enzyme revealed that the anti-alpha immunoreactive component was more sensitive to trypsinization than the anti-beta immunoreactive component of the Mr 60,000 protein band. Thus, the Mr 60,000 protein band of the mung bean PFP appears to be heterogeneous and contains both alpha and beta-like proteins. The above observations indicate that the alpha and beta subunits of PFP are two distinct polypeptides and that alpha acts as a regulatory protein in regulating both the catalytic activity and the Fru-2,6-P2-binding affinity of the beta subunit.     

A Change in Tubulin Synthesis in the Process of Tracheary Element Differentiation and Cell Division of Isolated Zinnia Mesophyll Cells

Changes in tubulin synthesis in the process of cytodifferentiationinto tracheary elements and cell division were investigatedusing a culture of single cells isolated from the mesophyllof Zinnia elegans. The tubulin content was measured by a sensitiveimmunoblotting method using a mouse monoclonal antibody to -or ß-tubulin as a probe and mung bean tubulin as astandard. Freshly isolated mesophyll cells had only small amountsof tubulin, but the content increased rapidly between 24 and48 h of culture before morphological differentiation and celldivision. The content rose more than sixfold during 48 h cultureand then decreased slightly. This pattern of increase closelyresembled that of the increase in cortical microtubules (MTs)estimated by electron microscopic analysis. The - and ß-tubulincontents in the cultured cells were almost the same and changedin coordination during culture. The activity of tubulin synthesis was determined by densitometricscanning of spots corresponding to tubulin subunits on an autoradiogramof a two-dimensional polyacrylamide gel of [³⁵S]-methionine-labeledproteins. Tubulin synthesis began as early as between 4 and8 h of culture and its rate increased similarly to the increasein the tubulin content, with the former always preceding thelatter, indicating that the increase in content resulted fromnew tubulin synthesis. (Received December 16, 1986; Accepted February 25, 1987)     

ADP-ribosylation of microtubule proteins as catalyzed by cholera toxin

Incubation of purified rat brain tubulin with cholera toxin and radiolabeled [32P] or [8-3H]NAD results in the labeling of both alpha and beta subunits as revealed on sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). Treatment of these protein bands with snake venom phosphodiesterase resulted in quantitative release of labeled 5'-AMP, respectively labeled with the corresponding isotope. Two-dimensional separation by isoelectric focusing and SDS-PAGE of labeled and native tubulin revealed that labeling occurs at least in four different isotubulins. The isoelectric point of the labeled isotubulins was slightly lower than that of native purified tubulin. This shift in mobility is probably due to additional negative charges involved with the incorporation of ADP-ribosyl residues into the tubulin subunits. SDS-PAGE of peptides derived from [32P]ADP-ribosylated alpha and beta tubulin subunits by Staphylococcus aureus protease cleavage showed a peptide pattern identical with that of native tubulin. Microtubule-associated proteins (MAP1 and MAP2) of high molecular weight were also shown to undergo ADP-ribosylation. Incubation of permeated rat neuroblastoma cells in the presence of [32P]NAD and cholera toxin results in the labeling of only a few cell proteins of which tubulin is one of the major substrates.     

Microtubular proteins of Chlamydomonas reinhardtii. An immunochemical study based on the use of an antibody specific for the beta-tubulin subunit.

An immunochemical assay for tubulin subunits is described. The method is applied directly to homogenates of Chlamydomonas reinhardtii solubilized in sodium dodecyl sulfate (Na dodecyl-SO4), and it makes use of a two-dimensional electrophoresis system; the first separation is carried out by Na dodecyl-SO4-polyacrylamide gel electrophoresis and the second by electrophoresis into an agarose gel containing antibodies. Tubulin is precipitated in the form of a "rocket" and the method is made quantitative through the use of cells labeled with [35S]sulfate. The antiserum used in this assay was prepared in rabbits using beta subunit of tubulin purified from Chlamydomonas flagella by two preparative Na dodecyl-SO4-polyacrylamide gel electrophoreses. This antiserum and an antiserum to alpha subunit of tubulin from porcine brain, prepared for comparative study, were extensively characterized. Both antisera show specificity for the polypeptide used as antigen and react with the native dimeric tubulin. The antiserum to beta subunit from Chlamydomonas flagella also forms immunoprecipitates with native brain tubulin and its beta subunit when used at high titer. In contrast, the antiserum to alpha subunit from porcine brain does not cross-react with Chlamydomonas tubulin. The immunochemical assay was applied to Chlamydomonas cells synchronized by a 12-h light/dark cycle. In cells collected during the light period (late G1), after removal of flagella, the content of tubulin is estimated to be 0.3% of total protein. As cells enter the dark period there is a striking increase in tubulin content which reaches a maximum just before cell division.     

Interaction of maize (Zea mays) protein phosphatase 2A with tubulin

Immunological and biochemical evidence has been obtained for an interaction of maize protein phosphatase 2A (PP2A) holoenzyme with tubulin. Tubulin co-purifies with maize seedling PP2A. Affinity chromatography of the maize PP2A preparation on immobilized tubulin revealed two peaks of phosphorylase alpha phosphatase activity. In one of the peaks, the catalytic (C) and constant regulatory (A) subunits of PP2A were identified by Western blotting. The subunits (C and A) of PP2A were co-immunoprecipitated from maize seedlings homogenate by an anti-alpha-tubulin antibody. The interaction of plant PP2A with tubulin indicates a possible role of reversible protein phosphorylation in the dynamic structure of plant cytoskeleton.     

Interactions of antimitotic peptides and depsipeptides with tubulin

Tubulin is the target for an ever increasing number of structurally unusual peptides and depsipeptides isolated from a wide range of organisms. Since tubulin is the subunit protein of microtubules, the compounds are usually potently toxic to mammalian cells. Without exception, these (depsi)peptides disrupt cellular microtubules and prevent spindle formation. This causes cells to accumulate at the G2/M phase of the cell cycle through inhibition of mitosis. In biochemical assays, the compounds inhibit microtubule assembly from tubulin and suppress microtubule dynamics at low concentrations. Most of the (depsi)peptides inhibit the binding of Catharanthus alkaloids to tubulin in a noncompetitive manner, GTP hydrolysis by tubulin, and nucleotide turnover at the exchangeable GTP site on beta-tubulin. In general, the (depsi)peptides induce the formation of tubulin oligomers of aberrant morphology. In all cases tubulin rings appear to be formed, but these rings differ in diameter, depending on the (depsi)peptide present during their formation.     

Increased microtubule assembly in bovine brain tubulin lacking the type III isotype of beta-tubulin

Tubulin, the major constituent protein of microtubules, is a heterodimer of alpha and beta subunits. Both alpha and beta exist in multiple isotypic forms. It is not clear if different isotypes perform different functions. In order to approach this question, we have made a monoclonal antibody specific for the beta III isotype of tubulin. This particular isotype is neuron-specific and appears to be phosphorylated near the C terminus. We have used immunoaffinity depletion chromatography to prepare tubulin lacking the beta III subunit. We find that removal of the beta III isotype results in a tubulin mixture able to assemble much more rapidly than is unfractionated tubulin when reconstituted with either of the two microtubule-associated proteins (MAPs), tau or MAP 2. Our results suggest that the different isotypes of tubulin differ from each other in their ability to polymerize into microtubules. We have also found that the anti-beta III antibody can stimulate microtubule assembly when reconstituted with tubulin and either tau or MAP 2. When reconstituted with tubulin lacking the beta III isotype, the antibody causes the tubulin to polymerize into a polymer that is a microtubule in the presence of MAP 2 and a ribbon in the presence of tau.     

Cloning and sequencing of V-ATPase subunit d from mung bean and its function in passive proton transport

We have previously shown that vacuolar H+-ATPase subcomplex V_o from mung bean contains subunit d, however, its sequence and function were unknown. In the present study, we report the cloning and recombinant over expression of subunit d from mung bean in E. coli. To study the function of subunit d, two vacuolar H+-ATPase subcomplexes V_o from mung bean were purified-one containing subunits a and c(c’,c”) and the other containing subunits a, c(c’,c”) and d. After reconstitution of the purified V_o subcomplexes into liposomes, the proton translocation was studied. Our results show that the V_o subcomplex in the absence of subunit d is a passive proton channel, while the V_o subcomplex in the presence of the subunit d is not. Taken together, our data supports the conclusion that the subunit d of the plant vacuolar H⁺-ATPase from mung bean is positioned at the central stalk and involved in the proton translocation across the tonoplast membrane.     

Phylogenetic relationships and chromosomal location of five distinct glycine receptor subunit genes in the teleost Danio rerio

Glycine receptors mediating synaptic inhibition are heteromeric proteins constituted of alpha and beta subunits. The mammalian GlyR subunits constitute a subgroup in the superfamily of ligand-gated ionic channels. To compare the evolutionary events in the mammalian and teleostean lineages for the receptor family, we first undertook systematic cloning of the constitutive subunits of the zebrafish glycine receptor. The isolation of two alpha subunits (alphaZ1 and alphaZ2) and one beta subunit (betaZ) has been reported previously and we report here the characterization of two novel alpha subunits, alphaZ3 and alphaZ4, increasing the known zebrafish subunits number to four alpha and one beta. Establishment of phylogenetic relationships reveals that alphaZ1, alphaZ3 and betaZeta are orthologous to mammalian alpha1, alpha3 and beta subunits. However, two zebrafish GlyRalpha subunit genes are orthologous to the unique avian and mammalian alpha4 subunit revealing a duplication of the alpha4 gene in zebrafish. Whole-mount in situ hybridization in 24-hours post fertilization (hpf) and 52-hpf embryos of the daughter gene products display very different expression patterns indicating distinct functions of the duplicated genes. Gene mapping reveals that the two duplicated genes are localized on two different linkage groups (LG5 and LG22) as would be daughter genes resulting from a large-scale duplication of the ancestral genome. Finally, we report that a linked pair of genes on human chromosome 4 (alpha3 and beta) is also linked on linkage group 1 in zebrafish (alphaZ3 and betaZ) as a consequence of a mosaic conserved syntheny.     

Posttranslational modifications of tubulin in cultured mouse brain neurons and astroglia

Posttranslational modifications of tubulin were analyzed in mouse brain neurons and glia developing in culture. Purified tubulin was resolved by isoelectric focusing. After 3 weeks of culture, neurons were shown to express a high degree of tubulin heterogeneity (8 alpha and 10 beta isoforms), similar to that found in the brain at the same developmental stage. Astroglial tubulin exhibits a less complex pattern consisting of 4 alpha and 4 beta isoforms. After incubation of neuronal and glial cells with 3H-acetate in the presence of cycloheximide, a major posttranslational label was found associated with alpha-tubulin and a minor one with beta-tubulin. The acetate-labeled isotubulins of neurons were resolved by isoelectric focusing into as many as 6 alpha and 7 beta isoforms, while those of astroglia were resolved into only 2 alpha and 2 beta isoforms. The same alpha isoforms were also shown to react with a monoclonal antibody recognizing selectively the acetylated form(s) of alpha-tubulin. Whether acetate-labeling of alpha-tubulin in these cells corresponds to the acetylation of Lys40, as reported for Chlamydomonas reinhardtii, is discussed according to very recent data obtained by protein sequence analysis. Tubulin phosphorylation was analyzed by incubation of cell cultures with 32PO4. No phosphorylation of alpha-tubulin isoforms was detected. A single beta-tubulin isoform (beta'2), expressed only in neurons, was found to be phosphorylated. This isoform is similar to that previously identified in differentiated mouse neuroblastoma cells.