Carboxy-terminal regions on the surface of tubulin and microtubules. Epitope locations of YOL1/34, DM1A and DM1B

Microtubule assembly in cold-adapted organisms: functional properties and structural adaptations of tubulins from antarctic fishes

Fishes native to the coastal waters of the Antarctic have adapted to habitat and body temperatures in the range -1.8 to +2 degrees C. Their cytoplasmic microtubules, unlike those of mammals and temperate poikilotherms, have evolved to assemble efficiently at these low temperatures. To learn about the underlying molecular adaptations, my laboratory is studying microtubule proteins [tubulin alpha beta dimers and microtubule-associated proteins (MAPs)] and tubulin genes from several Antarctic fishes, including the rockcods Notothenia coriiceps and Gobionotothen gibberifrons. We find that the assembly-enhancing adaptations of the fish microtubule proteins are intrinsic to the tubulin subunits themselves. Furthermore, microtubule formation by Antarctic fish tubulins is strongly entropy driven, due in part to an increased reliance, relative to tubulins from other species, on hydrophobic interactions. Based on analyses of tubulin polypeptides and cDNAs, we suggest that the structural adaptations of Antarctic fish tubulins most likely involve alterations in the primary sequences of tubulin isotypes. With respect to neural beta tubulins from other vertebrates, for example, the class II beta-tubulin isotype of N. coriiceps brain contains seven unique amino acid substitutions and one novel insertion in its 446-residue primary sequence. Most of these changes are located in a structural domain that forms contacts between tubulin dimers during microtubule assembly and would be expected to enhance polypeptide flexibility, thereby facilitating addition of tubulin to microtubule ends. The acidic carboxy-terminal tails of the alpha and beta tubulins, by contrast, appear not to be sites of cold adaptation of polymerization. We have also found that brain and egg tubulins from Antarctic fishes differ strikingly in their polymerization efficiencies, which demonstrates, in agreement with the multitubulin hypothesis, that tissue-specific tubulin isoforms can possess distinct functional properties. Thus, study of microtubule proteins from organisms, such as the Antarctic fishes, that have adapted to extreme thermal regimes should contribute significantly to an understanding of the quaternary interactions that control microtubule assembly in all eukaryotes.

Idiotypic vaccine for treatment of human B-cell lymphoma. Construction of IgG variable regions from single malignant B cells

Immunoglobulin idiotypes (Id) of malignant B cells represent highly specific markers which can be used for vaccination. PCR-amplification of immunoglobulin genes enables the rapid production of large amounts of Id vaccines. However, the separate amplification and subsequent recombination of heavy and light chains can lead to a loss of the relevant Id. To preserve the original chain pairs, we used single malignant B cells derived from an immunocytoma patient. Cytoplasm was extracted and the mRNA transcribed into cDNA. The VH and VL genes were then amplified by PCR and cloned into a vector for expression in E. coli. Id production was checked using an anti-Id mouse monoclonal Ab raised against the patient's tumor-specific IgG. One out of 3 constructs expressed the relevant Id. Analysis of the first 31 light chain residues revealed an identical sequence for the malignant B cells' IgG and the recombinant Id construct. Exchange of either the heavy or light chain with an unrelated chain resulted in loss of the Id. An unrelated sequence derived from the c-myc protein is coupled to the Id vaccine. The lymphoma patient was shown to have Abs to the c-myc sequence. This sequence therefore, increases the Id+ Ab's antigenicity. CD spectroscopy showed an alpha-helical structure for the c-myc epitope. In conclusion, a B-cell lymphoma autovaccine was produced containing immunogenic sequences that do not alter the steric conformation of the tumor-specific Id.

Primary structure and functional scFv antibody expression of an antibody against the human protooncogen c-myc

The immunoglobulin heavy- and light-chain variable region (Vh and Vl) genes were isolated from Myc1-9E10 hybridoma cells, which secreted monoclonal antibody against human oncogen c-myc. The expression vector pOPE52-c-myc was constructed for the recombinant production in E. coli. A 30 kDa single chain fragment (scFv) expression product was found in the periplasmic space by SDS-PAGE and immunoblotting. A significant fraction was processed correctly as demonstrated with an antiserum recognizing the processed aminoterminus only. The specific binding of the scFv fragment to the peptide epitope of the maternal monoclonal antibody was demonstrated and the primary sequence of the variable regions was determined. Sequence comparison with previously published partial Vh and Vl sequences from this hybridoma cell line revealed a genetic heterogeneity for the light chain variable region. The potential use of this scFv as a new tool for detection and purification of tagged proteins, for adding costimulatory signals to the surface of cancer cells as well as for analyzing c-myc function in the living cell by cytoplasmic expression is discussed.

Probing the kinesin-microtubule interaction

Kinesin is a mechanoenzyme that couples adenosine triphosphate hydrolysis to the generation of force and movement along microtubules. To gain insight into the interactions of kinesin and microtubules, cross-linking, mapping, and proteolysis experiments were executed. The motor domain of kinesin was consistently cross-linked to both alpha- and beta-tubulin subunits. Initial mapping of the cross-linked kinesin suggested that amino acids within the N- and C-terminal cyanogen bromide fragments of the motor domain formed cross-links to both alpha- and beta-tubulin subunits. Mapping of the cross-linked tubulin suggested that cross-linking to kinesin motors occurred within the negatively charged, C-terminal cyanogen bromide fragments of alpha- and beta-tubulin subunits. Treatment of microtubules with subtilisin, a protease that cleaves C-terminal fragments from alpha- and beta-tubulin, reduced their ability to be cross-linked to kinesin motors supporting the idea that C-terminal sequences of alpha- and beta-tubulin may interact with kinesin motors. Finally, of three synthetic peptides, a peptide consisting of the last 12 C-terminal amino acids of beta-tubulin competitively interfered with the microtubule-stimulated adenosine triphosphatase activity of the kinesin motor, further suggesting that C-terminal sequences of beta-tubulin may be involved in kinesin binding.

Microinjection of anti-alpha-tubulin antibody (DM1A) inhibits progesterone-induced meiotic maturation and deranges the microtubule array in follicle-enclosed oocytes of the frog, Rana pipiens

Microinjection of anti-alpha-tubulin (Dm1A) inhibited progesterone-induced meiotic maturation in large follicle-enclosed oocytes of the frog, Rana pipiens. DM1A (46 nl; 10 mg/ml) injection significantly increased the ED50 value for progesterone as determined by germinal vesicle dissolution (GVD) bioassay. By contrast, low doses of microinjected DM1A (46 nl; 2.5 mg/ml), anti-actin (clone KJ43A), anti-cytokeratin (C-11), anti-intermediate filament antibody (IFA), generic IgG (46 nl; 20 mg/ml) or sodium azide (46 nl; 1 mg/ml), an antibody preservative, were without inhibitory effect in this bioassay. Microinjected, affinity-purified DM1A (46 nl; 7.5 mg/ml) was also inhibitory, but preabsorption with pure tubulin prior to injection significantly reduced the inhibitory effect. DM1A injection had no effect on centrifugation-induced germinal vesicle migration (GVM). Previous work indicated that drugs (e.g. demecolcine and nocodazole), which destabilise microtubules, enhance both centrifugation-induced GVM and progesterone-induced GVD in Rana oocytes. Taking these results together, it is suggested that DM1A injection may have differential effects on microtubules in this cell. Thus, while the majority of microtubules were apparently depolymerised by DM1A (46 nl; 10 mg/ml) injection, a small subpopulation appeared to be stabilised as bundles. Confocal immunofluorescence microscopy of follicle-enclosed oocytes after DM1A injection revealed a major loss of microtubules throughout the cell; however, apparent sparse bundles of microtubules arranged in an approximately 600 microns shell were associated with the injectate region 24 h post-injection. By contrast, control follicle-enclosed oocytes topically labelled with DM1A post-fixation had extensive microtubule arrays similar to those previously reported in Xenopus oocytes. Intracellular recording after DM1A injection and progesterone treatment yielded an intermediate membrane potential (Vm = -31.8 mV) compared with control (immature) DM1A-injected cells (Vm = -44.7 mV) or potassium balanced salt solution (KBS)-injected cells matured with progesterone (Vm = -13.9 mV). These results suggest that DM1A injection does not completely inhibit electrophysiological changes initiated by progesterone. Working hypotheses are proposed that suggest a role for microtubules in the action of progesterone which normally lifts the prophase I block in the Rana follicle-enclosed oocyte.

Mapping surface sequences of the tubulin dimer and taxol-induced microtubules with limited proteolysis

Native tubulin alpha beta dimers and microtubules have been subjected to limited proteolysis with trypsin, chymotrypsin, elastase, clostripain, proteinase lysine-C, thermolysin, protease V8, papain, subtilisin, proteinase K, proteinase aspartic-N, and bromelain. Eighty nicking points have been mapped onto the alpha- and beta-tubulin sequences with the aid of site-directed antibodies, of which 18 sites have been exactly determined by N-terminal sequencing, and the probable position of 6 others deduced from protease specificities. Proteolytic sites cluster into five characteristic zones, including the C termini of both chains. Residues accessible to proteases in the tubulin dimer include alpha-tubulin Lys40-Thr41-Ile42, Glu168-Phe169-Ser170, Ser178-Thr179-Ala180-Val181, Lys280-Ala281, Glu290-Ile291, Ala294-Cys295, Arg339-Ser340 (plus probably Lys60-His61 and Glu183-Pro184) and beta-tubulin Gly93-Gln94, Lys174-Val175, Gly277-Ser278, Tyr281-Arg282-Ala283, Cys354-Asp355 (plus probably Arg121-Lys122, Phe167-Ser168, Tyr183-Asn184, and Glu426-Asp427 or Ala430-Asp431). While the majority of these sites remain accessible at the outer surface of taxol-induced microtubules, alpha-tubulin Lys280-Ala281, Arg339-Ser340 and beta-tubulin Tyr281-Arg282-Ala283 (and probably Arg121-Lys122) become protected from limited proteolysis, suggesting that they are close to or at intermolecular contacts in the assembled structure. The protease nicking points constitute sets of surface constraints for any three-dimensional model structures of tubulin and microtubules. The dimer tryptic site at alpha-tubulin 339-340 jumps approximately 12-22 residues upstream (probably to Lys326-Asp327 or Lys311-Tyr312) in taxol microtubules, suggesting a tertiary structural change. The cleavage of the approximately 10 C-terminal residues of alpha-tubulin by protease V8, papain, and subtilisin is inhibited in taxol microtubules compared to tubulin dimers, while the approximately 20 C-terminal residues of beta-tubulin are similarly accessible to protease V8, subtilisin, proteinase K, proteinase AspN, and bromelain and show enhanced papain cleavage. This is consistent with models in which the alpha-tubulin C-terminal zone is near the interdimer contact zone along the protofilaments, whereas the C terminus of beta is near the interface between both subunits.

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

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

Stable, detyrosinated microtubules function to localize vimentin intermediate filaments in fibroblasts

Separate populations of microtubules (MTs) distinguishable by their level of posttranslationally modified tubulin subunits and by their stability in vivo have been described. In polarized 3T3 cells at the edge of an in vitro wound, we have found a striking preferential coalignment of vimentin intermediate filaments (IFs) with detyrosinated MTs (Glu MTs) rather than with the bulk of the MTs, which were tyrosinated MTs (Tyr MTs). Vimentin IFs were not stabilizing the Glu MTs since collapse of the IF network to a perinuclear location, induced by microinjection of monoclonal anti-IF antibody, had no noticeable effect on the array of Glu MTs. To test whether Glu MTs may affect the organization of IFs we regrew MTs in cells that had been treated with nocodazole to depolymerize all the MTs and to collapse IFs; the reextension of IFs into the lamella lagged behind the rapid regrowth of Tyr MTs, but was correlated with the slower reformation of Glu MTs. Similar realignment of IFs with newly formed Glu MTs was observed in serum-starved cells treated with either serum or taxol to induce the formation of Glu MTs. Next, we microinjected affinity purified antibodies specific for Glu tubulin (polyclonal SG and monoclonal 4B8) and specific for Tyr tubulin (polyclonal W2 and monoclonal YL1/2) into 3T3 cells. Both injected SG and 4B8 antibodies labeled the subset of endogenous Glu MTs; W2 and YL1/2 antibodies labeled virtually all of the cytoplasmic MTs. Injection of SG or 4B8 resulted in the collapse of IFs to a perinuclear region. This collapse was comparable to that observed after complete MT depolymerization by nocodazole. Injection of W2, YL1/2, or nonspecific control IgGs did not result in collapse of the IFs. Taken together, these results show that Glu MTs localize IFs in migrating 3T3 fibroblasts and suggest that detyrosination of tubulin acts as a signal for the recruitment of vimentin IFs to MTs.

Axonal transport of mitochondria along microtubules and F-actin in living vertebrate neurons

A large body of evidence indicates that microtubules (MTs) conduct organelle transport in axons, but recent studies on extruded squid axoplasm have suggested that actin microfilaments (MFs) may also play a role in this process. To investigate the separate contributions to transport of each class of cytoskeletal element in intact vertebrate axons, we have monitored mitochondrial movements in chick sympathetic neurons experimentally manipulated to eliminate MTs, MFs, or both. First, we grew neurons in the continuous presence of: (a) cytochalasin E to create neurites which had never contained MFs; or (b) nocodazole or vinblastine to produce neurites which had never contained MTs. Mitochondria moved bidirectionally at normal velocities along the length of neurites which contained MTs and lacked MFs, but did not even enter neurites grown without MTs but containing MFs. In a second approach, we treated established neuronal cultures with cytoskeletal drugs to disrupt either MTs or MFs in axons already containing mitochondria. In cytochalasin-treated cells, which retained MTs but lacked MFs, average mitochondrial velocity increased in both directions, but net directional transport decreased. In vinblastine-treated cells, which lacked MTs but retained essentially normal levels of MFs, mitochondria continued to move bidirectionally but the average mitochondrial velocity and excursion length were reduced for both directions of movement, and the mitochondria spent threefold as much time moving in the retrograde as in the anterograde direction, resulting in net retrograde transport. Treatment of established cultures with both drugs produced neurites lacking MTs and MFs but still rich in neurofilaments; these showed a striking absence of any mitochondrial motility. These data indicate that axonal organelle transport can occur along both MTs and MFs in vivo, but with different velocities and net transport properties.

Monoclonal and polyclonal antibodies detect a new type of post-translational modification of axonemal tubulin

Polyclonal (PAT) and monoclonal (AXO 49) antibodies against Paramecium axonemal tubulin were used as probes to reveal tubulin heterogeneity. The location, the nature and the subcellular distribution of the epitopes recognized by these antibodies were, respectively, determined by means of: (i) immunoblotting on peptide maps of Paramecium, sea urchin and quail axonemal tubulins; (ii) immunoblotting on ciliate tubulin fusion peptides generated in E. coli to discriminate antibodies directed against sequential epitopes (reactive) from post-translational ones (non reactive); and (iii) immunofluorescence on Paramecium cells, using throughout an array of antibodies directed against tubulin sequences and post-translational modifications as references. AXO 49 monoclonal antibody and PAT serum were both shown to recognize epitopes located near the carboxyl-terminal end of both subunits of Paramecium axonemal tubulin, whereas the latter recognized additional epitopes in alpha-tubulin; AXO 49 and a fraction of the PAT serum proved to be unreactive over fusion proteins; both PAT and AXO 49 labelled a restricted population of very stable microtubules in Paramecium, consisting of axonemal and cortical ones, and their reactivity was sequentially detected following microtubule assembly; finally, both antibodies stained two upward spread bands in Paramecium axonemal tubulin separated by SDS-PAGE, indicating the recognition of various alpha- and beta-tubulin isoforms displaying different apparent molecular masses. These data, taken as a whole, definitely establish that PAT and AXO 49 recognize a post-translational modification occurring in axonemal microtubules of protozoa as of metazoa. This modification appears to be distinct from the previously known ones, and all the presently available evidence indicates that it corresponds to the very recently discovered polyglycylation of Paramecium axonemal alpha- and beta-tubulin.

ncd and kinesin motor domains interact with both alpha- and beta-tubulin

Motor domains of the Drosophila minus-end-directed microtubule (MT) motor protein ncd, were found to saturate microtubule binding sites at a stoichiometry of approximately one motor domain per tubulin dimer. To determine the tubulin subunit(s) involved in binding to ncd, mixtures of ncd motor domain and MTs were treated with the zero-length cross-linker 1-ethyl-3-(3-dimethylaminopropyl-carbodiimide) (EDC). EDC treatment generated covalently cross-linked products of ncd and alpha-tubulin and of ncd and beta-tubulin, indicating that the ncd motor domain interacts with both alpha- and beta-tubulin. When the Drosophila kinesin motor domain protein was substituted for the ncd motor domain, cross-linked products of kinesin and alpha-tubulin and of kinesin and beta-tubulin were produced. EDC treatment of mixtures of ncd motor domain and unassembled tubulin dimers or of kinesin motor domain and unassembled tubulin dimers produced the same motor-tubulin products generated in the presence of MTs. These results indicate that kinesin family motors of opposite polarity interact with both tubulin monomers and support a model in which some portion of each protein's motor domain overlaps adjacent alpha- and beta-tubulin subunits.

Bacterial expression and refolding of single-chain Fv fragments with C-terminal cysteines

Two antibody single-chain Fv (scFv) fragments carrying five C-terminal histidine residues were expressed in Escherichia coli as periplasmic inclusion bodies. Their variable heavy (VH) and light (VL) domains are derived from the mouse monoclonal antibody 215 (MAb215), specific for the largest subunit of RNA polymerase II of Drosophila melanogaster and rat MAb Yol1/34, specific for pig brain alpha-tubulin. ScFv-215 contains an additional cysteine residue near to its C-terminus. After solubilization of inclusion bodies followed by immobilized metal affinity chromatography (IMAC) in 6M urea and a renaturation procedure, scFv monomers, noncovalent dimers, and aggregated antibody fragments were separated by size exclusion chromatography. In addition, a fraction of disulfide-bonded scFv-215 homodimers (scFv')2 was also isolated. The various antibody forms appear to be in equilibrium after renaturation since first peak composed mainly of aggregates could be resolved into a similar pattern of aggregates, dimers, and monomers after repeating the denaturation/renaturation procedure. All fractions of the recombinant scFv-215 demonstrated high antigen-binding activity and specificity as shown by enzyme-linked immunosorbent assay (ELISA) and Western blot analysis. Affinity measurements carried out by competitive immunoassays showed that covalently linked (scFv')2 have binding constants quite close to those of the parental MAbs and fourfold higher than scFv' monomers. ScFv derivatives, specifically biotinylated through the free sulfhydryl group, recognize the corresponding antigen in ELISA and Western blot analysis, thus demonstrating the possibility of using chemically modified scFv antibodies for immunodetection.

Organization of the cytoskeleton in brine shrimp setal cells is molt-dependent

Fluorescence microscopy was used to examine the cytoskeleton in setal cells and antennae of the brine shrimp Artemia franciscana. Each setal cell has an elongated apical process that contains bundles of microtubules and microfilaments. When the organism molts, the apical process telescopes reversibly through the setal cell body into the hemocoel of the antenna. Staining of larval-stage Artemia with four monoclonal anti-tubulin antibodies (DM1 A, TAT, YL1/2, KMX) and with rhodamine–phalloidin indicated that the cytoskeletal elements were stable, remaining assembled as co-localized bundles in telescoping setal cells. Microtubule stability was suggested by previous observations of detyrosinated tubulin in setal cell extensions, but the microtubules were not completely detyrosinated, as shown by their interaction with YL1/2. Foci of tubulin staining within the antenna, enrichment of the microfilaments associated with the invaginating setal cell membrane, and the spatial distribution of other cytoskeletal elements were indicative of dynamic processes used in shape change during molting. Fluorescent labelling also revealed microtubules and microfilaments in tendinal cells, specialized epidermal cells that attach muscle in the antenna to the overlying cuticle.

Bifunctional and multimeric complexes of streptavidin fused to single chain antibodies (scFv)

Multivalent and multispecific antibodies with defined stoichiometry could provide valuable tools for biological and medical research and for the diagnosis and therapy of cancer. We have therefore fused single chain antibodies (scFv) with core-streptavidin. This chimeric protein, expressed by the vector pSTE-215 (plasmid for streptavidin-tagged expression), can form tetrameric complexes, binds antigen and contains the biotin binding site which may be used for further complex formation. An additional cysteine was inserted near the carboxy terminus to facilitate the construction of covalently linked bifunctional molecules. The scFv fusion protein could be purified by affinity chromatography using biotin analogues. We have also shown that the scFv fusion protein could be used for direct detection of its antigen in ELISA and Western blots when stained with biotinylated horseradish peroxidase.

Monoclonal anti-dipeptide antibodies cross-react with detyrosinated and glutamylated forms of tubulins

Two monoclonal antibodies, GLU-1 and A1.6, raised against gamma-L-glutamyl-L-glutamic acid dipeptide (Glu-Glu) and Ca(2+)-dependent ATPase from Paramecium, respectively, recognized the dipeptide Glu-Glu sequence. Whereas the antibodies immunofluorescently stained very few, if any, cytoskeletal fibers in cultured mammalian cells, almost all interphase as well as mitotic spindle microtubules became visible after treatment of cells with carboxypeptidase A. Immunoblot analysis demonstrated intense cross-reaction of the antibodies to the alpha-tubulin subunit. alpha-Tubulin isotypes produced as fusion proteins in bacteria were labeled by both the antibodies only when the proteins did not contain a tyrosine residue at the C terminus, indicating that GLU-1 and A1.6 specifically recognize the detyrosinated form of alpha-tubulin. When microtubule protein purified from brain was probed, not only alpha-but also, to a lesser extent, beta-tubulin were revealed by the dipeptide antibodies. A synthetic tripeptide YED containing one glutamyl group linked to the second residue of the peptide via the gamma position was also recognized by the antibodies. Since this peptide sequence corresponds to the amino acid sequence of polyglutamyated class III beta isotype at amino acid position 437 to 439, it is suggested that GLU-1 and A1.6 are able to recognize the glutamylated form of beta-tubulin. These results indicate that the C-terminal Glu-Glu sequence displays strong antigenicity, and the antibodies recognize the sequence present in the C terminus of the detyrosinated form of alpha-tubulin and the glutamyl side chain of beta-tubulin. Particularly strong immunoreaction was detected with ciliary and flagellar microtubules; thus, stable axonemal microtubules appear to be rich in post-translationally modified tubulin subunits.

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

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

Isolation of IgG antibody Fv-DNA from various mouse and rat hybridoma cell lines using the polymerase chain reaction with a simple set of primers

To facilitate the isolation of IgG antibody Fv-DNA sequences from hybridoma cell lines, we have established a polymerase chain reaction (PCR) procedure requiring only a small number of primers. The sense primers homologous to DNA coding for the first framework sequences were designed to hybridize to all the known antibody sequences under conditions that permit a high number of mismatches. The antisense primers were homologous to DNA coding for the beginning of the constant regions of the gamma and kappa chains. Restriction sites introduced by the primers enable the DNA to be cloned into bacterial expression vectors. Only three sense VH primers and two sense VL primers paired with one backward primer for the heavy and light chains, respectively, were necessary for the amplification of Fv-DNA from a total of 17 rodent cell lines that we have so far worked with. These consisted of 12 mouse cell lines and five rat cell lines. This procedure will therefore probably be sufficient to isolate the Fv-DNA from most mouse cell lines and possibly also from most rat cell lines.

Method for rapid electrophoretic transfer of isoelectric focusing gels to polyvinylidene difluoride

We present here a protocol for unidirectional wet electrotransfer of isoelectric focusing (IEF) gels to polyvinylidene difluoride (PVDF) that can be carried out in < 2 h, including preequilibration steps. In developing this protocol, we systematically evaluated sodium dodecyl sulfate and methanol concentrations in the preequilibration washes and length of electrotransfer. We demonstrated that the method is useful for tubulin, a protein that resists usual semi-dry transfer procedures due to its limited solubility under IEF conditions. We also successfully used our method for transferring the microtubule-associated protein, tau, as well as carbamylyte standards, creatine phosphokinase and carbonic anhydrase.

Kinesin and tau bind to distinct sites on microtubules

We have used a fluorescent derivative of kinesin, AF-kinesin (kinesin conjugated with 5-(iodoacetamido)fluorescein), to investigate the binding site of kinesin on microtubules and to compare this site with that to which tau binds. Microtubules saturated with tau will bind AF-kinesin in the presence of the ATP analogue, 5′-[beta,gamma-imino]triphosphate (AdoPP[NH]P). This shows that there are distinct binding sites for the two proteins. Further evidence comes from digestion studies where taxol-stabilised microtubules were treated with subtilisin, resulting in the cleavage of C-terminal residues from both the alpha- and beta-tubulin subunits. These treated microtubules can no longer bind tau, but are able to bind AF-kinesin in the presence of AdoPP[NH]P. Finally, AF-kinesin will support the gliding of subtilisin-digested microtubules in the presence of ATP at rates comparable to those obtained with non-digested microtubules. These results show directly that the binding site for kinesin is outside the C-terminal region of tubulin that is removed by subtilisin and is distinct from the binding site of tau.

Molecular mechanism of colchicine action: induced local unfolding of beta-tubulin

Colchicine, the classic antimitotic poison, disrupts cell division by preventing proper assembly of microtubules in the mitotic spindle. Colchicine is known to act by binding to tubulin, the heterodimeric subunit of microtubules. How this binding to tubulin changes the structure of the protein and results in polymerization poisoning has not been characterized. The structural locus of spectroscopically detected conformational changes induced by colchicine is unknown. We report here that colchicine induces the unfolding of a small region in the carboxyl-terminal region of beta-tubulin, around Arg-390. This unfolding is detected by proteolysis with trypsin and chymotrypsin. Chymotrypsin cleaves this region after Phe-389, and trypsin cleaves after Lys-392. The unfolded region appears to be the carboxyl end of an amphipathic helix in the absence of colchicine, and we propose that this unfolding prevents contacts necessary for assembly. Our results suggest that beta-tubulin is exposed on the growing end of the microtubule, which provides a mechanism for coupling GTP hydrolysis to polymerization.

Characterization of the post-translational modifications in tubulin from the marginal band of avian erythrocytes

Tubulin purified from turkey erythrocytes was characterized by partial protein sequence data, high-resolution IEF and by its reaction with antibodies specific for certain post-translational modifications. The tubulin from the marginal band contains a single alpha and beta isotype, i.e. alpha 1 and beta 6. Partial protein sequences and immunoblotting with antibody 6-11B-1 show that erythrocyte alpha 1 tubulin is not acetylated at Lys40. The acidic carboxy-terminal peptides purified by Mono Q chromatography and reverse-phase HPLC were characterized by sequence analysis and mass spectrometry. Although erythrocyte alpha tubulin is almost completely detyrosinated it retains the penultimate glutamic acid residue, which is partially lost in brain tubulin. Thus erythrocyte tubulin is an excellent substrate for extensive in vitro tyrosination by tubulin-tyrosine ligase. Erythrocyte alpha and beta tubulin lack the side-chain polyglutamylation found in all major tubulins from adult brain. Finally we show that about 10% of the beta tubulin is phosphorylated at Ser441. Thus erythrocyte tubulin is an unusual homogeneous preparation. It contains the minimum possible number of tubulin isotypes and the only post-translational modifications detected (detyrosination and phosphorylation) are reversible.

A family of vectors for surface display and production of antibodies

Expression vectors for surface display and production of single-chain (Fv) antibodies (scAb) have been constructed based on the phagemid pSEX, which expresses DNA encoding a scAb fused to the gene III product of filamentous phage [Breitling et al., Gene 104 (1991) 147-153]. A smaller version of this phagemid, pSEX20, was made by removing an unnecessary cat. To produce a vector for the surface display of other proteins and peptides, the scAb of pSEX20 was substituted by a polycloning site (MCS) to give pSEX40. For the presentation of Ab on the surface of Escherichia coli, phagemid pAP10 was derived from pSEX20 by substituting gene III with a gene encoding the peptidoglycan-associated lipoprotein (PAL). Vectors for producing scAb that can be purified by antibody and metal affinity chromatography were constructed by substituting gene III in the vector pSEX20 with DNA encoding a peptide with a C-terminal epitope recognised by a monoclonal antibody (phagemid pOPE40) or with five C-terminal histidines (pOPE 90).

G protein binding and G protein activation by nucleotide transfer involve distinct domains on tubulin: regulation of signal transduction by cytoskeletal elements

Cytoskeletal proteins have been demonstrated to interact with elements which mediate neuronal signal transduction. This laboratory has shown that tubulin forms complexes with certain G proteins and transfers GTP to them, and such nucleotide transfer correlates well with the inhibition of adenylyl cyclase. A previous study showed that tubulin-G protein complex formation occurred at regions of tubulin which were likely to be involved in binding to other tubulin dimers during the process of microtubule polymerization [Wang, N., & Rasenick, M. M. (1991) Biochemistry 30, 10957-10965]. This study represents an attempt to investigate the regions of tubulin which orchestrate the transfer of GTP from tubulin to G protein. It is demonstrated that two monoclonal antibodies, with determinants in similar regions of alpha (DM1a) or beta (DM1b) tubulin, blocked (by 70-80%) the ability of tubulin dimers (with GppNHp bound) to promote a stable inhibition of adenylyl cyclase. Under the same conditions, a polyclonal antitubulin antibody caused only a slight reduction in adenylyl cyclase inhibition. None of the antibodies altered the inhibition of adenylyl cyclase induced by GppNHp in the absence of tubulin. Under conditions where tubulin-GppNHp inhibited synaptic membrane adenylyl cyclase, tubulin dimers with the photoaffinity GTP analog azidoanilido-GTP (32P-AAGTP) bound transferred that nucleotide to G alpha i1 on the membrane. DM1a and DM1b blocked this nucleotide transfer. Similarly, tubulin-AAGTP transferred AAGTP to purified G proteins in solution and DM1a and DM1b blocked this process as well. Despite their ability to block the activation of Gi1 by tubulin, neither antibody altered the ability of tubulin to bind to native Gi alpha affixed to nitrocellulose.(ABSTRACT TRUNCATED AT 250 WORDS)

Rapid and reversible tubulin tyrosination in human neutrophils stimulated by the chemotactic peptide, fMet-Leu-Phe

Neutrophil activation by specific stimuli, such as the oligopeptide chemotactic factor fMet-Leu-Phe (fMLF), is associated with an increased enzymatic addition of tyrosine to tubulin alpha-subunits, as measured by 14C tyrosine uptake. In studies using immunoblots we have found that this increased tyrosine uptake into tubulin in activated neutrophils reflects an increase in the proportion of cellular tubulin that is tyrosinated rather than simply an increase in the turnover of tyrosinated subunits. However, the increased accumulation of tyrosinated tubulin was also found to follow an initial depletion of tyrosinated tubulin and concomitant increase in detyrosinated tubulin between 0 and 60 sec following stimulation of neutrophils with fMLF. Immunogold electron microscopy studies of intact microtubules recovered from activated neutrophils demonstrated that these rapid changes in the relative content of tubulin isoforms in the cells were not associated with the formation or disappearance of microtubule microdomains composed of only one form of tubulin. Previously, we have shown that under conditions of fMLF-stimulated exocytosis there is an increased binding of neutrophil granules to endogenous microtubules. Since neutrophil activation by fMLF is associated with increased tyrosination of alpha-tubulin subunits, we speculated that rapid changes in the levels of tyrosinated tubulin in the microtubules of activated neutrophils might have a role in the regulation of granule-microtubule interactions. When the binding of purified neutrophil granules to reconstituted rat brain microtubules containing approximately 50% tyrosinated tubulin was measured by electron microscopy and compared with granule binding to microtubules that contained no detectable tyrosinated tubulin, granule-microtubule associations were found to be significantly favored by detyrosinated vs. tyrosinated tubulin. These findings indicate that interactions between cytoplasmic granules and microtubules in activated neutrophils may be modulated by rapid changes in the relative content of detyrosinated and tyrosinated tubulin in the microtubule network of the cells.

Multiple sites for subtilisin cleavage of tubulin: effects of divalent cations

Limited digestion of pig brain GDP-tubulin by subtilisin was carried out in the presence of Mg2+, Mn2+, Ca2+, Zn2+, or Be2+. Isoelectric focusing, followed by SDS-PAGE, revealed characteristic divalent cation-dependent changes in the alpha- and beta-tubulin cleavage patterns. Previous studies revealed that the beta-cleavage pattern is different for heterodimers and microtubules [Lobert and Correia, 1992: Arch. Biochem. Biophys. 296: 152-160]. Divalent cation effects on subtilisin digestion of tubulin indicate different classes of divalent cation binding sites. Western blot analysis locates the proteolytic zone at residue 430 or higher in both subunits for all conditions. Turbidity and electron microscopy reveal that GDP-tubulin cleaved by subtilisin in the presence of Mg2+, Ca2+, or Mn2+ forms sheets of rings. Mn2+ induces ring formation in uncleaved GDP-tubulin. Isotype-depleted tubulin was generated by the removal of class III beta-tubulin using immunoaffinity chromatography. Subtilisin digestion of the depleted fraction and the purified class III beta-tubulin demonstrates that cleavage occurs at three to four distinct sites. Thus, subtilisin-digested tubulin is more heterogeneous than was previously reported and the cleavage sites depend on solution conditions, divalent cations, and the state of assembly. This has important implications for experiments that utilize subtilisin-digested tubulin for studying microtubule-associated protein binding.

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)

Recombinant human monoclonal antibodies. Basic principles of the immune system transferred to E. coli

To produce human monoclonal antibodies in bacteria, a gene repertoire of IgM variable regions was isolated from human peripheral B lymphocytes by the polymerase chain reaction. Alternatively, synthetic antibody genes with random hypervariable regions are being generated that may provide libraries of even higher complexity. For the selection of specific monoclonal antibodies from these libraries, we have developed two E. coli vector systems that facilitate the surface display of an antibody physically linked to its own gene. The phagemid pSEX encodes a fusion protein of an antigen binding domain (Fv-antibody) with the docking protein (pIII) of filamentous phages. Specific antibody genes can therefore be enriched by antigen affinity chromatography. The plasmid pAP1 encodes a fusion protein of an Fv-antibody with a bacterial cell-wall protein. Bacteria carrying this plasmid express functional Fv-antibodies tightly bound to their surface. This should enable the selection of single cells with a fluorescence-assisted cell sorter (FACS) using labeled antigen or by adsorption to immobilized antigen. These vectors permit three major principles of the antibody response to be mimicked in E. coli: 1. Generation of a highly complex antibody repertoire; 2. Clonal selection procedures for library screening; and 3. The possibility of increasing a given affinity by repeated rounds of mutation and selection.

Regulated secretion and purification of recombinant antibodies in E. coli

A plasmid for optimized protein expression of recombinant Fv antibodies (pOPE) in E. coli was used to express the variable domains of the murine monoclonal antibody HD39 specific for the human B-cell surface antigen CD22. The production of Fv antibodies by pOPE can be regulated over a wide range by varying the IPTG concentration. Antibodies that can discriminate between secreted and nonsecreted Fv antibody fragments were used to show that secretion is the limiting step for the production of functional Fv antibodies. IPTG concentrations above 20 microM increased the total antibody production, but did not yield larger amounts of secreted Fv antibodies. The addition of five histidines to the C terminus facilitates an easy single-step enrichment procedure based on immobilized metal affinity chromatography.

A vector for the removal of deletion mutants from antibody libraries

To reduce the number of deletion mutants from antibody (Ab) libraries that had been amplified by PCR from peripheral blood lymphocytes, we constructed the Ab expression vector, pLAB, in which DNA coding for a single-chain Ab was inserted into the gene encoding beta-lactamase (Bla) at the 3'-terminus of its signal sequence. After transforming Escherichia coli with this vector, a fusion protein with a functional Bla domain was produced that was able to protect the bacteria from the action of ampicillin (Ap). Libraries can therefore be usefully propagated with this vector, since only those clones carrying inserts that are in frame with Bla will survive Ap selection, while others that carry out-of-frame deletions or internal stop codons are eliminated.

Targeting Recombinant Antibodies to the Surface of Escherichia coli: Fusion to a Peptidoglycan Associated Lipoprotein

To target recombinant antibodies to the surface of Escherichia coli, we have fused single-chain variable domains to its peptidoglycan associated lipoprotein (PAL). The fusion protein was able to bind antigen and was tightly bound to the murein layer of the cell envelope. Antibody-PAL had little effect on cell growth and viability. In contrast, the expression of single chain antibody alone eventually resulted in cell lysis. Immunofluorescence studies on unfixed cells showed that functional antibodies were accessible at the surface of intact bacteria. This could provide a means of isolating single cells producing specific antibodies from libraries in E. coli by fluorescence assisted cell sorting (FACS). Pal fusions may also be of general interest for the presentation of proteins at the surface of E. coli as, for example, in the production of live vaccines.

Post-translationally modified tubulins in Artemia: prelarval development in the absence of detyrosinated tubulin

The synthesis of post-translationally modified tubulins was examined during Artemia development. Tubulin, either purified to homogeneity or in cell-free extracts, was blotted to nitrocellulose and probed with a panel of antibodies. When purified tubulin was examined, tyrosinated tubulin underwent a large decrease as development progressed and this was accompanied by the appearance of detyrosinated tubulin in samples from organisms developed 24 hr. The inclusion of carboxypeptidase inhibitors had a small effect on the relative amounts of tyrosinated and detyrosinated tubulins in 24-hr preparations. The amount of alpha- and beta-tubulin in cell-free extracts of Artemia either remained relatively constant during development or increased slightly. The same result was obtained for acetylated and tyrosinated tubulin. Detyrosinated tubulin first appeared in 24-hr cell-free extracts and was only post-translationally modified tubulin to increase, relative to the total amount of tubulin, as the brine shrimp developed. As revealed by immunofluorescence staining, detyrosinated tubulin occurred in many cell types of developing nauplii and was prominently displayed in mitotic figures. Artemia, a complex metazoan animal, is thus able to grow for an extended period of time in the absence of detyrosinated tubulin. This isoform is however, synthesized in early larvae and may be required for the development of elongated cells including those which encircle the gut. Detyrosination remains as the only developmentally related change observed for brine shrimp tubulin.

A surface expression vector for antibody screening

To select specific antibodies (Ab) from large recombinant libraries using small amounts of antigen, we have constructed a phagemid that expresses a single-chain Ab fused to pIII, a coliphage protein product of gene III that initiates infection by binding to F pili. Surprisingly, the production of the fusion protein (Ab::pIII) was induced by wild-type (wt) phage fd in the absence of IPTG. Ab::pIII was identified by a monoclonal Ab to an epitope in the linker sequence between the heavy and light chains, and by antisera to their N-terminal sequences. It is able to bind antigen and be assembled into infectious phagemid particles that can be enriched on columns of immobilised antigen. The phagemid DNA is even smaller than that of wt fd phages and can easily be propagated in plasmid form. Most importantly, its Ab::pIII-encoding gene can be tightly repressed so that Ab libraries can be amplified without risk of being dominated by deletion mutants. After induction, however, large quantities of the fusion protein can be produced, thus greatly facilitating its analysis.

Alpha-, beta-, and gamma-tubulins: sequence comparisons and structural constraints

Comparison of congruent to 160 alpha-, beta-, and gamma-tubulins, and excluding the highly divergent C-terminal peptide, indicates that the three subclasses have similar tertiary structures. Conserved sequences within or between the subclasses have been identified, together with the locations of known epitopes, chemical modifications, and mutations. Evidence is also reviewed concerning the identity of the GTP-binding sites, about which residues are exposed in the assembled microtubule and at subunit:subunit interfaces. These characteristics constrain the possible tertiary structure of the tubulin subunit.

Analysis of beta-tubulin sequences reveals highly conserved, coordinated amino acid substitutions. Evidence that these 'hot spots' are directly involved in the conformational change required for dynamic instability

Vertebrate beta-tubulins have been classified into six classes on the basis of their C-terminal sequences [(1987) J. Cell Biol. 105, 1707-1720]. In particular, the sequences starting at residue 430 differ between isotypes of the same animal but are conserved between species. We extend this analysis and show that there are three 'hot spots', at residues 35, 55-57 and 124 which exhibit intra-species heterogeneity but inter-species conservation. There is a remarkable correlation between the identity of these residues and the C-terminal sequences, and suggests that the vertebrate beta-tubulins fall into three broad types. This correlation extends to those non-vertebrate organisms which have the Type 1 C-terminal sequence. We propose that these three 'hot spots' and the C-terminal peptide interact in the tertiary structure. We have also noted that the C-terminal peptide almost always contains a single phenylalanine or tyrosine residue, and that there is a strong correlation between this residue and the amino acids at positions 217/218, in both the vertebrate and non-vertebrate sequences. We propose that the C-terminal aromatic amino acid interacts with residues 217/218 in the tertiary structure. Analysis of conditions which stabilise microtubules and/or lower the steady state critical concentration strongly suggests that these two sets of coordinated amino acid substitutions are directly involved in effecting the conformational change associated with GTP hydrolysis which results in dynamic instability. We propose that there is an interaction between the highly acidic sequence between residue 430 and the aromatic amino acid (termed peptide A) and conserved basic amino acids located close to the 'hot spots'. We suggest that this interaction is altered in response to the assembly-dependent GTP hydrolysis, with the consequential increase in the subunit dissociation rate constant.

Two Drosophila beta tubulin isoforms are not functionally equivalent

We have tested the functional capacity of different beta tubulin isoforms in vivo by expressing beta 3-tubulin either in place of or in addition to beta 2-tubulin in the male germ line of Drosophila melanogaster. The testes-specific isoform, beta 2, is conserved relative to major metazoan beta tubulins, while the developmentally regulated isoform, beta 3, is considerably divergent in sequence. beta 3-tubulin is normally expressed in discrete subsets of cells at specific times during development, but is not expressed in the male germ line. beta 2-Tubulin is normally expressed only in the postmitotic germ cells of the testis, and is required for all microtubule-based functions in these cells. The normal functions of beta 2-tubulin include assembly of meiotic spindles, axonemes, and at least two classes of cytoplasmic microtubules, including those associated with the differentiating mitochondrial derivatives. A hybrid gene was constructed in which 5' sequences from the beta 2 gene were joined to protein coding and 3' sequences of the beta 3 gene. Drosophila transformed with the hybrid gene express beta 3-tubulin in the postmitotic male germ cells. When expressed in the absence of the normal testis isoform, beta 3-tubulin supports assembly of one class of functional cytoplasmic microtubules. In such males the microtubules associated with the membranes of the mitochondrial derivatives are assembled and normal mitochondrial derivative elongation occurs, but axoneme assembly and other microtubule-mediated processes, including meiosis and nuclear shaping, do not occur. These data show that beta 3 tubulin can support only a subset of the multiple functions normally performed by beta 2, and also suggest that the microtubules associated with the mitochondrial derivatives mediate their elongation. When beta 3 is coexpressed in the male germ line with beta 2, at any level, spindles and all classes of cytoplasmic microtubules are assembled and function normally. However, when beta 3-tubulin exceeds 20% of the total testis beta tubulin pool, it acts in a dominant way to disrupt normal axoneme assembly. In the axonemes assembled in such males, the doublet tubules acquire some of the morphological characteristics of the singlet microtubules of the central pair and accessory tubules. These data therefore unambiguously demonstrate that the Drosophila beta tubulin isoforms beta 2 and beta 3 are not equivalent in intrinsic functional capacity, and furthermore show that assembly of the doublet tubules of the axoneme imposes different constraints on beta tubulin function than does assembly of singlet microtubules.

Tubulin assembly probed with antibodies to synthetic peptides

Antibodies to synthetic peptides from the alpha and beta-tubulin sequences were employed to study zones of this protein active in microtubule assembly. In purified calf brain tubulin, six short sequences, selected according to their hydrophilicity and conservation, were found to be accessible to their affinity-purified immunoglobulin G (IgG) antibodies, in a competition radioimmunoassay performed under non-assembly native conditions. This indicated that the six sequences are exposed on the surface of the tubulin alpha beta heterodimer. IgG antibodies to the alpha(430-443) and beta(412-431) sequences perturbed substoichiometrically the assembly of purified tubulin, inducing microtubule bundling and the formation of opened up structures. These positions, which are close to the C termini, were accessible to the anti-peptide antibodies in taxol-induced microtubules, Zn2(+)-induced tubulin sheets, Mg2(+)-induced tubulin rings and in PtK2 cell microtubules. This, together with the comparison of the sizes and gross shapes of the antibody probes and microtubules, suggested that these sequences might be located at the protruding parts of the protofilaments. Antibodies to positions alpha(155-168) did not react with microtubules, while the equivalent zone beta(153-165) was accessible. The alpha(214-226) and beta(241-256) sequences were antigenically occluded in the taxol microtubules, Zn2(+)-induced sheets and Mg2(+)-induced ring arrays, as well as in native microtubules from PtK2 cells, though they became reactive by fixation. This result strongly suggested that these two zones are close to tubulin-tubulin contact sites. A working model is proposed in which the positions alpha(214-226) and beta(241-256) are close to the axial contacts between heterodimers, which lead to protofilament formation, while the positions alpha(241-256) and beta(214-226) are suggested to be related to the alpha-beta binding interface within the heterodimer.

Interaction of brain cytoplasmic dynein and MAP2 with a common sequence at the C terminus of tubulin

Two main types of microtubule-associated proteins (MAPs) have been identified in neuronal cells. The fibrous MAPs, including MAP2 and tau, serve to organize and regulate the assembly of microtubules. A second distinct class of force-producing MAPs, including kinesin, dynein and dynamin, are involved in microtubule-based movement. These proteins are mechanochemical ATPases which seem to be responsible for the bidirectional transport of organelles and perhaps also the movement of chromosomes. Here we report that MAP2 inhibits microtubule gliding on dynein-coated coverslips, as well as the microtubule-activated ATPase of dynein, indicating that MAP2 and other fibrous MAPs could be important modulators of microtubule-based motility in vivo. By proteolytic modification of tubulin, we found that dynein interacts with microtubules at the C termini of alpha- and beta-tubulin, the regions previously reported to be the sites for the interaction of MAP2. The use of site-directed antibodies implicates a small region of alpha- and beta-tubulin, containing the sequence Glu-Gly-Glu-Glu, as the site of the interaction of dynein and MAP2 with the microtubule.

Assembly properties of altered beta-tubulin polypeptides containing disrupted autoregulatory domains

beta-Tubulin synthesis in eucaryotic cells is subject to control by an autoregulatory posttranscriptional mechanism in which the first four amino acids of the beta-tubulin polypeptide act either directly or indirectly to control the stability of beta-tubulin mRNA. To investigate the contribution of this amino-terminal domain to microtubule assembly and dynamics, we introduced a series of deletions encompassing amino acids 2 to 5 of a single mammalian beta-tubulin isotype, M beta 1. Constructs carrying such deletions were inserted into an expression vector, and the ability of the altered polypeptide to coassemble into microtubules was tested by using an anti-M beta 1-specific antibody. We show that the M beta 1 beta-tubulin polypeptide was competent for coassembly into microtubules in transient transfection experiments and in stably transfected cell lines when it lacked either amino acid 2 or amino acids 2 and 3. The capacity of these mutant beta-tubulins to coassemble into polymerized microtubules was only slightly diminished relative to that of unaltered beta-tubulin, and their expression did not influence the viability or growth properties of cell lines carrying these deletions. However, more extensive amino-terminal deletions either severely compromised or abolished the capacity for coassembly. In analogous experiments in which alterations were introduced into the amino-terminal domain of a mammalian alpha-tubulin isotype, M alpha 4, deletion of amino acid 2 did not affect the ability of the altered polypeptide to coassemble, although removal of additional amino-terminal residues essentially abolished the capacity for competent coassembly. The stability of the altered assembly-competent alpha- and beta-tubulin polypeptides was measured in pulse-chase experiments and found to be indistinguishable from the stability of the corresponding unaltered polypeptides. An assembly-competent M alpha 4 polypeptide carrying a deletion encompassing the 12 carboxy-terminal amino acids also had a half-life indistinguishable from that of the wild-type alpha-tubulin molecule. These data suggest that the universally conserved amino terminus of beta-tubulin acts largely in a regulatory role and that the carboxy-terminal domain of alpha-tubulin is not essential for coassembly in mammalian cells in vivo.

Assembly properties of altered beta-tubulin polypeptides containing disrupted autoregulatory domains

beta-Tubulin synthesis in eucaryotic cells is subject to control by an autoregulatory posttranscriptional mechanism in which the first four amino acids of the beta-tubulin polypeptide act either directly or indirectly to control the stability of beta-tubulin mRNA. To investigate the contribution of this amino-terminal domain to microtubule assembly and dynamics, we introduced a series of deletions encompassing amino acids 2 to 5 of a single mammalian beta-tubulin isotype, M beta 1. Constructs carrying such deletions were inserted into an expression vector, and the ability of the altered polypeptide to coassemble into microtubules was tested by using an anti-M beta 1-specific antibody. We show that the M beta 1 beta-tubulin polypeptide was competent for coassembly into microtubules in transient transfection experiments and in stably transfected cell lines when it lacked either amino acid 2 or amino acids 2 and 3. The capacity of these mutant beta-tubulins to coassemble into polymerized microtubules was only slightly diminished relative to that of unaltered beta-tubulin, and their expression did not influence the viability or growth properties of cell lines carrying these deletions. However, more extensive amino-terminal deletions either severely compromised or abolished the capacity for coassembly. In analogous experiments in which alterations were introduced into the amino-terminal domain of a mammalian alpha-tubulin isotype, M alpha 4, deletion of amino acid 2 did not affect the ability of the altered polypeptide to coassemble, although removal of additional amino-terminal residues essentially abolished the capacity for competent coassembly. The stability of the altered assembly-competent alpha- and beta-tubulin polypeptides was measured in pulse-chase experiments and found to be indistinguishable from the stability of the corresponding unaltered polypeptides. An assembly-competent M alpha 4 polypeptide carrying a deletion encompassing the 12 carboxy-terminal amino acids also had a half-life indistinguishable from that of the wild-type alpha-tubulin molecule. These data suggest that the universally conserved amino terminus of beta-tubulin acts largely in a regulatory role and that the carboxy-terminal domain of alpha-tubulin is not essential for coassembly in mammalian cells in vivo.

The role of microtubules in platelet secretory release

The role of microtubules in platelet aggregation and secretion has been analyzed using platelets permeabilized with digitonin and monoclonal antibodies to alpha (DM1A) and beta (DM1B) subunits of tubulin. Permeabilized platelets were able to undergo aggregation and secretory release. However, threshold doses of agonists capable of eliciting a second wave of aggregation and the platelet release reaction were higher than in control platelets exposed to dimethyl sulfoxide, the solvent for digitonin. Both antibodies to alpha and beta tubulin caused a further increase in the threshold concentration of agonists and inhibited the secretory release of permeabilized platelets, but were ineffective using intact platelets. Neither monoclonal antibody inhibited polymerization or depolymerization of platelet tubulin in vitro. Antibodies to platelet actin and myosin also exhibited an inhibitory activity on platelet aggregation albeit less severe than that observed with the antibodies to alpha and beta tubulin. There was evidence of an interaction between DM1A and DM1B and the antibodies to actin and myosin. The interaction of platelet tubulin and myosin was investigated by two different methods. (1) Coprecipitation of the proteins at low ionic strength at which tubulin by itself did not precipitate and (2) affinity chromatography on columns of immobilized myosin. Tubulin freed of its associated proteins (MAPs) by phosphocellulose chromatography bound to myosin in a molar ratio which approached 2. Platelet actin competed with tubulin for 1 binding site on the myosin molecule. MAPs also reduced the binding stoichiometry of tubulin/myosin. Treatment of microtubule protein with p-chloromercuribenzoate or colchicine did not influence its binding to myosin. DM1A and DM1B inhibited the interaction of tubulin and myosin. This effect could also be demonstrated by reaction of electrophoretic transblots of extracted platelet tubulin with the respective proteins. We interpret these results as evidence for an interference of the two monoclonal antibodies to the tubulin subunits (DM1A and DM1B) with the translocation of microtubule protein from its submembranous site to a more central one during the activation process.

The distribution, abundance and subcellular localization of kinesin

An antiserum which binds kinesin specifically on Western blots was used to determine the distribution and abundance of chicken kinesin by correlated immunoblotting and immunolocalization. Quantitative immunoblotting showed that the abundance of kinesin varied widely in different cell and tissue types, from 0.039% of total protein in epidermal fibroblasts to 0.309% in sympathetic neurons; of the types examined, only red blood cells lacked detectable kinesin. The molar ratio of tubulin/kinesin varied over a narrower range. To analyze the intracellular distribution of kinesin, cultured fibroblasts were fractionated by sequential extraction with saponin-, Triton X-100-, and SDS-containing buffer. Quantitative blotting of the resulting cell fractions indicated that 68% of fibroblast kinesin is in soluble form, 32% is membrane- or organelle-associated, and none is detectable in cytoskeletal fractions. To visualize this distribution, cells treated by the same extraction protocol were immunofluorescently stained with antikinesin and antitubulin. Without extraction, kinesin staining was located throughout cultured neurons and fibroblasts. However, when fibroblasts were extracted with saponin or Brij 58 before fixation, subsequent staining revealed that the remaining kinesin fraction was colocalized with interphase microtubules, but not with mitotic spindles. Prefixation extraction with Triton abolished antikinesin staining. These data suggest that kinesin may play a role in tubovesicular movement but provide no evidence for a role in mitosis.

Antibodies to synthetic peptides from the tubulin regulatory domain interact with tubulin and microtubules

The carboxyl-terminal region of tubulin alpha and beta subunits plays a major role in regulating its assembly into microtubules and constitutes an essential domain for the selective interaction of microtubule-associated proteins (MAPs). With the goal of understanding the structural basis of the regulatory function of the carboxyl-terminal domains of tubulin subunits, we have produced rabbit antisera against two MAP-interacting peptides Lys-Asp-Tyr-Glu-Glu-Val-Gly-Val-Asp-Ser-Val-Glu of alpha-tubulin and Tyr-Gln-Gln-Tyr-Gln-Asp-Ala-Thr-Ala-Asp-Glu-Gln-Gly of beta subunit. The affinity-purified alpha and beta anti-peptide antibodies interacted specifically with tubulin and with the respective peptide antigens but did not interact with MAPs. Substoichiometric amounts of both antibodies showed the capacity to inhibit in vitro MAP-induced tubulin assembly and to promote a fast depolymerization of preassembled microtubules. Taxol-promoted assembly of pure tubulin was not inhibited by the antibodies. In the presence of MAP-2 and taxol, the antibodies decreased the MAP-2 content of taxol-promoted microtubules. The interaction with microtubules was corroborated by immunofluorescence experiments in HeLa and NE-18 lung carcinoma cells. The epitopes recognized by the alpha and beta anti-peptide antibodies appear to be located in the outer surface of the microtubular structure.

Anti-idiotypic antibodies that react with microtubule-associated proteins are present in the sera of rabbits immunized with synthetic peptides from tubulin's regulatory domain

A fundamental question in microtubule research is how the interactions of tubulin subunits with microtubule-associated proteins (MAPs) are controlled. The answer should provide insight into the regulation of the cellular processes in which microtubules are implicated. Previous work demonstrated the interaction of MAPs with a 4-kDa C-terminal domain of tubulin alpha and beta subunits. Synthetic peptides from the variable region of the 4-kDa C-terminal moiety of tubulin subunits, alpha-(430-441) and beta-(422-434), bind to MAP-2 and to the MAP tau, and a preferential interaction of the beta peptide is observed. To define the regulatory significance of the substructure of the C-terminal tubulin domain, we produced rabbit antisera against these MAP-interacting peptides. We found that these antisera contained not only antibodies to the original synthetic peptides but also antibodies to MAPs. Here, we report that these antibodies, which react with MAP-1, MAP-2, and tau, appear to be a population of anti-idiotypic antibodies directed to the anti-peptide antibodies. They can inhibit MAP-induced tubulin assembly into microtubules in vitro, and the addition of MAPs overcomes the inhibition. The recognition by these anti-idiotypic antibodies of the tubulin-binding domain on MAPs provides unequivocal evidence that the tubulin region defined by the synthetic peptides is directly involved in the interaction with MAPs.

The alpha-tubulin gene family expressed during cell differentiation in Naegleria gruberi

Genes that direct the programmed synthesis of flagellar alpha-tubulin during the differentiation of Naegleria gruberi from amebae to flagellates have been cloned, and found to be novel with respect to gene organization, sequence, and conservation. The flagellar alpha-tubulin gene family is represented in the genome by about eight homologous DNA segments that are exceptionally similar and yet are neither identical nor arrayed in a short tandem repeat. The coding regions of three of these genes have been sequenced, two from cDNA clones and one from an intronless genomic gene. These three genes encode an identical alpha-tubulin that is conserved relative to the alpha-tubulins of other organisms except at the carboxyl terminus, where the protein is elongated by two residues and ends in a terminal glutamine instead of the canonical tyrosine. In spite of the protein conservation, the Naegleria DNA sequence has diverged markedly from the alpha-tubulin genes of other organisms, a counterexample to the idea that tubulin genes are conserved. alpha-Tubulin mRNA homologous to this gene family has not been detected in amebae. This mRNA increases markedly in abundance during the first hour of differentiation, and then decreases even more rapidly with a half-life of approximately 8 min. The abundance of physical alpha-tubulin mRNA rises and subsequently falls in parallel with the abundance of translatable flagellar tubulin mRNA and with the in vivo rate of flagellar tubulin synthesis, which indicates that flagellar tubulin synthesis is directly regulated by the relative rates of transcription and mRNA degradation.