Duplex microtubule is a new form of tubulin assembly induced by polycations

Tubulin Double Helix: Lateral and Longitudinal Curvature Changes of Tubulin Protofilament

By virtue of their native structures, tubulin dimers are protein building blocks that are naturally preprogrammed to assemble into microtubules (MTs), which are cytoskeletal polymers. Here, polycation-directed (i.e., electrostatically tunable) assembly of tubulins is demonstrated by conformational changes to the tubulin protofilament in longitudinal and lateral directions, creating tubulin double helices and various tubular architectures. Synchrotron small-angle X-ray scattering and transmission electron microscopy reveal a remarkable range of nanoscale assembly structures: single- and double-layered double-helix tubulin tubules. The phase transitions from MTs to the new assemblies are dependent on the size and concentration of polycations. Two characteristic scales that determine the number of observed phases are the size of polycation compared to the size of tubulin (≈4 nm) and to MT diameter (≈25 nm). This work suggests the feasibility of using polycations that have scissor- and glue-like properties to achieve "programmable breakdown" of protein nanotubes, tearing MTs into double-stranded tubulins and building up previously undiscovered nanostructures. Importantly, a new role of tubulins is defined as 2D shape-controllable building blocks for supramolecular architectures. These findings provide insight into the design of protein-based functional materials, for example, as metallization templates for nanoscale electronic devices, molecular screws, and drug delivery vehicles.

Tubulin-Based Nanotubes as Delivery Platform for Microtubule-Targeting Agents

Tubulin-based nanotubes (TNTs) to deliver microtubule-targeting agents (MTAs) for clinical oncology are reported. Three MTAs, docetaxel (DTX), laulimalide (LMD), and monomethyl auristatin E (MMAE), which attach to different binding sites in a tubulin, are loaded onto TNTs and cause structural changes in them, including shape anisotropy and tubulin layering. This drug-driven carrier transformation leads to changes in the drug-loading efficiency and stability characteristics of the carrier. TNTs coloaded with DTX and LMD efficiently deliver dual drug cargoes to cellular tubulins by the endolysosomal pathway, and results in synergistic anticancer and antiangiogenic action of the drugs in vitro. In in vivo tests, TNTs loaded with a microtubule-destabilizing agent MMAE suppress the growth of tumors with much higher efficacy than free MMAE did. This work suggests a new concept of using a drug's target protein as a carrier. The findings demonstrate that the TNTs developed here can be used universally as a delivery platform for many MTAs.

Transformation of taxol-stabilized microtubules into inverted tubulin tubules triggered by a tubulin conformation switch

Bundles of taxol-stabilized microtubules (MTs)--hollow tubules comprised of assembled αβ-tubulin heterodimers--spontaneously assemble above a critical concentration of tetravalent spermine and are stable over long times at room temperature. Here we report that at concentrations of spermine several-fold higher the MT bundles (B(MT)) quickly become unstable and undergo a shape transformation to bundles of inverted tubulin tubules (B(ITT)), the outside surface of which corresponds to the inner surface of the B(MT) tubules. Using transmission electron microscopy and synchrotron small-angle X-ray scattering, we quantitatively determined both the nature of the B(MT)-to-B(ITT) transformation pathway, which results from a spermine-triggered conformation switch from straight to curved in the constituent taxol-stabilized tubulin oligomers, and the structure of the B(ITT) phase, which is formed of tubules of helical tubulin oligomers. Inverted tubulin tubules provide a platform for studies requiring exposure and availability of the inside, luminal surface of MTs to MT-targeted drugs and MT-associated proteins.

YB-1 promotes microtubule assembly in vitro through interaction with tubulin and microtubules

Background

YB-1 is a major regulator of gene expression in eukaryotic cells. In addition to its role in transcription, YB-1 plays a key role in translation and stabilization of mRNAs.

Results

We show here that YB-1 interacts with tubulin and microtubules and stimulates microtubule assembly in vitro. High resolution imaging via electron and atomic force microscopy revealed that microtubules assembled in the presence of YB-1 exhibited a normal single wall ultrastructure and indicated that YB-1 most probably coats the outer microtubule wall. Furthermore, we found that YB-1 also promotes the assembly of MAPs-tubulin and subtilisin-treated tubulin. Finally, we demonstrated that tubulin interferes with RNA:YB-1 complexes.

Conclusion

These results suggest that YB-1 may regulate microtubule assembly in vivo and that its interaction with tubulin may contribute to the control of mRNA translation.

Regulated binding of adenomatous polyposis coli protein to actin

Adenomatous polyposis coli (APC) protein is a large tumor suppressor that is truncated in most colorectal cancers. The carboxyl-terminal third of APC protein mediates direct interactions with microtubules and the microtubule plus-end tracking protein EB1. In addition, APC has been localized to actin-rich regions of cells, but the mechanism and functional significance of this localization have remained unclear. Here we show that purified carboxyl-terminal basic domain of human APC protein (APC-basic) bound directly to and bundled actin filaments and associated with actin stress fibers in microinjected cells. Actin filaments and microtubules competed for binding to APC-basic, but APC-basic also could cross-link actin filaments and microtubules at specific concentrations, suggesting a possible role in cytoskeletal cross-talk. APC interactions with actin in vitro were inhibited by its ligand EB1, and co-microinjection of EB1 prevented APC association with stress fibers. Point mutations in EB1 that disrupted APC binding relieved the inhibition in vitro and restored APC localization to stress fibers in vivo, demonstrating that EB1-APC regulation is direct. Because tumor formation and metastasis involve coordinated changes in the actin and microtubule cytoskeletons, this novel function for APC and its regulation by EB1 may have direct implications for understanding the molecular basis of tumor suppression.

HURP wraps microtubule ends with an additional tubulin sheet that has a novel conformation of tubulin

HURP is a newly discovered microtubule-associated protein (MAP) required for correct spindle formation both in vitro and in vivo. HURP protein is highly charged with few predicted secondary and tertiary folding domains. Here we explore the effect of HURP on pure tubulin, and describe its ability to induce a new conformation of tubulin sheets that wrap around the ends of intact microtubules, thereby forming two concentric tubes. The inner tube is a normal microtubule, while the outer one is a sheet composed of tubulin protofilaments that wind around the inner tube with a 42.5 degrees inclination. We used cryo-electron microscopy and unidirectional surface shadowing to elucidate the structure and conformation of HURP-induced tubulin sheets and their interaction with the inner microtubule. These studies clarified that HURP-induced sheets are composed of anti-parallel protofilaments exhibiting P2 symmetry. HURP is a unique MAP that not only stabilizes and bundles microtubules, but also polymerizes free tubulin into a new configuration.

Microtubule protofilament number is modulated in a stepwise fashion by the charge density of an enveloping layer

Microtubules are able to adjust their protofilament (PF) number and, as a consequence, their dynamics and function, to the assembly conditions and presence of cofactors. However, the principle behind such variations is poorly understood. Using synchrotron x-ray scattering and transmission electron microscopy, we studied how charged membranes, which under certain conditions can envelop pre-assembled MTs, regulate the PF number of those MTs. We show that the mean PF number, <N>, is modulated primarily by the charge density of the membranes. <N> decreases in a stepwise fashion with increasing membrane charge density. <N> does not depend on the membrane-protein stoichiometry or the solution ionic strength. We studied the effect of taxol and found that <N> increases logarithmically with taxol/tubulin stoichiometry. We present a theoretical model, which by balancing the electrostatic and elastic interactions in the system accounts for the trends in our findings and reveals an effective MT bending stiffness of order 10-100 k(B)T/nm, associated with the observed changes in PF number.

Cationic liposome-microtubule complexes: pathways to the formation of two-state lipid-protein nanotubes with open or closed ends

Intermolecular interactions between charged membranes and biological polyelectrolytes, tuned by physical parameters, which include the membrane charge density and bending rigidity, the membrane spontaneous curvature, the biopolymer curvature, and the overall charge of the complex, lead to distinct structures and morphologies. The self-assembly of cationic liposome-microtubule (MT) complexes was studied, using synchrotron x-ray scattering and electron microscopy. Vesicles were found to either adsorb onto MTs, forming a "beads on a rod" structure, or undergo a wetting transition and coating the MT. Tubulin oligomers then coat the external lipid layer, forming a tunable lipid-protein nanotube. The beads on a rod structure is a kinetically trapped state. The energy barrier between the states depends on the membrane bending rigidity and charge density. By controlling the cationic lipid/tubulin stoichiometry it is possible to switch between two states of nanotubes with either open ends or closed ends with lipid caps, a process that forms the basis for controlled chemical and drug encapsulation and release.

The yeast DASH complex forms closed rings on microtubules

The Saccharomyces cerevisiae DASH complex is an essential microtubule-binding component of the kinetochore. We coexpressed all ten subunits of this assembly in Escherichia coli and purified a single complex, a approximately 210-kDa heterodecamer with an apparent stoichiometry of one copy of each subunit. The hydrodynamic properties of the recombinant assembly are indistinguishable from those of the native complex in yeast extracts. The structure of DASH alone and bound to microtubules was visualized by EM. The free heterodecamer is relatively globular. In the presence of microtubules, DASH oligomerizes to form rings and paired helices that encircle the microtubules. We discuss potential roles for such collar-like structures in maintaining microtubule attachment and spindle integrity during chromosome segregation.

Preparation and properties of pure tubulin S

Limited proteolysis of the tubulin dimer (alphabeta) by subtilisin occurs more rapidly with beta than with alpha tubulin. This leads to the formation of an intermediate hybrid dimer, alphabeta(s), before both C termini are cleaved to form tubulin S(alpha(s)beta(s)). The three forms of tubulin usually coexist in subtilisin-treated preparations and such cross-contamination can be reliably detected only by running SDS-polyacrylamide gels well beyond expulsion of the dye front. Previously published preparations have not ruled out such contamination or have formed poorly reversible polymers. Because ion exchange separation incurred substantial protein losses, we have developed a new protocol for rapid preparation of tubulin S (alpha(s)beta(s), free of alphabeta or alphabeta(s)) that is based on proteolysis at low ionic strength. This increases the relative rate of C terminal cleavage of beta tubulin. The product forms sheets, bundles, or rings that are depolymerized by cold, salt, and podophyllotoxin, partially depolymerized by Ca2+, and has a decreased critical concentration for polymerization that can be further decreased by taxol. We have also found a method for forming nearly pure alphabeta(s) dimers by using methods that retard proteolysis of the C terminus of alpha tubulin.

Scanning force microscopy of microtubules and polymorphic tubulin assemblies in air and in liquid

We have investigated microtubules (MTs) and polymorphic assemblies, formed in vitro from isolated microtubule protein, by scanning force microscopy (SFM) in air and in liquid. Immobilization of MTs was achieved by placing a drop of the assembly solution on a polylysine-coated coverslip. After washing with taxol and air drying, the characteristic microtubular fibrous morphology appeared in the SFM. The MTs formed a network similar to that obtained by transmission electron microscopy (TEM). A height of approximately 9.5 nm for dried MTs was computed from the surface topography. Glutaraldehyde fixation of the MTs yielded higher structures (approximately 14 nm), which swelled to approximately 20 nm after rehydration, a value close to the MT diameter of approximately 25 nm determined from TEM images of ultrathin sections. The protofilament pattern of the MTs and surface attached MT-associated proteins were not apparent from SFM, although the height along the long axis of the MTs appeared slightly modulated. In addition to MTs, various polymorphic tubulin assemblies including ribbons, hoops and double-walled MTs were visualized by SFM.

Effects of the fluorescence dye DAPI on microtubule structure in vitro: formation of novel types of tubulin assembly products

It has been found that the DNA fluorescence dye 4',6-diamidino-2-phenylindole (DAPI) is able to stain also microtubules. However, electron microscopy revealed that DAPI changed microtubule structure and induced the formation of a broad spectrum of polymorphic tubulin assembly products. Upon addition of DAPI to microtubules assembled from 10 to 15 mumol tubulin (molar DAPI/tubulin ratios of 10 to 40) in the presence of microtubule-associated proteins, most of the microtubules were decorated with additional protofilaments usually running parallel to the protofilaments of the microtubule wall (microtubule-protofilament complexes). When DAPI was already present during assembly, curved C- and S-shaped protofilament ribbons and microtubule-ribbon complexes with 6-shaped profiles were the most prominent products, beside microtubules. Additionally, protofilament bundles, some flat sheets, and hoops occurred. Electrophoresis revealed that DAPI lowered the amount of associated proteins, especially of tau-proteins, bound to the assembly products. Nevertheless, DAPI stimulated the assembly, enabled pure tubulin to assemble even at concentrations as low as 10 mumol, and stabilized the assembly products against cold. The microtubule-protofilament complexes, observed for the first time, are interpreted as the result of DAPI-induced protofilament linking as well as of activation of an additional tubulin-tubulin binding site which is possibly identical to that involved in the formation of microtubule doublets.

Tubulin protofilaments and kinesin-dependent motility

Microtubules are built of tubulin subunits assembled into hollow cylinders which consist of parallel protofilaments. Thus, motor molecules interacting with a microtubule could do so either with one or several tubulin subunits. This makes it difficult to determine the structural requirements for the interaction. One way to approach the problem is to alter the surface lattice. This can be done in several ways. Proto-filaments can be exposed on their inside (C-tubules or "sheets"), they can be made antiparallel (zinc sheets), or they can be rolled up (duplex tubules). We have exploited this polymorphism to study how the motor protein kinesin attached to a glass surface interacts and moves the various tubulin assemblies. Microtubules glide over the surface along straight paths and with uniform velocities. In the case of C-tubules, approximately 40% glide similarly to microtubules, but a major fraction do not glide at all. This indicates (a) that a full cylindrical closure is not necessary for movement, and (b) that the inside surface of microtubules does not support gliding. With zinc sheets, up to 70% of the polymers move, but the movement is discontinuous, has a reduced speed, and follows along a curved path. Since zinc sheets have protofilaments alternating in orientation and polarity, this result suggests that in principle a single protofilament can produce movement, even when its neighbors cannot. Duplex microtubules do not move because they are covered with protofilaments coiled inside out, thus preventing the interaction with kinesin. The data can be explained by assuming that the outside of one protofilament represents the minimal track for kinesin, but smooth gliding requires several parallel protofilaments. Finally, we followed the motion of kinesin-coated microbeads on sea-urchin sperm flagella, from the flagellar outer doublet microtubules to the singlet microtubule tips extending from the A-tubules. No change in behavior was detected during the transition. This indicates that even if these microtubules differ in surface lattice, this does not affect the motility.

Structural diversity and dynamics of microtubules and polymorphic tubulin assemblies

Tubulin, the main protein of microtubules (MTs), has the potency of forming a variety of other assembly products in vitro: rings, ring-crystals, C- and S-shaped ribbons, 10 nm fibres, hoops, sheets, heaped sheets, MT doublets, MT triplets, double-wall MTs, microtubules, curled ribbons, and paracrystals. The supramolecular subunits of all of them are the protofilaments which might be arranged either parallel to the axis (e.g., in MTs, ribbons) or curved (e.g., in hoops, microtubules). There is strong evidence that in the second case the protofilaments have an inside-out orientation compared to MTs. All assembly products mentioned are described structurally and their relevance to the in vivo situation is considered. Moreover, MTs and the other assemblies undergo permanent changes. These dynamics occurring in both individual assemblies and assembly populations are discussed from the structural point of view.

A basic protein from bovine brain that co-precipitates with tubulin in vitro

A 193 kDa protein consisting of 58 kDa subunits, which has pI values of 8.50 and 8.65, was purified from bovine brain cytosol. It formed heavy precipitates with tubulin, and the molar ratio of tubulin dimer to this protein in the precipitate was 3.2. In contrast to microtubules containing ordinary microtubule-associated proteins, these complexes remained stable against cold and 1 mM CaCl2.

Comparison of the microtubule formation induced by cytosynalin and MAPs

A comparison was made of cytosynalin- and MAPs-induced microtubule formation. Cytosynalin (a 35 kDa cytoskeleton-interacting and calmodulin-binding protein) facilitated more rapid and extensive microtubule formation than MAPs. Electron microscopic examination revealed that the cytosynalin-induced microtubules were 24 nm tubules surrounded by a periodic nap structure along their length. Consequently, the apparent diameter of microtubules was seen as 34 nm. Cytosynalin maximally bound to tubulin dimer at a molar ratio of 1:1. The effect of cytosynalin on the microtubule formation was found to be more potent than that of MAP2 or tau factor as determined by electron microscopy and cosedimentation assay.

Axonal microtubules are stained and cross-linked by highly cationic polyethyleneimine

Highly cationic polyethyleneimine was used as an electron microscopic tracer for anionic sites in the axoplasm of rat sciatic and optic nerve fibres. Microtubules showed a markedly increased electron density and aggregated to form large groups, mostly located in the immediate proximity of membranous axoplasmic organelles. Walls of adjacent microtubules were fused; there was also fusion of microtubules with the membranes of smooth axoplasmic reticulum and with axolemma. In contrast, neurofilaments had unaltered electron density and axoplasmic distribution. Staining and clustering of microtubules were interpreted as electrostatic binding of cationic polyethyleneimine to acidic tubulin. These findings may be relevant to the role of microtubules in fast axonal transport.

Identification of a distinct class of vinblastine binding sites on microtubules

Vinblastine, at concentrations above approximately 1 to 2 microM, causes depolymerization of steady-state bovine brain microtubules in vitro by a fraying of microtubule ends into protofilament-like spirals. Microtubule depolymerization is associated with the binding of vinblastine in approximately molar stoichiometry to tubulin in microtubules with apparent low affinity, as determined by binding experiments with radiolabeled vinblastine and by the ability of vinblastine to inhibit DEAE-dextran decoration of microtubule surfaces. Our data suggest that depolymerization occurs by a propagated mechanism, initially involving binding of vinblastine to a limited number of available sites on microtubule surfaces. This appears to cause loosening of protofilament associations which results in the exposure of new vinblastine-binding sites. Additional vinblastine binding in turn results in further loosening of protofilament associations. Such loosening, when it occurs at microtubule ends, results in protofilament-like splaying and end-wise depolymerization. Microtubule depolymerization appears mechanistically distinct from inhibition of microtubule polymerization by the drug, which is associated with the binding of vinblastine to small numbers of high-affinity binding sites on tubulin at one or both microtubule ends.

Assembly and three-dimensional image reconstruction of tubulin hoops

The three-dimensional structure of tubulin hoops has been determined by image reconstruction. The surface lattice of hoops is similar to that of microtubules, but in addition hoops possess a superstructure of protofilament triplets. The protofilaments differ mainly in their apparent volumes and lateral spacings. The volumes depend strongly on the orientation on the carbon support, while the spacings do not. The differences of appearance do not reflect changes of intrinsic subunit structure. They are explained by differential staining related to the orientation and packing of protofilament. Microtubule-associated proteins do not contribute to the average subunit structure. All apparent protofilament structures differ from that expected from X-ray patterns of microtubules in terms of subunit tilt and distribution of contrast. It is concluded that the negatively stained structure is a reliable representation of the arrangement of protein subunits, but not of their shape. Tubulin hoops occur in conditions of microtubule assembly near the critical concentration in a stabilizing buffer. Their formation depends on microtubule-associated proteins and on the initial presence of tubulin oligomers, which may associate into short protofilament triplets. If their elongation is rapid compared to lateral aggregation, they form closed hoops. The growth phase is followed by a redistribution phase, during which hoops disappear in favour of microtubules. This behaviour is explained by kinetic overshoot assembly. Each triplet resembles an incomplete microtubule wall so that the junction between two triplets may be compared to a junction between microtubule walls. Such junctions are formed by a closely spaced pair of protofilaments. They are analogous to junctions between microtubules and incomplete microtubule walls, and they have the same clockwise curvature when viewed at the growing end.

Structure of tubulin rings studied by X-ray scattering using synchrotron radiation

X-ray patterns have been obtained from solutions of microtubules and tubulin rings using synchrotron radiation. They are interpreted on the basis of the oriented fiber X-ray diagram of microtubules, and by comparing them with the calculated scattering traces of model structures. The data are consistent with models in which rings consist of coiled protofilaments. This is in agreement with some published ring models while it excludes others.

Microtubule turnover as a mechanism of mitosis and its possible evolution

Movement of chromosomes during mitosis appears to be coupled to the unidirectional turnover of spindle microtubules. This paper outlines a directional turnover model of mitosis which hypothesizes that: (1) Unidirectional turnover of tubulin subunits and microtubule-associated proteins occurs from an assembly site at one end to a disassembly site at the other end of mitotic apparatus microtubules. (2) The components of interpolar microtubules are continuously moving toward each pole due to their assembly in the opposite half spindle and disassembly in the near half spindle. (3) Mitotic chromosome movements are coupled to this directional turnover by selective formation of semi-stable lateral interactions or bridges between kinetochore microtubules and parallel interpolar microtubules. (4) The anaphase velocity of kinetochores relative to the poles is determined by the rate that kinetochore microtubules disassemble on their poleward ends minus the rate they assemble at their kinetochore ends. (5) Spindle elongation occurs when assembly of interpolar microtubules is more rapid than their disassembly or when interpolar microtubules slide in an anti-parallel arrangement. (6) The velocity of chromosome separation is the sum of spindle elongation and the poleward movements of sister chromosomes. Evidence for and against these hypotheses and some possible steps in the evolution of this type of mechanism are discussed.

Microtubules and coated vesicles in guard-cell protoplasts ofAllium cepa L

Protoplasts were prepared from the guard cells ofA. cepa. Epidermal peels taken from expanding green leaves and largely free of mesophyll were treated with Cellulysin, and protoplasts were harvested after 18 h of digestion. That the protoplasts were derived from guard cells was ascertained from their characteristic vacuolar autofluorescence and from observations showing that all other epidermal cells are killed in the peeling procedure. The protoplasts proved to be a good system with which to view the cell cortex and inner surface of the plasmalemma. The lysis of cells adhering to polylysine-treated, Formvar-coated grids, followed by negative staining in uranyl acetate, showed that many microtubules normally present in ordered arrays in situ remain closely applied to the inner surface of the plasmalemma in protoplasts. In addition, numerous vesiculate elements including coated vesicles and/or pits are present amongst the microtubules. Similar vesicles are evident in thin sections of fixed, embedded guard cells and protoplasts. The significance of these structures in the cell cortex is discussed.

Thyroid hormones and neurotubule assembly in vitro during brain development

A new model has been used to evaluate the effects of thyroid hormones on brain development. This model is based on the assumption that the major effect of thyroid hormones is in regulating the rate of neurite growth of the rat brain at early stages of postnatal development. Microtubules were chosen as markers of neurite growth. We tested, therefore, whether the rate of microtubule assembly in vitro is under thyroid hormone control. The following results were obtained: The rate of tubulin assembly into microtubules in vitro seems to be thyroid hormone dependent: (a) in 15-day-old hypothyroid rats the rates of tubulin assembly in vitro are low, comparable to those levels found in normal rats on day 3; (b) normal rates of assembly in vitro are restored upon addition of very small amounts of microtubule fragments which act as nucleating centers in the process of microtubule formation; (c) addition of microtubule-associated proteins to a hypothyroid preparation restores maximal assembly rates; similar results were obtained on adding one of the microtubule-associated proteins (purified tau protein); (d) physiological amounts of thyroid hormones completely restore normal assembly rates provided that they are administered very early after birth; (e) the ability of tubulin to assemble maximally does not seem to be permanently impaired, since normal assembly rates are spontaneously restored when hypothyroidism is maintained until an adult stage; (f) normal microtubule assembly is observed when hypothyroidism is produced at an adult stage. The model which may be constructed from these results implies that thyroid hormones are required briefly after birth to accelerate the rate of microtubule assembly thus allowing intensive neurite growth during the critical period of brain development.

In vitro reconstitution of calf brain microtubules: effects of macromolecules

A comparative study has been carried out of the ability to reconstitute into microtubules of tubulins prepared by the Weisenberg and the cycle procedures. It was found that further purification of cycle tubulin by phosphocellulose chromatography made its ability to polymerize identical with that of Weisenberg tubulin. By adding to either tubulin the isolated proteins which copurify with tubulin in the cycle prodecure, it is possible to reduce their critical concentrations of microtubule formation to a value identical with that of cycle tubulin. It was demonstrated quantitatively that the effect of these nontubulin proteins could be mimicked by a variety of polycationic molecules, the most effective one being poly(L-lysine). A possible mechanism is described by which growing microtubules could be stabilized by subsequent addition of the nontubulin proteins. The conclusion is drawn that, at present, it is not known whether the proteins which copurify with tubulin are specific components of the microtubule system, or simply artefactually coprecipitated impurities.

Tubulin assembly protein: immunochemical and immunofluorescent studies on its function and distribution in microtubules and cultured cells

Cytoplasmic microtubule assembly from tubulin monomers requires an accessory protein or proteins present is isolated microtubules. These proteins have been designated "tau" factors. One such factor, tubulin assembly protein (TAP), has been purified to homogeneity from calf brain microtubules. A precipitating, monospecific antibody against the protein has been prepared. The antibody has been used to investigate the mechanism of TAP action in microtubule assembly and the distribution of TAP in cellular microtubules. Immunochemical, immunofluorescent and electron microscopic studies indicate that TAP functions stoichiometrically by binding physically to tubulin to form a complex active in microtubule assembly. TAP is an elongation protein which is required throughout the growth of a microtubule and which is actually present along the entire microtubule. Immunofluorescence microscopy has been used to demonstrate that TAP is distributed throughout the cytoplasmic microtubule network of cultured human, hamster and rat cells-both normal and virally transformed. Immunofluorescence of cells in mitosis shows that TAP is present in the mitotic spindle. These results demonstrate the biological importance of tubulin assembly protein and suggest that it or immunologically related "tau" proteins represent ubiquitous cofactors in cytoplasmic microtubule assembly.

Modification of tubulin by tyrosylation in cells and extracts and its effect on assembly in vitro

A post-translational modification of tubulin with potential regulatory significance has been revealed by the discovery of an enzyme (tubulin-tyrosine ligase) in brain extracts which can add a tyrosine residue to the alpha chain, apparently through peptide bond linkage to a C-terminal glutamate. We have investigated whether this modification also occurs in vivo, and whether it alters the extent to which tubulin can assemble in vitro. Cytoplasmic tubulin purified from bovine brain by cycles of assembly was shown to be partially tyrosylated. Carboxypeptidase A digestion of isolated alpha chains liberated about 0.3 equivalent of tyrosine. Brief digestion of native tubulin increased the proportion of alpha chains which could be tyrosylated by ligase, from 25 to 45%. The tubulin assembled to the same extent before and after carboxypeptidase treatment. When tubulin was purified after introducing labeled tyrosine with ligase, the labeled species assembled in the same proportion as unlabeled. Thus tubulin can be incorporated into microbubules in vitro with or without C-terminal tyrosine. An apparent resolution of alpha chain into two components by hydroxylapatite chromatography was shown not to be due to the presence or absence of C-terminal tyrosine. Tubulin-tyrosine ligase was found in extracts of every rat tissue examined, but was not detected in sea urchin eggs before or after fertilization, in Tetrahymena cells or cilia, or in yeast. Cultured neuroblastoma cells fixed tyrosine into tubulin alpha chains under conditions where protein synthesis was inhibited; this in vivo fixation appeared to be into an insoluble moiety of tubulin. Incidental to these studies, a new assay utilizing an enamine substrate for carboxypeptidase was investigated.

In vitro reconstitution of calf brain microtubules: effects of solution variables

The effects of solution variables on the in vitro reconstitution of calf brain tubulin, purified by the method of Weisenberg et al. (Weisenberg, R. C., Borisy, G. G., and Taylor, E. W. (1968), Biochemistry 7, 4466-4479; Weisenberg, R. C., and Timasheff, S. N. (1970), Biochemistry 9, 4110-4116), as modified by Lee et al. (Lee, J. C., Frigon, R. P., and Timasheff, S. N. (1973), J. Biol. Chem. 248, 7253-7262), were investigated at pH 7.0. Reconstitution of microtubules was successful in a variety of buffer systems, the free energy of the propagation step of microtubule formation being little dependent on the buffer. Microtubule formation is promoted by magnesium ions and guanosine triphosphate, but inhibited by calcium ions. The dependence of the apparent association constant for microtubule formation on ligand concentration was analyzed by the linked function theory of Wyman (Wyman, J. (1964), Adv. Protein Chem. 19, 224-286), leading to the conclusion that the formation of a tubulin-tubulin contact involves the binding of one additional magnesium ion per tubulin dimer. Microtubule formation is also accompanied by the apparent binding of one additional proton and the release of water molecules, as suggested by the thermodynamic parameters determined. The reaction is entropy driven with an apparent heat capacity change, deltaCp, of -1500 +/- 500 cal/deg-mol. The enhancement of tubulin reassembly by glycerol is most likely due to nonspecific protein-solvent general thermodynamic interactions.

Polymorphism of tubulin assembly. In vitro formation of sheet, twisted ribbon and microtubule

Tubulin assembly in equilibrium has been investigated by electron microscopy at different pH values in the absence or presence of Ca2+ at 37 degrees C. In the presence of 1 mM EGTA, tubulin assembled into microtubules in the pH range 6.0 to 7.0 and into sheets in the pH range 5.8 to 6.2. The sheet was composed of 8 to 22 protofilaments and frequently curled in a lateral direction. Two-block-polymers of sheet and microtubule were present in the boundary of two pH ranges (6.0-6.2). In the presence of 1 mM CaCl2, rings were formed in the pH range 6.0 to 7.0: macrotubules of diameter 75 nm and microtubules were both formed in the pH range 6.2 to 6.4, and two types of twisted ribbons in the pH range 5.8 to 6.2. One type of twisted ribbons had a pitch of around 150 nm and amplitude of around 40 nm, the other had 250 nm and 90 nm, respectively. Microtubules formed in the presence of CaCl2 were not distinguishable from those formed in the absence of CaCl2 in electron-micrographs, although they had different properties. The formation of all of these polymers was inhibited by 0.2 M KCl and was temperature dependent; they disappeared at 0 degrees C and reappeared at 37 degrees C. Interconversion between these polymers was also investigated. Globular particles were formed in the absence of magnesium ions. Addition of 50 muM colchicine induced formation of filamentous polymers. On the basis of these experimental results, the structural relation among different forms of tubulin assembly was discussed.

Tubulin requires tau for growth onto microtubule initiating sites

Tubulin purified by phosphocellulose chromatography and free of accessory proteins will not form microtubules in the absence or presence of microtubule initiating sites (flagellar microtubules). The capacity for growth onto pre-existing "seeds" can be restored by the addition of small quantities of partially purified tau protein. Larger quantities restore the capacity for spontaneous assembly. These results suggest that tubulin requires tau for both initiation and growth of microtubules and that tau is incorporated into the microtubule throughout its length.

Polycation-induced assembly of purified tubulin

Several different polycations have been found that can substitute for the microtubule-associated proteins, or tau factor, in facilitating assembly of tubulin that has been purified by ion exchange chromatography. In low concentrations of the polycation diethylaminoethyl-dextran, 7 mg of tubulin is pelleted per 1 mg of polycation added. Under conditions favorable to microtubule assembly the entire pellet is seen by electron microscopy to consist of "double wall microtubules", which are essentially identical to normal microtubules in subunit structure and arrangement. When assembly is inhibited approximately the same amount of tubulin is pelleted, but it is in the form of clusters of curved sheets or filaments apparently related to tubulin rings. When conditions are changed to favor assembly, the tubulin within these clusters appears to reassemble to form the double wall microtubules.

Microtubule--synaptic vesicle associations in cultured rat spinal cord neurons

This paper describes new ultrastructural features of neural processes and of synapses in cultured CNS tissue treated with albumin before fixation using a modification of the technique recently introduced by Gray (1975). Nerve fibre bundles in explants of foetal spinal cord grown in vitro for 15-18 days were transected microsurgically. After transection the cultures were exposed to 20% albumin in distilled water and then fixed in unbuffered osmium tetroxide followed by unbuffered glutaraldehyde. In this material, but not in controls (injured but not exposed to albumin; exposed to albumin without injury) microtubules were found within many axonal varicosities, often situated close to presynaptic membrane specializations. These microtubules were closely associated with vesicles resembling synaptic vesicles, which were occasionally aligned in rows along the microtubules. Similar vesicle-microtubule associations were also found in non-terminal axons. Microtubules were also observed very close to some postsynaptic densities. The possiblity that the microtubule-vesicle associations are involved in vesicle movements (along axons and/or within axon terminals) is discussed. A more direct involvement of microtubules in terminals in the mechanism of transmitter release is also considered.

Effects of RNase and RNA on in vitro aster assembly

RNase alters the in vitro assembly of spindle asters in homogenates of meiotically dividing surf clam (Spisula solidissima) oocytes. Some effects of RNase, such as reduced astral fiber length, appear nonenzymatic and probably result from RNase binding to tubulin. However, RNase-induced changes in the microtubule organizing center are also observed. Since other polycations can mimic RNase effects, the existence of an RNA component of the spindle organizing center remains uncertain. Effects of RNase and other polycations on astral fiber length can be prevented and reversed by the RNase inhibitor, polyguanylic acid. Polyguanylic acid can also augment astral fiber length in the absence of added RNase or other polycations. Augmentation by polyguanylic acid is favored by high ionic strength, and can be duplicated by polyuridylic acid and, with less efficiency, by polyadenylic acid. Polycytidylic acid and unfractionated yeast RNA, however, are unable to augment aster assembly. Polyguanylic acid can also augment the length of astral fibers on complete spindles isolated under polymerizing conditions. These results demonstrate that specific polyribonucleotides can alter spindle assembly in vitro. The presence of an inhibitor of microtubule assembly in Spisula oocytes, which can be inactivated by specific RNAs, is suggested.