Coassembly of bovine and cod microtubule proteins: the ratio of the different tubulins within hybrid microtubules determines the ability to assemble at low temperatures, MAPs dependency and effects of Ca2+

Chaperone-mediated inhibition of tubulin self-assembly

Molecular chaperones are known to play an important role in facilitating the proper folding of many newly synthesized proteins. Here, we have shown that chaperone proteins exhibit another unique property to inhibit tubulin self-assembly efficiently. Chaperones tested include alpha-crystallin from bovine eye lenses, HSP16.3, HSP70 from Mycobacterium tuberculosis and alpha (s)-casein from milk. All of them inhibit polymerization in a dose-dependent manner independent of assembly inducers used. The critical concentration of MTP polymerization increases with increasing concentration of HSP16.3. Increase in chaperone concentration lowers the extent of polymerization and increases the lag time of self-assembly reaction. Although the addition of a chaperone at the early stage of elongation phase shows no effect on polymerization, the same concentration of chaperone inhibits polymerization completely when added before the initiation of polymerization. Bindings of HSP16.3 and alpha (s)-casein to tubulin have been confirmed using isothermal titration calorimetry. Affinity constants of tubulin are 5.3 xx 10(4) and 9.8 xx 10(5) M(-1) for HSP16.3 and alpha (s)-casein, respectively. Thermodynamic parameters indicate favourable entropy and enthalpy changes for both chaperones-tubulin interactions. Positive entropy change suggests that the interaction is hydrophobic in nature and desolvation occurring during formation of tubulin-chaperone complex. On the basis of thermodynamic data and observations made upon addition of chaperone at early elongation phase or before the initiation of polymerization, we hypothesize that chaperones bind tubulin at the protein-protein interaction site involved in the nucleation phase of self-assembly.

Meiosis-specific failure of cell cycle progression in fission yeast by mutation of a conserved beta-tubulin residue

The microtubule cytoskeleton is involved in regulation of cell morphology, differentiation, and cell cycle progression. Precisely controlled dynamic properties are required for these microtubule functions. To better understand how tubulin's dynamics are embedded in its primary sequence, we investigated in vivo the consequences of altering a single, highly conserved residue in beta-tubulin that lies at the interface between two structural domains. The residue differs between the cold-adapted Antarctic fish and temperate animals in a manner that suggests a role in microtubule stability. Fungi, like the Antarctic fish, have a phenylalanine in this position, whereas essentially all other animals have tyrosine. We mutated the corresponding residue in fission yeast to tyrosine. Temperature effects were subtle, but time-lapse microscopy of microtubule dynamics revealed reduced depolymerization rates and increased stability. Mitotic exit signaled by breakdown of the mitotic spindle was delayed. In meiosis, microtubules displayed prolonged contact to the cell cortex during horsetail movement, followed by completion of meiosis I but frequent asymmetric failure of meiosis II spindle formation. Our results indicate that depolymerization dynamics modulated through interdomain motion may be important for regulating a subset of plus-end microtubule complexes in Schizosaccharomyces pombe.

Cold adaptation of microtubule assembly and dynamics. Structural interpretation of primary sequence changes present in the alpha- and beta-tubulins of Antarctic fishes

The microtubules of Antarctic fishes, unlike those of homeotherms, assemble at very low temperatures (-1.8 degrees C). The adaptations that enhance assembly of these microtubules are intrinsic to the tubulin dimer and reduce its critical concentration for polymerization at 0 degrees C to approximately 0.9 mg/ml (Williams, R. C., Jr., Correia, J. J., and DeVries, A. L. (1985) Biochemistry 24, 2790-2798). Here we demonstrate that microtubules formed by pure brain tubulins of Antarctic fishes exhibit slow dynamics at both low (5 degrees C) and high (25 degrees C) temperatures; the rates of polymer growth and shortening and the frequencies of interconversion between these states are small relative to those observed for mammalian microtubules (37 degrees C). To investigate the contribution of tubulin primary sequence variation to the functional properties of the microtubules of Antarctic fishes, we have sequenced brain cDNAs that encode 9 alpha-tubulins and 4 beta-tubulins from the yellowbelly rockcod Notothenia coriiceps and 4 alpha-tubulins and 2 beta-tubulins from the ocellated icefish Chionodraco rastrospinosus. The tubulins of these fishes were found to contain small sets of unique or rare residue substitutions that mapped to the lateral, interprotofilament surfaces or to the interiors of the alpha- and beta-polypeptides; longitudinal interaction surfaces are not altered in the fish tubulins. Four changes (A278T and S287T in alpha; S280G and A285S in beta) were present in the S7-H9 interprotofilament "M" loops of some monomers and would be expected to increase the flexibility of these regions. A fifth lateral substitution specific to the alpha-chain (M302L or M302F) may increase the hydrophobicity of the interprotofilament interaction. Two hydrophobic substitutions (alpha:S187A in helix H5 and beta:Y202F in sheet S6) may act to stabilize the monomers in conformations favorable to polymerization. We propose that cold adaptation of microtubule assembly in Antarctic fishes has occurred in part by evolutionary restructuring of the lateral surfaces and the cores of the tubulin monomers.

Identification of betaIII- and betaIV-tubulin isotypes in cold-adapted microtubules from Atlantic cod (Gadus morhua): antibody mapping and cDNA sequencing

Isolated microtubule proteins from the Atlantic cod (Gadus morhua) assemble at temperatures between 8 and 30 degrees C. The cold-adaptation is an intrinsic property of the tubulin molecules, but the reason for it is unknown. To increase our knowledge of tubulin diversity and its role in cold-adaptation we have further characterized cod tubulins using alpha- and beta-tubulin site-directed antibodies and antibodies towards posttranslationally modified tubulin. In addition, one cod brain beta-tubulin isotype has been sequenced. In mammals there are five beta-tubulins (betaI, betaII, betaIII, betaIVa and betaIVb) expressed in brain. A cod betaIII-tubulin was identified by its electrophoretic mobility after reduction and carboxymethylation. The betaIII-like tubulin accounted for more than 30% of total brain beta-tubulins, the highest yield yet observed in any animal. This tubulin corresponds most probably with an additional band, designated beta(x), which was found between alpha- and beta-tubulins on SDS-polyacrylamide gels. It was found to be phosphorylated and neurospecific, and constituted about 30% of total cod beta-tubulin isoforms. The sequenced cod tubulin was identified as a betaIV-tubulin, and a betaIV-isotype was stained by a C-terminal specific antibody. The amount of staining indicates that this isotype, as in mammals, only accounts for a minor part of the total brain beta-tubulin. Based on the estimated amounts of betaIII- and betaIV-tubulins in cod brain, our results indicate that cod has at least one additional beta-tubulin isotype and that beta-tubulin diversity evolved early during fish evolution. The sequenced cod betaIV-tubulin had four unique amino acid substitutions when compared to beta-tubulin sequences from other animals, while one substitution was in common with Antarctic rockcod beta-tubulin. Residues 221, Thr to Ser, and 283, Ala to Ser, correspond in the bovine tubulin dimer structure to loops that most probably interact with other tubulin molecules within the microtubule, and might contribute to cold-adaptation of microtubules.

Microtubule aster formation by dynein-dependent organelle transport

The interplay between microtubules and the motor enzyme, cytoplasmic dynein, is essential for organisation of the cytoplasm, organelle transport, and cell division in eukaryotic cells. During mitosis, cytoplasmic dynein organises microtubules into two spindle pole asters, as well as the comparable multiple cytoplasmic asters induced by the microtubule-stabilising agent taxol. The mechanisms behind this cell cycle-regulated organisation are, however, not fully understood. We report here that the unidirectional dynein-dependent pigment organelle aggregation in taxol-treated melanophores from Atlantic cod, induces multiple microtubule asters. Usually, the pigment aggregates to a central pigment mass in the cell center, but pigment aggregation in taxol-treated cells induces formation of several peripheral pigment clusters that each localise to the center of a microtubule aster formation. When a cell with previously formed peripheral pigment clusters redisperse pigment, the asters disappear. Upon a subsequent reaggregation of the pigment, the aster formations reappear. The results indicate that the pigment aggregation process organises the microtubules into these formations. Immuno-electron microscopy of isolated pigment organelles indicates the presence of several dynein molecules on each pigment organelle, making it possible for each organelle to interact with several microtubules and thereby focusing microtubule minus ends. The possibility of unidirectional dynein-dependent organelle movement for organising microtubules into asters during cell division, and similarities in signal transduction between mitosis and pigment movement, are discussed.

Expression of cold-adapted beta-tubulins confer cold-tolerance to human cellular microtubules

Isolated microtubule proteins from the cold-adapted fish, Atlantic cod (Gadus morhua), assemble at temperatures between 8 and 30 degrees C, while avian and mammalian microtubules normally do not assemble at temperatures below 20 degrees C. Tubulin, the main component in microtubules, is expressed as many isotypes. Microtubules with different isotype composition have been shown to have different dynamic properties in vitro. Our hypothesis was that cold-tolerance of microtubules is caused by tubulin isotypes that differ in the primary sequence compared to mammalian tubulins. Here we show that transfection of human HepG2 cells with cod beta-tubulin induced cold-adaptation of the endogenous microtubules. Incorporation of one single tubulin isotype can induce cold-tolerance to cold-intolerant microtubules. Three cod beta-tubulin isotypes were tested and two of these (beta1 and beta2) transferred cold-tolerance to HepG2 microtubules, thus not all cod beta-tubulins were able to confer cold-stability.

Polyglutamylation of atlantic cod tubulin: immunochemical localization and possible role in pigment granule transport

In higher organisms, there is a large variety of tubulin isoforms, due to multiple tubulin genes and extensive post-translational modification. The properties of microtubules may be modulated by their tubulin isoform composition. Polyglutamylation is a post-translational modification that is thought to influence binding of both structural microtubule associated proteins (MAPs) and mechano-chemical motors to tubulin. The present study investigates the role of tubulin polyglutamylation in a vesicle transporting system, cod (Gadus morhua) melanophores. We did this by microinjecting an antibody against polyglutamylated tubulin into these cells. To put our results into perspective, and to be able to judge their universal application, we characterized cod tubulin polyglutamylation by Western blotting technique, and compared it to what is known from mammals. We found high levels of polyglutamylation in tissues and cell types whose functions are highly dependent on interactions between microtubules and motor proteins. Microinjection of the anti-polyglutamylation antibody GT335 into cultured melanophores interfered with pigment granule dispersion, while dynein-dependent aggregation was unaffected. Additional experiments showed that GT335-injected cells were able to aggregate pigment even when actin filaments were depolymerized, indicating that the maintained ability of pigment aggregation in these cells was indeed microtubule-based and did not depend upon actin filaments. The results indicate that dynein and the kinesin-like dispersing motor protein in cod melanophores bind to tubulin on slightly different sites, and perhaps depend differentially on polyglutamylation for their interaction with microtubules. The binding site of the dispersing motor may bind directly to the polyglutamate chain, or more closely than dynein.

Cloning and sequencing of an alpha-tubulin cDNA from Artemia franciscana: evidence for translational regulation of alpha-tubulin synthesis

The brine shrimp, Artemia franciscana, exhibits a limited number of tubulin isotypes which change little during early postgastrula growth. In order to better understand the synthesis of alpha-tubulins during Artemia development, a cDNA termed alphaAT1 was cloned and sequenced. Alignment analyses revealed that the polypeptide encoded by alphaAT1 is similar to alpha-tubulins from other species. Hybridization of alphaAT1 to restriction-digested DNA on Southern blots produced a simple banding pattern, indicating that Artemia have a small number of alpha-tubulin genes. Probing of Northern blots demonstrated an abundant supply of alpha-tubulin mRNA in dormant cysts, emerging nauplii and instar I larvae. However, it was not until instar I larvae were produced that the amount of polysomal alpha-tubulin mRNA increased, suggesting that synthesis of the tubulin corresponding to alphaAT1 is translationally controlled. This work provides one of the few examples where tubulin synthesis is thought to be translationally regulated. Moreover, when considered in the light of previous analyses, the findings imply that cell differentiation in postgastrula Artemia and the diversification of microtubule function certain to accompany this process occur with little or no change in alpha-tubulin composition.