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

Calcium-axonemal microtubuli interactions underlie mechanism(s) of primary cilia morphological changes

We have used cell culture of astrocytes aligned within microchannels to investigate calcium effects on primary cilia morphology. In the absence of calcium and in the presence of flow of media (10 μL.s^-1) the majority (90%) of primary cilia showed reversible bending with an average curvature of 2.1 ± 0.9 × 10^-4 nm^-1. When 1.0 mM calcium was present, 90% of cilia underwent bending. Forty percent of these cilia demonstrated strong irreversible bending, resulting in a final average curvature of 3.9 ± 1 × 10^-4 nm^-1, while 50% of cilia underwent bending similar to that observed during calcium-free flow. The average length of cilia was shifted toward shorter values (3.67 ± 0.34 μm) when exposed to excess calcium (1.0 mM), compared to media devoid of calcium (3.96 ± 0.26 μm). The number of primary cilia that became curved after calcium application was reduced when the cell culture was pre-incubated with 15 μM of the microtubule stabilizer, taxol, for 60 min prior to calcium application. Calcium caused single microtubules to curve at a concentration ≈1.0 mM in vitro, but at higher concentration (≈1.5 mM) multiple microtubule curving occurred. Additionally, calcium causes microtubule-associated protein-2 conformational changes and its dislocation from the microtubule wall at the location of microtubule curvature. A very small amount of calcium, that is 1.45 × 10¹¹ times lower than the maximal capacity of TRPPs calcium channels, may cause gross morphological changes (curving) of primary cilia, while global cytosol calcium levels are expected to remain unchanged. These findings reflect the non-linear manner in which primary cilia may respond to calcium signaling, which in turn may influence the course of development of ciliopathies and cancer.

Regulation of microtubule motors by tubulin isotypes and post-translational modifications

The ‘tubulin-code’ hypothesis proposes that different tubulin genes or posttranslational modifications (PTMs), which mainly confer variation in the carboxy-terminal tail (CTT), result in unique interactions with microtubule-associated proteins for specific cellular functions. However, the inability to isolate distinct and homogenous tubulin species has hindered biochemical testing of this hypothesis. Here, we have engineered 25 α/β tubulin heterodimers with distinct CTTs and PTMs and tested their interactions with four different molecular motors using single molecule assays. Our results show that tubulin isotypes and PTMs can govern motor velocity, processivity and microtubule depolymerization rates, with substantial changes conferred by even single amino acid variation. Revealing the importance and specificity of PTMs, we show that kinesin-1 motility on neuronal β-tubulin (TUBB3) is increased by polyglutamylation and that robust kinesin-2 motility requires detyrosination of α-tubulin. Our results also show that different molecular motors recognize distinctive tubulin “signatures”, which supports the premise of tubulin-code hypothesis.

The C terminus of tubulin, a versatile partner for cationic molecules: binding of Tau, polyamines, and calcium

The C-terminal region of tubulin is involved in multiple aspects of the regulation of microtubule assembly. To elucidate the molecular mechanisms of this regulation, we study here, using different approaches, the interaction of Tau, spermine, and calcium, three representative partners of the tubulin C-terminal region, with a peptide composed of the last 42 residues of α1a-tubulin. The results show that their binding involves overlapping amino acid stretches in the C-terminal tubulin region: amino acid residues 421-441 for Tau, 430-432 and 444-451 for spermine, and 421-443 for calcium. Isothermal titration calorimetry, NMR, and cosedimentation experiments show that Tau and spermine have similar micromolar binding affinities, whereas their binding stoichiometry differs (C-terminal tubulin peptide/spermine stoichiometry 1:2, and C-terminal tubulin peptide/Tau stoichiometry 8:1). Interestingly, calcium, known as a negative regulator of microtubule assembly, can compete with the binding of Tau and spermine with the C-terminal domain of tubulin and with the positive effect of these two partners on microtubule assembly in vitro. This observation opens up the possibility that calcium may participate in the regulation of microtubule assembly in vivo through direct (still unknown) or indirect mechanism (displacement of microtubule partners). The functional importance of this part of tubulin was also underlined by the observation that an α-tubulin mutant deleted from the last 23 amino acid residues does not incorporate properly into the microtubule network of HeLa cells. Together, these results provide a structural basis for a better understanding of the complex interactions and putative competition of tubulin cationic partners with the C-terminal region of tubulin.

Nondenaturing electrophoresis as a tool to investigate tubulin complexes

A protein molecule may exist as a monomer, homo-oligomer, or hetero-oligomer in a multiprotein complex. One-dimensional (1-D) native electrophoresis has long been used to characterize tubulins and their complexes. In this chapter, we describe the simplest way to identify the state of aggregation of commercial or homemade tubulins for further studies based on 1-D electrophoresis under nondenaturing conditions. We present a series of detailed protocols that can be used to analyze the maturation of alpha- and beta-tubulins and to identify the complexes formed during the folding and dimerization pathway as well as their stability.

T cell recognition in chronic beryllium disease

Chronic beryllium disease (CBD) is a granulomatous lung disorder caused by beryllium exposure in the workplace and is characterized by the accumulation of beryllium-specific CD4(+) T cells. Depending on genetic susceptibility and the nature of the exposure, CBD occurs in up to 20% of exposed workers. Genetic susceptibility has been associated with particular HLA-DP alleles, especially those possessing a negatively charged glutamic acid residue at the 69th position of the beta-chain. The mechanism for this association lies in the ability of these HLA-DP molecules to bind and present beryllium to pathogenic CD4(+) T cells. Large numbers of effector memory, beryllium-specific CD4(+) T cells are recruited to the lung of these subjects and secrete Th1-type cytokines upon beryllium recognition. The presence of circulating beryllium-specific CD4(+) T cells directly correlates with the severity of lymphocytic alveolitis. With the presence of a known antigenic stimulus, CBD serves as an important model of immune-mediated, organ destruction. Thus, our findings in CBD have important implications for studies in autoimmune diseases, in particular those with an unknown inciting antigen and an inaccessible target organ.

Identification of Ncd tail domain-binding sites on the tubulin dimer

The Drosophila non-claret disjunctional (Ncd) kinesin-like protein is required for spindle assembly in oocytes and spindle maintenance in early embryos. Through the action of ATP-dependent microtubule (MT)-binding sites in the head and ATP-independent MT-binding sites in the tail, Ncd may bundle and, perhaps, slide MTs relative to each other. Our previous work on the MT-binding site of the Ncd tail domain demonstrated that this site, like the MT-binding sites of tau, contains basic residues flanked by proline residues and can promote MT assembly and stability. Here, we characterize the interactions of a monomeric Ncd tail protein with subtilisin-digested MTs in order to identify sites on the tubulin dimer that interact with the Ncd tail. The results provide evidence for four such binding sites per tubulin dimer and support the hypothesis that each binding site consists of a cluster of acidic residues in the C-terminal regions of alpha- and beta-tubulin.

HIV-1 rev depolymerizes microtubules to form stable bilayered rings

We describe a novel interaction between HIV-1 Rev and microtubules (MTs) that results in the formation of bilayered rings that are 44-49 nm in external diameter, 3.4-4.2 MD (megadaltons) in mass, and have 28-, 30-, or 32-fold symmetry. Ring formation is not sensitive to taxol, colchicine, or microtubule-associated proteins, but requires Mg(2+) and is inhibited by maytansine. The interaction involves the NH(2)-terminal domain of Rev and the face of tubulin exposed on the exterior of the MTs. The NH(2)-terminal half of Rev has unexpected sequence similarity to the tubulin-binding portion of the catalytic/motor domains of the microtubule-destabilizing Kin I kinesins. We propose a model wherein binding of Rev dimers to MTs at their ends causes segments of two neighboring protofilaments to peel off and close into rings, circumferentially containing 14, 15, or 16 tubulin heterodimers, with Rev bound on the inside. Rev has a strong inhibitory effect on aster formation in Xenopus egg extracts, demonstrating that it can interact with tubulin in the presence of normal levels of cellular constituents. These results suggest that Rev may interact with MTs to induce their destabilization, a proposition consistent with the previously described disruption of MTs after HIV-1 infection.

Helicity of alpha(404-451) and beta(394-445) tubulin C-terminal recombinant peptides

We have investigated the solution conformation of the functionally relevant C-terminal extremes of alpha- and beta-tubulin, employing the model recombinant peptides RL52alpha3 and RL33beta6, which correspond to the amino acid sequences 404-451(end) and 394-445(end) of the main vertebrate isotypes of alpha- and beta-tubulin, respectively, and synthetic peptides with the alpha-tubulin(430-443) and beta-tubulin(412-431) internal sequences. Alpha(404-451) and beta(394-445) are monomeric in neutral aqueous solution (as indicated by sedimentation equilibrium), and have circular dichroism (CD) spectra characteristic of nearly disordered conformation, consistent with low scores in peptide helicity prediction. Limited proteolysis of beta(394-445) with subtilisin, instead of giving extensive degradation, resulted in main cleavages at positions Thr409-Glu410 and Tyr422-Gln423-Gln424, defining the proteolysis resistant segment 410-422, which corresponds to the central part of the predicted beta-tubulin C-terminal helix. Both recombinant peptides inhibited microtubule assembly, probably due to sequestration of the microtubule stabilizing associated proteins. Trifluoroethanol (TFE)-induced markedly helical CD spectra in alpha(404-451) and beta(394-445). A substantial part of the helicity of beta(394-445) was found to be in the CD spectrum of the shorter peptide beta(412-431) with TFE. Two-dimensional 1H-NMR parameters (nonsequential nuclear Overhauser effects (NOE) and conformational C alphaH shifts) in 30% TFE permitted to conclude that about 25% of alpha(404-451) and 40% of beta(394-451) form well-defined helices encompassing residues 418-432 and 408-431, respectively, flanked by disordered N- and C-segments. The side chains of beta(394-451) residues Leu418, Val419, Ser420, Tyr422, Tyr425, and Gln426 are well defined in structure calculations from the NOE distance constraints. The apolar faces of the helix in both alpha and beta chains share a characteristic sequence of conserved residues Ala,Met(+4),Leu(+7),Tyr(+11). The helical segment of alpha(404-451) is the same as that described in the electron crystallographic model structure of alphabeta-tubulin, while in beta(394-451) it extends for nine residues more, supporting the possibility of a functional coil --> helix transition at the C-terminus of beta-tubulin. These peptides may be employed to construct model complexes with microtubule associated protein binding sites.

Interaction of phomopsin A with normal and subtilisin-treated bovine brain tubulin

Tubulin, the major component of microtubules, has a tendency to lose its ability to assemble or to bind to ligands in a time-dependent process known as "decay." The decay process also causes tubulin to expose sulfhydryl groups and hydrophobic areas. The antimitotic drug phomopsin A strongly protects the tubulin molecule from decay. Here we have studied the interaction of phomopsin A with alpha beta tubulin and tubulin which has been treated with subtilisin to remove selectively the C-termini of the alpha and beta chains (alpha(s) beta(s)). The binding of phomopsin A to alpha beta tubulin decreases the sulfhydryl titer by approximately 1.0 mol/mol. Selective removal of the peptides from the C-terminal ends does not affect phomopsin A's interaction with tubulin. Moreover, the alpha(s) beta(s) tubulin-phomopsin A complex appears to be more stable than the alpha bet tubulin-phomopsin A complex as determined by the time-dependent increase in exposure of sulfhydryl groups and hydrophobic areas on tubulin. In fact, phomopsin A inhibits the decay process of alpha(s) beta(s) tubulin completely. This observation raises the possibility of determining the conformation of this configuration of tubulin.

Divalent cation and ionic strength effects on Vinca alkaloid-induced tubulin self-association

We present here a systematic study of ionic strength and divalent cation effects on Vinca alkaloid-induced tubulin spiral formation. We used sedimentation velocity experiments and quantitative fitting of weight-average sedimentation coefficients versus free drug concentrations to obtain thermodynamic parameters under various solution conditions. The addition of 50-150 mM NaCl to our standard buffer (10 mM piperazine-N,N'-bis(2-ethanesulfonic acid), 1 mM Mg, 50 microM GDP or GTP, pH 6.9) enhances overall vinblastine- or vincristine-induced tubulin self-association. As demonstrated in previous studies, GDP enhances overall self-association more than GTP, although in the presence of salt, GDP enhancement is reduced. For example, in 150 mM NaCl, GDP enhancement is 0.24 kcal/mol for vinblastine and 0.36 kcal/mol for vincristine versus an average enhancement of 0.87 (+/- 0.34) kcal/mol for the same drugs in the absence of salt. Wyman linkage analysis of experiments with vinblastine or vincristine over a range of NaCl concentrations showed a twofold increase in the change in NaCl bound to drug-induced spirals in the presence of GTP compared to GDP. These data indicate that GDP enhancement of Vinca alkaloid-induced tubulin self-association is due in part to electrostatic inhibition in the GTP state. In the absence of NaCl, we found that vinblastine and 1 mM Mn2+ or Ca2+ causes immediate condensation of tubulin. The predominant aggregates observed by electron microscopy are large sheets. This effect was not found with 1 mM Mg2+. At 100 microM cation concentrations (Mn2+, Mg2+, or Ca2+), GDP enhances vinblastine-induced spiral formation by 0.55 (+/- 0.26) kcal/mol. This effect is found only in K2, the association of liganded heterodimers at the ends of growing spirals. There is no GDP enhancement of K1, the binding of drug to heterodimer, although K1 is dependent upon the divalent cation concentration. NaCl diminishes tubulin condensation, probably by inhibiting lateral association, and allows an investigation of higher divalent cation concentrations. In the presence of 150 mM NaCl plus 1 mM divalent cations (Mn2+, Mg2+, or Ca2+) GDP enhances vinblastine-induced spiral formation by 0.35 (+/- 0.21) kcal/mol. Relaxation times determined by stopped-flow light scattering experiments in the presence of 150 mM NaCl and vincristine are severalfold longer than those in the presence of vinblastine, consistent with a mechanism involving the redistribution of longer polymers. Unlike previous results in the absence of NaCl, relaxation times in the presence of NaCl are only weekly protein concentration dependent, suggesting the absence of annealing or an additional rate-limiting step in the mechanism.

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.

Beta-tubulin folding is modulated by the isotype-specific carboxy-terminal domain

To investigate the contribution of the carboxy-terminal domain in the process of tubulin folding and dimer formation, we constructed a beta 1-beta 3 tubulin chimaera and two truncated carboxy-terminal beta 3-tubulins. The capacity of these altered polypeptides to incorporate into dimers and into microtubules was tested by non-denaturing electrophoresis and co-assembly experiments. The chimaera and the truncated protein with a deletion encompassing the last 12 amino acid residues (beta 3 delta C12) were incorporated into dimers and microtubules, though the level of incorporation was diminished compared to wild-type beta 3-tubulin. However, the level of incorporation of beta 3 delta C12 into subtilisin-digested dimers was similar to the incorporation of wild-type beta 3-tubulin. Since subtilisin deletes the carboxy-terminal region, these results suggest a regulatory role of the carboxy-terminal region in the folding process itself and not in the formation of the dimer.

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.

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.