Amino acid sequence of alpha- and beta-tubulins from pig brain: heterogeneity and regional similarity to muscle proteins

Tau phosphorylation in hippocampus results in toxic gain-of-function

The MAP (microtubule-associated protein) tau binds to tubulin, the main component of MTs (microtubules), which results in the stabilization of MT polymers. Tau binds to the C-terminal of tubulin, like other MAPs (including motor proteins such as kinesin) and it therefore may compete with these proteins for the same binding site in the tubulin molecule. In pathological conditions, tau is the main component of aberrant protein aggregates found in neurodegenerative disorders known as tauopathies where tau is present in its hyperphosphorylated form. GSK3 (glycogen synthase kinase 3, also known as tau kinase I) has been described as one of the main kinases involved in tau modifications. We have analysed the role of phospho-tau as a neurotoxic agent. We have analysed a transgenic mouse model which overexpresses GSK3beta. In this transgenic mouse, a clear degeneration of the dentate gyrus, which increases with age, was found. In a double transgenic mouse, which overexpresses GSK3 and tau at the same time, dentate gyrus degeneration was dramatically increased. This result may suggest that phospho-tau may be toxic inside neurons of the dentate gyrus. Once neuronal degeneration takes place, intracellular tau is secreted to the extracellular space. The present review discusses the toxicity of this extracellular tau for surrounding neurons.

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.

OP18/stathmin binds near the C-terminus of tubulin and facilitates GTP binding

It is has been previously suggested that the protein Op18/stathmin may interact with tubulin via the alpha-tubulin subunit [Larsson, N., Marklund, U., Melander Gradin, H., Brattsand, G. & Gullberg, M. (1997) Mol. Cell. Biol. 17, 5530-5539]. In this study we have used limited proteolysis and cross-linking analysis to localize further the stathmin-binding site on alpha-tubulin. Our results indicate that such a binding site is in a region close to the C-terminus of the molecule comprising residues 307 to the subtilisin-cleavage site on the alpha-tubulin subunit. Based on a recent model of the structure of tubulin [Nogales, E., Wolf, S.G. & Dowing, D.H. (1998) Nature (London) 391, 199-203], we found that this region contained the same areas that may be involved in longitudinal contacts of alpha-tubulin subunits within the microtubule. We also observed that the binding of stathmin to tubulin can modulate the binding of GTP to tubulin, as a consequence of a conformational change in the beta-tubulin subunit that occurs upon interaction of stathmin with tubulin.

Structure of the alpha beta tubulin dimer by electron crystallography

The alphabeta tubulin heterodimer is the structural subunit of microtubules, which are cytoskeletal elements that are essential for intracellular transport and cell division in all eukaryotes. Each tubulin monomer binds a guanine nucleotide, which is nonexchangeable when it is bound in the alpha subunit, or N site, and exchangeable when bound in the beta subunit, or E site. The alpha- and beta-tubulins share 40% amino-acid sequence identity, both exist in several isotype forms, and both undergo a variety of posttranslational modifications. Limited sequence homology has been found with the proteins FtsZ and Misato, which are involved in cell division in bacteria and Drosophila, respectively. Here we present an atomic model of the alphabeta tubulin dimer fitted to a 3.7-A density map obtained by electron crystallography of zinc-induced tubulin sheets. The structures of alpha- and beta-tubulin are basically identical: each monomer is formed by a core of two beta-sheets surrounded by alpha-helices. The monomer structure is very compact, but can be divided into three functional domains: the amino-terminal domain containing the nucleotide-binding region, an intermediate domain containing the Taxol-binding site, and the carboxy-terminal domain, which probably constitutes the binding surface for motor proteins.

Thermal transitions in the structure of tubulin. Environments of aromatic aminoacids

The environment of aromatic aminoacids in the thermal transition of brain tubulin has been studied by several spectroscopic techniques (Fourth Derivative, Difference Absorption, Fluorescence and Circular Dichroism), in order to study its denaturation. An irreversible, temperature-induced, structural transition was found at around 48 degrees C. In order to establish the relative degree of hydrophobicity of tubulin aromatic residues, before and after the thermal transition, difference and fourth derivative absorption spectra at different temperatures were compared with spectra of tyrosine and tryptophan model compounds in different media. It was found that at high temperatures, tubulin acquires a partially denatured stable state, with a significant amount of residual structure still preserved. This state is characterized by a general increase of the exposure of tyrosine residues to the medium, while the environment of tryptophans becomes more hydrophobic.

The characterization of phospholipids associated with microtubules, purified tubulin and microtubule associated proteins in vitro

1. Significant levels of total phospholipid phosphate were detected in highly purified microtubule protein preparations. 2. While the phospholipid profiles of total microtubule proteins and microtubule-associated proteins showed both similarities and differences to that of a whole brain homogenate, purified tubulin was associated only with phospholipids that were not detectable in the latter. 3. Phosphatidyl ethanolamine, found exclusively in a fraction of microtubule associated proteins, stimulated microtubule assembly in vitro.

Tubulin phosphorylation by casein kinase II is similar to that found in vivo

Purified brain tubulin subjected to an exhaustive phosphatase treatment can be rephosphorylated by casein kinase II. This phosphorylation takes place mainly on a serine residue, which has been located at the carboxy-terminal domain of the beta-subunit. Interestingly, tubulin phosphorylated by casein kinase II retains its ability to polymerize in accordance with descriptions by other authors of in vivo phosphorylated tubulin. Moreover, the V8 phosphopeptide patterns of both tubulin phosphorylated in vitro by casein kinase II and tubulin phosphorylated in vivo in N2A cells are quite similar, and different from that of tubulin phosphorylated in vitro by Ca/calmodulin-dependent kinase II. On the other hand, we have found an endogenous casein kinase II-like activity in purified brain microtubule protein that uses GTP and ATP as phosphate donors, is inhibited by heparin, and phosphorylates phosphatase-treated tubulin. Thus it appears that a casein kinase II-like activity should be considered a candidate for the observed phosphorylation of beta-tubulin in vivo in brain or neuroblastoma cells.

A model of the nucleotide-binding site in tubulin

Tubulin uses GTP to regulate microtubule assembly and is thought to be a member of a class of GDP/GTP-binding proteins (G-proteins) as defined by Hughes [(1983) Febs Lett. 164, 1-8]. How tubulin is structurally related to G-proteins is not known. We use a synthesis of sequence comparisons between tubulin, other G-proteins, and ADP/ATP-binding proteins and topological arguments to identify potential regions involved in nucleotide binding. We propose that the nucleotide-binding domain in the beta-subunit of tubulin is an alpha/beta structure derived from amino acid residues approximately 60-300. Five peptide sequences are identified which we suggest exist as 'loops' that extend from beta-strands and connect alpha-helices in this structure. We argue that GDP binds to four of the five loops in an Mg2+-independent manner while GTP binds in an Mg2+-dependent manner to a different combination of four loops. We propose that this switch between loops upon GTP binding induces a conformational change essential for microtubule assembly.

Location of two cysteines in brain beta 1-tubulin that can be cross-linked after removal of exchangeable GTP

Two cysteines of the major neuronal beta-tubulin subunit (beta 1) can be specifically cross-linked with the bifunctional sulfhydryl reagent N',N'-ethylenebis(iodoacetamide) after removal of GTP. A cysteine in position 12 cross-links with one of the cysteines in position 201 or 211. Although the two cross-linked cysteines are separated by at least 189 residues in the primary structure of tubulin, they are maximally 9 A apart in the tertiary structure.

Distinct localization and cell cycle dependence of COOH terminally tyrosinolated alpha-tubulin in the microtubules of Trypanosoma brucei brucei

alpha-Tubulin can be posttranslationally modified in that its COOH-terminal amino acid residue, tyrosine, can be selectively removed and replaced again. This reaction cycle involves two enzymes, tubulin carboxypeptidase and tubulin tyrosine ligase. The functional significance of this unusual modification is unclear. The present study demonstrates that posttranslational tyrosinolation of alpha-tubulin does occur in the parasitic hemoflagellate Trypanosoma brucei brucei and that posttranslational tyrosinolation can be detected in both alpha-tubulin isoforms found in this organism. Trypanosomes contain a number of microtubular structures: the flagellar axoneme; the subpellicular layer of singlet microtubules which are closely associated with the cell membrane; the basal bodies; and a cytoplasmic pool of soluble tubulin. Tyrosinolated alpha-tubulin is present in all these populations. However, immunofluorescence studies demonstrate a distinct localization of tyrosinolated alpha-tubulin within individual microtubules and organelles. This localization is subject to a temporal modulation that correlates strongly with progress of a cell through the cell cycle. Our results indicate that the presence of tyrosinolated alpha-tubulin is a marker for newly formed microtubules.

Subpellicular and flagellar microtubules of Trypanosoma brucei brucei contain the same alpha-tubulin isoforms

The cytoskeleton of the parasitic hemoflagellate Trypanosoma brucei brucei essentially consists of two microtubule-based structures: a subpellicular layer of singlet microtubules, which are in close contact with the cell membrane, and the flagellar axoneme. In addition, the cells contain a small pool of soluble tubulin. Two-dimensional gel electrophoretic analysis of the tubulins present in these subcellular compartments revealed two distinct electrophoretic isoforms of alpha-tubulin, termed alpha 1 and alpha 3. alpha 1-Tubulin most likely represents the primary translation product, while alpha 3-tubulin is a posttranslationally acetylated derivative of alpha 1-tubulin. In the pool of soluble cytoplasmic tubulin, alpha 1 is the predominant species, while the very stable flagellar microtubules contain almost exclusively the alpha 3-tubulin isoform. The subpellicular microtubules contain both isoforms. Neither of the two alpha-tubulin isoforms is organelle specific, but the alpha 3 isoform is predominantly located in stable microtubules.

Binding of mammalian brain microtubule-associated proteins (MAPs) to insect ovarian microtubules

In this study we have applied microtubule-associated proteins (MAPs) from mammalian brain to both native and reassembled insect ovarian microtubules. Such microtubules, which are normally smooth walled, become decorated with projections similar to those observed when mammalian brain MAPs are added back to assembling or assembled mammalian brain microtubules. The mammalian MAPs were also detected as components of insect microtubules when analyzed by polyacrylamide gel electrophoresis. Our observations suggest that mammalian brain MAPs have common binding sites on microtubules from two widely different sources and indicate the degree of evolutionary conservation of such sites.

Characterization of a membrane-specific tubulin isoform by peptide mapping

A membrane-specific tubulin-like protein, found in preparations of synaptic plasma membranes and brain mitochondria, was analyzed by chemical and proteolytic peptide mapping to determine which part of the molecule was different from cytoplasmic tubulin. The membrane polypeptide was identical to alpha tubulin in the first two-thirds of the molecule containing the amino terminal, as found by peptide mapping. However, some differences were observed in the peptide maps of the carboxy terminal one third of the molecule which includes a domain that is important in the regulation of tubulin self-assembly.

The interaction between subunits in the tubulin dimer

Limited proteolysis and chemical cross-linking techniques have been used to study the interaction between alpha- and beta-tubulin subunits. Trypsin digestion of tubulin dimer resulted in the cleavage of the alpha-subunit into two fragments, whereas chymotrypsin cleaved the beta-subunit into two distinct fragments. All of these fragments have been mapped on the tubulin subunits by further proteolysis with formic acid. Cross-linking of trypsin- and chymotrypsin-cleaved subunits has been performed with two different cross-linker agents of different cross-linking distance. The addition of formaldehyde resulted in the cross-linking of the alpha-tubulin N-terminal fragment with beta-tubulin C-terminal domain. The same result was obtained when methyl 4-mercaptobutyrimidate was used.

Involvement of the carboxyl-terminal domain of tubulin in the regulation of its assembly

Limited proteolysis of phosphocellulose-purified tubulin with subtilisin resulted in cleavage of both alpha and beta tubulin subunits, with the formation of two major fragments (S alpha, and S beta, 48 kDa) and a small peptide (4 kDa) containing the carboxyl-terminal region of tubulin. Interestingly, tubulin cleaved under the present conditions showed an increased ability to assemble into large polymers in the absence of MAPs and under conditions that do not promote assembly of undigested tubulin--i.e., low magnesium concentrations and the absence of taxol and polyalcohols. The critical concentrations for the subtilisin-cleaved tubulin assembly was similar to that of MAPs-promoted tubulin assembly. Assembly product from subtilisin-cleaved tubulin consisted mainly of protofilament bundles, hooked polymer, and open tubules, structures showing equatorial and longitudinal spacings of 50 and 40 A, respectively. The existence of junctions between polymer walls indicates that the carboxyl-terminal removal facilitates polymer-polymer interactions. These results, together with previous studies on the involvement of the carboxyl-terminal domain of tubulin in its interaction with MAP-2, suggest a regulatory role for this domain in tubulin assembly. Thus, in general terms the tubulin molecule can be analyzed as a protein containing two essential domains with functional significance, one domain playing a major role in self-association and the other (the carboxyl-terminal moiety) playing a regulatory role in modulating the interactions responsible for self-association.

Developmental and comparative aspects of brine shrimp tubulin

Tubulin from embryos of the brine shrimp Artemia has been purified to apparent homogeneity by chromatography on phosphocellulose P11 and DEAE-cellulose, (NH4)2SO4 fractionation and assembly-disassembly of microtubules. Peptide mapping indicated that Artemia and bovine brain tubulin were very similar in spite of differences in the electrophoretic behaviour of tubulin from these two organisms. Isoelectric focusing and two-dimensional gel electrophoresis were used to resolve and identify several Artemia isotubulins . The isotubulin composition and the quantity of tubulin did not change during pre-emergence development of Artemia embryos. Formation of microtubules with tubulin purified from embryos at different stages of development did not require glycerol or microtubule-associated proteins and formation of structurally normal microtubules was actually hindered by glycerol and Mg2+. The characteristics of Artemia tubulin, in concert with the unusual life history of Artemia, suggest that this organism will be very useful for the study of tubulin gene expression and tubulin utilization during embryo development.