Class II tubulin, the major brain beta tubulin isotype is polyglutamylated on glutamic acid residue 435

High-resolution separation of tubulin monomers on polyacrylamide minigels

High-resolution separation of alpha- and beta-tubulin by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) on minigels can be performed rapidly using simple modifications of the standard Laemmli procedure. Separation of the subunits can be observed even in high-protein loads (up to 40microg of protein).

Mass spectrometry analysis of C-terminal posttranslational modifications of tubulins

In mammalian brain and ciliary axonemes from ciliates, alpha- and beta-tubulins exhibit an extraordinary heterogeneity due to a combination of multigene family expression and numerous posttranslational modifications (PTMs). The combination of several PTMs located in the C-terminal tail of tubulins plays a major role in this important polymorphism of tubulin: polyglutamylation, polyglycylation, detyrosination, tyrosination, removal of the penultimate glutamate residue, and phosphorylation. In order to document the relationship and functions of these PTMs, we have developed a tubulin C-terminal Peptide Mass Fingerprinting (PMF) method. Using simplified microtubule proteins and tubulin C-terminal peptides purifications, direct matrix-assisted laser desorption ionization (MALDI) mass spectrometry (MS) analysis can generate a complete picture of all tubulin isotype-specific C-terminal peptides together with their respective PTMs. This chapter will illustrate the capability of this approach to compare tubulin isoform compositions and document the changes in PTMs between samples with different tubulin assembly properties or consecutively to inactivation of modification sites or modification enzymes. Complementary MS-based approaches useful to document the structure of the highly heterogeneous posttranslational polymodifications will also be presented.

Post-translational modifications to Toxoplasma gondii alpha- and beta-tubulins include novel C-terminal methylation

Toxoplasma gondii is an apicomplexan of both medical and veterinary importance which is classified as an NIH Category B priority pathogen. It is best known for its ability to cause congenital infection in immune competent hosts and encephalitis in immune compromised hosts. The highly stable and specialized microtubule-based cytoskeleton participates in the invasion process. The genome encodes three isoforms of both alpha- and beta-tubulin and we show that the tubulin is extensively altered by specific post-translational modifications (PTMs) in this paper. T. gondii tubulin PTMs were analyzed by mass spectrometry and immunolabeling using specific antibodies. The PTMs identified on alpha-tubulin included acetylation of Lys40, removal of the last C-terminal amino acid residue Tyr453 (detyrosinated tubulin) and truncation of the last five amino acid residues. Polyglutamylation was detected on both alpha- and beta-tubulins. An antibody directed against mammalian alpha-tubulin lacking the last two C-terminal residues (Delta2-tubulin) labeled the apical region of this parasite. Detyrosinated tubulin was diffusely present in subpellicular microtubules and displayed an apparent accumulation at the basal end. Methylation, a PTM not previously described on tubulin, was also detected. Methylated tubulins were not detected in the host cells, human foreskin fibroblasts, suggesting that this may be a modification specific to the Apicomplexa.

Characterization of anti-beta-tubulin antibodies

Tubulin antibodies are among the most extensively used immunological reagents in basic and applied cell and molecular biology. In this chapter, we provide a brief overview of the practices and reagents developed in our laboratory during the past 25 years for characterizing anti-beta-tubulin antibodies.

Roles of beta-tubulin residues Ala428 and Thr429 in microtubule formation in vivo

The C termini of beta-tubulin isotypes are regions of high sequence variability that bind to microtubule-associated proteins and motors and undergo various post-translational modifications such as polyglutamylation and polyglycylation. Crystallographic analyses have been unsuccessful in resolving tubulin C termini. Here, we used a stepwise approach to study the role of this region in microtubule assembly. We generated a series of truncation mutants of human betaI and betaIII tubulin. Transient transfection of HeLa cells with the mutants shows that mutants with deletions of up to 22 residues from betaIII and 16 from betaI can assemble normally. Interestingly, removal of the next residue (Ala(428)) results in a complete loss of microtubule formation without affecting dimer formation. C-terminal tail switching of human betaI and betaIII tubulin suggests that C-terminal tails are functionally equivalent. In short, residues outside of 1-429 of human beta-tubulins make no contribution to microtubule assembly. Ala(428), in the C-terminal sequence motif N-QQYQDA(428), lies at the end of helix H12 of beta-tubulin. We hypothesize that this residue is important for maintaining helix H12 structure. Deletion of Ala(428) may lead to unwinding of helix H12, resulting in tubulin dimers incapable of assembly. Thr(429) plays a more complex role. In the betaI isotype of tubulin, Thr(429) is not at all necessary for assembly; however, in the betaIII isotype, its presence strongly favors assembly. This result is consistent with a likely more complex function of betaIII as well as with the observation that evolutionary conservation is total for Ala(428) and frequent for Thr(429).

Axoneme-dependent tubulin modifications in singlet microtubules of the Drosophila sperm tail

Drosophila melanogaster sperm tubulins are posttranslationally glutamylated and glycylated. We show here that axonemes are the substrate for these tubulin C-terminal modifications. Axoneme architecture is required, but full length, motile axonemes are not necessary. Tubulin glutamylation occurs during or shortly after assembly into the axoneme; only glutamylated tubulins are glycylated. Tubulins in other testis microtubules are not modified. Only a small subset of total Drosophila sperm axoneme tubulins have these modifications. Biochemical fractionation of Drosophila sperm showed that central pair and accessory microtubules have the majority of poly-modified tubulins, whereas doublet microtubules have only small amounts of mono- and oligo-modified tubulins. Glutamylation patterns for different beta-tubulins experimentally assembled into axonemes were consistent with utilization of modification sites corresponding to those identified in other organisms, but surrounding sequence context was also important. We compared tubulin modifications in the 9 + 9 + 2 insect sperm tail axonemes of Drosophila with the canonical 9 + 2 axonemes of sperm of the sea urchin Lytichinus pictus and the 9 + 0 motile sperm axonemes of the eel Anguilla japonica. In contrast to Drosophila sperm, L. pictus sperm have equivalent levels of modified tubulins in both doublet and central pair microtubule fractions, whereas the doublets of A. japonica sperm exhibit little glutamylation but extensive glycylation. Tubulin C-terminal modifications are a prevalent feature of motile axonemes, but there is no conserved pattern for placement or amount of these

Polyglutamylation: a fine-regulator of protein function? 'Protein Modifications: beyond the usual suspects' review series

Polyglutamylation is a post-translational modification in which glutamate side chains of variable lengths are formed on the modified protein. It is evolutionarily conserved from protists to mammals and its most prominent substrate is tubulin, the microtubule (MT) building block. Various polyglutamylation states of MTs can be distinguished within a single cell and they are also characteristic of specific cell types or organelles. Polyglutamylation has been proposed to be involved in the functional adaptation of MTs, as it occurs within the carboxy-terminal tubulin tails that participate directly in the binding of many structural and motor MT-associated proteins. The discovery of a new family of enzymes that catalyse this modification has brought new insight into the mechanism of polyglutamylation and now allows for direct functional studies of the role of tubulin polyglutamylation. Moreover, the recent identification of new substrates of polyglutamylation indicates that this post-translational modification could be a potential regulator of diverse cellular processes.

Tubulin polyglutamylase enzymes are members of the TTL domain protein family

Polyglutamylation of tubulin has been implicated in several functions of microtubules, but the identification of the responsible enzyme(s) has been challenging. We found that the neuronal tubulin polyglutamylase is a protein complex containing a tubulin tyrosine ligase-like (TTLL) protein, TTLL1. TTLL1 is a member of a large family of proteins with a TTL homology domain, whose members could catalyze ligations of diverse amino acids to tubulins or other substrates. In the model protist Tetrahymena thermophila, two conserved types of polyglutamylases were characterized that differ in substrate preference and subcellular localization.

The proximal region of the β-tubulin C-terminal tail is sufficient for axoneme assembly

We have used Drosophila testis-specific beta2-tubulin to determine sequence requirements for different microtubules. The beta2-tubulin C-terminal tail has unique sperm-specific functions [Dev Biol 158:267-286 (2003)] and is also important for forming stable heterodimers with alpha-tubulin, a general function common to all microtubules [Mol Biol Cell 12(7):2185-2194 (2001)]. beta-tubulins utilized in motile 9 + 2 axonemes contain a C-terminal sequence "axoneme motif" [Science 275 (1997) 70-73]. C-terminal truncated beta2-tubulin cannot form the sperm tail axoneme. Here we show that a partially truncated beta2-tubulin (beta2Delta7) containing only the proximal portion of the C-terminal tail, including the axoneme motif, can support production of functional motile sperm. We conclude that these proximal eight amino acids specify the binding site for protein(s) essential to support assembly of the motile axoneme. Males that express beta2Delta7, although they are fertile, produce fewer sperm than wild type males. Beta2Delta7 causes a slightly increased error rate in spermatogenesis attributable to loss of stabilizing properties intrinsic to the full-length C-terminal tail. Therefore, beta2Delta7 males would be at a selective disadvantage and it is likely that the full-length C-terminus would be essential in the wild and in evolution.

The Centrosome in Normal and Transformed Cells

The centrosome is a unique organelle that functions as the microtubule organizing center in most animal cells. During cell division, the centrosomes form the poles of the bipolar mitotic spindle. In addition, the centrosomes are also needed for cytokinesis. Each mammalian somatic cell typically contains one centrosome, which is duplicated in coordination with DNA replication. Just like the chromosomes, the centrosome is precisely reproduced once and only once during each cell cycle. However, it remains a mystery how this protein-based structure undergoes accurate duplication in a semiconservative manner. Intriguingly, amplification of the centrosome has been found in numerous forms of cancers. Cells with multiple centrosomes tend to form multipolar spindles, which result in abnormal chromosome segregation during mitosis. It has therefore been postulated that centrosome aberration may compromise the fidelity of cell division and cause chromosome instability. Here we review the current understanding of how the centrosome is assembled and duplicated. We also discuss the possible mechanisms by which centrosome abnormality contributes to the development of malignant phenotype.

Interaction of STOP with neuronal tubulin is independent of polyglutamylation

In eukaryotes, the coordinated progress of the various cellular tasks along with the assembly of adapted cytoskeletal networks requires a tight regulation of the interactions between microtubules and their associated proteins. Polyglutamylation is the major post-translational modification of neuronal tubulin. Due to its oligomeric structure, polyglutamylation can serve as a potentiometer to modulate binding of diverse MAPs. In addition, it can exert a differential mode of regulation towards distinct microtubule protein partners. To find out to what extent polyglutamylation is a general regulator, we have analyzed its ability to affect the binding of STOPs, the major factors that confer cold- and nocodazole-resistance to microtubules. We have shown by blot overlay experiments that binding of STOP does not depend on the length of the polyglutamyl chains carried by tubulins. And contrary to the other microtubule-associated proteins tested so far, STOP can bind quantitatively to any tubulin isoform whatever its degree of polyglutamylation.

Structure of tubulin C-terminal domain obtained by subtilisin treatment The major α and β tubulin isotypes from pig brain are glutamylated

Limited subtilisin digestion of the tubulin alpha, beta heterodimer has been used in this work to reduce the total number of tubulin isotypes from 20 for native to 9 for subtilisin-cleaved tubulin. This indicates that the major part of tubulin heterogeneity is located at the C-terminus of the molecule. The C-terminal peptides of both alpha and beta subunits of tubulin were purified by anion-exchange HPLC. Combined use of Edman degradation chemistry and mass spectrometry on the isolated peptides shows that subtilisin cleavage occurs at position Asp-438 and His-406 of alpha and Gln-433 and His-396 of beta tubulin chains. Quantitative analysis of our data show that cleavage at positions His-406 (alpha) and His-396 (beta) occurs with a low efficiency and indicates that the major isotypes of pig brain tubulin are modified by sequential attachment of 1 to 5 glutamic acid residues at positions Glu-445 or -435 of alpha and beta tubulin, respectively.

Differential binding regulation of microtubule-associated proteins MAP1A, MAP1B, and MAP2 by tubulin polyglutamylation

The major neuronal post-translational modification of tubulin, polyglutamylation, can act as a molecular potentiometer to modulate microtubule-associated proteins (MAPs) binding as a function of the polyglutamyl chain length. The relative affinity of Tau, MAP2, and kinesin has been shown to be optimal for tubulin modified by approximately 3 glutamyl units. Using blot overlay assays, we have tested the ability of polyglutamylation to modulate the interaction of two other structural MAPs, MAP1A and MAP1B, with tubulin. MAP1A and MAP2 display distinct behavior in terms of tubulin binding; they do not compete with each other, even when the polyglutamyl chains of tubulin are removed, indicating that they have distinct binding sites on tubulin. Binding of MAP1A and MAP1B to tubulin is also controlled by polyglutamylation and, although the modulation of MAP1B binding resembles that of MAP2, we found that polyglutamylation can exert a different mode of regulation toward MAP1A. Interestingly, although the affinity of the other MAPs tested so far decreases sharply for tubulins carrying long polyglutamyl chains, the affinity of MAP1A for these tubulins is maintained at a significant level. This differential regulation exerted by polyglutamylation toward different MAPs might facilitate their selective recruitment into distinct microtubule populations, hence modulating their functional properties.

Identification by mass spectrometry of a new alpha-tubulin isotype expressed in human breast and lung carcinoma cell lines

The extensive C-terminal molecular heterogeneity of alpha- and beta-tubulin is a consequence of multiple isotypes, the products of distinct genes, that undergo several posttranslational modifications. These include polyglutamylation and polyglycylation of both subunits, reversible tyrosination and removal of the penultimate glutamate from alpha-tubulin, and phosphorylation of the beta III isotype. A mass spectrometry-based method has been developed for the analysis of the C-terminal diversity of tubulin from human cell lines. Total cell extracts are resolved by SDS--PAGE and transferred to nitrocellulose, and the region of the blot corresponding to tubulin (approximately 50 kDa) was excised and digested with CNBr to release the highly divergent C-terminal tubulin fragments. The masses of the human alpha- and beta-tubulin CNBr-derived C-terminal peptides are all in the 1500--4000 Da mass range and can be analyzed directly by MALDI-TOF mass spectrometry in the negative ion mode without significant interference from other released peptides. In this study, the tubulin isotype diversity in MDA-MB-231, a human breast carcinoma cell line, and A549, a human non-small lung cancer cell line, is reported. The major tubulin isotypes present in both cell lines are k-alpha 1 and beta 1. Importantly, we report a previously unknown alpha isotype present at significant levels in both cell lines. Moreover, the degree of posttranslational modifications to all isotypes was limited. Glu-tubulin, in which the C-terminal tyrosine of alpha-tubulin is removed, was not detected. In contrast to mammalian neuronal tubulin which exhibits extensive polyglutamylation, only low-level monoglutamylation of the k-alpha 1 and beta 1 isotypes was observed in these two human cell lines.

Incorporation of nitrotyrosine into alpha-tubulin by recombinant mammalian tubulin-tyrosine ligase

Tubulin-tyrosine ligase (TTL, EC 6.3.2.25) from porcine brain, which catalyses the readdition of tyrosine to the C-terminus of detyrosinated alpha-tubulin, was cloned and expressed in Escherichia coli as a glutathione S-transferase-fusion protein. Upon cleavage of the immobilised fusion protein, an electrophoretically homogeneous enzyme was obtained. Recombinant TTL, which exhibited similar catalytic properties as the mammalian enzyme purified from brain tissue, was capable of using nitrotyrosine as an alternative substrate in vitro. Incorporation of tyrosine into tubulin was competitively inhibited by nitrotyrosine with an apparent K(i) of 0.24 mM. The TTL-catalysed incorporation of nitrotyrosine as sole substrate into alpha-tubulin was clearly detectable at concentrations of 10 microM by immunological methods using nitrotyrosine specific antibodies. However, in competition with tyrosine 20-fold higher concentrations of nitrotyrosine were necessary before its incorporation became evident. Analysis of the C-terminal peptides of in vitro modified alpha-tubulin by MALDI-MS confirmed the covalent incorporation of nitrotyrosine into tubulin by TTL. In contrast to the C-terminal tyrosine, pancreatic carboxypeptidase A was incapable of cleaving nitrotyrosine from the modified alpha-tubulin.

Tubulin polyglycylation in Platyhelminthes: diversity among stable microtubule networks and very late occurrence during spermiogenesis

The distribution of glycylated tubulin has been analyzed in different populations of stable microtubules in a digenean flatworm, Echinostoma caproni (Platyhelminthes). Two cellular types, spermatozoa and ciliated excretory cells, have been analyzed by means of immunofluorescence, immunogold, and immunoblotting techniques using two monoclonal antibodies (mAbs), AXO 49, and TAP 952, specifically directed against differently glycylated isoforms of tubulin. The presence of glycylated tubulin in the two cell types was shown. However, the differential reactivities of TAP 952 and AXO 49 mAbs with the two axoneme types suggest a difference in their glycylation level. In addition, within a single cell, the spermatozoon, cortical microtubules underlying the flagellar membrane, and axonemal microtubules were shown to comprise different tubulin isoforms, the latter ones only being labelled with one of the antiglycylated tubulin mAbs, TAP 952. Similarly, the antiacetylated (6-11B-1) and polyglutamylated (GT335) tubulin mAbs decorated the two types of axonemal microtubules, but not the cortical ones. From these data, a subcellular sorting of posttranslationally modified tubulin isoforms within spermatozoa, on the one hand, and a cellular sorting of glycylated isoforms inside the whole organism, on the other hand, is demonstrated in the flatworm E. caproni. Last, a sequential occurrence of tubulin posttranslational modifications was observed in the course of spermiogenesis. Acetylation appears first, followed shortly by glutamylation; glycylation takes place at the extreme end of spermiogenesis and, specifically, in a proximo-distal process. Thus in agreement with, and extending other studies [Bré et al., 1996], glycylation appears to close the sequence of posttranslational events occurring in axonemal microtubules during spermiogenesis.

Energetics of vinca alkaloid interactions with tubulin isotypes: implications for drug efficacy and toxicity

A number of vinca alkaloids, including vincristine, vinblastine, and vinorelbine, are currently used in cancer chemotherapy. These three vinca alkaloids interact differently with a range of solid and hematologic tumors. To test the possibility that the tubulin isotype composition is an important determinant in antineoplastic efficacy, we determined thermodynamic parameters for vinca alkaloid interactions with purified beta-tubulin isotypes, alphabetaII or alphabetaIII, as well as mixtures of alphabetaII and alphabetaIII, alphabetaII and alphabetaI&IV, or alphabetaIII and alphabetaI&IV (referred to as isotype-depleted tubulin) by quantitative sedimentation velocity. Vincristine-, vinblastine-, or vinorelbine-induced isotype self-association was studied at 25 degrees C in 10 mM Pipes, pH 6.9, 1 mM MgSO4, and 2 mM EGTA in the presence of 50 microM GTP or GDP. For all three drugs, we observed no significant differences in overall affinities, K1K2, or in GDP enhancement of purified isotypes compared to unfractionated tubulin, suggesting that differential antitumor efficacy observed clinically for these vinca alkaloids is not determined by tissue isotype composition. Small, but significant differences in the individual binding parameters, K1 and K2, are found in the vincristine data. In the presence of vincristine and GTP, K1, the affinity of drug for tubulin heterodimers, tends to be larger for purified alphabetaII- or alphabetaIII-tubulin compared to unfractionated tubulin. Furthermore, the apparent dimerization constant, K2app, at physiologically significant drug concentrations is larger for these purified isotypes. When alphabetaII- and alphabetaIII-tubulin are combined, the cooperativity between drug binding and spiral formation approaches that of unfractionated PC-tubulin. These differences are not observed in the presence of vinblastine or vinorelbine. The differences found with vincristine may be implicated in the dose-limiting neurotoxicity found with this drug, but not found with vinblastine or vinorelbine.

Expression of glycylated tubulin during the differentiation of spermatozoa in mammals

Using quantitative immunogold analyses of tubulin isoforms we previously demonstrated a unique differential expression of glutamylated tubulin in the flagellum of mouse and man spermatozoa [Fouquet et al., 1997: Tissue Cell 29:573-583]. We have performed similar analyses for glycylated tubulin using two monoclonal antibodies, TAP 952 and AXO 49, directed to mono- and polyglycylated tubulin respectively. Glycylated tubulin was not found in centrioles and cytoplasmic microtubules (manchette) of germ cells. In mouse and man, axonemal tubulin was first monoglycylated and uniformly distributed in all doublets at all levels of the flagellum in elongating spermatids. In human mature spermatozoa axonemal microtubules were enriched in monoglycylated tubulin from the base to the tip of the flagellum. In mouse sperm flagellum a similar gradient of monoglycylated tubulin was also observed in addition to an opposite gradient of polyglycylated tubulin. In both species, monoglycylated tubulin labeling predominated in doublets 3-8 whereas glutamylated tubulin labeling [Fouquet et al., 1997] predominated in doublets 1-5-6. These differential labelings were suppressed after motility inhibition of mouse spermatozoa by sodium azide treatment and in non-motile human spermatozoa lacking dynein arms. The unique distribution of these tubulin isoforms and the known inhibition of motility induced by their specific antibodies are consistent with a complementary role of tubulin glycylation and glutamylation in the regulation of flagellar beating in mammalian spermatozoa.

Cold-adapted microtubules: characterization of tubulin posttranslational modifications in the Antarctic ciliate Euplotes focardii

In cold poikilotherm organisms, microtubule assembly is promoted at temperatures below 4 degrees C and cold-induced depolymerization is prevented. On the basis of the results of investigations on cold-adapted fishes, the property of cold adaptation is ascribed to intrinsic characteristics of the tubulins. To fully understand cold adaptation, we studied the tubulins of Euplotes focardii, an Antarctic ciliated protozoan adapted to temperatures ranging from -2 to +4 degrees C. In this organism, we had previously sequenced one beta-tubulin gene and, then identified three other genes (denoted as beta-T1, beta-T2, beta-T3 and beta-T4). Here we report that the amino acid sequence of the carboxy-terminal domain predicted from the beta-T3 gene (apparently the most expressed of the gene family) contains six modifications (five substitutions and one insertion) of conserved residues, unique with respect to all the other known beta-tubulin sequences. These modifications can change the structural conformation of the carboxy-terminal domain. Furthermore, in the variable terminal end of that domain, a consensus sequence for a phosphorylation site is present, and the residue Glu-438, the most frequent site for polyglutamylation in beta-tubulin, is substituted by Asp. Starting from these observations, we showed that in E. focardii only alpha-tubulin is polyglutamylated, while beta-tubulin undergoes phosphorylation. Polyglutamylated microtubules appear to colocalize with cilia and microtubular bundles, all structures in which microtubules undergo a sliding process. This finding supports the idea that alpha-tubulin polyglutamylation is involved in the interaction between tubulin and motor microtubule-associated proteins. Phosphorylation, usually a rare posttranslational modification of beta-tubulin, which is found extensively distributed in the beta-tubulin of this cold-adapted organism, may play a determinant role in the dynamic of polymerization and depolymerization at low temperatures.

Polyglycylation of tubulin is essential and affects cell motility and division in Tetrahymena thermophila

We analyzed the role of tubulin polyglycylation in Tetrahymena thermophila using in vivo mutagenesis and immunochemical analysis with modification-specific antibodies. Three and five polyglycylation sites were identified at glutamic acids near the COOH termini of alpha- and beta-tubulin, respectively. Mutants lacking all polyglycylation sites on alpha-tubulin have normal phenotype, whereas similar sites on beta-tubulin are essential. A viable mutant with three mutated sites in beta-tubulin showed reduced tubulin glycylation, slow growth and motility, and defects in cytokinesis. Cells in which all five polyglycylation sites on beta-tubulin were mutated were viable if they were cotransformed with an alpha-tubulin gene whose COOH terminus was replaced by the wild-type COOH terminus of beta-tubulin. In this double mutant, beta-tubulin lacked detectable polyglycylation, while the alpha-beta tubulin chimera was hyperglycylated compared with alpha-tubulin in wild-type cells. Thus, the essential function of polyglycylation of the COOH terminus of beta-tubulin can be transferred to alpha-tubulin, indicating it is the total amount of polyglycylation on both alpha- and beta-tubulin that is essential for survival.

Polyglutamylation of nucleosome assembly proteins

Polyglutamylation is an original posttranslational modification, discovered on tubulin, consisting in side chains composed of several glutamyl units and leading to a very unusual protein structure. A monoclonal antibody directed against glutamylated tubulin (GT335) was found to react with other proteins present in HeLa cells. After immunopurification on a GT335 affinity column, two prominent proteins of approximately 50 kDa were observed. They were identified by microsequencing and mass spectrometry as NAP-1 and NAP-2, two members of the nucleosome assembly protein family that are implicated in the deposition of core histone complexes onto chromatin. Strikingly, NAP-1 and NAP-2 were found to be substrates of an ATP-dependent glutamylation enzyme co-purifying on the same column. We took advantage of this property to specifically label and purify the polyglutamylated peptides. NAP-1 and NAP-2 are modified in their C-terminal domain by the addition of up to 9 and 10 glutamyl units, respectively. Two putative glutamylation sites were localized for NAP-1 at Glu-356 and Glu-357 and, for NAP-2, at Glu-347 and Glu-348. These results demonstrate for the first time that proteins other than tubulin are polyglutamylated and open new perspectives for studying NAP function.

Polyglutamylation and polyglycylation of alpha- and beta-tubulins during in vitro ciliated cell differentiation of human respiratory epithelial cells

Tubulins are the major proteins within centriolar and axonemal structures. In all cell types studied so far, numerous alpha- and beta-tubulin isoforms are generated both by expression of a multigenic family and various post-translational modifications. We have developed a primary culture of human nasal epithelial cells where the ciliated cell differentiation process has been observed and quantified. We have used this system to study several properties concerning polyglutamylation and polyglycylation of tubulin. GT335, a monoclonal antibody directed against glutamylated tubulins, stained the centriole/basal bodies and the axonemes of ciliated cells, and the centrioles of non-ciliated cells. By contrast, axonemal but not centriolar tubulins were polyglycylated. Several polyglutamylated and polyglycylated tubulin isotypes were detected by two-dimensional electrophoresis, using GT335 and a specific monoclonal antibody (TAP952) directed against short polyglycyl chains. Immunoelectron microscopy experiments revealed that polyglycylation only affected axonemal tubulin. Using the same technical approach, polyglutamylation was shown to be an early event in the centriole assembly process, as gold particles were detected in fibrogranular material corresponding to the first cytoplasmic structures involved in centriologenesis. In a functional assay, GT335 and TAP952 had a dose-dependent inhibitory effect on ciliary beat frequency. TAP952 had only a weak effect while GT335 treatment led to a total arrest of beating. These results strongly suggest that in human ciliated epithelial cells, tubulin polyglycylation has only a structural role in cilia axonemes, while polyglutamylation may have a function both in centriole assembly and in cilia activity.

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.

Tubulin polyglutamylase: isozymic variants and regulation during the cell cycle in HeLa cells

Polyglutamylation is a posttranslational modification of tubulin that is very common in neurons and ciliated or flagellated cells. It was proposed to regulate the binding of microtubule associated proteins (MAPs) and molecular motors as a function of the length of the polyglutamyl side-chain. Though much less common, this modification of tubulin also occurs in proliferating cells like HeLa cells where it is associated with centrioles and with the mitotic spindle. Recently, we partially purified tubulin polyglutamylase from mouse brain and described its enzymatic properties. In this work, we focused on tubulin polyglutamylase activity from HeLa cells. Our results support the existence of a tubulin polyglutamylase family composed of several isozymic variants specific for alpha- or beta-tubulin subunits. In the latter case, the specificity probably also concerns the different beta-tubulin isotypes. Interestingly, we found that tubulin polyglutamylase activity is regulated in a cell cycle dependent manner and peaks in G(2)-phase while the level of glutamylated tubulin peaks in mitosis. Consistent results were obtained by treating the cells with hydroxyurea, nocodazole or taxotere. In particular, in mitotic cells, tubulin polyglutamylase activity was always low while glutamylation level was high. Finally, tubulin polyglutamylase activity and the level of glutamylated tubulin appeared to be inversely related. This paradox suggests a complex regulation of both tubulin polyglutamylase and the reverse deglutamylase activity.

Affinity mass spectrometry-based approaches for the analysis of protein-protein interaction and complex mixtures of peptide-ligands

Combined applications of affinity purification procedures and mass-spectrometric analyses (affinity mass spectrometry or affinity-directed mass spectrometry) have gained broad interest in various fields of biological sciences. We have extended these techniques to the purification and analysis of closely related peptides from complex mixtures and to the characterization of binding motifs and relative affinities in protein-protein interactions. The posttranslational modifications in the carboxy-terminal region of porcine brain tubulin are used as an example for the applicability of affinity mass spectrometry in the characterization of complex patterns of related peptides. We also show that affinity mass spectrometry allows the mapping of sequential binding motifs of two interacting proteins. Using the ActA/Mena protein-protein complex as a model system, we show that we can selectively purify Mena-binding peptides from a tryptic digest of ActA. The results from this assay are compared to data sets obtained earlier by classical methods using synthetic peptides and molecular genetic experiments. As a further expansion of affinity mass spectrometry, we have established an internally standardized system that allows comparison of the affinities of related ligands for a given protein. Here the affinities of two peptide ligands for the monoclonal tubulin-specific antibody YL1/2 are determined in terms of half-maximal competition.

Synthetic peptides identify the minimal substrate requirements of tubulin polyglutamylase in side chain elongation

The minimal sequence requirement of Crithidia tubulin polyglutamylase is already fulfilled by tubulin-related peptides carrying a free alpha-carboxylate on a glutamic acid residue. Since the product of each glutamylation step fulfills the substrate requirements necessary for the next cycle, very long side chains are generated with brain tubulin as a substrate. Up to 70 mol of glutamic acid was incorporated per alphabeta-heterodimer. We speculate that the strict choice of a particular glutamate residue for the formation of the isopeptide bond initiating a novel side chain is made by a tubulin monoglutamylase which requires the entire tubulin as substrate.

Isolation of tubulin polyglutamylase from Crithidia; binding to microtubules and tubulin, and glutamylation of mammalian brain alpha- and beta-tubulins

Trypanosomatids have a striking cage-like arrangement of submembraneous microtubules. We previously showed that alpha- and beta- tubulins of these stable microtubules are extensively modified by polyglutamylation. Cytoskeletal microtubular preparations obtained by Triton extraction of Leishmania tarentolae and Crithidia fasciculata retain an enzymatic activity that incorporates radioactive glutamic acid in a Mg2+-ATP-dependent manner into alpha- and beta-tubulins. The tubulin polyglutamylase is extracted by 0.25 M salt. The Crithidia enzyme can be purified by ATP-affinity chromatography, glycerol-gradient centrifugation and ion-exchange chromatography. After extraction from the microtubular cytoskeleton the glutamylase forms a complex with alphabeta tubulin, but behaves after removal of tubulin as a globular protein with a molecular mass of 38x10(3). In highly enriched fractions a corresponding band is the major polypeptide visible in SDS-PAGE. The enzyme from Crithidia recognises mammalian brain tubulin, where it incorporates glutamic acid preferentially into the more acidic variants of both alpha- and beta-tubulins. Synthetic peptides with an oligoglutamyl side chain, corresponding to the carboxy-terminal end of brain alpha- and beta-tubulins, are accepted by the enzyme, albeit at low efficiency. The polyglutamylase elongates the side chain by up to 3 and 5 residues, respectively. Other properties of the tubulin polyglutamylase are also discussed.

Differentiation of human colon cancer cells changes the expression of beta-tubulin isotypes and MAPs

The human colon adenocarcinoma HT29-D4 cell line is an interesting model for studies on epithelial cell differentiation. Undifferentiated cells are malignant proliferating cells, whereas differentiated cells act like epithelial polarized cells. In the present study, we first characterized the action of taxoids on the microtubular network of HT29-D4 cells according to the state of differentiation. Microtubular bundles were found in undifferentiated cells but not in differentiated cells, even with 500-fold higher taxoid concentrations for 96 h. This finding led us to study changes in microtubules according to the polarity status of the cell. E-MAP-115 was expressed only in differentiated cells; expression of beta-tubulin isotypes was altered in them relative to undifferentiated cells. Classes I, II, III, IVa and IVb isotypes were expressed in both phenotypes; however, differentiated epithelial cells displayed a specific increase in class III beta-tubulin. Thus, the increase in expression of this beta-tubulin isotype in differentiated cells is not restricted to neuronal cells. Moreover, these expression changes may reflect a higher stability of microtubular network in differentiated cells, which may explain the lower activity of anti-microtubule agents, independently of the mitotic process. These results indicate that the composition of microtubules should be considered as one of the criteria involved in the response of tumour cells to chemotherapy with anti-microtubule agents.

Monoclonal antibody ID5: epitope characterization and minimal requirements for the recognition of polyglutamylated alpha- and beta-tubulin

A monoclonal antibody (ID5) raised against the synthetic tetradecapeptide corresponding to the C-terminal region of detyrosinated alpha-tubulin showed an unexpected cross-reactivity with beta-tubulin from pig brain tissue. The specificity and the minimal epitope requirements of ID5 were characterized by competitive enzyme-linked immunosorbent assay (ELISA) and spot blots using a series of synthetic peptides and the natural peptides of beta-tubulin and detyrosinated alpha-tubulin from brain. The epitope of ID5 is comprised of the carboxyterminal sequence -XEE carrying the terminal alpha-carboxylate group with X being a variable residue. All linkages in the epitope involve alpha-peptide bonds. This epitope is provided by the detyrosinated alpha-tubulin main chain and the polyglutamyl side chains of both brain alpha- and beta-tubulins. Affinity purification of beta-tubulin peptides and mass spectrometric characterization reveal that peptides carrying three to nine glutamyl residues in the side chain are recognized by ID5. These results show that except for the first gamma-peptide linkage the alpha-peptide bond is the preferred linkage type in the tubulin polyglutamyl side chains.

Posttranslational modifications of the C-terminus of alpha-tubulin in adult rat brain: alpha 4 is glutamylated at two residues

In adult mammalian brain, the C-terminus of alpha-tubulin exhibits a high degree of polymorphism due to a combination of four covalent posttranslational modifications: glutamylation, tyrosination, detyrosination, and removal of the penultimate glutamate residue (C-terminal deglutamylation). Glutamylation is the most abundant. To characterize the glutamylation of alpha-tubulin and its relationship with the other modifications, we developed a chromatographic procedure for purifying alpha-tubulin C-terminal peptides. The purified peptides were identified by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF) and amino acid sequencing. In this report, we provide a complete description of the glutamylation of tyrosinated, detyrosinated, and C-terminal deglutamylated isoforms of both alpha-tubulin isotypes (alpha1/2 and alpha4) expressed in adult rat brain. In particular, we describe for the first time the glutamylation of alpha4. More than 90% of the alpha-tubulin is glutamylated, and more than 75% of it is nontyrosinated. alpha4 is more extensively glutamylated than alpha1/2, containing as many as 11 posttranslationally added glutamate residues. The most abundant alpha4 isoform is nontyrosinated, containing five posttranslationally added glutamates, whereas the most abundant alpha1/2 isoforms are nontyrosinated, with only one or two posttranslationally added glutamates. In contrast to alpha1/2, alpha4 is glutamylated at two separate residues (Glu-443 and Glu-445) in the sequence 431DYEEVGIDSYEDEDEGEE448. This is the first evidence that glutamylation can occur on two different residues in the same mammalian tubulin isotype.

Tubulin polyglycylation: differential posttranslational modification of dynamic cytoplasmic and stable axonemal microtubules in paramecium

Polyglycylation, a posttranslational modification of tubulin, was discovered in the highly stable axonemal microtubules of Paramecium cilia where it involves the lateral linkage of up to 34 glycine units per tubulin subunit. The observation of this type of posttranslational modification mainly in axonemes raises the question as to its relationship with axonemal organization and with microtubule stability. This led us to investigate the glycylation status of cytoplasmic microtubules that correspond to the dynamic microtubules in Paramecium. Two anti-glycylated tubulin monoclonal antibodies (mAbs), TAP 952 and AXO 49, are shown here to exhibit different affinities toward mono- and polyglycylated synthetic tubulin peptides. Using immunoblotting and mass spectrometry, we show that cytoplasmic tubulin is glycylated. In contrast to the highly glycylated axonemal tubulin, which is recognized by the two mAbs, cytoplasmic tubulin reacts exclusively with TAP 952, and the alpha- and beta- tubulin subunits are modified by only 1-5 and 2-9 glycine units, respectively. Our analyses suggest that most of the cytoplasmic tubulin contains side chain lengths of 1 or 2 glycine units distributed on several glycylation sites. The subcellular partition of distinct polyglycylated tubulin isoforms between cytoplasmic and axonemal compartments implies the existence of regulatory mechanisms for glycylation. By following axonemal tubulin immunoreactivity with anti-glycylated tubulin mAbs upon incubation with a Paramecium cellular extract, the presence of a deglycylation enzyme is revealed in the cytoplasm of this organism. These observations establish that polyglycylation is reversible and indicate that, in vivo, an equilibrium between glycylating and deglycylating enzymes might be responsible for the length of the oligoglycine side chains of tubulin.

Posttranslational modifications of trichomonad tubulins; identification of multiple glutamylation sites

The alpha- and beta-tubulins present in cytoskeletons of Tritrichomonas mobilensis are extensively glutamylated. Automated sequencing and mass spectrometry of the carboxyterminal peptides identifies 4 glutamylation sites in alpha- and 2 sites in beta-tubulin. They are marked by asterisks in the terminal sequences GDE*E*E*E*DDG (alpha) and EGE*E*DEEAEA (beta). This is the first report that tubulin glutamylation can occur at multiple sites. Although T. mobilensis has four flagellae the tubulins lack polyglycylation. Thus glycylation is not necessary for formation or function of axonemal microtubules. Alpha-tubulin is completely acetylated at lysine 40 and shows no tyrosine cycle. Peptide sequences establish two distinct beta-tubulins.

Tubulin polyglutamylase: partial purification and enzymatic properties

In this work, we report on a novel enzyme, tubulin polyglutamylase, which catalyzes the posttranslational formation of polyglutamyl side chains onto alpha- and beta-tubulin. The length of the polyglutamyl side chain regulates the interaction between tubulin and various microtubule-associated proteins. We first developed an in vitro glutamylation assay. Activity measured in brain, a tissue particularly enriched with glutamylated tubulin, decreases during postnatal development. Thus, brains from 3-day-old mice were chosen as the starting material, and the enzyme was purified approximately 1000-fold. Its Mr was estimated to be 360K and its sedimentation coefficient 10 s. The enzyme catalyzes the MgATP-dependent addition of l-glutamate onto tubulin subunits. Microtubules are much better substrates than unpolymerized tubulin, and the reaction is very specific for glutamate, other amino acids or glutamate analogues not being substrates. Moreover, glutamyl units are added sequentially onto tubulin, leading to progressive elongation of the polyglutamyl side chains. Side chains of one to six or seven glutamyl units were obtained with microtubules, whereas much longer side chains (up to 15-20 units) were formed with unpolymerized tubulin. Interestingly, such very long polyglutamyl side chains were recently detected in some situations in vivo.

Multiple forms of tubulin: different gene products and covalent modifications

Tubulin, the subunit protein of microtubules, is an alpha/beta heterodimer. In many organisms, both alpha and beta exist in numerous isotypic forms encoded by different genes. In addition, both alpha and beta undergo a variety of posttranslational covalent modifications, including acetylation, phosphorylation, detyrosylation, polyglutamylation, and polyglycylation. In this review the distribution and possible functional significance of the various forms of tubulin are discussed. In analyzing the differences among tubulin isotypes encoded by different genes, some appear to have no functional significance, some increase the overall adaptability of the organism to environmental challenges, and some appear to perform specific functions including formation of particular organelles and interactions with specific proteins. Purified isotypes also display different properties in vitro. Although the significance of all the covalent modification of tubulin is not fully understood, some of them may influence the stability of modified microtubules in vivo as well as interactions with certain proteins and may help to determine the functional role of microtubules in the cell. The review also discusses isotypes of gamma-tubulin and puts various forms of tubulin in an evolutionary context.

Expression of tubulin isoforms during the differentiation of mammalian spermatozoa

Using the GT 335 mAb we have previously demonstrated a differential expression of glutamylated tubulin isoforms during spermatogenesis and in spermatooza of the mouse and human. Moreover, the proximodistal decrease of the immunolabeling and its predominance in doublets 1-5-6, corresponding to the plane of the flagellar wave, suggested that the glutamylated tubulin could be involved in a functional heterogeneity of microtubules in peripheral doublets of the sperm flagellum. In order to characterize further the importance of glutamylated tubulin in the sperm model, we analyzed tubulin isoforms by immunoblotting and quantitative immunogold, using antibodies to the C-terminal domain of both subunits including non-glutamylated and glutamylated epitopes. The unique differential immunolabeling of the glutamylated tubulin was confirmed with three mAbs 406-3, 392-2 and B3, in addition to GT 335. This differential labeling was interpreted as a differential accessibility of tubulin epitopes since it was greatly reduced in human spermatozoa lacking dynein arms and after motility inhibition of normal spermatozoa by azide pretreatment. We suggest that the glutamylated tubulin interacts with other axonemal and/or periaxonemal proteins which could be involved in flagellar beating and its regulation.

Taxol-resistant epithelial ovarian tumors are associated with altered expression of specific beta-tubulin isotypes

The treatment of advanced ovarian cancer with taxol is hindered by the development of drug resistance. The cellular target for taxol is the microtubule that is stabilized by the drug. Taxol preferentially binds to the beta subunit of tubulin of which there are six distinct isotypes in mammalian cells. We have used highly specific oligonucleotides and polymerase chain reaction to analyze expression of all six beta-tubulin genes. Human lung cancer cells (A549) were selected in 12 and 24 nM taxol resulting in cell lines that were 9- and 17-fold resistant, respectively. These cells displayed an altered ratio of classes I, II, III, and IVa beta-tubulin isotypes. Ovarian tumors, seven untreated primary and four taxol- resistant tumor-bearing ascites, displayed significant increases (P < 0.005) in classes I (3.6-fold), III (4.4-fold), and IVa (7.6-fold) isotypes in the taxol-resistant samples as compared with untreated primary ovarian tumors. The increased expression appears to be related to the resistance phenotype, as the basal levels of the class III and IVa isotypes in the untreated tumors were extremely low. This is the first report of altered expression of specific beta-tubulin genes in taxol-resistant ovarian tumors and we propose that the latter may play a role in clinical resistance to taxol.

Mammalian sperm tubulin: an exceptionally large number of variants based on several posttranslational modifications

Extraction of demembranated bull sperm flagella by SDS was used to maximize tubulin solubilization. The alpha- and beta-tubulin separated by SDS-PAGE were treated with endoproteinases LysC and AspN, respectively. Carboxy-terminal fragments were isolated by Mono Q chromatography and reversed-phase HPLC. Automated sequencing and mass spectrometry revealed an astonishingly high number of tubulin variants. Many variants were due to polyglutamylation and in particular to polyglycylation. The number of side-chain glycyl residues ranged from 0 to 28 in alpha and 0 to 15 in beta. Corresponding values for side-chain glutamyl residues were 0-6 in alpha and 0-3 in beta. Additional alpha variability was based on carboxy-terminal detyrosination and partial loss of the penultimate glutamate. A major glycylation site in alpha- and beta-tubulin was mapped. Some variants seem to display both glycyl and glutamyl side chains.

Profile of glutamylated tubulin expression during cerebellar granule cell development in vitro

The developmental regulation of tubulin and several of its posttranslational modifications was examined during the differentiation of rat cerebellar granule cells in vitro. In particular, we have noted that the glutamylation of alpha- and beta-tubulin subunits varies during development and becomes more prominent in differentiating neuronal processes. These results indicate that glutamylation of tubulin may be important in the stabilization of microtubules during the maturation of the neuronal cytoskeleton.

A brain protein (P30) that immunoreacts with a polyclonal anti-pancreatic carboxypeptidase A antibody shows properties that are shared with tubulin carboxypeptidase

A preparation of tubulin carboxypeptidase partially purified from bovine brain was found to contain a protein of molecular mass 30 kDa (P30) as determined by SDS-PAGE, that is recognized by a polyclonal anti-bovine pancreatic carboxypeptidase A. However, this protein is different from pancreatic carboxypeptidase A as judged by the isoelectric point and the pattern of peptides produced by trypsin digestion. The isoelectric point of P30 was similar to that found for tubulin carboxypeptidase (9 +/- 0.2). When the tubulin carboxypeptidase preparation was subjected to gel filtration chromatography under low salt concentration, P30 behaved as a protein of molecular mass 38 kDa whereas tubulin carboxypeptidase eluted at a position of 75 kDa molecular mass. However, when the chromatography was performed at relatively high salt concentration they behaved as proteins of 49 and 56 kDa, respectively. We considered that P30 may be an inactive monomeric form of the dimeric tubulin carboxypeptidase. However we can not rule out the possibility that it represents another carboxypeptidase not yet described.

Structurally similar Drosophila alpha-tubulins are functionally distinct in vivo

We used transgenic analysis in Drosophila to compare the ability of two structurally similar alpha-tubulin isoforms to support microtubule assembly in vivo. Our data revealed that even closely related alpha-tubulin isoforms have different functional capacities. Thus, in multicellular organisms, even small changes in tubulin structure may have important consequences for regulation of the microtubule cytoskeleton. In spermatogenesis, all microtubule functions in the postmitotic male germ cells are carried out by a single tubulin heterodimer composed of the major Drosophila alpha-84B tubulin isoform and the testis-specific beta 2-tubulin isoform. We tested the ability of the developmentally regulated alpha 85E-tubulin isoform to replace alpha 84B in spermatogenesis. Even though it is 98% similar in sequence, alpha 85E is not functionally equivalent to alpha 84B. alpha 85E can support some functional microtubules in the male germ cells, but alpha 85E causes dominant male sterility if it makes up more than one-half of the total alpha-tubulin pool in the spermatids. alpha 85E does not disrupt meiotic spindle or cytoplasmic microtubules but causes defects in morphogenesis of the two classes of singlet microtubules in the sperm tail axoneme, the central pair and the accessory microtubules. Axonemal defects caused by alpha 85E are precisely reciprocal to dominant defects in doublet microtubules we observed in a previous study of ectopic germ-line expression of the developmentally regulated beta 3-tubulin isoform. These data demonstrate that the doublet and singlet axoneme microtubules have different requirements for alpha- and beta-tubulin structure. In their normal sites of expression, alpha 85E and beta 3 are coexpressed during differentiation of several somatic cell types, suggesting that alpha 85E and beta 3 might form a specialized heterodimer. Our tests of different alpha-beta pairs in spermatogenesis did not support this model. We conclude that if alpha 85E and beta 3 have specialized properties required for their normal functions, they act independently to modulate the properties of microtubules into which they are incorporated.

Tubulin post-translational modifications--enzymes and their mechanisms of action

This review describes the enzymes responsible for the post-translational modifications of tubulin, including detyrosination/tyrosination, acetylation/deacetylation, phosphorylation, polyglutamylation, polyglycylation and the generation of non-tyrosinatable alpha-tubulin. Tubulin tyrosine-ligase, which reattaches tyrosine to detyrosinated tubulin, has been extensively characterized and its gene sequenced. Enzymes such as tubulin-specific carboxypeptidase and alpha-tubulin acetyltransferase, required, respectively, for detyrosination and acetylation of tubulin, have yet to be purified to homogeneity and examined in defined systems. This has produced some conflicting results, especially for the carboxypeptidase. The phosphorylation of tubulin by several different types of kinases has been studied in detail but drawing conclusions is difficult because many of these enzymes modify proteins other than their actual substrates, an especially pertinent consideration for in vitro experiments. Tubulin phosphorylation in cultured neuronal cells has proven to be the best model for evaluation of kinase effects on tubulin/microtubule function. There is little information on the enzymes required for polyglutamylation, polyglycylation, and production of non-tyrosinatable tubulin, but the available data permit interesting speculation of a mechanistic nature. Clearly, to achieve a full appreciation of tubulin post-translational changes the responsible enzymes must be characterized. Knowing when the enzymes are active in cells, if soluble or polymerized tubulin is the preferred substrate and the amino acid residues modified by each enzyme are all important. Moreover, acquisition of purified enzymes will lead to cloning and sequencing of their genes. With this information, one can manipulate cell genomes in order to either modify key enzymes or change their relative amounts, and perhaps reveal the physiological significance of tubulin post-translational modifications.

Subpellicular and flagellar microtubules of Trypanosoma brucei are extensively glutamylated

To determine the spectrum of tubulin variants in cytoskeletons of Trypanosoma brucei carboxy-terminal fragments of alpha- and beta-tubulin were isolated and characterized by sequencing and mass spectrometry. All variants arise by posttranslational modifications. We confirm the presence of tyrosinated and detyrosinated alpha-tubulin. Unexpectedly, but in line with its sequence, beta-tubulin also occurs with and without its carboxy-terminal tyrosine. Both tyrosinated and detyrosinated alpha- and beta-tubulins are extensively glutamylated. Unglutamylated tubulins are only trace components of the cytoskeletal microtubules. The maximal numbers of glutamyl residues in the lateral chain are 15 and 6 for alpha- and beta-tubulin, respectively. The oligoglutamyl side chain is linked via an isopeptide bond to glutamic acid residues 445 of alpha- and 435 of beta-tubulin. The same sites are used in glutamylated tubulins of mammalian brain. No tubulin variants based on polyglycylation are detected in cytoskeletal preparations or in isolated flagella. Tubulin specific incorporation of radioactive glutamate but not of glycine is observed when protein biosynthesis is completely inhibited in Trypanosoma cells. Possible reasons for the absence of polyglycylated tubulins from the trypanosomal axoneme are discussed. Finally we show that lysine 40 of the flagellar alpha-tubulin is completely acetylated.

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.

Polyglycylation of tubulin in the diplomonad Giardia lamblia, one of the oldest eukaryotes

We have searched for post-translational modifications in tubulin of the diplomonad Giardia lamblia, which is a representative of the earliest branches in eukaryotic evolution. The carboxyterminal peptide of alpha-tubulin was isolated and characterized by automated sequencing and mass spectrometry. Some 60% of the peptide is unmodified, while the remainder shows various degrees of polyglycylation. The number of glycyl residues in the lateral side chain ranges from 2 to 23. All peptide species encountered end with alanine-tyrosine, indicating the absence of a detyrosination/tyrosination cycle. We conclude that tubulin-specific polyglycylation could be as old as tubulin and axonemal structures.

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

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