A combination of posttranslational modifications is responsible for the production of neuronal alpha-tubulin heterogeneity

Mitochondrial impairment, decreased sirtuin activity and protein acetylation in dorsal root ganglia in Friedreich Ataxia models

Friedreich ataxia (FA) is a rare, recessive neuro-cardiodegenerative disease caused by deficiency of the mitochondrial protein frataxin. Mitochondrial dysfunction, a reduction in the activity of iron-sulfur enzymes, iron accumulation, and increased oxidative stress have been described. Dorsal root ganglion (DRG) sensory neurons are among the cellular types most affected in the early stages of this disease. However, its effect on mitochondrial function remains to be elucidated. In the present study, we found that in primary cultures of DRG neurons as well as in DRGs from the FXNI151F mouse model, frataxin deficiency resulted in lower activity and levels of the electron transport complexes, mainly complexes I and II. In addition, altered mitochondrial morphology, indicative of degeneration was observed in DRGs from FXNI151F mice. Moreover, the NAD+/NADH ratio was reduced and sirtuin activity was impaired. We identified alpha tubulin as the major acetylated protein from DRG homogenates whose levels were increased in FXNI151F mice compared to WT mice. In the mitochondria, superoxide dismutase (SOD2), a SirT3 substrate, displayed increased acetylation in frataxin-deficient DRG neurons. Since SOD2 acetylation inactivates the enzyme, and higher levels of mitochondrial superoxide anion were detected, oxidative stress markers were analyzed. Elevated levels of hydroxynonenal bound to proteins and mitochondrial Fe2+ accumulation was detected when frataxin decreased. Honokiol, a SirT3 activator, restores mitochondrial respiration, decreases SOD2 acetylation and reduces mitochondrial superoxide levels. Altogether, these results provide data at the molecular level of the consequences of electron transport chain dysfunction, which starts negative feedback, contributing to neuron lethality. This is especially important in sensory neurons which have greater susceptibility to frataxin deficiency compared to other tissues.

Glutamylation is a negative regulator of microtubule growth

Microtubules are noncovalent polymers built from αβ-tubulin dimers. The disordered C-terminal tubulin tails are functionalized with multiple glutamate chains of variable lengths added and removed by tubulin tyrosine ligases (TTLLs) and carboxypeptidases (CCPs). Glutamylation is abundant on stable microtubule arrays such as in axonemes and axons, and its dysregulation leads to human pathologies. Despite this, the effects of glutamylation on intrinsic microtubule dynamics are unclear. Here we generate tubulin with short and long glutamate chains and show that glutamylation slows the rate of microtubule growth and increases catastrophes as a function of glutamylation levels. This implies that the higher stability of glutamylated microtubules in cells is due to effectors. Interestingly, EB1 is minimally affected by glutamylation and thus can report on the growth rates of both unmodified and glutamylated microtubules. Finally, we show that glutamate removal by CCP1 and 5 is synergistic and occurs preferentially on soluble tubulin, unlike TTLL enzymes that prefer microtubules. This substrate preference establishes an asymmetry whereby once the microtubule depolymerizes, the released tubulin is reset to a less-modified state, while polymerized tubulin accumulates the glutamylation mark. Our work shows that a modification on the disordered tubulin tails can directly affect microtubule dynamics and furthers our understanding of the mechanistic underpinnings of the tubulin code.

A proteomic-informed view of the changes induced by loss of cellular adherence: The example of mouse macrophages

Except cells circulating in the bloodstream, most cells in vertebrates are adherent. Studying the repercussions of adherence per se in cell physiology is thus very difficult to carry out, although it plays an important role in cancer biology, e.g. in the metastasis process. In order to study how adherence impacts major cell functions, we used a murine macrophage cell line. Opposite to the monocyte/macrophage system, where adherence is associated with the acquisition of differentiated functions, these cells can be grown in both adherent or suspension conditions without altering their differentiated functions (phagocytosis and inflammation signaling). We used a proteomic approach to cover a large panel of proteins potentially modified by the adherence status. Targeted experiments were carried out to validate the proteomic results, e.g. on metabolic enzymes, mitochondrial and cytoskeletal proteins. The mitochondrial activity was increased in non-adherent cells compared with adherent cells, without differences in glucose consumption. Concerning the cytoskeleton, a rearrangement of the actin organization (filopodia vs sub-cortical network) and of the microtubule network were observed between adherent and non-adherent cells. Taken together, these data show the mechanisms at play for the modification of the cytoskeleton and also modifications of the metabolic activity between adherent and non-adherent cells.

Inhibition of cell migration and induction of apoptosis by a novel class II histone deacetylase inhibitor, MCC2344

Epigenetic modifiers provide a new target for the development of anti-cancer drugs. The eraser histone deacetylase 6 (HDAC6) is a class IIb histone deacetylase that targets various non-histone proteins such as transcription factors, nuclear receptors, cytoskeletal proteins, DNA repair proteins, and molecular chaperones. Therefore, it became an attractive target for cancer treatment. In this study, virtual screening was applied to the MicroCombiChem database with 1162 drug-like compounds to identify new HDAC6 inhibitors. Five compounds were tested in silico and in vitro as HDAC6 inhibitors. Both analyses revealed 1-cyclohexene-1-carboxamide, 2-hydroxy-4,4-dimethyl-N-1-naphthalenyl-6-oxo- (MCC2344) as the best HDAC6 inhibitor among the five ligands. The binding affinity of MCC2344 to HDAC6 was further confirmed by microscale thermophoresis. Additionally, the anti-cancer activity of MCC2344 was tested in several tumor cell lines. Leukemia cells were the most sensitive cells towards MCC2344, particularly the P-glycoprotein-overexpressing multidrug-resistant cell line CEM/ADR5000 exhibited remarkable collateral sensitivity towards MCC2344. Transcriptome analysis using microarray hybridization was performed for investigating downstream mechanisms of action of MCC2344 in leukemia cells. MCC2344 affected microtubule dynamics and suppressed cell migration in the wound healing assay as well as in a spheroid model by hyper-acetylation of tubulin and HSP-90. MCC2344 induced cell death in CEM/ADR5000 cells by activation of PARP, caspase-3, and p21 in addition to the downregulation of p62. MCC2344 significantly inhibited tumor growth in vivo in zebrafish larvae without mortality until 20 pM. We propose MCC2344 as a novel HDAC6 inhibitor for cancer treatment.

The tubulin code and its role in controlling microtubule properties and functions

Microtubules are core components of the eukaryotic cytoskeleton with essential roles in cell division, shaping, motility and intracellular transport. Despite their functional heterogeneity, microtubules have a highly conserved structure made from almost identical molecular building blocks: the tubulin proteins. Alternative tubulin isotypes and a variety of post-translational modifications control the properties and functions of the microtubule cytoskeleton, a concept known as the 'tubulin code'. Here we review the current understanding of the molecular components of the tubulin code and how they impact microtubule properties and functions. We discuss how tubulin isotypes and post-translational modifications control microtubule behaviour at the molecular level and how this translates into physiological functions at the cellular and organism levels. We then go on to show how fine-tuning of microtubule function by some tubulin modifications can affect homeostasis and how perturbation of this fine-tuning can lead to a range of dysfunctions, many of which are linked to human disease.

Posttranslational modification of plant microtubules

Microtubules in eukaryotes have a number of posttranslational modifications catalyzed by an array of enzymes. These modifications alter the properties of the microtubules and the ways in which they interact with partner proteins. In recent years many of the enzymes which modify the microtubules have been identified in animals and protozoans. Relatively little work has been done on their function in plants, however. This study uses bioinformatics to identify homologues of these enzymes in plant species from the green alga Chlamydomonas reiinhardtii to the angiosperm Arabidopsis thaliana. Many are conserved and this gives insight into the likely future direction of this dynamic field.

Tubulin posttranslational modifications in in vitro matured prepubertal and adult ovine oocytes

Microtubules (MTs), polymers of alpha/beta-tubulin heterodimers, are involved in crucial functions in eukaryotic cells. MTs physiology can be influenced by a variety of post-translational modifications (PTMs), including tyrosination, detyrosination, delta 2 modification, acetylation, polyglutamylation, polyglycylation. In mammalian oocytes, MTs are essential for meiosis, regulating the formation of meiotic spindle and chromosomes movements. Considering that the patterns of tubulin PTMs (tyrosination, detyrosination, acetylation, polyglutamylation and delta 2 modification) have not been investigated in ovine oocytes, this study has been designed to investigate their presence and quantification in in vitro matured (IVM) adult and prepubertal ovine oocytes. Oocytes from adult and lamb Sarda ewes, regularly slaughtered at the local abattoir, were in vitro matured, fixed, and processed by indirect immunofluorescence and confocal microscopy analyses at metaphase II stage. Our results revealed a well detectable signal for total, tyrosinated and acetylated α-tubulin in meiotic spindle of both sheep and lamb oocytes. On the other hand, no immunopositivity were appreciable for detyrosinated, polyglutamylated, and delta 2 tubulin in meiotic spindle of both sheep and lamb oocytes. As regard the tyrosinated and the acetylated α-tubulin PTMs, through the quantification of the fluorescence intensity, we did not find significant differences in their expression in meiotic spindle of sheep, while in lamb the acetylated tubulin levels were predominant in comparison with tyrosinated. Our results in addition to investigating for the first time the different tubulin PTMs in the spindle organization of ovine oocytes, showed a different microtubule pattern between adult and prepubertal oocytes. The microtubule cytoskeleton survey may thus suggest further cues to better understand skill-related problems in in the acquisition of oocyte competence.

Brassinosteroids regulate pavement cell growth by mediating BIN2-induced microtubule stabilization

Brassinosteroids (BRs), a group of plant steroid hormones, play important roles in regulating plant development. The cytoskeleton also affects key developmental processes and a deficiency in BR biosynthesis or signaling leads to abnormal phenotypes similar to those of microtubule-defective mutants. However, how BRs regulate microtubule and cell morphology remains unknown. Here, using liquid chromatography-tandem mass spectrometry, we identified tubulin proteins that interact with Arabidopsis BRASSINOSTEROID INSENSITIVE2 (BIN2), a negative regulator of BR responses in plants. In vitro and in vivo pull-down assays confirmed that BIN2 interacts with tubulin proteins. High-speed co-sedimentation assays demonstrated that BIN2 also binds microtubules. The Arabidopsis genome also encodes two BIN2 homologs, BIN2-LIKE 1 (BIL1) and BIL2, which function redundantly with BIN2. In the bin2-3 bil1 bil2 triple mutant, cortical microtubules were more sensitive to treatment with the microtubule-disrupting drug oryzalin than in wild-type, whereas in the BIN2 gain-of-function mutant bin2-1, cortical microtubules were insensitive to oryzalin treatment. These results provide important insight into how BR regulates plant pavement cell and leaf growth by mediating the stabilization of microtubules by BIN2.

Identification of DmTTLL5 as a Major Tubulin Glutamylase in the Drosophila Nervous System

Microtubules (MTs) play crucial roles during neuronal life. They are formed by heterodimers of alpha and beta-tubulins, which are subjected to several post-translational modifications (PTMs). Amongst them, glutamylation consists in the reversible addition of a variable number of glutamate residues to the C-terminal tails of tubulins. Glutamylation is the most abundant MT PTM in the mammalian adult brain, suggesting that it plays an important role in the nervous system (NS). Here, we show that the previously uncharacterized CG31108 gene encodes an alpha-tubulin glutamylase acting in the Drosophila NS. We show that this glutamylase, which we named DmTTLL5, initiates MT glutamylation specifically on alpha-tubulin, which are the only glutamylated tubulin in the Drosophila brain. In DmTTLL5 mutants, MT glutamylation was not detected in the NS, allowing for determining its potential function. DmTTLL5 mutants are viable and we did not find any defect in vesicular axonal transport, synapse morphology and larval locomotion. Moreover, DmTTLL5 mutant flies display normal negative geotaxis behavior and their lifespan is not altered. Thus, our work identifies DmTTLL5 as the major enzyme responsible for initiating neuronal MT glutamylation specifically on alpha-tubulin and we show that the absence of MT glutamylation is not detrimental for Drosophila NS function.

HDAC3 promotes meiotic apparatus assembly in mouse oocytes by modulating tubulin acetylation

Histone deacetylases (HDACs) have been shown to deacetylate numerous cellular substrates that govern a wide array of biological processes. HDAC3, a member of the Class I HDACs, is a highly conserved and ubiquitously expressed protein. However, its roles in meiotic oocytes are not known. In the present study, we find that mouse oocytes depleted of HDAC3 are unable to completely progress through meiosis, and are blocked at metaphase I. These HDAC3 knockdown oocytes show spindle/chromosome organization failure, with severely impaired kinetochore-microtubule attachments. Consistent with this, the level of BubR1, a central component of the spindle assembly checkpoint, at kinetochores is dramatically increased in metaphase oocytes following HDAC3 depletion. Knockdown and overexpression experiments reveal that HDAC3 modulates the acetylation status of α-tubulin in mouse oocytes. Importantly, the deacetylation mimetic mutant tubulin-K40R can partly rescue the defective phenotypes of HDAC3 knockdown oocytes. Our data support a model whereby HDAC3, through deacetylating tubulin, promotes microtubule stability and the establishment of kinetochore-microtubule interaction, consequently ensuring proper spindle morphology, accurate chromosome movement and orderly meiotic progression during oocyte maturation.

New HDAC6-mediated deacetylation sites of tubulin in the mouse brain identified by quantitative mass spectrometry

The post-translational modifications (PTMs) occurring on microtubules have been implicated in the regulation of microtubule properties and functions. Acetylated K40 of α-tubulin, a hallmark of long-lived stable microtubules, is known to be negatively controlled by histone deacetylase 6 (HDAC6). However, the vital roles of HDAC6 in microtubule-related processes such as cell motility and cell division cannot be fully explained by the only known target site on tubulin. Here, we attempt to comprehensively map lysine acetylation sites on tubulin purified from mouse brain tissues. Furthermore, mass spectrometry-based quantitative comparison of acetylated peptides from wild-type vs HDAC6 knockout mice allowed us to identify six new deacetylation sites possibly mediated by HDAC6. Thus, adding new sites to the repertoire of HDAC6-mediated tubulin deacetylation events would further our understanding of the multi-faceted roles of HDAC6 in regulating microtubule stability and cellular functions.

Cytoplasmic carboxypeptidase 5 regulates tubulin glutamylation and zebrafish cilia formation and function

Glutamylation is a functionally important tubulin posttranslational modification enriched on stable microtubules of neuronal axons, mitotic spindles, centrioles, and cilia. In vertebrates, balanced activities of tubulin glutamyl ligase and cytoplasmic carboxypeptidase deglutamylase enzymes maintain organelle- and cell type-specific tubulin glutamylation patterns. Tubulin glutamylation in cilia is regulated via restricted subcellular localization or expression of tubulin glutamyl ligases (ttlls) and nonenzymatic proteins, including the zebrafish TPR repeat protein Fleer/Ift70. Here we analyze the expression patterns of ccp deglutamylase genes during zebrafish development and the effects of ccp gene knockdown on cilia formation, morphology, and tubulin glutamylation. The deglutamylases ccp2, ccp5, and ccp6 are expressed in ciliated cells, whereas ccp1 expression is restricted to the nervous system. Only ccp5 knockdown increases cilia tubulin glutamylation, induces ciliopathy phenotypes, including axis curvature, hydrocephalus, and pronephric cysts, and disrupts multicilia motility, suggesting that Ccp5 is the principal tubulin deglutamylase that maintains functional levels of cilia tubulin glutamylation. The ability of ccp5 knockdown to restore cilia tubulin glutamylation in fleer/ift70 mutants and rescue pronephric multicilia formation in both fleer- and ift88-deficient zebrafish indicates that tubulin glutamylation is a key driver of ciliogenesis.

The chemical complexity of cellular microtubules: tubulin post-translational modification enzymes and their roles in tuning microtubule functions

Cellular microtubules are marked by abundant and evolutionarily conserved post-translational modifications that have the potential to tune their functions. This review focuses on the astonishing chemical complexity introduced in the tubulin heterodimer at the post-translational level and summarizes the recent advances in identifying the enzymes responsible for these modifications and deciphering the consequences of tubulin's chemical diversity on the function of molecular motors and microtubule associated proteins.

The ins and outs of tubulin acetylation: more than just a post-translational modification?

Microtubules are highly dynamic polymers of α/β tubulin heterodimers that play key roles in cell division and in organizing cell cytoplasm. Although they have been discovered more than two decades ago, tubulin post-translational modifications recently gained a new interest as their role was increasingly highlighted in neuron differentiation and neurodegenerative disorders. Here, we specifically focus on tubulin acetylation from its discovery to recent studies that provide new insights into how it is regulated in health and disease and how it impacts microtubule functions. Even though new mechanisms involving tubulin acetylation are regularly being uncovered, the molecular links between its location inside the microtubule lumen and its regulators and effectors is still poorly understood. This review highlights the emerging roles of tubulin acetylation in multiple cellular functions, ranging from cell motility, cell cycle progression or cell differentiation to intracellular trafficking and signalling. It also points out that tubulin acetylation should no longer be seen as a passive marker of microtubule stability, but as a broad regulator of microtubule functions.

Tubulin post-translational modifications: encoding functions on the neuronal microtubule cytoskeleton

In the past decades, a range of post-translational modifications has been discovered on tubulins, the major constituents of microtubules. Pioneering studies have described the occurrence and dynamics of these modifications and provided first insights into their potential functions in regulating the microtubule cytoskeleton. By contrast, several tubulin-modifying enzymes were only discovered in the last few years, and studies on molecular mechanisms and cellular functions of tubulin modifications are just beginning to emerge. This review highlights the roles of tubulin modifications in neurons. Recent studies are also discussed in relation to how the combinatorial use of tubulin modifications could generate a dynamic microtubule code, and how such a code might regulate basic as well as higher-order neuronal functions.

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.

Regulation of microtubule dynamics by inhibition of the tubulin deacetylase HDAC6

We studied the role of a class II histone deacetylase, HDAC6, known to function as a potent alpha-tubulin deacetylase, in the regulation of microtubule dynamics. Treatment of cells with the class I and II histone deacetylase inhibitor TSA, as well as the selective HDAC6 inhibitor tubacin, increased microtubule acetylation and significantly reduced velocities of microtubule growth and shrinkage. siRNA-mediated knockdown of HDAC6 also increased microtubule acetylation but, surprisingly, had no effect on microtubule growth velocity. At the same time, HDAC6 knockdown abolished the effect of tubacin on microtubule growth, demonstrating that tubacin influences microtubule dynamics via specific inhibition of HDAC6. Thus, the physical presence of HDAC6 with impaired catalytic activity, rather than tubulin acetylation per se, is the factor responsible for the alteration of microtubule growth velocity in HDAC6 inhibitor-treated cells. In support of this notion, HDAC6 mutants bearing inactivating point mutations in either of the two catalytic domains mimicked the effect of HDAC6 inhibitors on microtubule growth velocity. In addition, HDAC6 was found to be physically associated with the microtubule end-tracking protein EB1 and a dynactin core component, Arp1, both of which accumulate at the tips of growing microtubules. We hypothesize that inhibition of HDAC6 catalytic activity may affect microtubule dynamics by promoting the interaction of HDAC6 with tubulin and/or with other microtubule regulatory proteins.

INF1 is a novel microtubule-associated formin

Formin proteins, characterized by the presence of conserved formin homology (FH) domains, play important roles in cytoskeletal regulation via their abilities to nucleate actin filament formation and to interact with multiple other proteins involved in cytoskeletal regulation. The C-terminal FH2 domain of formins is key for actin filament interactions and has been implicated in playing a role in interactions with microtubules. Inverted formin 1 (INF1) is unusual among the formin family in having the conserved FH1 and FH2 domains in its N-terminal half, with its C-terminal half being composed of a unique polypeptide sequence. In this study, we have examined a potential role for INF1 in regulating microtubule structure. INF1 associates discretely with microtubules, and this association is dependent on a novel C-terminal microtubule-binding domain. INF1 expressed in fibroblast cells induced actin stress fiber formation, coalignment of microtubules with actin filaments, and the formation of bundled, acetylated microtubules. Endogenous INF1 showed an association with acetylated microtubules, and knockdown of INF1 resulted in decreased levels of acetylated microtubules. Our data suggests a role for INF1 in microtubule modification and potentially in coordinating microtubule and F-actin structure.

Mammalian Sir2-related protein (SIRT) 2-mediated modulation of resistance to axonal degeneration in slow Wallerian degeneration mice: a crucial role of tubulin deacetylation

It has been shown that Wallerian degeneration, an anterograde degeneration of transected axons, is markedly delayed in a mutant mouse called slow Wallerian degeneration (Wld(S)). These mice also show resistance to axonal degeneration caused by microtubule depolymerizing drugs, suggesting that axonal microtubules are stabilized. Here, we have focused on tubulin acetylation, a post-translational modification associated with microtubule stability. We found that the basal level of microtubule acetylation was increased in cultured cerebellar granule cells from Wld(S) mice. Nicotinamide but not 3-aminobenzamide, an inhibitor for poly(ADP)ribose polymerase, enhanced tubulin acetylation and resistance to axonal degeneration in cultured cerebellar granule cells from wild-type (WT) mice, suggesting that mammalian Sir2-related protein (SIRT) 2, a nicotinamide adenine dinucleotide (NAD)--dependent tubulin deacetylase, could modulate resistance to axonal degeneration. Indeed, the levels of NAD and SIRT2 were decreased in the cytoplasm from Wld(S) granule cells. Moreover, SIRT2 overexpression abrogated microtubule hyperacetylation and resistance to axonal degeneration in these cells. Conversely, SIRT2 knockdown by using a lentiviral vector expressing small interfering RNA, enhanced microtubule acetylation and resistance to axonal degeneration in WT granule cells. Taken together, these results suggest that SIRT2-mediated tubulin deacetylation is involved in both microtubule hyperacetylation and resistance to axonal degeneration in Wld(S) granule cells.

Non-lethal active caspase-3 expression in Bergmann glia of postnatal rat cerebellum

Caspase-3, an apoptotic executor, has been shown in recent years to mediate non-lethal events like cellular proliferation and differentiation, primarily in studies related to non-neural tissue. In central nervous system development, the role of active caspase-3 is still unclear. We provide the first evidence for a potential new role of active (cleaved) caspase-3 in promoting differentiation of Bergmann glia. This study was predicated on the hypothesis that active caspase-3 is important for the differentiation of glia. We addressed the hypothesis through the following specific aims: (1) to establish the expression of active caspase-3 in glia; (2) to determine the developmental phenotype of the active caspase-3-expressing glia; and (3) to confirm that active caspase-3 expression is not mediating an apoptotic event. Through a temporal investigation from postnatal day 8 to 21, we observed that Bergmann glia express active caspase-3 without compromising their survival. Potential apoptotic fate of active caspase-3-positive Bergmann glia were ruled out based on immunohistochemical exclusion of phosphatidylserine exposure (Annexin V), DNA fragmentation (TUNEL), and DNA compaction (TOPRO-3). More than 90% of the active caspase-3-positive cells lacked colabeling for one of the apoptotic markers. Correlative studies using a proliferation marker Ki67 and a differentiation marker brain lipid-binding protein suggest that the expression of active caspase-3 was mostly associated with differentiating rather than proliferating Bergmann glia at all ages. Thus, this study supports the hypothesis that active caspase-3 may be regulating both differentiation of Bergmann glia by allowing the cells to exit the cell cycle and their morphogenesis.

Isolation and characterization of modified species of a mutated (Cys125 -Ala) recombinant human interleukin-2

During purification of recombinant and mutated interleukin-2 (rhIL-2A125) by reversed-phase-high-performance liquid chromatography, more and less hydrophobic fractions named MHF and LHF, respectively are discarded due to the presence of some unidentified forms of rhIL-2Ala125. Using slow and linear gradients of acetonitrile, these fractions were further purified by RP-HPLC, analyzed by automatic Edman degradation, digested with trypsin and analyzed by electrospray ionization mass spectrometry. In all fractions, partial processing of the N-terminal Met residue was observed. In the LHF the Met104 was partially oxidized as sulfoxide. Combining the selective and reversible blocking of tryptic peptides and cation-exchange chromatography, two unexpected C-terminal peptides were selectively isolated. Automatic N-terminal sequencing showed that one of these corresponded to the C-terminal peptide of rhIL-2Ala125 linked to another 11 amino acids (AANDENYALAA) and the other corresponded to the C-terminal peptide of a truncated rhIL-2Ala125 without the C-terminal threonine residue and the extension of the 11 amino acids previously mentioned. MHF contained a mixture of four species of rhIL-2A125 monoacetylated at the N-terminus and at the epsilon-amino groups of internal Lys residues: 8, 32 and 48. Cys58 was found as free cysteine and also covalently linked to Mr 69 and 77 molecules. Covalent dimers of rhIL-2A125 linked through disulfide bridges between Cys58 and Cys105 of different monomers were also found.

Increase in Specific Proteins and mRNAs Following Transient Anoxia - Aglycaemia in Rat CA1 Hippocampal Slices

Incorporation of [35S]methionine into proteins and two-dimensional gel autoradiograms was used to characterize early post-anoxia - aglycaemia protein synthesis in the CA1 area of rat hippocampal slices maintained in vitro. We have compared the effects of 3 - 4 min and 5 - 10 min insults, since the former but not the latter produces a reversible block of synaptic transmission (see companion paper). An insult of between 3 min 30 s and 4 min induces a transient increase in the labelled proteins during the first hour of reoxygenation, as compared to control. The increase in protein synthesis is conspicuous for several proteins, including actin, alpha-tubulin and heat-shock proteins (hsp70c and hsp90), as determined by immunoblotting. In the case of alpha-tubulin, we show with in situ hybridization and polymerase chain reaction procedures that the increase in protein synthesis is associated with a marked increase in the expression of the corresponding messenger RNAs. The results demonstrate that, in addition to regulatory proteins such as hsps, the synthesis of several polypeptides, including those associated with the cytoskeleton, is altered in anoxic damage.

Polyglutamylated .alpha.-tubulin can enter the tyrosination/detyrosination cycle

We have previously identified a major modification of neuronal alpha-tubulin which consists of the posttranslational addition of a varying number of glutamyl units on the gamma-carboxyl group of glutamate residue 445. This modification, called polyglutamylation, was initially found associated with detyrosinated alpha-tubulin [Eddé, B., Rossier, J., Le Caer, J.P., Desbruyères, E., Gros, F., & Denoulet, P. (1990) Science 247, 83-85]. In this report we show that a lateral chain of glutamyl units can also be present on tyrosinated alpha-tubulin. Incubation of cultured mouse brain neurons with radioactive tyrosine, in the presence of cycloheximide, resulted in a posttranslational labeling of six alpha-tubulin isoelectric variants. Because both tyrosination and polyglutamylation occur in the C-terminal region of alpha-tubulin, the structure of this region was investigated. [3H]tyrosinated tubulin was mixed with a large excess of unlabeled mouse brain tubulin and digested with thermolysin. Five peptides, detected by their radioactivity, were purified by high-performance liquid chromatography. Amino acid sequencing and mass spectrometry showed that one of these peptides corresponds to the native C-terminal part of alpha-tubulin 440VEGEGEEEGEEY451 and that the remainders bear a varying number of glutamyl units linked to glutamate residue 445, which explains the observed heterogeneity of tyrosinated alpha-tubulin. A quantitative analysis showed that the different tyrosinated forms of alpha-tubulin represent a minor (13%) fraction of the total alpha-tubulin present in the brain and that most (80%) of these tyrosinated forms are polyglutamylated.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

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.

Evidence for a mouse brain-specific variant of alpha-tubulin

While studying the neural precursor cell intermediate filament protein known as nestin in the developing mouse brain, we observed a strong cross-reaction of our nestin antibody with a 50 kDa protein that appeared on embryonic day 10 and continued to accumulate until postnatal day 1. Here we report evidence that this protein is a brain-specific variant form of alpha-tubulin and discuss its implications.

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 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.

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.

Hysteretic behavior and differential apparent stability properties of microtubule species emerge from the regulation of post-translational modifications of microtubules

At the epigenetic level, microtubule diversity is generated by several mechanisms of reversible post-translational modifications of tubulin subunits. In most cases, modification enzymes preferentially act on the tubulin subunits of microtubules, whereas the substrate of the enzymes which ensure the reverse reaction is preferentially the alpha beta-dimer of nonpolymerized tubulin. Most modifications identified to date appear to be nearly ubiquitous within the animal kingdom. Moreover, modifications are generally not mutually exclusive, so that cellular microtubules often bear several distinct biochemical alterations whose biological role is yet unknown. Post-translational modifications often (but not always) occur on microtubule species with low turnover rate. However, in vitro comparison of the polymerization and depolymerization rates of modified or unmodified forms of tubulin did not reveal any significant difference between molecular species. Thus, post-translational modifications are thought to be the result rather than the cause of microtubule stability. We re-examine this contention in the light of a regulated kinetic scheme for multiple and non-exclusive enzymatic modifications of microtubules. This study shows that different apparent stability properties of microtubule species emerge under such a kinetic regulation, although all the species were assumed to have identical intrinsic stability properties. This model can be used to reinterpret the results of well-known studies bearing on the relationship between microtubule stability and post-translational modifications. Another important finding is that the existence of a regulation loop in one of the multiple pathways of enzymatic differentiation of microtubules endows the system with hysteretic properties. These properties may be viewed, under restrictive conditions, as a buffering mechanism for the transitions between microtubule growing and shrinking phases during fluctuations in the regulation of centrosomal nucleating activity.

Expression and posttranslational modification of class III beta-tubulin during neuronal differentiation of P19 embryonal carcinoma cells

We have used a combination of immunofluorescence microscopy, northern blotting, ELISA, and isoelectric focusing to characterize the expression of neuronal Class III beta-tubulin in P19 embryonal carcinoma cells induced to differentiate along a neuronal pathway by retinoic acid. Following 48 h differentiation, beta-III tubulin mRNA is evident and beta-III tubulin appears in the mitotic spindle of neuroblasts. Neurite outgrowth is obvious by day 3, and beta-III tubulin protein and mRNA levels increase concurrently until approximately day 7, when beta-III mRNA levels begin to decrease while protein levels remain high. In addition, increasingly acidic beta-III tubulin isoforms appear during neuronal differentiation. The expression of these isoelectric variants occurs concomitant with a temporal increase in the levels of beta-III tubulin present in the colchicine-stable microtubules. These results implicate posttranslational modifications of beta-III tubulin in the increased microtubule stability noted in differentiating P19 neurons.

Tubulin post-translational modifications in the primitive protist Trichomonas vaginalis

Using several specific monoclonal antibodies, we investigated the occurrence and distribution of different post-translationally modified tubulin during interphase and division of the primitive flagellated protist Trichomonas vaginalis. Immunoblotting and immunofluorescence experiments revealed that interphasic microtubular structures of T. vaginalis contained acetylated and glutamylated but non-tyrosinated and non-glycylated [Brugerolle and Adoutte, 1988: Bio Systems 21: 255-268] tubulin. Immunofluorescence studies performed on dividing cells showed that the extranuclear mitotic spindle (or paradesmosis) was acetylated and glutamylated, which contrast with the ephemeral nature of this structure. Newly formed short axostyles also contained acetylated and glutamylated tubulin suggesting that both post-translational modifications might take place very early after assembly of microtubular structures. Our results indicate that acetylation and glutamylation of tubulin appeared early in the history of eukaryotes and could reflect the occurrence of post-translational modifications of tubulin in the primitive eukaryotic cells. These cells probably had a highly ordered cross-linked microtubular cytoskeleton in which microtubules showed a low level of subunit exchange dynamics.

Post-translational modifications of tubulin suggest that dynamic microtubules are present in sensory cells and stable microtubules are present in supporting cells of the mammalian cochlea

Post-translational modifications to tubulin in the sensory and supporting cells of the cochlea were studied using antibodies specific to the tyrosinated, detyrosinated, acetylated and polyglutamylated isoforms. In the sensory cells, microtubules which label intensely with antibodies to tyrosinated tubulin are found in networks within the cytoplasm. Microtubules which label with antibodies to detyrosinated tubulin and polyglutamylated tubulin, but not acetylated tubulin, form a small component of the microtubules found in the cytoplasm only in the region below the cuticular plate. Microtubules in the supporting cells (inner and outer pillar cells and Deiters cells) are arranged in bundles and contain little tyrosinated tubulin. They are composed instead of predominantly post-translationally modified isoforms which include detyrosinated, acetylated and polyglutamylated tubulin. The findings suggest that microtubules in the sensory cells form dynamic structures, since microtubules that undergo cyclic polymerization and depolymerization predominantly contain tubulin that has not yet had its carboxy-terminal tyrosine residue removed. The presence of microtubules in the supporting cells in which the tubulin has been polymerized into microtubules long enough to be post-translationally modified, provides evidence that these microtubules are stable, long-lived and could contribute to the structural support of the sensory organ of Corti.

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.

Acetylation of lysine 40 in alpha-tubulin is not essential in Tetrahymena thermophila

In Tetrahymena, at least 17 distinct microtubule structures are assembled from a single primary sequence type of alpha- and beta-tubulin heterodimer, precluding distinctions among microtubular systems based on tubulin primary sequence isotypes. Tetrahymena tubulins also are modified by several types of posttranslational reactions including acetylation of alpha-tubulin at lysine 40, a modification found in most eukaryotes. In Tetrahymena, axonemal alpha-tubulin and numerous other microtubules are acetylated. We completely replaced the single type of alpha-tubulin gene in the macronucleus with a version encoding arginine instead of lysine 40 and therefore cannot be acetylated at this position. No acetylated tubulin was detectable in these transformants using a monoclonal antibody specific for acetylated lysine 40. Surprisingly, mutants lacking detectable acetylated tubulin are indistinguishable from wild-type cells. Thus, acetylation of alpha-tubulin at lysine 40 is non-essential in Tetrahymena. In addition, isoelectric focusing gel analysis of axonemal tubulin from cells unable to acetylate alpha-tubulin leads us to conclude that: (a) most or all ciliary alpha-tubulin is acetylated, (b) other lysines cannot be acetylated to compensate for loss of acetylation at lysine 40, and (c) acetylated alpha-tubulin molecules in wild-type cells contain one or more additional charge-altering modifications.

Immunocytochemical comparison of posttranslationally modified forms of tubulin in the vestibular end-organs of the gerbil: tyrosinated, acetylated and polyglutamylated tubulin

Specific antibodies against alpha-tubulin, acetylated alpha-tubulin, tyrosinated alpha-tubulin and polyglutamylated alpha- and beta-tubulin were used to compare the distribution of posttranslationally modified tubulin in the vestibular end-organs of the gerbil. Antibodies to acetylated tubulin labeled a dense network of microtubules in the hair cells and bundles of microtubule in the supporting cells. Nerve fibers within and below the epithelium were weakly labeled. This localization paralleled that seen with antibodies to alpha-tubulin which labeled all microtubules present in the cells. Antibodies to tyrosinated tubulin labeled networks and bundles of microtubules in both hair cells and supporting cells and in addition gave intense, diffuse labeling in the cytoplasm of both cell types. It also labeled the nerve fibers. Antibodies to polyglutamylated tubulin were localized mainly in nerve fibers, and in the calyces the labeled microtubules were found running circumferentially around the type I sensory hair cells. Thus, tyrosinated tubulin was found in the fine networks of microtubules in both the sensory and supporting cells. Acetylated tubulin was found in the dense networks and bundles of microtubules in the sensory and supporting cells, but did not colocalize with polyglutamylated tubulin, which was found predominantly in the nerve fibers. The labeling patterns for the tyrosinated tubulin and posttranslationally modified tubulins in the sensory and supporting cells of the vestibular end organs differ from that seen in the organ of Corti and may reflect differences in the stability of the microtubules and the mechanical properties of the sensory epithelium.

Monoclonal anti-dipeptide antibodies cross-react with detyrosinated and glutamylated forms of tubulins

Two monoclonal antibodies, GLU-1 and A1.6, raised against gamma-L-glutamyl-L-glutamic acid dipeptide (Glu-Glu) and Ca(2+)-dependent ATPase from Paramecium, respectively, recognized the dipeptide Glu-Glu sequence. Whereas the antibodies immunofluorescently stained very few, if any, cytoskeletal fibers in cultured mammalian cells, almost all interphase as well as mitotic spindle microtubules became visible after treatment of cells with carboxypeptidase A. Immunoblot analysis demonstrated intense cross-reaction of the antibodies to the alpha-tubulin subunit. alpha-Tubulin isotypes produced as fusion proteins in bacteria were labeled by both the antibodies only when the proteins did not contain a tyrosine residue at the C terminus, indicating that GLU-1 and A1.6 specifically recognize the detyrosinated form of alpha-tubulin. When microtubule protein purified from brain was probed, not only alpha-but also, to a lesser extent, beta-tubulin were revealed by the dipeptide antibodies. A synthetic tripeptide YED containing one glutamyl group linked to the second residue of the peptide via the gamma position was also recognized by the antibodies. Since this peptide sequence corresponds to the amino acid sequence of polyglutamyated class III beta isotype at amino acid position 437 to 439, it is suggested that GLU-1 and A1.6 are able to recognize the glutamylated form of beta-tubulin. These results indicate that the C-terminal Glu-Glu sequence displays strong antigenicity, and the antibodies recognize the sequence present in the C terminus of the detyrosinated form of alpha-tubulin and the glutamyl side chain of beta-tubulin. Particularly strong immunoreaction was detected with ciliary and flagellar microtubules; thus, stable axonemal microtubules appear to be rich in post-translationally modified tubulin subunits.

Developmental regulation of polyglutamylated alpha- and beta-tubulin in mouse brain neurons

Polyglutamylation is an important posttranslational modification of tubulin that is very active in nerve cells, where it accounts for the main factor responsible for tubulin heterogeneity. In the present work, we have analyzed quantitative and qualitative changes in glutamylated alpha- and beta-tubulin occurring during neuronal differentiation in culture. Glutamylated alpha- and beta-tubulin both markedly accumulate during this process with a time course remarkably similar to that observed in vivo during brain development. However, the characteristics of the glutamylation of the two subunits are not exactly the same. Glutamylated alpha-tubulin is already abundant in very young neurons and displays, at this stage, a wide range of its degree of glutamylation (1 to 6 glutamyl units present in the lateral polyglutamyl chain), which remains unchanged during the entire period of the culture. Glutamylated beta-tubulin is present at very low levels in young neurons and its accumulation during differentiation is accompanied by a progressive increase in its degree of glutamylation from 2 to 6 glutamyl units. Posttranslational incorporation of [3H]glutamate into alpha- and beta-tubulin decreases during differentiation, as well as the rate of the reverse deglutamylation reaction, suggesting that accumulation of glutamylated tubulin is accompanied by a decrease in the turnover of glutamyl units onto tubulin. Neuronal differentiation is also accompanied by an increase of other posttranslationally modified forms of tubulin, including acetylated and non-tyrosinatable alpha-tubulin, which can occur in combination with polyglutamylation and contributes to increase the complexity of tubulin in mature neurons.

Isolation of Escherichia coli synthesized recombinant eukaryotic proteins that contain epsilon-N-acetyllysine

Recombinant porcine (rpST) and bovine somatotropins (rbST) synthesized in Escherichia coli contain the amino acid, epsilon-N-acetyllysine. This amino acid was initially discovered in place of the normal lysine144 in a modified reversed-phase HPLC (RP-HPLC) species of rpST. Mass spectrometry and amino acid sequencing of a tryptic peptide isolated from this RP-HPLC purified protein were used to identify this altered residue as epsilon-N-acetyllysine. Ion-exchange chromatography was utilized to prepare low isoelectric point (pI) forms of rpST and rbST, which are enriched in epsilon-N-acetyllysine. Electrospray mass spectrometry demonstrated that the majority of the protein in these low pI fractions contained species 42 Da larger than normal. Immobilized pH gradient electrophoresis (IPG) of the ion-exchange purified low pI proteins was used to isolate several monoacetylated species of rpST and rbST. The location of the acetylated lysine in each IPG-purified protein was determined by tryptic peptide mapping and amino acid sequencing of the altered tryptic peptides. Amino acid analyses of enzymatic digests of rpST and rbST were also used to confirm the presence of epsilon-N-acetyllysine in these recombinant proteins. These data demonstrate that a significant portion of rpST and rbST produced in E. coli contain this unusual amino acid.

Differential distribution of glutamylated tubulin during spermatogenesis in mammalian testis

The distribution of glutamylated tubulin has been analyzed in mammalian testis using the specific mAb GT335 by immunoelectron microscopy and immunoblotting. In spermatozoa of various species, immunogold labeling showed the presence of glutamylated tubulin in all of the microtubules of axoneme and centrioles, whereas the microtubule network of the spermatid manchette was unlabeled. In earlier germ cells, centriole was the only microtubule structure to be labeled. A similar distribution was observed using the anti-acetylated tubulin antibody (6-11B-1), confirming previous results of Hermo et al. [Anat. Rec. 229:31-50, 1991]. However, among testicular somatic cells, microtubules of some Sertoli cell branches were not acetylated but glutamylated. 2-D PAGE of mouse and hamster sperm extracts showed a high level of alpha and beta-tubulin heterogeneity, comparable to that found in brain. Immunoblotting with GT335 revealed a large amount of glutamylated tubulin resolved into numerous alpha as well as beta-tubulin isoforms. This suggests that the major testis-specific tubulin isotypes (m alpha 3/7 and m beta 3) are also glutamylatable. These results show a subcellular sorting of posttranslationally modified tubulin isoforms in spermatids, glutamylation being associated with the most stable microtubule structures.

Glutamylated tubulin probed in ciliates with the monoclonal antibody GT335

Microtubular networks are extensively developed in many ciliate species. In several of them, we investigate the occurrence of the post-translational glutamylation of tubulin [Eddé et al., 1990: Science 247:82-85; Eddé et al., 1991: J. Cell. Biochem. 46:134-142] using as a probe for such modified tubulin, the monoclonal antibody GT335 [Wolff et al., 1992: Eur. J. Cell Biol. 59:425-432]. Results obtained in Paramecium strongly suggest that both axonemal and cytoplasmic tubulin are glutamylated. As in the vertebrate brain tubulin so far tested, the GT335 epitope is located at the carboxy-terminal fragment of cytoplasmic tubulin removed by subtilisin treatment. Immunoblotting and immunofluorescence experiments reveal that, unlike tubulin acetylation, glutamylation is not restricted to cold-resistant microtubules. In addition, immunofluorescence studies performed on dividing cells show that glutamylation takes place soon after the polymerization of microtubules. Finally, glutamylated tubulin is also detected in the ciliate species Euplotes, Tetrahymena, and Paraurostyla. Together with results obtained on flagellate species, this suggests that tubulin glutamylation came out early in the course of eukaryotic evolution and has been widely exploited in various cellular strategies.

Characterization of the post-translational modifications in tubulin from the marginal band of avian erythrocytes

Tubulin purified from turkey erythrocytes was characterized by partial protein sequence data, high-resolution IEF and by its reaction with antibodies specific for certain post-translational modifications. The tubulin from the marginal band contains a single alpha and beta isotype, i.e. alpha 1 and beta 6. Partial protein sequences and immunoblotting with antibody 6-11B-1 show that erythrocyte alpha 1 tubulin is not acetylated at Lys40. The acidic carboxy-terminal peptides purified by Mono Q chromatography and reverse-phase HPLC were characterized by sequence analysis and mass spectrometry. Although erythrocyte alpha tubulin is almost completely detyrosinated it retains the penultimate glutamic acid residue, which is partially lost in brain tubulin. Thus erythrocyte tubulin is an excellent substrate for extensive in vitro tyrosination by tubulin-tyrosine ligase. Erythrocyte alpha and beta tubulin lack the side-chain polyglutamylation found in all major tubulins from adult brain. Finally we show that about 10% of the beta tubulin is phosphorylated at Ser441. Thus erythrocyte tubulin is an unusual homogeneous preparation. It contains the minimum possible number of tubulin isotypes and the only post-translational modifications detected (detyrosination and phosphorylation) are reversible.

Reversible polyglutamylation of alpha- and beta-tubulin and microtubule dynamics in mouse brain neurons

The relationship between microtubule dynamics and polyglutamylation of tubulin was investigated in young differentiating mouse brain neurons. Selective posttranslational labeling with [3H]glutamate and immunoblotting with a specific monoclonal antibody (GT335) enabled us to analyze polyglutamylation of both alpha and beta subunits. Nocodazole markedly inhibited incorporation of [3H]glutamate into alpha- and beta-tubulin, whereas taxol had no effect for alpha-tubulin and a stimulating effect for beta-tubulin. These results strongly suggest that microtubule polymers are the preferred substrate for polyglutamylation. Chase experiments revealed the existence of a reversal reaction that, in the case of alpha-tubulin, was not affected by microtubule drugs, suggesting that deglutamylation of this subunit can occur on both polymers and soluble tubulin. Evidence was obtained that deglutamylation of alpha-tubulin operates following two distinct rates depending on the length of the polyglutamyl chain, the distal units (4th-6th) being removed rapidly whereas the proximal ones (1st-3rd) appearing much more resistant to deglutamylation. Partition of glutamylated alpha-tubulin isoforms was also correlated with the length of the polyglutamyl chain. Forms bearing four to six units were recovered specifically in the polymeric fraction, whereas those bearing one to three units were distributed evenly between polymeric and soluble fractions. It thus appears that the slow rate component of the deglutamylation reaction offers to neurons the possibility to maintain a basal level of glutamylated alpha-tubulin in the soluble pool independently of microtubule dynamics. Finally, some differences observed in the glutamylation of alpha- and beta-tubulin suggest that distinct enzymes are involved.