Class I and IVa beta-tubulin isotypes expressed in adult mouse brain are glutamylated

Disruption of tubulin-alpha4a polyglutamylation prevents aggregation of hyper-phosphorylated tau and microglia activation in mice

Dissociation of hyper-phosphorylated Tau from neuronal microtubules and its pathological aggregates, are hallmarks in the etiology of tauopathies. The Tau-microtubule interface is subject to polyglutamylation, a reversible posttranslational modification, increasing negative charge at tubulin C-terminal tails. Here, we asked whether tubulin polyglutamylation may contribute to Tau pathology in vivo. Since polyglutamylases modify various proteins other than tubulin, we generated a knock-in mouse carrying gene mutations to abolish Tuba4a polyglutamylation in a substrate-specific manner. We found that Tuba4a lacking C-terminal polyglutamylation prevents the binding of Tau and GSK3 kinase to neuronal microtubules, thereby strongly reducing phospho-Tau levels. Notably, crossbreeding of the Tuba4a knock-in mouse with the hTau tauopathy model, expressing a human Tau transgene, reversed hyper-phosphorylation and oligomerization of Tau and normalized microglia activation in brain. Our data highlight tubulin polyglutamylation as a potential therapeutic strategy in fighting tauopathies.

Pathologic oligomerization of hyper-phosphorylated Tau is a hallmark of tauopathies. Here the authors show that the loss of tubulin a4 polyglutamylation reverses tau hyperphosphorylation, oligomerization and microglia activation in a tauopathy mouse.

TTLL1 and TTLL4 polyglutamylases are required for the neurodegenerative phenotypes in pcd mice

Polyglutamylation is a dynamic posttranslational modification where glutamate residues are added to substrate proteins by 8 tubulin tyrosine ligase-like (TTLL) family members (writers) and removed by the 6 member Nna1/CCP family of carboxypeptidases (erasers). Genetic disruption of polyglutamylation leading to hyperglutamylation causes neurodegenerative phenotypes in humans and animal models; the best characterized being the Purkinje cell degeneration (pcd) mouse, a mutant of the gene encoding Nna1/CCP1, the prototypic eraser. Emphasizing the functional importance of the balance between glutamate addition and elimination, loss of TTLL1 prevents Purkinje cell degeneration in pcd. However, whether Ttll1 loss protects other vulnerable neurons in pcd, or if elimination of other TTLLs provides protection is largely unknown. Here using a mouse genetic rescue strategy, we characterized the contribution of Ttll1, 4, 5, 7, or 11 to the degenerative phenotypes in cerebellum, olfactory bulb and retinae of pcd mutants. Ttll1 deficiency attenuates Purkinje cell loss and function and reduces olfactory bulb mitral cell death and retinal photoreceptor degeneration. Moreover, degeneration of photoreceptors in pcd is preceded by impaired rhodopsin trafficking to the rod outer segment and likely represents the causal defect leading to degeneration as this too is rescued by elimination of TTLL1. Although TTLLs have similar catalytic properties on model substrates and several are highly expressed in Purkinje cells (e.g. TTLL5 and 7), besides TTLL1 only TTLL4 deficiency attenuated degeneration of Purkinje and mitral cells in pcd. Additionally, TTLL4 loss partially rescued photoreceptor degeneration and impaired rhodopsin trafficking. Despite their common properties, the polyglutamylation profile changes promoted by TTLL1 and TTLL4 deficiencies in pcd mice are very different. We also report that loss of anabolic TTLL5 synergizes with loss of catabolic Nna1/CCP1 to promote photoreceptor degeneration. Finally, male infertility in pcd is not rescued by loss of any Ttll. These data provide insight into the complexity of polyglutamate homeostasis and function in vivo and potential routes to ameliorate disorders caused by disrupted polyglutamylation.

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.

Crystal structure of tubulin tyrosine ligase-like 3 reveals essential architectural elements unique to tubulin monoglycylases

Glycylation and glutamylation, the posttranslational addition of glycines and glutamates to genetically encoded glutamates in the intrinsically disordered tubulin C-terminal tails, are crucial for the biogenesis and stability of cilia and flagella and play important roles in metazoan development. Members of the diverse family of tubulin tyrosine ligase-like (TTLL) enzymes catalyze these modifications, which are part of an evolutionarily conserved and complex tubulin code that regulates microtubule interactions with cellular effectors. The site specificity of TTLL enzymes and their biochemical interplay remain largely unknown. Here, we report an in vitro characterization of a tubulin glycylase. We show that TTLL3 glycylates the β-tubulin tail at four sites in a hierarchical order and that TTLL3 and the glutamylase TTLL7 compete for overlapping sites on the tubulin tail, providing a molecular basis for the anticorrelation between glutamylation and glycylation observed in axonemes. This anticorrelation demonstrates how a combinatorial tubulin code written in two different posttranslational modifications can arise through the activities of related but distinct TTLL enzymes. To elucidate what structural elements differentiate TTLL glycylases from glutamylases, with which they share the common TTL scaffold, we determined the TTLL3 X-ray structure at 2.3-Å resolution. This structure reveals two architectural elements unique to glycyl initiases and critical for their activity. Thus, our work sheds light on the structural and functional diversification of TTLL enzymes, and constitutes an initial important step toward understanding how the tubulin code is written through the intersection of activities of multiple TTLL enzymes.

Evidence for new C-terminally truncated variants of α- and β-tubulins

New C-terminally truncated α- and β-tubulin variants, both ending with an –EEEG sequence, are identified in vivo: αΔ3-tubulin, which has a specific neuronal distribution pattern (distinct from that of αΔ2-tubulin) and seems to be related to dynamic microtubules, and βΔ4-tubulin, corresponding to β2A/B-tubulin modified by truncation of four C-terminal residues, which is ubiquitously present in cells and tissues.

Cellular α-tubulin can bear various carboxy-terminal sequences: full-length tubulin arising from gene neosynthesis is tyrosinated, and two truncated variants, corresponding to detyrosinated and Δ2 α‑tubulin, result from the sequential cleavage of one or two C-terminal residues, respectively. Here, by using a novel antibody named 3EG that is highly specific to the –EEEG C-terminal sequence, we demonstrate the occurrence in neuronal tissues of a new αΔ3‑tubulin variant corresponding to α1A/B‑tubulin deleted of its last three residues (EEY). αΔ3‑tubulin has a specific distribution pattern: its quantity in the brain is similar to that of αΔ2-tubulin around birth but is much lower in adult tissue. This truncated α1A/B-tubulin variant can be generated from αΔ2-tubulin by the deglutamylases CCP1, CCP4, CCP5, and CCP6 but not by CCP2 and CCP3. Moreover, using 3EG antibody, we identify a C‑terminally truncated β-tubulin form with the same –EEEG C-terminal sequence. Using mass spectrometry, we demonstrate that β2A/B-tubulin is modified by truncation of the four C-terminal residues (EDEA). We show that this newly identified βΔ4-tubulin is ubiquitously present in cells and tissues and that its level is constant throughout the cell cycle. These new C-terminally truncated α- and β-tubulin variants, both ending with –EEEG sequence, are expected to regulate microtubule physiology. Of interest, the αΔ3-tubulin seems to be related to dynamic microtubules, resembling tyrosinated-tubulin rather than the other truncated variants, and may have critical function(s) in neuronal development.

The Altered Hepatic Tubulin Code in Alcoholic Liver Disease

The molecular mechanisms that lead to the progression of alcoholic liver disease have been actively examined for decades. Because the hepatic microtubule cytoskeleton supports innumerable cellular processes, it has been the focus of many such mechanistic studies. It has long been appreciated that α-tubulin is a major target for modification by highly reactive ethanol metabolites and reactive oxygen species. It is also now apparent that alcohol exposure induces post-translational modifications that are part of the natural repertoire, mainly acetylation. In this review, the modifications of the "tubulin code" are described as well as those adducts by ethanol metabolites. The potential cellular consequences of microtubule modification are described with a focus on alcohol-induced defects in protein trafficking and enhanced steatosis. Possible mechanisms that can explain hepatic dysfunction are described and how this relates to the onset of liver injury is discussed. Finally, we propose that agents that alter the cellular acetylation state may represent a novel therapeutic strategy for treating liver disease.

The tubulin code: molecular components, readout mechanisms, and functions

Microtubules are cytoskeletal filaments that are dynamically assembled from α/β-tubulin heterodimers. The primary sequence and structure of the tubulin proteins and, consequently, the properties and architecture of microtubules are highly conserved in eukaryotes. Despite this conservation, tubulin is subject to heterogeneity that is generated in two ways: by the expression of different tubulin isotypes and by posttranslational modifications (PTMs). Identifying the mechanisms that generate and control tubulin heterogeneity and how this heterogeneity affects microtubule function are long-standing goals in the field. Recent work on tubulin PTMs has shed light on how these modifications could contribute to a “tubulin code” that coordinates the complex functions of microtubules in cells.

Structural basis of tubulin tyrosination by tubulin tyrosine ligase

Structural analysis of a complex of tubulin and tubulin tyrosine ligase (TTL) reveals insights into TTL’s enzymatic mechanism, how it discriminates between α- and β-tubulin, and its possible evolutionary origin.

Tubulin tyrosine ligase (TTL) catalyzes the post-translational retyrosination of detyrosinated α-tubulin. Despite the indispensable role of TTL in cell and organism development, its molecular mechanism of action is poorly understood. By solving crystal structures of TTL in complex with tubulin, we here demonstrate that TTL binds to the α and β subunits of tubulin and recognizes the curved conformation of the dimer. Biochemical and cellular assays revealed that specific tubulin dimer recognition controls the activity of the enzyme, and as a consequence, neuronal development. The TTL–tubulin structure further illustrates how the enzyme binds the functionally crucial C-terminal tail sequence of α-tubulin and how this interaction catalyzes the tyrosination reaction. It also reveals how TTL discriminates between α- and β-tubulin, and between different post-translationally modified forms of α-tubulin. Together, our data suggest that TTL has specifically evolved to recognize and modify tubulin, thus highlighting a fundamental role of the evolutionary conserved tubulin tyrosination cycle in regulating the microtubule cytoskeleton.

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.

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.

Tubulin targets in the pathobiology and therapy of glioblastoma multiforme. I. Class III beta-tubulin

Glioblastoma multiforme (GBM) is the most common and deadliest form of primary brain cancer in adults. Despite advances in molecular biology and genetics of gliomas currently there is no effective treatment or promising molecularly targeted experimental therapeutic strategies for these tumors. In previous studies we have shown aberrant overexpression of the class III beta-tubulin isotype (betaIII-tubulin) in GBM and have proposed that this change may reflect perturbations in microtubule dynamics associated with glioma tumorigenesis, tumor progression and malignant transformation into GBM. This minireview focuses on microtubules and tubulin as emerging targets in potential therapy of GBM using a new class of betaIII-tubulin-targeted drugs in the light of recent developments concerning the function and potential role of this isotype in clinically aggressive tumor behavior, cancer stem cells, tumor hypoxia and chemoresistance to tubulin binding agents, principally taxanes.

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

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.

Coordination of posttranslational modifications of bovine brain alpha-tubulin. Polyglycylation of delta2 tubulin

Microtubules participate in a large number of intracellular events including cell division, intracellular transport and secretion, axonal transport, and maintenance of cell morphology. They are composed of tubulin, a heterodimeric protein, consisting of two similar polypeptides alpha and beta. In mammalian cells, both alpha- and beta-tubulin occur as seven to eight different genetic variants, which also undergo numerous posttranslational modifications that include tyrosination-detyrosination and deglutamylation, phosphorylation, acetylation, polyglutamylation, and polyglycylation. Tyrosination-detyrosination is one of the major posttranslational modifications in which the C-terminal tyrosine residue in alpha-tubulin is added or removed reversibly. Although this modification does not alter the assembly activity of tubulin in vitro, these two forms of tubulin have been found to be distributed differently in vivo and are also correlated with microtubule stability (Gunderson, G. G., Kalnoski, M. H., and Bulinski, J. C. (1984) Cell 38, 779-789). Thus, the question arises as to whether these two forms of tubulin differ in any other modifications. In an effort to answer this question, the tyrosinated and the nontyrosinated forms of the alpha1/2 isoform have been purified from brain tubulin by immunoaffinity chromatography. matrix-assisted laser desorption/ionization-time of flight mass spectrometric analysis of the C-terminal peptide revealed that the tyrosinated form is polyglutamylated with one to four Glu residues, while the Delta2 tubulin is polyglycylated with one to three Gly residues. These results indicate that posttranslational modifications of tubulin are correlated with each other and that polyglutamylation and polyglycylation of tubulin may have important roles in regulating microtubule assembly, stability, and function in vivo.

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.

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.

Purification and characterization of basal apparatuses from a flagellate green alga

Basal apparatuses consisting of two basal bodies and several attached fibers were isolated from the naked green flagellate Spermatozopsis similis by detergent extraction and mechanical disintegration. Sucrose density centrifugation yielded highly enriched basal apparatuses as shown by electron microscopy. SDS-PAGE revealed the absence of histones, indicating the removal of nuclear contaminations from the isolated basal apparatuses. A mass spectrometric analysis of the carboxyterminal peptides of alpha tubulin documented detyrosination and glutamylation as posttranslational modifications and showed that some 5% of the alpha tubulin carries a polyglutamyl side chain which can reach at least 17 residues in length. Monoclonal antibodies raised against the purified basal apparatuses were used to characterize novel components in the basal apparatus. A 210-kD component identified by mAB BAS (basal apparatus of Spermatozopsis) 1.4 was localized in the flagellar transitional region by immunogold electron microscopy. Antibody BAS 16.4 reacted with two high molecular weight bands (approximately 265 and 240 kD) in Western blotting and decorated a fiber attached to the proximal end of the basal bodies. Immunofluorescence staining of isolated cytoskeletons with these mABs demonstrated that the antigens are also present in the basal apparatuses of Chlamydomonas reinhardtii and Dunahella bioculata. These antibodies are useful tools for the molecular cloning of components from the basal apparatus.

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 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.

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.

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.

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.

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.

The A and B tubules of the outer doublets of sea urchin sperm axonemes are composed of different tubulin variants

The alpha beta-tubulin heterodimer, the structural unit of microtubules, comes in many variants. There are different alpha and beta isotypes encoded by multigene families. Additional heterogeneity is generated by a set of posttranslational modifications. Detyrosination of alpha-tubulin, removal of the carboxy-terminal Glu-Tyr dipeptide of alpha-tubulin, phosphorylation of some tubulins, polyglutamylation, and polyglycylation of alpha- and beta-tubulins all involve the acidic carboxy-terminal region. We have investigated the distribution of tubulin variants in the axonemal microtubules of sea urchin sperm flagella by immunological procedures and by direct sequence and mass spectrometric analysis of the carboxy-terminal peptides. The A and B tubules that comprise the nine outer doublets differ strongly in tubulin variants. A tubules contain over 95% unmodified, tyrosinated alpha beta-tubulin. In B tubules, alpha-tubulin is approximately 65% detyrosinated and both alpha- and beta-tubulin are 40-45% polyglycylated. These results show a segregation of tubulin variants between two different axonemal structures and raise the possibility that posttranslational modifications of tubulins reflect or specify structurally and functionally distinct microtubules.

Posttranslational modifications in the C-terminal tail of axonemal tubulin from sea urchin sperm

After proteolytic digestion of sperm tubulin from sea urchin Paracentrotus lividus, C-terminal peptides were isolated by chromatographic separations. The peptides were analyzed by Edman degradation and matrix-assisted laser desorption/ionization-time of flight mass spectrometry. About 70% of the isolated C-terminal peptides were unmodified. The remaining modified peptides have undergone a combination of numerous posttranslational modifications generating significant heterogeneity of sperm tubulin. alpha-Tubulin is modified by detyrosylation, release of the penultimate glutamate, polyglutamylation, and polyglycylation. Glycylation and glutamylation can coexist within one alpha-tubulin isoform. beta-Tubulin undergoes polyglycylation but was not observed to be polyglutamylated. The number of units posttranslationally added reaches 11 and 12 glycyl units on beta- and alpha-tubulin, respectively. This is different from the polyglycylation of axonemal tubulin in Paramecium cilia where up to 40 added glycyl units were observed both on alpha- and beta-tubulin.

Phosphorylation of beta III-tubulin

There is considerable evidence that mammalian beta-tubulin is phosphorylated. Specifically, of the seven beta isotypes, the phosphorylated one is beta III, the isotype found almost entirely in neurons. The phosphate is added at a serine and perhaps a tyrosine near the C-terminus. All the evidence to date has been gathered by growth of cells and tissues in the presence of radioactive inorganic phosphate followed by tubulin isolation and determination of the labeled tubulin; thus, the actual extent of phosphorylation of beta III is unknown. Nor is it known if alpha-tubulin and the other beta isotypes are phosphorylated by a mechanism which would not be revealed by previous experiments. In addition, the role of tubulin phosphorylation is unknown. We have purified the alpha beta II-, alpha beta III-, and alpha beta IV-tubulin dimers from bovine brain and have determined their phosphate content chemically. We have found that alpha-tubulin is not phosphorylated and neither are the beta II or beta IV isotypes. However, beta III is phosphorylated with a stoichiometry of about 1.52 mol/mol. We have found that the phosphate on beta III is resistant to a wide variety of phosphatases except for human erythrocyte phosphatase 2A and that removal of the phosphate inhibits microtubule assembly in vitro stimulated by microtubule-associated protein 2 (MAP 2). However such an inhibition was not evident when microtubule assembly was induced in the absence of microtubule-associated proteins. Our results suggest the possibility that beta III phosphorylation may play a role in regulating microtubule assembly in vivo.

Biology of the congenitally hypothyroid hyt/hyt mouse

The hyt/hyt mouse has an autosomal recessive, fetal onset, characterized by severe hypothyroidism that persists throughout life and is a reliable model of human sporadic congenital hypothyroidism. The hypothyroidism in the hyt/hyt mouse reflects the hyporesponsiveness of the thyroid gland to thyrotropin (TSH). This is attributable to a point mutation of C to T at nucleotide position 1666, resulting in the replacement of a Pro with Leu at position 556 in transmembrane domain IV of the G protein-linked TSH receptor. This mutation leads to a reduction in all cAMP-regulated events, including thyroid hormone synthesis. The diminution in T3/T4 in serum and other organs, including the brain, also leads to alterations in the level and timing of expression of critical brain molecules, i.e. selected tubulin isoforms (M beta 5, M beta 2, and M alpha 1), microtubule associated proteins (MAPs), and myelin basic protein, as well as to changes in important neuronal cytoskeletal events, i.e. microtubule assembly and SCa and SCb axonal transport. In the hyt/hyt mouse, fetal hypothyroidism leads to reductions in M beta 5, M beta 2, and M alpha 1 mRNAs, important tubulin isoforms, and M beta 5 and M beta 2 proteins, which comprise the microtubules. These molecules are localized to layer V pyramidal neurons in the sensorimotor cortex, a site of differentiating neurons, as well as a site for localization of specific thyroid hormone receptors. These molecular abnormalities in specific cells and at specific times of development or maturation may contribute to the observed neuroanatomical abnormalities, i.e. altered neuronal process growth and maintenance, synaptogenesis, and myelination, in hypothyroid brain. Abnormal neuroanatomical development in selected brain regions may be the factor underlying the abnormalities in reflexive, locomotor, and adaptive behavior seen in the hyt/hyt mouse and other hypothyroid animals.

Monoclonal and polyclonal antibodies detect a new type of post-translational modification of axonemal tubulin

Polyclonal (PAT) and monoclonal (AXO 49) antibodies against Paramecium axonemal tubulin were used as probes to reveal tubulin heterogeneity. The location, the nature and the subcellular distribution of the epitopes recognized by these antibodies were, respectively, determined by means of: (i) immunoblotting on peptide maps of Paramecium, sea urchin and quail axonemal tubulins; (ii) immunoblotting on ciliate tubulin fusion peptides generated in E. coli to discriminate antibodies directed against sequential epitopes (reactive) from post-translational ones (non reactive); and (iii) immunofluorescence on Paramecium cells, using throughout an array of antibodies directed against tubulin sequences and post-translational modifications as references. AXO 49 monoclonal antibody and PAT serum were both shown to recognize epitopes located near the carboxyl-terminal end of both subunits of Paramecium axonemal tubulin, whereas the latter recognized additional epitopes in alpha-tubulin; AXO 49 and a fraction of the PAT serum proved to be unreactive over fusion proteins; both PAT and AXO 49 labelled a restricted population of very stable microtubules in Paramecium, consisting of axonemal and cortical ones, and their reactivity was sequentially detected following microtubule assembly; finally, both antibodies stained two upward spread bands in Paramecium axonemal tubulin separated by SDS-PAGE, indicating the recognition of various alpha- and beta-tubulin isoforms displaying different apparent molecular masses. These data, taken as a whole, definitely establish that PAT and AXO 49 recognize a post-translational modification occurring in axonemal microtubules of protozoa as of metazoa. This modification appears to be distinct from the previously known ones, and all the presently available evidence indicates that it corresponds to the very recently discovered polyglycylation of Paramecium axonemal alpha- and beta-tubulin.

Beta tubulin of bull sperm is polyglycylated

Carboxy-terminal fragments of alpha and beta tubulin from bull sperm were isolated and characterized by automated sequencing and mass spectrometry. About 60% of sperm alpha tubulin is polyglycylated. The lateral chain, which can reach 13 residues in length, is covalently attached via an isopeptide bond. The fully detyrosinated sperm alpha tubulin lacks polyglycylation. Thus mammalian sperm microtubules differ from the ciliary axonemal microtubules of the protozoan Paramecium for which others have documented a complete polyglycylation of both alpha and beta tubulin.

Polyglycylation of tubulin: a posttranslational modification in axonemal microtubules

A posttranslational modification was detected in the carboxyl-terminal region of axonemal tubulin from Paramecium. Tubulin carboxyl-terminal peptides were isolated and analyzed by Edman degradation sequencing, mass spectrometry, and amino acid analysis. All of the peptides, derived from both alpha and beta tubulin subunits, were modified by polyglycylation, containing up to 34 glycyl units covalently bound to the gamma carboxyl group of glutamyl residues. This modification, present in one of the most stable microtubular systems, may influence microtubule stability or axoneme function, or both.