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

The detyrosination/re-tyrosination cycle of tubulin and its role and dysfunction in neurons and cardiomyocytes

Among the variety of post-translational modifications to which microtubules are subjected, the detyrosination/re-tyrosination cycle is specific to tubulin. It is conserved by evolution and characterized by the enzymatic removal and re-addition of a gene-encoded tyrosine residue at the C-terminus of α-tubulin. Detyrosinated tubulin can be further converted to Δ2-tubulin by the removal of an additional C-terminal glutamate residue. Detyrosinated and Δ2-tubulin are carried by stable microtubules whereas tyrosinated microtubules are present on dynamic polymers. The cycle regulates trafficking of many cargo transporting molecular motors and is linked to the microtubule dynamics via regulation of microtubule interactions with specific cellular effectors such as kinesin-13. Here, we give an historical overview of the general features discovered for the cycle. We highlight the recent progress toward structure and functioning of the enzymes that keep the levels of tyrosinated and detyrosinated tubulin in cells, the long-known tubulin tyrosine ligase and the recently discovered vasohibin-SVBP complexes. We further describe how the cycle controls microtubule functions in healthy neurons and cardiomyocytes and how deregulations of the cycle are involved in dysfunctions of these highly differentiated cells, leading to neurodegeneration and heart failure in humans.

Mechano-induced cell metabolism promotes microtubule glutamylation to force metastasis

Mechanical signals from the tumor microenvironment modulate cell mechanics and influence cell metabolism to promote cancer aggressiveness. Cells withstand external forces by adjusting the stiffness of their cytoskeleton. Microtubules (MTs) act as compression-bearing elements. Yet how cancer cells regulate MT dynamic in response to the locally constrained environment has remained unclear. Using breast cancer as a model of a disease in which mechanical signaling promotes disease progression, we show that matrix stiffening rewires glutamine metabolism to promote MT glutamylation and force MT stabilization, thereby promoting cell invasion. Pharmacologic inhibition of glutamine metabolism decreased MT glutamylation and affected their mechanical stabilization. Similarly, decreased MT glutamylation by overexpressing tubulin mutants lacking glutamylation site(s) decreased MT stability, thereby hampering cancer aggressiveness in vitro and in vivo. Together, our results decipher part of the enigmatic tubulin code that coordinates the fine-tunable properties of MT and link cell metabolism to MT dynamics and cancer aggressiveness.

Insights of the tubulin code in gametes and embryos: from basic research to potential clinical applications in humans†

Microtubules are intracellular filaments that define in space and in time a large number of essential cellular functions such as cell division, morphology and motility, intracellular transport and flagella and cilia assembly. They are therefore essential for spermatozoon and oocyte maturation and function, and for embryo development. The dynamic and functional properties of the microtubules are in large part defined by various classes of interacting proteins including MAPs (microtubule associated proteins), microtubule-dependent motors, and severing and modifying enzymes. Multiple mechanisms regulate these interactions. One of them is defined by the high diversity of the microtubules themselves generated by the combination of different tubulin isotypes and by several tubulin post-translational modifications (PTMs). This generates a so-called tubulin code that finely regulates the specific set of proteins that associates with a given microtubule thereby defining the properties and functions of the network. Here we provide an in depth review of the current knowledge on the tubulin isotypes and PTMs in spermatozoa, oocytes, and preimplantation embryos in various model systems and in the human species. We focus on functional implications of the tubulin code for cytoskeletal function, particularly in the field of human reproduction and development, with special emphasis on gamete quality and infertility. Finally, we discuss some of the knowledge gaps and propose future research directions.

Metabolite sensing and signaling in cell metabolism

Metabolite sensing is one of the most fundamental biological processes. During evolution, multilayered mechanisms developed to sense fluctuations in a wide spectrum of metabolites, including nutrients, to coordinate cellular metabolism and biological networks. To date, AMPK and mTOR signaling are among the best-understood metabolite-sensing and signaling pathways. Here, we propose a sensor-transducer-effector model to describe known mechanisms of metabolite sensing and signaling. We define a metabolite sensor by its specificity, dynamicity, and functionality. We group the actions of metabolite sensing into three different modes: metabolite sensor-mediated signaling, metabolite-sensing module, and sensing by conjugating. With these modes of action, we provide a systematic view of how cells sense sugars, lipids, amino acids, and metabolic intermediates. In the future perspective, we suggest a systematic screen of metabolite-sensing macromolecules, high-throughput discovery of biomacromolecule-metabolite interactomes, and functional metabolomics to advance our knowledge of metabolite sensing and signaling. Most importantly, targeting metabolite sensing holds great promise in therapeutic intervention of metabolic diseases and in improving healthy aging.

A simple, three-part model provides a systematic view of how cells sense sugars, lipids, amino acids and metabolic intermediates. Cells quickly and accurately perceive changes in intra- and extracellular molecules such as nutrients to respond to changing environments. Drawing on existing knowledge about AMPK and MTORC1 signaling, Yi-Ping Wang and Qun-Ying Lei at Fudan University in Shanghai propose a model in which three components: a sensor, transducer and effector enable metabolic sensing and signaling to proceed. The sensor detects the metabolite, and, through conjugation, conformational changes or protein–protein interactions, transmits this information to the transducer, which decides the appropriate response. The transducer then issues orders to effector proteins which coordinate the action. The future identification of novel metabolic sensors through systematic screening could lead to new therapeutic interventions for metabolic and age-related diseases.

Role of Cytosolic Carboxypeptidase 5 in Neuronal Survival and Spermatogenesis

Proteins may undergo a type of posttranslational modification – polyglutamylation, where a glutamate residue is enzymatically linked to the γ-carboxyl group of a glutamate in the primary sequence of proteins and additional glutamates are then sequentially added via α-carboxyl–linkages to the growing glutamate side chain. Nna1 (a.k.a. CCP1) defines the 6-member cytosolic carboxypeptidase (CCP) family that metabolizes polyglutamate side chain and its loss results in neurodegeneration and male infertility. Whereas most CCPs catalyze hydrolysis of α-carboxyl-linked glutamates, CCP5 uniquely metabolizes the γ-carboxyl linked, branch point glutamate. Using purified recombinant mouse CCP5, we confirmed that it metabolized γ-carboxyl-linked glutamate of synthetic substrates and tubulin. Despite this unique feature and its indispensible functions in lower species, we found that unlike Nna1, CCP5 is not essential for neuronal survival in mouse. CCP5 deficiency does cause male infertility. However, the mechanism by which this occurs is distinct from that of Nna1 loss. Instead, it is phenotypically reminiscent of the infertility of olt mice. Our findings suggest that Nna1 and CCP5 do not work coordinately in the same pathway in either the nervous system or spermatogenesis. This is the first study addressing the function of CCP5 in mammals.

Graded Control of Microtubule Severing by Tubulin Glutamylation

Microtubule-severing enzymes are critical for the biogenesis and maintenance of complex microtubule arrays in axons, spindles, and cilia where tubulin detyrosination, acetylation, and glutamylation are abundant. These modifications exhibit stereotyped patterns suggesting spatial and temporal control of microtubule functions. Using human-engineered and differentially modified microtubules we find that glutamylation is the main regulator of the hereditary spastic paraplegia microtubule severing enzyme spastin. Glutamylation acts as a rheostat and tunes microtubule severing as a function of glutamate number added per tubulin. Unexpectedly, glutamylation is a non-linear biphasic tuner and becomes inhibitory beyond a threshold. Furthermore, the inhibitory effect of localized glutamylation propagates across neighboring microtubules, modulating severing in trans. Our work provides the first quantitative evidence for a graded response to a tubulin posttranslational modification and a biochemical link between tubulin glutamylation and complex architectures of microtubule arrays such as those in neurons where spastin deficiency causes disease.

Microtubule Dynamics in Neuronal Development, Plasticity, and Neurodegeneration

Neurons are the basic information-processing units of the nervous system. In fulfilling their task, they establish a structural polarity with an axon that can be over a meter long and dendrites with a complex arbor, which can harbor ten-thousands of spines. Microtubules and their associated proteins play important roles during the development of neuronal morphology, the plasticity of neurons, and neurodegenerative processes. They are dynamic structures, which can quickly adapt to changes in the environment and establish a structural scaffold with high local variations in composition and stability. This review presents a comprehensive overview about the role of microtubules and their dynamic behavior during the formation and maturation of processes and spines in the healthy brain, during aging and under neurodegenerative conditions. The review ends with a discussion of microtubule-targeted therapies as a perspective for the supportive treatment of neurodegenerative disorders.

Comparison of the enzymatic and functional properties of three cytosolic carboxypeptidase family members

Nna1 (CCP1) defines a subfamily of M14 metallocarboxypeptidases (CCP1-6) and is mutated in pcd (Purkinje cell degeneration) mice. Nna1, CCP4, and CCP6 are involved in the post-translational process of polyglutamylation, where they catalyze the removal of polyglutamate side chains. However, it is unknown whether these three cytosolic carboxypeptidases share identical enzymatic properties and redundant biological functions. We show that like Nna1, purified recombinant CCP4 and CCP6 deglutamylate tubulin, but unlike Nna1, neither rescues Purkinje cell degeneration in pcd mice, indicating that they do not have identical functions. Using biotin-based synthetic substrates, we established that the three enzymes are distinguishable based upon individual preferences for glutamate chain length, the amino acid immediately adjacent to the glutamate chain, and whether their activity is enhanced by nearby acidic amino acids. Nna1 and CCP4 remove the C-terminal glutamate from substrates with two or more glutamates, whereas CCP6 requires four or more glutamates. CCP4 behaves as a promiscuous glutamase, with little preference for chain length or neighboring amino acid composition. Besides glutamate chain length dependence, Nna1 and CCP6 exhibit higher k(cat)/K(m) when substrates contain nearby acidic amino acids. All cytosolic carboxypeptidases exhibit a monoglutamase activity when aspartic acid precedes a single glutamate, which, together with their other individual preferences for flanking amino acids, greatly increases the potential substrates for these enzymes and the biological processes in which they act. Additionally, Nna1 metabolized substrates mimicking the C terminus of tubulin in a way suggesting that the tyrosinated form of tubulin will accumulate in pcd mice.

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.

A structural and functional analysis of Nna1 in Purkinje cell degeneration (pcd) mice

The axotomy-inducible enzyme Nna1 defines a subfamily of M14 metallocarboxypeptidases, and its mutation underlies the Purkinje cell degeneration (pcd) mouse. However, the relationship among its catalytic activity, substrate specificities, and the critical processes of neurodegeneration/axon regeneration is incompletely understood. Here we used a transgenic rescue strategy targeting expression of modified forms of Nna1 to Purkinje cells in pcd mice to determine structure-activity relationships for neuronal survival and in parallel characterized the enzymatic properties of purified recombinant Nna1. The Nna1 subfamily uniquely shares conserved substrate-determining residues with aspartoacylase that, when mutated, cause Canavan disease. Homologous mutations (D1007E and R1078E) inactivate Nna1 in vivo, as does mutation of its catalytic glutamate (E1094A), which implies that metabolism of acidic substrates is essential for neuronal survival. Consistent with reports that Nna1 is a tubulin glutamylase, recombinant Nna1-but not the catalytic mutants-removes glutamate from tubulin. Recombinant Nna1 metabolizes synthetic substrates with 2 or more C-terminal glutamate (but not aspartate) residues (V(max) for 3 glutamates is ∼7-fold higher than 2 glutamates although K(M) is similar). Catalysis is not ATP/GTP dependent, and mutating the ATP/GTP binding site of Nna1 has no effect in vivo. Nna1 is a monomeric enzyme essential for neuronal survival through hydrolysis of polyglutamate-containing substrates.

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.

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.

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

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

Polyglutamylation of nucleosome assembly proteins

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

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

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

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

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

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.

Tubulin tyrosine ligase: protein and mRNA expression in developing rat skeletal muscle

Alpha tubulin can be post-translationally tyrosinated at the carboxy-terminus by a specific enzyme: tubulin tyrosine ligase. The expression of tubulin tyrosine ligase mRNA and protein during the development of rat skeletal muscle was examined in the present study. A portion of the coding region of the rat ligase cDNA was isolated and sequenced. The nucleotide and amino acid sequences showed about 90% homology with previously reported porcine and bovine ligase sequences. In newborn rats, ligase mRNA and protein were highly expressed in skeletal muscle. During early postnatal development, however, both ligase mRNA and protein dropped down dramatically. Quantitative measurements revealed that ligase protein at postnatal day 20 represented only 10% or less of the level at postnatal day 1. Ligase mRNA expression was also examined during the myogenesis in vitro. A strong ligase mRNA signal was detected in both undifferentiated myoblasts and cross-striated, contractile myotubes. The present results suggest that, during muscle differentiation, ligase function may be regulated by the amount of available mRNA. The discrepancy in the ligase expression between the in vivo and in vitro myogenesis suggests that factors controlling the levels of mRNA in vivo are lost in vitro.

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.

Post-translational modifications and multiple tubulin isoforms in Nicotiana tabacum L. cells

Distribution of post-translationally modified tubulins in cells of Nicotiana tabacum L. was analysed using a panel of specific antibodies. Polyglutamylated, tyrosinated, nontyrosinated, acetylated and delta 2-tubulin variants were detected on alpha-tubulin subunits; polyglutamylation was also found on beta-tubulin subunits. Modified tubulins were detected by immunofluorescence microscopy in interphase microtubules, preprophase bands, mitotic spindles as well as in phragmoplasts. They were, however, located differently in the various microtubule structures. The antibodies against tyrosinated, acetylated and polyglutamylated tubulins gave uniform staining along all microtubules, while antibodies against nontyrosinated and delta 2-tubulin provided dot-like staining of interphase microtubules. Additionally, immunoreactivity of antibodies against acetylated and delta 2-tubulins was strong in the pole regions of mitotic spindles. High-resolution isoelectric focusing revealed 22 tubulin charge variants in N. tabacum suspension cells. Immunoblotting with antibodies TU-01 and TU-06 against conserved antigenic determinants of alpha- and beta-tubulin molecules, respectively, revealed that 11 isoforms belonged to the alpha-subunit and 11 isoforms to the beta-subunit. Whereas antibodies against polyglutamylated, tyrosinated and acetylated tubulins reacted with several alpha-tubulin isoforms, antibodies against nontyrosinated and delta 2-tubulin reacted with only one. The combined data demonstrate that plant tubulin is extensively post-translationally modified and that these modifications participate in the generation of plant tubulin polymorphism.

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.

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 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.

The carboxy-terminal peptide of detyrosinated alpha tubulin provides a minimal system to study the substrate specificity of tubulin-tyrosine ligase

The ATP-dependent tubulin-tyrosine ligase (TTL) restores the carboxy-terminal tyrosine of alpha tubulin in alpha beta tubulin that has been previously detyrosinated. Here we show that the carboxy-terminal tetradecapeptide of detyrosinated alpha tubulin is used by TTL as a substrate, albeit at 50-fold lower efficiency than alpha beta tubulin. The minimal system provided by the TTL/peptide combination mirrors the TTL/tubulin system in all aspects tested, and shows a pronounced substrate inhibition. Synthetic peptides varying in length and/or containing single amino acid replacements were used to analyze the TTL specificity for the carboxy-terminal sequence of detyrosinated alpha tubulin. Peptides ending like alpha tubulin with the sequence Gly-Glu-Glu are optimally tyrosinated once a peptide length of 12 residues is reached. Position -1 of this recognition sequence, to which the tyrosine is added, must be glutamic acid. Position -2 accepts only an acidic amino acid but glutamic acid is by far preferred over aspartic acid. These results explain why a subpopulation of brain alpha tubulin, which ends with the sequence Gly-Glu, is not tyrosinated by TTL. The carboxy-terminal dodecapeptide of brain alpha tubulin with its polyglutamyl side-chain on position -6 shows the same substrate activity as the corresponding synthetic peptide lacking the side-chain. We discuss the substrate specificity of TTL for different alpha tubulins and speculate why tubulin is a better substrate than the optimal peptide covering the carboxy-terminal of detyrosinated alpha tubulin.

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.

Tyrosinatable and non-tyrosinatable tubulin subpopulations in rat muscle in comparison with those in brain

Using immunobinding and enzymatic assays we determined in rat muscle extracts the proportion of tyrosinatable tubulin, that is, tubulin that participates in the tyrosination/detyrosination cycle. We found that in muscle, in contrast with nervous tissue, practically all tubulin molecules are tyrosinatable. In the case of rat brain the non-tyrosinatable tubulin pool accounts for about 50% of the tubulin. In addition, isolectrofocusing of 14C-tyrosinated tubulin from brain and muscle extracts revealed a different composition in tyrosinatable tubulin isotypes. One of the isotypes, which in muscle accounts for 86% of the 14C-tyrosinated tubulin species, was detyrosinated by the action of tubulin carboxypeptidase faster than the rest of the 14C-tyrosinated tubulin isotypes taken in whole. In the case of brain extract, that isotype accounts for only 16% of the labeled tubulin.

Mass spectrometric analysis of proteins

The past year has seen greatly increased acceptance and application of the analytical capabilities of mass spectrometry by the biochemical community. The technique has been used to provide accurate mass determinations of non-covalently bound protein complexes, rapid mapping of molecular weights of altered peptides in protease digests, sequencing by collisional activation in tandem mass spectrometry, characterization of glycosylation and other modifications, and quantitation of peptides used in clinical diagnostics.

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

Protein sequencing shows that porcine brain tubulin retains the N-terminal sequences of alpha and beta tubulin after a mild treatment with subtilisin. C-terminal peptides released by subtilisin were purified and characterized by automated Edman degradation and mass spectrometry. We confirm the polyglutamylation of alpha tubulin on glutamic acid residue 445 reported by others and show in addition that class II beta tubulin, the major beta tubulin isotype of adult brain, is also polyglutamylated. The substitution is restricted to glutamic acid residue 435. Thus all major tubulin isotypes of adult brain are subjected to polyglutamylation.