Tyrosyltubulin ligase activity in brain, skeletal muscle, and liver of the developing chick

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

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

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.

Regulation of cytoplasmic tubulin carboxypeptidase activity in vitro by cations and sulfhydryl-modifying compounds

alpha-tubulin subunits within microtubules (MTs) can be post-translationally detyrosinated by a tubulin-specific carboxypeptidase (TCP) activity to form biochemically distinct MTs. Attempts to characterize and purify TCP have suffered from the inability to detect low levels of activity and to distinguish TCP from other, competing enzyme activities. We recently developed an assay for TCP [Webster et al. (1992) Biochemistry 31:5849] that uses taxol-stabilized MTs as the substrate. In this study, we exploited the increased sensitivity and specificity of this new assay to explore the effects of various agents that might act to either stimulate or inhibit this enzyme in vitro. We tested a variety of both monovalent and divalent cations for their ability to affect TCP, and tested whether the cations were affecting the enzyme, the substrate, or both. We found that TCP displayed salt-sensitive binding to MTs, characteristic of other, more well characterized MT-associated proteins. While both calcium and magnesium stimulated TCP activity over a narrow concentration range (2-10 mM), they inhibited activity at higher concentrations. Other divalent cations tested, including zinc, copper, and cobalt, inhibited TCP at virtually all concentrations tested, but to different levels (zinc > copper > cobalt). Most of the zinc-induced TCP inhibition was attributed to the interference with the normal binding of TCP to MTs. In addition, we examined the involvement of free sulfhydryl groups (which are important for the activities of many types of enzymes) in TCP activity by the addition of sulfhydryl-modifying compounds during the assay, and found that their addition reduced TCP activity mainly (but not solely) by their action on the extract that contained the TCP. Finally, we tested the ability of DL-benzylsuccinic acid, a potent inhibitor of carboxypeptidase A, to inhibit TCP. While carboxypeptidase A has been found, in other studies, to be inhibited by micromolar concentrations, TCP was affected only at concentrations above 20 mM, adding another proof that carboxypeptidase A and TCP are distinct enzyme activities.

Tyrosination state of alpha-tubulin in regenerating peripheral nerve

Certain modifications of the neuronal cytoskeleton that are associated with development also occur during regeneration of adult mammalian peripheral nerve. The aim of the present study was to examine one such modification, the tyrosination of alpha-tubulin. Adult rats were anaesthetized and the left or right sciatic nerve randomly selected and crushed to induce regeneration. In certain instances nerves were crushed then ligatured about the crush, to prevent regeneration. Five days later the rats were killed and the regenerating (or ligatured) and the contralateral (control) nerves were removed. Quantitative immunoblotting of nerve homogenates with antibodies that recognize tyrosinated alpha-tubulin and total alpha-tubulin revealed a significant increase (p < 0.01) in the proportion of alpha-tubulin that was tyrosinated in nerve pieces distal (peripheral) to a nerve crush and to uncrushed nerve. No such difference occurred in ligatured (crushed but nonregenerating) nerve, implying that the increase was related to the presence of regenerating fibres; nor was there any gradient in tyrosination of alpha-tubulin in control nerves. This effect was confirmed by cytofluorimetric scanning and fluorescence confocal laser scanning microscopy of fixed sections of control and regenerating nerve, stained with antibodies directed against tyrosinated alpha-tubulin. When nerves were separated into fractions containing assembled and nonassembled tubulin, a significant (p < 0.01) increase was found in the proportion of tyrosinated alpha-tubulin in the nonassembled tubulin fraction in nerve pieces containing regenerating fibres. This occurred in the absence of a change in the proportion of assembled and nonassembled tubulin.(ABSTRACT TRUNCATED AT 250 WORDS)

Stabilization of post-translational modification of microtubules during cellular morphogenesis

This review discusses the possible role of alpha-tubulin detyrosination, a reversible post-translational modification that occurs at the protein's C-terminus, in cellular morphogenesis. Higher eukaryotic cells possess a cyclic post-translational mechanism by which dynamic microtubules are differentiated from their more stable counterparts; a tubulin-specific carboxypeptidase detyrosinates tubulin protomers within microtubules, while the reverse reaction, tyrosination, is performed on the soluble protomer by a second tubulin-specific enzyme, tubulin tyrosine ligase. In general, the turnover of microtubules in undifferentiated, proliferating cells is so rapid that the microtubules accumulate very little detyrosinated tubulin; that is, they are enriched in tyrosinated tubulin. However, an early event common to at least three well-studied morphogenetic events--myogenesis, neuritogenesis, and directed cell motility--is the elaboration of a polarized array of stable microtubules that become enriched in detyrosinated tubulin. The formation of this specialized array of microtubules in specific locations in cells undergoing morphogenesis suggests that it plays an important role in generating cellular asymmetries.

Generation of a stable, posttranslationally modified microtubule array is an early event in myogenic differentiation

Microtubules (MTs) have been implicated to function in the change of cell shape and intracellular organization that occurs during myogenesis. However, the mechanism by which MTs are involved in these morphogenetic events is unclear. As a first step in elucidating the role of MTs in myogenesis, we have examined the accumulation and subcellular distribution of posttranslationally modified forms of tubulin in differentiating rat L6 muscle cells, using antibodies specific for tyrosinated (Tyr), detyrosinated (Glu), and acetylated (Ac) tubulin. Both Glu and Ac tubulin are components of stable MTs, whereas Tyr tubulin is the predominant constituent of dynamic MTs. In proliferating L6 myoblasts, as in other types of proliferating cells, the level of Glu tubulin was very low when compared with the level of Tyr tubulin. However, when we shifted proliferating L6 cells to differentiation media, we observed a rapid accumulation of Glu tubulin in cellular MTs. By immunofluorescence, the increase in Glu tubulin was first detected in MTs of prefusion myoblasts and was specifically localized to MTs that were associated with elongating portions of the cell. MTs in the multinucleated myotubes observed at later stages of differentiation maintained the elevated level of Glu tubulin that was observed in the prefusion myoblasts. When cells at early stages of differentiation (less than 1 d after switching the culture medium) were immunostained for Glu tubulin and the muscle-specific marker, muscle myosin, we found that the increase in Glu tubulin preceded the accumulation of muscle myosin. Thus, the elaboration of Glu MTs is one of the very early events in myogenesis. Ac tubulin also increased during L6 myogenesis; however, the increase in acetylation occurred later in myogenesis, after fusion had already occurred. Because detyrosination was temporally correlated with early events of myogenesis, we examined the mechanism responsible for the accumulation of Glu tubulin in the MTs of prefusion myoblasts. We found that an increase in the stability of L6 cell MTs occurred at the onset of differentiation, suggesting that the early increase in detyrosination that we observed is a manifestation of a decrease in MT dynamics in elongating myoblasts. We conclude that the establishment of an oriented array of microtubules heightened in its stability and its level of posttranslationally modified subunits may be involved in the subcellular remodeling that occurs during myogenesis.

Characteristics of microtubules at the different stages of neuronal differentiation and maturation

The developing nervous system has proved to be a very powerful tool to analyze how MT are involved in basic biological processes such as cell proliferation, cell migration, cell shaping, and transport. A better knowledge of the basic events occurring during neurogenesis also affords us the possibility of establishing the basis of experiments and trying to solve unanswered and important questions. Despite the considerable value of cell culture, we need to use more discrete regions of the developing brain in situ in order to analyze the MT and their modifications into cells developing their "natural" environment. One major problem remains the question of the mode of assembly and disassembly, that is, the behavior of MT in living cells. Dynamic instability and/or treadmilling are accurate interpretations of the dynamics of MT at least in vitro or in cell culture, but we do need more information on what happens in situ and in vitro. One of the main tasks of cell biologists is to devise satisfactory tests to approach this fundamental question. In this view, pharmacological manipulation of embryos treated in whole-embryo culture systems might be a possible way. Microtubules are ubiquitous cell components. However, the extensive heterogeneity of MAP and tubulin in the CNS confers on the neurons a wide range of capabilities of assembly of these proteins and suggests that the neuron has a unique potential of a relation between MT composition and cell function. We have seen that each major event during neurogenesis is related to a specific series of modifications of the MT components. It remains to be determined if there is a causal or just a correlative relationship between the appearance of specific isotypes and the occurrence of specific events and/or functions. We have also to determine the exact spatial and temporal relations among the different isotypes of MT proteins, tubulin, and MAP. Is there a close correspondence between a tubulin and a MAP isotype? Can the appearance of one isotype of tubulin influence the appearance and the assembly of a specific MAP, or vice versa? Recent results obtained with the Tyr- and Glu-MT shed light on these questions and suggest a whole series of possibilities for cells to modulate the structure, behavior, and function of MT in specific domains of the neuron or in specific regions of the brain, by only a minute modification of the molecule of tubulin. Microtubule protein heterogeneity raises also a number of questions.(ABSTRACT TRUNCATED AT 400 WORDS)

Posttranslational tyrosination/detyrosination of tubulin

Tubulin can be posttranslationally modified at the carboxyl terminus of the alpha-subunit by the addition or release of a tyrosine residue. These reactions involve two enzymes, tubulin: tyrosine ligase and tubulin carboxypeptidase. The tyrosine incorporation reaction has been described mainly in nervous tissue but it has also been found in a great variety of tissues and different species. Molecular aspects of the reactions catalyzed by these enzymes are at present well known, especially the reaction carried out by the ligase. Several lines of evidence indicate that assembled tubulin is the preferred substrate of the carboxypeptidase, whereas nonassembled tubulin is preferred by the ligase. Apparently this posttranslational modification does not affect the capacity of tubulin to form microtubules but it generates microtubules with different degrees of tyrosination. Variation in the content of the carboxyterminal tyrosine of alpha-tubulin as well as changes in the activity of the ligase and the carboxypeptidase are manifested during development. Changes in the cellular microtubular network modify the turnover of the carboxyterminal tyrosine of alpha-tubulin. Different subsets of microtubules with different degrees of tyrosination have been detected in interphase cells and during the mitotic cycle. Data from biochemical, immunological, and genetic studies have been compiled in this review; these are presented, with pertinent comments, with the hope of facilitating the comprehension of this particular aspect of the microtubule field.

Generation of antisera that discriminate among mammalian alpha-tubulins: introduction of specialized isotypes into cultured cells results in their coassembly without disruption of normal microtubule function

To assay the functional significance of the multiple but closely related alpha-tubulin polypeptides that are expressed in mammalian cells, we generated three specific immune sera, each of which uniquely recognizes a distinct alpha-tubulin isotype. All three isotypes are expressed in a tissue-restricted manner: one (M alpha 3/7) only in mature testis, one (M alpha 4) mainly in muscle and brain, and the third (M alpha 6) in several tissues at a very low level. A fourth specific antiserum was also generated that distinguishes between the tyrosinated and nontyrosinated form of a single alpha-tubulin isotype. Because individual tubulin isotypes cannot be purified biochemically, these sera were raised using cloned fusion proteins purified from host Escherichia coli cells. To suppress the immune response to shared epitopes, animals were first rendered tolerant to fusion proteins encoding all but one of the known mammalian alpha-tubulin isotypes. Subsequent challenge with the remaining fusion protein then resulted in the elicitation of an immune response to unique epitopes. Three criteria were used to establish the specificity of the resulting sera: (a) their ability to discriminate among cloned fusion proteins representing all the known mammalian alpha-tubulin isotypes; (b) their ability to uniquely detect alpha-tubulin in whole extracts of tissues; and (c) their capacity to stain microtubules in fixed preparations of cells transfected with sequences encoding the corresponding isotype. The transfection experiments served to demonstrate (a) the coassembly of M alpha 3/7, M alpha 4, and M alpha 6 into both interphase and spindle microtubules in HeLa cells and NIH 3T3 cells, and (b) that the M alpha 4 isotype, which is unique among mammalian alpha-tubulins in that it lacks an encoded carboxy-terminal tyrosine residue, behaves like other alpha-tubulin isotypes with respect to the cycle of tyrosination/detyrosination that occurs in most cultured cells.

Enhanced stability of microtubules enriched in detyrosinated tubulin is not a direct function of detyrosination level

Interphase cultured monkey kidney (TC-7) cells contain distinct subsets of cellular microtubules (MTs) enriched in posttranslationally detyrosinated (Glu) or tyrosinated (Tyr) alpha tubulin (Gundersen, G. G., M. H. Kalnoski, and J. C. Bulinski. 1984. Cell. 38:779-789). To determine the relative stability of these subsets of MTs, we subjected TC-7 cells to treatments that slowly depolymerized MTs. We found Glu MTs to be more resistant than Tyr MTs to depolymerization by nocodazole in living cells, and to depolymerization by dilution in detergent-permeabilized cell models. However, in cold-treated cells, Glu and Tyr MTs did not differ significantly in their stability. Digestion of permeabilized cell models with pancreatic carboxypeptidase A, to generate Glu MTs from endogenous Tyr MTs, did not significantly alter the resistance of the endogenous Tyr MTs toward dilution-induced depolymerization. Furthermore, in human fibroblasts that contained no distinct Glu MTs, we observed a population of nocodazole-resistant MTs. These data suggest that Glu MTs possess enhanced stability against end-mediated depolymerization, yet detyrosination alone appears to be insufficient to confer this enhanced stability.

Postpolymerization detyrosination of alpha-tubulin: a mechanism for subcellular differentiation of microtubules

Tyrosinated (Tyr) and detyrosinated (Glu) alpha-tubulin, species interconverted by posttranslational modification, are largely segregated in separate populations of microtubules in interphase cultured cells. We sought to understand how distinct Tyr and Glu microtubules are generated in vivo, by examining time-dependent alterations in Tyr and Glu tubulin levels (by immunoblots probed with antibodies specific for each species) and distributions (by immunofluorescence) after microtubule regrowth and stabilization. When microtubules were allowed to regrow after complete depolymerization by microtubule antagonists, Glu microtubules reappeared with a delay of approximately 25 min after the complete array of Tyr microtubules had regrown. In these experiments, Tyr tubulin immunofluorescence first appeared as an aster of distinct microtubules, while Glu tubulin staining first appeared as a grainy pattern that was not altered by detergent extraction, suggesting that Glu microtubules were created by detyrosination of Tyr microtubules. Treatments with taxol, azide, or vinblastine, to stabilize polymeric tubulin, all resulted in time-dependent increases in polymeric Glu tubulin levels, further supporting the hypothesis of postpolymerization detyrosination. Analysis of monomer and polymer fractions during microtubule regrowth and in microtubule stabilization experiments were also consistent with postpolymerization detyrosination; in each case, Glu polymer levels increased in the absence of detectable Glu monomer. The low level of Glu monomer in untreated or nocodazole-treated cells (we estimate that Glu tubulin comprises less than 2% of the monomer pool) also suggested that Glu tubulin entering the monomer pool is efficiently retyrosinated. Taken together these results demonstrate that microtubules are polymerized from Tyr tubulin and are then rapidly converted to Glu microtubules. When Glu microtubules depolymerize, the resulting Glu monomer is retyrosinated. This cycle generates structurally, and perhaps functionally, distinct microtubules.

Tyrosination-detyrosination of the COOH-terminus of alpha-tubulin in oocytes and embryos of Bufo arenarum

Soluble tubulin from Bufo arenarum oocytes and early embryos was shown to be composed mainly of the non-tyrosinable species. The low proportion of tyrosinable tubulin was almost exclusively constituted by the tyrosinated form. Compared with oocytes and embryos, toad brain contained a higher proportion of tyrosinable tubulin constituted mainly by the non-tyrosinated form. Tubulin carboxypeptidase was detected in toad brain but not in oocytes and embryos.

Ultrastructural colocalization of tyrosinated and detyrosinated alpha-tubulin in interphase and mitotic cells

Immunofluorescence with specific peptide antibodies has previously established that tyrosinated (Tyr) and detyrosinated (Glu) tubulin, the two species generated by posttranslational modification of the COOH-terminus of alpha-tubulin, are present in distinct, but overlapping, subsets of microtubules in cultured cells (Gundersen, G. G., M. H. Kalnoski, and J. C. Bulinski, 1984, Cell, 38:779-789). Similar results were observed by light microscopic immunogold staining in the two cell types used in this study, CV1 and PtK2 cells: most microtubules were stained with the Tyr antibody, whereas only a few were stained with the Glu antibody. We have examined immunogold-stained preparations by electron microscopy to extend these results. In general, electron microscopic localization confirmed results obtained at the light microscopic level: the majority of the microtubules in CV1 and PtK2 cells were nearly continuously labeled with the Tyr antibody, whereas only a few were heavily labeled with the Glu antibody. However, in contrast to the light microscopic staining, we found that all microtubules of interphase and mitotic CV1 and PtK2 cells contained detectable Tyr and Glu immunoreactivity at the electron microscopic level. No specific localization of either species was observed in microtubules near particular organelles (e.g., mitochondria or intermediate filaments). Quantification of the relative levels of Glu and Tyr immunoreactivity in individual interphase and metaphase microtubules showed that all classes of spindle microtubules (i.e., kinetochore, polar, and astral) contained nearly the same level of Glu immunoreactivity; this level of Glu immunoreactivity was lower than that found in all interphase microtubules. Most interphase microtubules had low levels of Glu immunoreactivity, whereas a few had relatively high levels; the latter corresponded to morphologically sinuous microtubules. Quantification of the relative levels of Tyr and Glu immunoreactivity in segments along individual microtubules suggested that the level of Tyr (or Glu) tubulin in a given microtubule was uniform along its length. Understanding how microtubules with different levels of Tyr and Glu tubulin arise will be important for understanding the role of tyrosination/detyrosination in microtubule function. Additionally, the coexistence of microtubules with different levels of the two species may have important implications for microtubule dynamics in vivo.

Total tubulin and its aminoacylated and non-aminoacylated forms during the development of rat brain

The amount of total tubulin in the soluble fraction of rat brain was measured by a method based on the purification of tubulin previously labeled by incorporation of [14C]tyrosine in the C terminus of its alpha-chain. The tubulin content decreased from 2.01 to 1.30 nmol/mg protein when the animals passed from 4 to 30 days of age and then remained practically constant. The amounts of aminoacylated and non-aminoacylated tubulin present in the soluble brain extracts were determined from the incorporation of [14C]tyrosine into the free acceptor sites of tubulin preparations, that were preincubated without carboxypeptidase A or with this enzyme to eliminate tyrosine and phenylalanine from the C terminus of the alpha-chain of tubulin. The values obtained were corrected for the inactivation of tubulin to accept [14C]tyrosine that occurred during the isolation and incubation of the soluble fractions. The ratio non-aminoacylated/aminoacylated tubulin increased from 1.62 +/- 0.03 in the 4-day-old rats to 2.11 +/- 0.17 in the 120-day-old rats. The aminoacylatable tubulin, that is the sum of aminoacylated plus non-aminoacylated tubulin, decreased from 1.71 to 0.75 nmol/mg protein from 4-day-old to 30-day-old rats respectively and then remained practically constant. The amount of aminoacylatable tubulin is lower than that of total soluble tubulin. Therefore there is a fraction of tubulin that is unable to accept tyrosine. This non-aminoacylatable tubulin fraction increases with the age of the animal so that in the 120-day-old rats this tubulin species accounts for 48% of the total soluble tubulin.

On the mechanism of turnover of the carboxy-terminal tyrosine of the alpha chain of tubulin

The removal of tyrosine from the carboxyl terminus of the alpha chain of tubulin occurs predominantly from those tubulin dimers that are part of microtubules, and is dependent upon microtubular treadmill metabolism. A heretofore unrecognized factor is present in the high-speed supernatant fraction of rat brain homogenates that is required for detyrosination. This factor is neither tubulin:tyrosine ligase, the enzyme that catalyzes the addition of tyrosine to the carboxylterminal position of the alpha chain of tubulin, nor a carboxypeptidase-like activity. Pulse-chase experiments demonstrated that, in the reconstituted rat brain preparations, detyrosination takes place late in the transit of dimers through the microtubule and we suggest that dimer loss and detyrosination during treadmill metabolism in these systems are linked.

Tubulinyl-tyrosine carboxypeptidase from chicken brain: properties and partial purification

Tyrosine can be released from tubulinyl-tyrosine by the action of a brain carboxypeptidase. The molecular weight of this enzyme found by gel filtration through a column of Sephadex G-200 was 90,000. The enzyme was very unstable in a purified preparation in which the activity per milligram of protein was increased 250-fold with respect to the starting material. The precise magnitude of the purification cannot be stated because of the unknown amount of endogenous tubulinyl-tyrosine in the material to be assayed. A comparative study was done between tubulinyl-tyrosine carboxypeptidase (TTCP) activity and pancreatic carboxypeptidase A (CPA, EC 3.4.12.2) activity using tubulinyl-[14C]tyrosine as substrate. The most remarkable differences found are: MgCl2 (2 mM), phenyl acetate (10 mM), or EDTA (5 mM) increased the TTCP activity whereas the CPA activity was strongly inhibited by these compounds. Iodoacetate (2 mM) and ZnCl2 (0.1 mM) inhibited the TTCP activity more than the CPA activity. Contrarily, mercaptoethanol (50 mM) and dimethyl sulfoxide (5%) showed a stronger inhibitory effect on CPA than on TTCP. Of several N-carbobenzoxy dipeptides (Z-dipeptides) tested the greatest inhibitory effects on TTCP activity were obtained with Z-Glu-Tyr and Z-Glu-Phe, although strong inhibitory effects on CPA were also obtained with other Z-dipeptides.

The phylogenetic distribution of tubulin:tyrosine ligase

The post-translational addition of tyrosine to alpha-tubulin, catalyzed by tubulin:tyrosine ligase, has been previously reported in mammals and birds. The present study demonstrated that significant ligase activity was present in representative organisms from several other major vertebrate classes (chondrichthyes through reptiles) and that both substrate and enzyme from all vertebrates investigated were compatible with mammalian ligase and tubulin in the tyrosination reaction. None of the invertebrate tissues examined showed incorporation of tyrosine, phenylalanine or dihydroxyphenylalanine into alpha tubulin under conditions allowing significant incorporation of these compounds in vertebrate supernatant samples. The failure of invertebrate tubulin to incorporate tyrosine in vitro did not appear to be due to saturation of the carboxyl terminal position with tyrosine or the presence of a soluble inhibitor of ligase activity. Although tubulin amino acid composition has been highly conserved throughout evolution, a major evolutionary divergence is described based upon biochemical differences whereby invertebrate tubulin cannot be tyrosinated or post-translationally modified with phenylalanine or dihydroxyphenylalanine under conditions suitable for the incorporation of these compounds by vertebrate alpha tubulin.