Posttranslational tyrosination/detyrosination of tubulin

Differential distribution of glutamylated tubulin during spermatogenesis in mammalian testis

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

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)

Microtubule transport and assembly cooperate to generate the microtubule array of growing axons

MTs are major architectural elements in growing axons. MTs overlap with each other along the axon, forming an array that is continuous from the cell body to the tip of the axon. The MT array constitutes a scaffolding that mechanically supports the elongate shape of the axon and also contributes directly to its shape. MTs also direct the transport of vesicular organelles between the cell body and the axon, and thereby determine, in part, the composition of the axon. In this article, I have discussed mechanisms involved in the elaboration of the MT array in growing axons, and I have emphasized the distinct but complementary roles of polymer transport mechanisms and local assembly dynamics. MTs for the axon originate in the cell body, and they are delivered to the axon by the polymer transport mechanisms. These mechanisms thus contribute directly to the shape of the axon by supplying it with essential architectural elements. The shape of the axon is further modulated by dynamic processes that alter cytoskeletal structure locally along its length. These dynamic processes include the assembly/disassembly mechanisms which influence polymer length and possibly number locally along the axon by subunit exchange between the monomer and polymer pools. In addition, the polymer transport mechanisms themselves are subject to modulation along the axon, as demonstrated by the observation that transport rate of MTs varies along the length of individual axons (Reinsch et al., 1991). Such local variations can, in and of themselves, change the number of MTs along the axon, and thereby focally affect axon shape. Thus, the dynamic processes of polymer transport and local assembly act cooperatively to shape the MT array of the axon, and thereby contribute directly to the elaboration of axonal morphology.

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.

Newly assembled microtubules are concentrated in the proximal and distal regions of growing axons

We have investigated the sites of microtubule (MT) assembly in neurons during axon growth by taking advantage of the relationship between the proportion of tyrosinated alpha-tubulin (tyr-tubulin) in MTs and their age. Specifically, young (newly assembled) MTs contain more tyr-tubulin than older (more long-lived) MTs. To quantify the relative proportion of tyr-tubulin in MTs, cultured rat sympathetic neurons were permeabilized under conditions that stabilize existing MTs and remove unassembled tubulin. The MTs were then double-stained with antibodies to tyr-tubulin (as a measure of the amount of tyr-tubulin in MTs) and to beta-tubulin (as a measure of total MT mass), using immunofluorescence procedures. Cells were imaged with a cooled charge-coupled device camera and the relative proportion of tyr-tubulin in the MTs was quantified by computing the ratio of the tyr-tubulin fluorescence to the beta-tubulin fluorescence using a novel application of digital image processing and analysis techniques. The amount of tyr-tubulin in the MTs was highest in the cell body and at the growth cone; peak ratios in these two regions were approximately 10-fold higher than for the axon shaft. Moving out from the cell body into the axon, the tyr-tubulin content declined over an average distance of 40 microns to reach a constant low value within the axon shaft and then rose again more distally, over an average distance of 110 microns, to reach a peak at the growth cone (average axon length = 358 microns). These observations indicate that newly assembled MTs are concentrated in the proximal and distal regions of growing axons and therefore that the cell body and growth cone are the most active sites of MT assembly dynamics in neurons that are actively extending axons.

Slow transport of the cytoskeleton after axonal injury

The delivery of cytoskeletal proteins to the axon occurs by slow axonal transport. We examined how the rate of slow transport was altered after axonal injury. When retinal ganglion cell (RGC) axons regenerated through peripheral nerve grafts, an increase in the rate of slow transport occurred during regrowth of the injured axons. We compared these results to axonal injury in the optic nerve where no substantial regrowth occurs and found a completely different response. Slow transport was decreased approximately tenfold in rate in the proximal segment of crushed optic nerves. This decreased rate of slow transport was not induced immediately, but occurred about 1 week after injury. To explore whether a decrease in the rate of slow transport was induced when the regeneration of peripheral nerves was physically blocked, we examined slow transport in motor neurons after the sciatic nerve was transected and ligated. In this case, no change in the rate of the comigrating tubulin and neurofilament (NF) radioactive peaks were observed. We discuss how the changes in the rate of slow transport may reflect different neuronal responses to injury and speculate about the possible molecular changes in the expression of tubulin which may contribute to the observed changes.

Tyrosinated, detyrosinated and acetylated tubulin isotypes in rat brain membranes. Their proportions in comparison with those in cytosol

The heterogeneity of alpha-tubulin and the relative proportions of the tubulin isotypes were investigated in brain membranes of rats of 1, 25 and 180 days of age by using four anti-alpha-tubulin antibodies: a) the monoclonal DM1A antibody, specific for alpha-tubulin; b) the monoclonal 6-11B-1 antibody, specific for acetylated tubulin; c) a polyclonal antibody (Glu antibody), specific for detyrosinated tubulin; and d) a polyclonal antibody (Tyr antibody), specific for tyrosinated tubulin. We found that rat brain membranes contain the three tubulin isotypes mentioned above. The proportions of tyrosinated and detyrosinated tubulin relative to total alpha-tubulin were somewhat lower in membrane than in cytosol in animals of 25 and 180 days of age. At day one of development, the proportions in membrane were similar to those found in cytosol. With respect to the acetylated form, it was about 20 times higher in membrane than in cytosol at the three ages studied. The proportion of acetylated tubulin was determined in different subcellular fractions: myelin, synaptic vesicles, mitochondria, microsomes, and plasma membrane. While the amount of total tubulin differed between the different subcellular fractions, the proportion of acetylated tubulin relative to total alpha-tubulin was constant and similar to that found in total membranes. The proportion of acetylated tubulin was also investigated in non-neural tissues (kidney, liver and lung). Although values for cytosol were about 10-fold higher than that found in brain cytosol, no detectable values for membranes could be obtained in these organs.

Microtubule functions

Microtubules, with intermediate filaments and microfilaments, are the components of the cell skeleton which determinates the shape of a cell. Microtubules are involved in different functions including the assembly of mitotic spindle, in dividing cells, or axon extension, in neurons. In the first case, microtubules are highly dynamic, while in the second case microtubules are quite stable, suggesting that microtubule with different physical properties (stability) are involved in different functions. Thus, to understand the mechanisms of microtubule functions it is very important to understand microtubule dynamics. Historically, tubulin, the main component of microtubules, was first characterized as the major component of the mitotic spindle that binds to colchicine. Afterwards, it was found that tubulin is particularly more abundant in brain than in other tissues. Therefore, the roles of microtubules in mitosis, and in neurons, have been more extensively analyzed and, in this review, these roles will be discussed.

Aging and the neurocytoskeleton

It has been often demonstrated that during senescence some neurons undergo atrophic changes while others add new processes and terminals. Because microtubules form a substantial component of the dendritic and axonal cytoskeleton, we have studied the amount of tubulin and acetylated alpha-tubulin in three young (6 months) and three old (24 months) rats (Fischer 344). We have used sodium dodecyl sulfate (SDS) extracts of brain homogenates and Triton solubilized fractionated brain homogenates. With the first method we did not detect any age-related differences in total brain protein, total tubulin, or in relative amounts of acetylated alpha-tubulin. With the second method, we have observed a small systematic increase in relative amount of acetylated alpha-tubulin in the Ca2+/cold insoluble fraction. These results are similar to those reported in the literature, and they indicate a possible alteration in the cytoskeletal dynamics.

Characterization of a major brain tubulin variant which cannot be tyrosinated

Brain tubulin preparations contain an abundant type of tubulin which does not undergo the normal cycle of tyrosination-detyrosination, and whose nature is still unknown. We have used peptide sequence analysis and mass spectrometry combined with immunological procedures to show that this non-tyrosinatable tubulin has a specific primary structure. It differs from the tyrosinated isotype in that it lacks a carboxy-terminal glutamyl-tyrosine group on its alpha-subunit. Thus, non-tyrosinatable tubulin originates from a well-defined posttranslational modification of the tubulin primary structure which is located at the expected site of activity of tubulin tyrosine ligase. This probably accounts for the reason why it cannot be tyrosinated. The significance of this abundant brain isotubulin and the metabolic pathway involved in its formation remain to be elucidated. This should shed light on the relation between the structural diversity of the carboxy terminus of alpha-tubulin and the regulation of functional properties of microtubules.

Post-translational modification of microtubules is a component of synergic alterations of cytoskeleton leading to formation of cytoplasmic processes in fibroblasts

The protein kinase C activator phorbol 12-myristate 13-acetate (PMA) induces rapid and reversible shape changes in cultured fibroblasts: extension of motile lamellas is followed by transformation of these lamellas into nonmotile stalklike processes. This "lamella-to-stalk" transformation was found to be associated with the formation of microtubules enriched in detyrosinated alpha-tubulin. This change was local: microtubules in motile lamellas at the distal ends of the processes and in the cell bodies were not enriched in detyrosinated alpha-tubulin. Detyrosinated microtubules in the processes were more resistant to Colcemid treatment than other microtubules of PMA-treated and control cells. The effects of PMA were reversible and could be abolished by sphingosine, a specific inhibitor of protein kinase C. Besides modification of microtubules, lamella-to-stalk transformation is associated with the ingrowth of intermediate filaments into the extensions. Earlier it was found that this transformation is also associated with the profound reorganization of the system of actin microfilaments. Thus, all three cytoskeletal systems are altered simultaneously during PMA-induced formation of processes. Similar "cytoskeletal synergies" may play essential roles in many morphogenetic processes--e.g., in the growth of neurites.

Cytoskeletal specializations at the rod photoreceptor distal tip

We have examined microtubules and microtubule-like elements within the toad rod photoreceptor outer segment in order to define regional specializations of the photoreceptor cytoskeleton. "Ciliary" microtubules were localized within the rod outer segment (ROS) by using thin section electron microscopy, immunofluorescence, and rapid-freeze deep-etch microscopy. All three methods showed that ciliary microtubules stop short of the extreme ROS distal tip, although abundant microtubule-like structures distinct from the ciliary microtubules were found within the distal 10-15 microns of the ROS tip. These heretofore undescribed "distal ROS tubules" are clustered at the clefts or incisures of the disk membrane stack and resemble microtubules in overall size and shape, although they are not closely related antigenically to tubulin. The distal ROS tubules are more abundant in green rods than red rods and vary in number during the daily light/dark cycle. Quantitation of these tubules at two time points during the light/dark cycle suggests that there are three- to fourfold more tubules in the ROS tip one hour after light onset than one hour before light onset. Retinas prevented from normal disk membrane shedding by separation of the retina from the adjacent pigment epithelium, failed to develop increased numbers of tubules after light onset. This suggests that the newly described distal ROS tubules may modulate or be modulated by light-induced interactions between the photoreceptors and pigment epithelium, such as those that occur during the disk shedding phase of membrane turnover.

Tyrosinated and detyrosinated microtubules in axonal processes of cerebellar macroneurons grown in culture

We have used the monoclonal antibody YL 1/2 (Tyr) specific for tyrosinated tubulin, and a polyclonal antibody (Glu) specific for detyrosinated tubulin to visualize the distribution of microtubules and microtubule assembly sites during axonal outgrowth. Cerebellar macroneurons growing in culture initially extend several short and thin neurites which have the potential to differentiate either as axons or dendrites (Ferreira and Caceres: Developmental Brain Research 49:205-213, 1989). At the onset of axonal outgrowth the Tyr antibody labels the minor neurites, the axon, and its growth cone, while the Glu antibody only shows immunoreactivity in the axonal shaft. After nocodazole treatment, the Tyr staining disappears, whereas that produced by the Glu antibody remains practically unchanged. When nocodazole was removed, tyrosinated microtubules reappeared first at the tip of the axon, in a more distal region than that occupied by detyrosinated microtubules; another focal site of tyrosinated tubulin incorporation was detected in the cell body. Incorporation of tyrosinated tubulin into growing axons was also studied after taxol treatment. After long incubation periods in the presence of taxol, the Tyr staining disappeared from the axon but remained in the cell body; however, immunoreactivity in this site was negative when the cells were preincubated in the presence of protein synthesis inhibitors. Release from taxol results in the reappearance of Tyr immunoreactivity at the distal end of the axon. Taken collectively, the present results indicate 1) that in cerebellar macroneurons axonal differentiation is accompanied by a temporal and spatial differentiation of microtubules and 2) that there is an active site of tyrosinated tubulin assembly at the tip of axonal processes, and they suggest that the highly tyrosinated domain in this region is a consequence of rapid microtubule turnover and tubulin tyrosine ligase activity.

Specific antibodies for tyrosinated and detyrosinated tubulin recognize retina tubulin subpopulations that do not participate in the posttranslational tyrosination/detyrosination cycle

We have used the monoclonal YL 1/2 (Tyr antibody) and polyclonal (Glu antibody) antibodies, specific for tyrosinated and detyrosinated tubulin, respectively, to determine the levels and cellular distribution of these tubulin species in chick retina during development. At embryonic day 4, detyrosinated tubulin was restricted to the ganglion cells of the fundic region. As development progresses, immunofluorescence also appears, first, in the outermost zone of the retina and then in the plexiform layers. The Tyr antibody staining was found in the different layers and it was fairly homogeneous in distribution. Analysis by dot immunobinding showed that the ratios of tyrosinated to detyrosinated tubulin obtained at different ages do not agree with those obtained previously by an enzymatic method based on the incorporation of [14C]tyrosine. We found that the lack of coincidence is due to the fact that a fraction of the tubulin species determined by the Tyr and Glu antibodies does not participate in the posttranslational tyrosination/detyrosination cycle. This is a novel concept that should be considered in the interpretations of immunofluorescence studies concerning the cellular distribution of tyrosinated and detyrosinated tubulin.

Effect of polyanions and polycations on detyrosination of tubulin and microtubules at steady state

Microtubule protein preparations purified from rat brain were used to study the effect of polycations and polyanions on the release of the COOH-terminal tyrosine of the alpha-chain of tubulin catalyzed by tubulin carboxypeptidase. (1) Most of the polycations and polyanions tested, independently of the ionogenic group, inhibited the reaction in a concentration-dependent fashion. Under steady-state conditions, detyrosination of the microtubule pool was inhibited to the same degree as occurred with the non-assembled tubulin pool, except in the case of chondroitin sulphate. This compound inhibited detyrosination of the non-assembled tubulin pool, but not that of microtubules. (2) Heparin, the most potent inhibitor tested, produced the dissociation of the carboxypeptidase from microtubules. Many, but not all, of the other microtubule-associated polypeptides were also dissociated by heparin. (3) Polylysine counteracted the inhibitory and dissociating effects of heparin. (4) Heparin protected tubulin carboxypeptidase against inactivation. Our results and previous reports describing, in nervous tissue, the presence of proteoglycans, RNA and basic proteins that inhibit detyrosination, suggest that tubulin carboxypeptidase might be physiologically modulated by electrically charged macromolecules.

The non-tyrosinated M alpha 4 alpha-tubulin gene product is post-translationally tyrosinated in adult rat cerebellum

The distributions of the non-tyrosinated M alpha 4 alpha-tubulin gene product and its tyrosinated form M alpha 4 + Y were examined in immunoblots and sections of adult rat brain. In cerebellar sections, M alpha 4 and M alpha 4 + Y immunoreactivities were enriched in neurons, anti-M alpha 4 + Y labeling thus being more restricted in distribution than that of the general alpha-tubulin antibody YL1/2. The results indicate that the M alpha 4 gene product does not constitute the large non-tyrosinatable pool of alpha-tubulin in brain.

Amino acid modification of proteins in regenerating sciatic nerves of rats

Recent experiments have shown that Arg, Lys, and Leu can be incorporated posttranslationally into proteins of regenerating sciatic nerves of rats. The present experiments investigate a mixture of 15 radioactive amino acids to determine if additional amino acids can be conjugated posttranslationally to proteins of regenerating nerves. Proteins of regenerating sciatic nerves of rats were able to incorporate Arg, Lys, Leu, Pro, Val, Ala, Phe, and Ser in relatively large amounts and Asp, Glu, Thr, Gly, Ile, His, and Tyr in relatively low or undetectable amounts, in the most advanced portion of the regenerating nerves. Two-dimensional SDS PAGE showed incorporation of the amino acid mixture into distinct radioactive peaks with molecular weights in the 80-90 kD, 53-66 kD, 22-46 kD, and 17 kD ranges with isoelectric points between 5.0 and 7.9. Most of the amino acids were incorporated into proteins in all of the molecular weight ranges. But Ser was incorporated in highest amounts in the 17 kD range, and Val was most abundant in the 22-46 kD range. In some cases results indicated that single proteins were modified by several amino acids. While we do not yet know which amino acids modify specific nerve proteins or the function of the modifications in nerve regeneration, these studies demonstrate the participation of some but not all amino acids in posttranslational modification reactions and the selective modification of specific groups of nerve proteins by these amino acids.

Distribution of microtubules containing post-translationally modified alpha-tubulin during Drosophila embryogenesis

The distribution of microtubules (MTs) enriched in detyrosinated alpha-tubulin (Glu-tubulin) was studied in Drosophila embryos by immunofluorescence microscopy by using a monoclonal antibody (ID5) which was raised against a 14-residue synthetic peptide spanning the carboxyterminal sequence of Glu-tubulin (Wehland and Weber: J. Cell Sci. 88:185-203, 1987). While all MT arrays contained tyrosinated alpha-tubulin (Tyr-tubulin), MTs rich in Glu-tubulin were not found during early stages of development even by using an image intensification camera. Elevated levels of microtubular Glu-tubulin were first detected after CNS condensation in neurone processes. In addition, sperm tails, which remained remarkably stable inside the embryo until late stages of development, were decorated by ID5. This was in marked contrast to the distribution of microtubule arrays containing acetylated alpha-tubulin, which could already be detected during the cellular blastoderm stage. Additional experiments with taxol suggested that the absence of MTs rich in Glu-tubulin during early stages of development was not due to the rapid turnover rate of MTs, which would be too fast for alpha-tubulin to be detyrosinated. The possible significance of the differential detyrosination and acetylation of microtubules during development is discussed.

Tyrosination state of tubulin and the activity of tubulin:tyrosine ligase and tubulin carboxypeptidase in the developing retina of the chick

The tyrosination state of tubulin and the enzymes involved in the tubulin tyrosination/detyrosination cycle--tubulin:tyrosine ligase and tubulin carboxypeptidase--were determined in chick retina during development. The amount of tyrosinable (tyrosinated plus detyrosinated) tubulin increased approximately 110% from embryonic day 7 to 14. Then it decreased, and by day 19 it was similar to the value on day 7. This result did not change after hatching, at least up to day 20. The proportion of tyrosinated and detyrosinated tubulin significantly changed with the development of the animal. At embryonic day 7, these tubulin species were at a proportion of 70 and 30%, respectively, and after hatching, the values inverted, to 30 and 70%, respectively. This change did not correlate with the activity of the ligase relative to that of the carboxypeptidase, as measured in vitro. This observation suggested that a change in the turnover rate of microtubules, in the proportion of assembled and nonassembled tubulin pools, or in both had occurred. Coincident with the last possibility, the proportion of assembled tubulin was found to increase during the development of the animal. This finding suggests that the tyrosination state of tubulin may be determined, at least in part, by the assembly state.

The integrity of tubulin molecule is not required for the activity of tubulin carboxypeptidase

Tubulin dimer, alpha-tubulin subunit, and C-terminal peptides obtained from the alpha-tubulin subunit were compared in their capabilities to act as substrates of tubulin carboxypeptidase. The results obtained indicate that the enzyme does not require the beta-tubulin subunit to release tyrosine from alpha-tubulin. The 17-Kd C-terminal peptide of the alpha-tubulin subunit was obtained and it was detyrosinated at the same rate as tubulin dimer. A smaller C-terminal peptide of 2.8-3.7 Kd showed a lower capability to act as substrate. Similar results were obtained with pancreatic carboxypeptidase A. From the analysis of the results we consider that an optimal activity of the tubulin carboxypeptidase depends mainly on the accessibility of the C-terminal end of alpha-tubulin.