Phosphorylation of a neuronal-specific beta-tubulin isotype

Emerging roles for tubulin PTMs in neuronal function and neurodegenerative disease

Neurons are equipped with microtubules of different stability with stable and dynamic domains often coexisting on the same microtubule. While dynamic microtubules undergo random transitions between disassembly and assembly, stable ones persist long enough to serve as platforms for tubulin-modifying enzymes (known as writers) that attach molecular components to the α- or β-tubulin subunits. The combination of these posttranslational modifications (PTMs) results in a "tubulin code," dictating the behavior of selected proteins (known as readers), some of which were shown to be crucial for neuronal function. Recent research has further highlighted that disturbances in tubulin PTMs can lead to neurodegeneration, sparking an emerging field of investigation with numerous questions such as whether and how tubulin PTMs can affect neurotransmission and synaptic plasticity and whether restoring balanced tubulin PTM levels could effectively prevent or mitigate neurodegenerative disease.

Intrinsic and Extrinsic Factors Affecting Microtubule Dynamics in Normal and Cancer Cells

Microtubules (MTs), highly dynamic structures composed of α- and β-tubulin heterodimers, are involved in cell movement and intracellular traffic and are essential for cell division. Within the cell, MTs are not uniform as they can be composed of different tubulin isotypes that are post-translationally modified and interact with different microtubule-associated proteins (MAPs). These diverse intrinsic factors influence the dynamics of MTs. Extrinsic factors such as microtubule-targeting agents (MTAs) can also affect MT dynamics. MTAs can be divided into two main categories: microtubule-stabilizing agents (MSAs) and microtubule-destabilizing agents (MDAs). Thus, the MT skeleton is an important target for anticancer therapy. This review discusses factors that determine the microtubule dynamics in normal and cancer cells and describes microtubule-MTA interactions, highlighting the importance of tubulin isoform diversity and post-translational modifications in MTA responses and the consequences of such a phenomenon, including drug resistance development.

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.

A Journey through the Cytoskeleton with Protein Kinase CK2

Substrate pleiotropicity, a very acidic phosphorylation consensus sequence, and an apparent uncontrolled activity, are the main features of CK2, a Ser/Thr protein kinase that is required for a plethora of cell functions. Not surprisingly, CK2 appears to affect cytoskeletal structures and correlated functions such as cell shape, mechanical integrity, cell movement and division. This review outlines our current knowledge of how CK2 regulates cytoskeletal structures, and discusses involved pathways and molecular mechanisms.

Cytoskeletal dynamics during in vitro neurogenesis of induced pluripotent stem cells (iPSCs)

Patient-derived induced pluripotent stem cells (iPSCs) provide a novel tool to investigate the pathophysiology of poorly known diseases, in particular those affecting the nervous system, which has been difficult to study for its lack of accessibility. In this emerging and promising field, recent iPSCs studies are mostly used as "proof-of-principle" experiments that are confirmatory of previous findings obtained from animal models and postmortem human studies; its promise as a discovery tool is just beginning to be realized. A recent number of studies point to the functional similarities between in vitro neurogenesis and in vivo neuronal development, suggesting that similar morphogenetic and patterning events direct neuronal differentiation. In this context, neuronal adhesion, cytoskeletal organization and cell metabolism emerge as an integrated and unexplored processes of human neurogenesis, mediated by the lack of data due to the difficult accessibility of the human neural tissue. These observations raise the necessity to understand which are the players controlling cytoskeletal reorganization and remodeling. In particular, we investigated human in vitro neurogenesis using iPSCs of healthy subjects to unveil the underpinnings of the cytoskeletal dynamics with the aim to shed light on the physiologic events controlling the development and the functionality of neuronal cells. We validate the iPSCs system to better understand the development of the human nervous system in order to set the bases for the future understanding of pathologies including developmental disorders (i.e. intellectual disability), epilepsy but also neurodegenerative disorders (i.e. Friedreich's Ataxia). We investigate the changes of the cytoskeletal components during the 30days of neuronal differentiation and we demonstrate that human neuronal differentiation requires a (time-dependent) reorganization of actin filaments, intermediate filaments and microtubules; and that immature neurons present a finely regulated localization of Glu-, Tyr- and Acet-TUBULINS. This study advances our understanding on cytoskeletal dynamics with the hope to pave the way for future therapies that could be potentially able to target cytoskeletal based neurodevelopmental and neurodegenerative diseases.

Tissue-specific control of midbody microtubule stability by Citron kinase through modulation of TUBB3 phosphorylation

Cytokinesis, the physical separation of daughter cells at the end of cell cycle, is commonly considered a highly stereotyped phenomenon. However, in some specialized cells this process may involve specific molecular events that are still largely unknown. In mammals, loss of Citron-kinase (CIT-K) leads to massive cytokinesis failure and apoptosis only in neuronal progenitors and in male germ cells, resulting in severe microcephaly and testicular hypoplasia, but the reasons for this specificity are unknown. In this report we show that CIT-K modulates the stability of midbody microtubules and that the expression of tubulin β-III (TUBB3) is crucial for this phenotype. We observed that TUBB3 is expressed in proliferating CNS progenitors, with a pattern correlating with the susceptibility to CIT-K loss. More importantly, depletion of TUBB3 in CIT-K-dependent cells makes them resistant to CIT-K loss, whereas TUBB3 overexpression increases their sensitivity to CIT-K knockdown. The loss of CIT-K leads to a strong decrease in the phosphorylation of S444 on TUBB3, a post-translational modification associated with microtubule stabilization. CIT-K may promote this event by interacting with TUBB3 and by recruiting at the midbody casein kinase-2α (CK2α) that has previously been reported to phosphorylate the S444 residue. Indeed, CK2α is lost from the midbody in CIT-K-depleted cells. Moreover, expression of the nonphosphorylatable TUBB3 mutant S444A induces cytokinesis failure, whereas expression of the phospho-mimetic mutant S444D rescues the cytokinesis failure induced by both CIT-K and CK2α loss. Altogether, our findings reveal that expression of relatively low levels of TUBB3 in mitotic cells can be detrimental for their cytokinesis and underscore the importance of CIT-K in counteracting this event.

The Altered Hepatic Tubulin Code in Alcoholic Liver Disease

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

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

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

Cytoskeletal signaling: is memory encoded in microtubule lattices by CaMKII phosphorylation?

Memory is attributed to strengthened synaptic connections among particular brain neurons, yet synaptic membrane components are transient, whereas memories can endure. This suggests synaptic information is encoded and 'hard-wired' elsewhere, e.g. at molecular levels within the post-synaptic neuron. In long-term potentiation (LTP), a cellular and molecular model for memory, post-synaptic calcium ion (Ca²⁺) flux activates the hexagonal Ca²⁺-calmodulin dependent kinase II (CaMKII), a dodacameric holoenzyme containing 2 hexagonal sets of 6 kinase domains. Each kinase domain can either phosphorylate substrate proteins, or not (i.e. encoding one bit). Thus each set of extended CaMKII kinases can potentially encode synaptic Ca²⁺ information via phosphorylation as ordered arrays of binary 'bits'. Candidate sites for CaMKII phosphorylation-encoded molecular memory include microtubules (MTs), cylindrical organelles whose surfaces represent a regular lattice with a pattern of hexagonal polymers of the protein tubulin. Using molecular mechanics modeling and electrostatic profiling, we find that spatial dimensions and geometry of the extended CaMKII kinase domains precisely match those of MT hexagonal lattices. This suggests sets of six CaMKII kinase domains phosphorylate hexagonal MT lattice neighborhoods collectively, e.g. conveying synaptic information as ordered arrays of six "bits", and thus "bytes", with 64 to 5,281 possible bit states per CaMKII-MT byte. Signaling and encoding in MTs and other cytoskeletal structures offer rapid, robust solid-state information processing which may reflect a general code for MT-based memory and information processing within neurons and other eukaryotic cells.

Infectious delivery and long-term persistence of transgene expression in the brain by a 135-kb iBAC-FXN genomic DNA expression vector

Novel gene-based therapies for disease will depend in many cases on long-term persistent transgene expression. To develop gene therapy strategies for Friedreich's ataxia (FRDA), we have examined the persistence of transgene expression in the brain in vivo provided by the entire 135 kb FXN genomic DNA locus delivered as an infectious bacterial artificial chromosome (iBAC) herpes simplex virus type 1 (HSV-1)-based vector injected in the adult mouse cerebellum. We constructed genomic DNA-reporter fusion vectors carrying a complete 135 kb FXN genomic locus with an insertion of the Escherichia coli lacZ gene at the ATG start codon (iBAC-FXN-lacZ). SHSY5Y human neuroblastoma cells transduced by iBAC-FXN-lacZ showed high efficiency of vector delivery and LacZ expression. Direct intracranial injection of iBAC-FXN-lacZ into the adult mouse cerebellum resulted in a large number of easily detectable transduced cells, with LacZ expression driven by the FXN genomic locus, which persisted for at least 75 days. Green fluorescent protein expression driven from the same vector but by the strong HSV-1 IE4/5 promoter was transient. Our data demonstrate for the first time sustained transgene expression in vivo by infectious delivery of a genomic DNA locus >100 kb in size. Such an approach may be suitable for gene rescue strategies in neurological disease, such as FRDA.

"Memory bytes" - molecular match for CaMKII phosphorylation encoding of microtubule lattices

Learning, memory and long-term potentiation (LTP) are supported by factors including post-synaptic calcium ion flux activating and transforming the hexagonal calcium-calmodulin kinase II (CaMKII) holoenzyme. Upon calcium-induced activation, up to six kinase domains extend upward, and up to six kinase domains extend downward from the CaMKII association domain, the fully activated holoenzyme resembling a robotic insect 20 nanometers in length. Each extended kinase domain can be phosphorylated, and able to phosphorylate other proteins, thus potentially further encoding synaptic information at intraneuronal molecular sites for memory storage, processing and distribution. Candidate sites for phosphorylation-encoded molecular memory include microtubules, cylindrical lattice polymers of the protein tubulin. Using molecular modeling, we find spatial dimensions and geometry of the six extended CaMKII kinase domains can precisely match those of microtubule hexagonal lattice neighborhoods (both A- and B-lattices), and show two feasible phosphorylation mechanisms. In one, phosphorylation sites (e.g., valine 208) on a CaMKII extended kinase domain interact with serine 444 on a C-terminal "tail" of tubulin. In the second, the CaMKII kinase domain unfurls, enabling phosphorylation sites to contact threonine and serine sites on the tubulin surface. We suggest sets of six CaMKII kinase domains phosphorylate hexagonal microtubule lattice neighborhoods collectively, e.g., conveying synaptic information as ordered arrays of six "bits", and thus a "byte", with (minimally) 2⁶ (64) possible bit states per CaMKII-microtubule interaction. We model two levels of interaction between CaMKII and microtubules, suggesting a testable framework for molecular memory encoding.

Calcineurin and cytoskeleton in low-frequency depression

Transmitter release at high probability phasic synapses of crayfish neuromuscular junctions depresses by over 50% in 60 min when stimulated at 0.2 Hz. Inhibition of the protein phosphatase calcineurin by intracellular pre-synaptic injection of autoinhibitory peptide inhibited low-frequency depression (LFD) and resulted in facilitation of transmitter release. Since this inhibitor had no major effects when injected into the post-synaptic cell, only pre-synaptic calcineurin activity is necessary for LFD. To examine changes in phosphoproteins during LFD we performed a phosphoproteomic screen on proteins extracted from motor axons and nerve terminals after LFD induction or treatment with various drugs that affect kinase and phosphatase activity. Proteins separated by PAGE were stained with phospho-specific/total protein ratio stains (Pro-Q Diamond/SYPRO Ruby) to identify protein bands for analysis by mass spectrometry. Phosphorylation of actin and tubulin decreased during LFD, but increased when calcineurin was blocked. Tubulin and phosphoactin immunoreactivity in pre-synaptic terminals were also reduced after LFD. The actin depolymerizing drugs cytochalasin and latrunculin and the microtubule stabilizer taxol inhibited LFD. Therefore, dephosphorylation of pre-synaptic actin and tubulin and consequent changes in the cytoskeleton may regulate LFD. LFD is unlike long-term depression found in mammalian synapses because the latter requires in most instances post-synaptic calcineurin activity.Thus, this simpler invertebrate synapse discloses a novel pre-synaptic depression mechanism.

Roles of beta-tubulin residues Ala428 and Thr429 in microtubule formation in vivo

The C termini of beta-tubulin isotypes are regions of high sequence variability that bind to microtubule-associated proteins and motors and undergo various post-translational modifications such as polyglutamylation and polyglycylation. Crystallographic analyses have been unsuccessful in resolving tubulin C termini. Here, we used a stepwise approach to study the role of this region in microtubule assembly. We generated a series of truncation mutants of human betaI and betaIII tubulin. Transient transfection of HeLa cells with the mutants shows that mutants with deletions of up to 22 residues from betaIII and 16 from betaI can assemble normally. Interestingly, removal of the next residue (Ala(428)) results in a complete loss of microtubule formation without affecting dimer formation. C-terminal tail switching of human betaI and betaIII tubulin suggests that C-terminal tails are functionally equivalent. In short, residues outside of 1-429 of human beta-tubulins make no contribution to microtubule assembly. Ala(428), in the C-terminal sequence motif N-QQYQDA(428), lies at the end of helix H12 of beta-tubulin. We hypothesize that this residue is important for maintaining helix H12 structure. Deletion of Ala(428) may lead to unwinding of helix H12, resulting in tubulin dimers incapable of assembly. Thr(429) plays a more complex role. In the betaI isotype of tubulin, Thr(429) is not at all necessary for assembly; however, in the betaIII isotype, its presence strongly favors assembly. This result is consistent with a likely more complex function of betaIII as well as with the observation that evolutionary conservation is total for Ala(428) and frequent for Thr(429).

Effect of cortistatin on tau phosphorylation at Ser262 site

The development of intraneuronal lesions as a result of the progressive deposition of hyperphosphorylated tau at specific brain regions (such as hippocampus and cortex) plays a key role in the pathological process of Alzheimer's disease. However, the mechanisms by which tau phosphorylation is regulated, mainly in the pathology found in the cortex, are still poorly understood. Here, we analyzed the effect of cortistatin, a cortical neuropeptide related to somatostatin, on tau phosphorylation at Ser262 in cultures of murine cortical neurons. Both somatostatin and cortistatin induce tau phosphorylation at Ser262, a site modified in Alzheimer's disease, although with different kinetics in cortex. The effect of cortistatin likely is mediated by heterodimeric receptors composed of somatostatin receptor subtypes 2 and 4 and also by protein kinase C signaling. Cortistatin-deficient mice show decreased tau phosphorylation at Ser262 in the cortex but not in other brain regions tested. Our results suggest an important role for cortistatin in the regulation of tau phosphorylation that may be associated with the pathophysiology of Alzheimer's disease in regions such as the cerebral cortex.

Role of MAP1B in axonal retrograde transport of mitochondria

The MAPs (microtubule-associated proteins) MAP1B and tau are well known for binding to microtubules and stabilizing these structures. An additional role for MAPs has emerged recently where they appear to participate in the regulation of transport of cargos on the microtubules found in axons. In this role, tau has been associated with the regulation of anterograde axonal transport. We now report that MAP1B is associated with the regulation of retrograde axonal transport of mitochondria. This finding potentially provides precise control of axonal transport by MAPs at several levels: controlling the anterograde or retrograde direction of transport depending on the type of MAP involved, controlling the speed of transport and controlling the stability of the microtubule tracks upon which transport occurs.

Differentiation of a human neuroblastoma into neuron-like cells increases their susceptibility to transduction by herpesviral vectors

Gene transfer is a powerful tool for functional gene analysis in human cells. In this respect, there is a need to develop experimental models that involve homogeneous cultures of human neuron-like cells susceptible to gene transduction and that are easy to handle. Here we describe an optimized and reproducible procedure to differentiate human SH-SY5Y neuroblastoma cells into a homogeneous population of neuron-like cells. The fully differentiated cells are postmitotic and resemble primary cultured neurons in terms of their cytoskeletal polarity. Notably, differentiated SH-SY5Y cells are far more susceptible to transduction by herpes simplex virus (HSV-1)-based vectors than proliferating SH-SY5Y cells. This increase in transduction efficiency after neuronal differentiation may be due to the up-regulation of cell surface receptors for herpesvirus entry. In summary, we propose that fully differentiated human neuron-like cells obtained from the SH-SY5Y neuroblastoma may constitute an excellent and versatile experimental tool for gene transfer and functional genomic studies with HSV-1 vectors.

The shape of things to come: an emerging role for protein kinase CK2 in the regulation of cell morphology and the cytoskeleton

Protein kinase CK2 is a highly conserved, pleiotropic, protein serine/threonine kinase that is essential for life in eukaryotes. CK2 has been implicated in diverse cellular processes such as cell cycle regulation, circadian rhythms, apoptosis, transformation and tumorigenesis. In addition, there is increasing evidence that CK2 is involved in the maintenance of cell morphology and cell polarity, and in the regulation of the actin and tubulin cytoskeletons. Accordingly, this review will highlight published evidence in experimental models ranging from yeast to mammals documenting the emerging roles of protein kinase CK2 in the regulation of cell polarity, cell morphology and the cytoskeleton.

Identification of casein kinase 1, casein kinase 2, and cAMP-dependent protein kinase-like activities in Trypanosoma evansi

Trypanosoma evansi contains protein kinases capable of phosphorylating endogenous substrates with apparent molecular masses in the range between 20 and 205 kDa. The major phosphopolypeptide band, pp55, was predominantly localized in the particulate fraction. Anti-alpha and anti-beta tubulin monoclonal antibodies recognized pp55 by Western blot analyses, suggesting that this band corresponds to phosphorylated tubulin. Inhibition experiments in the presence of emodin, heparin, and 2,3-bisphosphoglycerate indicated that the parasite tubulin kinase was a casein kinase 2 (CK2)-like activity. GTP, which can be utilized instead of ATP by CK2, stimulated rather than inactivated the phosphorylation of tubulin in the parasite homogenate and particulate fraction. However, GTP inhibited the cytosolic CK2 responsible for phosphorylating soluble tubulin and other soluble substrates. Casein and two selective peptide substrates, P1 (RRKDLHDDEEDEAMSITA) for casein kinase (CK1) and P2 (RRRADDSDDDDD) for CK2, were recognized as substrates in T. evansi. While the enzymes present in the soluble fraction predominantly phosphorylated P1, P2 was preferentially labeled in the particulate fractions. These results demonstrated the existence of CK1-like and CK2-like activities primarily located in the parasite cytosolic and membranous fractions, respectively. Histone II-A and kemptide (LRRASVA) also behaved as suitable substrates, implying the existence of other Ser/Thr kinases in T. evansi. Cyclic AMP only increased the phosphorylation of histone II-A and kemptide in the cytosol, demonstrating the existence of soluble cAMP-dependent protein kinase-like activities in T. evansi. However, no endogenous substrates for this enzyme were identified in this fraction. Further evidences were obtained by using PKI (6-22), a reported inhibitor of the catalytic subunit of mammalian cAMP-dependent protein kinases, which specifically hindered the cAMP-dependent phosphorylation of histone II-A and kemptide in the parasite soluble fraction. Since the sum of the values obtained in the parasite cytosolic and particulate fractions were always higher than the values observed in the total T. evansi lysate, the kinase activities examined here appeared to be inhibited in the original extract.

Selectively enriched mRNAs in rat synaptoneurosomes

Differential display was used to identify synapse-enriched mRNAs. Of 15 mRNAs initially identified, all were found in multiple synaptoneurosome preparations; 58% were subsequently shown to be enriched in all the preparations by Northern blotting and semiquantitative RT-PCR. RNAs involved in signal transduction, vesicle trafficking, lipid modification and cell shape and remodeling were among these messages. Tip60a mRNA, recently found to associate with the fragile X mental retardation protein, was also identified. These data demonstrate the diversity of the local message pool at synapses.

A ubiquitous beta-tubulin disrupts microtubule assembly and inhibits cell proliferation

Vertebrate tubulin is encoded by a multigene family that produces distinct gene products, or isotypes, of both the alpha- and beta-tubulin subunits. The isotype sequences are conserved across species supporting the hypothesis that different isotypes subserve different functions. To date, however, most studies have demonstrated that tubulin isotypes are freely interchangeable and coassemble into all classes of microtubules. We now report that, in contrast to other isotypes, overexpression of a mouse class V beta-tubulin cDNA in mammalian cells produces a strong, dose-dependent disruption of microtubule organization, increased microtubule fragmentation, and a concomitant reduction in cellular microtubule polymer levels. These changes also disrupt mitotic spindle assembly and block cell proliferation. Consistent with diminished microtubule assembly, there is an increased tolerance for the microtubule stabilizing drug, paclitaxel, which is able to reverse many of the effects of class V beta-tubulin overexpression. Moreover, transfected cells selected in paclitaxel exhibit increased expression of class V beta-tubulin, indicating that this isotype is responsible for the drug resistance. The results show that class V beta-tubulin is functionally distinct from other tubulin isotypes and imparts unique properties on the microtubules into which it incorporates.

Direct Regulation of Microtubule Dynamics by Protein Kinase CK2

Microtubule dynamics is essential for many vital cellular processes such as morphogenesis and motility. Protein kinase CK2 is a ubiquitous protein kinase that is involved in diverse cellular functions. CK2 holoenzyme is composed of two catalytic alpha or alpha' subunits and two regulatory beta subunits. We show that the alpha subunit of CK2 binds directly to both microtubules and tubulin heterodimers. CK2 holoenzyme but neither of its individual subunits exhibited a potent effect of inducing microtubule assembly and bundling. Moreover, the polymerized microtubules were strongly stabilized by CK2 against cold-induced depolymerization. Interestingly, the kinase activity of CK2 is not required for its microtubule-assembling and stabilizing function because a kinase-inactive mutant of CK2 displayed the same microtubule-assembling activity as the wild-type protein. Knockdown of CK2alpha/alpha' in cultured cells by RNA interference dramatically destabilized their microtubule networks, and the destabilized microtubules were readily destructed by colchicine at a very low concentration. Further, over-expression of chicken CK2alpha or its kinaseinactive mutant in the endogenous CK2alpha/alpha'-depleted cells fully restored the microtubule resistance to the low dose of colchicine. Taken together, CK2 is a microtubule-associated protein that confers microtubule stability in a phosphorylation-independent manner.

Semaphorin 3C preserves survival and induces neuritogenesis of cerebellar granule neurons in culture

Semaphorins (sema) constitute a family of molecules sharing a common extracellular domain (semaphorin domain). This family includes several types of secreted and membrane-associated molecules that are grouped into eight subclasses (subclasses 1-7 and viral semaphorins). Subclass 3 semaphorins are secreted molecules involved in axonal guidance, mainly through repulsive gradients and induction of growth cone collapse. More recently sema 3 molecules have been identified as positive factors in dependence of the type of neurons. Besides their axonal guidance function, some semaphorins have been implicated in apoptosis and survival. We investigated the effect of sema3C on survival and neurite outgrowth of rat cerebellar granule neurons (CGNs) in culture. 3T3 cells were stably transfected with sema3C. Several clonal lines were established and tested for their neuritogenic activity and one, S3C-8, was selected for the bulk of experiments. S3C-8 was co-cultured with CGNs. Sema3C enhanced CGN viability as assessed in co-cultures of CGNs with monolayers of S3C-8 in comparison with co-cultures of CGNs with control mock-transfected 3T3 cells. Moreover sema3C induced neuritogenesis of cultured CGNs, which express neuropilin-1 and -2. S3C-8 cells, overexpressing sema3C, were significantly more neuritogenic for CGN than poly l-lysine (PLL), a positive substrate for CGNs, as assessed by the measurement of the length of neurites and confirmed by Tau expression along the time of culture. CGNs co-cultured with S3C-8, showed up-regulation of the expression of axonal microtubule-associated proteins (MAPs) such as Tau, phosphorylated MAP2C and mode I-phosphorylated MAP1B compared with neurons cultured on control 3T3 cells. We also found increased expression of a specific marker of neuronal cell bodies and dendrites, high molecular weight MAP2 (HMW-MAP2). Interestingly, there was no accompanying up-regulation of a marker enriched within the neuronal somatodendritic domain, mode II-phosphorylated MAP1B. These data support the idea that secreted sema3C favors survival and neuritogenesis of cultured CGNs.

Class III beta-tubulin isotype: a key cytoskeletal protein at the crossroads of developmental neurobiology and tumor neuropathology

The expression of the cytoskeletal protein class III beta-tubulin isotype is reviewed in the context of human central nervous system development and neoplasia. Compared to systemic organs and tissues, class III beta-tubulin is abundant in the brain, where it is prominently expressed during fetal and postnatal development. As exemplified in cerebellar neurogenesis, the distribution of class III beta-tubulin is neuron associated, exhibiting different temporospatial gradients in the neuronal progeny of the external granule layer versus the neuroepithelial germinal matrix of the velum medullare. However, transient expression of this protein is also present in the telencephalic subventricular zones comprising putative neuronal and/or glial precursor cells. This temporospatially restricted, potentially non-neuronal expression of class III beta-tubulin may have implications in the accurate identification of presumptive neurons derived from transplanted embryonic stem cells. In the adult central nervous system, the distribution of class III beta-tubulin is almost exclusively neuron specific. Altered patterns of expression are noted in brain tumors. In "embryonal"-type neuronal/neuroblastic tumors of the central nervous system, such as the medulloblastomas, class III beta-tubulin expression is associated with neuronal differentiation and decreased cell proliferation. In contrast, the expression of class III beta-tubulin in gliomas is associated with an ascending grade of histologic malignancy and with correspondingly high proliferative indices. Thus, class III beta-tubulin expression in neuronal or neuroblastic tumors is differentiation dependent, whereas in glial tumors, it is aberrant and/or represents "dedifferentiation" associated with the acquisition of glial progenitor-like phenotype(s). From a diagnostic perspective, the detection of class III beta-tubulin immunostaining in neoplastic cells should not be construed as categorical evidence of divergent neuronal differentiation in tumors, which are otherwise phenotypically glial. Because class III beta-tubulin is present in neoplastic but not in normal differentiated glial cells, the elucidation of molecular mechanisms responsible for the altered expression of this isotype may provide critical insights into the dynamics of the microtubule cytoskeleton in the growth and progression of gliomas.

Class III beta-tubulin in human development and cancer

The differential cellular expression of class III beta-tubulin isotype (betaIII) is reviewed in the context of human embryological development and neoplasia. As compared to somatic organs and tissues, betaIII is abundant in the central and peripheral nervous systems (CNS and PNS) where it is prominently expressed during fetal and postnatal development. As exemplified in cerebellar and sympathoadrenal neurogenesis, the distribution of betaIII is neuron-associated, exhibiting distinct temporospatial gradients according to the regional neuroepithelia of origin. However, transient expression of this protein is also present in the subventricular zones of the CNS comprising putative neuronal- and/or glial precursor cells, as well as in Kulchitsky neuroendocrine cells of the fetal respiratory epithelium. This temporally restricted, potentially non-neuronal expression may have implications in the identification of presumptive neurons derived from embryonic stem cells. In adult tissues, the distribution of betaIII is almost exclusively neuron-specific. Altered patterns of expression are noted in cancer. In "embryonal"- and "adult-type" neuronal tumors of the CNS and PNS, betaIII is associated with neuronal differentiation and decreased cell proliferation. In contrast, the presence of betaIII in gliomas and lung cancer is associated with an ascending histological grade of malignancy. Thus, betaIII expression in neuronal tumors is differentiation-dependent, while in non-neuronal tumors it is aberrant and/or represents "dedifferentiation" associated with the acquisition of progenitor-like phenotypic properties. Increased expression in various epithelial cancer cell lines is associated with chemoresistance to taxanes. Because betaIII is present in subpopulations of neoplastic, but not in normal differentiated glial or somatic epithelial cells, the elucidation of mechanisms responsible for the altered expression of this isotype may provide insights into the role of the microtubule cytoskeleton in tumorigenesis and tumor progression.

Immortalized olfactory ensheathing glia promote axonal regeneration of rat retinal ganglion neurons

Olfactory bulb ensheathing glia (OEG) have attracted special attention during the last few years because of their unique properties in promoting regeneration of adult mammalian central nervous system (CNS) components. However the molecular and cellular characteristics responsible for this capacity remain to be revealed. Such studies are presently hindered by the lack of a plentiful source of homogenous OEG. Thus the availability of immortalized OEG lines maintaining the regenerative characteristics of the primary cultures would represent an unlimited source of OEG for use not only in biochemical analyses of neuroregenerative mechanisms but also to characterize their regenerative properties in models in culture and in vivo. We have immortalized primary rat OEG using the SV40 large T antigen expressed from a constitutive cellular promotor, and report here the isolation and characterization of clonal lines. These OEG clonal lines were comparable to primary OEG and Schwann cells in the promotion of axonal regeneration of mature rat retinal ganglion neurons (RGN) but, significantly, this culture assay system more closely reflects the in vivo reparative properties of OEG on transected nerves than other assays of neuritogenesis in that it revealed OEG cells to promote the growth of a larger number of long axons than Schwann cells. Using this assay we were able to grade our OEG lines for their neuroregenerative capacity, opening the possibility of identifying molecules with correlative expression levels in these cells. Our preliminary characterization revealed that the expression level of a classical OEG marker, the p75-NGF receptor, does not correlate with neuroregenerative capacity.

Identification of sites of phosphorylation by G-protein-coupled receptor kinase 2 in beta-tubulin

G-protein-coupled receptor kinase 2 (GRK2) is known to specifically phosphorylate the agonist-bound forms of G-protein-coupled receptors (GPCRs). This strict specificity is due at least partly to activation of GRK2 by agonist-bound GPCRs, in which basic residues in intracellular regions adjacent to transmembrane segments are thought to be involved. Tubulin was found to be phosphorylated by GRK2, but it remains unknown if tubulin can also serve as both a substrate and an activator for GRK2. Purified tubulin, phosphorylated by GRK2, was subjected to biochemical analysis, and the phosphorylation sites in beta-tubulin were determined to be Thr409 and Ser420. In addition, the Ser444 in beta III-tubulin was also indicated to be phosphorylated by GRK2. The phosphorylation sites in tubulin for GRK2 reside in the C-terminal domain of beta-tubulin, which is on the outer surface of microtubules. Pretreatment of tubulin with protein phosphatase type-2A (PP2A) resulted in a twofold increase in the phosphorylation of tubulin by GRK2. These results suggest that tubulin is phosphorylated in situ probably by GRK2 and that the phosphorylation may affect the interaction of microtubules with microtubule-associated proteins. A GST fusion protein of a C-terminal region of beta I-tubulin (393-445 residues), containing 19 acidic residues but only one basic residue, was found to be a good substrate for GRK2, like full-length beta-tubulin. These results, together with the finding that GRK2 may phosphorylate synuclein and phosducin in their acidic domains, indicate that some proteins with very acidic regions but without basic activation domains could serve as substrates for GRK2.

Trypanosoma cruzi: in vitro phosphorylation of tubulin by a protein kinase CK2-like enzyme

One predominant 55-kDa polypeptide was phosphorylated in vitro in Trypanosoma cruzi homogenates prepared from three differentiation stages: epimastigotes, trypomastigotes, and spheromastigotes. Anti-alpha and anti-beta tubulin monoclonal antibodies immunoprecipitated the phosphorylated 55-kDa polypeptide from epimastigote extracts. Phosphoserine was the only residue phosphorylated in vitro in the 55-kDa polypeptide and in immunoprecipitated alpha tubulin. The phosphorylation of both the 55-kDa polypeptide and exogenously added casein was inhibited with GTP, heparin, and 2,3-bisphosphoglycerate in a dose-dependent manner, indicating the involvement of a CK2-like protein kinase. Moreover, when tubulin was isolated from an epimastigote homogenate by ultracentrifugation, followed by DEAE-Sephacel chromatography, a protein kinase that phosphorylated tubulin and casein co-purified with this cytoskeletal component. This result suggests an association between tubulin and its corresponding protein kinase in T. cruzi.

A pool of beta-tubulin is hyperphosphorylated at serine residues in Alzheimer disease brain

In Alzheimer disease (AD) brain, activities of protein phosphatase (PP)-2A/PP-1 which are known to be associated with microtubules are compromised and are probably a cause of neurofibrillary degeneration through hyperphosphorylation of microtubule proteins. In the present study, an increase of approximately 11 pmol phosphate/microg protein in 100,000 x g pellet from AD compared with age-matched control brains was found. Tau protein, which is hyperphosphorylated in AD can only account for approximately 4 pmol phosphate/microg protein, suggesting the presence of non-tau hyperphosphorylated proteins in the diseased brain. Western blot analysis with phosphoserine antibodies revealed a approximately 54 kDa non-tau protein to be significantly hyperphosphorylated in AD compared with age-matched control cases in the particulate fraction. The approximately 54 kDa protein was purified by preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis and identified as beta-tubulin by immunolabeling with specific antibodies, mass spectrometry analysis and by N-terminal amino acid sequencing. The purified protein was hyperphosphorylated at serine residues in AD.

Review: postchaperonin tubulin folding cofactors and their role in microtubule dynamics

The microtubule cytoskeleton consists of a highly organized network of microtubule polymers bound to their accessory proteins: microtubule-associated proteins, molecular motors, and microtubule-organizing proteins. The microtubule subunits are heterodimers composed of one alpha-tubulin polypeptide and one beta-tubulin polypeptide that should undergo a complex folding processing before they achieve a quaternary structure that will allow their incorporation into the polymer. Due to the extremely high protein concentration that exists at the cell cytoplasm, there are alpha- and beta-tubulin interacting proteins that prevent the unwanted interaction of these polypeptides with the surrounding protein pool during folding, thus allowing microtubule dynamics. Several years ago, the development of a nondenaturing electrophoretic technique made it possible to identify different tubulin intermediate complexes during tubulin biogenesis in vitro. By these means, the cytosolic chaperonin containing TCP-1 (CCT or TriC) and prefoldin have been demonstrated to intervene through tubulin and actin folding. Various other cofactors also identified along the alpha- and beta-tubulin postchaperonin folding route are now known to have additional roles in tubulin biogenesis such as participating in the synthesis, transport, and storage of alpha- and beta-tubulin. The future characterization of the tubulin-binding sites to these proteins, and perhaps other still unknown proteins, will help in the development of chemicals that could interfere with tubulin folding and thus modulating microtubule dynamics. In this paper, current knowledge of the above postchaperonin folding cofactors, which are in fact chaperones involved in tubulin heterodimer quaternary structure achievement, will be reviewed.

Activation of the AT(2) receptor of angiotensin II induces neurite outgrowth and cell migration in microexplant cultures of the cerebellum

Microexplant cultures from three-day-old rats were used to investigate whether angiotensin II (Ang II), through its AT(1) and AT(2) receptors, could be involved in the morphological differentiation of cerebellar cells. Specific activation of the AT(2) receptor during 4-day treatment induced two major morphological changes. The first was characterized by increased elongation of neurites. The second change was cell migration from the edge of the microexplant toward the periphery. Western blot analyses and indirect immunofluorescence studies revealed an increase in the expression of neuron-specific betaIII-tubulin, as well as an increase in expression of the microtubule-associated proteins tau and MAP2. These effects were demonstrated by co-incubation of Ang II with 1 microM DUP 753 (AT(1) receptor antagonist) or with 10 nM CGP 42112 (AT(2) receptor agonist) but abolished when Ang II was co-incubated with 1 microM PD 123319 (AT(2) receptor antagonist), indicating that differentiation occurs through AT(2) receptor activation and that the AT(1) receptor inhibits the AT(2) effect. Taken together, these results demonstrate that Ang II is involved in cerebellum development for both neurite outgrowth and cell migration, two important processes in the organization of the various layers of the cerebellum.

Isotype-specific changes in the amount of beta-tubulin RNA in synchronized tobacco BY2 cells

The 3'-ends of the beta-tubulin cDNA were amplified from tobacco BY2 polyA+ RNA. According to the differences in the predicted amino acid sequence at the extreme C-terminal, they were grouped into three different isotypes, NTB1 in which "EEGDYYEEDEEDLNEA", NTB2 in which "EEEYYEDEEEA QED" and NTB3 in which "DECEYEEEEEYDHEGN" follows the conservative "YQQYQDATAD" sequence. Using unique 3'-untranslated regions as probes, changes in the RNA levels of each beta-tubulin isotype were determined by dot-blot hybridization. The levels exhibited characteristic rhythms in the cell cycle. NTB1 RNA was highest in S phase in comparison to NTB2 RNA level which was highest in late G2. On the other hand, NTB3 RNA level was highest in early G2.

Downregulation of glycogen synthase kinase-3beta (GSK-3beta) protein expression during neuroblastoma IMR-32 cell differentiation

Glycogen synthase kinase-3gamma (GSK-3beta) is a multifunctional protein kinase that phosphorylates a variety of substrates including the neuronal-specific microtubule-associated protein tau. Here we report that the down-regulation of the GSK-3beta protein is an early event in the course of the differentiation of human neuroblastoma IMR-32 cells. This decline in GSK-3beta is accompanied by a significant decrease in the phosphorylation state of tau protein. A noteworthy increase in tau protein expression also takes place later during the differentiation of IMR-32 cells. The augmented expression and diminished phosphorylation of tau protein in differentiated IMR-32 cells can be correlated with increments in the assembly of microtubules and in the association of tau with microtubules. These results suggest a contribution of a decrease in GSK-3beta to molecular events leading to neuroblastoma cell differentiation. Among these, tau protein dephosphorylation might favor microtubule stabilization within neurites.

Binding and phosphorylation of tubulin by G protein-coupled receptor kinases

Although the beta-adrenergic receptor kinase (betaARK) mediates agonist-dependent phosphorylation and desensitization of G protein-coupled receptors, recent studies suggest additional cellular functions. During our attempts to identify novel betaARK interacting proteins, we found that the cytoskeletal protein tubulin could specifically bind to a betaARK-coupled affinity column. In vitro analysis demonstrated that betaARK and G protein-coupled receptor kinase-5 (GRK5) were able to stoichiometrically phosphorylate purified tubulin dimers with a preference for beta-tubulin and, under certain conditions, the betaIII-isotype. Examination of the GRK/tubulin binding characteristics revealed that tubulin dimers and assembled microtubules bind GRKs, whereas the catalytic domain of betaARK contains the primary tubulin binding determinants. In vivo interaction of GRK and tubulin was suggested by the following: (i) co-purification of betaARK with tubulin from brain tissue; (ii) co-immunoprecipitation of betaARK and tubulin from COS-1 cells; and (iii) co-localization of betaARK and GRK5 with microtubule structures in COS-1 cells. In addition, GRK-phosphorylated tubulin was found preferentially associated with the microtubule fraction during in vitro assembly assays suggesting potential functional significance. These results suggest a novel link between the cytoskeleton and GRKs that may be important for regulating GRK and/or tubulin function.

The G protein-coupled receptor kinase 2 is a microtubule-associated protein kinase that phosphorylates tubulin

The G protein-coupled receptor kinase 2 (GRK2) is a serine/threonine kinase that phosphorylates and desensitizes agonist-occupied G protein-coupled receptors (GPCRs). Here we demonstrate that GRK2 is a microtubule-associated protein and identify tubulin as a novel GRK2 substrate. GRK2 is associated with microtubules purified from bovine brain, forms a complex with tubulin in cell extracts, and colocalizes with tubulin in living cells. Furthermore, an endogenous tubulin kinase activity that copurifies with microtubules has properties similar to GRK2 and is inhibited by anti-GRK2 monoclonal antibodies. Indeed, GRK2 phosphorylates tubulin in vitro with kinetic parameters very similar to those for phosphorylation of the agonist-occupied beta2-adrenergic receptor, suggesting a functionally relevant role for this phosphorylation event. In a cellular environment, agonist occupancy of GPCRs, which leads to recruitment of GRK2 to the plasma membrane and its subsequent activation, promotes GRK2-tubulin complex formation and tubulin phosphorylation. These findings suggest a novel role for GRK2 as a GPCR signal transducer mediating the effects of GPCR activation on the cytoskeleton.

Expression of neuron-specific beta-III tubulin during olfactory neurogenesis in the embryonic and adult rat

The olfactory neuroepithelium retains the unique capacity to produce a new set of mature neurons every three to four weeks from a precursor population situated at the base of the epithelium. It is not known however, whether developing olfactory neurons in the adult rat follow the same program that is initiated embryonically. By tracking the expression of beta-III tubulin (by immunoreactivity to TuJ-1, an isoform-specific antibody) throughout embryogenesis, we have demonstrated a commitment to the olfactory neuron lineage in a subset of cells in the embryonic olfactory placode and followed their development into adulthood. We have also shown that this developmental pattern of beta-III tubulin expression is recapitulated in neurons undergoing a synchronized neurogenic response to either physical or chemical lesion in the adult neuroepithelium. The embryonic expression pattern reported here is similar to, but earlier than that reported for other markers of developing neurons, such as growth-associated protein-43 and neural cell adhesion molecule. The results of these studies suggest the retention of a conserved neurogenic program from embryonic to adult life in the olfactory neuron and, in addition, support the use of a readily accessible system such as the regenerating olfactory neuroepithelium as an alternative means of studying genes which may be crucial to normal neuronal development.

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.

Posttranslational modifications of alpha- and beta-tubulin in Giardia lamblia, an ancient eukaryote

Tubulin of Giardia lamblia, a representative of the oldest eukaryotes, was screened for posttranslational modifications. Mass spectrometry of the carboxy-terminal peptides documents a large number of variants. Both alpha- and beta-tubulin show polyglycylation with up to 20 and 15 extra glycyl residues respectively. Minor variants show a low level of glutamylation without or with glycylation. The glutamylation-specific antibody GT335 detects alpha- and beta-tubulin in immunoblots. The terminal tyrosine is fully retained in alpha-tubulin, which is completely acetylated at Lys-40. Thus except for the detyrosination/tyrosination cycle all posttranslational modifications known for higher eukaryotes are already present in Giardia.

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.

Characterization of microtubule-phosphofructokinase complex: specific effects of MgATP and vinblastine

Phosphofructokinase interacts with both microtubules and microtubules containing microtubule-associated proteins to produce bundling and periodical cross-bridging of tubules. Immunoelectron microscopy using anti-phosphofructokinase antibodies provided direct evidence that the kinase molecules are responsible for the cross-bridging of microtubules. Limited proteolysis by subtilisin, a procedure that cleaves the N-terminal segment of the free enzyme as well as the C-terminal "tails" of tubulin subunits exposed on microtubules, showed that while phosphofructokinase becomes resistant, tubulin retains sensitivity against proteolysis within the heterologous complex. These data suggest that the N-terminal segment of the enzyme, but not the C-terminal "tail" of tubulin subunits, is involved in the interaction between the microtubule and the kinase. The phosphorylation of phosphofructokinase or microtubules containing microtubule-associated proteins by the cAMP-dependent protein kinase did not interfere with the heterologous complex formation. MgATP prevents phosphofructokinase binding to the microtubules, and it can displace the enzyme from the single microtubules. However, the bundled microtubules are apparently resistant to the MgATP dissociation effect. Modelling of the assembly process suggests that the tubulin-kinase complex is able to polymerize as the free tubulin. Vinblastine, an anti-mitotic agent, inhibits tubulin assembly; however, its inhibitory effect is partially suppressed in the presence of phosphofructokinase. Fluorescence anisotropy measurements indicated that kinase and vinblastine compete for tubulin binding with no evidence for ternary complex formation. This competitive mechanism and the ability of the tubulin-enzyme complex to polymerize into microtubules may result in the resistance of the tubulin-enzyme complex against the inhibition of assembly induced by vinblastine. Microtubules formed in the presence of vinblastine plus phosphofructokinase can be visualized by electron microscopy. A molecular model is suggested that summarizes the effects of MgATP and vinblastine on the multiple equilibria in the tubulin/microtubules/phosphofructokinase system.

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

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

Phosphorylation of beta III-tubulin

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