The association of tubulin carboxypeptidase activity with microtubules in brain extracts is modulated by phosphorylation/dephosphorylation processes

Mathematical modeling of the microtubule detyrosination/tyrosination cycle for cell-based drug screening design

Microtubules and their post-translational modifications are involved in major cellular processes. In severe diseases such as neurodegenerative disorders, tyrosinated tubulin and tyrosinated microtubules are in lower concentration. We present here a mechanistic mathematical model of the microtubule tyrosination cycle combining computational modeling and high-content image analyses to understand the key kinetic parameters governing the tyrosination status in different cellular models. That mathematical model is parameterized, firstly, for neuronal cells using kinetic values taken from the literature, and, secondly, for proliferative cells, by a change of two parameter values obtained, and shown minimal, by a continuous optimization procedure based on temporal logic constraints to formalize experimental high-content imaging data. In both cases, the mathematical models explain the inability to increase the tyrosination status by activating the Tubulin Tyrosine Ligase enzyme. The tyrosinated tubulin is indeed the product of a chain of two reactions in the cycle: the detyrosinated microtubule depolymerization followed by its tyrosination. The tyrosination status at equilibrium is thus limited by both reaction rates and activating the tyrosination reaction alone is not effective. Our computational model also predicts the effect of inhibiting the Tubulin Carboxy Peptidase enzyme which we have experimentally validated in MEF cellular model. Furthermore, the model predicts that the activation of two particular kinetic parameters, the tyrosination and detyrosinated microtubule depolymerization rate constants, in synergy, should suffice to enable an increase of the tyrosination status in living cells.

Rab2 utilizes glyceraldehyde-3-phosphate dehydrogenase and protein kinase C{iota} to associate with microtubules and to recruit dynein

Rab2 requires glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and atypical protein kinase Ciota (aPKCiota) for retrograde vesicle formation from vesicular tubular clusters that sort secretory cargo from recycling proteins returned to the endoplasmic reticulum. However, the precise role of GAPDH and aPKCiota in the early secretory pathway is unclear. GAPDH was the first glycolytic enzyme reported to co-purify with microtubules (MTs). Similarly, aPKC associates directly with MTs. To learn whether Rab2 also binds directly to MTs, a MT binding assay was performed. Purified Rab2 was found in a MT-enriched pellet only when both GAPDH and aPKCiota were present, and Rab2-MT binding could be prevented by a recombinant fragment made to the Rab2 amino terminus (residues 2-70), which directly interacts with GAPDH and aPKCiota. Because GAPDH binds to the carboxyl terminus of alpha-tubulin, we characterized the distribution of tyrosinated/detyrosinated alpha-tubulin that is recruited by Rab2 in a quantitative membrane binding assay. Rab2-treated membranes contained predominantly tyrosinated alpha-tubulin; however, aPKCiota was the limiting and essential factor. Tyrosination/detyrosination influences MT motor protein binding; therefore, we determined whether Rab2 stimulated kinesin or dynein membrane binding. Although kinesin was not detected on membranes incubated with Rab2, dynein was recruited in a dose-dependent manner, and binding was aPKCiota-dependent. These combined results suggest a mechanism by which Rab2 controls MT and motor recruitment to vesicular tubular clusters.

Inhibitors of protein phosphatase 1 and 2A decrease the level of tubulin carboxypeptidase activity associated with microtubules

The association of tubulin carboxypeptidase with microtubules may be involved in the determination of the tyrosination state of the microtubules, i.e. their proportion of tyrosinated vs. nontyrosinated tubulin. We investigated the role of protein phosphatases in the association of carboxypeptidase with microtubules in COS cells. Okadaic acid and other PP1/PP2A inhibitors, when added to culture medium before isolation of the cytoskeletal fraction, produced near depletion of the carboxypeptidase activity associated with microtubules. Isolation of the native assembled and nonassembled tubulin fractions from cells treated and not treated with okadaic acid, and subsequent in vitro assay of the carboxypeptidase activity, revealed that the enzyme was dissociated from microtubules by okadaic acid treatment and recovered in the soluble fraction. There was no effect by nor-okadaone (an inactive okadaic acid analogue) or inhibitors of PP2B and of tyrosine phosphatases which do not affect PP1/PP2A activity. When tested in an in vitro system, okadaic acid neither dissociated the enzyme from microtubules nor inactivated it. In living cells, prior stabilization of microtubules with taxol prevented the dissociation of carboxypeptidase by okadaic acid indicating that dynamic microtubules are needed for okadaic acid to exert its effect. On the other hand, stabilization of microtubules subsequent to okadaic acid treatment did not reverse the dissociating effect of okadaic acid. These results suggest that dephosphorylation (and presumably also phosphorylation) of the carboxypeptidase or an intermediate compound occurs while it is not associated with microtubules, and that the phosphate content determines whether or not the carboxypeptidase is able to associate with microtubules.

Post-translational incorporation of the antiproliferative agent azatyrosine into the C-terminus of alpha-tubulin

Detyrosination/tyrosination of tubulin is a post-translational modification that occurs at the C-terminus of the alpha-subunit, giving rise to microtubules rich in either tyrosinated or detyrosinated tubulin which coexist in the cell. We hereby report that the tyrosine analogue, azatyrosine, can be incorporated into the C-terminus of alpha-tubulin instead of tyrosine. Azatyrosine is structurally identical to tyrosine except that a nitrogen atom replaces carbon-2 of the phenolic group. Azatyrosine competitively excluded incorporation of [14C]tyrosine into tubulin of soluble brain extract. A newly developed rabbit antibody specific to C-terminal azatyrosine was used to study incorporation of azatyrosine in cultured cells. When added to the culture medium (Ham's F12K), azatyrosine was incorporated into tubulin of glioma-derived C6 cells. This incorporation was reversible, i.e. after withdrawal of azatyrosine, tubulin lost azatyrosine and reincorporated tyrosine. Azatyrosinated tubulin self-assembled into microtubules to a similar degree as total tubulin both in vitro and in vivo. Studies by other groups have shown that treatment of certain types of cultured cancer cells with azatyrosine leads to reversion of phenotype to normal, and that administration of azatyrosine into animals harbouring human proto-oncogenic c-Ha- ras prevents tumour formation. These interesting observations led us to study this phenomenon in relation to tubulin status. Under conditions in which tubulin was mostly azatyrosinated, C6 cells remained viable but did not proliferate. After 7-10 days under these conditions, morphology changed from a fused, elongated shape to a rounded soma with thin processes. Incorporation of azatyrosine into the C-terminus of alpha-tubulin is proposed as one possible cause of reversion of the malignant phenotype.

Suppression of tubulin tyrosine ligase activity through reversible phosphorylation: a mechanism for inhibition of alpha-tubulin tyrosinylation

The tubulin tyrosinylation/detyrosinylation cycle is a well-established posttranslational modification, which is carried out by two enzymes: tubulin tyrosine ligase (TTL) and tubulin tyrosine carboxypeptidase (TTCP). In this paper, I present evidence suggesting that the cycle itself is under the hierarchical control of reversible phosphorylation and that proteinkinase C (PKC)-mediated phosphorylation of TTL inhibits its activity, thereby preventing tubulin tyrosinylation. Phosphorylation of TTL is postulated to occur in its presumed Mg(++)-ATP binding fold, leading to inhibition of Mg(++)/ATP binding and TTL mediated catalysis. The implications of such control are also discussed. Copyright 2001 Harcourt Publishers Ltd.

Phosphorylation of tubulin tyrosine ligase: a potential mechanism for regulation of alpha-tubulin tyrosination

The tubulin tyrosination/detyrosination cycle is a well-established posttranslational modification, which is carried out by two enzymes: Tubulin Tyrosine Ligase (TTL) and Tubulin Tyrosine Carboxypeptidase (TTCP). In this paper, I present evidence suggesting that the cycle itself is under the hierarchical control of reversible phosphorylation and that PKC mediated phosphorylation of TTL inhibits its activity, thereby preventing tubulin tyrosination. Phosphorylation of TTL is predicted to occur in a postulated Mg(++)/-ATP binding fold, leading to inhibition of Mg(++)/ATP binding and TTL mediated catalysis. The implications of such control are also discussed.

Tubulin carboxypeptidase assay based on the action of the enzyme on [14C]tyrosinated tubulin bound to nitrocellulose membrane

We have developed a method for the determination of tubulin carboxypeptidase activity which is based on the action of the enzyme on the substrate, [14C]tyrosinated tubulin, previously adsorbed on nitrocellulose membrane. In addition to being two to three times more sensitive than previous carboxypeptidase assays, this method allows the determination of dilute enzyme preparations even containing high salt (inhibitory) concentrations. This is a valuable property specially under circumstances in which numerous high salt-containing fractions with scarce activity should be analyzed (for example after certain chromatographic stages during enzyme purification). Our method is simpler, less time-consuming, and suitable for multiple, simultaneous determinations and the substrate bound to nitrocellulose can be stored for several months without significant alteration of its properties. Peptidases other than tubulin carboxypeptidase can act on [14C]tyrosinated tubulin bound to nitrocellulose, solubilizing radioactive compounds, suggesting the eventual applicability of this method to assay proteases in general. Other features and advantages of the assay as well as its limitations are discussed.

Tubulin carboxypeptidase/microtubules association can be detected in the distal region of neural processes

The association of tubulin carboxypeptidase with microtubules has been demonstrated in crude brain extracts and in living non-nervous cells. Here, we studied this phenomenon in cultured brain cells. To determine the association of the enzyme with neural microtubules we isolated the cytoskeletons (detergent-extraction under microtubule-stabilizing conditions) and measured the content of Tyr, Glu, and delta2 tubulin as a function of the in vitro incubation time of the cytoskeletons. The carboxypeptidase was found associated with microtubules in 2 days-cultured cells but not in 7 days-cultured cells. Quantitative analysis of digitized images after immunofluorescent staining revealed that detyrosination during the incubation of the cytoskeletons occurred preferentially in the distal regions of the neural processes. Prolonged taxol-treatment of the cells promoted higher detyrosination but Tyr tubulin was not depleted suggesting the existence of a subset of microtubules that has not associated carboxypeptidase and therefore cannot be detyrosinated even after prolonged taxol-treatment. This hypothesis was supported, although not conclusively, by additional experiments.

The third tubulin pool

Tubulin normally undergoes a cycle of detyrosination/tyrosination on the carboxy terminus of its alpha-subunit and this results in subpopulations of tyrosinated tubulin and detyrosinated tubulin. Brain tubulin preparations also contain a third major tubulin subpopulation which is non-tyrosinatable. This review describes the purification and the structural characterization of non-tyrosinatable tubulin. This tubulin variant lacks a carboxyterminal glutamyl-tyrosine group on its alpha-subunit (delta2-tubulin). Delta2-tubulin is generated from detyrosinated tubulin through an irreversible reaction. Delta2-tubulin accumulates in neurons and in stable microtubule assemblies. It also accumulates in some tumor cells due to the frequent loss of tubulin tyrosine ligase in such cells. Delta2-tubulin may be a useful marker of malignancy in human tumors.