Evidence Against Impaired Brain Microtubule Protein Polymerization at High Glucose Concentrations or During Diabetes Mellitus

Previous studies suggest that brain microtubule protein exposed to high glucose levels or isolated from diabetic rats can become glucosylated and that this impairs GTP-induced microtubule polymerization. We set out to extend that investigation to define the mechanistic basis for inhibition of microtubule assembly during diabetes or on incubation at high glucose levels. Rat and bovine brain microtubule protein was purified by cycles of polymerization/depolymerization. When microtubules were incubated for 1 h in either buffer or buffer containing glucose (up to 165 mM), there was no difference in polymerization, a finding contrary to the earlier study. Other rats were injected with vehicle or streptozotocin (90 mg/kg) to induce diabetes as evidenced by serum glucose in excess of 300 mg%, and at 4 weeks, brain microtubule protein was isolated by the polymerization cycling method. Again, there was no difference in the amount or purity of isolated microtubule protein between control or diabetic rats. We also observed no increase in microtubule glucosylation, and GTP-induced polymerization in vitro was indistinguishable for protein derived from brains of normal rats and rats with diabetes as measured by turbidity or electron microscopy. Our results suggest that in vitro incubation with glucose or in vivo elevation of glucose during diabetes fails to impair microtubule polymerization, pointing to other mechanisms for the neuropathy associated with diabetes.     

ch-TOGp is required for microtubule aster formation in a mammalian mitotic extract

Microtubules induced to polymerize with taxol in a mammalian mitotic extract organize into aster-like arrays in a centrosome-independent process that is driven by microtubule motors and structural proteins. These microtubule asters accurately reflect the noncentrosomal aspects of mitotic spindle pole formation. We show here that colonic-hepatic tumor-overexpressed gene (ch-TOGp) is an abundant component of these asters. We have prepared ch-TOGp-specific antibodies and show by immunodepletion that ch-TOGp is required for microtubule aster assembly. Microtubule polymerization is severely inhibited in the absence of ch-TOGp, and silver stain analysis of the ch-TOGp immunoprecipitate indicates that it is not present in a preformed complex and is the only protein removed from the extract during immunodepletion. Furthermore, the reduction in microtubule polymerization efficiency in the absence of ch-TOGp is dependent on ATP. These results demonstrate that ch-TOGp is a major constituent of microtubule asters assembled in a mammalian mitotic extract and that it is required for robust microtubule polymerization in an ATP-dependent manner in this system even though taxol is present. These data, coupled with biochemical and genetic data derived from analysis of ch-TOGp-related proteins in other organisms, indicate that ch-TOGp is a key factor regulating microtubule dynamics during mitosis.     

Enhancement of tubulin assembly as monitored by a rapid filtration assay

The early kinetics of microtubule formation from lamb brain tubulin isolated by affinity chromatography can be followed by a newly developed filter assay. The rapid collection of microtubules on glass fiber filters permits the calculation of the moles of tubulin polymerized. The filter assay gives both a rate and extent of polymerization that are identical to those obtained by turbidity or sedimentation analysis, respectively. The microtubules trapped by the filter are readily depolymerized by cold (t12= 3 min) and slowly by colchicine (t1/2= 32min). Tubulin purified by affinity chromatography requires a high protein concentration (>4 mg/ml) for polymerization. Although 5m glycerol allows polymerization to occur at tubulin concentrations below 2 mg/ml, the maximum amount of microtubule formation is observed at low tubulin concentration when microtubule-associated proteins are present. These proteins are not retained by the affinity resin; however, they can be eluted from diethylaminoethyl-Sephadex by solutions containing 0.3m KCl. Microtubule-associated proteins enhance both the rate of polymerization and the total amount of tubulin polymerized as assessed by the filter assay, suggesting that they are involved in both initiation and elongation of microtubules.     

A fluorescence-based high-throughput assay for antimicrotubule drugs

With the advent of combinatorial chemistry and the extensive libraries of potential drugs produced from it, there is a growing need for rapid sensitive, high-throughput screening for drug potency. Microtubules are important targets for anticancer agents, and new antimicrotubule compounds are of continued interest in drug development. The in vitro potency of antimicrotubule drugs may be evaluated by measuring the extent of tubulin assembly. The extent of polymerization is proportional to the turbidity of the solution, which usually has been measured as apparent absorption. The turbidity method has inherent problems that hinder its adaptation to a high-throughput format, such as a requirement for high protein concentrations and a high coefficient of variation. We present here a high-throughput assay for antimicrotubule activity in which fluorescence is used to monitor microtubule assembly. Both assembly-inhibiting and assembly-promoting compounds can be evaluated. The assay is rapid and easy to perform, and the data are reliable, with good accuracy and reproducibility.     

Fluorescence spot tests for DNA endonuclease, ligase, and topoisomerase activities

A rapid dilution cuvette has been designed for measurement of the rates of microtubule disassembly and the factors which stabilize tubules against dilution-induced depolymerization. The device minimizes microtubule shearing by obviating the passage of microtubules through small orifices, and optical measurements of the turbidity changes during disassembly are optimized by use of a long-path-length configuration. Details on the design and application of the rapid dilution cuvette to probe microtubule disassembly are described here. In principle, the method may be applied to study any indefinite polymerization process which is characterized by critical concentration phenomena.     

Computer modeling of fibrin polymerization kinetics correlated with electron microscope and turbidity observations: clot structure and assembly are kinetically controlled.

Although much is known about fibrin polymerization, because it is complex, the effects of various modifications are not intuitively obvious and many experimental observations remain unexplained. A kinetic model presented here that is based on information about mechanisms of assembly accounts for most experimental observations and allows hypotheses about the effects of various factors to be tested. Differential equations describing the kinetics of polymerization were written and then solved numerically. The results have been related to turbidity profiles and electron microscope observations. The concentrations of intermediates in fibrin polymerization, and fiber diameters, fiber and protofibril lengths have been calculated from these models. The simplest model considered has three steps; fibrinopeptide A cleavage, protofibril formation, and lateral aggregation of protofibrils to form fibers. The average number of protofibrils per fiber, which is directly related to turbidity, can be calculated and plotted as a function of time. The lag period observed in turbidity profiles cannot be accurately simulated by such a model, but can be simulated by modifying the model such that oligomers must reach a minimum length before they aggregate. Many observations, reported here and elsewhere, can be accounted for by this model; the basic model may be modified to account for other experimental observations. Modeling predicts effects of changes in the rate of fibrinopeptide cleavage consistent with electron microscope and turbidity observations. Changes only in the rate constants for initiation of fiber growth or for addition of protofibrils to fibers are sufficient to account for a wide variety of other observations, e.g., the effects of ionic strength or fibrinopeptide B removal or thrombospondin. The effects of lateral aggregation of fibers has also been modeled: such behavior has been observed in turbidity curves and electron micrographs of clots formed in the presence of platelet factor 4. Thus, many aspects of clot structure and factors that influence structure are directly related to the rates of these steps of polymerization, even though these effects are often not obvious. Thus, to a large extent, clot structure is kinetically determined.     

The sequence of a region of bacteriophage phiX174 DNA coding for parts of genes A and B

The kinetics of microtubule assembly were investigated by monitoring changes in turbidity which result from the scattering of incident light by the polymer. These studies indicated that assembly occurred by a pathway involving a nucleation phase, followed by an elongation phase as evidenced by a lag in the polymerization kinetics, followed by a psuedo-first-order exponential increase in turbidity. Analytical ultracentrifugation of solutions polymerized to equilibrium showed that 6 S tubulin was the only species detectable in equilibrium with microtubules. Investigation of the elongation reaction in mixtures of 6 S tubulin and microtubule fragments demonstrated that: (1) the net rate of assembly was the sum of the rates of polymerization and depolymerization; (2) the rate of polymerization was proportional to the product of the microtubule number concentration and the 6 S tubulin concentration; and (3) the rate of depolymerization was proportional to the number concentration of microtubules. These results demonstrate that microtubule assembly occurs by a condensation polymerization mechanism consisting of distinct nucleation and elongation steps. Microtubules are initiated in a series of protein association reactions in a pathway that has not been fully elucidated. Elongation proceeds by the consecutive association of 6 S tubulin subunits onto the ends of existing microtubules. Similarly, depolymerization occurs by dissociation of 6 S subunits from the ends of microtubules. The rate constants measured for polymerization and depolymerization at 30 °C were 4 × 10⁶m^?1 s^?1 and 7 s^?1, respectively.     

Bundling of microtubules in vitro by a high molecular weight protein prepared from the squid axon

A high molecular weight protein has been partially purified from sheaths of squid giant axons. This protein fraction was capable of restoring the membrane excitability of the squid axon which had been destroyed by internal perfusion of microtubule poison, when perfused along with microtubule proteins (Matsumoto et al. (1979) J. Biochem. 86, 1155-1158). This protein, designated as 260 K protein, was purified by gel filtration and Con A-Sepharose affinity chromatography. The apparent molecular weight of the axonal protein was estimated to be 260,000 by electrophoresis in the presence of sodium dodecylsulfate. This protein was revealed to be a glycoprotein. When phosphocellulose-purified tubulin was incubated with 260 K protein at 36 degrees C in the presence of dimethylsulfoxide, turbidity of the solution was much increased. 260 K protein co-sedimented with microtubles assembled from purified tubulin. Light microscopic and electron microscopic observations revealed that the high turbidity was due to bundling of microtubules which was caused by 260 K protein. On the other hand, the effect of this protein on the turbidity increase was not so prominent when microtubules were assembled from microtubule proteins consisting of tubulin and microtubule-associated proteins. High shear and low shear viscometry and co-sedimentation experiments revealed that 260 K protein had little effect on actin polymerization under the same medium conditions as used in tubulin polymerization.     

How calcium controls microtubule anisotropic phase formation in the presence of microtubule-associated proteins in vitro

Here we show a new effect of Ca²⁺ on microtubule morphology: Ca²⁺ can cause smooth curving of microtubules in the presence of microtubule-associated proteins (MAPs). In vitro, microtubules self-organize, forming complex dissipative structures. Such structures may be strongly affected by relatively weak external factors. A factor such as Ca²⁺ potentially influences spatiotemporal patterns of microtubule assembly, but the dynamics are unclear. We tested Ca²⁺ effects on microtuble formation. Using EM, microtubule length, curvature, and alignment and were measured in two systems: 2 mg/ml microtubule protein containing MAPs and 1 mM EGTA with and without 1 mM Ca²⁺. The two systems were then tested using light scattering. In low Ca²⁺, a birefringent microtubular pattern is seen, increasing with polymerization. When 1 mM Ca²⁺ is added to the solution. anisotropic phase is prevented without microtubule disruption. This demonstrates an additional mechanism by which Ca²⁺ can alter the dynamics and morphology of microtules.     

Prion protein inhibits microtubule assembly by inducing tubulin oligomerization

A growing body of evidence points to an association of prion protein (PrP) with microtubular cytoskeleton. Recently, direct binding of PrP to tubulin has also been found. In this work, using standard light scattering measurements, sedimentation experiments, and electron microscopy, we show for the first time the effect of a direct interaction between these proteins on tubulin polymerization. We demonstrate that full-length recombinant PrP induces a rapid increase in the turbidity of tubulin diluted below the critical concentration for microtubule assembly. This effect requires magnesium ions and is weakened by NaCl. Moreover, the PrP-induced light scattering structures of tubulin are cold-stable. In preparations of diluted tubulin incubated with PrP, electron microscopy revealed the presence of approximately 50 nm disc-shaped structures not reported so far. These unique tubulin oligomers may form large aggregates. The effect of PrP is more pronounced under the conditions promoting microtubule formation. In these tubulin samples, PrP induces formation of the above oligomers associated with short protofilaments and sheets of protofilaments into aggregates. Noticeably, this is accompanied by a significant reduction of the number and length of microtubules. Hence, we postulate that prion protein may act as an inhibitor of microtubule assembly by inducing formation of stable tubulin oligomers.     

RGS14 is a Microtubule-Associated Protein

Heterotrimeric G-proteins and their regulators are emerging as important players in modulating microtubule polymerization dynamics and in spindle force generation during cell division in C. elegans, D. melanogaster, and mammals. We recently demonstrated that RGS14 is required for completion of the first mitotic division of the mouse embryo, and that it regulates microtubule organization in vivo. Here, we demonstrate that RGS14 is a microtubule associated protein and a component of the mitotic spindle that may regulate microtubule polymerization and spindle organization. Taxol-stabilized tubulin, but not depolymerized tubulin co-immunoprecipitates with RGS14 from cell extracts. Furthermore, RGS14 co-purifies with tubulin from porcine brain following multiple rounds of microtubule polymerization/depolymerization and binds directly to microtubules formed in vitro from pure tubulin (KD=1.3 +/- 0.3 ?M). Both RGS14 and G?i1 in the presence of exogenous GTP promote tubulin polymerization, which is dependent on additional microtubule associated proteins. However, preincubation of RGS14 with G?i1-GDP precludes either from promoting microtubule polymerization, suggesting that a functional GTP/GDP cycle is necessary. Finally, we show that RGS14 is a component of mitotic asters formed in vitro from HeLa cell extracts and that depletion of RGS14 from cell extracts blocks aster formation. Collectively, these results show that RGS14 is a microtubule associated protein that may modulate microtubule dynamics and spindle formation.     

Decreased lateral aggregation of a variant recombinant fibrinogen provides insight into the polymerization mechanism

We analyzed the polymerization of BbetaA68T fibrinogen, the recombinant counterpart of fibrinogen Naples, a variant known to have decreased thrombin binding. When polymerized with equal thrombin concentrations, BbetaA68T fibrinogen had a longer lag time and lower rate of lateral aggregation, V(max), than normal recombinant fibrinogen, but a similar final turbidity. At thrombin concentrations that equalized the rates of fibrinopeptide A release, BbetaA68T fibrinogen polymerized with a lag time and V(max) similar to normal, but reached a significantly lower final turbidity. Similar results were produced when BbetaA68T was polymerized with Ancrod, which cleaves fibrinopeptide A at the same rate from either fibrinogen, and when BbetaA68T desA monomers were polymerized. The polymerization of desAB fibrin monomers, which circumvents fibrinopeptide release, was the same for both fibrinogens. We confirmed that turbidity was indicative of fiber thickness by scanning electron microscopy of fibrin clots. Here, we present the first experimental evidence of fibrin polymerization with a normal period of protofibril formation and rate of lateral aggregation, but with a significantly decreased extent of lateral aggregation. We conclude that the decreased lateral aggregation seen in BbetaA68T fibrinogen is due to an altered step in the enzymatic phase of its polymerization process. We propose that during normal polymerization a subtle conformational change in the E domain occurs, between the release of FpA and FpB, and that this change modulates the mechanism of lateral aggregation. Without this change, the lateral aggregation of BbetaA68T fibrinogen is impaired such that variant clots have thinner fibers than normal clots.     

Effects of vanadate on the assembly and disassembly of purified tubulin

Sodium-orthovanadate (100-700 microM) added to purified pig brain microtubule protein (molar ratios 13-90 moles vanadate/mole tubulin) inhibits to a considerable extent the assembly (up to 65%) and the disassembly rates (up to 60%) of microtubules, as determined by turbidimetry. Vanadate added to preformed microtubules did not appreciably alter the turbidity level of the samples, however, the disassembly rates were decreased in the same manner as when vanadate was added prior to polymerization. Microtubule protein kept on ice for 3-6 hours became more susceptible to vanadate than freshly prepared protein. The effect of vanadate was independent of the GTP concentration at which the polymerization assays were performed (0.025 to 1 mM GTP). In the presence of taxol, which increases the rate and extent of microtubule formation, vanadate had no effect on assembly rates. Disassembly was inhibited, however, much less than in the presence of vanadate alone. Electron microscopy and polyacrylamide gel electrophoresis did not reveal differences between microtubules prepared in the presence or in the absence of vanadate. This is consistent with the notion that vanadate does not interfere with the interaction between tubulin and the high-molecular weight microtubule-associated proteins. Apparently vanadate brings about an allosteric change of the microtubule protein(s) resulting in the abnormal polymerization kinetics of tubulin found in our study. The above results may be relevant for studies where the effects of vanadate on intracellular motility are interpreted as being solely due to a specific inhibition of ATPases.     

Structural thermal adaptation of β-tubulins from the Antarctic psychrophilic protozoan Euplotes focardii

Tubulin dimers of psychrophilic eukaryotes can polymerize into microtubules at 4°C, a temperature at which microtubules from mesophiles disassemble. This unique capability requires changes in the primary structure and/or in post-translational modifications of the tubulin subunits. To contribute to the understanding of mechanisms responsible for microtubule cold stability, here we present a computational structural analysis based on molecular dynamics (MD) and experimental data of three β-tubulin isotypes, named EFBT2, EFBT3, and EFBT4, from the Antarctic protozoon Euplotes focardii that optimal temperature for growth and reproduction is 4°C. In comparison to the β-tubulin from E. crassus, a mesophilic Euplotes species, EFBT2, EFBT3, and EFBT4 possess unique amino acid substitutions that confer different flexible properties of the polypeptide, as well as an increased hydrophobicity of the regions involved in microtubule interdimeric contacts that may overcome the microtubule destabilizing effect of cold temperatures. The structural analysis based on MD indicated that all isotypes display different flexibility properties in the regions involved in the formation of longitudinal and lateral contacts during microtubule polymerization. We also investigated the role of E. focardii β-tubulin isotypes during the process of cilia formation. The unique characteristics of the primary and tertiary structures of psychrophilic β-tubulin isotypes seem responsible for the formation of microtubules with distinct dynamic and functional properties.     

Separation of active tubulin and microtubule-associated proteins by ultracentrifugation and isolation of a component causing the formation of microtubule bundles

A new method for separating microtubule-associated proteins (MAPs) and tubulin, appropriate for relatively large-scale preparations, was developed. Most of the active tubulin was separated from the MAPs by centrifugation after selective polymerization of the tubulin was induced with 1.6 M 2-(N-morpholino)ethanesulfonate (Mes) and GTP. The MAPs-enriched supernatant was concentrated and subsequently clarified by prolonged centrifugation. The supernatant (total soluble MAPs) contained almost no tubulin, most of the nucleosidediphosphate kinase activity of the microtubule protein, good activity in promoting microtubule assembly in 0.1 M Mes, and proteins with the electrophoretic mobility of MAP-1, MAP-2, and tau factor. The pellet, inactive in supporting microtubule assembly, contained denatured tubulin, most of the ATPase activity of the microtubule protein, and significant amounts of protein with the electrophoretic mobility of MAP-2. Insoluble material at this and all previous stages, including the preparation of the microtubule protein, could be heat extracted to yield soluble protein active in promoting microtubule assembly and containing MAP-2 as a major constituent. The total soluble MAPs were further purified by DEAE-cellulose chromatography into bound and unbound components, both of which induced microtubule assembly. The bound component (DEAE-MAPs) contained proteins with the electrophoretic mobility of MAP-1, MAP-2, and tau factor. The polymerization reaction induced by the unbound component (flow-through MAPs) produced very high turbidity readings. This was caused by the formation of bundles of microtubules. Although the flow-through MAPs contained significantly more ATPase, tubulin-independent GTPase, and, especially, nucleosidediphosphate kinase activity than the DEAE-MAPs, preparation of a MAPs fraction without these enzymes required heat treatment.     

Involvement of the cytoskeleton in early grey crescent formation in axolotl oocytes

Summary Maturing axolotl oocytes which are treated with protein synthesis inhibitors or which are heat-shocked can be induced to reorganize their cytoplasm and to form an early grey crescent. The maturing axolotl oocyte has been used as a model system to study the role of the cytoskeleton in dorsoventral polarization as visualized by grey crescent formation. Results presented here provide evidence for the involvement of microtubules in the formation of the early grey crescent. Whereas inhibitors of microtubule polymerization and antibodies against tubulin both elicit early grey crescent formation, the effect of taxol shows that microtubule polymerization is required at a late stage in this event. The nucleus furnishes important factors required for early grey crescent formation and might play a role in microtubule polymerization.     

A porcine brain protein (35 K protein) which bundles microtubules and its identification as glyceraldehyde 3-phosphate dehydrogenase

A protein which binds to both tubulin and tubulin polymer was isolated from porcine brains. This protein has a molecular weight of 35,000 on SDS-polyacrylamide gel electrophoresis (designated as 35 K protein). The 35 K protein was purified through several steps of purification including ammonium sulfate fractionation, Sephadex G-100 gel filtration column chromatography, microtubule protein-agarose gel affinity column chromatography and phosphocellulose column chromatography. The 35 K protein caused pronounced enhancement of the turbidity increase produced by tubulin polymerization in the presence of DMSO, but did not have the ability to initiate polymerization of pure tubulin in the absence of DMSO. It was demonstrated that 35 K protein co-sediments with tubulin polymer in a concentration-dependent manner. Electron microscopic observation revealed the formation of bundles of tubulin polymer. Since the effect of 35 K protein was coupled with tubulin polymerization, 35 K protein did not cause the turbidity increase under conditions where tubulin polymerization was inhibited by Ca2+ or colchicine. The 35 K protein adsorbed on tubulin-Sepharose 4B was eluted by the addition of 2 mM ATP. ATP was shown to inhibit the interaction of 35 K protein with tubulin dimer or polymer. The 35 K protein was finally identified as glyceraldehyde 3-phosphate dehydrogenase from properties such as mobility on SDS-polyacrylamide gel electrophoresis, cleavage pattern on limited proteolysis, ability to bind to tubulin, and so on.     

Tubulin-colchicine complex (TC) inhibits microtubule depolymerization by a capping reaction exerted preferentially at the minus end

The effects of colchicine and tubulin-colchicine complex (TC) on microtubule depolymerization were studied using the axoneme-subunit system described previously [Bergen LG, Borisy GG; J Cell Biol 84:141-150, 1980]. This system allows the independent analysis of the polymerization kinetics at both the plus and minus ends of a microtubule. Depolymerization was induced by isothermal dilution with 10 volumes of an experimental solution containing colchicine, TC, or buffer alone. Colchicine alone (5-100 microM) blocked depolymerization at the minus end, whereas depolymerization at the plus end occurred at almost control rates. A similar effect was produced by TC (0.4:1-1:1 molar ratio to free tubulin). High molar ratios of TC to tubulin (10:1) blocked depolymerization at both plus and minus ends, and intermediate molar ratios of TC:T allowed depolymerization of the plus ends but at attenuated rates. The blockage was not readily reversible; TC-affected ends neither shortened upon dilution nor grew longer upon incubation with additional tubulin. We conclude that TC at suprastoichiometric ratios to tubulin inhibits microtubule depolymerization by a capping reaction and that this effect is exerted preferentially at the minus end.     

Microtubule nucleation and organization in dendrites

Dendrite branching is an essential process for building complex nervous systems. It determines the number, distribution and integration of inputs into a neuron, and is regulated to create the diverse dendrite arbor branching patterns characteristic of different neuron types. The microtubule cytoskeleton is critical to provide structure and exert force during dendrite branching. It also supports the functional requirements of dendrites, reflected by differential microtubule architectural organization between neuron types, illustrated here for sensory neurons. Both anterograde and retrograde microtubule polymerization occur within growing dendrites, and recent studies indicate that branching is enhanced by anterograde microtubule polymerization events in nascent branches. The polarities of microtubule polymerization events are regulated by the position and orientation of microtubule nucleation events in the dendrite arbor. Golgi outposts are a primary microtubule nucleation center in dendrites and share common nucleation machinery with the centrosome. In addition, pre-existing dendrite microtubules may act as nucleation sites. We discuss how balancing the activities of distinct nucleation machineries within the growing dendrite can alter microtubule polymerization polarity and dendrite branching, and how regulating this balance can generate neuron type-specific morphologies.     

Peroxynitrite oxidation of tubulin sulfhydryls inhibits microtubule polymerization

Considerable evidence both in vitro and in vivo implicates protein damage by peroxynitrite as a probable mechanism of cell death. Herein, we report that treatment of bovine brain microtubule protein, composed of tubulin and microtubule-associated proteins, with peroxynitrite led to a dose-dependent inhibition of microtubule polymerization. The extent of cysteine oxidation induced by peroxynitrite correlated well with inhibition of microtubule polymerization. Disulfide bonds between the subunits of the tubulin heterodimer were detected by Western blot as a result of peroxynitrite-induced cysteine oxidation. Addition of disulfide reducing agents including dithiothreitol and beta-mercaptoethanol restored a significant portion of the polymerization activity that was lost following peroxynitrite addition. Thus, peroxynitrite-induced disulfide bonds are at least partially responsible for the observed inhibition of polymerization. Sodium bicarbonate protected microtubule protein from the peroxynitrite-induced inhibition of polymerization. Tyrosine nitration of microtubule protein by 1 mM peroxynitrite increased approximately twofold when sodium bicarbonate was present whereas the extent of cysteine oxidation decreased from 7.5 to 6.3 mol cysteine/mol tubulin. These results indicate that cysteine oxidation of tubulin by peroxynitrite, rather than tyrosine nitration, is the primary mechanism of inhibition of microtubule polymerization.