Role of GTP hydrolysis in microtubule polymerization: evidence for a coupled hydrolysis mechanism

Mechanism of the Initial Tubulin Nucleation Phase

Tubulin nucleation is a highly frequent event in microtubule (MT) dynamics but is poorly understood. In this work, we characterized the structural changes during the initial nucleation phase of dynamic tubulin. Using size-exclusion chromatography-eluted tubulin dimers in an assembly buffer solution free of glycerol and tubulin aggregates enabled us to start from a well-defined initial thermodynamic ensemble of isolated dynamic tubulin dimers and short oligomers. Following a temperature increase, time-resolved X-ray scattering and cryo-transmission electron microscopy during the initial nucleation phase revealed an isodesmic assembly mechanism of one-dimensional (1D) tubulin oligomers (where dimers were added and/or removed one at a time), leading to sufficiently stable two-dimensional (2D) dynamic nanostructures, required for MT assembly. A substantial amount of tubulin octamers accumulated before two-dimensional lattices appeared. Under subcritical assembly conditions, we observed a slower isodesmic assembly mechanism, but the concentration of 1D oligomers was insufficient to form the multistranded 2D nucleus required for MT formation.

The effect and mechanism of millepachine-disrupted spindle assembly in tumor cells

Millepachine (MIL) is a bioactive natural product that shows great potential for cancer treatment. Previous studies showed that MIL was a novel cancer drug candidate with a special structure. To provide reference for the research and development of MIL, we further investigated the mechanism of MIL inducing G2/M arrest and found MIL disrupted spindle assembly in tumor cells. In this study, we investigated the disrupting spindle assembly effects of MIL with a focus on its potential mechanism of action. First, we indicated that MIL did not inhibit microtubule polymerization from the results of in-vivo microtubule nucleation assay and microtubule polymerization in-vitro assay but delayed this process by inhibiting the production of ATP in tumor cells. Thereafter, we investigated the effect of MIL on the mitotic spindle. We found that MIL induced multipolar spindles by inhibiting the activity of Eg5 and inhibited mitotic spindle formation and chromatin condensation by the activation of the spindle assembly checkpoint (SAC) in tumor cells. These results established a novel function of MIL in regulating the assembly of mitotic spindle. As Eg5 and SAC are antitumor targets, effect of MIL on the Eg5 protein and SAC activation hinted that MIL has novel application in the development of antitumor drugs.

Lessons from bacterial homolog of tubulin, FtsZ for microtubule dynamics

FtsZ, a homolog of tubulin, is found in almost all bacteria and archaea where it has a primary role in cytokinesis. Evidence for structural homology between FtsZ and tubulin came from their crystal structures and identification of the GTP box. Tubulin and FtsZ constitute a distinct family of GTPases and show striking similarities in many of their polymerization properties. The differences between them, more so, the complexities of microtubule dynamic behavior in comparison to that of FtsZ, indicate that the evolution to tubulin is attributable to the incorporation of the complex functionalities in higher organisms. FtsZ and microtubules function as polymers in cell division but their roles differ in the division process. The structural and partial functional homology has made the study of their dynamic properties more interesting. In this review, we focus on the application of the information derived from studies on FtsZ dynamics to study microtubule dynamics and vice versa. The structural and functional aspects that led to the establishment of the homology between the two proteins are explained to emphasize the network of FtsZ and microtubule studies and how they are connected.

Penrose-Hameroff orchestrated objective-reduction proposal for human consciousness is not biologically feasible

Penrose and Hameroff have argued that the conventional models of a brain function based on neural networks alone cannot account for human consciousness, claiming that quantum-computation elements are also required. Specifically, in their Orchestrated Objective Reduction (Orch OR) model [R. Penrose and S. R. Hameroff, J. Conscious. Stud. 2, 99 (1995)], it is postulated that microtubules act as quantum processing units, with individual tubulin dimers forming the computational elements. This model requires that the tubulin is able to switch between alternative conformational states in a coherent manner, and that this process be rapid on the physiological time scale. Here, the biological feasibility of the Orch OR proposal is examined in light of recent experimental studies on microtubule assembly and dynamics. It is shown that the tubulins do not possess essential properties required for the Orch OR proposal, as originally proposed, to hold. Further, we consider also recent progress in the understanding of the long-lived coherent motions in biological systems, a feature critical to Orch OR, and show that no reformation of the proposal based on known physical paradigms could lead to quantum computing within microtubules. Hence, the Orch OR model is not a feasible explanation of the origin of consciousness.

Microtubule assembly dynamics: new insights at the nanoscale

Although the dynamic self-assembly behavior of microtubule ends has been well characterized at the spatial resolution of light microscopy (~200 nm), the single-molecule events that lead to these dynamics are less clear. Recently, a number of in vitro studies used novel approaches combining laser tweezers, microfabricated chambers, and high-resolution tracking of microtubule-bound beads to characterize mechanochemical aspects of MT dynamics at nanometer scale resolution. In addition, computational modeling is providing a framework for integrating these experimental results into physically plausible models of molecular scale microtubule dynamics. These nanoscale studies are providing new fundamental insights about microtubule assembly, and will be important for advancing our understanding of how microtubule dynamic instability is regulated in vivo via microtubule-associated proteins, therapeutic agents, and mechanical forces.

Microtubule assembly dynamics at the nanoscale

BACKGROUND

The labile nature of microtubules is critical for establishing cellular morphology and motility, yet the molecular basis of assembly remains unclear. Here we use optical tweezers to track microtubule polymerization against microfabricated barriers, permitting unprecedented spatial resolution.

RESULTS

We find that microtubules exhibit extensive nanometer-scale variability in growth rate and often undergo shortening excursions, in some cases exceeding five tubulin layers, during periods of overall net growth. This result indicates that the guanosine triphosphate (GTP) cap does not exist as a single layer as previously proposed. We also find that length increments (over 100 ms time intervals, n = 16,762) are small, 0.81 +/- 6.60 nm (mean +/- standard deviation), and very rarely exceed 16 nm (about two dimer lengths), indicating that assembly occurs almost exclusively via single-subunit addition rather than via oligomers as was recently suggested. Finally, the assembly rate depends only weakly on load, with the average growth rate decreasing only 2-fold as the force increases 7-fold from 0.4 pN to 2.8 pN.

CONCLUSIONS

The data are consistent with a mechanochemical model in which a spatially extended GTP cap allows substantial shortening on the nanoscale, while still preventing complete catastrophe in most cases.

Nonequilibrium self-assembly of linear fibers: microscopic treatment of growth, decay, catastrophe and rescue

Many of the large structures of cells are constructed from fibers. These fibers self-assemble from individual proteins in a far-from-equilibrium fashion. Nonequilibrium self-assembly results in a highly dynamic process at the subcellular level that can be regulated and tuned to carry out many of the biological functions of the cell: growth, division and locomotion. We construct and analyze a nonequilibrium model of the dynamic end of a biological fiber that possesses site-resolved resolution. We solve for the steady states of this nonequilibrium system using a variational method. The results are compared to exact numerical solutions for systems with modest size. Using an effective reaction coordinate, we construct an effective potential from the steady-state distribution. The stochastic transitions of the system can be analyzed in this representation. We then apply this method to model microtubule systems. Predictions for macroscopic catastrophe, rescue and dynamic instability in the steady states are analyzed. We find that the length of the cap of the microtubule is small. The relations between the catastrophe/rescue rate and the growth rate are also discussed.

Steady states of a microtubule assembly in a confined geometry

We study the steady state of an assembly of microtubules in a confined volume, analogous to the situation inside a cell where the cell boundary forms a natural barrier to growth. We show that the dynamical equations for growing and shrinking microtubules predict the existence of two steady states, with either exponentially decaying or exponentially increasing distribution of microtubule lengths. We identify the regimes in parameter space corresponding to these steady states. In the latter case, the apparent catastrophe frequency near the boundary is found to be significantly larger than that in the interior. Both the exponential distribution of lengths and the increase in the catastrophe frequency near the cell margin is in excellent agreement with recent experimental observations.

G beta gamma mediates the interplay between tubulin dimers and microtubules in the modulation of Gq signaling

Agonist stimulation causes tubulin association with the plasma membrane and activation of PLC beta 1 through direct interaction with, and transactivation of, G alpha q. Here we demonstrate that G beta gamma interaction with tubulin down-regulates this signaling pathway. Purified G beta gamma, alone or with phosphatidylinositol 4,5-bisphosphate (PIP2), inhibited carbachol-evoked membrane recruitment of tubulin and G alpha q transactivation by tubulin. Polymerization of microtubules elicited by G beta gamma overrode tubulin translocation to the membrane in response to carbachol stimulation. G beta gamma sequestration of tubulin reduced the inhibition of PLC beta 1 observed at high tubulin concentration. G beta 1 gamma 2 interacted preferentially with tubulin-GDP, whereas G alpha q was transactivated by tubulin-GTP. Prenylation of the gamma 2 polypeptide was required for G beta gamma/tubulin interaction. Both confocal microscopy and coimmunoprecipitation studies revealed the spatiotemporal pattern of G beta gamma/tubulin interaction during carbachol stimulation of neuroblastoma SK-N-SH cells. In resting cells G beta gamma localized predominantly at the cell membrane, whereas tubulin was found in well defined microtubules in the cytosol. Within 2 min of agonist exposure, a subset of tubulin translocated to the plasma membrane and colocalized with G beta. Fifteen min post-carbachol addition, tubulin and G beta colocalized in vesicle-like structures in the cytosol. G beta/tubulin colocalization increased after pretreatment of cells with the microtubule-depolymerizing agent, colchicine, and was inhibited by taxol. Taxol also inhibited carbachol-induced PIP2 hydrolysis. It is suggested that G beta gamma/tubulin interaction mediates internalization of membrane-associated tubulin at the offset of PLC beta 1 signaling. Newly cytosolic G beta gamma/tubulin complexes might promote microtubule polymerization attenuating further tubulin association with the plasma membrane. Thus G protein-coupled receptors might evoke G alpha and G beta gamma to orchestrate regulation of phospholipase signaling by tubulin dimers and control of cell shape by microtubules.

Structural microtubule cap: stability, catastrophe, rescue, and third state

Microtubules polymerize from GTP-liganded tubulin dimers, but are essentially made of GDP-liganded tubulin. We investigate the tug-of-war resulting from the fact that GDP-liganded tubulin favors a curved configuration, but is forced to remain in a straight one when part of a microtubule. We point out that near the end of a microtubule, the proximity of the end shifts the balance in this tug-of-war, with some protofilament bending as result. This somewhat relaxes the microtubule lattice near its end, resulting in a structural cap. This structural cap thus is a simple mechanical consequence of two well-established facts: protofilaments made of GDP-liganded tubulin have intrinsic curvature, and microtubules are elastic, made from material that can yield to forces, in casu its own intrinsic forces. We explore possible properties of this structural cap, and demonstrate 1) how it allows both polymerization from GTP-liganded tubulin and rapid depolymerization in its absence; 2) how rescue can occur; 3) how a third, meta-stable intermediate state is possible and can explain some experimental results; and 4) how the tapered tips observed at polymerizing microtubule ends are stabilized during growth, though unable to accommodate a lateral cap. This scenario thus supports the widely accepted GTP-cap model by suggesting a stabilizing mechanism that explains the many aspects of dynamic instability.

Estimates of lateral and longitudinal bond energies within the microtubule lattice

We developed a stochastic model of microtubule (MT) assembly dynamics that estimates tubulin-tubulin bond energies, mechanical energy stored in the lattice dimers, and the size of the tubulin-GTP cap at MT tips. First, a simple assembly/disassembly state model was used to screen possible combinations of lateral bond energy (DeltaG(Lat)) and longitudinal bond energy (DeltaG(Long)) plus the free energy of immobilizing a dimer in the MT lattice (DeltaG(S)) for rates of MT growth and shortening measured experimentally. This analysis predicts DeltaG(Lat) in the range of -3.2 to -5.7 k(B)T and DeltaG(Long) plus DeltaG(S) in the range of -6.8 to -9.4 k(B)T. Based on these estimates, the energy of conformational stress for a single tubulin-GDP dimer in the lattice is 2.1-2.5 k(B)T. Second, we studied how tubulin-GTP cap size fluctuates with different hydrolysis rules and show that a mechanism of directly coupling subunit addition to hydrolysis fails to support MT growth, whereas a finite hydrolysis rate allows growth. By adding rules to mimic the mechanical constraints present at the MT tip, the model generates tubulin-GTP caps similar in size to experimental estimates. Finally, by combining assembly/disassembly and cap dynamics, we generate MT dynamic instability with rates and transition frequencies similar to those measured experimentally. Our model serves as a platform to examine GTP-cap dynamics and allows predictions of how MT-associated proteins and other effectors alter the energetics of MT assembly.

How motor proteins influence microtubule polymerization dynamics

The interplay between microtubules and microtubule-based motors is fundamental to basic aspects of cellular function, such as the intracellular transport of organelles and alterations in cellular morphology during cell locomotion and division. Motor proteins are unique in that they couple nucleotide hydrolysis to force production that can do work. The force transduction by proteins belonging to the kinesin and dynein superfamilies has been thought only to power movement of these motors along the surface of microtubules; however, a growing body of evidence, both genetic and biochemical, suggests that motors can also directly influence the polymerization dynamics of microtubules. For example, at the vertebrate kinetochore, motors interact directly with microtubule ends and modulate polymerization dynamics to orchestrate chromosome movements during mitosis. Although a role for motors in regulating microtubule length has been established, the mechanisms used by motors to promote microtubule growth or shrinkage are unclear, as is an understanding of why cells might choose motors to control dynamics rather than a variety of non-motor proteins known to affect microtubule stability. Elucidation of the exact mechanisms by which motors alter the exchange of tubulin subunits at microtubule ends in vitro may shed light on how microtubule stability is regulated to produce the array of dynamic behavior seen in cells.

Inhibition of acetyl CoA carboxylase by GTP gamma S

The effect of nonhydrolyzable guanine nucleotides on mammalian acetyl CoA carboxylase (ACC) activity was examined. Using porous rat adipocytes and crude fat cell homogenates to study metabolic pathway flux, GMPPNP and/or GTP gamma S inhibited [14C]fatty acid formation by up to 95% when either [6-14C]glucose-6-phosphate or [1-14C]acetyl CoA was used as substrate. If [2-14C]malonyl CoA initiated flux, however, no inhibition was apparent. These pathway flux studies suggested that ACC was the locus of inhibition, and that the mechanism might involve a disruption of guanine nucleotide hydrolysis by the nonhydrolyzable analogues. Using partially and avidin-sepharose-purified ACC preparations from rat fat, liver and mammary tissue, citrate-stimulated ACC activity was inhibited by 25-75% with 50 microM GTP gamma S. Related compounds and nucleotides had absent-to-minimal effects on ACC. ATP gamma S was inhibitory (10-30% at 5-15 microM), but always to a lesser degree than equimolar GTP gamma S. Filter binding assays with [alpha-32P]GTP or [35S]GTP gamma S were negative, but low-level GTPase activity was detected. Using photoaffinity labelling techniques, [alpha-32P]GTP was found to bind ACC and not pyruvate carboxylase. The hypothesis that citrate-responsive ACC activity may be modulated by an intrinsic or associated GTP binding site is explored. Since ACC forms polymers, as does the cytoskeletal protein beta-tubulin, amino acid sequence comparisons between ACC and atypical GTP binding domain of beta tubulin are presented.

Assembly and colchicine binding characteristics of tubulin with maximally tyrosinated and detyrosinated alpha-tubulins

The posttranslational removal and readdition of tyrosine at the C-terminus of alpha-tubulin is associated with generation of microtubule populations that differ in intracellular distributions, turnover rates, and sensitivities to microtubule-depolymerization agents. Here, we compared the in vitro assembly and colchicine binding characteristics of tubulin dimer preparations composed of alpha-tubulin that had been maximally tyrosinated (approximately 40% tyrosinated) by tubulin-tyrosine ligase and maximally detyrosinated (100% detyrosinated) by carboxypeptidase A. Maximally tyrosinated and detyrosinated tubulins had similar critical concentrations for polymerization and similar association constants for colchicine binding. Microtubules polymerized from the two tubulins also had similar steady-state mean lengths and length distributions. The growing and shortening dynamics (dynamic instability parameters) of individual microtubules made from maximally tyrosinated or detyrosinated alpha-tubulin as determined by video-enhanced dark-field microscopy were similar, but subtle differences in the growing and shortening rates were found. On balance, however, the dynamicity and thus the overall kinetic stability of the two microtubule populations were indistinguishable. The results support the idea that detyrosination of alpha-tubulin does not by itself generate stable microtubules.

Microtubule polymerization dynamics

The polymerization dynamics of microtubules are central to their biological functions. Polymerization dynamics allow microtubules to adopt spatial arrangements that can change rapidly in response to cellular needs and, in some cases, to perform mechanical work. Microtubules utilize the energy of GTP hydrolysis to fuel a unique polymerization mechanism termed dynamic instability. In this review, we first describe progress toward understanding the mechanism of dynamic instability of pure tubulin and then discuss the function and regulation of microtubule dynamic instability in living cells.

A metastable intermediate state of microtubule dynamic instability that differs significantly between plus and minus ends

The current two-state GTP cap model of microtubule dynamic instability proposes that a terminal crown of GTP-tubulin stabilizes the microtubule lattice and promotes elongation while loss of this GTP-tubulin cap converts the microtubule end to shortening. However, when this model was directly tested by using a UV microbeam to sever axoneme-nucleated microtubules and thereby remove the microtubule's GTP cap, severed plus ends rapidly shortened, but severed minus ends immediately resumed elongation (Walker, R.A., S. Inoué, and E.D. Salmon. 1989. J. Cell Biol. 108: 931-937). To determine if these previous results were dependent on the use of axonemes as seeds or were due to UV damage, or if they instead indicate an intermediate state in cap dynamics, we performed UV cutting of self-assembled microtubules and mechanical cutting of axoneme-nucleated microtubules. These independent methods yielded results consistent with the original work: a significant percentage of severed minus ends are stable after cutting. In additional experiments, we found that the stability of both severed plus and minus ends could be increased by increasing the free tubulin concentration, the solution GTP concentration, or by assembling microtubules with guanylyl-(alpha,beta)-methylene-diphosphonate (GMPCPP). Our results show that stability of severed ends, particularly minus ends, is not an artifact, but instead reveals the existence of a metastable kinetic intermediate state between the elongation and shortening states of dynamic instability. The kinetic properties of this intermediate state differ between plus and minus ends. We propose a three-state conformational cap model of dynamic instability, which has three structural states and four transition rate constants, and which uses the asymmetry of the tubulin heterodimer to explain many of the differences in dynamic instability at plus and minus ends.

How tubulin subunits are lost from the shortening ends of microtubules

Microtubules exhibit dynamic instability, switching between persistent states of growth and shortening at their ends. The switch between growth and shortening has been proposed to depend on end conformation where growing ends have "straight" tubulin protofilaments stabilized by a terminal cap of GTP-tubulin, while-shortening ends have lost their GTP-tubulin cap, allowing terminal GDP-tubulin dimers to curve inside-out and peel rapidly away from the microtubule lattice. This "conformational cap" model predicts that tubulin dissociation from shortening ends is a two-step process where the average lengths of curved GDP-tubulin protofilaments at a depolymerizing end will depend on the ratio of the rate of peeling to the rate of breakage of the longitudinal bonds between adjacent curved dimers. We have tested this model for the plus and minus ends of microtubules assembled with pure porcine tubulin off the ends of axoneme fragments in standard assembly buffer. Individual microtubule ends were imaged using video-enhanced differential interference contrast light microscopy. The rate of rapid shortening was systematically increased by isothermal dilution into assembly buffer containing various concentrations of Mg2+ or Ca2+ ions. At 1 mM Mg2+ and no Ca2+, shortening occurred at 20 (plus) and 45 (minus) microns/min. The ends appeared similar in contrast to growing ends and the core of the microtubule and the ends appeared blunt or slightly frayed by negative stain electron microscopy. Above 20 mM Mg2+ or above 5 mM Ca2+, microtubule shortening occurred at 60 (plus) and 115 (minus) microns/min or faster and "knobs" were distinctly visible at depolymerizing ends, particularly at the faster minus ends, and knob contrast remained constant during many micrometers of rapid shortening. Negative stain electron microscopy revealed that these knobs were "blossoms" of inside-out curved protofilaments, some extending for several helical turns (30 to 60 dimers in length) at constant curvature from the ends. At these high shortening velocities, the peeling of curved protofilaments was confined to within several dimers of the end of the microtubule cylinder, suggesting that dimer curling and protofilament peeling is constrained to the tip by interactions between adjacent straight protofilaments. Depolymerization is produced by conformational changes in GDP-tubulin since microtubules assembled with a slowly hydrolizable analog of GTP, GMPCPP, are stable even at 20 mM Mg2+ or 5 mM Ca2+. Monte Carlo simulations show that the ratio of the peeling to breakage rate constants can control the steady-state average length of curved GDP-tubulin protofilaments at the depolymerizing end.

Thermodynamic and structural analysis of microtubule assembly: the role of GTP hydrolysis

Different models have been proposed that link the tubulin heterodimer nucleotide content and the role of GTP hydrolysis with microtubule assembly and dynamics. Here we compare the thermodynamics of microtubule assembly as a function of nucleotide content by van't Hoff analysis. The thermodynamic parameters of tubulin assembly in 30-100 mM piperazine-N,N'-bis(2-ethanesulfonic acid), 1 mM MgSO4, 2 mM EGTA, pH 6.9, in the presence of a weakly hydrolyzable analog, GMPCPP, the dinucleotide analog GMPCP plus 2 M glycerol, and GTP plus 2 M glycerol were obtained together with data for taxol-GTP/GDP tubulin assembly (GMPCPP and GMPCP are the GTP and GDP nucleotide analogs where the alpha beta oxygen has been replaced by a methylene, -CH2-). All of the processes studied are characterized by a positive enthalpy, a positive entropy, and a large, negative heat capacity change. GMPCP-induced assembly has the largest negative heat capacity change and GMPCPP has the second largest, whereas GTP/2 M glycerol- and taxol-induced assembly have more positive values, respectively. A large, negative heat capacity is most consistent with the burial of water-accessible hydrophobic surface area, which gives rise to the release of bound water. The heat capacity changes observed with GTP/2 M glycerol-induced and with taxol-induced assembly are very similar, -790 +/- 190 cal/mol/k, and correspond to the burial of 3330 +/- 820 A2 of nonpolar surface area. This value is shown to be very similar to an estimate of the buried nonpolar surface in a reconstructed microtubule lattice. Polymerization data from GMPCP- and GMPCPP-induced assembly are consistent with buried nonpolar surface areas that are 3 and 6 times larger. A linear enthalpy-entropy and enthalpy-free energy plot for tubulin polymerization reactions verifies that enthalpy-entropy compensation for this system is based upon true biochemical correlation, most likely corresponding to a dominant hydrophobic effect. Entropy analysis suggests that assembly with GTP/2 M glycerol and with taxol is consistent with conformational rearrangements in 3-6% of the total amino acids in the heterodimer. In addition, taxol binding contributes to the thermodynamics of the overall process by reducing the delta H degree and delta S degree for microtubule assembly. In the presence of GMPCPP or GMPCP, tubulin subunits associate with extensive conformational rearrangement, corresponding to 10% and 26% of the total amino acids in the heterodimer, respectively, which gives rise to a large loss of configurational entropy. An alternative, and probably preferable, interpretation of these data is that, especially with GMPCP-tubulin, additional isomerization or protonation events are induced by the presence of the methylene moiety and linked to microtubule assembly. Structural analysis shows that GTP hydrolysis is not required for sheet closure into a microtubule cylinder, but only increases the probability of this event occurring. Sheet extensions and sheet polymers appear to have a similar average length under various conditions, suggesting that the minimum cooperative unit for closure of sheets into a microtubule cylinder is approximately 400 nm long. Because of their low level of occurrence, sheets are not expected to significantly affect the thermodynamics of assembly.

Differential effects of vinblastine on polymerization and dynamics at opposite microtubule ends

We have characterized the effects of vinblastine on the growing and shortening dynamics at opposite ends of individual bovine brain microtubules at steady state in vitro by video microscopy. Vinblastine exerted strikingly different effects on the dynamics and polymer mass at the plus and minus ends of microtubules. At concentrations between 0.1 and 0.4 microM, the drug strongly depolymerized microtubules at minus ends, whereas it did not significantly depolymerize microtubules at plus ends. Vinblastine stabilized plus ends by suppressing the rate and extent of growth and shortening, decreasing the catastrophe frequency, and increasing the rescue frequency. In contrast, vinblastine destabilized minus ends by increasing the catastrophe frequency and decreasing the rescue frequency, whereas it had no effect on the rate or extent of growth or shortening. Thus, vinblastine moderately increased the overall dynamicity at minus ends while strongly suppressing dynamicity at plus ends. Both the kinetic destabilization of microtubules at minus ends and the stabilization at plus ends may contribute to the altered function of mitotic spindle microtubules of cells blocked in mitosis by low concentrations of vinblastine.

Dynamic properties of nucleated microtubules: GTP utilisation in the subcritical concentration regime

Microtubule assembly kinetics have been studied quantitatively under solution conditions supporting microtubule dynamic instability. Purified GTP-tubulin (Tu-GTP) and covalently cross-linked short microtubule seeds (EGS-seeds; Koshland et al. (1988) Nature 331, 499) were used with and without biotinylation. Under sub-critical concentration conditions ([Tu-GTP] < 5.3 microM), significant microtubule growth of limited length was observed on a proportion of the EGS-seeds by immuno-electron microscopy. A sensitive fluorescence assay for microtubule GDP production was developed for parallel assessment of GTP utilisation. This revealed a correlation between the detected microtubule growth and the production of tubulin-GDP, deriving from the shortening phase of the dynamic microtubules. This correlation was confirmed by the action of nocodazole, a specific inhibitor of microtubule assembly, that was found to abolish the GDP release. The variation of the GDP release with tubulin concentration (Jh(c) plot) was determined below the critical concentration (Cc). The GDP production observed was consistent with the elongation of the observed seeded microtubules with an apparent rate constant of 1.5 × 10(6) M-1 second-1 above a threshold of approximately 1 microM tubulin. The form of this Jh(c) plot for elongation below Cc is reproduced by the Lateral Cap model for microtubule dynamic instability adapted for seeded assembly. The behaviour of the system is contrasted with that previously studied in the absence of detectable microtubule elongation (Caplow and Shanks (1990) J. Biol. Chem. 265, 8935–8941). The approach provides a means of monitoring microtubule dynamics at concentrations inaccessible to optical microscopy, and shows that essentially the same dynamic mechanisms apply at all concentrations. Numerical simulation of the subcritical concentration regime shows dynamic growth features applicable to the initiation of microtubule growth in vivo.

Microtubule dynamic turnover is suppressed during polarization and stimulated in hepatocyte growth factor scattered Madin-Darby canine kidney epithelial cells

The dynamic behavior of microtubules has been measured in non-polarized, polarized, and hepatocyte growth factor treated Madin-Darby canine kidney epithelial cells. In a nocodazole disassembly assay, microtubules in polarized cells were more resistant to depolymerization than microtubules in non-polarized cells; microtubules in scattered cells were nearly completely disassembled. Analysis of fluorescent microtubules in living cells further revealed that individual microtubules in polarized cells were kinetically stabilized and microtubules in scattered cells were highly dynamic. Individual microtubule behavior in polarized cells was characterized by a suppression of the average rate of shortening, an increase in the average duration of pause, a decrease in the frequency of catastrophe transitions, and an increase in the frequency of rescue transitions, when compared with microtubules in non-polarized cells. In contrast, microtubule behavior in epithelial cells treated with hepatocyte growth factor was characterized by increase in the average rates of microtubule growth and shortening, a decrease in the frequency of rescue transitions, and an increase in the frequency of catastrophe transitions, when compared with polarized cells. Dynamicity, a measure of the gain and loss of subunits from microtubule plus ends, was 2.7 microns/min in polarized cells and 11.1 microns/min in scattered cells. These results demonstrate that individual microtubule dynamic behavior is markedly suppressed in polarized epithelial cells. Our results further demonstrate that in addition to its previously characterized effects on cell locomotion, hepatocyte growth factor stimulates microtubule dynamic turnover in lamellar regions of living cells.

Force generation by microtubule assembly/disassembly in mitosis and related movements

In this article, we review the dynamic nature of the filaments (microtubules) that make up the labile fibers of the mitotic spindle and asters, we discuss the roles that assembly and disassembly of microtubules play in mitosis, and we consider how such assembling and disassembling polymer filaments can generate forces that are utilized by the living cell in mitosis and related movements.

Vinblastine suppresses dynamics of individual microtubules in living interphase cells

We have characterized the effects of vinblastine on the dynamic instability behavior of individual microtubules in living BS-C-1 cells microinjected with rhodamine-labeled tubulin and have found that at low concentrations (3-64 nM), vinblastine potently suppresses dynamic instability without causing net microtubule depolymerization. Vinblastine suppressed the rates of microtubule growth and shortening, and decreased the frequency of transitions from growth or pause to shortening, also called catastrophe. In vinblastine-treated cells, both the average duration of a pause (a state of attenuated dynamics where neither growth nor shortening could be detected) and the percentage of total time spent in pause were significantly increased. Vinblastine potently decreased dynamicity, a measure of the overall dynamic activity of microtubules, reducing this parameter by 75% at 32 nM. The present work, consistent with earlier in vitro studies, demonstrates that vinblastine kinetically caps the ends of microtubules in living cells and supports the hypothesis that the potent chemotherapeutic action of vinblastine as an antitumor drug is suppression of mitotic spindle microtubule dynamics. Further, the results indicate that molecules that bind to microtubule ends can regulate microtubule dynamic behavior in living cells and suggest that endogenous regulators of microtubule dynamics that work by similar mechanisms may exist in living cells.

Kinetic stabilization of microtubule dynamics at steady state in vitro by substoichiometric concentrations of tubulin-colchicine complex

We have analyzed the effects of tubulin-colchicine (TC)-complex on the dynamic instability behavior of bovine brain microtubules at steady state in vitro using video microscopy. Incorporation of low numbers of TC-complexes per microtubule strongly suppressed dynamics at the plus ends by reducing the rate and extent of growing and shortening and by increasing the time microtubules spent in an attenuated state, neither growing nor shortening detectably. In addition, TC-complex strongly suppressed the catastrophe frequency and increased the rescue frequency. At low concentrations (0.02-0.05 microM), TC-complex suppressed dynamics without reducing the polymer mass or the mean microtubule length. Such strong suppression of microtubule dynamics by low TC-complex concentrations in the absence of polymer mass changes demonstrates that microtubule dynamics are more sensitive to the actions of TC-complex than the polymer mass. Significant reduction of polymer mass occurred at relatively high TC-complex concentration (> 0.05 microM). However, the surviving microtubules were extremely stable. Thus, TC-complex stabilizes microtubules even though the microtubules can transiently depolymerize when TC-complex is added. The data also directly establish that kinetic suppression of dynamics by colchicine at low concentrations is effected by a low number of TC-complexes at the microtubule ends.

Interaction of the microtubule cytoskeleton with endocytic vesicles and cytoplasmic dynein in cultured rat hepatocytes

In a recent study (Goltz, J.S., Wolkoff, A.W., Novikoff, P.M., Stockert, R.J., and Satir, P. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 7026-7030), we found that ligand- and receptor-containing endocytic vesicles bind to endogenous microtubules in vitro after 60 min of receptor-mediated endocytosis of asialo-orosomucoid. In the presence of ATP, ligand-containing endocytic vesicles are released from microtubules, while those containing receptor are not. We hypothesized that cytoplasmic dynein may associate with ligand-containing, but not receptor-containing, domains of endocytic vesicles and might be involved in the movement of ligand-containing vesicles along microtubules during sorting of ligand from receptor. Direct evidence in support of this hypothesis has been obtained in the present study. Binding of ligand-containing vesicles to microtubules correlates highly (p < 0.001) with binding of dynein, but not kinesin, under a variety of conditions. Binding of receptor-containing vesicles to microtubules is independent of both cytoplasmic dynein and kinesin binding. Tight association of cytoplasmic dynein with a population of ligand-containing vesicles is seen directly by immunoprecipitation. These results support the view that in receptor-mediated endocytosis, ligand-containing vesicles become bound to microtubules by cytoplasmic dynein. While receptor domains of endosomes remain attached to microtubules in an ATP-independent manner, ligand-containing domains might be moved away toward pericentrosomal lysosomes by this motor molecule.

Structure of growing microtubule ends: two-dimensional sheets close into tubes at variable rates

Observation of microtubule growth at different rates by cryo-electron microscopy reveals that the ends range from blunt to long, gently curved sheets. The mean sheet length increases with the growth rate while the width of the distributions increases with the extent of assembly. The combination of a concentration dependent growth rate of the tubulin sheet with a variable closure rate of the microtubule cylinder, results in a model in which stochastic fluctuations in sheet length and tubulin conformation confine GTP-tubulins to microtubule ends. We propose that the variability of microtubule growth rate observed by video microscopy (Gildersleeve, R. F., A. R. Cross, K. E. Cullen, A. P. Fagen, and R. C. Williams. 1992. J. Biol. Chem. 267: 7995-8006, and this study) is due to the variation in the rate of cylinder closure. The curvature of the sheets at the end of growing microtubules and the small oligomeric structures observed at the end of disassembling microtubules, indicate that tubulin molecules undergo conformational changes both during assembly and disassembly.

Modulation of microtubule dynamic instability in vivo by brain microtubule associated proteins

Heat-stable brain microtubule associated proteins (MAPs) and purified microtubule associated protein 2 (MAP-2) were microinjected into cultured BSC-1 cells which had been previously injected with rhodamine-labeled tubulin. The dynamic instability behavior of individual microtubules was then examined using low-light-level fluorescence microscopy and quantitative microtubule tracking methods. Both MAP preparations suppressed microtubule dynamics in vivo, by reducing the average rate and extent of both growing and shortening events. The average duration of growing events was not affected. When measured as events/unit time, heat-stable MAPs and MAP-2 did not significantly alter the frequency of rescue; the frequency of catastrophe was decreased approximately two-fold by heat-stable MAPs and MAP-2. When transition frequencies were calculated as events/unit distance, both MAP preparations increased the frequency of rescue, without altering the frequency of catastrophe. The percentage of total time spent in the phases of growth, shrink and pause was determined. Both MAP-2 and heat-stable MAPs decreased the percentage of time spent shortening, increased the percentage of time spent paused, and had no effect on percentage of time spent growing. Heat-stable MAPs increased the average pause duration, decreased the average number of events per minute per microtubule and increased the probability that a paused microtubule would switch to growing rather than shortening. The results demonstrate that addition of MAPs to living cells reduces the dynamic behavior of individual microtubules primarily by suppressing the magnitude of dynamic events and increasing the time spent in pause, where no change in the microtubule length can be detected. The results further suggest that the expression of MAPs directly contributes to cell type-specific microtubule dynamic behavior.

Structural changes accompanying GTP hydrolysis in microtubules: information from a slowly hydrolyzable analogue guanylyl-(alpha,beta)-methylene-diphosphonate

We have used cryoelectron microscopy to try to understand the structural basis for the role of GTP hydrolysis in destabilizing the microtubule lattice. We have measured a structural difference introduced into microtubules by replacing GTP with guanylyl-(alpha,beta)-methylene-diphosphonate (GMPCPP). In a stable GMPCPP microtubule lattice, the moiré patterns change and the tubulin subunits increase in size by 1.5 A. This information provides a clue to the role of hydrolysis in inducing the structural change at the end of a microtubule during the transition from a growing to a shrinking phase.

The minimum GTP cap required to stabilize microtubules

BACKGROUND

Microtubules polymerized from pure tubulin show the unusual property of dynamic instability, in which both growing and shrinking polymers coexist at steady state. Shortly after its addition to a microtubule end, a tubulin subunit hydrolyzes its bound GTP. Studies with non-hydrolyzable analogs have shown that GTP hydrolysis is not required for microtubule assembly, but is essential for generating a dynamic polymer, in which the subunits at the growing tip have bound GTP and those in the bulk of the polymer have bound GDP. It has been suggested that loss of the 'GTP cap' through dissociation or hydrolysis exposes the unstable GDP core, leading to rapid depolymerization. However, evidence for a stabilizing cap has been very difficult to obtain.

RESULTS

We developed an assay to determine the minimum GTP cap necessary to stabilize a microtubule from shrinking. Assembly of a small number of subunits containing a slowly hydrolyzed GTP analog (GMPCPP) onto the end of dynamic microtubules stabilized the polymer to dilution. By labeling the subunits with rhodamine, we measured the size of the cap and found that as few as 40 subunits were sufficient to stabilize a microtubule.

CONCLUSIONS

On the basis of statistical arguments, in which the proportion of stabilized microtubules is compared to the probability that when 40 GMPCPP-tubulin subunits have polymerized onto a microtubule end, all protofilaments have added at least one GMPCPP-tubulin subunit, our measurements of cap size support a model in which a single GTP subunit at the end of each of the 13 protofilaments of a microtubule is sufficient for stabilization. Depolymerization of a microtubule may be initiated by an exposed tubulin-GDP subunit at even a single position. These results have implications for the structure of microtubules and their means of regulation.

XMAP from Xenopus eggs promotes rapid plus end assembly of microtubules and rapid microtubule polymer turnover

We have used video-enhanced DIC microscopy to examine the effects of XMAP, a Mr 215,000 microtubule-associated protein from Xenopus eggs (Gard, D.L., and M. W. Kirschner. 1987. J. Cell Biol. 105:2203-2215), on the dynamic instability of microtubules nucleated from axoneme fragments in vitro. Our results indicate that XMAP substantially alters the parameters of microtubule assembly at plus ends. Specifically, addition of 0.2 microM XMAP resulted in (a) 7-10-fold increase in elongation velocity, (b) approximately threefold increase in shortening velocity, and (c) near elimination of rescue (the switch from rapid shortening to elongation). Thus, addition of XMAP resulted in the assembly of longer, but more dynamic, microtubules from the plus ends of axonemes which upon catastrophe disassembled back to the axoneme nucleation site. In agreement with previous observations (Gard, D.L., and M. W. Kirschner. 1987. J. Cell Biol. 105:2203-2215), the effects of XMAP on the minus end were much less dramatic, with only a 1.5-3-fold increase in elongation velocity. These results indicate that XMAP, unlike brain MAPs, promotes both polymer assembly and turnover, and suggests that the interaction of XMAP with tubulin and the function of XMAP in vivo may differ from previously characterized MAPs.

The free energy for hydrolysis of a microtubule-bound nucleotide triphosphate is near zero: all of the free energy for hydrolysis is stored in the microtubule lattice

The standard free energy for hydrolysis of the GTP analogue guanylyl-(a,b)-methylene-diphosphonate (GMPCPP), which is -5.18 kcal in solution, was found to be -3.79 kcal in tubulin dimers, and only -0.90 kcal in tubulin subunits in microtubules. The near-zero change in standard free energy for GMPCPP hydrolysis in the microtubule indicates that the majority of the free energy potentially available from this reaction is stored in the microtubule lattice; this energy is available to do work, as in chromosome movement. The equilibrium constants described here were obtained from video microscopy measurements of the kinetics of assembly and disassembly of GMPCPP-microtubules and GMPCP-microtubules. It was possible to study GMPCPP-microtubules since GMPCPP is not hydrolyzed during assembly. Microtubules containing GMPCP were obtained by assembly of high concentrations of tubulin-GMPCP subunits, as well as by treating tubulin-GMPCPP-microtubules in sodium (but not potassium) Pipes buffer with glycerol, which reduced the half-time for GMPCPP hydrolysis from > 10 h to approximately 10 min. The rate for tubulin-GMPCPP and tubulin-GMPCP subunit dissociation from microtubule ends were found to be about 0.65 and 128 s-1, respectively. The much faster rate for tubulin-GMPCP subunit dissociation provides direct evidence that microtubule dynamics can be regulated by nucleotide triphosphate hydrolysis.

Interaction of tubulin with guanosine 5'-O-(1-thiotriphosphate) diastereoisomers: specificity of the alpha-phosphate binding region

The exchangeable nucleotide-binding site of tubulin has been studied using diastereoisomers A (Sp) and B (Rp) of guanosine 5'-O-(1-thiotriphosphate) (GTP alpha S) in which the phosphorus atom to which sulfur is attached is chiral. GTP alpha S(A) (10 microM) nucleated assembly of purified tubulin (20 microM) into microtubules in buffer containing 0.1 M 2-(N-morpholino)ethanesulfonic acid with 3 mM Mg2+ and 1 mM EGTA, pH 6.6 at 37 degrees C. With 0.2 mM GTP alpha S(A), the critical concentration (Cc; minimum protein concentration required for assembly) was 8 microM tubulin. Neither 0.2 mM GTP nor GTP alpha S(B) promoted microtubule assembly in buffer with 0.5-6.75 mM Mg2+ and 20-70 microM tubulin. The Cc values for GTP alpha S-(A)-induced assembly of tubulin in buffer with 30% glycerol and of microtubule protein (tubulin and microtubule-associated proteins) in buffer were lower than for GTP. GTP alpha S(A)-induced microtubules were more stable to the cold and to Ca2+. GTP alpha S(A) and GTP but not GTP alpha S(B) bound tightly to tubulin at 4 degrees C. Although GTP alpha S(B) did not nucleate assembly, it did bind to tubulin since it was incorporated into the growing microtubule. Both isomers were hydrolyzed in the microtubules. These studies show that GTP alpha S(A) promotes tubulin assembly better than GTP and GTP alpha S(B) and that there is stereoselectivity at the alpha-phosphate binding region of tubulin. The stereoselectivity may be due to different MgGTP alpha S(A) and -(B) interactions with tubulin.

Mutations in the Caenorhabditis elegans beta-tubulin gene mec-7: effects on microtubule assembly and stability and on tubulin autoregulation

We have sequenced 45 mutations in mec-7, a beta-tubulin gene required for the production of 15-protofilament microtubules in the nematode Caenorhabditis elegans, and have correlated sequence alterations with mutant phenotypes. The expression patterns of most alleles have also been determined by in situ hybridization and immunocytochemistry. Most (12/16) complete loss-of-function alleles, which are recessive, result from nonsense mutations, insertions, or deletions; three others disrupt a putative GTP-binding domain. Three of the four loss-of-function, missense mutations result in elevated mec-7 message levels, suggesting a defect in tubulin autoregulation that may be attributable to a loss in the ability to form heterodimers. Most (8/9) mild alleles are caused by missense mutations. Two mild alleles appear to increase microtubule stability and lead to the elaboration of ectopic neuronal processes in mec-7-expressing cells. Most (15/23) mutations that cause severe dominant or semidominant phenotypes are clustered into three discrete domains; four others occur in putative GTP-binding regions. Many of these dominant mutations appear to completely disrupt microtubule assembly.

A model of microtubule oscillations

Simulations of microtubule oscillations have been obtained by a kinetic model including nucleation of microtubules, elongation by addition of GTP-loaded tubulin dimers, disassembly into oligomers, and dissolution of oligomers followed by nucleotide exchange at the free dimers. Dynamic instability is described by the on and off rates for dimer association in the growth phase, the rate of rapid shortening, and the transition rates for catastrophe and rescue. The latter are assumed to be completely determined by the current state of the system ("short cap hypothesis"). Microtubule oscillations and normal polymerizations measured by time-resolved X-ray scattering were used to test the model. The model is able to produce oscillations without further assumptions. However, in order to obtain good fits to the experimental data one requires an additional mechanism which prevents rapid desynchronization of the microtubules. One of several possible mechanisms that will be discussed is the destabilization of microtubules by the products of disassembly.

Molecular mechanism of colchicine action: induced local unfolding of beta-tubulin

Colchicine, the classic antimitotic poison, disrupts cell division by preventing proper assembly of microtubules in the mitotic spindle. Colchicine is known to act by binding to tubulin, the heterodimeric subunit of microtubules. How this binding to tubulin changes the structure of the protein and results in polymerization poisoning has not been characterized. The structural locus of spectroscopically detected conformational changes induced by colchicine is unknown. We report here that colchicine induces the unfolding of a small region in the carboxyl-terminal region of beta-tubulin, around Arg-390. This unfolding is detected by proteolysis with trypsin and chymotrypsin. Chymotrypsin cleaves this region after Phe-389, and trypsin cleaves after Lys-392. The unfolded region appears to be the carboxyl end of an amphipathic helix in the absence of colchicine, and we propose that this unfolding prevents contacts necessary for assembly. Our results suggest that beta-tubulin is exposed on the growing end of the microtubule, which provides a mechanism for coupling GTP hydrolysis to polymerization.

Purification and biochemical characterization of tubulin from the budding yeast Saccharomyces cerevisiae

We describe a method for isolating milligram quantities of assembly-competent tubulin from the budding yeast Saccharomyces cerevisiae. The tubulin is > 95% purified and free of contaminating enzyme activities. As a result, it has been possible to determine the yeast tubulin equilibrium-binding constant for Mg-GTP and the tubulin GTPase activity under nonassembling and assembling conditions. We also determined the critical concentration for yeast tubulin polymerization and found it to be significantly lower than that for bovine brain tubulin under identical conditions. Similarly, the dynamic properties both of individual yeast microtubules and of bulk microtubule suspensions were significantly different from those of bovine brain microtubules free of microtubule-associated proteins. The data suggest that the properties of the yeast tubulin may reflect the particular functional requirements of the yeast cell. With this method, it is now possible to introduce any desired tubulin gene mutation into the budding yeast and correlate the phenotypic effects of the mutation in cells with the effects of the mutation on the biochemical and polymerization properties of the tubulin.

Kinetic stabilization of microtubule dynamic instability in vitro by vinblastine

The antiproliferative action of vinblastine at low concentrations appears to result from modulation of the polymerization dynamics of spindle microtubules rather than from depolarization of the microtubules [Jordan, M. A., Thrower, D., & Wilson, L. (1991) Cancer Res. 51, 2212-2222; (1992) J. Cell. Sci. 102, 401-416]. In the present study, we used differential interference contrast video microscopy to analyze the effects of vinblastine on the growing and shortening dynamics (dynamic instability) of individual bovine brain microtubules in vitro. With microtubules which were either depleted of microtubule-associated proteins (MAPs) or rich in MAPs, low concentrations of vinblastine (0.2 microM-1 microM) suppressed the growing and shortening rates and increased the percentage of time that the microtubules spent a state of attenuated activity, neither growing nor shortening detectably. Vinblastine also suppressed the duration of microtubule growing and shortening, and increased the duration of the attenuated state, during which the microtubules neither grew nor shortened detectably. Consistent with previous data obtained using radiolabeled nucleotide exchange in microtubule suspensions [Jordan, M. A., & Wilson, L. (1990) Biochemistry 29, 2730-2739], vinblastine suppressed growing and shortening dynamics at the kinetically more rapid plus ends. The results suggest that vinblastine kinetically stabilizes microtubule ends by modulating the gain and loss of the stabilizing GTP or GDP-Pi "cap", which is believed to be responsible for the transitions between the growing and shortening phases. The data support the hypothesis that (1) low concentrations of vinblastine inhibit mitosis by kinetically stabilizing the polymerization dynamics of spindle microtubules and that (2) the dynamics of spindle microtubules are critical for the proper progression of mitosis.

Regulation of microtubule dynamic instability

Characterization of microtubule assembly in vivo and in vitro has raised questions of how cells regulate dynamic instability. While dynamic instability is an intrinsic property of the tubulin molecule, factors are required to increase the plus-end elongation rate and increase the frequencies of both catastrophe and rescue to achieve cellular tubulin turnover rates. It is likely that the activities of the transition frequency regulators are themselves regulated during the cell cycle, but the mechanisms of regulation are not known. As we identify the proteins which alter microtubule assembly, new classes of MAPs will emerge. An understanding of how these proteins function may provide further insight into how cells organize the different arrays of microtubules used for such processes as vesicle transport, polarized organization of organelles, and chromosome movement.

Brain microtubule-associated proteins modulate microtubule dynamic instability in vitro. Real-time observations using video microscopy

We used video assays to study the dynamic instability behavior of individual microtubules assembled in vitro with purified tau, purified MAP2 or a preparation of unfractionated heat-stable MAPs. Axoneme-nucleated microtubules were assembled from pure tubulin at concentrations between 4 and 9 microM in the presence of MAPs, and observed by video-differential interference contrast microscopy. Microtubules co-assembled with each MAP preparation exhibited the elongation and rapid shortening phases and the abrupt transitions (catastrophe and rescue) characteristic of dynamic instability. Each MAP preparation increased the microtubule elongation rate above that for purified tubulin alone by decreasing the tubulin subunit dissociation rate during elongation. The brain MAPs used in this study reduced the rate of microtubule rapid shortening, but allowed significant loss of polymer during the shortening phase. Purified tau and MAP2 decreased the frequency of catastrophe and increased the frequency of rescue, while the heat-stable MAPs suppressed catastrophe at all but the lowest tubulin concentrations. Thus, each of these MAPs modulates, but does not abolish, dynamic instability behavior of microtubules. We propose a model to explain how MAP2 and tau bind to the microtubule lattice at sites along protofilaments so that the MAPs promote polymerization, but do not significantly block the mechanism of rapid shortening inherent in the tubulin lattice. Rapid shortening, when it occurs, proceeds primarily by the dissociation of short fragments of protofilaments, which contain the bound MAPs.

Modulation of the dynamic instability of tubulin assembly by the microtubule-associated protein tau

Microtubule-associated proteins (MAP), such as tau, modulate the extent and rate of microtubule assembly and play an essential role in morphogenetic processes, such as axonal growth. We have examined the mechanism by which tau affects microtubule polymerization by examining the kinetics of microtubule assembly and disassembly through direct observation of microtubules using dark-field microscopy. Tau increases the rate of polymerization, decreases the rate of transit into the shrinking phase (catastrophe), and inhibits the rate of depolymerization. Tau strongly suppresses the catastrophe rate, and its ability to do so is independent of its ability to increase the elongation rate. Thus, tau generates a partially stable but still dynamic state in microtubules. This state is perturbed by phosphorylation by MAP2 kinase, which affects all three activities by lowering the affinity of tau for the microtubule lattice.

Role of GTP hydrolysis in microtubule dynamics: information from a slowly hydrolyzable analogue, GMPCPP

The role of GTP hydrolysis in microtubule dynamics has been reinvestigated using an analogue of GTP, guanylyl-(alpha, beta)-methylene-diphosphonate (GMPCPP). This analogue binds to the tubulin exchangeable nucleotide binding site (E-site) with an affinity four to eightfold lower than GTP and promotes the polymerization of normal microtubules. The polymerization rate of microtubules with GMPCPP-tubulin is very similar to that of GTP-tubulin. However, in contrast to microtubules polymerized with GTP, GMPCPP-microtubules do not depolymerize rapidly after isothermal dilution. The depolymerization rate of GMPCPP-microtubules is 0.1 s-1 compared with 500 s-1 for GDP-microtubules. GMPCPP also completely suppresses dynamic instability. Contrary to previous work, we find that the beta--gamma bond of GMPCPP is hydrolyzed extremely slowly after incorporation into the microtubule lattice, with a rate constant of 4 x 10(-7) s-1. Because GMPCPP hydrolysis is negligible over the course of a polymerization experiment, it can be used to test the role of hydrolysis in microtubule dynamics. Our results provide strong new evidence for the idea that GTP hydrolysis by tubulin is not required for normal polymerization but is essential for depolymerization and thus for dynamic instability. Because GMPCPP strongly promotes spontaneous nucleation of microtubules, we propose that GTP hydrolysis by tubulin also plays the important biological role of inhibiting spontaneous microtubule nucleation.

Site-directed mutagenesis of the GTP-binding domain of beta-tubulin

Tubulin binds guanine nucleotides with high affinity and specificity. GTP, an allosteric effector of microtubule assembly, requires Mg2+ for its interaction with beta-tubulin and binds as the MgGTP complex. In contrast, GDP binding does not require Mg2+. The structural basis for this difference is not understood but may be of fundamental importance for microtubule assembly. We investigated the interaction of beta-tubulin with guanine nucleotides using site-directed mutagenesis. Acidic amino acid residues have been shown to interact with nucleotide in numerous nucleotide-binding proteins. In this study, we mutated seven highly conserved aspartic acid residues and one highly conserved glutamic acid residue in the putative GTP-binding domain of beta-tubulin (N-terminal 300 amino acids) to asparagine and glutamine, respectively. The mutants were synthesized in vitro using rabbit reticulocyte lysates, and their affinities for nucleotide determined by an h.p.l.c.-based assay. Our results indicate that the mutations can be placed in six separate categories on the basis of their effects on nucleotide binding. These categories range from having no effect on nucleotide binding to a mutation that apparently abolishes nucleotide binding. One mutation at Asp224 reduced the affinity of beta-tubulin for GTP in the presence but not in the absence of Mg2+. The specific effect of this mutation on nucleotide binding is consistent with an interaction of this amino acid with the Mg2+ moiety of MgGTP. This residue is in a region sharing sequence homology with the putative Mg2+ site in myosin and other ATP-binding proteins. As a result, tubulin belongs to a distinct class of GTP-binding proteins which may be evolutionarily related to the ATP-binding proteins.

Modulation of tubulin-nucleotide interactions by metal ions: comparison of beryllium with magnesium and initial studies with other cations

With microtubule-associated proteins (MAPs) BeSO4 and MgSO4 stimulated tubulin polymerization as compared to a reaction mixture without exogenously added metal ion, while beryllium fluoride had no effect (E. Hamel et al., 1991, Arch. Biochem. Biophys. 286, 57-69). Effects of both cations were most dramatic at GTP concentrations in the same molar range as the tubulin concentration. We have now compared effects of beryllium and magnesium on tubulin-nucleotide interactions in both unpolymerized tubulin and in polymer. Polymer formed with magnesium had properties similar to those of polymer formed without exogenous cation, except for a 20% lower stoichiometry of exogenous GTP incorporated into the latter. In both polymers the incorporated GTP was hydrolyzed to GDP. Stoichiometry of GTP incorporation into polymers formed with beryllium or magnesium was identical, but much of the GTP in the beryllium polymer was not hydrolyzed. The beryllium polymer was more stable than the magnesium polymer. Beryllium also differed from magnesium in only weakly enhancing the binding of GTP in the exchangeable site of unpolymerized tubulin, while neither cation affected GDP exchange at the site. If both cations were present in a reaction mixture, polymer stability was little changed from that of the beryllium polymer, but most of the GTP incorporated into polymer was hydrolyzed. Six additional metal salts (AlCl3, CdCl2, CoCl2, MnCl2, SnCl2, and ZnCl2) also stimulated MAP-dependent tubulin polymerization, but enhanced polymer stability did not correlate with polymer GTP content. We postulate that enhanced polymer stability is a consequence of cation binding directly to tubulin and/or polymer while deficient GTP hydrolysis in the presence of beryllium, as well as aluminum and tin, is a consequence of tight binding of cation to GTP in the exchangeable site.