GTP hydrolysis during microtubule assembly

The GTP cap model of dynamic instability [Mitchison, T., & Kirschner, M.W. (1984) Nature (London) 312, 237] postulates that a GTP cap at the end of most microtubules stabilizes the polymer and allows continuing assembly of GTP-tubulin subunits while microtubules without a cap rapidly disassemble. This attractive explanation for observed microtubule behavior is based on the suggestion that hydrolysis of GTP is not coupled to assembly but rather takes place as a first-order reaction after a subunit is assembled onto a polymer end. Carlier and Pantaloni [Carlier, M., & Pantaloni, D. (1981) Biochemistry 20, 1918] reported a lag of hydrolysis behind microtubule assembly and a first-order rate constant for hydrolysis (kh) of 0.25/min. A lag has not been demonstrated by other investigators, and a kh value that specifies such a slow rate of hydrolysis is difficult to reconcile with reported steady-state microtubule growth rates and frequencies of disassembly. We have looked for a lag using tubulin free of microtubule-associated protein at concentrations of 18.5-74 microM, assembly with and without glycerol, and two independent assays of GTP hydrolysis. No lag was observed under any of the conditions employed, with initial rates of hydrolysis increasing in proportion to rates of assembly. If hydrolysis is uncoupled from assembly, we estimate that kh must be at least 2.5/min and could be much greater, a result that we argue may be advantageous to the GTP cap model. We also describe a preliminary model of assembly coupled to hydrolysis that specifies formation and loss of a GTP cap, thus allowing dynamic instability.     

Dynamics of the microtubule oscillator: role of nucleotides and tubulin-MAP interactions.

Microtubules can be induced to perform synchronous and periodic cycles of assembly and disassembly at constant temperature. The process depends on GTP hydrolysis. Time-resolved X-ray scattering using synchrotron radiation shows a cyclic interconversion of tubulin subunits, microtubules and oligomers (= short protofilament fragments). Oscillations are correlated with conditions that stabilize polymers and destabilize oligomers, and others of opposite effect. Microtubule stabilizers include GTP, Mg2+ or microtubule-associated proteins (MAPs), destabilizers include GDP or elevated ionic strength. K+ at intracellular concentrations noticeably increases the stability of tubulin-MAP oligomers, in contrast to Na+. ATP and the non-hydrolyzable analogue AMP-PNP enhance oscillations by mechanisms that are not directly linked to the role of nucleotide hydrolysis in assembly. We propose a mechanism of oscillations that include oligomers as microtubule disassembly products which transiently lock the protein in an unpolymerizable state; this may point to a role of oligomers in controlling microtubule assembly cycles in cells.     

Hydrolysis of GTP associated with the formation of tubulin oligomers is involved in microtubule nucleation.

Hydrolysis of GTP is known to accompany microtubule assembly. Here we show that hydrolysis of GTP is also associated with the formation of linear oligomers of tubulin, which are precursors (prenuclei) in microtubule assembly. The hydrolysis of GTP on these linear oligomers inhibits the lateral association of GTP-tubulin that leads to the formation of a bidimensional lattice. Therefore GTP hydrolysis interferes with the nucleation of microtubules. Linear oligomers are also formed in mixtures of GTP-tubulin and GDP-tubulin. The hydrolysis of GTP associated with heterologous interactions between GTP-tubulin and GDP-tubulin in the cooligomer takes place at a threefold faster rate than upon homologous interactions between GTP-tubulins. The implication of these results in a model of vectorial GTP hydrolysis in microtubule assembly is discussed.     

Use of a GTP photoaffinity probe to resolve aspects of the mechanism of tubulin polymerization.

8-Azidoguanosine 5'-triphosphate (8-N3GTP) was used in a photoactivatable probe to examine the role of GTP in microtubule assembly. 8-N3GTP was able to substitute for GTP in the promotion of tubulin polymerization and was hydrolyzed at 37 degrees C in the presence or absence of colchicine or calcium. Photolysis of the analog in the presence of microtubular protein resulted in its covalent incorporation onto a GTP-specific site of the beta monomer. The efficiency of this incorporation was different when 8-N3GDP (which does not affect polymerization) was used in place of 8-N3GTP, implying a different orientation of the nucleoside diphosphate within the receptor site. During microtubule assembly, 8-N3GTP was hydrolyzed in situ at the tubulin-GTP exchangeable site in a process that was dependent upon polymerization. The use of [beta, gamma-32P]8-N3GTP and [gamma-32P]8-N3GTP indicated that this hydrolysis occurred concurrently with polymerization and that only nucleoside diphosphate remained bound to the polymerized tubulin.     

Assembly of pure tubulin in the absence of free GTP: effect of magnesium, glycerol, ATP, and the nonhydrolyzable GTP analogues

We describe in vitro microtubule assembly that exhibits, in bulk solution, behavior consistent with the GTP cap model of dynamic instability. Microtubules assembled from pure tubulin in the absence of free nucleotides could undergo one cycle of assembly, but could not sustain an assembly plateau. After the initial peak of assembly was reached and bound E-site GTP hydrolyzed to GDP, the microtubules gradually disassembled. We studied buffer conditions that maximized this disassembly while still allowing robust assembly to take place. While both glycerol and glutamate increased the rate of initial assembly and then slowed disassembly, magnesium promoted initial assembly and, surprisingly, enhanced disassembly. After cooling, a second cycle of assembly was unsuccessful unless GTP or the hydrolyzable GTP analogue GMPCPOP was readded. The nonhydrolyzable GTP analogues GMPPNP and GMPPCP could not support the second assembly cycle in the absence of E-site GTP. Analysis using HPLC found no evidence that GMPPNP, GMPPCP, or ATP could bind to free tubulin, and these nucleotides did not compete with GTP for the E-site. We have, however, demonstrated that the nonhydrolyzable GTP analogues and ATP do have an important effect on microtubule assembly. GMPPNP, GMPPCP, and ATP could each enhance the rate of assembly and stabilize the plateau of assembled microtubules against disassembly, while not binding appreciably to free tubulin. We conclude that these nucleotides, as well as GTP itself, enhance assembly by binding to a site on microtubules that is not present on free, unpolymerized tubulin. We estimate the affinity (KD) of the polymeric site for nucleotide triphosphates to be approximately 10(-4)M.     

Guanosine-5′-triphosphate hydrolysis and tubulin polymerization

Summary GTP hydrolysis associated with polymerization is a distinctive feature of microtubule assembly. This reaction may be fundamentally linked to the dynamic properties of microtubules in vivo. Kinetic analysis of the connection between microtubule assembly and associated GTP hydrolysis indicates that these two events are kinetically uncoupled, GTP hydrolysis occurring after tubulin incorporation in the microtubule. As a consequence, the combination of the diffusionnal incorporation of GTP in microtubules at steady-state and of subsequent GTP hydrolysis results in the formation of a steady-state GTP cap at microtubule ends. The interplay between GTP and GDP at microtubule ends is examined. Inhibition by GDP of steady-state GTP hydrolysis at microtubule ends and of microtubule elongation is understood within a tight reversible binding of GDP at microtubule ends generating inactive elongation sites. Nucleotides are freely exchangeable at microtubule ends. This result indicates that the nature of the nucleotide present at microtubule ends must be considered in a model for microtubule assembly.These data are pooled in order to define the general features of a model describing microtubule assembly and treadmilling in terms somewhat different from previously proposed models.     

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

The relationship between GTP hydrolysis and microtubule assembly has been investigated by using a rapid filtration method. Microtubules assembled from phosphocellulose-purified tubulin, double-labeled with [gamma-32P]- and [3H]GTP, were trapped and washed free of unbound nucleotide on glass fiber filters. The transient accumulation of microtubule-bound GTP predicted by uncoupled GTP hydrolysis models [Carlier & Pantaloni (1981) Biochemistry 20, 1918-1924; Carlier et al. (1987) Biochemistry 26, 4428-4437] during the rapid assembly of microtubules was not detectable under our experimental conditions. By calculating hypothetical time courses for the transient accumulation of microtubule-bound GTP, we demonstrate that microtubule-bound GTP would have been detectable even if the first-order rate constant for GTP hydrolysis were 4-5 times greater than the pseudo-first-order rate constant for tubulin subunit addition to microtubules. In a similar manner, we demonstrate that if GTP hydrolysis were uncoupled from microtubule assembly but were limited to the interface between GTP subunits and GDP subunits (uncoupled vectorial hydrolysis), then microtubule-bound GTP would have been detectable if GTP hydrolysis became uncoupled from microtubule assembly at less than 50 microM free tubulin, 5 times the steady-state tubulin concentration of our experimental conditions. In addition, during rapid microtubule assembly, we have not detected any microtubule-bound Pi, which has been proposed to form a stabilizing cap at the ends of microtubules [Carlier et al. (1988) Biochemistry 27, 3555-3559]. Also, several conditions that could be expected to increase the degree of potential uncoupling between GTP hydrolysis and microtubule assembly were examined, and no evidence of uncoupling was found. Our results are consistent with models that propose cooperative mechanisms that limit GTP hydrolysis to the terminal ring of tubulin subunits [e.g., O'Brien et al. (1987) Biochemistry 26, 4148-4156]. The results are also consistent with the hypothesis that a slow conformational change in tubulin subunits after GTP hydrolysis and Pi release occurs that results in destabilized microtubule ends when such subunits become exposed at the ends.     

Mechanism of the microtubule GTPase reaction

The rate of GTP hydrolysis by microtubules has been measured at tubulin subunit concentrations where microtubules undergo net disassembly. This was made possible by using microtubules stabilized against disassembly by reaction with ethylene glycol bis-(succinimidylsuccinate) (EGS) as sites for the addition of tubulin-GTP subunits. The tubulin subunit concentration was varied from 25 to 90% of the steady state concentration, and there was no net elongation of stabilized microtubule seeds. The GTPase rate with EGS microtubules was linearly proportional to the tubulin-GTP subunit concentration when this concentration was varied by dilution and by using GDP to compete with GTP for the tubulin E-site. The linear dependence of the rate is consistent with a GTP mechanism in which hydrolysis is coupled to the tubulin-GTP subunit addition to microtubule ends. It is inconsistent with reaction schemes in which: microtubules are capped by a single tubulin-GTP subunit, which hydrolyzes GTP when a tubulin-GTP subunit adds to the end; hydrolysis occurs primarily in subunits at the interface of a tubulin-GTP cap and the tubulin-GDP microtubule core; hydrolysis is not coupled to subunit addition and occurs randomly in subunits in a tubulin-GTP cap. It was also found that GDP inhibition of the microtubule GTPase rate results from GDP competition for GTP at the tubulin subunit E-site. There is no additional effect of GDP on the GTPase rate resulting from exchange into tubulin subunits at microtubule ends.     

A Thermodynamic Model of Microtubule Assembly and Disassembly

Microtubules are self-assembling polymers whose dynamics are essential for the normal function of cellular processes including chromosome separation and cytokinesis. Therefore understanding what factors effect microtubule growth is fundamental to our understanding of the control of microtubule based processes. An important factor that determines the status of a microtubule, whether it is growing or shrinking, is the length of the GTP tubulin microtubule cap. Here, we derive a Monte Carlo model of the assembly and disassembly of microtubules. We use thermodynamic laws to reduce the number of parameters of our model and, in particular, we take into account the contribution of water to the entropy of the system. We fit all parameters of the model from published experimental data using the GTP tubulin dimer attachment rate and the lateral and longitudinal binding energies of GTP and GDP tubulin dimers at both ends. Also we calculate and incorporate the GTP hydrolysis rate. We have applied our model and can mimic published experimental data, which formerly suggested a single layer GTP tubulin dimer microtubule cap, to show that these data demonstrate that the GTP cap can fluctuate and can be several microns long.     

Nucleotide release from tubulin and nucleoside-5'-diphosphate kinase action in microtubule assembly.

ATP and UTP support microtubule assembly through the action of brain nucleoside-5'-diphosphate kinase on GDP. Penningroth and Kirschner (1977) J. Mol. Biol. 115, 643-673) have proposed that microtubule assembly may occur by either of two mechanisms: indirectly, through nucleoside-5'-diphosphate kinase-catalyzed phosphorylation of uncomplexed GDP and directly by nucleoside-5'-diphosphate kinase-mediated transphosphorylation of tubulin-bound GDP at low tubulin concentrations. We find the rates of GDP and GTP release (0.68 and 0.32 min-1, respectively) are sufficiently fast relative to assembly to permit GDP release, phosphorylation, and GTP binding as the sole mechanism of nucleoside-5'-diphosphate kinase action in microtubule assembly. Computer simulation studies accord with the conclusion that GDP release is rapid relative to microtubule assembly. The specific activity of the nucleoside-5'-diphosphate kinase is 1.7 nmol/min/mg of microtubular protein under the conditions studied. Pulse-chase experiments with tubulin . [14C]GDP complex and the rapidity of GDP phosphorylation by the kinase are in agreement with this scheme. Finally, it was observed that the extent and rate of microtubule assembly depends upon the [ATP]/[ADP] ratio.     

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 [published erratum appears in J Cell Biol 1995 Apr;129(2):549]

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.     

Magnesium requirements for guanosine 5'-O-(3-thiotriphosphate) induced assembly of microtubule protein and tubulin

Two conflicting interpretations on the role of guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) in microtubule protein and tubulin assembly have been previously reported. One study finds that GTP gamma S promotes assembly while another study reports that GTP gamma S is a potent inhibitor of microtubule assembly. We have examined the potential role of Mg2+ to learn if the conflicting interpretations are due to a metal effect. Turbidity, electron microscopy, and nucleotide binding and hydrolysis were used to analyze the effect of the Mg2+ concentration on GTP gamma S-induced assembly of microtubule protein (tubulin + microtubule-associated proteins) in the presence of buffer +/- 30% glycerol and in buffer with GTP added before or after GTP gamma S. GTP gamma S substantially lowers the Mg2+ concentration required to induce cross-linked or clustered rings of tubulin. These cross-linked rings do not assemble well into microtubules, and GTP only partially restores microtubule assembly. However, taxol will promote GTP gamma S-induced cross-linked rings of microtubule protein to assemble into microtubules. The effect of GTP gamma S on microtubule protein assembly in the presence of Zn2+ with and without added Mg2+ suggests that GTP gamma S also effects the formation of Zn2+-induced sheet aggregates. Purified tubulin was used in assembly experiments with Mg2+, Zn2+, and taxol to better understand GTP gamma S interactions with tubulin. The optimal Mg2+ concentration for assembly of tubulin is lower with GTP gamma S than with GTP.(ABSTRACT TRUNCATED AT 250 WORDS)     

COPII coat assembly and selective export from the endoplasmic reticulum

The coat protein complex II (COPII) generates transport vesicles that mediate protein transport from the endoplasmic reticulum (ER). Recent structural and biochemical studies have suggested that the COPII coat is responsible for direct capture of membrane cargo proteins and for the physical deformation of the ER membrane that drives the transport vesicle formation. The COPII-coated vesicle formation at the ER membrane is triggered by the activation of the Ras-like small GTPase Sar1 by GDP/GTP exchange, and activated Sar1 in turn promotes COPII coat assembly. Subsequent GTP hydrolysis by Sar1 leads to disassembly of the coat proteins, which are then recycled for additional rounds of vesicle formation. Thus, the Sar1 GTPase cycle is thought to regulate COPII coat assembly and disassembly. Emerging evidence suggests that the cargo proteins modulate the Sar1 GTP hydrolysis to coordinate coat assembly with cargo selection. Here, I discuss the possible roles of the GTP hydrolysis by Sar1 in COPII coat assembly and selective uptake of cargo proteins into transport vesicles.     

Nucleotide binding and phosphorylation in microtubule assembly in vitro.

Two non-hydrolyzable analogs of GTP, guanylyl-β,γ-methylene diphosphonate and guanylyl imidodiphosphate, have been found to induce rapid and efficient microtubule assembly in vitro by binding at the exchangeable site (E-site) on tubulin. Characterization of microtubule polymerization by several criteria, including polymerization kinetics, nucleotide binding to depolymerized and polymerized microtubules, and microtubule stability, reveals strong similarities between microtubule assembly induced by GTP and non-hydrolyzable GTP analogs. Nucleoside triphosphates which bind weakly or not at all to tubulin, such as ATP, UTP and CTP, are shown to induce microtubule assembly by means of a nucleoside diphosphate kinase (NDP-kinase, EC 2.7.4.6.) activity which is not intrinsic to tubulin. The NDP-kinase mediates microtubule polymerization by phosphorylating tubulin-bound GDP in situ at the E-site. Although hydrolysis of exchangeably bound GTP occurs, it is found to be uncoupled from the polymerization reaction. The non-exchangeable nucleotide binding site on tubulin (N-site) is not directly involved in microtubule assembly in vitro. The N-site is shown to contain almost exclusively GTP which is not hydrolyzed during microtubule assembly. A scheme is presented in which GTP acts as an allosteric effector at the E-site during microtubule assembly in vitro.     

Inhibition of microtubule formation by DNA

In vitro assembly of porcine brain microtubular protein to microtubules is affected by calf thymus DNA. Dependent on mass ratio of DNA/MTP microtubule formation is partly inhibited or blocked. Microtubules formed in presence of DNA are not to be distinguished from those assembled without DNA by electron microscopy. Addition of DNA to microtubules in assembly buffer causes their disassembly.     

The association of heterotrimeric GTP-binding protein (Go) with microtubules

The heterotrimeric GTP-binding regulatory proteins (G proteins) play an important role in the regulation of membrane signal transduction. Recently, we identified the association of Go protein with mitotic spindles. Here we have investigated the relationship between Go protein and microtubules. We used temperature-dependent reversible assembly and taxol methods to purify microtubules from bovine brains. Goalpha and Gbeta proteins were identified in the microtubular fraction by both methods. The Goalpha subunit in the microtubular fraction could be ADP ribosylated by pertussis toxin. Co-immunoprecipitation data also revealed that Go protein can interact with microtubules. Exogenous Go protein could be incorporated into the assembled microtubular fraction, and 5 microg/ml (60 nM) of Go protein inhibited 40% of microtubule assembly. Western blot analysis of Goalpha-1 and Goalpha-2 in microtubular fractions showed that only Goalpha-1 is associated with microtubules. We conclude that the Goalpha-1betagamma proteins are associated with microtubules and may play some role in regulating the assembly and disassembly of microtubules.     

Metastability of microtubules induced by competing internal forces

Recent modeling efforts to estimate energies of tubulin-tubulin bonds shed light on a delicate balance between competing mechanical forces maintaining microtubule walls. Here we formulate two important refinements to the explanation of bond energetics. First, energy surface calculations in the elastic filament approximation reveal a finite stabilizing barrier assumed a simple Lennard-Jones-like potential for protein bonds. The presence of a guanosine triphosphate (GTP) cap represented by straight segments is necessary, as it is predicted for a long time. In the lack of such a cap, the protofilaments are either in an absolutely stable or absolutely unstable state. Second, our calculations show that this barrier appears only if the mechanical energy associated with the conformational change after GTP hydrolysis (curling energy) is larger than the strength of lateral bonds. The overall energy balance we propose supports continuous assembly of GTP dimers, a metastable state in the presence of a finite GTP cap and energetically driven disassembly of guanosine diphosphate protofilaments.     

GDP-Tubulin Incorporation into Growing Microtubules Modulates Polymer Stability

Microtubule growth proceeds through the endwise addition of nucleotide-bound tubulin dimers. The microtubule wall is composed of GDP-tubulin subunits, which are thought to come exclusively from the incorporation of GTP-tubulin complexes at microtubule ends followed by GTP hydrolysis within the polymer. The possibility of a direct GDP-tubulin incorporation into growing polymers is regarded as hardly compatible with recent structural data. Here, we have examined GTP-tubulin and GDP-tubulin incorporation into polymerizing microtubules using a minimal assembly system comprised of nucleotide-bound tubulin dimers, in the absence of free nucleotide. We find that GDP-tubulin complexes can efficiently co-polymerize with GTP-tubulin complexes during microtubule assembly. GDP-tubulin incorporation into microtubules occurs with similar efficiency during bulk microtubule assembly as during microtubule growth from seeds or centrosomes. Microtubules formed from GTP-tubulin/GDP-tubulin mixtures display altered microtubule dynamics, in particular a decreased shrinkage rate, apparently due to intrinsic modifications of the polymer disassembly properties. Thus, although microtubules polymerized from GTP-tubulin/GDP-tubulin mixtures or from homogeneous GTP-tubulin solutions are both composed of GDP-tubulin subunits, they have different dynamic properties, and this may reveal a novel form of microtubule “structural plasticity.”     

Effects of pH on tubulin-nucleotide interactions

Significant GTP-independent, temperature-dependent turbidity development occurs with purified tubulin stored in the absence of unbound nucleotide, and this can be minimized with a higher reaction pH. Since microtubule assembly is optimal at lower pH values, we examined pH effects on tubulin-nucleotide interactions. While the lowest concentration of GTP required for assembly changed little, GDP was more inhibitory at higher pH values. The amounts of exogenous GTP bound to tubulin at all pH values were similar, but the amounts of exogenous GDP bound and endogenous GDP (i.e., GDP originally bound in the exchangeable site) retained by tubulin rose as reaction pH increased. Endogenous GDP was more efficiently displaced by exogenous GTP than GDP at all pH values, but displacement by GTP was 10-15% greater at pH 6 than at pH 7. Dissociation constants for GDP and GTP were about 1.0 microM at pH 6 and 0.02 microM at pH 7. A small increase in the affinity of GDP relative to that of GTP occurs at pH 7 as compared to pH 6, together with a 50-fold absolute increase in the affinity of both nucleotides for tubulin at pH 7. The time courses of microtubule assembly and GTP hydrolysis were compared at pH 6 and pH 7. At pH 6, the two reactions were simultaneous in onset and initially stoichiometric. At pH 7, although the reactions began simultaneously, hydrolysis seemed to lag substantially behind assembly. Unhydrolyzed radiolabeled GTP was not incorporated into microtubules, however, indicating that GTP hydrolysis is actually closely coupled to assembly. The apparent lag in hydrolysis probably results from a methodological artifact rather than incorporation of GTP into the microtubule with delayed hydrolysis.     

Dephosphorylation of tubulin-bound guanosine triphosphate during microtubule assembly.

1. Tubulin purified from porcine brain in the presence of GTP contained 0.16 mole of GDP and 0.73 mole of GTP per 60,000 g of protein. 2. Microtubules reconstituted from the purified tubulin contained 0.43 mole of GDP and 0.41 mole of GTP per 60,000 g of protein. Guanine nucleotide bound to the exchangeable site of tubulin was converted to GDP during microtubule assembly, while GTP at the non-exchangeable site remained intact. 3. Guanine nucleotide which had been bound to the exchangeable site of tubulin before microtubule assembly was also exchangeable during disassembly.