The assembly of microtubule protein in vitro. The kinetic role in microtubule elongation of oligomeric fragments containing microtubule-associated proteins

Natural methylamine osmolytes, trimethylamine N-oxide and betaine, increase tau-induced polymerization of microtubules

The natural osmolyte trimethylamine N-oxide (TMAO) at 200 mM increases the extent and the rate of formation of polymerized microtubule (MT) complex caused by tau. TMAO at this concentration has no effect on tubulin alone. Urea at 200 mM blocks tubulin assembly caused by tau, but this inhibition can be reversed by an equal amount of TMAO. Besides TMAO, betaine, another natural osmolyte, was found to have the same effects on MT as TMAO. On the contrary, glycerol (a carbohydrate osmolyte) and glycine (an amino acid osmolyte) do not increase tau-induced MT assembly. The mechanism by which TMAO and betaine enhance tau's effectiveness is not known, but physical studies suggest that the secondary structure of tau is not appreciably changed by 200 mM TMAO. This is the first report showing that natural osmolytes, TMAO and betaine, at a near physiological concentration are able to stimulate tau-induced tubulin assembly.

Dynamic instability of microtubules: Monte Carlo simulation and application to different types of microtubule lattice

Dynamic instability is the term used to describe the transition of an individual microtubule, apparently at random, between extended periods of slow growth and brief periods of rapid shortening. The typical sawtooth growth and shortening transition behavior has been successfully simulated numerically for the 13-protofilament microtubule A-lattice by a lateral cap model (Bayley, P. M., M. J. Schilstra, and S. R. Martin. 1990. J. Cell Sci. 95:33-48). This kinetic model is now extended systematically to other related lattice geometries, namely the 13-protofilament B-lattice and the 14-protofilament A-lattice, which contain structural "seams". The treatment requires the assignment of the free energies of specific protein-protein interactions in terms of the basic microtubule lattice. It is seen that dynamic instability is not restricted to the helically symmetric 13-protofilament A-lattice but is potentially a feature of all A- and B-lattices, irrespective of protofilament number. The advantages of this general energetic approach are that it allows a consistent treatment to be made for both ends of any microtubule lattice. Important features are the predominance of longitudinal interactions between tubulin molecules within the same protofilament and the implication of a relatively favorable interaction of tubulin-GDP with the growing microtubule end. For the three lattices specifically considered, the treatment predicts the dependence of the transition behavior upon tubulin concentration as a cooperative process, in good agreement with recent experimental observations. The model rationalizes the dynamic properties in terms of a metastable microtubule lattice of tubulin-GDP, stabilized by the kinetic process of tubulin-GTP addition. It provides a quantitative basis for the consideration of in vitro microtubule behaviour under both steady-state and non-steady-state conditions, for comparison with experimental data on the dilution-induced disassembly of microtubules. Similarly, the effects of small tubulin-binding molecules such as GDP and nonhydrolyzable GTP analogues are readily treated. An extension of the model allows a detailed quantitative examination of possible modes of substoichiometric action of a number of antimitotic drugs relevant to cancer chemotherapy.

The effect of solution composition on microtubule dynamic instability

The exchange of tubulin dimer into steady-state microtubules was studied over a range of solution conditions, in order to assess the effects of various common buffer components on the dynamic instability of microtubules. In comparison with standard buffer conditions (100 mM-Pipes buffer, pH 6.5, containing 0.1 mM-EGTA, 1.8 mM-MgC12 and 1 M-glycerol), the rate and extent of exchange, and thus of dynamic instability, are suppressed by increasing the concentration of glycerol above 2 M. Exchange is enhanced by the addition of further Mg2+ (up to 17 mM) or by the addition of Ca2+ (up to 0.4 mM). Phosphate ion (150 mM) has relatively little effect on the dynamic behaviour of microtubules, as judged by the exchange method. The findings are interpreted within the framework of the Lateral Cap model for microtubule dynamic instability, in terms of the effects of these changes on the intrinsic rate constants of the system. By contrast, the extent of tubulin exchange depends selectively on the value of the dissociation rate constant for tubulin-GDP. A decrease in the extent of exchange, and hence in dynamic activity, is associated with a decreased value for this rate constant, and vice versa. The results also show good agreement of predictions of the model in treating the observed variations in the dynamic properties of individual microtubules, induced by different solution conditions.

Assembly of chick brain MAP2-tubulin microtubule protein. Characterization of the protein and the MAP2-dependent addition of tubulin dimers

The principle proteins present in twice-cycled chick brain microtubule protein were characterized. The protein consists of a stoichiometric mixture of MAP2 and tubulin, together with a number of minor components. Its composition remains unaltered after a third cycle of assembly in a buffer supplemented with 67 mM-NaCl, with the exception of the phosphorylation of MAP2 to a low level (congruent to 1 mol.mol-1). The inclusion of 67 mM-NaCl dissociates the MAP2-tubulin oligomers, and restricts the assembly to the MAP2-dependent addition and loss of tubulin dimers, such that the assembly kinetics approximate to a simple pseudo-first-order reaction. The assembled microtubules exhibit dynamic instability, with no evidence for end-to-end annealing.

Stoichiometry of estramustine phosphate binding to MAP2 measured by the disassembly of chick brain MAP2:tubulin microtubules

The concentration of estramustine phosphate required to inhibit the assembly or to induce the disassembly of chick brain MAP2:tubulin microtubules is markedly dependent upon the microtubule protein concentration. Analysis of this relationship shows that estramustine phosphate and tubulin compete for common MAP2 sites, that MAP2 can bind 5-6 moles.mole-1 estramustine phosphate, and that the Kd of these sites is congruent to 20 microM estramustine phosphate. It is proposed that two molecules of estramustine phosphate interact with each of the three tubulin-binding sites of MAP2 and inhibit the MAP2:tubulin interaction by neutralising two highly conserved basic residues.

Dynamic properties of microtubules at steady state of polymerisation

Microtubules of tubulin dimer, polymerised in vitro to steady-state are shown to incorporate tubulin rapidly and extensively. The method involves adding [3H]-GTP to microtubules at steady state, and analysing for non-exchangeable [3H]-GDP in the presence of a GTP regenerating system. The rate and extent of this exchange process is dependent on the length distribution of the microtubules, and is notably faster with sheared microtubules. We simulate all these features of the exchange kinetics, together with the length redistributions occurring at steady state of polymerisation, using a simple model based on a limited number of kinetic parameters deriving from the measurements of microtubule dynamics by Horio and Hotani. The observed exchange kinetics thus provide a direct experimental criterion of 'dynamic instability' of microtubules at steady state of polymerisation.

Microtubule assembly kinetics. Changes with solution conditions

The assembly kinetics of microtubule protein are altered by ionic strength, temperature and Mg2+, but not by pH. High ionic strength (I0.2), low temperature (T less than 30 degrees C) and elevated Mg2+ (greater than or equal to 1.2 mM) induce a transition from biphasic to monophasic kinetics. Comparison of the activation energy obtained for the fast biphasic step at low ionic strength (I0.069) shows excellent agreement with the values obtained at high ionic strength, low temperature and elevated Mg2+. From this observation it can be implied that the tubulin-containing reactant of the fast biphasic event is also the species that elongates microtubules during monophasic assembly. Second-order rate constants for biphasic assembly are 3.82(+/- 0.72) x 10(7) M-1.s-1 and 5.19(+/- 1.25) x 10(6) M-1.s-1, and for monophasic assembly the rate constant is 2.12(+/- 0.56) x 10(7) M-1.s-1. The microtubule number concentration is constant during elongation of microtubules for biphasic and monophasic assembly.

On the relationship between nucleotide hydrolysis and microtubule assembly: studies with a GTP-regenerating system

The assembly of pure tubulin dimer has been studied in two buffer systems (containing low and high glycerol/Mg), using a regeneration system protocol to assess the amount of GDP-tubulin in the assembling polymer. For both assembly systems studied, the GDP content is effectively stoichiometric with tubulin throughout assembly. This indicates a high degree of coupling between assembly and GTP-hydrolysis, giving a hydrolysis rate at least 10-fold faster than previously deduced. The steady state GTP hydrolysis rate is quantitatively consistent with this finding. We conclude that the extent of any GTP-tubulin cap is below the detectable limit, both during elongation and at steady state.

Magnesium ion effects on microtubule nucleation in vitro

Much interest has currently been attached to the length distribution of microtubules polymerized in vitro and the related question of their possible 'dynamic instability'. Fundamental to this question is the mechanism of microtubule nucleation, which controls the rates of assembly and disassembly of microtubule protein in vitro. These kinetics are affected by a number of factors, including both the guanine nucleotides, GTP and GDP, and magnesium ion. Mg2+ exerts complex effects, as indicated by the existence of an optimal Mg2+ concentration for the maximum assembly rate of microtubule protein, and we investigate these effects in this report. At [Mg2+] greater than 0.5 mM, the characteristic lag-phase is substantially increased and the rate of assembly is greatly reduced without affecting the critical concentration significantly. We show that increasing [Mg2+] has two effects on the assembly process: nucleation is less efficient and the intrinsic rate constant for the elongation reaction is reduced. Lowering [Mg2+] (less than 0.5 mM) also inhibits nucleation. These effects of varying [Mg2+] can be explained predominantly in terms of enhanced stability of the microtubule-associated protein-containing oligomeric species present in the microtubule protein preparation. [Mg2+] is thus found to be a further important factor in microtubule nucleation, and hence, in determining length distributions in assembling microtubules.

Effects of GDP on microtubules at steady state

We have re-examined the effect of varying GDP concentrations on the kinetics of GTP-induced assembly of microtubules from microtubule protein, and on the elongation of pre-existing microtubules subjected to a temperature jump relaxation from 21.5 to 37 degrees C. The assembly kinetics follow a simple model for assembly which involves a fast equilibrium of tubulin-GTP and tubulin-GDP coupled to the elongation process due to tubulin-GTP. The initial rate of the relaxation process is found to be dependent upon the GTP/GDP ratio, in confirmation of the results of Engelborghs and Van Houtte (Biophys. Chem. 14 (1981) 195). As an alternative to the interpretation previously advanced by them, involving modification of the reactivity of microtubule ENDs by GDP, we show that this result is consistent with the above model with one reasonable modification, namely, that the ratio of the affinities of tubulin for GTP and GDP should vary with temperature. The analysis shows that a decrease in this ratio of approx. 2-fold at 37 degrees C accounts for the observed effects. We conclude that more complex mechanisms involving consideration of modification of the reactivity of microtubule ENDs by GDP are not required to explain these results. This finding has important implications for current models of GDP-induced microtubule disassembly.

Is microtubule assembly a biphasic process? A fluorimetric study using 4',6-diamidino-2-phenylindole as a probe

The kinetics of microtubule assembly followed by turbidimetry usually describe an exponential process but hyperbolic or biphasic assembly curves have also been reported. A combination of turbidimetry, 4',6-diamidino-2-phenylindole (DAPI) fluorescence, electron microscopy, rapid sedimentation and wavelength dependence of scattered light have been used here to investigate these apparently biphasic kinetics of microtubule assembly. Experimental conditions have been developed under which the increase in DAPI fluorescence is a quantitative measurement of polymer formation which, in contrast to turbidity, does not depend on the size of the microtubules. Monophasic assembly curves were obtained when the increase in DAPI fluorescence was measured, whereas under the same conditions the turbidity change was a biphasic process. Biphasic turbidity curves are observed at low (less than 0.5 mM) magnesium ion concentration; the data are consistent with a rapid polymerization of tubulin into short polymers, followed by extensive length redistribution. The morphology of the early intermediates in microtubule assembly appears dependent on the concentration of Mg2+ ions.

Effects of ATP and cyclic AMP on the in vitro assembly and stability of mammalian brain microtubules

The relevance of protein phosphorylation, transphosphorylation and binding phenomena in the kinetics of the ATP-induced assembly of cycle-purified microtubule protein from mammalian brain were studied. ATP was able to induce the polymerization of microtubules of normal appearance. However, the assembled structures, were unstable and microtubules depolymerized after achievement of a transitory maximum. Cyclic AMP reduced the amplitude of the polymerization maximum in a concentration-dependent manner, correlating with the stimulation of the endogenous phosphorylation reaction. When microtubule assembly was induced by GTP, in the presence of various concentrations of ATP, the slope of the depolymerization phase was found to depend on the concentration of ATP. Fluoride ion inhibited the endogenous phosphorylation reaction and reduced the disassembly rate, in a concentration-dependent manner. Evidence is also presented indicating that ATP did not bind to phosphocellulose-purified tubulin. These results further contribute to indicate that ATP and cyclic AMP, acting coordinately to control the phosphorylation extent of microtubule proteins are important factors to determine microtubule stability within the cell. Some implications of this mechanism for the regulation by cAMP of the initiation of DNA synthesis and mitosis are considered.

Tubulin-nucleotide interactions. Effects of removal of exchangeable guanine nucleotide on protein conformation and microtubule assembly

The removal of tightly bound GDP from the exchangeable nucleotide-binding site of tubulin has been performed with alkaline phosphatase under conditions which essentially retain the assembly properties of the protein. When microtubule protein is treated with alkaline phosphatase, nucleotide is selectively removed from tubulin dimer rather than from MAP (microtubule-associated protein)-containing oligomeric species. Tubulin devoid of E-site (the exchangeable nucleotide-binding site of the tubulin dimer) nucleotide shows enhanced proteolytic susceptibility of the beta-subunit to thermolysin and decreased protein stability, consistent with nucleotide removal causing changes in protein tertiary structure. Pyrophosphate ion (3 mM) is able to promote formation of normal microtubules in the complete absence of GTP by incubation at 37 degrees C either with nucleotide-depleted microtubule protein or with nucleotide-depleted tubulin dimer to which MAPs have been added. The resulting microtubules contain up to 80% of tubulin lacking E-site nucleotide. In addition to its effects on nucleation, pyrophosphate competes weakly with GDP bound at the E-site. It is deduced that binding of pyrophosphate at a vacant E-site can promote microtubule assembly. The minimum structural requirement for ligands to induce tubulin assembly apparently involves charge neutralization at the E-site by bidentate ligation, which stabilizes protein domains in a favourable orientation for promoting the supramolecular protein-protein interactions involved in microtubule formation.

Direct incorporation of guanosine 5'-diphosphate into microtubules without guanosine 5'-triphosphate hydrolysis

Using highly purified calf brain tubulin bearing [8-14C]guanosine 5'-diphosphate (GDP) in the exchangeable nucleotide site and heat-treated microtubule-associated proteins (both components containing negligible amounts of nucleoside diphosphate kinase and nonspecific phosphatase activities), we have found that a significant proportion of exchangeable-site GDP in microtubules can be incorporated directly during guanosine 5'-triphosphate (GTP) dependent polymerization of tubulin, without an initial exchange of GDP for GTP and subsequent GTP hydrolysis during assembly. The precise amount of GDP incorporated directly into microtubules is highly dependent on specific reaction conditions, being favored by high tubulin concentrations, low GTP and Mg2+ concentrations, and exogenous GDP in the reaction mixture. Minimum effects were observed with changes in reaction pH or temperature, changes in concentration of microtubule-associated proteins, alteration of the sulfonate buffer, or the presence of a calcium chelator in the reaction mixture. Under conditions most favorable for direct GDP incorporation, about one-third of the GDP in microtubules is incorporated directly (without GTP hydrolysis) and two-thirds is incorporated hydrolytically (as a consequence of GTP hydrolysis). Direct incorporation of GDP occurs in a constant proportion throughout elongation, and the amount of direct incorporation probably reflects the rapid equilibration of GDP and GTP at the exchangeable site that occurs before the onset of assembly.

Control of nucleation in microtubule self-assembly

The inhibition of the rate and amplitude of assembly of microtubule protein at low GTP concentration is shown by measurement of microtubule length distributions to be due to the suppression of microtubule nucleation. This inhibitory effect is enhanced by GDP added before assembly, but can be overcome by a number of molecules such as pyrophosphate or ADP. The selective inhibition of nucleation by GDP in vitro, which occurs in addition to inhibition of elongation, could provide a mechanism for the control of spontaneous microtubule nucleation in vivo.

Inhibition of microtubule elongation by GDP

GDP reduces both the rate and amplitude of GTP-induced assembly of microtubules from tubulin dimer or from microtubule protein, and promotes disassembly from microtubules at the steady state. One interpretation postulates that added GDP modifies microtubule ENDS so that tubulin-GTP, the species involved in steady state elongation of microtubules, cannot bind to a microtubule END containing tubulin-GDP. This concept has been used in subsequent models of assembly which treat the 'dynamic instability' of microtubules. We question this interpretation on the basis of the published experimental data and the results reported here. Using a relatively simple model for microtubule assembly, we show by numerical simulation that the quantitative effects of GDP on the rate and amplitude of microtubule assembly and inhibition of steady state GTPase activity are well accounted for by the nucleotide exchange equilibrium of tubulin-GDP and tubulin-GTP. We therefore conclude that the effect of added GDP on elongation of MAP-containing microtubules and on steady state GTPase activity does not indicate modification of the activity of microtubule ENDs but depends on the tubulin-GTP/tubulin-GDP equilibrium. Additional evidence argues that microtubule ENDS containing GDP can indeed accept elongation by tubulin-GTP.

Assembly of microtubules from nucleotide-depleted tubulin

In vitro assembly of microtubules from tubulin is considered to have an absolute requirement for added GTP (or a non-hydrolysable GTP-analogue) involving binding at the E(exchangeable)-site located on the beta-subunit of the tubulin dimer. By contrast, GDP inhibits assembly. Nucleotide hydrolysis has been implicated in the dynamic properties of microtubules, treadmilling and mechanical coupling. Here we demonstrate that assembly is not necessarily dependent on the presence of GTP at the E-site; microtubules can be formed efficiently in the absence of GTP in the presence of pyrophosphate. These microtubules, which have normal morphology and lability at cold temperatures, contain N(non-exchangeable)-site GTP and a significant proportion of E-site GDP. This demonstrates the possibility of direct incorporation of GDP-containing tubulin dimer during assembly which probably derives from microtubule-associated protein (MAP)-containing oligomers. This finding has important implications for the mechanism of microtubule elongation. The effects of pyrophosphate suggest that charge neutralization by the bidentate ligand is an essential step in promoting microtubule assembly, and that this interaction involves only a minimal conformational change in the protein.

Incorporation of GDP-tubulin during elongation of microtubules in vitro

Removal of GDP from tubulin E-site is not obligatory for the in vitro assembly of microtubule protein in 0.5 mM GMPPCP. This assembly, which is significantly enhanced by glycerol, produces microtubules of normal morphology and with normal composition of microtubule-associated proteins (MAPs). [3H]-GDP initially present at the E-site is shown to be incorporated directly into microtubules during assembly; this incorporation, maximally 60% of the assembled polymer, is dependent upon MAPs. These results are consistent with oligomeric species composed principally of GDP-tubulin plus MAPs, being incorporated directly into microtubules. The finding that stoichiometric GTP-tubulin formation is not an essential prerequisite for microtubule assembly may have important implications for the energetics of microtubule formation.