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1.
《The Journal of cell biology》1995,129(5):1311-1328
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.  相似文献   

2.
Microtubule architecture can vary with eukaryotic species, with different cell types, and with the presence of stabilizing agents. For in vitro assembled microtubules, the average number of protofilaments is reduced by the presence of sarcodictyin A, epothilone B, and eleutherobin (similarly to taxol) but increased by taxotere. Assembly with a slowly hydrolyzable GTP analogue GMPCPP is known to give 96% 14 protofilament microtubules. We have used electron cryomicroscopy and helical reconstruction techniques to obtain three-dimensional maps of taxotere and GMPCPP microtubules incorporating data to 14 A resolution. The dimer packing within the microtubule wall is examined by docking the tubulin crystal structure into these improved microtubule maps. The docked tubulin and simulated images calculated from "atomic resolution" microtubule models show tubulin heterodimers are aligned head to tail along the protofilaments with the beta subunit capping the microtubule plus end. The relative positions of tubulin dimers in neighboring protofilaments are the same for both types of microtubule, confirming that conserved lateral interactions between tubulin subunits are responsible for the surface lattice accommodation observed for different microtubule architectures. Microtubules with unconventional protofilament numbers that exist in vivo are likely to have the same surface lattice organizations found in vitro. A curved "GDP" tubulin conformation induced by stathmin-like proteins appears to weaken lateral contacts between tubulin subunits and could block microtubule assembly or favor disassembly. We conclude that lateral contacts between tubulin subunits in neighboring protofilaments have a decisive role for microtubule stability, rigidity, and architecture.  相似文献   

3.
The microtubule assembly process has been extensively studied, but the underlying molecular mechanism remains poorly understood. The structure of an artificially generated sheet polymer that alternates two types of lateral contacts and that directly converts into microtubules, has been proposed to correspond to the intermediate sheet structure observed during microtubule assembly. We have studied the self-assembly process of GMPCPP tubulins into sheet and microtubule structures using thermodynamic analysis and stochastic simulations. With the novel assumptions that tubulins can laterally interact in two different forms, and allosterically affect neighboring lateral interactions, we can explain existing experimental observations. At low temperature, the allosteric effect results in the observed sheet structure with alternating lateral interactions as the thermodynamically most stable form. At normal microtubule assembly temperature, our work indicates that a class of sheet structures resembling those observed at low temperature is transiently trapped as an intermediate during the assembly process. This work may shed light on the tubulin molecular interactions, and the role of sheet formation during microtubule assembly.  相似文献   

4.
A wide range of small molecules, including alkaloids, macrolides and peptides, bind to tubulin and disturb microtubule assembly dynamics. Some agents inhibit assembly, others inhibit disassembly. The binding sites of drugs that stabilize microtubules are discussed in relation to the properties of microtubule associated proteins. The activities of assembly inhibitors are discussed in relation to different nucleotide states of tubulin family protein structures.  相似文献   

5.
The assembly/disassembly of biological macromolecules plays an important role in their biological functionalities. Although the dynamics of tubulin polymers and their super-assembly into microtubule structures is critical for many cellular processes, details of their cyclical polymerization/depolymerization are not fully understood. Here, we use a specially designed light scattering technique to continuously examine the effects of temperature cycling on the process of microtubule assembly/disassembly. We observe a thermal hysteresis loop during tubulin assembly/disassembly, consistently with earlier reports on the coexistence of tubulin and microtubules as a phase transition. In a cyclical process, the structural hysteresis has a kinetic component that depends on the rate of temperature change but also an intrinsic thermodynamic component that depends on the protein topology, possibly related to irreversible processes. Analyzing the evolution of such thermal hysteresis loops over successive cycles, we found that the assembly/disassembly ceases after some time, which is indicative of protein aging leading to its inability to self-assemble after a finite number of temperature cycles. The emergence of assembly-incompetent tubulin could have major consequences for human pathologies related to microtubules, including aging, neurodegenerative diseases and cancer.  相似文献   

6.
Estimation of the diffusion-limited rate of microtubule assembly.   总被引:2,自引:0,他引:2       下载免费PDF全文
Microtubule assembly is a complex process with individual microtubules alternating stochastically between extended periods of assembly and disassembly, a phenomenon known as dynamic instability. Since the discovery of dynamic instability, molecular models of assembly have generally assumed that tubulin incorporation into the microtubule lattice is primarily reaction-limited. Recently this assumption has been challenged and the importance of diffusion in microtubule assembly dynamics asserted on the basis of scaling arguments, with tubulin gradients predicted to extend over length scales exceeding a cell diameter, approximately 50 microns. To assess whether individual microtubules in vivo assemble at diffusion-limited rates and to predict the theoretical upper limit on the assembly rate, a steady-state mean-field model for the concentration of tubulin about a growing microtubule tip was developed. Using published parameter values for microtubule assembly in vivo (growth rate = 7 microns/min, diffusivity = 6 x 10(-12) m2/s, tubulin concentration = 10 microM), the model predicted that the tubulin concentration at the microtubule tip was approximately 89% of the concentration far from the tip, indicating that microtubule self-assembly is not diffusion-limited. Furthermore, the gradients extended less than approximately 50 nm (the equivalent of about two microtubule diameters) from the microtubule tip, a distance much less than a cell diameter. In addition, a general relation was developed to predict the diffusion-limited assembly rate from the diffusivity and bulk tubulin concentration. Using this relation, it was estimated that the maximum theoretical assembly rate is approximately 65 microns/min, above which tubulin can no longer diffuse rapidly enough to support faster growth.  相似文献   

7.
Microtubules assembled in vitro from pure tubulin can switch occasionally from growing to shrinking states or resume assembly, an unusual behavior termed "dynamic instability of microtubule growth". Its origin remains unclear and several models have been proposed, including occasional switching of the microtubules into energetically unfavorable configurations during assembly. In this study, we have asked whether the excess energy accumulated in these configurations would be of sufficient magnitude to destabilize the capping region that must exist at the end of growing microtubules. For this purpose, we have analyzed the frequency distribution of microtubules assembled in vitro from pure tubulin, and modeled the different mechanical constraints accumulated in their wall. We find that the maximal excess energy that the microtubule lattice can store is in the order of 11 kBT per dimer. Configurations that require distortions up to approximately 20 kBT are allowed at the expense of a slight conformational change, and larger distortions are not observed. Modeling of the different elastic deformations suggests that the excess energy is essentially induced by protofilament skewing, microtubule radial curvature change and inter-subunit shearing, distortions that must destabilize further the tubulin subunits interactions. These results are consistent with the hypothesis that unfavorable closure events may trigger the catastrophes observed at low tubulin concentration in vitro. In addition, we propose a novel type of representation that describes the stability of microtubule assembly systems, and which might be of considerable interest to study the effects of stabilizing and destabilizing factors on microtubule structure and dynamics.  相似文献   

8.
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.  相似文献   

9.
Several types of non-equilibrium phenomena have been observed in microtubule polymerization, including dynamic instability, assembly overshoot and oscillations. They can be interpreted in terms of interactions between tubulin subunits (= alpha, beta heterodimers), microtubules, and a third state, oligomers, which represent intermediates between microtubule disassembly and the regeneration of assembly-competent subunits by GTP. Here we examine the role of oligomers by varying conditions that stabilize or destabilize microtubules and/or oligomers. By varying their ratio one can drive tubulin assembly either into steady-state microtubules or oligomers. These regimens of assembly conditions are separated by a region where microtubules oscillate. The oscillations can be simulated by computer modelling, based on a reaction scheme involving the three states of tubulin and nucleotide exchange on tubulin subunits, but not on microtubules or oligomers.  相似文献   

10.
Using turbidometry, electron microscopy and immunofluorescent microscopy experiments we studied the effect of captan, a widely used pesticide on mammalian microtubules and microfilaments. Turbidometry at 350 nm showed a dose-dependent inhibition of tubulin assembly incubated with captan. The pesticide, given at equimolar concentration with tubulin (30 microM), caused the total inhibition of microtubule formation, while at lower concentrations (5-20 microM) the inhibition of tubulin polymerization was less extensive. At the same concentration range (5-30 microM), captan also promoted the disassembly of performed microtubules. The results of the in vitro effects of captan with microtubules were confirmed in parallel by electron microscopic studies. In vivo, captan caused also depolymerization of microtubules in cultured mouse fibroblasts as shown by indirect immunofluorescent staining of tubulin. The extent of microtubules disassembly was concentration- and time-dependent. While incubation of the cells with 10 microM captan for 3 h disturbs totally the microtubular structures, incubation with 5 microM captan needs 12 h for the same effect. Recovery of microtubules was observed, when preincubated cells were extensively washed. No interaction of this drug with equimolar concentration of G- or F-actin could be observed in vitro, as shown by polymerization experiments. In line with this, the fluorescent actin pattern in mouse fibroblasts incubated with 10 mM captan for up to 12 h did not seem to be altered. From these results it is concluded that captan interacts in equimolar concentrations with tubulin affecting the assembly and disassembly of microtubules in vitro and in cultures of mammalian cells.  相似文献   

11.
Tau, a microtubule-associated protein which copurifies with tubulin through successive cycles of polymerization and depolymerization, has been isolated from tubulin by phosphocellulose chromatography and purified to near homogeneity. The purified protein is seen to migrate during electrophoresis on acrylamide gels as four closely spaced bands of apparent molecular weights between 55,000 and 62,000. Specific activity for induction of microtubule formation from purified tubulin has been assayed by quantitative electron microscopy and is seen to be enhanced three- to fourfold in the purified tau when compared with the unfractionated microtubule-associated proteins. Nearly 90% of available tubulin at 1 mg/ml is found to be polymerizable into microtubules with elevated levels of tau. Moreover, the critical concentration for polymerization of the reconstituted tau + tubulin system is seen to be a function of tau concentration and may be lowered to as little as 30 μg of tubulin per ml. Under depolymerizing conditions, 50% of the tubulin at only 1 mg/ml may be driven into ring structures. A separate purification procedure for isolation of tau directly from cell extracts has been developed and data from this purification suggest that tau is present in the extract in roughly the same proportion to tubulin as is found in microtubules purified by cycles of assembly and disassembly. Tau is sufficient for both nucleation and elongation of microtubules from purified tubulin and hence the reconstituted tau + tubulin system defines a complete microtubule assembly system under standard buffer conditions. In an accompanying paper (Cleveland et al., 1977) the physical and chemical properties of tau are discussed and a model by which tau may function in microtubule assembly is presented.  相似文献   

12.
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.  相似文献   

13.
H W Detrich  L Wilson 《Biochemistry》1983,22(10):2453-2462
Tubulin was purified from unfertilized eggs of the sea urchin Strongylocentrotus purpuratus by chromatography of an egg supernatant fraction on DEAE-Sephacel or DEAE-cellulose followed by cycles of temperature-dependent microtubule assembly and disassembly in vitro. After two assembly cycles, the microtubule protein consisted of the alpha- and beta-tubulins (greater than 98% of the protein) and trace quantities of seven proteins with molecular weights less than 55 000; no associated proteins with molecular weights greater than tubulin were observed. When analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis on urea-polyacrylamide gradient gels, the alpha- and beta-tubulins did not precisely comigrate with their counterparts from bovine brain. Two-dimensional electrophoresis revealed that urchin egg tubulin contained two major alpha-tubulins and a single major beta species. No oligomeric structures were observed in tubulin preparations maintained at 0 degrees C. Purified egg tubulin assembled efficiently into microtubules when warmed to 37 degrees C in a glycerol-free polymerization buffer containing guanosine 5'-triphosphate. The critical concentration for assembly of once- or twice-cycled egg tubulin was 0.12-0.15 mg/mL. Morphologically normal microtubules were observed by electron microscopy, and these microtubules were depolymerized by exposure to low temperature or to podophyllotoxin. Chromatography of a twice-cycled egg tubulin preparation on phosphocellulose did not alter its protein composition and did not affect its subsequent assembly into microtubules. At concentrations above 0.5-0.6 mg/mL, a concentration-dependent "overshoot" in turbidity was observed during the assembly reaction. These results suggest that egg tubulin assembles into microtubules in the absence of the ring-shaped oligomers and microtubule-associated proteins that characterize microtubule protein from vertebrate brain.  相似文献   

14.
We have studied the self-association reactions of purified GDP-liganded tubulin into double rings and taxoid-induced microtubules, employing synchrotron time-resolved x-ray solution scattering. The experimental scattering profiles have been interpreted by reference to the known scattering profiles to 3 nm resolution and to the low-resolution structures of the tubulin dimer, tubulin double rings, and microtubules, and by comparison with oligomer models and model mixtures. The time courses of the scattering bands corresponding to the different structural features were monitored during the assembly reactions under varying biochemical conditions. GDP-tubulin essentially stays as a dimer at low Mg(2+) ion activity, in either the absence or presence of taxoid. Upon addition of the divalent cations, it associates into either double-ring aggregates or taxoid-induced microtubules by different pathways. Both processes have the formation of small linear (short protofilament-like) tubulin oligomers in common. Tubulin double-ring aggregate formation, which is shown by x-ray scattering to be favored in the GDP- versus the GTP-liganded protein, can actually block microtubule assembly. The tubulin self-association leading to double rings, as determined by sedimentation velocity, is endothermic. The formation of the double-ring aggregates from oligomers, which involves additional intermolecular contacts, is exothermic, as shown by x-ray and light scattering. Microtubule assembly can be initiated from GDP-tubulin dimers or oligomers. Under fast polymerization conditions, after a short lag time, open taxoid-induced microtubular sheets have been clearly detected (monitored by the central scattering and the maximum corresponding to the J(n) Bessel function), which slowly close into microtubules (monitored by the appearance of their characteristic J(0), J(3), and J (n) - (3) Bessel function maxima). This provides direct evidence for the bidimensional assembly of taxoid-induced microtubule polymers in solution and argues against helical growth. The rate of microtubule formation was increased by the same factors known to enhance taxoid-induced microtubule stability. The results suggest that taxoids induce the accretion of the existing Mg(2+)-induced GDP-tubulin oligomers, thus forming small bidimensional polymers that are necessary to nucleate the microtubular sheets, possibly by binding to or modifying the lateral interaction sites between tubulin dimers.  相似文献   

15.
Regulation of the microtubule steady state in vitro by ATP.   总被引:16,自引:0,他引:16  
R L Margolis  L Wilson 《Cell》1979,18(3):673-679
ATP increases microtubule steady state assembly and disassembly rates in vitro in a concentration-dependent manner. Bovine brain microtubules, composed of 75% tubulin and 25% high molecular weight microtubule-associated proteins (MAPs), were purified by three cycles of assembly and disassembly in the absence of ATP. When assembled to steady state, these microtubules add dimers at one end and lose them at the other in a unidirectional assembly-disassembly process. In the presence of 1.0 mM ATP the unidirectional flow of tubulin from one end of the microtubules to the other increases as much as 20 fold, as revealed by loss of 3H-GTP from uniformly labeled microtubules under GTP chase conditions and by the rate of disassembly following addition of 50 microM podophyllotoxin. UTP, CTP and 5' adenylylimidodiphosphate (AMP-PNP) cannot substitute for ATP in producing this effect. Furthermore, the increase in steady state flow rate persists afer ATP is removed. Thus microtubules assembled in ATP and centrifuged through sucrose cushions to separate them from nucleotides continue to exhibit increased rates in the next assembly cycle in the absence of ATP. It is possible that an ATP-dependent microtubule protein kinase is responsible for the observed increase in tubulin flow rate. A kinase activity associated with brain MAPs has been reported to be cAMP-dependent (Sloboda et al., 1975). We have found an adenylate cyclase activity associated with these microtubules. Whether the adenylate cyclase is a contaminant or due to a specific microtubules-associated protein, and whether its activity is functionally linked to the increased rate of assembly and disassembly in the presence of ATP, remain to be determined.  相似文献   

16.
Numerous isotypes of the structural protein tubulin have now been characterized in various organisms and their expression offers a plausible explanation for observed differences affecting microtubule function in vivo. While this is an attractive hypothesis, there are only a handful of studies demonstrating a direct influence of tubulin isotype composition on the dynamic properties of microtubules. Here, we present the results of experimental assays on the assembly of microtubules from bovine brain tubulin using purified isotypes at various controlled relative concentrations. A novel data analysis is developed using recursive maps which are shown to be related to the master equation formalism. We have found striking similarities between the three isotypes of bovine tubulin studied in regard to their dynamic instability properties, except for subtle differences in their catastrophe frequencies. When mixtures of tubulin isotypes are analyzed, their nonlinear concentration dependence is modeled and interpreted in terms of lower affinities of tubulin dimers belonging to the same isotype than those that represent different isotypes indicating hitherto unsuspected influences of tubulin dimers on each other within a microtubule. Finally, we investigate the fluctuations in microtubule assembly and disassembly rates and conclude that the inherent rate variability may signify differences in the guanosine-5′-triphosphate composition of the growing and shortening microtubule tips. It is the main objective of this article to develop a quantitative model of tubulin polymerization for individual isotypes and their mixtures. The possible biological significance of the observed differences is addressed.  相似文献   

17.
We describe preliminary results from two studies exploring the dynamics of microtubule assembly and organization within chromosomal spindle fibers. In the first study, we microinjected fluorescently labeled tubulin into mitotic PtK1 cells and measured fluorescence redistribution after photobleaching (FRAP) to determine the assembly dynamics of the microtubules within the chromosomal fibers in metaphase cells depleted of nonkinetochore microtubules by cooling to 23-24 degrees C. FRAP measurements showed that the tubulin throughout at least 72% of the microtubules within the chromosomal fibers exchanges with the cellular tubulin pool with a half-time of 77 sec. There was no observable poleward flux of subunits. If the assembly of the kinetochore microtubules is governed by dynamic instability, our results indicate that the half-life of microtubule attachment to the kinetochore is less than several min at 23-24 degrees C. In the second study, we used high-resolution polarization microscopy to observe microtubule dynamics during mitosis in newt lung epithelial cells. We obtained evidence from 150-nm-thick optical sections that microtubules throughout the spindle laterally associate for several sec into "rods" composed of a few microtubules. These transient lateral associations between microtubules appeared to produce the clustering of nonkinetochore and kinetochore microtubules into the chromosomal fibers. Our results indicate that the chromosomal fiber is a dynamic structure, because microtubule assembly is transient, lateral interactions between microtubules are transient, and the attachment of the kinetochores to microtubules may also be transient.  相似文献   

18.
MAP2C is a microtubule-associated protein abundant in immature nerve cells. We isolated a cDNA clone encoding whole mouse MAP2C of 467 amino acid residues. In fibroblasts transiently transfected with cDNA of MAP2C, interphase microtubule networks were reorganized into microtubule bundles. To reveal the dynamic properties of microtubule bundles, we analyzed the incorporation sites of exogenously introduced tubulin by microinjection of biotin-labeled tubulin and the turnover rate of microtubule bundles by photoactivation of caged fluorescein- labeled tubulin. The injected biotin-labeled tubulin was rapidly incorporated into distal ends of preexisting microtubule bundles, suggesting a concentration of the available ends of microtubules at this region. Although homogenous staining of microtubule bundles with antibiotin antibody was observed 2 h after injection, the photoactivation study indicated that turnover of microtubule bundles was extremely suppressed and < 10% of tubulin molecules would be exchanged within 1 h. Multiple photoactivation experiments provided evidence that neither catastrophic disassembly at the distal ends of bundles nor concerted disassembly due to treadmilling at the proximal ends could explain the observed rapid incorporation of exogenously introduced tubulin molecules. We conclude that microtubules bundled by MAP2C molecules are very stable while the abrupt increase of free tubulin molecules by microinjection results in rapid assembly from the distal ends within the bundles as well as free nucleation of small microtubules which are progressively associated laterally with preexisting microtubule bundles. This is the first detailed study of the function of MAPs on the dynamics of microtubules in vivo.  相似文献   

19.
Summary Depolymerization kinetics of microtubules assembled to steady-state by pod ophyllotoxin treatment show a dose-dependent effect of this mitotic poison on the net rate of microtubule disassembly. Pulse-chase experiments with microtubules at steady-state indicate that the depolymerization effect induced by superstoichiometric concentrations of podophyllotoxin relative to tubulin is polar and time-dependent, i.e. the rate of tubulin loss decreases along with the time of treatment in the presence of the drug. Under these conditions the rate of microtubule disassembly is much faster than one could expect from a unique effect of drug-tubulin complex on the microtubule assembly end. Podophyllotoxin-tubulin complex is not able to induce active depolymerization of microtubules, while free podophyllotoxin is. These results are consistent with the hypothesis that this drug acts on the microtubule assembly-disassembly process by two different mechanisms: 1) as a free drug, it actively promotes polar depolymerization of microtubules, and 2) as a drug-tubulin complex, it retards the addition of subunits into the microtubule ends.  相似文献   

20.
At alkaline pH, Ca2+ is no longer required for S-100 proteins to inhibit the assembly and to promote the disassembly of brain microtubules in vitro, though the presence of Ca2+ significantly favors the S-100 effects. These effects are inversely related to the microtubule protein concentration and directly related to the S-100 concentration and the pH. Ca2+-independent, pH-regulated inhibition of assembly of phosphocellulose-purified tubulin by S-100 is also described. The microtubule disassembling effect of S-100 is additive to that of alkali (used to raise the pH), and S-100 further disassembles microtubules after alkalinization. Thus the larger inhibitory effect of S-100 on microtubule assembly at alkaline versus acid pH depends on both a decrease in the assembly rate and an increase in the disassembly rate. Together with previous data on this topic, the present findings indicate that S-100 proteins act on microtubule protein in vitro primarily by binding to tubulin, this event being Ca2+-regulated at a given pH, and pH-regulated at a given free Ca2+ concentration.  相似文献   

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