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

Kinetic stability analysis of protein assembly on the center manifold around the critical point

Background

Non-linear kinetic analysis is a useful method for illustration of the dynamic behavior of cellular biological systems. To date, center manifold theory (CMT) has not been sufficiently applied for stability analysis of biological systems. The aim of this study is to demonstrate the application of CMT to kinetic analysis of protein assembly and disassembly, and to propose a novel framework for nonlinear multi-parametric analysis. We propose a protein assembly model with nonlinear kinetics provided by the fluctuation in monomer concentrations during their diffusion.

Results

When the diffusion process of a monomer is self-limited to give kinetics non-linearity, numerical simulations suggest the probability that the assembly and disassembly oscillate near the critical point. We applied CMT to kinetic analysis of the center manifold around the critical point in detail, and successfully demonstrated bifurcation around the critical point, which explained the observed oscillation.

Conclusions

The stability kinetics of the present model based on CMT illustrates a unique feature of protein assembly, namely non-linear behavior. Our findings are expected to provide methodology for analysis of biological systems.

Microtubule nucleation from stable tubulin oligomers

Microtubule assembly from purified tubulin preparations involves both microtubule nucleation and elongation. Whereas elongation is well documented, microtubule nucleation remains poorly understood because of difficulties in isolating molecular intermediates between tubulin dimers and microtubules. Based on kinetic studies, we have previously proposed that the basic building blocks of microtubule nuclei are persistent tubulin oligomers, present at the onset of tubulin assembly. Here we have tested this model directly by isolating nucleation-competent cross-linked tubulin oligomers. We show that such oligomers are composed of 10-15 laterally associated tubulin dimers. In the presence of added free tubulin dimers, several oligomers combine to form microtubule nuclei competent for elongation. We provide evidence that these nuclei have heterogeneous structures, indicating unexpected flexibility in nucleation pathways. Our results suggest that microtubule nucleation in purified tubulin solution is mechanistically similar to that templated by gamma-tubulin ring complexes with the exception that in the absence of gamma-tubulin complexes the production of productive microtubule seeds from tubulin oligomers involves trial and error and a selection process.

Molecular recognition of taxol by microtubules. Kinetics and thermodynamics of binding of fluorescent taxol derivatives to an exposed site

We have determined the kinetic scheme and the reaction rates of binding to microtubules of two fluorescent taxoids, 7-O-[N-(4'-fluoresceincarbonyl)-l-alanyl]Taxol (Flutax-1) and 7-O-[N-(2,7-difluoro-4'-fluoresceincarbonyl)-l-alanyl]Taxol (Flutax-2). Flutax-1 and Flutax-2 bind to microtubules with high affinity (K(a) approximately 10(7) m(-1), 37 degrees C). The binding mechanism consists of a fast bimolecular reaction followed by at least two monomolecular rearrangements, which were characterized with stopped-flow techniques. The kinetic constants of the bimolecular reaction were 6.10 +/- 0.22 x 10(5) m(-1) s(-1) and 13.8 +/- 1.8 x 10(5) m(-1) s(-1) at 37 degrees C, respectively. A second slow binding step has been measured employing the change of fluorescence anisotropy of the probe. The reversal of this reaction is the rate-limiting step of dissociation. A third step has been detected using small angle x-ray scattering and involves a 2-nm increase in the diameter of microtubules. It is suggested that the first step entails the binding of the Taxol moiety and the second a relative immobilization of the fluorescent probe. The equilibrium and some kinetic measurements required the use of stabilized cross-linked microtubules, which preserved taxoid binding. The results indicate that the Taxol binding site is directly accessible, in contrast with its location at lumen in the current model of microtubules. An alternative structural model is considered in which the binding site is located between protofilaments, accessible from the microtubule surface.

Phosphate release during microtubule assembly: what stabilizes growing microtubules?

The molecular mechanism underlying microtubule dynamic instability depends on the relationship between the addition of tubulin-GTP to a growing microtubule and its hydrolysis in the microtubule lattice to tubulin-GDP, with release of inorganic phosphate (Pi). Since this relationship remains controversial, we have re-examined the release of Pi upon microtubule assembly using a fluorometric assay for Pi, based on the phosphate-binding protein of Escherichia coli [Brune M., Hunter, J. L., Corrie, J. E. T., and Webb, M. R. (1994) Biochemistry 33, 8262-8271]. Microtubule assembly and Pi release were monitored simultaneously in a standard fluorimeter as an increase in the turbidity and fluorescence, respectively, in tubulin-GTP solutions assembled under conditions supporting dynamic instability. At the steady state of assembly, Pi release is nonlinear with respect to time, proceeding at a rate determined by the following: (a) the intrinsic GTPase activity of the nonpolymerized tubulin-GTP, and (b) the microtubule number concentration, which decreases progressively. Direct observation of the time course of nucleated microtubule assembly indicates that Pi release is closely coupled to microtubule elongation, even during the initial stages of assembly when uncoupling of tubulin-GTP addition and GTP hydrolysis would be most evident. Studies of the inhibition and reversal of the growth phase by cytostatic drugs show no evidence of a burst of Pi release. We conclude that nucleotide hydrolysis can keep pace with tubulin-GTP addition rates of 200 molecules per second per microtubule and that extended caps of tubulin-GTP or tubulin-GDP-Pi are not generated in normal assembly, nor are they required to stabilize growing microtubules or to support the phenomenon of dynamic instability of microtubules at the steady state.

Cytosolic free calcium and the cytoskeleton in the control of leukocyte chemotaxis

In response to a chemotactic gradient, leukocytes extravasate and chemotax toward the site of pathogen invasion. Although fundamental in the control of many leukocyte functions, the role of cytosolic free Ca2+ in chemotaxis is unclear and has been the subject of debate. Before becoming motile, the cell assumes a polarized morphology, as a result of modulation of the cytoskeleton by G protein and kinase activation. This morphology may be reinforced during chemotaxis by the intracellular redistribution of Ca2+ stores, cytoskeletal constituents, and chemoattractant receptors. Restricted subcellular distributions of signaling molecules, such as Ca2+, Ca2+/calmodulin, diacylglycerol, and protein kinase C, may also play a role in some types of leukocyte. Chemotaxis is an essential function of most cells at some stage during their development, and a deeper understanding of the molecular signaling and structural components involved will enable rational design of therapeutic strategies in a wide variety of diseases.