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Mentions: Although the preparations do not contain molecular motors, approximately 10 minutes after assembling the tubulin into microtubules, all the polystyrene particles in the field of view of the microscope (880 μm by 650 μm) start to move. They all travel in the same direction at a speed of about 1 μm per minute (Fig. 1) and progressively accelerate (Fig. 2) until about 90 minutes later they attain a maximum speed of approximately 5 μm per minute. During this time, the preparation in the observation area becomes uniformly birefringent (Fig. 1), indicating that the microtubules had adopted a uniform orientation. The microtubule orientation was determined as being the same as the direction of particle movement. After this, the striped arrangement described above develops by a process in which microtubules in certain regions progressively modify their orientation. These changes in orientation occur by a mechanism in which the microtubules partially dis-assemble and then re-assemble . During this time, the particle trajectories no longer all follow the same direction over the entire sample, but follow the changes in microtubule orientation as they occur during self-organisation. While this happens, particle movement progressively slows (Fig. 2). Practically no further transport occurs once self-organisation is complete after about 5 hours. Depending on conditions, the average particle speed is between 2 to 5 μm per minute and particles are transported a total distance of between 0.5 to 2 mm. Under highly reactive conditions, the particles sometimes attain velocities of from 10 μm to 20 μm per minute.
Microtubule self-organisation by reaction-diffusion processes causes collective transport and organisation of cellular particles
Bottom Line: The process also results in the macroscopic self-organisation of these particles.Self-organisation of this type provides a new mechanism by which intra cellular particles such as chromosomes and vesicles can be displaced and simultaneously organised by microtubules.It is plausible that processes of this type occur in vivo.
Affiliation: Commissariat à l'Energie Atomique, Département Réponse et Dynamique Cellulaires, Laboratoire d'Immunochimie, INSERM U548, D,S,V, CEA-Grenoble, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France. Gladeni@chartreuse.cea.fr
Abstract: The transport of intra-cellular particles by microtubules is a major biological function. Under appropriate in vitro conditions, microtubule preparations behave as a 'complex' system and show 'emergent' phenomena. In particular, they form dissipative structures that self-organise over macroscopic distances by a combination of reaction and diffusion.Here, we show that self-organisation also gives rise to a collective transport of colloidal particles along a specific direction. Particles, such as polystyrene beads, chromosomes, nuclei, and vesicles are carried at speeds of several microns per minute. The process also results in the macroscopic self-organisation of these particles. After self-organisation is completed, they show the same pattern of organisation as the microtubules. Numerical simulations of a population of growing and shrinking microtubules, incorporating experimentally realistic reaction dynamics, predict self-organisation. They forecast that during self-organisation, macroscopic parallel arrays of oriented microtubules form which cross the reaction space in successive waves. Such travelling waves are capable of transporting colloidal particles. The fact that in the simulations, the aligned arrays move along the same direction and at the same speed as the particles move, suggest that this process forms the underlying mechanism for the observed transport properties.This process constitutes a novel physical chemical mechanism by which chemical energy is converted into collective transport of colloidal particles along a given direction. Self-organisation of this type provides a new mechanism by which intra cellular particles such as chromosomes and vesicles can be displaced and simultaneously organised by microtubules. It is plausible that processes of this type occur in vivo.
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