Order by motion( dered structures such as the mitotic sp...)

dered structures such as the mitotic spindle are assembled in this dynamic state. In order to understand the principles governing the motor-driven dynamics and dynamic pattern formation by cytoskeletal filaments, researchers have to study biomimetic model systems such as motility assays, which contain only a few ingredients.

In such motility assays molecular motors are attached and immobilized on a surface, over which they pull cytoskeletal filaments. Scientists at the Max Planck Institute of Colloids and Interfaces have now developed a new theory which shows how this pulling motion in combination with collisions between different filaments can give rise to a spontaneous ordering into a parallel arrangement of filaments as demonstrated in a simulation snapshot in Figure 1. This ordering of filaments into parallel patterns is very similar to the ordering of rod-shaped molecules in the so-called nematic phase of a liquid crystal system, in which the rod-shaped molecules orient in a parallel fashion. But whereas the alignment of molecular rods in a liquid crystal system can only be triggered by increasing the density or the length of rods, the ordering in the motility assay can also be triggered by increasing the density of molecular motors which are moving the filaments. Surprisingly, the motor-driven motion of filaments enhances the tendency to order, which seems to defy basic physical principles according to which the microscopic motion of molecules usually destroys their order; a well-known example is provided by the melting of crystals.

The enhanced tendency of filaments for orientational order in the presence of motor activity is demonstrated in computer simulations as shown in Figure 1. In Figure 1(a), the motor density is low and filaments do not order. In Figure 1(b), only the motor density has been increased and the filaments start to align parallel to each other. This effect can be theoretically explained using the concept of an effective
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Contact: Dr. Jan Kierfeld
Jan.Kierfeld@mpikg.mpg.de
49-331-567-9620
Max-Planck-Gesellschaft
25-Jul-2006

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