Order by movement

Molecular motors create order in a filament fluid

Two snapshots of stubby filaments (blue) on a surface with anchored molecular motors (yellow). (a) At low engine density, the filaments show no order. (b) Above a threshold of engine density, the filaments spontaneously arrange in a parallel pattern. This "active nematic ordering" is caused by the interaction of motor driven motion and collisions of the filaments with each other. © Max Planck Institute for Colloids and Interfaces
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Molecules in biological cells maintain a high degree of spatial order, even though they are constantly in motion. At first glance, this seems to contradict fundamental physical principles that associate movement with disorder. Scientists have now shown the opposite effect on a simple biomimetic model system - the onset of spatial order through molecular motion. As the researchers report in the journal Physical Review Letters, this should help in the future to understand basic patterning processes in biological cells.

The cytoskeleton gives biological cells mechanical stability and plays a major role in intracellular transport and dynamics. On the filaments of the cytoskeleton, the movement of motor proteins can be directed, allowing rapid transport over long distances along the "filament rails" of the cytoskeleton. In addition to their role as nano-tractors, molecular motors are also actively involved in the ongoing reorganization of the cytoskeleton itself. This reorganization is necessary for cell motility and cell division. During these processes, the filaments of the cytoskeleton are in constant motion.

Orderly structures despite dynamics

Nevertheless, ordered structures such as the mitotic spindle are produced in this dynamic state. To understand the principles underlying this motor-driven dynamics and the associated patterning processes through filaments of the cytoskeleton, researchers use biomimetic model systems, such as 'motility assays', which contain only a few components of biological cells. In a motility assay, molecular motors are adsorbed and anchored to a surface through which they then pull cytoskeletal filaments.

The scientists at the Max Planck Institute for Colloids and Interfaces in Potsdam have now developed a new theory and tested it in computer simulations, which explain how the filaments spontaneously align themselves under the influence of this pulling motion and in interaction with collisions. This parallel arrangement of filaments is very similar to the so-called nematic ordering of rod-shaped molecules in a liquid crystal.

Increased tendency to order

But while the parallel order in the liquid crystal is triggered only by an increase in filament density or length, the filament order in the motility assay can also be achieved by increasing the engine density. Surprisingly, therefore, the motor-driven movement of the filaments increases the tendency to order. This seems to contradict a fundamental physical principle, according to which microscopic movement of molecules in general leads to a destruction of their order - a well-known example of this is the melting of a crystal. display

The larger orientation order of the filaments under the influence of the activity of molecular motors was shown in computer simulations (see Fig.) In Figure (a) the engine density is small and the filaments show no order. In figure (b), only the engine density has been increased and the filaments are arranged parallel to each other. Theoretically, this effect can be explained by the concept of an effective filament length, which increases due to motor activity: Because the motors move the filaments along their axis, they have a large Effektiveer effective length and already begin to interact with each other at lower densities and thus to arrange themselves in parallel.

This general theoretical concept should also apply to other engine-filament systems. Several experimental studies indicate that in filament and engine protein solutions - through the activity of molecular motors - filament orientation can also be achieved in three dimensions.

(Max Planck Institute for Colloids and Interfaces, 27.07.2006 - AHE)