Alignment of actin filament streams driven by myosin motors in crowded environments.

Background: Cellular dynamics depend on cytoskeletal filaments and motor proteins. Collective movements of filaments driven by motor proteins are observed in the presence of dense filaments in in vitro systems. As multiple macromolecules exist within cells and the physiological ionic conditions affect their interactions, crowding might contribute to ordered cytoskeletal architecture because of collective behavior.

Methods: Using an in vitro reconstituted system, we observed the emergence of stripe patterns resulting from collective actin filament streaming driven by myosin motors in the presence of the crowding agent, methylcellulose (MC).

Results: Although at high KCl concentrations (150mM), actin filaments tended to dissociate from a myosin-coated surface, 1% MC prevented this dissociation and enabled filament movement on myosin molecules. At concentrations of actin filaments above 0.2mg/mL, the moving filaments accumulated and progressively formed long, dense bands. The bands were spaced at about 10-μm intervals. Increasing the KCl concentration up to 300mM resulted in narrowing of the spacing between the aligned bands. On the other hand, low KCl concentrations (≤25mM) induced broad streams, where actin filaments exhibited bidirectional movement.

Conclusions: These results suggest that crowded environments can promote spatial patterning of the actin cytoskeleton, depending on the intensity of the myosin driving force and filament velocity, both modulated by the ionic strength.

General Significance: The mutual contribution of packing and driving forces provides insight into cytoskeleton organization in living cells, in which various macromolecules mingle.