Control of the pattern formation by adding crosslinking proteins
The pattern formation in the high density motility assay is highly sensible to the addition of crosslinking proteins. If the crosslinking protein fascin is added, highly polar structures are assembled on the timescale of seconds. At low material density, polar actin-fascin rings emerge (left) while the steric hindrance at high material densities results in elongated fibres (right). These structures are propelled at a speed of up to 6µm/sec and exceed the length of the individual filament by up to four orders of magnitude. The assembly of polar structures is the result of an upscaling of the microscopic binding properties of fascin: In vivo fascin is responsible for the fortification of cellular protrusions like filopodia and crosslinks filaments in a parallel or polar orientation only.
The structure formation in the presence of crosslinking proteins is an example for the emergence of frozen steady states in active systems. The emergence of frozen steady states is characterized by a successive elimination of fluctuations during a pattern forming instability while keeping the system in an active state. In driven systems frozen steady states can arise when particles are not only actively transported, but also integrated into new self-assembled higher-order structures where single particle fluctuations are arrested.
Such a frozen active state combines the rapid self-organization characteristics of driven systems with the robustness, stability and reproducibility of growth and self-assembly processes. Frozen-in structure directly document the history of the pattern forming instability, and thus are expected to strongly depend on the predominant nucleation and growth processes. In the present system these are governed by the filament and the crosslinker concentration.
The molecular nature of the approach with easily adjustable key parameters offers the possibility to develop tailored theoretical models and compare them to existing more generic theories. This is done in close collaboration with the group of Prof. Frey (LMU).