Fig. 2.

Ring curvature distributions and ring formation mechanisms. (A) shows the cumulative curvature radii distribution P(r) in the frozen steady state; the inset depicts the noncumulative distribution p(r). The distribution can be described by a double exponential decay according to P(r) ∝ A₁ exp(r/l₁) + A₂ exp(r/l₂) with decay lengths of l₁ = 3.3 μm and l₂ = 10.1 μm. This double exponential shape reflects the occurrence of two different ring morphologies: open and closed rings. These two different ring populations rely on distinct ring formation mechanisms that are related to the growth mechanisms in the system. While being transported, moving actin-fascin strings grow by merging with other strings of similar size. Merging events lead to elongated but still flexible strings that predominantly form closed ring if they cross their own tail (B). Open rings form upon a different mechanism (C): While moving, actin-fascin strings continuously pick up material—individual filaments or smaller actin-fascin strings. Thereby the strings grow predominately in width and get thicker and stiffer. If the strings are stiff enough the curvature freezes and the forces and fluctuations in the motility assay are not sufficient anymore to induce any change in curvature. While closed rings characteristically are small in size with radii of up to 30 μm, open rings are considerably broader and can have radii of up to 150 μm (A). The actin concentration was set to ρ = 3 μM and the fascin concentration was c = 0.2 μM. All scale bars are 50 μm. In (B) and (C) the investigated actin-fascin string is shown in red and its tip is marked by a yellow arrow.