Figure 1.

(A) Sketch of the feedback loop for MT growth regulation by Rac1 and stathmin in a cell. (B) Abstract scheme of the feedback loop. (C) Sketch of the one-dimensional model. The MT switches between growing (solid bar) or shrinking state (open bar) with rescue and catastrophe rates ω_r and ω_c, respectively. Rac1 proteins (circles) are located at the cell edge x = L in active or inactive form (solid and open circles, respectively). Rac1 is activated with a rate k_on,R only if the MT enters the shaded region II of size δ representing the cell edge, and deactivated at all times with the rate k_on,R. Stathmin deactivation only takes place in the shaded region II with a rate proportional to the fraction of active Rac1 k_off,Sr_on. The local concentration of active stathmin inhibits MT growth by tubulin sequestering, which reduces the MT growth velocity v₊ locally, or by direct catastrophe promotion.