Fig. 2.

Autocatalytic filament formation with and without negative feedback. Nucleation of new actin filaments by the Arp2/3 complex is autocatalytic, because the product of the reaction—a “daughter” filament—is also a substrate—an additional “mother” filament. a If autocatalysis were unregulated, filaments would beget filaments and the network density would grow exponentially. b In reality, however, the activity of the Arp2/3 complex is product inhibited, because growing filaments leech actin monomers away from WASP family proteins on the membrane surface, thus slowing the rate of nucleation. c Time evolution of filament density predicted by simple autocatalysis (Eq. 1, top curve) and autocatalysis with negative feedback (bottom curve)