Fig. 2
Linear and cross-filament nucleators. Complexes on the left represent early nucleation whilst complexes on the right show the subsequent seed elongation. Actin monomers are shown as arrow heads with the indent representing the barbed end (hydrophobic cleft). Actin monomers bound to each nucleator are shown in dark blue while actin subunits that are recruited post seed formation are shown in light blue. (a) Linear tandem nucleators require additional monomers (in grey) to bind laterally prior to completion of the seed, to form the start of the second F-actin filament: an inherently slow process because the off-rate for the first grey domain is fast compared to the rate of filament elongation. (b) Cross-filament nucleators hold the monomers for initiation of both filaments in place, shown here as an intramolecular arrangement. (c) Arp2/3 essentially acts as a cross-filament nucleator, with Arp2 (red) and Arp3 (orange) binding the pointed ends of a pair of new filaments. Based on31.
