Figure 2 - Filament polymerization by actin assembly factors.

(a) During actin-filament assembly from monomers, the dimerization and trimerization steps are unfavourable, whereas subsequent monomer additions are much more favourable. Nucleation factors overcome these unfavourable dimerization and/or trimerization steps. Actin filaments are polar structures, with the barbed end (B) being the sole site of elongation in non-muscle cells. Thus, physiologically relevant nucleation factors must allow barbed-end elongation. (b) The Arp2/3 complex (multi-coloured) nucleates a new filament from the side of an existing filament, causing filament branching at a 70° angle. The Arp2/3 complex remains at the branch point between the pointed end of the new filament and the side of the existing filament. Repeated branching results in the assembly of a dendritic network. The formin FH2 domain (semi-circles) nucleates a filament, and then moves processively with the barbed end as it elongates. Some formins can also bundle filaments. Spire (circles with black connectors) nucleates a filament by stabilizing a longitudinal tetramer. The current model is that spire dissociates from filaments soon after nucleation.