Fig. 5.

Model for FH1-mediated delivery of profilin actin to the FH2-bound barbed end. Cartoon representation of the addition of a profilin-bound actin subunit (dark blue and orange circles) to the barbed end of an actin filament bound by a representative formin FH1FH2 construct. The unstructured FH1 domains extend outwards from the FH2 dimer and contain multiple polyproline tracts (in this case, six green ovals per FH1 domain). The number of prolines in each polyproline tract tends to increase with the distance from the FH2 domain. Profilin-actin complexes bind the polyproline tracts and are delivered directly to the barbed end in a “loop closure” reaction via diffusion of the FH1 domain and formation of a ring complex (Vavylonis et al. 2006). Subsequent profilin and polyproline tract dissociation events occur rapidly and are not depicted