The mode of subunit addition regulates the processive elongation of actin filaments by formin.

Formins play crucial roles in actin polymerization by nucleating filaments and regulating their elongation. Formins bind the barbed ends of filaments via their dimeric FH2 domains, which step processively onto incoming actin subunits during elongation. Actin monomers can bind formin-bound barbed ends directly or undergo diffusion-mediated delivery through interactions with formin FH1 domains and profilin. Despite its fundamental importance, a clear mechanism governing processive FH2 stepping has remained elusive. In this study, we systematically characterized the polymerization behavior of the Saccharomyces cerevisiae formin Bni1p using in vitro reconstitution assays and stochastic simulations. We found that Bni1p assembles populations of filaments with lengths that depend nonlinearly on the rate of elongation. This processive behavior is dictated by a variable probability of dissociation that depends on the reaction conditions. Bni1p dissociates from barbed ends with a basal off-rate, which enables prolonged filament assembly over the course of a long lifetime at the barbed end. A bias toward FH1-mediated delivery as the dominant mechanism for polymerization curtails elongation by shortening the lifetime of the formin at the filament end. This facilitates the assembly of populations of filaments with similar average lengths, even when polymerization proceeds at different rates. Our results suggest a central role for formin FH1 domains in regulating processivity. The specific effects of FH1 domains on processivity are variable and likely tailored to the physiological function of each formin.