Formins are major regulators of actin networks. They enhance actin filament dynamics by remaining processively bound to filament barbed ends. How biochemical and mechanical factors affect formin processivity are open questions. Monitoring individual actin filaments in a microfluidic flow, we report that formins mDia1 and mDia2 dissociate faster under higher ionic strength and when actin concentration is increased. Profilin, known to increase the elongation rate of formin-associated filaments, surprisingly decreases the formin dissociation rate, by bringing formin FH1 domains in transient contact with the barbed end. In contrast, piconewton tensile forces applied to actin filaments accelerate formin dissociation by orders of magnitude, largely overcoming profilin-mediated stabilization. We developed a model of formin conformations showing that our data indicates the existence of two different dissociation pathways, with force favoring one over the other. How cells limit formin dissociation under tension is now a key question for future studies.

Formins are one of the central players in the assembly of most actin networks in cells. The sensitivity of these processive molecular machines to mechanical tension is now well established. However, how the activity of formins is affected by geometrical constraints related to network architecture, such as filament cross-linking and formin spatial confinement, remains largely unknown. Combining microfluidics and micropatterning, we reconstituted in vitro mDia1 formin-elongated filament bundles induced by fascin, with different geometrical constraints on the formins, and measured the impact of these constraints on formin elongation rate and processivity. When filaments are not bundled, the anchoring details of formins have only a mild impact on their processivity and do not affect their elongation rate. When formins are unanchored, we show that filament bundling by fascin reduces both their elongation rate and their processivity. Strikingly, when filaments elongated by surface-anchored formins are cross-linked together, formin elongation rate immediately decreases and processivity is reduced up to 24-fold depending on the cumulative impact of formin rotational and translational freedom. Our results reveal an unexpected crosstalk between the constraints at the filament and the formin levels. We anticipate that in cells the molecular details of formin anchoring to the plasma membrane strongly modulate formin activity at actin filament barbed ends.