(A) Sketch summarizing the conformations adopted by the FH2 dimer and the actin filament barbed end in our model, in the absence of profilin (for a complete description of the model, see Appendix 1). The system is in rapid equilibrium between an open and a closed state (depicted here as in the ‘stair-stepping’ model) and only the open state allows the addition of a new actin subunit at the barbed end. Following this elongation event, the system is in a transition state, which decays rapidly into a new open-closed rapid equilibrium. Formin dissociation from the barbed end can occur while the system is in the open state (with rate koffO) or in the transition state (rate koffT). The global, observable dissociation rate koff comprises these two routes. (B,C) Predictions of the model for the variation of the dissociation rate koff as a function of force, in log-linear representations. In both cases, koffT is the dominant contribution at zero force. In B, koffO increases more strongly than koffT when force is applied and thus becomes dominant at high force (computed with working distances δO=δ and δT=0, see Appendix 1). In C, koffT increases more strongly than koffO when force is applied (δO=0 and δT=δ, see Appendix 1).