Summary of VopL/F-mediated actin filament nucleation and association over a range of experimental conditions. Cartoon representation of VopL/F’s actin assembly properties with actin monomers (A), preassembled filaments only (B), and preassembled filaments with actin monomers and profilin (C). VopL/F are illustrated as a generalized dimer (A–C, left). (A) The VopL/F WH2 domains recruit actin monomers to the VCD, which organizes three monomers into an F-actin nucleus (red circles with bold outlines). The VCD domain remains briefly (∼100 s) associated with the pointed end, and VopL/F are unable to rebind or associate with preassembled filament ends. (B) In the absence of actin monomers, VopL/F can bind preassembled actin filament ends for ∼25–30 s. The VCD and WH2 domains mediate interaction with pointed and barbed ends, respectively. (C) In the presence of profilin-actin, VopL/F primarily (∼90%) nucleates actin filaments and then remains briefly (∼80 s) associated with the pointed end. VopL/F occasionally (10%) associates with preassembled filament barbed ends through their WH2 domains.