The bacterial virulence factors VopL and VopF nucleate actin from the pointed end

VopL and VopF (VopL/F) are tandem WH2-domain actin assembly factors used by infectious Vibrio species to induce actin assembly in host cells. There is disagreement about the filament assembly mechanism of VopL/F, including whether they associate with the filament barbed or pointed end. Here, we used multicolor total internal reflection fluorescence microscopy to directly observe actin assembly with fluorescently labeled VopL/F. In actin monomer assembly reactions, VopL/F exclusively nucleate actin filament assemblies, remaining only briefly associated with the pointed end. VopL/F do not associate with the ends of preassembled filaments. In assembly reactions with saturating profilin, ∼85% of VopL/F molecules also promote nucleation from the pointed end, whereas a smaller fraction (<15%) associate for ∼25 s with the barbed end of preassembled filaments, inhibiting their elongation. We conclude that VopL/F function primarily as actin nucleation factors that remain briefly (∼100 s) associated with the pointed end.