Movie S1.

Actin elongation in the presence of different PFN1 variants. mDia1 processivity and formin-induced filament barbed end elongation rates were assayed in a microfluidics assay (flow from left to right; illustrated in Figure 2A). Filament barbed end, initially exposed to mDia1, elongates from surface anchored spectrin-actin seeds, in the presence of 1 μM actin (10% alexa488-labelled) and 0.5 μM WT (left), C71G (middle) or G118V (right) PFN1 variants. Scale bar: 10 μm.