Movie S1.

Elongation and Xac2-induced filament severing of mDia1(-) F-actin (Fig. 1A). The barbed end of mDia1(-) F-actin grows freely in the presence of 1.5 μM actin and shrinks after reducing the actin concentration to 0.1 μM. Fifty nM Xac2 was added to the flow cell when the text of “Xac2” is displayed on the movie. Scale bar is 5 μm.