I. pauses during depolymerization

Pauses were observed during the depolymerization of individual filaments grown from ADP-, CrATPor MgATPactin, in the presence or in the absence of profilin. As discussed in the main text, these pauses did not affect the depolymerization traces up to the interruption. Moreover, as can be seen from Fig. S1 and Fig. S2, the occurrence of a pause did not affect the depolymerization velocity for segments depolymerizing before or after the pause. The mechanism underlying the pausing events will be elucidated elsewhere [T. Niedermayer, A. Jégou, E. Helfer, G. Romet-Lemonne, M.-F. Carlier, R. Lipowsky, in preparation].