Fig. 1: Effect of twinfilin and capping protein (CP) on processivity of formin.

a Schematic of the experimental strategy. Actin filaments were nucleated from coverslip-anchored formins by a flow containing 1 µM G-actin (15% Alexa-488 labeled) and 0.5 µM profilin. Filaments were then elongated in the presence of 1 µM unlabeled G-actin and 4 µM profilin to ensure insertional elongation between fluorescent fragments and surface-anchored formins. Filaments were then exposed to a flow containing 0.2 µM unlabeled G-actin and 0.7 µM profilin (PA) (control) with or without a range of concentrations of CP and/or mTwf1 (alone or together). The survival fraction of formin-bound filaments attached was monitored as a function of time. b Representative kymographs of a formin-anchored filament elongating from 0.2 µM unlabeled G-actin and 0.7 µM profilin (PA) (see Supplementary Movie 1). c Same conditions as (b) but supplemented with 50 nM CP (see Supplementary Movie 2). d Same conditions as (b) but supplemented with 1 µM mTwf1. e Same conditions as (b) but supplemented with 50 nM CP and 1 µM mTwf1 (see Supplementary Movie 3). f Survival fraction of formin-bound filaments (BF) as a function of time in the presence of PA supplemented with a range of CP concentrations. Experimental data (symbols) are fitted to a single-exponential decay function (lines) to determine the formin dissociation rate k_-F (BF → B + F). Number of filaments analyzed for each condition (0 to 1 µM CP): 70, 36, 75, 56, 26, 26. g Formin dissociation rate k-_F as a function of CP concentration, determined from data in (f). h Survival fraction of formin-bound filaments (BF) as a function of time in the presence of PA alone (black symbols,  n = 51 filaments) or supplemented with 1 µM mTwf1 (magenta symbols, n = 50 filaments), 50 nM CP (yellow symbols, n = 51 filaments) or 1 µM mTwf1, and 50 nM CP together (orange symbols, n = 50 filaments). Experimental data (symbols) are fitted to a single-exponential decay function (lines). i Formin dissociation rate k_-F as determined from data in (h). Error bars in g, i indicate 65% confidence intervals based on fits (see Methods). Source data are provided as a Source Data file.