Fig. 1

Nucleation geometry controls the actin network growth rate. a, b The growth of different restricted actin organizations. a Actin network emerging from 3 × 15 µm² GST-pWA-coated bar. Conditions: 6 µM G-actin Alexa-568 labeled, 18 µM Profilin, 120 nM Arp2/3 complex, 25 nM CP. b Same as a but for GST-pWA-coated sub-micron dot. c The growth rate of actin networks was calculated in µm s⁻¹ µM⁻¹ of available [G-actin]. Red bars represent mean speed values. Error bars show mean s.d. for n = 20 LMs from four experiments a and n = 13 LMs from five experiments b. ****p < 0.0001, multiple comparison Šídák method. d, e The growth of different unrestricted actin networks. d Actin assembly from 3 × 15 µm² GST-pWA-coated bar. Conditions: 1 µM G-actin Alexa-568 labeled, 3 µM Profilin, 80 nM Arp2/3 complex e same as d but for 1 × 1 µm² GST-pWA-coated dot. f The assembly of single actin filaments. Conditions: 1 µM G-actin Alexa-568 labeled, 3 µM Profilin. In f each filament is identified by a star. In a, b, c and d: purple shows the nucleation sites; in b, speckles in the comet tail indicated by colored stars were used for speed measurement. g For homogeneity and for each condition, the growth rate of actin filaments or actin organizations was calculated in µm s⁻¹ µM⁻¹ of available G-actin. Red bars represent mean speed values provided above. Error bars show mean s.d. for n patterns per condition, n = 26 from four experiments d, n = 11 from three experiments e and n = 26 from four experiments f. Adjusted p values were computed according to Šídák method to assess statistically significant difference in mean growth rates measured for each condition. h Cartoon for the reconstituted flat LMs: LMs grow from NPF-coated patterns (light red rectangle) printed on Silane-PEG-coated slide (darkened glass). The height between glass and coverslip was controlled by two pieces of calibrated tape (yellow walls). Schematic of actin assembly and the parameters used in the model equation: Network growth rate “V” depends on (i) the barbed end rate of assembly (k _on), (ii) the monomer size (δ), (iii) the local monomer concentration (C, factor 1) at the nucleation site, and (iv) a geometry/mechanical factor Φ that integrates the pulling force (black arrows, factor 2) and filament orientation with respect to the load that modulates the pushing force (yellow arrows, factor 3). Grey dots: actin monomers and subunits. Grey gradient: gradient of actin monomers (darker color for higher (C)). Red dots: transient actin attachment to the load through the NPFs—Arp2/3 complex—actin filament ternary complex (see also related Supplementary Figs 1 and 2). Scale bars in a, d are 15 µm. Scale bars in b, e, f are 10 µm