Fig. 4: Contractility and tension anisotropy gradient are coupled to anterograde flow of actin at negative curvature.

a, Laser ablation along the radial direction (green dashed line) in the centre of the actin structure (GFP-MRLC in white overlaid with the fibronectin pattern in magenta) at negative curvature reveals that the actin cables are under tension. Scale bars, 5 μm. b, Kymograph along the yellow dashed line in a shows recoil of actin cables after each ablation (six ablation time points are indicated by filled arrowheads). Horizontal scale bar, 3 μm, vertical scale bar, 1 min. c, Flow of TFs depends on myosin II contractility; anterograde flow of myosin stops after 25 μM blebbistatin treatment and recovers after washout. Top: images of MDCK cells transfected with mApple myosin IIA in each condition. Scale bars, 5 μm. Bottom: corresponding kymographs along the yellow dashed line in each image above. Horizontal scale bars, 1 μm; vertical scale bars, 5 min. Note that the same cell was imaged for the three conditions. d, Point laser ablation near the rear of the actin structure. Scale bar, 5 μm. The yellow circle shows recoil after 5 s. e, Point laser ablation at the front of the actin structure. Scale bar, 5 μm. The yellow circle shows recoil after 5 s. f,g, Tension anisotropy increases from the rear to the front. Relative distances within 0.2–0.4 are measured for the rear (n = 6), and 0.6–0.8 for the front (n = 9). Red line in f shows the fitted model (see Supplementary Information for detail). For g, a two-sided t-test was used, p = 0.0027. A normal distribution was tested and confirmed with Anderson–Darling test at a 0.01 confidence interval. h, Traction force microscopy (TFM) reveals a geometry-induced traction force pattern at negative curvature. Scale bar, 5 μm. i, Image showing an enlarged view at negative curvature, corresponding to the yellow dashed line box in h. Filled arrowheads show traction forces parallel to the edge of the cell and the open arrowhead shows forward-pointing traction forces at the rear of the actin structure. Scale bar, 5 μm. j, Simulation of tension within the transition region. Fitted traction forces of 450 pN applied at the front (red arrows) and fixed boundary condition applied at the rear (red curve). Brown arrows indicate the directions of the calculated principal stresses. The colour map indicates the magnitude of the first principal stress. See Supplementary Information for a detailed description of the simulation.