Formin inhibition blocks actin arc formation. (A, micrographs) 3D-SIM images of Jurkat T cells treated with DMSO, 10 µM SMIFH2, or 25 µM SMIFH2 and stained with phalloidin. Yellow arrowheads mark actin foci. (A, bottom) Mean phalloidin fluorescence in the pSMAC. n = 21–30 cells/condition. Box plots are centered on means and display upper and lower quartile ranges and min to max values. Bar graphs are means ± SEM. (B, top) Still image from a TIRF-SIM video of a Jurkat T cell expressing GFP–F-Tractin 30 s after SMIFH2 washout. Yellow arrowheads mark actin foci. (B, bottom) Still image from the same TIRF-SIM video 180 s after SMIFH2 washout. Yellow arrowheads mark linear actin filaments. SMAC zones are bracketed in A and B at top. See also Video 7. Bars, 5 µm. ***, P < 0.001; ****, P < 0.0001. a.u., arbitrary units; Fluor. Int., fluorescence intensity.