Myosin IIA inhibition disrupts actin arc organization. (A) 3D-SIM images of a Jurkat pretreated with DMSO and two Jurkats pretreated with 50 µM pnBB for 30 min and stained with phalloidin (Video 9). (B) Cells were scored for actin arc morphology as indicated. n = 100 cells/condition; three experiments per condition; data are means ± SD (see Materials and methods for details). (C) Boxed regions covering the pSMAC (yellow) were quantitated for actin arc anisotropy in D using FibrilTool. n = 22–26 cells/condition. (E) Model depicting formin-dependent actomyosin arc formation at the IS. Bars, 5 µm.