F-actin stabilization leads to contraction of the actomyosin network. (A–C) Time-lapse series of T cells spreading on OKT3-coated coverslips. Arrowheads indicate times when 1 µM JAS was added. Bars, 10 µm. (A) Jurkat T cells stably expressing GFP-actin (see Video 4). (B) Primary human T cell blasts transiently transfected with GFP-actin (see Video 5). (C) E6.1 Jurkat T cells transiently transfected with GFP–NMHC II-A (see Video 6). (D and E) Kymographs of F-actin and myosin dynamics, generated along the dashed lines in A and C, respectively. Arrowheads indicate the addition of 1 µM JAS into the media. Brackets represent 10 µm × 1 min.