Rho-ROCK pathway is involved in stress fiber formation induced by collagen I and GFOGER peptide. (A) MKs were preincubated with a control TAT peptide that contains the TAT sequence fused to a scrambled sequence of the GPVI tail (Fl-Tat VI-4-Scr), (B) the specific inhibitor of RhoGTPase TAT-C3 toxin (25 μg/ml, for 24 hours), or (C) ROCK inhibitor Y-27632 (10 μM, for 1 hour). Cells were then plated on collagen I or GFOGER peptide for 2 hours. A representative photomicrograph (A) shows actin stress fibers in the basal section (0.75 μm) of an MK pretreated by TAT-control, whereas pretreatment by TAT-C3 toxin or Y-27632 inhibited actin stress fibers assembly in MKs plated on collagen I (B,C) or GFOGER peptide (D). Scale bar equals 10 μm in panels A-D. (E) Each bar represents the mean ± SD of 300 cells showing stress fibers in collagen I or GFOGER peptide without and with TAT-C3 or Y-27632 treatment in 3 independent experiments.