Inhibition of Src kinase reverses the effect of Cdk5 inhibition on stress fiber stability and contraction. Cells were allowed to spread on fibronectin for 60 min; then incubated for 120 min in the absence or presence of PP1, olomoucine, or both; and double stained with phalloidin (panels A, C, E, and G) and pMRLC antibody (panels B, D, F, and H). (A) Control cells had concave boundaries and well-formed stress fibers. (B) Immunostaining of the same cell with pMRLC showed prominent immunofluorescence on stress fibers. (C) PP1 treatment did not change stress fiber organization. (D) PP1 also had no significant effect on pMRLC immunofluorescence. (E) Olomoucine-treated cells showed a significant loss of central stress fibers. (F) Olomoucine greatly reduced pMRLC immunofluorescence. (G) PP1 blocked the effect of olomoucine on stress fibers. Central stress fibers, which were lost with olomoucine treatment, were preserved. (H) PP1 blocked the effect of olomoucine on pMRLC immunofluorescence. Immunostaining of pMRLC on central stress fibers was especially prominent. Scale bar, 20 μm.