Pro-u-PA–induced actin cytoskeleton reorganization in RSMC. Cells were stimulated with 1 nmol/L pro-u-PA for different time periods at 37 °C, after which cells were fixed and stained with fluorescein-conjugated phalloidin. Quantification of actin cytoskeleton reorganization was performed by taking photographs at low magnification and by counting cells in each stage of cytoskeleton organization. (A) Examples of these stages are given (for more details see the text) briefly. Stage 1 corresponds to resting cells that exhibit stress fibers. Stage 2 coresponds to cells showing a reorganization of cytoskeleton: a decrease of stress fibers content, membrane ruffling, actin ring or actin semi-ring with elongated cell shape characteristic of motile cells. Stage 3 corresponds to cells that reform stress fibers but with a vinculin distribution different from that of cells in stage 1 (see Fig 3). (B) The percentage of cells exhibiting actin cytoskeleton reorganization was recorded after incubation for 5, 30, and 120 minutes with pro-u-PA.