MAPK/Erk1/2 kinase activation is required for optimal stress fiber formation on fibrillar collagen I. Confocal sections show F-actin distribution in MKs preincubated with DMSO (A), PD98059 (B), or UO126 (C), 20 μM for 30 minutes, seeded on collagen I for 2 hours, fixed, permeabilized, and stained as indicated, with rhodamine-phalloidin and monoclonal antivinculin FITC antibody. Note the presence of punctuate F-actin and very short and disorganized peripheral actin filaments in MKs treated by PD98059 (B) or UO126 (C). Also, in contrast with the control (Aiii), actin and vinculin are not any longer colocalized in MKs treated by the inhibitors (Biii,Ciii). (D) Confocal sections of a representative image of a primary MK infected by the retroviral vector MSCV-IRES-GFP used as a control (Di) or the retrovirus containing the dominant-negative (DN) form of MEK1 (Diii) and visualized through GFP expression. MKs were plated on collagen I for 2 hours, fixed, and stained for actin with rhodamine-labeled phalloidin (Dii,iv). Note the presence of stress fibers (Dii) in the MK infected by the retroviral vector control, whereas the MK infected by the DN-MEK1 shows the presence of dense bundles of actin aggregates (Div).