FIGURE 8.

On soft PA gels, slight overexpression of actin with GFP-actin transfection amplifies and overrides the weak optimum seen in spread area (A) and F-actin mass (B) over a range of collagen densities. Control cells were either transfected with bare GFP or nontransfected. For spread area, the solid bell-shaped curve through the control cells is the same as in Fig. 5 A; for F-actin mass measured as the integrated intensity of rhodamine phalloidin, the same curve shape is used to fit the control cells. As explained in the text, the dotted horizontal lines define baselines and the dotted, bell-shaped curves are stretched (2.6× or 3×) forms of the respective solid curves. Nonexpressing cells are excluded from analyses, as are completely rounded cells (<30%). Scale bar is 50 μm. (C) A schematic of F-actin assembly versus total actin pool is overlaid on hypothesized signaling curves. On optimum collagen, there is an impetus to assemble F-actin, which is amplified greatly with GFP-actin expression; on high or low collagen, the signal is effectively absent or inhibitory.