NRG-induced changes in G-actin concentration and cell area. (A and B) MCF-7 cells expressing Dp-actin (A) or Dp (B) were subjected to s-FDAP analysis. Cells were left untreated or were treated with NRG 2 min after the start of the measurements. Temporal changes in relative FDAP in NRG-treated (green) and untreated (blue) cells and changes in relative cell area in NRG-treated (red) and untreated (yellow) cells are plotted. Data are means ± SEM of 26–28 cells (Dp-actin) or 14–15 cells (Dp) from at least three independent experiments. (C) Quantitative analysis of the relative FDAP between 4 and 6 min (top) and the increase in relative cell area between 10 and 12 min (bottom) after the start of s-FDAP measurement. *, P < 0.05. (D) Time-lapse imaging of YFP-actin in an NRG-stimulated MCF-7 cell. Time is indicated as min:sec. Arrows indicate the positions of F-actin assembly (2:30) and lamellipodium extension (5:30). Bar, 20 µm. See Video 1.