Fig. 8. (A) High-speed FRAP measures. The cell is repeatedly imaged along the broken line at high frequency (400 Hz). Bleaching is performed along a strip covering the nucleus for 300 milliseconds. Fluorescence is corrected for background and divided by the cytoplasm fluorescence to compensate for image bleaching. (B) Recovery of fluorescence for the same cell before (red) and after stimulation with FGF4. Fluorescence has been normalized to the pre-bleach value. (C) Cumulative results. At this temporal resolution the recovery of GFP (starved cells) is clearly discernible and it almost overlaps with the recovery of ERK2-GFP measured in strongly overexpressing cells (blue, stimulation did not cause any change). Photobleaching in low-expressing cells is larger and the recovery is slower, indicating lower mobility, which is dependent on ERK activation.