hGAAP stimulates calpain activity. (A) U2-OS cells were transfected with the CFP/YFP calpain FRET biosensor, plated on fibronectin, and imaged live over 20 min in both FRET and IRM channels. Typical examples show stills from videos. Right images show enlargements of boxed areas in left images. Bars, 5 µm. (B) Summary results from the images collected at t = 0 (means ± SEM, for > 10 cells) show the percentages of low-FRET pixels (ratio