Caspase-3 (CPP32, apopain, YAMA), a member of asparate-specific cysteinyl proteases (or caspases) family, is a key mediator of apoptosis of mammalian cells [Kothakota et al., 1997]. Caspase-3 is activated during the early stages of apoptosis by self-proteolysis and/or cleavage by another protease. Active caspase-3 cleaves and activates caspases and many other cellular proteins, leading to apoptotic chromatin condensation and DNA fragmentation in all cell types examined [Porter and Janicke, 1999].

Casper3-BG is a FRET based sensor that can be used for detection of caspase-3 mediated apoptosis in living cells. The sensor consists of blue and green fluorescent proteins, TagBFP and TagGFP2, connected by the linker containing caspase-3 cleavage sequence, DEVD. Good overlap between the emission spectrum of TagBFP and the absorbance spectra of TagGFP2 ensures efficient FRET between these proteins [Subach et al., 2008].

The activation of caspase-3 during apoptosis leads to cleavage of DEVD sequence and elimination of FRET that can be detected as a decrease in green emission of TagGFP2 and a simultaneous increase in blue emission of TagBFP.