Venus dye trap catches executioners

Fluorescence resonance energy transfer (FRET) between coupled fluorescent proteins is a promising tool for studying caspase activation during apoptosis. Unfortunately, current FRET systems are acutely sensitive to changes in pH and chloride ion concentration, and since these fluctuate considerably during apoptosis the results of FRET experiments are often difficult to interpret. Takemoto et al., whose report appears on page 235, developed a new FRET system that is resistant to acidification and chloride, and used it to obtain a detailed view of caspase-3 and caspase-9 activation in apoptosis. The new technique should also enable studies of caspase activity in vivo that were previously impossible.To make a caspase-sensitive FRET system, two fluorescent proteins, a donor and an acceptor, are linked with a cleavage site that can be cut by a particular...