Caspase-9 swings both ways in the apoptosome

For nearly 2 decades, investigators have debated whether cysteinyl-aspartate-specific protease 9 (caspase-9) is activated within the apoptotic protease-activating factor 1 (Apaf-1) apoptosome through proximity-induced homodimerization or through formation of a holoenzyme. Recently, we have demonstrated that caspase-9 forms (and likely transitions between) both caspase-9 homo- and Apaf-1:caspase-9 heterodimers, each of which plays unique roles in the recruitment and activation of caspase-9.