Regulation of apoptotic protease activating factor-1 oligomerization and apoptosis by the WD-40 repeat region

Apoptotic protease activating factor-1 (Apaf-1) has been identified as a pr oximal activator of caspase-9 in cell death pathways that trigger mitochond rial damage and cytochrome c release. The mechanism of Apaf-1 action is unc lear but has been proposed to involve the clustering of caspase-9 molecules , thereby facilitating autoprocessing of adjacent zymogens, Here we show th at Apaf-1 can dimerize via the CED-4 homologous and linker domains of the m olecule providing a means by which Apaf-1 can promote the clustering of cas pase-9 and facilitate its activation Apaf-1 dimerization was repressed by t he C-terminal half of the molecule, which contains multiple WD-40 repeats, but this repression was overcome in the presence of cytochrome c and dATP. Removal of the WD-40 repeat region resulted in a constitutively active Apaf -1 that exhibited greater cytotoxicity in transient transfection assays whe n compared with full-length Apaf-1. These data suggest a mechanism for Apaf -1 function and reveal an important regulatory role for the WD-40 repeat re gion.