Plays a major role in programmed cell death (PCD, apoptosis). egl-1 binds to and directly inhibits the activity of ced-9, releasing the cell death activator ced-4 from a ced-9/ced-4 containing protein complex and allowing ced-4 to induce caspase ced-3 autoproteolytic cleavage and activation. Also forms an holoenzyme with processed ced-3 enhancing ced-3 activity. Component of the egl-1, ced-9, ced-4 and ced-3 apoptotic signaling cascade required for the initiation of programmed cell death in cells fated to die during embryonic and postembryonic development. During oogenesis, required for germline apoptosis downstream of ced-9 and upstream of ced-3 but independently of egl-1. May regulate germline apoptosis in response to DNA damage, probably downstream of let-60/ras and mpk-1 pathway. Regulates CEP neuron apoptosis in response to high Al(3+) levels. During male tail morphogenesis, promotes apoptosis of the tail-spike cell. During larval development, required for the elimination of transient presynaptic components downstream of egl-1 and ced-9 and upstream of ced-3 apoptotic pathway. Together with ain-1, a component of the miRNA-induced-silencing complex (miRISC), and probably upstream of ced-3, regulates temporal cell fate patterning during larval development. May play a role in resistance to S.typhimurium-mediated infection. Associates as an asymmetric homodimer with ced-9. Upon release from ced-9, forms an octamer, known as the apoptosome, and interacts with ced-3; the interaction results in ced-3 autoproteolytic cleavage and activation. The octamer (a tetramer of an asymmetric dimer) also interacts with two processed ced-3 to form a stable holoenzyme. Interacts with sex-determining protein fem-1. May form a complex composed of ced-3, ced-4 and mac-1 or of ced-9, ced-4 and mac-1. Within the complex, interacts with ced-4. In non cell death induced cells, ced-9 is required for mitochondrial localization. Perinuclear in cell death induced cells.