Acts as a cysteine protease in controlling programmed cell death (apoptosis) by proteolytically activating or inactivating a wide range of substrates. 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 ced-4 but independently of egl-1. By cleaving and activating ced-8, promotes phosphatidylserine exposure on the surface of apoptotic cells; phosphatidylserine is a specific marker only present at the surface of apoptotic cells and acts as a specific signal for engulfment. By cleaving and converting dcr-1 into a deoxyribonuclease (DNase), promotes apoptotic chromosomal DNA fragmentation. By cleaving mitochondrial fission protein drp-1, may regulate the removal of mitochondria during apoptosis. During germline apoptosis, cleaves translation initiation factor ifg-1 (isoform p170) promoting cap-independent translation. During male tail morphogenesis, promotes apoptosis of the tail-spike cell downstream of ced-4 but independently of egl-1 and ced-9. By cleaving cnt-1, prevents the activation of the prosurvival akt-1/2 signaling pathway and thus promotes apoptosis. Downstream of ced-4, may play a role in sex-specific cell apoptosis by cleaving sex-determining protein fem-1. May regulate germline apoptosis in response to DNA damage, probably downstream of let-60/ras and mpk-1 pathway. Cleaves ced-9 in vitro. Cleaves csp-2 isoform b resulting in the removal of the propeptide and the generation of csp-2 subunit p31 in vitro. Independently of its apoptotic role has additional functions. Probably by cleaving and thereby activating actin-severing protein gsnl-1, required for the elimination of transient presynaptic components during larval development downstream of egl-1, ced-9 and ced-4 pathway. Together with ain-1, a component of the miRNA-induced-silencing complex (miRISC), regulates temporal cell fate patterning during larval development. Acts in cell fate patterning by cleaving heterochronic protein lin-28, likely promoting its degradation. Also cleaves heterochronic protein lin-14 and exonuclease disl-2 in vitro. Downstream of calreticulin crt-1 and ced-4 and independently of egl-1 and ced-9, plays a role in the initial steps of axonal regrowth following axotomy. Cleaves 14-3-3-like protein ftt-2, tubulin tbb-2 and calreticulin crt-1 in vitro. Plays also a role in resistance to S.typhimurium-mediated infection. Strict requirement for an Asp residue at position P1 and has a preferred cleavage sequence of Asp-Glu-Val-Asp-|-. Octameric ced-4 activates zymogen autoprocessing and enhances activity of processed ced-3. Zymogen autoactivation is inhibited by csp-3. csp-3 has no effect on active ced-3. Zymogen autoactivation is inhibited by csp-2. Inhibited by cysteine protease inhibitor iodoacetic acid (CH3COOI). Inhibited by benzyloxycarbonyl-DEVD-fluoro-methyl ketone (zDEVD-fmk). Inhibited by benzyloxycarbonyl-VAD-fluoro-methyl ketone (zVAD-fmk). Not inhibited by N-[N-(L-3-transcarboxirane-2-carbonyl)-leucyl]-agmatine (E-64) or by the serine and cysteine protease inhibitor L-1-chloro-3-[4-to-osylamido]-7-amino-2-heptanone (TLCK). The active form is probably a heterodimer of the p17 subunit with either the p15 or p13 subunit which are all derived from the precursor by autocatalysis. Interacts with octameric ced-4 (two ced-3 zymogens per one ced-4 octamer); the interaction causes the autoproteolytic cleavage and activation of ced-3. Processed ced-3 also interacts with ced-4 octamer to form a stable holoenzyme. Interacts (via large subunit p17) with csp-3; the interaction prevents ced-3 autoactivation and delays ced-4-induced ced-3 processing. Interacts (via large subunit p17 or small subunit p13 or p15) with csp-2; the interaction inhibits ced-3 autoactivation. Interacts (via propeptide) with nucleoporin npp-14; the interaction tethers ced-3 to the nuclear membrane and prevents its autoprocessing in absence of ced-4. Interacts with dct-1. May form a complex composed of ced-3, ced-4 and mac-1. Colocalizes with nucleoporin npp-14 to the perinuclear region in germ cells. Becomes diffused in the cytoplasm in apoptotic germ cells. Localizes to axonal mitochondria and synapses of DD motor neurons. Synaptic localization is dependent on axonal mitochondria. The CARD domain is involved in ced-4 binding. Autocatalytic cleavage removes the propeptide and generates the catalytic subunit p17 and two non-catalytic subunits p15 and p13; autoproteolysis is induced by ced-4 oligomer. Cleaved by caspase csp-1 probably at Asp-146 and Asp-376. Belongs to the peptidase C14A family.