Acts as a cysteine protease in controlling programmed cell death (apoptosis) by proteolytically activating or inactivating a wide range of substrates (PubMed:8654923, PubMed:3955651, PubMed:18722182, PubMed:26074078, PubMed:27723735). 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 (PubMed:3955651, PubMed:17329362, PubMed:25432023, PubMed:27723735). During oogenesis, required for germline apoptosis downstream of ced-9 and ced-4 but independently of egl-1 (PubMed:9927601). 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 (PubMed:24225442). By cleaving and converting dcr-1 into a deoxyribonuclease (DNase), promotes apoptotic chromosomal DNA fragmentation (PubMed:20223951). By cleaving mitochondrial fission protein drp-1, may regulate the removal of mitochondria during apoptosis (PubMed:18722182). During germline apoptosis, cleaves translation initiation factor ifg-1 (isoform p170) promoting cap-independent translation (PubMed:21909434). During male tail morphogenesis, promotes apoptosis of the tail-spike cell downstream of ced-4 but independently of egl-1 and ced-9 (PubMed:17329362). By cleaving cnt-1, prevents the activation of the prosurvival akt-1/2 signaling pathway and thus promotes apoptosis (PubMed:25383666). Downstream of ced-4, may play a role in sex-specific cell apoptosis by cleaving sex-determining protein fem-1 (PubMed:10764728). May regulate germline apoptosis in response to DNA damage, probably downstream of let-60/ras and mpk-1 pathway (PubMed:21901106). Cleaves ced-9 in vitro (PubMed:17371877, PubMed:18776901, PubMed:19575016, PubMed:25432023, PubMed:27723735). Cleaves csp-2 isoform b resulting in the removal of the propeptide and the generation of csp-2 subunit p31 in vitro (PubMed:9857046). 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 (PubMed:26074078). Together with ain-1, a component of the miRNA-induced-silencing complex (miRISC), regulates temporal cell fate patterning during larval development (PubMed:25432023). In complex with ubr-1, which is E3 ubiquitin-protein ligase and component of the N-end rule pathway, acts in seam cell fate patterning during larval development by cleaving the heterochronic protein lin-28, and promoting its degradation (PubMed:25432023, PubMed:28602583). Also cleaves heterochronic protein lin-14 and exonuclease disl-2 in vitro (PubMed:25432023). 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 (PubMed:22629231). Cleaves 14-3-3-like protein ftt-2, tubulin tbb-2 and calreticulin crt-1 in vitro (PubMed:17371877). Also plays a role in resistance to S.typhimurium-mediated infection (PubMed:11226309). Strict requirement for an Asp residue at position P1 and has a preferred cleavage sequence of Asp-Glu-Val-Asp-|-. Octomeric ced-4 activates zymogen autoprocessing and enhances activity of processed ced-3 (PubMed:18776901, PubMed:19575016, PubMed:27723735, PubMed:24065769, PubMed:20434985). Zymogen autoactivation is inhibited by csp-3 (PubMed:18776901). csp-3 has no effect on active ced-3 (PubMed:18776901). Zymogen autoactivation is inhibited by csp-2 (PubMed:19575016). Inhibited by cysteine protease inhibitor iodoacetic acid (CH3COOI) (PubMed:8654923, PubMed:9857046, PubMed:18776901, PubMed:19575016, PubMed:27723735). Inhibited by benzyloxycarbonyl-DEVD-fluoro-methyl ketone (zDEVD-fmk) (PubMed:8654923, PubMed:9857046, PubMed:25432023). Inhibited by benzyloxycarbonyl-VAD-fluoro-methyl ketone (zVAD-fmk) (PubMed:17371877, PubMed:21909434). 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) (PubMed:8654923, PubMed:9857046). 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 (Probable). Interacts with octomeric ced-4 (two ced-3 zymogens per one ced-4 octamer); the interaction causes the autoproteolytic cleavage and activation of ced-3 (PubMed:20434985, PubMed:24065769). Processed ced-3 also interacts with ced-4 octamer to form a stable holoenzyme (PubMed:20434985). Interacts (via large subunit p17) with csp-3; the interaction prevents ced-3 autoactivation and delays ced-4-induced ced-3 processing (PubMed:18776901). Interacts (via large subunit p17 or small subunit p13 or p15) with csp-2; the interaction inhibits ced-3 autoactivation (PubMed:19575016). 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 (PubMed:27723735). Interacts with dct-1 (PubMed:11114722). May form a complex composed of ced-3, ced-4 and mac-1 (PubMed:10101135). Component of a complex containing at least ced-3, ubr-1 and possibly ate-1 (PubMed:28602583). Within complex interacts (via the p17 subunit) with ubr-1; this interaction is required for the ced-3-mediated cleavage and subsequent degradation of the heterochronic protein lin-28 (PubMed:28602583). Interacts with ate-1 (isoform a and isoform d); interaction with ate-1 (isoform a) is in the presence or absent of ubr-1 (PubMed:28602583). Colocalizes with nucleoporin npp-14 to the perinuclear region in germ cells (PubMed:27723735). Becomes diffused in the cytoplasm in apoptotic germ cells (PubMed:27723735). Localizes to axonal mitochondria and synapses of DD motor neurons (PubMed:26074078). Synaptic localization is dependent on axonal mitochondria (PubMed:26074078). Highly expressed in embryos and to a lesser extent in adults (PubMed:8242740). Expression is low throughout the larval stage (PubMed:8242740). Expressed in all cells, except intestinal cells and their precursors, starting at around 100-150 minutes post-fertilization and continuing throughout the comma stage of embryogenesis (PubMed:17329362). Not expressed after the 3-fold embryonic stage, and only expressed in 2-3 cells in larvae and adults (PubMed:17329362). In males, expressed in the tail at the L4 larval stage (PubMed:17329362). Expression in the tail-spike cell is restricted to the 3-fold embryonic stage prior to the tail-spike cell death (PubMed:17329362). 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 (PubMed:8654923, PubMed:9857046, PubMed:17371877, PubMed:18776901, PubMed:19575016, PubMed:27723735, PubMed:20434985). Cleaved by caspase csp-1 probably at Asp-144 and Asp-374 (PubMed:9857046). RNAi-mediated knockdown causes a rupture of the vulva and an increase in laid oocytes in a small proportion of animals. In an ain-1 mutant background, enhances the proportion of animals arrested at the larval stage, with egg-laying defects and with a ruptured vulva. Belongs to the peptidase C14A family.