Anti-apoptotic protein which functions as a caspase regulator, using its E3 ubiquitin-protein ligase activity to smother caspase activity. Binds, ubiquitinates and inactivates initiator caspase Dronc, and effector caspases Drice and Dcp-1. Acts as a Nedd8-E3 ubiquitin-protein ligase for Drice. Suppresses apoptosis by targeting the apoptosome for ubiquitination and inactivation. Plays an important role in cell motility. Overexpression suppresses rpr and hid-dependent cell death in the eye. Interaction of Diap1 with Dronc is required to suppress Dronc-mediated cell death through Diap1-mediated ubiquitination of Dronc. Acts as a positive regulator of Wnt signaling. S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + N(6)-ubiquitinyl-[acceptor protein]-L-lysine. Interacts (via BIR 2 domain) with Dronc (via residues 114-125). Rpr, hid and grim can outcompete Dronc for binding Diap1 therefore removing Diap1-mediated ubiquitination. Interacts (via BIR 2 domain) with HtrA2; this displaces any bound Dronc. Interacts with Strica (PubMed:11550090). The N-terminally cleaved form interacts with Ubr3 (via UBR-type zinc finger) (PubMed:25146930, PubMed:26383956); the interaction promotes the recruitment and uniquitination of substrate capases such as Dronc (PubMed:25146930). Detected in wing disks (at protein level). Ubiquitinated and degraded by HtrA2 in apoptotic cells; proteolytic cleavage at specific sites in the BIR domain linker region generating inactive fragments. Mutation of one site reduces but does not abolish cleavage as another site is selected by the protease. Belongs to the IAP family. Wrong choice of frame.