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Apoptosis is triggered by activation of initiator caspases upon complex-mediated clustering of the inactive zymogen, as occurs in the caspase-9-activating apoptosome complex. Activation of caspase-2 occurs in a complex that contains the death domain-containing protein PIDD, whose expression is induced by p53, and the adaptor protein RAIDD (1). RAIDD has a dual-domain structure similar to that of FADD. It has an NH2-terminal caspase homology domain that interacts with caspase-2 and a COOH-terminal DD that interacts with RIP. RAIDD is constitutively expressed in many tissues and thus could play a role in regulating apoptosis in mammalian cells (2). In metazoan cells there exists inactive polypeptide precursors (zymogens), each composed of a prodomain, which is cleaved to activate the protease, and a large and small catalytic subunit. The coupling of these proteases to signaling pathways is probably mediated by adaptor molecules that contain protein-protein interaction motifs such as the death domain (3).
