In vertebrates, X-linked inhibitor of apoptosis (XIAP) is a potent inhibitor of apoptosis. XIAP inhibits apoptosis by interacting with proapoptotic caspases via its baculovirus IAP repeat (BIR) domains and mediating caspase ubiquitination via its really interesting new gene (RING) domain and ubiquitin-associated (UBA) domain. In invertebrates, and mainly in arthropods, XIAP is also known as an apoptosis inhibitor. However, no studies on basal metazoan XIAP have been documented to date. In the present work, we examined the biological activity of XIAP from the jellyfish Aurelia coerulea (AcXIAP) and other non-bilaterians. AcXIAP has three BIRs and one RING domain but lacks a UBA domain. AcXIAP enhanced the apopto... More
In vertebrates, X-linked inhibitor of apoptosis (XIAP) is a potent inhibitor of apoptosis. XIAP inhibits apoptosis by interacting with proapoptotic caspases via its baculovirus IAP repeat (BIR) domains and mediating caspase ubiquitination via its really interesting new gene (RING) domain and ubiquitin-associated (UBA) domain. In invertebrates, and mainly in arthropods, XIAP is also known as an apoptosis inhibitor. However, no studies on basal metazoan XIAP have been documented to date. In the present work, we examined the biological activity of XIAP from the jellyfish Aurelia coerulea (AcXIAP) and other non-bilaterians. AcXIAP has three BIRs and one RING domain but lacks a UBA domain. AcXIAP enhanced the apoptosis-inducing activity of all four A. coerulea caspases identified in this study, including both initiator and effector clades. AcXIAP activated caspase via one of the BIRs, which bound and stabilized the caspase, and the RING domain, which mediated ubiquitination of the caspase p20 subunit in a lysine-independent manner. Similar caspase-activating properties were also observed in the XIAP of hydra, coral, and sponge. In hydra, XIAP knockdown markedly decreased cell death induced by an apoptosis inducer. Together these results revealed the unconventional function and working mechanism of XIAP in Cnidaria, and shed new light on the functional and structural evolution of XIAP.
