IAPs CARRY AN EVOLUTIONARILY CONSERVED UBIQUITIN-BINDING DOMAIN THAT IS INDISPENSABLE FOR NF-KB REGULATION AND CELL SURVIVAL.

The covalent attachment of ubiquitin to target proteins influences various cellular processes, including DNA repair, NF-κB signalling and cell survival1. The most common mode of regulation by ubiquitin-conjugation involves specialized ubiquitin-binding proteins that bind to ubiquitylated proteins and link them to downstream biochemical processes. Unravelling how the ubiquitin-message is recognized is essential because aberrant ubiquitin-mediated signalling contributes to tumour formation2. Recent evidence indicates that inhibitor of apoptosis (IAP) proteins are frequently overexpressed in cancer and their expression level is implicated in contributing to tumorigenesis, chemoresistance, disease progression and poor patient-survival3. Here, we have identified an evolutionarily conserved ubiquitin-associated (UBA) domain in IAPs, which enables them to bind to Lys 63-linked polyubiquitin. We found that the UBA domain is essential for the oncogenic potential of cIAP1, to maintain endothelial cell survival and to protect cells from TNF-α-induced apoptosis. Moreover, the UBA domain is required for XIAP and cIAP2–MALT1 to activate NF-κB. Our data suggest that the UBA domain of cIAP2–MALT1 stimulates NF-κB signalling by binding to polyubiquitylated NEMO. Significantly, 98% of all cIAP2– MALT1 fusion proteins retain the UBA domain, suggesting that ubiquitin-binding contributes to the oncogenic potential of cIAP2–MALT1 in MALT lymphoma. Our data identify IAPs as ubiquitin-binding proteins that contribute to ubiquitinmediated cell survival, NF-κB signalling and oncogenesis.