Receptor interacting protein is ubiquitinated by cellular inhibitor of apoptosis proteins (c-IAP1 and c-IAP2) in vitro

Receptor interacting protein (RIP) is recruited to tumor necrosis factor-alpha receptor 1 (TNFR1) complex upon stimulation and plays a crucial role in the receptor-mediated NF-kappaB activation. Among the components of the TNFR1 complex are proteins that possess ubiquitin-protein isopeptide ligase (E3) activities, such as TNFR1-associated factor 2 (TRAF2), cellular inhibitor of apoptosis proteins (c-IAPs) namely, c-IAP1 and c-IAP2. Here, we showed that ectopically expressed RIP is ubiquitinated, and either the intermediate or death domain of RIP is required for this modification. Expression of c-IAP1 and c-IAP2 decreased the steady-state level of RIP, which was blocked by inhibition of the 26S proteasome. RIP degradation requires intact c-IAP2 containing the RING domain. Our in vitro ubiquitination assay revealed that while TRAF2 had no effect, both c-IAP1 and c-IAP2-mediated RIP ubiquitination with similar efficiency, indicating that c-IAPs can function as E3 toward RIP.