Although ubiquitylation may regulate protein function through conformational changes, the most common mode of regulation by Ub-conjugation involves specific “Ub-receptors” that recognise ubiquitylated proteins and link them to downstream biochemical processes

We have discovered the presence of an evolutionarily conserved Ub-associated domain (UBA) in IAP proteins, suggesting that IAPs function as Ub-receptors in Ub-mediated signalling pathways. The UBA motif recognises Ub modifications on target proteins enabling Ub-mediated signalling. Interestingly, while cIAP1, cIAP2, XIAP and DIAP2 did not interact with mono-Ub (mUb), they all strongly interacted with K63-linked poly-Ub chains, particularly chains of four or more Ub moieties.  Point mutations in the UBA domain of these IAPs completely abrogates Ub-binding. Importantly, K63-poly-Ub conjugation of proteins does not target them for degradation but is essential in several signalling pathways, including those leading to NF-kB activation. Consistently, we find that XIAP’s ability to activate NF-kB relies on the presence of a functional UBA domain. Likewise, the ability of cIAP2-MALT to drive NF-kB activation critically depends on the UBA domain of the cIAP2 portion of the cIAP2-MALT fusion protein, which is responsible for emergence of MALT lymphoma. Consistent with the requirement of the UBA domain for cIAP2-MALT-mediated NF-kB activation, we find that more than 98% of all the breakpoints in cIAP2 occur downstream of exon 7, which encodes the UBA domain. This strongly suggests that the UBA domain is important for cIAP2-MALT function. Moreover, we find that the UBA domain of cIAP2-MALT selectively traps ubiquitylated NEMO thereby enabling NF-kB activation. Moreover, we find that the UBA domain is essential for cIAP1’s oncogenic potential, to maintain endothelial cell survival and to protect cells from TNFa-induced apoptosis.  We are currently exploring the mechanisms through which Ub-binding contributes to cIAP function.

Figure 1.  IAPs function as E3 ligases and Ub-receptors in TNF-Receptor signalling.

(A) Domain structures of cIAPs. BIR domains provide substrate interaction, while the Ub-binding UBA domain binds poly-Ub. The C-terminal RING domain is required for E3 ligase activity, dimersation and docking-site for the E2. A predicted structure of the three conserved α-helices of the UBA domain (green) of cIAP2 and the residues forming the hydrophobic interaction surface for Ub (MGF/LL) is shown. The structure of a cIAP2-RING dimer is shown on the right (grey and green depicts two respective cIAP2 molecules). (B) Suggested functions for cIAPs in TNF-Receptor (TNF-R) signalling. Stimulation with TNFα triggers the formation of ‘Complex-I’ at the TNF-R1, in which cIAP reportedly ubiquitylates RIP with K63-linked chains. These Ub-chains serve as docking and activation platforms for IKK, which leads to activation of the transcription factor NF-κB (p65/p50). cIAPs also seem to suppress the death-inducing ‘Complex-II’, which is the activation platform for caspase-8 that induces death by the extrinsic pathway.