Structural analysis shows that the BIR2 domain of E3 ligase XIAP binds across the RIP2 kinase dimer interface

NOD1 and NOD2 are innate immune system pattern recognition receptors that play a key role in the protection from bacterial infections and in the maintenance of gastro-intestinal homeostasis. Dysregulation of NOD signalling pathways promotes chronic inflammatory diseases such as inflammatory bowel disease. RIP2, which contains a kinase domain (RIP2K) and a CARD domain, is the immediate downstream signalling partner in the NOD pathway and inhibition of its kinase activity and ubiquitination are promising strategies to address these inflammatory diseases. However, recent work indicates that the phosphorylation activity of RIP2K is dispensable for signalling and that inhibitors of both RIP2K activity and RIP2 ubiquitination prevent the essential interaction between RIP2K and the BIR2 domain of XIAP, the key RIP2 ubiquitin E3 ligase. Moreover, XIAP BIR2 antagonists also block this interaction. To reveal the molecular mechanisms involved, we combined Native mass spectrometry, NMR, cryo-electron microscopy and Alphafold2 predictions to determine the structure of the RIP2K-XIAP BIR2 complex with 2:1 stoichiometry. The structure shows that complex formation requires that the kinase is in an active-like (i.e. αC-helix IN) and dimeric conformation and explains both inhibitory mechanisms. It also shows why phosphorylation of the kinase activation loop is dispensable for signalling and reveals the structural role of RIP2K-K209 residue in the RIP2K-XIAP BIR2 interaction. Importantly, our results clarify the features of the RIP2K conformation essential for its role as a scaffold protein for ubiquitination. ### Competing Interest Statement The authors have declared no competing interest.