Inhibitor of Apoptosis Proteins (IAPs) are the target of extensive research in the field of cancer therapy since they regulate apoptosis and cell survival. Smac-mimetics, the most promising IAP-targeting compounds specifically recognize the IAP-BIR3 domain and promote apoptosis, competing with caspases for IAP binding. Furthermore, Smac-mimetics interfere with the NF-κB survival pathway, inducing cIAP1 and cIAP2 degradation through an auto-ubiquitination process. It has been shown that the XIAP-BIR1 (X-BIR1) domain is involved in the interaction with TAB1, an upstream adaptor for TAK1 kinase activation, which in turn couples with the NF-κB survival pathway. Preventing X-BIR1 dimerization abolishes XIAP-mediated NF-κB activation, thus implicating a proximity-induced mechanism for TAK1 activation. In this context, in a systematic search for a molecule capable of impairing X-BIR1/TAB1 assembly, we identified the compound NF023. Here we report the crystal structure of the human X-BIR1 domain in the absence and in the presence of NF023, as a starting concept for the design of novel BIR1-specific compounds acting synergistically with existing pro-apoptotic drugs in cancer therapy.

Genetic alterations enhancing cell survival and suppressing apoptosis are hallmarks of cancer that significantly reduce the efficacy of chemotherapy or radiotherapy. The Inhibitor of Apoptosis Protein (IAP) family hosts conserved proteins in the apoptotic pathway whose over-expression, frequently found in tumours, potentiates survival and resistance to anticancer agents. In humans, IAPs comprise eight members hosting one or more structural Baculoviral IAP Repeat (BIR) domains. Cellular IAPs (cIAP1 and 2) indirectly inhibit caspase-8 activation, and regulate both the canonical and the non-canonical NF-κB signaling pathways. In contrast to cIAPs, XIAP (X chromosome-linked Inhibitor of Apoptosis Protein) inhibits directly the effector caspases-3 and -7 through its BIR2 domain, and initiator caspase-9 through its BIR3 domain; molecular docking studies suggested that Smac/DIABLO antagonizes XIAP by simultaneously targeting both BIR2 and BIR3 domains. Here we report analytical gel filtration, crystallographic and SAXS experiments on cIAP1-BIR3, XIAP-BIR3 and XIAP-BIR2BIR3 domains, alone and in the presence of compound 9a, a divalent homodimeric Smac mimetic. 9a is shown to bind two BIR domains inter- (in the case of two BIR3) and intra-molecularly (in the case of XIAP-BIR2BIR3), with higher affinity for cIAP1-BIR3, relative to XIAP-BIR3. Despite the different crystal lattice packing, 9a maintains a right handed helical conformation in both cIAP1-BIR3 and XIAP-BIR3 crystals, that is likely conserved in solution as shown by SAXS data. Our structural results demonstrate that the 9a linker length, its conformational degrees of freedom and its hydrophobicity, warrant an overall compact structure with optimal solvent exposure of its two active moieties for IAPs binding. Our results show that 9a is a good candidate for pre-clinical and clinical studies, worth of further investigations in the field of cancer therapy.

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