X-linked inhibitor of apoptosis protein BIR3 domain (XIAP BIR3 domain)

The X-linked inhibitor of apoptosis protein (XIAP) is a multi-domain protein that potently inhibits programmed cell death (apoptosis) primarily by direct binding and inhibition of caspases, the key effectors of apoptosis. The BIR3 domain is the third of three tandem baculoviral IAP repeat (BIR) domains within XIAP. Structurally, the BIR3 domain spans roughly 70 amino acids, forming a zinc finger motif required for ligand and caspase binding. Functionally, XIAP BIR3 binds to and inhibits caspase-9, holding it in an inactive state and thereby preventing the initiation of the caspase cascade central to apoptosis. This inhibition can be reversed by endogenous antagonists such as Smac/DIABLO, which binds the BIR3 domain at a conserved surface groove using an N-terminal AVPI motif, displacing caspase-9 and triggering cell death. As such, the XIAP BIR3 domain is a major therapeutic target for pro-apoptotic drugs—most notably "Smac mimetics," which are small molecules designed to replicate the interaction mode of Smac/DIABLO. The BIR3 domain also contributes to XIAP's roles in copper/zinc homeostasis, immune response regulation, and possibly protein–protein interaction networks relevant to survival signaling. Overexpression or dysregulation of XIAP BIR3 is strongly associated with therapy-resistant forms of multiple cancers, making it a focus of intense biomedical and drug discovery research.

Other names

XIAP BIR3XIAP baculoviral IAP repeat 3XIAP third BIR domainInhibitor of apoptosis protein BIR3 domain

Antagonism of XIAP BIR3–caspase-9 interaction by Smac mimetics, leading to caspase activation and induction of apoptosis in target cells