Inhibitor of apoptosis

The Inhibitors of Apoptosis (IAP) are a family of functionally- and structurally-related proteins, which serve as endogenous inhibitors of programmed cell death (apoptosis). A common feature of all IAPs is the presence of a BIR (Baculovirus IAP Repeat, a ~70 amino acid domain) in one to three copies. The human IAP family consists 8 members, and IAP homologs have been identified in numerous organisms.

The first members of the IAPs identified were from the baculovirus IAPs, Cp-IAP and Op-IAP, which bind to and inhibit caspases as a mechanism that contributes to its efficient infection and replication cycle in the host. Later, five more human IAPs were discovered which included XIAP, c-IAPl, C-IAP2, NAIP, and survivin.

The best characterized IAP is XIAP, which binds caspase-9, caspase-3 and caspase 7, thereby inhibiting their activation and preventing apoptosis. Also cIAP1 and cIAP2 have been shown to bind caspases, although how the IAPs inhibit apoptosis mechanistically at the molecular level is not completely understood.

Activity of XIAP is blocked by binding to DIABLO (Smac) and HTRA2 (Omi) proteins released from mitochondria after pro-apoptic stimuli.