Inhibitors of Apoptosis (IAPs) and Regulators
As their name implies, a primary role for members of the Inhibitors of Apoptosis (IAP) family lies in their ability to suppress apoptosis. This occurs through the inhibition of pro-apoptotic members of the caspase family. Some IAPs also act as ubiquitin ligases, targeting specific proteins for proteasomal degradation. IAP proteins are characterized structurally by the presence of at least one BIR (Baculoviral IAP Repeat) domain. Binding of the BIR domain of IAPs to active caspases inhibits caspase activity. Eight IAPs have been identified in humans including Apollon/BRUCE, cIAP-1, cIAP-2, ILP-2, Livin, NAIP, Survivin, and XIAP. The activities of IAPs are themselves suppressed by mitochondrial proteins released into the cytosol during apoptosis, such as HTRA2/Omi and Smac/Diablo. These proteins bind to the BIR domain of IAPs and reduce their ability to interact with caspases.