- The Inhibitor of Apoptosis (IAP) family comprises a group of intracellular proteins that regulate cell death, ubiquitination and innate immune signalling. Although originally identified for their ability to suppress apoptosis, IAPs are now recognised as multifunctional signalling molecules that influence inflammation, immunity, cell survival and cancer progression. Members of the IAP family are conserved across evolution, from insects to mammals, and share a characteristic structural feature: one or more BIR (Baculovirus IAP Repeat) domains that mediate protein–protein interactions.
- In mammals, the major IAP proteins include XIAP, cIAP1, cIAP2, ML-IAP (Livin), NAIP, and Survivin. Each member has distinct biological roles, but all contribute to the fine‑tuning of apoptosis and inflammatory signalling. The best‑studied IAPs—XIAP, cIAP1 and cIAP2—contain both BIR domains and a C‑terminal RING finger ubiquitin ligase domain, enabling them to regulate signalling pathways through ubiquitination. This dual functionality allows IAPs to act as both inhibitors of caspases and modulators of NF‑κB signalling.
- XIAP is the most potent direct inhibitor of caspases. Through its BIR2 and BIR3 domains, XIAP binds and suppresses caspase‑3, caspase‑7 and caspase‑9, preventing apoptosis even under strong pro‑death stimuli. This inhibition is relieved by mitochondrial proteins such as Smac/DIABLO, which bind XIAP and displace caspases. cIAP1 and cIAP2, in contrast, do not strongly inhibit caspases directly. Instead, they regulate cell survival by controlling ubiquitination of key signalling molecules such as RIPK1 in the TNF receptor pathway. By assembling K63‑linked ubiquitin chains, cIAPs promote pro‑survival NF‑κB signalling; when cIAPs are depleted, RIPK1 switches to a death‑inducing complex that triggers apoptosis or necroptosis.
- NAIP proteins play a specialised role in innate immunity. In humans, NAIP detects bacterial components such as the type III secretion system needle protein, while in mice different NAIP paralogues recognise flagellin or rod proteins. Upon detection, NAIP cooperates with NLRC4 to form the NAIP–NLRC4 inflammasome, leading to caspase‑1 activation, pyroptosis and IL‑1β maturation. This places NAIP at the intersection of pathogen sensing and inflammatory cell death.
- Survivin is the smallest IAP and has unique functions in cell division. It participates in chromosome segregation and cytokinesis as part of the chromosomal passenger complex. Survivin also contributes to apoptosis inhibition, although less directly than XIAP. Its expression is tightly regulated during development and is frequently upregulated in cancer, making it a major focus of therapeutic research.
- The IAP family is also deeply involved in cancer biology. Overexpression of XIAP, cIAPs or Survivin promotes tumour cell survival, resistance to chemotherapy and evasion of immune‑mediated killing. This has led to the development of Smac mimetics, small molecules that mimic the action of Smac/DIABLO and neutralise IAPs. Smac mimetics induce rapid degradation of cIAP1 and cIAP2, sensitising tumour cells to TNF‑mediated apoptosis and restoring proper cell death signalling.
- In summary, the IAP family comprises multifunctional regulators of apoptosis, ubiquitination and immune signalling. Through their BIR domains and RING ligase activity, IAPs control caspase activity, NF‑κB signalling, inflammasome activation and cell cycle progression. Their roles in cancer, inflammation and host defence make them central players in cell biology and attractive targets for therapeutic intervention.
