XIAP Protein, Human (His)

The XIAP protein serves as a multifunctional regulator with diverse roles spanning apoptosis, inflammatory signaling, immunity, copper homeostasis, mitogenic kinase signaling, cell proliferation, invasion, and metastasis. As a direct caspase inhibitor, XIAP impedes substrate entry into the active site pockets of CASP3 and CASP7, while also maintaining CASP9 in a monomeric, inactive state. Functioning as an E3 ubiquitin-protein ligase, XIAP orchestrates ubiquitination of several target proteins, including RIPK1, RIPK2, MAP3K2/MEKK2, DIABLO/SMAC, AIFM1, CCS, PTEN, and BIRC5/survivin, thereby regulating NF-kappa-B signaling and other crucial pathways. In the realm of innate immunity, XIAP mediates 'Lys-63'-linked polyubiquitination of RIPK2 downstream of NOD1 and NOD2, activating NF-kappa-B and MAP kinases signaling. Additionally, XIAP influences the BMP signaling pathway, SMAD and MAP3K7/TAK1 dependent pathways, and the Wnt signaling cascade. It plays a role in copper homeostasis by ubiquitinating COMMD1 and also functions as an E3 ubiquitin-protein ligase in the NEDD8 conjugation pathway. Moreover, XIAP protects cells from spontaneous ripoptosome formation, suppresses ripoptosome formation by ubiquitinating RIPK1 and CASP8, and positively regulates Wnt signaling by ubiquitinating TLE proteins. These diverse functions underscore the intricate and pivotal role of XIAP in governing cellular processes across multiple signaling pathways.