FLIP(L): the pseudo-caspase.

Possessing structural homology with their active enzyme counterparts but lacking catalytic activity, pseudoenzymes have been identified for all major enzyme groups. Caspases are a family of cysteine-dependent aspartate-directed proteases that play essential roles in regulating cell death and inflammation. Here, we discuss the only human pseudo-caspase, FLIP(L), a paralog of the apoptosis-initiating caspases, caspase-8 and caspase-10. FLIP(L) has been shown to play a key role in regulating the processing and activity of caspase-8, thereby modulating apoptotic signaling mediated by death receptors (such as TRAIL-R1/R2), TNF receptor-1 (TNFR1), and Toll-like receptors. In this review, these canonical roles of FLIP(L) are discussed. Additionally, a range of nonclassical pseudoenzyme roles are described, in which FLIP(L) functions independently of caspase-8. These nonclassical pseudoenzyme functions enable FLIP(L) to play key roles in the regulation of a wide range of biological processes beyond its canonical roles as a modulator of cell death.