Irreversible Inactivation

Abstract

Inhibitors of apoptosis (IAPs) are inactivated during apoptosis to allow cell death mediated by caspases to proceed. IAPs directly inhibit caspases. Inhibitors of IAPs include Smac (also known as DIABLO, called Smac/DIABLO) and Omi (also known as HtrA2, called Omi/HtrA2). Both are mitochondrial proteins released during the apoptotic process, which bind to the IAPs through a conserved motif, the IAP-binding motif, promoting the release of the bound caspases. Omi/HtrA2 is a serine protease, and its protease activity was known to be essential for its proapoptotic activity, but its apoptosis-relevant substrates have not been identified. Yang et al . report that in vitro Omi/HtrA2 cleaves six members of the IAP family; c-IAP1 is the most efficiently cleaved. Maximal cleavage activity required an active protease domain and the IAP-binding motif, which was only required for c-IAP1 cleavage and not β-casein cleavage. Addition of Omi/HtrA2 to a caspase assay demonstrated that lower concentrations of Omi/HtrA2 were required to stimulate caspase activation than were required for the stoichometric inhibitor Smac/DIABLO, which suggested a catalytic activation mechanism. Maximal caspase activation was observed in cells exposed to the proapoptotic stimulus TRAIL when those cells expressed c-IAP and Omi/HtrA2. Cells expressing modified c-IAP1 with mutated Omi/HtrA2 cleavage sites did not show maximal caspase activation. Reduction of endogenous Omi/HtrA2 with silencing RNAs showed that c-IAP1 was stabilized in those cells and that caspase activation in response to TRAIL was reduced compared with that in control cells. Thus, Omi/HtrA2 contributes to apoptosis through catalytic cleavage of IAPs; this provides an irreversible mechanism to commit to apoptosis. Q.-H. Yang, R. Church-Hajduk, J. Ren, M. L. Newton, C. Du, Omi/HtrA2 catalytic cleavage of inhibitor of apoptosis (IAP) irreversible inactivates IAPs and facilitates caspase activity in apoptosis. Genes Dev. 17 , 1487-1496 (2003). [Abstract] [Full Text]