Mitochondrial Ca(2+) influx targets cardiolipin to disintegrate respiratory chain complex II for cell death induction.

Massive Ca(2+) influx into mitochondria is critically involved in cell death induction but it is unknown how this activates the organelle for cell destruction. Using multiple approaches including subcellular fractionation, FRET in intact cells, and in vitro reconstitutions, we show that mitochondrial Ca(2+) influx prompts complex II of the respiratory chain to disintegrate, thereby releasing an enzymatically competent sub-complex that generates excessive reactive oxygen species (ROS) for cell death induction. This Ca(2+)-dependent dissociation of complex II is also observed in model membrane systems, but not when cardiolipin is replaced with a lipid devoid of Ca(2+) binding. Cardiolipin is known to associate with complex II and upon Ca(2+) binding coalesces into separate homotypic clusters. When complex II is deprived of this lipid, it disintegrates for ROS formation and cell death. Our results reveal Ca(2+) binding to cardiolipin for complex II disintegration as a pivotal step for oxidative stress and cell death induction.

De-ubiquitinating protease USP2a targets RIP1 and TRAF2 to mediate cell death by TNF.

Components of the TNFR1 complex are subject to dynamic ubiquitination that impacts on their effects as signalling factors. We have found that the ubiquitin-specific protease USP2a has a pivotal role in the decision for cell death or survival by the TNFR1 complex. This enzyme is a novel component of the TNFR1 complex that is recruited upon ligand binding and controls the signalling activity of the TNFR1-interacting protein RIP1 by removing its K63-linked ubiquitin chains. USP2a similarly de-ubiquitinates TRAF2, a ubiquitin-ligase recruited to the TNFR1 complex. During the TNF response the activity of USP2a on RIP1 and TRAF2 is required for the efficient reappearance of IκBα, which is essential to inactivate the anti-apoptotic transcription factor NF-κB. The effects of USP2a culminate in the conversion of the anti-apoptotic TNFR1 complex I into the pro-apoptotic TNFR1 complex II. Consequently, downregulation of USP2a promotes NF-κB activation and protects cells against TNF-induced cell death.