Intra-mitochondrial degradation of Tim23 curtails the survival of cells rescued from apoptosis by caspase inhibitors.

Caspase inhibition can extend the survival of cells undergoing apoptosis beyond the point of mitochondrial outer membrane permeabilisation (MOMP), but this does not confer long-term protection because caspase-independent death pathways emerge. Here, we describe a novel mechanism of mitochondrial self-destruction in caspase-inhibited cells, whose hallmark is the degradation of Tim23, the essential pore-forming component of the TIM23 inner membrane translocase. We show that Tim23 degradation occurs in cycling and post-mitotic cells, it is caspase-independent but Bax/Bak dependent, and it follows cytochrome c release. The proteolytic degradation of Tim23 is induced by MOMP and is mitochondrion-autonomous, as it also occurs in isolated mitochondria undergoing permeability transition. Degradation of Tim23 is selective, as expression of several other inner membrane proteins that regulate respiratory chain function is unaffected, and is not autophagic, as it occurs similarly in autophagy-proficient and -deficient (Atg-5 knockout) cells. Depleting Tim23 with siRNA is sufficient to inhibit cell proliferation and prevent long-term survival, while expression of degradation-resistant Tim23-GFP in mitochondria delays caspase-independent cell death. Thus, mitochondrial autodigestion of Tim23 joins the array of processes contributing to caspase-independent cell death. Because mitochondrial biogenesis requires a functional protein-import machinery, preventing Tim23 degradation might, therefore, be essential for repairing damaged mitochondria in chronic degenerative diseases.