Cardiolipin (CL), a mitochondria-specific glycerophospholipid, is required for diverse mitochondrial processes and orchestrates the function of various death-inducing proteins during apoptosis. Here, we identify a complex of the p53-regulated protein TRIAP1 (p53CSV) and PRELI in the mitochondrial intermembrane space (IMS), which ensures the accumulation of CL in mitochondria. TRIAP1/PRELI complexes exert lipid transfer activity in vitro and supply phosphatidic acid (PA) for CL synthesis in the inner membrane. Loss of TRIAP1 or PRELI impairs the accumulation of CL, facilitates the release of cytochrome c, and renders cells vulnerable to apoptosis upon intrinsic and extrinsic stimulation. Survival of TRIAP1- and PRELI-deficient cells is conferred by an excess of exogenously provided phosphatidylglycerol. Our results reveal a p53-dependent cell-survival pathway and highlight the importance of the CL content of mitochondrial membranes in apoptosis.

Mutations in subunits of mitochondrial m-AAA proteases in the inner membrane cause neurodegeneration in spinocerebellar ataxia (SCA28) and hereditary spastic paraplegia (HSP7). m-AAA proteases preserve mitochondrial proteostasis, mitochondrial morphology, and efficient OXPHOS activity, but the cause for neuronal loss in disease is unknown. We have determined the neuronal interactome of m-AAA proteases in mice and identified a complex with C2ORF47 (termed MAIP1), which counteracts cell death by regulating the assembly of the mitochondrial Ca uniporter MCU. While MAIP1 assists biogenesis of the MCU subunit EMRE, the m-AAA protease degrades non-assembled EMRE and ensures efficient assembly of gatekeeper subunits with MCU. Loss of the m-AAA protease results in accumulation of constitutively active MCU-EMRE channels lacking gatekeeper subunits in neuronal mitochondria and facilitates mitochondrial Ca overload, mitochondrial permeability transition pore opening, and neuronal death. Together, our results explain neuronal loss in m-AAA protease deficiency by deregulated mitochondrial Ca homeostasis.

The SPFH (stomatin, prohibitin, flotillin, HflC/K) superfamily is composed of scaffold proteins that form ring-like structures and locally specify the protein-lipid composition in a variety of cellular membranes. Stomatin-like protein 2 (SLP2) is a member of this superfamily that localizes to the mitochondrial inner membrane (IM) where it acts as a membrane organizer. Here, we report that SLP2 anchors a large protease complex composed of the rhomboid protease PARL and the i-AAA protease YME1L, which we term the SPY complex (for SLP2-PARL-YME1L). Association with SLP2 in the SPY complex regulates PARL-mediated processing of PTEN-induced kinase PINK1 and the phosphatase PGAM5 in mitochondria. Moreover, SLP2 inhibits the stress-activated peptidase OMA1, which can bind to SLP2 and cleaves PGAM5 in depolarized mitochondria. SLP2 restricts OMA1-mediated processing of the dynamin-like GTPase OPA1 allowing stress-induced mitochondrial hyperfusion under starvation conditions. Together, our results reveal an important role of SLP2 membrane scaffolds for the spatial organization of IM proteases regulating mitochondrial dynamics, quality control, and cell survival.