The transport of pyruvate, the end product of glycolysis, into mitochondria is an essential process that provides the organelle with a major oxidative fuel. Although the existence of a specific mitochondrial pyruvate carrier (MPC) has been anticipated, its molecular identity remained unknown. We report that MPC is a heterocomplex formed by two members of a family of previously uncharacterized membrane proteins that are conserved from yeast to mammals. Members of the MPC family were found in the inner mitochondrial membrane, and yeast mutants lacking MPC proteins showed severe defects in mitochondrial pyruvate uptake. Coexpression of mouse MPC1 and MPC2 in Lactococcus lactis promoted transport of pyruvate across the membrane. These observations firmly establish these proteins as essential components of the MPC.

Dynamin-Related Protein 1 (Drp1), a large GTPase of the dynamin superfamily, is required for mitochondrial fission in healthy and apoptotic cells. Drp1 activation is a complex process that involves translocation from the cytosol to the mitochondrial outer membrane (MOM) and assembly into rings/spirals at the MOM, leading to membrane constriction/division. Similar to dynamins, Drp1 contains GTPase (G), bundle signaling element (BSE) and stalk domains. However, instead of the lipid–interacting Pleckstrin Homology (PH) domain present in the dynamins, Drp1 contains the so-called B insert or variable domain that has been suggested to play an important role in Drp1 regulation. Different proteins have been implicated in Drp1 recruitment to the MOM, although how MOM-localized Drp1 acquires its fully functional status remains poorly understood. We found that Drp1 can interact with pure lipid bilayers enriched in the mitochondrion-specific phospholipid cardiolipin (CL). Building on our previous study, we now explore the specificity and functional consequences of this interaction. We show that a four lysine module located within the B insert of Drp1 interacts preferentially with CL over other anionic lipids. This interaction dramatically enhances Drp1 oligomerization and assembly-stimulated GTP hydrolysis. Our results add significantly to a growing body of evidence indicating that CL is an important regulator of many essential mitochondrial functions.

e Mammalian mitochondria may contain up to 1,500 different proteins, and many of them have neither been confidently identified nor characterized. In this study, we demonstrated that C11orf83, which was lacking experimental characterization, is a mitochondrial inner membrane protein facing the intermembrane space. This protein is specifically associated with the bc 1 complex of the electron transport chain and involved in the early stages of its assembly by stabilizing the bc 1 core complex. C11orf83 displays some overlapping functions with Cbp4p, a yeast bc 1 complex assembly factor. Therefore, we suggest that C11orf83, now called UQCC3, is the functional human equivalent of Cbp4p. In addition, C11orf83 depletion in HeLa cells caused abnormal crista morphology, higher sensitivity to apoptosis, a decreased ATP level due to impaired respiration and subtle, but significant, changes in cardiolipin composition. We showed that C11orf83 binds to cardiolipin by its ␣-helices 2 and 3 and is involved in the stabilization of bc 1 complex-containing supercomplexes, especially the III 2 /IV supercomplex. We also demonstrated that the OMA1 metalloprotease cleaves C11orf83 in response to mitochondrial depolarization, suggesting a role in the selection of cells with damaged mitochondria for their subsequent elimination by apoptosis, as previously described for OPA1. Mitochondria are membrane-enclosed organelles composed of several compartments which perform specialized and interconnected functions such as oxidative phosphorylation (OXPHOS), cell death, or carbohydrate and fatty acid metabolisms. OXPHOS, which provides most of the ATP used by the cell, takes place in the inner membrane (IM) and involves five complexes. Redox reactions are carried out by complex I (NADH: ubiquinone oxidoreductase; EC 1.6.5.3), complex II (succinate: ubiquinone oxidoreductase; EC 1.3.5.1), complex III (ubiquinol: ferricytochrome c oxidoreductase; EC 1.10.2.2), and complex IV (cytochrome c oxidoreductase; EC 1.9.3.1). Then, the ATP synthase, complex V (F 1 F 0 ATPase; EC 3.6.3.14), uses the energy released by respiration for ATP generation. The assembly of the OXPHOS complexes requires additional nuclear proteins called assembly factors (1). The physiological importance of these assembly factors is proven by the number of human diseases associated with mutations in genes encoding them (1).Complex III, also called cytochrome bc 1 complex, is a central component of the electron transport chain (ETC). It transfers electrons from coenzyme Q reduced either by complex I through NADH-linked substrates or by complex II through reduced flavin adenine dinucleotide (FADH 2 )-linked substrates to cytochrome c. The mammalian bc 1 complex, which forms as a stable dimer (2), is composed of 11 subunits, among which only MT-CYB is encoded by the mitochondrial genome (3, 4). Most of the work on the bc 1 complex assembly has been performed with Saccharomyces cerevisiae and has shown this to be a multistep process involving several subcomplexes and assembly factors (5...

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During apoptosis, proapoptotic BAX and BAK trigger mitochondrial outer membrane (MOM) permeabilization by a mechanism that is not yet fully understood. BH3-only proteins such as tBID, together with lipids of the MOM, are thought to play a key role in BAX and BAK activation. In particular, cardiolipin (CL) has been shown to stimulate tBID-induced BAX activation in vitro. However, it is still unclear whether this process also relies on CL in the cell, or whether it is more dependent on MTCH2, a proposed receptor for tBID present in the MOM. To address this issue, we deleted both alleles of cardiolipin synthase in human HCT116 cells by homologous recombination, which resulted in a complete absence of CL. The CL-deficient cells were fully viable in glucose but displayed impaired oxidative phosphorylation and an inability to grow in galactose. Using these cells, we found that CL was not required for either tBID-induced BAX activation, or for apoptosis in response to treatment with TRAIL. Downregulation of MTCH2 in HCT116 cells also failed to prevent recruitment of tBID to mitochondria in apoptotic conditions. However, when both CL and MTCH2 were depleted, a significant reduction in tBID recruitment was observed, suggesting that in HCT116 cells, CL and MTCH2 can have redundant functions in this process.