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Concept: Intermembrane space


Cardiolipin (CL) is a unique phospholipid found in mitochondrial inner membrane. It is a key component for mitochondrial function in both respiration and apoptosis. The level of CL is an important parameter for investigating these intracellular events and is a critical indicator of a number of diseases associated with mitochondrial respiratory functions. 10-Nonyl acridine orange (NAO) is the only fluorescent dye currently available for CL detection. However, the performance of NAO is far from satisfactory in terms of selectivity and sensitivity. In this work, we report an aggregation-induced emission-active fluorogen, TTAPE-Me, for CL detection and quantification. With improved sensitivity and excellent selectivity to CL over other major mitochondrial membrane lipids, TTAPE-Me could serve as a valuable fluorescent sensor for CL quantification. The use of TTAPE-Me for the quantification of isolated mitochondria is also demonstrated.

Concepts: Oxygen, Metabolism, Adenosine triphosphate, Mitochondrion, Organelle, Cellular respiration, Intermembrane space, Crista


The mitochondrial proteome comprises ~1000 (yeast)-1500 (human) different proteins, which are distributed into four different subcompartments. The sublocalization of these proteins within the organelle in most cases remains poorly defined. Here we describe an integrated approach combining stable isotope labeling, various protein enrichment and extraction strategies and quantitative mass spectrometry to produce a quantitative map of submitochondrial protein distribution in S. cerevisiae. This quantitative landscape enables a proteome-wide classification of 986 proteins into soluble, peripheral, and integral mitochondrial membrane proteins, and the assignment of 818 proteins into the four subcompartments: outer membrane, inner membrane, intermembrane space, or matrix. We also identified 206 proteins that were not previously annotated as localized to mitochondria. Furthermore, the protease Prd1, misannotated as intermembrane space protein, could be re-assigned and characterized as a presequence peptide degrading enzyme in the matrix.Protein localization plays an important role in the regulation of cellular physiology. Here the authors use an integrated proteomics approach to localize proteins to the mitochondria and provide a detailed map of their specific localization within the organelle.

Concepts: DNA, Protein, Cell, Metabolism, Mitochondrion, Intermembrane space, Mitochondrial matrix


The mitochondrial contact site and cristae organizing system (MICOS) is crucial for the formation of crista junctions and mitochondrial inner membrane architecture. MICOS contains two core components. Mic10 shows membrane-bending activity, whereas Mic60 (mitofilin) forms contact sites between inner and outer membranes. Here we report that Mic60 deforms liposomes into thin membrane tubules and thus displays membrane-shaping activity. We identify a membrane-binding site in the soluble intermembrane space-exposed part of Mic60. This membrane-binding site is formed by a predicted amphipathic helix between the conserved coiled-coil and mitofilin domains. The mitofilin domain negatively regulates the membrane-shaping activity of Mic60. Binding of Mic19 to the mitofilin domain modulates this activity. Membrane binding and shaping by the conserved Mic60-Mic19 complex is crucial for crista junction formation, mitochondrial membrane architecture and efficient respiratory activity. Mic60 thus plays a dual role by shaping inner membrane crista junctions and forming contact sites with the outer membrane.

Concepts: Mitochondrion, Cellular respiration, Membrane biology, Intermembrane space, Inner membrane, Mitochondrial matrix, Crista, Inner mitochondrial membrane


The mitochondrial inner membrane harbors three protein translocases. Presequence translocase and carrier translocase are essential for importing nuclear-encoded proteins. The oxidase assembly (OXA) translocase is required for exporting mitochondrial-encoded proteins; however, different views exist about its relevance for nuclear-encoded proteins. We report that OXA plays a dual role in the biogenesis of nuclear-encoded mitochondrial proteins. First, a systematic analysis of OXA-deficient mitochondria led to an unexpected expansion of the spectrum of OXA substrates imported via the presequence pathway. Second, biogenesis of numerous metabolite carriers depends on OXA, although they are not imported by the presequence pathway. We show that OXA is crucial for the biogenesis of the Tim18-Sdh3 module of the carrier translocase. The export translocase OXA is thus required for the import of metabolite carriers by promoting assembly of the carrier translocase. We conclude that OXA is of central importance for the biogenesis of the mitochondrial inner membrane.

Concepts: Cell, Metabolism, Cytoplasm, Citric acid cycle, International trade, Export, Intermembrane space, Import


Mitochondria are inherited maternally in most animals, but the mechanisms of selective paternal mitochondrial elimination (PME) are unknown. While examining fertilization in C. elegans, we observe that paternal mitochondria rapidly lose their inner membrane integrity. CPS-6, a mitochondrial endonuclease G, serves as a paternal mitochondrial factor that is critical for PME. The CPS-6 endonuclease relocates from the intermembrane space of paternal mitochondria to the matrix following fertilization to degrade mitochondrial DNA. It acts with maternal autophagy and proteasome machineries to promote PME. Loss of cps-6 delays breakdown of mitochondrial inner membranes, autophagosome enclosure of paternal mitochondria, and PME. Delayed removal of paternal mitochondria causes increased embryonic lethality, demonstrating that PME is important for normal animal development. Thus, CPS-6 functions as a paternal mitochondrial degradation factor during animal development.

Concepts: DNA, Mitochondrion, Organelle, Caenorhabditis elegans, Animal, Intermembrane space, Inner membrane, Mitochondrial matrix


Two protein translocases drive the import of β-barrel precursor proteins into the mitochondrial outer membrane: The translocase of the outer membrane (TOM complex) promotes transport of the precursor to the intermembrane space, whereas the sorting and assembly machinery (SAM complex) mediates subsequent folding of the β-barrel and its integration into the target membrane. The non-bilayer forming phospholipids phosphatidylethanolamine (PE) and cardiolipin (CL) are required for the biogenesis of β-barrel proteins. Whether bilayer-forming phospholipids such as phosphatidylcholine (PC), the most abundant phospholipid of the mitochondrial outer membrane, play a role in the import of β-barrel precursors is unclear. In this study we show that PC is required for stability and function of the SAM complex during the biogenesis of β-barrel proteins. PC further promotes the SAM-dependent assembly of the TOM complex, indicating a general role of PC for the function of the SAM complex. In contrast to PE-deficient mitochondria precursor accumulation at the TOM complex is not affected by depletion of PC. We conclude that PC and PE affect the function of distinct protein translocases in mitochondrial β-barrel biogenesis.

Concepts: Metabolism, Mitochondrion, Cell membrane, Cytoplasm, Phospholipid, Intermembrane space, Lecithin, Mitochondrial matrix


Mitochondria fulfill central functions in cellular energetics, metabolism, and signaling. The outer membrane translocator complex (the TOM complex) imports most mitochondrial proteins, but its architecture is unknown. Using a cross-linking approach, we mapped the active translocator down to single amino acid residues, revealing different transport paths for preproteins through the Tom40 channel. An N-terminal segment of Tom40 passes from the cytosol through the channel to recruit chaperones from the intermembrane space that guide the transfer of hydrophobic preproteins. The translocator contains three Tom40 β-barrel channels sandwiched between a central α-helical Tom22 receptor cluster and external regulatory Tom proteins. The preprotein-translocating trimeric complex exchanges with a dimeric isoform to assemble new TOM complexes. Dynamic coupling of α-helical receptors, β-barrel channels, and chaperones generates a versatile machinery that transports about 1000 different proteins.

Concepts: Protein, Amino acid, Metabolism, Mitochondrion, Cytosol, Cytoplasm, Intermembrane space, Mitochondrial matrix


The content of mitochondrial proteome is maintained through two highly dynamic processes, the influx of newly synthesized proteins from the cytosol and the protein degradation. Mitochondrial proteins are targeted to the intermembrane space by the mitochondrial intermembrane space assembly pathway that couples their import and oxidative folding. The folding trap was proposed to be a driving mechanism for the mitochondrial accumulation of these proteins. Whether the reverse movement of unfolded proteins to the cytosol occurs across the intact outer membrane is unknown. We found that reduced, conformationally destabilized proteins are released from mitochondria in a size-limited manner. We identified the general import pore protein Tom40 as an escape gate. We propose that the mitochondrial proteome is not only regulated by the import and degradation of proteins but also by their retro-translocation to the external cytosolic location. Thus, protein release is a mechanism that contributes to the mitochondrial proteome surveillance.

Concepts: Cell, Metabolism, Adenosine triphosphate, Mitochondrion, Cytosol, Cytoplasm, Intermembrane space, Mitochondrial matrix


Yeast Prx1 is a mitochondrial 1-Cys peroxiredoxin that catalyzes the reduction of endogenously generated H2O2 Prx1 is synthesized on cytosolic ribosomes as a preprotein with a cleavable N-terminal presequence that is the mitochondrial targeting signal, but the mechanisms underlying Prx1 distribution to distinct mitochondrial subcompartments are unknown. Here, we provide direct evidence of the following dual mitochondrial localization of Prx1: a soluble form in the intermembrane space and a form in the matrix weakly associated with the inner mitochondrial membrane. We show that Prx1 sorting into the intermembrane space likely involves the release of the protein precursor within the lipid bilayer of the inner membrane, followed by cleavage by the inner membrane peptidase (IMP). We also found that during its import into the matrix compartment, Prx1 is sequentially cleaved by mitochondrial processing peptidase (MPP) and then by octapeptidyl aminopeptidase 1 (Oct1). Oct1 cleaved eight amino acid residues from the N-terminal region of Prx1 inside the matrix, without interfering with its peroxidase activity in vitro Remarkably, the processing of Prx proteins by Oct1 appears to be an evolutionarily conserved process since yeast Oct1 could cleave the human mitochondrial peroxiredoxin Prx3 when expressed in Saccharomyces cerevisiae Altogether, the processing of peroxiredoxins by Imp2 or Oct1 likely represents systems that control the localization of Prxs into distinct compartments and thereby contribute to various mitochondrial redox processes..

Concepts: Protein, Bacteria, Amino acid, Metabolism, Enzyme, Mitochondrion, Organelle, Intermembrane space


Microscopy and mass spectrometry (MS) are complementary techniques: the former provides spatiotemporal information in living cells, but only for a handful of recombinant proteins, while the latter can detect thousands of endogenous proteins simultaneously, but only in lysed samples. Here, we introduce technology that combines these strengths by offering spatially and temporally resolved proteomic maps of endogenous proteins within living cells. The method relies on a genetically targetable peroxidase enzyme that biotinylates nearby proteins, which are subsequently purified and identified by MS. We used this approach to identify 495 proteins within the human mitochondrial matrix, including 31 not previously linked to mitochondria. The labeling was exceptionally specific and distinguished between inner membrane proteins facing the matrix versus the intermembrane space (IMS). Several proteins previously thought to reside in the IMS or outer membrane, including protoporphyrinogen oxidase, were reassigned to the matrix. The specificity of live-cell peroxidase-mediated proteomic mapping combined with its ease of use offers biologists a powerful tool for understanding the molecular composition of living cells.

Concepts: DNA, Protein, Cell, Adenosine triphosphate, Mitochondrion, Cytoplasm, Citric acid cycle, Intermembrane space