Single amino acid mutations in the Saccharomyces cerevisiae rhomboid peptidase,Pcp1p,alter mitochondrial morphology

Key to mitochondrial activities is the maintenance of mitochondrial morphology, specifically cristae structures formed by the invagination of the inner membrane that are enriched in proteins of the electron transport chain. In Saccharomyces cerevisiae , these cristae folds are a result of the membrane fusion activities of Mgm1p and the membrane‐bending properties of adenosine triphosphate (ATP) synthase oligomerization. An additional protein linked to mitochondrial morphology is Pcp1p, a serine protease responsible for the proteolytic processing of Mgm1p. Here, we have used hydroxylamine‐based random mutagenesis to identify amino acids important for Pcp1p peptidase activity. Using this approach we have isolated five single amino acid mutants that exhibit respiratory growth defects that correlate with loss of mitochondrial genome stability. Reduced Pcp1p protease activity was confirmed by immunoblotting with the accumulation of improperly processed Mgm1p. Ultra‐structural analysis of mitochondrial morphology in these mutants found a varying degree of defects in cristae organization. However, not all of the mutants presented with decreased ATP synthase complex assembly as determined by blue native polyacrylamide gel electrophoresis. Together, these data suggest that there is a threshold level of processed Mgm1p required to maintain ATP synthase super‐complex assembly and mitochondrial cristae organization.     

High resolution clear native electrophoresis for in-gel functional assays and fluorescence studies of membrane protein complexes

Clear native electrophoresis and blue native electrophoresis are microscale techniques for the isolation of membrane protein complexes. The Coomassie Blue G-250 dye, used in blue native electrophoresis, interferes with in-gel fluorescence detection and in-gel catalytic activity assays. This problem can be overcome by omitting the dye in clear native electrophoresis. However, clear native electrophoresis suffers from enhanced protein aggregation and broadening of protein bands during electrophoresis and therefore has been used rarely. To preserve the advantages of both electrophoresis techniques we substituted Coomassie dye in the cathode buffer of blue native electrophoresis by non-colored mixtures of anionic and neutral detergents. Like Coomassie dye, these mixed micelles imposed a charge shift on the membrane proteins to enhance their anodic migration and improved membrane protein solubility during electrophoresis. This improved clear native electrophoresis offers a high resolution of membrane protein complexes comparable to that of blue native electrophoresis. We demonstrate the superiority of high resolution clear native electrophoresis for in-gel catalytic activity assays of mitochondrial complexes I-V. We present the first in-gel histochemical staining protocol for respiratory complex III. Moreover we demonstrate the special advantages of high resolution clear native electrophoresis for in-gel detection of fluorescent labeled proteins labeled by reactive fluorescent dyes and tagged by fluorescent proteins. The advantages of high resolution clear native electrophoresis make this technique superior for functional proteomics analyses.     

Analysis of 953 Human Proteins from a Mitochondrial HEK293 Fraction by Complexome Profiling

Complexome profiling is a novel technique which uses shotgun proteomics to establish protein migration profiles from fractionated blue native electrophoresis gels. Here we present a dataset of blue native electrophoresis migration profiles for 953 proteins by complexome profiling. By analysis of mitochondrial ribosomal complexes we demonstrate its potential to verify putative protein-protein interactions identified by affinity purification – mass spectrometry studies. Protein complexes were extracted in their native state from a HEK293 mitochondrial fraction and separated by blue native gel electrophoresis. Gel lanes were cut into gel slices of even size and analyzed by shotgun proteomics. Subsequently, the acquired protein migration profiles were analyzed for co-migration via hierarchical cluster analysis. This dataset holds great promise as a comprehensive resource for de novo identification of protein-protein interactions or to underpin and prioritize candidate protein interactions from other studies. To demonstrate the potential use of our dataset we focussed on the mitochondrial translation machinery. Our results show that mitoribosomal complexes can be analyzed by blue native gel electrophoresis, as at least four distinct complexes. Analysis of these complexes confirmed that 24 proteins that had previously been reported to co-purify with mitoribosomes indeed co-migrated with subunits of the mitochondrial ribosome. Co-migration of several proteins involved in biogenesis of inner mitochondrial membrane complexes together with mitoribosomal complexes suggested the possibility of co-translational assembly in human cells. Our data also highlighted a putative ribonucleotide complex that potentially contains MRPL10, MRPL12 and MRPL53 together with LRPPRC and SLIRP.     

ATP synthase superassemblies in animals and plants: two or more are better

ATP synthases are part of the sophisticated cellular metabolic network and therefore multiple interactions have to be considered. As discussed in this review, ATP synthases form various supramolecular structures. These include dimers and homooligomeric species. But also interactions with other proteins, particularly those involved in energy conversion exist. The supramolecular assembly of the ATP synthase affects metabolism, organellar structure, diseases, ageing and vice versa. The most common approaches to isolate supercomplexes from native membranes by use of native electrophoresis or density gradients are introduced. On the one hand, isolated ATP synthase dimers and oligomers are employed for structural studies and elucidation of specific protein-protein interactions. On the other hand, native electrophoresis and other techniques serve as tool to trace changes of the supramolecular organisation depending on metabolic alterations. Upon analysing the structure, dimer-specific subunits can be identified as well as interactions with other proteins, for example, the adenine nucleotide translocator. In the organellar context, ATP synthase dimers and oligomers are involved in the formation of mitochondrial cristae. As a consequence, changes in the amount of such supercomplexes affect mitochondrial structure and function. Alterations in the cellular power plant have a strong impact on energy metabolism and ultimately play a significant role in pathophysiology. In plant systems, dimers of the ATP synthase have been also identified in chloroplasts. Similar to mammals, a correlation between metabolic changes and the amount of the chloroplast ATP synthase dimers exists. Therefore, this review focusses on the interplay between metabolism and supramolecular organisation of ATP synthase in different organisms.     

Yeast mitochondrial F1F0-ATP synthase exists as a dimer: identification of three dimer-specific subunits.

Using the technique of blue native gel electrophoresis, the oligomeric state of the yeast mitochondrial F1F0-ATP synthase was analysed. Solubilization of mitochondrial membranes with low detergent to protein ratios led to the identification of the dimeric state of the ATP synthase. Analysis of the subunit composition of the dimer, in comparison with the monomer, revealed the presence of three additional small proteins. These dimer-specific subunits of the ATP synthase were identified as the recently described subunit e/Tim11 (Su e/Tim11), the putative subunit g homolog (Su g) and a new component termed subunit k (Su k). Although, as shown here, these three proteins are not required for the formation of enzymatically active ATP synthase, Su e/Tim11 and Su g are essential for the formation of the dimeric state. Su e/Tim11 appears to play a central role in this dimerization process. The dimer-specific subunits are associated with the membrane bound F0-sector. The F0-sector may thereby be involved in the dimerization of two monomeric F1F0-ATP synthase complexes. We speculate that the F1F0-ATP synthase of yeast, like the other complexes of oxidative phosphorylation, form supracomplexes to optimize transduction of energy and to enhance the stability of the complex in the membrane.     

Characterization of domain interfaces in monomeric and dimeric ATP synthase

We disassembled monomeric and dimeric yeast ATP synthase under mild conditions to identify labile proteins and transiently stable subcomplexes that had not been observed before. Specific removal of subunits alpha, beta, oligomycin sensitivity conferring protein (OSCP), and h disrupted the ATP synthase at the gamma-alpha(3)beta(3) rotor-stator interface. Loss of two F(1)-parts from dimeric ATP synthase led to the isolation of a dimeric subcomplex containing membrane and peripheral stalk proteins thus identifying the membrane/peripheral stalk sectors immediately as the dimerizing parts of ATP synthase. Almost all subunit a was found associated with a ring of 10 c-subunits in two-dimensional blue native/SDS gels. We therefore postulate that c10a1-complex is a stable structure in resting ATP synthase until the entry of protons induces a breaking of interactions and stepwise rotation of the c-ring relative to the a-subunit in the catalytic mechanism. Dimeric subunit a was identified in SDS gels in association with two c10-rings suggesting that a c10a2c10-complex may constitute an important part of the monomer-monomer interface in dimeric ATP synthase that seems to be further tightened by subunits b, i, e, g, and h. In contrast to the monomer-monomer interface, the interface between dimers in higher oligomeric structures remains largely unknown. However, we could show that the natural inhibitor protein Inh1 is not required for oligomerization.     

The inhibitor protein (IF1) promotes dimerization of the mitochondrial F1F0-ATP synthase

The effect of increased expression or reconstitution of the mitochondrial inhibitor protein (IF1) on the dimer/monomer ratio (D/M) of the rat liver and bovine heart F1F0-ATP synthase was studied. The 2-fold increased expression of IF1 in AS-30D hepatoma mitochondria correlated with a 1.4-fold increase in the D/M ratio of the ATP synthase extracted with digitonin as determined by blue native electrophoresis and averaged densitometry analyses. Removal of IF1 from rat liver or bovine heart submitochondrial particles increased the F1F0-ATPase activity and decreased the D/M ratio of the ATP synthase. Reconstitution of recombinant IF1 into submitochondrial particles devoid of IF1 inhibited the F1F0-ATPase activity by 90% and restored partially the D/M ratio of the whole F1F0 complex as revealed by blue native electrophoresis and subsequent SDS-PAGE or glycerol density gradient centrifugation. Thus, the inhibitor protein promotes or stabilizes the dimeric form of the intact F1F0-ATP synthase. A possible location of the IF1 protein in the dimeric structure of the rat liver F1F0 complex is proposed. According to crystallographic and electron microscopy analyses, dimeric IF1 could bridge the F1-F1 part of the dimeric F1F0-ATP synthase in the inner mitochondrial membrane.     

Advantages and limitations of clear-native PAGE

Clear-native PAGE (CN-PAGE) separates acidic water-soluble and membrane proteins (pI < 7) in an acrylamide gradient gel, and usually has lower resolution than blue-native PAGE (BN-PAGE). The migration distance depends on the protein intrinsic charge, and on the pore size of the gradient gel. This complicates estimation of native masses and oligomerization states when compared to BN-PAGE, which uses negatively charged protein-bound Coomassie-dye to impose a charge shift on the proteins. Therefore, BN-PAGE rather than CN-PAGE is commonly used for standard analyses. However, CN-PAGE offers advantages whenever Coomassie-dye interferes with techniques required to further analyze the native complexes, e.g., determination of catalytic activities, as shown here for mitochondrial ATP synthase, or efficient microscale separation of membrane protein complexes for fluorescence resonance energy transfer (FRET) analyses. CN-PAGE is milder than BN-PAGE. Especially the combination of digitonin and CN-PAGE can retain labile supramolecular assemblies of membrane protein complexes that are dissociated under the conditions of BN-PAGE. Enzymatically active oligomeric states of mitochondrial ATP synthase previously not detected using BN-PAGE were identified by CN-PAGE.     

Two overlapping genes in bovine mitochondrial DNA encode membrane components of ATP synthase.

Two hydrophobic proteins have been purified to homogeneity from a mixture of about 13 proteins that are extracted from bovine mitochondria with a chloroform:methanol mixture. Sequence analysis shows that the smaller is a protein of 66 amino acids and is the product of a mitochondrial gene, A6L. The larger, a protein of 226 amino acids, is ATPase-6, a membrane component of ATP synthase, also encoded in mitochondrial DNA. The protein sequences determined establish that the genes for the two proteins overlap by 40 bases and indicate that translation of the second gene, ATPase-6, is initiated within the coding region of A6L. The A6L and the ATPase-6 proteins have also been isolated from the ATP synthase complex and so appear to be bona fide components of the enzyme. The function of A6L is unknown. However, weak structural homology suggests a functional similarity to the yeast mitochondrial protein, aapI, which is required for assembly of the fungal ATP synthase complex. Homologies between ATPase-6 and subunit a of the Escherichia coli ATP synthase complex indicate that the ATPase-6 protein has a similar role in the mitochondrial complex to its bacterial counterpart, being essential for the formation of an active proton channel.     

Oxidation-reduction respiratory chains and ATP synthase complex are localized in detergent-resistant lipid rafts

In order to detect and identify ubiquitous lipid raft marker proteins, we isolated lipid rafts from different mouse organs, including the liver, lung, large brain, and kidney, and analyzed their proteins via 2-DE. Many protein spots were determined to be ubiquitous in all of the lipid rafts, and were annotated via LC and MS/MS. Twelve proteins were identified as ubiquitous raft proteins, and most of these were determined to be mitochondrial proteins, including mortalin, prohibitin, voltage-dependent anion channel, ATP synthase, NADH dehydrogenase, and ubiquinol-cytochrome c reductase. Via immunoblotting, these proteins were shown to exist in detergent-resistant lipid rafts prepared using different organ tissues. Since these oxidation-reduction respiratory chains and ATP synthase complex were detected in detergent-resistant lipid raft fractions which had been isolated from the plasma membrane but not from the mitochondria, and found in the cell surface when determined by immunofluoresence and immunohistochemistry, we conclude that plasma membrane lipid rafts might contain oxidation-reduction respiratory chains and ATP synthase complex.     

Lipid raft proteome reveals ATP synthase complex in the cell surface

Since detergent-resistant lipid rafts are involved in pathogen invasion, cholesterol homeostasis, angiogenesis, neurodegenerative diseases and signal transduction, protein identification in the rafts could provide important information to study their function. Here, we analyzed detergent-resistant raft proteins isolated from rat liver by capillary liquid chromatography-tandem mass spectrometry. Out of 196 proteins identified, 32% belonged to the raft or plasma membrane, 24% to mitochondrial, 20% to microsomal, 7% to miscellaneous, and 17% are unknown proteins. For example, membrane-bound receptors, trimeric GTP-binding proteins, ATP-binding cassette transporters, and glycosylphosphatidylinositol-anchored proteins were identified in this analysis. Unexpectedly, there were many mitochondrial proteins, raising a new issue for the presence of mitochondrial rafts or the localization of mitochondrial proteins into plasma membrane rafts. We confirmed that ATP synthase alpha and beta were expressed on the surface of the plasma membrane in HepG2 hepatocytes by immunofluorescence, cell surface biotinylation, and cellular fractionation. They had two distinct biochemical properties, detergent insolubility and low density, suggesting that the ATP synthase complex might be located in plasma membrane rafts as well as in the mitochondria.     

Quantitation and origin of the mitochondrial membrane potential in human cells lacking mitochondrial DNA.

Mammalian mitochondrial DNA (mtDNA) encodes 13 polypeptide components of oxidative phosphorylation complexes. Consequently, cells that lack mtDNA (termed rho degrees cells) cannot maintain a membrane potential by proton pumping. However, most mitochondrial proteins are encoded by nuclear DNA and are still imported into mitochondria in rho degrees cells by a mechanism that requires a membrane potential. This membrane potential is thought to arise from the electrogenic exchange of ATP4- for ADP3- by the adenine nucleotide carrier. An intramitochondrial ATPase, probably an incomplete FoF1-ATP synthase lacking the two subunits encoded by mtDNA, is also essential to ensure sufficient charge flux to maintain the potential. However, there are considerable uncertainties about the magnitude of this membrane potential, the nature of the intramitochondrial ATPase and the ATP flux required to maintain the potential. Here we have investigated these factors in intact and digitonin-permeabilized mammalian rho degrees cells. The adenine nucleotide carrier and ATP were essential, but not sufficient to generate a membrane potential in rho degrees cells and an incomplete FoF1-ATP synthase was also required. The maximum value of this potential was approximately 110 mV in permeabilized cells and approximately 67 mV in intact cells. The membrane potential was eliminated by inhibitors of the adenine nucleotide carrier and by azide, an inhibitor of the incomplete FoF1-ATP synthase, but not by oligomycin. This potential is sufficient to import nuclear-encoded proteins but approximately 65 mV lower than that in 143B cells containing fully functional mitochondria. Subfractionation of rho degrees mitochondria showed that the azide-sensitive ATPase activity was membrane associated. Further analysis by blue native polyacrylamide gel electrophoresis (BN/PAGE) followed by activity staining or immunoblotting, showed that this ATPase activity was an incomplete FoF1-ATPase loosely associated with the membrane. Maintenance of this membrane potential consumed about 13% of the ATP produced by glycolysis. This work has clarified the role of the adenine nucleotide carrier and an incomplete FoF1-ATP synthase in maintaining the mitochondrial membrane potential in rho degrees cells.     

Each yeast mitochondrial F1F0-ATP synthase complex contains a single copy of subunit 8

The stoichiometry of subunit 8 in yeast mitochondrial F(1)F(0)-ATP synthase (mtATPase) has been evaluated using an immunoprecipitation approach. Single HA or FLAG epitopes were introduced at the N-terminus of subunit 8. Expression of each tagged subunit 8 variant in yeast cells lacking endogenous subunit 8 restored a respiratory phenotype and had little measurable effect on ATP hydrolase activity of the isolated enzyme. Moreover, the two epitope-tagged subunit 8 variants could be stably co-expressed in the same host cells and both of HA-Y8 and FLAG-Y8 could be detected in ATP synthase complexes isolated by native gel electrophoresis. Mitochondria isolated from each yeast strain were solubilized to release ATP synthase complexes in either the monomeric or dimeric forms. In each case, monoclonal antibodies directed against either the FLAG or HA epitope could immunoprecipitate intact ATP synthase complexes. When both HA-Y8 and FLAG-Y8 were co-expressed in cells, monomeric ATP synthases contained only a single subunit 8 variant after immunoprecipitation, corresponding to the particular antibody used (HA or FLAG). By contrast, both subunit 8 variants were recovered in samples of immunoprecipitated dimeric ATP synthase complexes, irrespective of the antibody used. We conclude that each monomeric yeast mitochondrial ATP synthase complex contains a single copy of subunit 8.     

Is there a relationship between the supramolecular organization of the mitochondrial ATP synthase and the formation of cristae?

Blue native polyacrylamide gel electrophoresis (BN-PAGE) analyses of detergent mitochondrial extracts have provided evidence that the yeast ATP synthase could form dimers. Cross-linking experiments performed on a modified version of the i-subunit of this enzyme indicate the existence of such ATP synthase dimers in the yeast inner mitochondrial membrane. We also show that the first transmembrane segment of the eukaryotic b-subunit (bTM1), like the two supernumerary subunits e and g, is required for dimerization/oligomerization of ATP synthases. Unlike mitochondria of wild-type cells that display a well-developed cristae network, mitochondria of yeast cells devoid of subunits e, g, or bTM1 present morphological alterations with an abnormal proliferation of the inner mitochondrial membrane. From these observations, we postulate that an anomalous organization of the inner mitochondrial membrane occurs due to the absence of ATP synthase dimers/oligomers. We provide a model in which the mitochondrial ATP synthase is a key element in cristae morphogenesis.     

Phosphorylation and kinetics of mammalian cytochrome c oxidase

The influence of protein phosphorylation on the kinetics of cytochrome c oxidase was investigated by applying Western blotting, mass spectrometry, and kinetic measurements with an oxygen electrode. The isolated enzyme from bovine heart exhibited serine, threonine, and/or tyrosine phosphorylation in various subunits, except subunit I, by using phosphoamino acid-specific antibodies. The kinetics revealed slight inhibition of oxygen uptake in the presence of ATP, as compared with the presence of ADP. Mass spectrometry identified the phosphorylation of Ser-34 at subunit IV and Ser-4 and Thr-35 at subunit Va. Incubation of the isolated enzyme with protein kinase A, cAMP, and ATP resulted in serine and threonine phosphorylation of subunit I, which was correlated with sigmoidal inhibition kinetics in the presence of ATP. This allosteric ATP-inhibition of cytochrome c oxidase was also found in rat heart mitochondria, which had been rapidly prepared in the presence of protein phosphatase inhibitors. The isolated rat heart enzyme, prepared from the mitochondria by blue native gel electrophoresis, showed serine, threonine, and tyrosine phosphorylation of subunit I. It is concluded that the allosteric ATP-inhibition of cytochrome c oxidase, previously suggested to keep the mitochondrial membrane potential and thus the reactive oxygen species production in cells at low levels, occurs in living cells and is based on phosphorylation of cytochrome c oxidase subunit I.     

Lipid,Detergent, and Coomassie Blue G-250 Affect the Migration of Small Membrane Proteins in Blue Native Gels: MITOCHONDRIAL CARRIERS MIGRATE AS MONOMERS NOT DIMERS*

Blue native gel electrophoresis is a popular method for the determination of the oligomeric state of membrane proteins. Studies using this technique have reported that mitochondrial carriers are dimeric (composed of two ∼32-kDa monomers) and, in some cases, can form physiologically relevant associations with other proteins. Here, we have scrutinized the behavior of the yeast mitochondrial ADP/ATP carrier AAC3 in blue native gels. We find that the apparent mass of AAC3 varies in a detergent- and lipid-dependent manner (from ∼60 to ∼130 kDa) that is not related to changes in the oligomeric state of the protein, but reflects differences in the associated detergent-lipid micelle and Coomassie Blue G-250 used in this technique. Higher oligomeric state species are only observed under less favorable solubilization conditions, consistent with aggregation of the protein. Calibration with an artificial covalent AAC3 dimer indicates that the mass observed for solubilized AAC3 and other mitochondrial carriers corresponds to a monomer. Size exclusion chromatography of purified AAC3 in dodecyl maltoside under blue native gel-like conditions shows that the mass of the monomer is ∼120 kDa, but appears smaller on gels (∼60 kDa) due to the unusually high amount of bound negatively charged dye, which increases the electrophoretic mobility of the protein-detergent-dye micelle complex. Our results show that bound lipid, detergent, and Coomassie stain alter the behavior of mitochondrial carriers on gels, which is likely to be true for other small membrane proteins where the associated lipid-detergent micelle is large when compared with the mass of the protein.     

Specific modification of mitochondrial protein thiols in response to oxidative stress: a proteomics approach

Mitochondria play a central role in redox-linked processes in the cell through mechanisms that are thought to involve modification of specific protein thiols, but this has proved difficult to assess. In particular, specific labeling and quantitation of mitochondrial protein cysteine residues have not been achieved due to the lack of reagents available that can be applied to the intact organelle or cell. To overcome these problems we have used a combination of mitochondrial proteomics and targeted labeling of mitochondrial thiols using a novel compound, (4-iodobutyl)triphenylphosphonium (IBTP). This lipophilic cation is accumulated by mitochondria and yields stable thioether adducts in a thiol-specific reaction. The selective uptake into mitochondria, due to the large membrane potential across the inner membrane, and the high pH of the matrix results in specific labeling of mitochondrial protein thiols by IBTP. Individual mitochondrial proteins that changed thiol redox state following oxidative stress could then be identified by their decreased reaction with IBTP and isolated by two-dimensional electrophoresis. We demonstrate the selectivity of IBTP labeling and use it to show that glutathione oxidation and exposure to an S-nitrosothiol or to peroxynitrite cause extensive redox changes to mitochondrial thiol proteins. In conjunction with blue native gel electrophoresis, we used IBTP labeling to demonstrate that thiols are exposed on the matrix faces of respiratory Complexes I, II, and IV. This novel approach enables measurement of the thiol redox state of individual mitochondrial proteins during oxidative stress and cell death. In addition the methodology has the potential to identify novel redox-dependent modulation of mitochondrial proteins.     

Yeast cells depleted in Atp14p fail to assemble Atp6p within the ATP synthase and exhibit altered mitochondrial cristae morphology

Within the yeast mitochondrial ATP synthase, subunit h is a small nuclear encoded protein belonging to the so-called "peripheral stalk" that connects the enzyme catalytic F(1) component to the mitochondrial inner membrane. This study examines the role of subunit h in ATP synthase function and assembly using a regulatable, doxycycline-repressible subunit h gene to overcome the strong instability of the mtDNA previously observed in strains lacking the native subunit h gene. Yeast cells expressing less than 3% of subunit h, but still containing intact mitochondrial genomes, grew poorly on respiratory substrates because of a major impairment of ATP synthesis originating from the ATP synthase, whereas the respiratory chain complexes were not affected. The lack of ATP synthesis in the subunit h-depleted (deltah) mitochondria was attributed to defects in the assembly/stability of the ATP synthase. A main feature of deltah-mitochondria was a very low content (<6%) in the mitochondrially encoded Atp6p subunit, an essential component of the enzyme proton channel, which was in large part because of a slowing down in translation. Interestingly, depletion of subunit h resulted in dramatic changes in mitochondrial cristae morphology, which further supports the existence of a link between the ATP synthase and the folding/biogenesis of the inner mitochondrial membrane.     

The F(1)F(0) ATP synthase and mitochondrial respiratory chain complexes are present on the plasma membrane of an osteosarcoma cell line: An immunocytochemical study

F(1)F(0) ATP synthase is ectopically expressed on the surface of several cell types, including endothelium and cancer cells. This study uses immunocytochemical detection methods via highly specific monoclonal antibodies to explore the possibility of plasma membrane localization of other mitochondrial proteins using an osteosarcoma cell line in which the location of the mitochondrial reticulum can be clearly traced by green fluorescent protein tagging of the organelle. We found that subunits of three of the four respiratory chain complexes were present on the surface of these cells. Additionally, we show for the first time that F(0) subunits d and OSCP of the ATP synthase are ectopically expressed. In all cases the OXPHOS proteins show a punctate distribution, consistent with data from proteome analysis of isolated lipid rafts that place the various mitochondrial proteins in plasma membrane microdomains. We also examined the cell surface for marker membrane proteins from several other intracellular organelles including ER, golgi and nuclear envelope. They were not found on the surface of the osteosarcoma cells. We conclude that mitochondrial membrane proteins are ectopically expressed, but not proteins from other cellular organelles. A specific mechanism by which the mitochondrion and plasma membrane fuse to deliver organellar proteins is suggested.     

Functional staging of ADP/ATP carrier translocation across the outer mitochondrial membrane.

The ADP/ATP carrier (AAC) is the major representative of the inner membrane carrier proteins of mitochondria that are synthesized without cleavable presequences. The characterization of the import pathway of AAC into mitochondria has mainly depended on an operational staging system. Here, we introduce two approaches for analyzing the import of AAC, blue native electrophoresis and folding-induced translocation arrest, that allow a functional staging of AAC transport across the outer membrane. (i) Blue native electrophoresis permits a direct monitoring of the receptor stage of AAC and its chase into mitochondria. Binding to this stage requires the receptor protein Tom70 but not Tom37 or Tom20. (ii) A fusion protein between AAC and dihydrofolate reductase can be selectively arrested in the general import pore complex of the outer membrane by ligand induced folding of the passenger protein. Cross-linking demonstrates that the arrested preprotein is in close contact not only with several receptors and Tim10 but also with the channel protein Tom40, providing the first direct evidence that cleavable preproteins and carrier preproteins interact with the same outer membrane channel. The staging system presented here permits a molecular dissection of AAC transport across the outer mitochondrial membrane, relates it to functional units of the translocases, and indicates a coordinated and successive cooperation of distinct translocase subcomplexes during transfer of the preprotein.