The import route of ADP/ATP carrier into mitochondria separates from the general import pathway of cleavable preproteins at the trans side of the outer membrane

The Mitochondrial Outer Membrane Protein Tom70-Mediator in Protein Traffic, Membrane Contact Sites and Innate Immunity

Tom70 is a versatile adaptor protein of 70 kDa anchored in the outer membrane of mitochondria in metazoa, fungi and amoeba. The tertiary structure was resolved for the Tom70 of yeast, showing 26 α-helices, most of them participating in the formation of 11 tetratricopeptide repeat (TPR) motifs. Tom70 serves as a docking site for cytosolic chaperone proteins and co-chaperones and is thereby involved in the uptake of newly synthesized chaperone-bound proteins in mitochondrial biogenesis. In yeast, Tom70 additionally mediates ER-mitochondria contacts via binding to sterol transporter Lam6/Ltc1. In mammalian cells, TOM70 promotes endoplasmic reticulum (ER) to mitochondria Ca2+ transfer by association with the inositol-1,4,5-triphosphate receptor type 3 (IP3R3). TOM70 is specifically targeted by the Bcl-2-related protein MCL-1 that acts as an anti-apoptotic protein in macrophages infected by intracellular pathogens, but also in many cancer cells. By participating in the recruitment of PINK1 and the E3 ubiquitin ligase Parkin, TOM70 can be implicated in the development of Parkinson's disease. TOM70 acts as receptor of the mitochondrial antiviral-signaling protein (MAVS) and thereby participates in the corresponding system of innate immunity against viral infections. The protein encoded by Orf9b in the genome of SARS-CoV-2 binds to TOM70, probably compromising the synthesis of type I interferons.

In vitro import and assembly of the nucleus-encoded mitochondrial subunit III of cytochrome c oxidase (Cox3)

The cox3 gene, encoding subunit III of cytochrome c oxidase (Cox3) is in mitochondrial genomes except in chlorophycean algae, where it is localized in the nucleus. Therefore, algae like Chlamydomonas reinhardtii, Polytomella sp. and Volvox carteri, synthesize the Cox3 polypeptide in the cytosol, import it into mitochondria, and integrate it into the cytochrome c oxidase complex. In this work, we followed the in vitro internalization of the Cox3 precursor by isolated, import-competent mitochondria of Polytomella sp. In this colorless alga, the precursor Cox3 protein is synthesized with a long, cleavable, N-terminal mitochondrial targeting sequence (MTS) of 98 residues. In an import time course, a transient Cox3 intermediate was identified, suggesting that the long MTS is processed more than once. The first processing step is sensitive to the metalo-protease inhibitor 1,10-ortophenantroline, suggesting that it is probably carried out by the matrix-located Mitochondrial Processing Protease. Cox3 is readily imported through an energy-dependent import pathway and integrated into the inner mitochondrial membrane, becoming resistant to carbonate extraction. Furthermore, the imported Cox3 protein was assembled into cytochrome c oxidase, as judged by the presence of a labeled band co-migrating with complex IV in Blue Native Electrophoresis. A model for the biogenesis of Cox3 in chlorophycean algae is proposed. This is the first time that the in vitro mitochondrial import of a cytosol-synthesized Cox3 subunit is described.

The Taz1p transacylase is imported and sorted into the outer mitochondrial membrane via a membrane anchor domain

Mutations in the mitochondrial transacylase tafazzin, Taz1p, in Saccharomyces cerevisiae cause Barth syndrome, a disease of defective cardiolipin remodeling. Taz1p is an interfacial membrane protein that localizes to both the outer and inner membranes, lining the intermembrane space. Pathogenic point mutations in Taz1p that alter import and membrane insertion result in accumulation of monolysocardiolipin. In this study, we used yeast as a model to investigate the biogenesis of Taz1p. We show that to achieve this unique topology in mitochondria, Taz1p follows a novel import pathway in which it crosses the outer membrane via the translocase of the outer membrane and then uses the Tim9p-Tim10p complex of the intermembrane space to insert into the mitochondrial outer membrane. Taz1p is then transported to membranes of an intermediate density to reach a location in the inner membrane. Moreover, a pathogenic mutation within the membrane anchor (V224R) alters Taz1p import so that it bypasses the Tim9p-Tim10p complex and interacts with the translocase of the inner membrane, TIM23, to reach the matrix. Critical targeting information for Taz1p resides in the membrane anchor and flanking sequences, which are often mutated in Barth syndrome patients. These studies suggest that altering the mitochondrial import pathway of Taz1p may be important in understanding the molecular basis of Barth syndrome.

A short C-terminal tail prevents mis-targeting of hydrophobic mitochondrial membrane proteins to the ER

Sdh3/Shh3, a subunit of mitochondrial succinate dehydrogenase, contains transmembrane domains with a hydrophobicity comparable to that of endoplasmic reticulum (ER) proteins. Here, we show that a C-terminal reporter fusion to Sdh3/Shh3 results in partial mis-targeting of the protein to the ER. This mis-targeting is mediated by the signal recognition particle (SRP) and depends on the length of the C-terminal tail. These results imply that if nuclear-encoded mitochondrial proteins contain strongly hydrophobic transmembrane domains and a long C-terminal tail, they have the potential to be recognized by SRP and mis-targeted to the ER.

Chloroplast β-barrel proteins are assembled into the mitochondrial outer membrane in a process that depends on the TOM and TOB complexes

Membrane-embedded β-barrel proteins are found in the outer membranes (OM) of Gram-negative bacteria, mitochondria and chloroplasts. In eukaryotic cells, precursors of these proteins are synthesized in the cytosol and have to be sorted to their corresponding organelle. Currently, the signal that ensures their specific targeting to either mitochondria or chloroplasts is ill-defined. To address this issue, we studied targeting of the chloroplast β-barrel proteins Oep37 and Oep24. We found that both proteins can be integrated in vitro into isolated plant mitochondria. Furthermore, upon their expression in yeast cells Oep37 and Oep24 were exclusively located in the mitochondrial OM. Oep37 partially complemented the growth phenotype of yeast cells lacking Porin, the general metabolite transporter of this membrane. Similarly to mitochondrial β-barrel proteins, Oep37 and Oep24 expressed in yeast cells were assembled into the mitochondrial OM in a pathway dependent on the TOM and TOB complexes. Taken together, this study demonstrates that the central mitochondrial components that mediate the import of yeast β-barrel proteins can deal with precursors of chloroplast β-barrel proteins. This implies that the mitochondrial import machinery does not recognize signals that are unique to mitochondrial β-barrel proteins. Our results further suggest that dedicated targeting factors had to evolve in plant cells to prevent mis-sorting of chloroplast β-barrel proteins to mitochondria.

Awaking TIM22, a Dynamic Ligand-gated Channel for Protein Insertion in the Mitochondrial Inner Membrane

Aqueous channels are at the core of the translocase of the outer membrane (TOM) and the translocase of the inner membrane for the transport of preproteins (TIM23), the translocases mediating the transport of proteins across the outer and inner mitochondrial membranes. Yet, the existence of a channel associated to the translocase of the inner membrane for the insertion of multitopic protein (TIM22) complex has been arguable, as its function relates to the insertion of multispanning proteins into the inner membrane. For the first time, we report conditions for detecting a channel activity associated to the TIM22 translocase in organelle, i.e. intact mitoplasts. An internal signal peptide in the intermembrane space of mitochondria is a requisite to inducing this channel, which is otherwise silent. The channel showed slightly cationic and high conductance activity of 1000 pS with a predominant half-open substate. Despite their different composition, the channels of the three mitochondrial translocases were thus remarkably similar, in agreement with their common task as pores transiently trapping proteins en route to their final destination. The opening of the TIM22 channel was a step-up process depending on the signal peptide concentration. Interestingly, low membrane potentials kept the channel fully open, providing a threshold level of the peptide is present. Our results portray TIM22 as a dynamic channel solely active in the presence of its cargo proteins. In its fully open conformation, favored by the combined action of internal signal peptide and low membrane potential, the channel could embrace the in-transit protein. As insertion progressed and initial interaction with the signal peptide faded, the channel would close, sustaining its role as a shunt that places trapped proteins into the membrane.

Preprotein Transport Machineries of Yeast Mitochondrial Outer Membrane Are not Required for Bax-induced Release of Intermembrane Space Proteins

The mitochondrial outer membrane contains protein import machineries, the translocase of the outer membrane (TOM) and the sorting and assembly machinery (SAM). It has been speculated that TOM or SAM are required for Bax-induced release of intermembrane space (IMS) proteins; however, experimental evidence has been scarce. We used isolated yeast mitochondria as a model system and report that Bax promoted an efficient release of soluble IMS proteins while preproteins were still imported, excluding an unspecific damage of mitochondria. Removal of import receptors by protease treatment did not inhibit the release of IMS proteins by Bax. Yeast mutants of each Tom receptor and the Tom40 channel were not impaired in Bax-induced protein release. We analyzed a large collection of mutants of mitochondrial outer membrane proteins, including SAM, fusion and fission components, but none of these components was required for Bax-induced protein release. The released proteins included complexes up to a size of 230 kDa. We conclude that Bax promotes efficient release of IMS proteins through the outer membrane of yeast mitochondria while the inner membrane remains intact. Inactivation of the known protein import and sorting machineries of the outer membrane does not impair the function of Bax at the mitochondria.

Mitochondrial Protein Sorting

The mitochondrial outer membrane contains two distinct machineries for protein import and protein sorting that function in a sequential manner: the general translocase of the outer membrane (TOM complex) and the sorting and assembly machinery (SAM complex), which is dedicated to beta-barrel proteins. The SAM(core) complex consists of three subunits, Sam35, Sam37, and Sam50, that can associate with a fourth subunit, the morphology component Mdm10, to form the SAM(holo) complex. Whereas the SAM(core) complex is required for the biogenesis of all beta-barrel proteins, Mdm10 and the SAM(holo) complex play a selective role in beta-barrel biogenesis by promoting assembly of Tom40 but not of porin. We report that Tom7, a conserved subunit of the TOM complex, functions in an antagonistic manner to Mdm10 in biogenesis of Tom40 and porin. We show that Tom7 promotes segregation of Mdm10 from the SAM(holo) complex into a low molecular mass form. Upon deletion of Tom7, the fraction of Mdm10 in the SAM(holo) complex is significantly increased, explaining the opposing functions of Tom7 and Mdm10 in beta-barrel sorting. Thus the role of Tom7 is not limited to the TOM complex. Tom7 functions in mitochondrial protein biogenesis by a new mechanism, segregation of a sorting component, leading to a differentiation of beta-barrel assembly.

Import of rat mitochondrial citrate carrier (CIC) at increasing salt concentrations promotes presequence binding to import receptor Tom20 and inhibits membrane translocation

Mitochondria contain a family of related carrier proteins that mediate transport of metabolites across the mitochondrial inner membrane. All members of this family are synthesized in the cytosol. We characterized the interactions of newly synthesized rat citrate carrier (CIC) precursor protein (pCIC) with the components of the mitochondrial protein import machinery. pCIC contains both a positively charged presequence of 13 amino acids and internal targeting sequences. We found that the pCIC presequence does not interfere with the import pathway and merely acts as an internal chaperone in the cytosol. Under conditions of increased ionic strength, the pCIC presequence binds to the import receptor Tom20 and accumulates at the mitochondrial surface, thereby delaying pCIC translocation across the mitochondrial outer membrane. Similarly, the presequence of the bovine phosphate carrier (PiC) precursor protein (pPiC) is arrested at the mitochondrial surface when salt concentrations are elevated. We conclude that presequences can only act as mediators of mitochondrial protein import if they allow rapid release from import receptor sites. Release from receptors sites may be rate-limiting in translocation.

A cryptic matrix targeting signal of the yeast ADP/ATP carrier normally inserted by the TIM22 complex is recognized by the TIM23 machinery

The yeast ADP/ATP carrier (AAC) is a mitochondrial protein that is targeted to the inner membrane via the TIM10 and TIM22 translocase complexes. AAC is devoid of a typical mitochondrial targeting signal and its targeting and insertion are thought to be guided by internal amino acid sequences. Here we show that AAC contains a cryptic matrix targeting signal that can target up to two thirds of the N-terminal part of the protein to the matrix. This event is coordinated by the TIM23 translocase and displays all the features of the matrix-targeting pathway. However, in the context of the whole protein, this signal is 'masked' and rendered non-functional as the polypeptide is targeted to the inner membrane via the TIM10 and TIM22 translocases. Our data suggest that after crossing the outer membrane the whole polypeptide chain of AAC is necessary to commit the precursor to the TIM22-mediated inner membrane insertion pathway.

The Phosphate Carrier Has an Ability to be Sorted to either the TIM22 Pathway or the TIM23 Pathway for Its Import into Yeast Mitochondria

Most mitochondrial proteins are synthesized in the cytosol, imported into mitochondria via the TOM40 (translocase of the mitochondrial outer membrane 40) complex, and follow several distinct sorting pathways to reach their destination submitochondrial compartments. Phosphate carrier (PiC) is an inner membrane protein with 6 transmembrane segments (TM1-TM6) and requires, after translocation across the outer membrane, the Tim9-Tim10 complex and the TIM22 complex to be inserted into the inner membrane. Here we analyzed an in vitro import of fusion proteins between various PiC segments and mouse dihydrofolate reductase. The fusion protein without TM1 and TM2 was translocated across the outer membrane but was not inserted into the inner membrane. The fusion proteins without TM1-TM4 were not inserted into the inner membrane but instead translocated across the inner membrane. Functional defects of Tim50 of the TIM23 complex caused either by depletion of the protein or the addition of anti-Tim50 antibodies blocked translocation of the fusion proteins without TM1-TM4 across the inner membrane, suggesting that lack of TM1-TM4 led to switch of its sorting pathway from the TIM22 pathway to the TIM23 pathway. PiC thus appears to have a latent signal for sorting to the TIM23 pathway, which is exposed by reduced interactions with the Tim9-Tim10 complex and maintenance of the import competence.

Biogenesis of the protein import channel Tom40 of the mitochondrial outer membrane: intermembrane space components are involved in an early stage of the assembly pathway

Tom40 forms the central channel of the preprotein translocase of the mitochondrial outer membrane (TOM complex). The precursor of Tom40 is encoded in the nucleus, synthesized in the cytosol, and imported into mitochondria via a multi-step assembly pathway that involves the mature TOM complex and the sorting and assembly machinery of the outer membrane (SAM complex). We report that opening of the mitochondrial intermembrane space by swelling blocks the assembly pathway of the beta-barrel protein Tom40. Mitochondria with defects in small Tim proteins of the intermembrane space are impaired in the Tom40 assembly pathway. Swelling as well as defects in the small Tim proteins inhibit an early stage of the Tom40 import pathway that is needed for formation of a Tom40-SAM intermediate. We propose that the biogenesis pathway of beta-barrel proteins of the outer mitochondrial membrane not only requires TOM and SAM components, but also involves components of the intermembrane space.

Mitochondria use different mechanisms for transport of multispanning membrane proteins through the intermembrane space

The mitochondrial inner membrane contains numerous multispanning integral proteins. The precursors of these hydrophobic proteins are synthesized in the cytosol and therefore have to cross the mitochondrial outer membrane and intermembrane space to reach the inner membrane. While the import pathways of noncleavable multispanning proteins, such as the metabolite carriers, have been characterized in detail by the generation of translocation intermediates, little is known about the mechanism by which cleavable preproteins of multispanning proteins, such as Oxa1, are transferred from the outer membrane to the inner membrane. We have identified a translocation intermediate of the Oxa1 preprotein in the translocase of the outer membrane (TOM) and found that there are differences from the import mechanisms of carrier proteins. The intermembrane space domain of the receptor Tom22 supports the stabilization of the Oxa1 intermediate. Transfer of the Oxa1 preprotein to the inner membrane is not affected by inactivation of the soluble TIM complexes. Both the inner membrane potential and matrix heat shock protein 70 are essential to release the preprotein from the TOM complex, suggesting a close functional cooperation of the TOM complex and the presequence translocase of the inner membrane. We conclude that mitochondria employ different mechanisms for translocation of multispanning proteins across the aqueous intermembrane space.

The delivery of ADP/ATP carrier protein to mitochondria probed by fusions with green fluorescent protein and ?-galactosidase

The import of proteins into mitochondria is an essential process, largely investigated in vitro with isolated mitochondria and radioactively labeled precursors. In this study, we used intact cells and fusions with genes encoding two reporter proteins, green fluorescent protein (GFP) and beta-galactosidase (lacZ), to probe the import of the ADP/ATP carrier (AAC). Typical mitochondrial fluorescence was observed with AAC-GFP fusions containing at least one complete transmembrane loop. This confirms the results of in vitro analysis demonstrating that an internal targeting signal was present in each one of the three transmembrane loops of the carrier. The fusions of AAC fragments to beta-galactosidase demonstrated that the targeting signal was capable of delivering the reporter molecule to the mitochondrial surface, but not to internalize it to a protease-inaccessible location. The delivery to a protease-inaccessible location required the presence of more distal sequences present within the third (C-terminal) transmembrane loop of the carrier molecule. The results of our study provide an alternative for investigation in a natural context of mitochondrial protein import in cells when the isolation of intact, functional mitochondria is not achievable.

The dynamic dimerization of the yeast ADP/ATP carrier in the inner mitochondrial membrane is affected by conserved cysteine residues

The ADP/ATP carrier (AAC) that facilitates the translocation of ATP made in mitochondria is inserted at the inner mitochondrial membrane by the TIM10-TIM22 protein import system. Here we addressed the state of the AAC precursor during insertion (stage IV of import) and identified residues of the carrier important for dimerization. By a combination of (i) import of a mix of His-tagged and untagged versions of AAC either 35S-labeled or unlabeled, (ii) import of a tandem covalent dimer AAC into wild-type mitochondria, and (iii) import of monomeric AAC into mitochondria expressing only the tandem covalent dimer AAC, we found that the stage IV intermediate is a monomer, and this stage is probably the rate-limiting step of insertion in the membrane. Subsequent dimerization occurs extremely rapidly (within less than a minute). The incoming monomer dimerizes with monomeric endogenous AAC suggesting that the AAC dimer is very dynamic. Conserved Cys residues were found not to affect insertion significantly, but they are crucial for the dimerization process to obtain a functional carrier.

Tom40, the import channel of the mitochondrial outer membrane, plays an active role in sorting imported proteins

The Translocase of the Outer Mitochondrial membrane (TOM complex) is centred on a channel, created by Tom40, serving as the only means of entry for proteins into the mitochondrion. Proteins destined for internal mitochondrial compartments interact subsequently with one of the two distinct protein Translocases of the Inner Mitochondrial membrane (TIM23 and TIM54 complexes) or follow specialized paths into the intermembrane space. To investigate the sorting of precursor proteins to these various sub-mitochondrial compartments, we created a library of tom40 mutants and screened for alleles selectively corrupt in protein sorting. One of the tom40 mutants, tom40-97, carries a single point mutation (W(243)R) resulting in an ineffective transfer of precursors to the TIM23 complex. There is no defect on transfer of precursors to the TIM54 complex or insertion of proteins into the outer membrane. The Tom40 channel is not a passive pore, but plays an active role in protein sorting for all sub-mitochondrial locations.

Trypanosoma brucei cytochrome c1 is imported into mitochondria along an unusual pathway

In most eukaryotic organisms, cytochrome c(1) is encoded in the nucleus, translated on cytosolic ribosomes, and directed to its final destination in the mitochondrial inner membrane by a bipartite, cleaved, amino-terminal presequence. However, in the kinetoplastids and euglenoids, the cytochrome c(1) protein has been shown to lack a cleaved presequence; a single methionine is removed from the amino terminus upon maturation, and the sequence upstream of the heme-binding site is generally shorter than that of the other eukaryotic homologs. We have used a newly developed mitochondrial protein import assay system from Trypanosoma brucei to demonstrate that the T. brucei cytochrome c(1) protein is imported along a non-conservative pathway similar to that described for the inner membrane carrier proteins of other organisms. This pathway requires external ATP and an external protein receptor but is not absolutely dependent on a membrane potential or on ATP hydrolysis in the mitochondrial matrix. We propose the cytochrome c(1) import in T. brucei is a two-step process first involving a membrane potential independent translocation across the outer mitochondrial membrane followed by heme attachment and a membrane potential-dependent insertion into the inner membrane.

Synthetic lethal interaction of the mitochondrial phosphatidylethanolamine biosynthetic machinery with the prohibitin complex of Saccharomyces cerevisiae

The majority of mitochondrial phosphatidylethanolamine (PtdEtn), a phospholipid essential for aerobic growth of yeast cells, is synthesized by phosphatidylserine decarboxylase 1 (Psd1p) in the inner mitochondrial membrane (IMM). To identify components that become essential when the level of mitochondrial PtdEtn is decreased, we screened for mutants that are synthetically lethal with a temperature-sensitive (ts) allele of PSD1. This screen unveiled mutations in PHB1 and PHB2 encoding the two subunits of the prohibitin complex, which is located to the IMM and required for the stability of mitochondrially encoded proteins. Deletion of PHB1 and PHB2 resulted in an increase of mitochondrial PtdEtn at 30 degrees C. On glucose media, phb1Delta psd1Delta and phb2Delta psd1Delta double mutants were rescued only for a limited number of generations by exogenous ethanolamine, indicating that a decrease of the PtdEtn level is detrimental for prohibitin mutants. Similar to phb mutants, deletion of PSD1 destabilizes polypeptides encoded by the mitochondrial genome. In a phb1Delta phb2Delta psd1(ts) strain the destabilizing effect is dramatically enhanced. In addition, the mitochondrial genome is lost in this triple mutant, and nuclear-encoded proteins of the IMM are assembled at a very low rate. At the nonpermissive temperature mitochondria of phb1Delta phb2Delta psd1(ts) were fragmented and aggregated. In conclusion, destabilizing effects triggered by low levels of mitochondrial PtdEtn seem to account for synthetic lethality of psd1Delta with phb mutants.

The novel presenilin-1-associated protein is a proapoptotic mitochondrial protein

Recent studies have suggested a possible role for presenilin proteins in apoptotic cell death observed in Alzheimer's disease. The mechanism by which presenilin proteins regulate apoptotic cell death is not well understood. Using the yeast two-hybrid system, we previously isolated a novel protein, presenilin-associated protein (PSAP) that specifically interacts with the C terminus of presenilin 1 (PS1), but not presenilin 2 (PS2). Here we report that PSAP is a mitochondrial resident protein sharing homology with mitochondrial carrier protein. PSAP was detected in a mitochondria-enriched fraction, and PSAP immunofluorescence was present in a punctate pattern that colocalized with a mitochondrial marker. More interestingly, overexpression of PSAP caused apoptotic death. PSAP-induced apoptosis was documented using multiple independent approaches, including membrane blebbing, chromosome condensation and fragmentation, DNA laddering, cleavage of the death substrate poly(ADP-ribose) polymerase, and flow cytometry. PSAP-induced cell death was accompanied by cytochrome c release from mitochondria and caspase-3 activation. Moreover, the general caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone, which blocked cell death, did not block the release of cytochrome c from mitochondria caused by overexpression of PSAP, indicating that PSAP-induced cytochrome c release was independent of caspase activity. The mitochondrial localization and proapoptotic activity of PSAP suggest that it is an important regulator of apoptosis.

The human mitochondrial ADP/ATP carriers: kinetic properties and biogenesis of wild-type and mutant proteins in the yeast S. cerevisiae

The mitochondrial adenine nucleotide carrier, or Ancp, plays a key role in the maintenance of the energetic fluxes in eukaryotic cells. Human disorders have been found associated to unusual human ANC gene (HANC) expression but also to direct inactivation of the protein, either by autoantibody binding or by mutation. However, the individual biochemical properties of the three HAncp isoforms have not yet been deciphered. To do so, the three HANC ORF were expressed in yeast under the control of the regulatory sequences of ScANC2. Each of the three HANC was able to restore growth on a nonfermentable carbon source of a yeast mutant strain lacking its three endogenous ANC. Their ADP/ATP exchange properties could then be measured for the first time in isolated mitochondria. HANC3 was the most efficient to restore yeast growth, and HAnc3p presented the highest V(M) (80 nmol ADP min(-1) mg protein(-1)) and K(ADP)(M)(8.4 microM). HAnc1p and HAnc2p presented similar kinetic constants (V(M) approximately 30-40 nmol ADP min(-(1) mg protein(-1) and K(ADP)(M) approximately 2.5-3.7 microM), whose values were consistent with HANC1's and HANC2's lower capacity to restore yeast growth. However, the HANC genes restored growth at a lower level than ScANC2, indicating that HAncp amount may be limiting in vivo. To optimize the HAncp production, we investigated their biogenesis into mitochondria by mutagenesis of two charged amino acids in the N-terminus of HAnc1p. Severe effects were observed with the D3A and D3K mutations that precluded yeast growth. On the contrary, the K10A mutation increased yeast growth complementation and nucleotide exchange rate as compared to the wild type. These results point to the importance of the N-terminal region of HAnc1p for its biogenesis and transport activity in yeast mitochondria.

Characterization of rat TOM70 as a receptor of the preprotein translocase of the mitochondrial outer membrane

We cloned a ∼70 kDa rat mitochondrial outer membrane protein (OM70)with a sequence identity of 28.1% and 20.1% with N. crassa and S. cerevisiae Tom70, respectively. Even with this low sequence identity,however, the proteins share a remarkable structural similarity: they have 7-10 tetratricopeptide repeat motifs and are anchored to the outer membrane through the N-terminal transmembrane domain with the bulk portion located in the cytosol. Antibodies against OM70 inhibited import of preproteins, such as the ADP/ATP carrier and rTOM40, that use internal targeting signals but not the import of cleavable presequence-containing preproteins. Blue native gel electrophoresis and immunoprecipitation of digitoninsolubilized mitochondrial outer membranes revealed that OM70 was loosely associated with the ∼400 kDa translocase complex of the mitochondrial outer membrane, which contains rTOM22 and rTOM40. A yeast two-hybrid system demonstrated that OM70 interacted with rTOM20 and rTOM22 through the cytoplasmic domains. Thus, OM70 is a functional homologue of fungal Tom70 and functions as a receptor of the preprotein import machinery of the rat mitochondrial outer membrane. Furthermore, the N-terminal 66 residue region of OM70, which comprises a hydrophilic 41 residue N-terminal domain, a 22 residue transmembrane domain and three arginine residues, is sufficient to act as a mitochondria-targeting signal, and the arginine cluster is crucial for this function.

Characterization of a human import component of the mitochondrial outer membrane, TOMM70A

Functional mitochondria require up to 1000 proteins to function properly, with 99% synthesized as precursors in the cytoplasm and transported into the mitochondria with the aid of cytosolic chaperones and mitochondrial translocators (import components). Proteins to be imported are chaperoned to the mitochondria by the cytosolic heat shock protein (cHSP70) and are immediately pursued by Translocators of the Outer Membrane (TOMs), followed by transient interactions of the unfolded proteins with Translocators of the Inner Membrane (TIMs). In the present study, we describe a human gene, TOMM70A, orthologous to the yeast Tom70 import component. TOMM70A is ubiquitously expressed in human tissues, maps on chromosome 3q13.1-q13.2 and consists of 12 coding exons spanning over 37 kb. TOMM70A localizes in the mitochondria of COS-7 cells, and in organello import assays confirmed its presence in the Outer Mitochondrial membrane (OM) of rat liver mitochondria. TOMM70A could play a significant role in the import of nuclear-encoded mitochondrial proteins with internal targeting sites such as ADP/ATP carriers and the uncoupling proteins.

Biogenesis of the dicarboxylate carrier (DIC): translocation across the mitochondrial outer membrane and subsequent release from the TOM channel are membrane potential-independent

The mitochondrial inner membrane of Saccharomyces cerevisiae contains a group of homologous carrier proteins that mediate the exchange of several metabolites. The members of this protein family are synthesized in the cytosol and reach their final topology after translocation across the mitochondrial outer membrane. Using the ADP/ATP carrier (AAC) as a model protein, previous studies have established four distinct steps of the import pathway (stages I-IV). In the absence of the mitochondrial membrane potential (deltapsi), the AAC accumulates at the inner surface of the outer membrane (stage IIIa) and remains bound to the outer membrane import channel. Only in the presence of the membrane potential, can a complex of small Tim proteins mediate transfer of the AAC to the inner membrane. In this study, we characterized the import pathway of the dicarboxylate carrier (DIC). Different from the AAC, the DIC showed complete deltapsi-independent translocation across the outer membrane, release from the import pore, and mainly accumulated in a soluble state in the intermembrane space, thus defining a new translocation intermediate (stage III*). The DIC should be a suitable model protein for the characterization of deltapsi-independent functions of the intermembrane space Tim proteins.

Versatility of the mitochondrial protein import machinery

The vast majority of mitochondrial proteins are synthesized in the cytosol and are imported into mitochondria by protein machineries located in the mitochondrial membranes. It has become clear that hydrophilic as well as hydrophobic preproteins use a common translocase in the outer mitochondrial membrane, but diverge to two distinct translocases in the inner membrane. The translocases are dynamic, high-molecular-weight complexes that have to provide specific means for the recognition of preproteins, channel formation and generation of import-driving forces.

The external calcium-dependent NADPH dehydrogenase from Neurospora crassa mitochondria

We have inactivated the nuclear gene coding for a putative NAD(P)H dehydrogenase from the inner membrane of Neurospora crassa mitochondria by repeat-induced point mutations. The respiratory rates of mitochondria from the resulting mutant (nde-1) were measured, using NADH or NADPH as substrates under different assay conditions. The results showed that the mutant lacks an external calcium-dependent NADPH dehydrogenase. The observation of NADH and NADPH oxidation by intact mitochondria from the nde-1 mutant suggests the existence of a second external NAD(P)H dehydrogenase. The topology of the NDE1 protein was further studied by protease accessibility, in vitro import experiments, and in silico analysis of the amino acid sequence. Taken together, it appears that most of the NDE1 protein extends into the intermembrane space in a tightly folded conformation and that it remains anchored to the inner mitochondrial membrane by an N-terminal transmembrane domain.

The protein import machinery of the mitochondrial membranes

Mitochondria are surrounded by two membranes that contain independent and non-related protein transport machineries. Remarkable progress was recently achieved in elucidating the structure of the outer membrane import channel and in the identification of new components involved in protein traffic across the intermembrane space and the inner membrane. Traditional concepts of protein targeting and sorting had to be revised. Here we briefly summarize the data on the mitochondrial protein import system with particular emphasis on new developments and perspectives.

The cytotoxic action of Bax on yeast cells does not require mitochondrial ADP/ATP carrier but may be related to its import to the mitochondria

The effect of the expression of murine Bax protein on growth and vitality was examined in Saccharomyces cerevisiae and compared with the effect of Bax in mutant cells lacking functional mitochondria. The cytotoxic effect of Bax on yeast does not require functional oxidative phosphorylation, respiration, or mitochondrial proteins (ADP/ATP carriers) implicated in the formation of the permeability transition pore in mammalian mitochondria. In the wild type S. cerevisiae the expression of Bax does not result in a severe effect on mitochondrial membrane potential and respiration. On the basis of Bax induced differences in the fluorescence of green fluorescent protein fused to mitochondrial proteins, it is proposed that Bax may interfere with one essential cellular process in yeast: the mitochondrial protein import pathway that is specific for the proteins of the mitochondrial carrier family.

Tim18p, a new subunit of the TIM22 complex that mediates insertion of imported proteins into the yeast mitochondrial inner membrane

Import of carrier proteins from the cytoplasm into the mitochondrial inner membrane of yeast is mediated by a distinct system consisting of two soluble 70-kDa protein complexes in the intermembrane space and a 300-kDa complex in the inner membrane, the TIM22 complex. The TIM22 complex contains the peripheral subunits Tim9p, Tim10p, and Tim12p and the integral membrane subunits Tim22p and Tim54p. We identify here an additional subunit, an 18-kDa integral membrane protein termed Tim18p. This protein is made as a 21.9-kDa precursor which is imported into mitochondria and processed to its mature form. When mitochondria are gently solubilized, Tim18p comigrates with the other subunits of the TIM22 complex on nondenaturing gels and is coimmunoprecipitated with Tim54p and Tim12p. Tim18p does not cofractionate with the TIM23 complex upon immunoprecipitation or nondenaturing gel electrophoresis. Deletion of Tim18p decreases the growth rate of yeast cells by a factor of two and is synthetically lethal with temperature-sensitive mutations in Tim9p or Tim10p. It also impairs the import of several precursor proteins into isolated mitochondria, and lowers the apparent mass of the TIM22 complex. We suggest that Tim18p functions in the assembly and stabilization of the TIM22 complex but does not directly participate in protein insertion into the inner membrane.

Mechanisms of protein translocation into mitochondria

Mitochondrial biogenesis utilizes a complex proteinaceous machinery for the import of cytosolically synthesized preproteins. At least three large multisubunit protein complexes, one in the outer membrane and two in the inner membrane, have been identified. These translocase complexes cooperate with soluble proteins from the cytosol, the intermembrane space and the matrix. The translocation of presequence-containing preproteins through the outer membrane channel includes successive electrostatic interactions of the charged mitochondrial targeting sequence with a chain of import components. Translocation across the inner mitochondrial membrane utilizes the energy of the proton motive force of the inner membrane and the hydrolysis of ATP. The matrix chaperone system of the mitochondrial heat shock protein 70 forms an ATP-dependent import motor by interaction with the polypeptide chain in transit and components of the inner membrane translocase. The precursors of integral inner membrane proteins of the metabolite carrier family interact with newly identified import components of the intermembrane space and are inserted into the inner membrane by a second translocase complex. A comparison of the full set of import components between the yeast Sacccharomyces cerevisiae and the nematode Caenorhabditis elegans demonstrates an evolutionary conservation of most components of the mitochondrial import machinery with a possible greater divergence for the import pathway of the inner membrane carrier proteins.

Import of carrier proteins into mitochondria

Carrier proteins located in the inner membrane of mitochondria are responsible for the exchange of metabolites between the intermembrane space and the matrix of this organelle. All members of this family are nuclear-encoded and depend on translocation machineries for their import into mitochondria. Recently many new translocation components responsible for the import of carrier proteins were identified. It is now possible to describe a detailed import pathway for this class of proteins. This review highlights the contribution made by translocation components to the process of carrier protein import into mitochondria.

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.

Biogenesis of Tim proteins of the mitochondrial carrier import pathway: differential targeting mechanisms and crossing over with the main import pathway

Two major routes of preprotein targeting into mitochondria are known. Preproteins carrying amino-terminal signals mainly use Tom20, the general import pore (GIP) complex and the Tim23-Tim17 complex. Preproteins with internal signals such as inner membrane carriers use Tom70, the GIP complex, and the special Tim pathway, involving small Tims of the intermembrane space and Tim22-Tim54 of the inner membrane. Little is known about the biogenesis and assembly of the Tim proteins of this carrier pathway. We report that import of the preprotein of Tim22 requires Tom20, although it uses the carrier Tim route. In contrast, the preprotein of Tim54 mainly uses Tom70, yet it follows the Tim23-Tim17 pathway. The positively charged amino-terminal region of Tim54 is required for membrane translocation but not for targeting to Tom70. In addition, we identify two novel homologues of the small Tim proteins and show that targeting of the small Tims follows a third new route where surface receptors are dispensable, yet Tom5 of the GIP complex is crucial. We conclude that the biogenesis of Tim proteins of the carrier pathway cannot be described by either one of the two major import routes, but involves new types of import pathways composed of various features of the hitherto known routes, including crossing over at the level of the GIP.

Transport of the ADP/ATP carrier of mitochondria from the TOM complex to the TIM22.54 complex

Members of the mitochondrial carrier family such as the ADP/ATP carrier (AAC) are composed of three structurally related modules. Here we show that each of the modules contains a mitochondrial import signal recognized by Tim10 and Tim12 in the intermembrane space. The first and the second module are translocated across the outer membrane independently of the membrane potential, DeltaDeltapsipsi, but they are not inserted into the inner membrane. The third module interacts tightly with the TOM complex and thereby prevents complete translocation of the precursor across the outer membrane. At this stage, binding of a TIM9.10 complex confers a topology to the translocation intermediate which reflects the modular structure of the AAC. The precursor is then transferred to the TIM9.10.12 complex, still interacting with the TOM complex. Release of the precursor from the TOM complex and insertion into the inner membrane by the TIM22.54 complex requires a DeltaDeltapsipsi-responsive signal in the third module.

The preprotein translocase of the mitochondrial inner membrane: function and evolution

Growing mitochondria acquire most of their proteins by the uptake of mitochondrial preproteins from the cytosol. To mediate this protein import, both mitochondrial membranes contain independent protein transport systems: the Tom machinery in the outer membrane and the Tim machinery in the inner membrane. Transport of proteins across the inner membrane and sorting to the different inner mitochondrial compartments is mediated by several protein complexes which have been identified in the past years. A complex containing the integral membrane proteins Tim17 and Tim23 constitutes the import channel for preproteins containing amino-terminal hydrophilic presequences. This complex is associated with Tim44 which serves as an adaptor protein for the binding of mtHsp70 to the membrane. mtHsp70, a 70 kDa heat shock protein of the mitochondrial matrix, drives the ATP-dependent import reaction of the processed preprotein after cleavage of the presequence. Preproteins containing internal targeting information are imported by a separate import machinery, which consists of the intermembrane-space proteins Tim9, Tim10, and Tim12, and the inner membrane proteins Tim22 and Tim54. The proteins Tim17, Tim22, and Tim23 have in common a similar topology in the membrane and a homologous amino acid sequence. Moreover, they show a sequence similarity to OEP16, a channel-forming amino acid transporter in the outer envelope of chloroplasts, and to LivH, a component of a prokaryotic amino acid permease, defining a new PRAT-family of preprotein and amino acid transporters.

Tim9, a new component of the TIM22.54 translocase in mitochondria

We have identified Tim9, a new component of the TIM22.54 import machinery, which mediates transport of proteins into the inner membrane of mitochondria. Tim9, an essential protein of Saccharomyces cerevisiae, shares sequence similarity with Tim10 and Tim12. Tim9 is located in the mitochondrial intermembrane space and is organized into two distinct hetero-oligomeric assemblies with Tim10 and Tim12. One complex contains Tim9 and Tim10. The other complex contains Tim9, Tim10 and Tim12 and is tightly associated with Tim22 in the inner membrane. The TIM9.10 complex is more abundant than the TIM9.10.12 complex and mediates partial translocation of mitochondrial carriers proteins across the outer membrane. The TIM9.10.12 complex assists further translocation into the inner membrane in association with TIM22.54.

Tim9p, an essential partner subunit of Tim10p for the import of mitochondrial carrier proteins

Tim10p, a protein of the yeast mitochondrial intermembrane space, was shown previously to be essential for the import of multispanning carrier proteins from the cytoplasm into the inner membrane. We now identify Tim9p, another essential component of this import pathway. Most of Tim9p is associated with Tim10p in a soluble 70 kDa complex. Tim9p and Tim10p co-purify in successive chromatographic fractionations and co-immunoprecipitated with each other. Tim9p can be cross-linked to a partly translocated carrier protein. A small fraction of Tim9p is bound to the outer face of the inner membrane in a 300 kDa complex whose other subunits include Tim54p, Tim22p, Tim12p and Tim10p. The sequence of Tim9p is 25% identical to that of Tim10p and Tim12p. A Ser67-->Cys67 mutation in Tim9p suppresses the temperature-sensitive growth defect of tim10-1 and tim12-1 mutants. Tim9p is a new subunit of the TIM machinery that guides hydrophobic inner membrane proteins across the aqueous intermembrane space.

Tim23p contains separate and distinct signals for targeting to mitochondria and insertion into the inner membrane

The Tim23 protein is an essential inner membrane (IM) component of the yeast mitochondrial protein import pathway. Tim23p does not carry an amino-terminal presequence; therefore, the targeting information resides within the mature protein. Tim23p is anchored in the IM via four transmembrane segments and has two positively charged loops facing the matrix. To identify the import signal for Tim23p, we have constructed several altered versions of the Tim23 protein and examined their function and import in yeast cells, as well as their import into isolated mitochondria. We replaced the positively charged amino acids in one or both loops with alanine residues and found that the positive charges are not required for import into mitochondria, but at least one positively charged loop is required for insertion into the IM. Furthermore, we find that the signal to target Tim23p to mitochondria is carried in at least two of the hydrophobic transmembrane segments. Our results suggest that Tim23p contains separate import signals: hydrophobic segments for targeting Tim23p to mitochondria, and positively charged loops for insertion into the IM. We therefore propose that Tim23p is imported into mitochondria in at least two distinct steps.