Sequential translocation of an artificial precursor protein across the two mitochondrial membranes.

We have constructed a chimeric mitochondrial precursor protein consisting of a mutant bovine pancreatic trypsin inhibitor coupled to the C terminus of a purified artificial precursor protein. This construct fails to complete its import into isolated mitochondria and becomes stuck across sites of close contact between the two mitochondrial membranes. When the mitochondria are then depleted of ATP and the intramolecular disulfide bridges of the trypsin inhibitor are cleaved by dithiothreitol, the trypsin inhibitor moiety is transported across the outer membrane into the intermembrane space. This translocation intermediate can be chased across the inner membrane by restoring the ATP levels in the matrix. These results show that translocation of pancreatic trypsin inhibitor across a biological membrane is prevented by its intramolecular disulfide bridges, that import into the matrix involves two distinct translocation system operating in tandem, and that ATP is required for protein translocation across the inner but not the outer membrane.     

Transport of proteins into yeast mitochondria

The amino-terminal sequences of several imported mitochondrial precursor proteins have been shown to contain all the information required for transport to and sorting within mitochondria. Proteins transported into the matrix contain a matrix-targeting sequence. Proteins destined for other submitochondrial compartments contain, in addition, an intramitochondrial sorting sequence. The sorting sequence in the cytochrome c1 presequence is a stop-transport sequence for the inner mitochondrial membrane. Proteins containing cleavable presequences can reach the intermembrane space by either of two pathways: (1) Part of the presequence is transported into the matrix; the attached protein, however, is transported across the outer but not the inner membrane (eg, the cytochrome c1 presequence). (2) The precursor is first transported into the matrix; part of the presequence is then removed, and the protein is reexported across the inner membrane (eg, the precursor of the iron-sulphur protein of the cytochrome bc1 complex). Matrix-targeting sequences lack primary amino acid sequence homology, but they share structural characteristics. Many DNA sequences in a genome can potentially encode a matrix-targeting sequence. These sequences become active if positioned upstream of a protein coding sequence. Artificial matrix-targeting sequences include synthetic presequences consisting of only a few different amino acids, a known amphiphilic helix found inside a cytosolic protein, and the presequence of an imported chloroplast protein. Transport of proteins across mitochrondrial membranes requires a membrane potential, ATP, and a 45-kd protein of the mitochondrial outer membrane. The ATP requirement for import is correlated with a stable structure in the imported precursor molecule. We suggest that transmembrane transport of a stably folded precursor requires an ATP-dependent unfolding of the precursor protein.     

Disrupted yeast mitochondria can import precursor proteins directly through their inner membrane

Import of precursor proteins into the yeast mitochondrial matrix can occur directly across the inner membrane. First, disruption of the outer membrane restores protein import to mitochondria whose normal import sites have been blocked by an antibody against the outer membrane or by a chimeric, incompletely translocated precursor protein. Second, a potential- and ATP-dependent import of authentic or artificial precursor proteins is observed with purified inner membrane vesicles virtually free of outer membrane components. Third, import into purified inner membrane vesicles is insensitive to antibody against the outer membrane. Thus, while outer membrane components are clearly required in vivo, the inner membrane contains a complete protein translocation system that can operate by itself if the outer membrane barrier is removed.     

Targeting of proteins to the outer envelope membrane uses a different pathway than transport into chloroplasts.

The chloroplastic envelope is composed of two membranes, inner and outer, each with a distinct set of polypeptides. Like proteins in other chloroplastic compartments, most envelope proteins are synthesized in the cytosol and post-translationally imported into chloroplasts. Considerable knowledge has been obtained concerning protein import proteins. We isolated a cDNA clone from pea that encodes a 14-kilodalton outer envelope membrane protein. The precursor form of this protein does not possess a cleavable transit peptide and its import into isolated chloroplasts does not require either ATP or a thermolysin-sensitive component on the chloroplastic surface. These findings, together with similar observations made with a spinach chloroplastic outer membrane protein, led us to propose that proteins destined for the outer membrane of the chloroplastic envelope follow an import pathway distinct from that followed by proteins destined for other chloroplastic compartments.     

Protein import into yeast mitochondria: the inner membrane import site protein ISP45 is the MPI1 gene product.

Protein import across both mitochondrial membranes is mediated by the cooperation of two distinct protein transport systems, one in the outer and the other in the inner membrane. Previously we described a 45 kDa yeast mitochondrial inner membrane protein (ISP45) that can be cross-linked to a partially translocated precursor protein (Scherer et al., 1992). We have now purified ISP45 to homogeneity and identified it as the product of the nuclear MPI1 gene. Identity of ISP45 with the MPI1 gene product was shown by microsequencing of three tryptic ISP45 peptides and by demonstrating that an antibody against an Mpi1p-beta-galactosidase fusion protein specifically recognizes ISP45. Antibodies monospecific for ISP45 inhibited protein import into right-side-out mitochondrial inner membrane vesicles, but not into intact mitochondria. On solubilizing mitochondria, ISP45 was rapidly converted to a 40 kDa proteolytic fragment unless mitochondria were first denatured with trichloroacetic acid. The combined genetic and biochemical evidence identifies ISP45/Mpi1p as a component of the protein import system of the yeast mitochondrial inner membrane.     

A new chloroplast protein import intermediate reveals distinct translocation machineries in the two envelope membranes: energetics and mechanistic implications

Chloroplast protein import presents a complex membrane traversal problem: precursor proteins must cross two envelope membranes to reach the stromal compartment. This work characterizes a new chloroplast protein import intermediate which has completely traversed the outer envelope membrane but has not yet reached the stroma. The existence of this intermediate demonstrates that distinct protein transport machineries are present in both envelope membranes, and that they are able to operate independently of one another under certain conditions. Energetic characterization of this pathway led to the identification of three independent energy-requiring steps: binding of the precursor to the outer envelope membrane, outer membrane transport, and inner membrane transport. Localization of the sites of energy utilization for each of these steps, as well as their respective nucleotide specificities, suggest that three different ATPases mediate chloroplast envelope transport.     

How mitochondria import hydrophilic and hydrophobic proteins

Most mitochondrial proteins are nuclear encoded and have to be transported into the organelle after synthesis on cytosolic ribosomes. Three multimeric protein complexes have been identified that import precursor proteins destined for the mitochondria: the TOM complex in the outer membrane and two TIM complexes in the inner membrane. Recent work has provided a detailed view of the different mechanisms operating during the import of the two major classes of mitochondrial proteins--hydrophilic proteins with cleavable presequences and hydrophobic proteins with multiple internal signals.     

Functional definition of outer membrane proteins involved in preprotein import into mitochondria

The role of plant mitochondrial outer membrane proteins in the process of preprotein import was investigated, as some of the principal components characterized in yeast have been shown to be absent or evolutionarily distinct in plants. Three outer membrane proteins of Arabidopsis thaliana mitochondria were studied: TOM20 (translocase of the outer mitochondrial membrane), METAXIN, and mtOM64 (outer mitochondrial membrane protein of 64 kD). A single functional Arabidopsis TOM20 gene is sufficient to produce a normal multisubunit translocase of the outer membrane complex. Simultaneous inactivation of two of the three TOM20 genes changed the rate of import for some precursor proteins, revealing limited isoform subfunctionalization. Inactivation of all three TOM20 genes resulted in severely reduced rates of import for some but not all precursor proteins. The outer membrane protein METAXIN was characterized to play a role in the import of mitochondrial precursor proteins and likely plays a role in the assembly of beta-barrel proteins into the outer membrane. An outer mitochondrial membrane protein of 64 kD (mtOM64) with high sequence similarity to a chloroplast import receptor was shown to interact with a variety of precursor proteins. All three proteins have domains exposed to the cytosol and interacted with a variety of precursor proteins, as determined by pull-down and yeast two-hybrid interaction assays. Furthermore, inactivation of one resulted in protein abundance changes in the others, suggesting functional redundancy. Thus, it is proposed that all three components directly interact with precursor proteins to participate in early stages of mitochondrial protein import.     

Protein translocation pathways of the mitochondrion

The biogenesis of mitochondria depends on the coordinated import of precursor proteins from the cytosol coupled with the export of mitochondrially coded proteins from the matrix to the inner membrane. The mitochondria contain an elaborate network of protein translocases in the outer and inner membrane along with a battery of chaperones and processing enzymes in the matrix and intermembrane space to mediate protein translocation. A mitochondrial protein, often with an amino-terminal targeting sequence, is escorted through the cytosol by chaperones to the TOM complex (translocase of the outer membrane). After crossing the outer membrane, the import pathway diverges; however, one of two TIM complexes (translocase of inner membrane) is generally utilized. This review is focused on the later stages of protein import after the outer membrane has been crossed. An accompanying paper by Lithgow reviews the early stages of protein translocation.     

Biogenesis of porin of the outer mitochondrial membrane involves an import pathway via receptors and the general import pore of the TOM complex

Porin, also termed the voltage-dependent anion channel, is the most abundant protein of the mitochondrial outer membrane. The process of import and assembly of the protein is known to be dependent on the surface receptor Tom20, but the requirement for other mitochondrial proteins remains controversial. We have used mitochondria from Neurospora crassa and Saccharomyces cerevisiae to analyze the import pathway of porin. Import of porin into isolated mitochondria in which the outer membrane has been opened is inhibited despite similar levels of Tom20 as in intact mitochondria. A matrix-destined precursor and the porin precursor compete for the same translocation sites in both normal mitochondria and mitochondria whose surface receptors have been removed, suggesting that both precursors utilize the general import pore. Using an assay established to monitor the assembly of in vitro-imported porin into preexisting porin complexes we have shown that besides Tom20, the biogenesis of porin depends on the central receptor Tom22, as well as Tom5 and Tom7 of the general import pore complex (translocase of the outer mitochondrial membrane [TOM] core complex). The characterization of two new mutant alleles of the essential pore protein Tom40 demonstrates that the import of porin also requires a functional Tom40. Moreover, the porin precursor can be cross-linked to Tom20, Tom22, and Tom40 on its import pathway. We conclude that import of porin does not proceed through the action of Tom20 alone, but requires an intact outer membrane and involves at least four more subunits of the TOM machinery, including the general import pore.     

Identification of intermediates in the pathway of protein import into chloroplasts and their localization to envelope contact sites

We have used a hybrid precursor protein to study the pathway of protein import into chloroplasts. This hybrid (pS/protA) consists of the precursor to the small subunit of Rubisco (pS) fused to the IgG binding domains of staphylococcal protein A. The pS/protA is efficiently imported into isolated chloroplasts and is processed to its mature form (S/protA). In addition to the mature stromal form, two intermediates in the pathway of pS/protA import were identified at early time points in the import reaction. The first intermediate represents unprocessed pS/protA bound to the outer surface of the chloroplast envelope and is analogous to a previously characterized form of pS that is specifically bound to the chloroplast surface and can be subsequently translocated in the stroma (Cline, K., M. Werner-Washburne, T. H. Lubben, and K. Keegstra. 1985. J. Biol. Chem. 260:3691-3696.) The second intermediate represents a partially translocated form of the precursor that remains associated with the envelope membrane. This form is processed to mature S/protA, but remains susceptible to exogenously added protease in intact chloroplasts. We conclude that the envelope associated S/protA is spanning both the outer and inner chloroplast membranes en route to the stroma. Biochemical and immunochemical localization of the two translocation intermediates indicates that both forms are exposed at the surface of the outer membrane at sites where the outer and inner membrane are closely apposed. These contact zones appear to be organized in a reticular network on the outer envelope. We propose a model for protein import into chloroplasts that has as its central features two distinct protein conducting channels in the outer and inner envelope membranes, each gated open by a distinct subdomain of the pS signal sequence.     

Coupling of protein synthesis and mitochondrial import in a homologous yeast in vitro system.

We made use of a homologous cell-free mitochondrial protein import system derived from the yeast Saccharomyces cerevisiae to investigate the coupling of protein synthesis and import. Mitochondrial precursor proteins were synthesized in a yeast lysate either in the presence or absence of isolated yeast mitochondria. We were, therefore, able to analyze protein import into mitochondria either in a strictly posttranslational reaction (when isolated mitochondria were added only after protein synthesis has been arrested by the addition of cycloheximide) or in a reaction in which synthesis and import were permitted to occur simultaneously. We found that the import of a precursor protein consisting of the amino-terminal mitochondrial targeting sequence of cytochrome oxidase subunit IV fused to mouse dihydrofolate reductase is very inefficient in a strictly posttranslational reaction, whereas efficient import is observed if precursor synthesis and import are coupled. The same result was obtained when we analyzed the import of bulk endogenous yeast mitochondrial proteins in this system. Finally, we found that the insertion of the yeast outer membrane protein porin is also several times more efficient when synthesis and insertion are coupled.     

Protein import into mitochondria: the requirement for external ATP is precursor-specific whereas intramitochondrial ATP is universally needed for translocation into the matrix.

ATP is needed for the import of precursor proteins into mitochondria. However, the role of ATP and its site of action have been unclear. We have now investigated the ATP requirements for protein import into the mitochondrial matrix. These experiments employed an in vitro system that allowed ATP levels to be manipulated both inside and outside the mitochondrial inner membrane. Our results indicate that there are two distinct ATP requirements for mitochondrial protein import. ATP in the matrix is always needed for complete import of precursor proteins into this compartment, even when the precursors are presented to mitochondria in an unfolded conformation. In contrast, the requirement for external ATP is precursor-specific; depletion of external ATP strongly inhibits import of some precursors but has little or no effect with other precursors. A requirement for external ATP can often be overcome by denaturing the precursor with urea. We suggest that external ATP promotes the release of precursors from cytosolic chaperones, whereas matrix ATP drives protein translocation across the inner membrane.     

Two Modular Forms of the Mitochondrial Sorting and Assembly Machinery Are Involved in Biogenesis of α-Helical Outer Membrane Proteins

The mitochondrial outer membrane contains two translocase machineries for precursor proteins—the translocase of the outer membrane (TOM complex) and the sorting and assembly machinery (SAM complex). The TOM complex functions as the main mitochondrial entry gate for nuclear-encoded proteins, whereas the SAM complex was identified according to its function in the biogenesis of β-barrel proteins of the outer membrane. The SAM complex is required for the assembly of precursors of the TOM complex, including not only the β-barrel protein Tom40 but also a subset of α-helical subunits. While the interaction of β-barrel proteins with the SAM complex has been studied in detail, little is known about the interaction between the SAM complex and α-helical precursor proteins. We report that the SAM is not static but that the SAM core complex can associate with different partner proteins to form two large SAM complexes with different functions in the biogenesis of α-helical Tom proteins. We found that a subcomplex of TOM, Tom5-Tom40, associates with the SAM core complex to form a new large SAM complex. This SAM-Tom5/Tom40 complex binds the α-helical precursor of Tom6 after the precursor has been inserted into the outer membrane in an Mim1 (mitochondrial import protein 1)-dependent manner. The second large SAM complex, SAM-Mdm10 (mitochondrial distribution and morphology protein), binds the α-helical precursor of Tom22 and promotes its membrane integration. We suggest that the modular composition of the SAM complex provides a flexible platform to integrate the sorting pathways of different precursor proteins and to promote their assembly into oligomeric complexes.     

The influence of depletion of voltage dependent anion selective channel on protein import into the yeast Saccharomyces cerevisiae mitochondria

The supply of substrates to the respiratory chain as well as of other metabolites (e.g. ATP) into inner compartments of mitochondria is crucial to preprotein import into these organelles. Transport of the compounds across the outer mitochondrial membrane is enabled by mitochondrial porin, also known as the voltage-dependent anion-selective channel (VDAC). Our previous studies led to the conclusion that the transport of metabolites through the outer membrane of the yeast Saccharomyces cerevisiae mitochondria missing VDAC (now termed YVDAC1) is considerably restricted. Therefore we expected that depletion of YVDAC1 should also hamper protein import into the mutant mitochondria. We report here that YVDAC1-depleted mitochondria are able to import a fusion protein termed pSu9-DHFR in the amount comparable to that of wild type mitochondria, although over a considerably longer time. The rate of import of the fusion protein into YVDAC1-depleted mitochondria is dis- tinctly lower than into wild type mitochondria probably due to restricted ATP access to the intermembrane space and is additionally influenced by the way the supporting respiratory substrates are transported through the outer membrane. In the presence of ethanol, diffusing freely through lipid membranes, YVDAC1-depleted mitochondria are able to import the fusion protein at a higher rate than in the presence of external NADH which is, like ATP, transported through the outer membrane by facilitated diffusion. It has been shown that transport of external NADH across the outer membrane of YVDAC1-depleted mitochondria is supported by the protein import machinery, i.e. the TOM complex (Kmita & Budzińska, 2000, Biochim. Biophys. Acta 1509, 86-94.). Since the TOM complex might also contribute to the permeability of the membrane to ATP, it seems possible that external NADH and ATP as well as the imported preprotein could compete with one another for the passage through the outer membrane in YVDAC1-depleted mitochondria.     

Role of ATP in mitochondrial protein import. Conformational alteration of a precursor protein can substitute for ATP requirement

The role of nucleoside triphosphates (NTPs) in the import of porin into the mitochondrial outer membrane was investigated with two forms of the porin precursor: the in vitro synthesized biosynthetic precursor (bs-porin) and a water-soluble form of porin (ws-porin) obtained by subjecting the membrane-derived porin to an acid-base treatment (exposure to trichloroacetic acid, followed by alkali and rapid neutralization). The import of ws-porin into mitochondria did not require NTPs, whereas the import of bs-porin required NTPs. In other characteristics, such as binding to a specific receptor protein on the mitochondrial surface, two-step insertion into the outer membrane, and formation of specific membrane channels, ws-porin was indistinguishable from bs-porin. Thus, the acid-base treatment applied in the preparation of ws-porin can substitute for the NTP-requiring step in mitochondrial protein import. We conclude that NTPs are required for unfolding mitochondrial precursor proteins ("translocation competent folding").     

Role of cytochrome c heme lyase in the import of cytochrome c into mitochondria

The import of cytochrome c into Neurospora crassa mitochondria was examined at distinct stages in vitro. The precursor protein, apocytochrome c, binds to mitochondria with high affinity and specificity but is not transported completely across the outer membrane in the absence of conversion to holocytochrome c. The bound apocytochrome c is accessible to externally added proteases but at the same time penetrates far enough through the outer membrane to interact with cytochrome c heme lyase. Formation of a complex in which apocytochrome c and cytochrome c heme lyase participate represents the rate-limiting step of cytochrome c import. Conversion from the bound state to holocytochrome c, on the other hand, occurs 10-30-fold faster. Association of apocytochrome c with cytochrome c heme lyase also takes place after solubilizing mitochondria with detergent. We conclude that the bound apocytochrome c, spanning the outer membrane, forms a complex with cytochrome c heme lyase from which it can react further to be converted to holocytochrome c and be translocated completely into the intermembrane space.     

The human mitochondrial import receptor, hTom20p, prevents a cryptic matrix targeting sequence from gaining access to the protein translocation machinery

Yeast Mas70p and NADH cytochrome b5 reductase are bitopic integral proteins of the mitochondrial outer membrane and are inserted into the lipid-bilayer in an Nin-Ccyto orientation via an NH2-terminal signal- anchor sequence. The signal anchor of both proteins is comprised of a short, positively charged domain followed by the predicted transmembrane segment. The positively charged domain is capable of functioning independently as a matrix-targeting signal in yeast mitochondria in vitro but does not support import into mammalian mitochondria (rat or human). Rather, this domain represents a cryptic signal that can direct import into mammalian mitochondria only if proximal components of the outer membrane import machinery are removed. This can be accomplished either by treating the surface of the intact mitochondria with trypsin or by generating mitoplasts. The import receptor Tom20p (Mas20p/MOM19) is responsible for excluding the cryptic matrix-targeting signal from mammalian mitochondria since replacement of yeast Tom20p with the human receptor confers this property to the yeast organelle while at the same time maintaining import of other proteins. In addition to contributing to positive recognition of precursor proteins, therefore, the results suggest that hTom20p may also have the ability to screen potential matrix-targeting sequences and exclude certain proteins that would otherwise be recognized and imported by distal components of the outer and inner membrane protein- translocation machinery. These findings also indicate, however, that cryptic signals, if they exist within otherwise native precursor proteins, may remain topogenically silent until the precursor successfully clears hTom20p, at which time the activity of the cryptic signal is manifested and can contribute to subsequent translocation and sorting of the polypeptide.     

Specific lipids influence the import capacity of the chloroplast outer envelope precursor protein translocon

Recent studies demonstrated that lipids influence the assembly and efficiency of membrane-embedded macromolecular complexes. Similarly, lipids have been found to influence chloroplast precursor protein binding to the membrane surface and to be associated with the Translocon of the Outer membrane of Chloroplasts (TOC). We used a system based on chloroplast outer envelope vesicles from Pisum sativum to obtain an initial understanding of the influence of lipids on precursor protein translocation across the outer envelope. The ability of the model precursor proteins p(OE33)titin and pSSU to be recognized and translocated in this simplified system was investigated. We demonstrate that transport across the outer membrane can be observed in the absence of the inner envelope translocon. The translocation, however, was significantly slower than that observed for chloroplasts. Enrichment of outer envelope vesicles with different lipids natively found in chloroplast membranes altered the binding and transport behavior. Further, the results obtained using outer envelope vesicles were consistent with the results observed for the reconstituted isolated TOC complex. Based on both approaches we concluded that the lipids sulfoquinovosyldiacylglycerol (SQDG) and phosphatidylinositol (PI) increased TOC-mediated binding and import for both precursor proteins. In contrast, enrichment in digalactosyldiacylglycerol (DGDG) improved TOC-mediated binding for pSSU, but decreased import for both precursor proteins. Optimal import occurred only in a narrow concentration range of DGDG.     

Yeast mitochondria lacking the phosphate carrier/p32 are blocked in phosphate transport but can import preproteins after regeneration of a membrane potential.

Two different functions have been proposed for the phosphate carrier protein/p32 of Saccharomyces cerevisiae mitochondria: transport of phosphate and requirement for import of precursor proteins into mitochondria. We characterized a yeast mutant lacking the gene for the phosphate carrier/p32 and found both a block in the import of phosphate and a strong reduction in the import of preproteins transported to the mitochondrial inner membrane and matrix. Binding of preproteins to the surface of mutant mitochondria and import of outer membrane proteins were not inhibited, indicating that the inhibition of protein import occurred after the recognition step at the outer membrane. The membrane potential across the inner membrane of the mutant mitochondria was strongly reduced. Restoration of the membrane potential restored preprotein import but did not affect the block of phosphate transport of the mutant mitochondria. We conclude that the inhibition of protein import into mitochondria lacking the phosphate carrier/p32 is indirectly caused by a reduction of the mitochondrial membrane potential (delta(gamma)), and we propose a model that the reduction of delta(psi) is due to the defective phosphate import, suggesting that phosphate transport is the primary function of the phosphate carrier/p32.