The role of the TIM8-13 complex in the import of Tim23 into mitochondria

Tim8 and Tim13 are non-essential, conserved proteins of the mitochondrial intermembrane space, which are organized in a hetero-oligomeric complex. They are structurally related to Tim9 and Tim10, essential components of the import machinery for mitochondrial carrier proteins. Here we show that the TIM8-13 complex interacts with translocation intermediates of Tim23, which are partially translocated across the outer membrane but not with fully imported or assembled Tim23. The TIM8-13 complex binds to the N-terminal or intermediate domain of Tim23. It traps the incoming precursor in the intermembrane space thereby preventing retrograde translocation. The TIM8-13 complex is strictly required for import of Tim23 under conditions when a low membrane potential exists in the mitochondria. The human homologue of Tim8 is encoded by the DDP1 (deafness/dystonia peptide 1) gene, which is associated with the Mohr-Tranebjaerg syndrome (MTS), a progressive neurodegenerative disorder leading to deafness. It is demonstrated that import of human Tim23 is dependent on a high membrane potential. A mechanism to explain the pathology of MTS is discussed.     

The C66W mutation in the deafness dystonia peptide 1 (DDP1) affects the formation of functional DDP1.TIM13 complexes in the mitochondrial intermembrane space

Mohr-Tranebjaerg syndrome is a progressive, neurodegenerative disorder caused by loss-of-function mutations in the DDP1/TIMM8A gene. DDP1 belongs to a family of evolutionary conserved proteins that are organized in hetero-oligomeric complexes in the mitochondrial intermembrane space. They mediate the import and insertion of hydrophobic membrane proteins into the mitochondrial inner membrane. All of them share a conserved Cys(4) metal binding site proposed to be required for the formation of zinc fingers. So far, the only missense mutation known to cause a full-blown clinical phenotype is a C66W exchange directly affecting this Cys(4) motif. Here, we show that the mutant human protein is efficiently imported into mitochondria and sorted into the intermembrane space. In contrast to wild-type DDP1, it does not complement the function of its yeast homologue Tim8. The C66W mutation impairs binding of Zn(2+) ions via the Cys(4) motif. As a consequence, the mutated DDP1 is incorrectly folded and loses its ability to assemble into a hetero-hexameric 70-kDa complex with its cognate partner protein human Tim13. Thus, an assembly defect of DDP1 is the molecular basis of Mohr-Tranebjaerg syndrome in patients carrying the C66W mutation.     

Neurological phenotype and reduced lifespan in heterozygous Tim23 knockout mice, the first mouse model of defective mitochondrial import

The Tim23 protein is the key component of the mitochondrial import machinery. It locates to the inner mitochondrial membrane and its own import is dependent on the DDP1/TIM13 complex. Mutations in human DDP1 cause the Mohr-Tranebjaerg syndrome (MTS/DFN-1; OMIM #304700), which is one of the two known human diseases of the mitochondrial protein import machinery. We created a Tim23 knockout mouse from a gene trap embryonic stem cell clone. Homozygous Tim23 mice were not viable. Heterozygous F1 mutants showed a 50% reduction of Tim23 protein in Western blot, a neurological phenotype and a markedly reduced life span. Haploinsufficiency of the Tim23 mutation underlines the critical role of the mitochondrial import machinery for maintaining mitochondrial function.     

Genetic and structural characterization of the human mitochondrial inner membrane translocase.

Translocation of nuclear-encoded mitochondrial preproteins is mediated by translocases in the outer and inner membranes. In the yeast Saccharomyces cerevisiae, translocation of preproteins into the matrix requires the membrane proteins Tim23, Tim17 and Tim44, which drive translocation in cooperation with mtHsp70 and its co-chaperone Mge1p. We have cloned and functionally analyzed the human homologues of Tim17, Tim23 and Tim44. In contrast to yeast, two TIM17 genes were found to be expressed in humans. TIM44, TIM23 and TIM17a genes were mapped to chromosomes 19p13.2-p13.3, 10q11. 21-q11.23 and 1q32. The TIM17b gene mapped to Xp11.23, near the fusion point where an autosomal region was proposed to have been added to the "ancient" part of the X chromosome about 80-130 MY ago. The primary sequences of the two proteins, hTim17a and hTim17b, are essentially identical, significant differences being restricted to their C termini. They are ubiquitously expressed in fetal and adult tissues, and both show expression levels comparable to that of hTim23. Biochemical characterization of the human Tim components revealed that hTim44 is localized in the matrix and, in contrast to yeast, only loosely associated with the inner membrane. hTim23 is organized into two distinct complexes in the inner membrane, one containing hTim17a and one containing hTim17b. Both TIM complexes display a native molecular mass of 110 kDa. We suggest that the structural organization of TIM23.17 preprotein translocases is conserved from low to high eukaryotes.     

The Tim8-Tim13 complex of Neurospora crassa functions in the assembly of proteins into both mitochondrial membranes

The Tim8 and Tim13 proteins in yeast are known to exist in the mitochondrial intermembrane space and to form a hetero-oligomeric complex involved in the import of the mitochondrial inner membrane protein Tim23, the central component of the TIM23 translocase. Here, we have isolated tim8 and tim13 mutants in Neurospora crassa and have shown that mitochondria lacking the Tim8-Tim13 complex were deficient in the import of the outer membrane beta-barrel proteins Tom40 and porin. Cross-linking studies showed that the Tom40 precursor contacts the Tim8-Tim13 complex. The complex is involved at an early point in the Tom40 assembly pathway because cross-links can only be detected during the initial stages of Tom40 import. In mitochondria lacking the Tim8-Tim13 complex, the Tom40 precursor appears in a previously characterized early intermediate of Tom40 assembly more slowly than in wild type mitochondria. Thus, our data suggest a model in which one of the first steps in Tom40 assembly may be interaction with the Tim8-Tim13 complex. As in yeast, the N. crassa Tim23 precursor was imported inefficiently into mitochondria lacking the Tim8-Tim13 complex when the membrane potential was reduced. Tim23 import intermediates could also be cross-linked to the complex, suggesting a dual role for the Tim8-Tim13 intermembrane space complex in the import of proteins found in both the outer and inner mitochondrial membranes.     

The Tim8-Tim13 complex has multiple substrate binding sites and binds cooperatively to Tim23

The Tim8-Tim13 complex, located in the mitochondrial intermembrane space, functions in the TIM22 import pathway that mediates the import of the mitochondrial carriers Tim23, Tim22, and Tim17 into the mitochondrial inner membrane. The Tim8-Tim13 complex assembles as a hexamer and binds to the substrate Tim23 to chaperone the hydrophobic Tim23 across the aqueous intermembrane space. However, both structural features of the Tim8-Tim13 complex and the binding interaction to Tim23 remain poorly defined. The crystal structure of the yeast Tim8-Tim13 complex, reported here at 2.6 Å resolution, reveals that the architecture of the Tim8-Tim13 complex is similar to those of other chaperones such as Tim9-Tim10, prefoldin, and Skp, in which long helices extend from a central body like tentacles from a jellyfish. Surface plasmon resonance was applied to investigate interactions between the Tim8-Tim13 complex and Tim23. The Tim8-Tim13 complex contained approximately six binding sites and showed a complex binding interaction indicative of positive cooperativity rather than a simple bimolecular interaction. By combining results from the structural and binding studies, we provide a molecular model of the Tim8-Tim13 complex binding to Tim23. The regions where the tentacle helices attach to the body of the Tim8-Tim13 complex contain six hydrophobic pockets that likely interact with specific sequences of Tim23 and possibly other substrates. Smaller hydrophobic patches on the tentacles themselves likely interact nonspecifically with the substrate's transmembrane helices, shielding it from the aqueous intermembrane space. The central region of Tim23, which enters the intermembrane space first, may serve to nucleate the binding of the Tim8-Tim13 complex, thereby initiating the chaperoned translocation of Tim23 to the mitochondrial inner membrane.     

Tim50 is a subunit of the TIM23 complex that links protein translocation across the outer and inner mitochondrial membranes

Based on the results of site-specific photocrosslinking of translocation intermediates, we have identified Tim50, a component of the yeast TIM23 import machinery, which mediates translocation of presequence-containing proteins across the mitochondrial inner membrane. Tim50 is anchored to the inner mitochondrial membrane, exposing the C-terminal domain to the intermembrane space. Tim50 interacts with the N-terminal intermembrane space domain of Tim23. Functional defects of Tim50 either by depletion of the protein or addition of anti-Tim50 antibodies block the protein translocation across the inner membrane. A translocation intermediate accumulated at the TOM complex is crosslinked to Tim50. We suggest that Tim50, in cooperation with Tim23, facilitates transfer of the translocating protein from the TOM complex to the TIM23 complex     

Tim50, a novel component of the TIM23 preprotein translocase of mitochondria

The preprotein translocase of the inner membrane of mitochondria (TIM23 complex) is the main entry gate for proteins of the matrix and the inner membrane. We isolated the TIM23 complex of Neurospora crassa. Besides Tim23 and Tim17, it contained a novel component, referred to as Tim50. Tim50 spans the inner membrane with a single transmembrane segment and exposes a large hydrophilic domain in the intermembrane space. Tim50 is essential for viability of yeast. Mitochondria from cells depleted of Tim50 displayed strongly reduced import kinetics of preproteins using the TIM23 complex. Tim50 could be cross-linked to preproteins that were halted at the level of the translocase of the outer membrane (TOM complex) or spanning both TOM and TIM23 complexes. We suggest that Tim50 plays a crucial role in the transfer of preproteins from the TOM complex to the TIM23 complex through the intermembrane space.     

The mitochondrial TIM22 preprotein translocase is highly conserved throughout the eukaryotic kingdom

The Mohr-Tranebjaerg syndrome (MTS), a neurodegenerative syndrome characterized by progressive sensorineural hearing loss, dystonia, mental retardation and blindness, is a mitochondrial disease caused by mutations in the deafness/dystonia peptide 1 (DDP1) gene. DDP1 shows similarity to the yeast proteins Tim9, Tim10 and Tim12, components of the mitochondrial import machinery for carrier proteins. Here, we show that DDP1 belongs to a large family of evolutionarily conserved proteins. We report the identification, chromosomal localization and expressional analysis of six human family members which represent further candidate genes for neurodegenerative diseases.     

Different import pathways through the mitochondrial intermembrane space for inner membrane proteins.

Earlier work on the protein import system of yeast mitochondria has identified two soluble 70 kDa protein complexes in the intermembrane space. One complex contains the essential proteins Tim9p and Tim10p and mediates transport of cytosolically-made metabolite carrier proteins from the outer to the inner membrane. The other complex contains the non-essential proteins Tim8p and Tim13p as well as loosely associated Tim9p; its function was unclear, but it interacted structurally or functionally with the Tim9p-Tim10p complex. We now show that the two 70 kDa complexes each mediate the import of a different subset of integral inner membrane proteins and that they can transfer these proteins to one of three different membrane insertion sites: the TIM22 complex, the TIM23 complex or an as yet uncharacterized insertion site. Yeast mitochondria thus use multiple pathways for escorting hydrophobic inner membrane proteins across the aqueous intermembrane space.     

Tim23–Tim50 pair coordinates functions of translocators and motor proteins in mitochondrial protein import

Mitochondrial protein traffic requires coordinated operation of protein translocator complexes in the mitochondrial membrane. The TIM23 complex translocates and inserts proteins into the mitochondrial inner membrane. Here we analyze the intermembrane space (IMS) domains of Tim23 and Tim50, which are essential subunits of the TIM23 complex, in these functions. We find that interactions of Tim23 and Tim50 in the IMS facilitate transfer of precursor proteins from the TOM40 complex, a general protein translocator in the outer membrane, to the TIM23 complex. Tim23–Tim50 interactions also facilitate a late step of protein translocation across the inner membrane by promoting motor functions of mitochondrial Hsp70 in the matrix. Therefore, the Tim23–Tim50 pair coordinates the actions of the TOM40 and TIM23 complexes together with motor proteins for mitochondrial protein import.     

Protein translocation into mitochondria: the role of TIM complexes

Import of nuclear-encoded mitochondrial preproteins is mediated by a general translocase in the outer membrane, the TOM complex, and by two distinct translocases in the mitochondrial inner membrane, the TIM23 complex and the TIM22 complex. Both TIM complexes cooperate with the TOM complex but facilitate import of different classes of precursor proteins. Precursors with an N-terminal presequence are imported via the TIM23 complex, whereas mitochondrial carrier proteins require the TIM22 complex for insertion into the inner membrane. This review discusses recent advances in understanding the structure and function of the translocases of the inner membrane and the possible role of Tim proteins in the development of the Mohr-Tranebjaerg syndrome, a mitochondrial disorder leading to neurodegeneration.     

The role of the Tim8p-Tim13p complex in a conserved import pathway for mitochondrial polytopic inner membrane proteins

Tim23p is imported via the TIM (translocase of inner membrane)22 pathway for mitochondrial inner membrane proteins. In contrast to precursors with an NH2-terminal targeting presequence that are imported in a linear NH2-terminal manner, we show that Tim23p crosses the outer membrane as a loop before inserting into the inner membrane. The Tim8p-Tim13p complex facilitates translocation across the intermembrane space by binding to the membrane spanning domains as shown by Tim23p peptide scans with the purified Tim8p-Tim13p complex and crosslinking studies with Tim23p fusion constructs. The interaction between Tim23p and the Tim8p-Tim13p complex is not dependent on zinc, and the purified Tim8p-Tim13p complex does not coordinate zinc in the conserved twin CX3C motif. Instead, the cysteine residues seemingly form intramolecular disulfide linkages. Given that proteins of the mitochondrial carrier family also pass through the TOM (translocase of outer membrane) complex as a loop, our study suggests that this translocation mechanism may be conserved. Thus, polytopic inner membrane proteins, which lack an NH2-terminal targeting sequence, pass through the TOM complex as a loop followed by binding of the small Tim proteins to the hydrophobic membrane spanning domains.     

The role of Hot13p and redox chemistry in the mitochondrial TIM22 import pathway

The small Tim proteins in the mitochondrial intermembrane space participate in the TIM22 import pathway for assembly of the inner membrane. Assembly of the small TIM complexes requires the conserved "twin CX3C" motif that forms juxtapositional intramolecular disulfide bonds. Here we identify a new intermembrane space protein, Hot13p, as the first component of a pathway that mediates assembly of the small TIM complexes. The small Tim proteins require Hot13p for assembly into a 70-kDa complex in the intermembrane space. Once assembled the small TIM complexes escort hydrophobic inner membrane proteins en route to the TIM22 complex. The mechanism by which the small Tim proteins bind and release substrate is not understood, and we investigated the affect of oxidant/reductant treatment on the TIM22 import pathway. With in organello import studies, oxidizing agents arrest the ADP/ATP carrier (AAC) bound to the Tim9p-Tim10p complex in the intermembrane space; this productive intermediate can be chased into the inner membrane upon subsequent treatment with reductant. Moreover, AAC import is markedly decreased by oxidant treatment in Deltahot13 mitochondria and improved when Hot13p is overexpressed, suggesting Hot13p may function to remodel the small TIM complexes during import. Together these results suggest that the small TIM complexes have a specialized assembly pathway in the intermembrane space and that the local redox state of the TIM complexes may mediate translocation of inner membrane proteins.     

The essential function of the small Tim proteins in the TIM22 import pathway does not depend on formation of the soluble 70-kilodalton complex

The TIM22 protein import pathway of the yeast mitochondrion contains several components, including a family of five proteins (Tim8p, -9p, -10p, -12p, and -13p [Tim, for translocase of inner membrane]) that are located in the intermembrane space and are 25% identical. Tim9p and Tim10p have dual roles in mediating the import of inner membrane proteins. Like the Tim8p-Tim13p complex, the Tim9p-Tim10p complex functions as a putative chaperone to guide hydrophobic precursors across the intermembrane space. Like membrane-associated Tim12p, they are members of the Tim18p-Tim22p-Tim54p membrane complex that mediates precursor insertion into the membrane. To understand the role of this family in protein import, we have used a genetic approach to manipulate the complement of the small Tim proteins. A strain has been constructed that lacks the 70-kDa soluble Tim8p-Tim13p and Tim9p-Tim10p complexes in the intermembrane space. Instead, a functional version of Tim9p (Tim9(S67C)p), identified as a second-site suppressor of a conditional tim10 mutant, maintains viability. Characterization of this strain revealed that Tim9(S67C)p and Tim10p were tightly associated with the inner membrane, the soluble 70-kDa Tim8p-Tim13p and Tim9p-Tim10p complexes were not detectable, and the rate of protein import into isolated mitochondria proceeded at a slower rate. An arrested translocation intermediate bound to Tim9(S67C)p was located in the intermembrane space, associated with the inner membrane. We suggest that the 70-kDa complexes facilitate import, similar to the outer membrane receptors of the TOM (hetero-oligomeric translocase of the outer membrane) complex, and the essential role of Tim9p and Tim10p may be to mediate protein insertion in the inner membrane with the TIM22 complex.     

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.     

Role of Tim50 in the Transfer of Precursor Proteins from the Outer to the Inner Membrane of Mitochondria

Transport of essentially all matrix and a number of inner membrane proteins is governed, entirely or in part, by N-terminal presequences and requires a coordinated action of the translocases of outer and inner mitochondrial membranes (TOM and TIM23 complexes). Here, we have analyzed Tim50, a subunit of the TIM23 complex that is implicated in transfer of precursors from TOM to TIM23. Tim50 is recruited to the TIM23 complex via Tim23 in an interaction that is essentially independent of the rest of the translocase. We find Tim50 in close proximity to the intermembrane space side of the TOM complex where it recognizes both types of TIM23 substrates, those that are to be transported into the matrix and those destined to the inner membrane, suggesting that Tim50 recognizes presequences. This function of Tim50 depends on its association with TIM23. We conclude that the efficient transfer of precursors between TOM and TIM23 complexes requires the concerted action of Tim50 with Tim23.     

The structural characteristics of human preprotein translocase of the inner mitochondrial membrane Tim23: implications for its physiological activities

The preprotein translocase of the inner mitochondrial membrane (TIM23 complex) is the main entry gate for proteins of the matrix and the inner membrane. Tim23 forms a pore for preprotein transportation in TIM23 complex, which spans the inner membrane with transmembrane segments and exposes a hydrophilic domain in the intermembrane space. In this study, we expressed and purified the intermembrane space (IMS) domain of human Tim23 (Tim23(IMS)). The far-UV CD spectra of Tim23(IMS) in native and denatured states revealed that the protein has a limited secondary structure and a not well-defined tertiary packing. Its Stokes radius was larger than both its expected size as a folded globular protein and the size determined by size exclusion chromatography. A large increase in 8-anilino-1-naphthalene-sulfonate (ANS) fluorescence (>50-fold) was observed, indicating that hydrophobic clusters are exposed at its surface. And GlobPlot/DisEMBL program predicted that the protein is in a loose folding state. We therefore conclude that, the non-bound hydrophilic domain of the human Tim23 is in a molten globule configuration with marginal stability. Furthermore, size exclusion chromatography and sedimentation equilibrium analysis showed that Tim23(IMS) exists as a dimer. And the results, showed by ANS binding and fluorescence quenching, indicated that a pH-dependent conformational change of Tim23(IMS) occurs, and at pH 4 and 3, it forms a compact structure.     

Functional reconstitution of the import of the yeast ADP/ATP carrier mediated by the TIM10 complex

Import of the ADP/ATP carrier (AAC) into mitochondria requires the soluble TIM10 complex to cross the intermembrane space. We report here that Tim9 and Tim10 purified from Escherichia coli can form a complex of the same size as the endogenous complex from yeast mitochondria. This shows that no other mitochondrial protein is required for the formation of the TIM10 complex. Co-expression of both proteins rendered Tim9 more soluble and allowed purification of the reconstituted complex in a single step. Urea/EDTA treatment of recombinant Tim10 allowed its import into tim10-ts mitochondria that lack endogenous Tim10 and cannot import AAC. In this way, we were able to (i) reconstitute the TIM10 complex in the intermembrane space and (ii) restore import of AAC to almost wild-type levels. The reconstituted TIM10 complex not only facilitated passage of AAC across the outer membrane but also ensured its accurate membrane insertion. We conclude that the TIM10 complex can be formed exclusively from Tim9 and Tim10 and that the reconstituted complex efficiently restores AAC import in a strain lacking the TIM10 complex.     

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.