Human deafness dystonia syndrome is a mitochondrial disease

Clinical aspects of hereditary hearing loss

Hearing loss is an etiologically diverse condition with many disease-related complications and major clinical, social, and quality of life implications. As the rate of acquired hearing loss secondary to environmental causes decreases and improvements in the diagnosis of abnormalities occur, the significance of genetic factors that lead to deafness increases. Advancements in molecular biology have led to improved detection and earlier intervention in patients with hearing loss. Subsequently, earlier implementation of educational services and cochlear implant technology in patients with profound hearing loss now results in superior communication skills and enhanced language development. The aim of this review is to provide a comprehensive framework underlying the causes of hearing impairment and to detail the clinical management for patients with hereditary hearing loss.

Mitochondrial protein import and human health and disease

The targeting and assembly of nuclear-encoded mitochondrial proteins are essential processes because the energy supply of humans is dependent upon the proper functioning of mitochondria. Defective import of mitochondrial proteins can arise from mutations in the targeting signals within precursor proteins, from mutations that disrupt the proper functioning of the import machinery, or from deficiencies in the chaperones involved in the proper folding and assembly of proteins once they are imported. Defects in these steps of import have been shown to lead to oxidative stress, neurodegenerative diseases, and metabolic disorders. In addition, protein import into mitochondria has been found to be a dynamically regulated process that varies in response to conditions such as oxidative stress, aging, drug treatment, and exercise. This review focuses on how mitochondrial protein import affects human health and disease.

Blepharospasm and limb dystonia caused by Mohr-Tranebjaerg syndrome with a novel splice-site mutation in the deafness/dystonia peptide gene

Mohr-Tranebjaerg syndrome (MTS) is an X-linked disorder characterized by childhood-onset progressive deafness, dystonia, spasticity, mental deterioration, and blindness. It is due to mutations in the deafness/dystonia peptide (DDP1) gene. We describe a sporadic 42-year-old man with MTS presenting with postlingual deafness, adult-onset progressive dystonia with marked arm tremor, mild spasticity of the legs, and visual disturbance due to a novel mutation (g to a transition at the invariant gt of the 5' splice donor site of exon 1) in the DDP1 gene. This case, and a review of previously reported cases, highlights a variety of potential diagnostic pitfalls in this condition.

Mitochondrial Transporters as Novel Targets for Intracellular Calcium Signaling

Ca2+signaling in mitochondria is important to tune mitochondrial function to a variety of extracellular stimuli. The main mechanism is Ca2+entry in mitochondria via the Ca2+uniporter followed by Ca2+activation of three dehydrogenases in the mitochondrial matrix. This results in increases in mitochondrial NADH/NAD ratios and ATP levels and increased substrate uptake by mitochondria. We review evidence gathered more than 20 years ago and recent work indicating that substrate uptake, mitochondrial NADH/NAD ratios, and ATP levels may be also activated in response to cytosolic Ca2+signals via a mechanism that does not require the entry of Ca2+in mitochondria, a mechanism depending on the activity of Ca2+-dependent mitochondrial carriers (CaMC). CaMCs fall into two groups, the aspartate-glutamate carriers (AGC) and the ATP-Mg/Picarriers, also named SCaMC (for short CaMC). The two mammalian AGCs, aralar and citrin, are members of the malate-aspartate NADH shuttle, and citrin, the liver AGC, is also a member of the urea cycle. Both types of CaMCs are activated by Ca2+in the intermembrane space and function together with the Ca2+uniporter in decoding the Ca2+signal into a mitochondrial response.

Assessing systems properties of yeast mitochondria through an interaction map of the organelle

Mitochondria carry out specialized functions; compartmentalized, yet integrated into the metabolic and signaling processes of the cell. Although many mitochondrial proteins have been identified, understanding their functional interrelationships has been a challenge. Here we construct a comprehensive network of the mitochondrial system. We integrated genome-wide datasets to generate an accurate and inclusive mitochondrial parts list. Together with benchmarked measures of protein interactions, a network of mitochondria was constructed in their cellular context, including extra-mitochondrial proteins. This network also integrates data from different organisms to expand the known mitochondrial biology beyond the information in the existing databases. Our network brings together annotated and predicted functions into a single framework. This enabled, for the entire system, a survey of mutant phenotypes, gene regulation, evolution, and disease susceptibility. Furthermore, we experimentally validated the localization of several candidate proteins and derived novel functional contexts for hundreds of uncharacterized proteins. Our network thus advances the understanding of the mitochondrial system in yeast and identifies properties of genes underlying human mitochondrial disorders.

The Tim9p/10p and Tim8p/13p complexes bind to specific sites on Tim23p during mitochondrial protein import

The import of polytopic membrane proteins into the mitochondrial inner membrane (IM) is facilitated by Tim9p/Tim10p and Tim8p/Tim13p protein complexes in the intermembrane space (IMS). These complexes are proposed to act as chaperones by transporting the hydrophobic IM proteins through the aqueous IMS and preventing their aggregation. To examine the nature of this interaction, Tim23p molecules containing a single photoreactive cross-linking probe were imported into mitochondria in the absence of an IM potential where they associated with small Tim complexes in the IMS. On photolysis and immunoprecipitation, a probe located at a particular Tim23p site (27 different locations were examined) was found to react covalently with, in most cases, only one of the small Tim proteins. Tim8p, Tim9p, Tim10p, and Tim13p were therefore positioned adjacent to specific sites in the Tim23p substrate before its integration into the IM. This specificity of binding to Tim23p strongly suggests that small Tim proteins do not function solely as general chaperones by minimizing the exposure of nonpolar Tim23p surfaces to the aqueous medium, but may also align a folded Tim23p substrate in the proper orientation for delivery and integration into the IM at the TIM22 translocon.

Hearing loss in mitochondrial disorders

Hearing loss is a common clinical feature in mitochondria-syndrome disorders. The underlining molecular etiology of hearing loss has not been fully investigated. In this study, 83 patients with mitochondrial syndromic hearing loss were evaluated clinically and their blood and tissue samples were examined molecularly. Using modified Walker's criteria, 31, 31, 14, and 7 patients had been classified as having definite, probable, possible, and unlikely diagnosis of mitochondrial disease, respectively. Deleterious mtDNA point mutations and/or abnormal mtDNA content or multiple deletions were identified in 20 patients with definite diagnosis and 2 patients with probable diagnosis. In addition to known, undisputed pathogenic mutations, several novel mutations believed to be clinically significant were found. Furthermore, abnormal mtDNA content and mtDNA deletions were found in some of the cases. Evaluation of clinical and diagnostic features associated with hearing loss revealed that cardiomyopathy, lactic acidosis, deficient respiratory chain enzyme complex activities, histochemical and ultrastructural abnormalities in mitochondria, and abnormal brain imaging results occurred significantly more frequently in patients with mtDNA alterations than in those without. This study revealed that the majority of the mtDNA defects in patients with mitochondrial syndromic hearing loss affect the overall mitochondrial gene expression.

Mammalian polynucleotide phosphorylase is an intermembrane space RNase that maintains mitochondrial homeostasis

We recently identified polynucleotide phosphorylase (PNPase) as a potential binding partner for the TCL1 oncoprotein. Mammalian PNPase exhibits exoribonuclease and poly(A) polymerase activities, and PNPase overexpression inhibits cell growth, induces apoptosis, and stimulates proinflammatory cytokine production. A physiologic connection for these anticancer effects and overexpression is difficult to reconcile with the presumed mitochondrial matrix localization for endogenous PNPase, prompting this study. Here we show that basal and interferon-beta-induced PNPase was efficiently imported into energized mitochondria with coupled processing of the N-terminal targeting sequence. Once imported, PNPase localized to the intermembrane space (IMS) as a peripheral membrane protein in a multimeric complex. Apoptotic stimuli caused PNPase mobilization following cytochrome c release, which supported an IMS localization and provided a potential route for interactions with cytosolic TCL1. Consistent with its IMS localization, PNPase knockdown with RNA interference did not affect mitochondrial RNA levels. However, PNPase reduction impaired mitochondrial electrochemical membrane potential, decreased respiratory chain activity, and was correlated with altered mitochondrial morphology. This resulted in FoF1-ATP synthase instability, impaired ATP generation, lactate accumulation, and AMP kinase phosphorylation with reduced cell proliferation. Combined, the data demonstrate an unexpected IMS localization and a key role for PNPase in maintaining mitochondrial homeostasis.

The many shapes of mitochondrial death

Mitochondria integrate apoptotic signalling by releasing cytochrome c and other proapoptotic cofactors needed for activation of effector caspases. Previously overlooked morphological changes, mitochondrial fragmentation and cristae remodelling, emerged as subroutines of the mitochondrial programme of apoptosis in mammalian cells, as well as in developmental cell death of Caenorhabditis elegans. Mitochondrial morphology results from fusion and fission processes, controlled by a growing set of 'mitochondria-shaping' proteins. Their levels and function appear to influence mitochondrial pathways of cell death, but mechanisms are largely unknown. An emerging model implicates different signals converging on mitochondria-shaping proteins to activate or deactivate them during apoptosis. In turn, these proteins can orchestrate changes in mitochondrial shape to insure cytochrome c release and progression of the apoptotic cascade. These therefore appear an appealing novel therapeutic target to modulate cell death in cancer.

Redox pathways of the mitochondrion

The mitochondrion houses a variety of redox pathways, utilized for protection from oxidative damage and assembly of the organelle. The glutathione/glutaredoxin and thioredoxin systems function in the mitochondrial matrix. The intermembrane space is protected from oxidative damage via superoxide dismutase and glutathione. Subunits in the cytochrome bc (1) complex utilize disulfide bonds for enzymatic activity, whereas cytochrome oxidase relies on disulfide linkages for copper acquisition. A redox pathway (Mia40p and Erv1p) mediates the import of intermembrane space proteins such as the small Tim proteins, Cox17p, and Cox19p, which have disulfide bonds. Many of the candidate proteins with disulfide bridges possess a twin CX3C motif or CX9C motif and utilize both metal binding and disulfide linkages for function. It may seem surprising that the intermembrane space has developed redox pathways, considering that the buffered environment should be reducing like the cytosol. However, the prokaryotic origin of the mitochondrion suggests that the intermembrane space may be akin to the oxidative environment of the bacterial periplasm. Although the players forming disulfide bonds are not conserved between mitochondria and prokaryotes, the mitochondrion may have maintained redox chemistry as an assembly mechanism in the intermembrane space for the import of proteins and metals and enzymatic activity.

The role of mitochondria in the pathogenesis of neurodegenerative diseases

A growing body of evidence indicates that mitochondrial dysfunction may play an important role in the pathogenesis of many neurodegenerative disorders. Because mitochondrial metabolism is not only the principal source of high energy intermediates, but also of free radicals, it has been suggested that inherited or acquired mitochondrial defects could be the cause of neuronal degeneration as a consequence of energy defects and oxidative damage. Mitochondrial respiratory chain dysfunction has been reported in association with primary mitochondrial DNA abnormalities, and also as a consequence of mutations in nuclear genes directly involved in mitochondrial functions, such as SURF1, frataxin, and paraplegin. Defects of oxidative phosphorylation and increased free radical production have also been observed in diseases that are not due to primary mitochondrial abnormalities. In these cases, the mitochondrial dysfunction is likely to be an epiphenomenon, which, nevertheless, could be of importance in precipitating a cascade of events leading to cell death. In either case, understanding the role of mitochondria in the pathogenesis of neurodegenerative diseases could be important for the development of therapeutic strategies in these disorders.

How is Mitochondrial Biogenesis Affected in Mitochondrial Disease?

Mitochondrial biogenesis occurs when the tissue energy demand is chronically increased to stress the ATP producing capacity of the preexisting mitochondria. In muscle, endurance training is a metabolic stress that is capable of inducing mitochondrial biogenesis, the consequence of which is improved performance during exercise. Expansion of the mitochondrial volume requires the coordinated response of the nuclear and mitochondrial genomes. During acute exercise, the initial signaling events are the perturbations in ATP turnover and calcium (Ca) concentrations caused by the contractile process. These alterations activate signal transduction pathways which target transcription factors involved in gene expression. Nuclear gene products are then posttranslationally imported into mitochondria. One of these, Tfam, is important for the regulation of mitochondrial DNA (mtDNA) gene expression. In muscle, a broad range of mitochondrial-specific diseases due to mutations in nuclear DNA or mtDNA exist, termed mitochondrial myopathies. These mutations result in dysfunctional mitochondrial assembly which ultimately leads to reduced ATP production. Mitochondrial myopathy patients exhibit a variety of compensatory responses which attempt to reconcile this energy deficiency, but the extent and the type of compensatory adaptations are disease-specific. Understanding the role of exercise in mediating these compensatory responses leading to mitochondrial biogenesis could help us in prescribing exercise designed to improve mitochondrial function in patients with mitochondrial myopathies. In addition, numerous other diseases (e.g., neurological disorders, cancer, diabetes, and cardiomyopathies), as well as the aging process, have etiologies or consequences attributed, in part, to mitochondrial dysfunction. Thus, insight gained by investigating the steps involved in exercise-induced mitochondrial biogenesis may help us to understand the underlying basis of these other disease states.

Mitochondrial assembly: protein import

The protein import process of mitochondria is vital for the assembly of the hundreds of nuclear-derived proteins into an expanding organelle reticulum. Most of our knowledge of this complex multisubunit network comes from studies of yeast and fungal systems, with little information known about the protein import process in mammalian cells, particularly skeletal muscle. However, growing evidence indicates that the protein import machinery can respond to changes in the energy status of the cell. In particular, contractile activity, a powerful inducer of mitochondrial biogenesis, has been shown to alter the stoichiometry of the protein import apparatus via changes in several protein import machinery components. These adaptations include the induction of cytosolic molecular chaperones that transport precursors to the matrix, the up-regulation of outer membrane import receptors, and the increase in matrix chaperonins that facilitate the import and proper folding of the protein for subsequent compartmentation in the matrix or inner membrane. The physiological importance of these changes is an increased capacity for import into the organelle at any given precursor concentration. Defects in the protein import machinery components have been associated with mitochondrial disorders. Thus, contractile activity may serve as a possible mechanism for up-regulation of mitochondrial protein import and compensation for mitochondrial phenotype alterations observed in diseased muscle.

GDAP1, the protein causing Charcot–Marie–Tooth disease type 4A, is expressed in neurons and is associated with mitochondria

Mutations in GDAP1, the ganglioside-induced differentiation-associated protein 1 gene, cause Charcot-Marie-Tooth (CMT) type 4A, a severe autosomal recessive form of neuropathy associated with either demyelinating or axonal phenotypes. Here, we demonstrate that GDAP1 has far greater expression in neurons than in myelinating Schwann cells. We investigated cell localization of GDAP1 in a human neuroblastoma cell line by means of transient overexpression and co-localization with organelle markers in COS-7 cells and by western blot analysis of subcell fractions with anti-GDAP1 polyclonal antibodies. We observed that GDAP1 is localized in mitochondria. We also show that C-terminal transmembrane domains are necessary for the correct localization in mitochondria; however, missense mutations do not change the mitochondrial pattern of the wild-type protein. Our findings suggest that CMT4A disease is in fact a mitochondrial neuropathy mainly involving axons and represents a disease belonging to the new category of mitochondrial disorders caused by mutations in nuclear genes. We postulate that GDAP1 may be related to the maintenance of the mitochondrial network.

New developments in mitochondrial assembly

The mitochondrion has developed an elaborate translocation system for the import of nuclear-coded proteins and the export of proteins coded on the mitochondrial genome. Precursor proteins contain targeting and sorting information to reach the mitochondrion, whereas the translocons recognize the information and direct the precursor to the correct compartment. The outer membrane contains the TOM (translocase of the outer membrane) complex for translocation and the SAM (sorting and assembly machinery) complex for assembly of outer membrane proteins with complex topologies. At the inner membrane, the TIM23 (translocase of the inner membrane) mediates the import of mitochondrial proteins with a typical N-terminal targeting sequence, and the TIM22 complex mediates the import of polytopic inner membrane proteins. Based on its prokaryotic origin, the inner membrane also contains several components that mediate the export and assembly of proteins from within the matrix. Together the translocation and assembly complexes coordinate assembly of the mitochondrion.

Defective mitochondrial protein translocation precludes normal Caenorhabditis elegans development

We demonstrate biochemically that the genes identified by sequence similarity as orthologs of the mitochondrial import machinery are functionally conserved in Caenorhabditis elegans. Specifically, tin-9.1 and tin-10 RNA interference (RNAi) treatment of nematodes impairs import of the ADP/ATP carrier into isolated mitochondria. Developmental phenotypes are associated with gene knock-down of the mitochondrial import components. RNAi of tomm-7 and ddp-1 resulted in mitochondria with an interconnected morphology in vivo, presumably due to defects in the assembly of outer membrane fission/fusion components. RNAi of the small Tim proteins TIN-9.1, TIN-9.2, and TIN-10 resulted in a small body size, reduced number of progeny produced, and partial embryonic lethality. An additional phenotype of the tin-9.2(RNAi) animals is defective formation of the somatic gonad. The biochemical demonstration that the protein import activity is reduced, under the same conditions that yield the defects in specific tissues and lethality in a later generation, suggests that the developmental abnormalities observed are a consequence of defects in mitochondrial inner membrane biogenesis.

Dystonia in the Mohr-Tranebjaerg syndrome responds to GABAergic substances

Mutations in the X-linked deafness-dystonia peptide 1 (DDP1) gene cause Mohr-Tranebjaerg syndrome (MTS), a rare form of deafness associated with dystonia. In the patient presented here, improvement of dystonic symptoms upon treatment with alcohol and GABAergic substances is demonstrated for the first time.

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.

Toxicity of Familial ALS-Linked SOD1 Mutants from Selective Recruitment to Spinal Mitochondria

One cause of amyotrophic lateral sclerosis (ALS) is mutation in ubiquitously expressed copper/zinc superoxide dismutase (SOD1), but the mechanism of toxicity to motor neurons is unknown. Multiple disease-causing mutants, but not wild-type SOD1, are now demonstrated to be recruited to mitochondria, but only in affected tissues. This is independent of the copper chaperone for SOD1 and dismutase activity. Highly preferential association with spinal cord mitochondria is seen in human ALS for a mutant SOD1 that accumulates only to trace cytoplasmic levels. Despite variable proportions that are successfully imported, nearly constant amounts of SOD1 mutants and covalently damaged adducts of them accumulate as apparent import intermediates and/or are tightly aggregated or crosslinked onto integral membrane components on the cytoplasmic face of those mitochondria. These findings implicate damage from action of spinal cord-specific factors that recruit mutant SOD1 to spinal mitochondria as the basis for their selective toxicity in ALS.

Organization and Function of the Small Tim Complexes Acting along the Import Pathway of Metabolite Carriers into Mammalian Mitochondria

Tim9, Tim10a, and Tim10b are members of the family of small Tim proteins located in the intermembrane space of mammalian mitochondria. In yeast, members of this family act along the TIM22 import pathway during import of metabolite carriers and other integral inner membrane proteins. Here, we show that the human small proteins form two distinct hetero-oligomeric complexes. A 70-kDa complex that contains Tim9 and Tim10a and a Tim9-10a-10b that is part of a higher molecular weight assembly of 450 kDa. This distribution among two complexes suggests Tim10b to be the functional homologue of yeast Tim12. Both human complexes are tightly associated with the inner membrane and, compared with yeast, soluble 70-kDa complexes appear to be completely absent in the intermembrane space. Thus, the function of soluble 70-kDa complexes as trans-site receptors for incoming carrier proteins is not conserved from lower to higher eukaryotes. During import, the small Tim complexes directly interact with human adenine nucleotide translocator (ANT) in transit in a metal-dependent manner. For insertion of carrier preproteins into the inner membrane, the human small Tim proteins directly interact with human Tim22, the putative insertion pore of the TIM22 translocase. However, in contrast to yeast, only a small fraction of Tim9-Tim10a-Tim10b complex is in a stable association with Tim22. We conclude that different mechanisms and specific requirements for import and insertion of mammalian carrier preproteins have evolved in higher eukaryotes.

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.

Flow cytometry and GFP: a novel assay for measuring the import and turnover of nuclear-encoded mitochondrial proteins in live PC12 cells

BACKGROUND

Mitochondrial protein import is typically measured by adding radiolabeled precursor proteins to isolated mitochondria. We have developed a novel, high-throughput method for measuring protein import in live differentiated PC12 cells using a tetracycline (Tet) regulated, nuclear encoded, mitochondrially-targeted GFP fusion protein and flow cytometry.

METHODS

We generated a PC12 cell line stably transfected with an inducible GFP fusion protein (GFPmt) targeted to mitochondria. GFPmt PC12 cells were treated with NGF for one week to induce neuronal differentiation in the presence of Tet to silence GFP expression. On day seven GFPmt expression was induced by removal of Tet and these "GFP-on" cells were exposed to sublethal levels of CCCP (2 microM) for 24 h. At 24 h, the cells were harvested in Ca(++)-free PBS and the GFPmt signal in live intact cells was measured using flow cytometry. Since GFPmt is not fluorescent prior to being imported into mitochondria, the GFPmt signal reflected only GFPmt imported to mitochondria. PI was used to gate out contributions from dead cells. Turnover of GFPmt in mitochondria was also assessed; in this case, Tet was added to arrest GFPmt expression in GFP-on cells, and the subsequent decline of the fluorescent signal, in the absence of any new GFP synthesis, was measured by flow cytometry.

RESULTS

Exposure to 2 microM CCCP for 24 h caused a 61% +/- 0.4 decline in GFPmt fluorescence compared to controls. This decline corresponded to a 30% +/- 7 decrease in GFPmt protein levels measured by Western blot of mitochondrial fractions, and a 72% +/- 5 decline in the import of newly synthesized GFPmt to mitochondria over a 1 h period 24-h after addition of 2 microM CCCP measured by autoradiography. CCCP partially depolarized mitochondria but was not lethal for up to five days.

CONCLUSIONS

This novel GFP-based flow cytometry assay is a rapid and sensitive technique for quantifying protein import to mitochondria in live neuronal cells.

Regulation of mitochondrial biogenesis in muscle by endurance exercise

Behavioural and hereditary conditions are known to decrease mitochondrial volume and function within skeletal muscle. This reduces endurance performance, and is manifest both at high- and low-intensity levels of exertion. A programme of regular endurance exercise, undertaken over a number of weeks, produces significant adaptations within skeletal muscle such that noticeable improvements in oxidative capacity are evident, and the related decline in endurance performance can be attenuated. Notwithstanding the important implications that this has for the highly trained endurance athlete, an improvement in mitochondrial volume and function through regular physical activity also endows the previously sedentary and/or aging population with an improved quality of life, and a greater functional independence. An understanding of the molecular and cellular mechanisms that govern the increases in mitochondrial volume with repeated bouts of exercise can provide insights into possible therapeutic interventions to care for those with mitochondrially-based diseases, and those unable to withstand regular physical activity. This review focuses on the recent developments in the molecular aspects of mitochondrial biogenesis in chronically exercising muscle. Specifically, we discuss the initial signalling events triggered by muscle contraction, the activation of transcription factors involved in both nuclear and mitochondrial DNA transcription, as well as the post-translational import mechanisms required for mitochondrial biogenesis. We consider the importance and relevance of chronic physical activity in the induction of mitochondrial biogenesis, with particular emphasis on how an endurance training programme could positively affect the age-related decline in mitochondrial content and delay the progression of age- and physical inactivity-related diseases.

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.

Reconstituted TOM core complex and Tim9/Tim10 complex of mitochondria are sufficient for translocation of the ADP/ATP carrier across membranes

Precursor proteins of the solute carrier family and of channel forming Tim components are imported into mitochondria in two main steps. First, they are translocated through the TOM complex in the outer membrane, a process assisted by the Tim9/Tim10 complex. They are passed on to the TIM22 complex, which facilitates their insertion into the inner membrane. In the present study, we have analyzed the function of the Tim9/Tim10 complex in the translocation of substrates across the outer membrane of mitochondria. The purified TOM core complex was reconstituted into lipid vesicles in which purified Tim9/Tim10 complex was entrapped. The precursor of the ADP/ATP carrier (AAC) was found to be translocated across the membrane of such lipid vesicles. Thus, these components are sufficient for translocation of AAC precursor across the outer membrane. Peptide libraries covering various substrate proteins were used to identify segments that are bound by Tim9/Tim10 complex upon translocation through the TOM complex. The patterns of binding sites on the substrate proteins suggest a mechanism by which portions of membrane-spanning segments together with flanking hydrophilic segments are recognized and bound by the Tim9/Tim10 complex as they emerge from the TOM complex into 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.

Compensatory responses of protein import and transcription factor expression in mitochondrial DNA defects

Defects in mitochondrial DNA (mtDNA) evoke distinctive responses in the nuclear genome, leading to altered mitochondrial biogenesis. We used C(2)C(12) cells depleted of mtDNA (rho(-) cells) and fibroblasts from a mitochondrial encephalopathy, lactic acidosis, and strokelike episodes (MELAS) patient to examine adaptations of the protein import machinery and transcription factors involved in mitochondrial biogenesis. In rho(-) cells, Tom20 and Tim23 protein levels were reduced by 25% and 59%, whereas mtHSP70 was induced by twofold relative to control cells. These changes were accompanied by a 21% increase in enhanced yellow fluorescent protein (EYFP) import into mitochondria in rho(-) cells (P < 0.05). In contrast, in MELAS cells mtHSP70 was elevated by 70%, whereas Tom20 and Tom34 protein levels were increased by 45% and 112% relative to control values. EYFP import was not altered in MELAS cells. In rho(-) cells, protein levels of the transcription factors nuclear respiratory factor-1 (NRF-1) and transcription factor A (Tfam) declined by 33% and 54%, whereas no change was observed for the coactivator peroxisome proliferator receptor-gamma coactivator-1alpha (PGC-1alpha). In contrast, Tfam was increased by 40% in MELAS cells. Rho(-) cells displayed reduced oxygen consumption (Vo(2)) and ATP levels, along with a twofold increase in lactate levels (P < 0.05). In electrically stimulated C(2)C(12) cells, 109%, 78%, 60%, and 67% increases were observed in mtDNA, Vo(2), cytochrome-c oxidase (COX) activity, and Tom34 levels, respectively (P < 0.05). Our findings suggest that compensatory adaptations occurred to maintain normal rates of protein import in response to mtDNA defects and support a role for contractile activity in reducing pathophysiology associated with mtDNA depletion. Because the expression of nuclear-encoded transcription factors and protein import machinery components was dependent on the type of mtDNA defect, these findings suggest involvement of distinct signaling cascades, each dependent on the type of mitochondrial defect, resulting in divergent changes in nuclear gene expression patterns.

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.

Transcription of TIM9, a new factor required for the petite-positive phenotype of Saccharomyces cerevisiae, is defective in spt7 mutants

TIM9 has been identified as an additional novel gene required for the petite-positive phenotype in Saccharomyces cerevisiae. tim9-1 was obtained through a screen for respiratory-deficient strains that are unable to survive in the absence of mitochondrial DNA. A point mutation found in the tim9-1 coding region converts codon 71 from Gly to Arg. Examination of genes encoding other Tim components indicated that the temperature-conditional alleles of essential genes for the viability of S. cerevisiae, TIM9, TIM10 and TIM12, are required for petite survival, while deletion of TIM8 and TIM13 has no notable effect on petite cell viability. Northern hybridization results suggested that the Spt7 transcription factor is strictly involved in transcription of TIM9 and that the synergistic lethality of tim9-1/spt7Delta dual mutations is due to the deficiency of TIM9 transcription together with defective function of the tim9-1 protein.

Genetic Features of Mitochondrial Respiratory Chain Disorders

ABSTRACT. Oxidative phosphorylation, i.e., ATP synthesis by the oxygen-consuming respiratory chain (RC), supplies most organs and tissues with a readily usable energy source, being functional before birth. Consequently, RC deficiencies can theoretically give rise to any symptom, in any organ or tissue, at any age and with any mode of inheritance, because of the twofold genetic origin of RC components (nuclear DNA and mitochondrial DNA). It was long wrongly considered that RC disorders originate from mutations of mitochondrial DNA, because for a long time only mutations or deletions of mitochondrial DNA were identified. However, the number of known disease-causing mutations in nuclear genes is steadily growing. These genes encode the various subunits of each complex, ancillary proteins functioning at different stages of holoenzyme biogenesis, including transcription, translation, chaperoning, addition of prosthetic groups, and protein assembly, and various enzymes involved in mitochondrial DNA metabolism.

Juxtaposition of the two distal CX3C motifs via intrachain disulfide bonding is essential for the folding of Tim10

The TIM10 complex, composed of the homologous proteins Tim10 and Tim9, chaperones hydrophobic proteins inserted at the mitochondrial inner membrane. A salient feature of the TIM10 complex subunits is their conserved "twin CX3C" motif. Systematic mutational analysis of all cysteines of Tim10 showed that their underlying molecular defect is impaired folding (demonstrated by circular dichroism, aberrant homo-oligomer formation, and thiol trapping assays). As a result of defective folding, clear functional consequences were manifested in (i) complex formation with Tim9, (ii) chaperone activity, and (iii) import into tim9ts mitochondria lacking both endogenous Tim9 and Tim10. The organization of the four cysteines in intrachain disulfides was determined by trypsin digestion and mass spectrometry. The two distal CX3C motifs are juxtaposed in the folded structure and disulfide-bonded to each other rather than within each other, with an inner cysteine pair connecting Cys44 with Cys61 and an outer pair between Cys40 and Cys65. These cysteine pairs are not equally important for folding and assembly; mutations of the inner Cys are severely affected and form wrong, non-native disulfides, in contrast to mutations of the outer Cys that can still maintain the native inner disulfide pair and display weaker functional defects. Taken together these data reveal this specific intramolecular disulfide bonding as the crucial mechanism for Tim10 folding and show that the inner cysteine pair has a more prominent role in this process.

Gastrointestinal manifestations of mitochondrial disease

Although non-specific gastrointestinal and hepatic symptoms are commonly found in most mitochondrial disorders, they are among the cardinal manifestations of several primary mitochondrial diseases, such as: mitochondrial neurogastrointestinal encephalomyopathy; mitochondrial DNA depletion syndrome; Alpers syndrome; and Pearson syndrome. Management of these heterogeneous disorders includes the empiric supplementation with various "mitochondrial cocktails," supportive therapies, and avoidance of drugs and conditions known to have a detrimental effect on the respiratory chain. There is a great need for improved methods of treatment and controlled clinical trials of existing therapies. Liver transplantation is successful in acquired cases; however neuromuscular involvement in primary mitochondrial disorders should be a contraindication for liver transplantation.

A heteroplasmic mitochondrial complex I gene mutation in adult-onset dystonia

Mitochondrial DNA (mtDNA) mutations can cause rare forms of dystonia, but the role of mtDNA mutations in other types of dystonia is not well understood. We now report identification by sequencing, restriction endonuclease analyses, and clonal analyses of a heteroplasmic missense A to G base pair substitution at nucleotide position 3796 (A3796G) in the gene encoding the ND1 subunit of mitochondrial complex I in a patient with adult-onset dystonia, spasticity, and core-type myopathy. The mutation converts a highly conserved threonine to an alanine. The same mutation subsequently was identified in 2 of 74 additional unrelated adult-onset dystonia patients. A muscle biopsy was obtained from 1 of these 2 subjects and this revealed abnormalities of electron transport chain (ETC) activities. The mutation was absent in 64 subjects with early onset dystonia, 82 normal controls, and 65 subjects with Parkinson's disease or multiple system atrophy. The A3796G mutation previously has been reported in 3 of 226 subjects from mitochondrial haplogroup H. Each of the 3 subjects in our study harboring the A3796G mutation was also from haplogroup H. However, a subgroup analysis of haplogroup H subjects from our study indicates that the A3796G mutation is significantly overrepresented among haplogroup H adult-onset dystonia subjects compared with haplogroup H controls (P<0.01). This difference remains significant even after excluding the index patient (P=0.04). These data suggest that, among haplogroup H subjects, the presence of the A3796G mutation increases the risk of developing adult-onset dystonia.

Spasmodic dysphonia in a patient with the A to G transition at nucleotide 8344 in mitochondrial DNA

Dystonia has been described in various diseases affecting mitochondrial function but spasmodic dysphonia, a form of focal dystonia, has not. We present a patient with action myoclonus affecting the hands and arms who carried the most common mutation in mitochondrial DNA causing the myoclonic epilepsy and ragged red fibers (MERRF) syndrome (the A-->G substitution at nucleotide 8344 in the tRNA(Lys) gene). This patient also had spasmodic dysphonia that was responsive to treatment with intralaryngeal botulinum toxin.

Interaction of the deafness-dystonia protein DDP/TIMM8a with the signal transduction adaptor molecule STAM1

The Mohr-Tranebjaerg-Jensen deafness-dystonia-optic atrophy protein DDP/TIMM8a is translated on cytoplasmic ribosomes but targeted ultimately to the mitochondrial intermembrane space, where it is involved in mitochondrial protein import. STAM1 is a cytoplasmic signal-transducing adaptor molecule implicated in cytokine signaling. We report here a direct interaction between DDP and STAM1, identified by yeast two-hybrid screening and confirmed by co-immunoprecipitation, fusion protein "pull downs," and nuclear redistribution assays. DDP coordinates Zn(2+), and Zn(2+) was found to stimulate the DDP-STAM1 interaction in vitro. Endogenous STAM1 localizes predominantly to early endosomes, and we found no evidence that STAM1 is imported into mitochondria in vitro. Thus, the DDP-STAM1 interaction likely occurs in the cytoplasm or at the mitochondrial outer membrane. The DDP-STAM1 interaction requires a coiled-coil region in STAM1 that overlaps with the immunoreceptor tyrosine-based activation motif (ITAM), a region previously shown to be important for interaction with Jak2/3 and hepatocyte growth factor-regulated tyrosine kinase substrate (Hrs). Thus, DDP binding may alter the interactions of STAM1 with several cytoplasmic proteins involved in cell signaling and endosomal trafficking.

Mechanisms of protein import into mitochondria

Apart from a handful of proteins encoded by the mitochondrial genome, most proteins residing in this organelle are nuclear-encoded and synthesised in the cytosol. Thus, delivery of proteins to their final destination depends on a network of specialised import components that form at least four main translocation complexes. The import machinery ensures that proteins earmarked for the mitochondrion are recognised and delivered to the organelle, transported across membranes, sorted to the correct compartment and assisted in overcoming energetic barriers.

The role of Tim9p in the assembly of the TIM22 import complexes

Tim9p is located in the soluble 70-kDa Tim9p-Tim10p complex and the 300-kDa membrane complex in the mitochondrial TIM22 protein import system, which mediates the import of inner membrane proteins. From a collection of temperature-sensitive mutants, we have analyzed two in detail. tim9-3 contained two mutations and tim9-19 contained one mutation, all located near the 'twin CX3C' motif that is conserved in the small Tim proteins. As a result, the import components in the tim9-3 mutant mitochondria were severely reduced and assembled into complexes of aberrant sizes. Protein import was severely reduced and Tim9p and Tim10p binding to in vitro imported ADP/ATP carrier was impaired. In the tim9-19 mutant mitochondria, the 300-kDa membrane complex was assembled, although the soluble 70-kDa Tim9p-Tim10p complex was not detectable. Protein import was decreased only two-fold. When coexpressed in Escherichia coli, tim9-19 and TIM10 proteins failed to assemble into a 70-kDa complex. Our findings suggest that residues near the 'twin CX3C' motif are important for the assembly of Tim9p in both the Tim9p-Tim10p complex and the 300-kDa membrane complex.

Insertion of hydrophobic membrane proteins into the inner mitochondrial membrane--a guided tour

Only a few mitochondrial proteins are encoded by the organellar genome. The majority of mitochondrial proteins are nuclear encoded and thus have to be transported into the organelle from the cytosol. Within the mitochondrion proteins have to be sorted into one of the four sub-compartments: the outer or inner membranes, the intermembrane space or the matrix. These processes are mediated by complex protein machineries within the different compartments that act alone or in concert with each other. The translocation machinery of the outer membrane is formed by a multi-subunit protein complex (TOM complex), that is built up by signal receptors and the general import pore (GIP). The inner membrane houses two multi-subunit protein complexes that each handles special subsets of mitochondrial proteins on their way to their final destination. According to their primary function these two complexes have been termed the pre-sequence translocase (or TIM23 complex) and the protein insertion complex (or TIM22 complex). The identification of components of these complexes and the analysis of the molecular mechanisms underlying their function are currently an exciting and fast developing field of molecular cell biology.

Mitochondrial biogenesis and the role of the protein import pathway

PURPOSE

The importance of the mitochondrial protein import pathway, discussed relative to other steps involved in the overall biogenesis of the organelle, are reviewed.

RESULTS

Mitochondrial biogenesis is a product of complex interactions between the nuclear and mitochondrial genomes. Signaling pathways, such as those activated by exercise, initiate the activation of transcription factors that increase the production of mRNA from nuclear and mitochondrial DNA. Nuclear gene products are translated in the cytosol as precursor proteins with inherent targeting signals. These precursor proteins interact with molecular chaperones that direct them to the import machinery of the outer membrane (Tom complex). The precursor is unfolded and transferred through the outer membrane, across the intermembrane space to the mitochondrial inner membrane translocases (Tim complex). Intramitochondrial components (mtHSP70) pull the precursor into the matrix, cleave off the targeting sequence (mitochondrial processing peptidase), and refold the protein (HSP60, cpn10) into its mature conformation. Physiological stressors such as contractile activity and thyroid hormone accelerate protein import into the mitochondria, coincident with an increase in the expression of some components of the import machinery. This is important for the overall expansion of the mitochondrial reticulum. Conversely, impairments in the import process can be a cause of mitochondrial dysfunction and disease.

CONCLUSIONS

Efforts to further characterize the components of the import machinery, to define the role of specific machinery components on the import rate, and to examine protein import function in a variety of mitochondrial diseases are warranted.

Functional consequences of thyroid hormone-induced changes in the mitochondrial protein import pathway

Thyroid hormone [3,5,3'-triiodo-l-thyronine (T(3))] induces phenotypic alterations in cardiac mitochondria, in part by influencing protein import and the expression of the import motor mitochondrial heat shock protein (mtHsp70). Here we examined the adaptability of translocases of the inner membrane (Tim) proteins, as well as the outer membrane receptor Tom34, to T(3). Administration of T(3) to rats for 5 days increased cardiac Tim23 and Tim44 mRNA levels by 55 and 50%, respectively, but had no effect on Tim17. T(3) treatment also induced a 45% increase in Tom34 mRNA, with no accompanying changes at the protein level, suggesting regulation at the posttranscriptional level. In H9c2 cardiac cells, Tim17 mRNA was elevated by 114% by 9 days of differentiation, whereas Tim23 and Tim44 declined by 25 and 29%, respectively. To determine the functional consequences of these T(3)-induced changes, malate dehydrogenase (MDH) import rates were measured in H9c2 cells stably overexpressing Tim44 and mtHsp70, either alone or in combination. MDH import remained unaltered in cells overexpressing Tim44 or in cells overexpressing both Tim44 and mtHsp70. However, when mtHsp70 was overexpressed alone, a 13% (P < 0.05) increase in MDH import rate was observed. These findings indicate that import machinery components are differentially regulated in response to stimuli that induce mitochondrial biogenesis, like T(3) and differentiation. In addition, the induction of an import machinery component in response to T(3) may not necessarily result in functional changes in protein import during mitochondrial biogenesis. Finally, mtHsp70 may play a regulatory role in the import process that is independent of its interaction with Tim44.

Mitochondrial disorders: clinical presentation and diagnostic dilemmas

The number of genes known to be involved in mitochondrial energy production and the elucidation of the function of their individual transcripts is still increasing. Although at this stage it is impossible to predict the number of human genes necessary for mitochondrial biogenesis and maintenance, the total number in humans will most probably exceed the number of mitochondrial genes found in, for example, the budding yeast, which is about 800. Without doubt we have only seen the tip of the iceberg of the clinical spectrum of mitochondrial disorders. Recent findings such as mutations in structural complex II genes in certain tumours emphasize the need to think outside the classical clinical presentation. We propose the consideration of a mitochondrial disorder in every chronic, intermittent or progressive disorder with single system or multisystem involvement, even if lactic acid is normal, and discuss such dilemmas as whether we should 'scrape the barrel' in every patient that are raised by this statement. The characterization of mitochondrial and nuclear DNA mutations in patients with enzymatically established mitochondrial defects has taught us that several of the current clinical and diagnostic assumptions have to be altered or even eliminated. The most challenging future task will be the development of new diagnostic criteria covering the expanding clinical spectrum of mitochondrial disorders.

Mitochondrial import of the ADP/ATP carrier: the essential TIM complex of the intermembrane space is required for precursor release from the TOM complex

The mitochondrial intermembrane space contains a protein complex essential for cell viability, the Tim9-Tim10 complex. This complex is required for the import of hydrophobic membrane proteins, such as the ADP/ATP carrier (AAC), into the inner membrane. Different views exist about the role played by the Tim9-Tim10 complex in translocation of the AAC precursor across the outer membrane. For this report we have generated a new tim10 yeast mutant that leads to a strong defect in AAC import into mitochondria. Thereby, for the first time, authentic AAC is stably arrested in the translocase complex of the outer membrane (TOM), as shown by antibody shift blue native electrophoresis. Surprisingly, AAC is still associated with the receptors Tom70 and Tom20 when the function of Tim10 is impaired. The nonessential Tim8-Tim13 complex of the intermembrane space is not involved in the transfer of AAC across the outer membrane. These results define a two-step mechanism for translocation of AAC across the outer membrane. The initial insertion of AAC into the import channel is independent of the function of Tim9-Tim10; however, completion of translocation across the outer membrane, including release from the TOM complex, requires a functional Tim9-Tim10 complex.

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.

Protein import into and across the mitochondrial inner membrane: role of the TIM23 and TIM22 translocons

Import of nuclear-encoded mitochondrial proteins requires the action of at least two different import machines, called translocons, in the mitochondrial inner membrane (IM). The TIM23 complex mediates the translocation of proteins into the mitochondria matrix, whereas the TIM22 complex is required for the insertion of polytopic proteins into the IM. While the two translocons are distinct and composed of separate subunits, the essential reactions in each complex are carried out by homologous proteins. In addition, the core components of both the TIM23 and TIM22 translocons have been shown to form aqueous pores in the mitochondrial IM. In this review, we summarize what is known about import of proteins across the mitochondrial IM.

Import and assembly of proteins into mitochondria of mammalian cells

Most of our knowledge regarding the process of protein import into mitochondria has come from research employing fungal systems. This review outlines recent advances in our understanding of this process in mammalian cells. In particular, we focus on the characterisation of cytosolic molecular chaperones that are involved in binding to mitochondrial-targeted preproteins, as well as the identification of both conserved and novel subunits of the import machineries of the outer and inner mitochondrial membranes. We also discuss diseases associated with defects in import and assembly of mitochondrial proteins and what is currently known about the regulation of import in mammals.

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.