Protein import into mammalian mitochondria

Mitochondrial association of myocilin, product of a glaucoma gene, in human trabecular meshwork cells

The trabecular meshwork (TM), an ocular tissue next to the cornea, is a major site for regulation of the aqueous humor outflow. Malfunctioning of this tissue is believed to be responsible for development of glaucoma, a major blinding disease. Myocilin is a gene directly linked to the most common form of glaucoma. Its protein product has been localized to both intra- and extra-cellular sites in TM cells. This study was to investigate the association of myocilin with mitochondria in TM cells. In vitro mitochondrial import assays showed that myocilin was imported to the TM mitochondria, targeting to mitochondrial membranes and/or the intermembrane space. The targeting was mediated mostly via the amino-terminal region of myocilin. When myocilin expression was induced either by treatment with dexamethasone or transfection with a myocilin construct, the mitochondrial membrane potential in TM cells, as assessed by JC-1 staining, was lowered. Subcellular fractionation and Western blot analyses confirmed that a portion of myocilin sedimented with the mitochondrial fractions. Upon anti-Fas treatment to provoke apoptosis, an increase of myocilin distribution in cytosolic fraction was observed, suggesting that myocilin was partially released from mitochondrial compartments. These results confirmed the association of myocilin with TM cell mitochondria and indicated that myocilin may have a proapoptotic role in TM cells.

Cell-penetrating peptides do not cross mitochondrial membranes even when conjugated to a lipophilic cation: evidence against direct passage through phospholipid bilayers

CPPs (cell-penetrating peptides) facilitate the cellular uptake of covalently attached oligonucleotides, proteins and other macromolecules, but the mechanism of their uptake is disputed. Two models are proposed: direct movement through the phospholipid bilayer and endocytic uptake. Mitochondria are a good model system to distinguish between these possibilities, since they have no vesicular transport systems. Furthermore, CPP-mediated delivery of macromolecules to the mitochondrial matrix would be a significant breakthrough in the study of mitochondrial function and dysfunction, and could also lead to new therapies for diseases caused by mitochondrial damage. Therefore we investigated whether two CPPs, penetratin and Tat, could act as mitochondrial delivery vectors. We also determined whether conjugation of the lipophilic cation TPP (triphenylphosphonium) to penetratin or Tat facilitated their uptake into mitochondria, since TPP leads to uptake of attached molecules into mitochondria driven by the membrane potential. Neither penetratin nor Tat, nor their TPP conjugates, are internalized by isolated mitochondria, indicating that these CPPs cannot cross mitochondrial phospholipid bilayers. Tat and TPP-Tat are taken up by cells, but they accumulate in endosomes and do not reach mitochondria. We conclude that CPPs cannot cross mitochondrial phospholipid bilayers, and therefore cannot deliver macromolecules directly to mitochondria. Our findings shed light on the mechanism of uptake of CPPs by cells. The lack of direct movement of CPPs through mitochondrial phospholipid bilayers, along with the observed endosomal accumulation of Tat and TPP-Tat in cells, makes it unlikely that CPPs enter cells by direct membrane passage, and instead favours cellular uptake via an endocytic pathway.

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

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

Import of proteins into mitochondria: a novel pathomechanism for progressive neurodegeneration

The vast majority of mitochondrial proteins are encoded as precursors by the nuclear genome. A major aspect of mitochondrial biogenesis is therefore the transfer of nuclear-encoded, cytosplasmically synthesized precursor proteins across and into the mitochondrial membranes. During the past years the use of simple model organisms such as the yeasts S. cerevisiae and N. crassa has helped considerably to identify and unravel the structure and function of a substantial number of components involved in targeting of nuclear-encoded preproteins to mitochondria. Several pathways and a number of components were characterized that are involved in guiding mitochondrial preproteins to their specific sites of function. In particular, import of nuclear-encoded precursor proteins into and across the mitochondrial inner membrane is mediated by two distinct translocases, the TIM23 complex and the TIM22 complex. Both TIM complexes cooperate with the general preprotein translocase of the outer membrane, TOM complex. The TIM complexes differ in the their substrate specificity. While the TIM23 complex mediates import of preproteins with a positively charged matrix targeting signal, the TIM22 complex facilitates the insertion of a class of hydrophobic proteins with internal targeting signals into the inner membrane. Most recently the rapid progress of research has allowed elucidation of a new mitochondrial disease on the molecular level. This rare X-linked progressive neurodegenerative disorder, named Mohr-Tranebjaerg (MT syndrome), is caused by mutations in the DDP1 gene and includes sensorineural deafness, blindness, mental retardation and a complex movement disorder. The analysis of the novel pathomechanism is based on the homology of the affected DDP1 protein to a family of conserved yeast components acting along the TIM22 pathway. This contribution briefly summarizes the current knowledge of the pathways of protein import and proposes a mechanism to explain how defective import leads to neurodegeneration.

New therapies for optic neuropathies: development in experimental models

Experimental models of human diseases have affected the design and direction of both basic and clinical research into understanding the pathogenesis and treatments of demyelinating disease, stroke, and hereditary disorders of the central nervous system. However, in spite of major advances in molecular research that have linked Leber Hereditary Optic Neuropathy to mutations in mitochondrial DNA, there has been relatively little focus in applying basic scientific methodologies to optic neuropathies other than glaucoma. The relative absence of detailed scientific knowledge about the basic mechanisms involved in the pathogenesis of optic nerve injury has contributed to the use of empiric therapies for neuro-ophthalmic optic neuropathies. Over the past decade major clinical trials, such as the Optic Neuritis Treatment Trial and Ischemic Optic Neuropathy Decompression Trial, have proven that currently available treatment options for demyelinating and ischemic optic neuropathies are ineffective and can even be harmful. Although the pathogenesis of visual failure in demyelinating, ischemic, and hereditary optic neuropathies appears diverse, a final common pathway for irreparable optic nerve injury may exist. This article reviews several models of experimental optic neuropathies that may aid in the development of novel treatments for neuro-ophthalmic disorders of the optic nerve during the 21st century.

Isoform-specific localization of A-RAF in mitochondria

RAF kinase is a family of isoforms including A-RAF, B-RAF, and C-RAF. Despite the important role of RAF in cell growth and proliferation, little evidence exists for isoform-specific function of RAF family members. Using Western analysis and immunogold labeling, A-RAF was selectively localized in highly purified rat liver mitochondria. Two novel human proteins, which interact specifically with A-RAF, were identified, and the full-length sequences are reported. These proteins, referred to as hTOM and hTIM, are similar to components of mitochondrial outer and inner membrane protein-import receptors from lower organisms, implicating their involvement in the mitochondrial transport of A-RAF. hTOM contains multiple tetratricopeptide repeat (TPR) domains, which function in protein-protein interactions. TPR domains are frequently present in proteins involved in cellular transport systems. In contrast, protein 14-3-3, an abundant cytosolic protein that participates in many facets of signal transduction, was found to interact with C-RAF but not with A-RAF N-terminal domain. This information is discussed in view of the important role of mitochondria in cellular functions involving energy balance, proliferation, and apoptosis and the potential role of A-RAF in regulating these systems.

Mitochondrial import of the long and short isoforms of human uncoupling protein 3

Human uncoupling protein 3 (UCP3) has two RNA transcripts that arise from the differential processing of the same gene product. One encodes the full length protein (UCP3L) while the other encodes a truncated version (UCP3S) lacking the sixth membrane spanning domain. The roles of the two isoforms are not known, but a mutation that decreases the proportion of UCP3L decreases fat oxidation and increases susceptibility to obesity. In the ADP/ATP carrier, a protein closely related to UCP3, the sixth membrane spanning domain is required for insertion into the inner membrane. Therefore, defective membrane insertion of UCP3S may account for the different effects of the two isoforms in vivo. We investigated mitochondrial import of the two UCP3 isoforms. When epitope-tagged versions of UCP3S and UCP3L were expressed in COS7 cells, both were inserted into the mitochondrial inner membrane. Translation in vitro followed by incubation with isolated mitochondria showed that both isoforms were inserted into the inner membrane, however, the insertion of UCP3S was significantly slower.

Identification of eukaryotic parvulin homologues: a new subfamily of peptidylprolyl cis-trans isomerases

We report here the existence of a subfamily of eukaryotic parvulin proteins that have strong sequence homology with E. coli parvulin, but lack the WW domain found in previously described eukarytoic parvulins. We hence term members of this subfamily EPVH (eukaryotic parvulin homologue). We describe the characterisation of hEPVH (human eukaryotic parvulin homologue). Immunogold labelling transmission electron microscopy reveals that hEPVH is preferentially localised in the mitochondrial matrix. The homology of hEPVH with its prokaryotic ancestor supports the hypothesis that this protein may have a mitochondrial function. An essential role in this organelle may explain the need for a high degree of conservation of this protein between distantly related species.

Recruitment of an alternatively spliced form of synaptojanin 2 to mitochondria by the interaction with the PDZ domain of a mitochondrial outer membrane protein

Synaptojanin 1 is an inositol 5'-phosphatase highly enriched in nerve terminals with a putative role in recycling of synaptic vesicles. We have previously described synaptojanin 2, which is more broadly expressed as multiple alternatively spliced forms. Here we have identified and characterized a novel mitochondrial outer membrane protein, OMP25, with a single PDZ domain that specifically binds to a unique motif in the C-terminus of synaptojanin 2A. This motif is encoded by the exon sequence specific to synaptojanin 2A. OMP25 mRNA is widely expressed in rat tissues. OMP25 is localized to the mitochondrial outer membrane via the C-terminal transmembrane region, with the PDZ domain facing the cytoplasm. Overexpression of OMP25 results in perinuclear clustering of mitochondria in transfected cells. This effect is mimicked by enforced expression of synaptojanin 2A on the mitochondrial outer membrane, but not by the synaptojanin 2A mutants lacking the inositol 5'-phosphatase domain. Our findings provide evidence that OMP25 mediates recruitment of synaptojanin 2A to mitochondria and that modulation of inositol phospholipids by synaptojanin 2A may play a role in maintenance of the intracellular distribution of mitochondria.

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.

Inhibition of protein import into mitochondria by amphiphilic cations: potential targets and mechanism of action

In this paper we describe for the first time the inhibitory effect of three amphiphilic cations, trifluoperazine, propranolol and dibucaine on mitochondrial protein import. The amphiphilic cations did not affect binding of mitochondrial precursor proteins to mitochondria. Import into mitoplasts was affected in a similar manner to intact mitochondria, indicating that the protein import machinery of the inner membrane of mitochondria was responsible for the observed effect. At concentrations which completely inhibited protein import, the amphiphilic cations did not affect the membrane potential (DeltaPsi) across the inner membrane. The inhibitory potency of amphiphilic cations reflects their lipid/water partition coefficient and relatively high concentrations of the drugs were required for complete inhibition, hence we propose that the mechanism of protein import inhibition by amphiphilic cations is due to membrane perturbing effects. We discuss the implications of our findings in view of the possible connection between various inner mitochondrial membrane channels and the protein import pore.

Characterization of mammalian translocase of inner mitochondrial membrane (Tim44) isolated from diabetic newborn mouse kidney

Mammalian translocase of mitochondrial inner membrane (mTim44) was isolated during representational difference analysis of cDNA from diabetic mouse kidney. Streptozotocin-induced diabetic mouse kidney cDNA was prepared and subtracted by normal mouse kidney cDNA. By using one of the isolated cDNA fragments as a screening probe, full-length cDNA of mTim44 was isolated from lambdaZAP kidney cDNA library. At the nucleotide level, mTim44 did not exhibit significant homology with any known genes; however, at the amino acid level, it had 50% similarity and 29% identity with yeast Tim44. C-terminal FLAG epitope-tagged mTim44 fusion protein was transiently expressed in COS7 cells. By using anti-FLAG epitope M2 monoclonal antibody, mTim44 was found to have its subcellular localization associated with mitochondria. By immunoelectron microscopy, mTim44 was seen in the paracrystalline structures within the mitochondria, as well as in their cristae. Mitochondrial import assay of in vitro translated mTim44 indicated that its precursor product ( approximately 50 kDa) was imported and proteolytically processed to a mature approximately 44-kDa protein, and its translocation was inner membrane potential (DeltaPsi)-dependent. Imported mTim44 was protected from protease digestion in which outer membranes were selectively permeabilized with digitonin. The mature form of mTim44 could be recovered in the supernatant of sonicated mitochondrial membrane fraction treated with 0.1 M Na2CO3, pH 11.5. The data indicate that mTim44 is a mitochondrial inner membrane protein, one of the members of the mammalian TIM complex and up-regulated in hyperglycemic states.