Mitochondrial Targeting Signal Research Articles

Role of polynucleotide kinase/phosphatase in mitochondrial DNA repair

Mutations in mitochondrial DNA (mtDNA) are implicated in a broad range of human diseases and in aging. Compared to nuclear DNA, mtDNA is more highly exposed to oxidative damage due to its proximity to the respiratory chain and the lack of protection afforded by chromatin-associated proteins. While repair of oxidative damage to the bases in mtDNA through the base excision repair pathway has been well studied, the repair of oxidatively induced strand breaks in mtDNA has been less thoroughly examined. Polynucleotide kinase/phosphatase (PNKP) processes strand-break termini to render them chemically compatible for the subsequent action of DNA polymerases and ligases. Here, we demonstrate that functionally active full-length PNKP is present in mitochondria as well as nuclei. Downregulation of PNKP results in an accumulation of strand breaks in mtDNA of hydrogen peroxide-treated cells. Full restoration of repair of the H2O2-induced strand breaks in mitochondria requires both the kinase and phosphatase activities of PNKP. We also demonstrate that PNKP contains a mitochondrial-targeting signal close to the C-terminus of the protein. We further show that PNKP associates with the mitochondrial protein mitofilin. Interaction with mitofilin may serve to translocate PNKP into mitochondria.

Hepatic and extrahepatic distribution of ornithine urea cycle enzymes in holocephalan elephant fish (Callorhinchus milii)

Cartilaginous fish comprise two subclasses, the Holocephali (chimaeras) and Elasmobranchii (sharks, skates and rays). Little is known about osmoregulatory mechanisms in holocephalan fishes except that they conduct urea-based osmoregulation, as in elasmobranchs. In the present study, we examined the ornithine urea cycle (OUC) enzymes that play a role in urea biosynthesis in the holocephalan elephant fish, Callorhinchus milii (cm). We obtained a single mRNA encoding carbamoyl phosphate synthetase III (cmCPSIII) and ornithine transcarbamylase (cmOTC), and two mRNAs encoding glutamine synthetases (cmGSs) and two arginases (cmARGs), respectively. The two cmGSs were structurally and functionally separated into two types: brain/liver/kidney-type cmGS1 and muscle-type cmGS2. Furthermore, two alternatively spliced transcripts with different sizes were found for cmgs1 gene. The longer transcript has a putative mitochondrial targeting signal (MTS) and was predominantly expressed in the liver and kidney. MTS was not found in the short form of cmGS1 and cmGS2. A high mRNA expression and enzyme activities were found in the liver and muscle. Furthermore, in various tissues examined, mRNA levels of all the enzymes except cmCPSIII were significantly increased after hatching. The data show that the liver is the important organ for urea biosynthesis in elephant fish, but, extrahepatic tissues such as the kidney and muscle may also contribute to the urea production. In addition to the role of the extrahepatic tissues and nitrogen metabolism, the molecular and functional characteristics of multiple isoforms of GSs and ARGs are discussed.

Abstract 9466: Recombinant Mitochondrial Transcriptional Factor A Protein Attenuates Pathological Remodeling in Cardiac Myocytes

Introduction: Mitochondrial transcriptional factor A (TFAM), a nucleoid protein of mitochondrial DNA (mtDNA), is essential not only for transcription and replication but also maintenance of mtDNA. We have previously reported that, in failing myocardium, TFAM expression and the myocardial mtDNA copy number were decreased. Moreover, in myocardial infarction, overexpression of TFAM attenuated the decrease in the mtDNA copy number, ameliorated cardiac remodeling, and markedly improved survival. Since exogenously-administrated recombinant TFAM protein has been shown to enter mitochondria in cultured cells, we examined our hypothesis that recombinant TFAM protein administration attenuates pathological remodeling in cardiac myocytes. Methods and Results: We prepared recombinant human TFAM including mitochondrial targeting signal by glutathione s-transferase fusion protein purification protocol. Recombinant TFAM was successfully recruited into mitochondria of rat neonatal cardiac myocytes, whereas TFAM without mitochondrial targeting signal was observed in nucleus. Treatment with TFAM increased the mtDNA copy number dose-dependently (100 nM: 1.8±0.1-folds), whereas it did not change morphology or population of mitochondria (electron microscopy). To elucidate the mechanism of TFAM on signaling pathways in cardiac hypertrophy, we investigated the effects of TFAM on the nuclear factor of activated T cell (NFAT) signaling, which is a major transcriptional factor regulating pathological hypertrophy and remodeling. TFAM totally abolished NFAT nuclear translocation induced by both angiotensin II (41±3% to 2±1%, p<0.01) and endothelin-1 (43±3% to 3±1%, p<0.01), suppressing respective NFAT transcriptional activity and NFAT-dependent gene expression. TFAM inhibited subsequent morphological hypertrophy of cardiac myocytes (angiotensin II: 1686±37 to 1167±20 μ m 2 , p<0.01, and endothelin-1: 1759±44 to 1206±21 μ m 2 , p<0.01). Conclusion: Recombinant TFAM increases the mtDNA copy number, and attenuates angiotensin II and endothelin-1-induced hypertrophy via inhibiting NFAT signaling. Recombinant TFAM would be an attractive, novel therapeutic strategy for cardiac hypertrophy and remodeling.

Bimodal targeting of cytochrome P450s to endoplasmic reticulum and mitochondria: the concept of chimeric signals

Targeting signals are critical for proteins to find their specific cellular destination. Signals for protein targeting to the endoplasmic reticulum (ER), mitochondria, peroxisome and nucleus are distinct and the mechanisms of protein translocation across these membrane compartments also vary markedly. Recently, however, a number of proteins have been shown to be present in multiple cellular sites such as mitochondria and ER, cytosol and mitochondria, plasma membrane and mitochondria, and peroxisome and mitochondria suggesting the occurrence of multimodal targeting signals in some cases. Cytochrome P450 monooxygenases (CYPs), which play crucial roles in pharmacokinetics and pharmacodynamics of drugs and toxins, are the prototype of bimodally targeted proteins. Several members of family 1, 2 and 3 CYPs have now been reported to be associated with mitochondria and plasma membrane in addition to the ER. This review highlights the mechanisms of bimodal targeting of CYP1A1, 2B1, 2E1 and 2D6 to mitochondria and ER. The bimodal targeting of these proteins is driven by their N-terminal signals which carry essential elements of both ER targeting and mitochondria targeting signals. These multimodal signals have been termed chimeric signals appropriately to describe their dual targeting property. The cryptic mitochondrial targeting signals of CYP2B1, 2D6, 2E1 require activation by protein kinase A or protein kinase C mediated phosphorylation at sites immediately flanking the targeting signal and/or membrane anchoring regions. The cryptic mitochondria targeting signal of CYP1A1 requires activation by endoproteolytic cleavage by a cytosolic endoprotease, which exposes the mitochondrial signal. This review discusses both mechanisms of bimodal targeting and toxicological consequences of mitochondria targeted CYP proteins.

Mitochondria as a sequestration site for incomplete TCRβ peptides: The TCRβ transmembrane domain is a sufficient mitochondrial targeting signal

The existence of incomplete T cell receptor (TCR) mRNA forms, including germline transcripts and products of unfruitful TCR rearrangements, has long been known. However, it is unclear whether these molecules are functional. We have previously shown that T cells also contain truncated TCRβ peptides that lack the N-terminal part and contain C-terminus sequences. These partial forms of TCRβ, target the mitochondrion and induce apoptosis, exhibiting a novel mode of TCR mediated cell death. Here we aimed at analyzing the minimal TCR sequences that direct the peptide to the mitochondrion. It is shown that truncated TCRβ, targets mitochondria and induces mitochondrial perinuclear clustering, in both monkey COS-7 and human 293 cells. These phenomena are mediated by the C-terminus of the molecule. Whereas the positively charged amino acids flanking the transmembrane domain (TMD) of TCRβ are beneficial for this process, they are not essential. Indeed, the isolated TMD of TCRβ serves as a sufficient mitochondrial targeting sequence. These results indicate that any given partial form of TCRβ, that contains the TMD, is bound to be sequestered by the mitochondrion. This may assure that incomplete TCR forms would not interfere with correct TCR complex formation.

N-acetylglutamate synthase deficiency: an insight into the genetics, epidemiology, pathophysiology, and treatment

The conversion of ammonia into urea by the human liver requires the coordinated function of the 6 enzymes and 2 transporters of the urea cycle. The initial and rate-limiting enzyme of the urea cycle, carbamylphosphate synthetase 1 (CPS1), requires an allosteric activator, N-acetylglutamate (NAG). The formation of this unique cofactor from glutamate and acetyl Coenzyme-A is catalyzed by N-acetylglutamate synthase (NAGS). An absence of NAG as a consequence of NAGS deficiency may compromise flux through CPS1 and result in hyperammonemia. The NAGS gene encodes a 528-amino acid protein, consisting of a C-terminal catalytic domain, a variable segment, and an N-terminal mitochondrial targeting signal. Only 22 mutations in the NAGS gene have been reported to date, mostly in the catalytic domain. NAGS is primarily expressed in the liver and intestine. However, it is also surprisingly expressed in testis, stomach and spleen, and during early embryonic development at levels not concordant with the expression of other urea cycle enzymes, CPS1, or ornithine transcarbamylase. The purpose of NAGS expression in these tissues, and its significance to NAGS deficiency is as yet unknown. Inherited NAGS deficiency is the rarest of the urea cycle disorders, and we review the currently reported 34 cases. Treatment of NAGS deficiency with N-carbamyglutamate, a stable analog of NAG, can restore deficient urea cycle function and normalize blood ammonia in affected patients.

Structure-Function Analysis of Diacylglycerol Acyltransferase Sequences from 70 Organisms

BackgroundDiacylglycerol acyltransferase families (DGATs) catalyze the final and rate-limiting step of triacylglycerol (TAG) biosynthesis in eukaryotic organisms. Understanding the roles of DGATs will help to create transgenic plants with value-added properties and provide clues for therapeutic intervention for obesity and related diseases. The objective of this analysis was to identify conserved sequence motifs and amino acid residues for better understanding of the structure-function relationship of these important enzymes.Results117 DGAT sequences from 70 organisms including plants, animals, fungi and human are obtained from database search using tung tree DGATs. Phylogenetic analysis separates these proteins into DGAT1 and DGAT2 subfamilies. These DGATs are integral membrane proteins with more than 40% of the total amino acid residues being hydrophobic. They have similar properties and amino acid composition except that DGAT1s are approximately 20 kDa larger than DGAT2s. DGAT1s and DGAT2s have 41 and 16 completely conserved amino acid residues, respectively, although only two of them are shared by all DGATs. These residues are distributed in 7 and 6 sequence blocks for DGAT1s and DGAT2s, respectively, and located at the carboxyl termini, suggesting the location of the catalytic domains. These conserved sequence blocks do not contain the putative neutral lipid-binding domain, mitochondrial targeting signal, or ER retrieval motif. The importance of conserved residues has been demonstrated by site-directed and natural mutants.ConclusionsThis study has identified conserved sequence motifs and amino acid residues in all 117 DGATs and the two subfamilies. None of the completely conserved residues in DGAT1s and DGAT2s is present in recently reported isoforms in the multiple sequences alignment, raising an important question how proteins with completely different amino acid sequences could perform the same biochemical reaction. The sequence analysis should facilitate studying the structure-function relationship of DGATs with the ultimate goal to identify critical amino acid residues for engineering superb enzymes in metabolic engineering and selecting enzyme inhibitors in therapeutic application for obesity and related diseases.

Nuclear translocation of glutathione S-transferase π is mediated by a non-classical localization signal

Glutathione S-transferase π (GSTπ), a member of the GST family of multifunctional enzymes, is highly expressed in human placenta and involved in the protection of cellular components against electrophilic compounds or oxidative stress. We have recently found that GSTπ is expressed in the cytoplasm, mitochondria, and nucleus in some cancer cells, and that the nuclear expression of GSTπ appears to correlate with resistance to anti-cancer drugs. Although the mitochondrial targeting signal of GSTπ was previously identified in the amino-terminal region, the mechanism of nuclear translocation remains completely unknown. In this study, we find that the region of GSTπ195–208 is critical for nuclear translocation, which is mediated by a novel and non-classical nuclear localization signal. In addition, using an in vitro transport assay, we demonstrate that the nuclear translocation of GSTπ depends on the cytosolic extract and ATP. Although further experiments are needed to understand in depth the precise mechanism of nuclear translocation of GSTπ, our results may help to establish more efficient anti-cancer therapy, especially with respect to resistance to anti-cancer drugs.

Identification of a Redox-Modulatory Interaction Between Uncoupling Protein 3 and Thioredoxin 2 in the Mitochondrial Intermembrane Space

Uncoupling protein 3 (UCP3) is a member of the mitochondrial solute carrier superfamily that is enriched in skeletal muscle and controls mitochondrial reactive oxygen species (ROS) production, but the mechanisms underlying this function are unclear. The goal of this work focused on the identification of mechanisms underlying UCP3 functions. Here we report that the N-terminal, intermembrane space (IMS)-localized hydrophilic domain of mouse UCP3 interacts with the N-terminal mitochondrial targeting signal of thioredoxin 2 (Trx2), a mitochondrial thiol reductase. Cellular immunoprecipitation and in vitro pull-down assays show that the UCP3-Trx2 complex forms directly, and that the Trx2 N-terminus is both necessary and sufficient to confer UCP3 binding. Mutation studies show that neither a catalytically inactivated Trx2 mutant, nor a mutant Trx2 bearing the N-terminal targeting sequence of cytochrome c oxidase (COXMTS-Trx2) bind UCP3. Biochemical analyses using permeabilized mitochondria, and live cell experiments using bimolecular fluorescence complementation show that the UCP3-Trx2 complex forms specifically in the IMS. Finally, studies in C2C12 myocytes stably overexpressing UCP3 (2.5-fold) and subjected to Trx2 knockdown show that Trx2 is required for the UCP3-dependent mitigation of complex III-driven mitochondrial ROS generation. UCP3 expression was increased in mice fed a high fat diet, leading to increased localization of Trx2 to the IMS. UCP3 overexpression also increased expression of the glucose transporter GLUT4 in a Trx2-dependent fashion. This is the first report of a mitochondrial protein-protein interaction with UCP3 and the first demonstration that UCP3 binds directly, and in cells and tissues with mitochondrial thioredoxin 2. These studies identify a novel UCP3-Trx2 complex, a novel submitochondrial localization of Trx2, and a mechanism underlying UCP3-regulated mitochondrial ROS production.

The MYB80 Transcription Factor Is Required for Pollen Development and the Regulation of Tapetal Programmed Cell Death inArabidopsis thaliana

Arabidopsis thaliana MYB80 (formerly MYB103) is expressed in the tapetum and microspores between anther developmental stages 6 and 10. MYB80 encodes a MYB transcription factor that is essential for tapetal and pollen development. Using microarray analysis of anther mRNA, we identified 404 genes differentially expressed in the myb80 mutant. Employing the glucocorticoid receptor system, the expression of 79 genes was changed when MYB80 function was restored in the myb80 mutant following induction by dexamethasone. Thirty-two genes were analyzed using chromatin immunoprecipitation, and three were identified as direct targets of MYB80. The genes encode a glyoxal oxidase (GLOX1), a pectin methylesterase (VANGUARD1), and an A1 aspartic protease (UNDEAD). All three genes are expressed in the tapetum and microspores. Electrophoretic mobility shift assays confirmed that MYB80 binds to all three target promoters, with the preferential binding site containing the CCAACC motif. TUNEL assays showed that when UNDEAD expression was silenced using small interfering RNA, premature tapetal and pollen programmed cell death occurred, resembling the myb80 mutant phenotype. UNDEAD possesses a mitochondrial targeting signal and may hydrolyze an apoptosis-inducing protein(s) in mitochondria. The timing of tapetal programmed cell death is critical for pollen development, and the MYB80/UNDEAD system may regulate that timing.

In silico survey of the mitochondrial protein uptake and maturation systems in the brown alga Ectocarpus siliculosus.

The acquisition of mitochondria was a key event in eukaryote evolution. The aim of this study was to identify homologues of the components of the mitochondrial protein import machinery in the brown alga Ectocarpus and to use this information to investigate the evolutionary history of this fundamental cellular process. Detailed searches were carried out both for components of the protein import system and for related peptidases. Comparative and phylogenetic analyses were used to investigate the evolution of mitochondrial proteins during eukaryote diversification. Key observations include phylogenetic evidence for very ancient origins for many protein import components (Tim21, Tim50, for example) and indications of differences between the outer membrane receptors that recognize the mitochondrial targeting signals, suggesting replacement, rearrangement and/or emergence of new components across the major eukaryotic lineages. Overall, the mitochondrial protein import components analysed in this study confirmed a high level of conservation during evolution, indicating that most are derived from very ancient, ancestral proteins. Several of the protein import components identified in Ectocarpus, such as Tim21, Tim50 and metaxin, have also been found in other stramenopiles and this study suggests an early origin during the evolution of the eukaryotes.

Characterization of the mitochondrial isoform of FASTK

The Fas‐Activated Serine/Threonine kinase (FASTK) is a protein that regulates alternative splicing in the nucleus (Simarro et al., 2007). This protein is also present in the cytosol where it is associated to stress granules and processing bodies (Kedersha et al., 2005). FASTK is the founding member of the FASTKD family, which contains five other proteins (1–5). These proteins localize to mitochondria (Simarro et al., 2009) and recently FASTKD2 has been reported to regulate cytochrome c oxidase activity (Ghezzi et al., 2008), whereas FASTKD3 is required for mitochondrial respiration. Importantly, FASTKD2 has been identified to be mutated in a case of mitochondrial encephalomyopathy.We have characterized the two different isoforms of FASTK and we have found that the isoform 1 of the protein contains an internal and atypical mitochondrial targeting signal. This isoform is in the mitochondrial inner membrane with its domains exposed to the matrix. Interestingly, we also found that in mitochondria FASTK is not equally distributed but is rather concentrated in particular foci within mitochondria. We are now investigating the function of FASTK in mitochondria, as well as of the FASTKDs.

The fission yeast Schizosaccharomyces pombe has two distinct tRNase ZLs encoded by two different genes and differentially targeted to the nucleus and mitochondria

tRNase Z is the endonuclease that is involved in tRNA 3'-end maturation by removal of the 3'-trailer sequences from tRNA precursors. Most eukaryotes examined to date, including the budding yeast Saccharomyces cerevisiae and humans, have a single long form of tRNase Z (tRNase ZL). In contrast, the fission yeast Schizosaccharomyces pombe contains two candidate tRNase ZLs encoded by the essential genes sptrz1+ and sptrz2+. In the present study, we have expressed recombinant SpTrz1p and SpTrz2p in S. pombe. Both recombinant proteins possess precursor tRNA 3'-endonucleolytic activity in vitro. SpTrz1p localizes to the nucleus and has a simian virus 40 NLS (nuclear localization signal)-like NLS at its N-terminus, which contains four consecutive arginine and lysine residues between residues 208 and 211 that are critical for the NLS function. In contrast, SpTrz2p is a mitochondrial protein with an N-terminal MTS (mitochondrial-targeting signal). High-level overexpression of sptrz1+ has no detectable phenotypes. In contrast, strong overexpression of sptrz2+ is lethal in wild-type cells and results in morphological abnormalities, including swollen and round cells, demonstrating that the correct expression level of sptrz2+ is critical. The present study provides evidence for partitioning of tRNase Z function between two different proteins in S. pombe, although we cannot rule out specialized functions for each protein.

Regional Specialization of Mitochondrial Ca 2+ Signaling in Cardiac Cells

Excitation-contraction coupling in cardiac cells is controlled mainly by ICa-gated release of Ca2+ from the sarcoplasmic reticulum (SR), but is also thought to be modulated by mitochondrial Ca+ signaling. Using field-stimulated rat ventricular cardiomyocytes, we have previously found that mitochondria that were primed by rapid beating or caffeine-induced SR-Ca2+ release subsequently were capable of rapid release of Ca2+ when stimulated by shear forces (Belmonte and Morad, 2008, J. Physiol. 586: 1379). To elucidate this finding we measured mitochondrial Ca2+ using Mitycam. The adenoviral Mitycam probe was originally created using the inverse pericam mutant M13-EYFP (V68L/Q69K)(145-238) and EYFP(V168L/Q69K)(1-144)-CaM, fused to the subunit VIII of human cytochrome c oxidase mitochondrial-targeting signal (Kettlewell et al., 2009, J Mol Cell Cardiol, 46:891). Short-time cultured adult feline cardiomyocytes were infected with the mitycam adenovirus at MOI of 200 virus particles per cell. After 3-4 days we measured the Mitycam fluorescence under voltage-clamp conditions using whole-cell fluorometry and 2-D confocal imaging. Supporting information was obtained using TMRE to assess mitochondrial depolarization and NCX-currents to assess cytosolic Ca2+ transients. The whole-cell Mitycam measurements suggested transfer of Ca2+ from the SR to the mitochondria during and following voltage-clamp depolarizations and caffeine-induced SR Ca2+ release. In confocal measurements the distribution of Mitycam-fluorescence showed longitudinal streaks with sarcomeric banding resembling the TMRE staining and a characteristic nuclear band (NB) of enhanced fluorescence that typically connected and extended from the two nuclei. Depolarization, caffeine, mechanical stimulation, and FCCP produced changes in Mitycam fluorescence that often resulted in redistribution between NB, general mitochondrial, and subsarcolemmal mitochondrial compartments. Similarly, injection of FCCP caused local decline in TMRE fluorescence (mitochondrial depolarization), but strongly enhanced subsarcolemmal fluorescence. These findings suggest that mitochondria in different regions of cardiac cells play different roles in Ca2+ signaling. NIH HL16152.

GENERATION OF VIRUS-LIKE PARTICLES IN PLANTS EXPRESSING THE CAPSID PROTEIN OF CYMBIDIUM RINGSPOT VIRUS

SUMMARY Agroinfiltration of Nicotiana benthamiana leaves with a construct expressing the coat protein of Cymbidium ringspot virus (CymRSV) resulted in the production of virus-like particles (VLPs) which showed a preferential localization within mitochondria with an apparently intact bounding membrane. VLPs were either non penetrated or penetrated by the negative stain. The former had the same outward aspect and size of wild type CymRSV and were assumed to encapsidate the CP messanger RNA, which was recovered from virus preparations purified from agroinfiltrated tissues. In addition, VLPs were shown to be able to encapsidate tombusviral satellite RNAs. Immunoblot analysis of dissociated VLPs showed that they were made up of a protein indistinguishable from the native CymRSV CP. A mitochondrial targeting signal was identified in the N-terminal region of CymRSV CP at amino acid (aa) position 10-27. The fusion protein CPMyc, containing the 10-aa Myc epitope fused to the CymRSV CP C-terminus, formed VLPs that were decorated by a Myc-specific antiserum.

Purification, molecular cloning, and some properties of a manganese-containing superoxide dismutase from Japanese flounder (Paralichthys olivaceus)

Manganese-superoxide dismutase (Mn-SOD) from Japanese flounder (Paralichthys olivaceus) hepatopancreas has been purified with high purification (781-fold) and recovery (10.8%). The molecular mass of the purified enzyme was estimated to be 26kDa by SDS-PAGE under reducing conditions. In activity staining by native-PAGE, the Japanese flounder Mn-SOD gave three active bands and exhibited KCN-insensitive activity. In addition, the electrophoretic mobility of this enzyme was observed to be faster than that of Japanese flounder Cu,Zn-SOD. On the other hand, the N-terminal amino acid sequence of this Mn-SOD was determined to be 16 amino acid residues, and the sequence showed high homology to other Mn-SODs but not Japanese flounder Cu,Zn-SOD. Analysis of nucleotide and deduced amino acid sequences revealed that the Mn-SOD cDNA consisted of a 64bp 5'-non-coding region, a 675bp open reading frame encoding 225 amino acids, and a 465bp 3'-non-coding region. The first 27 amino acids containing a mitochondria-targeting signal were highly conserved among other Mn-SODs.

The Human Cytomegalovirus Protein UL37 Exon 1 Associates with Internal Lipid Rafts

The human cytomegalovirus (HCMV) protein UL37 exon 1 (pUL37x1), also known as viral mitochondrion-localized inhibitor of apoptosis (vMIA), sequentially traffics from the endoplasmic reticulum (ER) through mitochondrion-associated membranes (MAMs) to the outer mitochondrial membrane (OMM), where it robustly inhibits apoptosis. Here, we report the association of pUL37x1/vMIA with internal lipid rafts (LRs) in the ER/MAM. The MAM, which serves as a site for lipid transfer and calcium signaling to mitochondria, is enriched in detergent-resistant membrane (DRM)-forming lipids, including cholesterol and ceramide, which are found in lower concentrations in the bulk ER. Sigma 1 receptor (Sig-1R), a MAM chaperone affecting calcium signaling to mitochondria, is anchored in the MAM by its LR association. Because of its trafficking through the MAM and partial colocalization with Sig-1R, we tested whether pUL37x1/vMIA associates with MAM LRs. Extraction with methyl-β-cyclodextrin (MβCD) removed pUL37x1/vMIA from lysed but not intact cells, indicating its association with internal LRs. Furthermore, the isolation of DRMs from purified intracellular organelles independently verified the localization of pUL37x1/vMIA within ER/MAM LRs. However, pUL37x1/vMIA was not detected in DRMs from mitochondria. pUL37x1/vMIA associated with LRs during all temporal phases of HCMV infection, indicating the likely importance of this location for HCMV growth. Although detected during its sequential trafficking to the OMM, the pUL37x1/vMIA LR association was independent of its mitochondrial targeting signals. Rather, it was dependent upon cholesterol binding. These studies suggest a conserved ability of UL37 proteins to interact with cholesterol and LRs, which is functionally distinguishable from their sequential trafficking to mitochondria.

Distinct regulation of mitochondrial localization and stability of two human Sirt5 isoforms

Seven human Sir2 homologues (sirtuin) have been identified to date. In this study, we clarified the mechanism of subcellular localization of two SIRT5 isoforms (i.e., SIRT5(iso1) and SIRT5(iso2) ) encoded by the human SIRT5 gene and whose C-termini slightly differ from each other. Although both isoforms contain cleavable mitochondrial targeting signals at their N-termini, we found that the cleaved SIRT5(iso2) was localized mainly in mitochondria, whereas the cleaved SIRT5(iso1) was localized in both mitochondria and cytoplasm. SIRT5ΔC, which is composed of only the common domain, showed the same mitochondrial localization as that of SIRT5(iso2) . These results suggest that the cytoplasmic localization of cleaved SIRT5(iso1) is dependent on the SIRT5(iso1) -specific C-terminus. Further analysis showed that the C-terminus of SIRT5(iso2) , which is rich in hydrophobic amino acid residues, functions as a mitochondrial membrane insertion signal. In addition, a de novo protein synthesis inhibition experiment using cycloheximide showed that the SIRT5(iso1) -specific C-terminus is necessary for maintaining the stability of SIRT5(iso1) . Moreover, genome sequence analysis from each organism examined indicated that SIRT5(iso2) is a primate-specific isoform. Taken together, these results indicate that human SIRT5 potentially controls various primate-specific functions via two isoforms with different intracellular localizations or stabilities.

The anti-apoptotic protein L * of Theiler's murine encephalomyelitis virus (TMEV) contains a mitochondrial targeting signal

L * protein of TMEV is out-of-frame with the viral polyprotein from an alternative initiation codon AUG, 13 nucleotides downstream from the authentic polyprotein AUG. Anti-apoptotic activity of L * was demonstrated by both ‘loss of function’ and ‘gain of function’ experiments. However, the precise mechanism(s) of anti-apoptotic activity of L * remains to be clarified. In this study, L * was demonstrated to be localized to mitochondria. It was also shown by the GFP fusion protein that N-terminal sequence of L * may contain a mitochondrial targeting signal (MTS). Surprisingly, L * (5–70)–GFP and L * (41–70)–GFP were localized to mitochondria although L * (1–70)–GFP was distributed in the cytosol, suggesting L * has an MTS between amino acid (AA) positions 41 and 70, and that L * (1–4) inhibits its mitochondrial targeting. Furthermore, L * (1–70)–GFP was localized to the mitochondria by co-expression of L * (65–156), indicating that L * (65–156) suppresses the inhibition of mitochondrial targeting by L * (1–4). These results suggest that the intra- or inter-molecular interaction of L * regulates its mitochondrial localization. It is also suggested that L * may inhibit the intrinsic apoptosis through the localization to mitochondria.

Betanodavirus B2 Causes ATP Depletion-induced Cell Death via Mitochondrial Targeting and Complex II Inhibition in Vitro and in Vivo

The betanodavirus non-structural protein B2 is a newly discovered necrotic death factor with a still unknown role in regulation of mitochondrial function. In the present study, we examined protein B2-mediated inhibition of mitochondrial complex II activity, which results in ATP depletion and thereby in a bioenergetic crisis in vitro and in vivo. Expression of protein B2 was detected early at 24 h postinfection with red-spotted grouper nervous necrosis virus in the cytoplasm. Later B2 was found in mitochondria using enhanced yellow fluorescent protein (EYFP) and immuno-EM analysis. Furthermore, the B2 mitochondrial targeting signal peptide was analyzed by serial deletion and specific point mutation. The sequence of the B2 targeting signal peptide ((41)RTFVISAHAA(50)) was identified and its presence correlated with loss of mitochondrial membrane potential in fish cells. Protein B2 also was found to dramatically inhibit complex II (succinate dehydrogenase) activity, which impairs ATP synthesis in fish GF-1 cells as well as human embryonic kidney 293T cells. Furthermore, when B2 was injected into zebrafish embryos at the one-cell stage to determine its cytotoxicity and ability to inhibit ATP synthesis, we found that B2 caused massive embryonic cell death and depleted ATP resulting in further embryonic death at 10 and 24 h post-fertilization. Taken together, our results indicate that betanodavirus protein B2-induced cell death is due to direct targeting of the mitochondrial matrix by a specific signal peptide that targets mitochondria and inhibits mitochondrial complex II activity thereby reducing ATP synthesis.