Signal anchor sequence insertion into the outer mitochondrial membrane. Comparison with porin and the matrix protein targeting pathway

EAE-induced upregulation of mitochondrial MnSOD is associated with increases of mitochondrial SGK1 and Tom20 protein in the mouse kidney cortex

Our previous demonstration that severe experimental autoimmune encephalomyelitis (EAE) increases MnSOD protein abundance in the mouse kidney cortex led this study to elucidate the underlying mechanism with monensin-treated HEK293 cells as a model. Severe EAE increases mitochondrial protein abundance of SGK1 kinase and Tom20, a critical subunit of mitochondrial translocase in the renal cortex. In HEK293 cells, catalase inhibits monensin-induced increases of mitochondrial SGK1 and Tom20 protein levels. Further, GSK650394, a specific inhibitor of SGK1 reduces monensin-induced increase of mitochondrial protein abundance of Tom20 and MnSOD. Finally, RNAi of Tom20 reduces the effect of monensin on MnSOD. MnSOD and Tom20 physically associate with each other. In conclusion, in HEK293 cells, mitochondrial reactive oxygen species increase protein abundance of mitochondrial SGK1, which leads to a rise of mitochondrial Tom20, resulting in importing MnSOD protein into the mitochondria. This could be a mechanism by which severe EAE up-regulates mitochondrial MnSOD in the kidney cortex.

Mechanistic Target of Rapamycin Complex 1 Promotes the Expression of Genes Encoding Electron Transport Chain Proteins and Stimulates Oxidative Phosphorylation in Primary Human Trophoblast Cells by Regulating Mitochondrial Biogenesis

Trophoblast oxidative phosphorylation provides energy for active transport and protein synthesis, which are critical placental functions influencing fetal growth and long-term health. The molecular mechanisms regulating trophoblast mitochondrial oxidative phosphorylation are largely unknown. We hypothesized that mechanistic Target of Rapamycin Complex 1 (mTORC1) is a positive regulator of key genes encoding Electron Transport Chain (ETC) proteins and stimulates oxidative phosphorylation in trophoblast and that ETC protein expression is down-regulated in placentas of infants with intrauterine growth restriction (IUGR). We silenced raptor (mTORC1 inhibition), rictor (mTORC2 inhibition) or DEPTOR (mTORC1/2 activation) in cultured term primary human trophoblast (PHT) cells. mTORC1 inhibition caused a coordinated down-regulation of 18 genes encoding ETC proteins representing all ETC complexes. Inhibition of mTORC1, but not mTORC2, decreased protein expression of ETC complexes I–IV, mitochondrial basal, ATP coupled and maximal respiration, reserve capacity and proton leak, whereas activation of mTORC1 had the opposite effects. Moreover, placental protein expression of ETC complexes was decreased and positively correlated to mTOR signaling activity in IUGR. By controlling trophoblast ATP production, mTORC1 links nutrient and O₂ availability and growth factor signaling to placental function and fetal growth. Reduced placental mTOR activity may impair mitochondrial respiration and contribute to placental insufficiency in IUGR pregnancies.

Characterization of the targeting signal in mitochondrial β-barrel proteins

Mitochondrial β-barrel proteins are synthesized on cytosolic ribosomes and must be specifically targeted to the organelle before their integration into the mitochondrial outer membrane. The signal that assures such precise targeting and its recognition by the organelle remained obscure. In the present study we show that a specialized β-hairpin motif is this long searched for signal. We demonstrate that a synthetic β-hairpin peptide competes with the import of mitochondrial β-barrel proteins and that proteins harbouring a β-hairpin peptide fused to passenger domains are targeted to mitochondria. Furthermore, a β-hairpin motif from mitochondrial proteins targets chloroplast β-barrel proteins to mitochondria. The mitochondrial targeting depends on the hydrophobicity of the β-hairpin motif. Finally, this motif interacts with the mitochondrial import receptor Tom20. Collectively, we reveal that β-barrel proteins are targeted to mitochondria by a dedicated β-hairpin element, and this motif is recognized at the organelle surface by the outer membrane translocase.

Mitochondrial β-barrel proteins are synthesized in the cytosol before being targeted to the organelle. Here, Jores et al. show that a specialized hydrophobic β-hairpin motif is the previously undefined targeting sequence and is recognized by the mitochondrial outer membrane translocase.

Characterization of the membrane-inserted C-terminus of cytoprotective BCL-XL

BCL-XL is a dominant inhibitor of apoptosis and a significant anti-cancer drug target. Endogenous BCL-XL is integral to the mitochondrial outer membrane (MOM). BCL-XL reconstituted in detergent-free lipid bilayer nanodiscs is anchored to the nanodisc lipid bilayer membrane by tight association of its C-terminal tail, while the N-terminal head retains the canonical structure determined for water-soluble, tail-truncated BCL-XL, with the surface groove solvent-exposed and available for BH3 ligand binding. To better understand the conformation and dynamics of this key region of BCL-XL we have developed methods for isolating the membrane-embedded C-terminal tail from its N-terminal head and for preparing protein suitable for structural and biochemical studies. Here, we outline the methods for sample preparation and characterization and describe previously unreported structural and dynamics features. We show that the C-terminal tail of BCL-XL forms a transmembrane α-helix that retains a significant degree of conformational dynamics. We also show that the presence of the intact C-terminus destabilizes the soluble state of the protein, and that the small fraction of soluble recombinant protein produced in Escherichia coli is susceptible to proteolytic degradation of C-terminal residues beyond M218. This finding impacts the numerous previous studies where recombinant soluble BCL-XL was presumed to be full-length. Nevertheless, the majority of recombinant BCL-XL produced in E. coli is insoluble and protected from proteolysis. This protein retains the complete C-terminal tail and can be reconstituted in lipid bilayers in a folded and active state.

Nuclear-encoded mitochondrial proteins and their role in cardioprotection

During myocardial ischemia/reperfusion, mitochondria are both a source and a target of injury. In cardioprotective maneuvers such as ischemic and pharmacological pre- and postconditioning mitochondria have a decisive role. Since about 99% of the mitochondrial proteins are encoded in the nucleus, deleterious and protective mitochondrial effects most likely comprise the import of cytosolic proteins. The present review therefore discusses the role of mitochondria in myocardial ischemia/reperfusion injury and protection from it, focusing on some cytosolic proteins, which are translocated into mitochondria before, during, or following ischemia/reperfusion. Both morphological and functional alterations are discussed at the level of the heart, the cardiomyocyte and/or the mitochondrion itself. This article is part of a Special Issue entitled: Mitochondria and Cardioprotection.

Incomplete T-cell receptor-beta peptides target the mitochondrion and induce apoptosis

The default pathway of cell-surface T-cell receptor (TCR) complex formation, and the subsequent transport to the membrane, is thought to entail endoplasmic reticulum (ER) localization followed by proteasome degradation of the unassembled chains. We show herein an alternative pathway: short, incomplete peptide versions of TCRbeta naturally occur in the thymus. Such peptides, which have minimally lost the leader sequence or have been massively truncated, leaving only the very C terminus intact, are sorted preferentially to the mitochondrion. As a consequence of the mitochondrial localization, apoptotic cell death is induced. Structure function analysis showed that both the specific localization and induction of apoptosis depend on the transmembrane domain (TMD) and associated residues at the COOH-terminus of TCR. Truncated forms of TCR, such as the short peptides that we detected in the thymus, may be products of protein degradation within thymocytes. Alternatively, they may occur through the translation of truncated mRNAs resulting from unfruitful rearrangement or from germline transcription. It is proposed that mitochondria serve as a subcellular sequestration site for incomplete TCR molecules.

How tails guide tail-anchored proteins to their destinations

A large group of diverse, functionally important, and differently localized transmembrane proteins shares a particular membrane topology, consisting of a cytosolic N-terminal region, followed by a transmembrane domain close to the C-terminus. Because of their structure, these C-tail-anchored (TA) proteins must insert into all their target membranes by post-translational pathways. Recent work, based on the development of stringent and sensitive biochemical assays, has demonstrated that novel unexplored mechanisms underlie these post-translational targeting and membrane insertion pathways. Unravelling these pathways will shed light on the biosynthesis and regulation of an important group of membrane proteins and is likely to lead to new concepts in the field of membrane biogenesis.

The C-terminus of cytochrome b5 confers endoplasmic reticulum specificity by preventing spontaneous insertion into membranes

The molecular mechanisms that determine the correct subcellular localization of proteins targeted to membranes by tail-anchor sequences are poorly defined. Previously, we showed that two isoforms of the tung oil tree [Vernicia (Aleurites) fordii] tail-anchored Cb5 (cytochrome b5) target specifically to ER (endoplasmic reticulum) membranes both in vivo and in vitro [Hwang, Pelitire, Henderson, Andrews, Dyer and Mullen (2004) Plant Cell 16, 3002-3019]. In the present study, we examine the targeting of various tung Cb5 fusion proteins and truncation mutants to purified intracellular membranes in vitro in order to assess the importance of the charged CTS (C-terminal sequence) in targeting to specific membranes. Removal of the CTS from tung Cb5 proteins resulted in efficient binding to both ER and mitochondria. Results from organelle competition, liposome-binding and membrane proteolysis experiments demonstrated that removal of the CTS results in spontaneous insertion of tung Cb5 proteins into lipid bilayers. Our results indicate that the CTSs from plant Cb5 proteins provide ER specificity by preventing spontaneous insertion into incorrect subcellular membranes.

Loss of ischemic preconditioning's cardioprotection in aged mouse hearts is associated with reduced gap junctional and mitochondrial levels of connexin 43

Connexin 43 (Cx43) is localized at left ventricular (LV) gap junctions and in cardiomyocyte mitochondria. A genetically induced reduction of Cx43 as well as blockade of mitochondrial Cx43 import abolishes the infarct size (IS) reduction by ischemic preconditioning (IP). With progressing age, Cx43 content in ventricular and atrial tissue homogenates is reduced. We now investigated whether or not 1) the mitochondrial Cx43 content is reduced in aged mice hearts and 2) IS reduction by IP is lost in aged mice hearts in vivo. Confirming previous results, sarcolemmal Cx43 content was reduced in aged (>13 mo) compared with young (<3 mo) C57Bl/6 mice hearts, whereas the expression levels of protein kinase C epsilon and endothelial nitric oxide synthase remained unchanged. Also in mitochondria isolated from aged mice LV myocardium, Western blot analysis indicated a 40% decrease in Cx43 content compared with mitochondria isolated from young mice hearts. In young mice hearts, IP by one cycle of 10 min ischemia and 10 min reperfusion reduced IS (% of area at risk) following 30 min regional ischemia and 120 min reperfusion from 67.7 +/- 3.3 (n = 17) to 34.2 +/- 6.6 (n = 5, P < 0.05). In contrast, IP's cardioprotection was lost in aged mice hearts, since IS in nonpreconditioned (57.5 +/- 4.0, n = 10) and preconditioned hearts (65.4 +/- 6.3, n = 8, P = not significant) was not different. In conclusion, mitochondrial Cx43 content is decreased in aged mouse hearts. The reduced levels of Cx43 may contribute to the age-related loss of cardioprotection by IP.

Prevention of the ischemia-induced decrease in mitochondrial Tom20 content by ischemic preconditioning

Preserved mitochondrial function (respiration, calcium handling) and integrity (cytochrome c release) is central for cell survival following ischemia/reperfusion. Mitochondrial function also requires import of proteins from the cytosol via the translocase of the outer and inner membrane (TOM and TIM complexes). Since mitochondrial function following ischemia/reperfusion is better preserved by ischemic preconditioning (IP), we now investigated whether expression of parts of the import machinery is affected by ischemia/reperfusion without or with IP in vivo. We analyzed the mitochondrial content of the presequence receptor Tom20, the pore forming unit Tom40 and Tim23. Goettinger minipigs were subjected to 90 min of low-flow ischemia without or with preconditioning by 10 min ischemia and 15 min reperfusion. Mitochondria were isolated from the ischemic or preconditioned anterior wall of the left ventricle and from the control posterior wall. Infarct size was significantly reduced by IP (20.1 +/- 1.6% of area at risk (non-preconditioned) vs. 6.5 +/- 2.5% of area at risk (IP)). Using Western blot analysis, the ratio of Tom20 (normalized to Ponceau S) between mitochondria isolated from the anterior ischemic and posterior control wall was reduced (0.72 +/- 0.11, a.u., n = 8), whereas the mitochondrial Tom20 content was preserved by IP (1.17 +/- 0.16 a.u., n = 7, P < 0.05). The mitochondrial Tom40, Tim23 and adenine nucleotide transporter (ANT) contents were not significantly different between non-preconditioned and preconditioned myocardium. The preservation of the mitochondrial Tom20 protein level may contribute to the improved mitochondrial function after IP.

Targeting of hepatitis C virus core protein to mitochondria through a novel C-terminal localization motif

The hepatitis C virus (HCV) core protein represents the first 191 amino acids of the viral precursor polyprotein and is cotranslationally inserted into the membrane of the endoplasmic reticulum (ER). Processing at position 179 by a recently identified intramembrane signal peptide peptidase leads to the generation and potential cytosolic release of a 179-amino-acid matured form of the core protein. Using confocal microscopy, we observed that a fraction of the mature core protein colocalized with mitochondrial markers in core-expressing HeLa cells and in Huh-7 cells containing the full-length HCV replicon. Subcellular fractionation confirmed this observation and showed that the core protein associates with purified mitochondrial fractions devoid of ER contaminants. The core protein also fractionated with mitochondrion-associated membranes, a site of physical contact between the ER and mitochondria. Using immunoelectron microscopy and in vitro mitochondrial import assays, we showed that the core protein is located on the mitochondrial outer membrane. A stretch of 10 amino acids within the hydrophobic C terminus of the processed core protein conferred mitochondrial localization when it was fused to green fluorescent protein. The location of the core protein in the outer mitochondrial membrane suggests that it could modulate apoptosis or lipid transfer, both of which are associated with this subcellular compartment, during HCV infection.

The tale of tail-anchored proteins: coming from the cytosol and looking for a membrane

A group of integral membrane proteins, known as C-tail anchored, is defined by the presence of a cytosolic NH2-terminal domain that is anchored to the phospholipid bilayer by a single segment of hydrophobic amino acids close to the COOH terminus. The mode of insertion into membranes of these proteins, many of which play key roles in fundamental intracellular processes, is obligatorily posttranslational, is highly specific, and may be subject to regulatory processes that modulate the protein's function. Although recent work has elucidated structural features in the tail region that determine selection of the correct target membrane, the molecular machinery involved in interpreting this information, and in modulating tail-anchored protein localization, has not been identified yet.

Bipartite signals mediate subcellular targeting of tail-anchored membrane proteins in Saccharomyces cerevisiae

Tail-anchored proteins have an NH(2)-terminal cytosolic domain anchored to intracellular membranes by a single, COOH-terminal, transmembrane segment. Sequence analysis identified 55 tail-anchored proteins in Saccharomyces cerevisiae, with several novel proteins, including Prm3, which we find is required for karyogamy and is tail-anchored in the nuclear envelope. A total of six tail-anchored proteins are present in the mitochondrial outer membrane and have relatively hydrophilic transmembrane segments that serve as targeting signals. The rest, by far the majority, localize via a bipartite system of signals: uniformly hydrophobic tail anchors are first inserted into the endoplasmic reticulum, and additional segments within the cytosolic domain of each protein can dictate subsequent sorting to a precise destination within the cell.

Characterization of the signal that directs Bcl-x(L), but not Bcl-2, to the mitochondrial outer membrane

It is assumed that the survival factors Bcl-2 and Bcl-x(L) are mainly functional on mitochondria and therefore must contain mitochondrial targeting sequences. Here we show, however, that only Bcl-x(L) is specifically targeted to the mitochondrial outer membrane (MOM) whereas Bcl-2 distributes on several intracellular membranes. Mitochondrial targeting of Bcl-x(L) requires the COOH-terminal transmembrane (TM) domain flanked at both ends by at least two basic amino acids. This sequence is a bona fide targeting signal for the MOM as it confers specific mitochondrial localization to soluble EGFP. The signal is present in numerous proteins known to be directed to the MOM. Bcl-2 lacks the signal and therefore localizes to several intracellular membranes. The COOH-terminal region of Bcl-2 can be converted into a targeting signal for the MOM by increasing the basicity surrounding its TM. These data define a new targeting sequence for the MOM and propose that Bcl-2 acts on several intracellular membranes whereas Bcl-x(L) specifically functions on the MOM.

Bcl-2 and porin follow different pathways of TOM-dependent insertion into the mitochondrial outer membrane

The bcl-2 gene encodes a 26kDa protein which functions as a central regulator of apoptosis. Here we investigated the pathway of Bcl-2alpha into the mitochondrial outer membrane using the yeast Saccharomyces cerevisiae as a model organism. We found that interactions of Bcl-2alpha with the mitochondrial import receptor Tom20 are dependent on two positively charged lysine residues in the immediate vicinity of the carboxy-terminal hydrophobic membrane anchor. The targeting function of these residues is independent of Tom22. Subsequent insertion of Bcl-2alpha into the mitochondrial outer membrane does not require Tom5 or Tom40, indicating that Bcl-2alpha bypasses the general import pore (GIP). Bcl-2alpha shows a unique pattern of interactions with the components of the mitochondrial TOM complex, demonstrating that at least two different pathways lead from the import receptor Tom20 into the mitochondrial outer membrane.

Mitochondrial targeting and membrane anchoring of a viral replicase in plant and yeast cells

Replication of the Carnation Italian ringspot virus genomic RNA in plant cells occurs in multivesicular bodies which develop from the mitochondrial outer membrane during infection. ORF1 in the viral genome encodes a 36-kDa protein, while ORF2 codes for the 95-kDa replicase by readthrough of the ORF1 stop codon. We have shown previously that the N-terminal part of ORF1 contains the information leading to vesiculation of mitochondria and that the 36-kDa protein localizes to mitochondria. Using infection, in vivo expression of green fluorescent protein fusions in plant and yeast cells, and in vitro mitochondrial integration assays, we demonstrate here that both the 36-kDa protein and the complete replicase are targeted to mitochondria and anchor to the outer membrane with the N terminus and C terminus on the cytosolic side. Analysis of deletion mutants indicated that the anchor sequence is likely to correspond approximately to amino acids 84 to 196, containing two transmembrane domains. No evidence for a matrix-targeting presequence was found, and the data suggest that membrane insertion of the viral proteins is mediated by an import receptor-independent signal-anchor mechanism relying on the two transmembrane segments and multiple recognition signals present in the N-terminal part of ORF1.

Biosynthesis of surfactant protein C (SP-C). Sorting of SP-C proprotein involves homomeric association via a signal anchor domain

Rat surfactant protein C (SP-C) is synthesized as a 194-amino acid propeptide (SP-C-(1-194)) that is directed to the distal secretory pathway and proteolytically processed as an integral membrane protein to yield its mature form. We had shown previously that trafficking of proSP-C is mediated both by a signal anchor domain contained within the mature SP-C sequence and by a targeting domain in the NH(2)-flanking propeptide. Based on evidence from other integral membrane proteins, we hypothesized that proSP-C targeting is effected by oligomerization of proSP-C monomers. To evaluate this in vitro, cDNA constructs encoding for either wild type proSP-C (pcDNA3/SP-C-(1-194)) or heterologous fusion proteins containing green fluorescent protein (EGFP) linked to SP-C-(1-194) (EGFP/SP-C-(1-194)), to mutant proSP-C lacking the NH(2) targeting domain (EGFP/SP-C-(24-194)), or to mature SP-C alone (EGFP/SP-C-(24-58)) were produced. In transfected A549 cells, fluorescence microscopy revealed that pcDNA3/SP-C-(1-194) and EGFP/SP-C-(1-194) were each expressed in CD63 (+), EEA1 (-) cytoplasmic vesicles. Expression of EGFP/SP-C-(24-194) or EGFP/SP-C-(24-58) resulted in translocation but retention in early compartments. When co-transfected with pcDNA3/SP-C-(1-194), both EGFP/SP-C-(24-194) and EGFP/SP-C-(24-58) were directed to CD63 (+) vesicles that also contained SP-C-(1-194). In contrast, trafficking of a folding mutant that forms juxtanuclear aggregates, EGFP/SP-C(C122/186G), was not corrected by cotransfection with pcDNA3/SP-C-(1-194). Chemical cross-linking studies of transfected cell lysates with bismaleimidohexane produced multimeric forms of both EGFP/SP-C-(1-194) and EGFP/SP-C-(24-58). These results indicate that sorting involves oligomeric association of proSP-C monomers mediated by the mature SP-C domain. Heteromeric assembly allows wild type proSP-C to facilitate trafficking of SP-C mutants with intact transmembrane domains but lacking targeting signals. We speculate that heterotypic oligomerization of wild type with SP-C folding mutants produces a dominant negative thus contributing to the pathology of chronic lung disease associated with patients heterozygous for mutant SP-C alleles.

Targeting of a tail-anchored protein to endoplasmic reticulum and mitochondrial outer membrane by independent but competing pathways

Many mitochondrial outer membrane (MOM) proteins have a transmembrane domain near the C terminus and an N-terminal cytosolic moiety. It is not clear how these tail-anchored (TA) proteins posttranslationally select their target, but C-terminal charged residues play an important role. To investigate how discrimination between MOM and endoplasmic reticulum (ER) occurs, we used mammalian cytochrome b(5), a TA protein existing in two, MOM or ER localized, versions. Substitution of the seven C-terminal residues of the ER isoform or of green fluorescent protein reporter constructs with one or two arginines resulted in MOM-targeted proteins, whereas a single C-terminal threonine caused promiscuous localization. To investigate whether targeting to MOM occurs from the cytosol or after transit through the ER, we tagged a MOM-directed construct with a C-terminal N-glycosylation sequence. Although in vitro this construct was efficiently glycosylated by microsomes, the protein expressed in vivo localized almost exclusively to MOM, and was nearly completely unglycosylated. The small fraction of glycosylated protein was in the ER and was not a precursor to the unglycosylated form. Thus, targeting occurs directly from the cytosol. Moreover, ER and MOM compete for the same polypeptide, explaining the dual localization of some TA proteins.

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

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

Targeting of C-terminal (tail)-anchored proteins: understanding how cytoplasmic activities are anchored to intracellular membranes

A class of integral membrane proteins, referred to as 'tail-anchored proteins', are inserted into phospholipid bilayers via a single segment of hydrophobic amino acids at the C-terminus, thereby displaying a large functional domain in the cytosol. This membrane attachment strategy allows eukaryotic cells to position a wide range of cytoplasmic activities close to the surface of an intracellular membrane. Tail-anchored proteins often, but not always, demonstrate a selective distribution to specific intracellular organelles. This membrane-specific distribution is required for the large number of targeting proteins that are tail-anchored, but may or may not be critical for the numerous tail-anchored pro-apoptotic and anti-apoptotic proteins of the Bcl-2 family. Recent work has begun to address the mechanism for targeting tail-anchored proteins to their resident membranes, but questions remain. What targeting signals determine each protein's intracellular location? Are there receptors for these signals and, if so, how do they function? What steps are required to integrate tail-anchored proteins into the phospholipid bilayers? In this Traffic interchange, we summarise what is known about tail-anchored proteins, and outline the areas that are currently under study.

Tom20-mediated mitochondrial protein import in muscle cells during differentiation

Mitochondrial biogenesis is accompanied by an increased expression of components of the protein import machinery, as well as increased import of proteins destined for the matrix. We evaluated the role of the outer membrane receptor Tom20 by varying its expression and measuring changes in the import of malate dehydrogenase (MDH) in differentiating C2C12 muscle cells. Cells transfected with Tom20 had levels that were twofold higher than in control cells. Labeling of cells followed by immunoprecipitation of MDH revealed equivalent increases in MDH import. This parallelism between import rate and Tom20 levels was also evident as a result of thyroid hormone treatment. Using antisense oligodeoxynucleotides, we inhibited Tom20 expression by 40%, resulting in 40-60% reductions in MDH import. In vitro assays also revealed that import into the matrix was more sensitive to Tom20 inhibition than import into the outer membrane. These data indicate a close relationship between induced changes in Tom20 and the import of a matrix protein, suggesting that Tom20 is involved in determining the kinetics of import. However, this relationship was dissociated during normal differentiation, since the expression of Tom20 remained relatively constant, whereas imported MDH increased 12-fold. Thus Tom20 is important in determining import during organelle biogenesis, but other mechanisms (e.g., intramitochondrial protein degradation or nuclear transcription) likely also play a role in establishing the final mitochondrial phenotype during normal muscle differentiation.

Targeting and insertion of C-terminally anchored proteins to the mitochondrial outer membrane is specific and saturable but does not strictly require ATP or molecular chaperones

A distinct class of proteins contain a C-terminal membrane anchor and a cytoplasmic functional domain. A subset of these proteins is targeted to the mitochondrial outer membrane. Here, to probe for the involvement of a saturable targeting mechanism for this class of proteins, and to elucidate the roles of chaperone proteins and ATP, we have utilized an in vitro targeting system consisting of in vitro-synthesized proteins and isolated mitochondria. To establish the specificity of targeting we have used a closely related protein pair. VAMP-1A and VAMP-1B are splice variants of the vesicle-associated membrane protein/synaptobrevin-1 (VAMP-1) gene. In intact cells VAMP-1B is targeted to mitochondria whereas VAMP-1A is targeted to membranes of the secretory pathway, yet these isoforms differ by only five amino acids at the extreme C-terminus. Here we demonstrate that, in vitro, VAMP-1B is imported into both intact mitochondria and mitochondrial outer-membrane vesicles with a 15-fold greater efficiency than VAMP-1A. We generated and purified bacterially expressed fusion proteins consisting of the C-terminal two-thirds of VAMP-1A or -1B proteins fused to glutathione S-transferase (GST). Using these fusion proteins we demonstrate that protein targeting and insertion is saturable and specific for the VAMP-1B membrane anchor. To elucidate the role of cytosolic chaperones on VAMP-1B targeting, we also used the purified, Escherichia coli-derived fusion proteins. (33)P-Labelled GST-VAMP-1B(61-116), but not GST-VAMP-1A(61-118), was efficiently targeted to mitochondria in a chaperone-free system. Thus the information required for targeting is contained within the targeted protein itself and not the chaperone or a chaperone-protein complex, although chaperones may be required to maintain a transport-competent conformation. Moreover, ATP was required for transport only in the presence of cytosolic chaperone proteins. Therefore the ATP requirement of transport appears to reflect the participation of chaperones and not any other ATP-dependent step. These data demonstrate that targeting of C-terminally anchored proteins to mitochondria is sequence specific and mediated by a saturable mechanism. Neither ATP nor chaperone proteins are strictly required for either specific targeting or membrane insertion.

Signals and receptors--the translocation machinery on the mitochondrial surface

Most proteins involved in mitochondrial biogenesis are encoded by the genome of the nucleus. They are synthesized in the cytosol and have to be transported toward and, subsequently, imported into the organelle. This targeting and import process is initiated by the specific mitochondrial targeting signal, which differs pending on the final localization of the protein. The preprotein will be recognized by cytosolic proteins, which function in transport toward the mitochondria and in maintaining the import competent state of the preprotein. The precursor will be transferred onto a multicomponent complex on the outer mitochondrial membrane, formed by receptor proteins and the general insertion pore (GIP). Some proteins are directly sorted into the outer membrane whereas the majority will be transported over the outer membrane through the import channel followed by further distribution of those proteins.

Mitochondrial proteins at unexpected cellular locations: export of proteins from mitochondria from an evolutionary perspective

Researchers in a wide variety of unrelated areas studying functions of different proteins are unexpectedly finding that their proteins of interest are actually mitochondrial proteins, although functions would appear to be extramitochondrial. We review the leading current examples of mitochondrial macromolecules indicated to be also present outside of mitochondria that apparently exit from mitochondria to arrive at their destinations. Mitochondrial chaperones, which have been implicated in growth and development, autoimmune diseases, cell mortality, antigen presentation, apoptosis, and resistance to antimitotic drugs, provide some of the best studied examples pointing to roles for mitochondria and mitochondrial proteins in diverse cellular phenomena. To explain the observations, we propose that specific export mechanisms exist by which certain proteins exit mitochondria, allowing these proteins to have additional functions at specific extramitochondrial sites. Several possible mechanisms by which mitochondrial proteins could be exported are discussed. Gram-negative proteobacteria, from which mitochondria evolved, contain a number of different mechanisms for protein export. It is likely that mitochondria either retained or evolved export mechanisms for certain specific proteins.

Direct membrane insertion of voltage-dependent anion-selective channel protein catalyzed by mitochondrial Tom20

Insertion of newly synthesized proteins into or across the mitochondrial outer membrane is initiated by import receptors at the surface of the organelle. Typically, this interaction directs the precursor protein into a preprotein translocation pore, comprised of Tom40. Here, we show that a prominent beta-barrel channel protein spanning the outer membrane, human voltage- dependent anion-selective channel (VDAC), bypasses the requirement for the Tom40 translocation pore during biogenesis. Insertion of VDAC into the outer membrane is unaffected by plugging the translocation pore with a partially translocated matrix preprotein, and mitochondria containing a temperature-sensitive mutant of Tom40 insert VDAC at the nonpermissive temperature. Synthetic liposomes harboring the cytosolic domain of the human import receptor Tom20 efficiently insert newly synthesized VDAC, resulting in transbilayer transport of ATP. Therefore, Tom20 transforms newly synthesized cytosolic VDAC into a transmembrane channel that is fully integrated into the lipid bilayer.

Functional and structural properties of the mitochondrial outer membrane receptor Tom20

Tom20 is an outer mitochondrial membrane protein that functions as a component of the import receptor complex for cytoplasmically synthesized mitochondrial precursor proteins. The human homologue, hTom20, consists of an N-terminal membrane anchor region predicted between aa5-25 and a soluble cytosolic domain from aa30 to 145. To analyze the properties of hTom20, we have expressed several truncations of the cytosolic domain as fusion proteins with glutathione S-transferase. Our studies reveal that the cytosolic region of hTom20 is a monomeric protein in solution containing two domains which are involved in different functions of the receptor. The N-terminal region is involved in membrane binding (aa30-60) and recognition of the cleavable matrix targeting signals (aa50-90). In addition, we have demonstrated that the receptor recognizes the alpha-helical state of the matrix targeting signal. The dissociation constant for this interaction in the presence of a detergent which induces this secondary structure is 0.6 microM, one-fifth the value in the absence of detergent. In aqueous solution, the region between aa30 and 60 is loosely folded and stabilized against proteolytic cleavage by interaction with detergents or a matrix targeting signal. Our work further shows that the remainder of the cytosolic domain of hTom20, aa60-145, is a compactly folded globular domain containing a region (aa90-145) that is critical for the recognition of proteins bearing internal signal sequences such as the uncoupling protein and porin.

A splice-isoform of vesicle-associated membrane protein-1 (VAMP-1) contains a mitochondrial targeting signal

Screening of a library derived from primary human endothelial cells revealed a novel human isoform of vesicle-associated membrane protein-1 (VAMP-1), a protein involved in the targeting and/or fusion of transport vesicles to their target membrane. We have termed this novel isoform VAMP-1B and designated the previously described isoform VAMP-1A. VAMP-1B appears to be an alternatively spliced form of VAMP-1. A similar rat splice variant of VAMP-1 (also termed VAMP-1B) has recently been reported. Five different cultured cell lines, from different lineages, all contained VAMP-1B but little or no detectable VAMP-1A mRNA, as assessed by PCR. In contrast, brain mRNA contained VAMP-1A but no VAMP-1B. The VAMP-1B sequence encodes a protein identical to VAMP-1A except for the carboxy-terminal five amino acids. VAMP-1 is anchored in the vesicle membrane by a carboxy-terminal hydrophobic sequence. In VAMP-1A the hydrophobic anchor is followed by a single threonine, which is the carboxy-terminal amino acid. In VAMP-1B the predicted hydrophobic membrane anchor is shortened by four amino acids, and the hydrophobic sequence is immediately followed by three charged amino acids, arginine-arginine-aspartic acid. Transfection of human endothelial cells with epitope-tagged VAMP-1B demonstrated that VAMP-1B was targeted to mitochondria whereas VAMP-1A was localized to the plasma membrane and endosome-like structures. Analysis of C-terminal mutations of VAMP-1B demonstrated that mitochondrial targeting depends both on the addition of positive charge at the C terminus and a shortened hydrophobic membrane anchor. These data suggest that mitochondria may be integrated, at least at a mechanistic level, to the vesicular trafficking pathways that govern protein movement between other organelles of the cell.

Interactions of the human mitochondrial protein import receptor, hTom20, with precursor proteins in vitro reveal pleiotropic specificities and different receptor domain requirements

Tom20 is part of a multiple component, dynamic complex that functions to import specific cytosolic proteins into or through the outer membrane of the mitochondrion. To analyze the contribution of Tom20 to precursor protein recognition, the cytosolic domain of the human mitochondrial import receptor, hTom20, has been expressed as a fusion protein with glutathione S-transferase and conditions established to measure specific interactions of the receptor component with precursor proteins in vitro. Reconstitution of receptor binding from purified components revealed that a prototypic matrix-destined precursor protein, pODHFR, interacts with Tom20 by a mechanism that is dependent on an active matrix targeting signal but does not require cytosolic components or ATP. Binding was influenced by both salt concentration and detergent. The effect of salt or detergent, however, varied for different precursor proteins. In particular, detergent selectively enhanced binding of pODHFR to receptor, possibly because of induced changes in the structure of the signal sequence. Finally, mutations were introduced into hTom20 which had a dramatic effect on binding of some precursor proteins but not on others. Taken together, the results suggest that hTom20 recognizes and physically interacts with precursor proteins bearing a diverse array of topogenic sequences and that such pleiotropic specificity for these precursor proteins may involve different domains within the receptor molecule.

Alteration of a mitochondrial outer membrane signal anchor sequence that permits its insertion into the inner membrane. Contribution of hydrophobic residues

Tom70p is targeted and inserted into the mitochondrial outer membrane in the Nin-Ccyto orientation, via an NH2-terminal signal anchor sequence. The signal anchor is comprised of two domains: an NH2-terminal hydrophilic region which is positively charged (amino acids 1-10) followed by the predicted transmembrane segment (amino acids 11-29). Substitution of the NH2-terminal domain with a matrix-targeting signal caused the signal anchor to adopt the reverse orientation in the outer membrane (Ncyto-Cin) or, if presented to mitoplasts, to arrest protein translocation at the inner membrane without insertion. Physically separating the transmembrane segment from the matrix-targeting signal by moving it downstream within the protein resulted in a failure to arrest in either membrane, and consequently the protein was imported to the matrix. However, if the mean hydrophobicity of the Tom70p transmembrane segment was increased in these constructs, the protein inserted into the inner membrane with an Nin-Cout orientation. Therefore we have determined conditions that allow the Tom70p transmembrane domain to insert in either membrane, pass through both membranes, or arrest without insertion in the inner membrane. These results identify the mean hydrophobicity of potential transmembrane domains within bitopic proteins as an important determinant for insertion into the mitochondrial inner membrane.