A signal-anchor sequence selective for the mitochondrial outer membrane

Translation rate underpins specific targeting of N-terminal transmembrane proteins to mitochondria

Protein biogenesis is a complex process, and complexity is greatly increased in eukaryotic cells through specific targeting of proteins to different organelles. To direct targeting, organellar proteins carry an organelle-specific targeting signal for recognition by organelle-specific import machinery. However, the situation is confusing for transmembrane domain (TMD)-containing signal-anchored (SA) proteins of various organelles because TMDs function as an endoplasmic reticulum (ER) targeting signal. Although ER targeting of SA proteins is well understood, how they are targeted to mitochondria and chloroplasts remains elusive. Here, we investigated how the targeting specificity of SA proteins is determined for specific targeting to mitochondria and chloroplasts. Mitochondrial targeting requires multiple motifs around and within TMDs: a basic residue and an arginine-rich region flanking the N- and C-termini of TMDs, respectively, and an aromatic residue in the C-terminal side of the TMD that specify mitochondrial targeting in an additive manner. These motifs play a role in slowing down the elongation speed during translation, thereby ensuring mitochondrial targeting in a co-translational manner. By contrast, the absence of any of these motifs individually or together causes at varying degrees chloroplast targeting that occurs in a post-translational manner.

Intracellular displacement of p53 using transactivation domain (p53 TAD) specific nanobodies

The tumor suppressor p53 is of crucial importance in the prevention of cellular transformation. In the presence of cellular stress signals, the negative feedback loop between p53 and Mdm2, its main negative regulator, is disrupted, which results in the activation and stabilization of p53. Via a complex interplay between both transcription-dependent and - independent functions of p53, the cell will go through transient cell cycle arrest, cellular senescence or apoptosis. However, it remains difficult to completely fathom the mechanisms behind p53 regulation and its responses, considering the presence of multiple layers involved in fine-tuning them. In order to take the next step forward, novel research tools are urgently needed. We have developed single-domain antibodies, also known as nanobodies, that specifically bind with the N-terminal transactivation domain of wild type p53, but that leave the function of p53 as a transcriptional transactivator intact. When the nanobodies are equipped with a mitochondrial-outer-membrane (MOM)-tag, we can capture p53 at the mitochondria. This nanobody-induced mitochondrial delocalization of p53 is, in specific cases, associated with a decrease in cell viability and with morphological changes in the mitochondria. These findings underpin the potential of nanobodies as bona fide research tools to explore protein function and to unravel their biochemical pathways.

Determining the target protein localization in 3D using the combination of FIB-SEM and APEX2

Determining the cellular localization of proteins of interest at nanometer resolution is necessary for elucidating their functions. Besides super-resolution fluorescence microscopy, conventional electron microscopy (EM) combined with immunolabeling or clonable EM tags provides a unique approach to correlate protein localization information and cellular ultrastructural information. However, there are still rare cases of such correlation in three-dimensional (3D) spaces. Here, we developed an approach by combining the focus ion beam scanning electron microscopy (FIB-SEM) and a promising clonable EM tag APEX2 (an enhanced ascorbate peroxidase 2) to determine the target protein localization within 3D cellular ultrastructural context. We further utilized this approach to study the 3D localization of mitochondrial dynamics-related proteins (MiD49/51, Mff, Fis1, and Mfn2) in the cells where the target proteins were overexpressed. We found that all the target proteins were located at the surface of the mitochondrial outer membrane accompanying with mitochondrial clusters. Mid49/51, Mff, and hFis1 spread widely around the mitochondrial surface while Mfn2 only exists at the contact sites.

VCA nanobodies target N-WASp to reduce invadopodium formation and functioning

Invasive cancer cells develop small actin-based protrusions called invadopodia, which perform a primordial role in metastasis and extracellular matrix remodelling. Neural Wiskott-Aldrich syndrome protein (N-WASp) is a scaffold protein which can directly bind to actin monomers and Arp2/3 and is a crucial player in the formation of an invadopodium precursor. Expression modulation has pointed to an important role for N-WASp in invadopodium formation but the role of its C-terminal VCA domain in this process remains unknown. In this study, we generated alpaca nanobodies against the N-WASp VCA domain and investigated if these nanobodies affect invadopodium formation. By using this approach, we were able to study functions of a selected functional/structural N-WASp protein domain in living cells, without requiring overexpression, dominant negative mutants or siRNAs which target the gene, and hence the entire protein. When expressed as intrabodies, the VCA nanobodies significantly reduced invadopodium formation in both MDA-MB-231 breast cancer and HNSCC61 head and neck squamous cancer cells. Furthermore, expression of distinct VCA Nbs (VCA Nb7 and VCA Nb14) in PC-3 prostate cancer cells resulted in reduced overall matrix degradation without affecting MMP9 secretion/activation or MT1-MMP localisation at invadopodial membranes. From these results, we conclude that we have generated nanobodies targeting N-WASp which reduce invadopodium formation and functioning, most likely via regulation of N-WASp—Arp2/3 complex interaction, indicating that this region of N-WASp plays an important role in these processes.

Newly born peroxisomes are a hybrid of mitochondrial and ER-derived pre-peroxisomes

Peroxisomes function together with mitochondria in a number of essential biochemical pathways, from bile acid synthesis to fatty acid oxidation. Peroxisomes grow and divide from pre-existing organelles, but can also emerge de novo in the cell. The physiological regulation of de novo peroxisome biogenesis remains unclear, and it is thought that peroxisomes emerge from the endoplasmic reticulum in both mammalian and yeast cells. However, in contrast to the yeast system, a number of integral peroxisomal membrane proteins are imported into mitochondria in mammalian cells in the absence of peroxisomes, including Pex3, Pex12, Pex13, Pex14, Pex26, PMP34 and ALDP. Overall, the mitochondrial localization of peroxisomal membrane proteins in mammalian cells has largely been considered a mis-targeting artefact in which de novo biogenesis occurs exclusively from endoplasmic reticulum-targeted peroxins. Here, in following the generation of new peroxisomes within human patient fibroblasts lacking peroxisomes, we show that the essential import receptors Pex3 and Pex14 target mitochondria, where they are selectively released into vesicular pre-peroxisomal structures. Maturation of pre-peroxisomes containing Pex3 and Pex14 requires fusion with endoplasmic reticulum-derived vesicles carrying Pex16, thereby providing full import competence. These findings demonstrate the hybrid nature of newly born peroxisomes, expanding their functional links to mitochondria.

Biogenesis of a Mitochondrial Outer Membrane Protein in Trypanosoma brucei: TARGETING SIGNAL AND DEPENDENCE ON A UNIQUE BIOGENESIS FACTOR

The mitochondrial outer membrane (OM) contains single and multiple membrane-spanning proteins that need to contain signals that ensure correct targeting and insertion into the OM. The biogenesis of such proteins has so far essentially only been studied in yeast and related organisms. Here we show that POMP10, an OM protein of the early diverging protozoan Trypanosoma brucei, is signal-anchored. Transgenic cells expressing variants of POMP10 fused to GFP demonstrate that the N-terminal membrane-spanning domain flanked by a few positively charged or neutral residues is both necessary and sufficient for mitochondrial targeting. Carbonate extraction experiments indicate that although the presence of neutral instead of positively charged residues did not interfere with POMP10 localization, it weakened its interaction with the OM. Expression of GFP-tagged POMP10 in inducible RNAi cell lines shows that its mitochondrial localization depends on pATOM36 but does not require Sam50 or ATOM40, the trypanosomal analogue of the Tom40 import pore. pATOM36 is a kinetoplastid-specific OM protein that has previously been implicated in the assembly of OM proteins and in mitochondrial DNA inheritance. In summary, our results show that although the features of the targeting signal in signal-anchored proteins are widely conserved, the protein machinery that mediates their biogenesis is not.

A new survivin tracer tracks, delocalizes and captures endogenous survivin at different subcellular locations and in distinct organelles

Survivin, the smallest member of the inhibitor of apoptosis protein family, plays a central role during mitosis and exerts a cytoprotective function. Survivin is highly expressed in most cancer types and contributes to multiple facets of carcinogenesis. The molecular mechanisms underlying its highly diverse functions need to be extensively explored, which is crucial for rational design of future personalized therapeutics. In this study, we have generated an alpaca survivin nanobody (SVVNb8) that binds with low nanomolar affinity to its target. When expressed as an intrabody in HeLa cells, SVVNb8 faithfully tracks survivin during different phases of mitosis without interfering with survivin function. Furthermore, coupling SVVNb8 with a subcellular delocalization tag efficiently redirects endogenous survivin towards the nucleus, the cytoplasm, peroxisomes and even to the intermembrane space of mitochondria where it presumably interacts with resident mitochondrial survivin. Based on our findings, we believe that SVVNb8 is an excellent instrument to further elucidate survivin biology and topography, and can serve as a model system to investigate mitochondrial and peroxisomal (survivin) protein import.

Positively charged residues within the MYO19 MyMOMA domain are essential for proper localization of MYO19 to the mitochondrial outer membrane

Myosins are well characterized molecular motors essential for intracellular transport. MYO19 copurifies with mitochondria, and can be released from mitochondrial membranes by high pH buffer, suggesting that positively-charged residues participate in interactions between MYO19 and mitochondria. The MYO19-specific mitochondria outer membrane association (MyMOMA) domain contains approximately 150 amino acids with a pI approximately 9 and is sufficient for localization to the mitochondrial outer membrane. The minimal sequence and specific residues involved in mitochondrial binding have not been identified. To address this, we generated GFP-MyMOMA truncations, establishing the boundaries for truncations based on sequence homology. We identified an 83-amino acid minimal binding region enriched with basic residues (pI ∼ 10.5). We sequentially replaced basic residues in this region with alanine, identifying residues R882 and K883 as essential for mitochondrial localization. Constructs containing the RK882-883AA mutation primarily localized with the endoplasmic reticulum (ER). To determine if ER-associated mutant MyMOMA domain and mitochondria-associated wild type MyMOMA display differences in kinetics of membrane interaction, we paired FRAP analysis with permeabilization activated reduction in fluorescence (PARF) analysis. Mitochondria-bound and ER-bound MYO19 constructs displayed slow dissociation from their target membrane when assayed by PARF; both constructs displayed exchange within their respective organelle networks. However, ER-bound mutant MYO19 displayed more rapid exchange within the ER network than did mitochondria-bound MYO19. Taken together these data indicate that the MyMOMA domain contains strong membrane-binding activity, and membrane targeting is mediated by a specific, basic region of the MYO19 tail with slow dissociation kinetics appropriate for its role(s) in mitochondrial network dynamics. © 2016 Wiley Periodicals, Inc.

Manipulation of the N-terminal sequence of the Borna disease virus X protein improves its mitochondrial targeting and neuroprotective potential

To favor their replication, viruses express proteins that target diverse mammalian cellular pathways. Due to the limited size of many viral genomes, such proteins are endowed with multiple functions, which require targeting to different subcellular compartments. One salient example is the X protein of Borna disease virus, which is expressed both at the mitochondria and in the nucleus. Moreover, we recently demonstrated that mitochondrial X protein is neuroprotective. In this study, we sought to examine the mechanisms whereby the X protein transits between subcellular compartments and to define its localization signals, to enhance its mitochondrial accumulation and thus, potentially, its neuroprotective activity. We transfected plasmids expressing fusion proteins bearing different domains of X fused to enhanced green fluorescent protein (eGFP) and compared their subcellular localization to that of eGFP. We observed that the 5-16 domain of X was responsible for both nuclear export and mitochondrial targeting and identified critical residues for mitochondrial localization. We next took advantage of these findings and constructed mutant X proteins that were targeted only to the mitochondria. Such mutants exhibited enhanced neuroprotective properties in compartmented cultures of neurons grown in microfluidic chambers, thereby confirming the parallel between mitochondrial accumulation of the X protein and its neuroprotective potential.-Ferré C. A., Davezac, N., Thouard, A., Peyrin, J. M., Belenguer, P., Miquel, M.-C., Gonzalez-Dunia, D., Szelechowski, M. Manipulation of the N-terminal sequence of the Borna disease virus X protein improves its mitochondrial targeting and neuroprotective potential.

Transport of rice cyclobutane pyrimidine dimer photolyase into mitochondria relies on a targeting sequence located in its C-terminal internal region

The cyclobutane pyrimidine dimer (CPD), which represents a major type of DNA damage induced by ultraviolet-B (UVB) radiation, is a principal cause of UVB-induced growth inhibition in plants. CPD photolyase is the primary enzyme for repairing CPDs and is crucial for determining the sensitivity of Oryza sativa (rice) to UVB radiation. CPD photolyase is widely distributed among species ranging from eubacteria to eukaryotes, and is classified into class I or II based on its primary structure. We previously demonstrated that rice CPD photolyase (OsPHR), which belongs to class II and is encoded by a single-copy gene, is a unique nuclear/mitochondrial/chloroplast triple-targeting protein; however, the location and nature of the organellar targeting information contained within OsPHR are unknown. Here, the nuclear and mitochondrial targeting signal sequences of OsPHR were identified by systematic deletion analysis. The nuclear and mitochondrial targeting sequences are harbored within residues 487-489 and 391-401 in the C-terminal region of OsPHR (506 amino acid residues), respectively. The mitochondrial targeting signal represents a distinct topogenic sequence that differs structurally and functionally from classical N-terminal pre-sequences, and this region, in addition to its role in localization to the mitochondria, is essential for the proper functioning of the CPD photolyase. Furthermore, the mitochondrial targeting sequence, which is characteristic of class-II CPD photolyases, was acquired before the divergence of class-II CPD photolyases in eukaryotes. These results indicate that rice plants have evolved a CPD photolyase that functions in mitochondria to protect cells from the harmful effects of UVB radiation.

Heidi McBride: mitochondria are well connected

McBride studies mitochondrial dynamics and mitochondrial trafficking with other organelles.

Stratifying fascin and cortactin function in invadopodium formation using inhibitory nanobodies and targeted subcellular delocalization

Invadopodia are actin-rich protrusions arising through the orchestrated regulation of precursor assembly, stabilization, and maturation, endowing cancer cells with invasive properties. Using nanobodies (antigen-binding domains of Camelid heavy-chain antibodies) as perturbators of intracellular functions and/or protein domains at the level of the endogenous protein, we examined the specific contribution of fascin and cortactin during invadopodium formation in MDA-MB-231 breast and PC-3 prostate cancer cells. A nanobody (K(d)~35 nM, 1:1 stoichiometry) that disrupts fascin F-actin bundling emphasizes the importance of stable actin bundles in invadopodium array organization and turnover, matrix degradation, and cancer cell invasion. Cortactin-SH3 dependent WIP recruitment toward the plasma membrane was specifically inhibited by a cortactin nanobody (K(d)~75 nM, 1:1 stoichiometry). This functional domain is shown to be important for formation of properly organized invadopodia, MMP-9 secretion, matrix degradation, and cancer cell invasion. Notably, using a subcellular delocalization strategy to trigger protein loss of function, we uncovered a fascin-bundling-independent role in MMP-9 secretion. Hence, we demonstrate that nanobodies enable high resolution protein function mapping in cells.

Reconstitution of mitochondria derived vesicle formation demonstrates selective enrichment of oxidized cargo

The mechanisms that ensure the removal of damaged mitochondrial proteins and lipids are critical for the health of the cell, and errors in these pathways are implicated in numerous degenerative diseases. We recently uncovered a new pathway for the selective removal of proteins mediated by mitochondrial derived vesicular carriers (MDVs) that transit to the lysosome. However, it was not determined whether these vesicles were selectively enriched for oxidized, or damaged proteins, and the extent to which the complexes of the electron transport chain and the mtDNA-containing nucloids may have been incorporated. In this study, we have developed a cell-free mitochondrial budding reaction in vitro in order to better dissect the pathway. Our data confirm that MDVs are stimulated upon various forms of mitochondrial stress, and the vesicles incorporated quantitative amounts of cargo, whose identity depended upon the nature of the stress. Under the conditions examined, MDVs did not incorporate complexes I and V, nor were any nucleoids present, demonstrating the specificity of cargo incorporation. Stress-induced MDVs are selectively enriched for oxidized proteins, suggesting that conformational changes induced by oxidation may initiate their incorporation into the vesicles. Ultrastructural analyses of MDVs isolated on sucrose flotation gradients revealed the formation of both single and double membranes vesicles of unique densities and uniform diameter. This work provides a framework for a reductionist approach towards a detailed examination of the mechanisms of MDV formation and cargo incorporation, and supports the emerging concept that MDVs are critical contributors to mitochondrial quality control.

The mitochondrial amidoxime-reducing component (mARC1) is a novel signal-anchored protein of the outer mitochondrial membrane

The mitochondrial amidoxime-reducing component (mARC) was recently discovered as the fifth eukaryotic molybdenum cofactor-containing enzyme. The human genome encodes two mARC proteins, mARC1 and mARC2, sharing significant homologies with respect to sequence and function. Whereas mARC2 was identified as a mitochondrial enzyme, the subcellular localization of mARC1 has remained uncharacterized, although the similarity of both proteins suggested identical subcellular localizations. In addition, neither mARC1 nor mARC2 could be attributed unambiguously to one of the four mitochondrial subcompartments. Accordingly, mechanisms triggering the subcellular distribution of both enzymes have been unexplored so far. Here, we shed light on the subcellular localization of mARC1 and demonstrate that it is integrated into the outer mitochondrial membrane. The C-terminal catalytic domain of the protein remains exposed to the cytosol and confers an N((in))-C((out)) membrane orientation of mARC1. This localization is triggered by the N terminus of the enzyme, being composed of a weak N-terminal mitochondrial targeting signal and a downstream transmembrane helix. We demonstrate the transmembrane domain of mARC1 to be sufficient for mitochondrial targeting and the N-terminal targeting signal to function as a supportive receptor for the outer mitochondrial membrane. According to its localization and targeting mechanism, we classify mARC1 as a novel signal-anchored mitochondrial protein. During mitochondrial import, mARC1 is not processed, and membrane integration proceeds membrane potential independently but requires external ATP, which finally results in the assembly of mARC1 into high oligomeric protein complexes.

A novel cell-free mitochondrial fusion assay amenable for high-throughput screenings of fusion modulators

Background

Mitochondria are highly dynamic organelles whose morphology and position within the cell is tightly coupled to metabolic function. There is a limited list of essential proteins that regulate mitochondrial morphology and the mechanisms that govern mitochondrial dynamics are poorly understood. However, recent evidence indicates that the core machinery that governs mitochondrial dynamics is linked within complex intracellular signalling cascades, including apoptotic pathways, cell cycle transitions and nuclear factor kappa B activation. Given the emerging importance of mitochondrial plasticity in cell signalling pathways and metabolism, it is essential that we develop tools to quantitatively analyse the processes of fission and fusion. In terms of mitochondrial fusion, the field currently relies upon on semi-quantitative assays which, even under optimal conditions, are labour-intensive, low-throughput and require complex imaging techniques.

Results

In order to overcome these technical limitations, we have developed a new, highly quantitative cell-free assay for mitochondrial fusion in mammalian cells. This assay system has allowed us to establish the energetic requirements for mitochondrial fusion. In addition, our data reveal a dependence on active protein phosphorylation for mitochondrial fusion, confirming emerging evidence that mitochondrial fusion is tightly integrated within the global cellular response to signaling events. Indeed, we have shown that cytosol derived from cells stimulated with different triggers either enhance or inhibit the cell-free fusion reaction.

Conclusions

The adaptation of this system to high-throughput analysis will provide an unprecedented opportunity to identify and characterize novel regulatory factors. In addition, it provides a framework for a detailed mechanistic analysis of the process of mitochondrial fusion and the various axis of regulation that impinge upon this process in a wide range of cellular conditions.

See Commentary: http://www.biomedcentral.com/1741-7007/8/99

A llama-derived gelsolin single-domain antibody blocks gelsolin-G-actin interaction

RNA interference has tremendously advanced our understanding of gene function but recent reports have exposed undesirable side-effects. Recombinant Camelid single-domain antibodies (VHHs) provide an attractive means for studying protein function without affecting gene expression. We raised VHHs against gelsolin (GsnVHHs), a multifunctional actin-binding protein that controls cellular actin organization and migration. GsnVHH-induced delocalization of gelsolin to mitochondria or the nucleus in mammalian cells reveals distinct subpopulations including free gelsolin and actin-bound gelsolin complexes. GsnVHH 13 specifically recognizes Ca(2+)-activated gelsolin (K (d) approximately 10 nM) while GsnVHH 11 binds gelsolin irrespective of Ca(2+) (K (d) approximately 5 nM) but completely blocks its interaction with G-actin. Both GsnVHHs trace gelsolin in membrane ruffles of EGF-stimulated MCF-7 cells and delay cell migration without affecting F-actin severing/capping or actin nucleation activities by gelsolin. We conclude that VHHs represent a potent way of blocking structural proteins and that actin nucleation by gelsolin is more complex than previously anticipated.

Antagonism of Beclin 1-dependent autophagy by BCL-2 at the endoplasmic reticulum requires NAF-1

In addition to mitochondria, BCL-2 is located at the endoplasmic reticulum (ER) where it is a constituent of several distinct complexes. Here, we identify the BCL-2-interacting protein at the ER, nutrient-deprivation autophagy factor-1 (NAF-1)-a bitopic integral membrane protein whose defective expression underlies the aetiology of the neurodegenerative disorder Wolfram syndrome 2 (WFS2). NAF-1 contains a two iron-two sulphur coordinating domain within its cytosolic region, which is necessary, but not sufficient for interaction with BCL-2. NAF-1 is displaced from BCL-2 by the ER-restricted BH3-only protein BIK and contributes to regulation of BIK-initiated autophagy, but not BIK-dependent activation of caspases. Similar to BCL-2, NAF-1 is found in association with the inositol 1,4,5-triphosphate receptor and is required for BCL-2-mediated depression of ER Ca(2+) stores. During nutrient deprivation as a physiological stimulus of autophagy, BCL-2 is known to function through inhibition of the autophagy effector and tumour suppressor Beclin 1. NAF-1 is required in this pathway for BCL-2 at the ER to functionally antagonize Beclin 1-dependent autophagy. Thus, NAF-1 is a BCL-2-associated co-factor that targets BCL-2 for antagonism of the autophagy pathway at the ER.

An alpaca single-domain antibody blocks filopodia formation by obstructing L-plastin-mediated F-actin bundling

L-plastin, a conserved modular F-actin bundling protein, is ectopically expressed in tumor cells and contributes to cell malignancy and invasion. The underlying molecular mechanisms involved remain unclear, in part, because specific inhibitors of L-plastin are lacking. We used recombinant alpaca-derived L-plastin single-domain antibodies (nanobodies) as effector of L-plastin function in cells.

Biogenesis of mitochondrial outer membrane proteins

Mitochondria are surrounded by two distinct membranes: the outer and the inner membrane. The mitochondrial outer membrane mediates numerous interactions between the mitochondrial metabolic and genetic systems and the rest of the eukaryotic cell. Proteins of this membrane are nuclear-encoded and synthesized as precursor proteins in the cytosol. They are targeted to the mitochondria and inserted into their target membrane via various pathways. This review summarizes our current knowledge of the sorting signals for this specific targeting and describes the mechanisms by which the mitochondrial import machineries recognize precursor proteins, mediate their membrane integration and facilitate assembly into functional complexes.

A molecular switch for targeting between endoplasmic reticulum (ER) and mitochondria: conversion of a mitochondria-targeting element into an ER-targeting signal in DAKAP1

dAKAP1 (AKAP121, S-AKAP84), a dual specificity PKA scaffold protein, exists in several forms designated as a, b, c, and d. Whether dAKAP1 targets to endoplasmic reticulum (ER) or mitochondria depends on the presence of the N-terminal 33 amino acids (N1), and these N-terminal variants are generated by either alternative splicing and/or differential initiation of translation. The mitochondrial targeting motif, which is localized between residues 49 and 63, is comprised of a hydrophobic helix followed by positive charges ( Ma, Y., and Taylor, S. (2002) J. Biol. Chem. 277, 27328-27336 ). dAKAP1 is located on the cytosolic surface of mitochondria outer membrane and both smooth and rough ER membrane. A single residue, Asp(31), within the first 33 residues of dAKAP1b is required for ER targeting. Asp(31), which functions as a separate motif from the mitochondrial targeting signal, converts the mitochondrial-targeting signal into a bipartite ER-targeting signal, without destroying the mitochondria-targeting signal. Therefore dAKAP1 possesses a single targeting element capable of targeting to both mitochondria and ER, with the ER signal overlapping the mitochondria signal. The specificity of ER or mitochondria targeting is determined and switched by the availability of the negatively charged residue, Asp(31).

An artificial mitochondrial tail signal/anchor sequence confirms a requirement for moderate hydrophobicity for targeting

Tail-anchored proteins are a group of membrane proteins oriented with their amino terminus in the cytoplasm and their carboxy terminus embedded in intracellular membranes. This group includes the apoptosis-mediating proteins of the Bcl-2 family as well as the vesicle targeting proteins of the SNARE group, among others. A stretch of hydrophobic amino acids at the extreme carboxy terminus of these proteins serves both as a membrane anchor and as a targeting signal. Tail-anchored proteins are differentially targeted to either the endoplasmic reticulum or the mitochondrial outer membrane and the mechanism which accomplishes this selective targeting is poorly understood. Here we define important characteristics of the signal/anchor region which directs proteins to the mitochondrial outer membrane. We have created an artificial sequence consisting of a stretch of 16 leucines bounded by positively charged amino acids. Using this template we demonstrate that moderate hydrophobicity distinguishes the mitochondrial tail-anchor sequence from that of the endoplasmic reticulum tail-anchor sequence. A change as small as introduction of a single polar residue into a sequence that otherwise targets to the endoplasmic reticulum can substantially switch targeting to the mitochondrial outer membrane. Further we show that a mitochondrially targeted tail-anchor has a higher propensity for the formation of alpha-helical structure than a sequence directing tail-anchored proteins to the endoplasmic reticulum.

Mitochondrial localization of antizyme is determined by context-dependent alternative utilization of two AUG initiation codons

Ornithine decarboxylase-antizyme (Az), a polyamine-induced protein that targets ornithine decarboxylase (ODC) to rapid degradation, is synthesized as two isoforms. Studies performed in vitro indicated that the 29 and 24.5 kDa isoforms originate from translation initiation at two alternative initiation codons. Using transient transfections we demonstrate here that also in cells the two isoforms are synthesized from two AUG codons with the second being utilized more efficiently. The more efficient utilization of the second AUG is due to its location within a better sequence context for translation initiation. By using immunostaining we demonstrate that only the less expressed long form of Az is localized in the mitochondria. Moreover, this long isoform of Az and not the more efficiently expressed short isoform is imported into mitochondria in an in vitro uptake assay. Our data therefore demonstrate that a single Az transcript gives rise to two Az proteins with different N-terminal sequence and that the longer Az form containing a potential N-terminal mitochondrial localization signal is transported to mitochondria.

Unfolding-resistant translocase targeting: a novel mechanism for outer mitochondrial membrane localization exemplified by the Bbeta2 regulatory subunit of protein phosphatase 2A

Heterotrimeric serine/threonine protein phosphatase 2A (PP2A) consists of scaffolding (A), catalytic (C), and variable (B, B', and B'') subunits. Variable subunits dictate subcellular localization and substrate specificity of the PP2A holoenzyme. The Bbeta regulatory subunit gene is mutated in spinocerebellar ataxia type 12, and one of its splice variants, Bbeta2, targets PP2A to mitochondria to promote apoptosis in PC12 cells (Dagda, R. K., Zaucha, J. A., Wadzinski, B. E., and Strack, S. (2003) J. Biol. Chem. 278, 24976-24985). Here, we report that Bbeta2 is localized to the outer mitochondrial membrane by a novel mechanism, combining a cryptic mitochondrial import signal with a structural arrest domain. Scanning mutagenesis demonstrates that basic and hydrophobic residues mediate mitochondrial association and the proapoptotic activity of Bbeta2. When fused to green fluorescent protein, the N terminus of Bbeta2 acts as a cleavable mitochondrial import signal. Surprisingly, full-length Bbeta2 is not detectably cleaved and is retained at the outer mitochondrial membrane, even though it interacts with the TOM22 import receptor, as shown by luciferase complementation in intact cells. Mutations that open the C-terminal beta-propeller of Bbeta2 facilitate mitochondrial import, indicating that this rigid fold acts as a stop-transfer domain by resisting the partial unfolding step prerequisite for matrix translocation. Because hybrids of prototypical import and beta-propeller domains recapitulate this behavior, we predict the existence of other similarly localized proteins and a selection against highly stable protein folds in the mitochondrial matrix. This unfolding-resistant targeting to the mitochondrial translocase is necessary but not sufficient for the proapoptotic activity of Bbeta2, which also requires association with the rest of the PP2A holoenzyme.

The protein import and assembly machinery of the mitochondrial outer membrane

The process of mitochondrial protein import has been studied for many years. Despite this attention, many processes associated with mitochondrial biogenesis are poorly understood. Insight into one of these processes, assembly of beta-barrel proteins into the mitochondrial outer membrane, will be discussed. This review focuses on recent data that suggest that assembly of beta-barrel proteins into the outer mitochondrial membrane is dependent on a newly identified protein complex termed the sorting and assembly machinery (SAM complex). Members of the SAM complex have been identified in both eukaryotic and prokaryotic organisms, suggesting that the process of beta-barrel assembly into membranes has been conserved through evolution.

Finding the right organelle. Targeting signals in mitochondrial outer-membrane proteins

The mitochondrial outer membrane contains a diverse set of proteins that includes enzymes, components of the preprotein translocation machinery, pore-forming proteins, regulators of programmed cell death, and those that control the morphology of the organelle. All these proteins, like the vast majority of mitochondrial proteins, are encoded in the nucleus, so they are synthesized in the cytosol and contain signals that are essential for their subsequent import into mitochondria. This review summarizes our current knowledge of the signals that target mitochondrial outer-membrane proteins to their correct intracellular location. In addition, the mechanisms by which these signals are decoded by the mitochondria are discussed.

Targeting and Assembly of Rat Mitochondrial Translocase of Outer Membrane 22 (TOM22) into the TOM Complex

Tom22 is a preprotein receptor and organizer of the mitochondrial outer membrane translocase complex (TOM complex). Rat Tom22 (rTOM22) is a 142-residue protein, embedded in the outer membrane through the internal transmembrane domain (TMD) with 82 N-terminal residues in the cytosol and 41 C-terminal residues in the intermembrane space. We analyzed the signals that target rTOM22 to the mitochondrial outer membrane and assembly into the TOM complex in cultured mammalian cells. Deletions or mutations were systematically introduced into the molecule, and the intracellular localization of the mutant constructs in HeLa cells was examined by confocal microscopy and cell fractionation. Their assembly into the TOM complex was also examined using blue native gel electrophoresis. These experiments revealed three separate structural elements: a cytoplasmic 10-residue segment with an acidic alpha-helical structure located 30 residues upstream of the TMD (the import sequence), TMD with an appropriate hydrophobicity, and a 20-residue C-terminal segment located 22 residues downstream of the TMD (C-tail signal). The import sequence and TMD were both essential for targeting and integration into the TOM complex, whereas the C-tail signal affected the import efficiency. The import sequence combined with foreign TMD functioned as a mitochondrial targeting and anchor signal but failed to integrate the construct into the TOM complex. Thus, the mitochondrial-targeting and TOM integration signal could be discriminated. A yeast two-hybrid assay revealed that the import sequence interacted with two intramolecular elements, the TMD and C-tail signal, and that it also interacted with the import receptor Tom20.

Engineered retargeting of viral RNA replication complexes to an alternative intracellular membrane

Positive-strand RNA virus replication complexes are universally associated with intracellular membranes, although different viruses use membranes derived from diverse and sometimes multiple organelles. We investigated whether unique intracellular membranes are required for viral RNA replication complex formation and function in yeast by retargeting protein A, the Flock House virus (FHV) RNA-dependent RNA polymerase. Protein A, the only viral protein required for FHV RNA replication, targets and anchors replication complexes to outer mitochondrial membranes in part via an N-proximal sequence that contains a transmembrane domain. We replaced the FHV protein A mitochondrial outer membrane-targeting sequence with the N-terminal endoplasmic reticulum (ER)-targeting sequence from the yeast NADP cytochrome P450 oxidoreductase or inverted C-terminal ER-targeting sequences from the hepatitis C virus NS5B polymerase or the yeast t-SNARE Ufe1p. Confocal immunofluorescence microscopy confirmed that protein A chimeras retargeted to the ER. FHV subgenomic and genomic RNA accumulation in yeast expressing ER-targeted protein A increased 2- to 13-fold over that in yeast expressing wild-type protein A, despite similar protein A levels. Density gradient flotation assays demonstrated that ER-targeted protein A remained membrane associated, and in vitro RNA-dependent RNA polymerase assays demonstrated an eightfold increase in the in vitro RNA synthesis activity of the ER-targeted FHV RNA replication complexes. Electron microscopy showed a change in the intracellular membrane alterations from a clustered mitochondrial distribution with wild-type protein A to the formation of perinuclear layers with ER-targeted protein A. We conclude that specific intracellular membranes are not required for FHV RNA replication complex formation and function.

Signal-anchor domains of proteins of the outer membrane of mitochondria: structural and functional characteristics

We have studied the topogenesis of a class of mitochondrial outer membrane proteins that expose a hydrophilic domain to the cytosol and are anchored to the membrane by a single transmembrane domain in the N-terminal region. To determine the role of these latter sequences in the targeting and insertion of such proteins we took two approaches. First, a functional complementation assay was used to define the structural elements that together with the anchor domain make up the topogenic signal. Moderate hydrophobicity of the transmembrane domain was found to be the most important requirement. Variants with a scrambled sequence of the membrane-spanning segment were only partially functional suggesting that specificity in the amino acid sequence is also of considerable importance. A net positive charge at both flanking regions of the transmembrane domain contributes to the efficiency of targeting and membrane integration but is not an essential structural feature of this signal. Second, chimeras of Tom20, Tom70, and OM45 were generated that contained the cytosolic domain of Tom20 or Tom70 and the anchor domain of one of the other members of the class. These hybrid proteins were able to rescue the growth of cells lacking Tom20 or Tom70. Thus, anchor domains of outer membrane proteins are functionally interchangeable. They play only a minor role in the specific function of these proteins, but have a decisive role in topogenic signaling.

Targeting and assembly of mitochondrial tail-anchored protein Tom5 to the TOM complex depend on a signal distinct from that of tail-anchored proteins dispersed in the membrane

Mitochondrial outer membrane proteins are synthesized without a cleavable presequence but instead contain segments responsible for mitochondrial targeting and membrane integration within the molecule: the transmembrane segment (TMS) and N- or C-terminal flanking segment. We analyzed targeting and integration of Tom5, a C-tail anchor protein associated with the preprotein translocase of the outer membrane, to the yeast mitochondrial outer membrane in vivo using green fluorescent protein as the reporter and compared the signal with other signals for proteins dispersed in the membrane. The functional assembly of Tom5 into the TOM complex was assessed by blue native PAGE and complementation of temperature-sensitive deltatom5 cells. Correct targeting and assembly required (i). an appropriate length TMS rather than hydrophobicity, (ii). a proline residue located at correct position in the TMS and specific residues near the proline, and (iii). that, in contrast to proteins dispersed in the outer membrane, the positive C-terminal segment was dispensable. Based on these findings, we constructed green fluorescent protein fusions with a C-terminal TMS in which the deduced sequences (minimum: Ser-Pro-Met) were inserted at an appropriate position within artificial Leu-Ala repeats. They were targeted to mitochondria and complemented the temperature-sensitive growth phenotype of deltatom5 yeast cells. The membrane-targeting mechanism of Tom5 appears to be distinct from that for proteins that are dispersed in the outer membrane.

Cellular distribution of Bcl-2 family proteins

Proteins synthesized in the cytosol can be inserted into or across organellar membranes depending on the signals present in their primary sequences. Proteins of the Bcl-2 family, which includes proteins that promote programmed cell death (apoptosis) and proteins that inhibit apoptosis, have signals that can target particular members to either the mitochondria or the endoplasmic reticulum (ER) or both. Germain and Shore discuss evidence for a role for the anti-apoptotic members of the family at the ER and describe an interaction with the FK506 binding protein, FKBP38, that may regulate the relative distribution of Bcl-2 proteins at the ER or mitochondria. This interaction may control the cell's sensitivity to apoptotic stimuli.

The liver isoform of carnitine palmitoyltransferase 1 is not targeted to the endoplasmic reticulum

Liver microsomal fractions contain a malonyl-CoA-inhibitable carnitine acyltransferase (CAT) activity. It has been proposed [Fraser, Corstorphine, Price and Zammit (1999) FEBS Lett. 446, 69-74] that this microsomal CAT activity is due to the liver form of carnitine palmitoyltransferase 1 (L-CPT1) being targeted to the endoplasmic reticulum (ER) membrane as well as to mitochondria, possibly by an N-terminal signal sequence [Cohen, Guillerault, Girard and Prip-Buus (2001) J. Biol. Chem. 276, 5403-5411]. COS-1 cells were transiently transfected to express a fusion protein in which enhanced green fluorescent protein was fused to the C-terminus of L-CPT1. Confocal microscopy showed that this fusion protein was localized to mitochondria, and possibly to peroxisomes, but not to the ER. cDNAs corresponding to truncated (amino acids 1-328) or full-length L-CPT1 were transcribed and translated in the presence of canine pancreatic microsomes. However, there was no evidence of authentic insertion of CPT1 into the ER membrane. Rat liver microsomal fractions purified by sucrose-density-gradient centrifugation contained an 88 kDa protein (p88) which was recognized by an anti-L-CPT1 antibody and by 2,4-dinitrophenol-etomoxiryl-CoA, a covalent inhibitor of L-CPT1. Abundance of p88 and malonyl-CoA-inhibitable CAT activity were increased approx. 3-fold by starvation for 24 h. Deoxycholate solubilized p88 and malonyl-CoA-inhibitable CAT activity from microsomes to approximately the same extent. The microsomal fraction contained porin, which, relative to total protein, was as abundant as in crude mitochondrial outer membranes fractions. It is concluded that L-CPT1 is not targeted to the ER membrane and that malonyl-CoA CAT in microsomal fractions is L-CPT1 that is derived from mitochondria, possibly from membrane contact sites.

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.

Flock house virus RNA polymerase is a transmembrane protein with amino-terminal sequences sufficient for mitochondrial localization and membrane insertion

Localization of RNA replication to intracellular membranes is a universal feature of positive-strand RNA viruses. Replication complexes of flock house virus (FHV), the best-studied alphanodavirus, are located on outer mitochondrial membranes in infected Drosophila melanogaster cells and are associated with the formation of membrane-bound spherules, similar to structures found for many other positive-strand RNA viruses. To further study FHV replication complex formation, we investigated the subcellular localization, membrane association, and membrane topology of protein A, the FHV RNA-dependent RNA polymerase, in the yeast Saccharomyces cerevisiae, a host able to support full FHV RNA replication and virion formation. Confocal immunofluorescence revealed that protein A localized to mitochondria in yeast, as in Drosophila cells, and that this mitochondrial localization was independent of viral RNA synthesis. Nycodenz gradient flotation and dissociation assays showed that protein A behaved as an integral membrane protein, a finding consistent with a predicted N-proximal transmembrane domain. Protease digestion and selective permeabilization after differential epitope tagging demonstrated that protein A was inserted into the outer mitochondrial membrane with the N terminus in the inner membrane space or matrix and that the C terminus was exposed to the cytoplasm. Flotation and immunofluorescence studies with deletion mutants indicated that the N-proximal region of protein A was important for both membrane association and mitochondrial localization. Gain-of-function studies with green fluorescent protein fusions demonstrated that the N-terminal 46 amino acids of protein A were sufficient for mitochondrial localization and membrane insertion. We conclude that protein A targets and anchors FHV RNA replication complexes to outer mitochondrial membranes, in part through an N-proximal mitochondrial localization signal and transmembrane domain.

Functions of outer membrane receptors in mitochondrial protein import

Most mitochondrial proteins are synthesized in the cytosol as precursor proteins and are imported into mitochondria. The targeting signals for mitochondria are encoded in the presequences or in the mature parts of the precursor proteins, and are decoded by the receptor sites in the translocator complex in the mitochondrial outer membrane. The recently determined NMR structure of the general import receptor Tom20 in a complex with a presequence peptide reveals that, although the amphiphilicity and positive charges of the presequence is essential for the import ability of the presequence, Tom20 recognizes only the amphiphilicity, but not the positive charges. This leads to a new model that different features associated with the mitochondrial targeting sequence of the precursor protein can be recognized by the mitochondrial protein import system in different steps during the import.

Gene activation by interaction of an inhibitor with a cytoplasmic signaling protein

Galactose-inducible genes (GAL genes) in yeast Saccharomyces cerevisiae are efficiently transcribed only when the sequence-specific transcription activator Gal4p is activated. Activation of Gal4p requires the interaction between the Gal4p inhibitory protein Gal80p and the galactokinase paralog, Gal3p. It has been proposed that Gal3p binds to a Gal80p-Gal4p complex in the nucleus to activate Gal4p. Here, we present evidence that the Gal3p-Gal80p interaction occurs in the cytoplasm, and concurrently, Gal80p is removed from Gal4p at the GAL gene promoter. We also show that GAL gene expression can be activated by heterologous protein-protein interaction in the cytoplasm that is independent of galactose and Gal3p function. These results indicate that galactose-triggered Gal3p-Gal80p association in the cytoplasm activates Gal4p in the nucleus.

Characterization of signal that directs C-tail-anchored proteins to mammalian mitochondrial outer membrane

We analyzed the signal that directs the outer membrane protein with the C-terminal transmembrane segment (TMS) to mammalian mitochondria by using yeast Tom5 as a model and green fluorescent protein as a reporter. Deletions or mutations were systematically introduced into the TMS or the flanking regions and their intracellular localization in COS-7 cells was examined using confocal microscopy and cell fractionation. 1) Three basic amino acid residues within the C-terminal five-residue segment (C-segment) contained the information required for mitochondrial-targeting. Reduction of the net positive charge in this segment decreased mitochondrial specificity, and the mutants were distributed throughout the intracellular membranes. 2) Elongation of the TMS interfered with the function of the C-segment and the mutants were delivered to the intracellular membranes. 3) Separation of the TMS and C-segment by linker insertion severely impaired mitochondrial targeting function, leading to mislocalization to the cytoplasm. 4) Mutations or small deletions in the region of the TMS flanking the C-segment also impaired the mitochondrial targeting. Therefore, the moderate length of the TMS, the positive charges in the C-segment, and the distance between or context of the TMS and C-segment are critical for the targeting signal. The structural characteristics of the signal thus defined were also confirmed with mammalian C-tail-anchored protein OMP25.

Characterization of rat TOM70 as a receptor of the preprotein translocase of the mitochondrial outer membrane

We cloned a ∼70 kDa rat mitochondrial outer membrane protein (OM70)with a sequence identity of 28.1% and 20.1% with N. crassa and S. cerevisiae Tom70, respectively. Even with this low sequence identity,however, the proteins share a remarkable structural similarity: they have 7-10 tetratricopeptide repeat motifs and are anchored to the outer membrane through the N-terminal transmembrane domain with the bulk portion located in the cytosol. Antibodies against OM70 inhibited import of preproteins, such as the ADP/ATP carrier and rTOM40, that use internal targeting signals but not the import of cleavable presequence-containing preproteins. Blue native gel electrophoresis and immunoprecipitation of digitoninsolubilized mitochondrial outer membranes revealed that OM70 was loosely associated with the ∼400 kDa translocase complex of the mitochondrial outer membrane, which contains rTOM22 and rTOM40. A yeast two-hybrid system demonstrated that OM70 interacted with rTOM20 and rTOM22 through the cytoplasmic domains. Thus, OM70 is a functional homologue of fungal Tom70 and functions as a receptor of the preprotein import machinery of the rat mitochondrial outer membrane. Furthermore, the N-terminal 66 residue region of OM70, which comprises a hydrophilic 41 residue N-terminal domain, a 22 residue transmembrane domain and three arginine residues, is sufficient to act as a mitochondria-targeting signal, and the arginine cluster is crucial for this function.

VDAC and the bacterial porin PorB of Neisseria gonorrhoeae share mitochondrial import pathways

The human pathogen Neisseria gonorrhoeae induces host cell apoptosis during infection by delivering the outer membrane protein PorB to the host cell's mitochondria. PorB is a pore-forming beta-barrel protein sharing several features with the mitochondrial voltage-dependent anion channel (VDAC), which is involved in the regulation of apoptosis. Here we show that PorB of pathogenic Neisseria species produced by host cells is efficiently targeted to mitochondria. Imported PorB resides in the mitochondrial outer membrane and forms multimers with similar sizes as in the outer bacterial membrane. The mitochondria completely lose their membrane potential, a characteristic previously observed in cells infected with gonococci or treated with purified PorB. Closely related bacterial porins of non-pathogenic Neisseria mucosa or Escherichia coli remain in the cytosol. Import of PorB into mitochondria in vivo is independent of a linear signal sequence. Insertion of PorB into the mitochondrial outer membrane in vitro depends on the activity of Tom5, Tom20 and Tom40, but is independent of Tom70. Our data show that human VDAC and bacterial PorB are imported into mitochondria by a similar mechanism.

Biogenesis of the mitochondrial TOM complex

The translocase at the outer membrane of mitochondria (TOM complex) mediates the initial steps of the import of preproteins into the organelle, which are essential for mitochondrial biogenesis and, therefore, for eukaryotic cell viability. The TOM complex is a multisubunit molecular machine with a dynamic structure. The biogenesis of TOM is of special interest because the complex is required for its own biogenesis. This article describes the mechanisms by which Tom components are targeted to the mitochondria and inserted into the outer membrane. The assembly of newly synthesized subunits into the functional TOM complex might occur via assembly intermediates that are in equilibrium with the mature complex.

Structural requirements of Tom40 for assembly into preexisting TOM complexes of mitochondria

Tom40 is the major subunit of the translocase of the outer mitochondrial membrane (the TOM complex). To study the assembly pathway of Tom40, we have followed the integration of the protein into the TOM complex in vitro and in vivo using wild-type and altered versions of the Neurospora crassa Tom40 protein. Upon import into isolated mitochondria, Tom40 precursor proteins lacking the first 20 or the first 40 amino acid residues were assembled as the wild-type protein. In contrast, a Tom40 precursor lacking residues 41 to 60, which contains a highly conserved region of the protein, was arrested at an intermediate stage of assembly. We constructed mutant versions of Tom40 affecting this region and transformed the genes into a sheltered heterokaryon containing a tom40 null nucleus. Homokaryotic strains expressing the mutant Tom40 proteins had growth rate defects and were deficient in their ability to form conidia. Analysis of the TOM complex in these strains by blue native gel electrophoresis revealed alterations in electrophoretic mobility and a tendency to lose Tom40 subunits from the complex. Thus, both in vitro and in vivo studies implicate residues 41 to 60 as containing a sequence required for proper assembly/stability of Tom40 into the TOM complex. Finally, we found that TOM complexes in the mitochondrial outer membrane were capable of exchanging subunits in vitro. A model is proposed for the integration of Tom40 subunits into the TOM complex.

EAT/mcl-1 expression in the human embryonal carcinoma cells undergoing differentiation or apoptosis

Differentiation and apoptosis are precisely regulated events in early embryogenesis. Retinoic acid-induced differentiation in the embryonal carcinoma (EC) cell line NCR-G3 triggers concurrent induction of apoptosis. Using this system, which serves as a model of early embryogenesis, the expression of various bcl-2-related genes was analyzed as these genes display either positive or negative regulatory effects on apoptosis. EAT/mcl-1, an antiapoptotic bcl-2-related gene and immediate early gene, was dramatically expressed at an early stage of NCR-G3 differentiation. Bcl-xL, another antiapoptotic gene, was induced at a middle stage of differentiation and then gradually decreased to basal level. Expression of Bax, a proapoptotic molecule, was detected at a high level and remained relatively constant. Meanwhile, Bcl-2 and Bcl-xS were below detectable levels throughout the various stages of differentiation. As the balance of bcl-2 genes is a crucial regulatory step in apoptosis, the results suggest that EAT and Bax likely regulate apoptosis in the early stages of differentiation. In later stages of differentiation, down-regulation of EAT was found to coincide with a gradual increase in apoptosis of NCR-G3 cells. Furthermore, use of the monoclonal antibody (3A2) specific to EAT revealed that EAT is localized to the outer mitochondrial membrane in human EC cells. In addition, EAT immunoreactivity was not detected in apoptotic NCR-G3 cells while it was observed in nearly all viable cells. The findings suggest that rapid induction of EAT may prevent NCR-G3 cells from undergoing apoptosis, thereby supporting viability at the early stage of differentiation.

The N-terminal domain of rat liver carnitine palmitoyltransferase 1 contains an internal mitochondrial import signal and residues essential for folding of its C-terminal catalytic domain

We have previously shown that the first 147 N-terminal residues of the rat liver carnitine palmitoyltransferase 1 (CPT1), encompassing its two transmembrane (TM) segments, specify both mitochondrial targeting and anchorage at the outer mitochondrial membrane (OMM). In the present study, we have identified the precise import sequence in this polytopic OMM protein. In vitro import studies with fusion and deletion CPT1 proteins demonstrated that none of its TM segments behave as a signal anchor sequence. Analysis of the regions flanking the TM segments revealed that residues 123-147, located immediately downstream of TM2, function as a noncleavable, matrix-targeting signal. They specify mitochondrial targeting, whereas the hydrophobic TM segment(s) acts as a stop-transfer sequence that stops and anchors the translocating CPT1 into the OMM. Heterologous expression in Saccharomyces cerevisiae of several deleted CPT1 proteins not only confirms the validity of the "stop-transfer" import model but also indicates that residues 1-82 of CPT1 contain a putative microsomal targeting signal whose cellular significance awaits further investigation. Finally, we identified a highly folded core within the C-terminal domain of CPT1 that is hidden in the entire protein by its cytosolic N-terminal residues. Functional analysis of the deleted CPT1 proteins indicates that this folded C-terminal core, which may belong to the catalytic domain of CPT1, requires TM2 for its correct folding achievement and is in close proximity to residues 1-47.

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.

The alpha and the beta: protein translocation across mitochondrial and plastid outer membranes

In the evolution of mitochondria and plastids from endosymbiotic bacteria, most of the proteins that make up these organelles have become encoded by nuclear genes and must therefore be transported across the organellar membranes, following synthesis in the cytosol. The core component of the protein translocation machines in both the mitochondrial and plastid outer membranes appears to be a beta-barrel protein, perhaps a relic from their bacterial ancestry, distinguishing these translocases from the alpha-helical-based protein translocation pores found in all other eukaryotic membranes.

The armadillo repeat region targets ARVCF to cadherin-based cellular junctions

The cytoplasmic domain of the transmembrane protein M-cadherin is involved in anchoring cytoskeletal elements to the plasma membrane at cell-cell contact sites. Several members of the armadillo repeat protein family mediate this linkage. We show here that ARVCF, a member of the p120 (ctn) subfamily, is a ligand for the cytoplasmic domain of M-cadherin, and characterize the regions involved in this interaction in detail. Complex formation in an in vivo environment was demonstrated in (1) yeast two-hybrid screens, using a cDNA library from differentiating skeletal muscle and part of the cytoplasmic M-cadherin tail as a bait, and (2) mammalian cells, using a novel experimental system, the MOM recruitment assay. Immunoprecipitation and in vitro binding assays confirmed this interaction. Ectopically expressed EGFP-ARVCF-C11, an N-terminal truncated fragment, targets to junctional structures in epithelial MCF7 cells and cardiomyocytes, where it colocalizes with the respective cadherins, beta-catenin and p120 (ctn). Hence, the N terminus of ARVCF is not required for junctional localization. In contrast, deletion of the four N-terminal armadillo repeats abolishes this ability in cardiomyocytes. Detailed mutational analysis revealed the armadillo repeat region of ARVCF as sufficient and necessary for interaction with the 55 membrane-proximal amino acids of the M-cadherin tail.

Characterization of the signal that directs Tom20 to the mitochondrial outer membrane

Tom20 is a major receptor of the mitochondrial preprotein translocation system and is bound to the outer membrane through the NH(2)-terminal transmembrane domain (TMD) in an Nin-Ccyt orientation. We analyzed the mitochondria-targeting signal of rat Tom20 (rTom20) in COS-7 cells, using green fluorescent protein (GFP) as the reporter by systematically introducing deletions or mutations into the TMD or the flanking regions. Moderate TMD hydrophobicity and a net positive charge within five residues of the COOH-terminal flanking region were both critical for mitochondria targeting. Constructs without net positive charges within the flanking region, as well as those with high TMD hydrophobicity, were targeted to the ER-Golgi compartments. Intracellular localization of rTom20-GFP fusions, determined by fluorescence microscopy, was further verified by cell fractionation. The signal recognition particle (SRP)-induced translation arrest and photo-cross-linking demonstrated that SRP recognized the TMD of rTom20-GFP, but with reduced affinity, while the positive charge at the COOH-terminal flanking segment inhibited the translation arrest. The mitochondria-targeting signal identified in vivo also functioned in the in vitro system. We conclude that NH(2)-terminal TMD with a moderate hydrophobicity and a net positive charge in the COOH-terminal flanking region function as the mitochondria-targeting signal of the outer membrane proteins, evading SRP-dependent ER targeting.