A novel insertion pathway of mitochondrial outer membrane proteins with multiple transmembrane segments

Mitochondrial protein import and the genesis of steroidogenic mitochondria

The principal site of regulation of steroid hormone biosynthesis is the transfer of cholesterol from the outer to inner mitochondrial membrane. Hormonal stimulation of steroidogenic cells promotes this mitochondrial lipid import through a multi-protein complex, termed the transduceosome, spanning the two membranes. The transduceosome complex is assembled from multiple proteins, such as the steroidogenic acute regulatory (STAR) protein and translocator protein (TSPO), and requires their targeting to the mitochondria for transduceosome function. The vast majority of mitochondrial proteins, including those participating in cholesterol import, are encoded in the nucleus. Their subsequent mitochondrial incorporation is performed through a series of protein import machineries located in the outer and inner mitochondrial membranes. Here we review our current knowledge of the mitochondrial cholesterol import machinery of the transduceosome. This is complemented with descriptions of mitochondrial protein import machineries and mechanisms by which these machineries assemble the transduceosome in steroidogenic mitochondria.

Tail-anchored membrane proteins: exploring the complex diversity of tail-anchored-protein targeting in plant cells

Tail-anchored (TA) proteins are special class of integral membrane proteins that in recent years have received a considerable amount of attention due to their diverse cellular functions and unique targeting and insertion mechanisms. Defined by the presence of a single, hydrophobic membrane-spanning domain at or near their C terminus, TA proteins must be inserted into membranes post-translationally and are orientated such that their larger N-terminal domain (most often the functional domain) faces the cytosol, while their shorter C-terminal domain faces the interior of the organelle. The C-terminal domain of TA proteins also usually contains the information responsible for their selective targeting to the proper subcellular membrane, a process that, based primarily on studies with yeasts and mammals, appears to be highly complex due to the presence of multiple pathways. Within this context, we discuss here the biogenesis of plant TA proteins and the potential for hundreds of new TA proteins identified via bioinformatics screens to contribute to the already remarkable number of roles that this class of membrane proteins participates in throughout plant growth and development.

Function of cytosolic chaperones in Tom70-mediated mitochondrial import

The great majority of mitochondrial proteins are synthesized by cytosolic ribosomes and then imported into the organelle post-translationally. The translocase of the outer membrane (TOM) is a proteinaceous machinery that contains surface receptors for preprotein recognition and also serves as the main entry gateway into mitochondria. Mitochondrial targeting requires various cytosolic factors, in particular the molecular chaperones Hsc70/Hsp70 and Hsp90. The chaperone activity of Hsc70/Hsp70 and Hsp90 occurs in coordinated cycles of ATP hydrolysis and substrate binding, and is regulated by a number of co-chaperone proteins. The import receptor Tom70 is a member of the tetratricopeptide repeat (TPR) co-chaperone family and contains a conserved TPR clamp domain for interaction with Hsc70 and Hsp90. Such interaction is essential for the initiation of the import process. This review will discuss the roles of Hsc70 and Hsp90 in mitochondrial import and summarize recent progress in understanding these pathways.

A TSPO-related protein localizes to the early secretory pathway in Arabidopsis, but is targeted to mitochondria when expressed in yeast

AtTSPO is a TspO/MBR domain-protein potentially involved in multiple stress regulation in Arabidopsis. As in most angiosperms, AtTSPO is encoded by a single, intronless gene. Expression of AtTSPO is tightly regulated both at the transcriptional and post-translational levels. It has been shown previously that overexpression of AtTSPO in plant cell can be detrimental, and the protein was detected in the endoplasmic reticulum (ER) and Golgi stacks, contrasting with previous findings and suggesting a mitochondrial subcellular localization for this protein. To ascertain these findings, immunocytochemistry and ABA induction were used to demonstrate that, in plant cells, physiological levels of AtTSPO colocalized with AtArf1, a mainly Golgi-localized protein in plant cells. In addition, fluorescent protein-tagged AtTSPO was targeted to the secretory pathway and did not colocalize with MitoTracker-labelled mitochondria. These results suggest that the polytopic membrane protein AtTSPO is cotranslationally targeted to the ER in plant cells and accumulates in the Trans-Golgi Network. Heterologous expression of AtTSPO in Saccharomyces cerevisiae, yeast devoid of TSPO-related protein, resulted in growth defects. However, subcellular fractionation and immunoprecipitation experiments showed that AtTSPO was targeted to mitochondria where it colocalized and interacted with the outer mitochondrial membrane porin VDAC1p, reminiscent of the subcellular localization and activity of mammalian translocator protein 18 kDa TSPO. The evolutionarily divergent AtTSPO appears therefore to be switching its sorting mode in a species-dependent manner, an uncommon peculiarity for a polytopic membrane protein in eukaryotic cells. These results are discussed in relation to the recognition and organelle targeting mechanisms of polytopic membrane proteins in eukaryotic cells.

Human mitochondrial import receptor Tom70 functions as a monomer

The mitochondrial import receptor Tom70 (translocase of the mitochondrial outer membrane 70) interacts with chaperone-preprotein complexes through two domains: one that binds Hsp70 (heat-shock protein 70)/Hsc70 (heat-shock cognate 70) and Hsp90, and a second that binds preproteins. The oligomeric state of Tom70 has been controversial, with evidence for both monomeric and homodimeric forms. In the present paper, we report that the functional state of human Tom70 appears to be a monomer with mechanistic implications for its function in mitochondrial protein import. Based on analytical ultracentrifugation, cross-linking, size-exclusion chromatography and multi-angle light scattering, we found that the soluble cytosolic fragment of human Tom70 exists in equilibrium between monomer and dimer. A point mutation introduced at the predicted dimer interface increased the percentage of monomeric Tom70. Although chaperone docking to the mutant was the same as to the wild-type, the mutant was significantly more active in preprotein targeting. Cross-linking also demonstrated that the mutant formed stronger contacts with preprotein. However, cross-linking of full-length wild-type Tom70 on the mitochondrial membrane showed little evidence of homodimers. These results indicate that the Tom70 monomers are the functional form of the receptor, whereas the homodimers appear to be a minor population, and may represent an inactive state.

Identification and characterization of mitochondrial targeting sequence of human apurinic/apyrimidinic endonuclease 1

Dually targeted mitochondrial proteins usually possess an unconventional mitochondrial targeting sequence (MTS), which makes them difficult to predict by current bioinformatics approaches. Human apurinic/apyrimidinic endonuclease (APE1) plays a central role in the cellular response to oxidative stress. It is a dually targeted protein preferentially residing in the nucleus with conditional distribution in the mitochondria. However, the mitochondrial translocation mechanism of APE1 is not well characterized because it harbors an unconventional MTS that is difficult to predict by bioinformatics analysis. Two experimental approaches were combined in this study to identify the MTS of APE1. First, the interactions between the peptides from APE1 and the three purified translocase receptors of the outer mitochondrial membrane (Tom) were evaluated using a peptide array screen. Consequently, the intracellular distribution of green fluorescent protein-tagged, truncated, or mutated APE1 proteins was traced by tag detection. The results demonstrated that the only MTS of APE1 is harbored within residues 289-318 in the C terminus, which is normally masked by the intact N-terminal structure. As a dually targeted mitochondrial protein, APE1 possesses a special distribution pattern of different subcellular targeting signals, the identification of which sheds light on future prediction of MTSs.

The Arabidopsis TSPO-related protein is a stress and abscisic acid-regulated, endoplasmic reticulum-Golgi-localized membrane protein

The Arabidopsis gene At2g47770 encodes a membrane-bound protein designated AtTSPO (Arabidopsis thaliana TSPO-related). AtTSPO is related to the bacterial outer membrane tryptophan-rich sensory protein (TspO) and the mammalian mitochondrial 18-kDa translocator protein (18 kDa TSPO), members of the group of TspO/MBR domain-containing membrane proteins. In this study we show that AtTSPO is mainly detected in dry seeds, but can be induced in vegetative tissues by osmotic or salt stress or abscisic acid (ABA) treatment, corroborating available transcriptome data. Using subcellular fractionation, immunocytochemistry and fluorescent protein tagging approaches we present evidence that AtTSPO is targeted to the secretory pathway in plants. Induced or constitutively expressed AtTSPO can be detected in the endoplasmic reticulum and the Golgi stacks of plant cells. AtTSPO tagged with fluorescent protein in transgenic plants (Arabidopsis and tobacco) was mainly detected in the Golgi stacks of leaf epidermal cells. Constitutive expression of AtTSPO resulted in increased sensitivity to NaCl, but not to osmotic stress, and in reduced greening of cultured Arabidopsis cells under light growing conditions. Transgenic Arabidopsis plants overexpressing AtTSPO were more sensitive to ABA-induced growth inhibition, indicating that constitutive expression of AtTSPO may enhance ABA sensitivity. AtTSPO is rapidly downregulated during seed imbibition, and the ABA-dependent induction in plant is transient. Downregulation of AtTSPO seems to be boosted by treatment with aminolevulinic acid. Taken together, these results suggest that AtTSPO is a highly regulated protein, induced by abiotic stress to modulate, at least in part, transient intracellular ABA-dependent stress perception and/or signalling.

Importing mitochondrial proteins: machineries and mechanisms

Most mitochondrial proteins are synthesized on cytosolic ribosomes and must be imported across one or both mitochondrial membranes. There is an amazingly versatile set of machineries and mechanisms, and at least four different pathways, for the importing and sorting of mitochondrial precursor proteins. The translocases that catalyze these processes are highly dynamic machines driven by the membrane potential, ATP, or redox reactions, and they cooperate with molecular chaperones and assembly complexes to direct mitochondrial proteins to their correct destinations. Here, we discuss recent insights into the importing and sorting of mitochondrial proteins and their contributions to mitochondrial biogenesis.

Targeting and insertion of the cholesterol-binding translocator protein into the outer mitochondrial membrane

Translocator protein (18 kDa, TSPO), previously known as the peripheral-type benzodiazepine receptor, is an outer mitochondrial membrane (OMM) protein necessary for cholesterol import and steroid production. We reconstituted the mitochondrial targeting and insertion of TSPO into the OMM to analyze the signals and mechanisms required for this process. Initial studies indicated the formation of a mitochondrial 66 kDa complex through Blue Native-PAGE analysis. The formation of this complex was found to be dependent on the presence of ATP and the cytosolic chaperone Hsp90. Through mutational analysis we identified two areas necessary for TSPO targeting, import, and function: amino acids 103-108 (Schellman motif), which provide the necessary structural orientation for import, and the cholesterol-binding C-terminus required for insertion. Although the translocase of the outer mitochondrial membrane (TOM) complex proteins Tom22 and Tom40 were present in the OMM, the TOM complex did not interact with TSPO. In search of proteins involved in TSPO import, we analyzed complexes known to interact with TSPO by mass spectrometry. Formation of the 66 kDa complex was found to be dependent on an identified protein, Metaxin 1, for formation and TSPO import. The level of import of TSPO into steroidogenic cell mitochondria was increased following treatment of the cells with cAMP. These findings suggest that the initial targeting of TSPO to mitochondria is dependent upon the presence of cytosolic chaperones interacting with the import receptor Tom70. The C-terminus plays an important role in targeting TSPO to mitochondria, whereas its import into the OMM is dependent upon the presence of the Schellman motif. Final integration of TSPO into the OMM occurs via its interaction with Metaxin 1. Import of TSPO into steroidogenic cell mitochondria is regulated by cAMP.

Multiple pathways for mitochondrial protein traffic

Mitochondria are two-membrane bounded organelles consisting of 1000–2000 different proteins, most of which are synthesized in the cytosol and subsequently imported into mitochondria. The imported proteins are further sorted to one of the four compartments, the outer membrane, intermembrane space, inner membrane, and matrix, mostly following one of the five major pathways. Mitochondrial protein import and sorting are mediated by the translocator complexes in the membranes and chaperones in the aqueous compartments operating along the import pathways. Here, we summarize the expanding knowledge on the roles of translocators, chaperones, and related components in the multiple pathways for mitochondrial protein trafficking.

Bacterial ortholog of mammalian translocator protein (TSPO) with virulence regulating activity

The translocator protein (TSPO), previously designated as peripheral-type benzodiazepine receptor, is a protein mainly located in the outer mitochondrial membrane of eukaryotic cells. TSPO is implicated in major physiological functions and functionally associated with other proteins such as the voltage-dependent anionic channel, also designated as mitochondrial porin. Surprisingly, a TSPO-related protein was identified in the photosynthetic bacterium Rhodobacter sphaeroides but it was initially considered as a relict of evolution. In the present study we cloned a tspO gene in Pseudomonas fluorescens MF37, a non-photosynthetic eubacterium and we used bioinformatics tools to identify TSPO in the genome of 97 other bacteria. P. fluorescens TSPO was recognized by antibodies against mouse protein and by PK 11195, an artificial ligand of mitochondrial TSPO. As in eukaryotes, bacterial TSPO appears functionally organized as a dimer and the apparent Kd for PK 11195 is in the same range than for its eukaryotic counterpart. When P. fluorescens MF37 was treated with PK 11195 (10(-5) M) adhesion to living or artificial surfaces and biofilm formation activity were increased. Conversely, the apoptotic potential of bacteria on eukaryotic cells was significantly reduced. This effect of PK11195 was abolished in a mutant of P. fluorescens MF37 deficient for its major outer membrane porin, OprF. The present results demonstrate the existence of a bacterial TSPO that shares common structural and functional characteristics with its mammalian counterpart. This protein, apparently involved in adhesion and virulence, reveals the existence of a possible new inter kingdom signalling system and suggests that the human microbiome should be involuntarily exposed to the evolutionary pressure of benzodiazepines and related molecules. This discovery also represents a promising opportunity for the development of alternative antibacterial strategies.

Mitochondrial targeting of cytochrome P450 proteins containing NH2-terminal chimeric signals involves an unusual TOM20/TOM22 bypass mechanism

Previously we showed that xenobiotic inducible cytochrome P450 (CYP) proteins are bimodally targeted to the endoplasmic reticulum and mitochondria. In this study, we investigated the mechanism of delivery of chimeric signal containing CYP proteins to the peripheral and channel-forming mitochondrial outer membrane translocases (TOMs). CYP+33/1A1 and CYP2B1 did not require peripheral TOM70, TOM20, or TOM22 for translocation through the channel-forming TOM40 protein. In contrast, CYP+5/1A1 and CYP2E1 were able to bypass TOM20 and TOM22 but required TOM70. CYP27, which contains a canonical cleavable mitochondrial signal, required all of the peripheral TOMs for its mitochondrial translocation. We investigated the underlying mechanisms of bypass of peripheral TOMs by CYPs with chimeric signals. The results suggested that interaction of CYPs with Hsp70, a cytosolic chaperone involved in the mitochondrial import, alone was sufficient for the recognition of chimeric signals by peripheral TOMs. However, sequential interaction of chimeric signal containing CYPs with Hsp70 and Hsp90 resulted in the bypass of peripheral TOMs, whereas CYP27A1 interacted only with Hsp70 and was not able to bypass peripheral TOMs. Our results also show that delivery of a chimeric signal containing client protein by Hsp90 required the cytosol-exposed NH(2)-terminal 143 amino acids of TOM40. TOM40 devoid of this domain was unable to import CYP proteins. These results suggest that compared with the unimodal mitochondrial targeting signals, the chimeric mitochondrial targeting signals are highly evolved and dynamic in nature.

Import of preproteins into the chloroplast inner envelope membrane

The chloroplast inner envelope membrane contains many integral proteins which differ in the number of alpha-helices that anchor the protein into the bilayer. For most of these proteins it is not known which pathway they engage to reach their final localisation within the membrane. In yeast mitochondria, two distinct sorting/insertion pathways have been described for integral inner membrane proteins, involving the Tim22 and Tim23 translocases. These routes involve on the one hand a conservative sorting, on the other hand a stop-transfer pathway. In this study we performed a systematic characterisation of the import behaviour of seven inner envelope proteins representing different numbers of predicted alpha-helices. We investigated their energy dependence, import rate, involvement of components of the chloroplast general import pathway and distribution between soluble and membrane fractions. Our results show the existence of at least two different families of inner envelope proteins that can be classified due to the occurrence of an intermediate processing form. Each of the proteins we investigated seems to use a stop-transfer pathway for insertion into the inner envelope.

Localization of the Carnation Italian ringspot virus replication protein p36 to the mitochondrial outer membrane is mediated by an internal targeting signal and the TOM complex

BACKGROUND

Carnation Italian ringspot virus (CIRV) is a positive-strand RNA virus that causes massive structural alterations of mitochondria in infected host cells, the most conspicuous being the formation of numerous internal vesicles/spherules that are derived from the mitochondrial outer membrane and serve as the sites for viral RNA replication. While the membrane-bound components of the CIRV replication complex, including a 36-kD RNA-binding protein (p36), are known to be essential for these changes in mitochondrial morphology and are relatively well characterized in terms of their roles in nascent viral RNA synthesis, how these proteins are specifically targeted and inserted into mitochondria is poorly defined.

RESULTS

Here we report on the molecular signal responsible for sorting p36 to the mitochondrial outer membrane. Using a combination of gain-of-function assays with portions of p36 fused to reporter proteins and domain-swapping assays with p36 and another closely-related viral RNA-binding protein, p33, that sorts specifically to the peroxisomal boundary membrane, we show that the mitochondrial targeting information in p36 resides within its two transmembrane domains (TMDs) and intervening hydrophilic loop sequence. Comprehensive mutational analysis of these regions in p36 revealed that the primary targeting determinants are the moderate hydrophobicity of both TMDs and the positively-charged face of an amphipathic helix within the intervening loop sequence. We show also using bimolecular fluorescence complementation (BiFC) that p36 interacts with certain components of the translocase complex in the mitochondrial outer membrane (TOM), but not with the sorting and assembly machinery (SAM).

CONCLUSION

Our results provide insight to how viruses, such as CIRV, exploit specific host-cell protein sorting pathways to facilitate their replication. The characterization of the targeting and insertion of p36 into the mitochondrial outer membrane also sheds light on the mechanisms involved in sorting of host-cell membrane proteins to mitochondria, a process that has been largely unexplored in plants.

An unusual TOM20/TOM22 bypass mechanism for the mitochondrial targeting of cytochrome P450 proteins containing N-terminal chimeric signals

Previously we showed that xenobiotic-inducible cytochrome P450 (CYP) proteins are bimodally targeted to the endoplasmic reticulum and mitochondria. In the present study, we investigated the mechanism of delivery of chimeric signal-containing CYP proteins to the peripheral and channel-forming mitochondrial outer membrane translocases (TOMs). CYP+33/1A1 and CYP2B1 did not require peripheral TOM70, TOM20, or TOM22 for translocation through the channel-forming TOM40 protein. In contrast, CYP+5/1A1 and CYP2E1 were able to bypass TOM20 and TOM22 but required TOM70. CYP27, which contains a canonical cleavable mitochondrial signal, required all of the peripheral TOMs for its mitochondrial translocation. We investigated the underlying mechanisms of bypass of peripheral TOMs by CYPs with chimeric signals. The results suggested that interaction of CYPs with Hsp70, a cytosolic chaperone involved in the mitochondrial import, alone was sufficient for the recognition of chimeric signals by peripheral TOMs. However, sequential interaction of chimeric signal-containing CYPs with Hsp70 and Hsp90 resulted in the bypass of peripheral TOMs, whereas CYP27 interacted only with Hsp70 and was not able to bypass peripheral TOMs. Our results also show that delivery of chimeric signal-containing client proteins by Hsp90 required the cytosol-exposed N-terminal 143 amino acids of TOM40. TOM40 devoid of this domain was unable to bind CYP proteins. These results suggest that, compared with the unimodal mitochondria-targeting signals, the chimeric mitochondria-targeting signals are highly evolved and dynamic in nature.

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.

Evidence for mitochondrial localization of P5, a member of the protein disulphide isomerase family

This report demonstrates for the first time that P5, a member of the protein disulphide isomerase (PDI) family, is present in the mitochondria. Various organelles were screened for proteins bearing the CGHC motif using an affinity column conjugated with the phage antibody 5E, which cross-reacts with PDI family proteins. P5 was found in bovine liver mitochondrial extract and identified by Western blot analysis using anti-P5 antibody and by mass spectrometric analysis. Results of cell fractionation, proteinase sensitivity experiments and immuno-electron microscopy supported the mitochondrial localization of P5 and also indicated the presence of ERp57, another PDI family protein, in mitochondria. Our findings will be useful for the elucidation of the translocation mechanism of PDI family proteins and their roles in mitochondria.

Sorting and assembly of mitochondrial outer membrane proteins

In the last years the picture of protein import into the mitochondria has become much more complicated in terms of new components and new sorting pathways. These novel findings have also changed views concerning the biogenesis pathway of mitochondrial outer membrane proteins. In addition to proteins anchored with transmembrane alpha-helices, the endosymbiotic origin of the mitochondria has resulted in the presence of transmembrane beta-barrels in this compartment. The sorting and assembly pathway of outer membrane proteins involves three machineries: the translocase of the outer membrane (TOM complex) the sorting and assembly machinery (SAM complex) and the MDM complex (mitochondrial distribution and morphology). Here we review recent developments on the biogenesis pathways of outer membrane proteins with a focus on Tom proteins, the most intensively studied class of these precursor proteins.

Tom20 and Tom22 share the common signal recognition pathway in mitochondrial protein import

Precise targeting of mitochondrial precursor proteins to mitochondria requires receptor functions of Tom20, Tom22, and Tom70 on the mitochondrial surface. Tom20 is a major import receptor that recognizes preferentially mitochondrial presequences, and Tom70 is a specialized receptor that recognizes presequence-less inner membrane proteins. The cytosolic domain of Tom22 appears to function as a receptor in cooperation with Tom20, but how its substrate specificity differs from that of Tom20 remains unclear. To reveal possible differences in substrate specificities between Tom20 and Tom22, if any, we deleted the receptor domain of Tom20 or Tom22 in mitochondria in vitro by introducing cleavage sites for a tobacco etch virus protease between the receptor domains and transmembrane segments of Tom20 and Tom22. Then mitochondria without the receptor domain of Tom20 or Tom22 were analyzed for their abilities to import various mitochondrial precursor proteins targeted to different mitochondrial subcompartments in vitro. The effects of deletion of the receptor domains on the import of different mitochondrial proteins for different import pathways were quite similar between Tom20 and Tom22. Therefore Tom20 and Tom22 are apparently involved in the same step or sequential steps along the same pathway of targeting signal recognition in import.