Tim23p contains separate and distinct signals for targeting to mitochondria and insertion into the inner membrane

Distinct structural motifs are necessary for targeting and import of Tim17 in Trypanosoma brucei mitochondrion

Nuclear-encoded mitochondrial proteins are correctly translocated to their proper sub-mitochondrial destination using location-specific mitochondrial targeting signals and via multi-protein import machineries (translocases) in the outer and inner mitochondrial membranes (TOM and TIMs, respectively). However, targeting signals of multi-pass Tims are less defined. Here, we report the characterization of the targeting signals of Trypanosoma brucei Tim17 (TbTim17), an essential component of the most divergent TIM complex. TbTim17 possesses a characteristic secondary structure including four predicted transmembrane (TM) domains in the center with hydrophilic N- and C-termini. After examining mitochondrial localization of various deletion and site-directed mutants of TbTim17 in T. brucei using subcellular fractionation and confocal microscopy, we located at least two internal targeting signals (ITS): (i) within TM1 (31-50 AAs) and (ii) TM4 + loop 3 (120-136 AAs). Both signals are required for proper targeting and integration of TbTim17 in the membrane. Furthermore, a positively charged residue (K122) is critical for mitochondrial localization of TbTim17. This is the first report of characterizing the ITS for a multipass inner membrane protein in a divergent eukaryote, like T. brucei.IMPORTANCEAfrican trypanosomiasis (AT) is a deadly disease in human and domestic animals, caused by the parasitic protozoan Trypanosoma brucei. Therefore, AT is not only a concern for human health but also for economic development in the vast area of sub-Saharan Africa. T. brucei possesses a single mitochondrion per cell that imports hundreds of nuclear-encoded mitochondrial proteins for its functions. T. brucei Tim17 (TbTim17), an essential component of the TbTIM17 complex, is a nuclear-encoded protein; thus, it is necessary to be imported from the cytosol to form the TbTIM17 complex. Here, we demonstrated that the internal targeting signals within the transmembrane 1 (TM1) and TM4 with loop 3, and residue K122 are required collectively for import and integration of TbTim17 in the T. brucei mitochondrion. This information could be utilized to block TbTim17 function and parasite growth.

Distinct structural motifs are necessary for targeting and import of Tim17 in Trypanosoma brucei mitochondrion

Nuclear-encoded mitochondrial proteins are correctly translocated to their proper sub-mitochondrial destination using location specific mitochondrial targeting signals (MTSs) and via multi-protein import machineries (translocases) in the outer and inner mitochondrial membranes (TOM and TIMs, respectively). However, MTSs of multi-pass Tims are less defined. Here we report the characterization of the MTSs of Trypanosoma brucei Tim17 (TbTim17), an essential component of the most divergent TIM complex. TbTim17 possesses a characteristic secondary structure including four predicted transmembrane (TM) domains in the center with hydrophilic N- and C-termini. After examining mitochondrial localization of various deletion and site-directed mutants of TbTim17 in T. brucei using subcellular fractionation and confocal microscopy we located at least two internal signals, 1) within TM1 (31-50 AAs) and 2) TM4 + Loop 3 (120-136 AAs). Both signals are required for proper targeting and integration of TbTim17 in the membrane. Furthermore, a positively charged residue (K 122 ) is critical for mitochondrial localization of TbTim17. This is the first report of characterizing the internal mitochondrial targeting signals (ITS) for a multipass inner membrane protein in a divergent eukaryote, like T. brucei .

Summary

Internal targeting signals within the TM1, TM4 with Loop 3, and residue K122 are required collectively for import and integration of TbTim17 in the T. brucei mitochondrion. This information could be utilized to block parasite growth.

Mitochondrial protein transport: Versatility of translocases and mechanisms

Biogenesis of mitochondria requires the import of approximately 1,000 different precursor proteins into and across the mitochondrial membranes. Mitochondria exhibit a wide variety of mechanisms and machineries for the translocation and sorting of precursor proteins. Five major import pathways that transport proteins to their functional intramitochondrial destination have been elucidated; these pathways range from the classical amino-terminal presequence-directed pathway to pathways using internal or even carboxy-terminal targeting signals in the precursors. Recent studies have provided important insights into the structural organization of membrane-embedded preprotein translocases of mitochondria. A comparison of the different translocases reveals the existence of at least three fundamentally different mechanisms: two-pore-translocase, β-barrel switching, and transport cavities open to the lipid bilayer. In addition, translocases are physically engaged in dynamic interactions with respiratory chain complexes, metabolite transporters, quality control factors, and machineries controlling membrane morphology. Thus, mitochondrial preprotein translocases are integrated into multi-functional networks of mitochondrial and cellular machineries.

Biogenesis of Mitochondrial Metabolite Carriers

: Metabolite carriers of the mitochondrial inner membrane are crucial for cellular physiology since mitochondria contribute essential metabolic reactions and synthesize the majority of the cellular ATP. Like almost all mitochondrial proteins, carriers have to be imported into mitochondria from the cytosol. Carrier precursors utilize a specialized translocation pathway dedicated to the biogenesis of carriers and related proteins, the carrier translocase of the inner membrane (TIM22) pathway. After recognition and import through the mitochondrial outer membrane via the translocase of the outer membrane (TOM) complex, carrier precursors are ushered through the intermembrane space by hexameric TIM chaperones and ultimately integrated into the inner membrane by the TIM22 carrier translocase. Recent advances have shed light on the mechanisms of TOM translocase and TIM chaperone function, uncovered an unexpected versatility of the machineries, and revealed novel components and functional crosstalk of the human TIM22 translocase.

The mitochondrial carrier pathway transports non-canonical substrates with an odd number of transmembrane segments

BACKGROUND

The mitochondrial pyruvate carrier (MPC) plays a central role in energy metabolism by transporting pyruvate across the inner mitochondrial membrane. Its heterodimeric composition and homology to SWEET and semiSWEET transporters set the MPC apart from the canonical mitochondrial carrier family (named MCF or SLC25). The import of the canonical carriers is mediated by the carrier translocase of the inner membrane (TIM22) pathway and is dependent on their structure, which features an even number of transmembrane segments and both termini in the intermembrane space. The import pathway of MPC proteins has not been elucidated. The odd number of transmembrane segments and positioning of the N-terminus in the matrix argues against an import via the TIM22 carrier pathway but favors an import via the flexible presequence pathway.

RESULTS

Here, we systematically analyzed the import pathways of Mpc2 and Mpc3 and report that, contrary to an expected import via the flexible presequence pathway, yeast MPC proteins with an odd number of transmembrane segments and matrix-exposed N-terminus are imported by the carrier pathway, using the receptor Tom70, small TIM chaperones, and the TIM22 complex. The TIM9·10 complex chaperones MPC proteins through the mitochondrial intermembrane space using conserved hydrophobic motifs that are also required for the interaction with canonical carrier proteins.

CONCLUSIONS

The carrier pathway can import paired and non-paired transmembrane helices and translocate N-termini to either side of the mitochondrial inner membrane, revealing an unexpected versatility of the mitochondrial import pathway for non-cleavable inner membrane proteins.

Cardiolipin mediates membrane and channel interactions of the mitochondrial TIM23 protein import complex receptor Tim50

The phospholipid cardiolipin mediates the functional interactions of proteins that reside within energy-conserving biological membranes. However, the molecular basis by which this lipid performs this essential cellular role is not well understood. We address this role of cardiolipin using the multisubunit mitochondrial TIM23 protein transport complex as a model system. The early stages of protein import by this complex require specific interactions between the polypeptide substrate receptor, Tim50, and the membrane-bound channel-forming subunit, Tim23. Using analyses performed in vivo, in isolated mitochondria, and in reductionist nanoscale model membrane systems, we show that the soluble receptor domain of Tim50 interacts with membranes and with specific sites on the Tim23 channel in a manner that is directly modulated by cardiolipin. To obtain structural insights into the nature of these interactions, we obtained the first small-angle x-ray scattering-based structure of the soluble Tim50 receptor in its entirety. Using these structural insights, molecular dynamics simulations combined with a range of biophysical measurements confirmed the role of cardiolipin in driving the association of the Tim50 receptor with lipid bilayers with concomitant structural changes, highlighting the role of key structural elements in mediating this interaction. Together, these results show that cardiolipin is required to mediate specific receptor-channel associations in the TIM23 complex. Our results support a new working model for the dynamic structural changes that occur within the complex during transport. More broadly, this work strongly advances our understanding of how cardiolipin mediates interactions among membrane-associated proteins.

Role of Tim17 Transmembrane Regions in Regulating the Architecture of Presequence Translocase and Mitochondrial DNA Stability

Mitochondrial life cycle and protein import are intricate cellular processes, which require precise coordination between the transport machineries of outer and inner mitochondrial membranes. Presequence translocase performs the indispensable function of translocating preproteins having N-terminal targeting sequences across the inner membrane. Tim23 forms the core of the voltage-gated import channel, while Tim17 is presumed to maintain the stoichiometry of the translocase. However, mechanistic insights into how Tim17 coordinates these regulatory events within the complex remained elusive. We demonstrate that Tim17 harbors conserved G/AXXXG/A motifs within its transmembrane regions and plays an imperative role in the translocase assembly through interaction with Tim23. Tandem motifs are highly essential, as most of the amino acid substitutions lead to nonviability due to the complete destabilization of the TIM23 channel. Importantly, Tim17 transmembrane regions regulate the dynamic assembly of translocase to form either the TIM23 (PAM)-complex or TIM23 (SORT)-complex by recruiting the presequence translocase-associated motor (PAM) machinery or Tim21, respectively. To a greater significance, tim17 mutants displayed mitochondrial DNA (mtDNA) instability, membrane potential loss, and defective import, resulting in organellar dysfunction. We conclude that the integrity of Tim17 transmembrane regions is critical for mitochondrial function and protein turnover.

Targeting of a polytopic membrane protein to the inner envelope membrane of chloroplasts in vivo involves multiple transmembrane segments

The inner envelope membrane (IEM) of the chloroplast plays crucial roles in forming an osmotic barrier and controlling metabolite exchange between the organelle and the cytosol. The IEM therefore harbours a number of membrane proteins and requires the import and integration of these nuclear-encoded proteins for its biogenesis. Recent studies have demonstrated that the transmembrane segment of single-spanning IEM proteins plays key roles in determining their IEM localization. However, few studies have focused on the molecular mechanisms by which polytopic membrane proteins are targeted to the IEM. In this study, we investigated the targeting mechanism of polytopic IEM proteins using the protein Cor413im1 as a model substrate. Cor413im1 does not utilize a soluble intermediate for its targeting to the IEM. Furthermore, we show that the putative fifth transmembrane segment of Cor413im1 is necessary for its targeting to the IEM. The C-terminal portion containing this transmembrane segment is also able to deliver Cor413im1 protein to the IEM. However, the fifth transmembrane segment of Cor413im1 itself is insufficient to target a fusion protein to the IEM. These data suggest that the targeting of polytopic membrane proteins to the chloroplast IEM in vivo involves multiple transmembrane segments and that chloroplasts have evolved a unique mechanism for the integration of polytopic proteins to the IEM.

Molecular insights revealing interaction of Tim23 and channel subunits of presequence translocase

Tim23 is an essential channel-forming subunit of the presequence translocase recruiting multiple components for assembly of the core complex, thereby regulating the protein translocation process. However, understanding of the precise interaction of subunits associating with Tim23 remains largely elusive. Our findings highlight that transmembrane helix 1 (TM1) is required for homodimerization of Tim23, while, together with TM2, it is involved in preprotein binding within the channel. Based on our evidence, we predict that the TM1 and TM2 from each dimer are involved in the formation of the central translocation pore, aided by Tim17. Furthermore, TM2 is also involved in the recruitment of Tim21 and the presequence-associated motor (PAM) subcomplex to the Tim23 channel, while the matrix-exposed loop L1 generates specificity in their association with the core complex. Strikingly, our findings indicate that the C-terminal sequence of Tim23 is dispensable for growth and functions as an inhibitor for binding of Tim21. Our model conceptually explains the cooperative function between Tam41 and Pam17 subunits, while the antagonistic activity of Tim21 predominantly determines the bound and free forms of the PAM subcomplex during import.

The mitochondrial protein import machinery has multiple connections to the respiratory chain

The mitochondrial inner membrane harbors the complexes of the respiratory chain and protein translocases required for the import of mitochondrial precursor proteins. These complexes are functionally interdependent, as the import of respiratory chain precursor proteins across and into the inner membrane requires the membrane potential. Vice versa the membrane potential is generated by the proton pumping complexes of the respiratory chain. Besides this basic codependency four different systems for protein import, processing and assembly show further connections to the respiratory chain. The mitochondrial intermembrane space import and assembly machinery oxidizes cysteine residues within the imported precursor proteins and is able to donate the liberated electrons to the respiratory chain. The presequence translocase of the inner membrane physically interacts with the respiratory chain. The mitochondrial processing peptidase is homologous to respiratory chain subunits and the carrier translocase of the inner membrane even shares a subunit with the respiratory chain. In this review we will summarize the import of mitochondrial precursor proteins and highlight these special links between the mitochondrial protein import machinery and the respiratory chain. This article is part of a Special Issue entitled: Respiratory complex II: Role in cellular physiology and disease.

Silencing of cardiac mitochondrial NHE1 prevents mitochondrial permeability transition pore opening

Inhibition of Na(+)/H(+) exchanger 1 (NHE1) reduces cardiac ischemia-reperfusion (I/R) injury and also cardiac hypertrophy and failure. Although the mechanisms underlying these NHE1-mediated effects suggest delay of mitochondrial permeability transition pore (MPTP) opening, and reduction of mitochondrial-derived superoxide production, the possibility of NHE1 blockade targeting mitochondria has been incompletely explored. A short-hairpin RNA sequence mediating specific knock down of NHE1 expression was incorporated into a lentiviral vector (shRNA-NHE1) and transduced in the rat myocardium. NHE1 expression of mitochondrial lysates revealed that shRNA-NHE1 transductions reduced mitochondrial NHE1 (mNHE1) by ∼60%, supporting the expression of NHE1 in mitochondria membranes. Electron microscopy studies corroborate the presence of NHE1 in heart mitochondria. Immunostaining of rat cardiomyocytes also suggests colocalization of NHE1 with the mitochondrial marker cytochrome c oxidase. To examine the functional role of mNHE1, mitochondrial suspensions were exposed to increasing concentrations of CaCl(2) to induce MPTP opening and consequently mitochondrial swelling. shRNA-NHE1 transduction reduced CaCl(2)-induced mitochondrial swelling by 64 ± 4%. Whereas the NHE1 inhibitor HOE-642 (10 μM) decreased mitochondrial Ca(2+)-induced swelling in rats transduced with nonsilencing RNAi (37 ± 6%), no additional HOE-642 effects were detected in mitochondria from rats transduced with shRNA-NHE1. We have characterized the expression and function of NHE1 in rat heart mitochondria. Because mitochondria from rats injected with shRNA-NHE1 present a high threshold for MPTP formation, the beneficial effects of NHE1 inhibition in I/R resulting from mitochondrial targeting should be considered.

Mitochondria of the yeasts Saccharomyces cerevisiae and Kluyveromyces lactis contain nuclear rDNA-encoded proteins

In eukaryotes, the nuclear ribosomal DNA (rDNA) is the source of the structural 18S, 5.8S and 25S rRNAs. In hemiascomycetous yeasts, the 25S rDNA sequence was described to lodge an antisense open reading frame (ORF) named TAR1 for Transcript Antisense to Ribosomal RNA. Here, we present the first immuno-detection and sub-cellular localization of the authentic product of this atypical yeast gene. Using specific antibodies against the predicted amino-acid sequence of the Saccharomyces cerevisiae TAR1 product, we detected the endogenous Tar1p polypeptides in S. cerevisiae (Sc) and Kluyveromyces lactis (Kl) species and found that both proteins localize to mitochondria. Protease and carbonate treatments of purified mitochondria further revealed that endogenous Sc Tar1p protein sub-localizes in the inner membrane in a N_in-C_out topology. Plasmid-versions of 5′ end or 3′ end truncated TAR1 ORF were used to demonstrate that neither the N-terminus nor the C-terminus of Sc Tar1p were required for its localization. Also, Tar1p is a presequence-less protein. Endogenous Sc Tar1p was found to be a low abundant protein, which is expressed in fermentable and non-fermentable growth conditions. Endogenous Sc TAR1 transcripts were also found low abundant and consistently 5′ flanking regions of TAR1 ORF exhibit modest promoter activity when assayed in a luciferase-reporter system. Using rapid amplification of cDNA ends (RACE) PCR, we also determined that endogenous Sc TAR1 transcripts possess heterogeneous 5′ and 3′ ends probably reflecting the complex expression of a gene embedded in actively transcribed rDNA sequence. Altogether, our results definitively ascertain that the antisense yeast gene TAR1 constitutes a functional transcription unit within the nuclear rDNA repeats.

Mechanisms regulating the susceptibility of hematopoietic malignancies to glucocorticoid-induced apoptosis

Glucocorticoids (GCs) are commonly used in the treatment of hematopoietic malignancies owing to their ability to induce apoptosis of these cancerous cells. Whereas some types of lymphoma and leukemia respond well to this drug, others are resistant. Also, GC-resistance gradually develops upon repeated treatments ultimately leading to refractory relapsed disease. Understanding the mechanisms regulating GC-induced apoptosis is therefore uttermost important for designing novel treatment strategies that overcome GC-resistance. This review discusses updated data describing the complex regulation of the cell's susceptibility to apoptosis triggered by GCs. We address both the genomic and nongenomic effects involved in promoting the apoptotic signals as well as the resistance mechanisms opposing these signals. Eventually we address potential strategies of clinical relevance that sensitize GC-resistant lymphoma and leukemia cells to this drug. The major target is the nongenomic signal transduction machinery where the interplay between protein kinases determines the cell fate. Shifting the balance of the kinome towards a state where Glycogen synthase kinase 3alpha (GSK3alpha) is kept active, favors an apoptotic response. Accumulating data show that it is possible to therapeutically modulate GC-resistance in patients, thereby improving the response to GC therapy.

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.

Quaternary structure of the mitochondrial TIM23 complex reveals dynamic association between Tim23p and other subunits

Tim23p is an essential channel-forming component of the multisubunit TIM23 complex of the mitochondrial inner membrane that mediates protein import. Radiolabeled Tim23p monocysteine mutants were imported in vitro, incorporated into functional TIM23 complexes, and subjected to chemical cross-linking. Three regions of proximity between Tim23p and other subunits of the TIM23 complex were identified: Tim17p and the first transmembrane segment of Tim23p; Tim50p and the C-terminal end of the Tim23p hydrophilic region; and the entire hydrophilic domains of Tim23p molecules. These regions of proximity reversibly change in response to changes in membrane potential across the inner membrane and also when a translocating substrate is trapped in the TIM23 complex. These structural changes reveal that the macromolecular arrangement within the TIM23 complex is dynamic and varies with the physiological state of the mitochondrion.

Protein kinase C isoform-dependent modulation of ATP-sensitive K+ channels in mitochondrial inner membrane

The ATP-sensitive K(+) (K(ATP)) channels in both sarcolemmal (sarcK(ATP)) and mitochondrial inner membrane (mitoK(ATP)) are the critical mediators in cellular protection of ischemic preconditioning (IPC). Whereas cardiac sarcK(ATP) contains Kir6.2 and sulfonylurea receptor (SUR)2A, the molecular identity of mitoK(ATP) remains elusive. In the present study, we tested the hypothesis that protein kinase C (PKC) may promote import of Kir6.2-containing K(ATP) into mitochondria. Fluorescence imaging of isolated mitochondria from both rat adult cardiomyocytes and COS-7 cells expressing recombinant Kir6.2/SUR2A showed that Kir6.2-containing K(ATP) channels were localized in mitochondria and this mitochondrial localization was significantly increased by PKC activation with phorbol 12-myristate 13-acetate (PMA). Fluorescence resonance energy transfer microscopy further revealed that a significant number of Kir6.2-containing K(ATP) channels were localized in mitochondrial inner membrane after PKC activation. These results were supported by Western blotting showing that the Kir6.2 protein level in mitochondria from COS-7 cells transfected with Kir6.2/SUR2A was enhanced after PMA treatment and this increase was inhibited by the selective PKC inhibitor chelerythrine. Furthermore, functional analysis indicated that the number of functional K(ATP) channels in mitochondria was significantly increased by PMA, as shown by K(ATP)-dependent decrease in mitochondrial membrane potential in COS-7 cells transfected with Kir6.2/SUR2A but not empty vector. Importantly, PKC-mediated increase in mitochondrial Kir6.2-containing K(ATP) channels was blocked by a selective PKCepsilon inhibitor peptide in both COS-7 cells and cardiomyocytes. We conclude that the K(ATP) channel pore-forming subunit Kir6.2 is indeed localized in mitochondria and that the Kir6.2 content in mitochondria is increased by activation of PKCepsilon. PKC isoform-regulated mitochondrial import of K(ATP) channels may have significant implication in cardioprotection of IPC.

The Tim9p/10p and Tim8p/13p complexes bind to specific sites on Tim23p during mitochondrial protein import

The import of polytopic membrane proteins into the mitochondrial inner membrane (IM) is facilitated by Tim9p/Tim10p and Tim8p/Tim13p protein complexes in the intermembrane space (IMS). These complexes are proposed to act as chaperones by transporting the hydrophobic IM proteins through the aqueous IMS and preventing their aggregation. To examine the nature of this interaction, Tim23p molecules containing a single photoreactive cross-linking probe were imported into mitochondria in the absence of an IM potential where they associated with small Tim complexes in the IMS. On photolysis and immunoprecipitation, a probe located at a particular Tim23p site (27 different locations were examined) was found to react covalently with, in most cases, only one of the small Tim proteins. Tim8p, Tim9p, Tim10p, and Tim13p were therefore positioned adjacent to specific sites in the Tim23p substrate before its integration into the IM. This specificity of binding to Tim23p strongly suggests that small Tim proteins do not function solely as general chaperones by minimizing the exposure of nonpolar Tim23p surfaces to the aqueous medium, but may also align a folded Tim23p substrate in the proper orientation for delivery and integration into the IM at the TIM22 translocon.

Mitochondrial Localization and Putative Signaling Function of Sucrose Synthase in Maize

In many organisms, an increasing number of proteins seem to play two or more unrelated roles. Here we report that maize sucrose synthase (SUS) is distributed in organelles not involved in sucrose metabolism and may have novel roles beyond sucrose degradation. Bioinformatics analysis predicts that among the three maize SUS isoforms, SH1 protein has a putative mitochondrial targeting peptide (mTP). We validated this prediction by the immunodetection of SUS in mitochondria. Analysis with isoform-specific antisera revealed that both SH1 and SUS1 are represented in mitochondria, although the latter lacks a canonical mTP. The SUS2 isoform is not detectable in mitochondria, despite its presence in the cytosol. In maize primary roots, the mitochondrion-associated SUS (mtSUS; which includes SH1 and SUS1) is present mostly in the root tip, indicating tissue-specific regulation of SUS compartmentation. Unlike the glycolytic enzymes that occur attached to the outside of mitochondria, SH1 and SUS1 are intramitochondrial. The low abundance of SUS in mitochondria, its high Km value for sucrose, and the lack of sucrose in mitochondria suggest that mtSUS plays a non-sucrolytic role. Co-immunoprecipitation studies indicate that SUS interacts with the voltage-dependent anion channel in an isoform-specific and anoxia-enhanced manner and may be involved in the regulation of solute fluxes into and out of mitochondria. In several plant species, at least one of the SUS proteins possesses a putative mTP, indicating the conservation of the noncatalytic function across plant species. Taken together, these observations suggest that SUS has a novel noncatalytic function in plant cells.

Translocation of mitochondrial inner-membrane proteins: conformation matters

Most of the mitochondrial inner-membrane proteins are generated without a presequence and their targeting depends on inadequately defined internal segments. Despite the numerous components of the import machinery identified by proteomics, the properties of hydrophobic import substrates remain poorly understood. Recent studies support several principles for these membrane proteins: first, they become organized into partially assembled forms within the translocon; second, they present noncontiguous targeting signals; and third, they induce conformational changes in translocase subunits, thereby mediating "assembly on demand" of the import machinery. It is possible that the energy needed for these proteins to pass across the outer membrane, to travel through the intermembrane space and to target the inner-membrane surface is provided by conformational changes involving import components that seem to have natively unfolded structures. Such structural malleability might render some of the translocase subunits more adept at driving the protein import process.

Estrogen receptors alpha and beta (ERalpha and ERbeta) and androgen receptor (AR) in human sperm: localization of ERbeta and AR in mitochondria of the midpiece

BACKGROUND

The central role of estrogens and androgens in the male reproductive system has focused attention on the presence and distribution of their cognate receptors [estrogen receptor (ER) alpha, ERbeta and androgen receptor (AR)] in male reproductive tissues and cells. Since the presence of steroid hormone receptors in mitochondria of mammalian cells has been well documented, we investigated the possibility of mitochondrial localization of sex steroid hormone receptors in sperm.

METHODS AND RESULTS

Applying immunofluorescence labelling and confocal laser scanning microscopy we show that the estrogen receptor beta and the AR of human sperm are specifically enriched in the midpiece, at the site of the mitochondria, which were visualized by labelling with the vital dye CMX. Nuclear and mitochondrial localization of AR was also detected in LnCap human prostate cancer cells. Differentially, most of the ERalpha immunostaining is in the form of a compact zone at a region corresponding to the equatorial segment of the upper post-acrosomal region of the sperm head. Immunoblotting experiments using sperm extracts revealed the presence of a 66 and a 45 kDa protein reacting with the ERalpha antibody, one 64 kDa protein reacting with the ERbeta antibody and a 110 and a 90 kDa protein reacting with the antibody against AR.

CONCLUSIONS

Our findings suggest that the differential localization of AR and ER isoforms in human sperm reveals distinct roles of these receptors in the physiology of sperm cells and, perhaps, also in the process of fertilization.

Differential expression and comparative subcellular localization of estrogen receptor beta isoforms in virally transformed and normal cultured human lens epithelial cells

A number of variants of the wild-type (wt) estrogen receptor beta (ERbeta-1) coexist in a wide range of tissues. In the human these include, together with others, the expression of several isoforms (ERbeta-2-ERbeta-5) due to alternative splicing of exons encoding the carboxy terminus. In this study, we determined whether virally transformed cell cultures of human lens epithelial cells (HLE-B3) express both full length (or wt) and variant isoforms of ERbeta in comparison to normal secondary cultures of human lens epithelial cells (nHLE) and furthermore, identify the subcellular localization of the wtERbeta-1 and ERbeta isoform variants in HLE-B3 and nHLE cells, as well as from human breast adenocarcinoma cells (MCF-7) which provided a positive control. ERbeta isoform mRNA expression was evaluated by coupled RT-PCR. Subcellular localization of ERbeta isoforms was determined on formaldehyde-fixed, Saponin-permeabilized cells using conventional immunofluorescence techniques and affinity purified polyclonal antibodies specific for ERbeta-1 as well as to two of the truncated carboxy terminus isoforms (beta-2 and beta-5). Total RNA was extracted from HLE-B3 and nHLE cells and lens tissue, as well as from human breast adenocarcinoma cells (MCF-7) and subjected to RT-PCR using specific estrogen receptor primers intended to distinguish ERbeta-1-ERbeta-5 mRNA. The PCR products corresponded to wtERbeta-1 as well as to the isoform variants beta-2 and beta-5. The proportional distribution of wtERbeta-1, beta-2 and beta-5 PCR products differed between the normal lens epithelial cells and the SV-40 transformed lens epithelial cell line; the nHLE being similar to lens tissue with respect to relative expression of ERbeta isoform cDNAs. Confocal microscopy and immunofluorescence revealed ERbeta-2 was distributed throughout the cytosol and was associated with the nucleus of all cells examined, although sporadic immunostaining was observed with the nuclei of MCF-7. Prominent immunostaining of ERbeta-1 appeared in the mitochondria (along with weaker staining in the nucleus) of all cell types as authenticated by co-localization with Mitotrack-633. ERbeta-5 immunostaining was diffuse in the cytosol and also associated with the nuclei of all cell types. The differential subcellular partitioning of ERbeta-1 to the mitochondria and ERbeta-2 to the nucleus suggests a new aspect of regulation and function of the estrogen signalling system.

Glucocorticoid receptor isoforms in human hepatocarcinoma HepG2 and SaOS-2 osteosarcoma cells: presence of glucocorticoid receptor alpha in mitochondria and of glucocorticoid receptor beta in nucleoli

In the context of a possible direct action of glucocorticosteroids on mitochondrial transcription and/or apoptosis by way of cognate mitochondrial receptors, the possible localization of glucocorticoid receptors alpha and beta (GRalpha and GRbeta) in mitochondria was explored in human hepatocarcinoma HepG2 and osteosarcoma SaOS-2 cells, in which glucocorticoids exert an anabolic and apoptotic effect, respectively. In both cell types, GRalpha was detected in mitochondria, in nuclei and in cytosol by immunofluorescence labeling and confocal scanning microscopy, by immunogold electron microscopy and by Western blotting. GRbeta was shown to be almost exclusively restricted to the nucleus of the two cell types, being particularly concentrated in nucleoli, pointing to a solely nuclear role of this receptor isoform and to a possible function in nucleoli related processes. Computer analysis identified a putative internal mitochondrial targeting sequence within the glucocorticoid receptor. The demonstration of mitochondrially localized GRalpha in HepG2 and SaOS-2 cells corroborates previous findings in other cell types and further supports a direct role of this receptor in mitochondrial functions.

New developments in mitochondrial assembly

The mitochondrion has developed an elaborate translocation system for the import of nuclear-coded proteins and the export of proteins coded on the mitochondrial genome. Precursor proteins contain targeting and sorting information to reach the mitochondrion, whereas the translocons recognize the information and direct the precursor to the correct compartment. The outer membrane contains the TOM (translocase of the outer membrane) complex for translocation and the SAM (sorting and assembly machinery) complex for assembly of outer membrane proteins with complex topologies. At the inner membrane, the TIM23 (translocase of the inner membrane) mediates the import of mitochondrial proteins with a typical N-terminal targeting sequence, and the TIM22 complex mediates the import of polytopic inner membrane proteins. Based on its prokaryotic origin, the inner membrane also contains several components that mediate the export and assembly of proteins from within the matrix. Together the translocation and assembly complexes coordinate assembly of the mitochondrion.

Evolutionary origin of a preprotein translocase in the periplastid membrane of complex plastids: a hypothesis

Plastids with four envelope membranes have evolved from red and green algae engulfed by phagotrophic protozoans. It is assumed that the Sec translocon resides in their outermost membrane, while in the two innermost membranes the Toc-Tic supercomplex is embedded. However, such a single Sec/single Toc-Tic model cannot explain the passage of proteins across the second (or periplastid) membrane which represents the endosymbiont plasmalemma. One of the most recent models postulates that this membrane contains the Toc75 channel which was relocated here from the endosymbiont plastid. Unfortunately, the precursor of this protein carries a bipartite presequence, which means that its insertion into the new membrane would require relocation and/or modification of two different processing peptidases. I suggest that these obstacles can be easily bypassed by the assumption that the mitochondrial Tim23 channel was inserted into the endosymbiont plasmalemma. In contrast to Toc75, this protein has an internal, uncleavable targeting signal and its insertion into the new membrane would require neither relocation nor modification of additional proteins. Besides, such a relocated Tim23 channel could import not only plastid, but also mitochondrial proteins. I hypothesize that from the latter proteins, initially directed to the endosymbiont mitochondrion, periplastid proteins have evolved which are now targeted to the former cytosol and/or nucleus of the eukaryotic algal endosymbiont.

Mitochondrial localization of ERalpha and ERbeta in human MCF7 cells

We observed previously that estrogen treatment increased the transcript levels of several mitochondrial DNA (mtDNA)-encoded genes for mitochondrial respiratory chain (MRC) proteins and MRC activity in rat hepatocytes and human Hep G2 cells. Others have reported detection of estrogen receptors (ER), ERalpha and ERbeta, in mitochondria of rabbit ovarian and uterine tissue. In this study, we have extended these observations. Using cellular fractionation and Western blot with ERalpha- and ERbeta-specific antibodies, we observed that ERalpha and ERbeta are present in mitochondria of human MCF7 cells and that the mitochondrial ERalpha and ERbeta account for 10 and 18%, respectively, of total cellular ERalpha and ERbeta in 17beta-estradiol (E(2))-treated MCF7 cells. We also found that E(2) significantly enhanced the amounts of mitochondrial ERalpha and ERbeta in a time- and concentration-dependent manner and that these effects are accompanied by a significant increase in the transcript levels of mtDNA-encoded genes, i.e., cytochrome c oxidase subunits I and II. Moreover, we demonstrated that these E(2)-mediated effects were inhibited by the pure ER antagonist, ICI-182780, indicating the involvement of ERs. Using immunohistochemistry with confocal microscopy and immunogold electron microscopy, we demonstrated that ERalpha and ERbeta are located within the MCF7 cell mitochondrial matrix. Computer analysis identified a putative internal mitochondrial targeting peptide signal within human ERbeta, suggesting an inherent potential for ERbeta to enter mitochondria. These findings confirm the observations of others and provide additional support for this novel localization of the ERs and for a potentially important role of the ER in the regulation of mtDNA transcription.

Move it on over: getting proteins across biological membranes

The translocation of proteins across membranes is a central problem in biology. Regardless of the system in question, delivering proteins across a given membrane relies on many of the same basic themes. At the same time, however, each membrane translocation system, be it signal-gated or signal-assembled, makes use of components unique to that system. The latest findings on protein translocation across a variety of biological membranes have been presented in a recent review article.

The Tim8-Tim13 complex of Neurospora crassa functions in the assembly of proteins into both mitochondrial membranes

The Tim8 and Tim13 proteins in yeast are known to exist in the mitochondrial intermembrane space and to form a hetero-oligomeric complex involved in the import of the mitochondrial inner membrane protein Tim23, the central component of the TIM23 translocase. Here, we have isolated tim8 and tim13 mutants in Neurospora crassa and have shown that mitochondria lacking the Tim8-Tim13 complex were deficient in the import of the outer membrane beta-barrel proteins Tom40 and porin. Cross-linking studies showed that the Tom40 precursor contacts the Tim8-Tim13 complex. The complex is involved at an early point in the Tom40 assembly pathway because cross-links can only be detected during the initial stages of Tom40 import. In mitochondria lacking the Tim8-Tim13 complex, the Tom40 precursor appears in a previously characterized early intermediate of Tom40 assembly more slowly than in wild type mitochondria. Thus, our data suggest a model in which one of the first steps in Tom40 assembly may be interaction with the Tim8-Tim13 complex. As in yeast, the N. crassa Tim23 precursor was imported inefficiently into mitochondria lacking the Tim8-Tim13 complex when the membrane potential was reduced. Tim23 import intermediates could also be cross-linked to the complex, suggesting a dual role for the Tim8-Tim13 intermembrane space complex in the import of proteins found in both the outer and inner mitochondrial membranes.

The WD-repeats of Net2p interact with Dnm1p and Fis1p to regulate division of mitochondria

The Net2, Fis1, and Dnm1 proteins are required for the division of mitochondria in the yeast Saccharomyces cerevisiae. Net2p has an amino-terminal region that contains predicted coiled-coil motifs and a carboxyl-terminal domain composed of WD-40 repeats. We found that the amino-terminal part of Net2p interacts with Fis1p, whereas the carboxyl-terminal region interacts with both Dnm1p and Fis1p. Overproduction of either domain of Net2p in yeast cells poisons mitochondrial fission, and the dominant-negative effect caused by the WD-repeats of Net2p is suppressed by increased levels of Dnm1p. Point mutations in the WD-region of Net2p or in the GTPase region of Dnm1p disrupt the normal Net2p-Dnm1p interaction, causing Net2p to lose its normal punctate distribution. Our results suggest that Dnm1p interacts with the WD-repeats of Net2p and in a GTP-dependent manner recruits Net2p to sites of mitochondrial division. Furthermore, our results indicate that Net2p is required for proper assembly of the mitochondrial fission components to regulate organelle division.

Protein insertion into the mitochondrial inner membrane by a twin-pore translocase

The mitochondrial inner membrane imports numerous proteins that span it multiple times using the membrane potential Deltapsi as the only external energy source. We purified the protein insertion complex (TIM22 complex), a twin-pore translocase that mediated the insertion of precursor proteins in a three-step process. After the precursor is tethered to the translocase without losing energy from the Deltapsi, two energy-requiring steps were needed. First, Deltapsi acted on the precursor protein and promoted its docking in the translocase complex. Then, Deltapsi and an internal signal peptide together induced rapid gating transitions in one pore and closing of the other pore and drove membrane insertion to completion. Thus, protein insertion was driven by the coordinated action of a twin-pore complex in two voltage-dependent steps.

The role of Tim9p in the assembly of the TIM22 import complexes

Tim9p is located in the soluble 70-kDa Tim9p-Tim10p complex and the 300-kDa membrane complex in the mitochondrial TIM22 protein import system, which mediates the import of inner membrane proteins. From a collection of temperature-sensitive mutants, we have analyzed two in detail. tim9-3 contained two mutations and tim9-19 contained one mutation, all located near the 'twin CX3C' motif that is conserved in the small Tim proteins. As a result, the import components in the tim9-3 mutant mitochondria were severely reduced and assembled into complexes of aberrant sizes. Protein import was severely reduced and Tim9p and Tim10p binding to in vitro imported ADP/ATP carrier was impaired. In the tim9-19 mutant mitochondria, the 300-kDa membrane complex was assembled, although the soluble 70-kDa Tim9p-Tim10p complex was not detectable. Protein import was decreased only two-fold. When coexpressed in Escherichia coli, tim9-19 and TIM10 proteins failed to assemble into a 70-kDa complex. Our findings suggest that residues near the 'twin CX3C' motif are important for the assembly of Tim9p in both the Tim9p-Tim10p complex and the 300-kDa membrane complex.

Protein translocons: multifunctional mediators of protein translocation across membranes

Protein translocation systems consist of complex molecular machines whose activities are not limited to unidirectional protein targeting. Protein translocons and their associated receptor systems can be viewed as dynamic modular units whose interactions, and therefore functions, are regulated in response to specific signals. This flexibility allows translocons to interact with multiple signal receptor systems to manage the targeting of topologically distinct classes of proteins, to mediate targeting to different suborganellar compartments, and to respond to stress and developmental cues. Furthermore, the activities of translocons are tightly coordinated with downstream events, thereby providing a direct link between targeting and protein maturation.

The role of the Tim8p-Tim13p complex in a conserved import pathway for mitochondrial polytopic inner membrane proteins

Tim23p is imported via the TIM (translocase of inner membrane)22 pathway for mitochondrial inner membrane proteins. In contrast to precursors with an NH2-terminal targeting presequence that are imported in a linear NH2-terminal manner, we show that Tim23p crosses the outer membrane as a loop before inserting into the inner membrane. The Tim8p-Tim13p complex facilitates translocation across the intermembrane space by binding to the membrane spanning domains as shown by Tim23p peptide scans with the purified Tim8p-Tim13p complex and crosslinking studies with Tim23p fusion constructs. The interaction between Tim23p and the Tim8p-Tim13p complex is not dependent on zinc, and the purified Tim8p-Tim13p complex does not coordinate zinc in the conserved twin CX3C motif. Instead, the cysteine residues seemingly form intramolecular disulfide linkages. Given that proteins of the mitochondrial carrier family also pass through the TOM (translocase of outer membrane) complex as a loop, our study suggests that this translocation mechanism may be conserved. Thus, polytopic inner membrane proteins, which lack an NH2-terminal targeting sequence, pass through the TOM complex as a loop followed by binding of the small Tim proteins to the hydrophobic membrane spanning domains.

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.

Protein import into and across the mitochondrial inner membrane: role of the TIM23 and TIM22 translocons

Import of nuclear-encoded mitochondrial proteins requires the action of at least two different import machines, called translocons, in the mitochondrial inner membrane (IM). The TIM23 complex mediates the translocation of proteins into the mitochondria matrix, whereas the TIM22 complex is required for the insertion of polytopic proteins into the IM. While the two translocons are distinct and composed of separate subunits, the essential reactions in each complex are carried out by homologous proteins. In addition, the core components of both the TIM23 and TIM22 translocons have been shown to form aqueous pores in the mitochondrial IM. In this review, we summarize what is known about import of proteins across the mitochondrial IM.

Tim22, the essential core of the mitochondrial protein insertion complex, forms a voltage-activated and signal-gated channel

The protein insertion complex of the mitochondrial inner membrane is crucial for import of the numerous multitopic membrane proteins with internal targeting signals. Little is known about the molecular mechanism of this complex, including whether it forms a real channel or merely acts as scaffold for protein insertion. We report the unexpected observation that Tim22 is the only essential membrane-integrated subunit of the complex. Reconstituted Tim22 forms a hydrophilic, high-conductance channel with distinct opening states and pore diameters. The channel is voltage-activated and specifically responds to an internal targeting signal, but not to presequences. Thus, a protein insertion complex can combine three essential functions, signal recognition, channel formation, and energy transduction, in one central component.

Transport of proteins into mitochondria

Most mitochondrial proteins are nuclear-encoded and synthesised as preproteins on polysomes in the cytosol. They must be targeted to and translocated into mitochondria. Newly synthesised preproteins interact with cytosolic factors until their recognition by receptors on the surface of mitochondria. Import into or across the outer membrane is mediated by a dynamic protein complex coined the translocase of the outer membrane (TOM). Preproteins that are imported into the matrix or inner membrane of mitochondria require the action of one of two translocation complexes of the inner membrane (TIMs). The import pathway of preproteins is predetermined by their intrinsic targeting and sorting signals. Energy input in the form of ATP and the electrical gradient across the inner membrane is required for protein translocation into mitochondria. Newly imported proteins may require molecular chaperones for their correct folding.

Versatility of the mitochondrial protein import machinery

The vast majority of mitochondrial proteins are synthesized in the cytosol and are imported into mitochondria by protein machineries located in the mitochondrial membranes. It has become clear that hydrophilic as well as hydrophobic preproteins use a common translocase in the outer mitochondrial membrane, but diverge to two distinct translocases in the inner membrane. The translocases are dynamic, high-molecular-weight complexes that have to provide specific means for the recognition of preproteins, channel formation and generation of import-driving forces.

Characterization of rat TOM40, a central component of the preprotein translocase of the mitochondrial outer membrane

We cloned a 38-kDa rat mitochondrial outer membrane protein (OM38) with structural homology to the central component of preprotein translocase of the fungal mitochondrial outer membrane, Tom40. Although it has no predictable alpha-helical transmembrane segments, OM38 is resistant to alkaline carbonate extraction and is inaccessible to proteases and polyclonal antibodies added from outside the mitochondria, suggesting that it is embedded in the membrane, probably in a beta-barrel structure, as has been similarly speculated for fungal Tom40. Immunoprecipitation demonstrated that OM38 is associated with the major import receptors rTOM20 and rTOM22, and several other unidentified components with molecular masses of 5-10 kDa in digitonin-solubilized membrane: OM10, OM7.5, and OM5. Blue native polyacrylamide gel electrophoresis revealed that OM38 is a component of a approximately 400-kDa complex, firmly associating with rTOM22 and loosely associating with rTOM20. The preprotein in transit to the matrix interacted with the TOM complex containing OM38, and immunodepletion of OM38 resulted in the loss of preprotein import activity of the detergent-solubilized and reconstituted outer membrane vesicles. Taken together, these results indicate that OM38 is a structural and functional homolog of fungal Tom40 and functions as a component of the preprotein import machinery of the rat mitochondrial outer membrane.

Membrane potential-driven protein import into mitochondria. The sorting sequence of cytochrome b(2) modulates the deltapsi-dependence of translocation of the matrix-targeting sequence

The transport of preproteins into or across the mitochondrial inner membrane requires the membrane potential Deltapsi across this membrane. Two roles of Deltapsi in the import of cleavable preproteins have been described: an electrophoretic effect on the positively charged matrix-targeting sequences and the activation of the translocase subunit Tim23. We report the unexpected finding that deletion of a segment within the sorting sequence of cytochrome b(2), which is located behind the matrix-targeting sequence, strongly influenced the Deltapsi-dependence of import. The differential Deltapsi-dependence was independent of the submitochondrial destination of the preprotein and was not attributable to the requirement for mitochondrial Hsp70 or Tim23. With a series of preprotein constructs, the net charge of the sorting sequence was altered, but the Deltapsi-dependence of import was not affected. These results suggested that the sorting sequence contributed to the import driving mechanism in a manner distinct from the two known roles of Deltapsi. Indeed, a charge-neutral amino acid exchange in the hydrophobic segment of the sorting sequence generated a preprotein with an even better import, i.e. one with lower Deltapsi-dependence than the wild-type preprotein. The sorting sequence functioned early in the import pathway since it strongly influenced the efficiency of translocation of the matrix-targeting sequence across the inner membrane. These results suggest a model whereby an electrophoretic effect of Deltapsi on the matrix-targeting sequence is complemented by an import-stimulating activity of the sorting sequence.

Identification of mammalian TOM22 as a subunit of the preprotein translocase of the mitochondrial outer membrane

A mitochondrial outer membrane protein of approximately 22 kDa (1C9-2) was purified from Vero cells assessing immunoreactivity with a monoclonal antibody, and the cDNA was cloned based on the partial amino acid sequence of the trypsin-digested fragments. 1C9-2 had 19-20% sequence identity to fungal Tom22, a component of the preprotein translocase of the outer membrane (the TOM complex) with receptor and organizer functions. Despite such a low sequence identity, both shared a remarkable structural similarity in the hydrophobicity profile, membrane topology in the Ncyt-Cin orientation through a transmembrane domain in the middle of the molecule, and the abundant acidic amino acid residues in the N-terminal domain. The antibodies against 1C9-2 inhibited the import of a matrix-targeted preprotein into isolated mitochondria. Blue native polyacrylamide gel electrophoresis of digitonin-solubilized outer membranes revealed that 1C9-2 is firmly associated with TOM40 in the approximately 400-kDa complex, with a size and composition similar to those of the fungal TOM core complex. Furthermore, 1C9-2 complemented the defects of growth and mitochondrial protein import in Deltatom22 yeast cells. Taken together, these results demonstrate that 1C9-2 is a functional homologue of fungal Tom22 and functions as a component of the TOM complex.

Two intermembrane space TIM complexes interact with different domains of Tim23p during its import into mitochondria

Tim23p (translocase of the inner membrane) is an essential import component located in the mitochondrial inner membrane. To determine how the Tim23 protein itself is transported into mitochondria, we used chemical cross-linking to identify proteins adjacent to Tim23p during its biogenesis. In the absence of an inner membrane potential, Tim23p is translocated across the mitochondrial outer membrane, but not inserted into the inner membrane. At this intermediate stage, we find that Tim23p forms cross-linked products with two distinct protein complexes of the intermembrane space, Tim8p-Tim13p and Tim9p-Tim10p. Tim9p and Tim10p cross-link to the COOH-terminal domain of the Tim23 protein, which carries all of the targeting signals for Tim23p. Therefore, our results suggest that the Tim9p-Tim10p complex plays a key role in Tim23p import. In contrast, Tim8p and Tim13p cross-link to the hydrophilic NH(2)-terminal segment of Tim23p, which does not carry essential import information and, thus, the role of Tim8p-Tim13p is unclear. Tim23p contains two matrix-facing, positively charged loops that are essential for its insertion into the inner membrane. The positive charges are not required for interaction with the Tim9p-Tim10p complex, but are essential for cross-linking of Tim23p to components of the inner membrane insertion machinery, including Tim54p, Tim22p, and Tim12p.

The central matrix loop drives import of uncoupling protein 1 into mitochondria

The uncoupling protein (UCP1) is a carrier protein of the inner mitochondrial membrane spanning the bilayer six times. It does not contain a typical amino-terminal targeting signal and the mechanism of targeting and insertion is unknown. Here we focus on the biogenesis of UCP1 by analysing the import signals contained within the three repeated units of the protein. The amino-terminal third of the protein can mediate insertion into the outer membrane and therefore acts as artificial targeting signal when fused to DHFR. However, in the context of full-length UCP, the targeting information contained within the first repeated unit is not sufficient to trigger insertion into the outer membrane. Deletion of either the first or third repeated unit from UCP1 did not reduce import into the inner membrane and bound to the outer membrane receptor protein hTom20 with the characteristics of full-length UCP1. Deletion of the second repeat of UCP1 completely abolished all import into the mitochondria. Consistent with this, the central repeat alone was efficiently imported to the inner membrane and bound hTom20 with the characteristics of UCP1. We conclude that the site for binding hTom20 is within the central repeat and that this domain contains the complete targeting signal for directing UCP1 to the inner membrane.

Tim23 links the inner and outer mitochondrial membranes

Tim23, a key component of the mitochondrial preprotein translocase, is anchored in the inner membrane by its C-terminal domain and exposes an intermediate domain in the intermembrane space that functions as a presequence receptor. We show that the N-terminal domain of Tim23 is exposed on the surface of the outer membrane. The two-membrane-spanning topology of Tim23 is a novel characteristic in membrane biology. By the simultaneous integration into two membranes, Tim23 forms contacts between the outer and inner mitochondrial membranes. Tethering the inner membrane translocase to the outer membrane facilitates the transfer of precursor proteins from the TOM complex to the TIM23 complex and increases the efficiency of protein import.

Tim18p, a new subunit of the TIM22 complex that mediates insertion of imported proteins into the yeast mitochondrial inner membrane

Import of carrier proteins from the cytoplasm into the mitochondrial inner membrane of yeast is mediated by a distinct system consisting of two soluble 70-kDa protein complexes in the intermembrane space and a 300-kDa complex in the inner membrane, the TIM22 complex. The TIM22 complex contains the peripheral subunits Tim9p, Tim10p, and Tim12p and the integral membrane subunits Tim22p and Tim54p. We identify here an additional subunit, an 18-kDa integral membrane protein termed Tim18p. This protein is made as a 21.9-kDa precursor which is imported into mitochondria and processed to its mature form. When mitochondria are gently solubilized, Tim18p comigrates with the other subunits of the TIM22 complex on nondenaturing gels and is coimmunoprecipitated with Tim54p and Tim12p. Tim18p does not cofractionate with the TIM23 complex upon immunoprecipitation or nondenaturing gel electrophoresis. Deletion of Tim18p decreases the growth rate of yeast cells by a factor of two and is synthetically lethal with temperature-sensitive mutations in Tim9p or Tim10p. It also impairs the import of several precursor proteins into isolated mitochondria, and lowers the apparent mass of the TIM22 complex. We suggest that Tim18p functions in the assembly and stabilization of the TIM22 complex but does not directly participate in protein insertion into the inner membrane.

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

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

Tim18p is a new component of the Tim54p-Tim22p translocon in the mitochondrial inner membrane

The mitochondrial inner membrane contains two separate translocons: one required for the translocation of matrix-targeted proteins (the Tim23p-Tim17p complex) and one for the insertion of polytopic proteins into the mitochondrial inner membrane (the Tim54p-Tim22p complex). To identify new members of the Tim54p-Tim22p complex, we screened for high-copy suppressors of the temperature-sensitive tim54-1 mutant. We identified a new gene, TIM18, that encodes an integral protein of the inner membrane. The following genetic and biochemical observations suggest that the Tim18 protein is part of the Tim54p-Tim22p complex in the inner membrane: multiple copies of TIM18 suppress the tim54-1 growth defect; the tim18::HIS3 disruption is synthetically lethal with tim54-1; Tim54p and Tim22p can be coimmune precipitated with the Tim18 protein; and Tim18p, along with Tim54p and Tim22p, is detected in an approximately 300-kDa complex after blue native electrophoresis. We propose that Tim18p is a new component of the Tim54p-Tim22p machinery that facilitates insertion of polytopic proteins into the mitochondrial inner membrane.