Sorting switch of mitochondrial presequence translocase involves coupling of motor module to respiratory chain

Mitochondrial complexome and import network

Mitochondria perform crucial functions in cellular metabolism, protein and lipid biogenesis, quality control, and signaling. The systematic analysis of protein complexes and interaction networks provided exciting insights into the structural and functional organization of mitochondria. Most mitochondrial proteins do not act as independent units, but are interconnected by stable or dynamic protein-protein interactions. Protein translocases are responsible for importing precursor proteins into mitochondria and form central elements of several protein interaction networks. These networks include molecular chaperones and quality control factors, metabolite channels and respiratory chain complexes, and membrane and organellar contact sites. Protein translocases link the distinct networks into an overarching network, the mitochondrial import network (MitimNet), to coordinate biogenesis, membrane organization and function of mitochondria.

The biogenesis and regulation of the plant oxidative phosphorylation system

Mitochondria are central organelles for respiration in plants. At the heart of this process is oxidative phosphorylation (OXPHOS) system, which generates ATP required for cellular energetic needs. OXPHOS complexes comprise of multiple subunits that originated from both mitochondrial and nuclear genome, which requires careful orchestration of expression, translation, import, and assembly. Constant exposure to reactive oxygen species due to redox activity also renders OXPHOS subunits to be more prone to oxidative damage, which requires coordination of disassembly and degradation. In this review, we highlight the composition, assembly, and activity of OXPHOS complexes in plants based on recent biochemical and structural studies. We also discuss how plants regulate the biogenesis and turnover of OXPHOS subunits and the importance of OXPHOS in overall plant respiration. Further studies in determining the regulation of biogenesis and activity of OXPHOS will advances the field, especially in understanding plant respiration and its role to plant growth and development.

Functional crosstalk between the TIM22 complex and YME1 machinery maintains mitochondrial proteostasis and integrity

TIM22 pathway cargos are essential for sustaining mitochondrial homeostasis as an excess of these proteins leads to proteostatic stress and cell death. Yme1 is an inner membrane metalloprotease that regulates protein quality control with chaperone-like and proteolytic activities. Although the mitochondrial translocase and protease machinery are critical for organelle health, their functional association remains unexplored. The present study unravels a novel genetic connection between the TIM22 complex and YME1 machinery in Saccharomyces cerevisiae that is required for maintaining mitochondrial health. Our genetic analyses indicate that impairment in the TIM22 complex rescues the respiratory growth defects of cells without Yme1. Furthermore, Yme1 is essential for the stability of the TIM22 complex and regulates the proteostasis of TIM22 pathway substrates. Moreover, impairment in the TIM22 complex suppressed the mitochondrial structural and functional defects of Yme1-devoid cells. In summary, excessive levels of TIM22 pathway substrates could be one of the reasons for respiratory growth defects of cells lacking Yme1, and compromising the TIM22 complex can compensate for the imbalance in mitochondrial proteostasis caused by the loss of Yme1.

Presequence translocase-associated motor subunits of the mitochondrial protein import apparatus are dual-targeted to mitochondria and plastids

The import and assembly of most of the mitochondrial proteome is regulated by protein translocases located within the mitochondrial membranes. The Presequence Translocase-Associated Motor (PAM) complex powers the translocation of proteins across the inner membrane and consists of Hsp70, the J-domain containing co-chaperones, Pam16 and Pam18, and their associated proteins Tim15 and Mge1. In Arabidopsis, multiple orthologues of Pam16, Pam18, Tim15 and Mge1 have been identified and a mitochondrial localization has been confirmed for most. As the localization of Pam18-1 has yet to be determined and a plastid localization has been observed for homologues of Tim15 and Mge1, we carried out a comprehensive targeting analysis of all PAM complex orthologues using multiple in vitro and in vivo methods. We found that, Pam16 was exclusively targeted to the mitochondria, but Pam18 orthologues could be targeted to both the mitochondria and plastids, as observed for the PAM complex interacting partner proteins Tim15 and Mge1.

Investigation of the Association between the Energy Metabolism of the Insect Vector Laodelphax striatellus and Rice Stripe Virus (RSV)

Viruses, as intracellular parasites, rely on the host organism to complete their life cycle. Although over 70% of plant viruses are transmitted by insect vectors, the role of vector energy metabolism on the infection process of insect-borne plant viruses is unclear. In this study, full-length cDNAs of three energy metabolism-related genes (LsATPase, LsMIT13 and LsNADP-ME) were obtained from the small brown planthopper (SBPH, Laodelphax striatellus), which transmits the Rice stripe virus (RSV). Expression levels of LsATPase, LsMIT13 and LsNADP-ME increased by 105%, 1120% and 259%, respectively, due to RSV infection. The repression of LsATPase, LsMIT13 or LsNADP-ME by RNAi had no effect on RSV nucleocapsid protein (NP) transcripts or protein levels. The repression of LsATPase caused a significant increase in LsMIT13 and LsNADP-ME transcript levels by 230% and 217%, respectively, and the repression of LsMIT13 caused a significant increase in LsNADP-ME mRNA levels. These results suggested that the silencing of LsATPase induced compensatory upregulation of LsMIT13 and LsNADP-ME, and silencing LsMIT13 induced compensatory upregulation of LsNADP-ME. Further study indicated that the co-silencing of LsATPase, LsMIT13 and LsNADP-ME in viruliferous SBPHs increased ATP production and RSV loads by 182% and 117%, respectively, as compared with nonviruliferous SBPHs. These findings indicate that SBPH energy metabolism is involved in RSV infection and provide insight into the association between plant viruses and energy metabolism in the insect vector.

Assembly of the Multi-Subunit Cytochrome bc1 Complex in the Yeast Saccharomyces cerevisiae

The cytochrome bc₁ complex is an essential component of the mitochondrial respiratory chain of the yeast Saccharomyces cerevisiae. It is composed of ten protein subunits, three of them playing an important role in electron transfer and proton pumping across the inner mitochondrial membrane. Cytochrome b, the central component of this respiratory complex, is encoded by the mitochondrial genome, whereas all the other subunits are of nuclear origin. The assembly of all these subunits into the mature and functional cytochrome bc₁ complex is therefore a complicated process which requires the participation of several chaperone proteins. It has been found that the assembly process of the mitochondrial bc₁ complex proceeds through the formation of distinct sub-complexes in an ordered sequence. Most of these sub-complexes have been thoroughly characterized, and their molecular compositions have also been defined. This study critically analyses the results obtained so far and highlights new possible areas of investigation.

Coupling to Pam16 differentially controls the dual role of Pam18 in protein import and respiratory chain formation

The presequence translocase (TIM23 complex) imports precursor proteins into the mitochondrial inner membrane and matrix. The presequence translocase-associated motor (PAM) provides a driving force for transport into the matrix. The J-protein Pam18 stimulates the ATPase activity of the mitochondrial Hsp70 (mtHsp70). Pam16 recruits Pam18 to the TIM23 complex to ensure protein import. The Pam16-Pam18 module also associates with components of the respiratory chain, but the function of the dual localization of Pam16-Pam18 is largely unknown. Here, we show that disruption of the Pam16-Pam18 heterodimer causes redistribution of Pam18 to the respiratory chain supercomplexes, where it forms a homodimer. Redistribution of Pam18 decreases protein import into mitochondria but stimulates mtHsp70-dependent assembly of respiratory chain complexes. We conclude that coupling to Pam16 differentially controls the dual function of Pam18. It recruits Pam18 to the TIM23 complex to promote protein import but attenuates the Pam18 function in the assembly of respiratory chain complexes.

Targeting and Insertion of Membrane Proteins in Mitochondria

Mitochondrial membrane proteins play an essential role in all major mitochondrial functions. The respiratory complexes of the inner membrane are key for the generation of energy. The carrier proteins for the influx/efflux of essential metabolites to/from the matrix. Many other inner membrane proteins play critical roles in the import and processing of nuclear encoded proteins (∼99% of all mitochondrial proteins). The outer membrane provides another lipidic barrier to nuclear-encoded protein translocation and is home to many proteins involved in the import process, maintenance of ionic balance, as well as the assembly of outer membrane components. While many aspects of the import and assembly pathways of mitochondrial membrane proteins have been elucidated, many open questions remain, especially surrounding the assembly of the respiratory complexes where certain highly hydrophobic subunits are encoded by the mitochondrial DNA and synthesised and inserted into the membrane from the matrix side. This review will examine the various assembly pathways for inner and outer mitochondrial membrane proteins while discussing the most recent structural and biochemical data examining the biogenesis process.

Role of Mitochondrial Protein Import in Age-Related Neurodegenerative and Cardiovascular Diseases

Mitochondria play a critical role in providing energy, maintaining cellular metabolism, and regulating cell survival and death. To carry out these crucial functions, mitochondria employ more than 1500 proteins, distributed between two membranes and two aqueous compartments. An extensive network of dedicated proteins is engaged in importing and sorting these nuclear-encoded proteins into their designated mitochondrial compartments. Defects in this fundamental system are related to a variety of pathologies, particularly engaging the most energy-demanding tissues. In this review, we summarize the state-of-the-art knowledge about the mitochondrial protein import machinery and describe the known interrelation of its failure with age-related neurodegenerative and cardiovascular diseases.

Role of the Mitochondrial Protein Import Machinery and Protein Processing in Heart Disease

Mitochondria are essential organelles for cellular energy production, metabolic homeostasis, calcium homeostasis, cell proliferation, and apoptosis. About 99% of mammalian mitochondrial proteins are encoded by the nuclear genome, synthesized as precursors in the cytosol, and imported into mitochondria by mitochondrial protein import machinery. Mitochondrial protein import systems function not only as independent units for protein translocation, but also are deeply integrated into a functional network of mitochondrial bioenergetics, protein quality control, mitochondrial dynamics and morphology, and interaction with other organelles. Mitochondrial protein import deficiency is linked to various diseases, including cardiovascular disease. In this review, we describe an emerging class of protein or genetic variations of components of the mitochondrial import machinery involved in heart disease. The major protein import pathways, including the presequence pathway (TIM23 pathway), the carrier pathway (TIM22 pathway), and the mitochondrial intermembrane space import and assembly machinery, related translocases, proteinases, and chaperones, are discussed here. This review highlights the importance of mitochondrial import machinery in heart disease, which deserves considerable attention, and further studies are urgently needed. Ultimately, this knowledge may be critical for the development of therapeutic strategies in heart disease.

Porins as helpers in mitochondrial protein translocation

Mitochondria import the vast majority of their proteins via dedicated protein machineries. The translocase of the outer membrane (TOM complex) forms the main entry site for precursor proteins that are produced on cytosolic ribosomes. Subsequently, different protein sorting machineries transfer the incoming preproteins to the mitochondrial outer and inner membranes, the intermembrane space, and the matrix. In this review, we highlight the recently discovered role of porin, also termed voltage-dependent anion channel (VDAC), in mitochondrial protein biogenesis. Porin forms the major channel for metabolites and ions in the outer membrane of mitochondria. Two different functions of porin in protein translocation have been reported. First, it controls the formation of the TOM complex by modulating the integration of the central receptor Tom22 into the mature translocase. Second, porin promotes the transport of carrier proteins toward the carrier translocase (TIM22 complex), which inserts these preproteins into the inner membrane. Therefore, porin acts as a coupling factor to spatially coordinate outer and inner membrane transport steps. Thus, porin links metabolite transport to protein import, which are both essential for mitochondrial function and biogenesis.

Interplay between Mitochondrial Protein Import and Respiratory Complexes Assembly in Neuronal Health and Degeneration

The fact that >99% of mitochondrial proteins are encoded by the nuclear genome and synthesised in the cytosol renders the process of mitochondrial protein import fundamental for normal organelle physiology. In addition to this, the nuclear genome comprises most of the proteins required for respiratory complex assembly and function. This means that without fully functional protein import, mitochondrial respiration will be defective, and the major cellular ATP source depleted. When mitochondrial protein import is impaired, a number of stress response pathways are activated in order to overcome the dysfunction and restore mitochondrial and cellular proteostasis. However, prolonged impaired mitochondrial protein import and subsequent defective respiratory chain function contributes to a number of diseases including primary mitochondrial diseases and neurodegeneration. This review focuses on how the processes of mitochondrial protein translocation and respiratory complex assembly and function are interlinked, how they are regulated, and their importance in health and disease.

Transmembrane Coordination of Preprotein Recognition and Motor Coupling by the Mitochondrial Presequence Receptor Tim50

Mitochondrial preproteins contain amino-terminal presequences directing them to the presequence translocase of the mitochondrial inner membrane (TIM23 complex). Depending on additional downstream import signals, TIM23 either inserts preproteins into the inner membrane or translocates them into the matrix. Matrix import requires the coupling of the presequence translocase-associated motor (PAM) to TIM23. The molecular mechanisms coordinating preprotein recognition by TIM23 in the intermembrane space (IMS) with PAM activation in the matrix are unknown. Here we show that subsequent to presequence recognition in the IMS, the Tim50 matrix domain facilitates the recruitment of the coupling factor Pam17. Next, the IMS domain of Tim50 promotes PAM recruitment to TIM23. Finally, the Tim50 transmembrane segment stimulates the matrix-directed import-driving force exerted by PAM. We propose that recognition of preprotein segments in the IMS and transfer of signal information across the inner membrane by Tim50 determine import motor activation.

Coupling of import and assembly pathways in mitochondrial protein biogenesis

Biogenesis and function of mitochondria depend on the import of about 1000 precursor proteins that are produced on cytosolic ribosomes. The translocase of the outer membrane (TOM) forms the entry gate for most proteins. After passage through the TOM channel, dedicated preprotein translocases sort the precursor proteins into the mitochondrial subcompartments. Many proteins have to be assembled into oligomeric membrane-integrated complexes in order to perform their functions. In this review, we discuss a dual role of mitochondrial preprotein translocases in protein translocation and oligomeric assembly, focusing on the biogenesis of the TOM complex and the respiratory chain. The sorting and assembly machinery (SAM) of the outer mitochondrial membrane forms a dynamic platform for coupling transport and assembly of TOM subunits. The biogenesis of the cytochrome c oxidase of the inner membrane involves a molecular circuit to adjust translation of mitochondrial-encoded core subunits to the availability of nuclear-encoded partner proteins. Thus, mitochondrial protein translocases not only import precursor proteins but can also support their assembly into functional complexes.

Mgr2 regulates mitochondrial preprotein import by associating with channel-forming Tim23 subunit

Mgr2, a newly identified subunit of the TIM23 complex, functions as a gatekeeper of presequence translocase and thereby maintains quality control of inner membrane preproteins sorting. However, precise recruitment of the Mgr2 subunit to the core channel and how it influences the assembly of the TIM23 complex during lateral sorting of preproteins are poorly understood. Present findings provide insights into a direct association of Mgr2 with the channel-forming Tim23 subunit. Furthermore, the mutational analysis uncovers the TM1 region of Mgr2 critically required for association with Tim23 and Tim21. On the other hand, the TM2 region of Mgr2 is involved in bridging respiratory complexes to the TIM23 complex via Tim21. Importantly, both TM regions of Mgr2 are essential for lateral sorting of preprotein into the inner membrane, as well as maintaining mitochondrial morphology. Together, our findings provide mechanistic insights into the role of Mgr2 in regulating the dynamicity of the TIM23 complex assembly required for preprotein import and coupling of respiratory pathways.

Mitochondrial proteins: from biogenesis to functional networks

Mitochondria are essential for the viability of eukaryotic cells as they perform crucial functions in bioenergetics, metabolism and signalling and have been associated with numerous diseases. Recent functional and proteomic studies have revealed the remarkable complexity of mitochondrial protein organization. Protein machineries with diverse functions such as protein translocation, respiration, metabolite transport, protein quality control and the control of membrane architecture interact with each other in dynamic networks. In this Review, we discuss the emerging role of the mitochondrial protein import machinery as a key organizer of these mitochondrial protein networks. The preprotein translocases that reside on the mitochondrial membranes not only function during organelle biogenesis to deliver newly synthesized proteins to their final mitochondrial destination but also cooperate with numerous other mitochondrial protein complexes that perform a wide range of functions. Moreover, these protein networks form membrane contact sites, for example, with the endoplasmic reticulum, that are key for integration of mitochondria with cellular function, and defects in protein import can lead to diseases.

The basic machineries for mitochondrial protein quality control

Mitochondria play pivotal roles in cellular energy metabolism, the synthesis of essential biomolecules and the regulation of cell death and aging. The proper folding, unfolding and degradation of the many proteins active within mitochondria is surveyed by the mitochondrial quality control machineries. Here, we describe the principal components of the mitochondrial quality control system and recent developments in the elucidation of the molecular mechanisms maintaining a functional mitochondrial proteome.

Mitochondrial presequence import: Multiple regulatory knobs fine-tune mitochondrial biogenesis and homeostasis

Mitochondria are pivotal organelles for cellular signaling and metabolism, and their dysfunction leads to severe cellular stress. About 60-70% of the mitochondrial proteome consists of preproteins synthesized in the cytosol with an amino-terminal cleavable presequence targeting signal. The TIM23 complex transports presequence signals towards the mitochondrial matrix. Ultimately, the mature protein segments are either transported into the matrix or sorted to the inner membrane. To ensure accurate preprotein import into distinct mitochondrial sub-compartments, the TIM23 machinery adopts specific functional conformations and interacts with different partner complexes. Regulatory subunits modulate the translocase dynamics, tailoring the import reaction to the incoming preprotein. The mitochondrial membrane potential and the ATP generated via oxidative phosphorylation are key energy sources in driving the presequence import pathway. Thus, mitochondrial dysfunctions have rapid repercussions on biogenesis. Cellular mechanisms exploit the presequence import pathway to monitor mitochondrial dysfunctions and mount transcriptional and proteostatic responses to restore functionality.

A mutagenesis analysis of Tim50, the major receptor of the TIM23 complex, identifies regions that affect its interaction with Tim23

Maintenance of the mitochondrial proteome depends on import of newly made proteins from the cytosol. More than half of mitochondrial proteins are made as precursor proteins with N-terminal extensions called presequences and use the TIM23 complex for translocation into the matrix, the inner mitochondrial membrane and the intermembrane space (IMS). Tim50 is the central receptor of the complex that recognizes precursor proteins in the IMS. Additionally, Tim50 interacts with the IMS domain of the channel forming subunit, Tim23, an interaction that is essential for protein import across the mitochondrial inner membrane. In order to gain deeper insight into the molecular function of Tim50, we used random mutagenesis to determine residues that are important for its function. The temperature-sensitive mutants isolated were defective in import of TIM23-dependent precursor proteins. The residues mutated map to two distinct patches on the surface of Tim50. Notably, mutations in both patches impaired the interaction of Tim50 with Tim23. We propose that two regions of Tim50 play a role in its interaction with Tim23 and thereby affect the import function of the complex.

Defective mitochondrial protein import contributes to complex I-induced mitochondrial dysfunction and neurodegeneration in Parkinson’s disease

Mitochondria are the prime energy source in most eukaryotic cells, but these highly dynamic organelles are also involved in a multitude of cellular events. Disruption of mitochondrial homeostasis and the subsequent mitochondrial dysfunction plays a key role in the pathophysiology of Parkinson’s disease (PD). Therefore, maintenance of mitochondrial integrity through different surveillance mechanisms is critical for neuronal survival. Here, we have studied the mitochondrial protein import system in in vitro and in vivo models of PD. Complex I inhibition, a characteristic pathological hallmark in PD, impaired mitochondrial protein import, which was associated with a downregulation of two key components of the system: translocase of the outer membrane 20 (TOM20) and translocase of the inner membrane 23 (TIM23), both in vitro and in vivo. In vitro, those changes were associated with OXPHOS protein downregulation, accumulation of aggregated proteins inside mitochondria and downregulation of mitochondrial chaperones. Most of these pathogenic changes, including mitochondrial dysfunction and dopaminergic cell death, were abrogated by TOM20 or TIM23 overexpression, in vitro. However, in vivo, while TOM20 overexpression exacerbated neurodegeneration in both substantia nigra (SN) pars compacta (pc) and striatum, overexpression of TIM23 partially protected dopaminergic neurons in the SNpc. These results highlight mitochondrial protein import dysfunction and the distinct role of two of their components in the pathogenesis of PD and suggest the need for future studies to further characterize mitochondrial protein import deficit in the context of PD.

Pathways to balance mitochondrial translation and protein import

Mitochondria contain their own genome that encodes for a small number of proteins, while the vast majority of mitochondrial proteins is produced on cytosolic ribosomes. The formation of respiratory chain complexes depends on the coordinated biogenesis of mitochondrially encoded and nuclear-encoded subunits. In this review, we describe pathways that adjust mitochondrial protein synthesis and import of nuclear-encoded subunits to the assembly of respiratory chain complexes. Furthermore, we outline how defects in protein import into mitochondria affect nuclear gene expression to maintain protein homeostasis under physiological and stress conditions.

Cox15 interacts with the cytochrome bc 1 dimer within respiratory supercomplexes as well as in the absence of cytochrome c oxidase

The heme a molecule is an obligatory cofactor in the terminal enzyme complex of the electron transport chain, cytochrome c oxidase. Heme a is synthesized from heme o by a multi-spanning inner membrane protein, heme a synthase (Cox15 in the yeast Saccharomyces cerevisiae). The insertion of heme a is critical for cytochrome c oxidase function and assembly, but this process has not been fully elucidated. To improve our understanding of heme a insertion into cytochrome c oxidase, here we investigated the protein-protein interactions that involve Cox15 in S. cerevisiae In addition to observing Cox15 in homooligomeric complexes, we found that a portion of Cox15 also associates with the mitochondrial respiratory supercomplexes. When supercomplex formation was abolished, as in the case of stalled cytochrome bc ₁ or cytochrome c oxidase assembly, Cox15 maintained an interaction with select proteins from both respiratory complexes. In the case of stalled cytochrome bc ₁ assembly, Cox15 interacted with the late-assembling cytochrome c oxidase subunit, Cox13. When cytochrome c oxidase assembly was stalled, Cox15 unexpectedly maintained its interaction with the cytochrome bc ₁ protein, Cor1. Our results indicate that Cox15 and Cor1 continue to interact in the cytochrome bc ₁ dimer even in the absence of supercomplexes or when the supercomplexes are destabilized. These findings reveal that Cox15 not only associates with respiratory supercomplexes, but also interacts with the cytochrome bc ₁ dimer even in the absence of cytochrome c oxidase.

Respiratory chain supercomplexes associate with the cysteine desulfurase complex of the iron-sulfur cluster assembly machinery

Mitochondrial cytochrome bc1 complex and cytochrome c oxidase associate in respiratory chain supercomplexes. We identified a specific association of the iron–sulfur cluster biogenesis desulfurase with the respiratory chain supercomplexes. Our finding reveals a novel link between respiration and iron–sulfur cluster formation.

Mitochondria are the powerhouses of eukaryotic cells. The activity of the respiratory chain complexes generates a proton gradient across the inner membrane, which is used by the F₁F_O-ATP synthase to produce ATP for cellular metabolism. In baker’s yeast, Saccharomyces cerevisiae, the cytochrome bc1 complex (complex III) and cytochrome c oxidase (complex IV) associate in respiratory chain supercomplexes. Iron–sulfur clusters (ISC) form reactive centers of respiratory chain complexes. The assembly of ISC occurs in the mitochondrial matrix and is essential for cell viability. The cysteine desulfurase Nfs1 provides sulfur for ISC assembly and forms with partner proteins the ISC-biogenesis desulfurase complex (ISD complex). Here, we report an unexpected interaction of the active ISD complex with the cytochrome bc1 complex and cytochrome c oxidase. The individual deletion of complex III or complex IV blocks the association of the ISD complex with respiratory chain components. We conclude that the ISD complex binds selectively to respiratory chain supercomplexes. We propose that this molecular link contributes to coordination of iron–sulfur cluster formation with respiratory activity.

Dynamic organization of the mitochondrial protein import machinery

Mitochondria contain elaborate machineries for the import of precursor proteins from the cytosol. The translocase of the outer mitochondrial membrane (TOM) performs the initial import of precursor proteins and transfers the precursors to downstream translocases, including the presequence translocase and the carrier translocase of the inner membrane, the mitochondrial import and assembly machinery of the intermembrane space, and the sorting and assembly machinery of the outer membrane. Although the protein translocases can function as separate entities in vitro, recent studies revealed a close and dynamic cooperation of the protein import machineries to facilitate efficient transfer of precursor proteins in vivo. In addition, protein translocases were found to transiently interact with distinct machineries that function in the respiratory chain or in the maintenance of mitochondrial membrane architecture. Mitochondrial protein import is embedded in a regulatory network that ensures protein biogenesis, membrane dynamics, bioenergetic activity and quality control.

Mitochondrial Machineries for Protein Import and Assembly

Mitochondria are essential organelles with numerous functions in cellular metabolism and homeostasis. Most of the >1,000 different mitochondrial proteins are synthesized as precursors in the cytosol and are imported into mitochondria by five transport pathways. The protein import machineries of the mitochondrial membranes and aqueous compartments reveal a remarkable variability of mechanisms for protein recognition, translocation, and sorting. The protein translocases do not operate as separate entities but are connected to each other and to machineries with functions in energetics, membrane organization, and quality control. Here, we discuss the versatility and dynamic organization of the mitochondrial protein import machineries. Elucidating the molecular mechanisms of mitochondrial protein translocation is crucial for understanding the integration of protein translocases into a large network that controls organelle biogenesis, function, and dynamics.

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.

The non-canonical mitochondrial inner membrane presequence translocase of trypanosomatids contains two essential rhomboid-like proteins

Mitochondrial protein import is essential for all eukaryotes. Here we show that the early diverging eukaryote Trypanosoma brucei has a non-canonical inner membrane (IM) protein translocation machinery. Besides TbTim17, the single member of the Tim17/22/23 family in trypanosomes, the presequence translocase contains nine subunits that co-purify in reciprocal immunoprecipitations and with a presequence-containing substrate that is trapped in the translocation channel. Two of the newly discovered subunits are rhomboid-like proteins, which are essential for growth and mitochondrial protein import. Rhomboid-like proteins were proposed to form the protein translocation pore of the ER-associated degradation system, suggesting that they may contribute to pore formation in the presequence translocase of T. brucei. Pulldown of import-arrested mitochondrial carrier protein shows that the carrier translocase shares eight subunits with the presequence translocase. This indicates that T. brucei may have a single IM translocase that with compositional variations mediates import of presequence-containing and carrier proteins.

The mitochondrial protein import machinery is crucial for eukaryotes but little is known about its evolutionary origin. Here, the authors characterize the translocase of the inner membrane (TIM) in trypanosomes, showing that it contains two rhomboid-like proteins essential for protein import.

Targeting mitochondria: how intravacuolar bacterial pathogens manipulate mitochondria

Manipulation of host cell function by bacterial pathogens is paramount for successful invasion and creation of a niche conducive to bacterial replication. Mitochondria play a role in many important cellular processes including energy production, cellular calcium homeostasis, lipid metabolism, haeme biosynthesis, immune signalling and apoptosis. The sophisticated integration of host cell processes by the mitochondrion have seen it emerge as a key target during bacterial infection of human host cells. This review highlights the targeting and interaction of this dynamic organelle by intravacuolar bacterial pathogens and the way that the modulation of mitochondrial function might contribute to pathogenesis.

Unraveling the complexity of mitochondrial complex I assembly: A dynamic process

Mammalian complex I is composed of 44 different subunits and its assembly requires at least 13 specific assembly factors. Proper function of the mitochondrial respiratory chain enzyme is of crucial importance for cell survival due to its major participation in energy production and cell signaling. Complex I assembly depends on the coordination of several crucial processes that need to be tightly interconnected and orchestrated by a number of assembly factors. The understanding of complex I assembly evolved from simple sequential concept to the more sophisticated modular assembly model describing a convoluted process. According to this model, the different modules assemble independently and associate afterwards with each other to form the final enzyme. In this review, we aim to unravel the complexity of complex I assembly and provide the latest insights in this fundamental and fascinating process. This article is part of a Special Issue entitled Respiratory complex I, edited by Volker Zickermann and Ulrich Brandt.

Tim62, a Novel Mitochondrial Protein in Trypanosoma brucei, Is Essential for Assembly and Stability of the TbTim17 Protein Complex

Trypanosoma brucei, the causative agent of human African trypanosomiasis, possesses non-canonical mitochondrial protein import machinery. Previously, we characterized the essential translocase of the mitochondrial inner membrane (TIM) consisting of Tim17 in T. brucei. TbTim17 is associated with TbTim62. Here we show that TbTim62, a novel protein, is localized in the mitochondrial inner membrane, and its import into mitochondria depends on TbTim17. Knockdown (KD) of TbTim62 decreased the steady-state levels of TbTim17 post-transcriptionally. Further analysis showed that import of TbTim17 into mitochondria was not inhibited, but its half-life was reduced >4-fold due to TbTim62 KD. Blue-native gel electrophoresis revealed that TbTim62 is present primarily in ∼150-kDa and also in ∼1100-kDa protein complexes, whereas TbTim17 is present in multiple complexes within the range of ∼300 to ∼1100 kDa. TbTim62 KD reduced the levels of both TbTim62 as well as TbTim17 protein complexes. Interestingly, TbTim17 was accumulated as lower molecular mass complexes in TbTim62 KD mitochondria. Furthermore, depletion of TbTim62 hampered the assembly of the ectopically expressed TbTim17-2X-myc into TbTim17 protein complex. Co-immunoprecipitation analysis revealed that association of TbTim17 with mHSP70 was markedly reduced in TbTim62 KD mitochondria. All together our results demonstrate that TbTim62, a unique mitochondrial protein in T. brucei, is required for the formation of a stable TbTim17 protein complex. TbTim62 KD destabilizes this complex, and unassembled TbTim17 degrades. Therefore, TbTim62 acts as a novel regulatory factor to maintain the levels of TIM in T. brucei mitochondria.

Translocase of inner mitochondrial membrane 44 alters the mitochondrial fusion and fission dynamics and protects from type 2 diabetes

OBJECTIVE

In obesity and type 2 diabetes, the impairment of mitochondrial function in white adipose tissue (WAT) is linked to a reduction in whole body insulin sensitivity. Timm44 is upregulated in the kidneys of streptozotocin-induced diabetic mice. In the inner mitochondrial membrane, Timm44 anchors mitochondrial heat-shock protein 70 (mtHsp70) to the translocase of inner mitochondrial membrane 23 (TIM23) complex and facilitates the import of mitochondria-targeted preproteins into the mitochondrial matrix dependent on the inner membrane potential and ATP hydrolysis on ATPase domain of mtHsp70.

METHODS

We generated the aP2-promoter driven Timm44 transgenic (Tg) mouse model and investigated whether Timm44 Tg mice fed high-fat/high-sucrose (HFHS) chow are protected from type 2 diabetes and obesity.

RESULTS

The body weight of aP2-promoter driven Timm44 Tg mice was lower than that of wild type mice, and insulin sensitivity was greater in Timm44 Tg mice than in wild type mice. Although WAT weight was not altered in Timm44 Tg mice fed HFHS chow, adipocyte size was reduced, and mitochondrial fusion associated with decreased expression of fission genes, such as Dnm1l and Fis1, was observed. In addition, when fed standard (STD) chow, the expressions of the fusion genes Opa1, Mfn1 and Mfn2, and Mfn1 were significantly increased in Timm44 Tg mice compared to wild type mice, and fused mitochondria were also observed in Timm44 Tg mice fed STD chow.

CONCLUSIONS

The Timm44 gene may be a new target for the treatment of type 2 diabetes.

Role of membrane contact sites in protein import into mitochondria

Mitochondria import more than 1,000 different proteins from the cytosol. The proteins are synthesized as precursors on cytosolic ribosomes and are translocated by protein transport machineries of the mitochondrial membranes. Five main pathways for protein import into mitochondria have been identified. Most pathways use the translocase of the outer mitochondrial membrane (TOM) as the entry gate into mitochondria. Depending on specific signals contained in the precursors, the proteins are subsequently transferred to different intramitochondrial translocases. In this article, we discuss the connection between protein import and mitochondrial membrane architecture. Mitochondria possess two membranes. It is a long-standing question how contact sites between outer and inner membranes are formed and which role the contact sites play in the translocation of precursor proteins. A major translocation contact site is formed between the TOM complex and the presequence translocase of the inner membrane (TIM23 complex), promoting transfer of presequence-carrying preproteins to the mitochondrial inner membrane and matrix. Recent findings led to the identification of contact sites that involve the mitochondrial contact site and cristae organizing system (MICOS) of the inner membrane. MICOS plays a dual role. It is crucial for maintaining the inner membrane cristae architecture and forms contacts sites to the outer membrane that promote translocation of precursor proteins into the intermembrane space and outer membrane of mitochondria. The view is emerging that the mitochondrial protein translocases do not function as independent units, but are embedded in a network of interactions with machineries that control mitochondrial activity and architecture.

Human mitochondrial COX1 assembly into cytochrome c oxidase at a glance

Mitochondria provide the main portion of cellular energy in form of ATP produced by the F1Fo ATP synthase, which uses the electrochemical gradient, generated by the mitochondrial respiratory chain (MRC). In human mitochondria, the MRC is composed of four multisubunit enzyme complexes, with the cytochrome c oxidase (COX, also known as complex IV) as the terminal enzyme. COX comprises 14 structural subunits, of nuclear or mitochondrial origin. Hence, mitochondria are faced with the predicament of organizing and controlling COX assembly with subunits that are synthesized by different translation machineries and that reach the inner membrane by alternative transport routes. An increasing number of COX assembly factors have been identified in recent years. Interestingly, mutations in several of these factors have been associated with human disorders leading to COX deficiency. Recently, studies have provided mechanistic insights into crosstalk between assembly intermediates, import processes and the synthesis of COX subunits in mitochondria, thus linking conceptually separated functions. This Cell Science at a Glance article and the accompanying poster will focus on COX assembly and discuss recent discoveries in the field, the molecular functions of known factors, as well as new players and control mechanisms. Furthermore, these findings will be discussed in the context of human COX-related disorders.

Assembly of β-barrel proteins in the mitochondrial outer membrane

Mitochondria evolved through endosymbiosis of a Gram-negative progenitor with a host cell to generate eukaryotes. Therefore, the outer membrane of mitochondria and Gram-negative bacteria contain pore proteins with β-barrel topology. After synthesis in the cytosol, β-barrel precursor proteins are first transported into the mitochondrial intermembrane space. Folding and membrane integration of β-barrel proteins depend on the mitochondrial sorting and assembly machinery (SAM) located in the outer membrane, which is related to the β-barrel assembly machinery (BAM) in bacteria. The SAM complex recognizes β-barrel proteins by a β-signal in the C-terminal β-strand that is required to initiate β-barrel protein insertion into the outer membrane. In addition, the SAM complex is crucial to form membrane contacts with the inner mitochondrial membrane by interacting with the mitochondrial contact site and cristae organizing system (MICOS) and shares a subunit with the endoplasmic reticulum-mitochondria encounter structure (ERMES) that links the outer mitochondrial membrane to the endoplasmic reticulum (ER).

Architecture of the TIM23 inner mitochondrial translocon and interactions with the matrix import motor

Translocation of proteins from the cytosol across the mitochondrial inner membrane is driven by action of the matrix-localized multi-subunit import motor, which is associated with the TIM23 translocon. The architecture of the import apparatus is not well understood. Here, we report results of site-specific in vivo photocross-linking along with genetic and coimmunoprecipitation analyses dissecting interactions between import motor subunits and the translocon. The translocon is composed of the two integral membrane proteins Tim23 and Tim17, each containing four membrane-spanning segments. We found that Tim23 having a photoactivatable cross-linker in the matrix exposed loop between transmembrane domains 1 and 2 (loop 1) cross-linked to Tim44. Alterations in this loop destabilized interaction of Tim44 with the translocon. Analogously, Tim17 having a photoactivatable cross-linker in the matrix exposed loop between transmembrane segments 1 and 2 (loop 1) cross-linked to Pam17. Alterations in this loop caused destabilization of the interaction of Pam17 with the translocon. Substitution of individual photoactivatable residues in Tim44 and Pam17 in regions we previously identified as important for translocon association resulted in cross-linking to Tim23 and Tim17, respectively. Our results are consistent with a model in which motor association is achieved via interaction of Tim23 with Tim44, which serves as a scaffold for association of other motor components, and of Tim17 with Pam17. As both Tim44 and Pam17 have been implicated as regulatory subunits of the motor, this positioning is conducive for responding to conformational changes in the translocon upon a translocating polypeptide entering the channel.

The mitochondrial ADP/ATP carrier associates with the inner membrane presequence translocase in a stoichiometric manner

The majority of mitochondrial proteins are synthesized with amino-terminal signal sequences. The presequence translocase of the inner membrane (TIM23 complex) mediates the import of these preproteins. The essential TIM23 core complex closely cooperates with partner protein complexes like the presequence translocase-associated import motor and the respiratory chain. The inner mitochondrial membrane also contains a large number of metabolite carriers, but their association with preprotein translocases has been controversial. We performed a comprehensive analysis of the TIM23 interactome based on stable isotope labeling with amino acids in cell culture. Subsequent biochemical studies on identified partner proteins showed that the mitochondrial ADP/ATP carrier associates with the membrane-embedded core of the TIM23 complex in a stoichiometric manner, revealing an unexpected connection of mitochondrial protein biogenesis to metabolite transport. Our data indicate that direct TIM23-AAC coupling may support preprotein import into mitochondria when respiratory activity is low.

Remodelling of the active presequence translocase drives motor-dependent mitochondrial protein translocation

Proteins with N-terminal targeting signals are transported across the inner mitochondrial membrane by the presequence translocase. To drive precursor translocation, the Hsp70-import motor associates with the protein-conducting channel of the TIM23 complex. It is unknown how the ATPase cycle of Hsp70 is regulated in the context of a translocating polypeptide chain. Here we establish an assay to monitor protein dynamics in the precursor-occupied presequence translocase and find that regulatory subunits of the import motor, such as the ATPase-stimulating J-protein Pam18, are recruited into the translocation intermediate. The presence of all Hsp70 co-chaperones at the import channel is not sufficient to promote matrix protein import, instead a recharging of the active translocase with Pam18 is required for motor activity. Thus, a replenishment cycle of co-chaperones at the TIM23 complex is an integral part of Hsp70's ATPase cycle at the channel exit site and essential to maintain motor-driven mitochondrial protein import.

DNAJC19, a mitochondrial cochaperone associated with cardiomyopathy, forms a complex with prohibitins to regulate cardiolipin remodeling

Prohibitins form large protein and lipid scaffolds in the inner membrane of mitochondria that are required for mitochondrial morphogenesis, neuronal survival, and normal lifespan. Here, we have defined the interactome of PHB2 in mitochondria and identified DNAJC19, mutated in dilated cardiomyopathy with ataxia, as binding partner of PHB complexes. We observed impaired cell growth, defective cristae morphogenesis, and similar transcriptional responses in the absence of either DNAJC19 or PHB2. The loss of PHB/DNAJC19 complexes affects cardiolipin acylation and leads to the accumulation of cardiolipin species with altered acyl chains. Similar defects occur in cells lacking the transacylase tafazzin, which is mutated in Barth syndrome. Our experiments suggest that PHB/DNAJC19 membrane domains regulate cardiolipin remodeling by tafazzin and explain similar clinical symptoms in two inherited cardiomyopathies by an impaired cardiolipin metabolism in mitochondrial membranes.

Unraveling the intricate organization of mammalian mitochondrial presequence translocases: existence of multiple translocases for maintenance of mitochondrial function

Mitochondria are indispensable organelles implicated in multiple aspects of cellular processes, including tumorigenesis. Heat shock proteins play a critical regulatory role in accurately delivering the nucleus-encoded proteins through membrane-bound presequence translocase (Tim23 complex) machinery. Although altered expression of mammalian presequence translocase components had been previously associated with malignant phenotypes, the overall organization of Tim23 complexes is still unsolved. In this report, we show the existence of three distinct Tim23 complexes, namely, B1, B2, and A, involved in the maintenance of normal mitochondrial function. Our data highlight the importance of Magmas as a regulator of translocase function and in dynamically recruiting the J-proteins DnaJC19 and DnaJC15 to individual translocases. The basic housekeeping function involves translocases B1 and B2 composed of Tim17b isoforms along with DnaJC19, whereas translocase A is nonessential and has a central role in oncogenesis. Translocase B, having a normal import rate, is essential for constitutive mitochondrial functions such as maintenance of electron transport chain complex activity, organellar morphology, iron-sulfur cluster protein biogenesis, and mitochondrial DNA. In contrast, translocase A, though dispensable for housekeeping functions with a comparatively lower import rate, plays a specific role in translocating oncoproteins lacking presequence, leading to reprogrammed mitochondrial functions and hence establishing a possible link between the TIM23 complex and tumorigenicity.

The protein import machinery of mitochondria-a regulatory hub in metabolism, stress, and disease

Mitochondria fulfill central functions in bioenergetics, metabolism, and apoptosis. They import more than 1,000 different proteins from the cytosol. It had been assumed that the protein import machinery is constitutively active and not subject to detailed regulation. However, recent studies indicate that mitochondrial protein import is regulated at multiple levels connected to cellular metabolism, signaling, stress, and pathogenesis of diseases. Here, we discuss the molecular mechanisms of import regulation and their implications for mitochondrial homeostasis. The protein import activity can function as a sensor of mitochondrial fitness and provides a direct means of regulating biogenesis, composition, and turnover of the organelle.

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.

Magmas overexpression inhibits staurosporine induced apoptosis in rat pituitary adenoma cell lines

Magmas is a nuclear gene that encodes for the mitochondrial import inner membrane translocase subunit Tim16. Magmas is overexpressed in the majority of human pituitary adenomas and in a mouse ACTH-secreting pituitary adenoma cell line. Here we report that Magmas is highly expressed in two out of four rat pituitary adenoma cell lines and its expression levels inversely correlate to the extent of cellular response to staurosporine in terms of apoptosis activation and cell viability. Magmas over-expression in rat GH/PRL-secreting pituitary adenoma GH4C1 cells leads to an increase in cell viability and to a reduction in staurosporine-induced apoptosis and DNA fragmentation, in parallel with the increase in Magmas protein expression. These results indicate that Magmas plays a pivotal role in response to pro-apoptotic stimuli and confirm and extend the finding that Magmas protects pituitary cells from staurosporine-induced apoptosis, suggesting its possible involvement in pituitary adenoma development.

A complex of Cox4 and mitochondrial Hsp70 plays an important role in the assembly of the cytochrome c oxidase

The biogenesis of Cox4 is unknown. Cox4, mtHsp70, and Mge1 form a complex that promotes the assembly of cytochrome c oxidase. In the absence of the mature cytochrome c oxidase, Cox4 arrests at the chaperone complex. This complex delivers Cox4 into the assembly line of complex IV when needed.

The formation of the mature cytochrome c oxidase (complex IV) involves the association of nuclear- and mitochondria-encoded subunits. The assembly of nuclear-encoded subunits like cytochrome c oxidase subunit 4 (Cox4) into the mature complex is poorly understood. Cox4 is crucial for the stability of complex IV. To find specific biogenesis factors, we analyze interaction partners of Cox4 by affinity purification and mass spectroscopy. Surprisingly, we identify a complex of Cox4, the mitochondrial Hsp70 (mtHsp70), and its nucleotide-exchange factor mitochondrial GrpE (Mge1). We generate a yeast mutant of mtHsp70 specifically impaired in the formation of this novel mtHsp70-Mge1-Cox4 complex. Strikingly, the assembly of Cox4 is strongly decreased in these mutant mitochondria. Because Cox4 is a key factor for the biogenesis of complex IV, we conclude that the mtHsp70-Mge1-Cox4 complex plays an important role in the formation of cytochrome c oxidase. Cox4 arrests at this chaperone complex in the absence of mature complex IV. Thus the mtHsp70-Cox4 complex likely serves as a novel delivery system to channel Cox4 into the assembly line when needed.